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THE HARVARD CLASSICS

The Five-Foot Shelf of Books

THE HARVARD CLASSICS
EDITED BY CHARLES W. ELIOT, LL.D.

The Origin of Species

By Charles Darwin

W//A Introductions and ^otes
Wo/ume II

P. F. Collier & Son Corporation

NEW YORK

Copyright, 1909
By p. F. Collier & Son

HANUTACTUKBD IN U. S. A.

"But with regard to the material world, we can at least go so
far as this — we can perceive that events are brought about not
by insulated interpositions of Divine power, exerted in each
particular case, but by the establishment of general laws."

Whewell: Bridgewater Treatise.

"The only distinct meaning of the word 'natural* is stated,
fixed, or settled; since what is natural as much requires and pre-
supposes an intelligent agent to render it so, i.e., to effect it
continually or at stated times, as what is supernatural or miracu-
lous does to effect it for once."

Butler: Analogy of Revealed Religion.

"To conclude, therefore, let no man out of a weak conceit of
sobriety, or an ill-applied moderation, think or maintain, that
a man can search too far or be too well studied in the book of
God's word, or in the book of God's works; divinity or philoso-
phy; but rather let men endeavour an endless progress or pro-
ficience in both."

Bacon: Advancement of Learning.

Down, Bec\enham, Kent,

First Edition, November 24th, l8§g.
Sixth Edition, January, i8'/2.

CONTENTS

PACE

EDITOR'S INTRODUCTION 5

AN HISTORICAL SKETCH

Of the Progress of Opinion on the Origin of Species .... 9

INTRODUCTION 19

CHAPTER I
Variation under Domestication 23

CHAPTER II
Variation under Nature 54

CHAPTER III
Struggle for Existence 71

CHAPTER IV
Natural Selection; or the Survival of the Fittest 87

CHAPTER V
Laws of Variation 138

CHAPTER VI
Difficulties op the Theory 169

CHAPTER VII
Miscellaneous Objections to the Theory of Natural Selection 209

CHAPTER VIII
Instinct 251

CHAPTER IX
Hybridism 285

4 coNTE^^^s

CHAPTER X
On the Imperfection of the Geological Record 319

CHAPTER XI
On the Geological Succession of Organic Beings 349

CHAPTER XII
Geographical Distribution 378

CHAPTER XIII
Geographical Distribution — continued 409

CHAPTER XIV

Mutual Affinities of Organic Beings: Morphology — Embry-
ology — Rudimentary Organs 431

CHAPTER XV
Recapitulation and Conclusion 478

Glossary 507

Index 517

INTRODUCTORY NOTE

Charles Robert Darwin, born at Shrewsbury, England, on February
12, 1809, came of a family of remarkable intellectual distinction which
is still sustained in the present generation. His father was a successful
physician with remarkable powers of observation, and his grandfather
was Erasmus Darwin, the well-known author of "The Botanic Garden."
He went to school at Shrewsbury, where he failed to profit from the
stria classical curriculum there in force; nor did the regular professional
courses at Edinburgh University, where he spent two years studying
medicine, succeed in rousing his interest. In 1827 he was entered at
Christ's College, Cambridge, to study for the B. A. degree, preparatory
to entering the Church; but while there his friendship with Henslow,
the professor of botany, led to his enlarging his general scientific knowl-
edge and finally to his joining the expedition of the "Beagle" in the
capacity of naturalist. From this Darwin returned after a voyage of five
years with a vast first-hand knowledge of geology and zoology, a reputa-
tion as a successful collector, and, most important of all, with the ger-
minal ideas of his theory of evolution. The next few years were spent in
working up the materials he had collected; but his health gave signs of
breaking, and for the rest of his life he suffered constandy, but without
complaint. With extraordinary courage and endurance he took up a life
of seclusion and methodical regularity, and accomplished his colossal
results in spite of the most severe physical handicap. He had married in
1839, and three years later he withdrew from London to the litde village
of Down, about sixteen miles out, where he spent the rest of his life. His
custom, which was almost a method, was to work till he was on the verge
of complete collapse, and then to take a holiday just sufficient to restore
him to working condition.

As early as 1842 Darwin had thrown into rough form the outlines of
his theory of evolution, but the enormous extent of the investigations he
engaged in for the purpose of testing it led to a constant postponing of
publication. Finally in June, 1858, A. R. Wallace sent him a manuscript
containing a statement of an identical theory of the origin of species,
which had been arrived at entirely independently. On the advice of Lyell,
the geologist, and Hooker, the botanist, Wallace's paf)er and a letter of
Darwin's of the previous year, in which he had outlined his theory to
Asa Gray, were read together on July i, 1858, and published by the
Linnzan Society. In November of the following year "The Origin of

b INTRODUCTORY NOTE

Species" was published, and the great battle was begun between the old
science and the new. This work was followed in 1868 by his "Variation
of Animals and Plants under Domestication," that in turn by the
"Descent of Man" in 1871, and that again by "The Expression of the
Emotions in Man and Animals." Each of these books was the elaboration
or complement of a section of its predecessor. The later years of Darwin's
life were chiefly devoted to botanical research, and resulted in a series
of treatises of the highest scientific value. He died at Down on April 19,
1882, and is buried in Westminster Abbey.

The idea of the evolution of organisms, so far from originating with
Darwin, is a very old one. Glimpses of it appear in the ancient Greek
philosophers, especially Empedocles and Aristotle; modern philosophy
from Bacon onward shows an increasing definiteness in its grasp of the
conception; and in the age preceding Darwin's, BufFon, Erasmus Darwin,
and Lamarck had given it a fairly concrete expression. As we approach
the date of the publication of "The Origin of Species" adherence to the
doctrine not only by naturalists but by poets, such as Goethe, becomes
comparatively frequent; and in the six years before the joint announce-
ment of Darwin and Wallace, Herbert Spencer had been supporting and
applying it vigorously in the field of psychology.

To these partial anticipations, however, Darwin owed little. When he
became interested in the problem, the doctrine of the fixity of sf)ecies
was still generally held; and his solution occurred to him mainly as the
result of his own observation and thinking. Speaking of the voyage of
the "Beagle," he says, "On my return home in the autumn of 1836 I
immediately began to prepare my journal for publication, and then saw
how many facts indicated the common descent of species. ... In July
(1837) I opened my first note-book for facts in relation to the Origin of
Species, about which I had long reflected, and never ceased working for
the next twenty years. . . . Had been greatly struck from about the
month of previous March on character of South American fossils, and
sjjecies on Galapagos ArchijDelago. These facts (especially latter) origin
of all my views." Again, "In October (1838), that is fifteen months after
I had begun my systematic inquiry, I happened to read for amusement
'Malthus on Population,' and being well prepared to appreciate the strug-
gle for existence which everywhere goes on from long-continued observa-
tion of the habits of animals and plants, it at once struck me that under
these circumstances favorable variations would tend to be preserved, and
unfavorable ones to be destroyed. The result of this would be the forma-
tion of new species. Here then I had at last got a theory by which to
work."

INTRODUCTORY NOTE 7

From these statements by Darwin himself we can see how far it is
from being the case that he merely gathered the ripe fruit of the labors
of his predecessors. All progress is continuous, and Darwin, like other
men, built on the foundations laid by others; but to say this is not to
deny him originality in the only vital sense of that word. And the impor-
tance of his contribution — in verifying the doctrine of descent, in inter-
preting and applying it, and in revealing its bearings on all departments
of the investigation of nature — is proved by the fact that his work opened
a new epoch in science and philosophy. As Huxley said, "Whatever be
the ultimate verdict of posterity upon this or that opinion which Mr.
Darwin has propounded; whatever adumbrations or anticipations of his
doctrines may be found in the writings of his predecessors; the broad
fact remains that, since the publication and by reason of the publication
of 'The Origin of Species' the fundamental conceptions and the aims of
the students of living Nature have been completely changed."

The present year (1909) has seen the celebration of the hundredth
anniversary of Darwin's birth and the fiftieth anniversary of the publica-
tion of his great work. Among the numerous expressions of honor and
gratitude which the world of science has poured upwn his memory, none
is more significant than the volume on "Darwin and Modern Science"
which has been issued by the press of his old University of Cambridge.
In this are collected nearly thirty papers by the leaders of modern science
dealing with the influence of Darwin upon various fields of thought and
research, and with the later developments and modifications of his con-
clusions. Biology, in many different departments. Anthropology, Geology,
Psychology, Philosophy, Sociology, Religion, Language, History, and
Astronomy are all represented, and the mere enumeration suggests the
colossal nature of his achievement and its results.

Yet the spirit of the man was almost as wonderful as his work. His
disinterestedness, his modesty, and his absolute fairness were not only
beautiful in themselves, but remain as a proof of the imjxirtance of
character in intellectual labor. Here is his own frank and candid sum-
ming up of his abilities: "My success as a man of science, whatever this
may have amounted to, has been determined, as far as I can judge, by
complex and diversified mental qualities and conditions. Of these, the
most important have been — the love of science — unbounded patience in
long reflecting over any subject — industry in observing and collecting
facts — and a fair share of invention as well as of common sense. With
such moderate abilities as I possess, it is truly surprising that I should
have influenced to a considerable extent the belief of scientific men on
some imfx)rtant pwints."

AN HISTORICAL SKETCH

OF THE PROGRESS OF OPINION ON
THE ORIGIN OF SPECIES,

PREVIOUSLY TO THE PUBLICATION OF
THE FIRST EDITION OF THIS WORK.

I WILL here give a brief sketch of the progress of opinion on the Origin
of Species. Until recently the great majority of naturalists believed that
sf>ecies were immutable productions, and had been separately created.
This view has been ably maintained by many authors. Some few natural-
ists, on the other hand, have believed that species undergo modification,
and that the existing forms of life are the descendants by true generation
of pre-existing forms. Passing over allusions to the subject in the classical
writers,' the first author who in modern times has treated it in a scientific
spirit was Buffon. But as his opinions fluctuated gready at different
{jeriods, and as he does not enter on the causes or means of the trans-
formation of species, I need not here enter on details.

Lamarck was the first man whose conclusions on the subject excited
much attention. This jusdy-celebrated naturalist first published his views
in 1801; he much enlarged them in 1809 in his 'Philosophie Zoologique,'

' Aristotle, in his 'Physicz Auscultationes' (lib. 2, cap. 8, s. 2), after remarking
that rain does not fall in order to make the corn grow, any more than it falls
to spoil the farmer's corn when threshed out of doors, applies the same argument
to organisation; and adds (as translated by Mr. Clair Grece, who first pointed out
the passage to me), "So what hinders the different parts [of the body] from having
this merely accidental relation in nature? as the teeth, for example, grow by necessity,
the front ones sharp, adapted for dividing, and the grinders flat, and serviceable for
masticating the food; since they were not made for the sake of this, but it was the
result of accident. And in like manner as to the other parts in which there appears
to exist an adaptation to an end. Wheresoever, therefore, all things together (that
is, all the parts of one whole) happened like as if they were made for the sake
of something, these were preserved, having been appropriately constituted by an
internal spontaneity; and whatsoever things were not thus constituted, perished, and
still perish." We here see the principle of natural selection shadowed forth, but how
little Aristotle fully comprehended the principle, is shown by his remarks on the
formation of the teeth.

10 HISTORICAL SKETCH

and subsequently, in 1815, in the Introduction to his 'Hist. Nat. des
Animaux sans Vertebres.' In tliese works he upholds the doctrine that
species, including man, are descended from other species. He first did
the eminent service of arousing attention to the probability of all change
in the organic, as well as in the inorganic world, being the result of law,
and not of miraculous interposition. Lamarck seems to have been chiefly
led to his conclusion on the gradual change of species, by the difficulty of
distinguishing species and varieties, by the almost perfect gradation of
forms in certain groups, and by the analogy of domestic productions.
With respect to the means of modification, he attributed something to
the direct action of the physical conditions of life, something to the
crossing of already existing forms, and much to use and disuse, that is,
to the effects of habit. To this latter agency he seems to attribute all the
beautiful adaptations in nature; such as the long neck of the giraffe for
browsing on the branches of trees. But he likewise believed in a law
of progressive development; and as all the forms of life thus tend to
progress, in order to account for the existence at the present day of simple
productions, he maintains that such forms are now spontaneously gener-
ated.*

Geoffroy Saint-Hilaire, as is stated in his 'Life,' written by his son,
suspected, as early as 1795, that what we call sf>ecies are various degenera-
tions of the same type. It was not until 1828 that he published his con-
viction that the same forms have not been perpetuated since the origin
of all things. Geoffroy seems to have relied chiefly on the condition of
life, or the "tnonde ambiant," as the cause of change. He was cautious
in drawing conclusions, and did not believe that existing species are now
undergoing modification; and, as his son adds, "C'est done un probleme
a r^server entierement a I'avenir, suppose meme que I'avenir doive avoir
prise sur lui."

' I have taken the date of the first publication of Lamarck from Isidore Geoffroy
Saint-Hilaire's ('Hist. Nat. G^n^ralc,' torn. iL p. 405, 1859) excellent history of
opinion on this subject. In this work a full account is foven of BufTon's conclusions
on the same subject. It b curious how largely my grandfather. Dr. Erasmus Darwin,
anticipated the views and erroneous grounds of opinion of Lamarck in his 'Zoonomia'
(vol. i. pp. 500-510), published in 1794. According to Isid. Geoffroy there is no
doubt that Goethe was an extreme partisan of similar views, as shown in the
Introduction to a work written in 1794 and 1795, but not published till long
afterwards; he has pointedly remarked ('Goethe als Naturforscher," von Dr. Karl
Meding, s. 34) that the future question for naturalists will be how, for instance,
cattle got their horns, and not for what they are used. It is rather a singular instance
of the manner in which similar views arise at about the same time, that Goethe in
Germany, Dr. Darwin in England, and Geoffroy Saint-Hilaire (as we shall immediately
see) in France, came to the same conclusion on the origin of species, in the yeari
'794-5-

HISTORICAL SKETCH 11

In 1 813, Dr. W. C. Wells read before the Royal Society 'An Account of
a White Female, part of whose skin resembles that of a Negro'; but his
paper was not published until his famous 'Two Essays upon Dew and
Single Vision' appeared in 1818. In this paper he distinctly recognises the
principle of natural selection, and this is the first recognition which has
been indicated; but he applies it only to the races of man, and to certain
characters alone. After remarking that negroes and mulattoes enjoy an
immunity from certain tropical diseases, he observes, firstly, that all ani-
mals tend to vary in some degree, and, secondly, that agriculturists im-
prove their domesticated animals by selection; and then, he adds, but
what is done in this latter case "by art, seems to be done with equal
efficacy, though more slowly, by nature, in the formation of varieties of
mankind, fitted for the country which they inhabit. Of the accidental
varieties of man, which would occur among the first few and scattered
inhabitants of the middle regions of Africa, some one would be better
fitted than the others to bear the diseases of the country. This race would
consequently multiply, while the others would decrease; not only from
their inability to sustain the attacks of disease, but from their incapacity
of contending with their more vigorous neighbours. The colour of this
vigorous race I take for granted, from what has been already said, would
be dark. But the same disposition to form varieties still existing, a darker
and a darker race would in the course of time occur: and as the darkest
would be the best fitted for the climate, this would at length become the
most prevalent, if not the only race, in the particular country in which
it had originated." He then extends these same views to the white
inhabitants of colder climates. I am indebted to Mr. Rowley, of the
United States, for having called my attention, through Mr. Brace, to the
above passage in Dr. Wells's work.

The Hon. and Rev. W. Herbert, afterwards Dean of Manchester, in
the fourth volume of the 'Horticultural Transactions,' 1822, and in his
work of the 'Amaryllidacea:' (1837, pp. 19, 339), declares that "horti-
cultural experiments have established, beyond the possibility of refuta-
tion, that botanical species are only a higher and more permanent class
of varieties." He extends the same view to animals. The Dean believes
that single species of each genus were created in an originally highly
plastic condition, and that these have produced, chiefly by intercrossing,
but likewise by variation, all our existing sf>ecies.

In 1826 Professor Grant, in the concluding paragraph in his well-
known paper ('Edinburgh Philosophical Journal,' vol. xiv. p. 283) on the
Spongilla, clearly declares his belief that species are descended from other

12 HISTORICAL SKETCH

sjjecies, and that they become improved in the course of modification.
This same view was given in his Fifty-fifth Lecture, published in the
'Lancet' in 1834.

In 1 83 1 Mr. Patrick Matthew published his work on 'Naval Timber
and Arboriculture,' in which he gives precisely the same view on the
origin of species as that (presendy to be alluded to) propounded by Mr.
Wallace and myself in the 'Linnean Journal,' and as that enlarged in the
present volume. Unfortunately the view was given by Mr. Matthew very
briefly in scattered passages in an appendix to a work on a different sub-
ject, so that it remained unnoticed until Mr. Matthew himself drew
attention to it in the 'Gardener's Chronicle,' on April 7th, i860. The
differences of Mr. Matthew's view from mine are not of much impor-
tance: he seems to consider that the world was nearly depopulated at
successive periods, and then re-stocked; and he gives as an alternative,
that new forms may be generated "without the presence of any mould
or germ of former aggregates." I am not sure that I understand some
passages; but it seems that he attributes much influence to the direct
action of the conditions of life. He clearly saw, however, the full force
of the principle of natural selection.

The celebrated geologist and naturalist. Von Buch, in his excellent
'Description Physique des Isles Canaries' (1836, p. 147), clearly expresses
his belief that varieties slowly become changed into permanent species,
which are no longer capable of intercrossing.

Rafinesque, in his 'New Flora of North America,' published in 1836,
wrote (p. 6) as follows: — "All species might have been varieties once, and
many varieties are gradually becoming species by assuming constant and
peculiar characters"; but farther on (p. 18) he adds, "except the original
types or ancestors of the genus."

In 1843-44 Professor Haldeman ('Boston Journal of Nat. Hist. U.
States,' vol. iv. p. 468) has ably given the arguments for and against the
hypothesis of the development and modification of species: he seems to
lean towards the side of change.

The 'Vestiges of Creation' appeared in 1844. In the tenth and much
improved edition (1853) the anonymous author says (p. 155): — "The
proposition determined on after much consideration is, that the several
series of animated beings, from the simplest and oldest up to the highest
and most recent, are, under the providence of God, the results, first, of an
impulse which has been imparted to the forms of life, advancing them,
in definite times, by generation, through grades of organisation terminat-
ing in the highest dicotyledons and vertebrata, these grades being few in

HISTORICAL SKETCH I3

number, and generally marked by intervals of organic character, which
we find to be a practical difficulty in ascertaining affinities; second, of
another impulse connected with the vital forces, tending, in the course of
generations, to modify organic structures in accordance with external
circumstances, as food, the nature of the habitat, and the meteoric
agencies, these being the 'adaptations' of the natural theologian." The
author apparently believes that organisation progresses by sudden leaps,
but that the effects produced by the conditions of life are gradual. He
argues with much force on general grounds that species are not immu-
table productions. But I cannot see how the two supposed "impulses"
account in a scientific sense for the numerous and beautiful co-adaptations
which we see throughout nature; I cannot see that we thus gain any
insight how, for instance, a woodpecker has become adapted to its peculiar
'habits of life. The work, from its powerful and brilliant style, though
displaying in the earlier editions little accurate knowledge and a great
want of scientific caution, immediately had a very wide circulation. In
my opinion it has done excellent service in this country in calling attention
to the subject, in removing prejudice, and in thus preparing the ground
for the reception of analogous views.

In 1846 the veteran geologist M. J. d'Omalius d'Halloy published in an
excellent though short paper ('Bulletins de I'Acad. Roy. Bruxelles,' tom.
xiii. p. 581) his opinion that it is more probable that new species have
been produced by descent with modification than that they have been
separately created: the author first promulgated this opinion in 1831.

Professor Owen, in 1849 ('Nature of Limbs,' p. 86), wrote as follows:
"The archetypal idea was manifested in the flesh under diverse such
modifications, upon this planet, long prior to the existence of those
animal species that actually exemplify it. To what natural laws or
secondary causes the orderly succession and progression of such organic
phenomena may have been committed, we, as yet, are ignorant." In his
address to the British Association, in 1858, he speaks (p. li.) of "the
axiom of the continuous operation of creative power, or of the ordained
becoming of living things." Farther on (p. xc), after referring to
geographical distribution, he adds, "These phenomena shake our confi-
dence in the conclusion that the Apteryx of New Zealand and the Red
Grouse of England were distinct creations in and for those islands
respectively. Always, also, it may be well to bear in mind that by the
word 'creation' the zoologist means 'a process he knows not what.' " He
amplifies this idea by adding that when such cases as that of the Red
Grouse are "enumerated by the zoologist as evidence of distinct creation

14 HISTORICAL SKETCH

of the bird in and for such islands, he chiefly expresses that he knows not
how the Red Grouse came to be there, and there exclusively; signifying
also, by this mode of expressing such ignorance, his belief that both the
bird and the islands owed their origin to a great first Creative Cause."
If we interpret these sentences given in the same address, one by the
other, it appears that this eminent philosopher felt in 1858 his confidence
shaken that the Apteryx and the Red Grouse Brst appeared in their
respective homes, "he knew not how," or by some process "he knew not
what."

This address was delivered after the papers by Mr. Wallace and myself
on the Origin of Species, presently to be referred to, had been read
before the Linnean Society. When the first edition of this work was
published, I was so completely deceived, as were many others, by such
expressions as "the continuous operation of creative power," that I
included Professor Owen with other palxontologists as being firmly con-
vinced of the immutability of species; but it appears ('Anat. of Verte-
brates,' vol. iii. p. 796) that this was on my part a preposterous error.
In the last edition of this work I inferred, and the inference still seems
to me perfectly just, from a passage beginning with the words "no
doubt the type-form," etc. (Ibid., vol. i. p. xxxv.), that Professor Owen
admitted that natural selection may have done something in the forma-
tion of a new species; but this it app>ears (Ibid., vol. iii. p. 798) is inac-
curate and without evidence. I also gave some extraas from a corre-
spondence between Professor Owen and the Editor of the 'Lxmdon
Review,' from which it appeared manifest to the Editor as well as to
myself, that Professor Owen claimed to have promulgated the theory of
natural selection before I had done so; and I expressed my surprise and
satisfaction at this announcement; but as far as it is possible to under-
stand certain recently published passages (Ibid., vol. iii. p. 798) I have
cither partially or wholly again fallen into error. It is consolatory to me
that others find Professor Owen's controversial writings as difficult to
understand and to reconcile with each other, as I do. As far as the
mere enunciation of the principle of natural selection is concerned, it is
quite immaterial whether or not Professor Owen preceded me, for both
of us, as shown in this historical sketch, were long ago preceded by Dr.
Wells and Mr. Matthews.

M. Isidore Geoflroy Saint-Hilaire, in his lectures delivered in 1850
(of which a r^sum^ appeared in the 'Revue et Mag. de Zoolog.,' Jan.
1851), briefly gives his reason for believing that specific characters "sont
fix^s, pour chaque esp^, tant qu'elle se pierp^tue au milieu des memes

HISTORICAL SKETCH I5

circonstances: ils se tnodifaent, si les circonstances ambiantes viennent k
changer." "En resume, l' observation des animaux sauvages demontre
d^ja la variabilite limitee des especes. Les experiences sur les animaux
sauvages devenus domestiques, et sur les animaux domestiques redevenus
sauvages, la demontrent plus clairctnent encore. Ces memes experiences
prouvent, de plus, que les differences produites peuvent etre de vaUur
generiqtte." In his 'Hist. Nat. Generale' (tom ii. p. 340, 1859) he ampli-
fies analogous conclusions.

From a circular lately issued it appears that Dr. Freke, in 1851
('Dublin Medical Press,' p. 322), propounded the doctrine that all organic
beings have descended from one primordial form. His grounds of belief
and treatment of the subject are wholly different from mine; but as Dr.
Freke has now (1861) published his Essay on the 'Origin of Species by
means of Organic AfHnity,' the dif&cult attempt to give any idea of his
views would be superfluous on my part.

Mr. Herbert Spencer, in an essay (originally published in the 'Leader,'
March, 1852, and republished in his 'Essays,' in 1858), has contrasted the
theories of the Creation and the Development of organic beings with
remarkable skill and force. He argues from the analogy of domestic
productions, from the changes which the embryos of many species
undergo, from the difficulty of distinguishing species and varieties,
and from the principle of general gradation, that species have been
modified; and he attributes the modification to the change of circum-
stances. The author (1855) has also treated Psychology on the principle
of the necessary acquirement of each mental power and capacity by
gradation.

In 1852 M. Naudin, a distinguished botanist, expressly stated, in an
admirable paper on the Origin of S{x;cies ('Revue Horticole,' p. 102;
since pardy republished in the 'Nouvelles Archives du Museum,' tom. i,
p. 171), his belief that species are formed in an analogous manner as
varieties are under cultivation; and the latter process he attributes to
man's power of selection. But he does not show how selection acts under
nature. He believes, like Dean Herbert, that species, when nascent, were
more plastic than at present. He lays weight on what he calls the prin-
ciple of finality, "puissance mysteriuse, indetermin^; fatalitc pour les
uns; pour les autres, volontd providentielle, dont Taction incessante sur
les etres vivants determine, a toutes les epoques de I'existence du monde,
la forme, le volume, et la dur^e de chacun d'eux en raison de sa destin^
dans I'ordre de choses dont il fait partie. C'est cette puissance qui
harmonise chaque membra a I'ensemble, en I'appropriam a la fonction

l6 HISTORICAL SKETCH

qu'il doit remplir dans I'organisme generale de la nature, fonction qui
est pour lui sa raison d'etre." '

In 1853 a celebrated geologist, Count Keyserling ('Bulletin de la Soc.
Geolog.,' 2nd ser., torn. x. p. 357), suggested that as new diseases, sup-
posed to have been caused by some miasma, have arisen and spread
over the world, so at certain periods the germs of existing species may
have been chemically affected by circumambient molecules of a par-
ticular nature, and thus have given rise to new forms.

In this same year, 1853, Dr. Schaaffhausen published an excellent
pamphlet ('Verhand. des Naturhist. Vereins der Preuss. Rheinlands,'
etc.), in which he maintains the development of organic forms on the
earth. He infers that many species have kept true for long periods,
whereas a few have become modified. The distinction of species he
explains by the destruction of intermediate graduated forms. "Thus living
plants and animals are not separated from the extinct by new creations,
but are to be regarded as their descendants through continued repro-
duction."

A well-known French botanist, M. Lecoq, writes in 1854 ('Etudes sur
Geograph. Bot.,' torn. i. p. 250), "On voit que nos recherches sur la
fixit^ ou la variation de I'espece, nous conduisent directement aux id^s
^mises, par deux hommes justement celebres, Geoffroy Saint-Hilaire et
Goethe." Some other passages scattered through M. Lecoq's large work,
make it a little doubtful how far he extends his views on the modification
of species.

The 'Philosophy of Creation' has been treated in a masterly manner
by the Rev. Baden Powell, in his 'Essays on the Unity of Worlds,' 1855.
Nothing can be more striking than the manner in which he shows that
the introduction of new species is "a regular, not a casual phenomenon,"
or, as Sir John Herschel expresses it, "a natural in contradistinction to
a miraculous process."

The third volume of the ']o\itnal of the Linnean Society' conuins
papers, read July ist, 1858, by Mr. Wallace and myself, in which, as

' From references in Bronn's 'Untcrsuchungen iibcr die EntwickcIunRs-Gesctze,'
it appears that the celebrated botanist and palzontologist, Unger, published, in 1852,
his belief that species undergo development and modification. Dalton, likewise, in
Pander and Dalton's work on Fossil Sloths, expressed, in 1821, a similar belief.
Similar views have, as is well known, been maintained by Oken in his mystical
'Natur-Philosophie.' From other references in Godron's work 'Sur I'Esp^e," it scemi
that Bory St. Vincent, Burdach, Poiret, and Fries, have all admitted that new species
are continually being produced.

I may add, that of the thirty-four authors named in this Historical Sketch, who
believe in the modification of species, or at least disbelieve in separate acts of creation,
twenty-seven have written on special branches of natural history or geology.

HISTORICAL SKETCH 1 7

stated in the introductory remarks to this volume, the theory of Natural
Selection is promulgated by Mr. Wallace with admirable force and
clearness.

Von Baer, towards whom all zoologists feel so profound a respect,
expressed about the year 1859 (see Prof. Rudolph Wagner, 'Zoologisch-
Anthropologische Untersuchungen,' 1861, s. 51) his conviction, chiefly
grounded on the laws of geographical distribution, that forms now
perfectly distinct have descended from a single parent-form.

In June, 1859, Professor Huxley gave a lecture before the Royal Institu-
tion on the 'Persistent Types of Animal Life.' Referring to such cases,
he remarks, "It is difficult to comprehend the meaning of such facts as
these, if we suppose that each sjjecies of animal and plant, or each great
type of organisation, was formed and placed upon the surface of the
globe at long intervals by a distinct act of creative power; and it is well
to recollect that such an assumption is as unsupported by tradition or
revelation as it is oppjosed to the general analogy of nature. If, on the
other hand, we view 'Persistent Types' in relation to that hypothesis
which supposes the sjiecies living at any time to be the result of the
gradual modification of pre-existing species, a hypothesis, which, though
unproven, and sadly damaged by some of its supporters, is yet the only
one to which physiology lends any countenance; their existence would
seem to show that the amount of modification which living beings have
undergone during geological time is but very small in relation to the
whole series of changes which they have suffered."

In December, 1859, Dr. Hooker published his 'Introduction to the
Australian Flora.' In the first part of this great work he admits the
truth of the descent and modification of species, and supports this doc-
trine by many original observations.

The first edition of this work was published on November 24th, 1859,
and the second edition on January 7th, i860.

INTRODUCTION

When on board H.M.S. 'Beagle,' as naturalist, I was much struck with
certain facts in the distribution of the organic beings inhabiting South
America, and in the geological relations of the present to the past inhabi-
tants of that continent. These facts, as will be seen in the latter chapters
of this volume, seemed to throw some light on the origin of species —
that mystery of mysteries, as it has been called by one of our greatest
philosophers. On my return home, it occurred to me, in 1837, that some-
thing might f)erhaps be made out on this question by patiendy accumu-
lating and reflecting on all sorts of facts which could possibly have any
bearing on it. After five years' work I allowed myself to speculate on the
subject, and drew up some short notes; these I enlarged in 1844 into a
sketch of the conclusions, which then seemed to me probable; from that
period to the present day I have steadily pursued the same object. I hope
that I may be excused for entering on these personal details, as I give
them to show that I have not been hasty in coming to a decision.

My work is now (1859) nearly finished; but as it will take me many
more years to complete it, and as my health is far from strong, I have
been urged to publish this Abstract. I have more especially been induced
to do this, as Mr. Wallace, who is now studying the natural history of
the Malay Archipelago, has arrived at almost exactly the same general
conclusions that I have on the origin of species. In 1858 he sent me a
memoir on this subject, with a request that I would forward it to Sir
Charles Lyell, who sent it to the Linnean Society, and it is published in
the third volume of the Journal of that Society. Sir C. Lyell and Dr.
Hooker, who both knew of my work — the latter having read my sketch
of 1844 — honoured me by thinking it advisable to publish, with Mr.
Wallace's excellent memoir, some brief extracts from my manuscripts.

This Abstract, which I now publish, must necessarily be imperfect. I
cannot here give references and authorities for my several statements; and
I must trust to the reader reposing some confidence in my accuracy. No
doubt errors will have crept in, though I hope I have always been cautious
in trusting to good authorities alone. I can here give only the general
conclusions at which I have arrived, with a few facts in illustration, but
which, I hope, in most cases will suffice. No one can feel more sensible
than I do of the necessity of hereafter publishing in detail all the facts,

19

20 INTRODUCTION

with references, on which my conclusions have been grounded; and I
hope in a future work to do this. For I am well aware that scarcely a
single point is discussed in this volume on which facts cannot be adduced,
often apparendy leading to conclusions directly opposite to those at which
I have arrived. A fair result can be obtained only by fully stating and
balancing the facts and arguments on both sides of each question; and
this is here impossible.

I much regret that want of space prevents my having the satisfaction of
acknowledging the generous assistance which I have received from very
many naturalists, some of them personally unknown to me. I cannot,
however, let this opportunity pass without expressing my deep obliga-
tions to Dr. Hooker, who, for the last fifteen years, has aided me in
every possible way by his large stores of knowledge and his excellent
judgment.

In considering the origin of species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings, on their
embryological relations, their geographical distribution, geological succes-
sion, and other such facts, might come to the conclusion that species have
not been independently created, but had descended, like varieties, from
other species. Nevertheless, such a conclusion, even if well founded,
would be unsatisfactory, until it could be shown how the innumerable
species inhabiting this world have been modified, so as to acquire that
perfection of structure and coadaptation which jusdy excites our admira-
tion. Naturalists continually refer to external conditions, such as climate,
food, etc., as the only possible cause of variation. In one limited sense, as
we shall hereafter see, this may be true; but it is preposterous to attribute
to mere external conditions, the structure, for instance, of the wood-
pecker, with its feet, tail, beak, and tongue, so admirably adapted to
catch insects under the bark of trees. In the case of the mistletoe, which
draws its nourishment from certain trees, which has seeds that must be
transported by certain birds, and which has flowers with separate sexes
absolutely requiring the agency of certain insects to bring pollen from one
flower to the other, it is equally prepwsterous to account for the structure
of this parasite, with its relations to several distinct organic beings, by
the effects of external conditions, or of habit, or of the vdition of the
plant itself.

It is, therefore, of the highest importance to gain a clear insight into the
means of modification and coadaptation. At the commencement of my
observations it seemed to me probable that a careful study of domesticated
animals and of cultivated plants would offer the best chance of making
out this obscure problem. Nor have I been disappointed; in this and in

INTRODUCTION 21

all other perplexing cases I have invariably found that our knowledge,
imperfect though it be, of variation under domestication, afforded the
best and safest clue. I may venture to express my conviction of the high
value of such studies, although they have been very commonly neglected
by naturalists.

From these considerations, I shall devote the first chapter of this
Abstract to Variation under Domestication. We shall thus see that a
large amount of hereditary modification is at least possible; and, what is
equally or more important, we shall see how great is the power of man in
accumulating by his Selection successive slight variations. I will then pass
on the variability of species in a state of nature; but I shall, unfortunately,
be compelled to treat this subject far too briefly, as it can be treated
properly only by giving long catalogues of facts. We shall, however, be
enabled to discuss what circumstances are most favourable to variation.
In the next chapter the Struggle for Existence amongst all organic beings
throughout the world, which inevitably follows from the high geometri-
cal ratio of their increase, will be considered. This is the doctrine of
Malthus, applied to the whole animal and vegetable kingdoms. As many
more individuals of each species are born than can possibly survive; and
as, consequently, there is a frequently recurrent struggle for existence, it
follows that any being, if it vary however slightly in any manner profit-
able to itself, under the complex and sometimes varying conditions of
life, will have a better chance of surviving, and thus be naturally selected.
From the strong principle of inheritance, any selected variety will tend
to propagate its new and modified form.

This fundamental subject of Natural Selection will be treated at some
length in the fourth chapter; and we shall then see how Natural Selec-
tion almost inevitably causes much Extinction of the less improved forms
of life, and leads to what I have called Divergence of Character. In the
next chapter I shall discuss the complex and little known laws of varia-
tion. In the five succeeding chapters, the most apparent and gravest
difficulties in accepting the theory will be given: namely, first, the diffi-
culties of transitions, or how a simple being or a simple organ can be
changed and f)erfected into a highly developed being or into an elab-
orately constructed organ; secondly, the subject of Instinct, or the mental
f)owers of animals; thirdly. Hybridism, or the infertility of species and
the fertility of varieties when intercrossed; and fourthly, the imperfection
of the Geological Record. In the next chapter I shall consider the geologi-
cal succession of organic beings throughout time; in the twelfth and
thirteenth, their geographical distribution throughout space; in the four-
teenth, their classification or mutual affinities, both when mature and in

22 INTRODUCTION

an embryonic condition. In the last chapter I shall give a brief recapitu-
lation of the whole work, and a few concluding remarks.

No one ought to feel surprise at much remaining as yet unexplained
in regard to the origin of species and varieties, if he make due allowance
for our profound ignorance in regard to the mutual relations of the many
beings which live around us. Who can explain why one species ranges
widely and is very numerous, and why another allied species has a narrow
range and is rare? Yet these relations are of the highest importance, for
they determine the present welfare, and, as I believe, the future success
and modification of every inhabitant of this world. Still less do we know
of the mutual relations of the innumerable inhabitants of the world dur-
ing the many past geological epochs in its history. Although much
remains obscure, and will long remain obscure, I can entertain no doubt,
after the most deliberate study and dispassionate judgment of which I am
capable, that the view which most naturalists until recently entertained,
and which I formerly entertained — namely, that each species has been
independendy created — is erroneous. I am fully convinced that species
are not immutable; but that those belonging to what are called the same
genera are lineal descendants of some other and generally extinct species,
in the same manner as the acknowledged varieties of any one species are
the descendants of that species. Furthermore, I am convinced that
Natural Selection has been the most important, but not the exclusive,
means of modification.

ORIGIN OF SPECIES

CHAPTER I

Variation under Domestication

Causes of variability — Effects of habit and the use or disuse of parts —
Correlated variation — Inheritance — Character of domestic varieties
— Difficulty of distinguishing between varieties and species — Origin
of domestic varieties from one or more species — Domestic pigeons,
their differences and origin — Principles of selection, anciently fol-
lowed, their effects — Methodical and unconscious selection — Un-
known origin of our domestic productions — Circumstances fav-
ourable to man's power of selection.

CAUSES OF VARIABILITY

WHEN we compare the individuals of the same variety or
sub-variety of our older cultivated plants and animals, one
of the first points which strikes us is, that they generally dif-
fer more from each other than do the individuals of any one species
or variety in a state of nature. And if we reflect on the vast diversity
of the plants and animals which have been cultivated, and which have
varied during all ages under the most different climates and treat-
ment, we are driven to conclude that this great variability is due to
our domestic productions having been raised under conditions of life
not so uniform as, and somewhat different from, those to which the
parent species had been exposed under nature. There is, also, some
probability in the view propounded by Andrew Knight, that this
variability may be partly connected with excess of food. It seems
clear that organic beings must be exposed during several generations
to new conditions to cause any great amount of variation; and that,
when the organisation has once begun to vary, it generally continues
varying for many generations. No case is on record of a variable or-
ganism ceasing to vary under cultivation. Our oldest cultivated
plants, such as wheat, still yield new varieties: our oldest domesticated
animals are still capable of rapid improvement or modification.

»3

24 ORIGIN OF SPECIES

As far as I am able to judge, after long attending to the subject,
the conditions of life appear to act in two ways, — directly on the
whole organisation or on certain parts alone, and indirectly by affect-
ing the reproductive system. With respect to the direct action, we
must bear in mind that in every case, as Professor Weismann has
lately insisted, and as I have incidentally shown in my work on
'Variation under Domestication,' there are two factors: namely, the
nature of the organism, and the nature of the conditions. The former
seems to be much the more important; for nearly similar variations
sometimes arise under, as far as we can judge, dissimilar conditions;
and, on the other hand, dissimilar variations arise under conditions
which appear to be nearly uniform. The effects on the offspring are
either definite or indefinite. They may be considered as definite when
all or nearly all the offspring of individuals exposed to certain con-
ditions during several generations are modified in the same manner.
It is extremely difficult to come to any conclusion in regard to the
extent of the changes which have been thus definitely induced. There
can, however, be little doubt about many slight changes, such as size
from the amount of food, colour from the nature of the food, thick-
ness of the skin and hair from climate, etc. Each of the endless varia-
tions which we see in the plumage of our fowls must have had some
efficient cause; and if the same cause were to act uniformly during a
long series of generations on many individuals, all probably would be
modified in the same manner. Such facts as the complex and extra-
ordinary outgrowths which variably follow from the insertion of a
minute drop of poison by a gall-producing insect, show us what
singular modifications might result in the case of plants from a
chemical change in the nature of the sap.

Indefinite variability is a much more common result of changed
conditions than definite variability, and has probably played a more
important part in the formation of our domestic races. We see in-
definite variability in the endless slight peculiarities which distinguish
the individuals of the same species, and which cannot be accounted
for by inheritance from either parent or from some more remote an-
cestor. Even strongly marked differences occasionally appear in the
young of the same litter, and in seedlings from the same seed capsule.
At long intervals of time, out of millions of individuals reared in the

VARIATION UNDER DOMESTICATION 2$

same country and fed on nearly the same food, deviations of struc-
ture so strongly pronounced as to deserve to be called monstrosities
arise; but monstrosities cannot be separated by any distinct line from
slighter variations. All such changes of structure, whether extremely
slight or strongly marked, which appear amongst many individuals
living together, may be considered as the indefinite effects of the con-
ditions of life on each individual organism, in nearly the same man-
ner as the chill affects different men in an indefinite manner, accord-
ing to their state of body or constitution, causing coughs or colds,
rheumatism or inflammation of various organs.

With respect to what I have called the indirect action of changed
conditions, namely, through the reproductive system of being
affected, we may infer that variability is thus induced, partly from
the fact of this system being extremely sensitive to any change in
the conditions, and partly from the similarity, as Kolreuter and others
have remarked, between the variability which follows from the
crossing of distinct species, and that which may be observed with
plants and animals when reared under new or unnatural conditions.
Many facts clearly show how eminently susceptible the reproductive
system is to very slight changes in the surrounding conditions. Noth-
ing is more easy than to tame an animal, and few things more diffi-
cult than to get it to breed freely under confinement, even when the
male and female unite. How many animals there are which will
not breed, though kept in an almost free state in their native coun-
try! This is generally, but erroneously, attributed to vitiated in-
stincts. Many cultivated plants display the utmost vigour, and yet
rarely or never seed! In some few cases it has been discovered that a
very trifling change, such as a little more or less water at some par-
ticular period of growth, will determine whether or not a plant will
produce seeds. I cannot here give the details which I have collected
and elsewhere published on this curious subject; but to show how
singular the laws are which determine the reproduction of animals
under confinement, I may mention that carnivorous animals,
even from the tropics, breed in this country pretty freely under
confinement, with the exception of the plantigrades or bear fam-
ily, which seldom produce young; whereas carnivorous birds, with
the rarest exceptions, hardly ever lay fertile eggs. Many exotic

26 ORIGIN OF SPECIES

plants have pollen utterly worthless, in the same condition as in
the most sterile hybrids. When, on the one hand, we see domesti-
cated animals and plants, though often weak and sickly, breeding
freely under confinement; and when, on the other hand, we see indi-
viduals, though taken young from a state of nature perfectly tamed,
long-lived and healthy (of which I could give numerous instances),
yet having their reproductive system so seriously af?ected by unper-
ceived causes as to fail to act, we need not be surprised at this system,
when it does act under confinement, acting irregularly, and produc-
ing offspring somewhat unlike their parents. I may add, that as
some organisms breed freely under the most unnatural conditions
(for instance, rabbits and ferrets kept in hutches), showing that their
reproductive organs are not easily affected; so will some animals
and plants withstand domestication or cultivation, and vary very
slightly — perhaps hardly more than in a state of nature.

Some naturalists have maintained that all variations are connected
with the act of sexual reproduction; but this is certainly an error; for
I have given in another work a long list of "sporting plants," as they
are called by gardeners; that is, of plants which have suddenly pro-
duced a single bud wi^h a new and sometimes widely different char-
acter from that of the other buds on the same plant. These bud varia-
tions, as they may be named, can be propagated by grafts, offsets,
etc., and sometimes by seed. They occur rarely under nature, but
are far from rare under culture. As a single bud out of the many
thousands, produced year after year on the same tree under uniform
conditions, has been known suddenly to assume a new charac-
ter; and as buds on distinct trees, growing under different condi-
tions, have sometimes yielded nearly the same variety — for in-
stance, buds on peach-trees producing nectarines, and buds on
common roses producing moss roses — we clearly see that the nature
of the condition is of subordinate importance in comparison with
the nature of the organism in determining each particular form of
variation; perhaps of not more importance than the nature of the
spark, by which a mass of combustible matter is ignited, has in
determining the nature of the flames.

VARIATION UNDER DOMESTICATION VJ

EFFECTS OF HABIT AND OF THE USE OR DISUSE OF PARTS;
CORRELATED VARIATION; INHERITANCE

Changed habits produce an inherited effect, as in the period of the
flowering of plants when transported from one climate to another.
With animals the increased use or disuse of parts has had a more
marked influence; thus I find in the domestic duck that the bones of
the wing weigh less and the bones of the leg more, in proportion to
the whole skeleton, than do the same bones in the wild duck; and this
change may be safely attributed to the domestic duck flying much
less, and walking more, than its wild parents. The great and in-
herited development of the udders in cows and goats in countries
where they are habitually milked, in comparison with these organs
in other countries, is probably another instance of the effects of use.
Not one of our domestic animals can be named which has not in
some country drooping ears; and the view which has been suggested
that the drooping is due to disuse of the muscles of the ear, from the
animals being seldom much alarmed, seems probable.

Many laws regulate variation, some few of which can be dimly
seen, and will hereafter be briefly discussed. I will here only allude
to what may be called correlated variation. Impwrtant changes in the
embryo or larva will probably entail changes in the mature animal.
In monstrosities, the correlations between quite distinct parts are
very curious; and many instances are given in Isidore Geoffroy St.
Hilaire's great work on this subject. Breeders believe that long limbs
are almost always accompanied by an elongated head. Some instances
of correlation are quite whimsical: thus cats which are entirely white
and have blue eyes are generally deaf; but it has been lately stated
by Mr. Tait that this is confined to the males. Colour and constitu-
tional jjeculiarities go together, of which many remarkable cases
could be given amongst animals and plants. From facts collected by
Heusinger, it appears that white sheep and pigs are injured by cer-
tain plants, whilst dark<oloured individuals escape: Professor
Wyman has recently communicated to me a good illustration of this
fact; on asking some farmers in Virginia how it was that all their
pigs were black, they informed him that the pigs ate the paintroot
(Lachnanthes), which colored their bones pink, and which caused the
hoofs of all but the black varieties to drop off; and one of the

28 ORIGIN OF SPECIES

"crackers" (;>., Virginia squatters) added, "We select the black
members of a litter for raising, as they alone have a good chance of
living." Hairless dogs have imperfect teeth; long-haired and coarse-
haired animals are apt to have, as is asserted, long or many horns;
pigeons with feathered feet have skin between their outer toes;
pigeons with short beaks have small feet, and those with long beaks
large feet. Hence if man goes on selecting, and thus augmenting, any
peculiarity, he will almost certainly modify unintentionally other
parts of the structure, owing to the mysterious laws of correlation.

The results of the various, unknown, or but dimly understood laws
of variation are infinitely complex and diversified. It is well worth
while carefully to study the several treatises on some of our old culti-
vated plants, as on the hyacinth, potato, even the dahlia, etc.; and it is
really surprising to note the endless points of structure and consti-
tution in which the varieties and sub-varieties differ slightly from
each other. The whole organisation seems to have become plastic,
and departs in a slight degree from that of the parental type.

Any variation which is not inherited is unimpKjrtant for us. But
the number and diversity of inheritable deviations of structure, both
those of slight and those of considerable physiological importance,
are endless. Dr. Prosper Lucas's treatise, in two large volumes, is
the fullest and the best on this subject. No breeder doubts how strong
is the tendency to inheritance; that like produces like, is his funda-
mental belief: doubts have been thrown on this principle only by
theoretical writers. When any deviation of structure often apjjears,
and we see it in the father and child, we cannot tell whether it may
not be due to the same cause having acted on both ; but when amongst
individuals, apparently exposed to the same conditions, any very rare
deviation, due to some extraordinary combination of circumstances,
appears in the parent — say, once amongst several million individuals
— and it reappears in the child, the mere doctrine of chances almost
compels us to attribute its reappearance to inheritance. Every one
must have heard of cases of albinism, prickly skin, hairy bodies, etc.,
appearing in several members of the same family. If strange and
rare deviations of structure are really inherited, less strange and com-
moner deviations may be freely admitted to be inheritable. Per-
haps the correct way of viewing the whole subject would be, to look

VARIATION UNDER DOMESTICATION 29

at the inheritance of every character whatever as the rule, and non-
inheritance as the anomaly.

The laws governing inheritance are for the most part unknown.
No one can say why the same peculiarity in different individuals of
the same species, or in different species, is sometimes inherited and
sometimes not so; why the child often reverts in certain characters to
its grandfather or grandmother or more remote ancestor; why a
peculiarity is often transmitted from one sex to both sexes, or to one
sex alone, more commonly but not exclusively to the like sex. It is
a fact of some importance to us, that peculiarities appearing in the
males of our domestic breeds are often transmitted, either exclusively
or in a much greater degree, to the males alone. A much more impor-
tant rule, which I think may be trusted, is that, at whatever period
of Ufe a peculiarity first appears, it tends to reappear in the off-
spring at a corresponding age, though sometimes earlier. In many
cases this could not be otherwise; thus the inherited f)eculiarities in
the horns of cattle could appear only in the offspring when nearly
mature; peculiarities in the silkworm are known to appear at the cor-
resfxjnding caterpillar or cocoon stage. But hereditary diseases and
some other facts make me believe that the rule has a wider extension,
and that, when there is no apparent reason why a peculiarity should
appear at any particular age, yet that it does tend to appear in the
offspring at the same period at which it first appeared in the parent.
I believe this rule to be of the highest importance in explaining the
laws of embryology. These remarks are of course confined to the first
appearance of the peculiarity, and not to the primary cause which
may have acted on the ovules or on the male element; in nearly the
same manner as the increased length of the horns in the offspring
from a short-horned cow by a long-horned bull, though appearing late
in life, is clearly due to the male element.

Having alluded to the subject of reversion, I may here refer to a
statement often made by naturalists — namely, that our domestic vari-
eties, when run wild, gradually but invariably revert in character to
their aboriginal stocks. Hence it has been argued that no deductions
can be drawn from domestic races to species in a state of nature. I
have in vain endeavoured to discover on what decisive facts the above
statement has so often and so boldly been made. There would be

3© ORIGIN OF SPECIES

great difficulty in proving its truth : we may safely conclude that very
many of the most strongly marked domestic varieties could not f)Os-
sibly live in a wild state. In many cases we do not know what the
aboriginal stock was, and so could not tell whether or not nearly per-
fect reversion had ensued. It would be necessary, in order to prevent
the effects of intercrossing, that only a single variety should have
been turned loose in its new home. Nevertheless, as our varieties
certainly do occasionally revert in some of their characters to ancestral
forms, it seems to me not improbable that if we could succeed in
naturalising, or were to cultivate, during many generations, the sev-
eral races, for instance, of the cabbage, in very poor soil (in which
case, however, some effect would have to be attributed to the definite
action of the poor soil), that they would, to a large extent, or even
wholly, revert to the wild aboriginal stock. Whether or not the ex-
periment would succeed, is not of great importance for our line of
argument; for by the experiment itself the conditions of life are
changed. If it could be shown that our domestic varieties mani-
fested a strong tendency to reversion, — that is, to lose their acquired
characters, whilst kept under the same conditions, and whilst kept in
a considerable body, so that free intercrossing might check, by blend-
ing together, any slight deviations in their structure, in such case, I
grant that we could deduce nothing from domestic varieties in regard
to species. But there is not a shadow of evidence in favour of this
view: to assert that we could not breed our cart and race horses, long
and short horned cattle, and pwultry of various breeds, and esculent
vegetables, for an unUmited number of generations, would be opposed
to all experience.

CHARACTER OF DOMESTIC VARIETIES; DIFFICULTY OF DISTINGUISHING BE-
TWEEN VARIETIES AND SPECIES; ORIGIN OF DOMESTIC VARIETIES FROM
ONE OR MORE SPECIES

When we look to the hereditary varieties or races of our domestic
animals and plants, and compare them with closely allied species, we
generally perceive in each domestic race, as already remarked, less
uniformity of character than in true species. Domestic races often
have a somewhat monstrous character; by which I mean, that,
although differing from each other, and from other species of the

CHARACTER OF DOMESTIC VARIETIES 3 1

same genus, in several trifling respects, they often differ in an extreme
degree in some one part, both when compared one with another,
and more especially when compared with the species under nature
to which they are nearest allied. With these exceptions (and with
that of the perfect fertility of varieties when crossed, — a subject here-
after to be discussed), domestic races of the same species differ from
each other in the same manner as do the closely allied species of the
same genus in a state of nature, but the differences in most cases are
less in degree. This must be admitted as true, for the domestic races
of many animals and plants have been ranked by some competent
judges as the descendants of aboriginally distinct species, and by other
competent judges as mere varieties. If any well-marked distinction
existed between a domestic race and a species, this source of doubt
would not so perpetually recur. It has often been stated that domestic
races do not differ from each other in characters of generic value. It
can be shown that this statement is not correct; but naturalists diffel
much in determining what characters are of generic value; all such
valuations being at present empirical. When it is explained how
genera originate under nature, it will be seen that we have no right
to expect often to find a generic amount of difference in our domesti-
cated races.

In attempting to estimate the amount of structural difference be-
tween allied domestic races, we are soon involved in doubt, from not
knowing whether they are descended from one or several parent
species. This point, if it could be cleared up, would be interesting; if,
for instance, it could be shown that the greyhound, bloodhound, ter-
rier, spaniel, and bulldog, which we all know propagate their kind
truly, were the offspring of any single species, then such facts would
have great weight in making us doubt about the immutability of the
many closely allied natural species — for instance, of the many foxes
— inhabiting different quarters of the world. I do not believe, as we
shall presently see, that the whole amount of difference between the
several breeds of the dog has been produced under domestication; I
believe that a small part of the difference is due to their being de-
scended from distinct species. In the case of strongly marked races of
some other domesticated species, there is presumptive or even strong
evidence, that all are descended from a single wild stock.

32 ORIGIN OF SPECIES

It has often been assumed that man has chosen for domestication
animals and plants having an extraordinary inherent tendency to
vary, and likewise to withstand diverse climates. I do not dispute
that these capacities have added largely to the value of most of our
domesticated productions; but how could a savage possibly know,
when he first tamed an animal, whether it would vary in succeeding
generations, and whether it would endure other climates? Has the
litde variability of the ass and goose, or the small power of endurance
of warmth by the reindeer, or of cold by the common camel, pre-
vented their domestication ? I cannot doubt that if other animals and
plants, equal in number to our domesticated productions, and be-
longing to equally diverse classes and countries, were taken from a
state of nature, and could be made to breed for an equal number of
generations under domestication, they would on an average vary as
largely as the parent species of our existing domesticated productions
have varied.

In the case of most of our anciently domesticated animals and
plants, it is not possible to come to any definite conclusion, whether
they are descended from one or several wild species. The argument
mainly relied on by those who believe in the multiple origin of our
domestic animals is, that we find in the most ancient times, on the
monuments of Egypt, and in the lake habitations of Switzerland,
much diversity in the breeds; and that some of these ancient breeds
closely resemble, or are even identical with, those still existing. But
this only throws far backwards the history of civilisation, and shows
that animals were domesticated at a much earlier period than has
hitherto been supposed. The lake inhabitants of Switzerland culti-
vated several kinds of wheat and barley, the pea, the poppy for oil,
and flax; and they possessed several domesticated animals. They also
carried on commerce with other nations. All this clearly shows, as
Heer has remarked, that they had at this early age progressed con-
siderably in civilisation; and this again implies a long continued
previous period of less advanced civilisation, during which the domes-
ticated animals, kept by different tribes in different districts, might
have varied and given rise to distinct races. Since the discovery of
flint tools in the superficial formations of many parts of the world,
all geologists believe that barbarian man existed at an enormously

CHARACTER OF DOMESTIC VARIETIES 33

remote period and we know that at the present day there is hardly
a tribe so barbarous, as not to have domesticated at least the dog.

The origin of most of our domestic animals will probably for ever
remain vague. But I may here state, that, looking to the domestic
dogs of the whole world, I have, after a laborious collection of all
known facts, come to the conclusion that several wild species of
Canidar have been tamed, and that their blood, in some cases mingled
together, flows in the veins of our domestic breeds. In regard to
sheep and goats I can form no decided opinion. From facts com-
municated to me by Mr. Blyth, on the habits, voice, constitution, and
structure of the humped Indian cattle, it is almost certain that they
are descended from a different aboriginal stock from our European
cattle and some competent judges believe that these latter have had
two or three wild progenitors, — whether or not these deserve to be
called species. This conclusion, as well as that of the specific distinc-
tion between the humped and common cattle, may, indeed, be looked
upon as established by the admirable researches of Professor Riiti-
meyer. With respect to horses, from reasons which I cannot here
give, I am doubtfully inclined to believe, in oppxssition to several au-
thors, that all the races belong to the same species. Having kept
nearly all the English breeds of the fowl alive, having bred and
crossed them, and examined their skeletons, it apfjcars to me almost
certain that all are the descendants of the wild Indian fowl, Gallus
bankiva; and this is the conclusion of Mr. Blyth, and of others who
have studied this bird in India. In regard to ducks and rabbits, some
breeds of which differ much from each other, the evidence is clear
that they are all descended from the common wild duck and rabbit.

The doctrine of the origin of our several domestic races from sev-
eral aboriginal stocks, has been carried to an absurd extreme by some
authors. They believe that every race which breeds true, let the dis-
tinctive characters be ever so slight, has had its wild prototype. At
this rate there must have existed at least a score of species of wild
cattle, as many sheep, and several goats, in Europe alone, and several
even within Great Britain. One author believes that there formerly
existed eleven wild species of sheep peculiar to Great Britain! When
we bear in mind that Britain has now not one peculiar mammal, and
France but few distinct from those of Germany, and so with Hun-

34 ORIGIN OF SPECIES

gary, Spain, etc., but that each of these kingdoms possesses several
peculiar breeds of cattle, sheep, etc., we must admit that many do-
mestic breeds must have originated in Europe; from whence other-
wise could they have been derived? So it is in India. Even in the
case of the breeds of the domestic dog throughout the world, which
I admit are descended from several wild species, it cannot be doubted
that there has been an immense amount of inherited variation; for
who will believe that animals closely resembUng the Italian grey-
hound, the bloodhound, the bulldog, pugdog, or Blenheim spaniel,
etc. — so unlike all wild Canida; — ever existed in a state of nature? It
has often been loosely said that all our races of dogs have been pro-
duced by the crossing of a few aboriginal species; but by crossing we
can only get forms in some degree intermediate between their par-
ents; and if we account for our several domestic races by this process,
we must admit the former existence of the most extreme forms, as
the Italian greyhound, bloodhound, bulldog, etc., in the wild state.
Moreover, the possibility of making distinct races by crossing has
been greatly exaggerated Many cases are on record, showing that a
race may be modified by occasional crosses, if aided by the careful se-
lection of the individuals which present the desired character; but to
obtain a race intermediate between two quite distinct races, would
be very difficult. Sir J. Sebright expressly experimented with this
object and failed. The offspring from the first cross between two pure
breeds is tolerably and sometimes (as I have found with pigeons)
quite uniform in character, and everything seems simple enough;
but when these mongrels are crossed one with another for several
generations, hardly two of them are alike, and then the difficulty of
the task becomes manifest.

BREEDS OF THE DOMESTIC PIGEON, THEIR DIFFERENCES AND ORIGIN

Believing that it is always best to study some special group, I have,
after deliberation, taken up domestic pigeons. I have kept every
breed which I could purchase or obtain, and have been most kindly
favoured with skins from several quarters of the world, more espe-
cially by the Hon. W. Elliot, from India, and by the Hon. C. Murray,
from Persia. Many treatises in different languages have been pub-
lished on pigeons, and some of them are very important, as being of

DOMESTIC PIGEONS 35

considerable antiquity. I have associated with several eminent fan-
ciers, and have been permitted to join two of the London Pigeon
Clubs. The diversity of the breeds is something astonishing. Com-
pare the English carrier and the short-faced tumbler, and see the won-
derful difference in their beaks, entailing corresponding differences
in their skulls. The carrier, more especially the male bird, is also re-
markable from the wonderful development of the carunculated skin
about the head; and this is accompanied by greatly elongated eye-
lids, very large external orifices to the nostrils, and a wide gape of
mouth. The short-faced tumbler has a beak in outhne almost like that
(if a finch; and the common tumbler has the singular inherited habit
of flying at a great height in a compact flock, and tumbling in the
air head over heels. The runt is a bird of great size, with long mas-
sive beak and large feet; some of the sub-breeds of runts have very
long necks, others very long wings and tails, others singularly short
tails. The barb is allied to the carrier, but, instead of a long beak,
has a very short and broad one. The pouter has a much elongated
body, wings, and legs; and its enormously developed crop, which it
glories in inflating, may well excite astonishment and even laughter.
The turbit has a short and conical beak, with a line of reversed
feathers down the breast; and it has the habit of continually expand-
ing, slightly, the upper part of the oesophagus. The Jacobin has the
feathers so much reversed along the back of the neck that they form
a hood; and it has, proportionally to its size, elongated wing and
tail feathers. The trumpxjter and laugher, as their names express,
utter a very different coo from the other breeds. The fantail has
thirty or even forty tail-feathers, instead of twelve or fourteen — the
normal number in all the members of the great pigeon family: these
feathers are kept expanded, and are carried so erect, that in good
birds the head and tail touch: the oil-gland is quite aborted. Several
other less distinct breeds might be specified.

In the skeletons of the several breeds, the development of the
bones of the face in length and breadth and curvature differs enor-
mously. The shape, as well as the breadth and length of the ramus
of the lower jaw, varies in a highly remarkable manner. The caudal
and sacral vertebra! vary in number; as does the number of the ribs,
together with their relative breadth and the presence of processes.

36 ORIGIN OF SPECIES

The size and shape of the apertures in the sternum are highly
variable; so is the degree of divergence and relative size of the two
arms of the furcula. The proportional width of the gape of mouth,
the proportional length of the eyelids, of the orifice of the nostrils,
of the tongue (not always in strict correlation with the length of
beak), the size of the crop and of the upper part of the oesophagus;
the development and abortion of the oil-gland; the number of the
primary wing and caudal feathers; the relative length of the wing
and tail to each other and to the body; the relative length of the
leg and foot; the number of scutelL-c on the toes, the development
of skin between the toes, are all points of structure which are vari-
able. The period at which the perfect plumage is acquired varies,
as does the state of the down with which the nestling birds are
clothed when hatched. The shape and size of the eggs vary. The
manner of flight, and in some breeds the voice and disposition, differs
remarkably. Lastly, in certain breeds, the males and females have
come to differ in a slight degree from each other.

Altogether at least a score of pigeons might be chosen, which, if
shown to an ornithologist, and he were told that they were wild
birds, would certainly be ranked by him as well-defined species.
Moreover, I do not believe that any ornithologist would in this case
place the English carrier, the short-faced tumbler, the runt, the barb,
[X)uter, and fantail in the same genus; more especially as in each of
these breeds several truly inherited sub-breeds, or species, as he would
call them, could be shown him.

Great as are the differences between the breeds of the pigeon, I
am fully convinced that the common opinion of naturalists is correct,
namely, that all are descended from the rock pigeon (Columba livia),
including under this term several geographical races or sub-species,
which differ from each other in the most trifling respects. As several
of the reasons which have led me to this belief are in some degree
applicable in other cases, I will here briefly give them. If the several
breeds are not varieties, and have not proceeded from the rock
pigeon, they must have descended from at least seven or eight aborigi-
nal stocks; for it is impossible to make the present domestic breeds
by the crossing of any lesser number: how, for instance, could a
pouter be produced by crossing two breeds unless one of the parent-

DOMESTIC PIGEONS 37

Stocks possessed the characteristic enormous crop? The supposed
aboriginal stocks must all have been rock pigeons, that is, they did
not breed or willingly perch on trees. But besides C. livia, with its
geographical sub-species, only two or three other species of rock
pigeons are known and these have not any of the characters of the
domestic breeds. Hence the supposed aboriginal stocks must either
still exist in the countries where they were originally domesticated,
and yet be unknown to ornithologists; and this, considering their
size, habits, and remarkable characters, seems improbable; or they
must have become extinct in the wild state. But birds breeding on
precipices, and good fliers, are unlikely to be exterminated; and the
common rock pigeon, which has the same habits with the domestic
breeds, has not been exterminated even on several of the smaller
British islets, or on the shores of the Mediterranean. Hence the
supposed extermination of so many species having similar habits
with the rock pigeon seems a very rash assumption. Moreover, the
several above-named domesticated breeds have been transported to
all parts of the world, and, therefore, some of them must have been
carried back again into their native country; but not one has become
wild or feral, though the dovecot pigeon, which is the rock pigeon
in a very slightly altered state, has become feral in several places.
Again, all recent experience shows that it is difficult to get wild ani-
mals to breed freely under domestication; yet on the hypothesis of
the multiple origin of our pigeons, it must be assumed that at least
seven or eight species were so thoroughly domesticated in ancient
times by half<ivilised man, as to be quite prolific under confinement.
An argument of great weight, and applicable in several other
cases, is, that the above-specified breeds, though agreeing generally
with the wild rock pigeon in constitution, habits, voice, colouring,
and in most parts of their structure, yet are certainly highly abnormal
in other parts; we may look in vain through the whole great family
of Columbidx for a beak like that of the English carrier, or that of
the short-faced tumbler, or barb; for reversed feathers like those of
the Jacobin; for a crop like that of the pouter; for tail-feathers like
those of the fantail. Hence it must be assumed not only that half-
civilised man succeeded in thoroughly domesticating several species,
but that he intentionally or by chance picked out extraordinarily

38 ORIGIN OF SPECIES

abnormal species; and further, that these very species have since all
become extinct or unknown. So many strange contingencies are
improbable in the highest degree.

Some facts in regard to the colouring of pigeons well deserve
consideration. The rock pigeon is of a slaty-blue, with white loins;
but the Indian sub-species, C. intermedia of Strickland, has this
part bluish. The tail has a terminal dark bar, with the outer feathers
externally edged at the base with white. The wings have two black
bars. Some semi-domestic breeds, and some truly wild breeds, have,
besides the two black bars, the wings chequered with black. These
several marks do not occur together in any other species of the whole
family. Now, in every one of the domestic breeds, taking thoroughly
well-bred birds, all the above marks, even to the white edging of
the outer tail-feathers, sometimes concur perfecdy developed. More-
over, when birds belonging to two or more distinct breeds are
crossed, none of which are blue or have any of the above-specified
marks, the mongrel offspring are very apt suddenly to acquire these
characters. To give one instance out of several which I have ob-
served: — I crossed some white fantails, which breed very true, with
some black barbs — and rt so happens that blue varieties of barbs are
so rare that I never heard of an instance in England; and the mon-
grels were black, brown, and mottled. I also crossed a barb with a
spot, which is a white bird with a red tail and red spot on the fore-
head, and which notoriously breeds very true; the mongrels were
dusky and mottled. I then crossed one of the mongrel barb-fantails
with a mongrel barb-spot, and they produced a bird of as beautiful
a blue colour, with the white loins, double black wing-bar, and barred
and white-edged tail-feathers, as any wild rock pigeon! We can
understand these facts, on the well-known principle of reversion to
ancestral characters, if all the domestic breeds are descended from
the rock pigeon. But if we deny this, we must make one of the two
following highly improbable suppositions. Either, first, that all the
several imagined aboriginal stocks were coloured and marked like
the rock pigeon, although no other existing species is thus coloured
and marked, so that in each separate breed there might be a tendency
to revert to the very same colours and markings. Or, secondly, that
each breed, even the purest, has within a dozen, or at most within a

DOMESTIC PIGEONS 39

score, of generations, been crossed by the rock pigeon; I say within
a dozen or twenty generations, for no instance is known of crossed
descendants reverting to an ancestor of foreign blood, removed by
a greater number of generations. In a breed which has been crossed
only once, the tendency to revert to any character derived from such
a cross will naturally become less and less, as in each succeeding gen-
eration there will be less of the foreign blood; but when there has
been no cross, and there is a tendency in the breed to revert to a
character which was lost during some former generation, this tend-
ency, for all that we can see to the contrary, may be transmitted
undiminished for an indefinite number of generations. These two
distinct cases of reversion are often confounded together by those
who have written on inheritance.

Lastly, the hybrids or mongrels from between all the breeds of the
pigeon are perfectly fertile, as I can state from my own observations,
purposely made, on the most distinct breeds. Now, hardly any cases
have been ascertained with certainty of hybrids from two quite
distinct species of animals being perfectly fertile. Some authors
believe that long-continued domestication eliminates this strong
tendency to sterility in species. From the history of the dog, and of
some other domestic animals, this conclusion is probably quite
correct, if applied to species closely related to each other. But to
extend it so far as to supjxjse that species, aboriginally as distinct as
carriers, tumblers, pouters, and fantails now are, should yield off-
spring perfectly fertile inter se, would be rash in the extreme.

From these several reasons, namely, — the improbability of man
having formerly made seven or eight supposed species of pigeons to
breed freely under domestication; — these supposed species being
quite unknown in a wild state, and their not having become any-
where feral; — these sp)ecies presenting certain very abnormal char-
acters, as compared with all other Columbidac, though so like the
rock pigeon in most respects; — the occasional re-appearance of the
blue colour and various black marks in all the breeds, both when
kept pure and when crossed; — and lastly, the mongrel offspring being
perfectly fertile; — from these several reasons, taken together, we may
safely conclude that all our domestic breeds are descended from the
rock pigeon or Columba livia with its geographical sub-species.

40 ORIGIN OF SPECIES

In favour of this view, I may add, firstly, that the wild C. livia
has been found capable of domestication in Europe and India; and
that it agrees in habits and in a great number of pxjints of structure
with all the domestic breeds. Secondly, that, although an English
carrier or a short-faced tumbler differs immensely in certain char-
acters from the rock pigeon, yet that, by comparing the several sub-
breeds of these two races, more especially those brought from distant
countries, we can make, between them and the rock pigeon, an
almost perfect series; so we can in some other cases, but not with
all the breeds. Thirdly, those characters which are mainly distinctive
of each breed are in each eminently variable, for instance the wattle
and length of beak of the carrier, the shortness of that of the tumbler,
and the number of tail-feathers in the fantail; and the explanation
of this fact will be obvious when we treat of Selection. Fourthly,
pigeons have been watched and tended with the utmost care, and
loved by many people. They have been domesticated for thousands
of years in several quarters of the world; the earliest known record
of pigeons is in the fifth ^tgyptian dynasty, about 3000 b.c^ as was
{X)inted out to me by Professor Lepsius; but Mr. Birch informs me
that pigeons are given in a bill of fare in the previous dynasty. In
the time of the Romans, as we hear from Pliny, immense prices
were given for pigeons; "nay, they are come to this pass, that they
can reckon up their pedigree and race." Pigeons were much valued
by Akber Khan, in India, about the year 1600; never less than 20,000
pigeons were taken with the court. "The monarchs of Iran and
Turan sent him some very rare birds"; and, continues the courtly
historian, "His Majesty, by crossing the breeds, which method was
never practised before, has improved them astonishingly." About
this same period the Dutch were as eager about pigeons as were the
old Romans. The paramount importance of these considerations in
explaining the immense amount of variation which pigeons have
undergone, will likewise be obvious when we treat of Selection. We
shall then, also, see how it is that the several breeds so often have a
somewhat monstrous character. It is also a most favourable circum-
stance for the production of distinct breeds, that male and female
pigeons can be easily mated for life; and thus different breeds can
be kept together in the same aviary.

SELECTION BY MAN 4 1

I have discussed the probable origin of domestic pigeons at some,
yet quite insufficient, length; because when I first kept pigeons and
watched the several kinds, well knowing how truly they breed, I
felt fully as much difficulty in believing that since they had been
domesticated they had all proceeded from a common parent, as any
naturalist could in coming to a similar conclusion in regard to the
many sjjecies of finches, or other groups of birds, in nature. One
circumstance has struck me much; namely, that nearly all the
breeders of the various domestic animals and the cultivators of plants,
with whom I have conversed, or whose treatises I have read, are
firmly convinced that the several breeds to which each has attended,
are descended from so many aboriginally distinct species. Ask, as
I have asked, a celebrated raiser of Hereford cattle, whether his cattle
might not have descended from Longhorns, or both from a common
parent-stock, and he will laugh you to scorn. I have never met a
pigeon, or poultry, or duck, or rabbit fancier, who was not fully
convinced that each main breed was descended from a distinct
sp>ecies. Van Mons, in his treatise on pears and apples, shows how
utterly he disbelieves that the several sorts, for instance a Ribston-
pippin or Codlin-apple, could ever have proceeded from the seeds
of the same tree. Innumerable other examples could be given. The
explanation, I think, is simple: from long-continued study they are
strongly impressed with the differences between the several races;
and though they well know that each race varies slightly, for they
win their prizes by selecting such slight differences, yet they ignore
all general arguments, and refuse to sum up in their minds slight
differences accumulated during many successive generations. May
not those naturalists who, knowing far less of the laws of inheritance
than does the breeder, and knowing no more than he does of the
intermediate links in the long lines of descent, yet admit that many
of our domestic races are descended from the same parents — may
they not learn a lesson of caution, when they deride the idea of species
in a state of nature being lineal descendants of other species?

PRINCIPLES OF SELECTION ANCIENTLY FOLLOWED, AND THEIR EFFECTS

Let US now briefly consider the steps by which domestic races have
been produced, either from one or from several allied species. Some

42 ORIGIN OF SPECIES

effect may be attributed to the direct and definite action of the
external conditions of hfe, and some to habit; but he would be a
bold man who would account by such agencies for the differences
between a dray and race horse, a greyhound and bloodhound, a
carrier and tumbler pigeon. One of the most remarkable features
in our domesticated races is that we see in them adaptation, not
indeed to the animal's or plant's own good, but to man's use or
fancy. Some variations useful to him have probably arisen suddenly,
or by one step; many botanists, for instance, believe that the fuller's
teasel, with its hooks, which cannot be rivalled by any mechanical
contrivance, is only a variety of the wild Dipsacus; and this amount
of change may have suddenly arisen in a seedling. So it has probably
been with the turnspit dog; and this is known to have been the
case with the ancon sheep. But when we compare the dray horse
and race horse, the dromedary and camel, the various breeds of
sheep fitted either for cultivated land or mountain pasture, with the
wool of one breed good for one purpose, and that of another breed
for another purpKJse; when we compare the many breeds of dogs,
each good for man in different ways; when we compare the game
cock, so pertinacious in battle, with other breeds so Uttle quarrelsome,
with "everlasting layers" which never desire to sit, and with the
bantam so small and elegant; when we compare the host of agricul-
tural, culinary, orchard, and flower garden races of plants, most
useful to man at different seasons and for different purposes, or so
beautiful in his eyes, we must, I think, look further than to mere
variability. We cannot suppose that all the breeds were suddenly
produced as perfect and as useful as we now see them; indeed,
in many cases, we know that this has not been their history. The
key is man's pwwer of accumulative selection: nature gives successive
variations; man adds them up in certain directions useful to him.
In this sense he may be said to have made for himself useful breeds.
The great jx)wer of this principle of selection is not hypothetical.
It is certain that several of our eminent breeders have, even within
a single lifetime, modified to a large extent their breeds of cattle and
sheep. In order fully to realise what they have done, it is almost
necessary to read several of the many treatises devoted to this subject,
and to inspect the animals. Breeders habitually speak of an animal's

SELECTION BY MAN 43

organisation as something plastic, which they can model almost as
they please. If I had space I could quote numerous passages to this
effect from highly competent authorities. Youatt, who was probably
better acquainted with the works of agriculturists than almost any
other individual, and who was himself a very good judge of animals,
speaks of the principle of selection as "that which enables the agri-
culturist, not only to modify the character of his flock, but to change
it altogether. It is the magician's wand, by means of which he
may summon into life whatever form and mould he pleases." Lord
Somerville, sf)eaking of what breeders have done for sheep, says: —
"It would seem as if they had chalked out upon a wall a form perfect
in itself, and then had given it existence." In Saxony the importance
of the principle of selection in regard to merino sheep is so fully
recognised, that men follow it as a trade; the sheep are placed on a
table and are studied, like a picture by a connoisseur; this is done
three times at intervals of months, and the sheep are each time
marked and classed, so that the very best may ultimately be selected
for breeding.

What English breeders have actually effected is proved by the
enormous prices given for animals with a good pedigree; and these
have been exported to almost every quarter of the world. The im-
provement is by no means generally due to crossing different breeds;
all the best breeders are strongly opposed to this practice, except some-
times amongst closely allied sub-breeds. And when a cross has been
made, the closest selection is far more indispiensable even than in
ordinary cases. If selection consisted merely in separating some very
distinct variety, and breeding from it, the principle would be so
obvious as hardly to be worth notice; but its importance consists in
the great effect produced by the accumulation in one direcdon,
during successive generations, of differences absolutely inappreciable
by an uneducated eye — differences which I for one have vainly
attempted to appreciate. Not one man in a thousand has accuracy
of eye and judgment suflScient to become an eminent breeder. If
gifted with these qualities, and he studies his subject for years, and
devotes his lifetime to it with indomitable perseverance, he will
succeed, and may make great improvements; if he wants any of
these qualities, he will assuredly fail. Few would readily believe

44 ORIGIN OF SPECIES

in the natural capacity and years of practice requisite to become even

a skilful pigeon fancier.

The same principles are followed by horticulturists; but the
variations are here often more abrupt. No one supposes that our
choicest productions have been produced by a single variation from
the aboriginal stock. We have proofs that this has not been so in
several cases in which exact records have been kept; thus, to give
a very trifling instance, the steadily increasing size of the common
gooseberry may be quoted. We see an astonishing improvement in
many florists' flowers, when the flowers of the present day are com-
pared with drawings made only twenty or thirty years ago. When
a race of plants is once pretty well established, the seed-raisers do not
pick out the best plants, but merely go over their seed-beds, and pull
up the "rogues," as they call the plants that deviate from the proper
standard. With animals this kind of selection is, in fact, likewise
followed; for hardly any one is so careless as to breed from his
worst animals.

In regard to plants, there is another means of observing the ac-
cumulated effects of selection — namely, by comparing the diversity
of flowers in the different varieties of the same species in the flower
garden; the diversity of leaves, pods, or tubers, or whatever part is
valued, in the kitchen garden, in comparison with the flowers of the
same varieties; and the diversity of fruit of the same species in the
orchard, in comparison with the leaves and flowers of the same set
of varieties. See how different the leaves of the cabbage are, and
how extremely alike the flowers; how unlike the flowers of the
heartsease are, and how alike the leaves; how much the fruit of the
different kinds of gooseberries differ in size, colour, shape, and hairi-
ness, and yet the flowers present very slight differences. It is not that
the varieties which differ largely in some one point do not differ at
all in other points; this is hardly ever, — I speak after careful observa-
tion, — perhaps never, the case. The law of correlated variation, the
importance of which should never be overlooked, will ensure some
differences; but, as a general rule, it cannot be doubted that the con-
tinued selection of slight variations, either in the leaves, the flowers,
or the fruit, will produce races differing from each other chiefly in
these characters.

UNCONSCIOUS SELECTION 45

It may be objected that the principle of selection has been reduced
to methodical practice for scarcely more than three-quarters of a
century; it has certainly been more attended to of late years, and
many treatises have been published on the subject; and the result
has been, in a corresponding degree, rapid and important. But it is
very far from true that the principle is a modern discovery. I could
give several references to works of high antiquity, in which the full
importance of the principle is acknowledged. In rude and barbarous
periods of English history choice animals were often imported, and
laws were passed to prevent their exportation: the destruction of
horses under a certain size was ordered, and this may be compared
to the "roguing" of plants by nurserymen. The principle of selection
I find distinctly given in an ancient Chinese encyclopsedia. Explicit
rules are laid down by some of the Roman classical writers. From
passages in Genesis, it is clear that the colour of domesticated animals
was at that early period attended to. Savages now sometimes cross
their dogs with wild canine animals, to improve the breed, and they
formerly did so, as is attested by passages in Pliny. The savages in
South Africa match their draught cattle by colour, as do some of
the Esquimaux their teams of dogs. Livingstone states that good
domestic breeds are highly valued by the negroes in the interior of
Africa who have not associated with Europeans. Some of these facts
do not show actual selection, but they show that the breeding of
domestic animals was carefully attended to in ancient times, and is
now attended to by the lowest savages. It would, indeed, have been
a strange fact, had attention not been paid to breeding, for the
inheritance of good and bad qualities is so obvious.

UNCONSCIOUS SELECTION

At the present time, eminent breeders try by methodical selection,
with a distinct object in view, to make a new strain or sub-breed,
superior to anything of the kind in the country. But, for our purpose,
a form of Selection, which may be called Unconscious, and which
results from every one trying to possess and breed from the best
individual animals, is more important. Thus, a man who intends
keeping pointers naturally tries to get as good dogs as he can, and
afterwards breeds from his own best dogs, but he has no wish or

46 ORIGIN OF SPECIES

expectation of permanently altering the breed. Nevertheless we may
infer that this process, continued during centuries, would improve
and modify any breed, in the same way as Bakewell, Collins, etc., by
this very same process, only carried on more methodically, did greatly
modify, even during their lifetimes, the forms and qualities of their
cattle. Slow and insensible changes of this kind can never be recog-
nised unless actual measurements or careful drawings of the breeds in
question have been made long ago, which may serve for comparison.
In some cases, however, unchanged, or but little changed individuals
of the same breed exist in less civilised districts, where the breed has
been less improved. There is reason to believe that King Charles'
spaniel has been unconsciously modified to a large extent since the
time of that monarch. Some highly competent authorities are con-
vinced that the setter is directly derived from the spaniel, and has
probably been slowly altered from it. It is known that the Enghsh
pointer has been greatly changed within the last century, and in this
case the change has, it is believed, been chiefly efleaed by crosses
with the foxhound; but what concerns us is, that the change has
been effected unconsciously and gradually, and yet so effectually, that,
though the old Spanish pointer certainly came from Spain, Mr. Bor-
row has not seen, as I am informed by him, any native dog in Spain
like our pointer. By a simple process of selection, and by careful
training, English race horses have come to surpass in fleetness and
size the parent Arabs, so that the latter, by the regulations for the
Goodwood Races, are favoured in the weights which they carry.
Lord Spencer and others have shown how the cattle of England have
increased in weight and in early maturity, compared with the stock
formerly kept in this country. By comparing the accounts given in
various old treatises of the former and present state of carrier and
tumbler pigeons in Britain, India, and Persia, we can trace the
stages through which they have insensibly passed, and come to differ
so greatly from the rock pigeon.

Youatt gives an excellent illustration of the effects of a course of
selection, which may be considered as unconscious, in so far that
the breeders could never have expected, or even wished, to produce
the result which ensued — namely, the production of two distinct
strains. The two flocks of Leicester sheep kept by Mr. Buckley and

UNCONSCIOUS SELECTION 47

Mr. Burgess, as Mr. Youatt remarks, "have been purely bred from
the original stock of Mr. Bakewell for upwards of fifty years. There
is not a suspicion existing in the mind of any one at all acquainted
with the subject, that the owner of either of them has deviated in
any one instance from the pure blood of Mr. Bakewell's flock, and
yet the difference between the sheep possessed by these two gentle-
men is so great that they have the appearance of being quite different
varieties."

If there exist savages so barbarous as never to think of the in-
herited character of the offspring of their domestic animals, yet any
one animal particularly useful to them, for any special purpose, would
be carefully preserved during famines and other accidents, to which
savages are so liable, and such choice animals would thus generally
leave more offspring than the inferior ones; so that in this case there
would be a kind of unconscious selection going on. We see the
value set on animals even by the barbarians of Tierra del Fuego, by
their killing and devouring their old women, in times of dearth, as of
less value than their dogs.

In plants the same gradual process of improvement, through the
occasional preservation of the best individuals, whether or not suffi-
ciently distinct to be ranked at their first appearance as distinct
varieties, and whether or not two or more species or races have
become blended together by crossing, may plainly be recognised in
the increased size and beauty which we now see in the varieties of
the heartsease, rose, pelargonium, dahlia, and other plants, when
compared with the older varieties or with their parent-stocks. No
one would ever expect to get a first-rate heartsease or dahlia from
the seed of a wild plant. No one would expect to raise a first-rate
melting pear from the seed of the wild f)ear, though he might succeed
from a poor seedling growing wild, if it had come from a garden-
stock. The pear, though cultivated in classical times, appears, from
Pliny's description, to have been a fruit of very inferior quality. I
have seen great surprise expressed in horticultural works at the
wonderful skill of gardeners, in having produced such splendid
results from such poor materials; but the art has been simple, and,
as far as the final result is concerned, has been followed almost un-
consciously. It has consisted in always cultivating the best known

48 ORIGIN OF SPECIES

variety, sowing its seeds, and, when a slightly better variety chanced
to appear, selecting it, and so onwards. But the gardeners of the
classical period, who cultivated the best pears which they could pro-
cure, never thought what splendid fruit we should eat; though we
owe our excellent fruit in some small degree to their having naturally
chosen and preserved the best varieties they could anywhere find.

A large amount of change, thus slowly and unconsciously accumu-
lated, explains, as I believe, the well-known fact, that in a number
of cases we cannot recognise, and therefore do not know, the wild
parent-stocks of the plants which have been longest cultivated in our
flower and kitchen gardens. If it has taken centuries or thousands
of years to improve or modify most of our plants up to their present
standard of usefulness to man, we can understand how it is that
neither Australia, the Cape of Good Hope, nor any other region
inhabited by quite uncivilised man, has afforded us a single plant
worth culture. It is not that these countries, so rich in species, do not
by a strange chance possess the aboriginal stocks of any useful plants,
but that the native plants have not been improved by continued
selection up to a standard of perfection comparable with that
acquired by the plants in countries anciently civilised.

In regard to the domestic animals kept by uncivilised man, it
should not be overlooked that they almost always have to struggle
for their own food, at least during certain seasons. And in two
countries very differently circumstanced, individuals of the same
species, having slightly different constitutions or structure, would
often succeed better in the one country than in the other; and thus
by a process of "natural selection," as will hereafter be more fully
explained, two sub-breeds might be formed. This, perhaps, partly
explains why the varieties kept by savnges, as has been remarked by
some authors, have more of the character of true species than the
varieties kept in civilised countries.

On the view here given of the important part which selection by
man has played, it becomes at once obvious, how it is that our
domestic races show adaptation in their structure or in their habits
to man's wants or fancies. We can, I think, further understand the
frequently abnormal character of our domestic races, and likewise
their differences being so great in external characters, and relatively

UNCONSCIOUS SELECTION 49

SO slight in internal parts or organs. Man can hardly select, or only
with much difficulty, any deviation of structure excepting such as is
externally visible; and indeed he rarely cares for what is internal.
He can never act by selection, excepting on variations which are first
given to him in some slight degree by nature. No man would ever
try to make a fantail till he saw a pigeon with a tail developed in
some slight degree in an unusual manner, or a pouter till he saw a
pigeon with a crop of somewhat unusual size; and the more ab-
normal or unusual any character was when it first appeared, the
more likely it would be to catch his attention. But to use such an
expression as trying to make a fantail, is, I have no doubt, in most
cases, utterly incorrect. The man who first selected a pigeon with
a slightly larger tail, never dreamed what the descendants of that
pigeon would become through long-continued, partly unconscious
and partly methodical, selection. Perhaps the parent-bird of all fan-
tails had only fourteen tail-feathers somewhat expanded, like the
present Java fantail, or like individuals of other and distinct breeds,
in which as many as seventeen tail-feathers have been counted.
Perhaps the first pwuter pigeon did not inflate its crop much more
than the turbit now does the upper part of its oesophagus, — a habit
which is disregarded by all fanciers, as it is not one of the points
of the breed.

Nor let it be thought that some great deviation of structure would
be necessary to catch the fancier's eye: he perceives extremely small
differences, and it is in human nature to fancy any novelty, however
slight, in one's own possession. Nor must the value which would
formerly have been set on any slight differences in the individuals of
the same species, be judged of by the value which is now set on
them, after several breeds have fairly been established. It is known
that with pigeons many slight variations now occasionally appear,
but these are rejected as faults or deviations from the standard of
perfection in each breed. The common goose has not given rise to
any marked varieties; hence the Toulouse and the common breed,
which differ only in colour, that most fleeting of characters, have
lately been exhibited as distinct at our poultry shows.

These views apj^jar to explain what has sometimes been noticed,
namely, that we know hardly anything about the origin or history

50 ORIGIN OF SPECIES

of any of our domestic breeds. But, in fact, a breed, like a dialect
of a language, can hardly be said to have a distinct origin. A man
preserves and breeds from an individual with some slight deviation
of structure, or takes more care than usual in matching his best
animals, and thus improves them, and the improved animals slowly
spread in the immediate neighbourhood. But they will as yet hardly
have a distinct name, and from being only slightly valued, their
history will have been disregarded. When further improved by the
same slow and gradual process, they will spread more widely, and
will be recognised as something distinct and valuable, and will then
probably first receive a provincial name. In semi-civilised countries,
with little free communication, the spreading of a new sub-breed
would be a slow process. As soon as the points of value are once
acknowledged, the principle, as I have called it, of unconscious
selection will always tend, — perhaps more at one period than at an-
other, as the breed rises or falls in fashion, — perhaps more in one
district than in another, according to the state of civilisation of the
inhabitants, — slowly to add to the characteristic features of the
breed, whatever they may be. But the chance will be infinitely small
of any record having been preserved of such slow, varying, and
insensible changes.

CIRCUMSTAN'CES FAVOURABLE TO MAN's POWER OF SEI.ECTION

I will now say a few words on the circumstances, favourable, or
the reverse, to man's power of selection. A high degree of variability
is obviously favourable, as freely giving the materials for selection
to work on; not that mere individual differences are not amply suffi-
cient, with extreme care, to allow of the accumulation of a large
amount of modification in almost any desired direction. But as
variations manifestly useful or pleasing to man appear only occasion-
ally, the chance of their appearance will be much increased by a large
number of individuals being kept. Hence, number is of the highest
importance for success. On this principle Marshall formerly re-
marked, with respect to the sheep of parts of Yorkshire, "as they
generally belong to poor people, and are mostly in small lots, they
never can be improved." On the other hand, nurserymen, from
keeping large stocks of the same plant, are generally far more success-

CIRCUMSTANCES FAVOURABLE TO SELECTION 5 1

ful than amateurs in raising new and valuable varieties. A large
number of individuals of an animal or plant can be reared only
where the conditions for its propagation are favourable. When the
individuals are scanty, all will be allowed to breed, whatever their
quality may be, and this will efTectually prevent selection. But
probably the most important element is that the animal or plant
should be so highly valued by man, that the closest attention is paid
to even the slightest deviations in its qualities or structure. Unless
such attention be paid nothing can be effected. I have seen it gravely
remarked, that it was most fortunate that the strawberry began to
vary just when gardeners began to attend to this plant. No doubt
the strawberry had always varied since it was cultivated, but the
slight varieties had been neglected. As soon, however, as gardeners
picked out individual plants with sHghtly larger, earlier, or better
fruit, and raised seedlings from them, and again picked out the best
seedlings and bred from them, then (with some aid by crossing
distinct species) those many admirable varieties of the strawberry
were raised which have appeared during the last half-century.

With animals, facility in preventing crosses is an important ele-
ment in the formation of new races, — at least, in a country which is
already stocked with other races. In this respea enclosure of the
land plays a part. Wandering savages or the inhabitants of open
plains rarely possess more than one breed of the same species. Pi-
geons can be mated for life, and this is a great convenience to the
fancier, for thus many races may be improved and kept true, though
mingled in the same aviary; and this circumstance must have largely
favoured the formation of new breeds. Pigeons, I may add, can be
propagated in great numbers and at a very quick rate, and inferior
birds may be freely rejected, as when killed they serve for food. On
the other hand, cats, from their nocturnal rambling habits, cannot
be easily matched, and, although so much valued by women and
children, we rarely see a distinct breed long kept up; such breeds as
we do sometimes see are almost always imported from some other
country. Although I do not doubt that some domestic animals vary
less than others, yet the rarity or absence of distinct breeds of the
cat, the donkey, peacock, goose, etc., may be attributed in main part
to selection not having been brought into play: in cats, from the

52 ORIGIN OF SPECIES

difficulty in pairing them; in donkeys, from only a few being kept
by poor people, and little attention paid to their breeding; for recently
in certain parts of Spain and of the United States this animal has
been surprisingly modified and improved by careful selection; in pea-
cocks, from not being very easily reared and a large stock not kept;
in geese, from being valuable only for two purposes, food and
feathers, and more especially from no pleasure having been felt in
the display of distinct breeds; but the goose, under the conditions
to which it is exposed when domesticated, seems to have a singularly
inflexible organisation, though it has varied to a slight extent, as I
have elsewhere described.

Some authors have maintained that the amount of variation in
our domestic productions is soon reached, and can never afterwards
be exceeded. It would be somewhat rash to assert that the limit has
been attained in any one case; for almost all our animals and plants
have been greatly improved in many ways within a recent period;
and this implies variation. It would be equally rash to assert that
characters now increased to their usual limit, could not, after remain-
ing fixed for many centuries, again vary under new conditions of
life. No doubt, as Mr. Wallace has remarked with much truth, a
limit will be at last reached. For instance, there must be a limit to
the fleetness of any terrestrial animal, as this will be determined by
the friction to be overcome, the weight of body to be carried, and the
power of contraction in the muscular fibres. But what concerns us
is that the domestic varieties of the same species differ from each
other in almost every character, which man has attended to and
selected, more than do the distinct species of the same genera. Isidore
Geoffroy St. Hilaire has proved this in regard to size, and so it is
with colour and probably with the length of hair. With respect to
fleetness, which depends on many bodily characters. Eclipse was far
fleeter, and a dray horse is incomparably stronger than any two
natural species belonging to the same genus. So with plants, the
seeds of the different varieties of the bean or maize probably differ
more in size, than do the seeds of the distinct species in any one
genus in the same two families. The same remark holds good in
regard to the fruit of the several varieties of the plum, and still more
strongly with the melon, as well as in many other analogous cases.

CIRCUMSTANCES FAVOURABLE TO SELECTION 53

To sum up on the origin of our domestic races of animals and
plants. Changed conditions of life are of the highest importance in
causing variability, both by acting directly on the organisation, and
indirectly by affecting the reproductive system. It is not probable
that variability is an inherent and necessary contingent, under all
circumstances. The greater or less force of inheritance and reversion
determine whether variations shall endure. Variability is governed
by many unknown laws, of which correlated growth is probably the
most important. Something, but how much we do not know, may
be attributed to the definite action of the conditions of life. Some,
perhaps a great, effect may be attributed to the increased use or dis-
use of parts. The final result is thus rendered infinitely complex.
In some cases the intercrossing of aboriginally distinct species appears
to have played an important part in the origin of our breeds. When
several breeds have once been formed in any country, their occasional
intercrossing, with the aid of selection, has, no doubt, largely aided
in the formation of new sub-breeds; but the importance of crossing
has been much exaggerated, both in regard to animals and to those
plants which are propagated by seed. With plants which are tempo-
rarily propagated by cuttings, buds, etc., the importance of crossing
is immense; for the cultivator may here disregard the extreme varia-
bility both of hybrids and of mongrels, and the sterility of hybrids;
but plants not propagated by seed are of little importance to us, for
their endurance is only temporary. Over all these causes of change,
the accumulative action of Selection, whether applied methodically
and quickly, or unconsciously and slowly, but more efficiently, seems
to have been the predominant power.

CHAPTER II

Variation Under Nature

Variability — Individual differences — Doubtful species — Wide ranging,
much diffused, and common sf>ecies, vary most — Species of the
larger genera in each country vary more frequendy than the species
of the smaller genera — Many of the sjjecies of the larger genera
resemble varieties in being very closely, but unequally, related to each
other, and in having restricted ranges.

"ORE applying the principles arrived at in the last chapter
to organic beings in a state of nature, we must briefly discuss
whether these latter are subject to any variation. To treat this
subject properly, a long catalogue of dry facts ought to be given; but
these I shall reserve for a future work. Nor shall I here discuss the
various definitions which have been given of the term species. No
one definition has satisfied all naturalists; yet every naturalist knows
vaguely what he means when he sjjeaks of a species. Generally the
term includes the unknown element of a distinct act of creation.
The term "variety" is almost equally difficult to define; but here
community of descent is almost universally implied, though it can
rarely be proved. We have also what are called monstrosities; but
they graduate into varieties. By a monstrosity I presume is meant
some considerable deviation of structure, generally injurious, or not
useful to the species. Some authors use the term "variation" in a
technical sense, as implying a modification directly due to the
physical conditions of life; and "variations" in this sense are sup-
posed not to be inherited; but who can say that the dwarfed condi-
tion of shells in the brackish waters of the Baltic, or dwarfed plants
on Alpine summits, or the thicker fur of an animal from far north-
wards, would not in some cases be inherited for at least a few genera-
tions.^ And in this case I presume that the form would be called a
variety.

It may be doubted whether sudden and considerable deviations of
structure such as we occasionally see in our domestic productions,

54

INDIVIDUAL DIFFERENCES 55

more especially with plants, are ever permanently propagated in a
state of nature. Almost every part of every organic being is so beau-
tifully related to its complex conditions of life that it seems as im-
probable that any part should have been suddenly produced perfect,
as that a complex machine should have been invented by man in a
perfect state. Under domestication monstrosities sometimes occur
which resemble normal structures in widely different animals. Thus
pigs have occasionally been born with a sort of proboscis, and if any
wild species of the same genus had naturally possessed a proboscis,
it might have been argued that this had appeared as a monstrosity;
but I have as yet failed to find, after diligent search, cases of mon-
strosities resembling normal structures in nearly allied forms, and
these alone bear on the question. If monstrous forms of this kind
ever do appear in a state of nature and are capable of reproduction
(which is not always the case), as they occur rarely and singly, their
preservation would depend on unusually favourable circumstances.
They would, also, during the first and succeeding generations cross
with the ordinary form, and thus their abnormal character would
almost inevitably be lost. But I shall have to return in a future
chapter to the preservation and perpetuation of single or occasional
variations.

INDIVIDUAL DIFFERENCES

The many slight differences which appear in the offspring from
the same parents, or which it may be presumed have thus arisen,
from being observed in the individuals of the same species inhabiting
the same confined locality, may be called individual differences.
No one supposes that all the individuals of the same species are cast
in the same actual mould. These individual differences are of the
highest imporunce for us, for they are often inherited, as must be
familiar to every one; and they thus afford materials for natural
selection to act on and accumulate, in the same manner as man
accumulates in any given direction individual differences in his
domesticated productions. These individual differences generally
affect what naturalists consider unimportant parts; but I could
show, by a long catalogue of faas, that parts which must be called
important, whether viewed under a physiological or dassificatory

56 ORIGIN OF SPECIES

point of view, sometimes vary in the individuals of the same species.
I am convinced that the most experienced naturaUst would be sur-
prised at the number of the cases of variability, even in important
parts of structure, which he could collect on good authority, as I
have collected, during a course of years. It should be remembered
that systematists are far from being pleased at finding variability in
important characters, and that there are not many men who will
laboriously examine internal and important organs, and compare
them in many specimens of the same species. It would never have
been expected that the branching of the main nerves close to the
great central ganglion of an insect would have been variable in the
same species; it might have been thought that changes of this nature
could have been effected only by slow degrees; yet Sir J. Lubbock
has shown a degree of variability in these main nerves in Coccus,
which may almost be compared to the irregular branching of the
stem of a tree. This philosophical naturalist, I may add, has also
shown that the muscles in the larvae of certain insects are far from
uniform. Authors sometimes argue in a circle when they state that
important organs never vary; for these same authors practically
rank those parts as important (as some few naturalists have hon-
estly confessed) which do not vary; and, under this point of view,
no instance will ever be found of an important part varying; but
under any other point of view many instances assuredly can be
given.

There is one point connected with individual differences, which
is extremely perplexing: I refer to those genera which have been
called "protean" or "polymorphic," in which the species present an
inordinate amount of variation. With respect to many of these
forms, hardly two naturalists agree whether to rank them as species
or as varieties. We may instance Rubus, Rosa, and Hieracium
amongst plants, several genera of insects and of Brachiopod shells.
In most polymorphic genera some of the species have fixed and
definite characters. Genera which are polymorphic in one country,
seem to be, with a few exceptions, polymorphic in other countries,
and likewise, judging from Brachiopod shells, at former periods of
time. These facts are very perplexing, for they seem to show that
this kind of variability is independent of the conditions of life. I

INDIVIDUAL DIFFERENCES 57

am inclined to suspect that we see, at least in some of these poly-
morphic genera, variations which are of no service or disservice to
the species, and which consequently have not been seized on and
rendered definite by natural selection, as hereafter to be explained.

Individuals of the same species often present, as is known to
every one, great differences of structure, independently of varia-
tion, as in the two sexes of various animals, in the two or three
castes of sterile female or workers amongst insects, and in the
immature and larval states of many of the lower animals.

There are, also, cases of dimorphism and trimorphism, both
with animals and plants. Thus, Mr. Wallace, who has lately called
attention to the subject, has shown that the females of certain species
of butterflies, in the Malayan Archipelago, regularly appeared under
two or even three conspicuously distinct forms, not connected by
intermediate varieties. Fritz Miiller has described analogous but
more extraordinary cases with the males of certain Brazilian crus-
taceans: thus, the male of a Tanais regularly occurs under two
distinct forms; one of these has strong and differently shaped pincers,
and the other has antenns much more abundantly furnished with
smelling-hairs. Although in most of these cases, the two or three
forms, both with animals and plants, are not now connected by
intermediate gradations, it is probable that they were once thus
connected. Mr. Wallace, for instance, describes a certain butterfly
which presents in the same island a great range of varieties con-
nected by intermediate links, and the extreme links of the chain
closely resemble the two forms of an allied dimorphic species in-
habiting another part of the Malay Archipelago. Thus also with
ants, the several worker-castes are generally quite distinct; but in
some cases, as we shall hereafter see, the castes are connected to-
gether by finely graduated varieties. So it is, as I have myself
observed, with some dimorphic plants. It certainly at first apf)ears
a highly remarkable fact that the same female butterfly should have
the power of producing at the same time three distinct female forms
and a male; and that an hermaphrodite plant should produce from
the same seed<apsule three distinct hermaphrodite forms, bearing
three different kinds of females and three or even six different kinds
of males. Nevertheless these cases are only exaggerations of the

58 ORIGIN OF SPECIES

common fact that the female produces offspring of two sexes which
sometimes differ from each other in a wonderful manner.

DOUBTFUL SPECIES

The forms which possess in some considerable degree the char-
acter of species, but which are so closely similar to other forms, or
are so closely linked to them by intermediate gradations, that natu-
ralists do not like to rank them as distinct species, are in several
respects the most important for us. We have every reason to beUeve
that many of these doubtful and closely aUied forms have perma-
nently retained their characters for a long time; for as long, as far
as we know, as have good and true species. Practically, when a
naturalist can unite by means of intermediate links any two forms,
he treats the one as a variety of the other; ranking the most common,
but sometimes the one first described, as the species, and the other
as the variety. But cases of great difficulty, which I will not here
enumerate, sometimes arise in deciding whether or not to rank one
form as a variety of another, even when they are closely connected
by intermediate links; nor will the conunonly assumed hybrid
nature of the intermediate forms always remove the difficulty. In
very many cases, however, one form is ranked as a variety of another,
not because the intermediate links have actually been found, but
because analogy leads the observer to suppose either that they do
now somewhere exist, or may formerly have existed; and here a
wide door for the entry of doubt and conjecture is opened.

Hence, in determining whether a form should be ranked as a
species or a variety, the opinion of naturalists having sound judg-
ment and wide experience seems the only guide to follow. We
must, however, in many cases, decide by a majority of natural-
ists, for few well-marked and well-known varieties can be named
which have not been ranked as species by at least some competent
judges.

That varieties of this doubtful nature are far from uncommon,
cannot be disputed. Compare the several floras of Great Britain, of
France, or of the United States, drawn up by different botanists,
and see what a surprising number of forms have been ranked by one
botanist as good species, and by another as mere varieties. Mr. H. C.

DOUBTFUL SPECIES 59

Watson, to whom I lie under deep obligation for assistance of all
kinds, has marked for me 182 British plants, which are generally
considered as varieties, but which have all been ranked by botanists
as species; and in making this list he has omitted many trifling
varieties, but which nevertheless have been ranked by some botanists
as species, and he has entirely omitted several highly polymorphic
genera. Under genera, including the most polymorphic forms, Mr.
Babington gives 251 species, whereas Mr. Bentham gives only 112, —
a difference of 139 doubtful forms! Amongst animals which unite
for each birth, and which are highly locomotive, doubtful forms,
ranked by one zoologist as a species and by another as a variety, can
rarely be found within the same country, but are common in sepa-
rated areas. How many of the birds and insects in North America
and Eurof)e, which differ very slightly from each other, have been
ranked by one eminent naturalist as undoubted species, and by
another as varieties, or, as they are often called, geographical races!
Mr. Wallace, in several valuable papers on the various animals,
especially on the Lepidoptera, inhabiting the islands of the great
Malayan Archipelago, show that they may be classed under four
heads, namely, as variable forms, as local forms, as geographical
races or sub-sp)ecies, and as true representative species. The first or
variable forms vary much within the limits of the same island. The
local forms are moderately constant and distinct in each separate
island; but when all from the several islands are compared together,
the differences are seen to be so slight and graduated, that it is
impossible to define or describe them, though at the same time the
extreme forms are sufficiently distinct. The geographical races or
sub-species are local forms completely fixed and isolated; but as
they do not differ from each other by strongly marked and impor-
tant characters, "There is no possible test but individual opinion to
determine which of them shall be considered as species and which
as varieties." Lastly, representative sf)ecies fill the same place in the
natural economy of each island as do the local forms and sub-
species; but as they are distinguished from each other by a greater
amount of difference than that between the local forms and sub-
species, they are almost universally ranked by naturalists as true
species. Nevertheless, no certain criterion can possibly be given by

60 ORIGIN OF SPECIES

which variable forms, local forms, sub-species, and representative
species can be recognised.

Many years ago, when comparing, and seeing others compare, the
birds from the closely neighbouring islands of the Galapagos
archip)elago, one with another, and with those from the American
mainland, I was much struck how entirely vague and arbitrary is the
distinction between species and varieties. On the islets of the little
Madeira group there are many insects which are characterised as
varieties in Mr. WoUaston's admirable work, but which would cer-
tainly be ranked as distinct species by many entomologists. Even
Ireland has a few animals, now generally regarded as varieties, but
which have been ranked as species by some zoologists. Several ex-
perienced ornithologists consider our British red grouse as only a
strongly marked race of Norwegian species, whereas the greater
number rank it as an undoubted species peculiar to Great Britain.
A wide distance between the homes of two doubtful forms leads
many naturahsts to rank them as distinct species; but what distance,
it has been well asked, will suffice; if that between America and
Europe is ample, will that between Europe and the Azores, or
Madeira, or the Canaries, or between the several islets of these small
archipelagos, be sufficient.'

Mr. B. D. Walsh, a distinguished entomologist of the United
States, has described what he calls Phytophagic varieties and Phyto-
phagic species. Most vegetable-feeding insects live on one kind of
plant or on one group of plants; some feed indiscriminately on many
kinds, but do not in consequence vary. In several cases, however,
insects found living on different plants, have been observed by Mr.
Walsh to present in their larval or mature state, or in both states,
slight, though constant differences in colour, size, or in the nature of
their secretions. In some instances the males alone, in other instances
both males and females, have been observed thus to differ in a slight
degree. When the differences are rather more strongly marked, and
when both sexes and all ages are affected, the forms are ranked by
all entomologists as good species. But no observer can determine
for another, even if he can do so for himself, which of these Phyto-
phagic forms ought to be called species and which varieties. Mr.
Walsh ranks the forms which it may be supposed would freely inter-

DOUBTFUL SPECIES 6 1

cross, as varieties; and those which appear to have lost this power,
as species. As the differences depend on the insects having long fed
on distinct plants, it cannot be expected that intermediate links
connecting the several forms should now be found. The naturalist
thus loses his best guide in determining whether to rank doubtful
forms as varieties or species. This likewise necessarily occurs with
closely allied organisms, which inhabit distinct continents or islands.
When, on the other hand, an animal or plant ranges over the same
continent, or inhabits many islands in the same archipelago, and
presents different forms in the different areas, there is always a good
chance that intermediate forms will be discovered which will link
together the extreme states; and these arc then degraded to the rank
of varieties.

Some few naturalists maintain that animals never present varie-
ties; but then these same naturalists rank the slightest difference as
of specific value; and when the same identical form is met with in
two distinct countries, or in two geological formations, they believe
that two distinct species are hidden under the same dress. The term
species thus comes to be a mere useless abstraction, implying and as-
suming a separate act of creation. It is certain that many forms, con-
sidered by highly competent judges to be varieties, resemble species so
completely in character, that they have been thus ranked by other
highly competent judges. But to discuss whether they ought to be
called species or varieties, before any definition of these terms has
been generally accepted, is vainly to beat the air.

Many of the cases of strongly-marked varieties or doubtful species
well deserve consideration; for several interesting lines of argu-
ment, from geographical distribution, analogical variation, hybridism,
etc., have been brought to bear in the attempt to determine their
rank; but space does not here permit me to discuss them. Close in-
vestigation, in many cases, will no doubt bring naturalists to agree
how to rank doubtful forms. Yet it must be confessed that it is in
the best known countries that we find the greatest number of them.
I have been struck with the fact, that if any animal or plant in a state
of nature be highly useful to man, or from any cause closely attracts
his attention, varieties of it will almost universally be found recorded.
These varieties, moreover, will often be ranked by some authors as

62 ORIGIN OF SPECIES

species. Look at the common oak, how closely it has been studied;
yet a German author makes more than a dozen species out of forms,
which are almost universally considered by other botanists to be varie-
ties; and in this country the highest botanical authorities and prac-
tical men can be quoted to show that the sessile and pedunculated
oaks are either good and distinct species or mere varieties.

I may here allude to a remarkable memoir lately published by
A. de CandoUe, on the oaks of the whole world. No one ever had
more ample materials for the discrimination of the species, or could
have worked on them with more zeal and sagacity. He first gives
in detail all the many points of structure which vary in the several
species, and estimates numerically the relative frequency of the
variations. He specifies above a dozen characters which may be
found varying even on the same branch, sometimes according to
age or development, sometimes without any assignable reason. Such
characters are not of course of specific value, but they are, as Asa
Gray has remarked in commenting on this memoir, such as gen-
erally enter into specific definitions. De Candolle then goes on to
say that he gives the rank of species to the forms that difTer by
characters never varying on the same tree, and never found con-
nected by intermediate states. After this discussion, the result of so
much labour, he emphatically remarks: "They are mistaken, who
repeat that the greater part of our species are clearly limited, and
that the doubtful s^iecies are in a feeble minority. This seemed to
be true, so long as a genus was imperfectly known, and its species
were founded upon a few specimens, that is to say, were provisional.
Just as we come to know them better, intermediate forms flow in,
and doubts as to specific limits augment." He also adds that it is
the best known species which present the greatest number of spon-
taneous varieties and sub-varieties. The Quercus robur has twenty-
eight varieties, all of which, excepting six, are clustered round three
sub-species, namely, Q. pedunculata, sessiliflora, and pubescens. The
forms which connect these three sub-species are comparatively rare;
and, as Asa Gray again remarks, if these connecting forms which
are now rare, were to become wholly extinct, the three sub-species
would hold exactly the same relation to each other, as do the four
or five provisionally admitted species which closely surround the

DOUBTFUL SPECIES 63

typical Quercus robur. Finally, De Candolie admits that out of the
300 species, which will be enumerated in his Prodromus as belong-
ing to the oak family, at least two-thirds are provisional species, that
is, are not known strictly to fulfil the definition above given of a
true species. It should be added that De Candolie no longer believes
that species are immutable creations, but concludes that the derivative
theory is the most natural one, "and the most accordant with the
known facts in paLtontology, geographical botany, and zoology, of
anatomical structure and classification."

When a young naturalist commences the study of a group of
organisms quite unknown to him, he is at first much perplexed in
determining what differences to consider as specific, and what as
varietal; for he knows nothing of the amount and kind of variation
to which the group is subject; and this shows, at least, how very
generally there is some variation. But if he confine his attention to
one class within one country, he will soon make up his mind how to
rank most of the doubtful forms. His general tendency will be to
make many species, for he will become impressed, just like the
pigeon or poultry fancier before alluded to, with the amount of
difference in the forms which he is continually studying; and he
has little general knowledge of analogical variation in other groups
and in other countries, by which to correct his first impressions. As
he extends the range of his observations, he will meet with more
cases of difBculty; for he will encounter a greater number of closely
allied forms. But if his observations be widely extended, he will in
the end generally be able to make up his own mind; but he will
succeed in this at the expense of admitting much variation, and the
truth of this admission will often be disputed by other naturalists.
When he comes to study allied forms brought from countries not
now continuous, in which case he cannot hope to find intermediate
links, he will be compelled to trust almost entirely to analogy, and
his difficulties will rise to a climax.

Certainly no clear line of demarcation has as yet been drawn be-
tween species and sub-species — that is, the forms which in the opinion
of some naturalists come very near to, but do not quite arrive at, the
rank of species: or, again, between sub-species and well-marked
varieties, or between lesser varieties and individual differences.

64 ORIGIN OF SPECIES

These differences blend into each other by an insensible series; and
a series impresses the mind with the idea of an actual passage.

Hence I look at individual differences, though of small interest
to the systematist, as of the highest importance for us, as being the
first steps towards such slight varieties as are barely thought worth
recording in works on natural history. And I look at varieties which
are in any degree more distinct and permanent, as steps towards
more strongly-marked and permanent varieties; and at the latter, as
leading to sub-species, and then to species. The passage from one
stage of difference to another may, in many cases, be the simple re-
sult of the nature of the organism and of the different physical con-
ditions to which it has long been exposed; but with respect to the
more important and adaptive characters, the passage from one stage
of difference to another, may be safely attributed to the cumulative
action of natural selection, hereafter to be explained, and to the
effects of the increased use or disuse of parts. A well-marked variety
may therefore be called an incipient species; but whether this belief
is justifiable must be judged by the weight of the various facts and
considerations to be given throughout this work.

It need not be supposed that all varieties or incipient species attain
the rank of species. They may become extinct, or they may endure
as varieties for very long periods, as has been shown to be the case
by Mr. Wollaston with the varieties of certain fossil land shells in
Madeira, and with plants by Gaston de Saporta. If a variety were
to flourish so as to exceed in numbers the parent species, it would
then rank as the species, and the species as the variety; or it might
come to supplant and exterminate the parent species; or both might
co-exist, and both rank as independent species. But we shall here-
after return to this subject.

From these remarks it will be seen that I look at the term species
as one arbitrarily given, for the sake of convenience, to a set of
individuals closely resembling each other, and that it does not es-
sentially differ from the term variety, which is given to less distinct
and more fluctuating forms. The term variety, again, in comparison
with mere individual differences, is also applied arbitrarily, for con-
venience' sake.

DOMINANT SPECIES VARY MOST 65

WIDE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES VARY MOST

Guided by theoretical considerations, I thought that some inter-
esting results might be obtained in regard to the nature and rela-
tions of the species which vary most, by tabulating all the varieties
in several well-worked floras. At first this seemed a simple task;
but Mr. H. C. Watson, to whom I am much indebted for valuable
advice and assistance on this subject, soon convinced me that there
were many difficulties as did subsequently Dr. Hooker, even in
stronger terms. I shall reserve for a future work the discussion of
these difficulties, and the tables of the propwrtional numbers of the
varying species. Dr. Hooker permits me to add that after having
carefully read my manuscript, and examined the tables, he thinks
that the following statements are fairly well established. The whole
subject, however, treated as it necessarily here is with much brevity,
is rather perplexing, and allusions cannot be avoided to the "struggle
for existence," "divergence of character," and other questions, here-
after to be discussed.

Alphonse de Candolle and others have shown that plants which
have very wide ranges generally present varieties; and this might
have been expected, as they are exposed to diverse physical condi-
tions, and as they come into competition (which, as we shall here-
after see, is an equally or more important circumstance) with dif-
ferent sets of organic beings. But my tables further show, that, in
any limited country, the species which are the most common, that
is, abound most in individuals, and the species which are most
widely diffused within their own country (and this is a different
consideration from wide range, and to a certain extent from com-
monness), oftenest give rise to varieties sufficiently well-marked to
have been recorded in botanical works. Hence it is the most flourish-
ing, or, as they may be called, the dominant species, — those which
range widely, are the most diffused in their own country, and are
the most numerous in individuals, — which oftenest produce well-
marked varieties, or, as I consider them, incipient species. And this,
perhaps, might have been anticipated; for, as varieties, in order to
become in any degree permanent, necessarily have to struggle with
the other inhabitants of the country, the species which are already

66 ORIGIN OF SPECIES

dominant will be the most likely to yield offspring, which, though
in some slight degree modified, still inherit those advantages that
enabled their parents to become dominant over their compatriots.
In these remarks on predominance, it should be understood that
reference is made only to the forms which come into competition
with each other, and more especially to the members of the same
genus or class having nearly similar habits of life. With respect to
the number of individuals, or commonness of species, the comparison
of course relates only to the members of the same group. One of
the higher plants may be said to be dominant if it be more numerous
in individuals and more widely diffused than the other plants of the
same country, which live under nearly the same conditions. A plant
of this kind is not the less dominant because some conferva inhabit-
ing the water or some parasitic fungus is infinitely more numerous
in individuals, and more widely diffused. But if the conferva or
parasitic fungus exceeds its allies in the above respects, it will then
be dominant within its own class.

SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY MORE FREQUENTLY
THAN THE SPECIES OF THE SMALLER GENERA

If the plants inhabiting a country, as described in any Flora, be
divided into two equal masses, all those in the larger genera (/>.,
those including many species) being placed on one side, and all those
in the smaller genera on the other side, the former will be found to
include a somewhat larger number of the very common and much
diffused or dominant species. This might have been anticipated; for
the mere fact of many sp>ecies of the same genus inhabiting any
country, shows that there is something in the organic or inorganic
conditions of that country favourable to the genus; and, consequently,
we might have expected to have found in the larger genera, or those
including many species, a larger proportional number of dominant
species. But so many causes tend to obscure this result, that I am
surprised that my tables show even a small majority on the side of
the larger genera. I will here allude to only two causes of obscurity.
Fresh-water and salt-loving plants generally have very wide ranges
and are much diffused, but this seems to be connected with the
nature of the stations inhabited by them, and has little or no relation

SPECIES OF LARGER GENERA VARIABLE d"]

to the size o£ the genera to which the species belong. Again, plants
low in the scale o£ organisation are generally much more widely
difTused than plants higher in the scale; and here again there is no
close relation to the size of the genera. The cause of lowly-organised
plants ranging widely will be discussed in our chapter on Geographi-
cal Distribution.

From looking at species as only strongly marked and well-defined
varieties, I was led to anticipate that the species of the larger genera
in each country would oftener present varieties, than the species of
the smaller genera; for wherever many closely related species (/>.,
species of the same genus) have been formed, many varieties or
incipient species ought, as a general rule, to be now forming. Where
many large trees grow, we expect to find saplings. Where many
species of a genus have been formed through variation, circumstances
have been favourable for variation; and hence we might exp>ect that
the circumstances would generally be still favourable to variation.
On the other hand, if we look at each sp)ecies as a special act of
creation, there is no apparent reason why more varieties should occur
in a group having many species, than in one having few.

To test the truth of this anticipation I have arranged the plants of
twelve countries, and the coleopterous insects of two districts, into
two nearly equal masses, the species of the larger genera on one side,
and those of the smaller genera on the other side, and it has invariably
proved to be the case that a larger proportion of the species on the
side of the larger genera presented varieties, than on the side of the
smaller genera. Moreover, the species of the large genera which
present any varieties, invariably present a larger average number of
varieties than do the species of the small genera. Both these results
follow when another division is made, and when all the least genera,
with from only one to four species, are altogether excluded from the
tables. These facts are of plain signification on the view that species
are only strongly-marked and permanent varieties; for wherever
many species of the same genus have been formed, or where, if we
may use the expression, the manufactory of species has been active,
we ought generally to find the manufactory still in action, more
especially as we have every reason to believe the process of manu-
facturing new species to be a slow one. And this certainly holds true,

68 ORIGIN OF SPECIES

if varieties be looked at as incipient species; for my tables clearly
show as a general rule that, wherever many species of a genus have
been formed, the species of that genus present a number of varieties,
that is, of incipient species, beyond the average. It is not that all large
genera are now varying much, and are thus increasing in the number
of their species, or that no small genera are now varying and increas-
ing; for if this had been so, it would have been fatal to my theory;
inasmuch as geology plainly tells us that small genera have in the
lapse of time often increased greatly in size; and that large genera
have often come to their maxima, decline, and disappeared. All that
we want to show is, that, where many sf)ecies of a genus have been
formed, on an average many are still forming; and this certainly
holds good.

MANY OF THE SPECIES INCLUDED WITHIN THE LARGER GENERA RESEMBLE
VARIETIES IN BEING VERY CLOSELY, BUT UNEQUALLY, RELATED TO EACH
OTHER, AND IN HAVING RESTRICTED RANGES

There are other relations between the sj^ecies of large genera and
their recorded varieties which deserve notice. We have seen that
there is no infallible criterion by which to distinguish species and
well-marked varieties; and when intermediate links have not been
found between doubtful forms, naturalists are compelled to come to
a determination by the amount of difference between them, judging
by analogy whether or not the amount suffices to raise one or both
to the rank of species. Hence the amount of difference is one very
important criterion in settling whether two forms should be ranked
as species or varieties. Now Fries has remarked in regard to plants,
and Westwood in regard to insects, that in large genera the amount
of difference between the species is often exceedingly small. I have
endeavoured to test this numerically by averages, and, as far as my
imperfect results go, they confirm the view. I have also consulted
some sagacious and experienced observers, and, after deliberation,
they concur in this view. In this respect, therefore, the species of the
larger genera resemble varieties, more than do the species of the
smaller genera. Or the case may be put in another way, and it may be
said, that in the larger genera, in which a number of varieties or incip-
ient species greater than the average are now manufacturing, many of

RESEMBLE VARIETIES 69

the species already manufactured still to a certain extent resemble
varieties, for they differ from each other by less than the usual amount
of difference.

Moreover, the species of the larger genera are related to each other
in the same manner as the varieties of any one species are related to
each other. No naturalist pretends that all the species of a genus are
equally distinct from each other; they may generally be divided
into sub-genera, or sections, or lesser groups. As Fries has well
remarked, little groups of species are generally clustered like satel-
lites around other species. And what are varieties but groups of
forms, unequally related to each other, and clustered round certain
forms — that is, round their parent-species? Undoubtedly there is one
most im[X)rtant point of difference between varieties and species;
namely, that the amount of difference between varieties, when com-
pared with each other or with their parent-species, is much less than
that between the species of the same genus. But when we come to dis-
cuss the principle, as I call it, of Divergence of Character, we shall see
how this may be explained, and how the lesser differences between
varieties tend to increase into the greater differences between
species.

There is one other point which is worth notice. Varieties generally
have much restricted ranges: this statement is indeed scarcely more
than a truism, for, if a variety were found to have a wider range than
that of its supposed parent-species, their denominations would be
reversed. But there is reason to believe that the species which are
very closely allied to other species, and in so far resemble varieties,
often have much restricted ranges. For instance, Mr. H. C. Watson
has marked for me in the well-sifted London Catalogue of Plants
(4th edition) sixty-three plants which are therein ranked as species,
but which he considers as so closely allied to other species as to be
of doubtful value: these sixty-three reputed species range on an
average over 6.9 of the provinces into which Mr. Watson has divided
Great Britain. Now, in this same catalogue, fifty-three acknowledged
varieties are recorded, and these range over 7.7 provinces; whereas,
the species to which these varieties belong range over 14.3 provinces.
So that the acknowledged varieties have nearly the same restricted
average range, as have the closely allied forms, marked for me by

70 ORIGIN OF SPECIES

Mr. Watson as doubtful species, but which are almost universally
ranked by British botanists as good and true species.

SUMMARY

Finally, varieties cannot be distinguished from species, — except,
first, by the discovery of intermediate linking forms; and, secondly,
by a certain indefinite amount of difference between them; for two
forms, if differing very little, are generally ranked as varieties, not-
withstanding that they cannot be closely connected; but the amount
of difference considered necessary to give to any two forms the rank
of species cannot be defined. In genera having more than the average
number of species in any country, the species of these genera have
more than the average number of varieties. In large genera the
species are aprt to be closely, but unequally, allied together, forming
little clusters round other species. Species very closely allied to other
species apparently have restricted ranges. In all these respects the
species of large genera present a strong analogy with varieties. And
we can clearly understand these analogies, if species once existed as
varieties, and thus originated; whereas, these analogies are utterly
inexplicable if species are independent creations.

We have, also, seen that it is the most flourishing or dominant
species of the larger genera within each class which on an average
yield the greatest number of varieties; and varieties, as we shall
hereafter see, tend to become converted into new and distinct species.
Thus the larger genera tend to become larger; and throughout nature
the forms of life which are now dominant tend to become still more
dominant by leaving many modified and dominant descendants.
But by steps hereafter to be explained, the larger genera also tend to
break up into smaller genera. And thus, the forms of life throughout
the universe become divided into groups subordinate to groups.

CHAPTER III

Struggle for Existence

Its bearing on natural selection — The term used in a wide sense — Geo-
metrical ratio of increase — Rapid increase of naturalized animals and
plants — Nature of the checks to increase — Competition universal —
Effects of climate — Protection from the number of individuals —
Complex relations of all animals and plants throughout nature —
Struggle for life most severe between individuals and varieties of the
same species: often severe between sjiecies of the same genus — The
relation of organism to organism the most important of all relations.

EFORE entering on the subject of this chapter, I must make
a few preliminary remarks, to show how the struggle for
existence bears on Natural Selection. It has been seen in the
last chapter that amongst organic beings in a state of nature there is
some individual variability: indeed I am not aware that this has ever
been disputed. It is immaterial for us whether a multitude of doubt-
ful forms be called species or sub-species or varieties; what rank,
for instance, the two or three hundred doubtful forms of British
plants are entitled to hold, if the existence of any well-marked vari-
eties be admitted. But the mere existence of individual variability
and of some few well-marked varieties, though necessary as the
foundation for the work, helps us but little in understanding how
species arise in nature. How have all those exquisite adaptations of
one part of the organisation to another part, and to the conditions
of life, and of one organic being to another being, been perfected?
We see these beautiful co-adaptations most plainly in the woodpecker
and the mistletoe; and only a little less plainly in the humblest para-
site which clings to the hairs of a quadruped or feathers of a bird: in
the structure of the beetle which dives through the water: in
the plumed seed which is wafted by the gentlest breeze; in short, we
see beautiful adaptations everywhere and in every part of the organic
world.
Again, it may be asked, how is it that varieties, which I have called

71

72 ORIGIN OF SPECIES

incipient species, become ultimately converted into good and distinct
species, which in most cases obviously differ from each other far more
than do the varieties of the same species ? How do those groups of
species, which constitute what are called distinct genera, and which
differ from each other more than do the species of the same genus,
arise? All these results, as we shall more fully see in the next chapter,
follow from the struggle for life. Owing to this struggle, variations,
however slight and from whatever cause proceeding, if they be in
any degree profitable to the individuals of a species, in their infinitely
complex relations to other organic beings and to their physical condi-
tions of life, will tend to the preservation of such individuals, and
will generally be inherited by the offspring. The offspring, also, will
thus have a better chance of surviving, for, of the many individuals
of any species which are periodically born, but a small number can
survive. I have called this principle, by which each slight variation,
if useful, is preserved, by the term Natural Selection, in order to mark
its relation to man's power of selection. But the expression often
used by Mr. Herbert Spencer, of the Survival of the Fittest, is more
accurate, and is sometimes equally convenient. We have seen that
man by selection can certainly produce great results, and can adapt
organic beings to his own uses, through the accumulation of slight
but useful variations, given to him by the hand of Nature. But
Natural Selection, as we shall hereafter see, is a power incessantly
ready for action, and is as immeasurably superior to man's feeble
efforts, as the works of Nature are to those of Art.

We will now discuss in a Httle more detail the struggle for ex-
istence. In my future work this subject will be treated, as it well
deserves, at greater length. The elder De Candolle and Lyell have
largely and philosophically shown that all organic beings are exposed
to severe competition. In regard to plants, no one has treated this
subject with more spirit and ability than W. Herbert, Dean of Man-
chester, evidently the result of his great horticultural knowledge.
Nothing is easier than to admit in words the truth of the universal
Struggle for life, or more difficult — at least, I have found it so — than
constantly to bear this conclusion in mind. Yet unless it be thor-
oughly engrained in the mind, the whole economy of nature, with
every fact on distribution, rarity, abundance, extinction, and vari-

STRUGGLE FOR EXISTENCE 73

ation, will be dimly seen or quite misunderstood. We behold the
face of nature bright with gladness, we often see superabundance
of food; we do not see, or we forget, that the birds which are idly
singing round us mostly live on insects or seeds, and are thus con-
stantly destroying life; or we forget how largely these songsters, or
their eggs, or their nestlings, are destroyed by birds and beasts of
prey; we do not always bear in mind, that, though food may be now
superabundant, it is not so at all seasons of each recurring year.

THE TERM, STRUGGLE FOR EXISTENCE, USED IN A LARGE SENSE

I should premise that I use this term in a large and metaphorical
sense including dependence of one being on another, and including
(which is more important) not only the life of the individual, but
success in leaving progeny. Two canine animals, in a time of dearth,
may be truly said to struggle with each other which shall get food
and live. But a plant on the edge of a desert is said to struggle for life
against the drought, though more properly it should be said to be
dependent on the moisture. A plant which annually produces a
thousand seeds, of which only one of an average comes to maturity,
may be more truly said to struggle with the plants of the same and
other kinds which already clothe the ground. The mistletoe is
dependent on the apple and a few other trees, but can only in a far-
fetched sense be said to struggle with these trees, for, if too many of
these parasites grow on the same tree, it languishes and dies. But
several seedling mistletoes, growing close together on the same
branch, may more truly be said to struggle with each other. As the
mistletoe is disseminated by birds, its existence depends on them;
and it may metaphorically be said to struggle with other fruit-bearing
plants, in tempting the birds to devour and thus disseminate its seeds.
In these several senses, which pass into each other, I use for con-
venience' sake the general term of Struggle for Existence.

GEOMETRICAL RATIO OF INCREASE

A Struggle for existence inevitably follows from the high rate at
which all organic beings tend to increase. Every being, which during
its natural lifetime produces several eggs or seeds, must suffer
destruction during some period of its Hfe, and during some season

74 ORIGIN OF SPECIES

or occasional year; otherwise, on the principle of geometrical increase,
its numbers would quickly become so inordinately great that no
country could support the product. Hence, as more individuals are
produced than can possibly survive, there must in every case be a
struggle for existence, either one individual with another of the same
species, or with the individuals of distinct species, or with the physical
conditions of life. It is the doctrine of Malthus applied with mani-
fold force to the whole animal and vegetable kingdoms; for in this
case there can be no artificial increase of food, and no prudential
restraint from marriage. Although some species may be now increas-
ing, more or less rapidly, in numbers, all cannot do so, for the world
would not hold them.

There is no exception to the rule that every organic being naturally
increases at so high a rate, that, if not destroyed, the earth would soon
be covered by the progeny of a single pair. Even slow-breeding man
has doubled in twenty-five years, and at this rate in less than a thou-
sand years, there would literally not be standing-room for his pro-
geny. Linna!us has calculated that if an annual plant produced only
two seeds — and there is no plant so unproductive as this — and their
seedlings next year produced two, and so on, then in twenty years
there would be a million plants. The elephant is reckoned the slow-
est breeder of all known animals, and I have taken some pains to
estimate its probable minimum rate of natural increase; it will be
safest to assume that it begins breeding when thirty years old, and
goes on breeding till ninety years old, bringing forth six young in
the interval, and surviving till one hundred years old; if this be so,
after a period of from 740 to 750 years there would be nearly nineteen
million elephants alive, descended from the first pair.

But we have better evidence on this subject than mere theoretical
calculations, namely, the numerous recorded cases of the astonish-
ingly rapid increase of various animals in a state of nature, when
circumstances have been favourable to them during two or three
following seasons. Still more striking is the evidence from our
domestic animals of many kinds which have run wild in several
parts of the world; if the statements of the rate of increase of slow-
breeding cattle and horses in South America, and latterly in Australia,
had not been well authenticated, they would have been incredible.

GEOMETRICAL RATIO OF INCREASE 75

So it is with plants; cases could be given of introduced plants which
have become common throughout whole islands in a period of less
than ten years. Several of the plants, such as the cardoon and a tall
thisde, which are now the commonest over the wide plains of La
Plata, clothing square leagues of surface almost to the exclusion of
every other plant, have been introduced from Europe; and there are
plants which now range in India, as I hear from Dr. Falconer, from
Cape Comorin to the Himalaya, which have been imported from
America since its discovery. In such cases, and endless others could
be given, no one supposes that the fertility of the animals or plants
has been suddenly and temporarily increased in any sensible degree.
The obvious explanation is that the conditions of life have been
highly favourable, and that there has constantly been less destruction
of the old and young, and that nearly all the young have been enabled
to breed. Their geometrical ratio of increase, the result of which
never fails to be surprising, simply explains their extraordinarily
rapid increase and wide diiTusion in their new homes.

In a state of nature almost every full-grown plant annually pro-
duces seed, and amongst animals there are very few which do not
annually pair. Hence we may confidently assert, that all plants and
animals are tending to increase at a geometrical ratio, — that all would
rapidly stock every station in which they could anyhow exist, — and
that this geometrical tendency to increase must be checked by
destruaion at some period of life. Our familiarity with the larger
domestic animals tends, I think, to mislead us: we see no great de-
struction falling on them, but we do not keep in mind that thousands
are annually slaughtered for food, and that in a state of nature an
equal number would have somehow to be disposed of.

The only difference between organisms which annually produce
eggs or seeds by the thousand, and those which produce extremely
few, is, that the slow-breeders would require a few more years to
people, under favourable conditions, a whole district, let it be ever
so large. The condor lays a couple of eggs and the ostrich a score,
and yet in the same country the condor may be the more numerous
of the two; the Fulmar petrel lays but one egg, yet it is believed to
be the most numerous bird in the world. One fly deposits hundreds
of eggs, and another, like the hippobosca, a single one; but this dif-

76 ORIGIN OF SPECIES

ference does not determine how many individuals of the two species
can be supported in a district. A large number of eggs is of some
importance to those species which depend on a fluctuating amount
of food, for it allows them rapidly to increase in number. But the
real importance of a large number of eggs or seeds is to make up for
much destruction at some period of life; and this period in the great
majority of cases is an early one. If an animal can in any way protect
its own eggs or young, a small number may be produced, and yet the
average stock be fully kept up; but if many eggs or young are
destroyed, many must be produced, or the species will become
extinct. It would suffice to keep up the full number of a tree, which
lived on an average for a thousand years, if a single seed were pro-
duced once in a thousand years, supposing that this seed were never
destroyed, and could be ensured to germinate in a fitting place. So
that, in all cases, the average number of any animal or plant depends
only indirectly on the number of its eggs or seeds.

In looking at Nature, it is most necessary to keep the foregoing
considerations always in mind — never to forget that every single
organic being may be said to be striving to the utmost to increase in
numbers; that each lives by a struggle at some period of its life;
that heavy destruction inevitably falls either on the young or old,
during each generation or at recurrent intervals. Lighten any check,
mitigate the destruction ever so little, and the number of the species
will almost instantaneously increase to any amount.

NATURE OF THE CHECKS TO INCREASE

The causes which check the natural tendency of each species to
increase are most obscure. Look at the most vigorous species; by as
much as it swarms in numbers, by so much will it tend to increase
still further. We know not exactly what the checks are even in a
single instance. Nor will this surprise any one who reflects how
ignorant we are on this head, even in regard to mankind, although
so incomparably better known than any other animal. This subject
of the checks to increase has been ably treated by several authors,
and I hope in a future work to discuss it at considerable length, more
especially in regard to the feral animals of South America. Here I
will make only a few remarks, just to recall to the reader's mind

NATURE OF THE CHECKS TO INCREASE 77

some of the chief points. Eggs or very young animals seem generally
to suffer most, but this is not invariably the case. With plants there
is a vast destruction of seeds, but, from some observations which I
have made, it appears that the seedlings suffer most from germi-
nating in ground already thickly stocked with other plants. Seed-
lings, also, are destroyed in vast numbers by various enemies; for
instance, on a piece of ground three feet long and two wide, dug and
cleared, and where there could be no choking from other plants, I
marked all the seedlings of our native weeds as they came up, and
out of 357 no less than 295 were destroyed, chiefly by slugs and
insects. If turf which has long been mown (and the case would be
the same with turf closely browsed by quadrupeds) be let to grow,
the more vigorous plants gradually kill the less vigorous, though
fully grown plants; thus out of twenty species growing on a little
plot of mown turf (three feet by four) nine species perished, from
the other species being allowed to grow up freely.

The amount of food for each species of course gives the extreme
limit to which each can increase; but very frequently it is not the
obtaining food, but the serving as prey to other animals, which
determines the average numbers of a species. Thus, there seems to
be little doubt that the stock of partridges, grouse and hares on any
large estate depends chiefly on the destruction of vermin. If not one
head of game were shot during the next twenty years in England,
and, at the same time, if no vermin were destroyed, there would, in
all probability, be less game than at present, although hundreds of
thousands of game animals are now annually shot. On the other
hand, in some cases, as with the elephant, none are destroyed by
beasts of prey; for even the tiger in India most rarely dares to attack
a young elephant protected by its dam.

Climate plays an important part in determining the average num-
bers of a species, and periodical seasons of extreme cold or drought
seem to be the most effective of all checks. I estimated (chiefly from
the greatly reduced numbers of nests in the spring) that the winter
of 1854-5 destroyed four-fifths of the birds in my own grounds; and
this is a tremendous destruction, when we remember that ten per
cent, is an extraordinarily severe mortality from epidemics with man.
The action of climate seems at first sight to be quite independent of

78 ORIGIN OF SPECIES

the struggle for existence; but in so far as climate chiefly acts in
reducing food, it brings on the most severe struggle between the
individuals, whether of the same or of distinct species, which subsist
on the same kind of food. Even when climate, for instance, extreme
cold, acts directly, it will be the least vigorous individuals, or those
which have got least food through the advancing winter, which will
suffer the most. When we travel from south to north, or from a
damp region to a dry, we invariably see some species gradually get-
ting rarer and rarer, and finally disappearing; and the change of
climate being conspicuous, we are tempted to attribute the whole
effect to its direct action. But this is a false view; we forget that each
species, even where it most abounds, is constantly suffering enormous
destruction at some period of its life, from enemies or from competi-
tors for the same place and food; and if these enemies or comjietitors
be in the least degree favoured by any slight change of climate, they
will increase in numbers; and as each area is already fully stocked
with inhabitants, the other species must decrease. When we travel
southward and see a species decreasing in numbers, we may feel sure
that the cause lies quite as much in other species being favoured, as
in this one being hurt. So it is when we travel northward, but in a
somewhat lesser degree, for the number of species of all kinds, and
therefore of competitors, decreases northwards; hence in going north-
wards, or in ascending a mountain, we far oftener meet with stunted
forms, due to the directly injurious action of climate, than we do in
proceeding southwards or in descending a mountain. When we
reach the Arctic regions, or snow<apped summits, or absolute deserts,
the struggle for life is almost exclusively with the elements.

That climate acts in main part indirectly by favouring other species,
we clearly see in the prodigious number of plants which in our
gardens can perfectly well endure our climate, but which never
became naturalised, for they cannot compete with our native plants
nor resist destruction by our native animals.

When a species, owing to highly favoured circumstances, increases
inordinately in numbers in a small tract, epidemics — at least, this
seems generally to occur with our game animals — often ensue; and
here we have a limiting check independent of the struggle for life.
But even some of these so-called epidemics appear to be due to para-

STRUGGLE FOR EXISTENCE 79

side worms, which have from some cause, possibly in part through
facility of diffusion amongst the crowded animals, been dispropor-
tionally favoured: and here comes in a sort of struggle between the
parasite and its prey.

On the other hand, in many cases, a large stock of individuals of
the same species, relatively to the numbers of its enemies, is absolutely
necessary for its preservation. Thus we can easily raise plenty of
corn and rape-seed, etc., in our fields, because the seeds are in great
excess, compared with the number of birds which feed on them; nor
can the birds, though having a superabundance of food at this one
season, increase in number proportionally to the supply of seed, as
their numbers are checked during winter; but any one who has tried,
knows how troublesome it is to get seed from a few wheat or other
such plants in a garden : I have in this case lost every single seed. This
view of the necessity of a large stock of the same species for its pres-
ervation, explains, I believe, some singular facts in nature such as that
of very rare plants being sometimes extremely abundant, in the few
spots where they do exist; and that of some social plants being social,
that is, abounding in individuals, even on the extreme verge of their
range. For in such cases, we may believe, that a plant could exist
only where the conditions of its life were so favourable that many
could exist together, and thus save the species from utter destruction.
1 should add that the good effects of intercrossing, and the ill effects
of close interbreeding, no doubt come into play in many of these
cases; but I will not here enlarge on this subject.

COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO EACH OTHER IN THE
STRUGGLE FOR EXISTENCE

Many cases are on record showing how complex and unexpected
are the checks and relations between organic beings, which have to
struggle together in the same country. I will give only a single
instance, which, though a simple one, interested me. In Stafford-
shire, on the estate of a relation, where I had ample means of inves-
tigation, there was a large and extremely barren heath, which had
never been touched by the hand of man; but several acres of exactly
the same nature had been enclosed twenty-five years previously and
planted with Scotch fir. The change in the native vegetation of the

8o ORIGIN OF SPECIES

planted part of the heath was most remarkable, more than is gen-
erally seen in passing from one quite different soil to another: not
only the proportional numbers of the heath plants were wholly
changed, but twelve species of plants (not counting grasses and
carices) flourished in the plantations, which could not be found on
the heath. The effect on the insects must have been still greater, for
six insectivorous birds were very common in the plantations, which
were not to be seen on the heath; and the heath was frequented by
two or three distinct insectivorous birds. Here we see how potent
has been the effect of the introduction of a single tree, nothing what-
ever else having been done, with the exception of the land having
been enclosed, so that cattle could not enter. But how important an
element enclosure is, I plainly saw near Farnham, in Surrey. Here
there are extensive heaths, with a few clumps of old Scotch firs on
the distant hilltops: within the last ten years large spaces have been
enclosed, and self-sown firs are now springing up in multitudes, so
close together that all cannot live. When I ascertained that these
young trees had not been sown or planted, I was so much surprised
at their numbers that I went to several points of view, whence I could
examine hundreds of acres of the unenclosed heath, and literally I
could not see a single Scotch fir, except the old planted clumps. But
on looking closely between the stems of the heath, I found a multi-
tude of seedlings and little trees which had been perpetually browsed
down by the cattle. In one square yard, at a point some hundred
yards distant from one of the old clumps, I counted thirty-two little
trees; and one of them, with twenty-six rings of growth, had, during
many years, tried to raise its head above the stems of the heath, and
had failed. No wonder that, as soon as the land was enclosed, it
became thickly clothed with vigorously growing young firs. Yet the
heath was so extremely barren and so extensive that no one would
ever have imagined that cattle would have so closely and effectually
searched it for food.

Here we see that cattle absolutely determine the existence of the
Scotch fir; but in several parts of the world insects determine the
existence of cattle. Perhaps Paraguay offers the most curious instance
of this; for here neither cattle nor horses nor dogs have ever run
wild, though they swarm southward and northward in a feral state;

STRUGGLE FOR EXISTENCE 8 1

and Azara and Rengger have shown that this is caused by the greater
number in Paraguay of a certain fly, which lays its eggs in the navels
of these animals when first born. The increase of these flies,
numerous as they are, must be habitually checked by some means,
probably by other parasitic insects. Hence, if certain insectivorous
birds were to decrease in Paraguay, the parasitic insects would prob-
ably increase; and this would lessen the number of the navel-
frequenting flies — then cattle and horses would become feral, and this
would certainly greatly alter (as indeed I have observed in parts of
South America) the vegetation: this again would largely af?ect the
insects; and this, as we have just seen in Staffordshire, the insectiv-
orous birds, and so onwards in ever-increasing circles of complexity.
Not that under nature the relations will ever be as simple as this.
Battle within battle must be continually recurring with varying suc-
cess; and yet in the long-run the forces are so nicely balanced, that
the face of nature remains for long periods of time uniform, though
assuredly the merest trifle would give the victory to one organic being
over another. Nevertheless, so profound is our ignorance, and so
high our presumption, that we marvel when we hear of the extinc-
tion of an organic being; and as we do not see the cause, we invoke
cataclysms to desolate the world, or invent laws on the duration of
the forms of life!

I am tempted to give one more instance showing how plants and
animals, remote in the scale of nature, are bound together by a web
of complex relations. I shall hereafter have occasion to show that
the exotic Lobelia fulgens is never visited in my garden by insects,
and consequently, from its peculiar structure, never sets a seed.
Nearly all our orchidaceous plants absolutely require the visits of
insects to remove their pollen-masses and thus to fertilise them. I find
from experiments that bumblebees are almost indispensable to the
fertilisation of the heartsease (Viola tricolor), for other bees do not
visit this flower. I have also found that the visits of bees are necessary
for the fertilisation of some kinds of clover; for instance, twenty
heads of Dutch clover (Trifolium repens) yielded 2,290 seeds, but
twenty other heads protected from bees produced not one. Again,
one hundred heads of red clover (T. pratense) produced 2,700 seeds,
but the same number of protected heads produced not a single seed.

82 ORIGIN OF SPECIES

Humblebees alone visit red clover, as other bees cannot reach the
nectar. It has been suggested that moths may fertiUse the clovers;
but I doubt whether they could do so in the case of the red clover,
from their weight not being sufficient to depress the wing petals.
Hence we may infer as highly probable that, if the whole genus of
humblebees became extinct or very rare in England, the heartsease
and red clover would become very rare, or wholly disappear. The
number of humblebees in any district depends in a great measure
upon the number of field mice, which destroy their combs and nests;
and Colonel Newman, who has long attended to the habits of hum-
blebees, believes that "more than two-thirds of them are thus de-
stroyed all over England." Now the number of mice is largely
dependent, as every one knows, on the number of cats; and Colonel
Newman says, "Near villages and small towns I have found the
nests of humblebees more numerous than elsewhere, which I attrib-
ute to the number of cats that destroy the mice." Hence it is quite
credible that the presence of a feline animal in large numbers in a
district might determine, through the intervention first of mice and
then of bees, the frequency of certain flowers in that district!

In the case of every species, many different checks, acting at
different periods of life, and during different seasons or years, prob-
ably come into play; some one check or some few being generally
the most potent; but all will concur in determining the average
number or even the existence of the species. In some cases it can
be shown that widely different checks act on the same species in
different districts. When we look at the plants and bushes clothing
an entangled bank, we are tempted to attribute their prop)ortional
numbers and kinds to what we call chance. But how false a view is
this! Every one has heard that when an American forest is cut down,
a very different vegetation springs up; but it has been observed that
ancient Indian ruins in the southern United States, which must for-
merly have been cleared of trees, now display the same beautiful
diversity and proportion of kinds as in the surrounding virgin forest.
What a struggle must have gone on during long centuries between
the several kinds of trees, each annually scattering its seeds by the
thousand; what war between insect and insect — between insects,
snails, and other animals with birds and beasts of prey — all striving

STRUGGLE FOR EXISTENCE 83

to increase, all feeding on each other, or on the trees, their seeds and
seedlings, or on the other plants which first clothed the ground and
thus checked the growth of the trees! Throw up a handful of
feathers, and all fall to the ground according to definite laws; but
how simple is the problem where each shall fall compared to that of
the action and reaction of the innumerable plants and animals which
have determined, in the course of centuries, the proportional num-
bers and kinds of trees now growing on the old Indian ruins!

The dependency of one organic being on another, as of a parasite
on its prey, lies generally between beings remote in the scale of
nature. This is likewise sometimes the case with those which may
be strictly said to struggle with each other for existence, as in the
case of locusts and grass-feeding quadrupeds. But the struggle will
almost invariably be most severe between the individuals of the same
species, for they frequent the same districts, require the same food,
and are exposed to the same dangers. In the case of varieties of the
same species, the struggle will generally be almost equally severe,
and we sometimes see the contest soon decided: for instance, if
several varieties of wheat be sown together, and the mixed seed be
resown, some of the varieties which best suit the soil or climate, or
are naturally the most fertile, will beat the others and so yield more
seed, and will consequently in a few years supplant the other vari-
eties. To keep up a mixed stock of even such extremely close varieties
as the variously-coloured sweet peas, they must be each year harvested
separately, and the seed then mixed in due proportion, otherwise the
weaker kinds will steadily decrease in number and disappear. So
again with the varieties of sheep; it has been asserted that certain
mountain varieties will starve out other mountain varieties, so that
they cannot be kept together. The same result has followed from
keeping together different varieties of the medicinal leech. It may
even be doubted whether the varieties of any of our domestic plants
or animals have so exactly the same strength, habits, and constitution,
that the original proportions of a mixed stock (crossing being pre-
vented) could be kept up for half-a-dozen generations, if they were
allowed to struggle together, in the same manner as beings in a state
of nature, and if the seed or young were not annually preserved in
due proportion.

04 ORIGIN OF SPECIES

STRUGGLE FOR LIFE MOST SEVERE BETWEEN INDIVIDUALS AND VARIETIES OF

THE SAME SPECIES

As the species of the same genus usually have, though by no means
invariably, much similarity in habits and constitution, and always in
structure, the struggle will generally be more severe between them,
if they come into competition with each other, than between the
species of distinct genera. We see this in the recent extension over
parts of the United States of one species of swallow having caused
the decrease of another species. The recent increase of the missel
thrush in parts of Scotland has caused the decrease of the song thrush.
How frequently we hear of one species of rat taking the place of
another species under the most different climates! In Russia the
small Asiatic cockroach has everywhere driven before it its great
congener. In Australia the imported hive bee is rapidly extermi-
nating the small, stingless native bee. One species of charlock has
been known to supplant another species; and so in other cases.
We can dimly see why the competition should be most severe be-
tween allied forms, which fill nearly the same place in the economy
of nature; but probably in no one case could we precisely say why
one species has been victorious over another in the great battle
of life.

A corollary of the highest importance may be deduced from the
foregoing remarks, namely, that the structure of every organic being
is related, in the most essential yet often hidden manner, to that of all
the other organic beings, with which it comes into compnitition for
food or residence, or from which it has to escape, or on which it
preys. This is obvious in the structure of the teeth and talons of the
tiger; and in that of the legs and claws of the parasite which clings
to the hair on the tiger's body. But in the beautifully plumed seed
of the dandelion, and in the flattened and fringed legs of the water
beetle, the relation seems at first confined to the elements of air and
water. Yet the advantage of plumed seeds no doubt stands in the
closest relation to the land being already thickly clothed with other
plants; so that the seeds may be widely distributed and fall on unoc-
cupied ground. In the water beetle, the structure of its legs, so well
adapted for diving, allows it to compete with other aquatic insects.

STRUGGLE FOR EXISTENCE 85

to huiM for its own prey, and to escape serving as prey to other
animals.

The store of nutriment laid up within the seeds of many plants
seems at first sight to have no sort of relation to other plants. But
from the strong growth of young plants produced from such seeds,
as peas and beans, when sown in the midst of long grass, it may be
suspected that the chief use of the nutriment in the seed is to favour
the growth of the seedlings, whilst struggling with other plants
growing vigorously all around.

Look at a plant in the midst of its range! Why does it not double
or quadruple its numbers? We know that it can perfectly well with-
stand a little more heat or cold, dampness or dryness, for elsewhere it
ranges into slightly hotter or colder, damper or drier districts. In this
case we can clearly see that if we wish in imagination to give the
plant the power of increasing in number, we should have to give it
some advantage over its competitors, or over the animals which prey
on it. On the confines of its geographical range, a change of consti-
tution with respect to climate would clearly be an advantage to our
plant; but we have reason to believe that only a few plants or animals
range so far, that they are destroyed exclusively by the rigour of the
climate. Not until we reach the extreme confines of life, in the
Arctic regions or on the borders of an utter desert, will competition
cease. The land may be extremely cold or dry, yet there will be
competition between some few species, or between the individuals
of the same species, for the warmest or dampest spots.

Hence we can see that when a plant or animal is placed in a new
country amongst new competitors, the conditions of its Ufe will gen-
erally be changed in an essential manner, although the climate may
be exactly the same as in its former home. If its average numbers
are to increase in its new home, we should have to modify it in a
different way to what we should have had to do in its native country;
for we should have to give it some advantage over a different set of
competitors or enemies.

It is good thus to try in imagination to give to any one species an
advantage over another. Probably in no single instance should we
know what to do. This ought to convince us of our ignorance on
the mutual relations of all organic beings; a conviction as necessary,

86 ORIGIN OF SPECIES

as it is difScult to acquire. All that we can do, is to keep steadily in
mind that each organic being is striving to increase in a geometrical
ratio; that each at some period of its life, during some season of the
year, during each generation, or at intervals, has to struggle for Ufe
and to suffer great destruction. When we reflect on this struggle, we
may console ourselves with the full belief, that the war of nature is
not incessant, that no fear is felt, that death is generally prompt, and
that the vigorous, the healthy, and the happy survive and multiply.

CHAPTER IV

Natural Selection; or the Survival of the Fittest

Natural Selection — its power compared with man's selection — its power
on characters of trifling importance — its power at all ages and on
both sexes — Sexual selection — On the generality of intercrosses
between individuals of the same species — Circumstances favourable
and unfavourable to the results of Natural Selection, namely, inter-
crossing, isolation, number of individuals — Slow action — Extinction
caused by Natural Selection — Divergence of Character, related to the
diversity of inhabitants of any small area, and to naturalisation —
Action of Natural Selection, through divergence of Character and
Extinction, on the descendants from a common parent — Explains the
grouping of all organic beings — Advance in organisation — Low
forms preserved — Convergence of Character — Indefinite multipli-
cation of species — Summary.

HOW will the struggle for existence, briefly discussed in the
last chapter, act in regard to variation? Can the principle
of selection, which we have seen is so potent in the hands
of man, apply under nature? I think we shall see that it can act
most efficiently. Let the endless number of slight variations and
individual differences occurring in our domestic productions, and,
in a lesser degree, in those under nature, be borne in mind; as well
as the strength of the hereditary tendency. Under domestication, it
may be truly said that the whole organisation becomes in some de-
gree plastic. But the variability, which we almost universally meet
with in our domestic productions, is not directly produced, as
Hooker and Asa Gray have well remarked, by man; he can neither
originate varieties, nor prevent their occurrence; he can only pre-
serve and accumulate such as do occur. Unintentionally he exposes
organic beings to new and changing conditions of life, and variabil-
ity ensues; but similar changes of conditions might and do occur
under nature. Let it also be borne in mind how infinitely complex
and close-fitting are the mutual relations of all organic beings to
each other and to their physical conditions of life; and consequently

87

88 ORIGIN OF SPECIES

what infinitely varied diversities of structure might be of use to
each being under changing conditions of Hfe. Can it, then, be
thought improbable, seeing that variations useful to man have un-
doubtedly occurred, that other variations useful in some way to each
being in the great and complex battle of life, should occur in the
course of many successive generations? If such do occur, can we
doubt (remembering that many more individuals are born than can
pxjssibly survive) that individuals having any advantage, however
slight, over others, would have the best chance of surviving and of
procreating their kind? On the other hand, we may feel sure that
any variation in the least degree injurious would be rigidly destroyed.
This preservation of favourable individual differences and variations,
and the destruction of those which are injurious, I have called
Natural Selection, or the Survival of the Fittest. Variations neither
useful nor injurious would not be affected by natural selection, and
would be left either a fluctuating element, as perhaps we see in cer-
tain polymorphic species, or would ultimately become fixed, owing
to the nature of the organism and the nature of the conditions.

Several writers have misapprehended or objected to the term
Natural Selection. Some have even imagined that natural selection
induces variability, whereas it implies only the preservation of such
variations as arise and are beneficial to the being under its conditions
of life. No one objects to agriculturists speaking of the potent effects
of man's selection; and in this case the individual differences given
by nature, which man for some object selects, must of necessity first
occur. Others have objected that the term selection implies con-
scious choice in the animals which become modified; and it has
even been urged that, as plants have no volition, natural selection is
not applicable to them! In the literal sense of the word, no doubt,
natural selection is a false term; but who ever objected to chemists
speaking of the elective affinities of the various elements? — and yet
an acid cannot strictly be said to elect the base with which it in
preference combines. It has been said that I speak of natural selection
as an active power or deity; but who objects to an author speaking
of the attraction of gravity as ruling the movements of the planets?
Every one knows what is meant and is implied by such metaphorical
expressions; and they are almost necessary for brevity. So again it

NATURAL SELECTION 89

is difficult to avoid personifying the word Nature; but I mean by
Nature, only the aggregate action and product of many natural
laws, and by laws the sequence of events as ascertained by us. With
a Uttle familiarity such superficial objections will be forgotten.

We shall best understand the probable course of natural selection
by taking the case of a country undergoing some slight physical
change, for instance, of climate. The proportional numbers of its
inhabitants will almost immediately undergo a change, and some
species will probably become extinct. We may conclude, from what
we have seen of the intimate and complex manner in which the
inhabitants of each country are bound together, that any change in
the numerical proportions of the inhabitants, independently of the
change of climate itself, would seriously affect the others. If the
country were open on its borders, new forms would certainly im-
migrate, and this would likewise seriously disturb the relations of
some of the former inhabitants. Let it be remembered how power-
ful the influence of a single introduced tree or mammal has been
shown to be. But in the case of an island, or of a country partly
surrounded by barriers, into which new and better adapted forms
could not freely enter, we should then have places in the economy
of nature which would assuredly be better filled up, if some of the
original inhabitants were in some manner modified; for, had the
area been open to immigration, these same places would have been
seized on by intruders. In such cases, slight modifications, which in
any way favoured the individuals of any species, by better adapting
them to their altered conditions, would tend to be preserved; and
natural selection would have free scope for the work of improvement.

We have good reason to believe, as shown in the first chapter,
that changes in the conditions of life give a tendency to increased
variability; and in the foregoing cases the conditions have changed,
and this would manifestly be favourable to natural selection, by
affording a better chance of the occurrence of profitable variations.
Unless such occur, natural selection can do nothing. Under the term
of "variations," it must never be forgotten that mere individual
differences are included. As man can produce a great result with
his domestic animals and plants by adding up in any given direc-
tion individual differences, so could natural selection, but far more

90 ORIGIN OF SPECIES

easily from having incomparably longer time for action. Nor do I
believe that any great physical change, as of climate, or any unusual
degree of isolation, to check immigration, is necessary in order that
new and unoccupied places should be left, for natural selection to
fill up by improving some of the varying inhabitants. For as all
the inhabitants of each country are struggling together with nicely
balanced forces, extremely slight modifications in the structure or
habits of one species would often give it an advantage over others;
and still further modifications of the same kind would often still
further increase the advantage, as long as the species continued under
the same conditions of life and profited by similar means of sub-
sistence and defence. No country can be named in which all the
native inhabitants are now so perfectly adapted to each other and
to the physical conditions under which they hve, that none of them
could be still better adapted or improved; for in all countries, the
natives have been so far conquered by naturalised productions, that
they have allowed some foreigners to take firm p)ossession of the
land. And as foreigners have thus in every country beaten some of
the natives, we may safely conclude that the natives might have been
modified with advantage, so as to have better resisted the intruders.
As man can produce, and certainly has produced, a great result
by his methodical and unconscious means of selection, what may
not natural selection effect? Man can act only on external and
visible characters: Nature, if I may be allowed to personify the
natural preservation or survival of the fittest, cares nothing for
appearances, except in so far as they are useful to any being. She
can act on every internal organ, on every shade of constitutional
difference, on the whole machinery of life. Man selects only for his
own good : Nature only for that of the being which she tends. Every
selected character is fully exercised by her, as is implied by the fact
of their selection. Man keeps the natives of many climates in the
same country; he seldom exercises each selected character in some
peculiar and fitting manner; he feeds a long and a short beaked
pigeon on the same food; he does not exercise a long-backed or
long-legged quadruped in any peculiar manner; he exposes sheep
with long and short wool to the same climate. He does not allow
the most vigorous males to struggle for the females. He does not

NATURAL SELECTION 9I

rigidly destroy all inferior animals, but protects during each varying
season, as far as lies in his power, all his productions. He often
begins his selection by some half-monstrous form; or at least by
some modification prominent enough to catch the eye or to be plainly
useful to him. Under nature, the slightest differences of structure or
constitution may well turn the nicely balanced scale in the struggle
for life, and so be preserved. How fleeting are the wishes and efforts
of man! how short his time! and consequently how poor will be
his results, compared with those accumulated by Nature during
whole geological periods? Can we wonder, then, that Nature's pro-
ductions should be far "truer" in character than man's productions;
that they should be infinitely better adapted to the most complex
conditions of life, and should plainly bear the stamp of far higher
workmanship?

It may metaphorically be said that natural selection is daily and
hourly scrutinising, throughout the world, the slightest variations;
rejecting those that are bad, preserving and adding up all that are
good; silently and insensibly working, whenever and wherever op-
portunity offers, at the improvement of each organic being in rela-
tion to its organic and inorganic conditions of life. We see nothing
of these slow changes in progress, until the hand of time has marked
the lapse of ages, and then so imperfect is our view into long-past
geological ages, that we see only that the forms of life are now
different from what they formerly were.

In order that any great amount of modification should be effected
in a species, a variety, when once formed, must again, perhaps after
a long interval of time, vary or present individual differences of the
same favourable nature as before; and these must be again pre-
served, and so onwards step by step. Seeing that individual differ-
ences of the same kind perpetually recur, this can hardly be con-
sidered as an unwarrantable assumption. But whether it is true, we
can judge only by seeing how far the hyf)othesis accords with and
explains the general phenomena of nature. On the other hand, the
ordinary belief that the amount of possible variation is a strictly
limited quantity is likewise a simple assumption.

Although natural selection can act only through and for the good
of each being, yet characters and structures, which we are apt to

92 ORIGIN OF SPECIES

consider as of very trifling importance, may thus be acted on. When
we see leaf -eating insects green, and bark-feeders mottled-grey; the
alpine ptarmigan white in winter, the red grouse the colour of
heather, we must believe that these tints are of service to these birds
and insects in preserving them from danger. Grouse, if not destroyed
at some period of their lives, would increase in countless numbers;
they are known to suffer largely from birds of prey; and hawks are
guided by eyesight to their prey — so much so, that on parts of the
Continent persons are warned not to keep white pigeons, as being
the most liable to destruction. Hence natural selection might be
effective in giving the proper colour to each kind of grouse, and in
keeping that colour, when once acquired, true and constant. Nor
ought we to think that the occasional destruction of an animal of
any particular colour would produce little effect: we should remem-
ber how essential it is in a flock of white sheep to destroy a lamb
with the faintest trace of black. We have seen how the colour of
the hogs, which feed on the "paint root" in Virginia, determines
whether they shall live or die. In plants, the down on the fruit and
the colour of the flesh are considered by botanists as characters of
the most trifling importance: yet we hear from an excellent horti-
culturist. Downing, that in the United States smooth-skinned fruits
suffer far more from a beetle, a Curculio, than those with down;
that purple plums suffer far more from a certain disease than yellow
plums, whereas another disease attacks yellow-fleshed f)eaches far
more than those with other coloured flesh. If, with all the aids of
art, these slight differences make a great difTerence in cultivating
the several varieties, assuredly, in a state of nature, where the trees
would have to struggle with other trees and with a host of enemies,
such differences would effectually settle which variety, whether a
smooth or downy, a yellow or purple fleshed fruit, should succeed.
In looking at many small points of difference between species,
which, as far as our ignorance permits us to judge, seem quite un-
impxjrtant, we must not forget that climate, food, etc., have no doubt
produced some direct effect. It is also necessary to bear in mind
that, owing to the law of correlation, when one part varies, and the
variations are accumulated through natural selection, other modifi-
cations, often of the most unexpected nature, will ensue.

NATURAL SELECTION 93

As we see that those variations which, under domestication, ap-
pear at any particular period of life, tend to reappear in the offspring
at the same period; for instance, in the shape, size, and flavour of
the seeds of the many varieties of our culinary and agricultural
plants; in the caterpillar and cocoon stages of the varieties of the
silkworm; in the eggs of poultry, and in the colour of the down of
their chickens; in the horns of our sheep and cattle when nearly
adult; so in a state of nature natural selection will be enabled to act
on and modify organic beings at any age, by the accumulation of
variations profitable at that age, and by their inheritance at a cor-
responding age. If it profit a plant to have its seeds more and more
widely disseminated by the wind, I can see no greater difficulty in
this being effected through natural selection, than in the cotton
planter increasing and improving by selection the down in the pods
on his cotton trees. Natural selection may modify and adapt the
larva of an insect to a score of contingencies, wholly different from
those which concern the mature insect; and these modifications may
effect, through correlation, the structure of the adult. So, conversely,
modifications in the adult may affect the structure of the larva; but
in all cases natural selection will ensure that they shall not be in-
jurious: for if they were so, the species would become extinct.

Natural selection will modify the structure of the young in rela-
tion to the parent, and of the parent in relation to the young. In
social animals it will adapt the structure of each individual for the
benefit of the whole community; if the community profits by the
selected change. What natural selection cannot do, is to modify the
structure of one species, without giving it any advantage, for the
good of another species; and though statements to this effect may
be found in works of natural history, I cannot find one case which
will bear investigation. A structure used only once in an animal's
life, if of high importance to it, might be modified to any extent by
natural selection; for instance, the great jaws possessed by certain
insects, used exclusively for opening the cocoon — or the hard tip to
the beak of unhatched birds, used for breaking the egg. It has been
asserted, that of the best short-beaked tumbler pigeons a greater
number perish in the egg than are able to get out of it; so that
fanciers assist in the act of hatching. Now, if nature had to make

94 ORIGIN OF SPECIES

the beak of a full-grown pigeon very short for the bird's own ad-
vantage, the process of modification would be very slow, and there
would be simultaneously the most rigorous selection of all the young
birds within the egg, which had the most powerful and hardest
beaks, for all with weak beaks would inevitably perish; or, more
delicate and more easily broken shells might be selected, the thick-
ness of the shell being known to vary like every other structure.

It may be well here to remark that with all beings there must be
much fortuitous destruction, which can have Uttle or no influence
on the course of natural selection. For instance a vast number of
eggs or seeds are annually devoured, and these could be modified
through natural selection only if they varied in some manner which
protected them from their enemies. Yet many of these eggs or
seeds would perhaps, if not destroyed, have yielded individuals bet-
ter adapted to their conditions of life than any of those which hap-
pened to survive. So again a vast number of mature animals and
plants, whether or not they be the best adapted to their conditions,
must be annually destroyed by accidental causes, which would not
be in the least degree mitigated by certain changes of structure or
constitution which would in other ways be beneficial to the species.
But let the destruction of the adults be ever so heavy, if the number
which can exist in any district be not wholly kept down by such
causes, — or again let the destruction of eggs or seeds be so great that
only a hundredth or a thousandth part are developed, — yet of those
which do survive, the best adapted individuals, supposing that there
is any variability in a favourable direction, will tend to propagate
their kind in larger numbers than the less well adapted. If the
numbers be wholly kept down by the causes just indicated, as will
often have been the case, natural selection will be powerless in cer-
tain beneficial directions; but this is no valid objection to its effi-
ciency at other times and in other ways; for we are far from having
any reason to suppose that many species ever undergo modification
and improvement at the same time in the same area.

SEXUAL SELECTION

Inasmuch as peculiarities often appear under domestication in one
sex and become hereditarily attached to that sex, so no doubt it will

SEXUAL SELECTION 95

be under nature. Thus it is rendered possible for the two sexes to
be modified through natural selection in relation to different habits
of life, as is sometimes the case; or for one sex to be modified in
relation to the other sex, as commonly occurs. This leads me to say
a few words on what I have called Sexual Selection. This form of
selection depends, not on a struggle for existence in relation to other
organic beings or to external conditions, but on a struggle between
the individuals of one sex, generally the males, for the possession of
the other sex. The result is not death to the unsuccessful competitor,
but few or no offspring. Sexual selection is, therefore, less rigorous
than natural selection. Generally, the most vigorous males, those
which are best fitted for their places in nature, will leave most
progeny. But in many cases, victory depends not so much on gen-
eral vigour, as on having special weapons, confined to the male sex.
A hornless stag or spurless cock would have a poor chance of leav-
ing numerous offspring. Sexual selection, by always allowing the
victor to breed, might surely give indomitable courage, length to
the spur, and strength to the wing to strike in the spurred leg, in
nearly the same manner as does the brutal cockfighter by the care-
ful selection of his best cocks. How low in the scale of nature the
law of battle descends, I know not; male alligators have been de-
scribed as fighting, bellowing, and whirling round, like Indians in
a war dance, for the possession of the females; male salmons have
been observed fighting all day long; male stag beetles sometimes
bear wounds from the huge mandibles of other males; the males of
certain hymenopterous insects have been frequently seen by that
inimitable observer M. Fabre, fighting for a particular female who
sits by, an apparently unconcerned beholder of the struggle, and
then retires with the conqueror. The war is, perhaps, severest be-
tween the males of polygamous animals, and these seem oftenest
provided with special weapons. The males of carnivorous animals
are already well armed; though to them and to others, special means
of defence may be given through means of sexual selection, as the
mane of the lion, and the hooked jaw to the male salmon; for the
shield may be as important for victory, as the sword or spear.

Amongst birds, the contest is often of a more peaceful character.
All those who have attended to the subject, believe that there is the

g6 ORIGIN OF SPECIES

severest rivalry between the males of many species to attract, by
singing, the females. The rock thrush of Guiana, birds of paradise,
and some others, congregate; and successive males display with the
most elaborate care, and show off in the best manner, their gorgeous
plumage; they likewise perform strange antics before the females,
which, standing by as spectators, at last choose the most attractive
partner. Those who have closely attended to birds in confinement
well know that they often take individual preferences and dislikes;
thus Sir R. Heron has described how a pied peacock was eminently
attractive to all his hen birds. I cannot here enter on the necessary
details; but if man can in a short time give beauty and an elegant
carriage to his bantams, according to his standard of beauty, I can
see no good reason to doubt that female birds, by selecting, during
thousands of generations, the most melodious or beautiful males,
according to their standard of beauty, might produce a marked
effect. Some well-known laws, with respect to the plumage of male
and female birds, in comparison with the plumage of the young,
can partly be explained through the action of sexual selection on
variations occurring at different ages, and transmitted to the males
alone or to both sexes at corresponding ages; but I have not space
here to enter on this subject.

Thus it is, as I believe, that when the males and females of any
animal have the same general habits of life, but differ in structure,
colour, or ornament, such differences have been mainly caused by
sexual selection: that is, by individual males having had, in succes-
sive generations, some slight advantage over other males, in their
weapons, means of defence, or charms, which they have transmitted
to their male offspring alone. Yet, I would not wish to attribute all
sexual differences to this agency : for we see in our domestic animals
peculiarities arising and becoming attached to the male sex, which
apparently have not been augmented through selection by man. The
tuft of hair on the breast of the wild turkey cock cannot be of any
use, and it is doubtful whether it can be ornamental in the eyes of
the female bird; indeed, had the tuft appeared under domestication,
it would have been called a monstrosity.

ACTION OF NATURAL SELECTION 97

ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR THE
SURVIVAL OF THE FITTEST

In order to make it clear how, as I believe, natural selection acts,
I must beg permission to give one or two imaginary illustrations.
Let us take the case of a wolf, which preys on various animals,
securing some by craft, some by strength, and some by fleetness;
and let us suppose that the fleetest prey, a deer for instance, had
from any change in the country increased in numbers, or that other
prey had decreased in numbers, during that season of the year when
the wolf was hardest pressed for food. Under such circumstances
the swiftest and slimmest wolves would have the best chance of
surviving and so be preserved or selected, — provided always that
they retained strength to master their prey at this or some other
period of the year, when they were compelled to prey on other
animals. I can see no more reason to doubt that this would be the
result, than that man should be able to improve the fleetness of his
greyhounds by careful and methodical selection, or by that kind of
unconscious selection which follows from each man trying to keep
the best dogs without any thought of modifying the breed. I may
add, that, according to Mr. Pierce, there are two varieties of the wolf
inhabiting the Catskill Mountains, in the United States, one with a
light greyhound-like form, which pursues deer, and the other more
bulky, with shorter legs, which more frequently attacks the shep-
herd's flocks.

It should be observed that, in the above illustration, I speak of
the slimmest individual wolves, and not of any single strongly-
marked variation having been preserved. In former editions of this
work I sometimes spoke as if this latter alternative had frequently
occurred. I saw the great importance of individual differences, and
this led me fully to discuss the results of unconscious selection by
man, which depends on the preservation of all the more or less
valuable individuals, and on the destruction of the worst. I saw,
also, that the preservation in a state of nature of any occasional
deviation of structure, such as a monstrosity, would be a rare event;
and that, if at first preserved, it would generally be lost by subsequent
intercrossing with ordinary individuals. Nevertheless, until reading

98 ORIGIN OF SPECIES

an able and valuable article in the 'North British Review' (1867),
I did not appreciate how rarely single variations, whether slight or
strongly-marked, could be perpetuated. The author takes the case
of a pair of animals, producing during their lifetime two hundred
oflspring, of which, from various causes of destruction, only two on
an average survive to procreate their kind. This is rather an extreme
estimate for most of the higher animals, but by no means so for
many of the lower organisms. He then shows that if a single in-
dividual were born, which varied in some manner, giving it twice
as good a chance of life as that of the other individuals, yet, the
chances would be strongly against its survival. Supf)osing it to sur-
vive and to breed, and that half its young inherited the favourable
variation; still, as the reviewer goes on to show, the young would
have only a slightly better chance of surviving and breeding; and
this chance would go on decreasing in the succeeding generations.
The justice of these remarks cannot, I think, be disputed. If, for in-
stance, a bird of some kind could procure its food more easily by
having its beak curved, and if one were born with its beak strongly
curved, and which consequently flourished, nevertheless there would
be a very poor chance of this one individual perpetuating its kind
to the exclusion of the common form; but there can hardly be a
doubt, judging by what we see taking place under domestication,
that this result would follow from the preservation during many
generations of a large number of individuals with more or less
strongly curved beaks, and from the destruction of a still larger
number with the straightest beaks.

It should not, however, be overlooked that certain rather strongly
marked variations, which no one would rank as mere individual
differences, frequently recur owing to a similar organisation being
similarly acted on — of which fact numerous instances could be given
with our domestic productions. In such cases, if the varying in-
dividual did not actually transmit to its offspring its newly acquired
character, it would undoubtedly transmit to them, as long as the
existing conditions remained the same, a still stronger tendency to
vary in the same manner. There can also be little doubt that the
tendency to vary in the same manner has often been so strong that
all the individuals of the same species have been similarly modified

ACTION OF NATURAL SELECTION 99

without the aid of any form of selection. Or only a third, fifth, or
tenth part of the individuals may have been thus affected, of which
fact several instances could be given. Thus Graba estimates that
about one-fifth of the guillemots in the Faroe Islands consist of a
variety so well marked, that it was formerly ranked as a distinct
species under the name of Uria lacrymans. In cases of this kind, if
the variation were of a beneficial nature, the original form would
soon be supplanted by the modified form, through the survival of
the fittest.

To the effects of intercrossing in eliminating variations of all
kinds, I shall have to recur; but it may be here remarked that most
animals and plants keep to their proper homes, and do not need-
lessly wander about; we see this even with migratory birds, which
almost always return to the same spot. Consequently each newly-
formed variety would generally be at first local, as seems to be the
common rule with varieties in a state of nature; so that similarly
modified individuals would soon exist in a small body together, and
would often breed together. If the new variety were successful in
its battle for life, it would slowly spread from a central district,
competing with and conquering the unchanged individuals on the
margins of an ever-increasing circle.

It may be worth while to give another and more complex illustra-
tion of the action of natural selection. Certain plants excrete sweet
juice, apparently for the sake of eliminating something injurious
from the sap: this is effected, for instance, by glands at the base of
the stipules in some Leguminosz, and at the backs of the leaves of
the common laurel. This juice, though small in quantity, is greedily
sought by insects; but their visits do not in any way benefit the
plant. Now, let us suppose that the juice or nectar was excreted
from the inside of the flowers of a certain number of plants of any
species. Insects in seeking the nectar would get dusted with pollen,
and would often transport it from one flower to another. The flow-
ers of two distinct individuals of the same species would thus get
crossed; and the act of crossing, as can be fully proved, gives rise to
vigorous seedlings, which consequently would have the best chance
of flourishing and surviving. The plants which produced flowers
with the largest glands or nectaries, excreting most nectar, would

100 ORIGIN OF SPECIES

oftenest be visited by insects, and would oftenest be crossed; and so
in the long run would gain the upper hand and form a local variety.
The flowers, also, which had their stamens and pistils placed, in
relation to the size and habits of the particular insect which visited
them, so as to favour in any degree the transportal of the pollen,
would likewise be favoured. We might have taken the case of in-
sects visiting flowers for the sake of collecting pollen instead of nec-
tar; and as pollen is formed for the sole purpose of fertilisation, its
destruction appears to be a simple loss to the plant; yet if a little
pollen were carried, at first occasionally and then habitually, by the
pollen-devouring insects from flower to flower, and a cross thus
effected, although nine-tenths of the pollen were destroyed, it might
still be a great gain to the plant to be thus robbed; and the in-
dividuals which produced more and more pollen, and had larger
anthers, would be selected.

When our plant, by the above process long continued, had been
rendered highly attractive to insects, they would, unintentionally on
their part, regularly carry pollen from flower to flower; and that
they do this effectually, I could easily show by many striking facts.
I will give only one, as likewise illustrating one step in the separation
of the sexes of plants. Some holly trees bear only male flowers, which
have four stamens producing a rather small quantity of pollen, and
a rudimentary pistil; other holly trees bear only female flowers;
these have a full-sized pistil, and four stamens with shrivelled
anthers, in which not a grain of pollen can be detected. Having
found a female tree exactly sixty yards from a male tree, I put the
stigmas of twenty flowers, taken from different branches, under the
microscope, and on all, without exception, there were a few pollen
grains, and on some a profusion. As the wind had set for several
days from the female to the male tree, the pollen could not thus
have been carried. The weather had been cold and boisterous, and
therefore not favourable to bees, nevertheless every female flower
which I examined had been effectually fertilised by the bees, which
had flown from tree to tree in search of nectar. But to return to
our imaginary case: as soon as the plant had been rendered so highly
attractive to insects that pollen was regularly carried from flower to
flower, another process might commence. No naturalist doubts the

ACTION OF NATURAL SELECTION 101

advantage of what has been called the "physiological division of
labour"; hence we may believe that it would be advantageous to a
plant to produce stamens alone in one flower or on one whole plant,
and pistils alone in another flower or on another plant. In plants
under culture and placed under new conditions of life, sometimes
the male organs and sometimes the female organs become more or
less impotent; now if we suppose this to occur in ever so slight a
degree under nature, then, as pollen is already carried regularly
from flower to flower, and as a more complete separation of the
sexes of our plant would be advantageous on the principle of the
division of labour, individuals with this tendency more and more
increased, would be continually favoured or selected, until at last
a complete separation of the sexes might be effected. It would take
up too much space to show the various steps, through dimorphism
and other means, by which the separation of the sexes in plants of
various kinds is apparently now in progress; but I may add that
some of the species of holly in North America, are, according to
Asa Gray, in an exactly intermediate condition, or, as he expresses
it, are more or less dioeciously polygamous.

Let us now turn to the nectar-feeding insects; we may suppose
the plant, of which we have been slowly increasing the nectar by
continued selection, to be a common plant; and that certain insects
depended in main part on its nectar for food. I could give many
facts showing how anxious bees are to save time: for instance, their
habit of cutting holes and sucking the nectar at the bases of certain
flowers, which with a very little more trouble, they can enter by the
mouth. Bearing such facts in mind, it may be believed that under
certain circumstances individual differences in the curvature or
length of the proboscis, etc., too slight to be appreciated by us, might
profit a bee or other insect, so that certain individuals would be able
to obtain their food more quickly than others; and thus the com-
munities to which they belonged would flourish and throw off many
swarms inheriting the same peculiarities. The tubes of the corolla
of the common red and incarnate clovers (Trifolium pratense and
incarnatum) do not on a hasty glance appear to differ in length;
yet the hive bee can easily suck the nectar out of the incarnate
clover, but not out of the common red clover, which is visited by

I02 ORIGIN OF SPECIES

humble bees alone; so that whole fields of the red ciover offer in
vain an abundant supply of precious nectar to the hive bee. That
this nectar is much liked by the hive bee is certain; for I have re-
peatedly seen, but only in the autumn, many hive bees sucking the
flowers through holes bitten in the base of the tube by humble bees.
The difference in the length of the corolla in the two kinds of clover,
which determines the visits of the hive bee, must be very trifling;
for I have been assured that when red clover has been mown, the
flowers of the second crop are somewhat smaller, and that these are
visited by many hive bees. I do not know whether this statement is
accurate; nor whether another published statement can be trusted,
namely, that the Ligurian bee, which is generally considered a mere
variety of the common hive bee, and which freely crosses with it, is
able to reach and suck the nectar of the red clover. Thus, in a coun-
try where this kind of clover abounded, it might be a great advan-
tage to the hive bee to have a slightly longer or differently constructed
proboscis. On the other hand, as the fertility of this clover absolutely
depends on bees visiting the flowers, if humble bees were to become
rare in any country, it might be a great advantage to the plant to
have a shorter or more deeply divided corolla, so that the hive bees
should be enabled to suck its flowers. Thus I can understand how a
flower and a bee might slowly become, either simultaneously or one
after the other, modified and adapted to each other in the most per-
fect manner, by the continued preservation of all the individuals
which presented slight deviations of structure mutually favourable
to each other.

I am well aware that this doctrine of natural selection, exempli-
fied in the above imaginary instances, is open to the same objections
which were first urged against Sir Charles Lyell's noble views on
"the modern changes of the earth, as illustrative of geology"; but
we now seldom hear the agencies which we see still at work, spoken
of as trifling or insignificant, when used in explaining the excavation
of the deepest valleys or the formation of long lines of inland cliffs.
Natural selection acts only by the preservation and accumulation of
small inherited modifications, each profitable to the preserved being;
and as modern geology has almost banished such views as the ex-
cavation of a great valley by a single diluvial wave, so will natural

ON THE INTERCROSSING OF INDIVIDUALS IO3

selection banish the belief of the continued creation of new organic
beings, or of any great and sudden modification in their structure.

ON THE INTERCROSSING OF INDIVIDUALS

I must here introduce a short digression. In the case of animals
and plants with separated sexes, it is of course obvious that two in-
dividuals must always (with the exception of the curious and not
well understood cases of parthenogenesis) unite for each birth; but
in the case of hermaphrodites this is far from obvious. Nevertheless
there is reason to believe that with all hermaphrodites two individ-
uals, either occasionally or habitually, concur for the reproduaion
of their kind. This view was long ago doubtfully suggested by
Sprengel, Knight, and Kolreuter. We shall presently see its im-
portance; but I must here treat the subject with extreme brevity,
though 1 have the materials prepared for an ample discussion. All
vertebrate animals, all insects, and some other large groups of
animals, pair for each birth. Modern research has much diminished
the number of supposed hermaphrodites, and of real hermaphrodites
a large number pair; that is, two individuals regularly unite for re-
production, which is all that concerns us. But still there are many
hermaphrodite animals which certainly do not habitually pair, and
a vast majority of plants are hermaphrodites. What reason, it may
be asked, is there for supposing in these cases that two individuals
ever concur in reproduction? As it is impossible here to enter 00
details, I must trust to some general considerations alone.

In the first place, I have collected so large a body of facts, and
made so many experiments, showing, in accordance with the almost
universal belief of breeders, that with animals and plants a cross
between different varieties, or between individuals of the same va-
riety but of another strain, gives vigour and fertility to the offspring;
and on the other hand, that close interbreeding diminishes vigour
and fertility; that these facts alone incline me to believe that it is a
general law of nature that no organic being fertilises itself for a
jjerpetuity of generations; but that a cross with another individual
is occasionally — perhaps at long intervals of time — indispensable.

On the belief that this is a law of nature, we can, I think, under-
stand several large classes of facts, such as the following, which on

104 ORIGIN OF SPECIES

any other view are inexplicable. Every hybridizer knows how un-
favourable exposure to wet is to the fertilisation of a flower, yet what
a multitude of flowers have their anthers and stigmas fully exposed
to the weather! If an occasional cross be indispensable, notwith-
standing that the plant's own anthers and pistil stand so near each
other as almost to insure self-fertilisation, the fullest freedom for
the entrance of pollen from another individual will explain the above
state of exposure of the organs. Many flowers, on the other hand,
have their organs of fructification closely enclosed, as in the great
papilionaceous or pea-family; but these almost invariably present
beautiful and curious adaptations in relation to the visits of insects.
So necessary are the visits of bees to many papilionaceous flowers,
that their fertiUty is greatly diminished if these visits be prevented.
Now, it is scarcely f>ossible for insects to fly from flower to flower,
and not to carry pollen from one to the other, to the great good of
the plant. Insects act like a camel-hair pencil, and it is suflicient, to
ensure fertilisation, just to touch with the same brush the anthers of
one flower and then the stigma of another; but it must not be sup-
posed that bees would thus produce a multitude of hybrids between
distinct species; for if a plant's own pollen and that from another
species are placed on the same stigma, the former is so prepotent that
it invariably and completely destroys, as has been shown by Gartner,
the influence of the foreign pollen.

When the stamens of a flower suddenly spring towards the pistil,
or slowly move one after the other towards it, the contrivance seems
adapted solely to ensure self-fertilisation; and no doubt it is useful
for this end: but the agency of insects is often required to cause the
stamens to spring forward, as Kolreuter has shown to be the case
with the barberry; and in this very genus, which seems to have a
special contrivance for self-fertilisation, it is well known that, if
closely-allied forms or varieties are planted near each other, it is
hardly (xjssible to raise pure seedlings, so largely do they naturally
cross. In numerous other cases, far from self-fertilisation being
favoured, there are sp)ecial contrivances which effectually prevent the
stigma receiving jxillen from its own flower, as I could show from
the works of Sprengel and others, as well as from my own observa-
tions: for instance, in Lobelia fulgens, there is a really beautiful and

ON THE INTERCROSSING OF INDIVIDUALS IO5

elaborate contrivance by which all the infinitely numerous pollen-
granules are swept out of the conjoined anthers of each flower, before
the stigma of that individual flower is ready to receive them; and as
this flower is never visited, at least in my garden, by insects, it never
sets a seed, though by placing pollen from one flower on the stigma
of another, I raise plenty of seedlings. Another species of Lobelia,
which is visited by bees, seeds freely in my garden. In very many
other cases, though there is no special mechanical contrivance to
prevent the stigma receiving pollen from the same flower, yet, as
Sprengcl, and more recently Hildebrand, and others, have shown,
and as I can confirm, either the anthers burst before the stigma is
ready for fertilisation, or the stigma is ready before the pollen of that
flower is ready, so that these so-named dichogamous plants have in
fact separated sexes, and must habitually be crossed. So it is with
the reciprocally dimorphic and trimorphic plants previously alluded
to. How strange are these facts! How strange that the pollen and
stigmatic surface of the same flower, though placed so close together,
as if for the very purpose of self-fertilisation, should be in so many
cases mutually useless to each other? How simply are these facts
explained on the view of an occasional cross with a distinct indi-
vidual being advantageous or indispensable!

If several varieties of the cabbage, radish, onion, and of some other
plants, be allowed to seed near each other, a large majority of the
seedlings thus raised turn out, as I have found, mongrels: for in-
stance, I raised 233 seedling cabbages from some plants of different
varieties growing near each other, and of these only seventy-eight
were true to their kind, and some even of these were not perfectly
true. Yet the pistil of each cabbage-flower is surrounded not only by
its own six stamens but by those of the many other flowers on the
same plant; and the pollen of each flower readily gets on its own
stigma without insect agency; for I have found that plants carefully
protected from insects produce the full number of pods. How, then,
comes it that such a vast number of the seedlings are mongrelized ?
It must arise from the pollen of a distinct variety having a prepotent
effect over the flower's own pollen; and that this is part of the general
law of good being derived from the intercrossing of distinct indi-
viduals of the same species. When distinct species are crossed the

I06 ORIGIN OF SPECIES

case is reversed, for a plant's own pollen is almost always prepotent
over foreign pollen; but to this subject we shall return in a future
chapter.

In the case of a large tree covered with innumerable flowers, it
may be objected that pollen could seldom be carried from tree to
tree, and at most only from flower to flower on the same tree; and
flowers on the same tree can be considered as distinct individuals
only in a limited sense. I believe this objection to be valid, but that
nature has largely provided against it by giving to trees a strong
tendency to bear flowers with separated sexes. When the sexes are
separated, although the male and female flowers may be produced
on the same tree, pollen must be regularly carried from flower to
flower; and this will give a better chance of pollen being occasionally
carried from tree to tree. That trees belonging to all orders have
their sexes more often separated than other plants, I find to be the
case in this country; and at my request Dr. Hooker tabulated the
trees of New 2^aland, and Dr. Asa Gray those of the United States,
and the result was as I anticipated. On the other hand, Dr. Hooker
informs me that the rule does not hold good in Australia: but if
most of the Australian trees are dichogamous, the same result would
follow as if they bore flowers with separated sexes. I have made
these few remarks on trees simply to call attention to the subject.

Turning for a brief space to animals: various terrestrial species
are hermaphrodites, such as the land Mollusca and earthworms;
but these all pair. As yet I have not found a single terrestrial animal
which can fertilise itself. This remarkable fact, which offers so
strong a contrast with terrestrial plants, is intelligible on the view
of an occasional cross being indispensable; for owing to the nature
of the fertihsing element there are no means, analogous to the action
of insects and of the wind with plants, by which an occasional cross
could be effected with terrestrial animals without the concurrence
of two individuals. Of aquatic animals, there are many self-fertilis-
ing hermaphrodites; but here the currents of water offer an obvious
means for an occasional cross. As in the case of flowers, I have as
yet failed, after consultation with one of the highest authorities,
namely. Professor Huxley, to discover a single hermaphrodite animal
with the organs of reproduction so perfectly enclosed that access

PRODUCTION OF NEW FORMS IO7

from without, and the occasional influence of a distinct individual,
can be shown to be physically impossible. Cirripedes long appeared
to me to present, under this point of view, a case of great difficulty;
but I have been enabled, by a fortunate chance, to prove that two
individuals, though both are self-fertilising hermaphrodites, do
sometimes cross.

It must have struck most naturalists as a strange anomaly that,
both with animals and plants, some species of the same family and
even of the same genus, though agreeing closely with each other in
their whole organisation, are hermaphrodites, and some unisexual.
But if, in fact, all hermaphrodites do occasionally intercross, the
difference between them and unisexual species is, as far as function
is concerned, very small.

From these several considerations and from the many special facts
which I have collected, but which I am unable here to give, it appears
that with animals and plants an occasional intercross between dis-
tinct individuals is a very general, if not universal, law of nature.

CIRCUMSTANCES FAVOURABLE FOR THE PRODUCTION OF NEW
FORMS THROUGH NATURAL SELECTION

This is an extremely intricate subject. A great amount of variabil-
ity, under which term individual differences are always included,
will evidently be favourable. A large number of individuals, by
giving a better chance within any given period for the appearance
of profitable variations, will compensate for a lesser amount of
variability in each individual and is, I beUeve, a highly important
element of success. Though Nature grants long periods of time for
the work of natural selection, she does not grant an indefinite period;
for as all organic beings are striving to seize on each place in the
economy of nature, if any one species does not become modified and
improved in a corresponding degree with its competitors, it will be
exterminated. Unless favourable variations be inherited by some at
least of the offspring, nothing can be effected by natural selection.
The tendency to reversion may often check or prevent the work;
but as this tendency has not prevented man from forming by selec-
tion numerous domestic races, why should it prevail against natural
selection ?

I08 ORIGIN OF SPECIES

In the case of methodical selection, a breeder selects for some
definite object, and if the individuals be allowed freely to intercross,
his work will completely fail. But when many men, without in-
tending to alter the breed, have a nearly common standard of [per-
fection, and all try to procure and breed from the best animals,
improvement surely but slowly follows from this unconscious proc-
ess of selection, notwithstanding that there is no separation of se-
lected individuals. Thus it will be under nature; for within a con-
fined area, with some place in the natural polity not perfectly occu-
pied, all the individuals varying in the right direction, though in
different degrees, will tend to be preserved. But if the area be large,
its several districts will almost certainly present different conditions
of life; and then, if the same species undergoes modification in dif-
ferent districts, the newly-formed varieties will intercross on the
confines of each. But we shall see in the sixth chapter that inter-
mediate varieties, inhabiting intermediate districts, will in the long
run generally be supplanted by one of the adjoining varieties. Inter-
crossing will chiefly affect those animals which unite for each birth
and wander much, and which do not breed at a very quick rate.
Hence with animals of this nature, for instance, birds, varieties will
generally be confined to separated countries; and this I find to be
the case. With hermaphrodite organisms which cross only occa-
sionally, and likewise with animals which unite for each birth, but
which wander little and can increase at a rapid rate, a new and im-
proved variety might be quickly formed on any one spot, and might
there maintain itself in a body and afterwards spread, so that the
individuals of the new variety would chiefly cross together. On this
principle, nurserymen always prefer saving seed from a large body
of plants, as the chance of intercrossing is thus lessened.

Even with animals which unite for each birth, and which do not
propagate rapidly, we must not assume that free intercrossing would
always eliminate the effects of natural selection; for I can bring
forward a considerable body of facts showing that within the same
area, two varieties of the same animal may long remain distinct,
from haunting different stations, from breeding at slightly different
seasons, or from the individuals of each variety preferring to pair
together.

PRODUCTION OF NEW FORMS IO9

Intercrossing plays a very important part in nature by keeping
the individuals of the same species, or of the same variety, true and
uniform in character. It will obviously thus act far more efficiently
with those animals which unite for each birth; but, as already stated,
we have reason to believe that occasional intercrosses take place with
all animals and plants. Even if these take place only at long intervals
of time, the young thus produced will gain so much in vigour and
fertility over the offspring from long<ontinued self-fertilisation,
that they will have a better chance of surviving and propagating
their kind; and thus in the long run the influence of crosses, even
at rare intervals, will be great. With respect to organic beings ex-
tremely low in the scale, which do not propagate sexually, nor con-
jugate, and which cannot possibly intercross, uniformity of character
can be retained by them under the same conditions of life, only
through the principle of inheritance, and through natural selection
which will destroy any individuals departing from the proper type.
If the conditions of life change and the form undergoes modification,
uniformity of character can be given to the modified offspring, solely
by natural selection preserving similar favourable variations.

Isolation, also, is an important element in the modification of
species through natural selection. In a confined or isolated area, if
not very large, the organic and inorganic conditions of life will
generally be almost uniform; so that natural selection will tend to
modify all the varying individuals of the same species in the same
manner. Intercrossing with the inhabitants of the surrounding dis-
tricts will, also, be thus prevented. Moritz Wagner has lately pub-
lished an interesting essay on this subject, and has shown that the
service rendered by isolation in preventing crosses between newly-
formed varieties is probably greater even than I supposed. But from
reasons already assigned I can by no means agree with this naturalist,
that migration and isolation are necessary elements for the formation
of new species. The importance of isolation is likewise great in pre-
venting, after any physical change in the conditions such as of cli-
mate, elevation of the land, etc., the immigration of better adapted
organisms; and thus new places in the natural economy of the dis-
trict will be left open to be filled up by the modification of the old
inhabitants. Lastly, isolation will give time for a new variety to be

no ORIGIN OF SPECIES

improved at a slow rate; and this may sometimes be of much im-
portance. If, however, an isolated area be very small, either from
being surrounded by barriers, or from having very peculiar physi-
cal conditions, the total number of the inhabitants will be small;
and this will retard the production of new species through natural
selection, by decreasing the chances of favourable variations aris-
ing.

The mere lapse of time by itself does nothing, either for or against
natural selection. I state this because it has been erroneously asserted
that the element of time has been assumed by me to play an all-
important part in modifying species, as if all the forms of Ufe were
necessarily undergoing change through some innate law. Lapse of
time is only so far important, and its importance in this respect is
great, that it gives a better chance of beneficial variations arising
and of their being selected, accumulated, and fixed. It likewise tends
to increase the direct action of the physical conditions of life, in re-
lation to the constitution of each organism.

If we turn to nature to test the truth of these remarks, and look
at any small isolated area, such as an oceanic island, although the
number of species inhabiting it is small, as we shall see in our chapter
on Geographical Distribution; yet of these species a very large pro-
portion are endemic, — that is, have been produced there and nowhere
else in the world. Hence an oceanic island at first sight seems to
have been highly favourable for the production of new species. But
we may thus deceive ourselves, for to ascertain whether a small
isolated area, or a large open area like a continent, has been most
favourable for the production of new organic forms, we ought to
make the comparison within equal times; and this we are incapable
of doing.

Although isolation is of great importance in the production of new
species, on the whole I am inclined to believe that largeness of area
is still more important, especially for the production of species which
shall prove capable of enduring for a long period, and of spreading
widely. Throughout a great and open area, not only will there be a
better chance of favourable variations, arising from the large num-
ber of individuals of the same species there supported, but the con-
ditions of life are much more complex from the large number of

PRODUCTION OF NEW FORMS III

already existing species; and if some of these many species become
modified and improved, others will have to be improved in a cor-
responding degree, or they will be exterminated. Each new form,
also, as soon as it has been much improved, will be able to spread
over the open and continuous area, and will thus come into com-
petition with many other forms. Moreover, great areas, though now
continuous, will often, owing to former oscillations of level, have
existed in a broken condition; so that the good effects of isolation
will generally, to a certain extent, have concurred. Finally, I con-
clude that, although small isolated areas have been in some respects
highly favourable for the production of new species, yet that the
course of modification will generally have been more rapid on large
areas; and what is more important, that the new forms produced
on large areas, which already have been victorious over many com-
p)etitors, will be those that will spread most widely, and will give
rise to the greatest number of new varieties and species. They will
thus play a more imjx)rtant part in the changing history of the
organic world.

In accordance with this view, we can, perhaps, understand some
facts which will be again alluded to in our chapter on Geographical
Distribution; for instance, the fact of the productions of the smaller
continent of Australia now yielding before those of the larger Eu-
ropxo-Asiatic area. Thus, also, it is that continental productions
have everywhere become so largely naturalised on islands. On a
small island, the race for life will have been less severe, and there
will have been less modification and less extermination. Hence, we
can understand how it is that the flora of Madeira, according to
Oswald Heer, resembles to a certain extent the extinct tertiary flora
of Europe. All fresh-water basins, taken together, make a small
area compared with that of the sea or of the land. Consequently,
the competition between fresh-water productions will have been less
severe than elsewhere; new forms will have been then more slowly
produced, and old forms more slowly exterminated. And it is in
fresh-water basins that we find seven genera of Ganoid fishes,
remnants of a once preponderant order : and in fresh water we find
some of the most anomalous forms now known in the world as the
Ornithorhynchus and Lepidosiren, which, like fossils, connect to a

112 ORIGIN OF SPECIES

certain extent orders at present widely sundered in the natural scale.
These anomalous forms may be called living fossils; they have en-
dured to the present day, from having inhabited a confined area,
and from having been exposed to less varied, and therefore less
severe, competition.

To sum up, as far as the extreme intricacy of the subject permits,
the circumstances favourable and unfavourable for the production
of new species through natural selection. 1 conclude that for ter-
restrial productions a large continental area, which has undergone
many oscillations of level, will have been the most favourable for
the production of many new forms of life, fitted to endure for a
long time and to spread widely. Whilst the area existed as a con-
tinent, the inhabitants will have been numerous in individuals and
kinds, and will have been subjected to severe competition. When
converted by subsidence into large separate islands, there will still
have existed many individuals of the same species on each island;
intercrossing on the confines of the range of each new species will
have been checked; after physical changes of any kind, immigration
will have been prevented, so that new places in the polity of each
island will have had to be filled up by the modification of the old
inhabitants; and time will have been allowed for the varieties in
each to become well modified and perfected. When, by renewed
elevation, the islands were reconverted into a continental area, there
will again have been very severe competition: the most favoured or
improved varieties will have been enabled to spread : there will have
been much extinction of the less improved forms, and the relative
proportional numbers of the various inhabitants of the reunited con-
tinent will again have been changed; and again there will have been
a fair field for natural selection to improve still further the inhabi-
tants, and thus to produce new species.

That natural selection generally acts with extreme slowness I fully
admit. It can act only when there are places in the natural jxiHty of
a district which can be better occupied by the modification of some
of its existing inhabitants. The occurrence of such places will often
depend on physical changes, which generally take place very slowly,
and on the immigration of better adapted forms being prevented.
As some few of the old inhabitants become modified, the mutual

EXTINCTION CAUSED BY NATURAL SELECTION II3

relations of others will often be disturbed; and this will create new
places, ready to be filled up by better adapted forms; but all this
will take place very slowly. Although all the individuals of the
same species differ in some slight degree from each other, it would
often be long before differences of the right nature in various parts
of the organisation might occur. The result would often be greatly
retarded by free intercrossing. Many will exclaim that these several
causes are amply sufficient to neutralise the power of natural selec-
tion. I do not believe so. But I do believe that natural selection will
generally act very slowly, only at long intervals of time, and only
on a few of the inhabitants of the same region. I further believe
that these slow, intermittent results accord well with what geology
tells us of the rate and manner at which the inhabitants of the world
have changed.

Slow though the process of selection may be, if feeble man can
do much by artificial selection, I can see no limit to the amount of
change, to the beauty and complexity of the co-adaptations between
all organic beings, one with another and with their physical con-
ditions of life, which may have been affected in the long course of
time through nature's power of selection, that is by the survival of
the fittest.

EXTINCTION CAUSED BY NATURAL SELECTION

This subject will be more fully discussed in our chapter on Geol-
ogy; but it must here be alluded to from being intimately connected
with natural selection. Natural selection acts solely through the
preservation of variations in some way advantageous, which con-
sequently endure. Owing to the high geometrical rate of increase
of all organic beings, each area is already fully stocked with in-
habitants; and it follows from this, that as the favoured forms in-
crease in number, so, generally, will the less favoured decrease and
become rare. Rarity, as geology tells us, is the precursor to extinction.
We can see that any form which is represented by few individuals
will run a good chance of utter extinction, during great fluctuations
in the nature of the seasons, or from a temporary increase in the
number of its enemies. But we may go further than this; for, as
new forms are produced, unless we admit that specific forms can go

114 ORIGIN OF SPECIES

on indefinitely increasing in number, many old forms must become
extinct. That the number of specific forms has not indefinitely in-
creased, geology plainly tells us; and we shall presently attempt to
show why it is that the number of species throughout the world has
not become immeasurably great.

We have seen that the species which are most numerous in in-
dividuals have the best chance of producing favourable variations
within any given period. We have evidence of this, in the facts
stated in the second chapter, showing that it is the common and
diffused or dominant species which offer the greatest number of
recorded varieties. Hence, rare species will be less quickly modified
or improved within any given period; they will consequently be
beaten in the race for life by the modified and improved descendants
of the commoner species.

From these several considerations I think it inevitably follows,
that as new sjjecies in the course of time are formed through natural
selection, others will become rarer and rarer, and finally extinct.
The forms which stand in closest competition with those undergoing
modification and improvement, will naturally suffer most. And we
have seen in the chapter on the Struggle for Existence, that it is the
most closely-allied forms, — varieties of the same species, and species
of the same genus or of related genera, — which, from having nearly
the same structure, constitution, and habits, generally come into the
severest competition with each other; consequently, each new variety
or species, during the progress of its formation, will generally press
hardest on its nearest kindred, and tend to exterminate them. We
see the same process of extermination amongst our domesticated
productions, through the selection of improved forms by man. Many
curious instances could be given showing how quickly new breeds
of cattle, sheep, and other animals, and varieties of flowers, take the
place of older and inferior kinds. In Yorkshire, it is historically
known that the ancient black cattle were displaced by the long-
horns, and that these "were swept away by the shorthorns" (I
quote the words of an agricultural writer) "as if by some murderous
pestilence."

DIVERGENCE OF CHARACTER II5

DIVERGENCE OF CHARACTER

The principle, which I have designated by this term, is of high
importance, and explains, as I believe, several important facts. In
the first place, varieties, even strongly marked ones, though having
somewhat of the character of species — as is shown by the hopeless
doubts in many cases how to rank them — yet certainly differ far
less from each other than do good and distinct species. Neverthe-
less, according to my view, varieties are species in the process of
formation, or are, as 1 have called them, incipient species. How,
then, does the lesser difference between varieties become augmented
into the greater difference between species? That this does habit-
ually happen, we must infer from most of the innumerable species
throughout nature presenting well-marked differences; whereas
varieties, the supposed prototypes and parents of future well-marked
species, present slight and ill-defined differences. Mere chance, as
we may call it, might cause one variety to differ in some character
from its parents, and the offspring of this variety again to differ
from its parent in the very same character and in a greater degree;
but this alone would never account for so habitual and large a de-
gree of difference as that between the species of the same genus.

As has always been my practice, I have sought light on this head
from our domestic productions. We shall here find something
analogous. It will be admitted that the production of races so dif-
ferent as shorthorn and Hereford cattle, race and cart horses, the
several breeds of pigeons, etc., could never have been effected by
the mere chance accumulation of similar variations during many
successive generations. In practice, a fancier is, for instance, struck
by a pigeon having a slightly shorter beak; another fancier is struck
by a pigeon having a rather longer beak; and on the acknowledged
principle that "fanciers do not and will not admire a medium
standard, but like extremes," they both go on (as has actually oc-
curred with the sub-breeds of the tumbler pigeon) choosing and
breeding from birds with longer and longer beaks, or with shorter
and shorter beaks. Again, we may suppose that at an early period
of history, the men of one nation or district required swifter horses,
whilst those of another required stronger and bulkier horses. The

Il6 ORIGIN OF SPECIES

early differences would be very slight; but, in the course of time,
from the continued selection of swifter horses in the one case, and
of stronger ones in the other, the differences would become greater,
and would be noted as forming two sub-breeds. Ultimately, after
the lapse of centuries, these sub-breeds would become converted
into two well-estabhshed and distinct breeds. As the differences
became greater, the inferior animals with intermediate characters,
being neither very swift nor very strong, would not have been used
for breeding, and will thus have tended to disappear. Here, then,
we see in man's productions the action of what may be called the
principle of divergence, causing differences, at first barely appre-
ciable, steadily to increase, and the breeds to diverge in character,
both from each other and from their common parent.

But how, it may be asked, can any analogous principle apply in
nature.? I believe it can and does apply most efficiently (though it
was a long time before I saw how), from the simple circumstance
that the more diversified the descendants from any one species be-
come in structure, constitution, and habits, by so much will they be
better enabled to seize on many and widely diversified places in the
polity of nature, and so be enabled to increase in numbers.

We can clearly discern this in the case of animals with simple
habits. Take the case of a carnivorous quadruped, of which the
number that can be supported in any country has long ago arrived
at its full average. If its natural power of increase be allowed to act,
it can succeed in increasing (the country not undergoing any change
in conditions) only by its varying descendants seizing on places at
present occupied by other animals; some of them, for instance, being
enabled to feed on new kinds of prey, either dead or alive; some
inhabiting new stations, climbing trees, frequenting water, and some
perhaps becoming less carnivorous. The more diversified in habits
and structure the descendants of our carnivorous animals become,
the more places they will be enabled to occupy. What applies to one
animal will apply throughout all time to all animals — that is, if they
vary — for otherwise natural selection can effect nothing. So it will
be with plants. It has been experimentally proved, that if a plot of
ground be sown with one species of grass, and a similar plot be sown
with several distinct genera of grasses, a greater number of plants

DIVERGENCE OF CHARACTER II7

and a greater weight of dry herbage can be raised in the latter than
in the former case. The same has been found to hold good when
one variety and several mixed varieties of wheat have been sown on
equal spaces of ground. Hence, if any one species of grass were to go
on varying, and the varieties were continually selected which dif-
fered from each other in the same manner, though in a very slight
degree, as do the distinct species and genera of grasses, a greater
number of individual plants of this species, including its modified
descendants, would succeed in living on the same piece of ground.
And we know that each species and each variety of grass is annually
sowing almost countless seeds; and is thus striving, as it may be
said, to the utmost to increase in number. Consequently, in the
course of many thousand generations, the most distinct varieties of
any one species of grass would have the best chance of succeeding
and of increasing in numbers, and thus of supplanting the less dis-
tinct varieties; and varieties, when rendered very distinct from each
other, take the rank of species.

The truth of the principle that the greatest amount of life can be
supported by great diversification of structure, is seen under many
natural circumstances. In an extremely small area, especially if
freely open to immigration, and where the contest between individual
and individual must be very severe, we always find great diversity
in its inhabitants. For instance, I found that a piece of turf, three
feet by four in size, which had been exposed for many years to
exacdy the same conditions, supported twenty species of plants, and
these belonged to eighteen genera and to eight orders, which shows
how much these plants differed from each other. So it is with the
plants and insects on small and uniform islets: also in small ponds
of fresh water. Farmers find that they can raise most food by a rota-
tion of plants belonging to the most different orders; nature follows
what may be called a simultaneous rotation. Most of the animals
and plants which live close round any small piece of ground, could
live on it (supposing its nature not to be in any way peculiar), and
may be said to be striving to the utmost to live there; but, it is seen,
that where they come into the closest competition, the advantages of
diversification of structure, with the accompanying differences of
habit and constitution, determine that the inhabitants, which thus

Il8 ORIGIN OF SPECIES

jostle each other most closely, shall, as a general rule, belong to what
we call different genera and orders.

The same principle is seen in the naturalisation of plants through
man's agency in foreign lands. It might have been expected that the
plants which would succeed in becoming naturalised in any land
would generally have been closely allied to the indigenes; for these
are commonly looked at as specially created and adapted for their
own country. It might also, perhaps, have been expected that natural-
ised plants would have belonged to a few groups more especially
adapted to certain stations in their new homes. But the case is very
different; and Alph. de Candolle has well remarked, in his great and
admirable work, that floras gain by naturalisation, proportionally
with the number of the native genera and species, far more in new
genera than in new species. To give a single instance: in the last
edition of Dr. Asa Gray's 'Manual of the Flora of the Northern
United States,' 260 naturalised plants are enumerated, and these
belong to 162 genera. We thus see that these naturalised plants are
of a highly diversified nature. They differ, moreover, to a large
extent, from the indigenes, for out of the 162 naturalised genera, no
less than 100 genera are not there indigenous, and thus a large pro-
portional addition is made to the genera now living in the United
States.

By considering the nature of the plants or animals which have in
any country struggled successfully with the indigenes, and have there
become naturalised, we may gain some crude idea in what manner
some of the natives would have to be modified, in order to gain an
advantage over their compatriots; and we may at least infer that
diversification of structure, amounting to new generic differences,
would be profitable to them.

The advantage of diversification of structure in the inhabitants
of the same region is, in fact, the same as that of the physiological
division of labor in the organs of the same individual body — a sub-
ject so well elucidated by Milne Edwards. No physiologist doubts
that a stomach adapted to digest vegetable matter alone, or flesh
alone, draws most nutriment from these substances. So in the gen-
eral economy of any land, the more widely and perfectly the animals
and plants are diversified for different habits of life, so will a greater

EFFECTS OF NATURAL SELECTION II9

number of individuals be capable of there supporting themselves.
A set of animals, with their organisation but little diversified, could
hardly compete with a set more perfectly diversified in structure. It
may be doubted, for instance, whether the Australian marsupials,
which are divided into groups differing but little from each other,
and feebly representing, as Mr. Waterhouse and others have re-
marked, our carnivorous, ruminant, and rodent mammals, could
successfully compete with these well-developed orders. In the Aus-
tralian mammals, we see the process of diversification in an early
and incomplete stage of development.

THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELECTION THROUGH
DIVERGENCE OF CHARACTER AND EXTINCTION, ON THE DESCENDANTS OF
A COMMON ANCESTOR

After the foregoing discussion, which has been much compressed,
we may assume that the modified descendants of any one species
will succeed so much the better as they become more diversified in
structure, and are thus enabled to encroach on places occupied by
other beings. Now let us see how this principle of benefit being
derived from divergence of character, combined with the principles
of natural selection and of extinction, tends to act.

The accompanying diagram will aid us in understanding this
rather perplexing subject. Let A to L represent the species of a
genus large in its own country; these species are supposed to resemble
each other in unequal degrees, as is so generally the case in nature,
and as is represented in the diagram by the letters standing at
unequal distances. I have said a large genus, because, as we saw in
the second chapter, on an average more species vary in large genera
than in small genera; and the varying species of the large genera
present a greater number of varieties. We have, also, seen that the
species, which are the commonest and the most widely diffused,
vary more than do the rare and restricted species. Let (A) be a
common, widely-diffused, and varying species, belonging to a genus
large in its own country. The branching and diverging dotted lines
of unequal lengths proceeding from (A), may represent its varying
offspring. The variations are supposed to be extremely slight, but of
the most diversified nature; they are not supposed all to appear

120

a'* 1'* />'*

ORIGIN OF SPECIES
b'-t f<*

i'*

i'* m"*

a'" f'o

m"> E"

>W \t^

\/-^'

>w 4/^*

k^ I' '-Is^Jm*

\U' .}:jjr ''••}/>»'

a'^!/ %'

Hk* \!-4*

■••A' Mivw*

A' '•■':■■■

(-* '''--Wfn-f

Ml..

j' Hv'w^ i

'■,!// 1 i

'••ft:

'/: i :

A

B

\ \

D

E
i

1

simultaneously, but often after long intervals of time; nor are they
all supposed to endure for equal periods. Only those variations
which are in some way profitable will be preserved or naturally
selected. And here the importance of the principle of benefit derived
from divergence of character comes in; for this will generally lead to
the most different or divergent variations (represented by the outer

EFFECTS OF NATURAL SELECTION 121

/«'* F "^ n'4 r'-f iv"f y^ y'4 z'4

XIII

XII

XI

X

IX

VIII

VII

VI

1 \ \ i / \ ^

1

; 1 \ i / \

/

1/

E" F"' w'" z'°

\ tu9\i/ --.i/'i'

j u' T^^''U-' '^ij/i^

i u'\/ ti>'--.^i/ \!^i'

1 ! UO^ \^

' 1 \./ .•/ ,

i i ■■•••-. ../"

\ j t^ -^jA'

\ ! i \ / . /

\ i 1 Mi' i

i \ \ \ ! ,' 1
: ! : \ \ 1 ; .' I J

• i 1 •«•■ i 1

H I K L

IV

III

II

dotted lines) being preserved and accumulated by natural selection.
When a dotted line reaches one of the horizontal lines, and is there
marked by a small numbered letter, a sufficient amount of variation
is supposed to have been accumulated to form it into a fairly well-
marked variety, such as would be thought worthy of record in a
systematic work.

122 ORIGIN OF SPECIES

The intervals between the horizontal lines in the diagram, may
represent each a thousand or more generations. After a thousand
generations, species (A) is supposed to have produced two fairly
well-marked varieties, namely a' and mK These two varieties will
generally still be exposed to the same conditions which made their
parents variable, and the tendency to variability is in itself hereditary;
consequently they will likewise tend to vary, and commonly in
nearly the same manner as did their parents. Moreover, these tAvo
varieties, being only slightly modified forms, will tend to inherit
those advantages which made their parent (A) more numerous than
most of the other inhabitants of the same country; they will also
partake of those more general advantages which made the genus to
which the parent-species belonged, a large genus in its own country.
And all these circumstances are favorable to the production of new
varieties.

If, then, these two varieties be variable, the most divergent of their
variations will generally be preserved during the next thousand
generations. And after this interval, variety a' is supposed in the
diagram to have produced variety a', which will, owing to the prin-
ciple of divergence, differ more from (A) than did variety a'.
Variety w' is supposed to have produced two varieties, namely m*
and /, differing from each other, and more considerably from their
common parent (A). We may continue the process by similar steps
for any length of time; some of the varieties, after each thousand
generations, producing only a single variety, but in a more and more
modified condition, some producing two or three varieties, and some
failing to produce any. Thus the varieties or modified descendants
of the common parent (A), will generally go on increasing in num-
ber and diverging in character. In the diagram the process is repre-
sented up to the ten-thousandth generation, and under a condensed
and simpHfied form up to the fourteen-thousandth generation.

But I must here remark that I do not suppose that the process ever
goes on so regularly as is represented in the diagram, though in itself
made somewhat irregular, nor that it goes on continuously; it is far
more probable that each form remains for long periods unaltered,
and then again undergoes modification. Nor do I suppose that the
most divergent varieties are invariably preserved; a medium form

EFFECTS OF NATURAL SELECTION 1 23

may often long endure, and may or may not produce more than one
modified descendant; for natural selection will always act according
to the nature of the places which are either unoccupied or not per-
fectly occupied by other beings; and this will depend on infinitely
complex relations. But as a general rule, the more diversified in
structure the descendants from any one species can be rendered, the
more places they will be enabled to seize on, and the more their
modified progeny will increase. In our diagram the line of succession
is broken at regular intervals by small numbered letters marking the
successive forms which have become sufEciendy distinct to be
recorded as varieties. But these breaks are imaginary, and might
have been inserted anywhere, after intervals long enough to allow
the accumulation of a considerable amount of divergent varia-
tion.

As all the modified descendants from a common and widely-
diffused species, belonging to a large genus, will tend to partake of
the same advantages which made their parent successful in life, they
will generally go on multiplying in number as well as diverging
in character; this is represented in the diagram by the several
divergent branches proceeding from (A). The modified off-
spring from the later and more highly improved branches in the
lines of descent, will, it is probable, often take the place of, and so
destroy, the earlier and less improved branches: this is represented
in the diagram by some of the lower branches not reaching to the
upper horizontal lines. In some cases, no doubt, the process of
modification will be confined to a single line of descent, and the
number of modified descendants will not be increased; although the
amount of divergent modification may have been augmented. This
case would be represented in the diagram, if all the lines proceeding
from (A) were removed, excepting that from a' to a'". In the same
way the English race horse and English pointer have apparently
both gone on slowly diverging in character from their original stocks,
without either having given off any fresh branches or races.

After ten thousand generations, sp)ecies (A) is supposed to have
produced three forms, a'", /"* and m^", which, from having diverged
in character during the successive generations, will have come to
differ largely, birt perhaps unequally, from each other and from their

124 ORIGIN OF SPECIES

common parent. If we suppose the amount of change between each
horizontal Une in our diagram to be excessively small, these three
forms may still be only well-marked varieties; but we have only to
suppose the steps in the process of modification to be more numerous
or greater in amount, to convert these three forms into doubtful or
at least into well-defined sp)ecies. Thus the diagram illustrates the
steps by which the small dilTerences distinguishing varieties are in-
creased into the larger differences distinguishing species. By con-
tinuing the same process for a greater number of generations (as
shown in the diagram in a condensed and simplified manner), we
get eight species, marked by the letters between a" and m", all de-
scended from (A). Thus, as I believe, species are multiplied, and
genera are formed.

In a large genus it is probable that more than one species would
vary. In the diagram I have assumed that a second species (I) has
produced, by analogous steps, after ten thousand generations, either
two well-marked varieties («/'" and z'°) or two species, according to
the amount of change supposed to be represented between the
horizontal lines. After fourteen thousand generations, six new
species, marked by the letters n" to 2", are supposed to have been
produced. In any genus, the species which are already very different
in character from each other, will generally tend to produce the
greatest number of modified descendants; for these will have the
best chance of seizing on new and widely different places in the
polity of nature: hence in the diagram I have chosen the extreme
species (A), and the nearly extreme species (I), as those which
have largely varied, and have given rise to new varieties and species.
The other nine species (marked by capital letters) of our original
genus, may for long but unequal periods continue to transmit
unaltered descendants; and this is shown in the diagram by the
dotted lines unequally prolonged upwards.

But during the process of modification, represented in the diagram,
another of our principles, namely that of extinction, will have played
an important part. As in each fully stocked country natural selection
necessarily acts by the selected form having some advantage in the
struggle for life over other forms, there will be a constant tendency
in the improved descendants of any one species to supplant and

EFFECTS OF NATURAL SELECTION 1 25

exterminate in each stage of descent their predecessors and their
original progenitor. For it should be remembered that the compe-
tition will generally be most severe between those forms which are
most nearly related to each other in habits, constitution, and structure.
Hence all the intermediate forms between the earlier and later states,
that is between the less and more improved states of the same species,
as well as the original parent-species itself, will generally tend to
become extinct. So it probably will be with many whole collateral
lines of descent which will be conquered by later and improved lines.
If, however, the modified offspring of a species get into some distinct
country, or become quickly adapted to some quite new station, in
which offspring and progenitor do not come into competition, both
may continue to exist.

If, then, our diagram be assumed to represent a considerable
amount of modification, species (A) and all the earlier varieties
will have become extinct, being replaced by eight new species (a"
to /»"); and species (I) will be replaced by six («'* to 2") new
species.

But we may go further than this. The original species of our
genus were supposed to resemble each other in unequal degrees, as
is so generally the case in nature; species (A) being more nearly
related to B, C, and D, than to the other species; and species (I) more
to G, H, K, L, than to the others. These two species (A) and (I)
were also supposed to be very common and widely diffused species,
so that they must originally have had some advantage over most of
the other species of the genus. Their modified descendants, fourteen
in number at the fourteen-thousandth generation, will probably have
inherited some of the same advantages; they have also been modified
and improved in a diversified manner at each stage of descent, so
as to have become adapted to many related places in the natural
economy of their country. It seems, therefore, extremely probable
that they will have taken the places of, and thus exterminated, not
only their parents (A) and (I), but likewise some of the original
species which were most nearly related to their parents. Hence very
few of the original sf)ecies will have transmitted offspring to the
fourteen-thousandth generation. We may suppose that only one,
(F), of the two species (E) and (F) which were least closely related

126 ORIGIN OF SPECIES

to the other nine original species, has transmitted descendants to this
late stage of descent.

The new species in our diagram, descended from the original
eleven species, will now be fifteen in number. Owing to the divergent
tendency of natural selection, the extreme amount of difference in
character between species a" and z" will be much greater than that
between the most distinct of the original eleven species. The new
species, moreover, will be allied to each other in a widely different
manner. Of the eight descendants from (A) the three marked a'*,
q^*, p'*, will be nearly related from having recently branched off
from a'"; i", and /", from having diverged at an earlier period from
a', will be in some degree distinct from the three first-named species;
and lastly, o", P* and m" will be nearly related one to the other, but,
from having diverged at the first commencement of the process of
modification, will be widely different from the other five species,
and may constitute a sub-genus or a distinct genus.

The six descendants from (I) will form two sub-genera or genera.
But as the original species (I) differed largely from (A), standing
nearly at the extreme end of the original genus, the six descendants
from (I) will, owing to inheritance alone, differ considerably from
the eight descendants from (A); the two groups, moreover, are
supposed to have gone on diverging in different directions. The
intermediate species, also (and this is a very important consideration),
which connected the original species (A) and (I), have all become,
excepting (F), extinct, and have left no descendants. Hence the six
new species descended from (I), and the eight descendants from
(A), will have to be ranked as very distinct genera, or even as distinct
sub-families.

Thus it is, as I believe, that two or more genera are produced by
descent with modification, from two or more species of the same
genus. And the two or more parent-species are supposed to be
descended from some one species of an earlier genus. In our diagram,
this is indicated by the broken lines, beneath the capital letters, con-
verging in sub-branches downwards towards a single point; this
point represents a species, the supposed progenitor of our several
sub-genera and genera.

It is worth while to reflect for a moment on the character of the

EFFECTS OF NATURAL SELECTION 127

new species f", which is supposed not to have diverged much in
character, but to have retained the form of (F), either unaltered or
altered only in a slight degree. In this case, its affinities to the other
fourteen new species will be of a curious and circuitous nature. Being
descended from a form which stood between the parent-species (A)
and (I), now supposed to be extinct and unknown, it will be in some
degree intermediate in character between the two groups descended
from these two species. But as these two groups have gone on diverg-
ing in character from the type of their parents, the new species (f")
will not be directly intermediate between them, but rather between
types of the two groups; and every naturalist will be able to call
such cases before his mind.

In the diagram, each horizontal line has hitherto been supposed
to represent a thousand generations, but each may represent a million
or more generations; it may also represent a section of the successive
strata of the earth's crust including extinct remains. We shall, when
we come to our chapter on Geology, have to refer again to this
subject, and I think we shall then see that the diagram throws light
on the affinities of extinct beings, which, though generally belonging
to the same orders, families, or genera, with those now living, yet
are often, in some degree, intermediate in character between existing
groups; and we can understand this fact, for the extinct species lived
at various remote epochs when the branching lines of descent had
diverged less.

I see no reason to limit the process of modification, as now ex-
plained, to the formation of genera alone. If, in the diagram, we
suppose the amount of change represented by each successive group
of diverging dotted lines to be great, the forms marked a** to />",
those marked ^" and /", and those marked o" to w", will form three
very distinct genera. We shall also have two very distinct genera
descended from (I), differing widely from the descendants of (A).
Those two groups of genera will thus form two distinct families, or
orders, according to the amount of divergent modification supposed
to be represented in the diagram. And the two new families, or
orders, are descended from two species of the original genus, and
these are supposed to be descended from some still more ancient and
unknown form.

128 ORIGIN OF SPECIES

We have seen that in each country it is the species belonging to
the larger genera which oftenest present varieties or incipient species.
This, indeed, might have been expected; for, as natural selection acts
through one form having some advantage over other forms in the
struggle for existence, it will chiefly act on those which already have
some advantage; and the largeness of any group shows that its
sp)ecies have inherited from a common ancestor some advantage in
common. Hence, the struggle for the production of new and modi-
fied descendants will mainly lie between the larger groups which
are all trying to increase in number. One large group will slowly
conquer another large group, reduce its numbers, and thus lessen
its chance of further variation and improvement. Within the same
large group, the later and more highly perfected sub-groups, from
branching out and seizing on many new places in the poUty of
Nature, will constantly tend to supplant and destroy the earlier and
less improved sub-groups. Small and broken groups and sub-groups
will finally disappear. Looking to the future, we can predict that
the groups of organic beings which are now large and triumphant,
and which are least broken up, that is, which have as yet suffered
least extinction, will, for a long period, continue to increase. But
which groups will ultimately prevail, no man can predict; for we
know that many groups, formerly most extensively developed, have
now become extinct. Looking still more remotely to the future, we
may predict that, owing to the continued and steady increase of the
larger groups, a multitude of smaller groups will become utterly
extinct, and leave no modified descendants; and consequently, that,
of the sf)ecies living at any one period extremely few will transmit
descendants to a remote futurity. I shall have to return to this subject
in the chapter on Classification, but I may add that as, according
to this view, extremely few of the more ancient species have trans-
mitted descendants to the present day, and, as all the descendants of
the same species form a class, we can understand how it is that there
exist so few classes in each main division of the animal and vegetable
kingdoms. Although few of the most ancient species have left modi-
fied descendants, yet, at remote geological periods, the earth may
have been almost as well peopled with species of many genera,
families, orders, and classes, as at the present time.

EFFECTS OF NATURAL SELECTION 1 29

ON THE DECREE TO WHICH ORGANISATION TENDS TO ADVANCE

Natural Selection acts exclusively by the preservation and accumu-
lation of variations, which are beneficial under the organic and
inorganic conditions to which each creature is exposed at all periods
of life. The ultimate result is that each creature tends to become more
and more improved in relation to its conditions. This improvement
inevitably leads to the gradual advancement of the organisation of
the greater number of Uving beings throughout the world. But here
we enter on a very intricate subject, for naturalists have not defined
to each other's satisfaction what is meant by an advance in organisa-
tion. Amongst the vertebrata the degree of intellect and an approach
in structure to man clearly come into play. It might be thought that
the amount of change which the various parts and organs pass
through in their development from the embryo to maturity would
suffice as a standard of comparison; but there are cases, as with
certain parasitic crustaceans, in which several parts of the structure
become less perfect, so that the mature animal cannot be called higher
than its larva. Von Baer's standard seems the most widely appli-
cable and the best, namely, the amount of differentiation of the
parts of the same organic being, in the adult state, as I should be
inclined to add, and their specialisation for different functions; or,
as Milne Edwards would express it, the completeness of the division
of physiological labour. But we shall see how obscure this subject
is if we look, for instance, to fishes, amongst which some naturalists
rank those as highest which, like the sharks, approach nearest to
amphibians; whilst other naturalists range the common bony or
teleostean fishes as the highest, inasmuch as they are most stricdy
fish-like, and differ most from the other vertebrate classes. We see
still more plainly the obscurity of the subject by turning to plants,
amongst which the standard of intellect is of course quite excluded;
and here some botanists rank those plants as highest which have
every organ, as sepals, petals, stamens, and pistils, fully developed in
each flower; whereas other botanists, probably with more truth, look
at the plants which have their several organs much modified and
reduced in number, as the highest.

If we take as the standard of high organisation, the amount of

130 ORIGIN OF SPECIES

differentiation and specialisation of the several organs in each being
when adult (and this will include the advancement of the brain for
intellectual purposes), natural selection clearly leads towards this
standard : for all physiologists admit that the specialisation of organs,
inasmuch as in this state they perform their functions better, is an
advantage to each being; and hence the accumulation of variations
tending towards specialisation is within the scope of natural selection.
On the other hand, we can see, bearing in mind that all organic
beings are striving to increase at a high ratio and to seize on every
unoccupied or less well occupied place in the economy of nature,
that it is quite possible for natural selection gradually to Bt a being
to a situation in which several organs would be superfluous or
useless: in such cases there would be retrogression in the scale of
organisation. Whether organisation on the whole has actually
advanced from the remotest geological periods to the present day
will be more conveniently discussed in our chapter on Geological
Succession.

But it may be objected that if all organic beings thus tend to rise
in the scale, how is it that throughout the world a multitude of the
lowest forms still exist; and how is it that in each great class some
forms are far more highly developed than others? Why have not
the more highly developed forms everywhere supplanted and ex-
terminated the lower? Lamarck, who believed in an innate and
inevitable tendency towards perfection in all organic beings, seems
to have felt this difficulty so strongly, that he was led to suppose that
new and simple forms are continually being produced by spontaneous
generation. Science has not as yet proved the truth of this belief,
whatever the future may reveal. On our theory the continued exist-
ence of lowly organisms offers no difficulty; for natural selection,
or the survival of the fittest, does not necessarily include progressive
development — it only takes advantage of such variations as arise
and are beneficial to each creature under its complex relations of
life. And it may be asked what advantage, as far as we can see, would
it be to an infusorian animalcule — to an intestinal worm — or even to
an earthworm, to be highly organised. If it were no advantage, these
forms would be left, by natural selection, unimproved or but Htde
improved, and might remain for indefinite ages in their present lowly

EFFECTS OF NATURAL SELECTION I3I

condition. And geology tells us that some of the lowest forms, as
the infusoria and rhizopods, have remained for an enormous period
in nearly their present state. But to suppose that most of the many
now existing low forms have not in the least advanced since the first
dawn of life would be extremely rash; for every naturalist who has
dissected some of the beings now ranked as very low in the scale,
must have been struck with their really wondrous and beautiful
organisation.

Nearly the same remarks are applicable if we look to the different
grades of organisation within the same great group; for instance, in
the Vertebrata, to the co-existence of mammals and fish — amongst
Mammalia, to the co-existence of man and the ornithorhynchus —
amongst fishes, to the co-existence of the shark and the lancelet
(Amphioxus), which latter fish in the extreme simplicity of its
structure approaches the invertebrate classes. But mammals and fish
hardly come into competition with each other; the advancement o£
the whole class of mammals, or of certain members in this class, to
the highest grade, would not lead to their taking the place of fishes.
Physiologists believe that the brain must be bathed by warm blood
to be highly active, and this requires aerial respiration; so that warm-
blooded mammals when inhabiting the water lie under a disadvan-
tage in having to come continually to the surface to breathe. With
fishes, members of the shark family would not tend to supplant the
lancelet; for the lancelet, as I hear from Fritz Miiller, has as sole
companion and competitor on the barren sandy shore of South Brazil,
an anomalous annelid. The three lowest orders of mammals, namely,
marsupials, edentata, and rodents, co-exist in South America in the
same region with numerous monkeys, and probably interfere little
with each other. Although organisation, on the whole, may have
advanced and be still advancing throughout the world, yet the scale
will always present many degrees of perfection; for the high advance-
ment of certain whole classes, or of certain members of each class,
does not at all necessarily lead to the extinction of those groups with
which they do not enter into close competition. In some cases, as we
shall hereafter see, lowly organised forms appear to have been pre-
served to the present day, from inhabiting confined or peculiar sta-
tions, where they have been subjected to less severe competition, and

132 ORIGIN OF SPECIES

where their scanty numbers have retarded the chance of favorable
variations arising.

Finally, I believe that many lowly organised forms now exist
throughout the world, from various causes. In some cases variations
or individual differences of a favorable nature may never have arisen
for natural selection to act on and accumulate. In no case, probably,
has time sufficed for the utmost possible amount of development.
In some few cases there has been what we must call retrogression of
organisation. But the main cause lies in the fact that under very
simple conditions of life a high organisation would be of no service,
— possibly would be of actual disservice, as being of a more delicate
nature, and more liable to be put out of order and injured.

Looking to the first dawn of life, when all organic beings, as we
may believe, presented the simplest structure, how, it has been asked,
could the first steps in the advancement or differentiation of parts
have arisen ? Mr. Herbert Spencer would probably answer, that, as
soon as simple unicellular organism came by growth or division to
be compounded of several cells, or became attached to any supporting
surface, his law "that homologous units of any order became differ-
entiated in proportion as their relations to incident forces became
different" would come into action. But as we have no facts to guide
us, speculation on the subject is almost useless. It is, however, an
error to suppose that there would be no struggle for existence, and,
consequently, no natural selection, until many forms had been pro-
duced: variations in a single species inhabiting an isolated station
might be beneficial, and thus the whole mass of individuals might
be modified, or two distinct forms might arise. But, as I remarked
towards the close of the Introduction, no one ought to feel surprise
at much remaining as yet unexplained on the origin of species, if
we make due allowance for our profound ignorance on the mutual
relations of the inhabitants of the world at the present time, and still
more so during past ages.

CONVERGENCE OF CHARACTER

Mr. H. C. Watson thinks that I have overrated the importance
of divergence of character (in which, however, he apparently be-
lieves), and that convergence, as it may be called, has likewise played

CONVERGENCE OF CHARACTER 1 33

a part. If two species, belonging to two distinct though allied genera,
had both produced a large number of new and divergent forms, it is
conceivable that these might approach each other so closely that they
would have all to be classed under the same genus; and thus the
descendants of two distinct genera would converge into one. But it
would in most cases be extremely rash to attribute to convergence a
close and general similarity of structure in the modified descendants
of widely distinct forms. The shape of a crystal is determined solely
by the molecular forces, and it is not surprising that dissimilar sub-
stances should sometimes assume the same form; but with organic
beings we should bear in mind that the form of each depends on an
infinitude of complex relations, namely on the variations which have
arisen, these being due to causes far too intricate to be followed out, —
on the nature of the variations which have been preserved or selected,
and this depends on the surrounding physical conditions, and in a
still higher degree on the surrounding organisms with which each
being has come into competition, — and lastly, on inheritance (in itself
a fluctuating element) from innumerable progenitors, all of which
have had their forms determined through equally complex relations.
It is incredible that the descendants of two organisms, which had
originally differed in a marked manner, should ever afterwards con-
verge so closely as to lead to a near approach to identity throughout
their whole organisation. If this had occurred, we should meet with
the same form, independently of genetic connection, recurring in
widely separated geological formations; and the balance of evidence
is opposed to any such an admission.

Mr. Watson has also objected that the continued action of natural
selection, together with divergence of character, would tend to make
an indefinite number of specific forms. As far as mere inorganic
conditions are concerned, it seems probable that a sufficient number
of species would soon become adapted to all considerable diversities
of heat, moisture, etc.; but I fully admit that the mutual relations of
organic beings are more important; and as the number of species in
any country goes on increasing, the organic conditions of life must
become more and more complex. Consequently there seems at first
sight no limit to the amount of profitable diversification of structure,
and therefore no limit to the number of species which might be

134 ORIGIN OF SPECIES

produced. We do not know that even the most prolific area is fully
stocked with specific forms: at the Caf)e of Good Hope and in Aus-
tralia, which support such an astonishing number of species, many
European plants have become naturalised. But geology shows us,
that from an early part of the tertiary period the number of species
of shells, and that from the middle part of this same period the
number of mammals, has not greatly or at all increased. What then
checks an indefinite increase in the number of sp>ecies ? The amount
of life (I do not mean the number of specific forms) supported on
an area must have a Hmit, depending so largely as it does on physical
conditions; therefore, if an area be inhabited by very many species,
each or nearly each species will be represented by few individuals;
and such species will be liable to extermination from accidental
fluctuations in the nature of the seasons or in the number of their
enemies. The process of extermination in such cases would be rapid,
whereas the production of new species must always be slow. Imagine
the extreme case of as many species as individuals in England, and
the first severe winter or very dry summer would exterminate
thousands on thousands of species. Rare species, and each species
will become rare if the number of species in any country becomes
indefinitely increased, will, on the principle often explained, present
within a given period few favorable variations; consequently, the
process of giving birth to new sp)ecific forms would thus be retarded.
When any species becomes very rare, close interbreeding will help
to exterminate it; authors have thought that this comes into play in
accounting for the deterioration of the aurochs in Lithuania, of red
deer in Scotland, and of bears in Norway, etc. Lastly, and this I am
inclined to think is the most important element, a dominant species,
which has already beaten many competitors in its own home, will
tend to spread and supplant many others. Alph. de Candolle has
shown that those species which spread widely, tend generally to
spread very widely; consequently, they will tend to supplant and
exterminate several species in several areas, and thus check the in-
ordinate increase of specific forms throughout the world. Dr. Hooker
has recently shown that in the southeast corner of Australia, where,
apparently, there are many invaders from different quarters of the
globe, the endemic Australian species have been greatly reduced in

SUMMARY OF CHAPTER 1 35

number. How much weight to attribute to these several considera-
tions I will not pretend to say; but conjointly they must limit in each
coimtry the tendency to an indefinite augmentation of specific forms.

SUMMARY OF CHAPTER

If under changing conditions of life organic beings present indi-
vidual differences in almost every part of their structure, and this
cannot be disputed; if there be, owang to their geometrical rate of
increase, a severe struggle for life at some age, season, or year, and
this certainly cannot be disputed; then, considering the infinite com-
plexity of the relations of all organic beings to each other and to their
conditions of life, causing an infinite diversity in structure, constitu-
tion, and habits, to be advantageous to them, it would be a most
extraordinary fact if no variations had ever occurred useful to each
being's own welfare, in the same manner as so many variations have
occurred useful to man. But if variations useful to any organic being
ever do occur, assuredly individuals thus characterised will have the
best chance of being preserved in the struggle for life; and from the
strong principle of inheritance, these will tend to produce offspring
similarly characterised. This principle of preservation, or the sur-
vival of the fittest, I have called Natural Selection. It leads to the
improvement of each creature in relation to its organic and inorganic
conditions of life; and consequently, in most cases, to what must be
regarded as an advance in organisation. Nevertheless, low and simple
forms will long endure if well fitted for their simple conditions of
life.

Natural selection, on the principle of qualities being inherited at
corresponding ages, can modify the egg, seed, or young, as easily as
the adult. Amongst many animals, sexual selection will have given
its aid to ordinary selection, by assuring to the most vigorous and
best adapted males the greatest number of offspring. Sexual selection
will also give characters useful to the males alone, in their struggles
or rivalry with other males; and these characters will be transmitted
to one sex or to both sexes, according to the form of inheritance
which prevails.

Whether natural selection has really thus acted in adapting the
various forms of life to their several conditions and stations, must be

136 ORIGIN OF SPECIES

judged by the general tenor and balance of evidence given in the
following chapters. But we have already seen how it entails extinc-
tion; and how largely extinction has acted in the world's history,
geology plainly declares. Natural selertion, also, leads to divergence
of character; for the more organic beings diverge in structure, habits,
and constitution, by so much the more can a large number be sup-
ported on the area, — of which we see proof by looking to the inhabi-
tants of any small spot, and to the productions naturalised in foreign
lands. Therefore, during the modification of the descendants of any
one sp)ecies, and during the incessant struggle of all species to increase
in numbers, the more diversified the descendants become, the better
will be their chance of success in the battle for life. Thus the small
differences distinguishing varieties of the same species, steadily tend
to increase, till they equal the greater differences between species of
the same genus, or even of distinct genera.

We have seen that it is the common, the widely diffused and
widely ranging sjjecies, belonging to the larger genera within each
class, which vary most; and these tend to transmit to their modified
offspring that superiority which now makes them dominant in their
own countries. Natural selection, as has just been remarked, leads to
divergence of character and to much extinction of the less improved
and intermediate forms of life. On these principles, the nature of
the affinities, and the generally well-defined distinctions between the
innumerable organic beings in each class throughout the world,
may be explained. It is a truly wonderful fact — the wonder of which
we are apt to overlook from familiarity — that all animals and all
plants throughout all time and space should be related to each other
in groups, subordinate to groups, in the manner which we every-
where behold — namely, varieties of the same specie most closely
related, species of the same genus less closely and unequally related,
forming sections and sub-genera, sp)ecies of distinct genera much less
closely related, and genera related in different degrees, forming sub-
families, families, orders, sub-classes, and classes. The several sub-
ordinate groups in any class cannot be ranked in a single file, but
seem clustered round points, and these round other points, and so on
in almost endless cycles. If species had been independently created,
no explanation would have been possible of this kind of classification;

SUMMARY OF CHAPTER 1 37

but it is explained through inheritance and the complex action of
natural selection, entailing extinction and divergence of character,
as we have seen illustrated in the diagram.

The affinities of all the beings of the same class have sometimes
been represented by a great tree. 1 believe this simile largely speaks
the truth. The green and budding twigs may represent existing
species; and those produced during former years may represent the
long succession of extinct sp)ecies. At each period of growth all the
growing twigs have tried to branch out on all sides, and to overtop
and kill the surrounding twigs and branches, in the same manner as
species and groups of species have at all times overmastered other
species in the great battle for life. The limbs divided into great
branches, and these into lesser and lesser branches, were themselves
once, when the tree was young, budding twigs; and this connection
of the former and present buds, by ramifying branches, may well
represent the classification of all extinct and living species in groups
subordinate to groups. Of the many twigs which flourished when
the tree was a mere bush, only two or three, now grown into great
branches, yet survive and bear the other branches; so with the species
which lived during long-past geological periods, very few have left
living and modified descendants. From the first growth of the tree,
many a limb and branch has decayed and dropped off; and these
fallen branches of various sizes may represent those whole orders,
families, and genera which have now no living representatives, and
which are known to us only in a fossil state. As we here and there
see a thin straggling branch springing from a fork low down in a
tree, and which by some chance has been favoured and is still alive
on its summit, so we occasionally see an animal like the Ornitho-
rhynchus or Lepidosiren, which in some small degree connects by
its affinities two large branches of life, and which has apparently been
saved from fatal comf)etition by having inhabited a protected station.
As buds give rise by growth to fresh buds, and these, if vigorous,
branch out and overtop on all sides many a feebler branch, so by
generation I believe it has been with the great Tree of Life, which
fills with its dead and broken branches the crust of the earth, and
covers the surface with its ever-branching and beautiful ramifications.

CHAPTER V
Laws of Variation

EHects of changed conditions — Use and disuse, combined with natural
selection; organs of flight and of vision — Acclimatisation — Corre-
lated variation — Compensation and economy of growth — False corre-
lations — Multiple, rudimentary, and lowly organised structures
variable — Parts developed in an unusual manner are highly variable;
specific characters more variable than generic: secondary sexual
characters variable — Species of the same genus vary in an analogous
manner — Reversions to long-lost characters — Summary.

I HAVE hitherto sometimes spoken as if the variations — so com-
mon and multiform with organic beings under domestication,
and in a lesser degree with those under nature — were due to
chance. This, of course, is a wholly incorrect expression, but it
serves to acknowledge plainly our ignorance of the cause of each
particular variation. Some authors believe it to be as much the func-
tion of the reproductive system to produce individual differences, or
slight deviations of structure, as to make the child like its parents.
But the fact of variations and monstrosities occurring much more
frequently under domestication than under nature, and the greater
variability of species having wide ranges than of those with restricted
ranges, lead to the conclusion that variability is generally related to
the conditions of life to which each sjjecies has been exposed during
several successive generations. In the first chapter I attempted to
show that changed conditions act in two ways, directly on the whole
organisation or on certain parts alone, and indirectly through the
reproductive system. In all cases there are two factors, the nature
of the organism, which is much the most important of the two, and
the nature of the conditions. The direct action of changed conditions
leads to definite or indefinite results. In the latter case the organisa-
tion seems to become plastic, and we have much fluctuating varia-
bility. In the former case the nature of the organism is such that it
yields readily, when subjected to certain conditions, and all, or nearly
all the individuals become modified in the same way.

138

EFFECTS OF CHANGED CONDITIONS I39

It is very difficult to decide how far changed conditions, such as
of climate, food, etc., have acted in a definite manner. There is
reason to believe that in the course of time the effects have been
greater than can be proved by clear evidence. But we may safely
conclude that the innumerable complex coadaptations of structure,
which we see throughout nature between various organic beings,
cannot be attributed simply to such action. In the following cases
the conditions seem to have produced some slight definite effect:
E. Forbes asserts that shells at their southern limit, and when living
in shallow water, are more brightly coloured than those of the same
species from further north or from a greater depth; but this certainly
does not always hold good. Mr. Gould believes that birds of the same
species are more brightly coloured under a clear atmosphere, than
when living near the coast or on islands; and Wollaston is con-
vinced that residence near the sea affects the colours of insects.
Moquin-Tandon gives a list of plants which, when growing near the
seashore, have their leaves in some degree fleshy, though not else-
where fleshy. These slightly varying organisms are interesting in as
far as they present characters analogous to those possessed by the
species which are confined to similar conditions.

When a variation is of the slightest use to any being, we cannot
tell how much to attribute to the accumulative action of natural
selection, and how much to the definite action of the conditions of
life. Thus, it is well known to furriers that animals of the same
species have thicker and better fur the further north they live; but
who can tell how much of this difference may be due to the warmest
clad individuals having been favoured and preserved during many
generations, and how much to the action of the severe climate.' For
it would appear that climate has some direct action on the hair of our
domestic quadrupeds.

Instances could be given of similar varieties being produced from
the same species under external conditions of life as different as can
well be conceived; and, on the other hand, of dissimilar varieties be-
ing produced under apparently the same external conditions. Again,
innumerable instances are known to every naturalist, of species keep-
ing true, or not varying at all, although hving under the most oppo-
site climates. Such considerations as these incline me to lay less

140 ORIGIN OF SPECIES

weight on the direa action of the surrounding conditions, than on a
tendency to vary, due to causes of which we are quite ignorant.

In one sense the conditions of Ufe may be said, not only to cause
variability, either directly or indirectly, but likewise to include natural
selection, for the conditions determine whether this or that variety
shall survive. But when man is the selecting agent, we clearly see
that the two elements of change are distinct; variability is in some
manner excited, but it is the will of man which accumulates the
variations in certain directions; and it is this latter agency which
answers to the survival of the fittest under nature.

EFFECTS OF THE INCREASED USE AND DISUSE OF PARTS,
AS CONTROLLED BY NATURAL SELECTION

From the facts alluded to in the first chapter, I think there can
be no doubt that use in our domestic animals has strengthened and
enlarged certain parts, and disuse diminished them; and that such
modifications are inherited. Under free nature, we have no standard
of comparison, by which to judge of the effects of long-continued
use or disuse, for we know not the parent-forms; but many animals
jwssess structures which can be best explained by the effects of disuse.
As Professor Owen has remarked, there is no greater anomaly in
nature than a bird that cannot fly; yet there are several in this state.
The logger-headed duck of South America can only flap along the
surface of the water, and has its wings in nearly the same condition
as the domestic Aylesbury duck: it is a remarkable fact that the
young birds, according to Mr. Cunningham, can fly, while the adults
have lost this power. As the larger ground-feeding birds seldom take
flight except to escape danger, it is probable that the nearly wingless
condition of several birds, now inhabiting or which lately inhabited
several oceanic islands, tenanted by no beasts of prey, has been
caused by disuse. The ostrich indeed inhabits continents, and is
exposed to danger from which it cannot escape by flight, biH it can
defend itself by kicking its enemies, as efficiently as many quadru-
peds. We may believe that the progenitor of the ostrich genus had
habits like those of the bustard, and that, as the size and weight of its
body were increased during successive generations, its legs were used
more, and its wings less, until they became incapable of flight.

EFFECTS OF USE AND DISUSE I4I

Kirby has remarked (and I have observed the same fact) that
the anterior tarsi, or feet, of many male dung-feeding beetles are
often broken off; he examined seventeen specimens in his own col-
lection, and not one had even a relic left. In the Onites apelles the
tarsi are so habitually lost, that the insect has been described as not
having them. In some other genera they are present, but in a rudi-
mentary condition. In the Ateuchus or sacred beetle of the Egyp-
tians, they are totally deficient. The evidence that accidental mutila-
tions can be inherited is at present not decisive; but the remarkable
cases observed by Brown-Sequard in guinea pigs, of the inherited
effects of operations, should make us cautious in denying this
tendency. Hence it will perhaps be safest to look at the entire ab-
sence of the anterior tarsi in Ateuchus, and their rudimentary con-
dition in some other genera, not as cases of inherited mutilations,
but as due to the effects of long-continued disuse; for as many dung-
feeding beetles are generally found with their tarsi lost, this must
happen early in life; therefore the tarsi cannot be of much im-
portance or be much used by these insects.

In some cases we might easily put down to disuse modifications
of structure which are wholly, or mainly, due to natural selection.
Mr. Wollaston has discovered the remarkable fact that 200 beetles,
out of the 550 species (but more are now known) inhabiting Madeira,
are so far deficient in wings that they cannot fly; and that, of the
twenty-nine endemic genera, no less than twenty-three have all their
species in this condition! Several facts, — namely, that beetles in many
parts of the world are frequently blown to sea and perish; that the
beetles in Madeira, as observed by Mr. Wollaston, lie much con-
cealed, until the wind lulls and the sun shines; that the proportion of
wingless beetles is larger on the expwsed Desertas than in Madeira
itself; and especially the extraordinary fact, so strongly insisted on
by Mr. Wollaston, that certain large groups of beetles, elsewhere
excessively numerous, which absolutely require the use of their
wings, are here almost entirely absent; — these several considerations
make me believe that the wingless condition of so many Madeira
beetles is mainly due to the action of natural selection, combined
probably with disuse. For during many successive generations each
individual beetle which flew least, either from its wings having been

142 ORIGIN OF SPECIES

ever so little less perfectly developed or from indolent habit, will
have had the best chance of surviving from not being blown out to
sea; and, on the other hand, those beetles which most readily took
to flight would oftenest have been blown to sea, and thus destroyed.

The insects in Madeira which are not ground-feeders, and which,
as certain flower-feeding coleoptera and lepidoptera, must habitually
use their wings to gain their subsistence, have, as Mr. Wollaston
suspects, their wings not at all reduced, but even enlarged. This is
quite compatible with the action of natural selection. For when a
new insect first arrived on the island, the tendency of natural selec-
tion to enlarge or to reduce the wings, would def)end on whether a
greater number of individuals were saved by successfully batding
with the winds, or by giving up the attempt and rarely or never
flying. As with mariners shipwrecked near a coast, it would have
been better for the good swimmers if they had been able to swim still
further, whereas it would have been better for the bad swimmers if
they had not been able to swim at all and had stuck to the wreck.

The eyes of moles and of some burrowing rodents are rudimentary
in size, and in some cases are quite covered by skin and fur. This
state of the eyes is probably due to gradual reduction from disuse,
but aided perhaps by natural selection. In South America, a burrow-
ing rodent, the tucutucu, or Ctenomys, is even more subterranean in
its habits than the mole; and 1 was assured by a Spaniard, who had
often caught them, that they were frequently blind. One which I
kept aUve was certainly in this condition, the cause, as appeared on
dissection, having been inflammation of the nictitating membrane.
As frequent inflammations of the eyes must be injurious to any
animal, and as eyes are certainly not necessary to animals having
subterranean habits, a reduction in their size, with the adhesion of
the eyelids and growth of fur over them, might in such case be an
advantage; and if so, natural selection would aid the effects of disuse.

It is well known that several animals, belonging to the most
different classes, which inhabit the caves of Carniola and of Ken-
tucky, are blind. In some of the crabs the footstalk for the eye
remains, though the eye is gone; — the stand for the telescope is there,
though the telescope with its glasses has been lost. As it is difficult
to imagine that eyes, though useless, could be in any way injurious

EFFECTS OF USE AND DISUSE I43

to animals living in darkness, their loss may be attributed to disuse.
In one of the blind animals, namely, the cave rat (Neotoma), two of
which were captured by Professor Silliman at above half a mile
distance from the mouth of the cave, and therefore not in the pro-
foundest depths, the eyes were lustrous and of large size; and these
animals, as I am informed by Professor Silliman, after having been
exposed for about a month to a graduated light, acquired a dim
perception of objects.

It is difficult to imagine conditions of Ufe more similar than
deep limestone caverns under a nearly similar climate; so that, in
accordance with the old view of the blind animals having been
separately created for the American and European caverns, very close
similarity in their organisation and affinities might have been ex-
pected. This is certainly not the case if we look at the two whole
faunas; and with respect to the insects alone, Schiodte has remarked,
"We are accordingly prevented from considering the entire phe-
nomenon in any other light than something purely local, and the
similarity which is exhibited in a few forms between the Mammoth
cave (in Kentucky) and the caves in Carniola, otherwise than as a
very plain expression of that analogy which subsists generally be-
tween the fauna of Europe and of North America." On my view
we must suppose that American animals, having in most cases ordi-
nary powers of vision, slowly migrated by successive generations from
the outer world into the deeper and deeper recesses of the Kentucky
caves, as did European animals into the caves of Europe. We have
some evidence of this gradation of habit; for, as Schiodte remarks,
"We accordingly look upon the subterranean faunas as small rami-
fications which have penetrated into the earth from the geographi-
cally limited faunas of the adjacent tracts, and which, as they ex-
tended themselves into darkness, have been accommodated to sur-
rounding circumstances. Animals not far remote from ordinary
forms, prepare the transition from light to darkness. Next follow
those that are constructed for twilight; and, last of all, those destined
for total darkness, and whose formation is quite peculiar." These
remarks of Schiodte's, it should be understood, apply not to the
same, but to distinct species. By the time that an animal had reached,
after numberless generations, the deepest recesses., disuse will on

144 ORIGIN OF SPECIES

this view have more or less perfectly obliterated its eyes, and natural
selection will often have effeaed other changes, such as an increase
in the length of the antennae or palpi, as a compensation for blindness.
Notwithstanding such modifications, we might expect still to see in
the cave animals of America, affinities to the other inhabitants of that
continent, and in those of Europe to the inhabitants of the European
continent. And this is the case with some of the American cave
animals, as I hear from Professor Dana; and some of the European
cave insects are very closely allied to those of the surrounding country.
It would be difficult to give any rational explanation of the affinities
of the blind cave animals to the other inhabitants of the two con-
tinents on the ordinary view of their independent creation. That
several of the inhabitants of the caves of the Old and New Worlds
should be closely related, we might expect from the well-known
relationship of most of their other productions. As a blind species
of Bathyscia is found in abundance on shady rocks far from caves,
the loss of vision in the cave species of this one genus has probably
had no relation to its dark habitation; for it is natural that an insect
already deprived of vision should readily become adapted to dark
caverns. Another blind genus (Anophthalmus) offers this remark-
able peculiarity, that the species, as Mr. Murray observes, have not
as yet been found anywhere except in caves, yet those which inhabit
the several caves of Eurof)e and America are distinct; but it is possible
that the progenitors of these several species, whilst they were fur-
nished with eyes, may formerly have ranged over both continents,
and then have become extinct, excepting in their present secluded
abodes. Far from feeling surprise that some of the cave animals
should be very anomalous, as Agassiz has remarked in regard to the
blind fish, the Amblyopsis, and as is the case with the blind Proteus
with reference to the reptiles of Europe, I am only surprised that
more wrecks of ancient life have not been preserved, owing to the
less severe competition to which the scanty inhabitants of these dark
abodes will have been exposed.

ACCLIMATISATIOK

Habit is hereditary with plants, as in the period of flowering, in
the time of sleep, in the amount of rain requisite for seeds to germi-

ACCLIMATISATION 1 45

nate, etc^ and this leads me to say a few words on acclimatisation.
As it is extremely common for distinct species belonging to the same
genus to inhabit hot and cold countries, if it be true that all the
species of the same genus are descended from a single parent-form,
acclimatisation must be readily effected during a long course of
descent. It is notorious that each species is adapted to the chmate of
its own home: species from an arctic or even from a temperate region
cannot endure a tropical climate, or conversely. So again, many
succulent plants cannot endure a damp climate. But the degree of
adaptation of species to the climates under which they live is often
overrated. We may infer this from our frequent inability to predict
whether or not an imported plant will endure our climate, and
from the number of plants and animals brought from different
countries which are here perfectly healthy. We have reason to believe
that species in a state of nature are closely limited in their ranges
by the competition of other organic beings quite as much as, or
more than, by adaptation to particular climates. But whether or not
this adaptation is in most cases very close, we have evidence with
some few plants, of their becoming, to a certain extent, naturally
habituated to different temf)eratures; that is, they become accli-
matised; thus the pines and rhododendrons, raised from seed col-
lected by Dr. Hooker from the same species growing at different
heights on the Himalaya, were found to possess in this country
different constitutional powers of resisting cold. Mr. Thwaites in-
forms me that he has observed similar facts in Ceylon; analogous
observations have been made by Mr. H. C. Watson on European
species of plants brought from the Azores to England; and I could
give other cases. In regard to animals, several authentic instances
could be adduced of species having largely extended, within historical
times, their range from warmer to cooler latitudes, and conversely;
but we do not positively know that these animals were stricdy
adapted to their native climate, though in all ordinary cases we
assume such to be the case; nor do we know that they have subse-
quently become specially acclimatised to their new homes, so as to
be better fitted for them than they were at first.

As we may infer that our domestic animals were originally chosen
by uncivilised man because they were useful and because they bred

146 ORIGIN OF SPECIES

readily under confinement, and not because they were subsequently
found capable of far-extended transportation, the common and ex-
traordinary capacity in our domestic animals of not only withstand-
ing the most different climates, but of being perfectly fertile (a far
severer test) under them, may be used as an argument that a large
proportion of other animals now in a state of nature could easily be
brought to bear widely different climates. We must not, however,
push the foregoing argument too far, on account of the probable
origin of some of our domestic animals from several wild stocks; the
blood, for instance, of a tropical and arctic wolf may perhaps be
mingled in our domestic breeds. The rat and mouse cannot be con-
sidered as domestic animals, but they have been transported by man
to many parts of the world, and now have a far wider range than
any other rodent; for they live under the cold climate of Faroe in
the north and of the Falklands in the south, and on many an island
in the torrid zones. Hence adaptation to any special climate may be
looked at as a quality readily grafted on an innate wide flexibility
of constitution, common to most animals. On this view, the capacity
of enduring the most different climates by man himself and by his
domestic animals, and the fact of the extinct elephant and rhinoceros
having formerly endured a glacial climate, whereas the living species
are now all tropical or sub-tropical in their habits, ought not to be
looked at as anomalies, but as examples of a very common flexi-
bility of constitution, brought, under peculiar circumstances, into
action.

How much of the acclimatisation of species to any peculiar cli-
mate is due to mere habit, and how much to the natural selection of
varieties having different innate constitutions, and how much to
both means combined, is an obscure question. That habit or cus-
tom has some influence, I must believe, both from analogy and from
the incessant advice given in agricultural works, even in the ancient
encyclopaedias of China, to be very cautious in transporting animals
from one district to another. And as it is not likely that man should
have succeeded in selecting so many breeds and sub-breeds with con-
stitutions specially fitted for their own districts, the result must, I
think, be due to habit. On the other hand, natural selection would
inevitably tend to preserve those individuals which were born with

CORRELATED VARIATION 1 47

constitutions best adapted to any country which they inhabited. In
treatises on many kinds of cultivated plants, certain varieties are said
to withstand certain climates better than others; this is strikingly
shown in works on fruit trees pubUshed in the United States, in
which certain varieties are habitually recommended for the northern
and others for the southern States; and as most of these varieties are
of recent origin, they cannot owe their constitutional differences to
habit. The case of the Jerusalem artichoke, which is never propagated
in England by seed, and of which consequently new varieties have
not been produced, has even been advanced, as proving that ac-
climatisation cannot be effected, for it is now as tender as ever it was!
The case, also, of the kidney bean has been often cited for a similar
purpose, and with much greater weight; but imtil someone will sow,
during a score of generations, his kidney beans so early that a very
large proportion are destroyed by frost, and then collect seed from the
few survivors, with care to prevent accidental crosses, and then again
get seed from these seedlings, with the same precautions, the experi-
ment cannot be said to have been tried. Nor let it be supposed that
differences in the constitution of seedling kidney beans never appear,
for an account has been published how much more hardy some seed-
hngs are than others; and of this fact I have myself observed striking
instances.

On the whole, we may conclude that habit, or use and disuse, have,
in some cases, played a considerable part in the modification of the
constitution and structure; but that the effects have often been largely
combined with, and sometimes overmastered by, the natural selection
of innate variations.

CORRELATED VARIATION

I mean by this expression that the whole organisation is so tied to-
gether, during its growth and development, that when slight varia-
tions in any one part occur, and are accumulated through natural se-
lection, other parts become modified. This is a very important
subject, most imperfectly understood, and no doubt wholly different
classes of facts may be here easily confounded together. We shall
presently see that simple inheritance often gives the false appearance
of correlation. One of the most obvious real cases is, that variations

148 ORIGIN OF SPECIES

of Structure arising in the young or larvx naturally tend to affect the
structure of the mature animal. The several parts of the body which
are homologous, and which, at an early embryonic jseriod, are identi-
cal in structure, and which are necessarily exposed to similar con-
ditions, seem eminently liable to vary in a like manner: we see this
in the right and left sides of the body varying in the same manner; in
the front and hind legs, and even in the jaws and limbs, varying
together, for the lower jaw is believed by some anatomists to be
homologous with the limbs. These tendencies, I do not doubt, may
be mastered more or less completely by natural selection; thus a
family of stags once existed with an antler only on one side; and if
this had been of any great use to the breed, it might probably have
been rendered permanent by selection.

Homologous parts, as has been remarked by some authors, tend to
cohere; this is often seen in monstrous plants: and nothing is more
common than the union of homologous parts in normal structures,
as in the union of the jjetals into a tube. Hard parts seem to affect
the form of adjoining soft parts; it is believed by some authors that
with birds the diversity in the shape of the pelvis causes the remark-
able diversity in the shape of their kidneys. Others believe that the
shape of the pelvis in the human mother influences by pressure the
shape of the head of the child. In snakes, according to Schlegel, the
form of the body and the manner of swallowing determine the posi-
tion and form of several of the most important viscera.

The nature of the bond is frequently quite obscure. M. Is. Geoffroy
St. Hilaire has forcibly remarked, that certain malcon formations
frequently, and that others rarely, co-exist, without our being able
to assign any reason. What can be more singular than the relation
in cats between complete whiteness and blue eyes with deafness, or
between the tortoise-shell colour and the female sex; or in pigeons be-
tween their feathered feet and skin betwixt the outer toes, or between
the presence of more or less down on the young pigeon when first
hatched, with the future colour of its plumage; or, again, the rela-
tion between the hair and teeth in the naked Turkish dog, though
here no doubt homology comes into play? With respect to this latter
case of correlation, I think it can hardly be accidental, that the two
orders of mammals which are most abnormal in their dermal cover-

CORRELATED VARIATION I49

ings, viz^ Cetacea (whales) and Edentata (armadilloes, scaly ant-
eaters, etc.), are likewise on the whole the most abnormal in their
teeth; but there are so many exceptions to this rule, as Mr. Mivart
has remarked, that it has little value.

I know of no case better adapted to show the importance of the
laws of correlation and variation, independently of utility and there-
fore of natural selection, than that of the difTerence between the outer
and inner flowers in some compositous and umbelliferous plants.
Every one is familiar with the difference between the ray and central
florets of, for instance, the daisy, and this difference is often accom-
panied with the partial or complete abortion of the reproductive
organs. But in some of these plants, the seeds also differ in shape and
sculpture. These differences have sometimes been attributed to the
pressure of the involucra on the florets, or to their mutual pressure,
and the shape of the seeds in the ray-florets of some Composita:
countenances this idea; but with the Umbelliferaj, it is by no means,
as Dr. Hooker informs me, the sfjecies with the densest heads which
most frequently differ in their inner and outer flowers. It might have
been thought that the development of the ray-petals by drawing
nourishment from the reproductive organs causes their abortion;
but this can hardly be the sole cause, for in some Composite the
seeds of the outer and inner florets differ, without any difference in
the corolla. Possibly these several differences may be connected with
the different flow of nutriment towards the central and external
flowers: we know, at least, that with irregular flowers, those nearest
to the axis are most subject to peloria, that is to become abnormally
symmetrical. I may add, as an instance of this fact, and as a striking
case of correlation, that in many pelargoniums, the two upper petals
in the central flower of the truss often lose their patches of darker
colour; and when this occurs, the adherent nectary is quite aborted;
the central flower thus becoming peloric or regular. When the colour
is absent from only one of the two upper pietals, the nectary is not
quite aborted but is much shortened.

With respect to the development of the corolla, Sprengel's idea that
the ray-florets serve to attract insects, whose agency is highly ad-
vantageous or necessary for the fertilisation of these plants, is highly
probable; and if so, natural selection may have come into play. But

150 ORIGIN OF SPECIES

with respect to the seeds, it seems impossible that their differences in
shape, which are not always correlated with any difference in the
corolla, can be in any way beneficial; yet in the Umbellifera: these
differences are of such apparent importance — the seeds being some-
times orthospermous in the exterior flowers and coelospermous in
the central flowers, — that the elder De Candolle founded his main
divisions in the order on such characters. Hence modifications of
structure, viewed by systematists as of high value, may be wholly due
to the laws of variation and correlation, without being, as far as we
can judge, of the slightest service to the species.

We may often falsely attribute to correlated variation structures
which are common to whole groups of species, and which in truth
are simply due to inheritance; for an ancient progenitor may have
acquired through natural selection some one modification in struc-
ture, and, after thousands of generations, some other and independent
modification; and these two modifications, having been transmitted
to a whole group of descendants with diverse habits, would naturally
be thought to be in some necessary manner correlated. Some other
correlations are apparently due to the manner in which natural se-
lection can alone act. For instance, Alph. de Candolle has remarked
that winged seeds are never found in fruits which do not open; I
should explain this rule by the impossibility of seeds gradually be-
coming winged through natural selection, unless the capsules were
open: for in this case alone could the seeds, which were a httle better
adapted to be wafted by the wind, gain an advantage over others less
well fitted for wide dispersal.

COMPENSATION AND ECONOMY OF GROWTH

The elder Geoffroy and Goethe propounded, at about the same
time, their law of compensation or balancement of growth; or, as
Goethe expressed it, "in order to sp)end on one side, nature is forced
to economise on the other side." I think this holds true to a certain
extent with our domestic productions: if nourishment flows to one
part or organ in excess, it rarely flows, at least in excess, to another
part; thus it is difficult to get a cow to give much milk and to fatten
readily. The same varieties of the cabbage do not yield abundant and
nutritious foliage and a copious supply of oil-bearing seeds. When

COMPENSATION AND ECONOMY OF GROWTH I5I

the seeds in our fruits become atrophied, the fruit itself gains largely
in size and quality. In our pwultry, a large tuft of feathers on the
head is generally accompanied by a diminished comb and a large
beard by diminished wattles. With species in a state of nature it
can hardly be maintained that the law is of universal application; but
many good observers, more especially botanists, believe in its truth. I
will not, however, here give any instances, for I see hardly any way of
distinguishing between the effects, on the one hand, of a part being
largely developed through natural selection and another and adjoin-
ing part being reduced by this same process or by disuse, and, on the
other hand, the actual withdrawal of nutriment from one part owing
to the excess of growth in another and adjoining part.

I suspect, also, that some of the cases of compensation which have
been advanced, and likewise some other facts, may be merged under
a more general principle, namely, that natural selection is continually
trying to economise every part of the organisation. If under changed
conditions of life a structure, before useful, becomes less useful, its
diminution will be favoured, for it will profit the individual not to
have its nutriment wasted in building up a useless structure. I can
thus only understand a fact with which I was much struck when ex-
amining cirripedes, and of which many analogous instances could
be given: namely, that when a cirripede is parasitic within another
cirrif)ede and is thus protected, it loses more or less completely its own
shell or carapace. This is the case with the male Ibla, and in a truly
extraordinary manner with the Proteolepas: for the carapace in all
other cirripedes consists of the three highly important anterior seg-
ments of the head enormously developed, and furnished with great
nerves and muscles; but in the parasitic and protected Proteolepas,
the whole anterior part of the head is reduced to the merest rudiment
attached to the bases of the prehensile antennae. Now the saving of a
large and complex structure, when rendered superfluous, would be a
decided advantage to each successive individual of the species; for in
the struggle for life to which every animal is exposed, each would
have a better chance of supporting itself, by less nutriment being
wasted.

Thus, as I believe, natural selection will tend in the long run to re-
duce any part of the organisation, as soon as it becomes, through

152 ORIGIN OF SPECIES

changed habits, superfluous, without by any means causing some
other part to be largely developed in a corresponding degree. And,
conversely, that natural selection may perfectly well succeed in largely
developing an organ without requiring as a necessary compensation
the reduction of some adjoining part.

MULTIPLE, RUDIMENTARY, AND LOWLY ORGANISED STRUCTURES
ARE VARIABLE

It seems to be a rule, as remarked by Is. Geoflroy St. Hilaire, both
with varieties and species, that when any part or organ is repeated
many times in the same individual (as the vertebrae in snakes, and
the stamens in f)olyandrous flowers) the number is variable; whereas
the same part or organ, when it occurs in lesser numbers, is constant.
The same author as well as some botanists have further remarked
that multiple parts are extremely liable to vary in structure. As
"vegetative repetition," to use Prof. Owen's expression, is a sign of
low organisation, the foregoing statements accord with the common
opinion of naturalists, that beings which stand low in the scale of
nature are more variable than those which are higher. I presume
that lowness here means that the several parts of the organisation
have been but little specialised for particular functions; and as long
as the same part has to perform diversified work, we can perhaps see
why it should remain variable, that is, why natural selection should
not have preserved or rejected each little deviation of form so care-
fully as when the part has to serve for some one special purpose. In
the same way that a knife which has to cut all sorts of things may be
of almost any shape; whilst a tool for some particular purpose must
be of some particular shape. Natural selection, it should never be for-
gotten, can act solely through and for the advantage of each being.

Rudimentary parts, as it is generally admitted, are apt to be highly
variable. We shall have to recur to this subject; and I will here only
add that their variability seems to result from their uselessness, and
consequently from natural selection having had no power to check
deviations in their structure.

STRUCTURES VARIABLE 153

A PAKT DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY DEGREE OR
MANNER, IN COMPARISON WITH THE SAME PART IN ALLIED SPECIES,
TENDS TO BE HIGHLY VARIABLE

Several years ago I was much struck by a remark, to the above
effect, made by Mr. Waterhouse. Professor Owen, also, seems to
have come to a nearly similar conclusion. It is hopeless to attempt to
convince any one of the truth of the above proposition without giving
the long array of facts which 1 have collected, and which cannot pos-
sibly be here introduced. I can only state my conviction that it is a
rule of high generality. I am aware of several causes of error, but I
hope that I have made due allowances for them. It should be under-
stood that the rule by no means applies to any part, however un-
usually developed, unless it be unusually developed in one species
or in a few species in comparison with the same part in many closely
allied species. Thus, the wing of a bat is a most abnormal struc-
ture in the class of mammals, but the rule would not apply here,
because the whole group of bats possesses wings; it would apply only
if some one species had wings developed in a remarkable manner in
comparison with the other species of the same genus. The rule
applies very strongly in the case of secondary sexual characters, when
displayed in any unusual manner. The term, secondary sexual char-
acters, used by Hunter, relates to characters which are attached to one
sex, but are not directly connected with the act of reproduction. The
rule applies to males and females; but more rarely to the females, as
they seldom offer remarkable secondary sexual characters. The rule
being so plainly applicable in the case of secondary sexual characters,
may be due to the great variability of these characters, whether or not
displayed in any unusual manner — of which fact I think there can
be little doubt. But that our rule is not confined to secondary sexual
characters is clearly shown in the case of hermaphrodite cirripedes;
I particularly attended to Mr. Waterhouse's remark, whilst investi-
gating this Order, and I am fully convinced that the rule almost
always holds good. I shall, in a future work, give a list of all the
more remarkable cases; I will here give only one, as it illustrates the
rule in its largest application. The opercular valves of sessile cirri-
pedes (rock barnacles) are, in every sense of the word, very impor-

154 ORIGIN OF SPECIES

tant structures, and they differ extremely little even in distinct genera;
but in the several species of one genus, Pyrgoma, these valves pre-
sent a marvellous amount of diversification; the homologous valves
in the different sjjecies being sometimes wholly unlike in shape; and
the amount of variation in the individuals of the same species is
so great, that it is no exaggeration to state that the varieties of the
same species differ more from each other in the characters derived
from these important organs, than do the species belonging to other
distinct genera.

As with birds the individuals of the same species, inhabiting the
same country, vary extremely little, I have particularly attended to
them; and the rule certainly seems to hold good in this class. I can-
not make out that it applies to plants, and this would have seriously
shaken my belief in its truth, had not the great variability in plants
made it particularly difficult to compare their relative degrees of
variability.

When we see any part or organ develof)ed in a remarkable degree
or manner in a species, the fair presumption is that it is of high im-
portance to that species: nevertheless it is in this case eminently liable
to variation. Why should this be so? On the view that each species
has been independently created, with all its parts as we now see them,
I can see no explanation. But on the view that groups of species are
descended from some other sfjecies, and have been modified through
natural selection, I think we can obtain some light. First let me make
some preliminary remarks. If, in our domestic animals, any part or
the whole animal be neglected, and no selection be applied, that part
(for instance, the comb in the Dorking fowl) or the whole breed
will cease to have a uniform character: and the breed may be said to
be degenerating. In rudimentary organs, and in those which have
been but little specialised for any particular purpose, and perhaps in
polymorphic groups, we see a nearly parallel case; for in such cases
natural selection either has not or cannot have come into full play,
and thus the organisation is left in a fluctuating condition. But what
here more particularly concerns us is, that those points in our domes-
tic animals, which at the present time are undergoing rapid change by
continued selection, are also eminently liable to variation. Look at
the individuals of the same breed of the pigeon, and see what a

STRUCTURES VARIABLE 155

prodigious amount of difference there is in the beaks of tumblers, in
the beaks and wattle of carriers, in the carriage and tail of fantails,
etc^ these being the points now mainly attended to by English fan-
ciers. Even in the same sub-breed, as in that of the short-faced tum-
bler, it is notoriously difficult to breed nearly perfect birds, many
departing widely from the standard. There may truly be said to be
a constant struggle going on between, on the one hand, the tendency
to reversion to a less perfect state, as well as an innate tendency to
new variations, and, on the other hand, the power of steady selec-
tion to keep the breed true. In the long run selection gains the day,
and we do not expect to fail so completely as to breed a bird as coarse
as a common tumbler pigeon from a good short-faced strain. But
as long as selection is rapidly going on, much variability in the
parts undergoing modification may always be expected.

Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other
species of the same genus, we may conclude that this part has under-
gone an extraordinary amount of modification since the period when
the several species branched off from the common progenitor of the
genus. This period will seldom be remote in any extreme degree, as
species rarely endure for more than one geological period. An extraor-
dinary amount of modification implies an unusually large and long-
continued amount of variability, which has continually been accumu-
lated by natural selection for the benefit of the species. But as the
variability of the extraordinarily developed part or organ has been
so great and long-continued within a period not excessively remote,
we might, as a general rule, still expect to find more variability in
such parts than in other parts of the organisation which have re-
mained for a much longer period nearly constant. And this, I am
convinced, is the case. That the struggle between natural selection
on the one hand, and the tendency to reversion and variability on
the other hand, will in the course of time cease; and that the most
abnormally developed organs may be made constant, I see no reason
to doubt. Hence, when an organ, however abnormal it may be, has
been transmitted in approximately the same condition to many modi-
fied descendants, as in the case of the wing of the bat, it must have
existed, according to our theory, for an immense period in nearly

156 ORIGIN OF SPECIES

the same state; and thus it has come not to be more variable than
any other structure. It is only in those cases in which the modifica-
tion has been comparatively recent and extraordinarily great that we
ought to find the generative variability, as it may be called, still
present in a high degree. For in this case the variability will seldom
as yet have been fixed by the continued selection of the individuals
varying in the required manner and degree, and by the continued
rejection of those tending to revert to a former and less modified
condition.

SPECIFIC CHARACTERS MORE VARIABLE THAN GENERIC CHARACTERS

The principle discussed under the last heading may be applied to
our present subject. It is notorious that specific characters are more
variable than generic. To explain by a simple example what is
meant: if in a large genus of plants some species had blue flowers
and some had red, the colour would be only a specific character, and
no one would be surprised at one of the blue sf)ecies varying into red,
or conversely; but if all the species had blue flowers, the colour would
become a generic character, and its variation would be a more un-
usual circumstance. I have chosen this example because the explana-
tion which most naturalists would advance is not here applicable,
namely, that specific characters are more variable than generic, be-
cause they are taken from parts of less physiological importance than
those commonly used for classing genera. I believe this explanation
is partly, yet only indirectly, true; I shall, however, have to return to
this fKjint in the chapter on Classification. It would be almost super-
fluous to adduce evidence in suppwrt of the statement, that ordinary
specific characters are more variable than generic; but with respect
to important characters, I have repeatedly noticed in works on
natural history, that when an author remarks with surprise that some
important organ or part, which is generally very constant throughout
a large group of species, differs considerably in closely allied species,
it is often variable in the individuals of the same species. And this
fact shows that a character, which is generally of generic value, when
it sinks in value and becomes only of specific value, often becomes
variable, though its physiological importance may remain the same.
Something of the same kind applies to monstrosities: at least Is.

SECONDARY SEXUAL CHARACTERS VARIABLE I57

Geoffroy St. Hilaire apparently entertains no doubt, that the more
an organ normally differs in the different species of the same group,
the more subject it is to anomalies in the individuals.

On the ordinary view o£ each sp)ecies having been independently
created, why should that part of the structure, which differs from
the same part in other independently created species of the same
genus, be more variable than those parts which are closely alike in
the several species? I do not see that any explanation can be given.
But on the view that species are only strongly marked and fixed varie-
ties, we might expect often to find them still continuing to vary in
those parts of their structure which have varied within a moderately
recent period, and which have thus come to differ. Or to state the
case in another manner: the points in which all the species of a genus
resemble each other, and in which they differ from allied genera, are
called generic characters; and these characters may be attributed to
inheritance from a common progenitor, for it can rarely have hap-
pened that natural selection will have modified several distinct spe-
cies, fitted to more or less widely different habits, in exactly the same
manner: and as these so-called generic characters have been inherited
from before the period when the several species first branched off from
their common progenitor, and subsequently have not varied or come
to differ in any degree, or only in a slight degree, it is not probable
that they should vary at the present day. On the other hand, the
points in which species differ from other sf)ecies of the same genus
are called specific characters; and as these specific characters have
varied and come to differ since the period when the species branched
off from a common progenitor, it is probable that they should still
often be in some degree variable, — at least more variable than those
parts of the organisation which have for a very long period remained
constant.

SECONDARY SEXUAL CHARACTERS VARIABLE

I think it will be admitted by naturalists, without my entering on
details, that secondary sexual characters are highly variable. It will
also be admitted that sf>ecies of the same group differ from each
other more widely in their secondary sexual characters, than in other
parts of their organisation: compare, for instance, the amount of

158 ORIGIN OF SPECIES

difference between the males of gallinaceous birds, in which second-
ary sexual characters are strongly displayed, with the amount of
difference between the females. The cause of the original variabil-
ity of these characters is not manifest; but we can see why they
should not have been rendered as constant and uniform as others,
for they are accumulated by sexual selection, which is less rigid in
its action than ordinary selection, as it does not entail death, but only
gives fewer offspring to the less favoured males. Whatever the cause
may be of the variability of secondary sexual characters, as they are
highly variable, sexual selection will have had a wide scope for action,
and may thus have succeeded in giving to the species of the
same group a greater amount of difference in these than in other
respects.

It is a remarkable fact, that the secondary differences between the
two sexes of the same species are generally displayed in the very same
parts of the organisation in which the spwcies of the same genus
differ from each other. Of this fact I will give in illustration the two
first instances which happen to stand on my list; and as the differ-
ences in these cases are of a very unusual nature, the relation can
hardly be accidental. The same number of joints in the tarsi is a
character common to very large groups of beetles, but in the Engidac,
as Westwood has remarked, the number varies greatly; and the
number likewise differs in the two sexes of the same species. Again
in the fossorial hymenoptera, the neuration of the wings is a char-
acter of the highest importance, because common to large groups;
but in certain genera the neuration differs in the different species,
and likewise in the two sexes of the same species. Sir J. Lubbock
has recently remarked, that several minute crustaceans offer excellent
illustrations of this law. "In Pontella, for instance, the sexual charac-
ters are afforded mainly by the anterior antennae and by the fifth pair
of legs: the specific differences also are principally given by these
organs." This relation has a clear meaning on my view: I look at
all the species of the same genus as having as certainly descended
from a common progenitor, as have the two sexes of any one spe-
cies. Consequently, whatever part of the structure of the common
progenitor, or of its early descendants, became variable, variations of
this part would, it is highly probable, be taken advantage of by

DISTINCT SPECIES PRESENT VARIATIONS 1 59

natural and sexual selection, in order to fit the several species to their
several places in the economy of nature, and likewise to fit the two
sexes of the same species to each other, or to fit the males to struggle
with other males for the possession of the females.

Finally, then, I conclude that the greater variabihty of specific char-
acters, or those which distinguish species from species, than of
generic characters, or those which are possessed by all the species;
that the frequent extreme variability of any part which is developed
in a species in an extraordinary manner in comparison with the same
part in its congeners; and the slight degree of variability in a part,
however extraordinarily it may be developed, if it be common to a
whole group of species; that the great variability of secondary sexual
characters, and their great difference in closely allied species; that
secondary sexual and ordinary specific differences are generally dis-
played in the same parts of the organisation, — are all principles closely
connected together. All being mainly due to the species of the same
group being the descendants of a common progenitor, from whom
they have inherited much in common, to parts which have recently
and largely varied being more Hkely still to go on varying than parts
which have long been inherited and have not varied, to natural selec-
tion having more or less completely, according to the lapse of time,
overmastered the tendency to reversion and to further variability, —
to sexual selection being less rigid than ordinary selection, — and to
variations in the same parts having been accumulated by natural and
sexual selection, and having been thus adapted for secondary sexual,
and for ordinary purposes.

DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS, SO THAT A VARIETY
OF ONE SPECIES OFTEN ASSUMES A CHARACTER PROPER TO AN ALLIED
SPECIES, OR REVERTS TO SOME OF THE CHARACTERS OF AN EARLY
PROGENITOR

These propositions will be most readily understood by looking to
our domestic races. The most distinct breeds of the pigeon, in coun-
tries widely apart, present sub-varieties with reversed feathers on
the head, and with feathers on the feet, characters not possessed by
the aboriginal rock pigeon; these then are analogous variations in

l60 ORIGIN OF SPECIES

two or more distinct races. The frequent presence of fourteen or even
sixteen, tail-feathers in the pouter may be considered as a variation
representing the normal structure of another race, the fantail. I
presume that no one will doubt that all such analogous variations are
due to the several races of the pigeon having inherited from a com-
mon parent the same constitution and tendency to variation, when
acted on by similar unknown influences. In the vegetable kingdom
we have a case of analogous variation, in the enlarged stems, or as
commonly called roots, of the Swedish turnip and rutabaga, plants
which several botanists rank as varieties produced by cultivation from
a common parent: if this be not so, the case will then be one of
analogous variation in two so-called distinct sf)ecies; and to these a
third may be added, namely, the common turnip. According to the
ordinary view of each species having been independently created, we
should have to attribute this similarity in the enlarged stems of these
three plants, not to the vera causa of community of descent, and a con-
sequent tendency to vary in a like manner, but to three separated
yet closely related acts of creation. Many similar cases of analogous
variation have been observed by Naudin in the great gourd family,
and by various authors in our cereals. Similar cases occurring with
insects under natural conditions have lately been discussed with much
ability by Mr. Walsh, who has grouped them under his law of
Equable Variability.

With pigeons, however, we have another case, namely, the oc-
casional appearance in all the breeds, of slaty-blue birds with two
black bars on the wings, white loins, a bar at the end of the tail, with
the outer feathers externally edged near their basis with white. As
all these marks are characteristic of the parent rock pigeon, I presume
that no one will doubt that this is a case of reversion, and not of a
new yet analogous variation appearing in the several breeds. We
may, I think, confidently come to this conclusion, because, as we
have seen, these coloured marks are eminently liable to appear in the
crossed offspring of two distinct and differently coloured breeds;
and in this case there is nothing in the external conditions of life to
cause the reappearance of the slaty-blue, with the several marks, be-
yond the influence of the mere act of crossing on the laws of
inheritance.

DISTINCT SPECIES PRESENT VARIATIONS l6l

No doubt it is a very surprising fact that characters should re-
appear after having been lost for many, probably for hundreds of
generations. But when a breed has been crossed only once by some
other breed, the offspring occasionally show for many generations
a tendency to revert in character to the foreign breed — some say, for
a dozen or even a score of generations. After twelve generations, the
proportion of blood, to use a common expression, from one ancestor,
is only one in 2,048; and yet, as we see, it is generally believed that a
tendency to reversion is retained by this remnant of foreign blood. In
a breed which has not been crossed but in which both parents have
lost some character which their progenitor possessed, the tendency,
whether strong or weak, to reproduce the lost character might, as
was formerly remarked, for all that we can see to the contrary, be
transmitted for almost any number of generations. When a charac-
ter which has been lost in a breed, reappears after a great number of
generations, the most probable hypothesis is, not that one individual
suddenly takes after an ancestor removed by some hundred genera-
tions, but that in each successive generation the character in question
has been lying latent, and at last, under unknown favourable con-
ditions, is developed. With the barb pigeon, for instance, which very
rarely produces a blue bird, it is probable that there is a latent
tendency in each generation to produce blue plumage. The abstract
improbability of such a tendency being transmitted through a vast
number of generations, is not greater than that of quite useless or
rudimentary organs being similarly transmitted. A mere tendency
to produce a rudiment is indeed sometimes thus inherited.

As all the species of the same genus are supposed to be descended
from a common progenitor, it might be expected that they would
occasionally vary in an analogous manner; so that the varieties of two
or more species would resemble each other, or that a variety of one
species would resemble in certain characters another and distinct
species, this other species being, according to our view, only a well-
marked and permanent variety. But characters exclusively due to
analogous variation would probably be of an unimportant nature,
for the preservation of all functionally important characters will have
been determined through natural selection, in accordance with the
different habits of the species. It might further be expected that

l62 ORIGIN OF SPECIES

the species of the same genus would occasionally exhibit reversions
to long lost characters. As, however, we do not know the common
ancestor of any natural group, we cannot distinguish between re-
visionary and analogous characters. If, for instance, we did not know
that the parent rock pigeon was not feather-footed or turn<rowned,
we could not have told, whether such characters in our domestic
breeds were reversions or only analogous variations; but we might
have inferred that the blue colour was a case of reversion from the
number of the markings, which are correlated with this tint, and
which would not probably have all appeared together from simple va-
riation. More especially we might have inferred this, from the blue
colour and the several marks so often appearing when differently
coloured breeds are crossed. Hence, although under nature it must
generally be left doubtful, what cases are reversions to formerly exist-
ing characters, and what are new but analogous variations, yet we
ought, on our theory, sometimes to find the varying offspring of a
species assuming characters which are already present in other mem-
bers of the same group. And this undoubtedly is the case.

The difficulty in distinguishing variable species is largely due to
the varieties mocking, as it were, other species of the same genus. A
considerable catalogue, also, could be given of forms intermediate
between two other forms, which themselves can only doubtfully be
ranked as species; and this shows, unless all these closely allied forms
be considered as independendy created species, that they have in
varying assumed some of the characters of the others. But the best
evidence of analogous variations is afforded by parts or organs which
are generally constant in character, but which occasionally vary so as
to resemble, in some degree, the same part or organ in an allied
species. I have collected a long list of such cases; but here, as before,
I lie under the great disadvantage of not being able to give them. I
can only repeat that such cases certainly occur, and seem to me very
remarkable.

I will, however, give one curious and complex case, not indeed as
affecting any important character, but from occurring in several
species of the same genus, partly under domestication and partly
under nature. It is a case almost certainly of reversion. The ass some-
times has very distinct transverse bars on its legs, like those on the

DISTINCT SPECIES PRESENT VARIATIONS 1 63

legs of the zebra. It has been asserted that these are plainest in the
foal, and, from inquiries which I have made, I believe this to be
true. The stripe on the shoulder is sometimes double, and is very
variable in length and outline. A white ass, but not an albino, has
been described without either spinal or shoulder stripe: and these
stripes are sometimes very obscure, or actually quite lost, in dark-
coloured asses. The koulan of Pallas is said to have been seen with
a double shoulder stripe. Mr. Blyth has seen a specimen of the
hemionus with a distinct shoulder stripe, though it properly has
none; and I have been informed by Colonel Poole that the foals of
this species are generally strijjed on the legs, and faintly on the
shoulder. The quagga, though so plainly barred like a zebra over
the body, is without bars on the legs; but Dr. Gray has figured one
specimen with very distinct zebra-like bars on the hocks.

With resp)ect to the horse, I have collected cases in England of the
spinal stripe in horses of the most distinct breeds, and of all colours:
transverse bars on the legs are not rare in duns, mouse-duns, and in
one instance in a chestnut; a faint shoulder stripe may sometimes be
seen in duns, and I have seen a trace in a bay horse. My son made a
careful examination and sketch for me of a dun Belgian cart horse
with a double strif)e on each shoulder and with leg stripes; I have
myself seen a dun Devonshire pony, and a small dun Welsh pony
has been carefully described to me, both with three parallel stripes on
each shoulder.

In the northwest part of India the Kattywar breed of horses is so
generally striped, that, as I hear from Colonel Poole, who examined
this breed for the Indian Government, a horse without strif)es is not
considered as purely bred. The spine is always striped; the legs are
generally barred, and the shoulder strifje, which is sometimes double
and sometimes treble, is common; the side of the face, moreover, is
sometimes striped. The strijjes are often plainest in the foal, and
sometimes quite disappear in old horses. Colonel Poole has seen
both gray and bay Kattywar horses striped when first foaled. I have
also reason to suspect, from information given me by Mr. W. W.
Edwards, that with the English race horse the spinal stripe is much
commoner in the foal than in the full-grown animal. I have myself
recently bred a foal from a bay mare (offspring of a Turkoman horse

164 ORIGIN OF SPECIES

and a Flemish mare) by a bay English race horse; this foal when a
week old was marked on its hinder quarters and on its forehead with
numerous, very narrow, dark, zebra-like bars, and its legs were feebly
striped. All the stripes soon disappeared completely. Without here
entering on further details, I may state that I have collected cases of
leg and shoulder stripes in horses of very different breeds in various
countries from Britain to Eastern China; and from Norway in the
north to the Malay Archipelago in the south. In all parts of the
world these stripes occur far oftenest in duns and mouse-duns; by the
term dun a large range of colour is included, from one between
brown and black to a close approach to cream colour.

I am aware that Colonel Hamilton Smith, who has written on this
subject, believes that the several breeds of the horse are descended
from several aboriginal species, one of which, the dun, was striped;
and that the above-described appearances are all due to ancient crosses
with the dun stock. But this view may be safely rejected, for it is
highly improbable that the heavy Belgian cart horse, Welsh p)onies,
Norwegian cobs, the lanky Kattywar race, etc., inhabiting the most
distant parts of the world, should all have been crossed with one sup-
posed aboriginal stock.

Now let us turn to the effects of crossing the several species of the
horse-genus. Rollin asserts, that the common mule from the ass
and horse is particularly apt to have bars on its legs; according to
Mr. Gosse, in certain parts of the United States, about nine out of
ten mules have stripjed legs. I once saw a mule with its legs so much
striped that any one might have thought that it was a hybrid zebra;
and Mr. W. C. Martin, in his excellent treatise on the horse, has
given a figure of a similar mule. In four coloured drawings, which
I have seen, of hybrids between the ass and zebra, the legs were much
more plainly barred than the rest of the body; and in one of them
there was a double shoulder stripe. In Lord Morton's famous hybrid,
from a chestnut mare and male quagga, the hybrid, and even the pure
offspring subsequently produced from the same mare by a black
Arabian sire, were much more plainly barred across the legs than is
even the pure quagga. Lastly, and this is another most remarkable
case, a hybrid has been figured by Dr. Gray (and he informs me
that he knows of a second case) from the ass and the hemionus; and

DISTINCT SPECIES PRESENT VARIATIONS 1 65

this hybrid, though the ass only occasionally has stripes on his legs
and the hemionus has none and has not even a shoulder stripe,
nevertheless had all four legs barred, and had three short shoulder
stripes, like those on the dun Devonshire and Welsh ponies, and even
had some zebra-like stripes on the sides of its face. With respect to
this last fact, I was so convinced that not even a stripe of colour ap-
pears from what is commonly called chance, that I was led solely
from the occurrence of the face stripes on this hybrid from the ass
and hemionus to ask Colonel Poole whether such face stripes ever
occurred in the eminently strip)ed Kattywar breed of horses, and
was, as we have seen, answered in the affirmative.

What now are we to say to these several facts ? We see several dis-
tinct species of the horse-genus becoming, by simple variation, striped
on the legs like a zebra, or striped on the shoulders like an ass. In the
horse we see this tendency strong whenever a dun tint appears — a
tint which approaches to that of the general colouring of the other
species of the genus. The appearance of the stripes is not accom-
panied by any change of form or by any other new character. We see
this tendency to become striped most strongly displayed in hybrids
from between several of the most distinct species. Now observe the
case of the several breeds of pigeons: they are descended from a
pigeon (including two or three sub-species or geographical races)
of a bluish colour, with certain bars and other marks; and when any
breed assumes by simple variation a bluish tint, these bars and other
marks invariably reappear; but without any other change of form
or character. When the oldest and truest breeds of various colours
are crossed, we see a strong tendency for the blue tint and bars and
marks to reappear in the mongrels. I have stated that the most prob-
able hypothesis to account for the reappearance of very ancient char-
acters, is — that there is a tendency in the young of each successive
generation to produce the long-lost character, and that this tendency,
from unknown causes, sometimes prevails. And we have just seen
that in several species of the horse-genus the stripes are either plainer
or appear more commonly in the young than in the old. Call the
breeds of pigeons, some of which have bred true for centuries, sjje-
cies; and how exactly parallel is the case with that of the species of
the horse-genus! For myself, I venture confidently to look back thou-

1 66 ORIGIN OF SPECIES

sands on thousands of generations, and I see an animal striped like
a zebra, but perhaps otherwise very differently constructed, the com-
mon parents of our domestic horse (whether or not it be descended
from one or more wild stocks) of the ass, the hemionus, quagga, and
zebra.

He who believes that each equine species was independently
created, will, I presume, assert that each sp)ecies has been created with
a tendency to vary, both under nature and under domestication, in
this particular manner, so as often to become striped like the other
species of the genus; and that each has been created with a strong
tendency, when crossed with species inhabiting distant quarters of
the world, to produce hybrids resembling in their stripes, not their
own parents, but other species of the genus. To admit this view is,
as it seems to me, to reject a real for an unreal, or at least for an
unknown, cause. It makes the works of God a mere mockery and
deception; I would almost as soon believe with the old and ignorant
cosmogonists, that fossil shells had never lived, but had been created
in stone so as to mock the shells living on the seashore.

SUMMARY

Our ignorance of the laws of variation is profound. Not in one
case out of a hundred can we pretend to assign any reason why this
or that part has varied. But whenever we have the means of insti-
tuting a comparison, the same laws appear to have acted in produc-
ing the lesser differences between varieties of the same species, and
the greater differences between species of the same genus. Changed
conditions generally induce mere fluctuating variability, but some-
times they cause direct and definite effects; and these may become
strongly marked in the course of time, though we have not sufficient
evidence on this head. Habit in producing constitutional peculiarities
and use in strengthening and disuse in weakening and diminishing
organs, appear in many cases to have been potent in their
effects. Homologous parts tend to vary in the same manner, and
homologous parts tend to cohere. Modifications in hard parts and in
external parts sometimes affect softer and internal parts. When one
part is largely developed, perhaps it tends to draw nourishment from
the adjoining parts; and every part of the structure which can be

SUMMARY 167

saved without detriment will be saved. Changes of structure at an
early age may affect parts subsequently developed; and many cases of
correlated variation, the nature of which we are unable to under-
stand, undoubtedly occur. Multiple parts are variable in number
and in structure, perhaps arising from such parts not having been
closely specialised for any particular function, so that their modi-
fications have not been closely checked by natural selection. It fol-
lows probably from this same cause, that organic beings low in the
scale are more variable than those standing higher in the scale, and
which have their whole organisation more specialised. Rudimentary
organs, from being useless, are not regulated by natural selection, and
hence are variable. Specific characters — that is, the characters which
have come to dif?er since the several species of the same genus
branched off from a common parent — are more variable than generic
characters, or those which have long been inherited, and have not dif-
fered within this same period. In these remarks we have referred to
special parts or organs being still variable, because they have recently
varied and thus come to differ; but we have also seen in the second
chapter that the same principle applies to the whole individual; for in
a district where many sp)ecies of a genus are found — that is, where
there has been much former variation and differentiation, or where
the manufactory of new specific forms has been actively at work — in
that district and amongst these species, we now find, on an average,
most varieties. Secondary sexual characters are highly variable, and
such characters differ much in the species of the same group. Vari-
ability in the same parts of the organisation has generally been taken
advantage of in giving secondary sexual differences to the two sexes
of the same sf)ecies, and sp)ecific differences to the several sjjecies of
the same genus. Any part or organ developed to an extraordinary
size or in an extraordinary manner, in comparison with the same
part or organ in the allied sp)ecies, must have gone through an ex-
traordinary amount of modification since the genus arose; and thus
we can understand why it should often still be variable in a much
higher degree than other parts; for variation is a long-continued and
slow process, and natural selection will in such cases not as yet have
had time to overcome the tendency to further variability and to re-
version to a less modified state. But when a species with any ex-

1 68 ORIGIN OF SPECIES

traordinarily developed organ has become the parent of many modi-
fied descendants — which on our view must be a very slow process,
requiring a long lapse of time — in this case, natural selection has suc-
ceeded in giving a fixed character to the organ, in however extraordi-
nary a manner it may have been develof)ed. Species inheriting nearly
the same constitution from a common parent, and exposed to similar
influences, naturally tend to present analogous variations, or these
same species may occasionally revert to some of the characters of
their ancient progenitors. Although new and important modifica-
tions may not arise from reversion and analogous variation, such
modifications will add to the beautiful and harmonious diversity of
nature.

Whatever the cause may be of each slight difference between the
offspring and their parents — and a cause for each must exist — we
have reason to believe that it is the steady accumulation of beneficial
differences which has given rise to all the more important modifica-
tions of structure in relation to the habits of each species.

CHAPTER VI
Difficulties of the Theory

Difficulties of the theory of descent with modification — Absence or rarity
of transitional varieties — Transitions in habits of life — Diversified
habits in the same species — Species with habits widely different from
those of their allies— Organs of extreme perfection — Modes of transi-
tion — Cases of difficulty — Natura non facit saltum — Organs of small
importance — Organs not in all cases absolutely perfect — The law of
Unity of Tyf)e and of the Conditions of Existence embraced by the
theory of Natural Selection.

10NG before the reader has arrived at this part of my work, a
crowd of difficulties will have occurred to him. Some o£
-^ them are so serious that to this day I can hardly reflect on
them without being in some degree staggered; but, to the best of
my judgment, the greater number are only apparent, and those that
are real are not, I think, fatal to theory.

These difficulties and objections may be classed under the follow-
ing heads: — First, why, if sf)ecies have descended from other sp>ecies
by fine gradations, do we not everywhere see innumerable transi-
tional forms? Why is not all nature in confusion, instead of the
species being, as we see them, well defined ?

Secondly, is it possible that an animal having, for instance, the
structure and habits of a bat, could have been formed by the modi-
fication of some other animal with widely different habits and struc-
ture? Can we believe that natural selection could produce, on the
one hand, an organ of trifling importance, such as the tail of a giraffe,
which serves as a fly-flapper, and, on the other hand, an organ so
wonderful as the eye ?

Thirdly, can instincts be acquired and modified through natural
selection? What shall we say to the instinct which leads the bee to
make cells, and which has practically anticipated the discoveries of
profound mathematicians?

Fourthly, how can we account for species, when crossed, being

169

170 ORIGIN OF SPECIES

Sterile and producing sterile offspring, whereas, when varieties are
crossed, their fertility is unimpaired?

The two first heads will here be discussed; some miscellaneous
objections in the following chapter; Instinct and Hybridism in the
two succeeding chapters.

ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES

As natural selection acts solely by the preservation of profitable
modifications, each new form will tend in a fully-stocked country to
take the place of, and finally to exterminate, its own less improved
parent-form and other less-favoured forms with which it comes into
competition. Thus extinction and natural selection go hand in hand.
Hence, if we look at each species as descended from some unknown
form, both the parei>t and all the transitional varieties will generally
have been exterminated by the very process of the formation and
perfection of the new form.

But, as by this theory innumerable transitional forms must have
existed, why do we not find them embedded in countless numbers
in the crust of the earth ? It will be more convenient to discuss this
question in the chapter on the Imperfection of the Geological Record;
and I will here only state that I believe the answer mainly lies in the
record being incomparably less perfect than is generally supposed.
The crust of the earth is a vast museum; but the natural collections
have been imperfectly made, and only at long intervals of time.

But it may be urged that when several closely-allied species inhabit
the same territory, we surely ought to find at the present time many
transitional forms. Let us take a simple case: in travelling from
north to south over a continent, we generally meet at successive inter-
vals with closely allied or representative sp>ecies, evidently filling
nearly the same place in the natural economy of the land. These
representative species often meet and interlock; and as the one
becomes rarer and rarer, the other becomes more and more frequent,
till the one replaces the other. But if we compare these species where
they intermingle, they are generally as absolutely distinct from each
other in every detail of structure as are specimens taken from the
metropolis inhabited by each. By my theory these allied species are
descended from a common parent; and during the process of modi-

TRANSITIONAL VARIETIES I7I

fication, each has become adapted to the conditions of hfe of its own
region, and has supplanted and exterminated its original parent-form
and all the transitional varieties between its past and present states.
Hence we ought not to expect at the present time to meet with
numerous transitional varieties in each region, though they must have
existed there, and may be embedded there in a fossil condition. But
in the intermediate region, having intermediate conditions of life,
why do we not now find closely-linking intermediate varieties? This
difficulty for a long time quite confounded me. But I think it can
be in large part explained.

In the first place we should be extremely cautious in inferring,
because an area is now continuous, that it has been continuous during
a long period. Geology would lead us to believe that most continents
have been broken up into islands even during the later tertiary
periods; and in such islands distinct species might have been sep-
arately formed without the possibility of intermediate varieties exist-
ing in the intermediate zones. By changes in the form of the land
and of climate, marine areas now continuous must often have existed
within recent times in a far less continuous and uniform condition
than at present. But I will pass over this way of escaping from the
difficulty; for I believe that many perfectly defined species have been
formed on strictly continuous areas; though I do not doubt that the
formerly broken condition of areas now continuous, has played an
important part in the formation of new sjiecies, more especially with
freely-crossing and wandering animals.

In looking at species as they are now distributed over a wide area,
we generally find them tolerably numerous over a large territory,
then becoming somewhat abruptly rarer and rarer on the confines,
and finally disappearing. Hence the neutral territory between two
representative species is generally narrow in comparison with the
territory proper to each. We see the same fact in ascending moun-
tains, and sometimes it is quite remarkable how abruptly, as
Alph. de Candolle has observed, a common alpine species disappears.
The same fact has been noticed by E. Forbes in sounding the depths
of the sea with the dredge. To those who look at climate and the
physical conditions of life as the all-important elements of distribu-
tion, these facts ought to cause surprise, as climate and height or

172 ORIGIN OF SPECIES

depth graduate away insensibly. But when we bear in mind that
almost every species, even in its metropolis, would increase im-
mensely in numbers, were it not for other competing species; that
nearly all either prey on or serve as prey for others; in short, that each
organic being is either directly or indirectly related in the most im-
portant manner to other organic beings, — we see that the range of
the inhabitants of any country by no means exclusively depends
on insensibly changing physical conditions, but in a large part on the
presence of other species, on which it lives, or by which it is destroyed,
or with which it comes into comf)etition; and as these sf)ecies are
already defined objects, not blending one into another by insensible
gradations, the range of any one sf)ecies, depending as it does on the
range of others, will tend to be sharply defined. Moreover, each
species on the confines of its range, where it exists in lessened num-
bers, will, during fluctuations in the number of its enemies or of its
prey, or in the nature of the seasons, be extremely liable to utter
extermination; and thus its geographical range will come to be still
more sharply defined.

As allied or representative species, when inhabiting a continuous
area, are generally distributed in such a manner that each has a wide
range, with a comparatively narrow neutral territory between them,
in which they become rather suddenly rarer and rarer; then, as
varieties do not essentially differ from species, the same rule will
probably apply to both; and if we take a varying species inhabiting
a very large area, we shall have to adapt two varieties to two large
areas, and a third variety to a narrow intermediate zone. The inter-
mediate variety, consequently, will exist in lesser numbers from in-
habiting a narrow and lesser area; and practically, as far as I can
make out, this rule holds good with varieties in a state of nature.
I have met with striking instances of the rule in the case of varie-
ties intermediate between well-marked varieties in the genus Balanus.
And it would appear from information given me by Mr. Watson,
Dr. Asa Gray, and Mr. WoUaston, that generally, when varieties
intermediate between two other forms occur, they are much rarer
numerically than the forms which they connect. Now, if we may
trust these facts and inferences, and conclude that varieties linking
two other varieties together generally have existed in lesser numbers

TRANSITIONAL VARIETIES 1 73

than the forms which they connect, then we can understand why
intermediate varieties should not endure for very long periods: —
why, as a general rule, they should be exterminated and disappear,
sooner than the forms which they originally linked together.

For any form existing in lesser numbers would, as already re-
marked, run a greater chance of being exterminated than one exist-
ing in large numbers; and in this particular case the intermediate
form would be eminently liable to the inroads of closely-allied forms
existing on both sides of it. But it is a far more important con-
sideration, that during the process of further modification, by which
two varieties are supposed to be converted and perfected into two
distinct species, the two which exist in larger numbers, from in-
habiting larger areas, will have a great advantage over the inter-
mediate variety, which exists in smaller numbers in a narrow and
intermediate zone. For forms existing in larger numbers will have
a better chance, within any given period, of presenting further
favourable variations for natural selection to seize on, than will the
rarer forms which exist in lesser numbers. Hence, the more common
forms, in the race for life, will tend to beat and supplant the less
common forms, for these will be more slowly modified and im-
proved. It is the same principle which, as I believe, accounts for the
common species in each country, as shown in the second chapter,
presenting on an average a greater number of well-marked varieties
than do the rarer species, I may illustrate what I mean by supposing
three varieties of sheep to be kept, one adapted to an extensive
mountainous region; a second to a comparatively narrow, hilly tract;
and a third to the wide plains at the base; and that the inhabitants
are all trying with equal steadiness and skill to improve their stocks
by selection; the chances in this case will be strongly in favour of
the great holders on the mountains or on the plains, improving their
breeds more quickly than the small holders on the intermediate
narrow, hilly tract; and consequently the improved mountain or
plain breed will soon take the place of the less improved hill breed;
and thus the two breeds, which originally existed in greater num-
bers, will come into close contact with each other, without the inter-
position of the supplanted, intermediate hill variety.

To sum up, I believe that species come to be tolerably well-defined

174 ORIGIN OF SPECIES

objects, and do not at any one period present an inextricable chaos
of varying and intermediate links: first, because new varieties are
very slowly formed, for variation is a slow process, and natural
selection can do nothing until favourable individual differences or
variations occur, and until a place in the natural polity of the country
can be better filled by some modification of some one or more of its
inhabitants. And such new places will depend on slow changes of
climate, or on the occasional immigration of new inhabitants, and,
probably, in a still more important degree, on some of the old in-
habitants becoming slowly modified, with the new forms thus pro-
duced and the old ones acting and reacting on each other. So that,
in any one region and at any one time, we ought to see only a few
species presenting slight modifications of structure in some degree
permanent; and this assuredly we do see.

Secondly, areas now continuous must often have existed within
the recent period as isolated portions, in which many forms, more
especially amongst the classes which unite for each birth and wander
much, may have separately been rendered sufficiently distinct to rank
as representative species. In this case, intermediate varieties between
the several representative sf)ecies and their common parent, must
formerly have existed within each isolated portion of the land, but
these links during the process of natural selection will have been
supplanted and exterminated, so that they will no longer be found
in a living state.

Thirdly, when two or more varieties have been formed in differ-
ent portions of a strictly continuous area, intermediate varieties will,
it is probable, at first have been formed in the intermediate zones,
but they will generally have had a short duration. For these inter-
mediate varieties will, from reasons already assigned (namely from
what we know of the actual distribution of closely allied or repre-
sentative species, and likewise of acknowledged varieties), exist in
the intermediate zones in lesser numbers than the varieties which
they tend to connect. From this cause alone the intermediate varieties
will be liable to accidental extermination; and during the process
of further modification through natural selection, they will almost
certainly be beaten and supplanted by the forms which they con-
nect; for these from existing in greater numbers will, in the aggre-

TRANSITIONS OF ORGANIC BEINGS 175

gate, present more varieties, and thus be further improved through
natural selection and gain further advantages.

Lastly, looking not to any one time, but to all time, if my theory
be true, numberless intermediate varieties, linking closely together
all the species of the same group, must assuredly have existed; but
the very process of natural selection constantly tends, as has been
so often remarked, to exterminate the parent-forms and the inter-
mediate links. Consequently evidence of their former existence
could be found only amongst fossil remains, which are preserved, as
we shall attempt to show in a future chapter, in an extremely im-
perfect and intermittent record.

ON THE ORIGIN AND TRANSITIONS OF ORGANIC BEINGS WITH
PECULIAR HABITS AND STRUCTURE

It has been asked by the opponents of such views as I hold, how,
for instance, could a land carnivorous animal have been converted
into one with aquatic habits; for how could the animal in its tran-
sitional state have subsisted? It would be easy to show that there
now exist carnivorous animals presenting close intermediate grades
from strictly terrestrial to aquatic habits; and as each exists by a
struggle for life, it is clear that each must be well adapted to its place
in nature. Look at the Mustela vison of North America, which has
webbed feet, and which resembles an otter in its fur, short legs,
and form of tail. During the summer this animal dives for and
preys on fish, but during the long winter it leaves the frozen waters,
and preys, like other pole-cats, on mice and land animals. If a
different case had been taken, and it had been asked how an in-
sectivorous quadruped could possibly have been converted into a
flying bat, the question would have been far more difficult to answer.
Yet I think such difficulties have little weight.

Here, as on other occasions, I lie under a heavy disadvantage,
for, out of the many striking cases which I have collected, I can
give only one or two instances of transitional habits and structures
in allied species; and of diversified habits, either constant or occa-
sional, in the same sjjecies. And it seems to me that nothing less
than a long list of such cases is sufficient to lessen the difficulty in
any particular case like that of the bat.

176 ORIGIN OF SPECIES

Look at the family of squirrels; here we have the finest gradation
from animals with their tails only slightly flattened, and from others,
as Sir J. Richardson has remarked, with the posterior part of their
bodies rather wide and with the skin on their flanks rather full, to
the so<alled flying squirrels; and flying squirrels have their limbs
and even the base of the tail united by a broad expanse of skin, which
serves as a parachute and allows them to glide through the air to
an astonishing distance from tree to tree. We cannot doubt that
each structure is of use to each kind of squirrel in its own country,
by enabling it to escape birds or beasts of prey, to collect food more
quickly, or, as there is reason to believe, to lessen the danger from
occasional falls. But it does not follow from this fact that the struc-
ture of each squirrel is the best that it is possible to conceive under
all possible conditions. Let the climate and vegetation change, let
other competing rodents or new beasts of prey immigrate, or old
ones become modified, and all analogy would lead us to believe that
some at least of the squirrels would decrease in numbers or become
exterminated, unless they also became modified and improved in
structure in a corresponding manner. Therefore, I can see no
difficulty, more especially under changing conditions of life, in
the continued preservation of individuals with fuller and fuller
flank-membranes, each modification being useful, each being propa-
gated, until, by the accumulated effects of this process of natural
selection, a {perfect so-called flying squirrel was produced.

Now look at the Galeopithecus or so<alled flying lemur, which
formerly was ranked amongst bats, but is now believed to belong to
the Insectivora. An extremely wide flank-membrane stretches from
the corners of the jaw to the tail, and includes the limbs with the
elongated fingers. This flank-membrane is furnished with an ex-
tensor muscle. Although no graduated links of structure, fitted for
gliding through the air, now connect the Galeopithecus with the
other Insectivora, yet there is no difficulty in supposing that such
links formerly existed, and that each was developed in the same
manner as with the less perfectly gliding squirrels; each grade of
structure having been useful to its pxDssessor. Nor can I see any
insuperable difficulty in further believing that the membrane con-
nected fingers and fore-arm of the Galeopithecus might have been

TRANSITIONS OF ORGANIC BEINGS 1 77

greatly lengthened by natural selection; and this, as far as the organs
of flight are concerned, would have converted the animal into a bat.
In certain bats in which the wing-membrane extends from the top
of the shoulder to the tail and includes the hind-legs, we perhaps
see traces of an apparatus originally fitted for gliding through the
air rather than for flight.

If about a dozen genera of birds were to become extinct, who
would have ventured to surmise that birds might have existed which
used their wings solely as flappers, like the logger-headed duck
(Micropterus of Eyton); as fins in the water and as front-legs on
the land, like the penguin; as sails, like the ostrich; and function-
ally for no purpose, like Apteryx? Yet the structure of each of
these birds is good for it, under the conditions of life to which it
is exposed, for each has to live by a struggle; but it is not necessarily
the best possible under all possible conditions. It must not be in-
ferred from these remarks that any of the grades of wing-structure
here alluded to, which perhaps may all be the result of disuse, indi-
cate the steps by which birds actually acquired their perfect power
of flight; but they serve to show what diversified means of transi-
tion are at least possible.

Seeing that a few members of such water-breathing classes as
the Crustacea and Mollusca are adapted to live on the land; and
seeing that we have flying birds and mammals, flying insects of the
most diversified types, and formerly had flying reptiles, it is con-
ceivable that flying-fish, which now glide far through the air, slightly
rising and turning by the aid of their fluttering fins, might have been
modified into perfectly winged animals. If this had been effected,
who would have ever imagined that in an early transitional state
they had been the inhabitants of the open ocean, and had used their
incipient organs of flight exclusively, as far as we know, to escape
being devoured by other fish?

When we see any structure highly perfected for any particular
habit, as the wings of a bird for flight, we should bear in mind that
animals displaying early transitional grades of the structure will
seldom have survived to the present day, for they will have been
supplanted by their successors, which were gradually rendered more
perfect through natural selection. Furthermore, we may conclude

178 ORIGIN OF SPECIES

that transitional states between structures fitted for very different
habits of life will rarely have been developed at an early period in
great numbers and under many subordinate forms. Thus, to return
to our imaginary illustration of the flying-fish, it does not seem
probable that fishes capable of true flight would have been developed
under many subordinate forms, for taking prey of many kinds in
many ways, on the land and in the water, until their organs of flight
had come to a high state of perfection, so as to have given them
a decided advantage over other animals in the battle for life. Hence
the chance of discovering species with transitional grades of struc-
ture in a fossil condition will always be less, from their having
existed in lesser numbers, than in the case of species with fully de-
veloped structures.

I will now give two or three instances both of diversified and
of changed habits in the individuals of the same species. In either
case it would be easy for natural selection to adapt the structure
of the animal to its changed habits, or exclusively to one of its
several habits. It is, however, difficult to decide, and immaterial
for us, whether habits generally change first and structure after-
wards; or whether slight modifications of structure lead to changed
habits; both probably often occurring almost simultaneously. Of
cases of changed habits it will suffice merely to allude to that of
the many British insects which now feed on exotic plants, or ex-
clusively on artificial substances. Of diversified habits innumerable
instances could be given: I have often watched a tyrant flycatcher
(Saurophagus sulphuratus) in South America, hovering over one
spot and then proceeding to another, Hke a kestrel, and at other
times standing stationary on the margin of water, and then dashing
into it like a kingfisher at a fish. In our own country the larger
titmouse (Parus major) may be seen climbing branches, almost
hke a creeper; it sometimes, like a shrike, kills small birds by blows
on the head; and I have many times seen and heard it hammering
the seeds of the yew on a branch, and thus breaking them like a
nuthatch. In North America the black bear was seen by Hearne
swimming for hours with widely open mouth, thus catching, almost
like a whale, insects in the water.

As we sometimes see individuals following habits different from

TRANSITIONS OF ORGANIC BEINGS I79

those proper to their species and to the other species of the same
genus, we might exjject that such individuals would occasionally
give rise to new species, having anomalous habits, and with their
structure either slightly or considerably modified from that of their
type. And such instances occur in nature. Can a more striking in-
stance of adaptation be given than that of a woodfjecker for climb-
ing trees and seizing insects in the chinks of the bark? Yet in North
America there are woodpeckers which feed largely on fruit, and
others with elongated wings which chase insects on the wing. On
the plains of La Plata, where hardly a tree grows, there is a wood-
pecker (Colaptes campestris) which has two toes before and two
behind, a long pointed tongue, pointed tail-feathers, sufficiently stifl
to support the bird in a vertical position on a post, but not so stiff
as in the typical woodpeckers, and a straight strong beak. The beak,
however, is not so straight or so strong as in the typical woodpeckers,
but it is strong enough to bore into wood. Hence this Colaptes in all
the essential parts of its structure is a woodpecker. Even in such
trifling characters as the colouring, the harsh tone of the voice, and
undulatory flight, its close blood-relationship to our common wood-
pecker is plainly declared; yet, as I can assert, not only from my
own observations, but from those of the accurate Azara, in certain
large districts it does not climb trees, and it makes its nest in holes
in banks! In certain other districts, however, this same woodpecker,
as Mr. Hudson states, frequents trees, and bores holes in the trunk
for its nest. I may mention as another illustration of the varied
habits of this genus, that a Mexican Colaptes has been described
by De Saussure as boring holes into hard wood in order to lay
up a store of acorns.

Petrels are the most aerial and oceanic of birds, but in the quiet
sounds of Tierra del Fuego, the Puffinuria berardi, in its general
habits, in its astonishing power of diving, in its manner of swim-
ming and of flying when made to take flight, would be mistaken
by any one for an auk or a grebe; nevertheless it is essentially a
petrel, but with many parts of its organisation profoundly modified
in relation to its new habits of life; whereas the woodpecker of La
Plata has had its structure only slightly modified. In the case of
the water-ouzel, the acutest observer, by examining its dead body,

l8o ORIGIN OF SPECIES

would never have suspected its sub-aquatic habits; yet this bird,
which is allied to the thrush family, subsists by diving — using its
wings under water, and grasping stones with its feet. All the mem-
bers of the great order of Hymenopterous insects are terrestrial,
excepting the genus Proctotrupes, which Sir John Lubbock has
discovered to be aquatic in its habits; it often enters the water and
dives about by the use not of its legs but of its wings, and remains
as long as four hours beneath the surface; yet it exhibits no modi-
fication in structure in accordance with its abnormal habits.

He who believes that each being has been created as we now
see it, must occasionally have felt surprise when he has met with
an animal having habits and structure not in agreement. What
can be plainer than that the webbed feet of ducks and geese are
formed for swimming? Yet there are upland geese with webbed
feet which rarely go near the water; and no one, except Audubon,
has seen the frigate-bird, which has all its four toes webbed, alight
on the surface of the ocean. On the other hand, grebes and coots are
eminently aquatic, although their toes are only bordered by mem-
brane. What seems plainer than that the long toes, not furnished
with membrane of the Grallatores, are formed for walking over
swamps and floating plants? — the water-hen and land-rail are mem-
bers of this order, yet the first is nearly as aquatic as the coot, and
the second is nearly as terrestrial as the quail or partridge. In such
cases, and many others could be given, habits have changed without
a correspHDnding change of structure. The webbed feet of the up-
land goose may be said to have become almost rudimentary in
function, though not in structure. In the frigate-bird, the deeply
scoofjed membrane between the toes shows that structure has begun
to change.

He who believes in separate and innumerable acts of creation
may say, that in these cases it has pleased the Creator to cause a being
of one type to take the place of one belonging to another type; but
this seems to me only re-stating the fact in dignified language. He
who believes in the struggle for existence and in the principle of
natural selection, will acknowledge that every organic being is
constantly endeavouring to increase in numbers; and that if any
one being varies ever so litde, either in habits or structure, and thus

ORGANS OF EXTREME PERFECTION l8l

gains an advantage over some other inhabitant of the same country,
it will seize on the place of that inhabitant, however different that
may be from its own place. Hence it will cause him no surprise that
there should be geese and frigate-birds with webbed feet, living on
the dry land and rarely alighting on the water, that there should be
long-toed corncrakes, living in meadows instead of in swamps;
that there should be woodpeckers where hardly a tree grows; that
there should be diving thrushes and diving Hymenoptera, and
petrels with the habits of auks.

ORGANS OF EXTREME PERFECTION AND COMPLICATION

To suppose that the eye with all its inimitable contrivances for
adjusting the focus to different distances, for admitting different
amounts of light, and for the correction of spherical and chromatic
aberration, could have been formed by natural selection, seems, I
freely confess, absurd in the highest degree. When it was first said
that the sun stood still and the world turned round, the common
sense of mankind declared the doctrine false; but the old saying
of Vox populi, vox Dei, as every philosopher knows, cannot be
trusted in science. Reason tells me, that if numerous gradations from
a simple and imperfect eye to one complex and perfect can be shown
to exist, each grade being useful to its possessor, as is certainly the
case; if further, the eye ever varies and the variations be inherited,
as is likewise certainly the case; and if such variations should be
useful to any animal under changing conditions of life, then the
difficulty of believing that a perfect and complex eye could be
formed by natural selection, though insuperable by our imagination,
should not be considered as subversive of the theory. How a nerve
comes to be sensitive to light, hardly concerns us more than how
life itself originated; but I may remark that, as some of the lowest
organisms, in which nerves cannot be detected, are capable of per-
ceiving light, it does not seem impossible that certain sensitive
elements in their sarcode should become aggregated and developed
into nerves, endowed with this special sensibility.

In searching for the gradations through which an organ in any
species has been perfected, we ought to look exclusively to its lineal
progenitors; but this is scarcely ever possible, and we are forced to

1 82 ORIGIN OF SPECIES

look to other species and genera of the same group, that is to the
collateral descendants from the same parent-form, in order to see
what gradations are possible, and for the chance of some gradations
having been transmitted in an unaltered or little altered condition.
But the state of the same organ in distinct classes may incidentally
throw light on the steps by which it has been perfected.

The simplest organ which can be called an eye consists of an optic
nerve, surrounded by pigment<ells and covered by translucent skin,
but without any lens or other refractive body. We may, however,
according to M. Jourdain, descend even a step lower and find aggre-
gates of pigment-cells, apparently serving as organs of vision, without
any nerves, and resting merely on sarcodic tissue. Eyes of the above
simple nature are not capable of distinct vision, and serve only to
distinguish light from darkness. In certain star-fishes, small de-
pressions in the layer of pigment which surrounds the nerve are
filled, as described by the author just quoted, with transparent
gelatinous matter, projecting with a convex surface, like the cornea
in the higher animals. He suggests that this serves not to form an
image, but only to concentrate the luminous rays and render their
perception more easy. In this concentration of the rays we gain the
first and by far the most important step towards the formation of
a true, picture-forming eye; for we have only to place the naked
extremity of the optic nerve, which in some of the lower animals
lies deeply buried in the body, and in some near the surface, at the
right distance from the concentrating apparatus, and an image will
be formed on it.

In the great class of the Articulata, we may start from an optic
nerve simply coated with pigment, the latter sometimes forming
a sort of pupil, but destitute of a lens or other optical contrivance.
With insects it is now known that the numerous facets on the cornea
of their great compound eyes form true lenses, and that the cones
include curiously modified nervous filaments. But these organs in
the Articulata are so much diversified that Miiller formerly made
three main classes with seven subdivisions, besides a fourth main
class of aggregated simple eyes.

When we reflect on these facts, here given much too briefly, with

ORGANS OF EXTREME PERFECTION 183

respect to the wide, diversified, and graduated range of structure
in the eyes of the lower animals; and when we bear in mind how
small the number of all living forms must be in comparison with
those which have become extinct, the difficulty ceases to be very
great in believing that natural selection may have converted the
simple apparatus of an optic nerve, coated with pigment and
invested by transparent membrane, into an optical instrument as
perfect as is pxjssessed by any member of the Articulate class.

He who will go thus far, ought not to hesitate to go one step
further, if he finds on finishing this volume that large bodies of
facts, otherwise inexplicable, can be explained by the theory of
modification through natural selection; he ought to admit that
a structure even as perfect as an eagle's eye might thus be formed,
although in this case he does not know the transitional states. It
has been objected that in order to modify the eye and still preserve
it as a perfect instrument, many changes would have to be effected
simultaneously, which, it is assumed, could not be done through
natural selection; but as I have attempted to show in my work
on the variation of domestic animals, it is not necessary to suppose
that the modifications were all simultaneous, if they were extremely
shght and gradual. Different kinds of modification would, also,
serve for the same general purpose: as Mr. Wallace has remarked,
"If a lens has too short or too long a focus, it may be amended either
by an alteration of curvature, or an alteration of density; if the
curvature be irregular, and the rays do not converge to a point,
then any increased regularity of curvature will be an improvement.
So the contraction of the iris and the muscular movements of the
eye are neither of them essential to vision, but only improvements
which might have been added and {perfected at any stage of the
construction of the instrument." Within the highest division of
the animal kingdom, namely, the Vertebrata, we can start from an
eye so simple, that it consists, as in the lancelet, of a little sack of
transparent skin, furnished with a nerve and lined with pigment,
but destitute of any other apparatus. In fishes and reptiles, as Owen
has remarked, "the range of gradations of dioptric structures is
very great." It is a significant fact that even in man, according to

184 ORIGIN OF SPECIES

the high authority of Virchow, the beautiful crystalline lens is
formed in the embryo by an accumulation of epidermic cells, lying
in a sack-like fold of the skin; and the vitreous body is formed
from embryonic sub-cutaneous tissue. To arrive, however, at a just
conclusion regarding the formation of the eye, with all its mar-
vellous yet not absolutely perfect characters, it is indispensable that
the reason should conquer the imagination; but I have felt the diffi-
culty far too keenly to be surprised at others hesitating to extend the
principle of natural selection to so startling a length.

It is scarcely possible to avoid comparing the eye with a telescope.
We know that this instrument has been perfected by the long-
continued efforts of the highest human intellects; and we naturally
infer that the eye has been formed by a somewhat analogous process.
But may not this inference be presumptuous? Have we any right
to assume that the Creator works by intellectual powers like those
of man? If we must compare the eye to an optical instrument,
we ought in imagination to take a thick layer of transparent tissue,
with spaces filled with fluid, and with a nerve sensitive to light be-
neath, and then suppose every part of this layer to be continually
changing slowly in density, so as to separate in<o layers of different
densities and thicknesses, placed at different distances from each
other, and with the surfaces of each layer slowly changing in form.
Further we must supjwse that there is a power, represented by
natural selection or the survival of the fittest, always intently watch-
ing each slight alteration in the transparent layers; and carefully
preserving each which, under varied circumstances, in any way
or in any degree, tends to produce a distincter image. We must
suppose each new state of the instrument to be multiplied by the
million; each to be preserved until a better one is produced, and
then the old ones to be all destroyed. In living bodies, variation will
cause the slight alterations, generation will multiply them almost
infinitely, and natural selection will pick out with unerring skill
each improvement. Let this process go on for millions of years;
and during each year on millions of individuals of many kinds;
and may we not believe that a living optical instrument might
thus be formed as superior to one of glass, as the works of the
Creator are to those of man?

MOOES OF TRANSITION 1 85

MODES OF TRANSITION

If it could be demonstrated that any complex organ existed, which
could not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can
find out no such case. No doubt many organs exist of which we do
not know the transitional grades, more especially if we look to
much-isolated species, round which, according to the theory, there
has been much extinrtion. Or again, if we take an organ common
to all the members of a class, for in this latter case the organ must
have been originally formed at a remote period, since which all
the many members of the class have been developed; and in order
to discover the early transitional grades through which the organ
has passed, we should have to look to very ancient ancestral forms,
long since become extinct.

We should be extremely cautious in concluding that an organ
could not have been formed by transitional gradations of some
kind. Numerous cases could be given amongst the lower animals
of the same organ performing at the same time wholly distinct
functions; thus in the larva of the dragon-fly and in the fish Cobites
the alimentary canal respires, digests, and excretes. In the Hydra,
the animal may be turned inside out, and the exterior surface will
then digest and the stomach respire. In such cases natural selection
might specialise, if any advantage were thus gained, the whole or
part of an organ, which had previously performed two functions,
for one function alone, and thus by insensible steps greatly change
its nature. Many plants are known which regularly produce at the
same time differently constructed flowers; and if such plants were
to produce one kind alone, a great change would be effected with
comparative suddenness in the character of the species. It is, how-
ever, probable that the two sorts of flowers borne by the same plant
were originally differentiated by finely graduated steps, which may
still be followed in some few cases.

Again, two distinct organs, or the same organ under two very
different forms, may simultaneously perform in the same individual
the same function, and this is an extremely important means of
transition : to give one instance, — there are fish with gills or branchiz

1 86 ORIGIN OF SPECIES

that breathe the air dissolved in the water, at the same time that
they breathe free air in their swim bladders, this latter organ being
divided by highly vascular partitions and having a ductus pneumati-
cus for the supply of air. To give another instance from the vegetable
kingdom; plants climb by three distinct means, by spirally twining,
by clasping a support with their sensitive tendrils, and by the emis-
sion of aerial roodets; these three means are usually found in distinct
groups, but some few species exhibit two of the means, or even all
three, combined in the same individual. In all such cases one of the
two organs might readily be modified and perfected so as to perform
all the work, being aided during the progress of modification by the
other organ; and then this other organ might be modified for some
other and quite distinct purpose, or be wholly obliterated.

The illustration of the swim bladder in fishes is a good one, be-
cause it shows us clearly the highly important fact that an organ
originally constructed for one purpose, namely, flotation, may be
converted into one for a widely different purpose, namely, respira-
tion. The swim bladder has, also, been worked in as an accessory to
the auditory organs of certain fishes. All physiologists admit that the
swim bladder is homologous, or "ideally similar" in position and
structure with the lungs of the higher vertebrate animals: hence
there is no reason to doubt that the swim bladder has actually
been converted into lungs, or an organ used exclusively for res-
piration.

According to this view it may be inferred that all vertebrate ani-
mals with true lungs are descended by ordinary generation from an
ancient and unknown prototype, which was furnished with a float-
ing apparatus or swim bladder. We can thus, as 1 infer from Owen's
interesting description of these parts, understand the strange fact
that every particle of food and drink which we swallow has to pass
over the orifice of the trachea, with some risk of falling into the lungs,
notwithstanding the beautiful contrivance by which the glottis is
closed. In the higher Vertebrata the branchii have wholly disap-
peared — but in the embryo the slits on the sides of the neck and
the loop-like course of the arteries still mark their former position.
But it is conceivable that the now utterly lost branchiae might have
been gradually worked in by natural selection for some distinct pur-

MODES OF TRANSITION 1 87

pose: for instance, Landois has shown that the wings of insects
are developed from the trachex; it is therefore highly probable that
in this great class organs which once served for respiration have
been actually converted into organs for flight.

In considering transitions of organs, it is so important to bear
in mind the probability of conversion from one function to another,
that I will give another instance. Pedunculated cirripedes have two
minute folds of skin, called by me the ovigerous frena, which serve,
through the means of a sticky secretion, to retain the eggs until they
are hatched within the sack. These cirripedes have no branchix,
the whole surface of the body and of the sack, together with the
small frena, serving for respiration. The Balanidz or sessile cir-
ripedes, on the other hand, have no ovigerous frena, the eggs lying
loose at the bottom of the sack, within the well-enclosed shell; but
they have, in the same relative position with the frena, large, much-
folded membranes, which freely communicate with the circulatory
lacunae of the sack and body, and which have been considered by
all naturalists to act as branchix. Now I think no one will dispute
that the ovigerous frena in the one family are strictly homologous
with the branchii of the other family; indeed, they graduate into
each other. Therefore it need not be doubted that the two little
folds of skin, which originally served as ovigerous frena, but which,
likewise, very slightly aided in the act of respiration, have been
gradually converted by natural selection into branchix, simply
through an increase in their size and the obliteration of their ad-
hesive glands. If all pedunculated cirripedes had become extinct,
and they have suffered far more extinction than have sessile cirri-
pedes, who would ever have imagined that the branchix in this
latter family had originally existed as organs for preventing the
ova from being washed out of the sack ?

There is another possible mode of transition, namely, through the
acceleration or retardation of the period of reproduction. This has
lately been insisted on by Professor Cope and others in the United
States. It is now known that some animals are capable of reproduc-
tion at a very early age, before they have acquired their perfect
characters; and if this pxjwer became thoroughly well developed in
a species, it seems probable that the adult stage of development

1 88 ORIGIN OF SPECIES

would sooner or later be lost; and in this case, especially if the larva
differed much from the mature form, the character of the species
would be greatly changed and degraded. Again, not a few animals,
after arriving at maturity, go on changing in character during nearly
their whole lives. With mammals, for instance, the form of the
skull is often much altered with age, of which Dr. Murie has given
some striking instances with seals; every one knows how the
horns of stags become more and more branched, and the plumes
of some birds become more finely developed, as they grow older.
Professor Cope states that the teeth of certain lizards change much
in shap)e with advancing years; with crustaceans not only many
trivial, but some important parts assume a new character, as recorded
by Fritz Miiller, after maturity. In all such cases, — and many could
be given, — if the age for reproduction were retarded, the character
of the species, at least in its adult state, would be modified; nor is
it improbable that the previous and earlier stages of development
would in some cases be hurried through and finally lost. Whether
species have often or ever been modified through this comparatively
sudden mode of transition, I can form no opinion; but if this has
occurred, it is probable that the differences between the young and
the mature, and between the mature and the old, were primordially
acquired by graduated steps.

SPECIAL DIFFICULTIES OF THE THEORY OF NATURAL SELECTION

Although we must be extremely cautious in concluding that any
organ could not have been produced by successive, small transitional
gradations, yet undoubtedly serious cases of difficulty occur.

One of the most serious is that of neuter insects, which are often
differently constructed from either the males or fertile females;
but this case will be treated of in the next chapter. The electric
organs of fishes offer another case of special difficulty; for it is im-
possible to conceive by what steps these wondrous organs have
been produced. But this is not surprising, for we do not even
know of what use they are. In the Gymnotus and Torpedo they no
doubt serve as powerful means of defence, and perhaps for securing
prey; yet in the ray, as observed by Matteucci, an analogous organ
in the tail manifests but little electricity even when the animal is

DIFFICULTIES OF THE THEORY 189

greatly irritated; so little, that it can hardly be of any use for the
above purposes. Moreover, in the ray, besides the organ just referred
to, there is, as Dr. R. McDonnell has shown, another organ near the
head, not known to be electrical, but which appears to be the real
homologue of the electric battery in the torpedo. It is generally
admitted that there exists between these organs and ordinary muscle
a close analogy, in intimate structure, in the distribution of the
nerves, and in the manner in which they are acted on by various
reagents. It should, also, be especially observed that muscular con-
traction is accompanied by an electrical discharge; and, as Dr. Rad-
cliffe insists, "in the electrical apparatus of the torpedo during rest,
there would seem to be a charge in every respect like that which is
met with in muscle and nerve during rest, and the discharge of the
torpedo, instead of being peculiar, may be only another form of the
discharge which attends upon the action of muscle and motor
nerve." Beyond this we cannot at present go in the way of ex-
planation; but as we know so little about the uses of these organs,
and as we know nothing about the habits and structure of the pro-
genitors of the existing electric fishes, it would be extremely bold to
maintain that no serviceable transitions are possible by which these
organs might have been gradually develof>ed.

These organs appear at first to offer another and far more serious
difficulty; for they occur in about a dozen kinds of fish, of which
several are widely remote in their affinities. When the same organ
is found in several members of the same class, especially if in mem-
bers having very different habits of life, we may generally attribute
its presence to inheritance from a common ancestor; and its absence
in some of the members to loss through disuse or natural selection.
So that, if the electric organs had been inherited from some one
ancient progenitor, we might have exjiected that all electric fishes
would have been specially related to each other; but this is far
from the case. Nor does geology at all lead to the belief that most
fishes formerly possessed electric organs, which their modified de-
scendants have now lost. But when we look at the subject more
closely, we find in the several fishes provided with electric organs,
that those are situated in different parts of the body, — that they
differ in construction, as in the arrangement of the plates, and,

190 ORIGIN OF SPECIES

according to Pacini, in the process or means by which the elearicity
is excited — and lastly, in being supplied with nerves proceeding
from different sources, and this is perhaps the most important of
all the differences. Hence in the several fishes furnished with
electric organs, these cannot be considered as homologous, but only
as analogous in function. Consequently there is no reason to sup-
pose that they have been inherited from a common progenitor;
for had this been the case they would have closely resembled each
other in all respects. Thus the difficulty of an organ, apparently
the same, arising in several remotely allied species, disappears, leav-
ing only the lesser yet still great difficulty; namely, by what gradu-
ated steps these organs have been developed in each separate group
of fishes.

The luminous organs which occur in a few insects, belonging to
widely different families, and which are situated in different parts
of the body, offer, under our present state of ignorance, a difficulty
almost exactly parallel with that of the electric organs. Other similar
cases could be given; for instance in plants, the very curious con-
trivance of a mass of pollen-grains, borne on a foot-stalk with an
adhesive gland, is apparently the same in Orchis and Asclepias, —
genera almost as remote as is possible amongst flowering plants;
but here again the parts are not homologous. In all cases of beings,
far removed from each other in the scale of organisation, which
are furnished with similar and peculiar organs, it will be found that
although the general appearance and function of the organs may
be the same, yet fundamental differences between them can always
be detected. For instance, the eyes of cephalopods or cuttle-fish and
of vertebrate animals appear wonderfully alike; and in such widely
sundered groups, no part of this resemblance can be due to in-
heritance from a common progenitor. Mr. Mivart has advanced
this case as one of sp)ecial difficulty, but I am unable to see the force
of his argument. An organ for vision must be formed of trans-
parent tissue, and must include some sort of lens for throwing an
image at the back of a darkened chamber. Beyond this superficial
resemblance, there is hardly any real similarity between the eyes
of cuttle-fish and vertebrates, as may be seen by consulting Hensen's
admirable memoir on these organs in the Cephalopoda. It is im-
possible for me here to enter on details, but I may specify a few

DIFFICULTIES OF THE THEORY I9I

of the points of difference. The crystalline lens in the higher cuttle-
fish consists of two parts, placed one behind the other like two
lenses, both having a very different structure and disposition to
what occurs in the vertebrata. The retina is wholly different, with
an actual inversion of the elemental parts, and with a large nervous
ganglion included within the membranes of the eye. The relations
of the muscles are as different as it is possible to conceive, and so in
other points. Hence it is not a little difficult to decide how far even
the same terms ought to be employed in describing the eyes of the
Cephalopoda and Vertebrata. It is, of course, open to any one to
deny that the eye in either case could have been developed through
the natural selection of successive slight variations; but if this be
admitted in the one case, it is clearly possible in the other; and
fundamental differences of structure in the visual organs of two
groups might have been anticipated, in accordance with this view
of their manner of formation. As two men have sometimes inde-
pendently hit on the same invention, so in the several foregoing
cases it appears that natural selection, working for the good of each
being, and taking advantage of all favourable variations, has pro-
duced similar organs, as far as function is concerned, in distinct
organic beings, which owe none of their structure in common to
inheritance from a common progenitor.

Fritz Miiller, in order to test the conclusions arrived at in this
volume, has followed out with much care a nearly similar line of
argument. Several families of crustaceans include a few species,
possessing an air-breathing apparatus and fitted to live out of the
water. In two of these families, which were more especially examined
by Miiller, and which are nearly related to each other, the species
agree most closely in all important characters; namely in their
sense organs, circulating system, in the position of the tufts of hair
within their complex stomachs, and lastly in the whole structure of
the water-breathing branchiae, even to the microscopical hooks by
which they are cleansed. Hence it might have been expected that in
the few species belonging to both families which live on the land,
the equally-impKjrtant air-breathing apparatus would have been the
same; for why should this one apparatus, given for the same purpose,
have been made to differ, whilst all the other important organs were
closely similar, or rather, identical.

192 ORIGIN OF SPECIES

Fritz Miiller argues that this close similarity in so many points
of structure must, in accordance with the views advanced by me,
be accounted for by inheritance from a common progenitor. But as
that vast majority of the sf^ecies in the above two families, as well as
most other crustaceans, are aquatic in their habits, it is improbable in
the highest degree that their common progenitor should have been
adapted for breathing air. Miiller was thus led carefully to examine
the apparatus in the air-breathing sf)ecies; and he found it to differ
in each in several important points, as in the position of the orifices,
in the manner in which they are opened and closed, and in some
accessory details. Now such differences are intelligible, and might
even have been expected, on the supposition that species belonging
to distinct families had slowly become adapted to live more and
more out of water, and to breathe the air. For these species, from
belonging to distinct families, would have differed to a certain
extent, and in accordance with the principle that the nature of each
variation depends on two factors, viz., the nature of the organism
and that of the surrounding conditions, their variability assuredly
would not have been exactly the same. Consequently natural selec-
tion would have had different materials or variations to work on,
in order to arrive at the same functional result; and the structures
thus acquired would almost necessarily have differed. On the
hypothesis of separate acts of creation the whole case remains unin-
telligible. This line of argunxent seems to have had great weight
in leading Fritz Miiller to accept the views maintained by me in this
volume.

Another distinguished zoologist, the late Professor Claparede,
has argued in the same manner, and has arrived at the same result.
He shows that there are parasitic mites (Acaridjc), belonging to
distinct sub-families and families, which are furnished with hair-
claspers. These organs must have been independently developed,
as they could not have been inherited from a common progenitor;
and in the several groups they are formed by the modification of
the fore-legs, — of the hind-legs, — of the maxillae or lips, — and of
appendages on the under side of the hind part of the body.

In the foregoing cases, we see the same end gained and the same
function performed, in beings not at all or only remotely allied,

DIFFICULTIES OF THE THEORY 1 93

by organs in appearance, though not in development, closely similar.
On the other hand, it is a common rule throughout nature that
the same end should be gained, even sometimes in the case of closely-
related beings, by the most diversified means. How differently con-
structed is the feathered wing of a bird and the membrane-covered
wing of a bat; and still more so the four wings of a butterfly, the
two wings of a fly, and the two wings with the elytra of a beetle.
Bivalve shells are made to open and shut, but on what a number of
patterns is the hinge constructed, — from the long row of neatly
interlocking teeth in a Nucula to the simple ligament of a Mussel!
Seeds are disseminated by their minuteness, — by their capsule being
converted into a light balloon-like envelope, — by being embedded
in pulp or flesh, formed of the most diverse parts, and rendered
nutritious, as well as conspicuously coloured, so as to attract and
be devoured by birds, — by having hooks and grapnels of many kinds
and serrated awns, so as to adhere to the fur of quadrupeds, — and
by being furnished with wings and plumes, as different in shape as
they are elegant in structure, so as to be wafted by every breeze.
I will give one other instance; for this subject of the same end being
gained by the most diversified means well deserves attention. Some
authors maintain that organic beings have been formed in many
ways for the sake of mere variety, almost like toys in a shop, biK
such a view of nature is incredible. With plants having separated
sexes, and with those in which, though hermaphrodites, the pollen
does not spontaneously fall on the stigma, some aid is necessary for
their fertilisation. With several kinds this is effected by the pollen-
grains, which are light and incoherent, being blown by the wind
through mere chance on to the stigma; and this is the simplest plan
which can well be conceived. An almost equally simple, though
very different, plan occurs in many plants in which a symmetrical
flower secretes a few drops of nectar, and is consequently visited by
insects; and these carry the pollen from the anthers to the stigma.

From this simple stage we may pass through an inexhaustible
nimiber of contrivances, all for the same purpose and effected in
essentially the same manner, birt entailing changes in every part of
the flower. The nectar may be stored in variously shaped receptacles,
with the stamens and pistils modified in many ways, sometimes
forming trap-like contrivances, and sometimes capable of neady

194 ORIGIN OF SPECIES

adapted movements through irritability or elasticity. From such
structures we may advance till we come to such a case of extraor-
dinary adaptions as that lately described by Dr. Criiger in the Qjry-
anthcs. This orchid has part of its labellum or lower lip hollowed
out into a great bucket, into which drops of almost pure water con-
tinually fall from two secreting horns which stand above it; and
when the bucket is half full, the water overflows by a spKJut on one
side. The basal part of the labellum stands over the bucket, and is
itself hollowed out into a sort of chamber with two lateral en-
trances; within this chamber there are curious fleshy ridges. The
most ingenious man, if he had not witnessed what takes place, could
never have imagined what purpose all these parts serve. But Dr.
Criiger saw crowds of large humble-bees visiting the gigantic flowers
of this orchid, not in order to suck nectar, but to gnaw ofl the ridges
within the chamber above the bucket; in doing this they frequently
pushed each other into the bucket, and their wings being thus
wetted they could not fly away, but were compelled to crawl out
through the passage formed by the spout or overflow. Dr. Criiger
saw a "continual procession" of bees thus crawling out of their
involuntary bath. The passage is narrow, and is roofed over by the
column, so that a bee, in forcing its way out, first rubs its back against
the viscid stigma and then against the viscid glands of the pollen-
masses. The pollen-masses are thus glued to the back of the bee which
first happens to crawl out through the passage of a lately expanded
flower, and are thus carried away. Dr. Criiger sent me a flower
in spirits of wine, with a bee which he had killed before it had quite
crawled out, with a prollen-mass still fastened to its back. When the
bee, thus provided, flies to another flower, or to the same flower a
second time, and is pushed by its comrades into the bucket and then
crawls out by the passage, the pollen-mass necessarily comes first into
contact with the viscid stigma, and adheres to it, and the flower is
fertilised. Now at last we see the full use of every part of the flower,
of the water-secreting horns, of the bucket half full of water, which
prevents the bees from flying away, and forces them to crawl out
through the sp)out, and rub against the properly placed viscid f)ollen-
masses and the viscid stigma.
The construction of the flower in another closely allied orchid,

DIFFICULTIES OF THE THEORY I95

namely the Catasetum, is widely different, though serving the same
end; and is equally curious. Bees visit these flowers, like those o£
the Coryanthes, in order to gnaw the labellum; in doing this they
inevitably touch a long, tapering, sensitive projection, or, as I have
called it, the antenna. This antenna, when touched, transmits a
sensation or vibration to a certain membrane which is instantly rup-
tured; this sets free a spring by which the pollen-mass is shot forth,
like an arrow, in the right direction, and adheres by its viscid ex-
tremity to the back of the bee. The pollen-mass of the male plant
(for the sexes are separate in this orchid) is thus carried to the
flower of the female plant, where it is brought into contact with the
stigma, which is viscid enough to break certain elastic threads, and
retain the pwllen, thus effecting fertilisation.

How, it may be asked, in the foregoing and in innumerable
other instances, can we understand the graduated scale of complex-
ity and the multifarious means for gaining the same end. The
answer no doubt is, as already remarked, that when two forms vary,
which already differ from each other in some slight degree, the
variability will not be of the same exact nature, and consequendy
the results obtained through natural selection for the same general
purpose will not be the same. We should also bear in mind that
every highly developed organism has passed through many changes;
and that each modified structure tends to be inherited, so that each
modification will not readily be quite lost, but may be again and
again further altered. Hence the structure of each part of each
species, for whatever purpose it may serve, is the sum of many in-
herited changes, through which the species has passed during its
successive adaptations to changed habits and conditions of Ufe.

Finally then, although in many cases it is most difficult even to
conjecture by what transitions organs have arrived at their present
state; yet, considering how small the proportion of living and
known forms is to the extinct and unknown, I have been astonished
how rarely an organ can be named, towards which no transitional
grade is known to lead. It certainly is true, that new organs appear-
ing as if created for some sf)ecial purpose, rarely or never appear
in any being; — as indeed is shown by that old, but somewhat exag-
gerated, canon in natural history of "Natura non facit saltum."

196 ORIGIN OF SPECIES

We meet with this admission in the writings of almost every ex-
perienced naturalist; or as Milne Edwards has well expressed it,
"Nature is prodigal in variety, but niggard in innovation." Why, on
the theory of Creation, should there be so much variety and so little
real novelty? Why should all the parts and organs of many inde-
pendent beings, each supposed to have been separately created for
its proper place in nature, be so commonly linked together by gradu-
ated steps? Why should not Nature take a sudden leap from struc-
ture to structure? On the theory of natural selection, we can clearly
understand why she should not; for natural selection acts only by
taking advantage of slight successive variations; she can never
take a great and sudden leap, but must advance by short and sure,
though slow steps.

ORGANS OF LITTLE APPARENT IMPORTANCE, AS AFFECTED
BY NATURAL SELECTION

As natural selection acts by life and death, — by the survival of
the fittest, and by the destruction of the less well-fitted individuals,
— I have sometimes felt great difficulty in understanding the origin
or formation of parts of little importance; almost as great, though
of a very different kind, as in the case of the most perfect and com-
plex organs.

In the first place, we are much too ignorant in regard to the whole
economy of any one organic being, to say what slight modifications
would be of importance or not. In a former chapter I have given
instances of very trifling characters, such as the down on fruit and
the colour of its flesh, the colour of the skin and hair of quadrupeds,
which, from being correlated with constitutional differences or
from determining the attacks of insects, might assuredly be acted
on by natural selection. The tail of the giraffe looks like an arti-
ficially constructed fly-flapper; and it seems at first incredible that
this could have been adapted for its present purpxjse by successive
slight modifications, each better and better fitted, for so trifling an
object as to drive away flies; yet we should pause before being too
positive even in this case, for we know that the distribution and ex-
istence of cattle and other animals in South America absolutely de-
pend on their power of resisting the attacks of insects: so that indi-

ORGANS AFFECTED 197

viduals which could by any means defend themselves from these
small enemies, would be able to range into new pastures and thus
gain a great advantage. It is not that the larger quadrupeds are
actually destroyed (except in some rare cases) by flies, but they are
incessantly harassed and their strength reduced, so that they are
more subject to disease, or not so well enabled in a coming dearth
to search for food, or to escape from beasts of prey.

Organs now of trifling importance have probably in some cases
been of high importance to an early progenitor, and, after having
been slowly perfected at a former period, have been transmitted to
existing species in nearly the same date, although now of very slight
use; but any actually injurious deviations in their structure would
of course have been checked by natural selection. Seeing how im-
portant an organ of locomotion the tail is in most aquatic animals,
its general presence and use for many purposes in so many land
animals, which in their lungs or modified swim bladders betray their
aquatic origin, may perhaps be thus accounted for. A well-developed
tail having been formed in an aquatic animal, it might subsequently
come to be worked in for all sorts of purposes, — as a fly-flapper, an
organ of prehension, or as an aid in turning, as in the case of the
dog, though the aid in this latter respect must be slight, for the hare,
with hardly any tail, can double still more quickly.

In the second place, we may easily err in attributing importance
to characters, and in believing that they have been developed through
natural selection. We must by no means overlook the effects of the
definite action of changed conditions of life, of so<alled spontaneous
variations, which seem to depend in a quite subordinate degree on
the nature of the conditions, of the tendency to reversion to long-lost
characters, of the complex laws of growth, such as of correlation,
compensation, of the pressure of one part on another, etc., and finally
of sexual selection, by which characters of use to one sex are often
gained and then transmitted more or less perfectly to the other sex,
though of no use to this sex. But structures thus indirectly gained,
although at first of no advantage to a species, may subsequently
have been taken advantage of by its modified descendants, under
new conditions of life and newly acquired habits.

If green woodpeckers alone had existed, and we did not know

198 ORIGIN OF SPECIES

that there were many black and pied kinds, I dare say that we should
have thought that the green colour was a beautiful adaptation to
conceal this tree-frequenting bird from its enemies; and conse-
quently that it was a character of importance, and had been ac-
quired through natural selection; as it is, the colour is probably in
chief part due to sexual selection. A trailing palm in the Malay
Archipelago climbs the loftiest trees by the aid of exquisitely con-
structed hooks clustered around the ends of the branches, and this
contrivance, no doubt, is of the highest service to the plant; but as
we see nearly similar hooks on many trees which are not climbers,
and which, as there is reason to believe from the distribution of the
thorn-bearing species in Africa and South America, serve as a defence
against browsing quadrupeds, so the spikes on the palm may at
first have been developed for this object, and subsequently have been
improved and taken advantage of by the plant, as it underwent
further modification and became a climber. The naked skin on the
head of a vulture is generally considered as a direct adaptation for
wallowing in putridity; and so it may be, or it may possibly be due
to the direct action of putrid matter; but we should be very cautious
in drawing any such inference, when we see that the skin on the
head of the clean-feeding male turkey is likewise naked. The sutures
in the skulls of young mammals have been advanced as a beautiful
adaptation for aiding parturition, and no doubt they facilitate, or
may be indispensable for this act; but as sutures occur in the skulls
of young birds and reptiles, which have only to escape from a
broken egg, we may infer that this structure has arisen from the
laws of growth, and has been taken advantage of in the parturition
of the higher animals.

We are profoundly ignorant of the cause of each slight variation
or individual difference; and we are immediately made conscious
of this by reflecting on the differences between the breeds of our
domesticated animals in different countries, more especially in the
less civilised countries where there has been but little methodical
selection. Animals kept by savages in different countries often have
to struggle for their own subsistence, and are exposed to a certain
extent to natural selection, and individuals with slightly different

UTILITARIAN DOCTRINE 1 99

constitutions would succeed best under different climates. With
catde susceptibility to the attacks of flies is correlated with colour,
as is the liability to be poisoned by certain plants; so that even colour
would be thus subjected to the action of natural selection. Some
observers are convinced that a damp climate affects the growth of
the hair, and that with the hair the horns are correlated. Mountain
breeds always differ from lowland breeds; and a mountainous
country would probably affect the hind limbs from exercising them
more, and possibly even the form of the pelvis; and then by the law
of homologous variation, the front limbs and the head would prob-
ably be affected. The shaf)e, also, of the pelvis might affect by
pressure the shape of certain parts of the young in the womb. The
laborious breathing necessary in high regions tends, as we have
good reason to believe, to increase the size of the chest; and again
correlation would come into play. The effects of lessened exercise
together with abundant food on the whole organisation is probably
still more important; and this, as H. von Nathusius has lately shown
in his excellent treatise, is apparently one chief cause of the great
modification which the breed of swine have undergone. But we are
far too ignorant to sjjeculate on the relative imf)ortance of the several
known and unknown causes of variation; and I have made these
remarks only to show that, if we are unable to account for the
characteristic differences of our several domestic breeds, which never-
theless are generally admitted to have arisen through ordinary
generation from one or a few parent-stocks, we ought not to lay
too much stress on our ignorance of the precise cause of the slight
analogous differences between true species.

UTILITARIAN DOCTRINE, HOW FAR TRUE: BEAUTY, HOW
ACQUIRED

The foregoing remarks lead me to say a few words on the protest
lately made by some naturalists, against the utilitarian doctrine that
every detail of structure has been produced for the good of its
possessor. They believe that many structures have been created
for the sake of beauty, to delight man or the Creator (but this latter
point is beyond the scope of scientific discussion), or for the sake

200 ORIGIN OF SPECIES

of mere variety, a view already discussed. Such doctrines, if true,
would be absolutely fatal to my theory. I fully admit that many
structures are now of no direct use to their possessors, and may
never have been of any use to their progenitors; but this does not
prove that they were formed solely for beauty or variety. No doubt
the definite action of changed conditions, and the various causes of
modifications, lately sjjecified, have all produced an effect, probably
a great effect, independently of any advantage thus gained. But a
still more important consideration is that the chief part of the organ-
isation of every living creature is due to inheritance; and conse-
quently, though each being assuredly is well fitted for its place in
nature, many structures have now no very close and direct relation
to present habits of life. Thus, we can hardly believe that the webbed
feet of the upland goose or of the frigate-bird are of special use to
these birds; we cannot believe that the similar bones in the arm of
the monkey, in the fore-leg of the horse, in the wing of the bat, and
in the flipper of the seal, are of special use to these animals. We may
safely attribute these structures to inheritance. But webbed feet
no doubt were as useful to the progenitor of the upland goose and of
the frigate-bird, as they now are to the most aquatic of living birds.
So we may believe that the progenitor of the seal did not possess
a flipper, but a foot with five toes fitted for walking or grasping; and
we may further venture to believe that the several bones in the limbs
of the monkey, horse, and bat, were originally developed, on the
principle of utiUty, probably through the reduction of more numer-
ous bones in the fin of some ancient fish-like progenitor of the whole
class. It is scarcely possible to decide how much allowance ought
to be made for such causes of change, as the definite action of
external conditions, scxalled spontaneous variations, and the com-
plex laws of growth; but with these important exceptions, we
may conclude that the structure of every Uving creature either
now is, or was formerly, of some direct or indirect use to its pos-
sessor.

With respect to the belief that organic beings have been created
beautiful for the delight of man, — a belief which it has been pro-
nounced is subversive of my whole theory, — I may first remark that
the sense of beauty obviously depends on the nature of the mind.

UTILITARIAN DOCTRINE 201

irrespective of any real quality in the admired object; and that the
idea of what is beautiful, is not innate or unalterable. We see this,
for instance, in the men of different races admiring an entirely dif-
ferent standard of beauty in their women. If beautiful objects had
been created solely for man's gratification, it ought to be shown
that before man appeared, there was less beauty on the face of the
earth than since he came on the stage. Were the beautiful volute
and cone shells of the Eocene epoch, and the gracefully sculptured
ammonites of the Secondary period, created that man might ages
afterwards admire them in his cabinet? Few objects are more beau-
tiful than the minute siliceous cases of the diatomaceae: were these
created that they might be examined and admired under the higher
powers of the microscope? The beauty in this latter case, and in
many others, is apparently wholly due to symmetry of growth.
Flowers rank amongst the most beautiful productions of nature;
but they have been rendered conspicuous in contact with the green
leaves, and in consequence at the same time beautiful, so that they
may be easily observed by insects. I have come to this conclusion
from finding it an invariable rule that when a flower is fertiUsed
by the wind it never has a gaily<oloured corolla. Several plants
habitually produce two kinds of flowers; one kind open and col-
oured so as to attract insects; the other closed, not coloured, destitute
of nectar, and never visited by insects. Hence we may conclude
that, if insects had not been developed on the face of the earth, our
plants would not have been decked with beautiful flowers, but
would have produced only such poor flowers as we see on our fir,
oak, nut and ash trees, on grasses, spinach, docks, and nettles, which
are all fertilised through the agency of the wind. A similar line of
argument holds good with fruits; that a ripe strawberry or cherry
is as pleasing to the eye as to the palate — that the gaily<oloured
fruit of the spindle-wood tree and the scarlet berries of the holly are
beautiful objects, — will be admitted by every one. But this beauty
serves merely as a guide to birds and beasts, in order that the fruit
may be devoured and the matured seeds disseminated: I infer that
this is the case from having as yet found no exception to the rule
that seeds are always thus disseminated when embedded within a
fruit of any kind (that is within a fleshy or pulpy envelope), if it

202 ORIGIN OF SPECIES

be coloured of any brilliant tint, or rendered conspicuous by being
white or black.

On the other hand, I wiUingly admit that a great number of male
animals, as all our most gorgeous birds, some fishes, reptiles, and
mammals, and a host of magnificently coloured butterflies, have
been rendered beautiful for beauty's sake; but this has been effected
through sexual selection, that is, by the more beautiful males having
been continually preferred by the females, and not for the delight of
man. So it is with the music of birds. We may infer from all this
that a nearly similar taste for beautiful colours and for musical sounds
runs through a large part of the animal kingdom. When the female
is as beautifully coloured as the male, which is not rarely the case
with birds and butterflies, the cause apparently lies in the colours
acquired through sexual selection having been transmitted to both
sexes, instead of to the males alone. How the sense of beauty in its
simplest form — that is, the reception of a peculiar kind of pleasure
from certain colours, forms, and sounds — was first developed in the
mind of man and of the lower animals, is a very obscure subject.
The same sort of difficulty is presented, if we enquire how it is
that certain flavours and odours give pleasure, and others displeasure.
Habit in all these cases appears to have come to a certain extent
into play; but there must be some fundamental cause in the consti-
tution of the nervous system in each species.

Natural selection cannot possibly produce any modification in a
species exclusively for the good of another species; though through-
out nature one species incessantly takes advantage of, and profits
by, the structures of others. But natural selection can and does often
produce structures for the direct injury of other animals, as we
see in the fang of the adder, and in the ovif)ositor of the ichneumon,
by which its eggs are deposited in the Uving bodies of other insects.
If it could be proved that any part of the structure of any one species
had been formed for the exclusive good of another species, it would
annihilate my theory, for such could not have been produced through
natural selection. Although many statements may be found in
works on natural history to this effect, I cannot find even one which
seems to me of any weight. It is admitted that the rattlesnake has a
poison-fang for its own defence, and for the destruction of its prey;

UTILITARIAN DOCTRINE 2O3

but some authors suppose that at the same time it is furnished with
a rattle for its own injury, namely, to warn its prey. I would almost
as soon believe that the cat curls the end of its tail when preparing
to spring, in order to warn the doomed mouse. It is a much more
probable view that the rattlesnake uses its rattle, the cobra expands
its frill, and the puff-adder swells whilst hissing so loudly and
harshly, in order to alarm the many birds and beasts which are
known to attack even the most venomous species. Snakes act on
the same principle which makes the hen ruffle her feathers and
expand her wings when a dog approaches her chickens. But I have
not space here to enlarge on the many ways by which animals en-
deavour to frighten away their enemies.

Natural selection will never produce in a being any structure
more injurious than beneficial to that being, for natural selection
acts solely by and for the good of each. No organ will be formed,
as Paley has remarked, for the purpose of causing pain or for
doing an injury to its possessor. If a fair balance be struck
between the good and evil caused by each part, each will be found
on the whole advantageous. After the lapse of time, under changing
conditions of life, if any part comes to be injurious, it will be modi-
fied; or if it be not so, the being will become extinct as myriads
have become extinct.

Natural selection tends only to make each organic being as per-
fect as, or slightly more perfect than, the other inhabitants of the
same country with which it comes into competition. And we see
that this is the standard of perfection attained under nature. The en-
demic productions of New Zealand, for instance, are perfect, one
compared with another; but they are now rapidly yielding before
the advancing legions of plants and animals introduced from Eurojje.
Natural selection will not produce absolute perfection, nor do we
always meet, as far as we can judge, with this high standard under
nature. The correction for the aberration of light is said by Miiller
not to be perfect even in that most perfect organ, the human eye.
Helmholtz, whose judgment no one will dispute, after describing
in the strongest terms the wonderful power of the human eye, adds
these remarkable words: "That which we have discovered in the
way of inexactness and imperfection in the optical machine and in

204 ORIGIN OF SPECIES

the image on the retina, is as nothing in comparison with the incon-
gruities which we have just come across in the domain of the sen-
sations. One might say that nature has taken delight in accumulat-
ing contradictions in order to remove all foundations from the theory
of a pre-existing harmony between the external and internal worlds."
If our reason leads us to admire with enthusiasm a multitude of in-
imitable contrivances in nature, this same reason tells us, though we
may easily err on both sides, that some other contrivances are less
perfect. Can we consider the sting of the bee as perfect, which,
when used against many kinds of enemies, cannot be withdrawn,
owing to the backward serratures, and thus inevitably causes the
death of the insect by tearing out its viscera?

If we look at the sting of the bee, as having existed in a remote
progenitor, as a boring and serrated instrument, like that in so
many members of the same great order, and that it has since been
modified, but not perfected for its present purpose, with the poison
originally adapted for some other object, such as to produce galls,
since intensified, we can perhaps understand how it is that the use
of the sting should so often cause the insect's own death : for if on
the whole the power of stinging be useful to the social community,
it will fulfil all the requirements of natural selection, though it may
cause the death of some few members. If we admire the truly
wonderful power of scent by which the males of many insects find
their females, can we admire the production for this single purpose
of thousands of drones, which are utterly useless to the community
for any other purpose, and which are ultimately slaughtered by
their industrious and sterile sisters? It may be difficult, but we ought
to admire the savage instinctive hatred of the queen-bee, which urges
her to destroy the young queens, her daughters, as soon as they are
born, or to perish herself in the combat; for undoubtedly this is for
the good of the community; and maternal love or maternal hatred,
though the latter fortunately is most rare, is all the same to the in-
exorable principles of natural selection. If we admire the several
ingenious contrivances by which orchids and many other plants are
fertilised through insect agency, can we consider as equally perfect
the elaboration of dense clouds of pollen by our fir-trees, so that a
few granules may be wafted by chance on to the ovules ?

SUMMARY 205

summary: the law oh unity of type and of the conditions of
existence embraced by the theory of natural selection

We have in this chapter discussed some of the difficuhies and ob-
jections which may be urged against the theory. Many of them are
serious; but I think that in the discussion light has been thrown on
several facts, which on the belief of indep)endent acts of creation
are utterly obscure. We have seen that species at any one period are
not indefinitely variable, and are not linked together by a multitude
of intermediate gradations, partly because the process of natural
selection is always very slow, and at any one time acts only on a
few forms; and partly because the very process of natural selection
implies the continual supplanting and extinction of preceding and
intermediate gradations. Closely allied species, now living on a con-
tinuous area, must often have been formed when the area was not
continuous, and when the conditions of life did not insensibly grad-
uate away from one part to another. When two varieties are formed
in two districts of a continuous area, an intermediate variety will
often be formed, fitted for an intermediate zone; but from reasons
assigned, the intermediate variety will usually exist in lesser num-
bers than the two forms which it connects; consequently the two
latter, during the course of further modification, from existing in
greater numbers, will have a great advantage over the less numerous
intermediate variety, and will thus generally succeed in supplanting
and exterminating it.

We have seen in this chapter how cautious we should be in con-
cluding that the most different habits of life could not graduate into
each other; that a bat, for instance, could not have been formed by
natural selection from an animal which at first only glided through
the air.

We have seen that a species under new conditions of life may
change its habits; or it may have diversified habits, with some very
unlike those of its nearest congeners. Hence we can understand,
bearing in mind that each organic being is trying to live wherever
it can live, how it has arisen that there are upland geese with webbed
feet, ground woodpeckers, diving thrushes, and petrels with the
habits of auks.

206 ORIGIN OF SPECIES

Although the belief that an organ so perfect as the eye could have
been formed by natural selection, is enough to stagger any one; yet
in the case of any organ, if we know of a long series of gradations in
complexity, each good for its possessor, then, under changing condi-
tions of life, there is no logical impossibility in the acquirement of
any conceivable degree of jjerfection through natural selection. In
the cases in which we know of no intermediate or transitional states,
we should be extremely cautious in concluding that none can have
existed, for the metamorphoses of many organs show what wonder-
ful changes in function are at least possible. For instance, a swim
bladder has apparently been converted into an air-breathing lung.
The same organ having performed simultaneously very different
functions, and then having been in part or in whole specialised for
one function; and two distinct organs having performed at the
same time the same function, the one having been perfected whilst
aided by the other, must often have largely facilitated transitions.

We have seen that in two beings widely remote from each other
in the natural scale, organs serving for the same purpose and in
external appearance closely similar may have been separately and
independently formed; but when such organs are closely examined,
essential differences in their structure can almost always be detected;
and this naturally follows from the principle of natural selection.
On the other hand, the common rule throughout nature is infinite
diversity of structure for gaining the same end; and this again
naturally follows from the same great principle.

In many cases we are far too ignorant to be enabled to assert that
a part or organ is so unimportant for the welfare of a species, that
modifications in its structure could not have been slowly accumu-
lated by means of natural selection. In many other cases, modifica-
tions are probably the direct result of the laws of variation or of
growth, independently of any good having been thus gained. But
even such structures have often, as we may feel assured, been sub-
sequently taken advantage of, and still further modified, for the
good of sfjecies under new conditions of life. We may, also, believe
that a part formerly of high importance has frequently been retained
(as the tail of an aquatic animal by its terrestrial descendants),

SUMMARY 207

though it has become of such small importance that it could not,
in its present state, have been acquired by means of natural selec-
tion.

Natural selection can produce nothing in one species for the ex-
clusive good or injury of another; though it may well produce
parts, organs, and excretions highly useful or even indispensable,
or again highly injurious to another species, but in all cases at the
same time useful to the possessor. In each well-stocked country
natural selection acts through the competition of the inhabitants,
and consequently leads to success in the battle for life, only in ac-
cordance with the standard of that particular country. Hence the
inhabitants of one country, generally the smaller one, often yield to
the inhabitants of another and generally the larger country. For in
the larger country there will have existed more individuals and more
diversified forms, and the competition will have been severer, and
thus the standard of perfection will have been rendered higher.
Natural selection will not necessarily lead to absolute perfection;
nor, as far as we can judge by our limited faculties, can absolute
perfection be everywhere predicated.

On the theory of natural selection we can clearly understand the
full meaning of that old canon in natural history, "Natura non facit
saltum." This canon, if we look to the present inhabitants alone of
the world, is not strictly correct; but if we include all those of past
times, whether known or unknown, it must on this theory be stricdy
true.

It is generally acknowledged that all organic beings have been
formed on two great laws — Unity of Type, and the Conditions of
Existence. By unity of type is meant that fundamental agreement in
structure which we see in organic beings of the same class, and which
is quite independent of their habits of life. On my theory, unity of
type is explained by unity of descent. The expression of conditions
of existence, so often insisted on by the illustrious Cuvier, is fully
embraced by the principle of natural selection. For natural selection
acts by either now adapting the varying parts of each being to its
organic and inorganic conditions of life; or by having adapted them
during past periods of time: the adaptations being aided in many

2o8 ORIGIN OF SPECIES

cases by the increased use or disuse of parts, being affected by the
direct action of the external conditions of Hfe, and subjected in all
cases to the several laws of growth and variation. Hence, in fact, the
law of the Conditions of Existence is the higher law; as it includes,
through the inheritance of former variations and adaptations, that
of Unity of Type.

CHAPTER VII

Miscellaneous Objections to the Theory of Natural Selection

Longevity — Modifications not necessarily simultaneous — Modifications
apparently of no direct service — Progressive developnient — Charac-
ters of small functional importance, the most constant — Supposed
incompetence of natural selection to account for the incipient stages
of useful structures — Causes which interfere with the acquisition
through natural selection of useful structures — Gradations of struc-
ture with changed functions — Widely different organs in members
of the same class, developed from one and the same source — Reasons
for disbelieving in great and abrupt modifications.

I WILL devote this chapter to the consideration of various mis-
cellaneous objections which have been advanced against my
views, as some of the previous discussions may thus be made
clearer; but it would be useless to discuss all of them, as many have
been made by writers who have not taken the trouble to understand
the subject. Thus a distinguished German naturalist has asserted
that the weakest part of my theory is, that I consider all organic
beings as imperfect: what I have really said is, that all are not as
{perfect as they might have been in relation to their conditions; and
this is shown to be the case by so many native forms in many
quarters of the world having yielded their places to intruding
foreigners. Nor can organic beings, even if they were at any one
time perfectly adapted to their conditions of life, have remained so,
when their conditions changed, unless they themselves likewise
changed; and no one will dispute that the physical conditions of
each country, as well as the numbers and kinds of its inhabitants,
have undergone many mutations.

A critic has lately insisted, with some parade of mathematical
accuracy, that longevity is a great advantage to all species, so that
he who believes in natural selection "must arrange his genealogical
tree" in such a manner that all the descendants have longer lives than
their progenitors! Cannot our critic conceive that a biennial plant
or one of the lower animals might range into a cold climate and

209

210 ORIGIN OF SPECIES

perish there every winter; and yet, owing to advantages gained
through natural selection, survive from year to year, by means of its
seeds or ova? Mr. E. Ray Lankester has recently discussed this
subject, and he concludes, as far as its extreme complexity allows him
to form a judgment, that longevity is generally related to the stand-
ard of each species in the scale of organisation, as well as to the
amount of expenditure in reproduction and in general activity. And
these conditions have, it is probable, been largely determined through
natural selection.

It has been argued that, as none of the animals and plants of
Egypt, of which we know anything, have changed during the last
three or four thousand years, so probably have none in any part of
the world. But, as Mr. G. H. Lewes has remarked, this line of
argument proves too much, for the ancient domestic races figured on
the Egyptian monuments, or embalmed, are closely similar or even
identical with those now living; yet all naturalists admit that such
races have been produced through the modification of their original
types. The many animals which have remained unchanged since
the commencement of the glacial period, would have been an
incomparably stronger case, for these have been exposed to great
changes of climate and have migrated over great distances; whereas,
in Egypt, during the last several thousand years, the conditions of
life, as far as we know, have remained absolutely uniform. The fact
of little or no modification having been effected since the glacial
period would have been of some avail against those who believe in
an innate and necessary law of development, but is powerless against
the doctrine of natural selection or the survival of the fittest, which
implies that when variations or individual differences of a beneficial
nature happen to arise, these will be preserved; but this will be
effected only under certain favourable circumstances.

The celebrated palarontologist, Bronn, at the close of his German
translation of this work, asks, how, on the principle of natural
selection, can a variety live side by side with the parent species? If
both have become fitted for slightly different habits of life or con-
ditions, they might live together; and if we lay on one side poly-
morphic species, in which the variability seems to be of a peculiar
nature, and all mere temporary variations, such as size, albinism,

THEORY OF NATURAL SELECTION 211

etc, the more permanent varieties are generally found, as far as I
can discover, inhabiting distinct stations, — such as high land or low
land, dry or moist districts. Moreover, in the case of animals which
wander much about and cross freely, their varieties seem to be
generally confined to distinct regions.

Bronn also insists that distinct species never differ from each other
in single characters, but in many parts; and he asks, how it always
comes that many parts of the organisation should have been modified
at the same time through variation and natural selection? But there
is no necessity for supposing that all the parts of any being have
been simultaneously modified. The most striking modifications,
excellently adapted for some purpose, might, as was formerly
remarked, be acquired by successive variations, if slight, first in one
part and then in another; and as they would be transmitted all
together, they would appear to us as if they had been simultaneously
developed. The best answer, however, to the above objection is
afforded by those domestic races which have been modified, chiefly
through man's power of selection, for some special purpose. Look at
the race and dray horse, or at the grey-hound and mastiff. Their
whole frames and even their mental characteristics have been modi-
fied; but if we could trace each step in the history of their trans-
formation, — and the latter steps can be traced, — we should not see
great and simultaneous changes, but first one part and then another
slightly modified and improved. Even when selection has been
applied by man to some one character alone, — of which our culti-
vated plants offer the best instances, — it will invariably be found that
although this one part, whether it be the flower, fruit, or leaves, has
been greatly changed, almost all the other parts have been slightly
modified. This may be attributed partly to the principle of correlated
growth, and partly to so<alled spontaneous variation.

A much more serious objection has been urged by Bronn, and
recently by Broca, namely, that many characters appear to be of no
service whatever to their possessors, and therefore cannot have been
influenced through natural selection. Bronn adduces the length of
the ears and tails in the different species of hares and mice, — the com-
plex folds of enamel in the teeth of many animals, and a multitude
of analogous cases. With respect to plants, this subject has been

212 ORIGIN OF SPECIES

discussed by Nageli in an admirable essay. He admits that natural
selection has effected much, but he insists that the families of plants
differ chiefly from each other in morphological characters, which
appear to be quite unimportant for the welfare of the species. He
consequently believes in an innate tendency towards progressive and
more perfect development. He specifies the arrangement of the cells
in the tissues, and of the leaves on the axis, as cases in which natural
selection could not have acted. To these may be added the numerical
divisions in the parts of the flower, the position of the ovules, the
shaf>e of the seed, when not of any use for dissemination, etc.

There is much force in the above objection. Nevertheless, we
ought, in the first place, to be extremely cautious in pretending to
decide what structures now are, or have formerly been, of use to
each species. In the second place, it should always be borne in mind
that when one part is modified, so will be other parts, through certain
dimly seen causes, such as an increased or diminished flow of nutri-
ment to a part, mutual pressure, an early developed part affecting one
subsequently developed, and so forth, — as well as through other
causes which lead to the many mysterious cases of correlation, which
we do not in the least understand. These agencies may be all grouped
together, for the sake of brevity, under the expression of the laws of
growth. In the third place, we have to allow for the direct and
definite action of changed conditions of life, and for so<alled spon-
taneous variations, in which the nature of the conditions apparently
plays a quite subordinate part. Bud variations, such as the appearance
of a moss-rose on a common rose, or of a nectarine on a peach-tree,
offer good instances of spontaneous variations; but even in these
cases, if we bear in mind the power of a minute drop of pxaison in
producing complex galls, we ought not to feel too sure that the
above variations are not the effect of some local change in the nature
of the sap, due to some change in the conditions. There must be
some efficient cause for each slight individual difference, as well as
for more strongly marked variations which occasionally arise; and
if the unknown cause were to act persistently, it is almost certain
that all the individuals of the species would be similarly modified.

In the earlier editions of this work I under-rated, as it now seems
probable, the frequency and importance of modifications due to

THEORY OF NATURAL SELECTION 213

spontaneous variability. But it is impossible to attribute to this cause
the innumerable structures which are so well adapted to the habits
of life of each species. I can no more believe in this, than that the
well-adapted form of a race-horse or greyhound, which before the
principle of selection by man was well understood, excited so much
surprise in the minds of the older naturalists, can thus be ex-
plained.

It may be worth while to illustrate some of the foregoing remarks.
With respect to the assumed inutility of various parts and organs, it
is hardly necessary to observe that even in the higher and best-known
animals many structures exist, which are so highly developed that
no one doubts that they are of importance, yet their use has not
been, or has only recently been, ascertained. As Bronn gives the
length of the ears and tail in the several species of mice as instances,
though trifling ones, of differences in structure which can be of no
special use, I may mention that, according to Dr. Schobl, the external
ears of the common mouse are supplied in an extraordinary manner
with nerves, so that they no doubt serve as tactile organs; hence the
length of the ears can hardly be quite unimportant. We shall, also,
presently see that the tail is a highly useful prehensile organ to
some of the species; and its use would be much influenced by its
length.

With respect to plants, to which on account of Niigeli's essay I
shall confine myself in the following remarks, it will be admitted
that the flowers of orchids present a multitude of curious structures,
which a few years ago would have been considered as mere morpho-
logical differences without any special function; but they are now
known to be of the highest importance for the fertilisation of the
sf)ecies through the aid of insects, and have probably been gained
through natural selection. No one until lately would have imagined
that in dimorphic and trimorphic plants the different lengths of the
stamens and pistils, and their arrangement, could have been of any
service, but now we know this to be the case.

In certain whole groups of plants the ovules stand erect, and in
others they are suspended; and within the same ovarium of some
few plants, one ovule holds the former and a second ovule the latter
position. These positions seem at first purely morphological, or of

214 ORIGIN OF SPECIES

no physiological signification; but Dr, Hooker informs me that
within the same ovarium, the upper ovules alone in some cases, and
in other cases the lower ones alone are fertilised; and he suggests
that this probably depends on the direction in which the pollen-tubes
enter the ovarium. If so, the position of the ovules, even when one
is erect and the other suspended within the same ovarium, would
follow from the selection of any slight deviations in position which
favoured their fertilisation, and the production of seed.

Several plants belonging to distinct orders habitually produce
flowers of two kinds, — the one open of the ordinary structure, the
other closed and imperfect. These two kinds of flowers sometimes
differ wonderfully in structure, yet may be seen to graduate into
each other on the same plant. The ordinary and open flowers can
be intercrossed; and the benefits which certainly are derived from
this process are thus secured. The closed and imperfect flowers are,
however, manifestly of high importance, as they yield with the
utmost safety a large stock of seed, with the expenditure of wonder-
fully little pollen. The two kinds of flowers often differ much, as
just stated, in structure. The petals in the imperfect flowers almost
always consist of mere rudiments, and the pollen-grains are reduced
in diameter. In Ononis columnse five of the alternate stamens are
rudimentary; and in some species of Viola three stamens are in this
state, two retaining their proper function, but being of very small
size. In six out of thirty of the closed flowers in an Indian violet
(name unknown, for the plants have never produced with me per-
fect flowers), the sepals are reduced from the normal number of
five to three. In one section of the Malpighiacex the closed flowers,
according to A. de Jussieu, are still further modified, for the five
stamens which stand opposite to the sepals are all aborted, a sixth
stamen standing opposite to a petal being alone developed; and this
stamen is not present in the ordinary flowers of these sjjecies; the
style is aborted; and the ovaria are reduced from three to two. Now
although natural selection may well have had the power to prevent
some of the flowers from expanding, and to reduce the amount of
pollen, when rendered by the closure of the flowers superfluous, yet
hardly any of the above special modifications can have been thus
determined, but must have followed from the laws of growth.

THEORY OF NATURAL SELECTION 21 5

including the functional inactivity of parts, during the progress of
the reduction of the pollen and the closure of the flowers.

It is so necessary to appreciate the impwrtant effects of the laws
of growth, that I will give some additional cases of another kind,
namely of differences in the same part or organ, due to differences
in relative position on the same plant. In the Spanish chestnut, and
in certain fir-trees, the angles of divergence of the leaves differ,
according to Schacht, in the nearly horizontal and in the upright
branches. In the common rue and some other plants, one flower,
usually the central or terminal one, opens first, and has five sepals
and petals, and five divisions to the ovarium; whilst all the other
flowers on the plant are tetramerous. In the British Adoxa the upper-
most flower generally has two calyx-lobes with the other organs
tetramerous, whilst the surrounding flowers generally have three
calyx-lobes with the other organs f)entamerous. In many Com-
posita: and Umbelliferx (and in some other plants) the circum-
ferential flowers have their corollas much more developed than those
of the centre; and this seems often connected with the abortion of
the reproductive organs. It is a more curious fact, previously referred
to, that the achenes or seeds of the circumference and centre some-
times differ greatly in form, colour, and other characters. In Cartha-
mus and some other Compositas the central achenes alone are fur-
nished with a pappus; and in Hyoseris the same head yields achenes
of three different forms. In certain Umbelliferac the exterior seeds,
according to Tausch, are orthospermous, and the central one coelo-
spermous, and this is a character which was considered by De
Candolle to be in other species of the highest systematic importance.
Prof. Braun mentions a Fumariaceous genus in which the flowers
in the lower part of the spike bear oval, ribbed, one-seeded nutlets;
and in the upper part of the spike, lanceolate, two-valved, and two-
seeded siliques. In these several cases, with the exception of that of
the well developed ray-florets, which are of service in making the
flowers conspicuous to insects, natural selection cannot, as far as we
can judge, have come into play, or only in a quite subordinate
manner. All these modifications follow from the relative position
and inter-action of the parts; and it can hardly be doubted that if
all the flowers and leaves on the same plant had been subjected to

2l6 ORIGIN OF SPECIES

the same external and internal condition, as are the flowers and
leaves in certain positions, all would have been modified in the
same manner.

In numerous other cases we find modifications of structure, which
are considered by botanists to be generally of a highly important
nature, affecting only some of the flowers on the same plant, or
occurring on distinct plants, which grow close together under the
same conditions. As these variations seem of no special use to the
plants, they cannot have been influenced by natural selection. Of
their cause we are quite ignorant; we cannot even attribute them,
as in the last class of cases, to any proximate agency, such as relative
position. I will give only a few instances. It is so common to observe
on the same plant, flowers indifferently tetramerous, pentamerous,
etc., that I need not give examples; but as numerical variations are
comparatively rare when the parts are few, I may mention that,
according to De Candolle, the flowers of Papaver bracteatum offer
either two sepals with four petals (which is the common type with
poppies), or three sepals with six petals. The manner in which the
petals are folded in the bud is, in most groups, a very constant
morphological character; but Professor Asa Gray states that with
some species of Mimulus, the estivation is almost as frequently that
of the Rhinanthideac as of the Antirrhinideae, to which latter tribe
the genus belongs. Aug. St. Hilaire gives the following cases: the
genus Zanthoxylon belongs to a division of the Rutacex with a single
ovary, but in some species flowers may be found on the same plant,
and even in the same panicle, with either one or two ovaries. In
Helianthemum the capsule has been described as unilocular or 3-
locular; and in H. mutabile, "Une lame, plus ou mains large,
s etend entre le pericarpe et le placenta." In the flowers of Saponaria
officinalis. Dr. Masters has observed instances of both marginal and
free central placentation. Lastly, St. Hilaire found towards the
southern extreme of the range of Gomphia olexformis two forms
which he did not at first doubt were distinct species, but he subse-
quently saw them growing on the same bush; and he then adds,
"Voila done dans un meme individu des loges et un style qui se
rattachent tantot a un axe verticale et tantot a un gynobase."

We thus see that with plants many morphological changes may be

THEORY OF NATURAL SELECTION 21 7

attributed to the laws of growth and the inter-action of parts, inde-
pendently of natural selection. But with respect to Niigeli's doctrine
of an innate tendency towards perfection or progressive develop-
ment, can it be said in the case of these strongly pronounced varia-
tions, that the plants have been caught in the act of progressing
towards a higher state of development ? On the contrary, I should
infer from the mere fact of the parts in question differing or varying
greatly on the same plant, that such modifications were of extremely
small importance to the plants themselves, of whatever importance
they may generally be to us for our classifications. The acquisition
of a useless part can hardly be said to raise an organism in the natural
scale; and in the case of the imperfect, closed flowers above described,
if any new principle has to be invoked, it must be one of retrogres-
sion rather than of progression; and so it must be with many
parasitic and degraded animals. We are ignorant of the exciting
cause of the above specified modifications; but if the unknown cause
were to act almost uniformly for a length of time, we may infer
that the result would be almost uniform; and in this case all
the individuals of the species would be modified in the same
manner.

From the fact of the above characters being unimportant for the
welfare of the species, any slight variations which occurred in them
would not have been accumulated and augmented through natural
selection. A structure which has been developed through long-
continued selection, when it ceases to be of service to a species,
generally becomes variable, as we see with rudimentary organs; for
it will no longer be regulated by this same power of selection. But
when, from the nature of the organism and of the conditions, modi-
fications have been induced which are unimportant for the welfare
of the species, they may be, and apparently often have been, trans-
mitted in nearly the same state to numerous, otherwise modified,
descendants. It cannot have been of much importance to the greater
number of mammals, birds, or reptiles, whether they were clothed
with hair, feathers, or scales; yet hair has been transmitted to almost
all mammals, feathers to all birds, and scales to all true reptiles. A
structure, whatever it may be, which is common to many allied
forms, is ranked by us as of high systematic importance, and con-

2l8 ORIGIN OF SPECIES

sequently is often assumed to be of high vital importance to the
species. Thus, as I am inclined to believe, morphological differences,
which we consider as important — such as the arrangement of the
leaves, the divisions of the flower or of the ovarium, the position of
the ovules, etc. — first appeared in many cases as fluctuating varia-
tions, which sooner or later became constant through the nature of
the organism and of the surrounding conditions, as well as through
the intercrossing of distinct individuals, but not through natural
selection; for as these morphological characters do not affect the
welfare of the species, any slight deviations in them could not have
been governed or accumulated through this latter agency. It is a
strange result which we thus arrive at, namely that characters of
slight vital importance to the species, are the most important to the
systematist; but, as we shall hereafter see when we treat of the genetic
principle of classification, this is by no means so paradoxical as it
may at first appjear.

Although we have no good evidence of the existence in organic
beings of an innate tendency towards progressive development, yet
this necessarily follows, as I have attempted to show in the fourth
chapter, through the continued action of natural selection. For the
best definition which has ever been given of a high standard of
organisation is the degree to which the parts have been speciaHsed
or differentiated; and natural selection tends towards this end, inas-
much as the parts are thus enabled to perform their functions more
efficiendy.

A distinguished zoologist, Mr. St. George Mivart, has recently
collected all the objections which have ever been advanced by myself
and others against the theory of natural selection, as propounded by
Mr. Wallace and myself, and has illustrated them with admirable
art and force. When thus marshalled, they make a formidable array;
and as it forms no part of Mr. Mivart's plan to give the various facts
and considerations opposed to his conclusions, no slight effort of
reason and memory is left to the reader, who may wish to weigh the
evidence on both sides. When discussing sp)ecial cases, Mr. Mivart
passes over the effects of the increased use and disuse of parts, which
I have always maintained to be highly important, and have treated

THEORY OF NATURAL SELECTION 219

in my 'Variation under Domestication' at greater length than, as I
believe, any other writer. He likewise often assumes that I attribute
nothing to variation, independently of natural selection, whereas in
the work just referred to I have collected a greater number of well-
established cases than can be found in any other work known to me.
My judgment may not be trustworthy, but after reading with care
Mr. Mivart's book, and comparing each section with what I have
said on the same head, I never before felt so strongly convinced of
the general truth of the conclusions here arrived at, subject, of course,
in so intricate a subject, to much partial error.

All Mr. Mivart's objections will be, or have been, considered in
the present volume. The one new point which appears to have
struck many readers is, "that natural selection is incompetent to
account for the incipient stages of useful structures." This subject
is intimately connected with that of the gradation of characters, often
accompanied by a change of function, — for instance, the conversion
of a swim bladder into lungs, — points which were discussed in the
last chapter under two headings. Nevertheless, I will here consider
in some detail several of the cases advanced by Mr. Mivart, selecting
those which are the most illustrative, as want of space prevents me
from considering all.

The girafTe, by its lofty stature, much elongated neck, fore legs,
head and tongue, has its whole frame beautifully adapted for brows-
ing on the higher branches of trees. It can thus obtain food beyond
the reach of the other Ungulata or hoofed animals inhabiting the
same country; and this must be a great advantage to it during
dearths. The Niata cattle in South America show us how small a
difference in structure may make, during such periods, a great dif-
ference in preserving an animal's life. These cattle can browse as
well as others on grass, but from the projection of the lower jaw
they cannot, during the often recurrent droughts, browse on the
twigs of trees, reeds, etc., to which food the common cattle and
horses are then driven; so that at these times the Niatas perish, if not
fed by their owners. Before coming to Mr. Mivart's objections, it
may be well to explain once again how natural selection will act in
all ordinary cases. Man has modified some of his animals, without
necessarily having attended to special points of structure, by simply

220 ORIGIN OF SPECIES

preserving and breeding from the fleetest individuals, as with the
race-horse and greyhound, or as with the game-cock, by breeding
from the victorious birds. So under nature with the nascent giraffe,
the individuals which were the highest browsers and were able dur-
ing dearths to reach even an inch or two above the others, will often
have been preserved; for they will have roamed over the whole
country in search of food. That the individuals of the same species
often differ slightly in the relative lengths of all their parts may be
seen in many works of natural history, in which careful measure-
ments are given. These slight proportional differences, due to the
laws of growth and variation, are not of the slightest use or impor-
tance to most species. But it will have been otherwise with the
nascent giraffe, considering its probable habits of life; for those indi-
viduals which had some one part or several parts of their bodies
rather more elongated than usual, would generally have survived.
These will have intercrossed and left offspring, either inheriting the
same bodily peculiarities, or with a tendency to vary again in the
same manner; whilst the individuals, less favoured in the same
respects, will have been the most liable to perish.

We here see that there is no need to separate single pairs, as man
does, when he methodically improves a breed; natural selection will
preserve and thus separate all the superior individuals, allowing them
freely to intercross, and will destroy all the inferior individuals. By
this process long-continued, which exactly corresponds with what
I have called unconscious selection by man, combined no doubt in
a most important manner with the inherited effects of the increased
use of parts, it seems to me almost certain that an ordinary hoofed
quadruped might be converted into a giraffe.

To this conclusion Mr. Mivart brings forward two objections.
One is that the increased size of the body would obviously require
an increased supply of food, and he considers it as "very problemati-
cal whether the disadvantages thence arising would not, in times of
scarcity, more than counterbalance the advantages." But as the
giraffe does actually exist in large numbers in South Africa, and as
some of the largest antelopes in the world, taller than an ox, abound
there, why should we doubt that, as far as size is concerned, inter-
mediate gradations could formerly have existed there, subjected as

THEORY OF NATURAL SELECTION 221

now to severe dearths? Assuredly the being able to reach, at each
stage of increased size, to a supply of food, left untouched by the
other hoofed quadrupeds of the country, would have been of some
advantage to the nascent giraffe. Nor must we overlook the fact,
that increased bulk would act as a protection against almost all
beasts of prey excepting the lion; and against this animal, its tall
neck, — and the taller the better, — would, as Mr. Chauncey Wright
has remarked, serve as a watch-tower. It is from this cause, as Sir
S. Baker remarks, that no animal is more difficult to stalk than the
giraffe. This animal also uses its long neck as a means of offence or
defence, by violently swinging its head armed with stump-like horns.
The preservation of each species can rarely be determined by any
one advantage but by the union of all, great and small.

Mr. Mivart then asks (and this is his second objection), if natural
selection be so potent, and if high browsing be so great an advantage,
why has not any other hoofed quadruped acquired a long neck and
lofty stature, besides the giraffe, and, in lesser degree, the camel,
guanaco, and macrauchenia? Or, again, why has not any member
of the group acquired a long proboscis? With respect to South
Africa, which was formerly inhabited by numerous herds of the
giraffe, the answer is not difficult, and can best be given by an illus-
tration. In every meadow in England in which trees grow, we see
the lower branches trimmed or planed to an exact level by the brows-
ing of the horses or cattle; and what advantage would it be, for
instance, to sheep, if kept there, to acquire slightly longer necks? In
every district some one kind of animal will almost certainly be able
to browse higher than the others; and it is almost equally certain that
this one kind alone could have its neck elongated for this purpose,
through natural selection and the effects of increased use. In South
Africa the competition for browsing on the higher branches of the
acacias and other trees must be between giraffe and giraffe, and not
with the other ungulate animals.

Why, in other quarters of the world, various animals belonging
to this same order have not acquired either an elongated neck or a
proboscis, cannot be distinctly answered; but it is as unreasonable to
exfject a distinct answer to such a question, as why some event in
the history of mankind did not occur in one country, whilst it did in

222 ORIGIN OF SPECIES

another. We are ignorant with resp)ect to the conditions which
determine the numbers and range of each species; and we cannot
even conjecture what changes of structure would be favourable to
its increase in some new country. We can, however, see in a general
manner that various causes might have interfered with the develop-
ment of a long neck or proboscis. To reach the foliage of a con-
siderable height (without climbing, for which hoofed animals are
singularly ill-constructed) implies greatly increased bulk of body;
and we know that some areas suppwrt singularly few large quadru-
peds, for instance South America, though it is so luxuriant; whilst
South Africa abounds with them to an unparalleled degree. Why
this should be so, we do not know; nor why the later tertiary periods
should have been much more favourable for their existence than the
present time. Whatever the causes may have been, we can see that
certain districts and times would have been much more favourable
than others for the development of so large a quadruped as the
giraffe.

In order that an animal should acquire some structure specially
and largely developed, it is almost indispensable that several other
parts should be modified and coadapted. Although every part of
the body varies slightly, it does not follow that the necessary parts
should always vary in the right direction and to the right degree.
With the different species of our domesticated animals we know
that the parts vary in a different manner and degree; and that some
species are much more variable than others. Even if the fitting vari-
ations did arise, it does not follow that natural selection would be
able to act on them, and produce a structure which apparently would
be beneficial to the species. For instance, if the number of indi-
viduals existing in a country is determined chiefly through destruc-
tion by beasts of prey, — by external or internal parasites, etc., — as
seems often to be the case, then natural selection will be able to do
little, or will be greatly retarded, in modifying any particular struc-
ture for obtaining food. Lastly, natural selection is a slow process,
and the same favourable conditions must long endure in order that
any marked effect should thus be produced. Except by assigning
such general and vague reasons, we cannot explain why, in many
quarters of the world, hoofed quadrupeds have not acquired much

THEORY OF NATURAL SELECTION 22$

elongated necks or other means for browsing on the higher branches
of trees.

Objections of the same nature as the foregoing have been advanced
by many writers. In each case various causes, besides the general
ones just indicated, have probably interfered with the acquisition
through natural selection of structures, which it is thought would be
beneficial to certain species. One writer asks, why has not the ostrich
acquired the power of flight? But a moment's reflection will show
what an enormous supply of food would be necessary to give to this
bird of the desert force to move its huge body through the air.
Oceanic islands are inhabited by bats and seals, but by no terrestrial
mammals; yet as some of these bats are peculiar species, they must
have long inhabited their present homes. Therefore Sir C. Lyell
asks, and assigns certain reasons in answer, why have not seals and
bats given birth on such islands to forms fitted to live on the land?
But seals would necessarily be first converted into terrestrial car-
nivorous animals of considerable size, and bats into terrestrial insec-
tivorous animals; for the former there would be no prey; for the
bats ground insects would serve as food, but these would already be
largely preyed on by the reptiles or birds, which first colonise and
abound on most oceanic islands. Gradations of structure, with each
stage beneficial to a changing species, will be favoured only under
certain p)eculiar conditions. A strictly terrestrial animal, by occasion-
ally hunting for food in shallow water, then in streams or lakes,
might at last be converted into an animal so thoroughly aquatic as
to brave the open ocean. But seals would not find on oceanic islands
the conditions favourable to their gradual reconversion into a terres-
trial form. Bats, as formerly shown, probably acquired their wings
by at first gliding through the air from tree to tree, like the so-called
flying squirrels, for the sake of escaping from their enemies, or for
avoiding falls; but when the power of true flight had once been
acquired, it would never be reconverted back, at least for the above
purposes, into the less efficient power of gliding through the air.
Bats might, indeed, like many birds, have had their wings greatly
reduced in size, or completely lost, through disuse; but in this case
it would be necessary that they shoiJd first have acquired the power
of running quickly on the ground, by the aid of their hind legs alone,

224 ORIGIN OF SPECIES

SO as to compete with birds or other ground animals; and for such
a change a bat seems singularly ill-fitted. These conjectural remarks
have been made merely to show that a transition of structure, with
each step beneficial, is a highly complex affair; and that there is
nothing strange in a transition not having occurred in any particular
case.

Lastly, more than one writer has asked, why have some animals
had their mental powers more highly developed than others, as such
development would be advantageous to all? Why have not apes
acquired the intellectual powers of man? Various causes could be
assigned; but as they are conjectural, and their relative probability
cannot be weighed, it would be useless to give them. A definite
answer to the latter question ought not to be expected, seeing that
no one can solve the simpler problem why, of two races of savages,
one has risen higher in the scale of civilisation than the other; and
this apparently implies increased brain-power.

We will return to Mr. Mivart's other objections. Insects often
resemble for the sake of protection various objects, such as green or
decayed leaves, dead twigs, bits of lichen, flowers, spines, excrement
of birds, and living insects; but to this latter point I shall hereafter
recur. The resemblance is often wonderfully close, and is not con-
fined to colour, but extends to form, and even to the manner in
which the insects hold themselves. The caterpillars which project
motionless like dead twigs from the bushes on which they feed,
offer an excellent instance of a resemblance of this kind. The cases
of the imitation of such objects as the excrement of birds, are rare
and exceptional. On this head, Mr. Mivart remarks, "As, according
to Mr. Darwin's theory, there is a constant tendency to indefinite
variation, and as the minute incipient variations will be in all direc-
tions, they must tend to neutralise each other, and at first to form
such unstable modifications that it is difficult, if not impossible, to
see how such indefinite oscillations of infinitesimal beginnings can
ever build up a sufficiently appreciable resemblance to a leaf, bamboo,
or other object, for Natural Selection to seize ujxin and perpetuate."

But in all the foregoing cases the insects in their original state no
doubt presented some rude and accidental resemblance to an object
commonly found in the stations frequented by them. Nor is this at

THEORY OF NATURAL SELECTION 225

all improbable, considering the almost infinite number of surround-
ing objects and the diversity in form and colour of the hosts of
insects which exist. As some rude resemblance is necessary for the
first start, we can understand how it is that the larger and higher
animals do not (with the exception, as far as I know, of one fish)
resemble for the sake of protection special objects, but only the
surface which commonly surrounds them, and this chiefly in colour.
Assuming that an insect originally happened to resemble in some
degree a dead twig or a decayed leaf, and that it varied slightly in
many ways, then all the variations which rendered the insect at all
more like any such object, and thus favoured its escape, would be
preserved, whilst other variations would be neglected and ultimately
lost; or, if they rendered the insect at all less like the imitated object,
they would be eliminated. There would indeed be force in Mr.
Mivart's objection, if we were to attempt to account for the above
resemblances, independently of natural selection, through mere
fluctuating variability; but as the case stands there is none.

Nor can I see any force in Mr. Mivart's difficulty with respect to
"the last touches of perfection in the mimicry;" as in the case given
by Mr. Wallace, of a walking stick insect (Ceroxylus laceratus),
which resembles "a stick grown over by a creeping moss or junger-
mannia." So close was this resemblance, that a native Dyak main-
tained that the foliaceous excrescences were really moss. Insects are
preyed on by birds and other enemies, whose sight is probably
sharper than ours, and every grade in resemblance which aided an
insect to escape notice or detection, would tend towards its preserva-
tion; and the more perfect the resemblance so much the better for the
insect. Considering the nature of the differences between the species
in the group which includes the above Ceroxylus, there is nothing
improbable in this insect having varied in the irregularities on its
surface, and in these having become more or less green-coloured;
for in every group the characters which differ in the several species
are the most apt to vary, whilst the generic characters, or those
common to all the species, are the most constant.

The Greenland whale is one of the most wonderful animals in the
world, and the baleen, or whalebone, one of its greatest peculiarities.

226 ORIGIN OF SPECIES

The baleen consists of a row, on each side, of the upper jaw, of about
300 plates or lamina:, which stand close together transversely to the
longer axis of the mouth. Within the main row there are some
subsidiary rows. The extremities and inner margins of all the plates
are frayed into stiff bristles, which clothe the whole gigantic palate,
and serve to strain or sift the water, and thus to secure the minute
prey on which these great animals subsist. The middle and longest
lamina in the Greenland whale is ten, twelve, or even fifteen feet
in length; but in the different species of Cetaceans there are grada-
tions in length; the middle lamina being in one species, according
to Scoresby, four feet, in another three, in another eighteen inches,
and in the Balacnoptera rostrata only about nine inches in length.
The quality of the whalebone also differs in the different species.

With respect to the baleen, Mr. Mivart remarks that if it "had once
attained such a size and development as to be at all useful, then its
preservation and augmentation within serviceable limits would be
promoted by natural selection alone. But how to obtain the begin-
ning of such useful development?" In answer, it may be asked, why
should not the early progenitors of the whales with baleen have
possessed a mouth constructed something like the lamellated beak
of a duck? Ducks, like whales, subsist by sifting the mud and
water; and the family has sometimes been called Criblatores, or
sifters. I hope that I may not be misconstrued into saying that the
progenitors of whales did actually possess mouths lamellated like the
beak of a duck. I wish only to show that this is not incredible, and
that the immense plates of baleen in the Greenland whale might
have been developed from such lamella: by finely graduated steps,
each of service to its possessor.

The beak of a shoveller duck (Spatula dypeata) is a more beauti-
ful and complex structure than the mouth of a whale. The upper
mandible is furnished on each side (in the specimen examined by
me) with a row or comb formed of 188 thin, elastic lamella:,
obliquely bevelled so as to be pointed, and placed transversely to the
longer axis of the mouth. They arise from the palate, and are
attached by flexible membrane to the sides of the mandible. Those
standing towards the middle are the longest, being about one-third
of an inch in length, and they project .14 of an inch beneath the

THEORY OF NATURAL SELECTION 1TJ

edge. At their bases there is a short subsidiary row of obliquely
transverse lamellae. In these several respects they resemble the plates
of baleen in the mouth of a whale. But towards the extremity of the
beak they differ much, as they project inwards, instead of straight
downwards. The entire head of the shoveller, though incomparably
less bulky, is about one-eighteenth of the length of the head of a
moderately large Balacnoptera rostrata, in which species the baleen is
only nine inches long; so that if we were to make the head of the
shoveller as long as that of the Balanoptera, the lamellae would be
six inches in length, — that is, two-thirds of the length of the baleen
in this sp>ecies of whale. The lower mandible of the shoveller-duck
is furnished with lamella: of equal length with those above, but
finer; and in being thus furnished it differs conspicuously from the
lower jaw of a whale, which is destitute of baleen. On the other
hand, the extremities of these lower lamella: are frayed into fine
bristly points, so that they thus curiously resemble the plates o£
baleen. In the genus Prion, a member of the distinct family of the
Petrels, the upper mandible alone is furnished with lamellae, which
are well developed and project beneath the margin; so that the beak
of this bird resembles in this respect the mouth of a whale.

From the highly developed structure of the shoveller's beak we
may proceed (as I have learnt from information and specimens sent
to me by Mr. Salvin), without any great break, as far as fitness for
sifting is concerned, through the beak of the Merganetta armata, and
in some respects through that of the Aix sponsa, to the beak of the
common duck. In this latter species, the lamellae are much coarser
than in the shoveller, and are firmly attached to the sides of the
mandible; they are only. about fifty in number on each side, and
do not project at all beneath the margin. They are square-topped,
and are edged with translucent hardish tissue, as if for crushing food.
The edges of the lower mandible are crossed by numerous fine ridges,
which project very little. Although the beak is thus very inferior as
a sifter to that of the shoveller, yet this bird, as every one knows,
constantly uses it for this purpose. There are other species, as I hear
from Mr. Salvin, in which the lamellae are considerably less developed
than in the common duck; but I do not know whether they use
their beaks for sifting the water.

228 ORIGIN OF SPECIES

Turning to another group of the same family. In the Egyptian
goose (Chenalopex) the beak closely resembles that of the common
duck; but the lamellae are not so numerous, nor so distinct from each
other, nor do they project so much inwards; yet this goose, as I am
informed by Mr. E. Bartlett, "uses its bill like a duck by throwing
the waters out at the corners." Its chief food, however, is grass,
which it crops like the common goose. In this latter oird, the lamellx
of the upper mandible are much coarser than in the common duck,
almost confluent, about twenty-seven in number on each side, and
terminating upwards in teeth-like knobs. The palate is also covered
with hard rounded knobs. The edges of the lower mandible are
serrated with teeth much more prominent, coarser, and sharper than
in the duck. The common goose does not sift the water, but uses
its beak exclusively for tearing or cutting herbage, for which pur-
pose it is so well fitted, that it can crop grass closer than almost any
other animal. There are other species of geese, as I hear from Mr.
Bartlett, in which the lamellae are less developed than in the common
goose.

We thus see that a member of the duck family, with a beak con-
structed like that of the common goose and adapted solely for
grazing, or even a member with a beak having less well-developjed
lamellae, might be converted by small changes into a species like the
Egyptian goose, — this into one like the common duck, — and, lastly,
into one like the shoveller, provided with a beak almost exclusively
adapted for sifting the water; for this bird could hardly use any part
of its beak, except the hooked tip, for seizing or tearing solid food.
The beak of a goose, as I may add, might also be converted by small
changes into one provided with prominent, recurved teeth, like those
of the Merganser (a member of the same family), serving for the
widely different purpose of securing live fish.

Returning to the whales. The Hyperoodon bidens is destitute of
true teeth in an efficient condition, but its palate is roughened,
according to Lacejjede, with small, unequal, hard points of horn.
There is, therefore, nothing improbable in supposing that some
early cetacean form was provided with similar points of horn on the
palate, but rather more regularly placed, and which, like the knobs
on the beak of the goose, aided it in seizing or tearing its food. If so,

THEORY OF NATURAL SELECTION 229

it will hardly be denied that the points might have been converted
through variation and natural selection into lamellae as well-
developed as those of the Egyptian goose, in which case they would
have been used both for seizing objects and for sifting the water;
then into lamellae like those of the domestic duck; and so onwards,
until they became as well constructed as those of the shoveller, in
which case they would have served exclusively as a sifting apparatus.
From this stage, in which the lamellae would be two-thirds of the
length of the plates of baleen in the Balaenoptera rostrata, gradations,
which may be observed in still-existing Cetaceans, lead us onwards
to the enormous plates of baleen in the Greenland whale. Nor is
there the least reason to doubt that each step in this scale might
have been as serviceable to certain ancient Cetaceans, with the func-
tions of the parts slowly changing during the progress of develop-
ment, as are the gradations in the beaks of the dif?erent existing
members of the duck family. We should bear in mind that each
species of duck is subjected to a severe struggle for existence, and
that the structure of every part of its frame must be well adapted
to its conditions of life.

The Pleuronectidaf, or flatfish, are remarkable for their asymmetri-
cal bodies. They rest on one side, — in the greater number of species
on the left, but in some on the right side; and occasionally reversed
adult specimens occur. The lower, or resting-surface, resembles at
first sight the ventral surface of an ordinary fish: it is of a white
color, less developed in many ways than the upper side, with the
lateral fins often of smaller size. But the eyes offer the most remark-
able peculiarity; for they are both placed on the upper side of the
head. During early youth, however, they stand opposite to each
other, and the whole body is then symmetrical, with both sides
equally coloured. Soon the eye proper to the lower side begins to
glide slowly round the head to the upper side; but does not pass
right through the skull, as was formerly thought to be the case. It
is obvious that unless the lower eye did thus travel round, it could
not be used by the fish whilst lying in its habitual position on one
side. The lower eye would, also, have been liable to be abraded by
the sandy bottom. That the Pleuronectidae are admirably adapted
by their flattened and asymmetrical structure for their habits of life,

230 ORIGIN OF SPECIES

is manifest from several species, such as soles, flounders, etc., being
extremely common. The chief advantages thus gained seem to be
protection from their enemies, and facility for feeding on the ground.
The different members, however, of the family present, as Schiodte
remarks, "a long series of forms exhibiting a gradual transition from
Hippoglossus pinguis, which does not in any considerable degree
alter the shape in which it leaves the ovum, to the soles, which are
entirely thrown to one side."

Mr. Mivart has taken up this case, and remarks that a sudden
spontaneous transformation in the position of the eyes is hardly con-
ceivable, in which I quite agree with him. He then adds: "If the
transit was gradual, then how such transit of one eye a minute
fraction of the journey towards the other side of the head could
benefit the individual is, indeed, far from clear. It seems, even, that
such an incipient transformation must rather have been injurious."
But he might have found an answer to this objection in the excel-
lent observations published in 1867 by Malm. The Pleuronectidac,
whilst very young and still symmetrical, with their eyes standing on
opposite sides of the head, cannot long retain a vertical position,
owing to the excessive depth of their bodies, the small size of their
lateral fins, and to their being destitute of a swim bladder. Hence
soon growing tired, they fall to the bottom on one side. Whilst thus
at rest they often twist, as Malm observed, the lower eye upwards,
to see above them; and they do this so vigorously that the eye is
pressed hard against the upper part of the orbit. The forehead
between the eyes consequently becomes, as could be plainly seen,
temporarily contracted in breadth. On one occasion Malm saw a
young fish raise and depress the lower eye through an angular dis-
tance of about seventy degrees.

We should remember that the skull at this early age is car-
tilaginous and flexible, so that it readily yields to muscular action.
It is also known with the higher animals, even after early youth,
that the skull yields and is altered in shape, if the skin or muscles
be f)ermanently contracted through disease or some accident. With
long-eared rabbits, if one ear lops forwards and downwards, its
weight drags forward all the bones of the skull on the same side,
of which I have given a figure. Malm states that the newly hatched

THEORY OF NATURAL SELECTION 23 1

young of perches, salmon, and several other symmetrical fishes, have
the habit of occasionally resting on one side at the bottom; and he
has observed that they often then strain their lower eyes so as to look
upwards; and their skulls are thus rendered rather crooked. These
fishes, however, are soon able to hold themselves in a vertical posi-
tion, and no permanent effect is thus produced. With the Pleuro-
nectidac, on the other hand, the older they grow the more habitually
they rest on one side, owing to the increasing flatness of their bodies,
and a permanent effect is thus produced on the form of the head,
and on the position of the eyes. Judging from analogy, the tendency
to distortion would no doubt be increased through the principle of
inheritance. Schiodte believes, in opposition to some other natural-
ists, that the Pleuronectidx are not quite symmetrical even in the
embryo; and if this be so, we could understand how it is that certain
species, whilst young, habitually fall over and rest on the left side,
and other species on the right side. Malm adds, in confirmation of
the above view, that the adult Trachypterus arcticus, which is not a
member of the Pleuronectidx, rests on its left side at the bottom, and
swims diagonally through the water; and in this fish, the two sides
of the head are said to be somewhat dissimilar. Our great authority
on Fishes, Dr. Giinther, concludes his abstract of Malm's paper, by
remarking that "the author gives a very simple explanation of the
abnormal condition of the Pleuronectoids."

We thus see that the first stages of the transit of the eye from one
side of the head to the other, which Mr. Mivart considers would be
injurious, may be attributed to the habit, no doubt beneficial to the
individual and to the species, of endeavouring to look upwards with
both eyes, whilst resting on one side at the bottom. We may also
attribute to the inherited effects of use the fact of the mouth in sev-
eral kinds of flat-fish being bent towards the lower surface, with the
jaw bones stronger and more effective on this, the eyeless side of the
head, than on the other, for the sake, as Dr. Traquair supposes, of
feeding with ease on the ground. Disuse, on the other hand, will
account for the less developed condition of the whole inferior half of
the body, including the lateral fins; though Yarrell thinks that the
reduced size of these fins is advantageous to the fish, as "there is so
much less room for their action, than with the larger fins above."

232 ORIGIN OF SPECIES

Perhaps the lesser number of teeth in the proportion of four to seven
in the upper halves of the two jaws of the plaice, to twenty-five to
thirty in the lower halves, may likewise be accounted for by disuse.
From the colourless state of the ventral surface of most fishes and of
many other animals, we may reasonably suppose that the absence of
colour in flat-fish on the side, whether it be the right or left, which is
undermost, is due to the exclusion of light. But it cannot be supposed
that the peculiar speckled appearance of the upper side of the sole, so
like the sandy bed of the sea, or the power in some sfjecies, as recently
shown by Pouchet, of changing their colour in accordance with the
siu-rounding surface, or the presence of bony tubercles on the upper
side of the turbot, are due to the action of the light. Here natural
selection has probably come into play, as well as in adapting the gen-
eral shape of the body of these fishes, and many other peculiarities, to
their habits of life. We should keep in mind, as I have before insisted,
that the inherited effects of the increased use of parts, and perhaps of
their disuse, will be strengthened by natural selection. For all spon-
taneous variations in the right direction will thus be preserved; as
will those individuals which inherit in the highest degree the effects
of the increased and beneficial use of any part. How much to attrib-
ute in each particular case to the effects of use, and how much to
natural selection, it seems impossible to decide.

I may give another instance of a structure which apparently owes
its origin exclusively to use or habit. The extremity of the tail in
some American monkeys has been converted into a wonderfully
perfect prehensile organ, and serves as a fifth hand. A reviewer who
agrees with Mr. Mivart in every detail, remarks on this structure:
"It is impossible to believe that in any number of ages the first slight
incipient tendency to grasp could preserve the lives of the individuals
possessing it, or favour their chance of having and of rearing off-
spring." But there is no necessity for any such belief. Habit, and
this almost implies that some benefit great or small is thus derived,
would in all probability suffice for the work. Brehm saw the young
of an African monkey (Cercopithecus) clinging to the under sur-
face of their mother by their hands, and at the same time they
hooked their little tails round that of their mother. Professor Hens-
low kept in confinement some harvest mice (Mus messorius) which

THEORY OF NATURAL SELECTION 233

do not possess a structurally prehensile tail; but he frequently
observed that they curled their tails round the branches of a bush
placed in the cage, and thus aided themselves in climbing. I have
received an analogous account from Dr. Giinther, who has seen a
mouse thus suspend itself. If the harvest mouse had been more
strictly arboreal, it would perhaps have had its tail rendered struc-
turally prehensile, as is the case with some members of the same
order. Why Cercopithecus, considering its habits whilst young, has
not become thus provided, it would be difficult to say. It is, however,
possible that the long tail of this monkey may be of more service to
it as a balancing organ in making its prodigious leaps, than as a
prehensile organ.

The mammary glands are common to the whole class of mammals,
and are indispensable for their existence; they must, therefore, have
been developed at an extremely remote period, and we can know
nothing positively about their manner of development. Mr. Mivart
asks: "Is it conceivable that the young of any animal was ever saved
from destruction by accidentally sucking a drop of scarcely nutritious
fluid from an accidentally hypertrophied cutaneous gland of its
mother? And even if one was so, what chance was there of the
perpetuation of such a variation?" But the case is not here put fairly.
It is admitted by most evolutionists that mammals are descended
from a marsupial form; and if so, the mammary glands will have
been at first developed within the marsupial sack. In the case of the
fish (Hippicampus) the eggs are hatched, and the young are reared
for a time, within a sack of this nature; and an American naturalist,
Mr. Lxxkwood, believes from what he has seen of the development
of the young, that they are nourished by a secretion from the cuta-
neous glands of the sack. Now with the early progenitors of
mammals, almost before they deserved to be thus designated, is it
not at least possible that the young might have been similarly
nourished ? And in this case, the individuals which secreted a fluid,
in some degree or manner the most nutritious, so as to partake of
the nature of milk, would in the long run have reared a larger num-
ber of well-nourished offspring, than would the individuals which
secreted a poorer fluid; and thus the cutaneous glands, which are the

234 ORIGIN OF SPECIES

homologues of the mammary glands, would have been improved or
rendered more effective. It accords with the widely extended prin-
ciple of specialisation, that the glands over a certain space of the
sack should have become more highly developed than the remainder;
and they would then have formed a breast, but at first without a
nipple, as we see in the Ornithorhyncus, at the base of the mam-
malian series. Through what agency the glands over a certain space
became more highly specialised than the others, 1 will not pretend to
decide, whether in part through compensation of growth, the effects
of use, or of natural selection.

The development of the mammary glands would have been of no
service, and could not have been effected through natural selection,
unless the young at the same time were able to partake of the secre-
tion. There is no greater difficulty in understanding how young
mammals have instinctively learnt to suck the breast, than in under-
standing how unhatched chickens have learnt to break the egg-shell
by tapping against it with their specially adapted beaks; or how a
few hours after leaving the shell they have learnt to pick up grains
of food. In such cases the most probable solution seems to be, that
the habit was at first acquired by practice at a more advanced age,
and afterwards transmitted to the offspring at an earlier age. But the
young kangaroo is said not to suck, only to cling to the nipple of its
mother, who has the power of injecting milk into the mouth of her
helpless, half -formed offspring. On this head Mr. Mivart remarks:
"Did no special provision exist, the young one must infallibly be
choked by the intrusion of the milk into the windpipe. But there is
a sjjecial provision. The larynx is so elongated that it rises up into
the posterior end of the nasal passage, and is thus enabled to give
free entrance to the air for the lungs, while the milk passes harm-
lessly on each side of this elongated larynx, and so safely attains the
gullet behind it." Mr. Mivart then asks how did natural selection
remove in the adult kangaroo (and in most other mammals, on the
assumption that they are descended from a marsupial form), "this
at least perfectly innocent and harmless structure?" It may be sug-
gested in answer that the voice, which is certainly of high impor-
tance to many animals, could hardly have been used with full force
as long as the larynx entered the nasal passage; and Professor Flower

THEORY OF NATURAL SELECTION 235

has suggested to me that this structure would have greatly interfered
with an animal swallowing solid food.

We will now turn for a short space to the lower divisions of the
animal kingdom. The Echinodermata (starfishes, sea urchins, etc.)
are furnished with remarkable organs, called pedicellarii, which
consist, when well developed, of a tridactyle forceps — that is, of one
formed of three serrated arms, neatly fitting together and placed on
the summit of a flexible stem, moved by muscles. These forceps can
seize firmly hold of any object; and Alexander Agassiz has seen an
Echinus or sea urchin rapidly passing particles of excrement from
forceps to forceps down certain lines of its body, in order that its
shell should not be fouled. But there is no doubt that besides remov-
ing dirt of all kinds, they subserve other functions; and one of these
apparently is defence.

With respect to these organs, Mr, Mivart, as on so many previous
occasions, asks: "What would be the utiUty of the first rudimentary
beginnings of such structures, and how could such incipient bud-
dings have ever preserved the life of a single Echinus?" He adds,
"Not even the sudden development of the snapping action could
have been beneficial without the freely moveable stalk, nor could the
latter have been efficient without the snapping jaws, yet no minute
merely indefinite variations could simultaneously evolve these com-
plex co-ordinations of structure; to deny this seems to do no less than
to affirm a startling paradox." Paradoxical as this may appear to
Mr. Mivart, tridactyle forcepses, immovably fixed at the base, but
capable of a snapping action, certainly exist on some starfishes; and
this is intelligible if they serve, at least in part, as a means of defence.
Mr. Agassiz, to whose great kindness I am indebted for much
information on the subject, informs me that there are other star-
fishes, in which one of the three arms of the forceps is reduced to a
support for the other two; and again, other genera in which the
third arm is completely lost. In Echinoneus, the shell is described
by M. Perrier as bearing two kinds of pedicellariac, one resembling
those of Echinus, and the other those of Spatangus; and such cases
are always interesting as affording the means of apparently sud-
den transitions, through the abortion of one of the two states of an
organ.

236 ORIGIN OF SPECIES

With respect to the steps by which these curious organs have been
evolved, Mr. Agassiz infers from his own researches and those of
MiJlier, that both in starfishes and sea urchins the {jedicellarix must
undoubtedly be looked at as modified spines. This may be inferred
from their manner of development in the individual, as well as from
a long and perfect series of gradations in different species and genera,
from simple granules to ordinary spines, to perfect tridactyle pedi-
cellariac. The gradation extends even to the manner in which
ordinary spines and the pedicellaria: with their supporting calcareous
rods are articulated to the shell. In certain genera of starfishes, "the
very combinations needed to show that the pedicellariae are only
modified branching spines" may be found. Thus we have fixed
spines, with three equi-distant, serrated, moveable branches, articu-
lated to near their bases; and higher up, on the same spine, three
other moveable branches. Now when the latter arise from the
summit of a spine they form in fact a rude tridactyle pedicellaria,
and such may be seen on the same spine together with the three
lower branches. In this case the identity in nature between the arms
of the pedicellaria and the moveable branches of a spine, is unmis-
takable. It is generally admitted that the ordinary spines serve as a
protection; and if so, there can be no reason to doubt that those
furnished with serrated and moveable branches likewise serve for
the same purpwse; and they would thus serve still more effectively
as soon as by meeting together they acted as a prehensile or snapping
apparatus. Thus every gradation, from an ordinary fixed spine to a
fixed pedicellaria, would be of service.

In certain genera of starfishes, these organs, instead of being fixed
or borne on an immovable support, are placed on the summit of a
flexible and muscular, though short, stem; and in this case they
probably subserve some additional function besides defence. In the
sea urchins the steps can be followed by which a fixed spine becomes
articulated to the shell, and is thus rendered moveable. I wish I had
space here to give a fuller abstract of Mr. Agassiz's interesting ob-
servations on the development of the pedicellaria:. All possible
gradations, as he adds, may likewise be found between the pedicel-
lariac of the starfishes and the hooks of the ophiurans, another group
of the Echinodermata; and again between the pedicellarix of sea

THEORY OF NATURAL SELECTION 237

urchins and the anchors of the Holothurix, also belonging to the
same great class.

Certain compound animals, or zoophytes, as they have been
termed, namely the Polyzoa, are provided with curious organs called
avicularia. These difler much in structure in the different species.
In their most perfect condition, they curiously resemble the head
and beak of a vulture in miniature, seated on a neck and capable of
movement, as is likewise the lower jaw or mandible. In one species
observed by me all the avicularia on the same branch often moved
simultaneously backwards and forwards, with the lower jaw widely
open, through an angle of about 90°, in the course of five seconds;
and their movement caused the whole polyzoary to tremble. When
the jaws are touched with a needle they seize it so firmly that the
branch can thus be shaken.

Mr. Mivart adduces this case, chiefly on account of the supposed
difficulty of organs, namely the avicularia of the Polyzoa and the
pedicellarijE of the Echinodermata, which he considers as "essentially
similar," having been developed through natural selection in widely
distinct divisions of the animal kingdom. But, as far as structure is
concerned, I can see no similarity between tridactyle pedicellariac and
avicularia. The latter resemble somewhat more closely the chelx or
pincers of crustaceans; and Mr. Mivart might have adduced with
equal appropriateness this resemblance as a special difficulty; or even
their resemblance to the head and beak of a bird. The avicularia
are believed by Mr. Busk, Dr. Smitt, and Dr. Nitsche — naturalists
who have carefully studied this group — to be homologous with the
zooids and their cells which compose the zoophyte; the moveable
lip or lid of the cell corresponding with the lower and moveable
mandible of the avicularium. Mr. Busk, however, does not know of
any gradations now existing between a zooid and an avicularium.
It is therefore impossible to conjecture by what serviceable gradations
the one could have been converted into the other: but it by no means
follows from this that such gradations have not existed.

As the cheli of crustaceans resemble in some degree the avicularia
of Polyzoa, both serving as pincers, it may be worth while to show
that with the former a long series of serviceable gradations still

238 ORIGIN OF SPECIES

exists. In the first and simplest stage, the terminal segment of a
limb shut down either on the square summit of the broad penulti-
mate segment or against one whole side; and is thus enabled to catch
hold of an object; but the limb still serves as an organ of locomotion.
We next find one corner of the broad penultimate segment slightly
prominent, sometimes furnished with irregular teeth; and against
these the terminal segment shuts down. By an increase in the size
of this projection, with its shaf)e, as well as that of the terminal seg-
ment, slightly modified and improved, the pincers are rendered more
and more perfect until we have at last an instrument as efficient
as the chela: of a lobster; and all these gradations can be actually
traced.

Besides the avicularia, the Polyzoa possess curious organs called
vibracula. These generally consist of long bristles, capable of move-
ment and easily excited. In one species examined by me the vibracula
were slightly curved and serrated along the outer margin; and all of
them on the same polyzoary often moved simultaneously; so that,
acting like long oars, they swept a branch rapidly across the object
glass of my microscope. When a branch was placed on its face, the
vibracula became entangled, and they made violent efforts to free
themselves. They are supposed to serve as a defence, and may be
seen, as Mr. Busk remarks, "to sweep slowly and carefully over the
surface of the polyzoary, removing what might be noxious to the
delicate inhabitants of the cells when their tentacula are protruded."
The avicularia, like the vibracula, probably serve for defence, but
they also catch and kill small living animals, which it is believed are
afterwards swept by the currents within reach of the tentacula of the
zooids. Some sf)ecies are provided with avicularia and vibracula;
some with avicularia alone, and a few with vibracula alone.

It is not easy to imagine two objects more widely different in
appearance than a bristle or vibraculum, and an avicularium like
the head of a bird; yet they are almost certainly homologous and
have been developed from the same common source, namely a zooid
with its cell. Hence we can understand how it is that these organs
graduate in some cases, as I am informed by Mr. Busk, into each
other. Thus with the avicularia of several species of Lepralia, the
moveable mandible is so much produced and is so like a bristle,

THEORY OF NATURAL SELECTION 239

that the presence of the upper or fixed beak alone serves to determine
its avicularian nature. The vibracula may have been directly de-
veloped from the hps of the cells, without having passed through
the avicularian stage; but it seems more probable that they have
passed through this stage, as during the early stages of the trans-
formation, the other parts of the cell with the included zooid could
hardly have disappeared at once. In many cases the vibracula have
a grooved support at the base, which seems to represent the fixed
beak; though this support in some species is quite absent. This
view of the development of the vibracula, if trustworthy, is interest-
ing; for supposing that all the species provided with avicularia had
become extinct, no one with the most vivid imagination would ever
have thought that the vibracula had originally existed as part of an
organ, resembling a bird's head or an irregular box or hood. It is
interesting to see two such widely different organs developed from
a common origin; and as the moveable Up of the cell serves as a
protection to the zooid, there is no difficulty in believing that all the
gradations, by which the lip became converted first into the lower
mandible of an avicularium and then into an elongated bristle, like-
wise served as a protection in different ways and under different
circumstances.

In the vegetable kingdom Mr. Mivart only alludes to two cases,
namely the structure of the flowers of orchids, and the movements
of climbing plants. With respect to the former, he says, "The ex-
planation of their origin is deemed thoroughly unsatisfactory — ut-
terly insufficient to explain the incipient, infinitesimal beginnings
of structures which are of utility only when they are considerably
developed." As I have fully treated this subject in another work, I
will here give only a few details on one alone of the most striking
peculiarities of the flowers of orchids, namely their pollinia. A pol-
linium when highly developed consists of a mass of pollen-grains,
affixed to an elastic foot-stalk or caudicle, and this to a little mass of
extremely viscid matter. The pollinia are by this means transported
by insects from one flower to the stigma of another. In some orchids
there is no caudicle to the pollen-masses, and the grains are merely
•tied together by fine threads; but as these are not confined to orchids,

240 ORIGIN OF SPECIES

they need not here be considered; yet I may mention that at the
base of the orchidaceous series, in Cypripedium, we can see how the
threads were probably first developed. In other orchids the threads
cohere at one end of the pollen-masses; and this forms the first or
nascent trace of a caudicle. That this is the origin of the caudicle,
even when of considerable length and highly developed, we have
good evidence in the aborted pollen-grains which can sometimes be
detected embedded within the central and solid parts.

With respect to the second chief peculiarity, namely, the little
mass of viscid matter attached to the end of the caudicle, a long
series of gradations can be specified, each of plain service to the
plant. In most flowers belonging to other orders the stigma secretes
a little viscid matter. Now in certain orchids similar viscid matter
is secreted, but in much larger quantities, by one alone of the three
stigmas; and this stigma, perhaps in consequence of the copious
secretion, is rendered sterile. When an insect visits a flower of this
kind, it rubs off some of the viscid matter and thus at the same time
drags away some of the |X)llen-grains. From this simple condition,
which differs but little from that of a multitude of common flowers,
there are endless gradations, — to species in which the pollen-mass
terminates in a very short, free caudicle, — to others in which the
caudicle becomes firmly attached to the viscid matter, with the
sterile stigma itself much modified. In this latter case we have a
poUinium in its most highly developed and perfect condition. He
who will carefully examine the flowers of orchids for himself will
not deny the existence of the above series of gradations — from a
mass of pollen-grains merely tied together by threads, with the
stigma differing but little from that of an ordinary flower, to a highly
complex jx)llinium, admirably adapted for transportal by insects;
nor will he deny that all the gradations in the several species are
admirably adapted in relation to the general structure of each flower
for its fertilisation by different insects. In this, and in almost every
other case, the enquiry may be pushed further backwards; and it
may be asked how did the stigma of an ordinary flower become
viscid, but as we do not know the full history of any one group of
beings, it is as useless to ask, as it is hopeless to attempt answering,
such questions.

THEORY OF NATURAL SELECTION 24 1

We will now turn to climbing plants. These can be arranged in
a long series, from those which simply twine round a sup()ort, to
those which I have called lea£<limbers, and to those provided with
tendrils. In these two latter classes the stems have generally, but
not always, lost the power of twining, though they retain the power
of revolving, which the tendrils likewise possess. The gradations
from leaf<limbers to tendril-bearers are wonderfully close, and cer-
tain plants may be indifferently placed in either class. But in ascend-
ing the series from simple twiners to leaf-climbers, an important
quality is added, namely sensitiveness to a touch, by which means
the foot-stalks of the leaves or flowers, or these modified and con-
verted into tendrils, are excited to bend round and clasp the touching
object. He who will read my memoir on these plants will, I think,
admit that all the many gradations in function and structure be-
tween simple twiners and tendril-bearers are in each case beneficial
in a high degree to the species. For instance, it is clearly a great
advantage to a twining plant to become a leaf -climber; and it is
probable that every twiner which possessed leaves with long foot-
stalks would have been developed into a leaf-climber, if the foot-
stalks had possessed in any slight degree the requisite sensitiveness
to a touch.

As twining is the simplest means of ascending a support, and
forms the basis of our series, it may naturally be asked how did
plants acquire this power in an incipient degree, afterwards to be
improved and increased through natural selection. The power of
twining depends, firstly, on the stems whilst young being extremely
flexible (but this is a character common to many plants which are
not climbers) ; and, secondly, on their continually bending to all
points of the compass, one after the other in succession, in the same
order. By this movement the stems are inclined to all sides, and
are made to move round and round. As soon as the lower part of
a stem strikes against any object and is stopped, the upper part still
goes on bending and revolving, and thus necessarily twines round
and up the support. The revolving movement ceases after the early
growth of each shoot. As in many widely separated families of
plants, single species and single genera possess the power of re-
volving, and have thus become twiners, they must have independ-

242 ORIGIN OF SPECIES

ently acquired it, and cannot have inherited it from a common
progenitor. Hence 1 was led to predict that some shght tendency
to a movement of this kind would be found to be far from uncom-
mon with plants which did not chmb; and that this had alTorded
the basis for natural selection to work on and improve. When I
made this prediction, I knew of only one imperfect case, namely of
the young flower-peduncles of a Maurandia which revolved slightly
and irregularly, like the stems of twining plants, but without mak-
ing any use of this habit. Soon afterwards Fritz Miiller discovered
that the young stems of an Alisma and of a Linum, — plants which
do not climb and are widely separated in the natural system, — re-
volved plainly, though irregularly; and he states that he has reason
to suspect that this occurs with some other plants. These slight
movements appear to be of no service to the plants in question; any-
how, they are not of the least use in the way of climbing, which is
the point that concerns us. Nevertheless we can see that if the
stems of these plants had been flexible, and if under the conditions
to which they are expwsed it had profited them to ascend to a height,
then the habit of sHghtly and irregularly revolving might have been
increased and utihsed through natural selection, until they had be-
come converted into well-developed twining species.

With respect to the sensitiveness of the foot-stalks of the leaves
and flowers, and of tendrils, nearly the same remarks are applicable
as in the case of the revolving movements of twining plants. As a
vast number of species, belonging to widely distinct groups, are
endowed with this kind of sensitiveness, it ought to be found in a
nascent condition in many plants which have not become climbers.
This is the case: I observed that the young flower-peduncles of the
above Maurandia curved themselves a little towards the side which
was touched. Morren found in several species of Oxalis that the
leaves and their foot-stalks moved, especially after exposure to a hot
sun, when they were gently and repeatedly touched, or when the
plant was shaken. I repeated these observations on some other
species of Oxalis with the same result; in some of them the move-
ment was distinct, but was best seen in the young leaves; in others
it was extremely slight. It is a more important fact that according
to the high authority of Hofmeister, the young shoots and leaves

THEORY OF NATURAL SELECTION 243

of all plants move after being shaken; and with climbing plants it
is, as we know, only during the early stages of growth that the foot-
stalks and tendrils are sensitive.

It is scarcely possible that the above slight movements, due to a
touch or shake, in the young and growing organs of plants, can be
of any functional importance to them. But plants possess, in
obedience to various stimuli, powers of movement, which are of
manifest importance to them; for instance, towards and more rarely
from the light, — in opposition to, and more rarely in the direction
of, the attraction of gravity. When the nerves and muscles of an
animal are excited by galvanism or by the absorption of strychnine,
the consequent movements may be called an incidental result, for
the nerves and muscles have not been rendered specially sensitive
to these stimuli. So with plants it appears that, from having the
power of movement in obedience to certain stimuli, they are ex-
cited in an incidental manner by a touch, or by being shaken. Hence
there is no great difficulty in admitting that in the case of leaf-
climbers and tendril-bearers, it is this tendency which has been
taken advantage of and increased through natural selection. It is,
however, probable, from reasons which I have assigned in my
memoir, that this will have occurred only with plants which had
already acquired the power of revolving, and had thus become
twiners.

I have already endeavoured to explain how plants became twiners,
namely, by the increase of a tendency to slight and irregular re-
volving movements, which were at first of no use to them; this
movement, as well as that due to a touch or shake, being the in-
cidental result of the power of moving, gained for other and
beneficial purposes. Whether, during the gradual development of
climbing plants, natural selection has been aided by the inherited
effects of use, I will not pretend to decide; but we know that cer-
tain periodical movements, for instance the so<alled sleep of plants,
are governed by habit.

I have now considered enough, perhaps more than enough, of
the cases, selected with care by a skilful naturalist, to prove that
natural selection is incompetent to account for the incipient stages

244 ORIGIN OF SPECIES

of useful structures; and I have shown, as I hope, that there is no
great difficulty on this head. A good opportunity has thus been
afforded for enlarging a little on gradations of structure, often as-
sociated with changed functions, — an important subject, which was
not treated at sufficient length in the former editions of this work.
I will now briefly recapitulate the foregoing cases.

With the giraffe, the continued preservation of the individuals of
some extinct high-reaching ruminant, which had the longest necks,
legs, etc., and could browse a little above the average height, and
the continued destruction of those which could not browse so high,
would have sufficed for the production of this remarkable quad-
ruped; but the prolonged use of all the parts together with inherit-
ance will have aided in an important manner in their co-ordination.
With the many insects which imitate various objects, there is no
improbability in the belief that an accidental resemblance to some
common object was in each case the foundation for the work of
natural selection, since perfected through the occasional preservation
of slight variations which made the resemblance at all closer; and
this will have been carried on as long as the insect continued to vary,
and as long as a more and more perfect resemblance led to its escape
from sharp-sighted enemies. In certain species of whales there is a
tendency to the formation of irregular little points of horn on the
palate; and it seems to be quite within the scope of natural selection
to preserve all favourable variations, until the points were converted
first into lamellated knobs or teeth, like those on the beak of a goose,
— then into short lamellae, like those of the domestic ducks — and
then into lamellae, as perfect as those of the shoveller-duck, — and
finally into the gigantic plates of baleen, as in the mouth of the
Greenland whale. In the family of the ducks, the lamellse are first
used as teeth, then partly as teeth and partly as a sifting apparatus,
and at last almost exclusively for this latter purpose.

With such structures as the above lamella; of horn or whale-bone,
habit or use can have done little or nothing, as far as we can judge,
towards their development. On the other hand, the transportal of
the lower eye of a flat-fish to the upper side of the head, and the
formation of a prehensile tail, may be attributed almost wholly to
continued use, together with inheritance. With respect to the mam-

THEORY OF NATURAL SELECTION 245

mx of the higher animals, the most probable conjecture is that
primordially the cutaneous glands over the whole surface of a
marsupial sack secreted a nutritious fluid; and that these glands
were improved in function through natural selection, and concen-
trated into a confined area, in which case they would have formed
a mamma. There is no more difficulty in understanding how the
branched spines of some ancient Echinoderm, which served as a
defence, became developed through natural selection into tridactyle
pedicellarii, than in understanding the development of the pincers
of crustaceans, through slight, serviceable modifications in the ulti-
mate and penultimate segments of a limb, which was at first used
solely for locomotion. In the avicularia and vibracula of the Polyzoa
we have organs widely different in appearance developed from the
same source; and with the vibracula we can understand how the
successive gradations might have been of service. With the pollinia
of orchids, the threads which originally served to tie together the
pollen-grains, can be traced cohering into caudicles; and the steps
can likewise be followed by which viscid matter, such as that secreted
by the stigmas of ordinary flowers, and still subserving nearly but
not quite the same purpose, became attached to the free ends of the
caudicles; — all these gradations being of manifest benefit to the
plants in question. With respect to climbing plants, I need not re-
peat what has been so lately said.

It has often been asked, if natural selection be so potent, why has
not this or that structure been gained by certain species, to which it
would apparently have been advantageous? But it is unreasonable
to expect a precise answer to such questions, considering our igno-
rance of the past history of each species, and of the conditions which
at the present day determine its numbers and range. In most cases
only general reasons, but in some few cases special reasons, can be
assigned. Thus to adapt a species to new habits of life, many co-
ordinated modifications are almost indisp)ensable, and it may often
have happened that the requisite parts did not vary in the right
manner or to the right degree. Many species must have been pre-
vented from increasing in numbers through destructive agencies,
which stood in no relation to certain structures, which we imagine
would have been gained through natural selection from appearing

246 ORIGIN OF SPECIES

to us advantageous to the species. In this case, as the struggle for
life did not depend on such structures, they could not have been
acquired through natural selection. In many cases complex and
long-enduring conditions, often of a p)eculiar nature, are necessary
for the development of a structure; and the requisite conditions may
seldom have concurred. The belief that any given structure, which
we think, often erroneously, would have been beneficial to a species,
would have been gained under all circumstances through natural
selection, is opposed to what we can understand of its manner of
action. Mr. Mivart does not deny that natural selection has effected
something; but he considers it as "demonstrably insufficient" to ac-
count for the phenomena which I explain by its agency. His chief
arguments have now been considered, and the others will hereafter
be considered. They seem to me to partake little of the character
of demonstration, and to have little weight in comparison with
those in favour of the power of natural selection, aided by the other
agencies often specified. I am bound to add, that some of the facts
and arguments here used by me, have been advanced for the same
purpose in an able article lately published in the 'Medico-Chirurgical
Review.'

At the present day almost all naturalists admit evolution under
some form. Mr. Mivart believes that sp)ecies change through "an
internal force or tendency," about which it is not pretended that
anything is known. That species have a capacity for change will be
admitted by all evolutionists; but there is no need, as it seems to me,
to invoke any internal force beyond the tendency to ordinary va-
riability, which through the aid of selection by man has given rise
to many well-adapted domestic races, and which through the aid
of natural selection would equally well give rise by graduated steps
to natural races or species. The final result will generally have been,
as already explained, an advance, but in some few cases a retrogres-
sion, in organisation.

Mr. Mivart is further inclined to believe, and some naturalists
agree with him, that new species manifest themselves "with sudden-
ness and by modifications appearing at once." For instance, he sup-
poses that the differences between the extinct three-toed Hipparion
and the horse arose suddenly. He thinks it difficult to believe that

THEORY OF NATURAL SELECTION 247

the wing of a bird "was developed in any other way than by a com-
paratively sudden modification of a marked and important kind;"
and apparently he would extend the same view to the wings of bats
and pterodactyles. This conclusion, wfcich implies great breaks or
discontinuity in the series, appears to me improbable in the highest
degree.

Every one who believes in slow and gradual evolution, will of
course admit that specific changes may have been as abrupt and as
great as any single variation which we meet with under nature, or
even under domestication. But as species are more variable when
domesticated or cultivated than under their natural conditions, it is
not probable that such great and abrupt variations have often
occurred under nature, as are known occasionally to arise under
domestication. Of these latter variations several may be attributed
to reversion; and the characters which thus reappear were, it is
probable, in many cases at first gained in a gradual manner. A still
greater number must be called monstrosities, such as six-fingered
men, porcupine men, Ancon sheep, Niata cattle, &c.; and as they
are widely different in character from natural species, they throw
very little light on our subject. Excluding such cases of abrupt
variations, the few which remain would at best constitute, if found
in a state of nature, doubtful species, closely related to their parental
types.

My reasons for doubting whether natural species have changed
as abrupdy as have occasionally domestic races, and for entirely dis-
believing that they have changed in the wonderful manner indicated
by Mr. Mivart, are as follows. According to our experience, abrupt
and strongly marked variations occur in our domesticated produc-
tions, singly and at rather long intervals of time. If such occurred
under nature, they would be hable, as formerly explained, to be
lost by accidental causes of destruction and by subsequent inter-
crossing; and so it is known to be under domestication, unless
abrupt variations of this kind are specially preserved and separated
by the care of man. Hence, in order that a new species should sud-
denly appear in the manner supposed by Mr. Mivart, it is almost
necessary to believe, in opposition to all analogy, that several won-
derfully changed individuals appeared simultaneously within the

248 ORIGIN OF SPECIES

same district. This difficulty, as in the case of unconscious selection
by man, is avoided on the theory of gradual evolution, through the
preservation of a large number of individuals, which varied more
or less in any favourable direction, and of the destruction of a large
number which varied in an opposite manner.

That many species have been evolved in an extremely gradual
manner, there can hardly be a doubt. The species and even the
genera of many large natural families are so closely allied together,
that it is difficult to distinguish not a few of them. On every con-
tinent in proceeding from north to south, from lowland to upland,
etc., we meet with a host of closely related or representative species;
as we likewise do on certain distinct continents, which we have
reason to believe were formerly connected. But in making these
and the following remarks, I am compelled to allude to subjects
hereafter to be discussed. Look at the many outlying islands round
a continent, and see how many of their inhabitants can be raised
only to the rank of doubtful species. So it is if we look to past
times, and compare the species which have just passed away with
those still living within the same areas; or if we compare the fossil
species embedded in the sub-stages of the same geological formation.
It is indeed manifest that multitudes of species are related in the
closest manner to other species that still exist, or have lately existed;
and it will hardly be maintained that such species have been de-
veloped in an abrupt or sudden manner. Nor should it be forgotten,
when we look to the special parts of allied species, instead of to
distinct species, that numerous and wonderfully fine gradations can
be traced, connecting together widely different structures.

Many large groups of facts are intelligible only on the principle
that species have been evolved by very small steps. For instance, the
fact that the species included in the larger genera are more closely
related to each other, and present a greater number of varieties than
do the species in the smaller genera. The former are also grouped
in Utde clusters, like varieties round species; and they present other
analogies with varieties, as was shown in our second chapter. On
this same principle we can understand how it is that specific charac-
ters are more variable than generic characters; and how the parts
which are developed in an extraordinary degree or manner are more

THEORY OF NATURAL SELECTION 249

variable than other parts of the same species. Many analogous facts,
all pointing in the same direction, could be added.

Although very many species have almost certainly been produced
by steps not greater than those separating fine varieties; yet it may
be maintained that some have been developed in a different and
abrupt manner. Such an admission, however, ought not to be made
without strong evidence being assigned. The vague and in some
respects false analogies, as they have been shown to be by Mr.
Chauncey Wright, which have been advanced in favour of this
view, such as the sudden crystallisation of inorganic substances, or
the falling of a facetted spheroid from one facet to another, hardly
deserve consideration. One class of facts, however, namely, the
sudden appearance of new and distinct forms of life in our geo-
logical formations, supports at first sight the beUef in abrupt de-
velopment. But the value of this evidence depends entirely on the
perfection of the geological record, in relation to periods remote in
the history of the world. If the record is as fragmentary as many
geologists strenuously assert, there is nothing strange in new forms
appearing as if suddenly developed.

Unless we admit transformations as prodigious as those advocated
by Mr. Mivart, such as the sudden development of the wings of
birds or bats, or the sudden conversion of a Hipparion into a horse,
hardly any light is thrown by the belief in abrupt modifications on
the deficiency of connecting links in our geological formations. But
against the belief in such abrupt changes, embryology enters a
strong protest. It is notorious that the wings of birds and bats, and
the legs of horses or other quadrupeds, are undistinguishable at an
early embryonic period, and that they become differentiated by in-
sensibly fine steps. Embryological resemblances of all kinds can be
accounted for, as we shall hereafter see, by the progenitors of our
existing species having varied after early youth, and having trans-
mitted their newly acquired characters to their offspring, at a cor-
responding age. The embryo is thus left almost unaffected, and
serves as a record of the past condition of the species. Hence it is
that existing species during the early stages of their development
so often resemble ancient and extinct forms belonging to the same
class. On this view of the meaning of embryological resemblances.

250 ORIGIN OF SPECIES

and indeed on any view, it is incredible that an animal should have
undergone such momentous and abrupt transformations, as those
above indicated; and yet should not bear even a trace in its embryonic
condition of any sudden modification, every detail in its structure
being developed by insensibly fine steps.

He who believes that some ancient form was transformed sud-
denly through an internal force or tendency into, for instance, one
furnished with wings, will be almost compelled to assume, in opposi-
tion to all analogy, that many individuals varied simultaneously.
It cannot be denied that such abrupt and great changes of structure
are widely different from those which most species apparendy have
undergone. He will further be compelled to believe that many
structures beautifully adapted to all the other parts of the same
creature and to the surrounding conditions, have been suddenly
produced; and of such complex and wonderful co-adaptations, he
will not be able to assign a shadow of an explanation. He will be
forced to admit that these great and sudden transformations have
left no trace of their action on the embryo. To admit all this is, as
it seems to me, to enter into the realms of miracle, and to leave
those of Science.

CHAPTER VIII

Instinct

Instincts comparable with habits, but different in their origin — Instincts
graduated — Aphides and ants — Instincts variable — Domestic in-
stincts, their origin — Natural instincts of the cuckoo, molothrus,
ostrich, and parasitic bees — Slave-making ants — Hive-bee, its cell-
making instinct — Changes of instinct and structure not necessarily
simultaneous — Difficulties of the theory of the Natural Selection of
instincts — Neuter or sterile insects — Summary.

MANY instincts are so wonderful that their development
will probably appear to the reader a difficulty sufficient to
overthrow my whole theory. I may here premise, that I
have nothing to do with the origin of the mental powers, any more
than I have with that of life itself. We are concerned only with the
diversities of instinct and of the other mental faculties in animals of
the same class.

I will not attempt any definition of instinct. It would be easy to
show that several distinct mental actions are commonly embraced by
this term; but every one understands what is meant, when it is said
that instinct impels the cuckoo to migrate and to lay her eggs in other
birds' nests. An action, which we ourselves require experience to en-
able us to perform, when performed by an animal, more especially by
a very young one, without experience, and when performed by many
individuals in the same way, without their knowing for what pur-
pose it is performed, is usually said to be instinctive. But I could
show that none of these characters are universal. A little dose of
judgment or reason, as Pierre Huber expresses it, often comes into
play, even with animals low in the scale of nature.

Frederick Cuvier and several of the older metaphysicians have com-
pared instinct with habit. This comparison gives, I think, an accu-
rate notion of the frame of mind under which an instinctive action
is performed, but not necessarily of its origin. How unconsciously
many habitual actions are performed, indeed not rarely in direct

251

252 ORIGIN OF SPECIES

opposition to our conscious will! yet they may be modified by the
will or reason. Habits easily become associated with other habits,
with certain periods of time, and states of the body. When once
acquired, they often remain constant throughout life. Several other
points of resemblance between instincts and habits could be pointed
out. As in repeating a well-known song, so in instincts, one action
follows another by a sort of rhythm; if a person be interrupted in a
song, or in repeating anything by rote, he is generally forced to go
back to recover the habitual train of thought; so P. Huber found it
was with a caterpillar, which makes a very complicated hammock;
for if he took a caterpillar which had completed its hammock up
to, say, the sixth stage of construction, and put it into a hammock
completed up only to the third stage, the caterpillar simply re-per-
formed the fourth, fifth, and sixth stages of construction. If, how-
ever, a caterpillar were taken out of a hammock made up, for in-
stance, to the third stage, and were put into one finished up to the
sixth stage, so that much of its work was already done for it, far from
deriving any benefit from this, it was much embarrassed, and in
order to complete its hammock, seemed forced to start from the
third stage, where it had left off, and thus tried to complete the
already finished work.

If we suppose any habitual action to become inherited — and it can
be shown that this does sometimes happen — then the resemblance
between what originally was a habit and an instinct becomes so close
as not to be distinguished. If Mozart, instead of playing the piano-
forte at three years old with wonderfully little practice, had played a
tune with no practice at all, he might truly be said to have done so
instinctively. But it would be a serious error to suppose that the
greater number of instincts have been acquired by habit in one gen-
eration, and then transmitted by inheritance to succeeding genera-
tions. It can be clearly shown that the most wonderful instincts with
which we are acquainted, namely, those of the hive-bee and of many
ants, could not possibly have been acquired by habit.

It will be universally admitted that instincts are as important as
corporeal structures for the welfare of each species, under its present
conditions of life. Under changed conditions of life, it is at least
possible that slight modifications of instinct might be profitable to a

INSTINCT 253

species; and if it can be shown that instincts do vary ever so little,
then I can see no difficulty in natural selection preserving and con-
tinually accumulating variations of instinct to any extent that was
profitable. It is thus, as I believe, that all the most complex and won-
derful instincts have originated. As modifications of corporeal struc-
ture arise from, and are increased by, use or habit, and are diminished
or lost by disuse, so I do not doubt it has been with instincts. But I
believe that the effects of habit are in many cases of subordinate im-
portance to the effects of the natural selection of what may be called
spontaneous variations of instincts; — that is of variations produced
by the same unknown causes which produce slight deviations of
bodily structure.

No complex instinct can possibly be produced through natural
selection, except by the slow and gradual accumulation of numerous
slight, yet profitable, variations. Hence, as in the cases of corporeal
structures, we ought to find in nature, not the actual transitional
gradations by which each complex instinct has been acquired — for
these could be found only in the lineal ancestors of each species — but
we ought to find in the collateral lines of descent some evidence of
such gradations; or we ought at least be able to show that gradations
of some kind are possible; and this we certainly can do. I have been
surprised to find, making allowance for the instincts of animals hav-
ing been but little observed except in Europe and North America,
and for no instinct being known amongst extinct species, how very
generally gradations, leading to the most complex instincts, can be dis-
covered. Changes of instinct may sometimes be facilitated by the
same species having different instincts at different periods of life, or
at different seasons of the year, or when placed under different cir-
cumstances, etc.; in which case either the one or the other instinct
might be preserved by natural selection. And such instances of
diversity of instinct in the same species can be shown to occur in
nature.

Again, as in the case of corporeal structure, and conformably to
my theory, the instinct of each species is good for itself, but has
never, as far as we can judge, been produced for the exclusive good
of others. One of the strongest instances of an animal apparently
performing an action for the sole good of another, with which I am

254 ORIGIN OF SPECIES

acquainted, is that of aphides voluntarily yielding, as was first ob-
served by Huber, their sweet excretion to ants: that they do so vol-
untarily, the following facts show: I removed all the ants from a
group of about a dozen aphides on a dock-plant, and prevented their
attendance during several hours. After this interval, I felt sure that
the aphides would want to excrete. I watched them for some time
through a lens, but not one excreted; 1 then tickled and stroked them
with a hair in the same mani\er, as well as I could, as the ants do
with their antennx; but not one excreted. Afterwards 1 allowed an
ant to visit them, and it immediately seemed, by its eager way of
running about, to be well aware what a rich flock it had discovered;
it then began to play with its antenna; on the abdomen first of one
aphis and then of another; and each, as soon as it felt the antennae,
immediately lifted up its abdomen and excreted a limpid drop of
sweet juice, which was eagerly devoured by the ant. Even the
quite young aphides behaved in this manner, showing that the action
was instinctive, and not the result of experience. It is certain, from
the observations of Huber, that the aphides show no dislike to the
ants: if the latter be not present they are at last compelled to eject
their excretion. But as the excretion is extremely viscid, it is no doubt
a convenience to the aphides to have it removed; therefore probably
they do not excrete solely for the good of the ants. Although there is
no evidence that any animal performs an action for the exclusive
good of another sjjecies, yet each tries to take advantage of the
instincts of others, as each takes advantage of the weaker bodily
structure of other species. So again certain instincts cannot be con-
sidered as absolutely perfect; but as details on this and other such
points are not indispensable, they may be here passed over.

As some degree of variation in instincts under a state of na-
ture, and the inheritance of such variations, are indispensable for
the action of natural selection, as many instances as possible ought to
be given; but want of space prevents me. I can only assert that in-
stincts certainly do vary — for instance, the migratory instinct, both in
extent and direction, and in its total loss. So it is with the nests of
birds, which vary partly in dependence on the situations chosen, and
on the nature and temperature of the country inhabited, but often
from causes wholly unknown to us: Audubon has given several re-

CHANGES OF HABIT OR INSTINCT 255

markable cases of diflerences in the nests of the same species in the
northern and southern United States. Why, it has been asked, if
instinct be variable, has it not granted to the bee "the abiUty to use
some other material when wax was deficient"? But what other
natural material could bees use? They will work, as I have seen,
with wax hardened with vermilion or softened with lard. Andrew
Knight observed that his bees, instead of laboriously collecting prop-
olis, used a cement of wax and turpentine, with which he had cov-
ered decorticated trees. It has lately been shown that bees, instead
of searching for pollen, will gladly use a very different substance,
namely oatmeal. Fear of any particular enemy is certainly an instinc-
tive quahty, as may be seen in nestling birds, though it is strengthened
by experience, and by the sight of fear of the same enemy in other
animals. The fear of man is slowly acquired, as 1 have elsewhere
shown, by the various animals which inhabit desert islands; and we
see an instance of this even in England, in the greater wildness of
all our large birds in comparison with our small birds; for the large
birds have been most persecuted by man. We may safely attribute the
greater wildness of our large birds to this cause; for in uninhabited
islands large birds are not more fearful than small; and the magpie,
so wary in England, is tame in Norway, as is the hooded crow in
Egypt.

That the mental qualities of animals of the same kind, born in a
state of nature, vary much, could be shown by many facts. Several
cases could also be adduced of occasional and strange habits in wild
animals, which, if advantageous to the species, might have given rise,
through natural selection, to new instincts. But I am well aware that
these general statements, without the facts in detail, will produce but
a feeble effect on the reader's mind. I can only repeat my assurance,
that I do not speak without good evidence.

INHERITED CHANGES OF HABIT OR INSTINCT IN
DOMESTICATED ANIMALS

The possibility, or even probability, of inherited variations of in-
stinct in a state of nature will be strengthened by briefly considering
a few cases under domestication. We shall thus be enabled to see
the part which habit and the selection of so-called spontaneous varia-

256 ORIGIN OF SPECIES

tions have played in modifying the mental qualities of our domestic
animals. It is notorious how much domestic animals vary in their
mental qualities. With cats, for instance, one naturally takes to
catching rats, and another mice, and these tendencies are known to
be inherited. One cat, according to Mr. St. John, always brought
home game-birds, another hares or rabbits, and another hunted on
marshy ground and almost nightly caught woodcocks or snipes. A
number of curious and authentic instances could be given of various
shades of disposition and of taste, and likewise of the oddest tricks,
associated with certain frames of mind or periods of time, being in-
herited. But let us look to the familiar case of the breeds of the dogs:
it cannot be doubted that young pointers (I have myself seen a strik-
ing instance) will sometimes point and even back other dogs the very
first time that they are taken out; retrieving is certainly in some de-
gree inherited by retrievers; and a tendency to run round, instead of
at, a flock of sheep, by shepherd dogs. I cannot see that these actions,
performed without experience by the young, and in nearly the same
manner by each individual, performed with eager delight by each
breed, and without the end being known — for the young pointer can
no more know that he points to aid his master, than the white butter-
fly knows why she lays her eggs on the leaf of the cabbage — I cannot
see that these actions differ essentially from true instincts. If we were
to behold one kind of wolf, when young and without any training,
as soon as it scented its prey, stand motionless like a statue, and then
slowly crawl forward with a peculiar gait; and another kind of wolf
rushing round, instead of at, a herd of deer, and driving them to a
distant point, we should assuredly call these actions instinctive.
Domestic instincts, as they may be called, are certainly far less Exed
than natural instincts; but they have been acted on by far less rigorous
selection, and have been transmitted for an incomparably shorter
period, under less fixed conditions of life.

How strongly these domestic instincts, habits, and dispositions are
inherited, and how curiously they become mingled, is well shown
when different breeds of dogs are crossed. Thus it is known that
a cross with a bull-dog has affected for many generations the courage
and obstinacy of greyhounds; and a cross with a greyhound has
given to a whole family of shepherd-dogs a tendency to hunt hares.

CHANGES OF HABIT OR INSTINCT 257

These domestic instincts, when thus tested by crossing, resemble
natural instincts, which in a like manner become curiously blended
together, and for a long period exhibit traces of the instincts of either
parent: for example, Le Roy describes a dog, whose great-grandfather
was a wolf, and this dog showed a trace of its wild parentage only in
one way, by not coming in a straight line to his master when called.

Domestic instincts are sometimes spoken of as actions which have
become inherited solely from long-continued and compulsory habit;
but this is not true. No one would ever have thought of teaching, or
probably could have taught, the tumbler-pigeon to tumble, — an action
which, as I have witnessed, is performed by young birds that have
never seen a pigeon tumble. We may believe that some one pigeon
showed a slight tendency to this strange habit, and that the long-con-
tinued selection of the best individuals in successive generations made
tumblers what they now are; and near Glasgow there are house-tum-
blers, as I hear from Mr. Brent, which cannot fly eighteen inches high
without going head over heels. It may be doubted whether any one
would have thought of training a dog to point, had not some one dog
naturally shown a tendency in this line; and this is known occasion-
ally to happen, as I once saw, in a pure terrier: the act of pointing
is probably, as many have thought, only the exaggerated pause of an
animal preparing to spring on its prey. When the first tendency to
point was once displayed, methodical selection and the inherited
effects of compulsory training in each successive generation would
soon complete the work; and unconscious selection is still in progress,
as each man tries to procure, without intending to improve the breed,
dogs which stand and hunt best. On the other hand, habit alone in
some cases has sufficed; hardly any animal is more difficult to tame
than the young of the wild rabbit; scarcely any animal is tamer
than the young of the tame rabbit; but J can hardly suppose that
domestic rabbits have often been selected for lameness alone; so that
we must attribute at least the greater part of the inherited change
from extreme wildness to extreme lameness, to habit and long>
continued close confinement.

Natural instincts are lost under domestication: a remarkable in-
stance of this is seen in those breeds of fowls which very rarely or
never become "broody," that is, never wish to sit on their eggs. Famil-

258 ORIGIN OF SPECIES

iarity alone prevents our seeing how largely and how permanently the
minds of our domestic animals have been modified. It is scarcely pos-
sible to doubt that the love of man has become instinctive in the dog.
All wolves, foxes, jackals, and species of the cat genus, when kept
tame, are most eager to attack poultry, sheep, and pigs; and this
tendency has been found incurable in dogs which have been brought
home as puppies from countries such as Tierra del Fuego and Aus-
tralia, where the savages do not keep these domestic animals. How
rarely, on the other hand, do our civilised dogs, even when quite
young, require to be taught not to attack poultry, sheep, and pigs!
No doubt they occasionally do make an attack, and are then beaten;
and if not cured, they are destroyed; so that habit and some degree of
selection have probably concurred in civilising by inheritance our
dogs. On the other hand, young chickens have lost, wholly by habit,
that fear of the dog and cat which no doubt was originally instinc-
tive in them; for I am informed by Captain Hutton that the young
chickens of the parent-stock, the Gallus bankiva, when reared in
India under a hen, are at first excessively wild. So it is with young
pheasants reared in England under a hen. It is not that chickens have
lost all fear, but fear only of dogs and cats, for if the hen gives the
danger-chuckle, they will run (more especially young turkeys) from
under her, and conceal themselves in the surrounding grass or
thickets; and this is evidently done for the instinctive purpose of
allowing, as we see in wild ground-birds, their mother to fly away.
But this instinct retained by our chickens has become useless under
domestication, for the mother-hen has almost lost by disuse the
power of flight.

Hence, we may conclude that under domestication instincts have
been acquired, and natural instincts have been lost, partly by habit,
and partly by man selecting and accumulating, during successive gen-
erations, peculiar mental habits and actions, which at first appeared
from what we must in our ignorance call an accident. In some cases
compulsory habit alone has sufficed to produce inherited mental
changes; in other cases, compulsory habit has done nothing, and all
has been the result of selection, pursued both methodically and un-
consciously: but in most cases habit and selection have probably
concurred.

INSTINCTS OF THE CUCKOO 259

SPECIAL INSTINCTS

We shall, perhaps, best understand how instincts in a state of na-
ture have become modified by selection, by considering a few cases.
I will select only three, — namely, the instinct which leads the cuckoo
to lay her eggs in other birds' nests; the slave-making instinct o£
certain ants; and the cell-making power of the hive-bee. These two
latter instincts have generally and justly been ranked by naturalists
as the most wonderful of all known instincts.

Instincts of the Ct(c/(oo. — It is supposed by some naturahsts that the
more immediate cause of the instinct of the cuckoo is, that she lays
her eggs, not daily, but at intervals of two or three days; so that, i£
she were to make her own nest and sit on her own eggs, those first
laid would have to be left for some time unincubated, or there would
be eggs and young birds of different ages in the same nest. If this
were the case, the process of laying and hatching might be inconven-
iently long, more especially as she migrates at a very early period;
and the first hatched young would probably have to be fed by the
male alone. But the American cuckoo is in this predicament; for
she makes her own nest, and has eggs and young successively hatched,
all at the same time. It has been both asserted and denied that the
American cuckoo occasionally lays her eggs in other birds' nests; but
I have lately heard from Dr. Merrell, of Iowa, that he once found in
Illinois a young cuckoo together with a young jay in the nest of a
Blue jay (Garrulus cristatus) ; and as both were nearly full feathered,
there could be no mistake in their identification. I could also give
several instances of various birds which have been known occasion-
ally to lay their eggs in other birds' nests. Now let us suppose that
the ancient progenitor of our European cuckoo had the habits of the
American cuckoo, and that she occasionally laid an egg in another
bird's nest. If the old bird profited by this occasional habit through
being enabled to migrate earlier or through any other cause; or if the
young were made more vigorous by advantage being taken of the
mistaken instinct of another s^iecies than when reared by their own
mother, encumbered as she could hardly fail to be by having eggs and
young of different ages at the same time; then the old birds or the
fostered young would gain an advantage. And analogy would lead

26o ORIGIN OF SPECIES

US to believe, that the young thus reared would be apt to follow by in-
heritance the occasional and aberrant habit of their mother, and in
their turn would be apt to lay their eggs in other birds' nests, and thus
be more successful in rearing their young. By a continued process of
this nature, I believe that the strange instinct of our cuckoo has been
generated. It has, also, recently been ascertained on sufficient evi-
dence, by Adolf Miiller, that the cuckoo occasionally lays her eggs on
the bare ground, sits on them, and feeds her young. This rare event
is probably a case of reversion to the long-lost, aboriginal instinct of
nidification.

It has been objected that I have not noticed other related instincts
and adaptations of structure in the cuckoo, which are spoken of as
necessarily co-ordinated. But in all cases, speculation on an instinct
known to us only in a single species, is useless, for we have hitherto
had no facts to guide us. Until recently the instincts of the Euro-
pean and of the non-parasitic American cuckoo alone were known;
now, owing to Mr. Ramsay's observations, we have learnt something
about three Australian species, which lay their eggs in other birds'
nests. The chief points to be referred to are three: first, that the
common cuckoo, with rare exceptions, lays only one egg in a nest,
so that the large and voracious young bird receives ample food.
Secondly, that the eggs are remarkably small, not exceeding those
of the skylark, — a bird about one-fourth as large as the cuckoo. That
the small size of the egg is a real case of adaptation we may infer
from the fact of the non-parasitic American cuckoo laying full-sized
eggs. Thirdly, that the young cuckoo, soon after birth, has the in-
stinct, the strength, and a properly shaped back for ejecting its foster-
brothers, which then perish from cold and hunger. This has been
boldly called a beneficial arrangement, in order that the young
cuckoo may get sufficient food, and that its foster-brothers may perish
before they had acquired much feeling!

Turning now to the Australian species; though these birds gener-
ally lay only one egg in a nest, it is not rare to find two and even
three eggs in the same nest. In the Bronze cuckoo the eggs vary
greatly in size, from eight to ten lines in length. Now if it had been
of an advantage to this species to have laid eggs even smaller than
those now laid, so as to have deceived certain foster-parents, or, as is

INSTINCTS OF THE CUCKOO 261

more probable, to have been hatched within a shorter period (for it
is asserted that there is a relation between the size of eggs and the
period of their incubation), then there is no difficulty in believing
that a race or species might have been formed which would have
laid smaller and smaller eggs; for these would have been more safely
hatched and reared. Mr. Ramsay remarks that two of the Australian
cuckoos, when they lay their eggs in an open nest, manifest a decided
preference for nests containing eggs similar in colour to their own.
The European species apparently manifests some tendency towards
a similar instinct, but not rarely departs from it, as is shown by her
laying her dull and pale-coloured eggs in the nest of the Hedge-
warbler with bright greenish-blue eggs. Had our cuckoo invariably
displayed the above instinct, it would assuredly have been added to
those which it is assumed must all have been acquired together.
The eggs of the Australian Bronze cuckoo vary, according to Mr.
Ramsay, to an extraordinary degree in colour; so that in this respect,
as well as in size, natural selection might have secured and fixed any
advantageous variation.

In the case of the European cuckoo, the offspring of the foster-
parents are commonly ejected from the nest within three days after
the cuckoo is hatched; and as the latter at this age is in a most
helpless condition, Mr. Gould was formerly inclined to believe that
the act of ejection was performed by the foster-parents themselves.
But he has now received a trustworthy account of a young cuckoo
which was actually seen, whilst still blind and not able even to hold
up its own head, in the act of ejecting its foster-brothers. One of
these was replaced in the nest by the observer, and was again thrown
out. With respect to the means by which this strange and odious
instinct was acquired, if it were of great impxDrtance for the young
cuckoo, as is probably the case, to receive as much food as possible
soon after birth, I can see no special difficulty in its having gradually
acquired, during successive generations, the blind desire, the strength,
and structure necessary for the work of ejection; for those young
cuckoos which had such habits and structure best developed would
be the most securely reared. The first step towards the acquisition
of the proper instinct might have been mere unintentional restless-
ness on the part of the young bird, when somewhat advanced in

262 ORIGIN OF SPECIES

age and strength; the habit having been afterwards improved, and
transmitted to an earher age. I can see no more difficulty in this,
than in the unhatched young of other birds acquiring the instinct
to break through their own shells; — or than in young snakes acquir-
ing in their upper jaws, as Owen has remarked, a transitory sharp
tooth for cutting through the tough egg-shell. For if each part is
liable to individual variations at all ages, and the variations tend to
be inherited at a corresfxinding or earlier age, — propositions which
cannot be disputed, — then the instincts and structure of the young
could be slowly modified as surely as those of the adult; and both
cases must stand or fall together with the whole theory of natural
selection.

Some species of Molothrus, a widely distinct genus of American
birds, allied to our starlings, have parasitic habits like those of the
cuckoo; and the species present an interesting gradation in the per-
fection of their instincts. The sexes of Molothrus badius are stated
by an excellent observer, Mr. Hudson, sometimes to live promiscu-
ously together in flocks, and sometimes to pair. They either build
a nest of their own, or seize on one belonging to some other bird,
occasionally throwing out the nestlings of the stranger. They either
lay their eggs in the nest thus appropriated, or oddly enough build
one for themselves on the top of it. They usually sit on their own
eggs and rear their own young; but Mr. Hudson says it is probable
that they are occasionally parasitic, for he has seen the young of this
species following old birds of a distinct kind and clamouring to be
fed by them. The parasitic habits of another species of Molothrus,
the M. bonariensis, are much more highly developed than those
of the last, but are still far from perfect. This bird, as far as it is
known, invariably lays its eggs in the nests of strangers; but it is
remarkable that several together sometimes commence to build an
irregular untidy nest of their own, placed in singularly ill-adapted
situations, as on the leaves of a large thistle. They never, however, as
far as Mr. Hudson has ascertained, complete a nest for themselves.
They often lay so many eggs — from fifteen to twenty — in the same
foster-nest, that few or none can possibly be hatched. They have,
moreover, the extraordinary habit of pecking holes in the eggs.

INSTINCTS OF THE MOLOTHRUS 263

whether of their own species or of their foster-parents, which they
find in the appropriated nests. They drop also many eggs on the bare
ground, which are thus wasted. A third species, the M. pecoris o£
North America, has acquired instincts as perfect as those of the
cuckoo, for it never lays more than one egg in a foster-nest, so that
the young bird is securely reared. Mr. Hudson is a strong dis-
believer in evolution, but he app)ears to have been so much struck
by the imperfect instincts of the Molothrus bonariensis that he quotes
my words, and asks, "Must we consider these habits, not as espe-
cially endowed or created instincts, but as small consequences of one
general law, namely, transition?"

Various birds, as has already been remarked, occasionally lay their
eggs in the nests of other birds. This habit is not very uncommon
with the Gallinaceac, and throws some Ught on the singular instinct
of the ostrich. In this family several hen birds unite and lay first
a few eggs in one nest and then in another; and these are hatched by
the males. This instinct may probably be accounted for by the fact
of the hens laying a large number of eggs, but, as with the cuckoo,
at intervals of two or three days. The instinct, however, of the
American ostrich, as in the case of the Molothrus bonariensis, has
not as yet been perfected; for a surprising number of eggs lie strewed
over the plains, so that in one day's hunting I picked up no less than
twenty lost and wasted eggs.

Many bees are parasitic, and regularly lay their eggs in the nests
of other kinds of bees. This case is more remarkable than that of the
cuckoo; for these bees have not only had their instincts but their
structure modified in accordance with their parasitic habits; for they
do not possess the f)ollen-collecting apparatus which would have
been indispensable if they had stored up food for their own young.
Some sf)ecies of Sphegidse (wasp-like insects) are likewise parasitic;
and M. Fabre has lately shown good reason for believing that,
although the Tachytes nigra generally makes its own burrow and
stores it with paralysed prey for its own larvae, yet that, when this
insect finds a burrow already made and stored by another sphex, it
takes advantage of the prize, and becomes for the occasion parasitic
In this case, as with that of the Molothrus or cuckoo, I can see no

264 ORIGIN OF SPECIES

difficulty in natural selection making an occasional habit permanent,
if of advantage to the species, and if the insect whose nest and stored
food are feloniously appropriated, be not thus exterminated.

Slave-making instinct. — This remarkable instinct was first dis-
covered in the Formica (Polyerges) rufescens by Pierre Huber, a
better observer even than his celebrated father. This ant is absolutely
dependent on its slaves; without their aid, the species would certainly
become extinct in a single year. The males and fertile females do
no work of any kind, and the workers or sterile females, though most
energetic and courageous in capturing slaves, do no other work.
They are incapable of making their own nests, or of feeding their
own larvae. When the old nest is found inconvenient, and they have
to migrate, it is the slaves which determine the migration, and
actually carry their masters in their jaws. So utterly helpless are the
masters, that when Huber shut up thirty of them without a slave,
but with plenty of the food which they like best, and with their own
larvae and pupae to stimulate them to work, they did nothing; they
could not even feed themselves, and many perished of hunger.
Huber then introduced a single slave (F. fusca), and she instandy
set to work, fed and saved the survivors; made some cells and tended
the larvar, and put all to rights. What can be more extraordinary
than these well-ascertained facts.? If we had not known of any
other slave-making ant, it would have been hopeless to speculate
how so wonderful an instinct could have been perfected.

Another species, Formica sanguinea, was likewise first discovered
by P. Huber to be a slave-making ant. This species is found in the
southern parts of England, and its habits have been attended to by
Mr. F. Smith, of the British Museum, to whom I am much indebted
for information on this and other subjects. Although fully trusting
to the statements of Huber and Mr. Smith, I tried to approach the
subject in a sceptical frame of mind, as any one may well be excused
for doubting the existence of so extraordinary an instinct as that of
making slaves. Hence, I will give the observations which I made in
some little detail. I opened fourteen nests of F. sanguinea, and found
a few slaves in all. Males and fertile females of the slave species (F.
fusca) are found only in their own proper communities, and have
never been observed in the nests of F. sanguinea. The slaves

SLAVE-MAKING INSTINCT 265

are black and not above half the size of their red masters, so that
the contrast in their appearance is great. When the nest is slightly
disturbed, the slaves occasionally come out, and like their masters
are much agitated and defend the nest: when the nest is much
disturbed, and the larva: and pupae are exposed, the slaves work
energetically together with their masters in carrying them away to
a place of safety. Hence, it is clear, that the slaves feel quite at home.
During the months of June and July, on three successive years, I
watched for many hours several nests in Surrey and Sussex, and
never saw a slave either leave or enter a nest. As, during these
months, the slaves are very few in number, I thought that they might
behave differently when more numerous; but Mr. Smith informs me
that he has watched the nests at various hours during May, June, and
August, both in Surrey and Hampshire, and has never seen the
slaves, though present in large numbers in August, either leave or
enter the nest. Hence, he considers them as strictly household slaves.
The masters, on the other hand, may be constantly seen bringing in
materials for the nest, and food of all kinds. During the year i860,
however, in the month of July, I came across a community with an
unusually large stock of slaves, and I observed a few slaves mingled
with their masters leaving the nest, and marching along the same
road to a tall Scotch-fir tree, twenty-five yards distant, which they
ascended together, probably in search of aphides or cocci. According
to Huber, who had ample opportunities for observation, the slaves
in Switzerland habitually work with their masters in making the
nest, and they alone open and close the doors in the morning and
evening; and, as Huber expressly states, their principal office is to
search for aphides. This difTerence in the usual habits of the masters
and slaves in the two countries, probably depends merely on the
slaves being captured in greater numbers in Switzerland than in
England.

One day I fortunately witnessed a migration of F. sanguinea from
one nest to another, and it was a most interesting spectacle to behold
the masters carefully carrying their slaves in their jaws instead of
being carried by them, as in the case of F. rufescens. Another day
my attention was struck by about a score of the slave-makers haunt-
ing the same spot, and evidently not in search of food; they ap-

266 ORIGIN OF SPECIES

preached and were vigorously repulsed by an independent com-
munity of the slave-species (F. f usca) ; sometimes as many as three
of these ants clinging to the legs of the slave-making F. sanguinea.
The latter ruthlessly killed their small opponents, and carried their
dead bodies as food to their nest, twenty-nine yards distant; but
they were prevented from getting any pupae to rear as slaves. I then
dug up a small parcel of the pupae of F. fusca from another nest,
and put them down on a bare spot near the place of combat; they
were eagerly seized and carried off by the tyrants, who perhaps
fancied that, after all, they had been victorious in their late com-
bat.

At the same time I laid on the same place a small parcel of the
pupae of another species, F. flava, with a few of these little yellow ants
still clinging to the fragments of their nest. This species is some-
times, though rarely, made into slaves, as has been described by
Mr. Smith. Although so small a species, it is very courageous, and I
have seen it ferociously attack other ants. In one instance I found
to my surprise an independent community of F. flava under a stone
beneath a nest of the slave-making F. sanguinea; and when I had
accidentally disturbed both nests, the little ants attacked their big
neighbours with surprising courage. Now I was curious to ascertain
whether F. sanguinea could distinguish the pupae of F. fusca, which
they habitually make into slaves, from those of the little and furious
F. flava, which they rarely capture, and it was evident that they did
at once distinguish them; for we have seen that they eagerly and
instantly seized the pupx of F. fusca, whereas they were much terri-
fied when they came across the pupae, or even the earth from the nest,
of F. flava, and quickly ran away; but in about a quarter of an
hour, shortly after all the little yellow ants had crawled away, they
took heart and carried off the pups.

One evening I visited another community of F. sanguinea, and
found a number of these ants returning home and entering their
nests, carrying the dead bodies of F. fusca (showing that it was not
a migration) and numerous pupae. I traced a long file of ants
burthened with booty, for about forty yards back, to a very thick
clump of heath, whence I saw the last individual of F. sanguinea
emerge, carrying a pupa; but I was not able to find the desolated nest
in the thick heath. The nest, however, must have been close at hand,

SLAVE-MAKING INSTINCT 267

for two or three individuals of F. fusca were rushing about in the
greatest agitation, and one was perched motionless with its own pupa
in its mouth on the top of a spray of heath, an image of despair over
its ravaged home.

Such are the facts, though they did not need confirmation by me,
in regard to the wonderful instinct of making slaves. Let it be
observed what a contrast the instinctive habits of F. sanguinea present
with those of the continental F. rufescens. The latter does not build
its own nest, does not determine its own migrations, does not collect
food for itself or its young, and cannot even feed itself: it is absolutely
dependent on its numerous slaves. Formica sanguinea, on the other
hand, possesses much fewer slaves, and in the early part of the
summer extremely few: the masters determine when and where a
new nest shall be formed, and when they migrate, the masters carry
the slaves. Both in Switzerland and England the slaves seem to have
the exclusive care of the larvx, and the masters alone go on slave-
making exf)editions. In Switzerland the slaves and masters work
together, making and bringing materials for the nest; both, but
chiefly the slaves, tend, and milk, as it may be called, their aphides;
and thus both collect food for the community. In England the
masters alone usually leave the nest to collect building materials
and food for themselves, their slaves and larvx. So that the masters
in this country receive much less service from their slaves than they
do in Switzerland.

By what steps the instinct of F. sanguinea originated I will not
pretend to conjecture. But as ants which are not slave-makers will,
as I have seen, carry off the pupa: of other species, if scattered near
their nests, it is possible that such pupa? originally stored as food
might become developed; and the foreign ants thus unintentionally
reared would then follow their proper instincts, and do what work
they could. If their presence proved usefu! to the species which had
seized them — if it were more advantageous to this species to capture
workers than to procreate them — the habit of collecting pupae, origi-
nally for food, might by natural selection be strengthened and ren-
dered permanent for the very different purpose of raising slaves.
When the instinct was once acquired, if carried out to a much less ex-
tent even than in our British F. sanguinea, which, as we have seen, is
less aided by its slaves than the same species in Switzerland, natural

268 ORIGIN OF SPECIES

selection might increase and modify the instinct — always supposing
each modification to be of use to the species — until an ant was
formed as abjectly dependent on its slaves as is the Formica rufescens.

CELL-MAKING INSTINCT OF THE HIVE BEE

I will not here enter on minute details on this subject, but will
merely give an outline of the conclusions at which I have arrived.
He must be a dull man who can examine the exquisite structure
of a comb, so beautifully adapted to its end, without enthusiastic
admiration. We hear from mathematicians that bees have practically
solved a recondite problem, and have made their cells of the proper
shape to hold the greatest possible amount of honey, with the least
possible consumption of precious wax in their construction. It has
been remarked that a skilful workman with fitting tools and
measures, would find it very difficult to make cells of wax of the true
form, though this is effected by a crowd of bees working in a dark
hive. Granting whatever instincts you please, it seems at first quite
inconceivable how they can make all the necessary angles and planes,
or even perceive when they are correctly made. But the difficulty is
not nearly so great as it at first appears: all this beautiful work can
be shown, I think, to follow from a few simple instincts.

I was led to investigate this subject by Mr. Waterhouse, who has
shown that the form of the cell stands in close relation to the presence
of adjoining cells; and the following view may, fjerhaps, be con-
sidered only as a modification of his theory. Let us look to the great
principle of gradation, and see whether Nature does not reveal to
us her method of work. At one end of a short series we have humble-
bees, which use their old cocoons to hold honey, sometimes adding
to them short tubes of wax, and likewise making separate and very
irregular rounded cells of wax. At the other end of the series we have
the cells of the hive bee, placed in a double layer: each cell, as is
well known, is an hexagonal prism, with the basal edges of its six
sides bevelled so as to join an inverted pyramid, of three rhombs.
These rhombs have certain angles, and the three which form the
pyramidal base of a single cell on one side of the comb enter into
the composition of the bases of three adjoining cells on the oppxjsite
side. In the series between the extreme perfection of the cells of the

CELL-MAKING INSTINCT 269

hive-bee and the simpHcity of those of the humble-bee we have the
cells of the Mexican Melipona domestica, carefully described and
figured by Pierre Huber. The Melipona itself is intermediate in
structure between the hive and humble-bee, but more nearly related
to the latter; it forms a nearly regular waxen comb of cylindrical
cells, in which the young are hatched, and, in addition, some large
cells of wax for holding honey. These latter cells are nearly spherical
and of nearly equal sizes, and are aggregated into an irregular mass.
But the important point to notice is, that these cells are always made
at that degree of nearness to each other that they would have inter-
sected or broken into each other if the spheres had been completed;
but this is never permitted, the bees building perfectly flat walls of
wax between the spheres which thus tend to intersect. Hence, each
cell consists of an outer spherical portion, and of two, three, or more
flat surfaces, according as the cell adjoins two, three, or more other
cells. When one cell rests on three other cells, which, from the
spheres being nearly of the same size, is very frequently and neces-
sarily the case, the three flat surfaces are united into a pyramid; and
this pyramid, as Huber has remarked, is manifestly a gross imitation
of the three-sided pyramidal base of the cell of the hive-bee. As in
the cells of the hive-bee, so here, the three plane surfaces in any one
cell necessarily enter into the construction of three adjoining cells.
It is obvious that the Melipona saves wax, and what is more impor-
tant, labour, by this manner of building; for the flat walls between
the adjoining cells are not double, but are of the same thickness as
the outer spherical portions, and yet each flat portion forms a part of
two cells.

Reflecting on this case, it occurred to me that if the Melipona had
made its spheres at some given distance from each other, and had
made them of equal sizes and had arranged them symmetrically in
a double layer, the resulting structure would have been as perfect as
the comb of the hive-bee. Accordingly I wrote to Professor Miller
of Cambridge, and this geometer has kindly read over the following
statement, drawn up from his information, and tells me that it is
strictly correct: —

If a number of equal spheres be described with their centres
placed in two parallel layers; with the centre of each sphere at the

270 ORIGIN OF SPECIES

distance of radius X V2, or radius X 141421 (or at some lesser
distance), from the centres of the six surrounding spheres in the
same layer; and at the same distance from the centres of the adjoin-
ing spheres in the other and parallel layer; then, if planes of inter-
section between the several spheres in both layers be formed, there
will result a double layer of hexagonal prisms united together by
pyramidal bases formed of three rhombs; and the rhombs and the
sides of the hexagonal prisms will have every angle identically the
same with the best measurements which have been made of the cells
of the hive-bee. But I hear from Professor Wyman, who has made
numerous careful measurements, that the accuracy of the workman-
ship of the bee has been greatly exaggerated; so much so, that what-
ever the typical form of the cell may be, it is rarely, if ever, realised.

Hence, we may safely conclude that, if we could slightly modify
the instincts already possessed by the Melipona, and in themselves
not very wonderful, this bee would make a structure as wonderfully
perfect as that of the hive-bee. We must suppose the Melipona to
have the power of forming her cells truly spherical, and of equal
sizes; and this would not be very surprising, seeing that she already
does so to a certain extent, and seeing what perfectly cyhndrical
burrows many insects make in wood, apparently by turning round
on a fixed point. We must suppose the Melipona to arrange her cells
in level layers, as she already does her cylindrical cells; and we must
further suppose, and this is the greatest difficulty, that she can some-
how judge accurately at what distance to stand from her fellow-
labourers when several are making their spheres; but she is already
so far enabled to judge of distance, that she always describes her
spheres so as to intersect to a certain extent; and then she unites
the points of intersection by perfectly flat surfaces. By such modi-
fications of instincts which in themselves are not very wonder-
ful — hardly more wonderful than those which guide a bird to make
its nest, — I believe that the hive-bee has acquired, through natural
selection, her inimitable architectural powers.

But this theory can be tested by experiment. Following the
example of Mr. Tegetmeier, I separated two combs, and put between
them a long, thick, rectangular strip of wax: the bees instantly began
to excavate minute circular pits in it; and as they deepened these

CELL-MAKING INSTINCT 27 1

little pits, they made them wider and wider until they were converted
into shallow basins, appearing to the eye perfectly true or parts o£
a sphere, and of about the diameter of a cell. It was most interesting
to observe that, wherever several bees had begun to excavate these
basins near together, they had begun their work at such a distance
from each other, that by the time the basins had acquired the above-
stated width (». e. about the width of an ordinary cell), and were in
depth about one-sixth of the diameter of the sphere of which they
formed a part, the rims of the basins intersected or broke into each
other. As soon as this occurred, the bees ceased to excavate, and
began to build up flat walls of wax on the Hnes of intersection
between the basins, so that each hexagonal prism was built upon the
scalloped edge of a smooth basin, instead of on the straight edges
of a three-sided pyramid as in the case of ordinary cells.

I then put into the hive, instead of a thick, rectangular piece of wax,
a thin and narrow, knife-edged ridge, coloured with vermilion.
The bees instantly began on both sides to excavate litde basins near
to each other, in the same way as before; but the ridge of wax was
so thin, that the bottoms of the basins, if they had been excavated to
the same depth as in the former experiment, would have broken into
each other from the opposite sides. The bees, however, did not suffer
this to happen, and they stopped their excavations in due time; so
that the basins, as soon as they had been a little deef)ened, came to
have flat bases; and these flat bases, formed by thin little plates of
the vermilion wax left ungnawed, were situated, as far as the eye
could judge, exactly along the planes of imaginary intersection be-
tween the basins on the opposite sides of the ridge of wax. In some
parts, only small portions, in other parts, large portions of a rhombic
plate were thus left between the opposed basins, but the work, from
the unnatural state of things, had not been neady performed. The
bees must have worked at very nearly the same rate in circularly
gnawing away and deepening the basins on both sides of the ridge
of vermilion wax, in order to have thus succeeded in leaving flat
plates between the basins, by stopping work at the planes of inter-
section.

Considering how flexible thin wax is, I do not see that there is
any difficulty in the bees, whilst at work on the two sides of a strip

272 ORIGIN OF SPECIES

of wax, perceiving when they have gnawed the wax away to the
proper thinness, and then stopping their work. In ordinary combs
it has appeared to me that the bees do not always succeed in working
at exactly the same rate from the opposite sides; for I have noticed
half<ompleted rhombs at the base of a just commenced cell, which
were slightly concave on one side, where I suppose that the bees
had excavated too quickly, and convex on the opposed side where
the bees had worked less quickly. In one well marked instance, I
put the comb back into the hive, and allowed the bees to go on
working for a short time, and again examined the cell, and I found
that the rhombic plate had been completed, and had become perfectly
fiat: it was absolutely impossible, from the extreme thinness of the
little plate, that they could have effected this by gnawing away the
convex side; and I suspect that the bees in such cases stand on oppo-
site sides and push and bend the ductile and warm wax (which as I
have tried is easily done) into its proper intermediate plane, and thus
flatten it.

From the experiment of the ridge of vermilion wax we can see
that, if the bees were to build for themselves a thin wall of wax, they
could make their cells of the proper shape, by standing at the proper
distance from each other, by excavating at the same rate, and by
endeavouring to make equal spherical hollows, but never allowing
the spheres to break into each other. Now bees, as may be clearly
seen by examining the edge of a growing comb, do make a rough,
circumferential wall or rim all round the comb; and they gnaw this
away from the opposite sides, always working circularly as they
deepen each cell. They do not make the whole three-sided pyramidal
base of any one cell at the same time, but only that one rhombic
plate which stands on the extreme growing margin, or the two
plates, as the case may be; and they never complete the upper edges
of the rhombic plates, until the hexagonal walls are commenced.
Some of these statements differ from those made by the justly cele-
brated elder Huber, but I am convinced of their accuracy; and if
I had space, I could show that they are conformable with my theory.

Huber's statement, that the very first cell is excavated out of a little
parallel-sided wall of wax, is not, as far as I have seen, strictly cor-
rect; the first commencement having always been a little hood of

CELL-MAKING INSTINCT 273

wax; but I will not here enter on details. We see how important a
part excavation plays in the construction of the cells; but it would
be a great error to suppose that the bees cannot build up a rough
wall of wax in the proper position — that is, along the plane of inter-
section between two adjoining spheres. I have several sjjecimens
showing clearly that they can do this. Even in the rude circum-
ferential rim or wall of wax round a growing comb, flexures may
sometimes be observed, corresponding in position to the planes of
the rhombic basal plates of future cells. But the rough wall of wax
has in every case to be finished ofl, by being largely gnawed away
on both sides. The manner in which the bees build is curious; they
always make the first rough wall from ten to twenty times thicker
than the excessively thin finished wall of the cell, which will ulti-
mately be left. We shall understand how they work, by supposing
masons first to pile up a broad ridge of cement, and then to begin
cutting it away equally on both sides near the ground, till a smooth,
very thin wall is left in the middle; the masons always piling up the
cut-away cement, and adding fresh cement on the summit of the
ridge. We shall thus have a thin wall steadily growing upward but
always crowned by a gigantic coping. From all the cells, both those
just commenced and those completed, being thus crowned by a
strong coping of wax, the bees can cluster and crawl over the comb
without injuring the delicate hexagonal walls. These walls, as Profes-
sor Miller has kindly ascertained for me, vary greatly in thickness;
being, on an average of twelve measurements made near the border
of the comb, ^-J^ of an inch in thickness; whereas the basal rhom-
boidal plates are thicker, nearly in the proportion of three to two,
having a mean thickness, from twenty-one measurements, of -rir
of an inch. By the above singular manner of building, strength is
continually given to the comb, with the utmost ultimate economy
of wax.

It seems at first to add to the difficulty of understanding how the
cells are made, that a multitude of bees all work together; one bee
after working a short time at one cell going to another, so that, as
Huber has stated, a score of individuals work even at the commence-
ment of the first cell. I was able practically to show this fact, by
covering the edges of the hexagonal walls of a single cell, or the

274 ORIGIN OF SPECIES

extreme margin of the circumferential rim of a growing comb, with
an extremely thin layer of melted vermilion wax; and I invariably
found that the colour was most delicately diffused by the bees — as
delicately as a painter could have done it with his brush — by atoms
of the coloured wax having been taken from the spot on which it
had been placed, and worked into the growing edges of the cells all
round. The work of construction seems to be a sort of balance
struck between many bees, all instinctively standing at the same
relative distance from each other, all trying to sweep equal spheres,
and then building up, or leaving ungnawed, the planes of intersec-
tion between these spheres. It was really curious to note in cases of
difficulty, as when two pieces of comb met at an angle, how
often the bees would pull down and rebuild in different ways the
same cell, sometimes recurring to a shape which they had at first
rejected.

When bees have a place on which they can stand in their proper
positions for working, — for instance, on a slip of wood, placed directly
under the middle of a comb growing downwards, so that the comb
has to be built over one face of the sHp — in this case the bees can lay
the foundations of one wall of a new hexagon, in its strictly proper
place, projecting beyond the other completed cells. It suffices that
the bees should be enabled to stand at their proper relative distances
from each other and from the walls of the last completed cells, and
then, by striking imaginary spheres, they can build up a wall inter-
mediate between two adjoining spheres; but, as far as I have seen,
they never gnaw away and finish of? the angles of a cell till a large
part both of that cell and of the adjoining cells has been built.
This capacity in bees of laying down under certain circumstances
a rough wall in its proper place between two just-commenced cells,
is important, as it bears on a fact, which seems at first subversive of
the foregoing theory; namely, that the cells on the extreme margin
of wasp-combs are sometimes strictly hexagonal; but I have not
space here to enter on this subject. Nor does there seem to me any
great difficulty in a single insect (as in the case of a queen-wasp)
making hexagonal cells, if she were to work alternately on the inside
and outside of two or three cells commenced at the same ume, always
standing at the proper relative distance from the parts of the cells

CELL-MAKING INSTINCT 275

just begun, sweeping spheres or cylinders, and building up inter-
mediate planes.

As natural selection acts only by the accumulation of slight modi-
fications of structure or instinct, each profitable to the individual
under its conditions of life, it may reasonably be asked, how a long
and graduated succession of modified architectural instincts, all
tending towards the present perfect plan of construction, could have
profited the progenitors of the hive-bee? I think the answer is not
difficult: cells constructed like those of the bee or the wasp gain in
strength, and save much in labour and space, and in the materials
of which they are constructed. With respect to the formation of
wax, it is known that bees are often hard pressed to get sufficient
nectar, and I am informed by Mr. Tegetmeier that it has been experi-
mentally proved that from twelve to fifteen f)ounds of dry sugar are
consumed by a hive of bees for the secretion of a pound of wax; so
that a prodigious quantity of fluid nectar must be collected and
consumed by the bees in a hive for the secretion of the wax necessary
for the construction of their combs. Moreover, many bees have to
remain idle for many days during the process of secretion. A large
store of honey is indispensable to support a large stock of bees during
the winter; and the security of the hive is known mainly to depend
on a large number of bees being supported. Hence the saving of wax
by largely saving honey and the time consumed in collecting the
honey must be an important element of success to any family
of bees. Of course the success of the sp>ecies may be dependent on
the number of its enemies, or parasites, or on quite distinct causes,
and so be altogether independent of the quantity of honey which the
bees can collect. But let us suppose that this latter circumstance
determined, as it probably often has determined, whether a bee allied
to our humble-bees could exist in large numbers in any country;
and let us further suppose that the community lived through the
winter, and consequently required a store of honey: there can in
this case be no doubt that it would be an advantage to our imaginary
humble-bee, if a slight modification in her instincts led her to make
her waxen cells near together, so as to intersect a Httle; for a wall in
common even to two adjoining cells would save some little labour
and wax. Hence it would continually be more and more advan-

276 ORIGIN OF SPECIES

tageous to our humble-bees, if they were to make their cells more
and more regular, nearer together, and aggregated into a mass, like
the cells of the Melipona; for in this case a large part of the bounding
surface of each cell would serve to bound the adjoining cells, and
much labour and wax would be saved. Again, from the same cause,
it would be advantageous to the Melipona, if she were to make her
cells closer together, and more regular in every way than at present;
for then, as we have seen, the spherical surfaces would wholly dis-
apf)ear and be replaced by plane surfaces; and the Melijxjna would
make a comb as perfect as that of the hive-bee. Beyond this stage of
perfection in architecture, natural selection could not lead; for the
comb of the hive-bee, as far as we can see, is absolutely perfect in
economising labour and wax.

Thus, as I believe, the most wonderful of all known instincts, that
of the hive-bee, can be explained by natural selection having taken
advantage of numerous, successive, slight modifications of simpler
instincts; natural selection having, by slow degrees, more and more
perfectly led the bees to sweep equal spheres at a given distance from
each other in a double layer, and to build up and excavate the wax
along the planes of intersection; the bees, of course, no more know-
ing that they swept their spheres at one particular distance from each
other, than they know what are the several angles of the hexagonal
prisms and of the basal rhombic plates; the motive power of the
process of natural selection having been the construction of cells of
due strength and of the proper size and shape for the larvae, this being
effected with the greatest possible economy of labour and wax; that
individual swarm which thus made the best cells with least labour,
and least waste of honey in the secretion of wax, having succeeded
best, and having transmitted their newly acquired economical in-
stincts to new swarms, which in their turn will have had the best
chance of succeeding in the struggle for existence.

OBJECTIONS TO THE THEORY OF NATURAL SELECTION AS APPLIED TO
instincts: neuter and sterile INSECTS

It has been objected to the foregoing view of the origin of instincts
that "the variations of structure and of instinct must have been
simultaneous and accurately adjusted to each other as a modification

OBJECTIONS TO THE THEORY 277

in the one without an immediate corresponding change in the other
would have been fatal." The force of this objection rests entirely
on the assumption that the changes in the instincts and structure
are abrupt. To take as an illustration the case of the larger titmouse
(Parus major) alluded to in a previous chapter; this bird often holds
the seeds of the yew between its feet on a branch, and hammers with
its beak till it gets at the kernel. Now what special difficulty would
there be in natural selection preserving all the slight individual vari-
ations in the shape of the beak, which were better and better adapted
to break open the seeds, until a beak was formed, as well constructed
for this purpose as that of the nuthatch, at the same time that habit,
or compulsion, or spontaneous variations of taste, led the bird to
become more and more of a seed-eater? In this case the beak is
supposed to be slowly modified by natural selection, subsequently
to, but in accordance with, slowly changing habits or taste; but let
the feet of the titmouse vary and grow larger from correlation with
the beak, or from any other unknown cause, and it is not improbable
that such larger feet would lead the bird to climb more and more
until it acquired the remarkable climbing instinct and power of the
nuthatch. In this case a gradual change of structure is supposed to
lead to changed instinctive habits. To take one more case: few in-
stincts are more remarkable than that which leads the swift of the
Eastern Islands to make its nest wholly of inspissated saliva. Some
birds build their nests of mud, believed to be moistened with saliva;
and one of the swifts of North America makes its nest (as I have
seen) of sticks agglutinated with saliva, and even with flakes of this
substance. Is it then very improbable that the natural selection of
individual swifts, which secreted more and more saliva, should at
last produce a sf)ecies with instincts leading it to neglect other mate-
rials, and to make its nest exclusively of inspissated saliva? And so
in other cases. It must, however, be admitted that in many instances
we cannot conjecture whether it was instinct or structure which
first varied.

No doubt many instincts of very difficult explanation could be
opposed to the theory of natural selection — cases, in which we cannot
see how an instinct could have originated; cases, in which no inter-
mediate gradations are known to exist; cases of instincts of such

278 ORIGIN OF SPECIES

trifling importance, that they could hardly have been acted on by
natural selection; cases of instincts almost identically the same in
animals so remote in the scale of nature, that we cannot account for
their similarity by inheritance from a common progenitor, and conse-
quently must believe that they were independendy acquired through
natural selection. I will not here enter on these several cases, but
will confine myself to one special difficulty, which at first appeared
to me insuperable, and actually fatal to the whole theory. I allude
to the neuters or sterile females in insect-communities; for these
neuters often differ widely in instinct and in structiu-e from both
the males and fertile females, and yet, from being sterile, they cannot
propagate their kind.

The subject well deserves to be discussed at great length, but I will
here take only a single case, that of working or sterile ants. How the
workers have been rendered sterile is a difficulty; but not much
greater than that of any other striking modification of structure;
for it can be shown that some insects and other articulate animals
in a state of nature occasionally become sterile; and if such inserts
had been social, and it had been profitable to the community that a
number should have been annually born capable of work, but in-
capable of procreation, I can see no esf>ecial difficulty in this having
been effected through natural selection. But I must pass over this
preliminary difficulty. The great difficulty Ues in the working ants
differing widely from both the males and the fertile females in
structure, as in the shape of the thorax, and in being destitute of
wings and sometimes of eyes, and in instinct. As far as instinct alone
is concerned, the wonderful difference in this respect between the
workers and the perfect females, would have been better exemplified
by the hive-bee. If a working ant or other neuter insect had been an
ordinary animal, I should have unhesitatingly assumed that all its
characters had been slowly acquired through natural selection;
namely, by individuals having been born with slight profitable modi-
fications, which were inherited by the offspring; and that these again
varied and again were selected, and so onwards. But with the work-
ing ant we have an insect differing greatly from its parents, yet
absolutely sterile, so that it could never have transmitted successively
acquired modifications of structure or instinct to its progeny. It may

OBJECTIONS TO THE THEORY 279

well be asked how is it possible to reconcile this case with the theory
of natural selection?

First, let it be remembered that we have innumerable instances,
both in our domestic productions and in those in a state of nature, of
all sorts of dilTerences of inherited structure which are correlated with
certain ages, and with either sex. We have differences correlated
not only with one sex, but with that short period when the reproduc-
tive system is active, as in the nuptial plumage of many birds, and
in the hooked jaws of the male salmon. We have even slight dif-
ferences in the horns of different breeds of cattle in relation to an
artificially imperfect state of the male sex; for oxen of certain breeds
have longer horns than the oxen of other breeds, relatively to the
length of the horns in both the bulls and cows of these same breeds.
Hence I can see no great difficulty in any character becoming cor-
related with the sterile condition of certain members of insect-
communities: the difficulty lies in understanding how such cor-
related modifications of structure could have been slowly accumulated
by natural selection.

This difficulty, though appearing insuperable, is lessened, or, as I
believe, disappears, when it is remembered that selection may be
applied to the family, as well as to the individual, and may thus
gain the desired end. Breeders of cattle wish the flesh and fat to
be well marbled together: an animal thus characterised has been
slaughtered, but the breeder has gone with confidence to the same
stock and has succeeded. Such faith may be placed in the power of
selection, that a breed of catde, always yielding oxen with extraor-
dinarily long horns, could, it is probable, be formed by carefully
watching which individual bulls and cows, when matched, produce
oxen with the longest horns; and yet no one ox would ever have
propagated its kind. Here is a better and real illustration : according
to M. Verlot, some varieties of the double annual stock, from having
been long and carefully selected to the right degree, always produce
a large proportion of seedlings bearing double and quite sterile
flowers; but they likewise yield some single and fertile plants. These
latter, by which alone the variety can be propagated, may be com-
pared with the fertile male and female ants, and the double sterile
plants with the neuters of the same community. As with the varieties

28o ORIGIN OF SPECIES

of the stock, so with social insects, selection has been applied to the
family, and not to the individual, for the sake of gaining a service-
able end. Hence, we may conclude that slight modifications of struc-
ture or of instinct, correlated with the sterile condition of certain
members of the community, have proved advantageous: consequently
the fertile males and females have flourished, and transmitted to
their fertile offspring a tendency to produce sterile members with
the same modifications. This process must have been repeated many
times, until that prodigious amount of difference between the fertile
and sterile females of the same species has been produced which we
see in many social insects.

But we have not as yet touched on the acme of the difficulty;
namely, the fact that the neuters of several ants differ, not only from
the fertile females and males, but from each other, sometimes to an
almost incredible degree, and are thus divided into two or even three
castes. The castes, moreover, do not commonly graduate into each
other, but are perfectly well defined; being as distinct from each
other as arc any two species of the same genus, or rather as any two
genera of the same family. Thus, in Eciton, there are working and
soldier neuters, with jaws and instincts extraordinarily different: in
Cryptocerus, the workers of one caste alone carry a wonderful sort
of shield on their heads, the use of which is quite unknown: in the
Mexican Myrmecocystus, the workers of one caste never leave the
nest; they are fed by the workers of another caste, and they have an
enormously developed abdomen which secretes a sort of honey, sup)-
plying the place of that excreted by the aphides, or the domestic cattle
as they may be called, which our European ants guard and imprison.

It will indeed be thought that I have an overweening confidence
in the principle of Natural Selection, when I do not admit that such
wonderful and well-established facts at once annihilate the theory.
In the simpler case of neuter insects all of one caste, which, as I
believe, have been rendered different from the fertile males and
females through natural selection, we may conclude from the analogy
of ordinary variations, that the successive, slight, profitable modifica-
tions did not first arise in all the neuters in the same nest, but in
some few alone; and that by the survival of the communities with
females which produced most neuters having the advantageous modi-

OBJECTIONS TO THE THEORY 28 1

fication, all the neuters ultimately came to be thus characterized.
According to this view we ought occasionally to find in the same
nest neuter insects, presenting gradations of structure; and this we
do find, even not rarely considering how few neuter insects out of
Europe have been carefully examined. Mr. F. Smith has shown that
the neuters of several British ants difTer surprisingly from each other
in size and sometimes in colour; and that the extreme forms can
be linked together by individuals taken out of the same nest: I have
myself compared perfect gradations of this kind. It sometimes hap-
pens that the larger or the smaller sized workers are the most numer-
ous; or that both large and small are numerous, whilst those of an
intermediate size are scanty in numbers. Formica flava has larger
and smaller workers, with some few of intermediate size; and, in
this species, as Mr. F. Smith has observed, the larger workers have
simple eyes (ocelli), which though small can be plainly distinguished,
whereas the smaller workers have their ocelli rudimentary. Having
carefully dissected several specimens of these workers, I can affirm
that the eyes are far more rudimentary in the smaller workers than
can be accounted for merely by their proportionally lesser size; and
I fully believe, though I dare not assert so positively, that the workers
of intermediate size have their ocelli in an exactly intermediate con-
dition. So that here we have two bodies of sterile workers in the
same nest, differing not only in size, but in their organs of vision,
yet connected by some few members in an intermediate condition.
I may digress by adding, that if the smaller workers had been the
most useful to the community, and those males and females had
been continually selected, which produced more and more of the
smaller workers, until all the workers were in this condition; we
should then have had a species of ant with neuters in nearly the
same condition as those of Myrmica. For the workers of Myrmica
have not even rudiments of ocelli, though the male and female ants
of this genus have well-developed ocelli.

I may give one other case: so confidently did I expect occasionally
to find gradations of important structures between the different
castes of neuters in the same species, that I gladly availed myself
of Mr. F. Smith's offer of numerous sp)ecimens from the same nest
of the driver ant (Anomma) of West Africa. The reader will per-

282 ORIGIN OF SPECIES

haps best appreciate the amount of diflerence in these workers by
my giving not the actual measurements, but a strictly accurate illus-
tration: the difference was the same as if we were to see a set of
workmen building a house, of whom many were five feet four inches
high, and many sixteen feet high; but we must in addition suppose
that the larger workmen had heads four instead of three times as
big as those of the smaller men, and jaws nearly five times as big.
The jaws, moreover, of the working ants of the several sizes differed
wonderfully in shape, and in the form and number of the teeth. But
the important fact for us is, that, though the workers can be grouped
into castes of different sizes, yet they graduate insensibly into each
other, as does the widely-different structure of their jaws. I speak
confidently on this latter point, as Sir J. Lubbock made drawings
for me, with the camera lucida, of the jaws which I dissected from
the workers of the several sizes. Mr. Bates, in his interesting 'Natural-
ist on the Amazons,' has described analogous cases.

With these facts before me, I believe that natural selection, by
acting on the fertile ants or parents, could form a species which should
regularly produce neuters, all of large size with one form of jaw, or
all of small size with widely different jaws; or lastly, and this is the
greatest difficulty, one set of workers of one size and structure, and
simultaneously another set of workers of a different size and struc-
ture; — a graduated series having first been formed, as in the case of
the driver ant, and then the extreme forms having been produced in
greater and greater numbers, through the survival of the parents
which generated them, until none with an intermediate structure
were produced.

An analogous explanation has been given by Mr. Wallace, of the
equally complex case, of certain Malayan butterflies regularly appear-
ing under two or even three distinct female forms; and by Fritz
Miiller, of certain Brazilian crustaceans likewise appearing under
two widely distinct male forms. But this subject need not here be
discussed.

I have now explained how, I believe, the wonderful fact of two
distinctly defined castes of sterile workers existing in the same nest,
both widely different from each other and from their parents, has
originated. We can see how useful their production may have been

SUMMARY 283

to a scx:ial community of ants, on the same principle that the division
of labour is useful to civilised man. Ants, however, work by in-
herited instincts and by inherited organs or tools, whilst man works
by acquired knowledge and manufactured instruments. But I must
confess, that, with all my faith in natural selection, I should never
have anticipated that this principle could have been efficient in so
high a degree, had not the case of these neuter insects led me to this
conclusion. I have, therefore, discussed this case, at some little but
wholly insufficient length, in order to show the power of natural
selection, and likewise because this is by far the most serious special
difficulty which my theory has encountered. The case, also, is very
interesting, as it proves that with animals, as with plants, any amount
of modification may be effected by the accumulation of numerous,
shght, spontaneous variations, which are in any way profitable, with-
out exercise or habit having been brought into play. For pecuUar
habits confined to the workers or sterile females, however long they
might be followed, could not possibly af?ect the males and fertile
females, which alone leave descendants. I am surprised that no one
has hitherto advanced this demonstrative case of neuter insects,
against the well-known doctrine of inherited habit, as advanced by
Lamarck.

SUMMARY

I have endeavoured in this chapter briefly to show that the mental
qualities of our domestic animals vary, and that the variations are
inherited. Still more briefly I have attempted to show that instincts
vary slightly in a state of nature. No one will dispute that instincts
are of the highest importance to each animal. Therefore there is no
real difficulty, under changing conditions of life, in natural selection
accumulating to any extent slight modifications of instinct which
are in any way useful. In many cases habit or use and disuse have
probably come into play. I do not pretend that the facts given in this
chapter strengthen in any great degree my theory; but none of the
cases of difficulty, to the best of my judgment, annihilate it. On the
other hand, the fact that instincts are not always absolutely perfect
and are liable to mistakes; — that no instinct can be shown to have
been produced for the good of other animals, though animals take

284 ORIGIN OF SPECIES

advantage of the instincts of others; — that the canon in natural
history, of "Natura non facit saltum," is applicable to instincts as
well as to corporeal structure, and is plainly explicable on the fore-
going views, but is otherwise inexplicable, — all tend to corroborate
the theory of natural selection.

This theory is also strengthened by some few other facts in regard
to instincts; as by that common case of closely allied, but distinct,
species, when inhabiting distant parts of the world and living under
considerably different conditions of life, yet often retaining nearly
the same instincts. For instance, we can understand, on the principle
of inheritance, how it is that the thrush of tropical South America
lines its nest with mud, in the same peculiar manner as does our
British thrush; how it is that the Hornbills of Africa and India have
the same extraordinary instinct of plastering up and imprisoning the
females in a hole in a tree, with only a small hole left in the plaster
through which the males feed them and their young when hatched;
how it is that the male wrens (Troglodytes) of North America build
"cock-nests," to roost in, like the males of our Kitty-wrens, — a habit
wholly unlike that of any other known bird. Finally, it may not be
a logical deduction, but to my imagination it is far more satisfactory
to look at such instincts as the young cuckoo ejecting its foster-
brothers, — ants making slaves, — the larvae of ichneumonidac feeding
within the live bodies of caterpillars, — not as spxjcially endowed or
created instincts, but as small consequences of one general law
leading to the advancement of all organic beings, — namely, multiply,
vary, let the strongest live and the weakest die.

CHAPTER IX

Hybridism

Distinction between the sterility of first crosses and of hybrids — Sterility
various in degree, not universal, aflected by close interbreeding,
removed by domestication — Laws governing the sterility of hybrids
— Sterility not a special endowment, but incidental on other difler-
ences, not accumulated by natural selection — Causes of the sterility
of first crosses and of hybrids — Parallelism between the effects of
changed conditions of life and of crossing — Dimorphism and Tri-
morphism — Fertility of varieties when crossed, and of their mongrel
offspring not universal — Hybrids and mongrels compared independ-
ently of their fertility — Summary.

THE view commonly entertained by naturalists is that species,
when intercrossed, have been specially endowed with ste-
rility, in order to prevent their confusion. This view certainly
seems at first highly probable, for species living together could hardly
have been kept distinct had they been capable of freely crossing.
The subject is in many ways impxjrtant for us, more especially as the
sterility of species when first crossed, and that of their hybrid off-
spring, cannot have been acquired, as I shall show, by the preserva-
tion of successive profitable degrees of sterility. It is an incidental
result of differences in the reproductive systems of the parent-species.

In treating this subject, two classes of facts, to a large extent
fundamentally different, have generally been confounded; namely,
the sterility of species when first crossed, and the sterility of the
hybrids produced from them.

Pure species have of course their organs of reproduction in a
perfect condition, yet when intercrossed they produce either few or
no offspring. Hybrids, on the other hand, have their reproductive
organs functionally impotent, as may be clearly seen in the state of
the male element in both plants and animals; though the formative
organs themselves are perfect in structure, as far as the microscope
reveals. In the first case the two sexual elements which go to form
the embryo are perfect; in the second case they are either not at all

285

286 ORIGIN OF SPECIES

developed, or are imperfectly developed. This distinction is impor-
tant, when the cause of the sterility, which is common to the two
cases, has to be considered. The distinction probably has been slurred
over, owing to the sterility in both cases being looked on as a special
endowment, beyond the province of our reasoning powers.

The fertility of varieties, that is of the forms known or believed to
be descended from common parents, when crossed, and likewise the
fertility of their mongrel offspring, is, with reference to my theory,
of equal importance with the sterility of sfjecies; for it seems to make
a broad and clear distinction between varieties and species.

DEGREES OF STERILITY

First, for the sterility of species when crossed and of their hybrid
offspring. It is impossible to study the several memoirs and works
of those two conscientious and admirable observers, Kolreuter and
Gartner, who almost devoted their lives to this subject, without being
deeply impressed with the high generality of some degree of sterility.
Kolreuter makes the rule universal; but then he cuts the knot, for in
ten cases in which he found two forms, considered by most authors
as distinct species, quite fertile together, he unhesitatingly ranks them
as varieties. Gartner, also, makes the rule equally universal; and he
disputes the entire fertility of Kolreuter's ten cases. But in these and
in many other cases, Gartner is obliged carefully to count the seeds,
in order to show that there is any degree of sterility. He always com-
pares the maximum number of seeds produced by two species when
first crossed, and the maximum produced by their hybrid offspring,
with the average number produced by their pure parent-species in a
state of nature. But causes of serious error here intervene: a plant,
to be hybridised, must be castrated, and, what is often more impor-
tant, must be secluded in order to prevent pollen being brought to it
by insects from other plants. Nearly all the plants experimented on
by Gartner were potted, and were kept in a chamber in his house.
That these processes are often injurious to the fertility of a plant
cannot be doubted; for Gartner gives in his table about a score of
cases of plants which he castrated, and artificially fertilised with their
own pollen, and (excluding all cases such as the Leguminosac, in
which there is an acknowledged difficulty in the manipulation) half

DEGREES OF STERILITY 287

o£ these twenty plants had their fertility in some degree impaired.
Moreover, as Gartner repeatedly crossed some forms, such as the
common red and blue pimpernels (Anagallis arvensis and cceulea),
which the best botanists rank as varieties, and found them absolutely
sterile, we may doubt whether many species are really so sterile,
when intercrossed, as he believed.

It is certain, on the one hand, that the sterility of various species
when crossed is so different in degree and graduates away so insen-
sibly, and, on the other hand, that the fertility of pure species is so
easily affected by various circumstances, that for all practical pur-
poses it is most difficult to say where perfect fertiUty ends and ste-
rility begins. 1 think no better evidence of this can be required than
that the two most experienced observers who have ever lived, namely
Kolreuter and Gartner, arrived at diametrically opposite conclusions
in regard to some of the very same forms. It is also most instructive
to compare — but I have not space here to enter into details — the
evidence advanced by our best botanists on the question whether
certain doubtful forms should be ranked as species or varieties, with
the evidence from fertility adduced by different hybridisers, or by
the same observer from experiments made during different years. It
can thus be shown that neither sterility nor fertility affords any cer-
tain distinction between species and varieties. The evidence from
this source graduates away, and is doubtful in the same degree as
is the evidence derived from other constitutional and structural
differences.

In regard to the sterility of hybrids in successive generations;
though Gartner was enabled to rear some hybrids, carefully guarding
them from a cross with either pure parent, for six or seven, and in
one case for ten generations, yet he asserts positively that their fer-
tility never increases, but generally decreases greatly and suddenly.
With respect to this decrease, it may first be noticed that when any
deviation in structure or constitution is common to both parents,
this is often transmitted in an augmented degree to the offspring;
and both sexual elements in hybrid plants are already affected in
some degree. But I believe that their fertility has been diminished
in nearly all these cases by an independent cause, namely, by too
close interbreeding. I have made so many experiments and collected

288 ORIGIN OF SPECIES

so many facts, showing on the one hand that an occasional cross with
a distinct individual or variety increases the vigour and fertility of
the offspring, and on the other hand that very close interbreeding
lessens their vigour and fertility, that I cannot doubt the correctness
of this conclusion. Hybrids are seldom raised by experimentalists
in great numbers; and as the parent-species, or other allied hybrids,
generally grow in the same garden, the visits of insects must be
carefully prevented during the flowering season; hence hybrids, if
left to themselves, will generally be fertilised during each generation
by pollen from the same flower; and this would probably be injuri-
ous to their fertility, already lessened by their hybrid origin. I am
strengthened in this conviction by a remarkable statement repeatedly
made by Gartner, namely, that if even the less fertile hybrids be
artificially fertilised with hybrid pollen of the same kind, their fer-
tility, notwithstanding the frequent ill effects from manipulation,
sometimes decidedly increases, and goes on increasing. Now, in the
process of artificial fertilisation, pollen is as often taken by chance
(as I know from my own experience) from the anthers of another
flower, as from the anthers of the flower itself which is to be fer-
tilised; so that a cross between two flowers, though probably often
on the same plant, would be thus effected. Moreover, whenever
complicated experiments are in progress, so careful an observer as
Gartner would have castrated his hybrids, and this would have
ensured in each generation a cross with pollen from a distinct flower,
either from the same plant or from another plant of the same hybrid
nature. And thus, the strange fact of an increase of fertility in the
successive generations of artificially fertilised hybrids, in contrast
with those spontaneously self-fertilised, may, as I believe, be accounted
for by too close interbreeding having been avoided.

Now let us turn to the results arrived at by a third most experi-
enced hybridiser, namely, the Hon. and Rev. W. Herbert. He is as
emphatic in his conclusion that some hybrids are perfectly fertile —
as fertile as the pure parent-species — as are Kolreuter and Gartner
that some degree of sterility between distinct species is a universal
law of nature. He experimented on some of the very same species
as did Gartner. The difference in their results may, I think, be in
part accounted for by Herbert's great horticultural skill, and by his

DEGREES OF STERILITY 289

having hot-houses at his command. Of his many important state-
ments I will here give only a single one as an example, namely, that
"every ovule in a pod of Crinum capense fertilised by C. revolutum
produced a plant, which I never saw to occur in a case of its natural
fecundation." So that here we have perfect or even more than
commonly perfect fertility, in a first cross between two distinct
species.

This case of the Crinum leads me to refer to a singular fact,
namely, that individual plants of certain species of Lobelia, Ver-
bascum and Passiflora, can easily be fertilised by pollen from a dis-
tinct species, but not by pollen from the same plant, though this
pollen can be proved to be perfectly sound by fertilising other plants
or species. In the genus Hippeastrum, in Corydalis, as shown by
Professor Hildebrand, in various orchids as shown by Mr. Scott and
Fritz Miiller, all the individuals are in this peculiar condition. So
that with some species, certain abnormal individuals, and in other
species all the individuals, can actually be hybridised much more
readily than they can be fertilised by pollen from the same individual
plant! To give one instance, a bulb of Hippeastrum aulicum pro-
duced four flowers; three were fertilised by Herbert with their own
pollen, and the fourth was subsequently fertilised by the pollen of a
compound hybrid descended from three distinct species: the result
was that "the ovaries of the three first flowers soon ceased to grow,
and after a few days p)erished entirely, whereas the pod impregnated
by the pollen of the hybrid made vigorous growth and rapid progress
to maturity, and bore good seed, which vegetated fredy." Mr.
Herbert tried similar experiments during many years, and always
with the same result. These cases serve to show on what slight and
mysterious causes the lesser or greater fertility of a species sometimes
depends.

The practical experiments of horticulturists, though not made
with scientific precision, deserve some notice. It is notorious in how
complicated a manner the species of Pelargonium, Fuchsia, Calceo-
laria, Petunia, Rhododendron, etc., have been crossed, yet many of
these hybrids seed freely. For instance, Herbert asserts that a hybrid
from Calceolaria integrifolia and plantaginea, species most widely
dissimilar in general habit, "reproduces itself as perfecdy as if it had

290 ORIGIN OF SPECIES

been a natural species from the mountains of Chili." I have taken
some pains to ascertain the degree of fertiHty of some of the complex
crosses of Rhododendrons, and I am assured that many of them
are perfectly fertile. Mr. C. Noble, for instance, informs me that he
raises stocks for grafting from a hybrid between Rhod. ponticum
and catawbiense, and that this hybrid "seeds as freely as it is possible
to imagine." Had hybrids, when fairly treated, always gone on
decreasing in fertility in each successive generation, as Gartner be-
lieved to be the case, the fact would have been notorious to nursery-
men. Horticulturists raise large beds of the same hybrid, and such
alone are fairly treated, for by insect-agency the several individuals
are allowed to cross freely with each other, and the injurious influ-
ence of close interbreeding is thus prevented. Any one may readily
convince himself of the efficiency of insect-agency by examining the
flowers of the more sterile kinds of hybrid Rhododendrons, which
produce no pollen, for he will find on their stigmas plenty of pollen
brought from other flowers.

In regard to animals, much fewer experiments have been carefully
tried than with plants. If our systematic arrangements can be trusted,
that is, if the genera of animals are as distinct from each other as are
the genera of plants, then we may infer that animals more widely
distinct in the scale of nature can be crossed more easily than in the
case of plants; but the hybrids themselves are, I think, more sterile.
It should, however, be borne in mind that, owing to few animals
breeding freely under confinement, few experiments have been fairly
tried: for instance, the canary bird has been crossed with nine distinct
species of finches, but, as not one of these breeds freely in confine-
ment, we have no right to expect that the first crosses between them
and the canary, or that their hybrids, should be perfectly fertile.
Again, with respect to the fertility in successive generations of the
more fertile hybrid animals, I hardly know of an instance in which
two families of the same hybrid have been raised at the same time
from different parents, so as to avoid the ill effects of close inter-
breeding. On the contrary, brothers and sisters have usually been
crossed in each successive generation, in opposition to the constantly
repeated admonition of every breeder. And in this case, it is not at

DEGREES OF STERILITY 29I

all surprising that the inherent sterility in the hybrids should have
gone on increasing.

Although I know of hardly any thoroughly well-authenticated
cases of perfectly fertile hybrid animals, I have reason to believe that
the hybrids from Cervulus vaginalis and Reevesii, and from Phasi-
anus colchicus with P. torquatus, are perfectly fertile. M. Quatre-
fages states that the hybrids from two moths (Bombyx cynthia and
arrindia) were proved in Paris to be fertile inter se for eight genera-
tions. It has lately been asserted that two such distinct species as the
hare and rabbit, when they can be got to breed together, produce
offspring which are highly fertile when crossed with one of the
parent-species. The hybrids from the common and Chinese geese
(A. cygnoides), species which are so different that they are generally
ranked in distinct genera, have often bred in this country with either
pure parent, and in one single instance they have bred inter se. This
was effected by Mr. Eyton, who raised two hybrids from the same
parents, but from different hatches; and from these two birds he
raised no less than eight hybrids (grandchildren of the pure geese)
from one nest. In India, however, these cross-bred geese must be far
more fertile; for 1 am assured by two eminendy capable judges,
namely Mr. Blyth and Captain Hutton, that whole flocks of these
crossed geese are kept in various parts of the country; and as they
are kept for profit, where neither pure parent-species exists, they
must certainly be highly or perfectly fertile.

With our domesticated animals, the various races when crossed
together are quite fertile; yet in many cases they are descended from
two or more wild sp)ecies. From this fact we must conclude either
that the aboriginal parent-species at first produced perfectly fertile
hybrids, or that the hybrids subsequently reared under domestica-
tion became quite fertile. This latter alternative, which was first
propounded by Pallas, seems by far the most probable, and can,
indeed, hardly be doubted. It is, for instance, almost certain that our
dogs are descended from several wild stocks; yet, with perhaps the
exception of certain indigenous domestic dogs of South America,
all are quite fertile together; but analogy makes me greatly doubt,
whether the several aboriginal species would at first have freely bred

292 ORIGIN OF SPECIES

together and have produced quite fertile hybrids. So again I have
lately acquired decisive evidence that the crossed offspring from the
Indian humped and common cattle are inter se perfectly fertile; and
from the observations by Riitimeyer on their important osteological
differences, as well as from those by Mr. Blyth on their differences
in habits, voice, constitution, etc., these two forms must be regarded
as good and distinct species. The same remarks may be extended
to the two chief races of the pig. We must, therefore, either give up
the belief of the universal sterility of species when crossed; or we
must look at this sterility in animals, not as an indelible character-
istic, but as one capable of being removed by domestication.

Finally, considering all the ascertained facts on the intercrossing of
plants and animals, it may be concluded that some degree of sterility,
both in first crosses and in hybrids, is an extremely general result;
but that it cannot, under our present state of knowledge, be consid-
ered as absolutely universal.

LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND OF HYBRIDS

We will now consider a little more in detail the laws governing
the sterility of first crosses and of hybrids. Our chief object will be
to see whether or not these laws indicate that species have been
specially endowed with this quality, in order to prevent their crossing
and blending together in utter confusion. The following conclusions
are drawn up chiefly from Gartner's admirable work on the hybridi-
sation of plants. I have taken much pains to ascertain how far they
apply to animals, and, considering how scanty our knowledge is in
regard to hybrid animals, I have been surprised to find how generally
the same rules apply to both kingdoms.

It has been already remarked, that the degree of fertility, both of
first crosses and of hybrids, graduates from zero to perfect fertility.
It is surprising in how many curious ways this gradation can be
shown; but only the barest outline of the facts can here be given.
When pollen from a plant of one family is placed on the stigma of
a plant of a distinct family, it exerts no more influence than so much
inorganic dust. From this absolute zero of fertility, the pollen of
different species applied to the stigma of some one species of the
same genus, yields a perfect gradation in the number of seeds pro-

LAWS GOVERNING THE STERILITY 293

duced, up to nearly complete or even quite complete fertility; and,
as we have seen, in certain abnormal cases, even to an excess of
fertility, beyond that which the plant's own pollen produces. So in
hybrids themselves, there are some which never have produced, and
probably never would produce, even with the pollen of the pure
parents, a single fertile seed: but in some of these cases a first trace
of fertility may be detected, by the pollen of one of the pure parent-
species causing the flower of the hybrid to wither earlier than it
otherwise would have done; and the early withering of the flower
is well known to be a sign of incipient fertilisation. From this
extreme degree of sterility we have self-fertilised hybrids producing
a greater and greater number of seeds up to perfect fertility.

The hybrids raised from two species which are very difficult to
cross, and which rarely produce any offspring, are generally very
sterile; but the parallelism between the difficulty of making a first
cross, and the sterility of the hybrids thus produced — two classes of
facts which are generally confounded together — is by no means
strict. There are many cases, in which two pure species, as in the
genus Verbascum, can be united with unusual facility, and produce
numerous hybrid-offspring, yet these hybrids are remarkably sterile.
On the other hand, there are species which can be crossed very rarely,
or with extreme difficulty, but the hybrids, when at last produced,
are very fertile. Even within the limits of the same genus, for
instance in Dianthus, these two opposite cases occur.

The fertility, both of first crosses and of hybrids, is more easily
affected by unfavorable conditions, than is that of pure species. But
the fertility of first crosses is likewise innately variable; for it is not
always the same in degree when the same two species are crossed
under the same circumstances; it depends in part upon the constitu-
tion of the individuals which happen to have been chosen for the
experiment. So it is with hybrids, for their degree of fertility is often
found to differ greatly in the several individuals raised from seed out
of the same capsule and exposed to the same conditions.

By the term systematic affinity is meant, the general resemblance
between species in structure and constitution. Now the fertility
of first crosses, and of the hybrids produced from them, is largely
governed by their systematic affinity. This is clearly shown by

294 ORIGIN OF SPECIES

hybrids never having been raised between species ranked by system-
atists in distinct families; and on the other hand, by very closely
allied species generally uniting with facility. But the correspondence
between systematic affinity and the facility of crossing is by no means
strict. A multitude of cases could be given of very closely allied
sfjecies which will not unite, or only with extreme difficulty; and on
the other hand of very distinct species which unite with the utmost
faciUty. In the same family there may be a genus, as Dianthus, in
which very many species can most readily be crossed; and another
genus, as Silene, in which the most persevering efforts have failed
to produce between extremely close species a single hybrid. Even
within the Umits of the same genus, we meet with this same differ-
ence; for instance, the many species of Nicotiana have been more
largely crossed than the species of almost any other genus; but
Gartner found that N. acuminata, which is not a particularly distinct
sjjecies, obstinately failed to fertiUse, or to be fertilised by no less
than eight other species of Nicotiana. Many analogous facts could
be given.

No one has been able to point out what kind or what amount of
difference, in any recognisable character, is sufficient to prevent two
species crossing. It can be shown that plants most widely different
in habit and general appearance, and having strongly marked differ-
ences in every part of the flower, even in the pollen, in the fruit, and
in the cotyledons, can be crossed. Annual and perennial plants,
deciduous and evergreen trees, plants inhabiting different stations
and fitted for extremely different climates, can often be crossed
with ease.

By a reciprocal cross between two species, I mean the case, for
instance, of a female ass being first crossed by a stallion, and then a
mare by a male ass; these two sp)ecies may then be said to have been
reciprocally crossed. There is often the widest possible difference
in the facility of making reciprocal crosses. Such cases are highly
important, for they prove that the capacity in any two species to
cross is often completely independent of their systematic affinity,
that is of any difference in their structure or constitution, excepting
in their reproductive systems. The diversity of the result in recipro-
cal crosses between the same two species was long ago observed by

LAWS GOVERNING THE STERILITY 295

Kolreuter. To give an instance: Mirabilis jalapa can easily be fer-
tilised by the pollen of M. longiflora, and the hybrids thus produced
are sufficiendy fertile; but Kolreuter tried more than two hundred
times, during eight following years, to fertilise reciprocally M. longi-
flora with the pollen of M. jalapa, and utterly failed. Several other
equally striking cases could be given. Thuret has observed the same
fact with certain sea-weeds or Fuci. Gartner, moreover, found that
this difference of facility in making reciprocal crosses is extremely
common in a lesser degree. He has observed it even between closely
related forms (as Matthiola annua and glabra) which many bot-
anists rank only as varieties. It is also a remarkable fact, that hybrids
raised from reciprocal crosses, though of course compounded of the
very same two sjjecies, the one sp>ecies having first been used as the
father and then as the mother, though they rarely differ in external
characters, yet generally differ in fertility in a small, and occasionally
in a high degree.

Several other singular rules could be given from Gartner: for
instance, some species have a remarkable power of crossing with
other species; other species of the same genus have a remarkable
power of impressing their likeness on their hybrid offspring; but
these two powers do not at all necessarily go together. There are
certain hybrids which, instead of having, as is usual, an intermediate
character between their two parents, always closely resemble one of
them; and such hybrids, though externally so like one of their pure
parent-species, are with rare exceptions extremely sterile. So again
amongst hybrids which are usually intermediate in structure between
their parents, exceptional and abnormal individuals sometimes are
born, which closely resemble one of their pure parents; and these
hybrids are almost always utterly sterile, even when the other hybrids
raised from seed from the same capsule have a considerable degree
of fertility. These facts show how completely the fertility of a hybrid
may be independent of its external resemblance to either pure parent.

Considering the several rules now given, which govern the fer-
tility of first crosses and of hybrids, we see that when forms, which
must be considered as good and distinct species, are united, their
fertility graduates from zero to perfect fertility, or even to fertility
under certain conditions in excess; that their fertility, besides being

296 ORIGIN OF SPECIES

eminently susceptible to favourable and unfavourable conditions, is
innately variable; that it is by no means always the same in degree in
the first cross and in the hybrids produced from this cross; that the
fertility of hybrids is not related to the degree in which they
resemble in external appearance either parent; and lastly, that the
facility of making a first cross between any two species is not always
governed by their systematic affinity or degree of resemblance to
each other. This latter statement is clearly proved by the difference
in the result of reciprocal crosses between the same two species, for,
according as the one species or the other is used as the father or the
mother, there is generally some difference, and occasionally the
widest possible difference, in the facility of effecting an union. The
hybrids, moreover, produced from reciprocal crosses often differ in
fertility.

Now do these complex and singular rules indicate that species
have been endowed with sterility simply to prevent their becoming
confounded in nature? I think not. For why should the sterility
be so extremely different in degree, when various species are crossed,
all of which we must suppwse it would be equally important to keep
from blending together? Why should the degree of sterility be
innately variable in the individuals of the same species? Why should
some SfHxies cross with facility, and yet produce very sterile hybrids;
and other species cross with extreme difficulty, and yet produce fairly
fertile hybrids? Why should there often be so great a difference in
the result of a reciprocal cross between the same two species ? Why,
it may even be asked, has the production of hybrids been permitted ?
To grant to species the special power of producing hybrids, and then
to stop their further propagation by different degrees of sterility,
not strictly related to the facility of the first union between their
parents, seenis a strange arrangement.

The foregoing rules and facts, on the other hand, appear to me
clearly to indicate that the sterility both of first crosses and of hybrids
is simply incidental or dependent on unknown differences in their
reproductive systems; the differences being of so peculiar and limited
a nature, that, in reciprocal crosses between the same two species, the
male sexual element of the one will often freely act on the female
sexual element of the other, but not in a reversed direction. It will

LAWS GOVERNING THE STERILITY 297

be advisable to explain a little more fully by an example what I mean
by sterility being incidental on other differences, and not a sjjecially
endowed quality. As the capacity of one plant to be grafted or
budded on another is unimportant for their welfare in a state o£
nature, I presume that no one will suppose that this capacity is a
specially endowed quality, but will admit that it is incidental on
differences in the laws of growth of the two plants. We can some-
times see the reason why one tree will not take on another, from
differences in their rate of growth, in the hardness of their wood, in
the period of the flow or nature of their sap, etc.; but in a multitude
of cases we can assign no reason whatever. Great diversity in the
size of two plants, one being woody and the other herbaceous, one
being evergreen and the other deciduous, an adaptation to widely
different climates, do not always prevent the two grafting together.
As in hybridisation, so with grafting, the capacity is limited by sys-
tematic affinity, for no one has been able to graft together trees
belonging to quite distinct families; and, on the other hand, closely
allied species, and varieties of the same species, can usually, but not
invariably, be grafted with ease. But this capacity, as in hybridisation,
is by no means absolutely governed by systematic affinity. Although
many distinct genera within the same family have been grafted
together, in other cases sfjecies of the same genus will not take on
each other. The pear can be grafted far more readily on the quince,
which is ranked as a distinct genus, than on the apple, which is a
member of the same genus. Even different varieties of the pear take
with different degrees of facility on the quince; so do different vari-
eties of the apricot and peach on certain varieties of the plum.

As Gartner found that there was sometimes an innate difference
in different individuals of the same two species in crossing; so
Sageret believes this to be the case with different individuals of the
same two species in being grafted together. As in reciprocal crosses,
the facility of effecting an union is often very far from equal, so it
sometimes is in grafting; the common gooseberry, for instance, can-
not be grafted on the currant, whereas the currant will take, though
with difficulty, on the gooseberry.

We have seen that the sterility of hybrids, which have their repro-
ductive organs in an imperfect condition, is a different case from

298 ORIGIN OF SPECIES

the difficulty of uniting two pure species, which have their repro-
ductive organs perfect; yet these two distinct classes of cases run to a
large extent parallel. Something analogous occurs in grafting; for
Thouin found that three species of Robinia, which seeded freely on
their own roots, and which could be grafted with no great difficulty
on a fourth species, when thus grafted were rendered barren. On the
other hand, certain species of Sorbus, when grafted on other species
yielded twice as much fruit as when on their own roots. We are
reminded by this latter fact of the extraordinary cases of Hip-
peastrum, Passiflora, etc., which seed much more freely when fer-
tilised with the pollen of a distinct species, than when fertilised with
pollen from the same plant.

We thus see, that, although there is a clear and great difference
between the mere adhesion of grafted stocks, and the union of the
male and female elements in the act of reproduction, yet that there
is a rude degree of parallelism in the results of grafting and of
crossing distinct species. And as we must look at the curious and
complex laws governing the facility with which trees can be grafted
on each other as incidental on unknown differences in their vegeta-
tive systems, so I believe that the still more complex laws governing
the facility of first crosses are incidental on unknown differences in
their reproductive systems. These differences in both cases, follow
to a certain extent, as might have been expected, systematic affinity,
by which term every kind of resemblance and dissimilarity between
organic beings is attempted to be expressed. The facts by no means
seem to indicate that the greater or lesser difficulty of either grafting
or crossing various sp)ecies has been a sp)ecial endowment; although
in the case of crossing, the difficulty is as important for the endurance
and stability of specific forms, as in the case of grafting it is unim-
portant for their welfare.

ORIGIN AND CAUSES OF THE STERILITY OF FIRST CROSSES AND OF HYBRIDS

At one time it appeared to me probable, as it has to others, that the
sterility of first crosses and of hybrids might have been slowly
acquired through the natural selection of slighdy lessened degrees
of fertiUty, which, like any other variation, spontaneously appeared
in certain individuals of one variety when crossed with those of

CAUSES OF THE STERILITY 299

another variety. For it would clearly be advantageous to two vari-
eties or incipient species, if they could be kept from blending, on
the same principle that, when man is selecting at the same time two
varieties, it is necessary that he should keep them separate. In the
first place, it may be remarked that species inhabiting distinct
regions are often sterile when crossed; now it could clearly have been
of no advantage to such separated species to have been rendered
mutually sterile, and consequently this could not have been effected
through natural selection; but it may perhaps be argued, that, if a
species was rendered sterile with some one compatriot, sterility with
other species would follow as a necessary contingency. In the second
place, it is almost as much opposed to the theory of natural selection
as to that of special creation, that in reciprocal crosses the male
element of one form should have been rendered utterly impotent
on a second form, whilst at the same time the male element of this
second form is enabled freely to fertilise the first form; for this
peculiar state of the reproductive system could hardly have been
advantageous to either species.

In considering the probability of natural selection having come
into action, in rendering species mutually sterile, the greatest dif-
ficulty will be found to lie in the existence of many graduated steps
from slightly lessened fertility to absolute sterility. It may be admit-
ted that it would profit an incipient species, if it were rendered
in some slight degree sterile when crossed with its parent form or
with some other variety; for thus fewer bastardised and deteriorated
offspring would be produced to commingle their blood with the new
sf)ecies in process of formation. But he who will take the trouble to
reflect on the steps by which this first degree of sterility could be
increased through natural selection to that high degree which is
common with so many species, and which is universal with species
which have been differentiated to a generic or family rank, will find
the subject extraordinarily complex. After mature reflection it seems
to me that this could not have been effected through natural selec-
tion. Take the case of any two sjjecies which, when crossed, produced
few and sterile offspring; now, what is there which could favour
the survival of those individuals which happened to be endowed in
a slightly higher degree with mutual infertility, and which thus

300 ORIGIN OF SPECIES

approached by one small step towards absolute sterility? Yet an
advance of this kind, if the theory of natural selection be brought
to bear, must have incessantly occurred with many species, for a
multitude are mutually quite barren. With sterile neuter insects we
have reason to believe that modifications in their structure and
fertility have been slowly accumulated by natural selection, from an
advantage having been thus indirectly given to the community to
which they belonged over other communities of the same species;
but an individual animal not belonging to a social community, if
rendered slightly sterile when crossed with some other variety, would
not thus itself gain any advantage or indirectly give any advantage
to the other individuals of the same variety, thus leading to their
preservation.

But it would be superfluous to discuss this question in detail; for
with plants we have conclusive evidence that the sterility of crossed
species must be due to some principle, quite independent of natural
selection. Both Gartner and Kolreuter have proved that in genera
including numerous species, a series can be formed from sjjecies
which when crossed yield fewer and fewer seeds, to species which
never produce a single seed, but yet are affected by the pollen of
certain other species, for the germen swells. It is here manifestly
impossible to select the more sterile individuals, which have already
ceased to yield seeds; so that this acme of sterility, when the germen
alone is affected, cannot have been gained through selection; and
from the laws governing the various grades of sterility being so
uniform throughout the animal and vegetable kingdoms, we may
infer that the cause, whatever it may be, is the same or nearly the
same in all cases.

We will now look a little closer at the probable nature of the
differences between species which induce sterility in first crosses and
in hybrids. In the case of first crosses, the greater or less difficulty
in effecting an union and in obtaining offspring apparently depends
on several distinct causes. There must sometimes be a physical
impossibility in the male element reaching the ovule, as would be the
case with a plant having a pistil too long for the pollen-tubes to
reach the ovarium. It has also been observed that when the pollen

CAUSES OF THE STERILITY 301

of one sjjecies is placed on the stigma of a distantly allied species,
though the pollen-tubes protrude, they do not penetrate the stigmatic
surface. Again, the male element may reach the female element but
be incapable of causing an embryo to be developed, as seems to
have been the case with some of Thuret's experiments on Fuci. No
explanation can be given of these facts, any more than why certain
trees cannot be grafted on others. Lasdy an embryo may be devel-
oped, and then perish at an early period. This latter alternative has
not been suiTiciently attended to; but I believe, from observations
communicated to me by Mr. Hewitt, who has had great experience
in hybridising pheasants and fowls, that the early death of the
embryo is a very frequent cause of sterility in first crosses. Mr. Salter
has recently given the results of an examination of about 500 eggs
produced from various crosses between three species of Gallus and
their hybrids; the majority of these eggs had been fertilised; and in
the majority of the fertilised eggs, the embryos had either been par-
tially developed and had then perished, or had become nearly
mature, but the young chickens had been unable to break through
the shells. Of the chickens which were born, more than four-fifths
died within the first few days, or at latest weeks, "without any
obvious cause, apparently from mere inability to live"; so that from
the 500 eggs only twelve chickens were reared. With plants, hy-
bridised embryos probably often perish in a Uke manner; at least it
is known that hybrids raised from very distinct species are sometimes
weak and dwarfed, and perish at an early age; of which fact Max
Wichura has recently given some striking cases with hybrid willows.
It may be here worth noticing that in some cases of parthenogenesis,
the embryos within the eggs of silk moths which had not been
fertilised, pass through their early stages of development and then
perish like the embryos produced by a cross between distinct species.
Until becoming acquainted with these facts, I was unwilling to
believe in the frequent early death of hybrid embryos; for hybrids,
when once born, are generally healthy and long-lived, as we see in
the case of the common mule. Hybrids, however, are differendy
circumstanced before and after birth; when born and living in a
country where their two parents live, they are generally placed under
suitable conditions of life. But a hybrid partakes of only half of the

302 ORIGIN OF SPECIES

nature and constitution of its mother; it may therefore before birth,
as long as it is nourished within its mother's womb, or within the
egg or seed produced by the mother, be exposed to conditions in
some degree unsuitable, and consequently be liable to perish at an
early jjeriod; more especially as all very young beings are eminently
sensitive to injurious or unnatural conditions of life. But after all,
the cause more probably lies in some imperfection in the original
act of impregnation, causing the embryo to be imperfectly devel-
oped, rather than in the conditions to which it is subsequently ex-
posed.

In regard to the sterility of hybrids, in which the sexual elements
are imperfectly developed, the case is somewhat different. I have
more than once alluded to a large body of facts showing that, when
animals and plants are removed from their natural conditions, they
are extremely liable to have their reproductive systems seriously
affected. This, in fact, is the great bar to the domestication of
animals. Between the sterility thus superinduced and that of hybrids,
there are many points of similarity. In both cases the steriUty is
independent of general health, and is often accompanied by excess
of size or great luxuriance. In both cases the sterility occurs in vari-
ous degrees; in both, the male element is the most liable to be
affected; but sometimes the female more than the male. In both,
the tendency goes to a certain extent with systematic affinity, for
whole groups of animals and plants are rendered imf)otent by the
same unnatural conditions; and whole groups of species tend to
produce sterile hybrids. On the other hand, one Sf)ecies in a group
will sometimes resist great changes of conditions with unimpaired
fertility; and certain species in a group will produce unusually fer-
tile hybrids. No one can tell till he tries, whether any particular
animal will breed under confinement, or any exotic plant seed freely
under culture; nor can he tell till he tries, whether any two species
of a genus will produce more or less sterile hybrids. Lastly, when
organic beings are placed during several generations under condi-
tions not natural to them, they are extremely liable to vary, which
seems to be partly due to their reproductive systems having been
specially affected, though in a lesser degree than when sterility
ensues. So it is with hybrids, for their offspring in successive genera-

CAUSES OF THE STERILITY 303

tions are eminently liable to vary, as every experimentalist has
observed.

Thus we see that when organic beings are placed under new and
unnatural conditions, and when hybrids are produced by the un-
natural crossing of two species, the reproductive system, independ-
ently of the general state of health, is affected in a very similar
manner. In the one case, the conditions of life have been disturbed,
though often in so slight a degree as to be inappreciable by us; in
the other case, or that of hybrids, the external conditions have
remained the same, but the organisation has been disturbed by two
distinct structures and constitutions, including of course the repro-
ductive systems, having been blended into one. For it is scarcely
possible that two organisations should be compxjunded into one,
without some disturbance occurring in the development, or peri-
odical action, or mutual relations of the different parts and organs
one to another or to the conditions of life. When hybrids are able to
breed inter se, they transmit to their offspring from generation to
generation the same compounded organisation, and hence we need
not be surprised that their sterility, though in some degree variable,
does not diminish; it is even apt to increase, this being generally the
result, as before explained, of too close interbreeding. The above
view of the sterility of hybrids being caused by two constitutions
being compounded into one has been strongly maintained by Max
Wichura.

It must, however, be owned that we cannot understand, on the
above or any other view, several facts with respect to the sterility of
hybrids; for instance, the unequal fertility of hybrids produced from
reciprocal crosses; or the increased sterility in those hybrids which
occasionally and exceptionally resemble closely either pure parent.
Nor do I pretend that the foregoing remarks go to the root of the
matter; no explanation is offered why an organism, when placed
under natural conditions, is rendered sterile. All that I have at-
tempted to show is, that in two cases, in some respeas allied, sterility
is the common result, — in the one case from the conditions of life
having been disturbed, in the other case from the organisation having
been disturbed by two organisations being compounded into one.

A similar parallelism holds good with an allied yet very different

304 ORIGIN OF SPECIES

class of facts. It is an old and almost universal belief founded on a
considerable body of evidence, which I have elsewhere given, that
slight changes in the conditions of life are beneficial to all living
things. We see this acted on by farmers and gardeners in their
frequent exchanges of seed, tubers, etc., from one soil or climate to
another, and back again. During the convalescence of animals, great
benefit is derived from almost any change in their habits of life.
Again, both with plants and animals, there is the clearest evidence
that a cross between individuals of the same species, which differ to
a certain extent, gives vigour and fertility to the offspring; and that
close interbreeding continued during several generations between
the nearest relations, if these be kept under the same conditions of
life, almost always leads to decreased size, weakness, or sterility.

Hence it seems that, on the one hand, slight changes in the condi-
tions of life benefit all organic beings, and on the other hand, that
slight crosses, that is, crosses between the males and females of the
same species, which have been subjected to slightly different condi-
tions, or which have slightly varied, give vigour and fertility to the
offspring. But, as we have seen, organic beings long habituated to
certain uniform conditions under a state of nature, when subjected,
as under confinement, to a considerable change in their conditions,
very frequently are rendered more or less sterile; and we know that
a cross between two forms, that have become widely or specifically
different, produce hybrids which are almost always in some degree
sterile. I am fully f)ersuaded that this double parallelism is by no
means an accident or an illusion. He who is able to explain why
the elephant and a multitude of other animals are incapable of
breeding when kept under only partial confinement in their native
country, will be able to explain the primary cause of hybrids being
so generally sterile. He will at the same time be able to explain how
it is that the races of some of our domesticated animals, which have
often been subjected to new and not uniform conditions, are quite
fertile together, although they are descended from distinct species,
which would probably have been sterile if aboriginally crossed. The
above two parallel series of facts seem to be connected together by
some common but unknown bond, which is essentially related to
the principle of life; this principle, according to Mr. Herbert Spencer,

DIMORPHISM AND TRIMORPHISM 305

being that life depends on, or consists in, the incessant action and
reaction of various forces, which, as throughout nature, are always
tending towards an equiHbrium; and when this tendency is sHghtly
disturbed by any change, the vital forces gain in jwwer.

RECIPROCAL DIMORPHISM AND TRIMORPHISM

This subject may be here briefly discussed, and will be found to
throw some light on hybridism. Several plants belonging to distinct
orders present two forms, which exist in about equal numbers and
which differ in no respect except in their reproductive organs; one
form having a long pistil with short stamens, the other a short pistil
with long stamens; the two having differently sized pollen-grains.
With trimorphic plants there are three forms likewise differing in the
lengths of their pistils and stamens, in the size and colour of the
pollen-grains, and in some other respects; and as in each of the three
forms there are two sets of stamens, the three forms pnDssess altogether
six sets of stamens and three kinds of pistils. These organs are so
proportioned in length to each other, that half the stamens in two
of the forms stand on a level with the stigma of the third form.
Now I have shown, and the result has been confirmed by other
observers, that, in order to obtain full fertility with these plants, it is
necessary that the stigma of the one form should be fertilised by
pollen taken from the stamens of corresponding height in another
form- So that with dimorphic species two unions, which may be
called legitimate, are fully fertile; and two, which may be called
illegitimate, are more or less infertile. With trimorphic species six
unions are legitimate, or fully fertile, — and twelve are illegitimate,
or more or less infertile.

The infertility which may be observed in various dimorphic and
trimorphic plants, when they are illegitimately fertilised, that is, by
pollen taken from stamens not corresponding in height with the
pistil, differs much in degree, up to absolute and utter sterility; just
in the same manner as occurs in crossing distinct species. As the
degree of sterility in the latter case depends in an eminent degree on
the conditions of life being more or less favourable, so I have found
it with illegitimate unions. It is well known that if pollen of a dis-
tinct species be placed on the stigma of a flower, and its own pollen

306 ORIGIN OF SPECIES

be afterwards, even after a considerable interval of time, placed on
the same stigma, its action is so strongly prepotent that it generally
annihilates the effect of the foreign pollen; so it is with the pollen
of the several forms of the same species, for legitimate pollen is
strongly prepotent over illegitimate pollen, when both are placed on
the same stigma. I ascertained this by fertilising several flowers,
first illegitimately, and twenty-four hours afterwards legitimately,
with pollen taken from a peculiarly coloured variety, and all the seed-
lings were similarly coloured; this shows that the legitimate pollen,
though applied twenty-four hours subsequently, had wholly de-
stroyed or prevented the action of the previously applied illegitimate
pollen. Again, as in making reciprocal crosses between the same two
species, there is occasionally a great difference in the result, so the
same thing occurs with trimorphic plants; for instance, the mid-
styled form of Lythrum salicaria was illegitimately fertilised with
the greatest ease by pollen from the longer stamens of the short-
styled form, and yielded many seeds; but the latter form did not
yield a single seed when fertilised by the longer stamens of the
mid-styled form.

In all these respects, and in others which might be added, the
forms of the same undoubted species when illegitimately united be-
have in exactly the same manner as do two distinct species when
crossed. This led me carefully to observe during four years many
seedlings, raised from several illegitimate unions. The chief result
is that these illegitimate plants, as they may be called, are not fully
fertile. It is possible to raise from dimorphic species, both long-
styled and short-styled illegitimate plants, and from trimorphic
plants all three illegitimate forms. These can then be properly united
in a legitimate manner. When this is done, there is no apparent
reason why they should not yield as many seeds as did their parents
when legitimately fertilised. But such is not the case. They are all
infertile, in various degrees; some being so utterly and incurably
sterile that they did not yield during four seasons a single seed or
even seed<apsule. The sterility of these illegitimate plants, when
united with each other in a legitimate manner, may be strictly com-
pared with that of hybrids when crossed inter se. If, on the other
hand, a hybrid is crossed with either pure parent-species, the sterility

DIMORPHISM AND TRIMORPHISM 307

is usually much lessened; and so it is when an illegitimate plant is
fertilised by a legitimate plant. In the same manner as the sterility
of hybrids does not always run parallel with the difficulty of making
the first cross between the two parent-species, so the sterility of certain
illegitimate plants was unusually great, whilst the sterility of the
union from which they were derived was by no means great. With
hybrids raised from the same seed-capsule the degree of sterility is
innately variable, so it is in a marked manner with illegitimate
plants. Lasdy, many hybrids are profuse and persistent flowerers,
whilst other and more sterile hybrids produce few flowers, and are
weak, miserable dwarfs; exactly similar cases occur with the illegiti-
mate offspring of various dimorphic and trimorphic plants.

Altogether there is the closest identity in character and behaviour
between illegitimate plants and hybrids. It is hardly an exaggeration
to maintain that illegitimate plants are hybrids, produced within the
limits of the same species by the improper union of certain forms,
whilst ordinary hybrids are produced from an improper union be-
tween so-called distinct species. We have also already seen that there
is the closest similarity in all respects between first illegitimate unions
and first crosses between distinct species. This will perhaps be made
more fully apparent by an illustration; we may suppose that a
botanist found two well-marked varieties (and such occur) of the
long-styled form of the trimorphic Lythrum salicaria, and that he
determined to try by crossing whether they were specifically distinct.
He would find that they yielded only about one-fifth of the projjer
number of seed, and that they behaved in all the other above speci-
fied respects as if they had been two distinct species. But to make
the case sure, he would raise plants from his supposed hybridized
seed, and he would find that the seedlings were miserably dwarfed
and utterly sterile, and that they behaved in all other respects like
ordinary hybrids. He might then maintain that he had actually
proved, in accordance with the common view, that his two varieties
were as good and as distinct species as any in the world; but he would
be completely mistaken.

The facts now given on dimorphic and trimorphic plants are
important, because they show us, first, that the physiological test of
lessened fertility, both in first crosses and in hybrids, is no safe

308 ORIGIN OF SPECIES

criterion o£ specific distinction; secondly, because we may conclude
that there is some unknown bond which connects the infertility of
illegitimate unions with that of their illegitimate offspring, and we
are led to extend the same view to first crosses and hybrids; thirdly,
because we find, and this seems to me of especial importance, that
two or three forms of the same sp)ecies may exist and may differ in
no respect whatever, either in structure or in constitution, relatively
to external conditions, and yet be sterile when united in certain ways.
For we must remember that it is the union of the sexual elements of
individuals of the same form, for instance, of two long-styled forms,
which results in sterility; whilst it is the union of the sexual elements
proper to two distinct forms which is fertile. Hence the case appears
at first sight exactly the reverse of what occurs, in the ordinary unions
of the individuals of the same species and with crosses between dis-
tinct species. It is, however, doubtful whether this is really so; but I
will not enlarge on this obscure subject.

We may, however, infer as probable from the consideration of
dimorphic and trimorphic plants, that the sterility of distinct species
when crossed and of their hybrid progeny, depends exclusively on
the nature of their sexual elements, and not on any difference in
their structure or general constitution. We are also led to this same
conclusion by considering reciprocal crosses, in which the male of
one species cannot be united, or can be united with great difficulty,
with the female of a second species, whilst the converse cross can be
effected with [jerfeot facility. That excellent observer, Gartner, like-
wise concluded that species when crossed are sterile owing to differ-
ences confined to their reproductive systems.

FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR MONGREL
OFFSPRING, NOT UNIVERSAL

It may be urged, as an overwhelming argument, that there must
be some essential distinction between species and varieties, inasmuch
as the latter, however much they may differ from each other in
external appearance, cross with perfect facility, and yield {perfectly
fertile offspring. With some exceptions, presently to be given, I fully
admit that this is the rule. But the subject is surrounded by difficul-
ties, for, looking to varieties produced under nature, if two forms

FERTILITY OF VARIETIES 309

hitherto reputed to be varieties be found in any degree sterile
together, they are at once ranked by most naturaUsts as species. For
instance, the blue and red pimpernel, which are considered by most
botanists as varieties, are said by Gartner to be quite sterile when
crossed, and he consequently ranks them as undoubted species. If
we thus argue in a circle, the fertility of all varieties produced under
nature will assuredly have to be granted.

If we turn to varieties, produced, or supposed to have been pro-
duced, under domestication, we are still involved in some doubt.
For when it is stated, for instance, that certain South American
indigenous domestic dogs do not readily unite with European dogs,
the explanation which will occur to every one, and probably the true
one, is that they are descended from aboriginally distinct sp)ecies.
Nevertheless the f)erfect fertility of so many domestic races, differing
widely from each other in appearance, for instance those of the
pigeon, or of the cabbage, is a remarkable fact; more especially when
we reflect how many species there are, which, though resembling
each other most closely, are utterly sterile when intercrossed. Several
considerations, however, render the fertility of domestic varieties
less remarkable. In the first place, it may be observed that the amount
of external difference between two species is no sure guide to their
degree of mutual sterility, so that similar differences in the case of
varieties would be no sure guide. It is certain that with species the
cause lies exclusively in differences in their sexual constitution. Now
the varying conditions to which domesticated animals and cultivated
plants have been subjected, have had so little tendency towards modi-
fying the reproductive system in a manner leading to mutual
sterility, that we have good grounds for admitting the directly oppo-
site doctrine of Pallas, namely, that such conditions generally elim-
inate this tendency; so that the domesticated descendants of species,
which in their natural state probably would have been in some degree
sterile when crossed, become perfectly fertile together. With plants,
so far is cultivation from giving a tendency towards sterility between
distinct species, that in several well-authenticated cases already
alluded to, certain plants have been affected in an opposite manner,
for they have become self-impotent whilst still retaining the capacity
of fertilising, and being fertilised by, other species. If the Pallasian

310 ORIGIN OF SPECIES

doctrine of the elimination of sterility through long-continued domes-
tication be admitted, and it can hardly be rejected, it becomes in
the highest degree improbable that similar conditions long con-
tinued should likewise induce this tendency; though in certain cases,
with sp)ecies having a p)eculiar constitution, sterility might occasion-
ally be thus caused. Thus, as I believe, we can understand why with
domesticated animals varieties have not been produced which are
mutually sterile; and why with plants only a few such cases, imme-
diately to be given, have been observed.

The real difficulty in our present subject is not, as it appears to
me, why domestic varieties have not become mutually infertile
when crossed, but why this has so generally occurred with natural
varieties, as soon as they have been permanently modified in a
sufficient degree to take rank as species. We are far from precisely
knowing the cause; nor is this surprising, seeing how profoundly
ignorant we are in regard to the normal and abnormal action of the
reproductive system. But we can see that sp)ecies, owing to their
struggle for existence with numerous competitors, will have been ex-
posed during long periods of time to more uniform conditions, than
have domestic varieties; and this may well make a wide difference in
the result. For we know how commonly wild animals and plants,
when taken from their natural conditions and subjected to captivity,
are rendered sterile; and the reproductive functions of organic
beings which have always lived under natural conditions would
probably in like manner be eminently sensitive to the influence of
an unnatural cross. Domesticated productions, on the other hand,
which, as shown by the mere fact of their domestication, were not
originally highly sensitive to changes in their conditions of life, and
which can now generally resist with undiminished fertility repeated
changes of conditions, might be expected to produce varieties, which
would be little liable to have their reproductive powers injuriously
affected by the act of crossing with other varieties which had orig-
inated in a like manner.

I have as yet spoken as if the varieties of the same species were
invariably fertile when intercrossed. But it is impossible to resist
the evidence of the existence of a certain amount of sterility in the
few following cases, which I will briefly abstract. The evidence is

FERTILITY OF VARIETIES 3 II

at least as good as that from which we beHeve in the sterility of a
multitude of species. The evidence is, also, derived from hostile
witnesses, who in all other cases consider fertility and sterility as safe
criterions of specific distinction. Gartner kept during several years
a dwarf kind of maize with yellow seeds, and a tall variety with
red seeds growing near each other in his garden; and although these
plants have separated sexes, they never naturally crossed. He then
fertilised thirteen flowers of the one kind with pollen of the other;
but only a single head produced any seed, and this one head pro-
duced only five grains. Manipulation in this case could not have
been injurious, as the plants have separated sexes. No one, I believe,
has suspected that these varieties of maize are distinct species; and it
is important to notice that the hybrid plants thus raised were them-
selves perfectly fertile; so that even Gartner did not venture to con-
sider the two varieties as specifically distinct.

Girou de Buzareingues crossed three varieties of gourd, which
like the maize has separated sexes, and he asserts that their mutual
fertilisation is by so much the less easy as their differences are
greater. How far these experiments may be trusted, I know not;
but the forms experimented on are ranked by Sageret, who mainly
founds his classification by the test of infertility, as varieties, and
Naudin has come to the same conclusion.

The following case is far more remarkable, and seems at first
incredible; but it is the result of an astonishing number of experi-
ments made during many years on nine species of Verbascum, by
so good an observer and so hostile a witness as Gartner: namely
that the yellow and white varieties when crossed produce less
seed than the similarly coloured varieties of the same species.
Moreover, he asserts that when yellow and white varieties of one
species are crossed with yellow and white varieties of a distinct
species, more seed is produced by the crosses between the similarly
coloured flowers, than between those which are differently coloured.
Mr. Scott also has experimented on the species and varieties of
Verbascum; and although unable to confirm Gartner's results on
the crossing of the distinct species, he finds that the dissimilarly
coloured varieties of the same sf)ecies yield fewer seeds, in the pro-
portion of eighty-six to lOO, than the similarly coloured varieties.

312 ORIGIN OF SPECIES

Yet these varieties differ in no respect except in the colour of their
flowers; and one variety can sometimes be raised from the seed of
another.

Kolreuter, whose accuracy has been confirmed by every subsequent
observer, has proved the remarkable fact, that one particular variety
of the common cobacco was more fertile than the other varieties,
when crossed with a widely distinct species. He experimented on
five forms which are commonly reputed to be varieties, and which
he tested by the severest trial, namely, by reciprocal crosses, and he
found their mongrel offspring perfectly fertile. But one of these
five varieties, when used either as the father or mother, and crossed
with the Nicotiana glutinosa, always yielded hybrids not so sterile
as those which were produced from the four other varieties when
crossed with N. glutinosa. Hence the reproductive system of this
one variety must have been in some manner and in some degree
modified.

From these facts it can no longer be maintained that varieties
when crossed are invariably quite fertile. From the great difficulty
of ascertaining the infertility of varieties in a state of nature, for a
supposed variety, if proved to be infertile in any degree, would
almost universally be ranked as a species; — from man attending only
to external characters in his domestic varieties, and from such varie-
ties not having been exposed for very long periods to uniform con-
ditions of life; — from these several considerations we may conclude
that fertility does not constitute a fundamental distinction between
varieties and sf>ecies when crossed. The general sterility of crossed
species may safely be looked at, not as a special acquirement or en-
dowment, but as incidental on changes of an unknown nature in
their sexual elements.

HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF
THEIR FERTILITY

Independently of the question of fertility, the offspring of species
and of varieties when crossed may be compared in several other
respects. Gartner, whose strong wish it was to draw a distinct line
between species and varieties, could find very few, and, as it seems
to me, quite unimportant differences between the so<alled hybrid

HYBRIDS AND MONGRELS COMPARED 313

offspring of species, and the so<allecl mongrel offspring of varieties.
And, on the other hand, they agree most closely in many important
respects.

I shall here discuss this subject with extreme brevity. The most
important distinction is, that in the first generation mongrels are
more variable than hybrids; but Gartner admits that hybrids from
species which have long been cultivated are often variable in the
first generation; and I have myself seen striking instances of this
fact. Gartner further admits that hybrids between very closely allied
sf)ecies are more variable than those from very distinct sp)ecies; and
this shows that the difference in the degree of variability graduates
away. When mongrels and the more fertile hybrids are propagated
for several generations, an extreme amount of variability in the off-
spring in both cases is notorious; but some few instances of both
hybrids and mongrels long retaining a uniform character could be
given. The variability, however, in the successive generations of
mongrels is, perhaps, greater than in hybrids.

This greater variability in mongrels than in hybrids does not
seem at all surprising. For the parents of mongrels are varieties,
and mostly domestic varieties (very few experiments having been
tried on natural varieties), and this implies that there has been recent
variability, which would often continue and would augment that
arising from the act of crossing. The slight variability of hybrids in
the first generation, in contrast with that in the succeeding genera-
tions, is a curious fact and deserves attention. For it bears on the
view which I have taken of one of the causes of ordinary variability;
namely, that the reproductive system from being eminently sensitive
to changed conditions of life, fails under these circumstances to per-
form its proper function of producing offspring closely similar in
all respects to the parent-form. Now, hybrids in the first generation
are descended from species (excluding those long cultivated) which
have not had their reproductive systems in any way affected, and
they are not variable; but hybrids themselves have their reproduc-
tive systems seriously affected, and their descendants are highly
variable.

But to return to our comparison of mongrels and hybrids: Gartner
states that mongrels are more liable than hybrids to revert to either

314 ORIGIN OF SPECIES

parent-form; but this, if it be true, is certainly only a difference in
degree. Moreover, Gartner expressly states that hybrids from long
cultivated plants are more subject to reversion than hybrids from
species in their natural state; and this probably explains the singular
difference in the results arrived at by different observers: thus Max
Wichura doubts whether hybrids ever revert to their parent-forms,
and he experimented on uncultivated species of willows; whilst
Naudin, on the other hand, insists in the strongest terms on the
almost universal tendency to reversion in hybrids, and he experi-
mented chiefly on cultivated plants. Gartner further states that
when any two sjiecies, although most closely allied to each other,
are crossed with a third species, the hybrids are widely different from
each other; whereas if two very distinct varieties of one species are
crossed with another sp)ecies, the hybrids do not differ much. But
this conclusion, as far as I can make out, is founded on a single ex-
periment; and seems directly opposed to the results of several ex-
periments made by Kolreuter.

Such alone are the unimportant differences which Gartner is
able to point out between hybrid and mongrel plants. On the other
hand, the degrees and kinds of resemblance in mongrels and in
hybrids to their respective parents, more especially in hybrids pro-
duced from nearly related species, follow, according to Gartner,
the same laws. When two species are crossed, one has sometimes a
prepotent power of impressing its likeness on the hybrid. So I believe
it to be with varieties of plants; and with animals one variety cer-
tainly often has this prepotent power over another variety. Hybrid
plants produced from a reciprocal cross, generally resemble each
other closely; and so it is with mongrel plants from a reciprocal
cross. Both hybrids and mongrels can be reduced to either pure
parent-form, by repeated crosses in successive generations with either
parent.

These several remarks are apparendy applicable to animals; but
the subject is here much complicated, partly owing to the existence
of secondary sexual charaaers; but more especially owing to prepo-
tency in transmitting likeness running more strongly in one sex than
in the other, both when one species is crossed with another, and

HYBRIDS AND MONGRELS COMPARED 315

when one variety is crossed with another variety. For instance, I
think those authors are right who maintain that the ass has a pre-
potent power over the horse, so that both the mule and the hinny
resemble more closely the ass than the horse; but that the prepotency
runs more strongly in the male than in the female ass, so that the
mule, which is the offspring of the male ass and mare, is more like
an ass, than is the hinny, which is the offspring of the female ass
and stallion.

Much stress has been laid by some authors on the supposed fart,
that it is only with mongrels that the offspring are not intermediate
in character, but closely resemble one of their parents; but this does
sometimes occur with hybrids, yet I grant much less frequently
than with mongrels. Looking to the cases which I have collected of
cross-bred animals closely resembling one parent, the resemblances
seem chiefly confined to characters almost monstrous in their nature,
and which have suddenly appeared — such as albinism, melanism,
deficiency of tail or horns, or additional fingers and toes; and do not
relate to characters which have been slowly acquired through selec-
tion. A tendency to sudden reversions to the perfect character of
either parent would, also, be much more likely to occur with mon-
grels, which are descended from varieties often suddenly produced
and semi-monstrous in character, than with hybrids, which are de-
scended from species slowly and naturally produced. On the whole,
I entirely agree with Dr. Prosper Lucas, who, after arranging an
enormous body of facts with respect to animals, comes to the con-
clusion that the laws of resemblance of the child to its parents are
the same, whether the two parents differ little or much from each
other, namely, in the union of individuals of the same variety, or of
different varieties, or of distinct sjiecies.

Independently of the question of fertiHty and sterility, in all other
respects there seems to be a general and close similarity in the off-
spring of crossed species, and of crossed varieties. If we look at
species as having been specially created, and at varieties as having
been produced by secondary laws, this similarity would be an
astonishing fact. But it harmonises perfectly with the view that there
is no essential distinction between species and varieties.

3l6 ORIGIN OF SPECIES

SUMMARY OF CHAPTER

First crosses between forms, sufficiently distinct to be ranked as
sf)ecies, and their hybrids, are very generally, but not universally,
sterile. The sterility is of all degrees, and is often so slight that the
most careful experimentalists have arrived at diametrically opposite
conclusions in ranking forms by this test. The sterility is innately
variable in individuals of the same species, and is eminently sus-
ceptible to the action of favourable and unfavourable conditions.
The degree of sterility does not strictly follow systematic affinity,
but is governed by several curious and complex laws. It is generally
different, and sometimes widely different in reciprocal crosses be-
tween the same two species. It is not always equal in degree in a
first cross and in the hybrids produced from this cross.

In the same manner as in grafting trees, the capacity in one species
or variety to take on another, is incidental on differences, generally
of an unknown nature, in their vegetative systems, so in crossing, the
greater or less facility of one sfjecies to unite with another is inci-
dental on unknown differences in their reproductive systems. There
is no more reason to think that species have been specially endowed
with various degrees of sterility to prevent their crossing and blending
in nature, than to think that trees have been specially endowed with
various and somewhat analogous degrees of difficulty in being
grafted together in order to prevent their inarching in our forests.

The sterility of first crosses and of their hybrid progeny has not
been acquired through natural selection. In the case of first crosses
it seems to depend on several circumstances; in some instances in
chief part on the early death of the embryo. In the case of hybrids,
it apparently depends on their whole organisation having been dis-
turbed by being compounded from two distinct forms; the sterility
being closely allied to that which so frequently affects pure sjiecies,
when exposed to new and unnatural conditions of life. He who will
explain these latter cases will be able to explain the sterility of hy-
brids. This view is strongly supported by a parallelism of another
kind: namely, that, firstly, slight changes in the conditions of life
add to the vigour and fertility of all organic beings; and secondly,
that the crossing of forms, which have been exposed to slightly

SUMMARY 317

difTerent conditions of life or which have varied, favours the size,
vigour, and fertihty of their offspring. The facts given on the sterility
of the illegitimate unions of dimorphic and trimorphic plants and
of their illegitimate progeny, perhaps render it probable that some
unknown bond in all cases connects the degree of fertility of first
unions with that of their offspring. The consideration of these facts
on dimorphism, as well as of the results of reciprocal crosses, clearly
leads to the conclusion that the primary cause of the sterility of
crossed species is confined to differences in their sexual elements.
But why, in the case of distinct species, the sexual elements should
so generally have become more or less modified, leading to their
mutual infertility, we do not know; but it seems to stand in some
close relation to species having been exposed for long periods of time
to nearly uniform conditions of life.

It is not surprising that the difficulty in crossing any two species,
and the sterility of their hybrid offspring, should in most cases cor-
respond, even if due to distinct causes: for both depend on the
amount of difference between the species which are crossed. Nor
is it surprising that the facility of effecting a first cross, and the
fertility of the hybrids thus produced, and the capacity of being
grafted together — though this latter capacity evidently depends on
widely different circumstances — should all run, to a certain extent,
parallel with the systematic affinity of the forms subjected to experi-
ment; for systematic affinity includes resemblances of all kinds.

First crosses between forms known to be varieties, or sufficiently
alike to be considered as varieties, and their mongrel offspring, are
very generally, but not, as is so often stated, invariably fertile. Nor
is this almost universal and perfect fertility surprising, when it is
remembered how liable we are to argue in a circle with respect to
varieties in a state of nature; and when we remember that the
greater number of varieties have been produced under domestication
by the selection of mere external differences, and that they have not
been long exposed to uniform conditions of life. It should also be
esf)ecially kept in mind, that long<ontinued domestication tends to
eliminate sterility, and is therefore little likely to induce this same
quality. Independently of the question of fertility, in all other
respects there is the closest general resemblance between hybrids and

3l8 ORIGIN OF SPECIES

mongrels, — in their variability, in their power of absorbing each
other by repeated crosses, and in their inheritance of characters from
both parent-forms. Finally, then, although we are as ignorant of
the precise cause of the sterility of first crosses and of hybrids as we
are why animals and plants removed from their natural conditions
become sterile, yet the facts given in this chapter do not seem to me
opposed to the belief that species aboriginally existed as varieties.

CHAPTER X

On the Imperfection of the Geological Record

On the absence of intermediate varieties at the present day — On the
nature of extinct intermediate varieties; on their number — On the
lapse of time, as inferred from the rate of denudation and of deposi-
tion — On the lapse of time as estimated by years — On the poorness
of our palaeontological collections — On the intermittence of geologi-
cal formations — On the denudation of granitic areas — On the absence
of intermediate varieties in any one formation — On the sudden
appearance of groups of sjjecies— On their sudden appearance in the
lowest known fossiliferous strata — Antiquity of the habitable earth.

IN the sixth chapter I enumerated the chief objections which
might be justly urged against the views maintained in this
volume. Most of them have now been discussed. One, namely
the distinaness of specific forms, and their not being blended to-
gether by innumerable transitional links, is a very obvious difficulty.
I assigned reasons why such links do not commonly occur at the
present day under the circumstances apparently most favourable for
their presence, namely on an extensive and continuous area with
graduated physical conditions. 1 endeavoured to show, that the life
of each species depends in a more important manner on the presence
of other already defined organic forms, than on climate, and, there-
fore, that the really governing conditions of life do not graduate
away quite insensibly like heat or moisture. I endeavoured, also, to
show that intermediate varieties, from existing in lesser numbers
than the forms which they connect, will generally be beaten out and
exterminated during the course of further modification and im-
provement. The main cause, however, of innumerable intermediate
links not now occurring everywhere throughout nature, depends on
the very process of natural selection, through which new varieties
continually take the places of and supplant their parent-forms. But
just in proportion as this process of extermination has acted on an
enormous scale, so must the number of intermediate varieties, which
have formerly existed, be truly enormous. Why then is not every

319

320 ORIGIN OF SPECIES

geological formation and every stratum full of such intermediate
links? Geology assuredly does not reveal any such finely-graduated
organic chain; and this, perhaps, is the most obvious and serious
objection which can be urged against the theory. The explanation
lies, as I believe, in the extreme imperfection of the geological
record.

In the first place, it should always be borne in mind what sort of
intermediate forms must, on the theory, have formerly existed. I
have found it difficult, when looking at any two species, to avoid
picturing to myself forms directly intermediate between them. But
this is a wholly false view; we should always look for forms inter-
mediate between each species and a common but unknown progeni-
tor; and the progenitor will generally have differed in some respects
from all its modified descendants. To give a simple illustration:
the fantail and pouter pigeons are both descended from the rock-
pigeon; if we possessed all the intermediate varieties which have
ever existed, we should have an extremely close series between both
and the rock-pigeon; but we should have no varieties directly in-
termediate between the fantail and pouter; none, for instance, com-
bining a tail somewhat expanded with a crop somewhat enlarged,
the characteristic features of these two breeds. These two breeds,
moreover, have become so much modified, that, if we had no his-
torical or indirect evidence regarding their origin, it would not have
been possible to have determined, from a mere comparison of their
structure with that of the rock-pigeon, C. livia, whether they had
descended from this species or from some other allied form, such
as C. CEnas.

So, with natural species, if we look to forms very distinct, for
instance to the horse and tapir, we have no reason to suppose that
links directly intermediate between them ever existed, but between
each and an unknown common parent. The common parent will
have had in its whole organisation much general resemblance to
the tapir and to the horse; but in some points of structure may have
differed considerably from both, even perhaps more than they differ
from each other. Hence, in all such cases, we should be unable to
recognise the parent-form of any two or more species, even if we
closely compared the structure of the parent with that of its modified

THE LAPSE OF TIME 321

descendants, unless at the same time we had a nearly perfect chain
of the intermediate links.

It is just possible by the theory, that one of two living forms might
have descended from the other; for instance, a horse from a tapir;
and in this case direct intermediate links will have existed between
them. But such a case would imply that one form had remained
for a very long period unaltered, whilst its descendants had under-
gone a vast amount of change; and the principle of competition
between organism and organism, between child and parent, will
render this a very rare event; for in all cases the new and improved
forms of life tend to supplant the old and unimproved forms.

By the theory of natural selection all living species have been con-
nected with the parent-species of each genus, by differences not
greater than we see between the natural and domestic varieties of
the same species at the present day; and these parent-species, now
generally extinct, have in their turn been similarly connected with
more ancient forms; and so on backwards, always converging to
the common ancestor of each great class. So that the number of
intermediate and transitional links, between all living and extinct
species, must have been inconceivably great. But assuredly, if this
theory be true, such have lived upon the earth.

ON THE LAPSE OF TIME, AS INFERRED FROM THE RATE OF
DEPOSITION AND EXTENT OF DENUDATION

Independently of our not finding fossil remains of such infinitely
numerous connecting links, it may be objected that time cannot
have sufficed for so great an amount of organic change, all changes
having been effected slowly. It is hardly possible for me to recall to
the reader who is not a practical geologist, the facts leading the
mind feebly to comprehend the lapse of time. He who can read
Sir Charles Lyell's grand work on the Principles of Geology, which
the future historian will recognise as having produced a revolution
in natural science, and yet does not admit how vast have been the
past periods of time, may at once close this volume. Not that it
suffices to study the Principles of Geology, or to read special treatises
by difTerent observers on separate formations, and to mark how
each author attempts to give an inadequate idea of the duration of

322 ORIGIN OF SPECIES

each formation, or even of each stratum. We can best gain some
idea of past time by knowing the agencies at work, and learning
how deeply the surface of the land has been denuded, and how much
sediment has been deposited. As Lyell has well remarked, the extent
and thickness of our sedimentary formations are the result and the
measure of the denudation which the earth's crust has elsewhere
undergone. Therefore a man should examine for himself the great
piles of superimposed strata, and watch the rivulets bringing down
mud, and the waves wearing away the sea<liffs, in order to com-
prehend something about the duration of past time, the monuments
of which we see all around us.

It is good to wander along the coast, when formed of moderately
hard rocks, and mark the process of degradation. The tides in most
cases reach the cliffs only for a short time twice a day, and the waves
eat into them only when they are charged with sand or pebbles; for
there is good evidence that pure water effects nothing in wearing
away rock. At last the base of the cliff is undermined, huge frag-
ments fall down, and these, remaining fixed, have to be worn away
atom by atom, until after being reduced in size they can be rolled
about by the waves, and then they are more quickly ground into
pebbles, sand, or mud. But how often do we see along the bases of
retreating cliffs rounded boulders, all thickly clothed by marine pro-
ductions, showing how little they are abraded and how seldom they
are rolled about! Moreover, if we follow for a few miles any line of
rocky cliff, which is undergoing degradation, we find that it is only
here and there, along a short length or round a promontory, that
the cliffs are at the present time suffering. The appearance of the
surface and the vegetation show that elsewhere years have elapsed
since the waters washed their base.

We have, however, recently learnt from the observations of Ram-
say, in the van of many excellent observers — of Jukes, Geikie, Croll,
and others, that subaerial degradation is a much more important
agency than coast-action, or the power of the waves. The whole
surface of the land is exposed to the chemical action of the air and
of the rain-water with its dissolved carbonic acid, and in colder
countries to frost; the disintegrated matter is carried down even
gentle slopes during heavy rain, and to a greater extent than might

THE LAPSE OF TIME 323

be supposed, especially in arid districts, by the wind; it is then
transported by the streams and rivers, which when rapid deepen
their channels, and triturate the fragments. On a rainy day, even in
a gently undulating country, we see the effects of subaerial degrada-
tion in the muddy rills which flow down every slope. Messrs.
Ramsay and Whitaker have shown, and the observation is a most
striking one, that the great lines of escarpment in the Wealden dis-
trict and those ranging across England, which formerly were looked
at as ancient sea-coasts, cannot have been thus formed, for each line
is composed of one and the same formation, whilst our sea<liffs
are everywhere formed by the intersection of various formations.
This being the case, we are compelled to admit that the escarpments
owe their origin in chief part to the rocks of which they are com-
posed having resisted subaerial denudation better than the surround-
ing surface; this surface consequently has been gradually lowered,
with the lines of harder rock left projecting. Nothing impresses
the mind with the vast duration of time, according to our ideas of
time, more forcibly than the conviction thus gained that subaerial
agencies which apparently have so little power, and which seem to
work so slowly, have produced great results.

When thus impressed with the slow rate at which the land is
worn away through subaerial and littoral action, it is good, in order
to appreciate the past duration of time, to consider on the one hand,
the masses of rock which have been removed over many extensive
areas, and on the other hand the thickness of our sedimentary
formations. I remember having been much struck when viewing
volcanic islands, which have been worn by the waves and pared all
round into perpendicular cliffs of one or two thousand feet in
height; for the gentle slope of the lava-streams, due to their formerly
Uquid state, showed at a glance how far the hard, rocky beds had
once extended into the open ocean. The same story is told still more
plainly by faults, — those great cracks along which the strata have
been upheaved on one side, or thrown down on the other, to the
height or depth of thousands of feet; for since the crust cracked, and
it makes no great difference whether the upheaval was sudden, or,
as most geologists now believe, was slow and effected by many
starts, the surface of the land has been so completely planed down

324 ORIGIN OF SPECIES

that no trace of these vast dislocations is externally visible. The
Craven fault, for instance, extends for upwards of thirty miles, and
along this line the vertical displacement of the strata varies from
600 to 3000 feet. Professor Ramsay has published an account of a
downthrow in Anglesea of 2,300 feet; and he informs me that he
fully believes that there is one in Merionethshire of 12,000 feet; yet
in these cases there is nothing on the surface of the land to show
such prodigious movements; the pile of rocks on either side of the
crack having been smoothly swept away.

On the other hand, in all parts of the world the piles of sedi-
mentary strata are of wonderful thickness. In the Cordillera I
estimated one mass of conglomerate at ten thousand feet; and al-
though conglomerates have probably been accumulated at a quicker
rate than finer sediments, yet from being formed of worn and
rounded fjebbles, each of which bears the stamp of time, they are
good to show how slowly the mass must have been heap)ed together.
Professor Ramsay has given me the maximum thickness, from actual
measurement in most cases, of the successive formations in different
parts of Great Britain; and this is the result: —

Feet

Palaeozoic strata (not including igneous beds) 57.154

Secondary strata •3ii9o

Tertiary strata 2,240

— making altogether 72,584 feet; that is, very nearly thirteen and
three-quarters British miles. Some of the formations, which are
represented in England by thin beds, are thousands of feet in thick-
ness on the Continent. Moreover, between each successive formation,
we have, in the opinion of most geologists, blank periods of enormous
length. So that the lofty pile of sedimentary rocks in Britain gives
but an inadequate idea of the time which has elapsed during their
accumulation. The consideration of these various facts impresses
the mind almost in the same manner as does the vain endeavour to
grapple with the idea of eternity.

Nevertheless this impression is partly false. Mr. Croll, in an in-
teresting paper, remarks that we do not err "in forming too great a
conception of the length of geological periods," but in estimating
them by years. When geologists look at large and complicated

THE LAPSE OF TIME 325

phenomena, and then at the figures representing several million
years, the two produce a totally different effect on the mind, and the
figures are at once pronounced too small. In regard to subaerial
denudation, Mr. CroU shows, by calculating the known amount of
sediment annually brought down by certain rivers, relatively to
their areas of drainage, that 1,000 feet of solid rock, as it became
gradually disintegrated, would thus be removed from the mean
level of the whole area in the course of six million years.

This seems an astonishing result, and some considerations lead
to the suspicion that it may be too large, but even if halved or
quartered it is still very surprising. Few of us, however, know
what a million really means: Mr. Croll gives the following illustra-
tion: Take a narrow strip of pajx;r, eighty-three feet four inches in
length, and stretch it along the wall of a large hall; then mark off
at one end the tenth of an inch. This tenth of an inch will represent
one hundred years, and the entire strip a million years. But let it
be borne in mind, in relation to the subject of this work, what a
hundred years implies, represented as it is by a measure utterly in-
significant in a hall of the above dimensions. Several eminent
breeders, during a single lifetime, have so largely modified some of
the higher animals, which propagate their kind much more slowly
than most of the lower animals, that they have formed what well
deserves to be called a new sub-breed. Few men have attended
with due care to any one strain for more than half a century, so
that a hundred years represents the work of two breeders in succes-
sion. It is not to be supposed that species in a state of nature ever
change so quickly as domestic animals under the guidance of
methodical selection. The comparison would be in every way fairer
with the effects which follow from unconscious selection, that is
the preservation of the most useful or beautiful animals, with no
intention of modifying the breed; but by this process of unconscious
selection, various breeds have been sensibly changed in the course
of two or three centuries.

Species, however, probably change much more slowly, and within
the same country only a few change at the same time. This slow-
ness follows from all the inhabitants of the same country being
already so well adapted to each other, that new places in the polity

326 ORIGIN OF SPECIES

of nature do not occur until after long intervals, due to the occur-
rence of physical changes of some kind, or through the immigration
of new forms. Moreover variations or individual differences of the
right nature, by which some of the inhabitants might be better
fitted to their new places under the altered circumstances, would
not always occur at once. Unfortunately we have no means of
determining, according to the standard of years, how long a period
it takes to modify a species; but to the subject of time we must
return.

ON THE POORNESS OF PALjEONTOLOCICAL COLLECTIONS

Now let us turn to our richest geological museums, and what a
paltry display we behold! That our collections are imperfect, is
admitted by every one. The remark of that admirable palaeontologist,
Edward Forbes, should never be forgotten, namely, that very many
fossil species are known and named from single and often broken
specimens, or from a few specimens collected on some one spot.
Only a small portion of the surface of the earth has been geologically
explored, and no part with sufficient care, as the important dis-
coveries made every year in Europe prove. No organism wholly
soft can be preserved. Shells and bones decay and disappear when
left on the bottom of the sea, where sediment is not accumulating.
We probably take a quite erroneous view, when we assume that
sediment is being deposited over nearly the whole bed of the sea, at
a rate sufficiently quick to embed and preserve fossil remains.
Throughout an enormously large proportion of the ocean, the bright
blue tint of the water bespeaks its purity. The many cases on record
of a formation conformably covered, after an immense interval of
time, by another and later formation, without the underlying bed
having suffered in the interval any wear and tear, seem explicable
only on the view of the bottom of the sea not rarely lying for ages
in an unaltered condition. The remains which do become embedded,
if in sand or gravel, will, when the beds are upraised, generally be
dissolved by the percolation of rain-water charged with carbonic
acid. Some of the many kinds of animals which live on the beach
between high and low water mark seem to be rarely preserved. For
instance, the several species of the Chthamalinx (a sub-family of

PAL.EONTOLOGICAL COLLECTIONS 327

sessile cirripedes) coat the rocks all over the world in infinite num-
bers; they are all strictly littoral, with the exception of a single
Mediterranean species, which inhabits deep water, and this has been
found fossil in Sicily, whereas not one other species has hitherto
been found in any tertiary formation; yet it is known that the genus
Chthamalus existed during the Chalk period. Lastly, many great
deposits requiring a vast length of time for their accumulation, are
entirely destitute of organic remains, without our being able to as-
sign any reason: one of the most striking instances is that of the
Flysch formation, which consists of shale and sandstone, several
thousand, occasionally even six thousand feet in thickness, and ex-
tending for at least 300 miles from Vienna to Switzerland; and
although this great mass has been most carefully searched, no fos-
sils, except a few vegetable remains, have been found.

With respect to the terrestrial productions which lived during the
Secondary and Palarozoic periods, it is superfluous to state that our
evidence is fragmentary in an extreme degree. For instance, until
recently not a land-shell was known belonging to either of these
vast periods, with the exception of one species discovered by Sir C.
Lyell and Dr. Dawson in the carboniferous strata of North America;
but now land-shells have been found in the lias. In regard to mam-
miferous remains, a glance at the historical table published in Lyell's
Manual will bring home the truth, how accidental and rare is their
preservation, far better than pages of detail. Nor is their rarity
surprising, when we remember how large a proportion of the bones
of tertiary mammals have been discovered either in caves or in
lacustrine deposits; and that not a cave or true lacustrine bed is
known belonging to the age of our secondary or palaeozoic forma-
tions.

But the imperfection in the geological record largely results from
another and more important cause than any of the foregoing;
namely, from the several formations being separated from each
other by wide intervals of time. This doctrine has been emphatically
admitted by many geologists and palarontologists, who, like E.
Forbes, entirely disbelieve in the change of species. When we see
the formations tabulated in written works, or when we follow them
in nature, it is difficult to avoid believing that they are closely con-

328 ORIGIN OF SPECIES

secutive. But we know, for instance, from Sir R. Murchison's great
work on Russia, what wide gaps there are in that country between
the superimposed formations; so it is in North America, and in
many other parts of the world. The most skilful geologist, if his
attention had been confined exclusively to these large territories,
would never have suspected that, during the periods which were
blank and barren in his own country, great piles of sediment,
charged with new and peculiar forms of life, had elsewhere been
accumulated. And if, in each separate territory, hardly any idea
can be formed of the length of time which has elapsed between the
consecutive formations, we may infer that this could nowhere be
ascertained. The frequent and great changes in the mineralogical
composition of consecutive formations, generally implying great
changes in the geography of the surrounding lands, whence the
sediment was derived, accord with the belief of vast intervals of
time having elapsed between each formation.

We can, I think, see why the geological formations of each region
are almost invariably intermittent; that is, have not followed each
other in close sequence. Scarcely any fact struck me more when
examining many hundred miles of the South American coasts,
which have been upraised several hundred feet within the recent
fjeriod, than the absence of any recent deposits sufficiently extensive
to last for even a short geological period. Along the whole west
coast, which is inhabited by a {jeculiar marine fauna, tertiary beds
are so poorly developed that no record of several successive and
peculiar marine faunas will probably be preserved to a distant age.
A little reflection will explain why, along the rising coast of the
western side of South America, no extensive formations with recent
or tertiary remains can anywhere be found, though the supply of
sediment must for ages have been great, from the enormous degrada-
tion of the coast-rocks and from muddy streams entering the sea.
The explanation, no doubt, is, that the littoral and sub-littoral de-
posits are continually worn away, as soon as they are brought up
by the slow and gradual rising of the land within the grinding
action of the coast-waves.

We may, I think, conclude that sediment must be accumulated in
extremely thick, solid, or extensive masses, in order to withstand

PAL/EONTOLOGICAL COLLECTIONS 329

the incessant action of the waves, when first upraised and during
successive oscillations of level, as well as the subsequent subaerial
degradation. Such thick and extensive accumulations of sediment
may be formed in two ways; either in profound depths of the sea,
in which case the bottom will not be inhabited by so many and
such varied forms of life, as the more shallow seas; and the mass
when upraised will give an imperfect record of the organisms which
existed in the neighbourhood during the period of its accumulation.
Or, sediment may be deposited to any thickness and extent over a
shallow bottom, if it continue slowly to subside. In this latter case,
as long as the rate of subsidence and the supply of sediment nearly
balance each other, the sea will remain shallow and favourable for
many and varied forms, and thus a rich fossiliferous formation,
thick enough, when upraised, to resist a large amount of denudation,
may be formed.

I am convinced that nearly all our ancient formations, which are
throughout the greater part of their thickness rich in fossils, have
thus been formed during subsidence. Since publishing my views on
this subject in 1845, I have watched the progress of Geology, and
have been surprised to note how author after author, in treating of
this or that great formation, has come to the conclusion that it was
accumulated during subsidence. I may add, that the only ancient
tertiary formation on the west coast of South America, which has
been bulky enough to resist such degradation as it has as yet suf-
fered, but which will hardly last to a distant geological age, was
deposited during a downward oscillation of level, and thus gained
considerable thickness.

All geological facts tell us plainly that each area has undergone
numerous slow oscillations of level, and apparently these oscillations
have affected wide spaces. Consequently, formations rich in fossils
and sufficiently thick and extensive to resist subsequent degradation,
will have been formed over wide spaces during periods of subsidence,
but only where the supply of sediment was sufficient to keep the
sea shallow and to embed and preserve the remains before they had
time to decay. On the other hand, as long as the bed of the sea
remains stationary, thicl{^ deposits cannot have been accumulated in
the shallow parts, which are the most favourable to life. Still less

330 ORIGIN OF SPECIES

can this have happened during the alternate periods of elevation;
or, to speak more accurately, the beds which were then accumulated
will generally have been destroyed by being upraised and brought
within the limits of the coast-action.

These remarks apply chiefly to littoral and sub-littoral deposits.
In the case of an extensive and shallow sea, such as that within a
large part of the Malay Archipelago, where the depth varies from
thirty or forty to sixty fathoms, a widely extended formation might
be formed during a period of elevation, and yet not suffer excessively
from denudation during its slow upheaval; but the thickness of the
formation could not be great, for owing to the elevatory movement
it would be less than the depth in which it was formed; nor would
the deposit be much consolidated, nor be capped by overlying forma-
tions, so that it would run a good chance of being worn away by
atmospheric degradation and by the action of the sea during sub-
sequent oscillations of level. It has, however, been suggested
by Mr. Hopkins, that if one part of the area, after rising and be-
fore being denuded, subsided, the deposit formed during the
rising movement, though not thick, might afterwards become pro-
tected by fresh accumulations, and thus be preserved for a long
period.

Mr. Hopkins also expresses his belief that sedimentary beds of
considerable horizontal extent have rarely been completely destroyed.
But all geologists, excepting the few who believe that our present
metamorphic schists and plutonic rocks once formed the primordial
nucleus of the globe, will admit that these latter rocks have been
stript of their covering to an enormous extent. For it is scarcely
possible that such rocks could have been solidified and crystallized
whilst uncovered; but if the metamorphic action occurred at pro-
found depths of the ocean, the former protecting mantle of rock
may not have been very thick. Admitting then that gneiss, mica-
schist, granite, diorite, etc., were once necessarily covered up, how
can we account for the naked and extensive areas of such rocks in
many parts of the world, except on the belief that they have sub-
sequently been completely denuded of all overlying strata? That
such extensive areas do exist cannot be doubted; the granitic region
of Parime is described by Humboldt as being at least nineteen times

PAL^ONTOLOGICAL COLLECTIONS 33 1

as large as Switzerland. South of the Amazon, Boue colours an
area composed of rocks of this nature as equal to that of Spain,
France, Italy, part of Germany, and the British Islands, all con-
joined. This region has not been carefully explored, but from the
concurrent testimony of travellers, the granitic area is very large;
thus. Von Eschwege gives a detailed section of these rocks, stretch-
ing from Rio de Janeiro for 260 geographical miles inland in a
straight line; and I travelled for 150 miles in another direction, and
saw nothing but granitic rocks. Numerous specimens, collected
along the whole coast from near Rio Janeiro to the mouth of the
Plata, a distance of 1,100 geographical miles, were examined by me,
and they all belonged to this class. Inland, along the whole northern
bank of the Plata, I saw, besides modern tertiary beds, only one
small patch of slightly metamorphosed rock, which alone could
have formed a part of the original capping of the granitic series.
Turning to a well-known region, namely, to the United States and
Canada, as shown in Professor H. D. Rogers's beautiful map, I have
estimated the areas by cutting out and weighing the paper, and I
find that the metamorphic (excluding "the semi-metamorphic")
and granitic rocks exceed, in the proportion of 19 to 12.5, the whole
of the newer Paleozoic formations. In many regions the meta-
morphic and granitic rocks would be found much more widely
extended than they appear to be, if all the sedimentary beds were
removed which rest unconformably on them, and which could not
have formed part of the original mantle under which they were
crystallized. Hence it is probable that in some parts of the world
whole formations have been completely denuded, with not a wreck
left behind.

One remark is here worth a passing notice. During p>eriods of
elevation the area of the land and of the adjoining shoal parts of
the sea will be increased, and new stations will often be formed: —
all circumstances favourable, as previously explained, for the forma-
tion of new varieties and species; but during such periods there
will generally be a blank in the geological record. On the other
hand, during subsidence, the inhabited area and number of in-
habitants will decrease (excepting on the shores of a continent when
first broken up into an archipelago), and consequently, during sub-

332 ORIGIN OF SPECIES

sidence, though there will be much extinction, few new varieties or
species will be formed; and it is during these very periods of sub-
sidence that the deposits which are richest in fossils have been
accumulated.

ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIETIES
IN ANY SINGLE FORMATION

From these several considerations, it cannot be doubted that the
geological record, viewed as a whole, is extremely imperfect; but if
we confine our attention to any one formation, it becomes much
more difficult to understand why we do not therein find closely
graduated varieties between the allied species which lived at its
commencement and at its close. Several cases are on record of the
same species presenting varieties in the upper and lower parts of
the same formation; thus, Trautschold gives a number of instances
with Ammonites; and Hilgendorf has described a most curious case
of ten graduated forms of Planorbis multiformis in the successive
beds of a fresh-water formation in Switzerland. Although each
formation has indisputably required a vast number of years for its
deposition, several reasons can be given why each should not com-
monly include a graduated series of links between the sf)ecies which
lived at its commencement and close; but I cannot assign due pro-
portional weight to the following considerations.

Although each formation may mark a very long lapse of years,
each probably is short compared with the period requisite to change
one species into another. I am aware that two paleontologists, whose
opinions are worthy of much deference, namely Bronn and Wood-
ward, have concluded that the average duration of each formation is
twice or thrice as long as the average duration of specific forms.
But insuperable difficulties, as it seems to me, prevent us from com-
ing to any just conclusion on this head. When we see a species first
apf>earing in the middle of any formation, it would be rash in the
extreme to infer that it had not elsewhere previously existed. So
again when we find a species disappearing before the last layers have
been deposited, it would be equally rash to suppose that it then
became extinct. We forget how small the area of Europe is com-
pared with the rest of the world; nor have the several stages of the

ABSENCE OF INTERMEDIATE VARIETIES 333

same formation throughout Europe been correlated with perfect
accuracy.

We may safely infer that with marine animals of all kinds there
has been a large amount of migration due to climatal and other
changes; and when we see a species first appearing in any formation,
the probability is that it only then first immigrated into that area.
It is well known, for instance, that several species appear somewhat
earlier in the palaeozoic beds of North America than in those of
Eurof)e; time having apparently been required for their migration
from the American to the European seas. In examining the latest
deposits in various quarters of the world, it has everywhere been
noted, that some few still existing species are common in the deposit,
but have become extinct in the immediately surrounding sea; or,
conversely, that some are now abundant in the neighbouring sea, but
are rare or absent in this particular deposit. It is an excellent lesson
to reflect on the ascertained amount of migration of the inhabitants
of Europe during the glacial epoch, which forms only a part of one
whole geological period; and likewise to reflect on the changes of
level, on the extreme change of climate, and on the great lapse of
time, all included within the same glacial period. Yet it may be
doubted whether, in any quarter of the world, sedimentary deposits,
including fossil remains, have gone on accumulating within the
same area during the whole of this period. It is not, for instance,
probable that sediment was deposited during the whole of the
glacial fjeriod near the mouth of the Mississippi, within that limit of
depth at which marine animals can best flourish: for we know that
great geographical changes occurred in other parts of America during
this space of time. When such beds as were deposited in shallow
water near the mouth of the Mississippi during some part of the
glacial period shall have been upraised, organic remains will prob-
ably first appear and disapf)ear at different levels, owing to the
migrations of species and to geographical changes. And in the dis-
tant future, a geologist, examining those beds, would be tempted to
conclude that the average duration of life of the embedded fossils
had been less than that of the glacial period, instead of having been
really far greater, that is, extending from before the glacial epoch to
the present day.

334 ORIGIN OF SPECIES

In order to get a perfect gradation between two forms in the upper
and lower parts of the same formation, the deposit must have gone
on continuously accumulating during a long period, sufficient for
the slow process of modification; hence the deposit must be a very
thick one; and the species undergoing change must have lived in the
same district throughout the whole time. But we have seen that a
thick formation, fossiliferous throughout its entire thickness, can
accumulate only during a jjeriod of subsidence; and to keep the
depth approximately the same, which is necessary that the same
marine species may live on the same space, the supply of sediment
must nearly counterbalance the amount of subsidence. But this
same movement of subsidence will tend to submerge the area whence
the sediment is derived, and thus diminish the supply, whilst the
downward movement continues. In fact, this nearly exact balancing
between the supply of sediment and the amount of subsidence is
probably a rare contingency; for it has been observed by more than
one paleontologist, that very thick deposits are usually barren of
organic remains, except near their upper or lower limits.

It would seem that each separate formation, like the whole pile
of formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation
composed of beds of widely different mineralogical composition, we
may reasonably suspect that the process of deposition has been more
or less interrupted. Nor will the closest inspection of a formation
give us any idea of the length of time which its deposition may have
consumed. Many instances could be given of beds only a few feet
in thickness, representing formations, which are elsewhere thousands
of feet in thickness, and which must have required an enormous
period for their accumulation; yet no one ignorant of this fact would
have even susfjected the vast lapse of time represented by the thinner
formation. Many cases could be given of the lower beds of a forma-
tion having been upraised, denuded, submerged, and then re-covered
by the upper beds of the same formation, — facts, showing what wide,
yet easily overlooked, intervals have occurred in its accumulation.
In other cases we have the plainest evidence in great fossilised trees,
still standing upright as they grew, of many long intervals of time
and changes of level during the process of deposition, which would

ABSENCE OF INTERMEDIATE VARIETIES 335

not have been suspected, had not the trees been preserved: thus Sir
C. Lyell and Dr. Dawson found carboniferous beds 1,400 feet thick
in Nova Scotia, with ancient root-bearing strata, one above the other
at no less than sixty-eight different levels. Hence, when the same
species occurs at the bottom, middle, and top of a formation, the
probability is that it has not lived on the same spot during the whole
period of deposition, but has disappeared and reappeared, perhaps
many times, during the same geological period. Consequently if it
were to undergo a considerable amount of modification during the
deposition of any one geological formation, a section would not in-
clude all the fine intermediate gradations which must, on our theory,
have existed, but abrupt, though perhaps slight, changes of form.

It is all-important to remember that naturaUsts have no golden rule
by which to distinguish species and varieties; they grant some litde
variability to each species, but when they meet with a somewhat
greater amount of difference between any two forms, they rank both
as species, unless they are enabled to connect them together by the
closest intermediate gradations; and this, from the reasons just as-
signed, we can seldom hope to effect in any one geological section.
Supposing B and C to be two species, and a third, A, to be found in
an older and underlying bed; even if A were strictly intermediate
between B and C, it would simply be ranked as a third and distinct
species, unless at the same time it could be closely connected by inter-
mediate varieties with either one or both forms. Nor should it be
forgotten, as before explained, that A might be the actual progenitor
of B and C, and yet would not necessarily be strictly intermediate be-
tween them in all respects. So that we might obtain the parent-sp)ecies
and its several modified descendants from the lower and upper beds
of the same formation, and unless we obtained numerous transitional
gradations, we should not recognise their blood-relationship, and
should consequently rank them as distinct species.

It is notorious on what excessively slight differences many
palaeontologists have founded their species; and they do this the
more readily if the specimens come from different substages of the
same formation. Some experienced conchologists are now sinking
many of the very fine species of D'Orbigny and others into the rank
of varieties; and on this view we do find the kind of evidence of

336 ORIGIN OF SPECIES

change which on the theory we ought to find. Look again at the
later tertiary deposits, which include many shells believed by the
majority of naturalists to be identical with existing species; but some
excellent naturahsts, as Agassiz and Pictet, maintain that all these
tertiary species are specifically distinct, though the distinction is ad-
mitted to be very slight; so that here, unless we believe that these
eminent naturalists have been misled by their imaginations, and
that these late tertiary species really present no difference whatever
from their living representatives, or unless we admit, in opposition
to the judgment of most naturalists, that these tertiary species are all
truly distinct from the recent, we have evidence of the frequent oc-
currence of slight modifications of the kind required. If we look to
rather wider intervals of time, namely, to distinct but consecutive
stages of the same great formation, we find that the embedded fossils,
though universally ranked as specifically different, yet are far more
closely related to each other than are the species found in more widely
separated formations; so that here again we have undoubted evidence
of change in the direction required by the theory; but to this latter
subject I shall return in the following chapter.

With animals and plants that propagate rapidly and do not wan-
der much, there is reason to suspect, as we have formerly seen, that
their varieties are generally at first local; and that such local varieties
do not spread widely and supplant their parent-forms until they
have been modified and perfected in some considerable degree. Ac-
cording to this view, the chance of discovering in a formation in any
one country all the early stages of transition between any two forms,
is small, for the successive changes are supposed to have been local
or confined to some one spot. Most marine animals have a wide
range; and we have seen that with plants it is those which have the
widest range, that oftenest present varieties; so that, with shells and
other marine animals, it is probable that those which had the widest
range, far exceeding the limits of the known geological formations in
Europe, have oftenest given rise, first to local varieties and ultimately
to new species; and this again would greatly lessen the chance of our
being able to trace the stages of transition in any one geological
formation.

It is a more important consideration, leading to the same result, as

ABSENCE OF INTERMEDIATE VARIETIES 337

lately insisted on by Dr. Falconer, namely, that the period during
which each species underwent modification, though long as measured
by years, was probably short in comparison with that during which
it remained without undergoing any change.

It should not be forgotten, that at the present day, with perfect
specimens for examination, two forms can seldom be connected by
intermediate varieties, and thus proved to be the same species, until
many specimens are collected from many places; and with fossil spe-
cies this can rarely be done. We shall, perhaps, best perceive the im-
probability of our being enabled to connect species by numerous,
fine, intermediate, fossil links, by asking ourselves whether, for in-
stance, geologists at some future period will be able to prove that our
different breeds of cattle, sheep, horses, and dogs are descended from
a single stock or from several aboriginal stocks; or, again, whether
certain sea-shells inhabiting the shores of North America, which are
ranked by some conchologists as distinct species from their European
representatives, and by other conchologists as only varieties, are really
varieties, or are, as it is called, specifically distinct. This could be
effected by the future geologist only by his discovering in a fossil
state numerous intermediate gradations; and such success is improb-
able in the highest degree.

It has been asserted over and over again, by writers who believe
in the immutability of species, that geology yields no linking forms.
This assertion, as we shall see in the next chapter, is certainly errone-
ous. As Sir J. Lubbock has remarked, "Every species is a link between
other allied forms." If we take a genus having a score of species,
recent and extinct, and destroy four-fifths of them, no one doubts that
the remainder will stand much more distinct from each other. If the
extreme forms in the genus happen to have been thus destroyed, the
genus itself will stand more distinct from other allied genera. What
geological research has not revealed, is the former existence of in-
finitely numerous gradations, as fine as existing varieties, connecting
together nearly all existing and extinct sjjecies. But this ought not
to be expected; yet this has been repeatedly advanced as a most seri-
ous objection against my views.

It may be worth while to sum up the foregoing remarks on the
causes of the imperfection of the geological record under an imagi-

338 ORIGIN OF SPECIES

nary illustration. The Malay Archipelago is about the size of Europe
from the North Cape to the Mediterranean, and from Britain to Rus-
sia; and therefore equals all the geological formations which have
been examined with any accuracy, excepting those of the United
States of America. I fully agree with Mr. Godwin-Austen, that the
present condition of the Malay Archipelago, with its numerous large
islands separated by wide and shallow seas, probably represents the
former state of Europe, whilst most of our formations were accumu-
lating. The Malay Archipelago is one of the richest regions in organic
beings; yet if all the species were to be collected which have ever
lived there, how imperfectly would they represent the natural
history of the world!

But we have every reason to believe that the terrestrial productions
of the archip)elago would be preserved in an extremely imperfect
manner in the formations which we suppose to be there accumulat-
ing. Not many of the strictly littoral animals, or of those which lived
on naked submarine rocks, would be embedded; and those embedded
in gravel or sand would not endure to a distant epoch. Wherever
sediment did not accumulate on the bed of the sea, or where it did
not accumulate at a sufficient rate to protect organic bodies from
decay, no remains could be preserved.

Formations rich in fossils of many kinds, and of thickness sufficient
to last to an age as distant in futurity as the secondary formations Ue
in the past, would generally be formed in the archipelago only dur-
ing periods of subsidence. These periods of subsidence would be
separated from each other by immense intervals of time, during
which the area would be either stationary or rising; whilst rising, the
fossiliferous formations on the steeper shores would be destroyed,
almost as soon as accumulated, by the incessant coast-action, as we
now see on the shores of South America. Even throughout the ex-
tensive and shallow seas within the archipelago, sedimentary beds
could hardly be accumulated of great thickness during the periods of
elevation, or become capped and protected by subsequent deposits, so
as to have a good chance of enduring to a very distant future. During
the periods of subsidence, there would probably be much extinction
of life; during the periods of elevation, there would be much varia-
tion, but the geological record would then be less perfect.

ABSENCE OF INTERMEDIATE VARIETIES 339

It may be doubted whether the duration of any one great period
of subsidence over the whole or part of the archipelago, together with
a contemporaneous accumulation of sediment, would exceed the
average duration of the same specific forms; and these contingencies
are indispensable for the preservation of all the transitional gradations
between any two or more sf)ecies. If such gradations were not all
fully preserved, transitional varieties would merely appear as so many
new, though closely allied species. It is also probable that each great
period of subsidence would be interrupted by oscillations of level, and
that slight climatal changes would intervene during such lengthy
periods; and in these cases the inhabitants of the archipelago would
migrate, and no closely consecutive record of their modifications
could be preserved in any one formation.

Very many of the marine inhabitants of the archipelago now range
thousands of miles beyond its confines; and analogy plainly leads to
the belief that it would be chiefly these far-ranging species, though
only some of them, which would oftenest produce new varieties;
and the varieties would at first be local or confined to one place, but
if possessed of any decided advantage, or when further modified and
improved, they would slowly spread and supplant their parent-forms.
When such varieties returned to their ancient homes, as they would
differ from their former state in a nearly uniform, though perhaps
extremely slight degree, and as they would be found embedded in
slightly different sub-stages of the same formation, they would, ac-
cording to the principles followed by many palaeontologists, be ranked
as new and distinct species.

If then there be some degree of truth in these remarks, we have no
right to expect to find, in our geological formations, an infinite num-
ber of those fine transitional forms which, on our theory, have con-
nected all the past and present species of the same group into one
long and branching chain of life. We ought only to look for a few
links, and such assuredly we do find — some more distandy, some
more closely, related to each other; and these links, let them be ever
so close, if found in different stages of the same formation, would, by
many paleontologists, be ranked as distinct species. But I do not
pretend that I should ever have suspected how f>oor was the record
in the best preserved geological sections, had not the absence of

340 ORIGIN OF SPECIES

innumerable transitional links between the species which lived at
the commencement and close of each formation, pressed so hardly
on my theory.

ON THE SUDDEN APPEARANCE OF WHOLE CROUPS OF
ALLIED SPECIES

The abrupt manner in which whole groups of sp)ecies suddenly
appear in certain formations, has been urged by several palxontolo-
gists — for instance, by Agassiz, Pictet, and Sedgwick — as a fatal ob-
jection to the belief in the transmutation of species. If numerous
species, belonging to the same genera or families, have really started
into life at once, the fact would be fatal to the theory of evolution
through natural selection. For the development by this means of a
group of forms, all of which are descended from some one progenitor,
must have been an extremely slow process; and the progenitors must
have lived long before their modified descendants. But we continu-
ally overrate the perfection of the geological record, and falsely infer,
because certain genera or families have not been found beneath a
certain stage, that they did not exist before that stage. In all cases
positive palacontological evidence may be implicitly trusted; nega-
tive evidence is worthless, as experience has so often shown. We con-
tinually forget how large the world is, compared with the area over
which our geological formations have been carefully examined; we
forget that groups of species may elsewhere have long existed, and
have slowly multiplied, before they invaded the ancient archipela-
goes of Europe and the United States. We do not make due allow-
ance for the intervals of time which have elapsed between our con-
secutive formations, — longer perhaps in many cases than the time
required for the accumulation of each formation. These intervals will
have given time for the multiplication of species from some one par-
ent-form: and in the succeeding formation, such groups or species
will appear as if suddenly created.

I may here recall a remark formerly made, namely, that it might re-
quire a long succession of ages to adapt an organism to some new
and peculiar line of life, for instance, to fly through the air; and con-
sequently that the transitional forms would often long remain con-
fined to some one region; but that, when this adaptation had once

APPEARANCE OF WHOLE GROUPS 34 1

been effected, and a few species had thus acquired a great advantage
over other organisms, a comparatively short time would be necessary
to produce many divergent forms, which would spread rapidly and
widely, throughout the world. Professor Pictet, in his excellent re-
view of this work, in commenting on early transitional forms, and
taking birds as an illustration, cannot see how the successive modifi-
cations of the anterior limbs of a supposed prototype could possibly
have been of any advantage. But look at the penguins of the South-
ern Ocean; have not these birds their front limbs in this precise in-
termediate state of "neither true arms nor true wings".? Yet these
birds hold their place victoriously in the battle for life; for they exist
in infinite numbers and of many kinds. I do not suppose that we
here see the real transitional grades through which the wings of
birds have passed; but what special difficulty is there in believing that
it might profit the modified descendants of the penguin, first to be-
come enabled to flap along the surface of the sea like the logger-
headed duck, and ultimately to rise from its surface and glide through
the air.'

I will now give a few examples to illustrate the foregoing remarks,
and to show how liable we are to error in supposing that whole
groups of species have suddenly been produced. Even in so short an
interval as that between the first and second editions of Pictet's great
work on Pala;ontology, published in 1844-46 and 1853-57, '^^ <^°"'
elusions on the first appearance and disappearance of several groups
of animals have been considerably modified; and a third edition
would require still further changes. I may recall the well-known fact
that in geological treatises, published not many years ago, mammals
were always spoken of as having abruptly come in at the commence-
ment of the tertiary series. And now one of the richest known accu-
mulations of fossil mammals belongs to the middle of the secondary
series; and true mammals have been discovered in the new red sand-
stone at nearly the commencement of this great series. Cuvier used
to urge that no monkey occurred in any tertiary stratum; but now
extinct sf)ecies have been discovered in India, South America, and in
Europe, as far back as the miocene stage. Had it not been for the
rare accident of the preservation of footsteps in the new red sandstone
of the United States, who would have ventured to suppose that no less

342 ORIGIN OF SPECIES

than at least thirty different bird-like animals, some of gigantic size,
existed during that period? Not a fragment of bone has been dis-
covered in these beds. Not long ago, palxontologists maintained that
the whole class of birds came suddenly into existence during the
eocene period; but now we know, on the authority of Professor
Owen, that a bird certainly lived during the deposition of the upper
greensand; and still more recently, that strange bird, the Arche-
opteryx, with a long lizard-like tail, bearing a pair of feathers on
each joint, and with its wings furnished with two free claws, has
been discovered in the oolitic slates of Solenhofen. Hardly any recent
discovery shows more forcibly than this, how little we as yet know
of the former inhabitants of the world.

I may give another instance, which, from having passed under
my own eyes, has much struck me. In a memoir on Fossil Sessile
Cirripedes, I stated that, from the large number of existing and ex-
tinct tertiary species; from the extraordinary abundance of the indi-
viduals of many species all over the world, from the arctic regions to
the equator, inhabiting various zones of depths from the upper tidal
limits to fifty fathoms; from the perfect manner in which specimens
are preserved in the oldest tertiary beds; from the ease with which
even a fragment of a valve can be recognized; from all these cir-
cumstances, I inferred that, had sessile cirripedes existed during the
secondary periods, they would certainly have been preserved and
discovered; and as not one species had then been discovered in beds
of this age, I concluded that this great group had been suddenly de-
veloped at the commencement of the tertiary series. This was a sore
trouble to me, adding as I then thought one more instance of the
abrupt appearance of a great group of species. But my work had
hardly been published, when a skilful palarontologist, M. Bosquet,
sent me a drawing of a perfect specimen of an unmistakeable sessile
cirripede, which he had himself extracted from the chalk of Belgium.
And, as if to make the case as striking as possible, this cirripede was
a Chthamalus, a very common, large, and ubiquitous genus, of which
not one species has as yet been found even in any tertiary stratum.
Still more recently, a Pyrgoma, a member of a distinct sub-family of
sessile cirripedes, has been discovered by Mr. Woodward in the upp)er
chalk; so that we now have abundant evidence of the existence of this
group of animals during the secondary period.

APPEARANCE OF WHOLE GROUPS 343

The case most frequently insisted on by palaeontologists of the ap-
parently sudden appearance of a whole group of sp)ecies, is that of
the teleostean fishes, low down, according to Agassiz, in the Chalk
period- This group includes the large majority of existing species.
But certain Jurassic and Triassic forms are now commonly admitted
to be teleostean; and even some palaeozoic forms have thus been
classed by one high authority. If the teleosteans had really appeared
suddenly in the northern hemisphere at the commencement of the
chalk formation, the fact would have been highly remarkable; but
it would not have formed an insup>erable difficulty, unless it could
Ukewise have been shown that at the same period the species were
suddenly and simultaneously develofjed in other quarters of the
world. It is almost superfluous to remark that hardly any fossil-fish
are known from south of the equator; and by running through
Pictet's Paleontology it will be seen that very few sjjecies are known
from several formations in Europe. Some few families of fish now
have a confined range; the teleostean fishes might formerly
have had a similarly confined range, and after having been largely
developed in some one sea, have spread widely. Nor have we any
right to suppose that the seas of the world have always been so freely
open from south to north as they are at present. Even at this day,
if the Malay Archipelago were converted into land, the tropical parts
of the Indian Ocean would form a large and {perfectly enclosed
basin, in which any great group of marine animals might be midti-
plied; and here they would remain confined, until some of the species
became adapted to a cooler climate, and were enabled to double the
Southern capes of Africa or Australia, and thus reach other and
distant seas.

From these considerations, from our ignorance of the geology of
other countries beyond the confines of Europe and the United States,
and from the revolution in our palxontological knowledge effected
by the discoveries of the last dozen years, it seems to me to be about as
rash to dogmatize on the succession of organic forms throughout the
world, as it would be for a naturalist to land for five minutes on a
barren point in Australia, and then to discuss the number and range
of its productions.

344 ORIGIN OF SPECIES

ON THE SUDDEN APPEARANCE OF CROUPS OF ALLIED SPECIES IN THE
LOWEST KNOWN FOSSILIFEROUS STRATA

There is another and allied difficulty, which is much more serious.
I allude to the manner in which species belonging to several of the
main divisions of the animal kingdom suddenly appear in the lowest
known fossiliferous rocks. Most of the arguments which have con-
vinced me that all the existing species of the same group are de-
scended from a single progenitor, apply with equal force to the earli-
est known sf)ecies. For instance, it cannot be doubted that all the
Cambrian and Silurian trilobites are descended from some one
crustacean, which must have lived long before the Cambrian age,
and which probably differed greatly from any known animal. Some
of the most ancient animals, as the Nautilus, Lingula, etc., do not
differ much from living sf)ecies; and it cannot on our theory be suf>-
posed, that these old species were the progenitors of all the species
belonging to the same groups which have subsequently appeared, for
they are not in any degree intermediate in character.

Consequently, if the theory be true, it is indisputable that before
the lowest Cambrian stratum was deposited long periods elapsed,
as long as, or probably far longer than, the whole interval from the
Cambrian age to the present day; and that during these vast periods
the world swarmed with living creatures. Here we encounter a for-
midable objection; for it seems doubtful whether the earth, in a fit
state for the habitation of living creatures, has lasted long enough.
Sir W. Thompson concludes that the consolidation of the crust can
hardly have occurred less than twenty or more than 400 million
years ago, but probably not less than ninety-eight or more than 200
million years. These very wide limits show how doubtful the data
are; and other elements may have hereafter to be introduced into
the problem. Mr. Croll estimates that about sixty million years have
elapsed since the Cambrian period, but this, judging from the small
amount of organic change since the commencement of the Glacial
epoch, appears a very short time for the many and great mutations of
life, which have certainly occurred since the Cambrian formation;
and the previous 140 million years can hardly be considered as suffi-
cient for the development of the varied forms of life which already

SUDDEN APPEARANCE OF GROUPS 345

existed during the Cambrian period. It is, however, probable, as Sir
WiHiam Thompson insists, that the world at a very early period
was subjected to more rapid and violent changes in its physical
conditions than those now occurring; and such changes would have
tended to induce changes at a corresponding rate in the organisms
which then existed.

To the question why we do not find rich fossiliferous deposits
belonging to these assumed earliest periods prior to the Cambrian
system, I can give no satisfactory answer. Several eminent geolo-
gists, with Sir R. Murchison at their head, were until recently con-
vinced that we beheld in the organic remains of the lowest Silurian
stratum the first dawn of life. Other highly competent judges, as
Lyell and E. Forbes, have disputed this conclusion. We should not
forget that only a small portion of the world is known with accuracy.
Not very long ago M. Barrande added another and lower stage,
abounding with new and peculiar sjjecies, beneath the then known
Silurian system; and now, still lower down in the Lower Cambrian
formation, Mr. Hicks has found in South Wales beds rich in tri-
lobites, and containing various molluscs and annelids. The presence
of phosphatic nodules and bituminous matter, even in some of the
lowest azoic rocks, probably indicates life at these periods; and the
existence of the Eozoon in the Laurentian formation of Canada is
generally admitted. There are three great series of strata beneath the
Silurian system in Canada, in the lowest of which the Eozoon is
found. Sir W. Logan states that their "united thickness may possi-
bly far surpass that of all the succeeding rocks, from the base of the
palaeozoic series to the present time. We are thus carried back to a
period so remote that the appearance of the so-called primordial
fauna (of Barrande) may by some be considered as a comparatively
modern event." The Eozoon belongs to the most lowly organised of
all classes of animals, but is highly organised for its class; it existed in
countless numbers, and, as Dr. Dawson has remarked, certainly
preyed on other minute organic beings, which must have lived in
great numbers. Thus the words, which I wrote in 1859, about the ex-
istence of living beings long before the Cambrian period, and which
are almost the same with those since used by Sir W. Logan, have
proved true. Nevertheless, the difficulty of assigning any good reason

346 ORIGIN OF SPECIES

for the absence of vast piles of strata rich in fossils beneath the Cam-
brian system is very great. It does not seem probable that the most
ancient beds have been quite worn away by denudation, or that their
fossils have been wholly obliterated by metamorphic action, for if
this had been the case we should have found only small remnants of
the formations next succeeding them in age, and these would always
have existed in a partially metamorphosed condition. But the descrip-
tions which we possess of the Silurian deposits over immense ter-
ritories in Russia and in North America, do not support the view, that
the older a formation is, the more invariably it has suffered extreme
denudation and metamorphism.

The case at present must remain inexplicable; and may be truly
urged as a valid argument against the views here entertained. To
show that it may hereafter receive some explanation, I will give the
following hypothesis. From the nature of the organic remains which
do not appear to have inhabited profound depths, in the several for-
mations of Europe and of the United States; and from the amount of
sediment, miles in thickness, of which the formations are composed,
we may infer that from first to last large islands or tracts of land,
whence the sediment was derived, occurred in the neighbourhood of
the now existing continents of Europe and North America. The
same view has since been maintained by Agassiz and others. But we
do not know what was the state of things in the intervals between the
several successive formations; whether Europe and the United States
during these intervals existed as dry land, or as a submarine surface
near land, on which sediment was not deposited, or as the bed on an
open and unfathomable sea.

Looking to the existing oceans, which are thrice as extensive as the
land, we see them studded with many islands; but hardly one truly
oceanic island (with the exception of New Zealand, if this can be
called a truly oceanic island) is as yet known to afford even a rem-
nant of any palaeozoic and secondary formation. Hence, we may per-
haps infer that during the palxozoic and secondary periods, neither
continents nor continental islands existed where our oceans now
extend; for had they existed, pala;ozoic and secondary formations
would in all probability have been accumulated from sediment de-
rived from their wear and tear; and these would have been at least

SUDDEN APPEARANCE OF GROUPS 347

partially upheaved by the oscillations of level, which must have inter-
vened during these enormously long periods. If then we may infer
anything from these facts, we may infer that, where our oceans now
extend, oceans have extended from the remotest period of which we
have any record; and on the other hand, that where continents now
exist, large tracts of land have existed, subjected no doubt to great
oscillations of level, since the Cambrian period. The colored map ap-
pended to my volume on Coral Reefs, led me to conclude that the
great oceans are still mainly areas of subsidence, the great archipela-
goes still areas of oscillations of level, and the continents areas of ele-
vation. But we have no reason to assume that things have thus re-
mained from the beginning of the world. Our continents seem to
have been formed by a preponderance, during many oscillations of
level, of the force of elevation ; but may not the areas of preponderant
movement have changed in the lapse of ages ? At a period long ante-
cedent to the Cambrian epoch, continents may have existed where
oceans are now spread out; and clear and open oceans may have ex-
isted where our continents now stand. Nor should we be justified in
assuming that if, for instance, the bed of the Pacific Ocean were now
converted into a continent we should there find sedimentary forma-
tions in a recognisable condition older than the Cambrian strata, sup-
posing such to have been formerly deposited; for it might well hap-
pen that strata which had subsided some miles nearer to the centre of
the earth, and which had been pressed on by an enormous weight of
superincumbent water, might have undergone far more meta-
morphic action than strata which have always remained nearer to the
surface. The immense areas in some parts of the world, for instance
in South America, of naked metamorphic rocks, which must have
been heated under great pressure, have always seemed to me to re-
quire some special explanation; and we may perhaps believe that we
see in these large areas, the many formations long anterior to the
Cambrian epoch in a completely metamorphosed and denuded con-
dition.

The several difficulties here discussed, namely — that, though we
find in our geological formations many links between the species
which now exist and which formerly existed, we do not find infinitely
numerous fine transitional forms closely joining them all together; —

348 ORIGIN OF SPECIES

the sudden manner in which several groups of species first appear in
our European formations; — the almost entire absence, as at present
known, of formations rich in fossils beneath the Cambrian strata, —
are all undoubtedly of the most serious nature. We see this in the fact
that the most eminent palaeontologists, namely, Cuvier, Agassiz, Bar-
rande, Pictet, Falconer, E. Forbes, etc., and all our greatest geologists,
as Lyell, Murchison, Sedgwick, etc., have unanimously, often vehe-
mently, maintained the immutability of species. But Sir Charles Lyell
now gives the support of his high authority to the other side; and
most geologists and palxontologists are much shaken in their for-
mer belief. Those who believe that the geological record is in any
degree perfect, will undoubtedly at once reject the theory. For my
part, following out Lyell's metaphor, I look at the geological record
as a history of the world imperfectly kept, and written in a changing
dialect; of this history we jx)ssess the last volume alone, relating
only to two or three countries. Of this volume, only here and there
a short chapter has been preserved; and of each page, only here and
there a few lines. Each word of the slowly-changing language, more
or less different in the successive chapters, may represent the forms of
life, which are entombed in our consecutive formations, and which
falsely apf>ear to have been abruptly introduced. On this view, the
difficulties above discussed are greatly diminished, or even disappear.

CHAPTER XI

On the Geological Succession of Organic Beings

On the slow and successive apf)earance of new species — On their different
rates of change — Species once lost do not reappear — Groups of
species follow the same general rules in their appearance and dis-
appearance as do single species — On extinction—On simultaneous
changes in the forms of life throughout the world — On the affinities
of extinct species to each other and to living species — On the state
of development of ancient forms — On the succession of the same
types within the same areas — Summary of preceding and present
chapter.

IET us now see whether the several facts and laws relating to
the geological succession of organic beings accord best with
_^ the common view of the immutability of species, or with
that of their slow and gradual modification, through variation and
natural selection.

New species have appeared very slowly, one after another, both on
the land and in the waters. Lyell has shown that it is hardly possible
to resist the evidence on this head in the case of the several tertiary
stages; and every year tends to fill up the blanks between the stages,
and to make the proportion between the lost and existing forms more
gradual. In some of the most recent beds, though undoubtedly of
high antiquity if measured by years, only one or two species are
extinct, and only one or two are new, having appeared there for the
first time, either locally, or, as far as we know, on the face of the
earth. The secondary formations are more broken; but, as Bronn has
remarked, neither the appearance nor disappearance of the many
species embedded in each formation has been simultaneous.

Species belonging to different genera and classes have not changed
at the same rate, or in the same degree. In the older tertiary beds
a few living shells may still be found in the midst of a multitude
of extinct forms. Falconer has given a striking instance of a similar
fact, for an existing crocodile is associated with many lost mammals

349

350 ORIGIN OF SPECIES

and reptiles in the sub-Himalayan deposits. The Silurian Lingula
differs but little from the living species of this genus; whereas most
of the other Silurian molluscs and all the crustaceans have changed
gready. The productions of the land seem to have changed at a
quicker rate than those of the sea, of which a striking instance has
been observed in Switzerland. There is some reason to beUeve that
organisms high in the scale, change more quickly than those that
are low: though there are exceptions to this rule. The amount of
organic change, as Pictet has remarked, is not the same in each
successive so<alled formation. Yet if we compare any but the most
closely related formations, all the species will be found to have
undergone some change. When a species has once disappeared from
the face of the earth, we have no reason to believe that the same
identical form ever reappears. The strongest apparent exception to
this latter rule is that of the so-called "colonies" of M. Barrande,
which intrude for a period in the midst of an older formation, and
then allow the preexisting fauna to reappear; but Lyell's explanation,
namely, that it is a case of temporary migration from a distinct
geographical province, seems satisfactory.

These several facts accord well with our theory, which includes no
fixed law of development, causing all the inhabitants of an area to
change abruptly, or simultaneously, or to an equal degree. The
process of modification must be slow, and will generally affect only a
few species at the same time; for the variability of each species is
independent of that of all others. Whether such variations or indi-
vidual differences as may arise will be accumulated through natural
selection in a greater or less degree, thus causing a greater or less
amount of (permanent modification, will dep)end on many complex
contingencies — on the variations being of a beneficial nature, on the
freedom of intercrossing, on the slowly changing physical conditions
of the country, on the immigration of new colonists, and on the
nature of the other inhabitants with which the varying species come
into competition. Hence it is by no means surprising that one species
should retain the same identical form much longer than others; or,
if changing, should change in a less degree. We find similar relations
between the existing inhabitants of distinct countries; for instance,
the land shells and coleopterous insects of Madeira have come to

GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 35 1

differ considerably from their nearest allies on the continent of
Europe, whereas the marine shells and birds have remained un-
altered. We can perhaps understand the apparently quicker rate
of change in terrestrial and in more highly organised productions
compared with marine and lower productions, by the more complex
relations of the higher beings to their organic and inorganic condi-
tions of life, as explained in a former chapter. When many of the
inhabitants of any area have become modified and improved, we can
understand, on the principle of competition, and from the all-
important relations of organism to organism in the struggle for life,
that any form which did not become in some degree modified and
improved, would be liable to extermination. Hence we see why all
the species in the same region do at last, if we look to long enough
intervals of time, become modified, for otherwise they would become
extinct.

In members of the same class the average amount of change during
long and equal periods of time, may, perhaps, be nearly the same;
but as the accumulation of enduring formation, rich in fossils,
depends on great masses of sediment being deposited on subsiding
areas, our formations have been almost necessarily accumulated at
wide and irregularly intermittent intervals of time; consequently
the amount of organic change exhibited by the fossils embedded in
consecutive formations is not equal. Each formation, on this view,
does not mark a new and complete act of creation, but only an
occasional scene, taken almost at hazard in an ever slowly changing
drama.

We can clearly understand why a species when once lost should
never reappear, even if the very same conditions of life, organic and
inorganic, should recur. For though the offspring of one species
might be adapted (and no doubt this has occurred in innumerable
instances) to fill the place of another species in the economy of
nature, and thus supplant it; yet the two forms — the old and the new
— would not be identically the same; for both would almost certainly
inherit different characters from their distinct progenitors; and
organisms already differing would vary in a different manner. For
instance, it is possible, if all our fantail pigeons were destroyed, that
fanciers might make a new breed hardly distinguishable from the

352 ORIGIN OF SPECIES

present breed; but if the parent rock pigeon were likewise destroyed,
and under nature we have every reason to believe that parent-forms
are generally supplanted and exterminated by their improved off-
spring, it is incredible that a fantail, identical with the existing
breed, could be raised from any other species of pigeon, or even from
any other well-established race of the domestic pigeon, for the succes-
sive variations would almost certainly be in some degree different,
and the newly-formed variety would probably inherit from its pro-
genitor some characteristic differences.

Groups of species, that is, genera and families, follow the same
general rules in their appearance and disappearance as do single
sp)ecies, changing more or less quickly, and in a greater or lesser
degree. A group, when it has once disappeared, never reappears;
that is, its existence, as long as it lasts, is continuous. I am aware
that there are some apparent exceptions to this rule, but the excep-
tions are surprisingly few, so few that E. Forbes, Pictet, and Wood-
ward (though all strongly opposed to such views as I maintain)
admit its truth; and the rule strictly accords with the theory. For all
the species of the same group, however long it may have lasted, are
the modified descendants one from the other, and all from a com-
mon progenitor. In the genus Lingula, for instance, the species which
have successively appeared at all ages must have been connected by
an unbroken series of generations, from the lowest Silurian stratum
to the present day.

We have seen in the last chapter that whole groups of species some-
times falsely appear to have been abruptly developed; and I have
attempted to give an explanation of this fact, which if true would be
fatal to my views. But such cases are certainly exceptional; the
general rule being a gradual increase in number, until the group
reaches its maximum, and then, sooner or later, a gradual decrease.
If the number of the species included within a genus, or the number
of the genera within a family, be represented by a vertical line of
varying thickness, ascending through the successive geological
formations, in which the species are found, the line will sometimes
falsely appear to begin at its lower end, not in a sharp point, but
abruptly; it then gradually thickens upwards, often keeping of
equal thickness for a space, and ultimately thins out in the upper

EXTINCTION 353

beds, marking the decrease and final extinction of the species. This
gradual increase in number of the species of a group is strictly con-
formable with the theory, for the species of the same genus, and the
genera of the same family, can increase only slowly and progressively;
the process of modification and the production of a number of allied
forms necessarily being a slow and gradual process, — one species first
giving rise to two or three varieties, these being slowly converted
into species, which in their turn produce by equally slow steps other
varieties and species, and so on, like the branching of a great tree
from a single stem, till the group becomes large.

ON EXTINCTION

We have as yet only spoken incidentally of the disappearance of
species and of groups of species. On the theory of natural selection,
the extinction of old forms and the production of new and improved
forms are intimately connected together. The old notion of all the
inhabitants of the earth having been swept away by catastrophes at
successive periods is very generally given up, even by those geologists,
as Elie de Beaumont, Murchison, Barrande, etc., whose general views
would naturally lead them to this conclusion. On the contrary, we
have every reason to believe, from the study of the tertiary forma-
tions, that species and groups of species gradually disappear, one
after another, first from one spot, then from another, and finally
from the world. In some few cases, however, as by the breaking of
an isthmus and the consequent irruption of a multitude of new
inhabitants into an adjoining sea, or by the final subsidence of an
island, the process of extinction may have been rapid. Both single
species and whole groups of species last for very unequal periods;
some groups, as we have seen, have endured from the earliest known
dawn of life to the present day; some have disappeared before the
close of the palxozoic period. No fixed law seems to determine the
length of time during which any single species or any single genus
endures. There is reason to believe that the extinction of a whole
group of species is generally a slower process than their production :
if their appearance and disappearance be represented, as before, by
a vertical line of varying thickness the line is found to tap)er more
gradually at its upper end, which marks the progress of extermina-

354 ORIGIN OF SPECIES

tion, than at its lower end, which marks the first appearance and the
early increase in number of the species. In some cases, however, the
extermination of whole groups, as of ammonites, towards the close
of the secondary period, has been wonderfully sudden.

The extinction of species has been involved in the most gratuitous
mystery. Some authors have even supposed that, as the individual
has a definite length of life, so have species a definite duration. No
one can have marvelled more than I have done at the extinction of
species. When I found in La Plata the tooth of a horse embedded
with the remains of Mastodon, Megatherium, Toxodon, and other
extinct monsters, which all co-existed with still living shells at a very
late geological f)eriod, I was filled with astonishment; for, seeing that
the horse, since its introduction by the Spaniards into South America,
has run wild over the whole country and has increased in numbers
at an unparalleled rate, 1 asked myself what could so recently have
exterminated the former horse under conditions of hfe apparently
so favourable. But my astonishment was groundless. Professor Owen
soon perceived that the tooth, though so like that of the existing
horse, belonged to an extinct species. Had this horse been still living,
but in some degree rare, no naturalist would have felt the least
surprise at its rarity; for rarity is the attribute of a vast number of
species of all classes, in all countries. If we ask ourselves why this or
that species is rare, we answer that something is unfavourable in its
conditions of life; but what that something is we can hardly ever tell.
On the supposition of the fossil horse still existing as a rare species,
we might have felt certain, from the analogy of all other mammals,
even of the slow-breeding elephant, and from the history of the
naturalisation of the domestic horse in South America, that under
more favourable conditions it would in a very few years have stocked
the whole continent. But we could not have told what the unfavour-
able conditions were which checked its increase, whether some one
or several contingencies, and at what period of the horse's life, and
in what degree they severally acted. If the conditions had gone on,
however slowly, becoming less and less favourable, we assuredly
should not have perceived the fact, yet the fossil horse would cer-
tainly have become rarer and rarer, and finally extinct; — its place
being seized on by some more successful competitor.

EXTINCTION 355

It is most difficult always to remember that the increase of every
creature is constantly being checked by unperceived hostile agencies;
and that these same unperceived agencies are amply sufficient to
cause rarity, and finally extinction. So little is this subject understood,
that I have heard surprise repeatedly expressed at such great mon-
sters as the Mastodon and the more ancient Dinosaurians having
become extinct; as if mere bodily strength gave victory in the batde
of life. Mere size, on the contrary, would in some cases determine,
as has been remarked by Owen, quicker extermination from the
greater amount of requisite food. Before man inhabited India or
Africa, some cause must have checked the continued increase of the
existing elephant. A highly capable judge, Dr. Falconer, believes
that it is chiefly insects which, from incessantly harassing and weak-
ening the elephant in India, check its increase; and this was Bruce's
conclusion with respect to the African elephant in Abyssinia. It is
certain that insects and blood-sucking bats determine the existence
of the larger naturalized quadrupeds in several parts of South
America.

We see in many cases in the more recent tertiary formations, that
rarity precedes extinction; and we know that this has been the
progress of events with those animals which have been exterminated,
either locally or wholly, through man's agency. I may repeat what
I published in 1845, namely, that to admit that species generally
become rare before they become extinct — to feel no surprise at the
rarity of a species, and yet to marvel greatly when the species ceases
to exist, is much the same as to admit that sickness in the individual
is the forerunner of death — to feel no surprise at sickness, but, when
the sick man dies, to wonder and to suspect that he died by some
deed of violence.

The theory of natural selection is grounded on the belief that each
new variety and ultimately each new sjjecies, is produced and main-
tained by having some advantage over those with which it comes
into competition; and the consequent extinction of the less favoured
forms almost inevitably follows. It is the same with our domestic
productions; when a new and slightly improved variety has been
raised, it at first supplants the less improved varieties in the same
neighbourhood; when much improved it is transported far and near.

356 ORIGIN OF SPECIES

like our short-horn cattle, and takes the place of other breeds in other
countries. Thus the appearance of new forms and the disappearance
of old forms, both those naturally and those artificially produced, are
bound together. In flourishing groups, the number of new specific
forms which have been produced within a given time has at some
periods probably been greater than the number of the old specific
forms which have been exterminated; but we know that species have
not gone on indefinitely increasing, at least during the later geologi-
cal epochs, so that, looking to later times, we may believe that the
production of new forms has caused the extinction of about the
same number of old forms.

The competition will generally be most severe, as formerly
explained and illustrated by examples, between the forms which
are most like each other in all respects. Hence the improved and
modified descendants of a species will generally cause the extermi-
nation of the parent species; and if many new forms have been
developed from any one species, the nearest allies of that species, />.,
the species of the same genus, will be the most liable to extermina-
tion. Thus, as I believe, a number of new species descended from
one species, that is a new genus, comes to supplant an old genus,
belonging to the same family. But it must often have happened that
a new species belonging to some one group has seized on the place
occupied by a species belonging to a distinct group, and thus have
caused its extermination. If many allied forms be developed from
the successful intruder, many will have to yield their places; and it
will generally be the allied forms, which will suffer from some
inherited inferiority in common. But whether it be sf)ecies belong-
ing to the same or to a distinct class, which have yielded their places
to other modified and improved species, a few of the sufferers may
often be preserved for a long time, from being fitted to some peculiar
line of life, or from inhabiting some distant and isolated station,
where they will have escaped severe competition. For instance, some
sjjecies of Trigonia, a great genus of shells in the secondary forma-
tions, survive in the Australian seas; and a few members of the great
and almost extinct group of Ganoid fishes still inhabit our fresh
waters. Therefore the utter extinction of a group is generally, as we
have seen, a slower process than its production.

FORMS OF LIFE CHANGING 357

With respect to the apparently sudden extermination of whole
families or orders, as of trilobites at the close of the palaeozoic period
and of ammonites at the close of the secondary period, we must
remember what has been already said on the probable wide intervals
of time between our consecutive formations; and in these intervals
there may have been much slow extermination. Moreover, when, by
sudden immigration or by unusually rapid development, many
species of a new group have taken possession of an area, many of
the older species will have been exterminated in a correspondingly
rapid manner; and the forms which thus yield their places will
commonly be allied, for they will partake of the same inferiority in
common.

Thus, as it seems to me, the manner in which single species and
whole groups of species become extinct accord well with the theory
of natural selection. We need not marvel at extinction; if we must
marvel, let it be at our own presumption in imagining for a moment
that we understand the many complex contingencies on which the
existence of each species depends. If we forget for an instant that
each species tends to increase inordinately, and that some check is
always in action, yet seldom perceived by us, the whole economy of
nature will be utterly obscured. Whenever we can precisely say why
this species is more abundant in individuals than that; why this
species and not another can be naturalised in a given country; then,
and not until then, we may justly feel surprise why we cannot
account for the extinction of any particular species or group of
species.

ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY
THROUGHOUT THE WORLD

Scarcely any palxontological discovery is more striking than the
fact that the forms of life change almost simultaneously throughout
the world. Thus our European Chalk formation can be recognised
in many distinct regions, under the most different climates, where
not a fragment of the mineral chalk itself can be found; namely in
North America, in equatorial South America, in Tierra del Fuego,
at the Cape of Good Hope, and in the peninsula of India. For at
these distant points, the organic remains in certain beds present an

358 ORIGIN OF SPECIES

unmistakeable resemblance to those of the Chalk. It is not that the
same species are met with; for in some cases not one species is
identically the same, but they belong to the same families, genera,
and sections of genera, and sometimes are similarly characterised in
such trifling points as mere superficial sculpture. Moreover, other
forms, which are not found in the Chalk of Europe, but which occur
in the formations either above or below, occur in the same order at
these distant points of the world. In the several successive palxozoic
formations of Russia, Western Europe, and North America, a similar
parallelism in the forms of life has been observed by several authors;
so it is, according to Lyell, with the European and North American
tertiary deposits. Even if the few fossil species which are common to
the Old and New Worlds were kept wholly out of view, the general
parallelism in the successive forms of life, in the paleozoic and
tertiary stages, would still be manifest, and the several forma-
tions could be easily correlated.

These observations, however, relate to the marine inhabitants of
the world: we have not sufficient data to judge whether the pro-
ductions of the land and of fresh water at distant points change in
the same parallel manner. We may doubt whether they have thus
changed: if the Megatherium, Mylodon, Macrauchenia, and Toxo-
don had been brought to Europe from La Plata, without any infor-
mation in regard to their geological position, no one would have
suspected that they had co-existed with sea-shells all still living; but
as these anomalous monsters co-existed with the Mastodon and
Horse, it might at least have been inferred that they had lived during
one of the later tertiary stages.

When the marine forms of life are spoken of as having changed
simultaneously throughout the world, it must not be supposed that
this expression relates to the same year, or to the same country, or
even that it has a very strict geological sense; for if all the marine
animals now living in Europe, and all those that lived in Europe
during the pleistocene period (a very remote period as measured by
years, including the whole glacial epoch) were compared with those
now existing in South America or in Australia, the most skilful
naturalist would hardly be able to say whether the present or the
pleistocene inhabitants of Europe resembled most closely those of

FORMS OF LIFE CHANGING 359

the southern hemisphere. So, again, several highly competent
observers maintain that the existing productions of the United
States are more closely related to those which lived in Europe during
certain late tertiary stages, than to the present inhabitants of Europe;
and if this be so, it is evident that fossiliferous beds now deposited
on the shores of North America would hereafter be liable to be
classed with somewhat older European beds. Nevertheless, looking
to a remotely future epoch, there can be little doubt that all the
more modern marine formations, namely, the upper pliocene, the
pleistocene and strictly modern beds of Europe, North and South
America, and Australia, from containing fossil remains in some
degree allied, and from not including those forms which are found
only in the older underlying deposits, would be correctly ranked as
simultaneous in a geological sense.

The fact of the forms of life changing simultaneously, in the above
large sense, at distant parts of the world, has greatly struck those
admirable observers, MM. de Verneuil and d'Archiac. After refer-
ring to the parallelism of the palarozoic forms of life in various parts
of Europe, they add, "If, struck by this strange sequence, we turn our
attention to North America, and there discover a series of analogous
phenomena, it will appear certain that all these modifications of
species, their extinction, and the introduction of new ones, cannot
be owing to mere changes in marine currents or other causes more
or less local and temporary, but depend on general laws which
govern the whole animal kingdom." M. Barrande has made forcible
remarks to precisely the same effect. It is, indeed, quite futile to
look to changes of currents, climate, or other physical conditions, as
the cause of these great mutations in the forms of life throughout the
world, under the most different climates. We must, as Barrande has
remarked, look to some sp)ecial law. We shall see this more clearly
when we treat of the present distribution of organic beings, and find
how slight is the relation between the physical conditions of various
countries and the nature of their inhabitants.

This great fact of the parallel succession of the forms of life
throughout the world, is explicable on the theory of natural selection.
New species are formed by having some advantage over older forms;
and the forms, which are already dominant, or have some advantage

360 ORIGIN OF SPECIES

over the other forms in their own country, give birth to the greatest
number of new varieties or incipient species. We have distinct evi-
dence on this head, in the plants which are dominant, that is, which
are commonest and most widely diffused, producing the greatest
number of new varieties. It is also natural that the dominant, vary-
ing, and far-spreading species, which have already invaded to a
certain extent the territories of other species, should be those which
would have the best chance of spreading still further, and of giving
rise in new countries to other new varieties and species. The process
of diffusion would often be very slow, depending on climatal and
geographical changes, on strange accidents, and on the gradual
acclimatisation of new species to the various climates through which
they might have to pass, but in the course of time the dominant
forms would generally succeed in spreading and would ultimately
prevail. The diffusion would, it is probable, be slower with the
terrestrial inhabitants of the distinct continents than with the marine
inhabitants of the continuous sea. We might therefore exfject to
find, as we do find, a less strict degree of parallelism in the succession
of the productions of the land than with those of the sea.

Thus, as it seems to me, the parallel, and, taken in a large sense,
simultaneous, succession of the same forms of life throughout the
world, accords well with the principle of new spjecies having been
formed by dominant species spreading widely and varying; the new
sf)ecies thus produced being themselves dominant, owing to their
having had some advantage over their already dominant parents, as
well as over other species, and again spreading, varying, and pro-
ducing new forms. The old forms which are beaten and which
yield their places to the new and victorious forms, will generally be
allied in groups, from inheriting some inferiority in common; and
therefore, as new and improved groups spread throughout the world,
old groups disappear from the world; and the succession of forms
everywhere tends to correspond both in their first appearance and
final disappearance.

There is one other remark connected with this subject worth
making. I have given my reasons for believing that most of our
great formations, rich in fossils, were deposited during periods of
subsidence; and that blank intervals of vast duration, as far as fossils

FORMS OF LIFE CHANGING 361

are concerned, occurred during the periods when the bed of the sea
was either stationary or rising, and hkewise when sediment was not
thrown down quickly enough to embed and preserve organic
remains. During these long and blank intervals I suppose that the
inhabitants of each region underwent a considerable amount of
modification and extinction, and that there was much migration
from other parts of the world. As we have reason to believe that
large areas are affected by the same movement, it is probable that
strictly contemporaneous formations have often been accumulated
over very wide spaces in the same quarter of the world; but we are
very far from having any right to conclude that this has invariably
been the case, and that large areas have invariably been affected by
the same movements. When two formations have been deposited in
two regions during nearly, but not exactly, the same period, we
should find in both, from the causes explained in the foregoing
paragraphs, the same general succession in the forms of life; but
the species would not exactly correspond; for there will have been
a little more time in the one region than in the other for modifica-
tion, extinction, and immigration.

I suspect that cases of this nature occur in Europe. Mr. Prestwich,
in his admirable Memoirs on the eocene deposits of England and
France, is able to draw a close general parallelism between the suc-
cessive stages in the two countries; but when he compares certain
stages in England with those in France, although he finds in both
a curious accordance in the numbers of the species belonging to the
same genera, yet the species themselves differ in a manner very diffi-
cult to account for considering the proximity of the two areas, —
unless, indeed, it be assumed that an isthmus separated two seas
inhabited by distinct, but contemporaneous, faunas. Lyell has made
similar observations on some of the later tertiary formations. Bar-
rande, also, shows that there is a striking general parallelism in the
successive Silurian deposits of Bohemia and Scandinavia; neverthe-
less he finds a surprising amount of difference in the species. If the
several formations in these regions have not been deposited during
the same exact periods, — a formation in one region often corre-
sponding with a blank interval in the other, — and if in both regions
the species have gone on slowly changing during the accumulation

362 ORIGIN OF SPECIES

of the several formations and during the long intervals of time
between them; in this case the several formations in the two regions
could be arranged in the same order, in accordance with the general
succession of the forms of life, and the order would falsely appear
to be strictly parallel; nevertheless the species would not be all the
same in the apparently corresponding stages in the two regions.

ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER,
AND TO LIVING FORMS

Let US now look to the mutual affinities of extinct and living
species. All fall into a few grand classes; and this fact is at once
explained on the principle of descent. The more ancient any form
is, the more, as a general rule, it differs from hving forms. But, as
Buckland long ago remarked, extinct species can all be classed either
in still existing groups, or between them. That the extinct forms of
life help to fill up the intervals between existing genera, families,
and orders, is certainly true; but as this statement has often been
ignored or even denied, it may be well to make some remarks on this
subject, and to give some instances. If we confine our attention
either to the Hving or to the extinct species of the same class, the
series is far less perfect than if we combine both into one general
system. In the writings of Professor Owen we continually meet
with the expression of generalised forms, as applied to extinct ani-
mals; and in the writings of Agassiz, of prophetic or synthetic types;
and these terms imply that such forms are in fact intermediate or
connecting links. Another distinguished paleontologist, M. Gaudry,
has shown in the most striking manner that many of the fossil
mammals discovered by him in Attica serve to break down the inter-
vals between existing genera. Cuvier ranked the Ruminants and
Pachyderms as two of the most distinct orders of mammals: but
SO many fossil links have been disentombed that Owen has had to
alter the whole classification, and has placed certain pachyderms in
the same sub-order with ruminants; for example, he dissolves by
gradations the apparently wide interval between the pig and the
camel. The Ungulata or hoofed quadrupeds are now divided into
the even-toed or odd-toed divisions; but the Macrauchenia of South
America connects to a certain extent these two grand divisions. No

AFFINITIES OF EXTINCT SPECIES 363

one will deny that the Hippahon is intermediate between the
existing horse and certain older ungulate forms. What a wonderful
connecting link in the chain of mammals is the Typotherium from
South America, as the name given to it by Professor Gervais
expresses, and which cannot be placed in any existing order. The
Sirenia form a very distinct group of mammals, and one of the
most remarkable peculiarities in the existing dugong and lamentin is
the entire absence of hind limbs without even a rudiment being left;
but the extinct Halitherium had, according to Professor Flower, an
ossified thigh-bone "articulated to a well-defined acetabulum in the
pelvis," and it thus makes some approach to ordinary hoofed quad-
rupeds, to which the Sirenia are in other respects aUied. The ceta-
ceans or whales are widely different from all other mammals, but
the tertiary 2^uglodon and Squalodon, which have been placed by
some naturalists in an order by themselves, are considered by Profes-
sor Huxley to be undoubtedly cetaceans, "and to constitute con-
necting links with the aquatic carnivora."

Even the wide interval between birds and reptiles has been shown
by the naturalist just quoted to be partially bridged over in the
most unexpected manner, on the one hand, by the ostrich and extinct
Archeopteryx, and on the other hand, by the Compsognathus, one
of the dinosaurians — that group which includes the most gigantic of
all terrestrial reptiles. Turning to the Invertebrata, Barrande asserts
(a higher authority could not be named) that he is every day
taught that, although paleozoic animals can certainly be classed
under existing groups, yet that at this ancient period the groups were
not so distinctly separated from each other as they now are.

Some writers have objected to any extinct species, or group of
species, being considered as intermediate between any two living
species, or groups of species. If by this term it is meant that an
extinct form is directly intermediate in all its characters between two
living forms or groups, the objection is probably valid. But in a
natural classification many fossil species certainly stand between
living species, and some extinct genera between living genera, even
between genera belonging to distinct families. The most common
case, especially with respect to very distinct groups, such as fish and
reptiles, seems to be, that, supposing them to be distinguished at the

364 ORIGIN OF SPECIES

present day by a score of characters, the ancient members are sep-
arated by a somewhat lesser number of characters; so that the two
groups formerly made a somewhat nearer approach to each other
than they now do.

It is a common belief that the more ancient a form is, by so much
the more it tends to connect by some of its characters groups now
widely separated from each other. This remark no doubt must be
restricted to those groups which have undergone much change in the
course of geological ages; and it would be difficult to prove the
truth of the proposition, for every now and then a Uving animal,
as the Lepidosiren, is discovered having affinities directed towards
very distinct groups. Yet if we compare the older reptiles and batra-
chians, the older fish, the older cephalopods, and the eocene mam-
mals, with the more recent members of the same classes, we must
admit that there is truth in the remark.

Let us see how far these several facts and inferences accord with
the theory of descent with modification. As the subject is somewhat
complex, I must request the reader to turn to the diagram in the
fourth chapter. We may suppose that the numbered letters in italics
represent genera, and the dotted lines diverging from them the
species in each genus. The diagram is much too simple, too few
genera and too few species being given, but this is unimportant for
us. The horizontal lines may represent successive geological forma-
tions, and all the forms beneath the uppermost line may be con-
sidered as extinct. The three existing genera a", ly'*, p'*, will form
a small family; ^'* and /" a closely allied family or sub-family; and
0", ir", m", a third family. These three families, together with the
many extinct genera on the several lines of descent diverging from
the parent-form (A) will form an order, for all will have inherited
something in common from their ancient progenitor. On the prin-
ciple of the continued tendency to divergence of character, which
was formerly illustrated by this diagram, the more recent any form
is, the more it will generally differ from its ancient progenitor.
Hence we can understand the rule that the most ancient fossils differ
most from existing forms. We must not, however, assume that
divergence of character is a necessary contingency; it depends solely
on the descendants from a species being thus enabled to seize on

AFFINITIES OF EXTINCT SPECIES 365

many and different places in the economy of nature. Therefore it is
quite possible, as we have seen in the case of some Silurian forms,
that a species might go on being slightly modified in relation to its
slightly altered conditions of life, and yet retain throughout a vast
period the same general characteristics. This is represented in the
diagram by the letter f'*.

All the many forms, extinct and recent, descended from (A),
make, as before remarked, one order; and this order, from the con-
tinued effects of extinction and divergence of character, has become
divided into several sub-families and families, some of which are
supposed to have perished at different periods, and some to have
endured to the present day.

By looking at the diagram we can see that if many of the extinct
forms supposed to be imbedded in the successive formations, were
discovered at several points low down in the series, the three existing
families on the uppermost line would be rendered less distinct from
each other. If, for instance, the genera a', a", d", f, m^, m', w', were
disinterred, these three families would be so closely linked together
that they probably would have to be united into one great family, in
nearly the same manner as has occurred with ruminants and certain
pachyderms. Yet he who objected to consider as intermediate the
extinct genera, which thus link together the living genera of three
families, would be partly justified, for they are intermediate, not
directly, but only by a long and circuitous course through many
widely different forms. If many extinct forms were to be discovered
above one of the horizontal lines or geological formations — for in-
stance, above No. VI. — but none from beneath this line, then only
two of the families (those on the left hand, a'*, etc., and ^'*, etc.)
would have to be united into one; and there would remain two
families, which would be less distinct from each other than they
were before the discovery of the fossils. So again if the three families
formed of eight genera (a'* to w'*), on the uppermost line, be sup-
posed to differ from each other by half-a-dozen important char-
acters, then the families which existed at the period marked VI.
would certainly have differed from each other by a less number of
characters; for they would at this early stage of descent have diverged
in a less degree from their common progenitor. Thus it comes that

366 ORIGIN OF SPECIES

ancient and extinct genera are often in a greater or less degree inter-
mediate in character between their modified descendants, or between
their collateral relations.

Under nature the process will be far more complicated than is
represented in the diagram; for the groups will have been more
numerous; they will have endured for extremely unequal lengths
of time, and will have been modified in various degrees. As we
possess only the last volume of the geological record, and that in a
very broken condition, we have no right to expect, except in rare
cases, to fill up the wide intervals in the natural system, and thus to
unite distinct families or orders. All that we have a right to expect
is, that those groups which have, within known geological periods,
undergone much modification, should in the older formations make
some slight approach to each other; so that the older members should
differ less from each other in some of their characters than do the
existing members of the same groups; and this by the concurrent
evidence of our best palaeontologists is frequently the case.

Thus, on the theory of descent with modification, the main facts
with respect to the mutual affinities of the extinct forms of life to
each other and to living forms, are explained in a satisfactory man-
ner. And they are wholly inexplicable on any other view.

On this same theory, it is evident that the fauna during any one
great period in the earth's history will be intermediate in general
character between that which preceded and that which succeeded it.
Thus the sp)ecies which lived at the sixth great stage of descent in
the diagram are the modified offspring of those which lived at the
fifth stage, and are the parents of those which became still more
modified at the seventh stage; hence they could hardly fail to be
nearly intermediate in character between the forms of life above and
below. We must, however, allow for the entire extinction of some
preceding forms, and in any one region for the immigration of new
forms from other regions, and for a large amount of modification
during the long and blank interval between the successive forma-
tions. Subject to these allowances, the fauna of each geological
period undoubtedly is intermediate in character, between the pre-
ceding and succeeding faunas. I need give only one instance, namely,
the manner in which the fossils of the Devonian system, when this

AFFINITIES OF EXTINCT SPECIES 367

system was first discovered, were at once recognised by palaeontolo-
gists as intermediate in character between those of the overlying
carboniferous, and underlying Silurian systems. But each fauna is
not necessarily exactly intermediate, as unequal intervals of time
have elapsed between consecutive formations.

It is no real objection to the truth of the statement that the fauna
of each f)eriod as a whole is nearly intermediate in character between
the preceding and succeeding faunas, that certain genera offer excep-
tions to the rule. For instance, the species of mastodons and ele-
phants, when arranged by Dr. Falconer in two series, — in the first
place according to their mutual affinities, and in the second place
according to their periods of existence, — do not accord in arrange-
ment. The sp)ecies extreme in character are not the oldest or the
most recent; nor are those which are intermediate in character, inter-
mediate in age. But supposing for an instant, in this and other such
cases, that the record of the first appearance and disappearance of
the species was complete, which is far from the case, we have no
reason to believe that forms successively produced necessarily endure
for corresponding lengths of time. A very ancient form may occa-
sionally have lasted much longer than a form elsewhere subsequently
produced, especially in the case of terrestrial productions inhabiting
separated districts. To compare small things with great; if the
principal living and extinct races of the domestic pigeon were
arranged in serial affinity, this arrangement would not closely accord
with the order in time of their production, and even less with the
order of their disappearance; for the parent rock pigeon still lives;
and many varieties between the rock pigeon and the carrier have
become extinct; and carriers which are extreme in the important
character of length of back originated earlier than short-beaked
tumblers, which are at the opposite end of the scries in this respect.

Closely connected with the statement, that the organic remains
from an intermediate formation are in some degree intermediate in
character, is the fact, insisted on by all palaeontologists, that fossils
from two consecutive formations are far more closely related to each
other, than are the fossils from two remote formations. Pictet gives
as a well-known instance, the general resemblance of the organic
remains from the several stages of the Chalk formation, though the

368 ORIGIN OF SPECIES

species are distinct in each stage. This fact alone, from its generality,
seems to have shaken Professor Pictet in his belief in the immuta-
bility of species. He who is acquainted with the distribution of
existing species over the globe, will not attempt to account for the
close resemblance of distinct species in closely consecutive forma-
tions, by the physical conditions of the ancient areas having remained
nearly the same. Let it be remembered that the forms of life, at
least those inhabiting the sea, have changed almost simultaneously
throughout the world, and therefore under the most different cli-
mates and conditions. Consider the prodigious vicissitudes of climate
during the pleistocene period, which includes the whole glacial
epoch, and note how little the specific forms of the inhabitants of the
sea have been affected.

On the theory of descent, the full meaning of the fossil remains
from closely consecutive formations being closely related, though
ranked as distinct species, is obvious. As the accumulation of each
formation has often been interrupted, and as long blank intervals
have intervened between successive formations, we ought not to
expect to find, as I attempted to show in the last chapter, in any one
or in any two formations, all the intermediate varieties between the
species which appeared at the commencement and close of these
periods: but we ought to find after intervals, very long as measured
by years, but only moderately long as measured geologically, closely
allied forms, or, as they have been called by some authors, repre-
sentative species; and these assuredly we do find. We find, in short,
such evidence of the slow and scarcely sensible mutations of specific
forms, as we have the right to expect.

ON THE STATE OF DEVELOPMENT OF ANCIENT COMPAKED
WITH LIVING FORMS

We have seen in the fourth chapter that the degree of differentia-
tion and specialisation of the parts in organic beings, when arrived
at maturity, is the best standard, as yet suggested, of their degree of
perfection or highness. We have also seen that, as the specialisation
of parts is an advantage to each being, so natural selection will tend
to render the organisation of each being more specialised and per-
fect, and in this sense higher; not but that it may leave many crea-

STATE OF DEVELOPMENT COMPARED 369

tures with simple and unimproved structures fitted for simple
conditions of life, and in some cases will even degrade or simplify
the organisation, yet leaving such degraded beings better fitted for
their new walks of life. In another and more general manner, new
species become superior to their predecessors; for they have to beat in
the struggle for life all the older forms, with which they come into
close competition. We may therefore conclude that if under a nearly
similar climate the eocene inhabitants of the world could be put into
competition with the existing inhabitants, the former would be
beaten and exterminated by the latter, as would the secondary by
the eocene, and the palaeozoic by the secondary forms. So that by
this fundamental test of victory in the battle for life, as well as by
the standard of the specialisation of organs, modern forms ought,
on the theory of natural selection, to stand higher than ancient forms.
Is this the case? A large majority of palaeontologists would answer
in the affirmative; and it seems that this answer must be admitted
as true, though difficult of proof.

It is no valid objection to this conclusion, that certain Brachiopods
have been but slightly modified from an extremely remote geological
epoch; and that certain land and fresh-water shells have remained
nearly the same, from the time when, as far as is known, they first
appeared. It is not an insuperable difficulty that Foraminifera have
not, as insisted on by Dr. Carpenter, progressed in organisation since
even the Laurentian ejXKh; for some organisms would have to
remain fitted for simple conditions of life, and what could be better
fitted for this end than these lowly organised Protozoa? Such objec-
tions as the above would be fatal to my view, if it included advance
in organisation as a necessary contingent. They would Ukewise be
fatal, if the above Foraminifera, for instance, could be proved to have
first come into existence during the Laurentian epoch, or the above
Brachiof)ods during the Cambrian formation; for in this case, there
would not have been time sufficient for the development of these
organisms up to the standard which they had then reached. When
advanced up to any given point, there is no necessity, on the theory
of natural selection, for their further continued progress; though
they will, during each successive age, have to be slightly modified,
so as to hold their places in relation to slight changes in their condi-

370 ORIGIN OF SPECIES

tions. The foregoing objections hinge on the question whether we
really know how old the world is, and at what period the various
forms of life first appeared; and this may well be disputed.

The problem whether organisation on the whole has advanced is
in many ways excessively intricate. The geological record, at all
times imperfect, does not extend far enough back, to show with
unmistakeable clearness that within the known history of the world
organisation has largely advanced. Even at the present day, looking
to members of the same class, naturalists are not unanimous which
forms ought to be ranked as highest : thus, some look at the selaceans
or sharks, from their approach in some important points of structure
to reptiles, as the highest fish; others look at the teleosteans as the
highest. The ganoids stand intermediate between the selaceans and
teleosteans; the latter at the present day are largely preponderant in
number; but formerly selaceans and ganoids alone existed; and in
this case, according to the standard of highness chosen, so will it be
said that fishes have advanced or retrograded in organisation. To
attempt to compare members of distinct types in the scale of high-
ness seems hopeless; who will decide whether a cuttle-fish be higher
thaa a bee — that insect which the great Von Baer believed to be "in
fact more highly organised than a fish, although upon another type"?
In the complex struggle for life it is quite credible that crustaceans,
not very high in their own class, might beat cephalopods, the highest
molluscs; and such crustaceans, though not highly developed,
would stand very high in the scale of invertebrate animals, if judged
by the most decisive of all trials — the law of battle. Beside these
inherent difficulties in deciding which forms are the most advanced
in organisation, we ought not solely to compare the highest mem-
bers of a class at any two periods — though undoubtedly this is one
and perhaps the most important element in striking a balance — but
we ought to compare all the members, high and low, at two periods.
At an ancient epoch the highest and lowest molluscoidal animals,
namely, cephalopods and brachiopods, swarmed in numbers; at the
present time both groups are greatly reduced, whilst others, inter-
mediate in organisation, have largely increased; consequently some
naturalists maintain that molluscs were formerly more highly
developed than at present; but a stronger case can be made out on

STATE OF DEVELOPMENT COMPARED yjl

the opposite side, by considering the vast reduction of the brachio-
pods, and the fact that our existing cephalopods, though few in
number, are more highly organised than their ancient representa-
tives. We ought also to compare the relative proportional numbers
at any two periods of the high and low classes throughout the world:
if, for instance, at the present day fifty thousand kinds of vertebrate
animals exist, and if we knew that at some former period only ten
thousand kinds existed, we ought to look at this increase in number
in the highest class, which implies a great displacement of lower
forms, as a decided advance in the organisation of the world. We
thus see how hopelessly difficult it is to compare with perfect fairness
under such extremely complex relations, the standard of organisation
of the imperfectly known faunas of successive periods.

We shall appreciate this difficulty more clearly, by looking to
certain existing faunas and floras. From the extraordinary manner
in which Eurojiean productions have recently spread over New
Zealand, and have seized on places which must have been previously
occupied by the indigenes, we must believe, that if all the animals
and plants of Great Britain were set free in New Zealand, a multi-
tude of British forms would in the course of time become thoroughly
naturalised there, and would exterminate many of the natives. On
the other hand, from the fact that hardly a single inhabitant of the
southern hemisphere has become wild in any part of Eurojje, we
may well doubt whether, if all the productions of New Zealand were
set free in Great Britain, any considerable number would be enabled
to seize on places now occupied by our native plants and animals.
Under this point of view, the productions of Great Britain stand
much higher in the scale than those of New Zealand. Yet the most
skilful naturalist, from an examination of the species of the two
countries, could not have foreseen this result.

Agassiz and several other highly competent judges insist that
ancient animals resemble to a certain extent the embryos of recent
animals belonging to the same classes; and that the geological suc-
cession of extinct forms is nearly parallel with the embryological
development of existing forms. This view accords admirably well
with our theory. In a future chapter I shall attempt to show that the
adult differs from its embryo, owing to variations having super-

372 ORIGIN OF SPECIES

vened at a not early age, and having been inherited at a correspond-
ing age. This process, whilst it leaves the embryo almost unaltered,
continually adds, in the course of successive generations, more and
more difference to the adult. Thus the embryo comes to be left as a
sort of picture, preserved by nature, of the former and less modified
condition of the species. This view may be true, and yet may never
be capable of proof. Seeing, for instance, that the oldest known
mammals, reptiles, and fishes strictly belong to their proper classes,
though some of these old forms are in a slight degree less distinct
from each other than are the typical members of the same groups at
the present day, it would be vain to look for animals having the
common embryological character of the Vertebrata, until beds rich
in fossils are discovered far beneath the lowest Cambrian strata — a
discovery of which the chance is small.

ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME
AREAS, DURING THE LATER TERTIARY PERIODS

Mr. Clift many years ago showed that the fossil mammals from
the Australian caves were closely allied to the living marsupials of
that continent. In South America, a similar relationship is mani-
fest, even to an uneducated eye, in the gigantic pieces of armour,
like those of the armadillo, found in several parts of La Plata; and
Professor Owen has shown in the most striking manner that most
of the fossil mammals, buried there in such numbers, are related to
South American types. This relationship is even more clearly seen
in the wonderful collection of fossil bones made by MM. Lund and
Clausen in the caves of Brazil. I was so much impressed with these
facts that I strongly insisted, in 1H39 and 1845, on this "law of the
succession of types," — on "this wonderful relationship in the same
continent between the dead and the living." Professor Owen has
subsequently extended the same generalisation to the mammals of
the Old World. We see the same law in this author's restorations of
the extinct and gigantic birds of New Zealand. We see it also in the
birds of the caves of Brazil. Mr. Woodward has shown that the
same law holds good with sea shells, but, from the wide distribution
of most molluscs, it is not well displayed by them. Other cases could
be added, as the relation between the extinct and living land shells

SUCCESSION OF SAME TYPES 373

of Madeira; and between the extinct and living brackish water
shells of the Aralo-Caspian Sea.

Now what does this remarkable law of the succession of the same
types within the same areas mean ? He would be a bold man who,
after comparing the present climate of Australia and of parts of
South America, under the same latitude, would attempt to account,
on the one hand through dissimilar physical conditions, for the dis-
similarity of the inhabitants of these two continents; and, on the
other hand through similarity of conditions, for the uniformity of
the same types in each continent during the later tertiary periods.
Nor can it be pretended that it is an immutable law that marsupials
should have been chiefly or solely produced in Australia; or that
Edentata and other American types should have been solely produced
in South America. For we know that Europe in ancient times was
peopled by numerous marsupials; and I have shown in the publica-
tions above alluded to, that in America the law of distribution of
terrestrial mammals was formerly different from what it now is.
North America formerly partook strongly of the present character
of the southern half of the continent; and the southern half was
formerly more closely allied, than it is at present, to the northern
half. In a similar manner we know, from Falconer and Cautley's
discoveries, that Northern India was formerly more closely related in
its mammals to Africa than it is at the present time. Analogous
facts could be given in relation to the distribution of marine
animals.

On the theory of descent with modification, the great law of the
long enduring, but not immutable, succession of the same types
within the same areas, is at once explained; for the inhabitants of
each quarter of the world will obviously tend to leave in that quarter,
during the next succeeding period of time, closely allied though in
some degree modified descendants. If the inhabitants of one conti-
nent formerly differed greatly from those of another continent, so
will their modified descendants still differ in nearly the same manner
and degree. But after very long intervals of time, and after great
geographical changes, permitting much intermigration, the feebler
will yield to the more dominant forms, and there will be nothing
immutable in the distribution of organic beings.

374 ORIGIN OF SPECIES

It may be asked in ridicule, whether I suppose that the mega-
therium and other allied huge monsters, which formerly lived in
South America, have left behind them the sloth, armadillo, and
anteater, as their degenerate descendants. This cannot for an instant
be admitted. These huge animals have become wholly extinct, and
have left no progeny. But in the caves of Brazil, there are many
extinct species which are closely allied in size and in all other char-
acters to the species still living in South America; and some of these
fossils may have been the actual progenitors of the living species.

It must not be forgotten that, on our theory, all the species of the
same genus are the descendants of some one species; so that, if six
genera, each having eight species, be found in one geological forma-
tion, and in a succeeding formation there be six other allied or repre-
sentative genera each with the same number of species, then we may
conclude that generally only one species of each of the older genera
has left modified descendants, which constitute the new genera con-
taining the several species; the other seven species of each old genus
having died out and left no progeny. Or, and this will be a far
commoner case, two or three species in two or three alone of the six
older genera will be the parents of the new genera : the other species
and the other old genera having become utterly extinct. In failing
orders, with the genera and species decreasing in numbers as is the
case with the Edentata of South America, still fewer genera and
species will leave modified blood-descendants.

SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS

I have attempted to show that the geological record is extremely
imperfect; that only a small portion of the globe has been geo-
logically explored with care; that only certain classes of organic
beings have been largely preserved in a fossil state; that the number
both of specimens and of species, preserved in our museums, is abso-
lutely as nothing compared with the number of generations which
must have passed away even during a single formation; that, owing
to subsidence being almost necessary for the accumulation of deposits
rich in fossil species of many kinds, and thick enough to outlast
future degradation, great intervals of time must have elapsed between
most of our successive formations; that there has probably been more

SUMMARY 375

extinction during the periods of subsidence, and more variation
during the periods of elevation, and during the latter the record will
have been least perfectly kept; that each single formation has not
been continuously deposited; that the duration of each formation is
probably short compared with the average duration of specific forms;
that migration has played an important part in the first appearance
of new forms in any one area and formation; that widely ranging
species are those which have varied most frequently, and have often-
est given rise to new species; that varieties have at first been local;
and lastly, although each species must have passed through numer-
ous transitional stages, it is probable that the periods, during which
each underwent modification, though many and long as measured
by years, have been short in comparison with the periods during
which each remained in an unchanged condition. These causes,
taken conjointly, will to a large extent explain why — though we do
find many links — we do not find interminable varieties, connecting
together all extinct and existing forms by the finest graduated steps.
It should also be constantly borne in mind that any linking variety
between two forms, which might be found, would be ranked, unless
the whole chain could be perfectly restored, as a new and distinct
species; for it is not pretended that we have any sure criterion by
which species and varieties can be discriminated.

He who rejects this view of the imperfection of the geological
record, will rightly reject the whole theory. For he may ask in vain
where are the numberless transitional links which must formerly
have connected the closely allied or representative species, found in
the successive stages of the same great formation? He may dis-
believe in the immense intervals of time which must have elapsed
between our consecutive formations; he may overlook how impor-
tant a part migration has played, when the formations of any one
great region, as those of Europe, are considered; he may urge the
apparent, but often falsely apparent, sudden coming in of whole
groups of sjjecies. He may ask where are the remains of those
infinitely numerous organisms which must have existed long before
the Cambrian system was deposited? We now know that at least
one animal did then exist; but I can answer this last question only
by supposing that where our oceans now extend they have extended

376 ORIGIN OF SPECIES

for an enormous period, and where our oscillating continents now
stand they have stood since the commencement of the Cambrian
system; but that, long before that epoch, the world presented a
widely different aspect; and that the older continents, formed of
formations older than any known to us, exist now only as remnants
in a metamorphosed condition, or Ue still buried under the ocean.

Passing from these difficulties, the other great leading facts in
paljcontology agree admirably with the theory of descent with modi-
fication through variation and natural selection. We can thus under-
stand how it is that new species come in slowly and successively; how
species of different classes do not necessarily change together, or at
the same rate, or in the same degree; yet in the long run that all
undergo modification to some extent. The extinction of old forms is
the almost inevitable consequence of the production of new forms.
We can understand why, when a species has once disappeared, it
never reappears. Groups of species increase in numbers slowly, and
endure for unequal periods of time; for the process of modification is
necessarily slow, and depends on many complex contingencies. The
dominant species belonging to large and dominant groups tend to
leave many modified descendants, which form new sub-groups and
groups. As these are formed, the species of the less vigorous groups,
from their inferiority inherited from a common progenitor, tend to
become extinct together, and to leave no modified offspring on the
face of the earth. But the utter extinction of a whole group of
species has sometimes been a slow process, from the survival of a
few descendants, lingering in protected and isolated situations.
When a group has once wholly disappeared, it does not reappear;
for the link of generation has been broken.

We can understand how it is that dominant forms which spread
widely and yield the greatest number of varieties tend to people the
world with allied, but modified, descendants; and these will gener-
ally succeed in displacing the groups which are their inferiors in the
struggle for existence. Hence, after long intervals of time, the pro-
ductions of the world appear to have changed simultaneously.

We can understand how it is that all the forms of life, ancient and
recent, make together a few grand classes. We can understand,
from the continued tendency to divergence of character, why, the

SUMMARY 377

more ancient a form is, the more it generally differs from those now
living; why ancient and extinct forms often tend to fill up gaps
between existing forms, sometimes blending two groups, previously
classed as distinct, into one; but more commonly bringing them only
a little closer together. The more ancient a form is, the more often
it stands in some degree intermediate between groups now distinct;
for the more ancient a form is, the more nearly it will be related to,
and consequently resemble, the common progenitor of groups, since
become widely divergent. Extinct forms are seldom directly inter-
mediate between existing forms; but are intermediate only by a long
and circuitous course through other extinct and different forms. We
can clearly see why the organic remains of closely consecutive forma-
tions are closely allied; for they are closely linked together by genera-
tion. We can clearly see why the remains of an intermediate
formation are intermediate in character.

The inhabitants of the world at each successive period in its history
have beaten their predecessors in the race for life, and are, in so far,
higher in the scale, and their structure has generally become more
specialised; and this may account for the common belief held by so
many palaeontologists, that organisation on the whole has progressed.
Extinct and ancient animals resemble to a certain extent the embryos
of the more recent animals belonging to the same classes, and this
wonderful fact receives a simple explanation according to our views.
The succession of the same types of structure within the same areas
during the later geological periods ceases to be mysterious, and is
intelligible on the principle of inheritance.

If, then, the geological record be as imperfect as many believe,
and it may at least be asserted that the record cannot be proved to
be much more perfect, the main objections to the theory of natural
selection are greatly diminished or disappear. On the other hand,
all the chief laws of palaeontology plainly proclaim, as it seems to
me, that species have been produced by ordinary generation: old
forms having been supplanted by new and improved forms of life,
the products of Variation and the Survival of the Fittest.

CHAPTER XII

Geographical Distribution

Present distribution cannot be accounted for by differences in physical
conditions — Importance of barriers — Affinity of the productions of
the same continent — Centres of creation — Means of dispersal, by
changes of chmate and of the level of the land, and by occasional
means — Dispersal during the Glacial period — Alternate Glacial
periods in the North and South.

IN considering the distribution of organic beings over the face
of the globe, the first great fact which strikes us is, that neither
the similarity nor the dissimilarity of the inhabitants of various
regions can be wholly accounted for by climatal and other physical
conditions. Of late, almost every author who has studied the subject
has come to this conclusion. The case of America alone would
almost suffice to prove its truth; for if we exclude the arctic and
northern temperate parts, all authors agree that one of the most
fundamental divisions in geographical distribution is that between
the New and Old Worlds; yet if we travel over the vast American
continent, from the central parts of the United States to its extreme
southern point, we meet with the most diversified conditions; humid
districts, arid deserts, lofty mountains, grassy plains, forests, marshes,
lakes, and great rivers, under almost every temperature. There is
hardly a climate or condition in the Old World which cannot be
paralleled in the New — at least as closely as the same species generally
require. No doubt small areas can be pointed out in the Old World
hotter than any in the New World; but these are not inhabited by a
fauna different from that of the surrounding districts; for it is rare
to find a group of organisms confined to a small area, of which the
conditions are peculiar in only a slight degree. Notwithstanding
this general parallelism in the conditions of the Old and New
Worlds, how widely different are their living productions!

In the southern hemisphere, if we compare large tracts of land in
Australia, South Africa, and western South America, between latl-

378

GEOGRAPHICAL DISTRIBUTION 379

tudes 25° and 35^, we shall find parts extremely similar in all their
conditions, yet it would not be possible to point out three faunas
and floras more utterly dissimilar. Or, again, we may compare the
productions of South America south of latitude 35^ with those north
of 25 , which consequently are separated by a space of ten degrees of
latitude, and are exposed to considerably different conditions; yet
they are incomparably more closely related to each other than they
are to the productions of Australia or Africa under nearly the same
climate. Analogous facts could be given with respect to the inhabi-
tants of the sea.

A second great fact which strikes us in our general review is, that
barriers of any kind, or obstacles to free migration, are related in a
close and important manner to the differences between the produc-
tions of various regions. We see this in the great differences in nearly
all the terrestrial produaions of the New and Old Worlds, excepting
in the northern parts, where the land almost joins, and where, under
a slightly different climate, there might have been free migration
for the northern temperate forms, as there now is for the strirtly
arctic productions. We see the same fact in the great difference be-
tween the inhabitants of Australia, Africa, and South America under
the same latitude; for these countries are almost as much isolated
from each other as is possible. On each continent, also, we see the
same fact; for on the opposite sides of lofty and continuous moun-
tain-ranges, of great deserts and even of large rivers, we find different
productions; though as mountain-chains, deserts, etc., are not as
impassable, or likely to have endured so long, as the oceans separat-
ing continents, the differences are very inferior in degree to those
characteristic of distinct continents.

Turning to the sea, we find the same law. The marine inhabitants
of the eastern and western shores of South America are very distinct,
with extremely few shells, Crustacea, or echinodermata in common;
but Dr. Giinther has recently shown that about thirty per cent, of the
fishes are the same on the opposite sides of the isthmus of Panama;
and this fact has led naturalists to believe that the isthmus was for-
merly open. Westward of the shores of America, a wide space of
open ocean extends, with not an island as a halting-place for emi-
grants; here we have a barrier of another kind, and as soon as this

380 ORIGIN OF SPECIES

is passed we meet in the eastern islands of the Pacific with another
and totally distinct fauna. So that three marine faunas range far
northward and southward in parallel lines not far from each other,
under corresponding climates; but from being separated from each
other by impassable barriers, either of land or open sea, they are
almost wholly distinct. On the other hand, proceeding still farther
westward from the eastern islands of the tropical parts of the Pacific,
we encounter no impassable barriers, and we have innumerable
islands as halting-places, or continuous coasts, until, after travelling
over a hemisphere, we come to the shores of Africa; and over this
vast space we meet with no well-defined and distinct marine faunas.
Although so few marine animals are common to the above-named
three approximate faunas of Eastern and Western America and the
eastern Pacific islands, yet many fishes range from the Pacific into
the Indian Ocean, and many shells are common to the eastern islands
of the Pacific and the eastern shores of Africa on almost exactly
opposite meridians of longitude.

A third great fact, partly included in the foregoing statement, is
the affinity of the productions of the same continent or of the same
sea, though the species themselves are distinct at different points and
stations. It is a law of the widest generality, and every continent
offers innumerable instances. Nevertheless, the naturalist, in travel-
ling, for instance, from north to south, never fails to be struck by
the manner in which successive groups of beings, specifically distinct,
though nearly related, replace each other. He hears from closely
allied, yet distinct kinds of birds, notes nearly similar, and sees their
nests similarly constructed, but not quite alike, with eggs coloured
in nearly the same manner. The plains near the Straits of Magellan
are inhabited by one species of Rhea (American ostrich), and north-
ward the plains of La Plata by another species of the same genus;
and not by a true ostrich or emu, like those inhabiting Africa and
Australia under the same latitude. On these same plains of La Plata
we see the agouti and bizcacha, animals having nearly the same
habits as our hares and rabbits, and belonging to the same order of
rodents, but they plainly display an American type of structure.
We ascend the lofty peaks of the Cordillera, and we find an alpine

GEOGRAPHICAL DISTRIBUTION 38 1

species of bizcacha; we look to the waters, and we do not find the
beaver or musk-rat, but the coypu and capybara, rodents of the South
American type. Innumerable other instances could be given. If
we look to the islands off the American shore, however much they
may differ in geological structure, the inhabitants are essentially
American, though they may be all peculiar species. We may look
back to past ages, as shown in the last chapter, and we find Ameri-
can types then prevailing on the American continent and in the
American seas. We see in these facts some deep organic bond,
throughout space and time, over the same areas of land and water,
independently of physical conditions. The naturalist must be dull
who is not led to inquire what this bond is.

The bond is simply inheritance, that cause which alone, as far as
we positively know, produces organisms quite like each other, or, as
we see in the case of varieties, nearly alike. The dissimilarity of the
inhabitants of different regions may be attributed to modification
through variation and natural selection, and probably in a sub-
ordinate degree to the definite influence of different physical con-
ditions. The degrees of dissimilarity will depend on the migration
of the more dominant forms of life from one region into another
having been more or less effectually prevented, at periods more or
less remote; — on the nature and number of the former immigrants;
— and on the action of the inhabitants on each other in leading to the
preservation of different modifications; the relation of organism to
organism in the struggle for life being, as I have already often re-
marked, the most important of all relations. Thus the high im-
portance of barriers comes into play by checking migration; as does
time for the slow process of modification through natural selection.
Widely ranging species, abounding in individuals, which have
already triumphed over many competitors in their own widely ex-
tended homes, will have the best chance of seizing on new places,
when they spread out into new countries. In their new homes they
will be exposed to new conditions, and will frequently undergo
further modification and improvement; and thus they will become
still further victorious, and will produce groups of modified descend-
ants. On this principle of inheritance with modification we can

382 ORIGIN OF SPECIES

understand how it is that sections of genera, whole genera, and even
families, are confined to the same areas, as is so commonly and
notoriously the case.

There is no evidence, as was remarked in the last chapter, of the
existence of any law of necessary development. As the variability
of each species is an independent property, and will be taken ad-
vantage of by natural selection, only so far as it profits each indi-
vidual in its complex struggle for hfe, so the amount of modifica-
tion in different species will be no uniform quantity. If a number
of species, after having long competed with each other in their old
home, were to migrate in a body into a new and afterwards isolated
country, they would be little liable to modification; for neither
migration nor isolation in themselves effect any thing. These prin-
ciples come into play only by bringing organisms into new relations
with each other and in a lesser degree with the surrounding physical
conditions. As we have seen in the last chapter that some forms
have retained nearly the same character from an enormously remote
geological (period, so certain species have migrated over vast spaces,
and have not become greatly or at all modified.

According to these views, it is obvious that the several species of
the same genus, though inhabiting the most distant quarters of the
world, must originally have proceeded from the same source, as they
are descended from the same progenitor. In the case of those species
which have undergone during whole geological periods little modifi-
cation, there is not much difficulty in believing that they have mi-
grated from the same region; for during the vast geographical and
climatal changes which have supwrvened since ancient times, almost
any amount of migration is possible. But in many other cases, in
which we have reason to believe that the species of a genus have
been produced within comparatively recent times, there is great diffi-
culty on this head. It is also obvious that the individuals of the same
species, though now inhabiting distant and isolated regions, must
have proceeded from one spot, where their parents were first pro-
duced: for, as has been explained, it is incredible that individuals
identically the same should have been produced from parents spe-
cifically distinct.

CENTRES OF SUPPOSED CREATION 383

SINGLE CENTRES OF SUPPOSED CREATION

We are thus brought to the question which has been largely dis-
cussed by naturalists, namely, whether species have been created at
one or more points of the earth's surface. Undoubtedly there are
many cases of extreme difficulty in understanding how the same
species could possibly have migrated from some one point to the
several distant and isolated points, where now found. Nevertheless
the simplicity of the view that each species was first produced within
a single region captivates the mind. He who rejects it, rejects the vera
causa of ordinary generation with subsequent migration, and calls
in the agency of a miracle. It is universally admitted, that in most
cases the area inhabited by a species is continuous; and that when
a plant or animal inhabits two pxiints so distant from each other,
or with an interval of such a nature, that the space could not have
been easily passed over by migration, the fact is given as something
remarkable and exceptional. The incapacity of migrating across a
wide sea is more clear in the case of terrestrial mammals than per-
haps with any other organic beings; and, accordingly, we find no
inexplicable instances of the same mammals inhabiting distant
points of the world. No geologist feels any difficulty in Great Britain
possessing the same quadrupeds with the rest of Europe, for they
were no doubt once united. But if the same species can be pro-
duced at two separate points, why do we not find a single mammal
common to Europe and Australia or South America? The condi-
tions of life are nearly the same, so that a multitude of European
animals and plants have become naturalised in America and Aus-
traUa; and some of the aboriginal plants are identically the same at
these distant points of the northern and southern hemispheres. The
answer, as I believe, is, that mammals have not been able to mi-
grate, whereas some plants, from their varied means of dispersal,
have migrated across the wide and broken interspaces. The great
and striking influence of barriers of all kinds, is intelligible only on
the view that the great majority of species have been produced on
one side, and have not been able to migrate to the opposite side.
Some few families, many sub-families, very many genera, and a still
greater number of sections of genera, are confined to a single region;

384 ORIGIN OF SPECIES

and it has been observed by several naturalists that the most natural
genera, or those genera in which the species are most closely related
to each other, are generally confined to the same country, or if they
have a wide range that their range is continuous. What a strange
anomaly it would be, if a directly opposite rule were to prevail,
when we go down one step lower in the series, namely, to the indi-
viduals of the same species, and these had not been, at least at first,
confined to some one region!

Hence it seems to me, as it has to many other naturalists, that the
view of each species having been produced in one area alone, and
having subsequently migrated from that area as far as its powers
of migration and subsistence under past and present conditions f)er-
mitted, is the most probable. Undoubtedly many cases occur, in
which we cannot explain how the same species could have passed
from one point to the other. But the geographical and climatal
changes which have certainly occurred within recent geological
times, must have rendered discontinuous the formerly continuous
range of many species. So that we are reduced to consider whether
the exceptions to continuity of range are so numerous and of so
grave a nature, that we ought to give up the belief, rendered prob-
able by general considerations, that each species has been produced
within one area, and has migrated thence as far as it could. It would
be hopelessly tedious to discuss all the exceptional cases of the same
species, now living at distant and separated points, nor do I for a
moment pretend that any explanation could be offered of many in-
stances. But, after some preliminary remarks, I will discuss a few of
the most striking classes of facts; namely, the existence of the same
species on the summits of distant mountain ranges, and at distant
points in the arctic and antarctic regions; and secondly (in the
following chapter), the wide distribution of fresh-water produc-
tions; and thirdly, the occurrence of the same terrestrial species on
islands and on the nearest mainland, though separated by hundreds
of miles of open sea. If the existence of the same species at distant
and isolated points of the earth's surface, can in many instances be
explained on the view of each species having migrated from a single
birthplace; then, considering our ignorance with respect to former
climatal and geographical changes and to the various occasional

CENTRES OF SUPPOSED CREATION 385

means of transport, the belief that a single birthplace is the law,
seems to me incomparably the safest.

In discussing this subject, we shall be enabled at the same time
to consider a point equally important for us, namely, whether the
several species of a genus which must on our theory all be descended
from a common progenitor, can have migrated, undergoing modi-
fication during their migration, from some one area. If, when most
of the species inhabiting one region are different from those of
another region, though closely allied to them, it can be shown that
migration from the one region to the other has probably occurred at
some former period, our general view will be much strengthened;
for the explanation is obvious on the principle of descent with modi-
fication. A volcanic island, for instance, upheaved and formed at
the distance of a few hundreds of miles from a continent, would
probably receive from it in the course of time a few colonists, and
their descendants, though modified, would still be related by in-
heritance to the inhabitants of that continent. Cases of this nature
are common, and are, as we shall hereafter see, inexplicable on the
theory of independent creation. This view of the relation of the
species of one region to those of another, does not differ much from
that advanced by Mr. Wallace, who concludes that "every species
has come into existence coincident both in space and time with a
preexisting closely allied species." And it is now well known that
he attributes this coincidence to descent with modification.

The question of single or multiple centres of creation differs from
another though allied question, — namely, whether all the indi-
viduals of the same species are descended from a single pair, or
single hermaphrodite, or whether, as some authors suppose, from
many individuals simultaneously created. With organic beings which
never intercross, if such exist, each species must be descended from
a succession of modified varieties, that have supplanted each other,
but have never blended with other individuals or varieties of the
same species; so that, at each successive stage of modification, all
the individuals of the same form will be descended from a single
parent. But in the great majority of cases, namely, with all organ-
isms which habitually unite for each birth, or which occasionally
iiKercross, the individuals of the same species inhabiting the same

386 ORIGIN OF SPECIES

area will be kept nearly uniform by intercrossing; so that many indi-
viduals will go on simultaneously changing, and the whole amount
of modification at each stage will not be due to descent from a single
parent. To illustrate what I mean: our English race-horses differ
from the horses of every other breed; but they do not owe their
difference and superiority to descent from any single pair, but to
continued care in the selecting and training of many individuals
during each generation.

Before discussing the three classes of facts, which I have selected
as presenting the greatest amount of difficulty on the theory of
"single centres of creation," I must say a few words on the means
of dispersal.

MEAN'S OF DISPERSAL

Sir C. Lyell and other authors have ably treated this subject. I
can give here only the briefest abstract of the more important facts.
Change of climate must have had a powerful influence on migration.
A region now impassable to certain organisms from the nature of
its climate, might have been a high road for migration, when the
climate was different. I shall, however, presently have to discuss
this branch of the subject in some detail. Changes of level in the land
must also have been highly influential: a narrow isthmus now
separates two marine faunas; submerge it, or let it formerly have been
submerged, and the two faunas will now blend together, or may
formerly have blended. Where the sea now extends, land may at a
former period have connected islands or possibly even continents
together, and thus have allowed terrestrial productions to pass from
one to the other. No geologist disputes that great mutations of level
have occurred within the period of existing organisms. Edward
Forbes insisted that all the islands in the Atlantic must have been
recently connected with Europe or Africa, and Europe likewise with
America. Other authors have thus hypothetically bridged over every
ocean, and united almost every island with some mainland. If indeed
the arguments used by Forbes are to be trusted, it must be admitted
that scarcely a single island exists which has not recently been united
to some continent. This view cuts the Gordian knot of the dispersal
of the same species to the most distant points, and removes many a

MEANS OF DISPERSAL 387

difficulty; but to the best of my judgment we are not authorised in
admitting such enormous geographical changes within the period of
existing species. It seems to me that we have abundant evidence of
great oscillations in the level of the land or sea; but not of such vast
changes in the position and extension of our continents, as to have
united them within the recent period to each other and to the several
intervening oceanic islands. I freely admit the former existence of
many islands, now buried beneath the sea, which may have served
as halting-places for plants and for many animals during their migra-
tion. In the coral-producing oceans such sunken islands are now
marked by rings of coral or atolls standing over them. Whenever it
is fully admitted, as it will some day be, that each species has pro-
ceeded from a single birthplace, and when in the course of time we
know something definite about the means of distribution, we shall
be enabled to speculate with security on the former extension of the
land. But 1 do not believe that it will ever be proved that within the
recent period most of our continents which now stand quite separate,
have been continuously, or almost continuously united with each
other, and with the many existing oceanic islands. Several facts in
distribution — such as the great difference in the marine faunas on
the opposite sides of almost every continent, — the close relation of
the tertiary inhabitants of several lands and even seas to their present
inhabitants, — the degree of affinity between the mammals inhabiting
islands with those of the nearest continent, being in part determined
(as we shall hereafter see) by the depth of the intervening ocean, —
these and other such facts are opposed to the admission of such pro-
digious geographical revolutions within the recent period, as are
necessary on the view advanced by Forbes and admitted by his
followers. The nature and relative proportions of the inhabitants of
oceanic islands are likewise opposed to the beUef of their former
continuity with continents. Nor does the almost universally volcanic
composition of such islands favour the admission that they are the
wrecks of sunken continents; — if they had originally existed as con-
tinental mountain ranges, some at least of the islands would have
been formed, like other mountain summits, of granite, metamorphic
schists, old fossiliferous and other rocks, instead of consisting of
mere piles of volcanic matter.

388 ORIGIN OF SPECIES

I must now say a few words on what are called accidental means,
but which more properly should be called occasional means of dis-
tribution. I shall here confine myself to plants. In botanical works,
this or that plant is often stated to be ill adapted for wide dissemi-
nation; but the greater or less facilities for transport across the sea
may be said to be almost wholly unknown. Until I tried, with Mr.
Berkeley's aid, a few experiments, it was not even known how far
seeds could resist the injurious action of sea water. To my surprise
I found that out of eighty-seven kinds, sixty-four germinated after
an immersion of twenty-eight days, and a few survived an immer-
sion of 137 days. It deserves notice that certain orders were far more
injured than others: nine Leguminosae were tried, and, with one
exception, they resisted the salt water badly; seven species of the
allied orders, Hydrophyllacese and Polemoniacex, were all killed by
a month's immersion. For convenience' sake I chiefly tried small
seeds without the capsule or fruit; and as all of these sank in a few
days they could not have been floated across wide spaces of the sea,
whether or not they were injured by the salt water. Afterwards I
tried some larger fruits, capsules, etc., and some of these floated for
a long time. It is well known what a difference there is in the buoy-
ancy of green and seasoned timber; and it occurred to me that floods
would often wash into the sea dried plants or branches with seed-
capsules or fruit attached to them. Hence I was led to dry the stems
and branches of ninety-four plants with ripe fruit, and to place them
on sea water. The majority sank quickly, but some which, whilst
green, floated for a very short time, when dried floated much longer;
for instance, ripe hazelnuts sank immediately, but when dried they
floated for ninety days, and afterwards when planted germinated;
an asparagus-plant with ripe berries floated for twenty-three days,
when dried it floated for eighty-five days, and the seeds afterwards
germinated; the ripe seeds of Helosciadium sank in two days, when
dried they floated for above ninety days, and afterwards germinated.
Altogether, out of the ninety-four dried plants, eighteen floated for
above twenty-eight days; and some of the eighteen floated for a very
much longer period. So that as f4 kinds of seeds germinated after
an immersion of twenty-eight days; and as H distinct species with
ripe fruit (but not all the same species as in the foregoing experi-

MEANS OF DISPERSAL 389

ment) floated, after being dried, for above twenty-eight days, we
may conclude, as far as anything can be inferred from these scanty
facts, that the seeds of ^Vir kinds of plants of any country might be
floated by sea currents during twenty-eight days, and would retain
their power of germination. In Johnston's Physical Atlas, the average
rate of the several Atlantic currents is thirty-three miles per diem
(some currents running at the rate of sixty miles per diem) ; on this
average, the seeds of iVir plants belonging to one country might be
floated across 924 miles of sea to another country, and when stranded,
if blown by an inland gale to a favourable spot, would germinate.

Subsequently to my experiments, M. Martens tried similar ones,
but in a much better manner, for he placed the seeds in a box in the
actual sea, so that they were alternately wet and exposed to the air
like really floating plants. He tried ninety-eight seeds, mostly dif-
ferent from mine; but he chose many large fruits and likewise seeds
from plants which live near the sea; and this would have favoured
both the average length of their flotation and their resistance to the
injurious action of the salt water. On the other hand, he did not
previously dry the plants or branches with the fruit; and this, as we
have seen, would have caused some of them to have floated much
longer. The result was that if of his seeds of different kinds floated
for forty-two days, and were then capable of germination. But I do
not doubt that plants exposed to the waves would float for a less
time than those protected from violent movement as in our experi-
ments. Therefore it would pwrhaps be safer to assume that the seeds
of about iV^ plants of a flora, after having been dried, could be
floated across a space of sea 900 miles in width, and would then
germinate. The fact of the larger fruits often floating longer than
the small, is interesting; as plants with large seeds or fruit which, as
Alph. de Candolle has shown, generally have restricted ranges, could
hardly be transported by any other means.

Seeds may be occasionally transported in another manner. Drift
timber is thrown up on most islands, even on those in the midst of
the widest oceans; and the natives of the coral islands in the Pacific
procure stones for their tools, solely from the roots of drifted trees,
these stones being a valuable royal tax. I find that when irregularly
shaped stones are embedded in the roots of trees, small parcels of

390 ORIGIN OF SPECIES

earth are frequently enclosed in their interstices and behind them, —
so perfectly that not a particle could be washed away during the
longest transport: out of one small portion of earth thus completely
enclosed by the roots of an oak about 50 years old, three dicoty-
ledonous plants germinated; I am certain of the accuracy of this
observation. Again, I can show that the carcases of birds, when
floating on the sea, sometimes escape being immediately devoured:
and many kinds of seeds in the crops of floating birds long retain
their vitality: peas and vetches, for instance, are killed by even a
few days' immersion in sea water; but some taken out of the crop
of a pigeon, which had floated on artificial sea water for thirty days,
to my surprise nearly all germinated.

Living birds can hardly fail to be highly effective agents in the
transportation of seeds. 1 could give many facts showing how
frequently birds of many kinds are blown by gales to vast distances
across the ocean. We may safely assume that under such circum-
stances their rate of flight would often be thirty-five miles an hour;
and some authors have given a far higher estimate. I have never
seen an instance of nutritious seeds passing through the intestines
of a bird; but hard seeds of fruit pass uninjured through even the
digestive organs of a turkey. In the course of two months, I picked
up in my garden twelve kinds of seeds, out of the excrement of small
birds, and these seemed perfect, and some of them, which were tried,
germinated. But the following fact is more important: the crops
of birds do not secrete gastric juice, and do not, as I know by trial,
injure in the least the germination of seeds; now, after a bird has
found and devoured a large supply of food, it is positively asserted
that all the grains do not pass into the gizzard for twelve or even
eighteen hours. A bird in this interval might easily be blown to the
distance of five hundred miles, and hawks are known to look out for
tired birds, and the contents of their torn crops might thus readily
get scattered. Some hawks and owls bolt their prey whole, and, after
an interval of from twelve to twenty hours, disgorge pellets, which,
as I know from experiments made in the Zoological Gardens, include
seeds capable of germination. Some seeds of the oat, wheat, millet,
canary, hemp, clover, and beet germinated after having been from
twelve to twenty-one hours in the stomachs of different birds of

MEANS OF DISPERSAL 39 1

prey; and two seeds of beet grew after having been thus retained for
two days and fourteen hours. Fresh-water fish, I find, eat seeds of
many land and water plants; fish are frequently devoured by birds,
and thus the seeds might be transported from place to place. I forced
many kinds of seeds into the stomachs of dead fish, and then gave
their bodies to fishing-eagles, storks, and pelicans; these birds, after
an interval of many hours, either rejected the seeds in pellets or
passed them in their excrement; and several of these seeds retained
the power of germination. Certain seeds, however, were always
killed by this process.

Locusts are sometimes blown to great distances from the land; I
myself caught one 370 miles from the coast of Africa, and have
heard of others caught at greater distances. The Rev. R. T. Lowe
informed Sir C. Lyell that in November 1844 swarms of locusts
visited the island of Madeira. They were in countless numbers, as
thick as the flakes of snow in the heaviest snowstorm, and extended
upwards as far as could be seen with a telescope. During two or three
days they slowly careered round and round in an immense ellipse,
at least five or six miles in diameter, and at night alighted on the
taller trees, which were completely coated with them. They then
disappeared over the sea, as suddenly as they had appeared, and have
not since visited the island. Now, in parts of Natal it is believed
by some farmers, though on insufficient evidence, that injurious
seeds are introduced into their grassland in the dung left by the
great flights of locusts which often visit that country. In consequence
of this belief Mr. Weale sent me in a letter a small packet of the dried
pellets, out of which I extracted under the microscope several seeds,
and raised from them seven grass plants, belonging to two species,
of two genera. Hence a swarm of locusts, such as that which visited
Madeira, might readily be the means of introducing several kinds of
plants into an island lying far from the mainland.

Although the beaks and feet of birds are generally clean, earth
sometimes adheres to them: in one case I removed sixty-one grains,
and in another case twenty-two grains of dry argillaceous earth from
the foot of a partridge, and in the earth there was a pebble as large
as the seed of a vetch. Here is a better case: the leg of a woodcock
was sent to me by a friend, with a little cake of dry earth attached to

392 ORIGIN OF SPECIES

the shank, weighing only nine grains; and this contained a seed of
the toad-rush (Juncus bufonius) which germinated and flowered.
Mr. Swaysland, of Brighton, who during the last forty years has paid
close attention to our migratory birds, informs me that he has often
shot wagtails (Motacillae), wheatears, and whincats (Saxicolx), on
their first arrival on our shores, before they had alighted; and he has
several times noticed litde cakes of earth attached to their feet. Many
facts could be given showing how generally soil is charged with
seeds. For instance. Prof. Newton sent me the leg of a red-legged
partridge (Caccabis rufa) which had been wounded and could not
fly, with a ball of hard earth adhering to it, and weighing six and a
half ounces. The earth had been kept for three years, but when
broken, watered and placed under a bell-glass, no less than eighty-
two plants sprung from it: these consisted of twelve monocoty-
ledons, including the common oat, and at least one kind of grass,
and of seventy dicotyledons, which consisted, judging from the
young leaves, of at least three distinct species. With such facts before
us, can we doubt that the many birds which are annually blown by
gales across great spaces of ocean, and which annually migrate — for
instance, the millions of quails across the Mediterranean — must
occasionally transport a few seeds embedded in dirt adhering to
their feet or beaks? But I shall have to recur to this subject.

As icebergs are known to be sometimes loaded with earth and
stones, and have even carried brushwood, bones, and the nest of a
land-bird, it can hardly be doubted that they must occasionally, as
suggested by Lyell, have transported seeds from one part to another
of the arctic and antarctic regions; and during the Glacial period
from one part of the now temperate regions to another. In the
Azores, from the large number of plants common to Europe, in
comparison with the species on the other islands of the Atlantic,
which stand nearer to the mainland, and (as remarked by Mr. H. C.
Watson) from their somewhat northern character in comparison
with the latitude, I suspected that these islands had been partly
stocked by ice-borne seeds, during the Glacial epoch. At my request
Sir C. Lyell wrote to M. Hartung to inquire whether he had observed
erratic boulders on these islands, and he answered that he had found
large fragments of granite and other rocks, which do not occur in

MEANS OF DISPERSAL 393

the archipelago. Hence we may safely infer that icebergs formerly
landed their rocky burthens on the shores of these mid-ocean islands,
and it is at least possible that they may have brought thither some
few seeds of northern plants.

Considering that these several means of transport, and that other
means, which without doubt remain to be discovered, have been in
action year after year for tens of thousands of years, it would, I
think, be a marvellous fact if many plants had not thus become
widely transported. These means of transport are sometimes called
accidental, but this is not strictly correct: the currents of the sea are
not accidental, nor is the direction of prevalent gales of wind. It
should be observed that scarcely any means of transport would carry
seeds for very great distances: for seeds do not retain their vitality
when exposed for a great length of time to the action of sea water;
nor could they be long carried in the crops or intestines of birds.
These means, however, would suffice for occasional transport across
tracts of sea some hundred miles in breadth, or from island to island,
or from a continent to a neighbouring island, but not from one
distant continent to another. The floras of distant continents would
not by such means become mingled; but would remain as distinct
as they now are. The currents, from their course, would never bring
seeds from North America to Britain, though they might and do
bring seeds from the West Indies to our western shores, where, if
not killed by their long immersion in salt water, they could not
endure our climate. Almost every year, one or two land birds are
blown across the whole Atlantic Ocean, from North America to
the western shores of Ireland and England; but seeds could be
transported by these rare wanderers only by one means, namely, by
dirt adhering to their feet or beaks, which is in itself a rare accident.
Even in this case, how small would be the chance of a seed falling
on favourable soil, and coming to maturity! But it would be a great
error to argue that because a well-stocked island, like Great Britain,
has not, as far as is known (and it would be very difficult to prove
this), received within the last few centuries, through occasional
means of transport, immigrants from Europe or any other continent,
that a poorly-stocked island, though standing more remote from the
mainland, would not receive colonists by similar means. Out of a

394 ORIGIN OF SPECIES

hundred kinds of seeds or animals transported to an island, even if
far less well-stocked than Britain, perhaps not more than one would
be so well fitted to its new home, as to become naturalised. But this
is no valid argument against what would be effected by occasional
means of transport, during the long lapse of geological time, whilst
the island was being upheaved, and before it had become fully
stocked with inhabitants. On almost bare land, with few or no
destructive insects or birds living there, nearly every seed which
chanced to arrive, if fitted for the climate, would germinate and
survive.

DISPERSAL DURING THE GLACIAL PERIOD

The identity of many plants and animals, on mountain-summits,
separated from each other by hundreds of miles of lowlands, where
Alpine sjjecies could not fxjssibly exist, is one of the most striking
cases known of the same species living at distant points, without the
apparent possibility of their having migrated from one point to the
other. It is indeed a remarkable fact to see so many plants of the
same species living on the snowy regions of the Alps or Pyrenees,
and in the extreme northern parts of Europe; but it is far more
remarkable, that the plants on the White Mountains, in the United
States of America, are all the same with those of Labrador, and
nearly all the same, as we hear from Asa Gray, with those on the
loftiest mountains of Europe. Even as long ago as 1747, such facts
led Gmelin to conclude that the same species must have been in-
dependently created at many distinct points; and we might have
remained in this same belief, had not Agassiz and others called
vivid attention to the Glacial period, which, as we shall immediately
see, affords a simple explanation of these facts. We have evidence
of almost every conceivable kind, organic and inorganic, that, within
a very recent geological period, central Europe and North America
suffered under an arctic climate. The ruins of a house burnt by fire
do not tell their tale more plainly than do the mountains of Scotland
and Wales, with their scored flanks, polished surfaces, and perched
boulders, of the icy streams with which their valleys were lately
filled. So greatly has the climate of Europe changed, that in north-
ern Italy, gigantic moraines, left by old glaciers, are now clothed

DISPERSAL DURING GLACIAL PERIOD 395

by the vine and maize. Throughout a large part of the United
States, erratic boulders and scored rocks plainly reveal a former cold
period.

The former influence of the glacial climate on the distribution
of the inhabitants of Europe, as explained by Edward Forbes, is
substantially as follows. But we shall follow the changes more
readily, by supposing a new glacial period slowly to come on, and
then pass away, as formerly occurred. As the cold came on, and
as each more southern zone became fitted for the inhabitants of
the north, these would take the places of the former inhabitants of
the temperate regions. The latter, at the same time, would travel
further and further southward, unless they were stopped by barriers,
in which case they would perish. The mountains would become
covered with snow and ice, and their former Alpine inhabitants
would descend to the plains. By the time that the cold had reached
its maximum, we should have an arctic fauna and flora, covering
the central parts of Europe, as far south as the Alps and Pyrenees,
and even stretching into Spain. The now temperate regions of the
United States would likewise be covered by arctic plants and animals
and these would be nearly the same with those of Europe; for the
present circumpolar inhabitants, which we suppose to have every-
where travelled southward, are remarkably uniform round the
world.

As the warmth returned, the arctic forms would retreat north-
ward, closely followed up in their retreat by the productions of the
more temperate regions. And as the snow melted from the bases
of the mountains, the arctic forms would seize on the cleared and
thawed ground, always ascending, as the warmth increased and the
snow still further disappeared, higher and higher, whilst their
brethren were pursuing their northern journey. Hence, when the
warmth had fully returned, the same species, which had lately lived
together on the European and North American lowlands, would
again be found in the arctic regions of the Old and New Worlds,
and on many isolated mountain summits far distant from each other.

Thus we can understand the identity of many plants at points so
immensely remote as the mountains of the United States and those
of Europe. We can thus also understand the fact that the Alpine

396 ORIGIN OF SPECIES

plants of each mountain range are more especially related to the
arctic forms living due north or nearly due north of them: for the
first migration when the cold came on, and the re-migration on the
returning warmth, would generally have been due south and north.
The Alpine plants, for example, of Scotland, as remarked by Mr.
H. C. Watson, and those of the Pyrenees, as remarked by Ramond,
are more especially allied to the plants of northern Scandinavia;
those of the United States, to Labrador; those of the mountains of
Siberia to the arctic regions of that country. These views, grounded
as they are on the perfectly well-ascertained occurrence of a former
Glacial period, seem to me to explain in so satisfactory a manner
the present distribution of the Alpine and arctic productions of
Europe and America, that when in other regions we find the same
species on distant mountain summits, we may almost conclude,
without other evidence, that a colder climate formerly permitted
their migration across the intervening lowlands, now become too
warm for their existence.

As the arctic forms moved first southward and afterwards back-
wards to the north, in unison with the changing climate, they will
not have been exposed during their long migrations to any great
diversity of temperature; and as they all migrated in a body together,
their mutual relations will not have been much disturbed. Hence,
in accordance with the principles inculcated in this volume, these
forms will not have been liable to much modification. But with the
Alpine productions, left isolated from the moment of the returning
warmth, first at the bases and ultimately on the summits of the
mountains, the case will have been somewhat different; for it is
not likely that all the same arctic species will have been left on
mountain ranges far distant from each other, and have survived
there ever since; they will also in all probability, have become
mingled with ancient Alpine species, which must have existed on
the mountains before the commencement of the Glacial epoch, and
which during the coldest period will have been temporarily driven
down to the plains; they will, also, have been subsequently exposed
to somewhat different climatal influences. Their mutual relations
will thus have been in some degree disturbed; consequently they
will have been liable to modification; and they have been modified;

DISPERSAL DURING GLACIAL PERIOD 'iff]

for if we compare the present Alpine plants and animals of the
several great European mountain ranges one with another, though
many of the species remain identically the same, some exist as
varieties, some as doubtful forms or sub-species, and some as dis-
tinct yet closely allied species representing each other on the several
ranges.

In the foregoing illustration I have assumed that at the com-
mencement of our imaginary Glacial period, the arctic productions
were as uniform round the polar regions as they are at the present
day. But it is also necessary to assume that many sub-arctic and
some few temf)erate forms were the same round the world, for
some of the species which now exist on the lower mountain-slof)es
and on the plains of North America and Europe are the same; and
it may be asked how I account for this degree of uniformity in the
sub-arctic and temf)erate forms round the world, at the commence-
ment of the real Glacial period. At the present day, the sub-arctic
and northern temperate productions of the Old and New Worlds
are separated from each other by the whole Atlantic Ocean and by
the northern part of the Pacific. During the Glacial period, when
the inhabitants of the Old and New Worlds lived farther southwards
than they do at present, they must have been still more completely
separated from each other by wider spaces of ocean; so that it may
well be asked how the same species could then or previously have
entered the two continents. The explanation, I believe, lies in the
nature of the climate before the commencement of the Glacial
period. At this, the newer Pliocene period, the majority of the
inhabitants of the world were specifically the same as now, and we
have good reason to believe that the climate was warmer than at
the present day. Hence we may suppose that the organisms which
now live under latitude 60°, lived during the Pliocene period, farther
north under the Polar Circle, in latitude 66°-67°; and that the
present arctic productions then lived on the broken land still nearer
to the pole. Now, if we look at a terrestrial globe, we see under the
Polar Circle that there is almost continuous land from western
Europe, through Siberia, to eastern America. And this continuity
of the circumpolar land, with the consequent freedom under a more
favourable climate for intermigration, will account for the supposed

398 ORIGIN OF SPECIES

uniformity of the sub-arctic and temperate productions of the Old
and New Worlds, at a period anterior to the Glacial epoch.

Believing, from reasons before alluded to, that our continents
have long remained in nearly the same relative position, though
subjected to great oscillations of level, I am strongly incUned to
extend the above view, and to infer that during some still earlier
and still warmer period, such as the older Pliocene period, a large
number of the same plants and animals inhabited the almost con-
tinuous circumpolar land; and that these plants and animals, both
in the Old and New Worlds, began slowly to migrate southwards
as the climate became less warm, long before the commencement
of the Glacial period. We now see, as I believe, their descendants,
mostly in a modified condition, in the central parts of Europe and
the United States. On this view we can understand the relationship
with very little identity, between the productions of North America
and Europe, — a relationship which is highly remarkable, considering
the distance of the two areas, and their separation by the whole
Atlantic Ocean. We can further understand the singular fact re-
marked on by several observers that the productions of Europe and
America during the later tertiary stages were more closely related
to each other than they are at the present time; for during these
warmer periods the northern parts of the Old and New Worlds
will have been almost continuously united by land, serving as a
bridge, since rendered impassable by cold, for the intermigration of
their inhabitants.

During the slowly decreasing warmth of the Pliocene period, as
soon as the species in common, which inhabited the New and Old
Worlds, migrated south of the Polar Circle, they will have been
completely cut off from each other. This separation, as far as the
more temperate productions are concerned, must have taken place
long ages ago. As the plants and animals migrated southward, they
will have become mingled in the one great region with the native
American productions, and would have had to compete with them;
and in the other great region, with those of the Old World. Con-
sequently we have here everything favourable for much modification,
— for far more modification than with the Alpine productions, left
isobted, within a much more recent period, on the several mountain-

ALTERNATE GLACIAL PERIODS 399

ranges and on the arctic lands of Europe and North America. Hence,
it has come, that when we compare the now Hving productions of
the temperate regions of the New and Old Worlds, we find very
few identical species (though Asa Gray has lately shown that more
plants are identical than was formerly supposed), but we find in
every great class many forms, which some naturalists rank as geo-
graphical races, and others as distinct species; and a host of closely
allied or representative forms which are ranked by all naturalists as
specifically distinct.

As on the land, so in the waters of the sea, a slow southern migra-
tion of a marine fauna, which, during the Pliocene or even a some-
what earlier p)eriod, was nearly uniform along the continuous shores
of the Polar Circle, will account, on the theory of modification, for
many closely allied forms now living in marine areas completely
sundered. Thus, I think, we can understand the presence of some
closely allied, still existing and extinct tertiary forms, on the eastern
and western shores of temperate North America; and the still more
striking fact of many closely allied crustaceans (as described in
Dana's admirable work), some fish and other marine animals,
inhabiting the Mediterranean and the seas of Japan, — these two
areas being now completely separated by the breadth of a whole
continent and by wide spaces of ocean.

These cases of close relationship in species either now or formerly
inhabiting the seas on the eastern and western shores of North
America, the Mediterranean and Japan, and the temperate lands of
North America and Europe, are inexplicable on the theory of crea-
tion. We cannot maintain that such species have been created alike,
in correspondence with the nearly similar physical conditions of the
areas; for if we compare, for instance, certain parts of South America
with parts of South Africa or Australia, we see countries closely
similar in all their physical conditions, with their inhabitants utterly
dissimilar.

ALTERNATE GLACIAL PERIODS IN THE NORTH AND SOUTH

But we must return to our more immediate subject. I am con-
vinced that Forbes' view may be largely extended. In Europe we
meet with the plainest evidence of the Glacial period, from the

400 ORIGIN OF SPECIES

western shores of Britain to the Oural range, and southward to the
Pyrenees. We may infer from the frozen mammals and nature of
the mountain vegetation, that Siberia was similarly affected. In the
Lebanon, according to Dr. Hooker, perj^etual snow formerly covered
the central axis, and fed glaciers which rolled 4,000 feet down the
valleys. The same observer has recently found great moraines at a
low level on the Atlas range in North Africa. Along the Himalaya,
at points 900 miles apart, glaciers have left the marks of their former
low descent; and in Sikkim, Dr. Hooker saw maize growing on
ancient and gigantic moraines. Southward of the Asiatic continent,
on the opposite side of the equator, we know, from the excellent
researches of Dr. J. Haast and Dr. Hector, that in New Zealand
immense glaciers formerly descended to a low level; and the same
plants found by Dr. Hooker on widely separated mountains in this
island tell the same story of a former cold period. From facts com-
municated to me by the Rev. W. B. Clarke, it appears also that there
are traces of former glacial action on the mountains of the south-
eastern corner of Australia.

Looking to America: in the northern half, ice-borne fragments
of rock have been observed on the eastern side of the continent, as
far south as latitude 36°-37'', and on the shores of the Pacific,
where the climate is now so different, as far south as lat. 46°,
Erratic boulders have, also, been noticed on the Rocky Mountains.
In the Cordillera of South America, nearly under the equator,
glaciers once extended far below their present level. In Central
Chile I examined a vast mound of detritus with great boulders,
crossing the Portillo valley, which, there can hardly be a doubt,
once formed a huge moraine; and Mr. D. Forbes informs me that
he found in various parts of the Cordillera, from latitude 13° to 30°
S., at about the height of 12,000 feet, deeply-furrowed rocks, re-
sembling those with which he was familiar in Norway, and likewise
great masses of detritus, including grooved pebbles. Along this
whole space of the Cordillera true glaciers do not now exist even
at much more considerable heights. Farther south on both sides of
the continent, from lat. 41 ° to the southernmost extremity, we have
the clearest evidence of former glacial action, in numerous immense
boulders transported far from their parent source.

ALTERNATE GLACIAL PERIODS 4OI

From these several facts, namely from the glacial action having
extended all round the northern and southern hemispheres — from
the period having been in a geological sense recent in both hemi-
spheres — from its having lasted in both during a great length of
time, as may be inferred from the amount of work effected — and
lastly from glaciers having recently descended to a low level along
the whole line of the Cordillera, it at one time appeared to me that
we could not avoid the conclusion that the temperature of the whole
world had been simultaneously lowered during the Glacial period.
But now, Mr. CroU, in a series of admirable memoirs, has attempted
to show that a glacial condition of climate is the result of various
physical causes, brought into operation by an increase in the eccen-
tricity of the earth's orbit. All these causes tend towards the same
end; but the most powerful appears to be the indirect influence of
the eccentricity of the orbit upon oceanic currents. According to
Mr. Croll, cold periods regularly recur every ten or fifteen thousand
years; and these at long intervals are extremely severe, owing to
certain contingencies, of which the most important, as Sir C. Lyell
has shown, is the relative position of the land and water. Mr. Croll
believes that the last great Glacial period occurred about 240,000
years ago, and endured with slight alterations of climate for about
160,000 years. With respect to more ancient Glacial periods, several
geologists are convinced from direct evidence that such occurred
during the Miocene and Eocene formations, not to mention still
more ancient formations. But the most important result for us,
arrived at by Mr. Croll, is that whenever the northern hemisphere
passes through a cold period the tempwrature of the southern hemi-
sphere is actually raised, with the winters rendered much milder,
chiefly through changes in the direction of the ocean currents. So

diversely it will be with the northern hemisphere, whilst the
xjuthern passes through a Glacial period. This conclusion throws
so much light on geographical distribution that I am strongly in-
clined to trust in it; but I will first give the facts, which demand an
explanation.

In South America, Dr. Hooker has shown that besides many
closely allied species, between forty and fifty of the flowering plants
of Tierra del Fuego, forming no inconsiderable part of its scanty

402 ORIGIN OF SPECIES

flora, are common to North America and Europe, enormously
remote as these areas in opposite hemispheres are from each other.
On the lofty mountains of equatorial America a host of peculiar
species belonging to European genera occur. On the Organ moun-
tains of Brazil, some few temperate European, some Antarctic, and
some Andean genera were found by Gardner, which do not exist
in the low intervening hot countries. On the Silla of Caraccas, the
illustrious Humboldt long ago found species belonging to genera
characteristic of the Cordillera.

In Africa, several forms characteristic of Europe and some few
representatives of the flora of the Cape of Good Hope occur on the
mountains of Abyssinia. At the Cape of Good Hope a very few
European species, believed not to have been introduced by man,
and on the mountains several representative European forms are
found, which have not been discovered in the intertropical parts of
Africa. Dr. Hooker has also lately shown that several of the plants
living on the upper parts of the lofty island of Fernando Po and on
the neighbouring Cameroon mountains, in the Gulf of Guinea, are
closely related to those on the mountains of Abyssinia, and likewise
to those of temperate Europe. It now also appears, as I hear from
Dr. Hooker, that some of these same temperate plants have been
discovered by the Rev. R. T. Lowe on the mountains of the Cape
Verde Islands. This extension of the same temperate forms, almost
under the equator, across the whole continent of Africa and to the
mountains of the Cape Verde archipelago, is one of the most astonish-
ing facts ever recorded in the distribution of plants.

On the Himalaya, and on the isolated mountain-ranges of the
peninsula of India, on the heights of Ceylon, and on the volcanic
cones of Java, many plants occur, either identically the same or
representing each other, and at the same time representing plants
of Europe not found in the intervening hot lowlands. A list of the
genera of plants collected on the loftier peaks of Java, raises a pic-
ture of a collection made on a hillock in Europe! Still more striking
is the fact that p)eculiar Australian forms are represented by certain
plants growing on the summits of the mountains of Borneo. Some
of these Australian forms, as I hear from Dr. Hooker, extend along
the heights of the peninsula of Malacca, and are thinly scattered on

ALTERNATE GLACIAL PERIODS 403

the one hand over India, and on the other hand as far north as
Japan.

On the southern mountains of Australia, Dr. F. Miiller has dis-
covered several European species; other species, not introduced by
man, occur on the lowlands; and a long list can be given, as I am
informed by Dr. Hooker, of European genera, found in Australia,
but not in the intermediate torrid regions. In the admirable 'Intro-
duction to the Flora of New Zealand,' by Dr. Hooker, analogous
and striking facts are given in regard to the plants of that large
island. Hence we see that certain plants growing on the more lofty
mountains of the tropics in all parts of the world, and on the tem-
perate plains of the north and south, are either the same sf>ecies or
varieties of the same species. It should, however, be observed that
these plants are not strictly arctic forms; for, as Mr. H. C. Watson
has remarked, "in receding from polar towards equatorial latitudes,
the Alpine or mountain floras really become less and less arctic."
Besides these identical and closely allied forms, many species in-
habiting the same widely sundered areas, belong to genera not now
found in the intermediate tropical lowlands.

These brief remarks apply to plants alone; but some few analogous
facts could be given in regard to terrestrial animals. In marine pro-
ductions, similar cases likewise occur; as an example, I may quote
a statement by the highest authority, Professor Dana, that "it is
certainly a wonderful fact that New Zealand should have a closer
resemblance in its Crustacea to Great Britain, its antipode, than to
any other part of the world." Sir J. Richardson, also, speaks of the
reappearance on the shores of New Zealand, Tasmania, etc., of
northern forms of fish. Dr. Hooker informs me that twenty-five
species of Algx are common to New Zealand and to Europe, but
have not been found in the intermediate tropical seas.

From the foregoing facts, namely, the presence of temperate forms
on the highlands across the whole of equatorial Africa, and along
the peninsula of India, to Ceylon and the Malay Archipelago, and
in a less well-marked manner across the wide expanse of tropical
South America, it appears almost certain that at some former period,
no doubt during the most severe part of a Glacial period, the low-
lands of these great continents were everywhere tenanted under the

404 ORIGIN OF SPECIES

equator by a considerable number of temperate forms. At this
period the equatorial climate at the level of the sea was probably
about the same with that now experienced at the height of from
five to six thousand feet under the same latitude, or perhaps even
rather cooler. During this, the coldest period, the lowlands under
the equator must have been clothed with a mingled tropical and
temf)erate vegetation, like that described by Hooker as growing
luxuriantly at the height of from four to five thousand feet on the
lower slopes of the Himalayas, but with perhaps a still greater pre-
ponderance of temperate forms. So again in the mountainous
islands of Fernando Po, in the Gulf of Guinea, Mr. Mann found
temperate European forms beginning to appear at the height of
about five thousand feet. On the mountains of Panama, at the
height of only two thousand feet, Dr. Seemann found the vegetation
like that of Mexico, "with forms of the torrid zone harmoniously
blended with those of the temperate."

Now let us see whether Mr. Croll's conclusion that when the
northern hemisphere suffered from the extreme cold of the great
Glacial period, the southern hemisphere was actually warmer, throws
any clear light on the present apparently inexplicable distribution
of various organisms in the temperate parts of both hemispheres,
and on the mountains of the tropics. The Glacial period, as measured
by years, must have been very long; and when we remember over
what vast spaces some naturalised plants and animals have spread
within a few centuries, this period will have been ample for any
amount of migration. As the cold became more and more intense,
we know that arctic forms invaded the temperate regions; and,
from the facts just given, there can hardly be a doubt that some of
the more vigorous, dominant and widest-spreading temperate forms
invaded the equatorial lowlands. The inhabitants of these hot low-
lands would at the same time have migrated to the tropical and
sub-tropical regions of the south, for the southern hemisphere was
at this period warmer. On the decline of the Glacial f)eriod, as both
hemispheres gradually recovered their former temf)eratures, the
northern temperate forms living on the lowlands under the equator,
would have been driven to their former homes or have been de-
stroyed, being replaced by the equatorial forms returning from the

ALTERNATE GLACIAL PERIODS 405

south. Some, however, of the northern temperate forms would
almost certainly have ascended any adjoining high land, where, if
suiliciently lofty, they would have long survived like the arctic forms
on the mountains of Europe. They might have survived, even if
the climate was not perfectly fitted for them, for the change of
temperature must have been very slow, and plants undoubtedly
possess a certain capacity for acclimatisation, as shown by their
transmitting to their offspring difJerent constitutional powers ol
resisting heat and cold.

In the regular course of events the southern hemisphere would
in its turn be subjected to a severe Glacial period, with the northern
hemisphere rendered warmer; and then the southern temperate
forms would invade the equatorial lowlands. The northern forms
which had before been left on the mountains would now descend
and mingle with the southern forms. These latter, when the warmth
returned, would return to their former homes, leaving some few
,sj)ecies on the mountains, and carrying southward with them some
of the northern temf)crate forms which had descended from their
mountain fastnesses. Thus, we should have some few species
identically the same in the northern and southern temperate zones
and on the mountains of the intermediate tropical regions. But the
species left during a long time on these mountains, or in opposite
hemispheres, would have to compete with many new forms and
would be exposed to somewhat different physical conditions; hence
they would be eminently liable to modification, and would generally
now exist as varieties or as representative species; and this is the
case. We must, also, bear in mind the occurrence in both hemi-
spheres of former Glacial periods; for these will account, in accord-
ance with the same principles, for the many quite distinct species
inhabiting the same widely separated areas, and belonging to genera
not now found in the intermediate torrid zones.

It is a remarkable fact strongly insisted on by Hooker in regard
to America, and by Alph. de CandoUe in regard to Australia, that
many more identical or slightly modified species have migrated
from the north to the south, than in a reversed direction. We see,
however, a few southern forms on the mountains of Borneo and
Abyssinia. I suspect that this preponderant migration from the

406 ORIGIN OF SPECIES

north to the south is due to the greater extent of land in the north,
and to the northern forms having existed in their own homes in
greater numbers, and having consequently been advanced through
natural selection and competition to a higher stage of perfection, or
dominating power, than the southern forms. And thus, when the
two sets became commingled in the equatorial regions, during the
alternations of the Glacial periods, the northern forms were the more
pHJwerful and were able to hold their places on the mountains, and
afterwards to migrate southward with the southern forms; but not
so the southern in regard to the northern forms. In the same man-
ner at the present day, we see that very many European productions
cover the ground in La Plata, New Zealand, and to a lesser degree
in Australia, and have beaten the natives; whereas extremely few
southern forms have become naturaUsed in any part of the northern
hemisphere, though hides, wool, and other objects likely to carry
seeds have been largely imported into Europe during the last two
or three centuries from La Plata and during the last forty or fifty
years from Australia. The Neilgherrie mountains in India, however,
ofler a partial exception; for here, as I hear from Dr. Hooker,
Australian forms are rapidly sowing themselves and becoming
naturalised. Before the last great Glacial period, no doubt the inter-
tropical mountains were stocked with endemic Alpine forms; but
these have almost everywhere yielded to the more dominant forms
generated in the larger areas and more efficient workshops of the
north. In many islands the native productions are nearly equalled,
or even outnumbered, by those which have become naturalised; and
this is the first stage towards their extinction. Mountains are islands
on the land, and their inhabitants have yielded to those produced
within the larger areas of the north, just in the same way as the
inhabitants of real islands have everywhere yielded and are still
yielding to continental forms naturalised through man's agency.

The same principles apply to the distribution of terrestrial animals
and of marine productions, in the northern and southern temperate
zones, and on the intertropical mountains. When, during the height
of the Glacial period, the ocean currents were widely different to
what they now are, some of the inhabitants of the temperate seas
might have reached the equator; of these a few would perhaps at

ALTERNATE GLACIAL PERIODS 407

once be abie to migrate southward, by keeping to the cooler currents,
whilst others might remain and survive in the colder depths until
the southern hemisphere was in its turn subjected to a glacial climate
and permitted their further progress; in nearly the same manner as,
according to Forbes, isolated spaces inhabited by arctic productions
exist to the present day in the deeper parts of the northern temperate
seas.

I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which
now live so widely separated in the north and south, and sometimes
on the intermediate mountain-ranges, are removed on the views
above given. The exact lines of migration cannot be indicated. We
cannot say why certain species and not others have migrated; why
certain species have been modified and have given rise to new forms,
whilst others have remained unaltered. We cannot hope to explain
such facts, until we can say why one species and not another becomes
naturalised by man's agency in a foreign land; why one species
ranges twice or thrice as far, and is twice or thrice as common, as
another species within their own homes.

Various special difficulties also remain to be solved; for instance,
the occurrence, as shown by Dr. Hooker, of the same plants at points
so enormously remote as Kerguelen Land, New Zealand, and
Fuegia; but icebergs, as suggested by Lyell, may have been con-
cerned in their dispersal. The existence at these and other distant
points of the southern hemisphere, of species, which, though distinct,
belong to genera exclusively confined to the south, is a more remark-
able case. Some of these species are so distinct, that we cannot sup-
pose that there has been time since the commencement of the last
Glacial period for their migration and subsequent modification to
the necessary degree. The facts seem to indicate that distinct species
belonging to the same genera have migrated in radiating lines from
a common genera; and I am inclined to look in the southern, as in
the northern hemisphere, to a former and warmer period, before the
commencement of the last Glacial period, when the antarctic lands,
now covered with ice, supported a highly peculiar and isolated flora.
It may be suspected that before this flora was exterminated during
the last Glacial epoch, a few forms had been already widely dispersed

408 ORIGIN OF SPECIES

to various points of the southern hemisphere by occasional means of
transport, and by the aid as halting-places, of now sunken islands.
Thus the southern shores of America, Australia, and New Zealand
may have become slightly tinted by the same pecuUar forms of life.
Sir C. Lyell in a striking passage has speculated, in language
almost identical with mine, on the effects of great alterations of
climate throughout the world on geographical distribution. And we
have now seen that Mr. Croll's conclusion that successive Glacial
periods in the one hemisphere coincide with warmer periods in the
opposite hemisphere, together with the admission of the slow modi-
fication of species, explains a multitude of facts in the distribution
of the same and of the allied forms of life in all parts of the globe.
The living waters have flowed during one period from the north
and during another from the south, and in both cases have reached
the equator; but the stream of life has flowed with greater force
from the north than in the opposite direction, and has consequently
more freely inundated the south. As the tide leaves its drift in
horizontal lines, rising higher on the shores where the tide rises
highest, so have the living waters left their living drift on our moun-
tain summits, in a line gently rising from the arctic lowlands to
a great altitude under the equator. The various beings thus left
stranded may be compared with savage races of man, driven up and
surviving in the mountain fastnesses of almost every land, which
serves as a record, full of interest to us, of the former inhabitants of
the surrounding lowlands.

CHAPTER XIII

Geographical Distribution — continued

Distribution of fresh-water productions — On the inhabitants of oceanic
islands — Absence of batrachians and of terrestrial mammals — On the
relation of the inhabitants of islands to those of the nearest mainland
— On colonisation from the nearest source with subsequent modifica-
tion — Summary of the last and present chapter.

FRESII-WATER PRODUCTIONS

AS lakes and river systems are separated from each other by bar-
/ \ riers of land, it might have been thought that fresh-water
jL .m. productions would not have ranged widely within the same
country, and as the sea is apparently a still more formidable barrier,
that they would never have extended to distant countries. But the
case is exactly the reverse. Not only have many fresh-water species,
belonging to different classes, an enormous range, but allied species
prevail in a remarkable manner throughout the world. When first
collecting in the fresh waters of Brazil, I well remember feeUng much
surprise at the similarity of the fresh-water insects, shells, etc., and
at the dissimilarity of the surrounding terrestrial beings, compared
with those of Britain.

But the wide ranging power of fresh-water productions can, I
think, in most cases be explained by their having become fitted, in
a manner highly useful to them, for short and frequent migrations
from pond to [X)nd, or from stream to stream, within their own coun-
tries; and liability to wide dispersal would follow from this capacity
as an almost necessary consequence. We can here consider only a few
cases; of these, some of the most difficult to explain are presented by
fish. It was formerly believed that the same fresh-water species never
existed on two continents distant from each other. But Dr. Giin-
ther has lately shown that the Galaxias attenuatus inhabits Tasmania,
New Zealand, the Falkland Islands, and the mainland of South
America. This is a wonderful case, and probably indicates dispersal

4«9

410 ORIGIN OF SPECIES

from an antarctic centre during a former warm period. This case,
however, is rendered in some degree less surprising by the species of
this genus having the power of crossing by some unknown means
considerable spaces of open ocean : thus there is one species common
to New Zealand and to the Auckland Islands, though separated by a
distance of about 230 miles. On the same continent fresh-water fish
often range widely, and as if capriciously; for in two adjoining river-
systems some of the species may be the same, and some wholly
different.

It is probable that they are occasionally transported by what- may
be called accidental means. Thus fishes still alive are not very rarely
dropped at distant points by whirlwinds; and it is known that the ova
retain their vitality for a considerable time after removal from the
water. Their dispersal may, however, be mainly attributed to changes
in the level of the land within the recent period, causing rivers to flow
into each other. Instances, also, could be given of this having oc-
curred during floods, without any change of level. The wide differ-
ence of the fish on the opposite sides of most mountain ranges, which
are continuous, and which consequently must from an early period
have completely prevented the inosculation of the river system on
the two sides, leads to the same conclusion. Some fresh-water fish
belong to very ancient forms, and in such cases there will have been
ample time for great geographical changes, and consequendy time
and means for much migration. Moreover Dr. Gunther has recently
been led by several considerations to infer that with fishes the same
forms have a long endurance. Salt-water fish can with care be slowly
accustomed to live in fresh water; and, according to Valenciennes,
there is hardly a single group of which all the members are confined
to fresh water, so that a marine species belonging to a fresh-water
group might travel far along the shores of the sea, and could, it is
probable, become adapted without much difficulty to the fresh waters
of a distant land.

Some species of fresh-water shells have very wide ranges, and allied
species which, on our theory, are descended from a common parent,
and must have prcxeeded from a single source, prevail throughout
the world. Their distribution at first perplexed me much, as
their ova are not likely to be transported by birds; and the ova,

FRESH-WATER PRODUCTIONS 4II

as well as the adults, are immediately killed by sea water. I
could not even understand how some naturalised sjjecies have spread
rapidly throughout the same country. But two facts, which I have
observed — and many others no doubt will be discovered — throw some
light on this subject. When ducks suddenly emerge from a pond
covered with duckweed, 1 have twice seen these little plants adhering
to their backs; and it has happened to me, in removing a little duck-
weed from one aquarium to another, that I have unintentionally
stocked the one with fresh-water shells from the other. But another
agency is perhaps more effectual : I suspended the feet of a duck in
an aquarium, where many ova of fresh-water shells were hatching;
and I found that numbers of the extremely minute and just-hatched
shells crawled on the feet, and clung to them so firmly that when
taken out of the water they could not be jarred off, though at a some-
what more advanced age they would voluntarily drop off. These
just-hatched molluscs, though aquatic in their nature, survived on
the duck's feet, in damp air, from twelve to twenty hours; and in this
length of time a duck or heron might fly at least six or seven hun-
dred miles, and if blown across the sea to an oceanic island, or to any
other distant point, would be sure to alight on a pxxsl or rivulet. Sir
Charles Lyell informs me that a Dytiscus has been caught with an
Ancylus (a fresh-water shell like a limpet) firmly adhering to it;
and a water-beetle of the same family, a Colymbetes, once flew on
board the 'Beagle', when forty-five miles distant from the nearest
land: how much farther it might have been blown by a favouring
gale no one can tell.

With respect to plants, it has long been known what enormous
ranges many fresh-water, and even marsh species, have, both over
continents and to the most remote oceanic islands. This is strikingly
illustrated, according to Alph. de CandoUe, in those large groups of
terrestrial plants, which have very few aquatic members; for the
latter seem immediately to acquire, as if in consequence, a wide range.
I think favourable means of dispersal explain this fact. I have be-
fore mentioned that earth occasionally adheres in some quantity to
the feet and beaks of birds. Wading birds, which frequent the muddy
edges of ponds, if suddenly flushed, would be the most likely to have
muddy feet. Birds of this order wander more than those of any

412 ORIGIN OF SPECIES

Other; and they are occasionally found on the most remote and barren
islands of the open ocean; they would not be Ukely to alight on the
surface of the sea, so that any dirt on their feet would not be washed
off; and when gaining the land, they would be sure to fly to their
natural fresh-water haunts. I do not believe that botanists are aware
how charged the mud of ponds is with seeds; I have tried several lit-
tle experiments, but will here give only the most striking case: I took
in February three table-spoonfuls of mud from three different points,
beneath water, on the edge of a little pond: this mud when dried
weighed only six and three-fourths ounces; I kept it covered up in my
study for six months, pulling up and counting each plant as it grew;
the plants were of many kinds, and were altogether 537 in number;
and yet the viscid mud was ail contained in a breakfast cup! Consid-
ering these facts, I think it would be an inexplicable circumstance if
water-birds did not transport the seeds of fresh-water plants to un-
stocked ponds and streams, situated at very distant points. The same
agency may have come into play with the eggs of some of the smaller
fresh-water animals.

Other and unknown agencies probably have also played a part. I
have stated that fresh-water fish eat some kinds of seeds, though they
reject many other kinds after having swallowed them; even small fish
swallow seeds of moderate size, as of the yellow water-lily and
Potamogeton. Herons and other birds, century after century, have
gone on daily devouring fish; they then take flight and go to other
waters, or are blown across the sea; and we have seen that seeds retain
their power of germination, when rejected many hours afterwards
in {jellets or in the excrement. When I saw the great size of the
seeds of that fine water-lily, the Nelumbium, and remembered Alph.
de CandoIIe's remarks on the distribution of this plant, I thought that
the means of its dispersal must remain inexplicable; but Audu-
bon states that he found the seeds of the great southern water-lily
(probably, according to Dr. Hooker, the Nelumbium luteum) in a
heron's stomach. Now this bird must often have flown with its
stomach thus well stocked to distant ponds, and then getting a hearty
meal of fish, analogy makes me believe that it would have rejected
the seeds in a pellet in a fit state for germination.

In considering these several means of distribution, it should be re-

INHABITANTS OF OCEANIC ISLANDS 413

membered that when a pond or stream is first formed, for instance,
on a rising islet, it will be unoccupied; and a single seed or egg will
have a good chance of succeeding. Although there will always be a
struggle for life between the inhabitants of the same pond, however
few in kind, yet as the number even in a well-stocked pond is small
in comparison with the number of species inhabiting an equal area of
land, the competition between them will probably be less severe than
between terrestrial species; consequently an intruder from the waters
of a foreign country would have a better chance of seizing on a new
place, than in the case of terrestrial colonists. We should also remem-
ber that many fresh-water productions are low in the scale of nature,
and we have reason to believe that such beings become modified
more slowly than the high; and this will give time for the migration
of aquatic species. We should not forget the probability of many
fresh-water forms having formerly ranged continuously over im-
mense areas, and then having become extinct at intermediate points.
But the wide distribution of fresh-water plants and of the lower ani-
mals, whether retaining the same identical form or in some degree
modified, apparently depends in main part on the wide dispersal of
their seeds and eggs by animals, more especially by fresh-water birds,
which have great powers of flight, and naturally travel from one piece
of water to another.

ON THE INHABITANTS OF OCEANIC ISLANDS

We now come to the last of the three classes of facts, which I have
selected as presenting the greatest amount of difficulty with respect
to distribution, on the view that not only all the individuals of the
same species have migrated from some one area, but that allied spe-
cies, although now inhabiting the most distant points, have proceeded
from a single area, — the birthplace of their early progenitors. I have
already given my reason for disbelieving in continental extensions
within the (period of existing species, on so enormous a scale that all
the many islands of the several oceans were thus stocked with their
present terrestrial inhabitants. This view removes many difficulties,
but it does not accord with all the facts in regard to the productions
of islands. In the following remarks I shall not confine myself to the
mere question of dispersal, but shall consider some other cases bearing

414 ORIGIN OF SPECIES

on the truth o£ the two theories of independent creation and of

descent with modification.

The species of ^11 kinds which inhabit oceanic islands are few in
number compared with those on equal continental areas: Alph. de
Candolle admits this for plants, and WoUaston for insects. New
Zealand, for instance, with its lofty mountains and diversified sta-
tions, extending over 780 miles of latitude, together with the outlying
islands of Auckland, Campbell, and Chatham, contain altogether
only 960 kinds of flowering plants; if we compare this moderate num-
ber with the species which swarm over equal areas in southwestern
Australia or at the Cape of Good Hope, we must admit that some
cause, independently of different physical conditions, has given rise
to so great a difference in number. Even the uniform county of
Cambridge has 847 plants, and the litde island of Anglesea 764, but
a few ferns and a few introduced plants are included in these num-
bers, and the comparison in some other respects is not quite fair. We
have evidence that the barren island of Ascension aboriginally pos-
sessed less than half-a-dozen flowering plants; yet many species have
now become naturalised on it, as they have in New Zealand and on
every other oceanic island which can be named. In St. Helena there
is reason to believe that the naturalised plants and animals have nearly
or quite exterminated many native productions. He who admits the
doctrine of the creation of each separate species, will have to admit
that a sufficient number of the best adapted plants and animals were
not created for oceanic islands; for man has unintentionally stocked
them far more fully and perfectly than did nature.

Although in oceanic islands the species are few in number, the pro-
portion of endemic kinds (/.r. those found nowhere else in the world)
is often extremely large. If we compare, for instance, the number of
endemic land shells in Madeira, or of endemic birds in the Galapagos
archipelago, with the number found on any continent, and then com-
pare the area of the island with that of the continent, we shall see
that this is true. This fact might have been theoretically exfjected,
for, as already explained, sfiecies occasionally arriving after long in-
tervals of time in the new and isolated district, and having to compete
with new associates, would be eminently liable to modification, and
would often produce groups of modified descendants. But it by no

INHABITANTS OF OCEANIC ISLANDS 415

means follows that, because in an island nearly all the species of one
class are peculiar, those of another class, or of another section of the
same class, are peculiar; and this difference seems to depend partly
on the species which are not modified having immigrated in a body,
so that their mutual relations have not been much disturbed; and
pardy on the frequent arrival of unmodified immigrants from the
mother country, with which the insular forms have intercrossed. It
should be borne in mind that the offspring of such crosses would cer-
tainly gain in vigour; so that even an occasional cross would produce
more effect than might have been anticipated. 1 will give a few illus-
trations of the foregoing remarks : in the Galapagos Islands there are
twenty-six land birds; of these twenty-one (or perhaps twenty-three)
are peculiar, whereas of the eleven marine birds only two are pecu-
liar; and it is obvious that marine birds could arrive at these islands
much more easily and frequendy than land birds. Bermuda, on the
other hand, which lies at about the same distance from North Amer-
ica as the Galapagos Islands do from South America, and which has
a very pecuHar soil, does not possess a single endemic land bird; and
we know from Mr. J. M. Jones' admirable account of Bermuda, that
very many North American birds occasionally or even frequently
visit this island. Almost every year, as I am informed by Mr. E. V.
Harcourt, many European and African birds are blown to Madeira;
this island is inhabited by ninety-nine kinds, of which one alone is
peculiar, though very closely related to a European form; and three
or four other species are confined to this island and to the Canaries.
So that the Islands of Bermuda and Madeira have been stocked from
the neighbouring continents with birds, which for long ages have
there struggled together, and have become mutually co-adapted.
Hence when settled in their new homes, each kind will have been
kept by the others to its proper place and habits, and will conse-
quently have been but little liable to modification. Any tendency to
modification will also have been checked by intercrossing with the
unmodified immigrants, often arriving from the mother country.
Madeira again is inhabited by a wonderful number of peculiar land
shells, whereas not one species of sea shell is peculiar to its shores;
now, though we do not know how sea shells are dispersed, yet we
can see that their eggs or larvx, perhaps attached to seaweed or float-

4l6 ORIGIN OF SPECIES

ing timber, or to the feet of wading-birds, might be transported across
three or four hundred miles of open sea far more easily than land
shells. The different orders of insects inhabiting Madeira present
nearly parallel cases.

Oceanic islands are sometimes deficient in animals of certain whole
classes, and their places are occupied by other classes; thus in the
Galapagos Islands reptiles, and in New Zealand gigantic wingless
birds, take, or recently took, the place of mammals. Although New
Zealand is here spoken of as an oceanic island, it is in some degree
doubtful whether it should be so ranked; it is of large size, and is not
separated from Australia by a profoundly deep sea; from its geologi-
cal character and the direction of its mountain ranges, the Rev. W. B.
Clarke has lately maintained that this island, as well as New Cale-
donia, should be considered as appurtenances of Australia. Turning
to plants. Dr. Hooker has shown that in the Galapagos Islands the
prop)ortional numbers of the different orders are very different from
what they are elsewhere. All such differences in number, and the ab-
sence of certain whole groups of animals and plants, are generally
accounted for by supposed differences in the physical conditions of the
islands; but this explanation is not a little doubtful. Facility of immi-
gration seems to have been fully as important as the nature of the
conditions.

Many remarkable little facts could be given with respect to the
inhabitants of oceanic islands. For instance, in certain islands
not tenanted by a single mammal, some of the endemic plants
have beautifully hooked seeds; yet few relations are more mani-
fest than that hooks serve for the transportal of seeds in the
wool or fur of quadrupeds. But a hooked seed might be carried
to an island by other means; and the plant then becoming modi-
fied would form an endemic species, still retaining its hooks,
which would form a useless appendage like the shrivelled wings
under the soldered wingcovers of many insular beetles. Again,
islands often possess trees or bushes belonging to orders which
elsewhere include only herbaceous sp)ecies; now trees, as Alph.
de Candolle has shown, generally have, whatever the cause may
be, confined ranges. Hence trees would be little likely to reach
distant oceanic islands; and an herbaceous plant, which had no chance

ABSENCE OF BATRACHIANS 417

of successfully competing with the many fully developed trees grow-
ing on a continent, might, when established on an island, gain an
advantage over other herbaceous plants by growing taller and taller
and overtopping them. In this case, natural selection would tend to
add to the stature of the plant, to whatever order it belonged, and
thus first convert it into a bush and then into a tree.

ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS OK
OCEANIC ISLANDS

With respect to the absence of whole orders or animals on oceanic
islands, Bory St. Vincent long ago remarked that batrachians (frogs,
toads, newts) are never found on any of the many islands with which
the great oceans are studded. I have taken pains to verify this asser-
tion, and have found it true, with the exception of New Zealand,
New Caledonia, the Andaman Islands, and perhaps the Solomon
Islands and the Seychelles. But I have already remarked that it is
doubtful whether New Zealand and New Caledonia ought to be
classed as oceanic islands; and this is still more doubtful with respect
to the Andaman and Solomon groups and the Seychelles. This gen-
eral absence of frogs, toads, and newts on so many true oceanic
islands cannot be accounted for by their physical conditions: indeed
it seems that islands are peculiarly fitted for these animals; for frogs
have been introduced into Madeira, the Azores, and Mauritius, and
have multiplied so as to become a nuisance. But as these animals and
their spawn are immediately killed (with the exception, as far as
known, of one Indian species) by sea water, there would be great
difficulty in their transportal across the sea, and therefore we can see
why they do not exist on strictly oceanic islands. But why, on the
theory of creation, they should not have been created there, it would
be very difficult to explain.

Mammals offer another and similar case. I have carefully searched
the oldest voyages, and have not found a single instance, free from
doubt, of a terrestrial mammal (excluding domesticated animals kept
by the natives) inhabiting an island situated above 300 miles from a
continent or great continental island; and many islands situated at a
much less distance are equally barren. The Falkland Islands, which
are inhabited by a wolf-like fox, come nearest to an exception; but

41 8 ORIGIN OF SPECIES

this group cannot be considered as oceanic, as it lies on a bank in con-
nection with the mainland at the distance of about 280 miles; more-
over, icebergs formerly brought boulders to its western shores, and
they may have formerly transported foxes, as now frequently hap-
pens in the arctic regions. Yet it cannot be said that small islands
will not support at least small mammals, for they occur in many parts
of the world on very small islands, when lying close to a continent;
and hardly an island can be named on which our smaller quadrupeds
have not become naturalised and greatly multiplied. It cannot be
said, on the ordinary view of creation, that there has not been time for
the creation of mammals; many volcanic islands are sufficiently an-
cient, as shown by the stupendous degradation which they have suf-
fered, and by their tertiary strata: there has also been time for the
production of endemic sf)ecies belonging to other classes; and on con-
tinents it is known that new spjecies of mammals app)ear and disap-
pear at a quicker rate than other and lower animals. Although
terrestrial mammals do not occur on oceanic islands, aerial mammals
do occur on almost every island. New Zealand possesses two bats
found nowhere else in the world : Norfolk Island, the Viti archipel-
ago, the Bonin Islands, the Caroline and Marianne archipelagoes,
and Mauritius, all possess their peculiar bats. Why, it may be asked,
has the supposed creative force produced bats and no other mammals
on remote islands? On my view this question can easily be answered;
for no terrestrial mammal can be transpwrted across a wide space of
sea, but bats can fly across. Bats have been seen wandering by day
far over the Atlantic Ocean; and two North American species either
regularly or occasionally visit Bermuda, at the distance of 600 miles
from the mainland. I hear from Mr. Tomes, who has specially
studied this family, that many species have enormous ranges, and are
found on continents and on far distant islands. Hence we have only
to suppose that such wandering species have been modified in their
new homes in relation to their new position, and we can understand
the presence of endemic bats on oceanic islands, with the absence of
all other terrestrial mammals.

Another interesting relation exists, namely between the depth of
the sea separating islands from each other or from the nearest conti-
nent, and the degree of affinity of their mammalian inhabitants. Mr.

ABSENCE OF BATRACHIANS 419

Windsor Earl has made some striking observations on this head,
since greatly extended by Mr. Wallace's admirable researches, in re-
gard to the great Malay Archipelago, which is traversed near Celebes
by a space of deep ocean, and this separates two widely distinct mam-
mahan faunas. On either side the islands stand on a moderately
shallow submarine bank, and these islands are inhabited by the same
or by closely allied quadrupeds. I have not as yet had time to follow
up this subject in all quarters of the world; but as far as I have gone,
the relation holds good. For instance, Britain is separated by a shal-
low channel from Europe, and the mammals are the same on both
sides; and so it is with all the islands near the shores of Australia.
The West Indian Islands, on the other hand, stand on a deeply sub-
merged bank, nearly 1000 fathoms in depth, and here we find Ameri-
can forms, but the species and even the genera are quite distinct. As
the amount of modification which animals of all kinds undergo
partly depends on the lapse of time, and as the islands which are
separated from each other or from the mainland by shallow channels,
are more likely to have been continuously united within a recent
period than the islands separated by deeper channels, we can under-
stand how it is that a relation exists between the depth of the sea
separating two mammaUan faunas, and the degree of their affinity, —
a relation which is quite inexplicable on the theory of independent
acts of creation.

The foregoing statements in regard to the inhabitants of oceanic
islands, — namely, the fewness of the sf)ecies, with a large proportion
consisting of endemic forms — the members of certain groups, but not
those of other groups in the same class, having been modified — the
absence of certain whole orders, as of batrachians and of terrestrial
mammals, notwithstanding the presence of aerial bats, — the singular
proportions of certain orders of plants, — herbaceous forms having
been develop)ed into trees, etc., — seem to me co accord better with the
belief in the efficiency of occasional means of transport, carried on
during a long course of time, than with the belief in the former con-
nection of all oceanic islands with the nearest continent; for on this
latter view it is probable that the various classes would have immi-
grated more uniformly, and from the species having entered in a body
their mutual relations would not have been much disturbed, and

420 ORIGIN OF SPECIES

consequently they would either not have been modified, or all the
species in a more equable manner.

I do not deny that there are many and serious difficulties in under-
standing how many of the inhabitants of the more remote islands,
whether still retaining the same specific form or subsequently modi-
fied, have reached their present homes. But the probability of other
islands having once existed as halting-places, of which not a wreck
now remains, must not be overlooked. I will specify one difficult
case. Almost all oceanic islands, even the most isolated and smallest,
are inhabited by land shells, generally by endemic species, but some-
times by species found elsewhere — striking instances of which have
been given by Dr. A. A. Gould in relation to the Pacific. Now it
is notorious that land shells are easily killed by sea water; their
eggs, at least such as I have tried, sink in it and are killed. Yet there
must be some unknown, but occasionally efficient, means for their
transportal. Would the just-hatched young sometimes adhere to the
feet of birds roosting on the ground, and thus get transported? It
occurred to me that land shells, when hybernating and having a
membranous diaphragm over the mouth of the shell, might be floated
in chinks of drifted timber across moderately wide arms of the sea.
And I find that several species in this state withstand uninjured an
immersion in sea water during seven days: one shell, the Helix
pomatia, after having been thus treated and again hybernating was
put into sea water for twenty days, and perfectly recovered. During
this length of time the shell might have been carried by a marine
current of average swiftness, to a distance of 660 geographical miles.
As this Helix has a thick calcareous operculum, I removed it, and
when it had formed a new membranous one, I again immersed it
for fourteen days in sea water, and again it recovered and crawled
away. Baron Aucapitaine has since tried similar experiments; he
placed 100 land shells, belonging to ten species, in a box pierced with
holes, and immersed it for a fortnight in the sea. Out of the hundred
shells, twenty-seven recovered. The presence of an operculum seems
to have been of importance, as out of twelve specimens of Cyclostoma
elegans, which is thus furnished, eleven revived. It is remarkable,
seeing how well the Helix pomatia resisted with me the salt-water,
that not one of fifty-four specimens belonging to four other species of

INHABITANTS OF ISLANDS 421

Helix tried by Aucapitaine, recovered. It is, however, not at all prob-
able that land shells have often been thus transported; the feet of
birds offer a more probable method.

ON THE RELATIONS OF THE INHABITANTS OF ISLANDS TO
THOSE OF THE NEAREST MAINLAND

The most striking and important fact for us is the affinity of the
species which inhabit islands to those of the nearest mainland, with-
out being actually the same. Numerous instances could be given.
The Galapagos archipelago, situated under the equator, lies at the
distance of between 500 and 600 miles from the shores of South
America. Here almost every product of the land and of the water
bears the unmistakeable stamp of the American continent. There are
twenty-six land birds; of these, twenty-one, or perhaps twenty-three,
are ranked as distinct species, and would commonly be assumed to
have been here created : yet the close affinity of most of these birds to
American species is manifest in every character, in their habits, ges-
tures, and tones of voice. So it is with the other animals, and with
a large proportion of the plants, as shown by Dr. Hooker in his
admirable Flora of this archipelago. The naturalist, looking at the
inhabitants of these volcanic islands in the Pacific, distant several
hundred miles from the continent, feels that he is standing on
American land. Why should this be so? why should the species
which are supposed to have been created in the Galapagos archi-
pelago, and nowhere else, bear so plainly the stamp of affinity to
those created in America.? There is nothing in the conditions of
life, in the geological nature of the islands, in their height or climate,
or in the proportions in which the several classes are associated to-
gether, which closely resembles the conditions of the South American
coast : in fact, there is a considerable dissimilarity in all these respects.
On the other hand, there is a considerable degree of resemblance in
the volcanic nature of the soil, in the climate, height and size of the
islands, between the Galapagos and Cape Verde archif)elagoes: but
what an entire and absolute difference in their inhabitants! The
inhabitants of the Cape Verde Islands are related to those of Africa,
like those of the Galapagos to America. Facts such as these, admit
of no sort of explanation on the ordinary view of independent crea-

422 ORIGIN OF SPECIES

rion: whereas on the view here maintained, it is obvious that the
Galapagos Islands would be likely to receive colonists from America,
whether by occasional means of transport or (though I do not believe
in this doctrine) by formerly continuous land, and the Cape Verde
Islands from Africa; such colonists would be liable to modification,
— the principle of inheritance still betraying their original birthplace.

Many analogous facts could be given: indeed it is an almost uni-
versal rule that the endemic productions of islands are related to
those of the nearest continent, or of the nearest large island. The
exceptions are few, and most of them can be explained. Thus, al-
though Kerguelen Land stands nearer to Africa than to America,
the plants are related, and that very closely, as we know from Dr.
Hooker's account, to those of America: but on the view that this
island has been mainly stocked by seeds brought with earth and
stones on icebergs, drifted by the prevailing currents, this anomaly
disappears. New Zealand in its endemic planes is much more closely
related to Australia, the nearest mainland, than to any other region :
and this is what might have been expected; but it is also plainly
related to South America, which, although the next nearest conti-
nent, is so enormously remote, that the fact becomes an anomaly.
But this difficulty partially disappears on the view that New Zealand,
South America, and the other southern lands have been stocked in
part from a nearly intermediate though distant point, namely from
the antarctic islands, when they were clothed with vegetation, during
a warmer tertiary period, before the commencement of the last Gla-
cial period. The affinity, which though feeble, I am assured by Dr.
Hooker is real, between the flora of the south-western corner of
Australia and of the Cape of Good Hope, is a far more remarkable
case; but this affinity is confined to the plants, and will, no doubt,
some day be explained.

The same law which has determined the relationship between the
inhabitants of islands and the nearest mainland, is sometimes dis-
played on a small scale, but in a most interesting manner, within the
limits of the same archipelago. Thus each separate island of the
Galapagos archipelago is tenanted, and the fact is a marvellous one,
by many distinct species; but these species are related to each other
in a very much closer manner than to the inhabitants of the Ameri-

INHABITANTS OF ISLANDS 423

can continent, or of any other quarter of the world. This is what
might have been expected, for islands situated so near to each other
would almost necessarily receive immigrants from the same original
source, and from each other. But how is it that many of the immi-
grants have been differently modified, though only in a small degree,
in islands situated within sight of each other, having the same
geological nature, the same height, climate, etc.? This long appeared
to me a great difficulty: but it arises in chief part from the deeply-
seated error of considering the physical conditions of a country as
the most important; whereas it cannot be disputed that the nature
of the other species with which each has to compete, is at least as
important, and generally a far more important element of success.
Now if we look to the sf)ecies which inhabit the Galapagos archi-
pelago, and are likewise found in other parts of the world, we find
that they differ considerably in the several islands. This difference
might indeed have been expected if the islands have been stocked
by occasional means of transport — a seed, for instance, of one plant
having been brought to one island, and that of another plant to
another island, though all proceeding from the same general source.
Hence, when in former times an immigrant first setded on one of
the islands, or when it subsequently spread from one to another,
it would undoubtedly be exposed to different conditions in the
different islands, for it would have to compete with a different set
of organisms; a plant, for instance, would find the ground best fitted
for it occupied by somewhat different species in the different islands,
and would be exposed to the attacks of somewhat different enemies.
If, then, it varied, natural selection would probably favour different
varieties in the different islands. Some species, however, might
spread and yet retain the same character throughout the group, just
as we see some species spreading widely throughout a continent and
remaining the same.

The really surprising fatt in this case of the Galapagos Archipel-
ago, and in a lesser degree in some analogous cases, is that each new
species after being formed in any one island, did not spread quickly
to the other islands. But the islands, though in sight of each other,
are separated by deep arms of the sea, in most cases wider than the
British Channel, and there is no reason to suppose that they have at

424 ORIGIN OF SPECIES

any former period been continuously united. The currents of the
sea are rapid and sweep between the islands, and gales of wind are
extraordinarily rare; so that the islands are far more effectually
separated from each other than they appear on a map. Nevertheless
some of the species, both of those found in other parts of the world
and of those confined to the archipelago, are common to the several
islands; and we may infer from their present manner of distribution,
that they have spread from one island to the others. But we often
take, I think, an erroneous view of the probability of closely-allied
sf)ecies invading each other's territory, when put into free intercom-
munication. Undoubtedly, if one species has any advantage over
another, it will in a very brief time wholly or in part supplant it;
but if both are equally well fitted for their own places, both will
probably hold their separate places for almost any length of time.
Being familiar with the fact that many species, naturalised through
man's agency, have spread with astonishing rapidity over wide
areas, we are apt to infer that most species would thus spread; but
we should remember that the species which become naturalised in
new countries are not generally closely allied to the aboriginal in-
habitants, but are very distinct forms, belonging in a large proportion
of cases, as shown by Alph. de Candolle, to distinct genera. In the
Galapagos archipelago, many even of the birds, though so well
adapted for flying from island to island, differ on the different islands;
thus there are three closely-allied species of mocking-thrush, each
confined to its own island. Now let us suppose the mocking-thrush
of Chatham Island to be blown to Charles Island, which has its own
mocking-thrush; why should it succeed in establishing itself there?
We may safely infer that Charles Island is well stocked with its
own sf)ecies, for annually more eggs are laid and young birds hatched,
than can possibly be reared; and we may infer that the mocking-
thrush peculiar to Charles Island is at least as well fitted for its home
as is the species peculiar to Chatham Island. Sir C. Lyell and Mr.
Wollaston have communicated to me a remarkable fact bearing on
this subject; namely, that Madeira and the adjoining islet of Porto
Santo possess many distinct but representative species of land shells,
some of which live in crevices of stone; and although large quantities
of stone are annually transported from Porto Santo to Madeira, yet

INHABITANTS OF ISLANDS 425

this latter island has not become colonised by the Porto Santo species;
nevertheless both islands have been colonised by European land shells,
which no doubt had some advantage over the indigenous species.
From these considerations I think we need not greatly marvel at the
endemic species which inhabit the several islands of the Galapagos
archipelago, not having all spread from island to island. On the
same continent, also, preoccupation has probably played an important
part in checking the commingling of the species which inhabit
different districts with nearly the same physical conditions. Thus, the
south-east and south-west corners of Australia have nearly the same
physical conditions, and are united by continuous land, yet they are
inhabited by a vast number of distinct mammals, birds, and plants;
so it is, according to Mr. Bates, with the butterflies and other animals
inhabiting the great, open, and continuous valley of the Amazons.
The same principle which governs the general character of the
inhabitants of oceanic islands, namely, the relation to the source
whence colonists could have been most easily derived, together with
their subsequent modification, is of the widest application throughout
nature. We see this on every mountain summit, in every lake and
marsh. For Alpine species, excepting in as far as the same species
have become widely spread during the Glacial epoch, are related to
those of the surrounding lowlands; thus we have in South America,
Alpine humming birds, Alpine rodents, Alpine plants, etc., all
strictly belonging to American forms; and it is obvious that a
mountain, as it became slowly upheaved, would be colonised from
the surrounding lowlands. So it is with the inhabitants of lakes
and marshes, excepting in so far as great facility of transport has
allowed the same forms to prevail throughout large fxjrtions of the
world. We see this same principle in the character of most of the
blind animals inhabiting the caves of America and of Europe. Other
analogous facts could be given. It will, I believe, be found universally
true, that wherever in two regions, let them be ever so distant, many
closely allied or representative species occur, there will likewise be
found some identical species; and wherever many closely allied
species occur, there will be found many forms which some naturalists
rank as distinct sfjecies, and others as mere varieties; these doubtful
forms showing us the steps in the progress of modification.

426 ORIGIN OF SPECIES

The relation between the power and extent of migration in certain
species, either at the present or at some former period, and the
existence at remote points of the world of closely allied species, is
shown in another and more general way. Mr. Gould remarked to
me long ago, that in those genera of birds which range over the
world, many of the species have very wide ranges. I can hardly doubt
that this rule is generally true, though difficult of proof. Amongst
mammals, we see it strikingly displayed in Bats, and in a lesser degree
in the Felidx and Canidx. We see the same rule in the distribution
of butterflies and beetles. So it is with most of the inhabitants of
fresh water, for many of the genera in the most distinct classes
range over the world, and many of the species have enormous ranges.
It is not meant that all, but that some of the species have very wide
ranges in the genera which range very widely. Nor is it meant that
the species in such genera have on an average a very wide range;
for this will largely depend on how far the process of modification
has gone; for instance, two varieties of the same species inhabit
America and Europe, and thus the species has an immense range;
but, if variation were to be carried a little further, the two varieties
would be ranked as distinct species, and their range would be greatly
reduced. Still less is it meant, that species which have the capacity
of crossing barriers and ranging widely, as in the case of certain
powerfully winged birds, will necessarily range widely; for we should
never forget that to range widely implies not only the pwwer of cross-
ing barriers, but the more important power of being victorious in
distant lands in the struggle for life with foreign associates. But
according to the view that all the species of a genus, though dis-
tributed to the most remote points of the world, are descended from
a single progenitor, we ought to find, and I believe as a general rule
we do find, that some at least of the species range very widely.

We should bear in mind that many genera in all classes are of
ancient origin, and the species in this case will have had ample time
for disp>ersal and subsequent modification. There is also reason to
believe, from geological evidence, that within each great class the
lower organisms change at a slower rate than the higher; conse-
quently they will have had a better chance of ranging widely and of
still retaining the same specific character. This fact, together with

SUMMARY 427

that of the seeds and eggs of most lowly organised forms being very
minute and better fitted for distant transportal, probably accounts
for a law which has long been observed, and which has lately been
discussed by Alph. de Candolle in regard to plants, namely, that the
lower any group of organisms stands, the more widely it ranges.

The relations just discussed, — namely, lower organisms ranging
more widely than the higher, — some of the species of widely ranging
genera themselves ranging widely, — such facts, as alpine, lacustrine,
and marsh productions being generally related to those which live
on the surrounding low lands and dry lands, — the striking relation-
ship between the inhabitants of islands and those of the nearest main-
land — the still closer relationship of the distinct inhabitants of the
islands in the same archipelago — are inexplicable on the ordinary
view of the independent creation of each species, but are explicable
if we admit colonisation from the nearest or readiest source, together
with the subsequent adaptation of the colonists to their new homes.

SUMMARY OF THE LAST AND PRESENT CHAPTERS

In these chapters I have endeavoured to show, that if we make
due allowance for our ignorance of the full effects of changes of
climate and of the level of the land, which have certainly occurred
within the recent period, and of other changes which have probably
occurred, — if we remember how ignorant we are with respect to the
many curious means of occasional transport, — if we bear in mind,
and this is a very impxirtant consideration, how often a sp)ecies may
have ranged continuously over a wide area, and then have become
extinct in the intermediate tracts, — the difficulty is not insuperable in
beheving that all the individuals of the same species, wherever found,
are descended from common parents. And we are led to this con-
clusion, which has been arrived at by many naturalists under the
designation of single centres of creation, by various general consider-
ations, more especially from the importance of barriers of all kinds,
and from the analogical distribution of sub-genera, genera, and
families.

With respect to distinct species belonging to the same genus, which
on our theory have spread from one parent-source; if we make the
same allowances as before for our ignorance, and remember that

428 ORIGIN OF SPECIES

some forms of life have changed very slowly, enormous periods of
time having been thus granted for their migration, the difficulties
are far from insuperable; though in this case, as in that of the
individuals of the same species, they are often great.

As exemplifying the effects of climatal changes on distribution, I
have attempted to show how important a part the last Glacial period
has played, which affected even the equatorial regions, and which,
during the alternations of the cold in the north and south, allowed the
productions of opposite hemispheres to mingle, and left some of
them stranded on the mountain summits in all parts of the world.
As showing how diversified are the means of occasional transport, I
have discussed at some little length the means of dispersal of fresh-
water productions.

If the difficulties be not insuperable in admitting that in the long
course of time all the individuals of the same species, and likewise
of the several species belonging to the same genus, have proceeded
from some one source; then all the grand leading facts of geo-
graphical distribution are explicable on the theory of migration,
together with subsequent modification and the multiplication of
new forms. We can thus understand the high importance of barriers,
whether of land or water, in not only separating, but in apparently
forming the several zoological and botanical provinces. We can thus
understand the concentration of related species within the same
areas; and how it is that under different latitudes, for instance, in
South America, the inhabitants of the plains and mountains, of the
forests, marshes, and deserts, are linked together in so mysterious a
manner, and are likewise linked to the extinct beings which formerly
inhabited the same continent. Bearing in mind that the mutual rela-
tion of organism to organism is of the highest importance, we can
see why two areas having nearly the same physical conditions should
often be inhabited by very different forms of life; for according to
the length of time which has elapsed since the colonists entered one
of the regions, or both; according to the nature of the communication
which allowed certain forms and not others to enter, either in greater
or lesser numbers; according or not, as those which entered hap-
pened to come into more or less direct competition with each other
and with the aborigines: and according as the immigrants were

SUMMARY 429

capable of varying more or less rapidly, there would ensue in the
two or more regions, independently of their physical conditions,
infinitely diversified conditions of life; there would be an almost
endless amount of organic action and reaction; and we should find
some groups of beings greatly, and some only slightly modified;
some developed in great force, some existing in scanty numbers — and
this we do find in the several great geographical provinces of the
world.

On these same principles we can understand, as I have endeavoured
to show, why oceanic islands should have few inhabitants, but that
of these, a large proportion should be endemic or peculiar; and why,
in relation to the means of migration, one group of beings should
have all its species peculiar, and another group, even within the
same class, should have all its species the same with those in an
adjoining quarter of the world. We can see why whole groups of
organisms, as batrachians and terrestrial mammals, should be absent
from oceanic islands, whilst the most isolated islands should possess
their own p)eculiar sp)ecies of aerial mammals or bats. We can see
why, in islands, there should be some relation between the presence
of mammals, in a more or less modified condition, and the depth
of the sea between such islands and the mainland. We can clearly
see why all the inhabitants of an archipelago, though specifically
distinct on the several islets, should be closely related to each other;
and should likewise be related, but less closely, to those of the nearest
continent, or other source whence immigrants might have been
derived. We can see why, if there exist very closely allied or repre-
sentative species in two areas, however distant from each other, some
identical species will almost always there be found.

As the late Edward Forbes often insisted, there is a striking
parallelism in the laws of life throughout time and space; the laws
governing the succession of forms in past times being nearly the same
with those governing at the present time the differences in different
areas. We see this in many facts. The endurance of each species
and group of species is continuous in time; for the apparent excepv
tions to the rule are so few, that they may fairly be attributed to our
not having as yet discovered in an intermediate deposit certain forms
which are absent in it, but which occur both above and below: so in

430 ORIGIN OF SPECIES

space, it certainly is the general rule that the area inhabited by a
single species, or by a group of species, is continuous, and the excep-
tions, which are not rare, may, as I have attempted to show, be
accounted for by former migrations under different circumstances,
or through occasional means of transport, or by the species having
become extinct in the intermediate tracts. Both in time and space
species and groups of species have their points of maximum develop-
ment. Groups of species, living during the same period of time, or
living within the same area, are often characterised by trifling features
in common, as of sculpture or colour. In looking to the long suc-
cession of past ages, as in looking to distant provinces throughout
the world, we find that species in certain classes differ little from each
other, whilst those in another class, or only in a different section of
the same order, differ greatly from each other. In both time and space
the lowly organised members of each class generally change less than
the highly organised; but there are in both cases marked exceptions
to the rule. According to our theory, these several relations through-
out time and space are intelligible; for whether we look to the allied
forms of life which have changed during successive ages, or to
those which have changed after having migrated into distant
quarters, in both cases they are connected by the same bond of ordi-
nary generation; in both cases the laws of variation have been the
same, and modifications have been accumulated by the same means
of natural selection.

CHAPTER XIV

Mutual Affinities of Organic Beings : Morphology — Embryology
— Rudimentary Organs

Qusification, groups subordinate to groups — Natural system — Rules and
difficulties in classification, explained on the theory of descent with
modification — Classification of varieties — Descent always used in
classification — Analogical or adaptive characters — Affinities, general,
complex, and radiating — Extinction separates and defines groups —
Morphology, between members of the same class, between parts of
the same individual — Embryology, laws of, explained by variations
not supervening at an early age, and being inherited at a correspond-
ing age — Rudimentary organs; their origin explained — Summary.

CLASSIFICATION

FROM the most remote period in the history of the world
organic beings have been found to resemble each other in
descending degrees, so that they can be classed in groups
under groups. This classification is not arbitrary like the grouping of
the stars in constellations. The existence of groups would have been
of simple significance, if one group had been exclusively fitted to in-
habit the land, and another the water; one to feed on flesh, another
on vegetable matter, and so on; but the case is widely difTerent, for
it is notorious how commonly members of even the same sub-group
have different habits. In the second and fourth chapters, on Variation
and on Natural Selection, I have attempted to show that within each
country it is the widely ranging, the much diffused and common,
that is the dominant species, belonging to the larger genera in each
class, which vary most. The varieties, or incipient species, thus pro-
duced, ultimately become converted into new and distinct species;
and these, on the principle of inheritance, tend to produce other new
and dominant species. Consequently the groups which are now
large, and which generally include many dominant species, tend to
go on increasing in size. I further attempted to show that from the
varying descendants of each species trying to occupy as many and

43 «

432 ORIGIN OF SPECIES

as different places as possible in the economy of nature, they con-
stantly tend to diverge in character. This latter conclusion is sup-
ported by observing the great diversity of forms which, in any small
area, come into the closest competition, and by certain facts in
naturalisation.

I attempted also to show that there is a steady tendency in the
forms which are increasing in number and diverging in character,
to supplant and exterminate the preceding, less divergent, and less
improved forms. I request the reader to turn to the diagram illus-
trating the action, as formerly explained, of these several principles;
and he will see that the inevitable result is, that the modified de-
scendants proceeding from one progenitor become broken up into
groups subordinate to groups. In the diagram each letter on the
uppermost line may represent a genus including several species; and
the whole of the genera along this upper line form together one class,
for all are descended from one ancient parent, and, consequently,
have inherited something in common. But the three genera on the
left hand have, on this same principle, much in common, and form
a sub-family, distinct from that containing the next two genera on
the right hand, which diverged from a common parent at the fifth
stage of descent. These five genera have also much in common,
though less than when grouped in sub-families; and they form a
family distinct from that containing the three genera still farther to
the right hand, which diverged at an earlier period. And all these
genera, descended from (A), form an order distinct from the genera
descended from (I). So that we here have many species descended
from a single progenitor grouped into genera; and the genera into
sub-families, families, and orders, all under one great class. The
grand fact of the natural subordination of organic beings in groups
under groups, which, from its familiarity, does not always sufficiently
strike us, is in my judgment thus explained. No doubt organic
beings, like all other objects, can be classed in many ways, either
artificially by single characters, or more naturally by a number of
characters. We know, for instance, that minerals and the elemental
substances can be thus arranged. In this case there is of course no
relation to genealogical succession, and no cause can at present be
assigned for their falling into groups. But with organic beings the

CLASSIFICATION 433

case is different, and the view above given accords with their natural
arrangement in group under group; and no other explanation has
ever been attempted.

Naturalists, as we have seen, try to arrange the species, genera, and
families in each class, on what is called the Natural System. But
what is meant by this system? Some authors look at it merely as a
scheme for arranging together those living objects which are most
alike, and for separating those which are most unlike; or as an arti-
ficial method of enunciating, as briefly as possible, general proposi-
tions, — that is, by one sentence to give the characters common, for
instance, to all mammals, by another those common to all carnivora,
by another those common to the dog-genus, and then, by adding a
single sentence, a full description is given of each kind of dog. The
ingenuity and utility of this system are indisputable. But many
naturalists think that something more is meant by the Natural
System; they believe that it reveals the plan of the Creator; that
unless it be specified whether order in time or space, or both, or
what else is meant by the plan of the Creator, it seems to me that
nothing is thus added to our knowledge. Expressions such as that
famous one by Linnxus, which we often meet with in a more or
less concealed form, namely, that the characters do not make the
genus, but that the genus gives the characters, seem to imply that
some deeper bond is included in our classifications than mere resem-
blance. I believe that this is the case, and that community of descent
— the one known cause of close similarity in organic beings — is the
bond, which though observed by various degrees of modification,
is partially revealed to us by our classifications.

Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification
either gives some unknown plan of creation, or is simply a scheme
for enunciating general propositions and of placing together the
forms most like each other. It might have been thought (and was in
ancient times thought) that those parts of the structure which deter-
mined the habits of life, and the general place of each being in the
economy of nature, would be of very high importance in classifica-
tion. Nothing can be more false. No one regards the external simi-
larity of a mouse to a shrew, of a dugong to a whale, of a whale to

434 ORIGIN OF SPECIES

a fish, as of any importance. These resemblances, though so inti-
mately connected with the whole life of the being, are ranked as
merely "adaptive or analogical characters"; but to the consideration
of these resemblances we shall recur. It may even be given as a
general rule, that the less any part of the organisation is concerned
with special habits, the more important it becomes for classification.
As an instance: Owen, in speaking of the dugong, says, "The gener-
ative organs, being those which are most remotely related to the
habits and food of an animal, I have always regarded as affording
very clear indications of its true affinities. We are least likely in
the modifications of these organs to mistake a merely adaptive for
an essential character." With plants how remarkable it is that the
organs of vegetation, on which their nutrition and life depend, are
of litde signification; whereas the organs of reproduction, with their
product the seed and embryo, are of paramount importance! So
again in formerly discussing certain morphological characters which
are not functionally important, we have seen that they are often of
the highest service in classification. This depends on their con-
stancy throughout many allied groups; and their constancy chiefly
depends on any slight deviations not having been preserved and
accumulated by natural selection, which acts only on serviceable
characters.

That the mere physiological importance of an organ does not
determine its dassificatory value, is almost proved by the fact, that
in allied groups, in which the same organ, as we have every reason
to suppose, has nearly the same physiological value, its dassificatory
value is widely different. No naturalist can have worked long at
any group without being struck with this fact; and it has been fully
acknowledged in the writings of almost every author. It will suffice
to quote the highest authority, Robert Brown, who, in speaking of
certain organs in the Proteaceae, says their generic importance, "like
that of all their parts, not only in this, but, as I apprehend, in every
natural family, is very unequal, and in some cases seems to be
entirely lost." Again, in another work he says, the genera of the
Connaraceae "differ in having one or more ovaria, in the existence
or absence of albumen, in the imbricate or valvular estivation. Any
one of these characters singly is frequently of more than generic

CLASSIFICATION 435

importance, though here even when all taken together they appear
insufficient to separate Cnestis from Connarus." To give an example
amongst insects: in one great division of the Hymenoptera, the
anteniue, as West wood has remarked, are most constant in structure;
in another division they differ much, and the differences are of quite
subordinate value in classification; yet no one will say that the
antennz in these two divisions of the same order are of unequal
physiological importance. Any number of instances could be given
of the varying importance for classification of the same important
organ within the same group of beings.

Again, no one will say that rudimentary or atrophied organs are
of high physiological or vital importance; yet, undoubtedly, organs
in this condition are often of much value in classification. No one
will dispute that the rudimentary teeth in the upper jaws of young
ruminants, and certain rudimentary bones of the leg, are highly
serviceable in exhibiting the close affinity between ruminants and
pachyderms. Robert Brown has strongly insisted on the fact that the
position of the rudimentary florets is of the highest importance in
the classification of the grasses.

Numerous instances could be given of characters derived from
parts which must be considered of very trifling physiological im-
portance, but which are universally admitted as highly serviceable
in the definition of whole groups. For instance, whether or not
there is an open passage from the nostrils to the mouth, the only
character, according to Owen, which absolutely distinguishes fishes
and reptiles — the inflection of the angle of the lower jaw in Mar-
supials — the manner in which the wings of insects are folded — mere
colour in certain Alga: — mere pubescence on parts of the flower
in grasses — the nature of the dermal covering, as hair or feath-
ers, in the Vertebrata. If the Ornithorhynchus had been covered
with feathers instead of hair, this external and trifling character
would have been considered by naturalists as an imp)ortant aid
in determining the degree of affinity of this strange creature to
birds.

The importance, for classification, of trifling characters, mainly
depends on their being correlated with many other characters of
more or less importance. The value indeed of an aggregate of char-

436 ORIGIN OF SPECIES

acters is very evident in natural history. Hence, as has often been
remarked, a species may depart from its allies in several characters,
both of high physiological importance, and of almost universal
prevalence, and yet leave us in no doubt where it should be ranked.
Hence, also, it has been found that a classification founded on any
single character, however important that may be, has always failed;
for no part of the organisation is invariably constant. The importance
of an aggregate of characters, even when none are important, alone
explains the aphorism enunciated by Linnaeus, namely, that the
characters do not give the genus, but the genus gives the characters;
for this seems founded on the appreciation of many trifling points of
resemblance, too slight to be defined. Certain plants, belonging to
the Malpighiaceac, bear perfect and degraded flowers; in the latter,
as A. de Jussieu has remarked, "The greater number of the charac-
ters proper to the species, to the genus, to the family, to the class,
disappear, and thus laugh at our classification." When Aspicarpa
produced in France, during several years, only these degraded
flowers, departing so wonderfully in a number of the most im-
portant points of structure from the proper type of the order, yet
M. Richard sagaciously saw, as Jussieu observes, that this genus
should still be retained amongst the Malpighiaceae. This case well
illustrates the spirit of our classifications.

Practically, when naturalists are at work, they do not trouble
themselves about the physiological value of the characters which they
use in defining a group or in allocating any particular species. If they
find a character nearly uniform, and common to a great number of
forms, and not common to others, they use it as one of high value;
if common to some lesser number, they use it as of subordinate value.
This principle has been broadly confessed by some naturalists to
be the true one; and by none more clearly than by that excellent
botanist, Aug. St. Hilaire. If several trifling characters are always
found in combination, though no apparent bond of connection can
be discovered between them, especial value is set on them. As in
most groups of animals, important organs, such as those for pro-
pelling the blood, or for aerating it, or those for propagating the
race, are found nearly uniform, they are considered as highly service-
able in classification; but in some groups all these, the most impor-

CLASSIFICATION 437

tant vital organs, are found to offer characters of quite subordinate
values. Thus, as Fritz Miiller has lately remarked, in the same group
of crustaceans, Cypridina is furnished with a heart, whilst in too
closely allied genera, namely Cypris and Cytherea, there is no such
organ; one species of Cypridina has well-developed branchiae, whilst
another sf)ecies is destitute of them.

We can see why characters derived from the embryo should be of
equal importance with those derived from the adult, for a natural
classification of course includes all ages. But it is by no means ob-
vious, on the ordinary view, why the structure of the embryo should
be more important for this purpose than that of the adult, which
alone plays its full part in the economy of nature. Yet it has been
strongly urged by those great naturalists, Milne Edwards and
Agassiz, that embryological characters are the most important of
all; and this doctrine has very generally been admitted as true. Never-
theless, their importance has sometimes been exaggerated, owing to
the adaptive characters of larva; not having been excluded; in order
to show this, Fritz Miiller arranged by the aid of such characters
alone the great class of crustaceans, and the arrangement did not
prove a natural one. But there can be no doubt that embryonic,
excluding larval characters, are of the highest value for classification,
not only with animals but with plants. Thus the main divisions of
flowering plants are founded on differences in the embryo, — on the
number and position of the cotyledons, and on the mode of develop-
ment of the plumule and radicle. We shall immediately see why
these characters possess so high a value in classification, namely, from
the natural system being genealogical in its arrangement.

Our classifications are often plainly influenced by chains of affini-
ties. Nothing can be easier than to define a number of characters
common to all birds; but with crustaceans, any such definition has
hitherto been found impossible. There are crustaceans at the oppo-
site ends of the series, which have hardly a character in common;
yet the species at both ends, from being plainly allied to others, and
these to others, and so onwards, can be recognised as unequivocally
belonging to this, and to no other class of the Articulata.

Geographical distribution has often been used, though perhaps
not quite logically, in classification, more especially in very large

438 ORIGIN OF SPECIES

groups of closely allied forms. Temminck insists on the utility or
even necessity of this practice in certain groups of birds; and it has
been followed by several entomologists and botanists.

Finally, with respect to the comparative value of the various groups
of species, such as orders, sub-orders, families, sub-families, and
genera, they seem to be, at least at present, almost arbitrary. Several
of the best botanists, such as Mr. Bentham and others, have strongly
insisted on their arbitrary value. Instances could be given amongst
plants and insects, of a group first ranked by practised naturalists
as only a genus, and then raised to the rank of a sub-family or family;
and this has been done, not because further research has detected
important structural differences, at first overlooked, but because
numerous allied species with slightly different grades of difference,
have been subsequently discovered.

All the foregoing rules and aids and difficulties in classification
may be explained, if I do not greatly deceive myself, on the view that
the Natural System is founded on descent with modification; — that
the characters which naturalists consider as showing true affinity
between any two or more species, are those which have been inherited
from a common parent, all true classification being genealogical; —
that community of descent is the hidden bond which naturalists have
been unconsciously seeking, and not some unknown plan of creation,
or the enunciation of general propositions, and the mere putting
together and separating objects more or less alike.

But I must explain my meaning more fully. I believe that the
arrangement of the groups within each class, in due subordination
and relation to each other, must be strictly genealogical in order to
be natural; but that the amount of difference in the several branches
or groups, though allied in the same degree in blood to their common
progenitor, may differ greatly, being due to the different degrees of
modification which they have undergone; and this is expressed by
the forms being ranked under different genera, families, sections, or
orders. The reader will best understand what is meant, if he will
take the trouble to refer to the diagram in the fourth chapter. We
will suppose the letters A to L to represent allied genera existing
during the Silurian epoch, and descended from some still earlier
form. In three of these genera (A, F, and I), a species has trans-

CLASSIFICATION 439

mitted modified descendants to the present day, represented by the
fifteen genera (a'* to z") on the uppermost horizontal Hne. Now all
these modified descendants from a single species are related in blood
or descent in the same degree; they may metaphorically be called
cousins to the same millionth degree; yet they differ widely and in
different degrees from each other. The forms descended from A,
now broken up into two or three families, constitute a distinct order
from those descended from I, also broken up into two families. Nor
can the existing sp)ecies, descended from A, be ranked in the same
genus with the parent A; or those from I, with the parent I. But the
existing genus F" may be supposed to have been but slightly modi-
fied; and it will then rank with the parent-genus F; just as some
few still Uving organisms belong to Silurian genera. So that the
comparative value of the differences between these organic beings,
which are all related to each other in the same degree in blood, has
come to be widely different. Nevertheless their genealogical arrange-
ment remains strictly true, not only at the present time, but at each
successive {Deriod of descent. All the modified descendants from A
will have inherited something in common from their common parent,
as will all the descendants from I; so will it be with each subordinate
branch of descendants, at each successive stage. If, however, we su[>
pose any descendant of A, or of I, to have become so much modified
as to have lost all traces of its parentage, in this case, its place in the
natural system will be lost, as seems to have occurred with some few
existing organisms. All the descendants of the genus F, along its
whole line of descent, are supposed to have been but little modified,
and they form a single genus. But this genus, though much isolated,
will still occupy its proper intermediate position. The representation
of the groups, as here given in the diagram on a flat surface, is much
too simple. The branches ought to have diverged in all directions.
If the names of the groups had been simply written down in a linear
series, the representation would have been still less natural; and it
is notoriously not possible to represent in a series, on a flat surface,
the affinities which we discover in nature amongst the beings of the
same group. Thus, the natural system is genealogical in its arrange-
ment, like a pedigree: but the amount of modification which the
different groups have undergone has to be expressed by ranking

440 ORIGIN OF SPECIES

them under different scxalled genera, sub-families, families, sections,
orders, and classes.

It may be worth while to illustrate this view of classification, by
taking the case of languages. If we possessed a perfect pedigree of
mankind, a genealogical arrangement of the races of man would
afford the best classification of the various languages now spoken
throughout the world; and if all extinct languages, and all inter-
mediate and slowly changing dialects, were to be included, such an
arrangement would be the only possible one. Yet it might be that
some ancient languages had altered very little and had given rise to
few new languages, whilst others had altered much owing to the
spreading, isolation, and state of civilisation of the several co-
descended races, and had thus given rise to many new dialects and
languages. The various degrees of difference between the languages
of the same stock, would have to be expressed by groups subordinate
to groups; but the prop)er or even the only possible arrangement
would still be genealogical; and this would be strictly natural, as
it would connect together all languages, extinct and recent, by the
closest affinities, and would give the filiation and origin of each
tongue.

In confirmation of this view, let us glance at the classification of
varieties, which are known or believed to be descended from a single
species. These are grouped under the species, with the sub-varieties
under the varieties; and in some cases, as with the domestic pigeon,
with several other grades of difference. Nearly the same rules are
followed as in classifying species. Authors have insisted on the
necessity of arranging varieties on a natural instead of an artificial
system; we are cautioned, for instance, not to class two varieties of
the pineapple together, merely because their fruit, though the most
important part, happens to be nearly identical; no one puts the
Swedish and common turnip together, though the esculent and
thickened stems are so similar. Whatever part is found to be most
constant, is used in classing varieties; thus the great agriculturist
Marshall says the horns are very useful for this purpose with cattle,
because they are less variable than the shap)e or colour of the body,
etc.; whereas with sheep the horns are much less serviceable, because
less constant. In classing varieties, I apprehend that if we had a

CLASSIFICATION 44 1

real pedigree, a genealogical classification would be universally pre-
ferred; and it has been attempted in some cases. For we might feel
sure, whether there had been more or less modification, that the
principle of inheritance would keep the forms together which were
allied in the greatest number of points. In tumbler pigeons, though
some of the sub-varieties differ in the important character of the
length of the beak, yet all are kept together from having the common
habit of tumbling; but the short-faced breed has nearly or quite lost
this habit; nevertheless, without any thought on the subject, these
tumblers are kept in the same group, because allied in blood and
alike in some other respects.

With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade,
that of species, the two sexes; and how enormously these sometimes
differ in the most important characters, is known to every naturalist:
scarcely a single fact can be predicated in common of the adult males
and hermaphrodites of certain cirrip)edes, and yet no one dreams of
separating them. As soon as the three Orchidean forms, Monachan-
thus, Myanthus, and Catasetum, which had previously been ranked
as three distinct genera, were known to be sometimes produced on
the same plant, they were immediately considered as varieties; and
now I have been able to show that they are the male, female, and
hermaphrodite forms of the same species. The naturalist includes
as one sjiccies the various larval stages of the same individual, how-
ever much they may differ from each other and from the adult, as
well as the so-called alternate generations of Steenstrup, which can
only in a technical sense be considered as the same individual. He
includes monsters and varieties, not from their partial resemblance
to the parent-form, but because they are descended from it.

As descent has universally been used in classing together the
individuals of the same species, though the males and females and
larva! are sometimes extremely different; and as it has been used
in classing varieties which have undergone a certain, and sometimes
a considerable, amount of modification, may not this same element
of descent have been unconsciously used in grouping species under
genera, and genera under higher groups, all under the so-called
natural system.' I believe it has been unconsciously used; and thus

442 ORIGIN OF SPECIES

only can I understand the several rules and guides which have been
followed by our best systematists. As we have no written pedigrees,
we are forced to trace community of descent by resemblances of
any kind. Therefore we choose those characters which are the least
likely to have been modified, in relation to the conditions of life to
which each species has been recently exposed. Rudimentary struc-
tures on this view are as good as, or even sometimes better than,
other parts of the organisation. We care not how trifling a character
may be — let it be the mere inflection of the angle of the jaw, the
manner in which an insect's wing is folded, whether the skin be
covered by hair or feathers — if it prevail throughout many and differ-
ent species, especially those having very different habits of life, it
assumes high value; for we can account for its presence in so many
forms with such different habits, only by inheritance from a common
parent. We may err in this respect in regard to single points of
structure, but when several characters, let them be ever so trifling,
concur throughout a large group of beings having different habits,
we may feel almost sure, on the theory of descent, that these char-
acters have been inherited from a common ancestor; and we know
that such aggregated characters have especial value in classifica-
tion.

We can understand why a species or a group of species may depart
from its allies, in several of its most important characteristics, and
yet be safely classed with them. This may be safely done, and is
often done, as long as a sufficient number of characters, let them be
ever so unimportant, betray the hidden bond of community of de-
scent. Let two forms have not a single character in common, yet, if
these extreme forms are connected together by a chain of interme-
diate groups, we may at once infer their community of descent, and
we put them all into the same class. As we find organs of high
physiological importance — those which serve to preserve life under
the most diverse conditions of existence — are generally the most con-
stant, we attach especial value to them; but if these same organs, in
another group or section of a group, are found to differ much, we
at once value them less in our classification. We shall presently see
why embryological characters are of such high classificatory impor-
tance. Geographical distribution may sometimes be brought usefully

ANAL(X5ICAL RESEMBLANCES 443

into play in classing large genera, because all the species of the same
genus, inhabiting any distinct and isolated region, are in all proba-
bility descended from the same parents.

ANALOGICAL RESEMBLANCES

We can understand, on the above views, the very important dis-
tinction between real affinities and analogical or adaptive resem-
blances. Lamarck first called attention to this subject, and he has
been ably followed by Macleay and others. The resemblance in the
shape of the body and in the fin-like anterior limbs between dugongs
and whales, and between these two orders of mammals and fishes
are analogical. So is the resemblance between a mouse and a shrew-
mouse (Sorex), which belong to different orders; and the still closer
resemblance, insisted on by Mr. Mivart, between the mouse and a
small marsupial animal (Antechinus) of Australia. These latter
resemblances may be accounted for, as it seems to me, by adaptation
for similarly active movements through thickets and herbage, to-
gether with concealment from enemies.

Amongst insects there are innumerable similar instances; thus
Linnxus, misled by external appearances, actually classed an homop-
terous insect as a moth. We see something of the same kind even
with our domestic varieties, as in the strikingly similar shape of the
body in the improved breeds of the Chinese and common pig, which
are descended from distinct species; and in the similarly thickened
stems of the common and specifically distinct Swedish turnip. The
resemblance between the greyhound and the racehorse is hardly
more fanciful than the analogies which have been drawn by some
authors between widely different animals.

On the view of characters being of real importance for classifica-
tion, only in so far as they reveal descent, we can clearly understand
why analogical or adaptive characters, although of the utmost im-
portance to the welfare of the being, are almost valueless to the
systematist. For animals, belonging to two most distinct lines of
descent, may have become adapted to similar conditions, and thus
have assumed a close external resemblance; but such resemblances
will not reveal — will rather tend to conceal their blood-relationship.
We can thus also understand the apparent paradox, that the very

444 ORIGIN OF SPECIES

same characters are analogical when one group is compared with
another, but give true affinities when the members of the same group
are compared together: thus, the shape of the body and fin-like limbs
are only analogical when whales are compared with fishes, being
adaptations in both classes for swimming through the water; but
between the several members of the whale family, the shape of the
body and the fin-like limbs offer characters exhibiting true affinity;
for as these parts are so nearly similar throughout the whole family,
we cannot doubt that they have been inherited from a common
ancestor. So it is with fishes.

Numerous cases could be given of striking resemblances in quite
distinct beings between single parts or organs, which have been
adapted for the same functions. A good instance is afforded by the
close resemblance of the jaws of the dog and Tasmanian wolf or
Thylacinus, — animals which are widely sundered in the natural
system. But this resemblance is confined to general appearance, as
in the prominence of the canines, and in the cutting shap)e of the
molar teeth. For the teeth really differ much: thus the dog has on
each side of the upper jaw four pre-molars and only two molars;
whilst the Thylacinus has three pre-molars and four molars. The
molars also differ much in the two animals in relative size and
structure. The adult dentition is preceded by a widely different milk
dentition. Any one may of course deny that the teeth in either case
have been adapted for tearing flesh, through the natural selection of
successive variations; but if this be admitted in the one case, it is
unintelligible to me that it should be denied in the other. I am glad
to find that so high an authority as Professor Flower has come to
this same conclusion.

The extraordinary cases given in a former chapter, of widely
different fishes possessing electric organs, — of widely different insects
possessing luminous organs, — and of orchids and asclepiads having
pollen-masses with viscid discs, come under this same head of ana-
logical resemblances. But these cases are so wonderful that they were
introduced as difficulties or objections to our theory. In all such cases
some fundamental difference in the growth or development of the
parts, and generally in their matured structure, can be detected. The
end gained is the same, but the means, though appearing superficially

ANALOGICAL RESEMBLANCES 445

to be the same, are essentially different. The principle formerly
alluded to under the term of analogical variation has probably in
these cases often come into play; that is, the members of the same
class, although only distantly allied, have inherited so much in com-
mon in their constitution, that they are apt to vary under similar
exciting causes in a similar manner; and this would obviously aid in
the acquirement through natural selection of parts or organs, strik-
ingly like each other, independently of their direct inheritance from
a common progenitor.

As species belonging to distinct classes have often been adapted
by successive slight modifications to live under nearly similar cir-
cumstances, — to inhabit, for instance, the three elements of land, air,
and water, — we can perhaps understand how it is that a numerical
parallelism has sometimes been observed between the sub-groups of
distinct classes. A naturaHst, struck with a parallelism of this nature,
by arbitrarily raising or sinking the value of the groups in several
classes (and all our experience shows that their valuation is as yet
arbitrary), could easily extend the parallelism over a wide range; and
thus the sejjtenary, quinary, quaternary and ternary classifications
have probably arisen.

There is another and curious class of cases in which close external
resemblance does not depend on adaptation to similar habits of life,
but has been gained for the sake of protection. I allude to the won-
derful manner in which certain butterflies imitate, as first described
by Mr. Bates, other and quite distinct species. This excellent observer
has shown that in some districts of South America, where, for in-
stance, an Ithomia abounds in gaudy swarms, another butterfly,
namely, a Leptalis, is often found mingled in the same flock; and the
latter so closely resembles the Ithomia in every shade and stripe of
colour and even in the shape of its wings, that Mr. Bates, with his
eyes sharpened by collecting during eleven years, was, though always
on his guard, continually deceived. When the mockers and the
mocked are caught and compared, they are found to be very different
in essential structure, and to belong not only to distinrt genera, but
often to distinct families. Had this mimicry occurred in only one or
two instances, it might have been passed over as a strange coinci-
dence. But, if we proceed from a district where one Leptalis imi-

446 ORIGIN OF SPECIES

tates an Ithomia, another mocking and mocked sf)ecies belonging to
the same two genera, equally close in their resemblance, may be
found. Altogether no less than ten genera are enumerated, which in-
clude species that imitate other butterflies. The mockers and mocked
always inhabit the same region; we never find an imitator living re-
mote from the form which it imitates. The mockers are almost
invariably rare insects; the mocked in almost every case abound in
swarms. In the same district in which a species of Leptalis closely
imitates an Ithomia, there are sometimes other Lepidoptera mimick-
ing the same Ithomia: so that in the same place, species of three
g;enera of butterflies and even a moth are found all closely resembling
a butterfly belonging to a fourth genus. It deserves especial notice that
many of the mimicking forms of the Leptalis, as well as of the mim-
icked forms, can be shown by a graduated series to be merely varieties
of the same species; whilst others are undoubtedly distinct species.
But why, it may be asked, are certain forms treated as the mimicked
and others as the mimickers? Mr. Bates satisfactorily answers this
question, by showing that the form which is imitated keeps the usual
dress of the group to which it belongs, whilst the counterfeiters have
changed their dress and do not resemble their nearest allies.

We are next led to inquire what reason can be assigned for certain
butterflies and moths so often assuming the dress of another and
quite distinct form; why, to the perplexity of naturalists, has nature
condescended to the tricks of the stage? Mr. Bates has, no doubt,
hit on the true explanation. The mocked forms, which always
abound in numbers, must habitually escape destruction to a large ex-
tent, otherwise they could not exist in such swarms; and a large
amount of evidence has now been collected, showing that they are
distasteful to birds and other insect-devouring animals. The mocking
forms, on the other hand, that inhabit the same district, are com-
paratively rare, and belong to rare groups; hence they must suffer
habitually from some danger, for otherwise, from the number of eggs
laid by all butterflies, they would in three or four generations swarm
over the whole country. Now if a member of one of these jserse-
cuted and rare groups were to assume a dress so like that of a well-
protected species that it continually deceived the practised eyes of an
entomologist, it would often deceive predaceous birds and insects,

ANALOGICAL RESEMBLANCES 447

and thus often escape destruction. Mr. Bates may almost be said to
have actually witnessed the process by which the mimickers have
come so closely to resemble the mimicked; for he found that some of
the forms of Leptalis which mimic so many other butterflies, varied
in an extreme degree. In one district several varieties occurred, and
of these one alone resembled to a certain extent, the common
Ithomia of the same district. In another district there were two or
three varieties, one of which was much commoner than the others,
and this closely mocked another form of Ithomia. From facts of this
nature, Mr. Bates concludes that the LeptaHs first varies; and when a
variety happens to resemble in some degree any common butterfly
inhabiting the same district, this variety, from its resemblance to a
flourishing and little persecuted kind, has a better chance of escaping
destruction from predaceous birds and insects, and is consequently
oftener preserved; — "the less perfect degrees of resemblance being
generation after generation eliminated, and only the others left to
propagate their kind." So that we have an excellent illustration of
natural selection.

Messrs. Wallace and Trimen have Hkewise described several
equally striking cases of imitation in the Lepidoptera of the Malay
Archipelago and Africa, and with some other insects. Mr. Wallace
has also detected one such case with birds, but we have none with
the larger quadrupeds. The much greater frequency of imitation
with insects than with other animals, is probably the consequence of
their small size; insects cannot defend themselves, excepting indeed
the kinds furnished with a sting, and I have never heard of an in-
stance of such kinds mocking other insects, though they are mocked;
insects cannot easily escape by flight from the larger animals which
prey on them; therefore, sp)eaking metaphorically, they are reduced,
hke most weak creatures, to trickery and dissimulation.

It should be observed that the process of imitation probably never
commenced between forms widely dissimilar in colour. But starting
with species already somewhat like each other, the closest re-
semblance, if beneficial, could readily be gained by the above means;
and if the imitated form was subsequently and gradually modified
through any agency, the imitating form would be led along the same
track, and thus be altered to almost any extent, so that it might ulti-

448 ORIGIN OF SPECIES

mately assume an appearance or colouring wholly unlike that of the
other members of the family to which it belonged. There is, how-
ever, some difficulty on this head, for it is necessary to suppose in
some cases that ancient members belonging to several distinct groups,
before they had diverged to their present extent, accidentally resem-
bled a member of another and protected group in a sufficient degree
to afford some slight protection, this having given the basis for the
subsequent acquisition of the most perfect resemblance.

ON THE NATURE OF THE AFFINFnES CONNECTING ORGANIC BEINGS

As the modified descendants of dominant species, belonging to
the larger genera, tend to inherit the advantages which made the
groups to which they belong large and their parents dominant, they
are almost sure to spread widely, and to seize on more and more
places in the economy of nature. The larger and more dominant
groups within each class thus tend to go on increasing in size; and
they consequently supplant many smaller and feebler groups. Thus
we can account for the fact that all organisms, recent and extinct, are
included under a few great orders, and under still fewer classes. As
showing how few the higher groups are in number, and how widely
they are spread throughout the world, the fact is striking that the dis-
covery of Australia has not added an insect belonging to a new class;
and that in the vegetable kingdom, as I learn from Dr. Hooker, it
has added only two or three families of small size.

In the chapter on Geological Succession I attempted to show, on
the principle of each group having generally diverged much in
character during the long<ontinued process of modification, how it
is that the more ancient forms of life often present characters in some
degree intermediate between existing groups. As some few of the
old and intermediate forms have transmitted to the present day de-
scendants but little modified, these constitute our so-called osculant
or aberrant species. The more aberrant any form is, the greater
must be the number of connecting forms which have been extermi-
nated and utterly lost. And we have some evidence of aberrant
groups having suffered severely from extinction, for they are almost
always represented by extremely few species; and such species as do
occur are generally very distinct from each other, which again implies

AFFINITIES CONNECTING ORGANIC BEINGS 449

extinction. The genera Ornithorhynchus and Lepidosiren, for ex-
ample, would not have been less aberrant had each been represented
by a dozen species, instead of as at present by a single one, or by two
or three. We can, I think, account for this fact only by looking at
aberrant groups as forms which have been conquered by more
successful competitors, with a few members still preserved under
unusually favourable conditions.

Mr. Waterhouse has remarked that, when a member belonging to
one group of animals exhibits an affinity to a quite distinct group,
this affinity in most cases is general and not special; thus, accord-
ing to Mr. Waterhouse, of all rodents, the bizcacha is most nearly
related to marsupials; but in the points in which it approaches this
order, its relations are general, that is, not to any one marsupial
species more than to another. As these points of affinity are believed
to be real and not merely adaptive, they must be due in accordance
with our view to inheritance from a common progenitor. Therefore
we must suppose either that all rodents, including the bizcacha,
branched off from some ancient marsupial, which will naturally have
been more or less intermediate in character with respect to all existing
marsupials; or that both rodents and marsupials branched off from a
common progenitor, and that both groups have since undergone
much modification in divergent directions. On either view we must
suppose that the bizcacha has retained, by inheritance, more of the
characters of its ancient progenitor than have other rodents; and
therefore it will not be specially related to any one existing marsupial,
but indirectly to all or nearly all marsupials, from having partially
retained the character of their common progenitor, or of some early
member of the group. On the other hand, of all marsupials, as Mr.
Waterhouse has remarked, the Phascolomys resembles most nearly,
not any one species, but the general order of rodents. In this case,
however, it may be strongly suspected that the resemblance is only
analogical, owing to the Phascolomys having become adapted to
habits like those of a rodent. The elder De CandoUe has made
nearly similar observations on the general nature of the affinities of
distinct families of plants.

On the principle of the multiplication and gradual divergence in
character of the species descended from a common progenitor, to-

450 ORIGIN OF SPECIES

gether with their retention by inheritance of some characters in com-
mon, we can understand the excessively complex and radiating
affinities by which all the members of the same family or higher
group are connected together. For the common progenitor of a
whole family, now broken up by extinction into distinct groups and
sub-groups, will have transmitted some of its characters, modified in
various ways and degrees, to all the species; and they will conse-
quently be related to each other by circuitous lines of affinity of vari-
ous lengths (as may be seen in the diagram so often referred to),
mounting up through many predecessors. As it is difficult to show the
blood relationship between the numerous kindred of any ancient and
noble family even by the aid of a genealogical tree, and almost im-
possible to do so without this aid, we can understand the extraordi-
nary difficulty which naturalists have experienced in describing,
without the aid of a diagram, the various affinities which they p)er-
ceive between the many living and extinct members of the same great
natural class.

Extinction, as we have seen in the fourth chapter, has played an
important part in defining and widening the intervals between the
several groups in each class. We may thus account for the distinct-
ness of whole classes from each other — for instance, of birds from all
other vertebrate animals — by the belief that many ancient forms of
life have been utterly lost, through which the early progenitors of
birds were formerly connected with the early progenitors of the other
and at that time less differentiated vertebrate classes. There has
been much less extinction of the forms of life which once connected
fishes with batrachians. There has been still less within some whole
classes, for instance the Crustacea, for here the most wonderfully di-
verse forms are still linked together by a long and only partially
broken chain of affinities. Extinction has only defined the groups:
it has by no means made them ; for if every form which has ever lived
on this earth were suddenly to reappear, though it would be quite
impossible to give definitions by which each group could be dis-
tinguished, still a natural classification, or at least a natural arrange-
ment, would be possible. We shall see this by turning to the diagram;
the letters, A to L, may represent eleven Silurian genera, some of
which have produced large groups of modified descendants, with

AFFINITIES CONNECTING ORGANIC BEINGS 45 1

every link in each branch and sub-branch still alive; and the links not
greater than those between existing varieties. In this case it would
be quite impossible to give definitions by which the several members
of the several groups could be distinguished from their more immedi-
ate parents and descendants. Yet the arrangement in the diagram
would still hold good and would be natural; for, on the principle of
inheritance, all the forms descended, for instance, from A, would
have something in common. In a tree we can distinguish this or
that branch, though at the actual fork the two unite and blend to-
gether. We could not, as I have said, define the several groups; but
we could pick out types, or forms, representing most of the char-
acters of each group, whether large or small, and thus give a general
idea of the value of the differences between them. This is what we
should be driven to, if we were ever to succeed in collecting all the
forms in any one class which have lived throughout all time and
space. Assuredly we shall never succeed in making so perfect a col-
lection: nevertheless, in certain classes, we are tending towards this
end; and Milne Edwards has lately insisted, in an able paper, on the
high importance of looking to types, whether or not we can separate
and define the groups to which such types belong.

Finally, we have seen that natural selection, which follows from
the struggle for existence, and which almost inevitably leads to ex-
tinction and divergence of character in the descendants from any one
parent-sp)ecies, explains that great and universal feature in the affin-
ities of all organic beings, namely, their subordination in group under
group. We use the element of descent in classing the individuals of
both sexes and of all ages under one species, although they may
have but few characters in common; we use descent in classing ac-
knowledged varieties, however different they may be from their par-
ents; and I believe that this element of descent is the hidden bond of
connexion which naturalists have sought under the term of the
Natural System. On this idea of the natural system being, in so far
as it is has been perfected, genealogical in its arrangement, with the
grades of difference expressed by the terms genera, families, orders,
etc., we can understand the rules which we are compelled to follow
in our classification. We can understand why we value certain resem-
blances far more than others; why we use rudimentary and useless

452 ORIGIN OF SPECIES

organs, or others of trifling physiological importance; why, in finding
the relations between one group and another, we summarily reject
analogical or adaptive characters, and yet use these same characters
within the limits of the same group. We can clearly see how it is that
all living and extinct forms can be grouped together within a few
great classes; and how the several members of each class are con-
nected together by the most complex and radiating lines of affinities.
We shall never, probably, disentangle the inextricable web of the
affinities between the members of any one class; but when we have
a distinct object in view, and do not look to some unknown plan
of creation, we may hope to make sure but slow progress.

Professor Hiickel in his 'Generelle Morphologic,' and in other
works, has recently brought his great knowledge and abilities to bear
on what he calls phylogeny, or the lines of descent of all organic
beings. In drawing up the several series he trusts chiefly to em-
bryological characters, but receives aid from homologous and rudi-
mentary organs, as well as from the successive periods at which the
various forms of life are believed to have first appeared in our geologi-
cal formations. He has thus boldly made a great beginning, and
shows us how classification will in the future be treated.

MORPHOLOGY

We have seen that the members of the same class, independently of
their habits of life, resemble each other in the general plan of their
organisation. This resemblance is often expressed by the term "unity
of type"; or by saying that the several parts and organs in the differ-
ent sjjecies of the class are homologous. The whole subject is included
under the general term of Morphology. This is one of the most inter-
esting departments of natural history, and may almost be said to be
its very soul. What can be more curious than that the hand of a man,
formed for grasping, that of a mole for digging, the leg of the horse,
the paddle of the porpoise, and the wing of the bat, should all be
constructed on the same pattern, and should include similar bones,
in the same relative positions? How curious it is, to give a subor-
dinate though striking instance, that the hind feet of the kangaroo,
which are so well fitted for bounding over the of)en plains, — those of
the climbing, leaf-eating koala, equally well fitted for grasping the

MORPHOLOGY 453

branches of trees, — those of the ground-dwelling, insect or root-eat-
ing, bandicoots, — and those of some other Australian marsupials, —
should all be constructed on the same extraordinary type, namely
with the bones of the second and third digits extremely slender and
enveloped within the same skin, so that they appear like a single toe
furnished with two claws. Notwithstanding this similarity of pat-
tern, it is obvious that the hind feet of these several animals are used
for as widely different purposes as it is possible to conceive. The case
is rendered all the more striking by the American opossums, which
follow nearly the same habits of life as some of their Australian rela-
tives, having feet constructed on the ordinary plan. Professor Flower,
from whom these statements are taken, remarks in conclusion: "We
may call this conformity to type, without getting much nearer to an
explanation of the phenomenon"; and he then adds, "but is it not
powerfully suggestive of true relationship, of inheritance from a
common ancestor?"

Geoffroy St. Hilaire has strongly insisted on the high importance
of relative position or connexion in homologous parts; they may
differ to almost any extent in form and size, and yet remain con-
nected together in the same invariable order. We never find, for in-
stance, the bones of the arm and fore-arm, or of the thigh and leg,
transposed. Hence the same names can be given to the homologous
bones in widely different animals. We see the same great law in the
construction of the mouths of insects: what can be more different
than the immensely long spiral proboscis of a sphinx-moth, the curi-
ous folded one of a bee or bug, and the great jaws of a beetle? — yet
all these organs, serving for such widely different purposes, are
formed by infinitely numerous modifications of an upper lip, mandi-
bles, and two pairs of maxilljc. The same law governs the construc-
tion of the mouths and limbs of crustaceans. So it is with the flowers
of plants.

Nothing can be more hopeless than to attempt to explain this
similarity of pattern in members of the same class, by utility or by the
doctrine of final causes. The hojjelessness of the attempt has been
expressly admitted by Owen in his most interesting work on the 'Na-
ture of Limbs.' On the ordinary view of the independent creation of
each being, we can only say that so it is; — that it has pleased the

454 ORIGIN OF SPECIES

Creator to construct all the animals and plants in each great class on
a uniform plan; but this is not a scientific explanation.

The explanation is to a large extent simple on the theory of the
selection of successive slight modifications, each modification being
profitable in some way to the modified form, but often affecting by
correlation other parts of the organisation. In changes of this nature,
there will be little or no tendency to alter the original pattern, or to
transpose the parts. The bones of a limb might be shortened and flat-
tened to any extent, becoming at the same time enveloped in thick
membrane, so as to serve as a fin; or a webbed hand might have all
its bones, or certain bones, lengthened to any extent, with the mem-
brane connecting them increased, so as to serve as a wing; yet all these
modifications would not tend to alter the framework of the bones or
the relative connexion of the parts. If we suppose that an early
progenitor — the archetype as it may be called — of all mammals, birds,
and reptiles, had its limbs constructed on the existing general pat-
tern, for whatever purpose they served, we can at once perceive the
plain signification of the homologous construction of the limbs
throughout the class. So with the mouths of insects, we have only to
suppose that their common progenitor had an upper lip, mandibles,
and two pairs of maxilLx, these parts being perhaps very simple in
form; and then natural selection will account for the infinite diversity
in the structure and functions of the mouths of insects. Nevertheless,
it is conceivable that the general pattern of an organ might become
so much obscured as to be finally lost, by the reduction and ultimately
by the complete abortion of certain parts, by the fusion of other parts,
and by the doubling or multiplication of others, — variations which
we know to be within the limits of possibility. In the paddles of the
gigantic extinct sea-lizards, and in the mouths of certain suctorial
crustaceans, the general pattern seems thus to have become partially
obscured.

There is another and equally curious branch of our subject;
namely, serial homologies, or the comparison of the different parts
or organs in the same individual, and not of the same parts or organs
in different members of the same class. Most physiologists believe
that the bones of the skull are homologous — that is, correspond in
number and in relative connexion — with the elemental parts of a

MORPHOLOGY 455

certain number of vertebra:. The anterior and posterior limbs in all
the higher vertebrate classes are plainly homologous. So it is with
the wonderfully complex jaws and legs of crustaceans. It is familiar
to almost every one, that in a flower the relative position of the sepals,
petals, stamens, and pistils, as well as their intimate structure, are
intelligible on the view that they consist of metamorphosed leaves
arranged in a spire. In monstrous plants, we often get direct evidence
of the possibility of one organ being transformed into another; and
we can actually see, during the early or embryonic stages of develop-
ment in flowers, as well as in crustaceans and many other animals,
that organs, which when mature become extremely different are at
first exactly alike.

How inexplicable are the cases of serial homologies on the ordinary
view of creation! Why should the brain be enclosed in a box com-
posed of such numerous and such extraordinarily shaped pieces of
bone, apparently representing vertebrae ? As Owen has remarked, the
benefit derived from the yielding of the separate pieces in the act o£
parturition by mammals, will by no means explain the same con-
struction in the skulls of birds and reptiles. Why should similar
bones have been created to form the wing and the leg of a bat, used
as they are for such totally different purposes, namely flying and
walking? Why should one crustacean, which has an extremely com-
plex mouth formed of many parts, consequently always have fewer
legs; or conversely, those with many legs have simpler mouths.?
Why should the sepals, petals, stamens, and pistils, in each flower,
though fitted for such distinct purposes, be all constructed on the
same pattern?

On the theory of natural selection, we can, to a certain extent, an-
swer these questions. We need not here consider how the bodies of
some animals first became divided into a series of segments, or how
they became divided into right and left sides, with corresponding or-
gans, for such questions are almost beyond investigation. It is, how-
ever, probable that some serial structures are the result of cells multi-
plying by division, entailing the multiplication of the parts developed
from such cells. It must suffice for our purpose to bear in mind that
an indefinite repetition of the same part or organ is the common
characteristic, as Owen has remarked, of all low or little specialised

456 ORIGIN OF SPECIES

forms; therefore the unknown progenitor of the Vertebrata prob-
ably possessed many vertebrae; the unknown progenitor of the Articu-
lata, many segments; and the unknown progenitor of flowering
plants, many leaves arranged in one or more spires. We have also for-
merly seen that parts many times repeated are eminently liable to
vary, not only in number, but in form. Consequently such parts,
being already present in considerable numbers, and being highly
variable, would naturally afford the materials for adaptation to the
most different purposes; yet they would generally retain, through the
force of inheritance, plain traces of their original or fundamental re-
semblance. They would retain this resemblance all the more, as the
variations, which afforded the basis for their subsequent modification
through natural selection, would tend from the first to be similar;
the parts being at an early stage of growth alike, and being subjected
to nearly the same conditions. Such parts, whether more or less
modified, unless their common origin became wholly obscure, would
be serially homologous.

In the great class of molluscs, though the parts in distinct species
can be shown to be homologous, only a few serial homologies, such as
the valves of Chitons, can be indicated; that is, we are seldom enabled
to say that one part is homologous with another part in the same indi-
vidual. And we can understand this fact; for in molluscs, even in the
lowest members of the class, we do not find nearly so much indefinite
repetition of any one part as we find in the other great classes of the
animal and vegetable kingdoms.

But morphology is a much more complex subject than it at first ap-
pears, as has lately been well shown in a remarkable paf)er by Mr. E.
Ray Lankester, who has drawn an important distinction between cer-
tain classes of cases which have all been equally ranked by natural-
ists as homologous. He proposes to call the structures which resemble
each other in distinct animals, owing to their descent from a com-
mon progenitor with subsequent modification, homogenous; and the
resemblances which cannot thus be accounted for, he proposes to call
homoplastic. For instance, he believes that the hearts of birds and
mammals are as a whole homogenous, — that is, have been derived
from a common progenitor; but that the four cavities of the heart in
the two classes are homoplastic, — that is, have been independently de-

DEVELOPMENT AND EMBRYOLOGY 457

veloped. Mr. Lankester also adduces the close resemblance of the
parts on the right and left sides of the body, and in the successive seg-
ments of the same individual animal; and here we have parts com-
monly called homologous, which bear no relation to the descent of
distinct species from a common progenitor. Homoplastic structures
are the same with those which I have classed, though in a very imper-
fect manner, as analogous modifications or resemblances. Their for-
mation may be attributed in part to distinct organisms, or to distinct
parts of the same organism, having varied in an analogous manner;
and in part to similar modifications, having been preserved for the
same general purpose or function, of which many instances have
been given.

Naturalists frequently speak of the skull as formed of metamor-
phosed vertebrae; the jaws of crabs as metamorphosed legs; the sta-
mens and pistils in flowers as metamorphosed leaves; but it would
in most cases be more correct, as Professor Huxley has remarked, to
speak of both skull and vertebrae, jaws and legs, etc., as having been
metamorphosed, not one from the other, as they now exist, but from
some common and simpler element. Most naturaUsts, however,
use such language only in a metaphorical sense; they are far from
meaning that during a long course of descent, primordial organs of
any kind — vertebrx in the one case and legs in the other — have actu-
ally been converted into skulls or jaws. Yet so strong is the appear-
ance of this having occurred, that naturalists can hardly avoid em-
ploying language having this plain signification. According to the
views here maintained, such language may be used literally; and the
wonderful fact of the jaws, for instance, of a crab retaining numerous
characters, which they probably would have retained through inherit-
ance, if they had really been metamorphosed from true though
extremely simple legs, is in part explained.

DEVELOPMENT AND EMBRYOLOGY

This is one of the most important subjects in the whole round of
natural history. The metamorphoses of insects, with which every one
is familiar, are generally effected abruptly by a few stages; but the
transformations are in reality numerous and gradual, though con-
cealed. A certain ephemerous insect (Chloeon) during its develop-

458 ORIGIN OF SPECIES

ment, moults, as shown by Sir J. Lubbock, above twenty times, and
each time undergoes a certain amount of change; and in this case we
see the act of metamorphosis {performed in a primary and gradual
manner. Many insects, and especially certain crustaceans, show us
what wonderful changes of structure can be effected during develop-
ment. Such changes, however, reach their acme in the so-called
alternate generations of some of the lower animals. It is, for in-
stance, an astonishing fact that a delicate branching coralline, studded
with polypi and attached to a submarine rock, should produce, first
by budding and then by transverse division, a host of huge floating
jelly-fishes; and that these should produce eggs, from which are
hatched swimming animalcules, which attach themselves to rocks
and become developed into branching corallines; and so on in an end-
less cycle. The belief in the essential identity of the process of alter-
nate generation and of ordinary metamorphosis has been greatly
strengthened by Wagner's discovery of the larva or maggot of a fly,
namely the Cecidomyia, producing asexually other larvae, and these
others, which finally are developed into mature males and females,
propagating their kind in the ordinary manner by eggs.

It may be worth notice that when Wagner's remarkable discovery
was first announced, I was asked how was it possible to account for
the larvx of this fly having acquired the pwwer of asexual reproduc-
tion. As long as the case remained unique no answer could be given.
But already Grimm has shown that another fly, a Chironomus, re-
produces itself in nearly the same manner, and he beUeves that this
occurs frequently in the order. It is the pupa, and not the larva, of the
Chironomus which has this power; and Grimm further shows that
this case, to a certain extent, "unites that of the Cecidomyia with the
parthenogenesis of the Coccidae," the term parthenogenesis implying
that the mature females of the Coccidae are capable of producing fer-
tile eggs without the concourse of the male. Certain animals belong-
ing to several classes are now known to have the power of ordinary
reproduction at an unusually early age; and we have only to accel-
erate parthenogenetic reproduction by gradual steps to an earlier and
earlier age, — Chironomus showing us an almost exactly intermedi-
ate stage, viz., that of the pupa — and we can perhaps account for the
marvellous case of the Cecidomyia.

DEVELOPMENT AND EMBRYOLOGY 459

It has already been stated that various parts in the same individual
which are exactly alike during an early embryonic period, become
widely different and serve for widely different purposes in the adult
state. So again it has been shown that generally the embryos of the
most distinct species belonging to the same class are closely similar,
but become, when fully developed, widely dissimilar. A better proof
of this latter fact cannot be given than the statement of Von Baer that
"the embryos of mammaha, of birds, lizards, and snakes, probably
also of chelonia, are in their earliest states exceedingly like one an-
other, both as a whole and in the mode of development of their parts;
so much so, in fact, that we can often distinguish the embryos only
by their size. In my possession are two little embryos in spirit, whose
names I have omitted to attach, and at present I am quite unable to
say to what class they belong. They may be lizards or small birds, or
very young mammalia, so complete is the similarity in the mode of
formation of the head and trunk in these animals. The extremities,
however, are still absent in these embryos. But even if they had ex-
isted in the earliest stage of their development we should learn noth-
ing, for the feet of lizards and mammals, the wings and feet of
birds, no less than the hands and feet of man, all arise from the same
fundamental form." The larvae of most crustaceans, at corresponding
stages of development, closely resemble each other, however different
the adults may become; and so it is with very many other animals.
A trace of the law of embryonic resemblance occasionally lasts till a
rather late age: thus birds of the same genus, and of allied genera,
often resemble each other in their immature plumage; as we see in
the spotted feathers in the young of the thrush group. In the cat
tribe, most of the species when adult are striped or spotted in lines;
and stripes or spots can be plainly distinguished in the whelp of the
lion and the puma. We occasionally though rarely see something of
the same kind in plants; thus the first leaves of the ulex or furze, and
the first leaves of the phyllodineous acacias, are pinnate or divided
like the ordinary leaves of the Leguminosz.

The points of structure, in which the embryos of widely different
animals within the same class resemble each other, often have no
direct relation to their condition of existence. We cannot, for in-
stance, suppose that in the embryos of the vertebrata the peculiar loop-

460 ORIGIN OF SPECIES

like courses of the arteries near the branchial slits are related to sim-
ilar conditions, — in the young mammal which is nourished in the
womb of its mother, in the egg of the bird which is hatched in a nest,
and in the spawn of a frog under water. We have no more reason to
believe in such a relation, than we have to believe that the similar
bones in the hand of a man, wing of a bat, and fin of a porpoise, are
related to similar conditions of life. No one supposes that the stripes
on the whelp of a lion, or the spots on the young blackbird, are of
any use to these animals.

The case, however, is different when an animal, during any part
of its embryonic career, is active, and has to provide for itself. The
period of activity may come on earlier or later in life; but whenever
it comes on, the adaptation of the larva to its conditions of life is just
as perfect and as beautiful as in the adult animal. In how important
a manner this has acted, has recently been well shown by Sir J. Lub-
bock in his remarks on the close similarity of the larvi of some in-
sects belonging to very different orders, and on the dissimilarity of
the larvae of other insects within the same order, according to their
habits of life. Owing to such adaptations, the similarity of the larvz
of allied animals is sometimes greatly obscured; especially when there
is a division of labour during the different stages of development, as
when the same larva has during one stage to search for food, and
during another stage has to search for a place of attachment. Cases
can even be given of the larvae of allied species, or groups of species,
differing more from each other than do the adults. In most cases,
however, the larvar, though active, still obey, more or less closely, the
law of common embryonic resemblance. Cirripedes afford a good in-
stance of this; even the illustrious Cuvier did not perceive that a bar-
nacle was a crustacean; but a glance at the larva shows this in an
unmistakable manner. So again the two main divisions of cirripedes,
the pedunculated and sessile, though differing widely in external
appearance, have larvae in all their stages barely distinguishable.

The embryo in the course of development generally rises in organi-
sation; I use this expression, though I am aware that it is hardly
possible to define clearly what is meant by the organisation being
higher or lower. But no one probably will dispute that the butterfly is
higher than the caterpillar. In some cases, however, the mature ani-

DEVELOPMENT AND EMBRYOLOGY 461

mal must be considered as lower in the scale than the larva, as with
certain parasitic crustaceans. To refer once again to cirrip)edes: the
larva: in the first stage have three pairs of locomotive organs, a sim-
ple single eye, and a probosciformed mouth, with which they feed
largely, for they increase much in size. In the second stage, answering
to the chrysalis stage of butterflies, they have six pairs of beautifully
constructed natatory legs, a pair of magnificent compound eyes, and
extremely complex antennas; but they have a closed and imperfect
mouth, and cannot feed: their function at this stage is, to search out
by their well-developed organs of sense, and to reach by their active
powers of swimming, a proper place on which to become attached
and to undergo their final metamorphosis. When this is completed
they are fixed for life: their legs are now converted into prehensile
organs; they again obtain a well<onstructed mouth; but they have no
antennae, and their two eyes are now reconverted into a minute,
single, simple eye-spot. In this last and complete state, cirripedes may
be considered as either more highly or more lowly organised than
they were in the larval condition. But in some genera the larvse
become developed into hermaphrodites having the ordinary struc-
ture, and into what I have called complemental males; and in the
latter the development has assuredly been retrograde, for the male is
a mere sack, which lives for a short time and is destitute of mouth,
stomach, and every other organ of importance, excepting those for
reproduction.

We are so much accustomed to see a difference in structure between
the embryo and the adult, that we are tempted to look at this differ-
ence as in some necessary manner contingent on growth. But there
is no reason why, for instance, the wing of a bat, or the fin of a por-
poise, should not have been sketched out with all their parts in proper
proportion, as soon as any part became visible. In some whole groups
of animals and in certain members of other groups this is the case,
and the embryo does not at any period differ widely from the adult:
thus Owen has remarked in regard to cuttlefish, "there is no meta-
morphosis; the cephalopodic character is manifested long before the
parts of the embryo are completed." Land shells and fresh-water
crustaceans are born having their proper forms, whilst the marine
members of the same two great classes pass through considerable

462 ORIGIN OF SPECIES

and often great changes during their development. Spiders, again,
barely undergo any metamorphosis. The larvae of most insects pass
through a worm-like stage, whether they are active and adapted to
diversified habits, or are inactive from being placed in the midst of
proper nutriment or from being fed by their parents; but in some few
cases, as in that of Aphis, if we look to the admirable drawings of
the development of this insect, by Professor Huxley, we see hardly
any trace of the vermiform stage.

Sometimes it is only the earlier developmental stages which fail.
Thus Fritz Miiller has made the remarkable discovery that certain
shrimp-like crustaceans (allied to Penoeus) first appear under the sim-
ple nauplius-form, and after passing through two or more zoea-
stages, and then through the mysis-stage, finally acquire their mature
structure: now in the whole great malacostracan order, to which
these crustaceans belong, no other member is as yet known to be
first developed under the nauplius-form, though many appear as
zoeas; nevertheless Miiller assigns reasons for his belief, that if there
had been no suppression of development, all these crustaceans would
have appeared as nauplii.

How, then, can we explain these several facts in embryology, —
namely, the very general, though not universal, difference in struc-
ture between the embryo and the adult; the various parts in the same
individual embryo, which ultimately become very unlike and serve
for diverse purposes, being at an early period of growth alike; the
common, but not invariable, resemblance between the embryos or
larvae of the most distinct species in the same class; the embryo often
retaining whilst within the egg or womb, structures which are of no
service to it, either at that or at a later period of life; on the other
hand, larvae which have to provide for their own wants, being per-
fectly adapted to the surrounding conditions; and lastly the fact of
certain larvae standing higher in the scale of organisation than the
mature animal into which they are developed? I believe that all
these facts can be explained, as follows.

It is commonly assumed, perhaps from monstrosities affecting the
embryo at a very early period, that slight variations or individual
differences necessarily appear at an equally early period. We have
litde evidence on this head, but what we have certainly points the

DEVELOPMENT AND EMBRYOLOGY 463

Other way; for it is notorious that breeders of cattle, horses, and vari-
ous fancy animals, cannot positively tell, until some time after birth,
what will be the merits or demerits of their young animals. We see
this plainly in our own children; we cannot tell whether a child will
be tall or short, or what its precise features will be. The question
is not, at what period of life each variation may have been caused,
but at what period the effects are displayed. The cause may have
acted, and I believe often has acted, on one or both parents before
the act of generation. It deserves notice that it is of no importance
to a very young animal, as long as it remains in its mother's womb
or in the egg, or as long as it is nourished and protected by its parent,
whether most of its characters are acquired a little earlier or later
in life. It would not signify, for instance, to a bird which obtained
its food by having a much-curved beak whether or not whilst young
it possessed a beak of this shape, as long as it was fed by its parents.

I have stated in the first chapter, that at whatever age a variation
first appears in the parent, it tends to reappear at a corresponding
age in the offspring. Certain variations can only appear at corre-
sponding ages; for instance, peculiarities in the caterpillar, cocoon, or
imago states of the silk-moth: or, again, in the full-grown horns of
cattle. But variations, which, for all that we can see might have first
appeared either earlier or later in life, likewise tend to reappear at a
corresponding age in the offspring and parent. I am far from mean-
ing that this is invariably the case, and I could give several exceptional
cases of variations (taking the word in the largest sense) which have
supervened at an earlier age in the child than in the parent.

These two principles, namely, that slight variations generally ap-
pear at a not very early period of life, and are inherited at a corre-
sponding not early period, explain, as I believe, all the above specified
leading facts in embryology. But first let us look to a few analogous
cases in our domestic varieties. Some authors who have written on
dogs, maintain that the greyhound and bulldog, though so different,
are really closely allied varieties, descended from the same wild stock;
hence I was curious to see how far their puppies differed from each
other: I was told by breeders that they differed just as much as their
parents, and this, judging by the eye, seemed almost to be the case;
but on actually measuring the old dogs and their six-days-old pup-

464 ORIGIN OF SPECIES

pies, I found that the puppies had not acquired nearly their full
amount of proportional difference. So, again, I was told that the
foals of cart and race horses — breeds which have been almost wholly
formed by selection under domestication — differed as much as the
full-grown animals; but having had careful measurements made of
the dams and of the three-days-old colts of race and heavy cart horses,
I find that this is by no means the case.

As we have conclusive evidence that the breeds of the pigeon are
descended from a single wild species, I compared the young within
twelve hours after being hatched; I carefully measured the propor-
tions (but will not here give the details) of the beak, width of mouth,
length of nostril and of eyelid, size of feet and length of leg, in the
wild parent-species, in pouters, fantails, runts, barbs, dragons, carriers,
and tumblers. Now, some of these birds, when mature, differ in so
extraordinary a manner in the length and form of beak, and in other
characters, that they would certainly have been ranked as distinct
genera if found in a state of nature. But when the nestling birds of
these several breeds were placed in a row, though most of them could
just be distinguished, the proportional differences in the above speci-
fied points were incomparably less than in the full-grown birds.
Some characteristic points of difference — for instance, that of the
width of mouth — could hardly be detected in the young. But there
was one remarkable exception to this rule, for the young of the short-
faced tumbler differed from the young of the wild rock-pigeon and
of the other breeds, in almost exactly the same proportions as in the
adult state.

These facts are explained by the above two principles. Fanciers
select their dogs, horses, pigeons, etc., for breeding, when nearly
grown up: they are indifferent whether the desired qualities are ac-
quired earlier or later in life, if the full-grown animal possesses them.
And the cases just given, more especially that of the pigeons, show
that the characteristic differences which have been accumulated by
man's selection, and which give value to his breeds, do not generally
appear at a very early period of life, and are inherited at a correspond-
ing not early period. But the case of the short-faced tumbler, which
when twelve hours old possessed its proper characters, proves that
this is not the universal rule; for here the characteristic differences

DEVELOPMENT AND EMBRYOLOGY 465

must either have appeared at an earlier period than usual, or, if not so,
the differences must have been inherited, not at a corresponding, but
at an earlier, age.

Now, let us apply these two principles to species in a state of nature.
Let us take a group of birds, descended from some ancient form and
modified through natural selection for different habits. Then, from
the many slight successive variations having supervened in the several
species at a not early age, and having been inherited at a correspond-
ing age, the young will have been but little modified, and they will
still resemble each other much more closely than do the adults, — just
as we have seen with the breeds of the pigeon. We may extend this
view to widely distinct structures and to whole classes. The fore-
limbs, for instance, which once served as legs to a remote progenitor,
may have become, through a long course of modification, adapted in
one descendant to act as hands, in another as paddles, in another as
wings; but on the above two principles the fore-limbs will not have
been much modified in the embryos of these several forms; although
in each form the fore-limb will differ greatly in the adult state. What-
ever influence long<ontinued use or disuse may have had in modify-
ing the limbs or other parts of any species, this will chiefly or solely
have affected it when nearly mature, when it was compelled to use
its full powers to gain its own living; and the effects thus produced
will have been transmitted to the offspring at a corresponding nearly
mature age. Thus the young will not be modified, or will be modified
only in a slight degree, through the effects of the increased use or
disuse of parts.

With some animals the successive variations may have supervened
at a very early period of life, or the steps may have been inherited at
an earlier age than that at which they first occurred. In either of
these cases, the young or embryo will closely resemble the mature
parent-form, as we have seen with the short-faced tumbler. And
this is the rule of development in certain whole groups, or in certain
sub-groups alone, as with cuttlefish, land shells, fresh-water crus-
taceans, spiders, and some members of the great class of insects. With
respect to the final cause of the young in such groups not passing
through any metamorphosis, we can see that this would follow from
the following contingencies; namely, from the young having to pro-

466 ORIGIN OF SPECIES

vide at a very early age for their own wants, and from their following
the same habits of life with their parents; for in this case, it would be
indispensable for their existence that they should be modified in the
same manner as their parents. Again, with respect to the singular
fact that many terrestrial and fresh-water animals do not undergo
any metamorphosis, whilst marine members of the same groups pass
through various transformations, Fritz Miiller has suggested that the
process of slowly modifying and adapting an animal to Uve on the
land or in fresh water, instead of in the sea, would be greatly simpU-
fied by its not passing through any larval stage; for it is not probable
that places well adapted for both the larval and mature stages, under
such new and greatly changed habits of life, would commonly be
found unoccupied or ill-occupied by other organisms. In this case
the gradual acquirement at an earlier and earlier age of the adult
structure would be favoured by natural selection; and all traces of
former metamorphoses would finally be lost.

If, on the other hand, it profited the young of an animal to follow
habits of life slightly different from those of the parent-form, and
consequently to be constructed on a slightly different plan, or if it
profited a larva already different from its parent to change still fur-
ther, then, on the principle of inheritance at corresponding ages, the
young or the larva: might be rendered by natural selection more and
more different from their parents to any conceivable extent. Differ-
ences in the larva might, also, become correlated with successive
stages of its development; so that the larva, in the first stage, might
come to differ greatly from the larva in the second stage, as is the case
with many animals. The adult might also become fitted for sites or
habits, in which organs of locomotion or of the senses, etc., would be
useless; and in this case the metamorphosis would be retrograde.

From the remarks just made we can see how by changes of struc-
ture in the young, in conformity with changed habits of hfe, to-
gether with inheritance at corresponding ages, animals might come
to pass through stages of development, perfectly distinct from the
primordial condition of their adult progenitors. Most of our best
authorities are now convinced that the various larval and pupal
stages of insects have thus been acquired through adaptation, and not
through inheritance from some ancient form. The curious case of

DEVELOPMENT AND EMBRYOL(X5Y 467

Sitaris — a beetle which passes through certain unusual stages of de-
velopment — will illustrate how this might occur. The first larval
form is described by M. Fabre, as an active, minute insect, furnished
with six legs, two long antennx, and four eyes. These larvae are
hatched in the nests of bees; and when the male-bees emerge from
their burrows, in the spring, which they do before the females, the
larvae spring on them, and afterwards crawl on to the females whilst
paired with the males. As soon as the female bee deposits her eggs
on the surface of the honey stored in the cells, the larva; of the Sitaris
leap on the eggs and devour them. Afterwards they undergo a
complete change; their eyes disappear; their legs and antennas become
rudimentary, and they feed on honey; so that they now more closely
resemble the ordinary larvae of insects; ultimately they undergo a
further transformation, and finally emerge as the perfect beetle.
Now, if an insect, undergoing transformations like those of the
Sitaris, were to become the progenitor of a whole new class of insects,
the course of development of the new class would be widely different
from that of our existing insects; and the first larval stage certainly
would not represent the former condition of any adult and ancient
form.

On the other hand, it is highly probable that with many animals
the embryonic or larval stages show us, more or less completely, the
condition of the progenitor of the whole group in its adult state. In
the great class of the Crustacea, forms wonderfully distinct from
each other, namely, suctorial parasites, cirripedes, entomostraca, and
even the malacostraca, appear at first as larva: under the naupUus-
form; and as these larvar live and feed in the open sea, and are not
adapted for any peculiar habits of life, and from other reasons as-
signed by Fritz Miillcr, it is probable that at some very remote f)eriod
an independent adult animal, resembhng the Nauplius, existed, and
subsequently produced, along several divergent Unes of descent, the
above-named great crustacean groups. So again, it is probable, from
what we know of the embryos of mammals, birds, fishes, and reptiles,
that these animals are the modified descendants of some ancient
progenitor, which was furnished in its adult state with branchiae, a
swim-bladder, four fin-like limbs, and a long tail, all fitted for an
aquatic life.

468 ORIGIN OF SPECIES

As all the organic beings, extinct and recent, which have ever lived,
can be arranged within a few great classes; and as all within each
class have, according to our theory, been connected together by fine
gradations, the best, and, if our collections were nearly perfect, the
only possible arrangement, would be genealogical; descent being the
hidden bond of connexion which naturalists have been seeking under
the term of the Natural System. On this view we can understand
how it is that, in the eyes of most naturalists, the structure of the
embryo is even more important for classification than that of the
adult. In two or more groups of animals, however much they may
differ from each other in structure and habits in their adult condition,
if they pass through closely similar embryonic stages, we may feel
assured that they all are descended from one parent-form, and are
therefore closely related. Thus, community in embryonic structure re-
veals community of descent; but dissimilarity in embryonic develop-
ment does not prove discommunity of descent, for in one of two groups
the developmental stages may have been suppressed, or may have been
so greatly modified through adaptation to new habits of life, as to be
no longer recognisable. Even in groups in which the adults have been
modified to an extreme degree, community of origin is often revealed
by the structure of the larvar; we have seen, for instance, that cirri-
pedes, though externally so like shellfish, are at once known by their
larvx to belong to the great class of crustaceans. As the embryo often
shows us more or less plainly the structure of the less modified and
ancient progenitor of the group, we can see why ancient and extinct
forms so often resemble in their adult state the embryos of existing
species of the same class. Agassiz believes this to be a universal law
of nature; and we may hope hereafter to see the law proved true. It
can, however, be proved true only in those cases in which the ancient
state of the progenitor of the group has not been wholly obliterated,
either by successive variations having supervened at a very early
period of growth, or by such variations having been inherited at an
earlier age than that at which they first appeared. It should also be
borne in mind, that the law may be true, but yet, owing to the geolog-
ical record not extending far enough back in time, may remain for a
long period, or for ever, incapable of demonstration. The law will not
stricdy hold good in those cases in which an ancient form became

RUDIMENTARY ORGANS 469

adapted in its larval state to some special line of life, and transmitted
the same larval state to a whole group of descendants; for such larva:
will not resemble any still more ancient form in its adult state.

Thus, as it seems to me, the leading facts in embryology, which are
second to none in importance, are explained on the principle of varia-
tions in the many descendants from some one ancient progenitor,
having appeared at a not very early period of life, and having been
inherited at a corresponding period. Embryology rises greatly in in-
terest, when we look at the embryo as a picture, more or less obscured,
of the progenitor, either in its adult or larval state, of all the members
of the same great class.

RUDIMENTARY, ATROPHIED, AND ABORTED ORGANS

Organs or parts in this strange condition, bearing the plain stamp
of inutihty, are extremely common, or even general, throughout na-
ture. It would be impxDssible to name one of the higher animals in
which some part or other is not in a rudimentary condition. In the
mammalia, for instance, the males possess rudimentary mammje; in
snakes one lobe of the lungs is rudimentary; in birds the "bastard-
wing" may safely be considered as a rudimentary digit, and in some
species the whole wing is so far rudimentary that it cannot be used
for flight. What can be more curious than the presence of teeth in
foetal whales, which when grown up have not a tooth in their heads;
or the teeth, which never cut through the gums, in the upper jaws
of unborn calves?

Rudimentary organs plainly declare their origin and meaning in
various ways. There are beetles belonging to closely allied species,
or even to the same identical species, which have either full-sized and
perfect wings, or mere rudiments of membrane, which not rarely lie
under wing<overs firmly soldered together; and in these cases it is
impossible to doubt, that the rudiments represent wings. Rudimen-
tary organs sometimes retain their potentiality: this occasionally oc-
curs with the mammse of male mammals, which have been known to
become well developed and to secrete milk. So again in the udders
in the genus Bos, there are normally four developed and two rudi-
mentary teats; but the latter in our domestic cows sometimes become
well developed and yield milk. In regard to plants the petals are

470 ORIGIN OF SPECIES

sometimes rudimentary, and sometimes well-developed in the indi-
viduals of the same species. In certain plants having separated sexes
Kolreuter found that by crossing a species, in which the male flowers
included a rudiment of a pistil, with an hermaphrodite species, hav-
ing of course a well-developed pistil, the rudiment in the hybrid off-
spring was much increased in size; and this clearly shows that the
rudimentary and perfect pistils are essentially alike in nature. An
animal may possess various parts in a perfect state, and yet they may
in one sense be rudimentary, for they are useless: thus the tadpole of
the common salamander or water newt, as Mr. G. H. Lewes remarks,
"has gills, and passes its existence in the water; but the Salamandra
atra, which lives high up among the mountains, brings forth its
young full-formed. This animal never lives in the water. Yet if we
open a gravid female, we find tadpoles inside her with exquisitely
feathered gills; and when placed in water they swim about like the
tadpoles of the water newt. Obviously this aquatic organisation
has no reference to the future life of the animal, nor has it any
adaptation to its embryonic condition; it has solely reference to
ancestral adaptations, it repeats a phase in the development of its
progenitors."

An organ, serving for two purposes, may become rudimentary or
utterly aborted for one, even the more important purpose, and remain
perfectly efficient for the other. Thus, in plants, the office of the
pistil is to allow the pollen-tubes to reach the ovules within the ova-
rium. The pistil consists of a stigma supported on a style; but in some
Compositac, the male florets, which of course cannot be fecundated,
have a rudimentary pistil, for it is not crowned with a stigma; but the
style remains well develop)ed and is clothed in the usual manner with
hairs, which serve to brush the pollen out of the surrounding and con-
joined anthers. Again, an organ may become rudimentary for its
propter purpose, and be used for a distinct one: in certain fishes the
swim bladder seems to be rudimentary for its proper function of giv-
ing buoyancy, but has become converted into a nascent breathing
organ or lung. Many similar instances could be given.

Useful organs, however little they may be developed, unless we
have reason to suppose that they were formerly more highly de-
veloped, ought not to be considered as rudimentary. They may be

RUDIMENTARY ORGANS 471

in a nascent condition, and in progress towards further development.
Rudimentary organs, on the other hand, are either quite useless, such
as teeth which never cut through the gums, or almost useless, such as
the wings of an ostrich, which serve merely as sails. As organs in this
condition would formerly, when still less developed, have been of
even less use than at present, they cannot formerly have been pro-
duced through variation and natural selection, which acts solely by
the preservation of useful modifications. They have been partially
retained by the power of inheritance, and relate to a former state of
things. It is, however, often difficult to distinguish between rudimen-
tary and nascent organs; for we can judge only by analogy whether a
part is capable of further development, in which case alone it de-
serves to be called nascent. Organs in this condition will always be
somewhat rare; for beings thus provided will commonly have been
supplanted by their successors with the same organ in a more per-
fect state, and consequently will have become long ago extinct. The
wing of the penguin is of high service, acting as a fin; it may, there-
fore, represent the nascent state of the wing; not that I believe this to
be the case; it is more probably a reduced organ, modified for a new
function; the wing of the Apteryx, on the other hand, is quite use-
less, and is truly rudimentary. Owen considers the simple filamentary
limbs of the Lepidosiren as the "beginnings of organs which attain
full functional development in higher vertebrates"; but, according to
the view lately advocated by Dr. Giinther, they are probably rem-
nants, consisting of the persistent axis of a fin, with the lateral rays
or branches aborted. The mammary glands of the Ornithorhynchus
may be considered, in comparison with the udders of a cow, as in a
nascent condition. The ovigerous frena of certain cirripedes, which
have ceased to give attachment to the ova and are feebly developed,
are nascent branchiae.

Rudimentary organs in the individuals of the same species are very
liable to vary in the degree of their development and in other re-
spects. In closely allied species, also, the extent to which the same or-
gan has been reduced occasionally differs much. This latter fact is
well exemplified in the state of the wings of female moths belonging
to the same family. Rudimentary organs may be utterly aborted; and
this implies, that in certain animals or plants, parts are entirely ab-

472 ORIGIN OF SPECIES

sent which analogy would lead us to expect to find in them, and
which are occasionally found in monstrous individuals. Thus in most
of the Scrophulariacea: the fifth stamen is utterly aborted; yet we may
conclude that a fifth stamen once existed, for a rudiment of it is found
in many species of the family, and this rudiment occasionally becomes
perfectly developed, as may sometimes be seen in the common snap-
dragon. In tracing the homologies of any part in different members
of the same class, nothing is more common, or, in order fully to un-
derstand the relations of the parts, more useful than the discovery
of rudiments. This is well shown in the drawings given by Owen of
the leg bones of the horse, ox, and rhinoceros.

It is an important fact that rudimentary organs, such as teeth in
the upper jaws of whales and ruminants, can often be detected in the
embryo, but afterwards wholly disappear. It is also, I believe, a uni-
versal rule, that a rudimentary part is of greater size in the embryo
relatively to the adjoining parts, than in the adult; so that the organ
at this early age is less rudimentary, or even cannot be said to be in
any degree rudimentary. Hence rudimentary organs in the adult
are often said to have retained their embryonic condition.

I have now given the leading facts with respect to rudimentary
organs. In reflecting on them, every one must be struck with aston-
ishment; for the same reasoning power which tells us that most parts
and organs are exquisitely adapted for certain purposes, tells us with
equal plainness that these rudimentary or atrophied organs are im-
perfect and useless. In works on natural history, rudimentary or-
gans are generally said to have been created "for the sake of sym-
metry," or in order "to complete the scheme of nature." But this
is not an explanation, merely a re-statement of the fact. Nor is it con-
sistent with itself: thus the boa<onstrictor has rudiments of hind-
limbs and of a pelvis, and if it be said that these bones have been
retained "to complete the scheme of nature," why, as Professor Weis-
mann asks, have they not been retained by other snakes, which do not
possess even a vestige of these same bones ? What would be thought
of an astronomer who maintained that the satellites revolve in ellip-
tic courses round their planets "for the sake of symmetry"; because
the planets thus revolve round the sun.' An eminent physiologist
accounts for the presence of rudimentary organs, by supposing that

RUDIMENTARY ORGANS 473

they serve to excrete matter in excess, or matter injurious to the sys-
tem; but can we suppose that the minute papilla, which often repre-
sents the pistil in male flowers, and which is formed of mere cellular
tissue, can thus act ? Can we suppose that rudimentary teeth, which
are subsequently absorbed, are beneficial to the rapidly growing em-
bryonic calf by removing matter so precious as phosphate of lime?
When a man's fingers have been amputated, imperfect nails have
been known to appear on the stumps, and I could as soon beheve that
these vestiges of nails are developed in order to excrete horny mat-
ter, as that the rudimentary nails on the fin of the manatee have been
developed for this same purpose.

On the view of descent with modification, the origin of rudimen-
tary organs is comparatively simple; and we can understand to a
large extent the laws governing their imperfect development. We
have plenty of cases of rudimentary organs in our domestic produc-
tions, as the stump of a tail in tailless breeds, the vestige of an ear in
earless breeds of sheep, the reappearance of minute dangling horns in
hornless breeds of cattle, more esf)ecially, according to Youatt, in
young animals, and the state of the whole flower in the cauliflower.
We often see rudiments of various parts in monsters; but I doubt
whether any of these cases throw light on the origin of rudimentary
organs in a state of nature, further than by showing that rudiments
can be produced; for the balance of evidence clearly indicates that
species under nature do not undergo great and abrupt changes. But
we learn from the study of our domestic productions that the disuse
of parts leads to their reduced size; and that the result is inherited.

It appears probable that disuse has been the main agent in render-
ing organs rudimentary. It would at first lead by slow steps to the
more and more complete reduction of a part, until at last it became
rudimentary, — as in the case of the eyes of animals inhabiting dark
caverns, and of the wings of birds inhabiting oceanic islands, which
have seldom been forced by beasts of prey to take flight, and have ulti-
mately lost the power of flying. Again, an organ, useful under cer-
tain conditions, might become injurious under others, as with the
wings of beetles living on small and exposed islands; and in this
case natural selection will have aided in reducing the organ, until
it was rendered harmless and rudimentary.

474 ORIGIN OF SPECIES

Any change in structure and function, which can be effected by
small stages, is within the power of natural selection; so that an or-
gan rendered, through changed habits of life, useless or injurious for
one pur(X)se, might be modified and used for another purpose. An
organ might, also, be retained for one alone of its former functions.
Organs, originally formed by the aid of natural selection, when ren-
dered useless may well be variable, for their variations can no longer
be checked by natural selection. All this agrees well with what we see
under nature. Moreover, at whatever period of life either disuse or
selection reduces an organ, and this will generally be when the being
has come to maturity and has to exert its full powers of action, the
principle of inheritance at corresponding ages will tend to reproduce
the organ in its reduced state at the same mature age, but will seldom
affect it in the embryo. Thus we can understand the greater size of
rudimentary organs in the embryo relatively to the adjoining parts,
and their lesser relative size in the adult. If, for instance, the digit
of an adult animal was used less and less during many generations,
owing to some change of habits, or if an organ or gland was less and
less functionally exercised, we may infer that it would become re-
duced in size in the adult descendants of this animal, but would
retain nearly its original standard of development in the embryo.

There remains, however, this difficulty. After an organ has ceased
being used, and has become in consequence much reduced, how can
it be still further reduced in size until the merest vestige is left; and
how can it be finally quite obliterated? It is scarcely possible that dis-
use can go on producing any further effect after the organ has once
been rendered functionless. Some additional explanation is here
requisite which I cannot give. If, for instance, it could be proved
that every part of the organisation tends to vary in a greater degree
towards diminution than towards augmentation of size, then we
should be able to understand how an organ which has become use-
less would be rendered, independently of the effects of disuse, rudi-
mentary, and would at last be wholly suppressed; for the variations
towards diminished size would no longer be checked by natural se-
lection. The principle of the economy of growth, explained in a
former chapter, by which the materials forming any part, if not use-

SUMMARY 475

£ul to the possessor, are saved as far as is possible, will perhaps come
into play in rendering a useless part rudimentary. But this principle
will almost necessarily be confined to the earlier stages of the process
of reduction; for we cannot suppose that a minute papilla, for in-
stance, representing in a male flower the pistil of the female flower,
and formed merely of cellular tissue, could be further reduced or
absorbed for the sake of economising nutriment.

Finally, as rudimentary organs, by whatever steps they may have
been degraded into their present useless condition, are the record of
a former state of things, and have been retained solely through the
power of inheritance, — we can understand, on the genealogical view
of classification, how it is that systematists, in placing organisms in
their proper places in the natural system, have often found rudimen-
tary parts as useful as, or even sometimes more useful than, parts
of high physiological importance. Rudimentary organs may be com-
pared with the letters in a word, still retained in the spelling, but
become useless in the pronunciation, but which serve as a clue for its
derivation. On the view of descent with modification, we may con-
clude that the existence of organs in rudimentary, imperfect, and use-
less condition, or quite aborted, far from presenting a strange diffi-
culty, as they assuredly do on the old doctrine of creation, might even
have been anticipated in accordance with the views here explained.

SUMMARY

In this chapter I have attempted to show, that the arrangement of
all organic beings throughout all time in groups under groups — that
the nature of the relationships by which all living and extinct or-
ganisms are united by complex, radiating, and circuitous lines of
affinities into a few grand classes, — the rules followed and the diffi-
culties encountered by naturalists in their classifications, — the value
set upon characters, if constant and prevalent, whether of high or of
the most trifling importance, or, as with rudimentary organs, of no
importance, — the wide opposition in value between analogical or
adaptive characters, and characters of true affinity; and other such
rules; — all naturally follow if we admit the common parentage of al-
lied forms, together with their modification through variation and

476 ORIGIN OF SPECIES

natural selection, with the contingencies of extinction and divergence
of character. In considering this view of classification, it should be
borne in mind that the element of descent has been universally used
in ranking together the sexes, ages, dimorphic forms, and acknowl-
edged varieties of the same species, however much they may differ
from each other in structure. If we extend the use of this element of
descent, — the one certainly known cause of similarity in organic be-
ings, — we shall understand what is meant by the Natural System : it
is genealogical in its attempted arrangement, with the grades of
acquired difference marked by the terms, varieties, species, genera,
famiUes, orders, and classes.

On this same view of descent with modification, most of the great
facts in Morphology become intelligible, — whether we look to the
same pattern displayed by the different species of the same class in
their homologous organs, to whatever purpose applied; or to the
serial and lateral homologies in each individual animal and plant.

On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being in-
herited at a corresponding period, we can understand the leading
facts in Embryology; namely, the close resemblance in the individual
embryo of the parts which are homologous, and which when matured
become widely different in structure and function; and the resem-
blance of the homologous parts or organs in allied though distinct
species, though fitted in the adult state for habits as different as is
possible. Larvae are active embryos, which have been specially modi-
fied in a greater or less degree in relation to their habits of life, with
their modifications inherited at a corresponding early age. On these
same principles, and bearing in mind that when organs are reduced
in size, either from disuse or through natural selection, it will gener-
ally be at that period of life when the being has to provide for its
own wants, and bearing in mind how strong is the force of inherit-
ance — the occurrence of rudimentary organs might even have been
anticipated. The importance of embryological characters and of
rudimentary organs in classification is intelligible, on the view that a
natural arrangement must be genealogical.

Finally, the several classes of facts which have been considered in

SUMMARY 477

this chapter, seem to me to proclaim so plainly, that the innumerable
species, genera and families, with which this world is peopled, are all
descended, each within its own class or group, from common par-
ents, and have all been modified in the course of descent, that I should
without hesitation adopt this viewr even if it were unsupported by
other facts or arguments.

CHAPTER XV

Recapitulation and Conclusion

Recapitulation of the objections to the theory of Natural Selection —
Recapitulation of the general and special circumstances in its favour
— Causes of the general belief in the immutability of species — How
far the theory of Natural Selection may be extended — Effects of its
adoption on the study of Natural History — Concluding remarks.

AS this whole volume is one long argument, it may be conven-
/ \ ient to the reader to have the leading facts and inferences
X .m, briefly recapitulated.

That many and serious objections may be advanced against the the-
ory of descent with modification through Variation and Natural Se-
lection, I do not deny. I have endeavoured to give to them their full
force. Nothing at first can appear more difficult to believe than that
the more complex organs and instincts have been perfected, not by
means superior to, though analogous with, human reason, but by the
accumulation of innumerable slight variations, each good for the in-
dividual possessor. Nevertheless, this difficulty, though appearing to
our imagination insuperably great, cannot be considered real if we
admit the following propositions, namely, that all parts of the organ-
isation and instincts offer, at least, individual differences — that there
is a struggle for existence leading to the preservation of profitable
deviations of structure or instinct — and, lastly, that gradations in the
state of perfection of each organ may have existed, each good of its
kind. The truth of these propositions cannot, I think, be disputed.

It is, no doubt, extremely difficult even to conjecture by what grada-
tions many structures have been perfected, more especially amongst
broken and failing groups of organic beings, which have suffered
much extinction; but we see so many strange gradations in nature,
that we ought to be extremely cautious in saying that any organ or
instinct, or any whole structure, could not have arrived at its present
state by many graduated steps. There are, it must be admitted, cases
of special difficulty opposed to the theory of Natural Selection; and

478

RECAPITULATION AND CONCLUSION 479

one of the most curious of these is the existence in the same com-
munity of two or three defined castes of workers or sterile female
ants; but I have attempted to show how these difficulties can be
mastered.

With respect to the almost universal steriUty of species when first
crossed, which forms so remarkable a contrast with the almost uni-
versal fertility of varieties when crossed, I must refer the reader to
the recapitulation of the facts given at the end of the ninth chapter,
which seem to me conclusively to show that this sterility is no more
a special endowment than is the incapacity of two distinct kinds of
trees to be grafted together; but that it is incidental on differences
confined to the reproductive systems of the intercrossed species. We
see the truth of this conclusion in the vast difference in the results of
crossing the same two species reciprocally, — that is, when one species
is first used as the father and then as the mother. Analogy from the
consideration of dimorphic and trimorphic plants clearly leads to the
same conclusion, for when the forms are illegitimately united, they
yield few or no seed, and their offspring are more or less sterile; and
these forms belong to the same undoubted species, and differ from
each other in no respect except in their reproductive organs and func-
tions.

Although the fertility of varieties when intercrossed and of their
mongrel offspring has been asserted by so many authors to be uni-
versal, this cannot be considered as quite correct after the facts given
on the high authority of Gartner and Kotreuter. Most of the varieties
which have been experimented on have been produced under domes-
tication; and as domestication (I do not mean mere confinement)
almost certainly tends to eliminate that sterility which, judging from
analogy, would have affected the parent-species if intercrossed, we
ought not to expect that domestication would likewise induce sterihty
in their modified descendants when crossed. This elimination of
sterility apparently follows from the same cause which allows our
domestic animals to breed freely under diversified circumstances; and
this again apparently follows from their having been gradually
accustomed to frequent changes in their conditions of life.

A double and parallel series of facts seems to throw much light on
the sterility of species, when first crossed, and of their hybrid off-

480 ORIGIN OF SPECIES

spring. On the one side, there is good reason to believe that slight
changes in the conditions of life give vigour and fertility to all or-
ganic beings. We know also that a cross between the distinct in-
dividuals of the same variety, and between distinct varieties,
increases the number of their offspring, and certainly gives to them
increased size and vigour. This is chiefly owing to the forms which
are crossed having been exposed to somewhat different conditions of
life; for 1 have ascertained by a laborious series of experiments that
if all the individuals of the same variety be subjected during several
generations to the same conditions, the good derived from crossing is
often much diminished or wholly disappears. This is one side of
the case. On the other side, we know that species which have long
been exposed to nearly uniform conditions, when they are sub-
jected under confinement to new and greatly changed conditions,
either perish, or if they survive, are rendered sterile, though retaining
perfect health. This does not occur, or only in a very slight degree,
with our domesticated productions, which have long been exposed
to fluctuating conditions. Hence when we find that hybrids pro-
duced by a cross between two distinct species are few in number, ow-
ing to their p)erishing soon after conception or at a very early age, or if
surviving that they are rendered more or less sterile, it seems highly
probable that this result is due to their having been in fact subjected
to a great change in their conditions of life, from being compounded
of two distinct organisations. He who will explain in a definite man-
ner why, for instance, an elephant or a fox will not breed under con-
finement in its native country, whilst the domestic pig or dog will
breed freely under the most diversified conditions, will at the same
time be able to give a definite answer to the question why two dis-
tinct species, when crossed, as well as their hybrid offspring, are gen-
erally rendered more or less sterile, whilst two domesticated varieties
when crossed and their mongrel offspring are jierfectly fertile.

Turning to geographical distribution, the difficulties encountered
on the theory of descent with modification are serious enough. All
the individuals of the same species, and all the species of the same
genus, or even higher group, are descended from common parents;
and therefore, in however distant and isolated parts of the world they
may now be found, they must in the course of successive generations

RECAPITULATION AND CONCLUSION 48 1

have travelled from some one point to all the others. We are often
wholly unable even to conjecture how this could have been effected.
Yet, as we have reason to believe that some species have retained the
same specific form for very long periods of time, immensely long as
measured by years, too much stress ought not to be laid on the oc-
casional wide diffusion of the same species; for during very long pe-
riods there will always have been a good chance for wide migra-
tion by many means. A broken or interrupted range may often be
accounted for by the extinction of the species in the intermediate
regions. It cannot be denied that we are as yet very ignorant as to the
full extent of the various climatal and geographical changes which
have affected the earth during modern periods; and such changes
will often have facilitated migration. As an example, I have at-
tempted to show how potent has been the influence of the Glacial
period on the distribution of the same and of aUied sfjecies through-
out the world. We are as yet profoundly ignorant of the many
occasional means of transport. With respect to distinct species of
the same genus inhabiting distant and isolated regions, as the process
of modification has necessarily been slow, all the means of migration
will have been possible during a very long period; and consequendy
the difficulty of the wide diffusion of the species of the same genus is
in some degree lessened.

As according to the theory of natural selection an interminable
number of intermediate forms must have existed, linking together
all the species in each group by gradations as fine as are our existing
varieties, it may be asked. Why do we not see these linking forms all
around us? Why are not all organic beings blended together in an
inextricable chaos? With respect to existing forms, we should re-
member that we have no right to expect (excepting in rare cases) to
discover directly connecting links between them, but only between
each and some extinct and supplanted form. Even on a wide area,
which has during a long period remained continuous, and of which
the climatic and other conditions of life change insensibly in proceed-
ing from a district occupied by one species into another district occu-
pied by a closely allied species, we have no just right to expect often
to find intermediate varieties in the intermediate zones. For we have
reason to believe that only a few species of a genus ever undergo

482 ORIGIN OF SPECIES

change; the other species becoming utterly extina and leaving no
modified progeny. Of the species which do change, only a few within
the same country change at the same time; and all modifications are
slowly effected. I have also shown that the intermediate varieties
which probably at first existed in the intermediate zones, would be
liable to be supplanted by the allied forms on either hand; for the lat-
ter, from existing in greater numbers, would generally be modified
and improved at a quicker rate than the intermediate varieties, which
existed in lesser numbers; so that the intermediate varieties would,
in the long run, be supplanted and exterminated.

On this doctrine of the extermination of an infinitude of con-
necting links, between the living and extinct inhabitants of the
world, and at each successive period between the extinct and still
older species, why is not every geological formation charged with
such links.? Why does not every collection of fossil remains afford
plain evidence of the gradation and mutation of the forms of
life? Although geological research has undoubtedly revealed the
former existence of many links, bringing numerous forms of life
much closer together, it does not yield the infinitely many fine grada-
tions between past and present species required on the theory; and
this is the most obvious of the many objections which may be uiged
against it. Why, again, do whole groups of allied sp)ecies apf)ear,
though this apfjearance is often false, to have come in suddenly on
the successive geological stages.' Although we now know that
organic beings appeared on this globe, at a period incalculably remote,
long before the lowest bed of the Cambrian system was deposited,
why do we not find beneath this system great piles of strata stored
with the remains of the progenitors of the Cambrian fossils? For
on the theory, such strata must somewhere have been deposited at
these ancient and utterly unknown epochs of the world's history.

I can answer these questions and objections only on the supposition
that the geological record is far more imperfect than most geologists
believe. The number of specimens in all our museums is absolutely
as nothing compared with the countless generations of countless
Sf)ecies which have certainly existed. The parent-form of any two or
more species would not be in all its characters directly intermediate
between its modified offspring, any more than the rock pigeon is

RECAPITULATION AND CONCLUSION 483

directly intermediate in crop and tail between its descendants, the
pouter and fantail pigeons. We should not be able to recognise a
species as the parent of another and modified species, if we were to
examine the two ever so closely, unless we possessed most of the
intermediate links; and owing to the imperfection of the geological
record, we have no just right to expect to find so many Unks. If two
or three, or even more linking forms were discovered, they woidd
simply be ranked by many naturalists as so many new species, more
esp)ecially if found in diflerent geological sub-stages, let their differ-
ences be ever so slight. Numerous existing doubtful forms could be
named which are probably varieties; but who will pretend that in
future ages so many fossil links will be discovered, that naturalists
will be able to decide whether or not these doubtful forms ought to
be called varieties.'' Only a small portion of the world has been
geologically explored. Only organic beings of certain classes can be
preserved in a fossil condition, at least in any great number. Many
species when once formed never undergo any further change but
become extinct without leaving modified descendants; and the
[Deriods, during which species have undergone modification, though
long as measured by years, have probably been short in comparison
with the periods during which they retained the same form. It is
the dominant and widely ranging species which vary most frequently
and vary most, and varieties are often at first local — both causes
rendering the discovery of intermediate hnks in any one formation
less likely. Local varieties will not spread into other and distant
regions until they are considerably modified and improved; and
when they have spread, and are discovered in a geological formation,
they apf)ear as if suddenly created there, and will be simply classed
as new species. Most formations have been intermittent in their
accumulation; and their duration has probably been shorter than
the average duration of specific forms. Successive formations are
in most cases separated from each other by blank intervals of time
of great length; for fossiliferous formations thick enough to resist
future degradation can as a general rule be accumulated only where
much sediment is deposited on the subsiding bed of the sea. During
the alternate periods of elevation and of stationary level the record
will generally be blank. During these latter periods there will prob-

484 ORIGIN OF SPECIES

ably be more variability in the forms of life; during periods of

subsidence, more extinction.

With respect to the absence of strata rich in fossils beneath the
Cambrian formation, I can recur only to the hypothesis given in
the tenth chapter; namely, that though our continents and oceans
have endured for an enormous period in nearly their present relative
positions, we have no reason to assume that this has always been the
case; consequently formations much older than any now known
may lie buried beneath the great oceans. With respect to the lapse
of time not having been sufficient since our planet was consolidated
for the assumed amount of organic change, and this objection, as
urged by Sir William Thompson, is probably one of the gravest as
yet advanced, I can only say, firstly, that we do not know at what
rate species change as measured by years, and secondly, that many
philosophers are not as yet willing to admit that we know enough of
the constitution of the universe and of the interior of our globe to
speculate with safety on its past duration.

That the geological record is imperfect, all will admit; but that it
is imperfect to the degree required by our theory, few will be inclined
to admit. If we look to long enough intervals of time, geology plainly
declares that species have all changed; and they have changed in the
manner required by the theory, for they have changed slowly and in
a graduated manner. We clearly see this in the fossil remains from
consecutive formations invariably being much more closely related
to each other, than are the fossils from widely separated formations.

Such is the sum of the several chief objections and difficulties
which may be justly urged against the theory; and I have now briefly
recapitulated the answers and explanations which, as far as I can
see, may be given. I have felt these difficulties far too heavily during
many years to doubt their weight. But it deserves especial notice
that the more important objections relate to questions on which we
are confessedly ignorant; nor do we know how ignorant we are.
We do not know all the possible transitional gradations between the
simplest and the most perfect organs; it cannot be pretended that we
know all the varied means of Distribution during the long lapse of
years, or that we know how imperfect is the Geological Record.
Serious as these several objections are, in my judgment they are by

RECAPITULATION AND CONCLUSION 485

no means sufficient to overthrow the theory of descent with subse-
quent modification.

Now let us turn to the other side of the argument. Under domesti-
cation we see much variability, caused, or at least excited, by changed
conditions of life; but often in so obscure a manner, that we are
tempted to consider the variations as sfKjntaneous. Variability is
governed by many complex laws, by correlated growth, compensa-
tion, the increased use and disuse of parts, and the definite action
of the surrounding conditions. There is much difficulty in ascertain-
ing how largely our domestic productions have been modified; but
we may safely infer that the amount has been large, and that modi-
fication can be inherited for long periods. As long as the conditions
of life remain the same, we have reason to believe that a modification,
which has already been inherited for many generations, may continue
to be inherited for an almost infinite number of generations. On the
other hand, we have evidence that variability when it has once come
into play, does not cease under domestication for a very long period;
nor do we know that it ever ceases, for new varieties are still occa?
sionally produced by our oldest domesticated productions.

Variability is not actually caused by man; he only unintentionally
exposes organic beings to new conditions of life, and then nature acts
on the organisation and causes it to vary. But man can and does
select the variations given to him by nature, and thus accumulates
them in any desired manner. He thus adapts animals and plants
for his own benefit or pleasure. He may do this methodically, or he
may do it unconsciously by preserving the individuals most useful
or pleasing to him without any intention of altering the breed. It
is certain that he can largely influence the character of a breed by
selecting, in each successive generation, individual differences so
slight as to be inappreciable except by an educated eye. This uncon-
scious process of selection has been the great agency in the formation
of the most distinct and useful domestic breeds. That many breeds
produced by man have to a large extent the character of natural
species, is shown by the inextricable doubts whether many of them
are varieties or aboriginally distinct sp>ecies.

There is no reason why the principles which have acted so effi-

486 ORIGIN OF SPECIES

ciently under domestication should not have acted under nature. In
the survival of favoured individuals and races, during the constantly-
recurrent Struggle for Existence, we see a powerful and ever-acting
form of Selection. The struggle for existence inevitably follows
from the high geometrical ratio of increase which is common to all
organic beings. This high rate of increase is proved by calculation, —
by the rapid increase of many animals and plants during a succession
of peculiar seasons, and when naturalised in new countries. More
individuals are born than can possibly survive. A grain in the balance
may determine which individuals shall live, and which shall die, —
which variety or species shall increase in number, and which shall
decrease, or finally become extinct. As the individuals of the same
species come in all respects into the closest competition with each
other, the struggle will generally be most severe between them; it
will be almost equally severe between the varieties of the same
species, and next in severity between the species of the same genus.
On the other hand the struggle will often be severe between beings
remote in the scale of nature. The slightest advantage in certain
individuals, at any age or during any season, over those with which
they come into competition, or better adaptation in however slight a
degree to the surrounding physical conditions, will, in the long run,
turn the balance.

With animals having separated sexes, there will be in most cases
a struggle between the males for the possession of the females. The
most vigorous males, or those which have most successfully struggled
with their conditions of life, will generally leave most progeny. But
success will often depend on the males having sf)ecial weapons, or
means of defence, or charms; and a slight advantage will lead to
victory.

As geology plainly proclaims that each land has undergone great
physical changes, we might have expected to find that organic beings
have varied under nature, in the same way as they have varied under
domestication. And if there has been any variability under nature,
it would be an unaccountable fact if natural selection had not come
into play. It has often been asserted, but the assertion is incapable
of proof, that the amount of variation under nature is a strictly
limited quantity. Man, though acting on external characters alone

RECAPITULATION AND CONCLUSION 487

and often capriciously, can produce within a short period a great
result by adding up mere individual differences in his domestic
productions; and every one admits that species present individual
differences. But, besides such differences, all naturalists admit that
natural varieties exist, which are considered sufficiently distinct to be
worthy of record in systematic works. No one has drawn any clear
distinction between individual differences and slight varieties; or
between more plainly marked varieties and sub-species and species.
On separate continents, and on different parts of the same continent
when divided by barriers of any kind, and on outlying islands, what
a multitude of forms exist, which some experienced naturalists rank
as varieties, others as geographical races or sub-species, and others as
distinct, though closely allied species!

If then, animals and plants do vary, let it be ever so slightly or
slowly, why should not variations or individual differences, which
are in any way beneficial, be preserved and accumulated through
natural selection, or the survival of the fittest? If man can by
patience select variations useful to him, why, under changing and
complex conditions of life, should not variations useful to nature's
living products often arise, and be preserved or selected ? What limit
can be put to this power, acting during long ages and rigidly scruti-
nising the whole constitution, structure, and habits of each creature,
— favouring the good and rejecting the bad? I can see no limit to this
power, in slowly and beautifully adapting each form to the most
complex relations of life. The theory of natural selection, even if
we look no farther than this, seems to be in the highest degree
probable. 1 have already recapitulated, as fairly as I could, the
opposed difficulties and objeaions: now let us turn to the special
facts and arguments in favour of the theory.

On the view that species are only strongly marked and permanent
varieties, and that each species first existed as a variety, we can see
why it is that no line of demarcation can be drawn between species,
commonly supposed to have been produced by special acts of crea-
tion, and varieties which are acknowledged to have been produced
by secondary laws. On this same view we can understand how it is
that in a region where many species of a genus have been produced.

488 ORIGIN OF SPECIES

and where they now flourish, these same species should present many
varieties; for where the manufactory of species has been active, we
might expect, as a general rule, to find it still in action; and this is
the case if varieties be incipient species. Moreover, the species of the
larger genefa, which afford the greater number of varieties or incipi-
ent species, retain to a certain degree the character of varieties; for
they differ from each other by a less amount of difference than do
the species of smaller genera. The closely allied species also of the
larger genera apparently have restricted ranges, and in their affinities
they are clustered in little groups round other species — in both
respects resembling varieties. These are strange relations on the
view that each species was independently created, but are intelligible
if each existed first as a variety.

As each species tends by its geometrical rate of reproduction to
increase inordinately in number; and as the modified descendants of
each species will be enabled to increase by as much as they become
more diversified in habits and structure, so as to be able to seize on
many and widely different places in the economy of nature, there
will be a constant tendency in natural selection to preserve the most
divergent offspring of any one species. Hence, during a long-
continued course of modification, the slight differences characteristic
of varieties of the same species, tend to be augmented into the greater
differences characteristic of the species of the same genus. New and
improved varieties will inevitably supplant and exterminate the
older, less improved, and intermediate varieties; and thus species are
rendered to a large extent defined and distinct objects. Dominant
species belonging to the larger groups within each class tend to give
birth to new and dominant forms; so that each large group tends to
become still larger, and at the same time more divergent in char-
acter. But as all groups cannot thus go on increasing in size, for the
world would not hold them, the more dominant groups beat the less
dominant. This tendency in the large groups to go on increasing in
size and diverging in character, together with the inevitable con-
tingency of much extinction, explains the arrangement of all the
forms of life in groups subordinate to groups, all within a few great
classes, which has prevailed throughout all time- This grand fact of

RECAPITULATION AND CONCLUSION 489

the grouping of all organic beings under what is called the Natural
System, is utterly inexplicable on the theory of creation.

As Natural Selection acts solely by accumulating slight, successive,
favourable variations, it can produce no great or sudden modifica-
tions; it can act only by short and slow steps. Hence, the canon of
"Natura non facit saltum," which every fresh addition to our knowl-
edge tends to confirm, is on this theory intelligible. We can see why
throughout nature the same general end is gained by an almost
infinite diversity of means, for every peculiarity when once acquired
is long inherited, and structures already modified in many different
ways have to be adapted for the same general purpose. We can, in
short, see why nature is prodigal in variety, though niggard in inno-
vation. But why this should be a law of nature if each species has
been independently created no man can explain.

Many other facts are, as it seems to me, explicable on this theory.
How strange it is that a bird, under the form of a woodpecker,
should prey on insects on the ground; that upland geese which rarely
or never swim, should possess webbed feet; that a thrush-like bird
should dive and feed on sub-aquatic insects; and that a petrel should
have the habits and structure fitting it for the life of an auk! and so
in endless other cases. But on the view of each species constantly
trying to increase in number, with natural selection always ready to
adapt the slowly varying descendants of each to any unoccupied or
ill-occupied place in nature, these facts cease to be strange, or might
even have been anticipated.

We can to a certain extent understand how it is that there is so
much beauty throughout nature; for this may be largely attributed
to the agency of selection. That beauty, according to our sense of it,
is not universal, must be admitted by every one who will look at
some venomous snakes, at some fishes, and at certain hideous bats
with a distorted resemblance to the human face. Sexual selection
has given the most brilliant colours, elegant patterns, and other
ornaments to the males, and sometimes to both sexes, of many birds,
butterflies, and other animals. With birds it has often rendered the
voice of the male musical to the female, as well as to our ears.
Flowers and fruit have been rendered conspicuous by brilliant

490 ORIGIN OF SPECIES

colours in contrast with the green foHage, in order that the flowers
may be easily seen, visited, and fertilised by insects, and the seeds
disseminated by birds. How it comes that certain colours, sounds,
and forms should give pleasure to man and the lower animals, — that
is, how the sense of beauty in its simplest form was first acquired, —
we do not know any more than how certain odours and flavours
were first rendered agreeable.

As Natural Selection acts by competition, it adapts and improves
the inhabitants of each country only in relation to their co-inhabi-
tants; so that we need feel no surprise at the species of any one
country, although on the ordinary view supposed to have been
created and specially adapted for that country, being beaten and
supplanted by the naturalised productions from another land. Nor
ought we to marvel if all the contrivances in nature be not, as far as
we can judge, absolutely perfect, as in the case even of the human
eye; or if some of them be abhorrent to our ideas of fitness. We
need not marvel at the sting of the bee, when used against an enemy,
causing the bee's own death; at drones being produced in such great
numbers for one single act, and being then slaughtered by their
sterile sisters; at the astonishing waste of pollen by our fir trees; at
the instinctive hatred of the queen bee for her own fertile daughters;
at ichneumonida: feeding within the living bodies of caterpillars; or
at other such cases. The wonder indeed is, on the theory of natural
selection, that more cases of the want of absolute perfection have not
been detected.

The complex and little known laws governing the produaion of
varieties are the same, as far as we can judge, with the laws which
have governed the production of distinct species- In both cases
physical conditions seem to have produced some direct and definite
effect, but how much we cannot say. Thus, when varieties enter any
new station, they occasionally assume some of the charaaers proper
to the species of th.it station. With both varieties and species, use
and disuse seem to have produced a considerable effect; for it is
impossible to resist this conclusion when we look, for instance, at
the loggerheaded duck, which has wings incapable of flight, in
nearly the same condition as in the domestic duck; or when we look
at the burrowing tucu tucu, which is occasionally blind, and then at

RECAPITULATION AND CONCLUSION 49I

certain moles, which are habitually blind and have their eyes covered
with skin; or when we look at the blind animals inhabiting the
dark caves of America and Europe. With varieties and species,
correlated variation seems to have played an important part, so that
when one part has been modified other parts have been necessarily
modified. With both varieties and species, reversions to long-lost
characters occasionally occur. How inexplicable on the theory of
creation is the occasional appearance of stripes on the shoulders and
legs of the several species of the horse-genus and of their hybrids!
How simply is this fact explained if we believe that these species
are all descended from a striped progenitor, in the same manner as
the several domestic breeds of the pigeon are descended from the
blue and barred rock pigeon!

On the ordinary view of each species having been independently
created, why should specific characters, or those by which the species
of the same genus differ from each other, be more variable than
generic characters in which they all agree ? Why, for instance, should
the colour of a flower be more likely to vary in any one species of a
genus, if the other species possess differently coloured flowers, than
if all possessed the same coloured flowers? If species are only well-
marked varieties, of which the characters have become in a high
degree permanent, we can understand this fact; for they have already
varied since they branched off from a common progenitor in certain
characters, by which they have come to be specifically distinct from
each other; therefore these same characters would be more likely
again to vary than the generic characters which have been inherited
without change for an immense period. It is inexplicable on the
theory of creation why a part developed in a very unusual manner
in one species alone of a genus, and therefore, as we may naturally
infer, of great importance to that species, should be eminently liable
to variation; but, on our view, this part has undergone, since the
several species branched off from a common progenitor, an unusual
amount of variability and modification, and therefore we might
expect the part generally to be still variable. But a part may be
developed in the most unusual manner, like the wing of a bat, and
yet not be more variable than any other structure, if the part be
common to many subordinate forms, that is, if it has been inherited

492 ORIGIN OF SPECIES

for a very long period; for in this case it will have been rendered
constant by long<ontinued natural selection.

Glancing at instincts, marvellous as some are, they offer no greater
difficulty than do corporeal structures on the theory of the natural
selection of successive, slight, but profitable modifications. We can
thus understand why nature moves by graduated steps in endowing
different animals of the same class with their several instincts. I
have attempted to show how much light the principle of gradation
throws on the admirable architectural powers of the hive bee. Habit
no doubt often comes into play in modifying instincts; but it
certainly is not indispensable, as we see in the case of neuter insects,
which leave no progeny to inherit the effects of long-continued
habit. On the view of all the species of the same genus having
descended from a common parent, and having inherited much in
common, we can understand how it is that allied species, when
placed under widely different conditions of life, yet follow nearly
the same instincts; why the thrushes of tropical and temperate South
America, for instance, line their nests with mud like our British
species. On the view of instincts having been slowly acquired
through natural selection, we need not marvel at some instincts being
not j^rfect and liable to mistakes, and at many instincts causing
other animals to suffer.

If species be only well-marked and permanent varieties, we can at
once see why their crossed offspring should follow the same complex
laws in their degrees and kinds of resemblance to their parents, — in
being absorbed into each other by successive crosses, and in other
such points, — as do the crossed offspring of acknowledged varieties.
This similarity would be a strange fact, if species had been inde-
pendently created and varieties had been produced through sec-
ondary laws.

If we admit that the geological record is imperfect to an extreme
degree, then the facts, which the record does give, strongly support
the theory of descent with modification. New sjjecies have come on
the stage slowly and at successive intervals; and the amount of
change, after equal intervals of time, is widely different in different
groups. The extinction of species and of whole groups of sf)ecies,
which has played so conspicuous a part in the history of the organic

RECAPITULATION AND CONCLUSION 493

world, almost inevitably follows from the principle of natural selec-
tion; for old forms are supplanted by new and improved forms.
Neither single species nor groups of species reappear when the chain
of ordinary generation is once broken. The gradual diffusion of
dominant forms, with the slow modification of their descendants,
causes the forms of life, after long intervals of time, to appear as if
they had changed simultaneously throughout the world. The fact
of the fossil remains of each formation being in some degree inter-
mediate in character between the fossils in the formations above and
below, is simply explained by their intermediate position in the
chain of descent. The grand fact that all extinct beings can be classed
with all recent beings, naturally follows from the living and the
extinct being the offspring of common parents. As species have
generally diverged in character during their long course of descent
and modification, we can understand why it is that the more ancient
forms, or early progenitors of each group, so often occupy a position
in some degree intermediate between existing groups. Recent forms
are generally looked upon as being, on the whole, higher in the
scale of organisation than ancient forms; and they must be higher,
in so far as the later and more improved forms have conquered the
older and less improved forms in the struggle for life; they have
also generally had their organs more specialised for different func-
tions. This fact is perfectly compatible with numerous beings still
retaining simple and but little improved structures, fitted for simple
conditions of life; it is likewise compatible with some forms having
retrograded in organisation, by having become at each stage of
descent better fitted for new and degraded habits of life. Lastly, the
wonderful law of the long endurance of allied forms on the same
continent, — of marsupials in Australia, of edentata in America, and
other such cases, — is intelligible, for within the same country the
existing and the extinct will be closely allied by descent.

Looking to geographical distribution, if we admit that there has
been during the long course of ages much migration from one
part of the world to another, owing to former climatal and geo-
graphical changes and to the many occasional and unknown means
of disf)ersal, then we can understand, on the theory of descent with
modification, most of the great leading facts in Distribution. We

494 ORIGIN OF SPECIES

can see why there should be so striking a parallelism in the distribu-
tion of organic beings throughout space, and in their geological suc-
cession throughout time; for in both cases the beings have been con-
nected by the bond of ordinary generation, and the means of modifi-
cation have been the same. We see the full meaning of the wonderful
fact, which has struck every traveller, namely, that on the same conti-
nent, under the most diverse conditions, under heat and cold, on
mountain and lowland, on deserts and marshes, most of the inhabit-
ants within each great class are plainly related; for they are the
descendants of the same progenitors and early colonists. On this
same principle of former migration, combined in most cases with
modification, we can understand, by the aid of the Glacial period,
the identity of some few plants, and the close aUiance of many
others, on the most distant mountains, and in the northern and
southern temperate zones; and likewise the close alliance of some
of the inhabitants of the sea in the northern and southern temperate
latitudes, though separated by the whole intertropical ocean. Al-
though two countries may present physical conditions as closely
similar as the same species ever require, we need feel no surprise
at their inhabitants being widely different, if they have been for a
long period completely sundered from each other; for as the relation
of organism to organism is the most impnartant of all relations, and
as the two countries will have received colonists at various periods
and in different proportions, from some other country or from each
other, the course of modification in the two areas will inevitably
have been different.

On this view of migration, with subsequent modification, we see
why oceanic islands are inhabited by only few sfjecies, but of these,
why many are peculiar or endemic forms. We clearly see why
species belonging to those groups of animals which cannot cross
wide spaces of the ocean, as frogs and terrestrial mammals, do not
inhabit oceanic islands; and why, on the other hand, new and
peculiar species of bats, animals which can traverse the ocean, are
often found on islands far distant from any continent. Such cases
as the presence of peculiar species of bats on oceanic islands and the
absence of all other terrestrial mammals, are facts utterly inexplicable
on the theory of independent acts of creation.

RECAPITULATION AND CONCLUSION 495

The existence of closely allied or representative species in any
two areas, implies, on the theory of descent with modification, that
the same parent-forms formerly inhabited both areas: and we almost
invariably find that wherever many closely aUied species inhabit two
areas, some identical species are still common to both. Wherever
many closely allied yet distinct species occur, doubtful forms and
varieties belonging to the same groups likewise occur. It is a rule
of high generality that the inhabitants of each area are related to the
inhabitants of the nearest source whence immigrants might have
derived. We see this in the striking relation of nearly all the plants
and animals of the Galapagos archipelago, of Juan Fernandez, and
of the other American islands, to the plants and animals of the
neighbouring American mainland; and of those of the Cape de
Verde archipelago, and of the other African islands to the African
mainland. It must be admitted that these facts receive no explana-
tion on the theory of creation.

The fact, as we have seen, that all past and present organic beings
can be arranged within a few great classes, in groups subordinate to
groups, and with the extinct groups often falling in between the
recent groups, is intelligible on the theory of natural selection with
its contingencies of extinction and divergence of character. On these
same principles we see how it is, that the mutual affinities of the
forms within each class are so complex and circuitous. We see why
certain characters are far more serviceable than others for classifica-
tion; why adaptive characters, though of paramount importance to
the beings, are of hardly any importance in classification; why
characters derived from rudimentary parts, though of no service to
the beings, are often of high classificatory value; and why embryo-
logical characters are often the most valuable of all. The real affini-
ties of all organic beings, in contradistinction to their adaptive
resemblances, are due to inheritance or community of descent. The
Natural System is a genealogical arrangement, with the acquired
grades of difference, marked by the terms, varieties, species, genera,
families, etc.; and we have to discover the lines of descent by the
most permanent characters whatever they may be and of however
slight vital importance.

The similar framework of bones in the hand of a man, wing of

49^ ORIGIN OF SPECIES

a bat, fin of a porpoise, and leg of the horse, — the same number of
vertebrae forming the neck of the giraffe and of the elephant,— and
innumerable other such facts, at once explain themselves on the
theory of descent with slow and slight successive modifications. The
similarity of pattern in the wing and in the leg of a bat, though used
for such different purpose,— in the jaws and legs of a crab,— in the
petals, stamens, and pistils of a flower, is likewise, to a large extent,
intelligible on the view of the gradual modification of parts or
organs, which were aboriginally alike in an early progenitor in each
of these classes. On the principle of successive variations not always
supervening at an early age, and being inherited at a corresponding
not early period of life, we clearly see why the embryos of mammals,
birds, reptiles, and fishes should be so closely similar, and so unhke
the adult forms. We may cease marvelling at the embryo of an air-
breathing mammal or bird having branchial sUts and arteries run-
ning in loops, like those of a fish which has to breathe the air dis-
solved in water by the aid of well-developed branchix.

Disuse, aided sometimes by natural selection, will often have
reduced organs when rendered useless under changed habits or
conditions of life; and we can understand on this view the meaning
of rudimentary organs. But disuse and selection will generally act
on each creature, when it has come to maturity and has to play its
full part in the struggle for existence, and will thus have little power
on an organ during early life; hence the organ will not be reduced
or rendered rudimentary at this early age. The calf, for instance,
has inherited teeth, which never cut through the gums of the upper
jaw, from an early progenitor having well-developed teeth; and we
may believe, that the teeth in the mature animal were formerly
reduced by disuse, owing to the tongue and palate, or lips, having
become excellently fitted through natural selection to browse with-
out their aid; whereas in the calf, the teeth have been left unaffected,
and on the principle of inheritance at corresponding ages have been
inherited from a remote period to the present day. On the view
of each organism with all its separate parts having been specially
created, how utterly inexplicable is it that organs bearing the plain
stamp of inutility, such as the teeth in the embryonic calf or the
shrivelled wings under the soldered wing<overs of many beetles.

RECAPITULATION AND CONCLUSION 497

should so frequently occur. Nature may be said to have taken pains
to reveal her scheme of modification, by means of rudimentary
organs, of embryological and homologous structures, but we are too
blind to understand her meaning.

I have now recapitulated the facts and considerations which have
thoroughly convinced me that species have been modified, during
a long course of descent. This has been effected chiefly through the
natural selection of numerous successive, slight, favourable varia-
tions; aided in an important manner by the inherited effects of the
use and disuse of parts; and in an unimportant manner, that is in
relation to adaptive structures, whether past or present, by the direct
action of external conditions, and by variations which seem to us
in our ignorance to arise spontaneously. It app)ears that I formerly
underrated the frequency and value of these latter forms of variation,
as leading to permanent modifications of structure independently
of natural selection. But as my conclusions have lately been much
misrepresented, and it has been stated that I attribute the modifica-
tion of species exclusively to natural selection, I may be permitted
to remark that in the first edition of this work, and subsequently,
I placed in a most conspicuous position — namely, at the close of the
Introduction — the following words: "I am convinced that natural
selection has been the main but not the exclusive means of modi-
fication." This has been of no avail. Great is the power of steady
misrepresentation; but the history of science shows that fortunately
this fxjwer does not long endure.

It can hardly be supposed that a false theory would explain, in so
satisfactory a manner as does the theory of natural selection, the
several large classes of facts above specified. It has recently been
objected that this is an unsafe method of arguing; but it is a method
used in judging of the common events of life, and has often been used
by the greatest natural philosophers. The undulatory theory of light
has thus been arrived at; and the belief in the revolution of the earth
on its own axis was until lately supported by hardly any direct evi-
dence. It is no valid objection that science as yet throws no light on
the far higher problem of the essence or origin of life. Who can
explain what is the essence of the attraction of gravity? No one
now objects to following out the results consequent on this unknown

498 ORIGIN OF SPECIES

element of attraction; notwithstanding that Leibnitz formerly
accused Newton of introducing "occult qualities and miracles into
philosophy."

I see no good reason why the views given in this volume should
shock the religious feelings of any one. It is satisfactory, as showing
how transient such impressions are, to remember that the greatest
discovery ever made by man, namely, the law of the attraction of
gravity, was also attacked by Leibnitz, "as subversive of natural, and
inferentially of revealed, religion." A celebrated author and divine
has written to me that "he has gradually learnt to see that it is just as
noble a conception of the Deity to believe that He created a few
original forms capable of self-development into other and needful
forms, as to believe that He required a fresh act of creation to
supply the voids caused by the action of His laws."

Why, it may be asked, until recently did nearly all the most
eminent Uving naturalists and geologists disbelieve in the muta-
bility of species. It cannot be asserted that organic beings in a state
of nature are subject to no variation; it cannot be proved that the
amount of variation in the course of long ages is a limited quantity;
no clear distinction has been, or can be, drawn between species and
well-marked varieties. It cannot be maintained that species when
intercrossed are invariably sterile, and varieties invariably fertile; or
that sterility is a special endowment and sign of creation. The belief
that species were immutable productions was almost unavoidable
as long as the history of the world was thought to be of short
duration; and now that we have acquired some idea of the lapse of
time, we are too apt to assume, without proof, that the geological
record is so perfect that it would have afforded us plain evidence
of the mutation of species, if they had undergone mutation.

But the chief cause of our natural unwillingness to admit that one
species has given birth to other and distinct species, is that we are
always slow in admitting great changes of which we do not see the
steps. The difficulty is the same as that felt by so many geologists,
when Lyell first insisted that long lines of inland cliffs had been
formed, and great valleys excavated, by the agencies which we see
still at work. The mind cannot possibly grasp the full meaning of
the term of even a million years; it cannot add up and perceive

RECAPITULATION AND CONCLUSION 499

the full effects of many slight variations, accumulated during an
almost infinite number of generations.

Although I am fully convinced of the truth of the views given in
this volume under the form of an abstract, I by no means expect
to convince experienced naturalists whose minds are stocked with a
multitude of facts all viewed, during a long course of years, from a
point of view directly opposite to mine. It is so easy to hide our
ignorance under such expressions as the "plan of creation," "unity
of design," etc., and to think that we give an explanation when we
only restate a fact. Any one whose disposition leads him to attach
more weight to unexplained difficulties than to the explanation of a
certain number of facts will certainly reject the theory. A few
naturalists, endowed with much flexibiUty of mind, and who have
already begun to doubt the immutability of species, may be influ-
enced by this volume; but I look with confidence to the future, to
young and rising naturalists, who will be able to view both sides
of the question with impartiality. Whoever is led to believe that
species are mutable will do good service by conscientiously express-
ing his conviction; for thus only can the load of prejudice by which
this subject is overwhelmed be removed.

Several eminent naturalists have of late published their belief that
a multitude of reputed species in each genus are not real species; but
that other species are real, that is, have been independently created.
This seems to me a strange conclusion to arrive at. They admit that
a multitude of forms, which till lately they themselves thought were
special creations, and which are still thus looked at by the majority
of naturalists, and which consequently have all the external char-
acteristic features of true species, — they admit that these have been
produced by variation, but they refuse to extend the same view to
other and slightly different forms. Nevertheless they do not pretend
that they can define, or even conjecture, which are the created forms
of life, and which are those produced by secondary laws. They
admit variation as a vera causa in one case, they arbitrarily reject it
in another, without assigning any distinction in the two cases. The
day will come when this will be given as a curious illustration
of the blindness of preconceived opinion. These authors seem no
more startled at a miraculous act of creation than at an ordinary

500 ORIGIN OF SPECIES

birth. But do they really believe that at innumerable periods in
the earth's history certain elemental atoms have been commanded
suddenly to flash into living tissues? Do they believe that at each
supposed act of creation one individual or many were produced?
Were all the infinitely numerous kinds of animals and plants created
as eggs or seed, or as full grown ? and in the case of mammals, were
they created bearing the false marks of nourishment from the
mother's womb ? Undoubtedly some of these same questions cannot
be answered by those who believe in the appearance or creation of
only a few forms of life, or of some one form alone. It has been
maintained by several authors that it is as easy to believe in the
creation of a million beings as of one; but Maupertuis' philosophical
axiom "of least action" leads the mind more willingly to admit the
smaller number; and certainly we ought not to believe that innu-
merable beings within each great class have been created with plain,
but deceptive, marks of descent from a single parent.

As a record of a former state of things, I have retained in the
foregoing paragraphs, and elsewhere, several sentences which imply
that naturalists believe in the separate creation of each species; and
I have been much censured for having thus expressed myself. But
undoubtedly this was the general belief when the first edition of
the present work appeared. I formerly spoke to very many natural-
ists on the subject of evolution, and never once met with any sympa-
thetic agreement. It is probable that some did then believe in
evolution, but they were either silent, or expressed themselves so
ambiguously that it was not easy to understand their meaning. Now,
things are wholly changed, and almost every naturalist admits the
great principle of evolution. There are, however, some who still
think that species have suddenly given birth, through quite unex-
plained means, to new and totally different forms: but, as I have
attempted to show, weighty evidence can be opposed to the admis-
sion of great and abrupt modifications. Under a scientific point of
view, and as leading to further investigation, but little advantage
is gained by believing that new forms are suddenly developed in an
inexplicable manner from old and widely different forms, over the
old belief in the creation of species from the dust of the earth.

It may be asked how far I extend the doctrine of the modification

RECAPITULATION AND CONCLUSION 5OI

of species. The question is difficult to answer, because the more
distinct the forms are which we consider, by so much the arguments
in favour of community of descent become fewer in number and less
in force. But some arguments of the greatest weight extend very
far. All the members of whole classes are connected together by a
chain of affinities, and all can be classed on the same principle, in
groups subordinate to groups. Fossil remains sometimes tend to
fill up very wide intervals between existing orders.

Organs in a rudimentary condition plainly show that an early
progenitor had the organ in a fully developed condition; and this
in some cases implies an enormous amount of modification in the
descendants. Throughout whole classes various structures are formed
on the same pattern, and at a very early age the embryos closely
resemble each other. Therefore 1 cannot doubt that the theory
of descent with modification embraces all the members of the same
great class or kingdom. I believe that animals are descended from
at most only four or five progenitors, and plants from an equal or
lesser number.

Analogy would lead me one step farther, namely, to the belief
that all animals and plants are descended from some one prototype.
But analogy may be a deceitful guide. Nevertheless all living things
have much in common, in their chemical composition, their cellular
structure, their laws of growth, and their liability to injurious influ-
ences. We see this even in so trifling a fact as that the same f)oison
often similarly affects plants and animals; or that the f)oison secreted
by the gallfly produces monstrous growths on the wild rose or oak
tree. With all organic beings, excepting perhaps some of the very
lowest, sexual reproduction seems to be essentially similar. With all,
as far as is at present known, the germinal vesicle is the same; so
that all organisms start from a common origin. If we look even
to the two main divisions — namely, to the animal and vegetable
kingdoms — certain low forms are so far intermediate in character
that naturalists have disputed to which kingdom they should be
referred. As Professor Asa Gray has remarked, "the spores and
other reproductive bodies of many of the lower algae may claim to
have first a characteristically animal, and then an unequivocal
vegetable, existence." Therefore, on the principle of natural selection

502 ORIGIN OF SPECIES

with divergence of character, it does not seem incredible that, from
some such low and intermediate form, both animals and plants may
have been developed; and, if we admit this, we must likewise admit
that all the organic beings which have ever lived on this earth may
be descended from some one primordial form. But this inference
is chiefly grounded on analogy, and it is immaterial whether or not
it be accepted. No doubt it is possible, as Mr. G. H. Lewes has
urged, that at the first commencement of life many different forms
were evolved; but if so, we may conclude that only a very few
have left modified descendants. For, as I have recently remarked in
regard to the members of each great kingdom, such as the Verte-
brata, Articulata, etc., we have distinct evidence in their embryo-
logical, homologous, and rudimentary structures, that within each
kingdom all the members are descended from a single progenitor.

When the views advanced by me in this volume, and by Mr.
Wallace, or when analogous views on the origin of species are gen-
erally admitted, we can dimly foresee that there will be a consider-
able revolution in natural history. Systematists will be able to
pursue their labours as at present; but they will not be incessantly
haunted by the shadowy doubt whether this or that form be a true
species. This, I feel sure and I speak after experience, will be tio
slight relief. The endless disputes whether or not some fifty species
of British brambles are good species will cease. Systematists will
have only to decide (not that this will be easy) whether any form
be sufficiently constant and distinct from other forms, to be capable
of definition; and if definable, whether the differences be sufficiently
important to deserve a specific name. This latter point will become a
far more essential consideration than it is at present ; for differences,
however slight, between any two forms, if not blended by inter-
mediate gradations, are looked at by most naturalists as sufficient
to raise both forms to the rank of species.

Hereafter we shall be compelled to acknowledge that the only
distinction between species and well-marked varieties is, that the
latter are known, or believed, to be connected at the present day by
intermediate gradations whereas species were formerly thus con-
nected. Hence, without rejecting the consideration of the present
existence of intermediate gradations between any two forms, we

RECAPITULATION AND CONCLUSION 503

shall be led to weigh more carefully and to value higher the actual
amount of difference between them. It is quite possible that forms
now generally acknowledged to be merely varieties may hereafter
be thought worthy of specific names; and in this case scientific and
common language will come into accordance. In short, we shall
have to treat species in the same manner as those naturalists treat
genera, who admit that genera are merely artificial combinations
made for convenience. This may not be a cheering prospect; but
we shall at least be freed from the vain search for the undiscovered
and undiscoverable essence of the term species.

The other and more general departments of natural history will
rise greatly in interest. The terms used by naturalists, of affinity,
relationship, community of type, paternity, morphology, adaptive
characters, rudimentary and aborted organs, etc., will cease to be
metaphorical, and will have a plain signification. When we no
longer look at an organic being as a savage looks at a ship, as some-
thing wholly beyond his comprehension; when we regard every
production of nature as one which has had a long history; when we
contemplate every complex structure and instinct as the summing
up of many contrivances, each useful to the possessor, in the same
way as any great mechanical invention is the summing up of the
labour, the experience, the reason, and even the blunders of numer-
ous workmen; when we thus view each organic being, how far
more interesting — I speak from experience — does the study of natural
history become!

A grand and almost untrodden field of inquiry will be opened,
on the causes and laws of variation, on correlation, on the effects
of use and disuse, on the direct action of external conditions, and
so forth. The study of domestic productions will rise immensely in
value. A new variety raised by man will be a more important and
interesting subject for study than one more species added to the
infinitude of already recorded species. Our classifications will come
to be, as far as they can be so made, genealogies; and will then
truly give what may be called the plan of creation. The rules for
classifying will no doubt become simpler when we have a definite
object in view. We possess no pedigrees or armorial bearings; and
we have to discover and trace the many diverging lines of descent

504 ORIGIN OF SPECIES

in our natural genealogies, by characters of any kind which have
long been inherited. Rudimentary organs will speak infallibly with
respect to the nature of long-lost structures. Sf)ecies and groups of
species which are called aberrant, and which may fancifully be
called living fossils, will aid us in forming a picture of the ancient
forms of life. Embryology will often reveal to us the structure, in
some degree obscured, of the prototypes of each great class.

When we can feel assured that all the individuals of the same
species, and all the closely allied species of most genera, have, within
a not very remote period descended from one parent, and have
migrated from some one birth-place; and when we better know the
many means of migration, then, by the light which geology now
throws, and will continue to throw, on former changes of climate
and of the level of the land, we shall surely be enabled to trace in
an admirable manner the former migrations of the inhabitants of
the whole world. Even at present, by comparing the differences
between the inhabitants of the sea on the opposite sides of a con-
tinent, and the nature of the various inhabitants on that continent
in relation to their apparent means of immigration, some light can
be thrown on ancient geography.

The noble science of Geology loses glory from the extreme im-
f)erfection of the record. The crust of the earth with its imbedded
remains must not be looked at as a well-filled museum, but as a
poor collection made at hazard and at rare intervals. The accumu-
lation of each great fossiliferous formation will be recognised as
having depended on an unusual concurrence of favourable circum-
stances, and the blank intervals between the successive stages as
having been of vast duration. But we shall be able to gauge with
some security the duration of these intervals by a comparison of the
preceding and succeeding organic forms. We must be cautious in
attempting to correlate as strictly contemporaneous two formations,
which do not include many identical species, by the general succes-
sion of the forms of life. As species are produced and exterminated
by slowly acting and still existing causes, and not by miraculous
acts of creation; and as the most important of all causes of organic
change is one which is almost independent of altered and perhaps
suddenly altered physical conditions, namely, the mutual relation

RECAPITULATION AND CONCLUSION 505

o£ organism to organism, — the improvement o£ one organism entail-
ing the improvement or the extermination of others; it follows, that
the amount of organic change in the fossils of consecutive formations
probably serves as a fair measure of the relative, though not actual
lapse of time. A number of species, however, keeping in a body
might remain for a long period unchanged, whilst within the same
period, several of these sp)ecies by migrating into new countries and
coming into competition with foreign associates, might become
modified; so that we must not overrate the accuracy of organic
change as a measure of time.

In the future I see open fields for far more important researches.
Psychology will be securely based on the foundation already well
laid by Mr. Herbert Spencer, that of the necessary acquirement of
each mental power and capacity by gradation. Much light will be
thrown on the origin of man and his history.

Authors of the highest eminence seem to be fully satisfied with
the view that each species has been independently created. To my
mind it accords better with what we know of the laws impressed
on matter by the Creator, that the production and extinction of
the past and present inhabitants of the world should have been due
to secondary causes, like those determining the birth and death of
the individual. When I view all beings not as special creations, but
as the lineal descendants of some few beings which lived long before
the first bed of the Cambrian system was deposited, they seem to
me to become ennobled. Judging from the past, we may safely infer
that not one living species will transmit its unaltered likeness to a
distant futurity. And of the species now living very few will trans-
mit progeny of any kind to a far distant futurity; for the manner in
which all organic beings are grouped, shows that the greater number
of species in each genus, and all the sfjecies in many genera, have
left no descendants, but have become utterly extinct. We can so far
take a prophetic glance into futurity as to foretell that it will be the
common and widely spread species, belonging to the larger and
dominant groups within each class, which will ultimately prevail
and procreate new and dominant species. As all the living forms of
life are the Hneal descendants of those which lived long before the
Cambrian epoch, we may feel certain that the ordinary succession

506 ORIGIN OF SPECIES

by generation has never once been broken, and that no cataclysm
has desolated the whole world. Hence we may look with some con-
fidence to a secure future of great length. And as natural selection
works solely by and for the good of each being, all corporeal and
mental endowments will tend to progress towards perfection.

It is interesting to contemplate a tangled bank, clothed with many
plants of many kinds, with birds singing on the bushes, with various
insects flitting about, and with worms crawling through the damp
earth, and to reflect that these elaborately constructed forms, so
different from each other, and dependent upon each other in so
complex a manner, have all been produced by laws acting around
us. These laws, taken in the largest sense, being Growth with
Reproduction; Inheritance which is almost implied by reproduction;
Variability from the indirect and direct action of the conditions of
life, and from use and disuse: a Ratio of Increase so high as to lead
to a Struggle for Life, and as a consequence to Natural Selection,
entailing Divergence of Character and the Extinction of less im-
proved forms. Thus, from the war of nature, from famine and
death, the most exalted object which we are capable of conceiving,
namely, the production of the higher animals, directly follows.
There is grandeur in this view of life, with its several powers, having
been originally breathed by the Creator into a few forms or into one;
and that, whilst this planet has gone cycling on according to the
fixed law of gravity, from so simple a beginning endless forms most
beautiful and most wonderful have been, and are being, evolved.

GLOSSARY

OF THE

PRINCIPAL SCIENTIFIC TERMS USED IN THE PRESENT

VOLUME*

Aberrant — Forms or groups of animals or plants which deviate in important
characters from their nearest allies, so as not to be easily included in the
same group with them, are said to be aberrant.

Aberration (in Optics) — In the refraction of light by a convex lens the rays passing
through different parts of the lens are brought to a focus at slightly different
distances — this is called spherical aberration; at the same time the coloured
rays are separated by the prismatic action of the lens and likewise brought to
a focus at different distances — this is chromatic aberration.

Abnormal — Contrary to the general rule.

Aborted — An organ is said to be aborted when its development has been arrested at
a very early stage.

Albinism — Albinos arc animals in which the usual colouring matters characteristic
of the species have not t>ccn produced in the skin and its appendages.
Albinism is the state of being an albino.

Algte — A class of plants including the ordinary seaweeds and the filamentous fresh-
water weeds.

Alternation of Generations — This term is applied to a peculiar mode of reproduction
which prevails among many of the lower animals, in which the egg produces
a living form quite different from its parent, but from which the parent-form
is reproduced by a process of budding, or by the division of the substance of
the first product of the egg.

Ammonites — A group of fossil, spiral, chambered shells, allied to the existing pearly
Nautilus, but having the partitions between the chambers waved in compli-
cated patterns at their junction with the outer wall of the shell.

Analogy — The resemblance of structures which depends upon similarity of function,
as in the wings of insects and birds. Such structures are said to be analogous,
and to be analogues of each other.

Animalcule — A minute animal: generally applied to those visible only by the
microscope.

Annelids — A class of worms in which the surface of the body exhibits a more or
less distinct division into rings or segments, generally provided with appendages
for locomotion and with gills. It includes the ordinary marine worms, the
earthworms, and the leeches.

Aniennte — Jointed organs apjicnded to the head in Insects, Crustacea, and Centipedes,
and not belonging to the mouth.

Anthers — The summits of the stamens of flowers, in which the pollen or fertilizing
dust is produced.

Aplacentalia, Aplacenlata or Aplacental Mammals — See Mammalia.

* I am indebted to the kindness of Mr. W. S. Dallas for this Glossary, which has

been given because several readers have complained to me that some of the terms

used were unintelligible to them. Mr. Dallas has endeavoured to give the explanations

of the terms in as popular a form as possible.

507

5o8

GLOSSARY

Archetypal — Of or belonging to the Archetype, or ideal primitive form upon which

all the beings of a group seem to be organized.
Articulata — A great division of the Animal Kingdom characterized generally by having

the surface of the body divided into rings called segments, a greater or less

number of which are furnished with jointed legs (such as Insects, Crustaceans,

and Centipedes).
Atymmetrical — Having the two sides unlike.
Atrophied — Arrested in development at a very early stage.

Balanus — ^The genus including the common Acorn shells which live in abundance

on the rocks of the seacoast.
Batrachiam — A class of animals allied to the Reptiles, but undergoing a peculiar

metamorphosis, in which the young animal is generally aquatic and breathes

by gills. (Examples, Frogs, Toads, and Newts.)
Boulders — Large transported blocks of stone generally embedded in clays or gravels.
Brachiopoda — A class of marine Mollusca, or soft-bodied animals, furnished with a

bivalve shell, attached to submarine objects by a stalk which passes through

an aperture in one of the valves, and furnished with fringed arms, by the

action of which food is carried to the mouth.
Branchi<r — Gills or organs for respiration in water.
Branchial — Pertaining to gills or branchia;.

Cambrian System — A scries of very ancient Paliozoic rocks, between the Laurentian
and the Silurian. Until recently these were regarded as the oldest fossiliferous
rocks.

Canidx — ^The Dog-family, including the Dog, Wolf, Fox, Jackal, etc.

Carapace — ^The shell enveloping the anterior part of the body in Crustaceans gen-
erally; applied also to the hard shelly pieces of the Cirripedcs.

Carboniferous — This term is applied to the great formation which includes, among
other rocks, the coal measures. It belongs to the oldest, or Palzozoic, system
of formations.

Caudal — Of or fwlonging to the tail.

Cephalopods — ^The highest class of the Mollusca, or soft-bodied animals, characterized
by having the mouth surrounded by a greater or less number of fleshy arms
or tentacles, which, in most living species, are furnished with sucking<ups.
(Examples, Cuttle-fish, Nautilus.)

Cetacea — An order of Mammalia, including the Whales, Dolphins, etc., having the
form of the body fish-like, the skin naked, and only the fore liml>s developed.

Chelonia — An order of Reptiles, including the Turtles, Tortoises, etc.

Cirripedes — An order of Crustaceans including the Barnacles and Acorn shells. Their
young resemble those of many other Crustaceans in form; but when mature
the>' arc always attached to other objects, either directly or by means of a
stalk, and their bodies are enclosed by a calcareous shell composed of several
pieces, two of which can open to give issue to a bunch of curled, jointed
tentacles, which represent the limbs.

Coccus — ^The genus of Insects including the Cochineal. In these the male is a minute,
winged fly, and the female generally a motionless, berry-like mass.

Cocoon — A case usually of silky material, in which insects are frequently enveloped
during the second or resting stage (pupa) of their existence. The term
"cocoon-stage" is here used as equivalent to "pupa-stage."

Ccelospermous — A term applieti to those fruits of the Umbellifen which have the
seed hollowed on the inner face.

Coleoptera — Beetles, an order of Insects having a biting mouth and the first pair of
wings more or less horny, forming sheaths for the second pair, and usually
meeting in a straight line down the middle of the back.

GLOSSARY 509

Column — A peculiar organ in the flowers of Orchids, in which the stamens, style and

stigma (or the reproductive parts) are united.
Compostlir or Composiious Plants — Plants in which the inflorescence consists o£

numerous small flowers (florets) brought together into a dense head, the base

of which is enclosed by a common envelope. (Examples, the Daisy, Dande-
lion, etc.)
Conjervir — The filamentous weeds of fresh water.
Conglomerate — A rock made up of fragments of rock or pebbles, cemented togethef

by some other material.
Corolla — The second envelope of a flower, usually composed of coloured, leaf-like

organs (|x^tals), which may be united by their edges either in the basal part

or throughout.
Correlation — The normal coincidence of one phenomenon, character, etc., with

another.
Corymb — A bunch of flowers in which those springing from the lower part of the

flower stalk are supported on long stalks so as to be nearly on a level with the

upper ones.
Cotyledons — The first or seed-leaves of plants.
Crustaceans — A class of articulated animals, having the skin of the body generally

more or less hardened by the deposition of calcareous matter, breathing by

means of gills. (Examples, Crab, Lobster, Shrimp, etc.)
Curculio — The old generic term for the Beetles known as Weevils, characterized by

their four jointed feet, and by the head being produced into a sort of beak

upon the sides of which the antenna: are inserted.
Cutaneous— Oi or belonging to the skin.

JJegradation — ^The wearing down of land by the action of the sea or of meteoric
agencies.

Denudation — The wearing away of the surface of the land by water.

Devonian System or Formation — A series of Palzozoic rocks, including the Old Red
Sandstone.

Dicotyledons or Dicotyledonous Plants — A class of plants characterized by having two
seed leaves, by the formation of new wood between the bark and the old wood
(exogenous growth) and by the reticulation of the veins of the leaves. The
parts of the flowers are generally in multiples of five.

Differentiation — ^The separation or discrimination of parts or orgaiu which in simpler
forms of life are more or less united.

Dimorphic — Having two distinct forms. — Dimorphism is the condition of the appear-
ance of the same species under two dissimilar forms.

Dioecious — Having the organs of the sexes upon distinct individuals.

Diorite — A peculiar form of Greenstone.

Dorsal — Of or belonging to the back.

Edentata — A peculiar order of Quadrupeds, characterized by the absence of at least
the middle incisor (front) teeth in both jaws. (Examples, the Sloths and
Armadillos.)

Elytra — ^The hardened fore-wings of Beetles, serving as sheaths for the membranous
hind-wings, which constitute the true organs of flight.

Embryo — The young animal undergoing development within the egg or womb.

Embryology — The study of the -development of the embryo.

Endemic — Peculiar to a given locality.

Entomostraca — A division of the class Crustacea, having all the segments of the body
usually distinct, gills attached to the feet or organs of the mouth, and the feet
fringed with fine hairs. They are generally of small size.

510 GLOSSARY

Eocene — The earliest of the three divisions of the Tertiary epoch of geologists. Rocks
of this age contain a small proportion of shells identical with species now living.
Ephemerous Insecti — ^Insects allied to the May fly.

Fauna — ^The totality of the animals naturally inhabiting a certain country or region,

or which have lived during a given geological period
Feluix—rhe Cat family.

feral — Having become wild from a state of cultivation or domestication.
Flora — The totality of the plants growing naturally in a country, or during a given

geological period.
Florets — Flowers imperfectly developed in some respects, and collected into a dense

spike or head, as in the Grasses, the Dandelion, etc.
Foetal — Of or belonging to the foetus, or embryo in course of development.
Foraminifera — A class of animals of very low organization and generally of small size,

having a jelly-like body, from the surface of which delicate filaments can be

given off and retracted for the prehension of external objects, and having a

calcareous or sandy shell, usually divided into chambers, and (wrfurated with

small apertures.
FostUijerous — Containing fossils.
Fostorial — Having a faculty of digging. The Fossorial Hymenoptera are a group of

Wasp-like Insects, which burrow in sandy soil to make nests for their young.
Frenum (pi. Frena) — A small band or fold of skin.
Fungi (sing. Fungus) — A class of cellular plants, of which Mushrooms, Toadstools,

and Moulds are familiar examples.
Furcula — The forked bone formed by the union of the collar bones in many birds, such

as the common Fowl.

Gallinaceous Birds — An order of Birds of which the common Fowl, Turkey, and
Pheasant are well-known examples.

Callus — The genus of birds which includes the common Fowl.

Ganglion — A swelling or knot from which nerves are given oR as from a centre.

Ganoid Fishes — Fishes covered with peculiar enamelled bony scales. Most of them
are extinct.

Germinal Vesicle — A minute vesicle in the eggs of animals, from which (he develop-
ment of the embryo proceeds.

Glacial Period — A period of great cold and of enormous extension of ice upon the
surface of the earth. It is believed that glacial periods have occurred repeat-
edly during the geological history of the earth, but the term is generally
applied to the close of the Tertiary epoch, when nearly the whole of Europe
was subjected to an arctic climate.

Gland — An organ which secretes or separates some peculiar product from the blood
or sap of animals or plants.

Glottis — The opening of the windpipe into the crsophagus or gullet.

Gneiss — A rock approaching granite in composition, but more or less laminated, and
really produced by the alteration of a sedimentary deposit after its consolidation.

Grallatores — ^The so<allcd Wading-birds (Storks, Cranes, Snipes, etc.), which are
generally furnished with long legs, bare of feathers above the heel, and have no
membranes between the toes.

Granite — A rock consisting essentially of crystals of felspar and mica in a mass of
quartz.

Habitat — ^The locality in which a plant or animal naturally lives.

Hemiptera — An order or sub-order of Insects, characterized by the possession of a
jointed beak or rostrum, and by having the fore-wings horny in the basal
portion and membranous at the extremity, where they cross each other. This
group includes the various species of Bugs.

GLOSSARY 511

Hermaphrodite — Possessing the organs of both sexes.

Homology — That relation between parts which results from their development from
corresponding embryonic parts, either in different animals, as in the case of the
arm of man, the fore-leg of a quadruped, and the wing of a bird; or in the
same individual, as in the case of the fore and hind legs in quadrupeds, and
the segments or rings and their appendages of which ttie body of a worm, a
centipede, etc., is composed. The latter is called serial homology. The parts
which stand in such a relation to each other are said to be homologous, and
one such part or organ is called the homologue of the other. In different
plants the parts of the flower are homologous, and in general these parts are
regarded as homologous with leaves.

Homoptera — An order or sub-order of Insects having (like the Hemiptera) a jointed
beak, but in which the fore-wings are either wholly membranous or wholly
leathery. The Cicadte, Frog-hoppers, and Aphides, are well-known examples.

Hybrid — The offspring of the union of two distinct species.

Hymenoplera — An order of Insects possessing biting jaws and usually four mem-
branous wings in which there are a few veins. Bees and Wasps are familiar
examples of this group.

Hyperlrophied — Excessively developed.

Ichneumonidit — A family of Hymenopterous insects, the members of which lay their
eggs in the txxlies or eggs of other insects.

Imago — The perfect (generally winged) reproductive state of an insect.

Indigens — The atmriginal animal or vegetable inhabitants of a country or region.

Inflorescence — The mode of arrangement of the flowers of plants.

Infusoria — A class of microscopic Animalcules, so called from their having originally
been observed in infusions of vegetable matters. They consist of a gelatinous
material enclosed in a delicate membrane, the whole or part of which is fur-
nished with short vibrating hairs (called cilia), by means of which the animal-
cules swim through the water or convey the minute particles of their food to
the orifice of the mouth.

Insectivorous — Feeding on Insects.

Invertebrala, or Invertebrate Animals — ^Those animals which do not possess a back-
bone or spinal column.

Lacuna — Spaces left among the tissues in some of the lower animals, and serving in
place of vessels for the circulation of the fluids of the body.

Lamellated — Furnished with lamellx or little plates.

iMTva (pi. Larvtr) — ^The first condition of an insect at its issuing from the egg, when it
is usually in the form of a grub, caterpillar, or maggot.

Larynx — ^The upper part of the windpipe opening into the gullet.

Laurentian — A group of greatly altered and very ancient rocks, which is gready devel-
oped along the course of the St. Laurence, whence the name. It is in these
that the earliest known traces of organic tmdies have been found.

Leguminosx — An order of plants represented by the common Peas and Beans, having
an irregular flower in which one petal stands up like a wing, and the stamens
and pistil are enclosed in a sheath formed by two other petals. The fruit is a
pod (or legume).

Lemuridit — A group of four-handed animals, distinct from the Monkeys and ap-
proaching the Insectivorous Quadrupeds in some of their characters and habits.
Its members have the nostrils curved or twisted, and a claw instead of a nail
upon the first finger of the hind hands.

Lepidoptera — ^An order of Insects, characterized by the possession of a spiral pro-
boscis, and of four large more or less scaly wings. It includes the well-known
Butterflies and Moths.

512 GLOSSARY"

Littoral — Inhabiting the seashore.

Loest — A marly deposit of recent (Post-Tertiary) date, which occupies a great part of

the valley of the Rhine.

Ualacostraca — ^The higher division of the Crustacea, including the ordinary Crabs,
Lobsters, Shrimps, etc., together with the Woodlice and Sandhoppers.

Mammalia — The highest class of animals, including the ordinary hairy quadrupeds,
the Whales, and Man, and characterized by the production of livmg younj;
which are nourished after birth by milk from the teats (Mammx, Mammary
glands) of the mother. A striking difference in embryonic development has
led to the division of this class into two great groups, in one of these, when the
embryo has attained a certain stage, a vascular connection, called the placenta,
is formed between the embryo and the mother; in the other this is wanting,
and the young are produced in a very incomplete state. The former, includ-
ing the greater part of the class, are called Placental mammals; the latter,
or Aplacental mammals, include the Marsupials and Monotremes (Orni:ho-
rhynchus).

Mammiferous — Having mammx or teats (see Mammalia).

Mandibles, in Insects — The first or uppermost pair of jaws, which are generally solid,
horny, biting organs. In Birds the term is applied to both jaws with their
horny coverings. In Quadrupeds the mandible is properly the lower jaw.

Marsupials — An order of Mammalia in which the young arc born in a very incomplete
state of development, and carried by the mother, while sucking, in a ven-
tral pouch (marsupium), such as the Kangaroos, Opossums, etc. (see Mam-
malia).

Maxilla, in Insects — The second or lower pair of jaws, which are composed of
several joints and furnished with peculiar jointed appendages called palpi,
or feelers.

Melanism — The opposite of albinism; an undue development of colouring material
in the skin and its appendages.

Metamorphic Koc^s — Sedimentary rocks which have undergone alteration, generally
by the action of heat, subsequently to their deposition and consolidation.

Mollusca — One of the great divisions of the Animal Kingdom, including those animals
which have a soft body, usually furnished with a shell, and in which the
nervous ganglia, or centres, present no definite general arrangement. They
are generally known under the denomination of "shell-fish"; the cuttle-fish,
and the common snails, whelks, oysters, mussels, and cockles, may serve as
examples of them.

Monocotyledons, or Monocotyledonous Plants — Plants in which the seed sends up
only a single seed-leaf (or cotyledon); characterized by the absence of con-
secutive layers of wood in the stem (endogenous growth), by the veins of the
leaves being generally straight, and by the parts of the flowers being generally
in multiples of three. (Examples, Grasses, Lilies, Orchids, Palms, etc.)

Moraines — ^The accumulations of fragments of rock brought down by glaciers.

Morphology — ^The law of form or structure independent of function.

Mysis-stage — A stage in the development of certain Crustaceans (Prawns), in which
they closely resemble the adults of a genus (\lysis) belonging to a slightly
lower group.

Nascent — Commencing development.

Natatory — Adapted for the purpose of swimming.

Nauplius-form — The earliest stage in the development of many Crustacea, especially
belonging to the lower groups. In this stage the animal has a short body,
with indistinct indications of a division into segments, and three pairs of
fringed limbs. This form of the common fresh-water Cyclops was described
as a distinct genus under the name of Naiiplius.

GLOSSARY 513

Ncuration — The arrangement of the veins or nervures in the wings of Insects.
Neuters — Imjiertcctly develo)Xxl females of certain social insects (such as Ants and

Bees), which |K.'rform all the labours of the community. Hence they are

also called wor/(ers.
Nictitating Membrane — A semi-transparent membrane, which can be drawn acrou

the eye in Birds and Reptiles, either to moderate ihc effects of a strong light

or to sweep particles of dust, etc., from the surface of the eye.

Owlli — The simple eyes or stcmmata of Insects, usually situated on the crown erf

the head between the great compound eyes.
(Esophagus — The gulleL
Oolitic — A great series of secondary rocks, so called from the texture of some of its

members, which appear to be made up of a mass of small egg-likje calcareous

bodies.
Operculum — A calcareous plate employed by many Mollusca to close the aperture of

their shell. Ihc opercular valves of Cirripedes are those which close the

aperture of the shell.
Orbit — The bony cavity for the reception of the eye.
Organism — An organized being, whether plant or animal.
Orihospermous — A term applied to tho^ fruits of the Umbelliferz which have the

seed straight.
Osculant — Forms or groups apparently intermediate between and connecting other

groups are said to be osculant.
Ova — Eggs.
Ovarium or Ovary (in Plants) — ^The lower part of the pistil or female organ of the

flower, containing the ovules or incipient seeds; by growth after the other

organs of the flower have fallen, it usually becomes converted into the fruit.
Ovigerous — Egg -bearing.
Ovules (of Plants) — ^The seeds in the earliest condition.

Pachyderms — A group of Mammalia, so called from their thick skins, and including

the Elephant, Rhinoceros, Hippopotamus, etc.
Palaozoic — The oldest system of fossil ifcrous rocks.
Palpi — Jointed appendages to some of the organs of the mouth in Insects and

Crustacea.
Papilionacetr — An order of Plants (see Lecuminos*). The flowers of these plants

are called papilionaceous, or butterfly-like, from the fancied resemblance of

the expanded superior petals to the wings of a butterfly.
Parasite — An animal or plant living upon or in, and at the expense of, another

organism.
Parthenogenesis — ^The production of living organisms from unimpregnated eggs

or seeds.
Pedunculated — Supported upon a stem or stalk. The pedunculated oak has its acorns

borne upon a footstool.
Peloria or Pelorism — The appearance of regularity of structure in the flowers of

plants which normally bear irregular flowers.
Pelvis — The bony arch to which the hind limbs of vertebrate animals are articulated.
Petals — The leaves of the corolla, or second circle of organs in a flower. They are

usually of delicate texture and brightly coloured.
Phyllodineoiis — Having flattened, leaf-like twigs or leafstalks instead of true leaves.
Pigment — The colouring material produced generally in the superficial pans of

animals. The cells secreting it are called pigment-cells.
Pinnate — Bearing leaflets on each side of a central stalk.
Pistils — The female organs of a flower, which occupy a position in the centre of the

other floral organs. The pistil is generally divisible into the ovary or germen,

the style and the stigma.

514 GLOSSARY

Placentalia, Placmtata, or Placental Mammalt — See Mammalia.

Plantigradet — Quadrupeds which walk upon the whole sole of the foot, like
the Bears.

Plastic Period — The latest portion of the Tertiary epoch.

Plumule (in Plantt) — ^The minute bud between the seed-leaves of newly-germinated
plants.

Plutonic Rock,t — Rocks supposed to have been produced by igneous action in the
depths of the earth.

Pollen — ^The male clement in flowering plants; usually a fine dust produced by the
anthers, which, by contact with the stigma, effects the fecundation of the
seeds. This impregnation is brought about by means of tubes (pollen-tubes)
which issue from the pollen-grams adhering to the stigma, and penetrate
through the tissues until they reach the ovary.

Polyandrous (Flowers) — Flowers having many stamens.

Polygamous Plants— V\aMs in which some flowers are unisexual and others her-
maphrodite. The unisexual (male and female) flowen may be on the same
or on different plants.

Polymorphic — Presenting many forms.

Polyzoary — ^The common structure formed by the cells of the Polyzoa, such as the
well-known Sea-mats.

Prehensile — Capable of grasping.

Prepotent — Having a superiority of power.

Primaries — ^The feathers forming the tip of the wing of a bird, and inserted upon
that part which represents the hand of man.

Processes — Projecting portions of bones, usually for the attachment of muscles,
ligaments, etc.

Propolis — A resinous material collected by the Hive-Bees from the opening buds of
various trees.

Protean — Exceedingly variable.

Protozoa — The lowest great division of the Animal Kingdom. These animals ?re
composed of a gelatinous material, and show scarcely any trace of distinct
organs. The Infusoria, Foraminifera, and Sponges, with some other forms,
belong to this division.

Pupa (pi. Pupce) — The second stage in the development of an Insect, from which it
emerges in the perfect (winged) reproductive form. In most insects the pupal
stage is passed in perfect repose. The chrysalis is the pupal state of butterflies.

Radicle — ^The minute root of an embryo plant.

Ramus — One-half of the lower jaw in the Mammalia. The portion which rises to

articulate with the skull is called the ascending ramus.
Range — The extent of country over which a plant or animal is naturally spread. Range

in time expresses the distribution of a species or group through the fossiliferous

beds of the earth's crust.
Retina — ^The delicate inner coat of the eye, formed by nervous filaments spreading

from the optic nerve, and serving for the perception of the impressions

produced by light.
Retrogression — Backward development. When an animal, as it approaches maturity,

becomes less perfectly organized than might be expected from its early suges

and known relationships, it is said to undergo a retrograde development or

metamorphosis.
Rhizopods — A class of lowly organized animals (Protozoa), having a gelatinous

body, the surface of which can be protruded in the form of root-like processes

or filaments, which serve for locomotion and the prehension of food. The

most important order is that of the Foraminifera.

GLOSSARY 515

Rodentf — ^The frnawing Mammalia, such as the Rats, Rabbits, and Squirrels. They
are especially characterized by the possession of a single pair of chisel-like
cutting teeth in each jaw, between which and the grinding teeth there
is a great gap.

Rubus — The Bramble Genus.

Rudimenlary — Very imperfectly developed.

Ruminanu — The group of Quadrupeds which ruminate or chew the cud, such at
Oxen, Sheep, and Deer. They have divided hoofs, and are destitute of front
teeth in the upper jaw.

Sacral — Belonging to the sacrum, or the bone composed usually of two or more

united vertebrz to which the sides of the pelvis in vertebrate animals are

attached.
Sarcode — ^I'he gelatinous material of which the bodies of the lowest animals

(Protozoa) are composed.
Scutell/c — The horny plates with which the feet of birds are generally more or less

covered, especially in front.
Sedimentary Formations — Rocks deposited as sediments from water.
Segments — ^The traverse rings of which the body of an articulate animal or

Annelid is composed.
Sepals — The leaves or segments of the calyx, or outermost envelope of an ordinary

flower. They are usually green, but sometimes brighUy coloured.
Serratures — Teeth like those of a saw.
Sessile — Not supported on a stem or footstalk.
Silurian System — A very ancient system of fossiliferous rocks belonging to the earlier

part of the Palxozoic series.
Specialization — The setting apart of a particular organ for the performance of a

particular function.
Spinal Chord — The central portion of the nervous system in the Vertebrata, which

descends from the brain through the arches of the vertebrz, and gives off

nearly all the nerves to the various organs of the body.
Stamens — The male organs of flowering plants, standing in a circle within the petals.

They usually consist of a filament and an anther, the anther being the essential

part in which the pollen, or fecundating dust, is formed.
Sternum — The breast-bone.

Stigma — ^I'he apical portion of the pistil in flowering plants.
Stipules — Small leafy organs placed at the base of the footstalks of the leaves in

many plants.
Style— "Vhc middle portion of the perfect pistil, which rises like a column from the

ovary and supports the stigma at its summit.
Subcutaneous — Situated beneath the skin.
Suctorial — Adapted for sucking.
Sutures (in the skull) — ^The lines of junction of the bones of which the skull it

composed.

Tarsus (pi. Tarsi) — ^The pointed feet of articulate animals, such as Insects.
Teteostean Fishes — Fishes of the kind familiar to us in the present day, having the

skeleton usually completely ossified and the scales horny.
Tentacula or Tentacles — Delicate fleshy organs of prehension or touch possessed by

many of the lower animals.
Tertiary — The latest geological epoch, immediately preceding the establishment ol

the present order of things.
Trachea — The windpipe or passage for the admission of air to the lungs.

5i6

GLOSSARY

Tridaclyle — ^Three-fingered, or composed of three movable parts attached to a
common base.

Trilobiles — A peculiar group of extinct Crustaceans, somewhat resembling the
Woodlicc in external form, and, like some of them, capable of rolling them-
selves up into a ball. Their remains arc found only in the Palzozoic rocks,
and most abundantly in those of Silurian age.

Trimorphic — Presenting three distinct forms.

Umbelli/erir — An order of plants in which the flowers, which contain five stamens
and a pistil with two styles, are supported upon footstalks which sprmg from
the top of the flower stem and spread out like the wires of an umbrella, so
as to bring all the flowers in the same head (umbel) nearly to the same level.
(Examples, Parsley and Carrot.)

Ungulata — Hoofed quadru[>eds.

Unicellular — Consisting of a single cell.

Vascular — Containing blood-vessels.

Vermiform — Like a worm.

Vertebrata; or Vertebrate Animals — ^The highest division of the animal kingdom,
JO called from the presence in most cases of a backbone composed of numerous
joints or veriebne, which constitutes the centre of the skeleton and at the
same time supports and protects the central parts of the nervous system.

Whorls — The circles or spiral lines in which the parts of plants are arranged upon

the axis of growth.
Worl^ers — See NEtrrERS.

Zoea-stage — The earliest stage in the development of many of the higher Crustacea,
so called from the name of Zoea, applied to these young animals when they
were supposed to constitute a peculiar genus.

Zooids — In many of the lower animals (such as the Corals, Medusz, etc.) reproduction
takes place in two ways, namely, by means of eggs an<l by a process of budding
with or without separation from the parent of the product of the latter, which
is often very different from that of the egg. The individuality of the species
is represented by the whole of the form produced between two sexual
reproductions; and these forms, which are apparently individual animals, have
been called zooids.

INDEX

Aberrant groups, 448.

Abyssinia, plants of, 40$.

Acclimatisation, 144.

Adoxa, 215.

Affinities of extinct species, 363.

, of organic beings, 448.

Agassiz, on Amblyopsis, 144.

, on groups of species suddenly ap-
pearing, 348.

, on prophetic forms, 362.

, on embryological succession, 371.

, on the Glacial period, 394.

, on embryological characters, 437.

, on the latest tertiary forms, 336.

, on parallelism of embryological

development and geological succession,
468.

, Alex., on pedicellariz, 236.

Algz of New Zealand, 403.

Alligators, males, fighting, 95.

Alternate generations, 4;8.

Amblyopsis, blind fish, 144.

America, North, productions allied to
those of Europe, 398.

, , boulders and glaciers of, 400.

, South, no modern formations on

west coast, 328.

Ammonites, sudden extinction of, 357.

Anagallis, sterility of, 287.

Analogy of variations, 161.

Ancylus, 411.

Andaman Islands inhabited by a toad, 417.

Animals, not domesticated from being
variable, 32.

, domestic, descended from several

. stocks, 33.

, , acclimatisation of, 146.

Animals of Australia, 119.

with thicker fur in cold climates,

139-
, blind, in caves, 142.

extinct, of Australia, 372.

Anomma, 281.

Antarctic islands, ancient flora of, 422.

Antechinus, 443.

Ants attending aphides, 254.

Ants, slave-making instinct, 264.

, neuters, structure of, 280.

Apes, not having acquired intellectual

powers, 224.
Aphides, attended by ants, 254.
Aphis, development of, 462.
Apteryx, 177.
Arab horses, 46.
Aralo-Caspian Sea, 373.
Archeopteryx, 342.
Archiac, M. de, on the succession of

species, 359.
Artichoke, Jerusalem, 147.
Ascension, plants of, 414.
Asclepias, pollen of, 190.
Asparagus, 388.
Aspicarpa, 436.
Asses, striped, 162.

, improved by selection, 52.

Ateuchus, 141.

Aucapitaine, on land-shells, 420.

Audubon, on habits of frigate-bird, 183.

, on variation in birds' nests, 254.

, on heron eating seeds, 412.

Australia, animals of, 119.

, dogs of, 258.

, extinct animals of, 372.

, European plants in, 403.

, glaciers of, 400.

Azara, on Hies destroying cattle, 81.
Azores, Bora of, 392.

B

Babington, Mr., on British plants, 59.

Bacr, Von, standard of Highness, 129.

, comparison of bee and fish, 370.

, embryonic similarity of the Verte-

brata, 459.

Baker, Sir S., on the giraffe, 221.

Balancement of growth, i;o.

Baleen, 226.

Barberry, flowers of, 104.

Barrande, M., on Silurian colonies, 350.

, on the succession of species, 359.

, on parallelism of palzozoic forma-
tions, 361.

, on affinities of ancient species, 363.

517

5i8

INDEX

Barriers, importance of, 379.

Bates, Mr., on mimetic butterflies, 445,
446.

Batrachians on islands, 417.

Bats, how structure acquired, 177.

, distribution of, 418.

Bear, catching water-insects, 178.

Beauty, how acquired, 199, 489.

Bee, sting of, 204.

, queen, killing rivals, 204.

, Australian, extermination of, 84.

Bees fertilising flowers, 81.

, hive, not sucking the red clover,

loi.

, Ligurian, 102.

, hive, cell-making instinct, 268.

, variation in habits, 254.

, parasitic, 263.

, humble, cells of, 269.

Beedes, wingless, in Madeira, 141.

with deficient tarsi, 141.

Bentham, Mr., on British plants, 59.

, on classification, 438.

Berkeley, Mr., on seeds in salt water, 388.

Bermuda, birds of, 415.

Birds acquiring fear, 255.

, beauty of, 202.

annually cross the Atlantic, 392.

, colour of, on continents, 139.

, footsteps, and remains of, in sec-
ondary rocks, 341.

, fossil, in caves of Brazil, 372.

, of Madeira, Bermuda, and Gala-
pagos, 415.

Birds, song of males, 96.

transporting seeds, 392.

, waders, 411.

, wingless, 140, 177, 178.

Bizcacha, 380.

, affinities of, 449.

Bladder for swimming, in fish, 186.

Blindness of cave animals, 142.

BIyth, Mr., on distinctness of Indian
catde, 33.

, on striped hemionus, 163.

, on crossed geese, 291.

Borrow, Mr., on the Spanish pointer, 46.

Bory St. Vincent, on Batrachians. 417.

Bosquet, M., on fossil Chthamalus, 342.

Boulders, erratic, on the Azores, 392.

Branchiz, 186, 187.

of crustaceans, 191.

Braun, Prof., on the seeds of Fuma-
riaccz, 215.

Brent, Mr., on house-tumblers, 257.
Britain, mammals of, 419.
Broca, Prof., on Natural Selection, 211.
Bronn, Prof., on duration of specific

forms, 332.

, various objections by, 211.

Brown, Robert, on classification, 434.
, S^uard, on inherited mutilations,

141-
Busk, Mr., on the Potyzoa, 237.
Butterflies, mimetic, 445, 446.
Buzareingues, on sterility of varieties, 311.

Cabbage, varieties of, crossed, 105.
Calceolaria, 289.

Canary-birds, sterility of hybrids, 290.
Cape de Verde islands, productions of,

421.

, plants of, on mountains, 402.

Cape of Good Hope, plants of, 134, 414.

Carpenter, Dr., on foraminifera, 369.

Carthamus, 215.

Catasetum, 195, 441.

Cats, with blue eyes, deaf, 27.

, variation in habits of, 256.

curling tail when going to spring,

203.
Cattle destroying fir-trees, 80.

destroyed by flies in Paraguay, 81.

.breeds of, locally extinct, 114.

, fertility of Indian and European

breeds, 291.

, Indian, 33, 392.

Cave, inhabitants of, blind, 142.
Cccidomyia, 458.

Celts, proving antiquity of man, 32.
Centres of Creation, 383.
Cephalopodz, structures of eyes, 190.

.development of, 461.

Ccrcopithecus, tail of, 232.
Ceroxylus laccratus, 225.
Cervulus, 291.
Cctacea, teeth and hair, 149.

, development of the whalebone, 226.

Cetaceans, 226.
Ceylon, plants of, 402.
Chalk formation, 357.
Characters, divergence of, 115.

.sexual, variable, 153. 157.

, adaptive or analogical, 443.

Charlock. 84.

Checks to increase. 77.

, mutual, 79.

INDEX

519

Chelz of Crustaceans, 257.

Chickens, instinctive lameness of, 258.

Chironomus, its asexual reproduction,
458.

Chthamalinz, 326.

Chthamalus, cretacean species of, 342.

Circumstances favourable to selection of
domestic products, jo.

to natural selection, 107.

Cirripedes capable of crossing, 107.
, carapace aborted, 151.

, their ovigerous f rena, 1 87.

, fossil, 342.

, larvsr of, 461.

ClaparMe, Prof., on the bair-claspcrs of
the Acaridz, 192.

Clarke, Rev. W. B., on old glaciers in
Australia, 400.

Classification, 431.

Clift, Mr., on the succession of types,
37i-

Climate, effects of, in checking increase
of beings, 77.

.adaptation of, to organisms, 145.

Climbing plants, 186.

, developments of, 241.

Clover visited by bees, 104.

Cobites, intestine of, 185.

Cockroach, 84.

Collections, palzontological, poor, 326.

Colour, influenced by climate, 139.

, in relation to attack by flics, 1 99.

Columba livia, parent of domestic pig-
eons, 36.

Colymbetes, 411.

Compensation of growth, 150.

Compositx. flowers and seeds of, 149.

, outer and inner florets of, 215.

, male flowers of, 470.

Conclusion, general, 497

Conditions, slight changes in, favourable
to fertility, 304.

Convergence of genera, 133.

Coot, 180.

Cope, Prof., on the acceleration or re-
tardation of the period of reproduction,
188.

Coral-islands, seeds drifted to, 389-90.

reefs, indicating movements of

earth. 387.

Corncrake, 181.

Correlated variation in domestic produc-
tions, 27.

Coryanthes, 194.

Creation, single centres of, 383.

Crinum, 289.

Croll, Mr., on subaerial denudation, 325.

, on the age of our oldest forma-
tions, 344.

, on alternate Glacial periods in the

North and South, 401.

Crosses, reciprocal, 293.

Crossing of domestic animals, importance
in altering breeds, 33.

, advantages of, 103.

, unfavourable to selection, 108.

Criiger, Dr., on Coryanthes, 194.

Crustacea of New Zealand, 403.

Crustacean, blind, 142.

air-breathers, 192.

Crustaceans, their chelz, 237-8.

Cryptocerus, 280.

Ctenomys, blind, 142.

Cuckoo, instinct of, 251, 259.

Cunningham, Mr., on the flight of the
logger-headed duck, 140.

Currants, grafts of, 297.

Currents of sea, rate of, 389.

Cuvier, on fossil monkeys, 341.

, Fred., on instinct, 251.

Cyclostoma, resisting salt water, 420.

Dana, Prof., on blind cave-animals, 144.
, on relations of crustaceans of

Japan, 399.
, on crustaceans of New Zealand,

403.
Dawson, Dr., on eozoon, 345.
De Candolle, Aug. Pyr., on struggle for

existence, 72.

, on umbelliferz, 1 50.

, on general aflinities, 449.

, Alph., on the variability of oaks,

, on low plants, widely dispersed,

427-

, on widely-ranging plants being va-
riable, 65.

, on naturalisation, 118.

, on winged seeds, 150.

, on Alpine species suddenly becom-
ing rare, 171.

, on distribution of plants with large

seeds, 389.

, on vegetation of Australia, 405.

, on fresh-water plants, 412.

, on insular plants, 4 1 4.

520 INDEX

Degradation of rocks, 322.

Denudation, rate of, 323.

of oldest rocks, 345.

of granitic areas, 330-1.

Development of ancient forms, 368.

Devonian system, 366-7.

Dianthus, fertility of crosses, 293.

Dimorphism in plants, 57, 305.

Dirt on feet of birds, 391.

Dispersal, means of, 386.

during Glacial period, 394.

Distribution, geographical, 378.

, means of, 386.

Disuse, effect of, under nature, 140.

Divergence of character, 115.

Diversification of means for same gen-
eral purpose, 193.

Division, physiological, of labour, 11 8-9.

Dog, resemblance of jaw to that of the
Thylacinus, 444.

Dogs, hairless, with imperfect teeth, 28.

descended from several wild stocks,

32-

, domestic instincts of, 256-7.

, inherited civilisation of, 256.

, fertility of breeds together, 291.

, of crosses, 309.

, proportions of body in different

breeds, when young, 463-4.
Domestication, variation under, 23.
Double flowers, 279.
Downing, Mr., on fruit-trees in America,

Dragon-flies, intestines of, 185.

Drift-timf>er, 389.

Driver ant, 281-2.

Drones killed by other bees, 204.

Duck, domestic, wings of, reduced, 27.

, beak of, 226.

.logger-headed, 177.

Duckweed, 411.
Dugong, affinities of, 434.
Dung-beetles with deficient tarsi, 141.
Dytiscus, 411.

Earl, Mr. W., on the Malay Archipel-
ago, 419.
Ears, drooping, in domestic animals, 27.

, rudimentary, 473.

Earth, seeds in roots of uees, 389-90.

charged with seeds, 392.

Echinodcrmata, their pedicellariz, 235.
Eciton, 280.

Economy of organisation, 151.

Edentia, teeth and hair, 148-9.

, fossil species of, 493.

Edwards, Milne, on physiological division
of labour, 118.

, on gradations of structure, 196.

, on embryological characters, 437.

£kS>> young birds escaping from, 93.

Eg)'pl, productions of, not modified, 210.

Electric organs, 188-9.

Elephant, rate of increase, 74.

, of Glacial period, 146.

Embryology, 457.

Eozoon Canadense, 345.

Existence, struggle for, 71.

, condition of, 207.

Extinction, as bearing on natural selec-
tion, 128.

of domestic varieties, 114.

. 353-

Eye, structure of, 182.

, correction for aberration, 203.

Eyes, reduced in moles, 142.

Fabrc, M., on hymcnoptera fighting, 95.

, on parasitic sphcx, 263.

, on Silaris, 467.

Falconer, Dr., on naturalisation of plants
in India, 75.

on elephants and mastodons, 367.

and Cautlcy, on mammals of sub-
Himalayan beds, 373.

Falkland Islands, wolf of, 417.

Faults, 323-4.

Faunas, marine, 380.

Fear, instinctive, in birds, 258.

Feet of birds, young molluscs adhering
to, 411.

Fertilisation variously effected, 193, 201.

Fertility of hybrids, 289.

, from slight changes in conditions,

3<'4-

of crossed varieties, 308.

Fir-trees destroyed by cattle, 80.

, pollen of, 204.

Fish, flying, 178.

, teleostean, sudden appearance of,

343-

, eating seeds, 391, 412.

, fresh- water, distribution of, 409-10.

Fishes, Ganoid, now confined to fresh

water, iii, 356.
.electric organs of, 188-9.

INDEX

521

Fishes, of southern hembphere, 403.
Flatfish, their structure, 229.
Flight, powers of, how acquired, 177.
Flint tools, proving antiquity of man,

31-3-

Flower, Prof., on the larynx, 234-5.

, on Halitherium, 363.

, on the resemblance between the

jaws of the dog and Thylacinus, 444.

, on the homology of the feel of

certain marsupials, 453.

Flowers, structure of, in relation to cross-
ing, 99-100.

, of compositx and umbelliferz, 1 49-

50,215.

, beauty of, 20 1 .

, double, 279.

Flysch formation, destitute of organic re-
mains, 327.

Forbes, Mr. D., on glacial action in the
Andes, 400.

, E., on colours of shells, 139.

, on abrupt range of shells in depth,

171.

, on poorness of palzontological col-
lections, 326.

, on continuous succession of genera,

351-

, on continental extensions, 386.

, on distribution during Glacial pe-
riod, 395.

, on parallelism in time and space,

429.

Forests, changes in, in America, 82.

Formation, Devonian, 366-7.

, Cambrian, 345.

Formations, thickness of, in Britain, 324.

, intermittent, 334.

Formica, rufcscens, 265.

, sanguinea, 264.

, flava, neuter of, 281.

Forms, lowly organised, long enduring,

131-
Frena, ovigerous, of cirripedes, 187.
Fresh-water productions, dispersal of,

409.
Fries, on species in large genera being

closely allied to other species, 68.
Frigate-bird, 180.
Frogs on islands, 417.
Fruit trees, gradual improvement of,

47-8.

in United States, 92.

, varieties of, acclimatised in United

Sutes, 147.

Fuci, crossed, 395, 301.

Fur, thicker in cold climates, 139.

Galapagos Archipelago, birds of, 414.

, productions of, 421.

Galaxias, its wide range, 409.

Galeopithecus, 176.

Game, increase of, checked by vermin,

78-9.
Gartner, on sterility of hybrids, 286-7,

292.

, on reciprocal crosses, 295.

, on crossed maize and Verbascum,

3'i.

, on comparison of hybrids and mon-
grels, 3>2-4.

Gaudry, Prof., on intermediate genera of
fossil mammals in Attica, 362.

Geese, fertility when crossed, 291.

, upland, 180.

Gcikie, Mr., on subaerial denudation,
322.

Genealogy, important in classification,
438.

Generations, alternate, 458.

Gcoffroy St. Hilaire, on balancement,
150.

, on homologous organs, 453.

, Isidore, on correlation, in mon-
strosities, 27.

, on correlation, 148.

, on variability of repeated parts,

"52.

, on variable parts being often mon-
strous, 156-7.

Geographical distribution, 378.

Geology, future progress of, 504.

, imperfection of the record, 504.

Gervais, Prof., on Typotherium, 363.

Giraffe, uil of, 196.

, structure of, 219.

Glacial period, 394.

, affecting the North and South, 397.

Glands, mammary, 233.

Gmclin, on distribution, 394.

Godwin-Austen, Mr., on the Malay Archi-
pelago, 338.

Goethe, on compensation of growth, 150.

Gomphia, 216.

Gooseberry, grafts of, 297.

Gould, Dr. Aug. A., on land shells, 420.

, Mr., on colours of birds, 139.

, on instincts of cuckoo. 261.

522 INDEX

Gould, on distribution of genera of birds,

426.
Gourds, crossed, 311.
Graba, on the Uria lacr)'mas, 99.
Grafting, capacity of, 297.
Granite, areas of denuded, 330-1.
Grasses, varieties of, 116-7.
Gray, Dr. Asa, on the variability of oaks,

62.

, on man not causing variability, 87.

^.on sexes of the holly, 10 1.

, on trees of the United States, 106.

, on naturalised plants in the United

Sutes, 118.

, on zstivation, 216.

, on Alpine plants, 394.

, on rarity of intermediate varieties,

172.

, Dr. J. E., on striped mule, 163.

Grebe, 179.

Grimm, on asexual reproduction, 458.

Groups, aberrant, 448.

Grouse, colours of, 92.

, red, a doubtful species, 60.

Growth, compensation of, 150.
Giinthcr, Dr., on flat-fish, 231.

, on prehensile tails, 233.

, on the fishes of Panama, 379.

, on the range of fresh-water fishes,

409.
, on the limbs of Lepidosiren, 471.

H

Haast, Dr., on glaciers of New Zealand,
400.

Habit, effect of, under domestication, 27.

.effect of, under nature, 140-1.

.diversified, of same species, 178.

Hackel, Prof., on classification and the
lines of descent, 452.

Hair and teeth, correlated, 148.

Halitherium, 363.

Harcourt, Mr. E. V., on the birds of Ma-
deira, 415.

Hartung, M., on boulders in the Azores,

392-3-
Hazel nuts, 388.
Hearne, on habits of bears. 178.
Heath, changes in vegetation, 79-80.
Hector, Dr., on glaciers of New Zealand,

400.
Heer, Oswald, on ancient cultivated

plants, 32.
-, on plants of Madeira, 1 1 1

Helianthemum, 216.
Helix pomatia, 420.

, resisting salt water, 420.

Hclmholtz, M., on the imperfection of

the human eye, 203-4.
Helosciadium, 388.
Hcmionus, striped, 165.
Hensen, Dr., on the eyes of Cephalopods,

190.
Herbert, W., on struggle for existence, 7a.

, on sterility of hybrids, 288-9.

Hermaphrodites crossing, 103.
Heron eating seed, 412.
Heron, Sir R., on peacocks, 96.
Hcusingcr, on white animals poisoned by

certain plants, 27.
Hewitt, Mr., on sterility of first crosses,

301.
Hildebrand, Prof., on the self-sterility of

Corydalis, 289.
Hilgendorf, on intermediate varieties

332-
Himalaya, glaciers of. 400.

, plants of, 402.

Hippeastrum, 289.
Hippicampus, 233.
Hofmeister, Prof., on the movements ol

plants, 242-3.
Holly-trees, sexes of, 100.
Hooker, Dr., on man not causing varia-
bility, 87.

, on trees of New Zealand, 106.

, on acclimatisation of Himalayan

trees, 145.

, on flowers of umbelliferz, 149.

.on the position of ovules, 214.

, on glaciers of Himalaya, 400.

. on glaciers of the Lebanon, 400.

, on plants of Tierra del Fuego.

401-2.
, on plants of mountains of Fernando

Po, 402.

, on algx of New Zealand, 403.

, on Australian plants, 403, 422.

, on vegetation at the base of the

Himalaya, 404.
, on relations of flora of America.

405.
, on flora of the Antarctic lands, 407.

422.
.on the plants of the Galapagos,

416. 421.
Hooks on palms, 198.

on seeds, on islands. 416.

Hopkins, Mr., on denudation, 330.

Hornbill, remarkable instinct of, 284.
Horns, rudimentary, 473.
Horse, fossil, in La Plata, 354.

, proportions of, when younj?, 464.

Horses, destroyed by flics in Paraguay,
80-1.

, striped, 163.

Horticulturists, selection applied by, 43-4-

Huber, on cells of bees, 272.

, P., on reason blended with instinct,

151-

, on habitual nature of instincts, 252.

, on slave-making ants, 264.

, on Melipona domestica, 269.

Hudson, Mr., on the Ground-Wood-
pecker of La Plata, 179.

, on the Molothrus, 262.

Humble-bees, cells of, 268.

Hunter, J., on secondary sexual charac-
ters, 153.

Hutton, Captain, on crossed geese, 291.

Huxley, Prof., on structure of hermaphro-
dites, 106-7.

, on the affinities of the Sirenia, 363.

, on forms connecting birds and rep-
tiles, 364.

, on homologous organs, 457.

, on the development of aphis, 462.

Hybrids and mongrels compared, 312.

Hybridism, 285.

Hydra, structure of, 185.

Hymenopiera, fighting, 95.

Hymenopterous insects, diving, 180.

Hyoscris, 215.

INDEX 523

Instinct, not varying simultaneously with

structure, 278.
Instincts, domestic, 255.
Intercrossing, advantages of, 103, 304.
Islands, oceanic, 413.
Isolation favourable to selection, 109.

J

Japan, productions of, 399.

Java, plants of, 402.

Jones, Mr. J. M., on the birds of Bermuda,

4'5-

Jourdain, M., on the eye-spots of star-
fishes, 182.

Jukes, Prof., on subaerial denudation,

322-
Jussieu, on classification, 436.

K

Kentucky, caves of, 142-3.
Kerguelen Land, flora of, 407, 422.
Kidney bean, acclimatisation of, 147.
Kidneys of birds, 148.
Kirby, on tarsi deficient in beetles,

141.
Knight, Andrew, on cause of variation,

Kiilreuter, on Intercrossing, 103.

, on the barberry, 104.

, on sterility of hybrids, 286—7.

, on reciprocal crosses, 294-5.

, on crossed varieties of nicotiana,

312.

, on crossing male and hermaphro-
dite flowers, 470.

Ibla, 151.

Icebergs transporting seeds, 392.

Increase, rate of, 73.

Individuals, large numbers of favourable
to selection, 107.

, many, whether simultaneously cre-
ated, 385.

Inheritance, laws of, 29.

, at corresponding ages, 29, 96.

Insects, colour of, fitted for their sta-
tions, 92.

, sea-side, colours of, 139.

, blind, in caves, 143.

, luminous, 190.

, their resemblance to certain objects,

224.

, neuter, 279-80

Instinct, 2;i.

Lamarck, on adaptive characters, 443.
Lancelet, 131.

, eyes of, 183.

Landois, on the development of the wings

of insects, 187.
Land shells, distribution of. 421.

, of Madeira, naturalised, 424-5.

, resisting salt water, 420.

Languages, classification of, 440.
Lankester, Mr. £. Ray, on Longevity,

210.

, on homologies, 457.

Lapse, great, of time, 321.

Larvx, 460.

Laurel, nectar secreted by the leaves, 99.

Laurentian formation, 345.

524 INDEX

Laws of variation, 138.
Leech, varieties of, 83.
Leguminosz, nectar secreted by glands,

99-

Leibnitz' attack on Newton, 498.

Lepidosiren, 1 1 1—2, 364.

, limbs in a nascent condition, 471.

Lewes, Mr. G. H., on species not having
changed in Egypt, 210.

, on the Salamandra atra, 470.

, on many forms of life having been

at first evolved, 502.

Life, struggle for, 73.

Lingula, Silurian, 344.

Linnzus, aphorism of, 433.

Lion, name of, 95.

, young of, striped, 459.

Lobelia fulgens, 81, 104-5.

, sterility of crosses, 289.

Lockwood, Mr., on the ova of the Hip-
pocampus, 233.

Locusts transporting seeds, 391.

Logan, Sir W., on Laurentian formation,

345-

Lowe, Rev. R. T., on locusts visiting
Madeira, 391.

Lowncss of structure connected with
variability, 152.

, related to wide distribution, 427.

Lubbock, Sir J., on the nerves of Coc-
cus, 56.

, on secondary sexual characters, 1 59.

, on a diving hymenopterous insect,

180.

, on affinities, 337.

, on metamorphoses, 460.

Lucas, Dr. P., on inheritance, 28.

, on resemblance of child to parent,

3>5-
Lund and Clausen, on fossils of Brazil,

372-

Lyell, Sir C, on the struggle for ex-
istence, 72.

, on modern changes of the earth,

102.

, on terrestrial animals not having

been developed on islands, 223.

, on a carboniferous land shell, 327.

, on strata beneath Silurian system,

345-

, on the imperfection of the geologi-
cal record, 348.

, on the appearance of species, 349.

, on Barrandc's colonies, 350.

Lyell, Sir C, on tertiary formatiom of
Europe and North America, 358.

, on parallelism of tertiary forma-
tions, 361.

, on transport of seeds by icebergs,

392.

, on great alterations of climate, 408.

, on the distribution of fresh-water

shells, 411.

, on land shells of Madeira, 424.

Lyell and Dawson, on fossilized trees in
Nova Scotia, 334-5.

Lythrum salicaria, trimorphic, 307.

M

Macleay, on analogical characters, 443.

Macrauchenia, 362.

McDonnell, Dr., on electric organs, 189.

Madeira, plants of, iti.

, beetles of, wingless, 141.

, fossil land shells of, 372—3.

, birds of, 415.

Magpie tame in Norway, 255.

Males fighting, 95.

Maize, crossed, 311.

Malay Archipelago compared with Eu-
rope, 338.

.mammals of, 419.

Malm, on flatfish, 230.

Malpighiacex, small imperfect flowers of,
214.

. 436-

Mammz, their development, 233.

, rudimentary, 469.

Mammals, fossil, in secondary formation,

34"-

, insular, 416.

Man, origin of, 505.

Manatee, rudimentary nails of, 473.

Marsupials of Australia, 119.

, structure of their feet, 452—3.

, fossil species of, 372.

Martens, M., experiment on seeds, 389.
Martin, Mr. W. C, on striped mules, 164.
Masters, Dr., on Saponaria, 216.
Matteucci, on the electric organs of rays,

l88r-9.

Matthiola, reciprocal crosses of, 295.

Maurandia, 242.

Means of dispersal, 386.

Melipona domestica, 269.

Merrell, Dr., on the American cuckoo,

259-
Metamorphism of oldest rocks, 345.

Mke destroying bees, 82.

, acclimatisation of, 146.

, tails of, 232-3.

Miller, Prof., on the cells of bees, 269-70,

273;

Mirabilis, crosses of, 295.

Missel -thrush, 84.

Mistletoe, complex relations of, 20.

Mivart, Mr., on the relation of hair and
teeth, 148-9.

, on the eyes of cephalopods, 1 90.

, various objections to natural selec-
tion, 219.

, on abrupt modifications, 246-7.

, on the resemblance of the mouse

and Antechinus, 443.

Mocking-thrush of the Galapagos, 424.

Modificalion of species not abrupt, 500.

Moles, blind, 142.

Molothrus, habits of, 262.

Mongrels, fertility and sterility of, 308.

and hybrids compared, 312.

Monkeys, fossil, 341.

Monachanthus, 441.

Mens, Van, on the origin of fruit-trees,
4'.

Monstrosities, 54.

Moquin-Tandon, on sea-side plants, 139.

Morphology, 452.

Morren, on the leaves of Oxalis, 242.

Moths, hybrid, 291.

Mozart, musical powers of, 252.

Mud, seeds in, 412.

Mules, striped, 164.

Miiller, Adolf, on the instincts of the
cuckoo, 260.

Miiller, Dr. Ferdinand, on Alpine Aus-
tralian plants, 403.

Miiller, Fritz, on dimorphic crustaceans,
57. 282.

, on the lancelet, 131.

, on air-breathing crustaceans, 192.

, on climbing plants, 242.

, on the self-sterility of orchids, 289.

, on embryology in relation to classi-
fication, 437.

, on the metamorphoses of crusta-
ceans, 462, 467.

, on terrestrial and fresh-water or-
ganisms not undergoing any meta-
morphosis, 466.

Multiplication of species not indefinite,

133-4.
Murchison, Sir R., on the formations of
Russia, 328.

INDEX 525

Murchison, on azoic formations, 345.

, on extinction, 353.

Murie, Dr., on the modification of ths

skull in old age, 188.
Murray, Mr. A., on cave-insects, 144.
Mustcia vision, 175.
Myanthus, 441.
Myrmecocystus, 280.
Myrmica, eyes of, 281.

N

Nageli, on morphological characters, 212.

Nails, rudimentary, 473.

Nathusius, Von, on pigs, 199.

Natural history, future progress of, 503.

selection, 87.

system, 433.

Naturalisation of forms distinct from
the indigenous species, 1 1 8.

Naturalisation in New Zealand, 203.

Naudin, on analogous variations in
gourds, 160.

, on hybrid gourds, 311.

, on reversion, 314.

Nautilus, Silurian, 344.

Necur of plants, 99-100.

Nectaries, how formed, 99.

Nelumbium luteum, 412.

Nests, variations in, 254, 277, 284.

Neuter insects, 280-1.

Newman, Col., on bumblebees, 82.

New Zealand, productions of, not per-
fect, 203.

, naturalised products of, 371.

, fossil birds of, 372.

, glaciers of, 400.

, crustaceans of, 403.

, algjt of, 403.

, number of plants of, 414.

, flora of, 422.

Newton, Sir I., attacked for irreligion,
498.

, Prof., on earth attached to a part-
ridge's foot, 392.

Nicotiana, crossed varieties of, 312.

, certain species very sterile, 294.

Nitsche, Dr., on the Polyzoa, 237.

Noble, Mr., on fertility of Rhododendron,
290.

Nodules, phosphatic, in azoic rocks, 345.

Oaks, variability of, 62-3.
Onites apelles, 141.

526

INDEX

Ononis, small imperfect flowers of, 214.

Orchids, fertilisation of, 194-5.

, the development of their flowers,

239-40.

, forms of, 441.

Orchis, pollen of, 190.

Organisation, tendency to advance, 129-
30.

Organs of extreme perfection, 181.

.electric, of fishes, 190.

, of little importance, 196.

, homologous, 454-5.

, rudiments of, and nascent, 469.

Ornithorhynchus, iii, 435.

, mamma: of, 234.

Ostrich not capable of flight, 223.

, habit of laying eggs together, 263.

, American, two species of, 380.

Otter, habits of, how acquired, 175.

Ouzel, water, 179.

Owen, Prof., on birds not flying, 140.

, on vegetative repetition, 152.

, on variability of unusually de-
veloped parts, 153.

, on the eyes of fishes, 183.

, on the swim bladder of fishes, 186.

, on fossil horse of La Plata, 354.

, on generalised form, 362.

, on relation of Ruminants and

Pachyderms, 362.

, on fossil birds of New Zealand, 372.

, on succession of types, 372.

, on affinities of the dugong, 434.

, on homologous organs, 453.

, on the metamorphosis of cephalo-

pods, 461.

Pacific Ocean, faunas of, 380.

Pacini, on electric organs, 189-90.

Paley, on no organ formed to give pain,
203.

Pallas, on the fertility of the domesti-
cated descendants of wild stocks, 291.

Palm with hooks, 198.

Papaver bracteatum, 216.

Paraguay, cattle destroyed by flies, 80-1.

Parasites, 262-3.

Partridge, with ball of earth attached to
foot, 391.

Parts greatly developed, variable, 153.

Parus major, 178.

Passiflora, 289.

Peaches in United States, 92.

Pear, grafts of, 297.
Pedicellariz, 236.
Pelargonium, flowers of, 149.

, sterility of, 289.

Pelvis of women, 148.
Pcloria, 149.
Period, Glacial, 394.
Petrels, habits of, 179.
Phasianus, fertility of hybrids, 291.
Pheasant, young, wild, 258.
Pictet, Prof., on groups of species sud-
denly appearing, 341.

, on rate of organic change, 350.

, on continuous succession of genera,

351.

, on change in latest tertiary forms,

336.

, on close alliance of fossils in con-
secutive formations, 367-8.

, on early transitional links, 341.

Pierce, Mr., on varieties of wolves, 97.

Pigeons with feathered feet and skin be-
tween toes, 28.

, breeds described, and origin of, 34.

, breeds of, how produced, 49.

, tumbler, not being able to get out

of egg, 93.

, reverting to blue colour, 162.

, instinct of tumbling, 257.

, young of, 464.

Pigs, black, not affected by the paint-
root, 27.

, modified by want of exercise, 199.

Pistil, rudimentary, 470.

Plants, poisonous, not affecting certain
coloured animals, 27.

, selection applied to, 47.

, gradual improvement of, 48.

, not improved in barbarous coun-
tries, 48.

, dimorphic, 57.

, destroyed by insects, 78-9.

, in midst of range, have to strug-
gle with other plants, 85.

, nectar of, 99.

, fleshy, on sea-shores, 139.

, climbing, 186, 241.

, fresh-water, distribution of, 411.

, low in scale, widely distributed,

427.

Pleuronectidz, their structure, 229.

Plumage, laws of change in sexes of birds.
96.

Plums in the United States, 92.

Pointer dog, origin of, 46.

INDEX

527

Pointer, habits of, 257.

Poison not affecting certain coloured ani-
mals, 27.

, similar effect of, on animals and

plants, SOI.

Pollen of fir-trees, 204.

transported by various means, 193-4,

201.

Pollinia, their development, 239.

Polyzoa, their avicularia, 237.

Poole, Col., on striped hemionus, 165.

Potamogeton, 412.

Pouchet, on the colours of flatfish, 232.

Prestwich, Mr., on English and French
eocene formations, 361.

Proctotrupes, 180.

Proteolcpas, 151.

Proteus, 144.

Psychology, future progress of, 505.

Prygoma, found in the chalk, 342.

Quagga, striped, 166.

Quatrefages, M., on hybrid moths, 291.

Quercus, variability of, 62.

Quince, grafts of, 397.

Rabbits, disposition of young, 257.

Races, domestic, characters of, 30.

Race horses, Arab, 46.

, English, 386.

Radcliffc, Dr., the electrical organs of
the torpedo, 189.

Ramond, on plants of Pyrenees, 306.

Ramsay, Prof., on subaerial denudation,
323-

, on thickness of the British forma-
tions, 324.

, on faults, 324.

, Mr., on instincts of cuckoo, 261.

Ratio of increase, 73.

Rats supplanting each other, 84.

, acclimatisation of, 146.

, blind, in cave, 143.

Rattlesnake, 202.

Reason and instinct, 251.

Recapitulation, general, 478.

Reciprocity of crosses, 294.

Record, geological, imperfect, 319.

Rengger, on flics destroying cattle, 80-1.

Reproduction, rate of, 73-4.

Resemblance, protective of insects, 224.

to parents in mongrels and hybrids,

3>4-5-

Reversion, law of inheritance, 29.

, in pigeons, to blue colour, 162.

Rhododendron, sterility of, 290.

Richard, Prof., on Aspicarpa, 436.

Richardson, Sir J., on structure of squir-
rels, 176.

, on fishes of the southern hemis-
phere, 403.

Robinia, grafts of, 298.

Rodents, blind, 142.

Rogers, Prof., Map of N. America, 331.

Rudimentary organs, 470.

Rudiments important for classification,

435-
Riitimeycr, on Indian cattle, 33, 292.

Salamandra atra, 470.

Saliva used in nests, 277.

Salvin, Mr., on the beaks of ducks,
227.

Sageret, on grafts, 297.

Salmons, males fighting, and hooked
jaws of, 95.

Salt water, how far injurious to seeds,
388-9.

, not destructive to landshells, 420.

Salter, Mr., on early death of hybrid em-
bryos, 301.

Saurophagus sulphuratus, 178.

Schacht, Prof., on phylloUxy, 215.

Schiiidtc, on blind insects, 143.

, on flatfish, 230.

Schlegel, on snakes, 148.

SchobI, Dr., on the cars of mice, 213.

Scott, J., Mr., on the self-sterility of or-
chids, 289.

, on the crossing of varieties of Ver-

bascum, 311.

Seawater, how far injurious to seeds,
388-9.

not destructive to landshells, 420.

Sebright, Sir J., on crossed animals, 34.

Sedgwick, Prof., on groups of species
suddenly appearing, 340.

Seedlings destroyed by insects, 77.

Seeds, nutriment in, 85.

(Winged, 150.

, means of dissemination, 193, 201,

390.

, power of resisting salt water, 389.

, in crops and intestines of birds,

390-

.eaten by fish, 391, 413.

, in mud, 413.

528

INDEX

Seeds, hooked, on islands, 416.
Selection of domestic products, 40.

, principle not of recent origin, 45.

, unconscious, 45.

, natural, 87.

, sexual, 94.

, objections to term, 88.

natural, has not induced sterility,

298-9.
Sexes, relations of, 95.
Sexual characters variable, I $7.

selection, 94.

Sheep, merino, their selection, 43.

■ , two sub-breeds, unintentionally

produced, 46.

, mountain varieties of, 83.

Shells, colours of, 139.

, hinges of, 193.

, littoral, seldom embedded, 327.

, fresh-water, long retain the same

forms, 369.

, , dispersal of, 410.

, of Madeira, 414.

, land, distribution of, 421.

, , resisting salt water, 420.

Shrew-mouse, 443.

Silene, infertility of crosses, 293.

Silliman, Prof., on blind rat, 143.

Sirenia, their affinities, 363.

Sitaris, metamorphosis of, 468.

Skulls of young mammals, 198, 455.

Slave-making instinct, 264.

Smith, Col. Hamilton, on striped horses,

164.
, Mr. Fred., on slave-making ants,

264.

, — on neuter ants, 281.

Smitt, Dr., on the Polyzoa, 237.

Snake with tooth for cutting through

egg-shell, 262.
Somerville, Lord, on selection of sheep,

43-
Sorbus, grafts of, 298.
Sorex, 443.

Spaniel, King Charles's breed, 46.
Specialisation of organs, 129-30.
Species, polymorphic, 56.

, dominant, 66.

, common, variable, 65.

in large genera variable, 66.

, groups of, suddenly appearing,

340. 344-

beneath Silurian formations, 345.

Species successively appearing, 349.

Species changing simultaneously through-
out the world, 357.

Spencer, Lord, on increase in size of cat-
de, 46.

, Herbert, Mr., on the first steps in

differentiation, 132.

, on the tendency to an equilibrium

in all forces, 304-5.

Sphex, parasitic, 263.

Spiders, development of, 462.

Sports in plants, 26.

Sprengcl, C. C, on crossing, 103.

, on ray-florets, 149.

Squalodon, 363.

Squirrels, gradations in structure, 176.

Suffordshire, heath, changes in, 79.

Stag beetles, fighting, 95.

Star-fishes, eyes of, 182.

, their pedicellariz, 236.

Sterility from changed conditions of life,

25-

of hybrids, 287.

, laws of, 292.

, causes of, 298.

, from unfavourable conditions, 303.

not induced through natural selec-
tion, 299.

St. Helena, productions of, 414.

St. Hilaire, Aug., on variability of cer-
tain plants, 216.

, on classification, 436.

St. John, Mr., on habits of cats, 256.

Sting of bee, 204.

Stocks, aboriginal, of domestic animals,

32-

Strata, thickness of, in Britain, 324.

Stripes on horses, 164.

Structure, degrees of utility of, 199.

Struggle for existence, 71.

Succession, geological, 349.

of types in same areas, 372.

Swallow, one species supplanting an-
other, 84.

Swaysland, Mr., on earth adhering to the
feet of migratory birds, 392.

Swifts, nests of, 277.

Swim bladder, 186.

Switzerland, lake habitations of, 32.

System, lutural, 433.

Tail of giraffe, 196.

of aquatic animals,

, prehensile, 233.

197.

469.

Tail, rudimentary, 473.

Tanais, dimorphic, 57.

Tarsi, deficient, 141.

Tausch, Dr., on Umbclliferx, 215.

Teeth and hair correlated, 148.

, rudimentary, embryonic calf,

496.
Tegetmeier, Mr., on cells of bee», 270,

275-

Temminck, on distribution aiding classi-
fication, 437-8.

Tendrils, their development, 241.

Thompson, Sir W., on the age of the
habitable world, 345.

, on the consolidation of the crust

of the earth, 484.

Thouin, on grafts, 298.

Thrush, aquatic species of, 180.

, mocking, of the Galapagos, 424.

, young of, spotted, 459.

, nest of, 284.

Thwaites, Mr., on acclimatisation, 145.

Thylacinus, 444.

Tierra del Fuego, dogs of, 258.

Timber, drift, 389.

Time, lapse of, 321.

by itself not causing modification,

107.

Titmouse, 178.

Toads on islands, 417.

Tobacco, crossed varieties of, 312.

Tomes, Mr., on the distribution of bats,
418.

Transitions in varieties rare, 171.

Traquair, Dr., on flatfish, 231.

Trautschold, on intermediate varieties,

332-
Trees on islands belong to peculiar orders,

416.

with separated sexes, 1 06.

Trifolium pratense, 81, loi.

incarnatum, i o i .

Trigonia, 356.
Trilobites, 345.

, sudden extinction of, 357.

Trimen, Mr., on imitating-insects, 447.
Trimorphism in plants, 57, 307.
Troglodytes, 284.
Tucu tucu, blind, 142.
Tumbler-pigeoiu, habits of, hereditary,

257-
Tumbler, young of, 464.
Turkey cock, tuft of hair on breast, 96.
, naked skin on head, 198.

INDEX 529

Turkey, young of, instinctively wild, 258.
Turnip and cabbage, analogous variation!

of, 160.
Type, unity of, 207.

Types, succession of, in same areas, 372,
Typotherium, 363.

U

Udders enlarged by use, 37.

, rudimentary, 469.

Ulex, young leaves of, 459.
Umbelliferz, flowers and seeds of, 150.

.outer and inner florets of, 215.

Unity of type, 207.
Uria lacrymans, 99.
Use, effects of, under domesticatioa, 27.

, effects of, in a state of nature, 140.

Utility, how far important in the con-
struction of each part, 199.

Valenciennes, on fresh-water fish, 410.
Variability of mongrels and hybrids,

3'3.

Variation under domestication, 23.

caused by reproductive system be-
ing affected by conditions of life, 24.

under nature, 54.

, laws of, 138.

, correlated, 27, 147, 199.

Variations appear at corresponding ages,

29. 93-

analogous in distinct species, 159.

Varieties, natural, 54.

, struggle between, 84.

, domestic, extinction of, 114.

.transitional, rarity of, 171.

.when crossed, fertile, 312.

, , sterile, 310.

, classification of, 440.

Vcrbascum, sterility of, 289.

.varieties of crossed, 311.

Verlot, M., on double stocks, 279.

Verneuil. M. de, on the succession of
species. 359.

Vibracula of the Polyzoa, 238.

Viola, small imperfect flowers of, 214.

, tricolor, 8 1 .

Virchow. on the structure of the crystal-
line lens. 183-4.

Virginia, pigs of, 92.

Volcanic islands, denudation of. 323.

Vulture, naked skin on head. 198.

530

INDEX

W

Wading-birds, 41 1-2.

Wagner, Dr., on Cecidomyia, 458.

, Moritz, on the importance of ijola-

tion, 109.

Wallace, Mr., on origin of species, 19.

, on the limit of variation under do-
mestication, 52.

, on dimorphic lepidoptera, 57, 282.

, on races in the Malay Archipelago,

59-
, on the improvement of the eye,

183.

, on the walking stick insect, 225.

, on laws of geographical distribu-
tion, 385.

, on the Malay Archipelago, 419.

, on mimetic animals, 447.

Walsh, Mr. B. D., on Phytophagic forms,
60.

, on equal variability, 160.

Water, fresh, productions of, 409.

Water-hen, 180.

Watcrhouse, Mr., on Australian mar-
supials, 119.

, on gready developed parts being

variable, 153.

, on the cells of bees, 268.

, on general affinities, 449.

Watson, Mr. H. C, on range of varieties
of British plants, 58-9, 69.

, on acclimatisation, 145.

, on flora of Azores, 392.

, on Alpine plants, 396.

, on rarity of intermediate varieties,

172.

, on convergence, 132.

, on the indefinite multiplication of

species, 133.

Wealc, Mr., on locusts transporting seeds,
391.

Web of feet in water-birds, 181.

Weismann, Prof., on the causes of vari-
ability, 24.

, on rudimentary organs, 472.

West Indian Islands, mammals of, 419.

Westwood, on species in large genera be-
ing closely allied to others, 68.

, on the tarsi of Engidx, 158.

, on the antennz of Hymenopterous

insects, 435.

Whales, 225-6.

Wheat, varieties of, 117.

White Mountains, flora of, 394.
Whitaker, Mr., on lines of escarpment,

323-

Wichura, Max, on hybrids, 301, 314.

Wings, reduction of size, 140.

of insects homologous with branchiz,

186, 187.

, rudimentary, in insects, 469.

Wolf crossed with dog, 257.

of Falkland Isles, 417.

Wollaston, Mr., on varieties of insects, 60.

, on fossil varieties of shells in Ma-
deira, 64.

, on colours of insects on seashore,

139.

, on wingless beetles, 141.

, on rarity of intermediate varieties,

172.

, on insular insects, 414.

, on land shells of Madeira natural-
ised, 424.

Wolves, varieties of, 97.

Woodcock with earth attached to leg,
391-2.

Woodpecker, habits of, 179.

, green colour of, 197-8.

Woodward, Mr., on the duration of
specific forms, 332.

, on Pyrgoma, 342.

, on the continuous succession of

genera, 352.

, on the succession of types, 372.

World, species changing simultaneously
throughout, 359.

Wrens, nest of, 284.

Wright, Mr. Chauncey, on the giraffe,
221.

, on abrupt modifications, 249.

Wyman, Prof., on correlation of colour
and effects of poison, 27.

, on the cells of the bee, 270.

Youatt, Mr., on selection, 43.

, on sub-breeds of sheep, 46.

, on rudimentary horns in young cat-
tle. 473-

Zanthoxylon, 216.
Zebra, stripes on, 162-3,
Zeuglodon, 363.