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The Origin of Species 

By Means of A T atural Selection 




new york HURST & COMPANY publishers 

'''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 Diviue power, 
exerted in each particular case, but by the establishment 
of general laws." — Whewell ; Bridgewater Treatise. 

" The only distinct meaning of the word l natural 9 is 
stated, fixed or settled ; since what is natural as much re- 
quires and presupposes an intelligent agent to render it so, 
i. e., to effect it continually or at stated times, as what is 
supernatural or miraculous does to effect it for once." — 
Butler : Analog?/ of Revealed Religion. 

" To conclude, therefore, let no man out of a weak conceit 
of sobriety, or an ill-applied moderation, think or maintain, 
chat 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 endeavor an endless prog- 
ress or proncience in both." — Bacon: Advancement of 





I will here give a brief sketch of the progress of opin- 
ion on the Origin of Species. Until recently the great 
.najority of naturalists believed that species were immut- 
able productions, and had been separately created. 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-exist- 
ing forms. Passing over allusions to the subject in the 
classical writers,* the first author who in modern times 
has treated it in a scientific spirit was Buffon. But as his 
opinions fluctuated greatly at different periods, and as he 

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


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 en- 
larged them in 1809 in his " Philosophic Zoologique," and 
subsequently, 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 spec'eo. He first did the eminent service of 
arousing attention to the probability of all change in the 
•rganic, as well as in the inorganic world, being the result 
of law, and not of miraculous interposition. Lamarck 
seems to have been chiefly led to his conclusion on the 
gradual change of species, by the difficulty of distinguish- 
ing species and varieties, by the almost perfect gradation 
of forms in certain groups, and by the analogy of domestic 
productions. With respect to the means of modification, 
he attributed something to the direct action of the physical 
conditions of life, something to the crossing of already 
existing forms, and much to use and disuse, that is, to the 
effects of habit. To this latter agency he seems to attribute 
all the beautiful adaptations in nature; such as the long 
neck of the giraffe for browsing on the branches of trees. 
But he likewise believed in a law of progressive develop- 
ment ; and as all the forms of life thus tend to progress, in 
order to account for the existence at the present day of 
simple productions, he maintains that such forms are now 
spontaneously generated.* 

Geoffroy Saint-Hilaire, as is stated in his " Life," written 

* I have ken the date of the first publication of Lamarck from 
Isidore Geotfroy Saint-Hilaire' s (" Hist. Nat. Generate, " torn. ii. p. 
405, 1859) excellent history of opinion on this subject. In this work a 
full account is given of Buff on' 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 " Zoono- 
mia" (vol. i. pp. 509-510), published in 1794. According to Isid. Geof- 
froy there is no doubt that Goethe was an extreme partisan of similar 
views, as shown in the introduction to a work written in 1794 and Vl?\ 
but not published till long afterward: he has pointedly remarked 
(" Goethe als Naturforscher," von Dr. Karl Meding, s. 34) that the 
future question for naturalists will be how, for instance, cattle got their 
horns, and not for what they are used. It is rat. r a singular insijuce 
of the manner in which similar views arise at about the same time, that 
Goethe in Germany, Dr. Darwin in England, and Geoffroy Saint-Hilair* 
(as we shall immediately see) in France, came to the same conclusion 
on the origin of species, in th - "*ars 1794-95. __ u _ 


by his son, suspected, as early as 1795, that what we call 
species are various degenerations of the same type. It was 
not until 1828 that he published his conviction that the same 
forms have not been perpetuated 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 a reserver entierement 
a l'avenir, suppose meme que l'avenir doive avoir prise 
sur lui." 

In 1813 Dr. W. C. Wells read before the Royal Society 
"An Account of a White Female, part of whose skin resem- 
bles that of a Negro ; " bnt his paper was not published 
until his famous "Two Essays upon Dew and Single Vision" 
appeared in 1818. In this paper he distinctly recognizes 
the principle of natural selection, and this is the first 
recognition which has been indicated ; but he applies it 
only to the races of man, and to certain characters alone. 
After remarking that negroes and mulattoes enjoy an im- 
munity from certain tropical diseases, he observes, firstly, 
that all animals tend to vary in some degree, and, secondly, 
that agriculturists improve their domesticated animals by 
selection ; and then, he adds, but what is done in this latter 
case "by art, seems to be done with equal efficacy, though 
more slowly, by nature, in the formation of varieties of 
mankind, fitted for the country which they inhabit. Of the 
accidental varieties of man, which would occur among the 
first few and scattered inhabitants of the middle regions of 
Africa, some one would be better fitted than 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 
neighbors. The color 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 origi- 
nated." He then extends these same views to the white in- 
habitants of colder climates. I am indebted to Mr. Rowley, 
of the United States, for having called my attention, through 
Mr. Brace, to the above £;*sstt£e of Dr. Wells's work. 


The Hon. and Rev. W. Herbert, afterward Dean ot Man- 
chester, in the fourth volume of the " Horticultural Trans- 
actions," 1822, and in his work on the " Amaryllidaceae " 
(1837, pp. 19, 339), declares that " horticultural experiments 
nave 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 paragraph in 
his well-known paper (" Edinburgh Philosophical Journal," 
vol. xiv. p. 283) on the Spongilla, clearly declares his belief 
that species are descended from other species, and that they 
become improved in the course of modification. This same 
view was given in his Fifty-fifth Lecture, published in the 
" Lancet " in 1834. 

In 1831 Mr. Patrick Matthew published his work on 
"Naval Timber and Arboriculture," in which he gives pre- 
cisely the same view on the origin of species as that (pres- 
ently to be alluded to) propounded by Mr. Wallace and 
myself in the " Linnean Journal," and as that enlarged 
in the present volume. Unfortunately the view was given 
by Mr. Matthew very briefly in scattered passages, in an 
appendix to a work on a different subject, so that it remained 
unnoticed until Mr. Matthew himself drew attention to it 
in the " Gardeners' Chronicle," on April 7, 1860. The dif- 
ferences of Mr. Matthew's views from mine are not of much 
importance : he seems to consider that the world was nearly 
depopulated at successive periods, and then restocked ; 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 pas- 
sages ; 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 

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 be- 
come changed into permanent species, which are no longer 
capable of intercrossing. 

Rafinesque, in his " New Flora of North America," put* 


Fished in 1836, wrote (p. 6) as follows : " All species might 
have been varieties )nce, and many varieties are gradually 
becoming species by assuming constant and peculiar charac- 
ters ; " but further on (p. 18), he adds, " except the original 
types or ancestors of the genus." 

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

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 organization terminating in the highest dicotyle- 
dons and vertebrata, these grades being few in number, and 
generally marked by intervals of organic character, which we 
find to be a practical difficulty in ascertaining affinities ; sec- 
ond, of another impulse connected with the vital forces, tend- 
ing, 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 appar- 
ently believes that organization 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 cannot 
see how the two supposed " impulses " account in a scientific 
sense for the numerous and beautiful coadaptations 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 early editions 
little accurate knowledge and a great want of scientific cau- 
tion, immediately had a very wide circulation. In my opin- 
ion it has done excellent service in this country in calling 
attention to the subject, in removing prejudice, and in thus 
preparing the ground for the reception of analogous views. 

In 1846 the veteran geologist M. J. d'Omalius d'Halloy 
published in an excellent though short paper ("Bulletin 


de PAcad. Roy. Bruxelles," torn. xiii. p. 581) his opinion 
that it is more probable that new species have been pro- 
duced by descent with modification than that they have been 
separately created : the author first promulgated this opinion 
in 1831. 

Professor Owen, in 1849 (" Nature of Limbs," p. 86) ; 
wrote as follows : " The archetypal idea was manifested in 
the flesh under diverse such modifications, upon this planet, 
long prior to the existence of those animal species that 
actually exemplify it. To what natural laws or secondary 
causes the orderly succession and progression of such organic 
phenomena may have been committed, we, as yet, are igno- 
rant." 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." Further on (p. xc), after referring to geographical 
distribution, he adds, "These phenomena shake our confi- 
dence in the conclusion that the Apteryx of New Zealand 
and the Red Grouse of England were distinct creations in 
and for those islands respectively. Always, also, it may be 
well to bear in mind that by the word ' creation ' the zoolo- 
gist means ' a process he knows not what.' " He amplifies 
this idea by adding that when such cases as that of the Red 
Grouse are " enumerated by the zoologist as evidence of dis- 
tinct creation of the bird in and for such islands, he chiefly 
expresses that he knows not how the Red Grouse came to be 
there, and there exclusively ; signifying also, by this mode 
of expressing such ignorance, his belief that both the bird 
and the islands owed their origin to a great first Creative 
Cause." If we interpret these sentences given in the sam© 
address, one by the other, it appears that this eminent 
philosopher felt in 1858 his confidence shaken that the 
Apteryx and the Red Grouse first appeared in their respec* 
tive homes " he knew not how," or by some process " he 
knew not what." 

This address was delivered after the papers t\v Mr. Wallace 
and myself on the Origin of Species, presently to be referred 
to, had been read before the Linnean Society. When the 
first edition of this work was published, I was so completely 
deceived, as were many others, by such expressions as " the 
continuous operation of creative power," that I included 
Professor Owen with other palaeontologists as being firmly 
convinced of the immutability of species ; but it appears 
("Anat. of VertebrateSj'Lvol. iii. p. 796) that this was on 


my part a preposterous error. In the last edition o? this 
work I inferred, and the inference still seems to. m» per- 
fectly just, from a passage beginning with the words "no 
doubt the type-form," etc. (Ibid., vol. i. p. xxxv.), that Pro- 
fessor Owen admitted that natural selection may have done 
something in the formation of a new species ; but this it 
appears (Ibid., vol iii. p. 798) is inaccurate and without evi- 
dence. I also gave some extracts from a correspondence 
between Professor Owen and the editor of the "London Re- 
view," from which it appeared manifest to the editor as 
well as to myself, that Professor Owen claimed to have pro- 
mulgated the theory of natural selection before I had done 
so ; and I expressed my surprise and satisfaction at this 
announcement; but as far as it is possible to 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 contro- 
versial writings as difficult to understand and to reconcile 
with each other, as I do. As far as the mere enunciation of 
the principle of natural selection is concerned, it is quite 
immaterial whether or not Professor Owen preceded me, for 
both of us, as shown in this historical sketch, were long ago 
preceded by Dr. Wells and Mr. Matthews. 

M. Isidore Geoffroy Saint-Hilaire, in his lectures delivered 
in 1850 (of which a resume appeared in the "Revue 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, V observation des animaux sauvages demontre 
deja la variabilite limitee des especes. Les experiences sur 
les animaux sauvages devenus domestiques, et sur les ani- 
maux domestiques redevenus sauvages, la demontrent 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. 322), propounded the doc- 
trine that all organic beings have descended from one pri- 
mordial 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 Spe- 
*"°s by means of Organic Affinity," the difficult attempt to 


give any idea of his views would be superfluous on my 

Mr. Herbert Spencer, in an essay (originally published ia 
the "Leader," March, 1852, and republished in his "Essays," 
in 1858), has contrasted the theories of the Creation and the 
Development of organic beings with remarkable skill and 
$DPrce. He argues from the analogy of domestic productions, 
from the changes which the embryos of many species un- 
dergo, from the difficulty of distinguishing species and varie- 
ties, 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 acquire- 
ment of each mental power and capacity by gradation. 

In 1852 M. Naudin, a distinguished botanist, expressly 
stated, in an admirable paper on the Origin of Species 
(" Revue Horticole," p. 102 ; since partly republished in the 
"Nouvelles Archives du Museum," torn. i. p. 171), his belief 
that species are formed in an analogous manner as varieties 
are under cultivation ; and the latter process he attributes to 
man's power of selection. But he does not show how selec- 
tion 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 Faction, 
incessante sur les etres vivantes determine, a toutes les 
epoques de Pexistence du monde, la forme, le volume, ex la 
duree de chacun d'eux, en raison de sa destinee dans l'ordre 
de choses dont il fait partie. C'est cette puissance qui har- 
monise chaque membre a l'ensembie, en l'appropriant a la 
fonction qu'il doit remplir dans l'organisme generale de la 
nature, fonction qui est pour lui sa raison d'etre." * 

* From references in Bronn's " Untersuchungen fiber die Entwickel- 
ungs-Gesetze," it appears that the celebrated botanist and palaeontolo- 
gist Unger published, in 1852, his belief that species undergo development 
and modification. Dalton, likewise, in Pander and Dalton's work on 
Fossil Sloths, expressed, in 1821, a similar belief. Similar views have, 
as is well known, been maintained by Oken in Lis mystical " Natur- 
Philosophie." From other references in Godron's work " Sur l'Espece," 
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 sepa- 
rate acts of creation, twenty-seven have written on special branches of 
natural history or geology. 

• •• 


In 1853 a celebrated geologist, Count Keyserling ("Bul- 
letin de la Soc. Geolog.," 2d 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 chemi- 
cally affected by circumambient molecules of a particular 
nature, and thus have given rise to new forms. 

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

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

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

The third volume of the "Journal of the Linnean 
Society " contains papers, read July 1, 1858, by Mr. 
Wallace and myself, in which, as stated in the introductory 
remarks to this volume, the theory of Natural Selection Is 
promulgated by Mr. Wallace with admirable force and 

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


In June, 1859, Professor Huxley gave a lecture before 
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 organization, 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 unproven, and 
sadly damaged by some of its supporters, is yet the only 
one to which physiology lends any countenance ; their 
existence would seem to show that the amount of modifica- 
tion which living beings have undergone during geological 
time is but very small in relation to the whole series of 
changes which they have suffered." 

In December, 1859, Dr. Hooker published his " Introduc- 
tion to the Australian Flora." In the first part of this o reat 
work he admits the truth of the descent and modification of 
species, and supports this doctrine by many original observe 

The first edition of this work was published on November 
24, 1859, and the second edition on January 7, 1860. 




Introduction •••••••• 1 



Causes of variability — Effects of habit and the use or disuse of 
parts — Correlated variation — Inheritance — Character of 
domestic varieties — Difficulty of distinguishing between 
varieties and species — Origin of domestic varieties from 
one or more species — Domestic pigeons, their differences 
and origin — Principles of selection, anciently followed, their 
effects — Methodical and unconscious selection — Unknown 
origin of our domestic productions — Circumstances favor- 
able to man's power of selection 6 



Variability — Individual differences — Doubtful species — Wide 
ranging, much diffused, and common species, vary most — 
Species of the larger genera in each country vary more fre- 
quently 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 ,. 



Its bearing on natural selection — The term used in a wide sense 
— Geometrical ratio of increase — Rapid increase of natural- 
ized animals and plants — Nature of the checks to increase-^ 
Competition universal — Effects of climate — Protection from 
the number of individuals — Complex relations of all animals 
and plants throughout nature — Struggle for life most severe 
between individuals and varieties of the same species: often 
severe between species of the same genus — The Relation of 
organism i\> organism the most important of all relations • • 






Hatural 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 gene- 
iality of intercrosses between individuals of the same species 

— Circumstances favorable and unfavorable to the results of 
Natural Selection, namely, intercrossing, isolation, number of 
individuals — Slow action — Extinction caused by Natural 
Selection — Divergence of Character, related to the diversity 
of inhabitants of any small area and to naturalization — 
Action of Natural Selection, through Divergence of Character 
and Extinction, on the descendants from a common parent — 
Explains the grouping of all organic beings — Advance in 
organization — Low forms preserved — Convergence of char- 
acter — Indefinite multiplication of species — Summary. . . 69 



Effects of changed conditions — Use and disuse, combined with 
natural selection; organs of flight and of vision — Acclimatiza- 
tion — Correlated variation — Compensation and economy of 
growth — False correlations — Multiple, rudimentary, and 
lowly organized 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 119 



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 Natural Selection 149 




Longevity — Modifications not necessarily simultaneous — Modifi- 
cations apparently of no direct service — Progressive develop- 



ment — Characters of small functional importance, the most 
constant — Supposed incompetence of natural selection to 
account for the incipiert stages of useful structures — Causes 
which interfere with the acquisition through natural selection 
of useful structures — Gradations of structure with changed 
functions — Widely different organs in members of the same 
class, developed from one and the same source — Reasons for 
disbelieving in great and abrupt modifications ...... 187 



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 227 



Distinction between the sterility of first crosses and of hybrids— 
Sterility various in degree, not universal, affected by close 
interbreeding, 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 hy- 
brids — Parallelism between the effects of changed conditions 
of life and of crossing — Dimorphism and Trimorphism — 
Fertility of varieties when crossed and of their mongrel off- 
spring not universal — Hybrids and mongrels compared inde- 
pendently of their fertility — Summary 260 



On the absence ©f 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 denuda- 
tion 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 denu- 
dation of granitic areas — On the absence of intermediate 
varieties in anyone formation — On the sudden appearance of 
groups of species — On tb°*r sudden appearance in the lowest 
known fossiliferous strata -Antiquitv of the habitable earth. 293 





On the slow and successive appearance of new species — On their 
different rates of change — Species once lost do not reappear 
— Groups of species follow the same general rules in their 
appearance and disappearance as do single species — On ex- 
tinction — 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 develop- 
ment of ancient forms — On the succession of the same types 
within the same areas — Summary of preceding and present 
chapter • • 32? 



Present distribution cannot be accounted for by differences in 
physical conditions — Importance of barriers — Affinity of the 
productions of the same continent — Centres of creation — 
Means of dispersal by changes of climate and of the level of 
the laud, and by occasional means — Dispersal during the 
Glacial period — Alternate Glacial periods in the north and 
south 350 



Distribution of fresh-water productions — On the inhabitants of 
oceanic islands — Absence of Batrachians and of terrestrial 
Mammals — On the relation of the inhabitants of islands to 
those cf *he nearest mainland — On colonization from the 
nearest «ource with subsequent modification — Summary of 
the las* 'tiid present chapter 380 



Classification, groups subordinate to groups — Natural system — 
Ruies and difficulties in classification, explained on the theory 
of descent with modification — Classification of varieties — 
Descent always used in classification — Analogical or adaptive 
characters — Affinities, general, complex and radiating — Ex- 
tinction separates and defines groups — Morphology, between 
members of the same class, between parts of the same indi- 
vidual — Embryology, laws of, explained by variations not 
supervening at an early age, and being inherited at a corre- 
sponding age — Rudimentary organs, their origin explained 
— Summary ••• • 40* 





Recapitulation of the objections to the theory of Natura. Selection 
— Recapitulation of the general and special circumstances iD 
its favor — Causes of the general belief in the inimutability o\ 
species — How far the theory of Natural Selection may be 
extended — Effects of its adoption on the study of Natural 
History — Concluding remarks 441 

6 t ossaky of Scientific Terms .•••• 475 



Whkn 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 rela- 
tions of the present to the past inhabitants of that conti- 
nent. These facts, as will be seen in the latter chapters of 
this volume, seemed to throw some light on the origin of 
species — that mystery of mysteries, as it has been called 
by one of our greatest philosophers. On my return home it 
occurred to me, in 1837, that 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 allowed myself 
to speculate on the subject, and drew up some short notes ; 
these I enlarged in 1844 into a sketch of the conclusions 
which then seemed to me probable : from that period to the 
present day I have steadily pursued the same object. I 
hope that I may be excused for entering on these personal 
details, as I give them to show that I have not been hasty 
in coming to a decision. 

My work is now (1859) nearly finished; but as it will 
take me many more years to complete it, and as my health 
is far from strong, I have been urged to publish this abstract. 
I have more especially been induced to do this, as Mr. Wal- 
lace, who is now studying the natural history of the Malay 
Archipelago, has arrived at almost exactly the same general 
conclusions that I have on the origin of species. In 1858 
he sent me a memoir on this subject, with a request that I 
would forward it to Sir Charles Lyell, who sent it to the 
Linnean Society, and it is published in the third volume of 
the Journal of that Society. Sir C. Lyell and Dr. Hooker, 



who both knew of my work — the latter having read my 
sketch of 1844 — honored me by thinking it advisable to 
publish, with Mr. Wallace's excellent memoir, some brief 
extracts from my manuscripts. 

This abstract, which I now publish, must necessarily be 
imperfect. I cannot here give references and authorities 
for my several statements ; and I must trust to the reader 
reposing some confidence in my accuracy. No doubt errors 
may have crept in, though I hope I have always been cau« 
tious 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 references, on which my conclusions have been 
grounded; and I hope in a future work to do this. For I 
am well aware that scarcely a single point is discussed in 
this volume on which facts cannot be adduced, often appar- 
ently 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 conceiv- 
able that a naturalist, reflecting on the mutual affinities of 
organic beings, on their embryological relations, their geo- 
graphical 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. Nevertheless, such a conclusion, even 
if well founded, would be unsatisfactory, until It could be 
shown how the innumerable species, inhabiting this world, 
have been modified, so as to acquire that perfection of 
structure and coadaptation which justly excites our admira- 
tion. Naturalists continually refer to external conditions, 
such as climate, food, etc., as the only possible cause of 
variation. In one limited sense, as we shall hereafter see # 


this may be true ; but it is preposterous to attribute to mere 
external conditions, the structure, for instance, of the wood- 
pecker, with its feet, tail, beak, and tongue, so admirably 
adapted to catch insects under the bark of trees. In the 
case of the mistletoe, which draws its nourishment from cer- 
tain trees, which has seeds that must be transported by 
certain birds, and which has flowers with separate sexes 
absolutely requiring the agency of certain insects to bring 
pollen from one flower to the other, it is equally preposter- 
ous 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 coadaptation. 
At the commencement of my observations it seemed to me 
probable that a careful study of domesticated animals and 
of cultivated plants would offer the best chance of making 
out this obscure problem. Nor have I been disappointed; 
in this and in all other perplexing cases I have invariably 
found that our knowledge, imperfect though it be, of vari- 
ation under domestication, afforded the best and safest clew. 
I may venture to express my conviction of the high value of 
such studies, although they have been very commonly neg- 
lected by naturalists. 

From these considerations, I shall devote the first chapter 
of this abstract to variation under domestication. We shall 
thus see that a large amount of hereditary modification is at 
least possible; and, what is equally or more important, we 
shall see how great is the power of man in accumulating, by 
his selection successive slight variations. I will then pass 
on 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 favorable to variation. In the 
next chapter the struggle for existence among all organic 
beings throughout the world, which inevitably follows from 
the high geometrical ratio of their increase, will be considered. 
This is the doctrine of Malthus, applied to the whole animal 
and vegetable kingdoms. As many more individuals of each 
species are born than can possibly survive; and as, conse- 
quently, there is a, frequently recurring struggle for exist- 
ence, it follows that any be_iri£» '# it vary however slightly 

4 UN'l-KODUC'ilON, 

in any mauuui profitable to itself, under the complex ana 
sometimes varying conditions of life, will have a better 
chance of surviving, and thus be naturally selected. From 
the strong principle of inheritance, any selected variety 
will tend to propagate its new and modified form. 

This fundamental subject of natural selection will be 
treated at some length in the fourth chapter ; and we shall 
then see how natural selection almost inevitably causes 
much extension of the less improved forms of life, anc 
leads to what I have called divergence of character. In the 
next chapter I shall discuss the complex and little known laws 
of variation. In the five succeeding chapters, the most appar- 
ent 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 perfected 
into a highly developed being or into an elaborately con- 
structed organ ; secondly, the subject of instinct, or the mental 
powers of animals ; thirdly, hybridism, or the infertility of 
species and the fertility of varieties when intercrossed ; and 
fourthly, the imperfection of the geological record. In the 
next chapter I shall consider the geological succession of 
organic beings throughout time; in the twelfth and thirteenth, 
their geographical distribution throughout space ; in the 
fourteenth, 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 wel- 
fare and, as I believe, the future success and modification of 
every inhabitant of this world. Still less do we know of 
the mutual relations of the innumerable inhabitants of the 
world 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* thai 


each species has been independently created — is erroneous 
I am fully convinced that species are not immutable; but 
that those belonging to what are called the same genera are 
lineal descendants of some other and generally extinct 
species, in the same manner as the acknowledged varieties 
of any one species are the descendants of that species. 
Furthermore, 1 am convinced that natural selection has been 
the most important, but not the exclusive, means of modifi- 




Causes of Variability — Effects of Habit and the Use or Disuse of Parts 

— Correlated Variation — Inheritance — Character of Domestic 
Varieties — Difficulty of distinguishing between Varieties and 
Species — Origin of Domestic Varieties from one or more Specie? 

— Domestic Pigeons, their Differences and Origin — Principles of 
Selection, anciently followed, their Effects — Methodical and Un- 
conscious Selection — Unknown Origin of our Domestic Produc- 
tions — Circumstances favorable to Man's Power of Selection. 


When we compare the individuals of the same variety or 
sub-variety of our older cultivated plants and animals, one of 
the first points which strikes us is, that they generally differ 
more from each other than do the individuals of any one 
species or variety in a state of nature. And if we reflect on 
the vast diversity of the plants and animals which have been 
cultivated, and which have varied during all ages under the 
most different climates and treatment, we are driven to con- 
clude that this great variability is due to our domestic pro- 
ductions having been raised under conditions of life not so 
uniform as. 3rd somewhat different from, those to which the 
parent species had been exposed under nature. There is. 
also, some probability in the view propounded by Andrew 
Knight, that this variability may be partly connected with 
excess of food. It seems clear that organic beings must be 
exposed during several generations to new conditions to ^luse 
any great amount of variation; and that, when the organiza- 
tion 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 cul- 
tivated plants, such as wheat, still yield new varieties ; our 
oldest domesticated animals are still capable of rapid improve- 
ment or modification. 

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 organization 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 Weismann has lately insisted, and 
as I have incidentally shown in my work on " Variation 
under Domestication," there are two factors : namely, the 
nature of the organism and the nature of the conditions. 
The former seems to be much the more important ; for nearly 
similar variations sometimes arise under, as far as we car 
judge, dissimilar conditions; and, on the other hand, dissimi 
tar variations arise under conditions which appear to be nearly 
uniform. The effects on the offspring are either definite or 
indefinite. They may be considered as definite when all or 
nearly all the offspring of individuals exposed to certain con- 
ditions during several generations are modified in the same 
manner. It is extremely difficult to come to any conclusion 
in regard to the extent of the changes which have been thus 
definitely induced. There can, however, be little doubt about 
many slight changes, such as size from the amount of food, 
color from the nature of the food, thickness of the skin and 
hair from climate, etc. Each of the endless variations which 
we see in the plumage of our fowls must have had some effi- 
cient cause ; and if the same cause were to act uniformly 
during a long series of generations on many individuals, all 
probably would be modified in the Game manner. Such facts 
as the complex and extraordinary outgrowths which variably 
follow from the insertion of a minute drop of poison by a 
gall-producing insect, show us what singular modifications 
might result in the case of plants from a chemical change in 
the nature of the sap. 

Indefinite variability is a much more common result of 
changed conditions than definite variability, and has proba- 
bly played a more important part in the formation of our 
domestic races. We see indefinite variability in the endless 
slight peculiarities which distinguish the individuals of the 
same species, and which cannot be accounted for by inherit- 
ance 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 country 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 varia- 
tions. All such changes of structure, whether extremely 


slight or strongly marked, which appear among many indi- 
viduals 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, according to their state of body or 
constitution, causing coughs or colds, rheumatism, or inflam- 
mation of various organs. 

With respect to what I have called the indirect action of 
•hanged conditions, namely, through the reproductive sys^ 
tern being affected, we may infer that variability is thus 
induced, partly from the fact of this system being extremely 
sensitive to any change in the conditions, and partly from 
the similarity, as Kolreuter and others have remarked, 
between the variability which follows from the crossing of 
distinct species, and that which may be observed with plants 
and animals when reared under new or unnatural conditions. 
Many facts clearly show how eminently susceptible the 
reproductive system is to very slight changes in the sur- 
rounding conditions. Nothing is more easy than to tame an 
animal, and few things more difficult than to get it to breed 
freely under confinement, even when the male and female 
unite. How many animals there are which will not breed, 
though kept in an almost free state in their native country ! 
This is generally, but erroneously, attributed to vitiated 
instincts. Many cultivated plants display the utmost vigor, 
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 deter- 
mine whether or not a plant will produce seeds. I cannot 
here give th^ details which I have collected and elsewhere 
published on this curious subject ; but to show how singular 
the laws are which determine the reproduction of animals 
under confinement, I may mention that carnivorous animals, 
even from the tropics, breed in this country pretty freely 
under confinement, with the exception of the plantigrades or 
bear family, which seldom produce young ; whereas carniv- 
orous birds, with the rarest exceptions, hardly ever lay fertile 
eggs. Many exotic plants have pollen utterly worthless, in 
the same condition 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 confine- 
ment ; and when, on the other hand, we see individuals, though 
takpn young from a state of nature perfectly tamed, long-lived 
aud healthy (of which I could g\ve numerous instances), yet 


having their reproductive 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 reproduc- 
tive organs are not easily affected ; so will some animals and 
plants withstand domestication or cultivation, and vary very 
slightly — perhaps hardly more than in a state of nature. 

Some naturalists have maintained that all variations are 
connected with the act of sexual reproduction ; but this is 
certainly an error ; for I have given in another work a long 
list of "sporting plants," as they are called by gardeners; 
that is, of plants which have suddenly produced a single 
bud with a new and sometimes widely different character 
from that of the other buds on the same plant. These bud 
variations, as they may be named, can be propagated by 
grafts, offsets, etc., and sometimes by seed. They occur 
rarely under nature, but are far from rare under culture. 
As a single bud out of 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 conditions, have 
sometimes yielded nearly the same variety — for instance, 
buds on peach-trees producing nectarines, and buds on 
common roses producing moss-roses — we clearly see that 
the nature of the conditions is of subordinate importance in 
comparison wivh the nature of the organism in determining 
each particular form of variation ; perhaps of not more im- 
portance than the nature of the spark, by which a mass of 
combustible matter is ignited, has in determining the nature 
of the flames. 


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 comparison with these organs in other countries, 
is probably another instance of the effects of use. Not one 
of our domestic animals can be named which has not in 
soma country drooping ears ; and the view which has been 
s, gested 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 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 Saint 
Hilaire's great work on this subject. Breeders believe that 
long limbs are almost always accompanied by an elongated 
head. Some instances of correlation are quite whimsical ; 
thus cats which are entirely white and have blue eyes are 
generally deaf ; but it has been lately stated by Mr. Tait 
that this is confined to the males. Color and constitutional 
peculiarities go together, of which many remarkable cases 
could be given among animals and plants. From facts col- 
lected by Heusinger, it appears that white sheep and pigs 
are injured by certain plants, while dark-colored individuals 
escape: Professor Wyman has recently communicated to 
me a good illustration of this fact ; on asking some farmers 
in Virginia how it was that all their pigs were black, they 
informed him that the pigs ate the paint-root (Lachnanthes), 
which colored their bones pink, and which caused the hoots 
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. 
Hence if man goes on selecting, and thus augmenting, any 
peculiarity, he will almost certainly modify unintentionally 
other parts of the structure, owinc-to the mysterious laws of 


The results of the various, unknown, or but dimly under* 
stood laws of variation are infinitely complex and diversified. 
i*.t 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, etc. ; 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 organization seems to have become 
plastic, and departs in a slight degree from that of the 
parental type. 

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' 
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 among individuals, apparently 
exposed to the same conditions, any very rare deviation, due 
to some extraordinary combination of circumstances, appears 
in the parent — say, once among several million individu- 
als — 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 al- 
binism, prickly skin, hairy bodies, etc.. appearing in several 
members of the same family. If strange and rare deviations 
of structure are really inherited, less strange and commoner 
deviations may be freely admitted to be inheritable. Per- 
haps the correct way of viewing the whole subject 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 different 
species, is sometimes inherited and sometimes not so; why 
the child often reverts in certain characteristics to its grand- 
father or grandmother or more remote ancestor; why a 
peculiarity is often transmitted from one sex to both sexes, 
or to one sex alone, more commonly but not exclusively to 
the like sex. It is a fact of some importance to us, that 


peculiarities appearing in the males of our domestic breeds 
are often transmitted, either exclusively or in a much 
greater degree, to the males alone. A much more impor- 
tant rule, which I think may be trusted, is that, at whatever 
period of life a peculiarity first appears, it tends to reappear 
in the offspring at a corresponding age, though sometimes 
earlier. In many cases 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 appear 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 em- 
bryology. These remarks are of course confined to the first 
appearance of the peculiarity, and not to the primary cause 
which may have acted on the ovules or on the male element ; 
in nearly the same manner as the increased length of the 
horns in the offspring from a short-horned cow by a long- 
horned bull, though appearing late in life, is clearly due to 
the male element. 

Having alluded to the subject of reversion, I may here 
refer to a statement often made by naturalists — namely, 
that our domestic varieties, when run wild, gradually but 
invariably revert in character to their aboriginal stock. 
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 endeavored 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 aboriginal 
stock was, and so could not tell whether or not nearly per- 
fect reversion had ensued. It would be necessary, in order 
to prevent the effects of intercrossing, that only a single 
variety should have been turned loose in its new home. 
Nevertheless, as our varieties certainly do occasionally revert 
in some of their characters to ancestral forms, it seems to 
me not improbable that if we could succeed in naturalizing, 
or were to cultivate, during many generations, the several 


races, for instance, of the cabbage, in very poor s^U. — in 
which case, however, some effect would have to be attributed 
to the definite action of the poor soil — that they would, to 
a large extent, or even wholly, revert to the wild aboriginal 
stock. Whether or not the experiment would succeed is not 
of great importance for our line of argument; for by the 
experiment itself the conditions of life are changed. If it 
could be shown that our domestic varieties manifested a 
strong tendency to reversion — that is, to lose their acquired 
characters, while kept under the same conditions and while 
kept in a considerable 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* favor 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 escu- 
lent vegetables, for an unlimited number of generations, 
would be opposed to all experience. 


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 mon- 
strous character ; by which I mean, that, although differing 
from each other and from other species of the same genus, 
in several trifling respects, they often differ in an extreme 
degree in some one part, both when compared one with 
another, and more especially when compared with the species 
unaer nature to which they are nearest allied. With these 
exceptions (and with that of the perfect fertility of varie- 
ties 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 casts 
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 vari- 


eties. If any well-marked distinction existed between a 
domestic race and a species, this source of doubt would not 
so perpetually recur. It has often been stated that domestic 
races do not differ from each other in characters of generic 
value. It can be shown that this statement is not correct j 
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 domesticated 

In attempting to estimate the amount of structural dif- 
ference 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 the different 
quarters of the world. I do not believe, as we shall pres- 
ently 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 domesticated species, 
there is presumptive or even strong evidence that all are 
descended from a single wild stock. 

It has often been assumed that man has chosen for domes- 
tication animals and plants having an extraordinary inherent 
tendency to vary, and likewise to withstand diverse cli- 
mates. I do not dispute that these capacities have added 
largely to the value of most of our domesticated produc- 
tions ; 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 conclu- 
sion, 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 backward the his- 
tory of civilization, and shows that animals were domesti- 
cated 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 ani- 
mals. 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 civilization; 
and this again implies a long continued previous period of 
less advanced civilization, during which the domesticated 
animals, kept by different tribes in different districts, might 
have varied and given rise to distinct races. Since the dis- 
covery of flint tools in the superficial formations of many 
parts of the world, all geologists believe that barbarian men 
existed at an enormously remote period ; and we know that 
at the present day there is hardly a tribe so barbarous as 
not to have domesticated at least the dog. 

The origin of most of our domestic animals will probably 
forever remain vague. But I may here state that, looking 
to the domestic dogs of the whole w r orld, I have, after a 
laborious collection of all known facts, come to the conclu- 
sion that several wild species of Canidse have been tamed, 
and that their blood, in some cases mingled together, flow* 
in the veins of cur domestic breeds. In regard to sheep 
and goats I can form no decided opinion. From facts com- 
municated to me by Mr. Blyth, on the habits, voice, consti- 
tution, 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 com- 
petent 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 differ much from each other, the evidence is clear 
that they are all descended from the common duck and wild 

The doctrine of the origin of our several domestic races 
from several aboriginal stocks, has been carried to an absurd 
extreme by some authors. They 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, etc., 
but that each of these kingdoms possesses several peculiar 
breeds of cattle, sheep, etc., 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 through- 
out 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, etc. 
— so urflike all wild Canidse — 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 abori- 
ginal species ; but by crossing we can only get forms in 
Home dsflrr^ rt 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, etc., in the 
wild state. Moreover, the possibility of making distinct 
races by crossing has been greatly exaggerated. Many cases 
are on record showing that a race may be modified by occa- 
sional crosses if aided by the careful selection of the 
individuals which present the desired character; but to 
obtain a race intermediate between two quite distinct races 
would be very difficult. Sir J. Sebright expressly experi- 
mented with this object and failed. The offspring from the 
first cross between two pure breeds is tolerably and some- 
times (as I have found with pigeons) quite uniform in 
character, and every thing seems simple enough ; but when 
these mongrels are crossed one with another for several gen- 
erations, hardly two of them are alike, and then the difficulty 
of the task becomes manifest. 



Believing that it is always best to study some special 
group, I have, after deliberation, taken up domestic pigeons. 
I have kept every breed which I could purchase or obtain, 
and have been most kindly favored 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 some- 
thing astonishing. Compare the English carrier and the 
short-faced tumbler, and see the wonderful difference in 
their beaks, entailing corr >sponding differences in their 
skulls. The carrier, more specially the male bird, is also 
remarkable from the wonderful development of the carun- 
culated 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 tum- 
bler has a beak in outline almost like that of a finch; and 
the common tumbler has the singular inherited habit of 
flying at a great height in a compact flock and tumbling in 
the air head over heels. The runt is a bird of great size, 


with long 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, lias 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 ex- 
panded and are carried so erect that in good birds the head 
and tail touch ; the oil-gland is quite aborted. Several other 
less distinct breeds might be specified. 

In the skeletons of the several breeds, the development 
of the bones of the face, in length and breadth and curva- 
ture, 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 presence 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 correlation 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 rela- 
tive length of the leg and foot; the number of scutellse 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 snape and size of the eggs vary. The man* 


Her 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 

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 fan- 
tail in the same genus ; more especially as in each of these 
breeds several truly-inherited sub-breeds, or species, as he 
would call them, could be shown him. 

Great as are the differences between the breeds of the 
pigeon, I am fully convinced that the common opinion of 
naturalists is correct, namely, that all are descended from 
the rock-pigeon (Columba livia), including under this term 
several geographical races or sub-species, which differ from 
each other in the most trifling respects. As several of the 
reasons which have led me to this belief are in some degree 
applicable in other cases, I will here briefly give them. If 
the several breeds are not varieties, and have not proceeded 
from the rock-pigeon, they must have descended from at 
least seven or eight 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 pos- 
sessed the characteristic enormous crop ? The supposed 
aboriginal stocks must all have been rock-pigeons, that is, 
they did not breed or willingly perch on trees. But besides 
C. livia, with its geographical sub-species, only two or three 
other species of rock-pigeons are known ; and these have not 
any of the characters of the domestic breeds. Hence the 
supposed aboriginal stocks must either still exist in the 
countries where they were originally domesticated, and yet 
be unknown to ornithologists ; and this, considering their 
size, habits, and remarkable characters, seems improbable ; 
or they must have become extinct in the wild state. But 
birds breeding on precipices, and good flyers, are unlikely 
to be exterminated ; and the common rock-pigeon, which has 
the same habits with the domestic breeds, has not been 
exterminated even on several of the smaller British islets, 
or on the shores of the Mediterranean. Hence the supposed 
extermination of so many species having similar habits with 


the rock-pigeon seems a very rash assumption. Moreover, 
the several above-named domesticated breeds have been 
transported to all parts of the world, and, therefore, some 
of them must have been carried back again into their native 
country ; but not one has become wild or feral, though the 
dovecot-pigeon, which is the rock-pigeon in a very slightly 
altered state, has become feral in several places. Again, all 
•ecent experience shows that it is difficult to get wild 
urimals to breed freely under domestication; yet on the 
iiypothesis of the multiple origin of our pigeons, it must be 
assumed that at least seven or eight species were so thor- 
oughly domesticated in ancient times by half-civilized man 
as to be quite prolific under confinement. 

An argument of great weight, and applicable in several 
other cases, is, that the above-specified breeds, though agree- 
ing generally with the wild rock-pigeon in constitution, 
habits, voice, coloring, and in most parts of their structure, 
yet are certainly highly abnormal in other parts ; we may 
look in vain through the whole great family of Columbidae 
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-civilized man succeeded in thoroughly 
domesticating several species, but that he intentionally or 
by chance picked out extraordinarily abnormal species ; and 
further, that these very species have since all become ex- 
tinct or unknown. So many strange contingencies are 
improbable in the highest degree. 

Some facts in regard to the coloring 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 checkered 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. Moreover, 
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 observed : I crossed some white fan- 
tails, 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 color, with the white 
loins, double black wing-bar, and barred and white-edged 
tail-feathers, as any wild rock-pigeon ! We can understand 
these facts, on the well-known principle of reversion to 
ancestral characters, if all the domestic breeds are descended 
from the rock-pigeon. But, if we deny this, we must make 
one of the two following highly improbable suppositions. 
Either, first, that all the several imagined aboriginal stocks 
were colored and marked like the rock-pigeon, although no 
other existing species is thus colored and marked, so that in 
each separate breed there might be a tendency to revert to 
the very same colors and markings. Or, secondly, that each 
breed, even the purest, has within a dozen, or at most within 
a score, of generations, been crossed by the rock-pigeon : I 
say within a dozen or twenty generations, for no instance is 
known of crossed descendants reverting to an ancestor of 
foreign blood, removed by a greater number of generations. 
In a breed which has been crossed only once the tendency to 
revert to any character derived from such a cross will natur, 
ally become less and less, as in each succeeding generation 
there will be less of the foreign blood; but when there ha* 
been no cross, and there is a tendency in the breed to revert 
to a character which was lost during some former generation, 
this tendency, for all that we can see to the contrary, may 
be transmitted undiminished for an indefinite number of 
generations. These two distinct cases of reversion are 
often confounded together by those who have written on 

Lastly, the hybrids or mongrels from between all the 
breeds of the pigeon are perfectly fertile, as I can state 
from my own observations, purposely made, on the most 
distinct breeds. Now, hardiy any cases nave oeen asce*- 
tained with certainty of hybrids from two quite distinct 


species of animals being perfectly fertile. Some authors 
believe that long-continued domestication eliminates this 
strong tendency to sterility in species. From the history 
of the dog, and of some other domestic animals, this con- 
clusion is probably quite correct, if applied to species 
closely related to each other. But to extend it so far as to 
suppose that species, aboriginally as distinct as carriers, 
tumblers, pouters, and fan tails now are, should yield off- 
spring perfectly fertile inter se, would be rash in the 

From these several reasons, namely, the improbability 
of man having formerly made seven or eight supposed 
species of pigeons to breed freely under domestication — 
these supposed species being quite unknown in a wild state, 
and their not having become anywhere feral — these species 
presenting certain very abnorma.1 characters, as compared 
with a)l other Columbidse, though so like the rock-pigeon 
in most respects — the occasional reappearance of the blue 
color and various black marks in all the breeds, both when 
kept pure and when crossed — and lastly, the mongrel off' 
spring being perfectly fertile — from these several reasons, 
taken together, we may safely conclude that all our 
domestic breeds are descended from the rock-pigeon or 
Columba livia with its geographical sub-species. 

In favor 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 
f rom distant countries, we can make, between them and 
tne 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 j the earliest knows 


record 01 ^iget>ns is in the fifth Egyptian dynasty, about 
3000 fc.<j., 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 
Romans, as we lwar 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 historian, 
" His Majesty, by crossing the breeds, which method was 
never practised before, has improved them astonishingly." 
About this same period the Dutch were as eager about 
pigeons as were the old Romans. The paramount impor- 
tance of these considerations in explaining the immense 
amount of variation which pigeons have undergone, will 
likewise be obvious when we treat of selection. We shaD 
then, also, see how it is that the several breeds so often 
have a somewhat monstrous character. It is also a most 
favorable 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 insufficient, length ; because when I first 
kept pigeons and watched the several kinds, well knowing 
how truly they breed, I felt fully as much difficulty in 
believing that since they had been domesticated they had 
all proceeded from a common parent, as any naturalist could 
in coming to a similar conclusion in regard to the many 
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 culti- 
vators 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 Ribston-pippin or Codiin-apple, could 
ever have proceeded from the seeds of the same tree. In- 
numerable other examples could be given. The explanation, 
I think, is simple : from long-continued study they are 
strongly impressed with the differences between the several 
races ; and though they well know that each race varies 
slightly, for they win their prizes by selecting such slight 
differences, yet they ignore all general arguments, and refuse 
to sum up in their minds slight differences accumulated dur 
ing many successive generations. May not those naturalists 
who, knowing far less of the laws of inheritance thai) does 
the breeder, and knowing no more than he does of the inter- 
mediate links in the long lines of descent, yet admit that 
many of our domestic races are descended from the same 
parents — may they not learn a lesson of caution, when they 
deride the idea of species in a state of nature being lineal 
descendants of other species ? 



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 attributed 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 ac- 
count by such agencies for the differences between a dray 
and race horse, a greyhound and bloodhound, a carrier and 
tumbler pigeon. One of the most remarkable features in 
our domesticated races is that we see in them adaptation, 
not indeed to the animal's or plant's own good, but to man's 
use or fancy. Some variations useful to him have probably 
arisen suddenly, or by one step ; many botanists, fo: instance, 
believe that the fuller's teasel, with its hooks, which cannot 
be rivalled by any mechanical contrivance, is only a variety 
of the wild Dipsacus ; and this amount of change may have 
suddenly arisen in a seedling. So it has probably been with 
the turnspit dog ; and this is known to have been the case 
with the ancon sheep. But when we compare the dray- 
horse and race-horse, the dromedary and camel, the various 
breeds of sheep fitted either for cultivated land or mountain 
pasture, with the wool of one breed good for one purpose, 
and that of another breec 1 for another purpose j when we 


compare the many breeds of dogs, each good for man in dif- 
ferent ways; when we compare the game-cock, so pertina- 
cious 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 compare the host of 
agricultural, culinary, orchard, and flower-garden races of 
plants, most useful to man at different seasons and for differ 
ent 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 i 
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 
variations ; man adds them up in certain directions useful 
to him. In this sense he may be said to have made for him- 
self useful breeds. 

The great power of this principle of selection is not hypo- 
thetical. It is certain that several of our eminent breeders 
have, even within a single lifetime, modified to a large ex- 
tent their breeds of cattle and sheep. In order fully to 
realize 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 ani- 
mal's organization as something plastic, which they can 
model almost as they please. If I had space I could quote 
numerous passages to this effect from highly competent 
authorities. Youatt, who was probably better acquainted 
with the works of agriculturists than almost any other in- 
dividual, 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 
importance of the principle of selection in regard to merino ' 
sheep is so fully recognized that men follow it as a trade: 
the sheep are placed on a table and are studied, like a pic- 
ture 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 some- 
times among closely allied sub-breeds. And when a cross 
has been made, the closest selection is far more indispens- 
able even than in ordinary cases. If selection consisted 
merely in separating some very distinct variety, and breed- 
ing 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 inapprecia- 
ble by an uneducated eye — differences which I for one 
have vainly attempted to appreciate. Not one man in a 
thousand has accuracy of eye and judgment sufficient to 
become an eminent breeder. If gifted with these quali- 
ties, and he studies his subject for years, and devotes his 
lifetime to it with indomitable perseverance, he will succeed, 
and may make great improvements ; if he wants any of 
these qualities, he will assuredly fail. Few would readily 
believe 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 sup- 
poses that our choicest productions have been produced 
by a single variation from the aboriginal stock. We have 
proofs that this has not been so in several cases in which 
exact records have been kept ; thus, to give a very trifling 
instance, the steadily increasing size of the common goose- 
berry 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 observing 
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 a 


br tubers, or whatever part is valued, in the kitchen-garden, 
In comparison with the flowers of the same varieties ; and 
the diversity of fruit of the same species in the orchard, in 
comparison with the leaves and flowers of the same set of 
varieties. See how different the leaves of the cabbage are, 
and how extremely alike the flowers ; how unlike the 
flowers of the heartsease are, and how alike the leaves ; how 
much the fruit of the different kinds of gooseberries differ 
in size, color, 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 corre- 
lated variation, the importance of which should never be 
overlooked, will insure some differences ; but, as a general 
rule, it cannot 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 pub- 
lished on the subject ; and the result has been, in a corre- 
sponding degree, rapid and important. But it is very far 
from true that the principle is a modern discovery. I could 
give several references to works of high antiquity, in which 
the full importance of the principle is acknowledged. In 
rude and barbarous periods of English history choice animals 
were often imported, and laws were passed to prevent 
their exportation : the destruction of horses under a cer- 
tain 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 ency- 
clopaedia. Explicit rules are laid down by some of the 
Roman classical writers. From passages in Genesis, it is 
clear that the color 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 
color, as do some of the Esquimaux their team of dogs- 
Livingstone states that good domestic breeds are highly 
valued by the negroes in the interior of Africa who have 


not associated with Europeans. Some of these facts do 
not show actual selection, but they show that the breeding 
of domestic animals was carefully attended to in ancient 
times, and \j now attended to by the lowest savages. It 
would, indeed, have been a strange fact, had attention not 
been paid to breeding, for the inheritance of good and bad 
qualities is so obvious. 


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, tor 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 afterward 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, etc., by this very same 
process, only carried on more methodically, did greatly 
modify, even during their lifetimes, the forms and qualities 
of their cattle. Slow and insensible changes of this kind 
can never be recognized unless actual measurements or care- 
ful drawings of the breeds in question have been made long 
ago, which may serve for comparison. In some cases, how- 
ever, unchanged, or but little changed, individuals of the 
same breed exist in less civilized districts, where the breed 
has been less improved. There is reason to believe that 
King Charles' spaniel has been unconsciously modified to a 
large extent since the time of that monarch. Some highly 
competent authorities are convinced that the setter is 
directly derived from the spaniel, and has probably 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 Id 
Spain like our pointer. 


By a similar process of selection, and by careful training, 
English race-horses have come to surpass in fleetness and 
size the parent Arabs, so that the latter, by the regulations 
for the Goodwood Races, are favored in the weights which 
they carry. Lord Spencer and others have shown how the 
cattle of England have increased in weight and in edrly 
maturity, compared with the stock formerly kept in this 
country. By comparing the accounts given in various old 
treatises of the former and present state of carrier and 
tumbler pigeons in Britain, India, and Persia, we can trace 
the stages through which they have insensibly passed, and 
come to differ so greatly from the rock-pigeon. 

Youatt gives an excellent illustration of the effects of a 
course of selection which may be considered as unconscious, 
in so far that the breeders could never have expected or 
even wished, to produce the result which ensued — namely, 
the production of the distinct strains. 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 upward 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. BakewelFs 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 

If there exist savages so barbarous as never to think of 
the inherited character of the offspring of their domestic 
animals, yet any one animal particularly useful to them, 
for any special purpose, would be carefully preserved dur- 
ing famines and other accidents, to which savages are so 
liable, and such choice animals would thuj 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 

In plants the same gradual process of improvement 
through the occasional preservation of the best individuals, 
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 recognized in the increased size 
and beauty which we now see in the varieties of the heart's* 
ease, 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 heart's- 
ease 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- 
«tock. The pear, though cultivated in classical times, 
appears, from Pliny's description, to have been a fruit of 
very inferior quality. I have seen great surprise expressed 
in horticultural works at the wonderful skill of gardeners 
in having produced such splendid results from such poor 
materials ; but the art has been simple, and, as far as the 
final result is concerned, has been followed almost uncon- 
sciously. It has consisted in always cultivating the best 
known variety, sowing its seeds, and, when a slightly better 
variety chanced to appear, selecting it, and so onward. 
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 natur- 
ally chosen and preserved the best varieties they could any- 
where find. 

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

In regard to the domestic animals kept by uncivilized 
man, it should not be overlooked that they almost always 
have to struggle for their own food, at least during cer- 


tain seasons. And in two countries very differently cir- 
cumstanced, individuals of the same species, having slightly 
different constitutions or structure, would often succeed 
better in the one country than in the other ; and thus by a 
process of " natural selection," as will hereafter be more 
fully explained, two sub-breeds might be formed. This, per- 
haps, partly explains why the varieties kept by savages, as 
lias been remarked by some authors, have more of the char- 
acter of true species than the varieties kept in civilized 

On the view here given of the important part which 
selection by man has played, it becomes at once obvious, 
how it is that our domestic races show adaptation in their 
structure or in their habits to man's wants or fancies. We 
can, I think, further understand the frequently abnormal 
character of our domestic races, and likewise their differ- 
ences 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 struc- 
ture excepting such as is externally visible ; and indeed he 
rarely cares for what is internal. He can never act by 
selection, excepting on variations which are first given to 
him in some slight degree by nature. No man would ever 
try to make a fan tail till he saw a pigeon with a tail 
developed in some slight degree in an unusual manner, 
or a pouter till he saw a pigeon with a crop of somewhat 
unusual size ; and the more abnormal or unusual any char- 
acter 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 fan tail is, I have no doubt, in most 
cases utterly incorrect. The man who first selected a 
pigeon with a slightly larger tail, never dreamed what the 
descendants of that pigeon would become through long- 
continued, partly unconscious and partly methodical, selec- 
tion. Perhaps the parent-bird of all fantails had only 
fourteen tail-feathers somewhat expanded, like the present 
Java fantail, or like individuals of other and distinct breeds, 
in which as many as seventeen tail-feathers have been 
counted. Perhaps the first pouter-pigeon did not inflate 
its crop much more than the turbid now does the upper 
part of its oesophagus — a habit which is disregarded by all 
fanciers, as it is not one of the points of the breed. 

Nor let it be thought that some great deviation of struc- 
ture would be necessary to catch the fancier's eye j he per- 


ceives extremely small differences, and it is in human 
nature to value any novelty, however slight, in one's own 
possession. Nor must the value which would formerly 
have been set on any slight differences in the individuals 
of the same species, be judged of by the value which is 
now set on them, after several breeds have fairly been 
established. It is known that with pigeons many slight 
variations now occasionally appear, but these are rejected 
as faults or deviations from the standard of perfection in 
each breed. The common goose has not given rise to any 
marked varieties ; hence the Toulouse and the common 
breed, which differ only in color, 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 neighborhood. 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 recog- 
nized as something distinct and valuable, and will then 
probably first receive a provincial name. In semi-civilized 
countries, with little free communication, the spreading of 
a new sub-breed would be a slow process. As soon as the 
points of value are once acknowledged, the principle, as 1 
have called it, of unconscious 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 another, according to the state of civilization 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 sucb 
tlow, varying;, and insensible changes. 




I will now say a few words on the circumstances, favor 
able or the reverse, to man's power of selection. A high 
degree of variability is obviously favorable, as freely giving 
the materials for selection to work on ; not that mere indi- 
vidual differences are not amply sufficient, with extreme 
care, to allow of the accumulation of a large amount of 
modification in almost any desired direction. But as vari- 
ations 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 part 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 
fai more successful than amateurs in raising new and val- 
uable varieties. A large number of individuals of an ani- 
mal or plant can be reared only where the conditions for 
its propagation are favorable. When 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 qual- 
ities or structure. Unless such attention be paid, nothing 
can be effected. I have seen it gravely remarked, that it 
was most fortunate that the strawberry began to vary just 
when gardeners began to attend to this plant. No doubt the 
strawberry had always varied since it was cultivated, but 
the slight variations had been neglected. As soon, however, 
as gardeners picked out individual plants with slightly 
Varger, earlier, or better fruit, and raised seedlings from 
them, and again picked out the best seedlings and bred from 
them, then (with some aid by crossing distinct species) 
those many admirable varieties of the strawberry were 
raised which have appeared during the last half-century. 

With animals, facility in preventing crosses is an impor- 
tant element in the formation of new races — at least, in a 
country which is already stocked with other races. In thia 


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 favored 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 chil- 
dren, we rarely see a distinct breed long kept up ; such breeds 
as we do sometimes see are almost always imported from 
some other country. Although I do not doubt that some 
domestic animals vary less than others, yet the rarity or 
absence of distinct breeds of the cat, the donkey, peacock, 
goose, etc., may be attributed in main part to selection not 
having been brought into play : in cats, from the difficulty 
in pairing them ; in donkeys, from only a few being kept by 
poor people, and little attention paid to their breeding ; for 
recently in certain parts of Spain and of the United States 
this animal has been surprisingly modified and improved by 
careful selection ; in peacocks, from not being very easily 
reared and a large stock not kept; in geese, from being valu- 
able 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 sin- 
gularly inflexible organization, though it has varied to a 
slight extent, as I have elsewhere described. 

Some authors have maintained that the amount of varia- 
tion in our domestic productions is soon reached, and can 
never afterward 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 im- 
proved 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. No doubt, as Mr. Wallace has 
remarked with much truth, a limit will be at last reached. 
For instance, there must be a limit to the fleetness of any 
terrestrial animal, as this will be determined by the friction 


to be overcome, the weight of the 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 spe- 
cies differ from each other in almost every character, which 
man has attended to and selected, more than do the distinct 
species of the same genera. Isidore Geoffroy Saint Hilaire 
has proved this in regard to size, and so it is with color, 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 comparably stronger, than any 
two natural species belonging to the same genus. So with 
plants, the seeds of the different varieties of the bean or 
maize probably differ more in size than do the seeds of the 
distinct species in any one genus in the same two families. 
The same remark holds good in regard to the fruit of the 
several varieties of the plum, and still more strongly with the 
melon, as well as in many other anagolous 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 organization, 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 condi- 
tions of life. Some, perhaps a great, effect may be attributed 
to the increased use or disuse of parts. The final result is 
thus rendered infinitely complex. In some cases the inter- 
crossing of aboriginally distinct species appears to have 
played an important part in the origin of our breeds. When 
several breeds have once been formed in any country, their 
occasional intercrossing, with the aid of selection, has, no 
doubt, largely aided in the formation of new sub-breeds; 
but the importance of crossing has been much exaggerated, 
both in regard to animals and to those plants which are 
propagated by seed. With plants which are temporarily 
propagated by cuttings, buds, etc., the importance of cross- 
ing is immense ; for the cultivator may here disregard the 
extreme variability both of hybrids and of mongrels, and 
the sterility of hybrids ; but plants not propagated by seed 


are of little importance to us, for their endurance is only 
temporary. Over all these causes of change, the accumu- 
lative action of selection, whether applied methodically and 
quickly, or unconsciously and slowly, but more efficiently, 
seems to have been the predominant power. 




Variability — Individual Differences — Doubtful Species — Wide ran- 
ging, 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. 

Before applying the principles arrived at in the last 
chapter to organic beings in a state of nature, we must 
briefly discuss whether these latter are subject to any vari- 
ation. To treat this subject properly, a long catalogue of 
dry facts ought to be given ; but these I shall reserve for 
a future work. Nor shall I here discuss the various defini- 
tions 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 Alpine summits, or the thicker fur of an animal from far 
northward, 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 considerable 
deviations of structure, such as we occasionally see 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 beautifully 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 some- 
times occur which resemble normal structures in widely" 
different animals. Thus pigs have occasionally been born 
with a sort of proboscis, and if any wild species of the same 
genus had naturally possessed a proboscis, it might have 
been argued that this had appeared as a monstrosity ; but 
I have as yet failed to find, after diligent search, cases of 
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 singly, their preservation would 
depend on unusually favorable circumstances. They would, 
also, during the first and succeeding generations, cross with 
the ordinary form, and thus their abnormal character would 
almost inevitably be lost. But L shall have to return in a 
future chapter to the preservation and perpetuation of single 
or occasional variations. 


The many slight differences which appear in the offspring 
from the same parents, or which it may be presumed have 
thus arisen, from being observed in the individuals of the 
same species inhabiting the same confined locality, may be 
called individual differences. No one supposes that all 
the individuals of the same species are cast in the same 
actual mould. These individual differences are of the highest 
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 indi- 
vidual differences in his domesticated productions. These 
individual differences generally affect what naturalists con- 
sider unimportant parts ; but I could show, by a long cata- 
logue of facts, that parts which must be called important, 
whether viewed under a physiological or classificatory point 
of view, sometimes vary in the individuals of the same 


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

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

Individuals of the same species often present, as is known 
to every one, great differences of structure, independently 
of variation, as in the two sexes of various animals, in the 
two or three castes of sterile females or workers among 
insects, and in the immature and larval states of many of 
the lower animals. There are, also, cases of dimorphism 
and trimorphism, both with animals and plants. Thus, 
Mr. Wallace, who has lately called attention to the subject, 
has shown that the females of certain species of butterflies, 
in the Malayan Archipelago, regularly appeared under two 
or even three conspicuously distinct forms, not connected 
by intermediate varieties. Fritz Muller has described anal- 
ogous but more extraordinary cases with the males of cer- 
tain Brazilian Crustaceans : thus, the male of a Tanais regu- 
larly occurs under two distinct forms ; one of these has strong 
and differently shaped pincers, and the other has anten- 
nae much more abundantly furnished with smelling-hairs. 
Although in most of these cases, the two or three forms, 
both with animals and plants, are not now connected by 
intermediate gradations, it is probable that they were once 
thus connected. Mr. Wallace, for instance, describes a cer- 
tain butterfly which presents in the same island a great 
range of varieties connected by intermediate links, and the 
extreme links of the chain closely resemble the two forms 
of an allied dimorphic species inhabiting another part of 
the Malay Archipelago. Thus also with ants, the several 
worker-castes are generally quite distinct ; but in some 
cases, as we shall hereafter see, the castes are connected 
together by finely graduated varieties. So it is, as I have 
myself observed, with some dimorphic plants. It certainly 
at first appears a highly remarkable fact that the same 
female butterfly should have the power of producing at the 
same time three distinct female forms and a male ; and 
that an hermaphrodite plant should produce from the same 
seed-capsule three distinct hermaphrodite forms, bearing 
three different kinds of females and three or even six dif- 
ferent kinds of males. Nevertheless these cases are only 
exaggerations of the common fact that the female produces 
offspring of two sexes which sometimes differ from each 
other in a wonderful manner. 



The forms which possess in some considerable degree 
the character of species, but which are so closely similar 
to other forms, or are so closely linked to them by interme- 
diate gradations, that naturalists do not like to rank them 
as distinct species, are in several respects the most impor- 
tant for us. We have every reason to believe that many of 
these doubtful and closely allied forms have permanently 
retained their characters for a long time ; for as long, as far 
as we know, as have good and true species. Practically, 
when a naturalist can unite by means of intermediate links 
any two forms, he treats the one as a variety of the other ; 
ranking the most common, but sometimes the one first 
described, as the species, and the other as the variety. 
But cases of great difficulty, which I will not here enu- 
merate, sometimes arise in deciding whether or not to rank 
one form as a variety of another, even when they are 
closely connected by intermediate links ; nor will the com- 
monly 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 

Hence, in determining whether a form should be ranked 
as a species or a variety, the opinion of naturalists 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 competent judges. 

That varieties of this doubtful nature are far from uncom- 
mon, 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 num- 
ber of forms have been ranked by one botanist as good 
species, and by another as mere varieties. Mr. H. C. Wat- 
son, to whom I lie under deep obligation for assistance of all 
kinds, has marked for me 182 British plants, which are gen- 
erally considered as varieties, but which have all been ranked 


6y botanists as species; and in making this list lie has 
omitted many trifling varieties, but which nevertheless have 
been ranked b}^ some botanists as species, and he lias entirely 
omitted several highly polymorphic genera. Under genera, 
including the most polymorphic forms, Mr. Babington gives 
251 species, whereas Mr. Bentham gives only 112 — a differ- 
ence of 139 doubtful forms ! Among animals which unite 
for each birth, and which are highly locomotive, doubtful 
forms, ranked by one zoologist as a species and by another 
as a variety, can rarely be found within the same country, 
but are common in separated areas. How many of the birds 
and insects in North America and Europe, which differ very 
slightly from each other, have been ranked by one eminent 
naturalist as undoubted species, and by another as varieties, 
or, as they are often called, geographical races ! Mr. Wallace, 
in several valuable papers on the various animals, especially 
on the Lepidoptera, inhabiting the islands of the great 
Malayan Archipelago, 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 differ- 
ences are seen to be so slight and graduated that it is impos- 
sible to define or describe them, though at the same time 
the extreme forms are sufficiently distinct. The geograph- 
ical races or sub-species are local forms completely fixed and 
isolated ; 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.' 2 
Lastly, representative species fill the same place in the nat- 
ural economy of each island as do the local forms and sub- 
species ; but as they are distinguished from each other by a 
greater amount of difference than that between the local 
forms and sub-species, they are almost universally ranked 
by naturalists as true species. Nevertheless, no certain cri- 
terion can possibly be given by which variable forms, local 
forms, sub-species, and representative species can be recog- 

Many years ago, when comparing, and seeing others com- 
pare, the birds from the closely neighboring islands of the 
Galapagos Archipelago* )ne with another, and with thosa 


from the American mainland, I was much struck bow en- 
tirely vague and arbitrary is the distinction between species 
and varieties. On the islets of the little Madeira group 
there are many insects which are characterized as varieties 
in Mr. Wollaston's admirable work, but which would cer- 
tainly be ranked as distinct species by many entomologists. 
Even Ireland has a few animals, now generally regarded as 
varieties, but which have been ranked as species by some 
zoologists. Several experienced ornithologists consider our 
British red grouse as only a strongly marked race of a Nor- 
wegian species, whereas the greater number rank it as an 
undoubted species peculiar to Great Britain. A wide dis- 
tance between the homes of two doubtful forms leads many 
naturalists to rank them as distinct species ; but what dis- 
tance, 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 sev- 
eral islets of these small archipelagoes, 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 color, 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 differ- 
ences are rather more strongly marked, and when both sexes 
and all ages are affected, the forms are ranked by all ento- 
mologists as good species. But no observer can determine 
for another, even if he can do so for himself, which of these 
Phytophagic forms ought to be called species and which 
varieties. Mr. W T alsh ranks the forms which it may be sup- ( 
posed would freely intercross, as varieties ; and those which 
appear to have lost this power, as species. As the differ- 
ences depend on the insects having long fed on distinct 
plants, it cannot be expected that intermediate links con- 
necting the several forms should now be found. The natur- 
alist thus loses his best guide in determining whether to 
rank doubtful forms as varieties or species. This likewise 
necessarily occurs with closely allied organisms, which, in- 


habit distiact continents or islands. When, on the other 
hand, an animal or plant ranges over the same continent, 
or inhabits many islands in the same archipelago, and pre- 
sents different forms in the different areas, there is always ?, 
good chance that intermediate forms will be discovered which 
will link together the extreme states ; and these are then 
degraded to the rank of varieties. 

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 geologi- 
cal formations, they believe that two distinct species are 
hidden under the same dress. The term species thus comes 
to be a mere useless abstraction, implying and assuming a 
separate act of creation. It is certain that many forms, con- 
sidered by highly competent judges to be varieties, resemble 
species so completely in character that they have been thus 
ranked by other highly competent judges. But to discuss 
whether they ought to be 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 distribution, analogical 
variation, hybridism, etc., have been brought to bear in the 
attempt to determine their rank ; but space does not here 
permit me to discuss them. Close 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 closely it has been studied ; 
yet a German author makes more than a dozen species out 
of forms, which are almost universally considered by other 
botanists to be 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 pub- 
lished by A. de Candolle, on the oaks of the whole world 


No one ever had more ample materials for the discrimina- 
tion 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 vari- 
ations. 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 labor, 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 pro- 
visional. Just as we come to know them better, inter- 
mediate 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. pedunculata, sessiliflora, and 
pubescens. The forms which connect these three sub- 
species sire comparatively rare ; and, as Asa Gray again 
remarks, if these connecting forms which are now rare were 
to become totally 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 robur. 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 definibion 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 palaeontology, geographi- 
cal botany, and zoology, of anatomical structure and classifi- 


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 sub- 
ject ; and this shows, at least, how very generally there is 
some variation. But if he confine his attention to one class 
within one country he will soon make up his mind how to 
rank most of the doubtful forms. His general tendency 
will be to make many species, for he will become impressed, 
just like the pigeon or poultry fancier before alluded to, 
with the amount of difference in the forms which he is 
continually studying; and he has little general knowledge 
of analogical variation in other groups and in other coun- 
tries by which to correct his first impressions. As he 
extends the range of his observations he will meet wit 
more cases of difficulty ; for he will encounter a greater 
number of closely allied forms. But if his observations be 
widely extended he will in the end generally be able to 
make up his own mind ; but he will succeed in this at the 
expense of admitting much variation, and the truth of this 
admission will often be disputed by other naturalists. 
When he comes to study allied forms brought from coun- 
tries not now continuous, in which case he cannot hope to 
find intermediate links, he will be compelled to trust almost 
entirely to analogy, and his difficulties will rise to a climax. 

Certainly no clear line ol 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 

Hence I look at individual differences, though of small 
interest to the systematist, as of the highest importance for 
us, as being the first steps toward 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 toward more strongly 
marked and permanent varieties ; and at the latter, as 
leading to sub-species and then to species. The pass* - ' 
from one stage of difference to another may, in many casesj 


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 cumulative action 
of natural selection, hereafter to be explained, and to the 
effects of the increased use or disuse of parts. A well- 
marked variety may therefore be called an incipient species; 
but whether this belief is justifiable must be judged by the 
weight of the various facts and considerations to be given 
throughout this work. 

It need not be supposed that all varieties or incipient 
species attain the rank of species. They may become ex- 
tinct, or they may endure as varieties for very long periods, 
as has been shown to be the case by Mr. Wollaston with 
he 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 

From these remarks it will be seen that I look at the 
term species as one arbitrarily given, for the sake of con- 
venience, 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 comparison with mere 
individual differences, is also applied arbitrarily, for con- 
venience' sake. 




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 difficulties, 
as did subsequently Dr. Hooper, 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. Hooper permits me to add that 
After having carefully read my manuscript, and examined 
the tables, he thinks that the following statements are 
fairly well established. The whole subject, however, treated 
as it necessarily here is with much brevity, is rather per- 
plexing, and allusions cannot be avoided to the " struggle 
for existence," " divergence of character," and other ques« 
tions, hereafter to be discussed. 

Alphonso de Candolle and others have shown that plants 
which have very wide ranges generally present varieties ; 
and this might have been expected, as they are exposed to 
diverse physical conditions, and as they come into competi- 
tion (which, as we shall hereafter see, is an equally or more 
important circumstance) with different sets of organic beings. 
But my tables further show, that, in any limited coun- 
try, the species which are the most common, that is, abound 
most in individuals, and the species which are most widely 
diffused within their own country (and this is a different 
consideration from wide range, a^d to a certain extent from 
commonness), oftenest give rise to varieties sufficiently 
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 country, and are the most numer- 
ous in individuals — which oftenest produce well-marked vari- 
eties, or, as I consider them, incipient species. And this, 
perhaps, might have been anticipated; for, as varieties, in 
order to become in any degree permanent, necessarily have 
to struggle with the other inhabitants of the country, the 
species which are already dominant will be the more likely 
to yield offspring, which, though in some slight degree 
modified, still inherit those advantages that enabled their 
parents to become dominant over their compatriots. In 
these remarks on predominance, it should be understood 
that reference is made only to the forms which come into 
competition with each other, and more especially to the 
members of the same genus or class having nearly similar 
habits of life. With respect to the number of individuals, 
or commonness of species, the comparison of course relates 
only too the members of the same group. One of the higher 
plants may be said to be dominant if it be more numerous 
in individuals and more widely diffused than the ptho* 
plants of the same country, which lWe under nearly im* 


same conditions. A plant of this kind is not the less domi« 
nant because some conferva inhabiting the water or some 
parasitic fungus is infinitely more numerous in individuals, 
and more widely diffused. But if the conferva or parasitic 
fungus exceeds its allies iu the above respects, it will then 
be dominant within its own class. 


If the plants inhabiting a country, as described in any 
Flora, be divided into two equal masses, all those in the 
larger genera (i. 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 some- 
what 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 favorable to the genus ; 
and, consequently, we might have expected to have found in 
the larger genera, or those including many species, a larger 
proportional number of dominant species. But so many 
causes tend to obscure this result, that I am surprised that 
my tables show even a small majority on the side of the 
larger genera. I will here allude to only two causes of 
obscurity. Fresh-water and salt-loving plants generally 
have very wide ranges and are much diffused, but this seems 
to be connected with the nature of the stations inhabited by 
them, and has little or no relation to the size of the genera 
to which the species belong. Again, plants low in the scale 
of organization 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 
organized plants ranging widely will be discussed in our i 
chapter on Geographical Distribution. 

From looking at species as only strongly marked and 
well-defined varieties, I was led to anticipate that the species 
of the larger genera in each country would oftener present 
varieties, than the species of the smaller genera ; for wher- 
ever many closely related species (i. e., species of the same 
genus) have been formed, many varieties or incipient species 
•ught, as a general rule, to be now forming. Where many 


large trees grow, we expect to find saplings. Where many 
Species of a genus have been formed through variation, 
circumstances have been favorable for variation ; and hence 
We might expect that the circumstances would generally still 
be favorable 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, ihan in one having few. 

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



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 anal- 
ogy whether or not the amount suffices tc raise one or bcth 
to the rank of species. Hence the amount of difference is 
one very important criterion in settling whether two forms 
should be ranked as species or varieties. Now Fries has 
remarked in regard to plants, and West wood in regard to 
insects, that in large genera the amount of difference 
between the species is often exceedingly small. I have 
endeavored to test this numerically by averages, and, as far 
as my imperfect results go, they confirm the view. I have 
also consulted some sagacious and experienced observers, 
and, after deliberation, they concur in this view. In this 
respect, therefore, the species of the larger genera resemble 
varieties, more than do the species of the smaller genera. 
Or the case may be put in another way, and it may be said >( 
that in the larger genera, in which a number of varieties or 
incipient species greater than the average are now manu- 
facturing, 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 difference. 

Moreover, the species of the larger genera are related to 
each other, in the same manner as the varieties of any one 
species are related to each other. No naturalist pretends 
that all the species of a genus are equally distinct from each 
other; they may generally be divided into sub-genera, or 
sections, or lesser groups. As Fries has well remarked, little 
groups of species are generally clustered like 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 difference 
between varieties and species, namely, that the amount ol 


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 varieties tend to increase into the greater 
differences between species. 

There is one other point which is worth notice. Varieties 
generally have much restricted ranges. This statement is 
indeed scarcely more than a truism, for, if a variety were 
found to have a wider range than that of its supposed parent 
species, their denominations would be reversed. But there 
is reason to believe that the species which are very closely 
allied to other species, and in so far resemble varieties, often 
have much restricted ranges. For instance, Mr. H. C. Wat- 
son has marked for me in the well-sifted London Catalogue 
of Plants (4th edition) sixty-three plants which are therein 
ranked as species, but which he considers as so closely allied 
to other species as to be of doubtful value : these sixty -three 
reputed species range on an average over 6.9 of the provinces 
into which Mr. Watson has divided Great Britain. Now, in 
this same catalogue, fifty-three acknowledged varieties are 
recorded, and these range over 7.7 provinces ; whereas, the 
species to which these varieties belong range over 14.3 prov- 
inces. 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 difference 
between them ; for two forms, if differing ve: r little, are 
generally ranked as varieties, notwithstanding that they 
cannot be closely connected ; but the amount of difference 
considered necessary to give to any two forms the rank of 
species cannot be defined. In genera having more than the 
average number of species in any country, the species of 
these genera have more than the average number of varie- 
ties. 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 species appar* 
ently have restricted ranges. In all these respects the spe- 
cies 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 

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




Its Bearing on Natural Selection — The Term used in a Wide Sense- 
Geometrical Ratio of Increase — Rapid Increase of Naturalized 
Animals and Plants — Nature of the Checks to Increase — Com- 
petition Universal — Effects of Climate — Protection from the 
number of Individuals — Complex Relations of all Animals and 
Plants throughout Nature — Struggle for Life most Severe between 
Individuals and Varieties of the Same Species: often severe be- 
tween 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 struggle 
for existence bears on natural selection. It has been seen 
in the last chapter that among organic beings in a state of 
nature there is some individual variability : indeed, I am not 
aware that this has ever been disputed. It is immaterial for 
us whether a multitude of 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 enti- 
tled to hold, if the existence of any well-marked varieties be 
admitted. But the mere existence of individual variability 
and of some few well-marked varieties, though necessary as 
the foundation for the work, helps us but little in under- 
standing how species arise in nature. How have all those 
exquisite adaptations of one part of the organization to an- 
other part, and to the conditions of life, and of one organic 
being to another being, been perfected ? We see these beau- 
tiful coadaptations most plainly in the woodpecker and the 
mistletoe ; and onty a little less plainly in the humblest par- 
asite 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 gen- 
tlest breeze ; in short, we see beautiful adaptations eveiy 
where and in every part of the organic world. 

Again, t 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 cases obviously 


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

We will now discuss in a 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 Candolle and Lyell have largely and philosophically 
shown that all organic beings are exposed to severe compe- 
tition. In regard to plants, no one has treated this subject 
with more spirit and ability than W. Herbert, Dean of Man- 
chester, evidently the result of his great horticultural knowl- 
edge. Nothing is easier than to admit in words the truth of 
the universal struggle for life, or more difficult — at least I 
found it so — than constantly to bear this conclusion in 
mind. Yet unless it be thoroughly ingrained in the mind, 
the whole economy of nature, with every fact on distribution, 
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 de- 
stroyed by birds and beasts of prey ; we do not always bear 
in mind, that, though food may be now superabundant, it is 
not so at all seasons of each recurring year. 



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


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 occasional year; 
otherwise, on the principle of geometrical increase, its num- 
bers would quickly become so inordinately great that nt 


country could support the product. Hence, as more in- 
dividuals are produced than can possibly survive, there 
must in every case be a struggle for existence, either one 
individual with another of the same species, or with the 
individuals of distinct species, or with the physical con- 
ditions of life. It is the doctrine of Malthus applied with 
manifold force to the whole animal and vegetable king- 
doms; for in this case there can be no artificial ncrease of 
food, and no prudential restraint from marriage. Although 
some species may be now increasing, more or le^ rapidly, 
in numbers, all cannot do so, for the world would not hold 

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. Lin- 
naeus has calculated that if an annual plant produced only 
two seeds — and there is no plant so unproductive as this — 
and their seedlings next year produced two, and so on, then 
in twenty years there would be a million plants. The 
elephant is reckoned the slowest breeder of all known ani- 
mals, 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 
tb re would be nearly nineteen million elephants alive de- 
scended 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 circumstances have been favorable 
to them during two or three following seasons Still mope 
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 Aus- 
tralia,* 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 through- 
out whole islands in a period of less than ten years. Sev» 


eral of the plants, such as the cardoon and a tall thistle, 
which are now the commonest over the wide plains of La 
Plata, clothing square leagues of surface almost to the ex 
elusion 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 Coniorin 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 o 
plants has been suddenly and temporarily increased in an 
sensible degree. The obvious explanation is that the con 
ditions of life have been highly favorable, 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 extraor- 
dinarily rapid increase and wide diffusion in their new homes. 

In a state of nature almost every full-grown plant an- 
nually produces seed, and among animals there are very 
few which do not annually pair. Hence we may confi- 
dently assert that all plants and animals are tending to in- 
crease 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 animals tends, I think, to mislead us ; 
we see no great destruction failing on them, but we do not 
keep in mind that thousands are annually slaughtered for 
food, and that in a state of nature an equal number would 
have somehow to be disposed of. 

The only difference between organisms which annually 
produce eggs or seeds by the thousand, and those which 
produce extremely few, is, that the slow breeders would re- 
quire a few more years to people, under favorable condi- 
tions, a whole district, let it be ever so large. The condoi 
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 be- 
lieved to be the most numerous bird in the world. One 
fly deposits hundreds of eggs, and another, like the hippo- 
bosca, 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 impor- 
tance to those species which depend on a fluctuating amount 


of food, for it allows them rapidly to increase in Timbers. 
But the real importance of a large number of eggs jr seed 
is to make up for much destruction at some period of J' "3; 
and this period in the great majority o cases is an early 
one. If an animal can in any way protect its own eggs or 
youn'", a 6mall number may be produced, and yet the 
average stock be fully kept up ; but if many eggs or young 
are destroyed, many must be produced, or the species will 
beconn extinct. It would suffice to keep up the full num 
ber 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 insured 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 

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


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. 
Chis subject of the checks to increase has been ably treated 
$y 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 t will make only a 
few remarks, just to recall to the readers mind some of the 
chief points. Eggs or very young p^imals seem generally 
to suffer moct, but this is^ not invariably the case. With 
plants there is a vast destruction of ,<*eeds, but from som« 


observations which I have made it appears that the seed- 
lings suffer most from germinating- 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 de- 
stroyed, 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 grown on a 
little plot of mown turf (three feet by tour), nine species 
perished, from the other species being allowed to grow up 

The amount of food for each species, of course, gives 
the extreme limit to which each can increase ; but very fre- 
quently it is not the obtaining food, but the serving as prey 
to other animals, which determines the average number of 
a species. Thus, there seems to be little doubt that the 
stock of partridges, grouse, and hares on any large estate 
depends chiefly on the destruction of vermin. If not one 
head of game were shot during the next twenty years in 
England, and, at the same time, if no vermin were de- 
stroyed, 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 numbers of a species, and periodical seasons of ex- 
treme 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 1854-55 
destroyed four fifths of the birds in my own grounds; and 
this is a tremendous destruction, when we remember that 
ten per cent is an extraordinarily severe mortality from 
epidemics with man. The action of climate seems at first 
sight to be quite independent of the struggle for existence ; 
but in so far as climate chiefly acts in reducing food, 
^t brings on the most severe struggle between the indi- 
viduals, whether of the same or of distinct species, which 


subsist on the same kind of food. Even when climate, 
for instance, extreme cold, acts directly, it will be the least 
vigorous individuals, or those which have got least food 
through the advancing winter, which will suffer the most. 
When we travel from south to north, or from a damp 
region to a dry, we invariably see some species gradually 
getting rarer and rarer, and finally disappearing; and the 
change of climate being conspicuous, we are tempted to 
attribute the whole effect to its direct action. But this is 
a false view ; we forget that each species, even where it 
most abounds, is constantly suffering enormous destruc- 
tion at some period of its life, from enemies or from com- 
petitors for the same place and food ; and if these enemies 
or competitors be in the least degree favored by any slight 
change of climate, they will increase in numbers ; and as 
each area is already fully stocked with inhabitants, the 
other species must decrease. When we travel southward 
and see a species decreasing in numbers, we may feel sure 
that the cause lies quite as much in other species being 
favored, as in this one being hurt. So it is when we 
travel northward, but in a somewhat lesser degree, for 
the number of species of all kinds, and therefore of 
competitors, decreases northward, or in ascending a moun- 
tain, we far oftener meet with stunted forms, due to the 
directly injurious action of climate, than we do in proceed- 
ing southward or in descending a mountain. When we 
reach the arctic regions, or snow-capped summits, or abso- 
lute deserts, the struggle for life is almost exclusively with 
the elements. 

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

When a species, owing to highly favorable circumstances, 
increases inordinately in numbers in a small tract, epidem- 
ics — at least, this seems generally to occur with our game 
animals — often ensue ; and here ws have a limiting check 
independent of the struggle for lite. But even some of 
these so-called epidemics appear to by due to parasitic 
worms, which have from some cause, possibly in part 
through facility of diffusion among the crowded animals, 
been disproportionally favored : and here comes in a sort of 
Struggle between the ^aiaaite and iu prey. 


On the other hand, in many cases, a large stock of iniv» 
viduals of the same species, relatively to the numbers of it* 
enemies, is absolutely necessary for its preservation. Thus 
we can easily raise plenty of corn and rape-seed, etc., in 
our fields, because the seeds are in great excess compared 
with the number of birds which feed on them ; nor can the 
birds, though having a superabundance of food at this one 
season, increase in number proportionally to the supply of 
seed, as their numbers are checked during 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 preser- 
vation, 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 favorable that many 
could exist together, and thus save the species from utter 
destruction. I should add that the good effects of inter- 
crossing, 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. 


Many cases are on record showing how complex and unex- 
pected are the checks and relations between organic beings, 
which have to struggle together in the same country. I 
will give only a single instance, which, though a simple 
one, interested me. In 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 pass- 
ing from one quite different soil to another : not only the 
proportional numbers of the heath-plants were wholly 
changed, but twelve species of plants (not counting grasses 


and carices) flourished in the plantations, which could not 
be found on the heath. The effect on the insects must 
have been still greater, for six insectivorous birds were very 
common in the plantations, which were not to be seen on 
the heath ; and the heath was frequented by two or three 
distinct insectivorous birds. Here we see how potent has 
been the effect of the introduction of a single tree, nothing 
whatever else having been done, with the exception of the 
land having been enclosed, so that cattle could not enter. 
But how important an element enclosure is, I plainly saw 
near Farnham, in Surrey. Here there are extensive heaths, 
with a few clumps of old Scotch firs on the distant hill- 
tops : within the last ten years large spaces have been 
enclosed, and self-sown firs are now springing up in multi- 
tudes, 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 steins of the 
heath, I found a multitude of seedlings and little trees 
which had been perpetually browsed down by the cattle. 
In one square yard, at a point some hundred yards distant 
from one of the old clumps, I counted thirty-two little 
trees ; and one of them, with twenty-six rings of growth, 
had, during many years, tried to raise its head above the 
stems of the heath, and had failed. No wonder that, as 
soon as the land was enclosed, it became thickly clothed 
with vigorously growing young firs. Yet the heath was so 
extremely barren and so extensive that no one would ever 
have imagined that cattle would have so closely and effect- 
ually searched it for food. 

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


certain insectivorous birds were to decrease in Paraguay, the 
parasitic insects would probably increase ; and this would 
lessen the number of the navel-frequenting flies — then 
cattle and horses would become feral, and this would cer- 
tainly 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 Stafford- 
shire, the insectivorous birds, and so onward in ever-increas- 
ing circles of complexity. Not that under nature the 
relations will ever be as simple as this. Battle within 
battle must be continually recurring with varying 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 life ! 

I am tempted to give one more instance showing how 
plants and animals, remote in the scale of nature, are bound 
together by a web of complex relations. I shall hereafter 
have occasion to show that the exotic Lobelia folgens is 
never visited in my garden by insects, and consequently, from 
its peculiar structure, never sets a seed. Nearly all our 
orchidaceous plants absolutely require the visits of insects 
to remove their pollen-masses and thus to fertilize them. I 
find from experiments that humble-bees are almost indispens- 
able to the fertilization of the heart's-ease (Viola tricolor), 
for other bees do not visit this flower. I have also found 
that the visits of bees are necessary for the fertilization of 
some kinds of clover ; for instance, twenty heads of Dutch 
clover (Trifolium repens) yielded 2,290 seeds, but twenty 
other heads, protected from bees, produced not one. Again, 
one hundred heads of red clover (T. pratense) produced 2,700 
seeds, but the same number of protected heads produced not 
a single seed. Humble-bees alone visit red clover, as other 
bees cannot reach the nectar. It has been suggested that 
moths may fertilize the clovers ; but I doubt whether they 
could do so in the case of the red clover, from their weight 
not being sufficient to depress the wing petals. Hence we 
may infer as highly probable, that, if the whole genus of 
inamble-bees became extinct or very rare in England, the 


heart's-ease and red clover would become very rare, or wholly 
disappear. 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 Colonel Newman, 
who has long attended to the habits of humble-bees, believes 
that " more than two-thirds of them are thus destroyed all 
over England." Now the number of mice is largely depend- 
ent, as every one knows, on the number of cats ; and Colonel 
Newman says, " Near villages and small towns I have found 
the nests of humble-bees more numerous than elsewhere, 
which I attribute to the number of cats that destroy the 
mice." Hence it is quite credible that the presence of a 
feline animal in large numbers in a district might determine, 
through the intervention first of mice and then of bees, the 
frequency of certain flowers in that district ! 

In the case of every species, many different checks, act- 
ing 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 checks act on the same species in different dis- 
tricts. When we look at the plants and bushes clothing 
an entangled bank, we are tempted to attribute their pro- 
portional 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 beautiful 
diversity and proportion of kinds as in the surrounding 
virgin forests. What a struggle must have gone on during 
long centuries between the several kinds of trees, each 
annually scattering its seeds by the thousand ; what war 
between insect and insect — between insects, snails, and 
other animals, with birds and beasts of prey — all striving to 
increase, all feeding on each other, or on the trees, their 
seeds and seedlings, or on the other plants which first clothed 
the ground and thus checked the growth of the trees ! 
Throw up a handful of feathers, and all fall to the ground 
according to definite laws; but how simple is the problem 
where each shall fall, compared to that of the action and 
reaction of the innumerable plants and animals which have 
determined; in the course of centuries, the proportional 


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 sometimes the case 
with those which may be strictly said to struggle with each 
other for existence, a? in the case of locusts and grass-feed- 
ing quadrupeds. But the struggle will almost invariably be 
most severe between th-3 individuals of the same species, for 
they frequent the same districts, require the same food, and 
are exposed to the same dangers. In the case of varieties of 
the same species, the struggle will generally be almost 
equally severe, and we sometimes see the contest soon 
decided: for instance, if several varieties of wheat be sown 
together and the mixed seed be resown, some of the varie- 
ties 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 colored sweet-pea^e, 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 varie- 
ties of sheep ; it has been asserted that certain mountain 
varieties will starve out other mountain varieties, so that 
they cannot be kept together. The same result has followed 
from keeping together different varieties of the medicinal 
leech. It may even be doubted whether the varieties of any 
of our domestic plants or animals have so exactly the same 
strength, habits, and constitution, that the original propor- 
tions of a mixed stock (crossing being prevented) couM be 
kept up for half a dozen generations, if they were allowed to 
struggle together, in the same manner as beings in a state 
of nature, and if the seed or young wsre not annually pre- 
served in due proportion. 


As the species of the same genus usually have, though by 
no means invariably, much similarity in habits and constitu* 
tion, and always in structure, the struggle will generally be 
more severe between them, if they come into competition 
with each other, than between the species of distinct genera 


We see this in the recent extension over parts of the United 
States of one species of swallow, having caused the decrease 
of another species. The recent increase of the saissel-thrush 
m parts of Scotland has caused the decrease of the song- 
hrush. How frequently we hear of one species of rat taking 
/he place of another species under the most different cli- 
mates ! In Russia the small Asiatic cockroach has every- 
where driven before it its great congener. In Australia the 
imported hive-bee is rapidly exterminating the small, sting- j 
less native bee. One species of charlock has been known ! 
to supplant another species ; and so in other cases. We 
can dimly see why the competition should be most severe 
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 another 
in the great battle of life. 

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

Look at a plant in the midst of its range ! Why does it 


not double or quadruple its numbers ? We know that \% 
can perfectly well withstand a little more heat or cold, 
dampness or dryness, for elsewhere it ranges into slightly 
hotter or colder, damper or dryer districts. In this case we 
can clearly see that if we wish in imagination to give the 
plant the power of increasing in numbers, we should have 
to give it some advantage over its competitors, or over the 
animals which prey on it. On the confines of its geographi- 
cal 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 rigor 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, among 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 
competitors or enemies. 

It is good thus to try in imagination to give 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 striving to increase in a geometrical ratio ; that 
each, at some period of its life, during some season of the 
year, during each generation, or at intervals, has to struggle 
for life and to suffer pjT?nt 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, t&Q 
healthy, and *he happy survive and multiply. 




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 Inter- 
crosses between Individuals of the Same Species — Circumstances 
Favorable and Unfavorable to the Results of Natural Selection, 
namely, Intercrossing, Isolation, Number of Individuals — Slow 
Action — Extinction caused by Natural Selection — Divergence of 
Character, related to the Diversity of Inhabitants of any Small Area 
and to Naturalization — Action of Natural Selection, through Diver- 
gence of Character and Extinction, on the Descendants from a Com- 
mon Parent, explains the Grouping of all Organic Beings — Advance 
in Organization — 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 prin- 
ciple 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 truly be 
said that the whole organization becomes in some degree 
plastic. But the variability, which we almost universally 
meet with in our domestic productions, is not directly pro- 
duced, as Hooker and Asa Gray have well remarked, by man; 
he can neither originate varieties nor prevent their occur- 
rence ; he can only 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 infinitely complex 
and close-fitting are the mutual relations of all organic 
beings to each other and to their physical conditions of life ; 
and consequently what infinitely varied diversities of struc- 
ture might be of use to each being under changing condi* 

70 natural selection. 

tions of life. Can it then be thought improbable, seeing 
that variations useful to man have undoubtedly occurred, 
that other variations useful in some way to each being in the 
great and complex battle of life, should occur in the course 
of many successive generations ? If such do occur, can we 
doubt (remembering that many more individuals are born 
than can possibly survive) that individuals having any advan- 
tage, however slight, over others, would have the best chance 
of surviving and 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 
favorable individual differences and variations, and the 
destruction of those which are injurious, I have called 
Natural Selection, or the Survival of the Fittest. Varia- 
tions neither useful nor injurious would not be affected by 
natural selection, and would be left either a fluctuating ele- 
ment, as perhaps we see in certain polymorphic species, or 
would ultimately become fixed, owing to the nature of the 
organism and the nature of the conditions. 

Several writers have misapprehended or objected to the 
term Natural Selection. Some have even imagined that 
natural selection induces variability, whereas it implies only 
the preservation of such variations as arise and are benefi- 
cial to the being under its conditions of life. No one 
objects to agriculturists speaking of the potent effects of 
man's selection ; and in this case the individual differences 
given by nature, which man for some object selects, must of 
necessity first occur. Others have objected that the term 
selection implies conscious choice in the animals which 
become modified ; aud it has even been urged, that, as plants 
have no volition, natural selection is not applicable to them ! 
In the literal sense of the word, no doubt, natural selection 
is a false terra ; but who ever objected to chemists speaking 
of the elective affinities of the various elements ? — and yet 
an acid cannot strictly be said to elect the base with which 
:t 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 expres- 
sions ; and they are almost necessary for brevity. So again 
it is difficult to avoid personifying the word Nature ; but I 
mean by nature, only the aggregate action and product of 
many natural laws, and by laws the sequence of events as 


ascertained by us. With a 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 undergoing some 
slight physical change, for instance, of climate. The pro- 
portional numbers of its inhabitants will almost immediately 
undergo a change, and some species will probably become 
extinct. We may conclude, from what we have seen of the 
intimate and complex manner in which the inhabitants of 
each country are bound together, that any change in the 
numerical proportions of the inhabitants, independently of 
the change of climate itself, would seriously affect the 
others. If the country were open on its borders, new forms 
would certainly immigrate, and this would likewise seri- 
ously disturb the relations of some of the former inhabit- 
ants. 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 favored 
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 

We have good reason to believe, as shown in the first 
chapter, that changes in the conditions of life give a tend- 
ency to increased variability ; and in the foregoing cases 
the conditions have changed, and this would manifestly be 
favorable to natural selection, by affording a better chance 
of the occurrence of profitable variations. Unless such 
occur, natural selection can do nothing. Under the term 
of "variations," it must never be forgotten that mere indi- 
vidual differences are included. As man can produce a 
great result with his domestic animals and plants by 
adding up in any given direction individual 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 immigration, is necessary iu 


order that new and unoccupied places should be left for 
naftiral selection to fill up by improving some of the vary- 
ingunhabitants. For as all the inhabitants of each country 
are struggling together with nicely balanced forces, ex- 
tremely slight modifications in the structure or habits of 
one species would often give it an advantage over others ; 
and still further modifications of the same kind would often 
still further increase the advantage, as long as the species 
continued under the same conditions of life and profited by 
similar means of 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 condi- 
tions 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 naturalized produc- 
tions 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 ? Man can act only 
on external and visible characters ; Nature, if I may be 
allowed to personify the natural preservation or survival of 
the fittest, cares nothing for appearances, except in so far 
as they are useful to any being. She can act on every 
internal organ, on every shade of constitutional difference, 
on the whole machinery of life. Man selects only for his 
own good ; Nature, only for that of the being which she 
tends. Every selected character is fully exercised by her, 
as is implied by the fact of their selection. Man keeps the 
natives of many climates in the same country. He seldom 
exercises each selected character in some peculiar and fitting 
manner ; he feeds a long and a short-beaked pigeon on the 
same food; he does not exercise a long-backed or long- 
legged quadruped in any peculiar manner ; he exposes 
sheep with long and short wool to the same climate ; does 
not allow the most vigorous males to struggle for t&m 
females; he does not rigidly destroy all inferior animals, 
but protects during each varying season, as far as lies in his 
power, all his productions. He often begins his selection 
by some half-monstrous form, or at least by some modifica- 
tion 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, com* 
pared with those accumulated 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 scrutinizing, throughout the world, the 
slightest variations ; rejecting those that are bad, preserv- 
ing and adding up all that are good; silently and insensibly 
working, whenever and wherever opportunity 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 fi-om 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 intervd of time, vary or pre- 
sent individual differences of the same favorable nature as 
before ; and these must again be preserved, and so onward, 
step by step. Seeing that individual differences of the 
same kind perpetually recur, this can hardly be considered 
as an unwarrantable assumption. 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 pos- 
sible variation is a strictly limited quantity, is likewise a 
simple assumption. 

Although natural selection can act only through and for 
the good of each being, yet characters and structures, which 
we are apt to consider as of very trifling importance, may 
thus be acted on. When we see leaf-eating insects green, 
and bark-feeders mottled-gray; the alpine ptarmigan white 
in winter, the red grouse the color of heather, we must 
believe that these tints are of service to these birds and 
insects in preserving them from danger (J rouse, if not 
destroyed at some period of their lives, would increase in 


countless numbers ; they are known to suffer largely from 
birds of prey ; and hawks are guided by eyesight to their 
prey — so much so that on parts of the Continent persons 
are warned not to keep white pigeons, as being the most 
liable to destruction. Hence natural selection might be 
effective in giving the proper color to each kind of grouse, 
and in keeping that color, when once acquired, true and 
constant. Nor ought we to think that the occasional 
Jestruction of an animal of any particular color would pro- 
duce 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 color of hogs, which 
feed on the "paint-root" in Virginia, determines whether 
they shall live or die. In plants, the down on the fruit and 
the color of the flesh are considered by botanists as charac- 
ters of the most trifling importance; yet we hear from an 
excellent horticulturist, Downing, that in the United States 
the 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 colored flesh. If, with all the aids of art, 
these slight differences make a great difference in cultivating 
the several varieties, assuredly, in a state of nature, where 
the trees would have to struggle with other trees and with a 
host of enemies, such differences would effectually settle 
which variety, whether a smooth or downy, a yellow or a 
purple fleshed fruit, should succeed. 

In looking at many small points of difference between 
species, which, as far as our ignorance permits us to judge, 
seem quite unimportant, we must not forget that climate, 
food, etc., have no doubt produced some direct effect. It 
is also necessary to bear in mind, that, owing to the law of 
correlation, when one part varies and the variations are 
accumulated through natural selection, other modifications, 
often of the most unexpected nature, will ensue. 

As we see that those variations which, under domestica- 
tion, appear at any particular period of life, tend to reappear 
in the offspring at the same period; for instance, in the 
shape, size, and flavor of the seeds of the many varieties of 
our culinary and agricultural plants ; in the caterpillar and 
cocoon stages of the varieties of the silkworm ; in the eggs 
of poultry, and in the color of the down of their chickens ; 
in the horns of our sheep and cattle when nearly adult j so 


fn a state of nature natural selection will be enabled to act 
on and modify organic beings at any age, by the accumula- 
tion of variations profitable at that age, and by their inher- 
itance at a corresponding age. If it profit a plant to have 
its seeds more and more widely disseminated by the wind, 
L 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 modifica- 
tions 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 insure that they shall not be injurious : for if they were 
so, the species would become 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 profits by the selected change. What 
natural selection cannot do, is to modify the structure of 
one species, without giving it any advantage, for the good 
of another species ; and though statements to this effect 
may be found in works of natural history, I cannot find one 
case which will bear investigation. A structure used only 
once in an animal's life, if of high importance to it, might 
be modified to any extent by natural selection ; for instance, 
the great jaws possessed by certain insects, used exclusively 
for opening the cocoon — or the hard tip to the beak of 
unhatched birds, used for breaking the eggs. It has been 
asserted, that of the best short-beaked tumbler-pigeons a 
greater number perish in the pgg than are able to get out of 
it ; so that fanciers assist in the act of 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 modifica- 
tion would be very slow, and there would be simultaneously 
the most rigorous selection of all the young birds within the 
egg, which had the most powerful and hardest beaks, for all 
with weak beaks would inevitably perish ; or, more delicate 
and more easily broken shells might be selected, the thick- 
ness of the shell being known to vary like every other 

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 de- 
voured, and these could be modified through natural selec- 
tion only if they varied in some manner which protected, 
them from their enemies. Yet many of these eggs or seeds 
would perhaps, if not destroyed, have yielded ind viduals 
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 
accidental causes, which would not be in the least degree 
mitigated by certain changes of structure or constitution 
which would in other ways be beneficial to the species. But 
let the destructiou 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 favorable direction, will tend to propagate their kind in 
larger numbers than the less well adapted. If the numbers 
be wholly kept down by the causes just indicated, as will 
often have been the case, natural selection will be powerless 
in 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 improvement at the 
same time in the same area. 


Inasmuch as peculiarities often appear under domestica- 
tion 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 some- 
times the case ; or for one sex to be modified in relation to 
the other sex, as commonly occurs. This leads me to say a 
few words on what I have called sexual selection. This 
form of selection depends, not on a struggle for existence in 
relation to other organic beingb or to external conditions, 
but on a struggle between tin individuals of one sex, gener* 
ally the males, for the possession of the other sex; The 

sexual SKLafttroft. 77 

result is not death to the unsuccessful competitor, but few 
or no ofispring. Sexual selection is, therefore, less rigorous 
than natural selection. Generally, the most vigorous males, 
those which are beist 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 of spur, and 
strength to the wing to strike in the spurred leg, in nearly 
the same manner as does the brutal cockfighter by the care- 
ful selection of his best cocks. How low in the scale of 
nature the law of battle descends, I know not ; male alli- 
gators have been described as fighting, bellowing, and whirl- 
ing round, like Indians in a war-dance, for the possession of 
the females ; male salmons have been observed fighting all 
day long ; male stag beetles sometimes bear wounds from 
the huge mandibles of other males ; the males of certain 
hymenopterous insects have been frequently seen by that 
inimitable observer M. Fabre, fighting for a particular 
female who sits by, an apparently unconcerned beholder of 
the struggle, and then retires with the conqueror. The war 
is, perhaps, severest between the males of polygamous ani- 
mals, 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. 
Among birds, the contest is often of a more peaceful 
character. All those who have attended to the subject, 
Delieve 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 spectators, at last 
choose the most attractive partner. Those who have closely 
attended to birds in confinement well know that they often 
take individual preferences and dislikes : thus Sir R. Heron 
has described how a pied peacock was eminently attractive 
to all his hen birds. I cannot here enter on the necessary 


details ; but if man can in a short time give beauty ana 
an elegant carriage to his bantams, according to his standard 
of beauty, I can see no good reason to doubt that female 
birds, by selecting, during thousands of generations, the most 
melodious or beautiful males, according to their 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 females 
of any animal have the same general habits of life, but 
differ in structure, color, or ornament, such differences have 
been mainly caused by sexual selection : that is, by individ- 
ual males having had, in successive generations, some slight 
advantage over other males, in their weapons, means of de- 
fence, or charms, which they have transmitted to their male 
offspring alone. Yet I would not wish to attribute all 
sexual differences to this agency : for we see in our domestic 
animals peculiarities arising and becoming attached to the 
male sex, which apparently have not been augmented 
through selection by man. The tuft of hair on the breast 
of the wild turkey-cock cannot be of any use, and it is 
doubtful whether it can be ornamental in the eyes of the 
female bird ; indeed, had the tuft appeared under domestica- 
tion it would have been called a monstrosity. 


In order to make it clear how, as I believe, natural seleo- 
tion acts, I must beg permission to give one or two ima- 
ginary 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 have the 
best chance of surviving, and so being preserved or selected, 
provided always that they retain teagth to master their 


prey at this or some other period of the year, when the/ 
were compelled to prey on other animals. I can see nc 
more reason to doubt that this would be the result, thaD 
that man should be able to improve the fleetness of his grey- 
hounds by careful and methodical selection, or by that kind 
of unconscious selection which follows from each man try- 
ing 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 Mourn 
tains, in the United States, one with a light greyhound-like 
form, which pursues deer, and the other more bulky, with 
shorter legs, which mere frequently attacks the shepherd's 

It should be observed that in the above illustration, I 
speak of the slimmest individual wolves, and not of any 
single strongly marked variation having been preserved. 
In former editions of this work I sometimes spoke as if this 
latter alternative had frequently occurred. I saw the great 
importance of individual differences, and this led me fully 
to discuss the results of unconscious selection by man, which 
depends on the preservation of all the more or less valuable 
individuals, and on the destruction of the worst. I saw, 
also, that the preservation in a state of nature of any occa- 
sional 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 valua- 
ble article in the North British Review (1867), I did not 
appreciate how rarely single variations, whether slight or 
strongly marked, could be perpetuated. The author takes 
the case of a pair of animals, producing during their life- 
time two hundred offspring, of which, from various causes 
Df destruction, only two on an average survive to procreate 
their kind. This is rather an extreme estimate for most of 
the higher animals, but by no means so for many of the 
lower organisms. He then shows that if a single individ* 
ual 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. Sup- 
posing it to survive and to breed, and that half its young 
inherited the favorable variation ; still, as the reviewer goes 
on to show, the young would have only a slightly better 
chance of surviving and breeding ; and this chance would 
go on decreasing in the succeeding generations. The justice 


of these remarks cannot, I think, be disputed. If, foi 
instance, a bird of some kind could procure its food more 
easily by having its beak curved, and if one were born with 
its beak strongly curved, and which consequently flourished, 
nevertheless there would be a very poor chance of this one 
individual perpetuating its kind to the exclusion of the com- 
mon 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 gen- 
arations 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 organization being similarly acted on — of which 
fact numerous instances could be given with our domestic 
productions. In such cases, if the varying individual did 
not actually transmit to its offspring its newly acquired 
character, it would undoubtedly transmit to them, as long as 
the existing conditions remained the same, a still stronger 
tendency to vary in the same manner. There can also be 
little doubt that the tendency to vary in the same manner 
has often been so strong that all the individuals of the same 
species have been similarly modified 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 laery- 
mans. In cases of this kind, if the variation were of a 
beneficial nature, the original form would soon be supplanted 
by the modified form, through the survival of the fittest. 

To the effects of intercrossing in eliminating variations of 
all kinds, I shall have to recur: but it may be here remarked 
that most animals and plants keep to their proper homes, 
and do not needlessly wander about; we see this even with 
migratory birds, which almost always return to the same 
*pot. Consequently each newly-formed variety would gen- 
erally be at first local, as seems to be the common rule with 
varieties in a state or nature; so that similarly modified indi- 
viduals 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 dis< 
trict, competing with and conquering the unchanged individ- 
uals on the margins of an ever-increasing circle. 

It may be worth while to give another and more complex 
illustration of the action of natural selection. Certain 
plants excrete sweet juice, apparently for the sake of elim- 
inating something injurious from the sap : this is effected, 
for instance, by glands at the base of the stipules in some 
Leguminosse, 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 ben- 
efit 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 otten trans- 
port it from one flower to another. The flowers of two dis- 
tinct individuals of the same species would thus get crossed; 
and the act of crossing, as can be fully proved, gives rise to 
vigorous seedlings, which consequently would have the best 
chance of flourishing and surviving. The plants which pro- 
duced flowers with the largest glands or nectaries, 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 insect which visited 
them, so as to favor in any degree the transportal of the 
pollen, would likewise be favored. 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 fertilization, 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 continued, had 
been rendered highly attractive to insects, they would, unin- 
tentionally 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 like- 
wise illustrating one step in the separation o£ the sexes 


of plants. Some holly-trees bear only male flowers, which 
have four stamens producing a rather small quantity of pol- 
len, and a rudimentary pistil ; other holly-trees bear only 
female flowers, these have a lull-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, 1 put the stigmas of twenty 
flowers, taken from different branches, under the micro- 
scope, 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 favorable to bees, 
nevertheless every female flower which I examined had been 
effectually fertilized 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 nat- 
uralist doubts the advantage of what has been called the 
" physiological division of labor ; " 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 cul- 
ture and placed under new conditions of life, sometimes the 
male organs and sometimes the female organs become more 
or less impotent ; now if we suppose this to occur in ever 
so slight a degree under nature, then, as pollen is already 
carried regularly from flower to flower, and as a more com- 
plete separation of the sexes of our plant would be advan- 
tageous on the principle of the division of labor, individuals 
with this tendency more and more increased would be con- 
tinually favored or selected, until at last a complete separa* 
tion of the sexes might be effected. It would take up too 
much space to show the various steps, through dimorphism 
and other means, by which the separation of the sexes in 
plants of various kinds is apparently now in progress ; but 
I may add that some of the species of holly in North Amer- 
ica are, according to Asa Gray, in an exactly intermediate 
condition, or, as he expresses it, are more less dioeciously 

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. 1 could give many facts snowing 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 thev can enter 
by the mouth. Bearing such facts in mind, it may be be- 
lieved that under certain circumstances individual differences 
in the curvature or length of the proboscis, etc., coo 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 communities to 
which they belonged would flourish and tni-ow off many 
swarms inheriting the same peculiarities. Tne tubes of the 
corolla of the common red or 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 the 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 repeat- 
edly seen, but only in the autumn, many hive-bees sucking 
the flowers through holes bitten in the base of the tube by 
humble-bees. The difference in the length of the corolla 
in the two kinds of clover, which determines the visits of 
the hive-bee, must be very trifling ; for I have been assured 
that when red clover has been mown, the flowers of the 
second crop are somewhat smaller, and that these are visited 
by many hive-bees. I do not know whether this statement 
is accurate ; nor whether another published statement can 
be trusted, namely, that the Ligurian bee, which is generally 
considered a mere variety of the common hive-bee, and which 
freely crosses with it, is able to reach and suck the nectar 
of the red clover. Thus, in a 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 pro- 
boscis. 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 structure 
mutually favorable to each other. 

I am well aware that this doctrine of natural selection, 
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 modern changes of the 
earth, as illustrative of geology ; " but we now seldom hear 
the agencies which we see still at work, spoken of as trifling 
or insignificant, when used in explaining the excavation of 
the deepest valleys or the formation of long lines of inland 
cliffs. Natural selection acts only by the preservation and 
accumulation of small inherited modifications, each profitable 
to the preserved being; and as modern geology has almost 
banished such views as the excavation of a great valley by 
a single diluvial 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. 


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 excep- 
tion of the curious and not well understood cases of partheno- 
genesis) unite for each birth ; but in the case of hermaphro- 
dites this is far from obvious. Nevertheless there is reason 
to believe that with all hermaphrodites two individuals, 
either occasionally or habitually, concur for the reproduction 
of their kind. This view was long ago doubtfully suggested 
by Sprengel, Knight, and Kolreuter. We shall presently see 
its importance; but I must here treat the subject with 
extreme 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 number of sup- 
posed hermaphrodites, and of real hermaphrodites a large 
number pair ; that is, two individuals regularly 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 hermaphro- 
dites. What reason, it may be asked, is there for supposing 
in these cases that two individuals ever concur in reproduc- 
tion ? As it is impossible here to enter on details, I must 
trust to some general considerations alone. 


In the first place, I have collected so large a body of 
facts, and made so many experiments, showing, in accord 
ance 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 vigor and fertility to the offspring; and on the 
other hand, that close interbreeding diminishes vigor and 
fertility ; that these facts alone incline me to believe that it 
is a general law of nature that no organic being fertilizes 
itself for a perpetuity of generations; but that a cross with 
another individual is occasionally — perhaps at long intervals 
of time — indispensable. 

On the belief tha-t this is a law of nature, we can, I think, 
understand several large classes of facts, such as the follow- 
ing, which on any other view are inexplicable. Every 
hybridizer knows how unfavorable exposure to wet is to 
the fertilization of a flower, yet what a multitude of flowers 
have their anthers and stigmas fully exposed to the weather ! 
If an occasional cross be indispensable, notwithstanding that 
the plant's own anthers and pistil stand so near each other 
as almost to insure self-fertilization, the fullest freedom for 
the entrance of pollen from another individual will explain 
the above state of exposure of the organs. Man}' flowers, 
on the other hand, have their organs of fructification closely 
enclosed, as in the great papilionaceous or pea-family ; but 
these almost invariably present beautiful and curious adapta- 
tions in relation to the visits of insects. So necessary 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 insure fertilization, just to 
touch with the same brush the anthers 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 

When the stamens of a flower suddenly spring toward 
the pistil, or slowly move one after the other toward it, the 
contrivance seems adapted solely to insure self-fertilization f 


and no doubt it is useful for this end : but the agency of 
insects is often required to cause the stamens to spring for- 
ward, as Kolreuter has shown to be the case with the bar- 
berry ; and in this very genus, which seems to have a special 
contrivance for self-fertilization, it is well known chat, 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-fertilization being favored, there are special contrivances 
which effectually 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 instance, 
in Lobelia fulgens, there is a really beautiful and elaborate 
contrivance by which all the infinitely numerous pollen- 
granules are swept out of the conjoined anthers of each 
flower, before the stigma of that individual flower is ready 
to receive them ; and as this flower is never visited, at least 
in my garden, by insects, it never sets a seed, though by 
placing pollen from one flower on the stigma of another, I 
raise plenty of seedlings. Another species of Lobelia, which 
is visited by bees, seeds freely in my garden. In very many 
other cases, though there is no special mechanical contrivance 
to prevent the stigma receiving pollen from the same flower, 
yet, as Sprengel and more recently Hildebrand and others 
have shown, and as 1 can confirm, either the anthers burst 
before the stigma is ready for fertilization, 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 recipro- 
cally 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-fertilization, 
should be in so many cases mutually useless to each other ! 
How simply are these facts explained on the view of an 
Dccasional cross with a distinct individual being advantageous 
or indispensable ! 

If several varieties of the cabbage, radish, onion, and of 
some other plants, be allowed to seed near each other, a 
large majority of the seedlings thus raised turn out, as I 
found, mongrels : for instance, I raised 233 seedling cab- 
bages from some plants of different varieties growing near 
each other, and of these only 78 were true to their kind, tnd 
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 
stigma without insect agency ; for I have found that plants 
carefully protected from insects produce the full number of 
pods. How, then, comes it that such a vast number of the 
seedlings are mongrelized ? It must arise from the pollen 
of a distinct -variety having a prepotent effect over the 
flower's own pollen ; and that this is part of the general 
law of good being derived from the intercrossing of distinct 
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 limited sense. 
I believe this oljection to be valid, but that nature has 
largely provided against it by giving to trees a strong tend- 
ency 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 
dichogamous, the same result would follow as if they bore 
flowers with separated sexes. I have made these few re* 
marks on trees simply to call attention to the subject. 

Turning for a brief space to animals : various terrestrial 
species are hermaphrodites, such as the land-mollusca and 
earth-worms ; but these all pair. As yet I have not found 
a single terrestrial animal which can fertilize itself. This 
remarkable fact, which offers so strong a contrast with ter- 
restrial plants, is intelligible on the view of an occasional 
cross being indispensable; for owing to the nature of the 
fertilizing element there are no means, analogous to ths 


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-fertilizing hermaphrodites ; but 
here the currents of water offer an obvious means for an occa- 
sional cross. As in the case of flowers, I have as yet failed, 
after consultation with one of the highest authorities, viz., 
Professor Huxley, to discover a single hermaphrodite animal 
with the organs of reproduction so perfectly enclosed that 
access from without, and the occasional influence of a dis- 
tinct individual, can be shown to be physically impossible. 
Cirrij: ;?des 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 of self-fertilizing hermaphrodites, do sometimes cross. 

It must have struck most naturalists as a strange anomaly 
\hat, both with animals and plants, some species of the same 
family and even of the same genus, though agreeing closely 
with each other in their whole organization, are hermaphro- 
dites, and some unisexual. But if, in fact, a 1 hermaphrodites 
do occasionally intercross, the difference between them and 
unisexual species is, as far as function is concerned, very 

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 individuals is a very 
general, if not universal, law of nature. 


This is an extremely intricate subject. A great amount 
of variability, under which term individual differences are 
always included, wi-l evidently be favorable. A large num- 
ber of individuals, by giving a better chance within any given 
period for the appearance of profitable variations, will com- 
pensate for a lesser amount of variability in each individual, 
and is, I believe, a highly important element of success. 
Though nature grants long periods uf time for the work of 
natural selection, she does not grant an indefinite period, for 
as all organic beings are striving to seize on each place in 
the economy of nature, if any one species does not become 
modified and improved in a corresponding degree with its 


competitors it will be exterminated. Unless favorable vari- 
ations be inherited by some at least of the offspring, nothing 
can be effected by natural selection. The tendency to re- 
version 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 allowed 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, improvement surely but slowly 
follows from this unconscious process of selection, notwith- 
standing 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 districts, the newly 
formed varieties will intercross on the confines of each. 
But we shall see in the sixth chapter that intermediate 
varieties, inhabiting intermediate districts, will in the long- 
run generally be supplanted by one of the adjoining vari- 
eties. 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 con- 
fined to separated countries ; and this I find to be the case. 
With hermaphrodite organisms which cross only occasion- 
ally, and likewise for 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 after- 
ward 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 j for J can bring forward a considerable body of 


facts showing that within the same area two varieties of 
the same animal may long remain distinct, from haunting 
different stations, from breeding at slightly different sea- 
sons, 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 animals which unite 
for each birth ; but, as already stated, we have reason tc 
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 vigor and fertility over the offspring from long-continued 
self-fertilization, that they will have a better chance of 
surviving and propagating their kind ; and thus in the long- 
run the influence of crosses, even at rare intervals, will be 
great. With respect to organic beings 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 conditions of life, 
only through the principle of inheritance, and through 
natural sel 3ction which will destroy any individuals depart- 
ing from the proper type. If the conditions of life change, 
and the form undergoes modification, uniformity of character 
can be given to the modified offspring, solely by natural 
selection preserving similar favorable variations. 

Isolation also is an important element in the modification 
of species through natural selection. In a confined or 
isolated area, if not very large, the organic and inorganic 
conditions of life will generally be almost uniform ; so that 
natural selection will tend to modify all the varying indi- 
viduals of the same species in the same manner. Inter- 
crossing with the inhabitants of the surrounding districts 
will also be thus prevented. Moritz Wagner has lately pub- 
lished an interesting essay on this subject, and has shown 
that the service rendered by isolation in preventing crosses 
between newly-formed varieties is probably greater even than 
I supposed. But from reasons already assigned I can by no 
means agree with this naturalist, that migration and isola- 
tion are necessary elements for the formation of new species. 
The importance of isolation is likewise great in preventing, 
after any physical change in the conditions, . such as of 
climate, elevation of the land, etc., the immigration of bette* 


adapted organisms ; and thus new places in the natural 
economy of the district A/ill be left open to be filled up by 
the modification of the old inhabitants. Lastly, isolatioa 
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 number of the inhabitants will be small ; 
and this will retard the production of new species through 
natural selection, by decreasing the chances of favorable 
variations arising. 

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 under- 
going change through some innate law. LapSe of time is 
only so far important, and its importance in this respect is 
great, that it gives a better chance of beneficial variations 
arising and of their being selected, accumulated, and fixed. 
It likewise tends to increase the direct action of the physical 
conditions of life, in relation to the constitution of each 

If we turn to nature to test the truth of these remarks, 
and look at any small isolated area, such as an oceanic 
island, although the number of species inhabiting it is small, 
as we shall see in our chapter on Geographical Distribution; 
yet of these species a very large 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 favorable for the production of new species. 
But we may thus deceive ourselves, for to ascertain whether 
a small isolated area, or a large open area like a continent, 
has been most favorable for the production of new organic 
forms, wr ought to make the comparison within equal times j 
and this we are incapable of doing. 

Although isolation is of great importance in the produc- 
tion 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 favorable variations, arising from the larg* 
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 species 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, and will thus come into competition 
with many other forms. Moreover, great areas, though now 
continuous, will often, owing to former oscillations of level, 
have existed in a broken condition ; so that the good effects 
of isolation will generally, to a certain extent, have con- 
curred. Finally, I conclude that, although small isolated 
areas have been in some respects highly favorable for the 
production of new species, yet that the course of modifica- 
tion will generally have been more rapid on large areas ; and 
what is more important, that the new forms produced on 
large areas, which already have been victorious over many 
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, understand 
some facts which will be again alluded to in our chapter on 
Geographical Distribution ; for instance, the fact of the 
productions of the smaller continent of Australia now yield- 
ing before those of the larger Europaeo- Asiatic area. Thus, 
also, it is that continental productions have everywhere 
become so largely naturalized on islands. On a small island, 
the race for life will have been less severe, and there will 
have been less modification and less extermination. Hence, 
we can understand how it is that the flora of Madeira, 
according to Oswald Heer, resembles to a certain extent the 
extinct tertiary flora of Europe. All fresh-water basins, 
taken together, make a small area compared with that of the 
sea or of the land. Consequently, the competition between 
fresh-water productions will have been less severe than else- 
where, new forms will have been then more slowly produced, 
and old forms more slowly exterminated. And it is in fresh- 
water basins that we find seven genera of Ganoid fishes, 
remnants of a once preponderant order : and in fresh water 
we find some of the most anomalous forms now known in 
the world as the Ornithorhynchus and Lepidosiren, which, 
like fossils, connect to a certain extent orders at present 
widely sundered in tbe natural scale. These anomalous 


forms may be called living fossils ; 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, 

To sum up, as far as the extreme intricacy of the subject 
permits, the circumstances favorable and unfavorable for 
the production of new species through natural selection. 
I conclude that for terrestrial productions a large continental 
area, which has undergone many oscillations of level, will 
have been the most favorable for the production of many 
new forms of life, fitted to endure for a long time and to 
spread widely. While 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 converted by subsistence into large separate islands, 
there will still have existed many individuals of the same 
species on each island : intercrossing on the confines of the 
range of each new species will have been checked : after 
physical changes of any kind, immigration will have been 
prevented, so that new places in the polity of each island 
will have had to be filled up by the modification of the old 
inhabitants ; and time will have been allowed for the 
varieties in each to become well modified and perfected. 
When, by renewed elevation, the islands were reconverted 
into a continental area, there will again have been very 
severe competition ; the most favored or improved varieties 
will have been enabled to spread ; there will have been much 
extinction of the less improved forms, and the relative pro- 
portional 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 slow- 
ness, I fully admit. It can ac*" only when there are places 
in the natural polity of a district which can be better occu- 
pied by the modification of some of its existing inhabitants. 
The occurrence of such place* will often depend on physi- 
cal changes, which generally take place ver}* slowly, and 
on the immigration of better adapted forms being pre- 
vented. As some few of the old inhabitants become modi- 
fied, 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 specie J 


differ in some slight degree from each other, it would often 
be long before differences of the right nature in various 
parts of the organization might occur. The result would 
often be greatly retarded by free intercrossing. Many will 
exclaim that these several causes are amply sufficient to 
neutralize the power of natural selection. I do not believe 
so. But I do believe that natural selection will generally 
act very slowly, only at long intervals of time, and only on 
a few of the inhabitants of the same region. I further 
believe that these slow, intermittent results accord well 
with what geology tells us of the rate and manner at which 
the inhabitants of the world have changed. 

Slow though the process of selection may be, if feeble 
man can do much by artificial selection, I can see no limit 
to the amount of change, to the beauty and complexity of 
the 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 


This subject will be more fully discussed in our chap 
ter on Geology; but it must here be alluded to from bein^ 
intimately connected with natural selection. Natural selec- 
tion acts solely through the preservation of variations in 
pome way advantageous, which consequently endure. Owing 
to the high geometrical rate of increase of all organic beings, 
each area is already fully stocked with inhabitants ; and it 
follows from this, that as the favored forms increase in 
number, so, generally, will the less-favored decrease and 
become rare. Rarity, as geology tells us, is the precursor to 
extinction. We can see that any form which is represented 
by few individuals will run a good chance of utter extinc- 
tion, during great fluctuations in the nature of the seasons, 
or from a temporary increase in the number of its enemies. 
But we may go further than this ; for, as new forms are 
produced, unless we admit that specific forms can go on 
indefinitely increasing in number, many old forms must 
become extinct. That the number of specific forms has not 
indefinitely increased, geology plainly tells us ; and we shall 
presently attempt to show why it is that the number of 
species throughout the world has not become immeasurably 


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

From these several considerations I think it inevitably 
follows, that as new species in the course of time are 
formed through natural selection, others will become rarer 
and rarer, and finally extinct. The forms which stand in 
closest competition with those undergoing modification 
and improvement, will naturally suffer most. And we 
have seen, in the chapter on the Struggle for Existence, 
that it is the most closely allied forms — varieties of the 
same species, and species of the same genus or related 
genera — which, from having nearly the same structure, 
constitution, and habits, generally come into the severest 
competition with each other ; consequently, each new vari- 
ety 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 extermina- 
tion among our domesticated productions, through the 
selection of improved forms by man. Many curious in- 
stances 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 agricul- 
tural writer) " as if by some murderous pestilence." 


The principle, which I have designated by this term, is 
of high importance, and explains, as I believe, several im- 
portant 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 difference between 
species ? That this does habitually happen, we must infer 
from most of the innumerable species throughout nature 
presenting well-marked differences ; whereas varieties, the 
supposed prototypes and parents of future well-marked 
species, present slight and ill-defined differences. Mere 
chance, as we may call it, might cause one variety to differ 
in some character from its parents, and the offspring of this 
variety again to differ from its parent in the very same 
character and in a greater degree ; but this alone would 
never account for so habitual and large a 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 Here- 
ford cattle, race and cart horses, the several breeds of 
pigeons, etc., could never have been effected by the mere 
chance accumulation of similar variations during many 
successive generations. In practice, a fancier is, for in- 
stance, struck by a pigeon having a slightly shorter beak; 
another fancier is struck by a pigeon having a rather longer 
beak; and on the acknowledged principle that "fanciers 
do not and will not admire a medium standard, but like 
extremes," they both go on (as has actually occurred with 
the sub-breeds of the tumbler-pigeon) choosing and breed- 
ing from birds with longer and longer beaks, or witb 
shorter and shorter beaks. Again, we may suppose thau 
at an early period of history, the men of one nation or dis- 
trict required swifter horses, while those of another re- 
quired 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 dis- 
tinct breeds. As the differences became greater, the in- 
ferior animals with intermediate characters, being neither 
very swift nor very strong, would not have been used for 
breeding, and will thus have tended to disappear. Here, 


then, we see in man's productions the action of what may be 
called the principle of divergence, causing differences, at 
first barely 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 principle 
apply in nature ? I believe it can and does apply most 
efficiently (though it was a long time before I saw how), 
from the simple circumstance that the more diversified the 
descendants from any one species become in structure, 
constitution, and habits, by so much will they be better 
enabled to seize on many and widely diversified places in 
the polity of nature, and so be enabled to increase in 

We can clearly discern this in the case of animals with 
simple habits. Take the case of a carnivorous quadruped, 
of which the number that can be supported in any country 
has long ago arrived at its full average. If its natural 
power of increase be allowed to act, it can succeed in in- 
creasing (the country not undergoing any change in con- 
ditions) 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, climb- 
ing trees, frequenting water, and some perhaps becoming 
less carnivorous. The more diversified in habits and struc- 
ture the descendants of our carnivorous animals become, 
the more places they will be enabled to occupy. What 
applies to one animal will apply throughout all time to all 
animals — that is, if they vary — for otherwise natural 
selection can effect nothing. So it will be with plants. It 
has been experimentally proved, that if a plot of ground 
be sown with one species of grass, and a similar plot be 
sown with several distinct genera of grasses, a greater 
number of plants and a greater weight of dry herbage can 
be raised in the latter than 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 
differed from each other in the same manner, though in a 
very slight degree, as do the distinct species and genera of 
grasses, a greater number of individual plants of this 
species, including its modified descendants, would succeed 


m living on the same piece of ground. And we know that 
each species and each variety of grass is annually sowing 
almost countless seeds ; and is thus striving, as it may be 
said, to the utmost to increase in number. Consequently, 
in the course of many thousand generations, the most dis* 
tinct 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 structure, 
is seen under many natural circumstances. In an extremely 
small area, especially if freely open to immigration, and 
where the contest between individual and individual must 
be very severe, we always find great diversity in its inhab- 
itants. 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 more 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 (suppos- 
ing its nature not to be in any way peculiar), and may be 
said to be striving to the utmost to live there ; but it i^ 
seen, that where they come into the closest competition, the 
advantages of diversification of structure, with the accom- 
panying 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 naturalization of plants 
through man's agency in foreign lands. It might have 
been expected that the plants which would succeed in 
becoming naturalized 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 
county. It might also, perhaps, have been expected that nat- 
uralized plants woujd have belonged to a few groups more 


especially adapted to certain stations in their new homes. 
But the case is very different; and Alph. de Candolle has 
well remarked, in his great and admirable work, that floras 
gain by naturalization, 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 naturalized plants are enumerated, and 
these belong to 162 genera. We thus see that these natur- 
alized plants are of a highly diversified nature. They differ, 
moreover, to a large extent, from the indigenes, for out of 
the 162 naturalized genera, no less than 100 genera are not 
there indigenous, and thus a large proportional addition is 
made to the genera now living in the United States. 

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

The advantage of diversification of structure in the 
inhabitants of the same region is, in fact, the same as that 
of the physiological division of labor in the organs of the 
same individual body — a subject so well elucidated by 
Milne Edwards. No physiologist doubts that a stomach 
adapted to digest vegetable matter alone, or flesh alone, 
draws most nutriment from these substances. So in the 
general economy of any land, the more widely and per- 
fectly the animals and plants are diversified for different 
habits of life, so will a greater number of individuals be 
capable of there supporting themselves. A set of animals, 
with their organization but little diversified, could hardly 
compete with a set more perfectly diversified in structure. 
It may be doubted, for instance, whether the Australian 
marsupials, which are divided into groups differing but 
little from each other, and feebly representing, as Mr. 
Waterhouse and others have remarked, our carnivorous, 
ruminant, and rodent mammals, could successfully com- 
pete with these well-developed orders. In the Australian 
mammals, we see the process of diversification in an early 
and incomplete stage of improvement. 



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

The accompanying diagram will aid us in understanding 
this rather perplexing subject. Let A to L represent the 
species of a genus large in its own country ; these species are: 
supposed to resemble each other in unequal degrees, as is so 
generally the case in nature, and as is represented in the 
diagram by the letters standing at unequal distances. I 
have said a large genus, because, as we saw in the second 
chapter, on an average more species vary in large genera 
than in small genera; and the varying species of the large 
genera present a greater number of varieties. We have, 
also, seen that the species, which, are the commonest and 
most widely diffused, vary more than do the rare and 
restricted species. Let (A) be a common, widely-diffused, 
and varying species, belonging to a genus large in its own 
country. The branching and diverging dotted lines of 
unequal lengths proceeding from (A) may represent its 
varying offspring. The variations are supposed to be ex- 
tremely 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 variations (repre- 
sented by the outer dotted lines) being preserved and ac- 
cumulated by natural selection. When a dotted line reaches 
one of the horizontal lines, and is there marked by a small 
numbered letter, a sufficient amount of variation is supposed 
to have been accumulated to form it into a fairly well-marked 


variety, such as would be thought worthy of record in a 
systematic work. 

The intervals between the horizontal lines in the diagram 
may represent each a thousand or more generations. After 
a thousand generations, species (A) is supposed to have pro- 
duced two fairly well-marked varieties, namely a 1 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 
numerous than most of the other inhabitants of the same 
country ; they will also partake of those more general 
advantages which made the genus to which the parent 
species belonged, a large genus in its own country. And all 
these circumstances are favorable to the production of new 

If, then, these two varieties be variable, the most diver- 
gent of their variations will generally be preserved during 
the next thousand generations. And after this interval, 
variety a 1 is supposed in the diagram to have produced 
variety a 2 , which will, owing to the principle of divergence, 
differ more from (A) than did variety a 1 . Variety m 1 is 
supposed to have produced two varieties, namely m 2 and s 2 , 
differing from each other, and more considerably from their 
common parent (A). We may continue the process by simi- 
lar steps for any length of time ; some of the varieties, 
after each thousand generations, producing only a single 
variety, but in a more and more modified condition, some 
producing two or three varieties, and some failing to produce 
any. Thus the varieties or modified descendants of the 
common parent (A), will generally go on increasing in 
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 irregular, nor that 
it goes on continuously ; it is far more probable that each 
form remains for long periods unaltered, and then again 
undergoes modification. Nor do I suppose that the mosfc 







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T 3 






i ! J / 

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V! / 



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"w. 1 

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divergent varieties are invariably preserved: 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 nature of the places which 
are either unoccupied or not perfectly 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 *. re 
places they will be enabled to seize on, and the more heir 
modified progeny will increase. In our diagram the line of 
succession is broken at regular intervals by small numbered 
letters marking the successive forms which have become 
sufficiently 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 accumulation 
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 multiplying in 
nunjber as well as diverging in character : this is represented 
in the diagram by the several divergent branches proceeding 
from (A). The modified offspring 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 dia- 
gram 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 in- 
creased ; although the amount of divergent modification may 
have been augmented. This case would be represented In 
the diagram, if all the lines proceeding from (A) were 
removed, excepting that from a 1 to a 10 . 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 supposed to 
have produced three forms, a 10 , f 10 , and m 10 , which, from 
having diverged in character during the successive genera- 
tions, will have come to differ largely, but perhaps unequally, 
from each other and from their common parent. If we 
suppose the amount of change between each horizontal line 


in our diagram to be excessively small, these three forms 
may still be only well-marked varieties ; but we have only to 
suppose the steps in the process of modification to be more 
numerous or greater in amount, to conveit these three forms 
into doubtful or at least into well-defined species. Thus the 
diagram illustrates the steps by which the small differences 
distinguishing varieties are increased into the larger differ 
ences distinguishing species. By continuing the same pro- 
cess 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 14 and m u , all 
descended from (A). Thus, as I believe, species are multi- 
plied, 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 thou- 
sand generations, either two well-marked varieties (w 1Q and 
£ 10 ) or two species, according to the amount of change 
supposed to be represented between the horizontal lines. 
After fourteen thousand generations, six new species, marked 
by the letters n u to z li , are supposed to have been produced. 
In any genus, the species which are already very different in 
character from each other, will generally tend to produce the 
greatest number of modified descendants ; for these will 
have the best chance of seizing on new and widely different 
places in the polity of nature ; hence in the diagram I have 
chosen the extreme species (A), and the nearly extreme 
species (I), as those which have largely varied, and have 
given rise to new varieties and species. The other nine 
species (marked by capital letters) of our original genus, 
may for long but unequal periods continue to transmit 
unaltered descendants ; and this is shown in the diagram by 
the dotted lines unequally prolonged upward. 

But during the process of modification, represented in the 
diagram, another of our principles, namely that of extinction, 
will have played an important part. As in each fully 
stocked country natural selection necessarily acts by the 
selected form having some advantage in the struggle for life 
over other forms, there will be a constant tendency in the 
improved descendants of any one species to supplant and 
exterminate in each stage of descent 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 
between the earlier and later states, that is between the less 
and more improved states of the same species, as well as the 
original parent species itself, will generally tend to become 
extinct. So it probably will be with many whole collateral 
lines of descent, which will be conquered by later and im- 
proved lines. If, however, the moditied offspring of a species 
get into some distinct country, or become quickly adapted to 
some quite new station, in which offspring and progenitor do 
not come into competition, both may continue to exist. 

If, then, our diagram be assumed to represent a consider- 
able amount of modification, species (A) and all the earlier 
varieties will have become extinct, being replaced by eight 
new species (a 14 to ra 14 ), and species (I) will be replaced by 
six (n u to s 14 ) new species. 

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

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 14 and z 1 * will be 
much greater than that between the most distinct of the 


ariVinal 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 li , </ 14 , p u , will 
be nearly related from having recently branched off from a 10 ; 
6 14 and/ 14 , from having diverged at an earlier period from 
a 5 , will be in some degree distinct from the three first-named 
species ; and lastly, o u , e u , and m u will be nearly related 
one to the other, but, from being diverged at the first com- 
mencement of the process of modification, will be widely 
different from the other five species, and may constitute a 
sub-genus or a distinct genus. 

The six descendants from (I) will form two sub-genera or 
genera. But as the original species (I) differed largely from 
(A), standing nearly at the extreme end of the original genus, 
the six descendants from (I) will, owing to inheritance alone, 
differ considerably from the eight descendants from (A) ; the 
two groups, moreover, are supposed to have gone on diver- 
ging in different directions. The intermediate species, also 
' this is a very important consideration), which connected 
the original species (A) and (1), have all become, except (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. 

Thu^ it is, as I believe, that two or more genera are pro- 
ancect by aescent, with modification, from two or more species 
of the same <>enus. 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 i&tt-ers, converging in sub-branches down- 
ward toward a single point : this point represents a species, 
the supposed progenitor of our several new sub-genera and 

It is worth while to reflect for a moment on the charactei 
31 the new species f 14 . which is supposed not to have diverged 
much in character, but to have retained the form of (F), 
either unaltered or altered onlv in a slight degree. In this 
case it: affinities to the other fourteen new species will be of 
a curiou- and circuitous nature. Being descended from a 
form that stood between tt^ parent-species (A) and (I), now 
supposed to be extinct and unknown, it will be in some degree 
intermediate in character between the two groups descended 
trom these two species. But as these two groups have gone 
on diverging in character from the type of their parents, the 


new species (f 14 ) will not be directly intermediate between 
them, but rather between types of the two groups ; and every 
naturalist will be able to call such cases before his mind. 

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

I see no reason to limit the process of modification, as now 
explained, to the formation of genera alone. If, in the dia- 
gram, we suppose the amount ot change represented by each 
successive group of diverging dotted lines to be great, the 
forms marked a u to p 1 *, those marked b 1 * and / 14 , and those 
marked « 14 to m 14 , will form three very distinct genera. We 
shall also have two very distinct 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 belong- 
ing 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 com/ 
mon. Hence, the struggle for the production of new and 
modified descendants will mainly lie between the larger 
groups which are all trying to increase in number. One 
large group will slowly conquer another large group, reduce 
its number, and thus lessen its chance of further variation 
and improvement. Within the same large group, the later and 


more highly perfected sub-groups, from branching out and 
seizing on many new places in the 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 pre- 
vail, no man can predict ; for we know that many groups 
formerly most extensively developed, have now become ex- 
tinct. Looking still more remotely to the future, we may 
predict that, owing to the continued and steady increase of 
the larger groups, a multitude of smaller groups will become 
utterly extinct, and leave no modified descendants ; and 
consequently, that, of the species living at any one period, 
extremely few will transmit descendants to a remote futurity. 
I shall have to return to this subject in the chapter on clas- 
sification, but I may add that as, according to this view, 
extremely few of the more ancient species have transmitted 
descendants to the presen* 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 n\ tet 
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. 



Natural selection acts exclusively by the preservation and 
accumulation of variations, which are beneficial under the 
organic and inorganic conditions to which each creature is 
exposed at all periods of life. The ultimate result is that 
^ach creature tends to become more and more improved in 
relation to its conditions. This improvement inevitably leads 
co the gradual advancement of the organization 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 organization. Among the vertebrata the degree 
of intellect and an approach in structure to man clearly come 


into play. It might be thought that the amount of change 
which the various ' parts and organs pass through in their 
development from embryo to maturity would suffice as a 
standard of comparison ; but there are cases, as with certain 
parasitic crustaceans, in which several parts of the structure 
become less perfect, so that the mature animal cannot be 
called higher than its larva. Von Haer's standard seems the 
most widely applicable and the best, namely, the amount of 
differentiation of the parts of the same organic being, in the 
adult state, as I should be inclined to add, and their special- 
ization for different functions ; or, as Milne Edwards would 
express it, the completeness of the division of physiological 
labor. But we shall see how obscure this subject is if we 
look, for instance, to fishes, among which some naturalists 
rank those as highest which, like the sharks, approach near- 
est to amphibians ; while other naturalists rank the common 
bony or teleostean fishes as the highest, inasmuch as they 
are most strictly fish-like, and differ most from the other 
vertebrate classes. We see still more plainly the obscurity 
of the subject by turning to plants, among which the stand- 
ard of intellect is of course quite excluded ; and here some 
botanists rank those plants as highest which have every 
organ, as sepals, petals, stamens, and pistils, fully developed 
in each flower ; whereas other botanists, probably with more 
truth, look at the plants which have their several organs 
much modified and reduced in number, as the highest. 

If we take as the standard of high organization, the amount 
of differentiation and specialization of the several organs in 
each being when adult (and this will include the advance- 
ment of the brain for intellectual purposes), natural selection 
clearly leads toward this standard : for all physiologists 
admit that the specialization of organs, inasmuch as in this 
state they perform their functions better, is an advantage to 
each being ; and hence the accumulation of variations tend- 
ing toward specialization is within the scope of natural 
selection. On the other hand, we can see, bearing in mind 
that all organic beings are striving to increase at a high 
ratio and to seize on every unoccupied or less well occupied 
place in the economy of nature, that it is quite possible 
for natural selection gradually to 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 
organization. Whether organization on the whole has actu- 
ally advanced from the remotest geological periods to the 


present day, will be more conveniently discussed in our chap- 
ter on Geological Succession. 

But it may be objected that if all organic beings thus 
tend to rise in the sealo, 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 inevi- 
table tendency toward 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 progres- 
sive development — it only takes advantage of such varia- 
tions as arise and are beneficial to each creature under its 
complex relations of life. And it may be asked what advan- 
tage, 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 organized. If it were no advantage, these 
forms would be left, by natural selection, unimproved 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 pres- 
ent state. But to suppose that most of the many now exist- 
ing low forms have not in the least advanced since the first 
dawn of life would be extremely rash ; for every naturalist 
who has dissected some of the beings now ranked as very 
low in the scale, must have been struck with their really 
wondrous and beautiful organization. 

Nearly the same remarks are applicable, if we look to the 
different grades of organization within the same great group; 
for instance, in the vertebrata, to the co-existence of mam- 
mals and fish — among mammalia, to the co-existence of man 
and the ornithorhynchus — among fishes, to the co-existence 
of the shark and the lancelet (Amphioxus), which latter fieh 
in the extreme simplicity of its structure approaches the 
invertebrate classes. But mammals and fish hardly come 
into competition with each other ; the advancement of the 
whole class of mammals, or of certain members in this classy 

112 ON the'degree to which 

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 disadvantage in hav- 
ing to come continually to the surface to breathe. With 
fishes, members of the shark family would not tend to sup- 
plant the lancelet ; for the lancelet, as I hear from Fritz 
Miiller, has as sole companion and competitor on the barren 
sandy shore of South Brazil, an anomalous annelid. The 
three lowest orders of mammals, namely, marsupials, eden- 
tata, and rodents, co-exist in South America in the same 
region with numerous monkeys, and probably interfere little 
with each other. Although organization, on the whole, may 
have advanced and be still advancing throughout the world, 
yet the scale will always present many degrees of perfection ; 
for the high advancement of certain whole classes, or of cer- 
tain 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 organized 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 favorable variations arising. 

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

Looking to the first dawn of life, when all organic beings, 
as we may believe, presented the simplest structure, how, 
it has been asked, could the first step in the advancement or 
differentiation of parts have arisen ? Mr. Herbert Spencer 
would probably answer, that, as soon as simple unicellular 
organism came by growth or division to be compounded of 
several cells, or became attached to any supporting surface^ 


his law "that homologous units of any order become differ- 
entiated 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 produced : variations 
in a single species inhabiting an isolated station might be 
beneficial, and thus the whole mass of individuals might 
be modified, or two distinct forms might arise. But, as I 
remarked toward the close of the introduction, no one ought 
to feel surprise at much remaining as yet unexplained on 
the origin of species, if we make due allowance for our pro- 
found ignorance on the mutual relations of the inhabitants 
of the world at the present time, and still more so during 
past ages. 


Mr. H. C. Watson thinks that I have overrated the im- 
portance of divergence of character (in which, however, 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 convergence a close and general similarity of 
structure in the modified descendants of widely distinct 
forms. The shape of a crystal is determined solely by the 
molecular forces, and it is not surprising that dissimilar sub- 
stances should sometimes assume the same form ; but with 
organic beings we should bear in mind that the form of each \ 
depends on an infinitude of complex relations, namely on , 
the variations which have arisen, these being due to causes ' 
far too intricate to be followed out — on the nature of the 
variations which have been preserved or selected, and this 
depends on the surrounding physical conditions, and in a 
still higher degree on the surrounding organisms with which 
each being has come into competition — = and lastly, on inher- 
itance (in itself a fluctuating element) from innumerable 
progenitors, all of which have had their forms determined 


through equally complex relations. It is incredible that thg 
descendants of two organisms, which had originally differed 
in a marked manner, should ever afterward converge so 
closely as to lead to a near approach to identity through- 
out their whole organization. If this had occurred, we should 
meet with the same form, independently of genetic connec- 
tion, recurring in widely separated geological formations; 
and the balance of evidence is opposed to any such an 

Mr. Watson has also objected that the continued action 
of natural selection, together with divergence of character, 
would tend to make an indefinite number of specific forms. 
As far as mere inorganic conditions are concerned, it seems 
probable that a sufficient number of species would soon 
become adapted to all considerable diversities of heat, 
moisture, etc. ; but I fully admit that the mutual relations 
of organic beings are more important ; and as the number 
of species in any country goes on increasing, the organic 
conditions of life must become more and more complex. 
Consequently there seems at first 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 
Australia, which support such an astonishing number of 
species, many European plants have become naturalized. 
But geology shows us, that from an early part of the tertiary 
period the number of species of shells, and that from the 
middle part of this same period the number of mammals, 
has not greatly or at all increased. What then checks an 
indefinite increase in the number of species ? The amount 
of life (I do not mean the number of specific forms) sup- 
ported on an area must have a limit, depending so largely as 
it does on physical conditions ; therefore, if an area be 
inhabited by very many species, each or nearly each species 
will be represented by few individuals ; and such species 
will be liable to extermination from accidental fluctuations 
in the nature of the seasons or in the number of their 
enemies. The process of extermination in such cases would 
oe rapid, whereas the production of new species must always 
be slow. Imagine the extreme case of as many species as 
pdividuals in England, and the first severe winter or very 
dry summer would exterminate thousands on thousands of 
species. Rare species, and each species will become rare if 


the number of species in any country becomes indefinitely 
increased, will, on the principle often explained, present 
within a given period few favorable variations ; conse- 
quently, the process of giving birth to new epecific forms 
would thus be retarded. When any species becomes very 
rare, close interbreeding will help to exterminate it; authors 
have thought that this comes into play in accounting for the 
leterioration of the aurochs in Lithuania, of red deer in 
Scotland, and of bears in Norway, etc. Lastly, and this I 
am inclined to think is the most important element, a domi- 
nant species, which has alreadv beaten *nany competitors in 
its own home, will tend to spre. J and supplant many others. 
Alph. de Candolle has shown that those species which 
spread widely tend generally to spread very widely, conse- 
quently they will tend to supplant and exterminate several 
species in several areas, and thus check the inordinate in- 
crease of specific forms throughout the world. Dr. Hooker 
has recently shown that in the south-east corner of Australia, 
where, apparently, there are many invaders from different 
quarters of the globe, the endemic Australian species have 
been greatly reduced in number. How much weight to at- 
tribute to these several considerations I will not pretend to 
jay ; but conjointly they must limit in each country the ten 
dency to an indefinite augmentation of specific forms. 


If under changing conditions of life organic beings pre 
sent 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 infinite complexity of the 
relations of all organic beings to each other and to their 
conditions of life, causing an infinite diversity in structure, 
constitution, and habits, to be advantageous to them, it 
would be a most extraordinary fact if no variations had ever 
occurred useful to each being's own welfare, in the same 
manner as so many variations have occurred useful to 
man. But if variations useful to any organic being ever do 
occur, assuredly individuals thus characterized 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 characterized. This principle of 


preservation, or the survival of the fittest, I have called 
natural selection. It leads to the improvement of each 
creature in relation to its organic and inorganic conditions 
of life ; and consequently, in most cases, to what must be 
regarded as an advance in organization. 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 in- 
herited at corresponding ages, can modify the egg, seed, or 
young, as easily as the adult. Among many animals sexual 
selection will have given its aid to ordinary selection by 
assuring to the most vigorous and best adapted males the 
greatest number of offspring. Sexual selection will also 
give characters useful to the males alone in their struggles 
or rivalry with other males ; and these characters will be 
transmitted to one sex or to both sexes, according to the 
form of inheritance which prevails. 

Whether natural selection has really thus acted in adapt- 
ing the various forms of life to their several conditions and 
stations, must be judged by the general tenor and balance of 
evidence given in the following chapters. But we have 
already s?en how it entails extinction ; and how largely ex- 
tinction has acted in the world's history, geology plainly 
declares. Natural selection, also, leads to divergence of 
character ; for the more organic beings diverge in structure, 
habits, and constitution, by so much the more can a large 
number be supported on the area, of which we see proof by 
looking to the inhabitants of any small spot, and to the pro- 
ductions naturalized in foreign lands. Therefore, during the 
modification of the descendants of any one species, and dur- 
ing the incessant struggle of all species to increase in num- 
bers, the more diversified the descendants become, the better 
will be their chance of success in the battle for life. Thus 
the small differences distinguishing varieties of the same 
species, steadily tend to increase, till they equal the greater 
differences between species of the same genus, or even of 
distinct genera. 

We have seen that it is the common, the widely diffused, 
and widely ranging species, belonging to the larger genera 
within each class, which vary most ; and these tend to trans- 
mit to their modified offspring that superiority which now 
makes them dominant in their own countries. Natural se- 
lection, as has just been remarked, leads to divergence of 
fcharacter and to much extinction of the less improved and 


intermediate forms of life. On these principles, the nature 
of the affinities, and the generally well defined distinctions 
between the innumerable organic beings in each class through- 
out the world, may be explained. It is a truly wonderful 
fact — the wonder of which we are apt to overlook from famil- 
iarity — that all animals and all plants, throughout all time 
and space, should be related to each other in groups, subordi- 
nate to groups, in the manner which we everywhere behold 
- — namely, varieties of the same species 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 related, 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 
endless 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 some- 
times been represented by a great tree. T 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 sur- 
rounding twigs and branches, in the same manner as species 
and groups of species have at all times overmastered other 
species in the great battle for life. The limbs divided into 
great branches, and these into lesser and lesser branches, 
were themselves once, when the tree was young, budding 
twigs ; and this connection of the former and present buds, 
by ramifying branches, may well represent the classification 
of all extinct and living species in groups subordinate to 
groups. Of the many twigs which flourished when the tree 
was a mere bush, only two or three, now grown into great 
branches, yet survive and bear the other branches ; so with 
the species which lived during long-past geological periods, 
very few have left living and modified descendants. From 
the first growth of the tree, many a limb and branch has 
decayed and dropped off j 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 favored and is 
still alive on its summit, s.- we occasionally see an animal 
like the Ornithorhynchus or Lepidosiren, which in some 
small degree connects by its affinities two large branches of 
life, and which has apparently been saved from fatal compe- 
tition by having inhabited a protected station. As buds give 
rise by growth to fresh buds, and these, if vigorous, branch 
out and overtop on all sides many a feebler branch, so by 
generation I believe it has been with the great Tree of Life, 
which fills with its dead and broken branches the crust of 
the earth, and covers the surface with its ever-branching and 
beautiful ramifications. 




Effects of Changed Conditions — Use and Disuse, combined with Nat- 
ural Selection ; Organs of Flight and of Vision — Acclimatization 

— Correlated Variation — Compensation and Economy of Growth. 

— False Correlations — Multiple, Rudimentary, and Lowly Organ- 
ized Structures Variable — Parts developed in an Unusual Man- 
ner are highly Variable: Specific Characters more Variable than. 
Generic; Secondary Sexual Characters Variable — Species of f he 
Same Genus vary in an Analogous Manner — Reversions to Long- 
Tost Characters — Summary. 

I have hitherto sometimes spoken as if the variations — 
bo common and multiform with organic beings under domes- 
tication, and in a lesser degree with those under nature — 
were due to chance. This, of course, is a wholly incorrect 
expression, but it serves to acknowledge plainly our ignor- 
ance of the cause of each particular variation. Some 
authors believe it to be as much the function of the repro- 
ductive system to produce individual differences, or slight 
deviations of structure, as to make the child like its parents. 
But the fact of variations and monstrosities occurring much 
more frequently under domestication than under nature, and 
the greater variability of species having wide ranges than of 
those with restricted ranges, lead to the conclusion that vari- 
ability is generally related to the conditions of life to which 
each species has been exposed during several successive gen- 
erations. In the first chapter I attempted to show that 
changed conditions act in two ways, directly on the whole or 
ganizacion or on certain parts alone, and indirectly through 
the reproductive system. In ail 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 organization seems to become 
plastic, and we have much fluctuating variability. 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. 


ft is very difficult to decide how far changed conditions, 
such as of climate, food, etc., have acted in a definite 
manner. There is reason to believe that in the course of 
time the effects have been greater than can be proved by 
clear evidence. Rut we may satelv conclude that the innu- 
merable complex coadaptations of structure, which we set 
throughout nature bet.veen 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 colored 
than those of the same species from farther 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 colored under a clear atmosphere/ than when 
living near the coast or on islands ; and Wollaston is con- 
vinced that residence near the sea affects the colors of insects. 
Moquin-Tandon g».ves a list of plants which, when growing 
near the sea-shore, have their leaves in some degree fleshy, 
though not elsewhere fleshy. These slightly varying organ- 
isms are interesting in as far as they present characters analo- 
gous to those possessed by the species which are confined to 
similar conditions. 

When a variation is of the slightest use to any being, we 
cannot tell how much to attribute to the accumulative action 
of natural selection, and how much to the definite action of 
the conditions of life. Thus, it is well known to furriers 
that animals of the same species have thicker and better fur 
the farther north they live ; but who can tell how much of 
this difference may be due to the warmest clad individuals 
having been favored 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 pro- 
duced from the same species under external conditions 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 considerations as these incline me to lay 
less weight on the direct action of the surrounding condi- 
tions, than on a tendency to vary, due to causes of which w* 
are Siuite ignorant. #w -.. 


In one sense the conditions of life may be said, not only 
to cause variability, either direct! v or indirectlv. but like- 
wise to include natural selection, for the conditions deter- 
mine whether this or that variety shall survive. But when 
man is the selecting agent, we (dearly see that the two ele- 
ments of change are distinct; variability is in some manner 
excited, but it is the will of man which accumulates the 
variations in certain direction; and it is this latter agency 
which answers to the survival of the fittest under nature. 


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 dimin- 
ished them ; and that such modifications are inherited. 
Under free nature we have no standard of comparison by 
which to judge of the effects of long-continued use or disuse, 
for we know not the parent-forms ; but many animals possess 
structures which can be best explained by the effects d. dis- 
use. As Professor Owen has remarked, there is no greater 
anomaly in nature than a bird that cannot fly ; yet there are 
several in this state. The logger-headed duck of South 
America can only flap along the surface of the water, and 
has its wings in nearly the same condition as the domestic 
Aylesbury duck : it is a remarkable fact that the young birds, 
according to Mr, Cunningham, can fly, while the adults have 
lost this power. As the larger ground-feeding birds seldom 
take flight except to escape danger, it is probable that the 
nearly wingless condition of several birds, now inhabiting 
or which lately inhabited several oceanic islands, tenanted 
by no beasts of prey, has been caused by disuse. The ostrich 
indeed inhabits continents, and is exposed to danger from 
which it cannot escape by flight, 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 increased during successive genera- 
tions, its legs were used more and its wings less, until they 
became incapable of flight. . 

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 speoi- 


mens 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 Egyp- 
tians, they are totally deficient. The evidence that acci- 
dental mutilations can be inherited is at present not 
decisive ; but the remarkable cases observed by Brown- 
Sequard in guinea-pigs, of the inherited effects of opera- 
tions, should make us cautious in denying this tendenc}'. 
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 dis- 
use ; for, as many dung-feeding beetles are generally found 
with their tarsi lost, tlrs must happen early in life ; there- 
fore the tarsi cannot be of much importance or be much 
used by these insects. 

In some cases we might easily put down to disuse modifi- 
cations of structure which are wholly or mainly due to 
natural selection. Mr. Wollaston has discovered the remark- 
able 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, else- 
where excessively numerous, which absolutely require the 
use of their wings, are here almost entirely absent. Thes^ 
several considerations make me believe that the winglegJ 
condition of so many Madeira beetles is mainly due to the 
action of natural selection, combined probably with disuse. 
For during many successive generations each individual 
beetle which flew least, either from its wings having been 
ever so little less perfectly developed or from indolent habit, 
will have had the best chance of surviving from not being 
blown out to sea; and, on the other hand, those beetles 
which most readily took to flight would oftenest have been 
blown to sea, and thus destroyed*. 


The insects in Madeira which are not ground-feeders, and 
which, as certain flower-feeding coleoptera and lepidoptera, 
must habitually use their wings to gK.n 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 tei. .ency 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 swimmers if they had not been 
able to swim at all and had stuck to the wreck. 

The eyes of moles and of some burrowing rodents are 
rudimentary in size, and in some cases are quite covered by 
skin and fur. This state of the eyes is probably due to 
gradual reduction from disuse, but aided perhaps by natural 
selection. In South America, a burrowing 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 appeared 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 cer- 
tainly not necessary to animals having subterranean habits, 
a reduction in their size, with the adhesion of the eyelids 
and growth of fur over them, might in such case be an 
advantage ; and if so, natural selection would aid the effects 
of disuse. 

It is well known that several animals, belonging to the 
most different classes, which inhabit the caves of Carniola 
and Kentucky, are blind. In some of the crabs Jae foot- 
stalk for the eyes 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 living in darkness, their loss may be attributed to 
disuse. In one of the blind animals, namely, the cave- 
rat (Neotoma), two of which were captured by Professor 
Silliman at above half a mile distance from the mouth of 
the cave, and therefore not in the profoundest depths, the 


eyes were lustrous and of large size ; and these animals, as 
I am informed by Professor Si Hi man, 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 similai 
than deep limestone caverns under a nearly similar climate ; 
so that, in accordance with the old view of the blind ani- 
mals having been separately created for the American and 
European caverns, very close similarity in their organiza- 
tion and affinities might have been expected. This is cer- 
tainly 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 Car- 
niola. otherwise than as a very plain expression of that 
analogy which subsists generally between the faun a of 
Europe and of North America.^ On my view we must sup- 
pose that American animals, having in most cases 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; tor, as Schiodte remarks: "We accordingly look 
upon the subterranean faunas as small ramifications which 
have penetrated into the earth from the geographically 
limited faunas of the adjacent tracts, and which, as they 
extended themselves into darkness, have been accommodated 
to surrounding circumstances. Animals not far remote from 
ordinary 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 darkness, and whose 
formation is quite peculiar." These remarks of Schiodte's, 
it should be understood, apply not to the same, but to dis- 
tinct species. By the time that an animal had reached, 
after numberless generations, the deepest recesses, disuse 
will on this view have more or less perfectly obliterated its 
eyes, and natural selection will otten have effected other 
changes, such as an increase in the length of the antennse 
or palpi, as a compensation for blindness. Notwithstanding 
such modifications, we might expect still to see in the cave- 
animals of America, affinities to the other inhabitants of 
that continent, and in those of Europe to the iuhaUitaats 


of the European continent. And this is the case with some 
of the American cave-animals, as I hear from Professor 
Dana; and some of the European cave-insects are very 
closely allied to those of the surrounding country. It 
would be difficult to give any rational explanation of the 
affinities of the blind cave-animals to the other inhabitants 
of the two continents on the ordinary view of their inde- 
pendent creation. That several cf the inhabitants of the 
caves of the Old and New Worlds should be closely related^ 
we might expect from the well-known relationship of most 
of their other productions. As a blind species of Bathyscia 
is found in abundance on shadv rocks far from caves, the 
losvj of vision in the cave species of this one genus has 
probably had no relation to its dark habitation; for it is 
natural that an insect already deprived of vision should 
readily become adapted to dark caverns. Another blind 
genus (Anophthalmus) offers this remarkable peculiarity, 
that the species, as Mr. Murray observes, have not as yet 
been found anywhere except in caves ; yet those which 
inhabit the several caves of Europe and America are dis- 
tinct ; but it is possible that the progenitors of these several 
species, while they were furnished with eyes, may formerly 
have ranged over both continents, and then have become ex- 
tinct, excepting in their present secluded abodes. Far from 
feeling surprise that some of the cave-animals should be 
very anomalous, as Agassiz has remarked in regard to the 
blind fish, the Amblyopsis, and as is the case with the blind 
Proteus, with reference to the reptiles of Europe, I am only 
surprised that more wrecks of ancient life have not been 
preserved, owing to the less severe competition to which 
the scanty inhabitants of these dark abodes will have been 


Habit is hereditary with plants, as in the period ef 
flowering, in the time of sleep, in the amount of rain 
requisite for seeds to germinate, etc.. and this leads me to 
say a few words on acclimatization. As it is extremely 
common for distinct species belonging to the same genus 
to inhabit hot and cold countries, if it be true that all the 
species of the same genus are descended from a single 
parent-form, acclimntizHtion must be readily effected dur- 
ing a long course of descent, it is notorious ihat eacb 


species is adapted to the climate of its own home : species 
from an arctic or even from a temperate region cannot 
endure a tropical climate, or conversely. So again, many 
succulent plants cannot endure a damp climate. But the 
degree of adaptation of species to the climates under which 
they live is often overrated. We may infer this from our 
frequent inability to predict whether or not an imported 
plant will endure our climate, and from the number of plants 
and animals brought from different countries which are here 
perfectly healthy. We have reason to believe that species 
in a state of nature are closely limited in their ranges by 
the competition of other organic beings quite as much as, or 
more than, by adaptation to particular climates. But whether 
or not this adaptation is in most cases very close, we have 
evidence with some few plants, of their becoming, to a cer- 
tain extent, naturally habituated to different temperatures; 
that is, they become acclimatized ; thus the pines and rhodo- 
dendrons, raised from seed collected by Dr. Hooker from 
the same species growing at different heights on the Hima- 
layas, were found to possess in this country different 
constitutional powers of resisting cold. Mr. Thwaites in- 
forms me that he has observed similar facts in Ceylon ; 
analogous observations have been made by Mr. H. C. Watson 
on European species of plants brought from the Azores to 
England ; and I could give other cases. In regard to ani- 
mals, several authentic instances could be adduced of 
species having largely extended, within historical times, 
their range from warmer to colder latitudes, and conversely ; 
but we do not positively know that these animals were 
strictly adapted to their native climate, though in all ordi- 
nary cases we assume such to be the case ; nor do we know 
that they have subsequently become specially acclimated to 
their new homes, so as to be better fitted for them than they 
were at first. 

As we may infer that our domestic animals were origin- 
ally chosen by uncivilized man because they were useful, 
and because they bred readily under confinement, and not be- 
cause they were subsequently found capable of far-extended 
transportation, the common and extraordinary 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 


tnust not, however, push the foregoing argument too far, on 
account of the probable origin of some of our domestic 
animals from several wild stocks ; the blood, for instance, 
of a tropical and arctic wolf may perhaps be mingled in our 
domestic breeds. The rat and mouse canuot be considered 
as domestic animals, but they have been transported by man 
to many parts of the world, and now have a far wider range 
than any other rodent ; for they live under the cold climate 
of Faroe in the north and of the Falklands in the south, and 
on many an island in the torrid zones. Hence adaptation to 
any special climate may be looked at as quality readily grafted 
on an innate wide flexibility of constitution, common to most 
animals. On this view, the capacity of enduring the most 
different climates by man himself and by his domestic ani- 
mals, and the fact of the extinct elephant and rhinoceros 
having formerly endured a glacial climate, whereas the living 
species are now all tropical or sub-tropical in their habits, 
ought not to be looked at as anomalies, but as examples 
of a very common flexibility of constitution brought, under 
peculiar circumstances, into action. 

How much of the acclimatization of species to any 
peculiar climate is due to mere habit, and how much to the 
natural selection of varieties having different innate con- 
stitutions, 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 
Encyclopaedias of China, to be very cautious in transporting 
animals from one district to another. And as it is not 
likely that man should have succeeded in selecting so many 
breeds and sub-breeds with 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 preserve those individuals which were born with constitu- 
tions best adapted to any country which they inhabited. In 
treatises on many kinds of cultivated plants, certain varieties 
are said to withstand certain climates better than others ; 
this is strikingly shown in works on fruit-trees 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 prop- 
agated in England by seed, and of which, consequently, new 


varieties have not been produced, has even been advanced, 
as proving that acclimatization cannot be effected, for it 
is now as tender as ever it was ! The case, also, of the 
kidney-bean has been often cited for a similar purpose, and 
with much greater weight ; but until some one 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 seed- ' 
iings, with the same precautions, the experiment cannot be 
said to have been tried. Nor let it be supposed that differ- 
ences 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 cases, played a considerable part in 
the modification of the constitution and structure ; but that 
the effects have often been largely combined with, and some- 
times overmastered by, the natural selection of innate vari- 


I mean by this expression that the whole organization is 
so tied together, during its growth and development, that 
when slight variations in any one part occur and are accu- 
mulated through natural selection, other parts become modi- 
fied. This is a very important subject, most imperfectly 
understood, and no doubt wholly different classes of facts 
may be here easily confounded together. We shall presently 
see that simple inheritance often gives the false appearance 
of correlation. One of the most real obvious cases is that 
variations of structure arising in the young or larvie natur- 
ally tend to affect the structure of the mature animal. The 
several parts which are homologous, and which, at an early 
embryonic period, are identical in structure, and which are 
necessarily exposed to similar conditions, seem eminently 
liable to vary in a like manner : we see this in the right and 
left sides of the body varying in the same manner ; in the 
front and hind legs, and even in the jaws and limbs, varying 
together, for the lower jaw is believed by some anatomists to 
be homologous with the limbs. These tendencies, I do not 
doubt, may be mastered more or less completely by natural 


selection ; thus a family of stags once existed with an antler 
only on one side ; and if this had been of any great use to 
the breed, it might probably have been rendered permanent 
by selection. 

Homologous parts, as has been remarked by some authors, 
tend to cohere ; this is often seen in monstrous plants : and 
nothing is more common than the union of homologous 
pnrts 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 authors 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, ac- 
cording to Schlegel, the form of the body and the manner 
of swallowing determine the position and form of several of 
the most important viscera. 

The nature of the bond is frequently quite obscure. M. Is. 
Geoifroy Saint-Hilaire has forcibly remarked that certain 
malconformations frequently, and that others rarely, coexist 
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 tor- 
toise-shell color and the female sex ; or in pigeons, between 
their feathered feet and skin betwixt the outer toes, or 
between the presence of more or less down on the young 
pigeon, when first hatched, with the future color of its 
plumage ; or again, the relation between the hair and the 
teeth in the naked Turkish dog, though here no doubt 
homology comes into play ? With respect to this latter 
case of correlation, I think it can hardly be accidental that 
the two orders of mammals which are most abnormal in 
their dermal covering, viz., cetacea (whales) and edentata 
(armadilloes, scaly ant-eaters, etc. ), are likewise on the whole 
the most abnormal in their teeth, but there are so many ex- 
ceptions to this rule, as Mr. Mivart has remarked, that it has 
little value. 

I know of no case better adapted to show the importance 
of the laws of correlation and variation, independently of 
utility, and therefore of natural selection, than that of the 
difference between the outer and inner flowers in some com* 
positous and umbelliferous plants. Every one is familiar 
with the difference between the ray and central florets of, 
for instance, the daisy, and this difference is often accoia» 


panied with the partial or complete abortion of the repro- 
ductive organs. But in some of these plants the seeds also 
differ in shape and sculpture. These differences have some- 
times been attributed to the pressure of the involucra on the 
florets, or to their mutual pressure, and the shape of the 
seeds in the ray florets of some compositse countenances this 
idea; but with the umbellifera* it is by no means, as Dr. 
Hooker informs me, the species with the densest heads which 
most frequently differ in their .nner and outer flowers. It 
might have been thought that >h j 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 compositse the seeds of the outer and inner flo- 
rets differ, without any difference in the corolla. Possibly 
these several differences maybe connected witn the different 
flow of nutriment toward the central and external flowers. 
We know, at least, that with irregular flowers those nearest 
to the axis are most subject to peloria, that is, to become 
abnormally symmetrical. I may add, as an instance of this 
fact, and as a striking case of correlation, that in many 
pelargoniums the two upper petals in the central flower of 
the truss often lose their patches of darker color ; and when 
this occurs, the adherent nectary is quite aborted, the central 
flower thus becoming peloric or regular. When the color is 
absent from only one of the two upper petals, the nectary is 
not quite aborted but is much shortened. 

With respect to the development of the corolla, SprengePs 
idea that the ray-florets serve to attract insects, whose agency 
is highly advantageous, or necessary for the fertilization of 
these plants, is highly probable ; and if so, natural selection 
may have come into play. But witn respect to the seeds, it 
seems impossible that their differences in shape, which are 
not always correlated with any difference in the corolla, can 
be in any way beneficial ; yet in the um belli ferae these dif- 
ferences are of such apparent importance — the seeds being 
sometimes orthospermous in the exterior flowers and coelo- 
spermous in the central flowers — that the elder De Candolle 
founded his main divisions in the order on such characters. 
Hence modifications of structure, viewed by systematists as 
of high value, may be wholly due to the laws of variation 
and correlation, without being, as far as we can judge, of the 
slightest service to the species. 

We may often falsely attribute to correlated variation 
structures which are common to whole groups of specie^ 


and which in truth are simply due to inheritance; for an 
ancient progeaitor may have acquired through natural selec- 
tion 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 nat- 
urally be thought to be in some necessary manner correlated. 
Some other correlations are apparently due to the manner in 
which natural selection can alone act. For instance, Alph. : 
de Candolle has remarked that winged seeds are never found 
in fruits which do not open ; I should explain this rule by 
the impossibility of seeds gradually becoming winged through 
natural selection, unless the capsules were open : for in this 
case 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. 


The elder Geoffroy and Goethe propounded, at about the 
same time, their law of compensation or balancement of 
growth; or, as Goethe expressed it, "in order to spend on 
one side, nature is forced to economize on the other side." 
I think this holds true to a certain extent with our domes- 
tic productions : if nourishment flows to one part or organ 
in excess, it rarely flows, at least in excess, to another part ; 
thus it is difficult to get a cow to give much milk and to 
fatten readily. The same varieties of the cabbage do not 
yield abundant and nutritious foliage and a copious supply 
of oil-bearing seeds. When the seeds in our fruits become 
atrophied, the fruit itself gains largely in size and quality. 
In our poultry, a large tuft of feathers on the. head is gen- 
erally 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 tho law is of universal appli- 
cation ; 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 devel- 
oped through natural selection and another and adjoining 
part being reduced by the 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 


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

Thus, as I believe, natural selection will tend in the long- 
run to reduce any part of the organization, as soon as it 
becomes, through changed habits, superfluous, without by 
any means causing some other part to be largely developed 
in a corresponding degree. And conversely, that natural 
selection may perfectly well succeed in largely developing 
an organ without requiring as a necessary compensation the 
reduction of some adjoining part. 


It seems to be a rule, as remarked by Is. Geoffroy Saint 
Hilaire, both with varieties and species, that when any pare 
or organ is repeated many times in the same individual (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. Ttia 


same author, as well as some botanists, have further re* 
marked that multiple parts are extremely liable to vary 
in structure. As "vegetable repetition," to use Professor 
Owen's expression, is a sign of low organization, the fore- 
going statements accord with the common opinion of natur- 
alists, 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 
organization have been but little specialized for particular 
functions ; and as long as the same part has to perform 
diversified work, we can perhaps see wh} r it should remain 
variable, that is, why natural selection should not have 
preserved or rejected each little deviation of form so care- 
fully as when the part has to serve for some one special 
purpose. In the same way that a knife which has to cut all 
sorts of things may be of almost any shape ; while a tool 
for some particular purpose must be of some particular 
shape. Natural selection, it should never be forgotten, can 
act solely through and for the advantage of each being. 

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


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 allowances for 
them. It should be understood that the rule by no means 
applies to any part, however unusually developed, unless it 
be unusually developed in one species or in a few species 
in comparison with the same part in many closely allied 
species. Thus, the wing of the bat is a most abnormal 


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 species had wings 
developed in a remarkable manner in comparison with the 
other species of the same genus. The rule applies very 
strongly in the case of secondary sexual characters, when 
displayed in any unusual manner. The term, secondary 
sexual characters, used by Hunter, relates to characters 
which are attached to one sex, but are not directly con- 
nected with the act of reproduction. The rule applies to 
males and females ; but more rarely t< '"he females, as they 
seldom offer remarkable secondary sexual characte. . The 
rule being so plainly applicable in the case of secondary 
sexual characters, may be due to the great variability of 
these characters, whether or not displayed in any unusual 
manner — of which fact I think there can be little doubt. 
But that our rule is not confined to secondary sexual charac- 
ters is clearly shown in the case of hermaphrodite cirri- 
pedes; I particularly attended to Mr. Waterhouse's remark, 
while 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 application. The opercular valves of sessile cirri- 
pedes (rock barnacles) are, in every sense of the word, very 
important 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, in- 
habiting the same country, vary extremely little, I have 
particularly attended to them ; and the rule certainly seems 
to hold good in this class. I cannot make out that it 
applies to plants, and this would have seriously shaken my 
belief in its truth, had not the great variability in plants 
made it particularly difficult to compare their relative de- 
grees of variability. 

When we see any part or organ developed in a remark' 


able degree or manner in a species, the fair presumption is 
that it is of high importance to that species : neverthe- 
less it is in this case eminently liable to variation. Why 
should this be so ? On the view that each species has been 
independently created, with all its parts as we now see 
them, I can see no explanation. But on the view that 
groups of species are descended from some other species 
and have been modified through natural selection, I think 
we can obtain some light. First let me make some pre- 
liminary remarks. If, in our domestic animals, any part 
or the whole animal be neglected, and no selection be 
applied, that part (for instance, the comb in the Dorking 
fowl) or the whole breed will cease to have a uniform 
character ; and the breed may be said to be degenerating. 
In rudimentary organs, and in those which have been but 
little specialized for any particular purpose, and perhaps 
in polymorphic groups, we see a nearly parallel case ; for 
in such cases natural selection e' J jher has not or cannot 
have come into full play, and tb'^ the organization is left 
in a fluctuating condition. But A r hat 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, etc., 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 going on between, on the 
one hand, the tendency to reversion to a less perfect state, 
as well as an innate tendency to new variations, and, on 
the other hand, the power of steady 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 
Boarse 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 de- 
veloped in an extraordinary manner in any one species, 
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 common progenitor of the 
genus. This period will seldom be remote in any extreme 
degree, as species rarely endure for more than one geologi- 
cal period. An extraordinary amount of modification im- 
plies an unusually large and long-continued amount of 
variability, which has continually been accumulated by nat- 
ural selection for the benefit of the species. Hut 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 organization which have remained for a much 
longer period nearly constant. And this, I am convinced, 
is the case. That the struggle between natural selec- 
tion 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, nas 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 con- 
tinued selection of the individuals varying in the required 
manner and degree, and by the continued rejection of those 
tending to revert to a former and less modified condition. 



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 color 
would be only a specific character, and no one would be sur- 


prised at one of the blue species varying into red, or con- 
versely ; but if all the species had blue flowers, the color 
would become a generic character, and its variation would 
be a more unusual circumstance. I have chosen this exam- 
ple because the explanation which most naturalists would 
advance is not here applicable, namely, that specific charac 
ters are more variable than generic, because they are taken 
from parts of less physiological importance than those com- 
monly used for classing genera. I believe 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 superfluous to adduce evidence in support 
of the statement, that ordinary specific characters are more 
variable than generic ; but with respect to important char- 
acters, I have repeatedly noticed in works on natural history, 
that when an author remarks with surprise that some impor- 
tant organ or part, which is generally very constant through- 
out a large group of species, differs considerably in closely 
allied species, it is often variable in the individuals of the 
same species. And this fact shows that a character, which 
is generally of generic value, when it sinks in value and 
becomes only of specific value, often becomes variable, 
though its physiological importance may remain the same. 
Something of the same kind applies to monstrosities: at least 
Is. Geoffroy Saint-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 anoma- 
lies in the individuals. 

On the ordinary view of each species having been inde- 
pendently created, why should that part of the structure, 
which differs from the same part in other independently 
created species of the same genus, be more variable than 
those parts which are closely alike in the several species ? 
I do not see that any explanation can be given. But on 
the view that species are only strongly marked and fixed 
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 those so-called generic 
characters have been inherited from before the period when 
the several species first branched off from their common 
progenitor, and subsequently have not varied or come to 
differ in any degree, or only in a slight degree, it is not 
probable that they should vary at the present day. On the 
other hand, the points in which species differ from other 
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 com- 
mon progenitor, it is probable that they should still often 
be in some degree variable — at least more variable than 
those parts of the organization which have for a very long 
period remained constant. 


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 ut 
the same group differ from each other more widely in their 
secondary sexual characters, than in other parts of their or- 
ganization : compare, for instance, the amount of difference 
between the males of gallinaceous birds, in which secondary 
sexual characters are strongly displayed, with the amount 
of difference between the females. The cause of the origi- 
nal 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 
offspring to the less favored 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 giv- 
ing 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 organization 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 gu u^list^aud as the differences uj 


these cases are of a very unusual nature, the relation cao 
hardly be accidental. The same number of joints in the 
tarsi is a character common to very large groups of beetles, 
bat in the Engidae, 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 hymen- 
optera, 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 offer excellent illustrations 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 principally given by 
these organs." This relation has a clear meaning on my 
view : I look at all the species of the same genus as having 
as certainly descended from a common progenitor, as have 
the two sexes of any one 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 natu- 
ral and sexual selection, in order to fit the 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 pos- 
sessed by all the species ; that the frequent extreme varia- 
bility 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 com- 
mon to a whole group of species ; that the great variability 
of secondary sexual characters and their great difference in 
closely allied species ; that secondary sexual and ordinary 
specific differences are generally displayed in the same parts 
of the organization, — 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 inh^rit^d 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 having more or 
less completely, according to the lapse of time, overmastered 
the tendency to reversion and to further variability, to sex- 
ual 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. 


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 considered as a variation 
representing the normal structure of another race, the fan^ 
tail. I presume that no one will doubt that all such analo- 
gous variations are due to the several races of the pigeon 
having inherited from a common parent the same constitu- 
tion and tendency to variation, when acted on by similar 
unknown influences. In the vegetable kingdom we have a 
case of analogous variation, in the enlarged stems, or as com- 
monly called roots, of the Swedish turnip and ruta-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 added, namely, 
the common turnip. According to the ordinary view of 
each species having been independently created, we should 
have to attribute this similar^ in the enlarged stems of 
these three plants, not to the vera causa of community of 
descent, and a consequent tendency 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 insects under 
natural conditions have lately been discussed with much 


ability by Mr. Walsh, who has grouped them under his law 
of equable variability. 

With pigeons, however, we have another case, namely, 
the 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 externally 
edged near their basis with white. As all these marks are 
characteristic of the parent rock-pigeon, I presume that no 
one will doubt that this is a case of reversion, and not of 
a new yet analogous variation appearing in the several 
breeds. We may, I think, confidently come to this con- 
clusion, because, as we have seen, these colored marks are 
eminently liable to appear in the crossed offspring of two 
distinct and differently colored 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 influence of the mere act of crossing on the laws 
of inheritance. 

No doubt it is a very surprising fact that characters 
should reappear after having been lost for many, probably 
for hundreds of generations. But when a breed has been 
crossed only once by some other breed, the offspring occa- 
sionally tshow 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 pro- 
portion of blood, to use a common expression, from one 
ancestor, is only one in 2048 ; and yet, as we see, it is gen- 
erally believed that a tendency to reversion is retained by 
this remnant of foreign blood. In a breed which has not 
been crossed, but in which both parents have lost some 
character which their progenitor possessed, the tendency, 
whether strong or weak, to reproduce the lost character 
might, as was formerly remarked, for all that we can see to 
the contrary, be transmitted for almost any number of gen- 
erations. When a character which has been lost in a breed, 
reappears after d great number of generations, the most 
probable hypothesis is, not that one individual suddenly 
takes after an ancestor removed by some hundred genera- 
tions, but that in each successive generation the character in 
question has been lying latent, and at last, under unknown, 
favorable 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. nrodjice blue plumage, Xhe abstract imprg^biiit^ of 


suck a tendency being transmitted through a vast numbei 
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 

As all the species of the same genus are supposed to be 
descended from a common progenitor, it might be expected 
that they would occasionally vary in an analogous manner; 
so that the varieties of two or more species would resemble 
each other, or that a variety of one species would resemble in 
certain characters another and distinct species, this other 
species being, according to our view, only a well-marked and 
permanent variety But characters exclusively due to anal- 
ogous 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 color 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 color and the several marks so often appearing 
when differently colored 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 assum- 
ing characters which are already present in other members 
of the same group. And this undoubtedl)* is the cas^. 

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 them- 
selves can only doubtfully be ranked as species ; and this 
shows, unless all these QlgseiF ftuifid iQrnxs be considered a? 



independently created species, that they have in varying 
assumed some of the characters of the others. But the best 
evidence of analogous variations is afforded by parts or 
organs which are generally constant in character, but which 
occasionally vary so as to resemble, in some degree, the 
same part or organ in an allied species. 1 have collected a 
long list of such cases ; but here, as before, I lie under the 
great disadvantage of not being able to give them. I can 
only repeat that such cases certainly occur, and seem to me 
very remarkable. 

I will, however, give one curious and complex case, not 
indeed as affecting any important character, but from occur- 
ring in several species of the same genus, partly under 
domestication and partly under nature. It is a case 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-colored asses. The koulan of 
Pallas is said to have been seen with a double shoulder- 
stripe. Mr. Blyth has seen a specimen of the hemionus 
with a distinct shoulder-stripe, though it properly has none; 
and I have been informed by Colonel Poole that the foals of 
this species are generally striped on the legs and faintly on 
the shoulder. The quagga, though so plainly barred like a 
zebra over the body, is without bars on the legs ; but Dr. 
Gray has figured one specimen with very distinct zebra-like 
bars on the hocks. 

With respect to the horse, I have collected cases in 
England of the spinal stripe in horses of the most distinct 
breeds and of all colors ; transverse bars on the legs are not 
rare in duns, mouse-duns, and in one instance in a chestnut ; 
a faint shoulder-stripe may sometimes be seen in duns, and 
I have seen a trace in a bay horse. My son made a careful 
examination and sketch for me of a dun Belgian cart-horse 
with a double 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 T 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 some- 
times treble, is common , the side of the face, moreover, is 
gometimes striped. The stripes are often plainest in the 
foal, and sometimes quite disappear in old horses. Colonel 
Poole has seen both gray and bay Kattywar horses striped 
when first foaled. I have also reason to suspect, from 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 myself 
recently bred a foal from a bay mare (offspring of a Turko- 
man 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 ail parts of the world these stripes occur far 
oftenest in duns and mouse-duns. By the term dun a large 
range of color is' included, from one between brown and 
black to a close approach to cream color. 

I am aware that Colonel Hamilton Smith, who has written 
on this subject, believes that the several breeds of the horse 
are descended from several aboriginal species, one of which, 
the dun, wa~ striped ; and that the above-described appear- 
ances 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 
cobs, the lanky Kattywar race, etc.. inhabiting the most 
distant 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. Rollin asserts that the common 
mule from the ass and horse is particularly apt to have bars 
on its legs ; according to Mr. Gosse, in certain parts of the 
United States, about nine out of ten mules have 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 j and 


Mr. W. C. Martin, in his excellent treatise on the horse, has 
given a figure of a similar mule. In lour colored 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 hemionus ; and this hybrid, 
though the ass only occasionally has stripes on his legs and 
the hemionus has none and has not even a shoulder-stripe, 
nevertheless had all four legs barred, and had three short 
shoulder-stripes, like those on the dun Devonshire and 
Welsh ponies, and even had some zebra-like stripes on the 
sides of its face. With respect to this last fact, I was so 
convinced that not even a stripe of color 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 f »ce-stripes 
ever occurred in the eminently striped Kattywr r breed of 
horses, and was, as we have seen, answered ii? the affirm- 

What now are we to say to these several fac ; ? We see 
several distinct species of the horse genus becoming, by 
simple variation, striped on the legs like a zebra, ^r 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 coloring 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. Now observe the case of the several 
breeds of pigeons : they are descended from a pigeon (in- 
cluding two or three sub-species or geographical races) of a 
bluish color, 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 colors a*t; yrossed, 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 reappearance of very ancient 
characters, is — that there is a tendency in the young of each 
successive generation to produce the long-lost character, and 
that this tendency, from unknown causes, sometimes pre- 
vails. 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 generations, and I see 
an animal striped like a zebra, but perhaps otherwise very 
differently constructed, the common 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 independ- 
ently 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 inhabiting distant quarters of the 
world, to produce hybrids resembling in their stripes, not 
their own parents, but other species of the genus. To 
admit this view, is, as it seems to me, to reject a real for an 
unreal, or at least for an unknown cause. It makes the 
works of God a mere mockery and deception ; I would 
almost as soon believe, with the old and ignorant cosmog- 
onists, that fossil shells had never lived, but had been 
created in stone so as to mock the shells living on the sea- 


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 differ- 
ences between species of the same genus Changed condi- 
tions generally Uiduce mere fluctuating variability, |^| 


sometimes they cause direct and definite effects; and these 
may become strongly marked in the course <■! time, though 
we have not sufficient evidence on this head. Hi hit in pro- 
ducing constitutional peculiarities, and use in strengthening 
and disuse in weakening and diminishing organs, appear in 
many cases to have been potent in their effects Homol- 
ogous parts tend to vary in the same manner, and homolo- 
gous parts tend to cohere. Modifications in hard parts and 
in external parts sometimes affect softer and internal parts. 
When one part is largely developed, perhaps it tends to 
draw nourishment from the adjoining parts; and every part 
of the structure which can be 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, 
undoubtedly occur. Multiple parts are variable in number 
and in structure, perhaps arising from such parts not having 
been closely specialized 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 scale, and which have their whole 
organization more specialized. Rudimentary organs, from 
being useless, are not regulated by natural selection, and 
hence are variable. Specific characters — that is, the char- 
acters which have come to differ since the several species of 
the same genus branched off from a common parent — are 
more variable than generic characters, or those which have 
long been inherited, and have not 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 wtrk — in that 
district and among these species, we now find, on an average, 
most varieties. Secondary sexual characters are highly vari- 
able, and such characters differ much in the species of the 
same group. Variability in the same parts of the organiza- 
tion has generally been taken advantage of in giving sec- 
ondary sexual differences to the two sexes of the same 
8p§GlgSj a^ specific differences to tj^e several species of the 


game genus. Any part or organ developed to an extraordi. 
nary size or in an extraordinary manner, 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 selection will in such cases not as yet have had 
time to overcome the tendency to further variability and to 
reversion to a less modified state. But when a species with 
an 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 char- 
acter 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 varia- 
tions, 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 rever- 
sion and analogous variation, such modifications will add to 
the beautiful and harmonious diversity of nature. 

Whatever the cause may be of each slight difference be- 
tween 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 aL the more important modifications of structure in rela 
&on to **Ue habits of each species. 




Difficulties of the Theory of Descent with Modification — Absence 01 
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 Natural 

Long before the reader has arrived at this part of my 
work, a crowd of difficulties will have occurred 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 species being, as we see "•"hem, well 
defined ? 

Secondly, is it possible that an animal having, fv,^ lu^^ance, 
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 impor- 
tance, such as the tail of a giraffe, which serves as a fly- 
flapper, and, on the other hand, an organ so wonderful as 
the eye ? 

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

Fourthly, b,ow can we account for species, w^eo crossed 


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

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


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 extermi- 
nate, its own less improved parent-form and other less- 
favored forms with which il comes into competition. Thus 
extinction and natural selection go hand in hand. Hence, 
if we look at each species as descended from some unknown 
form, both the 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 imbedded in 
countless numbers in the crust of the earth ? It will be 
more convenient to discuss this question in the chapter on 
the Imperfection of the Geological Record ; and I will here 
only state that I believe the answer mainly lies in the record 
being incomparably less perfect than is generally supposed. 
The crust of the earth is a vast museum ; but the natural 
collections have been imperfectly made, and only at long 
intervals of time. 

But it may be urged that when several closely allied 
species inhabit the same territory, wo 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 intervals with 
closely allied or representati/? species evidently filling 
nearly the same place in the natural economy of the land. 
These representative species often- meet and interlock; and 
as the one becomes rarer and rarer the other becomes more 
and more frequent, till the one re places the other. But if 
we compare these species where thfy intermingle, they are 
generally as absolutely distinct horn each other in every 
detail of structure as are specimens taken from the metropo- 
lis inhabited by each. By my theory these allied species 
arp descended from a common parent; and during the pro 
v*ess of modification, each has become adapted to the con- 


ditions of life of its own region, and has supplanted and 
exterminated its original parent-form and all the transi- 
tional 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 imbedded there in a 
fossil condition. But in the intermediate region, having 
intermediate conditions of life, why ao we not now find 
closely linking intermediate varieties ? This difficulty for 
a long time quite confounded me. But I think it can be in 
large part explained. 

In the first place we should be extremely cautious in in- 
ferring, 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 intermediate 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 continu- 
ous 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 wandering animals. 

In looking at species as they are now distributed over a 
wide area, we generally find them tolerably numerous over 
a large territory, then becoming somewhat abruptly rarer 
and rarer on the confines, and finally disappearing. Henc* 
the neutral territory between two representative species is 
generally narrow in comparison with the territory proper 
to each. We see the same fact in ascending mountains, 
and sometimes it is quite remarkable how abruptly, as 
Alph. de Candolle has observed, a common alpine species 
disappears. The same fact has been noticed by E. Forbes 
in sounding the depths of the sea with the dredge. To 
those who look at climate and the physical conditions of 
life as the all-important elements of distribution, these facts 
ought to cause surprise, as climate and height or depth gradu- 
«4;e away insensibly. But when we bear in mind that almost 
wmj species, even in its metropolis, would increase iiu- 


m^nsely in numbers, were it not for other competing species; 
that nearly all either prey on or serve as prey for others ; in 
short, that each organic being is either directly or indirectly 
related in the most important manner to other organic beings 
— we see that the range of the inhabitants of any country 
by no means exclusively depends on insensibly changing 
physical conditions, but in a large part on the presence of 
other species, on which it lives, or by which it is destroyed., 
or with which it comes into competition ; and as these 
species are already defined objects, not blending one into 
another by insensible gradations, the range of any one spe- 
cies, depending as it does on the range of others, will tend 
to be sharply defined. Moreover, each species on the con- 
fines of its range, where it exists in lessened numbers, will, 
during fluctuations in the number of its enemies or of its 
prey, or in the nature of the seasons, be extremely liable to 
utter extermination ; and thus its geographical range will 
come to be still more sharply defined. 

As allied or representative species, when inhabiting a 
continuous area, are generally distributed in such a manner 
that each has a wide range, with a comparatively narrow 
neutral territory between them, in which they become 
rather suddenly rarer and rarer ; then, as varieties do not 
essentially differ from species, the same rule will probably 
apply to both ; and if we take a varying species inhabiting a 
very large area, we shall have to adapt two varieties to two 
large areas, and a third variety to a narrow intermediate 
zone. The intermediate variety, consequently, will exist in 
lesser numbers 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 varieties inter- 
mediate between well-marked varieties in the genus Balanus. 
And it would appear from information given me by Mr. 
Watson, Dr. Asa Gray and Mr. Wollaston, that generally > 
when varieties intermediate between two other forms occur, 
they are much rarer numerically than the forms which they 
connect. Now, if we may trust these facts and inferences, 
and conclude that varieties linking two other varieties to- 
gether generally 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 exterminated and 
disappear, sooner than the forms which they originally linked 



For any form existing in lesser numbers would, as already 
remarked, run a greater chance of being exterminated 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 important 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 inhabit- 
ing larger areas, will have a great advantage over the inter- I 
mediate variety, which exists in smaller numbers in a narrow 
and intermediate zone. For forms existing in larger num- 
bers will have a better chance, within any given period, of 
presenting further favorable variations for natural selection 
to seize on, than will the rarer forms which exist in lesser 
numbers. Hence, the more common forms, in the race for 
life, will tend to beat and supplant the less common forms, 
for these will be more slowly modified and 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 inhabit- 
ants are all trying with equal steadiness and skill to im- 
prove their stocks by selection ; the chances in this case 
will be strongly in favor of the great holders on the moun- 
tains or on the plains, improving their breeds more quickly 
than the small holders on the intermediate narrow, hilly 
tract; and consequently the improved mountain or plain 
breed will soon take the place cf the less improved hill 
breed; and thus the two breeds, which originally existed in 
greater numbers, will come into close contact with each other- 
without the interposition of the supplanted, intermediate 
hill variety. 

To sum up, T believe that species come to be tolerably 
well-defined objects, and do not at anyone period present 
an inextricable chaos of varying and intermediate links : 
first, because new varieties are very slowly formed, for vari- 
ation is a slow process, and natural selection can do noth- 
ing until favorable individual differences Or variations occur, 
and until a place in the natural polity of the country can 


be better filled by some modification of some one or more 
of its inhabitants. And such new places will depend on 
slow changes of climate, or on the occasional immigration 
of new inhabitants, and, probably, in a still more important 
degree, on some of the old 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 structure in some 
degree permanent ; and this assuredly we do see. 

Secondly, areas now continuous must often have existed 
within the recent period as isolated portions, in which many 
forms, more especially among the classes which unite for 
each birth and wander much, may have separately been 
rendered sufficiently distinct to rank as representative species. 
In this case, intermediate varieties between the several 
representative species and their common parent, must for- 
merly 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, interme- 
diate varieties will, it is probable, at first have been formed 
in the intermediate zones, but they will generally have had 
a short duration. For these intermediate varieties will, 
from reasons already assigned (namely from what we know 
of the actual distribution of closely allied or representa- 
tive species, and likewise of acknowledged varieties), exist 
in the intermediate zones in lesser numbers than the varie- 
ties which they tend to connect. From this cause alone 
the intermediate varieties will be liable to accidental exter- 
mination ; and during the process of further modification 
through natural selection, they will almost certainly be 
beaten and supplanted by the forms which they connect; 
for these, from existing in greater numbers, will, in the 
aggregate, present more varieties, and thus be further im- 
proved through natural selection and gain further advan- 

Lastly, looking not to any one time, but at all time, if my 
theory be true, numberless intermediate varieties, linking 
closely together all the species of the same group, must 
assuredly have existed ; but the very process of natural 
selection constantly tends, as has been so often remarked f 


to exterminate the parent-forms and the intermediate links. 
Consequently evidence of their former existence could be 
found among fossil remains, which are preserved, as we shall 
attempt to show in a future chapter, in an extremely imper- 
fect and intermittent record. 


It has been asked by the opponents of such views as 1 
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 strug- 
gle for life, it is clear that each must be well adapted to 
its place in nature. Look at the Mustela vision 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 dif- 
ferent 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 disadvan- 
tage, for, out of the many striking cases which I have 
collected, I can give only one or two instances of transitional 
habits and structures in allied species ; and of diversified 
habits, either constant or 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 partiC' 
ular 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 flat- 
tened, and from others, as Sir J. Richardson has remarked, 
with the posterior part of their bodies rather wide and with 
the skin on their flanks rather full, to the so-called flying 
squirrels ; and flying squirrels have their limbs and even th« 
base of the tail united by a broad expanse of skin, whinfc 
serves as a parachute and^llows them to glide through tht 


air to an astonishing distance from tree to tree. We can* 
not 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 vegeta- 
tion change, let other competing rodents or new beasts of 
prey immigrate, or old ones become modified, and all analogy 
would lead us to believe that some, at least, of the squirrels 
would decrease in numbers or become exterminated, unless 
they also become modified and improved in structure in a 
corresponding manner. Therefore, I can see no difficulty, 
more especially under changing conditions of life, in the con- 
tinued 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 

Now look at the Galeopithecus or so-called flying lemur, 
which was formerly ranked among bats, but is now believed 
to belong to the Insectivora. An extremely wide flank- 
membrane stretches from the corners of the jaw to the tail, 
and includes the limbs with the elongated fingers. This 
flank-membrane is furnished with an extensor muscle. 
Although no graduated links of structure, fitted for gliding 
through the air, now connect the Galeopithecus with the 
other Insectivora, yet there is no difficulty in supposing that 
such links formerly existed, and that each was developed in 
the same manner as with the less perfectly gliding squirrels ; 
each grade of structure having been useful to its possessor. 
Nor can I see any insuperable difficulty in further believing 
that the membrane connected fingers and forearm of the 
Galeopithecus might have been greatly lengthened by natural 
selection ; and this, as far as the organs of flight are con- 
cerned, would have converted the animal into a bat. In 
certain bats in which the wing-membrane extends from the 
top of the shoulder to the tail and includes the hind-legs, we 
perhaps see traces of an apparatus originally fitted for glid- 
ing 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 th« 


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 aptervx ? Yet the structure of each of these oirds 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 transition 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, so far as we 
know, to escape being devoured by other fish ? 

When we see any structure highly perfected for any par- 
ticular habit, as the wings of a bird for flight, we should bear 
in mind that animals displaying early transitional grades of 
the structure will seldom have survived to the present day, 
for they will have been supplanted by their successors, which 
were gradually rendered more perfect through natural selec- 
tion. Furthermore, 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 num- 
bers 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 wa3^s, on r,he land and in the water, 
until their organs of flight had come to a high stage of per- 
fection, 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 having 
existed in lesser numbers, than in the case of species with 
fully developed structiwSfr 


I will now give two or three instances, both of diversified 
and of changed habits, in the individuals of the same species. 
In either case it would be easy for natural selection to adapt 
the structure of the animal to its changed habits, or exclu- 
sively to one of its several habits. It is, however, difficult to 
decide and immaterial for us, whether habits generally 
change first and structure afterward ; or whether slight mod 
itications 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 exclu- 
sively on artificial substances. Of diversified habits innu- 
merable instances could be given : I have often watched a 
tyrant flycatcher (Saurophagus sulphuratus) in South Amer- 
ica, hovering over one spot and then proceeding to another, 
like a kestrel, and at other times standing stationary on th» 
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 Hearne swimming for hours with widely open mouth, thus 
catching, almost like a whale, insects in the water. 

As we sometimes see individuals following habits differ- 
ent from those proper to their species and to the other 
species of the same genus, we might expect that such indi- 
viduals would occasionally give rise to new species, having 
anomalous habits, and with their structure either slightly or 
considerably modified from that of their type. And such 
instances occur in nature. Can a more striking 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 insects 
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 in the 
typical woodpeckers, and a straight, strong beak. The beak, 
however, is not so straight or so strong as in the typical 
Woodpeckers, but it i& ssfcrcmg enough to bore into wocx^ 


Hence thk Colaptes, in all the essential parts of its structure, 
is a woodpecker. Even in such tritiing characters as the col- 
oring, the harsh tone of the voice, and undulatory flight, its 
close blood-relationship to our common woodpecker is plainly 
declared ; yet, as I can assert, not only from my own obser- 
vations, but from those of the accurate Azara, in certain 
iarge districts it does not climb trees, and it makes its nest 
in holes in banks ! In certain other districts, however, this 
same woodpecker, as Mr Hudson states, frequents trees, and 
bores holes in the trunk for its nest. I may mention as 
another illustration of the varied hab.ts 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, 
m 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 organization profoundly modified in relation to its 
new habits of life ; whereas the woodpecker of La Plata 
has had its structure only slightly modified. In the case of 
the water-ouzel, the acutest observer, by examining its dead 
body, 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 Hymen- 
opterous insects are terrestrial, excepting the genus Procto- 
trupes, 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 modifi- 
cation 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 agree- 
ment. What can be plainer than that the webbed feet of 
ducks and geese are formed for swimming ? Yet there are 
upland geese with webbed feet which rarely go near the 
water; and no one, except Audubon, has seen the frigate- 
bird, which has all its four toes webbed, alight on the sur- 
face 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 furnished 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 is 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 rudimen' 
tary in function, though not in structure. In the frigate- 
bird, the deeply scooped membrane between the toes shows 
that structure has begun to change. 

He who believes in separate and innumerable acts of 
creation may say, that in these cases it has pleased the 
Creator to cause a being of one type to take the place of one 
belonging to another type ; but this seems to me only restat- 
ing the fact in dignified language. He who believes in the 
struggle for existence and in the principle of natural selec- 
tion, will acknowledge that every organic being is constantly 
endeavoring to increase 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 inhabitant, 
however different that may be from its own place. Hence 
it will cause him no surprise that there should be geese and 
frigate-birds with webbed feet, living on the dry land and 
rarely alighting on the water, that there should be long-toed 
corncrakes, living in meadows instead of in swamps ; that 
there should be woodpeckers where hardly a tree grows; 
that there should be diving thrushes and diving Hymen- 
optera, and petrels with the habits of auks. 


To suppose that the eye with all its inimitable contri- 
vances for adjusting the focus to different distances, for 
admitting different amounts of light, and for the correction 
of spherical and chromatic aberration, could have been 
formed by natural selection, seems, I freely confess, absurd 
in the highest degree. When it was first said that the sun 
stood still and the world turned round, the common-sense of 
mankind declared the doctrine false ; but the old saying of 
Vox populi, vox Dei, as every philosopher knows, cannot be 
trusted in science. Reason tells me, that if numerous gra- 


dations 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 evei 
varies and the variations be inherited, as is likewise cer- 
tainly the case ; and if such variations should be useful to 
any animal under changing conditions of life, then the diffi- 
culty of believing that a perfect and complex eye could be 
formed by natural selection, though insuperable by our 
imagination, should not be considered as subversive of the 
theory. How a nerve comes to be sensitive to light, hardly 
concerns us more than how life itself originated ; but I may 
remark that, as some of the lowest organisms in which 
nerves cannot be detected, are capable cf perceiving light, it 
does not seem impossible that certain sensitive elements in 
their sarcode should become aggregated and developed into 
nerves, endowed with this special sensibility. 

In searching for the gradations through which an organ in 
any species has been perfected, we ought to look exclusively 
to its lineal progenitors ; but this is scarcely ever possible, 
and we are forced to 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 possi- 
ble, 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 incident- 
ally throw light on the steps by which it has bean perfected. 

The simplest organ which can be called an eye consists 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, appar- 
ently serving as organs of vision, without any nerves, and 
resting merely on sarcodic tissue. Eyes of the above simple 
nature are not capable of distinct vision, and serve only to 
distinguish light from darkness. In certain star-fishes, small 
depressions in the layer of pigment which surrounds the 
nerve are filled, as described by the author just quoted, with 
transparent gelatinous matter, projecting with a convex sur- 
face, like the cornea in the higher animals. He suggests 
that this serves not to form an image, but only to concen- 
trate 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 toward the formation of 
a true, picture-forming eye - 9 lor we have only to place the 


naked extremity of the optic nerve, which in some of thd 
iower animals lies deeply buried in the body, and in some 
near the surface, at the right distance from the concentrat- 
ing 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 some- 
times forming a sort of pupil, but destitute of lens or other 
optical contrivance. With insects it is now known that the 
numerous facets on the cornea of their great compound eyes 
form true lenses, and that the cones include curiously modi- 
fied nervous filaments. But these organs in the Articulata 
are so much diversified that M tiller 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 appara- 
tus of an optic nerve, coated with pigment and invested by 
transparent membrane, into an optical instrument as perfect 
as is possessed by any member of the Articulata 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 simul- 
taneously, which, it is assumed, could not be done through 
natural selection; but as I have attempted to show in my 
work on the variation of domestic animals, it is not neces- 
sary to suppose that the modifications were all simultaneous, 
if they were extremely slight and gradual. Different kinds 
of modification would, also, serve for the same general pur- 
pose : as Mr. Wallace has remarked, " If a len has too short 
or too long a focus, it may be amended either by an altera- 
tion of curvature, or an alteration of density ; if the curva- 
ture* o^ irregular, and the rays do not converge to a point 
tbeu any increased regularity of curvature will be an iut 


provement. So the contraction of the iris and the muscular 
movements of the eye are neither of them essential to vision, 
but only improvements which might have been added and 
perfected at any stage of the construction of the instrument.*' 
Within the highest division of the animal kingdom, namely, 
the Vertebrata, we can start from an eye so simple, that it 
consists, as in the lancelet, of a little sack of transparent 
skin, furnished with a nerve and lined with pigment, but 
destitute of any other apparatus. In fishes and reptiles, as 
Owen has remarked, the range of graduation of dioptric 
structures is very great." It is a significant fact that even 
in caan, according to the high authority of Virchow, the 
beautiful crystalline lens is formed in the embryo by an 
accumulation of epidermic cells, lying in a sack-like fold of 
the skin ; and the vitreous body is formed from embryonic 
subcutaneous tissue. To arrive, however, at a just conclu- 
sion regarding the formation of the eye, with all its marvel- 
lous 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 length. 

It is scarcely possible to avoid comparing the eye with 
a telescope. We know that this instrument has been per- 
fected by the long-continued efforts of the highest human 
intellects ; and we naturally infer that the eye has been 
formed by a somewhat analogous process. But may not 
this inference be presumptuous ? Have we any right to 
assume that the Creator works by intellectual powers like 
those of man ? If we must compare the eye t j 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 den- 
sity, so as to separate into layers of different densities and 
thicknesses, placed at different distances from each other, 
and with the surfaces of each layer slowly changing in 
form. Further we must suppose that there is a power, 
represented by natural selection or the survival of the fit-; 
test, always intently watching each slight alteration in the. 
transparent layers ; and carefully preserving each which,-, 
under varied -circumstances, in any way or degree, tends to 
rjroduce 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, anft 
then the old ones to be all destroyed. In living bodies, 
variation will cause the slight alteration, generation will 
multiply them almost infinitely, and natural selection will 
pick out with unerring skill each improvement. Let this 
process go on for millions of years ; and during each year 
on millions of individuals of many kinds ; and may we not 
believe that a living optical instrument might thus be 
formed as superior to one of glass, as the works of th ! 
Creator are to those of man ? 


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, around which, according 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 extinct. 

We should be extremely cautious in concluding that an 
organ could not have been formed by transitional gradations 
of some kind. Numerous cases could be given among the 
lower animals of the same organ performing at the same time 
wholly distinct functions ; thus in the larva of the dragon- 
fly and in the fish Cobites the alimentary canal respires, 
digests and excretes. In the Hydra, the animal may be 
turned inside out, and the exterior surface will then digest 
and the stomach respire. In such cases natural selection 
might specialize, if any advantage were thus gained, the 
whole or part of an organ, which had previously performed 
two functions, for one function alone, and thus by insensible 
steps greatly change its nature. Many plants are known 
which regularly produce at the same time differently con- 
structed flowers ; and if such plants were to produce one 
kind alone, a great change would be effected with compaiar 


tlve suddenness in the character of the species. It is, how- 
ever, probable that the two sorts of flowers borne by the 
same plant were originally differentiated by finely graduated 
steps, which may still be followed in some few cases. 

Again, two distinct organs, or the same organ under two 
very different forms, may simultaneously perform in the 
same individual the same function, and this is an extremely 
important means of transition: to give one instance — there 
are fish with gills or branchiae that breathe the air dissolved 
in the water, at the same time that they breathe free air 
in their swim-bladders, this latter organ being divided by 
highly vascular partitions and having a ductus pneumaticus 
for the supply of air. To give another instance from the 
vegetable kingdom : plants climb by three distinct means, 
by spirally twining, by clasping a support with their sensi- 
tive 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, com- 
bined in the same individual. In all such cases one of the 
two organs might readily be modified and perfected so as to 
perform all the work, being aided during the progress of 
modification by the other organ ; and then this other organ 
might be modified for some other and quite distinct purpose, 
or be wholly obliterated. 

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

According to this view it may be inferred that all verte- 
brate animals with true lungs are descended by ordinary 
generation from an ancient and unknown prototype, which 
was furnished with a floating apparatus or swim-bladder 
We can thus, as I infer from Owen's interesting description 
of these parts, understand the strange fact that every par- 
ticle 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 — bnt in the embryo the slits on 
the sides of the neck and the loop-like course of the arteries 
still mark their former position. Hut it is conceivable that 
the now utterly lost branchiae might have been gradually 
worked in by natural selection for some distinct purpose : 
for instance, Landois has shown that the wings of insects 
are developed from the trachea, it is therefore highly prob- 
able that in this great class organs which once served for 
respiration, have been actually converted into organs for 

In considering transitions of organs, it is so important to 
bear in mind the probability nf conversion from one func- 
tion to another, that I will give another instance. Pedun- 
culated 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 branchiae, the 
whole surface of the body and of the sack, together with 
the small frena, serving ior respiration. The Balanidae or 
sessile cirripedes, 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 mem- 
branes, which freely communicate with the circulatory 
lacunae of the sack and body, and which have been consid- 
ered by all naturalists to act as branchiae Now I think no 
one will dispute that the cvigerous frena in the one family 
are strictly homologous with the branchiae of the other 
family ; indeed, they graduate into each other. Therefore 
it need not be doubted that the two little folds of skin, 
which originally served as ovigerous frena, but which, like- 
wise, very slightly aided in the act of respiration, have been 
gradually converted by natural selection into branchiae, 
simply through an increase in their size and the oblitera- 
tion of their adhesive glands. If all pedunculated cirripedes 
had become extinct, and they have suffered far more extinc- 
tion than have sessile cirripedes, who would ever have im- 
agined 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 Professor 
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 developed 
in a species, it seems probable that the adult stage of devel- 
opment 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 
iegraded. Again, not a few animals, after arriving at 
maturity, go on changing in character during nearly their 
whole lives. With mammals, for instance, the form of the 
skull is often much altered with age, of which Dr. Murie 
has given some striking instances with seals. Every one 
knows how the horns of stags become more and more 
branched, and the plumes of some birds become more finely 
developed, as they grow older. Professor Cope states that 
the teeth of certain lizards change much in shape with 
advancing years. With crustaceans not only many trivial, 
but some important, parts assume a new character, as re- 
corded by Fritz Miiller, after maturity. In all such cases 
— and many could be given — if the age for reproduction 
were retarded, the character of the species, at least in its 
adult state, would be modified ; nor is it improbable that 
the previous and earlier stages of development would in 
some cases be hurried through and finally lost. Whether 
species have often or ever been modified through this 
comparatively sudden mode of transition, I can form no 
opinion ; but if this has occurred, it is probable that the 
differences between the young and the mature, and between 
the mature and the old, were primordially acquired by 
graduated steps. 



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

One of the most serious is that of nenter 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 ray, as observed by Matteucci, an analogous organ 
in the tail manifests but little electricity, even when the 
animal is greatly irritated; so little that it can hardly be of 
any use for the above purposes. Moreover, in the ray, 
besides the organ just referred to, there is, as Dr. R. McDon- 
nell 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. Radcliffe 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 the rest, and the discharge of the 
torpedo, instead of being peculiar, may be only another form 
of the discharge which attends upon the action of muscle 
and motor nerve." Beyond this we cannot at present go in 
the way of 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 progenitors of the existing 
electric fishes, it would be extremely bold to maintain that 
no serviceable transitions are possible by which these organs 
might have been gradually 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 t« 
inheritance from a common ancestor ; and its absence in 
some of the members to loss through disuse or natural selec- 
tion. So that, if the electric organs had been inherited from 
some one ancient progenitor, we might have expected that 
all electric fishes would have been specially related to each 
other ; but this is far from the case. Nor does geology at 
all lead to tfhe belief that most fishes formerly possessed 


electric organs, which their modified descendants have now 
lost. But when we look at the subject more closely, we find 
in the several fishes provided with electric organs, that these 
aie 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 proces: or means by which the 
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 func- 
tion. Consequently there is no reason to suppose that they 
have been inherited from a common progenitor ; for had this 
been the case they would have closely resembled each other 
in all respects. Thus the difficulty of an organ, apparently 
the same, arising in several remotely allied species, disap- 
pears, leaving only the lesser yet still great difficulty : 
namely, by what graduated steps these organs have been 
developed in each separate group of fishes. 

The luminous organs which occur in a few insects, belong- 
ing to widely different families, and which are situated in 
different parts of the body, offer, under our present state of 
ignorance, a difficulty almost exactly parallel with that of 
the electric organs. Other similar cases could be given ; for 
instance in plants, the very curious 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 remote as is possible among flowering 
olants ; but here again the parts are not homologous. In 
all cases of beings, far removed from each other in the scale 
of organization, which are furnished with similar and peculiar 
organs, it will be found that although the general appearance 
and function of the organs may be the same, yet fundamental 
differences between them can always be detected. For in- 
stance, the eyes of Cephalopods or cuttle-fish and of verte- 
brate animals appear wonderfully alike ; and in such widely 
sundered groups, no part of this resemblance can be due to 
inheritance from a common progenitor. Mr. 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 resemblanee, 
there is hardly any real similarity between the eyes of cuttle 


fish and vertebrates, as may be seen by consulting TTensen'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 included within the membranes of the eye. The 
relations of the muscles are as different as it is possible to 
conceive, and so in other points. Hence it is not a little 
difficult k) decide how far even the same terms ougnt to be 
employed in describing the eyes of the Cepnalopoda and 
Vertebrata. It is, of course, open to any one to deny that 
the eye in either case could have been developed through the 
natural selection of successive slight variations ; but if this 
be admitted in the one case it is clearly possible in the 
other; and fundamental differences of structure in the visual 
organs of two groups might have been anticipated, in accord- 
ance 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 favorable 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 progenitor. 

Fritz Muller, 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 Muller, and which 
are nearly related to each other, the species agree most 
closely in all important characters : namely in their sense 
organs, circulating systems, in the position of the tufts of 
hair within their complex stomachs, and lastly in the whole 
structure of the water-breathing branchiae, even to the 
microscopical hooks by which they are cleansed. Hence it 
might have been expected that in the few species belonging 
to both families which live on the land, the equally impor- 
tant air-breathing apparatus would have been the same ; for 
why should this one apparatus, given for the same purpose 


have been made to differ, while all the other important organs 
were closely similar, or rather, identical. 

Fritz Mailer 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 improbable in the highest 
degree that their common progenitor should have been 
adapted for breathing air. Muller was thus led carefully tc 
examine the apparatus in the air-breathing species ; and he 
found it to differ in each in several important points, as in 
the position of the orifices, in the manner in which they are 
opened and closed, and in some accessory details. Now 
such differences are intelligible, and might even have been 
expected, on the supposition that species belonging to dis- 
tinct families had slowly become adapted to live more and 
more out of water, and to breathe the air. For these species, 
from belonging to distinct families, would have differed to 
a certain extent, and in accordance with the principle that 
the nature of each variation depends on two factors ; viz., the 
nature of the organism and that of the surrounding condi- 
tions, their variability assuredly would not have been 
exactly the same. Consequently natural selection would 
ave had different materials or variations to work on, in 
Order to arrive at the same functional result; and the struc- 
tures 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 Muller to accept 
the views maintained by me in this volume. 

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

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


remotely allied, by organs in appearance, though not ill 
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. Huw differently con^ 
structed is the feathered wing of a bird and the membrane* 
covered wing of a batr, and still more so the four wings oi 
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 con- 
structed, 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 baloon-like envelope, by being 
embedded in pulp or flesh, formed of the most diverse parts, 
and rendered nutritious, as well as conspicuously colored, 60 
as to attract and be devoured by birds, by having hooks and 
grapnels of many kinds and serrated awns, so as to adhere 
to the fur of quadrupeds, and by being furnished with 
wings and plumes, as different in shape as they are elegant 
in structure, so as to be wafted by every breeze. I will give 
one other instance : for this subject of the same end being 
gained by the most diversified means well deserves attention. 
Some authors maintain that organic beings have been formed 
in many ways for the sake of mere variety, almost like toys 
in a shop, but such a view of nature is incredible. With 
plants having separated sexes, and with those in which, 
though hermaphrodites, the pollen does not spontaneously 
fall on the stigma, some aid is necessary for their fertiliza- 
tion. 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 inex« 
haustible number oi contrivances, all for the same purpose 
and effected in essentially the same manner, but entailing 
changes in every part of th flower. The nectar may be 
stored in variously shaped receptacles, with the stamens and 
pistils modified in many ways, sometimes forming trap-lik» 
contrivances, and sometimes capable of neatly adapbo^ 


movements through irritability or elasticity. From such 
structures we may advance till we come to such a case ot 
extraordinary adaptation as that lately described by Dr 
Cruger in the (Joryanthes. 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 o,bove the bucket ; in doing this they frequently 
pushed each other into the bucket, and their wings being thus 
wetted they oould not fly away, but were compelled to crawl 
out through the passage formed by the spout or overflow 
Dr. Cruger saw a " continual procession " of bees thus crawl- 
ing out of their involuntary bath. The passage is narrow, 
and is roofed over by the column, so that a bee, in forcing 
its way out, first rubs its back against the viscid stigma and 
then against the viscid glands of the pollen-masses. The 
pollen-masses are thus glued to the back of the bee which 
first happens to crawl out through the passage of a latelv 
expanded fl /wer, and are thus carried away. Dr. Cruger 
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 comrades into the bucket and then crawls 
Dut by the passage, the pollen-mass necessarily comes first 
nto contact with the viscid stigma, and adheres to it, and 
*"he flower is fertilized. Now, at last we see the full use of 
every part of the flower, of the water-secreting horns of the 
bucket half-lull 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 endj and ij» equally curious. Bees visit 


these flowers, like those of the Coryanthes, in order to gnaw 
the labellum ; in doing this they inevitably touch a long, 
tapering, sensitive projection, or, as I have called it, the 
antenna. This antenna, when touched, transmits a sensa- 
tion 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 
polien-mass of the male plant (tor the sexes are separate in 
this orchid) is thus carried to the flower of the female plant, 
where it is brought into contact with the stigma, which is 
viscid enough to break certain elastic threads, and retain the 
pollen, thus effecting fertilization. 

How, it may be asked, in the foregoing and in innumer- 
able other instances, can we understand the graduated 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 struc- 
ture 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 adapta- 
tions to changed habits and conditions 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 pro- 
portion of living and known forms is to the extinct and 
unknown, I have been astonished how rarely an organ can 
be named, toward 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 indeed is shown by that old, but somewhat 
exaggerated, canon in natural history of "Natura non facit 
saltum." We meet with this admission in the writings of 
almost every experienced naturalist; or as Milne Edwards 
has well expressed it, u Nature is prodigal in variety, but 
niggard in innovation. " Why, on the theory of Creatim^ 


should there be so much variety and so little real novelty ? 
Why should all the parts and organs of many independent 
beings, each supposed to have been separately created for 
its own proper place in nature, be so commonly linked to- 
gether by graduated steps ? Why should not Nature take 
a sudden leap from structure to structure ? On the theory 
of natural selection, we can clearly understand why she 
should not; for natural selection acts only by taking advan 
ta»e of slight successive variations ; she can never take a 
great and sudden leap, but must advance by short and sure, 
though slow steps. 



As natural selection acts by life and death, by the sur- 
vival 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 color of its 
flesh, the color of the skin and hair of quadrupeds, which, 
from being correlated with constitutional differences, or 
from determining the attacks of insects might assuredly 
be acted on by natural selection. The tail of the giraffe 
looks like an artificially constructed fly-flapper; and it 
seems at first incredible that this could have been adapted 
for its present purpose by successive slight modifications, 
each better and better fitted, for so trifling an object as to 
drive away flies ; yet we should pause before being too 
positive even in this case, for we know that the distribution 
and 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 tmese small enemies, would be able 
to range into new pastures and thus gain a great advantage. 
It is not that the larger quadrupeds are actually destroyed 
(except in some rare cases) by flies, but they are in- 
cessantly harassed and their strength refuged, so that they 


are more subject to disease, or not so well enabled in a 
coming dearth to search for food, or to escape from beasts 
of prey. 

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

In the second place, we may easily err in attributing 
importance to characters, and in believing that they have 
been developed through natural selection. We must by no 
means overlook the effects of the definite action of changed 
conditions of life, of so-called spontaneous variations, which 
seem to depend in a quite subordinate degree on the nature 
of the conditions, of the tendency to reversion to long^ost 
characters, of the complex laws of growth, such as of correla- 
tion, comprehension, of the pressure of one part on another, 
etc., and finally of sexual selection, by which characters of 
use to one sex are often gained and then transmitted more 
or less perfectly to the other sex, though of no use to the 
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 color 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 selec- 
tion ; as it is, the color is probably in ch>ef part due to 
sexual selection, A trailing palm in the Malay Archipelago 


climbs the loftiest trees by the aid of exquisitely con- 
structed hooks clustered around the ends of the branches, 
and this contrivance, no doubt, is of the highest service to 
the plant ; but as we see nearly similar hooks on many trees 
which are not climbers, and which, as there is reason to 
believe from the distribution of the thorn-bearing species in 
Africa and South America, serve as a defence against brows- 
ing quadrupeds, so the spikes on the palm may at first 
nave been developed for this object, and subsequently have 
been improved and taken advantage of by the plant as it 
underwent further modification and became a climber. 
The naked skin on the head of a vulture is generally con- 
sidered as a direct adaptation for wallowing in putridity ; 
and so it may be, or it may possibly be due to the direct 
action of putrid matter ; but we should be very cautious in 
drawing any such inference, when we see that the skin on 
the head of the clean-feeding male turkey is likewise naked. 
The sutures in the skulls of young mammals have been 
advanced as a beautiful adaptation for aiding parturition, 
and no doubt they facilitate, or may be indispensable for 
this act : but as sutures occur in the skulls of young birds 
and reptiles, which have only to escape from a broken egg, 
we may infer that this structure has arisen from the laws of 
growth, and has been taken advantage of m the parturition 
of the higher animals. 

We are profoundly ignorant of the cause of each slight 
variation or individual difference ; and we are immediately 
made conscious of this by reflecting on the differences 
between the breeds of our domesticated animals in different 
countries, more especially in the less civilized countries, 
where there has been but little methodical selection. 
Animals kept by savages in different countries often have 
to struggle for their own subsistence, and are exposed to a 
certain extent to natural selection, and individuals with 
slightly different constitutions would succeed best under 
different climates. With cattle susceptibility to the attacks 
of flies is correlated with color, as is the liability to be pois- 
oned by certain plants ; so that even color would be thus 
subjected to the action of natural selection. Some observ- 
ers are convinced that a damp climate affects the growth of 
the hair, and that with the hair the horns are correlated. 
Mountain breeds always differ from lowland breeds : and a 
mountainous country would probably affect the hind limbs 
from exercising them more, and possibly even the form of 


the pelvis, and then by the law of homologous variation, 
the front limbs and the head would probably be affected. 
Yhe 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 Borrelation would come into play. The 
effects of lessened exercise, together with abundant food, on 
the whole organization 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 under- 
gone. But we are far too ignorant to speculate on the rela- 
tive importance of the several known and unknown causes 
of variation ; and I have made these remarks only to show 
that, if we are unable to account for the characteristic 
differences of our several domestic breeds, which neverthe- 
less are generally admitted to have arisen through ordinary 
generation from one or a few parent-stocks, we ought not to 
lay too much stress on our ignorance of the precise cause of 
the slight analogous differences between true species. 



The foregoing remarks lead me to say a few words on the 
protest lately made by some naturalists against the utilitarian 
doctrine that every detail of structure has been produced for 
the good of its possessor. They believe that many structures 
have been created for the sake of beauty, to delight man or the 
Creator (but this latter point is beyond the scope of scientific 
discussion), or for the sake 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 many 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, independently of any advan- 
tage thus gained. But a still more important consideration 
is that the chief part ot the organization of every living 
creature is due to inheritance ; and consequently, though 
tach being assuredly is well fitted for its place in nature^ 


many structures have now no very close and direct relation 
to present habits of life. Thus, we can hardly believe that 
the webbed feet of the upland goose, or of the frigate-bird, 
are of special use to these birds ; we cannot believe that the 
similar bones in the arm of the monkey, in the fore leg of the 
horse, in the wing of the bat, and in the flipper of the seal, 
are of special use to these animals. We may safely attribute 
these structures to inheritance. But webbed feet no doubt 
were as useful to the progenitor of the upland goose and of 
the frigate-bird, as they now are to the most aquatic of living 
birds. So we may believe that the progenitor of the seal did 
not possess a flipper, but a foot with five toes fitted for walk- 
ing or grasping; and we may further venture to believe that 
the several bones in the limbs of the monkey, horse and bat, 
were originally developed, on the principle of utility, prob- 
ably 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 ex- 
ceptions, we may conclude that the structure of every living 
creature either now is, or was formerly, of some direct or 
indirect use to its possessor. 

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 quality 
in the admired object ; and that the idea of what is beautiful, 
is not innate or unalterable. We see this, for instance, in 
the men of different races admiring an entirely 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 ammonites of the Secondary period, 
created that man might ages afterward admire them in his 
cabinet ? Few objects are more beautiful than the minute 
siliceous cases 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 apparent- y wholly due to symmetry 


of growth. Flowers rank among the most beautiful pra 
duetions of nature ; but they have been rendered conspicuous 
in contrast with the green leaves, and m 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 fertilized by the wind 
it never has a gayly-colored corolla. Several plants habitu- 
ally produce two kinds of flowers ; one kind open and colored 
so as to attract insects ; the other closed, not colored, destitute 
of nectar, and never visited by insects. Hence, we may con- 
clude that, if insects had not been developed on the face of 
the earth, our plants would not have been decked with beauti- 
ful flowers, but would have produced only such poor flowers as 
we see on our fir, oak, nut, and ash trees, on grasses, spinach, 
docks and nettles, which are all fertilized 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 gayly-colored 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 dis- 
seminated. I infer that this is the case from having as yet 
found no exception to the rule that seeds are always thus 
disseminated when embedded within a fruit of any kind 
(that is within a fleshy or pulpy envelope), if it be colored 
of any brilliant tint, or rendered conspicuous by being white 
or black. 

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 magnificently colored 
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 pre- 
ferred by the females, and not for the delight of man. So 
it is with the music of birds. We may infer from all this 
that a nearly similar taste for beautiful colors and for musi- 
cal sounds runs through a large part of the animal kingdom. 
When the female is as beautifully colored as the male, which 
is not rarely the case with birds and butterflies, the cause 
apparently lies in the colors acquired through sexual selec- 
tion having been transmitted to both sexes, instead of to the 
males alone. How the sense of beauty in its simplest form 
— that is ; the reception of a peculiar kind of pleasure from 


certain colors, forms, and sounds — was first developed in the 
mind of man and of the lower animals, is a very obscure sub- 
ject. The same sort of difficulty Is presented if we inquire 
how it is that certain flavors and odors 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 species. 

Natural selection cannot possibly produce any modification 
in a species exclusively for the good of another species, 
though throughout nature one species incessantly takes 
advantage of and profits by the structures of others. But 
natural selection can and does often produce structures for 
the direct injury of other animals, as we see in the fang of 
the adder, and in the ovipositor of the ichneumon, by which 
its eggs are deposited 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 fur- 
nished 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 
tne doomed mouse. It is a much more probable view that the 
rattlesnake uses its rattle, the cobra expands its frill and 
the puff-adder swells while hissing so loudly and harshly, in 
«rder 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 endeavor to frighten away their enemies. 

Natural selection will never produce in a being any 
structure more injurious than beneficial to that being, for 
natural selection acts solely by and for the good of each. 
No organ will be formed, as Paley has remarked, for the 
purpose of causing pain or for doing an injury to its pos- 
sessor. If a fair balance D© struck between the good ao4 


evil caused by each part, each will be fouri 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 in- 
habitants of the same country with wliLh it conies into 
competition. And we see that this is the standard of per- 
fection attained under nature. The endemic productions of 
New Zealand, for instance, are perfect, one compared with 
another ; but they are now rapidly yielding before the ad- 
vancing legions of plants and animals introduced from 
Europe. Natural selection will not produce absolute per- 
fection, nor do we always meet, as far as we can judge, 
with this high standard under nature. The correction for 
the aberration of light is said by Miiller not to be perfect 
even in that most perfect organ, the human eye. Helm- 
hoitz, whose judgment no oue will dispute, after describ- 
ing 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 imper- 
fection 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 sen- 
sations. One might say that nature has taken delight in 
accumulating contradictions in order to remove all founda- 
tion 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 con- 
trivances in nature, this same reason tells us, though we 
may easily err on both sides, that some other contrivances 
are less perfect. Can we consider the sting of the bee as 
perfect, which, when used against many kinds of enemies, 
cannot be withdrawn, owing to the backward serratures, 
and thus inevitably causes the death of the insect by tear- 
ing out its viscera ? 

If we look at the sting of the bee, as having existed in a 
remote progenitor, as a boring and serrated instrument, 
like that in so many members of the same great order, and 
that f J . has since been modifies 7 but not perfected for 
its present purpose, with the poison originally adapted for 
some other object, such as to produce galls, since inten- 
sified, we can perhaps understand bow it is that the use of 


the sting should so often cause the insect'.- l:wii death: for 
li on the whole the power of stinging be useful to the social 
eommunity, it will fulfil all the requirements of natural 
selection, though it may cause the death of some few mem- 
bers. If we admire the truly wonderful power of scent by 
which the males of many insects find their females, can we 
admire the production for this single purpose of thousands 
of drones, which are utterly useless to the community for 
any other purpose, and which are ultimately slaughtered by 
their industrious and sterile sisters ? It may be difficult, 
but we ought to admire the savage instinctive hatred of the 
queen-bee, which urges her to destroy the young queens, her 
daughters, as soon as they are born, or to perish herself in 
the combat; for undoubtedly this is for the good of the 
community ; and maternal love or maternal hatred, though 
the latter fortunately is most rare, is all the same to the 
inexorable principles of natural selection. If we admire 
the several ingenious contrivances by which orchids and 
many other plants are fertilized through insect agency, can 
we consider as equally perfect the elaboration of dense 
clouds of pollen by our fir-trees, so that a few granules 
may be wafted by chance on to the ovules ? 

summary: the law of unity of type and of the con- 
ditions of existence embraced by the theory of 
natural selection. 

We have in this chapter discussed some of the difficulties 
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 gradations, partly because the process of 
natural selection is always very slow, and at any one time 
acts only on a few forms ; and partly because the very pro- 
cess of natural selection implies the continual supplanting 
and extinction of preceding and intermediate gradations. 
Closely 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 insensi- 
bly graduate away from one part to another. When two 
varieties are formed in t^o districts of a continuous area, 


an intermediate variety will often be formed, fitted for aft 
intermediate zone ; but from reasons assigned, the interme- 
diate variety will usually exist in lesser numbers than 
the two forms which it connects , consequently the two 
latter, during the course of further modification, from exist- 
ing in greater numbers, will have a great advantage over 
the less numerous intermediate variety, and will thus 
generally succeed in supplanting and exterminating it. 

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

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

Although the belief that an organ so perfect as the eye 
could have been formed by natural selection, is enough to 
6tagger 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 conditions of life, there 
is no logical impossibility in the acquirement of any con- 
ceivable degree of perfection through natural selection. 
In the cases in which we know of no intermediate or transi- 
tional 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 swim-bladder has appar- 
ently 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 special- 
ized for one function ; and two distinct organs having per- 
formed at the same time the same function, the one having 
been perfected while aided by the other, must often have, 
largely facilitated transitions. 

We have seen that in two beings widely remote from each 
other in the natural scale, organs serving for the same pur- 
pose and in external appearance closely similar may have 
been separately and independently formed j but when suck 

SUMMAR\. 185 

organs are closely examined, essential differences in their 
structure can almost always be detected ; and this naturally 
follows from the principle of natural selection. On the 
other hand, the common rule throughout nature is infinite 
diversity of structure for gaining the same end ; and this 
again naturally follows from the same great principle. 

In many cases we are far too ignorant to be enabled to 
assert that a part or organ is so unimportant for the welfare 
of a species, that modifications in its structure could no* 
have been slowly accumulated by means of natural selec- 
tion. In many other cases, modifications are probably the 
direct result of the laws of variation or of growth, indepen- 
dently of any good having been thus gained. But even 
such structures have often, as we may feel assured, been 
subsequently taken advantage of, and still further modified, 
for the good of species under new conditions of life. We 
may, also, believe that a part formerly of high importance 
has frequently been retained (as the tail of an aquatic animal 
by its terrestrial descendants), 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 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 
Che inhabitants of another and generally the larger country. 
For in the larger country there will have existed more indi' 
viduals and more diversified forms, and the competition will 
have been severer, and thus the standard of perfection 
will have been rendered higher. Natural selection will not 
necessarily lead to absolute perfection ; nor, as far as we 
can judge by our limited faculties, can absolute perfection 
be everywhere predicated. 

On the theory of natural selection we can clearly under- 
stand 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 cor- 
rect, but if we include all those of past times, whether 


known or unknown, it must on this theory be strictly 

It is generally acknowledged that all organic being have 
been formed on. two great laws — Unity of Type, aud the 
Conditions of Existence. By unity of type is meant that 
fundamental agreement in structure which we see in organic 
beings of the same class, and which is quite independent 
of their habits of life. On my theory, unity of type is 
explained by unity of descent. The expression of conditions 
of existence, so often insisted on by the illustrious Cuvier, 
is fully embraced by the principle of natural selection. For 
natural selection acts by either now adapting the varying 
parts of each being to its organic and inorganic conditions 
of life; or by having adapted them during past periods of 
time : the adaptations being aided in many cases by the 
increased use or disuse of parts, being affected by the direct 
action of the external conditions of life, and subjected in 
all cases to the several laws of growth and variation. Hence, 
in fact, the law of the Conditions of Existence is the higher 
law ; as it includes, through the inheritance of former vari- 
ations p nd adaptations, that rf Unity of Type. 





Longevity — Modifications not necessarily Simultaneous — Modifica- 
tions apparently of no Direct Service — Progressive Development — 
Characters of Small Functional Importance, the most Constant — 
Supposed Incompetence of Natural Selection to account for the 
Incipient Stages of Useful Structures — Causes which interfere 
with the Acquisition through Natural Selection of Useful Struc- 
tures — Gradations of Structure with Changed Functions — Widely 
Different Organs in Members of the Same Class, developed from 
One and the Same Source — Reasons for disbelieving in Great and 
Abrupt Modifications. 

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

A critic has lately insisted, with some parade of mathe- 
matical accuracy, that longevity is a great advantage to all 
species, so that he who believes in natural selection " must 
arrange his genealogical tree " in such a manner that all 
the descendants have longer lives than their progenitors ! 
Cannot ~iur critics conceive that a biennial plant or one ot 


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

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

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 to- 
gether; and if we lay on one side polymorphic species, in 
which the variability seems to be of a peculiar nature, and 
all mere temporary variations, such as size, albinism, etc., 
the more permanent varieties are generally found, as far a* 


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

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 
organization 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 varia- 
tions, if slight, first in one part and then in another ; and 
as they would be transmitted all together, they would ap- 
pear to us as if they had been simultaneously developed. 
The best answer, however, to the above objection is afforded 
by those domestic races which have been modified, chiefly 
through man's power of selection, for some special purpose. 
Look at the race and dray horse, or at the greyhound and 
mastiff. Their whole frames, and even their mental char- 
acteristics, have been modified ; but if we could trace each 
step in the history of their transformation — and the latter 
steps can be traced — we should not see great and simulta- 
neous changes, but first one part and then another slightly 
modified and improved. Even when selection has been ap- 
plied 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 attributed partly to the principle of correlated growth, 
ani partly to so-called spontaneous variation. 

A much more serious objection has been urged by Bronn, 
and recently by Broca, namely, that many characters 
appear to be of no service whatever to their possessors, and 
therefore cannot have been influenced through natural 
selection. Bronn adduces the length of the ears and tails 
in the different species of hares and mice — the complex 
folds of enamel in the teeth of many animals, and a multi- 
tude of analogous cases. With respect to plants, this sub- 
ject has been discussed by Nageli in an admirable essay. 
He admits that natural selection has effected much ; but b$ 


insists that the families of plants differ chiefly from each 
other in morphological characters, which appear to be quite 
unimportant for the welfare of the species. He conse- 
quently believes in an innate tendency toward progressive 
and more perfect development. He specifies the arrange- 
ment of the cells in the tissues, and of the leaves on the 
axis, as cases in which natural selection could not have 
acted. To these may be added the numerical divisions in 
the parts of the flower, the position of the ovules, the 
shape of the seed, when not of any use for dissemination, 

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

In the earlier editions of this work I underrated, as it 
now seems probable, the frequency and importance of 
modifications due to spontaneous variability, But it is in> 


possible to attribute to this cause the innumerable struc- 
tures 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, ex* 
cited so much surprise in the minds of the older naturalists, 
can thus be explained. 

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

With respect to plants, to which on account of Nageli's 
essay I shall confine myself in the following remarks, it 
will be admitted that the flowers of the orchids present a 
multitude of curious structures, which a few years ago 
would have been considered as mere morphilogical differ- 
ences without any special fuuction ; but they are now 
known to be of the highest importance for the fertilization 
of the species through the aid of insects, and have prob- 
ably been gained through natural selection. No one until 
lately would have imagined that in dimorphic and tri- 
morphic plants the different lengths of the stamens and 
pistils, and their arrangement, could have been of any ser- 
vice, but now we know this to be the case. 

In certain whole groups of plants the ovules stand erect, 
and in others they are suspended ; and within the same 
ovarium of some few plants, one ovule holds the former 
and a second ovule the latter position. These positions 
seem at first purely morphological, or of no physiological 
signification ; but Dr. Hooker informs me that within the 
same ovarium, the upper ovules alone in some cases, and 


in others the lower ones alone are fertilized ; and he sug- 
gests 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 the selection of any 
slight deviations in position which favored their fertiliza- 
tion, and the production of seed. 

Several plants belonging to distinct orders habitually 
produce flowers of two kinds — the one open, of the ordi- 
nary structure, the other closed and imperfect. These two 
kinds of flowers sometimes differ wonderfully in structure, 
yet may be seen to graduate into each other on the same 
plant. The ordinary and open flowers can be intercrossed ; 
and the benefits which certainly are derived from this pro- 
cess are thus secured. The closed and imperfect flowers 
are, however, manifestly of high importance, as they yield 
with the utmost safety a large stock of seed, with the ex- 
penditure 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 columnse five of the alternate sta- 
mens are rudimentary ; and in some species of Viola three 
stamens are in this state, two retaining their proper func- 
tion, 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 flow r ers), 
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 modi- 
fied, for the five stamens which stand opposite to the sepals 
are all aborted, a sixth stamen standing opposite to a petal 
being alone developed ; and this stamen is not present in 
the ordinary flowers of this 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 closure of the 
flowers superfluous, yet hardly any of the above special 
modifications can have been thus determined, but must 
have followed from the laws of growth, including the func- 
tional inactivity of parts, during the progress of the reduc- 
tion of the pollen and the closure of the flowers. 

It is go 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 ; 
while 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, while 
the surrounding flowers generally have three calyx-lobes 
with the other organs pentamerous. In many compositae 
and umbelliferae (and in some other plants) the circum- 
ferential flowers have their corollas much more developed 
than those of the centre; and this seems often connected 
with the abortion of tne reproductive organs. It is a more 
curious fact, previously referred to, that the acheues or seeds 
of the circumference and centre sometimes differ greatly in 
form, color, and other characters. In Carthamus and some 
other compositae the central achenes alone are furnished 
with a pappus ; and in Hyoseris the same head yields 
achenes of three different forms. In certain umbelliferae 
the exterior seeds, according to Tausch, are orthospermous, 
and the central one coelospermous, and this is a character 
which was considered by De Candolle to be in other species 
of the highest systematic importance. Professor Braun 
mentions a Fumariaceous genus, in which the flowers in the 
lower part of the spike bear oval, ribbed, one-seeded nutlets ; 
and in the upper part of the spike, lanceolate, two-valved, 
and two-seeded siliques. In these several cases, with the 
exception of that of the well-developed ray-florets, which are 
of service in making the flowers conspicuous to insects, 
natural selection cannot, as far as we can judge, have come 
into play, or only in a quite subordinate manner. All these 
modifications follow from the relative position and inter- 
action of the parts ; and it can hardly be doubted that if all 
the flowers and leaves on the same plant had been subjected 
to the same external and internal condition, as ape the 
flowers and leaves in certain positions, all would have been 
modified in the same manner. 

In numerous other cases we find modifications of struc- 
fcure, which are considered by botanists to be generally of a 


highly important nature, affecting only some of the flowers 
on the same plant, or occurring on distinct plants, which 
grow close together under the same conditions. As these 
variations seem of no special use to the plants, they cannot 
have been influenced by natural selection. Of their cause 
we are quite ignorant ; we cannot even attribute them, as 
in the last class of cases, to any proximate agency, suoh aa 
relative position. I will give only a few instances. It is 
so common to observe on the same plant, flowers inditfer 
ently tetramerous, pentamerous, etc., that I need not givi 
examples; but as numerical variations are comparatively 
rare when the parts are few, I may mention that, accord 
ing to De Candolle, the flowers of Papaver bracteatun, 
offer either two sepals with four petals (which is the 
common type with poppies), or three sepals with six petals. 
The manner in which the petals are folded in the bud is, 
in most groups, a very constant morphological character ; 
but Professor Asa Gray states that with some species of 
Mimulus, the aestivation is almost as frequently that of 
the Rhinanthideae as of the Antirrhinidese, to which latter 
tribe the genus belongs. Aug. Saint-Hilaire gives the fol- 
lowing cases : the genus Zanthoxylon belongs to a division 
of the Rutaceae 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 Helian- 
themum the capsule has been described as unilocular or tri- 
locular ; and in H. mutabile, " Une lame plus ou moins large 
s'etend entre le pericarpe et le placenta." In the flowers of 
Saponaria officinalis Dr. Masters has observed instances of 
both marginal and free central placentation. Lastly, Saint- 
Hilaire found toward the southern extreme of the range 
of Gomphia oleaeformis two forms which he did not at first 
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 rat- 
tachent tantot a un axe verticale et tantot a un gynobase." 

We thus see that with plants many morphological changes 
may be attributed to the laws of growth and the interaction 
of parts, independently of natural selection. But with re- 
spect to Nageli's doctrine of an innate tendency toward 
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 toward 
ft higher state of development ? Ou the contrary, X should 


fnfer from the mere fact of the parts in question differing 
or varying greatly on the same plant, that such modifica- 
tions were of extremely small importance to the plants 
themselves, of whatever importance they may generally 
be to us for our classifications. The acquisition of a use- 
less part can hardly be said to raise an organism in the 
natural scale; and in the case of the imperfect, closed 
flowers, above described, if any new principle has to be 
invoked, it must be one of retrogression rather than of 
progression; and so it must be with many parasitic ana 
degraded animals. We are ignorant of the exciting cause 
of the above specified modifications ; but if the unknown 
cause were to act almost uniformly for a length of time, 
we may infer that the result would be almost uniform ; and 
in this case all the individuals of the species would be 
modified in the same manner. 

From the fact of the above characters being unimpor- 
tant for the welfare of the species, any slight variations 
which occurred in them would not have been accumulated 
and augmented through natural selection. A structure 
which has been developed through long-continued selec- 
tion, when it ceases to be of service to a species, generally 
becomes variable, as we see with rudimentary organs ; for 
it will no longer be regulated by this same power of selec- 
tion. But when, from the nature of the organism and of 
the conditions, modifications 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 is 
often assumed to be of high vital importance to the species. 
Thus, as I am inclined to believe, morphological differences, 
which we consider as important — such as the arrangement 
of the leaves, the divisions of the flower or of the ovarium, 
the position of the ovules, etc., first appeared in many ^.ses 
as fluctuating variations, which sooner or later became v_ jn- 
stant through the nature of the organism and of the sur- 
rounding conditions; as well as through the intercrossing of 


distinct individuals, but not through natural selection ; for 
as these morphological characters do not affect the welfare 
of the species, any slight deviations in them could not have 
been governed or accumulated through this latter agency. 
It is a strange result which we thus arrive at, namely, that 
characters of slight vital importance to the species are the 
most important to the systematist ; but, as we shall here- 
after see when we treat of the genetic principle of classifica- 
tion, this is b} r no means so paradoxical as it may at first 

Although we have no good evidence of the existence in 
organic beings of an innate tendency toward progressive 
development, yet this necessarily follows, as I have at- 
tempted to show in the fourth chapter, through the con- 
tinued action of natural selection. For the best definition 
which has ever been given of a high standard of organiza- 
tion, is the degree to which the parts have been specialized 
or differentiated ; and natural selection tends toward 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. Mivart passes over the effects 
off 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 believe, any other writer. He likewise often 
assumes that I attribute nothing to variation, independently 
of natural selection, whereas in the work just referred to 
I have collected a greater number of well-established cases 
than can be found in any other work known to me. My 
judgment may not be trustworthy, but after reading with 
care Mr. Mivart's book, and comparing each section with 
what I have said on the same head, I never before felt so 
strongly convinced of the general truth of the conclusions 
here arrived at, subject, of course, in so intricate asubject* 
to much partial error* 


All Mr. Mivait's objections will be, or have been, con. 
sidered in the present volume The one new point whick 
appears to have struck many readers is, " That natural selec- 
tion is incompetent to account for the incipient stages of 
useful structures." This subject is intimately connected 
with that of the gradation of the characters, often accom- 
panied 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, 1 
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 beautifully 
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 Niata 
cattle in South 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, etc., to which food the common 
cattle and horses are then driven ; so that at these times the 
Niatas perish, if not fed by their owners. Before ooming 
to Mr. Mivart's objections, it may be well to explain once 
again how natural selection will act in all ordinary cases. 
Man has modified some of his animals, without necessarily 
having attended to special points of structure, by simply 
preserving and breeding from the fleetest individuals, as 
with the race-horse and greyhound, or as with the game-cock, 
by breeding from the victorious birds. So under nature with 
the nascent giraffe, the individuals which were the highest 
browsers and were able 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 whick careful measure- 
ments are given. These slight proportional differences, 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 probablo 


habits of life; for those individuals which had some one 
part or several parts of their bodies rather more elongated 
than usual, would generally have survived. These will have 
intercrossed and left offspring, either inheriting the same 
bodily peculiarities, or with a tendency to vary again in the 
same manner; while the individuals less favored in the same 
respects will have been the most liable to perish. 

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

To this conclusion Mr. Mivart brings forward two ob- 
jections. One is that the increased size of the body would 
obviously require an increased supply of food, and he con- 
siders 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 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 beiug 
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 de- 
fence, by violently swinging its head armed with stump-like 
horns. The preservation of each species can rarely be 
determined by any one advantage, but by the union of all, 
great and small. 


Mr. Mivart then asks (and this is his second objection), 
if natural selection be so potent, and if high browsing be 
so great an advantage, why has not any other hoofed quad- 
ruped 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 South 
Africa, which was formerly inhabited 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 advantage would it be, for instance, to sheep, if kept 
there, to acquire slightly longer necks ? In every district 
some one kind of animal will almost certainly be able to 
browse higher than the others ; and it is almost equally 
certain that this one kind alone could have its neck elon- 
gated for this purpose, through natural selection and the 
effects of increased use. In South Africa the competition 
for browsing on the higher branches of the acacias and other 
trees must be between giraffe and giraffe, and not with the 
other ungulate animals. 

Why, in other quarters of the world, various animals 
belonging to this same order have not acquired either an 
elongated neck or a proboscis, cannot be distinctly answered; 
but it is as unreasonable to expect a distinct answer to such 
a question as why some event in the history of mankind did 
not occur in one country while 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 con- 
jecture what changes of structure would be favorable 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 
South America, though it is so luxuriant, while South Africa 
abounds with them to an unparalleled degree. Why this 
should be so, we do not know ; nor why the later tertiary 
periods should have been much more favorable for their 
existence than the present time. Whatever the causes may 
&ave beem, we can see that certain districts and times woullj 


have been much more favorable than others for the develop- 
ment of so large a quadruped as the giraffe. 

In order that an animal should acquire some structure 
specially and largely developed, it is almost indispensable 
that several other parte should be modified and coadapted. 
Although every part of the body varies slightly, it does not 
follow that the necessary parts should always vary in the 
right direction and to the right degree. With the different 
species of our domesticated animals we know that the parts 
vary in a different manner and degree, and that some species 
are much more variable than others. Even if the fitting 
Variations did arise, it does not follow that natural selection 
would be able to act on them and produce a structure which 
apparently would be beneficial to the species. For instance, 
if the number of individuals existing in a country is deter- 
mined chiefly through destruction by beasts of prey — by 
external or internal parasites, etc. — as seems often to be 
the case, then natural selection will be able to do little, or 
will be greatly retarded, in modifying any particular struc- 
ture for obtaining food. Lastly, natural selection is a slow 
process, and the same favorable conditions must long endure 
in order that any marked effect should thus be produced. 
Except by assigning such general and vague reasons, we 
cannot explain why, in many quarters of the world, hoofed 
quadrupeds have not acquired much 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 inter- 
fered 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 tc 
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 
fche land ? But seals would necessarily be first converted 
into terrestrial carnivorous animals of considerable size, and 
into terrestrial insectivorous animals j for th.e form§Jf 


ifiere 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 colonize and abound on 
most oceanic islands. Gradations of structure, with each 
stage beneficial to a changing species, will be favored only 
under certain peculiar conditions. A strictly terrestrial 
animal, by occasionally hunting for food in shallow water, 
then in streams or lakes, ni;ght at last be converted into an 
animal so thoroughly aquatic as to brave the open ocean. 
But seals would not find on oceanic islands the conditions 
favorable to their gradual reconversion into a terrestrial 
form. Bats, as formerly shown, probably acquired their 
wings by at first gliding through the air tree to tree, 
like the so-called flying Lquirrels, for th^ 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 re- 
duced in size, or completely lost, through disuse ; but in 
this case it would be necessary that they should first have 
acquired the power of running 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 developed 
than others, as such development would be advantageous to 
all ? Why have not apes acquired the intellectual powers 
cf 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 
civilization than the other; and this apparently implies 
increased brain power. 

We will return to Mr. Mivart's other objections. Insects 
often resemble, for the sake of protection, various objects, 
such as green, or decayed leaves, dead twigs, bits of lichen, 


flowers, spines, excrement of birds, and living insects ; but 
to this latter point I shall hereafter recur. The resemblance 
is often wonderfully close, and is not confined to color, 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 neutralize each other, and at first to form such unstable 
modifications that it is difficult, if not impossible, to see 
how such indefinite oscillation, 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 perpetuate." 

But in all the foregoing cases the insects in their original 
state no doubt presented some rude and accidental resem- 
blance to an object commonly found in the stations fre- 
quented by them. Nor is this at all improbable, consider- 
ing the almost infinite number of surrounding objects and 
the diversity in form and color of the hosts of insects which 
exist. As some rude resemblance is necessary for the first 
start, we can understand how it is that the larger and higher 
animals do not (with the exception, as far as I know, of one 
fish) resemble for the sake of protection special objects, but 
only the surface which commonly surrounds them, and this 
chiefly in color. Assuming that an insect originally hap- 
pened 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 favored its escape, would be preserved, 
while other variations would be neglected and ultimately 
lost; or, if they rendered the insect at all less like the 
imitated object, they would be eliminated. There would 
indeed be force in Mr. Mivart's objection, if we were to 
attempt to account for the above resemblances, independ- 
ently of natural selection, through mere fluctuating varia- 
bility ; but as the case stands there is none. 

Nor can I see any force in Mr. Mivart's difficulty with 
respect to "the last touches of perfection in the mimicry;" 
as in the case given b# Mr. Wallace, of a walking-stick u* 


sect (Ceroxylus lacerattis), which resembles "a stick grown 
over by a creeping moss or jungeriiiaiiiiia." So close was 
this resemblance, that a native Dyak maintained that the 
foliaceons 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 
toward its preservation ; and the more perfect the resem- 
blance, so much the better for the insect. Considering the 
nature of the differences between the species in the group 
which includes the above Ceroxylus, there is nothing im- 
probable in this insect having varied in the irregularities on 
its surface, and in these having become more or less green- 
colored ; for in every group the characters which differ in 
the several species are the most apt to vary, while the 
generic characters, or those common to all the species, are 
the most constant. 

The Greenland whale is one of the most wonderful ani- 
mals in the world, and the baleen, or whalebone, one of its 
greatest peculiarities. The baleen consists of a row, on each 
side of the upper jaw, of about 300 plates or lamime, 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 r,he whole gigantic 
palate, and serve to strain or tift the water, and thus to 
secure the minute prey on which these great animals subsist. 
The middle and lonsrest lamina in the Greenland whale is 
ten, twelve, or even fifteen feet in length ; but in the differ- 
ent species of Cetaceans there are gradations in length ; the 
middle lamina being in one species, according to Scoresby, 
four feet, in another three, in another eighteen inches, and 
in the Balsenoptera rostrata only about nine inches in length. 
The quality of the whalebone also differs in the different 

With respect to the baleen, Mr. Mivart remarks that if it 
" had once attained such a size and development as to be 
at all useful, then its preservation and augmentation within 
serviceable limits would be promoted by natural selection 
alone. But how to obtain the beginning of such useful 
development ? " In answer, it may be asked, why should 
not the early progenitors of the whales with baleen have 
possessed a mouth constructed something like the laniet 


iated beak of a duck ? Ducks ; 1 lke whales, subsist by sift- 
ing the mud and water ; and tho family has sometimes been 
called Criblatores, or sifters. T hope that I may not be 
misconstrued into saying that the progenitors of whales did 
actually possess mouthb 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 lamellae by finely graduated 
steps, each of service to its possessor. 

The beak of the shoveller-duck (Spatula elypeata) is 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 or comb formed 
of 188 thin, elastic lamellae, obliquely beveled 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 fourteen 
one-hundredths of an inch beneath the edge. At their 
bases there is a short subsidiary row of. obliquely transverse 
lamellae. In these several respects they resemble the plates 
of baleen in the mouth of a whale. But toward the ex- 
tremity of the beak they differ much, as they project in- 
ward, instead of 'straight downward. 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 Balaenoptera, the lam- 
ellae would be six inches in length, that is. two-thirds of 
the length of the baleen in this species of whale. The 
lower mandible of the shoveller-duck is furnished with 
lamellae of equal length with these 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 curiously 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 lamellae, which a*re well developed and pro- 
ject beneath the margin ; so that the beak of this bird 
Tesembles in this respect the mouth of a whale. 

From the highly developed structure of the shoveller's 


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

Turning to another group of the same family. In the 
Egyptian goose (Chenalopex) the beak closely resembles 
that of the common duck ; but the lamellae are not so numer- 
ous, nor so distinct from each other, nor do they project so 
much inward ; 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 common goose. In this latter bird the 
lamellae of the upper mandible are much coarser than in the 
common duck, almost confluent, about twenty-seven in num- 
ber on each side, and terminating upward 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 devel- 
oped than in the common goose. 

We thus see that a member of the duck family, with a 
beak constructed like that of a common goose and adapted 
solely for grazing, or even a member with a beak having 
less well developed lamellse, might be converted by small 
changes into a species like the Egyptian goose — this, into 


one like the common duck — and, lastly, into one like the 
shoveller, provided with a beak almost exclusively adapted 
for sifting the water ; for this bird could hardly use any 
part of its beak, except the hooked tip, for seizing or tear- 
ing solid food. The beak of a goose, as I may add, might 
also be converted by small changes into one provided with 
prominent, recurved teeth, like those of the Merganser (a 
member of the same family), serving for the widely different 
purpose of securing live fish. 

Returning to the whales. The Hyperoodon bidens is 
destitute of true teeth in an efficient condition, but its palate 
is roughened, according to Lacepede, with small, unequal, 
hard points of horn. There is, therefore, nothing improb- 
able 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, aiding 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 lamellae like 
those of the domestic duck ; and so onward, until they be- 
came as well constructed as those of the shoveller, in which 
case they would have served exclusively as a sifting ap- 
paratus. From this stage, in which the lamellae would be 
two-thirds of the length of the plates of baleen in the 
Balaenoptera rostrata, gradations, which may be observed in 
still-existing Cetaceans, lead us onward to the enormous 
plates of baleen in the Greenland whale. Nor is there the 
least reason to doubt that each step in this scale might have 
been as serviceable to certain ancient Cetaceans, with the 
functions of the parts slowly changing during the progress 
of development, as are the gradations in the beaks of the 
different existing members of the duck-family. We should 
bear in mind that each species of duck is subjected to a 
severe struggle for existence, and that the structure of every 
part of its frame must be well adapted to its conditions of 

The Pleuronectidae, or Flat-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 color, 
less developed in many ways than the upper side, with the 
lateral fins often of smaller size. Hut the eyes offer the 
siost 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 colored. Soon 
the eye proper to the lower side begins to glide slowly round 
the head to the upper side ; but does not pass right through 
the skull, as was formerly thought to be the case. It is 
obvious that unless the lower eye did thus travel round, it 
could not be used by the fish while lying in its habitual 
position on one side. The lower eye would, also, have been 
liable to be abraded by the sandy bottom. That the Pleu- 
ronectidae are admirably adapted by their flattened and 
asymmetrical structure for their habits of life, is manifest 
from several species, such as soles, flounders, etc., being 
extremely common. The chief advantages thus gained seem 
to be protection from their enemies, and facility for freeding 
on the ground. The different members, however, of the 
family present, as Schiodte remarks, " a long series of forms 
exhibiting a gradual transition from Hippoglossus pinguis, 
which does not in any considerable degree alter the shape in 
which it leaves the ovum, to the soles, which are entirely 
thrown to one side." 

Mr. Mivart has taken up this case, and remarks that a 
sudden spontaneous transformation in the position of the 
eyes is hardly 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 toward 
the other side of the head could benefit the individual is, 
indeed, far from clear. It seems, even, that such an in- 
cipient 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 
Pleuronectidae, while very young and still symmetrical, with 
their eyes standing on opposite sides of the head, cannot 
long retain a vertical position, owing to the excessive depth 
of their bodies, the small size of their lateral fins, and to 
their being destitute of a swim-bladder. Hence, soon grow- 
ing tired, they fall to the bottom on one side. While thus 
at rest they often twist, as Malm observed, the lower eye up- 
ward, to see above them ; and they do this so vigorously 
that the eye is pressed hard against the upper part of tiie 


orbit. The forehead between the eyes eonsequently becomes, 
as could be plainly seen, temporarily contracted in breadth. 
On one occasion Malm saw a youn^ fish raise and depress the 
lower eye through an angular distance of about seventy 

We should remember that the skull at this early age is 
cartilaginous and flexible, so that it readily yields to muscu- 
lar action. It is also known with the higher animals, ever 
after early youth, that the skull yields and is altered in shape, 
if the skin or muscles be permanently contracted through 
disease or some accident. With long-eared rabbits, if one 
ear flops forward and downward, 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 upward ; and their skulls are 
thus rendered rather crooked. These fishes, however, are 
soon able to hold themselves in a vertical position, 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 permanent effect is thus produeed on the form 
of the head, and on the position of the eyes. Judging from 
analogy, the tendency to distortion would no doubt be in- 
creased through the principle of inheritance. Schiodte 
believes, in opposition to some other naturalists, that the 
Pleuronectidae are not quite symmetrical even in the embryo; 
and if this be so, we could understand how it is that certain 
species, while young, habitually fall over and rest on the left 
side, and other species on the right, side. Malm adds, in 
confirmation of the above view, that the adult Trachypterus 
arcticus, which is not a member of the Pleuronectidae, rests 
on its left side at the bottom, and swims diagonally through 
the water; and in this fish, the two sides of the head are said 
to be somewhat dissimilar. Our great authority on Fishes, 
Dr. Gtinther, concludes his abstract of Malm's paper, by 
remarking that "the author gives a very simple explanation 
of the abnormal condition of the Pleuronectoids." 

We thus see that the first stages of the transit of the eye 
from one side of the head to the other, whieh Mr. Mivart 
considers would be injurious, may be attributed to the habit, 
no doubt beneficial to the individual and to the species, of 


endeavoring to look upward with both eyes, while resting on 
one aide at the bottom. We may also attribute to the inher- 
ited effects of use the fact of the mouth in several kinds of 
flat-fish being bent toward 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 feediug with ease on the ground. Disuse, on 
the jther hand, will account for the less developed condition 
of the whole inferior half of the body, including the lateral 
fins ; though Yarrel 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 colorless state of the ventral surface 
of most fishes and of many other animals, we may reasonably 
suppose that the absence of color in flat-fish on the side, 
whether it be the right or left, which is undermost, is due to 
the exclusion of light. But it cannot be supposed that the 
peculiar speckled appearance of the upper side of the sole, so 
like the sandy bed of the sea, or the power in some species, 
as recently shown by Pouchct, of changing their color in 
accordance with the surrounding 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 man}' other peculiarities, to their 
habits of life. We should keep in mind, as I have before 
insisted, that the inherited effects of the increased use of 
parts, and perhaps of their disuse, will be strengthened by 
natural selection. For all spontaneous variations in the 
right direction will thus be preserved ; as will those individ- 
uals which inherit in the highest degree 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 number of ages the first slight incipient tendency 


to grasp could preserve the lives of the individuals possess* 
ing it ; or favor 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 (Cer- 
copithecus) 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 Hen slow kept 
in confinement some harvest mice (Mus messorius) which do 
not possess a structurally prehensile tail ; but he frequently 
observed that they curled their tails round the branches 
of a bush placed in the cage, and thus aided themselves in 
climbing. I have received an analogous account from Dr. 
Giinther, who has seen a mouse thus suspend itself. If the 
harvest mouse had been more strictly arboreal, it would per- 
haps have had its tail rendered structurally prehensile, as is 
the case with some members of the same order. Why Cer- 
copithecus, considering its habits while young, has not become 
thus provided, it would be difficult to say. It is, however, 
possible that the long tail of this monkey may be of more 
service to it as a balancing organ in making its prodigious 
leaps, than as a prehensile organ. 

The mammary glands are common to the whole class of 
mammals, and are indispensable for their existence ; they 
must, therefore, have been developed at an extremely remote 
period, and we can know nothing positively about their man- 
ner of development. Mr. Mivart asks : " Is it conceivable 
that the young of any animal was ever saved from destruc- 
tion by accidentally sucking a drop of scarcely nutritious 
fluid from an accidentally hypertrophied cutaneous gland of 
its mother ? And even if one was so, what chance was there 
of the perpetuation of such a variation ? ' But the case is 
not here put fairly. It is admitted by most evolutionists 
that mammals are descended from a marsupial form ; and if 
so, the mammary glands will have been at first developed 
within the marsupial sack. In the case of the fish (Hippo- 
campus) the eggs are hatched, and the young are reared for 
a time, within a sack of this nature ; and an American natur- 
alist, Mr. Lockwood, believes from what he has seen of the 
development of the young, that they are nourished by a secre- 
tion from the cutaneous glands of the sack. Now, with the 
early progenitors of mammals, almost before they daserved 


to be thus designated, is it not at least possible that the 
young might have been similarly nourished ? And in this 
case, the individuals which secreted a fluid, in some degree 
or manner the most nutritious, so as to partake of the nature 
of milk, would in the long-run have reared a larger number 
of well-nourished offspring, 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 specialization, 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 Ornithorhvnchus, at the base of the mammalian series. 
Through what agency the glands over a certain space became 
more highly specialized 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 nat- 
ural selection, unless the young at the same time were able 
to partake of the secretion. There is no greater difficulty 
in understanding how young mammals have instinctively 
learned to suck the breast, than in understanding how un- 
matched chickens have learned to break the egg-shell by tap- 
ping against it with their specially adapted beaks ; or how 
a few hours after leaving the shell they have learned 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 afterward transmitted to the 
offspring at an earlier age. But the young kangaroo is said 
not to suck, only to cling to the nipple of its mother, who 
has the power of injecting milk into the mouth of her help- 
less, half-formed offspring. On this head Mr. Mivart re- 
marks : " Did no special provision exist, the young one must 
infallibly be choked by the intrusion of the milk into the wind- 
pipe. 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 selection 
remove in the adult kangaroo (and in most other mammals, 
on the assumption that they are descended from a marsupial 


form), " this at least perfectly innocent and harmless struc- 
ture ?" It may be suggested in answer, 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 divisions 
of the animal kingdom. The Echinodermata (star-fishes, 
sea-urchins, etc.) are furnished with remarkable organs called 
pedicellariae, which consist, when well developed, of a tri- 
dactyle forceps — that is, of one formed of three serrated 
arms, neatly fitting together and placed on the summit of a 
flexible stem, moved by muscles. These forceps can se*ize 
firmly hold of any object; and Alexander Agassiz has seen 
an Echinus or sea-urchin rapidly passing particles of excre- 
ment from forceps to forceps down certain lines of its body, 
in order that its shell should not be foaled. But there is no 
doubt that besides removing dirt of all kinds, they subserve 
other functions ; and one of these apparently is defence. 

With respect to these organs, Mr. Mivart, as on so many 
previous occasions, asks : " What would be the utility of the 
first rudimentary beginnings of such structures, and how could 
such incipient buddings have ever preserved the life of a 
single Echinus ? " He adds, " Not even the sudden develop- 
ment of the snapping action could have been beneficial with- 
out the freely movable stalk, nor could the latter have been 
efficient without the snapping jaws, yet no minute, merely 
indefinite variations could simultaneously evolve these com- 
plex coordinations 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 lorcepses, 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 tne 
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 pedicellariae, one re- 
sembling those of Echinus, and the other those of Spatangus; 
and such cases are always interesting as affording the means 
of apparently sudden transitions, through the abortion of one 
Of the two states of an oi»«9* - 


With respect to the steps by which these curious organs 
iiave been evolved, Mr. Agassiz infers from his own re- 
searches and those of Mr. Miiller, that both in star-fishes 
and sea-urchins the pedicellanae must undoubtedly be looked 
at as modified spines. This may be inferred from their man- 
ner 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 pedicellariae. The gradation extends even to the 
manner in which ordinary spines and the pedicellarise, with 
their supporting calcareous 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, movable branches, articu- 
lated to near their bases ; and higher up, on the same spine, 
three other movable branches. Now when the latter arise 
from the summit of a spine they form, in fact, a rude tridac- 
tyle 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 movable branches of a spine, is unmistakable. It is gen- 
erally admitted that the ordinary spines serve as a protec- 
tion ; and if so, there can be no reason to doubt that those 
furnished with serrated and movable 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 snapping 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 immovable support, are placed on 
the summit of a flexible and muscular, though short, stem ; 
and in this case they probably subserve some additional 
function besides defence. In the sea-urchins the steps can 
be followed by which a fixed spine becomes articulated to 
the shell, and is thus rendered movable. I wish I had space 
here to give a fuller abstract of Mr. Agassiz's interesting 
observations on the development of the pedicellariae. All 
possible gradations, as he adds, may likewise be found be- 
tween the pedicellariae of the star-fishes and the hooks of 
the Ophiurians, another group of the Echinodermata ; and 
again between the pedicellaria? of sea-urchins and the anchors 
©f the Holothiiriee, also belonging to the same great clas§, 


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 minia- 
ture, seated on a neck and capable of movement, as is like- 
wise the lower jaw or mandible. In one species observed by 
me, all the avicularia on the same branch often moved simul- 
taneously backward and forward, with the lower jaw widely 
open, through an angle of about 90 degrees, in the course of 
five seconds ; and their movement caused the whole polyzo- 
ary 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 pedicellariae of the Echinodermata, which 
he considers as "essentially similar," having been developed 
through natural selection in widely distinct divisions of the 
animal kingdom. But, as far as structure is concerned, I 
can see no similarity between tridactyle pedicellariae and 
avicularia. The latter resembles somewhat more closely the 
chelae or pincers of Crustaceans ; and Mr. Mivart might have 
adduced, with equal appropriateness, this resemblance as a 
special difficulty, or even their resemblance to the head and 
beak of a bird. The avicularia are believed by Mr. Busk, 
Dr. Smitt, and Dr. Nitsche — naturalists who have carefully 
studied this group — to be homologous with the zooids and 
their cells which compose the zoophyte, the movable lip or 
lid of the cell corresponding with the lower and movable 
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 con- 
verted into the other, but it by no means follows from this 
that sueh 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 penultimate 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 locomo- 
&}uu- \ye next find one corner of the broad penultimate 


segment slightly prominent, sometimes furnished with irreg- 
ular 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 mod- 
ified 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 simultaneously ; so that, acting like 
long oars, they swept a branch rapidly across the object-glass 
of my microscope. When a branch was placed on its face, 
the vibracula became entangled, and they made violent 
efforts to free themselves. They are supposed to serve as a 
defence, and may be seen, as Mr. Busk remarks, "to sweep 
slowly and carefully over the surface of the polyzoary, 
removing what might be noxious to the delicate inhabitants 
of the cells when their tentacula are protruded." The 
avicularia, like the vibracula, probably serve for defence, but 
they also catch and kill small living animals, which, it is 
believed, are afterward 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 avicula- 
rium 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 under- 
stand 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 movable 
mandible is so much produced and is so like a bristle that 
the presence of the upper or fixed beak alone serves to deter- 
mine 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 
eell, with the included zooid, could hardly have disappeared 


at once. In many cases the vibracula have a grooved sup» 
port at the base, which seems to represent the fixed beak ; 
though this support in some species is quite absent. This 
view of the development of the vibracula, if trustworthy, is 
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 movable lip of the cell serves as a protec- 
tion to the zooid, there is no difficulty in believing that al] 
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 different 
ways and under different circumstances. 

In the vegetable kingdom Mr. Mivart only alludes to two 
cases, namely the structure of the flowers of orchids, and 
the movements of climbing plants. With respect to the 
former, he says: "The explanation of their origin is deemed 
thoroughly unsatisfactory, — utterly insufficient to explain 
the incipient, infinitesimal beginnings of structures which 
are of utility only when they are considerably developed." 
As 1 have fully treated this subject in another work, I will 
here give only a few details on one alone of the most strik- 
ing peculiarities of the flowers of orchids, namely, their 
pollinia. A pollinium, when highly developed, consists of a 
mass of pollen-grains, affixed to an elastic foot-stalk or 
caudicle, and this to a little mass of extremely viscid matter. 
The pollinia are by this means transported by insects from 
one flower to the stigma of another. In some orchids there 
is no caudicle to the pollen-masses, and the grains are merely 
tied together by fine threads ; but as these are not confined 
to orchids, they need not here be considered ; yet I may 
inention that at the base of the orchidaceous series, in Cypri- 
pedium, we can see how the threads were probably first 
developed. In other orchids the threads cohere at one end 
of the pollen-masses ; and this forms the first or nascent 
trace of a caudicle. That this is the origin of the caudicle, 
even when of considerable length and highly developed, we 
have good evidence in the aborted pollen-grains which can 
sometimes be detected embedded within the central and 
solid parts* 


With respect to the second chief peculiarity, namely, the 
little mass of viscid matter attached to the end of the cau- 
dicle, a long series of gradations can be specified, each of 
plain service to the plant. In most flowers belonging to 
other orders the stigma secretes a little viscid matter. Now, 
in certain orchids similar viscid matter is secreted, but in 
much larger quantities, by one alone of the three stigmas ; 
and this stigma, perhaps in consequence of the copious 
secretion, is rendered sterile. When an insect visits a flower 
of this kind, it rubs off some of the viscid matter, and thus 
at the same time drags away some of the pollen-grains. 
From this simple condition, which differs but little from that 
of a multitude of common flowers, there are endless grada- 
tions — 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 to- 
gether by threads, with the stigma differing but little from 
that of an ordinary flower, to a highly complex pollinium, 
admirably adapted for transportal by insects ; nor will he 
deny that all the gradations in the several species are admir- 
ably adapted in relation to the general structure of each 
flower for its fertilization by different insects. In this, and 
in almost every other case, the inquiry may be pushed fur- 
ther backward ; and it may be asked how did the stigma of 
an ordinary flower become viscid; but as we do not know the 
full history of any one group of beings, it is as useless to 
ask, as it is hopeless to attempt answering, such questions. 

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 stems have generally, but not always, lost the 
power of twining, though they retain the power of revolv- 
ing, which the tendrils likewise possess. The gradations 
from leaf-climbers to tendril bearers are wonderfully close, 
and certain plants may be indifferently placed in either 
class. But in ascending the series from simple twiners to 
leaf -climbers, an important quality is added, namely sensi' 
tiveness to a touch, by which means the foot-stalks of the 


leaves or flowers, or these modified and converted into ten- 
drils, 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 beneficial in a high degree to the species. For 
instance, it is clearly a great advantage to a twining plant to 
become a leaf-climber ; and it is probable that every twiuer 
which possessed leaves with long foot-stalks would have been 
developed into a leaf -climber, if the foot-stalks had possessed 
in any slight degree the requisite sensitiveness to a touch. 

As twining is the simplest means of ascending a support, 
and forms the basis of our series, it may naturally be asked 
how did plants acquire this power in an incipient degree, 
afterward to be improved and increased through natural 
selection. The power of twining depends, firstly, on the 
stems while young being extremely flexible (but this is a 
character common to many plants which are not climbers) ; 
and, secondly, on their continually bending to all points of 
the compass, one after the other in succession, in the same 
order. By this movement the stems are inclined to all sides, 
and are made to move round and round. As soon as the 
lower part of a stem strikes against any object and is stopped, 
the upper part still goes on bending and revolving, and thus 
necessarily twines round and up the support. The revolving 
movement ceases after the early growth of each shoot. As 
in many widely separated families of plants, single species 
and single genera possess the pover of revolving, and have 
thus become twiners, they must have independently acquired 
it, and cannot have inherited 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 uncom- 
mon with plants which did not climb; and that this had 
afforded the basis for natural selection to work on and 
improve. When 7. made this prediction, I knew of only one 
imperfect oase, i?umely, of the young flower-peduncles of a 
Maurandia which revolved slightly and irregularly, like the 
stems of twini:ig plants, but without making any use of this 
habit. Soon afterward Fritz Miiller discovered that the 
young stem? of an Alisma and of a Linum — plants which 
do not climb and are widely separated in the natural system 
— revolved plainly, though irregularly : and he states that 
he has re' suspect that this occurs with some other 
plants. Tkese slight movements appear to be of no serriv# 


to the plants in question ; any 1 tow, they are not of the least 
use in the way of climbing, which is the point that concerns 
us. Nevertheless we can see that if the steins of these 
plants had been flexible, and if under the conditions to 
which they are exposed it had profited them to ascend to 
a height, then the habit of slightly and irregularly revolv- 
ing might have been increased and utilized through natural 
selection, until they had become converted into well-developed 
owining species. 

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

It is scarcely possible that the above slight movements, 
due to a touch or shake in the young and growing organs of 
plants, can be of any functional importance to them. But 
plants possess, in obedience to various stimuli, powers of 
movement, which are of manifest importance to them ; for 
instance, toward and more rarely from the light — in oppo- 
sition to, and more rarely in the direction of, the attraction 
of gravity. When the nerves and muscles of an animal are 
excited by galvanism or by the absorption of strychnine, the 
consequent movements may be called an incidental result, 
for the nerves and muscles have not been rendered specially 
sensitive to these stimuli. So with plants it appears that, 
from having the power of movement in obedience to certaio 


stimuli, they are excited in an incidental manner by a touch 
or by being shaken. Hence there is no great difficulty in 
admitting that in the case of leaf-climbers and tendril-bear- 
ers, it is this tendency which has been taken advantage of 
and increased through natural selection. It is, however, 
probable, from reasons which I have assigned in my memoir, 
that this will have occurred only with plants which had 
already acquired the power of revolving, and had thus 
become twiners. 

I have already endeavored 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 difficulty on this head. A 
good opportunity has thus been afforded for enlarging a 
little on gradations of structure, often associated with strange 
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 indi- 
viduals of some extinct high-reaching ruminant, which had 
the longest necks, legs, etc., and could browse a little above 
the average height, and the continued destruction of those 
which could not browse so high, would have sufficed for the 
production of this remarkable quadruped ; but the prolonged 
use of all the parts, together 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 common object was in each case the foundation for 
the work of natural selection, since perfected through the 
occasional preservation of slight variations which made the 


resemblance at all closer ; and this will have been carried on 
as long as the insect continued to vary, and as long as a 
more and more perfect resemblance led to its escape from 
sharp-sighted enemies. In certain species of whales there is 
a tendency to the formation of irregular little points of horn 
on the palate ; and it seems to be quite within the scope of 
natural selection to preserve all favorable variations, until the 
points were converted, rirst into lamellated knobs or teeth like 
those on the beak of a goose — then into short lamellae, 
like those of the domestic ducks — and then into lamellae as 
perfect as those of the snoveller-duck — and finally into the 
gigantic plates of baleen, as in the mouth of the Greenland 
whale. In the family 01 the ducks, the lamellae are first 
used as teeth, then partly as teeth and partly as a silting 
apparatus, and at last almost exclusively for this latter 

With such structures as the above lamellae of horn or 
whalebone, habit or use can have done little or nothing, as 
far as we can judge, toward 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 prehen- 
sile tail, may, be attributed almost wholly to continued use, 
together with inheritance. Witn respect to the mamma? 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 tn rough natural selection, 
and concentrated into a confined area, in which case they 
would have formed a mamma. There is no more difficulty 
in understanding how the branched spines of some ancient 
Echinoderm, which served as a defence, became developed 
through natural selection into tridactyle 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 first used solely 
for locomotion. In the avicularia and vibracula of the 
Polyzoa we have organs widely different in appearance de- 
veloped from the same source ; and with the vibracula we 
can understand how the successive gradations might have 
been of service. With the pollinia of orchids, the threads 
which originally served to tie together the pollen grains can 
be traced cohering into caudicles ; and the steps can 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, became attached tc 
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 

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 advanta- 
geous ? But it is unreasonable to expect a precise answer 
to such questions, considering our ignorance of the past 
history of each species, and of the conditions which at the 
present day determine its numbers and range. In most 
cases only general reasons, but in some few cases special 
reasons, can be assigned. Thus, to adapt a species to new 
habits of life, many co-ordinated modifications are almost 
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-enduring conditions, often 
of a peculiar nature, are necessary for the development of a 
structure ; and the requisite conditions may seldom have 
concurred. The belief that any given structure, which we 
think, often erroneously, would have been beneficial to a 
species, would have been gained under all circumstances 
through natural selection, is opposed to what we can under- 
stand of its manner of action. Mr. Mivart does not deny 
that natural selection has effected something ; but he consid- 
ers it as " demonstrably insufficient " to account for the phe- 
nomena 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 char- 
acter of demonstration, and to havt little weight in compariv 
■on with those in favor of the power of natural selection, 
aided by the other agencies often specified. I am bound to 
add, that some of the facts and arguments here used by me, 
have been advanced for the same purpose in an able article 
lately published in the " Medico-Chirurgical Review." 

At the present day almost all naturalists admit ^olutioii 


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 in- 
ternal force beyond the tendency to ordinary variability, 
which through the aid of selection by man has given rise 
to many well-adapted domestic races, and which, through 
the aid of natural selection, would equally well give rise by 
graduated steps to natural races or species. The final result 
will generally have been, as already explained, an advance, 
but in some few cases a retrogression, in organization. 

Mr. Mivart is further inclined to believe, and some natur- 
alists agree with him, that new species manifest themselves 
"with suddenness and by modifications appearing 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 
improbable in the highest degree. 

Every one who believes in slow and gradual evolution, 
will of course admit that specific changes may have been 
as abrupt and as great as any single variation which we 
meet with under nature, or even under domestication. But 
as species are more variable when domesticated or culti- 
vated than under their natural conditions, it is not probable 
that such great and abrupt variations have often occurred 
under nature, as are known occasionally to arise under 
domestication. Of these latter variations several may be 
attributed to reversion ; and the characters which thus re- 
appear 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, etc. ; and as they are widely 
different in character from natural species, they throw very 
little light on our subject. Excluding such cases of abrupt 
variations, the few which remain would at best constitute, if 
found in a state of nature, doubtful species, closely related 
to their parental types. 

My reasons for doubting^ whether Q&to&l species havd 


changed as abruptly as have occasionally domestic races, 
and for entirely disbelieving that they have changed in the 
wonderful manner indicated by Mr. Mivart, are as follows: 
According to our experience, abrupt and strongly marked 
variations occur in our domesticated 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 
intercrossing ; and so it is known to be under domestication, 
unless abrupt variations of this kind are specially preserved 
and separated by the care of man. Hence, in order that a 
new species should suddenly appear in the manner supposed 
by Mr. Mivart, it is almost necessary to believe, in opposi- 
tion to all analogy, that several wonderfully changed indi- 
viduals appeared simultaneously within the same district. 
This difficulty, as in the case of unconscious selection by 
man, is avoided on the theory of gradual evolution, through 
the preservation of a large number of individuals, which 
varied more or less in any favorable direction, and of the 
destruction of a large number which varied in an opposite 

That many species have been evolved in an extremely 
gradual manner, there can hardly be a doubt. The species 
and even the genera of many large natural families are so 
closely allied together that it is difficult to distinguish not 
a few of them. On every continent, in proceeding from 
north to south, from lowland to upland, etc., we meet with 
a host of closely related or representative species ; as we 
likewise do on certain distinct continents, which we have 
reason to believe were formerly connected. But in making 
these and the following remarks, I am compelled to allude 
to subjects hereafter to be discussed. Look at the many 
outlying islands round a continent, and see how many of 
their inhabitants can be raised only to the rank of doubtful 
species. So it is if we look to past times, and compare the 
species which have just passed away with those still living 
within the same areas ; or if we compare the fossil species 
embedded in the sub-stages of the same geological 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 main- 
tained 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, instead of to 4i^ 


tinct species, that numerous and wonderfully fine grada- 
tions can be traced, connecting together widely different 

Many large groups of facts are intelligible only on the 
principle that species have been evolved by very small 
steps. For instance, the fact that the species included in 
the larger genera are more closely related to each other, and 
present a greater number of varieties, than do the species 
in the smaller genera. The former are also grouped in 
little clusters, like varieties round species, and they pre- 
sent other analogies with varieties, as was shown in our 
second chapter. On this same principle 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 
tacts, 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 favor of 
this view, such as the sudden crystallization 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, sup- 
ports at first sight the belief in abrupt development. But 
the value of this evidence depends entirely on the perfec- 
tion of the geological record, in relation to periods remote 
in the history of the world. If the record is as frag- 
mentary as many geologists strenuously assert, there is 
nothing strange in new forms appearing as if suddenly 

Unless we admit transformations as prodigious as those 
advocated by Mr. Mivart, such as the sudden development 
of the wings of birds or bats, or the sudden conversion of 
a Hipparion into a horse, hardly any light is thrown by the 
belief in abrupt modifications on the deficiency of connect- 
ing links in our geological formations. But against the 
belief in such abrupt changes, embryology enters a strong 


protest. It is notorious that the wings of birds and bats, 
and the legs of horses or other quadrupeds, are undistin- 
guishable at an early embryonic period, and that they be- 
come 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 cor- 
responding age. The embryo is thus left almost unaffected, 
and serves as a record of the past condition of the species. 
Hence it is that existing species during the early stages of 
their development so often resemble ancient and extinct 
forms belonging to the same class. On this view of the 
meaning of embryological resemblances, and indeed on any 
view, it is incredible that an animal should have undergone 
such momentous and abrupt transformations as those above 
indicated, and yet should not bear even a trace in its 
embryonic condition of any sudden modification, every 
detail in its structure being developed by insensibly fine 

He who believes that some ancient form was transformed 
suddenly through an internal force or tendency into, for 
instance, one furnished with wings, will be almost com- 
pelled to assume, in opposition to all analogy, that many 
individuals varied simultaneously. It cannot be denied 
that such abrupt and great changes of structure are widely 
different from those which most species 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 produced ; and of such complex and 
wonderful coadaptations, he will not be able to assign a 
shadow of an explanation. He will be forced to admit that 
these great and sudden transformations have left no trace 
of their action on the embryo. To admit all tnis is, as it 
seems to me, to enter into the realms of miracle, and to 
leave those of science. 




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 — 

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

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

Frederick Cuvier and several of the older metaphysicians 
have 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 unconsciously many habitual actions 
are performed, indeed not rarely in direct opposition to our 
conscious wilU yet they may be modified by the wiD or 


reason. Habits easily become associate with other habits, 
with certain periods of time and states of the body. When 
once acquired, they often remain ecnstant throughout life. 
Several other points of resemblance cetween instincts and 
habits could be pointed out. As in repeating a well-known 
song, so in instincts, one action follows another by a sort of 
rhythm ; if a person be interrupted in a song, or in repeat- 
ing anything by rote, he is generally forced to go back to 
recover the habitual train of thought, so P Huber found it 
was with a caterpillar, which makes a very complicated 
hammock ; for if he took a caterpillar which had completed 
its hammock up to, say, the sixth stage of construction, and 
put it into a hammock completed up only to the third stage, 
the caterpillar simply re-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, it was much 
embarrassed, and in order to complete its hammock, seemed 
forced to start from the third stage, where it had left off, 
and thus tried to complete the already finished work. 

If we suppose any habitual aetion to become inherited — 
and it can be shown that this does sometimes happen — 
then the resemblance between what originally was a habit 
and an instinct becomes so close as not to be distinguished. 
If Mozart, instead of playing the piano-forte at three years 
old with wonderfully little practice, had played a tune with 
no practice at all, he might truly be said to have done so 
instinctively. But it would be a serious error to suppose 
that the greater number of instincts have been acquired by 
habit in one generation, and then transmitted by inherit- 
ance to succeeding generations It can be clearly shown 
that the most wonderful instincts with which we are ac- 
quainted, 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 im- 
portant 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 modifica,- 
tions 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 selection preserving and con- 
tinually accumulating variations of instinct to any ^xte:$ 


that was profitable. It is thus, as I believe, that all the 
most complex and wonderful instiucts have originated. As 
modifications of corporeal structure arise from, and are in- 
creased by, use or habit, and are diminished or lost by dis- 
use, so I do not doubt it has been with instincts. But I 
believe that the effects of habit are in many cases of sub- 
ordinate 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 accumu- 
lation of numerous slight, yet profitable, variations. Hence, 
as in the case of corporeal structures, we ought to find in 
nature, not the actual transitional gradations by which each 
complex instinct has been acquired — for these could be 
found only in the lineal ancestors of each species — but we 
ought to find in the collateral lines of descent some evidence 
of such gradations ; or we ought at least to be able to show 
that gradations of some kind zve possible ; and this we cer- 
tainly can do. I have been surprised to find, making allow- 
ance for the instincts of animals having been but little 
observed, except in Europe and North America, and for no in- 
stinct being known among extinct species, how very generally 
gradations, leading to the most complex instincts, can be dis- 
covered. Changes of instinct may sometimes be facilitated 
by the same species having different instincts at different 
periods of life, or at different seasons of the year, or when 
placed under different circumstances, etc. ; in which case 
either the one or the other instinct might be preserved by 
natural selection. And such instances of diversity of in- 
stinct in the same species can be shown to occur in nature. 

Again, as in the case of corporeal structure, and conform- 
ably to my theory, the instinct of each species is good for 
itself; but has never, as far as we can judge, been produced 
for the exclusive good of others. One of the strongest in- 
stances of an animal apparently performing an action for 
the sole good of another, with which I am acquainted, is 
that of aphides voluntarily yielding, as was firsv observed by 
Huber, their sweet excretion to ants ; that they do so volun- 
tarily, the following facts show : I removed all the ants from 
a group of about a dozen aphides on a dock-plant, and pre- 
vented 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 on* 
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 atennse; but not one excreted. Afterward, 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 begun to play with its antennae on 
the abdomen first of one aphis and then of another ; and 
each, as soon as it felt the antennae, immediately lifted up 
its abdomen and excreted a limpid drop of sweet juice, which 
was eagerly devoured by the ant. Even the quite young 
aphides behaved in this manner, showing that the action was 
instinctive, and not the result of experience. It is certain, 
from the observations of Huber, that the aphides show no 
dislike to the ants : if the latter be not present they are at 
last compelled to eject their excretion. But as the excretion 
is extremely viscid, it is no doubt a convenience to the 
aphides to have it removed ; therefore probably they do not 
excrete solely for the good of the ants. Although there is 
no evidence that any animal performs an action for the 
exclusive good of another species, yet each tries to take 
advantage of the instincts of others as each takes advan- 
tage of the weaker bodily 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 indis- 
pensable 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 direc- 
tion, and in its total loss. So it is with the nests of birds, 
which vary partly in independence 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 collect- 


fng propolis, used a cement of wax and turpentine, with 
which he had covered decorated trees. It has lately been 
shown that bees, instead of searching for pollen, will gladly 
use a very different substance, namely, oatmeal. Fear ot 
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 Eng- 
land, in the greater wildness of all our large birds in com- 
parison with our small birds ; for the large birds have been 
most persecuted by man. We may safely attribute the 
greater wildness of our large birds to this cause ; for in 
uninhabited islands large birds are not more fearful than 
small ; and the magpie, so wary in England, is tame in 
Norway, as is the hooded crow in Egypt. 

That the mental qualities of animals of the same kind, 
born in a state of nature, vary much, could be shown by 
many facts. Several cases could also be adduced of occa- 
sional and strange habits in wild animals, which, if advan- 
tageous 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. 


The possibility, or even probability, of inherited varia- 
tions of instinct in a state of nature will be strengthened 
by briefly considering a few cases under domestication. 
We shall thus be enabled to see the part which habit and 
the selection of so-called spontaneous variations have played 
in modifying the mental qualities of our domestic animals. 
It is notorious how much domestic animals vary in their 
mental qualities. With cats, for instance, one naturally 
takes to catching rats, and another mice, and these ten- 
dencies are known to be inherited. One cat, according to 
Mr. St. John, always brought home game birds, another 
hares or rabbits, and another hunted on marshy ground and 
almost nightly caught woodcocks or snipes. A number of 
curious and authentic instances could be given of various 


shades of disposition and of taste, and likewise of the oddest 
tricks, associated with certain frames of minds or periods 
of time, being inherited. 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 striking instances) will 
sometimes point and even back other dogs the very first 
time that they are taken out ; retrieving is certainly in 
some degree inherited by retrievers ; and a tendency to run 
round, instead of at, a flock of sheep, by shepherd dogs 
I cannot see that these actions, performed without experi 
ence by the young, and in nearly the same manner by each 
individual, performed with eager delight by each breed, and 
without the end being known — for the young pointer can 
no more know that he points to aid his master, than the 
white butterfly knows why she lays her eggs on the leaf of 
the cabbage — I cannot see that these actions differ essen- 
tially 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 instincts; 
but they have been acted on by far less rigorous selections, 
and have been transmitted for an incomparably shorter 
period, under less fixed conditions of life. 

How strongly these domestic instincts, habits, and dispo- 
sitions are inherited, and how curiously they become mingled, 
is well shown when different breeds of dogs are crossed. 
Thus it is known that a cross with a bull-dog has affected 
for many generations the courage and obstinacy of grey- 
hounds ; and a cross with a greyhound 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 Roy describes a 
dog, whose great-grandfather was a wolf, and this dog showed 
a trace of its wild parentage only in one way, by not coming 
in a straight line to his master when called. 

Domestic instincts are sometimes spoken of as actions 
which have become inherited solely from long-continued 
$nd compulsory habit j but this is pot true. £To ope 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 selection 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 successive generation 
would soon complete the work ; and unconscious selection is 
still in progress, as each man tries to procure, without 
intending to improve the breed, dogs which stand and hunt 
best. On the other hand, habit alone in some cases has 
sufficed ; hardly any animal is more difficult to tame than 
the young of the wild rabbit ; scarcely any animal is tamer 
than the young of the tame rabbit; but I can hardly sup- 
pose that domestic 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 con- 
finement. . 

Natural instincts are lost under domestication : a remark- 
able instance of this is seen in thoje breeds of fowls which 
very rarely or never become "brocdy," that is, never wish 
to sit on their eggs. Familiarity alone prevents our seeing 
how largely and how permanently 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 cat genus, when 
kept tame, are most eager to attack poultry, sheep, and 
pigs ; and this tendency has been found incurable in dogs 
which have been brought home as puppies from countries 
such as Tierra del Fuego and Australia, where the savages 
do not keep these domestic animals. How rarely, on the 
other hand, do our civilized dogs, even when quite youngs 


require to be taught not to attack poultry, sheep, and pigs! 
No doubt they occasionally do make an attack, and are then 
beaten ; and if not cured, they are destroyed ; so that habit 
and some degree of selection have probably concurred in 
civilizing by inheritance our dogs. On the other hand, 
young chickens have lost wholly by habit that fear of the 
dog and cat which no doubt was originally instinctive in 
them, for I am informed by Captain Huttou 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 pur- 
pose of allowing, as we see in wild ground-birds, their 
mother to fly away. But this instinct retained by our 
chickens has become useless under domestication, for the 
mother hen has almost lost by disuse the power of flight. 

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


We shall, perhaps, best understand how instincts in a 
state of nature have become modified by selection, by con- 
sidering a few cases. I will select only three, namely, the 
instinct which leads the cuckoo to lay her eggs in other birds' 
pests ; the slave-making instinct of certain ants ; and the 
eell-making power of the hive-bee. These two latter instincts 
have generally and justly been ranked by naturalists as tho 
tnost wonderful of all known instincts. 



It is supposed by some nattualist', that the more imme- 
diate cause of the instinct ot the cucko > ii that she lays her 
eggs, not daily, but at interval < f two or three days, so that 
it* 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 different 
ages in the same nest. If this were the case, the process of 
laying and hatching aright 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, 
tor she makes her own nest and has egg.; and young succes- 
sively 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. Merrill, of Iowa, that he once found in Illinois a 
young cuckoo, together with a young jay, in the nest of a 
blue jay (Garrulus cristatus) ; and as both were nearly full 
feathered, there could be no mistake in their identification. 
I could also give several instances of various birds which 
have been known occasionally to lay their eggs in other birds' 
nests. Now let us suppose that the ancient progenitor of 
our European cuckoo had the habits of the American cuckoo, 
and that she occasionally laid an egg in another bird's nest. 
If the old bird profited by this occasional habit through 
being enabled to emigrate earlier or through any other 
cause ; or if the young were made more vigorous by advan- 
tage 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 ot ' 
their mother, and in their turn would be apt to lay their eggs 
in other birds' nests, and thus be more successful in rearing 
their young. By a continued process of this nature, I believe 
that the strange instinct of our cuckoo has been generated. 
It has, also, recently been ascertained on sufficient evidence, 
by Adolf Miiller, that the cuckoo occasionally lays her eggs 
OB the bare ground, sits on them and feeds her young. Thi# 


rare event is probably the case of reversion to the long-lost, 
aboriginal instinct of nidification. 

It has been ejected that I have not noticed other related 
instincts and adaptations of structure in the cuckoo, which 
are spoken "A as necessarily co-ordinated. But in all cases 
speculation on an instinct known to us only in a single spe^ 
Mes, 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. E,am»«*y's observations, we have learned something 
about three Australian species, which lay their eggs in other 
birds' nests. The chief points to be referred to are three : 
first, that the common cuckoo, with rare exceptions, lays 
only one egg in a nest, so that the large and voracious young 
bird receives ample food. Secondly, that the eggs are re- 
markably small, not exceeding those of the skylark — a bird 
about one-fourth as large as the cuckoo. That the small size 
of the egg is a real case of adaptation we may infer from the 
fact of the non-parasitic American cuckoo laying full-sized 
eggs. Thirdly, that the young cuckoo, soon after birth, has 
the 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 arrange- 
ment, 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 advantage to this 
species to have laid eggs even smaller than those now laid, 
so as to have deceived certain foster-parents, or, as is more 
probable, to have been hatched within a shorter period (for 
ifc is asserted that there is a relation between the size of eggs 
and the period of their incubation), then there is no difficulty 
in believing that a race or species might have been formed 
which would have laid smaller and smaller eggs ; for these 
would have been more safely hatched and reared. Mr. 
Ramsay remarks that two of the Australian cuckoos, when 
they lay their eggs in an open nest, manifest a decided pref- 
erence for nests containing eggs similar in color to their 
own. The European species apparently manifests some tend- 
ency toward a similar instinpt, but aot rarely departs from 


it, as is shown by her laying her dull and pale-colored eggs 
in the nest of the hedge-warbler with bright greenish-blue 
eggs. Had our cuckoo invariably displayed the above in- 
stinct, it would assuredly have been added to those which it 
is assumed must all have been acc'rired together. The eggs 
of the Australian bronze cuckoo vary, according to Mr. Kam- 
say, to an extraordinary degree in color; so that in this 
respect, as well as in size, natural selection might have 
secured and fixed any advantageous variation. 

In the case of the European cuckoo, the offspring of the 
foster-parents are commonly ejected from the nest within 
three days after the cuckoo is hatched ; and as the latter 
at this age is in a most helpless condition, Mr. Gould was 
formerly inclined to believe that the act of ejection was 
performed by the foster-parents themselves. But he has 
now received a trustworthy account of a young cuckoo 
which was actually seen, while still blind and not able even 
to hold up its own head, in the act of ejecting its foster- 
brothers. One of these was replaced in the nest by the 
observer, and was again thrown out. With respect to the 
means by which this strange and odious instinct was 
acquired, if it were of great importance for the young cuckoo, 
u,3 is probably the case, to receive as much food as possible 
soon aft«r birth, I can see no special difficulty in its having 
gradually acquired, during successive generations, the blind 
desire, the strength, and structure necessary for the work 
of ejection ; for those cuckoos which had such habits and 
structure best developed would be the most securely reared. 
The first step toward the acquisition of the proper instinct 
might have been mere unintentional restlessness ou the part 
of the young bird, when somewhat advanced in age and 
strength ; the habit having been afterward improved, and 
transmitted to an earlier age. I can see no more difficulty 
in this than in the unhatched young of other birds acquiring 
the instinct to break through their own shells ; or than in 
young snakes acquiring in their upper jaws, as Owen has 
remarked, a transitory sharp tooth for cutting through the 
tough egg-shell. For if each part is liable to individual 
variations at ail ages, and the variations tend to be inherited 
at a corresponding or earlier age — propositions which can- 
ned 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 faD. together with 
the whole theory of natural section. 


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 interest- 
ing gradation in the perfection of their instincts. 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 following 
old birds of a distinct kind and clamoring to be fed by them. 
The parasitic habits of another species of Molothurs, 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 com- 
mence to build an irregular untidy nest of their own, placed 
in singular ill-adapted situations, as on the leaves of a large 
thistle. They never, however, as far as Mr. Hudsou has 
ascertained, complete a nest for themselves. They often lay 
so many eggs — from fifteen to twenty — in the same foster- 
nest, that few or none can possibly be hatched. They 
have, moreover, the extraordinary habit of pecking holes in 
the eggs, 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 dis- 
believer in evolution, but he appears to have been so mucn 
struck by the imperfect instincts of the Molothrus bonarien- 
sis 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, 
transition ? " 

Various birds, as has already been remarked, occasionally 
lay their eggs in the nests of other birds. This habit is 
not very uncommon with the Gallinaceae, and throws some 
light on the singular instil* * the ostrich. In this family 


several hevi 6irds unite and lay first a few eggs in one nest 
and then in another ; and these are hatched by the males. 
This instinct may probably be accounted for by the fact of 
the hens laying a large number of eggs, but, as with the 
cuckoo, at intervals of two or three days. The instinct, 
however, of the American ostrich, as in the case of the 
Molothrus bonariensis, has not as yet been perfected ; for a 
surprising number of eggs lie strewed over the plains, so that 
in one day's hunting I picked up no less than twenty lost 
and wasted eggs. 

Many bees are parasitic, and regularly lay their eggs in 
the nests of other kinds of bees. This case is more remark- 
able 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 indis- 
pensable 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 Tachytes nigra generally makes 
its own burrow and stores it with paralyzed prey for its own 
larvse, 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 Molothr jz cuckoo, I can see no diffi- 
culty in natural selection making an occasional habit perma- 
nent, if of advantage to the apecies, and if the insect whose 
nest and stored food are feloniously appropriated, be not 
thus exterminated. 


This remarkable instinct was first discovered in the 
Formica (Polyerges) rufescens by Pierre Huber, a better 
observer even than his celebrated father. This ant is 
absolutely dependent on its slaves ; without their aid, the 
species would certainly become extinct in a single year. 
The males and fertile females do no work of any kind, and 
the workers or sterile females, though most energetic and 
courageous in capturing slaves, do no other work. They are 
incapable of making their own nests, or of feeding their 
own larvae. When the old nest is found inconvenient, and 
they have to migrate, it is the slaves which determine the 
migration, and actually carry xneir masters in their jawa» 


So utterly helpless are the masters, that when Huber shut 
up thirty of them without a slave, but with plenty of food 
which they liked 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 
wonderful an instinct could have been perfected. 

Another species, Formica sanguinea, was likewise first 
discovered by P. Huber to be a slave-making ant. This 
species is found in the southern parts of England, and its 
habits have been attended to by Mr. F. Smith, of the British 
Museum, to whom I am much indebted for information 
on this and other subjects. Although fully trusting to the 
statements of Huber and Mr. Smith, I tried to approach 
the subject in a sceptical frame of mind, as any one may 
well be excused for doubting the existence of so extraordi- 
nary an instinct as that of making slaves. Hence, I will 
give the observations which I made in some little detail. 
I opened fourteen nests of F. sanguinea, and found a few 
slaves in all. Males and fertile females of the slave species 
(F. fusca) are found only in their own proper communities, 
and have never been observed in the nests of F. sanguinea. 
The slaves 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 energetically 
together with their masters in carrying them away to a 
place of safety. Hence, it is clear that the slaves feel quite 
at home. I/uring the months of June and July, on three 
successive years, I watched for many hours several nests in 
Surrey aJnd 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 nuxnerous ; 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 community with an unusually large 
stock of slaves, and I observed a few slaves mingled with 
their masters leaving the nest, and marching along the same 
road to a tall Scotch fir-tree, twenty-five yards distant, which 
they ascended together, probably in search of aphides 01 
cocci. According to Huber, who had ample opportunities 
for observation, the slaves in Switzerland habitually work 
with their masters in making the nest, and they alone open 
and close the doors in the morning and evening; and, as 
Huber expressly states, their principal office is to search for 
aphides. This difference in the usual habits of the masters 
and slaves in the two countries, probably depends merely 
on the slaves being captured in greater numbers in 
Switzerland than in England. 

One day I fortunately witnessed a migration of F. san- 
guinea from one nest to another, and it was a most interest- 
ing spectacle to behold the masters carefully carrying their 
slaves in their jaws instead of being carried by them, as in 
the case of F. rufescens. Another day my attention was 
struck by about a score of the slave-makers haunting the 
same spot, and evidently not in search of food ; they 
approached and were vigorously repulsed by an indepen- 
dent community of the slave-species (F. fusca) ; sometimes 
as many as three of these ants clinging to the legs of the 
slave-making F. sanguinea. The latter ruthlessly killed 
their small opponents and carried their dead bodies as food 
to their nest, twenty-nine yards distant ; but they were pre- 
vented 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 pupae of another species, F. flava, with a few of 
these little yellow ants still clinging to the fragments of 
their nest. This species is sometimes, though rarely, made 
into slaves, as has been described by Mr. Smith. Although 
so small a species, it is very courageous, and' I have seen it 
ferociously attack other ants. In one instance I found to 


my surprise an independent community of F. flava under & 
stone beneath a nest of the slave-making F. sanguinea ; and 
when I had accidentally disturbed both nests, the little ants 
attacked their big neighbors with surprising courage. Now 
I was curious to ascertain whether F. sanguinea could dis- 
tinguish the pupae of F. fusca, which they habitually make 
into slaves, from those of the little and furious F. flava, 
whLlt they rarely capture, and it was evident that they did 
it once distinguish them ; for we have seen that they 
eagerly and instantly seize the pupae of F. 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 pupae. 

One evening I visited another community of F. san- 
guinea, and found a number of these ants returning home 
and entering their nests, carrying the dead bodies of F. 
fusca (showing that it was not a migration) and numerous 
pupae. I traced a long file of ants burdened with booty, 
for about forty yards back, to a very thick clump of heath, 
whence I saw the last individual of F. sanguinea emerge, 
carrying a pupa ; but I was not able to find the desolated 
nest in the thick heath. The nest, however, must have 
been close at hand, for two or three individuals of F. fusca 
were rushing about in the greatest agitation, and one was 
porched motionless with its own pupa in its mouth on the 
top of a spray of heath, an image of despair over its ravaged 

Such are the facts, though they did not need confirma- 
tion by me, in regard to the wonderful instinct of making 
slaves. Let it be observed what a contrast the instinctive 
habits of F. sanguinea present with those of the conti- 
nental F. rufescens. The latter does not build its own 
nest, does not determine its own migrations, does not col- 
lect food for itself or its young, and cannot ^ en feed 
itself: it is absolutely dependent on its numerous slaves. 
Formica sanguinea, on the other hand, possesses much 
fewer slaves, ana in the early part of the summer extremely 
few: the masters determine when and where a new nest 
shall be formed, and when they migrate, the masters carry 
tlis- 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 col- 
lect food for the community. In England the masters alone 
usually leave the nest to collect building materials and fooa 
for themselves, their slaves and larvae. So that the masters 
in this country receive much less service from their slaves 
than they do in Switzerland. 

By what steps the instinct of F. sanguinea originated 1 
will not pretend to conjecture. But as ants which are not 
slave-makers will, as I have seen, carry off the pupae oi 
other species, if scattered near their nests, it is possible that 
such pupae originally stored as food might become developed ; 
and the foreign ants thus unintentionally reared would then 
follow their proper instincts, and do what work they could. 
If their presence proved 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 cf 
collecting pupae, originally for food, might by natural selec- 
tion be strengthened and rendered permanent for the very 
different purpose of raising slaves. When the instinct was 
once acquired, if carried out to a much less extent even than 
in our British F. sanguinea, which, as we have seen, is less 
aided by its slaves than the same species in Switzerland, 
natural selection might increase and modify the instinct — 
always supposing each modification to be of use to the species 
— until an ant was formed as abjectly dependent on its 
slaves as is the Formica rufescens. 


I will not here enter on minute details on this subject, 
but will merely give an outline of the conclusions at which 
I have arrived. He must be a dull man who can examine 
the exquisite structure of a comb, so beautifully adapted 
to its end, without enthusiastic admiration. We hear from 
mathematicians that bees have practically solved a recondite 
problem, and have made their cells of the proper shape to 
hold the greatest possible amount of honey, with the least 
possible consumption of precious wax in their construction. 
It has been remarked that a skilful workman with fitting 
tools and measures, would find it very difficult to make cells 
of wax of the true form, though this is effected by a crowd 
of bees working in a dark hive. Granting whatever in- 


stincts 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 at first appears : all this beautiful 
work can be shown, I think, to follow from a few simple 

I was led to investigate this subject by Mr. Waterhouse, 
who has shown that the form of the cell stands in close 
relation to the presence of adjoining cells ; and the following 
view may, 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 ad- 
joining 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 Mexican Melipona domestica, carefully described and 
figured by Pierre Huber. The Melipona itself is interme- 
diate 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 hold- 
. ing honey. These latter cells are nearly spherical and of 
nearly equal sizes, and are aggregated into an irregular 
mass. But the important point to notice is, that these 
cells are always made at that degree of nearness to each 
other that they w r ould have intersected or broken into each 
other if the spheres had been completed ; but this is never 
permitted, the bees building perfectly flat walls of wax be. 
tween 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 


#«*e 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 imi- 
tation 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 con- 
struction of three adjoining cells. It is obvious that the 
Melipona saves wax, and what is more important, labor, by 
this manner of building; for the flat walls between the 
adjoining cells are not double, but are of the same thickness 
as the outer spherical portions, and yet each flat portion 
forms a part of two cells. 

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

If a number of equal spheres be described with their 
centres placed in two parallel layers ; with the centre of 
each sphere at the distance of radius x V 2, or 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 to- 
gether by pyramidal bases formed of three rhombs ; and the 
rhombs and the sides of the hexagonal prisms will have 
every angle identically the same with the best measurements 
which have been made of the cells of the hive-bee. But I 
hear from Professor Wyman, who has made numerous care- 
ful measurements, that the accuracy of the workmanship of 
the bee has been greatly exaggerated ; so much so, that 
whatever the typical form of the cell may be, it is rarely, 
if ever, realized. 

Hence we may safely conclude that, if we could slightly 
modify the instincts already possessed by the Melipona, and 
in themselves not very wonderful, this bee would make 
a structure as wonderfully perfect as that of the hive-bee. 
W§ wust suppose the Melinona to have the power of form* 


ing her cells truly spherical, and of equal sizes ; and this 
would not be very surprising, seeing that she already does 
so to a certain extent, and seeing what perfectly cylindrical 
burrows many insects make in wood, apparently oy turning 
round on a fixed point. We must suppose the Melipona to 
arrange her cells in level layers, as she already does her 
cylindrical cells ; and we must further suppose, and this is 
the greatest difficulty, that she can somehow judge accu- 
rately at what distance to stand from her fellow-laborers 
when several are making their spheres ; but she is already 
so far enabled to judge of distance, that she always describes 
her spheres so as to intersect to a certain extent ; and then 
she unites the points of intersection by perfectly flat sur- 
faces. Bv such modifications of instincts which in them- 
selves are not very wonderful — hardly more wonderful than 
those which guide a bird to make its nest — I believe that 
the hive-bee has acquired, through natural selection, her 
inimitable architectural powers. 

But this theory can be tested by experiment. Following 
the example of Mr. Tegetmeier, I separated two combs, and 
put between them a long, thick, rectangular strip of wax : 
the bees instantly began to excavate minute circular pits in 
it; and as they deepened these little pits, they made them 
wider and wider until they were converted into shallow 
basins, appearing to the eye perfectly true or parts of a 
sphere, and of about the diameter of a cell. It was most 
interesting to observe that, wherever several bees had begun 
to excavate tnese basins near together, they had begun their 
work at such a distance from each other that by the time 
the basins had acquired the above-stated width (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 excavate, 
and began to build up flat walls of wax on the lines of inter- 
section between the basins, so that each hexagonal prism 
was built upon the scalloped edge of a smooth basin, instead 
of on the straight edges of a three-sided pyramid as in the 
case of ordinary cells. 

I then put into the hive, instead of a thick, rectangular 
piece of wax, a thin and narrow, knife-edged ridge, colored 
with vermilion. The bees instantly began on both sides 
to excavate little basins near to each other, in the same 
way as before j but the ridge of wax was so thin, that the 


bottoms of the basins, if they had been excavated to the 
same depth as in the former experiment, would have broken 
into each other from the opposite sides. The bees, however, 
did not suffer this to happen, and they stopped their excava- 
tions 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 side 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 inter- 

Considering how flexible thin wax is, I do not see that 
there is any difficulty in the bees, while 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 appeared to me that 
the bees do not always succeed in working at exactly the 
same rate from the opposite sides ; for I have noticed half- 
completed rhombs at the base of a just commenced cell, 
which were slightly concave on one side, where I suppose 
that the bees had excavated too quickly, and convex on the 
opposed side where the bees had worked less quickly. In 
one well-marked instance, I put the comb back into the hive, 
and allowed the bees to go on working for a short time, and 
again examined the cell, and I found that the rhombic plate 
had been completed, and had become perfectly 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 
ehape, by standing at the prooer distance from each other, 


by excavating at the same rate, and by endeavoring 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 rhom- 
bic plates, until the hexagonal walls are commenced. Some 
of these statements differ from those made by the justly 
celebrated elder Huber, but I am convinced of their accuracy ; 
and if I had space, I could show that they are conformable 
with my theory. 

Huber's statement, that the very first cell is excavated 
out of a little parallel-sided wall of wax, is not, as far as I 
have seen, strictly correct ; the first commencement 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 adjoining spheres. I have several 
specimens showing clearly that they can do this. Even in 
the rude circumferential rim or wall of wax round a growing 
comb, flexures may sometimes be observed, corresponding in 
position to the planes of the rhombic basal plates of future 
cells. But the rough wall of wax has in every case to be 
finished off, by being largely gnawed away on both sides. 
The manner in which the bees build is curious ; they always 
make the first rough wall from ten to twenty times thicker 
than the excessively thin finished wall of the cell, which 
will 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 add- 
ing fresh cement on the summit of the ridge. We shall thus 
have a thin wall steadily growing upward, but always crowned 
by a gigantic coping. From all the cells, both those just 
commenced and those completed, being thus crowned by a 
strong coping of wax, the bees can cluster and crawl over 


the comb without injuring the delicate hexagonal walls. 
These walls, as Professor Miller has kindly ascertained for 
me, vary greatly in thickness ; being, on an average of twelve 
measurements made near the border of the comb, 3 £ ? 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 5 .J^ of 
an inch. By the above singular manner of building, strength 
is continually given to the comb, with the utmost ultimate 
economy of wax. 

It seems at first to add to the difficulty of understanding 
how the cells are made, that a multitude of bees all work 
together; one bee after working a short time at one cell 
going to another, so that, as Huber has stated, a score of 
individuals work even at the 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 ex- 
treme margin of the circumferential rim of a growing comb, 
with an extremely thin layer of melted vermilion wax ; and 
I invariably found that the color was most delicately dif- 
fused by the bees — as delicately as a painter could have 
done it with his brush — by atoms of the colored 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 rejected. 

When bees have a place on which they can stand in their 
proper positions for working — for instance, on a slip of 
wood, placed directly under the middle of a comb growing 
downward, 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, project- 
ing beyond the other completed cells. It suffices that the 
bees should be enabled to stand at their proper relative dis- 
tances from each other and from the walls of the last com- 
pleted cell's, and then, by striking imaginary, spheres, they 

. am build up a wall intermediate between two -adjoioing 


spheres ; but as far as I have seen, they never gnaw away 
and finish off the angles of a cell till a large part both of 
that cell and of the adjoining cells has been built. This 
capacity in bees of laying down under certain circumstances 
a rough wall in its proper place between two just commenced 
cells, is important, as it bears on a fact, which seems at first 
subversive of the foregoing theory ; namely, that the cells 
on the extreme margin of wasp-combs are sometimes strictly 
hexagonal ; but I have not space here to enter on this sub- 
ject. Nor does there seem to me any great difficulty in a 
single insect (as in the case of a queen-wasp) making hex- 
agonal 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 cylin- 
ders, and building up intermediate planes. 

As natural 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 reason- 
ably be asked, how a long and graduated succession of modi- 
fied architectural instincts, all tending toward the present 
perfect plan of construction, could have profited the pro- 
genitors of the hive-bee ? I think the answer is not difficult : 
cells constructed like those of the bee or the wasp gain in 
strength, and save much in labor and space, and in the mate- 
rials of which they are constructed. With respect to the 
formation of wax, it is known that bees are often hard 
pressed to get sufficient nectar, and I am informed by Mr. 
Tegetmeier that it has been 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 con- 
sumed by the bees in a hive for the secretion of the wax 
necessary for the construction of their combs. Moreover, 
many bees have to remain idle for many days during the 
process of secretion. A large store of honey is indispensa- 
ble to support a large stock of bees during the winter ; and 
the security of the hive is known mainly to depend on a 
large number of bees being supported. Hence the saving of 
wax by largely saving honey, and the time consumed in col- 
lecting the honey, must be an important element of success 
to any family of bees. Of course the success of the species 
may be dependent on the number of its enemies, or para- 
sites, or on quite distinct causes, and so be altogether inde- 


pendent of the quantity of honey which the bees can collect. 
But let us suppose that this latter circumstance determined, 
as it probably often has determined, whether a bee allied to 
our humble-bees could exist in large numbers in any coun- 
try ; and let us further suppose that the community lived 
through the winter, and consequently required a store of 
honey : there can in this case be no doubt that it would be 
an advantage to our imaginary humble-bee if a slight modi- 
fication in her instincts led her to make her waxen cells near 
together, so as to intersect a little ; for a wall in common 
even to two adjoining cells would save some little labor and 
wax. Hence, it would continually be more and more advan- 
tageous to our humble-bees, if they were to make their cells 
more and more regular, nearer together, and aggregated into 
a mass, like the cells of the Melipona ; for in this case a 
large part of the bounding surface of each cell would serve 
to bound the adjoining cells, and much labor and wax would 
be saved. Again, from the same cause, it would be advan- 
tageous to the Melipona, if she were to make her cells closer 
together, and more regular in every way, than at present ; for 
then, as we have seen, the spherical surfaces would wholly 
disappear and be replaced by plane surfaces ; and the Meli- 
pona would make a comb as perfect as that of the hive-bee. 
Beyond this stage of perfection in architecture, natural selec- 
tion could not lead ; for the comb of the hive-bee, as far as 
we can see, is absolutely perfect in economizing labor and 

Thus, as I believe, the most wonderful of all known in- 
stincts, that of the hive-bee, can be explained by natural 
selection having taken advantage of numerous, successive, 
slight modifications of simpler instincts ; natural selection 
having, by slow degrees, more and more perfectly led the 
bees to sweep equal spheres at a given distance from each 
other in a double layer, and to build up and excavate the 
wax along the planes of intersection ; the bees, of course, no 
more knowing that they swept their spheres at one particu- 
lar 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 labor and 
wax ; that individual swarm which thus made the best cells 
With least labor, 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 of succeeding in the 
struggle for existence. 


It has been objected to the foregoing view of the origin 
of instincts, that "the variations of structure and of instinct 
must have been simultaneous and accurately adjusted to each 
other, as a modification in the one without an immediate 
corresponding change in the other would have been fatal." 
The force of this objection rests entirely on the assumption 
that the changes in the instincts and structure are abrupt. 
To take as an illustration the case of the larger titmouse 
(Parus major), alluded to in a previous chapter ; this bird 
often holds the seeds of the yew between its feet on a branch, 
and hammers with its beak till it gets at the kernel. Now 
what special difficulty would there be in natural selection 
preserving all the slight individual 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 nut-hatch, at the same time that 
habit, or compulsion, or spontaneous variations of taste, led 
the bird to become more and more of a seed-eater ? In this 
case the beak is supposed to be slowly modified by natural 
selection, subsequently to, but in accordance with, slowly 
changing habits or taste ; but let the feet of the titmouse 
vary and grow larger from correlation with the beak, or from 
any other unknown cause, and it is not improbable that such 
larger feet would lead the bird to climb more and more until 
it acquired the remarkable climbing instinct and power of 
the nut-hatch. In this case a gradual change of structure is 
supposed to lead to changed instinctive habits. To take one 
more case: few instincts 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 


ft to neglect other materials and to make its nest exclu- 
sively of inspissated saliva? And so in other cases. It 
must, however, be admitted that in many instances we cannot 
conjecture whether it was instinct or structure which first 

No doubt many instincts of very difficult explanation 
could be opposed to the theory of natural selection — cases,, 
in which we cannot see how an instinct could have ongi 
nated; cases, in which no intermediate gradations are known 
to exist ; cases of instincts of such trifling importance, that 
they could hardly have been acted upon 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 consequently must believe that they were independ- 
ently acquired through natural selection. I will not here 
enter on these several cases, but will confine myself to one 
special difficulty, which at first appeared to me insuper- 
able and actually fatal to the whole theory. I allude to the 
neuters or sterile females in insect communities ; for these 
neuters often differ widely in instinct and in structure from 
both the males and fertile females, and yet, from being 
sterile, they cannot propagate their kind. 

The subject well deserves to be discussed at great length, 
but I will here take only a single case, that of working or 
sterile ants. How the workers have been rendered sterile 
is a difficulty ; but not much greater than that of any 
other striking modification of structure ; for it can be 
shown that some insects and other articulate animals 
in a state of nature occasionally become sterile ; and if 
such insects had been social, and it had been profitable 
to the community that a number should have been annually 
born capable of work, but incapable of procreation, I can 
see no especial difficulty in this having been effected through 
natural selection. But I must pass over this preliminary diffi 
culty. The great difficulty lies in the working ants differ- 
ing widely from both the males and the fertile females in 
structure, as in the shape of the thorax, and in being desti- 
tute of wings and sometimes of eyes, and in instinct- As 
far as instinct alone is concerned, the wonderful difference 
in this respect between the workers and the perfect females 
would have been better exemplified by the hive-bee. If a 
working ant or other neuter insect had been an ordinary 
animal; I should have unhesitatingly assumed that aJ* it* 


characters had been slowly acquired through natural selec. 
tion ; namely, by individuals having been born with slight 
profitable modifications, which were inherited by the off- 
spring, and that these again varied and again were selected, 
and so onward. 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 pro- 
geny. It may well be asked how it is possible to reconcile 
this case with the theory of natural selection ? 

First, let it be remembered that we have innumerable in- 
stances, both in our domestic productions and in those in 
a state of nature, of all sorts of differences of inherited 
structure which are correlated with certain ages and with 
either sex. We have differences correlated not only with 
one sex, but with that short period when the 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 artificially imperfect state of the male 
sex. for oxen of certain breeds have longer horns than the 
oxen of other breeds, relatively to the length of the horns 
in both the bulls and cows of these same breeds. Hence, 
I can see no great difficulty in any character becoming cor- 
related with the sterile condition of certain members of 
insect communities ,- the difficulty lies in understanding how 
such correlated modifications of structure could have been 
slowly accumulated by natural selection. 

This difficulty, though appearing insuperable, is less- 
ened, or, as I believe, disappears, when it is remembered 
that selection may be applied to the family, as well 
as to the individual, and may thus gam the desired end- 
Breeders of cattle wish the flesh and fat to be well marbled 
together. An animal thus characterized has been slaugh- 
tered, 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 yield- 
ing oxen with extraordinarily long horns, could, it is prob- 
able, be formed by carefully watching which individual 
bulls and cows, when matched, produced oxen with the 
longest horns; and yet no one ox would ever have propa- 
gated its kind. Here is a better and real illustration: 
According to M, Verlot, some varieties of the double 
annual stock, from having been long and carefully selected 


to the right degree, always produce a large proportion oi 
seedlings bearing double and quite sterile flowers, but they 
likewise yield some single and fertile plants. These latter, 
by which alone the variety can be propagated, may be 
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 social 
insects, selection has been applied to the family, and not 
to the individual, for the sake of gaining a serviceable end. 
Hence, we may conclude that slight modifications of 
structure or of instinct, correlated with the sterile condi- 
tion of certain members of the community, have proved 
advantageous ; consequently the fertile males and females 
have flourished, and transmitted to their fertile offspring a 
tendency to produce sterile members with the same modifi- 
cations- This process must have been repeated many times, 
until that prodigious amount of difference between the 
fertile ard sterile females of the same species nas been 
producer which we see in many social instincts. 

But we have not as yet touched on the acme of the dif- 
ficulty ; namely, the fact that the neuters of several ants 
differ, not only from the fertile females and males, but 
from each other, sometimes to an almost incredible degree, 
and are thus divided into two or even three castes. The 
castes, moreover, do not commonly graduate into each 
other, but are perfectly well defined ; being as distinct from 
each other as 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 different : in Cryptocerus, the 
workers of one caste alone carry a wonderful sort of shield 
on their heads, the use of which is quite unknown ; in the 
Mexican Myrmecocystus, the workers of one caste never 
leave the nest; they are fed. by the workers of another 
caste, and they have an enormously developed abdomen 
which secretes a sort of honey, supplying the place of that 
excreted by the aphides, or the domestic cattle as they may 
be called, which our Eu. opean 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 diff erent from the fertile males and females 


through natural selection, we may conclude from the 
analogy of ordinary variations, that the successive, slight, 
profitable modifications did not first arise in all the neuters 
in the same nest, but in some lew alone ; and that by the 
survival of the communities with females which produced 
most neuters having the advantageous modification, ail 
the neuters ultimately came to be thus characterized. Ac- 
cording to this view we ought occasionally to find in the 
same nest neuter insects, presenting gradations of struc- 
ture ; and this we do find, even not rarely, considering how 
few neuter insects out of Europe have been carefully ex- 
amined. Mr. F. Smith has shown that the neuters of 
several British ants differ surprisingly from each other in 
size and sometimes in color ; 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, while 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 hat observed, the larger 
workers have simple eyes (ocelli), which, though small, can 
be plainly distinguished, whereas the smaller workers have 
their ocelli rudimentary. Having carefully dissected sev- 
eral specimens of these workers, I can affirm that the 
eyes are far more rudimentary in the smaller workers than 
can be accounted for merely by their proportionately 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 intermediate con- 
dition. I may digress by adding, that if the smaller 
workers had been the most useful to the community, and 
those males and females had been continually selected, 
which produced more and more of the smaller workers, 
until all the workers were in this condition, we should then 
have had a species of ant with neuters in nearly the same 
condition as those of Myrmica. For the workers of 
Myrmica have not even rudiments of oceli, though the male 
and female ants of this genus have well-developed oceli. 
J may give one other case : so confidently did I expect 


occasionally to find gradations of important structures 
between the different castes of neuters in the same species, 
that I gladly availed myself of Mr. F. Smith's offer of 
numerous 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 measurements, 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, moreover, of the working ants of the several 
sizes differed wonderfully in shape, and in the form and 
number of the teeth. But the important fact for us is 
that, though the workers can be grouped into castes of 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 " Naturalist on the Amazons," 
has described analogous cases. 

With these facts before me, I believe that natural selec- 
tion, by acting on the fertile ants or parents, could form a 
species which should regularly produce neuters, all of large 
size with one form of jaw, or all of small size with widely 
different jaws ; or lastly, and this is the greatest difficulty, 
one set of workers of one size and structure, and simulta- 
neously 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 M tiller, of certain Brazilian crustaceans 
likewise appearing under two widely distinct male forms. 
But this subject need not here be discussed. 

I have now explained how, I believe, the wonderful fact 
of two distinctly defined castes of sterile workers existing 


in the same nest, both widely different from each other 
and from their parents, has originated. We can see how 
useful their production may have been to a social commu- 
nity of ants, on the same principle that the division of 
labor is useful to civilized man. Ants, however, work by 
inherited instincts, and by inherited organs or tools, while 
man works by acquired knowledge and manufactured in- 
struments. But I must confess, that, with all my faith 
in natural selection, I should never have anticipated that 
this principle could have been efficient in so high a degree, 
had not the case of these neuter instincts 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 lias 
encountered. The case, also, is very interesting, as it 
proves that with animals, as with plants, any amount of 
modification may be effected by the accumulation of 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 of 
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 inherited habit, as ad- 
vanced by Lamarck. 


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


always absolutely perfect ana are liatfte to mistakes ; tnat 
no instinct een be shown to have been proauced for the 
good of other animals, though animals take advantage of 
the instincts of others; that the canon in natural history 
or "Natura non facrt saltum," is applicable to instincts as 
weii as to corporeal structure, and is plainly explicable on 
the foregoing views, but is otherwise inexplicable — all tend 
to corroborate the theory of natural selection. 

This theory is also strengthened by some few other factl 
in regard to instincts ; as by that common case of closel;- [ 
allied, but distinct, species, when inhabiting distant parts 
of the world and living under considerable different con- 
ditions of life, yet often retaining nearly the same instincts. 
For instance, we can understand, on the principle of inherit- 
ance, how it is that the thrush of tropical South America 
lines its nest with mud, in the same peculiar manner as 
does our British thrush ; how it is that the Hornbills of 
Africa and India have the same extraordinary instinct of 
plastering up and imprisoning the females in a hole in a 
tree, with only a small hole left in the plaster through 
which the males feed them and their young when hatched ; 
how it is that the male wrens (Troglodytes) of North 
America build "cock-nests," to roost in, like the males of 
our Kitty-wrens — a habit wholly unlike that of any other 
known bird. Finally, it may not be a logical deduction, 
but to my imagination it is far more satisfactory, to look at 
such instincts as the young cuckoo ejecting its foster- 
brothers, ants making slaves, the larvae of ichneumonidae 
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, affected by Close Inter- 
breeding, removed by Domestication — Laws governing the Steril- 
ity 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 — Parallel- 
ism 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 Universal — 
Hybrids and Mongrels compared independently of their Fertility 
— Summary. 

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

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

Pure species have of course their organs of reproduction 
in a perfect condition, yet when intercrossed they produce " 
either few or no offspring. Hybrids, on the other hand, 
have their reproductive organs functionally impotent, as 
may be clearly seen in the state of the male element in 
both plants and animals ; though the formative organs 
themselves are perfect in structure, as far as the micro- 
scope reveals. In the first case the two sexual elements 
which go to form the embryo are perfect $ in the second 


case they are either not at all developed, or are imperfectly 
developed. This distinction 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 sterilitj' in both cases being looked on as a 
special endowment, beyond the province of our reasoning 

The fertility of varieties, that is of the forms known 01 
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 importance 
with the sterility of species ; for it seems to make a broad 
and clear distinction between varieties and species. 


First, for the sterility of species when crossed and of 
their hybrid offspring. It is impossible to study the several 
memoirs and works of those two conscientious and admira- 
ble observers, Kolreuter and Gartner, who almost devoted 
their lives to this subject, without being deeply impressed 
with the high generality of some degree of sterility. Kol- 
reuter makes the rule universal ; but then he cuts the knot, 
for in ten cases in which he found two forms, considered by 
most authors as distinct species, quite fertile together, he 
unhesitatingly ranks them as varieties. Gartner, also, makes 
the rule equally universal ; and he disputes the entire fer- 
tility 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 ajy degree of sterility. He 
always compares the maximum number of seeds produced 
by two species when first crossed, and the maximum pro- 
duced 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 bo 
hybridized, 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 fertilized with their own 
pollen, and (excluding all cases such as the Leguininosse, in 


which there is an acknowledged difficulty in the manipula- 
tion) half of these twenty plants had their fertility in some 
degree impaired. Moreover, as Gartner repeatedly crossed 
some forms, such as the common red and blue pimpernels 
(Anagallis arvensis and ccerulea), which the best botanists 
rank as varieties, and found them absolutely sterile, we may 
doubt whether many species are really so sterile, when 
intercrossed, as he believed. 

It is certain, on the one hand, that the sterility of various 
species when crossed is so different in degree and graduates 
away so 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 experienced observers who have ever lived, namely, 
Kolreuter and Gartner, arrived at diametrically opposite 
conclusions in regard to some of the very same forms. It is 
also most instructive to compare — but I have not space 
here to enter 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 hybridizers, or by the 
same observer from experiments made during different 
years. It can thus be shown that neither sterility nor fer- 
tility 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 constitutional and structural differences. 

In regard to the sterility of hybrids in successive genera- 
tions; 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 generation s ? 
yet he asserts positively that their fertility never increases^ 
but generally decreases greatly and suddenly. With respect 
to this decrease, it may first be noticed that when any devia- 
tion in structure or constitution is common to both parents, 
this is often transmitted in an augmented degree to the off- 
spring; and both sexual elements in hybrid plants are 
already affected in some degree. But I believe that their 
fertility has been diminished in nearly all these cases by an 
independent cause, namely, by too close interbreeding. I 
have made so many experiments and collected so many 
facts, showing on the one hand that an occasional cross witli 


a distinct individual or variety increases the vigor and fer- 
tility of the offspring, and on the other hand that very close 
interbreeding lessens their vigor and fertility, that I cannot 
doubt the correctness of this conclusion. Hybrids are sel- 
dom 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 pre- 
vented during the flowering season : hence hybrids, if left to 
themselves, will generally be fertilized during each genera- 
tion by pollen from the same flower ; and this would prob- 
ably 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 fertilized 
with hybrid pollen of the same kind, their fertility, notwith- 
standing the frequent ill effects from manipulation, some- 
times decidedly increases, and goes on increasing. Now. in 
the process of artificial fertilization, pollen is as often taken 
by chance (as I know from my own experience) from the 
anthers of another flower, as from the anthers of the flower 
itself which is to be fertilized; 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 insured in each 
generation a cross with pollen from a distinct flower, either 
from the same plant or from another plant of the same 
hybrid nature. And thus, the strange fact of an increase of 
fertility in the successive generations of artificially fertilized 
hybrids, in contrast with those spontaneously self-fertilized, 
may, as I believe, be accounted for by too close interbreed- 
ing having been avoided. 

Now let us turn to the results arrived at by a third most 
experienced hybridizer, namely, the Hon. and Rev. W. 
Herbert. He is as emphatic in his conclusion that some 
hybrids are perfectly fertile — as fertile as the pure parent- 
species — as are Kolreuter and Gartner tha'. 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 horticul- 
tural 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 example, namely, that K every ovule in a 


pod of Crinum capense fertilized by C. revohitura produced 
a plant, which I never saw to occur in a case of its natural 
fecundation." So that here we have perfect, or even more 
than commonly perfect, fertility, in a first cross between 
two distinct species. 

This case of the Crinum leads me to refer to a singular 
fact, namely, that individual plants of certain species of 
Lobelia, Verbascum, and Passiflora, can easily be fertilized 
by the pollen from a distinct species, but not by pollen from 
the same plant, though this pollen can be proved to be 
perfectly sound by fertilizing other plants or species. In 
the genus Hippeastrum, in Corydalis, as shown by Professor 
Hildebrand, in various orchids as shown by Mr. Scott and 
Fritz Muller, all the individuals are in this peculiar condi- 
tion. So that with some species certain abnormal individ- 
uals, and in other species all the individuals, can actually be 
hybridized much more readily than they can be fertilized by- 
pollen from the same individual plant ! To give one 
instance, a bulb of Hippeastrum aulicum produced four 
flowers ; three were fertilized by Herbert with their own 
pollen, and the fourth was subsequently fertilized 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 matur- 
ity, and bore good seed, which vegetated 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 sometimes depends. 

The practical experiments of horticulturists, though not 
made with scientific precision, deserve some notice. It is 
notorious in how complicated a manner the species of 
Pelargonium, Fuchsia, Calceolaria, Petunia, Rhododendron, 
etc., have been crossed, yet many of these hybrids seed 
freely. For instance, Herbert asserts that a hybrid from 
Calceolaria integrifolia and plantaginea, species most widely 
dissimilar in general habit, " reproduces itself as 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 Rhododendrons, 
an I I am assured that many of them are perfectly fertile. 
&i- Q Noble, for instance, informs me that he raises stocks 


for grafting from a hybrid between Khod. 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 nurserymen. Horticulturists 
raise large beds of the same hybrid, and such alone are 
fairly treated, for by insect agency the several individuals 
are allowed to cross freely with each other, and the injurious 
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 most sterile kinds 
of hybrid Rhododendrons, which produce no pollen, for he 
will find on their stigmas plenty of pollen brought from 
other flowers. 

In regard to animals, much fewer experiments have been 
carefully tried than with plants. If our systematic arrange- 
ments 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 borm .n mind, that, 
owing to few animals breeding freely un'ie-v confinement, 
few experiments have been fairly tried ; for instance, the 
canary bird has been crossed with nine distinct species of 
finches, but, as not one of these breeds freely in confine- 
ment, we have no right to expect that the first crosses be- 
tween them and the canary, or that their hybrids, should be 
perfectly fertile. Again, with respect to the fertility in 
successive generations of the more fertile hybrid animals, I 
hardly know of an instance in which two families of the 
same hybrid have been raised at the same time from differ- 
ent parents, so as to avoid the ill effects of close inter- 
breeding. On the contrary, brothers and sisters have 
usually been crossed in each successive generation, in oppo- 
sition to the constantly 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 

Although I know of hardly any thoroughly well-authen- 
ticated cases of perfectly fertile hybrid animals, I have 
reason to believe that the hybrids from Cervulus vaginalis 
afld Keevesii, and from Phasianus colchicus with P. tor* 


quatus, are perfectly fertile. M. Quatrefages states that 
the hybrids from two moths (Bombyx cynthia and arrindia) 
yere proved in Paris to be fertile Inter se for eight genera- 
tions. It has lately been asserted that two such distinct 
species as the hare and rabbit, when they can be got to 
breed together, produce offspring which are highly fertile 
when crossed with one of the parent-species. The hybrids 
from the common and Chinese geese (A. cygnoides), species, 
which are so different that they are generally ranked in 
distinct genera, have often bred in this country with either 
pure parent, and in one single instance they have bred 
inter se. This was effected by Mr. Eyton, who raised two 
hybrids from the same parents, but from different hatches; 
and from these two birds he raised no less than eight 
hybrids (grandchildren of the pure geese) from one nest. 
In India, however, these cross-bred geese must be far more 
fertile ; for I am assured by two eminently capable judges, 
namely, Mr. Blyth and Captain Hutton, that whole Hocks 
of these crossed geese are kept in various parts of the coun- 
try; 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 aboriginal parent- 
speciec ^t first produced perfectly fertile hybrids, or that 
the hybnJs subsequently reared under domestication be- 
came quite fertile. This latter alternative, which was first 
propounded by Pallas, seems by far the most probable, and 
can, indeed, hardly be doubted. It is, for instance, almost 
certain that our dogs are descended from several wild stocks ; 
yet, with perhaps the exception of certain indigenous domes- 
tic dogs of South America, all are quite fertile together ; 
but analogy makes me greatly doubt, whether the several 
aboriginal species would at first have freely bred together 
and have produced quite fertile hybrids. So again I have 
lately acquired decisive evidence that the crossed offspring 
from the Indian humped and common cattle are inter se 
perfectly fertile ; and from the observations by R itimeyer on 
their important osteological differences, as well as from those 
by Mr. Blyth on their differences in habits, voice, constitu- 
tion, etc., these two forms must be regarded as good and 
distinct species. The same remarks may be extended to 


ihe 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 inter- 
crossing 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 considered as absolutely 



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, 
m order tc prevent their crossing and blending together in 
utter confusion. The following conclusions are drawn up 
chiefly from Gartner's admirable work on the hybridization 
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 sur- 
prised to find how generally the same rules apply to both 

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 onlv the barest outline of 
the facts can here be given. When pollen from a plant of 
one family is placed on the stigma of a plant of a distinct 
family, it exerts no more influence than so much inorganic dust. 
From this absolute zero of fertility, the pollen of different 
species applied to the stigma of some one species of the 
same genus, yields a perfect gradation in the number of 
seeds 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 fer» 


tility may be detected, by the pollen of one of the pure 
parent-species causing the flower of the hybrid to wither 
earlier than it otherwise would have done ; and the early 
withering of the flower is well known to be a sign of incip- 
ient fertilization. From this extreme degree of sterility we 
have self-fertilized hybrids producing a greater and greater 
number of seeds up to perfect fertility. 

The hybrids raised from two species which are very diffi- 
cult to cross, and which rarely produce any offspring, are 
generally very sterile ; but the parallelism between the diffi- 
culty of making a first cross, and the sterility of the hybrids 
thus produced — two classes of facts which are generally 
confounded together — is by no means strict. There are 
many cases, in which two pure species, as in the genus Ver- 
bascum, 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 with- 
in the limits of the same genus, for instance in Dianthus, 
these two opposite cases occur. 

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

By the term systematic affinity is meant, the general re- 
semblance between species in structure and constitution. 
Now the fertility of first crosses, and of the hybrids pro- 
duced from them, is largely governed by their systematic 
affinity. This is clearly shown by hybrids never having 
been raised between species ranked by systematists in dis- 
tinct families ; and on the other hand, by very closely allied 
species generally uniting with facility. But the correspond- 
ence between systematic affinity and the facility of crossing 
is' by no means strict. A multitude of cases could be gi ven 
of very closely allied sp«cies which will not unite, or only 
with extreme difficulty \ and ou the othec hand of very dia* 


ttnct species which unite with the utmost facility. Tn the 
same family there may be a genus, as Dianthus, in which 
very many species can most readily be crossed; and another 
^enus, as Silene, in which the most persevering efforts have 
tailed to produce between extremely olose species a single 
nybrid. Even within the limits of the same genus, we meet 
with *,his same difference; for instance, the many species of 
Nicniana 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, ob- 
stinately failed to fertilize, or to be fertilized by, no less 
than eight other species of Nicotiana. Many analogous 
facts could be given. 

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

By a reciprocal cross between two species, I mean the 
case, for instance, of a female ass being first crossed by a 
stallion, and then a mare by a male ass ; these two specie? 
may then be said to have been reciprocally crossed. There 
is often the widest possible difference in the facility of mak- 
ing reciprocal crosses. Such cases are highly important, for 
they prove that the capacity in any two species to cross is 
often completely independent of their systematic affinity, 
that is, of any difference in their structure or constitution, 
excepting in their reproductive systems. The diversity of 
the result in reciprocal crosses between the same two species 
was long ago observed by Kolreuter. To give an instance ; 
Mirabilis jalapa can easily be fertilized by the pollen of M. 
iongiflora, and the hybrids thus produced are sufficiently 
fertile ; but Kolreuter tried more than two hundred times, 
during eight following years, to fertilize reciprocally M. 
Iongiflora with the pollen of M. jalapa, and utterly failed. 
Several other equally striking cases could be given. Thuret 
has observed the same fact with certain sea-weeds or Fuci. 
Gartner, moreover, found that this difference of facility in 
making reciprocal crosses is extremely common in a lesser 


degree. He has observed it even between closely related 
forms (as Matthiola annua and glabra) whitfh 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 species 
having first been used as the father and then as the mother, 
though they rarely differ in external characters, yet generally 
differ in fertility in a small, and occasionally in a high, 

Several other singular rules could be given from Gartner : 
for instance, some species have a- remarkable power of cross- 
ing 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 among hybrids which are usually interme- 
diate in structure between their parents, exceptional and 
abnormal individuals sometimes are born, which closely re- 
semble 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 sus- 
ceptible to favorable and unfavorable conditions, is innately 
variable ; that it is by no means always the same in degree 
in the first cross and in the hybrids produced from this cross; 
that the fertility of hybrids is not related to the degree in 
which they resemble in external appearance either parent ; 
and lastly, that the facility of making a first cross between 
any two species is not always governed by their systematic 
affinity or degree of resemblance to each other. This latter 
statement is clearly proved by the difference in the result of 
reciprocal crosses between the same two species, for, accords 


ing as the one species or the other is used as the father or the 
mother, there is generally some difference, and occasionally 
the widest possible difference, in the facility of effecting an 
union. The hybrids, moreover, produced from reciprocal 
crosses often differ in fertility. 

Now, do these complex and singular rules indicate that 
species have been endowed with sterility simply to prevent 
their becoming confounded in nature ? I think not. For 
why should the sterility be so extremely different in degree, 
when various species are crossed, all of which we must sup- 
pose it would be equally important to keep from blending 
together ? Why should the degree of sterility be innately 
vstriable in the individuals of the same species ? Why 
sl.ould some species cross with facility, and yet produce very 
sterile hybrids ; and other species cross with extreme diffi- 
culty, 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 per- 
mitted ? To grant to species the special power of producing 
hybrids, and then to stop their further propagation by dif- 
ferent degrees of sterility, not strictly related to the facil- 
ity of the first union between their parents, seems a strange 

The foregoing rules and facts, on the other hand, appear- 
to me clearly to indicate that the sterility, both of first 
crosses and of hybrids, is simply incidental or dependent 
on unknown differences in their reproductive systems ; the 
differences being of so peculiar and limited a nature, that, 
in reciprocal crosses between the same two species, the male 
sexual element of the one will often freely act on the female 
sexual element of the other, but not in a reversed direction. 
It will be advisable 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 
unimportant for their welfare in a state of nature, I presume 
that no one will suppose that this capacity is a specially 
endowed quality, but will admit that it is incidental on dif- 
ferences 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, etc. \ but in a multitude of cases we can assign 


no reason whatever. Great diversity in the size of two 
plants, one being woody and the other herbaceous, one being 
evergreen and the other deciduous, and adaptation to widely 
different climates, do not always prevent the two grafting 
together. As in hybridization, so with grafting, the capa- 
city 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 hybridization, 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 apple, which is a member of the same 
genus. Even different varieties of the pear take with differ- 
ent degrees 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 differ- 
ent individuals of the same two species in being grafted 
together. As in reciprocal crosses, the facility of effecting 
an union is often very far from equal, so it sometimes is in 
grafting. The common gooseberry, for instance, cannot be 
grafted on the currant, whereas the currant will take, though 
with difficulty, on the gooseberry. 

We have seen that the sterility of hybrids which have 
their reproductive organs in an imperfect condition, is a 
different case from the difficulty of uniting two pure species 
which have their reproductive organs perfect ; yet these two 
distinct classes of cases run to a large extent parallel. Some- 
thing analogous occurs in grafting; for Thouin found that 
three species of Robinia, which seeded freely on their own 
roots, and which could be grafted with no great difficulty on 
a fourth species, when thus grafted were rendered barren. 
On the other hand, certain species of Sorbus, when grafted 
on other species, yielded twice as much fruit as when on 
their own roots. We are reminded by this latter fact of the 
extraordinary cases of hippeastrum, passiflora, etc., which 
seed much mors freely when fertilized with the pollen of a 
distinct species than when fertilized with pollen from the 
same plaui. 


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



At one time it appeared to me probable, as it has to others, 
that the sterility of first crosses and of hybrids might have 
been slowly acquired through the natural selection of slightly 
lessened degrees of fertility, which, like any other variation, 
spontaneously appeared in certain individuals of one variety 
when crossed with those of another variety. For it would 
clearly be advantageous 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 dis- 
tinct regions are often sterile when crossed ; now it could 
clearly have been of no advantage to such separated species 
to have been rendered mutually sterile, and consequently 
this could not have been effected through natural selection ; 
but it may perhaps be argued, that, if a species was rendered 
sterile with some one compatriot, sterility with other species 
would follow as a necessary contingency. In the second 
place, it is almost as much opposed to. the theory of natural 


selection as to that of special creation, that in reciprocal 
crosses the male element of one form should have been ren- 
dered utterly impotent on a second form, while at the same 
time the male element of this second form is enabled freely 
to fertilize the first form ; for this peculiar state of the 
reproductive system could hardly have been advantageous to 
either species. 

In considering the probability of natural selection having 
come into action, in rendering species mutually sterile, the 
greatest 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 incipient species, if it were rendered in some slight degree 
sterile when crossed with its parent form or with some other 
variety ; for thus fewer bastardized and deteriorated off- 
spring would be produced to commingle their blood with 
the new species in process of formation. But he who will 
take the trouble to reflect on the steps by which this first 
degree of sterility could be increased through natural selec- 
tion to that high degree which is common with so many spe- 
cies, 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 favor 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 toward absolute sterility ? Yet an advance of 
this kind, if the theory of natural selection be brought to 
bear, must have incessantly occurred with many species, for 
a multitude are mutually quite barren. With sterile neuter 
insects we have reason to believe that modifications in their 
structure and fertility have been slowly accumulated by 
natural selection, from an advantage having been thus indi 
rectly given to the community to which they belonged over 
other communities of the same species ; but an individual 
animal not belonging to a social community, if rendered 
slightly sterile when crossed with some other variety, would 
not thus itself gain any advantage or indirectly give any 
advantage to the other individuals of the same variety, thus 
leading to their preservation. 

But it would be superfluous to discuss this question in 


detail : for with plants we have conclusive evidence that 
the sterility of crossed species must be due to some princi- 
ple, quite independent of natural selection. Both Gartner 
and Kolreuter have proved that in genera including numer- 
ous 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 throughout 
the animal and vegetable kingdoms, we may infer that the 
cause, whatever it may be, is the same or nearly the same in 
all cases. 

We will now look a little closer at the probable nature 
of the differences between species which induce sterility in 
first crosses and in hybrids. In the case of first crosses, 
the greater or less difficulty in effecting an union and in 
obtaining offspring apparently depends on several distinct 
causes. There must sometimes be a physical impossibility 
in the male element reaching the ovule, as would be the 
case with a plant having a pistil too long for the pollen- 
tubes to reach the ovarium. It has also been observed that 
when the pollen of one species 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 inca- 
pable of causing an embryo to be developed, as seems to 
have been the case with some of Thuret's experiments on 
Fuci. No explanation can be given of these facts, any 
more than why certain trees cannot be grafted on others. 
Lastly, an embryo may be developed, and then perish at an 
early period. This latter alternative has not been suf- 
ficiently attended to ; but I believe, from observations com- 
municated to me by Mr. Hewitt, who has had great ex- 
perience in hybridizing 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 fertilized j and in the 


majority of the fertilized eggs, the embryos had either been 
partially developed and had then perished, or had become 
nearly mature, Out 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, "without any obvious cause, apparently from 
mere inability to live ; " so that from the 500 eggs only 
twelve chickens were reared. With plants, hybridized em- 
bryos probably often perish in a like manner; at least it is 
known that hybrids raised from very distinct species are 
sometimes weak and dwarfed, and perish at an early age ; 
of which fact Max Wichura has recently given some strike 
ing 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 fertilized, 
pass through their early stages of development and then 
perish like the embryos produced by a cross betwee.i dis- 
tinct species. Until becoming acquainted with these facts, 
I was unwilling to believe in the frequent early death of 
hybrid embryos ; for hybrids, when once born, are generally 
healthy and long-lived, as we see in the case of the common 
mule. Hybrids, however, are differently circumstanced 
before and after birth : when born and living in a com. try 
where their two parents live, they are generalty placed under 
suitable conditions of life. But a hybrid partakes of only 
half of the nature and constitution ot its mother; it may 
therefore, before birth, as long as it is nourished within its 
mother's womb, or within the egg or seed produced by the 
mother, be exposed to conditions in some degree unsuitable, 
and consequently be liable to perish at an early 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 tae embryo to 
be imperfectly developed, rather than in the conditions to 
which it is subsequently exposed. 

In regard to the sterility of hybrids, in which the sexuai 
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 thy 
sterility is independent 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 affec f ed ; but some- 
times the female more than the male. In botn, the tendency 
goes to a ceicain extent with systematic affinity, for ''hole 
groups of animals and plants are rendered impotent ^y 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 fertility ; and certain species in 
a group will produce unusually fertile hybrids. No one can 
tell till he tries, whether any particular animal will breed 
under confinement, or any exotic plant seed freely under 
culture ; nor can he tell till he tries, whether any two 
species of a genus will produce more or less sterile hybrids. 
.Lastly, when organic beings are placed during several gen- 
erations under conditions not natural to them, they are 
extremely liable tc 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 

Thus we see that when organic beings are placed under 
new and unnatural conditions, and when hybrids are pro- 
duced by the unnatural crossing of two species, the repro- 
ductive 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 disturbed, though often 
in so slight a degree as to be inappreciable by us ; in the 
other case, or that of hybrids, the external conditions have 
remained the same, but the organization has been disturbed 
by two distinct structures and constitutions, induing 
of course the reproductive systems, having been blended 
into one. For it is scarcely possible that two organizations 
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 offspring from generation to 
generation the same compounded organization, and hence we 
£eed not be surprised that their sterility, though in some 


degree variable, does not diminish ; it is even apt to 
increase, this being generally the result, as before explained, 
of too close interbreeding. The above view of the sterility 
of hybrids being caused by two constitutions being com- 
pounded into one has been strongly maintained by Max 

It must, however, be owned that we cannot understand, 
on the above or any other view, several facts with respect 
to the sterility of hybrids ; for instance, the unequal fer- 
tility of hybrids produced from reciprocal crosses; or the 
increased sterility in those hybrids which occasionally and 
exceptionally resemble closely either pure parent. Nor do 
I pretend that the foregoing remarks go to the root of the 
matter; no explanation is offered why an organism, when 
placed under unnatural 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 organization having been disturbed 
by two organizations 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 evidence, which I 
have elsewhere given, that slight changes in the conditions 
of life are beneficial to all living things. We see this acted 
on by farmers and gardeners in their frequent exchanges of 
seed, tubers, etc., from one soil or climate to another, and 
back again. During the convalescence of animals, great 
benefit is derived from almost any change in their habits of 
life. Again, both with plants and animals, there is the 
clearest evidence that a cross between individuals of the 
same species, which differ to a certain extent, gives vigor 
and fertility to the offspring ; and that close interbreeding 
continued during several generations between the nearest 
relations, if these be kept under the same conditions of life, 
almost always leads to decreased size, weakness, or sterility. 

Hence it seems, that, on the one hand, slight changes in 
the conditions of life benefit all organic beings, and on the 
other hand, that slight crosses, that is, crosses between the 
males and females of the same species, which have been 
subjected to slightly different conditions, or which have 
slightly varied, give vigor and fertility to the offspring. 
But, as we have seen, organic beings long habituated to 
certain uniform conditions under a state of nature, when 


subjected, as under confinement, to a considerable change in 
their conditions, very frequently are rendered more or less 
sterile ; and we know that a cross between two forms that 
have become widely or specifically different, produce hybrids 
which are almost always in some degree sterile. I am fully 
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 generally sterile. He will at the same time 
be able to explain how ; ~ is that the races of some of our 
domesticated animals, which have often been subjected to 
new and not uniform conditions, are quite fertile together, 
although they are descended from distinct species, which 
would probably have been sterile if aboriginally crossed. 
The above two parallel series of facts seem to be connected 
together by some common but unknown bond, which is essen- 
tially related to the principle of life ; this principle, accord- 
ing 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 
toward an equilibrium ; and when this tendency is slightly 
disturbed by any change, the vital forces gain in power. 


This subject may be here briefly discussed, and will be 
found to throw some light on hybridism. Several plants 
belonging to distinct orders present two forms, which exist 
in about equal numbers and which differ in no respect except 
in their reproductive organs ; one form having a long pistil 
with short stamens, the other a short pistil with long 
stamens ; the two having differently sized pollen-grains. With 
trimorphic plants there are three forms likewise differing in 
the lengths of their pistils and stamens, in the size ana color 
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 ana three 
kinds of pistils. These. organs are so proportioned in length 
to each other that half the stamens in two of the forms 
stand on a level with the stigma of the third form. Now I 
have shown, and the result has been confirmed by other 


observers, that in order to obtain full fertility with these 
plants, it is necessary that the stigma of the one form should 
be fertilized by pollen taken from the stamens of correspond- 
ing height in another form. So that with dimorphic species 
two unions, which may be called legitimate, are fully fertile ; 
and two, which may be called illegitimate, are more or less 
infertile. With trimorphic species six unions are legiti- 
mate, or fully fertile, and twelve are illegitimate, or more 
or less infertile. 

The infertility which may be observed in various dimor- 
phic and trimorphic plants, when they are illegitimately 
fertilized, that is, by pollen taken from stamens not corre- 
sponding in height with the pisuiJ, differs much in degree, 
up to absolute and utter sterility ; just in the same manner 
as occurs in crossing distinct species. As the degree of 
sterility in the latter case depends in an eminent degree on 
the conditions of life being more or less favorable, 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 afterward, even after a 
considerable interval of time, placed on the same stigma, its 
action is so strongly prepotent that it generally annihilates 
the effect of the foreign pollen ; so it is with the pollen of 
the several forms of the same species, for legitimate pollen 
is strongly prepotent over illegitimate pollen, when both are 
placed on the same stigma. I ascertained this by fertilizing 
several flowers, first illegitimately and twenty-four hours 
afterward legitimately, with pollen taken from a peculiarly 
colored variety, and all the seedlings were similarly colored ; 
this shows that the legitimate pollen, though applied twenty- 
four hours subsequently, had wholly destroyed or pre- 
vented the action of the previously applied illegitimate 
pollen. Again, as in making reciprocal crosses between the 
same two species, there is occasionally a great difference in 
the result, so the same thing occurs with trimorphic plants ; 
for instance, the mid-styled form of Lythrum salicaria was 
illegitimately fertilized 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 fertilized by the longer stamens of the mid-styled form. 

In all these respects, and in others which might be added, 
the forms of the same undoubted species, when illegiti- 
mately 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 sev- 
eral illegitimate unions. The chief result is that these 
legitimate 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 
fertilized. 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 illegiti- 
mate plants, when united with each other in a 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 fertilized 
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> 
that sterility of certain illegitimate plants was unusually 
great, while 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, while other and more sterile hybrids produce few 
flowers, and are weak, miserable dwarfs ; exactly similar 
cases occur with the illegitimate offspring of various dimor- 
phic and trimorphic plants. 

Altogether there is the closest identity in character and 
behavior 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, while 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 respects between first 
illegitimate unions and first crosses between distinct species. 
This will perhaps be made more fully apparent by an illus- 
tration; 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. Hf» 
would find that they yielded only about one-fifth of the 
proper number of seed, and that they behaved in all the 
other above specified respects as if they had been two dis- 
tinct species. But to make the case sure, he would raise 
plants from his supposed hybridized seed, and he would find 
that the seedlings were miserably dwarfed and utterly sterile, 
and that they behaved in all other respects like ordinary 
hybrids. He might then maintain that he had actually 
proved, in accordance with the common view, that his two 
varieties were as good and as distinct species as any in the 
world ; but he would be completely mistaken. 

The facts now given on dimorphic and trimorphic plants 
are important, because they show us, first, that the physio- 
logical 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 infertility of illegitimate unions with 
that of their illegitimate offspring, and we are led to extend 
the same view to first crosses and hybrids ; thirdly, because 
we find, and this seems to me of especial importance, that 
two or three forms of the same species may exist and may 
differ in no respect whatever, either in structure or in con- 
stitution, 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 ; while it is the union of the sexual ele- 
ments 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, doubtful whether this is really so ; but I will not 
enlarge on this obscure subject. 

We may, however, infer as probable from the considera- 
tion 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 con- 
stitution. We are also led to this same conclusion by. con- 
sidering 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, while the converse cross can 
b<* effected with perfect facility. That excellent observe^ 


Gartner, likewise concluded that species when crossed are 
sterile owing to differences confined to their reproductive 


It may be urged as an overwhelming argument that there 
must be some essential distinction between species and 
rarieties, inasmuch as the latter, however much they may 
differ from each other in external appearance, cross with 
perfect facility, and yield perfectly fertile offspring. With 
some exceptions, presently to be given, I fully admit that, 
this is the rule. But the subject is surrounded by difficul- 
ties, for, looking to varieties produced under nature, if two 
forms hitherto reputed to be varieties D3 found in any 
degree sterile together, they are at once ranked by most 
naturalists as species. For instance, the blue and red pim- 
pernel, which are considered by most botanists as varieties, 
are said by Gartner to be quite sterile when crossed, and he 
consequently ranks them as undoubted species. If we thus 
argue in a circle, the fertility of all varieties produced under 
nature will assuredly have to be granted. 

If we turn to varieties, produced, or supposed to have 
been produced, under domestication, we are still involved 
in some doubt. For when it is stated, for instance, that 
certain South American indigenous domestic dogs do not 
readily unite with European dogs, the explanation which 
will occur to every one, and probably the true one, is 
that they are descended from aboriginally distinct species. 
Nevertheless the perfect fertility of so many domestic races, 
differing widely from each other in appearance, for instance, 
those of the pigeon, or of the cabbage, is a remarkable fact ; 
more especially when we reflect how many species there 
are, which, though resembling each other most closely, are 
utterly sterile when intercrossed. Several considerations, 
however, render the fertility of domestic varieties less 
remarkable. In the first place, it may be observed that 
the amount of external difference between two species is 
no sure guide to their degree of mutual sterility, so that 
similar differences in the case of varieties would be no sure 
guide. It is certain that with species the cause lies exclu- 
sively in differences in their sexual constitution. Now the 
varying conditions to which domesticated animals and cul« 


tivated plants have been subjected, have had so little ten* 
dency toward modifying the reproductive system in a uiannei 
leading to mutual sterility, that we have good grounds foi 
admitting 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 fertile together. With 
plants, so far is cultivation from giving a tendency toward 
sterility between distinct species, that in several well-authen- 
ticated cases already alluded to, certain plants have been 
affected in an opposite manner, for they have become self- 
impotent, while still retaining the capacity of fertilizing, 
and being fertilized by, other species. If the Pallasian 
doctrine of the elimination of sterility through long-con- 
tinued 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 gener- 
ally 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 igno- 
rant we are in regard to the normal and abnormal action 
of the reproductive system. But we can see that species, 
owing to their struggle So* existence with numerous compet- 
itors, will have been exposed, 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 cap- 
tivity, are rendered sterile ; and the reproductive functions 
of organic beings which have always lived under natural 
conditions would probably in like manner be eminently 
sensitive to the influence of an unnatural cross. Domesti- 
cated productions, on the other hand, which, as shown by 


tRe mere fact of their domestication, were not originally 
highly sensitive to changes in their conditions of life, and 
which can now generally resist with undiminished fertility 
repeated changes of conditions, might be expected to pro- 
duce varieties, which would be little liable to have their 
reproductive powers injuriously affected by the act of cross- 
ing with other varieties which had originated in a like 

I have as yet spoken as if the varieties of the same 
"tpecies 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 wit- 
nesses, who in all other cases consider fertility and sterility 
as safe criterions of specific distinction. Gartner kept, 
during several years, a dwarf kind of maize with yellow 
seeds, and a tall variety with red seeds growing near each 
other in his garden ; and although these plants have separ- 
ated sexes, they never naturally crossed. He then fertilized 
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 separated sexes. 
No one, I believe, has suspected that these varieties of maize 
are distinct species ; and it is important to notice that the 
hybrid plants thus raised were themselves perfectly fertile; 
so that even Gartner did not venture to consider the two 
varieties as specifically distinct. 

Girou de Buzareingues crossed three varieties of gourd> 
which like the maize has separate sexes, and he asserts that 
their mutual fertilization 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 classifi- 
cation by the test of infertility, as varieties, and Naadin 
has come to the same conclusion. 

The following case is far more remarkable, and seems at 
first incredible ; but it is the result of an astonishing num- 
ber of experiments made during many years on nine species 
of Verbascum, by so good an observer and so hostile a wit- 
ness as Gartner : namely, that the yellow and white varie- 
ties when crossed produce less seed than the similarly 


colored varieties of the same species. Moreover, he asserts 
that, when yellow and white varieties of one species are 
crossed with yellow and white varieties of a distinct species, 
more seed is produced by the crosses between the similarly 
colored flowers, than between those which are differently 
colored. Mr. Scott also has experimented on the species 
and varieties of Verbascum ; and although unable to confirm 
Gartner's results on the crossing of the distinct species, 
he finds that the dissimilarly colored varieties of the same 
species yield fewer seeds, in the proportion of eighty-six 
to one hundred, than the similarly colored varieties. Yet 
these varieties differ in no respect, except in the color 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 fer- 
tile than the other varieties, when crossed with a widely 
distinct species. He experimented on five forms whicn are 
commonly reputed to be varieties, and which he tested by 
the severest trial, namely, by reciprocal crosses, and he 
found their mongrel offspring perfectly fertile. But one ol 
these five varieties, when used either as the father or mother, 
and crossed with the Nicotian a 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 vari^ 
eties when crossed are invariably quite fertile. From the 
great difficulty of ascertaining the infertility of varieties in 
a state of nature, for a supposed variety, if proved to be 
infertile in any degree, would almost universally be ranked 
as a species ; from man attending only to external charac- 
ters in his domestic varieties, and from such varieties not 
having been exposed for very long periods to uniform condi- 
tions of life ; from these several considerations we may com 
elude that fertility does not constitute a fundamental dis- 
tinction between varieties and species when crossed. The 
general sterility of crossed species may safely be looked at, 
not as a special acquirement or endowment, but as inciden- 
tal on changes of an unknown nature in their sexual elements. 




Independently of the question of fertility, the offspring 
of species and of varieties when crossed may be compared in 
several other respects. Gartner, whose strong wish it was to 
draw a distinct line between species and varieties, could find 
very few, and, as it seems to me, quite unimportant differ- 
ences 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 

I shall here discuss this subject with extreme brevity. 
The most important distinction is, that in the first genera- 
tion mongrels are more variable than hybrids ; but Gartner 
admits that hybrids from species which have long been cul- 
tivated 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 spe- 
cies are more variable than those from very distinct species } 
and this shows that the difference in the degree of vanabil- 
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 noto- 
rious ; but some few instances of both hybrids and mongrels 
long retaining a uniform character could be given. The 
variability, however, in the successive generations of mon- 
grels 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 experi- 
ments having been tried on natural varieties), and this 
implies that there has been recent variability, which would 
often continue and would augment that arising from the act 
of crossing. The slight variability of hybrids in the first 
generation, in contrast with that in the succeeding genera- 
tions, is a curious fact and deserves attention. For it bears 
on the view which I have taken of one of the causes of 
ordinary variability, namely, that the reproductive system, 
from being eminently sensitive to changed conditions of life, 
fails under these circumstances to perform its proper func- 
tion of producing offspring closely similar in all respects to 
the parent form. Now, hybrids in the first generation are 


descended from species (excluding those long cultivated) 
which have not had their reproductive systems in any way 
affected, and they are not variable ; but hybrids themselves 
have the 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 that the 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, while Naudin, on the other hand, insists 
in the strongest terms on the almost universal tendency to 
reversion in hybrids, and he experimented chiefly on culti- 
vated 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 differ- 
ent 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 opposed to the results of several experiments made 
by Kolreuter. 

Such alone are the unimportant differences which Gartner 
is able to point out between hybrid and mongrel plants. 
On the other hand, the degrees and kinds of resemblance in 
mongrels and in hybrids to their respective parents, more 
especially in hybrids produced 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 
eertainly 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 re- 
peated crosses in successive generations with either parent. 

These several remarks are apparently applicable to ani- 
mals, but the subject is here much complicated, partly owing 


to the existence of secondary sexual characters, but more 
especially owing to prepotency in transmitting likeness run- 
ning 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 
prepotency runs more strongly m the male than in the 
female ass, so that the mule, which is an offspring of the 
male ass and mare, is more like an ass than is the hinny, 
which is the offspring of the female ass and stallion. 

Much stress has been laid by some authors on the sup- 
posed 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 
sudden reversions to the perfect character of either parent 
would, also, be much more likely to occur with mongrels, 
which are descended from varieties often suddenly 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 resem- 
blance 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 differ- 
ent varieties, or of distinct species. 

Independently of the question of fertility and sterility, in 
all other respects there seems to be a general and close simi- 
larity 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 produced by sec- 
ondary laws, this similarity would be an astonishing fact. 
But it harmonizes perfectly with the view that there is no 
essential distinction between species and varieties. 



First crosses between forms, sufficiently distinct to be 
ranked as species, and their hybrids, are very generally, 
but not universally, sterile. The sterility is of all degrees, 
and is often so slight that the most careful experimental- 
ists have arrived at diametrically opposite conclusions in 
ranking forms by this test. The sterility is innately vari- 
able in individuals of the same species, and is eminently 
susceptible to action of favorable and unfavorable condi- 
tions. The degree of sterility does not strictly follow 
systematic affinity, but is governed by several curious and 
complex laws. It is generally different, and sometimes 
widely different, in reciprocal crosses 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 capacity in 
one species or variety to take on another, is incidental on 
differences, generally of an unknown nature, in their vege- 
tative systems, so in crossing, the greater or less facility of 
one species to unite with another is incidental on unknown 
differences in their reproductive systems. There is no 
more reason to think that species have been specially en- 
dowed with various degrees of sterility to prevent their 
crossing and blending in nature, than to think that trees 
have been specially endowed with various and somewhat 
analogous degrees of difficulty in being grafted together in 
order to prevent their inarching in our forests. 

The sterility of first crosses and of their hybrid progeny 
has not been acquired through natural selection. In the 
case of first crosses it seems to depend on several circum- 
stances ; in some instances, in chief part on the early death 
of the embryo. In the case of hybrids, it apparently de- 
pends on their whole organization having been disturbed 
by being compounded from two distinct forms ; the ster- 
ility being closely allied to that which so frequently affects 
pure species, when exposed to new and unnatural condi- 
tions 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 tli3 vigor and fertility of all organic beings ; and 
secondly, that the crossing of forms which have been ex« 


posed to slightly different conditions of life, or which have 
varied, favors the size, vigor, and fertility of their offspring. 
The facts given on the sterility of the illegitimate unions 
of dimorphic and trimorphic plants and of their illegitimate 
progeny, perhaps render it probable that some unknown 
bond in all cases connects the degree of fertility of first; 
unions with that of their offspring. The consideration of 
these facts on dimorphism, as well as of the results of re- 
ciprocal crosses, clearly leads to the conclusion that th^ 
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 gen' 
erally have become more or less modified, leading to thei? 
mutual infertility, we do not know ; but it seems to stano* 
in some close relation to species having been exposed fo* 
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 be' 
tween the species which are crossed. Nor is it surprising 
that the facility of effecting a first cross, and the fertility 
of the hybrids thus produced, and the capacity of being 
grafted together — though this latter capacity evidently 
depends on widely different circumstances — should all run, 
to a certain extent, parallel with the systematic affinity of 
the forms subjected to experiment; for systematic affinity 
includes resemblances of all kinds. 

First crosses between forms known to be varieties, or 
sufficiently alike to be considered as varieties, and their 
mongrel offspring, are very generally — but not, as is so 
often stated, invariably — fertile. Nor is this almost uni- 
versal and perfect fertility surprising, when it is remem- 
bered 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 lon^ exposed to uniform con- 
ditions of life. It should also be especially kept in mind, 
that long-continued domestication tends to eliminate ster- 
ility, and is therefore little likely to induce this same 
quality. Independently of the question of fertility, in all 
other respects there is the closest general resemblance be- 
tween hybrids and mongrels, in their variability, in theif 


power of absorbing each other by repeated crosses, and in 
their inheritance of characters from both parent forms. 
Finally, then, although we are as ignorant of the precise 
cause of the sterility of first crosses and of hybrids as we 
are why animals and plants removed from their natural con- 
ditions become sterile, yet the facts given in this chapter do 
not seem to me opposed to the belief that species aboriginally 
existed as varieties. 




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 Intermit- 
tence 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 objections 
which might be justly urged against the views maintained in 
this volume. Most of them have now been discussed. One, 
namely, the distinctness of specific forms and their not being 
blended together by innumerable transitional links, is a very 
obvious difficulty. I assigned reasons why such links do not 
commonly occur at the present day under the circumstances 
apparently most favorable for their presence, namely, on an 
extensive and continuous area with graduated physical con- 
ditions. I endeavored to show that the life of each species 
depends in a more important manner on the presence of other 
already defined organic forms, than on climate, and, therefore, 
that the really governing conditions of life do not graduate 
away quite insensibly like heat or moisture. I endeavored, 
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 occur- 
ring everywhere throughout nature, depends on the very 
process of natural selection, through which new varieties 
continually take the places of and supplant their parent- 
forms. But just in proportion as this process of extermina- 
tion 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 stratum full of such intermediate links / Geology 
assuredly does not reveal any such finely-graduated organic 
chain ; and this, perhaps, is the most obvious and serious 
objection which can be urged against the theory. The ex- 
planation lies, as I believe, in the extreme imperfection of 
the geological record. 

In the first place, it should always be borne in mind what 
3ort of intermediate forms must, on the theory, have formerly 
existed. I have found it difficult, when looking at any two 
species, to avoid picturing to myself forms directly inter- 
mediate 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 pro- 
genitor will generally have differed in some respects from all 
its modified descendants. To give a simple illustration : the 
fantail and pouter pigeons are both descended from the rock- 
pigeon ; if we possessed all the intermediate varieties which 
have ever existed, we should have an extremely close series 
between both and the rock-pigeon ; but we should have no 
varieties directly intermediate 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 evi- 
dence regarding their origin, it would not have been possible 
to have determined, from a mere comparison of their struc- 
ture with that of the rock-pigeon, C. livia, whether they had 
descended from this species or from some other allied form, 
such as C. cenas. 

So, with natural species, if we look to forms very distinct, 
for instance to the horse and tapir, we have no reason to 
suppose that links directly intermediate between them ever 
existed, but between each and an unknown common parent. 
The common parent will have had in its whole organization 
much general resemblance to the tapir and to the horse ; but 
in some points of structure may have differed considerably 
from both, even perhaps more than they differ from each 
other. Hence, m all such cases, we should be unable to 
recognize the purent form of any two or more species, 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 intermediate links. 

It is just possible, by the theory, that one of two living 
fo^ms might have descended from the other j for instance, a 


horse from a tapir ; and in this case direct intermediate links 
will have existed between them. But such a ease would 
imply that one form had remained for a very long period 
unaltered, while its descendants had undergone a vast amount 
of change ; and the principle of competition between organ- 
ism and organism, between child and parent, will render th\s 
a very rare event ; for in all cases the new and improved 
forms of life tend to supplant the old and unimproved forms. 
By the theory of natural selection all living species have 
been 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 generally extinct, have in their 
turn been similarly connected with more ancient forms ; and 
so on backward, always converging to the common ancestor 
of each great class. So that the number of intermediate and 
transitional links, between all living and extinct species, must 
have been inconceivably great. But assuredly, if this theory 
be true, such have lived upon the earth. 


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 histo- 
rian will recognize as having produced a revolution in nat- 
ural science, and yet does not admit how vast have been the 
past periods of time, may at once close this volume. Not 
that it suffices to study the Principles of Geology, or to read 
special treatises by different observers on separate formations, 
and to mark how each author attempts to give an inade- 
quate idea of the duration of each formation, or even of each 
stratum. We can best gain some idea of past time by know- 
ing 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 fos 
himself the great piles of superimposed 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 us. 

It is good to wander along the coast, when formed oi 
moderately hard rocks, and mark the process of degradation 
The tides in most cases reach the cliffs only for a short time 
twice a day, and the waves eat into them only when they 
are charged with sand or pebbles ; for there is good evidence 
that pure water effects nothing in wearing away rock. At 
last the base of the cliff is undermined, huge 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 bowlders, all 
thickly clothed by marine productions, showing how little 
they are abraded, and how seldom they are rolled about ! 
Moreover, if we follow for a few miles any line of rocky 
cliff, which is undergoing degradation, we find that it is 
only here and there, along a short length or round a prom- 
ontory, 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 

We have, however, recently learned from the observations 
of Kamsay, in the van of many excellent observers — of 
Jukes, Geikie, Croll, and others, that subaerial degradation 
is a much more important agency than coast-action, or the 
power of the waves. The whole surface of the land is 
exposed to the chemical action of the air and of the rain- 
water, with its dissolved carbonic acid, and in colder coun- 
tries to frost ; the disintegrated 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 frag- 
ments. On si rainy day. even in a gently undulating coun- 
try, 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 strik- 
ing one, that the great lines of escarpment in the Wealde^ 


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 for- 
mation, while 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 
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 pro- 
jecting. Nothing impresses the mind with the vast dura- 
tion of time, according to our ideas of time, more forcibly 
than the conviction thus gained that subaerial agencies, 
which apparently have so little power, and which seem to 
work so slowly, have produced great results. 

When thus impressed with the slow rate at which the 
land is worn away through subaerial and littoral action, it is 
good, in order to appreciate the past duration of time, to 
consider, on the one hand, the masses of rock which have 
been removed over many extensive areas, and on the other 
hand the thickness of our sedimentary formations. I re- 
member having been much struck when viewing volcanic 
islands, which have been worn by the waves and ^nred all 
round into perpendicular cliffs of one or two thousand feet 
in height ; for the gentle slope of the lava streams, due to 
their formerly 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 upheaved on 
one side, or thrown down on the other, to the height or 
depth of thousands of feet ; for since the crust cracked, and 
it makes no great difference whether the upheaval was sudden, 
or, as most geologists now believe, was slow and effected by 
many starts, the surface of the land has been so completely 
planed down that no trace of these vast dislocations is 
externally visible. The Craven fault, for instance, extends 
for upward of thirty miles, and along this line the vertical 
displacement of the strata varies from 600 to 3,000 feet. 
Professor Ramsay has published an account of a downthrow 
in Anglesea of 2,300 feet ; and he informs me that he fully 
believes that there is one in Merionethshire of 12,000 feet ; 
yet in these cases there is nothing on the surface of the land 
to show such prodigious movements ; the pile of rocks on 
either side of the crack having been smoothly swept away. 


On the other hand, in all parts of the world the piles of 
sedimentary strata are of wonderful thickness. In the Cor- 
dillera, I estimated one mass of conglomerate at 10,000 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 Ramsay 
has given me the maximum thickness, from actual measure- 
ment 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 2,240 

— making altogether 72,584 feet; that is, very nearly thir- 
teen and three-quarters British miles. Some of the forma- 
tions, which are represented in England by thin beds, are 
thousands of feet in thickness on the Continent. Moreover, 
between each successive formation we have, in the opinion 
of most geologists, blank periods of enormous length. So 
that the lofty pile of sedimentary rocks in Britain gives but 
an inadequate idea of the time which has elapsed during 
their accumulation. The consideration of these various 
facts impresses the mind almost in the same manner as 
does the vain endeavor to grapple with the idea of eternity. 
Nevertheless this impression is partly false. Mr. Croll, 
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 phenomena, and then at the 
figures representing several million years, the two produce a 
totally different effect on the mind, and the figures are at 
once pronounced too small. In regard to subaerial denuda- 
tion, Mr. Croll shows, by calculating the known amount of 
sediment annually brought down by certain rivers, relatively 
to their areas of drainage, that 1,000 feet of solid rock, as 
jt became gradually disintegrated, would thus be removed 
from the mean level of the whole area in the course of six 
million years. This seems an astonishing result, and some 
considerations lead to the suspicion that it may be too large, 
but if halved or quartered it is still very surprising. Few 
of us, however, know what a million really means. Mr. 
Croll gives the following illustration : Take a narrow strijp 


»f paper, eighty-three feet four inches in length, and stretch 
it along the wall of a large hall ; then mark off at one end 
the tenth of an inch. This tenth of an inch will represent 
one hundred years, and the entire strip a million years. But? 
let it be borne in mind, in relation to the subject of this 
work, what a hundred years implies, represented as it is by 
a measure utterly insignificant in a hall of the above dimen- 
sions. Several eminent breeders, during a single lifetime, 
have so largely modified some of the higher animals, which 
propagate their kind much more slowly than most of the 
tower animals, that they have formed what well deserves to 
be called a new sub-breed. Few men have attended with 
due care to any one strain for more than half a century, so 
that a hundred years represents the work of two breeders in 
succession. It is not to be supposed that species in a state 
of nature ever change so quickly as domestic animals under 
the guidance of methodical selection. The comparison would 
be in every way fairer with the effects which follow from 
unconscious selection, that is, the preservation of the most 
useful or beautiful animals, with no intention of modifying 
the breed ; but by this process of unconscious selection, vari- 
ous 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 circum- 
stances, 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. '' 


Now let us turn to our richest geological museums, and 
what a paltry display we behold ! That our collections are 
imperfect, is admitted by every one. The remark of that 
admirable palaeontologist Edward Forbes, should never be 


forgotten, namely, that very many fossil species are known 
and named from single and often broken specimens, or 
from a few specimens collected on some one spot. Only 
a small portion of the surface of the earth has been geologic 
cally explored, and no part with sufficient care, as the im- 
portant discoveries made every year in Europe prove. No 
organism wholly soft can be preserved. Shells and bones 
decay and disappear when left on the bottom of the sea 
where sediment is not accumulating. We probably take s 
quite erroneous view, when we assume that sediment is being , 
deposited over nearly the whole bed of the sea, at a rate 
sufficiently quick to embed, and preserve fossil remains. 
Throughout an enormously large proportion of the ocean, 
the bright blue tint of the water bespeaks its purity. The 
many cases on record of a formation conformably covered, 
after an immense interval of time, by another and late* 
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 em- 
bedded, if in sand or gravel, will, when the beds are up- 
raised, generally be dissolved by the percolation of rain 
water charged with carbolic 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 Chthamalinse (a sub-family of sessile 
cirripedes) coat the rocks all over the world in infinite num- 
bers : they are all strictly littoral, with the exception of a 
single Mediterranean species, which inhabits deep water, 
and this has been found fossil in Sicily, whereas not one 
other species has hitherto been found in any tertiary for- 
mation : 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 i 
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 vege- 
table remains, have been found. 

With respect to the terrestrial productions which lived 
during the Secondary and Palaeozoic periods, it is superflu* 


ous to state that our evidence is fragmentary in an extreme 
degree. For instance, until recently not a land-shell was 
known belonging to either of these vast periods, with the 
exception of one species discovered by Sir C. Lyell and Dr. 
Dawson in the carboniferous strata of North America ; but 
now land-shells have been found in the lias. In regard to 
mammiferous remains, a glance at the historical table pub- 
lished in Lycll's Manual will bring home the truth, how 
accidental and rare is their preservation, far better than 
pages of detail. Nor is their rarity surprising, when we 
remember how large a proportion of the bones of tertiary 
mammals have been discovered either in caves or in lacus- 
trine deposit):', ; and that not a cave or true lacustrine bed 
is known belonging to the age of our secondary or palaeozoic 

But the imperfection in the geological record largely re- 
sults from another and more important cause than any of 
the foregoing ; namely, from the several formations being 
separated from each other by wide intervals of time. This 
doctrine has been emphatically admitted by many geologists 
and palaeontologists, who, like E. Forbes, entirely disbe- 
lieve in the change of species. When we see the forma- 
tions 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 
R. Murchison's great work on Kussia, what wide gaps there 
are in that country between the superimposed formations ; 
so it is in North America, and in many other parts of the 
world. The most skilful geologist, if his attention had 
been confined exclusively to these large territories, would 
never have suspected that during the periods which were 
blank and barren in his own country, great piles of sedi- 
ment, charged with new and peculiar forms of life, had 
elsewhere been accumulated. And if, in every separate 
territory, hardly any idea can be formed of the length of 
time which has elaosed between the consecutive formations, 
we may infer that this could nowhere be ascertained. The 
frequent and great changes in the mineralogical composition 
of consecutive formations, generally implying great changes 
in the geography of the surrounding lands, whence the sedi- 
ment 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 invariablv intermittent; that is, have 


not allowed 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 deposits 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 suc- 
cessive and peculiar marine faunas will probabiy be pre- 
served to a distant age. A little reflection will explain 
why, along the rising coast of the western side of South 
America, no extensive formations with recent or tertiary 
remains can anywhere be found, though the supply of sedi- 
ment must for ages have been great, from the enormous 
degradation of the coast rocks and from the 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 

We may, I think, conclude that sediment must be ac- 
cumulated 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 sul sequent subaerial degradation. 
Such thick and extensive accumulations of sediment may 
be formed in two ways ; either in profound depths of the 
sea, in which case the bottom will not be inhabited by so 
many and such varied forms of life as the more shallow 
seas; and the mass when upraised will give an imperfect 
record of the organisms which existed in the neighborhood 
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 
favorable for many and varied forms, and thus a rich fossil- 
iferous formation, thick enough, when upraised, to resist a 
large amount of denudation, may be formed. 

I am convinced that nearly all our ancient formations, 
which are throughout the greater part of their thickness 
rich in fossils, have thus been formed during subsidence. 
Since publishing my views on this subject in 1845, I have 
Watched the progress of geology, and have been surprised 


note how author after author, in treating of this or that 
great formation, has come to the conclusion that it was 
accumulated during subsidence. I may add, that the only 
ancient tertiary formation on the west coast of South 
America, which has been bulky enough to resist such degra- 
dation as it has as yet suffered, but which will hardly last 
to a distant geological age, was deposited during a down- 
ward oscillation of level, and thus gained considerable 

All geological facts tell us plainly that each area has 
undergone numerous slow oscillations of level, and appar- 
ently these oscillations have affected wide spaces. Conse- 
quently 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 
cnly where the supply cf sediment was sufhcent to keep the 
sea shallow and to embc-d and preserve the remains before 
they had time to decay. On the other hand, as long as 
the bed of the sea remains stationary, thick deposits cannot 
have been accumulated in the shallow parts, which are the 
most favorable to life. Still less can this have happened 
during the alternate periods of elevation ; or, to speak more 
accurately, the beds which were then accumulated will gen- 
erally have been destroyed by being upraised and brought 
within the limits of the coast-action. 

These remarks apply chiefly to littoral and sub-littoral 
deposits. In the case of an extensive and shallow sea, such 
as that within a large part of the Malay Archipelago, where 
the depth varies from thirty or fort} r to sixty fathoms, a 
widely extended formation might be formed during a period 
of elevation, and yet not suffer excessively from denudation 
during its slow upheaval ; but the thickness of the formation 
could not be great, for owing to the elevatory movement it 
would be less than the depth in which it was formed ; nor 
would the deposit be much consolidated, nor be capped by 
overlying formations, so that it would run a good chance of 
being worn away by atmospheric degradation and by the 
action 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, sub- 
sided, the deposit formed during the rising movement, though 
not thick, might afterward become protected by fresh accu- 
mulations, and thus be preserved for a long period. 

Mr. Hopkins also expresses his belief that sedimentary 


beds of considerable horizontal extent have rarely been codU 
pletely destroyed. But all geologists, excepting the few 
who believe that our present metamorphic schists and 
plutonic rocks once formed the primordial nucleus of the 
globe, will admit that these latter rocks have been stripped 
of their covering to an enormous extent. For it is scarcely 
possible that such rocks could have been solidified and 
crystallized while uncovered ; but if the metamorphic action 
occurred at profound depths of the ocean, the former protect- 
ing mantle of rock may not have been very thick. Admit- 
ting then that gneiss, mica-schist, granite, diorite, etc., were 
once necessarily covered up, how can we account for the 
naked and extensive areas of such rocks in any 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 colors an area composed of rocks of this 
nature as equal to that of Spain, France, Italy, part of Ger- 
many, and the British Islands, all conjoined. This region 
has not been carefully explored, but from the concurrent 
testimony of travelers, the granitic area is very large : thus 
Von Eschwege gives a detailed section of these rocks, stretch- 
ing from Rio de Janeiro for 260 geographical miles inland 
in a straight line ; and I travelled for 150 miles in another 
direction, and saw nothing but granitic rocks. Numerous 
specimens, collected along the whole coast, from near Rio 
Janeiro to the mouth of the Plata, a distance of 1,100 geo- 
graphical miles, were examined by me, and they all belonged 
to this class. Inland, along the whole northern bank of the 
Plata, I saw, besides modern tertiary beds, only one small 
patch of slightly metamorphosed rock, which alone could have 
formed a part of the original capping of the granitic series 
Turning to a well-known region, namely, to the United 
States and Canada, as shown in Professor H. D. Rogers's 
beautiful map, I have estimated the areas by cutting out and 
weighing the paper, and I find that the metamorphic (exclud- 
ing the " semi-metamorphic") and granite rocks exceed, in the 
proportion of 19 to 12.5, the whole of the newer Palaeozoic 
formations. In many regions the metamorphic and granite 
rocks would be found much more widely extended than they 
appear to be, if all the sedimentary beds were removed which 
rest unconformably on them, and which could not have 


formed part of the original mantle under which they were 
crystallized. Hence, it is probable that in some parts of the 
world whole formations have been completely denuded, with 
not a wreck left behind. 

One remark is here worth a passing notice. During peri- 
ods of elevation, the area of the land and of the adjoining 
shoal parts of the sea will be increased, and new stations 
will often be formed — all circumstances favorable, as previ- 
ously 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 decrease (excepting on the shores of a continent when 
first broken up into an archipelago), and consequently, dur- 
ing subsidence, though there will be much extinction, few 
new varieties 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. 


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 forma- 
tion, it becomes much more difficult to understand why we 
do not therein find closely graduated varieties between the 
allied species which lived at its commencement and at its 
close. Several cases are on record of the same species pre- 
senting varieties in the upper and lower parts of the same 
formation. Thus Trautschold gives a number of instances 
with Ammonites, and Hilgendorf has described a most curi- 
ous case of ten graduated forms of Planorbis multiformis in 
the successive beds of a fresh-water formation in Switzerland. 
Although each formation has indisputably required a vast 
number of years for its deposition, several reasons can be i 
given why each should not commonly include a graduated 
series of links between the species which lived at its com- 
mencement and close, but I cannot assign due proportional 
weight to the following considerations. 

Although each formation may mark a very long lapse of 
years, each probably is short compared with the period requi- 
site to change one species into another. I am aware that 
two palaeontologists, whose opinions are worthy of much 


deference, namely Bronn and Woodward, have concluded! 
that the average iuration of each formation is twice or 
thrice as long as the average duration of specific forms. 
But insuperable difficulties, as it seems to me, prevent us 
from 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 else- 
where 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 ex- 
tinct. We forget how small the area of Europe is, compared 
with the rest of the world ; nor have the several 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 appear- 
ing in any formation, the probability is that it only then first 
immigrated into that area. It is well known, for instance, 
that several species appear somewhat earlier in the palseozic 
beds of North America than in those of Europe ; time hav- 
ing apparently been required for their migration from the 
American to the European seas. In examining the latest 
deposits, in various quarters of the world, it has everywhere 
been noted, that some few still existing species are common 
in the deposit, but have become extinct in the immediately 
surrounding sea ; or, conversely, that some are now abundant 
in the neighboring sea, but are rare or absent in this partic- 
ular deposit. It is an excellent lesson to reflect on the ascer- 
tained amount of migration of the inhabitants of Europe 
during the glacial epoch, which forms only a part of one 
whole geological period; and likewise to reflect on the 
changes of level, on the extreme change of climate, and on 
the great lapse of time, all included within this same glacial 
period. Yet it may be doubted whether, in any quarter of 
the world, sedimentary deposits, including fossil remcrins, 
have gone on accumulating within the same area during the 
whole of this period. It is not, for instance, probable that 
sediment was deposited during the whole of the glacial 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 Amer- 
ica during this space of time. When such beds as were de- 
posited in shallow water near the mouth of the Mississippi 


.tturing some part of the glacial period shall have been up 
raised, organic remains will probably first appear and disarm 
pear at different levels, owing to the migrations of species 
and to geographical changes. And in the distant future, & 
geologist, examining these beds, would be tempted to con- 
clude that the average duration of* life of the embedded 
fossils had been less than that of the glacial period, instead 
of having been really far greater, that is, extending from 
before the glacial epoch to the present day. 

In order to get a perfect gradation between two form? 
in the upper and lower parts of the same formation, the de- 
posit must have gone on continuously accumulating during a. 
long period, sufficient for the slow process of modification ; 
hence, the deposit must be a very thick one ; and the spe- 
cies undergoing change must have lived in the same district 
throughout the whole time. But we have seen that a thick 
formation, fossiliferous throughout its entire thickness, can 
accumulate only during a period of subsidence; and to keep 
the depth approximately the same, which is necessary that 
the same marine species may live on the same space, the 
supply of sediment must nearly counterbalance the amount 
of subsidence. But this same movement of subsidence will 
tend to submerge the area whence the sediment is derived,, 
and thus diminish the supply, while the downward move- 
ment continues. In fact, this nearly exact balancing be- 
tween the supply of sediment and the amount of subsidence 
is probably a rare contingency ; for it has been observed by 
more than one palaeontologist 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 formation in any country, has generally been inter- 
mittent in its accumulation. When we see, as is so often 
the case, a formation composed of beds of widely different 
mineralogical composition, we may reasonably suspect that 
the process of deposition has been more or less interrupted. 
Nor will the closest inspection of a formation give us any 
idea of the length of time which its deposition may have 
cousumed. Many instances could be given of beds, only a 
few feet in thickness, representing formations which are 
elsewhere thousands of feet in thickness, and which must 
have required an enormous period for their accumulation j 
yet no one ignorant of this fact would have even suspected 
the vast lapse of time represented by the thinner formation. 


Many cases could be given of the lower beds of a formation 
having been upraised, denuded, submerged, and then re-cov- 
ered by the upper beds of the same formation — facts, show- 
ing what wide, yet easily overlooked, intervals have occurred 
in its accumulation. In other cases we have the plainest 
evidence in great fossilized 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 preserved : thus Sir C. 
Lyell and Dr. Dawson found carboniferous beds 1,400 feet 
thick in Nova Scotia, with ancient root-bearing strata, one 
above the other, at no less than eighty-six 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 be- 
tween any two forms, they rank both as species, unless they 
are enabled to connect them together by the closest inter- 
mediate 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 intermediate 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 inter- 
mediate between them in all respects. So that we might 
obtain the parent-species and its several modified descend- 
ants from the lower and upper beds of the same formation, 
and unless we obtained numerous transitional gradations, we 
should not recognize their blood-relationship, and should 
consequently rank them as distinct species. 


Tt 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 con- 
chologists are now sinking many of the very tine 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 naturalists to be identical with existing species ; 
but some excellent naturalists, as Agassiz and Pictet, main- 
tain 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 naturalists have 
been misled by their imaginations, and that these late tertiary 
species really present no difference whatever from their liv- 
ing representatives, or unless we admit, in opposition to the 
judgment of most naturalists, that these tertiary species are 
all truly distinct from the recent, we have evidence of the 
frequent occurrence of slight modifications of the kind re- 
quired. If we look to rather wider intervals of time, namely, 
to distinct but consecutive stages of the same great forma- 
tion, 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 
tne 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-form until they have been modified 
and perfected in some considerable degree. According to 
this view, the chance of discovering in a formation in any 
one country all the early stages of transition between any 
two forms, is small, for the successive changes are supposed 
to have been local or confined to some one spot. Most marine 
animals have a wide range ; and we have seen that with 
plants it is those which have the widest range, that oftenest 
present varieties; so that, with shells and other marine 
animals, it is probable that those which had the widest 
range, far exceeding the limits of the known geological 


formations in Europe, have ofbenest given rise, first to local 
varieties and ultimately to new species ; and this again 
would greatly lessen the chance of our being able to trace 
the stages of transition in any one geological formation. 

It is a more important consideration, leading to the same 
result, as lately insisted on by Dr. Falconer, namely, that 
the period during which each species underwent modification, 
though long as measured by years, was probably short in 
comparison with that during which it remained without 
undergoing any change. 

It should not be forgotten, that at the present day, with 
perfect specimens for examination, two forms can seldom be 
connected by intermediate varieties, and thus proved to be 
the same species, until many specimens are collected from 
many places; and w;th fossil species this can rarely be 
done. We shall, perhaps, best perceive the improbability 
of our being enabled to connect 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 abo- 
riginal stocks ; or again, whether certain sea-shells inhabiting 
the shores of North America, which are ranked by some con- 
chologists as distinct species from their European represent- 
atives, and by other conchologists as only varieties, are really 
varieties, or are, as it is called, specifically distinct. This 
could be effected by the future geologist only by his discov- 
ering in a fossil state numerous intermediate gradations; 
and such success is improbable in the highest degree. 

It has been asserted over and over again, by writers who 
believe in the immutability of species, that geology yields 
no linking forms. This assertion, as we shall see in the 
next chapter, is certainly erroneous. As Sir J. Lubbock 
has remarked, " Every species is a link between other allied 
forms." If we take a genus having a score of species, 
recent and extinct, and destroy four-fifths of them, no one 
doubts that the remainder will stand much more distinct 
from each other. If the extreme forms in the genus hap- 
pen 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 infi- 
nitely numerous gradations, as fine as existing varieties, con- 
necting together nearly all existing and extinct species* 
But this ought not tp be expected; yet this has bees 


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 geological 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 Russia, and therefore 
equals all the geological formations which have been exam- 
ined with any accuracy, excepting those of the United States 
of America. I fully agree with Mr. Godwin-Austen, that 
the present condition of the Malay Archipelago, with its 
numerous large islands separated by wide and shallow seas, 
probably represents the former state of Europe, while most 
of our formations were accumulating. The Malay Archi- 
pelago is one of the richest regions in organic beings ; yet if 
all the species were to be collected which have ever lived 
there, how imperfectly would they represent the natural 
history of the world ! 

But we have every reason to believe that the terrestrial 
productions of the archipelago would be preserved in an 
extremely imperfect manner in the formations 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 endure to a distant epoch. Wherever sedi- 
ment 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 thick- 
ness sufficient to last to an age as distant in futurity 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 intervals of time, during which the area 
would be either stationary or rising ; while rising, the fos- 
siliferous formations on the steeper shores would be de- 
stroyed, almost 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 within the 
archipelago, sedimentary beds could hardly be accumulated 
of great thickness during the periods of elevation, or become 
capped and protected by subsequent deposits, so as to have 
a good «hance of enduring to a very distant future. During 
the periods of subsidence, there would probably be muc£ 


extinction of life ; during the periods of elevation, there 
would be much variation, but the geological record would 
then be less perfect. 

It may be doubted whether the duration of any one great 
period of subsidence over the whole or part of the archi- 
pelago, together with a contemporaneous accumulation of 
sediment, would exceed the average duration of the same 
specific forms ; and these contingencies are indispensable 
for the preservation of all the transitional gradations between 
any two or more 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 inter- 
rupted by oscillations of level, and that slight climatical 
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 modifica- 
tions could be preserved in any one formation. 

Very many of the marine inhabitants of the archipelago 
now range thousands of miles beyond its confines; and 
analogy plainly leads to the belief that it would be chiefly 
these far-ranging species, though only some of them, which 
would oftenest produce new varieties ; and the varieties 
would at first be local or confined to one place, but if pos- 
sessed of any decided advantage, or when further modified 
and improved, they would slowly spread and supplant their 
parent forms. When such varieties returned to their ancient 
homes, as they would differ from their former state in a 
nearly uniform, though perhaps extremely slight degree, and 
as they would be found embedded in slightly different sub- 
stages of tfue same formation, they would, according to the 
principles followed by many palaeontologists, be ranked as 
new and distinct species. 

If then there be some degree of truth in these remarks, 
we have no right to expect to find, in our geological forma* 
tions, an infinite number of those fine transitional forms 
which, on our theory, have connected all the past and pres- 
ent species of the same group into one long and branching 
chain of life. We ought only to look for a few links, and 
such assuredly we do find — some more 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 forma- 
tion, would, by many palaeontologists, be ranked as distinct 
species. But I do not pretend that I should ever hay© 


suspected how poor was the record in the best preserved 
geological sections, had not the absence of innumerable 
transitional links between the species which lived at the 
commencement and close of each formation, pressed so hardly 
on my theory. 



The abrupt manner in which whole groups of species sud- 
denly appear in certain formations, has been urged by sev- 
eral palaeontologists — for instance, by Agassiz, Pictet, and 
Sedgwick — as a fatal objection to the belief in the trans- 
mutation of species. If numerous species, belonging to the 
same genera or families, have really started into life at once, 
the fact would be fatal to the theory of evolution through 
natural selection. For the development by this means of a 
group of forms, all of which are descended from some one 
progenitor, must have been an extremely slow process ; and 
the progenitors must have lived long before their modified 
descendants. But we continually overrate the perfection 
of the geological record, and falsely infer, because certain 
genera or families have not been found beneath a certain 
stage, that they did not exist before that stage. In all cases 
positive palseontological evidence may be implicitly trusted ; 
negative evidence is worthless, as experience has so often 
shown. We continually forget how large the world is, com- 
pared with the area over which our geological formations 
have been carefully examined ; we forget that groups of 
species may elsewhere have long existed, and have slowly 
multiplied, before they invaded the ancient archipelagoes of 
Europe and the United States. We do not make due allow- 
ance 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 forma- 
tion. These intervals will have given time for the multipli- 
cation 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 confined to some one region * 


but that, when this adaptation 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 Pictet, in his 
excellent review of this work, in commenting on early 
transitional forms, and taking birds as an illustration, can- 
not see how the successive modifications of the anterior 
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 " 'I 
Yet these birds hold their place victoriously in the battle for 
life ; for they exist in infinite numbers and of many kinds. 
I do not suppose that we here see the real transitional grades 
through which the wings of birds have passed ; but what 
special difficulty is there in believing that it might profit the 
modified descendants of the penguin, first to 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 suppos- 
ing that whole groups of species have suddenly been pro- 
duced. Even in so short an interval as that between the 
first and second editions of Pictet's great work on Palaeon- 
tology, 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 require still further changes. I may recall 
the well-known fact that in geological treatises, published 
not many years ago, mammals were always spoken of as 
having abruptly come in at the commencement of the tertiary 
series. And now one of the richest known accumulations of 
fossil mammals belongs to the middle of the secondary 
series ; and true mammals have been discovered in the new 
red sandstone at nearly the commencement of this great 
series. Cuvier used to urge that no monkey occurred in any 
tertiary stratum ; but now extinct species hare been discov 
ered in India, South America, and in Europe, as far back as 
the miocene stage. Had it not been for the rare accident of 
the preservation of footsteps in the new red sandstone of the 
United States, who would have ventured to suppose that no 
less than at least thirty different bird-like animals, some of 


gigantic size, existed during that period ? Not 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 
green sand ; 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 forci- 
bly than this how little we as yet know of the former inhab- 
itants 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 num- 
ber of existing and extinct tertiary species ; from the ex- 
traordinary abundance of the individuals of many species 
all over the world, from the arctic regions to the equator, in- 
habiting various zones of depths, from the upper tidal limits 
to fi^ty fathoms ; from the perfect manner in which speci- 
mens are preserved in the oldest tertiary beds ; from the 
ease with which even a fragment of a valve can be recog- 
nizee. ; from all these circumstances, I inferred that, had 
sessile cirripedes existed during the secondary periods, they 
would certainly have been preserved and discovered ; and as 
not one species had then been discovered in beds of this age, 
I concluded that this great group had been suddenly de- 
veloped at the commencement of the tertiary series. This 
was a sore trouble to me, adding, as I then thought, one more 
instance of the abrupt appearance of a great group of species. 
But my work had hardly been published, when a skilful 
palaeontologist, M. Bosquet, sent me a drawing of a perfect 
specimen of an unmistakable sessile cirripede, which he had 
himself extracted from the chalk of Belgium. And, as if to 
make the case as striking as possible, this cirripede was a 
Chthamalus, a very common, large, and ubiquitous genus, of 
>*hich not one species has as yet been found even in any 
tertiary stratum. Still more recently, a Pyrgoma, a member 
of a distinct sub-family of sessile cirripedes, has been dis- 
covered by Mr. Woodward in the upper chalk ; so that we 
now have abundant evidence of the existence of this group 
of animals during the secondary period. 

The case most frequently iusisted on by palaeontologists^ 


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 commonly admitted to be teleostean; 
and even some palaeozoic forms have thus been classed by 
one high authority. If the teleosteans had really appeared 
suddenly in the northern hemisphere at the commencement 
of the chalk formation, the fact would have been highly 
remarkable ; but it would not have formed an 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 running through Pictet'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 confined range ; the teleostean fishes might 
formerly have had a similarly confined range, and after hav- 
ing 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 
enabled to double the southern capes of Africa or Australia 
and thus reach other and distant seas. 

From these considerations, from our ignorance of the 
geology of other countries beyond the confines of Europe 
and the United States, and from the revolution in our 
palaeontological knowledge effected by the discoveries of 
the last dozen years, it seems to me to be about as rash to 
dogmatize on the succession of organic forms throughout the 
world, as it would be for a naturalist to land for five minutes 
on a barren point in Australia, and v'uen to djscuss che nuva> 
btfi and range of its productions. 



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 sud 
denly appear in the lowest known fossiliferous rocks. Most 
of the arguments which have convinced me that ah the ex 
isting species of the same group are descended from a single 
progenitor, apply with equal force to the earliest known 
species. For instance, it cannot be doubted that all the Cam* 
brian and Silurian trilobites are descended from some one 
crustacean, which must have lived long before the Cambrian 
age, and which probably differed greatly from any known 
animal. Some of the most ancient animals, as the Nautilus, 
Lingula, etc., do not differ much from living 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 subsequently appeared, for they are not 
in an}' degree intermediate in character. 

Consequently, if the theory be true, it is indisputable that> 
before the lowest Cambrian stratum was deposited long 
periods elapsed, as long as, or probably far longer than, the 
whole interval from the Cambrian age to the present day ; 
and that during these vast periods the world swarmed with 
living creatures. Here we encounter a 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 twenty or more than four 
hundred million years ago, but probably not less than ninety- 
eight or more than two hundred million years. These very 
wide limits show how doubtful the data are ; and other ele- 
ments may have hereafter to be introduced into the problem. 
Mr. Croll estimates that about sixty million years have elapsed 
since the Cambrian period, but this, judging from the small 
amount of organic change since the commencement of the 
Glacial epoch, appears a very short time for the many and 
great mutations of life, which have certainly occurred since 
the Cambrian formation ; and the previous one hundred and 
forty 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 William 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 correspond- 
ing rate in the organisms which then existed. 

To the question why we do not rind 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 R Murchison at their 
bead, 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 disputed this conclusion. W should not forget 
than only a small portion of the world is known with accuracy. 
Not very long ago M. Barrande added another and lower 
stage, abounding with new and peculiar species, beneath the 
then known Silurian system ; and now, still lower down in 
the Lower Cambrian formation. Mr Hicks has found South 
Wales beds rich in trilobites. and containing various mollusks 
and annelids. The presence of phosphatic nodules and bitu- 
minous matter, even in some of the lowest azotic rocks, 
probably indicates life at these periods , and the existence 
of the Eozoon in the Laurentian formation of Canada is gen- 
erally admitted. There are three great series of strata 
beneath the Silurian system in Canada, in the lowest of 
which the Eozoon is found. Sir W Logan states that their 
"united thickness may possibly far surpass that of all the 
succeeding rocks, from the base of the palaeozoic series to 
the present time. We are thus carried back to a period so 
remote, that the appearance of the so-called primordial fauna 
(of Barrande) may by some be considered as a comparatively 
modern event." The Eozoon belongs to the most lowly 
organized of all classes of animals, but is highly organized 
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 difficulty 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 


Oeen wholly obliterated by metamorphic action, for if this had 
been the case we should have found only small remnants of 
the formations n^xt 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 Russia and in North America, do 
not support the view that the older a formation is, the more 
invariably it has suffered extreme denudation and metamor- 

The case at present must remain inexplicable, and may be 
truly urged as a valid argument against the views here enter- 
tained. To show that it may hereafter receive some expla- 
nation, I will give the following hypothesis. From the 
nature of the organic remains which do not appear to have in- 
habited profound depths, in the several formations of Europe 
and of the United States ; and from the amount of sedi- 
ment, miles in thickness, of which the formations are com- 
posed, we may infer that from first to last large islands or 
;racts of land, whence the sediment was derived, occurred in 
the neighborhood of the now existing continents of Europe 
ind North America. This same view has since been main- 
sained by Agassiz and others. But we do not know what 
was the state of things in the intervals between the several 
successive formations ; whether Europe and the United 
States during these intervals existed as dry land, or as a 
submarine surface near land, on which sediment was not 
deposited, or as the bed of an open and unfathomable sea. 

Looking to the existing oceans, which are thrice as exten- 
sive ijs 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 atford even a remnant of any palaeozoic or secondary 
formation. Hence, we may perhaps infer, that during the 
palaeozoic and secondary periods, neither continents nor con- 
tinental islands existed where our oceans now extend ; for 
had they existed, palaeozoic and secondary formations would 
in all probability have been accumulated from sediment de- 
rived from their wear and tear ; and these would have been 
at least partially upheaved by the oscillations of level, which 
must have intervened during these enormously long periods 
If, then, we may infer anything from these fr.cts, we may 
infer that, where our oceans now extend, oceans have ex- 
tended from the remotest period of which we have any record ; 
and on the other hand, that where continents now exist, large 


tracts of land have existed, subjected, no doubt, to great 
oscillations of level, since the Cambrian period. The colored 
map appended to my volume on Coral Reefs led me to con- 
clude that the great oceans are still mainly areas of sub- 
sidence, the great archipelagoes still areas of oscillations of 
level, and the continents areas of elevation. 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 preponderance, during many oscillations 
of level, of the force of elevation. But may not the areas of 
preponderant movement have changed in the lapse of ages ? 
At a period long antecedent to the Cambrian epoch, conti- 
nents may have existed where oceans are now spread out, 
and clear and open oceans may have existed where our con- 
tinents now stand. Nor should we be justified in assuming 
that if, for instance, the bed of the Pacific Ocean were now 
converted into a continent, we should there find sedimentary 
formations, in recognizable condition, older than the Cam- 
brian strata, supposing such to have been formerly deposited ; 
for it might well happen that strata which had subsided 
some miles nearer to the centre of the earth, and which had 
been pressed on by an enormous weight of superincumbent 
water, might have undergone far more metamorphic action 
than strata which have always remained nearer to the sur- 
face. 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 perhaps 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 difficulties here discussed, namely, that, though 
we find in our geological formations many links between the 
species which now exist and which formerly existed, we do 
not find infinitely numerous fine transitional forms closely 
joining them all together, the sudden manner in which several 
groups of species first appear in our European formations, 
the almost entire absence, as at present known, of formations 
rich in fossils beneath the Cambrian strata, are all undoubt- 
edly of the most serious nature. We see this in the fact 
that the most eminent pa.aeontologists, namely, Cuvier, 
Agassiz, Barrande, Pictet, Falconer, E. Forbes, etc., and all 
our greatest geologists, as Lyell, Murchison, Sedgwick, etc., 
have unanimously, often vehemently, maintained the immu- 


tability of species. But Sir Charles Lyell now gives the 
support of his hi^h authority to the opposite 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 tha 
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 
possess 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 eacri page, only here and 
there a few lines. Each word of the slowly-changing lan- 
guage, more or less different in the successive chapters, may 
represent the forms of life, which are entombed in our con- 
secutive formations, and which falsely appear to have been 
abruptly introduced. On this view the difficulties aboye 
discussed are greatly diminished or even disappear. 




On the Slow and Successive Appearance of New Species — On their 
Different Rates of Change — Species once lost do not reappear — 
Groups of Species follow the Same General Rules in Their Appear- 
ance 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 — Sum- 
mary of Preceding and Present Chapter. 

Let us now see whether the several facts and laws relat- 
ing to the geological succession of organic beings accord best 
with the common view of the immutability of species, or 
with that of their slow and gradual modification through 
variation and natural selection. 

New species have appeared very slowly, one after another, 
both on the land and in the waters. Lyell has shown that it 
is hardly possible to resist the evidence on this head in the 
case of the several tertiary stages ; and every year tends to 
fill up the blanks between the stages, and to make the pro- 
portion between the lost and existing forms more gradual. 
In some of the most recent beds, though undoubtedly of high 
antiquity if measured by years, only one or two species are 
extinct, and only one or two are new, having appeared there 
for the first time, either locally, or, as far as we know, on 
the face of the earth. The secondary formations are more 
broken ; but, as Bronn has remarked, neither the appearance 
nor disappearance of the many species embedded in each 
formation has been simultaneous. 

Species belonging to different genera and classes have not 
changed at the same rate, or in the same degree. In the 
older tertiary beds a few living shells may still be found in 
the midst of a multitude of extinct forms. Falconer has 
given a striking instance of a similar fact, for an existing 
crocodile is associated with many lost mammals 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 organic 
change, as Pictet has remarked, is not the same in each suc- 
cessive so-called formation. Yet if we compare any but the 
most closely related formations, all the species will be found 
to have undergone some change. When a species has once 
disappeared from the face of the earth, we have no reason 
to believe that the same identical form ever reappears. The 
strongest apparent exception to this latter rule is that of the 
so-called "colonies" of M. Barrande, which intrude for a 
period in the midst of an older formation, and then allow 
the pre-existing fauna to reappear ; but Lyell's explanation, 
namely, that it is a case of temporary migration from a dis- 
tinct geographical province, seems satisfactory. 

These several facts accord well with our theory, which 
includes no fixed law of development, causing all the inhabit- 
ants of an area to change abruptly, or simultaneously, or to 
an equal degree. The process of modification must be slow, 
and will generally affect only a few species at the same time ; 
for the variability of each species is independent of that of 
all others. Whether such variations or 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 benefi- 
cial nature, on the freedom of intercrossing, on the slowly 
changing physical conditions of the country, on the immi- 
gration of new colonists, and on the nature of the other 
inhabitants with which the varying species come into com- 
petition. Hence it is by no means surprising that one species 
should retain the same identical form much longer than 
others ; or, if changing, should change in a less degree. We 
find similar relations between the existing inhabitants of 
distinct countries ; for instance, the land-shells and coleop- 
terous rusects 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 aad in more highly organized productions com- 


pared with marine and lower productions, by the more com- 
plex 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 improved, we can understand, on the principle of 
competition, and from the all-important relations of organism 
to organism in the struggle for life, that any form which did 
not become in some degree modified and improved, would be 
liable to extermination. Hence, we see why all the species 
in the same region do at last, if we look to long enough in- 
tervals 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 sedi- 
ment being deposited on subsiding areas, our formations 
have been almost necessarily accumulated at wide and 
irregularly intermittent intervals of time ; consequently the 
amount of organic change exhibited by the fossils embedded 
in consecutive formations is not equal. Each formation, on 
this view, does not mark a new and complete act of creation, 
but only an occasional scene, taken almost at hazard, in an 
ever slowly changing drama. 

We can clearly understand why a species when once lost 
should never reappear, even if the very same conditions of 
life, organic and inorganic, should recur. For though the 
offspring of one species might be adapted (and no doubt this 
has occurred in innumerable instances) to fill the place of 
another species in the economy of nature, and thus supplant 
it ; yet the two forms — the old and the new — would not be 
identically the same ; for both would almost certainly inherit 
different characters from their distinct progenitors; and 
organisms already differing would vary in a different man- 
ner. 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 by their 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. ia 


some degree different, and the newly-formed variety would 
probably inherit from its progenitor some characteristic 

Groups of species, that is, genera and families, follow 
the same general rules in their appearance and disappear- 
ance as do single species, changing more or less quickly, and 
in a greater or lesser degree. A group, when it has once dis- 
appeared, never reappears ; that is, its existence, as long as 
it lasts, is continuous. I am aware that there are some 
apparent exceptions to this rule, but the exceptions are sur- 
prisingly 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 pres- 
ent day. 

We have seen in the last chapter that whole groups of 
species sometimes falsely appear to have been abruptly 
developed ; and I have attempted to give an explanation of 
this fact, which if true would be fatal to my views. But 
such cases are certainly exceptional ; the general rule being 
a gradual increase in number, until the group reaches its 
maximum, and then, sooner or later, a gradual decrease. If 
the number of the species included within a genus, or the 
number of the genera within a family, be represented by a 
vertical line of varying thickness, ascending through the 
successive geological formations in which the species are 
found, the line will sometimes falsely appear to begin at its 
lower end, not in a sharp point, but abruptly ; it then grad- 
ually thickens upward, 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 grad- 
ual 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 allied forms necessarily being 
a slow and gradual process, one species first giving rise to 
two or three varieties, these being slowly converted into 
species, which in their turn produce by equally slow steps 


other varieties and species, and so on, like the branching of 
a great tree from a single stem, till the group becomes large. 


We have as yet only spoken incidentally of the disap- 
pearance 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 con- 
nected together. The old notion of all the inhabitants of 
the earth having been swept away by catastrophes at succes- 
sive periods is very generally given up, even by those geolo- 
gists, as Elie de Beaumont, Murchison, Barrande, etc., whose 
general views would naturally lead them to this conclusion. 
On the contrary, we have every reason to believe, from the 
study of the tertiary 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 isthmus and 
the consequent irruption of a multitude of new inhabitants 
into an adjoining sea, or by the final subsidence of an island, 
the process of extinction may have been rapid. Both single 
species and whole groups of species last for very unequal 
periods ; some groups, as we have seen, have endured from 
the earliest known dawn of life to the present day ; some 
have disappeared before the close of the palaeozoic period. 
No fixed law seems to determine the length of time during 
which any single species or any single genus endui__>. There 
is reason to believe that the extinction of a whole group of 
species is generally a slower process than their production: 
if their appearance and disappearance be represented, as 
before, by a vertical line of varying thickness, the line is 
found to taper more gradually at its upper 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, toward the close of the 
secondary period, has been wonderfully sudden. 

The extinction of species has been involved in the most 
gratuitous mystery. Some authors have even supposed that, 
as the individual has a definite length of life, so have species 
a definite duration. No one can have marvelled more than 
I have done at the extinction of species. When I found in 
La Plata the tooth of a horse embedded with the remains of 


Mastodon, Megatherium, Toxodon, and other extinct mon- 
sters, which all co-existed with still living shells at a very 
late geological period, I was rilled with astonishment; for, 
seeing that the horse, since its introduction by the Spaniards 
into South America, has run wild over the whole country 
and has increased in numbers at an unparalleled rate, I asked 
myself what could so recently have exterminated the former 
horse under conditions of life apparently so favorable. But 
my astonishment was groundless. Professor Owen soon 
perceived that the tooth, though so like that of the existing 
horse, belonged to an extinct species. Had this horse been 
still living, but in some degree rare, no naturalist would have 
felt the least surprise at its rarity ; for rarity is the attribute 
of a vast number of species of all classes, in all countries. 
If we ask ourselves why tnis or that species is rare, we 
answer that something is unfavorable in its conditions of 
life ; but what that something is, we can hardly ever tell. 
On the supposition of the fossil horse still existing as a rare 
species, we might have felt certain, from the analogy of all 
other mammals, even of the slow-breeding elephant, and 
from the history of the naturalization of the domestic horse 
in South America, that under more favorable conditions it 
would in a very few years have stocked the whole continent. 
But we could not have told what the unfavorable conditions 
were which checked its increase, whether some one or several 
contingencies, and at what period of the horse's life, and in 
what degree, they severally acted. If the conditions had 
gone on, however slowly, becoming less and less favorable, 
we assuredly should not have 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 increase 
of every creature is constantly being checked by unper- 
ceived hostile agencies ; and that these same unperceived 
agencies are amply sufficient to cause rarity, and finally 
extinction. So little is this subject understood, that I have 
heard surprise repeatedly expressed at such great 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 requi- 
site food. Before man inhabited iaciia 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 weaking the elephant in India, check its increase ; and 
this was Bruce's conclusion with respect to the African 
elephant in Abyssinia. It is certain that insects and blood- 
sucking bats determine the existence of the larger natural- 
ized quadrupeds in several parts of South America. 

We see in many cases in the more recent tertiary forma- 
cions, that rarity precedes extinction ; and we know that 
this has been the progress of events with those animals 
which have been exterminated, either locally or wholly, 
through man's agency. I may repeat what I published in 
1845, namely, that to admit that species generally become 
rare before they become extinct — to feel no surprise at the 
rarity of a species, and yet to marvel greatly when the 
.species ceases to exist, is much the same as to admit that 
sickness in the individual is the forerunner of death — to 
feel no surprise at sickness, but, when the sick man dies, 
to wonder, and to suspect that he died by some deed of 

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 competition ; and the con- 
sequent extinction of the less-favored forms almost inevita- 
bly 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 
neighborhood ; 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 together. 
In flourishing groups, the number of new specific forms 
which have been produced within a given time has at some 
periods probably been greater than the number of the old 
specific forms which have been exterminated ; but we know 
that species have not gone on indefinitely increasing, at 
least during the later geological 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 few* 


merly explained and illustrated by examples, between the 
forms which are most like each other in all respects. Hence 
the improved and modified descendants of a species will 
generally cause the extermination of the parent-species ; 
and if many new forms have been developed from any 
■>ne species, the nearest allies of that species, i. e., the 
species of the same genus, will be the most liable to exter- 
mination. Thus, as I believe, a number of new species de- 
scended 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 
it« extermination. If many allied forms be developed from 
the successful intruder, many will have to yield their places ; 
and it will generally be the allied forms, which will suffer 
from some inherited inferiority in common. But whether 
it be species belonging to the same or to a distinct class, 
which have yielded their places to other modified and im« 
proved species, a few of the sufferers may often be pre- 
served 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. There- 
fore, the utter extinction of a group is generally, as we 
have seen, a slower process than its production. 

With respect to the apparently sudden extermination of 
whole families or orders, as of Trilobites at the close of 
the palaeozoic period, and of Ammonites at the close of the 
secondary period, we must remember what has been already 
said on the probable wide intervals of time between out 
consecutive formations ; and in these intervals there may 
have been much slow extermination. Moreover, when, by 
sudden immigration or by unusually rapid development, 
many species of a new group have taken possession of an 
area, many of the older species will have been exterminated 
in a correspondingly rapid manner; and the forms which 
thus yield their places will commonly be allied, for they will 
partake of the same inferiority in common. 

Thus, as it seems to me, the manner in which single 
species and whole groups of species become extinct 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 moment that we under- 
stand the many complex contingencies on which the exist- 
ence of each species depends. If we forget for an instant 
that each species tends to increase inordinately, and that some 
check is always in action, yet seldom perceived by us, the 
whole economy of nature wiii be utterly obscured. When- 
ever we can precisely say why this species is more abundant 
in individuals than that ; why this species and not another 
can be naturalized in a given country , then, and not until 
then, we may justly feel surprise why we cannot account 
for the extinction of any particular species or group of 


Scarcely any palaeontological discovery is more striking 
than the fact that the forms of life change almost simul- 
taneously throughout the world. Thus our European Chalk 
formation can be recognized 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 x'ierra 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 unmistakable resemblance to those of the Chalk. 
It is not that the same species are met with ; for in some 
cases not one species is identically the same ; but they 
belong to the same families, genera, and sections of genera, 
and sometimes are similarly characterized in such trifling 
points as mere superficial sculpture. Moreover, other forms, 
which are not found in the Chalk of Europe, but which 
occur in the formations either above or below, occur in 
the same order at these distant points of the world. In 
the several successive palaeozoic formations of Russia, West- 
em Europe, and North America, a similar parallelism in 
the forms of life has been observed by several authors ; so 
it is, according to Lyell, with the European and North 
American tertiary deposits. Even if the few fossil species 
which are common to the Old and New Worlds were kep\; 
wholly out of view, the general parallelism in the successive 
formp 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 inhab- 
itants 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 Mega- 
therium, Mylodon, 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 in- 
ferred that they had lived during one of the later tertiary 

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 pleis* 
tocene period (a very remote period as measured by years, 
including the whole glacial epochs were compared with those 
now existing in South America or in Australia, the most skil- 
ful naturalist would hardly be able to say whether the pres- 
ent or the pleistocene inhabitants of Europe resembled most 
closely those of the southern hemisphere. So, again, several 
highly competent observers maintain that the existing pro- 
ductions of the United States are more closely related to 
those which lived in Europe during certain late tertiary 
stages, than to the present inhabitants of Europe ; and if 
this be so, it is evident that fossiliferous beds now deposited 
on the shores of North America would hereafter be liable to 
be classed with somewhat older European beds. Neverthe- 
less, looking to a remotely future epoch, there can be little 
doubt that all the more modern marine formations, namely, 
the upper pliocene, the pleistocene, and strictly modern beds 
of Europe, North and South America, and Australia, from 
containing fossil remains in some degree allied, and from 
not including those forms which are found only in the older 
underlying deposits, would be correctly ranked as simulta- 
neous in a geological sense. 

The fact of the forms of life changing simultaneously in 
the above large sense, at distant parts of the world, has 


greatly struck those admirable observers, MM. de Vemeuil 
and d'Archiac. After referring to the parallelism of the 
palaeozoic 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 bo changes of cur- 
rents, climate, or other physical conditions, as the cause of 
these great mutations in the forms of life throughout the 
world, under the most different climates. We must, as 
Barrande has remarked, look to some 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 tb^ir own country, give birth to the greatest num- 
ber of ne\^ varieties or incipient species. We have distinct 
evidence on thiv; head, in the plants which are dominant, 
that is, which are commonest and most widely diffused, 
producing the greatest number of new varieties. It is also 
natural that the dominant, varying and far-spreading species, 
which have already invaded, to a certain extent, the terri- 
tories of other species, should be those which would have the 
best chance of spreading still farther, and of giving rise in 
new countries to other new varieties and species. The 
process of diffusion would often be very slow, depending on 
elimatal and geographical changes, on strange accidents, and 
on the gradual acclimatization of new species to the various 
climates through which they might have to pass, but in the 
course of time the dominant forms would generally succeed 
in spreading and would ultimately prevail. The diffusion 
would, it is probable, be slower with the terrestrial inhabit- 
ants of distinct continents than with the marine inhabitants 
of the continuous sea. We might therefore expect to find. 


as we do find, a less strict degree of parallelism in the suc- 
cession 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 dominant species spreading 
widely and varying; the new species thus produced being 
themselves dominant, owing to their having had some ad- 
vantage over their already 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 com- 
mon ; 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 sediment 
was not thrown down quickly enough to embed and preserve 
organic remains. During these long and blank intervals I 
suppose that the inhabitants of each region underwent a 
considerable amount of modification and extinction, and 
that there was much migration from other parts of the 
world. As we have reason to believe that large areas are 
affected by the same movement, it is probable that strictly 
contemporaneous formations have often been accumulated 
over very wide spaces in the same quarter of the world ; but 
We are very far from having any right to conclude that this 
has invariably been the case, and that large areas have invari- 
ably been affected by the same movements. When two for- 
mations have been deposited in two regions during nearly, 
but not exactly, the same period, we should find m both, from 
the causes explained in the foregoing paragraphs, the same 
general succession in Lhe forms of life; but the species 
would not exactly correspond ; for there will have been a 
little more time in the one region than in the other tot 
modification, extinction,, and immigration. 


I suspect that cases of this nature occur in Europe. 
Mr. Prestwich, in his admirable Memoirs on the eocene 
deposits of England and France, is able to draw a close 
general parallelism between the successive stages in the 
two countries ; but when he compares certain stages in 
England with those in France, although he rinds in both a 
curious accordance in the numbers of the species belonging 
to the same genera, yet the species themselves differ in a 
manner very difficult to account for considering the prox- 
imity 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 observa- 
tions 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; 
nevertheless 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 forma- 
tion 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 accumulation of the several 
formations and during the long intervals of time between 
them ; in this case the several formations in the two regions 
could be arranged in the same order, in accordance with the 
general succession of the forms of life, and the order would 
falsely appear to be strictly parallel ; nevertheless the species 
would not be all the same in the apparently corresponding 
stages in the two regions. 



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 intervals between existing 
genera, families, and orders, is certainly true ; but as this 
statement has often been ignored or even denied, it may 
be well to make some remarks on this subject, and to give 
gome instances. If we confine our attention either to th$ 


living or to the extinct species of the same class, the series 
is far less perfect than if we combine both into one general 
system. In the writings of Professor Owen we continually 
meet with the expression of generalized 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 palaeontologist, M. Gaudry, has shown 
in the most striking manner that many of the fossil mam- 
mals discovered by him in Attica serve to break down the 
intervals between existing genera. Cuvier ranked the Rumi- 
nants and Pachyderms as two of the most distinct orders of 
mammals; but so many fossil links have been disentombed 
that Owen has had to alter the whole classification, and has 
placed certain Pachyderms in the same sub-order with rumi- 
nants ; for example, he dissolves by gradations the appar- 
ently wide interval between the pig and the camel. The 
Ungulata or hoofed quadrupeds are now divided into the 
even-toed or odd-toed divisions ; but the Macrauchenia of 
South America connects to a certain extent these two grand 
divisions. No one will deny that the Hipparion is inter- 
mediate between the existing horse and certain other ungu- 
late forms. What a wonderful connecting link in the chain 
of mammals is the Typotherium from South America, as 
the name given to it by Professor Gervais expresses, and 
which cannot be placed in any existing order. The Sirenia 
form a very distinct group of the mammals, and one of the 
most remarkable peculiarities in existing dugong and lamen- 
tin is the entire absence of hind limbs, without even a rudi- 
ment being left ; but the extinct Halitherium had, according 
to Professor Flower, an ossified thigh-bone "articulated to 
a well-defined acetabulum in the pelvis," and it thus makes 
some approach to ordinary hoofed 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 
naturalists in an order by themselves, are considered by 
Professor Huxley to be undoubtedly cetaceans, " and to con- 
stitute 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 Archeopteryx, and on the 
Other hand by the Compsognathus, one of the Dinosauriaas 


— that group which includes the most gigantic of all te*. 
restrial reptiles. Turning to the Invertebrata, Barrande 
asserts (a higher authority could not be named) that he is 
every day taught that, although palaeozoic animals can cer- 
tainly be classed under existing groups, yet that at this 
ancient period the groups were not so distinctly separated 
from each other as they now are. 

Some writers have objected to any extinct species, or 
group of species, being considered as intermediate between 
any two living species, or groups of species. If by this 
term it is meant that an extinct form is directly interme- 
diate in all its characters between two living forms or 
groups, the objection is probably valid. But in a natural 
classification many fossil species certainly stand between 
living species, and some extinct genera between living 
genera, even between genera belonging to distinct families. 
The most common case, especially with respect to very dis- 
tinct groups, such as fish and reptiles, seems to be that, 
supposing them to be distinguished at the present day by a 
score of characters, 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 char- 
acters groups now widely separated from each other. This 
remark no doubt must be restricted to those groups which 
have undergone much change in the course of geological 
ages ; and it would be difficult to prove the truth of the 
proposition, for every now and then even a living animal, 
as the Lepidosiren, is discovered having affinities directed 
toward very distinct groups. Yet if we compare the older 
reptiles and Batrachians, the older fish, the older cephalo- 
pods, and the eocene mammals, with the recent members of 
fete same classes, we must admit that there is truth in the 

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 sup- 
pose that the numbered letters in Italics represent genera, 
and the dotted lines diverging from them the species in 
each genus. The diagram is much too simple, too few 
genera and too few species being given, but this is unim« 


portwrtt for us. The horizontal lines may represent succes- 
sive geological formations, and all the forms beneath the 
uppeimost line may be considered as extinct. The three 
existing genera a 14 , q 14 , p 14 , will form a small family ; b 14 and 
f 14 , a closely allied family or sub-family; and o 14 , e 14 , m 14 , 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 progen- 
itor. O.i the principle of the continued tendency to diver- 
gence of character, which was formerly illustrated by this 
diagram, Jie more recent any form is, the more it will gener- 
ally differ from its ancient progenitor. Hence, we can 
understand the rule that the most ancient fossils differ most 
from existing forms. We must not, however, assume that 
divergence of character is a necessary contingency ; it 
depends solely on the descendants from a species being 
thus enabled to seize on 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 14 . 

All the many forms, extinct and recent, descended from 
(A), make, as before remarked, one order ; and this order, 
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 

By looking at the diagram we can see that if many of the 
extinct forms supposed to be embedded in the successive 
formations, were discovered lit 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 1 , a b , a* , f*, m s , m 6 , m 9 , were disin- 
terred, these three families would be so closely linked 
together that they probably would have to be united into 
one great family, in nearly the same manner as has occurred 
with ruminants and certain pachyderms. Yet he who 
objected to consider as intermediate the extinct genera, 
which thus link together the living genera of three fami- 
lies, would be partly justified, for they are intermediate, not 


directly, but only by a long and circuitous course through 
many widely different forms. If many extinct forms were 
to be discovered above one of the middle horizontal lines 
or geological formations — for instance, above No. VI. — 
but none from beneath this line, then only two of the fami- 
lies (those on the left hand, a 14 , etc., and b u , etc.) would 
have to be united into one ; and there would remain two 
families, which would be less distinct from each other than 
they were before the discovery of the fossils. So, again, if 
the three families formed of eight genera (a 14 to ra 14 ), on 
the uppermost line, be supposed to differ from each other 
by half-a-dozen important characters, then the families 
which existed at a 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 relations. 

Under nature the process will be far more complicated 
Mian is represented in the diagram; for the groups will 
have been more numerous ; they will have endured for 
extremely unequal lengths of time, and will have been 
modified in various degrees. As we possess only the last 
volume of the geological record, and that in a very broken 
condition, we have no right to expect, except in rare cases, 
to fill up the wide intervals in the natural system, and thus 
to unite distinct families or orders. All that we have a 
right to expect is, that those groups which have, within 
known geological periods, undergone much modification, 
should in the older formations make some slight approach 
to each other; so that the older members should differ less 
from each other in some of their characters than do the 
existing members of the same groups ; and this by the con- 
current evidence of our best palaeontologists is frequently 
the case. 

Thus, on the theory of descent with modification, the 
main .facts with respect to the mutual affinities of the ex- 
tinct forms of life to each other and to living forms, are 
explained in a satisfactory manner. And they are wholly 
inexplicable. on any other view. 

: . On ;this;.same theory, it is evident that the fauna during 
any one great- period in the earth's history will be interme- 
diate in. general ohaj&cter between that which preceded aft<J 


that which succeeded it. Thus the species which lived at 
the sixth great stage of descent in the diagram are the modi- 
lied offspring of those which lived at the fifth stage, and are 
the parents of those which became still more modified at 
the seventh stage ; hence they could hardly fail to be nearly 
intermediate in character between the forms of life above 
and below. We must, however, allow for the entire extinc- 
tion of some preceding forms, and in any one region for the 
immigration of new forms from other regions, and for a 
large amount of modification during the long and blank 
intervals between the successive formations. Subject to 
these allowances, the fauna of each geological period un- 
doubtedly is intermediate in character, between the preced- 
ing 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 
recognized by palaeontologists as intermediate in character 
between those of the overlying carboniferous and underlying 
Silurian systems. But each fauna is not necessarily exactly 
intermediate, as unequal intervals of time have elapsed 
between consecutive formations. 

It is no real objection to the truth of the statement that 
the fauna of each period as a whole is nearly intermediate 
in character between the preceding and succeeding 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 existence — 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 char- 
acter, intermediate in age. But supposing for an instant, in 
this and other such cases, that the record of the first appear- 
ance and disappearance of the species was complete, which 
is far from the case, we have no reason to believe that forms 
successively produced necessarily endure for corresponding 
lengths of time. A very ancient form may occasionally have 
lasted much longer than a form elsewhere subsequently pro- 
duced, especially in the case of terrestrial productions inhab- 
iting separated districts. To compare small things with 
great; if the principal living and extinct races of the 
domestic pigeon were arranged in serial affinity, this arrange- 
ment would not closely accord with the order in time of 
their production, and even less with the order of their dis« 


appearance; for the parent rock-pigeon still lives; anu 
many varieties between the rock-pigeon and the carrier have 
become extinct ; and carriers which are extreme in the im- 
portant 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 organic 
'remains from an intermediate formation are in some degree 
intermediate in character, is the fact, insisted on by all 
palaeontologists, that fossils from two consecutive formations 
are far more closely related to each other, than are the fossils 
from two remote formations. Pictet gives as a well-known 
instance, the general resemblance of the jrganic remains 
from the several 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 acquainted 
with the distribution of existing species over the globe, will 
not attempt to account for the close resemblance of distinct 
species in closely consecutive formations, 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 vicissi- 
tudes of climate during the pleistocene period, which in- 
cludes the whole glacial epoch, and note how little the 
specific forms of the inhabitants of the sea have been 

On the theory of descent, the full meaning of the fossil 
remains from closely consecutive formations being closely 
related, though ranked as distinct species, is obvious. As 
the accumulation of each formation has often been inter- 
rupted, and as long blank intervals have intervened between 
successive formations, we ought not to expect to find, as I 
attempted to show in the last chapter, in any one or in any 
two formations, all the intermediate varieties between the 
species which appeared at the commencement and close of 
these periods : but we ought to find after intervals, very long 
as measured by years, but only moderately long as measured 
geologically, closely allied forms, or, as they have been called 
by some authors, representative species ; and these assuredly 
we do find. We find, in short, such evidence of the slow 
and scarcely sensible mutations of specific forms, as we have 
the right to expect. 




We have seen in the fourth chapter that the degree of 
differentiation and specialization of the parts in organic 
beings, when arrived at maturity, is the best standard, as 
yet suggested, of their degree of perfection or highness 
We have also seen, that, as the specialization of parts is an 
advantage to each being, so natural selection will tend to 
render the organization of each being more specialized and 
perfect, and in this sense higher; not but that it may leave 
many creatures with simple and unimproved structures fitted 
for simple conditions of life, and in some cases will even 
degrade or simplify the organization, yet leaving such de- 
graded beings better fitted for their new walks of life. In 
another and more general manner, new species become supe- 
rior to their predecessors ; for they have to beat in the strug- 
gle for life all the older forms, with which they come into close 
competition. We may therefore conclude that if under a 
nearly similar climate the eocene inhabitants of the world 
could be put into competition with the existing inhabitants, 
the former would be beaten and exterminated by the latter, as 
would the secondary by the eocene, and the palaeozic by the 
secondary forms. So that by this fundamental test of vic- 
tory in the battle for life, as well as by the standard of the 
specialization of organs, modern forms ought, on the theory 
of natural selection, to stand higher than ancient forms. Is 
this the case ? A large majority of palaeontologists would 
answer in the affirmative ; and it seems that this answer 
must be admitted as true, though difficult of proof. 

It is no valid objection to this conclusion, that certain 
Brachiopods have been but slightly modified from an ex- 
tremely remote geological epoch ; and that certain land rnd 
fresh-water shells have remained nearly the same, from tne 
time when, as far as is known, they first appeared. It is 
not an insuperable difficulty that Foraminifera have not, as 
insisted on by Dr. Carpenter, progressed in organization 
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 organized 
Protozoa ? Such objections as the above would be fatal to 
my view, if it included advance in organization as a neces- 
sary contingent. They would likewise be fatal, if the above 


Forarninifera, for instance, could be proved to have first come 
into existence during the Lauren tian epoch, or the above 
Brachiopods during the Cambrian formation ; for in this case, 
there would not have been time sufficient for the develop* 
ment of these organisms up to the standard which they had 
then reached. When advanced up to any given point, there 
is no necessity, on the theory of natural selection, for their 
further continued process ; though they will, during each 
successive age, have to be slightly modified, so as to hold 
their places in relation to slight changes in their conditions. 
The foregoing objections hinge on the question whether we 
really know how old the world is, and at what period the 
various forms of life first appeared; and this may well be 

The problem whether organization on the whole has ad- 
vanced is in many ways excessively intricate. The geological 
record, at all times imperfect, does not extend far enough 
back to show with unmistakable clearness that within the 
known history of the world organization has largely advanced. 
Even at the present day, looking to members of the same 
class, naturalists are not unanimous which forms ought to 
be ranked as highest: thus, some look at the selaceans or 
sharks, from their approach in some important points of 
structure to reptiles, as the highest fish ; others look at the 
teleosteans as the highest. The ganoids stand intermediate 
between the selaceans and teleosteans ; the latter at the 
present day are largely preponderant in number ; but for- 
merly 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 organization. 
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 believed to be " in fact more highly organ- 
ized than a fish, although upon another type " ? In the 
complex struggle for life it is quite credible that crustaceans, 
not very high in their own class, might beat cephalopods, 
the highest mollusks ; and such crustaceans, though not highly 
developed, would stand very high in the scale of invertebrate 
animals, if judged by the most decisive of all trials— -the 
law of battle. Beside these inherent difficulties in deciding 
which forms are the most advanced in organization, we ought 
not solely to compare the highest members of a class at any 
t,wo periods — though undoubtedly this is one and perhaps 


fche most important element in striking a balance — but we 
ought to compare all the members, high and low, at two 
periods. At an ancient epoch the highest and lowest mol- 
luscoidal animals, namely, cephalopods and brachiopods, 
swarmed in numbers; at the present time both groups are 
greatly reduced, while others, intermediate in organization, 
have largely increased ; consequently some naturalists main- 
tain that mollusks were formerly more highly developed 
than at present ; but a stronger case can be made out on the 
opposite side, by considering the vast reduction of brachio- 
pods, and the fact that our existing cephalopods, though 
few in number, are more highly organized than their ancient 
representatives. We ought also to compare the relative 
proportional numbers, at any two periods, of the high and 
low classes throughout the world : if, for instance, at the 
present day fifty thousand kinds of vertebrate animals exist, 
and if we knew that at some former period only ten thou- 
sand kinds existed, we ought to look at this increase in num- 
ber in the highest class, which implies a great displacement 
of lower forms, as a decided advance in the organization of 
the world. We thus see how hopelessly difficult it is to 
compare with perfect fairness", under such extremely com- 
plex relations, the standard of organization of the imper- 
fectly known faunas of successive periods. 

We shall appreciate this difficulty more clearly by look- 
ing to certain existing faunas and floras. From the ex- 
traordinary 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 Zealand, a multitude of British 
forms would in the course of time become thoroughly nat- 
uralized there, and would exterminate many of the natives. 
On the other hand, from the fact that hardly a single inhab- 
itant of the southern hemisphere has become wild in any 
part of Europe, we may well doubt whether, if all the pro- 
ductions of New Zealand were set free in Great Britain, any 
considerable number would be enabled to seize on places 
now occupied by our native plants and animals. Under 
this point of view, the productions of Great Britain stand 
much higher in the scale than those of New Zealand. Yet 
the most skilful naturalists, from an examination of the 
species of the two countries, could not have foreseen tkia 


Agassiz and several other highly competent judges insist 
that ancient animals resemble to a certain extent the em- 
bryos of recent animals belonging to the same classes ; and 
that the geological succession of extinct forms is nearly 
parallel with the embryological development of existing 
forms. This view accords admirably well with our theory. 
In a future chapter I shall attempt to show that the adult 
differs from its embryo, owing to variations having super- 
vened at a not early age, and having been inherited at a 
corresponding age. This process, while it leaves the em- 
bryo 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 pic- 
ture, preserved by nature, of the former and less modified 
condition of the species. This view may be true, a,nu ^et 
may never be capable of proof Seeing, for instance, that 
the oldest known mammals, reptiles, and fishes strictly be- 
long 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 commoik 
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. 


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 armor, like those of the armadillo, found 
in several parts of La Plata, and Professor Owen has 
shown in the most striking manner that most of the fossil 
mammals, buried there in such numbers, are related to 
South American types. This relationship is even more 
clearly seen in the wonderful collection of fossil bones 
made by MM. Lund and Clausen in the caves of Brazil. I 
was so much impressed with thes? facts that I strongly in- 
sisted, in 1839 and 1845, on this " law of the succession of 
types," — on "this wonderful relationship in the same con- 
tinent between the dead and the living." Professor Owen 
has subsequently extended the same generalization 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 mollusks, 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, 

Now, what does this remarkable law of the succession of I 
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 continents ; and, on the 
other hand through similarity of conditions, for the uni- 
formity of the same types in each continent during the 
later tertiary periods. Nor can it be pretended that it is 
an immutable law that marsupials should have been chiefly 
or solely produced in Australia ; or that Edentata and 
other American types should have been solely produced 
in South America. For we know that Europe in ancient 
times was peopled by numerous marsupials ; and I have 
shown, in the 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 par- 
took strongly of the present character of the southern half 
of the continent ; and the southern half was formerly 
more closely allied, than it is at present, to the northern 
half. In a similar manner we know, from Falconer and 
Cautley's discoveries, that Northern India was formerly 
more closely related in its mammals to Africa than it is at 
the present time. Analogous facts could be given in rela- 
tion to the distribution of marine animals. 

On the theory of descent with modification, the great law 
t»f the long-enduring, but not immutable, succession of the 
same types within the same areas, is at once explained ; for 
the inhabitants of each quarter of the world will obviously 
tend to leave in that quarter, during the next succeeding 
period of time, closely allied though in some degree modi- 
fied descendants. If the inhabitants of one continent for- 
merly 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 inter-migration, the feebler will yield to the more 
dominant 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 for- 
merly lived in South America, have left behind them the 
sloth, armadillo, and ant-eater, as their degenerate descend 
ants. 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 ex- 
tinct 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 progeni- 
tors 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 for- 
mation, and in a succeeding formation there be six other 
allied or representative genera, each with the same number 
of species, then we may conclude that generally only one 
species of each of the older genera has left modified 
descendants, which constitute the new genera 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 case, two or three species in two or 
three alone of the six older genera will be the parents of 
the new genera : the other species and the other old genera 
having become utterly extinct. In failing orders, with the 
genera and species decreasing in numbers as is the case 
with the Edentata of South America, still fewer genera and 
species will leave modified blood-descendants. 


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 
psssed away even during a single formation ; that,- owing 


to subsidence being almost necessary for the accumulation 
of deposits rich in fossil species of many kinds, and thick 
enough to outlast future degradation, great intervals of 
time must have elapsed between most of our successive 
formations ; that there has probably been more extinc- 
tion during the periods of subsidence, and more variation 
during the periods of elevation, and during the latter the 
record will have been least perfectly kept ; that each single 
formation has not been continuously deposited ; that the 
duration of each formation is probably short compared 
with the average duration of specific forms ; that migra- 
tion has played an important part in the first appear- 
ance 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 numerous transitional 
stager it is probable that the periods, during which each 
undeiv/ent modification, though many and long as meas- 
ured by years, have been short in comparison with the 
periods during which each remained in an unchanged con- 
dition. These causes, taken conjointly, will to a large 
extent explain why — though we do find many links — we 
do not find interminable varieties, connecting together all 
extinct and existing forms by the finest graduated steps. 
It should also be constantly borne in mind that any linking 
variety between two forms, which might be found, would 
be ranked, unless the whole chain could be perfectly 
restored, as a new and distinct species; for it is not pre- 
tended that we have any sure criterion by which species 
and varieties can be discriminated. 

He who rejects this view of the imperfection of the geo- 
logical record, will rightly reject the whole theory. For 
he may ask in vain where are the numberless transitional 
links which must formerly have connected the closely 
allied or representative species, found in the successive 
stages of the same great formation ? He may 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 forma- 
tions 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 n» 


merous 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 leading 
facts in palaeontology 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 ex- 
tinction of old forms is the almost inevitable consequence 
of the production of new forms. W r e can understand why, 
when a species has once disappeared, it never reappears. 
Groups of species increase in numbers slowly, and endure 
for unequal periods of time ; for the process of modifica- 
aon is necessarily slow, and depends on many complex 
jontingencies. The dominant species belonging to large 
>nd 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 modified off* 
spring on the face of the earth. But the utter extinction 
of a whole group of species has sometimes been a slow pro- 
cess, from the survival of a few descendants, lingering in 
protected and isolated situations. W T hen a group has once 
wholly disappeared, it does not reappear ; for the link of 
generation has been broken. 

We can understand how it is that dominant forms which 
spread widely and yield the greatest number of varieties 
tend to people the world with allied, but modified, descend- 
dants ; and these will generally succeed in displacing the 
groups which are their inferiors in the struggle for exist- 
ence. Hence, after long intervals of time, the productions 
of the world appear to have changed simultaneously. 


We can understand how it is that all the forms of life, 
ancient and recent, make together a few grand classes. 
We can understand, from the continued tendency tc di- 
vergence of character, why, the more ancient a form is, 
the more it generally differs from those now living ; why 
ancient and extinct forms often tend to fill up gaps be- 
tween existing forms, sometimes blending two groups, pre^ 
viously classed as distinct, into one ; but more commonly 
bringing them only a little closer together. The more 
ancient a form is, the more often it stands in some degree 
intermediate between groups now distinct ; for the more 
ancient a form is, the more nearly it will be related to, and 
consequently resemble, the common progenitor of groups, 
since become widely divergent. Extinct forms are seldom 
directly intermediate between existing forms ; but are in- 
termediate only by a long and circuitous course through 
other extinct 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 gen- 
eration. We can clearly see why the remains of an inter- 
mediate 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 specialized; and this 
may account for the common belief held by so many palae- 
ontologists, that organization on the whole has progressed. 
Extinct and ancient animals resemble to a certain extent the 
embryos of the more recent animals belonging to the same 
classes, and this wonderful fact receives a simple explana- 
tion according to our views. The succession of the same 
types of structure within the same areas during the later 
geological periods ceases to be mysterious, and is intelligible 
on the principle of inheritance. 

If, then, the geological record be as imperfect as many 
believe, and it may at least be asserted that the record can- 
not be proved to be much more perfect, the main objections 
to the theory of natural selection are greatly diminished or 
disappear. On the other hand, all the chief laws of 
palaeontology plainly proclaim, as it seems to me, that 
species have been produced by ordinary generation : old 
forms having been supplanted by new and improved forms 
of life, the products of Variation ajid the Survival of tka 




Present Distribution cannot be accounted for by Differences t» Y'hysi, 
cal Conditions — Importance of Barriers — Affinity of th^ Produc- 
tions 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 dissimilarity of 
the inhabitants of various regions can be wholly accounted 
for by climatal and other physical conditions. Of late, 
almost every author who has studied the subject has come 
to this conclusion. The case of America alone would 
almost suffice to prove its truth ; for if we exclude the 
arctic and northern temperate parts, all authors agree that 
one of the most fundamental divisions in geographical 
distribution is that between the New and the Old Worlds ; 
yet if we travel over the vast American continent, from 
the central parts of the United States to its extreme south- 
ern 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 con- 
dition in the Old World which cannot be paralleled in 
the New — at least so closely as the same species generally 
require. No doubt small areas can be pointed out in the 
Old World, hotter than any in the New World ; but these 
are not inhabited by a fauna different from that of the sur- 
rounding districts ; for it is rare to find a group of organ- 
isms confined to a small area, of which the conditions are 
peculiar in only a slight degree. Notwithstanding this 
general parallelism in the conditions of Old and New 
Worlds, how widely different are their living productions ! 

In the southern hemisphere, if we compare large tracts 
qf land in Australia, South Africa, and western Soutk 


America, between latitudes 25 and 35 degrees, we shall 
find parts extremely similar in all their conditions, yet it 
would not be possible to point out three faunas and floras 
more utterly dissimilar. Or, again, we may compare the 
productions of South America south of latitude 35 degrees 
with those north of 25 degrees, which consequently are sep- 
arated by a space of ten degrees of latitude, and are 
exposed to considerably different conditions ; yet they are 
incomparably more closely related to each other than they 
are to the productions of Australia or Africa under nearly 
the same climate. Analogous facts could be given with 
respect to the 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 climate, there 
might have been free migration for the northern temperate 
forms, as there now is for the strictly arctic productions. 
We see the same fact in the great difference 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, etc., 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 continents. 

Turning to the sea, we find the same law. The marine 
inhabitants of the eastern and western shores of South 
America are very distinct, with extremely few shells, Crus- 
tacea, 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. Westward 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 dis- 
tinct fauna. So that three marine faunas range northward 
and southward in parallel lines not far from each other, 
under corresponding climate ; but from being separated 
from each other by impassable barriers, either of land or 
open sea, they are almost wholly distinct. On the other 
hand, proceeding still farther westward from the eastern 
islands of the tropical parts of the Pacific, we encounter 
no impassable barriers, and we have innumerable islands as 
halting-places, or continuous coasts, until, after travelling 
over a hemisphere, we come to the shores of Africa ; and 
over this vast space we meet with no well-defined and dis- 
tinct marine faunas. Although so few marine animals are 
common to the above-named three approximate faunas of 
Eastern and Western America and the eastern Pacific 
islands, yet many fishes range from the Pacific into the 
Indian Ocean, and many shells are common to the eastern 
islands of the Pacific and the eastern shores of Africa on 
almost exactly opposite meridians of longitude. 

A third great fact, partly included in the foregoing state- 
ment, is the affinity of the productions of the same con- 
tinent or of the same sea, though the species themselves are 
distinct at different points and stations. It is a law of the 
widest generality, and every continent offers innumerable 
instances. Nevertheless, the naturalist, in travelling, for 
instance, from north to south, never fails to be struck by the 
manner in which successive groups of beings, specifically 
distinct, though nearly related, replace each other. He 
hears from closely allied yet distinct kinds of birds, notes 
nearly similar, and sees their nests similarly constructed, 
but not quite alike, with eggs colored in nearly the same 
manner. The plains near the Straits of Magellan are in« 
habited by one species of Rhea (American ostrich), and 
northward the plains of La Plata by another species of the 
same genus ; and not by a true ostrich or emu, like those 
inhabiting Africa and Australia under the same latitude. 
On these same plains of La Plata we see the agouti and 
bizcacha, animals having nearly the same habits as our 
hares and rabbits, and belonging to the same order of rodents, 
but they plainly display an American type of structure. 
We ascend the lofty peaks of the Cordillera, and we find an 
alpine species of bizcacha; we look to the waters, and we 
do not find the beaver or muskrat, but the coypu and capy- 
bara, rodents of the South American type. Innumerably 


other instances could be giveD II we look to the islands 
off the American shore, however much they may differ in 
geological structure, tk- inhabitants are essentially American, 
though they mav L^ all peculiar species. We may look back 
to past age? <*3 shown in the last chapter, and we find 
America**. «ypes then prevailing on the American continent 
and ii clie American seas. We see in these facts some deep 
or 6 aiiic bond, throughout space and time, over the same 
areas of land and water, independently of physical condi- 
tions. The naturalist must be dull who is not led to inquire 
what this bond is. 

The bond is simply inheritance, that cause which alone, 
as far as we positively know, produces organisms quite like 
each other, or, as we see in the case of varieties, nearly alike. 
The dissimilarity of the inhabitants of different regions 
may be attributed to modification through variation and nat- 
ural selection, and probably in a subordinate degree to the 
definite influence of different physical conditions. The de- 
grees of dissimilarity will depend on the migration of the 
itore 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 for- 
mer immigrants — and on the action of the inhabitants on 
each other in leading to the preservation of different modi- 
fications ; the relation of organism to organism in the strug- 
gle for life being, as I have already often remarked, the 
most important of all relations. Thus the high importance 
of barriers comes into play by checking migration ; as does 
time for the slow process of modification through natural 
selection. Widely ranging species, abounding in individ- 
uals, which have already triumphed over many competitors 
in their own widely extended homes, will have the best 
chance of seizing on new places, when they spread out into 
new countries. In their new homes they will be exposed 
to new conditions, and will frequently undergo further 
modification and improvement ; and thus they will become 
still further victorious, and will produce groups of modified 
descendants. On this principle of inheritance with modifi- 
cation we can understand how it is that sections of genera, 
whole genera, and even families, are confined to the same 
areas, as is so commonly and notoriously the case. 

There is no evidence, as w r as remarked in the last chapter, 
of the existence of any law of necessary development. As 
the variability of each species is an independent property, 


and will be taken advantage of by natural selection, only so 
far as it profits each individual 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 td 
migrate in a body into a new and afterward isolated country, 
they would be little liable to modification ; for neither migra 
tion nor isolation in themselves effect any thing. Thes« 
principles come into play only by bringing organisms info 
new relations with each other and in a lesser degree wit\ 
the surrounding physical conditions. As we have seen ii* 
the last chapter that some forms have retained nearly the 
same character from an enormously remote geological period, 
so certain species have migrated over vast spaces, and have 
not become greatly or at all modified. 

According to these views, it is obvious that the several 
species of the same genus, though inhabiting the most 
d.'stant quarters of the world, must originally have pro- 
ceeded 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 believing that 
they have migrated from the same region ; for during the 
vast geographical and climatical changes which have super- 
vened since ancient times, almost any amount of migration 
is possible. But in many other cases, in which we have 
reason to believe that the species of a genus have been 
produced within comparatively recent times, there is great 
difficulty on this head. It is also obvious that the individ- 
uals of the same species, 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 distinct. 


We are thus brought to the question which has been 
largely discussed by naturalists, namely, whether species 
have been created at one or more points of the earth's 
surface. Undoubtedly there are many cases of extreme 
difficulty in understanding how the same species could 
possibly have migrated from some one point to the several 
distant and isolated points where now found. Nevertheless 


the simplicity of the view that each species was first pro- 
duced within a single region captivates the mind. He who 
rejects it, rejects the vera causa of ordinary generation with 
subsequent migration, and calls in the agency of a miracle. 
It is universally admitted, that in most cases the area 
inhabited by a species is continuous , and that when a plant 
or animal inhabits two 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 inhabiting distant points of the world. 
No geologist feels any difficulty in Great Britain possessing 
the same quadrupeds with the rest of Europe, for they were 
no doubt once united. But if the same species can be pro- 
duced at two separate points, why do we not find a single 
mammal common to Europe and Australia or South America? 
The conditions of life are nearly the same, so that a multi- 
tude of European animals and plants have become natural- 
ized 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 mammals 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 intel- 
ligible 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, 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 naturalists,- 
tiiat the view of each SD§cies Uaviflg been produced in ons 


area alone, and having subsequently migrated from that 
area as far as its powers of migration and subsistence under 
past and present conditions permitted, 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 climatical changes 
which have certainly occurred within recent geological 
times, must have rendered discontinuous the formerly con- 
tinuous range of many species. So that we are reduced tc 
consider whether the exceptions to continuity of range are 
so numerous, and of so grave a nature, that we ought to give 
up the belief, rendered probable by general considerations, 
that each species has been produced within one area, and 
has migrated thence as far as it could. It would be hope- 
lessly tedious to discuss all the exceptional cases of the 
same species, now living at distant and separated points, 
nor do I for a moment pretend that any explanation could 
be offered of many 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 chapter), the wide distribution of fresh-water pro- 
ductions ; and thirdly, the occurrence of the same terrestrial 
species on islands and on the nearest mainland, though sep- 
arated by hundreds of miles of open sea. If the existence 
of the same species at distant and isolated points of the 
earth's surface can in many instances be explained on the 
view of each species having migrated from a single birth- 
place, then, considering our ignorance with respect to former 
climatical and geographical changes, and to the various 
occasional means of transport, the belief that a single birth- 
place is the law seems to me incomparably the safest. 

In discussing this subject we shall be enabled at the same 
time to consider a point equally important for us, namely, 
whether the several species of a genus which must on our 
theory all be descended from a common progenitor, can have 
migrated, undergoing modification during their migration 
from some one area. If, when most of the species inhabiting 
one region are different from those of another region, though 
closely allied to them, it can be shown that migration from 
the one region to the other has probably occurred at some 
former period, our general view will be much strengthened ,* 
for the explanation is obvious on the principle of descent 


with modification. A volcanic island, for instance, upheaved 
and formed at the distance of a few hundreds of miles from 
a continent, would probably receive from it in the course of 
time a few colonists, and their descendants, though modi- 
fied, would still be related 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 
independent creation. This view of i;he 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 species." 
And it is now well known that he attributes this coincidence 
to descent with modification. 

The question of single or multiple centres of creation 
differs from another though allied question ; namely, 
whether all individuals of the same species are descended 
from a single pair, or single hermaphrodite, or whether, as 
some authors suppose, from many individuals simultane- 
ously created. With organic beings which never intercross, 
if such exist, each species must be descended from a succes- 
sion of modified varieties, that have supplanted each other, 
but have never blended with other individuals or varieties 
of the same species ; so that, at each successive stage of 
modification, all the individuals of the same form will be 
descended from a single parent. But in the great majority 
of cases, namely, with all organisms which habitually unite 
for each birth, or which occasionally intercross, the individ- 
uals 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 individuals during each 

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



Sir C. Lyell and other authors have ably treated this sub* 
iect. I can give here only the briefest abstract of the more 
inportant facts. Change of climate must have had a power- 
fu influence on migration. A region now impassible to 
certain organisms from the nature of its climate, might have 
beei a high road for migration, when the climate was differ- 
ent, J shall, however, presently have to discuss this branch 
of tne subject in some detail. Changes of level in the land 
must also have been highly influential : a narrow isthmus 
now separates two marine faunas ; submerge it, or let it 
formerly have been submerged, and the two faunas will now 
blend together, or may formerly have blended. Where ths 
sea now extends, land may at a former period have con- 
nected 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 organ- 
isms. Edward Forbes insisted that all the islands in the 
Atlantic must have been recently connected with Europe or 
Africa, and Euiope likewise with America. Other authors 
have thus nypcthetically bridged over every ocean, and 
united almost every island with some mainland. If, iudeed, 
the arguments used by Forbes are to be trusted, it must be 
admitted that scarcely a single island exists which has not 
recently been united to some continent. This view cuts the 
Gordian knot of the dispersal of the same species to the 
most distant points and removes many a difficulty ; but to 
the best of my judgment we are not authorized in admit- 
ting 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 01 
sea ; but not of such vast changes in the position and exten- 
sion of our continents, as to have united them within the 
recent period to each other and to the several intervening 
oceanic islands. I freely admit the former existence of 
many islands, now buried beneath the sea, which may have 
served as halting-places for plants and for many animals 
during their 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 somethiug definite about the means of distribution, 
we shall be enabled to speculate with security on the former 
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 con- 
tinuously, or almost continuously united with each other, 
and with the many existing oceanic islands. Several facts 
in distribution — such as the great difference in the marine 
faunas on the opposite sides of almost every continent — 
the close relation of the tertiary inhabitants of severaJ 
lands and even seas to their present inhabitants — the 
dagree of affinity between the mammals inhabiting islands 
with those of the nearest continent, being in part deter 
mined (as we shall hereafter see) by the depth of the inter 
vening ocean — these and other such facts are apposed to 
the admission of such prodigious geographicaj revolutions 
within the recent period, as are necessary on the view 
advanced by Forbes and admitted by his followers. The 
nature and relative proportions of the inhabitants of oceanio 
islands are likewise opposed to the belief t,t their former 
continuity of continents. Nor does the almost universally 
volcanic composition of such islands favoj. che admission 
that they are the wrecks of sunken continei ts . if they bad 
originally existed as continental mountain anges, some at 
least of the islands would have been for ned, like other 
mountain summits, of granite, metamorp, ac schists, old 
fossiliferous and other rocks, instead of consisting of mere 
piles of volcanic matter. 

I must now say a few words on wnat are iallea accidental 
means, but which more properly should be ^aiJea occasional 
means of distribution. I shall here Routine myself to 
plants. In botanical works, this or thao plant is often 
stated to be ill adapted for wide dissemination, but the 
greater or less facilities for transport across the sea may be 
said to be almost wholly unknown. Until I tried, with Mr. 
Berkeley's aid, a few experiments, it was not even known 
how far seeds could resist the injurious action of sea-water. 
To my surprise I found that out of eighty-seven kinds, 
sixty-four germinated after an immersion of twenty-eight 
days, and a few survived an immersion 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 specief 


of the allied orders, Hyckophyllaceae and Polemoniacese, 
were all killed by a month's immersion. For convenience 
sake I chiefly tried small seeds without the capsules or 
fruit; and as all of these sunk in a few days, they could 
not have been floated across wide spaces of the sea, whether 
or not they were injured by salt water. Afterward I tried 
some larger fruits, capsules, etc., and some of these floated 
for a long time. It is well known what a difference there 
is in the 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 ninety-four plants with ripe fruit, and to place them on 
sea-water. The majority sunk quickly, but some which, 
while green, floated, for a very short time, when dried, 
floated much longer ; for instance, ripe hazel-nuts sunk 
immediately, but when dried they floated for ninety days, 
and afterward when planted germinated; an asparagus plant 
with ripe berries floated for twenty-three days, when dried 
it floated for eighty-five days, and the seeds afterward ger. 
minated; the ripe seeds of Helosciadium sunk in two days, 
when dried they floated for above ninety days, and after- 
ward germinated. Altogether, out of the ninety-four dried 
plants, eighteen floated for above twenty-eight days ; and 
some of the eighteen floated for a very much longer period. 
So that as f f kinds of seeds germinated after an immersion 
of twenty-eight days; and as ^f distinct species with ripe 
fruit (but not all the same species as in the foregoing ex- 
periment) floated, after being dried, for above twenty-eight 
days, we may conclude, as far as anything can be inferred 
from these scanty facts, that the seeds of ^-^ kinds of 
plants of any country might be floated by sea-currents 
during twenty-eight days, and would retain their power of 
germination. In Johnston's Physical Atlas, the average 
rate of the several Atlantic currents is thirty-three miles 
per diem (some currents running at the rate of sixty miles 
per diem) ; on this average, the seeds of ^fcfe plants belong- 
ing to one country might be floated across 924 miles of sea 
to another country, and when stranded, if blown by an 
inland gale to a favorable spot, would germinate. 

Subsequently to my experiments, M. Martens tried simi- 
lar ones, but in a much better manner, for he placed the 
seeds in a box in the actual sea, so that they were alter 
nately wet and exposed to the air like really floating plants, 


He twed ninety-eight seeds, mostly different from mine, 
but lie chose many large fruits, and likewise seeds, from 
plants which live near the sea; and this would have favored 
both the average length of their flotation and their resist- 
ance 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 J | of his seeds of different kinds floated for forty -two 
days, and were then capable of germination. But I do not 
doubt that plants exposed to the waves would float for a 
less time than those protected from violent movement as in 
our experiments. Therefore, it would perhaps be safer to 
assume that the seeds of about ^°q 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 facts of 
the larger fruits often floating longer than the small, is 
interesting; as plants with large seeds or fruit which, as 
Alph. de Candolle has shown, generally have restricted 
ranges, could hardly be transported by any other means. 

Seeds may be occasionally transported in another manner. 
Drift timber is thrown up on most islands, even on those in 
the midst of the widest oceans ; and the natives of the coral 
islands in the Pacific procure stones for their tools, solely 
from the roots of drifted trees, these stones being a valuable 
royal tax. I find that when irregularly shaped stones are 
embedded in the roots of trees, small parcels of earth are 
frequently enclosed in their interstices and behind them, so 
perfectly that not a particle could be washed away during 
the longest transport : out of one small portion of earth 
thus completely inclosed by the roots of an oak about fifty 
years old, three dicotyledonous plants germinated: I am 
certain of the accuracy of this observation. Again, I can 
show that the carcasses 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 : pease and vetches, for instance, are killed by even 
a few days' immersion in sea-water ; but some taken out of 
the crop of a pigeon, which had floated on artificial sea- 
water for thirty days, to my surprise nearly all germinated. 

Living birds can hardly fail to be highly effective agents 
in the transportation of seeds. I could give many facts 
showing how frequently birds of many kinds are blown by 
gales to vast distances across the ocean. We may safety 



assume that under such circumstances their rate of flight 
would often be thirty-five miles an hour ; and some authors 
have given a far higher estimate. I have never seen an 
instance of nutritious seeds passing through the intestines 
of a bird; but hard seeds of fruit pass uninjured through 
even the digestive organs of a turkey. In the course of two 
months, I picked up in my garden twelve kinds of seeds. 
out of the excrement of small birds, and these seemed per- 
fect, and some of them, which were tried, germinated. Bui 
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 posi- 
tively asserted that all the grains do not pass into the gizzard 
for twelve or even eighteen hours. A bird in this interval 
might easily be blown to the distance of five hundred miles, 
and hawks are known to look out for tired birds, and the 
contents of their torn crops might thus readily get scattered. 
{Some hawks and owls bolt their prey whole, and, after an 
interval of from twelve to twenty hours, disgorge pellets, 
which, as I know from experiment made in the Zoological 
Gardens, include seeds capable of germination. Some seeds 
of the oat, wheat, millet, canary, hemp, clover, and beet 
germinated after having been from twelve to twenty-one 
hours in the stomachs of different birds of prey ; and two 
seeds of beet grew after having been thus retained for two 
days and fourteen hours. Fresh-water fish, I find, eat seeds 
of many land and water plants ; fish are frequently devoured 
by birds, and thus the seeds might be transported from 
place to place. I forced many kinds of seeds into the 
stomachs of dead fish, and then gave their bodies to fishing- 
eagles, storks and pelicans ; these birds, after an interval of 
many hours, either rejected the seeds in pellets or passed 
them in their excrement ; and several of these seeds retained 
the power of germination. Certain seeds, however, were 
always killed by this process. 

Locusts are sometimes blown to great distances from the 
land. I myself caught one 370 miles from the coast of 
Africa, and have heard of others caught at greater distances. 
The Rev. R. T. Lowe informed Sir C. Lyell that in Novem- 
ber, 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 upward as far 
as could be seen with a telescope. During two or three 


days they slowly careered round and round in an immense 
ellipse, at least five or six miles in diameter, and at night 
alighted on the taller trees, which were completely coated 
with them. They then disappeared over the sea, as suddenly 
as they had appeared, and have not since visited the island. 
Now. in parts of Natal it is believed by some farmers, 
though on insufficient evidence, that injurious seeds are 
introduced into their grass-land in the dung left by the great 
flights of locusts which often visit that country. In conse- 
quence 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 (Juncus bufonius) which germinated 
and flowered. Mr. Swaysland, of Brighton, who during the 
last forty years has paid close attention to our migratory 
birds, informs me that he has often shot wagtails (Mota- 
cillse), wheatears, and whinchats (Saxicolse), on their first 
arrival on our snores, 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. Professor Newton 
■sent me the leg of a red-legged partridge (Caccabis rufa) 
which had been wounded and could not fly, with a ball of . 
hard earth adhering to it, and weighing six and a half . 
ounces. The earth had been kept for three years, but when 
broken, watered and placed under a bell glass, no less than 
eighty-two plants sprung from it : these consisted of twelve 
monocotyledons, including the common oat, and at least one 
kind of grass, and of seventy dicotyledons, which consisted, 
judging from the young leaves, of at least three .distinct 
species. With such facts before us, can we doubt .that the 
©any 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 arctie 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 com- 
parison with the latitude, I suspected that these islands had 
been partly stocked by ice-born seeds during the Glacial 
epoch. At my request Sir C. Lyell wrote to M. Hartung to 
inquire whether he had observed erratic bowlders 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 burdens on the shores of these 
mid-ocean islands, and it is at least possible that thej' may 
have brought thither some few seeds of Northern plants. 

Considering that these several means of transport, and 
that other means, which without doubt remain to be dis- 
covered, have been in action year after year for tens of 
thousands of years, it would, I think, be a marvellous fact 
if many plants had not thus become widely transported. 
These means of transport are sometimes called accidental ; 
but this is not strictly correct : the currents of the sea are 
not accidental, nor is the direction of prevalent gales of 
wind. It should be observed that scarcely any means of 
transport would carry seeds for very great distances : for 
seeds do not retain their vitality when exposed for a great 
length of time to the action of sea-water; nor could they 
be long carried in the crops or intestines of birds. These 
means, however, would suffice for occasional transport across 
tracts of sea some hundred miles in breadth, or from island 
to island, or from a continent to a neighboring island, but 
not from one distant continent to another. The floras of 
distant continents would not by such means become mingled j 


but would remain as distinct as they now are. The currents, 
from their course, would never bring seeds from North 
America to Britain, though they might and do bring seeds 
irom 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 falling 
on favorable soil, and coming to maturity ! But it would be 
a great error to argue that because a well-stocked island, like 
Great Britain, has not, as far as is known (and it would be 
very difficult to prove this), received within the last few 
centuries, through occasional means of transport, immigrants 
from Europe of 5iiy other continent, that a poorly stocked 
island, though standing more remote from the mainland, 
would not receive colonists by similar means. Out of a 
hundred kinds of seeds or animals transported to an island, 
even if far less well stocked than Britain, perhaps not more 
than one would be so well fitted to its new home, as to 
become naturalized. But this is no valid argument against 
what would be effected by occasional means of transport^ 
during the long i^pse of geological time, while the island 
was being upheaved, and before i>. had become fully stocked 
with inhabitants. On almost bare land, with few or no 
destructive insects or birds living there, nearly every seed 
which chanced to arrive, if fitted for the climate, would 
germinate and survive. 



The identity of many plants and animals, on mountain- 
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 extreme northern parts of Europe ; 
but it is far more remarkable, that the plants on the Whit© 


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 mount* 
ains 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 expla- 
nation 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 burned by fire do not tell their tale more plainly than 
do the mountains of Scotland and Wales, with their scored 
flanks, polished surfaces, and perched bowlders, of the icy 
streams with which their valleys were lately filled. So 
greatly has the climate of Europe changed, that in North- 
ern Italy, gigantic moraines, left by old glaciers, are now 
clothed by the vine and maize. Throughout a large part 
of the United States, erratic bowlders and scored rocks 
plainly reveal a former cold period. 

The former influence of the glacial climate on the dis- 
tribution 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 south- 
ern zone became fitted for the inhabitants of the north, 
these would take the places of the former inhabitants of 
the temperate regions. The latter, at the same time, 
would travel farther and farther southward, unless they 
were stopped by barriers, in which case they would perish. 
The mountains would become covered with snow and ice, 
and their former alpine inhabitants would descend to the 
plains. By the time that the cold had reached its maxi- 
mum, we should have an arctic fauna and flora, covering 
the central parts of Europe, as far south as the Alps and 
Pyrenees, and even stretching into Spain. The now 
temperate regions of the United States would likewise be 
covered by arctic plants and animals, and these would be 
nearly the same with those of Europe ; for the present 
circumpolar inhabitants, which we suppose to have every- 
where travelled southward, are remarkably uniform round 
the world. 


As the warmth returned, the arctic forms would retreat 
northward, closely followed up in their retreat by the pro- 
ductions 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, while their breth- 
ren 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 Amer- 
ican 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 return- 
ing 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 satis- 
factory 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 moun- 
tain summits, we may almost conclude, without other evidence, 
that a colder climate formerly permitted their migration 
across the intervening lowlands, now become too warm for 
their existence. 

As the arctic forms moved first southward and after- 
ward backward to the north, in unison with the changing 
climate, they will not have been exposed during their long 
migrations to any great diversity of temperature; and as 
they all migrated in a body together, their mutual rela- 
tions will not have been much disturbed. Hence, in 
accordance with the principles inculcated in this volume, 
these forms will not have bpen. liable to much modification. 


But with the alpine productions, left isolated from th« 
moment of the returning warmth, first at the bases and 
ultimately on the summits of the mountains, the case will 
have been somewhat different ; for it is not likely that all 
the same arctic species will have been left on mountain 
ranges far distant from each other, and have survived there 
ever since ; they will also, in all probability, have become 
mingled with ancient Alpine species, which must have 
existed on the mountains before the commencement of the 
Glacial epoch, and which during the coldest period will 
have been temporarily driven down to the plains; 'they 
will, also, have been subsequently exposed to somewhat 
different climatical influences. Their mutual relations will 
thus have been in some degree disturbed; consequently 
they will have been liable to modification ; and they have 
been modified, for if we compare the present alpine plants 
and animals of the several great European mountain 
ranges, one with another, though many of the species 
remain identically the same, some exist as varieties, some 
as doubtful forms of sub-species, and some as distinct yet 
closely allied species representing each other on the several 

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 are 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 mountain slopes and on the 
plains of North America and Europe are the same ; and it 
may be asked how I account for this degree of uniformity 
in the sub-arctic and temperate forms round the world, at 
the commencement of the real Glacial period. At the 
present day, the sub-arctic and northern temperate pro- 
ductions of the Old and New Worlds are separated from 
each other by the whole Atlantic Ocean and by the north- 
ern part of the Pacific. During the Glacial period, when 
the inhabitants of the Old and New Worlds lived further 
southward than they do at present, they must have been 
still more completely separated from each other b}^ wider 
spaces of ocean ; so that it may well be asked how the same 
species could then or previously have entered the two con- 
tinents. The explanation, I believe, lies in the nature of 
the climate before the commencement of the Glacial peried. 


At this, the newer Pliocene period, the majority of the in- 
habitants of the world were specifically the same as now, and 
we have good reason to believe that the climate was warmer 
than at the present day. Hence we may suppose that the 
organisms which now live under latitude 60 degrees, lived 
during the Pliocene period further north, under the Polar 
Circle, in latitude 66-67 degrees ; 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 undei 
the Polar Circle that there is almost continuous land from 
Western Europe througn Siberia, to Eastern America. And 
this continuity of the circumpolar land, with the conse- 
quent freedom under a more favorable climate for inter- 
migration, will account for the supposed uniformity of the 
sub-arctic and temperate productions of the Old and New 
Worlds, at a period anterior to the Glacial epoch. 

Believing, from reasons before alluded to, that our con- 
tinents have long remained in nearly the same relative posi- 
tion, 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 number of the same plants and 
animals inhabited the almost continuous circumpolar land ; 
and that these plants and animals, both in the Old and New 
Worlds, began slowly to migrate southward 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 under- 
stand the relationship, with very little identity, between the 
productions of North America and Europe, — a relationship 
which is highly remarkable, considering the distance of the 
two areas, and their separation by the whole Atlantic Ocean. 
We can further understand the singular fact remarked on by 
several observers, that the productions of Europe and Amer 
ica during the later tertiary stages were more closely relatec ( 
to each other than they are at the present time ; for during 
these warmer periods the northern parts of the Old and New 
Worlds will have been almost continuously united by land, 
serving as a bridge, since rendered impassable by cold, for 
inter-migration of their inhabitants. 

During the slowly decreasing warmth of the Pliocene 
period, as soon as the species in common, which inhabited 
the New and Old Worlds, migrated south of the Polar Circlei 


they will have been completely cut off from each othefc 
This separation, as far as the more temperate productions 
are concerned, must have taken place long a,ges ago. As 
the plants and animals migrated southward, they will have 
become mingled in the one great region with the native 
American productions, and would have had to compete with 
them ; and in the other great region, with those of the Old 
World. Consequently we have here everything favorable 
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 North America. Hence, it has 
come, that when we compare the now living productions of 
the temperate regions of the New and Old Worlds, we find 
very few identical species (though Asa Gray has lately 
shown that more plants are identical than was formerly sup- 
posed), but we find in every great class many forms, which 
some naturalists rank as geographical races, and others as 
distinct species ; and a host of closely allied or representa- 
tive forms which are ranked by all naturalists as specifically 

As on the land, so in the waters of the sea, a slow south, 
em 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 understand the presence of some closely allied, 
still existing and extinct tertiary forms, on the eastern and 
western shores of temperate North America; and the still 
more striking fact of many closely allied crustaceans (as 
described in Dana's admirable work), some fish and other 
marine animals, inhabiting the Mediterranean and the seas 
of Japan — these two areas being now completely separated 
by the breadth of a whole continent and by wide spaces of 

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. 


But we must return to our more immediate subject. I 
am convinced that Forbes' view may be largely extended. 
In Europe we meet with the plainest evidence of the Glacial 
period, from the western shores of Britain to the Ural 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, accord* 
ing to Dr. Hooker, perpetual snow formerly covered the 
central axis, and fed glaciers which rolled 4,000 feet down 
the valleys. The same observer has recently found great 
moraines at a low level on the Atlas range in North Africa. 
Along the Himalaya, at points 900 miles apart, glaciers have 
left the marks of their former low descent; and in Sikkim, 
Dr. Hooker saw maize growing on ancient and gigantic 
moraines. Southward of the Asiatic continent, on the 
opposite side of the equator, we know, from the excellent 
researches of Dr. J. Haast and Dr. Hector, that in New Zeal- 
and 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 Rev. W. B. 
Clarke, it appears also that there are traces of former gla- 
cial action on the mountains of the south-eastern corner of 

Looking to America: in the northern half, ice-borne 
fragments of rock have been observed on the eastern side 
of the continent, as far south as latitude thirty-six and 
thirty-seven degrees, and on the shores of the Pacific, where 
the climate is now so different, as far south as latitude forty- 
six degrees. Erratic bowlders have, also, been noticed on 
the Kockv Mountains. In the Cordillera of South America, 
nearly under the equator, glaciers once extended far below 
their present level. In Central Chili I examined a vast 
mound of detritus with great bowlders, crossing the Portillo 
Valley, which, there can hardly be a doubt, once formed a 
huge moraine ; and Mr. D. Forbes informs me that he found 
in various parts of the Cordillera, from latitude thirteen 
to thirty degrees south, at about the height of 12,000 feet, 


damply- 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 
Cordillera true glaciers do not now exist even at much more 
considerable heights. Further south, on both sides of the 
continent, from latitude forty-one degrees to the southern- 
most extremity, we have the clearest evidence of former 
glacial action, in numerous immense bowlders transported 
far from their parent source. 

From these several facts, namely, from the glacial action 
having extended all round the northern and southern hemi- 
spheres — 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 
avcid the conclusion that the temperature of the whole world 
had been simultaneously lowered during the Glacial period. 
But now, Mr. Croll, in a series of admirable memoirs, has 
attempted to show that a glacial condition of climate is the 
result of various physical causes, brought into operation by 
an increase in the eccentricity of the earth's orbit. All these 
causes tend toward the same end ; but the most powerful 
appears to be the indirect influence of the eccentricity of the 
orbit upon oceanic currents. According to Mr. Croll, cold 
periods regularly recur every ten or fifteen thousand years ; 
and these at long intervals are extremely severe, owing to 
certain contingencies, of which the most important, as Sir C. 
Lyell has shown, is the relative position of the land and 
water. Mr. Croll believes that the last great glacial period 
occurred about 240,000 years ago, and endured, with slight 
alterations of climate, for about 160,000 years. With re- 
spect 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 mosv important result for us, 
arrived at by Mr. Croll, is that whenever the northern hemi- 
sphere passes through a cold period the temperature of the 
southern hemisphere is actually raised, with the winters 
rendered much milder, chiefly through changes in the direc- 
tion of the ocean currents. So conversely it will be with the 
northern hemisphere, while the southern passes through a 
glacial period. This conclusion throws so much light oa 


geographical distribution, that I am strongly inclined to 
trust in it : but I will first give the facts which demand an 

In South America, Dr. Hooker has shown that besides many 
closely allied species, between forty and fifty of the flower- 
ing plants of Tierra del Fuego, forming no inconsiderable 
part of its scanty flora, are common to North America and 
Europe, enormously remote as these areas in opposite hemi- 
spheres 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 Carac- 
cas the illustrious Humboldt long ago found species belonging 
to genera characteristic of the Cordillera. 

In Africa, several forms characteristic of Europe, and some 
few representatives of the flora of the Cape of Good Hope, 
occur on the mountains of Abyssinia. At the Cape of Good 
Hope a very few European species, believed not to have been 
introduced by man, and on the mountains several representa- 
tive European forms, are found which have not been discov- 
ered 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 
neighboring Cameroon Mountains, in the Gulf of Guinea, are 
closely related to those on the mountains of Abyssinia, and 
likewise to those of temperate Europe. It now also appears, 
as I hear from Dr. Hooker, that some of these same temper- 
ate plants have been discovered by the Rev R. T. Lowe on 
the mountains of the Cape Verde Islands. This extension 
of the same temperate forms, almost under the equator, 
across the whole continent of Africa and to the mountains 
of the Gape Verde archipelago, is one of the most astonish- 
ing facts ever recorded in the distribution of plants. 

On the Himalaya, and on the isolated mountain ranges of 
the peninsula of India, on the heights of Ceylon and on the 
volcanic cones of Java, many plants occur either identically 
the same or representing each other, and at the same time 
representing plants of Europe not found in the intervening 
hot lowlands. A list of the genera of plants collected on the 
loftier peaks of Java, raises a picture of a collection made 
on a hillock in Europe. Still more striking is the fact that 
peculiar Australian forms are represented by certain plants 


growing on the summits of the mountains of Borneo. Some 
of these Australian forms, as I hear from Dr. Hooker, extend 
along the heights of the peninsula of Malacca, and are thinly 
scattered on 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 in- 
troduced by man, occur on the lowlands ; and a. long list can 
be given, as I am informed by Dr. Hooker, of European gen- 
era, found in Australia, but not in the intermediate torrid 
regions. In the admirable " Introduction to the Flora of 
New Zealand," by Dr. Hooker, analogous and striking facts 
are given in regard to the plants of that large island. Hence, 
we see that certain plants growing on the more lofty moun- 
tains of the tropics in all parts of the world, and on the tem- 
perate 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 toward equatorial latitudes, the alpine or mountain 
flora really become less and less arctic." Besides these 
identical and closely allied forms, many species inhabiting 
the same widely sundered areas, belong to genera not now 
found in the intermediate tropical lowlands. 

These brief remarks apply to plants alone ; but some few 
analogous facts could be given in regard to terrestrial ani- 
mals. In marine productions, similar cases likewise occur ; 
as an example, I may quote a statement by the highest 
authority, Professor Dana, that " it is certainly a wonderful 
fact that New Zealand should have a closer resemblance in 
its Crustacea to Great Britain, its antipode, than to any other 
part of the world." Sir J. Richardson, also, speaks of the 
reappearance on the shores of New Zealand, Tasmania, etc., 
of northern forms of fish. Dr. Hooker informs me that 
twenty-five species of 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 temper- 
ate 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 greafc 


continents were everywhere tenanted under the equator by a 
considerable number of temperate forms. At this period the 
equatorial climate at the level of the sea was probably about 
the same with that now experienced at the height of from 
five to six thousand feet under the same latitude, or perhaps 
even rather cooler. During this, the coldest period, the low- 
lands under the equator must have been clothed with a 
mingled tropical and temperate vegetation, like that de- 
scribed 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 
Fernando 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. CrolPs conclusion that when 
the northern hemisphere suffered from the extreme cold of 
the great Glacial period, the southern hemisphere was actu- 
ally warmer, throws any clear light on the present appar- 
ently inexplicable distribution of various organisms in the 
temperate parts of both hemispheres, and on the mountains 
of the tropics. The Glacial period, as measured by years, 
must have been very long ; and when we remember over what 
vast spaces some naturalized 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 tem- 
perate regions ; and, from the facts just given, there can 
hardly be a doubt that some of the more vigorous, dominant, 
and widest-spreading temperate forms invaded the equato- 
rial lowlands. The inhabitants of these hot lowlands would 
at the same time have migrated to the tropical and subtropi- 
cal regions of the south, for the southern hemisphere was at 
this period warmer. On the decline of the Glacial period, 
as both hemispheres gradually recovered their former tem- 
perature, the northern temperate forms living on the low- 
lands 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 sufficiently lofty, 


they would have long survived like the arctic forms on th© 
mountains of Europe. They might have survived, even if 
the climate was not perfectly litted for them, for the change 
j>i temperature must have been very slow, and plants un- 
do abtedly possess a certain capacity for acclimatization, as 
shown by their transmitting to their offspring different con- 
stitutional powers of resisting heat and cold. 

In the regular course of events the southern hemisphere 
would in its turn be subjected to a severe Glacial period, 
with the northern hemisphere rendered warmer ; and then 
the southern temperate forms would invade the equatorial 
lowlands. The northern forms which had before been left 
on the mountains would now descend and mingle with the 
southern forms. These latter, when the warmth returned, 
would return to their former homes, leaving some few spe- 
cies on the mountains, 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 south- 
ern temperate zones and on the mountains of the interme- 
diate tropical regions. But the species left during a long 
time on these mountains, or in opposite hemispheres, would 
have to compete with many new forms, and would be exposed 
to somewhat different physical conditions : hence, they would 
be eminently liable to modification, and would generally now 
exist as varieties or as representative species; and this is 
the case. We must, also, bear in mind the occurrence in 
both 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 sep- 
arated 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 Candolle 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 sus- 
pect that this preponderant 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 com- 
mingled in the equatorial regions, during the alternations 
of the Glacial periods, the northern forms were the more 
powerful and were able to hold their places on the moun- 
tains, and afterward 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 productions cover the ground in Lr 
Plata, New Zealand, and to a lesser degree in Australia, and 
have beaten the natives ; whereas extremely few southern 
forms have become naturalized in any part of the northern 
hemisphere, though hides, wool, and other objects likely to 
carry seeds have been largely imported into Europe during 
the last two or three centuries from La Plata, and during the 
last forty or fifty years from Australia. The Neilgherrie 
Mountains in India, however, offer a partial exception ; for 
here, as I hear from Dr. Hooker, Australian forms are rap- 
idly sowing themselves, and becoming naturalized. Before 
the last great Glacial period, no doubt the inter-tropical 
mountains were stocked with endemic alpine forms ; but 
these have almost everywhere yielded to the more dominant 
forms generated in the larger areas and more efficient work- 
shops of the north. In many islands the native productions 
are nearly equalled, or even outnumbered, by those which 
have become naturalized ; and this is the first stage toward 
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 inhab- 
itants of real islands have everywhere yielded and are still 
yielding to continental forms naturalized through man's 

The same principles apply to the distribution of terres- 
trial animals and of marine productions, in the northern and 
southern temperate zones, and on the inter-tropical moun- 
tains. When, during the height of the Glacial period, the 
ocean-currents were widely different to what they now are, 
some of the inhabitants of the temperate seas might have 
reached the equator ; of these a few would perhaps at once 
be able to migrate southward, by keeping to the cooler cur- 
rents^ while others might remain and survive in the colder 
depths until the southern hemisphere was in its turn sub- 
jected to a glacial climate and perinitted their further prog- 
ress ; in nearly the same manner as, according to Forbes, 
isolated spates inhabited by arctic productions exist to the 


present day in the deeper parts of the northern temperate 

I am far from supposing that all the difficulties in regard 
to the distribution and affinities of the identical and allied 
species, which now live so widely separated in the north and 
south, and sometimes on the intermediate mountain-ranges, 
are removed on the views above given. The exact lines o{ 
migration cannot he 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, while 
others have remained unaltered. We cannot hope to explain 
such facts, until we can say why one species and not anothei 
becomes naturalized by man's agency in a foreign land ; wiry 
one species ranges twice or thrice as far, and is twice oj 
thrice as common, as another species within their own homes, 

Various special difficulties also remain to be solved; fo* 
instance, the occurrence, as shown by Dr. Hooker, of the 
same plants at points so enormously remote as Kerguelen 
Land, New Zealand, and Fuegia; but icebergs, as suggested 
by Lyell, may have been concerned in thei iispersal. Tha 
existence at these and other distant points of the southern 
hemisphere, of species, whi^\ though distinct, belong to 
genera exclusively confined to the south, is a more remark- 
able case. Some of these species are so distin^ 4 -, that we 
cannot suppose that there has been time sincx. the com- 
mencement of the last Glacial period for their migration 
and subsequent modification to the necessary degree. The 
facts seem to indicate that distinct species belonging to the 
same genera have migrated in radiating lines from a common 
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 already widely dispersed to various 
points of the southern hemisphere by occasional means of 
transport, and by the aid, as halting-places, of now sunken 
islands. Thus the southern shores of America, Australia, 
and New Zealand may have become slightly tinted by the 
same peculiar forms of life. 

Sir C. Lyell in a striking passage has speculated, in lan- 
guage almost identical with mine, on the effects of great 
alterations of climate throughout the world on geographical 


distribution. And we have now seen that Mr. Croll's con- 
elusion that successive Glacial periods in the one hemisphere 
coincide with warnie*- periods in the opposite hemisphere, 
together with the admission of the slow modification of 
species, explains a multitude of facts in the distribution of 
the same and of the allied forms of life in all parts of the 
globe. The living waters have flowed during ono period 
from the north and during another from the south, „nd in 
both cases have reached the equator ; but the stream of life 
has flowed with greater force from the north than in the 
opposite direction,, and has consequently more freely inun- 
dated the south. As the tide leaves its drift in horizontal 
lines, rising higher on the shores where the tide rises high- 
est, so have the living waters left their living drift on our 
mountain summits, m a line gently rising from the arctic 
lowlands to a great altitude under the equator. The various 
beings thus left stranded may be compared with savage 
races of man, driven up and surviving in the mountain fast- 
nesses of almost every land, which serves as a record, full of 
interest to us, of *he former inhabitants*/ Mie Surrounding 



geographical distribution — continued. 

Distribution of Fresh-water Productions — On the Inhabitants of 
Oceanic Islands — Absence of Batrachians and of Terrestrial 
Mammals — On the Kelation of the Inhabitants of Islands to 
those of the Nearest Mainland — On Colonization from the Nearest 
Source with Subsequent Modification — Summary of the Last and 
Present Chapters. 


As lakes and river systems are separated from each other 
by barriers of land, it might have been thought that fresh- 
water productions would not have ranged widely within the 
same country, and as the sea is apparently a still more 
formidable barrier, that they would never have extended to 
distant countries. But the case is exactly the reverse. 
Not only have many fresh-water species belonging to dif- 
ferent classes, an enormous range, but allied species prevail 
in a remarkable manner throughout the world. When first 
collecting in the fresh waters of Brazil, I well remember 
feeling much surprise at the similarity of the fresh-water 
insects, shells, etc., and at the dissimilarity of the surround- 
ing terrestrial beings, compared with those of Britain. 

But the wide ranging power of fresh-water productions 
can, I think, in moat cases be explained by their having 
become fitted, in a manner highly useful to them, for short 
and frequent migrations from pond to pond, or from stream 
to stream, within their own countries ; and liability to wide 
dispersal would follow from this capacity as an almost 
necessary consequence. We can here consider only a few 
cases ; of these, some of the most difficult to explain are 
presented by fish. It was formerly believed that the same 
fresh-water species never existed on two continents distant 
from each other. But Dr. Gunther has lately shown that 
the Galaxias attenuatus inhabits Tasmania, New Zealand, 
the Falkland Islands, and the mainland of South America. 
This is a wonderful case, and probably indicates dispersaJ 
from an antarctic centre duriner a former warm period. 


This case, however, is rendered in some degree less sur- 
prising by the species of this genus having the power of 
crossing by some unknown means considerable spaces of 
open ocean : thus there is one species common to New 
Zealand and to the Auckland Islands, though separated by 
a distance of about 230 miles. On the same continent fresh- 
water fish often range widely, and as if capriciously ; for 
in two adjoining river systems some of the species may be 
the same and some wholly different. 

It is probable that they are occasionally transported b^ 
what may be called accidental means. Thus fishes still 
alive are not very rarely dropped at distant points by 
whirlwinds ; and it is known that the ova retain their 
vitality for a considerable time after removal from the 
water. Their dispersal may, however, be mainly attributed 
to changes in the level of the land within the recent period, 
causing rivers to flow into each other. Instances, also, 
could be given of this having occurred during floods, with- 
out any change of level. The wide differences of the fish 
on the opposite sides of most mountain-ranges, which are 
continuous and consequently must, from an early period, 
have completely prevented the inosculation of the river- 
systems on the two sides, leads to the same conclusion. 
Some fresh-water fish belong to very ancient forms, and in 
such cases there will have been ample time for great geo- 
graphical changes, and consequently time and means for 
much migration. Moreover, Dr. Gunther has recently been 
led by several considerations to infer that with fishes the 
same forms have a long endurance. Salt-water fish can 
with care be slowly accustomed to live in fresh water : and, 
according to Valenciennes, there is hardly a single group 
of which all the members are confined to fresh water, so 
that a marine species belonging to a fresh-water group might 
travel far along the shores of the sea, and could, it is prob- 
able, become adapted without much difficulty to the fresh 
waters of a distant land. 

Some species of fresh-water shells have very wide ranges, 
and allied species which, on our theory, are descended from 
a common parent, and must have proceeded from a single 
source, prevail throughout the world. Their distribution 
at first perplexed me much, as their ova are not likely to 
be transported by birds ; and the ova. as well as the adults, 
are immediately killed by sea-water. I could not even 
understand how some naturalized species have spread rap* 


idly throughout the same country. But two facts, which 
I have observed — aud many others no doubt will be dis- 
covered — throw some light on this subject. When ducks 
suddenly emerge from a pond covered with duck-weed, I 
lave twice seen these little plants adhering to their backs ; 
and it nas happened to me, in removing a little duck-weed 
"rom one aquarium to another, that I have unintentionally 
stocked the one with fresh-water shells from the other. 
But another agency is perhaps more effectual : I suspended 
the feet of a duck in an aquarium, where many ova of fresh- 
water shells were hatching; and I found that numbers of 
the extremely minute and just-hatched shells crawled on 
the feet, and clung to them so firmly that when taken out 
of the water they could not be jarred off, though at a some- 
what more advanced age they would voluntarily drop off. 
These just-hatched mollusks, though aquatic in their nature, 
survived on the duck's feet, in damp air, from twelve to 
twenty hours ; and in this length of time a duck or heron 
might fly at least six or seven hundred miles, and if blown 
across the sea to an oceanic island, or to any other distant 
point, would be sure to alight on a pool or rivulet. Sir 
Charles Lyell informs me that a dytiscus has been caught 
with an ancylus (a fresh-water shell like a limpet) firmly 
adhering to it; and a water-beetle of the same family, a 
colymbetes, once flew on board the Beagle, when forty- 
five miles distant from the nearest land : how much farther 
it might have been blown by a favoring gale, no one can 

With respect to plants, it has long been known what 
enormous ranges many fresh-water and even marsh species 
have, both over continents and to the most remote oceanic 
islands. This is strikingly illustrated, according to Alph. 
de Candolle, in those large groups of terrestrial plants, 
which have very few aquatic members ; for the latter seem 
immediately to acquire, as if in consequence, a wide range. 
I think favorable means of dispersal explain this fact. I 
have before mentioned that earth occasionally adheres in 
some quantity to the feet and beaks of birds. Wading 
birds, which frequent the muddy edges of ponds, if suddenly 
flushed, would be the most likely to have muddy feet. 
Birds of this order wander more than those of any other; 
and they are occasionally found on the most remote and 
barren islands of the open ocean ; they would not be likely 
to alight on the surface of the sea, so that any dirt on their 


feet would not be washed off : and when gaining the land, 
they would be sure to fly to their natural fresh-water haunts. 
I do not believe that botanists are aware how charged the 
mud of ponds is with seeds; I have tried* several little 
experiments, but will here give only the most striking case: 
I took in February three tablespoon fuls of mud from three 
different points, beneath water, on the edge of a little pond ; 
this mud when dried weighed only six and three-fourths 
ounces ; I kept it covered up in my study for six months, 
pulling up and counting each plant as it grew ; the plants 
were of many kinds, and were altogether 537 in number; 
and yet the viscid mud was all contained in a breakfast 
cup ! Considering these facts, I think it would be an 
inexplicable circumstance if water birds did not transport 
the seeds of fresh-water plants to unstocked ponds and 
streams, situated at very distant points. The same agency 
may have come into play with the eggs of some of the 
smaller fresh-water animals. 

Other and unknown agencies probably have also played 
a part. I have stated that fresh-water fish eat some kinds 
of seeds, though they reject many other kinds after having 
swallowed them ; even small fish swallow seeds of moderate 
size, as of the yellow water-lily and Potamogeton. Herons 
and other birds, century after century, have gone on daily 
devouring fish ; they then take flight and go to other 
waters, or are blown across the sea ; and we have seen that 
seeds retain their power of germination, when rejected 
many hours afterward in pellets or in the excrement. 
When I saw the great size of the seeds of that fine water- 
lily,, the Nelumbium, and remembered Alph. de Candolle's 
remarks on the distribution of this plant, I thought that 
the means of its dispersal must remain inexplicable ; but 
Audubon states that he found the seeds of the great southern 
water-lily (probably, according to Dr. Hooker, -the Nelum- 
bium luteuin) in a heron's stomach. Now this bird must 
often have flown with its stomach thus well stocked to 
distant ponds, and then, getting a hearty meal of fish, 
analogy makes me believe that it would have rejected the 
seeds in the pellet in a fit state for germination. 

in considering these several means of distribution, it 
should be remembered that when a pond or stream is first 
formed, for instance, on a rising islet, it will be unoccupied ; 
and a single seed or egg will have a good chance of succeed- 
ing. Although there will always be a struggle for Ufa 


between the inhabitants of the same pond, however few in 
kind, yet as the number even in a well-stocked pond is small 
in comparison with the number of species inhabiting an 
equal area of land, the competition between them will prob- 
ably be less severe than between terrestrial species ; conse- 
quently an intruder from the waters of a foreign country 
would have a better chance of seizing on a new place, than 
in the case of terrestrial colonists. We should also remem- 
ber that many fresh-water productions are low in the scale 
of nature, and we have reason to believe that such beings 
become modified more slowly than the high ; and this will 
give time for the migration of aquatic species. We should 
not forget the probability of many fresh-water forms having 
formerly ranged continuously over immense areas, and then 
having become extinct at intermediate points. But the wide 
distribution of fresh-water plants, and of the lower animals, 
whether retaining the same identical form, or in some degree 
modified, apparently depends in main part on the wide dis- 
persal of their seeds and eggs by animals, more especially 
by fresh-water birds, which have great powers of flight, and 
naturally travel from one piece of water to another. 


We now come to the last of the three classes of facts, 
which I have selected as presenting the greatest amount of 
difficulty with respect to distribution, on the view that not 
only all the individuals of the same species have migrated 
from some one area, but that allied species, although now 
inhabiting the most distant points, have proceeded from a 
single area, the birthplace of their early progenitors. 1 
have already given my reasons for disbelieving in conti- 
nental extensions within the period of existing species on 
so enormous a scale that all the many islands of the several 
oceans were thus stocked with their present terrestrial in- 
habitants. This view removes many difficulties, but it does 
not accord with all the facts in regard to the productions 
of islands. In the following remarks I shall not confine 
myself to the mere question of dispersal, but shall consider 
some other cases bearing on the truth of the two theories of 
independent creation and of descent with modification. 

The species of all kinds which inhabit oceanic islands are 
few in number compared with those on equal continental 
areas : Alph. de Candolle admits this for plants, and Wollas- 


ton for insects. New Zealand, for instance, with its lofty- 
mountains and diversified stations, extending over 780 miles 
of latitude, together with the outlying islands of Auckland, 
Campbell, and Chatham, contain altogether only 960 kinds 
of flowering plants ; if we compare this moderate number 
with the species which swarm over equal areas in South- 
western Australia or at the Cape of Good Hope, we must 
admit that some cause, independently of different physical 
conditions, has given rise to so great a difference in number. 
Even the uniform county of Cambridge has 847 plants, and 
the little island of Anglesea 764, but a few ferns and a few 
introduced plants are included in these numbers, and the 
comparison in some other respects is not quite fair. We 
have evidence that the barren island of Ascension aborigi- 
nally possessed less than half a dozen flowering plants ; yet 
many species have now become naturalized on it, as they 
have in New Zealand and on every other oceanic island 
which can be named. In St. Helena there is reason to 
believe that the naturalized plants and animals have nearly 
or quite exterminated many native productions. He who 
admits the doctrine of the creation of each separate species, 
will have to admit that a sufficient number of the best- 
adapted plants and animals were not created for oceanic 
islands ; for man has unintentionally stocked them far more 
fully and perfectly than did nature. 

Although in oceanic islands the species are few in number 
the proportion of endemic kinds (i. e., those found nowhere 
else in the world) is often extremely large. If we compare, 
for instance, the number of endemic land-shells in Madeira, 
or of endemic birds in the Galapagos Archipelago, with the 
number found on any continent, and then compare the area 
of the island with that of the continent, we shall see that 
this is true. This fact might have been theoretically 
expected, for, as already explained, species occasionally arriv- 
ing, after long intervals of time, in the new and isolated 
district, and having to compete with new associates, would 
be eminently liable to modification, and would often produce 
groups of modified descendants. But it by no means fol- 
lows that, because in an island nearly all the species of 
one class are peculiar, those of another class, or of another 
section of the same class, are peculiar ; and this diftereree 
seems to depend partly on the species which are not modified 
having immigrated in a body, so that their mutual relations 
have not been much disturbed j and partly on the frequent 


arrival of unmodified immigrants from the mother-country, 
with which the insular forms have intercrossed. It should 
be borne in mind that the offspring of such crosses would 
certainly gain in vigor ; so that even an occasional cross 
would produce more effect than might have been anticipated. 
I will give a few illustrations of the foregoing remarks : 
in the Galapagos Islands there are twenty-six land birds ; of 
these, twenty -one (or perhaps twenty-three ) are peculiar, 
whereas of the eleven marine birds only two are peculiar; 
and it is obvious that marine birds could arrive at those 
islands much more easily and frequently than land birds. 
Bermuda, on the other hand, which lies at about the same 
distance from North America as the Galapagos Islands do 
from South America, and which has a very peculiar soil, 
does not possess a single endemic land bird ; and we know 
from Mr. J. M. Jones' admirable account of Bermuda, that 
very many North American birds occasionally or even 
frequently visit this island. Almost every year, as I am 
informed by Mr. E. Harcourt, many European and African 
birds are blown to Madeira; this island is inhabited by 
ninety-nine kinds, of which one alone is peculiar, though 
very closely related to a European form ; and three or four 
other species are confined to this island and to the Canaries. 
So that the islands of Bermuda and Madeira have been 
stocked from the neighboring continents with birds, which 
for long ages have there struggled together, and have become 
mutually co-adapted. Hence, when settled in their new 
homes, each kind will have been kept by the others to its 
proper place and hab