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New York State Colleges 


Agriculture and Home Economics 

Cornell University 


924 052 394 107 

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tine Cornell University Library. 

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the United States on the use of the text. 





M. A., LL. D., F.R.S. 





p., 0^ . \ b o ^ 

Authorized Edition. 


Additions and Corrections, to the Sixth Edition . Page Ix 

Historical Sketch xiii 

Introduction 1 


variation under domestication. 

Causes of Variability — ^BfEects of Habit and the use or disuse of 
Parts — Correlated Variation — Inheritance — Character of Do- 
mestic 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 followed, their Effects — 
Methodical and Unconscious Selection — Unknown Origin of 
our Domestic Productions — Circumstances favourable to Man's 
power of Selection 7 



Variability — Individual differences — Doubtful species — Wide rang- 
ing, much diffused, and common species, vary most — Species of 
the larger genera in each country vary more frequently than 
the species of the smaller genera — Many of the species of the 
larger genera resemble varieties in being very closely, but 
unequally, related to each other, and in having restricted 

ranges 51 





Its bearing on natural selection — The term used in a wide sense — 
Geometrical ratio of increase — Rapid increase of naturalised 
animals and plants — Nature of the checks to increase — Com- 
petition universal — EflEects 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 species of the same genus — The relation of organism 
to organism the most important of all relations . . Page 75 



Natural Selection — its power compared with man's selection — its 
power on characters of trifling importance — its power at aU 
ages and on both sexes — Sexual selection — On the generality 
of intercrosses between individuals of the same species — Cir- 
cumstances favourable and unfavourable to the results of 
Natural Selection, namely, intercrossing, isolation, number of 
individuals — Slow action — Extinction caused by Natural Se- 
lection — 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 — Ex- 
plains the grouping of all organic beings — Advance in organ- 
isation — Low forms preserved — Convergence of Character — 
Indefinite multiplication of species — Summary , . .97 



EfEeets of changed conditions — Use and disuse, combined with 
natural selection ; organs of flight and of vision — Acclimatisa- 


tion — Correlated variation-^Compensation and economy of 
growth — False correlations — 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 .... Page 164 



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 transition — Cases of difficulty — Natura 
non facit saltum — Organs of small importance — Organs not in 
all cases absolutely perfect — The law of Unity of Type and of 
the Conditions of Existence embraced by the theory of Natu- 
ral Selection 207 



Longevity— Modifications not necessarily simultaneous— Modifica- 
tions apparently of no direct service- Progressive development 
—Characters of small functional importance, the most con- 
stant—Supposed incompetence of natural s^lection to account 
for the incipient stages of useful structures— Causes which in- 
terfere with the acquisition through natural selection of useful 
structures— Gradations of structure with changed functions— 
Widely different organs in members of the same class, de- 
veloped from one and the same source — Reasons for disbeliev- 
ing in great and abrupt modifications .... 263 




Instincts comparable with habits, but different in their origin- 
Instincts graduated — Aphides and ants — Instincts variable — 
Domestic instincts, 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 Page 319 



NuMEEOUS small corrections have been made in the 
last and present editions on various subjects, according 
as the evidence has become somewhat stronger or weaker. 
The more important corrections and some addaitens in 
the present volume are tabulated on the following page, 
for the convenience of those interested in the subject, 
and who possess the fifth edition. The second edition 
was little more than a reprint of the first, i- The third 
edition was largely corrected and added to, and the 
fourth and fifth still more largely. As copies of the 
present work will be sent abroad, it may be of use if I 
specify the state of the foreign editions. The third 
French and second German editions were from the third 
English, with some few of the additions given in the 
fourth edition. A new fourth French edition has been 
translated by Colonel Moulini6; of which the first half 
is from the fifth English, and the latter half from the 
present edition. A third German edition, under the 
superintendence of Professor Victor Carus, was from the 
fourth English edition; a fifth is now preparing by the 
same author from the present volume. The second 
American edition was from the English second, with a 


few of the additions given in the third; and a third 
American edition has been printed from the fifth Eng- 
lish edition. The Italian is from the third, the Dutch 
and three Eussian editions from the second English 
edition, and the Swedish from the fifth English edition. 






vol. 1. 


















TOl. 11. 


Chief Additions and Corrections. 

Influence of fortuitous destruction on natural se- 

On the convergence of specific forms. 

Account of the Ground- Woodpecker of La Plata 

On the modification of the eye. 

Transitions through the acceleration or retarda- 
tion of the period of reproduction. 

The account of the electric organ of fishes added to. 

Analogical resemblance between the eyes of Cepha- 
lopods and Vertebrates. 

ClaparSde on the analogical resemblance of the 
hair-claspers of the Acarid». 

The probable use of the rattle to the Rattle-snake. 

Helmholtz on the imperfection of the human eye. 

The first part of this new chapter consists of por- 
tions, in a much modified state, taken from chap, 
iv. of the former editions. The latter and larger 
part is new, and relates chiefly to the supposed 
incompetency of natural selection to account 
for the incipient stages of useful structures. 
There is also a discussion on the causes which 
prevent in many cases the acquisition through 
natural selection of useful structures. Lastly, 
reasons are given for disbelieving in great and 
sudden modifications. Gradations of character, 
often accompanied by changes of function, are 
likewise here incidentally considered. 

The statement with respect to young cuckoos 
ejecting their foster-brothers confirmed. 

On the cuckoo-like habits of the Molothrus. 

On fertile hybrid moths. 

The discussion on the fertility of hybrids not 

having been acquired through natural selection 

condensed and modified. 






vol. IL 
















Chief Additions and Corrections. 

On the causes of sterility c^ hybrids, added to and 

Pyrgoma found in the chalk. 

Extinct forms serving to connect existing groups. 

On earth adhering to the feet of migratory birds. 

On the wide geographical range of a species of 
Galaxias, a fresh-water fish. 

Discussion on analogical resemblances, enlarged 
and modified. 

Homological structure of the feet of certain mar- 
supial animals. 

On serial homologies, corrected. 

Mr. B. Ray Lankester on morphology. 

On the asexual reproduction of Chironomus. 

On the origin of rudimentary parts, corrected. 

Recapitulation on the sterility of hybrids, cor- 

Recapitulation on the absence of fossils beneath 
the Cambrian system, corrected. 

Natural selection not the exclusive agency in the 
modification of species, as always maintained 
in this work. 

The belief in the separate creation of species, gen- 
erally held by naturalists, until a recent period. 

" 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 
presupposes an intelligent agent to render it so, i. e., to eflect it 
continually or at stated times, as what is supernatural or miraculous 
does to effect it for once." 

BuTiEE : 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 philosophy ; but 
rather let men endeavour an endless progress or proficienoe in 

Bacon : Ad/vaneement of Ltaming. 

Down, Beckenham, Kent, 

First Edition, November Ziih, 1859. 
Sixth Edition, -Tan. 1873. 





I WILL here give a brief sketch of the progress of 
opinion on the Origin of Species. Until recent ly the 
great majority of naturalists believed that speeier were 
immutabl e produ ctions, and had been se.paratejyja:eated. 
This view has been ably maintained by many authors. 
Some few naturalists, 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 sub- 
ject in the classical writers,* the first author who in 

* Aristotle, in his ' Physieae Auscultationes ' (lib. S, cap. 8, s. 3), 
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 t 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 otherparts in 
which there appears to exist an adaptation to an end. Whereso- 
ever, therefore, all things together (that is all the parts of one 
whole) happened like as if they were made for the sake of some- 
thing, these were preserved, having been appropriately constituted 


modern times has treated it in a scientific spirit was 
Buffon. But as his opinions fluctuated greatly at dif- 
ferent periods, and as he does not enter on the causes 
or means of the transformation of species, I need not 
here enter on details. 

Lamarck was the first man whose conclusions on the 
subject excited much attention. This Justly-celebrated 
naturalist first published his views in 1801; he much 
enlarged them in 1809 in his ' Philosophie Zoologique/ 
and subsequently, in 1815, in the Introduction to his 
'Hist. Nat. des Animaux sans Vertebres.' In these 
works he upholds the doctrine that all species, including 
man, are descended from other species. He first did 
the eminent seryjce of arousing attention to the proba- 
bility of all change in the organic, as well as in the 
inorganic world, being the result of law, and not of 
miraculous interposition. Laim»rck 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 gracfetion of forms in 
certain groups, and by the analogy of domestic produc- 
tions. With respect to the means of modification, he 
attributed something to ^he direct action of the physical 
conditions of life,, something to the crossing of already 
existiiig 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 nec}{ of the giraffe for browsing on the 

by an internal spontaneity ; and whatsoever tilings were not tlius 
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. 


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 ex- 
istence at the present day of simple productions, he 
maintains that such forms are now spontaneously gen- 

Geoffrey Saint-Hilaire, as is stated in his ' Life,' 
written by his son, suspected, as early as 1795, that 
what we call species are various degenerations of the 
same type. It was not until 1838 that he published 
his conviction that the same forms have not been per- 
petuated since the origin of all things. Geoffroy seems 
to have relied chiefly on the conditions of life, or the 
" monde 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 k rfeerver 
entierement a I'avenir, suppose meme que I'avenir doive 
avoir prise sur lui." 

In 1813, Dr. W. C. Wells read before the Royal 

* I have taken the date of the first publication of Lamarck from 
Isld. Geoffroy Saint-Hilaire's ('Hist. Nat. Gfofirale,' tom. ii. p. 
405, 1859) excellent history of opinion on this subject. In this 
work a full account is given of BufEon's conclusions on the same 
subject. It is 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 doublf that Goethe 
was an extreme partisan of similar views, as shown in the Intro- 
duction to a work written in 1794 and 1795, but not published till 
long afterwards : he has pointedly remarked (' Goethe als Natur- 
forscher,' 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 snecies, in the years 1794^5. 


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 lirst recognition which has been indi- 
cated; but he applies it only to the races of man, and 
to certain characters alone. After remarking that ne- 
groes and mulattoes enjoy an immunity from certain 
tropical diseases, he observes, firstly, that all animals 
tend to vary in some degree, and, secondly, that agri- 
culturists improve their domesticated animals by selec- 
tion; and then, he adds, but what is done in this latter 
ease "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 inhabi- 
tants 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 neigh- 
bours. 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. Eowley, of the United 
States, for having called my attention, through Mr. 
Brace, to the above passage in Dr. Well's work. 

The Hon. and Eev. W. Herbert, afterwards Dean of 
Manchester, in the fourth volume of the ' Horticultural 
Transactions,' 1822, and in his work on the 'Amaryl- 
lidacese ' (1837, pp. 19, 339), declares that " horticultural 
experiments have established, beyond the possibility of 
refutation, 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 species. 

In 1826 Professor Grant, in the concluding para- 
graph in his well-known paper (' Edinburgh Philosophi- 
cal Journal,' vol. xiv. p. 283) on the Spongilla, clearly 
declares his belief that species are descended from other 
species, and that they become improved in the course of 
modification. This same view was given in his 55th 
Lecture, published in the ' Lancet ' in 1834. 

In 1831 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 
(presently to be alluded to) propounded by Mr. Wallace 
and myself in the 'Linnean Journal,' and as that en- 
larged 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, 1860. The differences of Mr. 


Matthew's view from mine are not of much importance: 
he seems to consider that the world was nearly de- 
populated 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. 

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

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 develop- 
ment 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 or- 
ganisation terminating in the highest dicotyledons and 
vertebrata, these grades being few in number, and gen- 
erally marked by intervals of organic character, which 
we find to be a practical difficulty in ascertaining affini- 
ties; 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 be- 
Ueves 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 immutable productions. But I can- 
not 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, imme- 
diately had a very wide circulation. In my opinion it 
has done excellent service in this country in calling at- 
tention to the subject, in removing prejudice, and in 
thus preparing the ground for the reception of analogous 

In 1846 the veteran geologist M. J. d'Omalius d'Hal- 
loy published in an excellent though short paper (' Bul- 
letins de I'Acad. Roy. Bruxelles,' torn. 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 divers 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 dis- 
tribution, he adds, " These phenomena shake our confi- 
dence in the conclusion that the Apteryx of New 
Zealand and the Eed Grouse of England were distinct 
creations in and for those islands respectively. Always, 
also, it may be well to hear 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 Eed Grouse are " enumerated 
by the zoologist as evidence of distinct creation of the 
bird in and for such islands, he chiefly expresses that 
he knows not how the Eed 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 Crea- 
tive 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 Eed Grouse first ap- 
peared 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 pub- 
lished, 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 palaeontologists as being firmly convinced of the 
immutability of species; but it appears (' Anat. of Ver- 
tebrates,' 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," &c. (Ibid. vol.. i. p. xxxv.), that 
Professor Owen admitted that natural selection may 
have done something in the formation of a new spe- 
cies; but this it appears (Ibid. vol. iii. p. 798) is in- 
accurate and without evidence. I also gave some ex- 
tracts from a correspondence between Professor Owen 
and the Editor of the " London Eeview,' 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 announce- 
ment; but as far as it is possible to understand certain 
recently published passages (Ibid. vol. iii. p. 798) I 
have either 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 con- 
cerned, 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 Geoffroy Saint-Hilaire, in his lectures de- 
livered in 1850 (of which a Eesume appeared in the 
'Eevue et Mag. de Zoolog.,' Jan. 1851), briefly gives 
his reason for believing that specific characters- " sont 
fixes, pour chaque espece, tant qu'elle se perpetue au 
milieu des memes circonstances: ils se modifient, si les 
circonstances ambiantes viennent a changer." "En- 
resume, I'observation des animaux sauvages demontre 
deja la variabilite limitee des espeees. Les experiences 
sur les animaux sauvages, devenus domestiques, et sur 
les animaux domestiques redevenus sauvages, la demon- 
trent plus clairement encore. Ces memes experiences 
prouvent, de plus, que les differences produites peuvent 
etre de valeur generique." In his ' Hist. Nat. Generale ' 
(torn. ii. p. 430, 1859) he amplifies analogous conclusions. 

From a circular lately issued it appears that Dr. 
Freke, in 1851 ('Dublin Medical Press,' p. 332), pro- 
pounded the doctrine that all organic beings have de- 
scended 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 Or- 
ganic Affinity,' the difficult attempt to give any idea of 
his views would be superfluous on my part. 

Mr. Herbert Spencer, in an Essay (originally pub- 
lished in the ' Leader,' March, 1853, 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 circumstances. The author (1855) has also 
treated Psychology on the principle of the necessary 
acquirement of each mental power and capacity by gra- 

In 1853 M. Naudin, a distinguished botanist, ex- 
pressly stated, in an admirable paper on the Origin of 
Species ('Eevue Horticole,' p. 102; since partly repub- 
lished in the ' Nouvelles Archives du Museum,' torn. i. 
p. 171), his belief that species are formed in an analo- 
gous manner as varieties are under cultivation; and 
the latter process he attributes to man's power of selec- 
tion. 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 principle of finality, 
"puissance mysterieuse, indeterminee; fatalite pour les 
uns; pour les autres, volonte providentielle, dont Tac- 
tion incessante sur les etres vivants determine, a toutes 
1^ ^poques de I'existence du monde, la forme, le vol- 
ume, et la duree de chacun d'eux, en raison de sa des- 
tines dans I'ordre de choses dont il fait partie. C'est 
cette puissance qui harmonise chaque membre a I'en- 
semble, en I'appropriant a la fonction qu'il doit remplir 
dans I'organisme general de la nature, fonction qui est 
pour lui sa raison d'etre." * 

• From references in Bronn's ' Untersuohungen Uber die Ent- 
wickelungs-Gesetze,' it appears tiiat the celebrated botanist and 


In 1853 a celebrated geologist, Count Keyserling 
(' Bulletin de la Soc. G6olog.,' 2nd Ser., torn. x. p. 357), 
suggested that as new diseases, supposed 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 circum- 
ambient molecules of a particular 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. Eheinlands,' &c.), 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 contin- 
ued reproduction." 

A well-known French botanist, M. Lecoq, writes in 
1854 ('Etudes sur Geograph. Bot.,' tom. i. p. 350), 
" On voit que nos recherches sur la fixite ou la variation 
de I'espece, nous conduisent directement aux idees 

palaeontologist 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 main- 
tained by Oken in his mystical ' Natur-Philosophie.' Prom other 
references in Godron's work 'Sur I'Bspece,' it seems 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. 


emises, par deux hommes justement celebres, GeofEroy 
Saint-Hilaire et Goethe." Some other passages scattered 
through M. Lecoq's large work, make it a little doubt- 
ful how far he extends his views on the modification of 

The ' Philosophy of Creation ' has been treated in a 
masterly manner by the Eev. 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 

The third volume of the ' Journal of the Linnean 
Society ' contains papers, read July 1st, 1858, by Mr. 
Wallace and myself, in which, as stated in the intro- 
ductory remarks to this volume, the theory of Natural 
Selection is promulgated by Mr. Wallace with admir- 
able force and clearness. 

Von Baer, towards whom all zoologists feel so pro- 
found a respect, expressed about the year 1859 (see 
Prof. Eudolph 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 be- 
fore the Royal Institution 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 species 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 opposed 
to the general analogy of nature. If^ on the other 
hand, we view ' Persistent Types ' in relation to that 
hypothesis which supposes the species living at any 
time to be the result of the gradual modification of 
pre-existing species a hypothesis which, though un- 
proven, 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 

In December, 1859, Dr. Hooker published his ' Intro- 
duction 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 doctrine by 
many original observations. 

The first edition of this work was published on No- 
vember 24th, 1859, and the second edition on January 
7th, 1860. 



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 in- 
habitants 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 mys- 
tery of mysteries, as it has been called by one of our 
greatest philosophers. On my return home, it occurred 
to me, in 1837, that something might perhaps be made 
out on this question by patiently accumulating and 
reflecting on all sorts of facts which could possibly 
have any bearing on it. After five years' work I al- 
lowed 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 proba- 
ble: from that period to the present day I have steadily 
pursued the same object. I hope that I may be ex- 
cused for entering on these personal details, as I give 
them to show that I have not been hasty in coming to a 


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 vol- 
ume of the Journal of that society. Sir C. Lyell 
and Dr. Plooker, who both knew of my work — the lat- 
ter 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 manu- 

This Abstract, which I now publish, must necessarily 
be imperfect. I cannot here give references and au- 
thorities 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, with refer- 
ences, 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 
apparently 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 
obligations 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 con- 
ceivable that a naturalist, reflecting on the mutual afiini- 
ties of organic beings, on their embryological relations, 
their geographical distribution, geological succession, 
and other such facts, might come to the conclusion 
that species had not been independently created, but 
had descended, like varieties, from other species. Never- 
theless, 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 justly excites our admiration. 
Naturalists continually refer to external conditions, 
such as climate, food, &c., as the only possible cause 
of variation.^ In one limited sense, as we shall here- 
after see, this may be true; but it is preposterous to 
attribute to mere external conditions, the structure, for 
instance, of the woodpecker, 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 
wliich has flowers with separate sexes absolutely re- 
quiring the agency of certain insects to bring pollen 
from one flower to the other, it is equally preposterous 
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 volition of 
the plant itself. 

It is, therefore, of the highest importance to gain a 
clear insight into the means of modification and co- 
adaptation. At the commencement of my observations 
it seemed to me probable that a careful study of domes- 
ticated 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 all other per- 
plexing cases I have invariably found that our knowl- 
edge, imperfect though it be, of variation under domes- 
tication, 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 ne- 
glected by naturalists. 

From these considerations, I .shall devote the first 
chapter of this Abstract to Variation under Domesti- 
cation. 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 to 
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 favour- 


able to variation. In the next chapter the Struggle for 
Existence amongst all organic beings throughout the 
world, which inevitably follows from the high geo- 
metrical ratio of their increase, will be considered. This 
is the doctrine of Malthus, applied to the whole animal 
and vegetable kingdoms. As many more indiv i duals 
o f each species ar e born thanjQan_possibly_survive; .^od^ 
as^_consequently, therej.s^ a_freg;B£atly---pe&iir-r-ing_&tr]iggle, 
Iot existenc g,^ it follows that any being, if it vary how - 
ev^T-slighiLy in an^ man.ner--piafiJ:abl£-.,taJAsLalf,.jind^ 
t he complex and sometimes varying conditions of life, 
win have a better c hance of surviving, and , thus be 
naturally selected. From the strong principle of inheri- 
tance, 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 Selection almost in- 
evitably causes much Extinction of the less improved 
forms of life, and leads to what I have called Diver- 
gence of Character. In the next chapter I shall discuss 
the complex and little known laws of variation. In 
the five succeeding chapters, the most apparent and 
gravest difficulties in accepting the theory will be given: 
namely, first, the difficulties of transitions, or how a 
simple being or a simple organ can be changed and per- 
fected into a highly developed being or into an elab- 
orately constructed organ; secondly, the subject of In- 
stinct, or the mental powers of animals; thirdly, Hybrid- 
ism, or the infertility of species and the fertility of 
varieties when intercrossed; and fourthly, the imperfec- 
tion of the Geological Record. In the next chapter I 
shall consider the geological 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 an embryonic condition. In the last 
chapter I shall give a brief recapitulation 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 rela- 
tions of the innumerable inhabitants of the world during 
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 enter- 
tained, and which I formerly entertained — ^namely, that 
each species has been independently created — is errone- 
ous. I am fully convinced that^speeies'arekTitit immuta- 
ble; but that those belonging to what arte called the 
same genera are lineal descendants of somje other and 
generally extinct species, in the same ma/nner as the 
acknowledged varieties of any one species\ are the de- 
scendants of that species.l^ Furthermore, I am convinced 
that Natural Selection has been the most important, hut 
not the exclusive, means of modificatibrsr 




Causes of Variability — BfEects of Habit and the use or disuse of 
Parts — Correlated Variation — Inheritance — Character of Do- 
mestic 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 followed, their Effects — 
Methodical and Unconscious Selection — Unknown Origin of 
our Domestic Productions — Circumstances favourable to Man's 
power of Selection. 

Causes of Yariahility. 

When we compare the individuals of the same vari- 
ety or sub- variety of our older cultivated plants and ani- 
mals, one of the first points which strikes us is, that they 
generally differ more from each other than do the in- 
dividuals 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 treatment, we are driven to 
conclude that this great variability is due to our 
domestic productions having been raised under condi- 
tions of life not so uniform as, and somewhat differ- 
ent from, those to which the parent species had been 
exposed under nature. There is, also, some probability 
3 ■ 


in the. view propounded by Andrew Knight, that this 
variability may be partly connected with excess of 
food.l^It seems clear that organic beings must be ex- 
posed 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 organism ceasing to vary under 
cultivation. Our oldest cultivated plants, such as wheat, 
still yield new varieties: our oldest domesticated ani- 
mals are still capable of rapid improvement or modifi- 

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 affecting the reproductive 
system. "With respect to the direct action, we must 
bear in mind that in every case, as Professor Weis- 
mann 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 organ- 
ism, and the nature of the conditions. The former seems 
to be much the more important; for nearly similar vari- 
ations 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 defi- 
nite when all or nearly all the offspring of individuals 
exposed to certain conditions 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, thickness of 
the skin and hair from climate, &c. Each of the endless 
variations 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 gen- 
erations on many individuals, all probably would be 
modified in the same manner. Such facts as the com- 
plex and extraordinary out-growths which invariably fol- 
low from the insertion of a minute drop of poison by a 
gall-producing insect, show us what singular modifica- 
tions 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 forma- 
tion of our domestic races. We see indefinite vari- 
ability 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 ancestor. 
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 same coun- 
try and fed on nearly the same food, deviations of 
structure 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 conditions of life on each 
individual organism, in nearly the same manner as the 
chill affects different men in an indefinite manner, ac- 
cording to their state of body or constitution, causing 
coughs or colds, rheumatism, or inflammation of various 

With respect to what I have called the indirect ac- 
tion of changed conditions, namely, through the repro- 
ductive system being affected, we may infer that vari- 
ability 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 ani- 
mals when reared under new or unnatural condi- 
tions. Many facts clearly show^ how eminently sus- 
ceptible the reproductive system is to very slight 
changes in the surrounding conditions. Nothing is 
more easy than to tame an animal, and few things more 
difficult than to get it to breed freely under confine- 
ment, 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 instincts. 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 
particular 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 pubhshed 
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 ani- 
mals, even from the tropics, breed in this country pretty 
freely under confinement, with the exception of the 
plantigrades or bear family, which seldom produce 
young; whereas carnivorous birds, with the rarest ex- 
ceptions, hardly ever lay fertile eggs. Many exotic 
plants have pollen utterly worthless, in the same con- 
dition as in the most sterile hybrids. When, on the 
one hand, we see domesticated animals and plants, 
though often weak and sickly, breeding freely under 
confinement; and when, on the other hand, we see 
individuals, though taken young from a state of nature 
perfectly tamed, -long-lived and healthy (of which I 
could give numerous instances), yet having their re- 
productive system so seriously affected by unperceived 
causes as to fail to act, we need not be surprised at this 
system, when it does act under confinement, acting 
irregularly, and producing 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 cultiva- 
tion, and vary very slightly — perhaps hardly more than 
in a state of nature. 

J^ Some naturalis ts have maintained th at all variations 
are connected with t he act of sexual r ep-od uction; but 
this i s certainly a n errorj for I have given in another 
■work along list of " sporting plants," as they are called 
by gardeners; — that is, of jlants which have suddenly 
produced a single bud with a new and sometim^sjddely 
different character from that of tTie other buds on the 


same plant. These bud variations, as they may be 
named, can be propagated by grafts, offsets, &c., 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 character; and as 
buds on distinct trees, growing under different con- 
ditions, have sometimes yielded nearly the same variety 
— for instance, buds on peach-trees producing nec- 
tarines, and buds on common roses producing moss- 
roses — we clearly see that the nature of the conditions 
is of subordinate importance in comparison with the 
nature of the organism in determining each particular 
form of variationj — perhaps of not more importance 
than the nature of the spark, by which a mass of com- 
bustible matter is ignited, has in determining the nature 
of the flames. 

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 
inherited development of the udders in cows and goats 


in countries where they are habitually milked, in com- 
parison with these organs in other countries, is proba- 
bly another instance of the effects of use. J^ot 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. Important 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 in- 
stances of correlation are quite whimsical: thus cats 
which are entirely white and have blue eyes are gen- 
erally deaf; but it has been lately stated by Mr. Tait 
that this is confined to the males. Colour and con- 
stitutional peculiarities go together, of which many re- 
markable cases could be given amongst animals and 
plants. From facts collected by Heusinger, it appears 
that white sheep and pigs are injured by certain plants, 
whilst dark-coloured individuals escape: Professor Wy- 
man has recently communicated to me a good illustra- 
tion 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 paint-root (Lachnanthes), 
which coloured their bones pink, and which caused the 
hoofs of all but the black varieties to drop off; and one 


of the " crackers " {i.e. 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.J^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 

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 cultivated 
plants, as on the hyacinth, potato, even the dahlia, &c.; 
and it is really surprising to note the endless points of 
structure and constitution 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 

Any variation which is not inherited is unimportant 
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 fundamental belief: doubts have 
been thrown on this principle only by theoretical 
writers. When any deviation of structure often appears, 


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, ap- 
parently 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, &c., appearing in several members of the 
same family. If strange and rare deviations of struc- 
ture are really inherited, less strange and commoner 
deviations may be freely admitted to be inheritable. 
Perhaps the correct way of viewing the whole sub- 
ject would be, to look 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 dif- 
ferent species, is sometimes inherited and sometimes not 
so; why the child often reverts in certain characters 
to its grandfather or grandmother or more remote an- 
cestor; why a peculiarity is often transmitted from 
one sex to both sexes, or to one sex alone, more com- 
monly 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 trans- 
mitted, either exclusively or in a much greater degree, 
to the males alone. A much more important rule, 
which I think may be trusted, is that, at whatever 
period of life a peculiarity first appears, it tends to 


reappear in the offspring at a corresponding age, though 
sometimes earlier. In many oases this could not be 
otherwise; thus the inherited peculiarities in the horns 
of cattle could appear only in the offspring when nearly 
mature; peculiarities in the silkworm are known to ap- 
pear at the corresponding 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 con- 
fined 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 varieties, 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 
great difficulty in proving its truth: we may safely 
conclude that very many of the most strongly marked 
domestic varieties could not possibly live in a wild 


state. In many cases we do not know what the aho- 
riginal stock was, and so could not tell whether or not 
nearly perfect reversion had ensued. It would he neces- 
sary, in order to prevent the effects of intercrossing, 
that only a single variety should have heen turned 
loose in its new home. Nevertheless, as our varieties 
certainly do occasionally revert in some of their charac- 
ters to ancestral forms, it seems to me not improbable 
that if we could succeed in naturalising, or were to 
cultivate, during many generations, the several races, 
for instance, of the cabbage, in very poor soil (in which 
case, however, some efiEect 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 experiment would 
succeed, is not of great importance for our line of argu- 
ment; for by the experiment itself the conditions of life 
are changed. If it could be shown that our domestic 
varieties manifested a strong tendency to reversion, — 
that is, to lose their acquired characters, whilst kept 
under the same conditions, and whilst kept in a con- 
siderable body, so that free intercrossing might check, 
by blending 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 
poultry of various breeds, and esculent vegetables, for 
an unlimited number of generations, would be opposed 
to all experience. 


Character of Domestic Varieties; difficulty of dis- 
tinguishing letween 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 same genus, in several triiiing 
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 hereafter 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 character of generic value. It can be shown 
that this statement is not correct; but naturalists differ 


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 do- 
mesticated races. 

In attempting to estimate the amount of structural 
difference between 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, terrier, spaniel, and bull-dog, 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 domesti- 
cation; I believe that a small part of the difference is 
due to their being descended from distinct species. In 
the case of strongly marked races of some other domes- 
ticated species, there is presumptive or even strong 
evidence, that all are descended from a single wild 

It has often been assumed that man has chosen for 
domestication animals and plants having an extraordi- 
nary 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 domes- 
ticated 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 little variability of 
the ass and goose, or the small power of endurance of 
warmth by the reindeer, or of cold by the common 
camel, prevented their domestication? I cannot doubt 
that if other animals and plants, equal in number to 
our domesticated productions, and belonging 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 cultivated 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 considerably 
in civilisation; and this again implies a long continued 


previous period of less advanced civilisation, during 
which the domesticated 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 remote period; and we know that at the pres- 
ent 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 
Canidse have been tamed, and that their blood, in some 
cases mingled together, flows in the veins of our do- 
mestic breeds. In regard to sheep and goats I can form 
no decided opinion. From facts communicated 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 distinction between the humped 
and common cattle, may, indeed, be looked upon as 
established by the admirable researches of Professor 
Riitimeyer. With respect to horses, from reasons 
which I cannot here give, I am doubtfully inclined 
to believe, in opposition to several authors, 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 appears 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 difEer 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 several aboriginal stocks, has been carried to 
an absurd extreme by some authors. ■XThey believe that 
every race which breeds true, let the distinctive 
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 Hungary, Spain, &c., but that 
each of these kingdoms possesses several peculiar breeds 
of cattle, sheep, &c., we must admit that many domestic 
breeds must have originated in Europe; for whence 
otherwise 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 
resembling the Italian greyhound, the bloodhound, the 
bull-dog, pug-dog, or Blenheim spaniel, &c. — so unlike 
all wild Canidae — ever existed in a state of nature? It 


has often been loosely said that all our races of dogs 
have been produced by the crossing of a few aboriginal 
species; but by crossing we can only get forms in some 
degree intermediate between their parents; 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, bull-dog, 
&c., 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 selection of the individuals which present 
the desired character; but to obtain a race intermediat e 
betw een 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 mon- 
grels are crossed one with another for several genera- 
tions, hardly two of them are alike, and then the diffi- 
culty of the task becomes manifest. 

Breeds of the Domestic Pigeon, their Differences and 

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 pur- 
chase or obtain, and have been most kindly favoured 
with skins from several quarters of the world, more 
especially by the Hon. W. Elliot from India, and by 
the Hon. C. Murray from Persia. Many treatises in 



different languages have been published on pigeons, 
and some of them are very important, as being of 
considerable antiquity. I have associated with several 
eminent fanciers, and have been permitted to join two 
of the London Pigeon Clubs. The diversity of the 
breeds is something astonishing. Compare the English 
carrier and the short-faced tumbler, and see the 
wonderful difference in their beaks, entailing corres- 
ponding differences in their skulls. The carrier, more 
especially the male bird, is also remarkable from the 
wonderful development of the carunculated skin about 
the head; and this is accompanied by greatly elongated 
eyelids, very large external orifices to the nostrils, and 
a wide gape of mouth. The short-faced tumbler has a 
beak in outline almost like that of a finch; and the 
common tumbler has the singular inherited habit of 
fiying 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 massive 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 expanding 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 trumpeter 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 develop- 
ment of the bones of the face in length and breadth and 
curvature differs enormously. 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 
vertebrae vary in number; as does the number of the 
ribs, together with their relative breadth and the pre- 
sence of processes. 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 cor- 
relation with the length of beak), the size of the crop 
and of the upper part of the oesophagus; , the develop- 
ment 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 
scutellae on the toes, the development of skin between 
the toes, are all points of structure which are variable. 
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, differ 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, pouter, and fantail in the same genus; more 
especially as in each of these breeds several truly-in- 
herited 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), in- 
cluding 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 aboriginal 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-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 aborigi-nal 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 ex- 
terminated; 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 animals to breed freely under domesti- 
cation; 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-civilised man, as to be quite pro- 
lific 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 abnor- 
mal in other parts; we may look in vain through the 
whole great family of Columbidse 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 intention- 
ally or by chance picked out extraordinarily abnormal 
species; and further, that these very species have since 
all become extinct or unknown. So many strange con- 
tigencies 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 perfectly 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 it so happens 
that blue varieties of barbs are so rare that I never 
heard of an instance in England; and the mongrels 
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 forehead, 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 under- 
stand 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, ok at most within a score, of generations, been 
crossed by \the rock-pigeon: I say within a dozen or 
twenty generjations, 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 natu/rally become less and less, as in each suc- 
ceeding ^jeneration there will be less of the foreign 
blood; bi'it when there has been no cross, and there 



is a tendency in the breed to revert to a character which 
w,as lost during some former generation, this tendency, 
for all that we can see to the contrary, may be trans- 
mitJ;ed undiminished for an indefinite number of gen- 
erations. These two distinct cases of reversion are 
often confounded together by those who have written 
on inheritance. 

Lastly, the h}fj)rids or mongrels from between all the 
breeds of the pigeon are perfectly fertile, as I can 
state from my own (^sl^servations, purposely made, on 
the most distinct breeds^ Now, hardly any cases have 
been ascertained with cer\tainty of hybrids from two 
quite distinct species of aninijals being perfectly fertile. 
Some authors believe that long;-continued domestication 
eliminates this strong tendencjr to sterility in species. 
Prom the history of the dog, and «f some other domestic 
animals, this conclusion is probaWj^ quite correct, if 
applied to species closely related to\aeh other. But 
to extend it so far as to suppose that species, aborigi- 
nally as distinct as carriers, tumblers, pout^ers, and fan- 
tails now are, should yield offspring perfeccily fertile 
inter se, would be rash in the extreme. / 

From these several reasons, namely, — thai improba- 
bility of man having formerly made seveA or eight 
supposed species of pigeons to breed freel/y under do- 
mestication; — these supposed species beiijig quite un- 
known in a wild state, and their not hajving become 
anywhere feral;— these species presenting i certain very 
abnormal characters, as compared withi all other 
Columbidffi, though so like the rock-pigeon in most 
respects;— the occasional re-appearance of \ the blue 
colour and various black marks in all the br^ieds, both 
when kept pure and when crossed;— and la,stly, 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- 

In favour of this view, I may add, firstly, that the 
wild C. livia has been found capable of domestication in 
Europe and in India; and that it agrees in habits and 
in a great number of points of structure with all the 
domestic breeds. Secondly, that, although an English 
carrier or a short-faced tumbler differs immensely in 
certain characters 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 Tecord of pigeons is in the fifth 
Egyptian dynasty, about 3000 b. c, as was pointed 
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 Eomans, 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 monarehs of Iran and Turan sent him 
some very rare birds; " and, continues the courtly his- 
torian, " 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 Eomans. The 
paramount importance of these considerations in ex- 
plaining 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 circumstance 
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 

I have discussed the probable origin of domestic 
pigeons at some, yet quite insufacient, 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 species 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 Long-horns, 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 species. 
Van Mons, in his treatise on pears and apples, shows 
how utterly he disbelieves that the several sorts, for 
instance a Eibston-pippin or Codlin-apple, could ever 
have proceeded from the seeds of the same tree. In- 
numerable other examples could be given. The ex- 
planation, 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 diilerences, 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 leam a lesson 
of caution, when they deride the idea of species in a 
state of nature being lineal descendants of other 


Principles of Selection anciently followed, and their 

Let us now briefly consider the steps by which 
domestic races have been produced, either from one or ■ 
from several allied species. Some effect may be at- 
tributed to the direct and definite action of the 
external conditions of life, 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, a 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 Dipsaeus; 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 ra^e-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 purpose; when we compare the 
many breeds of dogs, each good for inan in different 
ways; when we compare the game-cock, so pertinacious 
in. battle, with other breeds so little quarrelsome, with 
everlasting layers" which never desire to sit and 
with the bantam so small and elegant; when we 'com- 



pare the host of agricultural, culinary, orchard, and 
flower-garden races of plants, most useful to man at dif- 
ferent seasons and for difEerent 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 power 
of accumulative selection: nature gives successive varia- 
tions; 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 power 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 organisation as some- 
thing plastic, which they can model almost as they 
please. If I had space I could quote numerous pas- 
sages to this effect from highly competent authorities. 
Youatt, who was probably better acquainted with the 
works of. agriculturists than almost any other indi- 
vidual, and who was himself a very good judge of 
animals, speaks of the principle of selection as "that 
which enables the agriculturist, 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, speaking 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 im- 
portance 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 improvement 
is by no means generally due to crossing different 
breeds; all the best breeders are strongly opposed to 
this practice, except sometimes amongst closely allied 
sub-breeds. And when a cross has been made, the 
closest selection is far more indispensable even than in 
ordinary cases. If selection consisted merely in sepa- 
rating 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 direction, 
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 judg- 
ment sufficient 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 im- 
provements; if he wants any of these qualities, he will 
assuredly fail. Few would readily believe 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 pro- 
duced 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 compared 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 ob- 
serving the accumulated 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 differeht kinds of goose- 



berries differ in size, colour, shape, and hairiness,, 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 observation,— 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 can- 
not be doubted that the continued selection of slight 
variations,. either in the leaves, the flowers, or the fruit, 
will produce races differing from each other chiefly in 
these characters. 

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 ex- 
portation: 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 
encyclopaedia. Explicit rules are laid down by some 
of the Eoman classical writers. From passages in 
Genesis, it is clear that the colour of domestic 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 as- 
sociated 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 an- 
cient times, and is now attended to by the lowest sav- 
ages. 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 after- 
wards breeds from his own best dogs, but he has no 
wish or 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, &c., 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 recognised 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's spaniel has been unconsciously modified to a 
large extent since the time of that monarch. Some 
highly competent authorities are convinced that the 
setter is directly derived from the spaniel, and has pro- 
bably been slowly altered from it. It is known that the 
English pointer has been greatly changed within the last- 
century, and in this case the change has, it is believed, 
been chiefly effected 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. Borrow has not seen, as I am informed by 
him, any native dog in Spain like our pointer. 

By a similar process of selection, and by careful 
training, English racehorses have come to surpass in 
fleetness and size the parent Arabs, so that the latter, by 
the regulations for the Goodwood Eaces, 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 ac- 
counts 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 great- 
ly from the rock-pigeon. 


Youatt gives an excellent illustration of the effects 
01 a course of selection, which may he 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 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 gentlemen is so great that 
they have the appearance of being quite different varie- 

If there exist savages so barbarous as never to think 
of the inherited character of the offspring of their do- 
mestic animals, yet any one animal particularly useful to 
them, for any special purpose, would be carefully pre- 
served during famines and other accidents, to which sav- 
ages 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 indi- 
viduals, whether or not sufficiently 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 yarieties 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 pear, 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 result? 
from such poor materials; but the art has been simple, 
and, as far as the final result is concerned, has been 
followed almost unconsciously. It has consisted iij 
always cultivating the best known 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 procure, 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 

. A large amount of change, thus slowly and un- 
consciously accumulated, explains, ar I believe, the 
well-known fact, that in a number of cases we cannot 
recognise, and therefore do not know, the wild parent- 
Btocks of the plants which have been longest cultivated 
in our flower and kitchen gardens. If it has taken 

Chap.1.] unconscious SELECTION. 43 

centuries or thousands ~oi- years to improve gr^modify 
most of our plants up to their present stand ard o f use - 
lulness to man, we can understand how it is that neither 
Au^^E^iJ^j the Cape of Good Hope, nor any other region 
jnhahited by quite uncivilised man, has afforded us a 
single plant worth culture. It is not that these coun'- 
Tfies, so rich in species, do not by a strange chance pos- 
sess the aboriginal stocks of any useful plants, but that 
the native plants have not been improved by con- 
tinued selection up to a standard of perfection com- 
parable 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. "J^This, perhaps, partly 
explains why the varieties kept by savages, as has been 
remarked by some authors, have more of the character 
of true species than the varieties kept in civilised coun- 

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 adap- 
tation 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 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 iirst 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 de- 
veloped in some slight degree in an unusual manner, 
or a pouter till he saw a pigeon with a crop of some- 
what unusual size-jland the more abnormal 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 be- 
come through long-continued, partly unconscious and 
partly methodical, selection. Perhaps the parent-bird 
of all fantails had only fourteen tail-feathers somewhat 
expanded, like the present Java fantail, or like individ- 
uals of other and distinct breeds, in which as many as 
seventeen tail-feathers have been counted. Perhaps the 
first pouter-pigeon did not inflate its crop much more 
than the turbit now does the upper part of its oesopha- 
gi] s,— a habit which is disregarded by all fanciers, as it 
is not one of the points of the breed. 

ISTor 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 value any novelty, however slight, jn 
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 appear to explain what has sometimes 
been noticed — namely, that we know hardly anything 
about the origin or history 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 ac- 
knowledged, the principle, as I have called it, of un- 
conscious selection will always tend, — perhaps more 
at one period than at another, as the breed rises or falls 
in fashion, — perhaps more in one district than in an- 


other, according to the state of civilisation of the in- 
habitants,— 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 

Circumstances favourable to Man's Power of Selection. 

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 difEerences are not amply 
sufficient, with extreme care, to allow of the accumula- 
tion of a large amount of modification in almost any 
desired direction. But as variations manifestly useful 
or pleasing to man appear only occasionally, 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 remarked, 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 successful 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. 
■WTien the individuals are scanty, all will be allowed to 
breed, whatever their quality may be, and this will 
effectually 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 affected. 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 slightly largerj 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 varie- 
ties of the strawberry were raised which have appeared 
during the last half-century. 

With animals> facility in preventing crosses is an 
important element in the formation of new races, — at 
least, in a country which is already stocked with other 
races. In this respect 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. Pigeons 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 
^ong 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, 
&c., may be attributed in main part to selection not 
having been brought into play: in cats, from the 
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 sur- 
prisingly modified and improved by careful selection: 
in peacocks, 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 else- 
where 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 utmost limit, could not, after remaining fixed for 
many centuries, again vary under new conditions of 
life. Ko doubt, as Mr. Wallace, has remarked with 
much truth, a limit will be at last reached. For in- 
stance, there must be a limit to the fleetness of any 


terrestrial animal, as this will be determined by the 
inction 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 difEer 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 varie- 
ties of the bean or maize probably difEer 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. 

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 disuse of parts. The final re- 
sult 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 forma- 
tion 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 temporarily propagated by cut- 
tings, buds, &c., the importance of crossing is immense; 
for the cultivator may here disregard the extreme varia- 
bility both of hybrids and of mongrels, and the steril- 
ity 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 ac- 
cumulative action of Selection, whether applied method- 
ically and quickly, or unconsciously and slowly but 
more efiiciently seems to have been the predominant 




Variabilit J-— Individual diflEerenoes— Doubtful species— Wide rang- 
ing, much diffused, and common species, vary most— Species of 
tb^ larger genera in each country vary more frequently than 
the species of the smaller genera— Many of the species of the 
la ger genera resemble varieties in being very closely, but un- 
eqially, related to each other, and in having restricted ranges. 

Be/okb applying the principles arrived at in the last 
chapter .to organic beings in a state of nature, we must 
briefly ciscuss 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 TL^^erve 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 speaks 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 supposed not tO be inherited; but who 
can say that the dwarfed condition of shells in the 
brackish waters of the Baltic, or dwarfed plants on Al- 
pine summits, or the thicker fur of an animal from far 
northwards, would not in some cases be inherited for 
at least a few generations? and in this case I presume 
that the form would be called a variety. 

It may be doubted whether sudden and con&iderable 
deviations of structure such as we occasionally s^e in 
our domestic productions, more especially with plants, 
are ever permanently propagated in a state of nature. 
Almost every part of every organic being is so beauti- 
fully related to its complex conditions of life that it 
seems as improbable 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 soEt 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 monstrosities 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 singularly, their preservation 
would depend on unusually favourable circumstances. 
They would, also, during the first and succeeding gen- 



erations cross with the ordinary form, and thus their 
abnormal character would almost inevitably be lost. 
But I shall iave to return in a future chapter to the 
preservation and perpetuation of single or occasional 

Individual Differences. 

The many slight difEerences which appear in the 
ofEspring 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 difEerences. 
No one supposes that all the individuals of the same 
species are cast in the same actual mould. These in- 
dividual differences are of the highest importance 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 ifaimportant parts; but I could show by a 
long catalogue of facts, that parts which must be 
called important, whether viewed under a physiological 
or elassificatory point of view, sometimes vary in the 
individuals of the sanie species. I am convinced that 
the most experienced naturalist would be surprised at 
the number of the cases of variability, even in impor- 
tant 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 im- 
portant organs, and compare them in many specimens 
of the same species. It would never have been ex- 
pected 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 larvas of certain insects are far from uniform. 
Authors sometimes argue in a circle when they state 
that important organs never vary; for these same au- 
thors practically rank those parts as important (as some 
few naturalists have honestly 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 dif- 
ferences, which is extremely perplexing: I refer to 
those genera which have been called "protean" or 
" polymorphic," in which the species present an in- 
ordinate 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 
Eubus, Eosa, 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, poly- 


.^lorphie 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 am inclined to suspect that we see, at least 
in some of these polymorphic 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 he ex- 

Individuals of the same species often present, as is 
known to every one, great differences of structure, in- 
dependently of variation, as in the two sexes of various 
animals, in the two or three castes of sterile females 
or workers amongst insects, and in the immature and 
Larval sta|tes of many of the lower animals. There 
are, also, j cases of dimorphism and trimorphism, both 
''Fith aniihals and plants. Thus, Mr. Wallace, who 
fi'ls lately called attention to the subject, has shown 
''''i^'at the females of certain species of butterflies, in the 
i^-iialayan archipelago, regularly appear under two or 
'^'^'pn three conspicuously distinct forms, not connected 
'^y*, intermediate varieties. Fritz Miiller has described 
but more extraordinary cases with the males 
Brazilian Crustaceans: thus, the male of a 
Tanajs regularly occurs under two distinct forms; one 
of thgge :tias strong and differently shaped pincers, 
^^^ the ot !ier has antennas much more abundantly f ur- 
nisheq ^jy^ smelling-hairs. Although in most of these 
cases, the two or three forms, both with animals and 
plants,^ are! not now connected by intermediate gra- 
dations, it IS. probable that they were once thus con- 
nefited. Mr. Wallace, for instance, describes a certain 

of e)ertain 

56 DOUBTFUL SPECIES. [Chap, rf, 

butterfly which presents in the same island a goeat 
range of varieties connected by intermediate links,/ and 
the extreme links of the chain closely resemble thf3 two 
forms of an allied dimorphic species inhabiting an- 
other part of the Malay archipelago. Thus also with 
ants, the several worker-castes are generally/ quite 
distinct; but in some cases, as we shall hereafjter see, 
the castes are connected together by finely graiduated 
varieties. So it is, as I have myself observed, with some 
dimorphic plants. It certainly at first appears Sft highly 
remarkable fact that the same female butterfly 
should have the power of producing at ^the same 
time three distinct female forms and a malej and that 
an hermaphrodite plant should produce from; the same 
seed-capsule three distinct hermaphrodite fo;rms, bear- 
ing three diflEerent kinds of females and three or even 
six different kinds of males. Nevertheless these' 
cases are only exaggerations of the comf^mon far/^t 
that the female produces offspring of two sesi^s 
which sometimes differ from each other in a^l 

Doubtful Species. 

The forms which possess in some considertible de/?ree 
the character of species, but which are so closely siipil^r 
to other forms, or are so closely linked to them by inter- 
mediate gradations, that naturalists do not lii^e to 
rank them as distinct species, are in several respect-* i-he 
most important for us. We have every reason to be- 
lieve that many of these doubtful and closely allied 
forms have permanently retained their character^ for a 
long time; for as long, as far as we know, sis have good 
and true species. Practically, when a naturalist pan 



unite by means of intermediate links any two forms, 
ne 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, some- 
times arise in deciding whether or not to rank one form 
as a variety of another, even when they are closely con- 
nected by intermediate links; nor will the commonly- 
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 natural- 
ists having sound judgment and wide experience seems 
the only guide to follow. We must, however, in many 
cases, decide by a majority of naturalists, for few well- 
marked and well-known varieties can be named which 
have not been ranked as species by at least some com- 
petent 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 varie- 
ties. Mr. H. C. Watson, to whom I lie under deep 
obligation for assistance of all kinds, has marked for 
me 183 British plants, which are generally considered 

58 DOUBTFUL SPECIES. [Chap. 11. 

as varieties, but which have all- been ranked by bota- 
nists as species; and in inaking 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 351 species, whereas Mr. Bentham 
gives only 113, — 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 com- 
mon in separated areas. How many of the birds and 
insects in North America and Europe, which diffei 
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, shows that they may be classed 
under four heads, namely, as variable forms, as local 
forms, as geographical races or sub-species, 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 sufSciently distinct. The geographical racea 
or sub-species are local forms completely fixed and iso- 
lated; but as they do not differ from each other by 


strongly marked and important 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 species fill the same 
place in the natural economy of each island as do the 
local forms and sub-species; but as they are distin- 
guished from each other by a greater amount of difEer- 
ence 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 which variable forms, local 
forms, sub-species, and representative species can be 

Many years ago, when comparing, and seeing others 
compare, the birds from the closely neighbouring islands 
of the Galapagos archipelago, one with another, and 
with those from the American mainland, I was much 
struck how entirely vague and arbitrary is the dis- 
tinction between species and varieties. On the islets of 
the little Madeira group there are many insects which 
are charatiterised as varieties in Mr. Wollaston's ad- 
mirable work, but which would certainly 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 zo- 
ologists. Several experienced ornithologists consider 
our British red grouse as only a strongly-marked race 
of a 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 naturalists 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 Phytophagic 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 Phy- 
tophagic forms ought to be called species and which 
varieties. Mr. Walsh ranks the forms which it may 
be supposed would freely intercross, 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 in- 
termediate links connecting the several forms should 
now be found. The naturalist thus loses his best guide 
in determining whether to rank doubtful forms as varie- 
ties or species. This likewise necessarily occurs with 
closely allied organisms, which inhabit distinct conti- 
nents 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 pre- 
sents 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 are then degraded to the rank of varie- 

Some few naturalists maintain that animals never 
present varieties; but then these same naturalists rank 
the slightest difference as of specific value; and when 
the same identical form is met with in two distant 
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 use- 
less abstraction, implying and assuming a separate act 
of creation. It is certain that many forms, considered 
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 argument, from geographical dis- 
tribution, analogical variation, hybridism, &c., have 
been brought to bear in the attempt to determine their 
rank; but space does not here permit me to discuss 
them. Close investigation, 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 species. Look at the common oak, how close- 
ly 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 varieties; 
and in this country the highest botanical authorities 
and practical 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 Candolle, 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 generally enter 
into specific definitions. De Candolle then goes on to 
say that he gives the rank of species to the forms that 
differ by characters never varying on the same tree, and 
never found connected 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 species 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 spontaneous varieties 
and sub-varieties. Thus Quercus robur has twenty- 
eight varieties, all of which, excepting six, are clustered 
round three sub-species, namely, Q. peduneulata, sessili- 
flora, 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 typical Quercus ro- 
bur. Finally, De Candolle admits that out of the 300 
species, which will be enumerated in his Prodromus as 
belonging 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 Candolle 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 palaeontol- 
ogy, 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 doubt- 
ful 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 continu- 
ally studying; and he has little general knowledge of an- 
alogical variation in other groups and in other coun- 
tries, by which to correct his first impressions. As 
he extends the range of his observations, he will meet 
with more cases of difficulty; for he will encounter a 
greater number of closely-allied forms. But if his ob- 
servations be widely extended, he will in the end gen- 
erally be able to make up his own mind: but he will 
succeed in this at the expense of admitting much varia- 
tion, — 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 continu- 
ous, in which case he cannot hope to find intermediate 
links, he will be compelled to trust almost en- 
tirely to analogy, and his difficulties will rise to a 

Certainly no clear line of demarcation has as yet 
been drawn between 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. These differences blend into 
each other by an insensible series; and a series 


impresses the mind with the idea of an actual pas- 

Hence I look at individual differences, though of 
small interest to the systematist, as of the highest im- 
portance 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 
result of the nature of the organism and of the different 
physical conditions 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 cumulativ^e 
action of natural selection, hereafter to be explained, 
and to the effects of the increased use or disuse of parts. 
[a well-marked jariety may therefore be called an 
incipient speciesybut 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. Wollas- 
ton 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 hereafter 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 essentially differ from 
the term variety, which is given to less distinct and more 
fluctuating forms. The term variety, again, in com- 
parison with mere individual differences, is also applied 
arbitrarily, for convenience' sake. 

Wide-ranging, much diffused, and common Species vary 


Guided by theoretical considerations, I thought that 
some interesting results might be obtained in regard to 
the nature and relations 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 dif&culties, 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 proportional numbers 
of the varying species. Dr. Hooker permits me to ad(} 
that after having carefully read my manuscript, and 
examined the tables, he thinks that the following state- 
ments 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, hereafter to be discussed. 


Alphonse de CandoUe 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 conditions, and as they 
come into competition (which, as we shall hereafter 
see, is an equally or more important circumstance) with 
different sets of organic beings. But my tables fur- 
ther 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 dif- 
fused within their own country (and this is a different 
consideration from wide range, and to a certain extent 
from commonness), oftenest give rise to varieties suffi- 
ciently well-marked to have been recorded in botanical 
works. Hence it is the most flourishing, or, as. they 
may be called, the dominant species, — those which 
range widely, are the most diffused in their own coun- 
try, and are the most numerous in individuals, — which 
oftenest produce well-marked varieties, or, as I con- 
sider 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 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 tie 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 difEused 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 inhabiting the 
water or some parasitic fungus is infinitely more numer- 
ous in individuals and more widely difEused. 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 (t. e., 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 
species 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 re- 
sult, 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 to the size of the 
genera to which the species belong. Again, plants low 
in the scale of organisation are generally much more 
widely diffused 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 Geographical 

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 (i. e., 
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, circum- 
stances have been favourable for variation; and hence 
we might expect that the circumstances would generally 
be still favourable to variation. On the other hand, if 
we look at each species 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 hav- 
ing 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 in- 
variably proved to be the case that a larger propor- 
tion of the species on the side of the larger genera pre- 
sented varieties, than on the side of the smaller genera. 
Moreover, the species of the large genera which pre- 
sent any varieties, invariably present a larger average 
number of varieties than do the species of the small 
genera. Both these results follow when another divis- 
ion 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 manufacturing new species to be 
a slow one. And this certainly holds true, 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 increasing; 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 species 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 species 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 
diiference is one very important criterion in settling 
whether two forms should be ranked as species or 
varieties, l^^ow Pries 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 tett this numerically "by'^verages, 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 incipient 


species greater than the average are now manufactur- 
ing, many of the species already manufactured still to 
a certain extent resemble varieties, for they differ 
from each other by less than the usual amount of differ- 

Moreover, the species of the larger genera are re- 
lated to each other, in the same manner as the varieties 
of any one species are related to each other. No nat- 
uralist pretends that all the species of a genus are equal- 
ly 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 satellites 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 important point of dif- 
ference between varieties and species; namely, that the 
amount of difference between varieties, when compared 
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 discuss the principle, as I call it, 
of Divergence of Character, we shall see how this may be 
explained, and how the lesser differences between varie- 
ties tend to increase into the greater differences between 

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 re- 
stricted ranges. For instance, Mr. H. C. Watson has 
marked for me in the well-sifted London Catalogue of 
plants (4th edition) 63 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 63 
reputed species range on an average over 6.9 of the prov- 
inces into which Mr. Watson has divided Great Britain. 
Now, in this same Catalogue, 53 acknowledged varie- 
ties 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 
Mr. Watson as doubtful species, but which are almost 
universally ranked by British botanists as good and 
true species. 


Finally, varieties cannot be distinguished from 
species, — except, first, by the discovery of intermediate 
linking forms; and, secondly, by a certain indefinite 
amount of difEerence between them; for two forms, if 
differing very little, are generally ranked as varieties, 
notwithstanding that they cannot be closely connected; 
but the amount of difEerence 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 apt to be closely, but 
unequally, allied together, forming little clusters round 


other species. Species very closely allied to other spe- 
cies apparently have restricted ranges. In all these re- 
spects 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 divid- 
ed into groups subordinate to groups. 




Its bearing on natural selection— The term used in a wide sense- 
Geometrical ratio of increase— Rapid increase of naturalised 
animals and plants — Nature of the checks to increase— Com- 
petition universal— Efiects 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 species of the same genus — The relation of organism 
to organism the most important of all relations. 

Before entering on the subject of this chapter, I 
must make a few preliminary remarks, to show how the 
stru ggle for existence bears on JsTatural 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 mul- 
titude of doubtful 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 varieties 
be admitted. But the mere existence of individuafl 
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 con- 

76 STRUIgLB foe existence. [Chap. IIL 

ditions of life, and ^i one organic being to another 
being, been perfected;' We see these beautiful co-adap- 
tations most plainly in the woodpecker and the mistle- 
toe; and only a littlf/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 j)ea utiful ada p- 
tations everywhere and in every part of the organic 

Again, it may be asked, how is it that varieties, 
which I have called incipient species , become ultimately 
converted into good and distinct species which in most 
eases 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 sligh t and from 
whatever cause proceeding, if they be in any degre e 
profitab le to the individuals of a species, in their in- 
finitely complex relations to other organic beings and 
to their physical conditions of life, will tend to the 
pres ervation of such individuals , and will generally be 
inhe rited by the offsp ring. 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^n order to 
mark its relation to man's power of selection.'. But the 


expression often used by Mr. Herbert Spencer of the 
gurvival of the Fittest is more accurate, and is some- 
times equally nonmBient, 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 se6, 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 

We will now discuss in a little more detail the 
struggle for existence. In my future work this subject 
will be treated, as it well deserves, at greater length. 
The elder De CandoUe 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 Manchester, 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 thoroughly engrained in the mind, the 
whole economy of nature, with every fact on dis- 
tribution, rarity, abundance, extinction, and variation, 
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 constantly 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 superahundant, it is not so at all sea' 
sons 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, hut 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 lan- 
guishes and dies. But several seedling mistletoes, grow- 
ing 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 convenience' 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 life, and during some season or oc- 
casional year, otherwise, on the principle of geometri- 
cal increase, its numbers would quickly become so in- 
ordinately great that no country could support the pro- 
duct. Hence, as more individuals are produced than 
can possibly survive, there must in every case bea~ 
struggle for existence, either one individual with an- 
other 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 manifold 
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 increasing^ more or less rapid- 
ly> 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 thousand years, there would literally not be 
standing-room for his progeny. Linnaeus 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 should be a million plants. The 


elephant is reckoned the slowest 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 astonishingly rapid increase of 
various animals in a state of nature, when circum- 
stances 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. 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 thistle, which are now the com- 
monest over the whole 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 tem- 
porarily increased in any sensible degree. The ob- 
vious explanation is that the conditions of life have 
been highly favourable, and that there has consequently 
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 ex- 
traordinarily rapid increase and wide diffusion in their 
new homes. 

In a state of nature almost every full-grown plant 
annually produces 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 destruction at some period 
of life. Our familiarity with the larger domestic ani- 
mals 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 an- 
nually 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 favour- 
able 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 hippobosea, a single one; but this 
difference 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 oi 
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 produced 
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 bb 
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 in- 
stantaneously 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 cheeks 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 
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. Seedlings, 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 de- 
struction 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 number 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 ac- 
tion of climate seems at first sight to be quite indepen- 
dent of 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 most. When we 
travel from south to north, or from a damp region to a 
dry, we invariably see some species gradually getting 
rarer and rarer, and finally disappearing; and the change 
of climate being conspicuous, we are tempted to at- 
tribute 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 de- 
struction at some period of its life, from enemies or 
from competitors for the same place and food; and 
if these enemies or competitors 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 de- 
crease. 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 north- 
ward, but in a somewhat lesser degree, for the number 
of species of all kinds, and therefore of competitors, 
decreases northwards; hence in going northwards, 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-capped summits, or absolute deserts, 
the struggle for life is almost exclusively with the ele- 

That climate acts in main part indirectly by favour- 
ing other species, we clearly see in the prodigious num- 


ber of plants which in our gardens can perfectly well 
endure our climate, but which never become naturalised, 
for they cannot compete with our native plants nor re- 
sist destruction by our native animals. 

When a species, owing to highly favourable circum- 
stances, 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 parasitic worms, which have from 
some cause, possibly in part through facility of diffusion 
amongst the crowded animals, been disproportionally 
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 num- 
bers of its enemies, is absolutely necessary for its pres- 
ervation. Thus we can easily raise plenty of corn and 
rape-seed, &c., 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 super- 
abundance of food at this one season, increase in num- 
ber proportionally to the supply of seed, as their num- 
bers are cheeked during the 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 
preservation, 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 to- 
gether, and thus save the species from utter destruction. 
I 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 cheeks and relations between or- 
ganic 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 Staffordshire, 
on the estate of a relation, where I had ample means 
of investigation, there was a large and extremely barren 
heath, which had never been touched by the hand of 
man; but several hundred 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 planted part of the heath was most 
remarkable, more than is generally seen in passing from 
one,j:wute different soil to another: not only the pro- 
pojrtional numbers of the heath-plants were wholly 
chianged, but twelve species of plants (not counting 
grasses and earices) flourished in the plantations, which 
could not be found on the heath. The effect on the 
insects must have been still greater, for six insectivor- 
ous birds were very common in the plantations, which 
1 8 


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 in- 
troduction of a single tree, nothing whatever else having 
been done, with the exception of the land having been 
enclosed, so that cattle could not enter. But how im- 
portant 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 hill- 
tops: 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 multitude 
of seedlings and little trees which had been perpet- 
ually 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 filrs. 
Yet the heath was so extremely barren and so ext(to- 
sive that no one would ever have imagined tliiat 
cattle would have so closely and effectually searched it 
for food. 

Here we see that cattle absolutely determine tfhe 


existence of the Scotch fir; but in several parts of the 
world insects determine the existence of cattle. Per- 
haps 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; 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 Para- 
guay, the parasitic insects would probably increase; 
and this would lessen the number of the navel-frequent- 
ing 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 affect the insects; and this, 
as we have just seen in Staffordshire, the insectivorous 
birds, and so onwards in ever-increasing circles of com- 
plexity. Not that under nature the relations will ever 
be as simple as this. Battle within battle must be 
continually recurring with varying success; 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 extinction 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 


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 in- 
sects, 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 humble-bees are almost indispensable 
to the fertilisation of the heartsease (Violo 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, 20 heads of Dutch 
clover (Trifolium repens) yielded 3,390 seeds, but 30 
other heads protected from bees produced not one. 
Again, 100 heads of red clover (T. pratense) produced 
2,700 seeds, but the same number of protected heads 
produced not a single seed. Humble-bees alone visit 
red clover, as other bees cannot reach the nectar. It 
has been suggested that moths may fertilise the clovers; 
but I doubt whether they could do so in the case of the 
red clover, from their weight not being sufEcient to 
depress the wing petals. Hence we may infer as highly 
probable that, if the whole genus of humble-bees be- 
came extinct or very rare in England, the heartsease 
and red clover would become very rare, or wholly dis- 
appear. The number of humble-bees in any district 
depends in a great measure upon the number of field- 
mice, which destroy their combs and nests; and Col. 
Newman, who has long attended to the habits of hum- 
ble-bees, believes that "more than two-thirds of them 
are thus destroyed all over England." Now the num- 


ber. of mice is largely dependent, as every one knows, 
on the number of eats; and Col. Newman says, "Near 
villages and small towns I have found the nests of 
humble-bees more numerous than elsewhere, which I 
attrihute 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 deter- 
mine, through the intervention first of mice and 
then of bees, the frequency of certain flowers in that 

In the case of every species, many different checks, 
acting at different periods of life, and during different 
seasons or years, probably 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 cheeks 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 proportional 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 formerly 
have been cleared of trees, now display the same beauti- 
ful diversity and proportion of kinds as in the surround- 
ing 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 thou- 
sand; what war between insect and insect— between 
insects, snails, and other animals with birds and beasts 
of prey — all striving to increase, all feeding on each 


other, or on the trees, their seeds and seedKngs, or on 
the other plants which first clothed the ground and 
thus cheeked 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 re- 
action of the innumerable plants and animals which 
have determined, in the course of centuries, the pro- 
portional numbers 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 some- 
times 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 ex- 
posed to the same dangers. In the case of varieties 
of the same species, the struggle will generally be al- 
most 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 varieties. 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 constitu- 
tion, that the original proportions of a mixed stock 
(crossing being prevented) could be kept up for half-a- 
dozen generations, if they were allowed to struggle to- 
gether, in the same manner as beings in a state of nature, 
and if the seed or young were not annually preserved in 
due proportion. 

Struggle for Life most severe'ietween Individuals and 
Varieties of the same Species. 

As the species of the same genus usually have, 
though by no n^ans 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 be- 
tween 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 an- 
other 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 Eussia the small Asiatic cock- 
roach has everywhere driven before it its great con- 
gener. In Australia the imported hive-bee is rapidly 
exterminating the small, stingless native bee. One spe- 


cies 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 between allied 
forms, which fill nearly the same place in the economy 
of nature; but probably in no one case could we pre- 
cisely say why one species has been victorious over an- 
other in the great battle of life. 

A corollary of the highest importance may be de- 
duced 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 compe 
tition for food or residence, or from which it has t^ 
escape, or on which it preys. This is obvious in th? 
structure of the teeth and talons of the tiger; and in 
that of the legs and claws of the parasite which clingb 
to the hair on the tiger's body. But in the beautifully 
plumed seed of the dandeliop, 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 dis- 
tributed and fall on unoccupied ground. In the water- 
beetle, the structure of its legs, so well adapted for div- 
ing, allows it to compete with other aquatic insects, to 
hunt 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 re- 
lation to other plants. But from the strong o-rowth 
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 

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 withstand 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 im- 
agination 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 cf its geographical range, a change 
of constitution 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 life will generally 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 competi- 
tors 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, 
as it is difficult to acquire. All that we can do, is to 
keep steadily in mind that each organic being is striv- 
ing 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 life 
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 





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 — Cir- 
cumstances favourable and unfavourable to the results of 
Natural Selection, namely, intercrossing, isolation, number of 
individuals — Slow action — Extinction caused by Natural Se- 
lection — 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 — Ex- 
plains the grouping of all organic beings — Advance in organ- 
isation — Low forms preserved — Convergence of character — 
Indefinite multiplication 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 or- 
ganisation becomes in some degree 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 occur- 
rence; he can preserve and accumulate such as do 
occur. Unintentionally he exposes organic beings to 
new and changing conditions of life, and variability 
ensues; but similar changes of conditions might and 
do occur under nature. Let it also be borne in mind 
how iniinitely complex and close-fitting are the mutual 
relations of all organic beings to each other and to their 
physical conditions of life; and consequently what in- 
finitely varied diversities of structure might be of use to 
each being under changing conditions of life. Can it, 
then, be thought improbable, seeing that variations use- 
ful to man have undoubtedly occurred, that other varia- 
tions 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 possibly 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 per- 
haps we see in certain polymorphic species, or would ul- 
timately become fixed, owing to the nature of the organ- 
ism 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. ITo 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 conscious 
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 lit-' 
eral sense of the word, no doubt, natural selection is a 
felse term; but who ever objected to chemists speaking 
\i the elective affinities of the various elements? — ^and 
Vet an acid cannot strictly be said to elect the base with 
« hich 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 is difficult to avoid personify- 
ing 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. 
W^ith a little familiarity such superficial objections will 
be forgotten. 

We shall best understand the probable course of 
natural selection by taking the case of a country under- 
j,»oing 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 con- 
clude, from what we have seen of the intimate and com- 
plex 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 ailect the 
others. If the country were open on its borders, new 
forms would certainly immigrate, and this would like- 
wise seriously disturb the relations of some of the former 
inhabitants. Let it be remembered how powerful 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 fav- 
oured 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 mani- 
festly 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 forgot- 
ten 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 direction individ- 
ual differences, so could natural selection, but far more 
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 im- 
migration, 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 modifica- 
tions 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 subsistence 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 con- 
ditions under which they live, 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 possession 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? JS^a 
can act only on external and visible characters: Nature^ 


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 con- 
stitutional 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 charac- 
ter 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 rigidly destroy all inferior ani- 
mals, 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 ac- 
cumulated by Nature during whole geological periods ! 
Can we wonder, then, that Nature's productions 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 oppor- 
tunity offers, at the improvement of each organic being 
in relation 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 fa- 
vourable nature as before; and these must be again pre-l 
served, and so onwards step by step. Seeing that individ- 
ual differences of the same kind perpetually recur, this 
can hardly be considered as an unwarrantable assump- 
tion. But whether it is true, we can judge only by seeing 
how far the hypothesis 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 struc- 
tures, which we are apt to consider as of very trifling 
importance, may thus be acted on. When we see leaf- 
eating insects green, and bark-ieeders 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 mucb 
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 remember 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. Id 
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 peaches far more 
than those with other coloured flesh. If, with all the 
aids of art, these slight differences make a great differ- 
ence 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 differ- 
ences would effectually settle which variety, whether a 
smooth or downy, a yellow or purple fleshed fruit, should 

In looking at many small points of difference be- 
tween species, which, as far as our ignorance permits us 



to judge, seem quite unimportant, we must not forget 
that climate, food, &c., 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 modifications, often of the most unex- 
pected nature, will ensue. 

As we see that those variations which,' under domes- 
tication, appear at any particular period of life, tend to 
reappear in the offspring at the same period; — for in- 
stance, 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 silk-worm; 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 varia- 
tions profitable at that age, and by their inheritance at a 
corresponding 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 affect, 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 injurious: for if they were so, the species would be- 
come extinct. 

Natural selection will modify the structure of the 


young in relation 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 proiits by the se- 
lected 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 ani- 
mal'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 arc 
able to get out of it; so that fanciers assist in the act oi 
hatching. Now if nature had to make the beak of a full 
grown pigeon very short for the bird's own advantage, 
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 thickness 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 little 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 de- 
stroyed, have yielded individuals better adapted to their 
conditions of life than any of those which happened 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 accidentals 
causes, which would not be in the least degree mitigatedl | 
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 kepjfc 
down by the causes just indicated, as will often have 
been the case, natural selection will be powerless in 
certain beneficial directions; but this is no valid objec- 
tion to its efficiency at other times and in other ways; 
for we are far from having any reason to suppose that 
many species ever undergo modification and improve- 
ment at the same time in the same area. 

Sexual Selection. 

Inasmuch as peculiarities often appear under do- 
mestication in one sex and become hereditarily attached 
to that sex, so no doubt it will 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 oc- 
curs. This leads me to say a few words on what I have 
called Sexual Selection. _This form of selection de- 
pends, 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. Theje- 
sult is not death to the unsuccessful competitor, but Jew 
or no ,Qff spring. 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 general vigor, as 
on having special weapons, confined to the male sex. 
A. hornless stag or spurless cock would have a poor 
chance of leaving 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 careful se- 
lection of his best cocks. How low in the scale of 
nature the law of battle descends, I know not; male alli- 
gators have been described as fighting, bellowing, and 
whirling round, like Indians in a war-dance, for the pos- 
session 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 fre- 
quently seen by that inimitable observer M. Fabre, fight- 
ing 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 peace- 
ful character. All those who have attended to the sub- 
ject, believe that there is the 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 spec- 
tators, 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 E. 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 jgood reason to doubt that female birds, by 
selecting, during thousands of generations, the most 
melodious or beautiful males, according to their stand- 
ard 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 fe- 
males of any animal have the same general habits of life, 
but differ in structure, colour, or ornament, such dif- 
ferences have been mainly caused by sexual selection: 
that is, by individual males having had, in suc- 
cessive 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 dif- 
ferences to this agency: for we see in our domestic ani- 
mals 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 mon- 

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 fre- 
quently attacks the shepherd'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 se- 
lection by man, which depends on the preservation of all 
the more or less valuable individuals, and on the de- 
struction of the worst. I saw, also, that the preserva- 
tion 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 an able and valuable article 
in the ' North British Review ' (1867), I did not ap- 


predate 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 offspring, of which, from 
various causes of destruction, only two on an average 
survive to pro-create their kind. This is rather an ex- 
treme 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 individual 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. Supposing it to survive 
and to breed, and that half its young inherited the 
favourable variation; still, as the Eeviewer 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 dis- 
puted. If, for instance, a bird of some kind could pro- 
cure 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 eases, if the varying 
individual did not actually transmit to its offspring its 
newly-acquired character, it would undoubtedly trans- 
mit 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 tend- 
ency to vary in the same manner has often been so 
strong that all the individuals of the same species have 
been similarly modified 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 laerymans. In eases of this kind, if the variation 
were of a beneficial nature, the original form would soon 
be supplanted by the modified form, through the sur- 
vival of the fittest. 

To the effects of intercrossing in eliminating varia- 
tions 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 needlessly 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 com- 
plex illustration 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, hy glands at the base of the 
stipules in some Leguminosffi, 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 flowers of two distinct indi- 
viduals 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 nec- 
taries, excreting most nectar, would 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 insects 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 insects 
visiting flowers for the sake of collecting pollen instead 
of nectar; 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 individuals which produced more and more 
pollen, and had larger anthers, would be selected. 

When our plant, by the above process long con- 
tinued, 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 produ- 
cing 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, never- 
theless 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 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 im- 
potent; 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 advantage- 
ous on the principle of the division of labour, individ- 
uals with this tendency more and more increased, would 
be continually favoured or selected, until at last a com- 
plete 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 ap- 
parently 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. Bear- 
ing such facts in mind, it may be believed that under 
certain circumstances individual differences in the cur- 


vature or length of the proboscis, &e., 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 communi- 
ties to which they belonged would flourish and throw ofE 
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 humble-bees alone; so that whole fields of red clover 
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 repeatedly 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 com- 
mon hive-bee, and which freely crosses with it, is able to 
reach and suck the nectar of the red clover. Thus, in a 
country where this kind of clover abounded, it might be 
a great advantage 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 
perfect manner, by the continued preservation of all the 
individuals which presented slight deviations of struc- 
ture mutually favourable to each other. 

I am well aware that this doctrine of natural selec- 
tion, exemplified in the above imaginary instances, is 
open to the same objections which were first urged 
against Sir Charles Lyell's noble views on " the modem 
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 ex- 
plaining the excavation of the deepest valleys or the for- 
mation 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 excavation of a great valley by a single dilu- 
vial wave, so will natural 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 individuals 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 her- 
maphrodites two individuals, either occasionally or 
habitually, concur for the reproduction of their kind. 
This view was long ago doubtfully suggested by Spren- 
gel. Knight and Kolreuter. We shall presently see its 
importance; but I must here treat the subject with ex- 
treme brevity, though I 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 num- 
ber of supposed hermaphrodites, and of real hermaphro- 
dites a large number pair; that is, two individuals regu- 
larly unite for reproduction, 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 in- 
dividuals ever concur in reproduction? As it is impos- 
sible here to enter on details, I must trust to some gen- 
eral considerations alone. 

In the first place, I have collected so large a body of 
facts, and made so many experiments, showing, in ac- 
cordance with the almost universal belief of breeders, 
that with animals and plants a cross between different 
varieties, or between individuals of the same variety but 
of another strain, gives vigour and fertility to the off- 
spring; and on the other hand, that close interbreeding 
diminishes vigour and fertility; that these facts alone in- 
cline me to believe that it is a general law of nature that 
no organic being fertilises itself for a perpetuity of gen- 
erations; but that a cross with another individual is occa- 
sionally — perhaps at long intervals of time — indispen- 
sable. j„ 


On ths belief that this is a law of nature, we can. 
think, understand several large classes of facts, such a 
the following, which on any other view are inexplicable. 
Every hybridizer knows how unfavourable exposure to 
wet is to the fertilisation of a flower, yet what a multi. 
tude of flowers have their anthers and stigmas fully ex< 
posed to the weather ! If an occasional cross be indis- 
pensable, notwithstanding 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 en- 
closed, 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 neces« 
sary are the visits of bees to many papilionaceous flowers, 
that their fertility is greatly diminished if these visits be 
prevented. Now, it is scarcely possible 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 sufficient, to ensure 
fertilisation, just to touch with the same brush the an- 
thers of one flower and then the stigma of another ; but 
it must not be supposed 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 to- 
wards the pistil, or slowly move one after the other to- 
wards 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 
btamens 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 possible to raise 
pure seedlings, so largely do they naturally cross. In 
numerous other cases, far from self-fertilisation being 
favoured, there are special contrivances which effectu- 
ally prevent the stigma receiving pollen from its own 
flower, as I could show from the works of Sprengel and 
others, as well as from my own observations: for in- 
stance, in Lobelia fulgens, there is a really beautiful and 
elaborate contrivance by which all the infinitely numer- 
ous pollen-granules are swept out of the conjoined an- 
thers 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. An- 
other 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 
Sprengel, 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 individual being advantageous or indis- 

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 instance, I raised 233 seedling 
cabbages from some plants of different varieties growing 
near each other, and of these only 78 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 gen- 
eral law of good being derived from the intercrossing of 
distinct individuals of the same species. When distinct 
species are crossed the 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 lim- 
ited 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 Zealand, 
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 
diehogamous, 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 

Turning for a brief space to animals: various terres- 
trial species are hermaphrodites, such as the land-mol- 
lusea and earth-worms; 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 ow- 
ing to the nature of the fertilising 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 -fertilising hermaphrodites; but here the cur- 


rents 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 hermaph- 
rodite animal with the organs of reproduction so per- 
fectly enclosed that access from without, and the oc- 
casional 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 spe- 
cies 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 inter- 
cross, 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 distinct indi- 
viduals 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 variability, under which term individual dif- 
ferences are always included, will evidently be favour- 
able. 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 believe, a 
highly important element of success. Though Nature 
grants long periods of time for the work of natural selec- 
tion, 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 fa- 
vourable 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 selection numerous domestic races, 
why should it prevail against natural selection? 

In the case of methodical selection, a breeder selects 
for some definite object, and if the individuals be al- 
lowed freely to intercross, his work will completely fail. 
But when many men, without intending to alter the 
breed, have a nearly common standard of perfection, and 
all try to procure and breed from the best animals, im- 
provement surely but slowly follows from this uncon- 
scious , process of selection, notwithstanding that there 
is no separation of selected individuals. Thus it will be 
under nature; for within a confined area, with some 
place in the natural polity not perfectly occupied, 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 different dis- 
tricts, the newly-formed varieties will intercross on the 
confines of each. But we shall see in the sixth chapter 


that intermediate varieties, inhabiting intermediate dis- 
tricts, will in the long run generally be supplanted by one 
of the adjoining varieties. Intercrossing 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 coun- 
tries; and this I find to he the case. With hermaph- 
rodite organisms which cross only occasionally, and 
likewise with animals which unite for each birth, but 
which wander little and can increase at a rapid rate, a 
new and improved 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 consider- 
able body of facts showing that within the same area, 
two varieties of the same animal may long remain dis- 
tinct, from haunting different stations, from breeding 
at slightly different seasons, or from the individuals of 
each variety preferring to pair together. 

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 ani- 
mals 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 ofEspring from long-continued self-fertilisation, that 
they will have a better chance of surviving and propa- 
gating their kind; and thus in the long run the influ- 
ence of crosses, even at rare intervals, will be great. 
With respect to organic beings extremely low in the 
scale, which do not propagate sexually, nor conjugate, 
and which cannot possibly intercross, uniformity of 
character can be retained by them under the same con- 
ditions of life, only through the principle of inherit- 
ance, 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 pre- 
serving similar favourable variations. 

Isolation, also, is an important element in the modifi- 
cation of species through natural selection. In a con- 
fined 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 districts will, also, be thus prevented. 
Moritz Wagner has lately published 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 prevent- 


ing, after any physical change in the conditions, such as 
of climate, elevation of the land, &c., the immigration of 
better adapted organisms; and thus new places in the 
natural economy of the district 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 
improved at a slow rate; and this may sometimes be of 
much importance. If, however, an isolated area be very 
small, either from being surrounded by barriers, or from 
having very peculiar physical conditions, the total num- 
ber 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- 

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 life were 
necessarily undergoing change through some innate law. 
Lapse of time is only so far important, and- its impor- 
tance 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 relation to the constitution of each organism. 

If we turn to nature to test the truth of these re- 
marks, and look at any small isolated area, such as an 
oceanic island, although the number of species inhabit- 
ing it is small, as we shall see in our chapter on Geo- 
graphical Distribution; yet of these species a very large 
proportion 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 de- 
ceive 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 number of individuals 
of the same species there supported, but the conditions 
of life are much more complex from the large number of 
already existing species; and if some of these many spe- 
cies become modified and improved, others will have to 
be improved in a corresponding 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, a^d will thus come into com- 
petition with many other forms. Moreover, great areas, 
though now continuous, will often, owing to former os- 
cillations of level, have existed in a broken condition; so 
that the good effects of isolation will generally, to a cerr 
tain extent, have concurred. Finally, I conclude 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 al- 


ready have been victorious over many competitors, 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 important part in the changing 
history of the organic world. 

In accordance with this view, we can, perhaps, under- 
stand 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 Aus- 
tralia now yielding before those of the larger Europajo- 
Asiatic area. Thus, also, it is that continental produc- 
tions 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 modifica- 
tion 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 there 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 certain ex- 
tent orders at present widely sundered in the natural 
scale. These anomalous forms may be called living fos- 
sils; they have endured 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 cireumstanees favourable and un- 
favourable for the production of new species through 
natural selection. I conclude that for terrestrial produc- 
tions 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 continent, the inhabitants will 
have been numerous in individuals and kinds, and will 
have been subjected to severe competition. When con- 
verted 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 modi- 
fication 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 im- 
proved forms, and the relative proportional numbers of 
the various inhabitants of the reunited continent will 
again have been changed; and again there will have 
been a fair field for natural selection to improve still 
further the inhabitants, 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 polity of a district which can be 
better occupied by the modification of some of its exist- 


ing 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 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 gome slight de- 
gree from each other, it would often be long before dif- 
ferences of the right nature in various parts of the or- 
ganisation might occur. The result would often he 
greatly retarded by free intercrossing. Many will ex- 
claim that these several causes are amply sufficient to 
neutralise the power of natural selection. I do not be- 
lieve so. But I do believe that natural selection will gen- 
erally 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 ac- 
cord well with what geology tells us of the rate and man- 
ner 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 com- 
plexity of the coadaptations between all organic beings, 
one with another and with their physical conditions of 
life, which may have been effected 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 Geology; 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 consequent- 
ly endure. Owing to the high geometrical rate of in- 
crease of all organic beings, each area is already fully 
stocked with inhabitants; and it follows from this, that 
as the favoured forms increase in number, so, generally, 
will the less favoured decrease and become rare. Earity, 
as geology tells us, is the precursor to extinction. We 
can see that any form which is represented by few in- 
dividuals 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 ene- 
mies. But we may go further than this; for, as new 
forms are produced, unless we admit that specific forms 
can go on indefinitely increasing in number, many old 
forms must become extinct. That the number of spe- 
cific forms has not indefinitely increased, geology plainly 
tells us; and we shall presently attempt to show why it 
it is that the number of species throughout the world 
has not become immeasurably great. 

We have seen that the species which are most numer- 
ous in individuals 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 re- 
corded 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 modi- 
fied and improved descendants of the commoner species. 

From these several considerations I think it inevitably 
follows, that as new species in the course of time are 

134 DIVBKGEjSTCE op character. [Chap. In- 

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 do- 
mesticated productions, through the selection of im- 
proved 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 short-horns " (I quote the words of an 
agricultural writer) "as if by some murderous pestilence." 

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. 


Nevertheless, according to my view, varieties are species 
in the process of formation, or are, as I have called them, 
incipient species. How, then, does the lesser difference 
between varieties become augmented into the greater dif- 
ference between species? That this does habitually 
happen, we must infer from most of the innumerable 
species throughout nature presenting well-marked dif- 
ferences; 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 dif- 
fer 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 degree 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 different as short-horn 
and Hereford cattle, race and cart horses, the several 
breeds of pigeons, &c., could never have been effected by 
the mere chance accumulation of similar variations 
during many successive generations. In practice, a fan- 
cier is, for instance, struck by a pigeon having a slightly 
shorter beak; another fancier is struck by a pigeon hav- 
ing 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 occurred with the sub-breeds of the tum- 
bler-pigeon) choosing and breeding from birds with 
longer and longer beaks, or with shorter and shorter 
beaks. Again, we may suppose that at ah early period of 


history, the men of one nation or district required swift- 
er horses, whilst those of another required stronger 
and bulkier horses. The 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-established 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 dis- 
appear. Here, then, we see in man's productions the 
action of what may be called the principle of divergence, 
causing differences, at first barely appreciable, 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 prin- 
ciple apply in nature? I believe it can and does apply 
most efSeiently (though it was a long time before I saw 
how), from the simple circumstance that the more diver- 
sified the descendants from any one species become in 
structure, constitution, and habits, by so much will they 
be better enabled to seize on many and widely diver- 
sified 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 ease 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 undergoino- 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 diver- 
sified 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 affect 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 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 suc- 
ceed in living on the same piece of ground. And we 
know that each species and each variety of grass is annu- 
ally sowing almost countless seeds; and is thus striv- 
ing, as it may be said, to the utmost to increase in num- 
ber. 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 distinct 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 struc- 
ture, is seen under many natural circumstances. In an 
extremely small area, especially if freely open to immi- 
gration, 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 exactly 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 rotation 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 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 naturalised .plants would have be- 
longed to a few groups more especially adapted to certain 
stations in their new homes. But the case is very dif- 
ferent; and Alph. de CandoUe has well remarked, in his 
great and admirable work, that floras gain by naturalisa- 
tion, 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 natural- 
ised plants are of a highly diversified nature. They differ, 
moreover, to a large eztent, from the indigenes, for out 
of the 163 naturalised genera, no less than 100 genera are 
not there indigenous, and thus a large proportional addi- 
tion is made to thegenera nowlivingin 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 he 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 labour in the organs 
of the same individual body — a subject so well eluci- 
dated 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 general economy of any land, the more widely 
and perfectly the animals and plants are diversified for 
different habits of life, so will a greater number of indi- 
viduals 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 he doubted, for instance, whether the 
Australian marsupials, which are divided into groups dif- 
fering but little from each other, and feebly represent- 
ing, as Mr. Waterhouse and others have remarked, our 
carnivorous, ruminant, and rodent mammals, could suc- 
cessfully compete with these well-developed orders. In 
the Australian mammals, we see the process of diversifi- 
cation 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 descend- 
ants 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 extinc- 
tion, tends to act. 

The accompanying diagram will aid us in understand- 
ing 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, be- 
longing 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 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 varia- 
tions (represented by the outer dotted lines) being pre- 
served 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 accumu- 
lated to form it into a fairly well-marked variety, such as 
would be thought worthy of record in a systematic work. 
The intervals between the horizontal lines in the dia- 
gram, may represent each a thousand or more genera- 
tions. After a thousand generations, species (A) is sup- 
posed to have produced two fairly well-marked varieties, 
namely a^ and m^- 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 two varieties, 
being only slightly modified forms, will tend to inherit 
those advantages which made their parent (A) more nu- 
merous 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 favourable 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 principle 
of divergence, differ more from (A) than did variety a*. 
Variety m^ is supposed to have produced two varieties, 
namely m^ and s^, 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 genera- 
tions, 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 number and 
diverging in character. In the diagram the process is 
represented up to the ten-thousandth generation, and 
under a condensed and simplified form up to the four- 
teen-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 irregu- 
lar, nor that it goes on continuously; it is far more prob- 
able that each form remains for long periods unaltered, 
and then again undergoes modification. Nor do I sup- 
pose that the most divergent varieties are invariably pre- 
served: a medium form may often long endure, and may 
or may not produce more than one modified descendant; 
for natural selection will always act according to the na- 
ture 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 pro- 
geny 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 suffi- 
ciently distinct to be recorded as varieties. But these 
breaks are imaginary, and might have been inserted any- 
where, after intervals long enough to allow the accumu- 
lation of a considerable amount of divergent variation. 

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 mul- 
tiplying in number as well as diverging in character: 
this is represented in the diagram by the several diver- 
gent 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 modifica- 
tion 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, species (A) is sup- 
posed to have produced three forms, a^°, f^", and m^°, 
which, from having diverged in character during the suc- 
cessive generations, will have come to differ largely, but 
perhaps unequally, from each other and from their com- 
mon parent. If we suppose the amount of change be- 
tween each horizontal line in our diagram to be exces- 
sively 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 last into well-defined species. Thus the 
diagram illustrates the steps by which the small differ- 
ences distinguishing varieties are increased into the 
larger differences distinguishing species. By continuing 
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 descended 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 (w^" and 2") or two species, according 
to the amount of change supposed to be represented be- 
tween the horizontal lines. After fourteen thousand 
generations, six new species, marked by the letters n^* to 
z", 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 dif- 
ferent places in the polity of nature: hence in the dia- 
gram 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 con- 
tinue to transmit unaltered descendants; and this is 
shown in the diagram by the dotted lines unequally pro- 
longed 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 con- 
stant tendency in the improved descendants of any one 
species to supplant and exterminate in each stage of de- 
scent their predecessors and their original progenitor. 
For it should be remembered that the competition 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 be- 
tween the earlier and later states, that is between tTie 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 com- 
petition, both may continue to exist. 

If, then, our diagram be assumed to represent a con- 
siderable amount of modification, species (A) and all the 
earlier varieties will have become extinct, being replaced 
by eight new species {a^* to m^*); and species (I) will 
be replaced by six {n^* to z^*) new species. 

But we may go further than this. The original spe- 
cies 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 num- 
ber 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 extermi- 
nated not only their parents (A) and (I), but likewise 
some of the original species which were most nearly re- 
lated to their parents. Hence very few of the original 
species will have transmitted ofEspring to the fourteen- 
thousandth generation. We may suppose that only one, 
(F), of the two species (E and F) which were least closely 
related to the other nine original species, has trans- 
mitted 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 2" will be much greater than that be- 
tween 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^*, g^*, p^*, will be nearly 
related from having recently branched ofE from a^"; 6", 
and f^*, from having diverged at an earlier period from 
a', will be in some degree distinct from the three first- 
named species; and lastly, 0^*, e^*, 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) dif- 
fered 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, more- 
over, are supposed to have gone on diverging in dif- 


ferent directions. The intermediate species, also (and 
this is a very important consideration), which connected 
the original species (A) and (I), have all become, ex- 
cepting (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, 
converging in sub-branches downwards towards a single 
point; this point represents a species, the supposed pro- 
genitor of our several new sub-genera and genera. 

It is worth while to reflect for a moment on the 
character of the 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 be- 
tween the parent-species (A) and (I), now supposed to 
be extinct and unknown, it will be in some degree inter- 
mediate in character between the two groups descended 
from these two species. But as these two groups have 
gone on diverging in character from the type of their 
parents, the new species (f") will not be directly inter- 
mediate between them, but rather between types of the 
two groups; and every naturalist will be able to call such 
eases 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 afRnities of extinct be- 
ings, 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 explained, to the formation of genera alone. If, 
in the diagram, we suppose the amount of change, repre- 
sented by each successive group of diverging dotted lines 
to be great, the forms marked a^* to p^^, those marked 
6^* and /^*, and those marked o^* to m^*, will form three 
very distinct genera. We shall also have two very dis- 
tinct genera descended from (I), differing widely from 
the descendants of (A). These 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. 

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 species have inherited from a 
common ancestor some advantage in common. Hence, 
the struggle for the production of new and modified de- 
scendants 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 varia- 
tion 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 
polity 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 
steadyincrease of the larger groups, a multitude of small- 
er groups will become utterly extinct, and leave no modi- 
fied descendants; and consequently that, of the species 
living at any one period, extremely few will transmit de- 
scendants 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 transmitted 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 modified 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. 

On the Degree to which Organisation tends to advance. 

Natural Selection acts exclusively by the preserva- 
tion and accumulation of variations, which are benefi- 
cial under the organic and inorganic conditions to which 
each creature is exposed at all periods of life. The ulti- 
mate result is that each creature tends to become more 
and more improved in relation to its conditions. This 
improvement inevitably leads to the gradual advance- 
ment of the organisation of the greater number of living 
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 
organisation. Amongst the vertebrata the degree of in- 
tellect 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 suifiee as a standard of comparison; but there are 
cases, as with certain parasitic crustaceans, in which sev- 
eral 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 ap- 
plicable and the best, namely, the amount of differenti- 
ation of the parts of the same organic being, in the adult 


state as I should be inclined to add, and their specialisa- 
tion for different functions; or, as Milne Edwards would 
express it, the completeness of the division of physio- 
logical labour. But we shall see how obscure this sub- 
ject 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 rank the common bony or teleostean fishes as 
the highest, inasmuch as they are most strictly fish-like, 
and differ niost from the other vertebrate classes. We 
see still more plainly the obscurity of the subject by 
turning to plants, amongst which the standard of intel- 
lect 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 differentiation and specialieation of the sev- 
eral organs in each being when adult (and this will in- 
clude the advancement of the brain for intellectual pur- 
poses), natural selection clearly leads towards this 
standard: for all physiologists admit that the speciali- 
sation 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 se- 
lection. 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 occu- 
pied place in the economy of nature, that it is quite pos- 
sible for natural selection gradually to fit 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 conven- 
iently 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 exterminated 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 existence of lowly organisms offers 
no difficulty; for natural selection, or the survival of the 
fittest, does not necessarily include progressive develop- 
ment — 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 ani- 
malcule — ^to an intestinal worm — or even to an earth- 
worm, to be highly organised. If it were no advantage, 
these forms would be left, by natural selection, unim- 
proved or but little improved, and might remain for 
indefinite ages in their present lowly 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 dis- 
sected 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 difEerent 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 com- 
petition with each other; the advancement of 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 disad- 
vantage 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 lance- 
let, 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 nu- 
merous 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 per- 
fection; for the high advancement 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 preserved to the present day, 
from inhabiting confined or peculiar stations, where 
they have been subjected to less severe competition, and 
where their scanty numbers have retarded the chance 
of favourable variations arising. 

Finally', I beheve that many lowly organised forms 
now exist throughout the world, from various causes. 
In some cases variations or individual differences of a 
favourable nature may never have arisen for natural 
selection to act on and accumulate. In no case, prob- 
ably, 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 con- 
ditions 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 struc- 
ture, 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 unicelhilar 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 become differentiated in 
proportion as their relations to incident forces become 
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 iso- 
lated 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 spe- 
cies, if we make due allowance for our profound igno- 
rance on the mutual relations of the inhabitants of the 
world at the present time, and still more so during past 

Convergence of Character. 

Mr. H. C. Watson thinks that I have overrated the 
importance of divergence of character (in which, how- 
ever, he apparently believes), and that convergence, as 
it may be called, has likewise played 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 con- 
vergence 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 substances 
should sometimes assume the same form; but with or- 
ganic 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 phys- 
ical conditions, and in a still higher degree on the sur- 
rounding 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 dif- 
fered 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 oc- 
curred, we should meet with the same form, inde- 
pendently of genetic connection, recurring in widely 
separated geological formations; and the balance of evi- 
dence 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, &c.; but I fully admit that 
the mutual relations of organic beings are more im- 
portant; 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 produced. We 
do not know that even the most prolific area is fully- 
stocked with specific forms: at the Cape of Good Hope 
and in Austraha, 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 in- 
creased. What then checks an indefinite increase in 
the number of species? The amount of life (I do not 
mean the number of specific forms) supported on an 
area must have a limit, depending so largely as it doei» 
on physical conditions; therefore, if an area be inhab- 
ited 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 fluctua- 
tions in the naturie 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 exter- 
minate thousands on thousands of species. Eare spe- 
cies, 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 favourable variations; consequently, 
the process of giving birth to new specific forms would 
thus be retarded. When any species becomes very rare 
close interbreeding will help to exterminate it; authorr. 

Chap. IV.] SUMMARY. 159 

have thought that this comes into play in accounting 
for the deterioration of the Aurochs in Lithuania, of 
Bed Deer in Scotland, and of Bears in Norway, &c. 
Lastly, and this I am inclined to think is the most im- 
portant 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 inordinate increase of 
specific forms throughout the world. Dr. Hooker has 
recently shown that in the S.E. comer of Australia, 
where, apparently, there are many invaders from dif- 
ferent quarters of the globe, the endemic Australian 
species have been greatly reduced in number. How 
much weight to attribute to these several considerations 
I will not pretend to say; but conjointly they must 
limit in each country the tendency to an indefinite aug- 
mentation of specific forms. 

Summary of Chapter. 

If under changing conditions of life organic beings 
present individual differences in almost every part of 
their structure, and this cannot be disputed; if there be, 
owing 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 in- 
finite complexity of the relations of all organic beings to 
each other and to their conditions of life, causing an in- 
finite diversity in structure, constitution, and habits, to 
be advantageous to them, it would be a most extraor- 


dinary 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 survival of the fittest, I 
have called Natural Selection. It leads to the improve- 
ment of each creature in relation to its organic and inor- 
ganic conditions of life; and consequently, in most 
cases, to what must be regarded as an advance in organi- 
sation. 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 or- 
dinary 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 condi- 
tions and stations, must be judged by the general tenor 
and balance of evidence given in the following chapters. 
But we have already seen how it entails extinction; and 
how largely extinction has acted in the world's history 
geology plainly declares.- Natural selection, also, leads 

Chap. IV.] SUMMARY. 161 

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 supported on the 
area, — of which we see proof by looking to the in- 
habitants of any small spot, and to the productions 
naturalised in foreign lands. Therefore, during the 
modification of the descendants of any one species, and 
during the incessant struggle of all species to increase 
in numbers, the more diversified the descendants be- 
come, the better will be their chance of success in the 
battle for life. Thus the small differences distinguish- 
ing varieties of the same species, steadily tend, to in- 
crease, 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 species, 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 re- 
marked, leads to divergence of character and to much 
extinction of the less improved and intermediate forms 
of uf e. On these principles, the nature of the af&nities, 
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 fa- 
miliarity — 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 
everywhere behold — namely, varieties of the same spe- 
cies most closely related, species of the same genus less 
closely and unequally related, forming sections and sub- 


genera, species of distinct genera much less closely re- 
lated, and genera related in different degrees, forming 
sub-families, families, orders, sub-classes and classes. 
The several subordinate 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 end- 
less cycles. If species had been independently created, 
no explanation would have been possible of this kind of 
classification; 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. I 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 species. 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 flour- 
ished 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 

Chap. IV.] SUMMARY. 163 

living and modified descendants. From the fiist 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 occa- 
sionally see an animal like the Ornithorhynchus or 
Lepidosiren, which in some small degree connects by its 
aflBnities two large branches of life, and which has ap- 
parently been saved from fatal competition 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 raanifications. 




Effects of changed conditions — Use and disuse, combined with 
natural selection ; organs of flight and of vision — Acclimatisa- 
tion — Correlated variation — Compensation and economy of 
growth — False correlations — 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 varia- 
tions — so common 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 acknowl- 
edge plainly our ignorance of the cause of each par- 
ticular variation. Some authors believe it to be as 
'much the function of the reproductive system to pro- 
duce individual differences, or slight deviations of struc- 
ture, as to make the child like its parents. But the 
fact of variations and monstrosities occurring much 
more frequently under domestication than under na- 
ture, 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 species has been 
exposed during several successive generations. In the 

Chap. V]. LAWS OF VAfilATION. 165 

first chapter I attempted to show that changed con- 
ditions act in two ways, directly on the whole organi- 
sation 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 organisation seems to become plastic, and we 
have much fluctuating variahility. 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. 

It is very difficult to decide how far changed con- 
ditions, such as of climate, food, &c., 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 co-adaptations 
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 col- 
oured 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 WoUaston is convinced that residence 
near the sea affects the colours of insects. Moquin- 


Tandon gives a list of plants which, when growing 
near the sea-shore, have their leaves in some degree 
fleshy, though not elsewhere fleshy. These sHghtly va- 
rying 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 accumula- 
tive 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 condi- 
tions of life as different as can well be conceived; and, 
on the other hand, of dissimilar varieties being produced 
under apparently the same external conditions. Again, 
innumerable instances are known to every naturalist, of 
species keeping true, or not varying at all, although 
living under the most opposite climates. Such con- 
siderations as these incline me to lay less weight on 
the direct 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 life may be said, not 
only to cause variability, either directly or indirectly, 
but likewise to include natural selection, for the con- 


ditions 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 dis- 
tinct; 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 in- 
herited. 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 possess 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 sev- 
eral 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 es- 
cape danger, it is probable that the nearly wingless 
condition of several birds, now inhabiting or which 
lately inhabited several oceanic islands, tenanted by no 
beast of prey, has been caused by disuse. The ostrich 


indeed inhabits continents, and is exposed to danger 
from which it cannot escape by flight, but it can defend 
itself by kicking its enemies, as efficiently as many 
quadrupeds. 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 in- 
creased during successive generations, its legs were used 
more, and its wings less, until they became incapable of 

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 collection, 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 rudimentary condition. In the 
Ateuchus or sacred beetle of the Egyptians, 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 
absence of the anterior tarsi in Ateuchus, and their 
rudimentary condition 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 importance or be much used by these in- 

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 concealed, until the wind lulls and the sun 
' shines; that the proportion of wingless beetles is larger 
on the exposed 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 re- 
quire 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, com- 
bined probably with disuse. For during many suc- 
cessive generations each individual beetle which flew 
least, either from its wings having been 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-feed- 
ers, 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 selection to enlarge or to reduce the 
wings, would depend on whether a greater number of 
individuals were saved by successfully battling 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 virreck. 

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 per- 
haps by natural selection. In South America, a bur- 
rowing rodent, the tuco-tuco, or Ctenomys, is even more 
subterranean in its habits than the mole; and I was 
assured by a Spaniard, who had often caught them, 
that they were frequently blind. One which I kept 
alive was certainly in this condition, the cause, as ap- 
peared on dissection, having been inflammation of the 
nictitating membrane. As frequent inflammation of 
the eyes must be injurious to any animal, and as eyes 
are certainly not necessary to animals having subter- 
ranean habits, g, reduction in their size, with the adhe- 
sion of the eyelids and growth of fur over them, might 
in such case be an advantage; and if so, natural selec- 
tion would aid the effects of disuse. 

It is well known that several animals, belonging to 
the most different classes, which inhabit the caves of 
Camiola and of Kentucky, are blind. In some of the 
crabs the foot-stalk 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 to animals Hving 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 profoundest depths, the eyes were 
lustrous and of large size; and these animals, as I am 
informed by Professor Silliman, after having been ex- 
posed for about a month to a graduated light, acquired 
a dim perception of objects. 

It is difficult to imagine conditions of life 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 cre- 
ated for the American and European caverns, very 
close similarity in their organisation and affinities 
might have been expected. 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 phenomenon 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 gen- 
erally between the fauna of Europe and of North 
America." On my view we must suppose that Amer- 
ican animals, having in most eases ordinary 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 ac- 
cordingly look upon the subterranean faunas as small 
ramifications which have penetrated into the earth 
from the geographically limited faunas of the adja- 
cent tracts, and which, as they extended themselves 
into darkness, have been accommodated to surround- 
ing circumstances. Animals not far remote from or- 
dinary forms, prepare the transition from light to dark- 
ness. Next follow those that are constructed for 
twilight; and, last of all, those destined for total dark- 
ness, 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 gener- 
ations, the deepest recesses, disuse will on this view 
have more or less perfectly obliterated its eyes, and 
natural selection will often have effected other changes, 
such as an increase in the length of the antennse or 
palpi, as a compensation for blindness. ISTotwithstand- 
ing 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 Eu- 
ropean cave-insects are very closely allied to those of 
the surrounding country. It weuld 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 cre- 
ation. That several of the inhabitants of the caves 


of the Old and Xew Worlds should he 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) 
oilers this remarkable peculiarity, that the species, as 
Mr. Murray observes, have not as yet been found any- 
where except in caves; yet those which inhabit the 
several caves of Europe and America are distinct; but 
it is possible that the progenitors of these several 
species, whilst they were furnished with eyes, may 
formerly have ranged over both continents, and then 
have become extinct, excepting in their present se- 
cluded 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. 


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 germinate, &c., and this leads me 
to say a few words on acclimatisation. As it is ex- 


tremely 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 cli- 
mate 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 Hve 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 tem- 
peratures; that is, they become acclimatised: thus the 
pines and rhododendrons, raised from seed collected 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 informs 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 strictly 
adapted to their native climate, though in all ordinary 
cases we assume such to be the case; nor do we know 
that they have subsequently become specially acclima- 
tised to their new homes, so as to be better fitted for 
them than they were at first. f 

As we may infer that our domestic animals were 
originally chosen by uncivilised man because they were 
useful and because they bred 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 
withstanding 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, how- 
ever, 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 trans- 
ported 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 cli- 
mate 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 difEerent climates hy man himself and by his 
domestic animals, and the fact of the extinot elephant 
and rhinoceros having formerly endured a glacial cli- 
mate, whereas the living species are now all tropical 
or suh-tropical in their habits, ought not to be looked 
at as anomahes, but as examples of a very common 
flexibility of constitution, brought, under peculiar cir- 
•cumstances, into action. 

How much of the acclimatisation of species to any 
peculiar climate 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 custom has some 
influence, I must believe, both from analogy and from 
the incessant advice given in agricultural works, even 
in the ancient Encyclopagdias of China, to be very cau- 
tious in transporting animals from one district to an- 
other. And as it is not likely that man should have 
succeeded in selecting so many breeds and sub-breeds 
with constitutions 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 pre- 
serve those individuals which were born with consti- 
tutions best adapted to any country which they in- 
habited. 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 published 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 ad- 
vanced, as proving that acclimatisation cannot be ef- 
fected, 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 
until 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 experiment 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 seedlings 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 eases, played a considerable 
part in the modification of the constitution and struc- 
ture; but that the effects have often been largely com- 
bined with, and sometimes overmastered by, the natural 
selection of innate variations. 

Correlated Variation. 
I mean by this expression that the whole organisa- 
tion is so tied together during its growth and develop- 
ment, that when slight variations in any one part occur, 
and are accumulated through natural selection, other 
parts become modified. This is a very important sub- 
ject, most imperfectly understood, and no doubt wholly 
different classes of facts may be here easily confounded 


together. We shall presently see that simple inheri- 
tance often gives the false appearance of correlation. 
One of the most obvious real cases is, that variations of 
structure arising in the young or larva3 naturally tend 
to affect the structure of the mature animal. The sev- 
eral parts of the body which are homologous, and which, 
at an early embryonic period, are identical in structure, 
and which are necessarily exposed to similar conditions, 
seem eminently Hable 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 homolo- 
gous 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 ren- 
dered 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 petals into a tube. Hard parts seem to affect the 
form of adjoining soft parts; it is believed by some au- 
thors that with birds the diversity in the shape of the 
pelvis causes the remarkable 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 
Sehlegel, the form of the body and the manner of swal- 
lowing determine the position and form of several of the 
most important Tiseera. 


The nature of the bond is frequently quite obscure. 
M. Is. Geoffroy St. Hilaire has forcibly remarked, that 
certain malconformations 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 between their feathered feet 
and skin betwixt the outer toes, or between the pres- 
ence of more or less down on the young pigeon when 
first hatched, with the future colour of its plumage; 
or, again, the relation between the hair and teeth in 
the naked Turkish dog, though here no doubt homol- 
ogy 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 covering, viz., Cetacea (whales) 
and Edentata (armadilloes, scaly ant-eaters, &c.,) 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 impor- 
tance of the laws of correlation and variation, inde- 
pendently of utility and therefore of natural selection, 
than that of the difEerence between the outer and inner 
flowers in some Compositous and TJmbelliferous plants. 
Every one is familiar with the difference between the 
ray and central florets of, for instance, the daisy, and 
this difference is often accompanied with the partial or 
complete abortion of the reproductive organs. But in 
some of these plants, the seeds also differ in shape and 
structure. These differences have sometimes been at- 
tributed 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 Composits countenances this 
idea; hut with the UmbelliferEe, it is by no means, as 
Dr. Hooker informs me, the species with the densest 
heads which most frequently diSer 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 Compositee 
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 be- 
come abnormally symmetrical. I may add, as an in- 
stance 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 be- 
coming peloric or regular. When the colour is absent 
from only one of the two upper petals, the nectary is 
not qidte aborted but is much shortened. 

With respect to the development of the corolla, 
Sprengel's idea that the ray-florets serve to attract in- 
sects, whose agency is highly advantageous or necessary 
for the fertiUsation of these plants, is highly probable; 
and if so, natural selection may have come into play. 
But with respect to the seeds, it seems impossible that 
their differences in shape, which are not always cor- 
related with any difference in the corolla, can be in 
any way beneficial: yet in the Umbelliferae these dif- 


ferences are of such apparent importance — ^the seeds 
being sometimes orthospermous in the exterior iiowers 
and ccElospermous in the central flowers, — ^that the 
elder De Candolle founded his main divisions in the 
order on such characters. Hence modifications of struc- 
ture, 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 varia- 
tion structures which are common to whole groups of 
species, and which in truth are simply due to inheri- 
tance; for an ancient progenitor may have acquired 
through natural selection some one modification in 
structure, 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 man- 
ner in which natural selection can alone act. For in- 
stance, 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 becoming winged through natural selection, 
unless the capsules were open; for in this ease alone 
could the seeds, which were a little 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 Geoflroy and Goethe propounded, at about 
the same time, their law of compensation or balancement 
of growth; or, as Goethe expressed it, "in order to 
spend 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 difiieult to get a cow 
to give much milk and to fatten readily. The same 
vari,eties of the cabbage do not yield abundant and 
nutritious foliage and a copious supply of oil-bearing 
seeds. When ihe seeds in our fruits become atrophied, 
the fruit itself gains largely in size and quality. In 
our poultr}', 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 adjoining 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 compensa- 
tion which have been advanced, and likewise some other 
facts, may be merged under a more general principle, 
namely, that natural selection is continually trying t6 


economise every part of tlie 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 an useless structure. I can thus 
only understand a fact with which I was much struck 
when examining cirripedes, and of which many analo- 
gous instances could be given: namely, that when a 
cirripede ia parasitic within another cirripede 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 Proteo- 
lepas: for the carapace in all other cirripedes consists 
of the three highly-important anterior segments 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 re- 
duced to the merest rudiment attached to the bases of 
the prehensile antennae. ISTow the saving of a large 
and complex structure, when rendered superfluous, 
would be a decided advantage to each successive in- 
dividual of the species; for in the struggle for life to 
which every animal is exposed, each would have a bet- 
ter chance of supporting itself, by less nutriment being 

Thus, as I believe, natural selection will tend in the 
long run to reduce any part of the organisation, as soon 
as it becomes, through changed habits, superfluous, with- 
out 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 neces- 
sary 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. Geoffry St. 
Hilaire, both with varieties and species, that when any 
part or organ is repeated many times in the same in- 
dividual (as the vertebrae in snakes, and the stamens 
in polyandrous 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 organisa- 
tion, 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 re- 
jected each little deviation of form so carefully 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 par- 
ticular shape. Natural selection, it should never be 
forgotten, can act solely through and for the advantage 
of each being. 

Eudimentary 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 varia- 


bility seems to result from their uselessness, and conse- 
quently from natural selection having had no power to 
check deviations in their structure. 

A Part 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 I have collected, and 
which cannot possibly 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 allowance for them. It should be 
understood that the rule by no means applies to any 
part, however unusually developed, unless it be un- 
usually 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 ab- 
normal structure 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 spe- 
cies 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 man- 
ner. The term, secondary sexual characters, used by 
Hunter, relates to characters which are attached to one 
sex, but are not directly connected with the act of re- 


production. The rule applies to males and females; 
but more rarely to the females, as they seldom oSer 
remarkable secondary sexual characters. The rule be- 
ing 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 her- 
maphrodite cirripedes; I particularly attended to Mr. 
Waterhouse's remark, whilst investigating 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 applica- 
tion. The opercular valves of sessile cirripedes (rock 
barnacles) are, in every sense of the word, very im- 
portant structures, and they differ extremely little even 
in distinct genera; but in the several species of one 
genus, Pyrgoma, these valves present a marvellous 
amount of diversification; the homologous valves in the 
different species 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 cer- 
tainly seems to hold good in this class. I cannot make 
out that it applies to plants, and this would have seri- 

Chap, v.] HIGHLY VAEIABLB. 187 

ously 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 developed in a re- 
markable degree or manner in a species, the fair pre- 
sumption is that it is of high importance to that species: 
nevertheless it is in this case eminently liable to varia- 
tion. 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 species, 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 ani- 
mal 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 charac- 
ter: 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 or- 
ganisation is left in a fluctuating condition. But what 
here more particularly concerns us is, that those points 
in our domestic 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 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, &c., these being 


the points now mainly attended to by English fanciers. 
Even in the same sub-breed, as in that of the short- 
faced tumbler, 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 go- 
ing on between, on the one hand, the tendency to re- 
version to a less perfect state, as well as an innate 
tendency to new variations, and, on the other hand, 
the power of steady selection 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 spe- 
cies, compared with the other species of the same genus, 
we may conclude that this part has undergone an 
extraordinary amount of modification since the period 
when the several species branched off from the com- 
mon progenitor of the genus. This period will sel- 
dom be remote in any extreme degree, as species rarely 
endure for more than one geological period. An ex- 
traordinary amount of modification implies an unusu- 
ally large and long-continued amount of variability, 
which has continually been accumulated 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 remained for 

Chap, v.] HIGHLY VARIABLE. 189 

a much longer period nearly constant. And this, I am 
convinced, is the case. That the struggle hetween 
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 modified 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 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 modification 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 con- 
tinued rejection of those tending to revert to a former 
and less-modified condition. 

Specific Characters more Variable than Generic 

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 char- 
acter, and no one would be surprised at one of the 


blue species 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 
unusual circumstance. I have chosen this example be- 
cause the explanation which most naturalists would 
advance is not here applicable, namely, that specific 
characters are more variable than generic, because they 
are taken from parts of less physiological importance 
than those commonly used for classing genera. I be- 
lieve this explanation is partly, yet only indirectly, 
true; I shall, however, have to return to this point in 
the chapter on Classification. It would be almost su- 
perfluous to adduce evidence in support of the state- 
ment, that ordinary speciflc 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 con- 
stant throughout a large group of species, differs con- 
siderably 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 spe- 
cific value, often becomes variable, though its physio- 
logical importance may remain the same. Something 
of the same kind applies to monstrosities: at least Is. 
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 of each species 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 explana- 
tion can be given. But on the view that species are 
only strongly marked and fixed varieties, 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 happened that natural selection 
will have modified several distinct species, 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 pro- 
genitor, 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 species 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 or- 
ganisation which have for a very long period remained 

Secomdary 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 species 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 
difference between the males of gallinaceous birds, in 
which secondary sexual characters are strongly dis- 
played, with the amount of difference between the fe- 
males. The cause of the original variability 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 ofE- 
spring 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 species 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 differences 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 cLaraeter com- 
mon to very large groups of beetles, but in the 
Engidffi, 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 character 
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 ofEer excellent illustra- 
tions of this law. " In Pontella, for instance, the sexual 
characters are afforded mainly by the anterior antennae 
and by the fifth pair of legs: the specific differences 
also are printipally given by these organs." This re- 
lation 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 species. 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 natural and sexual selection, in order to fit the sev- 
eral 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 variability 
of specific characters, or those which distinguish species 
from species, than of generic characters, or those which 
are possessed by all the species; — ^that the frequent ex- 
treme 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 charac- 
ters, and their great difference in closely allied species; 
— that secondary sexual and ordinary specific diilerences 
are generally displayed in the same parts of the organisa- 
tion, — 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 likely 
still to go on varying than parts which have long been 
inherited and have not varied — to natural selection hav- 
ing 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 countries 
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 two or more 
distinct races. The frequent presence of fourteen or 
even sixteen tail-feathers in the pouter may be con- 
sidered 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 vari- 
ation, 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 Euta baga, 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 species; and to these a third may be add- 
ed, namely, the common turnip. According to the or- 
dinary 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 consequent ten 
dency to vary in a like manner, but to three separate 
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 insect? 
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, name- 
ly, the occasional 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 ex- 
ternally 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 re- 
version, and not of a new yet analogous variation ap- 
pearing in the several breeds. We may, I think, confi- 
dently 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, beyond the influ- 
ence of the mere act of crossing on the laws of inherit- 

No doubt it is a very surprising fact that characters 
should reappear after having "been lost for many, prob- 
ably 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 1 in 
2048; 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 hoth 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 gen- 
erations. When a character 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 hun- 
dred generations, but that in each successive generation 
the character in question has been lying latent and at 
last, under unknown favourable conditions, 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 in- 

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 anal- 
ogous 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 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 reversionary and analogous characters. If, for 
instance, we did not know that the parent rock-pigeon 
was not feather-footed or turn-crowned, 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 


variation. 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 existing 
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 members 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 independently cre- 
ated 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 col- 
lected 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 oc- 
cur, 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 ease almost certainly of reversion. The ass 
sometimes has very distinct transverse bars on its legs, 


like those on the 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 shoul- 
der-stripe. Mr. Blyth has seen a specimen of the hemi- 
onus 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 striped on the 
legs, and faintly on the shoulder. The quagga, though 
BO 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 respect to the horse, I have collected cases in 
England of the spinal stripe in horses of the most dis- 
tinct breeds, and of all colours: transverse bars on the 
legs are not rare in duns, mouse-duns, and in one in- 
stance in a chestnut: a faint shoulder-stripe may some- 
times 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 stripe 
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 north-west 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 stripes is not considered 



as purely-bred. The spine is always striped; the legs 
are generally barred; and the shoulder-stripe, which is 
sometimes double and sometimes treble, is common; 
the side of the face, moreover, is sometimes striped. 
The stripes are often plainest in the foal; and some- 
times 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 infor- 
mation 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 my- 
self recently bred a foal from a bay mare (offspring of a 
Turkoman horse 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 spe- 
cies — 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 ponies, Norwegian cohs, 
the lanky Kattywar race, &c., inhabiting the most dis- 
tant parts of the world, should all have been. crossed 
with one supposed aboriginal stock. 

Now let us turn to the effects of crossing the several 
species of the horse-genus. EoUin asserts, that the com- 
mon 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 striped 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 excel- 
lent 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 off- 
spring 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 hemio- 
nus; and this hybrid, though the ass only o.ccasionally 
has stripes on its 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 con- 


vinced that not even a stripe of colour appears 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 striped 
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 distinct 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 accompanied 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, liow ob- 
serve 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 probable hypothesis to account for the reappear- 
ance of very ancient characters, is — that there is a ten- 
dency in the yoiing 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, 
species; and how exactly parallel is the case with that 
of the species of the horse-genus! For myself, I venture 
confidently to look back thousands on thousands of gen- 
erations, and I see an animal striped like a zebra, but 
perhaps otherwise very differently constructed, the com- 
mon parent 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 inde- 
pendently created, will, I presume, assert that each 
species 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 in- 
habiting distant quarters of the world, to produce hy- 
brids 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 fos- 
sil shells had never lived, but had been created in stone 
so as to mock the shells living on the sea-shore. 

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 instituting 
a comparison, the same laws appear to have acted in 


producing the lesser differences between varieties of the 
same species, and the greater difHerences between species 
of the same genus. Changed conditions generally in- 
duce mere fluctuating variability, but sometimes 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 produc- 
ing 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 ad- 
joining parts; and every part of the structure which can 
be 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 understand, undoubt- 
edly 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 modifications have not been closely checked 
by natural selection. It follows probably from this 
same cause, that organic beings low in the scale are more 
variable than those standing higher in the 3cale, and 
which have their whole organisation more specialised. 
Eudimentary organs, from being useless, are not 
regulated by natural selection, and hence are variable. 
Specific characters — ^that is, the characters which have 
come to differ since the several species of the same genus 
branched off from a common parent — are more varia- 


ble than generic characters, or those which have long 
been inherited, and have not differed 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 species 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. Variability in the same parts of the organisa- 
tion has generally been taken advantage of in giving 
secondary sexual differences to the two sexes of the 
same species, and specific differences to the several spe- 
cies of the same genus. Any part or organ developed 
to an extraordinary size or in an extraordinary man- 
ner, in comparison with the same part or organ in the 
allied species, must have gone through an extraordinary 
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 selec- 
tion will in such cases not as yet have had time to over- 
come the. tendency to further variability and to rever- 
sion to a less modified state. But when a species with 
any extraordinarily-developed organ has become the 
parent of many modified descendants — which on our 
view must be a very slow process, requiring a long lapse 
of time — ^in this case, natural selection has succeeded 


in giving a fixed character to the organ, in however 
extraordinary a manner it may have been developed. 
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 modifications may not arise from 
reversion and analogous variation, such modifications 
will add to the beautiful and harmonious diversity of 

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 modifications 
of structure in relation to the habits of each species. 




Difficulties of the theory of descent with modiflcation— 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 transition — Cases of difficulty — Natura 
non facit saltum — Organs of small importance — Organs not in 
all cases absolutely perfect — The law of Unity of Type and of 
the Conditions of Existence embraced by the theory of Natu- 
ral Selection. 

Long before the reader has arrived at this part of my 
work, a crowd of difficulties will have ocdurred to him. 
Some of 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 the theory. 

These difficulties and objections may be classed 
under the following heads: — ^First, why, if species have 
descended from other species by fine gradations, do we 
not everywhere see innumerable transitional forms? 
Why is not all nature in confusion, instead of the spe- 
cies 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 modification of some other animal 
with widely different habits and structure? 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 he 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, wheii^ 
crossed, being sterile and producing sterile offspring, 
whereas, when varieties are crossed, their fertility is_ 

The two first heads will here be discussed; some 
miscellaneous objections in the following chapter; In- 
stinct 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 com- 
petition. Thus extinction and natural selection go 
hand in hand. Hence, if we look at each species as 
descended from some unknown form, both the parent 
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 Eecord; 


and I will here only state that I believe the answer 
mainly lies in the record being incomparably less per- 
fect 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 

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 species, evi- 
dently filling nearly the same place in the natural econ- 
omy 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 modification, each has become adapted to the 
conditions of life 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 em- 
bedded there in a fossil condition. But in the inter- 
mediate region, having intermediate conditions of life, 
why do we not now find closely-linking intermediate 
varieties? This difficulty for a long time quite con- 


founded me. But I think it can be in large part ex- 

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 
separately formed without the possibility of inter- 
mediate varieties existing 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 
species, more especially with freely-crossing and wander- 
ing 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 ter- 
ritory proper to each. We see the same fact in ascend- 
ing mountains, and sometimes it is quite remarkable 
how abruptly, as Alph. de CandoUe 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 ele- 
ments of distribution, these facts ought to cause surprise, 
as climate and height or depth graduate away insen- 
sibly. But when we bear in mind that almost every 
species, even in its metropolis, would increase immensely 
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 in- 
directly related in the most important manner to other 
organic beings, — we see that the range of the inhabi- 
tants 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 competition; and as these species are already de- 
fined objects, not blending one into another by insen- 
sible gradations, the range of any one species, 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 numbers, will, dur- 
ing 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 geo- 
graphical range will come to be still more sharply de- 

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 compara- 
tively 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 vary- 
ing 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 num- 
bers from inhabiting 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. Wol- 
laston, that generally, when varieties intermediate be- 
tween 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 gen- 
erally have existed in lesser numbers than the forms 
which they connect, then we can understand why in- 
termediate varieties should not endure for very long 
periods: — why, as a general rule, they should be ex- 
terminated and disappear, sooner than the forms which 
they originally linked together. 

For any form existing in lesser numbers would, as 
already remarked, run a greater chance of being exter- 
minated than one existing 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 impor- 
tant consideration, 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 inhabiting 
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 existiag 
m larger numbers will have a better chance, within any 
given period, of presenting further favourable varia- 
tions for natiiral 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 improved. 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 illus- 
trate 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 h\)lders 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, with- 
out the interposition of the supplanted, intermediate 
hill variety. 

To sum up, I believe that species come to be tolerably 
well-defined objects, and do not at any one period pre- 
sent 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, prob- 
ably, in a still more important degree, on some of the 
old inhabitants becoming slowly modified, with the 
new forms thus produced, 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 struc- 
ture 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, intermedi- 
ate varieties between the several representative species 
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 different portions of a strictly continuous area, inter- 
mediate varieties will, it is probable, at first have been 
formed in the intermediate zones, but they will gen- 
erally 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 representative 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 inter- 
mediate varieties will be liable to accidental extermina- 
tion; 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 aggregate, 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 pro- 
cess of natural selection constantly tends, as has been so 
often remarked, to exterminate the parent-forms and 
the intermediate 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 imperfect 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 transitional 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 stniggle 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 insectivorous 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 

Here, as on other occasions, I lie under a heavy 
disadvantage, for, out of the many striking cases which 
I have collected, I can only give one or two instances 
of transitional habits and structures in allied species; 
and of diversified habits, either constant or occasional, 
in the same species. 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. 

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. Eichardson 
has remarked, with the posterior part of their bodies 
rather wide and with the skin on their flanks rather 
full, to the so-called flying squirrels; and flying squir- 
rels 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 astonish- 
ing 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 structure 
of each squirrel is the best that it is possible to conceive 
under all possible conditions. Let the climate and vege- 
tation 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 exter- 
minated, unless they also became modified and improved 
in structure in a corresponding manner. Therefore, 
I can see no difficulty, more especially under chang- 
ing conditions of life, in the continued preservation of 
individuals with fuller and fuller flank-membranes, each 
modification being useful, each being propagated, until, 
by the accumulated effects of this process of natural 
selection, a perfect so-called flying squirrel was pro- 

Now look at the Galeopithecus or so-called flying 
lemur, which formerly was ranked amongst bats, but 
is now believed to belong to the Insectivora. An ex- 
tremely wide flank-membrane stretches from the corners 
of the jaw to the tail, and includes the limbs with the 
elongated fingers. This fiank-membrane is furnished 
with an extensor muscle. Although no graduated links 
of structure, fitted for gliding through the air, now con- 
nect the Galeopithecus with the other Insectivora, yet 
there is no difficulty in supposing that such links for- 
merly 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 possessor. 
Nor can I see any insuperable difficulty in further believ- 
ing that the membrane connected fingers and fore-arm 
of the Galeopithecus might have been greatly length- 


ened 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-mem- 
brane 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 
functionally for no purpose, like the 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 inferred from these remarks that any of the grades 
of wing-structure here alluded to, which perhaps may 
all be the result of disuse, indicate 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 conceivable 
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 tran- 
sitional 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. Fur- 
thermore, we may conclude 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 stage 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 structure 
in a fossil condition will always be less, from their hav- 
ing existed in lesser numbers, than in the case of species 
with fully developed structures. 

I will now give two or three instances both of diver- 
sifled and of changed habits in the individuals of the 
same species. In either case it would be easy for natu- 
ral 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 im- 


material for us, whether habits generally change first 
and structure afterwards; or whether slight modifica- 
tions 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 exclusively on artificial substances. Of diver- 
sified 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, like 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 like 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 Heame 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 those proper to their species and to the 
other species of the same genus, we might expect that 
such individuals would occasionally give rise to new 
species, having anomalous habits, and with their struc- 
ture either slightly or considerably modified from that 
of their type. And such instances occur in nature. 
Can a more striking instance of adaptation be given 
than that of a woodpecker for climbing 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 in- 
sects on the wing. On the plains of La Plata, where 
hardly a tree grows, there is a woodpecker (Colaptes 
campestris) which has two toes before and two behind, 
a long pointed tongue, pointed tail-feathers, sufficiently 
stiff to support the bird in a vertical position on a post, 
but not so stiff as ia 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 iu such trifling characters as the 
colouring, the harsh tone of the voice, and undulatory 
flight, its close blood-relationship to our conunon wood- 
pecker is plainly declared; yet, as I can assert, not only 
from my own observations, but from those of the ac- 
curate Azara, in certain large districts it does not climb 
trees, and it makes its nest in holes in banks! In cer- 
tain 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 

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 swimming 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 pro- 
foundly 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 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 members 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 modification 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 swim- 
ming? Yet there are upland geese with webbed feet 
which rarely go near the water; and no one except Au- 
dubon 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 membrane. 
What seems plainer than that the long toes, not fur- 
nished with membrane of the Grallatores are formed 
for walking over swamps and floating plants? — the 
water-hen and landrail are members of this order, yet 
the first is nearly as aquatic as the coot, and the second 
nearly as terrestrial as the quail or partridge. In such 
cases, and many others could be given, habits have 
changed without a corresponding change of structure. 


The webbed feet of the upland goose may be said to 
have become almost rudimentary in function, though 
not in structure. In the frigate-bird, the deeply scooped 
membrane between the toes shows that structure has be- 
gun 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 in- 
crease in numbers; and that if any one being varies 
ever so little, either in habits or structure, and thus 
gains an advantage over some other inhabitant of the 
same country, it will seize on the place of that in- 
habitant, 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 con- 
trivances 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, aa 
every philosopher knows, cannot be trusted in science. 
Eeason 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 like- 
wise 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 can- 
not be detected, are capable of perceiving light, it does 
not seem impossible that certain sensitive elements 
in their sarcode should become aggregated and de- 
veloped into nerves, endowed with this special sensi- 

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 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 con- 
sists of an optic nerve, surrounded by pigment-cells 
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 
aggregates 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 dis- 
tinguish light from darkness. In certain star-fishes, 
small depressions in the layer of pigment which sur- 
rounds the nerve are fillfid, as described by the author 
Just quoted, with transparent gelatinous matter, pro- 
jecting 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 curiouslj modified nervous filaments. 
But these organs in the Articulata are so much diver- 
sified 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 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 mem- 
brane, into an optical instrument as perfect as is pos- 
sessed 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 selec- 
tion; 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. 
it they were extremely slight 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 con- 
traction 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 per- 
fected at any stage of the construction of the instru- 
ment." 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 ap- 
paratus. 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, ac- 
cording to the high authority of Virchow, the beautiful 
crystalline lens is formed in the embryo by an accumu- 
lation of epidermic cells, lying in a sack-like fold of 
the skin; and the vitreous body is formed from embry- 
onic sub-cutaneous tissue. To arrive, however, at a just 
conclusion regarding the formation of the eye, with all 
its marvellous yet not absolutely perfect characters, it 
is indispensable that the reason should conquer the 
imagination; but I have felt the difficulty far too 
keenly to be surprised at others hesitating to extend 
the principle of natural selection to so startling a 

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 pro- 
cess. But may not this inference be presumptuous? 
Have we any right to assume that the Creator works by 
intellectual powers Hke 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 beneath, and then suppose every part of this 
layer to be continually changing slowly in density, so as 
to separate into layers of different densities and thick- 
nesses, placed at different distances from each other, 
and with the surfaces of each layer slowly changing in 
form. Further we must suppose that there is a power, 
represented by natural selection or the survival of the 
fittest, always intently watching 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 un- 
erring 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? 


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, accord- 
ing to the theory, there has been much extinction. 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 

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 Co- 
bites the alimentary canal respires, digests, and ex- 
cretes. In the Hydra, the animal may be turned in- 
side 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 per- 
formed 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 
he effected with comparative suddenness in the charac- 
ter of the species. It is, however, probable that the 
two sorts of flowers borne by the same plant were origi- 
nally 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 branchiae that 
breathe the air dissolved in the water, at the same time 
that they breathe free air in their swimhladders, this 
latter organ being divided by highly vascular partitions 
and having a ductus pneumatieus for the supply of air. 
To give another instance from the vegetable kingdom: 
plants climb by three distinct means, by spirally twin- 
ing, by clasping a support with their sensitive tendrils, 
and by the emission of aerial rootlets; 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 obliter- 

The illustration of the swimbladder in fishes is a 
good one, because it shows us clearly the highly im- 
portant fact that an organ originally constructed for 
one purpose, namely, flotation, may be converted into 


one for a widely different purpose, namely, respiration. 
The swimbladder has, also, been worked in as an acces- 
sory to the auditory organs of certain fishes. All phys- 
iologists admit that the swimbladder 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 swimbladder has actually 
been converted into lungs, or an organ used exclusively 
for respiration. 

According to this view it may be inferred that all 
vertebrate animals with true lungs are descended by 
ordinary generation from an ancient and unknown pro- 
totype, which was furnished with a floating apparatus 
or swimbladder. We can thus, as I 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 branchiae have wholly 
disappeared — ^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 gradu- 
ally worked in by natural selection for some distinct pur- 
pose: for instance, Landois has shown that the wings of 
insects are developed from the tracheae; 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 impor- 
tant 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 branchia3, the whole surface of 
the body and of the sack, together with the small frena, 
serving for respiration. The Balanidse or sessile cirri- 
pedes, 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 branchiae. ISTow I think no one 
will dispute that the ovigerous frena in the one family 
are strictly homologous with the branchiae of the other 
family; indeed, they graduate into each other. There- 
fore 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 respira- 
tion, have been gradually converted by natural selection 
into branchiae, simply through an increase in their size 
and the obliteration of their adhesive 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 branchiae 
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 Prof. 
Cope and others in the United States. It is now known 
that some animals are capable of reproduction at a very 


early age, before they have acquired their perfect charac- 
ters; and if this power became thoroughly well de- 
veloped in a species, it seems probable that the adult 
stage of development 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 ani- 
mals, after arriving at maturity, go on changing in char- 
acter 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 in- 
stances 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. Prof. Cope states that the teeth of certain 
lizards change much in shape with advancing years; with 
crustaceans not only many trivial, but some important 
parts assume a new character, as recorded by Fritz Miil- 
ler, 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 con- 
cluding that any organ could not have been produced 
by successive, small, transitional gradations, yet un- 
doubtedly 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 impossible 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 Eay, 
as observed by Matteucci, an analogous organ in the 
tail manifests but little electricity, even when the ani- 
mal is greatly irritated; so little, that it can hardly 
be of any use for the above purposes. Moreover, in the 
Eay, besides the organ just referred to, there is, as Dr. 
E. M'Donnell 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 
contraction is accompanied by an electrical discharge; 
and, as Dr. Eadcliffe 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 depends upon the action of 
muscle and motor nerve." Beyond this we cannot at 
present go in the way of explanation; but as we know 
so little about the uses of these organs, and as we know 
nothing about the habits and structure of the progeni- 
tors 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 gradu- 
ally developed. 

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 members 
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 an- 
cient progenitor, we might have expected that all elec- 
tric 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 pos- 
sessed electric organs, which their modified descend- 
ants have now lost. But when we look at the subject 
more closely, we find in the several fishes provided with 
electric organs, that these are situated in different parts 
of the body, — ^that they differ in construction, as in the 
arrangement of the plates, and, according to Pacini, in 
the process or means by which jthe electricity 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 suppose 
that they have been inherited from a common progeni- 
tor; for had this been the case they would have closely 
resembled each other in all respects. Thus the diffi- 
culty of an organ, apparently the same, arising in several 
remotely allied species, disappears, leaving only the 
lesser yet still great difficulty; namely, by what gradu- 
ated steps these organs have been developed in each sep- 
arate 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 
eases could be given; for instance in plants, the very 
curious contrivance 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 re- 
mote 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 or- 
ganisation, which are furnished with similar and pe- 
culiar organs, it will be found that although the gen- 
eral appearance and function of the organs may be the 
same, yet fundamental differences between them can 
always be detected. For instance, the eyes of cephalo- 
pods or cuttle-fish and of vertebrate animals appear won- 
derfully alike; and in. such widely sundered groups 
no part of this resemblance can be due to inheritance 



from a common progenitor, ilr. Mivart has advanced 
this case as one of special difficulty, but I am unable to 
see the force of his argument. An organ for vision 
must be formed of transparent tissue, and must include 
some sort of lens for throwing an image at the back 
of a darkened chamber. Beyond this superficial resem- 
blance, 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 impossible for me here to 
enter on details, but -I may specify a few 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 in- 
cluded within the membranes of the eye. The relations 
of the muscles are as different as it is possible to con- 
ceive, 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 de- 
veloped 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 independently hit on the same inven- 
tion, 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 
produced 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 pro- 

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 impor- 
tant characters; namely, in their sense organs, circulat- 
ing system, in the position of the tufts of hair within 
their complex stomachs, and lastly in the whole struc- 
ture of the water-breathing branchiae, even to the micro- 
scopical hooks by which they are cleansed. Hence it 
might have been expected that in the few species be- 
longing to both families which live on the land, the 
equally-important 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? 

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 the vast majority 
of the species in the above two families, as well as most 
other crustaceans, are aquatic in their habits, it is im- 
probable in the highest degree, that their common pro- 
genitor should have been adapted for breathing air. 


Miiller was thus led carefully to examine the apparatus 
in the air-breathing species; and he found it to difEer 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 difEered to a certain extent, and in ac- 
cordance 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 selection 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 unintelligible. This line of 
argument 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 
Clapar^de, has argued in the same manner, and has 
arrived at the same result. He shows that there are 
parasitic mites (Acaridae), belonging to distinct sub- 
families and families, which are furnished with hair- 
claspers. These organs must have been independent- 
ly 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 ap- 
pendages on the under side of the hind part of the 

In the foregoing cases, we see the same end gained 
and the same function performed, in beings not at all 
or only remotely allied, 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 constructed 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 ren- 
dered 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 fur- 
nished 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 diver- 
sified 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, but such a view of nature is incredible. With 
plants having separated sexes, and with those in which, 
though hermaphrodites, the pollen does not spontane- 
ously 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 in- 
exhaustible number of contrivances, all for the same 
purpose and effected in essentially the same manner, 
but 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 some- 
times capable of neatly adapted movements through 
irritability or elasticity. Prom such structures we may 
advance till we come to such a case of extraordinary 
adaptation as that lately described by Dr. Criiger in 
the Coryanthes. This orchid has part of its labellum 
or lower lip hollowed out into a great bucket, into 
which drops of almost pure water continually fall from 
two secreting horns which stand above it; and when 
the bucket is half full, the water overflows by a spout 
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 entrances; 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 off 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 involun- 
tary 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 pollen-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 com- 
rades into the bucket and then crawls out by the pas- 
sage, the pollen-mass necessarily comes flrst into eon- 
tact 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 spout, and rub against the properly placed viscid 
pollen-masses and the viscid stigma. 

The construction of the flower in another closely 


allied orchid, namely the Catasetum, is widely different, 
though serving the same end; and is equally curious. 
Bees visit these flowers, like those of the Coryanthes, in 
order to gnaw the labellum; in doing this they inevi- 
tably 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 ruptured; 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 
extremity 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 
retaining the pollen, fertilisation is effected. ' 

How, it may be asked, in the foregoing and in in- 
numerable other instances, can we understand the gradu- 
ated scale of complexity 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 consequently 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 inherited 
changes, through which the species has passed during 


its successive adaptations to changed habits and con- 
ditions of life. 

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 appearing as if created for some special 
purpose, rarely or never appear in any being; — as in- 
deed is shown by that old, but somewhat exaggerated, 
canon in natural history of " Natura non f acit saltum." 
We meet with this admission in the writings of almost 
every experienced naturalist; or as Milne Edwards has 
well expressed it, Nature is prodigal in variety, but nig- 
gard in innovation. Why, on the theory of Creation, 
should there be so much variety and so little real nov- 
elty? Why should all the parts and organs of many in- 
dependent beings, each supposed to have been separately 
created for its proper place in nature, be so commonly 
linked together by graduated steps? Why should not 
Nature take a sudden leap from structure to struc- 
ture? 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 


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 
complex 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 consti- 
tutional differences or from determining the attacks 
of insects, might assuredly be acted on by natural se- 
lection. The tail of the giraffe looks like an artifi- 
cially constructed fly-flapper; and it seems at first in- 
credible that this could have been adapted for its present 
purpose by successive slight modifications, each better 
and better fitted, for so trifiing an object as to drive 
away flies; yet we should pause before being too posi- 
tive even in this case, for we know that the distribution 
and existence of cattle and other animals in South 
America absolutely depend on their power of resisting 
the attacks of insects: so that individuals which could 
by any means defend themselves from these small ene- 
mies, 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 

Organs now of trifling importance have probably 
in some cases been of high importance to an early pro- 
genitor, and, after having been slowly perfected at a 
former period, have been transmitted to existing 
species in nearly the same state, 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 important an organ of locomo- 
tion the tail is in most aquatic animals, its general pres- 
ence and use for many purposes in so many land animals, 
which in their lungs or modified swimbladders 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 b.§ 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-called spon- 
taneous 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, 
&c., — and finally of sexual selection, by which charac- 
ters of use to one sex are often gained and then trans- 
mrtted'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 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 consequently 
that it was a character of importance, and had been 
acquired 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 feees by the aid of exquisitely constructed 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 ob- 
ject, 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 in- 
dispensable 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 struc- 
ture has arisen from the laws of growth, and has been 
taken advantage of in the parturition of the higher 

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 ani- 
mals in different countries, — more especially in the less 
civilised countries where there has been but little 
methodical selection. Animals kept by savages in dif- 
ferent countries often have to struggle for their own 
subsistence, and are exposed to a certain extent to natu- 
ral selection, and individuals with slightly different 
constitutions would succeed best under different cli- 
mates. With cattle 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 af' 
fects the growth of the hair, and that with the hair 
the horns are correlated. Mountain breeds always dif- 
fer 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 probably be affected. The 
shape, 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 breeds of 
swine have undergone. But we are far too ignorant to 
speculate on the relative importance of the several 
known and unknown causes of variation; and I have 
made these remarks only to show that, if we are un- 
able to account for the characteristic differences of 
our several domestic breeds, which nevertheless are gen- 
erally admitted to have arisen through ordinary gen- 
eration from one or a few parent-stocks, we ought not 
to lay too much stress on our ignorance of the pre- 
cise cause of the slight analogous differences between 
true species. 

Utilitarian Doctrine, how far true: Beauty, how 

The foregoing remarks lead me to say a few words or 
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 be- 
lieve 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 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 specified, 
have all produced an effect, probably a great effect, in- 
dependently of any advantage thus gained. But a still 
more important consideration is that the chief part 
of the organisation of every living creature is due to. 
inheritance; and consequently, though each being as- 
suredly is well fitted for its place in nature, many struc- 
tures 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 be- 
lieve 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 use- 
ful 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; but we may further 
venture to believe that the several bones in the limbs 
of the monkey, horse, and bat, were originally devel- 
oped, on the principle of utility, probably through the 
reduction of more numerous 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, so-called spontaneous 
variations, and the complex laws of growth; but with 
these important exceptions, we may conclude that 
the structure of every living creature either now is, 
or was formerly, of some direct or indirect use to its 

With respect to the belief that organic beings have 
been created beautiful for the delight of man, — a belief 
which it has been pronounced is subversive of my 
whole theory, — I may first remark that the sense of 
beauty obviously depends on the nature of the mind, 
irrespective of any real quahty 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 different 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 am- 
monites of the Secondary period, created that man 
might ages afterwards admire them in his cabinet? 
Few objeeis are more beautiful than the minute silice- 
ous eases of the, diatomacese: 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 contrast 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 fertilised by the wind it never has a 
gaily-coloured corolla. Several plants habitually pro- 
duce two kinds of flowers; one kind open and coloured 
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 pro- 
duced only such poor flowers as we see on our fir, oak, 
nut and ash trees, on grasses, spinach, docks, and net- 
tles, 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-coloured 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 fox(nd no exception to the 
rijle that seeds are always thus disseminated when 
embedded within a fruit of any kind (that is within 
a fleshy or pulpy envelope), if it be coloured of any 
brilliant tint, or rendered conspicuous by being white or 

On the other hand, I willingly admit that a great 
number of male animals, as all our most gorgeous birds, 
some fishes, reptiles, and mammals, and a host of mag- 
nificently 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 aU this that a nearly 
similar taste for beautifid 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 trasmitted to 
both sexes, instead of to the males alone. How the 
sense of beauty in its simplest form — ^that is, the re- 
ception of a peculiar kind of pleasure from certain col- 
ours, forms, and sounds — ^was first developed in the 
mind of man and of the lower animals, is a very ob- 
scure 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 constitution of the nervous system in each 

Natural selection cannot possibly produce any modi- 
fication in a species exclusively for the good of another 
species; though throughout nature one species inces- 
santly 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 ani- 
mals, as we see in the fang of the adder, and in the ovi- 
positor of the ichneumon, by which its eggs are de- 
posited in the living 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; 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 ex- 
pands 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 endeavour to frighten away their 

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. ISTo 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 modified; 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 perfect 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 Europe. Natural selec- 
tion will not produce absolute perfection, nor do we al- 
ways meet, as far as we can judge, with this high stand- 
ard 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 powers 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 the image on the retina, 
is as nothing in comparison with the incongruities which 
we have just come across in the domain of the sensa- 
tions. One might say that nature has taken delight 
in accumulating contradictions in order to remove all 
foundation 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 
inimitable 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 inevi- 
tably causes the death of the insect by tearing out its 

If we look at the sting of the bee, as having existed 
in a remote progenitor, as a boring and serrated instru- 
ment, like that in so many members of the same great 
order, and that it has since been modified but not per- 
fected for its present purpose, with the poison origi- 
nally adapted for some other object, such as to produce 
galls, since intensiiied, 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 ad- 
mire the truly wonderful power of scent by which the 
males of many insects find their females, can we ad- 
mire the production for this single purpose of thou- 
sands of drones, which are utterly useless to the com- 
munity 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 un- 
doubtedly 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 inexorable 
principle 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 

Chap. VI.] SUMMARY. 357 

dense clouds of pollen by our fir-trees, so that a few 
granules may be wafted by chance on to the 

Summary: the Law of Unity of Type and of the Con- 
ditions of Existence embraced by the Theory of Natu- 
ral Selection. 

We have in this chapter discussed some of the diffi- 
culties and objections 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 independent 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 grada- 
tions, 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 extinc- 
tion of preceding and intermediate gradations. Close- 
ly allied species, now living on a continuous area, must 
often have been formed when the area was not con- 
tinuous, and when the conditions of life did not insen- 
sibly graduate 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 
numbers than the two forms which it connects; conse- 
quently the two latter, during the course of further 
modification, from existing in greater numbers, will 
have a great advantage over the less numerous inter- 

258 SUMMARY. [CsAt. VL 

mediate variety, and will thus generally succeed in 
supplanting and exterminating it. ■ 

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

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 con- 
geners. 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. 

Although the belief that an organ so perfect as the 
eye could have been formed by natural selection, is 
enough t-o 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 con- 
ditions of life, there is no logical impossibility in the 
acquirement of any conceivable degree of perfection 
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 wonderful changes in function are at least 
possible. For instance, a swimbladder 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 

Chap. VI.] SUMMARY. 259 

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 
came 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 de- 
tected; 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 fol- 
lows 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 accumulated by means 
of natural selection. In many other cases, modifica- 
tions are probably the direct result of the laws of varia- 
tion or of growth, independently of any good having 
been thus gained. But even such structures have often, 
as we may feel assured, been subsequently taken ad- 
vantage of, and still further modified, for the good of 
species under new conditions of life. We may, also, be- 
lieve that a part formerly of high importance has fre- 
quently been retained (as the tail of an aquatic animal by 
its terrestrial descendants), though it has become of such 
small importance that it could not, in its present state, 
have been acquired by means of natural selection. 

Natural selection can produce nothing in one species 
for the exclusive good or injury of another; though it 
may well produce parts, organs, and excretions highly 

260 SUMMARY. [Chap. VL 

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 accordance 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 com- 
petition 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 
strictly 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 in- 
sisted 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 cases by the in- 
creased use or disuse of parts, being afEected by the di- 
rect action of the external conditions of life, and sub- 
jected 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. 




Longevity — Modifications not necessarily simultaneous — Modifica- 
tions apparently of no direct service — Progressive development 
— Characters of small functional importance, the most con- 
stant — Supposed incompetence of natural selection to account 
for the incipient stages of useful structures — Causes which in- 
terfere with the acquisition through natural selection of useful 
structures — Gradations of structure with changed functions — 
Widely different organs in members of the same class, de- 
veloped from one and the same source — Reasons for disbeliev- 
ing in great and abrupt modifications. 

I WILL devote this chapter to the consideration of 
various miscellaneous objections which have been ad- 
vanced against my views, as some of the previous discus- 
sions 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 sub- 
ject. Thus a distinguished German naturalist has as- 
serted that the weakest part of my theory is, that I con- 
sider 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 apy one time perfectly adapted to their conditions of 
life, ha\e 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 numhers 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 advan- 
tage 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 perish there every winter; and 
yet, owing to advantages gained through natural selec- 
tion, survive from year to year by means of its seeds or 
ova? Mr. E. Kay Lankester has recently discussed this 
subject, and he concludes, as far as its extreme com- 
plexity allows him to form a judgment, that longevity 
is generally related to the standard of each species in 
the scale of organisation, as well as to the amount of ex- 
penditure in reproduction and in general activity. And 
these conditions have, it is probable, been largely de- 
termined 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 de- 
velopment, but is powerless against the doctrine of natu- 
ral selection or the Survival of the fittest, which implies 
that when variations or individual differences of a bene- 
ficial nature happen to arise, these will be preserved; 
but this will be effected only under certain favourable 

The celebrated palaeontologist, 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 conditions, 
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, &c., 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 ease of animals 
which wander much about and cross freely, their 
varieties seem to be generally confined to distinct 

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 trans- 
mitted 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, chiefiy 
through man's power of selection, for some special pur- 
pose. Look at the race and dray horse, or at the grey- 
hound and mastiff. Their whole frames and even their 
mental characteristics have been modified; but if we 
could trace each step in the history of their transfor- 
mation, — 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 cultivated 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 at- 
tributed partly to the principle of correlated growth, and 
partly to so-called spontaneous variation. 

A much more serious objection has been urged by 
Bronn,and recently by Broca, namely, that many charac- 
ters appear to be of no service whatever to their pos- 


sessors, 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 complex folds of enamel in the teeth of many 
animals, and a multitude of analogous cases. With 
respect to plants, this subject has been discussed by 
Nageli in an admirable essay. He admits that natural 
selection has eilected much, but he insists that the 
families of plants differ chiefly from each other in mor- 
phological characters, which appear to be quite unim- 
portant for the welfare of the species. He. consequently 
believes in an innate tendency towards progressive 
and more perfect development. He specifies the ar- 
rangement of the cells in the tissues, and of the leaves 
on the axis, as eases in which natural selection could not 
have acted. To these may be added the numerical divi- 
sions in the parts of the flower, the position of the ovules, 
the shape of the seed, when not of any use for dissemina- 
tion, &c. 

There is much force in the above objection. Never- 
theless, 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 nutriment to a part, mutual pres- 
sure, 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 agen- 
cies 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-called 
spontaneous variations, in which the nature of the con- 
ditions 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, ofEer 
good instances of spontaneous variations; but even in 
these eases, if we hear in mind the power of a minute 
drop of poison 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 oc- 
casionally arise; and if the unknown cause were to act 
persistently, it Is almost certain that aU the individuals 
of the species would be similarly modified. 

In th e^arlier editions of this work I under-rated, as 
it now seems probable, the frequency and importance of 
modifications due to 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 explained. 

It may be worth while to illustrate some of the fore- 
going 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 triiling 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 in- 
fluenced by its length. 

With respect to plants, to which on account of Nage- 
li'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 morphological 
differences without any special function; but they are 
now known to be of the highest importance for the 
fertilisation of the species through the aid of insects, 
and have probably been gained through natural selec- 
tion. 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 

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 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 wonder- 
fully 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, mani- 
festly of high importance, as they yield with the utmost 
safety a large stock of seed, with the expenditure of 
wonderfully 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 columnas 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 perfect flowers), the sepals are reduced from 
the normal number of five to three. .In one section of 
the Malpighiaceae 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 ordi- 
nary flowers of these species; the style is aborted; and 
the ovaria are reduced from three to two. Now al' 
though 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 clos- 
ure 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, in-cluding 
the functional inactivity of parts, during the progress 
of the reduction of the pollen and the closure of the 

It is so necessary to appreciate the important 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 
uppermost flower generally has two calyx-lobes with the 
other organs tetramerous, whilst the surrounfling flowers 
generally have three calyx-lobes with the other organs 
pentamerous. In many Composite and Umbelliferffi 
(and in some other plants) the circumferential 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 sometimes dif- 
fer greatly in form, colour, and other characters. In Car- 
thamus and some other Composite the central achenes 
alone are furnished with a pappus; and in Hyoseris the 
same head yields achenes of three different forms. In 
certain Umbelliferse the exterior seeds, according to 
Tausch, are orthospermous, and the central one coelo- 
spermous, and this is a character which was considered 
hy De CandoUe to be in other species of the highest 
systematic importance. Prof. Braun mentions a Fu- 
mariaceous 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 con- 
spicuous to insects, natural selection cannot, as far as we 
can judge, have come into play, or only in a quite subor- 
dinate 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 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 gener- 
ally of a highly important nature, affecting only some 
of the flowers on the same plant, or occurring on dis- 
tinct 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 indifEerently tetram- 
erous, pentamerous, &c., 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 oSer 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 aestivation is almost as frequently that 
of the Rhinanthidese as of the Antirrhinidese, to which 
latter tribe the genus belongs. Aug. St. Hilaire gives 
the following cases: the genus Zanthoxylon belongs to 
a division of the Eutacese 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 de- 
scribed as unilocular or 3-locular; and in H. mutabile, 
"Une lame, plus ou mains large,s'eiend entre le pericarpe 
et le placenta." In the flowers of Saponaria ofiicinalis. 
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 
oleEeformis two forms which he did not at flrst doubt 
were distinct species, but he subsequently 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 

AVe thus see that with plants many morphological 
changes may be attributed to the laws of growth and 
the inter-action of parts, independently of natural selec- 
tion. But with respect to Nageli's doctrine of an innate 
tendency towards perfection or progressive development, 
can it be said in the case of these strongly pronounced 
variations, 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 classi- 
fications. The acquisition of a useless part can hard- 
ly 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 in- 
voked, it must be one of retrogression 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 uniform- 
ly for a length of time, we may infer that the result 
would be almost uniform; and in this case all the in- 
dividuals of the species would be modified in the same 

From the fact of the above characters being unim- 
portant for the welfare of the species, any slight varia- 
tions which occurred in them would not have been ac- 
cumulated 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 rudi- 
mentary organs; for it will no longer be regulated by 
this same power of selection. But when, from the na- 
ture of the organism and of the conditions, modifica- 
tions have been induced which are unimportant for the 
welfare of the species, they may be, and apparently 
often have been, transmitted 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 trans- 
mitted 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 consequently ia 
often assumed to be of high vital importance to the 
species. Thus, as I am inclined to believe, morpho- 
logical 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, 
&c. — 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 con- 
ditions, 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 para- 
doxical as it may at first appear. 

Although we have no good evidence of the existence 
in organic beings of an innate tendency towards pro- 
gressive 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 specialised or differentiated; and natural selection 
tends towards this end, inasmuch as the parts are thus 
enabled to perform their functions more efficiently. 

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 special cases, Mr. Mi- 
vart passes over the effects of the increased use and 
disuse of parts, which I have always maintained to be 
highly important, and have treated in my 'Variation 
under Domestication' at greater length than, as I be- 
lieve, any other writer. He likewise often assumes that 
I attribute nothing to variation, independently of natu- 
ral 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 judg- 


ment may not be trustworthy^ but after reading with 
care Mr. Miyart'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 conclu- 
sions 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 in- 
cipient stages of useful structures." This subject is 
intimately connected with that of the gradation of cha- 
racters, 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 giraffe, by its lofty stature, much elongated neck, 
fore-legs, head and tongue, has its whole frame beauti- 
fully adapted for browsing 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 ISTiata cattle in S. America show us how 
small a difference in structure may make, during such 
periods, a great difference 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, &c., to which food the common cattle and 
horses are then driven; so that at these times the ISTiatas 


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 neces- 
sarily having attended to special points of structure, 
by simply preserving and breeding from the fleetest in- 
dividuals, 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 girafEe, the in- 
dividuals which were the highest browsers, and were 
able during 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 
measurements are given. These slight proportional dif- 
ferences, due to the laws of growth and variation, are 
not of the slightest use or importance 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 pe- 
culiarities, or with a tendency to vary again in the same 
manner; whilst the individuals, less favoured in ihe 
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 in- 
tercross, and will destroy all the inferior individuals. 


By this process long-continued, which exactly corre- 
sponds with what I have called unconscious selection by 
man, combined no doubt in a mo^t 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 problematical 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 
S. 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, intermediate 
gradations could formerly have existed there, subjected 
as 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 in- 
creased 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 his 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 objec- 
tion), 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 a lesser degree, the 
camel, guanaco, and macrauchenia? Or, again, why has 
not any member of the group acquired a long proboscis? 
With respect to S. Africa, which was formerly in- 
habited by numerous herds of the giraffe, the answer is 
not difficult, and can best be given by an illustration. 
In every meadow in England in which trees grow, we see 
the lower branches trimmed or planed to an exact level 
by the browsing of the horses or cattle; and what ad- 
vantage 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 
slongated for this purpose, through natural selection and 
the effects of increased use. In S. Africa the competi- 
tion 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 expect 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 another. We are ignorant with respect 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 development of a long neck or proboscis. To reach 
the foliage at a considerable height (without climbing, 
for which hoofed animals are singularly ill-constructed) 
implies greatly increased bulk of body; and we know 
that some areas support singularly few large quadrupeds, 
for instance S. America, though it is so luxuriant; 
whilst S. 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 struc- 
ture specially and largely developed, it is almost indis- 
pensable that several other parts should be modified and 
co-adapted. 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 man- 
ner and degree; and that some species are much more 
variable than others. Even if the fitting variations did 
arise, it does not follow that natural selection would be 
able to act on them, and produce a structure which ap- 
parently would ■ be beneficial to the species. For in- 
stance, if the number of individuals existing in a country 
is determined chiefly through destruction by beasts of 
prey, — by external or internal parasites, &c., — as seems 
often to be the ease, then natural selection will be able 

Chap.VU.] theory of NATURAL SELECTION. 281 

to do little, or will he greatly retarded, in modifying any 
particular structure for obtaining food. Lastly, natural 
selection is a slow process, and the same favourable con- 
ditions 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 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 carnivorous animals of considerable size, 
and bats into terrestrial insectivorous 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. Grada- 
tions of structure, with each stage beneficial to a chang- 


ing species, will be favoured only under certain peculiar 
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 iind 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 should first have acquired the power of run- 
ning quickly on the ground, by the aid of their hind 
legs alone, 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 de- 
veloped 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 oth- 
er; and this apparently impHes increased brain-power. 

We will return to Mr. Mivart's other objections. In- 
sects 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 
confined 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 directions, 
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 upon and per- 

But in all the foregoing cases the insects in their 
original state no doubt presented some rude and ac- 
cidental resemblance to an object commonly found in 
the stations frequented by them. ISTor is this at all 
improbable, considering the almost infinite number of 


surrounding objects and the diversity in form and colour 
of the hosts of insects which exist. As some rude re- 
semblance is necessary for the first start, we can under- 
stand 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 origi- 
nally 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. Mi- 
vart's objection, if we were to attempt to account for the 
above resemblances, independently of natural selection, 
through mere fluctuating variability; but as the ease 
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 re- 
sembles " a stick grown over by a creeping moss or jun- 
germannia." So close was this resemblance, that a na- 
tive Dyak maintained 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 preser- 
vation; 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 in- 
cludes 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 whale-bone, one 
of its greatest peculiarities. The baleen consists of a 
row, on each side, of the upper jaw, of about 300 plates 
or laminae, 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 gradations in 
length; the middle lamina being in one species, accord- 
ing to Seoresby, four feet, in another three, in another 
eighteen inches, and in the Balssnoptera rostrata only 
about nine inches in length. The quality of the whale- 
bone 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 augmenta- 
tion within serviceable limits would be promoted by 
natural selection alone. But how to obtain the begin- 


ningof 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 CriUatores, or 
sifters. I hope that I may not be misconstrued into say- 
ing 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 lamellffi by finely graduated steps, 
each of service to its possessor. 

The beak of a shoveller-duck (Spatula clypeata) ifl 
a more beautiful 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 oi 
comb formed of 188 thin, elastic lamellae, obliquely be- 
velled 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 edge. At their 
bases there is a short subsidiary row of obliquely trans- 
verse 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 Balaenoptera 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 Balsenoptera, the lamellae would be 
six inches in length, — that is, two-thirds of the length 
of the baleen in this species of whale. The lower man- 
dible of the shoveller-duck is furnished with lamellae 
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 lamellae are 
frayed into fine bristly points, so that they thus curious- 
ly resemble the plates of baleen. In the genus Prion, a 
member of the distinct family of the Petrels, the upper 
mandible alone is furnished with lamella, 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 shovel- 
ler's beak we may proceed (as I have learnt from infor- 
mation and specimens sent to me by Mr. Salvin), with- 
out any great break, as far as fitness for sifting is con- 
cerned, 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 50 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 crush- 
ing food. The edges of the lower mandible are crossed 
by numerous fine ridges, which project very little. Al- 
though 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 


specieSj as I hear from Mr. Salvin, in which the lamellte 
are considerably less developed than in the common 
duck; but I do not know whether they use their beaks 
for sifting the water. 

Turning to another group of the same family. In 
the Egyptian goose (Chenalopex) the beak closely re- 
sembles 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 water out at the corners." Its 
chief food, however, is grass, which it crops like the com- 
mon goose. In this latter bird, the lamellae of the upper 
mandible are much coarser than in the common duck, 
almost confluent, about 27 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 
purpose 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 constructed like that of the common goose and 
adapted solely for grazing, or even a member with a beak 
having less well-developed 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 ex^ 
clusively 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 se- 
curing live fish. 

Eeturning to the whales. The Hyperoodon bidens 
is destitute of true teeth in an efficient condition, but 
its palate is roughened, according to Laeepede, with 
small, unequal, hard points of horn. There is, there- 
fore, 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, 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 lamellffi 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 ap- 
paratus. Prom this stage, in which the lamellae would 
be twQ-thirds of the length of the plates of baleen in the 
Balaenoptera rostrata, gradations, which may be ob- 
served in still-existing Cetaceans, lead us onwards to the 
enormous plates of baleen in the Greenland whale. ISTor 
is there the least reason to doubt that each step in this 
scale might have been as serviceable to certain ancient 
Cetaceans, with the functions of the parts slowly chang- 
ing during the progress of development, as are the grada- 


tions in the beaks of the different existing members of 
the duck-family. We should bear in mind that each 
siDecies of duck is subjected to a severe struggle for ex- 
istence, and that the structure of every part of its frame 
must be well adapted to its conditions of life. 

The Pleuronectidffi, or Plat-fish, are remarkable for 
their asymmetrical 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 colour, less developed in many ways than the 
upper side, with the lateral fins often of smaller size. 
But the eyes offer the most remarkable 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 propei 
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 habit- 
ual position on one side. The lower eye would, also, 
have been liable to be abraded by the sandy bottom. 
That the Pleuronectidse are admirably adapted by their 
fiattened and asymmetrical structure for their habits of 
life, is manifest from several species, such as soles, 
flounders, &c., 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 entire- 
ly 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 conceivable, 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 excellent observations 
published in 1867 by Malm. The Pleuronectidas, whilst 
irery young and still symmetrical, with their eyes stand- 
ing 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 swimbladder. Hence soon grow- 
ing tired, fhey 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 conse- 
quently 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 distance of about seventy degrees. 

We should remember that the skull at this early age 
is cartilaginous and flexible, so that it readily yields to 
muscular action. It is also known with the higher ani- 
nals, even after early youth, that the skull yields and 


is altered in shape, if the skin or muscles be perma- 
nently 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 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, how- 
ever, are soon able to hold themselves in a vertical posi- 
tion, and no permanent effect is thus produced. With 
the Pleuronectidse, 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 perma- 
nent 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 be- 
lieves, in opposition to some other naturalisfs, that the 
Pleuronectidse 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 mem- 
ber of the Pleuronectidse, 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 some- 
what dissimilar. Our great authority on Fishes, Dr. 
Glinther, concludes his abstract of Malm's paper, by 
remarking that " the author gives a very simple expla- 


nation of the abnormal condition of the Pleuronec- 

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 several 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." 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 sup- 
posed 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 species, as recently shown by Pouchet, of 
changing their colour in accordance with the surround- 
ing 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 general shape of the body 
of these fishes, and many other peculiarities, to their 
habits of life. We should keep in mind, as I have be- 
fore insisted, that the inherited effects of the increased 
use of parts, and perhaps of their disuse, will be strength- 
ened by natural selection. For all spontaneous varia- 
tions in the right direction will thus be preserved; as 
will those individuals which inherit in the highest de- 
gree the effects of the increased and beneficial use of any 
part. How much to attribute 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 num- 
ber 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 surface 
of their mother by their hands, and at the same time 
they hooked their little tails round that of their mother. 
Professor Henslow kept in confinement some harvest 
mice (Mus messorius) which do not possess a structu- 


rally 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 har- 
vest mouse had been more strictly arboreal, it vrould 
perhaps have had its tail rendered structurally prehen- 
sile, as is the case with some members of the same order. 
Why Cercopitheeus, 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 extreme- 
ly 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 hyper- 
trophied 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 mam- 
mary glands will have been at first developed within the 
marsupial sack. In the case of the fish (Hippocampus) 
the eggs are hatched, and the young are reared for a time, 
within a sack of this nature; and an American naturalist, 
Mr. Lockwood, believes from what he has seen of the 

development of the young, that, they are nourished by 


a secretion from the cutaneous glands of the sack. Now 
with the early progenitors of mammals, almost before 
they deserved to be thus designated, is it not at least pos- 
sible that the young might have been similarly nour- 
ished? 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 number of well-nourished off- 
spring, than would the individuals which secreted a 
poorer fluid; and thus the cutaneous glands, which are 
the homologues of the mammary glands, would have 
been improved or rendered more effective. It accords 
with the widely extended principle 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 mammalian series. Through what agency 
the glands over a certain space became more highly 
specialised than the others, I 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 secretion. There is no 
greater difficulty in understanding how young mam- 
mals have instinctively learnt to suck the breast, than in 
understanding 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 leav- 
ing 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 
ofEspring 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 in- 
trusion of the milk into the windpipe. But there is 
a special 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 harmlessly on each side of 
this elongated larynx, and so safely attains the gullet 
behind it." Mr. Mivart then asks how did natural selec- 
tion 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 suggested in an- 
swer that the voice, which is certainly of high importance 
to many animals, could hardly have been used with full 
force as long as the larynx entered the nasal passage; 
and Professor Flower 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 divi- 
sions of the animal kingdom. The Echinodermata (star- 
fishes, sea-urchins, &c.) are furnished with remarkable 
organs, called pedicellarise, which consist, when well 
developed, of a tridacytle forceps — that is, of one formed 
of three serrated arms, neatly fitting together and 
placed on the summit of a flexible stem, moved by mus- 
cles. 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 removing dirt of all kinds, they subserve 
other functions; and one of these apparently is de- 

"With respect to these organs, Mr. Mivart, as on so 
many previous occasions, asks: " "What would be the 
utility of the first rudimentary beginnings of such struc- 
tures, and how could such incipient buddings 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 complex 
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 star-fishes; 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 pedi- 
cellarias, one resembling those of Echinus, and the other 
those of Spatangus; and such cases are always inter- 
esting as affording the means of apparently sudden 


transitions, through the abortion of one of the two 
states of an organ. 

With respect to the steps by which these curious 
organs have been evolved, Mr. Agassiz infers from his 
own researches and those of Miiller, that both in star- 
fishes and sea-urchins the pedicellarise must undoubted- 
ly 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 pedicellariffi. The 
gradations extend even to the manner in which ordinary 
spines and pedicellarise with their supporting calcare- 
ous rods are articulated to the shell. In certain genera 
of star-fishes, " the very combinations needed to show 
that the pedicellarise 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. N^ow when the lat- 
ter 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 
pedicellarise and the moveable branches of a spine, is un- 
mistakable. 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 purpose; 
and they would thus serve still more effectively as soon as 
by meeting together they acted as a prehensible or snap- 
ping apparatus. Thus every gradation, from an ordinary 
fixed spine to a fixed pedicellaria, would be of service. 


In certain genera of star-fishes these organs, instead 
of being fixed or borne on an immoveable 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 ab- 
stract of Mr. Agassiz's interesting observations on the de- 
velopment of the pedicellariffi. All possible gradations, 
as he adds, may likewise be found between the pedi- 
cellarise of the star-fishes and the hooks of the Ophiuri- 
ans, another group of Bchinodermata; and again be- 
tween the pedicellariae of sea-urchins and the anchors of 
the Holothuriae, 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 differ 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 pedicellaris of the Echinoder- 


mata, which he considers as " essentially similar/' having 
been developed through natural selection in widely dis- 
tinct divisions of the animal kingdom. But, as far as 
structure is concerned, I can see no similarity between 
tridactyle pedicellarise and avicularia. The latter re- 
semble somewhat more closely the chelse or pincers of 
Crustaceans; and Mr. Mivart might have adduced with 
equal appropriateness this resemblance as a special diffi- 
culty; 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 move- 
able 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 chelae 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 exists. In the first 
and simplest stage, the terminal segment of a limb shuts 
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 promi- 
nent, sometimes furnished with irregular teeth; and 
against these the terminal segment shuts down. By an 


increase in the size of this projection, with its shape, as 
well as that of the terminal segment, slightly modified 
and improved, the pincers are rendered more and more 
perfect, until we have at last an instrument as efficient 
as the chelae 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 movement 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 simul- 
taneously; so that, acting like long oars, they swept a 
branch rapidly across the object-glass of my micro- 
scope. When a branch was placed on its face, the vibrac- 
ula 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 cai;efully over the surface of the poly- 
zoary, removing what might be noxious to the delicate 
inhabitants of the cells when their tentacula are pro- 
truded." The avicularia, like the vibracula, probably 
serve for defence, but they also catch and kill small liv- 
ing animals, which it is believed are afterwards swept 
by the currents within reach of the tentacula of the 
zooids. Some species 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, that the 
presence of the upper or fixed beak alone serves to de- 
termine its avicularian nature. The vibracula may 
have been directly developed from the lips 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 
transformation, the other parts of the cell with the in- 
cluded 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 trust- 
worthy, is interesting; 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 lip 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, likewise served as a protection in dif- 
ferent 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 explanation of their 
origin is deemed thoroughly unsatisfactory — utterly in- 
sufficient to explain the incipient, infinitesimal begin- 
nings 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 poUinia. A pollin- 
ium when highly developed consists of a mass of pollen- 
grains, aifixed 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, they need not here be con- 
sidered; yet I may mention that at the base of the or- 
chidaceous 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 con- 
siderable length and highly developed, we have good 
evidence in the aborted pollen-grains which can some- 
times be detected embedded within the central and 
soUd 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 be- 
longing 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 con- 
sequence of the copious secretion, is rendered sterile. 
Wben an insect visits a flower of this kind, it rubs o£E 
some of the viscid matter and thus at the same time 
drags away some of the pollen-grains. From this simple 
condition, which differs but little from that of a multi- 
tude 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 pollinium 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 pollinium, admirably adapted for trans- 
portal by insects; nor will he deny that all the grada- 
tions 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 use- 
less to ask, as it is hopeless to attempt answering, such 

We will now turn to climbing plants. These can be 
arranged in a long series, from those which simply twine 
round a support, to those which I have called leaf- 
climbers, and to those provided with tendrils. In these 


two latter classes the steins 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-climbers to tendril-bearers are 
wonderfully close, and certain plants may be indifferent- 
ly placed in either class. But in ascending the series 
from simple twiners to leaf -climbers, an important qual- 
ity is added, namely sensitiveness to a touch, by which 
means the foot-stalks of the leaves or flowers, or these 
modified and converted 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 between 
simple twiners and tendril-bearers are in each case bene- 
ficial 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 sup- 
port, and forms the basis of our series, it may naturally 
be asked how did plants acquire this power in an in- 
cipient degree, afterwards to be improved and increased 
through natural selection. The power of twining de- 
pends, 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 con- 
tinually 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 rcYolving, 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 revolving, and have thus become twiners, they must 
have independently acquired it, and cannot have in- 
herited it from a common progenitor. Hence I was led 
to predict that some slight tendency to a movement of 
this kind would be found to be far from uncommon with 
plants which did not climb; and that this had afforded 
the basis for natural selection to work on and improve. 
When I made this prediction, I knew of only one imper- 
fect case, namely, of the young flower-peduncles of a 
Maurandia which revolved slightly and irregularly, like 
the stems of twining plants, but without making 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, — revolved plainly, though irregular- 
ly; 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 climb- 
ing, 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 ex- 
posed it had profited them to ascend to a height, then the 
habit of slightly and irregularly revolving might have 
been increased and utilised through natural selection, 
until they had become 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 ease 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 be- 
come 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 ex- 
posure to a hot sun, when they were gently and repeat- 
edly 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 movement 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 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 move- 
ments, due to a touch or shake, in the young and grow- 
ing organs of plants, can be of any functional impor- 
tance to them. But plants possess, in obedience to vari- 
ous 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 rare- 
ly in the direction of, the attraction of gravity. When 
the nerves and muscles of an animal are excited by gal- 
vanism or by the absorption of strychnine, the conse- 


quent 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 cer- 
tain stimuli, they are excited in an incidental manner by 
a touch, or by being shaken. Hence there is no great dif- 
ficulty 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 as- 
signed 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 revolving movements, which were 
at first of no use to them; this movement, as well as that 
due to a touch or shake, being the incidental 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 certain periodical movements, for instance 
the so-called 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 of useful structures; 
and I have shown, as I hope, that there is no great diffi- 
culty on this head. A good opportunity has thus been 
afforded for enlarging a little on gradations of structure, 
often associated 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, &c., and could browse 
a little above the average height, and the continued de- 
struction of those which could not browse so high, would 
have sufficed for the production of this remarkable 
quadruped; but the prolonged use of all the parts to- 
gether with inheritance 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 com- 
mon object was in each case the foundation for the work 
of natural selection, since perfected through the occa- 
sional 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 fa- 
vourable variations, until the points were converted first 
into lamellated knobs or teeth, like those on the beak of 
a goose, — then into short lamellse, like those of the do- 
mestic ducks,— and then into lamellse, 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 appara- 
tus, and at last almost exclusively for this latter purpose. 


With such structures as the above lamellEe of horn or 
whalebone, 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 mammae 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 concentrated 
into a confined area, in which case they would have 
formed a mamma. There is no more difficulty in under- 
standing how the branched spines of some ancient Echi- 
noderm, which served as a defence, became developed 
through natural selection into tridactyle pedicellariae, 
than in understanding the development of the pincers 
of crustaceans, through slight, serviceable modifications 
in the ultimate 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 origin- 
ally served to tie together the pollen-grains, can be 
traced cohering into caudicles; and the steps can like- 
wise 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, be- 
came 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 repeat 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 ignor- 
ance of the past history of each species, and of the condi- 
tions 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-ordi- 
nated modifications are almost indispensable, 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 prevented 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 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-en- 
during conditions, often of a peculiar nature, are neces- 
sary for the development of a structure; and the re- 
quisite 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 con- 
siders it as " demonstrably insufficient " to account 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 ' Medieo-Chirurgical Review.' 

At the present day almost all naturalists admit evolu- 
tion under some form. Mr. Mivart believes that species 
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 variability, which through the aid of selec- 
tion by man has given rise to many well-adapted do- 
mestic races, and which through the aid of natural se- 
lection would equally well give rise by graduated steps 
to natural races or species. The final result will gener- 
ally have been, as already explained, an advance, but 
in some few cases a retrogression, in organisation. 

Mr. Mivart is further inclined to believe, and some 
naturalists agree with him, that new species manifest 
themselves "with suddenness and by modifications ap- 
pearing at once." For instance, he supposes that the 
differences between the extinct three-toed Hipparion 
and the horse arose suddenly. He thinks it difficult 
to believe that the wing of a bird " was developed in any 
other way than by a comparatively 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, which implies great 
breaks or discontinuity in the series, appears to me im- 
probable in -the highest degree. 

Every one who believes in slow and gradual evolu- 
tion, 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 maybe 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. Ex- 
cluding such cases of abrupt variations, the few which re- 
main would at best constitute, if found in a state of na- 
ture, doubtful species, closely related to their parental 

My reasons for doubting whether natural species 
have changed as abruptly as have occasionally domestic 
races, and for entirely disbelieving that they have 
changed in the wonderful manner indicated by Mr. Mi- 
vart,are as follows. According to our experience, abrupt 
and strongly marked variations occur in our domesti- 
cated productions, singly and at rather long intervals of 
time. If such occurred under nature, they would be 
liable, 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 ab- 
rupt variations of this kind are specially preserved and 
separated by the care of man. Hence in order that a 
new species should suddenly appear in the manner sup- 
posed by Mr. Mivart, it is almost necessary to believe, 
in opposition to all analogy, that several wonderfully 
changed individuals appeared simultaneously within the 
same district. This difficulty, as in the case of un- 
conscious 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 spe- 
cies and even the genera of many large natural families 
are so closely allied together, that it is difficult to dis- 
tinguish not a few of them. On every continent in pro- 
ceeding from north to south, from lowland to upland, 
&e., we meet with a host of closely related or representa- 
tive species; as we likewise do on certain distinct conti- 
nents, 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 inhabi- 
tants 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 with- 
in the same areas; or if we compare the fossil species 
embedded in the sub-stages of the same geological forma- 
tion. 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 developed in an 
abrupt or sudden manner. Nor should it be forgotten, 
when we look to the special parts of allied species, in- 
stead of to distinct species, that numerous and wonder- 
fully 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 in- 
cluded 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 little clusters, like varieties round 
species, and they present other analogies with varieties, 
as was shown in our second chapter. On this same prin- 
ciple we can understand how it is that specific characters 
are more variable than generic characters; and how the 
parts which are developed in an extraordinary degree or 
manner are more 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 geological formations supports at first sight 
the belief in abrupt development. But the value of this 
evidence depends entirely on the perfection of the geo- 
logical record, in relation to periods remote in the his- 
tory 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 de- 
velopment 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 forma- 
tions. 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 insensibly 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 transmitted their 
newly acquired characters to their offspring, at a corres- 
ponding 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 ex- 
tinct forms belonging to the same class. On this view 
of the meaning of embryological resemblances, and in- 
deed on any view, it is incredible that an animal should 


have undergone such momentous and abrupt transforma- 
tions, 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 de- 
veloped by insensibly fine steps. 

He who believes that some ancient form was trans- 
formed suddenly through an internal force or tendency 
into, for instance, one furnished with wings, will be al- 
most compelled to assume, in opposition to all analogy, 
that many individuals varied simultaneously. It can- 
not be denied that such abrupt and great changes of 
structure are widely different from those which most 
species apparently 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 pro- 
duced; and of such complex and wonderful co-adapta- 
tions, he will not be able to assign a shadow of an ex- 
planation. 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. 

Chap. VIIL] LNSTINCT. 319 



Instincts comparable, with habits, but different in their origin — 
Instincts graduated — Aphides and ants — Instincts variable — 
Domestic instincts, 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 develop- 
ment will probably appear to the reader a difficulty suffi- 
cient to overthrow my whole theory. I may here pre- 
mise that I have nothing to do with the origin of the 
mental powers, any more than I have with that of life it- 
self. "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 ac- 
tions are commonly embraced by this term; but every 
one understands what is meant, when it is said that in- 
stinct impels the cuckoo to migrate and to lay her eggs 
in other birds' nests. An action, which we ourselves re- 
quire experience to enable us to perform, when per- 
formed 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 

320 INSTINCT. [Chap. VIIL 

what purpose it. is performed, is usually said to be in- 
stinctive. But I could show that none of these charac- 
ters 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 meta- 
physicians have compared instinct with habit. This 
comparison gives, I think, an accurate notion of the 
frame of mind under which an instinctive action is 
performed, but not necessarily of its origin. How un- 
consciously many habitual actions are performed, in- 
deed not rarely in direct 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 in- 
stincts and habits could be pointed out. As in repeat- 
ing a well-known song, so in instincts, one action fol- 
lows another by a sort of rhythm; if a person be inter- 
rupted 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 eater- 
pillar, which makes a very complicated hammock; for 
if he took a caterpillar which had completed its ham- 
mock 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-performed the fourth, fifth, and 
sixth stages of construction. If, however, a caterpillar 
were taken out of a hammock made up, for instance, 
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, i'' 

Chap. VIII.] iNSTlNOT. 3^1 

was much emtarrassed, 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 al- 
ready finished work. 

If we suppose any habitual action to become in- 
herited — 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 
pianoforte 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 
generation, and then transmitted by inheritance to suc- 
ceeding generations. 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 
species; and if it can be shown that instincts do vary 
ever so little, then I can see no difficulty in natural se- 
lection preserving and continually accumulating vari- 
ations 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 structure 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 

322 INSTINCT. [Chap. VIIL 

that the effects of habit are in many cases of subordinate 
importance 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, varia- 
tions. Hence, as in the case of corporeal structures, we 
ought to find in nature, not the actual transitional gra- 
dations by which each complex instinct has been ac- 
quired — for these could be found only in the lineal an- 
cestors of each species — ^but we ought to find in the col- 
lateral lines of descent some evidence of such gradations; 
or we ought at least to 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 having been but little observed ex- 
cept in Europe and North America, and for no instinct 
being known amongst extinct species, how very gener- 
ally gradations, leading to the most complex instincts, 
can be discovered. Changes of instinct may sometimes 
be facilitated by the same species having different in- 
stincts at different periods of life, or at different seasons 
of the year, or when placed under different circum- 
stances, &c.; 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 spe- 
cies can be shown to occur in nature. 

Again, as in the case of corporeal structure, and con- 
formably 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 

Chap. VIII.] INSTINCT. 323 

the strongest instances of an animal apparently perform- 
ing an action for the sole good of another^ with which I 
am acquainted, is that of aphides voluntarily yielding, 
as was first observed by Huber, their sweet excretion to 
ants: that they do so voluntarily, 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; I then tickled and stroked them with a hair 
in the same manner, as well as I could, as the ants do 
with their antennae; but not one excreted. Afterwards 
I 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 antennae on the abdomen first of one 
aphis and then of another; and each, as soon as it felt 
the antennas, 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 cer- 
tain, 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; there- 
fore 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 
species, yet each tries to take advantage of the instincts 
of others, as each, takes advantage of the weaker bodily 

324 INSTINCT. [Chap. VIII. 

structure of other species. So again certain instincts 
cannot be considered 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 nature, 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 instincts 
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 de- 
pendence 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 remarkable cases of differences 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 ability 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 propolis, 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. Pear of any 
particular enemy is certainly an instinctive quality, 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 I 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 

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 ad- 
duced 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 

The possibility, or even probability, of inherited 
variations of instinct in a state of nature will be strength- 
ened by briefly considering a few cases under domesti- 
cation. We shall thus be enabled to see the part which 
habit and the selection of so-called spontaneous varia- 
tions have played in modifying the mental qualities of 
our domestic animals. It is i;otorious how much domes- 
tic animals vary in their mental qualities. With eats, 
for instance, one naturally takes to catching rats, and 
another mice, and these tendencies are known to be in- 


herited. 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 inheritedl 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 striking instance) 
will sometimes point and even back other dogs the very 
first time that they are taken out; retrieving is cer- 
tainly in some degree inherited by retrievers; and a ten- 
dency to run round, instead of at, a flock of sheep, by 
shepherd dogs. I cannot see that these actions, per- 
formed 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 theyoungpointer can no more know that he 
points to aid his master, than the white butterfly 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 fixed than natural in- 
stincts; but they have been acted on by far less rigorous 
selection, and have been transmitted for an incompar- 
ably shorter period, under less fixed conditions of life. 


How strongly these domestic instincts, habits, and 
dispositions are inherited, and how curiously they be- 
come mingled, is well shown when different breeds of 
dogs are crossed. Thus it is known that a cross with a 
bull-dog has ailected for many generations the courage 
and obstinacy of greyhounds; and a cross with a grey- 
hound has given to a whole family of shepherd-dogs a 
tendency to hunt hares. 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 Eoy 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 ac- 
tions which have become inherited solely from long-con- 
tinued 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-continued selec- 
tion of the best individuals in successive generations 
made tumblers what they now are; and near Glasgow 
there are house-tumblers, 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 occasionally 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 suc- 
cessive generation would soon complete the work; and 
unconscious selection is still in progress, as each man 
tries to procure, without intending to improve the hreed, 
dogs which stand and hunt best. On the other hand, 
habit alone in some cases has sufficed; hardly any ani- 
mal 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 I can hardly suppose that do- 
mestic rabbits have often been selected for tameness 
alone; so that we must attribute at least the greater 
part of the inherited change from extreme wildness to 
extreme tameness, to habit and long-continued close 

Natural instincts are lost under domestication: a 
remarkable instance 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. Familiarity 
alone prevents our seeing how largely and how perma- 
nently the minds of our domestic animals have been 
modified. It is scarcely possible to doubt that the love 
of man has become instinctive in the dog. All wolves, 
foxes, jackals, and species of the eat 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 coun- 
tries such as Tierra del Fuego and Australia, 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; sc that habit and some degree of se- 
lection have probably concurred in civilising by inherit- 
ance 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 instinctive in them; for I am in- 
formed 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 re- 
tained by our chickens has become useless under domes- 
tication, 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 generations, pe- 
culiar 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 suiSced to pro- 
duce inherited mental changes; in other cases, com- 
pulsory habit has done nothing, and all has been the 
result of selection, pursued both methodically and un- 
consciously: but in most eases habit and selection have 
probably concurred. 

330 SPECIAL INSTINCTS. [Chap. Vlll. 

Special Instincts. 

We shall, perhaps, best understand how instincts in 
a state of nature 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 of 
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 Cuckoo. — It is supposed by some 
naturalists that the more immediate cause of the in- 
stinct of the cuckoo is, that she lays her eggs, not daily, 
but at intervals of two or three days; so that, if she were 
to make her own nest and sit on her own eggs, those 
first laid would have to be left for some time unincu- 
bated, or there would be eggs and young birds of dif- 
ferent ages in the same nest. If this were the case, the 
process of laying and hatchiiig might be inconveniently 
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 to- 
gether with a young jay in the nest of a Blue jay (Gar- 
rulus 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 occasionally to lay their eggs in other 
birds' nests. Now let us suppose that the ancient pro- 
genitor 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 ear- 
lier or through any other cause; or if the young were 
made more vigorous by advantage being taken of the 
mistaken instinct of another species 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 us to believe, that the young thus reared would be 
apt to follow by inheritance 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 pro- 
cess of this nature, I believe that the strange instinct of 
our cuckoo has been generated. It has, also, recently 
been ascertained on sufficient evidence, by Adolf Miil- 
ler, 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 European and of the non-parasitic American 
cuckoo alone were known; now, owing to Mr. Eamsay's 


oLservations, 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 vora- 
cious 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 adapta- 
tion 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 instinct, 
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 beneficent ar- 
rangement, 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 generally 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 advan- 
tage to this species to have laid eggs even smaller than 
those now laid, so as to have deceived certain foster- 
parents, or, as is 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. Eam- 
say 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 invari- 
ably 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 Austral- 
ian Bronze cuckoo vary, according to Mr. Eamsay, 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 oifspring 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 hot 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 impor- 
tance 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 ejec- 
tion; 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 
restlessness on the part of the young bird, when some- 
what advanced in age and strength; the habit having 
been afterwards improved, and transmitted to an earlier 
age. I can see no more difficulty in this, than in the un- 
hatched young of other birds acquiring the instinct to 
break through their own shells; — or than in young 
snakes acquiring in their upper jaws, as Owen has re- 
marked, a transitory sharp tooth for cutting through 
the tough egg-shell. For if each part is liable to indi- 
vidual variations at all ages, and the variations tend to 
be inherited at a corresponding or earlier age, — propo- 
sitions 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 se- 

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 perfection of their in- 
stincts. The sexes of Molothrus badius are stated by 
an excellent observer, Mr. Hudson, sometimes to live 
promiscuously 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 fol- 
lowing 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 ascer- 
tained, 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, 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 of 
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 disbeliever in evolution, but he 
appears 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 especially endowed or created instincts, but as 
small consequences of one general law, namely, tran- 
sition? " 

Various birds, as has already been remarked, occa- 
sionally lay their eggs in the nests of other birds. This 
habit is not very uncommon with the Gallinaeese, and 
throws some light on the singular instinct of the ostrich. 


In this family seyeral 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 bona- 
riensis, 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 pollen-collecting apparatus which would 
have been indispensable if they had stored up food for 
their own young. Some species of Sphegidae (wasp-like 
insects) are likewise parasitic; and M. Fabre has lately 
shown good reason for believing that, although the 
Tach}'tes 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 difficulty 
in natural selection making an occasional habit perma- 
nent, 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 discovered in the Formica (Polyerges) ruf escens 


by Pierre Hiihcr, a better observer even than his cele- 
brated father. This ant is absolutely dependent on its 
slaves; without their aid, the species would certainly be- 
come extinct in a single year. The males and fertile fe- 
males do no work of any kind, and the workers or sterile 
females, though most energetic and courageous in cap- 
turing 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 instantly set to work, 
fed and saved the survivors; made some cells and tended 
the larvae, 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 wonder- 
ful 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 mak- 
ing 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 
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 larvae and pupae are exposed, the slaves work ener- 
getically 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 numer- 
ous; 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 1860, 
however, in the month of July, I came across a com- 
munity 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 opportuni- 
ties 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 even- 
ing; and, as Huber expressly states, their principal office 
is to search for aphides. This difference in the usual 
habits of the masters and slaves in the two countries, 
probably depends merely on the slaves being -cap- 
tured in greater numbers in Switzerland than in Eng- 

One day I fortunately witnessed a migration of P. 
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 car- 
ried by them, as in the case of P. rufescens. Another 
day my attention was struck by about a score of the 
slave-makers haunting the same spot, and evidently not 
in search of food; they approached and were vigorously 
repulsed by an independent community of the slave- 
species (F. fusca); sometimes as many as three of these 
ants clinging to the legs of the slave-making F. san- 
guinea. The latter ruthlessly killed their small oppo- 
nents, and carried their dead bodies as food to their 
nest, twenty-nine yards distant; but they were prevent- 
ed 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 combat. 

At the same time I laid on the same place a small 


parcel of the pupsE of another species, F. flava, with a 
few of these little yellow ants still clinging to the frag- 
ments of their nest. This species is sometimes, though 
rarely, made into slaves, as has been described by Mr. 
Smith. Although so small a species, it is very cour- 
ageous, and I have seen it ferociously attack other ants. 
In one instance I found to my surprise an independent 
community of P. flava under a stone beneath a nest of 
the slave-making P. sanguinea; and when I had acci- 
dentally disturbed both nests, the little ants attacked 
their big neighbours with surprising courage. Now I 
was curious to ascertain whether P. sanguinea could dis- 
tinguish the pupee of P. fusca, which they habitually 
make into slaves, from those of the little and furious 
P. 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 pupae of 
P. fusca, whereas they were much terrified 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 

One evening I visited another community of P. 
sanguinea, and found a number of these ants returning 
home and entering their nests, carrying the dead bodies 
of P. fusca (showing that it was not a migration) and 
numerous pups. 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 P. 
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, for two or three 

Chap. Vlll.] SLAVE-MAKllTa INSTINCT. 341 

individuals of F. fusea were rushing about in the great- 
est 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 confir- 
mation by me, in regard to the wonderful instinct of 
making slaves. Let it be observed what a contrast the 
instinctive habits of P. sanguinea present with those of 
the continental P. 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, pos- 
sesses 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 mi- 
grate, the masters carry the slaves. Both in Switzerland 
and England the slaves seem to have the exclusive care 
of the larvae, and the masters alone go on slave-making 
expeditions. 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 larvse. 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 P. sanguinea originated 
I will not pretend to conjecture. But as ants which 
are- not slave-makers will, as I have seen, carry off the 
pupae of other species, if scattered near their nests, it is 
possible that such pupae originally stored as food might 


betoine developed; and the foreign ants thus uninten- 
tionally reared would then follow their proper instincts^ 
and do what work they could. If their presence proved 
useful 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, 
originally for food, might by natural selection be 
strengthened and rendered permanent for the very dif- 
ferent purpose of raising slaves. When the instinct was 
once acquired, if carried out to a much less extent even 
than in our British P. sanguinea, which, as we have 
seen, is less aided by its slaves than the same species in 
Switzerland, natural selection might increase and modify 
the instinct — always supposing each modification to be 
of use to the species — until an ant was formed as ab- 
jectly dependent on its slaves as is the Formica rufes- 

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 re- 
condite 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 skil- 
ful 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 in- 

I was led to investigate this subject by Mr. Water- 
house, who has shown that the form of the cell stands in 
close relation to the presence of adjoining cells; and the 
following view may, perhaps, be considered 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 opposite side. In the series between the extreme 
perfection of the cells of the hive-bee and the simplicity 
of those of the humble-bee we have the cells of the Mex- 
ican 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 

344 SPECUL tl5Slii!TCTS. [Chap.VIIL 

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 necessarily 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 ob- 
vious that the Melipona saves wax, and what is more im- 
portant, 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. 

Eeflecting 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 numter of equal spheres be described with 
their centres placed in two parallel layers; with the cen- 
tre of each sphere at the distance of radius X V ^j "'^ 
radius X 1.41421 (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 
adjoining spheres in the other and parallel layer; then, 
if planes of intersection 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 prisms will have every angle identi- 
cally the same with the best measurements which have 
been made of the cells of the hive-bee. But I hear 
from Prof. Wyman, who has made numerous careful 
measurements, that the accuracy of the workmanship 
of the bee has been greatly exaggerated; so much so, 
that whateTer the typical form of the cell may be, it is 
rarely, if ever, realised. 

Eence we may safely conclude that, if we could 
sKghtly modify the instincts already possessed by the 
Mehpona, and in themselves not very wonderful, this 
bee would make a structure as wonderfully perfect as 
that of the hive-bee. We must suppose that Melipona 
to have the power of forming her cells truly spherical, 
and of equal sizes; and this would not be very surpris- 
ing, seeing that she already does so to a certain extent, 
and seeing what perfectly cylindrical burrows many in- 
sects 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 cylindri- 
cal cells; and we must further suppose, and this is the 
greatest difficulty, that she can somehow judge accu- 


rately at what distance to stand from her fellow-labour- 
ers 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 cer- 
tain extent; and then she unites the points of intersec- 
tion by perfectly flat surfaces. By such modifications 
of instincts which in themselves are not very wonderful, 
— hardly more wonderful than those which guide a bird 
to make its nest, — I believe that the hive-bee has ac- 
quired, through natural selection, her inimitable archi- 
tectural powers. 

But this theory can be tested by experiment. Fol- 
lowing 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 min- 
ute circular pits in it; and as they deepened these little 
pits, they made them wider and wider until they were 
converted into shallow basins, appearing to the eye per- 
fectly true or parts of a sphere, and of about the diam- 
eter 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 dis- 
tance from each other, that by the time the basins had 
acquired the above-stated width (i. 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 ex- 
cavate, and began to build up fiat walls of wax on the 
lines 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 or ordinary cells. 


I then put into the hive, instead of a thick, rectangu- 
lar piece of wax, a thin and narrow, knife-edged ridge, 
coloured with vermilion. The bees instantly began on 
both sides to excavate little 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 experi- 
ment, 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 
deepened, 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 
between 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 neatly 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 intersection. 

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 of wax, perceiving when they 
have gnawed the wax away to the proper thinness, and 
then stopping their work. In ordinary combs it has ap- 
peared 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-completed rhombs at the 


base of a just commenced cell, which were slightly con- 
cave on one side, where I suppose that the bees had ex- 
cavated 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 be(3ome perfectly 
flat: 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 opposite 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 


thijse made by the justly celebrated elder Huber, but 
I am convinced of their accuracy; and if I had 
space, I would show that they are conformable with 
my theory. 

Huber's statement that the very first cell is exca- 
vated out of a little parallel-sided wall of wax, is not, as 
far as I have seen, strictly correct; the first commence- 
ment having always been a little hood of 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 intersection between two ad- 
joining spheres. I have several specimens showing 
clearly that they can do this. Even in the rude circum- 
ferential rim or wall of wax round a growing comb, flex- 
ures may sometimes be observed, corresponding in posi- 
tion to the planes of the rhombic basal plates of future 
cells. But the rough wall of wax has in every case to be 
finished off, by being largely gnawed away on both sides. 
The manner in which the bees build is curious; they al- 
ways make the first rough wall from ten to twenty times 
thicker than the excessively thin finished wall of the cell, 
which will ultimately 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 tnd 
crawl over the comb without injuring the delicate hexrg- 
onal walls. These walls, as Professor Miller has kindly 
ascertained for me, vary greatly in thickness; being, on 
an average of twelve measurements made near the bor- 
der of the comb, -^ of an inch in thickness; whereas 
the basal rhomboidal plates are thicker, nearly in the 
proportion of three to two, having a mean thickness, 
from twenty-one measurements, of ^|^ of an inch. By 
the above singular manner of building, strength is con- 
tinually given to the comb, with the utmost ultimate 
economy of wax. 

It seems at first to add to the difficulty of under- 
standing 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 commencement 
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 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 intersection 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 

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 slip— 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 com- 
pleted 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 intermediate between two adjoining spheres; but, 
as far as I have seen, they never gnaw away and finish pflE 
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-com- 
menced cells, is important, as it bears on a fact, which 
seems at first subversive of the foregoingtheory; 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 time, always standing at 
the proper relative distance from the parts of the cells 
just begun, sweeping spheres or cylinders, and building 
up intermediate planes. 


**"SB-aist*iral selection acts only by the accumulation 
of slight modifications 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 diificult: 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 suffi- 
cient nectar, and I am informed by Mr. Tegetmeier that 
it has been experimentally proved that from twelve to 
fifteen pounds of dry sugar are consumed by a hive of 
bees for the secretion of a pound of wax; so that a pro- 
digious quantity of fluid nectar must be collected and 
consumed by the bees in a hive for the secretion of the 
wax necessary for the constru'ction 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 sup- 
ported. 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 species may be de- 
pendent on the number of its enemies, or parasites, or 
on quite distinct causes, and so be altogether independ- 
ent of the quantity of honey which the bees can collect. 
But let us suppose that this latter circumstance deter- 
mined, as it probably often has determined, whether a 


boe allied to our humble-bees could exist in large num- 
bers in any country; and let us further suppose that the 
community lived tlirough 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 inter- 
sect a little; 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 advantageous to 
our humble-hees, if they were to make their cells more 
and more regular, nearer together, and aggregated into 
a mass, like the cells of the Mehpona; for in this case a 
large part of the bounding surface of each cell would 
serve to bound the adjoining cells, and much lahour and 
wax would be saved. Again, from the same cause, it 
would he advantageous to the Melipona, if she were to 
make her cells closer together, and more regular in every 
waythan at present; for then, as we have seen, the spheri- 
cal surfaces would wholly disappear and he replaced hy 
plane surfaces; and the Melipona would make a comh 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 inter- 


section; the bees, of course, no more knowing that they 
swei^t 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 
instincts to new swarms, which in their turn will have 
had the best chance oi succeeding in the struggle for 

Oljedions to the Theory of Natural Selection- as applied 
to Instincts: Neuter and Sterile Insects. 

It has been objected to the foregoing view of. the ori- 
gin of instincts that " the variations of structure and of 
instinct must have been simultaneous and accurately 
adjusted to each other, as a modification in the one 
without an immediate corresponding change in the 
other would have been fatal." The force of this objec- 
tion 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 variations 
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 compul- 
sion, 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 selec- 
tion, 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 in- 
stinct and power of the nuthatch. In this case a grad- 
ual 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 species 
with instincts leading it to neglect other materials, and 
to make its nest exclusively of inspissated saliva? Ani 
so in other eases. 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 explana- 
tion could be opposed to the theory of natural selec- 
tion — cases, in which we cannot see how an instinct 
could have originated; cases, in which no intermediate 


gradations are known to exist; cases of instincts of such 
trifling importance, that they could hardly have been 
ccted 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 vfere independently ac- 
quired 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 insuper- 
able, and actually fatal to the whole theory. I allude 
to the neuters or sterile females in insect-communi- 
ties; for these neuters often differ widely in instinct and 
in structure from both the males and fertile females, 
and yet, from being sterile, they cannot propagate their 

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 struc- 
ture; for it can be shown that some insects and other 
articulate animals in a state of nature occasionally be- 
come sterile; and if such insects 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 especial difficulty in 
this having been effected through natural selection. 
But I must pass over this preliminary difficulty. The 
great difficulty lies 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 as- 
sumed that all its characters had been slowly acquired 
through natural selection; namely, by individuals hav- 
ing been bom with slight profitable modifications, 
which were inherited by the offspring; and that these 
again varied and again were selected, and so onwards. 
But with the working 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 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 innumera- 
ble instances, both in our domestic productions and in 
those in a state of nature, of all sorts of differences of in- 
herited 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 reproductive system is active, as in the nuptial 
plumage of many birds, and in the hooked jaws of the 
male salmon. We have even slight differences in the 
horns of different breeds of cattle in relation to an arti- 
ficially imperfect state of the male sex; for oxen of cer- 
tain 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 correlated 
with the sterile condition of certain members of insect - 
communities: the difficulty lies in understanding how 


such correlated 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 remem- 
bered 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 cattle, always yielding oxen with extraordi- 
narily long horns, could, it is probable, be formed by 
carefully watching which individual bulls and cows, 
when matched, produced oxen with the longest horns; 
and yet no ox would ever have propagated its Mnd. 
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 like- 
wise yield some single and fertile plants. These latter, 
by which alone the variety can be propagated, may be 
compared with the fertile male and female ants, and the 
double sterile plants with the neuters of the same com- 
munity. As with the varieties of the stock, so with so- 
cial 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 modi- 
fications of structure or of instinct, correlated with 
the sterile condition of certain members of the com- 
munity, have proved advantageous: consequently the 
fertile males and females have flourished, and trans- 


mitted 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 fer- 
tile and sterile females of the same species has been pro- 
duced, 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 gradu- 
ate into each other, but are perfectly well defined; be- 
ing as distinct from each other as are 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 dif- 
ferent: 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 Myrmeco- 
cystus, 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, supplying 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 succes- 
sive, slight, profitable modifications 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 ad- 
vantageous modification, all the neuters ultimately came 
to be thus characterised. According to this view we 
ought occasionally to find in the same nest neuter in- 
sects, 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 differ 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 happens that the larger or the 
smaller sized workers are the most numerous; 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 rudimen- 
tary. 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 ac- 
counted for merely by their proportionally lesser size; 
and I fully believe, though I dare not assert so posi- 
tively, that the workers of intermediate size have their 
ocelli in an exactly intermediate condition. 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 inter- 
mediate condition. I may digress by adding, that if the 
smaller workers had been the most useful to the com- 
munity, and those males and females had been continu- 
ally selected, which produced more and more of the 
smaller workers, until all the workers were in this con- 
dition; we should then have had a species of ant with 
neuters in nearly the same condition as those of Myr- 
miea. For the workers of Myrmica have not even rudi- 
ments 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 ex- 
pect occasionally to find gradations of important struc- 
tures between the different castes of neuters in the same 
species, that I gladly availed myself of Mr. F. Smith's 
offer of numerous specimens from the same nest of the 
driver ant (Anomma) of West Africa. The reader will 
perhaps best appreciate the amount of difference in 
these workers, by my giving not the actual measure- 
ments, but a strictly accurate illustration: 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, more- 
over, 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 dif- 
ferent 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 ' JSTaturalist 
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 difRculty, one set of workers of one size 
and structure, and simultaneously another set of workers 
of a different size and structure; — 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 appearing under two or even three 
distinct female forms; and by Fritz Miiller, of certain 
Brazilian crustaceans likewise appearing under two wide- 
ly distinct male forms. But this subject need not here 
be discussed. 

I have now explained how, as I believe, the wonder- 
ful 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 to a 
social community of ants, on the same principle that 
the division of labour is useful to civilised man. Ants, 
however, work by inherited instincts and by inherited 

Chap. VIII.] SUMMARY. 363 

organs or tools, whilst man works by acquired knowl- 
edge and manufactured instruments. But I must con- 
fess, 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 encoun- 
tered. The case, also, is very interesting, as it proves 
that with animals, as with plants, any amount of modi- 
fication may be effected by the accumulation of numer- 
ous, slight, spontaneous variations, which are in any way 
profitable, without exercise or habit having been brought 
into play. For peculiar habits confined to the workers 
or sterile females, however long they might be followed, 
could not possibly affect 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 in- 
herited habit, as advanced by Lamarck. 


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. There- 
fore there is no real difficulty, under changing condi- 
tions of life, in natural selection accumulating to any ex- 

364 SUMMARY. [Chap. VIIl. 

tent 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 de- 
gree 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 advantage of the instincts 
of others; — that the canon in natural history, of " Na- 
tura non faeit saltum," is applicable to instincts as well 
as to corporeal structure, and is plainly explicable on 
the foregoing views, but is otherwise inexplicable, 
— all tend to corroborate the theory of natural selec- 

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 dis- 
tant 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 im- 
prisoning 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 

Chap. VIII.] SUMMARY. 365 

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 ichneu- 
monidse feeding within the live bodies of caterpillars, — 
not as specially 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. 








Distinction between the sterility of first crosses and of hybrids — 
Sterility various in degree, not universal, aSected by close in- 
terbreeding, removed by domestication — Laws governing the 
sterility of hybrids — Sterility not a special endowment, but 
incidental on other differences, 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 trimorphism — Fertility of 
varieties when crossed and of their mongrel offspring not uni- 
versal — Hybrids and mongrels compared independently of their 
fertility — Summary Page 1 



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 deposition — On the lapse of time as estimated by years 
— On the poorness of our pal»ontological collections — On the 
intermittence of geological formations — On the denudation of 
granitic areas — On the absence of intermediate varieties in any 
one formation — On the sudden appearance of groups of species 
— On their sudden appearance in the lowest known fossiltferous 
strata — Antiquity of the habitable earth .... 48 




On the slow and successive appearance of new species — On their 
different rates of change — Species once lost do no'', reappear — 
Groups of species follow the same general rules in their ap- 
pearance and disappearance 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 . . Page 89 



Present distribution cannot be accounted for by differences ii> 
physical conditions — Importance of barriers — Affinity of tha 
productions of the same continent — Centres of creation — Means 
of dispersal by changes of climate and of the level of the land- 
and by occasional means — Dispersal during the Grlacial period 
— Alternate Qlacial periods in the North and South . . 124 


Distribution of fresh-water productions — On the inhabitants of 
oceanic islands — Absence of Batrachians and of terrestrial Mam- 
mals—On the relation of the inhabitants of islands to those of 
the nearest mainland — On colonisation from the nearest source 
with subsequent modification — Summary of the last and present 
chapter 171 



mutual affinities of organic beings t moepholoot : 
embryology: rudimentary organs. 

Classification, groups subordinate to groups — Natural system — 
Rules and diflSculties in classification, explained on the theory 
of descent with modification — Classification of varieties — De- 
scent always used in classification — Analogical or adaptive char- 
acters—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 super- 
vening at an early age, and being inherited at a correspond- 
ing age — Rudimentary organs : their prigin explained — Sum- 
mary Page 202 


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 ex- 
tended — Effects of its adoption on the study of Natural History 
— Concluding remarks ....... 267 

Glossary of Scientific Terms 307 

2ndez •!••• 323 




Distinction between the sterility of first crosses and of hybrids^ 
Sterility various in degree, not universal, affected by close in- 
terbreeding, removed by domestication — Laws governing the 
sterility of hybrids — Sterility not a special endowment, but 
incidental on other differences, 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 trjmorphism — Fertility of 
varieties when crossed and of their mongrel offspring not uni- 
versal — Hybrids and mongrels compared independently of their 
fertility — Summary. 

The view commonly entertained by naturalists is 
that species, when intercrossed, have been specially en- 
dowed with sterility, in order to prevent their confusion. 
This view certainly seems at first highly probable, for 
species living together could hardly have l)een kept dis- 
tinct had they been capable of freely crossing. The 
subject is in many ways important for us, more especia l-^ 
ly as the sterility _of sp ecies when firs t_ crossed,_ andthat 
"oTtheir hybrid.afifspringj^cannot have been acquiredT^T" 
shall show, by the preservation of suceissTve ^roStabl? 

2 HYBRIDISM. [Chap. IX. 

degrees of sterility. It js an incidental^ resjilt_of diir 
lereSiesTnTEe~reproductiYe systfiin.s_ of _the_ Earent^ 

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 reproduc- 
tion 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 per- 
fect; in the second case they are either not at all de- 
veloped, or are imperfectly developed. This distinc- 
tion is important, 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 en- 
dowment, beyond the province of our reasoning 

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 off- 
spring, is, with reference to my theory, of equal im- 
portance with the sterility of species; for it seems to 
make a broad and clear distinction between varieties 
and species. 

Degrees of Sterility. — First, for the sterility of spe- 


cies when crossed and of their hybrid offspring. It is 
impossible to study the several memoirs and works of 
those two conscientious and admirable observers, K61- 
reuter 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 eases in which he found two forms, considered 
by most authors as distinct species, quite fertile to- 
gether, he unhesitatingly ranks them as varieties. Gart- 
ner, also, makes the rule equally universal; and he dis- 
putes 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 compares 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 
both 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 
important, 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 (ex- 
cluding all cases such as the Leguminosae, in which there 
is an acknowledged difficulty in the manipulation) 
half of these twenty plants had their fertility in some 
degree impaired. Moreover, as Gartner repeatedly 

4 HYBRIDISM. [Chap. IX. 

crossed some forms, such as the common red and 
blue pimpernels (Anagallis arvensis and coerulea), 
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 be- 

It is certain, on the one hand, that the sterility of 
various species when crossed is so different in degree 
and graduates away so insensibly, and, on the other 
hand, that the fertility of pure species is so easily 
affected by various circumstances, that for all practical 
purposes it is most difficult to say where perfect fertility 
ends and sterility begins. I think no better evidence 
of this can be required than that the two most ex- 
perienced observers who have ever lived, namely Kol- 
reuter 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 on 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 certain 
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 con- 
stitutional 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 constitu- 
tion 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 di- 
minished in nearly all these cases by an independent 
cause, namely, by too close interbreeding. I have made 
so many experiments and collected so many facts, show- 
ing on the one hand that an occasional cross with a dis- 
tinct individual or variety increases the vigour and fer- 
tility 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 injurious 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 
fertility, notwithstanding the frequent ill effects from 
manipulation, sometimes decidedly increases, and goes 
on increasing. N^ow, in the process of artificial fertilisa- 
tion, pollen is as often taken by chance (as I know from 
my own experience) from the anthers of another flower. 

6 HYBRIDISM. [Chap. IX. 

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 genera- 
tions of artificially fertilised hybrids, in contrast with 
those spontaneously self-fertilised, may, as I believe, be 
accounted for by too close interbreeding having been 

Now let us turn to the results arrived at by a third 
most experienced hybridiser, namely, the Hon. and Eev. 
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 having 
hot-houses at his command. Of his many important 
statements I will here give only a single one as an ex- 
ample, 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 fecun- 
dation." 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, Verbascum and Passiflora, can easily 
be fertilised by pollen from a distinct 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 Cory- 
dalis as shown by Professor Hildebrand, in various or- 
chids 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 produced 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 
perished entirely, whereas the pod impregnated by the 
pollen of the hybrid made vigorous growth and rapid 
progress to maturity, and bore good seed, which vege- 
tated freely." 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 some- 
times 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, Calceolaria, Petunia, Rhodo- 
dendron, &c., have been crossed, yet many of these 
hybrids seed freely. For instance, Herbert asserts that 

8 HYBRIDISM. [Chap. IX. 

a hybrid from Calceolaria integrifolia and plantaginea, 
species most widely dissimilar in general habit, " re- 
produces itself as perfectly as if it had been a natural 
species from the mountains of Chili." I have taken 
some pains to ascertain the degree of fertility of some 
of the complex crosses of Ehododendrons, and I am 
assured that many of them are perfectly fertile. Mr. 
C. ISToble, 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 believed to be the 
case, the fact would have been notorious to nursery- 
men. Horticulturists raise large beds of the same hy- 
brid, and such alone are fairly treated, for by insect 
agency the several individuals are allowed to cross freely 
with each other, and the injurious influence 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 hy- 
brid Rhododendrons, which produce no pollen, fof 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 dis- 
tinct species of finches, but, as not one of these breeds 
freely in confinement, 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 in- 
stance 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 interbreeding. On the 
contrary, brothers and sisters have usually been crossed 
in each successive generation, in opposition to the con- 
stantly repeated admonition of every breeder. And in 
this case, it is not at all surprising that the inherent 
sterility in the hybrids should have gone on increas- 

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 Eeevesii, and from Phasianus colchicus 
with P. torquatus, are perfectly fertile. M. Quatrefages 
states that the hybrids from two moths (Bombyx cyn- 
thia and arrindia) were proved in Paris to be fertile inter 
se for eight generations. It has lately been asserted 
that two such distinct species as the hare and rabbit, 
when they can be got to breed together, produce off- 
spring, 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 int$r 

10 HYBRIDISM. [Chap. IX 

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 I am assured by two eminently 
capable judges, namely Mr. Blyth and Capt. 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 species. 
From this fact we must conclude either that the abo- 
riginal parent-species at first produced perfectly fertile 
hybrids, or that the hybrids subsequently reared under 
domestication became quite fertile. This latter alter- 
native, 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 sev- 
eral aboriginal species would at first have freely bred 
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 ob- 
servations by Eiitimeyer on their important osteologieal 
differences, as well as from those by Mr. Blyth on their 
differences in habits, voice, constitution, &c., 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 characteristic, 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 con- 
sidered as absolutely universal. 

Laws governing the Sterility of first Crosses and of 

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 ad- 
mirable work on the hybridisation 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 king- 

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 spe- 
cies applied to the stigma of some one species of the 
same genus, yields a perfect gradation in the number 
of seeds, produced, 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 with- 
ering of the flower is well known to be a sign of in- 
cipient fertilisation. From this extreme degree of 
sterility we have self-fertilised hybrids producing a 
greater and greater number of seeds up to perfect fer- 

The hybrids raised from two species which are very 
difficult to cross, and which rarely produce any off- 
spring, 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 generailly 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 unfavourable 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 constitution 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 condi- 

By the term systematic affinity is meant, the general 
resemblance between species in structure and constitu- 
tion. 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 hy- 
brids never having been raised between species ranked 
by systematists in distinct families; and on the other 
hand, by very closely allied species generally uniting 
with facility. But the correspondence between syste- 
matic affinity and the facility of crossing is by no means 
strict. A multitude of cases could be given of very 
closely allied species which will not unite, or only with 
extreme difficulty; and on the other hand of very dis- 
tinct species which unite with the utmost facility. 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 perse- 
vering efforts have failed to produce between extremely 
close species a single hybrid. Even within the limits 
of the same genus, we meet with this same difference; 
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 species, obstinately failed 
to fertilise, or to be fertilised by no less than eight other 
species of Nicotiana. Many analogous facts could be 

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. An- 
nual and perennial plants, deciduous and evergreen trees, 
plants inhabiting different stations and fitted for ex- 
tremely different climates, can often be crossed with 

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 
species 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 independ- 
ent of their systematic affinity, that is of any differ- 
ence in their structure or constitution, excepting in 


their reproductive systems. The diversity of the result 
in reciprocal crosses between the same tw^o species was 
long ago observed by Kolreuter. To give an instance: 
Mirabilis jalapa can easily be. fertilised by the pollen of 
M. longiflora, and the hybrids thus produced are suffi- 
ciently fertile; but Kolreuter tried more than two hun- 
dred times, during eight following years, to fertilise 
reciprocally M. longiflora with the pollen of M. jalapa, 
and utterly failed. S&veral 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 recipro- 
cal crosses is extremely common in a lesser degree. He 
has observed it even between closely related forms (as 
Matthiola annua and gilabra) which many botanists 
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 species, the one spe- 
cies having first been used as the father and then as 
the mother, though they rarely differ in external char- 
acters, yet generally differ in fertility in a small, and oc- 
casionally 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 fertility 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 eminently susceptible to favourable and unfa- 
vourable 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 syste- 
matic affinity or degree of resemblance to each other. 
This latter statement is clearly proved by the differ- 
ence 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 gen- 
erally some difference, and occasionally the widest pos- 
sible 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 suppose it would be equally im- 
portant to keep from blending together? Why should 
the degree of sterility be innately variable in the in- 
dividuals of the same species? Why should some spe- 
cies 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 re- 
ciprocal 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, seems 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 lim- 
ited 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 be ad- 
visable to explain a little more fully by an example what 
I mean by sterility being incidental on other differences, 
and not a specially endowed quality. As the capacity 
of one plant to be grafted or budded on another is un- 
important for their welfare in a state of nature, I pre- 


sume that no one will suppose that this capacity is a 
specially endowed quality, but will admit that it is inci- 
dental on differences in the laws of growth of the two 
plants. We can sometimes 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, &e.; but in a multitudo 
of cases we can assign no reason whatever. Great di- 
versity in the size of two plants, one being woody and 
the other herbaceous, one being evergreen and the other 
deciduous, and adaptation to widely different climates, 
do not always prevent the two grafting together. As in 
hybridisation, so with grafting, the capacity is limited by 
systematic 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 
species 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 ap- 
ple, which is a member of the same genus. Even dif- 
ferent varieties of the pear take with different de- 
grees of facility on the quince; so do different varie- 
ties of the apricot and peach on certain varieties of the 

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 spe- 



eies in being grafted together. As in reciprocal crosses, 
the facility of eflecting an union is often very far from 
equal, so it sometimes is in grafting; the common goose- 
berry, for instance, cannot be grafted on the currant, 
whereas the currant will take, though with difiaculty, on 
the gooseberry. 

We have seen that the sterility of hybrids, which 
have their reproductive organs in an imperfect con- 
dition, is a different case from the difliculty of uniting 
two pure species, which have their reproductive 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 
Eobinia^ 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 Hippeastrum, 
Passifiora, &c., which seed much more freely when fertil- 
ised 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 dis- 
tinct 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 differ- 
ences in their vegetative systems, so I believe that the 
still more complex laws governing the facility of first 


crosses are incidental on unknown diflEerences 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 resem- 
blance and dissimilarity between organic beings is at- 
tempted to be expressed. The facts by no means seem 
to indicate that the greater or lesser difficulty of either 
grafting or crossing various species has been a special 
endowment; although in the case of crossing, the diffi- 
culty is as important for the endurance and stability of 
specific forms, as in the case of grafting it is unimpor- 
tant 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 an3"of h^Sds. 
might have Tieenslowly acquired "through the natural 
selection of sliglitly lessened degrees of' fertility, which, 
like any other variation, spontaneously appeared in cer- 
tain individuals of one variety when crossed with those 
of another variety. For it would clearly be advantage- 
ous to two varieties 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 sepa- 
rated 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 com- 
patriot, 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 ad- 
vantageous to either species. 

In considering the probability of natural selection 
having come into action, in rendering species mutually 
sterile, the greatest difficulty will be found to lie in the 
existence of many graduated steps from slightly lessened 
fertility to absolute sterility. It may be admitted that_^ 
it would profit an incipienL speGies, if it were rendered 
in some slight degree sterile when crossed with its 

-parent form or with some other variety; for thus fewer 
"ibastardrsed'^and deteriorated ofEspring would be pro- 
duced to commingle their blood with the new species in 
process^F Rrmation. But he who will take the trouble 

"lo 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 selection. Take the case of 
any two species 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 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 modifica- 
tions 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 so- 
cial community, if rendered slightly sterile when crossed 
with some other variety, would not thus itself gain anj 
advantage or indirectly give any advantage to the other 
individuals of the same variety, thus leading to their 

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 in- 
cluding numerous species, a series can be formed from 
species 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 through- 
out 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 na- 
ture of the differences between species which induce 
sterility in first crosses and in hybrids. In the case of 
first crosses, the greater or less diificulty 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 of one spe- 
cies 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 
jvhy certain trees cannot be grafted on others. Lastly, 
an embryo may be developed, and then perish at an early 
period. This latter alternative has not been sufficiently 
attended to; but I believe, from observations communi- 
cated 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 eitlier been partially developed and had then per- 


isfiJcl;- ut^-iad. become nearly mature, but the young 
chickens had been unable to break through the shell. 
Of the chickens which were born, more than four-fifths 
died within the first few days, or at latest weeks, " with- 
out any obvious cause, apparently from mere inability 
to live; " so that from the 500 eggs only twelve chick- 
ens were reared. With plants, hybridised embryos prob- 
ably often perish in a like manner; at least it is known 
that hybrids raised from very distinct species are some- 
times 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 pro- 
duced by a cross between distinct species. Until becom- 
ing acquainted with these facts, I was unwilling to be- 
lieve 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. Hy- 
brids, however, are differently 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 par- 
takes of only half of the 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 condi- 
tions in some degree unsuitable, and consequently be 
liable to perish at an early period; 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 imper- 
fectly developed, rather than in the conditions to which 
it is subsequently exposed. 

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 sterility is inde- 
pendent of general health, and is often accompanied 
by excess of size or great luxuriance. In both cases the 
sterility occurs in various degrees; in both, the male 
element is the most liable to be affected; but some- 
times the female more than the male. In' both, the 
tendency goes to a certain extent with systematic affin- 
ity, for whole groups of animals and plants are rendered 
impotent by the same unnatural conditions; and whole 
groups of species tend to produce sterile hybrids. On 
the other hand, one species in a group will sometimes 
resist great changes of conditions with unimpaired fer- 
tility; and certain species in a group will produce un- 
usually fertile hybrids. No one can tell, till he tries, 
whether any particular animal will breed under confine- 
ment, 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 conditions 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 generations 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 unnatural crossing of two species, 
the reproductive system, independently of the general 
state of health, is affected in a very similar manner. 
In the one case, the conditions of life have been dis- 
turbed, though often in so slight a degree as to be in- 
appreciable 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 
reproductive systems, having been blended into one. 
For it is scarcely possible that two organisations should 
be compounded into one, without some disturbance 
occurring in the development, or periodical 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 off- 
spring from generation to generation the same com- 
pounded organisation, and hence we need not be sur- 
prised that their sterility, though in some degree varia- 
ble, 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 com- 
pounded into one has been strongly maintained by Max 


It must, however, be owned that we cannot under- 
stand, 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 recipro- 
cal crosses; or the increased sterility in those hybrids 
which occasionally and exceptionally resemble closely 
either pure parent. Nor do I pretend that the fore- 
going remarks go to the root of the matter; no explana- 
tion is offered why an organism, when placed under un- 
natural conditions, is rendered sterile. All that I have 
attempted to show is, that in two cases, in some respects 
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 dis- 
turbed bytwo organisations being compounded into one. 

A similar parallelism holds good with an allied yet 
very different class of facts. It is an old and almost 
universal belief founded on a considerable body of evi- 
dence, 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, &c., from one soil 
or climate to another, and back again. During the con- 
valescence of animals, great benefit is derived from al- 
most 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 fer- 
tility to the offspring; and that close interbreeding 
continued during several generations between the near- 
est 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 conditions of life benefit all organic be- 
ings, and on the other hand, that slight crosses, that is 
crosses between the males and females of the same spe- 
cies, which have been subjected to slightly different 
conditions, or which have slightly varied, give vigour 
and fertility to the offspring. But, as we have seen, or- 
ganic beings long habituated to certain uniform condi- 
tions under a state of nature, when subjected, as under 
confinement, to a considerable change in their condi- 
tions, 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 persuaded 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 gen- 
erally sterile. He will at the same time be able to ex- 
plain 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, al- 
though 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 con- 
nected together by some common but unknown bond, 
which is essentially related to the principle of life; this 
principle, according to Mr. Herbert Spencer, 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 equilibrium; and when 


this tendency is slightly disturbed by any change, the 
vital forces gain in power. 

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 dif- 
ferently 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 possess altogether six sets of stamens 
and three kinds of pistils. These organs are so pro- 
portioned in length to each other, that half the sta- 
mens 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 cor- 
responding height in another form. So that with di- 
morphic species two unions, which may be called legiti- 
mate, are fully fertile; and two, which may be called 
illegitimate, are more or less infertile. With trimor- 
phic species six unions are legitimate, or fully fer- 
tile, — and twelve are illegitimate, or more or less infer- 


The infertility which may be observed in various 
dimorphic and trimorphic plants, when they are il- 
legitimately fertilised, that is by pollen taken from 
stamens not corresponding in height with the pistil, 
differs much in degree, up to absolute and utter steril- 
ity; just in the same manner as occurs in crossing dis- 
tinct 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 distinct species be placed on the stigma of a 
flower, and its own pollen be afterwards, even after a 
considerable interval of time, placed on the same stigma, 
its action is so strongly prepotent that it generally anni- 
hilates 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 il- 
legitimately, and twenty-four hours afterwards legiti- 
mately with the pollen taken from a peculiarly coloured 
variety, and all the seedlings were similarly coloured; 
this shows that the legitimate pollen, though applied 
twenty-four hours subsequently, had wholly destroyed 
or prevented the action of the previously applied il- 
legitimate pollen. Again, as in making reciprocal 
crosses between the same two species, there is occasion- 
ally a great difference in the result, so the same thing 
occurs with trimorphic plants; for instance, the mid- 
styled form of Lythrum saliearia was illegitimately fer- 
tilised 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 ■ 

In all these respects, and in others which might be 
added, the forms of the same undoubted species when 
illegitimately united behave 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 re- 
sult 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-capsule. The sterility of these 
illegitimate plants, when united with each other in ? 
legitimate manner, may be strictly compared with that 
of hybrids when crossed inter se. If, on the other hand, 
a hybrid is crossed with either pure parent-species, the 
sterility 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. Lastly, many hybrids are profuse and persistent 
flowerers, whilst other and more sterile hybrids pro- 
duce few flowers, and are weak, miserable dwarfs; 
exactly similar cases occur with the illegitimate off- 
spring 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 between so-called distinct species. We have also 
already seen that there is the closest similarity in all re- 
spects 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 
proper number of seeds, and that they behaved in all 
the other above specified respects as if they had been two 
distinct species. But to make the case sure, he would 
raise plants from his supposed hybridised 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 main- 
tain 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 criterion of specific 
distinction; secondly, because we may conclude that 
there is some unknown bond which connects the in- 
fertility of illegitimate unions with that of their illegiti- 
mate 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 species 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 distinct species. It is, however, doubt- 
ful whether this is really so; but I will not enlarge on 
this obscure subject. 

We may, however, infer as probable from the con- 
sideration 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 efEected 
with perfect facility. That excellent observer, Gart- 
ner, likewise concluded that species when crossed are 
sterile owing to diSerences confined to their reproduc- 
tive 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 sub- 
ject is surrounded by difficulties, for, looking to varie- 
ties produced under nature, if two forms hitherto re- 
puted to be varieties be found in any degree sterile to- 
gether, they are at once ranked by most naturalists 
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 subsequently 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 

If we turn to varieties, produced, or supposed to have 
been produced, under domestication, we are still in- 
volved in some doubt. For when it is stated, for in- 
stance, that certain South American indigenous domes- 
tic dogs do not readily unite with European dogs, the 
explanation which will occur to every one, and probably 

Chap. IX.] WHEN CROSSED. 85 

the true one, is that they are descended from ahorigi- 
nally distinct species. Nevertheless the perfect fertil- 
ity 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 es- 
pecially when we reflect how many species there are, 
which, though resembling each other most closely, are 
utterly sterile when intercrossed. Several considera- 
tions, 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 ho sure guide to their degree of mutual steril- 
ity, 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 modify- 
ing the reproductive system in a manner leading to 
mutual sterility, that we have good grounds for admit- 
ting the directly opposite doctrine of Pallas, namely, 
that such conditions generally eliminate 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 fer- 
tile 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 op- 
posite manner, for they have become self-impotent 
whilst still retaining the capacity of fertilising, and 
being fertilised by, other species. If the Pallasian doc- 
trine of the elimination of sterility through long-con- 


tinned domestication be admitted, and it can hardly 
be rejected, it becomes in the highest degree improbable 
that similar conditions long-continued should likewise 
induce this tendency; though in certain cases, with 
species having a peculiar constitution, sterility might 
occasionally 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, immediately 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 species, owing to their struggle for ex- 
istence 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 man- 
ner be eminently sensitive to the influence of an un- 
natural cross. Domesticated productions, on the other 
hand, which, as shown by the mere fact of their domesti- 
cation, were not originally highly sensitive to changes 

Chap. IX.] WHEN CROSSED. 37 

in their conditions of life, and which can now generally 
resist with undiminished fertility repeated changes of 
conditions, might he 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 originated 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 
at least as good as that from which we believe in the 
sterility of a multitude of species. The evidence is, 
also, derived from hostile witnesses, who in all other 
cases consider fertihty 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 
produced only five grains. Manipulation in this case 
could not have been injurious, as the plants have sepa- 
rated sexes. No one, I beHeve, has suspected that these 
varieties of maize are distinct species; and it is impor- 
tant to notice that the hybrid plants thus raised were 
themselves perfectly fertile; so that even Gartner did 
not venture to consider the two varieties as specifically 

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. 

Th? following case is far more remarkable, and seems 
at first incredible; but it is the result of an astonishing 
number of experiments 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 varie- 
ties 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 Gart- 
ner's results on the crossing of the distinct species, he 
finds that the dissimilarly coloured varieties of the 
same species yield fewer seeds, in the proportion of 86 
to 100, than the similarly coloured varieties. 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 tobacco 
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 

Chap. IX.] WHEN CROSSED. 39 

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. 
Prom 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 attend- 
ing only to external characters in his domestic varieties, 
and from such varieties not having been exposed for 
very long periods to uniform conditions of life; — from 
these several considerations we may conclude that fer- 
tility does not constitute a fundamental distinction be- 
tween varieties and species when crossed. The gen- 
eral sterility of crossed species may safely be looked at, 
not as a special acquirement or endowment, but as in- 
cidental on changes of an unknown nature in their sex- 
ual elements. 

Hybrids and Mongrels compared, independently of their 

Independently of the question of fertility, the off- 
spring 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 spe- 
cies and varieties, could find very few, and, as it seems 


to ine, quite unimportant differences between the so- 
called hybrid offspring of species, and the so-called 
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 species are more 
variable than those from very distinct species; and 
this shows that the difference in the degree of variabil- 
ity graduates away. When mongrels and the more 
fertile hybrids are propagated for several generations, 
an extreme amount of variability in the offspring in 
both cases is notorious; but some few instances of both 
hybrids and mongrels long retaining a uniform charac- 
ter 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 generations, 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 con- 
ditions 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 hy- 
brids in the first generation are descended from spe- 
cies (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 reproductive 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 parent-form; but this, 
if it be true, is certainly only a difference in degree. 
Moreover, Gartner expressly states thai 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 experimented 
chiefly on cultivated plants. Gartner further states 
that when any two species, 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 species, the hybrids do not differ much. 
But this conclusion, as far as I can make out, is 
founded on a single experiment; and seems directly 


V??*«*4-tftJiLe. results of several experiments made by 

Such alone are the unimportant differences which 
Gartner is able to point out between hybrid and mon- 
grel 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 apparently applicable to 
animals; but the subject is here much complicated, 
partly owing to the existence of secondary sexual char- 
acters; but more especially owing to prepotency in 
transmitting likeness running more strongly in one sex 
than in the other, both when one species is crossed with 
another, and when one variety is crossed with another 
variety. For instance, I think those authors are right 
who maintain that the ass has a prepotent power over 
the horse, so that both the mule and the hinny resemble 
more closely the ass than the horse; but that the pre- 
potency 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 

Much stress has been laid hy some authors on the 
supposed fact, 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 selection. A tendency to sud- 
den 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 sud- 
denly produced and semi-monstrous in character, than 
with hybrids, which are descended 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 conclusion 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 species. 

Independently of the question of fertility and steril- 
ity, in all other respects there seems to be a general and 
close similarity in the offspring of crossed species, and 
of crossed varieties. If we look at species as having 
been specially created, and at varieties as having been 

4:4: SUMMARY. [Chap. IX 

pfoducetf' "Sy-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. 

Summary of Chapter. 

First crosses between forms, sufficiently distinct to 
be ranked as species, and their hybrids, are very gen- 
erally, 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 spe- 
cies, and is eminently susceptible to the action of fa- 
vourable 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 re- 
ciprocal crosses between 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 capac- 
ity in one species or variety to take on another, is inci- 
dental on differences, generally of an unknown nature, 
in their vegetative sjfstems, so in crossing, the greater 
or less facility of one species to unite with another is in- 
cidental on unknown differences in their reproductive 
systems. There is no more reason to think that species 
have been specially endowed with various degrees of 
steriHty to prevent their crossing and blending in na- 
ture, than to think that trees have been specially en- 
dowed with various and somewhat analogous degrees of 

Chap. IX.] SUMMARY. 45 

difficulty in being grafted together in order to preTent 
their inarching in our forests. 

The sterility of first crosses and of their hybrid 
progeny has not been acquired through natural se- 
lection. 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 ease of 
hybrids, it apparently depends on their whole organi- 
sation having been disturbed by being compounded 
from two distinct forms; the sterility being closely 
allied to that which so frequently affects pure species, 
when exposed to new and unnatural conditions of life. 
He who will explain these latter cases will be able to 
explain the sterility of hybrids. 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, thatjLhe_jcrossing_of^ forms, which have 
been exposed to slightly different conditions^ of life" 
or which have varied, favours the size, vigour, and fer- 
tility ^TtEeir offspring. , The facts given on the steril- 
ity of the illegitimate unions of dimorphic and trimor- 
phic 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 

46 SUMMARY. [Chap. IX. 

seeMs to stafia 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 correspond, even if due to distinct 
causes: for both depend on the amount of difference 
between the species which are crossed. Nor is it sur- 
prising 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 circum- 
stances — should all run, to a certain extent, parallel 
with the systematic affinity of the forms subjected to 
experiment; for systematic affinity includes resem- 
blances 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 especially kept in mind, that long-continued 
domestication tends to eliminate sterility, and ic there- 
fore little likely to induce this same quality. Inde- 
pendently of the question of fertility, in all other re- 
spects there is the closest general resemblance between 
hybrids and mongrels, — ^in their variability, in their 

Chap. IX.] SUMMARY. 47 

power of absorbing each other by repeated crosses, and 
in their inheritance of characters from both parent- 
forms. Finally, then, although Ve 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 varie- 




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 deposition — On the lapse of time as estimated by years 
— On the poorness of our palaeontological collections — On the 
intermittence of geological formations — On the denudation of 
granitic areas — On the absence of intermediate varieties in any 
one formation — On the sudden appearance of groups of species 
— On their sudden appearance in the lowest known fossiliferous 
strata — Antiquity of the habitable earth. 

In the sixth chapter I enumerated the chief objec- 
tions which might be justly urged against the views 
maintained in this volume. Most of them have now 
been discussed. One, namely the distinctness of spe- 
cific forms, and their not being blended together by in- 
numerable transitional links, is a very obvious difficulty. 
I assigned reasons why such hnks do not commonly oc- 
cur at the present day under the circumstances ap- 
parently most favourable for their presence, namely, on 
an extensive and continuous area with graduated phys- 
ical conditions. I 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 cUmate, and, therefore, that the really govern- 
ing conditions of life do not graduate away quite insen- 
sibly like heat or moisture. I endeavoured, also, to 


show that intermediate varieties, from existing in lesser 
numbers than the forms which they connect, will gen- 
erally be beaten out and exterminated during the course 
of further modification and improvement. 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 sup- 
plant 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 geological formation and every stra- 
tum full of such intermediate links? Geology assured- 
ly does not reveal any such finely-graduated organic 
chain; and this, perhaps, is the most obvious and seri- 
ous objection which can be urged against the theory. 
The explanation lies, as I believe, in the extreme imper- 
fection 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 difiicult, when 
looking at any two species, to avoid picturing to my- 
self forms directly intermediate between them. But 
this is a wholly false view; we should always look for 
forms intermediate between each species and a common 
but unknown progenitor; and the progenitor will gen- 
erally have differed in some respects from all its modi- 
fied 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- 


pigeoT^ui we should have no varieties directly in- 
termediate between the fantail and pouter; none, for 
instance, combining 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 historical 
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. oenas. 

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 re- 
semblance 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 spe- 
cies, even if we closely compared the structure of the 
parent with that of its modified descendants, unless at 
the same time we had a nearly perfect chain of the in- 
termediate 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 re- 
mained for a very long period unaltered, whilst its de- 
scendants had undergone 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 

By the theory of natural selection all living species 
have been connected 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 gen- 
erally extinct, have in their turn been similarly con- 
nected 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 spe- 
cies, must have been inconceivably great. But as- 
suredly, if this theory be true, such have lived upon the 

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 his- 
torian 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^sufhces to study the Principles of 
Geology, or to read special treatises by different ob- 
servers on separate formations, and to mark how each 
author attempts to give an inadequate idea of the dura- 
tion of 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 super- 
imposed strata, and watch the rivulets bringing down 
mud, and the waves wearing away the sea-cliffs, in 
order to comprehend something about the duration of 
past time, the monuments of which we see all around 

It is good to wander along the coast, when formed of 
moderately hard rocks, and mark the process of degra- 
dation. 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 noth- 
ing in wearing away rock. At last the base of the cliff 
is undermined, huge fragments 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 productions, 

Chap. X.] THE LAPSE OF TIME. 53 

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 deg- 
radation, 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 obser- 
vations of 'Eamsay, in the van of many excellent ob- 
servers — of Jukes, Geikie, Croll, and others,, that sub- 
aerial 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 car- 
bonic acid, and in colder countries to frost; the disin- 
tegrated matter is carried down even gentle slopes 
during heavy rain, and to a greater extent than might 
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 undulat- 
ing country, we see the effects of subaerial degradation 
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 escarp- 
ment in the Wealden district 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-cliffs are everywhere formed by the intersection 
of various formations. This being the case, we are 
compelled to admit that the escarpments owe their 

54 THE LAPSE OP TIME. [Chap. X. 

origin in chief part to the rocks of which they are com- 
posed having resisted subaerial denudation better than 
the surrounding 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 sub- 
aerial agencies which apparently have so little power, 
and which seem to work so slowly, have produced great 

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 dura- 
tion 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 sedi- 
mentary formations. I remember having been much 
struck when viewing volcanic islands, which have been 
worn by the waves and pared all round into perpen- 
dicular cliffs of one or two thousand feet in height; 
for the gentle slope of the lava-streams, due to their 
formerly liquid 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 up- 
heaved 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 sur- 
face of the land has been so completely planed down 
that no trace of these vast dislocations is externally 
visible. The Craven fault, for instance, extends for 

Chap. X.] THE LAPSE OP TIME. 55 

upwards of 30 miles, and along this line the vertical 
displacement of the strata varies from 600 to 3000 feet. 
Professor Eamsay has published an account of a down- 
throw in Anglesea of 3300 feet; and he informs me 
that he fully believes that there is one in Merioneth- 
shire of 12,000 feet; yet in these eases 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 sedimentary strata are of wonderful thickness. In 
the Cordillera I estimated one mass of conglomerate at 
ten thousand feet; and although conglomerates have 
probably been accumulated at a quicker rate than finer 
sediments, yet from being formed of worn and rounded 
pebbles, each of which bears the stamp of time, they 
are good to show how slowly the mass must have been 
heaped together. Professor Eamsay 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: — 


Palaeozoic strata (not including igneous beds) 57,154 

Secondary strata 13,190 

Tertiary strata 3,240 

— 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 thickness on the Con- 
tinent. 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 

56 THE LAPSE OP TIME. [Chap. X. 

idea of the time which has elapsed during their accumu- 
lation. The consideration of these various facts im- 
presses the mind almost in the same manner as does the 
vain endeavour to grapple with the idea of eternity. 

ISTevertheless this impression is partly false. Mr. 
CroU, in an interesting 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 phe- 
nomena, 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. Croll 
shows, by calculating the known amount of sediment 
annually brought down by certain rivers, relatively to 
their areas of drainage, that 1000 feet of solid rock, as 
it became gradually disintegrated, would thus be re- 
moved 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 fol- 
lowing illustration: take a narrow strip of paper, 83 
feet 4 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, 
wnat a hundred years implies, represented as it is by a 
measure utterly insignificant in a hall of the above 
dimensions. Several eminent breeders, during a single 
lifetime, have so largely modified some of the higher 

Chap. X.] THE LAPSE OF TIME. 57 

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 repre- 
sents the work of two breeders in succession. It is not 
to be supposed that species in a state of nature ever 
change so quickly as domestic animals under the guid- 
ance 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 uncon- 
scious 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 slowness follows from all the 
inhabitants of the same country being already so well 
adapted to each other, that new places in the polity of 
nature do not occur until after long intervals, due to the 
occurrence 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 Palceontological Collections. 

Now let us turn to our richest geological museums, 
and what a paltry display we behold! That our col- 
lections are imperfect is admitted by every one. The 
remark of that admirable palajontologist, 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 organ 
ism wholly soft can be preserved. Shells and bones de- 
cay 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 ia 
being deposited over nearly the whole bed of the sea^ at 
a rate sufficiently quick to embed and preserve fossil re- 
mains. 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 con- 
formably 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 oi 
gravel, will, when the beds are upraised, generally be 
dissolved by the percolation of rain-water charged -nith 
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 Chthamalinae (a sub-family of 
sessile cirripedes) coat the rocks all over the world in 
infinite numbers:- they are all strictly littoral, with the 
exception of a single Mediterranean species, which in- 
habits 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 assign 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 extending for at least 300 miles from 
Vienna to Switzerland; and although this great mass 
has been most carefully searched, no fossils, except a 
few' vegetable remains, have been found. 

With respect to the terrestrial productions which 
lived during the Secondary and Palaeozoic 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 has. In regard to mammiferous remains, a 
glance at the historical table published in Lyell's 
Manual wiH 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 palaeo- 
zoic formations. 

But the imperfection in the geological record largely 
results from another and more important cause than 
any of the foregoing; namely, from the several forma- 
tions being separated from each other by wide intervals 
of time. This doctrine has been emphatically admitted 
by many geologists and paleontologists, who, like E. 
Forbes, entirely disbeheve 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 consecutive. But 
we know, for instance, from Sir E. Murchison's great 
work on Eussia, what wide gaps there are in that coun- 
try 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 consecu- 
tive formations, we may infer that this could nowhere 
be ascertained. The frequent and great changes in 
the mineralogical composition of consecutive forma- 
tions, 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 period, than the absence of any recent de- 
posits sufficiently extensive to last for even a short 
geological period. Along the whole west coast, which is 
inhabited by a peculiar 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 ter- 
tiary remains can anywhere be found, though the supply 
of sediment must for ages have been great, from the 
enormous degradation of the coast-rocks and from 
muddy streams entering the sea. The explanation, no 
doubt, is, that the littoral and sub-littoral deposits 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 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 pro- 
found 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 up- 
raised 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 up- 
raised, to resist a large amount of denudation, may be 

I am convinced that nearly all our ancient forma- 
tions, which are throughout the greater part of their 
thickness rich in fossils, have thus been formed during 
subsidence. Since publishing my views on this subjeci 
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 subsi- 
dence. I may add, that the only ancient tertiary forma- 
tion on the west coast of South America, which has 
been bulky enough to resist such degradation as it has 
as yet suffered, but which will hardly last to a dis- 
tant geological age, was deposited during a downward 
oscillation of level, and thus gained considerable thick- 

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 de- 
cay. On the other hand, as long as the bed of the sea 
remains stationary, thick deposits cannot have been ac- 
cumulated in the "shallow parts, which are the most 
favourable to life. Still less can this have happened 
during the alternate periods of elevation; or, to speak 
more accurately, • the beds which were then accumu- 
lated will generally have been destroyed by being 
upraised and brought within the limits of the coast- 

These remarks apply chiefly to littoral and sub-lit- 
toral deposits. In the case of an extensive and shallow 
6oa, such as that within a large part of the Malay Archi- 
pelago, where the depth varies from 30 or 40 to 60 
fathoms, a widely extended formation might be formed 
during a period of elevation, and yet not suffer exces- 
liively from denudation during its slow upheaval; but 
the thickness of the formation could not be great, for 
fiwing to the elevatory movement it would be less than 
the depth in which it was formed; nor would the de- 
posit be much consolidated, nor be capped by overlying 
formations, so that it would run a good chance of being 
worn away by atmospheric degradation and by the ac- 
tion of the sea during subsequent oscillations of level. 
It has, however, been suggested by Mr. Hopkins, that 
if one part of the area, after rising and before being 
denuded, subsided, the deposit formed during the ris- 
ing movement, though not thick, might afterwards be- 
come protected by fresh accumulations, and thus be 
preserved for a long period. 

Mr. Hopkins also expresses his belief that sedimen- 
tary beds of considerable horizontal extent have rarely 
^•)een completely destroyed. But all geologists^ except- 


ing 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 ex- 
tent. For it is scarcely possible that such rocks could 
have been solidified and crystallized whilst uncovered; 
but if the metamorphic action occurred at profound 
depths of the ocean, the former protecting mantle of 
rock may not have been very thick. Admitting then 
that gneiss, mica-schist, granite, diorite, &c., 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 
subsequently 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 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 conjoined. This region has not 
been carefully explored, but from the concurrent testi- 
mony of travellers, the granitic area is very large: thus. 
Von Eschwege gives a detailed section of these rocks, 
stretching from Eio 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 Eio Janeiro to the mouth of the 
Plata, a distance of 1100 geographical miles, were ex- 
amined by me, and they all belonged to this class. In- 
land, 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-meta- 
" morphic ") and granitic rocks exceed, in the propor- 
tion of 19 to 12.5, the whole of the newer Palaeozoic 
formations. In many regions the metamorphic and 
granitic rocks would be found much more widely ex- 
tended 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 crystalUzed. Hence it 
is probable that in some parts of the world whole forma- 
tions have been completely denuded, with not a wreck 
left behind. 

One remark is here worth a passing notice. During 
periods of elevation the area of the land and of the ad- 
joining shoal parts of the sea will be increased, and new 
stations will often be formed: — all circumstances favour- 
able, as previously explained, for the formation 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 inhabitants will decreasie (excepting on 
the shores of a continent when first broken up into an 
archipelago), and consequently during subsidence, 
though there will be much extinction, few new varie- 
ties or species will be formed; and it is during these 
very periods of subsidence, 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 gradu- 
ated 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 Am- 
monites; 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 Swit- 
zerland. Although each formation has indisputably 
required a vast number of years for its deposition, sev- 
eral reasons can be given why each should not commonly 
include a graduated series of links between the species 
which lived at its commencement and close; but I 
cannot assign due proportional weight to the following 

Although each formation may mark a very long 
lapse of years, each probably is short compared with the 
period reqiiisite to change one species into another. I 
am aware that two palsontologists, 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 dura- 
tion of specific forms. But insuperable difficulties, 
as it seems to me, prevent us from coming to any just 
conclusion on this head. When we see a species first 


appearing 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 
compared with the rest of the world; nor have the sev- 
eral stages of the 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 probabil- 
ity is that it only then first immigrated into that area. 
It is well-known, for instance, that several species ap- 
pear somewhat earlier in the palaeozoic beds of North 
America than in those of Europe; time having appa- 
rently 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, con- 
versely, that some are now abundant in the neighbour- 
ing 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 dur- 
ing 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 this 
same glacial period. Yet it may be doubted whether, 
in any quarter of the world, sedimentary deposits, in- 


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 de- 
posited during the whole of the glacial period 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 disappear at different levels, ow- 
ing to the migrations of species and to geographical 
changes. And in the distant future, a geologist, ex- 
amining these 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 hav- 
ing been really far greater, that is, extending from be- 
fore the glacial epoch to the present day. 

In order to get a perfect gradation between two 
forms in the upper and lower parts of the same forma- 
tion, the deposit must have gone on continuously ac- 
cumulating 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 ac- 
cumulate only during a period of subsidence; and to 
keep the depth approximately the same, which is neces- 
sary that the same marine species may live on the same 
space, the supply of sediment must nearly counterbal- 
ance the amount of subsidence. But this same move- 


ment 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 palaaontologist, 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. Kor 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 else- 
where thousands of feet in thickness, and which must 
have required an enormous period for their accumula- 
tion; yet no one ignorant of this fact would have even 
suspected the vast lapse of time represented by the 
thinner formation. Maiiy cases could be given of the 
lower beds of a formation having been upraised, de- 
nuded, 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 ac- 
cumulation. In other cases we have the plainest evi- 
dence 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 


not have been suspected, had not the trees been pre- 
served: thus Sir C. Lyell and Dr. Dawson found carbon- 
iferous beds 1400 feet thick in Nova Scotia, with an- 
cient 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 include 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 naturalists have 
no golden rule by which to distinguish species and 
varieties; they grant some little 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 assigned, 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 interme- 
diate 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 between 


them in all respects. So that we might obtain the 
parent-species and its several modified descendants from 
the lower and upper beds of the same formation, and un- 
less 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 sub-stages 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 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 natu- 
ralists to be identical with existing species; but some 
excellent naturalists, as Agassiz and Pictet, maintain 
that all these tertiary species are specifically distinct, 
though the distinction is admitted to be very slight; so 
that here, unless we believe that these eminent natu- 
ralists have been misled by their imaginations, and 
that these late tertiary species really present no dif- 
ference whatever from their living representatives, or 
unless we admit, in opposition to the judgment of most 
naturalists, that these tertiary species are all truly dis- 
tinct from the recent, we have evidence of the frequent 
occurrence 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 wander 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 de- 
gree. According to this view, the chance of discov- 
ering 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 ani- 
mals 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 spe- 
cies; 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 lately insisted on by Dr. Falconer, name- 
ly, that the period during which each species under- 
went 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 

Chap.X.] in any single FORMATION. 73 

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 species 
this can rarely be done. We shall, perhaps, best per- 
ceive the improbability of our being enabled to con- 
nect species by numerous, fine, intermediate, fossil links, 
by asking ourselves whether, for instance, 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 abori- 
ginal stocks; or, again, whether certain sea-shells in- 
habiting the shores of North America, which are 
ranked by some conchologists as distinct species from 
their European representatives, and by other con- 
chologists 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 discover- 
ing in a fossil state numerous intermediate grada- 
tions; and such success is improbable in the highest de- 

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 erroneous. As Sir 
J. Lubbock has remarked, " Every species is a link 
"between other allied forms." If we take a genug 
having a score of species, recent and extinct, and de- 
stroy four-fifths of them, no one doubts that the re- 
mainder 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 infinitely 


numerous gradations^ as fine as existing varieties, con- 
necting together nearly all existing and extinct species. 
But this ought not to be expected; yet this has been 
repeatedly advanced as a most serious objection against 
my views. 

It may be worth while to sum up the foregoing 
remarks on the causes of the imperfection of the geo- 
logical record under an imaginary illustration. The 
Malay Archipelago is about the size of Europe from the 
North Cape to the Mediterranean, and from Britain to 
Eussia; and therefore equals all the geological forma- 
tions 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, prob- 
ably represents the former state of Europe, whilst 
most of our formations were accumulating. 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 ter- 
restrial productions of the archipelago would be pre- 
served in an extremely imperfect manner in the forma- 
tions which we suppose to be there accumulating. 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 en- 
dure 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 futu- 
rity as the secondary formations lie in the past, would 
generally be formed in the archipelago only during 
periods of subsidence. These periods of subsidence 
would be separated from each other by immense in- 
tervals of time, during which the area would be either 
stationary or rising; whilst rising, the fossiUferous for- 
mations on the steeper shores would be destroyed, al- 
most as soon as accumulated, by the incessant coast- 
action, as we now see on the shores of South America. 
Even throughout the extensive and shallow seas with- 
in the archipelago, sedimentary beds could' hardly be 
accumulated of great thickness during the periods of 
elpvation, or become capped and protected by subse- 
qu^.nt deposits, so as to have a good chance of enduring 
to a very distant future. During the periods of sub- 
sidence, there would probably be much extinction of life; 
during the periods of elevation, there would be much 
variation, but the geological record would then be less 

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 accumu- 
lation of sediment, would exceed the average duration of 
the same specific forms; and these contingencies are in- 
dispensable for the preservation of all the transitional 
gradations between any two or more species. 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 os- 
cillations 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 archi- 
pelago now range thousands of miles beyond its con- 
fines; 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 varie- 
ties; and the varieties would at first be local or eon- 
fined to one place, but if possessed of any decided ad- 
vantage, 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, accord- 
ing to the principles followed by many palffiontologists, 
be ranked as new and distinct species. 

If then there be some degree of truth in these re- 
marks, we have no right to expect to find, in our geo- 
logical formations, an infinite number of those fine 
transitional forms which, on our theory, have connected 
all the past and present species of the same group into 
cne long and branching chain of life. We ought only 

to look for a few links, and such assuredly we do find 

some more distantly, 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 palffiontologists, be ranked as distinct 
species. But I do not pretend that I should ever have 
suspected how poor was the record in the best preserved 

Chap.X.] in any single POEMATION. 77 

geological sections, had not the absence of innumerable 
transitional links between the species which lived at 
the commencement and close of each formation, pressed 
60 hardly on my theory. 

On the sudden Appearance of whole Groups of allied 

The abrupt manner in which whole groups of spe- 
cies suddenly appear in certain formations, has been 
urged by several palaeontologists — ^for instance, by Agas- 
siz, Pictet, and Sedgwick — as a fatal objection to the be- 
lief in the transmutation of species. If numerous spe- 
cies, 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 progeni- 
tors must have lived long before their modified descen- 
dants. But we continually overrate the perfection of 
the geological record, and falsely infer, because certain 
genera or families have not been found beneath a cer- 
tain stage, that they did not exist before that stage. 
In all cases positive palaeontological evidence may be 
implicitly trusted; negative evidence is worthless, as 
experience has so often shown. We continually 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 else- 
where have long existed, and have slowly multiplied, 
before they invaded the ancient archipelagoes of Europe 
and the United States. We do not make due allowance 


for the intervals of time which have elapsed between 
our consecutive 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 parent- 
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 require 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 consequently that 
the transitional forms would often long remain eon- 
fined to some one region; but that, when this adapta- 
tion had once 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 Pietet, in his 
excellent Review of this work, in commenting on early 
transitional forms, and taking birds as an illustration, 
cannot see how the successive modifications of the an- 
terior limbs of a supposed prototype could possibly 
have been of any advantage. But look at the penguins 
of the Southern Ocean; have not these birds their front 
limbs in this precise intermediate 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 sup- 
pose 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 become 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 Paleontology, published in 1844-46 and 
in 1853-57, the conclusions on the first appearance and 
disappearance of several groups of animals have been 
considerably modified; and a third edition would re- 
quire 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 commencement of the 
tertiary series. And now one of the richest known ac- 
cumulations of fossil mammals belongs to the middle of 
the secondary series; and true mammals have been dis- 
covered in the new red sandstone at nearly the com- 
mencement of this great series. Cuvier used to urge 
that no monkey occurred in any tertiary stratum; but 
now extinct species 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 pres- 
ervation of footsteps in the new red sandstone of the 
United States, who would have ventured to suppose that 
no less than at least thirty different bird-like animals, 
some of gigantic size, existed during that period? Kot 
a fragment of bone has been discovered in these beds. 
Not long ago, palaeontologists 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 greensandj and still more 
recently, that strange bird, the Archeopteryx, 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 extinct tertiary spe- 
cies; 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 50 fathoms; 
from the perfect manner in which specimens are pre- 
served in the oldest tertiary beds; from the ease with 
which evena fragment of avalve can be recognised; from 
all these circumstances, I inferred that, had sessile cirri- 
pedes 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 developed at the commencement of the ter- 
tiary series. This was a sore trouble to me, adding as 
I then thought one more instance of the abrupt ap- 
pearance of a great group of species. But my work had 
hardly been published, when a skilful palaeontologist, 
M. Bosquet, sent me a drawing of a perfect specimen 
of an unmistakable sessile cirripede, which he had him- 
self 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 ubiqui- 
tous genus, of which not one species has as yet been 
found even in any tertiary stratum. Still more re- 
cently, a Pyrgoma, a member of a distinct sub-family of 
sessile cirripedes, has been discovered by Mr. Wood- 
ward in the upper chalk; so that we now have abun- 
dant evidence of the existence of this group of animals 
during the secondary period. 

The case most frequently insisted on by palaeonto- 
logists of the apparently sudden appearance of a whole 
group of species, 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 com- 
monly admitted to be teleostean; and even some palaeo- 
zoic 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 insuperable difficulty, 
unless it could likewise have been shown that at the 
same period the species were suddenly and simultane- 
ously developed 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 run- 
ning through Pietet's Palaeontology it will be seen that 
very few species are known from several formations 
in Europe. Some few families of fish now have a con- 
fined 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 multiplied; and 
here they would remain confined, until some of the 
species became adapted to a cooler climate, and 
were enable to double the Southern capes of Af- 
rica or Australia, and thus reach other and distant 

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 palaeontological knowledge effected by the dis- 
coveries 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 natura- 
list to land for five minutes on a barren point in Aus- 
tralia, and then to discuss the number and range of its 

On the sudden Appearance of Groups 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 fossili- 
ferous rocks. Most of the arguments which have con- 
vinced me that all the existing species of the same group 
are descended from a single progenitor, apply with 
equal force to the earliest known species. For in- 
stance, it cannot be doubted that all the Cambrian and 


Silurian trilobites are descended from some one crusta- 
cean, 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, &c., do not differ much from 
living species; and it cannot on our theory be supposed, 
that these old species were the progenitors of all the 
species belonging to the same groups which have sub- 
sequently appeared, for they are not in any degree in- 
termediate 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 en- 
counter a formidable 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 20 or more than 
400 million years ago, but probably not less than 98 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 60 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 for- 
mation; and the previous 140 million years can hardly 
be considered as sufficient for the development of the 


varied forms of life which already existed during the 
Cambrian period. It is, however, probable, as Sir Wil- 
liam 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 

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 geologists, with Sir E. Mur- 
chison at their head, were until recently convinced 
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 dis- 
puted this conclusion. We should not forget that only 
a sriiall portion of the world is known with accuracy. 
Not very long ago M. Barrande added another and 
lower stage, abounding with new and peculiar species, 
beneath the then known Silurian system; and now, 
etill lower down in the Lower Cambrian formation, Mr. 
Hicks has found in South Wales beds rich in trilobites, 
and containing various molluscs and annelids. The 
presence of phosphatie nodules and bituminous matter, 
even in some of the lowest azoic rocks, probably indi- 
cates life at these periods; and the existence of the 
Eozoon in the Laurentian formation of Canada is gener- 
ally admitted. There are three great series of strata be- 
neath the Silurian system in Canada, in the lowest of 
which the Eozoon is found. Sir W. Logan states that 
their "united thickness may possibly far surpass that 
" of all the succeeding rocks, from the base of the palseo- 


"zoic 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 existence 
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 diffi- 
culty of assigning any good reason for the absence of 
vast piles of strata rich in fossils beneath the Cambrian 
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 ob- 
literated by metamorphic action, for if this had heen 
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 descriptions which we possess of 
the Silurian deposits over immense territories in Eussia 
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 hy- 
pothesis. From the nature of the organic remains 
which do not appear to have inhabited profound depths, 


in thie several formations 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. This same view has since 
been maintained by Agassiz and others. But we do not 
know what was the state of things in the intervals be- 
tween 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 of 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 remnant 
of any palaeozoic or secondary formation. Hence we 
may perhaps infer, that during the palaeozoic and sec- 
ondary periods, neither continents nor continental is- 
lands existed where our oceans now extend; for had 
they existed, palaeozoic and secondary formations would 
in all probability have been accumulated from sediment 
derived from their wear and tear; and these would have 
been at least partially upheaved by the oscillations of 
level, which must have intervened during these enor- 
mously 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 oscilla- 
tions of level, since the Cambrian period. The col- 
oured map appended to my volume on Coral Reefs, led 
me to conclude that the great oceans are still mainly 
areas of subsidence, the great archipelagoes still areas of 
oscillations of level, and the continents areas of eleva- 
tion. But we have no reason to assume that things have 
thus remained from the beginning of the world. Our 
continents seem to have been formed by a preponder- 
ance, during many oscillations of level, of the force of 
elevation; but may not the areas of preponderant move- 
ment have changed in the lapse of ages? At a period 
long antecedent to the Cambrian epoch, continents may 
have existed where oceans are now spread out; and clear 
and open oceans may have existed where our continents 
now stand. N'or should we be justified in assuming 
bhat if, for instance, the bed of the Pacific Ocean were 
now converted into a continent we should there find 
£edimentary formations in a recognisable condition older 
than the Cambrian strata, supposing such to have been 
formerly deposited; for it might well happen that strata 
which had subsided some milesnearerto the centre of the 
earth, and which had been pressed on by an enormous 
weight of superincumbent water, might have undergone 
far more metamorphic action than strata which have al- 
ways 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 require some special explanation; and we may per- 
haps believe that we see in these large areas, the many 
formations long anterior to the Cambrian epoch in a 
completely metamorphosed and denuded condition. 


The several difSculties 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 transi- 
tional forms closely joining them all together;— 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 fos- 
sils 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, Barrande, Pictet, Falconer, E. Forbes, &c., and ' 
all our greatest geologists, as Lyell, Murchison, Sedgwick, 
&c., have unanimously, often vehemently, maintained 
the immutability of species. But Sir Charles Lyell 
now gives the support of his high authority to the op- 
posite side; and most geologists and palaeontologists are 
much shaken in their former 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 pos- 
sess 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 chap- 
ters, may represent the forms of life, which are en- 
tombed in our consecutive formations, and which falsely 
appear to have been abruptly introduced. On this 
view, the difficulties above discussed are greatly dimin- 
ished, or even disappear. 




On the slow and successive appearance of new species — On their 
difEerent rates of change — Species once lost do not reappear — 
Groups of species follow the same general rules in their ap- 
pearance and disappearance 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. 

Let 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 modi- 
fication, 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 exist- 
ing 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 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 greatly. 
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 believe that organisms high in the scale, 
change more quickly than those that are low: though 
there are exceptions to this rule. The amount of or- 
ganic change, as Pictet has remarked, is not the same in 
each successive so-called formation. Yet if we compare 
any but the most closely related formations, all the spe- 
cies 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 
pre-existing 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 simul- 
taneously, or to an equal degree. The process of modi- 
fication must be slow, and will generally effect 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 individual 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 depend 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 sur- 
prising that one species should retain the same identi- 
cal form much longer than . others; or, if changing, 
should change in a less degree. We find similar rela- 
tions between the existing inhabitants of distinct coun- 
tries; for instance, the land-shells and coleopterous in- 
sects of Madeira have come to differ considerably from 
their nearest allies on the continent of Europe, whereas 
the marine shells and birds have remained unaltered. 
We can perhaps understand the apparently quicker rate 
of change in terrestrial and in more highly organised 
productions compared with marine and lower produc- 
tions, by the more complex relations of the higher 
beings to their organic and inorganic conditions of life, 
as explained in a former chapter. When many of the 
inhabitants of any area have become modified and im- 
proved, we can understand, on the principle of compe- 
tition, and from the all-important relations of organ- 


ism to organism in the struggle for life, that any form 
which did not become in some degree modified and im- 
proved, 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 formations, rich in fossils, depends on great 
masses of sediment being deposited on subsiding areas, 
our formations have* been almost necessarily accumu- 
lated at wide and irregularly intermittent intervals of 
time; consequently the amount of organic change ex- 
hibited by the fossils embedded in consecutive forma- 
tions 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 con- 
ditions 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 econ- 
omy 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 organ- 
isms 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 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 
Lytheir improved offspring,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 successive 
variations would almost certainly be in some degree dif- 
ferent, and the newly-formed variety would probably in- 
herit from its progenitor some characteristic differences. 

Groups of species, that is, genera and families, fol- 
low the same general rules in their appearance and dis- 
appearance as do single species, 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 excepti'ons to this 
rule, but the exceptions are surprisingly few, so few that 
E. Forbes, Pictet, and Woodward (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 common 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 sometimes falsely appear to have been abrupt- 
ly developed; and I have attempted to'give an explana- 
tion 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 vary- 
ing thickness, ascending through the successive geologi- 
cal 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 beds, 
marking the decrease and final extinction of the species. 
This gradual increase in number of the species of a 
group is strictly conformable 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 alUed forms necessarily being a slow and gradual pro- 
cess, — one species first giving rise to two or three varie- 
ties, these being slowly converted into species, which in 
their turn produce by equally slow steps other varieties 
and species, and so on, hke 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 dis- 
appearance 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 Eliede Beaumont, Mur- 

Chap. XI.] EXTINCTION. 95 

chison, Barrande, &c., whose general views would natu- 
rally lead them to this conclusion. On the contrary, 
we have every reason to believe, from the study of the 
tertiary formations, 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 isth- 
mus and the consequent irruption of a multitude of new 
inhabitants into an adjoining sea, or by the final subsi- 
dence 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 earhest known 
dawn of life to the present day; some have disappeared 
before the close of the palaeozoic 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 pro- 
duction: if their appearance and disappearance be rep- 
resented, as before, by a vertical line of varying thick- 
ness the line is found to taper more gradually at its up- 
per end, which marks the progress of extermination, 
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 sup- 
posed 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, Mega- 
therium, Toxodon, and other extinct monsters, which 
all co-existed with still living shells at a very late geo- 
logical period, I was filled with astonishment; for, see- 
ing that the horse, since its introduction by the Span- 
iards into South America, has run wild over the whole 
country and has increased in numbers at an unparal- 
leled rate, I asked myself what could so recently have 
exterminated the former horse under conditions of life 
apparently so favourable. But my astonishment was 
groundless. Professor Owen soon perceived that the 
tooth, though so like that of the existing horse, be- 
longed 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 cer- 
tain, 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 unfavourable condi- 
tions 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 

Chap. XI.] EXTINCTION. 97 

perceived the fact, yet the fossil horse would certainly 
have become rarer and rarer, and finally extinct; — ^its 
place being seized on by some more successful competitor. 

It is most difficult always to remember that the in- 
crease 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 under- 
stood, that I have heard surprise repeatedly expressed 
at such great monsters as the Mastodon and the more 
ancient Dinosaurians having become extinct; as if mere 
bodily strength gave victory in the battle 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 
weakening the elephant in India, cheek 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 
naturalised quadrupeds in several parts of S. America. 

We see in many eases in the more recent tertiary for- 
mations, that rarity precedes extinction; and we know 
that this has been the progress of events with those 
animals which have been 3xtermihated, 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 mar- 
vel 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 
species, is produced and maintained by having some 
advantage over those with which it comes into competi- 
tion; and the consequent extinction of the less-fa- 
voured 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, 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 to- 
gether. In flourishing groups, the number of new spe- 
cific 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 ex- 
terminated; but we know that species have not gone on 
indefinitely increasing, at least during the later geo- 
logical 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 hke each other in all respects. 
Hence the improved and modified descendants of a spe- 
cies will generally cause the extermination of the parent- 
species; and if many new forms have been developed 


from any one species, the nearest allies of that species, 
i.e. the species of the same genus, will be the most liable 
to extermination. 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 wi^l generally be 
the alhed forms, which will suffer from some inherited 
inferiority in common. But whether it be species be- 
longing to the same or to a distinct class, which have 
yielded their places to other modified and improved spe- 
cies, 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 species of Trigonia, a great genus of shells 
in the secondary formations, 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. 

With respect to the apparently sudden extermina- 
tion 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 unusu- 


ally 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 correspond- 
ingly rapid manner; and the forms which thus yield 
their places will commonly be allied, for 'they will par- 
take 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 
accords 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 mo- 
ment that we understand the many complex contin- 
gencies 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 econ- 
omy 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 an- 
other can be naturalised in a given country; then, and 
not until then, we may justly feel surprise why we can- 
not account for the extinction of any particular spe- 
cies or group of species. 

On the Forms of Life changing almost simultaneously 
throughout the World. 
Scarcely any palseontological 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 
distant 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 unmis- 
takeable 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, hut they he- 
long 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 fgund in the Chalk of Europe, 
but which occur in the formations either above or be- 
low, occur in the same order at these distant points 
of the world. In the several successive palaeozoic for- 
mations of Eussia, Western Europe, and North America, 
a similar parallelism in the forms of life has been ob- 
served by several authors; so it is, according to Lyell, 
with the European and North American tertiary de- 
posits. Even if the few fossil species which are com- 
mon to the Old and New Worlds were kept wholly out 
of view, the general parallelism in the successive forms 
of life, in the palaeozoic and tertiary stages, would still 
be manifest, and the several formations could be easily 

These observations, however, relate to the marine 
inhabitants of the world: we have not sufficient data 
to judge whether the productions 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, Myiodon, Macrauchenia, 
and Toxodon had been brought to Europe from La Plata, 
without any information 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 century, 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 gla- 
cial epoch) were compared with those now existing in 
South America or in Australia, the most skilful natu- 
ralist would hardly be able to say whether the present 
or the pleistocene inhabitants of Europe resembled most 
closely those of 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 inhabi- 
tants of Europe;' and if this be so, it is evident that fos- 
siliferous 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 Aus- 
tralia, 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 simultaneous- 


ly, in the above large sense, at distant parts of the world, 
has greatly struck these admirable observers, MM. de 
Verneuil and d'Archiac. After referring to the parallel- 
ism of the palajozoic 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 otlier physical con- 
ditions, as the cause of these great mutations in the 
forms of life throughout the world, under the most dif- 
ferent climates. We must, as Barrande has remarked, 
look to some special 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 over 
the other forms in their own country, give birth to the 
greatest number of new varieties or incipient species. 
We have distinct evidence on this head, in the plants 
which are domihant, that is, which are commonest and 
most widely diffused, producing the greatest number of 


new varieties. It is also natural that the dominant, 
varying, 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 acci- 
dents, 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 ulti- 
mately prevail. The diffusion would, it is probable, be 
slower with the terrestrial inhabitants of distinct conti- 
nents than with the marine inhabitants of the continu- 
ous sea. "We might therefore expect to find, as we do 
find, a less strict degree of parallelism in the succesion 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 species having been formed by domi- 
nant species spreading widely and varying; the new 
species thus produced being themselves dominant, ow- 
ing to their having had some advantage over their al- 
ready dominant parents, as well as over other species, 
and again spreading, varying, and producing 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 are concerned, 
occurred during the periods when the bed of the sea 
was either stationary or rising, and likewise when sedi- 
ment 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 be- 
lieve that large areas are affected by the same move- 
ment, it is probable that strictly contemporaneous for- 
mations 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 dur- 
ing nearly, but not exactly, the same period, we should 
find in both, from the causes explained in the fore- 
going paragraphs, the same general succession in the 
forms of life; but the species would not exactly cor- 
respond; for there will have been a little more time 
in the one region than in the other for modification, 
extinction, and immigration. 

I suspect that eases 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 


"■geuei ai—jjErctllelism between the successive 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 them- 
selves differ in a manner very difi&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. Barrande, also, shows that 
there is a striking general parallelism in the successive 
Silurian deposits of Bohemia and Scandinavia; never- 
theless 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 corresponding with a 
blank interval in the other, — and if in both regions the 
species have gone on slowly changing during the accu- 
mulation of the several formations and during the long 
intervals of time between them; in this ease 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 living 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 in- 
tervals 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 living or to the 
extinct species of the same class, the series is far less 
perfect than if we combine both into one general sys- 
tem. In the writings of Professor Owen we continu- 
ally meet with the expression of generalised forms, as 
applied to extinct animals; 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 palffibntolo- 
gist, 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 intervals between exist- 
ing genera. Cuvier ranked the Ruminants and Pachy- 
derms 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 Mac- 
rauchenia of S. America connects to a certain extent 
these two grand divisions. No one will deny that the 
Hipparion is intermediate between the existing horse 


and certain older ungulate forms. What a wonderful 
connecting link in the chain of mammals is the Typo- 
therium from S. 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 dis- 
tinct group of mammals, and one of the most remark- 
able peculiarities in the existing dugong and lamentin 
is the entire absence of hind limbs without even a rudi- 
ment being left; but the extinct Hahtherium had, ac- 
cording 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 
quadrupeds, to which the Sirenia are in other respects 
allied. The cetaceans or whales are widely different 
from all other mammals, but the tertiary Zeuglodon 
and Squalodon, which have been placed by some natu- 
rahsts in an order by themselves, are considered by Pro- 
fessor Huxley to be undoubtedly cetaceans, " and to 
constitute connecting 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 Archeo- 
pteryx, 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, and a higher author- 
ity 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 di- 
rectly intermediate in all its characters between two 
living forms or groups, the objection is probably valid. 
But in a natural classification many fossil species cer- 
tainly stand between living species, and some extinct 
genera between living genera, even between genera be- 
longing 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 present day by a score of charac- 
ters, the ancient members are separated 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 difiicult 
to prove the truth of the proposition, for every now and 
then even a living animal, as the Lepidosiren, is dis- 
covered having affinities directed towards very distinct 
groups. Yet if we compare the older Eeptiles and 
Batrachians, the older Fish, the older Cephalopods, and 
the eocene Mammals, 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 be- 
ing given, but this is unimportant for us. The hori- 
zontal lines may represent successive geological forma- 
tions, and all the forms beneath the uppermost line may 
be considered as extinct. The three existing genera a", 
5", p", will form a small family; &^* and /" a closely 
allied family or sub-family; and 0", e", 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 principle 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 char- 
acter is a necessary contingency; it depends solely on 
the descendants from a species being thus enabled to 
seize on 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 ordePj from the continued 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^, a^°, 
P, m^, m*, m", were disinterred, these three families 
would be so closely linked together that they probably 
would have to be united into one great family, in near- 
ly the same manner as has occurred with ruminants 
and certain pachyderms. Yet he who objected to con- 
sider as intermediate the extinct genera, which thus 
link together the living genera of three families, would 
be partly justified, for they are intermediate, not direct- 
ly, 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 middle hori- 
zontal lines or geological formations — for instance, 
above No. VI. — but none from beneath this line, then 
only two of the families (those on the left hand, a^*, &c., 
and h^*, &c.) 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 m"), on the uppermost line, be sup- 
posed to differ from each other by half-a-dozen impor- 
tant characters, 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 ancient and extinct genera are often in a 
greater or less degree intermediate in character between 
their modified descendants, or between their collateral 

Under nature the process will be far more compli- 
cated than is represented in the diagram; for the groups 
will have been more numerous; they will have en- 
dured for extreraely 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 for- 
mations 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 palseontologists 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 manner.' And 
they are wholly inexpUeable 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 spe- 


cies 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 forme of life above and below. 
We must, however, allow for the entire extinction of some 
preceding forms, and in any one region for the immi- 
gration of new forms from other regions, and for a 
large amount of modification during the long and blank 
intervals between the successive formations. Subject 
to these allowances, the fauna of each geological period 
undoubtedly is intermediate in character, between the 
preceding and succeeding faunas. I need give only one 
instance, namely, the manner in which the fossils of the 
Devonian system, when this system was first discovered, 
were at once recognised by palaeontologists as inter- 
mediate 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 conse- 
cutive formations. 

It is no real objection to the truth of the statement 
that the fauna of each period as a whole is nearly inter- 
mediate in character between the preceding and suc- 
ceeding faunas, that certain genera offer exceptions to 
the rule. For instance, the species of mastodons and 
elephants, 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 ex- 
istence, — do not accord in arrangement. The species 
extreme in character are not the oldest or the most 
recent; nor are those which are intermediate in charac- 


ter, intermediate in age. But supposing for an instant, 
in this and other such eases, that the record of the first 
appearance and disappearance of the species was com- 
plete, which is far from the case, we have no reason to 
believe that forms successively produced necessarily en- 
dure for corresponding lengths of time. A very an- 
cient form may occasionally have lasted much longer 
than a form elsewhere subsequently produced, especially 
in the case of terrestrial productions inhabiting sepa- 
rated districts. To compare small things with great; 
if the principal living and extinct races of the domestic 
pigeon were arranged in serial affinity, this arrange- 
ment 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 beak 
originated earlier than short-beaked tumblers, which 
are at the opposite end of the series in this respect. 

Closely connected with the statement, that the or- 
ganic remains from an intermediate formation are in 
some degree intermediate in character, is the fact, in- 
sisted 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 forma- 
tions. Pictet gives as a well-known instance, the gen- 
eral resemblance of the organic remains from the sev- 
eral stages of the Chalk formation, though the species 
are distinct in each stage. This fact alone, from its 
generality, seems to have shaken Professor Pictet in his 
belief in the immutability of species. He who is ac- 
quainted with the distribution of existing species over 


the globe, will not attempt to account for the close re- 
semblance of distinct species in closely consecutive for- 
mations, 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 climates 
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 be- 
ing 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 in- 
termediate 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 geologi- 
cally, closely allied forms, or, as they have been called 
by some authors, representative species; and these as- 
suredly 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 compared with 
Living Formes. 

We have seen in the fourth chapter that the degree 
of differentiation and specialisation of the parts in or- 
ganic 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 speciali- 
sation of parts is an advantage to each being, so natural 
selection will tend to render the organisation of each 
being more specialised and perfect, and in this sense 
higher; not but that it may leave many creatures with 
simple and unimproved structures fitted for simple con- 
ditions 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 con- 
clude 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 natu- 
ral 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 


It is no valid objection to this conclusion, that cer- 
tain 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 diflaculty that 
Foraminifera have not, as insisted on by Dr. Carpenter, 
progressed in organisation since even the Laurentian 
epoch; 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 objections as the above would be fatal to my view, 
if it included advance in organisation as a necessary 
contingent. They would likewise be fatal, if the above 
Foraminifera, for instance, could be proved to have 
first come into existence during the Laurentian epoch, 
or the above Brachiopods during the Cambrian forma- 
tion; for in this ease, 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', 
oh the theory of natural selection, for their further 
continued progress; though they will, during each suc- 
cessive age, have to be slightly modified, so as to hold 
their places in relation to slight changes in their con- 
ditions. The foregoing objections hinge on the ques- 
tion 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 ex- 
tend far enough back, to shew with unmistakeable clear- 


ness that within the known history of the world or- 
ganisation 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 struc- 
ture to reptiles, as the highest fish; others look at the 
teleosteans as the highest. The ganoids stand inter- 
mediate 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 highness seems 
hopeless; who will decide whether a cuttle-fish be higher 
than a bee — ^that insect which the great Von Baer be- 
lieved to be " in fact more highly organised than a fish, 
although upon another type " ? In the complex strug- 
gle 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. Besides these inherent 
difficulties in deciding which forms are the most ad- 
vanced in organisation, we ought not solely to compare 
the highest members 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 the two 
periods. At an ancient epoch the highest and lowest 
molluseoidal animals, namely, cephalopods and brachio- 


pods, swarmed in numbers; at the present time both 
groups are greatly reduced, whilst others, intermediate 
in organisation, have largely increased; consequently 
some naturalists maintain that molluscs were formerly 
more highly developed than at present; hut a stronger 
case can be made out on the opposite side, by consider- 
ing the vast reduction of brachiopods, 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 propor- 
tional 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 in- 
crease 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 difl&eult it is to compare with perfect fair- 
ness under such extremely complex relations, the stand- 
ard 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 European 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 Zea- 
land, a multitude 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 feet that hardly a single inhabitant of the 


southern hemisphere has become wild in any part of 
Europe, we may well doubt whether, if all the produc- 
tions 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 Zea- 
land. Yet the most skilful naturalist, from an exami- 
nation 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 succession of extinct 
forms is nearly parallel with the embryological de- 
velopment of existing forms. This view accords ad- 
mirably well with our theory. In a future chapter I 
shall attempt to show that the adult differs from its 
embryo, owing to variations having supervened at a not 
early age, and having been inherited at a corresponding 
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 manifest, 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 strik- 
ing 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 1839 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 of 
Madeira; and between the extinct and living brackish 
water-shells of the Aralo-Caspian Sea. 


Wow 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 dissimilarity of the inhabitants of these two con- 
tinents; 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 chieily 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 publications 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 con- 
tinent; 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 rela- 
tion to the distribution of marine animals. 

On the theory of descent with modification, the 
great law of the long enduring, but not immutable, suc- 
cession 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 continent 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 in- 
termigration, the feebler will yield to the more domi- 
nant forms, and there will be nothing immutable in the 
distribution of organic beings. 

It may be asked in ridicule, whether I suppose that 
the megatherium 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 characters 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 formation, and in a succeeding forma- 
tion there be six other allied or representative genera 
each with the same number of species, then we may con- 
clude that generally only one species of each of the older 
genera has left modified descendants, which constitute 
the new genera containing 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 ease, 
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 spe- 
cies 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 geologically 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 
absolutely as nothing compared with the number of 
generations which must have passed away even during a 
single formation; that, owing to subsidence being al- 
most 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 extinction during the 
periods of subsidence, and more variation during the 
periods of elevation, and during- the latter the record 
will have been less perfectly kept; that each single for- 
mation has not been continuously deposited; that the 
duration of each formation is probably short compared 
with the average duration of specific forms; that mi- 
gration has played an important part in the first ap- 
pearance of new forms in any one area and formation; 
that widely ranging species are those which have varied 
most frequently, and have oftenest given rise to new 
species; that varieties have at first been local; and lastly, 


although each species must have passed through nuimer- 
ous transitional stages, it is probable that the period 
during which each underwent modification, though" 
many and long as measured by years, have been short 
in comparison with the periods during which each re- 
mained 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 inter- 
minable varieties, connecting together all extinct and 
existing forms by the finest graduated steps. It should 

^algoJie.^constantly borne in noind that any linking va- 
riety between two forms, which might be'fbund, would 
'~T)e ranFe'd, unless the whole chain could be perfectly 
restored, as a new and distiiict species; for it_is not pre- 

/teintedthat we have any sure criterion by which species 
and varieties can be discriminated. 

IIe~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 tran- 
sitional links which must formerly have connected the 
closely allied or representative species, found in the 
successive stages pf the same great formation? He 
may disbelieve in the immense intervals of time which 
must have elapsed between our consecutive formations; 
he may overlook how important 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 species. 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 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 lie 
still buried under the ocean. 

Passing from these difficulties, the other great lead- 
ing facts in palseontology agree admirably with the 
theory of descent with modification through variation 
and natural selection. We can thus understand 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 dis- 
appeared, it never reappears. Groups of species in- 
crease in numbers slowly, and endure for unequal 
periods of time; for the process of modification is neces- 
sarily slow, and depends on many complex contingen- 
cies. The dominant species belonging to large and 
dominant groups tend to leave many modified descend- 
ants, 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 modi- 
fied 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 descend- 


ants, lingering iff 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 domi nant form s 
which spread widely and yield the, greatest n umber of 
varieties tend to people the world with a^llied^but modi- 
fied, descendants; and these will generally succeed in 
displacing the groups which are their inferiors m the 
struggle for existence. Hence, after long intervals of 
time, the productions of the world appear_ to .have 

We TfanTunderstandTiow it is that all the forms of 
life, ancient and recent, make together a few grand 
classes. We can understand, from the continued ten- 
dency to divergence of character, why the more ancient 
d 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 to- 
gether. 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 ex- 
tinct and different forms. We can clearly see why the 
organic remains of closely consecutive formations are 
closely allied; for they are closely linked together by 
generation. 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 special- 
ised; and this may account for the common belief held 
by so many palaeontologists, that organisation on the 
whole has progressed. Extinct and ancient animals re- 
semble to a certain extent the embryos of the more re- 
cent 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 geologi- 
cal 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 gen- 
eration: old forms having been supplanted by new and 
improved forms of life, the products of Variation and 
the Survival of the Fittest. 




Present distribution cannot be accounted for by difEerences in 
physical conditions— Importance of barriers— Affinity of the 
productions of the same continent — Centres of creation — Means 
of dispersal by changes of climate 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 dissimi- 
larity 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 divi- 
sions 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 re- 
quire. 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 con- 
ditions are peculiar in only a slight degree. Notwith- 
standing 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 latitudes 35° 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 dissimikr. Or, again, we may com- 
pare the productions of South America south of lat. 35° 
with those north of 25°, which consequently are sepa- 
rated by a space of ten degrees of latitude, and are ex- 
posed 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 inhabitants 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 productions of various 
regions. We see this in the great difference in nearly 
all the terrestrial productions of the New and Old 
Worlds, excepting in the northern parts, where the land 
almost joins, and where, under a slightly different cli- 
mate, there might have been free migration for the 


northern temperate forms, as there now is for the strict- 
ly arctic productions. We see the same fact in the 
great diflEerence between 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 mountain-ranges, of great deserts and 
even of large rivers, we find different productions; 
though as mountain-chains, deserts, &c., are not as 
impassable, or likely to have endured so long, as the 
oceans separating continents, the differences are very 
inferior in degree to those characteristic of distinct con- 

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 formerly open. West- 
ward of the shores of America, a wide space of open 
ocean extends, with not an island as a halting-place for 
emigrants; here we have a barrier of another kind, and 
as soon as this 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 sepa- 
rated 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 in- 
numerable 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 fau- 
nas. Although so few marine animals are common 
to the above-named three approximate faunas of East- 
ern and Western America and the Eastern Pacific is- 
lands, yet many fishes range from the Pacific into the 
Indian Ocean, and many shells are common to the east- 
ern islands of the Pacific and the eastern shores of 
Africa on almost exactly opposite meridians of longi- 

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 them- 
selves are distinct at different points and stations. It 
is a law of the widest generality, and every continent 
offers innumerable instances. Nevertheless the natural- 
ist, in travelling, for instance, from north to south, 
never fails to be struck by the manner in which suc- 
cessive groups of beings, specifically distinct, though 
nearly related, replace each other. He hears from close- 
ly 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 Ehea (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 lati- 
tude. 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 species of bizca- 
cha; we look to the waters, and we do not find the 
beaver or musk-rat, but the coypu and capybara, ro- 
dents of the S. American type. Innumerable other 
instances could be given. If we look to the islands ofE 
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 American types then prevailing on the 
American continent and in the American seas. We 
see in these facts some deep organic bond, through- 
out space and time, over the same areas of land and 
water, independently of physical conditions. The natu- 
raUst must be dull who is not led to enquire 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 varie- 
ties, nearly alike. The dissimilarity of the inhabitants 
of different regions may be attributed to modification 
through variation and natural selection, and probably 
in a subordinate degree to the definite influence of dif- 
ferent physical conditions. The degrees of dissimilar- 
ity 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 importance of barriers comes into play by check- 
ing migration; as does time for the slow process of 
modification through natural selection. Widely-rang- 
ing species, abounding in individuals, which have al- 
ready triumphed over many competitors in their own 
widely-extended homes, will have the best chance of 
seizing on new places, when they spread into new coun- 
tries. In their new homes they will be exposed to new 
conditions, and will frequently undergo ■ further modi- 
fication and improvement; and thus they will become still 
further victorious, and will produce groups of modified 
descendants. On this principle of inheritance with 
modification we can understand how it is that sec- 
tions of genera, whole genera, and even families, are 
confined to the same areas, as is so commonly and notori- 
ously the case. 

There is no evidence, as was remarked in the last 
chapter, of the existence of any law of necessary de- 
velopment. As the variability of each species is an 
independent property, and will be taken advantage of 
by natural selection, only so far as it profits each in- 
dividual in its complex struggle for life, so the amount 
of modification 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 principles come into play only by bring- 


ing organisms into new relations with each other and in 
a lesser degree with the surrounding physical condi- 
tions. 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 sev- 
eral 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 modification, there is not much difficulty in be- 
lieving that they have migrated from the same region; 
for during the vast geographical and climatal changes 
which have supervened since ancient times, almost any 
amount of migration is possible. But in many other 
cases, in which we have reason to believe that the spe- 
cies of a genus have been produced within comparative- 
ly recent times, there is great difficulty on this head. 
It is also obvious that the individuals of the same spe- 
cies, though now inhabiting distant and isolated regions, 
must have proceeded from one spot, where their parents 
were first produced: for, as has been explained, it is 
incredible that individuals identically the same should 
have been produced from parents specifically dis- 

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 pointy of the earth's sur- 
face. Undoubtedly there are many cases of extreme 


difficulty in understanding how the same species could 
possibly have migrated from some one point to the sev- 
eral distant and isolated points, where now found. 
Nevertheless the simplicity of the view that each spe- 
cies 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 ad- 
mitted, that in most eases the area inhabited by a spe- 
cies is continuous; and that when a plant or animal 
inhabits two points 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 perhaps with 
any other organic beings; and, accordingly, we find no 
inexplicable instances of the same mammals inhabit- 
ing distant points of the world. No geologist feels any 
difficulty in Great Britain possessing the same quad- 
rupeds with the rest of Europe, for they were no doubt 
once united. But if the same species can be produced 
at two separate points, why do we not find a single mam- 
mal common to Europe and Australia or South Amer^ 
ica? The conditions of life are nearly the same, so that 
a multitude of European animals and plants have be- 
come naturalised in America and Australia; 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 mam- 
mals have not been able to migrate, 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 intelli- 
gible 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; 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 individuals 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 natu- 
ralists, that the view of each species having been pro- 
duced in one area alone, and having subsequently mi- 
grated from that area as far as its powers of migration 
and subsistence under past and present conditions per- 
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 cer- 
tainly occurred within recent geological times, must 
have rendered discontinuous the formerly continuous 
range of many species. So that we are reduced to con- 
sider 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 probable by general con- 
siderations, that each species has been produced within 
one arer and has migrated thence as far as it could, 


It would be hopelessly tedious to discuss all the excep- 
tional 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 instances. 
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 chap- 
ter), the wide distribution of freshwater productions; 
and thirdly, the occurrence of the same terrestrial species 
on islands and on the nearest mainland, though sepa- 
rated by hundreds of miles of open sea. If the exist- 
ence 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 elimatal and geographical changes and 
to the various occasional means of transport, the belief 
that a single birthplace is the law, seems to me incom- 
parably 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 modifica- 
tion during their migration, from some one area. If, 
when most of the species inhabiting one region are dif- 
ferent 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 strength- 
ened; for the explanation is obvious on the principle of 


descent with modification. A volcanic island, for in- 
stance, upheaved and formed at the distance of a few 
hundreds of miles from a continent, would probably re- 
ceive from it in the course of time a few colonists, and 
their descendants, though modified, would still be re- 
lated by inheritance to the inhabitants of that continent. 
Cases of this nature are common, and are, as we shall 
hereafter see, inexplicable on the theory of independ- 
ent 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 pre-existing closely allied spe- 
cies." And it is now well known that he attributes this 
coincidence to descent with modification. 

The question of single or multiple centres of crea- 
tion differs from another though allied question, — 
namely, whether all the individuals of the same species 
are descended from a single pair, or single hermaphrodite, 
or whether, as some authors suppose, from many individ- 
uals simultaneously created. "With organic beings which 
never intercross, if such exist, each species must be de- 
scended 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, name- 
ly, with all organisms which habitually unite for each 
birth, or which occasionally intercross, the individuals 
of the same species inhabiting the same area will be kept 
nearly uniform by intercrossing; so that many individ- 
uals 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 in- 
dividuals during each generation. 

Before discussing the three classes of facts, which I 
have selected as presenting the greatest amount of diffi- 
culty on the theory of "single centres of creation," I 
must say a few words on the means of dispersal. 

Means 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, pres- 
ently 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 sepa- 
rates 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 haye thus hypothetically 
bridged over every ocean, and united almost every is- 
land 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 re- 
cently been united to some continent. This view cuts 
the Gordian knot of the dispersal of the same species to 
the more distant points, and removes many a 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 'ad- 
mit 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 migration. 
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 proceeded 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 for- 
mer extension of the land. But I 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 in- 
habitants of several lands and even seas to their present 
inhabitants, — the degree of affinity betueen the mam- 
mals inhabiting islands with those of the nearest conti- 
nent, 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 
prodigious geographical revolutions within the recent 
period, as are necessary on the view advanced by Forbes 
and admitted by his followers. The nature and rela- 
tive proportions of the inhabitants of oceanic islands are 
likewise opposed to the belief of their former continu- 
ity with continents. Nor does the almost universally 
volcanic composition of such islands favour the admis- 
sion that they are the wrecks of sunken continents; — if 
they had originally existed as continental mountain 
ranges, some at least of the islands would have been 
formed, like other mountain summits, of granite, meta- 
morphic schists, old fossiliferous and other rocks, instead 
of consisting of mere piles of volcanic matter. 

I must now say a few words on what are called 
accidental means, but which more properly should be 
called occasional means of distribution. I shall here 
confine myself to plants. In botanical works, this or 
that plant is often stated to be ill adapted for wide dis- 
semination; but the greater or less facilities for trans- 
port 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 sur- 
prise I found that out of 87 kinds, 64 germinated after 
an immersion of 28 days, and a few survived an imraer- 


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, Hy- 
drophyllaceae and Polemoniaceae, 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, &c., and some of these floated for 
a long time. It is well known what a difference there 
is in the buoyancy 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 94 plants with ripe fruit, and to place 
them on sea- water. The majority sank rapidly, but 
some which, whilst green, floated for a short time, when 
dried floated much longer; for instance, ripe hazel- 
nuts sank immediately, but when dried they floated for 
90 days, and afterwards when planted germinated; an 
asparagus-plant with ripe berries floated for 33 days, 
when dried it floated for 85 days, and the seeds after- 
wards germinated; the ripe seeds of Helosciadium sank 
in two days, when dried they floated for above 90 days, 
and afterwards germinated. Altogether, out of the 94 
dried plants, 18 floated for above 38 days; and some of 
the 18 floated for a very much longer period. So that 
as 14 kinds of seeds germinated after an immersion of 
38 days; and as -^ distinct species with ripe fruit (but 
not all the same species as in the foregoing experiment) 
floated, after being dried, for above 38 days, we may 


conclude, as far as anything can be inferred from these 
scanty facts, that the seeds of -^^^ kinds of plants of any 
country might be floated by sea-currents during 28 days, 
and would retain their power of germination. In 
Johnston's Physical Atlas, the average rate of the sev- 
eral Atlantic currents is 33 miles per diem (some cur- 
rents running at the rate of 60 miles per diem); on this 
average, the seeds of ^-g plants belonging to one coun- 
try 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 float- 
ing plants. He tried 98 seeds, mostly different 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 flota- 
tion 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 -^ of his 
seeds of different kinds floated for 42 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 
experiments. Therefore it would perhaps be safer to 
assume that the seeds of about ^\ 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 

Chap.XII.] means op dispersal. 145 

the small, is interesting; as plants with large seeds or 
fruit which, as Alph. de CandoUe 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 naidst 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 earth are frequently 
enclosed in their interstices and behind them, — so per- 
fectly 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 dicotyledonous 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 artiflcial sea- water for 30 days, to my surprise nearly 
all germinated. 

Living birds can hardly fail to be highly effective 
agents in the transportation of seeds. I 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 circumstances 
their rate of flight would often be 35 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 12 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 
500 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 germi- 
nated after having been from twelve to twenty-one hours 
in the stomachs of different birds of 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 fre- 
quently 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 re- 
jected the seeds in pellets or passed them in their excre- 


ment; 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 Eev. E. 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 ca- 
reered round and round in an immense ellipse, at least 
five or six miles in diameter, and at night ahghted 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. N"ow, in parts of Natal it is beUeved by some 
farmers, though on insufficient evidence, that injurious 
seeds are introduced into their grass-land 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 the shank, weighing only nine 
grains; and this contained a seed of the toad-rush (Jun- 
cus 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), wheat- 
ears, and whinchats (Saxicolae), on their first arrival on 
our shores, before they had alighted; and he has several 
times noticed little cakes of earth attached to their feet. 
Many facts could be given showing how generally soil 
is charged with seeds. For instance. Prof. ISTewton 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 82 plants sprung from it: these consisted of 
13 monocotyledons, including the common oat, and at 
least one kind of grass, and of 70 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 

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 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 

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 mar- 
vellous 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 direc- 
tion 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, how- 
ever, 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 ITorth America to Britain, though 
they might and do bring seeds from the West Indies 
to our western shores, where, if not killed by their very 
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 fall- 
ing 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 occa- 
sional 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 hun- 
dred 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 argu- 
ment against what would be effected by occasional 
means of transport, during the long lapse of geologi- 
cal 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 in- 
sects or birds living there, nearly every seed which 
chanced to arrive, if fitted for the climate, would ger- 
minate and survive. 

Dispersal during the Glacial Period. 

The identity of many plants and animals, on moun- 
tain-summits, separated from each other by hundreds 
of miles of lowlands, where Alpine species could not 
possibly 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 ex- 
treme northern parts of Europe; but it is far more re- 
markable, 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 independently 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 Are do 
not tell their tale more plainly than do the mountains 
of Scotland and Wales, with their scored flanks, pol- 
ished 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 Northern 
Italy, gigantic moraines, left by old glaciers, are now 
clothed 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 in- 
habitants 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 fur- 
ther southward, unless they were stopped by barriers, 
in which case they would perish. The mountains 
would become covered with snow and ice, and their for- 
mer 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 everywhere travelled southward, are remarkably 
uniform round the world. 

As the warmth returned, the arctic forms would 
retreat northward, 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 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 low- 
lands, now become too warm for their existence. 

As the arctic forms moved first southward and after- 
wards backwards to the north, in unison with the chang- 
ing climate, they will not have been exposed during 
their long migrations to any great diversity of tem- 
perature; and as they all migrated in a body together, 
their mutual relations will not have been much dis- 
turbed. Hence, in accordance with the principles in- 
culcated in this volume, these forms will not have been 
liable to much modification. But with the Alpine pro- 
ductions, left isolated from the moment of the return- 
ing 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 some- 
what different climatal influences. Their mutual rela- 
tions will thus have been in some degree disturbed; con- 
sequently they will have been liable to modification; and 
they have been modified; for if we compare the present 


Alpine plants and animals of the several great Euro- 
pean 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 distinct yet closely allied species representing 
each other on the several ranges. 

In the foregoing illustration I have assumed that at 
the commencement of our imaginary Glacial period, 
the arctic productions were as uniform round the polar 
regions as they aje at the present day. But it is also 
necessary to assume that many sub-arctic and some few 
temperate forms were the same round the world, for 
some of the species which now exist on the lower moun- 
tain-slopes and on the plains of North America and 
Europe are the same; and it may be asked how I ac- 
count for this degree of uniformity in the sub-arctic 
and temperate 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 in- 
habitants 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 specifi- 
cally the same as now, and we have good reason to be- 
lieve 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 wes- 
tern Europe, through Siberia, to eastern America. And 
this continuity of the circumpolar land, with the con- 
sequent freedom under a more favourable climate for in- 
termigration,will account for the supposed uniformity of 
the sub-arctic and temperate productions of the Old and 
ISTew 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 inclined 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 num- 
ber of the same plants and animals inhabited the al- 
most continuous 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, consider- 
ing the distance of the two areas, and their separation 
by the whole Atlantic Ocean. We can further under- 
stand the singular fact remarked on by several observers 


that the productions of Europe and America during 
the later tertiary stages were more closely related to 
Lach other than they are at the present time; for dur- 
ing 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 in- 

During the slowly decreasing warmth of the Plio- 
cene period, as soon as the species in common, which in- 
habited 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 mi- 
grated 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 isolated, within a much more 
recent period, on the several mountain-ranges and on 
the arctic lands of Europe and N. America. Hence it 
has come, that when we compare the now living pro- 
ductions 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. 


^ii-BiTIrthe land, so in the waters of the sea, a slow 
southern migration of a marine fauna, which, during 
the Pliocene or even a somewhat earlier period, 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 under- 
stand 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 de- 
scribed 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 creation. 
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 convinced that Forbes's view may be largely ex- 


tended. In Europe we meet with the plainest evi- 
dence of the Glacial period, from the 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, perpetual snow formerly covered the central 
axis, and fed glaciers which rolled 400 feet down the 
valleys. The same observer has recently found great 
moraines at a low level on the Atlas range in N. 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 con- 
tinent, 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 communicated to me by the Kev. 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 lat. 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 Eoeky 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 lat. 13° to 30° S., 
at about the height of 12,000 feet, deeply-furrowed 
rocks, resembling those with which he was familiar in 
Norway, and likewise great masses of detritus, including 
grooved pebbles. Along this whole space of the Cor- 
dillera 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 ex- 
tremity, we have the clearest evidence of former glacial 
action, in numerous immense boulders transported far 
from their parent source. 

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 hemispheres — 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 con- 
dition of climate is the result of various physical causes, 
brought into operation by an increase in the eccentricity 
of the earth's orbit. All these causes tend towards the 
same end; but the most powerful appears to be the in- 
direct influence of the eccentricity of the orbit upon 
oceanic currents. According to Mr. Croll, cold periods 

Chai. xil.j IN THE NORTH AND SOUTH. 161 


regularly recur every ten or fifteen thousand years; and 
these at long intervals are extremely severe, owing to cer- 
tain 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 iQlacial 
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 temperature 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 conversely it will be with the north- 
ern hemisphere, whilst the southern passes through a 
Glacial period. This conclusion throws so much light 
on geographical distribution that I am strongly inclined 
to trust in it; but I will first give the facts, which 
demand an explanation. 

In South America, Dr. Hooker has shown that be- 
sides many closely allied species, between forty and 
fifty of the flowering plants of Tierra del Fuego, form- 
ing no inconsiderable part of its scanty flora, are com- 
mon to Forth 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 mountains 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 Cordil- 

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, be- 
lieved not to have been introduced by man, and on the 
mountains several representative European forms are 
found, which have not been discovered in the inter- 
tropical 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 neigh- 
bouring Cameroon mountains, in the Gulf of Giiinea, 
are closely related to those on the mountains of Abys- 
sinia, 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 Eev. E. T. Lowe on the mountains of the Cape 
Verde islands. This extension of the same temper- 
ate 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 astonishing 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 picture of a collection made on 


a hillock in Europe! Still more striking is the fact 
that peculiar Australian forms are represented by cer- 
tain 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 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 discovered 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 admir- 
able ' Introduction to the Flora of New Zealand,' by Dr 
Hooker, analogous and striking facts are given in re- 
gard to the plants of that large island. Hence we see 
that certain plants growing on the more lofty moun- 
tains of the tropics in all parts of the world, and on the 
temperate plains of the north and south, are either the 
same species 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 inhabiting the same widely sun- 
dered areas, belong to genera not now found in the inter- 
mediate tropical lowlands. 

These brief remarks apply to plants alone; but some 
few analogous facts could be given in regard to terres- 
trial- animals. In marine productions, similar cases 
likewise occur; as an example, I may quote a statement 


by the highest authority, Prof. Dana, that "it is cer- 
tainly a wonderful fact that ISTew 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, &c., of northern 
forms of fish. Dr. Hooker informs me that twenty-five 
species of Algae 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 lowlands of these great continents 
were everywhere tenanted under the equator by a con- 
siderable number of temperate forms. At this period 
the equatorial climate at the level of the sea was prob- 
ably 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 
temperate 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 Himalaya, but 
with perhaps a still greater preponderance of temperate 
forms. So again in the mountainous island of Fer- 
nando Po, in the Gulf of Guinea, Mr. Mann found tem- 
perate 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 
n-hen the northern hemisphere sufEered from the ex- 
treme 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 hemi- 
spheres, 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 naturahsed 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 vigor- 
ous, dominant, and widest-spreading temperate forms in- 
vaded the equatorial lowlands. The inhabitants of 
these hot lowlands would at the same time have migrated 
to the tropical and subtropical regions of the south, for 
the southern hemisphere was at this period warmer. 
On the decline of the Glacial period, as both hemi- 
spheres gradually recovered their former temperatures, 
the northern temperate forms living on the lowlands 
under the equator, would have been driven to their 
former homes or have been destroyed, being replaced 
by the equatorial forms returning from the south. 
Some, however, of the northern temperate forms would 
almost certainly have ascended any adjoining high land, 


where, if suiEciently lofty, they would have long sur- 
vived 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 pos- 
sess a certain capacity for acclimatisation, as shown by 
their transmitting to their offspring different consti- 
tutional powers of resisting heat and cold. 

In the regular course of events the southern hemi- 
sphere would in its turn he 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 de- 
scend and mingle with the southern forms. These 
latter, when the warmth returned, would return to their 
former homes, leaving some few species on the moun- 
tains, and carrying southward with them some of the 
northern temperate 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 dur- 
ing 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 hemispheres of former Glacial periods; for these 
will account, in accordance 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 Can- 
dolle 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 preponder- 
ant migration from the 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 alter- 
nations of the Glacial periods, the northern forms were 
the more powerful 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 manner at 
the present day, we see that very many European pro- 
ductions 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 naturalised in any part of the northern hemi- 
sphere, 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, offer a 
partial exception; for here, as I hear from Dr. Hooker, 


Australian forms are rapidly sowing themselves and be- 
coming naturalised. Before the last great Glacial 
period, no doubt the intertropical mountains were 
stocked with endemic Alpine forms; but these have al- 
most everywhere yielded to the more dominant forms 
generated in the larger areas and more efficient work- 
shops of the north. In many islands the native pro- 
ductions 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 con- 
tinental forms naturalised through man's agency. 

The same principles apply to the distribution of ter- 
restrial animals and of marine productions, in the north- 
ern and southern temperate zones, and on the inter- 
tropical mountains. When, during the height of the 
Glacial period, the ocean-currents were widely differ- 
ent 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 once be able 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, iso- 
lated 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 
T-egard 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 inter- 
mediate 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 Ker- 
guelen Land, New Zealand, and Fuegia; but icebergs, 
as suggested by Lyell, may have been concerned 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 remarkable case. Some of these 
species are so distinct, that we cannot suppose that there 
has been time since the commencement of the last Gla- 
cial period for their migration and subsequent modi- 
fication 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 
centre; 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 al- 
ready widely dispersed to various points of the southern