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Chaules Darwin.
"But with regard to the material world, we can at least
go so far as this — we can perceive that events are brought
about not by insulated interpositions of Divine power,
exerted in each particular case, but by the establishment
of general laws." — Whewell : Bridgewater Treatise.
" The only distinct meaning of the word i natural ' 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 : Analogy of Revealed Religion.
" To conclude, therefore, let no man out of a weak conceit
of sobriety, or an ill-applied moderation, think or maintain,
that a man can search too far or be too well studied in the
book of God's word, or in the book of God's works ; divinity
or philosophy ; but rather let men endeavor an endless prog-
ress or proficience in both." — Bacon : Advancement of
Learning.
AN HISTOKICAL SKETCH
OF THE PROGRESS OF OPINION ON THE ORIGIN OF
SPECIES,
PREVIOUSLY TO THE PUBLICATION OP THE
FIRST EDITION OF THIS WORK.
I will here give a brief sketch of the progress of opin-
ion on the Origin of Species. Until ' recently the great
majority 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 " Physicse Auscultationes " (lib. 2, cap. 8, s. 2),
after remarking that rain does not fall in order to make the corn
grow, any more than it falls to spoil the farmer's corn when threshed
out of doors, applies the same argument to organization; and adds (as
translated by Mr. Clair Grece, who first pointed out tbe 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 how little Aristotle fully comprehended
the principle, is shown by his remarks on tbe formation of the teeth.
Vl HISTORICAL SKETCH.
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 " Philosophie 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 species. He first did the eminent service of
arousing attention to the probability of all change in the
organic, as well as in the inorganic world, being the result
of law, and not of miraculous interposition. Lamarck
seems to have been chiefly led to his conclusion on the
gradual change of species, by the difficulty of 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 taken the date of the first publication of Lamarck from
Isidore Geoffroy Saint-Hilaire's (" Hist. Nat. Generale," 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 1795.
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 rather a singular instance
of the manner in which similar views arise at about the same time, that
Goethe in Germany, Dr. Darwin in England, and Geoffroy Saint-Hilaire
(as we shall immediately see) in France, came to the same conclusion
on the origin of species, in the years 1794-95.
HISTORICAL SKETCH. VII
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 Pavenir 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 aby 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 passage of Dr. Wells's work.
viii HISTORICAL SKETCH.
The Hon. and Rev. W. Herbert, afterward Dean of Man-
chester, in the fourth volume of the " Horticultural Trans-
actions," 1822, and in his work on the " Auiaryllidaceae "
(1837, pp. 19, 339), declares that " horticultural experiments
have established, beyond the possibility of refutation, that
botanical species are only a higher and more permanent class
of varieties." He extends the same view to animals. The
dean believes that single species of each genus were created
in an originally highly plastic condition, and that these have
produced, chiefly by intercrossing, but likewise by variation,
all our existing species.
In 1826 Professor Grant, in the concluding 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
selection.
The celebrated geologist and naturalist, Von Buch, in his
excellent " Description Physique des Isles Canaries " (1836,
p. 147), clearly expresses his belief that varieties slowly be-
come changed into permanent species, which are no longer
capable of intercrossing.
Bafinesque, in his " New Flora of North America," pub*
HISTORICAL SKETCH. ix
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
change.
The " Vestiges of Creation " appeared in 1844. In the
tenth and much improved edition (1853) the anonymous
author says (p. 155) : " The proposition determined on after
much consideration is, that the several series of animated
beings, from the simplest and oldest up to the highest and
most recent, are, under the providence of God, the results,
first, of an impulse which has been imparted to the forms of
life, advancing them in definite times, by generation, through
grades of 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
1 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 ("Bulletins
r
X HISTORICAL SKETCH.
de l'Acad. 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. 8G),
wrote as follows : " The archetj^pal 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 same
address, one by the other, it appears that this eminent
philosopher felt in 1858 his confidence shaken that the
Apteryx and the Red Grouse 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 by Mr. Wallace
and myself on the Origin of Species, presently to be referred
to, had been read before the Linnean Society. When the
first edition of this work was published, I was so completely
deceived, as were many others, by such expressions as " the
continuous operation of creative power," that I included
Professor Owen with other palaeontologists as being firmly
convinced of the immutability of species ; but it appears
(" Anat. of Vertebrates," vol. iii. p. 796) that this was on
HISTORICAL SKETCH. xi
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 variability limitee des especes. Les experiences sur
les animaux sauvages devenus domestiques, et sur les ani-
maux domestiques redevenus sauvages, la d6montrent plus
clairement encore. Ces memes experiences prouvent, de
plus, que les differences produites peuvent etre de valeur
generique." In his " Hist. Nat. Gen6rale " (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-
cies by means of Organic Affinity," the difficult attempt to
Xii HISTORICAL SKETCH.
give any idea of his views would be superfluous on my
part.
Mr. Herbert Spencer, in an essay (originally published in
the "Leader," March, 1852, and republished in his "Essays,"
in 1858), has contrasted the theories of the Creation and the
Development of organic beings with remarkable skill and
force. He argues from the analogy of domestic productions,
from the changes which the embryos of many species 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 cle l'existence du nionde, la forme, le volume, et 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'ensemble, 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 iiber 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 his 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,
• • •
HISTORICAL SKETCH. Xiil
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
excellent 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 horames
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
clearness.
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 distiinclj
have descended from a single parent-form,
xiv HISTORICAL SKETCH.
In June, 1859, Professor Huxley gave a lecture before
the Boyal 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 modificav
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 great
work he admits the truth of the descent and modification of
species, and supports this doctrine by many original observa-
tions.
The first edition of this work was published on November
4, 1859, and the second edition on January 7, 1860.
o
C
/
CONTENTS.
paos
introduction 1
CHAPTER I.
VARIATION UNDER DOMESTICATION.
Causes of variability — Effects of habit and the use or disuse of
parts — Correlated variation — Inheritance — Character of
domestic varieties — Difficulty of distinguishing between
varieties and species — Origin of domestic varieties from
one or more species — Domestic pigeons, their differences
and origin — Principles of selection, anciently followed, their
effects — Methodical and unconscious selection — Unknown
origin of our domestic productions — Circumstances favor-
able to man's power of selection 6
CHAPTER II.
VARIATION UNDER NATURE.
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 37
restricted ranges
CHAPTER III.
STRUGGLE FOR EXISTENCE.
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 througbout nature — Struggle for life most severe
between individuals and varieties of the same species: often
severe between species of the same genus — The Relation of
organism to organism the most important of all relatione . •
xv
3tyi CONTENTS.
PAGE
CHAPTER IV.
NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
Natural Selection — its power compared with man's selection —
its power on characters of trifling importance — its power at
all ages and on both sexes — Sexual Selection — On the gene-
rality 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
CHAPTER V.
LAWS OF VARIATION.
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
CHAPTER VI.
DIFFICULTIES OF THE THEORY.
Difficulties of the theory of descent with modification — Absence
or rarity of transitional varieties — Transitions in habits of
life — Diversified habits in the same species — Species with
habits widely different from those of their allies — Organs of
extreme perfection — Modes of transition — Cases of difficulty
— Natura non facit saltum — Organs of small importance —
Organs not in all cases absolutely~perfeet — The law of Unity
of Type and of the Conditions of Existence embraced by the
theory of Natural Selection 149
CHAPTER VII.
MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL
SELECTION.
Longevity — Modifications not necessarily simultaneous — Modifi-
cations apparently of no direct service — Progressive develop-
CONTENTS, xvii
PAGE
ment — 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 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
CHAPTER VIIL
INSTINCT.
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
CHAPTER IX.
HYBRIDISM.
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 26G
CHAPTER X.
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day — On
the nature of extinct intermediate varieties; on their number
— On the lapse of time, as inferred from the rate of 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 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, 293
• •■
PAGE
XV1U CONTENTS.
CHAPTER XI.
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
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 322
CHAPTER XII.
GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in
physical conditions — Importance of barriers — Affinity of the
productions of the same continent — Centres of creation —
Means of dispersal by changes of 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
CHAPTER XIII.
geographical distribution — continued.
Distribution of fresh-water productions — On the inhabitants of
oceanic islands — Absence of Batrachians and of terrestrial
Mammals — On the relation of the inhabitants of islands to
those of the nearest mainland — On colonization from the
nearest source with subsequent modification — Summary of
the last and present chapter 380
CHAPTER XIV.
MUTUAL AFFINITIES OF ORGANIC BEINGS! MORPHOLOGY —
EMBRYOLOGY — RUDIMKNTARY ORGANS.
Classification, groups subordinate to groups — Natural system —
Rules and difficulties in classification, explained on the theory
of descent with modification — Classification of varieties —
Descent always used in classification — Analogical or adaptive
characters — Affinities, general, complex and radiating — 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 , 402
CONTENTS.
XIX
PAOS
CHAPTER XV.
RECAPITULATION AND CONCLUSION.
Recapitulation of the objections to the theory of Natural Selection
— Recapitulation of the general and special circumstances in
its favor — Causes of the general belief in the immutability of
species — How far the theory of Natural Selection may be
extended — Effects of its adoption on the study of Natural
History — Concluding remarks 447
gl088ary of scientific tebms 475
Index 487
ORIGIN OF SPECIES.
INTRODUCTION.
When on board H. M. S. Beagle, as naturalist, I was much
struck with certain facts in the distribution of the organic
beings inhabiting South America, and in the geological 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,
I
2 INTRODUCTION.
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,
INTRODUCTION. 3
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 being, if it vary however slightly
4 INTRODUCTION.
in any manner profitable to itself, under the complex and
sometimes varying conditions of life, will have a better
chance of surviving, and thus be naturally selected. From
the strong principle of inheritance, any selected variety
will tend to propagate its new and modified form.
This fundamental subject of natural selection will be
treated at some length in the fourth chapter ; and we shall
then see how natural selection almost inevitably causes
much extension of the less improved forms of life, and
leads to what I have called divergence of character. In the
next chapter I shall discuss the complex and little known laws
of 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, that
INTRODUCTION. 5
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, I am convinced that natural selection has been
the most important, but not the exclusive, means of modifi-
cation.
VARIATION UNDER DOMESTICATION.
CHAPTER I.
VARIATION UNDER DOMESTICATION.
Causes of Variability — Effects of Habit and the Use or Disuse of Parts
— Correlated Variation — Inheritance — Character of Domestic
Varieties — Difficulty of distinguishing between Varieties and
Species — Origin of Domestic Varieties from one or more Species
— Domestic Pigeons, their Differences and Origin — Principles of
Selection, anciently followed, their Effects — Methodical and Un-
conscious Selection — Unknown Origin of our Domestic Produc-
tions— Circumstances favorable to Man's Power of Selection.
CAUSES OF VARIABILITY.
When we compare the individuals of the same variety or
sub-variety of our older cultivated plants and animals, one of
the first points which strikes us is, that they generally 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, and somewhat different from, those to which the
parent species had been exposed under nature. There is,
also, some probability in the view propounded by Andrew
Knight, that this variability may be partly connected with
excess of food. It seems clear that organic beings must be
exposed during several generations to new conditions to cause
any great amount of variation ; and that, when the 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,
VARIATION UNDER DOMESTICATION. 7
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 can
judge, dissimilar conditions ; and, on the other hand, dissimi-
lar 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 same 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-eapsule. 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 moustrosities arise; but monstrosities
cannot be separated by any distinct line from slighter varia-
tions. All such changes of structure, whether extremely
8 VARIATION UNDER DOMESTICATION.
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
changed conditions, namely, through the reproductive sys-
tem 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 the details which I have collected and elsewhere
published on this curious subject ; but to show how singular
the laws are which determine the reproduction of animals
under confinement, I may mention that carnivorous animals,
even from the tropics, breed in this country pretty freely
under confinement, with the exception of the plantigrades or
bear 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
taken young from a state of nature perfectly tamed, long-lived
and healthy (of which I could give numerous instances), yet
variation under domestication. 9
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 with 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.
EFFECTS OF HABIT AND OF THE USE OR DISUSE OF PARTS ;
CORRELATED VARIATION; INHERITANCE.
Changed habits produce an inherited effect, as in the
period of the flowering of plants when transported from one
climate to another. With animals the increased use or
disuse of parts has had a more marked influence ; thus I
find in the domestic duck that the bones of the wing weigh
less and the bones of the leg more, in proportion to the
whole skeleton, than do the same bones in the wild duck ;
10 VARIATION UNDER DOMESTICATION.
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
some country drooping ears ; and the view which has been
suggested that the drooping is due to disuse of the muscles
of the ear, from the animals being seldom much alarmed,
seems probable.
Many laws regulate variation, some few of which can be
dimly seen, and will hereafter be briefly discussed. I will
here only allude to what may be called correlated variation.
Important changes in the embryo or larva will probably
entail changes in the mature animal. In monstrosities, the
correlations between quite distinct parts are very curious ;
and many instances are given in Isidore Geoffroy 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 hoof's
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, owing to the mysterious laws of
correlation.
VARIATION UNDER DOMESTICATION. 11
The results of the various, unknown, or but dimly under-
stood laws of variation are infinitely complex and diversified.
.Tt 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
12 VARIATION UNDER DOMESTICATION.
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 firsfe
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
CHARACTER OF DOMESTIC VARIETIES. 13
races, for instance, of the cabbage, in very poor soil — in.
which case, however, some effect would have to be attributed
to the definite action of the poor soil — that they would, to
a large extent, or even wholly, revert to the wild aboriginal
stock. Whether or not the 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.
CHARACTER OF DOMESTIC VARIETIES J DIFFICULTY OF DIS-
TINGUISHING BETWEEN VARIETIES AND SPECIES J ORIGIN"
OF DOMESTIC VARIETIES FROM ONE OR MORE SPECIES.
When we look to the hereditary varieties or races of our
domestic animals and plants, and compare them with closely
allied species, we generally perceive in each domestic race,
as already remarked, less uniformity of character than in
true species. Domestic races often have a somewhat 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
under 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 cases
are less in degree. This must be admitted as true, for the
domestic races of many animals and plants have been ranked
by some competent judges as the descendants of aboriginally
distinct species, and by other competent judges as mere vari-
14 CHARACTER OF DOMESTIC VARIETIES.
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 ;
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
races.
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 ns 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
CHARACTER OF DOMESTIC VARIETIES. 15
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 world, I have, after a
laborious collection of all known facts, come to the conclu-
sion that several wild species of Canidae have been tamed,
and that their blood, in some cases mingled together, flows
in the veins of our domestic breeds. In regard to sheep
and goats I can form no decided opinion. From facts com-
municated to me by Mr. Blyth, on the habits, voice, 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
16 CHARACTER OF DOMESTIC VARIETIES.
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
rabbit.
The doctrine of the origin of our several domestic races
from several aboriginal stocks, has been carried to an absurd
extreme by some authors. 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 unlike all wild Canidfe — 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
some degree intermediate between their parents j an4 if Tve
DOMESTIC PIGEONS. 17
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.
BREEDS OF THE DOMESTIC PIGEON, THEIR DIFFERENCES
AND ORIGIN.
Believing that it is always best to study some special
group, I have, after deliberation, taken up domestic pigeons.
I have kept every breed which I could purchase or obtain,
and have been most kindly 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 corresponding differences in their
skulls. The carrier, more especially 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
t$.e air head 9Yer feftlSt The runt is a bird of grea^ size.
18 DOMESTIC PIGEONS.
with long massive beak and large feet; some of the sub-
breeds of runts have very long necks, others very long wings
and tails, others singularly short tails. The barb is allied
to the carrier, but, instead of a long beak, has a very short
and broad one. The pouter has a much elongated body,
wings and legs ; and its enormously developed crop, which
it glories in inflating, may well excite astonishment and
even laughter. The turbit has a short and conical beak with
a line of reversed feathers down the breast ; and it has the
habit of continually expanding, slightly, the upper part of
the oesophagus. The Jacobin has the feathers so much
reversed along the back of the neck that they form a hood;
and it has, proportionally to its size, elongated wing and
tail feathers. The trumpeter and laugher, as their names
express, utter a very different coo from the other breeds.
The fantail has thirty or even forty tail-feathers, instead of
twelve or fourteen — the normal number in all the members
of the great pigeon family ; these feathers are kept 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 shape and size of the eggs vary. The man-
DOMESTIC PIGEONS. 10
ner of flight, and in some breeds the voice and disposition,
differ remarkably. Lastly, in certain breeds, the males and
females have come to differ in a slight degree from each
other.
Altogether at least a score of pigeons might be chosen
which, if shown to an ornithologist, and he were told that
they were wild birds, would certainly be ranked by him as
well-defined species. Moreover, I do not believe that any
ornithologist would in this case place the English carrier,
the short-faced tumbler, the runt, the barb, pouter, and 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
20 DOMESTIC PlGEOtfS.
tlie rock-pigeon seems a very rash assumption. Moreover,
the several above-named domesticated breeds have been
transported to all parts of the world, and, therefore, some
of them must have been carried back again into their native
country ; but not one has become wild or feral, though the
dovecot-pigeon, which is the rock-pigeon in a very slightly
altered state, has become feral in several places. Again, all
recent experience shows that it is difficult to get wild
animals to breed freely under domestication; yet on the
hypothesis of the multiple origin of our pigeons, it must be
assumed that at least seven or eight species were so 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-
DOMESTIC MG£ONS. 21
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 hai
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
inheritance.
Lastly, the hybrids or mongrels from between all the
breeds of the pigeon are perfectly fertile, as I can state
from my own observations, purposely made, on the most
distinct breeds. Now, hardly any cases have been ascer-
tained with certainty of hybrids from two quite distinct
<•*,»
22 DOMESTIC PIGEONS.
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 fantails now are, should yield off-
spring perfectly fertile inter se, would be rash in the
extreme.
From these several reasons, namely, the improbability
of man having formerly made seven or eight supposed
species of pigeons to breed freely under domestication —
these supposed species being quite unknown in a wild state,
and their not having become anywhere feral — these species
presenting certain very abnormal characters, as compared
with all other Columbidae, 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
from distant countries, we can make, between them and
the rock-pigeon, an almost perfect series ; so we can in
some other cases, but not with all the breeds. Thirdly,
those characters which are mainly distinctive of each breed
are in each eminently variable, for instance, the wattle and
length of beak of the carrier, the shortness of that of the
tumbler, and the number of tail-feathers in the fan tail :
and the explanation of this fact will be obvious when we
treat of selection. Fourthly, pigeons have been watched
and tended with the utmost care and loved by many
people. They have been domesticated for thousands of
years in several quarters of the world ; the earliest known
DOMESTIC PIGEONS. 23
record of pigeons is in the fifth Egyptian dynasty, about
3000 B.C., as was pointed out to me by Professor Lepsius ;
but Mr. Birch informs me that pigeons are given in a bill
of fare in the previous dynasty. In the time of the
Romans, as we hear from Pliny, immense prices were given
for pigeons ; " nay, they are come to this pass, that they
can reckon up their pedigree and race." Pigeons were
much valued by Akber Khan, in India, about the year
1600 ; never less than 20,000 pigeons were taken with the
court. " The monarchs of Iran and Turan sent him some
very rare birds ; " and, continues the courtly historian,
"His Majesty, by crossing the breeds, which method was
never practised before, has improved them astonishingly."
About this same period the Dutch were as eager about
pigeons as were the old Romans. The paramount 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 shall
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
aviary.
I have discussed the probable origin of domestic pigeons
at some, yet quite insufficient, length j 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 }~ou 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
24 SELECTION BY MAN.
and apples, shows how utterly he disbelieves that the several
sorts, for instance a Kibston-pippin or Codlin-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 than 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 ?
PRINCIPLES OF SELECTION ANCIENTLY FOLLOWED, AND
THEIR EFFECTS.
Let us now briefly consider the steps by which domestic
races have been produced, either from one or from several
allied species. Some 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, for instance,
believe that the fuller's teasel, with its hooks, which cannot
be rivalled by any mechanical contrivance, is only a variety
of the wild Dipsacus ; and this amount of change may have
suddenly arisen in a seedling. So it has probably been with
the turnspit dog ; and this is known to have been the case
with the ancon sheep. But when we compare the dray-
horse and race-horse, the dromedary and camel, the various
breeds of sheep fitted either for cultivated land or mountain
pasture, with the wool of one breed good for one purpose,
and that of another brec"1 for another purpose ; when ifQ
SELECTION BY MAN. 25
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
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
26 SELECTION BY MAN.
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
lias 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 j the diversity of leaves, pods,
SELECTION BY MAN. 27
ar 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 hav§
28 UNCONSCIOUS SELECTION.
not associated with Europeans. Some of these facts do
not show actual selection, but they show that the breeding
of domestic animals was carefully attended to in ancient
times, and is now attended to by the lowest savages. It
would, indeed, have been a strange fact, had attention not
been paid to breeding, for the inheritance of good and bad
qualities is so obvious.
UNCONSCIOUS SELECTION.
At the present time, eminent breeders try by methodical
selection, with a distinct object in view, to make a new
strain or sub-breed, superior to anything of the kind in the
country. But, for our purpose, a form of selection, which
may be called unconscious, and which results from every
one trying to possess and breed from the best individual
animals, is more important. Thus, a man who intends
keeping pointers naturally tries to get as good dogs as he
can, and 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 in.
ftpian like our pointer.
UNCONSCIOUS SELECTION. 29
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 early
maturity, compared with the stock formerly kept in this
country. By comparing the accounts given in various old
treatises of the former and present state of carrier and
tumbler pigeons in Britain, India, and Persia, we can trace
the stages through which they have insensibly passed, and
come to differ so greatly from the rock-pigeon.
Youatt gives an excellent illustration of the effects of a
course of selection which may be considered as unconscious,
in so far that the breeders could never have expected or
even wished, to produce the result which ensued — namely,
the production of 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. Bake well 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. Bakewell's flock, and yet the difference
between the sheep possessed by these two gentlemen is so
great that they have the appearance of being quite different
varieties."
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 thus generally leave
more offspring than the inferior ones ; so that in this case
there would be a kind of unconscious selection going on.
We see the value set on animals even by the barbarians
of Tierra del Fuego, by their killing and devouring their
old women, in times of dearth, as of less value than their
dogs.
In plants the same gradual process of improvement
through the occasional preservation of the best individuals,
whether or not 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
30 UNCONSCIOUS SELECTION.
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 hearts-
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-
stock. The pear, though cultivated in classical times,
appears, from Pliny's description, to have been a fruit of
very inferior quality. I have seen great surprise expressed
in horticultural works at the wonderful skill of gardeners j
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-
UNCONSCIOUS SELECTION. 31
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
has been remarked by some authors, have more of the char-
acter of true species than the varieties kept in civilized
countries.
On the view here given of the important part which
selection by man has played, it becomes at once obvious,
how it is that our domestic races show adaptation in their
structure or in their habits to man's wants or fancies. We
can, I think, further understand the frequently abnormal
character of our domestic races, and likewise their 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 fantail till he saw a pigeon with a tail
developed in some slight degree in an unusual manner,
or a pouter till he saw a pigeon with a crop of somewhat
unusual size ; and the more 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 fantail is, I have no doubt, in most
cases utterly incorrect. The man who first selected a
pigeon with a slightly larger tail, never dreamed what the
descendants of that pigeon would become through long-
continued, partly unconscious and partly methodical, 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-
32 UNCONSCIOUS SELECTION.
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 oi
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 such
slow, varying, and insensible changes.
CIRCUMSTANCES FAVORABLE TO SELECTION. 33
CIRCUMSTANCES FAVORABLE TO MAN'S POWER OP
SELECTION.
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
far 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
larger, earlier, or better fruit, and raised seedlings from
them, and again picked out the best seedlings and bred from
them, then (with some aid by crossing distinct species)
those many admirable varieties of the strawberry were
raised which have appeared during the last half-century.
With animals, facility in preventing crosses is an impor-
tant element in the formation of new races — at least, in a
country which is already stocked with other races. In this
3-i CIRCUMSTANCES FAVORABLE TO SELECTION.
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,
ior 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 a.uimal, as this will be determined by the friction
CIRCUMSTANCES FAVORABLE TO SELECTION. 35
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 teniporarity
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
36 CIRCUMSTANCES FAVORABLE TO SELECTION.
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.
\
VARIATION UtfDER NATURB. 37
1
CHAPTER II.
YARIATION UNDER NATURE.
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
38 INDIVIDUAL DIFFERENCES.
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 I shall have to return in a
future chapter to the preservation and perpetuation of single
or occasional variations.
INDIVIDUAL DIFFERENCES.
The many slight differences which appear in the offspring
from the same parents, or which it may be presumed have
thus arisen, from being observed in the individuals of the
same species inhabiting the same confined locality, may be
called individual differences. No one supposes that all
the individuals of the same species are cast in the same
actual mould. These individual differences are of the highest
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
INDIVIDUAL DIFFERENCES. 39
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 system a-
tists are far from being pleased at finding variability in
important characters, and that there are not many men
who will laboriously examine internal and important organs,
and compare them in many specimens of the same species.
It would never have been expected that the branching of
the main nerves close to the great central ganglion of an
insect would have been variable in the same species ; it
might have been thought that changes of this nature could
have been effected only by slow degrees ; yet Sir J. Lubbock
has shown a degree of variability in these main nerves in
Coccus, which may almost be compared to the irregular
branching of the stem of a tree. This philosophical natur-
alist, I may add, has also shown that the muscles in the
larvas of certain insects are far from uniform. Authors
sometimes argue in a circle when they state that important
organs never vary ; for these same 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
40 INDIVIDUAL Dlftfl&fcfitfCIS.
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.
DOUBT UOL SPECIES. 41
DOUBTFUL SPECIES.
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
opened.
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
42 DOUBTFUL SPECIES.
by botanists as species; and in making this list he has
omitted many trifling varieties, but which nevertheless have
been ranked by some botanists as species, and he has entirely
omitted several highly polymorphic genera. Under genera,
including the most polymorphic forms, Mr. Babington gives
251 species, whereas Mr. Bentham gives only 112 — a 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."
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-
nized.
Many years ago, when comparing, and seeing others com-
pare, the birds from the closely neighboring islands of the
Galapagos Archipelago, one with another, and with those
DOUBTFUL SPECIES. 43
from the American mainland, I was much struck how 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. Walsh 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-
44 DOUBTFUL SPECIES.
habit distinct continents or islands. When, on the other
hand, an animal or plant ranges over the same continent,
or inhabits many islands in the same archipelago, and pre-
sents different forms in the different areas, there is always a
good chance that intermediate forms will be discovered which
will link together the extreme states ; and these are then
degraded to the rank of 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, d§ Coolie, _on the oaks of the whole world-
DOUBTFUL SPECIES. 45
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 are 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 definition above given of a true species. It
should be added that De Candolle no longer believes that
species are immutable creations, but concludes that the
derivative theory is the most natural one, "and the most
accordant with the known facts in palaeontology, geographi-
cal botany, ancl zoology ? of anatomical structure &n4 classifi-
cation,"
46 DOUBTFUL SPECIES.
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,
Avith 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 with
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 of demarcation has as yet been
drawn between species and sub-species — that is, the forms
which in the opinion of some naturalists come very near
to, but do not quite arrive at, the rank of species ; or,
again, between sub-species and well-marked varieties, or
between lesser varieties and individual differences. These
differences blend into each other by an insensible series;
and a series impresses the mind with the idea of an actual
passage.
Hence I look at individual differences, though of small
interest to the systematist, as of the highest importance for
us, as being the first steps 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 passage
from one stage of difference to another may, in many cases,
DOMINANT SPECIES VARY MOST. 47
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
the varieties of certain fossil land-shells in Madeira, and
with plants by Gaston de Saporta. If a variety were to
nourish so as to exceed in numbers the parent species, it
would then rank as the species, and the species as the
variety ; or it might come to supplant and exterminate the
parent species ; or both might co-exist, and both rank as
independent species. But we shall hereafter return to this
subject.
From these remarks it will be seen that I look at the
term species as one arbitrarily given, for the sake of 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.
WIDE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES
VARY MOST.
Guided bjr 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 (lid subsequently Dr. Hooper, even in stronger terms. I
shall reserve for a future work the, discussion of these
48 DOMINANT SrECIES VARY MOST.
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, and 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 to the members of the same group. One of the higher
plants may be said to be dominant if it be more numerous
in individuals and mpre wic^ly diffused than the other
plants '' q£ ' 'tfo* fiame. go^rv, w]j jph Ijve pnfev peayjv $$
SPECIES OF LARGER GENERA VARIABLE. 49
game 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 in the above respects, it will then
be dominant within its own class.
SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY
MORE FREQUENTLY THAN THE SPECIES OF THE SMALLER
GENERA.
If the plants inhabiting a country, as described in any
Flora, be divided into two equal masses, all those in the
larger genera (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
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
pu.gbtj as a general rnle? to be. now forming f^bere many
50 SPECIES OF LARGER GENERA VARIABLE.
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, than in one having few.
To test the truth of this anticipation I have arranged the
plants of twelve countries, and the coleopterous insects of
two districts, into two nearly equal masses, the species of
the larger genera on one side, and those of the smaller
genera on the other side, and it has invariably proved to be
the case that a larger proportion of the species on the side
of the larger genera presented varieties, than on the side of
the smaller genera. Moreover, the species of the large
genera which present any varieties, invariably present a
larger average number of varieties than do the species of
the small genera. Both these results follow when another
division is made, and when all the least genera, with from
only one to four species, are altogether excluded from the
tables. These facts are of plain signification on the view
that species are only strongly marked and permanent
varieties ; for wherever many species of the same genus
have been formed, or where, if we may use the expression,
the manufactory of species has been active, we ought
generally to find the manufactory still in action, more
especially as we have every reason to believe the process of.
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 j
and this certainly holds good.
SPECIES OF LARGER GENERA. bl
MANY OF THE SPECIES INCLUDED WITHIN THE LARGER GUN-
ERA RESEMBLE VARIETIES IN BEltfG VERY CLOSELY, BUT
UNEQUALLY, RELATED TO EACH OTHER, AND IN HAVING
RESTRICTED RANGES.
There are other relations between the species of large
genera and their recorded varieties which deserve notice.
We have seen that there is no infallible criterion by which
to distinguish species and well-marked varieties ; and when
intermediate links have not been found between doubtful
forms, naturalists are compelled to come to a determination
by the amount of difference between them, judging by anal-
ogy whether or not the amount suffices to raise one or both
to the rank of species. Hence the amount of difference is
one very important criterion in settling whether two forms
should be ranked as species or varieties. Now Fries has
remarked in regard to plants, and Westwood in regard to
insects, that in large genera the amount of difference
between the species is often exceedingly small. I have
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 of
62 SPECIES OF LARGER GENERA.
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.
SUMMARY.
Finally, varieties cannot be distinguished from species, —
except, first, by the discovery of intermediate linking forms ;
and, secondly, by a certain indefinite amount of difference
between them; for two forms, if differing vei j 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
RESEMBLE VARIETIES. S3
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
creations.
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. Ajid thus the forms of life
throughout the universe become divided into groups subordi-
nate to groups.
54 STRUGGLE FOR EXISTENCE.
CHAPTER III.
STRUGGLE FOR EXISTENCE.
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 t>e
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 only 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 every-
where and in every part of the organic world.
Again, it may be asked, how is it that varieties, which I
have called incipient species, become ultimately converted
into good and distinct species, which in most cases obviously
\
STRUGGLE FOR EXISTENCE. 55
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 iw 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
56 STRUGGLE FOR EXISTENCE.
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.
THE TERM, STRUGGLE FOR EXISTENCE, USED IN A LARGE
SENSE.
I should premise that I use this term in a large and
metaphorical sense, including dependence of one being on
another, and including (which is more important) not only
the life of the individual, but success in leaving progeny.
Two canine animals, in a time of dearth, may be truly
said to struggle with each other which shall get food and
live. But a plant on the edge of a desert is said to struggle
for life against the drought, though more properly it should
be said to be dependent on the moisture. A plant which
annually produces a thousand seeds, of which only one of
an average comes to maturity, may be more truly said to
struggle with the plants of the same and other kinds which
already clothe the ground. The mistletoe is dependent on
the apple and a few other trees, but can only in a far-fetched
sense be said to struggle with these trees, for, if too many
of these parasites grow on the same tree, it languishes and
dies. But several seedling mistletoes, growing close together
on the same branch, may more truly be said to struggle
with each other. As the mistletoe is disseminated by birds,
its existence depends on them ; and it may metaphorically
be said to struggle with other fruit-bearing plants, in 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-
ence.
GEOMETRICAL RATIO OP INCREASE.
A struggle for existence inevitably follows from the high
rate at which all organic beings tend to increase. Every
being, which during its natural lifetime produces several
eggs or seeds, must suffer destruction during some period
of its life, and during some season or occasional year;
otherwise, on the principle 01 geometrical increase, its num-
bers would quickly become so inordinately great that no
GEOMETRICAL RATIO OF INCREASE. 57
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 increase of
food, and no prudential restraint from marriage. Although
some species may be now increasing, more or less rapidly,
in numbers, all cannot do so, for the world would not hold
them.
There is no exception to the rule that every organic being
naturally increases at so high a rate, that, if not destroyed,
the earth would soon be covered by the progeny of a single
pair. Even slow-breeding man has doubled in twenty-five
years, and at this rate, in less than a 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
there 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 moie
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, §ey
58 GEOMETRICAL RATIO OF INCREASE.
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-
clusion of every other plant, have been introduced from
Europe ; and there are plants which now range in India,
as I hear from Dr. Falconer, from Cape Comorin to the
Himalaya, which have been imported from America since
its discovery. In such cases, and endless others could be
given, no one supposes that the fertility of the animals or
plants has been suddenly and temporarily increased in any
sensible degree. The obvious explanation is that the 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. Heuce 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 falling on them, but we do not
keep in mind that thousands are annually slaughtered for
food, and that in a state of nature an equal number would
have somehow to be disposed of.
The only difference between organisms which annually
produce eggs or seeds by the thousand, and those which
produce extremely few, is, that the slow breeders would re-
quire a few more years to people, under favorable condi-
tions, a whole district, let it be ever so large. The condor
lays a couple of eggs and the ostrich a score, and yet in the
same country the condor may be the more numerous of the
two. The Fulmar petrel lays but one egg, yet it is 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
NATURE OF THE CHECKS TO INCREASE. 59
of food, for it allows them rapidly to increase in numbers.
But the real importance of a large number of eggs or seed
is to make up for much destruction at some period of life ;
and this period in the great majority of cases is an early
one. If an animal can in any way protect its own eggs or
young, a small number may be produced, and yet the
average stock be fully kept up ; but if many eggs or young
are destroyed, many must be produced, or the species will
become extinct. It would suffice to keep up the full 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
seeds.
In looking at Nature, it is most necessary to keep the
foregoing considerations always in mind — never to forget
that every single organic being may be said to be striving
to the utmost to increase in numbers ; that each lives by a
struggle at some period of its life ; that heavy destruction
inevitably falls either on the young or old during each gen-
eration or at recurrent intervals. Lighten any check, miti-
gate the destruction ever so little, and the number of the
species will almost instantaneously increase to any amount.
NATURE OF THE CHECKS TO INCREASE.
The causes which check the natural tendency of each
species to increase are most obscure. Look at the most
vigorous species ; by as much as it swarms in numbers, by
so much will it tend to increase still further. We know
not exactly what the checks are, even in a single instance.
Nor will this surprise any one who reflects how ignorant
we are on this head, even in regard to mankind, although
so incomparably better known than any other animal.
This subject of the checks to increase has been ably treated
by several authors, and I hope in a future work to discuss
it at considerable length, more especially in regard to the
feral animals of South America. Here I will make only a
few remarks, just to recall to the reader's mind some of the
chief points. Eggs or very young animals seem generally
to suffer most, but this is not invariably the case. With
plants there is a vast destruction of seeds, but from some
80 NATURE OE THE CHECKS TO ItfCIfcEASE.
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 four), nine species
perished, from the other species being allowed to grow up
freely.
The amount of food for each species, of course, gives
the extreme limit to which each can increase ; but very 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,
it brings on the most severe struggle between the indi-
viduals, whether of the same or of distinct species, which
NATURE OF THE CHECKS TO INCfcEASE. 61
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 we have a limiting check
independent of the struggle for life. But even some of
these so-called epidemics appear to be due to parasitic
worms, which have from some cause, possibly in part
through facility of diffusion among the crowded animals,
been disproportionally favored : -and here comes in a sort of
Struggle between the parasite and its prey.
62 STRUGGLE FOR EXISTENCE.
On the other hand, in many cases, a large stock of indi-
viduals of the same species, relatively to the numbers of its
enemies, is absolutely necessary for its preservation. Thus
Ave 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 manv
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.
COMPLEX RELATIONS OF ALL ANIMALS AND PLANTS TO
EACH OTHER IN THE STRUGGLE FOR EXISTENCE.
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, wThich, 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-
live 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
STRUGGLE FOR EXISTENCE. 63
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, T 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 stems 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 Paragua}7- offers
the most curious instance of this ; for here neither cattle nor
horses nor dogs have ever run wild, though they swarm
southward and northward in a feral state ; and Azara and
Rengger have shown that this is caused by the greater number
in Paraguay of a certain fly, which lays its eggs in the navels
of these animals when first born. The increase of these
flies, numerous as they are, must be habitually checked by
some means, probably by other parasitic insects. Hence, if
64 STRUGGLE EOU EXISTENCE.
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
humble-bees became extinct or very rare in England, the
STRUGGLE FOR EXISTENCE. 65
hearts-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
66 STRUGGLE FOR EXISTENCE.
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, as in the case of locusts and grass-feed-
ing quadrupeds. But the struggle will almost invariably be
most severe between the individuals of the same species, for
they frequent the same districts, require the same food, and
are exposed to the same dangers. In the case of varieties of
the same species, the struggle will generally be almost
equally severe, and we sometimes see the contest soon
decided : for instance, if several varieties of wheat be sown
together and the mixed seed be resown, some of the 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-pease, 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) could be
kept up for half a dozen generations, if they were allowed to
struggle together, in the same manner as beings in a state
of nature, and if the seed or young were not annually pre*
served in due proportion.
STRUGGLE FOR LIFE MOST SEVERE BETWEEN INDIVIDUALS
AND VARIETIES OF THE SAME SPECIES.
As the species of the same genus usually have, though by
no means invariably, much similarity in habits and 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.
STRUGGLE FOR EXISTENCE. 67
We see this in the recent extension over parts of the United
States of one species of swallow, having caused the decrease
of another species. The recent increase of the missel-thrush
in parts of Scotland has caused the decrease of the song-
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-
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 bod}^ 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
seedlings, while struggling with other plants growing vigor-
ously all around.
Look at a plant in the midst of its range ! Why does it
68 STRUGGLE FOR EXISTENCE.
not double or quadruple its numbers ? We know that it
can perfectly well withstand a little more heat or cold,
dampness or dryness, for elsewhere it ranges into slightly
hotter or colder, damper or 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 great destruction. When we reflect on
this struggle we may console ourselves with the full belief
that the war of nature is not incessant, that no fear is felt,
that death is generally prompt, and that the vigorous, tho
kealthy, and the happy survive and multiply.
NATURAL SELECTION. 69
CHAPTER IV.
NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
Natural Selection — Its Power compared with Man's Selection — It«
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 cond>
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 potymorphic species, or
would ultimately become fixed, owing t» 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 ; and it has even been urged, that, as plants
have no volition, natural selection is not applicable to them !
In the literal sense of the word, no doubt, natural selection
is a false term ; but who ever objected to chemists speaking
of the elective affinities of the various elements ? — and yet
an acid cannot strictly be said to elect the base with which
it in preference combines. It has been said that I speak of
natural selection as an active power or Deity ; but who
objects to an author speaking of the attraction of gravity as
ruling the movements of the planets ? Every one knows
what is meant and is implied by such metaphorical 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 a3
NATURAL SELECTION. 7T
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
improvement.
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 in
T2 NATURAL SELECTION.
order that new and unoccupied places should be left for
natural selection to fill up by improving some of the vary-
ing inhabitants. 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 the
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
NATURAL SELECTION. 73
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 from what they formerly were.
In order that any great amount of modification should be
effected in a species, a variety, when once formed, must
again, perhaps after a long interval of time, vary or 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. Grouse, if not
destroyed at some period of jbheir lives, would increase ill
74 NATURAL SELECTION.
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
destruction 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; so
NATURAL SELECTION. 75
in 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 hj the wind,
I can see no greater difficulty in this being effected through
natural selection, than in the cotton-planter increasing and
improving by selection the down in the pods on his cotton-
trees. Natural selection may modify and adapt the larva of
an insect to a score of contingencies, wholly different from
those which concern the mature insect ; and these 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 egg 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
.structure.
It may be well here to remark that with all beings there
T6 SEXUAL SELECTION.
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 individuals
better adapted to their conditions of life than any of those
which happened to survive. So again a vast number of
mature animals and plants, whether or not they be the best
adapted to their conditions, must be annually destroyed by
accidental causes, which would not be in the least degree
mitigated by certain changes of structure or constitution
which would in other ways be beneficial to the species. But
let the destruction of the adults be ever so heavy, if the
number which can exist in any district be not wholly kept
down by such causes — or again let the destruction of eggs
or seeds be so great that only a hundredth or a thousandth
part are developed — yet of those which do survive, the best
adapted individuals, supposing that there is any variability
in a 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.
SEXUAL SELECTION.
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 beings or to external conditions,
but on a struggle between the individuals of one sex, gener*
a)lv fcfte rnales, for $9 possession of ^e; ot^er sex, Tk§
SEXUAL SELECTION. 77
result is not death to the unsuccessful competitor, but few
or no offspring. Sexual selection is, therefore, less rigorous
than natural selection. Generally, the most vigorous males,
those which are best fitted for their places in nature, will
leave most progeny. But in many cases victory depends not
so much on 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
ft all bis hen &r<lsr I cannot hm eftfe? <?n $P RWWWy
78 ILLUSTRATIONS OF THE ACTION
details ; but if man can in a short time give beauty and
an elegant carriage to his bantams, according to his standard
of beauty, I can see no good reason to doubt that female
birds, by selecting, during thousands of generations, the most
melodious or beautiful males, according to their 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.
ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR
THE SURVIVAL OF THE FITTEST.
In order to make it clear how, as I believe, natural selec-
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 strength to master their
OF NATURAL SELECTION. 79
prey at this or some other period of the year, when they
were compelled to prey on other animals. I can see no
more reason to doubt that this would be the result, than
that man should be able to improve the fleetness of his grey-
hounds by careful and methodical selection, or by that kind
of unconscious selection which follows from each man try-
irig 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 Moun-
tains, in the United States, one with a light greyhound-like
form, which pursues deer, and the other more bulky, with
shorter legs, which more frequently attacks the shepherd's
flocks.
It should be observed that in the above illustration, I
speak of the slimmest individual wolves, and not of any
single strongly marked variation having been preserved.
In former editions of this work I sometimes spoke as if this
latter alternative had frequently occurred. I saw the great
importance of individual differences, and this led me fully
to discuss the results of unconscious 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
of destruction, only two on an average survive to procreate
their kind. This is rather an extreme estimate for most of
the higher animals, but by no means so for many of the
lower organisms. He then shows that if a single 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
80 ILLUSTRATIONS OF THE ACTION
of these remarks cannot, I think, be disputed. If, for
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-
erations 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 lacry-
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 of nature ; so that similarly modified indi-
viduals would soon exist in a small body together, and would
often breetf together, If the new variety were successful iu
OF NATURAL SELECTION. 81
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 often 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 Qne ^tep in the gepara^ipR of the, sexes
82 ILLUSTRATIONS OF THE ACTION
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 full-sized pistil, and four
stamens with shrivelled anthers, in which not a grain of
pollen can be detected. Having found a female tree exactly
sixty yards from a male tree, I put the stigmas of twenty
flowers, taken from different branches, under the 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 onr 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
polygamous.
Let us now turn to the nectar-feeding insects ; we may
suppose the plant, of which we have been slowly increasing
the nectar by continued selection, to be a common plant ;
OF NATURAL SELECTION. 83
and that certain insects depended in main part on its
nectar for food. I could give many facts showing how-
anxious bees are to save time : for instance, their habit of
cutting holes and sucking the nectar at the luses of certain
flowers, which with a very little more trouble they 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 o'oher 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 tnK>w off many
swarms inheriting the same peculiarities. Xne tubes of the
corolla of the common red or incarnate clovers (Tri folium
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
84 otf 'tm w? z&ctLOs&mG
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.
ON THE INTERCROSSING OF INDIVIDUALS.
I must here introduce a short digression. In the case of
animals and plants with separated sexes, it is of course
obvious that two individuals must always (with the 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.
Of INDIVIDUALS. 85
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 that 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. Many flowers,
on the other hand, have their organs of fructification closely
enclosed, as in the great papilionaceous or pea-family ; but
these almost invariably present beautiful and curious 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
pollen.
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-f ertilization j
86 ON THE INTERCROSSING
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 that, if
closely-allied forms or varieties are planted near each other,
it is hardly possible to raise pure seedlings, so largely do
they naturally cross. In numerous other cases, far from
self-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 flowTer 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 I 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
occasional 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, and
some even of these were not perfectly true. Yet the pistil
OF INDIVIDUALS. 87
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 objection 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 th9
38 CIRCUMSTANCES FAVORABLE TO THE
action of insects and of the wind with plants, by which an
occasional cross could be effected with terrestrial animals
without the concurrence of two individuals. Of aquatic
animals, there are many self-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.
Cirripedes long appeared to me to present, under this point
of view, a case of great difficulty ; but I have been enabled,
by a fortunate chance, to prove that two individuals, though
both of self-fertilizing hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange anomaly
that, both with animals and plants, some species of the same
family and even of the same genus, though agreeing closely
with each other in their whole organization, are hermaphro-
dites, and some unisexual. But if, in fact, all hermaphrodites
do occasionally intercross, the difference between them and
unisexual species is, as far as function is concerned, very
small.
From these several considerations and from the many
special facts which I have collected, but which I am unable
here to give, it appears that with animals and plants an
occasional intercross between distinct individuals is a very
general, if not universal, law of nature.
CIRCUMSTANCES FAVORABLE FOR THE PRODUCTION OF NEW
FORMS THROUGH NATURAL SELECTION.
This is an extremely intricate subject. A great amount
of variability, under which term individual differences are
always included, will 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 nighty important element of success.
Though nature grants long periods of time for the work of
natural selection, she does not grant an indefinite period, for
as all organic beings are striving to seize on each place in
the economy of nature, if any one species does not become
fftodified and improved in a corresponding degree with its
RESULTS OF NATURAL SELECTION. 89
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; for I can bring forward a considerable body of
90 CIRCUMSTANCES FAVORABLE TO THE
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 to
believe that occasional intercrosses take place with all
animals and plants. Even if these take place only at long
intervals of time, the young thus produced will gain so much
in 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 selection 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 better
RESULTS OF NATURAL SELECTION. 91
adapted organisms ; and thus new places in the natural
economy of the district will be left open to be filled up by
the modification of the old inhabitants. Lastly, isolation
will give time for a new variety to be improved at a slow
rate ; and this may sometimes be of much importance. If,
however, an isolated area be very small, either from being
surrounded by barriers, or from having very peculiar physical
conditions, the total 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.
Tt likewise tends to increase the direct action of the physical
conditions of life, in relation to the constitution of each
organism.
If we turn to nature to test the truth of these 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, we ought to make the comparison within equal times ;
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 large
number of individuals of the same species there supported^
§2 CIRCUMSTANCES FAVOftAfcLE TO THE
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 Europseo-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 the natural scale. These anomalous
RESULTS OF NATURAL SELECTION. $2
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,
competition.
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 act 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 places will often depend on physi-
cal changes, which generally take place very 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 species
94 EXTINCTION BY NATURAL SELECTION.
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
fittest.
EXTINCTION CAUSED BY NATURAL SELECTION.
This subject will be more fully discussed in our chap-
ter on Geology ; but it must here be alluded to from being
intimately connected with natural selection. Natural selec-
tion acts solely through the preservation of variations in
some 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
$reat
DIVERGENCE OF CHARACTER. 95
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."
DIVERGENCE OF CHARACTER.
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,
96 DIVERGENCE OE CHARACTER.
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 bj7" 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 thai,
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 havo tended to disappear. Here;
I DIVERGENCE OF CHARACTER. 97
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
numbers.
We can clearly discern this in the case of animals with
simple habits. Take the case of a carnivorous quadruped,
of which the number that can be supported in any country
has long ago arrived at its full average. If its natural
power of increase be allowed to act, it can succeed in 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
98 DIVERGENCE OF CHARACTER.
in living on the same piece of ground. And we know that
each species and each variety of grass is annually sowing
almost countless seeds ; and is thus striving, as it may be
said, to the utmost to increase in number. Consequently,
in the course of many thousand generations, the most 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 is
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 commonl}'
looked at as specially created and adapted for their own.
county. It might also, perhaps, have been expected that nat-
uralized plants would have belonged to a few groups more
DIVERGENCE OF CHARACTER. 99
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.
100 RESULT OF THE ACTION
THE PROBABLE EFFECTS OF THE ACTION OF NATURAL SELEC-
TION THROUGH DIVERGENCE OF CHARACTER AND EXTINC-
TION, ON THE DESCENDANTS OF A COMMON ANCESTOR.
After the foregoing discussion, which has been much
compressed, we may assume that the modified descendants
of any one species will succeed so much the better as they
become more diversified in structure, and are thus enabled
to encroach on places occupied by other beings. Now let
us see how this principle of benefit being derived from
divergence of character, combined with the principles of
natural selection and of extinction, tends to act.
The accompanying diagram will aid us in understanding
this rather perplexing subject. Let A to L represent the
species of a genus large in its own country ; these species are
supposed to resemble each other in unequal degrees, as is so
generally the case in nature, and as is represented in the
diagram by the letters standing at unequal distances. I
have said a large genus, because, as we saw in the second
chapter, on an average more species vary in large genera
than in small genera ; and the varying species of the large
genera present a greater number of varieties. We have,
also, seen that the species, which are the commonest and
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
OF NATURAL SELECTION. 101
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 a1 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
varieties.
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 a1 is supposed in the diagram to have produced
variety a2, which will, owing to the principle of divergence,
differ more from (A) than did variety a\ Variety m1 is
supposed to have produced two varieties, namely m2 and s2,
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 most
102
RESULT OF THE ACTION
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OF NATURAL SELECTION.
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104 RESULT OF THE ACTION
divergent varieties are invariably preserved: a medium
farm 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 more
places they will be enabled to seize on, and the more their
modified progeny will increase. In our diagram the line of
succession is broken at regular intervals by small numbered
letters marking the successive forms which have become
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
number 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 a1 to a10. 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, a10, /10, and m10, 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
OP NATURAL SELECTION. 105
in our diagram to be excessively small, these throe 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 eigltt
species, marked by the letters between au and ra14, 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 (w10 and
310) 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 w14 to 214, 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
analtered 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,,
106 RESULT OF THE ACTION
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 modified offspring of a species
get into some distinct country, or become quickly adapted to
some quite new station, in which offspring and progenitor do
not come into competition, both may continue to exist.
If, then, our diagram be assumed to represent a consider-
able amount of modification, species (A) and all the earlier
varieties will have become extinct, being replaced by eight
new species (a14 to m14), and species (I) will be replaced by
six (nu to s14) new species.
But we may go further than this. The original species of
our genus were supposed to resemble each other in unequal
degrees, as is so generally the case in nature ; species (A)
being more nearly related to B, C, and D than to the other
species ; and species (I) more to G, H, K, L, than to the
others. These two species (A and I) were also supposed to
be very common and widely diffused species, so that they
must originally have had some advantage over most of the
other species of the genus. Their modified descendants,
fourteen in number at the fourteen-thousandth generation,
will probably have inherited some of the same advantages ;
they have also been modified and improved in a diversified
manner at each stage of descent, so as to have become
adapted to many related places in the natural economy of
their country. It seems, therefore, extremely probable that
they will have taken the places of, and thus exterminated,
not only their parents (A) and (I), but likewise some of the
original species which were most nearly related to their
parents. Hence very few of the original species will have
transmitted offspring to the fourteen-thousandth generation.
We aiay 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
descent.
The new species in our diagram, descended from the original
eleven species, will now be fifteen in number. Owing to the
divergent tendency of natural selection, the extreme amount
of difference in character between species a14 and z1* will be
mucli greater than that between the most distinct of the
OF NATURAL SELECTION. 107
original eleven species. The new species, moreover, will be
allied to each other in a widely different manner. Of the
eight descendants from (A) the three marked a14, qu, pu, will
be nearly related from having recently branched off from a19 ;
bli and /14, from having diverged at an earlier period from
a5, will be in some degree distinct from the three first-named
species ; and lastly, o14, e14, and mu 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
(and this is a very important consideration), which connected
the original species (A) and (I), 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.
Thus it is, as I believe, that two or more genera are pro-
duced by descent, with modification, from two or more species
of the same genus. And the two or more parent-species are
supposed to be descended from some one species of an earlier
genus. In our diagram this is indicated by the broken lines
beneath the capital letters, converging in sub-branches down-
ward toward a single point : this point represents a species,
the supposed progenitor of our several new sub-genera and
genera.
It is worth while to reflect for a moment on the character
of the new species f14, which is supposed not to have diverged
much in character, but to have retained the form of (F),
either unaltered or altered only in a slight degree. In this
case its affinities to the other fourteen new species will be of
a curious and circuitous nature. Being descended from a
form that stood between the parent-species (A) and (I), now
supposed to be extinct and unknown, it will be in some degree
intermediate in character between the two groups descended
from these two species. But as these two groups have gone
on diverging in character from the type of their parents, the
108 RESULT OF THE ACTION
new species (f14) 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 of change represented by each
successive group of diverging dotted lines to be great, the
forms marked au to pu, those marked bli and fu, and those
marked o14 to m14, 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 l^ter an4
OF NATURAL SELECTION. 109
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 present day, and as all the descendants
of the same species form a class, we can understand how it
is that there exist so few classes in each main division of the
animal and vegetable kingdoms. Although few of the most
ancient species have left modified descendants, yet, at remote
geological periods, the earth may have been almost as well
peopled with species of many genera, families, orders, and
classes, as at the present time.
ON THE DEGREE TO WHICH ORGANIZATION TENDS TO
ADVANCE.
Natural selection acts exclusively by the preservation and
Hccumulation of variations, which are beneficial under the
organic and inorganic conditions to which each creature is
exposed at all periods of life. The ultimate result is that
each creature tends to become more and more improved in
relation to its conditions. This improvement inevitably leads
to the gradual advancement of the 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 mm eJe&rly come
110 ON THE DEGREE TO WHICH
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 Baer'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 econorny 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
ORGANIZATION TENDS TO ADVANCE. Ill
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 scale, how is it that throughout the
world a multitude of the lowest forms still exist ; and how-
is it that in each great class some forms are far more highly
developed than others ? Why have not the more highly
developed forms everywhere supplanted and exterminated
the lower ? Lamarck, who believed in an innate and 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 fish
in the extreme simplicity of its structure approaches the
invertebrate classes. But mammals and fish hardly come
into competition with each other ; the advancement of the
whole class of mammals, or of certain members in this class.
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
iishes, members of the shark family would not tend to sup-
plant the lancelet ; for the lancelet, as I hear from Fritz
M tiller, 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
yvqulcl probably answer, ^at, as goon as simple unicellular
organism same by growfcji or division tq be compounded of
Several pgjlfy QV became, attached tq anv's^ppprtjng surface,
ORGANIZATION TENDS TO ADVANCE. 113
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.
CONVERGENCE OF CHARACTER.
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) frorn innumerable
progenitors, all of wJaich have Jia4 their fe>nB§ SetermmecL
114 CONVERGENCE OF CHARACTER.
through equally complex relations. It is incredible that the
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
admission.
Mr. Watson has also objected that the continued action
of natural selection, together with divergence of character,
would tend to make an indefinite number of specific forms.
As far as mere inorganic conditions are concerned, it seems
probable that a sufficient number of species would soon
become adapted to all considerable diversities of heat,
moisture, etc. ; but I fully admit that the mutual relations
of organic beings are more important ; and as the number
of species in any country goes on increasing, the organic
conditions of life must become more and more complex.
Consequently there seems at first 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
be rapid, whereas the production of new species must alwa}^s
be slow. Imagine the extreme case of as many species as
individuals in England, and the first severe winter or very
dry summer would exterminate thousands on thousands of
species, Bare species, and each species will become rare if
CONVERGENCE OF CHARACTER. 115
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 specific forms
would thus be retarded. When any species becomes very
rare, close interbreeding will help to exterminate it; authors
have thought that this comes into play in accounting for the
deterioration of the aurochs in Lithuania, of red deer in
Scotland, and of bears in Norway, etc. Lastly, and this I
am inclined to think is the most important element, a domi-
nant species, which has already beaten many competitors in
its own home, will tend to spread and supplant many others.
Alph. de Candolle has shown that those species which
spread widely tend generally to spread very widely, 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
say; but conjointly they must limit in each country the ten-
dency to an indefinite augmentation of specific forms.
SUMMARY OF CHAPTER.
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
116 SUMMARY.
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 seen 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
character and to much extinction of the less improved and
StJMMARY. 117
intermediate forms of life. On these principles, the nature
of the affinities, and the generally well denned 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 family
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. I believe this simile
largely speaks the truth. The green and budding twigs may
represent existing species ; and those produced during former
years may represent the long succession of extinct species.
At each period of growth all the growing twigs have tried
to branch out on all sides, and to overtop and kill the 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 ; and these fallen branches of various
118
SUMMARY.
sizes may represent those whole orders, families, and genera
which have now no living representatives, and which are
known to ns 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, so 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. •
i ••
LAWS OF VARIATION. 119
CHAPTER V.
LAWS OF VARIATION.
]£ffects of Changed Conditions — Use and Disuse, combined with Nat
ural Selection; Organs of Flight and of Vision — Acclimatizatior
— Correlated Variation — Compensation and Economy of Growth
-—False Correlations — Multiple, Rudimentary, and Lowly Organ
ized Structures Variable — Parts developed in an Unusual ]VIan-
ner are highly Variable: Specific Characters more Variable that:
Generic; Secondary Sexual Characters Variable — Species of the
Same Genus vary in an Analogous Manner — Reversions to Long
lost Characters — Summary.
I have hitherto sometimes spoken as if the variations —
so 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-
ganization or on certain parts alone, and indirectly through
the reproductive system. In all cases there are two factors,
the nature of the organism, which is much the most important
of the two, and the nature of the conditions. The direct
action of changed conditions leads to definite or indefinite
results. In the latter case the 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.
120 LAWS OF VARIATION.
It is very difficult to decide how far changed conditions,
such as of climate, food, etc., have acted in a definite
manner. There is reason to believe that in the course of
time the effects have been greater than can be proved by
clear evidence. But we may safely conclude that the innu-
merable complex coadaptations of structure, which we see
throughout nature between various organic beings, cannot
be attributed simply to such action. In the following cases
the conditions seem to have produced some slight definite
effect : E. Forbes asserts that shells at their southern limit,
and when living in shallow water, are more brightly 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 gives 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 w^ell 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 we
are quite ignorant.
EFFECTS OF tfSE AND DISUSE. 121
In one sense the conditions of life may be said, not only
to cause variability, either directly or indirectly, 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 clearly 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.
EFFECTS OF THE INCREASED USE AND DISUSE OF PARTS, AS
CONTROLLED BY NATURAL SELECTION.
From the facts alluded to in the first chapter, I think there
can be no doubt that use in our domestic animals has
strengthened and enlarged certain parts, and disuse 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 of 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 specie
122 EFFECTS OF USE AND DISUSE.
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 tendency.
Hence, it will perhaps be safest to look at the entire absence
of the anterior tarsi in Ateuchus, and their rudimentary
condition in some other genera, not as cases of inherited
mutilations, but as due to the effects of long-continued dis-
use ; for, as many dung-feeding beetles are generally found
with their tarsi lost, this 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. These
several considerations make me believe that the wingless
condition of so many Madeira beetles is mainly due to the
action of natural selection, combined probably with disuse.
For during many successive generations each individual
beetle which flew least, either from its wings having been
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.
EFFECTS OF USE AND DISUSE. 123
The insects in Madeira which are not ground-feeders, and
which, as certain flower-feeding coleoptera and lepidoptera,
must habitually use their wings to gain their subsistence,
have, as Mr. Wollaston suspects, their wings not at all
reduced, but even enlarged. This is quite compatible with
the action of natural selection. For when a new insect first
arrived on the island, the tendency of natural 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 Cteuomys, 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 the 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
124 EFFECTS OF USE AND D1SUSF.
eyes were lustrous and of large size ; and these animals, as
I am informed by Professor Silliman, after having been ex-
posed for about a month to a graduated light, acquired a
dim perception of objects.
It is difficult to imagine conditions of life more similar
than deep limestone caverns under a nearly similar climate ;
so that, in accordance with the old view of the blind 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 fauna 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 ; for, 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 often have effected other
changes, such as an increase in the length of the antennas
or palpi, as a compensation for blindness. Notwithstanding
such modifications, we might expect still to see in the cave-
animals of America, affinities to the other inhabitants of
that continent, and in those of Europe to the inhabitants
Effects of use Atfb Misuse. 125
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
©f the two continents on the ordinary view of their inde-
pendent creation. That several of the inhabitants of the
caves of the Old and New Worlds should be closely related,
we might expect from the well-known relationship of most
of their other productions. As a blind species of Batfcyscia
is found in abundance on shady rocks far from caves, the
loss of vision in the cave species of this one genus has
probably had no relation to its dark habitation; for it is
natural that an insect already deprived of vision should
readily become adapted to dark caverns. Another blind
genus (Anophthalmus) offers this 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
exposed.
ACCLIMATIZATION.
Habit is hereditary with plants, as in the period of
flowering, in the time of sleep, in the amount of rain,
requisite for seeds to germinate, 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, acclimatization must be readily effected dur-
ing a long course of descent. It is notorious that each
126 ACCLIMATIZATION.
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 subsequent!}'' 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
ACCLIMATIZATION. 127
must not, however, push the foregoing argument too far, on
account of the probable origin of some of our domestic
animals from several wild stocks ; the blood, for instance,
of a tropical and arctic wolf may perhaps be mingled in our
domestic breeds. The rat and mouse cannot be 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 habituallv
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
128 CORRELATED VARIAtiOtf.
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-
lings, 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-
ations.
CORRELATED VARIATION.
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 larvae 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
CORRELATED VARIATION. 129
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.
Geoffroy Saint-Hilaire has forcibly remarked that certain
maleonformations 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 accon>
130 CORRELATED VARIATION.
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 umbelliferse it is by no means, as Dr.
Hooker informs me, the species with the densest heads which
most frequently differ in their inner and outer flowers. It
might have been thought that the development of the ray-
petals by drawing nourishment from the reproductive organs
causes their abortion ; but this can hardly be the sole cause,
for in some compositse the seeds of the outer and inner flo-
rets differ, without any difference in the corolla. Possibly
these several differences may be connected with 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
Dot quite aborted but is much shortened.
With respect to the development of the corolla, Sprengel's
idea that the ray-florets serve to attract insects, whose agency
is highly advantageous, or necessary for the fertilization of
these plants, is highly probable ; and if so, natural selection
may have come into play. But with respect to the seeds, it
seems impossible that their differences in shape, which are
not always correlated with any difference in the corolla, can
be in any way beneficial ; yet in the umbelliferae these dif-
ferences are of such apparent importance — the seeds being
sometimes orthospermous in the exterior 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 species,
COMPENSATION AND ECONOMY OF GROWTH. 131
and which in truth are simply due to inheritance ; for an
ancient progenitor 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.
COMPENSATION AND ECONOMY OF GROWTH.
The elder Geoffroy and Goethe propounded, at about the
same time, their law of compensation or balancement of
growth; or, as Goethe expressed it, "in order to 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 the 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
part.
132 MULTIPLE AND RUDIMENTARY.
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 to
have its nutriment wasted in building up a useless structure.
I can thus only understand a fact with which I was much
struck when 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.
MULTIPLE, RUDIMENTARY, AND LOWLY ORGANIZED
STRUCTURES ARE VARIABLE.
It seems to be a rule, as remarked by Is. Geoffroy Saint-
Hilaire, both with varieties and species, that when any part
or organ is repeated many times in the same individual (as
the vertebrae in snakes, and the stamens in polyandrous
flowers) the number is variable ; whereas the same part or
organ, when it occurs in lesser numbers, is constant. The
STRUCTURES VARIABLE. 133
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 why it should remain
variable, that is, why natural selection should not have
preserved or rejected each little deviation of form so care-
fully as when the part has to serve for some one special
purpose. In the same way that a knife which has to cut all
sorts of things may be of almost any shape ; 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
structure.
A PART DEVELOPED IN ANY SPECIES IN AN EXTRAORDINARY
DEGREE OR MANNER, IN COMPARISON WITH THE SAME
PART IN ALLIED SPECIES, TENDS TO BE HIGHLY VARIABLE.
Several years ago I was much struck by a remark to the
above effect made by Mr. Waterhouse. Professor Owen,
also, seems to have come to a nearly similar conclusion.
It is hopeless to attempt to convince any one of the truth
of the above proposition without giving the long array of
facts which I have collected, and which cannot possibly be
here introduced. I can only state my conviction that it is
a rule of high generality. I am aware of several causes of
error, but I hope that I have made due 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
134 UNUSUALLY DEVELOPED PARTS
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 to the females, as they
seldom offer remarkable secondary sexual characters. The
rule being so plainly applicable in the case of secondary
sexual characters, may be due to the great variability of
these characters, whether or not displayed in any unusual
manner — of which fact I think there can be little doubt.
But that our rule is not confined to secondary sexual 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-
HIGHLY VARIABLE. 135
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 specie9
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 either has not or cannot
have come into full play, and tb^o the organization is left
in a fluctuating condition. But what here more particularly
concerns us is, that those points in our domestic animals,
which at the present time are undergoing rapid change by
continued selection, are also eminently liable to variation.
Look at the individuals of the same breed of the pigeon,
and see what a prodigious amount of difference there is in
the beaks of tumblers, in the beaks and wattle of carriers,
in the carriage and tail of fantails, 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
coarse as a common tumbler pigeon from a good short-faced
strain. But as long as selection is rapidly going on, much
variability in the parts undergoing modification may always
be expected.
Now let us turn to nature. When a part has been de-
veloped in an extraordinary manner in any one species,
compared with the other species of the same genus, we may
136* UNUSUALLY DEVELOPED PARTS
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. But as the
variability of the extraordinarily developed part or organ
has been so great and long-continued, within a period
not excessively remote, we might, as a general rule, still
expect to find more variability in such parts than in other
parts of the 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, has been
transmitted in approximately the same condition to many
modified descendants, as in the case of the wing of the
bat, it must have existed, according to our theory, for an
immense period in nearly the same state ; and thus it has
come not to be more variable than any other structure.
It is only in those cases in which the modification has
been comparatively recent and extraordinarily great that
we ought to find the generative variability, as it may be
called, still present in a high degree. For in this case the
variability will seldom as yet have been fixed by the 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.
SPECIFIC CHARACTERS MORE VARIABLE THAN GENERIC
CHARACTERS.
The principle discussed under the last heading may be
applied to our present subject. It is notorious that specific
characters are more variable than generic. To explain by a
simple example what is meant : if in a large genus of plants
some species had blue flowers and some had red, the color
would be only a specific character, and no one would be sur-
klGHLt VARIABLE. 13?
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
138 SECONDARY SEXUAL CHARACTERS VARIABLE. ;
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.
SECONDARY SEXUAL CHARACTERS VARIABLE.
I think it will be admitted by naturalists, without my
entering on details, that secondary sexual characters are
highly variable. It will also be admitted that species of
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 on my list : and as the differences in
SECONDARY SEXUAL CHARACTERS VARIABLE. 139
these cases are of a very unusual nature, the relation can
hardly be accidental. The same number of joints in the
tarsi is a character common to very large groups of beetles,
but in the Engidee, 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 inherited much in common, to parts which
have recently and largely varied being more likely still to
140 DISTINCT SPECIES PRESENT
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.
DISTINCT SPECIES PRESENT ANALOGOUS VARIATIONS, SO THAT
A VARIETY OF ONE SPECIES OFTEN ASSUMES A CHARAC-
TER PROPER TO AN ALLIED SPECIES, OR REVERTS TO SOME
OF THE CHARACTERS OF AN EARLY PROGENITOR.
These propositions will be most readily understood by
looking to our domestic races. The most distinct breeds of
the pigeon, in countries widely apart, present sub-varieties
with reversed feathers on the head, and with feathers on the
feet, characters not possessed by the aboriginal rock-pigeon ;
these then are analogous variations in two or more distinct
races. The frequent presence of fourteen or even sixteen
tail-feathers in the pouter may be 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 similarity 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
ANALOGOUS VARIATIONS. 141
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 show for many generations a tendency to revert in
character to the foreign breed — some say, for a dozen or even
a score of generations. After twelve generations, the 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 formerl}' remarked, for all that we can see to
the contrary, be transmitted for almost any number of gen-
erations. When a character which has been lost in a breed,
reappears after a great number of generations, the most
probable hypothesis is, not that one individual suddenly
takes after an ancestor removed by some 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.
£0 po4uce blue plumage. The abstract improbability of
142 DISTINCT SPECIES PRESENT
such a tendency being transmitted through a vast number
of generations, is not greater than that of quite useless or
rudimentary organs being similarly transmitted. A mere
tendency to produce a rudiment is indeed sometimes thus
inherited.
As all the species of the same genus are supposed to be
descended from a common progenitor, it might be expected
that they would occasionally vary in an analogous manner ;
so that the varieties of two or more species would resemble
each other, or that a variety of one species would resemble in
certain characters another and distinct species, this other
species being, according to our view, only a well-marked and
permanent variety. But characters exclusively due to 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 undoubtedly is the case.
The difficulty in distinguishing variable species is largely
due to the varieties mocking, as it were, other species of the
same genus. A considerable catalogue, also, could be given
of forms intermediate between two other forms, which them-
selves can only doubtfully be ranked as species ; and this
shows, unless all these closely allied forms be considered as
ANALOGOUS VARIATIONS. 143
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. I have collected a
long list of such cases ; but here, as before, I lie under the
great disadvantage of not being able to give them. I can
only repeat that such cases certainly occur, and seem to me
very remarkable.
I will, however, give one curious and complex case, not
indeed as affecting any important character, but from 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 b}r 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
144 DISTINCT SPECIES PRESENT
horses is so generally striped, that, as I hear from Colonel
Poole, who examined this breed for the Indian Government,
a horse without stripes is not considered as purely bred.
The spine is always striped, the legs are generally barred,
and the shoulder-stripe, which is sometimes double and some-
times treble, is common ; the side of the face, moreover, is
sometimes striped. The stripes are often plainest in the
foal, and sometimes quite disappear in old horses. Colonel
Poole has seen both gray and ba}^ 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 all 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, was 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 highty 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 mi^ht have thought that it was a hybrid zebra ; a&4
ANALOGOUS VARIATIONS. 145
Mr. W. C. Martin, in his excellent treatise on the horse, has
given a figure of a similar mule. In four 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 face-stripes
ever occurred in the eminently striped Kattywar breed of
horses, and was, as we have seen, answered in the affirm-
ative.
What now are we to say to these several facts ? We see
several distinct species of the horse genus becoming, by
simple variation, striped on the legs like a zebra, or striped
on the shoulders like an ass. In the horse we see this
tendency strong whenever a dun tint appears — a tint which
approaches to that of the general 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 are crossed, we see a strong
146 SUMMARY.
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-
shore.
SUMMARY.
Our ignorance of the laws of variation is profound. Not
in one case out of a hundred can we pretend to assign any
reason why this or that part lias 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 induce mere ^ .fluctuating variability, but
SUMMARY. 147
sometimes they cause direct and definite effects ; and these
may become strongly marked in the course of time, though
we have not sufficient evidence on this head. Habit in 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 work — 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
species, and specific differences to the several species of the
148 , SUMMAitf.
same 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 all the more important modifications of structure in rela-
tion to the habits of each species.
DIFFICULTIES of the THEORY. 149
CHAPTER VI.
DIFFICULTIES OF THE THEORY.
Difficulties of the Theory of Descent with Modification — Absence or
Rarity of Transitional Varieties — Transitions in Habits of Life —
Diversified Habits in the Same Species — Species with Habits
Widely Different from Those of Their Allies — Organs of Extreme
Perfection — Modes of 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.
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 them, well
defined ?
Secondly, is it possible that an animal having, for instance,
the structure and habits of a bat, could have been formed by
the modification of some other animal with widely different
habits and structure ? Can we believe that natural selection
could produce, on the one hand, an organ of trifling 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, how can we account for species, when crossed,
150 ABSENCE OR RARITY
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.
ON THE ABSENCE OR RARITY OF TRANSITIONAL VARIETIES.
As natural selection acts solely by the preservation of
profitable modifications, each new form will tend in a fully
stocked country to take the place of, and finally to extermi-
nate, its own less improved parent-form and other less-
favored forms with which it comes into competition. Thus
extinction and natural selection go hand in hand. Hence,
if we look at each species as descended from some unknown
form, both the 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, we surely ought to find
at the present time many transitional forms. Let us take
a simple case: in travelling from north to south over a
continent, we generally meet at successive intervals with
closely allied or representative 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 replaces the other. But if
we compare these species where they intermingle, they are
generally as absolutely distinct from each other in every
detail of structure as are specimens taken from the metropo-
lis inhabited by each. By my theory these allied species
are descended from a common parent ; and during the pro-
cess of modification, each has become adapted to the con-
OF TRANSITIONAL VARIETIES. 151
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 do we not now find
closely linking intermediate varieties ? This difficulty for
a long time quite confounded me. But I think it can be in
large part explained.
In the first place we should be extremely cautious in 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. Hence
the neutral territory between two representative species is
generally narrow in comparison with the territory proper
to each. We see the same fact in ascending 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-
ate away insensibly. But when we bear in mind that almost
every species, even in its metropolis, would increase im-
152 ABSENCE OR RARITY
mensely in numbers, were it not for other competing species ;
that nearly all either prey on or serve as prey for others ; in
short, that each organic being is either directly or indirectly
related in the most 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 denned 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 denned. 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 linkecl
together.
OF TRANSITIONAL VARIETIES. 153
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 tlve
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-
mediate variety, which exists in smaller numbers in a narjow
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 of 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, I believe that species come to be tolerably
well-defined objects, and do not at any one period present
an inextricable chaos of varying and intermediate links :
first, because new varieties are very slowly formed, for 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
154 TRANSITIONAL VARIETIES.
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-
tages.
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,
TRANSITIONS OF ORGANIC BEINGS. 155
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.
ON THE ORIGIN AND TRANSITION OF ORGANIC BEINGS WITH
PECULIAR HABITS AND STRUCTURE.
It has been asked by the opponents of such views as I
hold, how, for instance, could a land carnivorous animal
have been converted into one with aquatic habits ; for how
could the animal in its transitional state have subsisted ?
It would be easy to show that there now exist carnivorous
animals presenting close intermediate grades from strictly
terrestrial to aquatic habits ; and as each exists by a 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
weight.
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, whiofc
serves as a parachute and allows them to glide through the
156 transitions of orga^c Barnes.
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, tojessen 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
produced.
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 the
TRANSITIONS OF ORGANIC BEINGS. 15?
logger-headed duck (Micropterus of Eyton) ; as fins in the
water and as front-legs on the land, like the penguin ; as
sails, like the ostrich ; and functionally for no purpose, like
the apteryx ? Yet the structure of each of these birds is
good for it, under the conditions of life to which it is exposed,
for each has to live by a struggle : but it is not necessarily
the best possible under all possible conditions. It must not
be inferred from these remarks that any of the grades of
wing-structure here alluded to, which perhaps may all be the
result of disuse, indicate the steps by which birds actually
acquired their perfect power of flight ; but they serve to show
what diversified means of 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 ways, on the 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 structures*
158 TRANSITIONS 01 ORGANIC BEINGS.
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-
ifications 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 the
margin of water, and then dashing into it like a kingfisher
at a fish. In our own country the larger titmouse (Parus
major) may be seen climbing branches, almost like a creeper ;
it sometimes, like a shrike, kills small birds by blows on the
head ; and I have many times seen and heard it hammering
the seeds of the yew on a branch, and thus breaking them
like a nuthatch. In North America the black bear was seen
by 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 is strong enough to bore into wood.
TRANSITIONS OF ORGANIC BEINGS. 159
Hence this Colaptes, in all the essential parts of its structure,
is a woodpecker. Even in such trilling 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
large districts it does not climb trees, and it makes its nest
in holes in banks ! In certain other districts, however, this
same woodpecker, as Mr. Hudson states, frequents trees, and
bores holes in the trunk for its nest. I may mention as
another illustration of the varied habits of this genus, that
a Mexican Colaptes has been described by De Saussure as
boring holes into hard wood in order to lay up a store of
acorns.
Petrels are the most aerial and oceanic of birds, but, in
the quiet sounds of Tierra del Fuego, the Puffinuria berardi,
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 ;
vet 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
160 OKGAtfS OF EXTREME PERFECTION.
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 rudiment
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.
ORGANS OF EXTREME PERFECTION AND COMPLICATION.
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-
ORGAtfS OF EXTREME PERFECTION. 161
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 ever
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 of 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 been 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 j for we have only to place the
162 ORGANS OF EXTREME PERFECTION.
naked extremity of the optic nerve, which in some of the
lower animals lies deeply buried in the body, and in some
near the surface, at the right distance from the 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 Mtiller 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 be irregular, and the rays do not converge to a point,
then any increased regularity of curvature will be an inv
ORGANS OF EXTREME PERFECTION. 163
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 man, 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 to an optical
instrument, we ought in imagination to take a thick layer
of transparent tissue, with spaces filled with fluid, and with
a nerve sensitive to light beneath, and then suppose every
part of this layer to be continually changing slowly in 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
produce a distincter image. We must suppose each new
state of the instrument to be multiplied by the million,-
164 MODES OF TRANSITION.
each to be preserved until a better one is produced, and.
then the old ones to be all destroyed. In living bodies,
variation will cause the slight 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 the
Creator are to those of man ?
MODES OF TRANSITION.
If it could be demonstrated that any complex organ
existed, which could not possibly have been formed by
numerous, successive, slight modifications, my theory would
absolutely break down. But I can find out no such case.
No doubt many organs exist of which we do not know the
transitional grades, *nore 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 compare
MODES OF TRANSITION. 165
tive 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
166 MODES OF TRANSITION.
lungs, notwithstanding the beautiful contrivance by which
the glottis is closed. In the higher vertebrata the branchiae
have wholly disappeared — but in the embryo the slits on
the sides of the neck and the loop-like course of the arteries
still mark their former position. But it is conceivable that
the now utterly lost branchiae might have been 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
flight.
In considering transitions of organs, it is so important to
bear in mind the probability of 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 for 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 ovigerous 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
MODES OF TRANSITION. 167
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
degraded. Again, not a few animals, after arriving at
maturity, go on changing in character during nearly their
whole lives. With mammals, for instance, the form of the
skull is often much altered with age, of which Dr. Murie
has given some striking instances with seals. Every one
knows how the horns of stags become more and more
branched, and the plumes of some birds become more finely
developed, as they grow older. Professor Cope states that
the teeth of certain lizards change much in shape with
advancing years. With crustaceans not only many trivial,
but some important, parts assume a new character, as re-
corded by Fritz Mtiller, after maturity. In all such cases
— and many could be given — if the age for reproduction
were retarded, the character of the species, at least in its
adult state, would be modified ; nor is it improbable that
the previous and earlier stages of development would in
some cases be hurried through and finally lost. Whether
species have often or ever been modified through this
comparatively sudden mode of transition, I can form no
opinion ; but if this has occurred, it is probable that the
differences between the young and the mature, and between
the mature and the old, were primordially acquired by
graduated steps.
SPECIAL DIFFICULTIES OF THE THEORY OF NATURAL
SELECTION.
Although we must be extremely cautious in concluding
that any organ could not have been produced by successive,
small, transitional gradations, yet undoubtedly serious cases
of difficulty occur.
One of the most serious is that of neuter insects, which
are often differently constructed from either the males or
fertile females ; but this case will be treated of in the next
chapter. The electric organs of fishes offer another case
168 DIFFICULTIES OP THE ?HEOE¥
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 to
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 t&e belief that most fishes formerly possessed
OF NATURAL SELECTION". 16&
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
are situated in different parts of the body, that they differ
in construction, as in the arrangement of the plates, and,
according to Pacini, in the process or means by which tiie
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
plants ; 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 resemblance,
there is hardly any real similarity between the eyes of cuttle-
170 DIFFICULTIES OF THE THEORY
fish, and vertebrates, as may be seen by consulting Hensen's
admirable memoir on these organs in the Cephalopoda. It
is impossible for me here to enter on details, but I may
specify a few of the points of difference. The crystalline
lens in the higher cuttle-fish consists of two parts, placed
one behind the other like two lenses, both having a very
different structure and disposition to what occurs in the
vertebrata. The retina is wholly different, with an actual
inversion of the elemental parts, and with a large nervous
ganglion included within tke membranes of the eye. The
relations of the muscles are as different as it is possible to
conceive, and so in other points. Hence it is not a little
difficult to decide how far even the same terms ought to be
employed in describing the eyes of the Cephalopoda and
Vertebrata. It is, of course, open to any one to deny that
the eye in either case could have been developed through the
natural selection of successive slight variations ; but if this
be admitted in the one case it is clearly possible in the
other ; and fundamental differences of structure in the visual
organs of two groups might have been anticipated, in 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 Mtiller, 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 Mtiller, 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.
ON NATURAL SELECTION. 171
have been made to differ, while all the other important organs
were closely similar, or rather, identical.
Fritz Miiller argues that this close similarity in so many
points of structure must, in accordance with the views
advanced by me, be accounted for by inheritance from a
common progenitor. But as the vast majority of the species
in the above two families, as well as most other crustaceans,
are aquatic in their habits, it is improbable in the highest
degree that their common progenitor should have been
adapted for breathing air. Miiller was thus led carefully to
examine the apparatus in the air-breathing 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 Miiller 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
(Acaridae), 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
172 DIFFICULTIES OF THE THEORY
remotely allied, by organs in appearance, though not in
development, closely similar. On the other hand, it is a
common rule throughout nature that the same end should
be gained, even sometimes in the case of closely related
beings, by the most diversified means. How differently con-
structed is the feathered wing of a bird and the membrane-
covered wing of a bat ; 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, so
as to attract and be devoured by birds, by having hooks and
grapnels of many kinds and serrated awns, so as to adhere
to the fur of quadrupeds, and by being furnished with
wings and plumes, as different in shape as they are elegant
in structure, so as to be wafted by every breeze. I will give
one other instance : for this subject of the same end being
gained by the most diversified means well deserves attention.
Some authors maintain that organic beings have been formed
in many ways for the sake of mere variety, almost like toys
in a shop, 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 of contrivances, all for the same purpose
and effected in essentially the same manner, but entailing
changes in every part of the flower. The nectar may be
stored in variously shaped receptacles, with the stamens and
pistils modified in many ways, sometimes forming trap-like
Contrivances, $#4 sometimes capable of RffttSly 34aptec|
OF NATURAL SELECTION. 173
movements through irritability or elasticity. From such
structures we may advance till we come to such a case of
extraordinary adaptation as that lately described by Dr.
Criiger in the Coryanthes. This orchid has part of its
labellum or lower lip hollowed out into a great bucket, into
which drops of almost pure water continually fall from two
secreting horns which stand above it ; and when the bucket
is half-full, the water overflows by a spout on one side.
The basal part of the labellum stands over the bucket, and
is itself hollowed out into a sort of chamber with two lateral
entrances ; within this chamber there are curious fleshy
ridges. The most ingenious man, if he had not witnessed
what takes place, could never have imagined what purpose
all these parts serve. But Dr. Criiger saw crowds of large
humble-bees visiting the gigantic flowers of this orchid, not
in order to suck nectar, but to gnaw off the ridges within
the chamber above the bucket ; in doing this they frequently
pushed each other into the bucket, and their wings being thus
wetted they ouuld not fly away, but were compelled to crawl
out through the passage formed by the spout or overflow.
Dr. Criiger saw a " continual procession " of bees thus 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, £ rst rubs its back against the viscid stigma and
then against the viscid glands of the pollen-masses. The
pollen-masses are thus glued to the back of the bee which
first happens to crawl out through the passage of a lately
expanded flower, and are thus carried away. Dr. Criiger
sent me a flower in spirits of wine, with a bee which he had
killed before it had quite crawled out, with a pollen-mass
still fastened to its back. When the bee, thus provided, flies
to another flower, or to the same flower a second time, and
is pushed by its comrades into the bucket and then crawls
out by the passage, the pollen-mass necessarily comes first
into contact with the viscid stigma, and adheres to it, and
the flower is fertilized. Now, at last we see the full use of
every part of the flower, of the water-secreting horns of the
bucket half-full of water, which prevents the bees from flying
awa}T, and forces them to crawl out through the spout, and
rub against the properly placed viscid pollen-masses and the
viscid stigma.
The construction of the flower in another closely allied
orchid, namely, the Catasetum, is widely different, though
serving the same end; an4 is equally curious, gees visit
174 DIFFICULTIES OF THE THEORY
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
pollen-mass of the male plant (for the sexes are separate in
this orchid) is thus carried to the flower of the female plant,
where it is brought into contact with the stigma, which is
viscid enough to break certain elastic threads, and retain the
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
uiggard in innovation." Why, on the theory of Creation,
OF NATURAL SELECTION. 175
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 theoiy
of natural selection, we can clearly understand why she
should not ; for natural selection acts only by taking advan-
tage of slight successive variations ; she can never take a
great and sudden leap, but must advance by short and sure,
though slow steps.
ORGANS OF LITTLE APPARENT IMPORTANCE, AS AFFECTED
BY NATURAL SELECTION.
As natural selection acts by life and death, by the 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 Ijhese 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 reduced, so that they
176 ORGANS OF LITTLE DiPORTANCE
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-lost
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 chief part due to
Sexual selection, A toiling pahn in the Malay Archipelago
AFFECTED BY NATURAL SELECTION. 177
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
have been developed for this object, and subsequently have
been improved and taken advantage of by the plant as it
underwent further modification and became a climber.
The naked skin on the head of a vulture is generally 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 in the parturition
of the higher animals.
We are profoundly ignorant of the cause of each slight
variation or individual difference ; and we are immediately
made conscious of this by reflecting on the differences
between the breeds of our domesticated animals in different
countries, more especially in the less 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 possiblv even the fgrm of.
178 UTILITARIAN DOCTRINE, HOW FAR TRUE :
the pelvis ; and then by the law of homologous variation,
the front limbs and the head would probably be affected.
The shape, also, of the pelvis might affect by pressure the
shape of certain parts of the young in the womb. The
laborious breathing necessary in high regions tends, as we
have good reason to believe, to increase the size of the
chest ; and again correlation would come into play. The
effects of lessened exercise, together with abundant food, on
the whole 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.
UTILITARIAN DOCTRINE, HOW FAR TRUE I BEAUTY, HOW
ACQUIRED.
The foregoing remarks lead me to say a few words on the
protest lately made by some naturalists against the utilitarian
doctrine that every detail of structure has been produced for
the good of its possessor. They believe that many structures
have been created for the sake of beaut}r, 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 of the organization of every living
creature is due to inheritance ; and consequently, though
each being assuredly is well fitted for its place in nature,
BEAUTY, HOW ACQUIRED. 179
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 oases of the diatomacese : were these created that
they might be examined and admired under the higher
powers of the microscope ? The beauty in this latter case,
and in many others, is apparently wholly due to symmetry
180 UTILITARIAN DOCTRINE, HOW FAR TRU£:
of growth. Flowers rank among the most beautiful pro-
ductions of nature ; but they have been rendered conspicuous
in contrast with the green leaves, and in consequence at the
same time beautiful, so that they may be easily observed by
insects. I have come to this conclusion from finding it an
invariable rule that when a flower is 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
BfiAtOT, ttOW ACQUlMD. 181
eertain 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
the doomed mouse. It is a much more probable view that the
rattlesnake uses its rattle, the cobra expands its frill and
the puff-adder swells while hissing so loudly and harshly, in
order to alarm the many birds and beasts which are known
to attack even the most venomous species. Snakes act on
the same principle which makes the hen ruffle her feathers
and expand her wings when a dog approaches her chickens.
But I have not space here to enlarge on the many ways by
which animals 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 be struck between the good aftid
182 UTILITARIAN DOCTRINE, HOW FAR TRUE:
evil caused by each part, each will be found on the whole
advantageous. After the lapse of time, under changing
conditions of life, if any part conies 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 which it comes 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-
holtz, whose judgment no one 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 it has since been modified but not perfected for
its present purpose, with the poison originally adapted for
some other object, such as to produce galls, since inten-
sified, we can perhaps understand how it is that the use of
BEAUTY, HOW ACQUIRED. 183
the sting should so often cause the insect's own death : for
if on the whole the power of stinging be useful to the social
community, it will fulfil all the requirements of natural
selection, though it may cause the death of some few 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 two districts of a continuous area,
184 SUMMARY.
an intermediate variety will often be formed, fitted for an
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 lire
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
stagger any one ; yet in the case of any organ, if we know
of a long series of gradations in complexity, each good for
its possessor, then under changing 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 ; but when such
SUMMARY. 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 non
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
the 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
186 SUMMARY.
known or unknown, it must on this theory be strictly
true.
It is generally acknowledged that all organic beings have
been formed on two great laws — Unity of Type, and the
Conditions of Existence. By unity of type is meant that
fundamental agreement in structure which we see in organic
beings of the same class, and which is quite independent
of their habits of life. On my theory, unity of type is
explained by unity of descent. The expression of conditions
of existence, so often 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 and adaptations, that of Unity of Type.
MISCELLANEOUS OBJECTIONS. 187
CHAPTER VII.
MISCELLANEOUS OBJECTIONS TO THE THEORY OP NATURAL
SELECTION.
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 the previous discussions may thus be
made clearer ; but it would be useless to discuss all of them,
as many have been made by writers who have not taken the
trouble to understand the subject. Thus a distinguished
German naturalist has asserted that the weakest part of my
theory is, that I consider all organic beings as imperfect :
what I have really said is, that all are not as perfect as they
might have been in relation to their conditions ; and this
is shown to be the case by so many native forms in many
quarters of the world having yielded their places to 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
mutations.
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 our critics conceive that a biennial plant or one of
188 MISCELLANEOUS OBJECTIONS TO TIUE
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 to
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 princi]3le
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 as
^HEOfrT OF NATURAL SELECTION. 189
I can discover, inhabiting distinct stations, such as high
land or low land, dry or moist districts. Moreover, in the
case of animals which wander much about and cross freely,
their varieties seem to be generally confined to distinct
regions.
Bronn also insists that distinct species never differ
from each other in single characters, but in many parts ;
and he asks, how it always comes that many parts of the
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
ilower, fruit, or leaves, has been greatly changed, almost
all the other parts have been slightly modified. This may
be attributed partly to the principle of correlated growth,
and partly to so-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 he
190 MISCELLANEOUS OBJECTIONS TO THE
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,
etc.
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
afi0 ecting 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 im-
THEORY OF NATURAL SELECTION. 191
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 function ; 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
192 MISCELLANEOUS OBJECTIONS TO THE
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 alwaj^s 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 flowers),
the sepals are reduced from the normal number of five to
three. In one section of the Malpighiaceae the closed
flowers, according to A. de Jussieu, are still further 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 so necessary to appreciate the important effects of
THEORY OP NATURAL SELECTION. 193
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 compositse
and umbelliferse (and in some other plants) the circum-
ferential flowers have their corollas much more developed
than those of the centre ; and this seems often connected
with the abortion of the reproductive organs. It is a more
curious fact, previously referred to, that the achenes or seeds
of the circumference and centre sometimes differ greatly in
form, color, and other characters. In Carthamus and some
other composite the central achenes alone are furnished
with a pappus ; and in Hyoseris the same head yields
achenes of three different forms. In certain umbelliferse
the exterior seeds, according to Tausch, are orthospermous,
and the central one 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 are the
flowers and leaves in certain positions, all would have been
modified in the same manner.
In numerous other cases we find modifications of struc-
ture, which are considered by botanists to be generally of a
194 MISCELLANEOUS OBJECTIONS TO THE
highly important nature, affecting only some of the flowers
on the same plant, or occurring on distinct plants, which
grow close together under the same conditions. As these
variations seem of no special use to the plants, they cannot
have been influenced by natural selection. Of their cause
we are quite ignorant ; we cannot even attribute them, as
in the last class of cases, to any proximate agency, such as
relative position. I will give only a few instances. It is
80 common to observe on the same plant, flowers indiffer
ently tetramerous, pentamerous, etc., that I need not givi,
examples ; but as numerical variations are comparative!}
rare when the parts are few, I may mention that, accord
ing to De Candolle, the flowers of Papaver braeteatun,
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 Rhinanthidese 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 Rutacese 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
a higher state of development ? On the contrary, I should
THEORY OF NATURAL SELECTION. 195
infer 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 and
degraded animals. We are ignorant of the exciting cause
of the above specified modifications ; but if the unknown
cause were to act almost uniformly for a length of time,
we may infer that the result would be almost uniform ; and
in this case all the individuals of the species would be
modified in the same manner.
From the fact of the above characters being 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 cases
as fluctuating variations, which sooner or later became con-
stant through the nature of the organism and of the sur-
rounding conditions, as well as through the intercrossing of
196 MISCELLANEOUS OBJECTIONS TO THE
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 by no means so paradoxical as it may at first
appear.
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
of the increased use and disuse of parts, which I have
always maintained to be highly important, and have treated
in my " Variation under Domestication " at greater length
than, as I believe, any other writer. He likewise often
assumes that I attribute nothing to variation, independently
of natural selection, whereas in the work just referred to
I have collected a greater number of well-established cases
than can be found in any other work known to me. My
judgment may not be trustworthy, but after reading with
care Mr. Mivart's book, and comparing each section with
what I have said on the same head, I never before felt so
strongly convinced of the general truth of the conclusions
here arrived at, subject, of course, in so intricate a subject^
to much partial error, *
THEORY OF NATURAL SELECTION. 197
All Mr. Mivart's objections will be, or have been, con-
sidered in the present volume. The one new point which
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 coming
to Mr. Mivart's objections, it may be well to explain once
again how natural selection will act in all ordinary cases.
Man has modified some of his animals, without necessarily
having attended to special points of structure, by simply
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 which careful measure-
ments are given. These slight proportional differences, due
to the laws of growth and variation, are not of the slightest
use or importance to most species. But it will have been
otherwise with the nascent giraffe, considering its probable
198 MISCELLANEOUS OBJECTIONS TO THE
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 being
able to reach, at each stage of increased size, to a supply
of food left untouched by the other hoofed quadrupeds of
the country, would have been of some advantage to the
nascent giraffe. Nor must we overlook the fact, that in-
creased bulk would act as a protection against almost all
beasts of prey excepting the lion ; and against this animal,
its tall neck — and the taller the better — would, as Mr.
Chauncey Wright has remarked, serve as a watch-tower.
It is from this cause, as Sir S. Baker remarks, that no
animal is more difficult to stalk than the giraffe. This
animal also uses its long neck as a means of offence or 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.
THEORY OF NATURAL SELECTION. 199
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
have been, we can see that certain districts and times would
200 MISCELLANEOUS OBJECTIONS TO THE
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 parts should be modified and coadapted.
Although every part of the body varies slightly, it does not
follow that the necessary parts should always vary in the
right direction and to the right degree. With the different
species of our domesticated animals we know that the parts
vary in a different manner and degree, and that some species
are much more variable than others. Even if the fitting
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 litfle, 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 to
give to this bird of the desert force to move its huge body
through the air. Oceanic islands are inhabited by bafcs and
seals, but by no terrestrial mammals ; yet as some of these
bats are peculiar species, they must have long inhabited
their present homes. Therefore Sir C. Lyell asks, and
assigns certain reasons in answer, why have not seals and
bats given birth on such islands to forms fitted to live on
the land ? But seals would necessarily be first converted
into terrestrial carnivorous animals of considerable size, and
bats into terrestrial insectivorous animals j for the former
THEORY OF tf ATtTtlAL SELECTION. 201
there would be no prey ; for the bats ground-insects would
serve as food, but these would already be largely preyed on
by the reptiles or birds, which first 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, might at last be converted into an
animal so thoroughly aquatic as to brave the open ocean.
But seals would not find on oceanic islands the conditions
favorable to their gradual reconversion into a terrestrial
form. Bats, as formerly shown, probably acquired their
wings by at first gliding through the air from tree to tree,
like the so-called flying squirrels, for the sake of escaping
from their enemies, or for avoiding falls ; but when the
power of true flight had once been acquired, it would never
be reconverted back, at least for the above purposes, into the
less efficient power of gliding through the air. Bats might,
indeed, like many birds, have had their wings greatly 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
of man ? Various causes could be assigned ; but as they
are conjectural, and their relative probability cannot be
weighed, it would be useless to give them. A definite
answer to the latter question ought not to be expected,
seeing that no one can solve the simpler problem, why, of
two races of savages, one has risen higher in the scale of
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,
202 MISCELLANEOUS OBJECTIONS TO THE
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 by Mr. Wallace, of a walking-stick in-
THEORY OF NATURAL SELECTION 20b
sect fCeroxylus laceratus), which resembles "a stick grown
over oy a creeping moss or jungermannia." So close was
this resemblance, that a native Dyak maintained that the
foliaceous excrescences were really moss. Insects are preyed
on by birds and other enemies whose sight is probably
sharper than ours, and every grade in resemblance which
aided an insect to escape notice or detection, would tend
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 lamina?, which
stand close together transversely to the longer axis of the
mouth. Within the main row there are some subsidiary
rows. The extremities and inner margins of all the plates
are frayed into stiff bristles, which clothe the whole gigantic
palate, and serve to strain or sift the water, and thus to
secure the minute prey on which these great animals subsist.
The middle and longest lamina in the Greenland whale is
ten, twelve, or even fifteen feet in length; but in the 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
species.
With respect to the baleen, Mr. Mivart remarks that if it
"had once attained such a size and development as to be
at all useful, then its preservation and augmentation within
serviceable limits would be promoted by natural selection
alone. But how to obtain the besrinnins: of such useful
development ? ' In answer, it may be asked, why should
not the early progenitors of the whales with baleen have
possessed a moufch constructed something like the lamel-
204 MISCELLANEOUS OBJECTIONS TO THE
lated beak of a duck ? Ducks, like whales, subsist by sift-
ing the mud and water ; and the family has sometimes been
called Criblatores, or sifters. I hope that I may not be
misconstrued into saying that the progenitors of whales did
actually possess mouths lamellated like the beak of a duck.
I wish only to show that this is not incredible, and that the
immense plates of baleen in the Greenland whale might
have been developed from such lamellae by finely graduated
steps, each of service to its possessor.
The beak of the shoveller-duck (Spatula clypeata) 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
line 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 are well developed and pro-
ject beneath the margin; so that the beak of this bird
resembles in this respect the mouth of a whale.
Fl'Pm the highly developed structure of tfre shoveller's
THEORY OF NATURAL SELECTION. 205
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 sifting is concerned, through
the beak of the Merganetta armata, and in some respects
through that of the Aix sponsa, to the beak of the common
duck. In this latter species the lamellae are much coarser
than in the shoveller, and are firmly attached to the sides
of the mandible ; they are only about fifty in number on
each side, and do not project at all beneath the margin.
They are square-topped, and are edged with translucent,
hardish tissue, as if for crushing food. The edges of the
lower mandible are crossed b}7- 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 lamellae, might be converted by small
Changes into a, species like the Egyptian goo§§ — this i»tt
206 MISCELLANEOUS OBJECTIONS TO THE
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
life.
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-stirface, resembles at first sight the
THEORY OF NATURAL SELECTION. 207
ventral surface of an ordinary fish ; it is of a white color,
less developed in many ways than the upper side, with the
lateral fins often of smaller size. But the eyes offer the
most 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-
ronectidaB 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
Pleuronectidse, 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 the
208 MISCELLANEOUS OBJECTIONS TO THE
orbit. The forehead between the eyes consequently becomes,
as could be plainly seen, temporarily contracted in breadth.
On one occasion Malm saw a young fish raise and depress the
lower eye through an angular distance of about seventy
degrees.
We should remember that the skull at this early age is
cartilaginous and flexible, so that it readily yields to muscu-
lar action. It is also known with the higher animals, even
after early youth, that the skull yields and is altered in shape,
if the skin or muscles be 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 Pleuronectides,
on the other hand, the older they grow the more habitually
they rest on one side, owing to the increasing flatness of their
bodies, and a permanent effect is thus produced on the form
of the head, and on the position of the eyes. Judging from
analogy, the tendency to distortion would no doubt be in-
creased through the principle of inheritance. Schiodte
believes, in opposition to some other naturalists, that the
Pleuronectidse are not quite symmetrical even in the embryo;
and if this be so, we could understand how it is that certain
species, 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, which Mr. Mivart
considers would be injurious, may be attributed to the habit,
no doubt beneficial to the individual and to the species, of
THEORY OF NATURAL SELECTION. 209
endeavoring to look upward with both eyes, while resting on
one side 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 feeding with ease on the ground. Disuse, on
the other hand, will account for the less developed condition
of the whole inferior half of the body, including the lateral
tins ; 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 man}' 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 Pouchet, 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 many other peculiarities, to their
habits of life. We should keep in mind, as I have before
insisted, that the inherited effects of the increased use of
parts, and perhaps of their disuse, will be strengthened by
natural selection. For all 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 pf ages the first slight incipient tendency
210 MISCELLANEOUS OBJECTIONS TO THE
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 Henslow 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 deserved
THEORY OF NATURAL SELECTION. 211
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 Ornithorhynchus, 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-
hatched 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 }roung 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
212 MISCELLANEOUS OBJECTIONS TO THE
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 seize
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 fouled. 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 forcepses, immovably
fixed at the base, but capable of a snapping action, certainly
exist on some star-fishes ; and this is intelligible if they serve,
at least in part, as a means of defence. Mr. Agassiz, to whose
great kindness I am indebted for much information on the
subject, informs me that there are other star-fishes, in which
one of the three arms of the forceps is reduced to a support
for the other two ; and again, other genera in which the third
arm is completely lost. In Echinoneus, the shell is described
by M. Perrier as bearing two kinds of 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 or^an* -<- ...
THEORY OF NATURAL SELECTION. 213
With respect to the steps by which these curious orgaus
have been evolved, Mr. Agassiz infers from his own re-
searches and those of Mr. Miiller, that both in star-fishes
and sea-urchins the pedicellariae 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 pedicellariae, with
their supporting calcareous rods, are articulated to the shell.
In certain genera of star-fishes, "the very combinations
needed to show that the pedicellariae are only modified
branching spines" may be found. Thus we have fixed spines,
with three equi-distant, serrated, 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 pedicellariae 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 pedicellariae of sea-urchins and the anchors
of the Holothuriae, also belonging to the same great class.
214 MISCELLANEOUS OBJECTIONS TO THE
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 tridactjde 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 zooicls 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 such gradations have not existed.
As the chelae of Crustaceans resemble in some degree the
avicularia of Polyzoa, both serving as pincers, it may be
worth while to show that with the former a long series of
serviceable gradations still exists. In the first and simplest
stage, the terminal segment of a limb shuts down either on
the square summit of the broad 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-
tion. We next find one corner of the broad penultimate
THEORY OF NATURAL SELECTION. 215
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
cell, with the included zooid, could hardly have disappeared
216 MISCELLANEOUS OBJECTIONS TO THE
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 all
the gradations, by which the lip became converted first into
the lower mandible of an avicularium, and then into an
elongated bristle, likewise served as a protection in 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 I 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
mention 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,
THEORY OF NATURAL SELECTION. 217
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
218 MISCELLANEOUS OBJECTIONS TO 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 twiner
which possessed leaves with long foot-stalks would have been
developed into a leaf-climber, if the foot-stalks had possessed
in any slight degree the requisite sensitiveness to a touch.
As twining is the simplest means of ascending a support,
and forms the basis of our series, it may naturally be asked
how did plants acquire this power in an incipient degree,
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 power 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 I made this prediction, I knew of only one
imperfect case, namely, of the young flower-peduncles of a
Maurandia which revolved slightly and irregularly, like the
stems of twining plants, but without making any use of this
habit. Soon afterward Fritz Miiller discovered that the
young stems of an Alisma and of a Linum — plants which
do not climb and are widely separated in the natural system
— revolved plainly, though irregularly : and he states that
he has reason to suspect that this occurs with some other
plants. These slight movements appear to be of no service
THEORY OF NATURAL SELECTION. 219
to the plants in question ; anyhow, they are not of the least
use in the way of climbing, which is the point that concerns
us. Nevertheless we can see that if the stems of these
plants had been flexible, and if under the conditions to
which they are 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
twining species.
With respect to the sensitiveness of the foot-stalks of the
leaves and flowers, and of tendrils, nearly the same remarks
are applicable as in the case of the revolving movements of
twining plants. As a vast number of species, belonging to
widely distinct groups, are endowed with this kind of 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
sensitive.
It is scarcely possible that the above slight movements,
due to a touch or shake in the young and growing organs of
plants, can be of any functional importance to them. But
plants possess, in obedience to various stimuli, powers of
movement, which are of manifest importance to them ; for
instance, 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 certain
/
220 MISCELLANEOUS OBJECTIONS TO THE
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 bv 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
THEOkY OF NATURAL SELECTION. 22i
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, first into lamellated knobs or teeth like
those on the beak of a goose — then into short lamellae,
like those of the domestic ducks — and then into lamellae as
perfect as those of the shoveller-duck — and finally into the
gigantic plates of baleen, as in the mouth of the Greenland
whale. In the family of the ducks, the lamellae are first
used as teeth, then partly as teeth and partly as a sifting
apparatus, and at last almost exclusively for this latter
purpose.
With such structures as the above 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. With respect to the mammae of
the higher animals, the most probable conjecture is that
primordially the cutaneous glands over the whole surface of
a marsupial sack secreted a nutritious fluid ; and that these
glands were improved in function through natural selection,
and concentrated into a confined area, in which case they
would have formed a mamma. There is no more difficulty
in 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
222 MISCELLANEOUS OBJECTIONS TO THE
nearly but not quite the same purpose, became attached to
the free ends of the caudicles — all these gradations being
of manifest benefit to the plants in question. With respect
to climbing plants, I need not repeat what has been so lately
said.
It has often been asked, if natural selection be so potent,
why has not this or that structure been gained by certain
species, to which it would apparently have been advanta-
geous ? But it is unreasonable to expect a precise answer
to such questions, considering our ignorance pf 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 have little weight in compari-
son 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 " Medieo-Chirurgieal -Review."
At the present day almost all naturalises admit -evolution
THEORY OF NATURAL SELECTION. 223
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 veiw
little light on our subject. Excluding such cases of abrupt
variations, the few which remain would at best constitute, if
found in a state of -nature}' doubtful species, closely related
to their parental types.
My reasons for doubting whether natural species have
224 MISCELLANEOUS OBJECTIONS TO THE
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
manner.
That many species have been evolved in an extremely
gradual manner, there can hardly be a doubt. The species
and even the genera of many large natural families are so
closely allied together that it is difficult to distinguish not
a few of them. On every 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 dis«
THEOftY OE NATURAL SELECTION. 225
tinct species, that numerous and wonderfully fine grada-
tions can be traced, connecting together widely different
structures.
Many large groups of facts are intelligible only on the
principle that species have been evolved by very small
steps. For instance, the fact that the species included in
the larger genera are more closely related to each other, and
present a greater number of varieties, than do the species
in the smaller genera. The former are also grouped in
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
facts, all pointing in the same direction, could be added.
Although very many species have almost certainly been
produced by steps not greater than those separating fine
varieties ; yet it may be maintained that some have been
developed in a different and abrupt manner. Such an
admission, however, ought not to be made without strong
evidence being assigned. The vague and in some respects
false analogies, as they have been shown to be by Mr.
Chauncey Wright, which have been advanced in 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
developed.
Unless we admit transformations as prodigious as those
advocated by Mr. Mivart, such as the sudden development
of the wings of birds or bats, or the sudden conversion of
a Hipparion into a horse, hardly any light is thrown by the
belief in abrupt modifications on the deficiency of connect-
ing links in our geological formations. But against the
belief in such abrupt changes, embryology enters a strong
226 MISCELLANEOUS OBJECTIONS.
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
steps.
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 miraclb, and to
leave those of science.
* !; ' ■;;" »v-..rjw*tff
INSTINCT. 227
CHAPTER VIIL
INSTINCT.
Instincts Comparable with Habits, but Different in Their Origin —
Instincts Graduated — Aphides and Ants — Instincts Variable —
Domestic Instincts, Their Origin — Natural Instincts of the Cuckoo,
Molothrus, Ostrich and Parasitic Bees — Slave-making Ants —
Hive-Bee, its Cell-making Instinct — Changes of Instinct and
Structure not Necessarily Simultaneous — Difficulties of the Theory
of the Natural Selection of Instincts — Neuter or Sterile Insects —
Summary.
Many instincts are so wonderful that their 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-
cerned 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
the 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 will or
228 INSTINCT.
reason. Habits easily become associate with other habits,
with certain periods of time and states of the body. When
once acquired, they often remain constant throughout life.
Several other points of resemblance between instincts and
habits could be pointed out. As in repeating a well-known
song, so in instincts, one action follows another by a sort of
rhythm ; if a person be interrupted in a song, or in 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 instinqt to any extent?
INSTINCT. " 229
that was profitable. It is thus, as I believe, that all the
most complex and wonderful instincts 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 are 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 first 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*
230 INSTINCT.
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-
INHERITED CHANGES OF HABIT OR INSTINCT. 231
ing 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 of
any particular enemy is certainly an instinctive quality, as
may be seen in nestling birds; though it is strengthened by
experience, and by the sight of fear of the same enemy in
other animals. The fear of man is slowly acquired, as I
have elsewhere shown, by the various animals which inhabit
desert islands ; and we see an instance of this even in 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.
INHERITED CHANGES OF HABIT OR INSTINCT IN DOMES-
TICATED ANIMALS.
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
232 CHANGES Of* HAfelT Ok iNSTtttCT
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
and compulsory habit ; but this is not true. No one would
W DOMESTICATED ANIMALS. 233
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 those breeds of fowls which
very rarely or never become "broody," that is, never wish
to sit on their eggs. Familiarity alone prevents our seeing
how largely and how 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 young,
234 SPECIAL INSTINCTS.
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 Hutton that the young
chickens of the parent stock, the Gallus bankiva, when
reared in India under a hen, are at first excessively wild.
So it is with young pheasants reared in England under a
hen. It is not that chickens have lost all fear, but fear only
of dogs and cats, for if the hen gives the clanger 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.
SPECIAL INSTINCTS.
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'
nests ; the slave-making instinct of certain ants ; and the
cell-making power of the hive-bee. These two latter instincts
have generally and justly been ranked by naturalists as the
most wonderful of all known instincts.
INSTINCTS OF THE CUCKOO. 235
INSTINCTS OF THE CUCKOO.
It is supposed by some naturalists that the more imme-
diate cause of the instinct of the cuckoo is that she lays her
eggs, not daily, but at intervals of two or three days, so that
if she were to make her own nest and sit on her own eggs,
those first laid would have to be left for some time unincu-
bated, or there would be eggs and young birds of different
ages in the same nest. If this were the case, the process of
laying and hatching might be inconveniently long, more
especially as she migrates at a very early period, and the
first hatched young would probably have to be fed by the
male alone. But the American cuckoo is in this predicament,
for she makes her own nest and has eggs and young 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 of
their mother, and in their turn would be apt to lay their eggs
in other birds' nests, and thus be more successful in rearing
their young. By a continued process of this nature, I believe
that the strange instinct of our cuckoo has been generated.
It has, also, recently been ascertained on sufficient evidence,
by Adolf Mtiller, that the cuckoo occasionally lays her eggs
gn the bare ground^ sits on them and feeds her young, Thi?
236 SPECIAL INSTINCTS.
rare eveut is probably the case of reversion to the long-lost,
aboriginal instinct of nidification.
It has been objected that I have not noticed other related
instincts and adaptations of structure in the cuckoo, which
are spoken of as necessarily co-ordinated. But in all cases,
speculation on an instinct known to us only in a single spe-
cies, 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. Ramsay'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
it is asserted that there is a relation between the size of eggs
and the period of their incubation), then there is no difficult}^
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 instinct, but not rarely departs from
INSTINCTS OF THE MOLOTHRUS. 237
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 acquired together. The eggs
of the Australian bronze cuckoo vary, according to Mr. Ram-
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,
as is probably the case, to receive as much food as possible
soon after birth, I can see no special difficulty in its having
gradually acquired, during successive generations, the blind
desire, the strength, and structure necessary for the work
of ejection ; for those 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 on 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 all ages, and the variations tend to be inherited
at a corresponding or earlier age — propositions which can-
not be disputed — then the instincts and structure of the
young could be slowly modified as surely as those of the
adult ; and both cases must stand or fall together with
the whole theory of natural selection.
238 SPECIAL INSTINCTS.
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. Hudson has
ascertained, complete a nest for themselves. They often lay
so many eggs — from fifteen to twenty — in the same foster-
nest, that few or none can possibly be hatched. They
have, moreover, the extraordinary habit of pecking holes in
the eggs, 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,
lias 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 much
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 GalHnacese, and throws some
light on the singular instinct of the ostrich. In this family
SLAVE-MAKING INSTINCT. 239
several hen birds unite and lay first a few eggs in one nest
and then in another ; and these are hatched by the males.
This instinct may probably be accounted for by the fact of
the hens laying a large number of eggs, but, as with the
cuckoo, at intervals of two or three days. The instinct,
however, of the American ostrich, as in the case of the.
Molothrus bonariensis, has not as yet been perfected ; for a
surprising number of eggs lie strewed over the plains, so that
in one day's hunting I picked up no less than twenty lost
and wasted eggs.
Many bees are parasitic, and regularly lay their eggs in
the nests of other kinds of bees. This case is more 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 Sphegida? (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
larvae, yet that, when this insect finds a burrow already
made and stored by another sphex, it takes advantage of the
prize, and becomes for the occasion parasitic. In this case
as with that of the Molothrus or cuckoo, I can see no diffi-
culty in natural selection making an occasional habit perma-
nent, if of advantage to the species, and if the insect whose
nest and stored food are feloniously appropriated, be not
thus exterminated.
SLAVE-MAKING INSTINCT.
This remarkable instinct was first discovered in the
Formica (Polyerges) 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 larvaa. When the old nest is found inconvenient, and
they have to migrate, it is the slaves which determine the
migration; and actually carry their masters in their jaws*
240 SPECIAL INSTINCTS.
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. During the months of June and July, on three
successive years, I watched for many hours several nests in
Surrey and Sussex, and never saw a slave either leave or enter
a nest. As, during these months, the slaves are very few in
number, I thought that they might behave differently when
more numerous ; but Mr. Smith informs me that he has
watched the nests at various hours during May, June, and
August, both in Surrey $n& Hani ush ire, and has never seen
tJie slav§§; though present in large numbers in August, either
SLAVE-MAKING INSTINCT. 241
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 or
cocci. According to Huber, who had ample opportunities
for observation, the slaves in Switzerland habitually work
with their masters in making the nest, and they alone open
and close the doors in the morning and evening; and, as
Huber expressly states, their principal office is to search for
aphides. This 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
242 SPECIAL INSTINCTS.
my surprise an independent community of F. flava under a
stone beneath a nest of the slave-making F. sanguinea ; and
when I had accidentally disturbed both nests, the little ants
attacked their big 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,
which they rarely capture, and it was evident that they did
at once distinguish them ; for we have seen that they
eagerly and instantly seize the pupae of F. fusca, whereas
they were much terrified when the}' 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
perched motionless with its own pupa in its mouth on the
top of a spray of heath, an image of despair over its ravaged
home.
Such are the facts, though they did not need 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 even feed
itself: it is absolutely dependent on its numerous slaves.
Formica sanguinea, on the other hand, possesses much
fewer slaves, and in the early part of the summer extremely
few : the masters determine when and where a new nest
shall be formed, and when they migrate, the masters carry
the slaves. Both in Switzerland and England the slaves
seem to have the exclusive care of the larvae, and the masters
alone go on slave-making expeditions. In Switzerland the
CELL-MAKING INSTINCT. 243
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 food
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 I
will not pretend to conjecture. But as ants which are not
slave-makers will, as I have seen, carry off the pupae of
other species, if scattered near their nests, it is possible that
such pupae originally stored as food might 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 of
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.
CELL-MAKING INSTINCT OF THE HIVE-BEE.
I will not here enter on minute details on this subject,
but will merely give an outline of the conclusions at which
I have arrived. He must be a dull man who can examine
the exquisite structure of a comb, so beautifully adapted
to its end, without enthusiastic admiration. We hear from
mathematicians that bees have practically solved a recondite
problem, and have made their cells of the proper shape to
hold the greatest possible amount of honey, with the least
possible consumption of precious wax in their construction.
It has been remarked that a skilful workman with fitting
tools and measures, would find it very difficult to make cells
of wax of the true form, though this is effected by a crowd
of bees working in a dark hive. Granting whatever in-
2-±i SPECIAL INSTINCTS.
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
instincts.
I was led to investigate this subject by Mr. Waterhouse,
who has shown that the form of the cell stands in close
relation to the presence of adjoining cells ; and the following
view may, 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 would 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
CELL-MAKING iKSttNCf. 245
#»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 rts 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.
We must suppose the Melipona to have the power of form-
246 SPECIAL INSTINCTS.
ing her cells truly spherical, and or 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. By 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 ; but the ridge of wax was so thin, that the
CELL-MAKING INSTINCT. 247
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
leepening the basins on both sides of the ridge of vermilion
wax, in order to have thus succeeded in leaving flat plates
between the basins, by stopping work at the planes of inter-
section.
Considering how flexible thin wax is, I do not see that
there is any difficulty in the bees, 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
shape, by standing at the proper distance from each other,
248 SPECIAL INSTINCTS.
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 trmes 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
CELL-MAKING INSTINCT. 249
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, ^\^ 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 5J^ 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 cells, and then, by striking imaginary spheres, they
c<m build up a wall intermediate between two adjoining
250 SPECIAL INSTINCTS.
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 circumstance*
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-
lites, or on quite distinct causes; and so be altogether inde-
CELL-MAKING INSTINCT. 251
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
wax.
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
252 OBJECTIONS TO tfHE THEOEY
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.
OBJECTIONS TO THE THEORY OF NATURAL SELECTION AS
APPLIED TO INSTINCTS : NEUTER AND STERILE INSECTS.
It has been objected to the foregoing view of the origin
of instincts, that " the variations of structure and of instinct
must have been simultaneous and accurately adjusted to each
other, as a modification 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
OF NATURAL SEL^CTIOtf. 253
it 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
varied.
No doubt many instincts of very difficult explanation
could be opposed to the theory of natural selection — cases,
in which we cannot see how an instinct could have origi-
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 all its
254 OBJECTIONS TO THE THEORY
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 gain 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
OF NATURAL SELECTION. 255
bo the right degree, always produce a large proportion of
seedlings bearing double and quite sterile flowers, but they
likewise yield some single and fertile plants. These latter,
by which alone the variety can be propagated, may be
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 and sterile females of the same species has been
produced 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 Euiopean ants guard and imprison.
It will indeed be thought that I have an overweening
confidence in the principle of natural selection, when I do
not admit that such wonderful and well-established facts
at once annihilate the theory. In the simpler case of
neuter insects all of one caste, which, as I believe, have
been rendered different from the fertile males and females
256 OBJECTIONS tO THE TiJEOkY
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 few alone ; and that by the
survival of the communities with females which produced
most neuters having the advantageous modification, all
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 has observed, the larger
workers have simple eyes (ocelli), which, though small, can
be plainly distinguished, whereas the smaller workers have
their ocelli rudimentary. Having carefully dissected 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,
whjch 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.
I may give one other case : so confidently did I expect
OF NATURAL SELECTION. 257
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 Miiller, of certain Brazilian crustaceans
likewise appearing under two widely distinct male forms.
But this subject need not here be discussed.
I have now explained how, I believe, the wonderful fact
of two distinctly defined castes of sterile workers existing
258 SUMMARY.
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 has
encountered. The case, also, is very interesting, as it
proves that with animals, as with plants, any amount of
modification may be effected by the accumulation of 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.
SUMMARY.
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
of instinct which are in any way useful. In many cases
habit or use and disuse have probably come into play. I
do not pretend that the facts given in this chapter
strengthen in any great degree my theory ; but none of the
cases of difficulty, to the best of my judgment, annihilate
it. On the other hand, the fact that instincts are not
SUMMARY. 259
always absolutely perfect ana are name to mistakes ; tnat
no instinct can be shown to have been produced for the
good of other animals, though animals take advantage of
the instincts of others ; that the canon in natural history
of "Natura non facit 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 facts
in regard to instincts ; as by that common case of closely
allied, but distinct, species, when inhabiting distant parts
of the world and living under 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.
260 HYBRIDISM.
CHAPTER IX.
HYBRIDISM.
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
DEGREES OF STERILITY. 261
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 sterility in both cases being looked on as a
special endowment, beyond the province of our reasoning
powers.
The fertility of varieties, that is of the forms known 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 distin«tion between varieties and species.
DEGREES OF STERILITY.
First, for the sterility of species when crossed and of
their hybrid offspring. It is impossible to study the several
memoirs and works of those two conscientious and 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 any 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 be
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 Leguminosee, in
262 HYBRIDISM.
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 coerulea), which the best botanists
rank as varieties, and found them absolutely sterile, we may
doubt whether many species are really so sterile, when
intercrossed, as he 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 generations,
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 with
DEGREES OF STERILITY. 268
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 that some degree
of sterility between distinct species is a universal law of
nature. He experimented on some of the very same species
as did Gartner. The difference in their results may, I
think, be in part accounted for by Herbert's great 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 " every ovule in a
264 HYBRIDISM.
pod of Crinum capense fertilized by C. revolutura 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 Miiller, 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,
and I am assured that many of them are perfectly fertile.
Mr. C. Noble, for instance, :.nforms me that ne raises stocks
DECREES OE STERILITY. 265
for grafting from a hybrid between Rhod. ponticum and
catawbiense, and that this hybrid " seeds as freely as it is
possible to imagine." Had hybrids, when fairly treated,
always gone on decreasing in fertility in each successive
generation, as Gartner believed to be the case, the fact
would have been notorious to 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 borne in mind, that,
owing to few animals breeding freely under confinement,
few experiments have been fairly tried; for instance, the
canary bird has been crossed with nine distinct species of
finches, but, as not one of these breeds freely in confine-
ment, we have no right to expect that the first crosses 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 oa
increasing.
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
and Reevesii, and from Phasianus colchicus with f . tor«
266 H rBRIDISM.
quatus, are perfectly fertile. M. Quatrefages states that
the hybrids from two moths (Bombyx cynthia and arrindia)
were proved in Paris to be fertile inter se for eight genera-
tions. It has lately been asserted that two such distinct
species as the hare and rabbit, when they can be got to
breed together, produce offspring which are highly fertile
when crossed with one of the parent-species. The hybrids
from the common and Chinese geese (A. cygnoides), species,
which are so different that they are generally ranked in
distinct genera, have often bred in this country with either
pure parent, and in one single instance they have bred
inter se. This was effected by Mr. Eyton, who raised two
hybrids from the same parents, but from different hatches ;
and from these two birds he raised no less than eight
hybrids (grandchildren of the pure geese) from one nest.
In India, however, these cross-bred geese must be far more
fertile ; for I am assured by two eminently capable judges,
namely, Mr. Blyth and Captain Hutton, that whole flocks
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 Rutimeyer 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
DEGREES OF STERILITY. 267
the two chief races of the pig. "We must, therefore, either
give up the belief of the universal sterility of species when
crossed ; or we must look at this sterility in animals, not
as an indelible characteristic, but as one capable of being
removed by domestication.
Finally, considering all the ascertained facts on the 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
universal.
LAWS GOVERNING THE STERILITY OF FIRST CROSSES AND
OF HYBRIDS.
We will now consider a little more in detail the laws
governing the sterility of first crosses and of hybrids. Our
chief object will be to see whether or not these laws indicate
that species have been specially endowed with this quality,
in order to prevent their crossing and blending together in
utter confusion. The following conclusions are drawn up
chiefly from Gartner's admirable work on the 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
kingdoms.
It has been already remarked, that the degree of fertility,
both of first crosses and of hybrids, graduates from zero to
perfect fertilit}^. It is surprising in how many curious ways
this gradation can be shown ; but only the barest outline of
the facts can here be given. When pollen from a plant of
one family is placed on the stigma of a plant of a distinct
family, it exerts no more influence than so much inorganic dust.
From this absolute zero of fertility, the pollen of different
species applied to the stigma of some one species of the
same genus, yields a perfect gradation in the number of
seeds 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*
268 LAWS GOVERNING THE STERILITY
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, ar©
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 to differ greatly in the several individuals
raised from seed out of the same capsule and exposed to the
same conditions.
By the term systematic affinity is meant, the general 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 given
of very closely allied species which will not unite, or only
with extreme difficulty | and on the other hand of very dia-
OF FIRST CROSSES AND OF HYBRIDS. 209
tinct species which unite with the utmost facility. In the
same family there may be a genus, as Dianthus, in which
very many species can most readily be crossed ; and another
genus, as Silene, in which the most persevering efforts have
failed to produce between extremely close species a single *
hybrid. Even within the limits of the same genus, we meet
with this same difference ; for instance, the many species of -
Nicotiana have been more largely crossed than the species
of almost any other genus; but Gartner found that N.
acuminata, which is not a particularly distinct species, ol> (
stinately failed to fertilize, or to be fertilized by, no less i
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 shown
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 species
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.
longiflora, and the hybrids thus produced are sufficiently
fertile ; but Kolreuter tried more than two hundred times,
during eight following years, to fertilize reciprocally M.
longiflora 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 commoa ia a lesser
270 LAWS GOVERNING THE STERILITY
•degree. He has observed it even between closely related
forms (as Matthiola annua and glabra) which many botanists
rank only as varieties. It is also a remarkable fact, that
hybrids raised from reciprocal crosses, though of course
compounded of the very same two species, the one 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,
degree.
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, accord-
OF FIRST CROSSES AND OF HYBRIDS. 271
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
variable in the individuals of the same species ? Why
should 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
arrangement.
The foregoing rules and facts, on the other hand, appear
to me clearly to indicate that the sterility, both of first
crosses and of hybrids, is simply incidental or dependent
on unknown differences in their reproductive systems ; the
differences being of so peculiar and limited a nature, that,
in reciprocal crosses between the same two species, the male
sexual element of the one will often freely act on the female
sexual element of the other, but not in a reversed direction.
It will 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. j but in a multitude of cases we can assign
272 LAWS GOVERNING THE STERILITY
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
plum.
As Gartner found that there was sometimes an innate
difference in different individuals of the same two species in
crossing ; so Sageret believes this to be the case with 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 more freely when fertilized with the pollen of a
distinct species than wb.ei} fertilized with pollen from, the
OF FIRST CROSSES AND OF HYBRIDS. 273
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.
ORIGIN AND CAUSES OF THE STERILITY OF FIRST CROSSES
AND OF HYBRIDS.
At one time it appeared to me probable, as it has to others,
that the sterility of first crosses and of hybrids might have
been slowly acquired through the natural selection of 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 a,§ much opposed tq the theory of natural
274 CAUSES OF THE STERILITY
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 man}' 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
OF FIRST CROSSES AND OF HYBRIDS. 275
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
276 CAtfSES 6f Tim STfcRILlT?
majority of the fertilized eggs, the embryos had either been
partially developed and had then perished, or had become
nearly mature, but the young chickens had been unable to
break through the shell. Of the chickens which were born,
more than four-fifths died within the first few days, or at
latest weeks, " 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 strik-
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 between 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 country
where their two parents live, they are generally placed under
suitable conditions of life. But a hybrid partakes of only
half of the nature and constitution of its mother; it may
therefore, before birth, as long as it is nourished within its
mother's womb, or within, the egg or seed produced by the
mother, be exposed to 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 the 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 sexual
elements are imperfectly developed, the case is somewhat
different. I have more than once alluded to a large body of
facts showing that, when animals and plants are removed
from their natural conditions, they are extremely liable to
have their reproductive systems seriously affected. This,
in fact, is the great bar to the domestication of animals.
Between the sterility thus superinduced and that of hybrids,
m FlitST CROSSES AND OF HYBRIDS. 277
/there are many points of similarity. In both cases the
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 affected; but some-
times the female more than the male. In both, the tendency
goes to a certain extent with systematic affinity, for whole
groups of animals and plants are rendered impotent by the
same unnatural conditions; and whole groups of species
tend to produce sterile hybrids. On the other hand, one
species in a group will sometimes resist great changes of
conditions with unimpaired 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 to vary, which seems to be partly due to
their reproductive systems having been specially affected,
though in a lesser degree than when sterility ensues. So it
is with hybrids, for their offspring in successive generations
are eminently liable to vary, as every experimentalist has
observed.
Thus we see that when organic beings are placed under
new and unnatural conditions, and when hybrids are 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, including
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
need not be surprised that their sterility, though in some
278 CAUSES OF THE STERILITY
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
Wichura.
It must, however, be owned that we cannot understand,
on the above or any other view, several facts with respect
to the sterility of hybrids ; for instance, the unequal 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
OF FIRST CROSSES AND OF HYBRIDS. 279
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 it is that the races of some of our
domesticated animals, which have often been subjected to
new and not uniform conditions, are quite fertile together,
although they are descended from distinct species, which
would probably have been sterile if aboriginally crossed.
The above two parallel series of facts seem to be connected
together by some common but unknown bond, which is 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.
RECIPROCAL DIMORPHISM AND TRIMORPHISM.
This subject may be here briefly discussed, and will be
found to throw some light on hybridism. Several plants
belonging to distinct orders present two forms, which exist
in about equal numbers and which differ in no respect except
in their reproductive organs ; one form having a long pistil
with short stamens, the other a short pistil with long
stamens ; the two having differently sized pollen-grains. With
trimorphic plants there are three forms likewise differing in
the lengths of their pistils and stamens, in the size and 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 and three
kinds of pistils. These organs are so proportioned in length
to each other that half the stamens in two of the forms
stand on a level with the stigma of the third form. Now I
have shown, and the result has been confirmed by other
280 RECIPROCAL DIMORPHISM
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 pistil, differs much in degree,
up to absolute and utter sterility ; just in the same manner
as occurs in crossing distinct species. As the degree of
sterility in the latter case depends in an eminent degree on
the conditions of life being more or less favorable, so I
have found it with illegitimate unions. It is well knowu
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.
AND TMMORPHISM. 281
observe during four years many seedlings, raised from sev-
eral illegitimate unions. The chief result is that these
illegitimate plants, as they may be called, are not fully
fertile. It is possible to raise from dimorphic species, both,
long-styled and short-styled illegitimate plants, and from
trimorphic plants all three illegitimate forms. These can
then be properly united in a legitimate manner. When this
is done, there is no apparent reason why they should not
yield as many seeds as did their parents when legitimately
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
282 RECIPROCAL DIMORPHISM AND TRIMORPHISM.
try by crossing whether they were specifically distinct. He
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
to effected with perfect facility. That excellent observer,
FERTILITY OF VARIETIES WHEN CROSSED. 283
Gartner, likewise concluded that species when crossed are
sterile owing to differences confined to their reproductive
systems.
FERTILITY OF VARIETIES WHEN CROSSED, AND OF THEIR
MONGREL OFFSPRING, NOT UNIVERSAL.
It may be urged as an overwhelming argument that there
must be some essential distinction between species and
varieties, inasmuch as the latter, however much they may
differ from each other in external appearance, cross with
perfect facility, and yield perfectly fertile offspring. _ With
some exceptions, presently to be given, I fully admit that
this is the rule. But the subject is surrounded by difficul-
ties, for, looking to varieties produced under nature, if two
forms hitherto reputed to be varieties be 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'
2§4 FERTILITY OF VARIETIES
tivated plants have been subjected, have had so little ten.
dency toward modifying the reproductive system in a manner
leading to mutual sterility, that we have good grounds for
admitting the directly opposite doctrine of Pallas, namely,
that such conditions generally eliminate this tendency ; su
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 for 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
WHEtf CttOSSEft. 285
the mete 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
manner.
I have as yet spoken as if the varieties of the same
species were invariably fertile when intercrossed. But it
is impossible to resist the evidence of the existence of a
certain amount of sterility in the few following cases, which
I will briefly abstract. The evidence is at least as good as
that from which we believe in the sterility of a multitude
of species. The evidence is also derived from hostile 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 Naiidin
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
286 FERTILITY OF VARIETIES WHEN CROSSED.
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 which are
commonly reputed to be varieties, and which he tested by
the severest trial, namely, by reciprocal crosses, and he
found their mongrel offspring perfectly fertile. But one of
these five varieties, when used either as the father or mother,
and crossed with the 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 con-
clude 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.
HYBRIDS AND MONGRELS COMPARED. 287
HYBRIDS AND MONGRELS COMPARED, INDEPENDENTLY OF
THEIR FERTILITY.
Independently of the question of fertility, the offspring
of species and of varieties when crossed may be compared in
several other respects. Gartner, whose strong wish it was to
draw a distinct line between species and varieties, could find
very few, and, as it seems to me, quite unimportant 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
respects.
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 variabil-
ity graduates away. When mongrels and the more fertile
hybrids are propagated for several generations, an extreme
amount of variability in the offspring in both cases is 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 funo*
tion of producing offspring closely similar in all respects to
the parent form. Now, hybrids in the first generation are
288 HYBRIDS AND MONGRELS COMPARED.
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
certainly 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
HYBRIDS AND MONGRELS COMPARED. 289
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 in 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,
290 SUMMARY.
SUMMARY OF CHAPTER.
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-
SUMMARY. 291
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 their
mutual infertility, we do not know ; but it seems to stand
in some close relation to species having been exposed fox
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 offspringr
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 tc
varieties in a state of nature ; and when we remember that
the greater number of varieties have been produced under
domestication by the selection of mere external differences,
and that they have not been long exposed to uniform 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 their
292 SUMMARY.
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.
IMPEKFECT10N OJ? GEOLOGICAL RECORD. 293
CHAPTER X.
ON THE IMPERFECTION OF THE GEOLOGICAL RECOBD.
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 thelntermit-
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
294 IMPERFECTION OF THE
every stratum full of such intermediate links ? Geology
assuredly does not reveal any such finely-graduated organic
chain ; and this, perhaps, is the most obviouB 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
sort of intermediate forms must, on the theory, have formerly
existed. I have found it difficult, when looking at any two
species, to avoid picturing to myself forms directly 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. oenas.
So, with natural species, if we look to forms very distinct,
for instance to the horse and tapir, we have no reason to
suppose that links directly intermediate between them ever
existed, but between each and an unknown common parent.
The common parent will have had in its whole 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, in all such cases, we should be unable to
recognize the parent 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
f o*ms might have descended from the other ; for instance, a
GEOLOGICAL RECORD. 295
horse from a tapir ; and in this case direct intermediate links
will have existed between them. But such a case would
imply that one form had remained for a very long period
unaltered, 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 this
a very rare event ; for in all cases the new and improved
forms of life tend to supplant the old and unimproved forms.
By the theory of natural selection all living species have
been 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.
ON THE LAPSE OF TIME, AS INFERRED FROM THE RATE OF
DEPOSITION AND EXTENT OF DENUDATION.
Independently of our not finding fossil remains of such
infinitely numerous connecting links, it may be objected that
time cannot have sufficed for so great an amount of organic
change, all changes having been effected slowly. It is hardly
possible for me to recall to the reader who is not a practical
geologist, the facts leading the mind feebly to comprehend
the lapse of time. He who can read Sir Charles Lyell's grand
work on the Principles of Geology, which the future 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
296 THft LA^Sti OP TlMfi.
elsewhere undergone. Therefore a man should examine fol
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 ol
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
base.
We have, however, recently learned from the observations
of Ramsay, 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 a 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 Wealden,
THE LAPSE OF TIME. 297
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 oue 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 pared all
round into perpendicular cliffs of one or two thousand feet
in height ; for the gentle slope of the lava streams, due to
their formerly 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.
298 THE LAPSE OF TIME.
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 :
Feet.
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 strip
THE LAPSE OF TIME. 299
of 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
lower animals, that they have formed what well deserves to
be called a new sub-breed. Few men have attended with
due care to any one strain for more than half a century, so
that a hundred years represents the work of two breeders in
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 j but
to the subject of time we must return.
ON THE POORNESS OF PAL^EONTOLOGICAL COLLECTIONS.
Now let us turn to our richest geological museums, and
what a paltry display we behold ! That our collections are
imperfect, is admitted by every one. The remark of that
admirable palaeontologist, Edward Forbes, should never be
300 THE POORNESS OF
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 geologi-
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 a
quite erroneous view, when we assume that sediment is being
deposited over nearly the whole bed of the sea, at a rate
sufficiently quick to embed and preserve fossil remains.
Throughout an enormously large proportion of the ocean,
the bright blue tint of the water bespeaks its purity. The
many cases on record of a formation conformably covered,
after an immense interval of time, by another and later
formation, without the underlying bed having suffered in
the interval any wear and tear, seem explicable only on the
view of the bottom of the sea not rarely lying for ages in
an unaltered condition. The remains which do become 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 been7 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
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
(luring the $e§oi}c[ary and Palaeozoic periods, jt js superflu-
PAL^CXNTOLOGICAL COLLECTIONS. 301
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 Lyell's Manual will bring home the truth, how
accidental and rare is their preservation, far better than
pages of detail. Nor is their rarity surprising, when we
remember how large a proportion of the bones of tertiary
mammals have been discovered either in caves or in lacus-
trine deposits ; and that not a cave or true lacustrine bed
is known belonging to the age of our secondary or palaeozoic
formations.
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 Russia, what wide gaps there
are in that country between the superimposed formations ;
so it is in North America, and in many other parts of the
world. The most skilful geologist, if his attention had
been confined exclusively to these large territories, would
never have suspected that during the periods which were
blank and barren in his own country, great piles of 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 elapsed between the consecutive formations,
we may infer that this could nowhere be ascertained. The
frequent and great changes in the mineralogical composition
of consecutive formations, generally implying great changes
in the geography of the surrounding lands, whence the 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
e&ch region are almost invariably intermittent) tih& is, have
302 THE POORNESS OF
not followed each other in close sequence. Scarcely any
fact struck me more when examining many hundred miles
of the South American coasts, which have been upraised
several hundred feet within the recent period, than the
absence of any recent 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 probably 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
waves.
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 subsequent subaerial degradation.
Such thick and extensive accumulations of sediment may
be formed in two ways ; either in profound depths of the
sea, in which case the bottom will not be inhabited by so
many and such varied forms of life as the more shallow
seas ; and the mass when upraised will give an imperfect
record of the organisms which existed in the 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
PAL^EONTOLOGICAL COLLECTIONS. 303
to note how author after author, in treating of this or that
great formation, has come to the conclusion that it was
accumulated during subsidence. I may add, that the only
ancient tertiary formation on the west coast of South
America, which has been bulky enough to resist such 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
thickness.
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
only where the supply of sediment was sufficent to keep the
sea shallow and to embed and preserve the remains before
they had time to decay. On the other hand, as long as
the bed of the sea remains stationary, 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 forty to sixty fathoms, a
widely extended formation might be formed during a period
of elevation, and yet not suffer excessively from denudation
during its slow upheaval ; but the thickness of the formation
could not be great, for owing to the elevatory movement it
Avould 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
nut 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
304 THE POORNESS OF
beds of considerable horizontal extent have rarely been com*
pletely destroyed. But all geologists, excepting the few
who believe that our present metaniorphic 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 metaniorphic 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 metamorphio and granite
rocks would be found mucli more widely extended than they
appear to, be< if all the Sftdhnentary beds were removed which
PALiEONTOLOGICAL COLLECTIONS. 305
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.
ON THE ABSENCE OF NUMEROUS INTERMEDIATE VARIE-
TIES IN ANY SINGLE FORMATION.
From these several considerations it cannot be doubted
that the geological record, viewed as a whole, is extremely
imperfect ; but if we confine our attention to any one 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
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. palaeontologist;^ whose opinions are worthy p,f iftuch.
306 ABSENCE OF INTERMEDIATE .VARIETIES
deference, namely Bronn and Woodward, have concluded
that the average iuration of each formation is twice 01
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 palaeozic
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, whi^h 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 remains,
have gone on accumulating within the same area during the
whole of this period. It is not, for instance, probable that
sediment was deposited during the whole of the glacial 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
IN ANY SINGLE FORMATION. 307
during some part of the glacial period shall have been up-
raised, organic remains will probably first appear and disap-
pear at different levels, owing to the migrations of species
and to geographical changes. And in the distant future, a
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 forms
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
consumed. Many instances could be given of beds, only a
few feet in thickness, representing formations which are
elsewhere thousands of feet in thickness, and which must
have required an enormous period for their accumulation ;
yet no one ignorant of this fact would have even suspected
the vast lapse of time represented by the thinner formation.
303 ABSENCE OF iNTERMEDiATE VARIETIES
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.
W ANY SINGLE FORMATION. S0&
It is notorious on what excessively slight differences many
palaeontologists have founded their species ; and they do
this the more readily if the specimens come from different
sub-stages of the same formation. Some experienced con-
chologists are now sinking many of the very fine species of
D'Orbigny and others into the rank of varieties ; and on
this view we do find the kind of evidence of change which
on the theory we ought to find. Look again at the later
tertiary deposits, which include many shells believed by the
majority of 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
the theory ; but to this latter subject I shall return in the
following chapter.
With animals and plants that propagate rapidly and do
not wander much, there is reason to suspect, as we have
formerly seen, that their varieties are generally at first
local ; and that such local varieties do not spread widely and
supplant their parent-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
310 ABSENCE OF INTERMEDIATE VARIETIES
formations in Europe, have of fcenest 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 with 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 to be expected ; yet this has been
IN ANY SINGLE FORMATION. 311
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 chance of enduring to a very distant future. During
the periods of subsidence, there would probably be much
312 ABSENCE OF INTERMEDIATE VARIETIES
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 the 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 have.
IN ANY SINGLE FORMATION. 313
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.
ON THE SUDDEN APPEARANCE OF WHOLE GROUPS OF
ALLIED SPECIES.
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
fprms wou!4 pften long remain confined! to some one region, j
314 SUDDEN APPEARANCE OF
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"?
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 Palseon-
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 have 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
GROUPS OF ALLIED SPECIES. 615
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 fifty 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-
nized ; 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
which not one species has as yet been found even in any
tertiary stratum. Still more recently, a Pyrgoma, a member
of a distinct sub-family of sessile cirripedes, has been 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 insisted on by palaeontologists,
S16 SUDDEN APPEARANCE OF ALLIED SPECIES.
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
palseontological knowledge effected by the discoveries of
the last dozen years, it seems to me to be about as rash to
dogmatize on the succession of organic forms throughout the
world, as it would be for a naturalist to land for five minutes
on a barren point in Australia, and then to discuss the nuni<
ber and range of its productions.
GROUPS OP ALLIED SPECIES. 317
ON THE SUDDEN APPEARANCE OF GROUPS OF ALLIED SPECIES
IN THE LOWEST KNOWN FOSSILIFEROUS STRATA.
There is another and allied difficulty, which is much more
serious. I allude to the manner in which species belonging
to several of the main divisions of the animal kingdom sud-
denly appear in the lowest known fossiliferous rocks. Most
of the arguments which have convinced me that all 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 any degree intermediate in character.
Consequently, if the theory be true, it is indisputable that
before the lowest Cambrian stratum was deposited long
periods elapsed, as long as, or probably far longer than, the
whole interval from the Cambrian age to the present day ;
and that during these vast periods the world swarmed with
living creatures. Here we encounter a 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,
318 GROUPS OF ALLIED SPECIES.
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 find rich fossiliferous
deposits belonging to these assumed earliest periods prior
to the Cambrian system, I can give no satisfactory answer.
Several eminent geologists, with Sir R. Murchison at their
head, were until recently convinced that we beheld in the
organic remains of the lowest Silurian stratum the first dawn
of life. Other highly competent judges, as Lyell and E.
Forbes, have disputed this conclusion. We 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
IN LOWEST FOSSILIFEROUS STRATA. 319
been wholly obliterated by metainorphic action, for if this had
been the case we should have found only small remnants of
the formations next succeeding them in age, and these would
always have existed in a partially metamorphosed condition.
But the 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-
phism.
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
tracts of land, whence the sediment was derived, occurred in
the neighborhood of the now existing continents of Europe
and North America. This same view has since been main-
tained 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 as the land, we see them studded with many islands ; but
hardly one truly oceanic island (with the exception of New
Zealand, if this can be called a truly oceanic island) is as yet
known to afford even a remnant of any palaeozoic or secondary
formation. Hence, we may perhaps infer, that during the
palaeozoic and 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 facts, 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
320 GftOtJl'S OF ALLIED SP^CtES
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 palaeontologists, namely, Cuvier,
Agassiz, Barrande, Pictet, Falconer, E. Forbes, etc., and all
our greatest geologists, as Lyell, Murchison, Sedgwick, etc.,
Jiave unanimously, often vehemently, maintained the immu*
IK Lowest FossiLiFEftous strata. 821
tability of species. But Sir Charles Lyell now gives the
support of his high 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 the
theory. For my part, following out Lyell's metaphor, I look
at the geological record as a history of the world imperfectly
kept and written in a changing dialect. Of this history we
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 each 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 above
discussed are greatly diminished or even disappear.
322 THE GEOLOGICAL SUCCESSION
CHAPTER XL
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
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
OF ORGANIC BEINGS. 328
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 insects of Madeira have come to differ considerably
from their nearest allies on the continent of Europe, whereas
the marine shells and birds have remained unaltered. We
can perhaps understand the apparently quicker rate of change
in terrestrial and in more highly organized productions com-
324 THE GEOLOGICAL SUCCESSION
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 woul4 almost certainly be in,
OF ORGANIC BEINGS. 325
some degree different, and the newly-formed variety would
probably inherit from its progenitor some characteristic
differences.
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 in crease 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
326 EXTINCTION.
other varieties and species, and so on, like the branching of
a great tree from a single stem, till the group becomes large.
ON EXTINCTION.
We have as yet only spoken incidentally of the 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 ttgether. 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 endures. There
is reason to believe that the extinction of a whole group of
species is generally a slower process than their production :
if their appearance and disappearance be represented, as
before, by a vertical line of varying thickness, the line is
found to 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 remain* of
EXTINCTION. 327
Mastodon, Megatherium, Toxodon, and other extinct mon-
sters, which all co-existed with still living shells at a very
late geological period, I was tilled 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 this 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 India or Africa, some cause
328 EX'flNCTlOtf.
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-
tions, that rarity precedes extinction ; and we know that
this has been the progress of events with those animals
which have been exterminated, either locally or wholly,
through man's agency. I may repeat what I published in
1845, namely, that to admit that species generally become
rare before they become extinct — to feel no surprise at the
rarity of a species, and yet to marvel greatly when the
species ceases to exist, is much the same as to admit that
sickness in the individual is the forerunner of death — to
feel no surprise at sickness, but, when the sick man dies,
to wonder, and to suspect that he died by some deed of
violence.
The theory of natural selection is grounded on the belief
that each new variety, and ultimately each new 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 for-
EXTINCTION. 323
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
"me 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
its extermination. If many allied forms be developed from
the successful intruder, many will have to yield their places ;
and it will generally be the allied forms, which will suffer
from some inherited inferiority in common. But whether
it be 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 Granoid 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 our
consecutive formations ; and in these intervals there may
have been much slow extermination. Moreover, when, by
sudden immigration or by unusually rapid development,
many species of a new group have taken possession of an
area, many of the older species will have been exterminated
in a correspondingly rapid manner; and the forms which
thus yield their places will commonly be allied, for they will
partake of the same inferiority in common.
Thus, as it seems to me, the manner in which single
species and whole groups of species become extinct accords
330 FORMS OF LIFE CHANGING
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 will 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
species.
ON THE FORMS OF LIFE CHANGING ALMOST SIMULTA-
NEOUSLY THROUGHOUT THE WORLD.
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 Tierra del Fuego, at the
Cape of Good Hope, and in the peninsula of India. For
at these distant points, the organic remains in certain beds
present an 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-
ern Europe, and North America, a similar parallelism in
the forms of life has been observed by several authors ; so
it is, according to Lyell, with the European and North
American tertiary deposits. Even if the few fossil species
which are common to the Old and New Worlds were kept
wholly out of view, the general parallelism in the successive
forms of life, in the palaeozoic and tertiary stages, would
THROUGHOUT THE WORLD. 331
still be manifest, and the several formations could be easily
correlated.
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
stages.
When the marine forms of life are spoken of as having
changed simultaneously throughout the world, it must not
be supposed that this expression relates to the same year,
or to the same century, or even that it has a very strict
geological sense ; for if all the marine animals now living in
Europe, and all those that lived in Europe during the pleis-
tocene period (a very remote period as measured by years,
including the whole glacial epoch) were compared with those
now existing in South America or in Australia, the most 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 worlcl, has
332 FORMS OF LIFE CHANGING
greatly struck those admirable observers, MM. de Verneuil
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 to 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 their own country, give birth to the greatest num-
ber of new varieties or incipient species. We have distinct
evidence on this head, in the plants which are dominant,
that is, which are commonest and most widely diffused,
producing the greatest number of new varieties. It is also
natural that the dominant, 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
climatal 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
Qf fcbe continuous sea^ 'We might therefore expect to fin^
THROUGHOUT THE WORLD. 333
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 in both, from
the causes explained in the foregoing paragraphs, the same
general succession in the forms of life ; but the species
would not exactly correspond ; for there will have been a
little more time in the one region than in th§ Q$\$V fof
wodi^cation, extinction, and immigration^
334 AFFINITIES OF EXTINCT SPECIES.
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 finds in both a
curious accordance in the numbers of the species belonging
to the same genera, yet the species themselves differ in a
manner very 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
Avould not be all the same in the apparently corresponding
stages in the two regions.
ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER,
AND TO LIVING FORMS.
Let us now look to the mutual affinities of extinct and
living species. All fall into a few grand classes; and this
fact is at once explained on the principle of descent.
The more ancient any form is, the more, as a general
jule, 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
some instances. If we, confine our attention either to the
AFFINITIES OF EXTINCT SPECIES. 335
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 Pinosaurians
836 AFFINITIES OF EXTINCT SPECIES.
— that group which includes the most gigantic of all ter-
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
the same classes, we must admit that there is truth in the
remark.
Let us see how far these several facts and inferences
accord with the theory of descent with modification. As
the subject is somewhat complex, I must request the reader
to turn to the diagram in the fourth chapter. We may 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, ^nim-
AFFINITIES OF EXTINCT SPECIES. 337
portant for us. The horizontal lines may represent succes-
sive geological formations, and all the forms beneath the
uppermost line may be considered as extinct. The three
existing genera au, qu, p14, will form a small family ; bli and
fu, a closely allied family or sub-family ; and ou, eu, mu, 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. On the principle of the continued tendency to diver-
gence of character, which was formerly illustrated by this
diagram, the 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 f14.
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
day.
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 at several points low down in
the series, the three existing families on the uppermost line
would be rendered less distinct from each other. If, for
instance, the genera a1, a5, a10, f8, ra3, m6, m*, 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 aud 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, nof(
338 AFFINITIES OF EXTINCT SPECIES.
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, a14, etc., and bu, 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 (a14 to mu), 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
than is represented in the diagram; for the groups will
have been more numerous ; they will have endured for
extremely unequal lengths of time, and will have been
modified in various degrees. As we possess only the last
volume of the geological record, and that in a very broken
condition, we have no right to expect, except in rare cases,
to fill up the wide intervals in the natural system, and thus
to unite distinct families or orders. All that we have a
right to expect is, that those groups which have, within
known geological periods, undergone much modification,
should in the older formations make some slight approach
to each other ; so that the older members should differ less
from each other in some of their characters than do the
existing members of the same groups ; and this by the 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 character between that which preceded and
AFFINITIES OF EXTINCT SPECIES. 339
that which succeeded it. Thus the species which lived at
the sixth great stage of descent in the diagram are the modi-
fied 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-
340 AFFINITIES OF tt&lftffCT SECIES.
appearance; for the parent rock-pigeon still lives: and
many varieties between the rock-pigeon and the carrier have
become extinct ; and carriers which are extreme in the 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
affected.
On the theory of descent, the full meaning of the fossil
remains from closely consecutive formations being closely
related, though ranked as distinct species, is obvious. As
the accumulation of each formation has often been 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. — - -
ANCIENT AND LIVING FORMS. 341
OK THE STATE OF DEVELOPMENT OF ANCIENT COMPARED
WITH LIVING FORMS.
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 and
fresh-water shells have remained nearly the same, from the
time when, as far as is known, they first appeared. It is
not an insuperable difficulty that Foraminifera have not, as
insisted on by Dr. Carpenter, progressed in 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
842 STATE OF DEVELOPMENT OP
Foraminifera, for instance, could be proved to have first come
into existence during the Laurentian 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
disputed.
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 highty
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
two periods — though undoubtedly this is one and perhaps
ANCIENT AND LIVING FORMS. 343
the most important element in striking a balance — but we
ought to compare all the members, high and low, at two
periods. At an ancient epoch the highest and lowest 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 this
result.
344 SUCCESSION OF THE
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, anu ^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 d*,jr,
it would be vain to look for animals having the commou
embryological character of the vertebrata, until beds rich in
fossils are discovered far beneath the lowest Cambrian
strata — a discovery of which the chance is small.
ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME
AREAS, DURING THE LATER TERTIARY PERIODS.
Mr. Clift many years ago showed that the fossil mammals
from the Australian caves were closely allied to the living
marsupials of that continent. In South America a similar
relationship is manifest, even to an uneducated eye, in the
gigantic pieces of 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 these 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
J)a5 subsequently extended the same generalization to tf*§
SAME TYPES IN SAME AREAS. 345
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
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 t}rpes 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
of the long-enduring, but not immutable, succession of the
same types within the same areas, is at once explained ; for
the inhabitants of each quarter of the world will obviously
tend to leave in that quarter, during the next succeeding
period of time, closely allied though in some degree 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 th©
346 SUMMARY OF 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.
SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.
I have attempted to show that the geological record
is extremely imperfect ; that only a small portion of the
globe has been geologically explored with care ; that only
certain classes of organic beings have been largely preserved
in a fossil state ; that the number both of specimens and of
species, preserved in our museums, is absolutely as nothing
compared with the number of generations which must have
psssed away even during a single formation ; that, owing
PRECEDING AND PRESENT CHAPTERS. 347
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
stages, it is probable that the periods, during which each
underwent 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 nu»
U& SUMMARY OF THE
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. We can understand why,
when a species has once disappeared, it never reappears.
Groups of species increase in numbers slowly, and endure
for unequal periods of time ; for the process of modifica-
tion is necessarily slow, and depends on many complex
contingencies. The dominant species belonging to large
and dominant groups tend to leave many modified descend-
ants, which form new sub-groups and groups. As these
are formed, the species of the less vigorous groups, from
their inferiority inherited from a common progenitor, tend
to become extinct together, and to leave no 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. When a group has once
wholly disappeared, it does not reappear ; for the link of
generation has been broken.
We can understand how it is that dominant forms which
spread widely and yield the greatest number of varieties
tend to people the world with allied, but modified, 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.
PRECEDING AKD PRESENT CHAFERS. Mb
We can understand how it is that all the forms of life,
ancient and recent, make together a few grand classes.
We can understand, from the continued tendency to 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-
i 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 and the Survival of the
Fittest,
350 GEOGRAPHICAL DISTRIBUTION.
CHAPTER XII.
GEOGRAPHICAL DISTRIBUTION.
Present Distribution cannot be accounted for by Differences in Physi-
cal Conditions — Importance of Barriers — Affinity of the 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
of land in Australia, South Africa, and western South
GEOGRAPHICAL DISTRIBUTION. 351
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. Gtinther lias
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
352 GEOGRAPHICAL DISTRIBUTION.
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 rauskrat, but the coypu and capy-
bara; rodents of the South American type. Innumerably
GEOGRAPHICAL mSTRlBUTtOtt *K
other instances could be given II ^e look to the islands
off the American shore. hov,ever much they may differ in
geological structure, tk- inhabitants are essentially American,
though they may L<* all peculiar species. We may look kick
to past age? <*s shown in the last chapter, and we find
America^ ^ypes then prevailing on the American continent
and Ji cne American seas. We see in these facts some deep
orsanic 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
ifeore 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 was remarked in the last chapter,
of the existence of any law of necessary development. As
the variability of each species is an independent property,
354 SINGLE CENTRES OF CREATION.
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 U,
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 intt
new relations with each other and in a lesser degree witv
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
distant 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.
SINGLE CENTRES OF SUPPOSED CREATION.
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
SINGLE CENTRES OF CREATION. 355
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 universall)'' 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 incapacit}1-
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,
that the view of each species having been produced in one
356 SINGLE CENTRES OF CREATION.
area alone, and having subsequently migrated from that
area as far as its powers of migration and subsistence under
past find 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 certainty occurred within recent geological
times, must have rendered discontinuous the formerly con-
tinuous range of many species. So that we are reduced to
consider whether the exceptions to continuity of range are
so numerous, and of so grave a nature, that we ought to give
up the belief, rendered 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
SINGLE CENTRES OF CREATION. 357
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 the relation of the
species of one region to those of another, does not differ
much from that advanced by Mr. Wallace, wrho 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 case^, 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
generation.
Before discussing the three classes of facts, which I have
selected as presenting the greatest amount of difficulty on
the theory of " single centres of creation," I must say a few
words on the means of dispersal
358 MEANS OF DISPERSAL.
MEANS OF DISPERSAL.
Sir C. Lyell and other authors have ably treated this sub-
ject. I can give here only the briefest abstract of the more
important facts. Change of climate must have had a power-
ful influence on migration. A region now impassible to
certain organisms from the nature of its climate, might have
been a high road for migration, when the climate was differ-
ent. I shall, however, presently have to discuss this branch
of the subject in some detail. Changes of level in the land
must also have been highly influential : a narrow isthmus
now separates two marine faunas ; submerge it, or let it
formerly have been submerged, and the two faunas will now
blend together, or may formerly have blended. Where the
sea now extends, land may at a former period have con-
nected islands or possibly even continents together, ant!
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 recentty connected with Europe or
Africa, and Euiope likewise with America. Other authors
have thus nypothetically bridged over every ocean, and
united almost every island with some mainland. If, indeed,
the argument--? used by Forbes are to be trusted, it must be
admitted thai, scarcely a single island exists which has not
recently been united to some continent. This view cuts the
Gordian knct 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 or
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
MEANS OF DISPERSAL. $60
a single birthplace, and when in the course of time ^e
know something 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 several
lands and even seas to their present inhabitants — the
degree of affinity between the mammals inhabiting islands
with those of the nearest continent, being in oart deter
mined (as we shall hereafter see) by the depth ox the inter
vening ocean — these and other such facts are opposed to
the admission of such prodigious geographical revolutions
within the recent period, as are necessary on the view
advanced by Forbes and admitted by his followers. The
nature and relative proportions of the inhabitants of oceanic
islands are likewise opposed to the belief of their former
continuity of continents. Nor does the almost universal!)'
volcanic composition of such islands favor the admission
that they are the wrecks of sunken continents ; if they had
originally existed as continental mountain ranges, some at
least of the islands would have been formed, like other
mountain summits, of granite, metamorphic schists, old
fossiliferous and other rocks, instead of consisting of mere
piles of volcanic matter.
I must now say a few words on what are called accidental
means, but which more properly should be called occasional
means of distribution. I shall here confine myself tr>
plants. In botanical works, this or that plant is oftei*
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 Leguminosse were tried, and, with one
exception, they resisted the salt water badly ; seven specie!
360 MfiANS OF DISPERSAL.
of the allied orders, Hydrophyllaceae 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 || 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 -j1^ 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 -^^ 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.
MEANS OF DISPERSAL. 36l
He tried ninety-eight seeds, mostly different from mine,
but he chose many large fruits, and likewise seeds, from
plants which live near the sea ; and this would have favored
both the average length of their notation and their resist-
ance to the injurious action of the salt water. On the othrjr
hand, he did not previously dry the plants or branches with
the fruit; and this, as we have seen, would have caused
some of them to have floated much longer. The result was
that if of his seeds of different kinds floated for forty -two
days, and were then capable of germination. But I do not
doubt that plants exposed to the waves would float for a
less time than those protected from violent movement as in
our experiments. Therefore, it would perhaps be safer to
assume that the seeds of about -j^ 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
862 MEANS OF DISPERSAL.
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. But
the following fact is more important : the crops of birds do
not secrete gastric juice, and do not, as I know by trial,
injure in the least the germination of seeds ; now, after a bird
has found and devoured a large supply of food, it is 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
MEANS OF DISPERSAL. 363
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-
cillae), wheatears, and whinchats (Saxicolse), on their first
arrival on our shores, before they had alighted ; and he
has several times noticed little cakes of earth attached to
their feet. Many facts could be given showing how generally
soil is charged with seeds. For instance, 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
many birds which are annually blown by gales across great
364 MEANS OF DISPERSAL.
spaces of ocean, and which annually migrate — for instance,
the millions of quails across the Mediterranean — must
occasionally transport a few seeds embedded in dirt adhering
to their feet or beaks ? But I shall have to recur to this
subject.
As icebergs are known to be sometimes loaded with earth
and stones, and have even carried brushwood, bones, and the
nest of a land-bird, it can hardly be doubted that they must
occasionally, as suggested by Lyell, have transported seeds
from one part to another of the arctic and antarctic regions ;
and during the Glacial period from one part of the now
temperate regions to another. In the Azores, from the large
number of plants common to Europe, in comparison with the
species on the other islands of the Atlantic, which stand
nearer to the mainland and (as remarked by Mr. H. C.
Watson) from their somewhat Northern character, in 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 they may
have brought thither some few seeds of Northern plants.
Considering that these several means of transport, and
that other means, which without doubt remain to be 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 mjngled j
MEANS OF DISPERSAL. 365
but would remain as distinct as they now are. The currents,
from their course, would never bring seeds from North
America to Britain, though they might and do bring seeds
from the West Indies to our western shores, where, if not
killed by their 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 or any other continent, that a poorly stocked
island, though standing more remote from the mainland,
would not receive colonists by similar means. Out of a
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 lapse of geological time, while the island
was being upheaved, and before it had become fully stocked
with inhabitants. On almost bare land, with few or no
destructive insects or birds living there, nearly every seed
which chanced to arrive, if fitted for the climate, would
germinate and survive.
DISPERSAL DURING THE GLACIAL PERIOD.
The identity of many plants and animals, on mountain-
summits, separated from each other by hundreds of miles of
lowlands, where alpine 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, Oft th§ Whito
866 DISPERSAL DURING
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.
THE GLACIAL PERIOD. 367
• 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,
fchese forms will not have been liable to much modification,
368 DISPERSAL DURING
But with the alpine productions, left isolated from the
moment of the returning warmth, first at the bases and
ultimately on the summits of the mountains, the case will
have been somewhat different; for it is not likely that all
the same arctic species will have been left on mountain
ranges far distant from each other, and have survived there
ever since ; they will also, in all probability, have become
mingled with ancient Alpine species, which must have
existed on the mountains before the commencement of the
Glacial epoch, and which during the coldest period will
have been temporarily driven down to the plains ; the}^
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
ranges.
In the foregoing illustration I have assumed that at the
commencement of our imaginary Glacial period, the arctic
productions were as uniform round the polar regions as
they 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 by 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, fceiore the c^mnience^nt; of \$$ glacial period
THE GLACIAL PERIOD. 369
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 under «■
the Polar Circle that there is almost continuous land from 7
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 identit}7, 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 related
to each other than they are at the present time ; for during
these warmer periods the northern parts of the Old and New
Worlds will have been almost continuously united by land,
serving as a bridge, since rendered impassable by cold, for
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 aiid Old Worlds, migrated south of tbf ?ola? Circle,
370 DISPERSAL DURING
they will have been completely cut off from each other.
This separation, as far as the more temperate productions
are concerned, must have taken place long ages ago. As
the plants and animals migrated southward, they will have
become mingled in the one great region with the native
American productions, and would have had to compete with
them ; and in the other great region, with those of the Old
World. 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
distinct.
As on the land, so in the waters of the sea, a slow south-
ern 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
mariue animals, inhabiting the Mediterranean and the seas
of Japan — these two areas being now completely separated
by the breadth of a whole continent and by wide spaces of
ocean.
These cases of close relationship in species either now or
formerly inhabiting the seas on the eastern and western
shores of North America, the Mediterranean and Japan,
and the temperate lands of North America and Europe, are
inexplicable on the theory of creation. We cannot maintain
that such species have been created alike, in correspondence
with the nearly similar physical conditions of the areas ; for
if we compare, for instance, certain parts of South America
THE GLACIAL PERIOD. 371
with parts of South Africa or Australia, we see countries
closely similar in all their physical conditions, with their
inhabitants utterly dissimilar.
ALTERNATE GLACIAL PERIODS IN THE NORTH AND SOUTH.
But we must return to our more immediate subject. I
am convinced that Forbes' 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
Australia.
Looking to America: in the northern half, ice-borne
fragments of rock have been observed on the eastern side
of the continent, as far south as latitude 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 Rocky 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,
372 ALTERNATE GLACIAL PERIODS
deeply-furrowed rocks, resembling those with which he was
familiar in Norway, and likewise great masses of detritus,
including grooved pebbles. Along this whole space of the
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
avoid 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
ocourred 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 most 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 on
IN THE NORTH AND SOUTM. 373
geographical distribution, that I am strongly inclined to
trust in it : but I will first give the facts which demand an
explanation.
In South America, Dr. Hooker has shown that 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 Cape Verde archipelago, is one of the most astonish-
ing facts ever recorded in the distribution of plants.
On the Himalaya, and on the isolated mountain ranges of
the. peninsula of India, on the heights of Ceylon and on the
volcanic cones of Java, many plants occur either identically
the same or representing each other, and at the same time
representing plants of Europe not found in the intervening
hot lowlands. A list of the genera of plants collected on the
loftier peaks of Java, raises a picture of a collection made
on a hillock in Europe. Still more striking is the fact that
peculiar Australian forms are represented by certain planU
374 ALTERNATE GLACIAL PERIODS
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 tern-,
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 great
IN THE NORTH AND SOUTH. 375
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 hei'ght 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. Croll's 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,
376 ALTERNATE GLACIAL PERIODS
they would have long survived like the arctic forms on the
mountains of Europe. They might have survived, even it
the climate was not perfectly fitted for them, for the change
of temperature must have been very slow, and plants un-
doubtedly 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
IN THE NORTH AND SOUTH. 377
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 La
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
agency.
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 permitted their further prog-
ress ; in nearly the same manner as, according to Forbes,
isolated spaces inhabited by arctic productions exist to the
378 ALTERNATE GLACIAL PERIODS
present day in the deeper parts of the northern temperate
seas.
I am far from supposing that all the difficulties in regard
to the distribution and affinities of the identical and allied
species, which now live so widely separated in the north and
south, and sometimes on the intermediate mountain-ranges,
are removed on the views above given. The exact lines or
migration cannot be indicated. We cannot say why certain
species and not others have migrated ; why certain species
have been modified and have given rise to new forms, while
others have remained unaltered. We cannot hope to explain
such facts, until we can say why one species and not another
becomes naturalized by man's agency in a foreign land ; why
one species ranges twice or thrice as far, and is twice or
thrice as common, as another species within their own homes.
Various special difficulties also remain to be solved; for
instance, the occurrence, as shown by Dr. Hooker, of the
same plants at points so enormously remote as Kerguelen
Land, New Zealand, and Fuegia; but icebergs, as suggested
by Lyell, may have been concerned in their dispersal. The
existence at these and other distant points of the southern
hemisphere, of species, which, though distinct, belong to
genera exclusively confined to the south, is a more remark-
able case. Some of these species are so distinct, that we
cannot suppose that there has been time since 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
IN THE NORTH AND SOUTH. 379
distribution. And we have now seen that Mr. CrolFs con-
clusion that successive Glacial periods in the one hemisphere
coincide with warmer 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 one period
from the north and during another from the south, and in
both cases have reached the equator ; but the stream of life
has flowed with greater force from the north than in the
opposite direction, and has consequently more freely 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, in a line gently rising from the arctic
lowlands to a great altitude under the equator. The various
beings thus left stranded may be compared with savage
races of man, driven up and surviving in the mountain fast-
nesses of almost every land, which serves as a record, full of
interest to us, of the former inhabitants of the surrounding
lowlands.
280 FRESH-WATER PRODUCTIONS.
CHAPTER XIII.
geographical distribution — continued.
Distribution of Fresh-water Productions — On the Inhabitants of
Oceanic Islands — Absence of Batrachians and of Terrestrial
Mammals — On the Relation of the Inhabitants of Islands to
those of the Nearest Mainland — On Colonization from the Nearest
Source with Subsequent Modification — Summary of the Last and
Present Chapters.
FRESH-WATER PRODUCTIONS.
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 most 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. Giinther has lately shown that
the Galaxias attenuatus inhabits Tasmania, New Zealand,
the Falkland Islands, and the mainland of South America.
This is a wonderful case, and probably indicates dispersal
from an antarctic centre during a former warm period.
FRESH-WATER PRODUCTIONS. S8l
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 by
what may be called accidental means. Thus fishes still
alive are not very rarely dropped at distant points by
whirlwinds ; and it is known that the ova retain their
vitality for a considerable time after removal from the
water. Their dispersal may, however, be mainly attributed
to changes in the level of the land within the recent period,
causing rivers to flow into each other. Instances, also,
could be given of this having 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. Giinther 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-
382 FRESH-WATER PRODUCTIONS.
idly throughout the same country. But two fact*, which
I have observed — and 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
have twice seen these little plants adhering to their backs ;
and it has happened to me, in removing a little duck-weed
from one aquarium to another, that I have unintentionally
stocked the one with fresh-water shells from the other.
But another agency is perhaps more effectual : I suspended
the feet of a duck in an aquarium, where many ova of fresh-
water shells were hatching ; and I found that numbers of
the extremely minute and just-hatched shells crawled on
the feet, and clung to them so firmly that when taken out
of the water they could not be jarred off, though at a some-
what more advanced age they would voluntarily drop off.
These just-hatched 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
tell.
With respect to plants, it has long been known what
enormous ranges many fresh-water and even marsh species
have, both over continents and to the most remote oceanic
islands. This is strikingly illustrated, according to Alph.
de 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
FRESH-WATER PRODUCTIONS. 38
o
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 tablespoonfuls 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 luteum) in a heron's stomach. Now this bird must
often have flown with its stomach thus well stocked to
distant ponds, and then, getting a hearty meal of fish,
analogy makes me believe that it would have rejected the
seeds in 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 life
384 ItfHAfelf AtttS OF OCEANIC ISLANDS.
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 b}^ animals, more especially
by fresh-water birds, which have great powers of flight, and
naturally travel from one piece of water to another.
ON THE INHABITANTS OF OCEANIC ISLANDS.
We now come to the last of the three classes of facts,
which I have selected as presenting the greatest amount of
difficulty with respect to distribution, on the view that not
only all the individuals of the same species have migrated
from some one area, but that allied species, although now
inhabiting the most distant points, have proceeded from a
single area, the birthplace of their early progenitors. I
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-
INHABITANT S Of OCEANIC ISLAND. 3SS
fcon 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 (L 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 difference
seems to depend partly on the species which are not modified
having immigrated in a body, so that their mutual relations
have not been much disturbed ; and partly on the frequent
386 INHABITANTS OF OCEANIC ISLANDS.
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 habits, and will consequently have been
but little liable to modification. Any tendency to modifica-
tion will also have been checked by intercrossing with the
unmodified immigrants, often arriving from the mother-
country. Madeira again is inhabited by a wonderful number
of peculiar land-shells, whereas not one species of sea-shell
is peculiar to its shores : now, though we do not know how
sea-shells are dispersed, yet we can see that their eggs or
larvae, perhaps attached to sea-weed or floating timber, or to
the feet of wading birds, might be transported across three
or four hundred miles of open sea far more easily than land-
shells. The different orders of insects inhabiting Madeira
present nearly parallel cases.
Oceanic islands are sometimes deficient in animals of
certain whole classes, and their places are occupied by other
classes ; thus in the Galapagos Islands reptiles, and in New
INHABITANTS OF OCEANIC ISLANDS. 387
Zealand gigantic wingless birds, take, or recently took, the
place of mammals. Although New Zealand is here spoken
of as an oceanic island, it is in some degree doubtful
whether it should be so ranked ; it is of large size, and is
not separated from Australia by a profoundly deep sea ; from
its geological character and the direction of its mountain
ranges, the Rev. W. B. Clarke has lately maintained that this
island, as well as New Caledonia, should be considered as
appurtenances of Australia. Turning to plants, Dr. Hooker
has shown that in the Galapagos Islands the proportional
numbers of the different orders are very different from what
they are elsewhere. All such differences in number, and the
absence of certain whole groups of animals and plants, are
generally accounted for by supposed differences in the phj^si-
cal conditions of the islands ; but this explanation is not a
little doubtful. Facility of immigration seems to have been
fully as important as the nature of the conditions.
Many remarkable little facts could be given with respect
to the inhabitants of oceanic islands. For instance, in
certain islands not tenanted by a single mammal, some of
the endemic plants have beautifully hooked seeds ; yet few
relations are more manifest than that hooks serve for the
transportal of seeds in the wool or fur of quadrupeds. But
a hooked seed might be carried to an island by other
means ; and the plant then becoming modified would form an
endemic species, still retaining its hooks, which would
form a useless appendage, like the shrivelled wings under
the soldered wing-covers of many insular beetles. Again,
islands often possess trees or bushes belonging to orders
which elsewhere include only herbaceous species ; now trees,
as Alph. de Candolle has shown, generally have, whatever the
cause may be, confined ranges. Hence trees would be little
likely to reach distant oceanic islands ; and an herbaceous
plant, which had no chance of successfully competing with
the many fully developed trees growing on a continent,
might, when established on an island, gain an advantage over
other herbaceous plants by growing taller and taller and
over-topping them. In this case, natural selection would
tend to add to the stature of the plant, to whatever order
it belonged, and thus first convert it into a bush and then
into a tree.
388 ABSENCE OE TERRESTRIAL
ABSENCE OF BATRACHIANS AND TERRESTRIAL MAMMALS ON
OCEANIC ISLANDS.
With respect to the absence of whole orders of animals
on oceanic islands, Bory St. Vincent long ago remarked that
Batrachians (frogs, toads, newts) are never found on any of
the many islands with which the great oceans are studded.
I have taken pains to verify this assertion, and have found
it true, with the exception of New Zealand, New Caledonia,
the Andaman Islands, and perhaps the Solomon Islands and
the Seychelles. But I have already remarked that it is
doubtful whether New Zealand and New Caledonia ought to
be classed as oceanic islands ; and this is still more doubtful
with respect to the Andaman and Solomon groups and the
Seychelles. This general absence of frogs, toads, and newts
on so many true oceanic islands cannot be accounted for by
their physical conditions : indeed, it seems that islands are
peculiarly fitted for these animals ; for frogs have been
introduced into Madeira, the Azores, and Mauritius, and
have multiplied so as to become a nuisance. But as these
animals and their spawn are immediately killed (with the
exception, as far as known, of one Indian species) by sea-
water, there would be great difficulty in their transportal
across the sea, and therefore we can see why they do not
exist on strictly oceanic islands. But why, on the theory of
creation, they should not have been created there, it would
be very difficult to explain.
Mammals offer another and similar case. I have carefully
searched the oldest voyages, and have not found a single
instance, free from doubt, of a terrestrial mammal (exclud-
ing domesticated animals kept by the natives) inhabiting an
island situated about 300 miles from a continent or great
continental island ; and many islands situated at a much less
distance are equally barren. The Falkland Islands, which
are inhabited by a wolf-like fox, come nearest to an excep-
tion ; but this group cannot be considered as oceanic, as it
lies on a bank in connection with the mainland at a distance
of about 280 miles ; moreover, icebergs formerly brought
bowlders to its western shores, and they may have formerly
transported foxes, as now frequently happens in the arctic
regions. Yet it cannot be said that small islands will not
support at least small mammals, for they occur in many
parts of the world on very small islands, when lying close
MAMMALS ON OCEANIC ISLANDS. 389
to a continent ; and hardly an island can be named on which
our smaller quadrupeds have not become naturalized and
greatly multiplied. It cannot be said, on the ordinary view
of creation, that there has not been time for the creation of
mammals ; many volcanic islands are sufficiently ancient, as
shown by the stupendous degradation which they have suf-
fered, and by their tertiary strata: there has also been time
for the production of endemic species belonging to other
classes ; and on continents it is known that new species of
mammals appear and disappear at a quicker rate than other
and lower animals. Although terrestrial mammals do not
occur on oceanic islands, aerial mammals do occur on almost
every island. New Zealand possesses two bats found nowhere
else in the world : Norfolk Island, the Viti Archipelago, the
Bonin Islands, the Caroline and Marianne Archipelagoes,
and Mauritius, all possess their peculiar bats. Why, it may
be asked, has the supposed creative force produced bats and
no other mammals on remote islands ? On my view this
question can easily be answered; for no terrestrial mammal
can be transported across a wide space of sea, but bats ean
fly across. Bats have been seen wandering by day far over
the Atlantic Ocean ; and two North American species, either
regularly or occasionally, visit Bermuda, at the distance of
600 miles from the mainland. I hear from Mr. Tomes, who
has specially studied this family, that many species have
enormous ranges, and are found on continents and on far
distant islands. Hence, we have only to suppose that such
wandering species have been modified in their new homes in
relation to their new position, and we can understand the
presence of endemic bats on oceanic islands, with the
absence of all other terrestrial mammals.
Another interesting relation exists, namely, between the
depth of the sea separating islands from each other, or from
the nearest continent, and the degree of affinity of their mam-
malian inhabitants. Mr. Windsor Earl has made some strik-
ing observations on this head, since greatly extended by Mr.
Wallace's admirable researches, in regard to the great Malay
Archipelago, which is traversed near Celebes by a space of
deep ocean, and this separates two widely distinct mam-
malian faunas. On either side, the islands stand on a mod-
erately shallow submarine bank, and these islands are
inhabited by the same or by closely allied quadrupeds. I
have not as yet had time to follow up this subject in all
quarters of the world -? but as far as I have gone, the rel«fr
390 ABSENCE OF TERRESTRIAL
tion holds good. For instance, Britain is separated by a
shallow channel from Europe, and the mammals are the
same on both sides ; and so it is with all the islands near
the shores of Australia. The West Indian Islands, on the
other hand, stand on a deeply submerged bank, nearly one
thousand fathoms in depth, and here we find American
forms, but the species and even the genera are quite distinct.
As the amount of modification which animals of all kinds
undergo partly depends on the lapse of time, and as the
islands which are separated from each other, or from the
mainland, by shallow channels, are more likely to have been
continuously united within a recent period than the islands
separated by deeper channels, we can understand how it is
that a relation exists between the depth of the sea separat-
ing two mammalian faunas, and the degree of their affinity,
a relation which is quite inexplicable on the theory of inde-
pendent acts of creation.
The foregoing statements in regard to the inhabitants of
oceanic islands, namely, the fewness of the species, with a
large proportion consisting of endemic forms — the mem-
bers of certain groups, but not those of other groups in the
same class, having been modified — the absence of certain
whole orders, as of batrachian« and of terrestrial mammals,
notwithstanding the presence of aerial bats, the singular
proportions of certain orders of plants, herbaceous forms
having been developed into trees, etc., seem to me to accord
better with the belief in the efficiency of occasional means
of transport, carried on during a long course of time, than
with the belief in the former connection of all oceanic
islands with the nearest continent ; for on this latter view
it is probable that the various classes would have immi-
grated more uniformly, and from the species having entered
in a body, their mutual relations would not have been much
disturbed, and, consequently, they would either have not
been modified, or all the species in a more equable manner.
I do not deny that there are many and serious difficulties
in understanding how many of the inhabitants of the more
remote islands, whether still retaining the same specific form
or subsequently modified, have reached their present homes.
But the probability of other islands having once existed as
halting-places, of which not a wreck now remains, must not
be overlooked. I will specify one difficult case. Almost
all oceanic islands, even the most isolated and smallest, are
inhabited by land-shells, generally by endemic species, but
MAMMALS ON OCEANIC ISLANDS. 391
sometimes by species found elsewhere, striking instances of
which have been given by Dr. A. A. Gould in relation to the
Pacific. Now it is notorious that land shells are easily killed
by sea-water ; their eggs, at least such as I have tried, sink
in it and are killed. Yet there must be some unknown, but
occasionally efficient, means for their transportal. Would
the just-hatched young sometimes adhere to the feet of
birds roosting on the ground and thus get transported ? It
occurred to me that land-shells, when hibernating and hav-
ing a membranous diaphragm over the mouth of the shell,
might be floated in chinks of drifted timber across moder-
ately wide arms of the sea. And I find that several species
in this state withstand uninjured an immersion in sea-water
during seven days. One shell, the Helix pomatia, after hav-
ing been thus treated, and again hibernating, was put into
sea-water for twenty days and perfectly recovered. During
this length of time the shell might have been carried by a
marine current of average swiftness to a distance of 660
geographical miles. As this Helix has a thick calcareous
operculum I removed it, and when it had formed a new
membranous one, I again immersed it for fourteen days in
sea-water, and again it recovered and crawled away. Baron
Aucapitaine has since tried similar experiments. He placed
100 land-shells, belonging to ten species, in a box pierced
with holes, and immersed it for a fortnight in the sea. Out
of the hundred shells twenty-seven recovered. The presence
of an operculum seems to have been of importance, as out of
twelve specimens of Cyclostoma elegans, which is thus
furnished, eleven revived. It is remarkable, seeing how
well the Helix pomatia resisted with me the salt water, that
not one of fifty-four specimens belonging to four other spe-
cies of Helix tried by Aucapitaine recovered. It is, how-
ever, not at all probable that land-shells have often been
thus transported; the feet of birds offer a more probable
method.
ON THE RELATIONS OF THE INHABITANTS OF ISLANDS TO
THOSE OF THE NEAREST MAINLAND.
The most striking and important fact for us is the affinity
of the species which inhabit islands to those of the nearest
mainland, without being actually the same. Numerous
instances could be given. The Galapagos Archipelago, situ-
ated under the equator, lies at the distance of between 500
392 RELATIONS OF THE INHABITANTS OF
and 600 miles from the shores of South America. Here
almost every product of the land and of the water bears the
unmistakable stamp of the American continent. There are
twenty-six land birds. Of these twenty-one, or perhaps
twenty-three, are ranked as distinct species, and would com-
monly be assumed to have been here created ; yet the close
affinity of most of these birds to American species is mani-
fest in every character in their habits, gestures, and tones of
voice. So it is with the other animals, and with a large
proportion of the plants, as shown by Dr. Hooker in his
admirable Flora of this archipelago. The naturalist, looking
at the inhabitants of these volcanic islands in the Pacilic,
distant several hundred miles from the continent, feels that
he is standing on American land. Why should this be so ?
Why should the species which are supposed to have been
created in the Galapagos Archipelago, and nowhere else,
bear so plainly the stamp of affinity to those created in
America? There is nothing in the conditions of life, in the
geological nature of the islands, in their height or climate,
or in the proportions in which the several classes are associ-
ated together, which closely resembles the conditions of the
South American coast. In fact, there is a considerable dis-
similarity in all these respects. On the other hand, there is
a considerable degree of resemblance in the volcanic nature
of the soil, in the climate, height, and size of the islands,
between the Galapagos and Cape Verde Archipelagoes : but
what an entire and absolute difference in their inhabitants !
The inhabitants of the Cape Verde Islands are related to
those of Africa, like those of the Galapagos to America.
[Facts, such as these, admit of no sort of explanation on
the ordinary view of independent creation ; whereas, on the
view here maintained, it is obvious that the Galapagos
Islands would be likely to receive colonists from America,
whether by occasional means of transport or (though I do
not believe in this doctrine) by formerly continuous land,
and the Cape Verde Islands from Africa; such colonists
would be liable to modification — the principle of inheritance
still betraying their original birthplace.
Many analogous facts could be given : indeed, it is an
almost universal rule that the endemic productions of islands
are related to those of the nearest continent, or of the near-
est large island. The exceptions are few, and most of them
can be explained. Thus, although Kerguelen Land stands
nearer to Africa than to America, the plants are related, and
ISLANDS to THOSE OF THE MAINLAND. 393
that very closely, as we know from Dr. Hooker's account, to
those of America : but on the view that this island has been
mainly stocked by seeds brought with earth and stones on
icebergs, drifted by the prevailing currents, this anomaly
disappears. New Zealand in its endemic plants is much
more closely related to Australia, the nearest mainland, than
to any other region : and this is what might have been
expected ; but it is also plainly related to South America,
which, although the next nearest continent, is so enormously
remote, that the fact becomes an anomaly. But this diffi-
culty partially disappears on the view that New Zealand,
South America, and the other southern lands, have been
stocked in part from a nearly intermediate though distant
point, namely, from the Antarctic Islands, when they were
clothed with vegetation, during a warmer tertiary period,
before the commencement of the last Glacial period. The
affinity, which, though feeble, I am assured by Dr. Hooker is
real, between the flora of the south-western corner of Aus-
tralia and of the Cape of Good Hope, is a far more remark-
able case ; but this affinity is confined to the plants, and will,
no doubt, some day be explained.
The same law which has determined the relationship be-
tween the inhabitants of islands and the nearest mainland,
is sometimes displayed on a small scale, but in a most inter-
esting manner, within the limits of the same archipelago.
Thus each separate island of the Galapagos Archipelago is
tenanted, and the fact is a marvellous one, by many distinct
species ; but these species are related to each other in a very
much closer manner than to the inhabitants of the American
continent, or of any other quarter of the world. This is
what might have been expected, for islands situated so near
to each other would almost necessarily receive immigrants
from the same original source and from each other. But
how is it that many of the immigrants have been differently
modified, though only in a small degree, the islands situated
within sight of each other, having the same geological
nature, the same height, climate, etc. ? This long appeared
to me a great difficulty : but it arises in chief part from the
deeply-seated error of considering the physical conditions of a
country as the most important ; whereas it cannot be disputed
that the nature of the other species with which each has to
compete, is at least as important, and generally a far more
important element of success. Now, if we look to the
species which inhabit the Galapagos Archipelago, and are
294 RELATIONS OF THE INHABITANTS OF
likewise found in other parts of the world, we find that they
differ considerably in the several islands. This difference
might indeed have been expected if the islands have been
stocked by occasional means of transport — a seed, for
instance, of one plant having been brought to one island,
and that of another plant to another island, though all
proceeding from the same general source. Hence, when in
former times an immigrant first settled on one of the
islands, or when it subsequently spread from one to another,
it would undoubtedly be exposed to different conditions in
the different islands, for it would have to compete with a
different set of organisms ; a plant, for instance, would find
the ground best fitted for it occupied by somewhat different
species in the different islands, and would be exposed to the
attacks of somewhat different enemies. If, then, it varied,
natural selection would probably favor different varieties in
the different islands. Some species, however, might spread
and yet retain the same character throughout the group, just
as we see some species spreading widely throughout a con-
tinent and remaining the same.
The really surprising fact in this case of the Galapagos
Archipelago, and in a lesser degree in some analogous cases,
is that each new species, after being formed in any one
island, did not spread quickly to the other islands. But the
islands, though in sight of each other, are separated by deep
arms of the sea, in most cases wider than the British Chan-
nel, and there is no reason to suppose that they have at any
former period been continuously united. The currents of
the sea are rapid and deep between the islands, and gales
of wind are extraordinarily rare ; so that the islands are far
more effectually separated from each other than they appear
on a map. Nevertheless, some of the species, both of those
found in other parts of the world and of those confined to
the archipelago, are common to the several islands ; and we
may infer from the present manner of distribution that they
have spread from one island to the others. But we often
take, I think, an erroneous view of the probability of closely
allied species invading each other's territory, when put into
free intercommunication. Undoubtedly, if one species has
any advantage over another, it will in a very brief time
wholly or in part supplant it ; but if both are equally well
fitted for their own places, both will probably hold their
separate places for almost any length of time. Being
familiar with the fact that many species, naturalized through
ISLANDS TO THOSE OF THE MAINLAND. 396
man's agency, have spread with astonishing rapidity over
wide areas, we are apt to infer that most species would thus
spread; but we should remember that the species which
become naturalized in new countries are not generally closely
allied to the aboriginal inhabitants, but are very distinct
forms, belonging in a large proportion of cases, as shown by
Alph. de Candolle, to distinct genera. In the Galapagos
Archipelago, many even of the birds, though so well adapted
for flying from island to island, differ on the different
islands ; thus there are three closely allied species of mock-
ing-thrush, each confined to its own island. Now let us
suppose the mocking-thrush of Chatham Island to be blown
to Charles Island, which has its own mocking-thrush; why
should it succeed in establishing itself there ? We may
safely infer that Charles Island is well stocked with its own
species, for annually more eggs are laid and young birds
hatched than can possibly be reared ; and we may infer that
the mocking-thrush peculiar to Charles Island is at least as
well fitted for its home as is the species peculiar to Chatham
Island. Sir C. Lyell and Mr. Wollaston have communicated
to me a remarkable fact bearing on this subject ; namely,
that Madeira and the adjoining islet of Porto Santo pos-
sess many distinct but representative species of land-
shells, some of which live in crevices of stone ; and
although large quantities of stone are annually transported
from Porto Santo to Madeira, yet this latter island has not
become colonized by the Porto Santo species ; nevertheless,
both islands have been colonized by European land-shells,
which no doubt had some advantage over the indigenous
species. Prom these considerations I think we need not
greatly marvel at the endemic species which inhabit the
several islands of the Galapagos Archipelago not having all
spread from island to island. On the same continent, also,
pre-occupation has probably played an important part in
checking the commingling of the species which inhabit
different districts with nearly the same physical conditions.
Thus, the south-east and south-west corners of Australia
have nearly the same physical conditions, and are united
by continuous land, yet they are inhabited by a vast number
of distinct mammals, birds, and plants ; so it is, according
to Mr. Bates, with the butterflies and other animals inhabit-
ing the great, open, and continuous valley of the Amazons.
The same principle which governs the general character
of the inhabitants of oceanic islands, namely, the relation to
396 RELATIONS OF THE INHABITANTS OF
the source whence colonists could have been most easily
derived, together with their subsequent modification, is of
the widest application throughout nature. We see this on
every mountain-summit, in every lake and marsh. For
alpine species, excepting in as far as the same species have
become widely spread during the Glacial epoch, are related
to those of the surrounding lowlands ; thus we have in South
America, alpine humming-birds, alpine rodents, alpine plants,
etc., all strictly belonging to American forms ; and it is
obvious that a mountain, as it became slowly upheaved, would
be colonized from the surrounding lowlands. So it is with
the inhabitants of lakes and marshes, excepting in so far as
great facility of transport has allowed the same forms to
prevail throughout large portions of the world. We see the
same principle in the character of most of the blind animals
inhabiting the caves of America and of Europe. Other
analogous facts could be given. It will, I believe, be found
universally true, that wherever in two regions, let them be
ever so distant, many closely allied or representative species
occur, there will likewise be found some identical species ;
and wherever many closely allied species occur, there will
be found many forms which some naturalists rank as distinct
species, and others as mere varieties ; these doubtful forms
snowing us the steps in the progress of modification.
The relation between the power and extent of migration
in certain species, either at the present or at some former
period, and the existence at remote points of the world of
closely allied species, is shown in another and more general
way. Mr. Gould remarked to me long ago, that in those
genera of birds which range over the world, many of the
species have very wide ranges. I can hardly doubt that
this rule is generally true, though difficult of proof. Among
mammals, we see it strikingly displayed in bats, and in a
lesser degree in the Felidse and Canidee. WTe see the same
rule in the distribution of butterflies and beetles. So it is
with most of the inhabitants of fresh water, for many of the
genera in the most distinct classes range over the world, and
many of the species have enormous ranges. It is not meant
that all, but that some of the species, have very wide ranges
in the genera which range very widely. Nor is it meant
that the species in such genera have, on an average, a very
wide range ; for this will largely depend on how far the pro-
cess of modification has gone ; for instance, two varieties of
th§ saine species inhabit America and! Ivtfop^ an$ thus th$
ISLANDS TO THOSE OF THE MAINLAND. 397
species has an immense range ; but, if variation were to be
carried a little further, the two varieties would be ranked as
distinct species, and their range would be greatly reduced.
Still less is it meant, that species which have the capacity
of crossing barriers and ranging widely, as in the case of
certain powerfully winged birds, will necessarily range
widely ; for we should never forget that to range widely
implies not only the power of crossing barriers, but the more
important power of being victorious in distant lands in the
struggle for life with foreign associates. But according to
the view that all the species of a genus, though distributed
to the most remote points of the world, are descended from
a single progenitor, we ought to find, and I believe as a gen-
eral rule we do find, that some at least of the species range
very widely.
We should bear in mind that many genera in all classes
are of ancient origin, and the species in this case will have
had ample time for dispersal and subsequent modification.
There is also reason to believe, from geological evidence,
that within each great class the lower organisms change at a
slower rate than the higher ; consequently they will have
had a better chance of ranging widely and of still retaining
the same specific character. This fact, together with that of
the seeds and eggs of most lowly organized forms being very
minute and better fitted for distant transportal, probably
accounts for a law which has long been observed, and which
has lately been discussed by Alph. de Candolle, in regard to
plants ; namely, that the lower any group of organisms
stands, the more widely it ranges.
The relations just discussed — namely, lower organisms
ranging more widely than the higher — some of the species
of widely ranging genera themselves ranging widely — such
facts, as alpine, lacustrine, and marsh productions being gen-
erally related to those which live on the surrounding low
lands and dry lands — the striking relationship between the
inhabitants of islands and those of the nearest mainland —
the still closer relationship of the distinct inhabitants of the
islauds in the same archipelago — are inexplicable on the
ordinary view of the independent creation of each species,
but are explicable if we admit colonization from the nearest
or readiest source, together with the subsequent ^da/ptation.
of the colonists to their new homes,
398 SUMMARY.
SUMMARY OF THE LAST AND PRESENT CHAPTERS.
In these chapters I have endeavored to show that if we
.make due allowance for our ignorance of the full effects of
changes of climate and of the level of the land, which have
certainly occurred within the recent period, and of other
changes which have probably occurred — if we remember
.how ignorant we are with respect to the many curious
means of occasional transport — if we bear in mind, and this
is a very important consideration, how often a species may
have ranged continuously over a wide area, and then have
.become extinct in the intermediate tracts — the difficulty is
not insuperable in believing that all the individuals of the
same species, wherever found, are descended from common
.parents. And we are led to this conclusion, which has been
arrived at by many naturalists under the designation of
single centres of creation, by various general considerations,
more especially from the importance of barriers of all kinds,
and from the analogical distribution of subgenera, genera,
and families.
With respect to distinct species belonging to the same
genus, which on our theory have spread from one parent-
source ; if we make the same allowances as before for our
ignorance, and remember that some forms of life have
changed very slowly, enormous periods of time having been
thus granted for their migration, the difficulties are far from
insuperable ; though in this case, as in that of the indi-
viduals of the same species, they are often great.
, As exemplifying the effects of climatical changes on dis-
tribution, I have attempted to show how important a part
the last Glacial period has played, which affected even the
equatorial regions, and which, during the alternations of
the cold in the north and the south, allowed the productions
of opposite hemispheres to mingle, and left some of them
stranded on the mountain-summits in all parts of the world.
As showing how diversified are the means of occasional
transport, I have discussed at some little length the means
of dispersal of fresh-water productions.
If the difficulties be not insuperable in admitting that in
the long course of time all the individuals of the same
species, and likewise of the several species belonging to the
same genus; have proceeded from some one source j then all
SUMMARY. 399
the grand leading facts of geographical distribution are
explicable on the theory of migration, together with subse-
quent modification and the multiplication of new forms.
We can thus understand the high importance of barriers,
whether of land or water, in not only separating but in
apparently forming the several zoological and botanical
provinces. We can thus understand the concentration of
related species within the same areas; and how it is that
under different latitudes, for instance, in South America, the
inhabitants of the plains and mountains, of the forests,
marshes and deserts, are linked together in so mysterious a
manner, and are likewise linked to the extinct beings which
formerly inhabited the same continent. Bearing in mind
that the mutual relation of organism to organism is of the
highest importance, we can see why two areas, having nearly
the same physical conditions, should often be inhabited by
very different forms of life ; for according to the length of
time which has elapsed since the colonists entered one of the
regions, or both ; according to the nature of the communi-
cation which allowed certain forms and not others to enter,
either in greater or lesser numbers ; according or not as
those which entered happened to come into more or less
direct competition with each other and with the aborigines ;
and according as the immigrants were capable of varying
more or less rapidly, there would ensue in the two or more
regions, independently of their physical conditions, infi-
nitely diversified conditions of life ; there would be an
almost endless amount of organic action and reaction, and
we should find some groups of beings greatly, and some only
: slightly, modified ; some developed in great force, some exist-
ing in scanty numbers — and this we do find in the several
great geographical provinces of the world.
On these same principles we can understand, as I have
endeavored to show, why oceanic islands should have few
inhabitants, but that, of these, a large proportion should be
'•. endemic or peculiar ; and why, in relation to the means of
migration, one group of beings should have all its species
peculiar, and another group, even within the same class,
should have all its species the same with those in an adjoin-
ing quarter of the world. We can see why whole groups of
organisms, as batrachians and terrestrial mammals, should
f be absent from oceanic islands, while the most isolated
islands should possess their own peculiar species of aerial
mammals or bats. We can see why, in islands, there should
400 SUMMARY.
be some relation between the presence of mammals, in a
more or less modified condition, and the depth of the sea
between such islands and the mainland. We can clearly see
why all the inhabitants of an archipelago, though specifically
distinct on the several islets, should be closely related to
each other ; and should likewise be related, but less closely,
to those of the nearest continent, or other source whence
immigrants might have been derived. We can see why, if
there exist very closely allied or representative species in
two areas, however distant from each other, some identical
species will almost always there be found.
As the late Edward Forbes often insisted, there is a strik-
ing parallelism in the laws of life throughout time and space;
the laws governing the succession of forms in past times
being nearly the same with those governing at the present
time the differences in different areas. We see this in many
facts. The endurance of each species and group of species
is continuous in time ; for the apparent exceptions to the
rule are so few that they may fairly be attributed to our not i
having as yet discovered in an intermediate deposit certain I
forms which are absent in it, but which occur both above and j
below : so in space, it certainly is the general rule that the
area inhabited by a single species, or by a group of species,
is continuous, and the exceptions, which are not rare, may,
as I have attempted to show, be accounted for by former i
migrations under different circumstances, or through occa-
sional means of transport, or by the species having become
extinct in the intermediate tracts. Both in time and space I
species and groups of species have their points of maximum I
development. Groups of species, living during the same
period of time, or living within the same area, are often
characterized by trifling features in common, as of sculpture i
or color. In looking to the long succession of past ages, as !
in looking to distant provinces throughout the world, we i
find that species in certain classes differ little from each
other, while those in another class, or only in a different
section of the same order, differ greatly from each other.
In both time and space the lowly organized members of each
class generally change less than the highly organized ; but ;
there are in both cases marked exceptions to the rule.
According to our theory, these several relations throughout i
time and space are intelligible ; for whether we look to the
allied forms, pf J4fe wJaicE have changed during sucpessjre
SUMMARY. 401
ages, or to those which have changed after having migrated
into distant quarters, in both cases they are connected by
the same bond of ordinary generation; in both cases the
laws of variation have been the same, and modifications have
been accumulated by the same means of natural selection.
102 CLASSIFICATION.
CHAPTER XIV.
MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY —
EMBRYOLOGY — RUDIMENTARY ORGANS.
Classification, Groups Subordinate to Groups — Natural System — Rules
and Difficulties in Classification, explained on the Theory of
Descent with Modification — Classification of Varieties — Descent
always used in Classification — Analogical or Adaptive Characters
— Affinities, General, Complex, and Radiating — Extinction sepa-
rates and defines Groups — Morphology, between Members of the
Same Class, between Parts of the Same Individual — Embryology,
Laws of, explained by Variations not supervening at an Early
Age, and being inherited at a Corresponding Age — Rudimentary
Organs, their Origin explained — Summary.
CLASSIFICATION.
From the most remote period in the history of the world,
organic beings have been found to resemble each other in
descending degrees, so that they can be classed in groups
under groups. This classification is not arbitrary like the
grouping of the stars in constellations. The existence of
groups would have been of simple significance, if one group
had been exclusively fitted to inhabit the land, and another
the water ; one to feed on flesh, another on vegetable matter,
and so on ; but the case is widely different, for it is notorious
how commonly members of even the same sub-group have
different habits. In the second and fourth chapters, on
Variation and on Natural Selection, I have attempted to
show that within each country it is the widely ranging, the
much diffused and common, that is the dominant species,
belonging to the larger genera in each class, which vary
most. The varieties, or incipient species, thus produced,
ultimately become converted into new and distinct species ;
and these, on the principle of inheritance, tend to produce
other new and dominant species. Consequently the groups
which are now large, and which generally include many
dominant species, tend to go on increasing in size. I further
attempted to show that from the varying descendants of each
species trying to occupy as many and as different places as
possible in the economy of nature, they constantly tend to
CLASSIFICATION. 403
diverge in character. This latter conclusion is supported by
observing the great diversity of forms, which, in any small
area, come into the closest competition, and by certain facts
in naturalization.
I attempted also to show that there is a steady tendency in
the forms which are increasing in number and diverging
in character, to supplant and exterminate the preceding, less
divergent, and less improved forms. I request the reader to
turn to the diagram illustrating the action, as formerly ex-
plained, of these several principles ; and he will see that
the inevitable result is, that the modified descendants pro-
ceeding from one progenitor become broken up into groups
subordinate to groups. In the diagram each letter on the
uppermost line may represent a genus including several
species ; and the whole of the genera along this upper
line form together one class, for all are descended from
one ancient parent, and, consequently, have inherited some-
thing in common. But the three genera on the left hand
have, on this same principle, much in common, and form a
sub-family distinct from that containing the next two genera
on the right hand, which diverged from a common parent at
the fifth stage of descent. These five genera have also
much in common, though less than when grouped in sub-
families ; and they form a family distinct from that contain-
ing the three genera still farther to the right hand, which
diverged at an earlier period. And all these genera, de-
scended from (A), form an order distinct from the genera
descended from (I). So that we here have many species
descended from a single progenitor, grouped into genera ;
and the genera into sub-families, families, and orders, all
under one great class. The grand fact of the natural sub-
ordination of organic beings in groups under groups, which,
from its familiarity, does not always sufficiently strike us,
is in my judgment thus explained. No doubt organic beings,
like all other objects, can be classed in many ways, either
artificially by single characters, or more naturally by a num-
ber of characters. We know, for instance, that minerals
and the elemental substances can be thus arranged. In this
case there is of course no relation to genealogical succession,
and no cause can at present be assigned for their falling into
groups. But with organic beings the case is different, and
the view above given accords with their natural arrangement
in group under group; and no other explanation has ever
been attempted.
404 CLASSlIlCAfiO^.
Naturalists, as we have seen, try to arrange the species,
genera, and families in each class, on what is called the
Natural System. But what is meant by this system ? Some
authors look at it merely as a scheme for arranging together
those living objects which are most alike, and for separating
those which are most unlike ; or as an artificial method of
enunciating, as briefly as possible, general propositions —
that is, by one sentence to give the characters common, for
instance, to all mammals, by another those common to all
carnivora, by another those common to the dog-genus, and
then, by adding a single sentence, a full description is given
of each kind of dog. The ingenuity and utility of this
system are indisputable. But many naturalists think that
something more is meant by the Natural System ; they be-
lieve that it reveals the plan of the Creator ; but unless it
be specified whether order in time or space, or both, or what
else is meant by the plan of the Creator, it seems to me that
nothing is thus added to our knowledge. Expressions such
as that famous one by Linnaeus, which we often meet with
in a more or less concealed form, namely, that the characters
do not make the genus, but that the genus gives the charac-
ters, seem to imply that some deeper bond is included in our
classifications than mere resemblance. I believe that this
is the case, and that community of descent — the one known
cause of close similarity in organic beings — is the bond,
which, though observed by various degrees of modification,
is partially revealed to us by our classifications.
Let us now consider the rules followed in classification, and
the difficulties which are encountered on the view that classi-
fication either gives some unknown plan of creation, or is
simply a scheme for enunciating general propositions and
of placing together the forms most like each other. It
might have been thought (and was in ancient times thought)
that those parts of the structure which determined the
habits of life, and the general place of each being in the
economy of nature, would be of very high importance in
classification. Nothing can be more false. No one regards
the external similarity of a mouse to a shrew, of a dugong
to a whale, of a whale to a fish, as of any importance.
These resemblances, though so intimately connected with
the whole life of the being, are ranked as merely " adaptive
or analogical characters : " but to the consideration of these
resemblances we shall recur. It may even be given as a
general rule, that the less any part of the organization is
CLASSIFICATION. 405
concerned with special habits, the more important it becomes
for classification. As an instance : Owen, in speaking of
the dugong, says, "The generative organs, being those which
are most remotely related to the habits and food of an ani-
mal, I have always regarded as affording very clear indica-
tions of its true affinities. We are least likely in the
modifications of these organs to mistake a merely adaptive
for an essential character." With plants how remarkable it
is that the organs of vegetation, on which their nutrition
and life depend, are of little signification ; whereas the
organs of reproduction, with their product the seed and
embryo, are of paramount importance ! So again, in for-
merly discussing certain morphological characters which are
not functionally important, we have seen that they are often
of the highest service in classification. This depends on
their constancy throughout many allied groups ; and their
constancy chiefly depends on any slight deviations not hav-
ing been preserved and accumulated by natural selection,
which acts only on serviceable characters.
That the mere physiological importance of an organ does
not determine its classificatory value, is almost proved by
the fact, that in allied groups, in which the same organ, as
we have every reason to suppose, has nearly the same physi-
ological value, its classificatory value is widely different.
No naturalist can have worked long at any group without
being struck with this fact ; and it has been fully acknowl-
edged in the writings of almost every author. It will suffice
to quote the highest authority, Robert Brown, who, in speak-
ing of certain organs in the Proteaceae, says their generic
importance, "like that of all their parts, not only in this,
but, as I apprehend, in every natural family, is very unequal,
and in some cases seems to be entirely lost." Again, in
another work he says, the genera of the Connaraceae " differ
in having one or more ovaria, in the existence or absence of
albumen, in the imbricate or valvular aestivation. Any one
of these characters singly is frequently of more than generic
importance, though here, even when all taken together, they
appear insufficient to separate Cnestis from Connarus." To
give an example among insects : in one great division of the
Hymenoptera, the antennas, as Westwood has remarked, are
most constant in structure ; in another division they differ
much, and the differences are of quite subordinate value in
classification ; yet no one will say that the antennae in these
two divisions of the same order are of unequal physiological
406 CLASSIFICATION.
importance. Any number of instances could be given of the
varying importance for classification of the same important
organ within the same group of beings.
Again, no one will say that rudimentary or atrophied
organs are of high physiological or vital importance ; yet,
undoubtedly, organs in this condition are often of much
value in classification. No one will dispute that the rudi-
mentary teeth in the upper jaws of young ruminants, and
certain rudimentary bones of the leg, are highly serviceable
in exhibiting the close affinity between ruminants and pachy-
derms. Robert Brown has strongly insisted on the fact
that the position of the rudimentary florets is of the high-
est importance in the classification of the grasses.
Numerous instances could be given of characters derived
from parts which must be considered of very trifling physi-
ological importance, but which are universally admitted as
highly serviceable in the definition of whole groups. For
instance, whether or not there is an open passage from the
nostrils to the mouth, the only character, according to Owen,
which absolutely distinguishes fishes and reptiles — the in-
flection of the angle of the lower jaw in Marsupials — the
manner in which the wings of insects are folded — mere
color in certain Algae — mere pubescence on parts of the
flower in grasses — the nature of the dermal covering, as
hair or feathers, in the Vertebrata. If the Ornithorhynchus
had been covered with feathers instead of hair, this external
and trifling character would have been considered by natural-
ists as an important aid in determining the degree of affinity
of this strange creature to birds.
The importance, for classification, of trifling characters,
mainly depends on their being correlated with many other
characters of more or less importance. The value indeed of
an aggregate of characters is very evident in natural history.
Hence, as has often been remarked, a species may depart
from its allies in several characters, both of high physiologi-
cal importance, and of almost universal prevalence, and yet
leave us in no doubt where it should be ranked. Hence,
also, it has been found that a classification founded on any
single character, however important that may be, has always
failed ; for no part of the organization is invariably constant.1
The importance of an aggregate of characters, even when
none are important, alone explains the aphorism enunciated
by Linnaeus, namely, that the characters do not give the genus,
but the genus gives the character ; for this seems founded on
CLASSIFICATION. 407
the appreciation of many trifling points of resemblance, too
slight to be defined. Certain plants belonging to the Mal-
pighiacese bear perfect and degraded flowers ; in the latter,
as A. de Jussieu has remarked, " The greater number of the
characters proper to the species, to the genus, to the family,
to the class, disappear, and thus laugh at our classification."
When Aspicarpa produced in France, during several years,
only these degraded flowers, departing so wonderfully in a
number of the most important points of structure from the
proper type of the order, yet M. Richard sagaciously saw, as
Jussieu observes, that this genus should still be retained
among the Malpighiaeeae. This case well illustrates the
spirit of our classifications.
Practically, when naturalists are at work, they do not
trouble themselves about the physiological value of the
characters which they use in defining a group or in allocat-
ing any particular species. If they find a character nearly
uniform, and common to a great number of forms, and not
common to others, they use it as one of high value ; if com-
mon to some lesser number, they use it as of subordinate
value. This principle has been broadly confessed by some
naturalists to be the true one ; and by none more clearly than
by that excellent botanist, Aug. Saint-Hilaire. If several
trifling characters are always found in combination, though
no apparent bond of connection can be discovered between
them, especial value is set on them. As in most groups of
animals, important organs, such as those for propelling the
blood, or for aerating it, or those for propagating the race,
are found nearly uniform, they are considered as highly ser-
viceable in classification ; but in some groups all these, the
most important vital organs, are found to offer characters of
quite subordinate value. Thus, as Fritz Miiller has lately
remarked, in the same group of crustaceans, Cypridina is
furnished with a heart, while in two closely allied genera,
namely Cypris and Cytherea, there is no such organ ; one
species of Cypridina has well-developed branchiae, while
another species is destitute of them.
We can see why characters derived from the embryo
should be of equal importance with those derived from the
adult, for a natural classification of course includes all ages.
But it is by no means obvious, on the ordinary view, why the
structure of the embryo should be more important for this
purpose than that of the adult, which alone plays its full
part in the economy of nature. Yet it has been strongly
408 CLASSIFICATION.
urged by those great naturalists, Milne Edwards and Agas-
siz, that embryological characters are the most important
of all; and this doctrine has very generally been admitted
as true. Nevertheless, their importance has sometimes been
exaggerated, owing to the adaptive characters of larvae not
having been excluded ; in order to show this, Fritz Mtiller
arranged, by the aid of such characters alone, the great class
of crustaceans, and the arrangement did not prove a natural
one. But there can be no doubt that embryonic, excluding
larval, characters, are of the highest value for classification,
not only with animals but with plants. Thus the main
divisions of flowering plants are founded on differences in
the embryo — on the number and position of the cotyledons,
and on the mode of development of the plumule and radicle.
We shall immediately see why these characters possess
so high a value in classification ; namely, from the natural
system being genealogical in its arrangement.
Our classifications are often plainly influenced by chains
of affinities. Nothing can be easier than to define a number
of characters common to all birds ; but with crustaceans, any
such definition has hitherto been found impossible. There
are crustaceans at the opposite ends of the series, which have
hardly a character in common ; yet the species at both ends,
from being plainly allied to others, and these to others, and
so onward, can be recognized as unequivocally belonging to
this, and to no other class of the Articulata.
Geographical distribution has often been used, though
perhaps not quite logically, in classification, more especially
in very large groups of closely allied forms. Temminck in-
sists on the utility or even necessity of this practice in cer-
tain groups of birds ; and it has been followed by several
entomologists and botanists.
Finally, with respect to the comparative value of the yari-
rious groups of species, such as orders, sub-orders, families,
sub-families, and genera, they seem to be, at least at present,
almost arbitrary. Several of the best botanists, such as
Mr. Bentham, and others, have strongly insisted on their
arbitrary value. Instances could be given among plants and
insects, of a group first ranked by practised naturalists as
only a genus, and then raised to the rank of a sub-family
or family ; and this has been done, not because further
research has detected important structural differences, ^ at
first overlooked, but because numerous allied species, with
slightly different grades of difference, have been subsequently
(iiscovered.
I
CLASSIFICATION. 409
All the foregoing rules and aids and difficulties in classifi-
cation may be explained, if I do not greatly deceive myself,
on the view that the Natural System is founded on descent
with modification — that the characters which naturalists
consider as showing true affinity between any two or more
species, are those which have been inherited from a common
parent, all true classification being genealogical — that com-
munity of descent is the hidden bond which naturalists have
been unconsciously seeking, and not some unknown plan of
creation, or the enunciation of general propositions, and the
mere putting together and separating objects more or less
alike.
But I must explain my meaning more fully. I believe
that the arrangement of the groups within each class, in due
subordination and relation to each other, must be strictly
genealogical in order to be natural ; but that the amount of
difference in the several branches or groups, though allied
in the same degree in blood to their common progenitor, may
differ greatly, being due to the different degrees of modifica-
tion which they have undergone ; and this is expressed by
the forms being ranked under different genera, families, sec-
tions, or orders. The reader will best understand what is
meant if he will take the trouble to refer to the diagram in
the fourth chapter. We will suppose the letters A and L
to represent allied genera existing during the Silurian epoch,
and descended from some still earlier form. In three of these
genera (A, F, and I) a species has transmitted modified
descendants to the present day, represented by the fifteen
genera (au to zu) on the uppermost horizontal line. Now,
all these modified descendants from a single species are
related in blood or descent in the same degree. They may
metaphorically be called cousins to the same millionth degree,
yet they differ widely and in different degrees from each
other. The forms descended from A, now broken up into two
or three families, constitute a distinct order from those de-
scended from I, also broken up into two families. Nor can
the existing species descended from A be ranked in the same,
genus with the parent A, or those from I with the parent I.
But the existing genus f14 may be supposed to have been but
slightly modified, and it will then rank with the parent genus
F, just as some few still living organisms belong to Silurian
genera. So that the comparative value of ^he differences be-
tween these organic beings, which are all related to each other
in the same degree in. blood, has come to be widely different
410 CLASSIFICATION.
Nevertheless, their genealogical arrangement remains strictly
true, not only at the present time, but at each successive
period of descent. All the modified descendants from A will
have inherited something in common from their common
parent, as will all the descendants from I ; so will it be with
each subordinate branch of descendants at each successive
stage. If, however, we suppose any descendant of A or of I to
have become so much modified as to have lost all traces of its
parentage in this case, its place in the natural system will be
lost, as seems to have occurred with some few existing organ-
isms. All the descendants of the genus F, along its whole
line of descent, are supposed to have been but little modified,
and they form a single genus. But this genus, though much
isolated, will still occupy its proper intermediate position.
The representation of the groups, as here given in the dia-
gram on a flat surface, is much too simple. The branches
ought to have diverged in all directions. If the names of
the groups had been simply written down in a linear series,
the representation would have been still less natural ; and it
is notoriously not possible to represent in a series, on a flat
surface, the affinities which we discover in nature among the
beings of the same group. Thus, the natural system is gene-
alogical in its arrangement, like a pedigree. But the amount
of modification which the different groups have undergone
has to be expressed by ranking them under different so-
called genera, sub-families, families, sections, orders, and
classes.
It may be worth while to illustrate this view of classifi-
cation, by taking the case of languages. If we possessed
a perfect pedigree of mankind, a genealogical arrangement
of the races of man would afford the best classification of
the various languages now spoken throughout the world ;
and if all extinct languages, and all intermediate and slowly
changing dialects, were to be included, such an arrangement
would be the only possible one. Yet it might be that some
ancient languages had altered very little and had given rise
to few new languages, while others had altered much, owing
to the spreading, isolation, and state of civilization of the
several co-descended races, and had thus given rise to many
new dialects and languages. The various degrees of differ-
ence between the languages of the same stock would have
to be expressed by groups subordinate to groups ; but the
proper or even the only possible arrangement would still be
genealogical ; and this would be strictly natural, as it would
CLASSIFICATION. 411
connect together all languages, extinct and recent, by the
closest affinities, and would give the filiation and origin of
each tongue.
In confirmation of this view, let us glance at the classifi-
cation of varieties, which are known or believed to be de-
scended from a single species. These are grouped under
the species, with the sub-varieties under the varieties ; and
in some cases, as with the domestic pigeon, with several
other grades of difference. Nearly the same rules are fol-
lowed as in classifying species. Authors have insisted on
the necessity of arranging varieties on a natural instead of
an artificial system ; we are cautioned, for instance, not to
class two varieties of the pineapple together, merely be-
cause their fruit, though the most important part, happens
to be nearly identical ; no one puts the Swedish and com-
mon turnip together, though the esculent and thickened
stems are so similar. Whatever part is found to be most
constant, is used in classing varieties ; thus the great agri-
culturist Marshall says the horns are very useful for this
purpose with cattle, because they are less variable than the
shape or color of the body, etc. ; whereas with sheep the
horns are much less serviceable, because less constant. In
classing varieties, I apprehend that if we had a real pedi-
gree, a genealogical classification would be universally pre-
ferred ; and it has been attempted in some cases. For we
might feel sure, whether there had been more or less modi-
fication, that the principle of inheritance would keep the
forms together which were allied in the greatest number
of points. In tumbler pigeons, though some of the sub-
varieties differ in the important character of the length of
the beak, yet all are kept together from having the common
habit of tumbling ; but the short-faced breed has nearly or
quite lost this habit ; nevertheless, without any thought on
the subject, these tumblers are kept in the same group,
because allied in blood and alike in some other respects.
With species in a state of nature, every naturalist has in
fact brought descent into his classification ; for he includes
in his lowest grade, that of species, the two sexes ; and how
enormously these sometimes differ in the most important
characters is known to every naturalist: scarcely a single
fact can be predicated in common of the adult males and
hermaphrodites of certain cirripedes, and yet no one dreams
of separating them. As soon as the three Orchidean forms,
Monachanthus, Myanthus, and Catasetum, which had pre-
412 CLASSIFICATION.
viously been ranked as three distinct genera, were known to
be sometimes produced on the same plant, they were imme-
diately considered as varieties ; and now I have been able
to show that they are the male, female, and hermaphrodite
forms of the same species. The naturalist includes as one
species the various larval stages of the same individual, how-
ever much they may differ from each other and from the
adult, as well as the so-called alternate generations of
Steenstrup, which can only in a technical sense be con-
sidered as the same individual. He includes monsters and
varieties, not from their partial resemblance to the parent-
form, but because they are descended from it.
As descent has universally been used in classing together
the individuals of the same species, though the males and
females and larvae are sometimes extremely different ; and
as it has been used in classing varieties which have under-
gone a certain, and sometimes a considerable, amount of
modification ; may not this same element of descent have
been unconsciously used in grouping species under genera,
and genera under higher groups, all under the so-called
natural system ? I believe it has been unconsciously used ;
and thus only can I understand the several rules and guides
which have been followed by our best systematists. As we
have no written pedigrees, we are forced to trace community
of descent by resemblances of any kind. Therefore, we
choose those characters which are the least likely to have
been modified, in relation to the conditions of life to which
each species has been recently exposed. Rudimentary
structures on this view are as good, or even sometimes
better than other parts of the organization. We care not
how trifling a character may be — let it be the mere inflec-
tion of the angle of the jaw, the manner in which an
insect's wing is folded, whether the skin be covered by hair
or feathers — if it prevail throughout many and different
species, especially those having very different habits of life,
it assumes high value ; for we can account for its presence
in so many forms with such different habits, only by inherit-
ance from a common parent. We may err in this respect
in regard to single points of structure, but when several
characters, let them be ever so trifling, concur throughout a
large group of beings having different habits, we may feel
almost sure, on the theory of descent, that these characters
have been inherited from a common ancestor ; and we know
that such aggregated characters have especial value in
classification.
ANALOGICAL RESEMBLANCES. 413
We can understand why a species or a group of species
may depart from its allies, in several of its most important
characteristics, and yet be safely classed with them. This
may be safely done, and is often done, as long as a suffi-
cient number of characters, let them be ever so unimpor-
tant, betray the hidden bond of community of descent,
i Let two forms have not a single character in common, yet,
if these extreme forms are connected together by a chain
of intermediate groups, we may at once infer their com-
munity of descent, and we put them all into the same class.
As we find organs of high physiological importance — those
which serve to preserve life under the most diverse condi-
tions of existence — are generally the most constant, we
attach especial value to them ; but if these same organs, in
another group or section of a group, are found to differ
much, we at once value them less in our classification. We
shall presently see why embryological characters are of
such high classificatory importance. Geographical distribu-
tion may sometimes be brought usefully into play in classing
large genera, because all the species of the same genus,
inhabiting any distinct and isolated region, are in all proba-
bility descended from the same parents.
ANALOGICAL RESEMBLANCES.
We can understand, on the above views, the very impor-
tant distinction between real affinities and analogical or
adaptive resemblances. Lamarck first called attention to
this subject, and he has been ably followed by Macleay and
others. The resemblances in the shape of the body and in
the fin-like anterior limbs between dugongs and whales, and
between these two orders of mammals and fishes, are ana-
logical. So is the resemblance between a mouse and a
shrew-mouse (Sorex), which belong to different orders ;
and the still closer resemblance, insisted on by Mr. Mivart,
between the mouse and a small marsupial animal (Ant-
echinus) of Australia. These latter resemblances may be
accounted for, as it seems to me, by adaptation for similarly
active movements through thickets and herbage, together
with concealment from enemies.
Among insects there are innumerable similar instances ;
thus Linnaeus, misled by external appearances, actually
classed an homopterous insect as a moth. We see some-
thing of the same kind even with our domestic varieties, as
414 ANALOGICAL RESEMBLANCES.
in the strikingly similar shape of the body in the improved
breeds of the Chinese and common pig, which are descended
from distinct species ; and in the similarly thickened stems
of the common and specifically distinct Swedish tnrnip. The
resemblance between the greyhound and the race-horse is
hardly more fanciful than the analogies which have been
drawn by some authors between widely different animals.
On the view of characters being of real importance for
classification, only in so far as they reveal descent, we can
clearly understand why analogical or adaptive characters,
although of the utmost importance to the welfare of the
being, are almost valueless to the systematist. For animals,
belonging to two most distinct lines of descent, may have
become adapted to similar conditions, and thus have assumed
a close external resemblance ; but such resemblances will not
reveal — will rather tend to conceal their blood-relationship.
We can thus also understand the apparent paradox, that the
very same characters are analogical when one group is com-
pared with another, but give true affinities when the mem-
bers of the same group are compared together: thus, the
shape of the body and fin-like limbs are only analogical when
whales are compared with fishes, being adaptations in both
classes for swimming through the water ; but between the
several members of the whale family, the shape of the body
and the fin-like limbs offer characters exhibiting true affinity ;
for as these parts are so nearly similar throughout the whole
family, we cannot doubt that they have been inherited from
a common ancestor. So it is with fishes.
Numerous cases could be given of striking resemblances
in quite distinct beings between single parts or organs,
which have been adapted for the same functions. A good
instance is afforded by the close resemblance of the jaws of
the dog and Tasmanian wolf or Thylacinus — animals which
are widely sundered in the natural system. But this resem-
blance is confined to general appearance, as in the prominence
of the canines, and in the cutting shape of the molar teeth.
For the teeth really differ much : thus the dog has on each
side of the upper jaw four pre-molars and only two molars ;
while the Thylacinus has three pre-molars and four molars.
The molars also differ much in the two animals in relative
size and structure. The adult dentition is preceded by a
widely different milk dentition. Any one may, of course,
deny that the teeth in either case have been adapted for
tearing flesh, through the natural selection of successive
ANALOGICAL RESEMBLANCES. 415
rariations ; but if this be admitted in the one case, it is
unintelligible to me that it should be denied in the other.
I am glad to find that so high an authority as Professor
Flower has come to this same conclusion.
The extraordinary cases given in a former chapter, of
widely different fishes possessing electric organs — of widely
different insects possessing luminous organs — and of orchids
and asclepiads having pollen-masses with viscid disks, come
under this same head of analogical resemblances. But these
cases are so wonderful that they were introduced as difficul-
ties or objections to our theory. In all such cases some
fundamental difference in the growth or development of the
parts, and generally in their matured structure, can be
detected. The end gained is the same, but' the means, though
appearing superficially to be the same, are essentially dif-
ferent. The principle formerly alluded to under the term of
analogical variation has probably in these cases often come
into play ; that is, the members of the same class, although
only distantly allied, have inherited so much in common in
their constitution, that they are apt to vary under similar
exciting causes in a similar manner; and this would obvi-
ously aid in the acquirement through natural selection of
parts or organs, strikingly like each other, independently of
their direct inheritance from a common progenitor.
As species belonging to distinct classes have often been
adapted by successive slight modifications to live under
nearly similar circumstances — to inhabit, for instance, the
three elements of land, air, and water — we can perhaps
understand how it is that a numerical parallelism has some-
times been observed between the sub-groups of distinct classes.
A naturalist, struck with a parallelism of this nature, by
arbitrarily raising or sinking the value of the groups in sev-
eral classes (and all our experience shows that their valuation
is as yet arbitrary), could easily extend the parallelism over
a wide range ; and thus the septenary, quinary, quaternary,
and ternary classifications have probably arisen.
There is another and curious class of cases in which close
external resemblance does not depend on adaptation to simi-
lar habits of life, but has been gained for the sake of pro-
tection. I allude to the wonderful manner in which certain
butterflies imitate, as first described by Mr. Bates, other and
quite distinct species. This excellent observer has shown
that in some districts of South America, where, for instance,
an Ithomia abounds in gaudy swarms, another butterfly,
416 ANALOGICAL RESEMBLANCES.
namely, a Leptalis, is often found mingled in the same flock ;
and the latter so closely resembles the Ithomia in every
shade and stripe of color, and even in the shape of its wings,
that Mr. Bates, with his eyes sharpened by collecting during
eleven years, was, though always on his guard, continually
deceived. When the mockers and the mocked are caught
and compared, they are found to be very different in essen-
tial structure, and to belong not only to distinct genera, but
often to distinct families. Had this mimicry occurred in
only one or two instances, it might have been passed over as
a strange coincidence. But, if we proceed from a district
where one Leptalis imitates an Ithomia, another mocking
and mocked species, belonging to the same two genera,
equally close in their resemblance, may be found. Alto-
gether no less than ten genera are enumerated, which include
species that imitate other butterflies. The mockers and
mocked always inhabit the same region ; we never find an
imitator living remote from the form which it imitates. The
mockers are almost invariably rare insects ; the mocked in
almost every case abounds in swarms. In the same district
in which a species of Leptalis closely imitates an Ithomia,
there are sometimes other Lepidoptera mimicking the same
Ithomia : so that in the same place, species of three genera
of butterflies and even a moth are found all closely resem-
bling a butterfly belonging to a fourth genus. It deserves
especial notice, that many of the mimicking forms of the
Leptalis, as well as of the mimicked forms, can be shown
by a graduated series to be merely varieties of the same
species ; while others are undoubtedly distinct species. But
why, it may be asked, are certain forms treated as the mim-
icked and others as the mimickers ? Mr. Bates satisfactorily
answers this question by showing that the form which is
imitated keeps the usual dress of the group to which it
belongs, while the counterfeiters have changed their dress
and do not resemble their nearest allies.
We are next led to inquire what reason can be assigned
for certain butterflies and moths so often assuming the
dress of another and quite distinct form ; why, to the per-
plexity of naturalists, has nature condescended to the tricks
of the stage ? Mr. Bates has, no doubt, hit on the true
explanation. The mocked forms, which always abound in
numbers, must habitually escape destruction to a large extent,
otherwise they could not exist in such swarms ; and a large
amount of evidence has now been collected, showing that
ANALOGICAL RESEMBLANCES. 417
they are distasteful to birds and other insect-devouring ani-
mals. The mocking forms, on the other hand, that inhabit
the same district, are comparatively rare, and belong to rare
groups ; hence, they must suffer habitually from some danger,
for otherwise, from the number of eggs laid by all butter-
flies, they would in three or four generations swarm over the
whole country. Now if a member of one of these persecuted
and rare groups were to assume a dress so like that of a well-
protected species that it continually deceived the practised
eyes of an entomologist, it would often deceive predaceous
birds and insects, and thus often escape destruction. Mr.
Bates may almost be said to have actually witnessed the
process by which the mimickers have come so closely to
resemble the mimicked; for he found that some of the forms
of Leptalis which mimic so many other butterflies, varied in
an extreme degree. In one district several varieties occurred,
and of these one alone resembled, to a certain extent, the
common Ithomia of the same district. In another district
there were two or three varieties, one of which was much
commoner than the others, and this closely mocked another
form of Ithomia. From facts of this nature, Mr. Bates
concludes that the Leptalis first varies ; and when a variety
happens to resemble in some degree any common butterfly
inhabiting the same district, this variety, from its resem-
blance to a flourishing and little persecuted kind, has a
better chance of escaping destruction from predaceous birds
and insects, and is consequently oftener preserved ; " the less
perfect degrees of resemblance being generation after gen-
eration eliminated, and only the others left to propagate
their kind." So that here we have an excellent illustration
of natural selection.
Messrs. Wallace and Trimen have likewise described sev-
eral equally striking cases of imitation in the Lepidoptera
of the Malay Archipelago and Africa, and with some other
insects. Mr. Wallace has also detected one such case with
birds, but we have none with the larger quadrupeds. The
much greater frequency of imitation with insects than with
other animals, is probably the consequence of their small
size ; insects cannot defend themselves, excepting indeed the
kinds furnished with a sting, and I have never heard of an
instance of such kinds mocking other insects, though they
are mocked ; insects cannot easily escape by flight from the
larger animals which prey on them ; therefore, speaking
metaphorically, they are reduced, like most weak creatures,
to trickery and dissimulation^
418 AFFINITIES CONNECTING
It should be observed that the process of imitation prob-
ably never commenced between forms widely dissimilar in
color. But, starting with species already somewhat like each
other, the closest resemblance, if beneficial, could readily be
gained by the above means, and if the imitated form was
subsequently and gradually modified through any agency,
the imitating form would be led along the same track, and
thus be altered to almost any extent, so that it might ulti-
mately assume an appearance or coloring wholly unlike that
of the other members of the family to which it belonged.
There is, however, some difficulty on this head, for it is
necessary to suppose in some cases that ancient members
belonging to several distinct groups, before they had diverged
to their present extent, accidentally resembled a member of
another and protected group in a sufficient degree to afford
some slight protection, this having given the basis for the
subsequent acquisition of the most perfect resemblance.
ON THE NATURE OF THE AFFINITIES CONNECTING ORGANIC
BEINGS.
As the modified descendants of dominant species, belong-
ing to the larger genera, tend to inherit the advantages
which made the groups to which they belong large and their
parents dominant, they are almost sure to spread widely,
and to seize on more and more places in the economy of
nature. The larger and more dominant groups within each
class thus tend to go on increasing in size, and they conse-
quently supplant many smaller and feebler groups. Thus,
we can account for the fact that all organisms, recent and
extinct, are included under a few great orders and under still
fewer classes. As showing how few the higher groups are
in number, and how widely they are spread thoughout the
world, the fact is striking that the discovery of Australia
lias not added an insect belonging to a new class, and that
in the vegetable kingdom, as I learn from Dr. Hooker, it has
added only two or three families of small size.
In the chapter on geological succession I attempted to
show, on the principle of each group having generally
diverged much in character during the long-continued pro-
cess of modification, how it is that the more ancient forms
of life often present characters in some degree intermediate
between existing groups. As some few of the old and inter-
mediate forms have transmitted to the present day descend-
ORGANIC BEINGS. 419
ants but little modified, these constitute our so-called osculant
or aberrant species. The more aberrant any form is, the
greater must be the number of connecting forms which have
been exterminated and utterly lost. And we have evidence
of aberrant groups having suffering severely from extinction,
for they are almost always represented by extremely few
species, and such species as do occur are generally very dis-
tinct from each other, which again implies extinction. The
genera Ornithorhynchus and Lepidosiren, for example, would
not have been less aberrant had each been represented by a
dozen species, instead of as at present by a single one, or by
two or three. We can, I think, account for this fact only
by looking at aberrant groups as forms which have been con-
quered by more successful competitors, with a few members
still preserved under unusually favorable conditions.
Mr. Waterhouse has remarked that when a member be-
longing to one group of animals exhibits an affinity to a
quite distinct group, this affinity in most cases is general
and not special; thus, according to Mr. Waterhouse, of all
Rodents, the bizcacha is most nearly related to Marsupials ;
but in the points in which it approaches this order, its rela-
tions are general, that is, not to any one Marsupial species
more than to another. As these points of affinity are believed
to be real and not merely adaptive, they must be due, in
accordance with our view, to inheritance from a common
progenitor. Therefore, we must suppose either that all
Rodents, including the bizcacha, branched off from some
ancient Marsupial, which will naturally have been more or
less intermediate in character with respect to all existing
Marsupials ; or that both Rodents and Marsupials branched
off from a common progenitor, and that both groups have
since undergone much modification in divergent directions.
On either view we must suppose that the bizcacha has re-
tained, by inheritance, more of the characters of its ancient
progenitor than have other Rodents ; and therefore it will
not be specially related to any one existing Marsupial, but
indirectly to all or nearly all Marsupials, from having par-
tially retained the character of their common progenitor, or
of some early member of the group. On the other hand, of
all Marsupials, as Mr. Waterhouse has remarked, the Phas-
colomys resembles most nearly, not any one species, but the
general order of Rodents. In this case, however, it may be
strongly suspected that the resemblance is only analogical,
owing to the Phascolomys having become adapted to habits
420 AFFINITIES CONNECTING
like those of a Rodent. The elder De Candolle has made
nearly similar observations on the general nature of the affin-
ities of distinct families of plants.
On the principle of the multiplication and gradual diver-
gence in character of the species descended from a common
progenitor, together with their retention by inheritance of
some characters in common, we can understand the exces-
sively complex and radiating affinities by which all the
members of the same family or higher group are connected
together. For the common progenitor of a whole family,
now broken up by extinction into distinct groups and sub-
groups, will have transmitted some of its characters, modified
in various ways and degrees, to all the species ; and they
will consequently be related to each other by circuitous lines
of affinity of various lengths (as may be seen in the diagram
so often referred to), mounting up through many predeces-
sors. As it is difficult to show the blood relationship be-
tween the numerous kindred of any ancient and noble family
even by the aid of a genealogical tree, and almost impossi-
ble to do so without this aid, we can understand the ex-
traordinary difficulty which naturalists have experienced in
describing, without the aid of a diagram, the various affinities
which they perceive between the many living and extinct
members of the same great natural class.
Extinction, as we have seen in the fourth chapter, has
played an important part in defining and widening the inter-
vals between the several groups in each class. We may
thus account for the distinctness of whole classes from each
other — for instance, of birds from all other vertebrate ani-
mals— by the belief that many ancient forms of life have
been utterly lost, through which the early progenitors of birds
were formerly connected with the early progenitors of the
other and at that time less differentiated vertebrate classes.
There has been much less extinction of the forms of life
which once connected fishes with Batrachians. There has
been still less within some whole classes, for instance the
Crustacea, for here the most wonderfully diverse forms are
still linked together by a long and only partially broken
chain of affinities. Extinction has only defined the groups :
it has by no means made them ; for if every form which has
ever lived on this earth were suddenly to reappear, though
it would be quite impossible to give definitions by which
each group could be distinguished, still a natural classifica-
tion, or at least a natural arrangement, would be possible.
ORGANIC BEINGS. 421
We shall see this by turning to the diagram ; the letters, A
to L, may represent "eleven Silurian genera, some of which
have produced large groups of modified descendants, with
every link in each branch and sub-branch still alive ; and the
links not greater than those between existing varieties. In
this case it would be quite impossible to give definitions by
which the several members of the several groups could be
distinguished from their more immediate parents and descend-
ants. Yet the arrangement in the diagram would still hold
good and would be natural ; for, on the principle of inherit-
ance, all the forms descended, for instance, from A, would
have something in common. In a tree we can distinguish
this or that branch, though at the actual fork the two unite
and blend together. We could not, as I have said, define
the several groups ; but we could pick out types, or forms,
representing most of the characters of each group, whether
large or small, and thus give a general idea of the value of
the differences between them. This is what we should be
driven to, if we were ever to succeed in collecting all the
forms in any one class which have lived throughout all time
and space. Assuredly we shall never succeed in making so
perfect a collection : nevertheless, in certain classes, we are
tending toward this end ; and Milne Edwards has lately
insisted, in an able paper, on the high importance of looking
to types, whether or not we can separate and define the
groups to which such types belong.
Finally, we have seen that natural selection, which fol-
lows from the struggle for existence, and which almost
inevitably leads to extinction and divergence of character
in the descendants from any one parent species, explains
that great and universal feature in the affinities of all or-
ganic beings, namely, their subordination in group under
group. We use the element of descent in classing the
individuals of both sexes and of all ages under one species,
although they may have but few characters in common ; we
use descent in classing acknowledged varieties, however
different they may be from their parents ; and I believe
that this element of descent is the hidden bond of connec-
tion which naturalists have sought under the term of the
Natural System. On this idea of the natural system being,
in so far as it has been perfected, genealogical in its
arrangement, with the grades of difference expressed by
the terms genera, families, orders, etc., we can under-
stand the rules which we are compelled to follow in our
422 MORPHOLOGY.
classification. We can understand why we value certain
resemblances far more than others ; why we use rudiment-
ary and useless organs, or others of trifling physiological
importance ; why, in finding the relations between one
group and another, we summarily reject analogical or
adaptive characters, and yet use these same characters
within the limits of the same group. We can clearly see
how it is that all living and extinct forms can be grouped
together within a few great classes ; and how the several
members of each class are connected together by the most
complex and radiating lines of affinities. We shall never,
probably, disentangle the inextricable web of the affinities
between the members of any one class ; but when we have a
distinct object in view, and do not look to some unknown
plan of creation, we may hope to make sure but slow
progress.
Professor Hackel in his "Generelle Morphologie," and
in other works, has recently brought his great knowledge
and abilities to bear on what he calls phylogeny, or the
lines of descent of all organic beings. In drawing up the
several series he trusts chiefly to embryological characters,
but receives aid from homologous and rudimentary organs,
as well as from the successive periods at which the various
forms of life are believed to have first appeared in our
geological formations. He has thus boldly made a great
beginning, and shows us how classification will in the future
be treated.
MORPHOLOGY.
We have seen that the members of the same class, in-
dependently of their habits of life, resemble each other in
the general plan of their organization. This resemblance
is often expressed by the term "unity of type;" or by say-
ing that the several parts and organs in the different species
of the class are homologous. The whole subject is included
under the general term of Morphology. This is one of the
most interesting departments of natural history, and may
almost be said to be its very soul. What can be more curi-
ous than that the hand of a man, formed for grasping, that
of a mole for digging, the leg of the horse, the paddle of the
porpoise, and the wing of the bat, should all be constructed
on the same pattern, and should include similar bones, in
the same relative positions ? How curious it is, to give a
MORPHOLOGY. 428
subordinate though striking instance, that the hind feet of
the kangaroo, which are so well fitted for bounding over
the open plains — those of the climbing, leaf-eating koala,
equally well fitted for grasping the branches of trees —
those of the ground-dwelling, insect or root eating, bandi-
coots— and those of some other Australian marsupials —
should all be constructed on the same extraordinary type,
namely with the bones of the second and third digits
extremely slender and enveloped within the same skin, so
that they appear like a single toe furnished with two claws.
Notwithstanding the similarity of pattern, it is obvious that
the hind feet of these several animals are used for as widely
different purposes as it is possible to conceive. The case is
rendered all the more striking by the American opossums,
which follow nearly the same habits of life as some of their
Australian relatives, having feet constructed on the ordinary
plan. Professor Flower, from whom these statements are
taken, remarks in conclusion : " We may call this conform-
ity to type, without getting much nearer to an explanation
of the phenomenon ; " and he then adds, "but is it not power-
fully suggestive of true relationship, of inheritance from a
common ancestor?"
Geoffroy Saint-Hilaire has strongly insisted on the high
importance of relative position or connection in homolo-
gous parts ; they may differ to almost any extent in form
and size, and yet remain connected together in the same
invariable order. We never find, for instance, the bones of
the arm and forearm, or of the thigh and leg, transposed.
Hence, the same names can be given to the homologous
bones in widely different animals. We see the same great
law in the construction of the mouths of insects : what can
be more different than the immensely long spiral proboscis
of a sphinx-moth, the curious folded one of a bee or bug,
and the great jaws of a beetle ? Yet all these organs, serv-
ing for such widely different purposes, are formed by in-
finitely numerous modifications of an upper lip, mandibles,
and two pairs of maxillae. The same law governs the con-
struction of the mouths and limbs of crustaceans. So it is
with the flowers of plants.
Nothing can be more hopeless than to attempt to explain
this similarity of pattern in members of the same class, by
utility or by the doctrine of final causes. The hopelessness
of the attempt has been expressly admitted by Owen in his
most interesting work on the " Nature of Limbs." On the
424 Morphology.
ordinary view of the independent creation of each being, we
can only say that so it is ; that it has pleased the Creator to
construct all the animals and plants in each great class on a
uniform plan ; but this is not a scientific explanation.
The explanation is to a large extent simple, on the theory
of the selection of successive slight modifications, each being-
profitable m some way to the modified form, but often
affecting by correlation other parts of the organization. In
changes of this nature, there will be little or no tendency to
alter the original pattern, or to transpose the parts. The
bones of a limb might be shortened and flattened to any
extent, becoming at the same time enveloped in thick mem-
brane, so as to serve as a fin ; or a webbed hand might have
all its bones, or certain bones, lengthened to any extent,
with the membrane connecting them increased, so as to
serve as a wing ; yet all these modifications would not tend
to alter the framework of the bones or the relative connec-
tion of the parts. If we suppose that an early progenitor —
the archetype, as it may be called — of all mammals, birds,
and reptiles, had its limbs constructed on the existing
general pattern, for whatever purpose they served, we can
at once perceive the plain signification of the homologous
construction of the limbs throughout the class. So with the
mouths of insects, we have only to suppose that their com-
mon progenitor had an upper lip, mandibles, and two pairs
of maxillae, these parts being perhaps very simple in form ;
and then natural selection will account for the definite
diversity in the structure and functions of the mouths of
insects. Nevertheless, it is conceivable that the general
pattern of an organ might become so much obscured as to be
finally lost, by the reduction and ultimately by the complete
abortion of certain parts, by the fusion of other parts, and by
the doubling or multiplication of others, variations which we
know to be within the limits of possibility. In the paddles
of the gigantic extinct sea-lizards, and in the mouths of
certain suctorial crustaceans, the general pattern seems thus
to have become partially obscured.
There is another and equally curious branch of our sub-
ject ; namely, serial homologies, or the comparison of the
different parts or organs in the same individual, and not of
the same parts or organs in different members of the same
class. Most physiologists believe that the bones of the
skull are homologous — that is, correspond in number and
relative connection — with the elemental parts of a certain
J MORPHOLOGY. 426
number of vertebrae. The anterior and posterior limbs in
all the higher vertebrate classes are plainly homologous. So
it is with the wonderfully complex jaws and legs of crusta-
ceans. It is familiar to almost every one, that in a flower
the relative position of the sepals, petals, stamens, and pistils,
as well as their intimate structure, are intelligible on the
view that they consist of metamorphosed leaves arranged in
a spire. In monstrous plants, we often get direct evidence
of the possibility of one organ being transformed into
another; and we can actually see, during the early or
embryonic stages of development in flowers, as well as in
crustaceans and many other animals, that organs which
when mature become extremely different are at first exactly
alike.
How inexplicable are the cases of serial homologies on
the ordinary view of creation ! Why should the brain be
enclosed in a box composed of such numerous and such ex-
traordinarily shaped pieces of bone, apparently representing
vertebrae ? As Owen has remarked, the benefit derived from
the yielding of the separate pieces in the act of parturition
by mammals, will by no means explain the same construc-
tion in the skulls of birds and reptiles. Why should similar
bones have been created to form the wing and the leg of a
bat, used as they are for such totally different purposes,
namely, flying and walking ? Why should one crustacean,
which has an extremely complex mouth formed of many
parts, consequently always have fewer legs ; or conversely,
those with many legs have simpler mouths ? Why should
the sepals, petals, stamens, and pistils, in each flower, though
fitted for such distinct purposes, be all constructed on the
same pattern ?
On the theory of natural selection, we can, to a certain
extent, answer these questions. We need not here consider
how the bodies of some animals first became divided into
a series of segments, or how they became divided into right
and left sides, with corresponding organs, for such questions
are almost beyond investigation. It is, however, probable
that some serial structures are the result of cells multiply-
ing by division, entailing the multiplication of the parts
developed from such cells. It must suffice for our purpose
to bear in mind that an indefinite repetition of the same
part or organ is the common characteristic, as Owen has re-
marked, of all low or little specialized forms ; therefore the
unknown progenitor of the Yertebrata probably possessed
426 MORPHOLOGY.
many vertebrae ; the unknown progenitor of the Articulata,
many segments ; and the unknown progenitor of flowering
plants, many leaves arranged in one or more spires. We
have also formerly seen that parts many times repeated are
eminently liable to vary, not only in number, but in form.
Consequently such parts, being already present in consider-
able numbers, and being highly variable, would naturally
afford the materials for adaptation to the most different pur-
poses ; yet they would generally retain, through the force of
inheritance, plain traces of their original or fundamental
resemblance. They would retain this resemblance all the
more, as the variations, which afforded the basis for their
subsequent modification through natural selection, would
tend from the first to be similar ; the parts being at an early
stage of growth alike, and being subjected to nearly the
same conditions. Such parts, whether more or less modified,
unless their common origin became wholly obscured, would
be serially homologous.
In the great class of mollusks, though the parts in dis-
tinct species can be shown to be homologous, only a few
serial homologies, such as the valves of Chitons, can be
indicated ; that is, we are seldom enabled to say that one
part is homologous with another part in the same individual.
A-nd we can understand this fact; for in mollusks, even in
the lowest members of the class, we do not find nearly so
much indefinite repetition of any one part as we find in the
other great classes of the animal and vegetable kingdoms.
But morphology is a much more complex subject than it
At first appears, as has lately been well shown in a remark-
able paper by Mr. E. Ray Lankester, who has drawn an im-
portant distinction between certain classes of cases which
have all been equally ranked by naturalists as homologous.
.He proposes to call the structures which resemble each other
in distinct animals, owing to their descent from a common
progenitor with subsequent modification, homogenous ; and
xhe resemblances which cannot thus be accounted for, he
proposes to call homoplastic. For instance, he believes that
the hearts of birds and mammals are as a whole homogenous
— that is, have been derived from a common progenitor;
but that the four cavities of the heart in the two classes are
homoplastic — that is, have been independently developed.
Mr. Lankester also adduces the close resemblance of the
parts on the right and left sides of the body, and in the suc-
cessive segments of the same individual animal ; and here
DEVELOPMENT AND EMBRYOLOGY. 427
we have parts commonly called homologous which bear no
relation to the descent of distinct species from a common
progenitor. Homoplastic structures are the same with those
which I have classed, though in a very imperfect manner, as
analogous modifications or resemblances. Their formation
may be attributed in part to distinct organisms, or to dis-
tinct parts of the same organism, having varied in an analo-
gous manner ; and in part to similar modifications, having
been preserved for the same general purpose or function, of
which many instances have been given.
Naturalists frequently speak of the skull as formed of
metamorphosed vertebrae ; the jaws of crabs as metamor-
phosed legs ; the stamens and pistils in flowers as metamor-
phosed leaves ; but it would in most cases be more correct,
as Professor Huxley has remarked, to speak of both skull
and vertebrae, jaws and legs, etc., as having been metamor-
phosed, not one from the other, as they now exist, but from
some common and simpler element. Most naturalists, how-
ever, use such language only in a metaphorical sense ; they
are far from meaning that during a long course of descent,
primordial organs of any kind — vertebrae in the one case
and legs in the other — have actually been converted into
skulls or jaws. Yet so strong is the appearance of this
having occurred, that naturalists can hardly avoid employing
language having this plain signification. According to the
views here maintained, such language may be used literally ;
and the wonderful fact of the jaws, for instance, of a crab,
retaining numerous characters, which they probably would
have retained through inheritance, if they had really been
metamorphosed from true though extremely simple legs, is
in part explained.
DEVELOPMENT AND EMBRYOLOGY.
This is one of the most important subjects in the whole
round of natural history. The metamorphoses of insects,
with which every one is familiar, are generally effected
abruptly by a few stages ; but the transformations are in
reality numerous and gradual, though concealed. A certain
ephemerous insect (Chloeon) during its development moults,
as shown by Sir J. Lubbock, above twenty times, and each
time undergoes a certain amount of change ; and in this case
we see the act of metamorphosis performed in a primary
and gradual manner. Many insects, and especially certain
428 DEVELOPMENT AND EMBRYOLOGY.
crustaceans, show us what wonderful changes of structure
can be effected during development. Such changes, how-
ever, reach their acme in the so-called alternate generations
of some of the lower animals. It is, for instance, an aston-
ishing fact that a delicate branching coralline, studded with
polypi, and attached to a submarine rock, should produce,
first by budding and then by transverse division, a host of
huge floating jelly-fishes ; and that these should produce
eggs, from which are hatched swimming animalcules, which
attach themselves to rocks and become developed into branch-
ing corallines ; and so on in an endless cycle. The belief in
the essential identity of the process of alternate generation
and of ordinary metamorphosis has been greatly strength-
ened by Wagner's discovery of the larva or maggot of a fly,
namely the Cecidomyia, producing asexually other larvae,
and these others which finally are developed into mature
males and females, propagating their kind in the ordinary
manner by eggs.
It may be worth notice, that when Wagner's remarkable
discovery was first announced, I was asked how was it pos-
sible to account for the larvae of this fly having acquired
the power of asexual reproduction. As long as the case
remained unique, no answer could be given. But already
Grimm has shown that another fly, a Chironomus, repro-
duces itself in nearly the same manner, and he believes that
this occurs frequently in the order. It is the pupa, and not
the larva, of the Chironomus which has this power; and
Grimm further shows that this case, to a certain extent,
" unites that of the Cecidomyia, with the parthenogenesis of
the Coccidae ; " the term parthenogenesis implying that the
mature females of the Coccidae are capable of producing
fertile eggs without the concourse of the male. Certain
animals belonging to several classes are now known to have
the power of ordinary reproduction at an unusually early
age ; and we have only to accelerate parthenogenetic repro-
duction by gradual steps to an earlier and earlier age —
Chironomus showing us an almost exactly intermediate stage,
viz., that of the pupa — and we can perhaps account for the
marvellous case of the Cecidomyia.
It has already been stated that various parts in the same
individual, which are exactly alike during an early embry-
onic period, become widely different and serve for widely
different purposes in the adult state. So again it has been
fhown that general^ tlie embryos, pf the most; distinct sp$-
DEVELOPMENT AND EMBRYOLOGY. 429
cies belonging to the same class are closely similar, but
become, when fully developed, widely dissimilar. A better
proof of this latter fact cannot be given than the statement
by Von Baer that "the embryos of mammalia, of birds,
lizards and snakes, probably also of chelonia, are in the
earliest states exceedingly like one another, both as a whole
and in the mode of development of their parts ; so much so,
in fact, that we can often distinguish the embryos only by
their size. In my possession are two little embryos in spirit,
whose names I have omitted to attach, and at present I am
quite unable to say to what class they belong. They may
be lizards or small birds, or very young mammalia, so com-
plete is the similarity in the mode of formation of the head
and trunk in these animals. The extremities, however, are
still absent in these embryos. But even if they had existed
in the earliest stage of their development we should learn
nothing, for the feet of lizards and mammals, the wings and
feet of birds, no less than the hands and feet of man, all
arise from the same fundamental form." The larvae of most
crustaceans, at corresponding stages of development, closely
resemble each other, however different the adults may be-
come ; and so it is with very many other animals. A trace
of the law of embryonic resemblance occasionally lasts till a
rather late age : thus birds of the same genus, and of allied
genera, often resemble each other in their immature plum-
age ; as we see in the spotted feathers in the young of the
thrush group. In the cat tribe, most of the species when
adult are striped or spotted in lines; and stripes or spots
can be plainly distinguished in the whelp of the lion and the
puma. We occasionally, though rarely, see something of
the same kind in plants ; thus the first leaves of the ulex or
furze, and the first leaves of the phyllodineous acacias, are
pinnate or divided, like the ordinary leaves of the leguini-
nosse.
The points of structure, in which the embryos of widely
different animals within the same class resemble each other,
often have no direct relation to their conditions of existence.
We cannot, for instance, suppose that in the embryos of the
vertebrata the peculiar, loop-like courses of the arteries near
the branchial slits are related to similar conditions — in the
young mammal which is nourished in the womb of its
mother, in the egg of the bird which is hatched in a nest, and
in the spawn of a frog under water. We have no more
reason to believe in such a relation thw we h&Y§ t/9 belie y^
430 DEVELOPMENT AND EMBRYOLOGY.
that the similar bones in the hand of a man, wing of a bat,
and fin of a porpoise, are related to similar conditions of
life. No one supposes that the stripes on the whelp of a
lion, or the spots on the young blackbird, are of any use to
these animals.
The case, however, is different when an animal, during
any part of its embryonic career, is active, and has to pro-
vide for itself. The period of activity may come on earlier
or later in life ; but whenever it comes on, the adaptation of
the larva to its conditions of life is just as perfect and as
beautiful as in the adult animal. In how important a man-
ner this has acted, has recently been well shown by Sir J.
Lubbock in his remarks on the close similarity of the larvae
of some insects belonging to very different orders, and on
the dissimilarity of the larvae of other insects within the
same order, according to their habits of life. Owing to such
adaptations the similarity of the larvae of allied animals is
sometimes greatly obscured; especially when there is a
division of labor during the different stages of development,
as when the same larva has during one stage to search for
food, and during another stage has to search for a place of
attachment. Cases can even be given of the larvae of allied
species, or groups of species, differing more from each other
than do the adults. In most cases, however, the larvae,
though active, still obey, more or less closely, the law of
common embryonic resemblance. Cirripedes afford a good
instance of this ; even the illustrious Cuvier did not perceive
that a barnacle was a crustacean ; but a glance at the larva
shows this in an unmistakable manner. So again the two
main divisions of cirripedes, the pedunculated and sessile,
though differing widely in external appearance, have larvae
in all their stages barely distinguishable.
The embryo in the course of development generally rises
in organization. I use this expression, though I am aware
that it is hardly possible to define clearly what is meant by
organization being higher or lower. But no one probably
will dispute that the butterfly is higher than the caterpillar.
In some cases, however, the mature animal must be consid-
ered as lower in the scale than the larva, as with certain
parasitic crustaceans. To refer once again to cirripedes :
the larvae in the first stage have three pairs of locomotive
organs, a simple single eye, and a probosciformed mouth,
with which they feed largely, for they increase much in
size. In the second stage, answering to the chrysalis stage
DEVELOPMENT AND EMBRYOLOGY. 431
of butterflies, they have six pairs of beautifully constructed
natatory legs, a pair of magnificent compound eyes, and
extremely complex antennae ; but they have a closed and
imperfect mouth and cannot feed: their function at this stage
is, to search out by their well-developed organs of sense, and
to reach by their active powers of swimming, a proper place
on which to become attached and to undergo their final
metamorphosis. When this is completed they are fixed for
life : their legs are now converted into prehensile organs ;
they again obtain a well-constructed mouth ; but they have
no antennae, and their two eyes are now reconverted into a
minute, single, simple eye-spot. In this last and complete
state, cirripedes may be considered as either more highly or
more lowly organized than they were in the larval condition.
But in some genera the larvae become developed into hermaph-
rodites having the ordinary structure, and into what I have
called complemental males ; and in the latter the develop-
ment has assuredly been retrograde, for the male is a mere
sac, which lives for a short time and is destitute of mouth,
stomach, and every other organ of importance, excepting
those for reproduction.
We are so much accustomed to see a difference in struc-
ture between the embryo and the adult, that we are tempted
to look at this difference as in some necessary manner
contingent on growth. But there is no reason why, for
instance, the wing of a bat, or the fin of a porpoise, should
not have been sketched out with all their parts in proper
porportion, as soon as any part became visible. In some
whole groups of animals and in certain members of other
groups this is the case, and the embryo does not at any
period differ widely from the adult : thus Owen has re-
marked, in regard to cuttle-fish, "there is no metamor-
phosis ; the cephalopodic character is manifested long before
the parts of the embryo are completed." Land-shells and
fresh-water crustaceans are born having their proper forms,
while the marine members of the same two great classes
pass through considerable and often great changes dur-
ing their development. Spiders, again, barely undergo any
metamorphosis. The larvae of most insects pass through
a worm-like stage, whether they are active and adapted to
diversified habits, or are inactive from being placed in the
midst of proper nutriment, or from being fed by their
parents ; but in some few cases, as in that of Aphis, if we
look to the admirable drawings of the development of this
432 DEVELOPMENT AXD EMBRYOLOGY.
insect, by Professor Huxley, we see hardly any trace of the
vermiform stage.
Sometimes it is only the earlier developmental stages
which fail. Thus, Fritz Miiller has made the remarkable
discovery that certain shrimp-like crustaceans (allied to
Penoeus) first appear under the simple nauplius-form, and
after passing through two or more zoea-stages, and then
through the mysis-stage, finally acquire their mature struc-
ture : now in the whole great malacostracan order, to which
these crustaceans belong, no other member is as yet known
to be first developed under the nauplius-form, though many
appear as zoeas ; nevertheless Miiller assigns reasons for
his belief, that if there had been no suppression of de-
velopment, all these crustaceans would have appeared as
nauplii.
How, then, can we explain these several facts in embry-
ology— namely, the very general, though not universal,
difference in structure between the embryo and the adult;
the various parts in the same individual embryo, which
ultimately become very unlike, and serve for diverse pur-
poses, being at an early period of growth alike ; the com-
mon, but not invariable, resemblance between the embryos
or larvae of the most distinct species in the same class ; the
embryo often retaining, while within the egg or womb,
structures which are of no service to it, either at that or at
a later period of life ; on the other hand, larvae which have
to provide for their own wants, being perfectly adapted to
the surrounding conditions ; and lastly, the fact of certain
larvae standing higher in the scale of organization than the
mature animal into which they are developed ? I believe
that all these facts can be explained as follows.
It is commonly assumed, perhaps from monstrosities af-
fecting the embryo at a very early period, that slight varia-
tions or individual differences necessarily appear at an
equally early period. We have little evidence on this head,
but what we have certainly points the other way ; for it is
notorious that breeders of cattle, horses, and various fancy
animals, cannot positively tell, until some time after birth,
what will be the merits and demerits of their young animals.
We see this plainly in our own children; we cannot tell
whether a child will be tall or short, or what its precise
features will be. The question is not, at what period of life
each variation may hav^ jueen caused, but at what period the
greets ty$ ^splayed, >Tte Q&j^j may frave a^ed, mi I
DEVELOPMENT AND EMBRYOLOGY. 433
believe often has acted, on one or both parents before the
act of generation. It deserves notice that it is of no
importance to a very young animal, as long as it remains in
its mother's womb or in the egg, or as long as it is nourished
and protected by its parent, whether most of its characters
are acquired a little earlier or later in life. It would not
signify, for instance, to a bird which obtained its food by
having a much-curved beak, whether or not while young it
possessed a beak of this shape, as long as it was fed by its
parents.
I have stated in the first chapter, that at whatever age a
variation first appears in the parent, it tends to reappear
at a corresponding age in the offspring. Certain variations
can only appear at corresponding ages ; for instance, pecul-
iarities in the caterpillar, cocoon, or imago states of the
silk-moth ; or, again, in the full-grown horns of cattle.
But variations which, for all that we can see, might have
first appeared either earlier or later in life, likewise tend to
reappear at a corresponding age in the offspring and parent.
I am far from meaning that this is invariably the case, and I
could give several exceptional cases of variations (taking
the word in the largest sense) which have supervened at an
earlier age in the child than in the parent.
These two principles, namely, that slight variations gen-
erally appear at a not very early period of life, and are
inherited at a corresponding not early period, explain, as I
believe, all the above specified leading facts in embryology.
But first let us look to a few analogous cases in our domes-
tic varieties. Some authors who have written on dogs main-
tain that the greyhound and bull-dog, though so different,
are really closely allied varieties, descended from the same
wild stock, hence I was curious to see how far their puppies
differed from each other. I was told by breeders that they
differed just as much as their parents, and this, judging by
the eye, seemed almost to be the case ; but on actually
measuring the old dogs and their six-days-old puppies, I
found that the puppies had not acquired nearly their full
amount of proportional difference. So, again, I was told
that the foals of cart and race horses — breeds which have
been almost whollv formed bv selection under domestication
— differed as much as the full-grown animals; but having
had careful measurements made of the dams and of three-
days-old colts of race and heavy cart jiorses; J fjRd thajj this
is by m means tb§ case,
434 DEVELOPMENT AND EMBRYOLOGY.
As we have conclusive evidence that the breeds of the
pigeon are descended from a single wild species, I compared
the young within twelve hours after being hatched. I care-
fully measured the proportions (but will not here give
the details) of the beak, width of mouth, length of nostril
and of eyelid, size of feet and length of leg, in the wild
parent species, in pouters, fantails, runts, barbs, dragons,
carriers, and tumblers. Now, some of these birds, when
j nature, differ in so extraordinary a manner in the length
and form of beak, and in other characters, that they would
certainly have been ranked as distinct genera if found in a
state of nature. But when the nestling birds of these
several breeds were placed in a row, though most of them
could just be distinguished, the proportional differences in
the above specified points were incomparably less than in
the full-grown birds. Some characteristic points of differ-
ence— for instance, that of the width of mouth — could
hardly be detected in the young. But there was one re-
markable exception to this rule, for the young of the
short-faced tumbler differed from the young of the wild
rock-pigeon, and of the other breeds, in almost exactly the
same proportions as in the adult stage.
These facts are explained by the above two principles
Fanciers select their dogs, horses, pigeons, etc., for breed-
ing, when nearly grown up. They are indifferent whether
the desired qualities are acquired earlier or later in life, if
the full-grown animal possesses them. And the cases just
given, more especially that of the pigeons, show that the
characteristic differences which have been accumulated by
man's selection, and which give value to his breeds, do not
generally appear at a very early period of life, and are
inherited at a corresponding not early period. But the
case of the short-faced tumbler, which when twelve hours
old possessed its proper characters, proves that this is not
the universal rule ; for here the characteristic differences
must either have appeared at an earlier period than usual]
or, if not so, the differences must have been inherited, not at
a corresponding, but at an earlier, age.
Now, let us apply these two principles to species in a state
of nature. Let us take a group of birds, descended from
some ancient form and modified through natural selection for
different habits. Then, from the many slight successive
.variations having supervened in the several species at a not
early age, and having been inherited at a corresponding age,
DEVELOPMENT AND EMBRYOLOGY. 435
the young will have been but little modified, and they will
still resemble each other much more closely than do the
adults, just as we have seen with the breeds of the pigeon.
We may extend this view to widely distinct structures and
to whole classes. The fore limbs, for instance, which once
served as legs to a remote progenitor, may have become,
through a long course of modification, adapted in one de-
scendant to act as hands, in another as paddles, in another as
wings ; but on the above two principles the fore limbs will
not have been much modified in the embryos of these sev-
eral forms ; although in each form the fore limb will differ
greatly in the adult state. Whatever influence long con-
tinued use or disuse may have had in modifying the limbs
or other parts of any species, this will chiefly or solely have
affected it when nearly mature, when it was compelled to
use its full powers to gain its own living ; and the effects
thus produced will have been transmitted to the offspring at
a corresponding nearly mature age. Thus the young will
not be modified, or will be modified only in a slight degree,
through the effects of the increased use or disuse of parts.
With some animals the successive variations may have
supervened at a very early period of life, or the steps may
have been inherited at an earlier age than that at which they
first occurred. In either of these cases, the young or dub^o
will closely resemble the mature parent-form, as we have
seen with the short-faced tumbler. And this is the rule of
development in certain whole groups, or in certain sub-groups
alone, as with cuttle-fish, land-shells, fresh-water crustaceans,
spiders, and some member of the great class of insects. With
respect to the final cause of the young in such groups not
passing through any^ metamorphosis, we can see that this
would follow from the following contingencies : namely,
from the young having to provide at a very early age for
their own wants, and from their following the same habits
of life with their parents ; for in this case it would be indis-
pensable for their existence that they should be modified in
the same manner as their parents. Again, with respect to
the singular fact that many terrestrial and fresh-water ani-
mals do not undergo any metamorphosis, while marine mem-
bers of the same groups pass through various transformations,
Fritz Miiller has suggested that the process of slowly modi-
fying and adapting an animal to live on the land or in fresh
water, instead of in the sea, would be greatly simplified by
its not passing through any larval stage ; for it is not prob-
43(3 DEVELOPMENT AND EMBRYOLOGY.
able that places well adapted for both the larval and mature
stages, under such new and greatly changed habits of life,
would commonly be found unoccupied or ill-occupied by other
organisms. In this case the gradual acquirement at an earlier
and earlier age of the adult structure would be favored by
natural selection ; and all traces of former metamorphoses
would finally be lost.
If, on the other hand, it profited the young of an animal
to follow habits of life slightly different from those of the
parent-form, and consequently to be constructed on a slightly
different plan, or if it profited a larva already different from
its parent to change still further, then, on the principle of
inheritance at corresponding ages, the young or the larvae
might be rendered by natural selection more and more dif-
ferent from their parents to any conceivable extent. Differ-
ences in the larva might, also, become correlated with
successive stages of its development ; so that the larva, in
the first stage, might come to differ greatly from the larva
in the second stage, as is the case with many animals. The
adult might also become fitted for sites or habits, in which
organs of locomotion or of the senses, etc., would be useless ;
and in this case the metamorphosis would be retrograde.
From the remarks just made we can see how by changes
of structure in the young, in conformity with changed habits
of life, together with inheritance at corresponding ages, ani-
mals might come to pass through stages of development,
perfectly distinct from the primordial condition of their adult
progenitors. Most of our best authorities are now convinced
that the various larval and pupal stages of insects have thus
been acquired through adaptation, and not through inheritance
from some ancient form. The curious case of Sitaris — a
beetle which passes through certain unusual stages of devel-
opment— will illustrate how this might occur. The first
larval form is described by M. Fabre, as an active, minute
insect, furnished with six legs, two long antennae, and four
eyes. These larvae are hatched in the nests of bees ; and
when the male bees emerge from their burrows, in the spring,
which they do before the females, the larvae spring on them,
and afterward crawl on to the females while paired with the
males. As soon as the female bee deposits her eggs on the
surface of the honey stored in the cells, the larvae of the Sit-
aris leap on the eggs and devour them. Afterward they
undergo a complete change ; their eyes disappear ; their
legs and antennae become rudimentary, and they feed on
DEVELOPMENT* AND EMMtfOLOGY. 43?
honey ; so that they now more closely resemble the ordinary
larvae of insects ; ultimately they undergo a further transfor-
mation, and finally emerge as the perfect beetle. Now, if an
insect, undergoing transformations like those of the Sitaris,
were to become the progenitor of a whole new class of in-
sects, the course of development of the new class would be
widely different from that of our existing insects ; and the
first larval stage certainly would not represent the former
condition of any adult and ancient form.
On the other hand, it is highly probable that with many
animals the embryonic or larval stages show us, more or less
completely, the condition of the progenitor of the whole
group in its adult state. In the great class of the Crustacea,
forms wonderfully distinct from each other, namely, suctorial
parasites, cirripedes, entomostraca, and even the malacostraca,
appear at first as larvae under the naupliusform ; and as these
larvae live and feed in the open sea, and are not adapted for
any peculiar habits of life, and from other reasons assigned
by Fritz Miiller, it is probable that at some very remote
period an independent adult animal, resembling the Nauplius,
existed, and subsequently produced, along several divergent
lines of descent, the above-named great Crustacean groups.
So again, it is probable, from what we know of the embryos
of mammals, birds, fishes, and reptiles, that these animals are
the modified descendants of some ancient progenitor, which
was furnished in its adult state with branchiae, a swim-bladder,
four fin-like limbs, and a long tail, all fitted for an aquatic
life.
As all the organic beings, extinct and recent, which have
ever lived, can be arranged within a few great classes ; and as
all within each class have, according to our theory, been con-
nected together by fine gradations, the best, and, if our col-
lections were nearly perfect, the only possible arrangement
would be genealogical ; descent being the hidden bond of
connection which naturalists have been seeking under the
term of the Natural System. On this view we can under-
stand how it is that, in the eyes of most naturalists, the
structure of the embryo is even more important for classifi-
cation than that of the adult. In two or more groups of
animals, however much they may differ from each other in
structure and habits in their adult condition, if they pass
through closely similar embryonic stages, we may feel assured
that they all are descended from one parent-form, and are
therefore closely related. Thus, community in embryonic
438 DEVELOPMENT AND EMBRYOLOGY]
structure reveals community of descent ; but dissimilarity in
embryonic development does not prove discommunity of
descent, for in one of two groups the developmental stages
may have been suppressed, or may have been so greatly modi-
fied through adaptation to new habits of life as to be no
longer recognizable. Even in groups in which the adults
have been modified to an extreme degree, community of
origin is often revealed by the structure of the larvae ; we
have seen, for instance, that cirripedes, though externally so
like shell-fish, are at once known by their larvae to belong
to the great class of crustaceans. As the embryo often shows
us more or less plainly the structure of the less modified and
ancient progenitor of the group, we can see why ancient
and extinct forms so often resemble in their adult state the
embryos of existing species of the same class. Agassiz be-
lieves this to be a universal law of nature ; and we may hope
hereafter to see the law proved true. It can, however, be
proved true only in those cases in which the ancient state of
the progenitor of the group has not been wholly obliterated,
either by successive variations having supervened at a very
early period of growth, or by such variations having been
inherited at an earlier age than that at which they first ap-
peared. It should also be borne in mind, that the law may
be true, but yet, owing to the geological record not extending
far enough back in time, may remain for a long period, or for-
ever, incapable of demonstration. The law will not strictly
hold good in those cases in which an ancient form became
adapted in its larvae state to some special line of life, and
transmitted the same larval state to a whole group of descend-
ants ; for such larval will not resemble any still more ancient
form in its adult state.
Thus, as it seems to me, the leading facts in embryology,
which are second to none in importance, are explained on
the principle of variations in the many descendants from
some one ancient progenitor, having appeared at a not very
early period of life, and having been inherited at a corre-
sponding period. Embryology rises greatly in interest,
when we look at the embryo as a picture, more or less ob-
scured, of the progenitor, either in its adult or larval state,
of all the members of the same great class.
RUDIMEMTARY AND ABORTED ORGANS. 439
RUDIMENTARY, ATROPHIED, AND ABORTED ORGANS.
Organs or parts in this strange condition, bearing the
plain stamp of inutility, are extremely common, or even
general, throughout nature. It would be impossible to name
one of the higher animals in which some part or other is
not in a rudimentary condition. In the mammalia, for
instance, the males possess rudimentary mammae ; in snakes
one lobe of the lungs is rudimentary; in birds the "bastard-
wing " may safely be considered as a rudimentary digit, and
in some species the whole wing is so far rudimentary that
it cannot be used for flight. What can be more curious than
the presence of teeth in fcetal whales, which when grown up
have not a tooth in their heads ; or the teeth, which never
cut through the gums, in the upper jaws of unborn calves ?
Rudimentary organs plainly declare their origin and mean-
ing in various ways. There are beetles belonging to closely
allied species, or even to the same identical species, which
have either full-sized and perfect wings, or mere rudiments
of membrane, which not rarely lie under wing-covers firmly
soldered together ; and in these cases it is impossible to
doubt that the rudiments represent wings. Rudimentary
organs sometimes retain their potentiality : this occasion-
ally occurs with the mammae of male mammals, which have
been known to become well developed and to secrete milk.
So again in the udders in the genus Bos, there are normally
four developed and two rudimentary teats ; but the latter in
our domestic cows sometimes become well developed and
yield milk. In regard to plants, the petals are sometimes
rudimentary, and sometimes well developed in the individ-
uals of the same species. In certain plants having separated
sexes, Kolreuter found that by crossing a species, in which
the male flowers included a rudiment of a pistil, with an
hermaphrodite species, having of course a well-developed
pistil, the rudiment in the hybrid offspring was much
increased in size ; and this clearly shows that the rudimen-
tary and perfect pistils are essentially alike in nature. An
animal may possess various parts in a perfect state, and yet
they may in one sense be rudimentary, for they are useless :
thus the tadpole of the common salamander or water-newt,
as Mr. G. H. Lewes remarks, "has gills, and passes its
existence in the water; but the Salamandra atra, which
lives high up among the mountains, brings forth its young
440 RUDIMENTARY, ATROPHIED,
fall-formed. This animal never lives in the water. Yet if
we open a gravid female, we find tadpoles inside her with
exquisitely feathered gills ; and when placed in water they
swim about like the tadpoles of the water-newt. Obviously
this aquatic organization has no reference to the future life
of the animal, nor has it any adaptation to its embryonic
condition ; it has solely reference to ancestral adaptations, it
repeats a phase in the development of its progenitors."
An organ, serving for two purposes, may become rudi-
mentary or utterly aborted for one, even the more important
purpose, and remain perfectly efficient for the other. Thus,
in plants, the office of the pistil is to allow the pollen tubes
to reach the ovules within the ovarium. The pistil consists
of a stigma supported on a style ; but in some Compositse,
the male florets, which of course cannot be fecundated, have
a rudimentary pistil, for it is not crowned with a stigma;
but the style remains well developed and is clothed in the
usual manner with hairs, which serve to brush the pollen
out of the surrounding and conjoined anthers. Again, an
organ may become rudimentary for its proper purpose, and
be used for a distinct one : in certain fishes the swim-bladder
seems to be rudimentary for its proper function of giving
buoyancy, but has become converted into a nascent beathing
organ or lung. Many similar instances could be given.
Useful organs, however little they may be developed,
unless we have reason to suppose that they were formerly
more highly developed, ought not to be considered as rudi-
mentary. They may be in a nascent condition, and in pro-
gress toward further development. Rudimentary organs,
on the other hand, are either quite useless, such as teeth
which never cut through the gums, or almost useless, such
as the wings of an ostrich, which serve merely as sails. As
organs in this condition would formerly, when still less
developed, have been of even less use than at present, they
cannot formerly have been produced through variation and
natural selection, which acts solely by the preservation of
useful modifications. They have been partially retained by
the power of inheritance, and relate to a former state of
things. It is, however, often difficult to distinguish between
rudimentary and nascent organs ; for we can judge only by
analogy whether a part is capable of further development,
in which case alone it deserves to be called nascent. Organs
in this condition will always be somewhat rare ; for beings
thus provided will commonly have been supplanted by thei?
AND ABORTED ORGANS. 441
successors with the same organ in a more perfect state, and
consequently will have become long ago extinct. The wing
of the penguin is of high service, acting as a fin ; it may,
therefore, represent the nascent state of the wing ; not that
I believe this to be the case ; it is more probably a reduced
organ, modified for a new function ; the wing of the Apteryx,
on the other hand, is quite useless, and is truly rudimentary.
Owen considers the simple filamentary limbs of the Lepido-
siren as the " beginnings of organs which attain full func-
tional development in higher vertebrates ; " but, according
to the view lately advocated by Dr. Giinther, they are prob-
ably remnants, consisting of the persistent axis of a fin,
with the lateral rays or branches aborted. The mammary
glands of the Ornithorhynchus may be considered, in com-
parison with the udders of a cow, as in a nascent condition.
The ovigerous frena of certain cirripedes, which have ceased
to give attachment to the ova and are feebly developed, are
nascent branchiae.
Rudimentary organs in the individuals of the same species
are very liable to vary in the degree of their development
and in other respects. In closely allied species, also, the
extent to which the same organ has been reduced occasion-
ally differs much. This latter fact is well exemplified in
the state of the wings of female moths belonging to the
same family. Rudimentary organs may be utterly aborted ;
and this implies, that in certain animals or plants, parts are
entirely absent which analogy would lead us to expect to
find in them, and which are occasionally found in monstrous
individuals. Thus in most of the Scrophulariacea? the fifth
stamen is utterly aborted ; yet we may conclude that a fifth
stamen once existed, for a rudiment of it is found in many
species of the family, and this rudiment occasionally becomes
perfectly developed, as may sometimes be seen in the com-
mon snap-dragon. In tracing the homologies of any part in
different members of the same class, nothing is more common,
or, in order fully to understand the relations of the parts,
more useful than the discovery of rudiments. This is well
shown in the drawings given by Owen of the leg bones of
the horse, ox and rhinoceros.
It is an important fact that rudimentary organs, such as
teeth in the upper jaws of whales and ruminants, can often
be detected in the embryo, but afterward wholly disappear.
It is also, I believe, a universal rule, that a rudimentary
part is of greater size in the embryo relatively to tfc§
442 RUDIMENTARY, ATROPHIED,
adjoining parts, than in the adult; so that the organ at
this early age is less rudimentary, or even cannot be said
to be in any degree rudimentary. Hence rudimentary organs
in the adult are often said to have retained their embryonic
condition.
I have now given the leading facts with respect to rudi-
mentary organs. In reflecting on them, every one must be
struck with astonishment ; for the same reasoning power
which tells us that most parts and organs are exquisitely
adapted for certain purposes, tells us with equal plainness
that these rudimentary or atrophied organs are imperfect
and useless. In works on natural history, rudimentary
organs are generally said to have been created "for the
sake of symmetry," or in order " to complete the scheme
of nature." But this is not an explanation, merely a re-
statement of the fact. Nor is it consistent with itself :
thus the boa-constrictor has rudiments of hind limbs and
of a pelvis, and if it be said that these bones have been re-
tained " to complete the scheme of nature," why, as Pro-
fessor Weismann asks, have they not been retained by other
snakes, which do not possess even a vestige of these same
bones ? What would be thought of an astronomer who
maintained that the satellites revolve in elliptic courses
round their planets "for the sake of symmetry," because
the planets thus revolve round the sun ? An eminent
physiologist accounts for the presence of rudimentary
organs, by supposing that they serve to excrete matter in
excess, or matter injurious to the system ; but can we sup-
pose that the minute papilla, which often represents the
pistil in male flowers, and which is formed of mere cellular
tissue, can thus act ? Can we suppose that rudimentary
teeth, which are subsequently absorbed, are beneficial to
the rapidly growing embryonic calf by removing matter so
precious as phosphate of lime ? When a man's fingers have
been amputated, imperfect nails have been known to appear
on the stumps, and I could as soon believe that these
vestiges of nails are developed in order to excrete horny
matter, as that the rudimentary nails on the fin of the
manatee have been developed for this same purpose.
On the view of descent with modification, the origin of
rudimentary organs is comparatively simple ; and we can
understand to a large extent the laws governing theii
imperfect development. We have plenty of cases of rudi-
mentary organs in our domestic productions, as the stump
AND ABORTED ORGANS. 443
of a tail in tailless breeds, the vestige of an ear in ear-
less breeds, of sheep — the re-appearance of minute dan-
gling horns in hornless breeds of cattle, more especially,
according to Youatt, in young animals — and the state of
the whole flower in the cauliflower. We often see rudi-
merits of various parts in monsters ; but I doubt whether
any of these cases throw light on the origin of rudimentary
organs in a state of nature, further than by showing that
rudiments can be produced ; for the balance of evidence
clearly indicates that species under nature do not undergo
great and abrupt changes. But we learn from the study of
our domestic productions that the disuse of parts leads to
their reduced size ; and that the result is inherited.
It appears probable that disuse has been the main agent
in rendering organs rudimentary. It would at first lead by
slow steps to the more and more complete reduction of a
part, until at last it became rudimentary — as in the case of
the eyes of animals inhabiting dark caverns, and of the
wings of birds inhabiting oceanic islands, which have sel-
dom been forced by beasts of pray to take flight, and have
ultimately lost the power of flying. Again, an organ,
useful under certain conditions, might become injurious
under others, as with the wings of beetles living on small
and exposed islands ; and in this case natural selection will
have aided in reducing the organ, until it was rendered
harmless and rudimentary.
Any change in structure and function, which can be
effected by small stages, is within the power of natural
selection ; so that an organ rendered, through changed
habits of life, useless or injurious for one purpose, might
be modified and used for another purpose. An organ,
might, also, be retained for one alone of its former func-
tions. Organs, originally formed by the aid of natural
selection, when rendered useless may well be variable, for
their variations can no longer be checked by natural selec-
tion. All this agrees well with what we see under nature.
Moreover, at whatever period of life either disuse or selection
reduces an organ, and this will generally be when the being
has come to maturity and has to exert its full powers of
action, the principle of inheritance at corresponding ages
will tend to reproduce the organ in its reduced state at the
same mature age, but will seldom affect it in the embryo.
Thus we can understand the greater size of rudimentary
organs in the embryo relatively to the adjoining parts, and
444 RUDIMENTARY, ATROPHIED
their lesser relative size in the adult, it, ior instance, the
digit of an adult animal was used less and less during
many generations, owing to some change of habits, or if an
organ or gland was less and less functionally exercised, we
may infer that it would become reduced in size in the
adult descendants of this animal, but would retain nearly its
original standard of development in the embryo.
There remains, however, this difficulty. After an organ
has ceased being used, and has become in consequence much
reduced, how can it be still further reduced in size until the
merest vestige is left ; and how can it be finally quite obliter-
ated ? It is scarcely possible that disuse can go on produ-
cing any further effect after the organ has once been rendered
functionless. Some additional explanation is here requisite
which I cannot give. If, for instance, it could be proved
that every part of the organization tends to vary in a greater
degree toward diminution than toward augmentation of size,
then we should be able to understand how an organ which
has become useless would be rendered, independently of the
effects of disuse, rudimentary, and would at last be wholly
suppressed ; for the variations toward diminished size would
no longer be checked by natural selection. The principle of
the economy of growth, explained in a former chapter, by
which the materials forming any part, if not useful to the
possessor, are saved as far as is possible, will perhaps come
into play in rendering a useless part rudimentary. But this
principle will almost necessarily be confined to the earlier
stages of the process of reduction ; for we cannot suppose
that a minute papilla, for instance, representing in a male
flower the pistil of the female flower, and formed merely of
cellular tissue, would be further reduced or absorbed for the
sake of economizing nutriment.
Finally, as rudimentary organs, by whatever steps they
may have been degraded into their present useless condition,
are the record of a former state of things, and have been
retained solely through the power of inheritance — we can
understand, on the genealogical view of classification, how
it is that systematists, in placing organisms in their proper
places in the natural system, have often found rudimentary
parts as useful as, or even sometimes more useful than, parts
of high physiological importance. Rudimentary organs may
be compared with the letters in a word, still retained in the
spelling, but become useless in the pronunciation, but which
serve as a clew for its derivation. On the view of descent
AtfD ABO£T££> ORGANS. 44T
with modification, we may conclude that tne existence of
organs in a rudimentary, imperfect, and useless condition, or
quite aborted, far from presenting a strange difficulty, as they
assuredly do on the old doctrine of creation, might even have
been anticipated in accordance with the views here explained.
SUMMARY.
In this chapter I have attempted to show that the arrange-
ment of all organic beings throughout all time in groups under
groups — that the nature of the relationships by which all
living and extinct organisms are united by complex, radiat-
ing, and circuituous lines of affinities into a few grand classes
— the rules followed and the difficulties encountered by nat-
uralists in their classifications — the value set upon charac-
ters, if constant and prevalent, whether of high or of the
most trifling importance, or, as with rudimentary organs, of
no importance — the wide opposition in value between ana-
logical or adaptive characters, and characters of true affinity ;
and other such rules ; — all naturally follow if we admit the
common parentage of allied forms, together with their modi-
fication through variation and natural selection, with the
contingencies of extinction and divergence of character. In
considering this view of classification, it should be borne in
mind that the element of descent has been universally used
in ranking together the sexes, ages, dimorphic forms, and
acknowledged varieties of the same species, however much
they may differ from each other in structure. If we extend
the use of this element of descent — the one certainly known
cause of similarity in organic beings — we shall understand
what is meant by the Natural System : it is genealogical in
its attempted arrangement, with the grades of acquired
difference marked by the terms, varieties, species, genera,
families, orders, and classes.
On this same view of descent with modification, most of
the great facts in Morphology become intelligible — whether
we look to the same pattern displayed by the different
species of the same class in their homologous organs, to
whatever purpose applied ; or to the serial and lateral
homologies in each individual animal and plant.
On the principle of successive slight variations, not neces-
sarily or generally supervening at a very early period of
life, and being inherited at a corresponding period, we can
understand the leading facts in embryology; namely, the
446 SUMMARY.
close resemblance in the individual embryo of the parts
which are homologous, and which when matured become
widely different in structure and function ; and the resem-
blance of the homologous parts or organs in allied though
distinct species, though fitted in the adult state for habits as
different as is possible. Larvae are active embryos, which
have been specially modified in a greater or less degree in
relation to their habits of life, with their modifications in-
herited at a corresponding early age. On these same prin-
ciples, and bearing in mind that when organs are reduced in
size, either from disuse or through natural selection, it will
generally be at that period of life when the being has to
provide for its own wants, and bearing in mind how strong
is the force of inheritance — the occurrence of rudimentary,
organs might even have been anticipated. The importance
of embryological characters and of rudimentary organs in
classification is intelligible, on the view that a natural
arrangement must be genealogical.
Finally, the several classes of facts which have been con-
sidered in this chapter, seem to me to proclaim so plainly,'
that the innumerable species, genera, and families, with
which this world is peopled, are all descended, each within
its own class or group, from common parents, and have all
been modified in the course of descent, that I should with-
out hesitation adopt this view, even if it were unsupported
by other facts or arguments.
RECAPITULATION. 447
CHAPTER XV.
RECAPITULATION AND CONCLUSION.
Recapitulation of the Objections to the Theory of Natural Selection —
Recapitulation of the General and Special Circumstances in its
Favor — Causes of the General Belief in the Immutability of Species
— How far the Theory of Natural Selection may be extended —
Effects of its Adoption on the Study of Natural History — Con-
cluding Remarks.
As this whole volume is one long argument, it may be
convenient to the reader to have the leading facts and
inferences briefly recapitulated.
That many and serious objections may be advanced against
the theory of descent with modification through variation
and natural selection, I do not deny. I have endeavored to
give to them their full force. Nothing at first can appear
more difficult to believe than that the more complex organs
and instincts have been perfected, not by means superior to,
though analogous with, human reason, but by the accumula-
tion of innumerable slight variations, each good for the
individual possessor. Nevertheless, this difficulty, though
appearing to our imagination insuperably great, cannot be
considered real if we admit the following propositions,
namely, that all parts of the organization and instincts offer,
at least, individual differences — that there is a struggle
for existence leading to the preservation of profitable devia-
tions of structure or instinct — and, lastly, that gradations
in the state of perfection of each organ may have existed,
each good of its kind. The truth of these propositions can-
not, I think, be disputed.
It is, no doubt, extremely difficult even to conjecture by
what gradations many structures have been perfected, more
especially among broken and failing groups of organic
beings, which have suffered much extinction; but we see
so many strange gradations in nature, that we ought to be
extremely cautious in saying that any organ or instinct, or
any whole structure, could not have arrived at its present
state by many graduated steps. There are, it must be
admitted* cases of special difficulty opposed to the theory of
448 ftECAHfULATIOtf.
natural selection : and one of the most curious of these is
the existence in the same community of two or three
denned castes of workers or sterile female ants ; but I have
attempted to show how these difficulties can be mastered.
With respect to the almost universal sterility of species
when first crossed, which forms so remarkable a contrast
with the almost universal fertility of varieties when crossed,
I must refer the reader to the recapitulation of the facts
given at the end of the ninth chapter, which seem to me
conclusively to show that this sterility is no more a special
endowment than is the incapacity of two distinct kinds of
trees to be grafted together; but that it is incidental on
differences confined to the reproductive systems of the inter-
crossed species. We see the truth of this conclusion in the
vast difference in the results of crossing the same two species
reciprocally — that is, when one species is first used as the
father and then as the mother. Analogy from the consider-
ation of dimorphic and trimorphic plants clearly leads to
the same conclusion, for when the forms are illegitimately
united, they yield few or no seed, and their offspring are
more or less sterile ; and these forms belong to the same
undoubted species, and differ from each other in no respect
except in their reproductive organs and functions.
Although the fertility of varieties when intercrossed, and
of their mongrel offspring, has been asserted by so many
authors to be universal, this cannot be considered as quite
correct after the facts given on the high authority of Gart-
ner and Kolreuter. Most of the varieties which have been
experimented on have been produced under domestication ;
and as domestication (I do not mean mere confinement)
almost certainly tends to eliminate that sterility which,
judging from analogy, would have affected the parent-species
if intercrossed, we ought not to expect that domestication
would likewise induce sterility in their modified descendants
when crossed. This elimination of sterility apparently fol-
lows from the same cause which allows our domestic animals
to breed freely under diversified circumstances ; and this
again apparently follows from their having been gradually
accustomed to frequent changes in their conditions of life.
A double and parallel series of facts seems to throw much
light on the sterility of species, when first crossed, and of
their hybrid offspring. On the one side, there is good reason
to believe that slight changes in the conditions cf life give
vigor and fertility to ail organic beings. We know also
RECAPITULATION. 449
»>
that a cross between the distinct individuals of the same
variety, and between distinct varieties, increases the number
of their offspring, and certainly gives to them increased size
and vigor. This is chiefly owing to the forms which are
crossed having been exposed to somewhat different condi-
tions of life ; for I have ascertained by a laborious series of
experiments that if all the individuals of the same variety
be subjected during several generations to the same con-
ditions, the good derived from crossing is often much dimin-
ished or wholly disappears. This is one side of the case.
On the other side, we know that species which have long
been exposed to nearly uniform conditions, when they are
subjected under confinement to new and greatly changed
conditions, either perish, or if they survive, are rendered
sterile, though retaining perfect health. This does not
occur, or only in a. very slight degree, with our domesticated
productions, which have long been exposed to fluctuating
conditions. Hence when we find that hybrids produced by
a cross between two distinct species are few in number,
owing to their perishing soon after conception or at a very
early age, or if surviving that they are rendered more or less
sterile, it seems highly probable that this result is due to
their having been in fact subjected to a great change in their
conditions of life, from being compounded of two distinct
organizations. He who will explain in a definite manner
why, for instance, an elephant or a fox will not breed under
confinement in its native country, whilst the domestic pig or
dog will breed freely under the most diversified conditions,
will at the same time be able to give a definite answer to the
question why two distinct species, when crossed, as well as
their hybrid offspring, are generally rendered more or less
sterile, while two domesticated varieties when crossed and
their mongrel offspring are perfectly fertile.
Turning to geographical distribution, the difficulties en-
countered on the theory of descent with modification are
serious enough. All the individuals of the same species,
and all the species of the same genus, or even higher group,
are descended from common parents ; and therefore, in how-
ever distant and isolated parts of the world they may now
be found, they must in the course of successive generations
have travelled from some one point to all the others. We
are often wholly unable even to conjecture how this could
have been effected. Yet, as we have reason to believe that
some species have retained the same specific form fox' yery
450 RECAPITULATION.
long periods of time, immensely long as measured by years,
too much stress ought not to be laid on the occasional wide
diffusion of the same species ; for during very long periods
there will always have been a good chance for wide migra-
tion by many means. A broken or interrupted range may
often be accounted for by the extinction of the species in the
intermediate regions. It cannot be denied that we are as
yet very ignorant as to the full extent of the various climati-
cal and geographical changes which have affected the earth
during modern periods ; and such changes will often have
facilitated migration. As an example, I have attempted to
show how potent has been the influence of the Glacial period
on the distribution of the same and of allied species through-
out the world. We are as yet profoundly ignorant of the
many occasional means of transport. With respect to dis-
tinct species of the same genus, inhabiting distant and isolated
regions, as the process of modification has necessarily been
slow, all the means of migration will have been possible
during a very long period ; and consequently the difficulty
of the wide diffusion of the species of the same genus is in
some degree lessened.
As according to the theory of natural selection an inter-
minable number of intermediate forms must have existed,
linking together all the species in each group by gradations
as fine as our existing varieties, it may be asked, Why do we
not see these linking forms all around us ? Why are not all
organic beings blended together in an inextricable chaos ?
With respect to existing forms, we should remember that
we have no right to expect (excepting in rare cases) to
discover directly connecting links between them, but only
between each and some extinct and supplanted form. Even
on a wide area, which has during a long period remained
continuous, and of which the climatic and other conditions
of life change insensibly in proceeding from a district occu-
pied by one species into another district occupied by a
closely allied species, we have no just right to expect often
to find intermediate varieties in the intermediate zones. For
we have reason to believe that only a few species of a genus
ever undergo change ; the other species becoming utterly
extinct and leaving no modified progeny. Of the species
which do change, only a few within the same country change
at the same time ; and all modifications are slowly effected.
I have also shown that the intermediate varieties which
probably at first existed in the intermediate zones, would be
RECAPITULATION. 451
liable to be supplanted by the allied forms on either hand ;
for the latter, from existing in greater numbers, would gen-
erally be modified and improved at a quicker rate than the
intermediate varieties, which existed in lesser numbers ; so
that the intermediate varieties would, in the long run, be
supplanted and exterminated.
On this doctrine of the extermination of an infinitude of
connecting links, between the living and extinct inhabitants
of the world, and at each successive period between the
extinct and still older species, why is not every geological
formation charged with such links ? Why does not every
collection of fossil remains afford plain evidence of the
gradation and mutation of the forms of life ? Although
geological research has undoubtedly revealed the former
existence of many links, bringing numerous forms of life
much closer together, it does not yield the infinitely many
fine gradations between past and present species required on
the theorj'-, and this is the most obvious of the many objec-
tions which may be urged against it. Why, again, do whole
groups of allied species appear, though this appearance is
often false, to have come in suddenly on the successive
geological stages ? Although we now know that organic
beings appeared on this globe, at a period incalculably re-
mote, long before the lowest bed of the Cambrian system
was deposited, why do we not find beneath this system great
piles of strata stored with the remains of the progenitors of
the Cambrian fossils ? For on the theory, such strata must
somewhere have been deposited at these ancient and utterly
unknown epochs of the world's history.
I can answer these questions and objections only on the
supposition that the geological record is far more imperfect
than most geologists believe. The number of specimens in
all our museums is absolutely as nothing compared with
the countless generations of countless species which have
certainly existed. The parent form of any two or more
species would not be in all its characters directly interme-
diate between its modified offspring, any more than the
rock-pigeon is directly intermediate in crop and tail be-
tween its descendants, the pouter and fantail pigeons. We
should not be able to recognize a species as the parent of
another and modified species, if we were to examine the two
ever so closely, unless we possessed most of the intermediate
links ; and owing to the imperfection of the geological record,
we have no just right to expect to find so many links. If
452 RECAPITULATION.
two or three, or even more linking forms were discovered,
they would simply be ranked by many naturalists as so many
new species, more especially if found in different geological
sub-stages, let their differences be ever so slight. Numerous
existing doubtful forms could be named which are probably
varieties ; but who will pretend that in future ages so many
fossil links will be discovered, that naturalists will be able
to decide whether or not these doubtful forms ought to be
called varieties? Only a small portion of the world has
been geologically explored. Only organic beings of certain
classes can be preserved in a fossil condition, at least in any
great number. Many species when once formed never un-
dergo any further change, but become extinct without leaving
modified descendants ; and the periods during which species
have undergone modification, though long as measured by
years, have probably been short in comparison with the peri-
ods during which they retained the same form. It is the
dominant and widely ranging species which vary most fre-
quently and vary most, and varieties are often at first local
— both causes rendering the discovery of intermediate links
in any one formation less likely. Local varieties will not
spread into other and distant regions until they are consid-
erably modified and improved; and when they have spread,
and are discovered in a geological formation, they appear as
if suddenly created there, and will be simply classed as new
species. Most formations have been intermittent in their
accumulation, and their duration has probably been shorter
than the average duration of specific forms. Successive
formations are in most cases separated from each other by
blank intervals of time of great length, for fossiliferous for-
mations thick enough to resist future degradation can, as a
general rule, be accumulated only where much sediment is
deposited on the subsiding bed of the sea. During the alter-
nate periods of elevation and of stationary level, the record
will generally be blank. During these latter periods there
will probably be more variability in the forms of life; during
periods of subsidence, more extinction.
With respect to the absence of strata rich in fossils be-
neath the Cambrian formation, I can recur only to the
hypothesis given in the tenth chapter; namely, that though
our continents and oceans have endured for an enormous
period in nearly their present relative positions, we have no
reason to assume that this has always been the case ; conse-
quently formations much older than any now known may lie
RECAPITULATION. 453
buried beneath the great oceans. With respect to the lapse
of time not having been sufficient since our planet was con-
solidated for the assumed amount of organic change, and this
objection, as urged by Sir William Thompson, is probably
one of the gravest as yet advanced, I can only say, firstly,
that we do not know at wThat rate species change, as meas-
ured by years, and secondly, that many philosophers are not
as yet willing to admit that we know enough of the consti-
tution of the universe and of the interior of our globe to
speculate with safety on its past duration.
That the geological record is imperfect, all will admit;
but that it is imperfect to the degree required by our theory,
few will be inclined to admit. If we look to long enough
intervals of time, geology plainly declares that species have
all changed ; and they have changed in the manner required
by the theory, for they have changed slowly and in a gradu-
ated manner. We clearly see this in the fossil remains from
consecutive formations invariably being much more closely
related to each other than are the fossils from widely sepa-
rated formations.
Such is the sum of the several chief objections and
difficulties which may be justly urged against the theo^ ;
and I have now briefly recapitulated the answers and ex-
planations which, as far as I can see, may be given. I
have felt these difficulties far too heavily during many
years to doubt their weight. But it deserves especial notice
that the more important objections relate to questions on
which we are confessedly ignorant; nor do we know how
ignorant we are. We do not know all the possible transi-
tional gradations between the simplest and the most perfect
organs ; it cannot be pretended that we know all the varied
means of Distribution during the long lapse of years, or that
we know how imperfect is the Geological Record. Serious
as these several objections are, in my judgment the}'" are by
no means sufficient to overthrow the theory of descent with
subsequent modification.
Now let us turn to the other side of the argument.
Under domestication we see much variability caused, or at
least excited, by changed conditions of life ; but often in so
obscure a manner, that we are tempted to consider the varia-
tions as spontaneous. Variability is governed by many com-
plex laws, by correlated growth, compensation, the increased
use and Misuse of parts? and the definite action of the sur-
454 RECAPITULATION.
•ounding conditions. There is much difficulty in ascer-
taining how largely our domestic productions have been
modified ; but we may safely infer that the amount has
been large, and that modifications can be inherited for long
periods. As long as the conditions of life remain the same,
we have reason to believe that a modification, which has
already been inherited for many generations, may continue
to be inherited for an almost infinite number of generations.
On the other hand we have evidence that variability, when
it has once come into play, does not cease under domesti-
cation for a very long period ; nor do we know that it ever
ceases, for new varieties are still occasionally produced by
our oldest domesticated productions.
Variabilit}'- is not actually caused by man ; he only unin-
tentionally exposes organic beings to new conditions of
life, and then nature acts on the organization and causes it
to Vary. But man can and does select the variations given
to him by nature, and thus accumulates them in any desired
manner. He thus adapts animals and plants for his own
benefit or pleasure. He may do this methodically, or he
may do it unconsciously by preserving the individuals most
useful or pleasing to him without any intention of altering
the breed. It is certain that he can largely influence the
character of a breed by selecting, in each successive genera-
tion, individual differences so slight as to be inappreciable
except by an educated eye. This unconscious process of
selection has been the great agency in the formation of the
most distinct and useful domestic breeds. That many
breeds produced by man have to a large extent the character
of natural species, is shown by the inextricable doubts
whether many of them are varieties or aboriginally distinct
species.
There is no reason why the principles which have acted
so efficiently under domestication should not have acted
under nature. In the survival of favored individuals and
races, during the constantly recurrent Struggle for Exist-
ence, we see a powerful and ever-acting form of Selection.
The struggle for existence inevitably follows from the high
geometrical ratio of increase which is common to all organic
beings. This high rate of increase is proved by calculation
— by the rapid increase of many animals and plants during
a succession of peculiar seasons, and when naturalized in
new countries. More individuals are born than can possibly
survive. A grain in the balance may determine which indi-
RECAPITULATION. 455
viduals shall live, and which shall die — which variety of
species shall increase in number, and which shall decrease,
or finally become extinct. As the individuals of the same
species come in all respects into the closest competition
with each other, the struggle will generally be most severe
between them ; it will be almost equally severe between the
varieties of the same species, and next in severity between
the species of the same genus. On the other hand the
struggle will often be severe between beings remote in the
scale of nature. The slightest advantage in certain individ-
uals, at any age or during any season, over those with which
they come into competition, or better adaptation in however
slight a degree to the surrounding physical conditions, will,
in the long-run, turn the balance.
With animals having separated sexes, there will be in
most cases a struggle between the males for the possession
of the females. The most vigorous males, or those which
have most successfully struggled with their conditions of
life, will generally leave most progeny. But success will
often depend on the males having special weapons or means
of defence or charms ; and a slight advantage will lead to
victory.
As geology plainly proclaims that each land has under-
gone great physical changes, we might have expected to
find that organic beings have varied under nature, in the
same way as they have varied under domestication. And
if there has been any variability under nature, it would be
an unaccountable fact if natural selection had not come
into play. It has often been asserted, but the assertion is
incapable of proof, that the amount of variation under
nature is a strictly limited quantity. Man, though acting
on external characters alone and often capriciously, can
produce within a short period a great result by adding up
mere individual differences in his domestic productions ;
and every one admits that species present individual differ-
ences. But, beside such differences, all naturalists admit
that natural varieties exist, which are considered suffi-
ciently distinct to be worthy of record in systematic works.
No one has drawn any clear distinction between individual
differences and slight varieties ; or between more plainly
marked varieties and sub-species and species. On separate
continents, and on different parts of the same continent,
when divided by barriers of any kind, and on out-lying
islands, what a multitude of forms exist, which some experi-
456 RECAPITULATION.
enced naturalists rank as varieties, others as geographical
races or sub-species, and others as distinct though closely
allied species !
If, then, animals and plants do vary, let it be ever so
slightly or slowly, why should not variations or individual
differences, which are in any way beneficial, be preserved
and accumulated through natural selection, or the survival
of the fittest ? If man can by patience select variations
useful to him, why, under changing and complex conditions
of life, should not variations useful to nature's living prod-
ucts often arise, and be preserved or selected ? What limit
can be put to this power, acting during long ages and rigidly
scrutinizing the whole constitution, structure, and habits of
each creature, favoring the good and rejecting the bad ? I
can see no limit to this power, in slowly and beautifully
adapting each form to the most complex relations of life.
The theory of natural selection, even if we look no further
than this, seems to be in the highest degree probable. I
have already recapitulated, as fairly as I could, the opposed
difficulties and objections : now let us turn to the special
facts and arguments in favor of the theory.
On the view that species are only strongly marked and
permanent varieties, and that each species first existed as a
variety, we can see why it is that no line of demarcation
can be drawn between species, commonly supposed to have
been produced by special acts of creation, and varieties
which are acknowledged to have been produced by second-
ary laws. On this same view we can understand how it
is that in a region where many species of a genus have been
produced, and where they now flourish, these same species
should present many varieties ; for where the manufactory
of species has been active, we might expect, as a general
rule, to find it still in action ; and this is the case if varie-
ties be incipient species. Moreover, the species of the
larger genera, which afford the greater number of varieties
or incipient species, retain to a certain degree the character
of varieties; for they differ from each other by a less
amount of difference than do the species of smaller genera.
The closely allied species also of a larger genera apparently
have restricted ranges, and in their affinities they are
clustered in little groups round other species — in both
respects resembling varieties. These are strange relations
on the view that each species was independently created,
but are intelligible if each existed first as a variety.
RECAPITULATION. 457
As each species tends by its geometrical rate of repro-
duction to increase inordinately in number ; and as the
modified descendants of each species will be enabled to
increase by as much as they become more diversified in
ha,bits and structure, so as to be able to seize on many and
widely different places in the economy of nature, there
will be a constant tendency in natural selection to preserve
the most divergent offspring of any one species. Hence,
during a long-continued course of modification, the slight
differences characteristic of varieties of the same species,
tend to be augmented into the greater differences character-
istic of the species of the same genus. New and improved
varieties will inevitably supplant and exterminate the older,
less improved, and intermediate varieties ; and thus species
are rendered to a large extent defined and distinct objects.
Dominant species belonging to the larger groups within each
class tend to give birth to new and dominant forms ; so
that each large group tends to become still larger, and at
the same time more divergent in character. But as all
groups cannot thus go on increasing in size, for the world
would not hold them, the more dominant groups beat the
less dominant. This tendency in the large groups to go on
increasing in size and diverging in character, together with
the inevitable contingency of much extinction, explains the
arrangement of all the forms of life in groups subordinate
to groups, all within a few great classes, which has prevailed
throughout all time. This grand fact of the grouping of all
organic beings under what is called the Natural System, is
utterly inexplicable on the theory of creation.
As natural selection acts solely by accumulating slight,
successive, favorable variations, it can produce no great or
sudden modifications ; it can act only by short and slow
steps. Hence, the canon of "Natura non facit saltum,"
which every fresh addition to our knowledge tends to con-
firm, is on this theory intelligible. We can see why through-
out nature the same general end is gained by an almost
infinite diversity of means, for every peculiarity when once
acquired is long inherited, and structures already modified
in many different ways have to be adapted for the same
general purpose. We can, in short, see why nature is
prodigal in variety, though niggard in innovation. But why
this should be a law of nature if each species has been
independently created, no man can explain.
Many other facts are, as it seems to me, explicable on thia
458 RECAPITULATION.
theory. How strange it is that a bird, under the form of a
woodpecker, should prey on insects on the ground; that
upland geese, which rarely or never swim, should possess
webbed feet ; that a thrush-like bird should dive and feed on
sub-aquatic insects ; and that a petrel should have the habits
and structure fitting it for the life of an auk ! and so in
endless other cases. But on the view of each species con-
stantly trying to increase in number, with natural selection
always ready to adapt the slowly varying descendants of
each to any unoccupied or ill-occupied place in nature, these
facts cease to be strange, or might even have been
anticipated.
We can to a certain extent understand how it is that
there is so much beauty throughout nature ; for this may be
largely attributed to the agency of selection. That beauty,
according to our sense of it, is not universal, must be
admitted by every one who will look at some venomous
snakes, at some fishes, and at certain hideous bats with a
distorted resemblance to the human face. Sexual selection
has given the most brilliant colors, elegant patterns, and
other ornaments to the males, and sometimes to both sexes,
of many birds, butterflies, and other animals. With bird*
it has often rendered the voice of the male musical to the
female, as well as to our ears. Flowers and fruit have been
rendered conspicuous by brilliant colors in contrast with the
green foliage, in order that the flowers may be easily seen,
visited and fertilized by insects, and the seeds disseminated
by birds. How it comes that certain colors, sounds, and
forms should give pleasure to man and the lower animals,
that is, how the sense of beauty in its simplest form was
first acquired, we do not know any more than how certain
odors and flavors were first rendered agreeable.
As natural selection acts by competition, it adapts and
improves the inhabitants of each country only in relation
to their co-inhabitants ; so that we need feel no surprise at
the species of any one country, although on the ordinary
view supposed to have been created and specially adapted
for that country, being beaten and supplanted by the
naturalized productions from another land. Nor ought we
to marvel if all the contrivances in nature be not, as far as
we can judge, absolutely perfect, as in the case even of the
human eye ; or if some of them be abhorrent to our ideas of
fitness. We need not marvel at the sting of the bee, when
used against an enemy, causing the bee's own death; at
RECAPITULATION. 459
drones being produced in such great numbers for one single
act, and being then slaughtered by their sterile sisters ; at
the astonishing waste of pollen by our fir-trees : at the
instinctive hatred of the queen bee for her own fertile
daughters ; at ichneumonidse feeding within the living bodies
of caterpillars ; or at other such cases. The wonder, indeed,
is, on the theory of natural selection, that more cases of the
want of absolute perfection have not been detected.
The complex and little known laws governing the produc-
tion of varieties are the same, as far as we can judge, with
the laws which have governed the production of distinct
species. In both cases physical conditions seem to have
produced some direct and definite effect, but how much we
cannot say. Thus, when varieties enter any new station,
they occasionally assume some of the characters proper to
the species of that station. With both varieties and
species, use and disuse seem to have produced a considerable
effect; for it is impossible to resist this conclusion when
we look, for instance, at the logger-headed duck, which has
wings incapable of flight, in nearly the same condition as
in the domestic duck ; or when we look at the borrowing
tucu-tucu, which is occasionally blind, and then at certain
moles, which are habitually blind and have their eyes cov-
ered with skin ; or when we look at the blind animals in-
habiting the dark caves of America and Europe. With
varieties and species, correlated variation seems to have
played an important part, so that when one part has been
modified other parts have been necessarily modified. With
both varieties and species, reversions to long-lost characters
occasionally occur. How inexplicable on the theory of
creation is the occasional appearance of stripes on the
shoulders and legs of the several species of the horse-genus
and of their hybrids ! How simply is this fact explained
if we believe that these species are all descended from a
striped progenitor, in the same manner as the several
domestic breeds of the pigeon are descended from the blue
and barred rock-pigeon !
On the ordinary view of each species having been inde-
pendently created, why should specific characters, or those
by which the species of the same genus differ from each
other, be more variable than generic characters in which
they all agree ? Why, for instance, should the color of a
flower be more likely to vary in any one species of a genus,
•if the other species possess differently colored flowers, than
460 RECAPITULATION.
if all possessed the same colored flowers ? If species are
only well-marked varieties, of which the characters have
become in a high degree permanent, we can understand this
fact ; for they have already varied since they branched off
from a common progenitor in certain characters, by which
they have come to be specifically distinct from each other ;
therefore these same characters would be more likely again
to vary than the generic characters which have been in-
herited without change for an immense period. It is inex-
plicable on the theory of creation why a part developed in
a very unusual manner in one species alone of a genus, and
therefore, as we may naturally infer, of great importance to
that species, should be eminently liable to variation ; but,
on our view, this part has undergone, since the several
species branched off from a common progenitor, an unusual
amount of variability and modification, and therefore we
might expect the part generally to be still variable. But a
part may be developed in the most unusual manner, like the
wing of a bat, and yet not be more variable than any other
structure, if the part be common to many subordinate forms,
that is, if it has been inherited for a very long period ; for
in this case it will have been rendered constant by long-
continued natural selection.
Glancing at instincts, marvellous as some are, they offer
no greater difficulty than do corporeal structures on the
theory of the natural selection of successive, slight, but
profitable modifications. We can thus understand why
nature moves by graduated steps in endowing different
animals of the same class with their several instincts. I
have attempted to show how much light the principle of
gradation throws on the admirable architectural powers of
the hive-bee. Habit no doubt often comes into play in
modifying instincts ; but it certainly is not indispensable,
as we see in the case of neuter insects, which leave no
progeny to inherit the effects of long-continued habit. On
the view of all the species of the same genus having de-
scended from a common parent, and having inherited much
in common, we can understand how it is that allied species,
when placed under widely different conditions of life, yet
follow nearly the same instincts ; why the thrushes of
tropical and temperate South America, for instance, line
their nests, with mud like our British species. On the view
of instincts having been slowly acquired through natural
f&lwtiQV, we nee^ gp|j marvel at §oine jj?s|i|jcts being no$
RECAPITULATION. 461
perfect and liable to mistakes, ana at many instincts causing
other animals to suffer.
If species be only well-marked and permanent varieties,
we can at once see why their crossed offspring should follow
the same complex laws in their degrees and kinds of resem-
blance to their parents — in being absorbed into each other
by successive crosses, and in other such points — as do the
crossed offspring of acknowledged varieties. This similar-
ity would be a strange fact, if species had been independently
created, and varieties had been produced through secondary
laws.
If we admit that the geological record is imperfect to an
extreme degree, then the facts, which the record does give,
strongly support the theory of descent with modification.
New species have come on the stage slowly and at successive
intervals ; and the amount of change, after equal intervals of
time, is widely different in different groups. The extinction of
species and of whole groups of species, which has played so
conspicuous a part in the history of the organic world,
almost inevitably follows from the principle of natural
selection ; for old forms are supplanted by new and improved
forms. Neither single species nor groups of species reap-
pear when the chain of ordinary generation is once broken.
The gradual diffusion of dominant forms, with the slow-
modification of their descendants, causes the forms of life,
after long intervals of time, to appear as if they had changed
simultaneously throughout the world. The fact of the fossil
remains of each formation being in some degree intermediate
in character between the fossils in the formations above and
below, is simply explained by their intermediate position in
the chain of descent. The grand fact that all extinct beings
can be classed with all recent beings, naturally follows from
the living and the extinct being the offspring of common
parents. As species have generally diverged in character
during their long course of descent and modification, we
can understand why it is that the more ancient forms, or
early progenitors of each group, so often occupy a position
in some degree intermediate between existing groups.
Recent forms are generally looked upon as being, on the
whole, higher in the scale of organization than ancient
forms ; and they must be higher, in so far as the later and
more improved forms have conquered the older and less
improved forms in the struggle for life ; they have also
generally ha4 j&eir organs more specialized £9? 3iff§re»tf
.462 RECAPITULATION.
functions. This fact is perfectly compatible with numerous
beings still retaining simple and but little improved struc-
tures, fitted for simple conditions of life ; it is likewise com-
patible with some forms having retrograded in organization,
by having become at each stage of descent better fitted for
new and degraded habits of life. Lastly, the wonderful
law of the long endurance of allied forms on the same con-
tinent— of marsupials in Australia, of edentata in America,
and other such cases — is intelligible, for within the same
country the existing and the extinct will be closely allied by
descent.
Looking to geographical distribution, if we admit that
there has been during the long course of ages much migra-
tion from one part of the world to another, owing to former
climatical and geographical changes and to the many occa-
sional and unknown means of dispersal, then we can under-
stand, on the theory of descent with modification, most of
the great leading facts in distribution. We can see why there
should be so striking a parallelism in the distribution of
organic beings throughout space, and in their geological suc-
cession throughout time ; for in both cases the beings have
been connected by the bond of ordinary generation, and the
means of modification have been the same. We see the full
meaning of the wonderful fact, which has struck every trav-
eller, namely, that on the same continent, under the most
diverse conditions, under heat and cold, on mountain and
lowland, on deserts and marshes, most of the inhabitants
within each great class are plainly related; for they are
the descendants of the same progenitors and early colonists.
On this same principle of former migration, combined in
most cases with modification, we can understand, by the aid
of the Glacial period, the identity of some few plants, and the
close alliance of many others, on the most distant mountains,
and in the northern and southern temperate zones ; and like-
wise the close alliance of some of the inhabitants of the sea
in the northern and southern temperate latitudes, though
separated by the whole intertropical ocean. Although two
countries may present physical conditions as closely similar
as the same species ever require, we need feel no surprise at
their inhabitants being widely different, if they have been
for a long period completely sundered from each other ; for
as the relation of organism to organism is the most impor-
tant of all relations, and as the two countries will have
received colonists at various periods and in different propor*
RECAPITU LATION. 463
tions, from some other country or from each other, the course
of modification in the two areas will inevitably have been
different.
On this view of migration, with subsequent modification,
we see why oceanic islands are inhabited by only few spe-
cies, but of these, why many are peculiar or endemic forms.
We clearly see why species belonging to those groups of
animals which cannot cross wide spaces of the ocean, as
frogs and terrestrial mammals, do not inhabit oceanic
islands ; and why, on the other hand, new and peculiar
species of bats, animals which can traverse the ocean, are
often found on islands far distant from any continent. Such
cases as the presence of peculiar species of bats on oceanic
islands and the absence of all other terrestrial mammals, are
facts utterly inexplicable on the theory of independent acts
of creation.
The existence of closely allied representative species in
any two areas, implies, on the theory of descent with modi-
fication, that the same parent forms formerly inhabited both
areas : and we almost invariably find that wherever many
closely allied species inhabit two areas, some identical species
are still common to both. Wherever many closely allied yet
distinct species occur, doubtful forms and varieties belong-
ing to the same groups likewise occur. It is a rule of high
generality that the inhabitants of each area are related to
the inhabitants of the nearest source whence immigrants
might have been derived. We see this in the striking rela-
tion of nearly all the plants and animals of the Galapagos
Archipelago, of Juan Fernandez, and of the other American
islands, to the plants and animals of those of the mainland ;
and of those of the Cape de Verde Archipelago, and of the
other African islands to the African mainland. It must be
admitted that these facts receive no explanation on the
theory of creation.
The fact, as we have seen, that all past and present
organic beings can be arranged within a few great classes,
in groups subordinate to groups, and with the extinct groups
often falling in between the recent groups, is intelligible
on the theory of natural selection with its contingencies of
extinction and divergence of character. On these same
principles we see how it is that the mutual affinities of
the forms within each class are so complex and circuitous.
We see why certain characters are far more serviceable than
others for classification, j why adaptive characters, though
464 RECAPITULATION.
of paramount importance to the beings, are of hardly any
importance in classification; why characters derived from
rudimentary parts, though of no service to the beings, are
often of high classificatory value ; and why embryological
characters are often the most valuable of all. The real
affinities of all organic beings, in contra-distinction to their
adaptive resemblances, are due to inheritance or community
of descent. The Natural System is a genealogical arrange-
ment with the acquired grades of difference, marked by the
terms, varieties, species, genera, families, etc. ; and we have
to discover the lines of descent by the most permanent char-
acters, whatever they may be, and of however slight vital
importance.
The similar framework of bones in the hand of a man,
wing of a bat, fin of the porpoise, and leg of the horse —
the same number of vertebrae forming the neck of the
giraffe and of the elephant — and innumerable other such
facts, at once explain themselves on the theory of descent
with slow and slight successive modifications. The simi-
larity of pattern in the wing and in the leg of a bat, though
used for such different purpose — in the jaws and legs of a
crab — in the petals, stamens, and pistils of a flower, is like-
wise, to a large extent, intelligible on the view of the grad-
ual modification of parts or organs, which were aboriginally
alike in an early progenitor in each of these classes. On
the principle of successive variations not always supervening
at an early age, and being inherited at a corresponding not
early period of life, we clearly see why the embryos of mam-
mals, birds, reptiles, and fishes should be so closely similar
and so unlike the adult forms. We may cease marvelling at
the embryo of an air-breathing mammal or bird having
branchial slits and arteries running in loops, like those of
a fish which has to breathe the air dissolved in water by the
aid of well-developed branchiae.
Disuse, aided sometimes by natural selection, will often
have reduced organs when rendered useless under changed
habits or conditions of life ; and we can understand on this
view the meaning of rudimentary organs. But disuse and
selection will generally act on each creature, when it has
come to maturity and has to play its full part in the strug-
gle for existence, and will thus have little power on an
organ during early life ; hence the organ will not be re-
duced or rendered rudimentary at this early age. The calf,
for instance, has inherited teeth, which never cut through
CONCLUSION. 465
the gums of the upper jaw, from an early progenitor having
well-developed teeth ; and we may believe, that the teeth in
the mature animal were formerly reduced by disuse, owing
to the tongue and palate, or lips, having become excellently
fitted through natural selection to browse without their
aid ; whereas in the calf, the teeth have been left unaffected,
and on the principle of inheritance at corresponding ages
have been inherited from a remote period to the present
day. On the view of each organism with all its separate
parts having been specially created, how utterly inexplica-
ble is it that organs bearing the plain stamp of inutility,
such as the teeth in the embryonic calf, or the shrivelled
wings under the soldered wing-covers of many beetles,
should so frequently occur. Nature may be said to have
taken pains to reveal her scheme of modification, by means
of rudimentary organs, of embryological and homologous
structures, but we are too blind to understand her meaning.
I have now recapitulated the facts and considerations
which have thoroughly convinced me that species have been
modified, during a long course of descent. This has been
effected chiefly through the natural selection of numerous
successive, slight, favorable variations; aided in an impor-
tant manner by the inherited effects of the use and disuse
of parts ; and in an unimportant manner, that is, in relation
to adaptive structures, whether past or present, by the
direct action of external conditions, and by variations
which seem to us in our ignorance to arise spontaneously.
It appears that I formerly underrated the frequency and
value of these latter forms of variation, as leading to per-
manent modifications of structure independently of natural
selection. But as my conclusions have lately been much
misrepresented, and it has been stated that I attribute the
modification of species exclusively to natural selection, I
may be permitted to remark that in the first edition of this
work, and subsequently, I placed in a most conspicuous
position — namely, at the close of the Introduction — the
following words : " I am convinced that natural selection
has been the main but not the exclusive means of modi-
fication." This has been of no avail. Great is the power
of steady misrepresentation ; but the history of science shows
that fortunately this power does not long endure.
It can hardly be supposed that a false theory would
explain, in so satisfactory a manner as does the theory
pi natural selection; tlie several large classes of facts above
466 CONCLUSION.
specified. It has recently been objected that this is an
unsafe method of arguing ; but it is a method used in
judging of the common events of life, and has often been
used by the greatest natural philosophers. The undulatory
theory of light has thus been arrived at ; and the belief in
the revolution of the earth on its own axis was until lately
supported by hardly any direct evidence. It is no valid
objection that science as yet throws no light on the far
higher problem of the essence or origin of life. Who can
explain what is the essence of the attraction of gravity ?
No one now objects to following out the results consequent
on this unknown element of attraction ; notwithstanding that
Leibnitz formerly accused Newton of introducing " occult
qualities and miracles into philosophy."
I see no good reasons why the views given in this volume
should shock the religious feelings of any one. It is satis-
factory, as showing how transient such impressions are, to
remember that the greatest discovery ever made by man,
namely, the law of the attraction of gravity, was also
attacked by Leibnitz, " as subversive of natural, and infer-
entially of revealed, religion." A celebrated author and
divine has written to me that " he has gradually learned to
see that it is just as noble a conception of the Deity to
believe that He created a few original forms capable of
self-development into other and needful forms, as to believe
that He required a fresh act of creation to supply the voids
caused by the action of His laws."
Why, it may be asked, until recently did nearly all the
most eminent living naturalists and geologists disbelieve in
the mutability of species ? It cannot be asserted that or-
ganic beings in a state of nature are subject to no varia-
tion ; it cannot be proved that the amount of variation in
the course of long ages is a limited quantity ; no clear dis-
tinction has been, or can be, drawn between species and
well-marked varieties. It cannot be maintained that spe-
cies when intercrossed are invariably sterile and varieties
invariably fertile; or that sterility is a special endowment
and sign of creation. The belief that species were immut-
able productions was almost unavoidable as long as the his-
tory of the world was thought to be of short duration ; and
now that we have acquired some idea of the lapse of time,
we are too apt to assume, without proof, that the geo-
logical record is so perfect that it would have afforded us
plain evidence of the mutation of species, if they had under
^one mutation.
CONCLUSION. 467
But the chief cause of our natural unwillingness to admit
that one species has given birth to other and distinct species,
is that we are always slow in admitting great changes of
which we do not see the steps. The difficulty is the same
as that felt by so many geologists, when Lyell first insisted
that long lines of inland cliffs had been formed, and great
valleys excavated, by the agencies which we still see at work.
The mind cannot possibly grasp the full meaniDg of the term
of even a million years ; it cannot add up and perceive the
full effects of many slight variations, accumulated during an
almost infinite number of generations.
Although I am fully convinced of the truth of the views
given in this volume under the form of an abstract, I by
no means expect to convince experienced naturalists whose
minds are stocked with a multitude of facts all viewed, dur-
ing a long course of years, from a point of view directly
opposite to mine. It is so easy to hide our ignorance under
such expressions as the " plan of creation," " unity of de-
sign," etc., and to think that we give an explanation when
we only restate a fact. Any one whose disposition leads him
to attach more weight to unexplained difficulties than to the
explanation of a certain number of facts will certainly reject
the theory. A few naturalists, endowed with much flexibil-
ity of mind, and who have already begun to doubt the immu-
tability of species, may be influenced by this volume ; but I
look with confidence to the future, to young and rising nat-
uralists, who will be able to view both sides of the question
with impartiality. Whoever is led to believe that species
are mutable will do good service by conscientiously express-
ing his conviction ; for thus only can the load of prejudice
by which this subject is overwhelmed be removed.
Several eminent naturalists have of late published their
belief that a multitude of reputed species in each genus are
not real species ; but that other species are real, that is, have
been independently created. This seems to me a strange
conclusion to arrive at. They admit that a multitude of
forms, which till lately they themselves thought were spe-
cial creations, and which are still thus looked at by the
majority of naturalists, and which consequently have all the
external characteristic features of true species — they admit
that these have been produced by variation, but they refuse
to extend the same view to other and slightly different forms.
Nevertheless, they do not pretend that they can define, or
eyen conjecture, which are the created forms of life, and
468 COtfCLtJSIOtf.
which are those produced by secondary laws. They admit
variation as a vera causa in one case, they arbitrarily reject
it in another, without assigning any distinction in the two
cases. The day will come when this will be given as a curi-
ous illustration of the blindness of preconceived opinion.
These authors seem no more startled at a miraculous act of
creation than at an ordinary birth. But do they really
believe that at innumerable periods in the earth's history
certain elemental atoms have been commanded suddenly to
flash into living tissues ? Do they believe that at each sup-
posed act of creation one individual or many were produced ?
Were all the infinitely numerous kinds of animals and plants
created as eggs or seed, or as full grown ? and in the case
of mammals, were they created bearing the false marks of
nourishment from the mother's womb ? Undoubtedly some
of these same questions cannot be answered by those who
believe in the appearance or creation of only a few forms of
life, or of some one form alone. It has been maintained by
several authors that it is as easy to believe in the creation of
a million beings as of one ; but Maupertuis' philosophical
axiom of " least action" leads the mind more willingly to
admit the smaller number ; and certainly we ought not to
believe that innumerable beings within each great class have
been created with plain, but deceptive, marks of descent from
a single parent.
As a record of a former state of things, I have retained in
the foregoing paragraphs, and elsewhere, several sentences
which imply that naturalists believe in the separate creation
of each species ; and I have been much censured for having
thus expressed myself. But undoubtedly this was the
general belief when the first edition of the present work
appeared. I formerly spoke to very many naturalists on
the subject of evolution, and never once met with any
sympathetic agreement. It is probable that some did then
believe in evolution, but they were either silent or expressed
themselves so ambiguously that it was not easy to understand
their meaning. Now, things are wholly changed, and almost
every naturalist admits the great principle of evolution.
There are, however, some who still think that species have
suddenly given birth, through quite unexplained means, to
new and totally different forms. But, as I have attempted
to show, weighty evidence can be opposed to the admission of
great and abrupt modifications. Under a scientific point of
view, and as leading to further investigation, but little ad van-
CONCLUSION. 469
tage is gained by believing that new forms are suddenly devel-
oped in an inexplicable manner from old and widely different
forms, over the old belief in the creation of species from the
dust of the earth.
It may be asked how far I extend the doctrine of the
modification of species. The question is difficult to answer,
because the more distinct the forms are which we consider,
by so much the arguments in favor of community of descent
become fewer in number and less in force. But some argu-
ments of the greatest weight extend very far. All the
members of whole classes are connected together by a chain
of affinities, and all can be classed on the same principle, in
groups subordinate to groups. Fossil remains sometimes
tend to fill up very wide intervals between existing orders.
Organs in a rudimentary condition plainly show that an
early progenitor had the organ in a fully developed condi-
tion, and this in some cases implies an enormous amount of
modification in the descendants. Throughout whole classes
various structures are formed on the same pattern, and at a
very early age the embryos closely resemble each other.
Therefore I cannot doubt that the theory of descent with
modification embraces all the members of the same great
class or kingdom. I believe that animals are descended
from at most only four or five progenitors, and plants from
an equal or lesser number.
Analogy would lead me one step further, namely, to the
belief that all animals and plants are descended from some
one prototype. But analogy may be a deceitful guide.
Nevertheless all living things have much in common, in
their chemical composition, their cellular structure, their
laws of growth, and their liability to injurious influences.
We see this even in so trifling a fact as that the same
poison often similarly affects plants and animals, or that the
poison secreted by the gall-fly produces monstrous growths
on the wild rose or oak tree. With all organic beings, ex-
cepting perhaps some of the very lowest, sexual reproduc-
tion seems to be essentially similar. With all, as far as is
at present known, the germinal vesicle is the same ; so that
all organisms start from a common origin. If we look even
to the two main divisions — namely, to the animal and vege-
table kingdoms — certain low forms are so far intermediate
in character that naturalists have disputed to which king-
dom they should be referred. As Professor Asa Gray has
remarked, u the spores and other reproductive bodies of manj
470 CONCLUSION.
of the lower algae may claim to have first a characteristically
animal, and then an unequivocally vegetable existence."
Therefore, on the principle of natural selection with
divergence of character, it does not seem incredible, that,
from some such low and intermediate form; both animals
and plants may have been developed ; and, if we admit this,
we must likewise admit that all the organic beings which
have ever lived on this earth may be. descended from some
one primordial form. But this inference is chiefly grounded
on analogy, and it is immaterial whether or not it be
accepted. No doubt it is possible, as Mr. G. H. Lewes
has urged, that at the first commencement of life many
different forms were evolved ; but if so, we may conclude
that only a very few have left modified descendants. For,
as I have recently remarked in regard to the members of
each great kingdom, such as the Vertebrata, Articulat^, etc.,
we have distinct evidence in their embryological, homol-
ogous, and rudimentary structures, that within each king-
dom all the members are descended from a single progenitor.
When the views advanced by me in this volume, and by
Mr. Wallace, or when analogous views on the origin of
species, are generally admitted, we can dimly foresee that
there will be a considerable revolution in natural history.
Systematists will be able to pursue their labors as at present ,*
but they will not be incessantly haunted by the shadowy
doubt whether this or that form be a true species. This, I
feel sure, and I speak after experience, will be no slight
relief. The endless disputes whether or not some fifty
species of British brambles are good species will cease.
Systematists will have only to decide (not that this will be
easy) whether any form be sufficiently constant, and distinct
from other forms, to be capable of definition ; and if defin-
able, whether the differences be sufficiently important to
deserve a specific name. This latter point will become a far
more essential consideration than it is at present ; for differ-
ences, however slight, between any two forms, if not blended
by intermediate gradations, are looked at by most naturalists
as sufficient to raise both forms to the rank of species.
Hereafter, we shall be compelled to acknowledge that the
only distinction between species and well-marked varieties
is, that the latter are known or believed to be connected at
the present day by intermediate gradations, whereas species
were formerly thus connected. Hence, without rejecting
the consideration of the present existence of intermediate
CONCLUSION. 471
gradations between any two forms, we shall be led to weigh
more carefully and to value higher the actual amount of
difference between them. It is quite possible that forms
now generally acknowledged to be merely varieties may
hereafter be thought worthy of specific names ; and in this
case scientific and common language will come into accord-
ance. In short, we shall have to treat species in the same
manner as those naturalists treat genera, who admit that
genera are merely artificial combinations made for conve-
nience. This may not be a cheering prospect ; but we shall
at least be freed from the vain search for the undiscovered
and undiscoverable essence of the term species.
The other and more general departments of natural history
will rise greatly in interest. The terms used by naturalists,
of affinity, relationship, community of type, paternity, mor-
phology, adaptive characters, rudimentary and aborted organs,
etc., will cease to be metaphorical, and will have a plain
signification. When we no longer look at an organic being
as a savage looks at a ship, as something wholly beyond his
comprehension ; when we regard every production of nature
as one which has had a long history ; when we contemplate
every complex structure and instinct as the summing up of
many contrivances, each useful to the possessor, in the same
way as any great mechanical invention is the summing
up of the labor, the experience, the reason, and even the
blunders of numerous workmen ; when we thus view each
organic being, how far more interesting — I speak from
experience — does the study of natural history become !
A grand and almost untrodden field of inquiry will be
opened, on the causes and laws of variation, on correlation,
on the effects of use and disuse, on the direct action of
external conditions, and so forth. The study of domestic
productions will rise immensely in value. A new variety
raised by man will be a more important and interesting sub-
ject for study than one more species added to the infinitude
of already recorded species. Our classifications will come
to be, as far as they can be so made, genealogies ; and will
then truly give what may be called the plan of creation.
The rules for classifying will no doubt become simpler when
we have a definite object in view. We possess no pedigree
or armorial bearings ; and we have to discover and trace the
many diverging lines of descent in our natural genealogies,
by characters of any kind which have long been inherited.
Rudimentary organs will speak infallibly with respect to
472 CONCLUSION.
the nature of long-lost structures. Species and groups of
species which are called aberrant, and which may fancifully
be called living fossils, will aid us in forming a picture of
the ancient forms of life. Embryology will often reveal to
us the structure, in some degree obscured, of the prototypes
of each great class.
When we can feel assured that all the individuals of the
same species, and all the closely allied species of most
genera, have, within a not very remote period, descended
from one parent, and have migrated from some one birth-
place ; and when we better know the many means of migra-
tion, then, by the light which geology now throws, and will
continue to throw, on former changes of climate and of the
level of the land, we shall surely be enabled to trace in an
admirable manner the former migrations of the inhabitants
of the whole world. Even at present, by comparing the
differences between the inhabitants of the sea on the oppo-
site sides of a continent, and the nature of the various
inhabitants on that continent in relation to their apparent
means of immigration, some light can be thrown on ancient
geography.
The noble science of geology loses glory from the extreme
imperfection of the record. The crust of the earth, with
its embedded remains, must not be looked at as a well-filled
museum, but as a poor collection made at hazard and at
rare intervals. The accumulation of each great fossiliferous
formation will be recognized as having depended on an
unusual occurrence of favorable circumstances, and the
blank intervals between the successive stages as having
been of vast duration. But we shall be able to guage with
some security the duration of these intervals by a compari-
son of the preceding and succeeding organic forms. We
must be cautious in attempting to correlate as strictly
contemporaneous two formations, which do not include
many identical species, by the general succession of the
forms of life. As species are produced and exterminated
by slowly acting and still existing causes, and not by
miraculous acts of creation ; and as the most important of
all causes of organic change is one which is almost inde-
pendent of altered and perhaps suddenly altered physical
conditions, namely, the mutual relation of organism to
organism — the improvement of one organism entailing the
improvement or the extermination of others ; it follows,
$hat; the amount of organic change in the fossils of qon
CONCLUSION. 473
secutive formations probably serves as a fair measure of
the relative, though not actual lapse of time. A number of
species, however, keeping in a body might remain for a long
period unchanged, while within the same period, several of
these species, by migrating into new countries and coming
into competition with foreign associates, might become
modified; so that we must not overrate the accuracy of
organic change as a measure of time.
In the future I see open fields for far more important
researches. Psychology will be securely based on the
foundation already well laid by Mr. Herbert Spencer, that
of the necessary acquirement of each mental power and
capacity by gradation. Much light will be thrown on the
origin of man and his history.
Authors of the highest eminence seem to be fully satisfied
with the view that each species has been independently
created. To my mind it accords better with what we know
of the laws impressed on matter by the Creator, that the
production and extinction of the past and present inhabit-
ants of the world should have been due to secondary
causes, like those determining the birth and death of the
individual. When I view all beings not as special creations,
but as the lineal descendants of some few beings which
lived long before the first bed of the Cambrian system was
deposited, the}' seem to me to become ennobled. Judging
from the past, we may safely infer that not one living species
will transmit its unaltered likeness to a distinct futurity.
And of the species now living, very few will transmit
progeny of any kind to a far distant futurity ; for the
manner in which all organic beings are grouped shows that
the greater number of species in each genus, and all the
species in many genera, have left no descendants, but
have become utterly extinct. We can so far take a pro-
phetic glance into futurity as to foretell that it will be the
common and widely spread species, belonging to the larger
and dominant groups within each class, which will ulti-
mately prevail and procreate new and dominant species.
As all the living forms of life are the lineal descendants of
those which lived long before the Cambrian epoch, we may
feel certain that the ordinary succession by generation has
never once been broken, and that no cataclysm has desolated
the whole world. Hence, we may look with some confidence
to a secure future of great length. And as natural selection
works solelv by and for the good of each being, all corpo-
474
CONCLUSION.
real and mental endowments will tend to progress toward
perfection.
It is interesting to contemplate a tangled bank, clothed
with many plants of many kinds, with birds singing on the
bushes, with various insects flitting about, and with worms
crawling through the damp earth, and to reflect that these
elaborately constructed forms, so different from each other,
and dependent upon each other in so complex a manner,
have all been produced by laws acting around us. These
laws, taken in the largest sense, being Growth with repro-
duction ; Inheritance which is almost implied by reproduc-
tion ; Variability from the indirect and direct action of the
conditions of life, and from use and disuse : a Ratio of
Increase so high as to lead to a Struggle for Life, and as a
consequence to Natural Selection, entailing Divergence of
Character and the Extinction of less improved forms.
Thus, from the war of nature, from famine and death, the
most exalted object which we are capable of conceiving,
namely, the production of the higher animals, directly fol-
lows. There is grandeur in this view of life with its sev-
eral powers, having been originally breathed by the Creator
into a few forms or into one ; and that, while this planet
has gone circling on according to the fixed law of gravity,
from so simple a beginning endless forms most beautiful
and most wonderful have been, and are being evolved.
GLOSSARY
OF THE
PRINCIPAL SCIENTIFIC TERMS USED IN THE
PRESENT VOLUME.1
Aberrant. — Forms or groups of animals or plants which deviate In
important characters from their nearest allies, so as not to be easily
included in the same group with them, are said to be aberrant.
Aberration (in Optics). — In the refraction of light by a convex lens
the rays passing through different parts of the lens are brought to
a focus at slightly different distances — this is called spherical aber-
ration; at the same time the colored rays are separated by the
prismatic action of the lens and likewise brought to a focus at
different distances — this is chromatic aberration.
Abnormal. — Contrary to the general rule.
Aborted. — An organ is said to be aborted, when its development has
been arrested at a very early stage.
Albinism. — Albinos are animals in which the usual coloring matters
characteristic of the species have not been produced in the skin and
its appendages. Albinism is the state of being an Albino.
Alg.e. — A class of plants including the ordinary sea-weeds and the
filamentous fresh-water weeds.
Alternation of Generations. — This term is applied to a peculiar
mode of reproduction which prevails among many of the lower
animals, in which the egg produces a living form quite different
from its parent, but from which the parent-form is reproduced by a
process of budding, or by the division of the substance of the first
product of the egg.
Ammonites. — A group of fossil, spiral, chambered shells, allied to
the existing pearly Nautilus, but having the partitions between the
chambers waved in complicated patterns at their junction with the
outer wall of the shell.
Analogy. — The resemblance of structures which depends upon simi-
larity of function, as in the wings of insects and birds. Such struc-
tures are said to be analogous, and to be analogues of each other.
Animalcule. — A minute animal: generally applied to those visible
only by the microscope.
1 I am indebted to the kindness of Mr. W. S. Dallas for this Glossary, which
has been given because several readers have complained to me that some of the
terms used were unintelligible to them. Mr. Dallas has endeavored to giv« th*
explanations of the terms in as popular a form as possible
475
4?6 GLOSSARY.
Annelids. — A class of worms in which the surface of the body
exhibits a more or less distinct division into rings or segments,
fenerally provided with appendages for locomotion and with gills,
t includes the ordinary marine worms, the earth-worms and the
leeches.
Antennae. — Jointed organs appended to the head in Insects. Crusta-
cea and Centipedes, and not belonging to the mouth.
Anthers. — The summits of the stamens of flowers, in which the
pollen or fertilizing dust is produced.
Ape ace nt alia, Aplacentata or Aplacental Mammals. See Mam-
malia.
Archetypal. — Of or belonging to the Archetype, or ideal primitive
form upon which all the beings of a group seem to be organized.
Articulata. — A great division of the Animal Kingdom characterized
generally by having the surface of the body divided into rings called
segments, a greater or less number of which are furnished with
jointed legs (such as Insects, Crustaceans and Centipedes).
Asymmetrical. — Having the two sides unlike.
Atrophied. — Arrested in development at a very early stage.
Balanus. — The genus including the common Acorn-shells which live
in abundance on the rocks of the seacoast.
Batrachians. — A class of animals allied to the Reptiles, but under-
going a peculiar metamorphosis, in which the young animal is
generally aquatic and breathes by gills. {Examples, Frogs, Toads,
and Newts.)
Bowlders. — Large transported blocks of stone generally embedded in
clays or gravels.
Brachiopoda. — A class of marine Mollusca, or soft-bodied animals,
furnished with a bivalve shell, attached to submarine objects by a
stalk which passes through an aperture in one of the valves, and
furnished with fringed arms, by the action of which food is carried
to the mouth.
Branchle. — Gills or organs for respiration in water.
Branchial. — Pertaining to gills or branchiae.
Cambrian System. — A series of very ancient Palaeozoic rocks, be-
tween the Laurentian and the Silurian. Until recently these were
regarded as the oldest fossiliferous rocks.
Canid^e. — The Dog-family, including the Dog, Wolf, Fox, Jackal,
etc.
Carapace. — The shell enveloping the anterior part of the body in
Crustaceans generally ; applied also to the hard shelly pieces of the
Cirri pedes.
Carboniferous. — This term is applied to the great formation which
includes, among other rocks, the coal-measures. It belongs to the
oldest, or Palaeozoic, system of formations.
Caudal. — Of or belonging to the tail.
Cephalopods. — The highest class of the Mollusca. or soft-bodied
animals, characterized by having the mouth surrounded by a greater
or less number of fleshy arms or tentacles, which, in most living
species are furnished with sucking-cups. (Examples, Cuttle-fish,
Nautilus.)
Cetacea. — An order of Mammalia, including the Whales, Dolphins,
etc., having the form of the body fish-like, the skin naked, and
only the fore limbs developed.
GLOSSARY. 477
ChelonIA.— An order of Reptiles including the Turtles, Tortoises,
etc.
Cirripedes. — An order of Crustaceans including the Barnacles and
Acorn-shells. Their young resemble those of many other Crusta-
ceans in form ; but when mature they are always attached to other
objects, either directly or by means of a stalk, and their bodies are
enclosed by a calcareous shell composed of several pieces, two of
which can open to give issue to a bunch of curled, jointed tentacles,
which represent the limbs.
Coccus. — The genus of Insects including the Cochineal. In these
the male is a minute, winged fly, and the female generally a motion-
less, berry-like mass.
Cocoon. — A case usually of silky material, in which insects are fre-
quently enveloped during the second or resting-stage (pupa) of their
existence. The term " cocoon-stage " is here used as equivalent to
"pupa-stage."
Ccelospermous. — A term applied to those fruits of the Umbelliferae
which have the seed hollowed on the inner face.
Coleoptera. — Beetles, an order of Insects having a biting mouth and
the first pair of wings more or less horny, forming sheaths for the
second pair, and usually meeting in a straight line down the
middle of the back.
Column. — A peculiar organ in the flowers of Orchids, in which the
stamens, style and stigma (or the reproductive parts) are united.
Composite or Compositous Plants. — Plants in which the inflores-
cence consists of numerous small flowers (florets) brought together
into a dense head, the base of which is enclosed by a common
envelope. (Examples, the Daisy, Dandelions, etc.)
Conferv^e. — The filamentous weeds of fresh water.
Conglomerate. — A rock made up of fragments of rock or pebbles,
cemented together by some other material.
Corolla. — The second envelope of a flower usually composed of col-
ored, leaf-like organs (petals), which may be united by their edges
either in the basal part or throughout.
Correlation. — The normal coincidence of one phenomenon, charac-
ter, etc., with another.
Corymb. — A bunch of flowers in which those springing from the
lower part of the flower-stalks are supported on long stalks so as to
be nearly on a level with the upper ones.
Cotyledons. — The first or seed-leaves of plants.
Crustaceans. — A class of articulated animals, having the skin of the
body generally more or less hardened by the deposition of calcareous
matter, breathing by means of gills. (Examples, Crab, Lobster,
Shrimp, etc.)
Curculio. — The old generic term for the Beetles known as Weevils,
characterized by their four jointed feet, and by the head being pro-
duced into a sort of beak, upon the sides of which the antennae are
inserted.
Cutaneous. — Of or belonging to the skin.
Degradation. — The wearing down of land by the action of the sea
or of meteoric agencies.
Denudation. — The wearing away of the surface of the land by
water.
Devonian System or Formation. — A series of Palaeozoic rocks,
including the Old Red Sandstone.
478 GLOSSARY.
Dicotyledons or Dicotyledonous Plants. — A class of plants
characterized by having two seed-leaves, by the formation of new
wood between the bark and the old wood (exogenous growth) and
by the reticulation of the veins of the leaves. The parts of the
flowers are generally in multiples of five.
Differentiation. — The separation or discrimination of parts or
organs which in simpler forms of life are more or less united.
Dimorphic. — Having two distinct forms — Dimorphism is the con-
dition of the appearance of the same species under two dissimilar
forms.
Dicecious. — Having the organs of the sexes upon distinct indi-
viduals.
Diorite. — A peculiar form of Greenstone.
Dorsal. — Of or belonging to the back.
Edentata. — A peculiar order of Quadrupeds, characterized by the
absence of at least the middle incisor (front) teeth in both jaws.
(Examples, the Sloths and Armadillos.)
Elytra. — The hardened fore-wings of Beetles, serving as sheaths for
the membraneous hind-wings, which constitute the true organs of
flight.
Embryo. — The young animal undergoing development within the egg
or womb.
Embryology. — The study of the development of the embryo.
Endemic. — Peculiar to a given locality.
Entomostraca. — A division of the class Crustacea, having all the
segments of the body usually distinct, gills attached to the feet
or organs of the mouth, and the feet fringed with fine hairs. They
are generally of small size.
Eocene. — The earliest of the three divisions of the Tertiary epoch of
geologists. Rocks of this age contain a small proportion of shells
identical with species now living.
Ephemerous Insects. — Insects allied to the May-fly.
Fauna. — The totality of the animals naturally inhabiting a certain
country or region, or which have lived during a given geological
period.
Felid js. — The Cat-family.
Feral, — Having become wild from a state of cultivation or domesti-
cation.
Flora. — The totality of the plants growing naturally in a country, or
during a given geological period.
Florets. — Flowers imperfectly developed in some respects, and col-
lected into a dense spike or head, as in the Grasses, the Dande-
lion, etc.
Fcetal. — Of or belonging to the foetus, or embryo in course of devel-
opment.
Foraminifera. — A class of animals of very low organization and
generally of small size, having a jelly-like body, from the surface
of which delicate filaments can be given off and retracted for the
prehension of external objects, and having a calcareous or sandy
shell, usually divided into chambers and perforated with small
apertures.
Fossiliferous. — Containing fossils.
GLOSSARY. 479
Fossorial. — Having a faculty of digging. The Fossorial Hymenop-
tera are a group of Wasp-like Insects, which burrow in sandy soil
to make nests for their young.
Frenum (pi. Frena). — A small band or fold of skin.
Fungi (sing. Fungus). — A class of cellular plants, of which Mush-
rooms, Toadstools, and Moulds are familiar examples.
Furcula. — The forked bone formed by the union of the collar-bones
in many birds, such as the common Fowl.
Gallinaceous Birds. — An order of Birds of which the common
Fowl, Turkey, and Pheasant are well-known examples.
Gallus. — The genus of birds which includes the common Fowl.
Ganglion. — A swelling or knot from which nerves are given off as
from a centre.
Ganoid Fishes. — Fishes covered with peculiar enamelled bony scales.
Most of them are extinct.
Germinal Vesicle. — A minute vesicle in the eggs of animals, from
which the development of the embryo proceeds.
Glacial Period. — A period of great cold and of enormous extension
of ice upon the surface of the earth. It is believed that glacial
periods have occurred repeatedly during the geological history of
the earth, but the term is generally applied to the close of the Ter-
tiary epoch, when nearly the whole of Europe was subjected to an
arctic climate.
Gland. — An organ which secretes or separates some peculiar product
from the blood or sap of animals or plants.
Glottis. — The opening of the windpipe into the oesophagus or
gullet.
Gneiss. — A rock approaching granite in composition, but more or
less laminated, and really produced by the alteration of a sediment-
ary deposit after its consolidation.
Grallatores. — The so-called Wading-birds (Storks, Cranes, Snipes,
etc.), which are generally furnished with long legs, bare of feathers
above the heel, and have no membranes between the toes.
Granite. — A rock consisting essentially of crystals of felspar and
mica in a mass of quartz.
Habitat. — The locality in which a plant or animal naturally lives.
Hemiptera. — An order or sub-order of Insects, characterized by the
possession of a jointed beak or rostrum, and by having the fore-wings
horny in the basal portion and membraneous at" the extremity,
where they cross each other. This group includes the various spe-
cies of Bugs.
Hermaphrodite. — Possessing the organ of both sexes.
Homology. — The relation between parts which results from their de-
velopment from corresponding embryonic parts, either in different
animals, as in the case of the arm of man, the fore-leg of a
quadruped, and the wing of a bird ; or in the same individual, as
in the case of the fore and hind legs in quadrupeds, and the seg-
ments or rings and their appendages of which the body of a worm,
a centipede, etc., is composed. The latter is called serial homology.
The parts which stand in such a relation to each other are said to
be homologous, and one such part or organ is called the homologue
of the other. In different plants the parts of the flower are homol-
ogous, and in general these parts are regarded as homologous with
leaves.
480 GLOSSARY.
Homoptera. — An order or sub-order of Insects having (like th«
Hemiptera) a jointed beak, but in which the fore-wings are eithei
wholly membranous or wholly leathery. The Cicadas, Frog-hop-,
pers, and Aphides, are well-known examples.
Hybrid. — The offspring of the union or two distinct species.
Hymenoptera. — An order of Insects possessing biting jaws and usu-
ally four membranous wings in which there are a few veins. Bees
and Wasps are familiar examples of this group.
Hypertrophied. — Excessively developed.
Ichneumonidje. — A family of Hyraenopterous insects, the members
of which lay their eggs in the bodies or eggs of other insects.
Imago. — The perfect (generally winged) reproductive state of an
insect.
Indigens. — The aboriginal animal or vegetable inhabitants of a coun-
try or region.
Inflorescence. — The mode of arrangement of the flowers of plants.
Infusoria. — A class of microscopic Animalcules, so called from their
having originally been observed in infusions of vegetable matters.
They consist of a gelatinous material enclosed in a delicate mem-
brane, the whole or part of which is furnished with short vibrating
hairs (called cilia), by means of which the animalcules swim through
the water or convey the minute particles of their food to the orifice
of the mouth.
Insectivorous. — Feeding on Insects.
Invertebrata or Invertebrate Animals. — Those animals which
do not possess a backbone or spinal column.
Lacunae. — Spaces left among the tissues in some of the lower animals,
and serving in place of vessels for the circulation of the fluids of
the body.
Lamellated. — Furnished with lamellae or little plates.
Larva (pi. Larvje). — The first condition of an insect at its issuing
from the egg, when it is usually in the form of a grub, caterpillar or
maggot.
Larynx. — The upper part of the windpipe opening into the gullet.
Laurentian. — A group of greatly altered and very ancient rocks,
which is greatly developed along the course of the St. Lawrence,
whence the name. It is in these that the earliest known traces of
organic bodies have been found.
Leguminos^e. — An order of plants represented by the common Pease
and Beans, having an irregular flower in which one petal stands up
like a wing, and the stamens and pistil are enclosed in a sheath
formed by two other petals. The fruit is a pod (or legume).
Lemuridje. — A group of four-handed animals, distinct from the Mon-
keys, and approaching the Insectivorous Quadrupeds in some of
their characters and habits. Its members have the nostrils curved
or twisted, and a claw instead of a nail upon the first finger of the
hind hands.
Lepidoptera. — An order of Insects, characterized by the possession
of a spiral proboscis, and of four large more or less scaly wings.
It includes the well-known Butterflies and Moths.
Littoral. — Inhabiting the seashore.
Loess. — A marly deposit of recent (Post-Tertiary) date, which occu-
pies a great part of the valley of the Khine.
GLOSSARY. 481
MalacostrAcA. — The higher division of the Crustacea, including the
ordinary Crabs, Lobsters, Shrimps, etc., together with the Wood-
lice and Sand-hoppers.
Mammalia. — The highest class of animals, including the ordinary
hairy quadrupeds, the Whales and Man, and characterized by the
production of living young which are nourished after birth by milk
from the teats (Mamma*, Mammary glands) of the mother. A
striking difference in embryonic development has led to the divis-
ion of this class into two great groups; in one of these, when the
embryo has attained a certain stage, a vascular connection, called
the placenta, is formed between the embryo and the mother; in
the other this is wanting, and the young are produced in a very
incomplete state. The former, including the greater part of the
class, are called Placental mammals ; the latter, or Aplacental
mammals, include the Marsupials and Monotremes (Ornitho-
rhynchus).
Mammifekous. — Having mammae or teats (see Mammalia).
Mandibles in Insects. — The first or uppermost pair of jaws, which
are generally solid, horny, biting organs. In Birds the term is
applied to both jaws with their horny coverings. In quadrupeds
the mandible is properly the lower jaw.
Marsupials. — An order of Mammalia in which the young are born in
a very incomplete state of development and carried by the mother,
while sucking, in a ventral pouch (marsupium), such as the Kan-
garoos, Opossums, etc. (see Mammalia).
Maxillae in Insects. — The second or lower pair of jaws, which are
composed of several joints and furnished with peculiar jointed
appendages called palpi or feelers.
Melanism. — The opposite of albinism; an undue development of
coloring material in the skin and its appendages.
Metamokphic Rocks. — Sedimentary rocks which have undergone
alteration, generally by the action of heat, subsequently to their
deposition and consolidation.
Mollusca. — One of the great divisions of the Animal Kingdom, in-
cluding those animals which have a soft body, usually furnished
with a shell, and in which the nervous ganglia, or centres, present
no definite general arrangement. They are generally known under
the denomination of " shell-fish; " the cuttle-fish, and the common
snails, whelks, oysters, mussels and cockles, may serve as examples
of them.
Monocotyledons, or Monocotyledonous Plants. — Plants in
which the seed sends up only a single seed-leaf (or cotyledon);
characterized by the absence of consecutive layers of wood in the
stem (endogenous growth), by the veins of the leaves being gener-
ally straight, and by the parts of the flowers being generally in mul-
tiples of three. (Examples, Grasses, Lilies, Orchids, Palms, etc.)
Moraines. — The accumulations of fragments of rock brought down
by glaciers.
Morphology. — The law of form or structure independent of function.
Mysis-stage. — A stage in the development of certain Crustaceans
(Prawns), in which they closely resemble the adults of a genus
(Mysis) belonging to a slightly lower group.
Nascent. — Commencing development.
Natatory. — Adapted for the purpose of swimming.
482 GLOSSARY.
Nauplius-form. — The earliest stage in the development of many
Crustacea, especially belonging to the lower groups. In this stage
the animal has a short body, with indistinct indications of a divis-
ion into segments, and three pairs of fringed limbs. This form of
the common fresh-water Cyclops was described as a distinct genus
under the name of Nauplius.
Neuration. — The arrangement of the veins or nervures in the wings
of Insects.
Nictitating Membrane. — A semi-transparent membrane, which
can be drawn across the eye in Birds and Reptiles, either to mod-
erate the effects of a strong light or to sweep particles of dust, etc.,
from the surface of the eye.
Neuters. — Imperfectly developed females of certain social insects
(such as Ants and Bees), which perform all the labors of the com-
munity. Hence they are also called workers.
Ocelli. — The simple eyes or stemmata of Insects, usually situated on
the crown of the head between the great compound eyes.
(Esophagus. — The gullet.
Oolitic. — A great series of secondary rocks, so called from the texture
of some of its members, which appear to be made up of a mass of
small egg-like calcareous bodies.
Operculum. — A calcareous plate employed by many Mollusca to close
the aperture of their shell. The opercular valves of Cirripedes
are those which close the aperture of the shell.
Orbit. — The bony cavity for the reception of the eye.
Organism. — An organized being, whether plant or animal.
Orthospermous. — A term applied to those fruits of the Umbelliferae
which have the seed straight.
Osculant. — Forms or groups apparently intermediate between and
connecting other groups are said to be osculant.
Ova. — Eggs.
Ovarium or Ovary (in plants). — The lower part of the pistil or
female organ of the flower, containing the ovules or incipient
seeds ; by growth after the other organs of the flower have fallen,
it usually becomes converted into the fruit.
Ovigerous. — Egg-bearing.
Ovules (of plants). — The seeds in the earliest condition.
Pachyderms. — A group of Mammalia so called from their thick
skins, and including the Elephant, Rhinoceros, Hippopotamus,
etc.
Palaeozoic. — The oldest system of fossiliferous rocks.
Palpi. — Jointed appendages to some of the organs of the mouth in
Insects and Crustacea.
Papilionace^e. — An order of Plants (see Leguminos^e). — The flowers
of these plants are called papilionaceous, or butterfly-like, from the.
fancied resemblance of the expanded superior petals to the wings
of a butterfly.
Parasite. — An animal or plant living upon or in, and at the expense
of, another organism.
Parthenogenesis. — The production of living organisms from unim-
pregnated eggs or seeds.
Pedunculated. — Supported upon a stem or stalk. The peduncul-
ated oak has its acorns borne upon a footstool.
GLOSSARY. 483
Peloria or Pelorism. — The appearance of regularity of structure
in the flowers of plants which normally bear irregular flowers.
Pelvis. — The bony arch to which the hind limbs of vertebrate animali
are articulated.
Petals. — The leaves of the corolla, or second circle of organs in j
flower. They are usually of delicate texture and brightly colored,
Phyllodineous. — Having flattened, leaf-like twigs or leaf-stalks ii
stead of true leaves.
Pigment. — The coloring material produced generally in the supe.-
ficial parts of animals. The cells secreting it are called pigmerd-
cells.
Pinnate. — Bearing leaflets on each side of a central stalk.
Pistils. — The female organs of a flower, which occupy a position in
the centre of the other floral organs. The pistil is generally divisi-
ble into the ovary or germen, the style and the stigma.
Placentalia, Placentata, or Placental Mammals. — See Mam-
malia.
Plantigrades. — Quadrupeds which walk upon the whole sole of the
foot, like the Bears.
Plastic. — Readily capable of change.
Pleistocene Period. — The latest portion of the Tertiary epoch.
Plumule (in plants). — The minute bud between the seed-leaves of
newly germinated plants.
Plutonic Rocks. — Rocks supposed to have been produced by igneous
action in the depths of the earth.
Pollen. — The male element in flowering plants; usually a fine dust
produced by the anthers, which, by contact, with the stigma effects
the fecundation of the seeds. This impregnation is brought about
by means of tubes (pollen-tubes) which issue 'from the pollen-grains
adhering to the stigma, and penetrate through the tissues until
they reach the ovary.
Polyandrous (flowers). — Flowers having many stamens.
Polygamous Plants. — Plants in which some flowers are unisexual
and others hermaphrodite. The unisexual (male and female)
flowers may be on the same or on different plants.
Polymorphic. — Presenting many forms.
Polyzoary. — The common structure formed by the cells of the
Polyzoa, such as the well-known Sea-mats.
Prehensile. — Capable of grasping.
Prepotent. — Having a superiority of power.
Primaries. — The feathers forming the tip of the wing of a bird, and
inserted upon that part which represents the hand of man.
Processes. — Projecting portions of bones, usually for the attachment
of muscles, ligaments, etc.
Propolis. — A resinous material collected by the Hive-Bees from the
opening buds of various trees.
Protean. — Exceedingly variable.
Protozoa. — The lowest great division of the Animal Kingdom.
These animals are composed of a gelatinous material and show
scarcely any trace of distinct organs. The Infusoria, Foramini-
fera and Sponges, with some other forms, belong to this division.
Pupa (pi. Pup js). — The second stage in the development of an In-
sect, from which it emerges in the perfect (winged) reproductive
form. In most insects the pupal stage is passed in perfect repose.
The chrysalis is the pupal state of Butterflies.
484 GLOSSARY.
Ramus. — One half of the lower jaw in the Mammalia. The portion
which rises to articulate with the skull is called the ascending
ramus.
Radicle. — The minute root of an embryo plant.
Range. — The extent of country over which a plant or animal is
naturally spread. Range in time expresses the distribution of a
species or group through the fossiliferous beds of the earth's
crust.
Retina. — The delicate inner coat of the eye, formed by nervous fila-
ments spreading from the optic nerve and serving for the perception
of the impressions produced by light.
Retrogression. — Backward development. When an animal, as it
approaches maturity, becomes less perfectly organized than might
be expected from its early stages and known relationships, it is said
to undergo a retrograde development or metamorphosis.
Rhizopods. — A class of lowly organized animals (Protozoa), having a
gelatinous body, the surface of which can be protruded in the
form of root-like processes or filaments, which serve for locomotion
and the prehension of food. The most important order is that of
the Foraminifera.
Rodents. — The gnawing Mammalia, such as the Rats, Rabbits and
Squirrels. They are especially characterized by the possession of a
single pair of chisel-like cutting teeth in each jaw, between which
and the grinding teeth there is a great gap.
Rubus. — The Bramble Genus.
Rudimentary. — Very imperfectly developed.
Ruminants. — The group of Quadrupeds which ruminate or chew the
cud, such as oxen, sheep and deer. They have divided hoofs, and
are destitute of front teeth in the upper jaw.
Sacral. — Belonging to the sacrum, or the bone composed usually of
two or more united vertebrae to which the sides of the pelvis in
vertebrate animals are attached.
Sarcode. — The gelatinous material of which the bodies of the lowest
animals (Protozoa) are composed.
Scutell^:. — The horny plates with which the feet of birds are gen-
erally more or less covered, especially in front.
Sedimentary Formations. — Rocks deposited as sediments from
water.
Segments. — The transverse rings of which the body of an articulate
animal or Annelid is composed.
Sepals. — The leaves or segments of the calyx, or outermost envelope
of an ordinary flower. They are usually green, but sometimes
brightly colored.
Serratures. — Teeth like those of a saw.
Sessile. — Not supported on a stem or footstalk.
Silurian System. — A very ancient system of fossiliferous rocks be-
longing to the earlier part of the Palaeozoic series.
Specialization. — The setting apart of a particular organ for the
performance of a particular function.
Spinal Cord. — The central portion of the nervous system in the
Vertebrata, which descends from the brain through the arches of
the vertebrae, and gives off nearly all the nerves tp the various
organs of the body.
GLOSSARY. 485
Stamens. — The male organs of flowering plants, standing in a circle
within the petals. They usually consist of a filament and an
anther, the anther being the essential part in which the pollen, or
fecundating dust, is formed.
Sternum. — The breast-bone.
Stigma. — The apical portion of the pistil in flowering plants.
Stipules. — Small leafy organs placed at the base of the footstalks of
the leaves in many plants.
Style. — The middle portion of the perfect pistil, which rises like a
column from the ovary and supports the stigma at its summit.
Subcutaneous. — Situated beneath the skin.
Suctorial. — Adapted for sucking.
Sutures (in the skull). — The lines of junction of the bones of which
the skull is composed.
Tarsus (pi. Tarsi). — The jointed feet of articulate animals, such as
insects.
Teleostean Fishes. — Fishes of the kind familiar to us in the
present day, having the skeleton usually completely ossified and
the scales horny.
Tentacula or Tentacles. — Delicate fleshy organs of prehension
or touch possessed by many of the lower animals.
Tertiary. — The latest geological epoch, immediately preceding the
establishment of the present order of things.
Trachea. — The windpipe or passage for the admission of air to the
lungs.
Tridactyle. — Three-fingered, or composed of three movable parts
attached to a common base.
Trilobites. — A peculiar group of extinct Crustaceans, somewhat
resembling the Wood-lice in external form, and, like some of them,
capable of rolling themselves up into a ball. Their remains are
found only in the Paleozoic rocks, and most abundantly in those of
Silurian age.
Trimorphic. — Presenting three distinct forms.
*»
Umbellifer^e. — An order of plants in which the flowers, which
contain five stamens and a pistil with two styles, are supported
upon footstalks which spring from the top of the flower stem and
spread out like the wires of an umbrella, so as to bring all the
flowers in the same head (umbel) nearly to the same level. {Ex-
amples, Parsley and Carrot. )
Ungulata. — Hoofed quadrupeds.
Unicellular. — Consisting of a single cell.
Vascular. — Containing blood-vessels.
Vermiform. — Like a worm.
Vertebrata ; or Vertebrate Animals. — The highest division of
the animal kingdom, so called from the presence in most cases of
a backbone composed of numerous joints or vertebrae, which con-
stitutes the centre of the skeleton and at the same time supports
and protects the central parts of the nervous system.
Whorls. — The circles or spiral lines in which the parts of plants are
arranged upon the axis of growth.
Workers. — See Neuters,
486 GLOSSARY.
Zoea-stage. — The earliest stage in the development of many of the
higher Crustacea, so called from the name of Zoea applied to these
young animals when they were supposed to constitute a peculiar
genus.
Zooids. — In many of the lower animals (such as the Corals Medusae,
etc. ) reproduction takes place in two ways, namely, by means of
eggs and by a process of budding with or without separation from
the parent of the product of the latter, which is often very differ-
ent from that of the egg. The individuality of the species is repre-
sented by the whole of the form produced between two sexual
reproductions; and these forms, which are apparently individual
animals, have been called zooids.
INDEX.
Aberrant groups, 419.
Abyssinia, plants of, 373.
Acclimatization, 125.
Adoxa, 193.
Affinities of extinct species, 334.
of organic beings, 414.
Agassiz, on Amblyopsis, 125.
on groups of species suddenly
appearing, 325.
on propbetic forms, 335.
on embryological succession, 344.
on the glacial period, 266.
on embryological characters, 408.
on the latest tertiary forms, 309.
on parallelism of embryological
development and geological
succession, 438.
Agassiz, Alex., on pedicellariae, 213.
Algas of New Zealand, 374.
Alligators, males, fighting, 77.
Alternate generations, 428.
Amblyopsis, blind fish, 125.
America, North, productions allied
to those of Europe, 370.
bowlders and glaciers of, 366.
America, South,- no modern forma-
tions on west coast, 311.
Ammonites, sudden extinction of,
329.
Anagallis, sterility of, 262.
Analogy of variations, 142.
Andaman Islands inhabited by a
toad, 388.
Ancylus, 382.
Animals, not domesticated from
being variable, 15.
domestic, descended from several
stocks, 16.
domestic, acclimatization of, 126.
Animals of Australia, 99.
with thicker fur in cold climates,
120.
blind, in caves, 123.
extinct, of Australia, 346.
Anomma, 260.
Antarctic islands, ancient flora of,
393.
Antechinus, 410.
Ants attending aphides, 229.
slave-making instinct, 239.
neuters, structure of, 255.
Apes, not having acquired intellec-
tual powers, 201.
Aphides, attended by ants, 229.
Aphis, development of, 431.
Apteryx, 157.
Arab horses, 29.
Aralo-Caspian Sea, 345.
Archeopteryx, 315.
Archiac, M. de, on the succession
of species, 332.
Artichoke, Jerusalem, 127.
Ascension, plants of, 385.
Asclepias, pollen of, 169.
Asparagus, 360.
Aspicarpa, 407.
Asses, striped, 143.
improved by selection, 34.
Ateuchus, 122.
Aucapitaine, on land-shells, 391.
Audubon, on habits of frigate-bird,
159.
on variation in birds' nests, 230.
on heron eating seeds, 383.
Australia, animals of, 99.
dogs of, 233.
extinct animals of, 346.
European plants in, 374.
glaciers of, 371.
Azara, on flies destroying cattle, 63.
Azores, flora of, 364.
Babington,Mr.,on British plants,42.
Baer, Von, standard of highness,
110.
comparison of bee and fish, 342.
embryonic similarity of the Ver-
tebrata, 429.
487
488
INDEX.
Baker, Sir S., on the giraffe, 198.
Balancement of growth, 131.
Baleen, 203.
Barberry, flowers of, 86.
Barrande, M. , on the Silurian colo-
nies, 323.
on the succession of species, 334.
on parallelism of palaeozoic for-
mations, 336.
on affinities of ancient species,
336.
Barriers, importance of, 355.
Bates, Mr., on mimetic butterflies,
315-317.
Batrachians on islands, 388.
Bats, how structure acquired, 156.
distribution of, 389.
Bear, catching water insects, 158.
Beauty, how acquired, 178, 358.
Bee, sting of, 182.
queen, killing rivals, 182.
Australian, extermination of, 67.
Bees, fertilizing flowers, 64.
hive, not sucking the red clover,
83.
hive, cell-making instinct, 243.
Ligurian, 83.
variation in habits, 230.
parasitic, 239.
humble, cells of, 244.
Beetles, wingless, in Madeira, 122.
with deficient tarsi, 121.
Bentham, Mr., on British plants,
42.
on classification, 408.
Berkeley, Mr., on seeds in salt
water, 359.
Bermuda, birds of, 386.
Birds acquiring fear, 231.
beauty of, 180.
annually cross the Atlantic, 365.
color of, on continents, 120.
footsteps and remains of, in sec-
ondary rocks, 314.
fossil, in caves of Brazil, 346.
of Madeira, Bermuda, and Gala-
pagos, 386.
song of males, 77.
transporting seeds, 365.
waders, 382.
wingless, 121, 157.
Bizcacha, 352.
affinities of, 419.
Bladder for swimming, in fish, 165.
Blindness of cave animals, 123.
Blyth, Mr., on distinctness of In-
dian cattle, 15.
on striped hemionus, 143.
on crossed geese, 266.
Borrow, Mr., on the Spanish
pointer, 28.
Bory St. Vincent, on Batrachians,
388.
Bosquet, M., on fossil Chthamalus,
315.
Bowlders, erratic, on the Azores,
364.
Branchiae, 165 166.
of crustaceans, 170.
Braun, Prof., on the seeds of
Fumariaceae, 193.
Brent, Mr., on house-tumblers, 233.
Britain, mammals of, 390.
Broca, Prof., on Natural Selection,
189.
Bronn, Prof., on duration of spe-
cific forms, 206.
various objections by, 189.
Brown, Robert, on classification j
405.
Brown-Sequard, on inherited muti-
lations, 122.
Busk, Mr., on the Polyzoa, 214.
Butterflies, mimetic, 416, 417.
Buzareingues, on sterility of varie-
ties, 285.
Cabbage, varieties of, crossed, 13.
Calceolaria, 264.
Canary-birds, sterility of hybrids,
265.
Cape de Verde Islands, produc-
tions of, 392.
plants of, on mountains, 373.
Cape of Good Hope, plants of, 114,
385.
Carpenter, Dr., on foraminifera,
341.
Carthamus, 193.
Catasetum, 173, 411.
Cats, with blue eyes, deaf, 10.
variation in habits of, 231.
curling tail when going to spring,
181.
Cattle destroying fir-trees, 63.
destroyed by flies in Paraguay,
63. '
breeds of, locally extinct, 95.
fertility of Indian and European
breeds, 266.
INDEX.
489
Cattle, Indian, 15, 266.
Cave, inhabitants of, blind, 123.
Cecidomyia, 428.
Celts, proving antiquity of man, 15.
Centres of creation, 357.
Cephalopodae, structures of eyes,169.
development of, 431.
Cercopitheeus, tail of, 210.
Ceroxylus laceratus, 203.
Cervulus, 265.
Cetacea, teeth and hair, 129.
development of the whalebone,
203.
Cetaceans, 203.
Ceylon, plants of, 366.
Chalk formation, 330.
Characters, divergence of, 95.
sexual, variable, 135, 138.
adaptive or analogical, 413.
Charlock, 67.
Checks to increase, 59.
mutual, 61.
Chelae of crustaceans, 214.
Ciiickens, instinctive tameness of,
234.
Chironomus, its asexual reproduc-
tion, 428.
Chthamalinae, 300.
Chthamalus, cretacean species of,
315.
Circumstances favorable to selec-
tion of domestic products, 33.
to natural selection, 88.
Cirri pedes capable of crossing, 88.
carapace aborted, 132.
their ovigerous frena, 166.
fossil, 315.
larvae of, 429.
Claparede, Prof., on the hair-clasp-
ers of the Acaridae, 171.
Clarke, Rev. W. B., on old glaciers
in Australia, 371.
Classification, 404.
Clift, Mr., on the succession of
types, 344.
Climate, effects of, in checking in-
crease of beings, 61.
adaptation of, to organisms, 126.
Climbing plants, 165.
development of, 217.
Clover visited by bees, 83.
Cobites, intestine of, 164.
Cockroach, 67.
Couections, palajontological, poor,
399,
Color, influenced by climate, 120.
in relation to attack by flies, 177.
Columba livia, parent of domestic
pigeons, 19.
Colym betes, 382.
Compensation of growth, 131.
Compos itae, flowers and seeds of, 130.
outer and inner florets of, 193.
male flowers of, 444.
Conclusion, general, 465.
Conditions, slight changes in.
favorable to fertility, 277.
Convergence of genera, 113.
Coot, 159.
Cope, Prof., on the acceleration or
retardation of the period of
reproduction, 166.
Coral islands, seeds drifted to, 361.
reefs, indicating movements of
earth, 361.
Corn-crake, 160.
Correlated variation in domestic
productions, 10.
Coryanthes, 173.
Creation, single centres of, 357.
Crinum, 264.
Croll, Mr., on subaerial denuda-
tion, 296.
on the age of our oldest forma-
tions, 318.
on alternate Glacial periods in
the North and South, 372.
Crosses, reciprocal, 269.
Crossing of domestic animals, im-
portance in altering breeds, 17.
advantages of, 84.
unfavorable to selection, 89.
Cruger, Dr., on Coryanthes, 173.
Crustacea of New Zealand, 374.
Crustacean, blind, 123.
air-breathers, 170.
Crustaceans, their chelae, 214.
Cryptocerus, 255.
Ctenomys, blind, 123.
Cuckoo, instinct of, 235.
Cunningham, Mr., on the flight of
the logger-headed duck, 121.
Currants, grafts of, 272.
Currents of sea, rate of, 360.
Cuvier, on conditions of existence,
228.
Cuvier, Fred., on instinct, 227.
on fossil monkeys, 314.
Cyclostoma, resisting salt water.
391.
490
INDEX.
Dana, Prof., on blind cave-animals,
125.
on relations of crustaceans of
Japan, 370,
on crustaceans of New Zealand,
374.
Dawson, Dr., on eozoon, 318.
De Candolle, Aug. Pyr., on strug-
gle for existence, 55.
on umbelliferse, 130.
on general affinities, 420.
De Candolle, Alph., on the varia-
bility of oaks, 44.
on low plants, widely dispersed,
397.
on widely-ranging plants being
variable, 48.
on naturalization, 99.
on winged seeds, 131.
on alpine species suddenly be-
coming rare, 151.
on distribution of plants with
large seeds, 361.
on vegetation of Australia, 376.
on fresh-water plants, 382.
on insular plants, 487.
Degradation of rocks, 296.
Denudation, rate of, 297.
of oldest rocks, 318.
of granite areas, 304.
Development of ancient forms,
341.
Devonian system, 339.
Dianthus, fertility of crosses, 268.
Dimorphism in plants, 40, 280.
Dirt on feet of birds, 363.
Dispersal, means of, 358.
during Glacial period, 365.
Distribution, goographical, 350.
means of, 358.
Disuse, effect of, under nature, 121.
Diversification of means for same
general purpose, 172.
Division, physiological, of labor, 99.
Divergence of character, 95.
Dog, resemblance of jaw to that of
the Thylacinus, 414.
Dogs, hairless, with imperfect
teeth, 10.
descended from several wild
stocks, 15.
domestic instincts of, 232.
inherited civilization of, 233.
fertility of breeds together, 266.
fertility of crosses, 283.
Dogs, proportions of body in dif-
ferent breeds, when young,
433.
Domestication, variation under, 6.
Double flowers, 255.
Downing, Mr., on fruit-trees in
America, 74.
Dragon flies, intestines of, 164.
Drift-timber, 361.
Driver-ant, 257.
Drones killed by other bees, 183.
Duck, domestic, wings of, reduced,
9.
beak of, 204.
logger-headed, 157.
Duckweed, 382.
Dugong, affinities of, 405.
Dung-beetles with deficient tarsi,
121.
Dytiscus, 382.
Earl, Mr. W., on the Malay Archi-
pelago, 389.
Ears, drooping, in domestic ani-
mals, 10.
rudimentary, 440.
Earth, seeds in roots of trees, 361.
charged with seeds, 363.
Echinodermata, their pedicellariaa,
212.
Eciton, 255.
Economy of organization, 132.
Edentata, teeth and hair, 129.
fossil species of, 462.
Edwards, Milne, on physiological
division of labor, 99.
on gradations of structure, 174.
Edwards, on embryological charac-
ters, 408.
Eggs, young birds escaping from,
75.
Egypt, productions of, not modi-
fied, 188.
Electric organs, 167.
Elephant, rate of increase, 57.
of Glacial period, 127.
Embryology, 427.
Eozoon Canadense, 318.
Epilepsy inherited, 121.
Existence, struggle for, 54.
condition of, 183.
Extinction, 326.
as bearing on natural selection,
105.
of domestic varieties, 108.
INDEX.
491
Syv, structure of, 162.
Eye, correction for aberration, 182.
Eyes, reduced, in moles, 123.
Fabre, M., on hymenoptera fight-
ing, 77.
on parasitic sphex, 239.
on Sitaris, 436.
Falconer, Dr., on naturalization of
plants in India, 58.
on elephants and mastodons, 339.
and Cautley, on mammals of
sub-Himalayan beds, 345.
Falkland Islands, wolf of, 388.
Faults, 299.
Faunas, marine, 352.
Fear, instinctive, in birds, 231.
Feet of birds, young mollusks ad-
hering to, 382.
Fertilization variously effected,
173 180
Fertility of hybrids, 268.
from slight changes in condi-
tions, 278.
of crossed varieties, 283.
Fir-trees destroyed by cattle, 63.
pollen of, 183.
Fish, flying, 157.
teleostean, sudden appearance
of, 316.
eating seeds, 362, 383.
fresh-water, distribution of, 382.
Fishes, ganoid, now confined to
fresh water, 92.
electric organs of, 167.
ganoid, living in fresh water, 329.
of southern hemisphere, 376.
Flat-fish, their structure, 206.
Flight, powers of, how acquired,
157.
Flint-tools, proving antiquity of
man, 15.
Flower, Prof., on the Larnyx, 212.
on Halitherium, 335.
on the resemblance between the
jaws of the dog and Thyla-
cinus, 414.
on the homology of the feet of
certain marsupials, 423.
Flowers, structure of, in relation to
crossing, 81.
of composite and umbelliferse,
130, 193.
beauty of, 180.
Rouble, 255,
Flysch formation, destitute of or-
ganic remains, 300.
Forbes, Mr. D., on glacial action
in the Andes, 371.
Forbes, E., on colors of shells, 120.
on abrupt range of shells in
depth, 151.
on poorness of palaeontological
collections, 299, 300.
on continuous succession of
genera, 325.
on continental extensions, 358,
359.
on distribution during Glacial
period, 366.
on parallelism in time and space,
400.
Forests, changes in, in America,
65.
Formation, Devonian, 339.
Cambrian, 317.
intermittent, 307.
Formations, thickness of, in
Britain, 297.
Formica rufescens, 239.
sanguinea, 240.
flava, neuter of, 256.
Forms, lowly organized, long en-
during, 104.
Frena, ovigerous, of cirripedes, 166.
Fresh-water productions, dispersal
of, 383.
Fries, on species in large genera
being closely allied to other
species, 51.
Frigate-bird, 159.
Frogs on islands, 388.
Fruit-trees, gradual improvement
of, 30.
in United States, 74.
varieties of, acclimatized in
United States, 127.
Fuci, crossed, 269, 275.
Fur thicker in cold climates, 120.
Furze, 429.
Galapagos Archipelago, birds of,
386.
productions of, 391, 343.
Galaxias, its wide range, 380.
Galeopithecus, 156.
Game, increase of, checked by
vermin, 60.
Gartner, on sterility of hybrids,
261, 262.
492
INDEX.
Gartner, on reciprocal crosses, 263.
on crossed maize and verbascum,
285.
on comparison of hybrids and
mongrels, 287, 288.
Gaudry, Prof., on intermediate
genera of fossil mammals in
Attica, 335.
Geese, fertility when crossed, 266.
upland, 159.
Geikie, Mr., on subaerial denuda-
tion, 296.
Genealogy, important in classifi-
cation, 403.
Generations, alternate, 428.
Geographical distribution, 350.
Geography, ancient, 472.
Geology, future progress of, 472.
imperfection of the record, 472.
Gervais, Prof., on Typotherium,
335.
Giraffe, tail of, 175.
structure of, 197.
Glacial period, 365.
affecting the North and South,
371.
Glands, mammary, 210.
Gmelin, on distribution, 366.
Godwin-Austen, Mr., on the Malay
Archipelago, 311.
Goethe, on compensation of
growth, 131.
Gomphia, 194.
Gooseberry, grafts of, 272.
Gould, Dr. Aug. A., on land shells,
391.
Gould, Mr., on colors of birds, 120.
on instincts of cuckoo, 237.
on distribution of genera of birds,
396.
Gourds, crossed, 285.
Graba, on the Uria lacrymas, 80.
Grafting, capacity of, 271, 272.
Granite, areas of denuded, 304.
Grasses, varieties of, 97.
Gray, Dr. Asa, on the variability
of oaks, 45.
on man not causing variability,
69.
on sexes of the holly, 82.
on trees of the United States,
87.
on naturalized plants in the
United States, 99.
on aestivation, 194.
Gray, Dr. Asa, on rarity of Inter-
mediate varieties, 152.
on Alpine plants, 366.
Gray, Dr. J. E., on striped mule,
143.
Grebe, 159.
Grimm, on asexual reproduction,
428.
Groups, aberrant, 419.
Grouse, colors of, 74.
red, a doubtful species, 43.
Growth, compensation of, 131.
Gunther, Dr., on flat-fish, 208.
on prehensile tails, 210.
on the fishes of Panama, 351.
on the range of fresh-water
fishes, 381.
on the limbs of Lepidosiren, 441.
Haast, Dr., on glaciers of New
Zealand, 371.
Habit, effect of, under domesti-
cation, 10.
effect of, under nature, 121.
diversified, of same species,
155.
Hackel, Prof., on classification and
the lines of descent, 422.
Hair and teeth, correlated, 129.
Halitherium, 335.
Harcourt, Mr. E. V., on the birds
of Madeira, 386.
Hartung, M., on bowlders in the
Azores, 364.
Hazel-nuts, 360.
Hearne, on habits of bears, 158.
Heath, changes in vegetation, 62.
Hector, Dr., on glaciers of New
Zealand, 371.
Heer, Oswald, on ancient culti-
vated plants, 15.
on plants of Madeira, 92.
Helianthemum, 194.
Helix pomatia, 391.
resisting salt water, 391.
Helmholtz, M., on the imperfection
of the human eye, 182.
Helosciadium, 360.
Hemionus, striped, 145.
Hensen, Dr., on the eyes of
Cephalopods, 169.
Herbert, W., on struggle for exist-
ence, 55.
on sterility of hybrids, 263.
Hermaphrodites crossing, 84,
INDEX.
493
Heron eating seed, 383.
Heron, Sir R., on peacocks, 77.
Heusinger, on white animals poi-
soned by certain plants, 10.
Hewitt, Mr., on sterility of first
crosses, 275.
Hildebrand, Prof., on the self-
sterility of Corydalis, 264.
Hilgendorf, on intermediate varie-
ties, 305.
Himalaya, glaciers of, 371.
plants of, 373.
Hippeastrum, 264.
Hippocampus, 210.
Hofmeister, Prof., on the move-
ments of plants, 219.
Holly-trees, sexes of, 82.
Hooker, Dr., on trees of New
Zealand, 87.
on acclimatization of Himalayan
trees, 126.
Hooker, Dr., on flowers of urn-
belliferae, 130.
on the position of ovules, 191,
192.
on glaciers of Himalaya, 371.
on algae of New Zealand, 374.
on vegetation at the base of the
Himalaya, 375.
on plants of Tierra del Fuego,
373.
on Australian plants, 374, 393.
on relations of flora of America,
373.
on flora of the Antarctic lands,
378, 393.
on the plants of the Galapagos,
387, 392.
on glaciers of the Lebanon,
371.
on man not causing variability,
69.
on plants of mountains of Fer-
nando Po, 373.
Hooks on palms, 177.
on seeds on islands, 387.
Hopkins, Mr., on denudation, 303.
Hornbill, remarkable instinct of,
259.
Horns, rudimentary, 443.
Horse, fossil, in La Plata, 326.
proportions of, when young, 432.
Horses destroyed by flies in Para-
guay, 63.
Striped, 143.
Horticulturists, selection applied
by, 26.
Huber, on cells of bees, 248.
Huber, P., on reason blended with
instinct, 227.
on habitual nature of instincts,
228.
on slave-making ants, 239.
on Melipona domestica, 244.
Hudson, Mr., on the ground-
woodpecker of La Plata, 158.
on the Molothrus, 238.
Humble-bees, cells of, 244.
Hunter, J., on secondary sexual
characters, 134.
Hutton, Captain, on crossed geese,
266.
Huxley, Prof., on structure of
hermaphrodites, 88.
on the affinities of the Sirenia,
335.
on forms connecting birds and
reptiles, 335.
on homologous organs, 427.
on the development of aphis,
431, 432.
Hybrids and mongrels compared,
287.
Hybridism, 260.
Hydra, structure of, 164.
Hymenoptera, fighting, 77.
Hymenopterous insect, diving, 159.
Hyoseris, 193.
Ibla, 132.
Icebergs transporting seeds, 364.
Increase, rate of, 56.
Individuals, numbers favorable to
selection, 88.
many, whether simultaneously-
created, 357.
Inheritance, laws of, 11.
at corresponding ages, 12.
Insects, color of, fitted for thei*
stations, 73.
sea-side, colors of, 120.
blind, in caves, 125.
luminous, 169.
their resemblance to certain ob-
jects, 201, 202.
neuter, 255.
Instinct, 227.
not varying simultaneously with
structure, 253.
Instincts, domestic, 232.
494
INDEX.
Intercrossing, advantages of, 85,
285.
Islands, oceanic, 384.
Isolation favorable to selection, 90.
Japan, productions of, 370.
Java, plants of, 373.
Jones, Mr. J. M., on the birds of
Bermuda, 386.
Jourdain, M., on the eye-spots of
star fishes, 161.
Jukes, Prof., on subaerial denuda-
tion, 296.
Jussieu, on classification, 407.
Kentucky, caves of, 124.
Kerguelen-land, flora of, 378, 392.
Kidney-bean, acclimatization of,
128.
Kidneys of birds, 129.
Kirby, on tarsi deficient in beetles,
121.
Knight, Andrew, on cause of vari-
ation, 6.
Kolreuter, on intercrossing, 84.
on the barberry, 86.
on sterility of hybrids, 262, 263.
on crossed varieties of nicotiana,
286.
on crossing male and hermaphro-
dite flowers, 439.
on reciprocal crosses, 269.
Lamarck, on adaptive characters,
413.
Lancelet, 112.
eyes of, 163.
Landois, on the development of the
wings of insects, 166.
Land shells, distribution of, 381.
of Madeira, naturalized, 386.
resisting salt water, 382.
Languages, classification of, 410.
Lankester, Mr. E. Ray, on longev-
ity, 188.
on homologies, 426.
Lapse, great, of time, 295.
Larvae, 429-431.
Laurel, nectar secreted by the
leaves, 81.
Laurentian formation, 318.
Laws of variation, 119.
Leech, varieties of, 66.
Legurninosae, D^ctar secreted by
glands, 81.
Leibnitz' attack on Newton, 460.
Lepidosiren, 92, 336.
limbs in a nascent condition, 441.
Lewes, Mr. G. H., on species not
having changed in Egypt, 188.
on the Salamandra atra, 439.
on many forms of life having been
at first evolved, 470.
Life, struggle for, 54.
Lingula, Silurian, 317.
Linnaeus, aphorism of, 404.
Lion, mane of, 77.
young of, striped, 429.
Lobelia fulgens, 64, 86.
sterility of crosses, 264.
Lock wood, Mr., on the ova of the
Hippocampus, 210.
Locusts transporting seeds, 363.
Logan, Sir W., on Laurentian for-
mation, 318.
Lowness of structure connected
with variability, 133.
related to wide distribution, 397.
Lowe, Rev. R. T., on locusts visit-
ing Madeira, 362.
Lubbock, Sir J., on the nerves of
coccus, 39.
on secondary sexual characters,
139.
on a diving hymenopterous in-
sect, 159.
on affinities, 310.
on metamorphoses, 427.
Lucas, Dr. P., on inheritance, 11.
on resemblance of child to parent,
289.
Lund and Clausen, on fossils of
Brazil, 344.
Lyell, Sir C, on the struggle for
existence, 55.
on modern changes of the earth,
84.
on terrestrial animals not having
been developed on islands, 200.
on a carboniferous land-shell, 301.
on strata beneath Silurian system,
364.
on the imperfection of the geo-
logical record, 321.
on the appearance of species, 322.
on Barrande's colonies, 323.
on tertiary formations of Europe
and North America, 330.
on parallelism of tertiary forma-
tions, 334.
INDEX.
495
Lyell, Sir Cv on transport of seeds
by icebergs, 364.
on great alterations of climate,
378.
on the distribution of fresh-water
shells, 382.
on land-shells of Madeira, 395.
Lyell and Dawson, on fossilized
trees in Nova Scotia, 308.
Lythrum salicaria, trimorphic, 280.
Macleay, on analogical characters,
413.
Macrauchenia, 335.
M'Donnell, Dr., on electric organs,
168.
Madeira, plants of, 92.
beetles of, wingless, 122.
fossil land-shells of, 345.
birds of, 386.
Magpie tame in Norway, 231.
Males fighting, 77.
Maize, crossed, 285.
Malay Archipelago compared with
Europe, 311.
mammals of, 389.
Malm, on flat-fish, 208.
Malpighiacese, 407.
small imperfect flowers of, 192.
Mammse, their development, 211.
rudimentary, 439.
Mammals, fossil, in secondary for-
mation, 314.
insular, 388, 389.
Man, origin of, 473.
Manatee, rudimentary nails of, 442.
Marsupials of Australia, 99.
structure of their feet, 423.
Marsupials, fossil species of, 344.
Martens, M., experiment on seeds,
360.
Martin, Mr. W. C, on striped
mules, 145.
Masters, Dr., on Saponaria, 194.
Matteucci, on the electric organs
of rays, 168.
Matthiola, reciprocal crosses of, 270.
Maurandia, 218.
Means of dispersal, 358.
Melipona domestica, 244.
Merrill, Dr., on the American
cuckoo, 235.
Metamorphism of oldest rocks, 319.
Mice destroying bees, 65.
acclimatization of, 127,
Mice, tails of, 210.
Miller, Prof., on the cells of bees,
245, 249.
Mirabilis, crosses of, 269.
Missel-thrush, 67.
Mistletoe, complex relations of, 3.
Mivart, Mr., on the relation of hair
and teeth, 129.
on the eyes of cephalopods, 169.
various objections to Natural Se-
lection, 196.
on abrupt modifications, 214.
on the resemblance of the mouse
and antechinus, 413.
Mocking-thrush of the Galapagos,
395.
Modification of species not abrupt,
468.
Moles, blind, 123.
Molothrus, habits of, 238.
Mongrels, fertility and sterility of,
2S4. '
and hybrids compared, 287.
Monkeys, fossil, 314.
Monachanthus, 411.
Mons, Van, on the origin of fruit
trees, 23.
Monstrosities, 37.
Moquin-Tandon, on sea-side plants,
120.
Morphology, 422.
Morren, on the leaves of Oxalis,
219.
Moths, hybrid, 266.
Mozart, musical powers of, 228.
Mud, seeds in, 383.
Mules, striped, 143.
Miiller, Adolph, on the instincts of
the cuckoo, 235.
Miiller, Dr. Ferdinand, on alpine
Australian plants, 374.
Miiller, Fritz, on dimorphic crusta-
ceans, 40, 257.
on the lancelet, 112.
on air-breathing crustaceans, 170.
on the self-sterility of orchids,
264.
on embryology in relation to clas-
sification, 407.
on the metamorphoses of crusta-
ceans, 432, 437.
on terrestrial and fresh-water
organisms not undergoing any
metamorphosis, 435.
on climbing plants, 218.
496
INDEX.
Multiplication of species not indefi-
nite, 115.
Murchison, Sir R., on the forma-
tions of Russia, 301.
on azoic formations, 318.
on extinction, 326.
Murie, Dr., on the modification of
the skull in old age, 166.
Murray, Mr. A., on cave-insects,
125.
Mustela, vision, 155.
Myanthus, 411.
Myrmecocystus, 255.
Myrmica, eyes of, 256.
Nageli, on morphological charac-
ters, 191.
Nails, rudimentary, 442.
Nathusius, Von, on pigs, 178.
Natural History, future progress
of, 470.
Natural Selection, 69.
Natural system, 404.
Naturalization of forms distinct
from the indigenous species,
96.
Naturalization in New Zealand, 182.
Naudin, on analogous variations in
gourds, 140.
on hybrid gourds, 285.
on reversion, 288.
Nautilus, Silurian, 317.
Nectar of plants, 81.
Nectaries, how formed, 81.
Nelumbium luteum, 383.
Nests, variations in, 230, 252, 259.
Neuter insects, 256, 257.
Newman, Col., on humble-bees,
65.
New Zealand, productions of, not
perfect, 182.
naturalized products of, 343.
fossil birds of, 345.
glaciers of, 371.
crustaceans of, 374.
algae of, 374.
number of plants of, 385.
flora of, 393.
Newton, Sir I., attacked for irreli-
gion, 466.
Newton, Prof. , on earth attached to
a partridge's foot, 363.
Nicotiana, crossed varieties of, 286.
pertain species very sterile, 269.
JNjfcsche, Dr., on ttye Polyzoa, 214.
Noble, Mr., on fertility of Rhodo-
dendron, 264.
Nodules, phosphatic, in azoic
rocks, 318.
Oaks, variability of, 45.
Onites appelles, 122.
Ononis, small imperfect flowers of,
192.
Orchids, fertilization of, 173.
the development of their flowers,
217.
forms of, 411.
Orchis, pollen of, 169.
Organization, tendency to advance,
109.
Organs of extreme perfection, 160.
electric, of fishes, 167.
of little importance, 175.
homologous, 423.
rudiments of, and nascent, 440.
Ornithorhynchus, 92, 406.
mammas of, 211.
Ostrich not capable of flight, 200.
habit of laying eggs together, 239.
American, two species of, 352.
Otter, habits of, how acquired, 155.
Ouzel, water, 159.
Owen, Prof., on birds not flying,
121.
on vegetative repetition, 133.
on variability of unusually de-
veloped parts, 133.
on the eyes of fishes, 163.
on the swim-bladder of fishes, 165.
on fossil horse of La Plata, 326.
on generalized form, 335.
on relation of ruminants and
pachyderms, 335.
on fossil birds of New Zealand,
345.
on succession of types, 345.
on affinities of the dugong, 405.
on homologous organs, 425.
on the metamorphosis of cephal-
opods, 431.
Pacific Ocean, faunas of, 352.
Pacini, on electric organs, 169.
Paley, on no organ formed to give
pain, 181.
Pallas, on the fertility of the do-
mesticated descendants of wild
stocks, 266.
Palm with hooks, 177,
INDEX.
497
Papaver bracteatum, 194.
Paraguay, cattle destroyed by flies,
63.
Parasites, 238.
Partridge, with ball of earth at-
tached to foot, 363.
Parts greatly developed, variable,
130.
Parus major, 158.
Passiflora, 264.
Peaches in United States, 74.
Pear, grafts of, 272.
Pedicellarise, 213.
Pelargonium, flowers of, 130.
sterility of, 264.
Pelvis of women, 129.
Peloria, 130.
Period, glacial, 365.
Petrels, habits of, 150.
Phasianus, fertility of hybrids, 265.
Pheasant, young, wild, 233.
Pictet, Prof., on groups of species
suddenly appearing, 313.
on rate of organic change, 320.
on continuous succession of
genera, 322.
on close alliance of fossils in con-
secutive formations, 340.
on change in latest tertiary forms,
313.
on early transitional links, 316.
Pierce, Mr., on varieties of wolves,
79.
Pigeons with feathered feet, and
skin between toes, 10.
breeds described, and origin of,
17.
breeds of, how produced, 32, 34.
tumbler, not being able to get
out of egg, 75.
reverting to blue color, 141.
instinct of tumbling, 233.
young of, 434.
Pigs, black, not affected by the
paint-root, 10.
modified by want of exercise, 178.
Pistil, rudimentary, 439.
Plants, poisonous, not affecting
certain colored animals, 10.
selection, applied to, 29.
gradual improvement of, 30.
not improved in barbarous coun-
tries, 30.
dimorphic, 40, 280.
destroyed by insects, 60.
Plants, in midst of range, have to
struggle with other plants, 67.
nectar of, 83.
fleshy, on seashores, 120.
climbing, 165, 217.
fresh-water, distribution of, 382.
low in scale, widely distributed,
397.
Pleuronectidse, their structure, 207.
Plumage, laws of changes in sexes
of birds, 78.
Plums in the United States, 74.
Pointer dog, origin of, 28.
habits of, 232.
Poison not affecting certain colored
animals, 10.
similar effect of, on animals and
plants, 469.
Pollen of fir-trees, 183.
transported by various means,
172, 180.
Pollinia, their development, 216.
Polyzoa, their avicularia, 214.
Poole, Col., on striped hemionus,
143.
Poteamogeton, 383.
Pouchet, on the colors of fiat-fish,
209.
Prestwich, Mr., on English and
French eocene formations, 334.
Proctotrupes, 159.
Proteolepas, 132.
Proteus, 125.
Psychology, future progress of, 473.
Pyrgoma, found in the chalk, 315.
Quagga, striped, 143.
Quatrefages, M., on hybrid moths,
266.
Quercus, variability of, 45.
Quince, grafts of, 272.
Rabbits, disposition of young, 233.
Races, domestic, characters of, 14.
Race-horses, Arab, 29.
English, 357.
Radcliffe, Dr., the electrical organs
of the torpedo, 168.
Raymond, on plants of Pyrenees,
367.
Ramsay, Prof., on subaerial denu-
dation, 296.
on thickness of the British forma-
tions, 297.
on faults, 297.
498
INDEX.
Ramsay, Mr., on instinct of cuckoo,
236.
Ratio of increase, 56.
Rats supplanting each other, 65.
acclimatization of, 127.
blind, in cave, 123.
Rattlesnake, 181.
Reason and instinct, 228.
Recapitulation, general, 447.
Reciprocity of crosses, 269.
Record, geological, imperfect, 293.
Rengger, on flies destroying cattle,
63.
Reproduction, rate of, 56.
Resemblance, protective, of insects,
201.
to parents in mongrels and
hybrids, 287.
Reversion, law of inheritance, 13.
in pigeons, to blue color, 141.
Rhododendron, sterility of, 264.
Richard, Prof., on Aspicarps, 407.
Richardson, Sir J., on structure of
squirrels, 155.
on fishes of the southern hemi-
sphere, 374.
Robin ia, grafts of, 272.
Rodents, blind, 123.
Rogers, Prof., map of N. America,
304.
Rudimentary organs, 439.
Rudiments important for classifica-
tion, 407.
Rutimeyer on Indian cattle, 16,
266.
Salamandre atra, 439.
Saliva used in nests, 252.
Salvin, Mr., on the beaks of ducks,
205.
Sageret, on grafts, 272.
Salmons, males fighting, and
hooked jaws of, 77.
Salt water, how far injurious to
seeds, 359.
not destructive to land shells, 391.
Salter, Mr. , on early death of hybrid
embryos, 275.
Saurophagus sulphuratus, 158.
Schacht, Prof., on Phyllotaxy, 193.
Schiodte, on blind insects, 124.
on flat-fish, 207.
Schlegel, on snakes, 129.
School, Dr., on the ears of mice,
191.
Scott, J., Mr., on the self-sterility
of orchids, 264.
on the crossing of varieties of
verbascum, 286.
Sea water, how far injurious to
seeds, 359.
not destructive to land shells,
391.
Seabright, Sir J., on crossed ani-
mals, 17.
Sedgwick, Prof., on groups of
species suddenly appearing,
313.
Seedlings destroyed by insects, 60.
Seeds, nutriment in, 67.
winged, 131.
means of dissemination, 172,
180.
power of resisting salt water, 359.
in crops and intestines of birds,
362.
eaten by fish, 362, 383.
in mud, 383.
hooked, on islands, 387.
Selection of domestic products, 23.
principle not of recent origin, 27.
unconscious, 28.
natural, 69.
sexual, 76.
objections to term, 70.
natural, has not induced sterility,
273.
Sexes, relations of, 76.
Sexual characters variable, 138.
selection, 70.
Sheep, Merino, their selection, 25.
two sub-breeds, unintentionally
produced, 29.
mountain varieties of, 66.
Shells, colors of, 120.
hinges of, 172.
littoral, seldom embedded, 300.
fresh-water, long retain the same
forms, 341.
fresh-water, dispersal of, 381.
of Madeira, 486.
land, distribution of, 486.
land, resisting salt water, 391.
Shrew mouse, 413.
Silene, infertility of crosses, 269.
Silliman, Prof., on blind rat, 123.
Sirenia, their affinities, 335.
Sitaris, metamorphosis of, 436.
Skulls of young mammals, 171,425.
Slave-making instinct, 239.
INDEX.
499
Smith, Col. Hamilton, on striped
horses, 144.
Smith, Mr. Fred., on slave-making
ants, 240.
on neuter ants, 256.
Smith, Dr., on the Polyzoa, 214.
Snake with tooth for cutting
through egg-shell, 237.
Somerville, Lord, on selection of
sheep, 25.
Sorbus, grafts of, 272.
So rex, 413.
Spaniel, King Charles breed, 28.
Specialization of organs, 110.
Species, polymorphic, 39.
dominant, 48.
common, variable, 47.
in large genera variable, 49.
groups of, suddenly appearing,
313, 316.
beneath Silurian formations, 317.
successively appearing, 320.
changing simultaneously through-
out the world, 330.
Spencer, Lord, on increase in size
of cattle, 29.
Spencer, Herbert, Mr., on the first
steps in differentiation, 112.
on the tendency to an equilibrium
in all forces, 279.
Sphex, parasitic, 239.
Spiders, development of, 431.
Sports in plants, 9.
Sprengel, C. C, on crossing, 84.
on ray-florets, 130.
Squalodon, 335.
Squirrels, gradations in structure,
155.
Staffordshire, heath, changes in,
62.
Stag-beetles, fighting, 77.
Starfishes, eyes of, 161.
their pedicellarire, 213.
Sterility from changed conditions
of life, 8.
of hybrids, 262.
of hybrids, laws of, 267.
of hybrids, causes of, 273.
from unfavorable conditions, 277.
not induced through natural se-
lection, 274.
St. Helena, productions of, 385.
St. Hilaire, Aug., on variability of
certain plants, 194.
on classification, 407.
St. Hilaire, Geoffroy, on balance-
ment, 131.
on homologous organs, 423.
St. Hilaire, Isidore, on variability
of repeated parts, 132.
on correlation, in monstrosities,
10.
on correlation, 129.
on variable parts being often
monstrous, 137.
St. John, Mr., on habits of cats,
231.
Sting of bee, 182.
Stocks, aboriginal, of domestic ani-
mals, 16.
Strata, thickness of, in Britain, 298.
Stripes on horses, 143.
Structure, degrees of utility of, 178.
Struggle for existence, 54.
Succession, geological, 322.
of types in same areas, 344.
Swallow, one species supplanting
another, 67.
Swaysland, Mr., on earth adhering
to the feet of migratory birds,
363.
Swifts, nests of, 252.
Swim-bladder, 165.
Switzerland, lake inhabitations of,
15.
System, natural, 404.
Tail of giraffe, 175.
of aquatic animals, 176.
prehensile, 209.
rudimentary, 441.
Tanais, dimorphic, 40.
Tarsi, deficient, 122.
Tausch, Dr., on umbelliferse, 193.
Teeth and hair correlated, 129.
rudimentary, in embryonic calf,
439, 464.
Tegetmeier, Mr., on cells of bees,
246, 250.
Temminck, on distribution aiding
classification, 408.
Tendrils, their development, 217.
Thompson, Sir W., on the age of
the habitable world, 317.
on the consolidation of the crust
of the earth, 453.
Thouin, on grafts, 272.
Thrush, aquatic species of, 159.
mocking, of the Galapagos, 395.
young of, spotted, 429.
50u
INDEX.
Thrush, nest of, 259.
Thuret, M.t on crossed fuci, 269.
Thwaites, Mr., on acclimatization,
126.
Thylacinus, 414.
Tierra del Fuego, dogs of, 233.
plants of, 478.
Timber-drift, 361.
Time, lapse of, 295.
by itself not causing modification,
90.
Titmouse, 158.
Toads on islands, 388.
Tobacco, crossed varieties of, 286.
Tomes, Mr., on the distribution of
bats, 389.
Transitions in varieties rare, 150.
Traquair, Dr., on flat-fish, 209.
Trautschold, on intermediate vari-
eties, 305.
Trees on islands belong to peculiar
orders, 387.
with separated sexes, 87.
Trifolium pratense, 64, 83.
incarnatum, 83.
Trigonia, 329.
Trilobites, 317.
sudden extinction of, 329.
Trimen, Mr., on imitating insects,
417.
Trimorphism in plants, 39, 279.
Troglodytes, 259.
Tuco-tuco-blind, 123.
Tumbler pigeons, habits of, heredi-
tary, 232.
Tumbler, young of, 434.
Turkey-cock, tuft of hair on breast,
78.
naked skin on head, 177.
young of, instinctively wild, 233.
Turnip and cabbage, analogous
variations of, 140.
Type, unity of, 186.
Types, succession of, in same areas,
344.
Typotherium, 335.
Udders, enlarged by use, 10.
rudimentary, 441.
Ulex, young leaves of, 429.
Umbelliferae, flowers and seeds of,
130.
outer and inner florets of, 193.
Unity of type, 186.
Uria lacrymans, 80.
Use, effects of, under domestica-
tion, 10.
effects of, in a state of nature,
121.
Utility, how far important in the
construction of each part, 178.
Valenciennes, on fresh-water fish,
381.
Variability of mongrels and hy-
brids, 287.
Variation, under domestication, 7.
caused by reproductive system
being affected by conditions of
life, 8.
under nature, 37.
laws of, 119.
correlated, 10, 128, 178.
Variations appear at correspond-
ing ages, 12, 75.
analogous in distinct species, 139.
Varieties, natural, 35.
struggle between, 66.
domestic, extinction of, 95.
transitional, rarity of, 150.
when crossed, fertile, 283.
Varieties, when crossed, sterile,
284.
classification of, 411.
Verbascum, sterility of, 264.
varieties of, crossed, 285.
Verlot, M., on double stocks, 254.
Verneuil, M. de, on the succession
of species, 332.
Vibracula of the Polyzoa, 215.
Viola, small imperfect -flowers of,
192.
tricolor, 64.
Virchow, on the structure of the
crystalline lens, 163.
Virginia, pigs of, 74.
Volcanic islands, denudation of,
297.
Vulture, naked skin on head, 177.
Wading-birds, 383.
Wagner, Dr., on Cecidomyia, 428.
Wagner, Moritz, on the importance
of isolation, 90.
Wallace, Mr., on origin of spe-
cies, 1.
on the limit of variation under
domestication, 33.
on dimorphic lepidoptera, 40,
257.
INDEX.
501
Wallace, Mr , on races in the Malay
Archipelago, 42.
on the improvement of the eye,
162.
on the walking-stick insect, 202.
on laws of geographical distribu-
tion, 356.
on the Malay Archipelago, 389.
on mimetic animals, 417.
Walsh, Mr. B. D., on phytophagic
forms, 43.
on equal variability, 141.
Water, fresh, productions of, 380.
Water-hen, 159.
Waterhouse, Mr., on Australian
marsupials, 99.
on greatly developed parts being
variable, 133.
on the cells of bees, 244.
on general affinities, 419.
Water-ouzel, 159.
Watson, Mr. H. C, on range of vari-
eties of British plants, 41, 52.
on acclimatization, 126.
on flora of Azores, 364.
xi rarity *>< intermediate varie-
ties ^52.
on alpine plants, 467.
on convergence, 113.
on the indefinite multiplication
of species, 114.
Weale, Mr., on locusts transporting
seeds, 363.
Web of feet in water-birds, 160.
Weismann, Prof., on the causes of
variability, 7.
on rudimentary organs, 442.
West Indian Islands, mammals of,
389.
Westwood, on species in large
genera being closely allied to
others, 51.
on the tarsi of Engidae, 139.
on the antenna? of hymenopter-
ous insects, 405.
Whales, 202.
Wheat, varieties of, 97.
White Mountains, flora of, 366.
Whittaker, Mr., on lines of escarp-
ment, 297.
Wichura, Max, on hybrids, 276,
278, 288.
Wings, reduction of size, 122.
of insects homologous with
branchiae, 165.
rudimentary, in insects, 439.
Wolf, crossed with dog, 232.
of Falkland Isles, 389.
Wollaston, Mr., on varieties of
insects, 42.
on fossil varieties of shells in
Madeira, 47. '
on colors of insects on sea-shore,
120.
on wingless beetles, 122.
on rarity of intermediate varie-
ties, 152.
on insular insects, 384.
on land-shells of Madeira natur-
alized, 395.
Wolves, varieties of, 79.
Woodcock with earth attached to
leg, 363.
Woodpecker, habits of, 158.
green color of, 176.
Woodward, Mr., on the duration of
specific forms, 306.
on Pyrgoma, 314.
on the continuous succession of
genera, 325.
on the succession of types, 344.
World, species changing simulta-
neously throughout, 330.
Wright, Mr. Chauncey, on the
giraffe, 198.
on abrupt modifications, 225.
Wrens, nest of, 259.
Wyman, Prof., on correlation of
color and effects of poison, 10.
on the cells of the bee, 245.
Youatt, Mr., on selection, 25.
on sub-breeds of sheep, 29.
on rudimentary horns in young
cattle, 443.
Zanthoxylon, 193.
Zebra, stripes on, 143.
Zeuglodon, 335.
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