EVOLUTION AND ADAPTATION
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EVOLUTION
AND ADAPTATION
BY
THOMAS HUNT MORGAN, Ph.D.
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THE MACMILLAN COMPANY
LONDON: MACMILLAN & CO., Ltd.
1908
All rights reserved
Copyright, 1903,
Bv THE MACMILLAN COMPANY.
Set up and electrotyped. Published October, 1903. Reprinted
January, 1908.
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Nortooati 19rcsB
J. S. Cushing Co. lierwick & Smith Co.
Norwood, Mass., U.S.A.
TO
professor URttilltam JUritlj Broofes;
AS A TOKEN OF SINCERE ADMIRATION
AND RESPECT
PREFACE
The adaptation of animals and plants to the conditions
under which they live has always excited the interest, and
also the imagination, of philosophers and scientists ; for this
relation between the organism and its environment is one of
the most characteristic features of living things. The ques-
tion at once suggests itself : How has such a relation been
brought about ? Is it due to something inherent in the liv-
ing matter itself, or is it something that has been, as it were,
superimposed upon it ? An example may make my meaning
clearer. No one will suppose that there is anything inherent
in iron and other metals that would cause them to produce
an engine if left to themselves. The particular arrangement
of the pieces has been superimposed upon the metals, so that
they now fulfil a purpose, or use. Have the materials of
which organisms are composed been given a definite arrange-
ment, so that they fulfil the purpose of maintaining the
existence of the organism ; and if so, how has this been
accomplished ? It is the object of the following pages to
discuss this question in all its bearings, and to give, as far
as possible, an idea of the present state of biological thought
concerning the problem. I trust that the reader will not
be disappointed if he finds in the sequel that many of the
most fundamental questions in regard to adaptation are still
unsettled.
In attempting to state the problem as clearly as possible,
I fear that it may appear that at times I have " taken sides,"
vii
viii Preface
when I should only have been justified in stating the different
aspects of the question. But this will do little harm provided
the issue has been sharply drawn. Indeed, it seems to me
that the only scientific value, that a discussion of what the
French call "les grands problemes de la Biologie" has, is to
get a clearer understanding of the relation of what is known
to what is unknown or only surmised.
In some quarters speculation concerning the origin of the
adaptation of living things is frowned upon, but I have failed
to observe that the critics themselves refrain entirely from
theorizing. They shut one door only to open another, which
also leads out into the dark. To deny the right to speculative
thought would be to deny the right to use one of the best
tools of research.
Yet it must be admitted that all speculation is not equally
valuable. The advance of science in the last hundred years
has shown that the kind of speculation that has real worth is
that which leads the way to further research and possible
discovery. Speculation that leads to this end must be recog-
nized as legitimate. It becomes useless when it deals with
problems that cannot be put to the actual test of observation
or experiment. It is in this spirit that I have approached the
topics discussed in the following pages.
The unsophisticated man believes that all other animals
exist to minister to his welfare ; and from this point of view
their adaptations are thought of solely in their relation to
himself. A step in advance was taken when the idea was
conceived that adaptations are for the good of the organisms
themselves. It seemed a further advance when the con-
clusion was reached that the origin of adaptations could be
accounted for, as the result of the benefit that they conferred
on their possessor. This view was the outcome of the accep-
tation of the theory of evolution, combined with Darwin's
theory of natural selection. It is the view held by most
biologists at the present time ; but I venture to prophesy
Preface ix
that if any one will undertake to question modern zoologists
and botanists concerning their relation to the Darwinian
theory, he will find that, while professing in a general way
to hold this theory, most biologists have many reservations
and doubts, which they either keep to themselves or, at any
rate, do not allow to interfere either with their teaching of
the Darwinian doctrine or with the applications that they
may make of it in their writings. The claim of the oppo-
nents of the theory that Darwinism has become a dogma
contains more truth than the nominal followers of this school
find pleasant to hear ; but let us not, therefore, too hastily
conclude that Darwin's theory is without value in relation
to one side of the problem of adaptation; for, while we can
profitably reject, as I believe, much of the theory of natural
selection, and more especially the idea that adaptations have
arisen because of their usefulness, yet the fact that living-
things must be adapted more or less well to their environ-
ment in order to remain in existence may, after all, account
for the widespread occurrence of adaptation in animals and
plants. It is this point of view that will be developed in
the following pages.
I am fully aware of the danger in attempting to cover
so wide a field as that of " Evolution and Adaptation," and
I cannot hope to escape the criticism that is certain to be
directed against a specialist who ventures nowadays beyond
the immediate field of his own researches ; yet, in my own
defence, I may state that the whole point of view under-
lying the position here taken is the immediate outcome of
my work on regeneration. One of the general questions
that I have always kept before me in my study of regenera-
tive phenomena is how such a useful acquirement as the
power to replace lost parts has arisen, and whether the
Darwinian hypothesis is adequate to explain the result.
The conclusion that I have reached is that the theory is
entirely inadequate to account for the origin of the power
x Preface
to regenerate; and it seemed to me, therefore, desirable to
reexamine the whole question of adaptation, for might it
not prove true here, also, that the theory of natural selection
was inapplicable ? This was my starting-point. The results
of my examination are given in the following pages.
I am deeply indebted to Professor G. H. Parker and to
Professor E. G. Conklin for advice and friendly' criticism ;
and in connection with the revision of the proof I am
under many obligations to Professor Joseph W. Warren
and to Professor E. A. Andrews. Without their generous
help I should scarcely have ventured into a field so full of
pitfalls.
Bryn Mawr, Penn., June 10, 1903.
CONTENTS
CHAPTER I
PAGE
The Problem of Adaptation i
Structural Adaptations i
Adjustments of the Individual to Changes in the Environment . 12
Adaptations for the Good of the Species 19
Organs of Little Use to the Individual 22
Changes in the Organism that are of No Use to the Individual
or to the Race 25
Comparison with Inorganic Phenomena 26
CHAPTER II
The Theory of Evolution 30
Evidence in Favor of the Transmutation Theory ... 32
Evidence from Classification and from Comparative Anat-
omy .......... 32
The Geological Evidence 39
Evidence from Direct Observation and Experiment . . 43
Modern Criticism of the Theory of Evolution ... 44
CHAPTER III
The Theory of Evolution (continued') 58
The Evidence from Embryology 58
The Recapitulation Theory ...... 58
Conclusions 84
CHAPTER IV
Darwin's Theories of Artificial and of Natural Selection 91
The Principle of Selection 91
Variation and Competition in Nature 104
The Theory of Natural Selection . . . . . .116
xi
XI 1
Contents
CHAPTER V
The Theory of Natural Selection {continued)
Objections to the Theory of Natural Selection .
Sterility between Species .....
Weismann's Germinal Selection
129
129
147
154
CHAPTER VI
Darwin's Theory of Sexual Selection .
Sexual Selection ......
General Criticism of the Theory of Sexual Selection
167
167
213
CHAPTER VII
The Inheritance of Acquired Characters .... 222
Lamarck's Theory . 222
Darwin's Hypothesis of Pangenesis 233
The Neo-Lamarckian School ....... 240
CHAPTER VIII
Continuous and Discontinuous Variation and Heredity
Continuous Variation
Heredity and Continuous Variation
Discontinuous Variation .
Mendel's Law ....
The Mutation Theory of De Vries
Conclusions ....
261
261
270
272
278
287
297
CHAPTER IX
Evolution as the Result of External and Internal Factors 300
The Effect of External Influences 300
Responsive Changes in the Organism that adapt it to the New
Environment . • • 3 X 9
Nageli's Perfecting Principle ....... 325
Contents
XI 1 1
CHAPTER X
The Origin of the Different Kinds of Adaptations
Form and Symmetry
Mutual Adaptation of Colonial Forms
Degeneration .....
Protective Coloration
Sexual Dimorphism and Trimorphism
Length of Life as an Adaptation
Organs of Extreme Perfection .
Secondary Sexual Organs as Adaptations
Individual Adjustments as Adaptations
Color Changes as Individual Adaptations
Increase of Organs through Use and Decrease throug
Reactions of the Organism to Poisons, etc.
Regeneration ......
PAGE
ONS .
340
.
34°
.
350
•
352
•
357
•
360
•
37°
•
37i
•
372
•
• 375
•
• 375
jh Disuse
■ 376
•
■ 377
.
• 379
CHAPTER XI
Tropisms and Instincts as Adaptations
382
CHAPTER XII
Sex as an Adaptation .
The Different Kinds of Sexual Individuals
The Determination of Sex
Sex as a Phenomenon of Adaptation
414
4U
422
439
CHAPTER XIII
Summary and General Conclusions
452
INDEX 465
EVOLUTION AND ADAPTATION
CHAPTER I
THE PROBLEM OF ADAPTATION
Between an organism and its environment there takes
place a constant interchange of energy and of material. This
is, in general, also true for all bodies whether living or lifeless ;
but in the living organism this relation is a peculiar one ; first,
because the plant or the animal is so constructed that it is
suited to a particular set of physical conditions, and, second,
because it may so respond to a change in the outer world
that it further adjusts itself to changing conditions, ie. the
response may be of such a kind that it better insures the
existence of the individual, or of the race. The two ideas
contained in the foregoing statement cover, in a general way,
what we mean by the adaptation of living things. The fol-
lowing examples will serve to illustrate some of the very
diverse phenomena that are generally included under this
head.
Structural Adaptations
The most striking cases of adaptations are those in which
a special, in the sense of an unusual, relation exists between
the individual and its surroundings. For example, the fore-
leg of the mole is admirably suited for digging underground.
A similar modification is found in an entirely different group
of the animal kingdom, namely, in the mole-cricket, in which
the first legs are also well suited for digging. By their use the
mole-cricket makes a burrow near the surface of the ground,
2 Evolution and Adaptation
similar to, but of course much smaller than, that made by the
mole. In both of these cases the adaptation is the more
obvious, because, while the leg of the mole is formed on the
same general p4an as that of other vertebrates, and the leg of
the mole-cricket has the same fundamental structure as that
of other insects, yet in both cases the details of structure and
the general proportions have been so altered, that the leg is
fitted for entirely different purposes from that to which the
legs of other vertebrates and of other insects are put. The
wing of the bat is another excellent case of a special adap-
tation. It is a modified fore-limb having a strong membrane
stretched between the fingers, which are greatly elongated.
Here we find a structure, which in other mammals is used
as an organ for supporting the body, and for progression on
the ground, changed into one for flying in the air.
The tails of mammals show a number of different adapta-
tions. The tail is prehensile in some of the monkeys ; and not
only can the monkey direct its tail toward a branch in order to
grasp it, but the tail can be wrapped around the branch and
hold on so firmly that the monkey can swing freely, hang-
ing by its tail alone. The animal has thus a sort of fifth
hand, one as it were in the middle line of the body, which can
be used as a hold-fast, while the fingered hands are put to
other uses. In the squirrels the bushy tail serves as a pro-
tection during the winter for those parts of the body not so
thickly covered by hair. The tail of the horse is used to
brush away the flies that settle on the hind parts of the body.
In other mammals, the dog, the cat, and the rat, for example,
the tail is of less obvious use, although the suggestion has
been made that it may serve as a sort of rudder when the
animal is running rapidly. In several other cases, as in the
rabbit and in the higher apes, the tail is very short, and is of
no apparent use ; and in man it has completely disappeared.
A peculiar case of adaptation is the so-called basket on the
third pair of legs of the worker honey-bee. A depression
The Problem of Adaptation 3
of the outer surface of the tibia is arched over by stiff
hairs. The pollen collected from the stamens of flowers
is stowed away in this receptacle by means of the other
pairs of legs. The structure is unique, and is not found in
any other insects except the bees. It is, moreover, present
only in the worker bees, and is absent in the queen and the
males.
The preceding cases, in which the adapted parts are used
for the ordinary purposes of life of the individual, are not
essentially different from the cases in which the organ is
used to protect the animal from its enemies. The bad taste
of certain insects is supposed to protect them from being-
eaten by birds. Cases like this of passive protection grade
off in turn into those in which, by some reflex or voluntary
act, the animal protects itself. The bad-smelling horns of
the caterpillar of the black swallow-tailed butterfly (Papilio
polyxenes) are thrust out when the animal is touched, and it
is believed that they serve to protect the caterpillar from
attack. The foetid secretion of the glands of the skunk is
believed to serve as a protection to the animal, although the
presence of the nauseous odor may lead finally to the exter-
mination of the skunk by man. The sting of bees and of
wasps serves to protect the individual from attack. The sting
was originally an ovipositor, and used in laying the eggs.
It has, secondarily, been changed into an organ of offence.
The special instincts and reflex acts furnish a striking
group of adaptations. The building of the spider's web is
one of the most remarkable cases of this kind. The con-
struction of the web cannot be the result of imitation, since,
in many instances, the young are born in the spring of the
year following the death of the parents. Each species of
spider has its own type of web, and each web has as char-
acteristic a form as has the spider itself. It is also important
to find that a certain type of web may be characteristic of
an entire family of spiders. Since, in many cases, the web
4 Evolution and Adaptation
is the means of securing the insects used for food, it fulfils
a purpose necessary for the welfare of the spider.
The making of the nests by birds appears to be also in
large part an instinctive act ; although some writers are
inclined to think that memory of the nest in which the
young birds lived plays a part in their actions, and imitation
of the old birds at the time of nest-building may, perhaps,
also enter into the result. It has been stated that the first
nest built by young birds is less perfect than that built by
older birds, but this may be due to the bird's learning some-
thing themselves in building their nests, i.e. to the perfecting
of the instinct in the individual that makes use of it. In any
case much remains that must be purely instinctive. The
construction of the comb by bees appears to be largely, per-
haps entirely, an instinctive act. That this is the case was
shown by isolating young workers as soon as they emerged
from the cell, and before they could have had any experience
in seeing comb built. When given some wax they set to
work to make a comb, and made the characteristic six-sided
structures like those made by the bees in a hive. The forma-
tion of so remarkable a structure as the comb is worthy of
admiration, for, with the greatest economy of material, a
most perfect storeroom for the preservation of the honey is
secured. This adaptation appears almost in the nature of
foresight, for the store of honey is used not only to feed the
young, but may be drawn on by the bees themselves in time
of need. It is true that a comparison with other kinds of
bees makes it probable that the comb was first made for the
eggs and larvae, and only later became used as a storehouse,
but so far as its form is concerned there is the same economy
of constructive materials in either case.
The behavior of young birds, more especially those that
take care of themselves from the moment they leave the egg,
furnishes a number of cases of instincts that are protective.
If, for example, a flock of young pheasants is suddenly dis-
The Problevi of Adaptation 5
turbed, the birds at once squat down on the ground, and
remain perfectly quiet until the danger is past. Their re-
semblance to the ground is so perfect that they are almost
invisible so long as they remain quiet. If, instead of remain-
ing still, they were to attempt to run away when disturbed,
they would be much more easily seen.
Certain solitary wasps {Ammophila) have the habit of
stinging caterpillars and spiders, and dragging them to their
nests, where they are stored away for the future use of the
young that hatch from the eggs laid by the wasp on the
body of the prey. As a result of the sting which the wasp
administers to the caterpillar, the latter is paralyzed, and
cannot escape from the hole in which it is stored, where it
serves as food for the young wasp that emerges from the
egg. It was originally claimed by Forel that the wasp stings
the caterpillar in such a way that the central nervous system
is always pierced, and many subsequent naturalists have mar-
velled at the perfection of such a wonderful instinct. But
the recent results of the Peckhams have made it clear that
the act of the wasp is not carried out with the precision
previously supposed, although it is true that the wasp pierces
the caterpillar on the lower surface where the ventral chain
of ganglia lies. The habit of this wasp is not very dissimilar
from that shown by many other kinds of wasps that sting
their captive in order to quiet it. We need not imagine in
this case that the act carries with it the consciousness that
the caterpillar, quieted in this way, will be unable to escape
before the young wasps have hatched.
The resemblance in color of many animals to their natural
backgrounds has in recent years excited the interest and
imagination of many naturalists. The name of protective
coloration has been given to this group of phenomena. The
following cases which have less the appearance of purely
imaginative writing may serve by way of illustration. A
striking example is that of the ptarmigan which has a pure
6 Evolution a) id Adaptation
white coat in winter, and a brown coat in summer. The
white winter plumage renders the animal less conspicuous
against the background of snow, while in summer the
plumage is said to closely resemble the lichen-covered
ground on which the bird rests. The snowy owl is a north-
ern bird, whose color is supposed to make it less conspicuous,
and may serve either as a protection against enemies, or
may allow the owl to approach its prey unseen. It should
not pass unnoticed, however, that there are white birds in
other parts of the world, where their white color cannot be
of any use to them as a protection. The white cockatoos,
for example, are tropical birds, living amongst green foliage,
where their color must make them conspicuous, rather than
the reverse.
The polar bear is the only member of the family that is
white, and while this can scarcely be said to protect it from
enemies, because it is improbable that it has anything to fear
from the other animals of the ice-fields, yet it may be claimed
that the color is an adaptation to allow the animal to ap-
proach unseen its prey.
In the desert many animals are sand-colored, as seen for
instance in the tawny color of the lion, the giraffe, the
antelopes, and of many birds that live on or near the ground.
It has been pointed out that in the tropics and temperate
zones there are many greenish and yellowish birds whose
colors harmonize with the green and yellow of the trees
amongst which they live ; but on the other hand we must
not forget that in all climes there are numbers of birds
brilliantly colored, and many of these do not appear to be
protected in any special way. The tanagers, humming-birds,
parrots, Chinese pheasants, birds of paradise, etc., are ex-
tremely conspicuous, and so far as we can see they must
be much exposed on account of the color of their plumage.
Whether, therefore, we are justified in picking out certain
cases as examples of adaptation, because of an agreement in
The Problem of Adaptation 7
color between the organism and its surroundings, and in
neglecting all others, is, as has been already said, a point to
be further examined.
Not only among mammals and birds have many cases of
protective coloration been described by writers dealing with
this subject, but in nearly every group of the animal kingdom
similar cases have been recognized. The green and brown
color of lizards may protect them, the green color of many
frogs is supposed to conceal them as they sit amongst the
plants on the edge of a stream or pond. The gray-brown
color of the toad has been described as a resemblance to the
dry ground, while the brilliant green of several tree-frogs
conceals them very effectively amongst the leaves. Many
fishes are brilliantly colored, and it has even been suggested
that those living amongst corals and sea-anemonies have
acquired their colors as a protection, but Darwin states that
they appeared to him very conspicuous even in their highly
colored environment.
Amongst insects innumerable cases of adaptive coloration
have been described. In fact this is the favorite group
for illustrating the marvels of protective coloration. A few
examples will here serve our purpose. The oft-cited case
of the butterfly Kallima is, apparently, a striking instance of
protective resemblance. When at rest the wings are held
together over the back, as in nearly all butterflies, so that
only the under surface is exposed. This surface has an
unquestionably close resemblance to a brown leaf. It is said
on no less authority than that of Wallace that when this
butterfly alights on a bush it is almost impossible to dis-
tinguish between it and a dead leaf. The special point in
the resemblance to which attention is most often called is the
distinct line running obliquely across the wings which looks
like the midrib of a leaf. Whether the need of such a close
resemblance to a leaf is requisite for the life of this butterfly,
we do not know, of course, and so long as we do not have
8 Evolution and Adaptation
this information there is danger that the case may prove too
much, for, if it should turn out that this remarkable case is
accidental the view in regard to the resemblance may be
endangered.
Amongst caterpillars there are many cases of remarkable
resemblances in color between the animal and its surroundings.
The green color of many of those forms that remain on the
leaves of the food-plant during the day will give, even to the
most casual observer, the impression that the color is for the
purpose of concealment ; and that it does serve to conceal
the animal there can be no doubt. But even from the point
of view of those who maintain that this color has been
acquired because of its protective value it must be admitted
that the color is insufficient, because some of these same green
caterpillars are marvellously armed with an array of spines
which are also supposed to be a protection against enemies.
Equally well protected are the brown and mottled geometrid
caterpillars. These have, moreover, the striking and unusual
habit of fixing themselves by the posterior pairs of false legs,
and standing still and rigid in an oblique position on the
twigs to which they are affixed. So close is their resemblance
to a short twig, that even when their exact position is known
it is very difficult to distinguish them.
Grasshoppers that alight on the ground are, in many cases,
so similar to the surface of the ground that unless their
exact location is known they easily escape attention, while the
green color of the katydid, a member of the same group of
orthoptera, protects it from view in the green foliage of the
trees where it lives. The veinlike wings certainly suggest a
resemblance to a leaf, but whether there is any necessity
for so close an imitation may be questioned.
There can be little doubt in some of these cases that the
color of the animal may be a protection to it, but as has
been hinted already, it is another question whether it
acquired these colors because of their usefulness. Never-
The Problem of Adaptation g
theless, if the color is useful to its possessor, it is an adapta-
tion in our sense of the word, without regard to the way in
which it has been acquired. Even, for instance, if the resem-
blance were purely the outcome of chance in the sense that
the color appeared without relation to the surroundings, it
would still be an adaptation if it were of use to the animal
under the ordinary conditions of life.
In the lower groups numerous cases in which animals
resemble their surroundings could be given. Such cases are
known in Crustacea, worms, mollusks, hydroids, etc., and the
possible value of these resemblances may be admitted in
many instances.
It is rather curious that so few cases of adaptive color-
ation have been described for plants. No one supposes
that the slate color of the lichen is connected with the color
of the rocks on which it grows, in the sense that the resem-
blance is of any use to the lichen. Nor does the color of the
marine red algae serve in any way to protect the plants so
far as is known. The green color of nearly all the higher
plants is obviously connected with the substance, chlorophyl,
that is essential for the processes of assimilation, and has
no relation to external objects. But when we come to the
colors of flowers we meet with curious cases of adaptation,
at least according to the generally accepted point of view.
For it is believed by many naturalists that the color of the
corolla of flowering plants is connected with the visits of
insects to the flowers, and these visits are in many cases
essential for the cross-fertilization of the flowers. This adap-
tation is one useful to the species, rather than the individual,
and belongs to another category.
The leaf of the Venus's fly-trap, which suddenly closes
together from the sides when a fly or other light body
comes to rest on it, is certainly a remarkable adaptation.
A copious secretion of a digestive fluid is poured out on the
surface of the leaf, and the products of digestion are absorbed.
io Evolution and Adaptation
There can be no question that this contrivance is of some
use to the plant. In other insectivorous plants, the pitcher
plants, the leaves are transformed into pitchers. In Nepenthes
a digestive fluid is secreted from the walls. A line of glands
secreting a sweet fluid serves to attract insects to the top
of the pitcher, whence they may wander or fall into the fluid
inside, and there being drowned, they are digested. A lidlike
cover projecting over the opening of the pitcher is supposed
to be of use to keep out the rain.
In Utricularia, a submerged water-plant, the tips of the
leaves are changed into small bladders, each having a small
entrance closed by an elastic valve opening inwards. Small
snails and crustaceans can pass into this opening, to which
they are guided by small outgrowths ; but once in the cup
they cannot get out again, and, in fact, small animals are
generally found in the bladders where they die and their
substance is absorbed by forked hairs projecting into the in-
terior of the bladder.
The cactus is a plant that is well suited to a dry climate.
Its leaves have completely disappeared, and the stem has
become swollen into a water-reservoir. " It has been esti-
mated that the amount of water evaporated by a melon
cactus is reduced to one six-hundredth of that given off
by any equally heavy climbing-plant."
Sachs gives the following account of the fertilization
process in AristolocJiia Clematitis, which he refers to as a
conspicuous and peculiar adaptation. In Figure i A a group
of flowers is shown, and in Figure i B and C a single
flower is split open to show the interior. In B a small fly
has entered, and has brought in upon its back some pol-
len that has stuck to it in another flower. The fly has
entered through the long neck which is beset with hairs
which are turned inwards so that the fly can enter but
cannot get out. In roaming about, the pollen that is stick-
ing to its back will be rubbed against the stigmatic surface.
The Problem of Adaptation
ii
" As soon as this has taken place the anthers, which have
been closed hitherto, dehisc and become freely accessible,"
as a result in the change in the stigma and of the collapse
of the hairs at the base of the enlargement which has
widened. The fly can now crawl under the anthers, and,
Fig. I. — The fertilization of Aristolochia Clematitis. A, portion of stem with
flowers in axil of leaf in different stages. B, longitudinal sections of two
flowers, before and after fertilization. (After Sachs.)
if it does so, new pollen may stick to its back. At this
time the hairs in the throat dry up, and the fly can leave
its prison house, Figure I C. If the fly now enters another
flower this is fertilized by repeating the process. The unfer-
tilized flowers stand erect with widely open mouths. As soon
as they have been fertilized they bend down, as seen in
12 Evolution and Adaptation
Figure i A, and at the same time the terminal flap bends over
the open mouth of the throat, " stopping the entrance to the
flies, which have now nothing more to do here."
Adjustments of the Individual to Changes in the
Environment
The most familiar cases of adjustments of the individual to
the environment are those that we recognize in our own
bodies. After violent exercise we breathe more rapidly, and
take deeper inspirations. Since during exercise our blood
loses more oxygen and takes in more carbon dioxide from the
muscles, it is clear that one result of more rapid breathing is
to get more oxygen into the blood and more carbon dioxide
out of it. The process of sweating, that also follows exercise,
may be also looked upon as an adaptive process, since by
evaporation the skin is kept cooler, and, in consequence, the
blood, which at this time flows in larger quantities to the skin,
is cooled also.
More permanent adaptive changes than these also take
place as the result of prolonged use of certain parts. If the
muscles work against powerful resistance, they become larger
after several days or weeks, and are capable of doing more
work than at first. Conversely, when any group of muscles is
not used, it becomes smaller than the normal and capable of do-
ing less work. It would be a nice point to decide whether this
latter change is also an adaptation. If so it is one in a some-
what different sense from that usually employed. The result
is of no direct advantage to the animal, except possibly in sav-
ing a certain amount of food, but since the same change will
take place when an abundance of food is consumed, the result
is, under these conditions, of no use.
The thickening of the skin on those parts of the body where
continued pressure is brought to bear on it is a change in a
useful direction. The thickening on the soles of the feet and
The Problem of Adaptation 13
on the palms of the hands is a case in point. Not only is the
skin thicker at birth in these parts, but it becomes thicker
through use. In other parts of the body also, the skin hardens
and becomes thicker if pressure is brought to bear on it. We
may regard this as a general property of the skin, which is
present even in those parts where, under ordinary circum-
stances, it can rarely or never be brought into use.
Even as complicated and as much used an organ as the
eye can become adaptively improved. It is said that the
lateral region of the field of vision can be trained to perceive
more accurately ; and every one who has used a microscope
is familiar with the fact that if one eye is habitually used it
becomes capable of seeing more distinctly and better than
the other eye. This seems to be due, in part at least, to the
greater contraction of the iris.
Another phenomenon, which, I think, must be looked upon
as an adaptation, is the immunity to certain poisons that can
be gradually brought about by slowly increasing the amount
introduced into the body. Nicotine is a most virulent poison,
and yet by slowly increasing the dose an animal can be
brought into a condition in which an amount of nicotine, fatal
to an ordinary individual, can be administered without any
ill effects at all resulting.
The same phenomenon has been observed in the case of
other poisons, not only in case of other alkaloids, such as
morphine and cocaine, but also in the case of caffein, alcohol,
and even arsenic. There is a curious phenomenon in regard
to arsenic, which appears to be well established, viz., that a
person who has gradually increased the dose to an amount
great enough to kill ten ordinary men, will die if he sud-
denly ceases altogether to take arsenic. He can, however, be
gradually brought back to a condition in which arsenic is not
necessary for his existence, if the dose is gradually decreased.
It is a curious case of adaptation that we meet with here,
since the man becomes so thoroughly adjusted to a poison
14 Evolution and Adaptation
that if he is suddenly brought back to the -normal condition
of the race he will die.
Immunity to the poison of venomous snakes can also be
acquired by slowly increasing the amount given to an animal.
It is possible to make a person so immune to the poison of
venomous snakes that he would become, in a sense, adapted
to live amongst them without danger to himself. It is to be
noted, moreover, that this result could be reached only by
quite artificial means, for, under natural conditions it is incon-
ceivable that the nicely graded series of doses of increasing
strength necessary to bring about the immunity could ever be
acquired. Hence we find here a case of response in an adap-
tive direction that could not have been the outcome of experi-
ence in the past. It is important to emphasize this capacity of
organisms to adapt themselves to certain conditions entirely
new to them.
These cases lead at once to cases of immunity to certain
bacterial diseases. An animal may become immune to a par-
ticular disease in several ways. First, by having the disease
itself, which renders it immune for a longer or a shorter
period afterwards ; or, second, by having a mild form of the
disease as in the case of smallpox, where immunity is brought
about by vaccination, i.e. by giving the individual a mild
form of smallpox ; or, third, by introducing into the blood
an antidote, in the form, for example, of antitoxin, which has
been made by another animal itself immune to the disease.
The first two classes of immunity may be looked upon as
adaptations which are of the highest importance to the or-
ganism ; the last case can scarcely be looked upon as an
adaptive process, since the injurious effect of the poison may
as well be neutralized outside of the body by mixing it with the
antitoxin. We may suppose, then, that in the body a similar
process goes on, so that the animal itself takes no active part
in the result.
When we consider that there are a number of bacterial
The Problem of Adaptation 15
diseases, in each of which a different poison is made by the
bacteria, we cannot but ask ourselves if the animal really
makes a counter-poison for each disease, or whether a single
substance may not be manufactured that counteracts all
alike ? That the latter is not the case is shown by the fact
that an animal made immune to one disease is not immune
to others. When we recall that the animal has also the
capacity to react in one way or another to a large number
of organic and inorganic poisons, to which it or its ancestors
can have had little or no previous experience, we may well
marvel at this wonderful regulative power.
The healing of wounds, which takes place in all animals,
forms another class of adaptive processes. The immense use-
fulness of this power is obvious when it is remembered how
exposed most animals are to injuries. By repairing the
injury the animal can better carry on its normal functions.
Moreover, the presence of the wound would give injurious
bacteria a ready means of entering the body. In fact, an
intact skin is one of the best preventives to the entrance
of bacteria.
Not only have most organisms the power of repairing
injuries, but many animals have also the closely related
power of regenerating new parts if the old ones are lost.
If a crab loses its leg, a new one is regenerated. If a fresh-
water worm (Lumbriculus) is cut into pieces, each piece makes
a new head at its anterior end and a new tail at the posterior
end. In this way as many new worms are produced as there
are pieces. And while in a strict sense it cannot be claimed
that this power of regeneration is of any use to the original
worm, since the original worm, as such, no longer exists,
yet since it has not died but has simply changed over into
several new worms, the process is of use inasmuch as by this
means the pieces can remain in existence.
We need not discuss here the relative importance to differ-
ent animals of this power of regeneration, but it may be stated,
1 6 Evolution and Adaptation
that, while in some cases it may be necessary to replace the
lost part if the animal is to remain in existence, as when
a new head is formed on an earthworm after the old one
was cut off, in other cases the replacement of the lost part
appears to be of minor importance, as in the case of the
leg of the crab. While we are not, for the moment, con-
cerned with the relative importance of the different adapta-
tions, this question is one of much importance in other
connections and will be considered later.
The protective coloration of some animals, which is the
direct result of a change in color of the animal in response
to the surroundings, furnishes us with some most striking
cases of adaptive coloration. A change of this sort has
been recorded in a number of fishes, more especially in the
flounders. The individuals found living on a dark back-
ground are darker than those living on a lighter background ;
and when the color of the background is changed it has
been observed that the color of the fish also changes in the
same direction. I have observed a change of this sort from
dark to light, or from light to dark, in the common minnow
{Fundulus) in accordance with a change of its background,
and the same sort of change appears to take place in many
other fishes.
The change from green to brown and from brown to green
in certain tree frogs and in the lizard (Anolis), which is
popularly supposed to take place according to whether the
background is green or brown, is not after all, it appears,
connected with the color of the background, but depends on
certain other responses of the animals that have not yet been
satisfactorily made out. If it be claimed that in summer
the animal would generally be warm, and therefore, often
green, and that this color would protect it at this time of
year when the surroundings are green, and in winter
brown, when this color is the prevailing one in temperate
regions, then it might appear that the change is of use to
The Problem of Adaptation 17
the animal ; but if it is true that the same change takes
place in some of the lizards that live in the tropics, where
the prevailing color is always green, it would appear that
the result may have no direct relation with the surroundings.
It has been shown in a number of well-authenticated cases
that the pupae of certain butterflies vary in color within
certain limits in response to the color of the background.
When the caterpillar fixes itself to some surface, and there
throws off the outer skin, and acquires a new one, the color
of the latter is influenced by the background. The result is a
better protection to the pupa. The change is not brought
about through the ocelli or eyes, but through the general sur-
face of the skin, for the same change takes place when the
eyes have been previously covered with a dark pigment.
The growth of plants toward the light may be looked
upon as an adaptive process, since only in the light can they
find the conditions necessary for their life. The extraor-
dinary elongation of shoots and young plants when grown
in the dark may also be considered an adaptation for finding
the light, since in this way a plant, deeply embedded in the
ground, may ultimately reach the surface. Thus while the
actual process of elongation in the dark is not in itself of any
use, yet under the ordinary conditions of its life, this response
may be of great benefit to the plant.
The closing together of the leaves of some plants has been
supposed to protect them from too rapid radiation of heat,
and incidentally this purpose may be fulfilled; but since
some tropical plants also close their leaves during the night,
it can hardly be maintained that the closing has been
acquired for this purpose. It has been suggested that the
opening of certain flowers under certain conditions of light
is connected with the visits of insects that bring about cross-
fertilization.
The preceding examples will suffice to give a general
idea of what is meant by adaptation in organisms. That
1 8 Evolution and Adaptation
the term includes a large number of phenomena of very
different kinds is apparent. When we have examined these
phenomena further we shall find, I think, that it will be nec-
essary to put some of them into different categories and
treat them differently. It is probably incorrect to suppose
that all processes useful to the organism have been acquired
in the same way, nevertheless, for the present the term
adaptation is sufficiently general, even if vague, to cover
these different groups of cases.
It may be asked, in what respects are these structures
and processes of adaptation different from the ordinary struc-
tures and changes that go on in the organism ? Why is the
leg of the mole more of an adaptation than that of a dog ?
The one is of as much use as the other to its possessor.
What reason can we give for citing the poison of the snake,
and not mentioning in the same connection the other glands
of the body ? In fact, the poison gland of the snake is sup-
posed to be a modified superior labial gland. Why, in short,
are not the processes of digestion, excretion, secretion, the
beating of the heart, the ordinary reflex acts of the nervous
system, and the action of the sense organs, as truly adapta-
tions as the special cases that have been selected for illustra-
tion. The answer is simply that we are more impressed by
those cases of adaptation that are more unusual, as when an
animal departs in the use of certain structures from the rest
of the group to which it belongs. For example, if all mam-
mals lived underground, ourselves included, and the fore-legs
or arms were used for burrowing, we should not think this
unusual; but if we found an animal using all four legs to
support the body and for purposes of progression, we
should, most likely, think this was an excellent illustration of
adaptation.
In other instances the condition is somewhat different.
The color of certain animals may unquestionably be of use
to them in concealing them from their enemies. In other
The Problem of Adaptation 19
cases the color may not serve this purpose, or any purpose
at all. Thus while in the former case we speak of the color
as an adaptation to the surroundings, in the latter we do not
think of it as having any connection at all with the environ-
ment. Even in the same animal the color of different parts
of the body may appear under this twofold relation. For
example, the green color of the skin of the frog renders it
less conspicuous amongst the green plants on the edge of
the stream, but the brilliant orange and black pigment in the
body-cavity cannot be regarded as of any use to the animal.
Adaptations for the Good of the Species
Aside from the class of adaptations that are for the good
of the individual, there is another class connected solely
with the preservation of the race. The organs for reproduc-
tion are the most important examples of this kind. These
organs are of no use to the individual for maintaining its own
existence, and, in fact, their presence may even be deleterious
to the animal. The instincts connected with the use of these
organs may lead inevitably to the death of the individual,
as in the case of the California salmon, which, on entering
fresh water in order to deposit its eggs, dies after performing
this act.
The presence of the organs of reproduction in the indi-
vidual is obviously connected with the propagation of other
individuals. Indeed in many organisms the life of the
individual appears to have for its purpose the continuation
of the race. In a large number of animals the individual
dies after it has deposited its eggs. The most striking case
is that of the May-flies, whose life, as mature individuals, may
last for only a few hours. The eggs are set free by the
bursting of the abdomen, and the insect dies. The male
bee also dies after union with the queen. In some annelids,
the body is also said to burst when the eggs are set free ;
20 Evolution and Adaptation
and in other forms those parts of the body containing the
eggs break off, and, after setting free the eggs, die. These
are extreme cases of what is seen in many animals, namely
the replacement of the old individuals by a new generation ;
and while in general there is only a loose connection between
the death of the individual and the consummation of its repro-
ductive power, yet the two run a course so nearly parallel
that several writers have attempted to explain this connection
as one of racial adaptation.
It has also been pointed out that in those higher animals
that take care of their young after birth, the life of the
individual does not end with the period of birth of the young,
but extends at least throughout the time necessary to care
for the young. It has even been suggested that this length-
ening of the life period has been acquired on account of its
use to the species. When, however, as in the case of the ver-
tebrates, the young are born at intervals either in great
numbers at a birth, as in fishes and amphibia, or in lots of
twos, threes, or fours, as in many birds and mammals, or
even only one at a time, as in a few birds and in man, it
will be evident that the relation cannot be so simple a
has been supposed. It cannot be assumed in these forms that
the end of the life of the individual is in any way connected
with the ripening of the last eggs, for, on the contrary,
hundreds, or even many thousands, of potential eggs may
be present in the ovaries when the animal is overtaken by
old age, and its power of reproduction lost.
In regard to several of the lower animals, we find, in a num-
ber of cases where there are accurate data, that the individ-
ual goes on year after year producing young. Whether
they ever grow old, in the sense of losing their power of
reproduction, has not been definitely determined, but there
is, so far as I know, no evidence to show that such a pro-
cess takes place, and these animals appear to have the power
of reproducing themselves indefinitely.
The Problem of Adaptation 21
The phenomenon of old age (apart from its possible con-
nection with the cessation of the power of reproduction),
which leads to the death of the individual, has been looked
upon by a few writers as an adaptation of the individual for
the good of the species. It has been pointed out by these
writers that the longer an individual lives, the more likely it
is to become damaged, and if along with this its powers of
reproduction diminish, as compared with younger individuals,
then it stands in the way and takes food that might be used
by other, younger individuals, that are better able to carry on
the propagation of the race. It is assumed, therefore, that
the life of the individual has been shortened for the benefit
of the race. Whether such a thing is probable is a question
that will also be discussed later. We are chiefly concerned
here only in recording the different groups of phenomena
that have been regarded by biologists as adaptations.
The so-called secondary sexual characters such as the
brighter colors of the males, ornaments of different kinds,
crests, color-pattern, tail feathers, etc., organs of offence and
of defence used in fighting members of the same species,
present a rather unique group of adaptations. These char-
acters are supposed to be of use to the individual in conquering
its rivals, or in attracting the females. They may be consid-
ered as useful to the individual in allowing it to propagate at
the expense of its rivals, but whether the race is thereby
benefited is a question that will be carefully considered later.
The colors of flowers, that is supposed to attract insects,
have been already mentioned. The sweet fluid, or nectar,
secreted by many flowers is sought by insects, which on enter-
ing the flowers bring about cross-fertilization. Thus while
the nectar seems to be of no immediate service to the plant it-
self, it is useful to the species in bringing about the fertiliza-
tion of the flowers. The odors of flowers also serve to attract
insects, and their presence is one of the means by which in-
sects find the flowers. This also is of advantage to the race.
22 Evolution and Adaptation
Organs of Little Use to the Individual
In every organism there are parts of the body whose
presence cannot be of vital importance to the individual.
We may leave out of consideration the reproductive organs,
since their presence, as has just been stated, is connected
with the continuation of the race. The rudimentary organs,
so-called, furnish many examples of structures whose pres-
ence may be of little or of no use to the individual ; in fact,
as in the case of the appendix in man, the organs may be a
source of great clanger to the individual. In this respect the
organism is a structure not perfectly adapted to its conditions
of life, since it contains within itself parts that are of little
or of no use, which may even lead to its destruction, and may
often expose it to unnecessary danger. Nevertheless such
parts are surprisingly infrequent, and their presence is usu-
ally accounted for on the supposition that in the past these
organs have been of use, and have only secondarily come to
play an insignificant part in the functions of the organism.
Another example of the same thing is found in the rudi-
mentary eyes of animals living in the dark, such as the mole
and several cave animals, fishes, amphibia, and insects.
There are still other organs, which cannot be looked upon
as rudimentary, yet whose presence can scarcely be consid-
ered as essential to the life of the individual. It is with this
class that we are here chiefly concerned. For instance, the
electric organs in some of the rays and fish can hardly
protect the animal from enemies, even when as highly devel-
oped as in the torpedo ; and we do not know of any other
essential service that they can perform. Whether the same
may be also said of the phosphorescent organs of many
animals is perhaps open in some cases to doubt, but there
can be little question that the light produced by most of the
small marine organisms, such as noctiluca, jellyfish, cteno-
phores, copepods, pyrosoma, etc., cannot be of use to these
The Problem of Adaptation 23
animals in protecting them from attack. In the case of cer-
tain bacteria it seems quite evident that the production of
light can be of no use as such to them. The production of
light may be only a sort of by-product of changes going on
in the organism, and have no relation to outside conditions.
In certain cases, as in the glowworm, it has been supposed
that the display may serve to bring the sexes together ; but
since the phosphorescent organs are also present in the larval
stages of the glowworm, and since even the egg itself is said
to be phosphorescent, it is improbable, in these stages at least,
that the presence of the light is of service to the organism.
It has been pointed out that the colors of certain animals
may serve to conceal them and may be regarded as an
adaptation ; but it is also true that in many cases the color of
the whole animal or the color of special parts can be of little
if any direct use. While it is difficult to show that the
wonderful patterns and magnificent coloration of many of the
larger animals are not of service to the animal, however
sceptical we may be on the subject, yet in the case of many
microscopical forms that are equally brilliantly colored there
can be little doubt that the coloration can be of no special
service to them. If it be admitted that in these small forms
the color and the color patterns are not protective, we should
at least be on our guard in ascribing off-hand to larger forms
a protective value in their coloration, unless there is actual
proof that it serves some purpose.
We also see in other cases that the presence of color need
not be connected with any use that it bears as such to the
animal. For instance, the beautiful colors on the inside of
the shells of many marine snails and of bivalve mollusks,
can be of no use to the animal that makes the shell, because
as long as the animal is alive this color cannot be seen from
the outside. This being the case let us not jump too readily
to the conclusion that when other shells are colored on the
outer surface that this must be of use to the mollusk.
24 Evolution and Adaptation
In regard to the colors of plants, there are many cases of
brilliant coloration, which so far as we can see can be of no
service to the organism. In such forms as the lichens and
the toadstools, many of which are brilliantly colored, it is
very doubtful if the color, as such, is of any use to the plant.
The splendid coloring of the leaves in the autumn is certainly
of no service to the trees.
It should not pass unnoticed in this connection that the
stems and the trunks of shrubs and of trees and also many
kinds of fruits and nuts are sometimes highly colored. It
is true that some of the latter have been supposed to owe
their color to its usefulness in attracting birds and other
animals which, feeding on the fruit, swallow the seeds, and
these, passing through the digestive tract and falling to the
ground, may germinate. The dissemination of the seeds of
such plants is supposed to be brought about in this way ; and
since they may be widely disseminated it may be supposed
that it is an advantage to the plant to have attracted the
attention of the fruit-eating birds. On the other hand one
of the most brilliantly colored seeds, the acorn, is too large
to pass through the digestive tracts of birds, and is, in fact,
ground to pieces in the gizzard, and in the case of several
mammals that feed on the acorns, the acorn is crushed by
the teeth. It would seem, therefore, that its coloration is
injurious to it rather than the reverse, as it leads to its
destruction. It has been suggested by Darwin that since
the acorns are for a time stored up in the crop of the
bird, the passenger pigeon for example, and since the birds
may be caught by hawks and killed, the seeds in the crop
thus become scattered. Consequently it may be, after all,
of use to the oak to produce colored acorns that attract the
attention of these pigeons. This suggestion seems too far-
fetched to consider seriously. In the case of the horse-
chestnut the rich brown color is equally conspicuous, but
the nut is too large to be swallowed by any of the ordinary
The Problem of Adaptation 25
seed-feeding birds or mammals. Shall we try to account for
its color on the grounds of the poisonous character of the
seed ? Has it been acquired as a warning to those animals
that have eaten it once, and been made sick or have died in
consequence ? I confess to a personal repugnance to imag-
inative explanations of this sort, that have no facts of experi-
ence to support them.
Changes in the Organism that are of No Use to the
Individual or to the Race
As an example of a change in the organism that is of no
use to it may be cited the case of the turning white of the
hair in old age in man and in several other mammals. The
absorption of bone at the angle of the chin in man, is another
case of a change of no immediate use to the individual. We
also find in many other changes that accompany old age,
processes going on that are of no use to the organism, and
which may, in the end, be the cause of its death. Such
changes, for instance, as the loss of the vigor of the muscles,
and of the nervous system, the weakening of the heart, and
partial failure of many of the organs to carry out their
functions. These changes lead sooner or later to the death
of the animal, in consequence of the breaking down of some
one essential organ, or to disease getting an easier foot-
hold in the body. We have already discussed the possible
relation of death as an adaptation, but the changes just men-
tioned take place independently of their relation to the death
of the organism as a whole, and show that some of the nor-
mal organic processes are not for the good of the individual
or of the race. In fact, the perversions of some of the most
deeply seated instincts of the species, as in infanticide, while
the outcome of definite processes in the organism, are of
obvious disadvantage to the individual, and the perversion of
so deeply seated a process as the maternal instinct, leading
26 Evohition and Adaptation
to the destruction of the young, is manifestly disadvantageous
to the race. As soon, however, as we enter the field of so-
called abnormal developments, the adaptive relation of the
organism to its environment is very obscure ; and yet, as in
the case of adaptation to poisons, we see that we cannot draw
any sharp line between what we call normal and what we call
abnormal development.
Comparison with Inorganic Phenomena
The preceding examples and discussion give some idea of
what is meant by adaptation in living things. In what respects,
it may be asked, do these adaptations differ from inorganic
phenomena ? The first group of inorganic bodies that chal-
lenges comparison are machines. These are so constructed
that they may be said to accomplish a definite purpose, and
the question arises whether this purpose can be profitably
compared with the purposefulness of the structure and
response of organisms. That the two cannot be profitably
compared is seen at once, when we recall the fact that the
activity of the machine is of no use to it, in the sense of
preserving its integrity. The object of the machine is, in
fact, to perform some useful purpose for the organism that
built it, namely, for man. Furthermore, the activity of the
machine only serves to wear it out, and, therefore, its actions
do not assist in preserving its integrity as do some, at least,
of the activities of an animal. It is true, of course, that in a
mechanical sense every action of the organism leads also to a
breaking down of its structure in the same way that a machine
is also worn out by use ; but the organism possesses another
property that is absent in the machine, namely, the power of
repairing the loss that it sustains.
One of the most characteristic features of the organism is
its power of self-adjustment, or of regulation, by which it
adapts itself to changes in the environment in such a way
The Problem of Adaptation 27
that its integrity is maintained. Most machines have no such
regulative power, although, in a sense, the fly-wheel of an
engine regulates the speed, and a water-bath, with a ther-
mostat, regulates itself to a fixed temperature ; but even this
comparison lacks one of the essential features of the regula-
tion seen in organisms, 'namely, in that the regulation does
not protect the machine from injury. It may be claimed,
however, that the safety valve of an engine does fulfil this
purpose, since it may prevent the engine from exploding.
Here, in fact, we do find better grounds for comparison, but,
when we take into account the relation of the regulations in
the organism to all the other properties of the organism,
we see that this comparison is not very significant. The
most essential difference between a machine and an organism
is the power of reproduction possessed by the latter, which is
absent in all machines. Here, however, we meet with a
somewhat paradoxical relation, since the reproductive power
of organisms cannot be looked upon as an adaptation for the
continuation of the individual, but rather for the preservation
of a series of individuals. Hence, in this respect also, we
cannot profitably compare the individual with a machine, but
if we make any comparison we should compare all the indi-
viduals that have come from a single one with a machine.
In this sense the power of reproduction is a sort of racial
regulation. A comparison of this sort is obviously empty of
real significance.
The regenerative power of the organism, by means of
which it may replace a lost part, or by means of which a
piece may become a new whole, is also something not
present in machines.
In using a machine for comparison we should not leave
out of sight the fact that machines are themselves the work
of organisms, and have been made for some purpose useful
to the organism. They may perform the same purpose for
which we would use our own hands, for they differ from
28 Evolution and Adaptation
parts of the body mainly in that they are made of different
compounds having different properties, as the above com-
parisons have shown. But the regulations of the machine
have been added to it by man on account of their useful-
ness to himself, and are not properties of the material of
which the machine itself is composed. This shows, I think,
the inappropriateness of making any comparison between
these two entirely different things.
If, then, we find the comparison between machines and
organisms unprofitable, can we find any other things in
inorganic nature that can be better compared with the
phenomenon of adaptation of the organism ? The following
phenomena have been made the subject of comparison from
time to time. The bendings, which are gradually made by
rivers often lead to a meeting of the loops, so that a direct,
new communication is established, and the course of the
river is straightened out. The water takes, therefore, a
more direct course to the sea. It cannot be said, however,
to be of any advantage to the river to straighten its course.
Again, a glacier moulds itself to its bed, and gradually
moves around obstacles to a lower level, but this adaptation
of the glacier to the form of its surroundings cannot be
said to be of advantage to the glacier. On the contrary,
the glacier reaches so much the sooner a lower level where
it is melted.
The unusual case of a solid being lighter than the liquid
from which it forms, as seen in the case of ice, has been
looked upon as a useful arrangement, since were the reverse
the case all rivers and ponds would become solid in winter
in cold climates, and the polar regions would become one
solid block of ice. But no one will suppose for a moment
that there is any relation between the anomalous condition
of the lightness of ice, and its relation to the winter freezing
of streams, ponds, etc. It has even been suggested that this
property of ice was given to it in order that the animals
The Problem of Adaptation 29
living in the water might not be killed, which would be the
case if the ice sank to the bottom, but such a method of
interpreting physical phenomena would scarcely commend
itself to a physicist.
The formation of a covering of oxide over the surface of
a piece of iron delays the further process of oxidation, but
who will imagine that this property of iron has been ac-
quired in order to prevent the iron from being destroyed by
oxygen ?
If a piece is broken from a crystal, and the crystal is
suspended in a saturated solution of the same substance,
new material is deposited over its whole surface, and, as it
grows larger, the broken side is completed and the crystal
assumes its characteristic form. But of what advantage is
it to the crystal whether it is complete or incomplete? In
the case of an animal it is of some importance to be able
to complete itself after injury, because it can then better
obtain the food necessary to keep it alive, or it can better
escape its enemies ; but this is not the case with the crystal.
In conclusion, therefore, it is obvious that the adaptations
of organisms are something peculiar to living things, and
their obvious purpose is to maintain the integrity of the indi-
vidual, or that of the species to which the individual
belongs. We are, therefore, confronted with the question
as to how this peculiarity has come to be associated with
the material out of which living things are made. In sub-
sequent chapters this will be fully discussed, but before we
take up this topic, it will be necessary to reach some under-
standing in regard to the theory of evolution, for the whole
subsequent issue will turn upon the question of the origin of
the forms of animals and plants living at the present time.
CHAPTER II
THE THEORY OF EVOLUTION
One of the most important considerations in connection
with the problem of adaptation is that in all animals and
plants the individuals sooner or later perish and new genera-
tions take their places. Each new individual is formed, in
most cases, by the union of two germ-cells derived one from
each parent. As a result of this process of intermixing,
carried on from generation to generation, all the individuals
would tend to become alike, unless something else should
come in to affect the result.
So far as our actual experience reaches, we find that the
succeeding generations of individuals resemble each other.
It is true that no two individuals are absolutely alike, but if a
sufficiently large number are examined at a given time, they
will show about the same variations in about the same pro-
portionate numbers. Such a group of similar forms, repeat-
ing itself in each generation, is the unit of the systematists,
and is called a species.
It has been said that within each species the individuals
differ more or less from each other, but our experience
teaches that in each generation the same kinds of variations
occur, and, moreover, that from any one individual there may
arise in the next generation any one of the characteristic
variations. Certain limitations will have to be made in re-
gard to this statement, but for the present it will suffice.
The Law of Biogenesis states that each living thing arises
from another living thing ; that there is no life without ante-
cedent life, i.e. spontaneous generation does not occur. The
30
The Theory of Evolution 31
law is not concerned with the likeness or unlikeness of the dif-
ferent individuals that descend from each other. The theory
of ■ evolution includes the same idea, but in addition it has
come to mean nowadays, that there have been changes, as
the succeeding generations have arisen. The transmutation
theory, and even the descent theory, have come to mean nearly
the same thing as the theory of evolution. It is unfortunate
that one of these terms cannot be used to signify simply the
repetition, generation after generation, of groups of similar
individuals. The theory of descent might be used to convey
only this idea, but unfortunately it too has come to include
also the idea of change. I shall attempt nevertheless to dis-
criminate between the descent and the transmutation theory,
and use the term descent theory when I do not wish to con-
vey the idea of change, and transmutation theory when I do
wish to emphasize this idea.
On the transmutation theory it is assumed that a group
(species) may give rise to one or more groups of forms differ-
ing from their ancestors ; the original group being now re-
placed by its new kinds of offspring, or the old and the new
may remain in existence at the same time. This process
repeating itself, each or some of the new groups giving rise in
turn to one or more new species, there will be produced a larger
group of species having certain similar characters which are
due to their common descent. Such a group of species is
called a genus. The resemblances of these species is
accounted for by their common descent ; but their differences
must be due to those factors that have caused them to depart
from the original type. We may now proceed to consider
the evidence on which this idea of transmutation rests.
32 Evolution and Adaptation
Evidence in Favor of the Transmutation Theory
evidence from classification and from comparative
ANATOMY
It does not require any special study to see that there are
certain groups of animals and of plants that are more like
each other than they are like the members of any other group.
It is obvious to every one that the group known as mammals
has a combination of characters not found in any other
group ; such, for instance, as a covering of hair, mammary
glands that furnish milk to the young, and a number of other
less distinctive features. These and other common character-
istics lead us to put the mammals into a single class. The
birds, again, have certain common characters such as feathers,
a beak without teeth, the development of a shell around the
egg, etc., and on account of these resemblances we put them
into another class. Everywhere in the animal and plant
kingdoms we find large groups of similar forms, such as the
butterflies, the beetles, the annelidan worms, the corals, the
snails, the starfishes, etc.
Within each of these groups we find smaller groups, in
each of which there are again forms more like each other
than like those of other groups. We may call these smaller
groups families. Within the families we find smaller groups,
that are more like each other than like any other groups in
the same family, and these we put into genera. Within the
genus we find smaller groups following the same rule, and
these are the species. Here we seem to have reached a limit
in many cases, for we do not always find within the species
groups of individuals more like each other than like other
groups. Although we find certain differences between the
individuals of a species, yet the differences are often incon-
stant in the sense that amongst the descendants of any in-
dividual there may appear any one of the other variations.
If this were the whole truth, it would seem that we had here
The Theory of Evolution 33
reached the limits of classification, the species being the
unit. This, however, is far from being the case, for, in
many species we find smaller groups, often confined to
special localities. These groups are called varieties.
In some cases it appears, especially in plants, these
smaller groups of varieties resemble in many ways the
groups of species in other forms, since they breed true to
their kind, even under changed conditions. They have been
recognized as " smaller species " by a number of botanists.
In this connection a point must be brought up that has
played an important role in all discussion as to what limits
can be set to a species. As a rule it is found that two dis-
tinct species cannot be made to cross with each other, i.e.
the eggs of an individual of one species cannot be fertilized
by spermatozoa derived from individuals of another species ;
or, at least, if fertilization takes place the embryo does not
develop. In some cases, however, it has been found possi-
ble to cross-fertilize two distinct species, although the off-
spring is itself more or less infertile. Even this distinction,
however, does not hold absolutely, for, in a few cases, the
offspring of the cross is fertile. It cannot be maintained,
therefore, that this test of infertility between species invari-
ably holds, although in a negative sense the test may apply,
for if two different forms are infertile, inter se, the result
shows that they are distinct species. If they cross they may
or may not be good species, and some other test must be
used to decide their relation.
We should always keep in mind the fact that the individual
is the only reality with which we have to deal, and that the
arrangement of these into species, genera, families, etc., is
only a scheme invented by man for purposes of classification.
Thus there is no such thing in nature as a species, except as
a concept of a group of forms more or less alike. In nature
there are no genera, families, orders, etc. These are inven-
tions of man for purposes of classification.
34 Evolution and Adaptation
Having discovered that it is possible to arrange animals
and plants in groups within groups, the question arises as to
the meaning of this relation. Have these facts any other
significance than that of a classification of geometric figures,
or of crystals according to the relations of their axes, or of
bodies as to whether they are solids, liquids, or gases, or even
whether they are red, white, or blue ?
If we accept the transmutation view, we can offer an
explanation of the grouping of living things. According to
the transmutation theory, the grouping of living things is due
to their common descent, and the greater or less extent to
which the different forms have diverged from each other. It
is the belief in this principle that makes the classification of
the biologist appear to be of a different order from that in
any other science ; and it is this principle that appears to give
us an insight into a large number of phenomena.
For example, if, as assumed in the theory, a group of
individuals (species) breaks up into two groups, each of these
may be supposed to inherit a large number of common char-
acteristics from their ancestors. These characters are, of
course, the resemblances, and from them we conclude that
the species are related and, therefore, we put them into the
same genus. The differences, as has been said, between the
species must be explained in some other way ; but the prin-
ciple of classification with which we are here concerned is
based simply on the resemblances, and takes no account of
the differences between species.
In this argument it has been tacitly assumed that the
transformation of one species into another, or into more
than one, takes place by adding one or more new characters
to those already present, or by changing over a few char-
acters without altering others. But when we come to examine
any two species whatsoever, we find that they differ, not only
in one or in a few characters, but in a large number of points ;
perhaps in every single character. It is true that sometimes
The Theory of Evolution 35
the differences are so small that it is difficult to distinguish
between two forms, but even in such cases the differences,
although small, may be as numerous as when they are more
conspicuous. If, then, this is what we really find when we
carefully examine species of animals or of plants, what is
meant when we claim that our classification is based on the
characters common to all of the forms that have descended
from the same ancestor? We shall find, if we press this
point that, in one sense, there is no absolute basis of this sort
for our classification, and that we have an unreal system.
If this is admitted, does our boasted system of classification,
based as it is on the principle of descent, give us anything
fundamentally different from an artificial classification ? A
few illustrations may make clearer the discussion that follows.
If, for example, we take a definition of the group of verte-
brates we read : " The group of craniate vertebrates includes
those animals known as Fishes, Amphibians, Reptiles, Birds,
and Mammals ; or in other words, Vertebrates with a skull, a
highly complex brain, a heart of three or four chambers, and
red blood corpuscles." If we attempt to analyze this defini-
tion, we find it stated that the skull is a characteristic of all
vertebrates, but if we ask what this thing is that is called
skull, we find not only that it is something different in dif-
ferent groups, being cartilaginous in sharks, and composed
of bones in mammals, but that it is not even identical in
any two species of vertebrates. If we try to define it as a
case of harder material around the brain, then it is not
something peculiar to the vertebrates, since the brain of the
squid is also encased in a cartilaginous skull. What has been
said of the skull may be said in substance of the brain, of the
heart, and even of the red blood corpuscles.
If we select another group, we find that the birds present
a sharply defined class with very definite characters. The
definition of the group runs as follows : " Birds are char-
acterized by the presence of feathers, their fore-limbs are
36 Evolution and Adaptation
used for flight, the breast-bone is large and serves for the
attachment of the muscles that move the wings ; outgrowths
from the lungs extend throughout the body and even into the
bones and serve as air sacs which make the body more buoy-
ant. Only one aortic arch is present, the right, and the right
ovary and oviduct are not developed. The eyes are large
and well developed. Teeth are absent. We have here a series
of strongly marked characteristics such as distinguish hardly
any other class. Moreover, the organization of existing birds
is, in its essential features, singularly uniform ; the entire
class presenting less diversity of structure than many orders
of Fishes, Amphibians, and Reptiles." x The feathers are
the most unique features of birds, and are not found in any
other group of the animal kingdom ; moreover the plan on
which they are formed is essentially the same throughout the
group, yet in no two species are the feathers identical, but
differ not only in form and proportions, but even in the char-
acter of the barbs' and hooks for holding the vane together.
The modification of the fore-limbs for flight is another char-
acteristic feature ; yet in some birds, as the ostrich and kiwi,
although the wing has the same general plan as in other
birds, it is not used for flight. In the latter it is so small that
it does not project beyond the feathers, and in some birds, as
in the penguins, the wings are used only as organs for swim-
ming.
In spite of these differences we have no difficulty in
recognizing throughout the group of birds a similarity of
plan or structure, modified though it be in a thousand
different ways.
Enough has been said to illustrate what is meant by the
similarities of organisms on which we base our system of
classification. When we conclude from the statement that
all vertebrates have a skull that they owe this to a common
descent, we do not mean that a particular structure has been
1 Parker and Haswell: "Text Book of Zoology."
The Theory of Evolution 37
handed down as a sort of entailed heirloom, but that the
descendants have followed the same plan of structure as that
of their ancestors, and have the brain enclosed in a covering
of harder material, although this material may not have
exactly the same form, or be made of the same substance in
all cases. Furthermore while we may recognize that the
cartilaginous skull of the shark is simpler in structure than
that of the cartilaginous-bony skull of the frog, and that
the skull of the frog is simpler than that of the rabbit,
yet we should not be justified in stating, except in a
metaphorical sense, that something has been added to the
skull of the shark to make that of the frog, and some-
thing to the latter to make that of the rabbit. On the con-
trary, while something may have been added, and the plan
made more complicated, the skull has also been changed
throughout in every single part.
There is another point of some importance to be taken
into account in this connection ; namely, that each new
generation begins life as a single cell or egg. The egg
does not contain any preformed adult structures that it
hands down unaltered, but it is so constructed that, under
constant conditions, the same, or nearly the same, kind of
structure is produced. Should something affect the egg f
we can imagine that it might form a new combination on
the same general plan as that of the old, yet one that differed
from the original in every detail of its structure. It is this
idea, I believe, that lies at the base of the transmutation
theory. On some such assumption as this, and on this
alone, can we bring the theory of transmutation into har-
mony with the facts of observation.
What has been said in regard to individuals as a whole
may be repeated also in respect to the study of the single
organs. Selecting any one group of the animal or plant
kingdom, we find the same organ, or the same combination
of organs present in whole groups of forms. We can often
38 Evolution and Adaptation
arrange these organs in definite series passing from the
simple to the complex, or, in case of degeneration, in the
reverse order. However convenient it may be to study
the structure of organisms from this point of view, the arti-
ficiality of the procedure will be obvious, since here also the
organs of any two species do not differ from each other in
only one point, but in many, perhaps in all. Therefore to
arrange or to compare them according to any one scheme
gives only an incomplete idea of their structure. We should
apply here the same point of view that we used above in
forming a conception of the meaning of the zoological and
botanical systems. We must admit that our scheme is only
an ideal, which corresponds to nothing real in nature, but
is an abstraction based on the results of our experience.
It might be a pleasing fancy to imagine that this ideal
scheme corresponds to the plan of structure or of organiza-
tion that is in every egg, and furnishes the basis for all the
variations that have come or may come into existence ; but
we should find no justification whatsoever for believing that
our fiction corresponds to any such real thing.
To sum up the discussion : we find that the resemblances
of animals and plants can be accounted for on the transmu-
tation theory, not in the way commonly implied, but in a some-
what different sense. We have found that the resemblances
between the different members of a group are only of a
very general sort, and the structures are not identically the
same in any two species — in fact, perhaps in no two indi-
viduals. This conclusion, however, does not stand in con-
tradiction to the transmutation hypothesis, because, since
each individual begins as an egg which is not a replica of
the original adult from which it is derived, there can be no
identity, but at most a very close similarity. Admitting, then,
that our scheme is an ideal one, we can claim, nevertheless,
that on this basis the facts of classification find a legitimate
explanation in the transmutation theory.
The Theory of Evolution 39
THE GEOLOGICAL EVIDENCE
On the theory of descent, as well as on the theory of
transmutation, the ancestors of all present forms are sup-
posed to have lived at some time in the past on the surface
of the earth. If, therefore, their remains should have been
preserved, we should expect on the descent theory to find
some, at least, of these remains to be like present forms,
while on the transmutation theory we should expect to find
most, if not all, of the ancestral forms to be different from
the present ones.
The evidence shows that fossil forms are practically all
different from living forms, and the older they are the
greater the difference from present forms. In general,
therefore, it may be said that the evidence is in favor of the
transmutation theory. It can scarcely be claimed that the
evidence is absolutely conclusive, however probable it may
appear, for the problem is complicated in a number of ways.
In the first place, there is convincing evidence that some
forms have been entirely exterminated. Other groups have
very few living representatives, as is the case in the group
containing nautilus, and in that of the crinoids. It is there-
fore always possible that a given fossil form may represent
an extinct line, and may be only indirectly connected with
forms alive at the present time. Again the historical record
is so broken and incomplete in all but a few cases that its
interpretation is largely a question of probability. We can
easily conceive that it would be only in very exceptional
cases that successive generations of the same form would be
buried one above the other, so that we should find the
series unbroken. This is evident not only because the condi-
tions that were at one time favorable for the preservation
of organic remains might not be favorable at another time,
but also because if the conditions remained the same the
organisms themselves might also remain unchanged. A new
40 Evolution and Adaptation
form, in fact, would be, ex hypotJiese, better suited to live
in a different environment, and consequently we should not
expect always to find its remains in the same place as that
occupied by the parent species. This possibility of migration
of new forms into a new locality makes the interpretation
of the geological record extremely hazardous.
Nevertheless, if the evolution of the entire animal and
plant kingdoms had taken place within the period between
the first deposits of stratified rocks and the present time, we
might still have expected to find, despite the imperfections
of the record, sufficient evidence to show how the present
groups have arisen, and how they are related to one another.
But, unfortunately, at the period when the history of the
rocks begins, nearly all the large groups of animals were
in existence, and some of them, indeed, as the trilobites
and the brachiopods, appear to have reached the zenith of.
their development.
On the other hand, the subdivisions of the group of verte-
brates have evolved during the period known to us. It is
true that the group was already formed when our knowledge
of it begins, but, from the fishes onwards, the history of the
vertebrates is recorded in the rocks. The highest group of
all, the mammals, has arisen within relatively modern times.
The correctness of the transmutation theory could be as well
established by a single group of geological remains as by the
entire animal kingdom. Let us, therefore, examine how far
the theory is substantiated by the paleontological record of the
vertebrates. We find that the earliest vertebrates were fishes,
and these were followed successively by the amphibians,
reptiles, birds, and mammals, one of the last species of all
to appear being man himself. There can be little doubt that
this series, with certain limitations to bespoken of in a moment,
represents a progressive series beginning with the simpler
forms and ending with the more complicated. Even did we
not know this geological sequence we would conclude, from
The Theory of Evolution 41
the anatomical evidence alone, that the progression had been
in some such order as the geological record shows. The
limitation referred to above is this : that while the mammals
arose later than the birds, we need not suppose that the
mammals arose from the birds, and not even perhaps from
the reptiles, or at least not from reptiles like those living
at the present day. The mammals may in fact, as some
anatomists believe, have come direct from amphibian-like
forms. If this is the case, we find the amphibians giving rise
on one hand to reptiles and these to birds, and on the other
hand to mammals.
This case illustrates how careful we should be in interpret-
ing the record, since two or more separate branches or orders
may arise independently from the same lower group. If the
mammals arose from the amphibians later than did the rep-
tiles, it would be easy to make the mistake, if the record was
incomplete at this stage, of supposing that the mammals had
come directly from the reptiles.
That the birds arose as an offshoot from reptile-like forms
is not only probable on anatomical grounds, but the geo-
logical record has furnished us with forms like archasop-
teryx, which in many ways appears to stand midway between
the reptiles and birds. This fossil, archaeopteryx, has a bird-
like form with feathered wings, and at the same time has a
beak with reptilian teeth, and a long, feathered tail with a
core of vertebrae.
From another point of view we see how difficult may be
the interpretation of the geological record, when we recall
that throughout the entire period of evolution of the verte-
brates the fishes, amphibians, reptiles, and birds remained still
in existence, although they, or some of them, may have at
one time given origin to new forms. In fact, all these groups
are alive and in a flourishing condition at the present time.
The fact illustrates another point of importance, namely, that
we must not infer that because a group gives rise to a higher
42 Evolution and Adaptation
one, that it itself goes out of existence, being exterminated
by the new form. There may be in fact no relation what-
soever between the birth of a new group and the extermina-
tion of an old one.
On the transmutation theory we should expect to find not
only a sequence of forms, beginning with the simplest and
culminating with the more complex, but also, in the beginning
of each new group, forms more or less intermediate in
structure. It is claimed by all paleontologists that such
forms are really found. For example, transitional forms
between the fishes and the amphibia are found in the group
of dipnoans, or lung-fishes, a few of which have survived to
the present day. There are many fossil forms that have
characters between those of amphibians and reptiles, which
if not the immediate ancestors of the reptiles, yet show
that at the time when this group is supposed to have
arisen intermediate forms were in existence. The famous
archaeopteryx remains have been already referred to above,
and it appears in this case that we have not only an inter-
mediate form, but possibly a transitional one. In the group
of mammals we find that the first forms to appear were the
marsupials, which are undoubtedly primitive members of the
group.
The most convincing evidence of transmutation is found in
certain series of forms that appear quite complete. The
evolution of the horse series is the most often cited. As this
case will be discussed a little later, we need not go into it
fully here. It will suffice to point out that a continuous
series of forms has been found, that connect the living
horses having a single toe through three-toed, with the five-
toed horses. Moreover, and this is important, this series
shows a transformation not only in one set of structures, but
in all other structures. The fossil horses with three toes are
found in the higher geological layers, and those with more
toes in the deeper layers progressively. In some cases, at
The Theory of Evolution 43
least, the fossils have been found in the same part of the
world, so that there is less risk of arranging them arbitrarily
in a series to fit in with the theory.
EVIDENCE FROM DIRECT OBSERVATION AND EXPERIMENT
Within the period of human history we do not know of a
single instance of the transformation of one species into
another one, if we apply the most rigid and extreme tests
used to distinguish wild species from each other. 1 It may
be claimed that the theory of descent is lacking, therefore,
in the most essential feature that it needs to place the
theory on a scientific basis. This must be admitted. On
the other hand, the absence of direct observation is not
fatal to the hypothesis, for several reasons. In the first
place, it is only within the last few hundred years that
an accurate record of wild animals and plants has been kept,
so that we do not know except for this period whether any
new species have appeared. Again, the chance of observing
the change might not be very great, especially if the change
were sudden. We would simply find a new species, and
could not state where it had come from. If, on the other
hand, the change were very slow, it might extend over so
many years that the period would be beyond the life of an
individual man. In only a few cases has it been possible
to compare ancient pictures of animals and plants with their
prototypes living at the present time, and it has turned out
in all cases that they are the same. But these have been
almost entirely domesticated forms, where, even if a change
had been found, it might have been ascribed to other fac-
tors. In other cases, as in the mummified remains of a few
Egyptian wild animals (which have also been found to be
exactly like the same animals living at the present day),
1 The transformation of " smaller species," described by De Vries, will be
described in a later chapter.
44 Evolution and Adaptation
it was pointed out by Geoffroy Saint-Hilaire that, since the
conditions of the Egyptian climate are the same to-day as
they were two thousand years ago, there is no reason to
expect any change would have taken place. But waiving
this assumption, we should not forget that the theory of evo-
lution does not postulate that a change must take place
in the course of time, but only that it may take place
sometimes.
The position that we have here taken in regard to the
lack of evidence as to the transformation of species is, per-
haps, extreme, for, as will be shown in some detail in later
chapters, there is abundant evidence proving that species
have been seen to change greatly when the conditions sur-
rounding them have been changed ; but never, as has been
stated, so far, or rather in such a way, that an actual new
species that is infertile with the original form has been pro-
duced. Whether, after all, these changes due to a change
in the environment are of the kind that makes new species,
is also a question to be discussed later.
The experimental evidence, in favor of the transformation
of species, relates almost entirely to domesticated forms, and
in this case the conscious agency of man seems, in some cases,
to have played an important part ; but here, even with the
aid of the factor of isolation, it cannot be claimed that a
single new species has been produced, although great
changes in form have been effected. It is clear, therefore,
that we must, at present, rely on other data, less satisfac-
tory in all respects, to establish the probability of the theory
of transformation.
MODERN CRITICISM OF THE THEORY OF EVOLUTION
Throughout the whole of the nineteenth century a steady
fire of criticism was directed against the theory of evolution ;
the names of Cuvier and of Louis Agassiz stand out preemi-
The Theory of Evolution 45
nent in this connection, yet the theory has claimed an ever
increasing number of adherents, until at the present time it
is rare to find a biologist who does not accept in one form or
another the general principle involved in the theory. The
storm of criticism aroused by the publication of Darwin's
" Origin of Species," was directed more against the doctrine
of evolution than against Darwin's argument for natural se-
lection. The ground has been gone over so often that there
would be little interest in going over it again. It will be more
profitable to turn our attention to the latest attack on the
theory from the ranks of the zoologists themselves.
Fleischmann, in his recent book, " Die Descendenztheorie,"
has made a new assault on the theory of evolution from the
three standpoints of paleontology, comparative anatomy, and
embryology. His general method is to try to show that the
recognized leaders in these different branches of biology
have been led to express essentially different views on
the same questions, or rather have compromised the doc-
trine by the examples they have given to illustrate it.
Fleischmann is fond of bringing together the antiquated
and generally exaggerated views of writers like Haeckel,
and contrasting them with more recent views on the same
subject, without making sufficient allowances for the ad-
vances in knowledge that have taken place. He selects
from each field a few specific examples, by means of
which he illustrates the weakness, and even, as he be-
lieves, the falsity of the deductions drawn for the par-
ticular case. For example, the plan of structure of the
vertebrates is dealt with in the following way : In this
group the limbs, consisting typically of a pair of fore-
legs and a pair of hind-legs, appear under the form of
cylindrical outgrowths of the body. In the salamander,
in the turtle, in the dog, the cylindrical legs, supporting
the body and serving to support it above the ground, are
used also for progression. The general purpose to which
46 Evolution and Adaptation
the limbs are put as organs of locomotion has not inter-
fered with an astonishing number of varieties of struc-
ture, adapted to different conditions of existence, such as
the short legs used for creeping in salamanders, lizards,
turtles, crocodiles ; the long and thin legs of good runners,
as the hoofed animals ; the mobile legs of the apes used for
climbing ; and the parachute legs of some squirrels used for
soaring. Even more striking is the great variety of hands
and feet, as seen in the flat, hairy foot of the bear; the
fore-foot of the armadillos, carrying long, sickle-shaped
claws ; the digging foot of the mole ; the plump foot of
the elephant, ending in a broad, flat pad with nails around
the border, and without division into fingers ; the hand of
man and of the apes ending with fine and delicate fingers
for grasping. To have discovered a general plan of struc-
ture running through such a great variety of forms was
proclaimed a triumph of anatomical study. 1
A study of the bony structure of the limb shows that typi-
cally it consists of a single proximal bone (the humerus in
the upper arm, the femur in the thigh), followed by two
bones running parallel to each other (the radius and ulna in
the arm and the tibia and fibula in the shank); these are
succeeded in the arm by the two series of carpal bones, and
in the leg by the two series of tarsal bones, and these are
followed in each by five longer bones (the metacarpals and
metatarsals), and these again by the series of long bones
that lie in the fingers and toes. Despite the manifold variety
of forms, Fleischmann admits that both the hind- and the
fore-limbs are constructed on the same plan throughout the
vertebrates. Even forms like the camel, in which there are
fewer terminal bones, may be brought into the same category
by supposing a reduction of the bones to have taken place,
so that three of the digits have been lost. In the leg of
the pig and of the reindeer, even a greater reduction may
1 This paragraph is a free translation of Fleischmann's text.
The Theory of Evolution 47
be supposed to have taken place. Fleischmann points out
that these facts were supposed to be in full harmony with
the theory of descent.
The analysis of the origin of the foot of the horse gave
even better evidence, it was claimed, in favor of the theory.
The foot consists of a single series of bones corresponding
to the middle finger and toe. When, as sometimes happens,
individual horses are found in which in addition to the single
middle finger two smaller lateral fingers with small hoofs
appear, the followers of the descent theory rejoiced to be able
to bring this forward as a confirmation of their doctrine.
The occurrence was explained as a sporadic return to an
ancestral form. The nai've exposition of the laws of in-
heritance that were supposed to control such phenomena
was accepted without question. And when finally a large
number of fossil remains were found by paleontologists, —
remains showing a gradual increase in the middle finger,
and a decrease in size of the lateral fingers, — it was sup-
posed that the proof was complete ; and anatomists even
went so far as to hold that the original ancestor of the
horse was a five-fingered animal.
This same law of type of structure was found to extend to
the entire vertebrate series, and the only plausible explana-
tion appeared to be that adopted by Darwin and his fol-
lowers, namely, that the resemblance is the result of the
blood-relationship of the different forms. But a simple com-
parison of the skeleton of the limbs if carried out without
theoretical prejudice would show, Fleischmann thinks, that
there is only a common style, or plan of structure, for the
vertebrates. This anatomical result has about the same
value as the knowledge of the different styles of historical
architecture — that, for instance, all large churches of the
Gothic period have certain general principles in common.
The believers in the theory of descent have, however, he
thinks, gone beyond the facts, and have concluded that the
48 Evolution and Adaptation
common plan in animals is the consequence of a common
descent. " I cannot see the necessity for such a conclusion,
and I certainly should unhesitatingly deny that the common
plan of the Gothic churches depended on a common archi-
tect. The illustration is, however, not perfect, because the
influence of the mediaeval school of stone-cutters on its wan-
dering apprentices is well known."
Fleischmann adds that if the descent theory is true we
should expect to find that if a common plan of structure is
present in one set of organs, as the limbs, it should be pres-
ent in all other organs as well, but he does not add that this
is generally the case.
The weakness of Fleischmann's argument is so apparent
that we need not attempt an elaborate refutation. When he
says there is no absolute proof that the common plan of
structure must be the result of blood-relationship, he is not
bringing a fatal argument against the theory of descent, for
no one but an enthusiast sees anything more in the explana-
tion than a very probable theory that appears to account for
the facts. To demand an absolute proof for the theory is to
ask for more than any reasonable advocate of the descent
theory claims for it. As I have tried to show in the preced-
ing pages, the evidence in favor of the theory of descent
is not absolutely demonstrative, but the theory is the most
satisfactory one that has as yet been advanced to account
for the facts. Fleischmann's reference to the common plan
of structure of the Gothic churches is not very fortunate for
his purpose, since he admits himself that this may be the
result of a common tradition handed down from man to man,
a sort of continuity that is not very dissimilar in principle from
that implied in the descent theory ; in the latter the continuity
of substance taking the place of the tradition in the other.
Had the plan for each, or even for many of the churches,
originated independently in the mind of each architect, then
the similarity in style would have to be accounted for by a
The Theory of Evolution 49
different sort of principle from that involved in the theory
of descent; but as a matter of fact the historical evidence
makes it probable that similar types of architecture are
largely the result of imitation and tradition. Certain varia-
tions may have been added by each architect, but it is just
the similarity of type or plan that is generally supposed to
be the outcome of a common tradition.
Fleischmann's attempt in the following chapter to belittle
Gegenbaur's theory of the origin of the five-fingered type of
hand from a fin, like that of a fish, need not detain us, since
this theory is obviously only a special application which like
any other may be wrong, without in the least injuring the
general principle of descent. That all phylogenetic questions
are hazardous and difficult is only too obvious to any one
familiar with the literature of the last thirty years.
Fleischmann devotes a long chapter to the geological evi-
dences in connection with the evolution of the horse, and
attempts to throw ridicule on the conclusions of the paleon-
tologists by emphasizing the differences of opinion that have
been advanced in regard to the descent of this form. After
pointing out that the horse, and its few living relatives, the
ass and the zebra, are unique in the mammalian series in
possessing a single digit, he shows that by the discovery of
the fossil horses the group has been simply enlarged, and
now includes horses with one, three, and five toes. The
discovery of the fossil forms was interpreted by the advocates
of the descent theory as a demonstration of the theory. The
series was arranged by paleontologists so that the five-toed
form came first, then those with three and one toe, the
last represented by the living horses. But the matter was
not so simple, Fleischmann points out, as it appeared to
be to the earlier writers, for example to Haeckel, Huxley,
Leidy, Cope, Marsh. Different authors came to express
different opinions in regard to the genealogical connection
between the fossil forms. Several writers have tried to show
E
50 Evolution and Adaptation
that the present genus, Equus, has not had a single line of
descent, but have supposed that the European horses and
the original American horses had different lines of ancestry,
which may have united only far back in the genus Epihippus.
Fleischmann points out that the arrangement of the series
is open to the criticism that it is arbitrary, and that we could
equally well make up an analogous series beginning with the
five-fingered hand of man, then that of the dog with the
thumb incompletely developed, then the four-fingered hind-
foot of the pig without a big toe and with a weak second and
fifth digit, then the foot of the camel with only two toes,
and lastly the foot of the horse with only one toe. It sounds
strange that Fleischmann should make such a trivial reply as
this, and deliberately ignore the all-important evidence with
which he is, of course, as is every zoologist, perfectly con-
versant. Not only are there a hundred other points of
agreement in the horse series, but also the geological
sequence of the strata, in which some at least of the series
have been found, shows that the arrangement is not arbitrary,
as he implies.
Fleischmann then proceeds to point out that when the
evidence from other parts of the anatomy is taken into
account, it becomes evident that all the known fossil re-
mains of horses cannot be arranged in a single line, but
that there are at least three families or groups recognizable.
Many of these forms are known only from fragments of their
skeletons — a few teeth, for instance, in the case of Mero-
hippus, which on this evidence alone has been placed at
the uniting point of two series. At present about eight dif-
ferent species of living horses are recognized by zoologists,
and paleontological evidence shows only that many other
species have been in existence, and that even three- and one-
toed forms lived together at the same time.
Fleischmann also enters a protest against the ordinary
arrangement of the fossil genera Eo-, Oro-, Meso-, Mero-
The Theory of Evolution 5 1
hippus in a series, for these names stand not for single species,
but for groups containing no less than six species under
Protohippus, fourteen under Equus, twelve under Mesohippus,
and twenty under Hipparion. Fleischmann concludes : " The
descent of the horses has not been made out with the precision
of an accurate proof, and it will require a great deal of work
before we get an exact and thorough knowledge of the fossil
forms. What a striking contrast is found on examination be-
tween the actual facts and the crude hopes of the apostles
of the descent theory ! . . ."
In so far as this criticism of Fleischmann's applies to the
difficulties of determining the past history of the horse, it may
be granted that he has scored a point against those who have
pretended that the evidence is simple and conclusive ; but we
should not fail to remember that this difficulty has been felt
by paleontologists themselves, who have been the first to call
attention to the complexity of the problem, and to the diffi-
culties of finding out the actual ancestors of the living
representative of the series. And while we may admit that
the early enthusiasts exaggerated, unintentionally, the im-
portance of the few forms known to them, and went too far
in supposing that they had found the actual series of ances-
tors of living horses, yet we need not let this blind us to the
importance of the facts themselves. Despite the fact that it
may be difficult and, perhaps, in most cases, impossible, to
arrange the fossil forms in their relations to one another and
to living forms, yet on an unprejudiced view it will be clear,
I think, that so far as the evidence goes it is in full harmony
with the theory of descent. This is especially evident if we
turn our attention to a part of the subject that is almost
entirely ignored by Fleischmann, and yet is of fundamental
importance in judging of the result. The series of forms
beginning with the five-toed horses and ending with those
having a single toe has not been brought together haphazard,
as Fleischmann's comparison might lead one to suppose, but
52 Evolution and Adaptation
the five-fingered forms are those from the older rocks, and
the three-toed forms from more recent layers. The value
of this kind of evidence might have been open to greater
doubt had the series been made up of forms found scattered
over the whole world, for it is well known how difficult it is
to compare in point of time the rocks of different continents.
But in certain parts of the world, especially in North America,
series of fossil horses have been found in sedimentary de-
posits that appear to be perfectly continuous. This series,
by itself, and without regard to the point as to whether in
other parts of the world other series may exist, shows exactly
those results which the theory of descent postulates, and we
find here, in all probability, a direct line of descent. While it
may be freely admitted that no such series can demonstrate
the theory of descent with absolute certainty, yet it would be
folly to disregard evidence as clear as this.
In regard to the other point raised by Fleischmann
concerning the large number of species of fossil horses that
have existed in past times, it is obvious that while this greatly
increases the difficulty of the paleontologist it is not an
objection to the descent theory. In fact, our experience with
living species would lead us to expect that many types have
been represented at each geological period by a number of
related species that may have inhabited the same country. On
the descent theory, one species only in each geological period
could have been in the line of descent of the present species
of horse. The difficulty of determining which species (if
there were several living in a given epoch) is the ancestor
of the horse is increased, but this is not in itself an objection
to the theory.
The descent of birds from flying reptiles is used by
Fleischmann as another point of attack on the transmuta-
tion theory. The theory postulates that the birds have come
from ancestors whose fore-legs have been changed into
highly specialized wings. The long vertebrated tail of the
The Theory of Evolution 53
ancestral form is supposed to have become very short, and
long feathers to have grown out from its stump which act
as a rudder during flight. Flying reptiles with winged fore-
legs and a long vertebrated tail have been actually found as
fossil remains, as seen in the pterodactyls and in the famous
archaeopteryx. The latter, which is generally regarded either
as the immediate ancestor of living birds, or at least as a
closely similar form, possessed a fore-leg having three fingers
ending in claws, and feathers on the forearm similar to those
of modern birds. It had a long tail, like that of a lizard, but
with well-developed feathers along its sides. It had pointed
teeth in the horn-covered jaws. Fleischmann proceeds to point
out that the resemblance of the hand of archaeopteryx to
that of the reptiles is not very close, for two fingers are
absent as in modern birds. The typical form of the foot is
that of the bird, and is not the simple reptilian type of struc-
ture. Feathers and not scales cover the body, and give no
clew as to how the feathers of birds have arisen. He con-
cludes, therefore, that archaeopteryx, having many true bird-
like characters, such as feathers, union of bones in the foot,
etc., has other characters not possessed by living birds,
namely, a long, vertebrated tail, a flat breastbone, biconcave
vertebrae, etc. Therefore, it cannot be regarded as an inter-
mediate form. Fleischmann does not point out that it is just
these characters that would be postulated on the descent
theory for the ancestor of the birds, if the latter arose from
reptiles. Even if it should turn out that archaeopteryx is
not the immediate forefather of living birds, yet the dis-
covery that a form really existed intermediate in many
characters between the reptiles and the birds is a gain for
the transmutation theory. It is from a group having such
characters that the theory postulates that the birds have been
evolved, and to have discovered a member of such a group
speaks directly and unmistakably in favor of the proba-
bility of the transmutation theory.
54 Evolution and Adaptation
Fleischmann again fails to point out that the geological
period in which the remains of archaeopteryx were found,
is the one just before that in which the modern group of
birds appeared, and, therefore, exactly the one in which the
theory demands the presence of intermediate forms. This
fact adds important evidence to the view that looks upon
archaeopteryx as a form belonging to a group from which
living birds have arisen. That a number of recent paleon-
tologists believe archaeopteryx to belong to the group of
birds, rather than to the reptiles, or to an intermediate group,
does not in the least lessen its importance, as Fleischmann
pretends it does, as a form possessing a number of reptilian
characters, such as the transmutation theory postulates for
the early ancestors of the birds.
The origin of the mammalian phylum serves as the text
for another attack on the transmutation theory. Fleischmann
points out that the discovery of the monotremes, including
the forms ornithorhynchus and echidna, was hailed at first as
a demonstration of the supposed descent of the mammals from
a reptilian ancestor. The special points of resemblance be-
tween ornithorhynchus and reptiles and birds are the com-
plete fusion of the skull bones, the great development of
the vertebrae of the neck region, certain similarities in the
shoulder girdle, the paired oviducts opening independently
into the last part of the digestive tract (cloaca), and the
presence of a parchment-like shell around the large, yolk-
bearing egg. These are all points of resemblance to reptiles
and birds, and were interpreted as intermediate stages be-
tween the latter groups and the group of mammals. In
addition to these intermediate characters, ornithorhynchus
possesses some distinctive, mammalian features --mammary
glands and hair, for instance. Fleischmann takes the ground,
in this case, that there are so many points of difference be-
tween the monotremes and the higher mammals, that it is
impossible to see how from forms like these the higher
The Theory of Evolution 55
groups could have arisen, and that ornithorynchus cannot
be placed as an intermediate form, a link between saurian s
and mammals, as the followers of the transmutation theory
maintain. He shows, giving citations, that anatomists them-
selves are by no means in accord as to the exact position of
ornithorhynchus in relation to the higher forms.
In reply to this criticism, the same answer made above for
archseopteryx may be repeated here, namely, that because cer-
tain optimists have declared the monotremes to be connecting
forms, it does not follow that the descent theory is untrue, and
not even that these forms do not give support to the theory,
if in a less direct way. I doubt if any living zoologist regards
either ornithorhynchus or echidna as the ancestral form from
which the mammals have arisen. But on the other hand it
may be well not to forget that these two forms possess many
characters intermediate between those of mammals and rep-
tiles, and it is from a group having such intermediate characters
that we should expect the mammals to have arisen. These
forms show, if they show nothing else, that it is possible for a
species to combine some of the characters of the reptiles with
those of the mammals ; and the transmutation theory does no
more than postulate the existence at one time of such a group,
the different species of which may have differed in a number
of points from the two existing genera of monotremes.
The origin of lung-bearing vertebrates from fishlike ances-
tors, in which the swim-bladder has been changed into lungs,
has been pointed to by the advocates of the transmutation
theory as receiving confirmation in the existence of animals
like those in the group of dipnoan fishes. In these animals
both gills and a swim-bladder, that can be used as a lung, are
present ; and through some such intermediate forms it is
generally supposed that the lung-bearing animals have arisen.
Fleischmann argues, however, that, on account of certain
trivial differences in the position of the duct of the swim-
bladder in living species, the supposed comparison is not to
56 Evolution and Adaptation
the point ; but the issue thus raised is too unimportant to
merit further discussion. Leaving aside also some even more
doubtful criticisms which are made by Fleischmann, and
which might be added to indefinitely without doing more
than showing the credulity of some of the more ardent
followers of the transmutation theory, or else the uncertainty
of some of the special applications of the theory, let us pass
to Fleischmann's criticism of the problem of development. 1
With fine scorn Fleischmann points to the crudity of the
ideas of Oken and of Haeckel in regard to the embryology
(or the ontogeny) repeating the ancestral history (or the
phylogeny). We may consider briefly (since we devote the
next chapter almost entirely to the same topic) the excep-
tions to this supposed recapitulation, which Fleischmann has
brought together. The young of beetles, flies, and butter-
flies creep out of the egg as small wormlike forms of appar-
ently simple organization. They have a long body, composed
of a series of rings ; the head is small and lacks the feelers,
and often the faceted eyes. The wings are absent, and the
legs are short. At first sight the larva appears to resemble a
worm, and this led Oken to conclude that the insects appear
first in the form of their ancestors, the segmented worms. If
we examine the structure of the larva more carefully, we shall
find that there are a great many differences between it and
the segmented worms ; and that even the youngest larva is
indeed a typical insect. The tracheae, so characteristic of the
group of insects, are present, the structure of the digestive
tract with its Malpighian tubes, the form of the heart, the
structure of the head, as well as the blastema of the repro-
ductive organs, show in the youngest larva the type of the
insects. In other words the body of the caterpillar is formed
on exactly the same fundamental plan as that of the butterfly.
1 The long argument of Fleischmann in regard to the origin of the fresh-
water snails, as illustrated by the planorbis series, and also the origin of the
nautiloid group, lias been recently dealt with fully by Plate, and, therefore, need
not be considered here.
The Theory of Evolution 57
In regard to the larval forms of other groups we find the
same relations, as, for example, in the amphibians. The young
of salamanders, toads, and frogs leave the egg not in the
completed form, but as small tadpoles adapted to life in the
water. A certain resemblance to fish cannot be denied.
They possess a broad tail, gills (rich in blood vessels) on
each side of the neck, and limbs are absent for a long time.
These are characters similar to those of fish, but a more care-
ful anatomical examination destroys the apparent resemblance.
The superficial resemblances are due to adaptation to the
same external conditions.
Fleischmann ridicules the idea that the young chick
resembles at any stage an adult, ancestral animal ; the pres-
ence of an open digestive tract shows how absurd such an
idea is. The obvious contradiction is explained away by
embryologists, by supposing that the ancestral adult stages
have been crowded together in order to shorten the period of
development ; and that, in addition, larval characters and pro-
visional organs have appeared in the embryo itself, which
confuse and crowd out the ancestral stages.
In regard to the presence of gill-slits in the embryo of
the higher vertebrates, in the chick, and in man, for example,
Fleischmann says : " I cannot see how it can be shown by
exact proof that the gill-slits of the embryos of the higher
vertebrates that remain small and finally disappear could
once have had the power of growing into functional slits."
With this trite comment the subject is dismissed.
On the whole, Fleischmann's attack cannot be regarded as
having seriously weakened the theory of evolution. He has
done, nevertheless, good service in recalling the fact that,
however probable the theory may appear, the evidence is
indirect and exact proof is still wanting. Moreover, as I
shall attempt to point out in the next chapter, we are far
from having arrived at a satisfactory idea of how the process
has really taken place.
CHAPTER III
THE THEORY OF EVOLUTION {Continued)
The Evidence from Embryology
the recapitulation theory
At the close of the eighteenth, and more definitely at the
beginning of the nineteenth, century a number of naturalists
called attention to the remarkable resemblance between the
embryos of higher animals and the adult forms of lower
animals. This idea was destined to play an important role
as one of the most convincing proofs of the theory of evolu-
tion, and it is interesting to examine, in the first place, the
evidence that suggested to these earlier writers the theory
that the embryos of the higher forms pass through the adult
stages of the lower animals.
The first definite reference 1 to the recapitulation view that
I have been able to find is that of Kielmeyer in 1793, which
was inspired, he says, by the resemblance of the tadpole of
the frog to an adult fish. 2 This suggested that the embryo
of higher forms corresponds to the adult stages of lower
ones. He adds that man and birds are in their first stages
plantlike.
Oken in 1805 gave the following fantastic account of this
relation: "Each animal 'metamorphoses itself through all
animal forms. The frog appears first under the form of a
mollusk in order to pass from this stage to a higher one.
1 The earlier references of a few embryologists are too vague to have any bear-
ing on the subject.
2 Autenrieth in 1797 makes the briefest possible reference to some such princi-
ple in speaking uf the way in which the nose of the embryo closes.
58
The Theory of Evolution 59
The tadpole stage is a true snail ; it has gills which hang
free at the sides of the body as is the case in Unio pictorum.
It has even a byssus, as in Mytilus, in order to cling to the
grass. The tail is nothing else than the foot of the snail.
The metamorphosis of an insect is a repetition of the whole
class, scolopendra, oniscus, julus, spider, crab."
Walther, in 1808, said: "The human foetus passes through
its metamorphosis in the cavity of the uterus in such a way
that it repeats all classes of animals, but, remaining perma-
nently in none, develops more and more into the innate
human form. First the embryo has the form of a worm.
It reaches the insect stage just before its metamorphosis.
The origin of the liver, the appearance of the different secre-
tions, etc., show clearly an advance from the class of the
worm into that of the mollusk."
Meckel first in 1808, again in 181 1, and more fully in 1821
made much more definite comparisons between the embryos
of higher forms and the adult stages of lower groups. He
held that the embryo of higher forms, before reaching its com-
plete development, passes through many stages that corre-
spond to those at which the lower animals appear to be
checked through their whole life. In fact the embryos of
higher animals, the mammals, and especially man, correspond
in the form of their organs, in their number, position, and
proportionate size to those of the animals standing below
them. The skin is at first, and for a considerable period of
embryonic life, soft, smooth, hairless, as in the zoophytes,
medusae, many worms, mollusks, fishes, and even in the
lower amphibians. Then comes a period in which it becomes
thicker and hairy, when it corresponds to the skin of the
higher animals. It should be especially noted here, that the
foetus of the negro is more hairy than that of the European.
The muscular system of the embryo, owing to its lack of
union in the ventral wall, corresponds to the muscles of the
shelled, headless mollusks, whose mantle is open in the same
60 Evolution and Adaptation
region. Meckel compares the bones of the higher verte-
brates with the simpler bones of the lower forms, and even
with the cartilages of the cephalopod. He points out that in
the early human embryo the nerve cord extends the whole
length of the spinal canal. He compares the simple heart of
the embryo with that of worms, and a later stage, when two
chambers are present, with that of the gasteropod mollusk.
The circulation of the blood in the placenta recalls, he savs,
the circulation in the skin of the lower animals. The lobu-
lated form of the kidney in the human embryo is compared
with the adult condition in the fishes and amphibians. The
internal position of the reproductive organs in the higher
mammals recalls the permanent position of these organs in
the lower animals. The posterior end of the body of the
human embryo extends backwards as a tail which later dis-
appears.
Some of these comparisons of Meckel sound very absurd
to us nowadays, especially his comparison between the em-
bryos of the higher vertebrates, and the adults of worms,
crustaceans, spiders, snails, bivalve mollusks, cephalopods, etc.
On the other hand, many of these comparisons are the same
as those that are to be found in modern text-books on embry-
ology ; and we may do well to ask ourselves whether these
may not sound equally absurd a hundred years hence. Why
do some of Meckel's comparisons seem so naive, while others
have a distinctly modern flavor? In a word, can we justify
the present belief of some embryologists that the embryos
of higher forms repeat the adult stages of lower members
of the same group ? It is important to observe that up to
this time the comparison had always been made between
the embryo of the higher form and the adult forms of
existing lower animals. The theory of evolution had, so
far, had no influence on the interpretation that was later
given to this resemblance.
Von Baer opposed the theory of recapitulation that had
The Theory of Evolution 61
become current when he wrote in 1828. According to Von
Baer, the more nearly related two animals are, or rather the
more nearly similar two forms are (since Von Baer did not
accept the idea of evolution), the more nearly alike is their
development, and so much longer in their development do
they follow in the same path. For example two similar
species of pigeons will follow the same method of develop-
ment up to almost the last stage of their formation. The
embryos of these two forms will be practically identical
until each produces the special characters of its own
species. On the other hand two animals belonging to
different families of the same phylum will have only the
earlier stages in common. Thus, a bird and a mammal
will have the first stages similar, or identical, and then
diverge, the mammal adding the higher characters of its
group. The resemblance is between corresponding em-
bryonic stages and not between the embryo of the mammal
and the adult form of a lower group.
Von Baer was also careful to compare embryos of the
same phylum with each other, and states explicitly that
there are no grounds for comparison between embryos of
different groups. 1
We shall return again to Von Baer's interpretation and
then discuss its value from our present point of view.
Despite the different interpretation that Von Baer gave
to this doctrine of resemblance the older view of recapitula-
tion continued to dominate the thoughts of embryologists
throughout the whole of the nineteenth century.
Louis Agassiz, in the Lowell Lectures of 1848, proposed
for the first time the theory that the embryo of higher
forms resembled not so much lower adult animals living
at the present time, as those that lived in past times.
Since Agassiz himself did not accept the theory of evolu-
1 In one place Von Baer raises the question whether the egg may not be a
form common to all the phyla.
62 Evolution and Adaptation
tion, the interpretation that he gave to the recapitulation
theory did not have the importance that it was destined
to have when the animals that lived in the past came to
be looked upon as the ancestors of existing animals. 1 But
with the acceptation of the theory of evolution, which was
largely the outcome of the publication of Darwin's " Origin
of Species" in 1859, this new interpretation immediately
blossomed forth. In fact, it became almost a part of the
new theory to believe that the embryo of higher forms
recapitulated the series of ancestral adult forms through
which the species had passed. The one addition of any
importance to the theory that was added by the Darwinian
school was that the history of the past, as exemplified by
the embryonic development, is often falsified.
Let us return once more to the facts and see which of
them are regarded at present as demanding an explanation.
These facts are not very numerous and yet sufficiently ap-
parent to attract attention at once when known.
The most interesting case, and the one that has most often
attracted attention, is the occurrence of gill-clefts in the
embryos of reptiles, birds, and mammals. These appear
on each side of the neck in the very early embryo. Each
is formed by a vertical pouch, that grows out from the wall
of the pharynx until it meets the skin, and, fusing with the
latter, the walls of the pouch separate, and a cleft is formed.
This vertical cleft, placing the cavity of the pharynx in com-
munication with the outside, is the gill-slit. Similar openings
in adult fishes put the pharynx in communication with the
exterior, so that water taken through the mouth passes out
at the sides of the neck between the gill filaments that border
the gill-slits. In this way the blood is aerated. The number
of gill-slits that are found in the embryos of different groups
1 Carl Vogt in 1S42 suggested that fossil species, in their historical succession,
pass through changes similar to those which the embryos of living forms
undergo.
The Theory of Evolution 63
of higher vertebrates, and the number that open to the ex-
terior are variable ; but the number of gill-openings that are
present in the adults of lower vertebrates is also variable.
No one who has studied the method of development of the
gill-slits in the lower and higher vertebrates will doubt for a
moment that some kind of relation must subsist between
these structures.
In the lowest adult form of the vertebrates, amphioxus,
the gill-system is used largely as a sieve for procuring food,
partly also, perhaps, for respiration. In the sharks, bony
fishes, and lower amphibians, water is taken in through the
mouth, and passes through the gill-slits to the exterior.
As it goes through the slits it passes over the gills, that
stand like fringes on the sides of the slits. The blood that
passes in large quantities through the gills is aerated in
this way. In the embryos of the higher vertebrates the
gill-slits may appear even before the mouth has opened,
but in no case is there a passage of water through the
gill-slits, nor is the blood aerated in the gill-region, although
it passes through this part on its way from the heart to
the dorsal side of the digestive tract. It is quite certain
that the gill-system of the embryo performs no respiratory
function. 1
In the higher amphibians, the frogs for example, we find
an interesting transition. The young embryo, when it
emerges from the egg-membranes, bears three pairs of
external gills that project from the gill-arches into the sur-
rounding water. Later these are absorbed, and a new
system of internal gills, like those of fishes, develops on
the gill-arches. These are used throughout the tadpole
stage for respiratory purposes. When the tadpole is about
to leave the water to become a frog, the internal gills are
1 This statement is not intended to prejudice the question as to whether the
presence of the gill-slits and arches may be essential to the formation of other
organs.
64 Evolution and Adaptation
also absorbed and the gill-clefts close. Lungs then develop
which become the permanent organs of respiration.
There are two points to be noticed in this connection.
First, the external gills, which are the first to develop, do not
seem to correspond to any permanent adult stage of a lower
group. Second, the transition from the tadpole to the frog
can only be used by way of analogy of what is supposed
to have taken place ancestrally in the reptiles, birds, and
mammals, since no one will maintain that the frogs represent
a group transitional between the amphibians and the higher
forms. However, since the salamanders also have gills and
gill-slits in the young stages, and lose them when they leave
the water to become adult land forms, this group will better
serve to illustrate how the gill-system has been lost in the
higher forms. Not that in this case either, need we suppose
that the forms living to-day represent ancestral, transitional
forms, but only that they indicate how such a remarkable
change from a gill-breathing form, living in the water,
might become transformed into a lung-breathing land form.
Such a change is supposed to have taken place when the
ancestors of the reptiles and the mammals left the water
to take up their abode on the land.
The point to which I wish to draw especial attention in
this connection is that in the higher forms the gill-slits ap-
pear at a very early stage ; in fact, as early in the mammal
as in the salamander or the fish, so that if we suppose
their appearance in the mammal is a repetition of the
adult amphibian stage, then, since this stage appears as early
in the development of the mammal as in the amphibians
themselves, the conclusion is somewhat paradoxical.
The history of the notochord in the vertebrate series gives
an interesting parallel. In amphioxus it is a tough and firm
cord that extends from end to end of the body. On each
side of it lie the plates of muscles. It appears at a very
early stage of development as a fold of the upper wall of
The Theory of Evolution 65
the digestive tract. In the cartilaginous fishes the notochord
also appears at a very early stage, and also from the dorsal
wall of the digestive tract. In later embryonic stages it
becomes surrounded by a cartilaginous sheath, or tube,
which then segments into blocks, the vertebrae. The noto-
chord becomes partially obliterated as the centra of the
vertebras are formed, but traces of it are present even in
adult stages. In the lower amphibians the notochord arises
also at an early stage over and perhaps, in part, from the
dorsal wall of the digestive tract. It is later almost entirely
obliterated by the development of the vertebrae. These
vertebrae first appear as a membraneous tube which breaks
up into cartilaginous blocks, and these are the structures
around and in which the bone develops to form the per-
manent vertebrae.
In higher forms, reptiles, birds, and mammals, the noto-
chord also appears at the very beginning of the develop-
ment, but it is not certain that we can call the material out
of which it forms the dorsal wall of the archenteron (the
amphibians giving, perhaps, intermediate stages). It be-
comes surrounded by continuous tissue which breaks up into
blocks, and these become the bases of the vertebrae. The
notochord becomes so nearly obliterated in later stages that
only the barest traces of it are left either in the spaces
between, or in, the vertebrae.
In this series we see the higher forms passing through
stages similar at first to those through which the lower forms
pass ; and it is especially worthy of note that the embryo
mammal begins to produce its notochord at the very begin-
ning of its development, at a stage, in fact, so far as compari-
son is possible, as early as that at which the notochord of
amphioxus develops.
The development of the skull gives a somewhat similar
case. The skulls of sharks and skates are entirely cartilagi-
nous and imperfectly enclose the brain. The ganoids
66 Evolution and Adaptation
have added to the cartilaginous skull certain plates in the
dermal layer of the skin. In the higher forms we find the
skull composed of two sets of bones, one set developing from
the cartilage of the first-formed cranium, and the other having
a more superficial origin ; the latter are called the membrane
bones, and are supposed to correspond to the dermal plates
of the ganoids.
In the development of the kidneys, or nephridia, we find,
perhaps, another parallel, although, owing to recent dis-
coveries, we must be very cautious in our interpretation. As
yet, nothing corresponding to the nephridia of amphioxus has
been discovered in the other vertebrates. Our comparison
must begin, therefore, higher up in the series. In the sharks
and bony fishes the nephridia lie at the anterior end of the
body-cavity. In the amphibia there is present in the
young tadpole a pair of nephridial organs, the head-kidneys,
also in the anterior end of the body-cavity. Later these are
replaced by another organ, the permanent mid-kidney, that
develops behind the head-kidney. In reptiles, birds, and
mammals a third nephridial organ, the hind-kidney, develops
later than and posterior to the mid-kidney, and becomes the
permanent organ of excretion. Thus in the development of
the nephridial system in the higher forms we find the same
sequence, more or less, that is found in the series of adult
forms mentioned above. The anterior end of the kidney
develops first, then the middle part, and then the most poste-
rior. The anterior part disappears in the amphibians, the
anterior and the middle parts in the birds and mammals, so
that in the latter groups the permanent kidney is the hind-
kidney alone.
The formation of the heart is supposed to offer certain
parallels. Amphioxus is without a definite heart, but there is
a ventral blood vessel beneath the pharynx, which sends blood
to the gill-system. This blood vessel corresponds in position
to the heart of other vertebrates. In sharks we find a thick-
The Theory of Evolution 67
walled muscular tube below the pharynx; the blood enters at
its posterior end, flows forward and out at the anterior end
into a blood vessel that sends smaller vessels up through the
gill-arches to the dorsal side.
In the amphibia the heart is a tube, so twisted on itself that
the original posterior end is carried forward to the anterior
end, and this part, the auricle, is divided lengthwise by a
partition into a right and a left side. In the reptiles the
ventricle is also partially separated into two chambers, com-
pletely so in the crocodiles. In birds and mammals the
auricular and ventricular septa are complete in the adult, and
the ventral aorta that carries the blood forward from the
heart is completely divided into two vessels, one of which now
carries blood to the lungs. When we examine the develop-
ment of the heart of a mammal, or of a bird, we find some-
thing like a parallel series of stages, apparently resembling
conditions found in the different groups just described. The
heart is, at first, a straight tube, it then bends on itself, and a
constriction separates the auricular part from the ventricular,
and another the ventricular from the ventral aorta. Vertical
longitudinal partitions then arise, one of which separates the
auricle into two parts, and another the ventricle into two
parts, and a third divides the primitive aorta into two parts.
In the early stages all the blood passes from the single
ventral aorta through the gill-arches to the dorsal side, and it
is only after the appearance of the lung-system that the gill-
system is largely obliterated.
We find here, then, a sort of parallel, provided we do not
inquire too particularly into details. This comparison may be
justified, at least so far that the circulation is at first through
the arches and is later partially replaced by the double cir-
culation, the systemic and the pulmonary.
A few other cases may also be added. The proverbial
absence of teeth in birds applies only to the adult condition,
for, as first shown by Geoff roy Saint-Hilaire, four thickenings,
68 Evolution and Adaptation
or ridges, develop in the mouth of the embryo ; two in the
upper, two in the lower, jaw. These ridges appear to corre-
spond to those of reptiles and mammals, from which the teeth
develop. It may be said, therefore, that the rudiments of
teeth appear in the embryo of the bird. This might be inter-
preted to mean that the embryo repeats the ancestral reptilian
stage, or, perhaps, the ancestral avian stage that had teeth in
the beak ; but since only the beginnings of teeth appear, and
not the fully formed structures, this interpretation would
clearly overshoot the mark.
The embryo of the baleen whale has teeth that do not
break through the gums and are later absorbed. Since the
ancestors of this whale probably had teeth, as have other
whales at the present time, the appearance of teeth in the
embryo has been interpreted as a repetition of the original
condition. Some of the ant-eaters are also toothless, but
teeth appear in the embryo and are lost later. In the rumi-
nants that lack teeth in the front part of the upper jaw, e.g.
the cow and the sheep, teeth develop in the embryo which
are subsequently lost.
One interpretation of these facts is that the ancestral
adult condition is repeated by the embryo, but as I have
pointed out above in the cases of the teeth in whales, since
the teeth do not reach the adult form, and do not even break
through the gums in some forms, it is obviously stretching
a point to claim that an adult condition is repeated. More-
over, in the case of the birds only the dental ridges appear,
and it is manifestly absurd to claim in this case that the
ancestral adult condition of the reptiles is repeated.
That a supposed ancestral stage may be entirely lost in
the embryo of higher forms is beautifully shown in the devel-
opment of some of the snakes. The snakes are probably
derived from lizardlike ancestors, which had four legs, yet
in the development the rudiments of legs do not appear, and
this is the more surprising since a few snakes have small
The Theory of Evolution 69
rudimentary legs. In these, of course, the rudiments of legs
must appear in the embryo, but in the legless forms even the
beginnings of the legs have been lost, or at any rate very
nearly so.
Outside the group of vertebrates there are also many
cases that have been interpreted as embryonic repetitions
of ancestral stages, but a brief examination will suffice to
show that many of these cases are doubtful, and others little
less than fanciful. A few illustrations will serve our pur-
pose. The most interesting case is that given by the history
of the nauplius theory.
The free-living larva of the lower crustaceans — water-
flees, barnacles, copepods, ostracods — emerges from the egg
as a small, flattened oval form with three pairs of append-
ages. This larva, known as the nauplius, occurs also in
some of the higher crustaceans, not often, it is true, as a free
form, as in penaeus, but as an embryonic stage. The occur-
rence of this six-legged form throughout the group was
interpreted by the propounders of the nauplius theory as
evidence sufficient to establish the view that it represented
the ancestor of the whole group of Crustacea, which ancestor
is, therefore, repeated as an embryonic form. This hypothe-
sis was accepted by a large number of eminent embryologists.
The history of the collapse of the theory is instructive.
It had also been found in one of the groups of higher
crustaceans, the decapods, containing the crayfish, lobster,
and crabs, that another characteristic larval form was
repeated in many cases. This larva is known as the zoea.
It has a body made up of a fused head and thorax carrying
seven pairs of appendages and of a segmented abdomen of
six segments. The same kind of evidence that justified
the formulation of the nauplius theory would lead us to infer
that the zoea is the ancestor of the decapods. The later
development of the zoea shows, however, that it cannot
be such an ancestral form, for, in order to reach the.
jo Evolution and Adaptation
full number of segments characteristic of the decapods,
new segments are intercalated between the cephalothorax
and abdomen. In fact, in many zoeas this intercalated
region is already in existence in a rudimentary condition,
and small appendages may even be present. A study of the
comparative anatomy of the crustaceans leaves no grounds
for supposing that the decapods with their twenty-one seg-
ments have been evolved from a thirteen-segmented form
like the zoea by the intercalation of eight segments in the
middle of the body. It follows, if this be admitted, and
it is generally admitted now, that the zoea does not repre-
sent an original ancestral form at all, but a highly modified
new form, as new, perhaps, as the group of decapods itself.
We are forced to conclude, then, that the presence of a larval
form throughout an entire group cannot be accepted as evi-
dence that it represents an ancestral stage. We can account
for the presence of the zoea, however, by making a single
supposition, namely, that the ancestor from which the group
of decapod has evolved had a larva like the zoea, and that
this larval form has been handed down to all of the de-
scendants.
The fate of the zoea theory cast a shadow over the
nauplius theory, since the two rested on the same sort of
evidence. The outcome was, in fact, that the nauplius
theory was also abandoned, and this was seen to be the
more necessary, since a study of the internal anatomy of the
lowest group of crustaceans, the phyllopods, showed that they
have probably come directly from many segmented, annelid-
ian ancestors. The presence of the nauplius is now gener-
ally accounted for by supposing that it was a larval form
of the ancestor from which the group of crustaceans arose.
The most extreme, and in many ways the most uncritical,
application of the recapitulation theory was that made by
Haeckel, more especially his attempt to reduce all the higher
animals to an ancestral double-walled sac with an opening
The Theory of Evolution 71
at one end, — the gastraea. He dignified the recapitulation
theory with an appellation of his own, " The Biogenetic
Law." Haeckel's fanciful and extreme application of the
older recapitulation theory has probably done more to bring
the theory into disrepute amongst embryologists than the
criticisms of the opponents of the theory.
In one of the recognized masterpieces of embryological
literature, His's " Unsere Korperform," we find the strongest
protest that has yet been made against the Haeckelian
pretension that the phylogenetic history is the "cause" of
the ontogenetic series. His writes : " In the entire series of
forms which a developing organism runs through, each form
is the necessary antecedent step of the following. If the
embryo is to reach the complicated end-forms, it must pass,
step by step, through the simpler ones. Each step of the
series is the physiological consequence of the preceding
stage and the necessary condition for the following. Jumps,
or short cuts, of the developmental process, are unknown in
the physiological process of development. If embryonic
forms are the inevitable precedents of the mature forms,
because the more complicated forms must pass through the
simpler ones, we can understand the fact that paleonto-
logical forms are so often like the embryonic forms of to-day.
The paleontological forms are embryonal, because they have
remained at the lower stage of development, and the present
embryos must pass also through lower stages in order to
reach the higher. But it is by no means necessary for the
later, higher forms to pass through embryonal forms because
their ancestors have once existed in this condition. To take
a special case, suppose in the course of generations a species
has increased its length of life gradually from one, two, three
years to eighty years. The last animal would have had
ancestors that lived for one year, two years, three years, etc.,
up to eighty years. But who would claim that because the
final eighty-year species must pass necessarily through one,
y 2 Evolution and Adaptation
two, three years, etc., that it does so because its ancestors
lived one year, two years, three years, etc. ? The descent
theory is correct so far as it maintains that older, simpler
forms have been the forefathers of later complicated forms.
In this case the resemblance of the older, simpler forms to
the embryos of later forms is explained without assuming
any law of inheritance whatsoever. The same resemblance
between the older and simpler adult forms, and the present
embryonic forms would even remain intelligible were there no
relation at all between them."
Interesting and important as is this idea of His, it will not,
I think, be considered by most embryologists as giving an
adequate explanation of many facts that we now possess. It
expresses, no doubt, a part of the truth but not the whole
truth.
We come now to a consideration of certain recently
ascertained facts that put, as I shall try to show, the whole
question of embryonic repetition in a new light.
A minute and accurate study of the early stages of
division or cleavage of the egg of annelids has shown a
remarkable agreement throughout the group. The work of
E. B. Wilson on nereis, and on a number of other forms, as
well as the subsequent work of Mead, Child, and Treadwell
on other annelids, has shown resemblances in a large number
of details, involving some very complicated processes. 1
Not only is the same method of cleavage found in most
annelids, but the same identical form of division is also pres-
ent in many of the mollusks, as shown especially by the work
of Conklin, Lillie, and Holmes. This resemblance has been
discussed at some length by those who have worked out these
results in the two groups. The general conclusion reached
by them is that the only possible interpretation of the
1 On the other hand it should not pass unnoticed that Eisigh as shown in one
form (in which, however, the eggs are under special conditions being closely
packed together) that the usual type of cleavage is altered.
The Theory of Evolution 73
phenomenon is that some sort of genetic connection must
exist between the different forms ; and while not explicitly
stated, yet there is not much doubt that some at least of
these authors have had in mind the view that the annelids
and mollusks are descended from common ancestors whose
eggs segmented as do those of most of the mollusks and
annelids of the present day. This conclusion is, I believe, of
more far-reaching importance than has been supposed, and
may furnish the key that will unlock the whole question of
the resemblance of embryos to supposed ancestral forms.
It is a most fortunate circumstance that in the case of this
cell lineage the facts are of such a kind as to preclude the
possibility that the stages in common could ever have been
ancestral adult stages. If this be granted then only two
interpretations are possible: the results are due either. to a
coincidence, or to a common embryonic form that is repeated
in the embryo of many of the descendants. That the simi-
larity is not due to a coincidence is made probable from the
number and the complexities of the cleavage stages.
I believe that we can extend this same interpretation to
all other cases of embryonic resemblance. It will explain
the occurrence of gill-slits in the embryo of the bird, and the
presence of a notochord in the higher forms in exactly the
same way as the cleavage stages are explained. But how,
it may be asked, can we explain the apparent resemblance
between the embryo of the higher form and the adult of
lower groups. The answer is that this resemblance is decep-
tive, and in so far as there is a resemblance it depends
on the resemblance of the adult of the lower form to its own
embryonic stages with which we can really make a compari-
son. The gill-slits of the embryo of the chick are to be com-
pared, not with those of the adult fish, but with those of the
embryo of the fish. It is a significant fact, in this connection,
that the gill-slits appear as early in the embryo of the fish as
they do in the bird ! The notochord of the embryo bird is
74 Evolution and Adaptation
comparable with that of the embryo of amphioxus, and not
with the persistent notochord in the adult amphioxus. Here
also it is of the first importance to find that the notochord
appears both in the embryo bird and in amphioxus at the very
beginning of the development. The embryo bird is not fish-
like except in so far as there are certain organs in the embryo
fish that are retained in the adult form. The embryo bird
bears the same relation to the embryo fish that the early
segmentation stages of the mollusk bear to the early seg-
mentation stages of the annelid. There are certain obvious
resemblances between this view and that of Von Baer, but
there are also some fundamental differences between the two
conceptions.
Von Baer thought that within each group the embryonic
development is the same up to a certain point. He supposed
that the characters of the group are the first to appear, then
those of the order, class, family, genus, and, finally, of the
species. He supposed that two similar species would follow
the same method of development until the very last stage was
reached, when each would then add the final touches that
give the individual its specific character. We may call this
the theory of embryonic parallelism. Here there is an impor-
tant difference between my view and that of Von Baer, for I
should not expect to find the two embryos of any two species
identical at any stage of their development, but at most there
might exist a close resemblance between them.
Von Baer's statement appears to be erroneous from a mod-
ern point of view in the following respects. We know that in
certain large groups some forms develop in a very different way
from that followed by other members of the group, as shown
by the cephalopods, for instance, in the group of mollusks.
Again, it is entirely arbitrary to assume that the group-
characters are the first to appear, and then successively
those of the order, family, genus, species. Finally, as has
been said above, we do not find the early embryos of a
The Theory of Evolution 75
group identical; for with a sufficient knowledge of the devel-
opment it is always possible to distinguish between the em-
bryos of different species, as well as between the adults, only
it is more difficult to do so, because the embryonic forms are
simpler. The most fundamental difference between the view
of Von Baer and modern views is due to our acceptation of
the theory of evolution which seems to make it possible to
get a deeper insight into the meaning of the repetition, that
carries us far ahead of Von Baer's position. For with the
acceptance of this doctrine we have an interpretation of how
it is possible for the embryonic stages of most members of a
group to have the same form, although they are not identi-
cal. There has been a continuous, although divergent, stream
of living material, carrying along with it the substance out of
which the similar embryonic forms are made. As the stream
of embryonic material divided into different paths it has also
changed many of the details, sometimes even all ; but never-
theless it has often retained the same general method of
development that is associated with its particular composition.
We find the likeness, in the sense of similarity of plan, ac-
counted for by the inheritance of the same sort of substance ;
the differences in the development must be accounted for in
some other way.
Among modern writers Hurst alone has advanced a view
that is similar in several respects to that which I have here
defended. It may be well to give his statement, since it
brings - out certain points of resemblance with, as well as cer-
tain differences from, my own view. 1 He says : " Direct
observation has shown that, when an animal species varies
{i.e. becomes unlike what it was before) in adult structure,
those stages in the development which are nearest the adult
undergo a similar, but usually smaller, change. This is shown
in domestic species by the observations of Darwin, and the
1 Hurst, C. H., "Biological Theories, III," "The Recapitulation Theory,"
Natural Science, Vol. ii., 1893.
76 Evolution and Adaptation
result is in exact harmony with the well-known law of Von
Baer, which refers to natural species, both nearly related and
widely dissimilar. Von Baer's observations as well as Dar-
win's, and as well as those of every student who has ever
compared the embryos of two vertebrate species, may be
summarized as follows : —
" Animals which, though related, are very similar in the
adult state, resemble each other more closely in early stages
of development, often, indeed, so closely as to be indistin-
guishable in those early stages. As development proceeds
in such species, the differences between the two embryos com-
pared become more and more pronounced." On this point,
which is an essential one, I cannot agree with Hurst ; for I
do not think that the facts show that the early stages of two
related forms are necessarily more and more alike the farther
back we go. The resemblance that is sometimes so striking in
the earlier stages is due to the fewer points there are for
comparison, and to the less development of the parts then
present. Hurst continues : " If similar comparisons could be
instituted between the ancestral species and its much modi-
fied descendants, there is no reason for doubting that a similar
result would be reached. This, indeed, has been done in the
case of some breeds of pigeons, which we have excellent
reasons for believing to be descended from Columba livia.
True, C. livia is not a very remote ancestor, but I do not
think that will vitiate the argument. Let me quote Darwin
verbatim : ' As we have conclusive evidence that the breeds
of the pigeon are descended from a single wild species, I
have compared the young within twelve hours after being
hatched ; I have carefully 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 mature differ in so extraordinary a manner in the
The Theory of Evolution Jj
length and form of the 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 the mouth — could
hardly be detected in the young. But there was one remark-
able 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 propor-
tions as in the adult state.' "
Hurst concludes that : " The more the adult structure
comes to be unlike the adult structure of the ancestors, the
more do the late stages of development undergo a modifica-
tion of the same kind. This is not mere dogma, but it is a
simple paraphrase of Von Baer's law. It is proved true not
only by the observations of Von Baer and of Darwin, already
referred to, but by the direct observation of every one who
takes the trouble to compare the embryos of any two verte-
brates, provided only he will be content to see what actually
lies before him and not the phantasms which the recapitu-
lation theory may have printed on his imagination."
The growth of the antlers of stags is cited by Hurst in
order to illustrate that what has been interpreted as a re-
capitulation may have a different interpretation. " Each
stag develops a new pair of antlers in each successive year,
and each pair of antlers is larger than the pair produced in
the previous year. This yearly increase in the size of the
antlers has been put forward as an example of an ontogenetic
record of past evolution. I, however, deny that it is such a
record."
" The series of ancestors may have possessed larger antlers
in each generation than in the generation before it. It is not
yS Evolution and Adaptation
an occasional accidental parallelism between the ontogeny
and the phylogeny which I deny, but the causal relation
between the two. Had the ancestors had larger antlers than
the existing ones, there is no justification for the assumption
that existing stags would acquire antlers of which each pair,
in later years, would be smaller than those of the previous
year."
Hurst concludes : " There are many breeds of hornless
sheep, but they do not bear large horns in early years and
then shed them. If a rudiment ever appears in the embryo
of such sheep, its growth is very early arrested." The case
of the appendix in man might have been cited here as
a case in point. It is supposed to have been larger in the
ancestors of man, but we do not find it appearing full size in
the embryo and later becoming rudimentary. The preceding
statements will show that, while Hurst's view is similar in some
respects to my own, yet it differs in one fundamental respect
from it, and in this regard he approaches more nearly to the
theory of Von Baer.
Hertwig has recently raised some new points of issue in
regard to the recapitulation theory, and since he may appear
to have penetrated farther than most other embryologists of
the present time, it will be necessary to examine his view-
somewhat carefully. He speaks of the germ-cell (egg, or
spermatozoon) as a species-cell, because it contains, in its
finer organization, the essential features of the species to
which it belongs. There are as many of these kinds of cells
as there are different kinds of animals and plants. Since the
bodies of the higher animals have developed from these
species-cells, so the latter must have passed in their phylogeny
through a corresponding development from a simple to a
more and more complex cell-structure. " Our doctrine is,
that the species-cell, even as the adult, many-celled representa-
tive of the species, has passed through a progressive, and,
indeed, in general a corresponding development in the course
The Theory of Evolution 79
of phylogeny. This view appears to stand in contradiction to
the biogenetic law. According to the formula that Haeckel
has maintained, the germ development is an epitome of the
genealogy ; or the ontogeny is a recapitulation of the
phylogeny ; or, more fully, the series of forms through which
the individual organism passes during its development from
the egg-cell to the finished condition is a short, compressed
repetition of the longer series of forms which the forefathers
of the same organism, or the stem-form of the species, has
passed through, from the earliest appearance of organisms to
the present time." " Haeckel admits that the parallel may
be obliterated, since much may be absent in the ontogeny
that formerly existed in the phylogeny. If the ontogeny were
complete, we could trace the whole ancestry." Hertwig states
further, that " The theory of biogenesis 1 makes it necessary
to change Haeckel's expression of the biogenetic law, so that
a contradiction contained in it may be removed. We must
drop the expression ' repetition of the form of extinct fore-
fathers,' and put in its place the repetition of forms which
are necessary for organic development, and lead from the
simple to the complex. This conception may be illustrated
by the egg-cell."
Since each organism begins its life as an egg we must not
suppose that the primitive conditions of the time, when only
single-celled amcebas existed on our planet, are repeated.
The egg-cell of a living mammal is not, according to Hert-
wig's hypothesis, an indifferent structure without much spe-
cialization like an amoeba, but is an extraordinarily complex
end-product of a long historical process, which the organ-
ized substance has passed through. If the egg of a mam-
mal is different from that of a reptile, or of an amphibian,
1 This term, by which Hertwig designates a particular view of his own, has
been already preoccupied in a much wider sense by Huxley to mean that all life
comes from preexisting life. Hertwig means by the theory of biogenesis that as
the egg develops there is a constant interchange between itself and its sur-
roundings.
8o Evolution and Adaptation
because in its organization it contains the basis of a mammal,
just so much more must it be different from the hypothetical
one-celled amoeba, which has no other characteristics than
those that go to make up an amoeba. Expressed more gen-
erally, the developmental process in the many-celled organ-
isms begins, not where it began in primitive times, but as the
representation of the highest point which the organization
has at present reached. The development commences with
the egg, because it is the elemental and fundamental form in
which organic life is represented in connection with the
reproductive process, and also because it contains in itself the
properties of the species in its primordia.
" The egg-cell of the present time, and its one-celled prede-
cessor in the phylogenetic history, the amoeba, are only
comparable in so far as they fall under the common definition
of the cell, but beyond this they are extraordinarily different
from each other."
" The phyletic series must be divided into two different kinds
of processes : — First. The evolution of the species-cell, which
is a steady advance from a simple to a complex organization.
Second. The periodically repeated development of the many-
celled individual out of the single cell, representative of the
species (or the individual ontogeny), which in general follows
the same rules as the preceding ontogeny, but is each time
somewhat modified according to the amount to which the
species-cell has itself been changed in the phytogeny.
Similar restricting and explanatory additions to the biogenetic
law, like those stated here for the one-celled stage, must be
made in other directions. Undoubtedly there exists in a
certain sense a parallel between the phylogenetic, and the
ontogenetic, development.
" On the basis of the general developmental hypothesis on
which we stand, all forms which in the chain of ancestors
were end-products of the individual development are now
passed through by their descendants as embryonic stages, and
The Theory of Evolution Si
so in a certain degree are recapitulated. We also admit that
the embryonic forms of higher animals have many points of
comparison with the mature forms of related groups standing
lower in the system.
" Nevertheless, a deeper insight into the conditions re-
lating to these resemblances shows that there are very im-
portant differences that should not be overlooked. Three
points need to be mentioned : i. The cell-material which in
the ancestral chain gives the basis for each ontogenetic process
is each time a different material as far as concerns its finer
organization and primordia. Indeed, the differences become
greater the farther apart the links of the original chain
become. This thought may be formulated in another way :
The same ontogenetic stages that repeat themselves periodi-
cally in the course of the phylogeny always contain at bottom
a somewhat different cell-material. From this the second rule
follows as a consequence. 2. Between the mature end-form
of an ancestor and the corresponding embryonic form of a
widely remote descendant (let us say between the phylo-
genetic gastraea and the embryonic gastrula stage of a living
mammal, according to the terminology of Haeckel) there
exists an important difference, namely, that the latter is sup-
plied with numerous primordia which are absent in the other,
and which force it to proceed to the realization of its develop-
mental process. The gastrula, therefore, as the bearer of
important latent forces, is an entirely different thing from the
gastraea, which has already reached the goal of its development.
3. In the third place, at each stage of the ontogeny outer and
inner factors are at work, in fact even more intensely than
in the fully formed organism. Each smallest change that acts
anew in this way at the beginning of the ontogeny can start
an impulse leading to more extensive changes in later stages.
Thus the presence of yolk and its method of distribution in
the egg alone suffice to bring about important changes in
the cleavage, and in the formation of the germ-layers, the
82 Evolution and Adaptation
blastula, and gastrula stages," etc. " Moreover, the embryo
may adapt itself to special conditions of embryonic life, and
produce organs of an ephemeral nature like the amnion,
chorion, and placenta."
" A comparison of ontogenetic with antecedent phylo-
genetic stages must always keep in view the fact that the
action of external and internal factors has brought about
considerable changes in the ontogenetic system, and, indeed,
in a generally advancing direction, so that in reality a later
condition can never correspond to a preceding one."
Hertwig sums up his conclusion in the statement that
ontogenetic stages give us, therefore, a greatly changed
picture of the phylogenetic series of adult ancestors. " The
two correspond not according to their actual contents but
only as to their form." Hertwig also repeats His's idea, that
the reason that certain kinds of form repeat themselves in
the. development of animals with a great constancy depends
principally on this, that they supply the necessary conditions
under which alone the following higher stage of the ontogeny
can be formed. The development, for instance, begins with
the division of the egg, because this is the only way that a
one-celled condition can give rise to a many-celled form.
Again, the organs can be formed only when groups of cells
have made a closer union with one another. Thus the gastrula
must begin with the antecedent blastula, etc. Definite forms
are, despite all modifying influences, held to firmly, because
by their presence the complicated end-stages can be reached
in the simplest and most suitable way.
Thus Hertwig adopts here a little from one doctrine and
there a little from another, and between his attempt to reinstate
the old biogenetic law of Hacckel, and to adopt a more modern
point of view, he brings together a rather curious collection of
statements which are not any too well coordinated. Take,
for example, his description of the relation between Haeckel's
gastraea and the embryonic gastrula stage. The latter he
The Theory of Evolution 83
maintains is a repetition of the other, but only in form, not in
actual contents. And in another connection we are told that
the cause of this repetition is that the gastrula is the simplest
way in which the later stages can be reached, and, therefore,
it has been retained. It seems to me that Hertwig has under-
taken an unnecessary and impossible task when he attempts
to adjust the old recapitulation theory to more modern
standards. His statement that the egg is entirely different
from its amoeba prototype is, of course, only the view generally
held by all embryologists. His mystical statement that the
embryonic form repeats the ancestral adult stage in its form,
bat not in its contents, will scarcely recommend itself as a
model of clear thinking. Can we be asked to believe for
instance that a young chick repeats the ancestral adult fish
form but not the contents of the fish ?
In conclusion, then, it seems to me that the idea that adult
ancestral stages have been pushed back into the embryo, and
that the embryo recapitulates in part these ancestral adult
stages is in principle false. The resemblance between the
embryos of higher forms and the adults of lower forms is
due, as I have tried to show, to the presence in the embryos
of the lower groups of certain organs that remain in the
adult forms of this group. It is only the embryonic stages of
the two groups that we are justified in comparing; and their
resemblances are explained on the assumption that there
has been an ancestral adult form having these embryonic
stages in its development and these stages have been handed
down to the divergent lines of its descendants.
Since we have come to associate with the name of the
recapitulation theory the idea of the recurrence of an ances-
tral adult form, it may be better to find a substitute for this
term. I suggest, therefore, for the view, that the embryos
of the higher group repeat the modified form of the embryos
of the lower groups, the term, the theory of embryonic
repetition, or, more briefly, the repetition theory.
84 Evolution and Adaptation
Conclusions
In the light of the preceding discussion concerning the
evidence in favor of the transmutation theory, we may now
proceed to sum up our general conclusions, and at the same
time discuss some further possibilities in regard to the
descent theory.
The most widely accepted view in regard to the theory of
organic evolution is that which looks upon the resemblances
between the members of a group as due to their common
descent from one original species that has broken up, as it
were, into a number of new forms. Strictly applied, this
means that all the vertebrates have come from one original
species, all the mollusks from another, the echinoderms from
a third, etc. Even farther back there may have been a com-
mon ancestral species for any two of the large groups, as,
for example, the annelids and the mollusks ; and if the re-
lationship of all the many-celled forms be looked upon as
probable, then they too have originated from one ancestral
species.
Many zoologists appear to hesitate to apply strictly this
fundamental idea contained in the transmutation theory, be-
cause, perhaps, they feel that it does not fit in with their gen-
eral experience of living forms. Yet there can be no doubt
that it is the primary conception of the transmutation
theory. This is, however, not the whole question, for we
must further consider the number of individuals of a species
that are involved.
In some species there are smaller groups of individuals
that are more like one another than like other individuals of
the same species. Such groups are called varieties, and are
often associated with certain localities, or with a special
environment. In the latter case they are called local varie-
ties. Some of these appear to breed true, not only when
kept under the same conditions, but even when transferred
The Theory of Evolution 85
to a new environment. Others change with the environ-
ment. It is not improbable that the varieties are of a dif-
ferent kind in these two cases, as shown by their different
behavior when put under new and different surroundings.
The variety that owes its peculiarities, not to the immediate
environment, but to some internal condition independent of
the surroundings, is recognized by some biologists as a
smaller species. Such species appear to be commoner in
plants than in animals, although it is possible that this only
means that more cases have been found by the botanists,
owing to the greater ease with which plants can be handled.
These smaller species, in contradistinction to the ordinary
Linnaean species, differ from the latter in the smaller amount
of differences between the groups, and probably also in that
they freely interbreed, and leave fertile descendants ; but
whether this is only on account of the smaller differences
between them than between larger species, or because of
some more fundamental difference in the kind of variation
that gives rise to these two kinds of groups, we do not know.
These smaller species, or constant varieties, as we may call
them, may be looked upon as incipient Linnaean species,
which, by further variations of the same, or of other sorts,
may end by giving rise to true species. A genus composed
of several species might be formed in this way, and then, if
each species again broke up into a number of new groups,
each such group would now be recognized as a genus, and
the group of genera would form a family, etc. The process
continuing, a whole class, or order, or even phylum, might be
the result of this process that began in a single species.
But we must look still farther, and inquire whether the
start was made from a single individual, that began to vary,
or from a number of individuals, or even from all the indi-
viduals, of a species. If we suppose the result to depend
on some external cause that affects all the individuals of a
species alike, then it might appear that the species, or at
86 Evolution and Adaptation
least as many individuals of a species as are affected, will
give the starting-point for the new group. But if the new
variation arises not directly as a response to some change in
the surroundings, then it might appear in one or in a few
individuals at a time. Let us consider what the results
might be under these two heads.
If amongst the descendants of a single individual a new
form or a number of new forms were to arise, then, if they
represented only a variety, they would cross with the other
forms like the parent species; and, under these conditions, it
is generally assumed that the new variety would be swamped.
If, however, the new forms have the value of new species,
then, ex JiypotJicse, they are no longer fertile with the original
forms, and might perpetuate themselves by self-fertilization,
as would be possible in some of the higher plants, and in
those animals that are bisexual. But as a rule even bisexual
forms are not self-fertilized, and, therefore, unless a number
of offspring arose from the same form the chance of propaga-
tion would be small.
If, however, a number of new forms appeared at the same
time and left a number of descendants, then the probability
that the new group might perpetuate itself is greater, and the
chance that such a group would arise is in proportion to the
number of individuals that varied in the same direction simul-
taneously. In this case the new species has not come from
a single individual or even from a pair of individuals, but from
a number of individuals that have varied more or less in the
same direction.
This point of view puts the descent theory in a somewhat
unforeseen light, for we cannot assume in such a case that the
similarities of the members of even the same species are due
to direct descent from an original ancestor, because there are
supposed to have been a number of ancestors that have
all changed in,. the same direction. The question is further
complicated by the fact that the new individuals begin to
The Theory of Evolution 8y
interbreed, so that their descendants come to have, after a
time, the common blood, so to speak, of all the new forms.
If with each union there is a blending of the substances of
the individuals, there will result in the end a common sub-
stance representing the commingled racial germ-plasm.
A new starting-point is then reached, and new species
may continue to be formed out of this homogeneous ma-
terial. Thus, in a sense, we have reached a position
which, although it appears at first quite different from
the ordinary view, yet, after all, gives us the same stand-
point as that assumed by the transmutation theory ; for, while
the latter assumes that the resemblances of the members
of a group are due to descent from the same original
form, and often by implication from a single individual,
we have here reached the conclusion that it is only a
common, commingled germ-plasm that is the common in-
heritance.
When we examine almost any group of living animals or
plants, whether they are low or high in organization, we
find that it is composed of a great many different species,
and so far as geology gives any answer, we find that this
must have been true in the past also. Why, then, do we
suppose that all the members of the higher groups have
come from a single original species or variety ? Why may
not all, or many, of the similar species of the lower group
have changed into the species of the higher group, — species
for species ? If this happened, the resemblance of the new
species of the group could be accounted for on the suppo-
sition that their ancestors were also like one another. The
likeness would not be due, then, to a common descent, and
it would be false to attempt to explain their likeness as. due to
a common inheritance. But before going farther, it may be
well to inquire to what the resemblances of the individuals of
the original species were due ; for, if they have come from an
older group that has given rise to divergent lines of descent,
88 Evolution and Adaptation
then we are only removing the explanation one step farther
back. If this original group has come from numerous species
of a still older group, and this, in turn, from an older one
still, then we must go back to the first forms of life that ap-
peared on the globe, and suppose that the individuals of these
primitive forms are the originals of the species that we find
living to-day. For instance, it is thinkable that each species
of vertebrate arose from a single group of the earliest forms
of life that appeared on the surface of the earth. If this
were the case, there must have been as many different kinds
of species of the original group as there are species alive at
the present time, and throughout all the past. This view finds
no support from our knowledge of fossil remains, and, al-
though it may be admitted that this knowledge is very in-
complete, yet, if the process of evolution had taken place as
sketched out above, we should expect, at least, to have found
some traces of it amongst fossil forms. Since this question
is an historical one, we can, at best, only expect to decide
which of all the possible suggestions is the more probable.
We conclude, then, that it is more probable that the verte-
brates, the mollusks, the insects, the crustaceans, the annelids,
the ccelenterates, and the sponges, etc., have come each from
a single original species. Their resemblances are clue to a
common inheritance from a common ancestral species. Even
if it be probable that at the time when the group of verte-
brates arose from a single species, there were in existence
other closely related species, yet we must suppose, if we
adhere to our point of view, that these other related species
have had nothing to do with the group of vertebrates, but that
they have died out. Moreover, we must suppose that each
order, each class of vertebrate, has come from a single origi-
nal species; each family has had a similar origin, as well as
each genus, but, of course, at different periods of time. Let
us not shrink from carrying this principle to its most extreme
point, for, unless the principle is absolutely true, then our
The Theory of Evolution 89
much boasted explanation of the resemblances of forms in
the same group will be thrown into hopeless confusion.
Let us ask another question in this connection. If a single
species gave rise to a group of new species that represented
the first vertebrates, they would have formed the first genus ;
and if the descendants of these diverged again so that new
genera were formed, then a group which we should call a
family would have been formed.
As the divergence went on, an order would be developed,
and then a class, and then a phylum. The common charac-
ters possessed by the members of this phylum would have
been present in the original species that began to diverge.
Hence, we find the definition of the phylum containing only
those points that are the features possessed by all of the de-
scendants, and in the same way we should try to construct
the definition of each of the subordinate groups. This is the
ideal of the principle of classification based on the theory of
descent with divergence. If we admit the possibility of the
other view that I have mentioned above, or of any other of
the numerous possibilities that will readily suggest them-
selves, then we must be prepared to give up some of the
most attractive features of the explanation of resemblance
as due to descent.
That all biologists believe strictly in divergent descent, to
the exclusion of any other processes, is not the case. And,
as I have said before, since we are dealing with an historical
question, it would be very unwise, in our present ignorance
on many points, to pretend that we have any direct proof of
the explanation that we find generally given to account for
the resemblances of the species of a group to each other.
At most we can claim that it is the simplest point of view,
and that most biologists believe it to be also the most prob-
able. It has been suggested that, in some cases, the new
forms that arise from two or more species run a parallel
course. If the original forms from which they came were
go Evolution and Adaptation
very much alike, it would soon be impossible to say what
the parentage of a particular form was ; that is, to which of
the two original forms it belonged. It has also been sug-
gested that even a convergence has at times taken place, so
that the descendants of different species have become more
alike than the original forms, at least in some one or more re-
spects. This last limitation is the saving clause, for species
differ in so many points that, even when they converge in a
few, it is unlikely that they will do so in all, and, therefore,
the deception may be discovered by the acute observer. One
famous paleontologist has gone so far even as to suppose that
a species may change its generic characters, so that it goes
over bodily into a new genus without losing its specific char-
acters. If such things do occur, then our classifications may
well be the laughing-stock of Nature.
CHAPTER IV
DARWIN'S THEORIES OF ARTIFICIAL AND OF NATURAL
SELECTION
The Principle of Selection
Darwin's theory of natural selection is preeminently a theory
of adaptation. It appears, in fact, better suited to explain
this phenomenon than that of the "origin of species." Dar-
win prepared his reader for the ideas contained in the theory
of natural selection by a brief consideration of the results of
artificial selection; and since the key to the situation is, I
believe, to be found in just this supposed resemblance, we
cannot do better than examine the theories in the order fol-
lowed by Darwin himself.
One of the means by which the artificial races of animals
and plants have been formed by man is selection. The
breeder picks out individuals having a certain peculiarity, and
allows them to breed together. He hopes to find among
the offspring, not only individuals like the parent forms, but
also some that have the special peculiarity even more strongly
developed. If such are found, they are isolated and allowed
to breed, and in the next generation it is hoped to find one or
more new individuals that show still more developed the
special character that is sought. This process, repeated
through a number of generations, is supposed to have led to
the formation of many of our various forms of domesticated
animals and plants.
This heaping up as a result of the union of similar individ-
uals cannot for a moment be supposed to be the outcome of
the addition of the two variations to each other. Such an
9i
92 Evolution and Adaptation
idea is counter to all the most familiar facts of inheritance.
For instance, when two similar forms unite, we do not find
that the young show all the characters of the mother plus all
those of the father, i.e. each peculiarity that is the same in
both, increased twofold. On the contrary, the young are in
the vast majority of cases not essentially different from either
parent.
A more thorough examination of the facts shows that the
problem is by no means so simple as the preceding general
statement might lead one to suppose, for our experience
shows that it is not always possible to increase all variations
by selection, and, furthermore, there is very soon found a
limit, even in favorable cases, to the extent to which the pro-
cess can be carried. The most important point appears to be
the nature of the variations themselves which may arise from
different causes, and which have different values in relation
to the possibility of their continuation.
We may begin, therefore, by following Darwin in his analy-
sis of variation, as given in the opening chapter of the " Ori-
gin of Species." He thinks that the great amount of
variation shown by domesticated animals and plants is due,
in the first place, to the new conditions of life to which they
are exposed, and also to the lack of uniformity of these con-
ditions. Darwin thinks, also, that there is some probability
that this variability is due, in part, to an excess of food. " It
seems clear that organic beings must be exposed during sev-
eral generations to new conditions to cause any great amount
of variation, and that when the organization has once begun
to vary, it generally continues varying for many generations.
No case is on record of a variable organism ceasing to vary
under cultivation. Our oldest cultivated plants, such as
wheat, still yield new varieties ; our oldest domesticated ani-
mals are still capable of rapid improvement or modification."
In this statement of Darwin, full of significance, we must
be careful to notice that he does not mean to imply, when he
Darwin's Artificial and Natural Selection 93
states that an organism that has once begun to vary con-
tinues to vary for many generations, that this continuous
variation is always in the same direction, but only that
new combinations, scattering in all directions, continue to
appear.
The nature of the organism seemed to Darwin to be a more
important factor in the origin of new variations than the
external conditions, " for nearly similar variations sometimes
arise under, as far as we can judge, dissimilar conditions ;
and, on the other hand, dissimilar variations arise under con-
ditions which appear to be nearly uniform." The following
statement is important in connection with the origin of
" definite " variations. " Each of the endless variations which
we see in the plumage of our fowls must have had some
efficient cause ; and if the same causes were to act uniformly
during a long series of generations on many individuals, all
probably would be modified in the same direction." Here
we find an explicit statement in regard to the accumulation of
variation in a given direction as the result of an external
agent, but Darwin hastens to add: "Indefinite variability is a
much more common result of changed conditions than definite
variability, and has probably played a more important part in
the formation of our domestic races. We see mdefinite vari-
ability in the endless slight peculiarities which distinguish the
individuals of the same species, and which cannot be accounted
for by inheritance from either parent or from some more
remote ancestor. Even strongly marked differences occa-
sionally appear in the young of the same litter, and in seed-
lings from the same seed capsule. At long intervals of time,
out of millions of individuals reared in the same country and
fed on nearly the same food, deviations of structure so strongly
pronounced as to deserve to be called monstrosities arise ;
but monstrosities cannot be separated by any distinct line
from slighter variations."
Another cause of variation, Darwin believes, is in the in-
94 Evolution and Adaptation
herited effect of " habit and of the use and disuse of parts,"
or what is generally known as the Lamarckian factor of
heredity. Darwin believes that changes in the body of the
parent, that are the result of the use or of the disuse of a part,
may be transmitted to the descendants, and cites a number
of cases which he credits to this process. As we shall deal
more fully with this topic in another chapter, we may treat it
here quite briefly. As an example of the inheritance of dis-
use, Darwin gives the following case : " I find in the domes-
tic duck that the bones of the wing weigh less and the bones
of the leg more in proportion to the whole skeleton than do
the same bones in the wild duck, and this change may be
safely attributed to the domestic cluck flying much less and
walking more than its wild parents." The great and in-
herited development of the udders of cows and of goats in
countries where they are habitually milked, in comparison
with these organs in other countries, is given as another
instance of the effect of use. " Not one of our domestic
animals can be named that in some country has not drooping
ears, and the view has been suggested that the drooping is
due to the disuse of the muscles of the ears from the animals
being seldom much alarmed."
It need scarcely be pointed out here, that, in the first case
given, those ducks would have been most likely to remain in
confinement that had less well-developed wings, and hence
at the start artificial selection may have served to bring
about the result. The great development of the udders of
cows and of goats is obviously connected with the greater
milk-giving qualities of these animals, which may have been
selected for this purpose.
Another " law " of variation recognized by Darwin is what
is called correlated variation. For example, it has been
found that cats which are entirely white and have blue eyes
are generally deaf, and this is stated to be confined to the
males. The teeth of hairless dogs are imperfect; pigeons
Darwin s Artificial and Natural Selection 95
with feathered feet have skin between the outer toes, and
those with short beaks have small feet, and vice versa.
Another source of variation is that of reversion, or the
reappearance in the offspring of characters once possessed
by the ancestors. Finally, Darwin thinks that a source of
variation is to be found in modifications due to the influence
of a previous union with another male, or, as it is generally
called, telegony. As an example Darwin cites the famous
case of Lord Morton's mare. " A nearly purely bred Ara-
bian chestnut mare bore a hybrid to a quagga. She subse-
quently produced two colts by a black Arabian horse. These
colts were partially dun-colored and were striped on the legs
more plainly than the real hybrid or even than the quagga." 1
This case, however, is not above suspicion, since it is well
known that stripes [often appear on young horses, and the
careful analysis made later by Ewart, as well as his other
experiments on the possibility of the transmission of influ-
ences of this sort, puts the whole matter in a very dubious
light.
These citations show that Darwin recognized quite a num-
ber of sources of variation, and, although he freely admits that
" our ignorance of the laws of variation is profound," yet
some at least of these sources of variation are very question-
able. Be this as it may, it is important to emphasize that
Darwin recognized two main sources of variation, — one of
which is the indefinite, or fluctuating, variability that appears
constantly in domesticated animals and plants, and the other,
definite variability, or a change in a definite direction, that can
often be traced to the direct action of the environment on
the parent or on its reproductive cells. It is the former,
i.e. the fluctuating variability, that, according to Darwin, has
been used by the breeder to produce most of our domestic
races. In regard to the other source of variation, the
definite kind, we must analyze the facts more closely.
1 " Animals and Plants under Domestication," Chap. IX.
96 Evolution and Adaptation
A definite change in the surroundings might bring about
a definite change in the next generation, because the new con-
dition acts either on the developing organism, or on the egg
itself from which the individual develops. The distinction
may be one of importance, for, if the new condition only
effects the developing organism directly, then, when the in-
fluence is removed, there should be a return to the former
condition ; but if the egg itself is affected, so that it is
fundamentally changed, then the effect might persist even if
the animal were returned to its former environment. More
important still is Darwin's recognition of the cumulative
effect in a given direction of external influences, for a new
variation, that was slight at first, might, through prolonged
action, continue to become more developed without any other
processes affecting the organism.
From the Darwinian point of view, however, the all-im-
portant source for the origin of new forms is the fluctuating
variation, which is made use of both in the process of arti-
ficial and of natural selection. We may now proceed to
inquire how this is supposed to take place.
It has been stated that, by means of artificial selection,
Darwin believes the breeder has produced the greater number
of domesticated animals and plants. The most important
question is what sort of variations he has made use of in
order to produce his result. Has he made use of the
fluctuating variations, or of the definite ones ? It is diffi-
cult, if not impossible, to answer this question in most
cases, because the breeder does not always distinguish be-
tween the two. There can be little question, however, that
he may sometimes have made use of the definite kinds,
whether these are the outcome of external or of internal
influences. The question has been seriously raised only in
recent years, and we are still uncertain how far we can accu-
mulate and fix a variation that is of the fluctuating kind. In
a few cases it has been found that the upper limit is soon
Darwin's Artificial and Natural Selection 97
reached, as shown by De Vries's experiments with clover,
and it is always possible that a definite variation of the
right sort may arise at any stage of the process. If this
should occur, then a new standard is introduced from which,
as from a new base, variations fluctuating in the desired
direction may be selected.
This question, before all others, ought to be settled before
we begin to speculate further as to what selection is able
to accomplish.
Darwin's theory is often stated in such a general way
that it would be applicable to either sort of variation ; but
if definite variation can go on accumulating without selec-
tion, then possibly we could account for evolution without
supposing any other process to intervene. Under these
circumstances all that could be claimed for selection would
be the destruction of those variations incapable of living,
or of competing with other forms. Hence the process of
selection would have an entirely negative value.
The way in which domesticated animals and plants have
originated is explained by Darwin in the following significant
passage : —
" 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
account by such agencies for the differences between a dray-
and race-horse, a greyhound and bloodhound, a carrier and
tumbler pigeon. One of the most remarkable features in
our domesticated races is that we see in them adaptation,
not indeed to the animal's or plant's own good, but to man's
use or fancy. Some variations useful to him have probably
arisen suddenly, or by one step ; many botanists, for instance,
believe that the fuller's-teasel, with its hooks, which cannot
be rivalled by any mechanical contrivance, is only a variety
H
98 Evolution and Adaptation
of the wild Dipsacus ; and this amount of change may have
suddenly arisen in a seedling. So it has probably been with
the turnspit dog; and this is known to have been the case
with the ancon sheep. But when we compare the dray-
horse and race-horse, the dromedary and camel, the various
breeds of sheep fitted either for cultivated land or mountain
pasture, with the wool of one breed good for one purpose,
and that of another breed for another purpose; when we
compare the many breeds of dogs, each good for man in
different 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
different purposes, or so beautiful in his eyes, we must, I
think, look further than to mere variability. We cannot
suppose that all the breeds were suddenly produced as per-
fect 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 direc-
tions useful to him. In this sense he may be said to have
made for himself useful breeds."
Darwin also gives the following striking examples, which
make probable the view that domestic forms have really
been made by man selecting those variations that are useful
to him : —
" In regard to plants, there is another means of observing
the accumulated effects of selection — namely, by comparing
the diversity of flowers in the different varieties of the same
species in the flower-garden ; the diversity of leaves, pods,
01 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
Darwin s Artificial and Natural Selection 99
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, — per-
haps never, the case. The law of correlated 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."
Exception may perhaps be taken to the concluding sen-
tence, for, interesting as the facts here recorded certainly
are, it does not necessarily follow that all domestic products
have arisen " by the continued selection of slight variations,"
however probable the conclusion may appear. Darwin also
believes that a process of " unconscious selection " has given
even more important " results than methodical selection." By
unconscious selection is meant the outcome of " every one
trying to possess and breed from best individual animals."
"Thus a man who intends keeping pointers naturally tries
to get as good dogs as he can, and afterwards breeds from
his own best dogs, but he has no wish, or expectation of per-
manently altering the breed. Nevertheless we may infer
that this process, continued during centuries, would improve
and modify any breed. . . . There is reason to believe that
the King Charles spaniel has been unconsciously modified
to a large extent since the time of that monarch."
The enormous length of time required to produce new
species by the selection of fluctuating variations is every-
where admitted by Darwin ; nowhere perhaps more strikingly
ioo Evolution and Adaptation
than in the following statement : " 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 uncivil-
ized man has afforded us a single plant worth culture. It is
not that these countries, so rich in species, do not by a
strange chance possess the aboriginal stocks of any useful
plants, but that the native plants have not been improved by
continued selection up to a standard of perfection comparable
with that acquired by the plants in countries anciently
civilized."
In reply to this, it may be said that if the selection of
fluctuating variations leads to an accumulation in the given
direction, it is not apparent why it should take thousands of
years to produce a new race, or require such a high degree
of skill as Darwin supposes the breeder to possess.
The conditions favorable to artificial selection are, accord-
ing to Darwin : I. The possession of a large number of in-
dividuals, for in this way the chance of the desired variation
appearing is increased. 2. Prevention of intercrossing, such
as results when the land is enclosed, so that new forms may
be kept apart. 3. Changed conditions, as introducing varia-
bility. 4. The intercrossing of aboriginally distinct species.
5. The intercrossing of new breeds, " but the importance
of intercrossing has been much exaggerated." 6. In plants
propagation of bud variations by means of cuttings. The
chapter concludes with the statement, " Over all these
causes of Change, the accumulative action of Selection,
whether applied methodically and quickly, or unconsciously
and slowly, but more efficiently, seems to have been the pre-
dominant Power."
Variability, Darwin says, is governed by many unknown
laws, and the final result is "infinitely complex." If this is
so, we may at least hesitate before we accept the statement
Darwin's Artificial and Natural Selection 101
that selection of fluctuating variations has been the only-
principle that has brought about these results. This is a
most important point, for, as we shall see, the central question
in the theory of natural selection has come to be whether
by the accumulation of fluctuating variations a new species
could ever be produced. If it be admitted that the evidence
from artificial selection is far from convincing, in showing
that selection of fluctuating variations could have been the
main source, even in the formation of new races, we need
not be prejudiced in favor of such a process, when we come
to examine the formation of species in nature.
There are still other questions raised in this same chap-
ter, that demand serious consideration. Darwin writes as
follows : —
" When we look to the hereditary varieties or races of our
domestic animals and plants, and compare them with closely
allied species, we generally perceive in each domestic race, as
already remarked, less uniformity of character than in true
species. Domestic races often have a somewhat monstrous
character ; by which I mean, that, although differing from
each other, and from other species of the same genus, in
several trifling respects, they often differ in an extreme de-
gree 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 excep-
tions (and with that of the perfect fertility of varieties when
crossed, — a subject hereafter to be discussed), domestic
races of the same species differ from each other in the same
manner as do the closely allied species of the same genus in
a state of nature, but the differences in most cases are less in
degree. This must be admitted as true, for the domestic
races of many animals and plants have been ranked by some
competent judges as the descendants of aboriginally distinct
species, and by other competent judges as mere varieties. If
any well-marked distinction existed between a domestic race
102 Evolution and Adaptation
and a species, this source of doubt would not so perpetually
recur."
The point here raised in regard to the systematic value of
the new forms is the question that first demands our attention.
We must exclude all those cases in which several original
species have been blended to make a new form, because the
results are too complicated to make use of at present. The
domesticated races of dogs appear to have had such a mul-
tiple origin, the origin of horses is in doubt ; but the domesti-
cated pigeons, ducks, rabbits, and fowls are supposed, by
Darwin, to have come each from one original wild species.
The great variety of the domestic pigeons gives perhaps the
most striking illustration of changes that have taken place
under domestication ; and Darwin lays great stress on the
evidence from this source.
It seems probable in this case, (i) that all the different
races of pigeons have come from one original species ; (2) that
the structural differences are in some respects as great as those
recognized by systematists as specifically distinct; (3) that the
different races breed true to their kind; (4) that the result
has been reached mainly by selecting and isolating variations
that have appeared under domestication, and that probably
some, at least, of these variations were fluctuating ones.
Does not this grant all that Darwin contends for ? In one
sense, yes ; in another, no ! The results appear to show that
by artificial selection of some kind a group of new forms may
be produced that in many respects resemble a natural family,
or a genus ; but if this is to be interpreted to mean that the
result is the same as that by which natural groups have
arisen, then I think that there are good reasons for dissenting
from such a conclusion. Moreover, we must not grant too
readily that the different races of pigeons have arisen by the
selection of fluctuating variations alone, for this is not estab-
lished with any great degree of probability by the evidence.
In regard to the first point we find that one of the most
Darwin's Artificial and Natural Selection 103
striking differences between species in nature is their infer-
tility, and the infertility of their offspring when intercrossed.
This is a very general rule, so far as we know. In regard to
the different races of domesticated forms, the most significant
fact is that, no matter how different they may be, they are
perfectly fertile inter se. In this respect, as well as in others,
there are important differences between domesticated races
and wild species. The further difference, that has been
pointed out by a number of writers, should also not pass
unnoticed, namely, that the domestic forms differ from each
other in the extreme development of some one character, and
not in a large number of less conspicuous characters, as is the
case in wild species.
These considerations show that, interesting and suggestive
as are the facts of artificial selection, they fail to demon-
strate the main point for which they are used by Darwin.
With the most rigorous attention to the process of artificial
selection, new species comparable in all respects to wild ones
have not been formed, even in those cases in which the
variation has been carried farthest (where the history of the
forms is most completely known).
There is another point on which emphasis should be laid.
If by selecting the most extreme forms in each generation
and breeding from them the standard can be raised, it might
appear that we could go on indefinitely in the same direction,
and produce, for instance, pigeons with legs five metres long,
and with necks of corresponding length. But experience has
shown that this cannot be done. As Darwin frequently re-
marks, the breeder is entirely helpless until the desired varia-
tion appears. It seems possible, by selecting the more extreme
of the fluctuating variations in each generation, that a higher
plane of variation is established, and even that more extreme
forms are likely to arise for a few generations ; but, even if
this is the case, a limit is soon reached beyond which it is
impossible to go.
104 Evolution and Adaptation
The facts of observation show, that when a new variety
appears its descendants are more likely, on the average, to
produce proportionately more individuals that show the same
variation, and some even that may go still farther in the same
direction. If these latter are chosen to be the parents of the
next generation, then once more the offspring may show the
same advance ; but little by little the advance slows down,
until before very long it may cease altogether. Unless, then, a
new kind of variation appears, or a new standard of variation
develops of a different kind, the result of selection of fluctu-
ating variations has reached its limit. Our experience seems,
therefore, to teach us that selection of fluctuating variations
leads us to only a certain point, and then stops in this direc-
tion. We get no evidence from the facts in favor of the
view that the process, if carried on for a long time, could
ever produce such great changes, or the kind of changes, as
those seen in wild animals and plants.
Variation and Competition in Nature
Darwin rests his theory on the small individual variations
which occur in nature, as the following quotation shows : —
" It may be doubted whether sudden and considerable
deviations of structure such as we occasionally see in our
domestic productions, more especially with plants, are ever
permanently propagated in a state of nature. Almost every
part of every organic being is so beautifully related to its
complex conditions of life that it seems as improbable that
any part should have been suddenly produced perfect, as that
a complex machine should have been invented by man in a
perfect state. Under domestication monstrosities sometimes
occur which resemble normal structures in widely different
animals. Thus pigs have occasionally been born with a sort
of proboscis, and if any wild species of the same genus had
naturally possessed a proboscis, it might have been argued
Darwin's Artificial and Natural Selection 105
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 unusu-
ally 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."
It is clear that Darwin does not think that the sudden and
large variations that sometimes occur furnish the basis for
natural selection, and the final statement in the last citation
(which was added in later editions of the " Origin of Species "),
to the effect that if such monstrous variations appeared as
single or occasional variations they would be lost by intercross-
ing implies that, in general, single variations would likewise
be lost unless they appeared in a sufficient number of indi-
viduals to maintain themselves against the swamping effects
of intercrossing.
It is necessary to quote again, in order to show that, in
some cases at least, Darwin believed selection plays little or
no part in the origin and maintenance of certain peculiarities
that are of no use to the species. " There is one point con-
nected with individual differences, which is extremely per-
plexing : I refer to those genera which have been called
protean or ' polymorphic,' in which the species present an
inordinate amount of variation. With respect to many of
these forms, hardly two naturalists agree, whether to rank
them as species or as varieties. We may instance Rubus,
Rosa, and Hieracium amongst plants, several genera of in-
sects and of Brachiopod shells. In most polymorphic genera
some of the species have fixed and definite characters. Gen-
era which are polymorphic in one country seem to be, with
106 Evolution and Adaptation
a few exceptions, polymorphic in other countries, and like-
wise, judging from Brachiopod shells, at former periods of
time. These facts are very perplexing, for they seem to show
that this kind of variability is independent of the conditions
of life. I am inclined to suspect that we see, at least in some
of these polymorphic genera, variations which are of no
service or disservice to the species, and which consequently
have not been seized on by selection to act on and accumulate,
in the same manner as man accumulates in any given direc-
tion individual differences in his domesticated productions.
These individual differences generally affect what naturalists
consider unimportant parts ; but I could show by a long cata-
logue of facts, that parts which must be called important,
whether viewed under a physiological or classificatory point
of view, sometimes vary in the individuals of the same species.
I am convinced that the most experienced naturalist would
be surprised at the number of cases of variability, even in
important parts of structure, which he could collect on good
authority, as I have collected, during a course of years."
After pointing out that naturalists have no definite stand-
ard to determine whether a group of individuals is a variety
or a species, Darwin makes the highly important admissions
contained in the following paragraph : " Hence, I look at indi-
vidual 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 re-
cording 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 subspecies, and then to species.
The passage from one stage of difference to another may, in
many cases, be the simple result of the nature of the organism
and of the different physical conditions to which it has long
been exposed ; but with respect to the more important and
adaptive characters, the passage from one stage of difference
Darwin s Artificial and Natural Selection 107
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."
In this paragraph attention should be called especially,
first, to the statement in respect to the origin of varieties,
which are said to arise through individual differences. It
is not clear whether these differences are supposed to have
appeared first in one, or in a few individuals, or in large
numbers at the same time. Again, especial note should
be made of the striking admission, that the passage from
one stage to another may, in many cases, be the simple
result of the nature of the organism and of the physical
conditions surrounding it ; but with respect to the more
important and adaptive differences, natural selection " may
safely " be supposed to have intervened. Is it to be won-
dered at that Darwin's critics have sometimes accused him
of playing fast and loose with the origin of varieties ? And
since this question is fundamental for the theory of natural
selection, it is much to be regretted that Darwin leaves the
matter in such a hazy condition. It may be said that, at
the time when he wrote, he made the best of the evidence
in regard to the origin of varieties. Be this as it may, a
theory standing on no better foundations than this is not
likely to be found satisfactory at the present time.
We come now to the most important chapters, the third
and the fourth, of the " Origin of Species," dealing with " the
struggle for existence," " natural selection," or the " survival
of the fittest." Behind these fatal phrases, which have become
almost household words, lurk many dangers for the unwary.
" It has been seen in the last chapter that amongst organic
beings in a state of nature there is some individual variability :
io8 Evolution and Adaptation
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 subspecies or varieties ; what rank, for in-
stance, the two or three hundred doubtful forms of British
plants are entitled to hold, if the existence of any well-marked
varieties be admitted. But the mere existence of individual
variability and of some few well-marked varieties, though
necessary as the foundation for the work, helps us but little
in understanding how species arise in nature. How have all
those exquisite adaptions of one part of the organization to
another part, and to the conditions of life, and of one organic
being to another being, been perfected ? We see these beau-
tiful coadaptions most plainly in the woodpecker and the
mistletoe ; and only a little less plainly in the humblest
parasite which clings to the hairs of a quadruped or feathers
of a bird ; in the structure of the beetle which dives through
the water ; in the plumed seed which is wafted by the
gentlest breeze ; in short, we see beautiful adaptions
everywhere 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
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 individ-
uals 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 gener-
ally be inherited by the offspring. The offspring, also, will
thus have a better chance of surviving, for, of the many
Darwin s Artificial and Natural Selection 109
individuals of any species which are periodically born, but a
small number can survive. I have called this principle, by
which each slight variation, if useful, is preserved, by the
term Natural Selection, in order to mark its relation to man's
power of selection. But the expression often used by Mr.
Herbert Spencer of the Survival of the Fittest is more
accurate, and is sometimes equally convenient. We have
seen that man by selection can certainly produce great re-
sults, and can adapt organic beings to his own uses, through
the accumulation of slight but useful variations, given to
him by the hand of Nature. But Natural Selection, as we
shall hereafter see, is a power incessantly ready for action,
and is as immeasurably superior to man's feeble efforts, as
the works of Nature are to those of Art."
Darwin gives the following explicit statement of the way
in which he intends the term "struggle for existence" to be
understood : " 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 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 mois-
ture. A plant which actually produces a thousand seeds of
which only one on 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 dis-
iio Evolution and Adaptation
seminated by birds, its existence depends on them, and it may
metaphorically be said to struggle with other fruit-bearing
plants, in tempting the birds to devour and thus disseminate
its seeds. In these several senses, which pass into each other,
I use for convenience' sake the general term ' Struggle for
Existence.' "
A number of writers have objected to the general and
often vague way in which Darwin makes use of this phrase ;
but it does not seem to me that this is a serious objection,
provided we are on our guard as to what the outcome will
be in each case. In each instance we must consider the
question on its own merits, and if it is found convenient to
have a sufficiently general and non-committal term, such as the
" struggle for existence," to include all cases, I see no serious
objection to the use of such an expression, although it is
true the outcome has been that it has become a catchword,
that is used too often by those who have no knowledge of its
contents.
Were it not that each animal and plant gives birth, on an
average, to more than two offspring, the species would soon
become exterminated by accidents, etc. We find in some of
the lower animals, and in some of the higher plants, that
thousands and even millions of eggs are produced by a
single individual in the course of its life. A single nematode
may lay sixty million eggs, and a tapeworm one thousand
million. A starfish may produce about thirty-nine million
eggs, a salmon may contain fifteen thousand, and a large shad
as many as one hundred thousand. The queen of a termite
nest is said to lay eighty thousand eggs a day.
In the higher vertebrates the number of young is con-
siderably less, but since the young stages are passed within
the body of the parent, proportionately more of them reach
maturity, so that even in man the population may be doubled
in twenty-five years, and in the elephant, slowest breeder of
all animals, Darwin has calculated that, if it begins breeding
Darwin s Artificial and Nattiral Selection in
when about thirty years old and goes on until ninety years,
bringing forth six young in the interval, after 750 years
there will be nearly nineteen million elephants alive which
have descended from the first pair.
Obviously, then, if all the descendants of all the individuals
of a species were to remain alive, the world would be over-
crowded in a very short time, and the want of room would in
itself lead to the destruction of countless individuals, if for
no other reason than lack of food. We can easily carry out
on a small scale an experiment that shows how the overstock-
ing, resulting from favorable conditions, comes about, and how
it checks itself. If we make a meat broth suitable for the
life of a particular bacterium, and sow in the broth a very
few individuals, we find in the course of several days the fluid
swarming with the descendants of the original individuals.
Thus it has been shown that, if we start with a few hundred
bacteria, there will be five thousand after twenty-four hours,
and twenty thousand, forty-eight hours later ; and after four
days they are beyond calculation.
Cohn found that a single bacterium produces two individ-
uals in one hour, and four in two hours, and if they continue
to multiply at this rate there will be produced at the end of
three days 4,772 billions of descendants. If these are reduced
to weight, they would weigh seventy-five hundred tons. Thus
when the conditions are favorable, bacteria are able to in-
crease at such an enormous rate that they could cover the
surface of the earth in a very few days. The reason that
they do not go on increasing at this rate is that they soon
exhaust the food supply, and the rate of increase slows
down, and will finally cease altogether. If the bacteria
were dependent on a continuous supply of food, they would
perish after the supply had been exhausted, so that the
rapid rate of multiplication would serve only to bring
the career of the organism to an untimely end. If the
weaker individuals were to die first, the products of their dis-
112 Evolution and Adaptation
integration might serve to nourish the stronger individuals ;
hunger coming on again, the next weakest might die ; and the
same process continuing, we might imagine that the bacteria
were finally reduced to a single one which would then die in
turn for lack of food. Like a starving shipload of men, re-
duced by hunger to cannibalism, the life of some and finally of
the last individual might be prolonged in the hope of rescue,
but if this did not arrive, the last and perhaps the strongest
individual would perish. But this is not what we find occur-
ring in these lower organisms, for, as a rule, they gradually
cease to increase when the food supply becomes lessened, and
their activities slow down. Finally, when the food is gone,
they pass into a resting stage, in which condition they can
remain dormant for a long time, even for years. If they
should again find themselves in favorable surroundings,
they become active, and begin once more their round of
multiplication. We cannot follow the individuals in such a
culture of bacteria, but there is nothing to be seen that
suggests a struggle for existence, if this idea conveys the
impression of the destruction of certain individuals by com-
petition with others. In fact, the results are in some respects
exactly the reverse. Millions of individuals are present at
the time when the food supply becomes exhausted, and they
all pass into a protected resting stage.
The enormous rate of increase in this case finds its coun-
terpart in higher animals when the food supply, or the ab-
sence of enemies, allows a species to multiply at its maximum
rate of increase. The introduction of rabbits into Australia
was followed by an enormous increase in a few years, and the
introduction of the English sparrow into the United States
has had a similar result. But in no country can such a
process continue beyond a certain point, because, in the first
place, the scarcity of food will begin to keep the birth-rate
down, and in the second place, the increase in numbers may
lead to an increase in the number of its enemies, or even
Darwin's Artificial and Natural Selection 113
induce other forms to feed on it. Crowding will also give
an opportunity for the spread of disease, which again may
check the increase. Sooner or later a sort of ever shifting
balance will be reached for each species, and after this, if the
conditions remain the same, the number of individuals will
keep approximately constant.
Darwin admits that the " causes which check the natural
tendency of each species to increase are most obscure." " We
know not exactly what the checks are even in a single in-
stance." This admission may well put us on our guard
against a too ready acceptation of a theory in which the whole
issue turns on just this very point, namely, the nature of the
checks to increase. Darwin gives the following general cases
to show what some of the checks to increase are. He states
that eggs and very young animals and seeds suffer more
than the adults ; that " the amount of food for each species of
course gives the extreme limit to which each can increase;
but very frequently it is not the obtaining food, but the serving
as prey to other animals which determines the average num-
bers of a species. Thus, there seems to be little doubt that
the stock of partridges, grouse, and hares on any large estate
depends largely on the destruction of the vermin." " On the
other hand, in some cases, as with the elephant, none are de-
stroyed by beasts of prey ; for even the tiger in India most
rarely dares to attack a young elephant protected by its
dam." "Climate plays an important part in determining
the average number of a species, and periodical seasons of
extreme cold or drought seem to be the most effective of all
checks." "The action of climate seems at first sight to be
quite independent of the struggle for existence ; but in so far
as climate acts in reducing food, it brings on the most severe
struggle between the individuals, whether of the same, or of
distinct species which subsist on the same kind of food."
We need not follow Darwin through his account of how
complex are the relations of all animals and plants to one
U4 Evolution and Adaptation
another in the struggle for existence, for, if true, it only
goes to show more plainly how impossible it is to establish
any safe scientific hypothesis, where the conditions are so
complex and so impossible to estimate. To show that the
young Scotch fir in an enclosed pasture is kept down by the
browsing of the cattle, and in other parts of the world, Para-
guay for instance, the number of cattle is determined by
insects, and that the increase of these flies is probably habitu-
ally checked by other insects, leads to a bewilderingly com-
plex set of conditions. We cannot do better than to quote
Darwin's conclusion : " Hence, if certain insectivorous birds
were to decrease in Paraguay, the parasitic insects would
probably increase ; and this would lessen the number of the
navel-frequenting flies — then cattle and horses would be-
come feral, and this would certainly greatly alter (as indeed
I have observed in parts of South America) the vegetation :
this again would largely affect the insects ; and this, as we
have just seen in Staffordshire, the insectivorous birds, and
so onwards in ever increasing circles of complexity. Not
that under nature the relations will ever be as simple as this.
Battle within battle must be continually recurring with vary-
ing 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 ! "
The effect of the struggle for existence in determining the
distribution of species is well illustrated in the following
cases : —
" As the species of the same genus usually have, though
by no means invariably, much similarity in habits and con-
Darwin s Artificial and Natural Selection 115
stitution, and always in structure, the struggle will generally
be more severe between them, if they come into competition
with each other, than between the species of distinct genera.
We see this in the recent extension over parts of the United
States of one species of swallow having caused the decrease
of another species. The recent increase of the missel-thrush
in parts of Scotland has caused the decrease of the song-
thrush. How frequently we hear of one species of rat taking
the place of another species under the most different 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
precisely say why one species has been victorious over
another in the great battle of life."
All this goes to show, if it really shows anything at all,
that the distribution of a species is determined, in part, by
its relation to other animals and plants — a truism that is
recognized by every naturalist. The statement has no neces-
sary bearing on the origin of new species through competi-
tion, as the incautious reader might infer. Not that I mean
in any way to imply that Darwin intended to produce this
effect on the reader ; but Darwin is not always careful to
discriminate as to the full bearing of the interesting illustra-
tions with which his book so richly abounds.
At the end of his treatment of the subject, Darwin empha-
sizes once more how little we know about the subject of the
struggle for existence.
" It is good thus to try in imagination to give to any one
species an advantage over another. Probably in no single
instance should we know what to do. This ought to con-
n6 Evolution and Adaptation
vince 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 geometri-
cal 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 vig-
orous, the healthy, and the happy survive and multiply."
The kindliness of heart that prompted the concluding sen-
tence may arouse our admiration for the humanity of the
writer, but need not/therefore, dull our criticism of his theory.
For whether no fear is felt, and whether death is prompt or
slow, has no bearing on the question at issue — except as it
prepares the gentle reader to accept the dreadful calamity of
nature, pictured in this battle for existence, and make more
contented with their .lot "the vigorous, the healthy, and the
happy."
The Theory of Natural Selection
We have already anticipated, to some extent, Darwin's
conclusion in regard to the outcome of the competition of
animals and plants. This result is supposed to lead to the
survival of the fittest. The competition is carried out by
nature, who is personified as selecting those forms for further
experiments that have won in the struggle for existence.
" Can the principle of selection, which we have seen is so
potent in the hands of man, apply under Nature ? I think
we shall see that it can act most efficiently. Let the endless
number of slight variations and individual differences occur-
ring 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. Can it, then, be thought im-
Darwin! s Artificial and Natural Selection 117
probable, seeing that variations useful to man have undoubt-
edly occurred, that other variations useful in some way to
each being in the great and complex battle for life, should
occur in the course of many successive generations ? If such
do occur can we doubt (remembering how many more indi-
viduals are born than can possibly survive) that individuals
having any advantage, however slight, over others, would
have the best chance of surviving and of procreating their
kind ? On the other hand, we may feel sure that any varia-
tion in the least degree injurious would be rigidly destroyed."
The process of natural selection is defined as follows,
"The preservation of favorable individual differences and
variations and the destruction of those that are injurious I
have called Natural Selection or the Survival of the Fittest."
And immediately there follows the significant statement,
that, "Variations neither useful nor injurious would not be
affected by natural selection, and would be left either a
fluctuating element, as perhaps we see in certain polymorphic
species, or would ultimately become fixed, owing to the
nature of the organism and the nature of the conditions."
It will be seen from this quotation, as well as from others
already given, that Darwin leaves many structures outside
of the pale of natural selection, and uses his theory to ex-
plain only those cases that are of sufficient use to be decisive
in the life and death struggle of the individuals with each
other and with the surrounding conditions.
Darwin states that we can best understand " the probable
course of natural selection by taking the case of a country
undergoing some slight physical change, for instance, of
climate. The proportional numbers of its inhabitants will
almost immediately undergo a change, and some species will
probably become extinct. We may conclude, from what we
have seen of the intimate and complex manner in which the
inhabitants of each country are bound together, that any
change in the numerical proportions of the inhabitants, in-
Ii8 Evolution and Adaptation
dependently of the change of climate itself, would seriously
affect the others. ... 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."
The first half of the first of these two quotations seems so
plausible, that without further thought we may be tempted
to give a ready assent to the second, yet the whole issue is
contained in this statement. In the abstract, it undoubtedly
appears true that any slightly useful modification might tend
to be preserved. Whether it will, in reality, be preserved
must depend on many things that should be taken into
account. This question will come up later for further con-
sideration ; but it should be pointed out here, that, even
assuming that one or more individuals happen to possess a
favorable variation, it by no means follows that natural
selection would have free scope for the work of improvement,
because the question of the inheritance of this variation,
and of its accumulation and building up through successive
generations, must be determined before we can be expected
to give assent to this argument, that appears so attractive
when stated in an abstract and vague way.
Darwin again makes the statement that under the term
variation it must never be forgotten that mere individual
differences are meant. " As a 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." Too much emphasis cannot be laid
on the fact that Darwin believed that selection takes place
amongst the small individual differences that we find in
animals and plants. Some of his followers, as we shall see,
are apt to put into the background this fundamental con-
ception of Darwin's view. His constant comparison between
Darwin's Artificial and Natural Selection 119
the results of artificial and natural selection leaves no room
for doubt as to his meaning. Darwin himself seems, at
times, not unconscious of the weakness of this comparison.
He says : " How fleeting are the wishes and efforts of man !
how short his time ! and consequently how poor will be his
results, compared with those accumulated by Nature during
whole geological periods. Can we wonder then that Nature's
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 ? " We should not
lose sight of the fact that even after the most rigorous selec-
tive process has been brought to bear on organisms, namely,
by isolation under domestication, we do not apparently find
ourselves gradually approaching nearer and nearer to the
formation of new species, but we find, on the contrary, that
we have produced something quite different. In the light of
this truth, the relation between the two selective theories
may appear quite different from the interpretation that Dar-
win gives of it. We may well doubt whether nature does
select so much better than does man, and whether she has
ever made new species in this way.
We come now to a point that touches the theory of natural
selection in a very vital spot.
" It may be well here to remark that with all beings there
must be much fortuitous destruction, which can have little or
no influence on the course of natural selection. For instance,
a vast number of eggs or seeds are annually devoured, and
these could be modified through natural selection only if they
varied in some manner which protected them from their
enemies. Yet many of these eggs or seeds would perhaps, if
not destroyed, have yielded individuals better adapted to their
conditions of life than any of those which happened to sur-
vive. 60 again a vast number of mature animals and plants,
whether or not they be the best adapted to their conditions,
1 20 Evolution and Adaptation
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 bene-
ficial to the species. But let the destruction of the adults
be ever so heavy, if the number which can exist in any dis-
trict 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 selec-
tion will be powerless in certain beneficial directions ; but this
is no valid objection to its efficiency at other times and in other
ways ; for we are far from having any reason to suppose
that many species ever undergo modification and improve-
ment at the same time in the same area."
Some of the admissions made in this paragraph have an
important bearing on the theory of natural selection. Far
from supposing that fortuitous destruction would have no
influence on the course of natural selection, it can be shown
that it would have a most disastrous effect. In many cases
the destruction comes in the form of a catastrophe to the
individuals, so that small differences in structure, whether
advantageous or not, are utterly unavailing. Our experience
shows us that a destruction of this sort is going on around
us all the time, and accounts in large part for the way in
which the majority of animals and plants are destroyed.
Unless, for example, a seed happen to fall on a place suitable
for its growth, it will perish without respect to a slight advan-
tage it may have over other seeds of its kind. Of the thou-
sands of eggs laid by one starfish, chance alone will decide
whether one or another embryo is destroyed by larger animals,
or if they escape this danger, the majority of them may be
Darwin s Artificial and Natural Selection 121
carried out to sea, where it will not be of the least avail if
one individual has a slight advantage over the others. Dar-
win admits this, but adds that, if only a thousandth part is
developed, yet of those that do survive the best adapted
individuals will tend to propagate their kind in larger 'num-
bers than the less well adapted. The argument is not, how-
ever, so simple as it appears to be on the surface. I pass
over, for the present, the apparent inconsequence in this
statement that the best adapted individuals will tend to prop-
agate their kind in larger numbers. It is not by any means
certain that this is the case. Darwin's meaning is, however,
fairly clear, and can be interpreted to mean this : after the
fortuitous destruction has finished, there will be a further
competition of the survivors amongst themselves and with
the surrounding conditions. In this higher competition, which
is less severe, small individual differences suffice to determine
the survival of certain individuals. These are, therefore,
selected.
In this argument it is assumed that a second competition
takes place after the first destruction of individuals has oc-
curred, and this presupposes that more individuals reach
maturity than there is room for in the economy of nature.
But we do not know to what extent this takes place. If only
as many mature as can survive, then the second competition
does not take place. If, on the other hand, fewer mature than
there is room for, then again competition does not take place.
And if at all times selection is not rigorously carried out,
everything may be lost that has been so laboriously gained.
We see then that the result that Darwin imagines would take
place, can be carried out only when more individuals reach
maturity than there is room for (if it is a case of competition
with one another), or that escape their enemies (if it is a
question of competition with other forms).
It is instructive to consider some of the examples that
Darwin has given to illustrate how the process of natural
122 Evolution and Adaptation
selection is carried out. The first example is the imaginary
case of a species of wolf, the individuals of which secure
their prey sometimes by craft, sometimes by strength, and
sometimes by fleetness. If the prey captured by the first
two methods should fail, then all the wolves would be obliged
to capture their food by fleetness, and consequently the fleet-
est alone would survive. " I can see no more reason to doubt
that this would be the result than that man should improve
the fleetness of his greyhounds." But even if the fleetness
of the race could be kept up in this way, it does not follow
that a new species of wolf would be formed in consequence,
as Darwin implies. His own comment on this illustration is,
perhaps, the best criticism that can be made.
" 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 alter-
native had frequently occurred. I saw the great importance
of individual differences, and this led me fully to discuss the
results of unconscious selection by man, which depends on
the preservation of all the more or less valuable individuals,
and on the destruction of the worst. I saw, also, that the
preservation in a state of nature of any occasional deviation
of structure, such as a monstrosity, would be a rare event ; and
that, if at first preserved, it would generally be lost by subse-
quent intercrossing with ordinary individuals. Nevertheless,
until reading an able and valuable article in the North Brit-
ish Review (1867), I did not appreciate how rarely single
variations, whether slight or strongly marked, could be per-
petuated. The author takes the case of a pair of animals,
producing during their lifetime two hundred offspring, of
which, from various causes of destruction, only two on an
average survive to 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
Darwin s Artificial and Natural Selection 123
shows that if a single individual were born, which varied in
some manner, giving it twice as good a chance of life as that
of the other individuals, yet the chances would be strongly
against its survival. Supposing it to survive and to breed,
and that half its young inherited the favourable variation ;
still, as the 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 genera-
tions. The justice of these remarks cannot, I think, be dis-
puted. 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 conse-
quently flourished, nevertheless there would be a very poor
chance of this one individual perpetuating its kind to the ex-
clusion of the common form; but there can hardly be a
doubt, judging by what we see taking place under domestica-
tion, that this result would follow from the preservation dur-
ing many generations of a large number of individuals with
more or less strongly curved beaks, and from the destruction
of a still larger number with the straightest beaks."
There then follows what, I believe, is one of the most sig-
nificant admissions in the "Origin of Species": —
" 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 numer-
ous instances could be given with our domestic productions.
In such cases, if the varying individual did not actually trans-
mit to its offspring its newly acquired character, it would
undoubtedly transmit to them, as long as the existing condi-
tions 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.
124 Evolution and Adaptation
Or only a third, fifth, or tenth part of the individuals may-
have been thus affected, of which fact several instances could
be given. Thus Graba estimates that about one-fifth of the
guillemots in the Faroe Islands consist of a variety so well
marked, that it was formerly ranked as a distinct species
under the name of Uria lacrymans. In cases of this kind, if
the variation were of a beneficial nature, the original form
would soon be supplanted by the modified form, through the
survival of the fittest."
Do not the admissions in this paragraph almost amount to
a withdrawal of much that has preceded in regard to the
survival of fluctuating, individual differences ? In the last
edition, from which we have just quoted, Darwin, in response
to the criticisms which his book met, inserted here and there
statements that are in many ways in contradiction to the
statements in the first edition, and yet the earlier statements
have been allowed to stand for the most part.
The next example is also worthy of careful examination,
since it appears to prove too much : —
" 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 eliminating
something injurious from the sap : this is effected, for in-
stance, by glands at the base of the stipules in some Legu-
minosae, and at the backs of the leaves of the common laurel.
This juice, though small in quantity, is greedily sought by
insects ; but their visits do not in any way benefit the plant.
Now, let us suppose that the juice or nectar was excreted
from the inside of the flowers of a certain number of plants
of any species. Insects in seeking the nectar would get
dusted with pollen, and would often transport it from one
flower to another. The flowers of two distinct individuals of
the same species would thus get crossed; the act of crossing,
as can be fully proved, gives rise to vigorous seedlings,
which consequently would have the best chance of flourish-
Darwin s Artificial and Natural Selection 125
ing and surviving. The plants which produced 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 reader will notice that the sweet juice or nectar
secreted by certain plants is supposed to have first appeared
independently of the action of natural selection. Why then
account for its presence in flowers as the outcome of an
entirely different process ? If the nectar is eagerly sought
for by insects, without the plant benefiting in any way by
their visitations, why give a different explanation of its origin
in flowers where it is of benefit to the plant ?
Darwin carries his illustration further: "When our plant,
by the above process long continued, had been rendered
highly attractive to insects, they would unintentionally, on
their part, regularly carry pollen from flower to flower ; and
that they do this effectually, I could easily show by many
striking facts. I will give only one, as likewise illustrating one
step in the separation of the sexes of plants. . . . As soon
as the plant had been rendered so highly attractive to insects
that pollen was regularly carried from flower to flower, another
process might commence. No naturalist doubts the advan-
tage of what has been called the ' physiological division of
labour ' ; hence we may believe that it would be advantageous
to a plant to produce stamens alone in one flower or on one
whole plant, and pistils alone in another flower or on another
plant. In plants under culture and placed under new con-
ditions of life, sometimes the male organs and sometimes the
female organs become more or less impotent ; now if we
suppose this to occur in ever so slight a degree under
nature, then, as pollen is already carried regularly from
flower to flower, and as a more complete separation of the
sexes of our plant would be advantageous on the principle
of the division of labour, individuals with this tendency
126 Evolution and Adaptation
more and more increased would be continually favoured or
selected, until at last a complete separation of the sexes
might be effected. It would take up too much space to
show the various steps, through dimorphism and other
means, by which the separation of the sexes in plants of
various kinds is apparently now in progress ; but I may add
that some of the species of holly in North America are,
according to Asa Gray, in an exactly intermediate con-
dition, or, as he expresses it, are more or less diceciously
polygamous."
From this it will be seen that Darwin supposes that the
separation of the sexes in some of the higher plants has been
brought about by natural selection. Despite the supposed
advantage of the so-called "division of labor," one may, I
venture to suggest, be sceptical as to whether the separation
of the sexes can be explained in this way. The whole case is
largely supposititious, since in most of the higher hermaphro-
ditic plants and in nearly all hermaphroditic animals the
sexual products ripen at different times in the same indi-
vidual. Hence there is no basis for the assumption that
unless the sexes are separated there will be self-fertilization.
Shall we assume that this difference in time of ripening
of the two kinds of sex-cells is also the outcome of natural
selection, and that there has existed an earlier stage in all
animals and plants, that now have different times for the
ripening of their sexual elements, a time when these products
ripened simultaneously ? I doubt if even a Darwinian would
give such loose rein to his fancy.
But this is not yet the whole story that Darwin has made
out in this connection, for he continues : —
" Let us now turn to the nectar-feeding insects ; we may
suppose the plant, of which we have been slowly increasing
the nectar by continued selection, to be a common plant ; and
that certain insects depended in main part on its nectar for
food. I could give many facts showing how anxious bees
Darwin s Artificial and Natural Selection 127
are to save time : for instance, their habit of cutting holes
and sucking the nectar at the bases of certain flowers, which
with a very little more trouble, they can enter by the mouth.
Bearing such facts in mind, it may be believed that under
certain circumstances individual differences in the curvature
or length of the proboscis, etc., too slight to be appreciated
by us, might profit a bee or other insect, so that certain indi-
viduals would be able to obtain their food more quickly than
others ; arid thus the communities to which they belonged
would flourish and throw off many swarms inheriting the
same peculiarities."
Aside from the general criticism that will suggest itself
here also, it should be pointed out that even if " certain indi-
viduals " of the bees had slightly longer proboscides, this
would, in the case of the hive-bees at least, be of no avail,
since they do not reproduce, and hence leave no descendants
with longer mouth-parts. Of course, it may be replied that
those colonies in which the queens produce more of the long-
proboscis kind of worker would have an advantage over other
colonies not having so many individuals of this sort. It
would then be a competition of one colony with another, as
Darwin supposes to take place in colonial forms. But whether
slight differences of this sort would lead to the elimination
of the least well-endowed colonies is entirely a matter of
speculation. Since there are flowers with corolla-tubes of
all lengths, we can readily suppose that if one kind of flower
excluded individuals of certain colonies, they would search
elsewhere for their nectar rather than perish. While differ-
ent races might arise in this way, the process would not be
the survival of the fittest, but a process of adaptation to a new
environment.
We come now to a topic on which Darwin lays much
stress : the divergence of character. He tries to show how
the " lesser differences between the varieties become aug-
mented into the greater differences between species."
128 Evolution and Adaptation
" Mere chance, as we may call it, might cause one variety
to differ in some character from its parents, and the off-
spring 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."
Then, after pointing out that under domestication two
different races, the race-horse and the dray-horse, for in-
stance, might arise by selecting different sorts of variations,
Darwin inquires : —
" 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 descen-
dants 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."
Here we touch on one of the fundamental principles of the
doctrine of evolution. It is intimated that the new form of
animal or plant first appears (without regard to any kind of
selection), and then finds that place in nature where it can
remain in existence and propagate its kind. Darwin refers
here, of course, only to the less extensive variations, the in-
dividual or fluctuating kind ; but as we shall discuss at greater
length in another place, this same process, if extended to
other kinds of variation, may give us an explanation of evolu-
tion without competition, or selection, or destruction of the
individuals of the same kind taking place at all.
CHAPTER V
THE THEORY OF NATURAL SELECTION {Continued)
Objections to the Theory of Natural Selection
Although in the preceding chapter a number of criticisms
have been made of the special parts of the theory of natural
selection, there still remain to be considered some further
objections that have been made since the first publication of
the theory. It is a fortunate circumstance from every point
of view that Darwin himself was able in the later editions of
the " Origin of Species " to reply to those criticisms that he
thought of sufficient importance. He says: —
" 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."
The first difficulty is this : " 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 ? "
The answer that Darwin gives is, that by competition the
new form will crowd out its own less-improved parent form,
and other less-favored forms. But is this a sufficient or satis-
factory answer? If we recall what Darwin has said on the
advantage that those forms will have, in which a great num-
ber of new variations appear to fit them to the great diversity
k 129
130 Evolution and Adaptation
of natural conditions, and if we recall the gradations that exist
in external conditions, I think we shall find that Darwin's
reply fails to give a satisfactory answer to the question.
It is well known, and Darwin himself has commented on
it, that the same species often remains constant under very
diverse external conditions, both inorganic and organic.
Hence I think the explanation fails, in so far as it is
based on the accumulation by selection of small individual
variations that are supposed to give the individuals some
slight advantage under each set of external conditions.
Darwin admits that " this difficulty for a long time quite
confounded me. But I think it can be in large part ex-
plained." The first explanation that is offered is that areas
now continuous may not have been so in the past. This
may be true in places, but the great continents have had
continuous areas for a long time, and Darwin frankly ac-
knowledges that he " will pass over this way of explaining
the difficulty." The second attempt is based on the sup-
posed narrowness of the area, where two species, descended
from a common parent, overlap. In this region the change
is often very abrupt, and Darwin adds : —
" 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 grad-
uate away insensibly. But when we bear in mind that almost
every species, even in its metropolis, would increase immensely
in numbers, were it not for other competing species ; that nearly
all either prey on or serve as prey for others ; in short, that
each organic being is either directly or 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 ex-
clusively depends on insensibly changing physical conditions,
but in a large part on the presence of other species, on which
it lives, or by which it is destroyed, or with which it comes
into competition ; and as these species are already defined ob-
Darwin s Artificial and Natural Selection 131
jects, not blending one into another by insensible gradations,
the range of any one species, depending as it does on the
range of others, will tend to be sharply defined."
Here we have a petitio principii. The sharp definition of
species, that we started out to account for, is explained by
the sharp definition of other species !
A third part of the explanation is that, owing to the rela-
tive fewness of individuals at the confines of the range dur-
ing the fluctuations of their enemies, or of their prey, or in
the nature of the seasons, they would be extremely liable to
utter extermination. If this were really the case, then new
species themselves which, on the theory, are at first few in
numbers ought to be exterminated. On the whole, then, it
does not appear that Darwin has been very successful in his
attempt to meet this objection to the theory.
Darwin tries to meet the objection, that organs of extreme
perfection and complication cannot be accounted for by nat-
ural selection, as follows : —
" To suppose that the eye with all its inimitable contriv-
ances for adjusting the focus to different distances, for ad-
mitting 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."
The following sketch that Darwin gives to show how he
imagined the vertebrate eye to have been formed is very
instructive, as illustrating how he supposed that natural se-
lection acts : —
" 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 sensi-
tive to light beneath, and then suppose every part of this
layer to be continually changing slowly in density, so as to
separate into layers of different densities and thicknesses,
placed at different distances from each other, and with the
132 Evolution and Adaptation
surfaces of each layer slowly changing in form. Further we
must suppose that there is a power, represented by natural
selection or the survival of the fittest, always intently watch-
ing each slight alteration in the transparent layers ; and care-
fully preserving each which, under varied circumstances, in
any way or in any degree, tends to produce a distincter
image. We must suppose each new state of the instru-
ment to be multiplied by the million ; each to be preserved
until a better one is produced, and then the old ones to be
all destroyed. In living bodies, variation will cause the
slight alterations, generation will multiply them almost infi-
nitely, 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."
We may conclude in Darwin's own words : —
" To arrive, however, at a just conclusion regarding the
formation of the eye, with all its marvellous yet not abso-
lutely 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."
The electric organs, present in several fish, offer a case
of special difficulty to the selection theory. When well
developed, as in the Torpedo and in Gymnotus, it is conceiv-
able that it may serve as an organ of defence, but in other
forms the shock is so weak that it is not to be supposed that
it can have any such function. Romanes, who in many ways
was one of the stanchest followers of Darwin, admits that, so
far as he can see, the evolution of the electric organs cannot
be explained by the selection theory. Darwin offers no
explanation, but bases his defence on the grounds that we do
not know of what use this organ can be to the animal.
Darwin's Artificial and Natural Selection 133
Darwin also refers to the phosphorescent, or luminous,
organs as a supposed case of difficulty for his theory.
" The luminous organs which occur in a few insects, be-
longing 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."
In this case also, as in that of the electric organs, the
structures appear in entirely different parts of the body of
the insect in different species, so that their occurrence in this
group cannot be accounted for on a common descent. In
whatever way they have arisen, they must have evolved in-
dependently in different species. Darwin advances no ex-
planation of the origin of the luminous organs, but states
that they "offer under our present state of "ignorance a diffi-
culty almost exactly parallel with that of the electric organs."
It will be noticed that the difficulty referred to rests on the
assumption that since the organs are well developed they
must have some important use !
We may next consider "organs of little apparent impor-
tance as affected by natural selection." Darwin says : —
"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 impor-
tance ; almost as great, though of a very different kind, as in
the case of the most perfect and complex organs." -
His answers to this difficulty are : (1) we are too ignorant
" in regard to the whole economy of any one organic being to
say what slight modifications would be of importance or not,"
— thus such apparently trifling characters as the down on fruit,
or the colors of the skin and hair of quadrupeds, which from
being correlated with constitutional differences or from de-
termining the attacks of insects might be acted on by nat-
ural selection ; (2) organs now of trifling importance have in
134 Evolution and Adaptation
some cases been of high importance to an early progenitor ;
(3) the changed conditions of life may account for some of
the useless organs ; (4) reversion accounts for others; (5) the
complex laws of growth account for still others, such as
correlation, compensation of the pressure of one part on
another, etc. ; (6) the action of sexual selection is responsible
for many characters not to be explained by natural selection.
Admitting that there may be cases that can be accounted for
on one or the other of these six possibilities, yet there can be
no doubt that there are still a considerable number of specific
characters that cannot be explained in any of these ways.
I do not think that Darwin has by any means met this
objection, even if all these six possibilities be admitted as
generally valid.
Amongst the '"miscellaneous objections" to his theory that
Darwin considers we may select the most important cases.
The following paragraph has been sometimes quoted by later
writers to show that Darwin saw, to a certain extent, the
insufficiency of fluctuating variations as a basis for selection.
What he calls here " spontaneous variability " refers to sudden
and extensive variations, or what we may call discontinuous
variations. " In the earlier editions of this work I under-
rated, as it now seems probable, the frequency and importance
of modifications due to spontaneous variability. But it is
impossible to attribute to this cause the innumerable struc-
tures which are so well adapted to the habits of life of each
species. I can no more believe in this, that the well-adapted
form of a race-horse or greyhound, which before the principle
of selection by man was well understood, excited so much
surprise in the minds of the older naturalists, can thus be
explained."
Darwin appears to mean by the latter part of this state-
ment, that he cannot believe that such sudden and great
variations as have caused a peach tree to produce nectarines
can account for the wonderful adaptations of organisms ; but
Darwin s Artificial and Natural Selection 135
it is not really necessary to suppose that this would often
occur, for the same result could be reached by several stages,
even if the discontinuous variations had been small, and had
appeared in many individuals simultaneously. After showing
that in a number of flowers, especially of the Compositae and
Umbelliferae, the individual flowers in the closely crowded
heads are sometimes formed on a different type, Darwin con-
cludes : " 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 interaction 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."
Further on we meet with the following remarkable state-
ment : " 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 can-
not have been of much importance to the greater number of
mammals, birds, or reptiles, whether they were clothed with
hair, feathers, or scales ; yet hair has been transmitted to
almost all mammals, feathers to all birds, and scales to all
true reptiles. A structure, whatever it may be, which is
common to many allied forms, is ranked by us as of high
systematic importance, and consequently is often assumed to
be of high vital importance to the species. Thus, as I am
inclined to believe, morphological differences, which we con-
sider 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
136 Evolution and Adaptation
variations, which sooner or later became constant through the
nature of the organism and of the surrounding conditions, as
well as through the intercrossing of distinct individuals, but
not through natural selection ; for as these morphological
characters do not affect the welfare of the species, any slight
deviations in them could not have been governed or accumu-
lated through this latter agency. It is a strange result which
we thus arrive at, namely, that characters of slight vital im-
portance to the species are the most important to the system-
atist ; but, as we shall hereafter see when we treat of the
genetic principle of classification, this is by no means so para-
doxical as it may at first appear."
If all this be granted, it is once more evident that the only
variations that come under the action of selection are the
limited number that are of vital importance to the organism.
How little the theory of natural selection can be used to
explain the origin of species will be apparent from the above
quotation. This is, of course, not an argument against the
theory itself, which would still be one of vast importance if it
explained adaptive characters alone ; but enough has been
said, I think, to show that it is improbable that the origin of
adaptive and non-adaptive characters are to be explained by
entirely different principles.
In reply to a criticism of Mivart, Darwin makes the
further admission as to the insufficiency of the theory of
natural selection : " 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 Domes-
tication' at greater length than, as I believe, any other
writer. He likewise often assumes that I attribute nothing
to variation, independent of natural selection, whereas in the
work just referred to I have collected a greater number of
well-established cases than is to be found in any other work
known to me." If this is admitted, and if it can be shown
Darwin's Artificial and Natural Selection 137
that the evidence in favor of the inheritance of acquired
characters is very doubtful at best, may we not conclude
that Mivart's criticisms have sometimes hit the mark ?
The following objection appears to be a veritable stum-
bling-block to the theory. Flatfishes and soles lie on one
side, and do not stand in a vertical position as do other fish.
Some species lie on one side and some on the other, and
some species contain both right-sided and left-sided indi-
viduals. In connection with this unusual habit we find a
striking change in the structure. The eye that would be on
the under side has shifted, so that it has come to lie on the
upper side of the head, i.e. both eyes lie on the same side, —
a condition found in no other vertebrate. As a result of the
shifting of the eye, the bones of the skull have also become
profoundly modified. The young fish that emerge from the
egg swim at first upright, as do ordinary fish, and only after
they have led a free existence for some time do they turn
to one side and sink to the bottom. Unless the under eye
moved to the upper side it would be of no use to the flatfish,
and might even be a source of injury. Mivart points out
that a sudden, spontaneous transformation in the position of
eye is hardly conceivable, and to this Darwin, of course,
assents. Mivart adds : " If the transit was gradual, then
how such transit of one eye a minute fraction of the journey
towards the other side of the head could benefit the indi-
vidual is, indeed, far from clear. It seems even that such an
incipient transformation must rather have been injurious."
Darwin's reply is characteristic : —
"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
endeavoring to look upwards with both eyes, whilst resting
on one side at the bottom. We may also attribute to the
inherited effects of use the fact of the mouth in several kinds
1 38 Evolution and Adaptation
of flatfish being bent towards the lower surface, with the jaw-
bones stronger and more effective on this, the eyeless side of
the head, than on the other side, for the sake, as Dr. Traquair
supposes, of feeding with ease on the ground. Disuse, on
the other hand, will account for the less developed condition
of the whole inferior half of the body, including the lateral
fins ; though Yarrell thinks that the reduced size of these fins
is advantageous to the fish, as 'there is so much less room
for their action, than with the larger fins above.' Perhaps
the lesser number of teeth in the proportion of four to seven
in the upper halves of the two jaws of the plaice, to twenty-
five to thirty in the lower halves, may likewise be accounted
for by disuse. From the colorless state of the ventral sur-
face of most fishes and of many other animals, we may
reasonably suppose that the absence of color in flatfish on
the side, whether it be the right or left, which is undermost,
is due to the exclusion of light."
By falling back on the theory of inheritance of acquired
characters Darwin tacitly admits the incompetence of natural
selection to explain the evolution of the flatfish. If the latter
theory prove incorrect, it must then be admitted that the evo-
lution of the flatfishes cannot be accounted for by either of
the two main theories on which Darwin relies.
Mivart further points out that the beginning stages of the
mammary glands cannot be explained by Darwin's theory. To
which Darwin replies, that an American naturalist, Mr. Lock-
wood, believes from what he has seen of the development of
the young of the pipe-fish (Hippocampus) that "they are nour-
ished by a secretion from the cutaneous glands of the sac" in
which the young are enclosed. This can scarcely be said to be
a satisfactory reply ; for, if it is true that this is the case for
the pipe-fish, — and I cannot find on inquiry that this state-
ment has been confirmed, — it is still rather speculative to
suppose that the ancestral mammals nourished their young by
secreting a fluid into the marsupial sac around the embryos.
Darwin s Artificial and Natural Selection 139
Darwin deals with instincts of animals in the same way as he
deals with their structures. After pointing out that instincts
are variable, and that the variations are hereditary, he pro-
ceeds to show how selection may act by picking out those
individuals possessing the more favorable instincts. In other
words, the theory of natural selection is applied to functions,
as well as to structure. Darwin makes use here also of the
Lamarckian factor of inheritance, and concludes that "in
most cases habit and selection have probably both occurred."
A few examples will sufficiently serve to illustrate Darwin's
meaning. The first case given is that of the cuckoo, which
lays its eggs in the nests of other birds, where they are
hatched and the young reared by their foster-parents. The
starting-point for such a perversion of the ordinary habits of
birds is to be found, he thinks, in the occasional deposi-
tion of eggs in the nests of other birds, which has at times
been observed for a number of species. For instance, this
has been seen in the American cuckoo, which ordinarily builds
a nest of its own. It is recorded and believed to be true
that the young English cuckoo, when only two or three days
old, ejects from the nest the offspring of its foster-parents,
and this " strange and odious instinct " is supposed by Darwin
to have been acquired in order that the young cuckoo might get
more food, and that the young bird has acquired during succes-
sive generations the strength and structure necessary for the
work of ejection. This is of course largely speculative, and
it is by no means obvious that it was a greater benefit to the
cuckoo to have other birds rear its young than to do so itself.
We can equally well imagine, since this is the turn the argu-
ment takes, that the occasional instinct to deposit eggs in the
nests of other birds would be disadvantageous, and could not
have been acquired by the selection of a fluctuating instinct
of this sort. We have no right to assume, that because a
new habit has been acquired, that it is a more advantageous
one than the one that has been lost. All that we can legit-
140 Evolution and Adaptation
imately infer is, that, although the normal instinct has been
changed into another, the race has still been able to remain
in existence. The same conclusion applies to the case of
MolotJirus bonariensis, cited by Darwin, and is here even more
obvious : —
" 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 MolotJirus 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 Molothrus, the M. bonariensis,
are much more highly developed than those of the last, but
are still far from perfect. This bird, as far as is known,
invariably lays its eggs in the nest of strangers ; but it is
remarkable that several together sometimes commence to
build an irregular untidy nest of their own, placed in singu-
larly 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."
Darwin s Artificial and Natural Selection 141
Can we possibly be expected to believe that it has been to
the advantage of this species to give up its original regular
method of incubating its own eggs, and acquire such a
haphazard, new method ? Does not the explanation prove
too much, rather than give support to Darwin's hypothesis ?
Is it not better to conclude, that despite the disadvantages
entailed by a change in the original instincts, the species
is still able to remain in existence ?
Darwin points out, in the case of the slave-making ants,
that the slave-making instinct may have arisen in the first
instance by ants carrying pupae, that they have captured,
into their own nests. Later this habit might become fixed,
and, finally, after passing through several stages of develop-
ment, the ants might become absolutely dependent on their
slaves. It is also supposed that those colonies in which this
instinct was better developed would survive in competition
with other colonies of the same species on account of the
supposed advantage of owning slaves. In this way natural
selection steps in and perfects the process.
It is far from proven, or even made probable, that a species
of ant that becomes gradually dependent on its slaves is
more likely to survive than other colonies that are not
so dependent. All we can be certain of is that with slaves
they have still been able to maintain their own. Moreover,
we must not forget that it is not enough to show that a
particular habit might be useful to a species, but it should
also be shown that it is of sufficient importance, at every
stage of its evolution, to give a decisive advantage in the
" struggle for existence." For unless a life and death
struggle takes place between the different colonies, natural
selection is powerless to bring about its supposed results.
And who will be bold enough to affirm that the presence of
slaves in a nest will give victory to that colony in competi-
tion with its neighbors ? Has the history of mankind taught
us that the slave-making countries have exterminated the
142 Evolution and Adaptation
countries without slaves ? Is the question so simple as this ?
May not the degeneration of the masters more than compen-
sate for the acquirement of slaves, and may not the loss
of life in obtaining slaves more than counterbalance the ad-
vantage of the slaves after they are captured? In the face
of these possibilities it is not surprising to find that Darwin,
when summing up the chapter, makes the following admis-
sion : " I do not pretend that the facts in this chapter
strengthen in any degree my theory ; but none of the cases
of difficulty, to the best of my judgment, annihilate it."
Darwin, with his usual frankness, adds : —
" No doubt many instincts of very difficult explanation
could be opposed to the theory of natural selection, — cases,
in which we cannot see how an instinct could have originated ;
cases, in which no intermediate gradations are known to
exist ; cases of instincts of such trifling importance, that they
could hardly have been acted on by natural selection ; cases
of instincts almost identically the same in animals so remote
in the scale of nature, that we cannot account for their
similarity by inheritance from a common progenitor, and
consequently must believe that they were independently
acquired through natural selection. I will not here enter on
these several cases, but will confine myself to one special
difficulty, which at first appeared to me insuperable, and
actually fatal to the whole theory. I allude to the neuters or
sterile females in insect communities ; for these neuters often
differ widely in instinct and in 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
Darwins Artificial and Natural Selection 143
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 difficulty. The great
difficulty lies in the working ants differing widely from both
the males and the fertile females in structure, as in the
shape of the thorax, and in being destitute of wings and
sometimes of eyes, and in instinct. As far as instinct alone
is concerned, the wonderful difference in this respect between
the workers and the perfect females, would have been better
exemplified by the hive-bee. If a working ant or other neuter
insect had been an ordinary animal, I should have unhesitat-
ingly assumed that all its characters had been slowly ac-
quired through natural selection ; namely, by individuals
having been born with slight profitable modifications, which
were inherited by the offspring ; and that these again varied
and again were selected, and so onwards. But with the
working ant we have an insect differing greatly from its
parents, yet absolutely sterile ; so that it could never have
transmitted successively acquired modifications of structure
or instinct to its progeny. It may well be asked, how is
it possible to reconcile this case with the theory of natural
selection ? "
Darwin's answer is that the differences of structure are
correlated with certain ages and with the two sexes, but this
is obviously only shifting the difficulty, not meeting it. He
concludes, " I can see no great difficulty in any character
becoming correlated with the sterile condition of certain
members of the insect communities, the difficulty lies in
understanding how such correlated modifications of structure
could have been slowly accumulated by natural selection."
" This difficulty, though appearing insuperable, is lessened, or,
as I believe, disappears, when it is remembered that selection
144 Evolution and Adaptation
may be applied to the family, as well as to the individual,
and may thus give the desired end."
Darwin did not fail to see that there is a further difficulty
even greater than the one just mentioned. He says: "But
we have not as yet touched on the acme of the difficulty ;
namely, the fact that the neuters of several ants differ, not
only from the fertile females and males, but from each other,
sometimes to an almost incredible degree, and are thus di-
vided 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 dif-
ferent : in Cryptocerus, the workers of one caste alone carry
a wonderful sort of shield on their heads, the use of which
is quite unknown : in the Mexican 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, supply-
ing the place of that excreted by the aphides, or the domes-
tic cattle as they may be called, which our European ants
guard and imprison."
"It will indeed be thought that I have an overweening con-
fidence in the principle of natural selection, when I do not
admit that such wonderful and well-established facts at once
annihilate the theory. In the simpler case of neuter insects
all of one caste, which, as I believe, have been rendered
different from the fertile males and females through natural
selection, we may conclude from the analogy of ordinary
variations, that the successive, slight, profitable 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 advan-
tageous modification, all the neuters ultimately came to be
Darwin s Artificial and Natural Selection 145
thus characterized. According to this view we ought occa-
sionally to find in the same nest neuter insects, presenting
gradations of structure ; and this we do find, even not rarely,
considering how few neuter insects out of Europe have been
carefully examined."
From this the conclusion is reached : —
" 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 dif-
ferent jaws; or lastly, and this is the greatest difficulty, one
set of workers of one size and structure, and simultaneously
another set of workers of a different size and structure ; — a
graduated series having first been formed, as in the case of
the driver ant, and then the extreme forms having been pro-
duced in greater and greater numbers, through the survival
of the parents which generated them, until none with an
intermediate structure were produced.
" I have now explained how, as I believe, the wonderful
fact of two distinctly defined castes of sterile workers exist-
ing in the same nest, both widely different from each other
and from their parents, has originated. We can see how
useful their production may have been to a social community
of ants, on the same principle that the division of labor is
useful to civilized man. Ants, however, work by inherited
instincts and by inherited organs or tools, whilst man works
by acquired knowledge and manufactured instruments. But
I must confess, that, with all my faith in natural selection, I
should never have anticipated that this principle could have
been efficient in so high a degree, had not the case of these
neuter insects led me to this conclusion. I have, therefore,
discussed this case, at some little but wholly insufficient
length, in order to show the power of natural selection, and
likewise because this is by far the most serious special diffi-
culty which my theory has encountered. The case, also, is
146 Evolution and Adaptation
very interesting, as it proves that with animals, as with
plants, any amount of modification may be effected by the
accumulation of numerous, slight, spontaneous variations,
which are in any way profitable, without exercise or habit
having been brought into play. For peculiar habits confined
to the workers or sterile females, however long they might
be followed, could not possibly affect the males and fertile
females, which alone leave descendants. I am surprised
that no one has hitherto advanced this demonstrative case
of neuter insects, against the well-known doctrine of inherited
habit, as advanced by Lamarck."
We may dissent at once from Darwin's statement which,
he thinks, " proves that any amount of modification may be
affected by the accumulation of numerous slight variations
which are in any way profitable without exercise or habit
having been brought into play"; we may dissent if for no
other reason than that this begs the whole point at issue, and
is not proven. It does not follow because in some colonies
all intermediate stages of neuters exist, that in other colo-
nies, where no such intermediate stages are present, these
have been slowly weeded out by natural selection, causing
to disappear all colonies slightly below the mark. It is this
that begs the question. Because we can imagine that
intermediate stages between the different castes may have
been present, it neither follows that such fluctuating varia-
tions have been the basis for the evolution of the more
sharply defined types, nor that the imagined advantage of
such a change would have led through competition to the
extermination of the other colonies. However much we
may admire the skill with which Darwin tried to meet this
difficulty, let us not put down the results to the good of the
theory, but rather repeat once more Darwin's own words at
the end of this chapter, to the effect that the facts do not
strengthen the theory.
Darwin s Artificial and Natural Selection 147
Sterility between Species
The care with which Darwin examined every bearing of
his theory is nowhere better exemplified than in his treat-
ment of the question of sterility between the individuals of
different species. It would be so obviously to the advantage
of the selection theory if it were true that sterility between
species had been acquired by selection in order to prevent
intercrossing, that it would have been easy for a less cautious
thinker to have fallen into the error of supposing that sterility
might have been acquired in this way. Tempting as such a
view appears, Darwin was not caught by the specious argu-
ment, as the opening sentence in the chapter of hybridism
shows : —
" 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 offspring, 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 dealing with this subject Darwin points out that we must
be careful to distinguish between "the sterility of species
when first crossed, and the sterility of hybrids produced from
them." In the former case, the reproductive organs of each
individual are in a perfectly normal condition, while hybrids
appear to be generally impotent owing to some imperfection
in the reproductive organs themselves. They are not perfectly
fertile, as a rule, either with each other, or with either of the
parent forms.
148 Evolution and Adaptation
In striking contrast to the sterility between species is the
fertility of varieties. If, as Darwin believes, varieties are
incipient species, we should certainly expect to find them
becoming less and less fertile with other fraternal varieties, or
with the parent forms in proportion as they become more
different. Yet experience appears to teach exactly the op-
posite ; but the question is not a simple one, and the results
are not so conclusive as appears at first sight. Let us first
see how Darwin met this obvious contradiction to his view.
In the first place, he points out that all species are not in-
fertile when crossed with other species. The sterility of
various species, when crossed, is so different in degree, and
graduates away so insensibly, and the fertility of pure species
is so easily affected by various circumstances, that it is most
difficult to say where perfect fertility ends and sterility be-
gins. " It can thus be shown that neither sterility nor fer-
tility afford any certain distinction between species and
varieties.'' Darwin cites several cases in plants in which
crosses between species have been successfully accomplished.
The following remarkable results are also recorded : " Indi-
vidual plants in certain species of Lobelia, Verbascum, and
Passiflora can easily be fertilized by pollen from a distinct
species, but not by pollen from the same plant, though this
pollen can be proved to be perfectly sound by fertilizing
other plants or species. In the genus Hippeastrum, in Co-
rydalis as shown by Professor Hildebrand, in various orchids
as shown by Mr. Scott and Fritz Muller, all the individuals
are in this peculiar condition. So that with some species,
certain abnormal individuals, and in other species all the
individuals, can actually be hybridized much more readily
than they can be fertilized by pollen from the same individual
plant!" 1
1 A somewhat parallel case has recently been discovered by Castle for the her-
maphroditic ascidian Ciona intestinalis. In this case the spermatozoa of any
individual fail to fertilize the eggs of the same individual, although they will fer-
tilize the eggs of any other individual.
Darwin's Artificial and Natural Selection 149
In regard to animals, Darwin concludes that " 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."
The most significant fact in this connection is that the
more widely different two species are, so that they are placed
in different families, so much the less probable is it that
cross-fertilization will produce any result. From this condi-
tion of infertility there may be traced a gradation between
less different forms of the same genus to almost complete,
or even complete, fertility between closely similar species.
Darwin further points out that: "The hybrids raised from
two species which are very difficult to cross, and which rarely
produce any offspring, are generally very sterile ; but the
parallelism between the difficulty of making a first cross, and
the sterility of the hybrids thus produced — two classes of
facts which are generally confounded together — is by no
means strict. There are many cases, in which two pure
species, as in the genus Verbascum, can be united with
unusual facility, and produce numerous hybrid offspring,
yet these hybrids are remarkably sterile. On the other
hand, there are species which can be crossed very rarely,
or with extreme difficulty, but the hybrids, when at last
produced, are very fertile. Even within the limits of the
same genus, for instance in Dianthus, these two opposite
cases occur."
In regard to reciprocal crosses Darwin makes the following
important statements : " The diversity of the result in re-
ciprocal crosses between the same two species was long ago
observed by Kol renter. To give an instance : Mirabilis
jalapa can easily be fertilized by the pollen of AT. longiflora,
and the hybrids thus produced are sufficiently fertile ; but
Kolreuter tried more than two hundred times, during eight
150 Evolution and Adaptation
following years, to fertilize reciprocally M. longiflora with the
pollen of M. jalapa, and utterly failed."
A formal interpretation of this difference can be easily
imagined. The infertility in one direction may be due to
some physical difficulty met with in penetrating the stigma,
or style. For instance, the tissue in one species may be too
compact, or the style too long. Pfliiger, who carried out a
large number of experiments by cross-fertilizing different
species of frogs, reached the conclusion that the spermatozoa
having small and pointed heads could cross-fertilize more
kinds of eggs, than could the spermatozoa with large blunt
heads. This is probably due to the ability of the smaller
spermatozoa to penetrate the jelly around the eggs, or the
pores in the surface of the egg itself. But there are also
other sides to this question, as recent results have shown, for,
even if a foreign spermatozoon can enter an egg, it does not
follow that the development of the egg will take place.
Here the difficulty is due to some obscure processes in the
egg itself. Now that we know more of the nicely balanced
combinations that take place during fertilization of the egg,
and during the process of cell division, we can easily see that
if the processes were in the least different in the two species
it might be impossible to combine them in a single act.
" 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 ? "
" 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
Darwin s Artificial and Natural Selection 151
crosses between the same two species, the male sexual ele-
ment of the one will often freely act on the female sexual
element of the other, but not in a reversed direction."
Does Darwin give here a satisfactory answer to the diffi-
culty that he started out to explain away ? On the whole,
the reader will admit, I think, that he has fairly met the sit-
uation, in so far as he has shown that there is no absolute
line of demarcation between the power of intercrossing of
varieties and races, and of species. It is also extremely im-
portant to have found that the difficulties increase, so to speak,
even beyond the limits of the species ; since species, belonging
to different genera, are as a rule more difficult to intercross
than when they belong to the same genus. The further
question, as to whether there are differences in respect to the
power of intercrossing between different kinds of varieties,
such as those dependent on selection of fluctuating varia-
tions, of local conditions, of mutations, etc., is far from being
settled at the present time.
That this property of species is useful to them, in the some-
what unusual sense that it keeps them from freely mingling
with other species, is true ; but, as has been said, this would be
a rather peculiar kind of adaptation. If, however, it be
claimed that this property is useful to species, as Darwin
himself claims, then, as he also points out, it is a useful
acquirement that cannot have arisen through natural selec-
tion. It is not difficult to show why this must be so. If two
varieties were to some extent at the start less fertile, inter se,
than with their own kind, the only way in which they could
become more infertile through selection would be by selecting
those individuals in each generation that are still more infer-
tile, but the forms of this sort would, ex hypothese, become
less numerous than the descendants of each species itself,
which would, therefore, supplant the less fertile ones.
Darwin's own statement in regard to this point is as fol-
lows : —
152 Evolution and Adaptation
" 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 in-
cipient 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 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 abso-
lute 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 offspring
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 selection to that
high degree which is common with so many species, and
which is universal with species which have been differentiated
to a generic or family rank, will find the subject extraordi-
narily 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, pro-
duced 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 infer-
tility, 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 inces-
Darwin s Artificial and Natural Selection 153
santly occurred with many species, for a multitude are mutu-
ally quite barren."
Darwin points out the interesting parallel existing between
the results of intercrossing, and those of grafting together
parts of different species.
"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 differences in the laws of growth of the two plants. We
can sometimes see the reason why one tree will not take on
another, from differences in their rate of growth, in the
hardness of their wood, in the period of the flow or nature
of their sap, etc. ; but in a multitude of cases we can assign
no reason whatever. Great diversity in the size of two
plants, one being woody and the other herbaceous, one
being evergreen and the other deciduous, and adapted to
widely different climates, do not always prevent the two
grafting together. As in hybridization, so with grafting,
the capacity is limited by systematic affinity, for no one has
been able to graft together trees belonging to quite distinct
families ; and, on the other hand, closely allied species, and
varieties of the same species, can usually, but not invariably,
be grafted with ease. But this capacity, as in 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 distant
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 varieties
of the apricot and peach on certain varieties of the plum."
" We thus see, that although there is a clear and great
difference between the mere adhesion of grafted stocks, and
154 Evolution and Adaptation
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 of distinct species.
And we must look at the curious and complex laws govern-
ing the facility with which trees can be grafted on each other
as incidental on unknown differences in their vegetative sys-
tems, so I believe that the still more complex laws governing
the facility of first crosses are incidental on unknown dif-
ferences in their reproductive systems. . . . The facts by
no means seem to indicate that the greater or lesser difficulty
of either grafting or crossing various species has been a
special endowment ; although in the case of crossing, the
difficulty is as important for the endurance and stability
of specific forms, as in the case of grafting it is unimpor-
tant for their welfare."
Weismann's Germinal Selection
We cannot do better, in bringing this long criticism of the
Darwinian theory to an end, than by considering the way in
which Weismann has attempted in his paper on " Germinal
Selection " to solve one of the "patent contradictions " of the
selection theory. He calls attention, in doing so, to what he
regards as a vital weakness of the theory in the form in
which it was left by Darwin himself. Weismann says : —
"The basal idea of the essay — the existence of Germinal
Selection — was propounded by me some time since, 1 but it is
here for the first time fully set forth and tentatively shown to
be the necessary complement of the process of selection.
Knowing this factor, we remove, it seems to me, the patent
contradiction of the assumption that the general fitness of
organisms, or the adaptations necessary to their existence, are
produced by accidental variations — a contradiction which
1 Neue Gedanken zur Vererbungsfrage, eine Antwort an Herbert Spencer,
Jena, 1895.
Darwiri s Artificial and Natural Selection 155
formed a serious stumbling-block to the theory of selection.
Though still assuming that the primary variations are ' acci-
dental,' I yet hope to have demonstrated that an interior
mechanism exists which compels them to go on increasing in
a definite direction, the moment selection intervenes. Defi-
nitely directed variation exists, but not predestined variation,
running on independently of the life conditions of the organ-
ism, as Nageli, to mention the most extreme advocate of this
doctrine, has assumed ; on the contrary, the variation is such
as is elicited and controlled by those corfditions themselves,
though indirectly."
" The real aim of the present essay is to rehabilitate the
principle of selection. If I should succeed in reinstating this
principle in its emperilled rights, it would be a source of
extreme satisfaction to me ; for I am so thoroughly convinced
of its indispensability as to believe that its demolition would
be synonymous with the renunciation of all inquiry concern-
ing the causal relation of vital phenomena. If we could un-
derstand the adaptations of nature, whose number is infinite,
only upon the assumption of a teleological principle, then, I
think, there would be little inducement to trouble ourselves
about the causal connection of the stages of ontogenesis, for
no good reason would exist for excluding teleological princi-
ples from this field. Their introduction, however, is the ruin
of science." 1
Weismann states that those critics who maintain that
selection cannot create, but only reject, "fail to see that pre-
cisely through this rejection its creative efficacy is asserted."
There is raised here, though not for the first time, a point
that is of no small importance for both Darwinians and anti-
Darwinians to consider ; for, without further examination, it
is by no meanc self-evident, as Weismann implies, that by
exterminating all variations that are below the average the
1 Translated by J. McCormack. The Open Court Publishing Company. The
following quotations are also taken from this translation.
156 Evolution and Adaptation
standard of successive generations could ever be raised be-
yond the most extreme fluctuating variation. At least this
appears to be the case if individual, fluctuating variations be
the sort selected, and it is to this kind of variation to which
Weismann presumably refers. Without discussing this point
here, let us examine further what Weismann has to say. He
thinks that while in each form there may be a very large
number of possible variations, yet there are also impossible
variations as well, which do not appear. " The cogency, the
irresistible cogency as I take it, of the principle of selection
is precisely its capacity of explaining why fit structures al-
ways arise, and this certainly is the great problem of life."
Weismann points out that it is a remarkable fact that to-day,
after science has been in possession of this principle for
something over thirty years, " during which time she has bus-
ily occupied herself with its scope, the estimation in which
the theory is held should be on the decline." " It would be
easy to enumerate a long list of living writers who assign to
it a subordinate part only in evolution, or none at all."
" Even Huxley implicitly, yet distinctly, intimated a doubt
regarding the principle of selection when he said : ' Even if
the Darwinian hypothesis were swept away, evolution would
still stand where it is.' Therefore he, too, regarded it as
not impossible that this hypothesis should disappear from
among the great explanatory principles by which we seek
to approach nearer to the secrets of nature."
Weismann is not, however, of this opinion, and believes
that the present depression is only transient, because it is only
a reaction against a theory that had been exalted to the
highest pinnacle. He thinks that the principle of selection
is not overestimated, but that naturalists imagined too quickly
that they understood its workings. " On the contrary, the
deeper they penetrated into its workings the clearer it ap-
peared that something was lacking, that the action of the
principle, though upon the whole clear and representable, yet
Darwiiis Artificial and Natural Selection 157
when carefully looked into encountered numerous difficulties,
which were formidable, for the reason that we were unsuc-
cessful in tracing out the actual details of the individual pro-
cess, and, therefore, in fixing the phenomenon as it actually
occurred. We can state in no single case how great a varia-
tion must be to have selective value, nor how frequently it
must occur to acquire stability. We do not know when and
whether a desired useful variation really occurs, nor on what
its appearance depends ; and we have no means of ascertain-
ing the space of time required for the fulfilment of the selec-
tive processes of nature, and hence cannot calculate the
exact number of such processes that do and can take place at
the same time in the same species. Yet all this is necessary
if we wish to follow out the precise details of a given case.
" But perhaps the most discouraging circumstance of all
is, that we can assert in scarcely a single actual instance in
nature whether an observed variation is useful or not — a
drawback that I distinctly emphasized some time ago. Nor
is there much hope of betterment in this respect, for think
how impossible it would be for us to observe all the individ-
uals of a species in all their acts of life, be their habitat ever
so limited — and to observe all this with a precision enabling
us to say that this or that variation possessed selective value,
that is, was a decisive factor in determining the existence of
the species."
" And thus it is everywhere. Even in the most indubitable
cases of adaptation as, for instance, in that of the striking pro-
tective coloring of many butterflies, the sole ground of infer-
ence that the species on the whole is adequately adapted to
its conditions of life, is the simple fact that the species is,
to all appearances, preserved undiminished, but the inference
is not at all permissible that just this protective coloring has
selective value for the species, that is, if it were lacking,
the species would necessarily have perished."
Few opponents of Darwinism could give a more pessimist'' ■•
158 Evolution and Adaptation
account of the accomplishments of the theory of natural se-
lection than this, by one of the leaders of the modern school :
" Discouraging, therefore, as it may be that the control of
nature in her minutest details is here gainsaid us, yet it were
equivalent to sacrificing the gold to the dross, if simply from
our inability to follow out the details of the individual case
we should renounce altogether the principle of selection, or
should proclaim it as only subsidiary, on the ground that we
believe the protective coloring of the butterfly is not a pro-
tective coloring, but a combination of colors inevitably result-
ing from internal causes. The protective coloring remains a
protective coloring whether at the time in question it is or is
not necessary for the species ; and it arose as protective col-
oring — arose not because it was a constitutional necessity of
the animal's organism that here a red and there a white,
black, or yellow spot should be produced, but because it was
advantageous, because it was necessary for the animal.
There is only one explanation possible for such patent adap-
tations, and that is selection. What is more, no other natural
way of their originating is conceivable, for we have no right
to assume teleological forces in the domain of natural phe-
nomena."
Weismann states that he does not accept Eimers's view that
the markings of the wings of the butterflies of the genus
Papilio are due to a process of evolution in a direct line, in-
dependent of external causes.
"On the contrary, I believe it can be clearly proved that
the wing of the butterfly is a tablet on which Nature has in-
scribed everything she has deemed advantageous to the pres-
ervation and welfare of her creatures, and nothing else ; or, to
abandon the simile, that these color patterns have not pro-
ceeded from inward evolutional forces but are the result of
selection. At least in all places where we do understand
their biological significance these patterns are constituted and
distributed over the wing exactly as utility would require."
Darwin s Artificial and Natural Selection 159
Again : " I should be far from maintaining that the mark-
ings arose unconformably to law. Here, as elsewhere, the
dominance of law is certain. But I take it, that the laws
involved, that is, the physiological conditions of the variation,
here are without exception subservient to the ends of a higher
power — utility; and that it is utility primarily that deter-
mines the kind of colors, spots, streaks, and bands that shall
originate, as also their place and mode of disposition. The
laws come into consideration only to the extent of conditioning
the quality of the constructive materials — the variations, out
of which selection fashions the designs in question. And this
also is subject to important restrictions, as will appear in the
sequel." This conclusion contains all that the most ardent
Darwinian could ask.
He rejects the idea that internal laws alone could have pro-
duced the result, because : —
" If internal laws controlled the markings on butterflies'
wings, we should expect that some general rule could be es-
tablished, requiring that the upper and under surfaces
of the wings should be alike or that they should be
different, or that the fore wings should be colored the same
as or differently from the hind wings, etc. But in reality all
possible kinds of combinations occur simultaneously, and no
rule holds throughout. Or, it might be supposed that bright
colors should occur only on the upper surface or only on the
under surface, or on the fore wings or only on the hind wings.
But the fact is they occur indiscriminately, now here, now
there, and no one method of appearance is uniform throughout
all the species. But the fitness of the various distributions of
colors is apparent, and the moment we apply the principle of
utility we know why in the diurnal butterflies the upper sur-
face alone is usually variegated and the under surface pro-
tectively colored, or why in the nocturnal butterflies the fore
wings have the appearance of bark, of old wood, or of a leaf,
whilst the hind wings, which are covered when resting, alone
160 Evolution and Adaptation
are brilliantly colored. On this theory we also understand
the exceptions to these rules. We comprehend why Danaids,
Heliconids, Euploids, and Acracids, in fact all diurnal butter-
flies offensive to the taste and smell, are mostly brightly marked
and equally so on both surfaces, whilst all species not thus
exempt from persecution have the protective coloring on the
under surface and are frequently quite differently colored
there from what they are on the upper.
" In any event, the supposed formative laws are not obliga-
tory. Dispensations from them can be issued and are issued
whenever utility requires it."
Dispensations from the laws of growth ! Does not a
philosophy of this sort seem to carry us back into the dark
ages ? Is this the best that the Darwinian school can do
to protect itself against the difficulties into which its chief
disciple confesses it has fallen ?
Weismann lays great emphasis on the case of the Indian
leaf -butterfly, Kallima inadiis ; and points out that the leaf
markings are executed " in absolute independence of the
other uniformities governing the wing."
"The venation of the wing is utterly ignored by the leaf
markings, and its surface is treated as a tabula rasa upon
which anything conceivable can be drawn. In other words,
we are presented here with a bilaterally symmetrical figure
engraved on a surface which is essentially radially symmetri-
cal in its divisions.
" I lay unusual stress upon this point because it shows that
we are dealing here with one of those cases which cannot be
explained by mechanical, that is, by natural means, unless
natural selection actually exists and is actually competent to
create new properties ; for the Lamarckian principle is ex-
cluded here ab initio, seeing that we are dealing with a for-
mation which is only passive in its effects : the leaf markings
are effectual simply by their existence and not by any func-
tion which they perform ; they are present in flight as well
Darwin's Artificial and Natural Selection 161
as at rest, during the absence of a danger, as well as during
the approach of an enemy.
" Nor are we helped here by the assumption of purely inter-
nal motive forces, which Nageli, Askenasy, and others have
put forward as supplying a mechanical force of evolution. It
is impossible to regard the coincidence of an Indian butterfly
with the leaf of a tree now growing in an Indian forest as
fortuitous, as a lusus natures. Assuming this seemingly me-
chanical force, therefore, we should be led back inevitably
to a teleological principle which produces adaptive characters
and which must have deposited the directive principle in the
very first germ of terrestrial organisms, so that after untold
ages at a definite time and place the illusive leaf markings
should be developed. The assumption of preestablished
harmony between the evolution of the ancestral line of the
tree with its prefigurative leaf, and that of the butterfly with
its imitating wing, is absolutely necessary here, as I pointed
out many years ago, but as is constantly forgotten by the
promulgators of the theory of internal evolutionary forces."
Weismann concludes, therefore, that for his present pur-
pose it suffices to show "that cases exist wherein all natural
explanations except that of selection fail us," and' he then
proceeds to point out that even the natural selection of Dar-
win and of Wallace also fail to give us a reasonable explana-
tion of how, for example, the markings on the wings of the
Kallima butterfly have come about. The main reason that
he gives to show that this is the case rests on the difficulty of
the assumption that the right variations should always be
present in the right place. Here "is the insurmountable
barrier for the explanatory power of the principle [natural
selection] for who, or what, is to be our guarantee that the
dark scales shall appear at the exact spots on the wing where
the midrib of the leaf must grow ? And that later dark
scales shall appear at the exact spots to which the midrib
must be prolonged ? And that still later such dark scales
M
1 62 Evolution and Adaptation
shall appear at the places whence the lateral ribs start, and
that here also a definite acute angle shall be preserved."
Thus the philosopher in his closet multiplies and magnifies
the difficulties for which he is about to offer a panacea. Had
the same amount of labor been spent in testing whether the
life of this butterfly is so closely dependent on the exact imi-
tation of the leaf, we might have been spared the pains of
this elaborate exordium. There are at least some grounds
for suspicion that the whole case of Kallima is " made up."
If this should prove true, it will be a bad day for the Darwin-
ians, unless they fall back on Weismann's statement that
their theory is insufficient to prove a single case !
Weismann has used Kallima only as the most instruc-
tive illustration. The objections that are here evident are
found not only in the cases of protective coloration, but " are
applicable in all cases where the process of selection is con-
cerned. Take, for example, the case of instincts that are
called into action only once in life, as the pupal performances
of insects, the fabrication of cocoons, etc. How is it that
the useful variations were always present here ? " Weismann
concludes that " something is still wanting to the selection
theory of Darwin and Wallace, which it is obligatory on us
to discover, if we possibly can, and without which selection
as yet offers no complete explanation of the phyletic processes
of transformation." Weismann's first step in the solution of
the difficulty is contained in the following statement: —
" My inference is a very simple one : if we are forced by
the facts on all hands to the assumption that the useful
variations which render selection possible are always present,
then, some profound connection must exist between the utility
of a variation and its actual appearance, or, in other words,
the direction of the variation of a part must be determined by
utility, and we shall have to see whether facts exist that con-
firm our conjecture."
Weismann finds the solution in the method by which the
Darwin s Artificial and Natural Selection 163
breeder has obtained his results in artificial selection. For
instance, the long-tailed variety of the domestic cock of Japan
owes its existence, it is claimed, to skilful selection, and not
at all to the circumstance that, at some period of the race's
history, a cock with tail-feathers six feet in length suddenly
and spasmodically appeared.
Weismann continues: "Now what does this mean? Simply
that the hereditary diathesis, the germinal constitution (the
Anlage) of the breed was changed in the respect in question,
and our conclusion from this and numerous similar facts of
artificial selection runs as follows : by the selection alone of the
plus or minus variations of a character is the constant modifi-
cation of that character in the plus or minus direction deter-
mined. Obviously the hereditary diminution of a part is also
effected by the simple selection of the individuals in each
generation possessing the smallest parts, as is proved, for
example, by the tiny bills and feet of numerous breeds of
doves. We may assert, therefore, in general terms : a defi-
nitely directed progressive variation of a given part is pro-
duced by continued selection in that definite direction. This
is no hypothesis, but a direct inference from the facts and
may also be expressed as follows : by a selection of the kind
referred to the germ is progressively modified in a manner
corresponding with the production of a definitely directed
progressive variation of the part."
So far there is nothing essentially new offered, since Darwin
often tacitly recognized that the standard of variation could
be raised in this way, and in some places he has made
definite statements that this will take place. Weismann
thinks that after each selection, fluctuation will then occur
around a higher average (mode). He says "that this is a
fact," and is proved by the case of the Japanese cock. It
need scarcely be pointed out that it is an assumption, based
on what is supposed to have taken place in this bird, and is
not a "fact."
164 Evolution and Adaptation
Weismann continues : " But the question remains, why is
this the fact ? " He believes his hypothesis of the existence
of determinants in the germ gives a satisfactory answer to
this "why." "According to this theory every independent
and hereditarily variable part is represented in the germ by
a determinant, whose size and power of assimilation corre-
sponds to the size and vigor of the part. These determinants
multiply as do all vital units by growth and division, and
necessarily they increase rapidly in every individual, and the
more rapidly the greater the quantity of the germinal cells
the individual produces. And since there is no more reason
for excluding irregularities of passive nutrition, and of the
supply of nutriment in these minute, microscopically invisible
parts, than there is in the larger visible parts of the cells,
tissues, and organs, consequently the descendants of a deter-
minant can never all be exactly alike in size and capacity of
assimilation, but they will oscillate in this respect to and fro
about the maternal determinant as about their zero point, and
will be partly greater, partly smaller, and partly of the same
size as that. In these oscillations, now, the material for
further selection is presented, and in the inevitable fluctu-
ations of the nutrient supply, I see the reason why every
step attained immediately becomes the zero point of new
fluctuations, and consequently why the size of a part can
be augmented or diminished by selection without limit, solely
by the displacement of the zero point of variation as the
result of selection."
The best illustration of this process of germinal selection
is found, Weismann believes, in the case of the degeneration
of organs. " For in most retrogressive processes active selec-
tion in Darwin's sense plays no part, and advocates of the
Lamarckian principle, as above remarked, have rightly
denied that active selection, that is, the selection of indi-
viduals possessing the useless organ in its most reduced
state, is sufficient to explain the process of degeneration. I,
Darwin s Artificial and Natural Selection 165
for my part, have never assumed this, and have on this very
account enunciated the principle of panmixia. Now, although
this, as I have still no reason for doubting, is a perfectly cor-
rect principle, which really does have an essential and indis-
pensable share in the process of retrogression, still it is not
alone sufficient for a full explanation of the phenomena.
My opponents, in advancing this objection, were right, to the
extent indicated, and as I expressly acknowledge, although
they were unable to substitute anything positive in its stead
or to render my explanation complete. The very fact of the
cessation of control over the organ is sufficient to explain its
degeneration, that is, its deterioration, the disharmony of its
parts, but not the fact which actually and always occurs
where an organ has become useless — viz., its gradual and
unceasing diminution continuing for thousands ana thousands
of years and culminating in its final and absolute effacement"
If then neither selection of persons nor the cessation of
personal selection can explain the phenomenon, we must
look elsewhere for the answer. This Weismann finds in
the application of Roux's hypothesis of the struggle of the
parts to obtain nourishment.
" The production of the long tail-feathers of the Japanese
cock does not repose solely on the displacement directly
effected by personal selection, of the zero point of variation
upward, but that it is also fostered and strengthened by
germinal selection. Were that not so, the phenomena of the
transmutation of species, in so far as fresh growth and the
enlargement and complication of organs already present are
concerned, would not be a zvliit more intelligible than they
were before."
Thus Weismann has piled up one hypothesis on another as
though he could save the integrity of the theory of natural
selection by adding new speculative matter to it. The most
unfortunate feature is that the new speculation is skilfully
removed from the field of verification, and invisible germs
1 66 Evolution and Adaptation
whose sole functions are those which Weismann's imagina-
tion bestows on them, are brought forward as though they
could supply the deficiencies of Darwin's theory. This is,
indeed, the old method of the philosophizers of nature. An
imaginary system has been invented which attempts to ex-
plain all difficulties, and if it fails, then new inventions are to
be thought of. Thus we see where the theory of the selection
of fluctuating germs has led one of the most widely knpwn
disciples of the Darwinian theory.
The worst feature of the situation is not so much that
Weismann has advanced new hypotheses unsupported by
experimental evidence, but that the speculation is of such a
kind that it is, from its very nature, unverifiable, and there-
fore useless. Weismann is mistaken when he assumes that
many zoologists object to his methods because they are
largely speculative. The real reason is that the speculation
is so often of a kind that cannot be tested by observation or
by experiment.
CHAPTER VI
DARWIN'S THEORY OF SEXUAL SELECTION
Sexual Selection
The theory of sexual selection was formulated by Darwin,
even in the first edition of the " Origin of Species," but was
developed at much greater length in " The Descent of Man."
" This form of selection depends, not on a struggle for exist-
ence in relation to other organic beings or to external condi-
tions, but on a struggle between the individuals of one sex,
o-enerally the males, for the possession of the other sex. The
result is not death to the unsuccessful competitor, but few or
no offspring. Sexual selection is, therefore, less rigorous
than natural selection. Generally the most vigorous males,
those which are best fitted for their place in nature, will leave
most progeny. But in many cases victory depends, not so
much on general vigor, as on having special weapons, con-
fined to the male sex. A hornless stag or spurless cock
would have a poor chance of leaving numerous offspring.
Sexual selection, by always allowing the victor to breed,
might surely give indomitable courage, length to the spur,
and strength to the wing to strike in the spurred leg in
nearly the same manner as the brutal cock-fighter by the
careful selection of his best cocks." It is important to no-
tice that the theory of sexual selection is admittedly an
extension of the selection principle into a new field. Having
accounted for domesticated animals and plants by artificial
selection, and for the adaptations of wild species by natural
selection, there remained only to account for the second-
167
1 68 Evolution and Adaptation
ary sexual differences between the sexes by the principle of
sexual selection.
There are two ways in which Darwin supposes sexual se-
lection to act : (i) through competition of the individuals of
the same sex with each other, — the strongest or best-equipped
for fighting or for finding the individuals of the other sex
gaining an advantage ; (2) through selection by the individu-
als of one sex of certain preferred individuals of the other sex.
The first category is natural selection applied to the members
of one sex in competition with each other, although the result
does not lead to the death of the unsuccessful individual,
but excludes it from leaving progeny. In the second cate-
gory a new element is introduced, namely, the selective pozver
of the individuals of one sex, usually the female. It is this
part that adds a distinctly new element to Darwin's other
two theories of selection, and it is this part that we naturally
think of as the theory of sexual selection par excellence.
Darwin makes, however, no sharp distinction between these
two sides of his theory, but includes both under the heading
of sexual selection.
In order to get the theory itself before us in as concrete
form as possible, let us examine some of the cases that
Darwin has given to show how he supposes the process to
be carried out.
" There are many other structures and instincts which must
have been developed through sexual selection — such as the
weapons of offence and the means of defence of the males for
fighting with and driving away their rivals — their courage
and pugnacity — their various ornaments — their contrivances
for producing vocal or instrumental music — and their glands
for emitting odors, most of these latter structures serving
only to allure or excite the female. It is clear that these
characters are the result of sexual and not of ordinary selec-
tion, since unarmed, unornamented, or unattractive males
would succeed equally well in the battle for life and in leaving
Darwin s Theory of Sexual Selection 169
a numerous progeny, but for the presence of better-endowed
males. We may infer that this would be the case, because
the females, which are unarmed and unornamented, are able to
survive and procreate their kind. Secondary sexual charac-
ters of the kind just referred to will be fully discussed in the
following chapters, as being in many respects interesting,
but especially as depending on the will, choice, and rivalry of
the individuals of either sex. When we behold two males
fighting for the possession of the female, or several male birds
displaying their gorgeous plumage, and performing strange
antics before an assembled body of females, we cannot doubt
that, though led by instinct, they know what they are about,
and consciously exert their mental and bodily powers."
This general statement gives an idea of the class of phe-
nomena that Darwin proposes to explain by the theory of sex-
ual selection. The close resemblance between this process
and that of artificial selection may be gathered from the fol-
lowing statement : —
" Just as man can improve the breed of his game-cocks by
the selection of those birds which are victorious in the cock-
pit, so it appears that the strongest and most vigorous males,
or those provided with the best weapons, have prevailed
under nature, and have led to the improvement of the natural
breed or species. A slight degree of variability leading to
some advantage, however slight, in reiterated deadly contests
would suffice for the work of sexual selection ; and it is certain
that secondary sexual characters are eminently variable. Just
as man can give beauty, according to his standard of taste, to
his male poultry, or more strictly can modify the beauty orig-
inally acquired by the parent species, can give to the Sebright
bantam a new and elegant plumage, an erect and peculiar car-
riage — so it appears that female birds in a state of nature have,
by a long selection of the more attractive males, added to their
beauty or other attractive qualities. No doubt this implies
powers of discrimination and taste on the part of the female
170 Evolution and Adaptation
which will at first appear extremely improbable ; but by the
facts to be adduced hereafter, I hope to be able to show that
the females actually have these powers. When, however, it
is said that the lower animals have a sense of beauty, it must
not be supposed that such sense is comparable with that of a
cultivated man, with his multiform and complex associated
ideas. A more just comparison would be between the taste
for the beautiful in animals, and that in the lowest savages,
who admire and deck themselves with any brilliant, glittering,
or curious object."
Darwin did not close his eyes to the difficulties which the
theory had to contend against. One of the most formidable
of these objections is described in the following words :
"Our difficulty in regard to sexual selection lies in under-
standing how it is that the males which conquer other males,
or those which prove the most attractive to the females,
leave a greater number of offspring to inherit their superi-
ority than their beaten and less attractive rivals. Unless
this result does follow, the characters which give to certain
males an advantage over others could not be perfected and
augmented through sexual selection. When the sexes exist
in exactly equal numbers, the worst-endowed males will
(except where polygamy prevails) ultimately find females,
and leave as many offspring, as well fitted for their general
habits of life, as the best-endowed males. From various
facts and considerations, I formerly inferred that with most
animals, in which secondary sexual characters are well
developed, the males considerably exceeded the females in
number ; but this is not by any means always true. If the
males were to the females as two to one, or as three to two,
or even in a somewhat lower ratio, the whole affair would be
simple ; for the better-armed or more attractive males would
leave the largest number of offspring. But after investi-
gating, as far as possible, the numerical proportion of the
sexes, I do not believe that any great inequality in number
Darwin s Theory of Sexual Selection i 7 1
commonly exists. In most cases 'sexual selection appears
to have been effective in the following manner.
" Let us take any species, a bird for instance, and divide
the females inhabiting a district into two equal bodies, the
one consisting of the more vigorous and better-nourished
individuals, and the other of the less vigorous and healthy.
The former, there can be little doubt, would be ready to
breed in the spring before the others ; and this is the
opinion of Mr. Tenner Weir, who has carefully attended to
the habits of birds during many years. There can also be
no doubt that the most vigorous, best-nourished and earliest
breeders would on an average succeed in rearing the largest
number of fine offspring. The males, as we have seen, are
generally ready to breed before the females ; the strongest,
and with some species the best-armed of the males, drive
away the weaker ; and the former would then unite with
the more vigorous and better-nourished females, because
they are the first to breed. Such vigorous pairs would
surely rear a larger number of offspring than the retarded
females, which would be compelled to unite with the con-
quered and less powerful males, supposing the sexes to be
numerically equal ; and this is all that is wanted to add, in
the course of successive generations, to the size, strength
and courage of the males, or to improve their weapons."
I shall comment later on the points here raised, but we
should not let this opportunity pass without noticing, that even
if the pairing were to follow according to the method here
imagined, still the argument breaks down at the critical
point, for there is no evidence that the more precocious
females would rear a larger number of offspring than the
more normal females, or even those that breed somewhat
later.
The greater eagerness of the males which has been ob-
served in so many different classes of animals is accounted
for as follows : —
172 Evolution and Adaptation
" But it is difficult to understand why the males of species,
of which the progenitors were primordially free, should in-
variably have acquired the habit of approaching the females,
instead of being approached by them. But in all cases, in
order that the males should seek efficiently, it would be
necessary that they should be endowed with strong passions ;
and the acquirement of such passions would naturally follow
from the more eager leaving a larger number of offspring
than the less eager."
Thus we are led to the rather complex conclusion, that
the more eager males will leave more descendants, and those
that are better endowed with ornaments will be the ones
selected. But unless it can be shown that there is some
connection between greater eagerness and better ornamenta-
tion, it might often occur that the less ornamented were the
more eager individuals, in which case there would be an
apparent conflict between the two acquirements.
After giving some cases of the greater variability of the
males, in respect to characters that are not connected with
sexual selection, and presumably not the result of any kind
of selection, Darwin concludes : " Through the action of
sexual and natural selection male animals have been rendered
in very many instances widely different from their females ;
but independently of selection the two sexes, from differing
constitutionally, tend to vary in a somewhat different man-
ner. The female has to expend much organic matter in the
formation of her ova, whereas the male expends much force
in fierce contests with his rivals, in wandering about in
search of the female, in exerting his voice, pouring out
odoriferous secretions, etc. : and this expenditure is gen-
erally concentrated within a short period. The great vigor
of the male during the season of love seems often to in-
tensify his colors, independently of any marked difference
from the female. In mankind, and even as low down in the
organic scale as in the Lepidoptera, the temperature of the
Darwin s Theory of Sexual Selection 173
body is higher in the male than in the female, accompanied
in the case of man by a slower pulse. On the whole, the
expenditure of matter and force by the two sexes is probably
nearly equal, though effected in very different ways and at
different rates."
Again: "From the causes just specified, the two sexes
can hardly fail to differ somewhat in constitution, at least
during the breeding season ; and although they may be
subjected to exactly the same conditions, they will tend to
vary in a different manner. If such variations are of no
service to either sex, they will not be accumulated and in-
creased by sexual or natural selection. Nevertheless, they
may become permanent if the exciting cause acts perma-
nently ; and in accordance with a frequent form of inheritance
they may be transmitted to that sex alone in which they
first appeared. In this case, the two sexes will come to
present permanent, yet unimportant, differences of character.
For instance, Mr. Allen shows that with a large number of
birds inhabiting the northern and southern United States,
the specimens from the south are darker-colored than those
from the north ; and this seems to be the direct result of
the difference in temperature, light, etc., between the two
regions. Now, in some few cases, the two sexes of the same
species appear to have been differently affected ; in the
Agel(B7is phceniceus the males have had their colors greatly
intensified in the south ; whereas with Cardinalis virginianus
it is the females which have been thus affected : with Quis-
calus major the females have been rendered extremely vari-
able in tint, whilst the males remain nearly uniform."
The admissions contained in this statement would seem
to jeopardize the entire question, for, if it is admitted that, on
account of the difference in the constitution of the two sexes,
the influence of the surrounding conditions would produce a
different effect on them, it would seem that there is no need
whatsoever for the theory of sexual selection. What Darwin
1 74 Evolution and Adaptation
is probably attempting to show is that the material for the
further action of sexual selection is already given ; but the
question may well be asked, if the external conditions have
done so much, why may they not have gone farther and pro-
duced the entire result ?
Darwin makes the following suggestion to account for those
cases in which the female is the more highly colored : —
" A few exceptional cases occur in various classes of
animals, in which the females instead of the males have
acquired well-pronounced secondary sexual characters, such
as brighter colors, greater size, strength, or pugnacity.
With birds there has sometimes been a complete transposi-
tion of the ordinary characters proper to each sex ; the
females having become the more eager in courtship, the
males remaining comparatively passive, but apparently select-
ing the more attractive females, as we may infer from the
results. Certain hen birds have thus been rendered more
highly colored or otherwise ornamented, as well as more
powerful and pugnacious than the cocks ; these characters
being transmitted to the female offspring alone."
Then follows immediately the discussion as to whether a
double process of sexual selection may not be supposed to go
on at the same time. " It may be suggested that in some
cases a double process of selection has been carried on ; that
the males have selected the more attractive females, and the
latter the more attractive males. This process, however,
though it might lead to the modification of both sexes, would
not make the one sex different from the other, unless indeed
their tastes for the beautiful differed ; but this is a supposition
too improbable to be worth considering in the case of any
animal, excepting man. There are, however, many animals
in which the sexes resemble each other, both being furnished
with the same ornaments, which analogy would lead us to
attribute to the agency of sexual selection. In such cases
it may be suggested with more plausibility, that there has
Darwin s Theory of Sexual Selection 175
been a double or mutual process of sexual selection ; the
more vigorous and precocious females selecting the more
attractive and vigorous males, the latter rejecting all except
the more attractive females. But from what we know of the
habits of animals, this view is hardly probable, for the male
is generally eager to pair with any female. It is more prob-
able that the ornaments common to both sexes were acquired
by one sex, generally the male, and then transmitted to the
offspring of both sexes. If, indeed, during a lengthened
period the males of any species were greatly to exceed the
females in number, and then during another lengthened
period, but under different conditions, the reverse were to
occur, a double but not simultaneous process of sexual selec-
tion might easily be carried on, by which the two sexes might
be rendered widely different."
The improbability of such a process is so manifest that
the suggestion can scarcely be looked upon as anything more
than pure speculation. We shall have occasion later to re-
turn to the same subject, and point out its bearing more
explicitly.
Nearly the whole animal kingdom is passed in review by
Darwin from the point of view of the sexual selection theory.
There is brought together a large number of extremely inter-
esting facts, and if the theory did no more than merely hold
them together, it has served, in this respect, a useful end.
We may select some of the most instructive cases by way of
illustrating the theory.
In many of the lower animals in which the sexes are sep-
arated, and these alone, of course, can be supposed to come
within the range of the theory, there are no striking differ-
ences between the sexes, in regard to ornamentation, although
in other respects differences may exist.
" Moreover it is almost certain that these animals have too
imperfect senses and much too low mental powers, to appre-
ciate each other's beauty or other attractions, or to feel rivalry.
176 Evolution and Adaptation
" Hence in these classes or subkingdoms, such as the Proto-
zoa, Ccelenterata, Echinodermata, Scolecida, secondary sexual
characters, of the kind which we have to consider, do not
occur ; and this fact agrees with the belief that such charac-
ters in the higher classes have been acquired through sexual
selection, which depends on the will, desire, and choice of
either sex."
There are some cases, however, in which animals low in
the scale show a difference in the ornamentation of the two
sexes. A few cases have been recorded in the roundworms,
where different shades of the same tint distinguish the sexes.
In the annelids the sexes are sometimes so different, that, as
Darwin remarks, they have been placed in different genera
and even families, "*yet the differences do not seem to be of
the kind which can be safely attributed to sexual selection."
In regard to the nemertian worms, although they "vie in
variety and beauty of coloring with any other group in the
invertebrate series," yet Mcintosh states that he "cannot
discover that these colors are of any service." In the cope-
pods, belonging to the group of lower Crustacea, Darwin
excludes those cases in which the males alone " are furnished
with perfect swimming legs, antennae, and sense organs ; the
females being destitute of these organs, with their bodies
often consisting of a mere distorted mass," because these
extraordinary differences between the two sexes are no doubt
related to their widely different habits of life. Nevertheless,
it is important to observe that such extreme differences may
exist in cases where sexual selection cannot come in, because
of the absence of eyes in the female.
In regard to another copepod, Saphirina, Darwin points
out that the males are furnished with minute scales, which
exhibit beautiful changing colors, and these are absent in
the females ; yet he states that it would be extremely rash
to conclude that these curious organs serve to attract
the females. Differences in the sexes are also found in one
Darwin's Theory of Sexual Selection 177
species of Squilla, and a species of Gelasimus. In the latter
case Darwin thinks that the difference is probably due to
sexual selection. In addition to these cases, recorded by
Darwin, there may be added the two remarkable cases,
shown in our Figure 2 A, B, of Calocalanus pavo, the female of
FlG. 2. — A male of the copepod, Calocalanus plumulosus. B and C, a male and
a female of Calocalanus pavo. (After Giesbrecht.)
which has a gorgeous tail worthy of a peacock, and of Calo-
calanus plumulosus, in which one of the setae of the tail is
drawn out into a long featherlike structure. In the former,
the male is much more modestly adorned, as shown in Fig-
ure 2 C ; in the latter species the male is unknown.
In spiders, where as a rule the sexes do not differ much
N
1 78 Evolution and Adaptation
from each other in color, the males are often of a darker shade
than the females. " In some species, however, the difference
is conspicuous ; thus the female of Sparassus smaragdulus is
dullish green, whilst the adult male has the abdomen of a fine
yellow with three longitudinal stripes of rich red." Darwin
believes that sexual selection must take place in this group,
because Canestrini has observed that the males fight for the
possession of the females. He has also stated that the
males pay court to the female, and that she rejects some of
the males who court her, and sometimes devours them, until
finally one is chosen. Darwin believed, on this evidence,
that the difference in color of the sexes had been acquired
by sexual selection, "though we have here not the best kind
of evidence — the display by the male of his ornaments."
This evidence has, however, now been supplied through
the interesting observations of Mr. and Mrs. Peckham.
These accurate observers have studied the courtship of the
male, and observed that during the process, he twists and
turns his body in such a way as to show to best advantage
his colors to the female. From their account this certainly
appears to be the result of his movements, but whether this
is really the case, and whether the female makes any choice
amongst her suitors, according to whether they are more or
less brilliantly marked, we are absolutely ignorant. The fol-
lowing account given by Darwin should not pass unnoticed: —
" The male is generally much smaller than the female,
sometimes to an extraordinary degree, and he is forced to be
extremely cautious in making his advances, as the female
often carries her coyness to a dangerous pitch. De Geer
saw a male that 'in the midst of his preparatory caresses
was seized by the object of his attentions, enveloped by her
in a web and then devoured, a sight which, as he adds, filled
him with horror and indignation.' The Rev. O. P. Cam-
bridge accounts in the following manner for the extreme
smallness of the male in the genus Nephila. ' M. Vinson
Dariviiis Theory of Sexual Selection 179
gives a graphic account of the agile way in which the dimin-
utive male escapes from the ferocity of the female, by gliding
about and playing hide and seek over her body and along her
gigantic limbs : in such a pursuit it is evident that the
chances of escape would be in favor of the smallest males,
while the larger ones would fall early victims ; thus gradually
a diminutive race of males would be selected, until at last
they would dwindle to the smallest possible size compatible
with the exercise of their generative functions, — in fact
probably to the size we now see them, i.e. so small as to be
a sort of parasite upon the female, and either beneath her
notice, or too agile and too small for her to catch without
great difficulty.' "
It is certainly surprising to find Darwin ascribing even
this difference in size between the sexes to the action of
selection. Is it not a little ludicrous to suppose that the
females have reduced the males to a size too small for them
to catch ?
There are many cases known in the animal kingdom where
there is a difference in size between the two sexes, especially
in the group of insects ; but I doubt very much if they are to
be accounted for as the result of sexual selection. In some
of these cases Darwin accounts for the larger size of the
female, on account of the large number of eggs which she
has to carry. In other insects where the male is larger,
as in the stag-beetle, the size is ascribed to the conflicts of
the males, leading to the survival of the larger individuals.
In still other cases, where the males are larger, but do not
fight, an explanation is admittedly wanting; but it is suggested
that here there would be no necessity for the males to be
smaller than the females in order to mature before them (as
is supposed to happen in other species), for in these cases
the individuals are not short-lived, and there would be ample
time for pairing. Again, although the males of nearly all
bees are smaller than the females, yet the reverse is true in
i So Evolution and Adaptation
those forms in which the females are fertilized during the
marriage flight. The explanation offered is that in these
forms the male carries the female, and this is assumed to
require greater size on his part. This loose way of guessing,
as to a possible explanation, is characteristic of the whole
hypothesis of sexual selection. First one, and then another,
guess is made as to the causes of the differences between the
sexes. It is not shown in a single one of the instances that
the postulated cause has really had anything to do with the
differences in question ; and the attempt to show that the
theory is probable, by pointing out the large number of cases
which it appears to account for, is weakened to a very great
degree by the number of exceptional cases, for which an
equally ready explanation of a different kind is forthcoming.
This way of giving loose rein to the imagination has been
the bane of the method that has followed hard on the track
of Darwin's hypothesis, and for which his example has been
in no small measure responsible. Thus, in the case just
quoted, there are no less than four distinct conjectures made
to account for the differences in size between the sexes, and
each guess involves an entirely different set of processes.
Considering the complicated relation of the life of organisms,
it may be doubted if any of the imagined processes could
bring about the result, and certainly not a single one has
been shown to be a real, or a sufficient, cause in the evolution-
ary process. Neither the actuality of the postulated causes,
nor their application to a particular case, has been shown
to exist.
In the Diptera, or flies, Wallace records one interesting
case of sexual difference in the genus Elaphomyia of New
Guinea, in which the males are furnished with horns, which
the females lack. Darwin writes : —
" The horns spring from beneath the eyes, and curiously
resemble those of a stag, being either branched or palmated.
In one of the species, they equal the whole body in length.
Darwin s Theory of Sexual Selection 181
They might be thought to be adapted for fighting, but as in
one species they are of a beautiful pink color, edged with
black, with a pale central stripe, and as these insects have
altogether a very elegant appearance, it is perhaps more
probable that they serve as ornaments."
Presumably, therefore, Darwin means these colored horns
have been acquired by sexual selection.
In the Hemiptera, or bugs, both sexes of some species are
"beautifully colored," and as the members of the group are
often unpalatable to other animals, the color in this case is
supposed to act as a warning signal.
In other cases it is stated, however, that "a small pink and
green species " could hardly be distinguished from the buds
on the trunks of the lime trees which this insect frequents.
In this case the color appears "to be directly protective."
Thus without any means of forming a correct judgment,
the color of one animal is supposed to be the result of
natural selection, since it resembles its surroundings, but
of sexual selection if the color is present or more pro-
nounced in one sex. Where neither view can easily be
applied, the color is ascribed in a general way to the nature
of the organism.
In respect to the group of Hymenoptera, or bees, Darwin
records the following cases : —
" In this order slight differences in color, according to
sex, are common, but conspicuous differences are rare except
in the family of bees ; yet both sexes of certain groups are so
brilliantly colored — for instance in Chrysis, in which ver-
milion and metallic greens prevail — that we are tempted to
attribute the result to sexual selection. In the Ichneumonidae,
according to Mr. Walsh, the males are almost universally
lighter-colored than the females. On the other hand, in the
Tenthredinidse the males are generally darker than the
females. In the Siricidas the sexes frequently differ ; thus
the male of Sirex juvencus is banded with orange, whilst the
1 82 Evolution and Adaptation
female is dark purple ; but it is difficult to say which sex is
the more ornamented."
In other families of bees, differences in the color of the
sexes have been recorded, and since the males have been seen
fighting for the possession (?) of the females, and since bees
are known to recognize differences in color, Darwin believes
that : —
" In some species the more beautiful males appear to have
been selected by the females ; and in others the more beauti-
ful females by the males. Consequently in certain genera,
the males of the several species differ much in appearance,
whilst the females are almost indistinguishable ; in other
genera the reverse occurs. H. Muller believes that the
colors gained by one sex through sexual selection have often
been transferred in a variable degree to the other sex, just as
the pollen-collecting apparatus of the female has often been
transferred to the male, to whom it is absolutely useless."
Although in beetles the sexes are generally colored alike,
yet in some of the longicorns there are exceptions to the rule.
" Most of these insects are large and splendidly colored. The
males in the genus Pyrodes, which I saw in Mr. Bates's
collection, are generally redder .but rather duller than the
females, the latter being colored of a more or less splendid
golden-green. On the other hand, in one species the male
is golden-green, the female being richly tinted with red and
purple. In the genus Esmeralda the sexes differ so greatly
in color that they have been ranked as distinct species ; in
one species both are of a beautiful shining green, but the
male has a red thorax. On the whole, as far as I could
judge, the females of those Prionidx, in which the sexes
differ, are colored more richly than the males, and this does
not accord with the common rule in regard to color, when
acquired through sexual selection."
The great horns that rise from the heads of many male
beetles are very striking cases of sexual difference, and
Darwiiis Theory of Sexual Selection 183
Darwin compares them to the horns of stags and of the
rhinoceros. They "are wonderful from their size and
shapes." Darwin offers the following conjecture as to their
meaning: "The extraordinary size of the horns, and their
widely different structure in closely allied forms, indicate
that they have been formed for some purpose ; but their
excessive variability in the males of the same species leads
to the inference that this purpose cannot be of a definite
nature. The horns do not show marks of friction, as if used
for any ordinary work. Some authors suppose that as the
males wander about much more than the females, they re-
quire horns as a defence against their enemies ; but as the
horns are often blunt, they do not seem well adapted for
defence. The most obvious conjecture is that they are
used by the males for fighting together ; but the males have
never been observed to fight ; nor could Mr. Bates, after a
careful examination of numerous species, find any sufficient
evidence, in their mutilated or broken condition, of their hav-
ing been thus used. If the males had been habitual fighters,
the size of their bodies would probably have been increased
through sexual selection, so as to have exceeded that of the
females ; but Mr. Bates, after comparing the two sexes in
above a hundred species of the Copridae, did not find any
marked difference in this respect amongst well-developed
individuals. In Lethrus, moreover, a beetle belonging to
the same great division of the lamellicorns, the males are
known to fight, but are not provided with horns, though their
mandibles are much larger than those of the female."
" The conclusion that the horns have been acquired as orna-
ments is that which best agrees with the fact of their having
been so immensely, yet not fixedly, developed, — as shown
by their extreme variability in the same species, and by their
extreme diversity in closely allied species. This view will at
first appear extremely improbable ; but we shall hereafter
find with many animals standing much higher in the scale,
184 Evolution and Adaptation
namely fishes, amphibians, reptiles and birds, that various
kinds of crests, knobs, horns and combs have been developed
apparently for this sole purpose."
It is asking a great deal to suppose that animals, so dull
and sluggish as these beetles, are endowed with a sufficient
aesthetic discrimination to select in each generation those
males whose horns are a little longer than the average. The
resemblance of the horns to those of stags is, as Darwin
points out, obvious, but in the latter case also it remains to
be proven that they are the result of sexual selection, as
Darwin believes to be the case ; but the evidence for this
belief is not much better, as we shall see in the case of the
antlers of deer, than it is in these beetles.
In regard to butterflies, the males and females are both
often equally brilliantly colored ; in other species the differ-
ences in the sexes are very striking. Darwin states : —
" Even within the same genus we often find species pre-
senting extraordinary differences between the sexes, whilst
others have their sexes closely alike." The fine colors of
the wings of many moths are also supposed by Darwin to
have arisen through sexual selection, although the colors
are usually on the lower wings, which are covered during
the day by the less ornamented upper wings. It is assumed
that, since the moths often begin to fly at dusk, their colors
might at this time be seen and appreciated by the other sex.
It should not be overlooked, however, that, in the case of
some of the most highly colored moths, it is known that the
males find the females through the sense of smell. More-
over, although moths are often finely colored, Darwin points
out that " it is a singular fact that no British moths which
are brilliantly colored, and, as far as I can discover, hardly
any foreign species, differ much in color according to sex ;
though this is the case with many brilliant butterflies."
Yet Darwin does not hesitate to conclude : " From the sev-
eral foregoing facts it is impossible to admit that the brilliant
Darwin s Theory of Sexual Selection 185
colors of butterflies, and of some few moths, have commonly
been acquired for the sake of protection. We have seen
that their colors and elegant patterns are arranged and ex-
hibited as if for display. Hence I am led to believe that the
females prefer or are most excited by the more brilliant
males ; for on any other supposition the males would, as far
as we can see, be ornamented to no purpose. We know that
ants and certain lamellicorn beetles are capable of feeling
an attachment for each other, and that ants recognize their
fellows after an interval of several months. Hence there is
no abstract improbability in the Lepidoptera, which probably
stand nearly or quite as high in the scale as these insects,
having sufficient mental capacity to admire bright colors.
They certainly discover flowers by color."
So far as the evidence of ants having an attachment for
each other is concerned, we may eliminate this part of the
argument, since the evidence on which the statement is based
is now regarded as only showing that ants recognize each
other by their sense of smell, which resides in the anten-
nae. Hence the so-called fondling means only that the
ants are trying by smell to determine the odor of the other
individual.
Darwin points out a number of cases in which the females
are more brightly colored than the males, and for such cases
he reverses the process of selection, supposing that the males
have been discriminating, and have not " gladly accepted any
female." No explanation is offered to account for this
reversal of instinct, in fact, no evidence to show that such a
reversal really exists. Darwin points out that in most cases
the male insect carries the female during the period of union,
while in two species of butterflies, Colias edusa and hyale, the
females carry the males, and the females are here the more
highly colored. He suggests that since in this case "the
females take the more active part in the final marriage cere-
mony, so we may suppose that they likewise do so in the
1 86 Evolution and Adaptation
wooing ; and in this case we can understand how it is that
they have been rendered the more beautiful."
A most significant fact in connection with the difference
in sexual coloration of butterflies did not escape Darwin's
attention.
"Whilst reflecting on the beauty of many butterflies, it
occurred tome that some caterpillars were splendidly colored ;
and as sexual selection could not possibly have here acted, it
appeared rash to attribute the beauty of the mature insect to
this agency, unless the bright colors of their larvae could be
somehow explained. In the first place, it may be observed
that the colors of caterpillars do not stand in any close corre-
lation with those of the mature insect. Secondly, their bright
colors do not serve in any ordinary manner as a protection.
Mr. Bates informs me, as an instance of this, that the most
conspicuous caterpillar which he ever beheld (that of a
Sphinx) lived on the large green leaves of a tree on the open
llanos of South America; it was about four inches in length,
transversely banded with black and yellow, and with its head,
legs, and tail of a bright red. Hence it caught the eye of any
one who passed by, even at the distance of many yards, and
no doubt that of every passing bird."
Darwin applied to Wallace for a solution of this difficulty,
and received the reply that he "thought it probable that con-
spicuously colored caterpillars were protected by having a
nauseous taste ; but as their skin is extremely tender, and as
their intestines readily protrude from a wound, a slight peck
from the beak of a bird would be as fatal to them as if they
had been devoured. Hence, as Mr. Wallace remarks, ' dis-
tastefulness alone would be insufficient to protect a caterpillar
unless some outward sign indicated to its would-be destroyer
that its prey was a disgusting morsel.' Under these circum-
stances it would be highly advantageous to a caterpillar to be
instantaneously and certainly recognized as unpalatable by all
birds and other animals. Thus the most gaudy colors would
Darwin's Theory of Sexual Selection 187
be serviceable, and might have been gained by variation and
the survival of the most easily recognized individuals."
It need scarcely be pointed out that an occasional peck
can scarcely be supposed to have led to the splendid develop-
ment of color shown by some caterpillars, and Darwin con-
fesses that at first sight this hypothesis appears bold, but
nevertheless he believes that it will be found to be true. He
adds, " We cannot, however, at present thus explain the
elegant diversity in the colors of many caterpillars."
A most important fact in this connection should not be over-
looked, namely, that in the caterpillar stage the sexual organs
are so little developed that it is generally impossible at this
time to distinguish between the sexes, unless a microscopic
examination is made. This gives us, perhaps, a clew as to the
difference between the mature sexual forms. These differ-
ences are connected with difference of sex itself. This con-
clusion also fits in well with the fact that during the period
when the sexual organs are at the height of their develop-
ment the individuals are most brilliantly colored. The pri-
mary cause of the brilliant color of many animals concerns
us here only secondarily, for, since it is known that many of
the lower forms are no less brilliantly and elaborately colored
than are the sexes of the higher forms, it is not surprising
that the sexes themselves sometimes differ in this respect.
Organs for producing sounds of different sorts are present
in some insects, and these organs Darwin includes under the
head of secondary sexual organs. In the group of Hemiptera,
or bugs, the cicadas are the most familiar species that pro-
duce sounds. The noise is made by the males ; the females
are quite mute.
"With respect to the object of the music, Dr. Hartman, in
speaking of the Cicada septemdecim of the United States,
says, 'the drums are now (June 6th and 7th, 1851) heard in
all directions. This I believe to be the marital summons
from the males. Standing in thick chestnut sprouts about
1 88 Evolution and Adaptation
as high as my head, where hundreds were around me, I
observed the females coming around the drumming males.'
He adds, 'this season (August, 1868) a dwarf pear-tree in my
garden produced about fifty larvae of C. pruiuosa ; and I
several times noticed the females to alight near a male while
he was uttering his clanging notes.' Fritz Miiller writes to
me from S. Brazil that he has often listened to a musical
contest between two or three males of a species with a par-
ticularly loud voice, seated at a considerable distance from
each other : as soon as one had finished his song, another
immediately began, and then another. As there is so much
rivalry between the males, it is probable that the females not
only find them by their sounds, but that, like female birds,
they are excited or allured by the male with the most attrac-
tive voice."
In the flies the following cases are given by Darwin : —
" That the males of some Diptera fight together is certain ;
for Professor Westwood has several times seen this with the
Tipulae. The males of other Diptera apparently try to win
the females by their music : H. Miiller watched for some
time two males of an Eristalis courting a female ; they hov-
ered above her, and flew from side to side, making a high
humming noise at the same time. Gnats and mosquitoes
(Culicidae) also seem to attract each other by humming ; and
Professor Mayer has recently ascertained that the hairs on
the antennae of the male vibrate in unison with the notes of a
tuning-fork, within the range of the sounds emitted by the
female."
In the crickets, grasshoppers, and locusts, the males "are
remarkable for their musical powers " ; and it is generally
assumed that the sounds serve to call or to excite the female.
In these forms the noise is made by rubbing the wings over
each other or the legs against the wing-covers.
In some of these forms both sexes have stridulating organs,
and in one case they differ to a certain extent from each
Darwin s Theory of Sexual Selection icS9
other. " Hence we cannot suppose that they have been
transferred from the male to the female, as appears to have
been the case with the secondary sexual characters of many
other animals. They must have been independently devel-
oped in the two sexes, which no doubt mutually call to each
other during the season of love."
Some beetles also possess rasping organs in different parts
of the body, but they cannot produce much noise by this
means.
"We thus see that in the different coleopterous families
the stridulating organs are wonderfully diversified in position,
but not much in structure. Within the same family some
species are provided with these organs, and others are desti-
tute of them. This diversity is intelligible, if we suppose
that originally various beetles made a shuffling or hissing
noise by the rubbing together of any hard and rough parts
of their bodies, which happened to be in contact ; and that
from the noise thus produced being in some way useful, the
rough surfaces were gradually developed into regular stridu-
lating organs. Some beetles as they move, now produce,
either intentionally or unintentionally, a shuffling noise, with-
out possessing any proper organs for the purpose."
Darwin says that he expected from analogy to find in this
group also differences in the sexes, but none such were found,
although in some cases the males alone possess certain char-
acters or have them more highly developed.
It is important not to forget, when considering all questions
connected with sexual selection, that in order for the result
to be successful it is not only necessary that the female
respond to the noises and music of the other sex, but that
she choose the suitor that makes the greatest, or the most
pleasing, noise. If the stridulating organs are only used by
the animals in finding each other, then the case might be
considered as coming under the head of natural selection.
If this be granted, then it may be claimed, and apparently
190 Evolution and Adaptation
Darwin is inclined to adopt this view, that those males that
make the most noise will be more likely to be heard, and
possibly approached. They will, therefore, be more likely to
leave descendants. We have already considered this question
when dealing with the theory of natural selection in the pre-
ceding chapter and need not go over the ground again. This
much may, however, be said again, that even if it is probable
that these organs are of use to the animals in finding each
other, and this seems not improbable, it does not follow that
the organs have been acquired through selection for this
purpose.
Darwin finds his best examples of secondary sexual charac-
ters in the group of vertebrates, and since in this group the
intelligence is of a higher order than in the other groups, the
argument that the female chooses the more pleasing suitor is
made to appear more plausible.
The elongation of the lower jaw that occurs in a few fishes
at the breeding season is regarded as a secondary sexual
character. On the other hand, Darwin recognizes the follow-
ing difficulty in regard to the size of the males : —
" In regard to size, M. Carbonnier maintains that the
female of almost all fishes is larger than the male ; and Dr.
Giinther does not know of a single instance in which the
male is actually larger than the female. With some cyprino-
donts the male is not even half as large. As in many kinds
of fishes the males habitually fight together, it is surprising
that they have not generally become larger and stronger than
the females through the effects of sexual selection. The
males suffer from their small size, for, according to M. Car-
bonnier, they are liable to be devoured by the females of
their own species when carnivorous, and no doubt by other
species. Increased size must be in some manner of more
importance to the females, than strength and size are to the
males for fighting with other males ; and this perhaps is to
allow of the production of a vast number of ova."
Darwin s Theory of Sexual Selection 191
The last sentence implies that this particular case is to
be explained by the females becoming larger on account of
the number of eggs that they are to produce. But why was
not the same explanation offered in the case of the spiders ?
It is this uncertain way of applying any explanation that sug-
gests itself, that puts the whole method in an unfortunate
light.
In many species of fish the males are brighter in color
than the females. In the case of Callionymus lyra, Darwin
states : —
" When fresh caught from the sea the body is yellow of
various shades, striped and spotted with vivid blue on the
head ; the dorsal fins are pale brown with dark longitudinal
bands, the ventral, caudal, and anal fins being bluish black.
The female, or sordid dragonet, was considered by Linnaeus,
and by many subsequent naturalists, as a distinct species ; it
is of a dingy reddish brown, with the dorsal fin brown and
the other fins white. The sexes differ also in the propor-
tional size of the head and mouth, and in the position of the
eyes ; but the most striking difference is the extraordinary
elongation in the male of the dorsal fin. Mr. W. Saville
Kent remarks that this ' singular appendage appears from
my observations of the species in confinement, to be subser-
vient to the same end as the wattles, crests, and other abnor-
mal adjuncts of the male in gallinaceous birds, for the purpose
of fascinating their mates.' "
In the case of another fish, Cottus scorpius, there is also a
great difference between the sexes, and here the males be-
come very brilliant only at the breeding season. In other
fishes, in which the sexes are colored alike, the males may
become more brilliant during the breeding season. This,
too, is explained by Darwin on the assumption that those
males that have varied at the breeding season, so as to be-
come more brightly colored, have been chosen in preference
to the other males.
192 Evolution and Adaptation
A few cases are cited in which it has been observed that
the males appear to exhibit themselves before the females, as
in the following case of the Chinese M acropus : —
" The males are most beautifully colored, more so than the
females. During the breeding season they contend for the
possession of the females ; and, in the act of courtship, ex-
pand their fins, which are spotted and ornamented with
brightly colored rays, in the same manner, according to M.
Carbonnier, as the peacock. They then also bound about
the females with much vivacity, and appear by Tetalage de
leurs vives couleurs chercher a attirer l'attention des femelles,
lesquelles ne paraissaient indifferentes a ce manege, elles
nageaient avec une molle lenteur vers les males et semblaient
se complaire dans leur voisinage.' "
In this connection Darwin makes the following general
statement : —
" The males sedulously court the females, and in one case,
as we have seen, take pains in displaying their beauty before
them. Can it be believed that they would thus act to no
purpose during their courtship ? And this would be the case,
unless the females exert some choice and select those males
which please or excite them most. If the female exerts such
choice, all the above facts on the ornamentation of the males
become at once intelligible by the aid of sexual selection."
While it may readily be granted that display of the male
may have for its purpose the excitement of the female, it is
another question as to whether she will be more excited by
the more beautiful suitor. The attentions of the male may
be supposed to have a purpose, even if the female does not
choose the more beautiful of her suitors. It is this last prop-
osition, so necessary for the theory of sexual selection, that
seems improbable. But even if it were probable, there are,
as we shall see, other difficulties to be overcome before we
should be justified in accepting Darwin's statement quoted
above, concerning the results of sexual selection.
Darwin s Theory of Sexual Selection 193
In regard to those species of fish in which both sexes are
equally ornamented, Darwin returns once more to his hy-
pothesis that the color of the male, acquired through sexual
selection, may be transmitted to the other sex, and then, as
if in doubt on this point, he adds, that it may be the result
of the " nature of the tissues and of the surrounding condi-
tions." He even makes the suggestion, somewhat further
on, that the colors may be warning, although it is confess-
edly unknown that these fish are distasteful to fish-devouring
animals.
In amphibians the crest on the back of the male triton,
which becomes colored along its edge, is described as a second-
ary sexual character. The vocal sacs, present in some species
of frogs, are found sometimes in both sexes, but more highly
developed in the males. In other species, as in the toad, it
is the male alone that sings. In the reptiles we find that the
two sexes of the turtles are colored alike, and this holds also
for the crocodiles. Some male turtles make sounds at the
breeding season, and the same is true for the crocodiles, the
males of which are said to make a " prodigous display." In
snakes the males are smaller, as a rule, than the females, and
the colors are more strongly pronounced, and although some
snakes are very brilliantly colored, Darwin puts this down
either to protective coloration, or to mimicry of other kinds
of snakes. But surely the extremely brilliant colors of many
snakes cannot be accounted for in any of these ways. The
cause of the color of the venomous kinds, that are supposed
to be imitated by the others, " remains to be explained and
this may perhaps be sexual selection."
" It does not, however, follow because snakes have some
reasoning power, strong passions and mutual affection, that
they should likewise be endowed with sufficient taste to
admire brilliant colors in their partners, so as to lead to the
adornment of the species through sexual selection. Never-
theless, it is difficult to account in any other manner for the
1 94 Evolution and Adaptation
extreme beauty of certain species ; for instance, of the coral-
snakes of South America, which are of a rich red with black
and yellow transverse bands."
In lizards the erectile crests of the male Anolis, the brilliant
throat patches of Sitaria minor, which is colored blue, black,
and red, the skinny appendages present on the throat of the
little lizards of the genus Draco, which in the beauty of their
colors baffle description, are given as cases of sexual adorn-
ment. In the last case cited the ornaments are present,
however, in both sexes. The remarkable horns in the males
of different species of chameleons are imagined to have been
acquired through the battle of the males with each other.
In the group of birds we find some of the most striking
cases of secondary sexual differences. The spurs, combs,
wattles, horns, air-filled sacs, topknots, feathers with naked
shafts, plumes, and greatly elongated feathers are all second-
ary sexual characters. The songs of the males, the rattling
together of the quills of the peacock, the drumming of the
grouse, and the booming sounds made by the night jars
while on the wing, are further examples of secondary sex-
ual differences. The odor of the male of the Australian
musk duck is also put in the same category.
The pugnacity of many male birds is well known, and it is
imagined that one of the results of the competition of the
individuals of the same sex with each other has led to the
development of the organs of defence and offence. The
males that have been successful in these battles are then sup-
posed to mate with the best females. In this way those
secondary sexual differences, connected with the encounters
of the males, are supposed to have been formed. Darwin
states in this connection : —
" Even with the most pugnacious species it is probable
that the pairing does not depend exclusively on the mere
strength and courage of the male ; for such males are gener-
ally decorated with various ornaments, which often become
Darwiiis Theory of Sexual Selection 195
more brilliant during the breeding season, and which are
sedulously displayed before the females. The males also
endeavor to charm or excite their mates by love-notes, songs,
and antics ; and the courtship is, in many instances, a pro-
longed affair. Hence it is not probable that the females are
indifferent to the charms of the opposite sex, or that they are
invariably compelled to yield to the victorious males."
Thus a double process of selection is imagined to take
place ; one, the outcome of a competition of the males with
each other, and the other, through a choice of the more suc-
cessful males by the females, the more beautiful being
supposed to be chosen.
It may be well not to lose sight of the fact that unless the
selection is severe in each generation, its good effects will be
lost, as has been stated in connection with the theory of nat-
ural selection. Still more important is the consideration
that unless the same variations appear at the same time, in
many of the surviving males, the results will be lost through
crossing. These statements will show that the difficulties of
the theory are by no means small, and when we are asked to
believe further that another process still has been superim-
posed on this one, namely, the selection of the more beautiful
males by the females, we can appreciate how great are the
difficulties that must be overcome in order that the process
may be carried out.
The love-antics and dances of male birds at the breeding
season furnish many curious data. The phenomena are
imagined by Darwin to be connected with sexual selection,
for in the dances the males are supposed to exhibit their or-
naments to the females who are imagined to choose the suitor
that is most to their taste.
Hudson, who has studied the habits of birds in the field,
asks some very pertinent questions in connection with their
performances of different kinds. "What relation that we
can see or imagine to the passion of love and the business of
196 Evolution and Adaptation
courtship have these dancing and vocal performances in nine
cases out of ten ? In such cases, for instance, as that of the
scissortail tyrant-bird, and its pyrotechnic displays, when a
number of couples leave their nests containing eggs and
young to join in a wild aerial dance ; the mad exhibitions of
ypecahas and ibises and the jacana's beautiful exhibition of
grouped wings ; the triplet dances of the spur-winged lapwing,
to perform which two birds already mated are compelled to
call in a third bird to complete the set ; the harmonious duets
of the oven-birds and the duets and choruses of nearly all
the wood-hewers, and the wing-slapping aerial displays of the
whistling widgeons, — will it be seriously contended that the
female of this species makes choice of the male able to ad-
minister the most vigorous and artistic slaps ? "
" The believer in the theory would put all these cases
lightly aside to cite the case of the male cow-bird practising
antics before the female, and drawing a wide circle of melody
around her, etc. . . . And this was in substance what Dar-
win did." " How unfair the argument is based on these
carefully selected cases gathered from all regions of the globe
and often not properly reported is seen when we turn to the
book of nature and closely consider the habits and actions
of all the species inhabiting any one district." Hudson con-
cludes that he is convinced that any one who will note the
actions of animals for himself will reach the conviction, that
" conscious sexual selection on the part of the female is not
the cause of music and dancing performances in birds, nor
of the brighter colors and ornaments that distinguish the
male."
The differences in color in the sexes of birds are classified
by Darwin as follows: (1) when the males are ornamented
exclusively or in a much higher degree than the females ;
(2) when both sexes are highly ornamented ; (3) when the
female is more brightly colored. A few examples of each
sort may be chosen for illustration.
Darwin s Theory of Sexual Selection 197
" In regard to color, hardly anything need here be said,
for every one knows how splendid are the tints of many
birds, and how harmoniously they are combined. The col-
ors are often metallic and iridescent. Circular spots are
sometimes surrounded by one or more differently shaded
zones, and are thus converted into ocelli. Nor need much be
said on the wonderful difference between the sexes of many
birds. The common peacock offers a striking instance.
Female birds of paradise are obscurely colored and destitute
of all ornaments, whilst the males are probably the most
highly decorated of all birds, and in so many different ways,
that they must be seen to be appreciated. The elongated
and golden-orange plumes which spring from beneath the
wings of the Paradisea apoda, when vertically erected and
made to vibrate, are described as forming a sort of halo, in
the centre of which the head 'looks like a little emerald sun,
with its rays formed by the two plumes.' "
Male humming-birds are almost as splendidly colored as
are the birds of paradise, some having the feathers modified
in a truly extraordinary way. "Almost every part of their
plumage has been taken advantage of, and modified ; and the
modifications have been carried, as Mr. Gould showed me, to
a wonderful extreme in some species belonging to nearly
every subgroup. Such cases are curiously like those which
we see in our fancy breeds, reared by man for the sake of
ornament : certain individuals originally varied in one charac-
ter, and other individuals of the same species in other charac-
ters ; and these have been seized on by man and much
augmented — as shown by the tail of the fantail pigeon, the
hood of the jacobin, the beak and wattle of the carrier, and
so forth. The sole difference between these cases is that
in the one the result is due to man's selection, whilst in
the other, as with humming-birds, birds of paradise, etc., it
is due to the selection by the females of the more beautiful
males."
198 Evolution and Adaptation
A remarkable bird of South America, the bell-bird, has a
peculiar note that "can be distinguished at the distance of
nearly three miles and astonishes every one who hears it.
. . . The male is pure white, whilst the female is dusky-
green ; and white is a very rare color in terrestrial species
of moderate size and inoffensive habits. The male, also, as
described by Waterton, has a spiral tube, nearly three inches
in length, which rises from the base of the beak. It is jet-
black, dotted over with minute downy feathers. This tube
can be inflated with air, through a communication with the
palate ; and when not inflated hangs down on one side. The
genus consists of four species, the males of which are very
distinct, whilst the females, as described by Mr. Sclater in a
very interesting paper, closely resemble each other, thus offer-
ins: an excellent instance of the common rule that within the
same group the males differ much more from each other than
do the females. In a second species (C. nudicollis) the male
is likewise snow-white, with the exception of a large space of
naked skin on the throat and round the eyes, which during
the breeding season is of a fine green color. In a third
species (C. tricarunculatus) the head and neck alone of the
male are white, the rest of the body being chestnut-brown,
and the male of this species is provided with three filamentous
projections half as long as the body — one rising from the
base of the beak, and the two others from the corners of the
mouth."
The most familiar case of sexual difference amongst North
American birds is that of the scarlet tanager, in which the
male is scarlet with jet-black wings, while the female is an
inconspicuous yellow-green color. Amongst domesticated
animals the peafowl shows the most beautiful case of sexual
differences. The magnificent tail of the male can be lifted
up, so as to be seen to best advantage when the male faces
the observer. Moreover the wild form, living in the forests
of India, has the same gorgeous train.
Darwin s Theory of Sexual Selection 199
The male Argus pheasant has a remarkable series of spots,
or ocelli, on the secondary wing-covers. They are concealed
until the male displays them before the female. Darwin
states that, while it may seem incredible that such elegant
shading and exquisite patterns could have been the outcome
of the taste of the female, yet the extraordinary attitude
assumed by the male during courtship appears entirely pur-
poseless, unless it be supposed that he is attempting to charm
the female by a display of his ornamentation.
Let us pass to the second class of cases, in which both
sexes are similarly and brightly colored, and in which the
young have a plumage different from the adults. For exam-
ple, the male and the female of the splendid scarlet ibis are
alike, whilst the young are brown. The males and females of
many finely colored herons are ornamented alike, and this
plumage, Darwin admits, has a nuptial character. He even
tries to explain this by the curious assumption, that while the
color has been acquired through the selection of the males
by the females, the results attained in this way have been
transmitted to both sexes. We find here another example of
the method so often employed by Darwin. When he meets
with facts that are not in conformity with the theory, he pro-
ceeds to make a new assumption without establishing its
validity. Thus, to assume that in all cases where the sexes
are colored differently, the characters acquired by the males
have been transmitted only to the same sex, and in those
cases where the sexes are colored alike the transmission has
been to both sexes, is most arbitrary.
In other cases, which are commoner than the last, the male
and female have the same color, and the young in their first
plumage resemble the adults. Darwin admits that here the
facts are so complex that his conclusions are doubtful. The
following account of the tree-sparrow shows how vague are
the principles involved in the entire discussion in relation to
transmission : —
200 Evolution and Adaptation
" Now with the tree-sparrow {P. montanus) both sexes and
the young closely resemble the male of the house-sparrow ;
so that they have all been modified in the same manner, and
all depart from the typical coloring of their early progenitor.
This may have been effected by a male ancestor of the tree-
sparrow having varied, firstly, when nearly mature ; or sec-
ondly, whilst quite young, and by having in either case
transmitted his modified plumage to the females and the
young ; or, thirdly, he may have varied when adult and trans-
mitted his plumage to both adult sexes, and, owing to the
failure of the law of inheritance at corresponding ages, at
some subsequent period to his young."
The further admissions made in the following quotation are
also significant : —
" The plumage of certain birds goes on increasing in
beauty during many years after they are fully mature ; this
is the case with the train of the peacock, with some of the
birds of paradise, and with the crest and plumes of certain
herons, for instance, the Ardea ludovicana. But it is doubt-
ful whether the continued development of such feathers is
the result of the selection of successive beneficial variations
(though this is the most probable view with birds of para-
dise) or merely of continuous growth. Most fishes continue
increasing in size, as long as they are in good health and
have plenty of food ; and a somewhat similar law may pre-
vail with the plumes of birds."
We need not follow Darwin through his discussion of
those cases in which the adults have a winter and a summer
dress and the young resemble the one or the other in plu-
mage, or are different from either. The discussion of these
cases, confessedly very complex, adds nothing to our under-
standing of the theory, and little but conjecture is offered
to account for the facts.
The extreme to which even conjecture can be carried may
be gathered from the following quotation, taken from the
Darwin s Theory of Sexual Selection 201
section dealing with cases in which the young in their first
plumage differ from each other according to sex, the young
males resembling more or less closely the adult males, and
the young females more or less closely the adult females :
"Two humming-birds belonging to the genus Eustepha-
nus, both beautifully colored, inhabit the small island of Juan
Fernandez, and have always been ranked as specifically dis-
tinct. But it has lately been ascertained that the one which
is of a rich chestnut-brown color with a golden-red head, is
the male, whilst the other, which is elegantly variegated with
green and white with a metallic-green head, is the female.
Now the young from the first somewhat resemble the adults
of the corresponding sex, the resemblance gradually becom-
ing more and more complete.
" In considering this last case, if as before we take the plu-
mage of the young as our guide, it would appear that both
sexes have been rendered beautiful independently ; and not
that one sex has partially transferred its beauty to the other.
The male apparently has acquired his bright colors through
sexual selection in the same manner as, for instance, the pea-
cock or pheasant in our first class of cases ; and the female
in the same manner as the female Rhynchaea or Turnix in
our second class of cases. But there is much difficulty in
understanding how this could have been effected at the same
time with the two sexes of the same species. Mr. Salvin
states, as we have seen in the eighth chapter, that with cer-
tain humming-birds the males greatly exceed the females in
number, whilst with other species inhabiting the same coun-
try the females greatly exceed the males. If, then, we might
assume that during some former lengthened period the males
of the Juan Fernandez species had greatly exceeded the
females in number, but that during another lengthened
period the females had far exceeded the males, we could
understand how the males at one time, and the females at
another, might have been rendered beautiful by the selection
202 Evolution and Adaptation
of the brighter-colored individuals of either sex ; both sexes
transmitting their characters to their young at a rather
earlier age than usual. Whether this is the true explanation
I will not pretend to say ; but the case is too remarkable to
be passed over without notice."
The third group of cases include those in which the fe-
males are more brightly colored, or more ornamented, than
the males. These cases are rare, and the differences between
the sexes are never so great as when the male is the more
highly colored. Wallace thinks that since in these cases the
male incubates the eggs his less conspicuous colors have
been acquired through natural selection. In the genus
Turnix the female is larger than the male, and lacks the
black on the throat and neck, and the plumage as a whole is
lighter than that of the male. The natives assert that the
females after laying their eggs associate in flocks, and leave
the males to do the incubating; and from other evidence
Darwin thinks that this is true.
In three species of painted snipe the females " are not only
larger but much more richly colored than the males," and the
trachea is more convoluted in some species. " There is also
reason to believe that the male undertakes the duty of incu-
bation." In the dotterel plover the female is larger and
somewhat more strongly colored. The males take at least
a share in the incubation. In the common cassowary the
female is larger and the skin of the head more brightly
colored than in the male. The female is pugnacious during
the breeding season and the male sits on the eggs. The
female emu is large and has a crest. She is more coura-
geous and pugilistic and makes a deep, hollow, guttural boom.
The male is more docile and can only hiss or croak. He
not only incubates the eggs, but defends the young against
their own mother. " So that with this emu we have a com-
plete reversal not only of the parental and incubating instincts,
but of the usual moral qualities of the two sexes ; the females
Darwin s Theory of Sexual Selection 203
being savage, quarrelsome, and noisy, the males gentle and
good. The case is very different with the African ostrich,
for the male is somewhat larger than the female and has
finer plumes with more strongly contrasted colors ; neverthe-
less he undertakes the whole duty of incubation."
Darwin attempts to explain these reversals of instincts on
the assumption that the males have turned the tables on the
females, and have themselves done the selecting ; and inci-
dentally, it may be pointed out in passing, they have had to
pay the penalty by incubating the eggs.
In the group of mammals, Darwin thinks that the male wins
the female by conquering other males rather than by charming
her through his display. The males, even when unarmed,
engage in desperate conflicts with each other, and sometimes
kill, but more often only wound, their fellows. The second-
ary sexual characters of the males have been acquired,
therefore, by natural selection applied to one sex, and less
frequently through the choice of the female. Since we are
here more especially concerned with the latter class of
phenomena, we may examine only a few cases under the
first head.
The horns of stags are used by them in their conflicts
with each other ; the tusks of the elephant make this animal
the most dangerous in the world, when in must. The horns
of bulls, the canine teeth of many mammals, the tusks of the
walrus, are further examples of organs which have been,
according to Darwin, acquired through the competitions of
the males with each other.
The voices of mammals are used for various purposes, " as
a signal of danger, as a call from one member of the troup to
another, and from the mother to her lost offspring, or from
the latter for protection."
"Almost all male animals use their voices much more dur-
ing the rutting season than at any other time ; and some, as
the giraffe and porcupine, are said to be completely mute
204 Evolution and Adaptation
excepting at this season. As the throats {i.e. the larynx and
thyroid bodies) of stags periodically become enlarged at the
beginning of the breeding season, it might be thought that
their powerful voices must be somehow of high importance
to them ; but this is very doubtful. From information given
to me by two experienced observers, Mr. McNeill and Sir P.
Egerton, it seems that young stags under three years old do
not roar or bellow ; and that the old ones begin bellowing at
the commencement of the breeding season, at first only occa-
sionally and moderately, whilst they restlessly wander about
in search of the females. Their battles are prefaced by loud
and prolonged bellowing, but during the actual conflict they
are silent. Animals of all kinds which habitually use their
voices utter various noises under any strong emotion, as
when enraged and preparing to fight ; but this may merely
be the result of nervous excitement, which leads to the spas-
modic contraction of almost all the muscles of the body, as
when a man grinds his teeth and clenches his fists in rage or
agony. No doubt stags challenge each other to mortal com-
bat by bellowing ; but those with the more powerful voices,
unless at the same time the stronger, better-armed, and more
courageous, would not gain any advantage over their rivals."
" Some writers suggest that the bellowing serves as a call
to the female ; but the experienced observers above quoted
inform me that female deer do not search for the male,
though the males search eagerly for the females, as indeed
might be expected from what we know of the habits of other
male quadrupeds. The voice of the female, on the other
hand, quickly brings to her one or more stags, as is well
known to the hunters who in wild countries imitate her cry.
"As the case stands, the loud voice of the stag during the
breeding season does not seem to be of any special service
to him, either during his courtship or battles, or in any other
way. But may we not believe that the frequent use of the
voice, under the strong excitement of love, jealousy, and rage,
Darwin s Theory of Sexual Selection 205
continued during many generations, may at last have pro-
duced an inherited effect on the vocal organs of the stag, as
well as of other male animals ? This appears to me, in our
present state of knowledge, the most probable view."
Here once more we find that Darwin makes use, as a sort
of last resort, of the principle of the inheritance of acquired
characters. As long as the theory of selection, in any of its
forms, appears to offer a satisfactory solution, we find the
facts used in support of this theory, but as soon as a diffi-
culty arises the Lamarckian theory is brought to the front.
It is this shifting, as we have already more than once pointed
out, that shows how little real basis there is for the theory of
sexual selection.
The male gorilla has a tremendous voice, and he has, as
has also the orang, a laryngeal sac. One species of gibbon
has the power of producing a correct octave of musical notes.
"The vocal organs of the American Mycetes caraya are
one-third larger in the male than in the female, and are won-
derfully powerful. These monkeys in warm weather make
the forests resound at morning and evening with their over-
whelming voices. The males begin the dreadful concert, and
often continue it during many hours, the females sometimes
joining in with their less powerful voices. An excellent
observer, Rengger, could not perceive that they were excited
to begin by any special cause ; he thinks that, like many
birds, they delight in their own music, and try to excel each
other. Whether most of the foregoing monkeys have acquired
their powerful voices in order to beat their rivals and charm
the females — or whether the vocal organs have been strength-
ened and enlarged through the inherited effects of long-
continued use without any particular good being thus gained
— I will not pretend to say ; but the former view, at least in
the case of the Hylobates agilis, seems the most probable."
The odor of some mammals is confined to, or more devel-
oped, in the males ; but in some forms, as in the skunk, it is
206 Evolution and Adaptation
present in both sexes. In the shrew mice, abdominal scent
glands are present, but since these mice are rejected by birds
of prey, their glands probably serve to protect them ; " never-
theless the glands become enlarged in the males during the
breeding season." In many other quadrupeds the scent
glands are of the same size in both sexes, and their func-
tion is unknown.
" In other species the glands are confined to the males, or
are more developed than in the females ; and they almost
always become more active during the rutting season. At
this period the glands on the sides of the face of the male
elephant enlarge, and emit a secretion having a strong musky
odor. The males, and rarely the females, of many kinds of
bats have glands and protrudable sacs situated in various
parts ; and it is believed that these are odoriferous.
" The rank effluvium of the male goat is well known, and
that of certain male deer is wonderfully strong and persist-
ent. Besides the general odor, permeating the whole body
of certain ruminants (for instance, Bos moschatus) in the
breeding season, many deer, antelopes, sheep, and goats,
possess odoriferous glands in various situations, more es-
pecially on their faces. The so-called tear-sacs, or subor-
bital pits, come under this head. These glands secrete a
semifluid fetid matter which is sometimes so copious as to
stain the whole face, as I have myself seen in an antelope.
They are ' usually larger in the male than in the female,
and their development is checked by castration.' According
to Desmarest they are altogether absent in the female of
Antilope subgutturosa. Hence, there can be no doubt that
they stand in close relation with the reproductive functions.
They are also sometimes present, and sometimes absent, in
nearly allied forms. In the adult male musk-deer {Moschus
moschifcrus), a naked space round the tail is bedewed with
an odoriferous fluid, whilst in the adult female and in the
male until two years old, this space is covered with hair, and
Darwin s Theory of Sexual Selection 207
is not odoriferous." Darwin believes in these cases that the
odor serves to attract the females. He admits that here,
" active and long-continued use cannot have come into
play as in the case of the vocal organs." He concludes,
therefore, that " the odor emitted must be of considerable
importance to the male, inasmuch as large and complex
glands, furnished with muscles for everting the sac, and for
closing or opening the orifice, have in some cases been
developed. The development of these organs is intelligible
through sexual selection, if the most odoriferous males are
the most successful in winning the females, and in leaving
offspring to inherit their gradually perfected glands and
colors."
There is sometimes a difference in the mammals in the
hair of the two sexes both in amount and in color. In some
species of goats the males have a beard, in others it is
present in both sexes. The bull, but not the cow, has curly
hair on the forehead. In some monkeys the beard is con-
fined to the male, as in the orang ; in other species it is only
larger in the males.
"The males of various members of the ox family (Bovidae),
and of certain antelopes, are furnished with a dewlap, or
great fold of skin on the neck, which is much less developed
in the female.
" Now, what must we conclude with respect to such sexual
differences as these ? No one will pretend that the beards
of certain male goats, or the dewlap of the bull, or the crests
of hair along the backs of certain male antelopes, are of any
use to them in their ordinary habits.
" Must we attribute all these appendages of hair or skin to
mere purposeless variability in the male ? It cannot be
denied that this is possible ; for in many domesticated quad-
rupeds, certain characters, apparently not derived through
reversion from any wild parent form, are confined to the
males, or are more developed in them than in the females
2o8 Evolution and Adaptation
— for instance, the hump on the male zebu cattle of India,
the tail of fat-tailed rams, the arched outline of the forehead
in the males of several breeds of sheep, and, lastly, the mane,
the long hairs on the hind-legs, and the dewlap of the male
of the Berbura goat."
In these cases and in others that Darwin cites, which seem
clearly to indicate that some of these secondary sexual charac-
ters are not the result of sexual selection, he concludes, "that
they must be due to simple variability, together with sexually
limited inheritance.
" Hence it appears reasonable to extend this same view to
all analogous cases with animals in a state of nature. Never-
theless I cannot persuade myself that it generally holds good,
as in the case of the extraordinary development of hair on
the throat and fore-legs of the male Ammotragus, or in
that of the immense beard of the male Pithecia. Such study
as I have been able to give to nature makes me believe that
parts or organs which are highly developed, were acquired
at some period for a special purpose. With those antelopes
in which the adult male is more strongly colored than the
female, and with those monkeys in which the hair on the
face is elegantly arranged and colored in a diversified
manner, it seems probable that the crests and tufts of hair
were gained as ornaments ; and this I know is the opinion of
some naturalists. If this be correct, there can be little doubt
that they were gained, or at least modified through sexual
selection ; but how far the same view may be extended to
other mammals is doubtful."
The astonishing colors in some of the monkeys cannot
be passed over without comment.
" In the beautiful Cercopithecus diana, the head of the
adult male is of an intense black, whilst that of the female
is dark gray ; in the former the fur between the thighs is of
an elegant fawn-color, in the latter it is paler.
" In the Cercopithecus cynosurus and griseoviridis one part
Darwin s Theory of Sexual Selection 209
of the body, which is confined to the male sex, is of the most
brilliant blue or green, and contrasts strikingly with the naked
skin on the hinder part of the body, which is vivid red.
" Lastly, in the baboon family, the adult male of Cyno-
cephalus hamadryas differs from the female not only by his
immense mane, but slightly in the color of the hair and of
the naked callosities. In the drill (C. IcucopJicziis) the females
and young are much paler-colored, with less green, than the
adult males. No other member in the whole class of mam-
mals is colored in so extraordinary a manner as the adult
male mandrill (C. mormon). The face at this age becomes
of a fine blue, with the ridge and tip of the nose of the most
brilliant red. According to some authors, the face is also
marked with whitish stripes, and is shaded in parts with
black, but the colors appear to be variable. On the fore-
head there is a crest of hair, and on the chin a yellow beard.
'Toutes les parties superieures de leurs cuisses et le grand
espace nu de leurs fesses sont egalement colores du rouge le
plus vif, avec un melange de bleu qui ne manque reellement
pas d'elegance.' When the animal is excited all the naked
parts become much more vividly tinted."
Darwin sums up the evidence in regard to the differences
in color between the male and female in the following
statement : —
" I have now given all the cases known to me of a differ-
ence in color between the sexes of mammals. Some of
these may be the result of variations confined to one sex
and transmitted to the same sex, without any good being
gained, and therefore without the aid of selection. We
have instances of this with our domesticated animals, as in
the males of certain cats being rusty-red, whilst the females
are tortoise-shell colored. Analogous cases occur in nature :
Mr. Bartlett has seen many black varieties of the jaguar,
leopard, vulpine phalanger, and wombat ; and he is certain
that all or nearly all these animals, were males. On the
p
210 Evolution and Adaptation
other hand, with wolves, foxes, and apparently American
squirrels, both sexes are occasionally born black. Hence it
is quite possible that with some mammals a difference in
color between the sexes, especially when this is congenital,
may simply be the result, without the aid of selection, of
the occurrence of one or more variations, which from the
first were sexually limited in their transmission. Neverthe-
less it is improbable that the diversified, vivid, and con-
trasted colors of certain quadrupeds, for instance, of the
above monkeys and antelopes, can thus be accounted for."
Finally, the case of man must be considered from the
point of view of sexual selection, for Darwin claims that
man has acquired a number of his secondary sexual char-
acters in this way. For instance, the beard is an excellent
case of a secondary sexual character. Darwin's interpretation
is that the beard has been retained, or even developed,
through the selection by the females of those males that
had this outgrowth best developed. Conversely, the absence
of hair on the face of the female is supposed by Darwin to
have been brought about by men selecting those women
having less hair on their faces. The greater intellect,
energy, courage, pugnacity, and size of man are the outcome
of the competition of the males with each other, since the
individual excelling in these qualities will be able to select
the most desirable wife, or wives, and it is assumed will,
therefore, leave more descendants. The standard of beauty
has been kept up by men selecting the most beautiful women
in each generation (the fate of the other married women is
ignored), and this beauty is supposed to have been transmitted
primarily to their daughters, but also to their sons.
Although all these forms of selection are imagined to be
acting in man, either alternately or simultaneously, yet Dar-
win recognizes in man a number of checks to the action
of sexual selection : amongst savages, the so-called com-
munal marriages ; second, infanticide, generally of the young
Darwiiis Theory of Sextcal Selection 2 1 1
females, which appears in some races to be practised to an
astonishing degree ; third, early betrothals ; fourth, the hold-
ing: of women as slaves.
When we recall that selection to be effective can only
be carried out under very exacting conditions, we cannot
but be appalled at the demands made here on our credulity.
The choice of the women has produced the beard of man,
the choice of man the absence of a beard in women ; the
competition of the males with each other is leading at the
same time to the development of at least half a dozen
qualities that are supposed to be male specialities, and
while all this is going on the results are being checked
sometimes by one means, sometimes by another. Moreover,
even this is not all that we are asked to accept, for there
are several other qualities of the male that are put down as
secondary sexual characters. For example, let us examine
what Darwin has to say in regard to the development of
the voice, and of singing in man.
In man the vocal cords are about a third longer than in
woman and his voice deeper. Emasculation arrests the de-
velopment of the vocal apparatus, and the voice remains like
that of a woman. This difference between the sexes, Dar-
win thinks, is due probably to long-continued use by the
male "under the excitement of love, rage, and jealousy."
In other words, an appeal is again made to the Lamarckian
theory, and in this case to explain the origin of an organ that
conforms to all the requirements of the secondary sexual
characters.
" The capacity and love for singing, or music, though not a
sexual character in man," in the sense of being confined to
one sex, yet is supposed to have arisen through sexual selec-
tion in the following way : " Human song is generally
admitted to be the basis or origin of instrumental music.
As neither the enjoyment nor the capacity of producing
musical notes are faculties of the least use to man in refer-
212 Evolution and Adaptation
ence to his daily habits of life, they must be ranked amongst
the most mysterious with which he is endowed."
Man is supposed to have possessed this faculty of song
from a very remote time, and even the most savage races
make musical sounds, although we do not enjoy their music,
or they ours.
" We see that the musical faculties, which are not wholly
deficient in any race, are capable of prompt and high de-
velopment, for Hottentots and Negroes have become excel-
lent musicians, although in their native countries they rarely
practise anything that we should consider music. Hence the
capacity for high musical development, which the savage
races of man possess, may be due either to the practice by
our semi-human progenitors of some rude form of music, or
simply to their having acquired the proper vocal organs for
a different purpose. But in this latter case we must assume,
as in the above instance of parrots, and as seems to occur
with many animals, that they already possessed some sense
of melody."
Darwin sums up the evidence in the two following state-
ments, the insufficiency of which to explain the phenomena
is I think only too obvious : " All these facts in respect to
music and impassioned speech become intelligible to a certain
extent, if we assume that musical tones and rhythm were used
by our half-human ancestors, during the season of courtship,
when animals of all kinds are excited not only by love, but
by the strong passions of jealousy, rivalry, and triumph.
From the deeply laid principle of inherited associations,
musical tones in this case would be likely to call up vaguely
and indefinitely the strong emotions of a long past age."
Thus the difficulty is shifted to the shoulders of our long
lost savage ancestors ; or even, in fact, to our simian fore-
fathers, as the following paragraph indicates : —
" As the males of several quadrumanous animals have
their vocal organs much more developed than in the females,
Darwin s Theory of Sexual Selection 213
and as a gibbon, one of the anthropomorphous apes, pours
forth a whole octave of musical notes and may be said to
sing, it appears probable that the progenitors of man, either
the males or females or both sexes, before acquiring the
power of expressing their mutual love in articulate language,
endeavored to charm each other with musical notes and
rhythm. So little is known about the use of the voice by
the Quadrumana during the season of love, that we have no
means of judging whether the habit of singing was first
acquired by our male or female ancestors. Women are
generally thought to possess sweeter voices than men, and as
far as this serves as any guide, we may infer that they first
acquired musical powers in order to attract the other sex.
But if so, this must have occurred long ago, before our ances-
tors had become sufficiently human to treat and value their
women merely as useful slaves. The impassioned orator,
bard, or musician, when with his varied tones and cadences
he excites the strongest emotions in his hearers, little
suspects that he uses the same means by which his half-
human ancestors long ago aroused each other's ardent pas-
sions during their courtship and rivalry."
We have now examined in some detail the evidence that
Darwin has brought forward in support of his hypothesis of
sexual selection. A running comment has been made while
considering the individual cases, but it may be well to sum
up the matter by briefly indicating the reasons why the hy-
pothesis seems incompetent to explain the facts.
General Criticism of the Theory of Sexual Selection
1. Some of the objections that apply to the theory of
natural selection apply also with equal force to the theory of
sexual selection in so far as the results in both cases are sup-
posed to be the outcome of the selection of individual, or
fluctuating, variations. If these variations appear in only
214 Evolution and Adaptation
a few individuals, their perpetuation is not possible, since
they will soon disappear through crossing. It would be, of
course, preposterous to suppose that at any one time only
those few individuals pair and leave descendants that have the
secondary sexual characters developed to the highest point,
but if something of this sort does not occur, the extreme of
fluctuating variations cannot be maintained. Even if half
of the individuals are selected in each generation, the accu-
mulation of a variation in a given direction could not go
very far. The assumption, however, that only half of all the
individuals that reach maturity breed, and that all of these
are chosen on account of the special development of their
secondary sexual characters, seems preposterous. Further-
more, if it is assumed that the high development of the new
character appears in a large number of individuals, then it
is not improbable that its continued appearance might be
accounted for without bringing in, at all, the hypothesis of
sexual selection.
2. But even supposing that the females select the most beau-
tiful males, then, since in the vast majority of higher animals
the males and the females are in equal numbers, the others will
also be able to unite with each other in pairs after this first
selection has taken place. Nothing will therefore be gained
in the next generation. It is interesting to see how Darwin
attempts to meet this argument. He tries to show in the
case of birds, that there are always unpaired individuals, but
since the few facts that he has been able to collect show that
there are as many additional females as males, the argument
proves too much. A few species are polygamous, one male
having a number of female birds ; but on this basis we can
only account, at best, for the development through com-
petition of the organs of offence and defence used to keep
away the weaker males. Yet it is just amongst these birds
that we often find the ornamental characters well developed.
In fact, since all the females in such cases are selected, and
Darwin s Theory of Sexual Selection 215
since they will transmit the characters of all the males, it is
evident that the secondary sexual characters could not be
formed in the way imagined.
3. If the female fails to select only the more ornamental
males, no result will follow. It has not been shown that she
is capable of making such a choice, and in the lower forms
particularly, it does not seem probable that this is done.
The argument that Darwin often employs, namely, that
unless she does select, the display of the males before her is
meaningless, is not to the point. So far as we can detect
the " cause " of the display of the male, it appears to be due
to his own excitement ; and even if we go so far as to admit
that the "purpose" is to attract the other sex, it still does
not in the least follow that the most ornamental male is se-
lected, and unless this occurs the display has no bearing on
the hypothesis of sexual selection.
4. The two forms of sexual selection, namely, competition
of the males with one another (really one form of natural
selection), and the selection of the most ornamental or gifted
individuals, are both used by Darwin to explain secondary
sexual characters, the one for organs of offence and defence,
and the other for ornamental characters. If we fully appre-
ciate the difficulties that any theory of selection meets with,
we shall realize how extraordinarily complex the action must
be, when two such processes are carried out at the same
time, or even during alternating periods.
5. It has been objected to Darwin's theory of sexual selec-
tion, that he suddenly reverses its mode of action to explain
those cases in which the female is the stronger and more
ornamented sex ; but if, as Darwin shows, the instincts of
the male have also changed, and have become more like those
of the female, I can see no inherent difficulty in this way of
applying the theory. A much more serious objection, it
seems to me, is that the male is supposed to select the female
for one set of characteristics, and the female to select the
216 Evolution and Adaptation
male for another set. It sounds a little strange to suppose
that women have caused the beard of man to develop by se-
lecting the best-bearded individuals, and the compliment has
been returned by the males selecting the females that have
the least amount of beard. It is also assumed that the results
of the selection are transmitted to one sex only. Unless, in
fact, the character in question were from the beginning
peculiar to only one sex as to its inheritance, the two sexes
might go on forever selecting at cross-purposes, and the result
would be nothing.
6. The development, or the presence, of the aesthetic feel-
ing in the selecting sex is not accounted for on the theory.
There is just as much need to explain why the females are
gifted with an appreciation of the beautiful, as that the beau-
tiful colors develop in the males. Shall we assume that still
another process of selection is going on, as a result of which
those females are selected by the males that appreciate their
unusual beauty, or that those females whose taste has soared
a little higher than that of the average (a variation of this
sort having appeared) select males to correspond, and thus the
two continue heaping up the ornaments on one side and the
appreciation of these ornaments on the other ? No doubt an
interesting fiction could be built up along these lines, but
would any one believe it, and, if he did, could he prove it ?
Darwin assumes that the appreciation on the part of the
female is always present, and he thus simplifies, in appearance,
the problem, but he leaves half of it unexplained.
7. It has been pointed out, that it is important to dis-
tinguish between the possible excitement of the female by
the display or antics of the male, and the selection of the
more beautiful or agile performer. Darwin himself records
a few cases, which plainly show that the more beautiful is
not always the more successful. It has also been suggested
that the battles of the males are sometimes sham performances,
and even when they are real, if the less vigorous do not remain
Darwin s Theory of Sexual Selection 217
to be destroyed but run away, they live to find mates of their
own. In fact, the conduct of the males at the breeding
season appears to be much more the outcome of their own
excitement than an attempt to attract the females.
8. There is another side to the question, the importance
of which is so great, that it is surprising that Darwin has
not taken any notice of it. If, in order to bring about, or
even maintain, the results of sexual selection, such a tre-
mendous elimination of individuals must take place, it is
surprising that natural selection would not counteract this
by destroying those species in which a process, so useless for
the welfare of the species, is going on. It is curious that this
has not been realized by those who believe in both of these
two hypotheses.
9. What has just been said applies also with almost equal
force to the development of such structures as the horns of
deer, bison, antelopes, and the brilliant colors of many insects
and birds. If in nature, competition between species takes
place on the scale that the Darwinian theory of natural selec-
tion postulates, such forms, if they are much exposed, would
be needlessly reduced in numbers in the process of acquiring
these structures. So many individuals would have been at
such a disadvantage in breeding, that if competition is as se-
vere as the theory of natural selection postulates, these species
could hardly be expected to compete successfully with other
species in which sexual selection was not taking place.
10. Darwin admits that, in certain cases, external condi-
tions may have acted directly to produce the colors in certain
forms, and if these were not injurious he thinks they might
have become constant. Such cases are left unexplained in
the sense that they are not supposed to be adaptations to any-
thing in particular. That colors produced in this way might
afterward be found useful, irrespective of how they arose, is
admitted as one of the ways in which sexual differences may
have arisen.
2i8 Evolution and Adaptation
ii. It is baffling to find Darwin resorting to the Lamarck-
ian explanation in those cases in which the improbability of
the hypothesis of sexual selection is manifest. If either prin-
ciple is true, we should expect it to apply to all phenomena of
the same sort ; yet Darwin makes use of the Lamarckian
principle, in the hypothesis of sexual selection, only when
difficulties arise.
12. In attempting to explain the development of the musi-
cal sense in man, it is clear that the hypothesis of sexual
selection fails to give a satisfactory explanation. To suppose
that the genius of a Beethoven or of a Mozart could have
been the result of a process of sexual selection is too absurd
to discuss. Neither the power of appreciation nor of expres-
sion in music could possibly have been the outcome of such a
process, and it does not materially help the problem to refer
it back to a troop of monkeys making the woods hideous
with their cries.
We come now to some of the special cases to which Dar-
win's hypothesis has been applied.
13. In one case at least, it is stated that a bird living on
the ground might have acquired the color of the upper sur-
face of the body through natural selection, while the under
surface of the males of the same species might have become
ornamented through the action of sexual selection. Thus in
one and the same individual the two processes are supposed
to have been at work, and it does not lessen the difficulty very
much by supposing the two processes to have been carried
out at different times, because it is evident that what had
been gained at one time by one process might become lost
while the color of certain parts was being acquired through
the other process.
14. Darwin points out that "the plumage of certain birds
goes on increasing in beauty during many years after they
are fully mature," as in the peacock, and in some of the birds
of paradise, and with the plumes and crests of some herons.
Darwin s Theory of Sexual Selection 2 1 9
This is explained as possibly merely the result of "continued
growth." The improbability of selection is manifest in these
cases, but if "continued growth " can accomplish this much,
why may not the whole process be also the outcome of such
growth ? At any rate, whatever the explanation is, it is im-
portant to find a case of a secondary sexual character that the
hypothesis obviously is insufficient to explain.
15. It is admitted in a number of cases, as in the stag for
instance, that, although the larynx of the male is enlarged,
this is not, in all probability, the outcome of sexual selection,
but in other forms this same enlargement is ascribed to the
selection process.
16. It is admitted that in none of the highly colored
British moths is there much difference according to sex,
although when a difference of color is found in butterflies
this is put down to the action of sexual selection. If such
wonderful colors as those of moths can arise without the
action of selection, why make a special explanation for those
cases in which this difference is associated with sex ?
17. It is well known that birds sing at other times of the
year than at the breeding season, and an attempt is made to
account for this in that birds take pleasure in practising those
instincts that they make use of at other times, as the cat
plays with the captive mouse. Does not this suggest that,
if they had certain instincts, they would- be more likely to
employ them at the times when their vitality or excitement
is at its highest without regard to the way in which they have
come by them ?
18. The color of the iris of the eyes of many species of
hornbills is said to be an intense crimson in the males, and
white in the females. In the male condor the eye is yellowish
brown, and in the female a bright red. Darwin admits that
it is doubtful if this difference is the result of sexual selec-
tion, since in the latter case the lining of the mouth is black
in the males, and flesh-colored in the females, which does not
220 Evolution and Adaptation
affect the external beauty. Yet if these colors were more
extensive and on the exterior, there can be little doubt that
they would have been explained as due to sexual selection.
19. When the females in certain species of birds differ
more from each other than they do from their respective
males, the case is compared to "those inexplicable ones,
which occur independently of man's selection in certain sub-
breeds of the game-fowl, in which the females are very dif-
ferent, whilst the males can hardly be distinguished." Here
then is a case of difference in color associated with sex, but
not the outcome of sexual selection.
20. The long hairs on the throat of the stag are said possi-
bly to be of use to him when hunted, since the dogs generally
seize him by the throat, " but it is not probable that the hairs
were specially developed for this purpose ; otherwise the
young and the females would have been equally protected."
Here also is a sexual difference that can scarcely be ascribed
to selection.
Some cases of differences in color between the sexes
" may be the result of variations confined to one sex, and
transmitted to the same sex without any good being gained,
and, therefore, without the aid of selection. We have
instances of this with our domesticated animals, as in the
males of certain cats being rusty-red while the females are
tortoise-shell colored. Analogous cases occur in nature :
Mr. Bartlett has seen many black varieties of the jaguar,
leopard, vulpine phalanger, and wombat ; and he is certain
that all or nearly all of these animals were males." If
changes of this sort occur, associated with one sex, why is
there any need of a special explanation in other cases of
difference ?
In the light of the many difficulties that the theory of
sexual selection meets with, I think we shall be justified in
rejecting it as an explanation of the secondary sexual differ-
Danviris Theory of Sexual Selection 221
ences amongst animals. Other attempts to explain these
differences have been equally unsuccessful. Thus Wallace
accounts for them as due to the excessive vigor of the male,
but Darwin's reply to Wallace appears to show that this is
not the cause of the difference. He points out that, while
the hypothesis might appear plausible in the case of color,
it is not so evident in the case of other secondary sexual
characters, such, for instance, as the musical apparatus of the
males of certain insects, and the difference in the size of the
larynx of certain birds and mammals.
Darwin's theory served to draw attention to a large num-
ber of most interesting differences between the sexes, and,
even if it prove to be a fiction, it has done much good in
bringing before us an array of important facts in regard to
differences in secondary sexual characters. More than this I
do not believe it has done. The theory meets with fatal ob-
jections at every turn.
In a later chapter the question will be more fully discussed
as to the sense in which these secondary sexual differences
may be looked upon as adaptations.
CHAPTER VII
THE INHERITANCE OF ACQUIRED CHARACTERS AS
A FACTOR IN EVOLUTION
Lamarck's Theory
One of the most striking and peculiar characteristics of
living things is that through use a part is able to carry out
a particular function better than before, and in some cases
the use of the part leads to its increase in size. Conversely,
disuse leads to the decrease of a part in size. We are per-
fectly familiar with this process in ourselves as applied to our
nervous system and muscles.
It is not surprising that the idea should have arisen that,
if the results of the use of a part are inherited by the next
generation, the adaptation of organisms might be explained
in this way. The presence of the organs of touch, in those
parts of the body that are more likely to come into contact
with foreign bodies, offers a striking parallel to the perfecting
of the sensation of touch that can be brought about through
the use of any part. The development of eyes only on the
exposed parts of the body, as on the tentacles of the seden-
tary annelids, or along the margin of the mantle of a bivalve
mollusk, suggests that there may be some direct connection
between their presence in these regions and the effect of
light on the parts. In fact, ever since the time of Lamarck,
there have been many zoologists who have claimed that many
of the adaptations of organisms have arisen in this way, that
is, through the inheritance of the characters acquired through
use. In general this theory is summed up in the phrase,
"the inheritance of acquired characters."
222
Inheritance of Acquired Characters 223
This view is prominently associated with the name of
Lamarck, who held, however, a different view in- regard
to the origin of some of the other structures of the organism.
Moreover, Erasmus Darwin, even before Lamarck, had sug-
gested the principle of the inheritance of acquired characters.
As has just been said, Lamarck held that the inheritance of
acquired characters was only one of the ways in which ani-
mals have become changed, and he clearly stated that in the
case of all plants and of some of the lower animals the change
(evolution) which he supposed them to undergo was due to
the general influence of the environment. Since plants and
the lower animals (as he supposed) have no central nervous
system, or at least no such well-defined nervous system as
have the higher animals, Lamarck thought that they could
not have evolved in the same way as have the higher animals.
We now know that, so far as the lower animals, at least, are
concerned, there was no need for such a distinction, since
many of their responses are like those of the higher animals.
This distinction that Lamarck made is responsible, no doubt,
for a misconception that was long held in regard to a part of
his views. It is often stated that he supposed the desire
of the animal for a particular part has led to the develop-
ment of that part ; while in reality he only maintained the
desire to use a particular organ to fulfil some want led to
its better development through exercise, and the result was
inherited. Lamarck also supposed that the decrease in use
of a part which leads to its decrease in size accounts for the
degeneration of organs.
Lamarck first advanced his theory in 1801, when he cited
the following examples in its favor. A bird, driven through
want to the water to find its food, will separate its toes when
they strike the water. The skin uniting the bases of the toes
will be stretched in consequence, and in this way the broad
membrane between the toes of ducks and geese has been
acquired. The toes of a bird that is in the habit of perching
224 Evolution and Adaptation
on a tree become elongated in consequence of becoming
stretched, hence has arisen the foot with the long toes char-
acteristic of arboreal birds.
Shore-birds, " which do not care to swim," but must
approach the water in order to obtain food, will be in danger
of sinking into the mud, "but, wishing to act so that their
body shall not fall into the liquid, they will contract the habit
of extending and lengthening their legs." Hence have arisen
the stiltlike legs of shore-birds.
These ideas were more fully elaborated in the following
year. He added the further examples : Our dray-horses
have arisen through the use to which they have been put,
and the race-horse also, which has been used in a different
way. Cultivated plants, on the contrary, are the result of the
new environment to which they have been subjected.
In the "Philosophie Zoologique," published in 1809, Lamarck
has much more fully developed his theory. Here he combats
strenuously the idea that species are fixed. His point of view
may be judged by the following propositions, which he be-
lieves can be established : —
1. That all organized bodies of our globe are veritable
productions of nature, which she has successively produced
in the course of a long time.
2. That in her progress nature began, and begins still
every day, to produce the simplest organisms, and that she
still produces directly the same primitive kinds of organiza-
tions. This process has been called spontaneous generation.
3. That the first beginning of animals and of plants takes
place in favorable localities and under favorable circum-
stances. An organic movement having once established
their production, they have of necessity gradually developed
their organs, and have become diversified in the course of
time.
4. That the power of growth of each part of the body
being inherited as a consequence of the first effect of life,
Inheritance of Acqttired Characters 225
different modes of multiplication and of regeneration have
arisen, and these have been conserved.
5. That with the aid of sufficient time and of favorable
circumstances the changes that have taken place on the sur-
face of the globe have called forth new structures and new
habits, and in consequence have modified the organs of the
body, and made animals and plants such as we see them at
the present day.
6. Finally, as a result of these changes that living bodies
have been forced to undergo, species have been formed, but
these species have only a relative constancy, and are not as
ancient as is nature herself. If the environment remains the
same, species also remain the same, as is exemplified by the
animals living at present in Egypt, which are exactly like
those living there in ancient times.
Lamarck concludes that the appearance of stability is
always mistaken by the layman for the reality, because, in
general, every one judges things relatively to himself. In
fact, species are not absolutely constant, but are so only
temporarily. " The influence of the environment is con-
tinuous and always active, but its effects may only be
recognized after a long time." The irregularity and the
complexity of the organization of animals is the outcome of
the infinitely diversified circumstances to which they have
been subjected. These changes, Lamarck claims, do not
directly cause modifications in the form of animals, 1 but
bring about changes in their needs, and changes in their
needs bring about changes in their actions. If the needs
remain the same, the acquired actions become habits. These
habitual actions lead to the use of certain parts in preference
to others, and this in turn to an alteration in form and struc-
ture. The individuals so changed breed together and leave
descendants that inherit the acquired modification.
Curiously enough, Lamarck follows up this argument by
1 This is clearly meant to be applied only in the case of higher animals.
Q
226 Evolution and Adaptation
citing some cases amongst plants that have been changed
directly by the action of the environment. He says that
since plants have no motions they have consequently no
habits, but they are developed by changes in their nutrition,
etc., and this brings about the superiority of some of the
vital movements over others.
Amongst domestic animals Lamarck cites the case of the
dog:, that has come from a wild form like the wolf, but hav-
ing been carried into different countries has acquired different
and new habits, and this has led to the formation of new
races, such as the bulldog, greyhound, pug-dog, spaniel, etc.
Lamarck's argument shifts so often back and forth from
animals to plants, that it is clear that in his own mind he did
not see any important difference between the action of the
environment on plants, and the use of the organs of the
animal. He gives in this same connection his oft-quoted
summary of what he calls the two laws of nature " which
observation always establishes."
First Law. In every animal, that has not passed beyond
the term of its development, the frequent and sustained use
of any organ strengthens it, develops it, increases its size,
and gives it strength proportionate to the length of time of its
employment. On the other hand, the continued lack of use of
the same organ sensibly weakens it ; it deteriorates, and its
faculties diminish progressively until at last it disappears.
Second Law. Nature preserves everything that she has
caused the individual to acquire or to lose by the influence
of the circumstances to which the race has been for a long
time exposed, and consequently by the influence of the pre-
dominant use of certain organs (or in consequence of its
continued disuse). She does this by the generation of new
individuals which are produced with the newly acquired
organs. This occurs, provided that the acquired changes
were common to the two sexes, or to the individuals that
produced the new forms.
Inheritance of Acquired Characters 227
These laws are, Lamarck says, fundamental truths which
cannot be misunderstood except by those who have never
observed or followed nature in her operations. He insists
that it is a mistake to suppose that the parts are responsible
for the functions, for it is easy to demonstrate that it is the
needs and uses of the organs that have caused the parts to
develop.
If it is supposed, he continues, that these laws are hypo-
thetical, they may be demonstrated by the following facts :
The adult baleen whale is without teeth, although in the
foetus teeth are present, concealed in the jaws. The loss
of the teeth is the result of the whale swallowing its food
without first masticating it. The ant-eater is also without
teeth, and has also the habit of swallowing its food without
chewing it. The mole has very small eyes, and this is the
result of its having made very little use of them, since its
habits are subterranean. Another animal, the aspalax, has
only the rudiments of eyes, and has almost completely lost
the power of sight. This animal also lives underground like
the mole.
Proteus, an aquatic salamander living in deep caves, has
only rudimentary eyes. In these latter cases it is the disuse
of the eye that has led to its degeneration. This is proven,
Lamarck adds, by the fact that the organs of hearing are
never in this condition, because sound vibrations penetrate
everywhere, even into the densest bodies.
It is a part of the plan of organization of the reptiles that
they have four legs ; but the snakes, although belonging to
this group, have no legs. This absence of legs is explained
by their having acquired the habit of gliding over the ground,
and of concealing themselves in the grass. Owing to their
repeated effort to elongate themselves, in order to pass
through narrow spaces, their bodies have become drawn out.
Under these circumstances legs would be useless, since long
ones would interfere with their motion, and short ones could
228 Evolution and Adaptation
not move their long bodies. Since the plan of organization
limits the snakes to only four legs, and since this number
would be useless, they have disappeared.
Many insects are destitute of wings, although wings are a
part of the plan of organization of this group. They are
absent only in those forms whose habits render wings useless,
consequently they have disappeared through disuse.
The preceding cases are those in which the disuse of an
organ has led to its degeneration. The following cases
are cited to show that by use an organ increases in size.
The formation of the web in the feet of water-birds has
already been given as a character which Lamarck supposes
to have been acquired through use ; also the case of shore-
birds, which, by an effort to elongate their legs, have actually
made them so in the course of time. The necks of water-
birds are also long on account of their having been stretched
in the efforts to catch fish. The long tongues of the ant-eater,
of the woodpecker, and of humming-birds are the result of
use, and the long, forked tongue of serpents has come from
their using their tongue to feel objects in front of them.
Fishes that have acquired the habit of living in shallow
water, flounders, soles, etc., have been forced to swim on their
sides in order to approach nearer to the shore. Since more
light comes from above than from below, the eye on the
under side, straining to turn to the light, has finally migrated
to the upper side.
The habit of eating great quantities of food, which distends
the digestive organs, has caused the bodies of herbivorous
quadrupeds to become large, as seen in the elephant, the
rhinoceros, oxen, horses, and buffaloes. The habit of stand-
ing for a long time on their feet has caused some animals to
develop hard, thick hoofs. Herbivorous animals, that inhabit
countries where they are constantly subjected to attack, as
deer and antelopes for example, are forced to escape by rapid
flight, and in consequence their bodies have become slen-
Inheritance of Acquired Characters 229
derer and their legs thinner. The horns, antlers, and pro-
tuberances that many of these animals possess are the results
of their butting each other when angered.
" The long neck and the form of the giraffe offer a curious
case. We know that the giraffe is the tallest of all animals.
It inhabits the centre of Africa, living in those localities
where the earth is nearly always dry and without herbage.
It is obliged to browse on the foliage of trees, and this leads
to its stretching continually upwards. As a result of this
habit, carried on for a long time, in all the individuals of the
race, the anterior limbs have become longer than the pos-
terior, and its neck has also lengthened, so that the giraffe
without rising on its hind-legs stretches up its neck and can
reach to the height of six metres."
The curved claws of the carnivora have arisen from the
necessity of grasping their prey. The power of retracting
the claws has also been acquired by the effort to draw them
in when running over hard ground. The abdominal pouch
of the kangaroo, in which the young are carried, opens an-
teriorly, and this has led to the animal standing erect so that
its young are not injured. In consequence, the fore-legs
have become shorter through disuse, and the hind-legs have
become stronger through use. The tail, which is also used as
a support, has become enormously thick at its base.
The sloth has been compelled to seek refuge in the trees,
and has taken up its abode permanently there, feeding on
leaves. Its movements are limited to those involved in
crawling along the limbs in order to reach the .leaves. After
feeding it remains inactive and sluggish, these habits being
provoked by the heat of the climate. The results of its mode
of life have been to cause the arms to become elongated due
to the habit of the sloth of grasping the limbs of the tree ;
the claws of the fingers and toes have also become long and
hooked in order to retain their hold. The digits that do not
make any individual movements have lost the power to do
230 Evolution and Adaptation
so, and have become fused, and can only be bent in and
straightened out. The thighs, being bent out to clasp the
larger branches, have caused the pelvis to widen, and, in con-
sequence, the cotyloid cavities have become directed back-
ward. Many of the bones of the skeleton have become
fused, as a result of the immobility of the animal.
Lamarck says, that " Nature, in producing, successively, all
the species of animals, beginning with the most imperfect, or
the most simple, and terminating with the most perfect, has
gradually complicated their organization. These animals
becoming scattered throughout the habitable regions of the
globe each species has received from the influences of its
surroundings its present habits, and the modifications of the
parts the use of which we recognize."
Such are Lamarck's views and a fairly complete statement
of the facts from which he draws his conclusions. His
illustrations appear naive, and often not a little ludicrous,
but it must be admitted that, despite their absurdities, his
theory appears in some cases to account wonderfully well for
the facts. The long legs of wading birds, the long neck and
disproportionately long fore-legs of the giraffe, the structure
of the sloth, and particularly the degeneration of the eyes of
animals living in the dark, seem to find a simple explanation
in the principle of the inheritance of acquired characters.
But the crucial point of the entire theory is passed over in
silence, or rather is taken for granted by Lamarck, namely, the
inheritance in the offspring of the characters acquired through
use or disuse in the parent. He does not even discuss this
topic, but in several places states unreservedly that the in-
crease or decrease of a part reappears in the next generation.
It is here that Lamarck's theory has been attacked in more
modern times, for as soon as experimental proof was de-
manded to show that the results of use or of disuse of an
organ is inherited, no such proof was forthcoming. Yet
the theory is one that has the great merit of being capable of
Inheritance of Acquired Characters 231
experimental test, and it is astonishing to find that, with
the immense amount that has been written by his followers,
so few attempts have been made to give the theory a thorough
test. The few results that have been obtained are not, how-
ever, favorable to the theory, but almost the only attempts at
experiment that have been made in this direction have been
those of mutilating certain parts ; and were it not for popu-
lar belief to the effect that such mutilations are inherited,
one would least expect to get evidence for or against the
theory in this direction. Lamarck himself believed that the
changes were slowly acquired, and I think modern Lamarck-
ians are justified in claiming that the validity of the theory
can only be tested by experiments in which the organism is
subjected to influences extending over a considerable period,
although Lamarck appears to have believed that the first
results may appear quite soon. Before expressing any
opinion in regard to the probability of the theory, let us
examine what the followers of Lamarck have contributed in
the way of evidence to the theory, rather than the applica-
tions that they have made of the theory. We shall also find
it profitable to consider some of the modern criticism, to which
the theory has been subjected.
Despite the contempt with which Darwin referred to
Lamarck's theory, he himself, as we have seen, often made
use of the principle of the inheritance of acquired characters,
and even employed the same illustrations cited by Lamarck.
Darwin seems to have misunderstood Lamarck's view, and
to have accepted the current opinion that Lamarck sup-
posed an animal acquired a new organ by desiring or need-
ing it. Darwin says, " Heaven forefend me from Lamarck's
nonsense of a tendency to progressive adaptation from
the slow willing of the animals." Darwin speaks of La-
marck as stating that animals will that the &gg shall be
a particular form so as to become attached to particular
objects. Lamarck's latest biographer, Packard, says he is
232 Evolution and Adaptation
unable to find any statements of this sort in Lamarck's
writings.
The following cases that Darwin tried to explain through
the inheritance of acquired characters are exactly like those
to which Lamarck applied his theory. The bones of the
wing of the domestic duck weigh less than those of the wild
duck, and the bones of the leg more. Darwin believes this
is due to the effects of the inheritance of acquired characters.
The drooping ears of many domestic mammals are also
explained by him as a result of disuse — "the animals being
seldom much alarmed." In speaking of the male of the
beetle, Onites apelles, Darwin quotes Kirby to the effect that
the tarsi are so habitually lost that the species has been
described without this part of the foot. In the sacred beetle
of Egypt the tarsus is totally absent. Hence he concludes
that the absence of tarsi in the sacred beetle, and the rudi-
mentary condition of the tarsus in others, is probably the
result of disuse, rather than a case of inheritance of a muti-
lation. Darwin grants that "the evidence that accidental
mutilations can be inherited is at present not decisive, but
the remarkable case observed by Brown-Sequard in guinea-
pigs of the inherited effects of operations should make us
cautious in denying this tendency."
The wingless condition of several insects inhabiting oceanic
islands has come about, Darwin thinks, through disuse. The
ostrich also, owing to its increase in size, made less use of its
wings and more use of its legs, with the result that its wings
degenerated and its legs got stronger. The rudimentary
condition of the eyes of the mole is the result of disuse,
" aided perhaps by natural selection." Many of the ani-
mals inhabiting the caves of Kentucky and of Carniola
are blind, and this is ascribed to disuse. " As it is diffi-
cult to imagine that the eyes, though useless, could be in
any way injurious to animals living in darkness, their loss
may be attributed to disuse." The long neck of the giraffe
Inheritance of Acquired Characters 233
Darwin attributes partly to natural selection and partly
to use.
These references will suffice to show that Darwin is in full
accord with the main argument of Lamarck. In fact, the
curious hypothesis of pangenesis that Darwin advanced was
invented partly to account for the inheritance of acquired
characters. Despite the hesitancy that Darwin himself felt
in advancing this view, and contrary to Huxley's advice, he
at last published his provisional hypothesis of pangenesis in
the twenty-seventh chapter of his " Animals and Plants
under Domestication."
Darwin's Hypothesis of Pangenesis
The study of bud variation, of the various forms of inheri-
tance, and of reproduction and of the causes of variation, led
him, Darwin says, to the belief that these subjects stand in
some sort of relation to each other. He says : " I have been
led, or rather forced, to form a view which to a certain extent
connects these facts by a tangible method. Every one would
wish to explain to himself, even in an imperfect manner, how
it is possible for a character possessed by some remote
ancestor suddenly to reappear in the offspring; how the
effects of increased or decreased use of a limb can be trans-
mitted to the child ; how the male sexual element can act not
solely on the ovules, but occasionally on the mother form ;
how a hybrid can be produced by the union of the cellular
tissue of two plants independently of the organs of genera-
tion ; how a limb can be reproduced on the exact line of
amputation, with neither too much nor too little added ; how
the same organism may be produced by such widely different
processes, as budding and true seminal generation ; and,
lastly, how of two allied forms, one passes in the course of
its development through the most complex metamorphoses,
and the other does not do so, though when mature both are
234 Evolution and Adaptation
alike in every detail of structure. I am aware that my view
is merely a provisional hypothesis or speculation ; but, until
a better one be advanced, it will serve to bring together a
multitude of facts which are at present left disconnected by
any efficient cause."
In presenting the hypothesis of pangenesis Darwin begins
by enumerating the different kinds of sexual and asexual
processes of reproduction, for which he hopes to offer a
provisional explanation. Here we find mentioned various
methods of budding and self-division, regeneration, partheno-
genesis, sexual reproduction, and the inheritance of acquired
characters. It is with the last only that we are here chiefly
concerned ; in fact, the need of an hypothesis of this sort to
explain the other kinds of inheritance is by no means evident.
There are, however, two other phenomena, besides that of the
supposed inheritance of acquired characters, to which the
hypothesis of pangenesis might appear to apply specially,
namely, the effect of foreign pollen on the tissues of the
mother plant, and the supposed influence of the union with
the first male on the subsequent young (telegony). It is,
however, far from being shown that any influence of this
latter kind really occurs, despite the fact that it is generally
believed in by breeders.
It is important to observe that Darwin proposes to explain
on the hypothesis of pangenesis, not only the inheritance of
characters acquired through use, but also the decrease of
structures through disuse ; and this applies, not only to the
structure, but to function as well, as when the intelligence of
the dog is explained through his association with man, and
the tameness of the domestic rabbits through their long con-
finement. In the following quotation these points are referred
to : " How can the use or disuse of a particular limb or of the
brain affect a small aggregate of reproductive cells, seated in
a distant part of the body, in such a manner that the being
developed from these cells inherits the characters of either
Inheritance of Acquired Characters 235
one or both parents ? Even an imperfect answer to this
question would be satisfactory."
Coming now to the theory, we find that it consists of one
chief assumption and several minor ones. " It is universally
admitted that the cells or units of the body increase by self-
division or proliferation, retaining the same nature, and that
they ultimately become converted into the various tissues and
substances of the body. But besides this means of increase
I assume that the units throw off minute granules which are
dispersed throughout the whole system ; that these, when
supplied with proper nutriment, multiply by self-division, and
are ultimately developed into units like those from which
they were originally derived. These granules may be called
gemmules. They are collected from all parts of the system
to constitute the sexual elements, and their development in
the next generation forms a new being ; but they are likewise
capable of transmission in a dormant state to future genera-
tions, and may then be developed. . . . Gemmules are sup-
posed to be thrown off by every unit, not only during the
adult state, but during each stage of development of every
organism ; but not necessarily during the continued existence
of the same unit. Lastly, I assume that the gemmules in
their dormant state have a mutual affinity for each other,
leading to their aggregation into buds, or into the sexual
elements. Hence, it is not the reproductive organs, or buds,
which generate new organisms, but the units of which each
individual is composed. These assumptions constitute the
provisional hypothesis which I have called Pangenesis."
It will be noticed that the first assumption is that the cells
throw off minute gemmules or granules. The second assump-
tion is that these are collected in the reproductive organs, or
in buds, or in regenerating parts ; the third assumption is
that the gemmules may lie dormant through several genera-
tions ; the fourth, that the development of the reproductive
cells is not so much the development of the cell itself, but of
236 Evolution and Adaptation
the gemmules that have collected in it. The fifth assumption
is that the gemmules are thrown off at all stages of develop-
ment ; the sixth, that in their dormant state they have a
mutual affinity for each other ; the seventh, that there may be
a sort of continual competition in the germ-cells between the
original gemmules and the new ones, and, according to which
win, the old or the new form develops. Thus we see on
closer analysis that the pangenesis hypothesis is made up of
a goodly number of different assumptions. At least half a
dozen imaginary properties are ascribed to the imaginary
gemmules, and these attributes are all essential to the
working of the hypothesis.
Some of the more obvious objections to the hypothesis
have been stated by Darwin himself. Such, for instance, as
our ignorance at what stage in their history the body-cells are
capable of throwing off gemmules, and whether they collect
only at certain times in the reproductive organs, as the
increased flow of blood to these organs at certain seasons
might seem to indicate. Nor have we any evidence that they
are carried by the blood at all. The experiment of Galton, of
transfusing the blood of one animal into another, and finding
that this produced no effect on the young that were born
later, might be interpreted to mean that gemmules are not trans-
ported by the blood ; but this kind of experiment is inconclu-
sive, especially in the light of recent results on the effect of
the blood of one animal on that of another.
A part of the evidence on which Darwin relied to support
his theory has been shown to be incorrect by later work.
Thus the assumption that more than a single pollen grain, or
more than one spermatozoon, is necessary in some cases for
fertilization, is certainly wrong. In most cases, in fact, the
entrance of more than one spermatozoon into the egg is dis-
astrous to the development. The cases referred to by Dar-
win can probably be explained by the difficulty that some of
the pollen grains, or spermatozoa, may have in penetrating
Inheritance of Acquired Characters 237
the egg, or to the immaturity or impotence of some of the
male germ-cells, and not to the need of more than one to
accomplish the true fertilization.
Darwin's idea that the small number of gemmules in the
unfertilized egg may account for the lack of power of such
eggs to develop until they are fertilized, has been shown to
be incorrect by recent results in experimental embryology.
We now know that many different kinds of stimuli have the
power to start the development of the egg. Moreover, we
also know that if a single spermatozoon is supplied with a
piece of egg-protoplasm without a nucleus, it suffices to cause
this piece of protoplasm to develop.
In the case of regeneration, which Darwin also tries to
explain on the pangenesis hypothesis, we find that there is
no need at all for an hypothesis of this sort ; and there are a
number of facts in connection with regeneration that are not
in harmony with the hypothesis. For instance, when a part is
cut off, the same part is regenerated ; but under these circum-
stances it cannot be imagined that the part removed supplies
the gemmules for the new part. Darwin tries to meet this
objection by the assumption that every part of the body con-
tains gemmules from every other part. But it has been
shown that if a limb of the newt is completely extirpated, a
new limb does not regenerate ; and there is no reason why it
should not do so on Darwin's assumption that germs of the
limb exist throughout the body.
The best-authenticated cases of the influence of the male
on the tissues of the female are those in plants, where one
species, or variety, is fertilized by another. Thus, if the
orange is fertilized by the pollen of the lemon, the fruit may
have the color and flavor of the lemon. Now the fruit is a
product of the tissues of the ovary of the female, and not a
part of the seedling that develops in the fruit from the cross-
fertilized egg-cell. Analogous cases are recorded for the
bean, whose pods may have their color influenced by fertil-
2 3S Evolution and Adaptation
izing the flower with pollen of another variety having pods
of a different color. In these cases we do not know whether
the color of the fruit is influenced directly by the foreign
pollen, or whether the influence is through the embryo that
develops from the egg-cell. The action may appear to be the
same, however, in either case ; but because it seems probable
here that there is some sort of influence of one tissue on
another, let us not too readily conclude that this is brought
about through any such imaginary bodies as gemmules. It
may be directly caused, for instance, by some chemical sub-
stance produced in the young hybrid plant. If this is the
case, the result would not be different in kind from that of
certain flowers whose color may be influenced by certain
chemical substances in the soil.
In the cases amongst animals, where the maternal tissues are
believed to be influenced by a previous union with the male, as
in the oft-cited case of Lord Morton's mare, a reexamination of
the evidence by Ewart has shown that the case is not demon-
strated, and not even probable. Several years ago I tried to
test this view in the case of mice. A white mouse was first
bred to a dark male house-mouse, and the next time to a
white mouse, but none of the offspring from the second union
showed any trace of black. If the spermatozoa of the dark
mouse are hypodermically injected into the body-cavity of the
female, the subsequent young from a white male show no evi-
dence that the male cells have had any influence on the ovary.
The following facts, spoken of by Darwin himself, are
not in favor of his hypothesis of pangenesis : " But it
appears at first sight a fatal objection to our hypothesis
that a part of an organ may be removed during several
successive generations, and if the operation be not followed
by disease, the lost part reappears in the offspring. Dogs
and horses formerly had their tails clocked during many gen-
erations without any inherited effect; although, as we have
seen, there is some reason to believe that the tailless condi-
Inheritance of Acquired Characters 239
tions of certain sheep-dogs is due to such inheritance." The
answer that Darwin gives is that the gemmules themselves,
that were once derived from the part, are still present in
other parts of the body, and it is from these that the organs
in the next generation may be derived. But Darwin fails to
point out that, if this were the case, it must also be true for
those cases in which an organ is no longer used. Its decrease
in size in successive generations cannot be due to its disuse,
for the rest of the body would supply the necessary gemmules
to keep it at its full state of development. Thus, in trying to
meet an obvious objection to his hypothesis, Darwin brings for-
ward a new view that is fatal to another part of his hypothesis.
The following cases, also given by Darwin, are admitted by
him to be inexplicable on his hypothesis : " With respect to
variations due to reversion, there is a similar difference be-
tween plants propagated from buds and seeds. Many varie-
ties can be propagated securely by buds, but generally or
invariably revert to their parent forms by seed. So, also,
hybridized plants can be multiplied to any extent by buds,
but are continually liable to reversion by seed, — that is, to
the loss of their hybrid or intermediate character. I can
offer no satisfactory explanation of these facts. Plants with
variegated leaves, phloxes with striped flowers, barberries
with seedless fruit, can all be securely propagated by buds
taken from the stem or branches ; but buds from the roots of
these plants almost invariably lose their character and revert
to their former condition. This latter fact is also inexplica-
ble, unless buds developed from the roots are as distinct from
those on the stem, as is one bud on the stem from another,
and we know that these latter behave like independent or-
ganisms.". As Darwin here states, these facts appear to be
directly contradictory to his hypothesis, and he makes no
effort to account for them.
The entire question of the possibility of the inheritance of
acquired characters is itself at present far from being on a
240 Evolution and Adaptation
satisfactory basis, as we shall try to show; and Darwin's
attempt at an explanation, in his chapter on pangenesis, does
not put the matter in a much more satisfactory condition.
The Neo-Lamarckian School
Let us now turn our attention to a school that has grown
up in modern times, the members of which call themselves
Neo-Lamarckians. Let us see if they have supplied the
essential evidence that is required to establish the Lamarck-
ian view, namely, that characters acquired by the individual
are transmitted to the offspring.
Lamarck's views were adopted by Herbert Spencer, and
play an important role in his " Principles of Biology " (1866-
1871), and even a more conspicuous part in his later writings.
In the former he cites, amongst other cases, that of " a puppy
taken from its mother at six weeks old who, although never
taught ' to beg ' (an accomplishment his mother had been
taught), spontaneously took to begging for everything he
wanted when about seven or eight months old." If tricks
like this are inheritable is it not surprising that more puppies
do not stand on their hind-legs ?
The larger hands of the laboring classes in England are
supposed to be inherited by their children, and the smaller
hands of the leisure classes are supposed to be the result of
the disuse of the hands by their ancestors ; but even if these
statements in regard to size are true, there are many other
conceivable causes that may have led to this result.
Short-sightedness appears more often, it is said, in those
classes of society that make most use of their eyes in reading
and in writing ; but if we ask for experimental evidence to
show that this is due to inheritance, and not due to the chil-
dren spoiling their eyes at school, there is none forthcoming.
The problem is by no means so simple as the uninitiated may
be led to believe.
Inheritance of Acquired Characters 241
Spencer thinks that " some of the best illustrations of
functional heredity are furnished by mental characteristics."
He cites the musical faculty as one that could not have been
acquired by natural selection, and must have arisen through
the inheritance of acquired modifications. The explanation
offered is " that the habitual association of certain cadences
of speech with certain emotions has clearly established in
the race an organized and inherited connection between
such cadences and such emotions, . . . and that by the con-
tinued hearing and practice of melody there has been gained
and transmitted an increasing musical sensibility." But a
statement that the results have been acquired in this way
does not supply the proof which the theory is in need of ;
neither does it follow that, because the results cannot be
explained by the theory of natural selection, therefore, they
must be explained by the Lamar ckian theory.
The clearest proofs that Spencer finds of the inheritance
of acquired characters are in the well-known experiments of
Brown-Sequard. These experiments will be more fully dis-
cussed below. Amongst the other morbid processes that
Spencer thinks furnish evidence in favor of this view, are
cases of a tendency to gout, the occurrence of mental tricks,
musical prodigies, liability to consumption, in all of which
cases the fundamental distinction between the inheritance of
an acquired character and the inherited tendency toward a
particular malady is totally ignored.
Twenty-seven years later (in 1893) Spencer took up the
open challenge of the anti-Lamarckian writers, and by bring-
ing forward a number of new arguments attempted to rein-
state the principle of the inheritance of acquired characters.
His first illustration is drawn from the distribution of the
sense of touch in different parts of our bodies. Weber's ex-
periments have shown that if the sharp points of a pair of
compasses are applied to the tips of the forefingers, the sen-
sation of two separate points is given when the points are
242 Evolution and Adaptation
only one-twelfth of an inch apart, and if the points are moved
nearer together, they give the sensation of only one point.
The inner surfaces of the second joints of the fingers can
only distinguish two points when they are one-sixth of an
inch apart. The innermost joints are less discriminating,
and are about equal in the power of discrimination to the tip
of the nose. The end of the big toe, the palm of v the hand,
and the cheek discriminate only about one-fifth as well as do
the tips of the fingers. The back of the hand and the top
of the head distinguish only about one-fifteenth as well as
the finger-tips. The front of the thigh, near the knee, is
somewhat less sensitive than the back of the hand. On the
breast the points of the compasses must be separated by more
than an inch and a half in order to give two sensations. In
the middle of the back the points must be separated by two
and a half inches, or more, in order to give two separate
impressions.
What is the meaning of these differences, Spencer a,§Jc£.*
If natural selection has brought about the result, then it must
be shown that " these degrees of endowment have advan-
taged the possessor to such an extent that not infrequently
life has been directly or indirectly preserved by it." He
asks if this, or anything approaching this, result could have
occurred.
That the superior perceptiveness of the forefinger-tip
might have arisen through selection is admitted by Spencer,
bat how could this have been the case, he asks, for the mid-
dle of the back, and for the face ? The tip of the nose has
three times more power of discrimination than the lower
part of the forehead. Why should the front of the thigh
near the knee be twice as perceptive as in the middle of
the thigh ; and why should the middle of the back and of
the neck and the middle of the forearm and of the thigh
stand at such low levels? Is it possible, Spencer asks again,
that natural selection has determined these relations, and
Inheritance of Acquired Characters 243
if not, how can they be explained? His reply is that the
differences can all be accounted for on the theory of the
inheritance of use, for it is evident that " these gradations
in tactile perceptiveness correspond with the gradations in
the tactual exercise of the parts." Except from contact
with the clothing the body receives hardly any touch sen-
sations from outside, and this accounts for its small power
of discrimination. The greater sensitiveness of the chest
and abdomen, as compared with the back, is due to these
regions being more frequently touched by the hands, and
is also owing to inheritance from more remote ancestors,
in which the lower surface of the body was more likely to
have come in contact with foreign objects than was the back.
The middle of the forearm and of the thigh are also less ex-
posed than the knee and the hand, and have correspondingly
the power of tactile discrimination less well developed.
Weber showed that the tip of the tongue is more sensitive
than any other part of the body, for it can distinguish be-
tween two points only one twenty-fourth of an inch apart.
Obviously, Spencer says, natural selection cannot account
for such extreme delicacy of touch, because, even if it were
useful for the tongue to distinguish objects by touch, this
power could never be of vital importance to the animal. It
cannot even be supposed that such delicacy is necessary for
the power of speech.
The sensitiveness of the tongue can be accounted for,
however, Spencer claims, as the result of the constant use
of the tongue in exploring the cavity of the mouth. It is
continually moving about, and touching now one part, and
now another, of the mouth cavity. " No advantage is gained.
It is simply that the tongue's position renders perpetual ex-
ploration almost inevitable." No other explanation of the
facts seemed possible to Spencer.
Two questions will at once suggest themselves. First, can
it be shown that the sensitiveness to touch in various parts of
244
Evolution and Adaptation
the body is the result of individual experience ? Have we
learned to discriminate in those parts of the body that are
most often brought into contact with surrounding objects ?
Even the power of discrimination in the tips of the fingers
can be improved, as Spencer himself has shown, in the case
of the blind, and of skilled compositors. Can we account in
this way for the power of discrimination in various parts of
the body ? In other words, if, beginning in infancy, the middle
of the back constantly came into contact with surrounding
objects, would this region become as sensitive as the tips of
the fingers ? The experiment has not, of course, been carried
out, but it is not probable that it would succeed. I venture
this opinion on the ground of the relative number of the
nerves and of the organs of touch on the back, as compared
with those of the finger-tips. But, it will be asked, will not
the number of the sense-organs become greater if a part is
continually used by the individual ? It is improbable that
much improvement could be brought about in this way. The
improvement that takes place through experience is probably
not so much the result of the development of more sense-
organs, as of better discrimination in the sensation, because
the increased power can be very quickly acquired.
An examination of the relative abundance of touch-spots in
the skin shows that they are much more numerous in regions
of greater sensitiveness. The following table, taken from
Sherrington's account of sense-organs in Schaefer's "Text-
book of Physiology," gives the smallest distance that two
points, simultaneously applied, can be recognized as such (and
not simply as one impression) in different regions.
Mm.
I.I
2-3
4-5
4-5
6.8
9.0
9.0
Tip of tongue .....
Volar surface of ungual phalanx of finger
Red surface of lip .
Volar face of second phalanx .
Dorsal face of third phalanx .
Side of tongue ....
Third line of tongue, 27 mm. from tip
Inheritance of Acquired Characters 245
Mm.
Plantar face of ungual phalanx of first toe 11.3
Palm 11. 3
Back of second phalanx of finger 1 1 .3
Forehead 22.6
Back of ankle 22.6
Back of hand 31.6
Forearm, leg ........... 40.6
Dorsum of foot 4°-6
Outer sternum 45.1
Back of neck 54.1
Middle of back 67.1
Upper arm, thigh 67.1
The great difference in the sensitiveness of the skin in
the different regions is very striking, and if, as seems probable,
about the same proportionate difference is found at birth, then
the degree of sensibility of the different regions is inborn,
and is not the result of each individual experience. Until it
can be shown that more of the sense-organs develop in any
special part, as the result of the increased use of the part, we
have no real basis on which to establish, even as probable,
the Lamarckian view.
But, after all, is the distribution of the sense-organs ex-
actly that which we should expect on the Lamarckian theory ?
Has not Spencer taken too much for granted in this direc-
tion ? The lower part of the forearm (represented by 15) we
should expect to be more sensitive than the protected surface
of the eyelid (11. 3), but this is not the case. The forehead
(22.6) is much less sensitive than the forearm, and only half
as sensitive as the eyelid. The knee (36.1) is still less sensi-
tive than any of these other parts, and this does not in the
least accord with the theory, since in its constant moving
forward it must be continually coming into contact with foreign
bodies. The fact that the back is as insensitive as the upper
arm (67.7) can hardly be accredited in favor of the theory.
The great difference between the lower third of the forearm
on the ulnar surface (15) and the upper arm (67.7) seems
246 Evolution and Adaptation
out of all proportion to what we should expect on the theory.
And is it not a little odd that the end of the nose should be
so highly sensitive ?
There is another point that we cannot afford to neglect in
this connection. It is known that in addition to touch-spots
there are warm and cold spots in the skin, which produce,
when touched, the sensation of warmth, or of cold, respec-
tively, and not the sensation of touch. The degree of
sensitiveness of different regions of the body throws an
interesting side-light on Spencer's argument.
The warm spots are much fewer than the cold spots. The
spots are arranged in short lines radiating from centres
which coincide with hairs. The number of these spots
varies a good deal, even in the same region of the skin. If
the sensitiveness of the skin is tested, the following results
will be obtained. The list includes twelve grades of sensitive-
ness, beginning with the places giving the lowest maximum
of intensity. About one hundred square areas were tested
in each region.
COLD SENSATIONS
i. Tips of fingers and toes, malleoli, ankle.
2. Other parts of digits, tip of nose, olecranon.
3. Glabella, chin, palm, gums.
4. Occiput, patella, wrist.
5. Clavicle, neck, forehead, tongue.
6. Buttocks, upper eyelid.
7. Lower eyelid, popliteal space, sole, cheek.
8. Inner aspect of thigh, arm above elbow.
9. The intercostal spaces along axillary line.
10. Mammary areola.
11. Nipple, flank.
12. Certain areas of the loins and abdomen.
WARMTH SENSATIONS
0. Lower gum, mucosa of cheek, cornea.
1. Tips of fingers and toes, cavity of mouth, conjunctiva, and
patella.
Inheritance of Acquired Characters 247
2. Remaining surface of digits, middle of forehead, olecranon.
3. Glabella, chin, clavicle.
4. Palm, buttock, popliteal space.
5. Neck.
6. Back.
7. Lower eyelid, cheek.
8. Nipple, loin.
These two tables show the great differences in the range
of sensitiveness to cold and to warmth in different parts of
the body. I doubt if any one will attempt to show that
these differences of range of sensation can be accounted for
either by natural selection or by the Lamarckian hypothesis.
Of course, it does not necessarily follow that, because this
is true for the warm and cold spots, that it must also be true
for the tactile organs; but I think that the fact of such a great
difference in the responsiveness to cold and to warmth in
different parts of the body should put us on our guard against
a too ready acceptation of Spencer's argument. More espe-
cially is this seen to be necessary, when, as has been shown
above, the distribution of the touch-organs themselves by no
means closely corresponds to what we should expect, if they
have developed in response to contact, as Spencer maintains.
The other main argument advanced by Spencer to fortify
the theory of the inheritance of acquired characters, and at
the same time to show the inadequacy of the theory of natu-
ral selection, is based on the idea of what he calls the " co-
operation of the parts " that is required in order to carry out
any special act. Spencer contends that " the relative powers
of cooperative parts cannot be adjusted solely by the sur-
vival of the fittest, and especially where the parts are nu-
merous and the cooperation complex."
Spencer illustrates his point by the case of the extinct
Irish elk, whose immensely developed horns weighed over
a 
