Evolution and adaptation

ALBERT R. MANN
LIBRARY
AT
GORNELL UNIVERSS1\

Evolution and adaptation,

TNL

Cornell University

Library

The original of this book is in
the Cornell University Library.

There are no known copyright restrictions in
the United States on the use of the text.

http://www.archive.org/details/cu31924003176066

EVOLUTION AND ADAPTATION

EVOLUTION

AND ADAPTATION

BY

THOMAS HUNT MORGAN, Pu.D.

Nets Work

THE MACMILLAN COMPANY
LONDON: MACMILLAN & CO., Lr.

1908

All rights reserved

Copvricut, 1903,
By THE MACMILLAN COMPANY.

Set up and electrotyped. Published October, 1903. Reprinted
January, 1908.

é Norwood ress
J. 8. Cushing Co. — Berwick & Smith Co.
Norwood, Mass., U.S.A.

TO

Professor William ikeith Brooks

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

*

vill 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 problémes 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 zz 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
reéxamine 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 Io, 1903.

CONTENTS

CHAPTER I

PAGE
THE PROBLEM OF ADAPTATION . : F . ‘ . I
Structural Adaptations . ‘ I
Adjustments of the Individual to Chases in the civioaniek = 12
Adaptations for the Good of the Species . ‘i 3 3 - 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 c 2 . ; . . . - 25

Comparison with Inorganic Phenomena . x 3 . - 26

CHAPTER II

THE THEORY OF EVOLUTION . é ; 7 . ‘ 7 + 30

Evidence in Favor of the Transmutation Theory. 3 - 32
Evidence from Classification and from Comparative Anat-

omy . . F = of os $a Tere eS. Bz

’ The Geological Evidence . ‘ i . . ‘i - 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 = F i A ‘ - 58
The Recapitulation Theory . a : is - . 58
Conclusions . z 3 s a . é - . 84

CHAPTER IV
Darwin’s THEORIES OF ARTIFICIAL AND OF NATURAL SELECTION QI
The Principle of Selection : ‘ - . 7 s - gt
Variation and Competition in Nature ‘i 7 é - 104

The Theory of Natural Selection 5 j j . . 16

xi

xii Contents

CHAPTER V

THE THEORY OF NATURAL SELECTION (continued)
Objections to the Theory of Natural Selection .
Sterility between Species
Weismann’s Germinal Selection

CHAPTER VI

DarRwIn’s THEORY OF SEXUAL SELECTION
Sexual Selection

General Criticism of the Theory of Sexual Selection .

CHAPTER VII

THE INHERITANCE OF ACQUIRED CHARACTERS .

Lamarck’s Theory . : 5 ; a .
Darwin’s Hypothesis of Pangenesis . : .
The Neo-Lamarckian School . i % .

CHAPTER VIII

CONTINUOUS AND DISCONTINUOUS VARIATION AND HEREDITY

Continuous Variation
Heredity and Continuous Variation
Discontinuous Variation

Mendel’s Law . 7 5 ‘ P ‘ i

w The Mutation Theory of De Vries. . é

Conclusions : ‘ i F ‘ é A
CHAPTER IX

EVOLUTION AS THE RESULT OF EXTERNAL AND INTERNAL FACTORS

The Effect of External Influences

Responsive Changes in the Organism that adapt it to the New

Environment .
Nageli’s Perfecting Principle

PAGE
129
129
147
154

167
213

222
222

233
240

261
261
270
272
278
287
297

300
300

319
325

Contents

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 hcg Disuse .

Reactions of the Organism to Poisons, etc.

Regeneration

CHAPTER XI

TROPISMS AND INSTINCTS AS ADAPTATIONS

CHAPTER XII

SEx AS AN ADAPTATION

The Different Kinds of Sexual Individuals

The Determination of Sex
Sex as a Phenomenon of Adaptation

CHAPTER XIII

SUMMARY AND GENERAL CONCLUSIONS .

INDEX . . . . . . .

xiii

PAGE
340
340
350
352
357
360
370
371
372
375
375
376
377
379

382

414
422

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, ze. 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,

B i

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 plan 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 poller: 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 (Papzlio
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 usec for food, it fulfils
a purpose necessary for the welfare of she 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, z.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 larve, 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 Problem 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. Asa 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 and 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 Ka//zma 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. Soclose 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 9

theless, if the color is useful.to its possessor, it is an adapta-
tion in our s-nse 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 algz 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.

10 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 Aristolochia Clematitis, which he refers to as a
conspicuous and peculiar adaptation. In Figure 1 A-a group
of flowers is shown, and in Figure 1 B and C a single
flower is split open to show the interior. In Ba 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.

Lhe 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,

Fic. 1.— 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 1 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 1 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 acase of response in an adap.
tive direction that could not have been the outcome of experi-
ence in the past. Itis 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, z.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 (Lumébriculus) 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,

16 Lvolution 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 ry

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 pupe 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

c

18 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 danger 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 tothem. 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 Evolution 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

thatits 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 Lvolution 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, z.e. spontaneous generation does not occur. The

30

The Theory of Evolution Si

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 Zransmutation 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

i

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 réle 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, 2.2.
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. ven 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, zzter 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.
D

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

‘

Lhe 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.” 1 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,
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 sbme 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 thé 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 hypothese, 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 be spoken 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 archzeop-
teryx, which in many ways appears to stand midway between
the reptiles and birds. This fossil, archeopteryx, 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 vertebre.

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
archzopteryx 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! 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 preémi-

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.?

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 naive 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 T heory 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 afe 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 51

hippus ina 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
archeopteryx. 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 archzopteryx 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 archzopteryx, 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
vertebra, 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 archzopteryx 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 archzopteryx 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
archzopteryx as a form belonging to a group from which
living birds have arisen. That a number of recent paleon-
tologists believe archzopteryx 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 vertebrz 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 saurians
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
archaeopteryx 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.!
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 aseries 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 tracheze, 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, has 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, inthe 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 EvmpENcE 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 réle
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! 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? 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 of 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 1811, 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,
medusz, 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 says,
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.!

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-

1In one place Von Baer raises the question whether the egg may not bea
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.!_ 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 1842 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.

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 gillarches. 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 ata very
early stage of development as a fold of the upper wall of

The T: heory 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 vertebre. The noto-
chord becomes partially obliterated as the centra of the
vertebree 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 vertebra. These
vertebrz 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 vertebree.

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 cancall 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 vertebrz.

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

F

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 Geoffroy 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, che 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 ege
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 penzeus, 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 zoéa.
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 zoéa is the ancestor of the decapods. The later
development of the zoéa shows, however, that it cannot
be such an ancestral form, for, in order to reach the

70 Evolution and Adaptation

full number of segments characteristic of the decapods,
new segments are intercalated between the cephalothorax
and abdomen. In fact, in many zoéas 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 zoéa 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 zoéa 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 zoéa, however, by making a single
supposition, namely, that the ancestor from which the group
of decapod has evolved had a larva like the zoéa, and that
this larval form has been handed down to all of the de-
scendants.

The fate of the zoéa 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 res

at one end,—the gastrea. 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,

72 Evolution and Adaptaton

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.!

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 ve

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 fat-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 isa 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 T: heory of Evolution oe

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.' He says: “ Direct
observation has shown that, when an animal species varies
(z.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. via 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 97

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

78 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
acase 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 willshow 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
spermatozoén) 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! 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 amcoebas 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 amceba, 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 preéxisting life. | Hertwig means by the theory of biogenesis that as
the egg develops there is « constant interchange between itself and its sur-

roundings.

80 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 ameeba. 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 phylogeny.
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 81

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: 1. 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 gastrzea 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
gastreea, which hasalready 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

G

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 Haeckel, 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
gastreea 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 amceba 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,
but 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 zdea that adult
ancestral stages have been pushed back into the embryo, and
that the embryo recapitulates in part these ancestral adult
Stages ts 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
Linnzan 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 Linnaan 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 hypothese, 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

Lhe Theory of Evolution 87

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 due 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 thisconnection. 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 im 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 preéminently 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

g!I

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, z.c. 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 indefinite vari-
ability in the endless slight peculiarities which distinguish the
individuals of the same species, and which cannot be accounted
for by inheritance from either parent or from some more
remote ancestor. Even strongly marked differences 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 duck 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 bya 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.” }
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,
ze. 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,
o. 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

LOO 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: 1. 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, (1) 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 zzzer 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 : —

“Tt 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.

“Tt has been seen in the last chapter that amongst organic
beings in a state of nature there is some individual variability :

108 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-

110 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 Natural Selection 111

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

I

114 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 she
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-

116 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-

Va eS

Darwin's Artificzal 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-

118 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

ny

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.

“Tt 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. So again a vast number of mature animals and plants,
whether or not they be the best adapted to their conditions,

120 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.

“Tt 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 bea 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-
minosz, 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; and 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

12
K < 9

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 petztzo principiiz. 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 : —

“Tf 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 Composite and
Umbelliferze, 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, z.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

138 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,
jis 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
Molothrus 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 Molothrus badius are stated by an excellent observer, Mr.
- Hudson, sometimes to live promiscuously together in flocks,
and sometimes to pair. They either build a nest of their own,
or seize on one belonging to some other bird, occasionally
throwing out the nestlings of the stranger. They either lay
their eggs in the nest thus appropriated, or oddly enough
build one for themselves on the top of it. They usually sit
on their own eggs and rear their own young; but Mr. Hudson
says it is probable that they are occasionally parasitic, for he
has seen the young of this species following old birds of a
distinct kind and clamoring to be fed by them. The parasitic
habits of another species of Molothrus, the JZ donariensis,
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 pupe, 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: “JI 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

Darwin's 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.”

“Tt 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.

“T 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

L

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 Miiller, all the individuals
are in this peculiar condition. So that with some species,
certain abnormal individuals, and in other species all the
individuals, can actually be hybridized much more readily
than they can be fertilized by pollen from the same individual
plant!”

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 Kélreuter. To give an instance: Mirabilis
jalapa can easily be fertilized by the pollen of MZ. longifora,
and the hybrids thus produced are sufficiently fertile ; -but
Kélreuter tried more than two hundred times, during eight

150 Evolution and Adaptation

following years, to fertilize reciprocally M7. dongifiora with the
pollen of M7. 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 limtts 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, zzer 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 1 53

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
spectally 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,! 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.

Darwin's Artificial and Natural Selection 155

formed a serious stumbling-block to the theory of selection.
Though still assuming that the przmary 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 conditions 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.’’}

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 means 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

Darwin's 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 itis 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 pessimisti:

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 : —

“Tf 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.

“Tn any event, the supposed formative laws are not obliga-
tory. Dispensations from them can be issued and are issued
whenever utility requires tt.”

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 inachis ; 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 utterlyignored 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 dzlaterally symmetrical figure
engraved on a surface which is essentially radially symmetri-
cal in its divisions.

“T 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 aé zuztio, 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 /usus nature. 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 preéstablished
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

162 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 tts 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: dy the selection alone of the
plus or minus variations of a character 1s 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: dy a selection of the kind
referred to the germ is progressively modified in a manner
corresponding with the production of a definitely directed
progressive vartation 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, w/y 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., z¢s 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 zt zs 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 whit 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

166 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 known
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

Tue 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 ona struggle between. the individuals. of one Sex,
generally thé 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

168 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: (1) 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 1 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 power
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 sar 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-.
HOD, since _unarmed, unornamented, or lnatttactive: moles

Darwin's Theory of Sexual Selection 169

a numerous progeny, but for the presence of better-endowed |
males. We may 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 réferred 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 171

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. Jenner 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
Agcleus pheniceus the males have had their colors greatly
intensified in the south; whereas with Cardzinalis virginianus
it is the females which have been thus affected : with Quzs-
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

174 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, Coelenterata, 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, antennz, 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

OS

Fic. 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 Cado-
calanus plumulosus, in which one of the sete 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

178 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

Darwin's 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, 7.¢. 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

180 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 bea 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 Ichneumonide,
according to Mr. Walsh, the males are almost universally
lighter-colored than the females. On the other hand, in the
Tenthredinide the males are generally darker than the
females. In the Siricide the sexes frequently differ; thus
the male of Sivex_ juvencus is banded with orange, whilst the

182 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. Miiller 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 Prionidz, 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

Darwin's 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 Copridz, 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
zesthetic 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-
nz. 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, Coldas 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

186 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 larvee 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

'

188 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 larva of C. pruznosa; 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 thére 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
Tipulaz. 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
(Culicidze) also seem to attract each other by humming; and
Professor Mayer has recently ascertained that the hairs on
the antennz 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 189

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 Linnzus,
and by many subsequent naturalists, as a distinct species; it
is of a dingy reddish brown, with the dorsal fn 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 Macropus :—

“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 ‘l’étalage de
leurs vives couleurs chercher a attirer l’attention des femelles,
lesquelles ne paraissaient indifférentes 4 ce manége, 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.”

“Tt 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

ce)

194 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 Azo/s, the brilliant
throat patches of Széaria 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

Darwin's 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 aérial 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 aérial 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 ove 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 Adaptateon

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-
ing 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. zudicollis) 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 Rhynchzea 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 (z.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 cavaya 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
—JI will not pretend to say; but the former view, at least in
the case of the Hy/obates 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
moschiferus), 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 (Bovida),
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

208 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. leucopheus) 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 supérieures de leurs cuisses et le grand
espace nu de leurs fesses sont également colorés du rouge le
plus vif, avec un mélange de bleu qui ne manque réellement
pas d’élégance.’ 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 :—

“T 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 selectidn, 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 othef, 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

Darwin's Theory of Sexual Selection 211

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 t the more ornamental

males, no no result will follow. It has not been shown that she
“WS capable ot of making ng 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
sn eanIe eee) is not to the point. So far as we can detect
the “cause” of the display of the male, i it appears ‘to be due
“to-his own excitement j and even i 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 or namental male is se-
Tected, and unless this occurs the display has no ) bearing on
the hypothesis of sexual selection. oe sae
eae 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 zsthetic 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.

g.. 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.

218 Evolution and Adaptation

11. 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 tefer
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 219

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-

Darwin's Theory of Sexual Selection oak

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 ptocess 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 a2)

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 Acquired 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,! 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.

Lnheritance of Acquired Characters ae7

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 throughuse. 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

L[iheritance 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 egg shall be
a particular form so as to become attached to particular
objects. Lamarck’s latest biographer, Packard, says he is

252 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, Onztes 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-Séquard 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 Acguired Characters 234

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 HyYPoTHESIS 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 ¢hzs 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 Acguived 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 atall. 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 Acguired 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 nota
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-

238 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 reéxamination 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 docked during many gen-
erations without any inherited effect; although, as we have
seen, there is some reason to believe that the tailless condi-

ee ae ay

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 ona

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 NeEo-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 réle 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 Lamarckian theory.

The clearest proofs that Spencer finds of the inheritance
of acquired characters are in the well-known experiments of
Brown-Séquard. 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

R

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 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 asks.
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,
but 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, c
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.
Tip of tongue . - : . : . a SEA
Volar surface of ungual phalanx of finger ‘i ‘ ‘ : s- 233
Red surface of lip . i é c ‘ . ‘ » 45
Volar face of second phalanx . Z ‘ 7 . é , © 45
Dorsal face of third ake : ‘ A : é 3 C - 68
Side of tongue ‘ ; F 2 : + 9.0

Third line of tongue, 27 mm. Been Gp ‘ : e - ‘ - 9.0

Inheritance of Acquired Characters 245

Mm
Plantar face of ungual phalanx of first toe . A ‘ A «EG
Palm ‘ : F é 7 . . 7 5 ‘ s - 11.3
Back of second phalanx of finger. 5 3 5 7 is + 113
Forehead . . F ‘ - . . Fi ‘i ; . 22.6
Back of ankle ‘ . 3 . . . ‘ 3 ‘ - 22.6
Back of hand . : . . . . ‘ . ‘ . - 31.6
Forearm, leg . - : . 3 3 : e $ ‘ » 40.6
Dorsum of foot : - . - . - : F - - 40.6
Outer sternum . 3 . - . . ‘ ‘ 7 + 45.1
Back of neck . $ é . ‘ . . ei . . + 54.1
Middle of back , : . ‘ ‘ F F ; . 67.1
Upper arm, thigh . : 4 7 3 . . . . . 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

Tips of fingers and toes, malleoli, ankle.
Other parts of digits, tip of nose, olecranon.
Glabella, chin, palm, gums.

Occiput, patella, wrist.

Clavicle, neck, forehead, tongue.
Buttocks, upper eyelid.

Lower eyelid, popliteal space, sole, cheek.
Inner aspect of thigh, arm above elbow.
9. The intercostal spaces along axillary line.

1o. Mammary areola.

11. Nipple, flank.

12. Certain areas of the loins and abdomen.

Sr aARaR YS om

WARMTH SENSATIONS

o. Lower gum, mucosa of cheek, cornea.

1. Tips of fingers and toes, cavity of mouth, conjunctiva, and
patella. ”

Inheritance of Acquired Characters 247

Remaining surface of digits, middle of forehead, olecranon.
Glabella, chin, clavicle.

Palm, buttock, popliteal space.

Neck.

Back.

Lower eyelid, cheek.

Nipple, loin.

COPS OR eG

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 codperative parts cannot be adjusted solely by the sur-
vival of the fittest, and especially where the parts are nu-
merous and the codperation complex.”

Spencer illustrates his point by the case of the extinct
Irish elk, whose immensely developed horns weighed over
a hundredweight. The horns, together with the massive
skull, could not have been supported by the outstretched

248 Evolution and Adaptation

neck without many and great changes of the muscles and
bones of the neck and of the fore-part of the body. Unless,
for instance, the fore-legs had been also strengthened, there
would be failure in fighting and in locomotion. Since “we
cannot assume spontaneous increase of all these parts pro-
portionate to the additional strains, we cannot suppose them
to increase by variations one at once, without supposing the
creature to be disadvantaged’ by the weight and nutrition of
the parts that were for a time useless, — parts, moreover,
which would revert to their original sizes before the other
needful variations occurred.”

The answer made to this argument was that codrdinating
parts vary together. In reply to which Spencer points to
the following cases, which show that this is not so: The
blind crayfish in the Kentucky caves have lost their eyes,
but not the stalks that carry them. Again, the normal
relation between the length of tongue and of beak in some
varieties of pigeons is lost. The greater decrease in the
jaws in some species of pet dogs than of the number of
their teeth has caused the teeth to become crowded! “I
then argued that if codperative parts, small in number,
and so closely associated as these are, do not vary together,
it is unwarrantable to allege that cooperative parts, which are
very numerous and remote from one another, vary together.”
Spencer puts himself here into the position of seriously main-
taining that, because some coéperative parts do. not vary to-
gether, therefore no codperative parts have ever done so, and
he has taken this position in the face of some well-known
cases in which certain parts have been found to vary together.

In this same connection Spencer brings up the familiar
piece de résistance of the Lamarckian school, the giraffe. He
recognizes that the chief traits in the structure of this animal
are the result of natural selection, since its efforts to reach

1It is curious that Spencer does not see that this case is as much against his
point as in favor of it, since the unused teeth did not also degenerate.

Inheritance of Acquired Characters 249

higher branches could not be the cause of the lengthening of
the legs. But “the coadaptation of the parts, required to make
the giraffe’s structure useful, is much greater than at first
appears.” For example, the bones and the muscles of the
hind-legs have been also altered, and Spencer argues that it
is “impossible to believe’’ that all parts of the hind-quarters
could have been coadapted to one another, and to all parts
of the fore-quarters. A lack of coadaptation of a single muscle
“would cause fatal results when high speed had to be main-
tained while escaping from an enemy.”

Spencer claims that, since 1886, when he first published
this argument, nothing like an adequate response has been
made; and I think he might have added that an adequate
answer is not likely to be forthcoming, since nothing short
of a demonstration of how the giraffe really evolved is
likely to be considered as sufficient. Wallace’s reply, that
the changes in question could have been brought about
by natural selection, since similar changes have been brought
about by artificial selection, is regarded as inadequate by
Spencer, since it assumes a parallel which does not exist.
Nevertheless, Wallace’s reply contains, in my opinion, the
kernel of the explanation, in so far as it assumes that con-
genital variation! may suffice to account for the origin of
a form even as bizarre as that of the giraffe. The ancon
ram and the turnspit dog were marked departures from the
normal types, and yet their parts were sufficiently codrdi-
nated for them to carry out the usual modes of progression.
It would not have been difficult, if we adopted Spencer’s
mode of arguing, to show that these new forms could not
possibly have arisen as the result of congenital variations.

Again, it might be argued that the large, powerful dray-
horse could not have arisen through a series of variations
from the ordinary horse, because, even if variations in the

1 Wallace assumes fluctuating variation to suffice, but in this I cannot agree
with him.

250 Evolution and Adaptation

right direction occurred in the fore-quarters, it is unlikely that
similar variations would occur in the hind-quarters, etc. Yet
the feat has been accomplished, and while it is difficult to
prove that the inheritance of acquired characters has not had
a hand in the process, it is improbable that this has been the
case, but rather that artificial selection of some kind of vari-
ations has been the factor at work.

So long as the Lamarckian theory is supported by argu-
ments like these, it can never hope to be established with any-
thing more than a certain degree of probability. If it is
correct, then its demonstration must come from experiment.
This brings us to a consideration of the experimental evidence
which has been supposed by some writers to give conclusive
proof of the validity of the theory.

The best direct evidence in favor of the Lamarckian argu-
ment is that furnished by the experiments of Brown-Séquard.
He found, as the result of injury to the nervous system of
guinea-pigs, that epilepsy appeared in the adult animal, and
that young born from these epileptic parents became also epi-
leptic. Still more important was his discovery that, after an
operation on the nerves, as a result of which certain organs,
the ear or the leg, for instance, are affected, the same affec-
tion appears in the young born from such parents. These
results of Brown-Séquard have been vouched for by two of
his assistants, and his results in regard to the inheritance of
epilepsy have been confirmed by Obersteiner, and by Luciani
on dogs. Equally important is their later confirmation, as
far as the main facts go, by Romanes.

Brown-Séquard gives the following summary of his results.
I follow Romanes’ translation in his book on “ Darwin and
after Darwin,” where there is also given a careful analysis of
Brown-Séquard’s results, as well as the outcome of the experi-
ments of Romanes himself. The summary is as follows : —

1. “Appearance of epilepsy in animals born of parents which
had been rendered epileptic by an injury to the spinal cord.

Inheritance of Acquired Characters 251

2. Appearance of epilepsy also in animals born of parents
which had been rendered epileptic by section of the sciatic
nerve.

3. A change in the shape of the ear in animals born of
parents in which such a change was the effect of a division
of the cervical sympathetic nerve.

4. Partial closure of the eyelids in animals born of parents
in which that state of the eyelids had been caused either by
section of the cervical sympathetic nerve, or the removal of
the superior cervical ganglion.

5. Exophthalmia in animals born of parents in which an
injury to the restiform body had produced that protrusion of
the eyeball. This interesting fact I have witnessed a good
many times, and seen the transmission of the morbid state of
the eye continue through four generations. In these animals
modified by heredity, the two eyes generally protruded,
although in the parents usually only one showed exophthalmia,
the lesion having been made in most cases only on one of
the corpora restiformia.

6. Hematoma and dry gangrene of the ears in animals
born of parents in which these ear alterations had been caused
by an injury to the restiform body near the nib of the calamus.

7. Absence of two toes out of the three of the hind-leg, and
sometimes of the three, in animals whose parents had eaten
up their hind-leg toes, which had become anzesthetic from a
section of the sciatic nerve alone, or of that nerve and also of
the crural. Sometimes, instead of complete absence of
the toes, only a part of one or two or three was missing in the
young, although in the parent not only the toes, but the
whole foot was absent (partly eaten off, partly destroyed by
inflammation, ulceration, or gangrene).

8. Appearance of various morbid states of the skin and
hair of the neck and face in animals born of parents having
had similar alterations in the same parts as effects of an
injury to the sciatic nerve.”

252 Evolution and Adaptation

Romanes, who later went over the same ground, in part
under the immediate direction of Brown-Séquard himself, has
made some important observations in regard to these results,
many of which he was able to confirm.

He did not repeat the experiment of cutting the cord, but
he found that, to produce epilepsy, it was only necessary to
cut the sciatic nerve. The “epileptiform habit’? does not
appear in the animal until some time after the operation; it
lasts for some weeks or months, and then disappears. The
attacks are not brought on spontaneously, but by “irritating
a small area of the skin behind the ear on the same side of
the body as that on which the sciatic nerve had been divided.”
The attack lasts for only a few minutes, and during it the
animal is convulsed and unconscious. -Romanes thinks that
the injury to the sciatic nerve, or to the spinal cord, produces
some sort of a change in the cerebral centres, “and that it is
this change — whatever it is, and in whatever part of the
brain it takes place —which causes the remarkable phenomena
in question.”

In regard to Brown-Séquard’s statements, made in the 3d
and the 4th paragraphs, in respect to the results of the
operation of cutting the cervical sympathetic, Romanes had
not confirmed the results when his manuscript went to press ;
but soon afterward, after Romanes’ death, a note was printed
in Mature by Dr. Hill, announcing that two guinea-pigs from
Romanes’ experiment had been born, “both of which ex-
hibited a well-marked droop of the upper eyelid. These
guinea-pigs were the offspring of a male and female in both
of which I had produced for Dr. Romanes, some months
earlier, a droop of the left upper eyelid by division of the
left cervical sympathetic nerve. This result is a corroboration
of the series of Brown-Séquard experiments on the inheritance
of acquired characters.”

Romanes states that he also found that injury to a par-
ticular spot of the restiform bodies is quickly followed by a

Inheritance of Acquired Characters 253

protrusion of the eye on the same side, and further, that he
had “also had many cases in which some of the progeny of
parents thus affected have shown considerable protrusion
of the eyeballs of both sidés, and this seemingly abnormal
protrusion has occasionally been transmitted to the next gen-
eration. Nevertheless, I am far from satisfied that this latter
fact is anything more than an accidental coincidence.” This
reservation is made on the ground that the protrusion in the
young is never so great as in the parents, and also because
there is amongst guinea-pigs a considerable amount of in-
dividual variation in the degree of prominence of the eye-
balls. Romanes, while unwilling to deny that an ‘“ obviously
abnormal amount of protrusion, due to the operation, may be
inherited in lesser degree,” is also unwilling to affirm so
important a conclusion on the basis of these experiments
alone.

In regard to Brown-S€quard’s 6th statement, Romanes found
after injury to the restiform body that hematoma and dry
gangrene may supervene, either several weeks after the
operation, or at any subsequent time, even many months
afterward. The disease usually affects the upper parts of
both ears, and may then gradually extend downward until
nearly the whole ear is involved. “As regards the progeny
of animals thus affected in some cases, but by no means in
all, a similarly morbid state of the ears may arise apparently
at any time in the life history of the individual. But I have
observed that in cases where two or more individuals of the
same litter develop this diseased condition, they usually do so
at about the same time, even though this may be months after
birth, and therefore after the animals are fully grown.” More-
over, the morbid process never extends so far in the young as
it does in the parents, and “it almost always affects the middle
third of the ear.” Several of the progeny from this first gener-
ation, which had apparently inherited the disease, but had not
themselves been directly operated upon, showed a portion of

254 Evolution and Adaptation

the ear consumed apparently by the same disease. Romanes
then gives the following significant analysis of this result.
Since a different part of the ear of the progeny is affected,
and also a “very much less quantity thereof,” it might seem
that the result was due either to a mere coincidence, or to
the transmission of microbes. But he goes on to say, that he
fairly well excluded both of these possibilities, for, in the first
place, he has never observed “the very peculiar process in
the ears, or in any other parts of guinea-pigs which have
neither themselves had the restiform bodies injured, nor
been born of parents thus mutilated.” In regard to mi-
crobes, Romanes tried to infect the ears of normal guinea-
pigs by first scarifying these parts, and then rubbing them
with the diseased surfaces of the ears of affected guinea-
pigs. In not a single case was the disease produced.

Romanes concludes that these “results in large measure
corroborate the statements of Brown-Séquard; and it.is only
fair to add that he told me they were the results which he
had himself obtained most frequently, but that he had also
met with many cases where the diseased condition of the
ears in parents affected the same parts in their progeny and
also occurred in more equal degrees.”

We come now.to the remarkable conclusion given in
Brown-Séquard’s 7th statement, in regard to the absence of
toes in animals whose parents had eaten off their own hind
toes and even parts of their legs: Romanes got neuroses in
the animals operated upon, and found that the toes might be
eaten off ; but none of the young showed any defect in these
parts. Furthermore, Romanes repeated the same operation
upon the descendants through six successive generations, so
as to produce, if possible, a cumulative effect, but no inheri-
tance of the mutilation was observed. ‘On the other hand,
Brown-Séquard informed me that he had observed this in-
herited absence of toes only in about one or two per cent of
cases.” It is possible, therefore, Romanes adds, that his

Inheritance of Acquired Characters 255

own experiments were not sufficiently numerous to have
obtained such cases.

In this connection I may give an account of some observa-
tions that I made while carrying out some experiments in
telegony with mice. I found in one litter of mice that when
the young came out of the nest they were tailless. The same
thing happened again when the second litter was produced,
but this time I made my observations sooner, and examined
the young mice immediately after birth. I found that the
mother had bitten off, and presumably eaten, the tails of her
offspring at the time of birth. Had I been carrying on a
series of experiments to see if, when the tails of the parents
were cut off, the young inherit the defect, I might have been
led into the error of supposing that I had found such a case
in these mice. If this idiosyncrasy of the mother had reap-
peared in any of her descendants, the tails might have disap-
peared in succeeding generations. This perversion of the
maternal instincts is not difficult to understand, when we
recall that the female mouse bites off the navel-string of each
of her young as they are born, and at the same time eats the
afterbirth. Her instinct was carried further in this case, and
the projecting tail was also removed.

Is it not possible that something of this sort took place in
Brown-Séquard’s experiment ? The fact that the adults had
eaten off their own feet might be brought forward to indicate
the possibility of a perverted instinct in this case also. At least
my observation shows a possible source of error that must be
guarded against in future work on this subject.

In regard to the 8th statement of Brown-Séquard, as
to various morbid states of the skin, Romanes did not test
this, because the facts which it alleges did not seem of a
sufficiently definite character.

These experiments of Brown-Séquard, and of those who
have repeated them, may appear to give a brilliant experi-
mental confirmation of the Lamarckian position; yet I think,

256 Evolution and Adaptation

if I were a Lamarckian, I should feel very uncomfortable to
have the best evidence in support of the theory come from
this source, because there are a number of facts in the results
that make them appear as though they might, after all, be the
outcome of a transmitted disease, as Weismann claims, rather
than the inheritance of an acquired character. Until we
know more of the pathology of epilepsy, it may be well not
to lay too great emphasis on these experiments. It should
not be overlooked that during the long time that the embryo
is nourished in the uterus of the mother, there is ample op-
portunity given for the transmission of material, or possibly
even of bacteria. If it should prove true that epilepsy is due
to some substance present in the nervous system, such sub-
stances could get there during the uterine life of the embryo.
Even if this were the case, it may be claimed that it does
not give an explanation of the local reappearance of the
disease in the offspring. But here also we must be on our
guard, for it is possible that only certain regions of the body
are susceptible to a given disease; and it has by no means
been shown that the local defect itself is inherited, but only
the disease. Romanes insists that a very special operation is
necessary to bring about certain forms of transmission.

It is well also to keep in mind the fact, that if this sort of
effect is inherited, then we must be prepared to accept as a
possibility that other kinds of injury to the parent may be
transmitted to the offspring. It would be of great disadvan-
tage to animals if they were to inherit the injuries that their
parents have suffered in the course of their lives. In fact,
we might expect to find many plants and animals born in a
dreadful state of mutilation as a result of inheritances of this
sort. Thus, while the Lamarckians try to show that, on their
principle, characters for the good of the species may be
acquired, they must also be prepared, if they accept this
kind of evidence, to grant that immense harm may also result
from its action. I do not urge this as an argument against

Inheritance of Acquired Characters 257

the theory itself, but point it out simply as one of the conse-
quences of the theory.

It has been shown quite recently, by Charrin, Delamare,
and Moussu, that when, after the operation of laparotomy on
a pregnant rabbit or guinea-pig, the kidney or the liver has
become diseased, the offspring sometimes show similar affec-
tions in the corresponding organs (kidney or liver). The
result is due, the authors think, to some substance set free |
from the diseased kidney of the parent that affects the kidney
of the young in the uterus. By injecting into the blood of a
pregnant animal fresh extracts from the kidney of another |
animal, the authors believe that the kidney of the young are
also affected. It will be observed that this transmission of
an acquired character appears to be different from that of
transmission through the egg; for it is the developing, or
developed organ itself, that is acted upon. The results throw
an interesting light on the cases of epilepsy described by
Brown-Séquard, since they show that the diseased condition
of the parent may be transmitted to the later embryonic
stages. May not, therefore, Brown-Séquard’s results be also ;
explained as due to direct transmission from the organs of the
parent to the similar organs of the young in the uterus?

There is another series of experiments of a different sort
that has been used as an argument in favor of the Lamarck-
jan view. These are the results that Cunningham has ob-
tained on young flatfish, He put the very young fish,
while still bilaterally symmetrical (in which stage the pigment |
is equally developed on both sides of the body) into aquaria
lighted from below. He found that when the young fish
begins to undergo its metamorphosis, the pigment gradually |
disappears on one side, as it would have done under normal
conditions, z.¢. when they are lighted from above. If, how-
ever, the fish are kept for some time longer, lighted from
below, the pigment begins to come back again. “The first
fact proves that the disappearance of the pigment-cells from

s

258 Evolution and Adaptation

the lower side in the metamorphosis is an hereditary charac-
ter, and not a change produced in each individual by the
withdrawal of the lower side from the action of light. On
the other hand, the experiments show that the absence of
pigment-cells from the lower side throughout life is due to
the fact that light does not act upon that side, for, when it is
allowed to act, pigment-cells appear. It seems to me that
the only reasonable conclusion from these facts is, that the
disappearance of pigment-cells was originally due to the
absence of light, and that the change has now become
hereditary. The pigment-cells produced by the action of
light on the lower side are in all respects similar to those
normally present on the upper side of the fish. If the dis-
appearance of the pigment-cells were due entirely to a
variation of the germ-plasm, no external influence could
cause them to reappear, and, on the other hand, if there
were no hereditary tendency, the coloration of the lower
side of the flatfsh when exposed would be rapid and
complete.” !

This evidence might be convincing were it not weakened
by two or three assumptions. In the first place, it is not
shown that if the loss of color on the lower side had been
the result of the inheritance of an acquired character that
the results seen in Cunningham’s experiment would follow
as a consequence. Thus one of the starting-points of the
argument really begs the whole question. In the second
place, it is unproven that, had the loss of color of the lower
side been the result of a variation of the germ-plasm, no ex-
ternal influence could cause it to reappear. In this connec-
tion there is another fact that has a bearing on the point here
raised. In some species of flatfish the right side is turned
down, and in other species the left. Occasionally an indi-
vidual is found in a right-sided species that is left-sided, and
in such cases the color is also reversed. Now, to explain this

1 Natural Science, October, 1893.

Inheritance of Acquired Characters 259

in the way suggested by Cunningham, we should be obliged
to assume that some of the ancestors acquired the loss of
pigment on one side of the body, and others on the other
side according to which side was turned down. This suppo-
sition might be appealed to to give us an explanation of the
occasional reversal of the symmetry as a rare occurrence
at the present time; but the argument is so transparently
improbable that, I believe, the Lamarckian school would
hesitate to make use of it, yet, in principle, it is about the
same as that Cunningham has followed above.

If, on the other hand, we suppose the difference in color of
the two sides to have been the result of a germ-variation, we
need only suppose that this was of such a kind that the color
of the under side is only in a latent condition, and if an
external factor can cause a reaction to take place on the light
side, it is not surprising that this should call forth the latent
color patterns. The result can be given at least a formal
explanation on the theory that the original change was a
germ-variation.

We come now to the evidence derived from paleontology.
A number of evolutionists, more especially of the American
school, have tried to show that the evolution of a number of
groups can best be accounted for on the theory of the
inheritance of acquired characters. A point that we must
always bear in mind is that evolution in a direct line need not
necessarily be the outcome of Lamarckian factors. Some of
our leading paleontologists, Cope, Hyatt, Scott, Osborn, have
been strongly impressed by the paleontological evidence in
favor of the view that evolution has often been in direct
lines; and some, at least, of these investigators have been
led to conclude that only the Lamarckian factor of the inheri-
tance of acquired characters can give a sufficient explana-
tion of the facts. Paleontologists have been much impressed
by the fact that evolution has been along the lines which we
might imagine that it would follow if the effects of use and

260 Evolution and Adaptation

of disuse are inherited. There is, however, no proof that
this is the case, although there are a number of instances to
which this mode of explanation appears to give the readiest
solution. But, as has been said before, it is not this kind
of evidence that the theory is in need of, since Lamarck him-
self gave an ample supply of illustrations. What we need is
clear evidence that this sort of inheritance is possible, and,
from the very nature of the case, it is just this evidence that
fossil remains can never supply.

The same criticism may be made of the work of Ryder,
Packard, Dall, Jackson, Eimer, Cunningham, Semper, De
‘ Varigny, and others of the Lamarckian school. Despite the
' large number of cases that they have collected, which appear
to them to be most easily explained on the assumption of
the inheritance of acquired characters, the proof that such
inheritance is possible is not forthcoming. Why not then
spend a small part of the energy, that has been used to
expound the theory, in demonstrating that such a thing is
really possible? One of the chief virtues of the Lamarckian
_ theory is that it is capable of experimental verification or con-
tradiction, and who can be expected to furnish such proof if
not the Neo-Lamarckians ?

We may fairly sum up our position in regard to the theory
of the inheritance of acquired characters in the verdict of
“not proven.” Iam not sure that we should not be justified
at present in claiming that the theory is unnecessary and
even improbable.

CHAPTER VIII

CONTINUOUS AND DISCONTINUOUS VARIATION AND
HEREDITY

THE two terms continuous and discontinuous variation refer
to the succession or inheritance of the variations rather than
to the actual conditions amongst a group of individuals liv-
ing at the same time; but this distinction has only a subor-
dinate value. The term fluctuating, or individual variation,
expresses more nearly the conditions of the individuals of a
species at any one time, and the continuation of this sort of
difference is the continuous variation spoken of above. The
discontinuous variations are probably of the same nature as
those that have been called mutations, and what Darwin some-
times called sports, or single variations, or definite variations.

CoNTINUOUS VARIATION

If we examine a number of individuals of the same species,
we find that no two of them are exactly alike in all particulars.
If, however, we arrange them according to some one character,
for example, according to the height, we find that there is a
gradation more or less perfect from one end of the series to the
other. Thus, if we were to take at random a hundred men, and
stand them in line arranged according to their height, the
tops of their heads, if joined, would form a nearly continuous
line; the line will, of course, incline downward from the
tallest to the shortest man. This illustrates individual varia-
tion. An arrangement of this kind fails to bring out one of
the most important facts connected with individual differences.

261

262 Evolution and Adaptation

If the line is more carefully examined, it will be found that
somewhere near the middle the men are much more nearly
of the same height, or rather there are more men having
about the same height than there are near the ends of the
line. Another arrangement will bring this out better. If we
stand in a line all the men from 60 to 61.9 inches, and in an-
other parallel line all those between 62 and 63.9, then those
between 64 and 65.9, then between 66 and 67.9 inches in
height, etc., it will be found that there are more men in some
of these lines than in others. The longest line will be that
containing the men of about 65 inches; the two lines formed
out of men on each side of this one will contain somewhat
fewer men, and the next ones fewer still, and soon. If we
looked at our new group of men from above, we should have
a figure triangular in outline, the so-called frequency polygon,
Figure 3 B. With a larger amount of data of this sort it is
possible to construct a curve, the curve of frequency, Figure
3 A. In order to obtain this curve of frequency, it is of
course not necessary to actually put the individuals in line,
but the curve can be drawn on paper from the measurements.
We sort out the measurements into classes as in the case
given above. The classes are laid off at regular intervals
along a base-line by placing points at definite intervals.
Perpendiculars are then erected at each point, the height of
each being proportional to the frequency with which each
class occurs. If now we join the tops of these perpendiculars,
the curve of frequency is the result.

“Jn arranging the individuals it will be found, as has been
said, that certain groups contain more individuals. They
will form the longest line. This value that occurs with the
greatest frequency is called the mode. The position of this
modal class in the polygon is one of the points of importance,
and the spread of the polygon at its base is another. A
polygon with a low mode and a broad range means great
variability. The range may, however, be much affected by a

Variation and Heredity 263

single individual standing far removed from the rest, so that
a polygon containing such an individual might appear to
‘show greater variation than really exists. Therefore we need
a measure of variability that shall take into account the

O
O
e@)
[ \ OO
[ \ Coe
OOl
eee
‘i \ eoeee
/ X eee
A N eooee®
i OCOCCOCO
B
at
fi i: an
]
i =O
x
iN fe
f P cE
Cc D

Fic, 3.— Curves of frequency, etc. A, normal curve. B, showing the method
of arranging individuals in lines containing similar kinds of individuals.
C, curve that is skew to the right. D, polygon of frequencies of horns of
rhinoceros beetles. (After Davenport.)

departures of all the individuals from the mode. One such
measure is the arithmetical average of all the departures from
the mean in both directions; and this measure has been
widely employed. At present another method is preferred,
namely, the square root of the squared departures. This
measure is called the standard deviation. The standard

264 Evolution and Adaptation

deviation is of great importance, because it is the index of
variability.”

Of the different kinds of polygons there are two main
sorts, the simple and the complex. The former have only a
single mode, the latter have more than one mode. Some
simple polygons lie symmetrically on each side of the mode,
Figure 3 A; others are unsymmetrical or skew, Figure 3 B.
' The skew polygon generally extends out on one side farther
than on the other. It has been suggested that when a poly-
gon is symmetrical the species is not changing, and when
skew that the species is evolving in the direction of the
longer base. This assumes that the sort of variation meas-
ured by these curves is of the kind of which evolution is
made up, but this is a question that we must further con-
sider. How far the change indicated by the skew curve
may be carried is also another point for further examination.

A complex polygon of variation, Figure 3 D, has been some-
times interpreted to mean that two subgroups exist in a
species, as is well shown in the case of the rhinoceros beetle
described by Bateson. Two kinds of male individuals exist,
some with long horns, others with short horns; each with a
mode of its own, the two polygons overlapping. Other com-
plex polygons may be due to changes occurring at different
times in the life of the individual, as old age, for example.

If, instead of examining the variations of the individuals of
the race, we study the variations in the different organs of the
same individual, we find in many cases that certain organs
vary together. Thus the right and the left leg nearly always
vary in the same direction, also the first joints of the index
and middle fingers, and the stature and the forearm. On the
other hand, the length of the clavicle and that of the humerus
do not vary together to the same extent; and the breadth and
height of the skull even less so.

1 Davenport, C. B., “The Statistical Study of Biological Problems,’ Popular
Science Monthly, September, 1900,

Variation and Heredity 265

We may also study those cases in which a particular
organ is repeated a number of times in the same individual,
as are the leaves of a tree. If the leaves of the same tree are
examined in respect, for example, to the number of veins that
each contains, we find that the number varies, and that the
results give a variation polygon exactly like that when differ-
ent individuals are compared with one another. Let us take
the illustration given by Pearson. He counted the veins on
each side of the midrib of the leaves of the beech. If a
number of leaves be collected from one tree, and the same
number from another, and if all those having fifteen veins are
put in one vertical column, and all those with sixteen in an-
other, as shown in the following table, it will be found that

No. of Veins . .| Io} 11} 12/13] 14) 15 | 16] 17| 18} 19| 20| a1 | 22
First Tree. . .{|—]/—/]—j}—|/—] 1] 4] 7] 9] 4} 1}/—J—
Second Tree . .{—|—/—]| 3] 4] 9] 8} 2]/—J—|—!—|—

each tree has a mode of its own. Thus in the first tree the
mode is represented by nine individuals having eighteen
veins, and in the second by nine individuals having fifteen
veins. So far as this character is concerned we might have
interchanged certain of the individual leaves, but we could
not have interchanged the two series. They are zxdividual
to the two trees. Now in what does this individuality con-
sist? Clearly there are most leaves in one tree with eighteen
ribs, and most in the other with fifteen ribs.

If we contrast these results with those obtained by picking
at random a large number of leaves from different beech trees,
we have no longer types of individuals, but racial characters.
Pearson has given the following table to illustrate these points:

FREQUENCY OF DIFFERENT TYPES OF BEECH LEAVES

No. of Veins} to} 11] 12] 13 | 14 | 15 | 16] 17 | 18 | 19 | 20] 21 | 22

(Frequency | 1 | 7 | 34] 110) 318] 479 | 595 | 516 | 307 | 181 | 36/15 | 1

266 Evolution and Adaptation

Thus the mode for beech trees in general is sixteen; but, as
shown in the other table, this mode does not correspond with
either of the two individual modes here ascertained. The
illustration shows that the racial mode may differ from the
individual mode. There are also cases known in which
the mode of a group of individuals living in one locality is
different from that of another group living in another locality.
This difference may be a constant one from year to year,
although so slight, that unless actual measurements are made,
the difference cannot be detected, because of the overlapping
of the individuals from different localities. If evolution took
place by slow changes of this sort, it might be possible to de-
tect its action, even when very slow, by means of measure-
ments made on a large number of individuals. At least this
has been suggested by those who believe new species may
result from changes of this sort.

There is some evidence showing that by selecting particular
individuals of a series, and breeding from them, the mode
may be changed in the direction of selection. Thus it has
been stated by Davenport that the descendants of twelve-
and thirteen-rayed daisies give a polygon with a skewness of
+1.92; while the descendants of twenty-one-rayed plants
give a polygon with a skewness of —.13.

Pearson has described very concisely the possibilities in-
volved in the selective action of the environment. He states
that if we examine the frequency distribution of a set of
organisms that have just become mature, and later make a
similar examination on the same number of individuals (but
not the same individuals) during the period of reproduction,
we shall probably find that a change has taken place which
may have been due to selection of some sort. The same
thing might be found in the next generation, and, if it did,
this would indicate that “selection does not necessarily mean
a permanent or a progressive change.” The selection in this
imaginary case would be purely periodic and suffice only to

Variation and Heredity 267

maintain a given race under given conditions. “Each new
adolescent generation is not the product of the entire preced-
ing generation, but only of selected individuals. This is cer-
tainly the case for civilized man, in which case twenty-six per
cent of the married population produce fifty per cent of the
next generation.”

Pearson believes that “if a race has been long under the
same environment it is probable that only periodic selection
is at work, maintaining its stability. Change the environ-
ment and a secular change takes place, the deviations from
the mode previously destroyed giving the requisite material.”
“Clearly periods of rapidly changing environment, of great
climatological and geological change, are likely to be asso-
ciated with most marked secular selection. To show that
there is little or no change year by year in the types of rab-
bit and wild poppy in our English fields, or of daphnia in our
English ponds, is to put forward no great argument for the
inefficiency of natural selection. Take the rabbit to Australia,
the wild poppy to the Cape, the daphnia into the laboratory,
and change their temperature, their food supply, and the
chemical constituents of water and air, and then the exist-
ence of no secular selection would indeed be a valid argument
against the Darwinian theory of evolution.’’ In regard to
the last point, it should be noted that, even if under the
changed conditions a change in the mode took place, as
Pearson assumes, it does not follow necessarily that selection
has had anything to do with it, but the environment may have
directly changed the forms. Furthermore, and this is the
essential point, even if selection does act to the extent of
changing the mode, we should not be justified in concluding
that this sort of change could go on increasing as long as the
selection lasts. All that might happen would be to keep the
species up to the highest point to which fluctuating variation
can be held. This need not lead to the formation of new
species, or direct the course of evolution.

268 Evolution and Adaptation

Pearson points out further that, even if we suppose that a
secular change is produced in a new environment, we cannot
explain how species may break up into two or more races
that are relatively infertile. Suppose two groups of individ-
uals, subjected to different environments, become isolated
geographically. Two local races will be produced. “ Isola-
tion may account for the origin of local races, but never for
the origin of species unless it is accompanied by a differen-
tial fertility.” In other words, Pearson thinks that, unless
the reproductive organs are correlated with other organs, in
such a way that as these organs change the interracial fer-
tility of the germ-cells is altered, so that in the two changed
groups the individuals are no longer interfertile, new species
cannot be accounted for, since their mutual infertility is one
of their most characteristic features. ‘Without a barrier to
intercrossing during differentiation the origin of species
seems inexplicable.”

We need not discuss the various suggestions that have
been made to explain this difficulty, none of which, as Pear-
son points out, have been satisfactory. He himself believes
that a process of segregation of like individuals must occur,
during the incipient stages at least, in the formation of species.
Afterwards a correlation may exist between the new organs
and the germ-cells, of such a sort that a relative or an abso-
lute sterility between the incipient species is attained. After
this condition has been reached the two new species may
freely intermix without a return to the primitive type, since
they are no longer fertile zzter se. It seems to me, also, that
this would be an essential requisite if we assume that species
are slowly formed out of races from individual differences, as
Pearson supposes to be the case. There are, however, other
possibilities that Pearson does not take into account, namely,
that from the very beginning the change may be so great
that the new form is not fertile with the original one; and
there is also another possibility as well, that, although the

Variation and Heredity 269

new and the old forms are fertile, the hybrids may be like
one or the other parent, as in several cases to be given later.
Not that I mean to say that in either of these two ways can we
really offer a solution of the question of infertility, for, from
the evidence that we possess, it appears improbable that the
infertility of species zwzer se has been the outcome of either
of these causes.

In support of his main thesis Pearson gives certain data in
respect to preferential mating in the human race. By this is
meant that selection of certain types of individuals is more
likely to take place, and also that the fertility of certain types
of individuals is greater than that of other types. The cal-
culations are based on stature, color of hair, and of eyes.
The results appear to show in all cases examined that there
is a slight tendency to form new races as the result of the
more frequent selection of certain kinds of individuals. But
even if this is the case, what more do the results show than
that local races may be formed,—races having a certain
mode for height, for color of eyes or of hair? That changes
of this kind can be brought about we knew already without
any elaborate measurements, yet we should not conclude from
this that new species will be formed by a continuation of the
process. .

Pearson writes: “As to the problem of evolution itself we
are learning to see it under a new light. Natural selection,
combined with sexual selection [by which Pearson means
segregation of certain types through individual selection ]
and heredity, is actually at work changing types. We have
quantitative evidence of its effects in many directions.” Yes!
but no evidence that selection of this sort can do anything
more than keep up the type to the upper limit attained in
each generation by fluctuating variations. Pearson adds,
“ Variations do not occur accidentally, or in isolated instances ;
autogamic and assortive mating are realities, and the problem
of the near future is not whether Darwinism is a reality, but

270 Evolution and Adaptation

what is quantitively the rate at which it is working and has
worked.” This statement expresses no more than Pearson’s
conviction that the process of evolution has taken place by
means of selection. He ignores other possibilities, which if
established may put the whole question in a very different
light.

HEREDITY AND CONTINUOUS VARIATION

It has been to a certain extent assumed in the preceding
pages that both parents are alike, or, if different, that they
have an equal influence on the offspring. This may be true
in many cases for certain characteristics. Thus a son from
a tall father and a short mother may be intermediate in
height, or if the father is white and. the mother black, the
children are mulattoes. But other characters rarely or never
blend. In such cases the offspring is more like one or the
other parent, in which case the inheritance is said to be
exclusive. Thus if one parent has blue eyes and the other
black, some of the children may have black eyes and others
blue. There are also cases of particular inheritance where
there may be patches of color, some like the color of one
parent, some like that of the other parent. The latter two
- kinds of inheritance will be more especially considered in the
subsequent part of this chapter; for the present we are here
chiefly concerned with blended characters.

How much in such cases does each parent contribute to the
offspring ? This has been expressed by Galton in his law of
ancestral heredity. This law takes into account not only the
two parents, but also the four grandparents, and the eight
great-grandparents, etc. There will be 1024 in the tenth
generation. These 1024 individuals may be taken as a fair
sample of the general population, provided there has not been
much interbreeding. Are we then to look upon the individ-
ual as the fused or blended product of the population a few
generations back? If this were true, should we not expect

Variation and Heredity 271

to find all the individuals of a community very much alike,
except for the fluctuating variations close around the mode?

As a result of his studies on the stature of man, and on
the coat color of the Basset hounds, Galton has shown that
the inheritance from the parents can be represented by the
fraction 4; that is one-half of the peculiarities of the individ-
ual comes from the two parents. The four grandparents
together count for } of the total inheritance, the great-grand-
parents }, and so on, giving the series 4, $, 4. Pearson, taking
certain other points into consideration, believes the following
series more fully represents the inheritance from the ances-
tors, .3, .15, .075, .0375, etc. He concludes that, “if Dar-
winism be the true view of evolution, z.e. if we are to describe
evolution by natural selection combined with heredity, then
the law which gives us definitely and concisely the type of
the offspring in terms of the ancestral peculiarities is at
once the foundation stone of biology and the basis upon
which heredity becomes an exact branch of science.”

The preceding statements give some idea of what would
occur in a community in which no selection was taking
place. The results will be quite different, although the same
general law of inheritance will hold, if selection takes place in
each generation. If, for instance, selection takes place, the
offspring after four generations will have .93 of the selected
character, and without further selection will not regress, but
breed true:to this type! “ After six generations of selection
the offspring will, selection being suspended, breed true to
under two per cent divergence from the previously selected
type.”

If, however, we do not assume that the ancestors were
mediocre, it is found that after six generations of selection the
offspring will breed true to the selected type within one per
cent of its value. Thus, if selection were to act on a race

1 In this statement the earlier ancestors are assumed to be identical with the
general type of the population.

272 Evolution and Adaptation

of men having a mode of 5 feet 9 inches, and the 6-foot men
were selected in each generation, then in six generations this
type would be permanently established, and this change
could be effected in two hundred years.!

Thus we have exact data as to what will happen on the
average when blended, fluctuating variations are selected.
Important as such data must always be to give us accurate
information as to what will occur if things are left to
“chance” variations, yet if it should prove true that evolu-
tion has not been the outcome of chance, then the method is
entirely useless to determine how evolution has occurred.

More important than a knowledge of what, according to
the theory of chances, fluctuating variations will do, will be
information that would tell us what changes will take place
in each individual. In this field we may hope to obtain data
no less quantitative than those of chance variations, but of a
different kind. A study of some of the results of discon-
tinuous variation will show my meaning more clearly.

DISCONTINUOUS VARIATION

Galton, in his book on “ Natural Inheritance,” points out
that “the theory of natural selection might dispense with
a restriction for which it is difficult to see either the need
or the justification, namely, that the course of evolution al-
ways proceeds by steps that are severally minute and that
become effective only through accumulation.” An apparent
reason, it is suggested, for this common belief “is founded on
the fact that whenever search is made for intermediate forms
between widely divergent varieties, whether they are of plants
or of animals, of weapons or utensils, of customs, religion, or
language, or of any other product of evolution, a long and
orderly series can usually be made out, each member of which
differs in an almost imperceptible degree from the adjacent

1 Quoted from Pearson’s “Grammar of Science.”

Variation and Heredity ans

specimens. But it does not at all follow because these inter-
mediate forms have been found to exist, that they were the
very stages that were passed through in the course of evolu-
tion. Counter evidence exists in abundance, not only of the
appearance of considerable sports, but of their remarkable
stability in hereditary transmission.” Comparing such an
apparently continuous series with machines, Galton con-
cludes, “If, however, all the variations of any machine that
had ever been invented were selected and arranged in a
museum, each would differ so little from its neighbors as to
suggest the fallacious inference that the successive inven-
tions of that machine had progressed by means of a very
large number of hardly discernible steps.”

Bateson, also, in his “ Materials for the Study of Variation,”
speaks of the two possible ways in which variations may arise.
He points out that it has been tacitly assumed that the tran-
sitions have been continuous, and that this assumption has
introduced many gratuitous difficulties. Chief of these is
the difficulty that in their initial and imperfect stages many
variations would be useless. ‘Of the objections that have
been brought against the Theory of Natural Selection, this
is by far the most serious.” He continues: “The same
objection may be expressed in a form which is more correct
and comprehensive. We have seen that the differences be-
tween species on the whole are Specific, and are differences
of kind forming a discontinuous Series, while the diversities
of environment to which they are subject are, on the whole,
differences of degree, and form a continuous Series; it is,
therefore, hard to see how the environmental differences can
thus be made in any sense the directing cause of Specific
differences, which by the Theory of Natural Selection they
should be. This objection of course includes that of the
utility of minimal Variations.”

“ Now the strength of this objection lies wholly in the sup-
posed continuity of the process of Variation. We see all

T

274 Evolution and Adaptation

organized nature arranged in a discontinuous series of groups
differing from each other by differences which are Specific;
on the other hand, we see the diverse environments to which
these forms are subject passing insensibly into each other.
We must admit, then, that if the steps by which the diverse
forms of life have varied from each other have been insensi-
ble, — if, in fact, the forms ever made up a continuous series,
—these forms cannot have been broken into a discontinuous
series of groups by a continuous environment, whether acting
directly as Lamarck would have, or as selective agent as Dar-
win would have. This supposition has been generally made
and admitted, but in the absence of evidence as to Variation
it is nevertheless a gratuitous assumption, and, as a matter of
fact, when the evidence as to Variation is studied, it will be
found to be in a great measure unfounded.”

There is a fair number of cases on record in which discon-
tinuous variations have been seen to take place. Darwin him-
self has given a number of excellent examples, and Bateson,
in the volume referred to above, has brought together a large
and valuable collection of facts of this kind.

Some of the most remarkable of these instances have been
already referred to and need only be mentioned here. The
black-shouldered peacock, the ancon ram, the turnspit dog,
the merino sheep, tailless and hornless animals, are all cases
in point. In several of these it has been discovered that the
young inherit the peculiarities of their parents if the new
variations are bred together; and what is more striking, if
the new variation is crossed with the parent form, the young
are like one or the other parent, and not intermediate in
character. This latter point raises a question of fundamental
importance in connection with the origin of species.

Darwin states that he knows of xo cases in which, when
different species or cven strongly markcd varieties are crossed,
the hybrids are like one form or the other. They show, he be-
lieves, always a blending of the peculiarities of the two parents.

Variation and Feredity 275

He then makes the following significant statement: “All
the characters above enumerated which are transmitted in
a perfect state to some of the offspring and not to others —
such as distinct colors, nakedness of skin, smoothness of
leaves, absence of horns or tail, additional toes, pelorism,
dwarfed structure, etc., have all been known to appear sud-
denly in individual animals or plants. From this fact, and
from the several slight, aggregated differences which dis-
tinguish domestic races and species from each other, not
being liable to this peculiar form of transmission, we may
conclude that it is in some way connected with the sudden
appearance of the characters in question.”

Darwin has, incidentally, raised here a question of the most
far-reaching import. If it should prove true, as he believes,
that inheritance of this kind of discontinuous variation is also
discontinuous, and that we do not get the same result when
distinct species are intercrossed, or even when well-marked
domestic races are interbred, then he has, indeed, placed a
great obstacle in the path of those who have tried to show
that new species have arisen through discontinuous variation
of this sort.

If wild species, when crossed, give almost invariably inter-
mediate forms, then it may appear that we are going against
the only evidence that we can hope to obtain if we claim
that discontinuous variation, of the kind that sports are made
of, has supplied the material for evolution. If, furthermore,
when distinct races of domesticated animals are crossed, we
do not get discontinuous inheritance, it might, perhaps, with
justness be claimed that this instance is paralleled by what
takes place when wild species are crossed. And if domesti-
cated forms have been largely the result of the selection of
fluctuating variations, as Darwin believes, then a strong case
is apparently made out in favor of Darwin’s view that con-
tinuous variation has given the material for the process of
_ evolution in nature. Whether selection or some other factor

276 Evolution and Adaptation

has directed the formation of the new species would not, of
course, be shown, nor would it make any difference in the
present connection.

Before we attempt to reach a conclusion on this point let
us analyze the facts somewhat more closely.

In the first place, a number of these cases of discontinuous
variation are of the nature of abnormalities. The appearance
of extra fingers or toes in man and other mammals is an ex-
ample of this sort. This abnormality is, if inherited at all,
inherited completely ; that is, if present the extra digit is per-
fect, and never appears in an intermediate condition, even
when one of the parents was without it. The most obvious
interpretation of this fact is that when the material out of
which the fingers are to develop is divided up, or separated
into its component parts, one more part than usual is laid
down. Similarly, when a flower belonging to the triradiate
type gives rise to a quadriradiate form,—as sometimes
occurs, — the new variation seems to depend simply on the
material being subdivided once more than usual; perhaps
because a little more of it is present, or because it has a
somewhat different shape. My reasons for making a sur-
mise of this sort are based on certain experimental facts in
connection with the regeneration of animals. It has been
shown in several cases that it is possible to produce more
than the normal number of parts by simply dividing the ma-
terial so that each part becomes more or less a new whole,
and the total number of parts into which the material becomes
subdivided is increased. It seems not improbable that phe-
nomena of this sort have occurred in the course of evolution,
although it is, of course, possible that those characters that
define species do not belong to this class of variation. To
take an example. There are nine neck-vertebre in some
birds, but in the swan the number is twenty-five. We cannot
suppose that the ancestor of the swan gradually added enough
materially to make up one new vertebra and then another,

Variation and Heredity a7

but at least one new whole vertebra was added at a time;
and we know several cases in which the number of vertebrze
in the neck has suddenly been increased by the addition
of one more than normal, and the new vertebra is perfectly
formed from the first.

In cases of this sort we can easily understand that the
inheritance must be either of one kind or the other, since
intermediate conditions are impossible, when it comes to
the question of one or not one; but if one individual had
one and another six vertebrze, then it would be theoretically
possible for the hybrid to have three.

This brings us to a question that should have been spoken
of before in regard to the inheritance of discontinuous varia-
tion. It sometimes occurs that a variation, which appears
in other respects to be discontinuous, is inherited in a blended
form. Thus the two kinds of variation may not always be
so sharply separated as one might be led to believe. There
may be two different kinds of discontinuous variation in re-
spect to inheritance, or there may be variations that are only
to a greater or a less extent inherited discontinuously ; and
it seems not improbable that both kinds occur.

This diversion may not appear to have brought us any
nearer to the solution of the difficulty that Darwin’s state-
ment has emphasized, except in so far as it may show that
the lines are not so sharply drawn as may have seemed to be
the case. The solution of the difficulty is, I believe, as
follows : —

The discontinuity referred to by Darwin relates to cases in
which only a single step (or mutation) has been taken, and tt
is a question of inheritance of one or not one. If, however, six
successive steps should be taken in the same direction, then
when such a form ts crossed with the original form, the hybrid
may inherit only three of the steps and stand exactly midway
between the parent forms; or it may inherit four, or five, or
three, or two steps and stand correspondingly nearer to the one

278 Evolution and Adaptation :

or to the other parent. Thus while it may not be possible to
halve a single step (hence one-sided inheritance), yet when more
than one step has been taken the inheritance may be divided.
There is every evidence that most of the Linnean (wild)
species that Darwin refers to have diverged from the parent
form, and from each other, by a number of successive steps ;
hence on crossing, the hybrid often stands somewhere between
the two parent forms. On this basis not only can we meet
Darwin's objection, but the point of view gives an interesting
insight into the problem of inheritance and the formation of
Spectes.

The whole question of inheritance has assumed a new
aspect; first on account of the work of De Vries in regard
to the appearance of discontinuous variation in plants; and
secondly, on account of the remarkable discoveries of Gregor
Mendel as to the laws of inheritance of discontinuous varia-
tions. Mendel’s work, although done in 1865, was long
neglected, and its importance has only been appreciated in
the last few years. We shall take up Mendel’s work first,
and then that of De Vries.

MENDEL’s Law!

The importance of Mendel’s results and their wide applica-
tion is apparent from the results in recent years of De Vries,
Correns, Tschermak, Bateson, Castle, and others. Mendel
carried out his experiments on the pea, Pisum sativum.
Twenty-two varieties were used, which had been proven by
experiment to be pure breeds. When crossed they gave per-
fectly fertile offspring. Whether they all have the value of
varieties of a single species, or are different subspecies, or
even independent species, is of little consequence so far as

1 Bateson, in his book on “ Mendel’s Principles of Heredity,” has given an
admirable presentation of Mendel’s results, I have relied largely on this in my
account.

Variation and Heredity 279

Mendel’s experiments are concerned. The flower of the pea
is especially suitable for experiments of this kind. It cannot
be accidentally fertilized by foreign pollen, because the repro-
ductive organs are inclosed in the keel of the flower, and, as
a rule, the anthers burst and cover the stigma of the same
flower with its own pollen before the flower opens. In order
to cross-fertilize the plants it is necessary to open the young
buds before the anthers are mature and carefully remove all
the anthers. Foreign pollen may be then, or later, introduced.

The principle involved in Mendel’s law may be first stated
in a theoretical case, from which a certain complication that
appears in the actual results may be removed.

If A represent a variety having a certain character, and
B another variety in which the same character is different,
let us say in color, and if these two individuals, one of each
kind, are crossed, the hybrid may be represented by H. If
a number of these hybrids are bred together, their descendants
will be of three kinds; some will be like the grandparent,
A,in regard to the special character that we are following,
some will be like the other grandparent, 2, and others will be
like the hybrid parent, H. Moreover, there will be twice as
many with the character /7, as with A, or with B.

ae B
yy

If now we proceed to let these A’s breed together, it will
be found that their descendants are all A, forever. If the

280 Evolution and Adaptation

B’s are bred together they produce only 4’s. But when the
H’s are bred together they give rise to H’s, A’s, and B’s, as
shown in the accompanying diagram. In each generation,
the 4’s will also breed true, the 4’s true, but the /7/’s will
give rise to the three kinds again, and always in the same
proportion.

Thus it is seen that the hybrid individuals continue to give
off the pure original forms, in regard to the special character
under consideration. The numerical relation between the
numbers is also a striking fact. Its explanation is, however,
quite simple, and will be given later.

In the actual experiment the results appear somewhat more
complicated because the hybrid cannot be distinguished from
one of the original parents, but the results really conform
exactly to the imaginary case given above. The accompany-
ing diagram will make clearer the account that follows.

A B

Ee ca

A mid
\

A(B)

The hybrid, A(B), produced by crossing A and B is like A so
far as the special character that we will consider is concerned.
In reality the character that A stands for is only dominant,
that is, it has been inherited discontinuously, while the other
character, represented by 8, is latent, or recessive as Mendel
calls it. Therefore, in the table, it is included in parentheses.
If the hybrids, represented by this form A(Z), are bred

Variation and Heredity 281

together, there are produced two kinds of individuals, A’s
and £’s, of which there are three times as many A’s as B’s.
It has been found, however, that some of these A’s are pure
forms, as indicated by the A on the left in our table, while
the others, as shown by their subsequent history, are hybrids,
A(B). There are also twice as many of these A(B)’s as of
the pure 4’s (or of the 4’s), Thus the results are really the
same as in our imaginary case, only obscured by the fact that
the A’s and the A(4)’s are exactly alike to us in respect to
the character chosen. We see also why there appear to be
three times as many A’s as B’s. In reality the results are
1 A, 2 A(B), 1 B.

In subsequent generations the results are the same as in
this one, the A’s giving rise only to A, the B’s to B, and the
A(B)’s continuing to split up into the three forms, as shown in
our diagram. , Mendel found the same law to hold for all the
characters he examined, including such different ones as the
form of the seed, color of seed-albumen, coloring of seed-coat,
form of the ripe pods, position of flowers, and length of stem.

Mendel also carried out a series of experiments in which
several differentiating characters are associated. In the first
experiment the parental plants (varieties) differed in the form
of the seed and in the color of the albumen. The two char-
acters of the seed plant are designated by the capital letters
A and &; and of the ‘pollen plant by small a@and 4. The
hybrids will be, of course, combinations of these, although
only certain characters may dominate. Thus in the experi-
ments, the parents are AB (seed plant) and aé (pollen plant),
with the following seed characters : —

Seed parent $ A form round Pollen parent § a form angular
AB B albumen yellow ab b albumen green

When these two forms were crossed the seeds appeared round
and yellow like those of the parent, AB, z.e. these two char-
acters dominated in the hybrid.

282 Evolution and Adaptation

The seeds were sown, and in turn yielded plants which when
self-fertilized gave four kinds of seeds (which frequently all
appeared in the same pod). Thus 556 seeds were produced
by 15 plants, having the following characters : —

AB 315 round and yellow
Aé i1o1 angular and yellow
aB 108 round and green
ab 32 angular and green

These figures stand almost in the relation of 9: 3:3:1.

These seeds were sown again in the following year and
gave :—

From the round yellow seeds : —

AB 38 round and yellow seeds

ABbé 65 round yellow and green seeds

AaB 60 round yellow and angular yellow seeds

AaB 138 round yellow and green, angular yellow
and green seeds

From the angular yellow seeds : —

aB 28 angular yellow seeds
aBb 68 angular yellow and green seeds

From the round green seeds : —

Ab 35 round green seeds
Aab 67 round angular seeds

From the angular green seeds : —

ab 30 angular green seeds

Thus there were 9 different kinds of seeds produced.
There had been separated out at this time 38 individuals
like the parent seed plant, AB, and 30 like the parent
pollen plant, 2. Since these had come from similar seeds
of the preceding generation they may be looked upon as
pure at this time. The forms 4é and a@B are also constant

Variation and Heredity 283

forms which do not subsequently vary. The remainder are
still mixed or hybrid in character. By successive self-fertili-
zations it is possible gradually to separate out from these the
pure types of which they are compounded.

Without going into further detail it may be stated that the
offspring of the parent hybrids, having two pairs of differen-
tiating characters, are represented by the series : —

AB Ab aB ab 2ABb 2aBb 2 Aab 2ABa 2 AaBo

This series is really a combination of the two series :—

A+2Aa+a
B+2£B6+6

Mendel even went farther, and used two parent varieties
having three differentiating characters, as follows :—

ABC seed parent abc pollen plant

A form round a form angular
{2 albumen yellow {i albumen green
C seed-coat grey brown c seed-coat white

The results, as may be imagined, were quite complex, but
can be expressed by combining these series : —

A+2Aa+a
B+2B80+6
C+2Cc +e

In regard to the two latter experiments, in which two and
three characters respectively were used, it is interesting to
point out that the form of the hybrid more nearly approaches
“to that one of the parental plants which possesses the great-
est number of dominant characters.”’ If, for instance, the seed
plant has short stem, terminal white flowers, and simply
inflated pods; the pollen plant, on the other hand, a long
stem, violet-red flowers distributed along the stem, and con-
stricted pods,—then the hybrid resembles the seed parent
only in the form of the pod; in its other characters it agrees

284 Evolution and Adaptation

with the pollen plant. From this we may conclude that, if two
varieties differing in a large number of characters are crossed,
the hybrid might get some of its dominant characters from
one parent, and other dominant characters from the other
parent, so that, unless the individual characters themselves
were studied, it might appear that the hybrids are interme-
diate between the two parents, while in reality they are only
combinations of the dominant characters of the two forms.
But even this is not the whole question.

Mendel points out that, from knowing the characters of
the two parent forms (or varieties), one could not prophesy
what the hybrid would be like without making the actual
trial. Which of the characters of the two parent forms
will be the dominant ones, and which recessive, can only be
determined by experiment. Moreover, the hybrid characters
are something peculiar to the hybrid itself, and to itself alone,
and not simply the combination of the characters of the two
forms. Thus in one case a hybrid from a tall and a short
variety of pea was even taller than the taller parent variety.
Bateson lays much emphasis on this point, believing it to be
an important consideration in all questions relating to hybri-
dization and inheritance. ,

The theoretical interpretation that Mendel has put upon
his results is so extremely simple that there can be little
doubt that he has hit on the real explanation. The results
can be accounted for if we suppose that the hybrid pro-
duces egg-cells and pollen-cells, each of which is the bearer
of only one of the alternative characters, dominant or reces-
sive as the case may be. If this is the case, and if on an
average there are the same number of egg-cells and pollen-
cells, having one or the other of these kinds of characters,
then on a random assortment meeting of egg-cells and
pollen-cells, Mendel’s law would follow. For, 25 per cent of
dominant pollen grains would meet with 25 per cent dominant
egg-cells; 25 per cent recessive pollen grains would meet with

Variation and Heredity 285

25 per cent recessive egg-cells; while the remaining 50 per
cent of each kind would meet each other. Or, as Mendel
showed by the following scheme : —

A SS a
A A a a
Or more simply by this scheme : —

A a
Lt

Mendel’s results have received confirmation by a number of
more recent workers, and while in some cases the results
appear to be complicated by other factors, yet there can
remain little doubt that Mendel has discovered one of the
fundamental laws of heredity.

It has been found that there are some cases in which the
sort of inheritance postulated by Mendel’s law does not seem
to hold, and, in fact, Mendel himself spoke of such cases.
He found that some kinds of hybrids do not break up in
later generations into the parent forms. He also points out
that in cases of discontinuity the variations in each character
must be separately regarded. In most experiments in cross-
ing, forms are chosen which differ from each other in a
multitude of characters, some of which are continuous and
others discontinuous, some capable of blending with their
contraries while others are not. The observer in attempting
to discover any regularity is confused by the complications
thus introduced. Mendel’s law could only appear in such
cases by the use of an overwhelming number of examples
which are beyond the possibilities of experiment.!

Let us now examine the bearing of these discoveries on

Pollen-cells

Egg-cells

1 This statement is largely taken from Bateson’s book.

286 Evolution and Adaptation

the questions of variation which were raised in the preceding
pages. It should be pointed out, however, that it would be
premature to do more than indicate, in the most general
way, the application of these conclusions. The chief value
of Mendel’s results lies in their relation to the theory of in-
heritance rather than to that of evolution.

In the first place, Mendel’s results indicate that we cannot
make any such sharp distinction as Darwin does between
the results of inheritance of discontinuous and of continu-
ous variations. As Mendel’s results show, it is the separate
characters that must be considered in each case, and not
simply the sum total of characters.

The more general objection that Darwin has made may
appear to hold, nevertheless. He thinks that the evolution of.
animals and plants cannot rest primarily on the appearance
of discontinuous variations, because they occur rarely and
would be swamped by intercrossing. If Mendel’s law ap-
plies to such cases, that is, if a cross were made between
such a sport and the original form, the hybrid in this case, if
self-fertilized, would begin to split up into the two original
forms. But, on the other hand, it could very rarely happen
that the hybrid did fertilize its own eggs, and, unless this oc-
curred, the hybrid, by crossing with the parent forms in each
generation, would soon lose all its characters inherited from
its “sport” ancestor. Unless, therefore, other individuals
gave rise to sports at the same time, there would be little
jchance of producing new species in this way. We see then
jthat discontinuity in itself, unless it involved infertility with
the parent species, of which there is no evidence, cannot be
made the basis for a theory of evolution, any more than can
individual differences, for the swamping effect of intercross-
ing would in both cases soon obliterate the new form. If,
however, a species begins to give rise to a large number of
individuals of the same kind through a process of discontinu-
ous variation, then it may happen that a new form may

Variation and Heredity 287

establish itself, either because it is adapted to live under con-
ditions somewhat different from the parent form, so that the
dangers of intercrossing are lessened, or because the new
form may absorb the old one. It is also clear, from what
has gone before, that the new form can only cease to be fer-
tile with the parent form, or with its sister forms, after it has
undergone such a number of changes that it is no longer
able to combine the differences in a new individual. This
result will depend both on the kinds of the new characters, as
well as the amounts of their difference. This brings us to
a consideration of the results of De Vries, who has studied
the first steps in the formation of new species in the “ muta-
tions”’ of the evening primrose.

THE Mutation THEORY OF DE VRIES

De Vries defines the mutation theory as the conception
that “the characters of the organism are made up of elements
(‘Einheiten’) that are sharply separated from each other.
These elements can be combined in groups, and in related
species the same combinations of elements recur. Transi-
tional forms like those that are so common in the external
features of animals and plants do not exist between the
elements themselves, any more than they do between the
elements of the chemist.”

This principle leads, De Vries says, in the domain of the
descent theory to the conception that species have arisen
from each other, not continuously, but by steps. Each new
step results from a new combination as compared with the old
one, and the new forms are thereby completely and sharply
separated from the species from which they havecome. The
new species is all at once there; it has arisen from the parent
form without visible preparation and without transitional steps.

The mutation theory stands in sharp contrast to the selec-
tion theory. The latter uses as its starting-point the com-

288 Evolution and Adaptation

mon form of variability known as individual or fluctuating
variation ; but according to the mutation theory there are two
kinds of variation that are entirely different from each other.
“The fluctuating variation can, as I hope to show, not over-
step the bounds of the species, even after the most prolonged
selection, — much less can this kind of variation lead to the
production of new, constant characters.” Each peculiarity
of the organism has arisen from a preceding one, not
through the common form of variation, but through a sudden
change that may be quite small but is perfectly definite.
This kind of variability that produces new species, De Vries
calls mutability ; the change itself he calls a mutation. The
best-known examples of mutations are those which Darwin
called “single variations” or “sports.”

De Vries recognizes the following kinds of variation : —

First, the polymorphic forms of the systematists. The
ordinary groups which, following Linnzeus, we call species,
are according to De Vries collective groups, which are the
outcome of mutations. Many such Linnzean species include
small series of related forms, and sometimes even large num-
bers of such forms. These are as distinctly and completely
separated from each other as are the best species. Generally
these small groups are called varieties, or subspecies, — va-
rieties when they are separated by a single striking char-
acter, subspecies when they differ in the totality of their
characters, in the so-called habitus.

These groups have already been recognized by some
investigators as elementary species, and have been given cor-
responding binary names. Thus there are recognized two
hundred elementary species of the form formerly called Draba
verna.

When brought under cultivation these elementary species
are constant in character and transmit their peculiarities
truly. They are not local races in the sense that they are
the outcome in each generation of special external conditions.

Variation and Heredity 289

Many other Linnzean species are in this respect like Drada
verna, and most varieties, De Vries thinks, are really element-
ary species.

Second, the polymorphism due to intercrossing is the
outcome of different combinations of hereditary qualities.
There are here, De Vries says, two important classes of facts
to be kept strictly apart, —scientific experiment, and the
results of the gardener and of the cultivator. The experi-
menter chooses for crossing, species as little variable as pos-
sible; the gardener and cultivator on the other hand prefer
to cross forms of which one at least is variable, because the
variations may be transmitted to the hybrid, and in this way
a new form be produced.

New elementary characters arise in experiments in crossing
only through variability, not through crossing itself.

Third, variability in the ordinary sense, that is, individual
variability, includes those differences between the individual
organs that follow Quetelet’s theory of chance. This kind
of variability is ‘characterized by its presence at all times, in
all groups of individuals.

De Vries recalls Galton’s apt comparison between variability
and a polyhedron which can roll from one face to another.
When it comes to rest on any particular face, it is in stable
equilibrium. Small vibrations or disturbances may make it
oscillate, but it returns always to the same face. These
oscillations are like the fluctuating variations. A greater
disturbance may cause the polyhedron to roll over on to a
new face, where it comes to rest again, only showing the
ever present fluctuations around its new centre. The new
position corresponds to a mutation. It may appear from our
familiarity with the great changes that we associate with the
idea of discontinuous variability, that a mutation must also
involve a considerable change. Such, however, De Vries says,
is not the case. In fact, numerous mutations are smaller
than the extremes of fluctuating variation. For example, the

U

290 Evolution and Adaptation

different elementary species of Drada verna are less different
from each other than the forms of leaves on a tree. The
essential differences between the two kinds of variation is
that the mutation is constant, while the continuous variation
fluctuates back and forth.

The following example is given by De Vries to illustrate the
general point of view in regard to varieties and species. The
species Oxalis corniculata is a “collective” species that lives
in New Zealand. It has been described as having seven well-
characterized varieties which do not live together or have in-
termediate forms. If we knew only this group, there would
be no question that there are seven good species. But in
other countries intermediate forms exist, which exactly bridge
over the differences between the seven New Zealand forms.
For this reason all the forms have been united in a single
species.

Another example is that of the fern, Lomaria procera, from
New Zealand, Australia, South Africa, and South America.
If the forms from only one country be considered, they appear
to be different species; but if all the forms from the different
parts of the world be taken into account, they constitute a
connected group, and are united into one large species.

It will be seen, therefore, that the limits of a collective
species are determined solely by the deficiencies in the ge-
nealogical tree of the elementary species. If all the element-
ary species in one country were destroyed, then the forms
living in other countries that had been previously held
together because of those which have now been destroyed,
would, after the destruction, become true species. In other
words: “The Linnean species are formed by the disap-
pearance of other elementary species, which at first connected
all forms. This mode of origin is a purely historical process,
and can never become the subject of experimental investi-
gation.” Spencer’s famous expression, the “survival of the
fittest,” is incomplete, and should read the “survival of the

Variation and Heredity 291

fittest species.” It is, therefore, not the study of Linnzan
species that has a physiological interest, but it is the study
of the elementary species of which the Linnzan species are
made up, that furnishes the all-important problem for experi-
mental study.

De Vries gives a critical analysis of a number. of cases in
which new races have been formed under domestication. He
shows very convincingly that, whenever the result has been
the outcome of the selection of fluctuating variations, the
product that is formed can only be kept to its highest point
of development by the most rigid and ever watchful care.
If selection ceases for only a few generations, the new form
sinks back at once to its original level. Many of our cul-
tivated plants have really arisen, not by selection of this
sort, but by mutations; and there are a number of recorded
cases where the first and sudden appearance of a new form
has been observed. In such cases as these there is no need
for selection, for if left to themselves there is no return to the
original form. If, however, after a new mutation has ap-
peared in this way, we subject its fluctuating variations to
selection, we can keep the new form up to its most extreme
limit, but can do nothing more.

Another means, frequently employed, by which new varie-
ties have been formed is by bringing together different ele-
mentary species under cultivation. For instance, there are a
large number of wild elementary species of apples, and De
Vries believes that our different races of apples owe their
origin in part to these different wild forms. Crossing, culti-
vation, and selection have done the rest.

De Vries points out some of the inconsistencies of those
who have attempted to discriminate between varieties and
species. The only rule that can be adhered to is that a
variety differs from a species to which it belongs in only one
or in a few characters. Most so-called varieties in nature are
really elementary species, which differ from their nearest

292 Evolution and Adaptation

relatives, not in one character only, but in nearly all their
characters. There is no ground, De Vries states, for believ-
ing them to be varieties. If it is found inconvenient to rank
them under the names of the old Linnzan species, it will be
better, perhaps, to treat them as subspecies, but De Vries
prefers to call them elementary species.

In regard to the distribution of species in nature, it may be
generally stated that the larger the geographical domain so
much the larger is the number of elementary species. They
are found to be heaped up in the centre of their area of dis-
tribution, but are more scattered at the periphery.

In any one locality each Linnzean species has as a rule
only one or a few elementary species. The larger the area
the more numerous the forms. From France alone Jordan
had brought together in his garden 50 elementary species of
Draba verna. From England, Italy, and Austria there could
be added 150 more. This polymorphism is, De Vries thinks,
a general phenomenon, although the number of forms is sel-
dom so great as in this case.

Amongst animals this great variety of forms is not often
met with, yet amongst the mammalia and birds of North
America there are many cases of local forms or races, some
of which at least are probably mutations. This can only be
proven, however, by actually transferring the forms to new
localities in order to find out if they retain their original char-
acters, or become changed into another form. It seems not
improbable that many of the forms are not the outcome of
the external conditions under which the animal now lives,
but would perpetuate themselves in a new environment.

From the evidence that his results have given, De Vries
believes it is probable that mutation has occurred in all direc-
tions. In the same way that Darwin supposed that individ-
ual or fluctuating variations are scattering, so also De Vries
believes that the new forms that arise through mutation are
scattering. On this point it seems to me that De Vries may

Variation and Heredity 293

be too much prejudiced by his results with the evening prim-
rose. If, as he supposes, many forms, generally ranked as
varieties, are really elementary species, it seems more proba-
ble that the mutation of a form may often be limited to the
production of one or of only a very few new forms. The
single variations, or sports, point even more strongly in favor
of this interpretation. Moreover, the general problem of
evolution from a purely theoretical point of view is very
much simplified, if we assume that the kinds of mutating
forms may often be very limited, and that mutations may
often continue to occur in a direct line. On this last point,
De Vries argues that the evidence from paleontology cannot
be trusted, for all that we can conclude from fossil remains
is that certain mutations have dominated, and have been suffi-
ciently abundant to leave a record. In other words, the con-
ditions may have been such that only certain forms could
find a foothold.

De Vries asks whether there are for each species periods
of mutation when many and great changes take place, and
periods when relatively little change occurs. The evidence
upon which to form an opinion is scanty, but De Vries is
inclined to think that such periods do occur. It is at least
certain from our experience that there are long periods when
we do not see new forms arising, while at other times,
although we know very few of them, epidemics of change may
take place. The mutative period which De Vries found in
the evening primrose is the best-known example of such a
period of active mutation. Equally important for the descent
theory is the idea that the same mutation may appear time
after time. There is good evidence to show that this really
occurs, and in consequence the chances for the perpetuation of
such a form are greatly increased. Delbceuf, who advocated
this idea of the repeated reappearance of a new form, has
also attempted to show that if this occurs the new form may
become established without selection of any kind taking place,

294 Evolution and Adaptation

—the time required depending upon the frequency with
which the new form appears. This law of Delboeuf, De Vries
believes, is correct from the point of view of the mutation
theory. It explains, in a very simple way, the existence of
numerous species-characters that are entirely useless, such,
for instance, as exist between the different elementary species
of Draba verna. “ According to the selection theory only
useful characters can survive; according to the mutation
theory, useless characters also may survive, and even those
that may be hurtful to a small degree.”

We may now proceed to examine the evidence from which
De Vries has been led to the general conclusions given in
the preceding pages. De Vries found at Hilversam, near
Amsterdam, a locality where a number of plants of the even-
ing primrose, nothera lamarckiana, grow in large numbers.
This plant is an American form that has been imported into
Europe. It often escapes from cultivation, as is the case at
Hilversam, where for ten years it had been growing wild.
Its rapid increase in numbers in the course of a few years
may be one of the causes that has led to the appearance of a
mutation period. The escaped plants showed fluctuating
variations in nearly all of their organs. They also had pro-
duced a number of abnormal forms. Some of the plants
came to maturity in one year, others in two, or in rare cases
in three, years.

A year after the first finding of these plants De Vries
observed two well-characterized forms, which he at once rec-
ognized as new elementary species. One of these was O. dre-
vistylis, which occurred only as female plants. The other new
species was a smooth-leafed form with a more beautiful foli-
age than O. lamarckiana. This is O. levifola. It was found
that both of these new forms bred true from self-fertilized
seeds. At first only a few specimens were found, each form
in a particular part of the field, which looks as though each
might have come from the seeds of a single plant.

Variation and Heredity 295

These two new forms, as well as the common O. lamarck-
tana, were collected, and from these plants there have
arisen the three groups or families of elementary species that
De Vries has studied. In his garden other new forms also
arose from those that had been brought under cultivation.
The largest group and the most important one is that from
the original O. Jamarckiana form. The accompanying table

GENOTHERA LAMARCKIANA

ELEMENTARY SPECIES

7

Generation | Gicas | ALBIDA ey - ~ eee es NanneLia | Lata Scr
8 Gener.
VIII} 1899 5 I ° 1700 21 I
annual x 4 -
7 Gener.
VII; 1898 9 fo) 3000 II
annual s v 2
6 Gener.
VI 1897 II 29 3 1800 9 5 I
X a JS
annual
5 Gener.
Vv 1896 25 135 20 8000 49 142 6
annual . v a
4 Gener. :
IV 1895 I 15 176 8 14000 60 73 I
annual ‘ Yv sd
3 Gener.
III {1890-91 I 10000 3 3
biennial s 4
2 Gener.
II {1888-89 15000 5 5
biennial \~———,
1 Gener.
I 1886-87 9
biennial

296 Evolution and Adaptation

shows the mutations that arose between 1887 and 1899
from these plants. The seeds were selected in each case
from self-fertilized plants of the /amarckiana form, so that the
new plants appearing in each horizontal line are the descend-
ants in each generation of /amarckiana parents. It will be
observed that the species, O. oblongata, appeared again and
again in considerable numbers, and the same is true for
several of the other forms also. Only the two species, O.
gigas and O. scintillans, appeared very rarely.

Thus De Vries had, in his seven generations, about fifty
thousand plants, and about eight hundred of these were muta-
tions. When the flowers of the new forms were artificially
fertilized with pollen from the flowers on the same plant, or
of the same kind of plant, they gave rise to forms like them-
selves, thus showing that they are true elementary species.’
It is also a point of some interest to observe that all these forms
differed from each other in a large number of particulars.

Only one form, O. scéztillans, that appeared eight times, is
not constant as are the other species. When self-fertilized
its seeds produce always three other forms, O. sczntillans,
O. oblongata, and O. lamarckiana. It differs in this respect
from all the other elementary species, which mutate not more
than once in ten thousand individuals.

From the seeds of one of the new forms, O. levzfolia,
collected in the field, plants were reared, some of which were
O. lamarckiana and others O. levifolia. They were allowed
to grow together, and their descendants gave rise to the same
forms found in the /amarckiana family, described above,
namely, O. lata, elliptica, nannella, rubrinervis, and also two
new species, O. spatulata and leptocarpa. .

In the Zaza family, only female flowers are produced, and,
therefore, in order to obtain seeds they were fertilized with

1 0. lata is always female, and cannot, therefore, be self-fertilized. When
crossed with O. /amarckiana there is produced fifteen to twenty per cent of
pure éafa individuals.

Variation and Heredity 207

pollen from other species. Here also appeared some of the
new species, already mentioned, namely, albida, nannella,
lata, oblongata, rubrinervis, and also two new species, elliptica
and subovata.

De Vries also watched the field from which the original
forms were obtained, and found there many of the new
species that appeared under cultivation. These were found,
however, only as weak young plants that rarely flowered.
Five of the new forms were seen either in the Hilversam
field, or else raised from seeds that had been collected there.
These facts show that the new species are not due to cultiva-
tion, and that they arise year after year from the seeds of
the parent form, O. /amarckiana.

CONCLUSIONS

From the evidence given in the preceding pages it ap-
pears that the line between fluctuating variations and muta-
tions may be sharply drawn. If we assume that mutations
have furnished the material for the process of evolution, the
whole problem appears in a different light from that in which
it was placed by Darwin when he assumed that the fluctuating
variations are the kind which give the material for evolution.

From the point of view of the mutation theory, species are
no longer looked upon as having been slowly built up through
the selection of individual variations, but the elementary
species, at least, appear at a single advance, and fully formed.
This need not necessarily mean that great changes have sud-
denly taken place, and in this respect the mutation theory is
in accord with Darwin’s view that extreme forms that rarely
appear, “sports,” have not furnished the material for the
process of evolution.

As De Vries has pointed out, each mutation may be different
from the parent form in only a slight degree for each pojnt,
although all the points may be different. The most unique

298 Evolution and Adaptation

feature of these mutations is the constancy with which the
new form is inherited. It is this fact, not previously fully ap-
preciated, that De Vries’s work has brought prominently into
the foreground. There is another point of great interest in
this connection. Many of the groups that Darwin recognized
as varieties correspond to the elementary species of De Vries.
These varieties, Darwin thought, are the first stages in the for-
mations of species, and, in fact, cannot be separated from
species in most cases. The main difference between the
selection theory and the mutation theory is that the one sup-
poses these varieties to arise through selection of individual
variations, the other supposes that they have arisen spontane-
ously and at once from the original form, The development
of these varieties into new species is again supposed, on the
Darwinian theory, to be the result of further selection, on the
mutation theory, the result of the appearance of new mutations.

In consequence of this difference in the two theories, it
will not be difficult to show that the mutation theory escapes
some of the gravest difficulties that the Darwinian theory
has encountered. Some of the advantages of the mutation
theory may be briefly mentioned here.

1. Since the mutations appear fully formed from the be-
ginning, there is no difficulty in accounting for the incipient
stages in the development of an organ, and since the organ
may persist, even when it has no value to the race, it may be-
come further developed by later mutations and may come to
have finally an important relation to the life of the individual.

2. The new mutations may appear in large numbers, and
of the different kinds those will persist that can get a foot-
hold. On account of the large number of times that the
same mutations appear, the danger of becoming swamped
through crossing with the original form will be lessened in
proportion to the number of new individuals that arise.

3. If the time of reaching maturity in the new form is
different from that in the parent forms, then the new species

Variation and Heredity 299

will be kept from crossing with the parent form, and since this
new character will be present from the beginning, the new form
will have much better chances of surviving than if a difference
in time of reaching maturity had to be gradually acquired.

4. The new species that appear may be in some cases
already adapted to live, in a different environment from that
occupied by the parent form; and if so, it will be isolated
from the beginning, which will be an advantage in avoiding
the bad effects of intercrossing.

5. It is well known that the differences between related
species consists largely in differences of unimportant organs,
and this is in harmony with the mutation theory, but one of
the real difficulties of the selection theory.

6. Useless or even slightly injurious characters may appear
as mutations, and if they do not seriously affect the perpetua-
tion of the race, they may persist.

In Chapters X and XI, an attempt will be made to point
out in detail the advantages which the mutation theory has
over the Darwinian theory.

CHAPTER IX

EVOLUTION AS THE RESULT OF EXTERNAL AND
INTERNAL FACTORS

WE come now to a consideration of other theories that
have been advanced to account for the evolution of new
forms; and in so far as these new forms are adapted to
their environment, the theories will bear directly on the
question of the origin of adaptive variations. One school
of transformationists has made the external world and the
changes taking place in it the source of new variations.
Another school believes that the changes arise within the
organism itself. We may examine these two points of view
in turn.

Tue EFFECT OF EXTERNAL INFLUENCES

We have already seen that Lamarck held as a part of his
doctrine of transformation that the changes in the external
world, the environment, bring about, directly, changes in the
organism, and he believed that all plants and many of
the lower animals have evolved as the result of a reaction of
this sort. This idea did not originate with Lamarck, how-
ever, since before him Buffon had advanced the same hy-
pothesis, and there cannot be much doubt that Lamarck
borrowed from his patron, Buffon, this part of his theory
of evolution.

This idea of the influence of the external world as a factor
inducing changes in the organism has come, however, to be
associated especially with the name of Geoffroy Saint-Hilaire,
whose period of activity, although overlapping, came after

300

External and Internal Factors in Evolution 301

that of Lamarck. The central idea of Geoffroy’s view was
that species of animals and plants undergo change as the
environment changes; and it is important to note, in passing,
that he did not suppose that these changes were always for
the benefit of the individual, z.e. they were not always adap-
tive. If they were not, the forms became extinct. So long
as the conditions remain constant, the species remains con-
stant; and he found an answer in this to Cuvier’s argument,
in respect to the similarity between the animals living at
present in Egypt and those discovered embalmed along with
mummies at least two thousand years old. Geoffroy Saint-
Hilaire said, that since the climatic conditions of Egypt
had remained exactly the same during all these years, the
animals of Egypt would also have remained unchanged.

Geoffroy’s views were largely influenced by his studies in
systematic zoology and by his conception of a unity of plan
running through the entire animal kingdom. His study of
embryology and paleontology had led him to believe that
present forms have descended from other organisms living in
the past, and in this connection his discovery of teeth in the
jaws of the embryo of the baleen whale and also his dis-
covery of the embryonic dental ridges in the upper and in
the lower jaws of birds, were used with effect in supporting
the theory of change or evolution. Lastly, his remarkable
work in the study of abnormal forms prepared the way for
his conception of sudden and great changes, which he be-
lieved organisms ‘capable of undergoing. He went so far in
fact, in one instance, as to suppose that it was not impossible
that a bird might have issued fully equipped from the egg of
a crocodile. Such an extreme statement, which seems to us
nowadays only laughable, need not prejudice us against the
more moderate parts of his speculation.

His study of the fossil gavials found near Caen led him
to believe that they are quite distinct from living crocodiles.
He asked whether these old forms may not represent a link

302 Evolution and Adaptation

in the chain that connects, without interruption, the older
inhabitants of the earth with animals living at the present
time. Without positively affirming that this is the case, he
did not hesitate to state that a transformation of this sort
seemed possible to him. He said: “I think that the process
of respiration constitutes an acquirement so important in the
‘disposition’ of the forms of animals, that it is not at all
necessary to suppose that the surrounding respiratory gases
become modified quickly and in large amount in order that
the animal may become slowly modified. The prolonged
action of time would ordinarily suffice, but if combined with
a cataclysm, the result would be so much the better.”

He supposed that in the course of time respiration becomes
difficult and finally impossible as far as certain systems of
organs are concerned. The necessity then arises and creates
another arrangement, perfecting or altering the existing struc-
tures. Modifications, fortunate or fatal, are created which
through propagation are continued, and which, if fortunate,
influence all the rest of the organization. But if the modifica-
tions are injurious to the animals in which they have appeared,
the animals cease to exist, and are replaced by others having
a different form, and one suited to the new circumstances.

The comparison between the stages of development of the
individual and the evolution of the species was strongly im-
pressed on the mind of Geoffroy. He says: “We see, each
year, the spectacle of the transformation in organization from
one class into another. A batrachian is at first a fish under
the name of a tadpole, then a reptile (amphibian) under that
of a frog.” “The development, or the result of the trans-
formation, is brought about by the combined action of light
and of oxygen; and the change in the body of the animal
takes place by the production of new blood-vessels, whose
development follows the law of the balancing of organs,
in the sense, that if the circulating fluids precipitate them-
selves into new channels there remains less in the old

External and Internal Factors in Evolution 303

vessels.” By preventing tadpoles from leaving the water,
Geoffroy claims that it has been shown that they can be
prevented from changing into frogs. The main point that
Geoffroy attempts to establish is no doubt fairly clear, but
the way in which he supposes the change to be effected is
not so clear, and his ideas as to the way in which new change
may be perpetuated in the next generation are, from our more
modern point of view, extremely hazy. It is perhaps not
altogether fair to judge his view from the standpoint of the
origin of adaptive structures, but rather as an attempt to
explain the causes that have brought about the evolution of
the organic world.

During the remainder of the nineteenth century there
accumulated a large number of facts in relation to the action
of the external conditions in bringing about changes in
animals and plants. Much of this evidence is of impor-
tance in dealing with the question of the origin of organic
adaptation.

The first class of facts in this connection is that of geo-
graphical variation in animals and plants. It will be im-
possible here to do more than select some of the most
important cases. De Varigny, in his book on “ Experimental
Evolution,” has brought together a large number of facts of
this kind, and from his account the following illustrations
have been selected. He says: “When the small brown
honey-bee from High Burgundy is transported into Bresse—
although not very distant—it soon becomes larger and
assumes a yellow color; this happens even in the second
generation.” It is also pointed out that the roots of the
beet, carrot, and radish are colorless in their wild natural
state, but when brought under cultivation they become red,
yellow, etc. Vilmorin has noted that the red, yellow, ane
violet colors of carrots appear only some time after the wild
forms have been brought under cultivation. Moquin-Tandon
has seen “gentians which are blue in valleys become white

304 Evolution and Adaptation

on mountains.” Other cases also are on record in which the
colors of a plant are dependent on external conditions. .

The sizes of plants and animals are also often directly trace-
able to certain external conditions; the change is generally
connected with the amount of food obtainable. ‘“ Generally
speaking,” De Varigny says, “insular animals are smaller
than their continental congeners. In the Canary Islands
the oxen of one of the smallest islands are smaller than
those on the others, although all belong to the same breed,
and the hdrses are also smaller, and the indigenous inhabitants
are in the same case, although belonging to a tall race. It
would seem that in Malta elephants were very small, — fossil
elephants, of course,—and that during the Roman period
the island was noted for a dwarf breed of dogs, which was
named after its birthplace, according to Strabo. In Corsica,
also, horses and oxen are very small, and Cervus corsicanus,
the indigenous deer, is quite reduced in dimensions; .. .
and lastly, the small dimensions of the Falkland horses —
imported from Spain in 1764-——are familiar to all. The
dwarf rabbits of Porto Santo described by Darwin may also
be cited as a case in point.”

These facts, interesting as they are, will, no doubt, have to
be more carefully examined before the evidence can have
great value, for it is not clear what factor or factors have
produced the decrease in size of these animals.

The -following cases show more clearly the immediate
effect of the environment: “Many animals, when trans-
ferred to warm climates, lose their wool, or their hairy cover-
ing is much reduced. In some parts of the warmer regions
of the earth, sheep have no wool, but merely hairs like those
of dogs. Similarly, as Roulin notices, poultry have, in
Columbia, lost their feathers, and while the young are at first
covered with a black and delicate down, they lose it in great
part as they grow, and the adult fowls nearly realize Plato’s
realistic description of man—a biped without feathers.

Lixternal and Internal Factors in Evolution 305

Conversely, many animals when transferred from warm to
cold climates acquire a thicker covering; dogs and horses, for
instance, becoming covered with wool.”

A number of kinds of snails that were supposed to belong
to different species have been found, on further examination,
to be only varieties due to the environment. “ Locard has
discovered through experiments that L. ¢wrgida and elophila
are mere varieties—due to environment— of the common
Lymnea stagnalis.” He says, “These are not new species,
but merely common aspects of a common type, which is
capable of modification and of adaptation according to the
nature of the media in which it has to live.” It has also
been shown by Bateson that similar changes occur in
Cardium edule, and other lamellibranchs are known to vary
according to the nature of the water in which they live.

In regard to plants, the influence of the environment has
long been known to produce an effect on the form, color,
etc., of the individuals. “The common dandelion (7arara-
cum densleonis) has in dry soil leaves which are much more
irregular and incised, while they are hardly dentate in marshy
stations, where it is called Zavaxracum palustre.

“Tndividuals growing near the seashore differ markedly
from those growing far inland. Similarly, species such as
some Ranunculi, which can live under water as well as in
air, exhibit marked differences when considered in their
different stations, as is well known to all. These differences
may be important enough to induce botanists to believe in
the existence of two different species when there is only
one.”

An interesting case is that of Daphnia rectirostris, a small
crustacean living sometimes in fresh water, at other times
in water containing salt and also in salt lakes. There are
two forms, corresponding to the conditions under which they
live, and it is said that the differences are of a kind that
suffice to separate species from each other. In another

x

306 Evolution and Adaptation

crustacean, Branchipus ferox, the form differs in a number
of points, according to whether it lives in salt or in fresh
water. Schmankewitsch says that, had he not found all
transitional forms, and observed the transformation in
cultures, he would have regarded the two forms as separate
species. The oft-quoted case of Artemia furnishes a very
striking example of the influence of the environment.
Artemia salina lives in water whose concentration varies
between 5 and 12 degrees of saltness. When the amount
of salt is increased to 12 degrees, the animal shows certain
characteristics like those of Artemia mitlhausenit, which
may live in water having 24 to 25 degrees of saltness. The
form A. salina may be further completely changed into that
of A. milhausenit by increasing the amount of salt to the latter
amount.

Among domesticated animals and plants—a few instances
of which have been already referred to— we find a large
number of cases in which a change in the environment
produces definite changes in the organism. Darwin has
made a most valuable collection of facts of this kind in his
“Animals and Plants under Domestication.” He believes
that domesticated forms are much more variable than wild
ones, and that this is due, in part, to their being protected
from competition, and to their having been removed from
their natural conditions and even from their native country.
“In conformity with this, all our domesticated productions
without exception vary far more than natural species. The
hive-bee, which feeds itself, and follows in most respects
its natural habits of life, is the least variable of all domesti-
cated animals.... Hardly a single plant can be named,
which has long been cultivated and propagated by seed, that
is not highly variable.” ‘‘ Bud-variation . . . shows us that
variability may be quite independent of seminal reproduc-
tion, and likewise of reversion to long-lost ancestral charac-
ters. No one will maintain that the sudden appearance of

External and Internal Factors in Evolution 307

a moss-rose on a Provence rose is a retutn to a former
state, ... nor can the appearance of nectarines on peach
trees be accounted for on the principle of reversion.” It is
said that bud-variations are also much more frequent on
cultivated than on wild plants.

Darwin adds: “These general considerations alone render
it probable that variability of every kind is directly or indi-
rectly caused by changed conditions of life. Or to put the
case under another point of view, if it were possible to expose
all the individuals of a species during many generations to
absolutely uniform conditions of life, there would be no
variability.”

In some cases it has been observed that, in passing from
one part of a continent to another, many or all of the forms
of the same group and even of different groups change in
the same way. Allen’s account of the variations in North
American birds and mammals furnishes a number of strik-
ing examples of this kind of change. He finds that, as
a rule, the birds and mammals of North America increase
in size from the south northward. This is true, not only
for the individuals of the same species, but generally the
largest species of each genus are in the north. There
are some exceptions, however, in which the increase in size
is in the opposite direction. The explanation of this is
that the largest individuals are almost invariably found in
the region where the group to which the species belongs
receives its greatest numerical development. This Allen
interprets as the hypothetical “centre of distribution of the
species,” which is in most cases doubtless also its original
centre of dispersal. If the species has arisen in the north,
then the northern forms are the largest; but if it arose in
the south, the reverse is the case. Thus, most of the species
of North America that live north of Mexico are supposed
to have had a northern origin, as shown by the circumpolar
distribution of some of them and by the relationship of

308 Evolution and Adaptation

others to Old World species; and in these the largest indi-
viduals of the species of a genus are northern. Conversely,
in the exceptional cases of increase in size toward the south,
it can be shown that the forms have probably had a southern

origin.
The Canidz (wolves and foxes) have their largest repre-
sentatives, the world over, in the north. ‘In North America

the family is represented by six species, the smallest of which
(speaking generally) are southern and the largest northern.”
The three species that have the widest ranges (the gray
wolf, the common fox, and the gray fox) show the most
marked differences in size. The skull, for instance, of ‘the
common wolf is fully one-fifth larger in the northern parts
of British America and Alaska than it is in northern
Mexico, where it finds the southern limit of its habitat.
Between the largest northern skull and the largest southern
skull there is a difference of about thirty-five per cent of
the mean size. Specimens from the intermediate region
show a gradual intergradation between the extremes, although
many of the examples from the upper Missouri country are
nearly as large as those from the extreme north.” The
common fox is about one-tenth larger, on the average, in
Alaska than it is in New England. The gray fox, whose
habitat extends from Pennsylvania southward to Yucatan,
has an average length of skull of about five inches in the
north, and less than four in Central America — about ten
per cent difference.

The Felidz, or cats, “reach their greatest development as
respects both the number and the size of the species in the
intertropical regions. This family has sent a single typical
representative, the panther (Fe/is concolor), north of Mexico,
and this ranges only to about the northern boundary of the
United States. The other North American representatives
of the family are the lynxes, which in some of their varieties
range from Alaska to Mexico.” Although they vary greatly

External and Internal Factors in Evolution 309

in different localities in color and in length and texture of
pelage, they do not vary as to the size of their skulls. On
the other hand the panther (and the ocelots) greatly increases
in size southward, “ or toward the metropolis of the family.”

Other carnivora that increase in size northward are the
badger, the marten, the fisher, the wolverine, and the ermine,
which are all northern types.

Deer are also larger in the north; in the Virginia deer the
annually deciduous antlers of immense size reach their great-
est development in the north. The northern race of flying
squirrels is one-half larger than the southern, “yet the two
extremes are found to pass so gradually one into the other,
that it is hardly possible to define even a southern and a
northern geographical race.” The species ranges from the
arctic regions to Central America.

In birds also similar relations exist, but there is less often
an increase in size northward. In species whose breeding
station covers a wide range of latitude, the northern birds
are not only smaller, but have quite different colors, as is
markedly the case in the common quail, the meadow-lark,
the purple grackle, the red-winged blackbird, the flicker, the
towhee bunting, the Carolina dove, and in numerous other
species. The same difference is also quite apparent in the
blue jay, the crow, in most of the woodpeckers, in the titmice,
numerous sparrows, and several warblers and thrushes.
The variation often amounts to from ten to fifteen per cent
of the average size of the species.

Allen also states that certain parts of the animal may vary
proportionately more than the general size, there being an
apparent tendency for peripheral parts to enlarge toward the
warmer regions, 7.¢. toward the south. “In mammals which
have the external ears largely developed —as in the wolves,
foxes, some of the deer, and especially the hares — the larger
size of this organ in southern as compared with northern in-
dividuals of the same species, is often strikingly apparent.”

310 Evolution and Adaptation

It is even more apparent in species inhabiting open plains.
The ears of the gray rabbit of the plains of western Arizona
are twice the size of those of the Eastern states.

In birds the bill especially, but also the claws and tail,
is larger in the south. In passing from New England
southward to Florida the bill in slender-billed forms be-
comes larger, longer, more attenuated, and more decurved;
while in short-billed forms the southern individuals have
thicker and larger bills, although the birds themselves are
smaller.

The remarkable changes and gradations of color in birds
in different parts of North America are very instructive, and
the important results obtained by American ornithologists
form an interesting chapter in zoology. The evidence would
convince the most sceptical of the difficulty of distinguishing
between Linnzean species. It is not surprising to find in this
connection a leading ornithologist exclaiming, “if there really
are such things as species.” The differences here noted are
mainly from east to west. We may briefly review here a
few striking cases selected from Coues’s “Key to North
American Birds.”

The flicker, or golden-winged woodpecker (Colaptes aura-
tus), has a wide distribution in eastern North America. It
is replaced in western North America (from the Rocky
Mountains to the Pacific) by C. mexicanus. In the inter-
mediate regions, Missouri and the Rocky Mountain region,
the characters of the two are blended in every conceivable
degree in different specimens. “ Perhaps it is a hybrid, and
perhaps it is a transitional form, and doubtless there are no
such things as species in Nature... . In the west you will
find specimens avratus on one side of the body, mexicanus
on the other.” There is a third form, C. chrysoides, with
the wings and tail as in auratus, and the head as in mext-
canus, that lives in the valley of the Colorado River, Lower
California, and southward. °

External and Internal Factors in Evolution 311

In regard to the song-sparrow (Melospiza), Coues writes:
“The type of the genus is the familiar and beloved song-
sparrow, a bird of constant characters in the east, but in the
west is split into numerous geographical races, some of them
looking so different from typical fasciata that they have been
considered as distinct species, and even placed in other
genera. This differentiation affects not only their color, but
the size, relative proportions of parts, and particularly the
shape of the bill; and it is sometimes so great, as in the case
of M. cinerea, that less dissimilar looking birds are commonly
assigned to different genera. Nevertheless the gradation is
complete, and affected by imperceptible degrees... . The
several degrees of likeness and unlikeness may be thrown
into true relief better by some such expressions as the follow-
ing, than by formal antithetical phrases: (1) The common
eastern bird commonly modified in the interior into the duller
colored (2) fallax. This in the Pacific watershed, more de-
cidedly modified by deeper coloration, — broader black
streaks in (3) hkermanni, with its diminutive local race
(4) samuelis, and more ruddy shades in (5) guttata north-
ward, increasing in intensity with increased size in (6) rafina.
Then the remarkable (7) czzerea, insulated much further
apart than any of the others. A former American school
would probably have made four ‘ Eee species,’ (1) fasciata,
(2) samuelis, (3) vafina, (4) cinerea.’

Somewhat similar relations are found in three other gen-
era of finches. Thus Passerella is “imperfectly differen-
tiated”; Junco is represented by one eastern species, but in
the west the stock splits up into numerous forms, “all of
which intergrade with each other and with the eastern bird.
Almost all late writers have taken a hand at Junco, shuffling
them about in the vain attempt to decide which are ‘species’
and which ‘ varieties.’ All are either or both, as we may
elect to consider them.” In the distribution of the genus
Pipilo similar relations are found. There is an eastern form

312 Lvolution and Adaptation

much more distinct from the western forms than these are
from each other.

Finally may be mentioned the curious variations in
screech-owls of the genus Scops. This owl has two strik-
ingly different plumages—a mottled gray and a reddish
brown, which, although very distinct when fully developed,
yet “are entirely independent of age, season, or sex.” There
is an eastern form, Scops asio, that extends west to the
Rocky Mountains. There is a northwestern form, S. kennt-
cotti, which in its red phase is quite different from S. aso,
but in its gray plumage is very similar. The California form,
S. benderit, is not known to have a red phase, and the gray
phase is quite different from that of S. aszo, but like the last
form. The Colorado form, S. maxwelle, has no red phase,
‘but on the contrary the whole plumage is very pale, almost
as if bleached, the difference evident in the nestlings even.”
The Texas form, S. masellz, has both phases, and is very
similar to S. aszo. The Florida form is smaller and colored
like S. asto. The red phase is the frequent, if not the
usual, one. The flammulated form, S. fiammula, is “a very
small species, with much the general aspect of an ungrown
S. asto.” This is the southwestern form, easily distinguished
on account of its small size and color from the other forms.

These examples might be greatly increased, but they will
suffice, I think, to convince one of the difficulty of giving a
sharp definition to “species.” The facts speak strongly in
favor of the transmutation theory, and show us how a species
may become separated under different conditions into a num-
ber of new forms, which would be counted as new different
species, if the intermediate forms were exterminated.

In discussing the nature of the changes that bring about
variability, Darwin remarks: ‘‘ From a remote period to the
present day, under climates and circumstances as different
as it is possible to conceive, organic beings of all kinds, when
domesticated or cultivated, have varied. We see this with

External and Internal Factors in Evolution 313

the many domestic races of quadrupeds and birds belonging
to different orders, with goldfish and silkworms, with plants
of many kinds, raised in various quarters of the world. In
the deserts of northern Africa the date-palm has yielded

thirty-eight varieties ; in the fertile plains of India it is noto- |

rious how many varieties of rice and of a host of other plants
exist; in a single Polynesian island, twenty-four varieties of
the breadfruit, the same number of the banana, and twenty-
two varieties of the arum, are cultivated by the natives. The
mulberry tree of India and Europe has yielded many varie-
ties serving as food for the silkworm; and in China sixty-
three varieties of the bamboo are used for various domestic
purposes. These facts, and innumerable others which could
be added, indicate that a change of almost any kind in the
conditions of life suffices to cause variability — different
changes acting on different organisms.”

Darwin thinks that a change in climate alone is not one of
the potent causes of variability, because the native country
of a plant, where it has been longest cultivated, is where it
has oftenest given rise to the greatest number of varieties.
He thinks it also doubtful that a change in food is an impor-
tant source of variability, since the domestic pigeon has
varied more than any other species of fowl, yet the food has
been always nearly the same. This is also true for cattle
and sheep, whose food is probably much less varied in kind
than in the wild species.

Another point of interest is raised by Darwin. He thinks,
as do others also, that the influence of a change in the con-
ditions is cumulative, in the sense that it may not appear
until the species has been subjected to it for several genera-
tions. Darwin states that universal experience shows that
when new plants are first introduced into gardens they do
not vary, but after several generations they will begin to
vary to a greater or less extent. In a few cases, as in that
of the dahlia, the zinnia, the Swan River daisy, and. the

314 Evolution and Adaptation

Scotch rose, it is known that the new variations only appeared
after a time. The following statement by Salter is then
quoted, “Every one knows that the chief difficulty is in
breaking through the original form and color of the species,
and every one will be on the lookout for any natural sport,
either from seed or branch; that being once obtained, how-
ever trifling the change may be, the result depends on him-
self.” Jonghe is also quoted to the effect that “there is
another principle, namely, that the more a type has entered
into a state of variation, the greater is the tendency to con-
tinue doing so, and the more it has varied from the original
type, the more is it disposed to vary still further.” Darwin
also quotes with approval the opinion of the most celebrated
horticulturist of France, Vilmorin, who maintained that “when
any particular variation is desired, the first step is to get the
plant to vary in any manner whatever, and to go on select-
ing the most variable individuals, even though they vary in
the wrong direction; for the fixed character of the species
being once broken, the desired variation will sooner or later
appear.”

Darwin also cites a few cases where animals have changed
quite quickly when brought under domestication. Turkeys
raised from the eggs of wild species lose their metallic tints,
and become spotted with white in the third generation. Wild
ducks lose their true plumage after a few generations. “The
white collar around the neck of the mallard becomes much
broader and more irregular, and white feathers appear in
the duckling’s wings. They increase also in size of body.”
In these cases it appears that several generations were
necessary in order to bring about a marked change in the
original type, but the Australian dingoes, bred in the Zoo-
logical Gardens, produced puppies which were in the first
generation marked with white and other colors.

The following cases from De Varigny are also very striking.
The dwarf trees from Japan, for the most part conifers, which

External and Internal Factors in Evolution ars

may be a hundred years old and not be more than three feet
high, are in part the result “of mechanical processes which
prevent the spreading of the branches, and in part of a starv-
ing process which consists in cutting most roots and in keeping |
the plant in poor soil.”

As an example of the sudden appearance of a new varia-
tion the following case is interesting. A variety of begonia
is recorded as having appeared quite suddenly at a number of
places at the same time. In another case a narcissus which
had met with adverse circumstances, and had then been
supplied with a chemical manure in some quantity, began to
bear double flowers.

Amongst animals the following cases of the appearance of
sudden variations are pointed out by De Varigny. “In Para-
guay, during the last century (1770), a bull was born without
horns, although his ancestry was well provided with these
appendages, and his progeny was also hornless, although at
first he was mated with horned cows. If the horned and the
hornless were met in fossil state, we would certainly wonder
at not finding specimens provided with semi-degenerate horns,
and representing the link between both, and if we were told
that the hornless variety may have arisen suddenly, we should
not believe it and we should be wrong. In South America
also, between the sixteenth and eighteenth centuries the niata
breed of oxen sprang into life, and this breed of bulldog
oxen has thriven and become a new race. So in the San
Paulo provinces of Brazil, a new breed of oxen suddenly
appeared which was provided with truly enormous horns, the
breed of franqueiros, as they are called. The mauchamp
breed of sheep owes its origin to a single lamb that was born
in 1828 from merino parents, but whose wool, instead of being
curly like that of its parents, remained quite smooth. This
sudden variation is often. met with, and in France has been
noticed in different herds.”

The ancon race of sheep originated in 1791 from a ram

316 Evolution and Adaptation

born in Massachusetts having short crooked legs and a long
back. From this one ram by crossing, at first with common
sheep, the ancon race has been produced. ‘ When crossed
with other breeds the offspring, with rare exception, instead
of being intermediate in character, perfectly resemble either
parent; even one of twins has resembled one parent and the
second the other.”

Two especially remarkable cases remain to be described.
These are the Porto Santo rabbit and the japanned peacock.
Darwin has given a full account of both of these cases.
“The rabbits which have become feral on the island of Porto
Santo, near Madeira, deserve a fuller account. In 1418 or
1419 J. Gonzales Zarco happened to have a female rabbit on
board which had produced young during the voyage, and he
turned them all out on the island. These animals soon
increased so rapidly that they became a nuisance, and actually
caused the abandonment of the settlement. Thirty-seven
years subsequently, Cada Mosto describes them as innumer-
able; nor is this surprising, as the island was not inhabited by
any beast of prey, or by any terrestrial mammal. We do not
know the character of the mother rabbit; but it was probably
the common domestic kind. The Spanish peninsula, whence
Zarco sailed, is known to have abounded with the common
wild species at the most remote historical period; and as these
rabbits were taken on board for food, it is improbable that
they should have been of any peculiar breed. That the breed
was well domesticated is shown by the doe having littered
during the voyage. Mr. Wollaston, at my request, brought
two of these feral rabbits in spirits of wine; and, subsequently,
Mr. W. Haywood sent home three more specimens in brine
and two alive. These seven specimens, though caught at
different periods, closely resemble each other. They were
full-grown, as shown, by the state of their bones. Although
the conditions of life in Porto Santo are evidently highly
favorable to rabbits, as proven by their extraordinarily rapid

External and Internal Factors in Evolution 317

increase, yet they differ conspicuously in their small size from
the wild English rabbit.... In color the Porto Santo
rabbit differs considerably from the common rabbit; the
upper surface is redder, and is rarely interspersed with any
black or black-tipped hairs. The throat and certain parts of
the under surface, instead of being pure white, are generally
gray or leaden color. But the most remarkable difference
is in the ears and tail. I have examined many fresh English
rabbits, and the large collection of skins in the British Museum
from various countries, and all have the upper surface of the
tail and the tips of the ears clothed with blackish gray fur;
and this is given in most works as one of the specific char-
acters of the rabbit. Now in the seven Porto Santo rabbits
the upper surface of the tail was reddish brown, and the tips
of the ears had no trace of the black edging. But here we
meet with a singular circumstance: in June, 1861, I examined
two of these rabbits recently sent to the Zoological Gardens
and their tails and ears were colored as just described; but
when one of their dead bodies was sent to me in February, 1863,
the ears were plainly edged, and the upper surface of the tail
was covered with blackish gray fur, and the whole body was
much less red; so that under the English climate this individ-
ual rabbit had recovered the proper color of its fur in rather
less than four years.”

Another striking case of sudden variation is found in the
peacock. It is all the more remarkable because this bird has
hardly varied at all under domestication, and is almost exactly
like the wild species living in India to-day. Darwin states:
“There is one strange fact with respect to the peacock,
namely, the occasional appearance in England of the ‘ja-
panned’ or ‘black-shouldered’ kind. This form has lately
been named, on the high authority of Mr. Slater, as a distinct
species, viz. Pavo nigripennis, which he believes will here-
after be found wild in some country, but not in India, where
it is certainly unknown. The males of these japanned birds

318 Evolution and Adaptation

differ conspicuously from the common peacock in the color
of their secondary wing-feathers, scapulars, wing-coverts, and
thighs, and are, I think, more beautiful; they are rather
smaller than the common sort, and are always beaten by
them in their battles, as I hear from the Hon. A. S. G. Can-
ning. The females are much paler-colored than those of
the common kind. ‘Both sexes, as Mr. Canning informs me,
are white when they leave the egg, and they differ from the
young of the white variety only in having a peculiar pinkish
tinge on their wings. These japanned birds, though appear-
ing suddenly in flocks of the common kind, propagate their
kind quite truly.”

In two cases, in which these birds had appeared quite sud-
denly in flocks of the ordinary kind, it is recorded that
“though a smaller and weaker bird, it increased to the ex-
tinction of the previously existing breed.” Here we have
certainly a remarkable case of a new species suddenly
appearing and replacing the ordinary form, although the
birds are smaller, and ave beaten in their battles.

Darwin has given an admirably clear statement of his
opinion as to the causes of variability in the opening para-
graph of his chapter dealing with this topic in his “ Animals
and Plants.” Some authors, he says, “look at variability as a
necessary contingent on reproduction, and as much an original
law as growth or inheritance. Others have of late encouraged,
perhaps unintentionally, this view by speaking of inheritance
and variability as equal and antagonistic principles. Pallas
maintained, and he has had some followers, that variability
depends exclusively on the crossing of primordially distinct
forms. Other authors attribute variability to an excess of
food, and with animals, to an excess relatively to the amount
of exercise taken, or again, to the effects of a more genial
climate. That these causes are all effective is highly probable.
But we must, I think, take a broader view, and conclude that
organic beings, when subjected during several generations to

External and Internal Factors in Evolution 319

any change whatever in their condition, tend to vary; the
kind of variation which ensues depending in most cases in a
far higher degree on the nature of the constitution of the
being, than on the nature of the changed conditions.”

Most naturalists will agree, in all probability, with this con-
clusion of Darwin’s. The examples cited in the preceding
pages have shown that there are several ways in which the
organisms may respond to the environment. In some cases
it appears to affect all the individuals in the same way; in
other cases it appears to cause them to fluctuate in many
directions ; and in still other cases, without any recognizable
change in the external conditions, new forms may suddenly
appear, often of a perfectly definite type, that depart widely
from the parent form.

For the theory of evolution it is a point of the first impor-
tance to determine which of these modes of variation has
supplied the basis for evolution. Moreover, we are here
especially concerned with the question of how adaptive vari-
ations arise. Without attempting to decide for the present
between these different kinds of variability, let us examine
certain cases in which an immediate and adaptive response
to the environment has been described as taking place.

RESPONSIVE CHANGES IN THE ORGANISM THAT ADAPT IT
TO THE NEw ENVIRONMENT

There is some experimental evidence showing that some-
times organisms respond directly and adaptively to certain
changes in the environment. Few as the facts are, they
require very careful consideration in our present examination.
The most striking, perhaps, is the acclimatization to different
temperatures. It has been found that while few active organ-
isms can withstand a temperature over 45 degrees C., and that
for very many 40 degrees is a fatal point, yet, on the other
hand, there are organisms that live in certain hot springs

320 Evolution and Adaptation

where the temperature is very high. Thus, to give a few
examples, there are some of the lower plants, nostocs and
protococcus forms, that live in the geysers of California at a
temperature of 93 degrees C., or nearly that of boiling water.
Leptothrix is found in the Carlsbad springs, that have a tem-
perature of 44 to 54 degrees. Oscillaria have been found in
the Yellowstone Park in water between 54 and 68 degrees,
and in the hot springs in the Philippines at 71 degrees, and
on Ischia at 85 degrees, and in Iceland at 98 degrees.

It is probable from recent observations of Setchel that
most of the temperatures are too high, since he finds that
the water at the edge of hot springs is many degrees lower
than that in the middle parts.

The snail, Physa acuta, has been found in France living at
a temperature of 35 to 36 degrees; another snail, Paludina,
at Abano, Padua, at sodegrees. Rotifers have been found at
Carlsbad at 45 to 54 degrees; Anguillide at Ischia at 81 de-
grees; Cypris balnearia, a crustacean at Hammam-Meckhou-
tin, at 81 degrees; frogs at the baths of “ Pise” at 38 degrees.

Now, there can be little doubt that these forms have had
ancestors that were like the other members of the group, and
would have been killed had they been put at once into water
of these high temperatures, therefore it seems highly prob-
able that these forms have become specially adapted to live
in these warm waters. It is, therefore, interesting to find that
it has been possible to acclimatize animals experimentally to
a temperature much above that which would be fatal to them
if subjected directly to it. Dutrochet (in 1817) found that if
the plant, nitella, was put into water at 27 degrees, the cur-
rents in the protoplasm were stopped, but soon began again.
If put now into water at 34 degrees they again stopped mov-
ing, but in a quarter of an hour began once more. If then
put into water at 40 degrees the currents again slowed down,
but began again later.

Dallinger (in 1880) made a most remarkable series of ex-

External and Internal Factors in Evolution 321

periments on flagellate protozoans. He kept them ina warm
oven, beginning at first at a temperature of 16.6 degrees C.
“ He employed the first four months in raising the tempera-
ture 5.5 degrees. This, however, was not necessary, since
the rise to 21 degrees can be made rapidly, but for success
in higher temperatures it is best to proceed slowly from the
beginning. When the temperature had been raised to 23
degrees, the organisms began dying, but soon ceased, and
after two months the temperature was raised half a degree.
more, and eventually to 25.5 degrees. Here the organisms
began to succumb again, and it was necessary repeatedly to
lower the temperature slightly, and then to advance it to
25.5 degrees, until, after several weeks, unfavorable appear-
ances ceased. For eight months the temperature could not
be raised from this stationary point a quarter of a degree
without unfavorable appearances. During several years,
proceeding by slow stages, Dallinger succeeded in raising
the organisms up to a temperature of 70 degrees C., at which
the experiment was ended by an accident.”

Davenport and Castle carried out a series of experiments
on the egg of the toad, in which they tried to acclimatize
the eggs to a temperature higher than normal. Recently
laid eggs were used; one lot kept at a temperature of 15
degrees C., the other at 24-25 degrees C. Both lots de-
veloped normally. At the end of four weeks the tempera-
ture point at which the tadpoles were killed was determined.
Those reared at a temperature of 15 degrees C. died at 41
degrees C., or below; those reared at 24-25 degrees C. sus-
tained a temperature 10 degrees higher; no tadpole dying
in this set under 43 degrees C. “This increased capacity
for resistance was not produced by the dying off of the less
resistant individuals, for no death occurred in these experi-
ments during the gradual elevation of the temperatures in
the cultures.” The increased resistance was due, therefore,

1 Quoted from Davenport’s “ Experimental Morphology.”
y

322 Evolution and Adaptation

to a change in the protoplasm of the individuals. It was
also determined that the acquired resistance was only very
gradually lost (after seventeen days’ sojourn in cooler water).
The explanation of this result may be due, in part, to the pro-
toplasm containing less water at higher temperatures, for it
is known that while the white of egg (albumen) coagulates
at 56 degrees C. in aqueous solution; with only 18 per cent
of water it coagulates between 80 degrees and 90 degrees C.;
and with 6 per cent, at 145 degrees C.; and without water
between 100 degrees and 170 degrees C.

It has long been known that organisms in the dry condi-
tion resist a much higher temperature. The damp uredo-
spore is killed at 58.5 degrees to 60 degrees C.; but dry
spores withstand’ 128 degrees C. It is also known that
organisms may become acclimatized to cold through loss of
water, but we lack exact experimental data to show to what
extent this can be carried.

There are also some experiments that go to show that ani-
mals may become attuned to certain amounts of light, but the
facts in this connection will be described in another chapter.

Some important results have been obtained by accustom-
ing organisms to solutions containing various amounts of salts.
A number of cases of this sort are given by De Varigny. It
has been found that littoral marine animals that live where
the water may become diluted by the rain, or by rivers, sur-
vive better when put into fresh water than do animals living
farther from the shore. Thus the oyster, the mussel, and the
snail, Patella, withstand immersion in fresh water better than
other animals that live farther out at sea. The reverse is
also true; fresh-water forms, such as Lymnza, Physa, Palu-
dina, and others may be slowly acclimatized to water contain-
ing more salt. The forms mentioned above could be brought
by degrees into water containing 4 per cent of salt, which
would have killed the animals if they had been brought sud-
denly into it. Similar results have been obtained for amceba.

External and Internal Factors in Evolution 323

It has been shown that certain rotifers and tardigrades,
and also some unicellular animals, that live in pools and
ponds that are liable to become dry, withstand desiccation,
while other members of the same groups, living in the sea,
do not possess this power of resistance. Cases of this sort
are usually explained as cases of adaptation, but it has not
been shown experimentally that resistance to drying can be
acquired by a process of acclimatization to this condition.
The case is also in some respects different from the preced-
ing, since intermediate conditions are less likely to be met
with, or to be of sufficiently long duration for the animal to
become acclimatized to them. It seems more probable, in
such cases, that these forms have been able to live in such
precarious conditions from the beginning because they could
resist the effects of drying, not that they have slowly acquired
this power. Finally, there must be discussed the question of
the acclimatization to poisons, to which an individual may be
rendered partially immune. The point of special importance
in this connection is that the animal may be said to respond
adaptively to a large number of substances, which it has
never met before in its individual history, or to which its
ancestors have never been subjected. It may become slowly
adapted to many different kinds of injurious substances.
These cases are amongst the most important adaptive indi-
vidual responses with which we are familiar, and the point
cannot be too much emphasized that organisms have this
latent capacity without ever having had an opportunity to
acquire it through experience.

The preceding groups of phenomena, included under
the general heading of individual acclimatization, have one
striking thing in common, namely, that a physiological
adaptation is brought about without a corresponding
change in form, although we must suppose that the struc-
ture has been altered in certain respects at least. The form
of the individual remains the same as before, but so far

324 Evolution and Adaptation

as its powers of resistance are concerned it is a very differ-
ent being.

In regard to the perpetuation of the advantages gained by
means of this power of adaptation, it is clear in those cases
in which the young are nourished during their embryonic
life by the mother, that, in this way, the young may be
rendered immune to a certain extent, and there are instances
of this sort recorded, especially in the case of some bacterial
diseases. Whether this power can also be transmitted through
the egg, in those instances in which the egg itself is set free
and development takes place outside the body, has not been
shown. In any case, the effect appears not to be a perma-
nent one and will wear off when the particular poison no
longer acts. It is improbable, therefore, that any permanent
contribution to the race could be gained in this way. Adap-
tations of this sort, while of the highest importance to the
individual, can have produced little direct effect on the evolu-
tion of new forms, although it may have been often of para-
mount importance to the individuals to be able to adapt
themselves, or rather to become able to resist the effect of
injurious substances. The important fact in this connection
is the wonderful latent power possessed by all animals. So
many, and of such different kinds, are the substances to
which they may become immune, that it is inconceivable
that this property of the organism could ever have been
acquired through experience, no matter how probable it may
be made to appear that this might have occurred in certain
cases of fatal bacterial diseases. And if not, in so many
other cases, why invent a special explanation for the few
cases ?

We may defer the general discussion of the réle that
external factors have played in the adaptation of organisms,
until we have examined some of the theories which attribute
changes to internal factors. The idea that something innate
in the living substance itself has served as the basis for evolu-

External and Internal Factors in Evolution 325

tion has given rise to a number of different hypotheses. That
of the botanist Nageli is one of the most elaborately worked
out theories of this sort that has been proposed, and may be
examined by way of illustration.

NAGELI’S PERFECTING PRINCIPLE

Nageli used the term completing principle (“Vervollkom-
mungsprincip”) to express a tendency toward perfection
and specialization. Short-sighted writers, he says, have
pretended to see in the use of this principle something
mystical, but on the contrary it is intended that the term
shall be employed in a purely physical sense. It represents
the law of inertia in the organic realm. Once set in motion,
the developmental process cannot stand still, but must
advance in its own direction. Perfection, or completion,
means nothing else than the advance to complicated struc-
ture, “but since persons are likely to attach more meaning
to the word ferfectzon than is intended, it would perhaps
be better to replace it with the less objectionable word pro-
gression.”

Nageli says that Darwin, having in view only the condition
of adaptation, designates that as more complete which gives
its possessor an advantage in the battle for existence. Nageli
claims that this is not the only criterion that applies to organ-
isms, and it leaves out the most important part of the phe-
nomenon. There are two kinds of completeness which we
should keep distinctly apart: (1) the completeness of organi-
zation characterized by the complication of the structure and
the most far-reaching specialization of the parts; (2) the
completeness of the adaptation, present at each stage in the
organization, which consists in the most advantageous devel-
opment of the organism (under existing conditions) that is
possible with a given complication of structure and a given
division of functions.

326 Evolution and Adaptation

The first of these conceptions Nageli always calls “complete-
ness” (Vollkommenheit), for want of a simpler and better
expression; the second he calls adaptation. By way of
illustrating the difference between the two, the following
examples may be given. The unicellular plants and the
moulds are excellently adapted each to its conditions of life,
but they are much less complete in structure than an apple
tree, or a grape vine. The rotifers and the leeches are well
adapted to their station, but in completeness of structure
they are much simpler than the vertebrates.

If we consider only organization and division of labor as
the work of the completing principle, and leave for the
moment adaptation out of account, we may form the following
picture of the rise of the organic world. From the inorganic
world there arose the simplest organic being thinkable, being
little more than a drop of substance. If this underwent any
change at all, it would have been necessarily in the direction
of greater complication of structure; and this would constitute
the first step in the upward direction. In this way Nageli
imagines the process once begun would continue. When the
movement has reached a certain point, it must continue in the
same direction. The organic kingdom consists, therefore, of
many treelike branches, which have had a common starting-
point. Not only does he suppose that organisms were once
spontaneously generated, and began their first upward course
of development, but the process has been repeated over and
over again, and each time new series have been started on
the upward course. The organic kingdom is made up, there-
fore, of all degrees of organization, and all these have had
their origins in the series of past forms that arose and began
their upward course at different times in the past. Those
that are the highest forms at the present time represent the
oldest series that successfully developed; the lowest forms
living at the present time are the last that have appeared on
the scene of action.

External and Internal Factors in Evolution 327

Organisms, as has been said, are distinguished from one
another, not only in that one is simpler and another more
complicated, but also in that those standing at the same stage
of organization are unequally differentiated in their functions
and in their structure, which is connected primarily with cer-
tain external relations which Nageli calls adaptations.

Adaptation appears at each stage of the organization, which
stage is, for a given environment, the most advantageous
expression of the main type that was itself produced by
internal causes. For this condition of adaptation, a suffi-
cient cause is demanded, and this is, as Nageli tries to show
later, the result of the inherited response to the environ-
ment. In many cases this cause will continue to act until
complete adaptation is gained; in other cases, the external
conditions give a direction only, and the organism itself con-
tinues the movement to its more perfect condition.

The difference between the conception of the organic king-
dom as the outcome of mechanical causes on the one hand, or of
competition and extermination on the other hand, can be best
brought out, Nageli thinks, by the following comparison of
the two respective methods of action. There might have been
no competition, and no consequent extermination in the plant
kingdom, if from the beginning the surface of the earth had
continually grown larger in proportion as living things
increased in numbers, and if animals had not appeared to
destroy the plants. Under these conditions each germ cou.d
then have found room and food, and have unfolded itself
without hinderance. If now, as is assumed to be the case
on the Darwinian theory, individual variations had been in
all directions, the developmental movement could not have
gone beyond its own beginnings, and the first-formed plants
would have remained swinging now on one side and now on
another of the point first reached. The whole plant kingdom
would have remained in its entirety at its first stage of evolu-
tion, that is, it would never have advanced beyond the stage

328 Lvolution and Adaptation

of a naked drop of plasma with or without amembrane. But,
according to the further Darwinian conception, competition,
leading to extermination, is capable of bringing such a condi-
tion toa higher stage of development, since it is assumed that
those individuals which vary in a beneficial direction would
have an advantage over those that have not taken such a
step, or have made a step backward.

If, on the other hand, under the above-mentioned conditions
of unrestricted development, without competition, variations
were determined by “ mechanical principles,” then, according
to Nageli’s view, all plant forms that now exist would still
have evolved, and would be found living at the present time,
but along with all those that now exist there would be still
other forms in countless numbers. These would represent
those forms which have been suppressed. On Nageli’s view
competition and suppression do not produce new forms, but
only weed out the intermediate forms. He says without com-
petition the plant kingdom would be like the Milky Way; in
consequence of competition the plant kingdom is like the
firmament studded with bright stars.

The plant kingdom may also be compared to a branched
tree, the ends of whose branches represent living species.
This tree has an inordinate power of growth, and if left to it-
self it would produce an impenetrable tangle of interwoven
branches. The gardener prevents this crowding by cutting
away some of the parts, and thus gives to the tree distinct
branches and twigs. The tree would be the same without the
watchful trimming of the gardener, but without definite form.

Nageli states: “ From my earlier researches I believe that
the external influences are small in comparison to the internal
ones. I shall speak here only of the influences of climate and
of food, which are generally described as the causes of change,
without however any one’s having really determined whether
or not a definite result can be brought about by these factors.
Later I shall speak of a special class of external influences

External and Internal Factors in Evolution 329

which, according to my view, bring forth beyond a doubt
adaptive changes.”

The external influence of climate and of food act only as
transitory factors. A rich food supply produces fat, lack
of food leads to leanness, a warm summer makes a plant more
aromatic, and its fruit sweeter; a cold year means less odor
and sour fruit. Of two similar seeds the one sown in rich
soil will produce a plant with many branches and abundance
of flowers; the other, planted in sandy soil, will produce a
plant without branches, with few flowers, and with small
leaves. The seeds from these two. plants will behave in
exactly the same way; they have inherited none of the
differences of their parents. Influences of this sort, even if
extending over many generations, have no permanent effect.
Alpine plants that have lived since the ice age under the
same conditions, and have the characters of true high-
mountain plants, lose these characters completely during the
first summer, if transplanted to the plains. Moreover, it
makes no difference whether the seed or the whole plant
itself be transferred. In place of the dwarfed, unbranched
growth, and the reduced number of organs, the plant when
transferred to the plains shoots upin height, branches strongly,
and produces numerous leaves and flowers. The plants retain
their new characters as long as they live in the plain without
any other new variation being observed in them.

Other characteristics also, which arise from different kinds
of external influences due to different localities, such as damp-
ness and shade, a swampy region, or different geological
substrata, last only so long as the external conditions last.

These transient peculiarities make up the characters of
local varieties. That they have no permanency is intelli-
gible, since they exhibit no new characters, but the change
consists mainly in the over- or under-development of those
peculiarities that are dependent on external influences. The
effect of these influences may be compared to an elastic rod,

330 Evolution and Adaptation ‘

which, however much it may be distorted by external circum-
stances, returns again to its original form as soon as released.

Besides these temporary changes, due to external influences,
there are many cases known in which the same plant lives
under very diverse conditions and yet remains exactly the same.
For example, the species of Rhododendron ferragineum lives
on archzean mountains and especially where the soil is poor in
calcium. Another species, Rhododendron hirsutum is found
especially on soil rich in calcium. The difference in the two
species has been supposed to depend on differences in the
soil, and if so, we would imagine that, if transplanted for a
long time, the one should change in the direction of the other.
Yet it is known that the rusty rhododendron may be found in
all sorts of localities, even on dry, sunny, calcareous rocks of
the Apennines and of the Jura, and despite its residence in
these localities, since the glacial epoch, no change whatever
has taken place.

Single varieties of the large and variable genus of Azera-
cium have lived since the glacial period in the high regions
of the Alps, Carpathians, and in the far north, and also in the
plains of different geological formations, but these varieties
have remained exactly the same, although on all sides there
are transitional forms leading from these to other varieties.

Some parasitic species also furnish excellent illustrations of
the same principle. Besides the several species of Oroban-
chia and of the parasitic moulds, the mistletoe deserves special
mention. It lives on both birch and apple trees and on both
presents exactly the same appearance; and even if it is true
that mistletoe growing on conifers presents certain small devi-
ations in its character, it is still doubtful whether, if trans-
ferred to the birch or apple tree, it would not lose these
differences, thus indicating that they are not permanent.

It is a fact of general observation that, on the one hand,
the same variety occurs in different localities and under dif-
ferent surroundings, and, on the other hand, that slightly

External and Internal Factors in Evolution 331

different varieties live together in the same place and there-
fore under the same external conditions. It is evident,
then, that food conditions have neither originated the dif-
ferences nor kept them up. The rarer cases in which in
different localities different varieties exist show nothing, be-
cause competition and suppression keep certain varieties from
developing where it would be possible otherwise for them
to exist.

Nageli says his conclusion may be tested from another
point of view. If food conditions, as is generally supposed,
have a definite, z.¢. a permanent, effect on the organism, then
all organisms living under the same conditions should show
the same characters. Indeed, it has been claimed in some
instances that this is actually the case. Thus it is stated
that dry localities cause plants to become hairy, and that
absence of hairiness is met with in shady localities. This
may apply to certain species, but in other cases exactly the
reverse is true, and even the same species behaves differently
in different regions, as in Mzeracizum. And so it is with all
characteristics which are ascribed to external influences. As
soon as it is supposed a discovery has been made in this di-
rection, we may rest assured that in other cases the reverse
will be found to hold. We have had, in respect to the influence
of the outer world on organisms, the same experience as with
the rules for the weather, — when we come to examine the
facts critically there are found to be as many exceptions as
confirmations of the rule.

If climatic influence has a definite effect, the entire flora of
a special locality ought to have the same peculiarities, but this
stands in contradiction to allthe results of experience. The
character of the vegetation is not determined by the envi-
ronment of the plants but by their prehistoric origin, and as
the result of competition. Nageli concludes his discussion
with the statement that all of our experience goes to show
that the effects of external influences (climate and food)

Cee Evolution and Adaptation

appear at once, and their results last only as long as the
influences themselves last, and are then lost, leaving nothing
permanent behind. This is true even when the external
influences have lasted for a long time, — since the glacial
epoch, for instance. We find, he claims, nothing that sup-
ports the view that such influences are inherited.

If we next examine the question of changes from zudernal
causes, Nageli claims that here also observation and research
fail to show the origin of a new species, or even of a new
variety from external causes. In the organic world little
change has taken place, he believes, since the glacial epoch.
Many varieties have even remained the same throughout the
whole intervening time; and while it cannot be doubted that
new varieties have also been formed, yet the cause of their
origin cannot be empirically demonstrated. The permanent,
hereditary characters, of whose origin we know something
from experience, belong to the individual changes which
have appeared under cultivation in the formation of domestic
races. These are for the most part the result of crossing.
So far as we have any definite information as to the origin of
the changes, they are the result of inner, and never of exter-
nal, causes. We recognize that this must be the case, since
under the same external conditions individuals behave differ-
ently — in the same flower-bud some seeds give rise to plants
- like the parent, others to altered ones. The strawberry with a
single leaflet, instead of three, arose in the last century in a
single individual amongst many other ordinary plants. From
the ten seeds of a pear Van Mons obtained as many different
kinds of pears. The most conclusive proof of the action
of inner causes is most clearly seen when the branches of
the same plant differ. In Geneva a horse-chestnut bore
a branch with “filled” flowers, and from this branch, by
means of cuttings, this variation has been carried over all
Europe. In the Botanic Garden at Munich there is a beech
with small divided leaves; but one of its branches produces

External and Internal Factors in Evolution 333

the common broad undivided leaves. Many such examples
have been recorded which can only be explained by assuming
that a cell, or a group of cells, like those from which the
other branches arose, have become changed in some unknown
way as the result of inner causes. The properties that are
permanent and inherited are contained in the idioplasm,
which the parent transmits to its offspring. A cause that per-
manently transforms the organism must also transform the
idioplasm. How powerless, in comparison to internal causes,
the external causes are is shown most conclusively in grafting.
The graft, although it receives its nourishment through the
stock, which may be another species, remains itself unchanged.

Nageli makes the following interesting comparison
between the development of the individual from an egg, and
the evolution, or development, of the phylum. No one will
doubt that the egg during the entire time of its process of
transformation is guided by internal factors. Each succes-
sive stage follows with mechanical necessity from the pre-
ceding. If an animal can develop from inner causes from
a drop of ‘plasma, why should not the entire evolutionary
process have also been the outcome of developmental inner
causes? He admits that there is a difference in the two
cases in that the plasma that forms the egg has come from
another animal, and contains all the properties of the indi-
vidual in a primordial condition. In the other case we must
suppose that the original drop of plasma did not contain at
first the primordium of definite structures, but only the
ability to form such. Logically the difference is unimpor-
tant. The main point is that in the primordium of the germ
a special peculiarity of the substance is present which by
forming new substances grows, and changes as it grows, and
the one change of necessity excites the next until finally a
highly organized being is the result.

Nageli discusses a question in this connection, which, he
says, has been unnecessarily confused in the descent theory.

334 Evolution and Adaptation

oO

Since we are entirely in the dark as to how much time has
been required for the formation of phyla, so also are we
ignorant as to how long it may have taken for each step in
advance. We may err equally in ascribing too much and
too little time to the process. It is, moreover, not necessary
that for every step the same amount of time should have
been required. On the contrary, the probability is that
recognizable changes may at times follow each other rapidly,
and then for a time come to a standstill, —just as in the
development of the individual there are periods of more rapid
and others of less rapid change.

A more difficult problem than that relating to the sort of
changes the external influences bring about in the organism,
is the question as to how they effect the organism, or how
they act on it mechanically. This, as is well known, was
answered by Darwin, who regards all organization as a prob-
lem of adaptation: only those chance variations surviving
which are capable of existence, the others being destroyed.
On this theory external influences have only a negative or a
passive action, namely, in setting aside the unadapted indi-
viduals. Ndageli, on the other hand, looks upon some kinds
of external conditions as directly giving rise to the adaptive
characters of the organism. This is accomplished, he sup-
poses, in the following ways: two kinds of influence are
recognized; the direct action, which, as in inorganic nature,
comes to an end when the external influences come to an
end, as when cold .diminishes the chemical actions in the
plant; and ¢he cudirect action, generally known as a stimulus,
which starts a series of molecular motions, invisible to us,
but which we recognize only in their effects. Very often
the stimulus starts only a reflex action, usually at the place
of application.

A stimulus acting for but a short time produces no last-
ing effect on the idioplasm. A person stung by a wasp
suffers no permanent effect from the injury. But if a stim-

External and Internal Factors in Evolution 335

ulus acts for a long time, and through a large number of
generations, then it may, even if of small strength, so change
the zdzoplasm, that a tendency or disposition capable of being
seen may be the result. This appears to be the case in
regard to the action of light, which causes certain parts of
the plant to turn toward it and others away from it; also
for the action of gravity, which determines the downward
direction of the roots. It may be claimed, perhaps, that
these are the results of direct influence and not of an
internal response, but this is not the case; for some plants
act in exactly the opposite way, and send a stem downward,
as in the case of the cleistogamous flowers of Cardamine
chenopodifolia ; and other plants turn away from the light.
This means that the idioplasm behaves differently in different
plants in response to the same stimulus.

Concerning the more visible effects of adaptation, Nageli
states that in regard to some of them there can be no ques-
tion as to how they must have arisen. Protection against
cold, by the formation of a thick coat of hair, is the direct
result of the action of the cold on the skin of the animal.
The different weapons of offence and of defence, horns,
spurs, tusks, etc., have arisen, he maintains, through stimulus
to those parts of the body where these structures arise.

The causes of the other adaptations, especially of those
occurring in plants, are less obvious. Land plants .protect
themselves from drying by forming a layer of cork over the
surface. The most primitive plants were water plants, which
acclimated themselves little by little to moist, and then to dry,
air. When they first emerged from the water the drying
acted as a.stimulus on the surface, and caused it to harden
in the same way as a drop of glue hardens. This harden-
ing in turn acted as a stimulus, causing a chemical transfor-
mation of the surface into a corky substance. This effect
was inherited, and in this way the power to form cork origi-
nated.

336 Evolution and Adaptation

Land plants have, in addition to the soft parts, the hard
bast and wood which serves the mechanical purpose of sup-
porting the soft tissues and protecting them from being
injured. The arrangement of the hard parts is such as to
suggest that they are the result of the action of pressures
and tensions on the plant, for the strongest cells are found
where there is most need for them. It is easy to imagine,
Nageli adds, that this important arrangement of the tissues
is the result of external forces which brought about the result
in these parts.

Nageli accounts for the origin of twining plants as follows.
Being overshadowed by other plants, the stem will grow
rapidly in the damp air. Coming in contact with the stems
of other plants, the delicate stem is stimulated on one side,
and grows around the point of contact. This tendency
becomes inherited, and the habit to twine is ultimately
established.

The difference in the two sides of leaves is explained by
Nageli as the result of the difference in the illumination of the
two sides. This influence of light on the leaf has been in-
herited. The formation of the tubular corolla that is seen in
many plants visited by insects is explained as the result of the
stimulus produced by the insects in looking for the pollen.
The increase in the length of the proboscis of the insect is
the result of the animal straining to reach the bottom of the
ever elongating tube of the corolla. ‘The tubular corolla
and the proboscis of the insect appear as though made for
each other. Both have slowly developed to their present
condition, the long tube from a short tube and the long
proboscis from a short one.” Thus, by purely Lamarckian
principles, Nageli attempts to account for many of the adap-
tations between the organism and the outer world. But if
this takes place, where is there left any room for the action
for his so-called perfecting principle? Nageli proceeds to
show how he supposes that the two work together.

°

External and Internal Factors in Evolution 337

As a result of inner causes the organism would pass
through a series of perfectly definite stages, J, J1, J% But
if, at any stage, external influences produced an effect on
the organism so that the arrangement of the idioplasm
changes in response, a new adaptation is produced. In this
way new characters, not inherent in the idioplasm, may be
added, and old ones be changed or lost. “In order not to
be misunderstood in regard to the completing or perfecting
principle I will add, that I ascribe to it no determinate action
in the organism, neither in producing the long neck of the
giraffe, nor the prehensile tail of the ape, neither the claws
of the crab, nor the decoration of the bird of paradise.
These structures are the outcome of both factors. I cannot
picture to myself how external causes alone, and just as little
how internal causes alone, could have changed a monad into
aman.” But Nageli goes on to say, that if at any stage
of organization one of the two causes should cease to act,
the other could only produce certain limited results. Thus,
if external causes alone acted, the organization would remain
at the same stage of completeness, but might become adapted
to all kinds of external conditions —a worm, for instance,
would not develop into a fish, but would remain a worm for-
ever, although it might change its worm structure in many
ways in response to external stimuli. If, on the other hand,
only the completing principle acted, then without changing
its adaptations the number of the cells and the size of the
organs might be increased, and functions that were formerly
united might become separated. Thus, without altering the
character of the organism, a more highly developed (in the
sense of being more specialized) organism would appear.

Nageli, as we have just seen, has attempted to build up a
conception of nature based on two assumptions, neither of
which has been demonstrated to be an actual principle of
development. His hypothesis appears, therefore, entirely
arbitrary and speculative to a high degree. Even if it were

Z

338 Evolution and Adaptation

conceivable that two such principles as these control the evo-
lution of organisms, it still requires a good deal of imagination
to conceive how the two go on working together. Moreover,
it is highly probable that whole groups have evolved in the
direction of greater simplification, as seen especially in the
case of those groups that have become degenerate. To
what principle can we refer processes of this sort ?

It is certainly a strange conclusion this, at which Nageli
finally arrives, for, after strenuously combating the idea that
the external factors of climate and of food have influence in
producing new species, he does not hesitate to ascribe all
sorts of imaginary influences to other external causes. The
apparent contradiction is due, perhaps, to the fact that his
experience with actual species led him to deny that the direct
action of the environment produces permanent changes, while
in theory he saw the necessity of adding to his perfecting
principle some other factor to explain the adaptations of the
new forms produced by inner causes. Nageli seems to have
felt strongly the impossibility of explaining the process of
evolution and of adaptation as the outcome of the selection
of chance variations, now in this direction, now in that. He
seems to have felt that there must be something within the
organism that is driving it ever upward, and he attempts to
avoid the teleological element, which such a conception is
almost certain to introduce, by postulating the inheritance of
the effects of long-continued action of the environment, in so
far as certain factors in the environment produce a response
in the organism. Nevertheless, this combination is not one
that is likely to commend itself, aside from the fact that the
assumptions have no evidence to support them. Despite
Nageli’s protest that his principles are purely physical, and
that there is nothing mystical in his point of view, it must be
admitted that his conception, as a whole, is so vague and

difficult in its application that it probably deserves the neglect
which it generally receives.

External and Internal Factors in Evolution 339

Nageli’s wide experience with living plants convinced him
that there is something in the organism over and beyond the in-
fluence of the external world that causes organisms to change;
and we cannot afford, I think, to despise his judgment on
this point, although we need not follow him to the length of
supposing that this internal influence is a “force” driving
the organism forward in the direction of ever greater com-
plexity. A more moderate estimate would be that the organ-
ism often changes through influences that appear to us to be
internal, and while some of the changes are merely fluctu-
ating or chance variations, there are others that appear to be
more limited in number, but perfectly definite and permanent
in character. It is the latter, which, I believe, we can safely
accredit to internal factors, and which may be compared to
Nageli’s internal causes, but this is far from assuming that
these changes are in the direction of greater completeness or
perfection, or that evolution would take place independently
of the action of external agencies.

CHAPTER X

THE ORIGIN OF THE DIFFERENT KINDS OF
ADAPTATIONS

In the present chapter we may first consider, from the
point of view of discontinuous variations as contrasted with
the theory of the selection of individual variations, the
structural adaptations of animals and plants, ze. those
cases in which the organism has a definite form that adapts
it to live in a particular environment. In the second place,
we may consider those adaptations that are the result of the
adjustment of each individual to its surroundings. In sub-
sequent chapters the adaptations connected with the
responses of the nervous system and with the process of
sexual reproduction will be considered.

It should be stated here, at the outset, that the term
mutation will be used in the following chapters in a very
general way, and it is not intended that the word shall
convey only the idea which De Vries attaches to it; it is
used rather as synonymous with discontinuous and also defi-
nite variation of all kinds. The term will be used to include
“the single variations” of Darwin, “sports,” and even ortho-
genic variation, if this has been definite or discontinuous.

Form AND SYMMETRY

Almost without exception, animals and plants have defi-
nite and characteristic forms. In other words, they are
not amorphous masses of substance. The members of each
species conform, more or less, to a sort of ideal type. Our

340

Origin of Different Kinds of Adaptations 341

first problem is to examine in what sense the form itself
may be looked upon as an adaptation to the surroundings.

It is a well-recognized fact that the forms of many animals
appear to stand in a definite relation to the environment.
For instance, animals that move in definite directions in
relation to their structure have the anterior and the pos-
terior ends quite different, and it is evident that these ends
stand in quite different relations to surrounding objects;
while, on the other hand, the two sides of the body which
are, as a rule, subjected to the same influences are nearly
exactly alike. The dorsal and the ventral surfaces of the —
body are generally exposed to very different external condi-
tions, and are quite different in structure.

The relation is so obvious in most cases that it might
lead one quite readily to conclude that the form of the ani-
mal had been moulded by its surroundings. Yet this first
impression probably gives an entirely wrong conception of
how such a relation has been acquired. Before we attempt to
discuss this question, let us examine some typical examples.

A radial type of structure is often found in fixed forms,
and in some floating forms, like the jellyfish. In a fixed
form, a sea-anemone, for instance, the conditions around the
free end and the fixed end of the body are entirely different,
and we find that these two ends are also different. The
free end contains the special sense organs, the mouth, ten-
tacles, etc.; while the fixed end contains the organ for attach-
ment. It is evident that the free end is exposed to the same
conditions in all directions, and it may seem probable that
this will account for the radial symmetry of the anemone.
There are also a few free forms, the sea-urchin for instance,
that have a radial symmetry. Whether their ancestors were
fixed forms, for which there is some evidence, we do not
know definitely; but, even if this is true, it does not affect
the main point, namely, that, although at present free to
move, the sea-urchin is radially symmetrical. But when we

342 Evolution and Adaptation

examine its method of locomotion, we find that it moves
indifferently in any direction over a solid surface; that is,
it keeps its oral face against a solid object, and moves over
the surface in any direction. Under these circumstances
the same external conditions will act equally upon all sides
of the body. In contrast to these common sea-urchins,
there are two other related groups, in which, although traces
of a well-marked radial symmetry are found, the external
form has been so changed that a secondary bilateral form
has been superimposed on it. These are the groups of the
clypeasters and the spatangoids, and it is generally supposed
that their forefathers were radially symmetrical forms like
the ordinary forms of sea-urchins. These bilateral forms
move in the direction of their plane of symmetry, but
we have no means of knowing whether they first became
bilateral and, in consequence, now move in the direction of
the median plane, or whether they acquired the habit of mov-
ing in one direction, and in consequence acquired a bilateral
symmetry. It seems more probable that the form changed
first, for otherwise it is difficult to see why a change of move-
ment in one direction should ever have taken place.

The radially symmetrical form is characteristic of many
flowers that stand on the ends of their stalks. They also
will be subjected to similar external influences in all direc-
tions. Many flowers, on the other hand, are bilaterally
symmetrical. Some of these forms are of such a sort that
they are generally interpreted as having been acquired in
connection with the visits of insects. Be this as it may,
it is still not clear why, if the flowers are terminal, insects
should not approach them equally from every direction. If
the flowers are not terminal, as, in fact, many of them are
not, their relation to the surroundings is bilateral with re--
spect to internal as well as to external conditions. The
former, rather than the latter, may have produced the
bilateral form of the flower. Here also we meet with the

Origin of Different Kinds of Adaptations 343

problem as to whether the flowers, being lateral in position,
have assumed a bilateral form because their internal re-
lations were bilateral; or whether an external relation, for
example, the visits of insects, has been the principle cause
of their becoming bilateral.

C
Fic. 4.—A, right and left claws of lobster; B, of the fiddler-crab; and C,
of Alpheus.

In some bilateral forms the right and left sides may be
unsymmetrical in certain organs. Right and left handedness
in man is the most familiar example, although the structural
difference on which this rests is not very obvious. More
striking is the difference in the two big claws of the lobster
(Fig. 4 A). One of the two claws is flat and has a fine saw-
toothed edge. The other is thicker and has rounded knobs
instead of teeth. It is said that these two claws are used by
the lobster for different purposes, —the heavy one for crush-
ing and for holding on, and the narrower for cutting up the

344 Evolution and Adaptation

food. If this is true, then we find a symmetrical organism
becoming unsymmetrical, and in consequence it takes advan-
tage of its asymmetry by using its right and left claws for
different purposes.

More striking still is the difference in the size of the right
and left claws in a related form, Alpheus —a crayfish-like
form that lives in the sea. With the larger claw (Fig. 4 C) it
makes a clicking sound that can be heard for a long distance.
In some of the crabs the difference in the size of the two
claws is enormous, as in the male fiddler-crab, for example
(Fig. 4 B). One of the claws is so big and unwieldy that it
must put the animal at a distinct disadvantage. Its use is
unknown, although it has been suggested that it is a second-
ary sexual character.

The asymmetry of the body of the snail is very conspicu-
ous, at least so far as certain organs are concerned. The
foot on which the animal crawls and the head have preserved
their bilaterality; but the visceral mass of the animal, con-
tained in the spirally wound shell, lying on the middle of the
upper surface of the foot, is twisted into a spiral form. Many
of the organs of one side of the body are atrophied. The
gill, the kidney, the reproductive organ, and one of the
auricles of the heart have completely, or almost completely,
disappeared. The cause of this loss seems to be connected
with the spiral twist of the visceral mass. One of the conse-
quences of the twisting has been to bring the organs of the
left side of the body around the posterior end until they come
to lie on the right side, the organs of the original right side
being carried forward and there atrophying.

There is another remarkable fact connected with the asym-
metry of the snail. In some species, Helix pomatia, for
example, the twist has been toward the right, ze. in the
direction which the hands of a watch follow when the face
is turned upward toward the observer. Individuals twisted
in this direction are called dextral. Occasionally there is

Origin of Different Kinds of Adaptations 345

found an individual with the spiral in the opposite direction
(sinistral), and in this the conditions of the internal organs
are exactly reversed. It is the left set of organs that is now
atrophied, and the right set that is functional. Such changes
appear suddenly. Organs of one side of the body that have
not been functional for many generations may become fully
developed. Moreover, Lang has shown that when a sinis-
tral form breeds with a normal dextral form, or even when
sinistral forms are bred with each other, the young are prac-
tically all of the ordinary type.

An attempt has been made to connect these facts with the
mode of development of the mollusks. It is known that the
eggs of a number of gasteropod mollusks segment in a per-
fectly definite manner. A sort of spiral cleavage is followed
by the formation of a large mesodermal cell from the left
posterior yolk-cell. From this mesodermal cell nearly all
the mesodermal organs of the body are formed. Thus it
may appear that the spiral form of the snail is connected
with the spiral form of the cleavage. In a few species of
marine and, fresh-water snails the cleavage spiral is reversed,
and the mesoderm arises from the right posterior yolk-cell.
It has been shown in several cases that the snail coming
from such an egg is twisted in the reverse direction from
that of ordinary snails.

It has been suggested, therefore, that the occasional sinis-
tral individual of Helix arises from an egg cleaving in the
reverse direction, and there is nothing improbable in an
assumption of this kind. No attempt has been made as yet
to explain why, in some cases, the cleavage spiral is turned
in one direction, and in other cases in the reverse direction ;
but even leaving this unaccounted for, the assumption of
the unusual form of Helix being the result of a reversal of the
cleavage throws some light as to how it is possible for the
complete reversal of the organs of the adult to arise. If it is
assumed that in the early embryo the cells on each side of

346 Evolution and Adaptation

the median line are alike, and at this time capable of forming
adult structures, a simple change of the spiral from right to
left might determine on which side of the middle line the
mesodermal cell would lie, and its presence on one side rather
than on the other might determine which side of the embryo
would develop, and which would not. This possibility removes
much of the mystery which may appear to surround a sudden
change of this sort.

It seems to me that we shall not go far wrong if we assume
that it is largely a matter of indifference whether an individ-
ual snail is a right-handed or a left-handed form, as far as its
relation to the environment is concerned. One form would
have as good a chance for existing as the other. If this is
granted, we may conclude that, while in most species a per-
fectly definite type is found, a right or a left spiral, yet
neither the one nor the other has been acquired on account
of its relation to the environment. This conclusion does not,
of course, commit us in any way as to whether the spiral
form of the visceral mass has been acquired in relation to
the environment, but only to the view that, if a spiral form
is to be produced, it is indifferent which way it turns. From
the evolutionary point of view this conclusion is of some
importance, since it indicates that one of the alternatives has
been adopted and has become practically constant in most
cases without selection having had anything to do with it.

Somewhat similar conditions are found in the flounders
and soles. As is well known, these fishes lie upon one side of
the body on the bottom of the ocean. Some species, with the
rarest exceptions to be mentioned in a moment, lie always on
the right side, others on the left side. A few species are
indifferently right or left. At rare intervals a left-sided form
is found in a right-sided species, and conversely, a right-sided
form in a left-sided species. In such cases the reversed type
is as perfectly developed in all respects as the normal form,
but with a complete reversal of its right and left sides,

Origin of Different Kinds of Adaptations 347

When the young flounders leave the egg, they swim in an
upright position, as do ordinary fishes, with both sides equally
developed. There cannot be any doubt that the ancestors
of these fish were bilaterally symmetrical. Therefore, within
the group, both right-handed and left-handed forms have
appeared. It seems to me highly improbable that if a right-
handed form had been slowly evolved through the selec-
tion of favorable variations in this direction, the end result
could be suddenly reversed, and a perfect left-sided form
appear. Moreover, as has been pointed out, the intermediate
stages. would have been at a great disadvantage as compared
with the parent, and this would lead to their extermination
on the selection theory. If, however, we suppose that a vari-
ation of this sort appeared at once, and was fixed, —a muta-
tion in other words, —and that whether or not it had an ad-
vantage over the parent form, it could still continue to exist,
and propagate its kind, then we avoid the chief difficulty of
the selection theory. . Moreover, we can imagine, at least,
that if this variation appeared in the germ and was, in its
essential nature, something like the relation seen in the snail,
the occasional reversal of the relations of the parts presents
no great difficulty.

In this same connection may be mentioned a curious fact
first discovered by Przibram and later confirmed by others.
If the leg carrying the large claw of a crustacean be removed,
then, at the next moult, the leg of the other side that had
been the smaller first leg becomes the new big one; and the
new leg that has regenerated from the place where the big
one was cut off becomes the smaller one.

Wilson has suggested that both claws in the young crusta-
cean have the power to become either sort. We do not know
what decides the matter in the adult, after the removal of one
of the claws. Some slight difference may turn the balance
one way or the other, so that the smaller claw grows into the
larger one. At any rate, there is seen a latent power like

348 Evolution and Adaptation

that in the egg of the snail. Zeleny has found a similar rela-
tion to exist for the big and the little opercula of the marine
worm, Hydroides.

Let us consider now the more general questions involved
in these symmetrical and asymmetrical relations between the
organism and its environment. In what sense, it may be
asked, is the symmetry of a form an adaptation to its environ-
ment? That the kind of symmetry gives to the animal in many
cases a certain advantage in relation to its environment is so
evident that I think it will not be questioned. The main
question is how this relation is supposed to have been attained.
Three points of view suggest themselves: First, that the form
has resulted directly from the action of the environment upon
the organism. This is the Lamarckian point of view, which
we rejected as improbable. Second, that the form has been
slowly acquired by selecting those individual variations that
best suited it to a given set of surrounding conditions. This
is the Darwinian view, which we also reject. The third,
that the origin of the form has had nothing to do with the
environment, but appeared independently of it. Having,
however, appeared, it has been able to perpetuate itself under
certain conditions. ,

It should be pointed out that the Darwinian view does not
suppose that the environment actually produces any of the new
variations which it selects after they have appeared, but in so
_ far as the environment selects individual differences it is sup-
posed to determine the direction in which evolution takes
place. On the theory that evolution has taken place indepen-
dently of selection, this latter is not supposed to be the case;
the finished products, so to speak, are offered to the environ-
ment ; and if they pass muster, even ever so badly, they may
continue to propagate themselves.

The asymmetrical form of certain animals living in a sym-
metrical environment might be used as an argument to
show that the relation of symmetry between an animal and

Origin of Different Kinds of Adaptations 349

its environment can easily be overstepped without danger.
The enormous claw of the fiddler-crab must throw the
animal out of all symmetrical relation with its environment,
and yet the species flourishes. The snail carries around a
spiral hump that is entirely out of symmetrical relation with
the surroundings of a snail.

These facts, few though they are, yet suffice to show, I
believe, that the relation of symmetry between the organism
and its environment may be, and is no doubt in many cases,
more perfect than the requirements of the situation demand.
The fact that animals made unsymmetrical through injuries
(as when a crab loses several legs on one side, or a worm its
head) can still remain in existence in their natural environ-
ment, is in favor of the view that I have just stated. By
this I do not mean to maintain that a symmetrical form does
not have, on the whole, an advantage over the same form
rendered asymmetrical, but that this relation need not have
in all forms a selective value, and if not, then. it cannot be
the outcome of a process of natural selection.

To sum up: it appears probable that the laws determining
the symmetry of a form are the outcome of internal factors,
and are not the result either of the direct action of the. en-
vironment, or of a selective process. The finished products
and not the different imperfect stages in such a process, are
what the inner organization offers to the environment.
While the symmetry or asymmetry may be one of the numer-
ous conditions which determine whether a form can _ per-
sist or not, yet we find that the symmetrical relations may
be in some cases more perfect than the environment actually
demands ; and in other cases, although the form may place
the organism at a certain disadvantage, it may still be able
to exist in certain localities.

350 Evolution and Adaptation

MutruaL ADAPTATION OF COLONIAL FORMS

In the white ants, true ants, and bees, we find certain in-
dividuals of the community specialized in such a way that
their modifications stand in certain useful relations to other
members of the community. Amongst the bees, the workers
collect the food, make the comb, and look after the young.
The queen does little more than lay eggs, and the drone’s
only function is to fertilize the queen. In the true ants there
are, besides the workers and the queen and the males, the
soldier caste. These have large thick heads and large strong
jaws. On the Darwinian theory it is assumed that this caste
must have an important rédle to play, for otherwise their pres-
ence as a distinct group of forms cannot be accounted for ;
but I do not believe it is necessary to find an excuse for
their existence in their supposed utility. From the point of
view of the mutation theory, their real value may be very
small, but so long as their actual presence is not entirely
fatal to the community they may be endured.

In regard to these forms, Sharp writes:! “The soldiers
are not alike in any two species of Termitidz, so far as we
know, and it seems impossible to ascribe the differences that
exist between the soldiers of different species of Termitidze
to special adaptations for the work they have to perform.”
“On the whole, it would be more correct to say that the
soldiers are very dissimilar in spite of their having to perform
similar work, than to state that they are dissimilar in con-
formity with the different tasks they carry on.” The sol-
diers have the same instincts as the workers, and do the
same kinds of things to a certain extent. ‘The soldiers are
not such effective combatants as the workers are.” State-
ments such as these indicate very strongly that the origin of
this caste can have very little to do with its importance as a
specialized part of the community.

1 “The Cambridge Natural History,” Vol. V, 1895.

Origin of Different Kinds of Adaptations 351

The differences between the castes have gone so far in
some of these groups that the majority of the members of the
community have even lost the power to reproduce their kind,
and this function has devolved upon the queen, whose sole
duty is to reproduce the different castes of which the com-
munity is composed. This specialization carries with it the
idea of the individuals being adapted to each other, so that,
taken all together, they form a whole, capable of maintaining
and reproducing itself. It does not seem that we must nec-
essarily look upon this union as the result of competition
leading to a death struggle between different colonies, so
that only those have survived in each generation that carried
the work of specialization one step farther. All that is re-
quired is to suppose that such specialization has appeared in
a group of forms living together, and the group has been able
to perpetuate itself. We do not find that all other members
of the two great groups to which the white ants and true ants
belong have been crowded out because these colonial forms
have been evolved. Neither need we suppose that during
the evolution of these colonial species there has been a death
struggle accompanying each stage in the evolution. If the
members of a colonial group began to give rise to different
forms through mutations, and if it happened that some of the
combinations formed in this way were capable of living to-
gether, and perpetuating the group, this is all that is required
for such a condition to persist.

The relation of the parents to the offspring presents in
some groups a somewhat parallel case to that of these colonial
forms. Not only are some of the fundamental instincts of
the parents changed, but structures may be present in the
parents whose only use is in connection with the young.
The marsupial pouch of the kangaroo, in which the immature
young are carried and suckled, is a case in point, and the
mammary glands of the Mammalia furnish another illustration.

Adaptations of these kinds are clearly connected with the

352 Evolution and Adaptation

perpetuation of the race. In the case of the mammals the
young are born so immature that they are dependent on
the parental organs, just spoken of, for their existence.,
Could we follow this relation through its evolutionary stages,
it would no doubt furnish us with important data, but un-
fortunately we can do no more than guess how this relation
became established. The changes in the young and in the
parent may have been intimately connected at each stage, or
more or less independent. If we suppose the mammary
glands to have appeared first, they might have been utilized
by the young in order to procure food. Their presence
would then make it possible for the young to be born in an
immature condition, as is the case with the young of many
of the mammals. But this is pure guessing, and until we
know more of the actual process of evolution in this case, it
is unprofitable to speculate.

DEGENERATION

In almost every group of the animal kingdom there are
forms that are recognized as degenerate. This degeneration
is usually associated with the habitat of the animal. In many
cases it can be shown with much probability that these degen-
erate forms have descended from members of the group that
are not degenerate. We find there is a loss of those organs
that are not useful to the organism in its new environment.
The degeneration may involve nearly the whole organization
(except as a rule the reproductive system), as seen in the
tapeworm, or only certain organs of the body, as the eyes in
cave animals. A few examples will bring the main facts
before us.

A parasitic existence is nearly always associated with de-
generation. Under these conditions, food can generally be
obtained without difficulty, at the expense of the host, and
apparently associated with this there is a degeneration, and

Origin of Different Kinds of Adaptations 353

even a complete loss of so important an organ as the diges-
tive tract. Thus the tapeworm has lost all traces of its
digestive tract, absorbing the already digested matter of its
host through its body wall. Some of the roundworms, that
live in the alimentary tracts of other animals, may have their
digestive organs reduced. In Trichina, this degeneration has
gone so far that the digestive tract is represented, in part,
by a single line of endoderm cells, pierced by a cavity. The
digestive organs are also absent in certain male rotifers,
which are parasitic on the females, and these organs are
also very degenerate in the male of Bonellia, a gephyrean
worm. A parasitic snail, Extoscolax ludwigit, has its diges-
tive apparatus reduced to a sucking tube ending in a blind
sac. The rest of the tract has completely degenerated. The
remarkable parasitic crustacean, Sacculina carcini, looks like
a tumor attached to the under surface of the abdomen of a
crab. It has neither mouth nor digestive tract, and absorbs
nourishment from the crab through rootlike outgrowths that
penetrate the body. From its development alone we know
that it is a degenerate barnacle.

“There seems to be in all these cases an apparent connec-
tion between the absence of the digestive tract and the
presence of an abundant supply of food, that has already
been partly digested:by the host. Put ina different way, we
may say that the presence of this food has furnished the
environment in which an animal may live that has a rudi-
mentary digestive tract.

An interesting case of degeneration is found in the rudi-
mentary mouth parts of the insects known as May-flies, or
ephemerids. Some of these species live in the adult con-
dition for only a few hours, only long enough to unite and
deposit their eggs. In the adult stage the insects do not
take any food. In this case the degeneration is obviously
not connected with the presence of food, but apparently
with the shortness of the adult life.

2A

354 Evolution and Adaptation

One of the most familiar cases of degeneration is blind-
ness, associated with life in the dark. The most striking
cases are those of cave animals, but this is only an extreme
example of what is found everywhere amongst animals that
live concealed during the day under stones, etc. The blind
fish and the blind crayfish of the Mammoth Cave, the
blind proteus of the caves of Carniola, the blind mole that
burrows underground, the blind larvae of many insects that
live in the dark, are examples most often cited. Some noc-
turnal animals, like the earthworm, have no eyes, although
they are still able to distinguish light ; and some of the deep-
sea animals, that live below the depth to which light pene-
trates, have degenerate eyes. The workers of some ants,
that remain in the nests, are blind, but the males and the
queens of these forms have well-developed eyes, although the
eyes may be of use to them at only one short period of their
life, namely, at the time of the marriage flight. This fact
is significant and is underestimated by those who believe
that disuse accounts for the degeneration of organs.

The wings of the ostrich and of the kiwi are rudimentary
structures no longer used for flight, and many insects, be-
longing to several different orders, have lost their wings, as
seen in fleas, some kinds of bugs, and moths, and even in
some grasshoppers.

A curious case of degeneration is found in the abdomen of
the hermit crab, which is protected by the appropriated shell
of a snail. The appendages of one side of the abdomen
have nearly disappeared in the male, although in the female
the abdominal appendages are used to carry the eggs as in
other decapod crustaceans. The abdomen, instead of being
covered by a hard cuticle, as in other members of this group,
is soft and unprotected except by the shell of the snail.

Cases of these kinds could be added to almost indefinitely,
and the explanation of these degenerate structures has been
a source of contention amongst zoologists for a long time.

Origin of Different Kinds of Adaptations 355

The most obvious interpretation is that the degeneration has
been the result of disuse. But as I have already discussed
this question, and given my reasons for regarding it as im-
probable that degeneration has arisen in this way, we need
not further consider this point here.

The selectionists have offered several suggestions to
account for degeneration. In fact, this has been one of the
difficulties that has given them most concern. They have
suggested, for example, that when an organ is no longer of
use to its possessor it would become a source of danger,
and hence would be removed through natural selection.
They have also suggested that since such organs draw on the
general food supply they would place their possessor at a
disadvantage, and hence would be removed. Weismann has
attempted to meet the difficulty by his theory of “ Panmixia,”
or universal crossing, by which means the useless structures
are imagined to be eliminated.

These attempts will suffice to point out the straits to
which the Darwinians have found themselves reduced, and
we have by no means exhausted the list of suggestions that
have been made. Let us see, if, on any other view, we can
avoid some of the difficulties that ‘the selection theory has
encountered.

In the first place we shall be justified, I think, in eliminat-
ing competition as a factor in the process, since the admis-
sion that an organ has become useless carries with it the
idea that it has no longer a selective value. If, in its useless
condition, it is no longer greatly injurious, as is probably,
though not necessarily always, the case, then selection can-
not enter into the problem. If in parasitism we assume that
an animal finds a lodgement in another animal, where it is
able to exist, we may have the first stage of the process
introduced at once. If under these conditions a mutation
appeared, involving some of the organs that are no longer
essential to the life of the individual in its new environment,

356 Evolution and Adaptation

the new mutation may persist. We need not suppose that
the original form becomes crowded out, but only that a more
degenerate form has come into existence. As a matter of
fact we find in most groups, in which degenerate forms exist,
a number of different stages in the degeneration in different
species. Mutation after mutation might follow until many of
the original organs have disappeared. The connection that
appears to exist between the degeneration of a special part
and the environment in which the animal lives finds its
explanation simply in the fact that the environment makes
possible the existence of that sort of mutation in it. We do
not know, as yet, whether through mutative changes an
organ can completely disappear, although this seems probable
from the fact that in a few cases mutations are known to
have arisen in which a given part is entirely functionless.
If we could assume that, a mutation in the direction of
degeneration being once established, further mutations in
the same direction would probably occur, the problem
would be much simplified; but we lack data, at present, to
establish this view.

In the case of blind animals it seems probable that the
transition has taken place in such forms as had already
established themselves in places more or less removed from
the light. Such forms as had the habit of hiding away under
stones, or in the ground, living partly in and partly out of
the light, might, if a mutation appeared of such a sort that
amongst other changes the eyes were less developed, still be
capable of leading an existence in the dark, while it might be
impossible for them to exist any longer with weakened vision
in the light. If such a process took place, the habitat of the
new form would be limited, or in other words it would be
confined to the locality to which it finds itself adapted ;
not that it has become adapted to the environment through
competition with the original species, or, in fact, with any
other.

Origin of Different Kinds of Adaptations 357

Thus, from the point of view that is here taken, an animal
does not become degenerate because it becomes parasitic,
but the environment being given, some forms have found
their way there; in fact, we may almost say, have been
forced there, for these degenerate forms can only exist under
such conditions.

In conclusion, this much at least can be claimed for the
mutation theory; that it meets with no serious difficulty in
connection with the phenomena of degeneration. It meets
with no difficulty, because it makes no pretence to, explain
the origin of adaptations, but can account for the occurrence
of degenerate forms, if it is admitted that these appear as
mutations, or as definite variations. Let us, however, not
close our eyes to the fact that there is still much to be
explained in respect to the degeneration of animals and
plants. It is far from my purpose to apply the mutation
theory to all adaptations; in fact, it will not be difficult to
show that there are many adaptations whose existence can
have nothing directly to do with the mutation theory.

PROTECTIVE COLORATION

That many species of animals are protected by their re-
semblance to their environment no one will probably deny.
That we are ignorant in all cases as to how far this protec-
tion is necessary for the maintenance of the species must be
admitted. That some of the resemblances that have been
pointed out have been given fictitious value, I believe very
probable.

Resemblance in color between the organism and its en-
vironment has given to the modern selectionist some of his.
most valuable arguments, but we should be on our guard
against supposing that, because an animal may be protected
by its color, the color has been acquired on this account.
On the supposition that the animal has become adapted by

358 Evolution and Adaptation

degrees, and through selection, we meet with all the objec-
tions that have been urged, in general, against the theory of
natural selection. But if we assume here also that muta-
tions have occurred without relation to the environment, and,
having once appeared, determined in some cases the distribu-
tion of the species, we have at least a simple hypothesis that
appears to explain the facts. If it be claimed that the re-
semblance is, in some cases, too close for us to suppose that
it has arisen independently of the environment, it may be
pointed out that it has not been shown that such a close
resemblance is at all necessary for the continued existence
of the species, and hence the argument is likely to prove
too much. For instance, the most remarkable case of re-
semblance is that of Kallima, but in the light of a recent
statement by Dean it may be seriously asked whether there
is absolute need of such a close resemblance to a leaf. Even
if it be admitted that to a certain extent the butterfly is at
times protected by its resemblance to a leaf, it is not improb-
able that it could exist almost equally well without such a
close resemblance. If this is true, natural selection could
never have brought about such a close imitation of a leaf.
Cases like these of over-adaptation are not unaccountable on
the theory of mutation, for on this view the adaptation may
be far ahead of what the actual requirements for protection
demand. We meet occasionally, I think, throughout the liv-
ing world with resemblances that can have no such inter-
pretation, and a number of the kinds of adaptations to be
described in this chapter show the same relation.

Some of the cases of mimicry appear also to fall under this
head ; although I do not doubt that many so-called cases of
mimicry are purely imaginary, in the sense that the resem-
blance has not been acquired on account of its relation to
the animal imitated. There is no need to question that in
some cases animals may be protected by their resemblance to
other animals, but it does not follow, despite the vigorous

Origin of Different Kinds of Adaptations 359

assertions of some modern Darwinians, that this imitation
has been the result of selection. Until it can be shown that ‘
the imitating species is dependent on its close imitation for
its existence, the evidence is unconvincing; and even if, in
some cases, this should prove to be the case, it does not
follow that natural selection has brought about the result, or
even that it is the most plausible explanation that we have
to account for the results. The mutation theory gives, in
such cases, an equally good explanation, and at the same time
avoids some of the difficulties that appear fatal to the selec-
tion theory.

What has been said against the theory of mimicry might

be repeated in much stronger terms against the hypothesis
of warning colors.
“It seems to me, in this connection, that the imagination
of the selectionist: has sometimes been allowed to “run
wild”; and while it may be true that in some cases the
colors may serve as a signal to the possible enemies of the
animal, it seems strange that it has been thought necessary
to explain the origin of such colors as the result of natural
selection. Indeed, some of these warning colors appear
unnecessarily conspicuous for the purpose they have to per-
form. In other words, it does not seem plausible that an
animal already protected should need to be so conspicuous.
If we stop for a moment to consider what an enormous
amount of destruction must have occurred, according to
Darwin’s theory, in order to bring this warning coloration
to its supposed state of perfection, we may well hesitate be-
fore committing ourselves to such an extreme view.

That gaudy colors have appeared or been present in ani-
mals that are protected in other ways is not improbable,
when we consider the réle that color plays everywhere in
nature. That the presence of such colors may, to a certain
limited extent, protect its possessor may be admitted without
in any degree supposing that natural selection has directed

360 Evolution and Adaptation

the evolution of such color, or that it has been acquired
through a life and death struggle of the individuals of the
species.

SexuaL DimorPHIsM! AND TRIMORPHISM

It has been found in a few species of animals and plants
that two or more forms of one sex may exist, and here we
find a condition that appears to be far more readily explained
on the mutation theory than on any other. The most impor-
tant cases, perhaps, are those in plants, but there are also
similar cases known amongst animals, and these will be given
first.

There is a North American butterfly, Papzlio turnus, that
appears under at least two forms. In the eastern United
States the male has yellow wings with black stripes. There
are two kinds of females, one of which resembles the male
except that she has also an orange “eye-spot”; the other
female is much blacker, and this variety is found particularly
in the south and west. The species is dimorphic, therefore,
mainly in the latter regions.

The cases of seasonal dimorphism offer somewhat similar
illustrations. The European butterfly, Vanessa levana-prorsa,
has a spring generation (/evana) with a yellow and black pat-
tern on the upper surface of the wings. The summer genera-
tion (prorsa) has black wings “with a broad white transverse
band, and delicate yellow lines running parallel to the margins.”
These two types are sharply separated, and their differences
in color do not appear to be associated with any special pro-
tection that it confers on the bearer. These facts in regard
to Vanessa seem to indicate that differences may arise that
are perfectly well marked and sharply defined, which yet
appear to be without any useful significance.

1 This term is used here in the sense employed by Darwin. The same term is
sometimes used for those cases in which the male departs very greatly from the
female in form.

Origin of Different Kinds of Adaptations 361

We meet with cases in which the same animal has at dif-
ferent times of year different colors, as seen in the summer
and winter plumage of the ptarmigan. There is no direct
evidence to show how this seasonable change has been
brought about; but from the facts in regard to Vanessa we
can see that it might have been at least possible for the
white winter plumage, for instance, to have appeared without
respect to any advantage it conferred on the animal, but after
it had appeared it may have been toa certain degree useful to
its possessor.

Amongst plants there are some very interesting cases of
dimorphism and trimorphism in the structure of the flowers.
Darwin has studied some of these cases with great care, and
has made out some important points in regard to their powers
of cross-fertilization.!1_ The common European cowslip, Prim-
ula veris, var. officinalis, is found under two forms, Figure 5
A and B, which are about equally abundant. In one the style
is long so that the stigma borne on its end comes to the
top of the tube of the corolla. The stamens in this form
stand about halfway up the tube. This is called the long-
styled form. The other kind, known as the short-styled form,
has a style only half as long as the tube of the corolla, and
the stamens are attached around the upper end of the tube
near its opening. In other words, the position of the end of
the style (the stigma) and that of the stamens is exactly
reversed in the two forms. The corolla is also somewhat
differently shaped in the two forms, and the expanded part
of the tube above the stamens is larger in the long-styled
‘than in the short-styled form. Another difference is found
in the stigma, which is globular in the long-styled, and
depressed on its top in the short-styled, form. The papillz

1 Many of the facts as to the occurrence of these cases were known before
Darwin worked on them; but very little had been ascertained in regard to the
sexual relation between the dimorphic and trimorphic forms, and it was here that
Darwin obtained his most interesting results,

362 Evolution and Adaptation

on the former are twice as long as those on the short-styled
form. The most important difference is found in the size of
the pollen grains. These are larger in the long-styled form,
being in the two cases in the proportion of 100 to 67, The
shape of the grains is also different. Furthermore, the long-

Fic. 5.— A, long-styled, and B, short-styled, forms of Primula veris. C, D, E,
the three forms of the trimorphic flower of Lythrum salicaria, with petals
and calyx removed on near side. (After Darwin.)

styled form tends to flower before the other kind, but the
short-styled form produces more seeds. The ovules in the
long-styled form, even when unfertilized, are considerably
larger than those of the short-styled, and this, Darwin sug-
gests, may be connected with the fact that fewer seeds are
produced, since there is less room for them. The important

Origin of Different Kinds of Adaptations 363

point for our present consideration is that intermediate forms
do not exist, although there are fluctuating variations about
the two types. Moreover, the two kinds of flowers never
appear on the same plant.

Darwin tried the effect of fertilizing the long-styled flowers
with the pollen from the same flower or from other long-
styled flowers. Unions of this sort he calls illegitimate, for
reasons that will appear later. He also fertilized the long-
styled flowers with pollen from short-styled forms. A union
of this sort is called legitimate. Conversely, the short-styled
forms were fertilized with their own pollen or with that from
another short-styled form. This is also an illegitimate union.
Short-styled forms fertilized with pollen from long-styled forms
give again legitimate unions.

The outcome of these different crossings are most curious.
In the table, page 364, the results of the four combinations
are given. It will be seen at once that the legitimate unions
give more capsules, and the seeds weigh more, than in the
illegitimate unions.

The behavior of the offspring from seeds of legitimate and
illegitimate origin is even more astonishing. Darwin found
in Primula veris (the form just described) that the seeds from
the short-styled form fertilized with pollen from the same form
germinated so badly that he obtained only 14 plants, of which
9 were short-styled and 5 long-styled. The long-styled form
fertilized with its own-styled pollen produced ‘in the first gen-
eration 3 long-styled plants. From their seed 53 long-styled
grandchildren were produced; from their seed 4 long-styled
great-grandchildren ; from their seed 20 long-styled great-
great-grandchildren ; and lastly, from their seed 8 long-styled
and 2 short-styled great-great-great-grandchildren.”

From other long-styled plants, fertilized with their own-
form pollen, 72 plants were raised, which were made up of
68 long-styled and 4 short-styled. In all, 162 illegitimate
unions of this sort produced 156 long-styled and 6 short-

364 Evolution and Adaptation

NuMBER Numser |Maximum oF/MinimuM oF| AVERAGE
NaTuRE OF UNION or FLowers| oF SEED |SEEDSIN ANy|SEEDS IN ANY|No. OF SEEDS
. FERTILIZED | CapsuLES [ONE CaPSULE|ONE CAPSULE|PER CAPSULE

Long-styled form by
pollen of short-
styled form :
Legitimate union.

10 6 62 34 46.5

Long-styled form by
own-form pollen : 20 4 49 2 27.7
Mlegitimate union.

Short-styled form by
pollen of long- 8 61
styled form : ve
Legitimate union.

37 47-7

Short-styled form by
own-form pollen : 17 3 19 6 12.1
Illegitimate union.

The two legitimate

6 I
unions together. - 14 2 37 47

The two illegitimate

unions together. 3° 7 49 2 35-5

styled plants. It is evident from these results that the long-
form pistils, fertilized with pollen from flowers of the same
pistil-form (from other individuals as a rule), tend to produce
the same form as their parents, although occasionally the
other form. The fertility of these plants from an illegitimate
union is found to be very low. Darwin observed that some-
times the male and female organs of these plants were in a
very deteriorated condition. It is interesting to notice, in
this connection, that in another species, Primula SUMENSTS,
illegitimate plants from long-styled parents were vigorous,
but the flowers were small and more like the wild form.
They were, however, perfectly fertile.

Origin of Different Kinds of Adaptations 365

Illegitimate plants from short-styled parents were dwarfed
in stature, and often had a weakly constitution. They were
not very fertile z#ter se, and remarkably infertile when legiti-
mately fertilized. This kind of result, where a difference in
the power of mutual intercrossing exists between two forms,
recalls in many ways the difference in the results of cross-
ing of different species of animals and plants, especially those
cases in which a cross can be made in one way more success-
fully than in the other.

The heterostyled trimorphic plants, of which Lythrum
salicaria, Figure 5 C, D, E, may be taken as an example, are
even more remarkable. There are three different kinds of
flowers: in one the pistil is long and there is a medium and
a short set of stamens; in another the pistil is of intermedi-
ate length and there is a long set and a short set of sta-
mens ; in the third kind the pistil is short, and there is a
medium and a long set of stamens. There are possible only
six sorts of legitimate unions between these three sets of
flowers. No less than twelve kinds of illegitimate unions
may occur. In regard to the difference in the sizes of the
pollen grains, those from the long-styled form are the largest,
from the mid-styled form next, and from the short-styled
form the smallest. The extreme difference is as 100 to 60.
“Nothing shows more clearly the extraordinary complexity
of the reproductive system of this plant than the necessity of
making eighteen distinct unions in order to ascertain the rela-
tive fertilizing power of the three forms.” Darwin tried the
effect of each of these combinations, making 223 unions in
all. The results are surprising. Comparing the outcome
of the six legitimate unions with the twelve illegitimate ones,
the following results were obtained : —

366 Evolution and Adaptation

NuMBER NuMBER AVERAGE AVERAGE
No. or SEEDS
NATuRE oF UNION oF FLowers | oF Capsuves | No. oF SEEDS eek POWER:
FERTILIZED Propucep | PER CAPSULE Rennnset
The 6 legitimate unions 75 56 96.29 71.89
The 12 illegitimate unions 146 36 44.72 11.03

This table shows that the fertility of the legitimate to that
of the illegitimate is as 100 to 33, as judged by the flowers
that produced capsules ; and as 100 to 46 as judged by the
average number of seeds per capsule. It is evident, there-
fore, that “it is only the pollen from the longest stamens
that can fully fertilize the longest pistil; only that from the
mid-length stamens, the mid-length pistil; and only that
from the shortest stamens, the shortest pistil.”

Darwin tries to connect this fact with the visits of insects
to the flowers. He says: ‘‘And now we can comprehend
the meaning of the almost exact correspondence in length
between the pistil in each form and of a set of six stamens
in two of the other forms; for the stigma of each form is
thus rubbed against that part of the insect’s body which be-
comes charged with the proper pollen.” A further conclusion
that Darwin draws is “that the greater the inequality in
length between the pistil and the set of stamens, the pollen
of which is employed for its fertilization, by so much is the
sterility the more increased.” Darwin also makes the fol-
lowing significant comment on the problem here involved:
“The correspondence in length between the pistil in each
form, and a set of stamens in the other two forms, is prob-
ably the direct result of adaptation, as it is of the highest
service to the species by leading to full and legitimate fer-
tilization.”” He points out, on the other hand, that the in-
creased sterility of the illegitimate unions, in proportion to
the inequality in length between the pistil and the stamens

Origin of Different Kinds of Adaptations 367

employed, can be of no service at all. Neither can this re-
lation have any connection with the facility for self-fertiliza-
tion. ‘We are led, therefore, to conclude that the rule of
increased sterility in accordance with increased inequality
in length between the pistils and stamens is a purposeless
result, incidental on those changes through which the species
has passed in acquiring certain characters fitted to insure
the legitimate fertilization of the three flowers.”

In regard to the plants that were raised from the seeds
from legitimate and illegitimate unions, Darwin found in
Lythrum that of twelve illegitimate unions two were com-
pletely barren, and nearly all showed lessened fertility ; only
one approached complete fertility. Darwin lays much em-
phasis on the close resemblance in the sterility of the illegiti-
mate unions, and the sterility of different species when
crossed. In both cases every degree of sterility is met with,
“from very slightly lessened fertility to absolute barrenness.”
The importance of this comparison cannot, I think, be over-
estimated, for, if admitted, it indicates clearly that the infer-
tility between species cannot be used as a criterion of their
distinctness, because here, in individuals belonging to the
same species, we find sterility between pistils and stamens
of different lengths. If, as I shall urge below, we must con-
sider these different forms of Primula the results of a muta-
tion, and not the outcome of selection as Darwin supposed,
then this relation in regard to infertility becomes a point of
great interest.

This brings us to the central point of our examination
of these cases of dimorphism and trimorphism. How have
these forms arisen? Darwin tries to account for them as
follows: Since heterostyled plants occur in fourteen different
families of plants, it is probable that this condition has been
acquired independently in each family, and “that it can be
acquired without any great difficulty.” The first step in the
process he imagines to have been due to great variability

368 Evolution and Adaptation

in the length of the pistil and stamens, or of the pistil alone.
Flowers in which there is a great deal of variation of this
sort are known. “As most plants are occasionally cross-
fertilized by the aid of insects, we may assume that this was
the case with our supposed varying plant; but that it would
have been beneficial to it to have been more regularly cross-
fertilized.” “This would have been better accomplished if the
stigma and the stamens stood at the same level; but as
the stamens and pistil are supposed to have varied much
in length, and to be still varying, it might well happen that
they could be reduced much more easily through natural
selection into two sets of different lengths in different indi-
viduals than all to the same length and level in all individuals.”
By means of these assumptions, improbable as they may
appear, Darwin tries to explain these cases of dimorphism.
But when we attempt to apply the same argument to the
trimorphic forms, it is manifestly absurd to pretend that three
such sharply defined types could ever have been formed as
the result of natural selection. But we have not even yet
touched the chief difficulty, as Darwin himself points out.
“The essential character of a heterostyled plant is that an
individual of one form cannot fully fertilize, or be fertilized
by, an individual of the same form, but only by one belonging
to another form.” This result Darwin admits cannot be ex-
plained by the selection theory, for, as he says, ‘“ How can
it be any advantage to a plant to be sterile with half of its
brethren, that is, with the individuals belonging to the same
form?” He concludes that this sterility between the indi-
viduals of the same form is an incidental and purposeless
result. “Inner constitutional differences” between the
individuals is the only suggestion that is offered to account
for the phenomenon. In other words, it is clearly apparent
that the attempt to apply the theory of selection has here
broken down, and it is a fortunate circumstance that the
Lamarckian theory cannot here be brought to the rescue, as

Origin of Different Kinds of Adaptations 369

it so often is in Darwin’s writings, when the theory of natural
selection fails to give a sufficient explanation.

On the other hand, this is one of the cases that seem to
fit in excellently with the mutation theory, for if these two
forms of the primrose should appear, as mutations, and if, as
is the case, they do not blend when crossed, but are equally
inherited, they would both continue to exist as we find them
to-day. Whether the similar forms were infertile with each
other would be determined at the outset by the nature of
the individual variation, and if, despite this obvious disadvan-
tage, the forms could still continue to propagate themselves,
the new dimorphic form would remain in existence. Darwin
cannot explain the origin of dimorphic forms and trimorphic
forms unless he can show that there is some advantage in
having two forms, and as we have seen, he fails completely
to show that there is an advantage. On the other hand, the
result might have been reached on the mutation theory, even
if the dimorphic and trimorphic forms were placed at a
greater disadvantage than were the parent forms. In such
a case fewer individuals might appear, or find a foothold; but
as long as the race could be kept up the new forms would
remain in existence. Thus, while no attempt is made to ex-
plain what has always been, and may possibly long remain,
inexplicable to us, namely, the origin of the new form itself,
yet granting that such new forms may sometimes appear
spontaneously, they may be able to establish themselves, re-
gardless of whether they are a little more or a little less well
adapted to the environment than were their parent forms.
If it should appear that the question is begged by the as-
sumption that mutations such as these may appear (at one
step or by a series of steps is immaterial), it should not be
forgotten that the whole Darwinian theory itself also rests
on the spontaneous appearance of fluctuating variations,
whose origin it does not pretend to explain. In this re-
spect both theories are on the same footing, but where the

28

370 Evolution and Adaptation

Darwinian theory meets with difficulties at every turn by
assuming that new forms are built up through the action
of selection, the mutation theory escapes most of these diffi-
culties, because it applies no such rigid test as that of selec-
tion to account for the presence of new forms.

LENGTH OF LIFE AS AN ADAPTATION

It has been pointed out in the first chapter that the length
of life of the individual has been supposed by some of the
most enthusiastic followers of Darwin to be determined by
the relation of the individual to the species as a whole. In
other words, the doctrine of utility has been applied here also,
on the ground that it would be detrimental to the species to
have part of the individuals live on to a time when they can
no longer propagate the race or protect the young. It is
assumed that those varieties or groups of individuals (unfor-
tunately not sharply defined) would have the best chance
to survive in which the parent forms died as soon as they
had lost the power to produce new individuals. Sometimes
interwoven with this idea there is another, namely, that death
ztself has been acquired because it was more profitable to
supplant the old and the injured individuals by new ones,
than to have the old forms survive, and thus deprive the
reproducing individuals of some of the common food supply.

This insidious form that the selection theory has taken in
the hands of its would-be advocates only serves to show to
what extremes its disciples are willing to push it. On the
whole it would be folly to pursue such a will-o’-the-wisp, when
the theory can be examined in much more tangible examples.
If in these cases it can be shown to be improbable, the re-
maining superstructure of quasi-mystical hypothesis will fall
without more ado.

That the problem of the length of life may be a real one
for physiological investigation will be granted, no doubt, with-

Origin of Different Kinds of Adaptations 371

out discussion, and that in some cases the length of life and
the coming to maturity of the germ-cell may be, in some way,
physiologically connected seems not improbable ; but that this
relation has been regulated by the competition of species
with each other can scarcely be seriously maintained. I will
not pretend to say whether the mutation theory can or can-
not be made to appear to give the semblance of an explana-
tion of the length of life in each species, but it seems to me
fairly certain that this is one of the questions which we are
not yet in a position to attempt to consider on any theory of
evolution.

OrGANS OF EXTREME PERFECTION

It has often been pointed out that certain organs may be
more perfectly developed than the requirements of the sur-
roundings strictly demand. At least we have no good reasons
to suppose in some cases that constant selection is keeping
certain organs at the highest possible point of development,
yet, on the Darwinian theory, as soon as selection ceases
to be operative the level of perfection must sink to that
which the exigencies of the situation demand. The prob-
lem may be expressed in a different way. Does the animal
or plant ever possess organs that are more perfectly adapted
than the absolute requirements demand? If such organs
are the result of fluctuating variations, they will be unable
to maintain themselves in subsequent generations without
a constant process of selection going on. If, on the other
hand, the organs have arisen as mutations, they may be-
come permanently established without respect to the degree
of perfection of their adaptation. We can see, therefore,
that cases of extreme perfection meet with no difficulty on
the mutation theory, while they have proven one of the
stumbling-blocks to the selection theory.

There are, in fact, many structures in the animal and plant
kingdoms that appear to be more perfect than the require-

57% Evolution and Adaptation

ments seem to demand. The exact symmetry of many forms
appears in some cases to be unnecessarily perfect. The per-
fection of the hand of man, the development of his vocal
organs, and certain qualities of his brain, as his musical and
mathematical powers, seem to go beyond the required limits.
It is not, of course, that these things may not be of some use,
but that their development appears to have gone beyond what
selection requires of these parts.

Closely related to this group of phenomena are those cases
in which certain organs are well developed, but which can
scarcely be of use to the animal in proportion to their elabo-
ration. The electric organs of several fishes and skates are
excellent examples of this sort of structures. The phospho-
rescent organs do not appear, in some forms at least, to be
useful in proportion to their development. The selection
theory fails completely to explain the building up of organs
of this kind, but on the mutation theory there is no difficulty
at all in accounting for the presence of even highly developed
organs that are of little or of no use to the individual. If the
organs appeared in the first place as mutations, and their
presence was not injurious to the extent of interfering seri-
ously with the existence and propagation of the new form,
this new form may remain in existence, and if the mutations
continued in the same direction, the organs might become
more perfect, and highly developed. The whole class of sec-
ondary sexual organs may belong to this category, but a discus-
sion of these organs will be deferred to the following section.

SECONDARY SEXUAL ORGANS AS ADAPTATIONS

In the sixth chapter we have examined at some length Dar-
win’s interpretation of the secondary sexual characters.
His explanation has been found insufficient in many cases to
account for the conditions. That these organs do play in
some cases a réle in the relation of the sexes to each other
may be freely admitted. In other words, in some animals the

Origin of Different Kinds of Adaptations 373

organs in one sex appear in the light of adaptations to certain
instincts in the other sex. It would, perhaps, appear to
simplify the problem to deny outright that any such relation
exists ; but I think, in the light of the evidence that we have,
this procedure would be like that of the proverbial ostrich,
which is supposed to stick its head in the sand in order to
escape an anticipated danger. If we assumed this agnostic
position, we might attempt to account for the appearance of
secondary sexual organs as mutations that had appeared in
one sex, and had no immediate connection with the other
sex; and, so long as these organs were not directly and
seriously injurious, we might assume that the animals in
which such structures had appeared might be able to exist.
But, on the other hand, I think that an examination of the
evidence will show that this way out of the difficulty is not-
very satisfactory, for the organs in question appear, in some
cases at least, to be closely connected with certain definite
responses in the other sex. Moreover, as Darwin has so
insistently pointed out, the action of the males is of such a
sort that it is evidently associated with the presence of the
secondary sexual organs which they often display before the
other sex. Furthermore, the greater and often exclusive
development of these organs during the sexual period dis-
tinctly points to them as in some way connected with the
relation of the sexes to each other. And finally, there is a
small, although not entirely convincing, body of evidence,
indicating that the female is influenced by the action of
the male; but I do not think that this evidence shows
that she selects one individual at the expense of all
other rivals. We meet here with a problem that is as pro-
foundly interesting as it is obscure. In fact, if we admit
that this relation exists we have a double set of conditions to
deal with: first, the development in the males of certain
secondary sexual organs; and secondly, the instinct to dis-
play these organs. The supposed influence of the display on

374 Evolution and Adaptation

the female may also have to be taken into account, although,
for all we know to the contrary, the same results might
follow were there no secondary sexual character at all, as
is, in fact, the case in most animals.

I have a strong suspicion that much that has been written
on this subject is imaginative, and in large part fictitious; so
that it may, after all, be the wisest course not to attempt to
explain how this relation has arisen until we have a more
definite conception of what we are really called upon to
explain. For example, when we see a gorgeously bedecked
male displaying himself before a female, we feel that his finery
must have been acquired for this very purpose. On the
other hand, when we see an unornamented male also making
definite movements before the female, we do not feel called
upon to explain the origin of his colors. Now, it is not im-
probable that the ornaments of the first individual have not
been acquired in order to display them before the female,
and this view seems to me the more probable. From this
standpoint our problem is at least much simplified. What
we need to account for is only that the male is excited to
undergo certain movements in the presence of the female,
and possibly that the female may be influenced by the re-
sult. That this view is the more profitable is indicated by
the occurrence of secondary sexual characters in the lower
forms, as in the insects and crustaceans, in which it appears
almost inconceivable that the ornamentation could have been
acquired in connection with the zsthetic taste of the other
sex. It does not seem to me that the conditions in the
higher animals call for any other explanation than that which
applies to these lower forms.

My position may be summed up in the statement, that,
while in some cases there appears to be a connection between
the presence in one sex of secondary sexual organs and their
effect on the other sex, yet their origin cannot be explained
on account of this connection.

Origin of Different Kinds of Adaptations 375

INDIVIDUAL ADJUSTMENTS AS ADAPTATIONS

As pointed out in the first chapter, there is a group of adap-
tations, obviously including several quite different kinds of
phenomena, that can at least be conveniently brought to-
gether under the general rubric of individual adjustments or
regulations. A few examples of these will serve to show in
what sense they may be looked upon as adaptations, and how
they may be regarded from the evolutionary point of view.

CoLor CHANGES AS INDIVIDUAL ADAPTATIONS

The change in color of certain fish in response to the color
of the background, the change in color of some chrysalides
also in response to their surroundings, appears to be of some
use to the animals in protecting them from their enemies.
The change in color from green to brown and from brown
back to green in several lizards and in some tree frogs is
popularly supposed to be in response to the color of the sur-
roundings, but a more searching examination has shown that,
in some cases at least, the response has nothing to do with
the color of the background.

In the first cases mentioned above, in which the response
appears to be of some advantage to the animal, the question
may be asked, how have such responses arisen? The selec-
tion theory assumes that those animals that responded at
first to a slight degree in a favorable direction have escaped,
and this process being repeated, the power to change has been
gradually built up. The mutation theory will also account
for the result by assuming the response to have appeared
as a new quality, but it has been preserved, not because it
has been of vital importance to its possessor, but simply
because the species possessing it has been able to survive,
perhaps in some cases even more easily, although this is not
essential. Even if the change were of no direct benefit,

376 Evolution and Adaptation

or even injurious to a slight degree, it might have been
retained, as appears in fact to be the case in the change of
color of the green lizards.

INCREASE OF ORGANS THROUGH USE AND DECREASE
THROUGH DISUSE

We meet here with one of the most characteristic and
unique features of living things as contrasted with non-living
things. We shall have to dismiss at once the idea that we
can explain this attribute of organisms by either the selec-
tion or the mutation theory; for we find animals possessing
this power that could never be supposed to have acquired it
by any experience to which they have been subjected; and
since it appears to be so universally present, we cannot
account for it as a chance mutation that may have appeared
in each species. No doubt Wolff had responses of this kind
in view when he made the rather sweeping statement that
purposeful adaptation is the most characteristic feature of
living things. The statement appears to contain a large
amount of truth, if confined to the present group of phe-
nomena.

This power of self-regulation may confer a great benefit
on its possessor. The increase in the size and strength of
the muscles through use may give the animal just those
qualities that make its existence easier. The increase in the
power of vision, or at least of visual discrimination through
use, of the power of smell and of taste, of hearing and of
touch, are familiar examples of this phenomenon.

However much we may be tempted to speculate as to how
this property of the animal may have been acquired, we lack
the evidence which would justify us in formulating even a
working hypothesis. It may be that when we come to know
more of what the process of contraction of the muscle in-
volves, the possibility of its development as a consequence

Origin of Different Kinds of Adaptations 377

of its use may be found to be a very simple phenomenon that
requires no special explanation at all to account for its exist-
ence in the individual, further than that the muscles are of
such a kind that this is a necessary physical result of their
action. But until we know more of the physiology in-
volved in the process, it is idle to speculate about the ori-
gin of the phenomenon.

REACTIONS OF THE ORGANISM TO POISONS, ETC.

In this case also we meet with a number of responses for
whose origin we can give not the shadow of an explanation.
On the other hand, the cases are significant in so far as a
number of them show quite clearly that the response cannot
have been acquired through the experience of the organism,
or the selection of those individuals that have best resisted
the particular poison. This is true, because in a number of
cases the poison is a substance that the animal cannot possi-
bly have met with during the ordinary course of its life, or
of that of its ancestors. It may be argued, it is true, that
in the case of the poisons produced by certain bacteria the
power of resistance has been acquired through the survival
of the less susceptible, or more resistant, individuals. Im-
probable as this may be in some cases, it does not, even if
it were true, alter the real issue, for it can be shown, as has
just been said, that the same power of responding adaptively
is sometimes shown in cases of poisons that are new to the
animal.

There is no question that different individuals respond in
very different degrees to these poisonous substances, and it
is easy to imagine in the case of contagious diseases that
a sort of selective process might go on that would bring
the race up to the highest point to which fluctuating varia-
tions could be carried, even to complete immunity ; but even
if this were the case, it seems to be true that the moment

378 Evolution and Adaptation

the selection stopped the race would sink back to the former
condition.

All this touches only indirectly the main point that we

have under consideration, namely, the existence of this
power of resistance in cases where it cannot have been the
result of any educative process. Since the responses to
new poisons do not appear to be in principle different from
the responses to those to which the organism may have pos-
sibly been subjected at times in the past, we shall probably
not go far wrong if we treat all cases on the same general
footing. Whether the power of adaptation to certain sub-
stances, such as nicotine, morphine, cocaine, arsenic, alcohol,
etc.,is brought about by the formation of a counter-substance
is as yet unproven. And while it seems not improbable
that in some of these instances it may turn out that this is
the case, especially for poisons of plant origin, it is better
to suspend judgment on this point until each case has been
established.
_ In recent years it has been shown that the animal body
has the power of making counter-substances when a very
large number of different kinds of things are introduced
into the blood. We seem to be here on the threshold of a
field for discovery which may, if opened up, give us an in-
sight into some of the most remarkable phenomena of adap-
tation shown by living things.

It has already been pointed out that it appears to be almost
a reductio ad absurdum to speak of animals adapting them-
selves to poisonous substances. It is curious, too, that in
man at least the use of these substances may arouse a craving
for the poison, or at any rate the individual may become so
dependent on the poison that the depression following its dis-
use may lead to a desire for a repetition of the dose. The
two questions that are raised here must be kept apart, for
the adaptation of the individual to the poison and the so-
called craving for it may depend on quite different factors.

Origin of Different Kinds of Adaptations 379

Nevertheless, it seems to be true in the case of morphine and
of arsenic, and probably for some other substances as well,
that if their use is suddenly stopped the individual may die
in consequence. In this respect the organism behaves ex-
actly as it does to an environment to which it has become
adapted.

REGENERATION

Many animals are able to replace lost parts, and all of
them can heal wounds and mend injuries. This power is
obviously of great advantage to them, and it has been sup-
posed by Darwin, and more especially by his followers, that
the power has been acquired through natural selection. It
is not difficult to show that regeneration could not, in many
cases, and presumably in none, have been acquired in this
way. Since I have treated this subject at some length
recently in my book on “Regeneration,” I shall attempt to
do no more here than indicate the outline of the argument.

The Darwinians believe that, if some individuals of a
species have the power to replace a part that is lost better
than have other individuals, it would follow that those would
survive that regenerate best, and in this way after a time the
power to regenerate perfectly would be acquired.

But the matter is by no means so simple as may appear
from this statement. In the first place, it is a matter of
common observation that all the individuals of a species are
never injured in the same part of the body at the same time.
In those cases in which it is known that a special part is
often injured, an examination has shown that there are not
more than ten per cent of individuals that are injured at any
one time, and in the case of the vast majority of animals
this estimate is much too great. Thus there will be very
little chance for competition of the injured individuals in
each generation with each other, and the effects that are
imagined to be gained as a result would be entirely lost

380 Evolution and Adaptation

by crossing with the uninjured individuals. But it is not
necessary to consider this possibility, since there is another
fact that shows at once that the power to regenerate could
not have been gained through selection. The number of un-
injured individuals in each generation will be much greater
than the injured ones, and these will have so great an ad-
vantage over the injured individuals that, if competition
approached the degree assumed by the selectionists, the in-
jured individuals should be exterminated. A slight ad-
vantage gained through better powers of regeneration would
be of little avail in competition, as compared with the com-
petition with the uninjured individuals. Since selection is
powerless to accomplish its end without competition, and
since with competition all the injured individuals would be
eliminated, it is clear that an appeal cannot be made to
selection to explain the power of regeneration.

In many cases the power of regeneration could not have
been slowly acquired through selection, since the interme-
diate steps would be of no use. Unless, for example, a
limb regenerated from the beginning almost completely, the
result would be of no use to the animal. If the limb did
regenerate completely the first time it was injured, then the
selection hypothesis becomes superfluous.

There are also a few cases known in which a process of re-
generation takes place that is of no use to the animal. If, for
instance, the earthworm (AJlolobophora fetida) be cut in two
in the middle, the posterior piece regenerates at its anterior
cut end, not a head, but a tail. Not by the widest stretch
of the imagination can such a result be accounted for on the
selection theory. Again, we find the reverse case, as it were,
in certain planarians. If the head of Planaria lugubris is
cut off just behind the eyes, there develops at the cut
surface of this head-piece another head turned in the opposite
direction. Here again we have the regeneration of a per-
fect structure, but one that is entirely useless to the in-

«

Origin of Different Kinds of Adaptations 381

dividual. The development of an antenna in place of an eye
in the shrimp, when the eye stalk is cut off near its base, is
another instance of the occurrence of a perfectly constant
process, but one that is of no use to the organism.

When we recall that in some organisms regeneration takes
place in almost every part of the body, it does not seem
possible that this power could have been acquired by selec-
tion. And when we find that many internal organs regener-
ate, that can rarely or never be injured without the animal
perishing, it seems impossible that this can be ascribed to
the principle of natural selection.

It has also been found that if the first two cells of the
egg of a number of animals, jellyfish, sea-urchins, salaman-
ders, etc., be separated, each will produce an entire animal.
In some of these cases it is inconceivable that the process
could ever have been acquired through selection, because the
cells themselves can be separated only by very special and
artificial means.

These, and other reasons, indicate with certainty that re-
generation cannot be explained by the theory of natural
selection.

CHAPTER XI
TROPISMS AND INSTINCTS'AS ADAPTATIONS

Or the different kinds of adaptation none are more re-
markable than those connected with the immediate responses
of organisms to external agents. These responses are usually
thought of as associated with the nervous system; and while
in the higher forms the nervous system plays an important
réle in the reaction, yet in many cases it is little more than
the shortest path between the point stimulated and the muscles
that contract ; and in the lower animals, where we find just as
definite responses, there may be no distinct nervous system,
as in the protozoa, for instance.

Many of the so-called instincts of animals have been shown
in recent years to be little more than direct responses to ex-
ternal agents. Many of these instincts are for the good of
the individual, and must be looked upon as adaptations. For
example: if a frog is placed in a jar of water, and the tem-
perature of the water lowered, the frog will remain at the
top until the water reaches 8 degrees C., when it will dive
down to the bottom of the jar; and, if the temperature is
further lowered, it will remain there until the water becomes
warmer again, when it will come to the surface again. It is
clear that, under the ordinary conditions of life of the frog,
this reaction is useful to it, since it leads the animal to go to
the bottom of the pond on the approach of cold weather,
and thus to avoid being frozen at the surface.

Another illustration of an instinct that is a simple response
to light is shown by the earthworm. During the day the
worm remains in its burrow, but on dark nights it comes out

382

vg vopisms and Instincts as Adaptations 383

of its hole, and lies stretched out on the surface of the ground.
It procures its food at this time, and the union of the individ-
uals takes place. In the early morning the worm retires into
its burrow.

This habit of the earthworm is the direct result of its re-
action to light. It crawls away from ordinary light as bright
as that of diffuse daylight, and, indeed, from light very
much fainter than that of daylight. If, however, the light
be decreased to a certain point, the worm will then turn and,
crawl toward the source of light. This lower limit has been
found by Adams to be about that of .oo1 candle-metre. This
corresponds to the amount of light of a dark night, and gives
an explanation of why the worm leaves its burrow only at
night, and also why it crawls back on the approach of dawn.
It is also obvious that this response is useful to the animal,
for if it left the burrow during the day, it would quickly fall
a prey to birds.

The blow-fly lays its eggs on decaying meat, on which the
larvee feed. The fly is drawn to the meat by its sense of
smell, a simple and direct response to a chemical compound
given off by the meat. The maggot that lives in the decay-
ing meat is also attracted by the same odor, as Loeb has
shown, and will not leave the meat, or even a spot on a
piece of glass that has been smeared with the juice of the
meat, so long as the odor remains. Here again the life of
the race depends on the proper response to an external
agent, and the case is all the more interesting, since the
response of the fly to the meat is of no immediate use to
the fly itself, but to the maggot that hatches from the egg
of the fly.

The movement toward or from a stimulating agent is, in
some cases, brought about in the following way. Suppose
an earthworm is lying in complete darkness, and light be
thrown upon it from one side. The worm turns its head,
as it thrusts it forward, to the side away from the light; and

384 Evolution and Adaptation

as it again moves forward, it continues to bend its head
away from the light, until it is crawling directly away from
the source. When the light first strikes the worm, the two
sides will be differently illuminated. This causes a bending
of the head, as it stretches forward, toward the side of less
illumination, and the bending is due to a stronger contrac-
tion of some of the muscles on the less illuminated side;
at least the reaction appears to be due to a simple response
of this kind. When the body has been so far turned that
the two sides are equally illuminated, the muscles of the two
sides will contract equally, and the movement will be straight
forward and away from the light. If the reaction is as simple
as this (which is in principle the explanation advanced by
Loeb), the result is a simple reflex act, and need not involve
any consciousness or intentional action on the part of the
worm to crawl away from the light. In fact, the same reac-
tion takes place when the brain is removed, not so quickly or
definitely, it is true, but this may be due to the removal of
the anterior segments of the worm, in which part the skin
appears to be more sensitive to light than elsewhere.

Another factor that plays an important réle in the habits
of the earthworm is the response to contact, — the so-called
stereotropism. If, in crawling over a flat surface, the worm
comes in contact with a crevice, it will crawl along it, and
refuse to leave until the end is reached. The contact holds
the worm as strongly as though it were actually pulled into
the crevice. It can be forced to leave a crevice only by
strong sunlight, and then it does not do so at once. If the
worm crawls into a small glass tube, it is also held there
by its response to contact, and the smaller the tube, the more
difficult is it to make the worm leave by throwing strong sun-
light upon it.

Loeb has found that when winged aphids, the sexual
forms, are collected in a tube, and the tube is kept in a
room, the aphids crawl toward the light. This happens in

Tropisms and Instincts as Adaptations 385

ordinary diffuse light, as well as in lamplight. It is stated
that the animals orientate themselves towards the light more
quickly when it is strong than when it is weak. They turn
their bodies toward the light, and then move forward in the
direction from which the rays come. It can be shown by a
simple experiment that the aphids are turned by the direction
of the light, and not by its intensity. If they are placed ina
tube, and the tube laid obliquely before a window in such a
way that the direct sunlight falls only on the inner end of the
tube, the aphids will, if started at the inner end of the tube,
first crawl toward the outer surface of the tube, and then wan-
der along this wall, passing out of the region of sunlight into
the end of the tube nearest the window, where they come to
rest at the end. They have moved constantly towards the
direction from which the rays come, passing, as it were, from
ray to ray, but each time toward a ray nearer the source of
the light.

If the tube be turned toward the window, and the window
end be covered with blue glass, the aphids crawl into this
end of the tube, as they would have done had the tube been
uncovered. If, on the other hand, the end of the tube be
covered with red glass, they do not crawl into the part of the
tube that is covered, unless they are very sensitive to light.
Even in the latter case they may remain scattered in the red
part, and do not all accumulate at the end, as they do when
blue glass is used. In other words, while they respond to blue
as they do to ordinary light, they behave toward red as they do
towards a very faint light.

In diffuse daylight the aphids, as has been said, crawl
toward the light, but if they come suddenly into the sun-
light they begin to fly. Thus they remain on the food plant
until the sun strikes it, and then they fly away.

The aphid also shows another response; it is negatively
geotropic, z.¢. it tends to crawl upward against gravity. If
placed on an inclined, or on a vertical, surface, it will crawl

2c

386 Evolution and Adaptation

upward. Such an experiment is best made in the dark,
since in the light the aphid also responds to the light. If
put on a window it crawls upward never downward.

Aphids are also sensitive to heat. If they are placed ina
darkened tube and put near a stove, they crawl away from
the warmer end; but if they are acted upon by the light at
the same time, they will be more strongly attracted by the
light than repulsed by the heat. We thus see that there are
at least three external agents that determine the movements
of this animal, and its ordinary behavior is determined by
a combination of these, or by that one that acts so strongly
as to overpower the others.

The swarming of the male and female ants is also largely
directed by the influence of light. Loeb observed that when
the direct sunlight fell full upon a nest in a wall the sexual
forms emerged, and then flew away. Other nests in the
ground were affected earlier in the day, because the sun
reached them first. These ants, when tested, were found to
respond to light in the same way as do the aphids. The
wingless forms, or worker ants, do not show this response,
and the winged forms soon lose their strong response to light
after they have left the nest. Thus we see that the helio-
tropism is here connected with a certain stage in the develop-
ment of the individual; and this is useful to the species, as it
leads the winged queens and males to leave the nest, and
form new colonies. Even the loss of response that takes
place later may be looked upon as beneficial to the species,
since the queens do not leave the nest after they have once
established it.

It is familiar to every one that many of the night-flying
insects are attracted to a lamplight, and since those that fly
most rapidly may be actually carried into the flame before
they can turn aside, it may seem that such a response is.
worse than useless to them. The result must be considered,
however, in connection with other conditions of their life.

Tropisms and Instincts as Adaptations 387

The following experiments carried out by Loeb on moths
show some of the responses of these insects to light.

Night-flying moths were placed in a box and exposed in a
room to ordinary light. As twilight approached the moths
became active and began to fly always toward the window
side of the box. They were positively heliotropic to light of
this intensity. If let out of the case, they flew toward the
window, where they remained even during the whole of the
next day, fully exposed to light. If the moth is disturbed in
the daytime, so that it flies, it goes always toward the light,
and never away from it. These facts show that the moth is
always positively heliotropic, and also that the flight toward
the lamp is a natural response, misapplied in this case.
That the moths do not fly by day is due to another factor,
namely, the alternation in the degree of their sensitiveness at
different times. But this condition alone does not seem to
account fully for all the facts.

If the moths are given the alternative of flying toward the
evening light, or toward the lamp, they always go toward
the brighter light. Thus if, when they swarm at dusk, they
are set free in the middle of the room, at the back of which
a lamp is burning, the moths fly toward the window. If,
however, they are set free within a metre of the lamp, they
fly toward it.

The explanation that Loeb offers of the habit of these
moths to fly only in the evening is, that, although they are
at all times positively heliotropic, they respond to light only
in the evening. In other words, it is assumed that there is a
periodic change in their sensitiveness to light, which corre-
sponds with the change from day to night. Loeb says that,
just as certain flowers open only at night, and others only
during the day, so do moths become more responsive in
the evening, and butterflies during the day. Both moths and
butterflies are positively heliotropic, and the sensitiveness of
moths to light may be even greater in the evening than is

388 Evolution and Adaptation

that of butterflies, for the light of the evening to which the
moth reacts is less than the minimal to which the butterfly
responds.

Moths appear to pass into a sort of sleep during the day,
while butterflies are quiescent only at night. The periodicity
of the sleeping time continues, at least for several days,
when the insects are kept in the dark. For instance, moths
kept in the dark become restless as the evening approaches,
as Réaumur observed long ago. It has been found in plants
that this sort of periodicity may continue for several days,
but gradually disappears if the plants are kept in the dark.
By using artificial light, and exposing the plants to it during
the night, and putting them in the dark during the day, a new
periodicity, alternating with the former one, may be induced ;
and this will continue for some days if the plants are then
kept continually in the dark.

Loeb tried the experiment of exposing the quiescent moths
suddenly to a lower intensity of light, in order to see if they
would respond equally well at any time of day. It was found
that if the change was made in the forenoon, between six
o'clock and noon, it was not possible to awaken the moths by
a sudden decrease in the intensity of the light. But it was
possible to do so in the afternoon, long before the appearance
of dusk. It appears, therefore, that in this species, Sphinx
euphorbi@, it is possible to influence the period of awaken-
ing by decreasing the intensity of light, but this can be done
only near the natural period of awakening. It seems to me
that this awaking of a positively heliotropic animal by decreas-
ing the light needs to be further investigated.

The day butterflies are also positively heliotropic. Butter-
flies of the species Papilio machaon, that have been raised
from the pupa, remain quietly on the window in the diffuse
daylight of a bright day. They can be carried around on
the finger without leaving it, but the moment they come into
the direct rays of the sun they fly away.

Tropisms and Instincts as Adaptations 389

Butterflies that have just emerged from their pupa case
exhibit a marked negative geotropic reaction, and this ap-
pears to be connected with the necessity of unfolding their
wings at this time. Loeb says that the same cause that
determines the direction of the falling stone and the paths of
the planets, namely, gravity, also directs the actions of the
butterfly that has just left its pupa case. The geotropic
response is especially strong at first. The animal wanders
around until it reaches a vertical wall, which it immediately
ascends, straight upward, and remains hanging at the top
until its wings have unfolded. A similar response occurs in
the final stage of the larva of the May-fly, which leaves the
water and crawls up a blade of grass, or other vertical sup-
port, and there, bursting the pupa skin, it dries its wings and
flies away. That this is a reaction to gravity and not to
light is shown by Loeb’s observation, that their empty skins
are sometimes observed under a bridge where the light does
not come from above. “This observation on the larva of
the May-fly contradicts the assumption that the ‘purpose’ of
the geotropic response of the butterfly is that it may the
better unfold its new wings, for in the ephemerid larva the
negative geotropism appears at a time when no wings are
present.” On the other hand, it should not be overlooked
that the reaction is important for the May-fly larva in other
ways, because it leads the larva to leave the water at the
right period, and come out into the air, where the flying
insect can more safely emerge.

It is not without interest to find that caterpillars ex-
hibit some of the same reaction shown by butterflies. Loeb
has made numerous experiments with the caterpillars of
Porthesia chrysorrhea. The caterpillars of this moth col-
lect together in the autumn and spin a web or nest in
which they pass the winter. If they are taken from the
nest and brought into a warm room, they will orientate
themselves to the light, and also crawl toward it. If

390 Evolution and Adaptation

placed in a tube, they crawl to the upper side of the glass
and then along this side toward the light. If a covering is
placed over the end of the tube that is turned toward the
window, the caterpillars will crawl only as far as the edge
of the cloth. They also react negatively to gravity. If
kept in a dark room, they will crawl upward to the top of
the receptacle in which they are enclosed. If subjected
to the influences of both light and gravity, they respond
more strongly to the light. The caterpillars also show a
contact reaction. They tend to collect on convex sides or
on corners and angles of solid bodies. They may even pile
up one on top of the other in response to this reaction; the
convex side of a quiescent animal acting on another animal
crawling over it as any convex surface would do and holding
the animal fast.

These three kinds of reactions determine the instincts of
these caterpillars. In the spring, when they become warm,
they leave the nest. Positive heliotropism and negative
geotropism compel them to crawl upward to the tops of the
branches of the trees, and there the contact reaction with the
small buds holds them fast in this place. That they are not
attracted to the end of the branches by the food that they
find there is shown by placing buds in the bottom of the
tubes in which the caterpillars are contained. The caterpillars
remain at the top of the tube, although food is within easy
reach. If, however, they are placed directly on the buds, the
contact reaction will hold them there, and they will not crawl
farther upward. Curiously enough, as soon as the cater-
pillars have fed and the time for shedding approaches, the
responsiveness to light and to gravity decreases, and at the
time of shedding they do not respond at all to these agents.
These same caterpillars react also to warmth above a certain
point. In a dark tube placed near a stove, the caterpillars
collect at the end farthest away from the source of the heat.
They react to light best at a temperature between 20 and 30

Tropisms and Instincts as Adaptations 391

degrees C., and above this temperature point they become
restless and wander about.

The very close connection between the reactions of this
caterpillar and its mode of life is perfectly obvious. The
entire series of changes seems to have for its “ purpose” the
survival of the individual by bringing it to the place where
it will find its food. It may seem natural to conclude that
these responses have been acquired for this very purpose,
but let us not too quickly jump at this obvious conclusion
until the whole subject has been more fully examined.

The upward and downward movements of some pelagic
animals have been shown to depend on certain tropic re-
sponses. Every student of marine zoology is familiar with |
the fact that many animals come to the surface at night,
and go down at the approach of daylight. It has been ‘
shown that this migration is due largely to a response to
light. Light can penetrate to only about four hundred
metres in sea-water, and there is complete darkness below
this level. It has been shown that the swimming larvze of
one of the barnacles is positively heliotropic in a weak light,
but negatively heliotropic in a stronger light. Animals
having responses like these will come to the surface as the
light fades away in the evening and remain there until the
light becomes too bright in the following morning. They
will then become negatively heliotropic and begin to go
down. When they reach a level where the intensity of the
light is such that they become positively heliotropic, they
will turn and start upward again. Thus during the day
they will keep below the surface, remaining in the region
where they change from positive to negative, and vzce versa.

It would not be difficult to imagine that this upward and
downward migration of pelagic animals is useful to them, but,
on the other hand, it may be equally well imagined that the
response may be injurious to them. Thus it might be sup-
posed that certain forms could procure their food by coming

392 Evolution and Adaptation

to the surface at night, and avoid their enemies by going
down during the day. But it is difficult to see why organ-
isms that serve as prey should not have acquired exactly the
opposite tropisms in order to escape.

Some of these marine forms are also geotropic. Loeb has
determined that “the same circumstances that make the
animals negatively heliotropic also make them positively
geotropic, and vice versa.” It was found, for instance, that
the larva of the marine worm Polygordius is negatively
geotropic at a low temperature, while at a higher tempera-
ture it is positively geotropic. This response would drive
the animals upward when the water becomes too cold, and
back again if the surface water becomes too warm; but
whether the response is so adjusted that the animals keep, as
far as possible, in water of that temperature that is best for
their development, we do not know. We can easily imagine
that within wide limits this is the case.

The change from positive to negative can also be brought
about in other ways. One of the most striking cases of
this sort is that described by Towle in one of the small
crustaceans, Cypridopsis vidua. It was found that after an
animal had been picked up in a pipette its response was al-
ways positive; that is, it swam toward the light, no matter
what its previous condition had been. The disturbance
caused by picking the animal up induced always a positive
response towards light. If the light were moved, the Cyp-
ridopsis followed the light. In this way it could be kept
positive for some time, but if it came to rest, or if it came into
contact with the sides or end of the trough, it became, after
a short time, negatively heliotropic, and remained negative as
long as it could be kept in motion, without being disturbed, or
coming into contact with a solid object. If when positive
it were allowed to reach the glass at the end of the trough, it
would swim about there, knocking against the glass, and then
soon turn and swim away from the light. If the light were

Tropisms and Instincts as Adaptations 393

shifted while the negative animal was in the middle of the
trough, it would turn and swim directly away, as before, from
the source of light. It could be kept in this negative state ‘as
long as it did not come into contact with the ends.

It appears that the positive condition in Cypridopsis is of
short duration, and ceases after a while either as a response
to contact or without any observable external factor causing
the change.

This crustacean lives at the bottom of pools, amongst water-
plants, and here also, no doubt, the same change from one to
the other reaction takes place. What possible advantage it
may be to the animal to be kept continually changing in this
way is not at all obvious, nor, in fact, are we obliged to
assume that this reaction may be of any special use to it.
Indeed, it is far from obvious how the change that causes the
animal to swim toward the light when it is disturbed could
be of the least advantage to it.

In another crustacean, one of the marine copepods,
Labidocera e@stiva, it has been shown by Parker that the
male and female react in a somewhat different way both to
light and to gravity. The females are strongly negatively
geotropic, and this sends them up to the top of the water.
The males are very slightly negatively geotropic. The
females are strongly positively heliotropic toward light of
low intensity; the males show the same response to a less
degree. To strong light the females are negative and the
males are indifferent. On the other hand, the males are
attracted to the females, probably in response to some
chemical substance diffusing from the females, since the
males show the same reaction when the females are en-
closed in an opaque tube through whose ends a diffusion
of substances may take place. This crustacean frequents
the surface of the ocean from sunset to sunrise. During the
day it retires to deeper water. Its migrations can be ex-
plained as follows: The females come to the surface at

394 Evolution and Adaptation

night, because they are positively heliotropic to weak light,
and also because they are negatively geotropic. They go
down during the day, because they react to bright light
more strongly than to gravity. The males follow the fe-
males, largely because they react positively chemotactically
toward the females.

Some other animals respond in a somewhat different way
to light, as shown by the fresh-water planarians. These
animals remain during the day under stones, where the
amount of light is relatively less than outside. If they are
placed in a dish in the light in front of a window, they crawl
away from the light, but when they reach the back of the
dish they do not come to rest, but continue to crawl around
the sides of the dish even toward the light. The light
makes the worms restless, and while they show a negative
response as long as they are perfectly free to move away
from the light, they will not come to rest when they come
to the back of the dish if they are there still in the light,
" because the irritating action of the light on them is stronger
than its directive action. If, however, in crawling about
they come accidentally into a place less bright than that in
which they have been, they stop, and will not leave this
somewhat darker spot for a brighter one, although they
might leave the newly found spot for one still less bright.

At night the planarians come out and wander around,
which increases their chance of finding food, although it
would not be strictly correct to say that they come out in
search of food. If, however, food is placed near them, a
piece of a worm, for example, they will turn toward it,
being directed apparently by a sense of smell, or rather of
taste.

The heliotropic responses of the planarians appear to be of
use to them, causing them to hide away in the daytime, and
to come out only after dark, when their motions will not dis-
cover them to possible enemies. But some of the planarians

Tropisms and Instincts as Adaptations 395

are protected in other ways, so that they will not be eaten by
fish, probably owing to a bad taste; so that it is not so appar-
ent that they are in real need of the protection that their
heliotropic response brings to them. Their turning towards
their food is, however, beyond question of great advantage to
them, for in this way they can find food that they cannot
detect in any other way.

The unicellular plants were amongst the first organisms
whose tropic responses were studied, and the classical work
of Strasburger gave the impetus to much of the later work.
In recent years the unicellular animals, the protozoans,
have been carefully studied, more especially by Jennings.
His results show that the reactions in these animals
are different in some important respects from those met
with in higher forms. For instance, most of the free-
swimming infusoria are unsymmetrical, as are also many of
the flagellate forms, and as they move forward they rotate
freely on a longitudinal axis. It is therefore impossible that
they could orientate themselves as do the higher animals that
have been described above, and we should not expect these
Protozoa to react in the same way. In fact, Jennings shows
that they exhibit a different mode of response. Paramoecium
offers a typical case. As it moves forward it rotates toward
the aboral side of the body. As a result of the asymmetry
of the body, the path followed, as it revolves on its own axis,
is that of a spiral. Did the animal not rotate, as it swims
forward, its asymmetrical form would cause it to move in a
circle, but its rotation causes, as has been said, the course to
be that of a spiral, and the general direction of movement is
forward.) The rotation of a paramcecium on its axis is in turn
caused by the oblique stroke of the cilia that cover the sur-
face of the body. Their action when reversed causes the
animal to rotate backward.

1 The same result is attained by a bullet that is caused by the rifling to rotate
as it moves forward.

396 Evolution and Adaptation

If a drop of weak acid be put into the water in which the
parameecia are swimming, — for instance, in the water be-
tween a cover-slip and a slide, —it will be found, after a
time, that many individuals have collected in the drop. It
was at first supposed that the parameecia are attracted by
the diffusion of the acid in the water, and turn toward the
source of the chemical stimulus ; but Jennings has shown that
this is not the way in which the aggregation is brought
about. If the individuals are watched, it will be found that
they swim forward in a spiral path without regard to the
position of the drop of acid. If one happens, by chance, to
run into the drop, there is no reaction as it enters, but when
it reaches the other side of the drop, and comes into contact
with the water on this side, it suddenly reacts. It stops,
backs into the middle of the drop, rotates somewhat toward
the aboral side (z.e. away from the vestibule), and then starts
forward again, only to repeat the action on coming into con-
tact with the edge of the drop again. The paramcecium has
been caught in a veritable trap. All paramcecia that chance
to swim into the drop will also be caught, until finally a large
number will accumulate in the region. The result shows,
that, in passing from ordinary water into a weak acid, no
reaction takes place; but having once entered the acid, the
animal reacts on coming into contact with the water again.

On the other hand, there are some substances to which the
paramcecium may be said to be negatively chemotropic. If
‘a drop of a weak alkaline solution be put into water in which
paramcecium is swimming, an individual that happens to run
against it reacts at once. It stops instantly, backs off,
revolving in the opposite direction, turns somewhat to one
side, and swims forward again. The chances are that it will
again hit the drop, in which case it repeats the same reaction,
turning again to one side. If it continues to react in this way,
it will, in the course of time, turn so far that when it swims
forward it will miss the edge of the drop, and then continue

Tropisms and Instincts as Adaptations 397

on its way. If an individual were put into an alkaline drop,
it would leave it, because it would not react when it passed
from inside the drop into the surrounding water.

Unicellular animals react to other things besides differences
in the chemical composition of different parts of a solution.
In many cases they react to light, swimming toward or away
from it according to whether they are positively or negatively
heliotropic. If they are positively heliotropic, and while
swimming run into a shadow, they react as they would on
coming into contact with a drop of acid. Since they rotate as
they swim forward, we cannot explain their orientation as in
the case of other animals that hold a fixed vertical position.
If we assume that the two ends of the body are differently
affected by the light, for which there is some evidence, we
can perhaps in this way account for their turning toward, or
away from, the source of light.

Changes in the osmotic pressure of the different parts of
the fluid, mechanical stimulation produced by jarring, ex-
tremes of heat and of cold, all cause this same characteristic
reaction in Paramcecium; and this accounts for their be-
havior toward these agents that are so different in other
respects.

Paramoecia, as well as other protozoans, show a contact
response. They fix themselves to certain kinds of solid
bodies. If, for example, a small bit of bacterial slime is put
into the water, the parameecia collect around it in crowds,
and eat the bacteria; but they will collect in the same way
around almost any solid. On coming in contact with bodies
having a certain physical texture, the cilia covering the para-
moecium stop moving, only those in the oral groove continu-
ing to strike backward. The animal comes to rest, pressed
against the solid body. If one or more parameecia remain in
the same place, they set free carbon dioxide, as a result of
their respiratory processes. There is formed around them a
region containing more of this acid than does the surround-

398 Evolution and Adaptation

ing water. If other moving paramcecia swim, by chance,
into this region, they are caught, and as a result an accu-
mulation of individuals will take place. The more that
collect the larger will the area become, and thus large num-
bers may be ultimately entrapped in a region where there is
formed a substance that, from analogy with other animals,
we should expect to be injurious.

The question as to how far these responses of the unicellu-
lar forms are of advantage to them is difficult to decide, for
while, as in the above case, the response appears to be
injurious rather than useful, yet under other conditions the
same response may be eminently advantageous. In other
cases, as when the parameecia back away, and then swim for-
ward again, only to repeat the process, the act appears to be
such a stupid way of avoiding an obstacle that the reaction
hardly appears to us in the light of a very perfect adaptation.
If we saw a higher animal trying to get around a wall by
butting its head into it until the end was finally reached, we
should probably not look upon that animal as well adapted
for avoiding obstacles.

Bacteria, which are generally looked upon as unicellular
plants, appear, despite the earlier statements to the contrary,
to react in much the same way as do the protozoans, according
to the recent work of Rothert, and of Jennings and Crosby.
The bacteria do not seem to turn toward or away from chem-
ical substances, but they collect in regions containing cer-
tain substances in much the same way as do the protozoans.
The collecting of bacteria in regions where oxygen is pres-
ent has been known for some time, but it appears from
more recent results that they are not attracted toward the
oxygen, but by accidentally swimming into a region containing
more oxygen they are held there in the same way as is para-
moecium in a drop of acid. On the other hand bacteria do
not enter a drop of salt solution, or of acids, or of alkalies.
They react negatively to all such substances. Some kinds of

Tropisms and Instincts as Adaptations 399

bacteria have a flagellum at each end, and swim indifferently
in either direction. If they meet with something that stimu-
lates them, as they move forward, they swim away in the
opposite direction, and continue to move in the new direction
until something causes again a reversal of their movement.
In this respect their mode of reaction seems of greater
advantage than that followed by paramoecium.

Another instinct, that appears to be due to a tropic
response, is the definite time of day at which some marine
animals deposit their eggs. The primitive fish, Amphioxus,
sets free its eggs and sperm only in the late afternoon. A
jellyfish, Gonionema, also lays its eggs as the light begins
to grow less in the late afternoon, and in this case it has
been found that the process can be hastened if the animals
are placed in the dark some hours before their regular ©
time of laying. There is no evidence that this habit is of
any advantage to the animal. We may imagine, if we like,
that the early stages may meet with less risk at night, but
this is not probable, for it is at this time that countless marine
organisms come to the surface, and it would seem that the
chance of the eggs being destroyed would then be much
greater. It is more probable that the response is of no
immediate advantage to the animals that exhibit it, although
in particular cases it may happen to be so.

This response recalls the diurnal opening and closing of
certain flowers. The flowers of the night-blooming cereus
open only in the dusk of evening, and then emit their strong
fragrance. Other flowers open only in the daytime, and
some only in bright sunlight. It is sometimes pointed out
that it is of advantage to some of these flowers to open at a
certain time, since the particular insects that are best suited
to fertilize them may then be abroad. This may often be the
case, but we cannot but suspect that in other cases it may
be a matter of little importance. In special instances it may
be that the time of opening of the flowers is of importance

400 Evolution and Adaptation

to the species; but even if this is so, there is no need to
assume that the response has been gradually acquired for
this particular purpose. If it were characteristic of a new
form to open at a particular time, and there were insects in
search of food at this time that would be likely to fertilize
the plant, then the plant would be capable of existing; but
this is quite different from supposing that the plant devel-
oped this particular response, because this was the most
advantageous time of day for the fertilization of its flowers.

We can apply this same point of view, I believe, to many
of the remarkable series of tropisms shown by plants, whose
whole existence in some cases is closely connected with
definite reactions to their environment. Let us examine
some of these cases.

When a seed germinates, the young stem is negatively
geotropic, and, in consequence, as it elongates it turns up-
ward towards the light that is necessary for its later growth.
The root, on the contrary, is positively geotropic, and, in
consequence, it is carried downward in the ground. Both
responses are in this case of the highest importance to the
seedling, for in this way its principal organs are carried into
that environment to which they are especially adapted. It
matters very little how the seed lies in the ground, since the
stem when it emerges will grow upward and the root down-
ward. The young stem, when it emerges from the soil, will
turn toward the light if the illumination comes from one
side, and this also may often be of advantage to the plant,
since it turns toward the source from which it gets its
energy. The leaves also turn their broad surfaces toward
the light, and as a result they are able to make use of a
greater amount of the energy of the sunlight. The turning is
due to one side of the stem growing more slowly than the
opposite side, and it is true, in general, that plants grow
faster at night than in the daylight. Very bright light will
in some cases actually stop all growth for atime. Thus we

Tropisms and Instincts as Adaptations 401

see that this bending of the stem toward the light and the
turning of the leaves to face the light. are only parts of the
general relation of the whole plant toward the light.

Negative heliotropism is much less frequent in plants. It
has been observed in aérial roots, in many roots that are ordi-
narily buried in the ground, in anchoring tendrils that serve
as holdfasts, and even in the stems of certain climbers. In
all of these cases, and more especially in the case of the
climbers, the reaction is obviously of advantage to the plant;
and it is significant to find, in plants that climb by tendrils
carrying adhering disks, that there is a reversal of the ordi-
nary heliotropism shown by homologous organs in other
plants. There is an obvious adaptation in the behavior of
the tendril, since its growth away from the more illuminated
side is just the sort of reaction that is likely to bring it into
contact with a solid body.

In this connection it is important to observe that these
reactions to light are perfectly definite, being either positive
or negative under given conditions, and therefore there is at
present nothing to indicate that there has been a gradual
transformation from positive to negative, or wzce versa. It
seems to me much more probable that when the structural
change took place, that converted the plant into a climber,
there appeared a new heliotropic response associated with the
other change. In other words, both appeared together in the
new organ, and neither was gradually acquired by picking
out fluctuating variations.

The leaves of plants also show a sort of transverse heliotropic
response. It has been found, for example, that the leaves of
Malva will turn completely over if illuminated by a mirror
from below. A curious case of change of heliotropism is found
in the flower stalks of Linaria. They are at first positively
heliotropic, but after the flower has been fertilized the stalk be-
comes negatively heliotropic. As the stalks continue to grow
longer, they push the fruits into the crevices of the rocks on

2D

402 Evolution and Adaptation

which the plants grow, and in this way insure the lodgement
of the seeds. Here we have an excellent example showing
that the negative heliotropism of the flower stalk could
scarcely have been acquired by slight changes in the final
direction, for only the complete change is useful to the plant.
Intermediate steps would have no special value.

As has been pointed out in the case of the seedling plant,
the main stem responds positively and the roots negatively
to gravity. In addition to this, the lateral position taken by
the lateral roots and branches and by underground stems are
also, in part, due to a geotropic response. In this case also
the effect is produced by the increased growth on the upper
side when the response is positive, and on the lower when it
is negative. Leaves also assume a transverse position in
response to the action of gravity, or at least they make a
definite angle with the direction of its action.

The most striking case of geotropic response is seen in
plants that climb up the stems of other plants. The twining
around the support is the result of a geotropic response of
the sides of the stem. The young seedling plant stands at
first erect. As its end grows it begins to curve to one side
in an oblique position, and this is due to an increase in growth
on one side of the apex of the shoot. As a result the stem
bends toward the other side. Not only does the end “sweep
round in a circle like the hands of a watch,” but it rotates on
its long axis as it revolves. Asa result of this rotation “the
part of the stem subjected to the action of the lateral geo-
tropism is constantly changing ; and the revolving movement
once begun, must continue, as no position of equilibrium can
be attained.” This movement will carry the end around any
support, not too thick, that the stem touches.

Most climbers turn to the left, ze. against the hands of a
watch, others are dextral, and a few climb either way.)

1 These cases recall the spiral growth of the shell of the snail, but the spiral
in the latter is due to some other factor,

Tropisms and Instincts as Adaptations 403

Strasburger states that whenever any external force, or sub-
stance, is important to the vital activity of the plant or any
of its organs, there will also be found to be developed a cor-
responding irritability to their influence. Roots in dry soil
are diverted to more favorable positions by the presence of
greater quantities of moisture. This may, I venture to sug-
gest, be putting the cart before the horse. The plant may
be only able to exist whose responses are suited to certain
external conditions, and these determine the limits of distribu-
tion of the plant or the places in which it is found.

A number of plants climb in a different way, and show
another sort of tropism. Those that climb by means of ten-
drils twist their tendrils about any support that they happen
to come in contact with, and thus the plant is able to lift its
weak stem, step by step, into the air. The twining of the
tendrils is due to contact, which causes a cessation of growth
at the points of contact. The growth of the opposite side
continues, and thus the tendril bends about its support. In
the grape and in ampelopsis the tendril is a modified branch.
The stalk of the leaves in a few plants, as in Lophospermum,
act as tendrils. Other climbers are able to ascend vertical
walls owing to the presence of disks, whose secretions hold
the tendril firmly against the support, as in ampelopsis.

It is interesting to find in practically all these cases that,
whatever the stimulus may be, the results are reached in the
same way, namely, by one part growing faster than another.
The fact of importance in this connection is that the plant is
so constructed that the response is often beneficial to the
organism.

Before leaving this subject there is one set of responses to
be referred to that is not the result of growth. Certain move-
ments are brought about by the change in the turgidity of
certain organs. The small lateral leaflets of Desmodium
gyrans make circling movements in one to three minutes.
No apparent benefit results from their action. The terminal

404 Evolution and Adaptation

leaflets of Trifolium pratense oscillate in periods of two to four
hours, but do so only in the dark; in the light the leaflets
assume a rigid position. There is nothing in the process to
suggest that the movement is useful to the plant, and yet it
appears to be as definite as are those cases in which the
response is of vital importance. Had these movements been
of use, their origin would, no doubt, have been explained be-
cause of their usefulness, and the conclusion would have been
wrong.

The leaves of the Mimosa respond, when touched, and it
cannot be supposed that this is of any great advantage to the
plant. The sleep movements of many plants are also due to
the effect of light. In some cases the leaflets are brought to-
gether with their upper surfaces in contact with one another ;
in other cases the lower surfaces are brought together. Dar-
win supposed that these sleep movements served to protect
the leaves from a too rapid loss of heat through radiation, but
it has been pointed out that tropical plants exhibit the same
responses. We have here another admirable instance of the
danger of concluding that because we can imagine an advan-

tage of a certain change, that the change has, therefore, been a

acquired because of the advantage. In the Mimosa not only
do the leaflets close together, but the whole leaf drops down
if the stimulus is strong. Other plants also show in a less
degree the same movements, Robinia and Oxalis for instance,
and certainly in these latter the result does not appear to be
of any advantage to the plants.

The preceding account of some of the tropisms in animals
and plants will serve to give an idea of how certain move-
ments are direct responses to the environment.’ Some of
the reactions appear to be necessary for the life of the
individual, others seem to be of less importance, and a few
of no use at all. Yet the latter appear to be as definite
and well-marked as are the useful responses. I think the
conviction will impress itself on any one who examines

Tropisms and Instincts as Adaptations 405

critically the facts, that we are not warranted in applying
one explanation to those responses that are of use, and
another to those that are of little or of no value. Inasmuch
as the Darwinian theory fails to account for the origin of
organs of little or of no value, it is doubtful if it is needed to
explain the origin of the useful responses. If, on the other
hand, we assume that the ovig7z of the responses has nothing
to do with their value to the organism, we meet with no diffi-
culty in those cases in which the response is of little or of
no use to the organism. That great numbers of responses
are of benefit to the organism that exhibits them can be
accounted for on the grounds that those new species, that
have appeared, that have useful responses, are more likely,
in the long run, to survive, than are those that do not re-
spond adaptively.

We may now examine some of the more complicated
responses and instincts, more especially those of the higher
animals. Some of these are pure tropisms, z.e. definite re-
sponses or reactions to an external exciting agent; others may
be, in part, the result of individual experience, involving
memory; others, combinations of the two; and still others
may depend on a more complex reaction in the central ner-
vous system of the animal. These cases can be best under-
stood by means of.a few illustrations.

As an example of a simple action may be cited a well-
known reflex after cutting the nerve-cord of the frog, or
after destroying the brain. If the frog is held up, and its
side tickled, the leg is drawn up to rub the place touched.
To accomplish this requires a beautifully adjusted system
of movements, yet the act seems to be a direct reflex, involv-
ing only the spinal cord.

An example of a somewhat more complex reflex is the
biting off of the navel-string by the mother in rodents
and other mammals; an act eminently useful to the young
animal, although of no importance to the mother herself.

406 Evolution and Adaptation

The protection of the young by their parents from the
attacks of other animals appears to be a somewhat com-
plex instinct, and it is interesting to note that the protection
is extended to the young only so long as they are in need
of it, and as soon as they are able to shift for themselves
the maternal protection is withdrawn.

The instinct of the young chick to seize in its beak any
small moving object is a simple and useful reflex action, but if
the object should happen to be a bee which stings the chick,
another bee or similar insect will not be seized. Here we see
that a reflex has been changed, and changed with amazing
quickness. Moreover, the chick has learnt to associate this
experience with a particular sort of moving object. It is this
power to benefit by the result of a brief experience that is one
of the most advantageous properties of the organism.

Young chicks first show a drinking reflex if by chance
their beaks are wet by water. At once the head is lifted
up, and the drop of water passes down the throat. In this
way the chick first learns the meaning of water, and no doubt
soon comes to associate it with its own condition of thirst.
The sight of water produces no effect on the inexperienced
chick, and it may even stand with its feet in the water with-
out drinking ; but as soon as it touches, by chance, the water
with its beak, the reflex, or rather the set of reflexes is
started.

A more complicated instinct is that shown by the spider
in making its web. In some cases the young are born from
eggs laid in the preceding summer, and can have had, there-
fore, no experience of what a web is like; and yet, when they
come to build this wonderfully complex structure, they do so
in a manner that is strictly characteristic of the species.

The formation of the comb by bees, in which process,
with a minimum of wax, they secure a maximum number of
small storehouses in which to keep their honey and rear their
young, is often cited as a remarkable case of adaptation.

Tropisms and Instincts as Adaptations 407

There has been some discussion as to whether birds build
their nests in imitation of the nest in which they were reared,
or whether they do so independently of any such experience.
There can be no doubt, however, that in some birds neither
memory nor imitation can play any important part in the
result, and that they build their nests as instinctively as spi-
ders make webs.

These instincts of spiders, bees, and birds appear to be
more complex than the reflexes and tropisms that were first
described. Whether they are really so, or only combina-
tions of simple responses, we do not yet know. That they
have come suddenly into existence as we now find them
does not seem probable, but this does not mean that they
must have been slowly acquired as the result of selection.
The mutation theory also assumes that the steps of advance
may have been small.

Our account may be concluded with the recital of some
instincts, chosen almost at random, that serve to show some
other adaptations which are the result of these inborn
responses.

It is known that ants travel long distances from their nests,
and yet return with unerring accuracy. It has been shown
that they are able to do this through a marvellous sense of
smell. The track left by the ant, as it leaves the nest, serves
as a trail in returning to the starting-point. Moreover, it
appears that the ant can pick out her own trail, even when
it has been crossed by that of other ants. This means that
she can distinguish the odor of her own trail from that of
other members of the colony. The sense-organs by means
of which the odor is detected lie in the antenna. This fact
accounts for certain actions of ants that have been described
as showing that they have an affection for each other. Two
ants, meeting, pat each other with their antennez. In this
way they are quickly able to distinguish members of their
own nest from those of other nests. If they are of the same

408 Evolution and Adaptation

nest, they separate quietly ; if of other nests, they may fight.
If an ant from one nest is put into another nest, it is instantly
attacked and killed —an act that appears to be injurious
rather than useful, for the ant might become a valuable mem-
ber of the new colony. If, however, an ant is first immersed
in the blood of a member of the community into which she
is to be introduced, she will not be attacked, and may soon
become a part of the new community. By her baptism of
blood she has no doubt acquired temporarily the odor of the
new nest, and by the time that this has worn off she will
have acquired this odor by association, and become thereby a
member of another colony.

Numerous stories have been related of cases in which an
ant, having found food, returns to the nest with as much of
it as she can carry, and when she comes out again brings with
her a number of other ants. This has been interpreted to
mean that in some mysterious way the ant communicates her
discovery to her fellow-ants. A simpler explanation is proba-
bly more correct. The odor of the food, or of the trail,
serves as a stimulus to other ants, that follow to the place
where the first ant goes for a new supply of the food. The
fact that the first individual returns to the supply of food
seems to indicate that the ant has memory, and this is obvi-
ously of advantage to her and to the whole colony.

The peculiar habits of some of the solitary wasps, of sting-
ing the caterpillar or other insect which they store up as
food for their young, is often quoted as a wonderful case
of adaptive instinct. The poison that is injected into the
wound paralyzes the caterpillar, but as a rule does not kill it,
so that it remains motionless, but in a fresh state to serve
as food for the young that hatch from the egg of the wasp.
A careful study of this instinct by Mr. and Mrs. Peckham
has shown convincingly that the act is not carried out with the
precision formerly supposed. It had been claimed that the
sting is thrust into the caterpillar on the lower side, a ventral

Tropisms and Instincts as Adaptations 409

ganglion being pierced, the poison acting with almost instan-
taneous effect. But it may be questioned whether this is
really necessary, and whether the same end might not be
gained, although not quite so instantaneously, if the cater-
pillar were pierced in almost any other part of the body. Can
we be seriously asked to believe that this instinct has been
perfected by the destruction of those individuals (or of their
descendants) that have not pierced the caterpillar in exactly
the middle of a segment of the anterior ventral surface? It
seems to me that the argument proves too much from the
selectionist’s point of view. If the wasp pierced the cater-
pillar in the middle of its back, we should have passed over
the act without comment; but since the injection is usually
made on the ventral side, and since we know that the nerv-
ous system lies in this position, it has been assumed that the
act is carried out in this way, in order that the poison may
penetrate the nervous system more quickly. Yet a fuller
knowledge may show that there is really no necessity for
such precision.

A curious response is the so-called death-feigning instinct
shown by a number of animals, especially by certain insects,
but even by some mammals and birds. Certain insects, if
touched, draw in their legs, let go their hold, and fall to the
ground, if they happen to be on a plant. It is not unusual
to meet with the statement that this habit has been acquired
because it is useful to the insect, since it may often escape in
this way from an enemy. This does not appear on closer
examination to be always the case, and sometimes as much
harm as good may result, or what is more probable, neither
much advantage, nor disadvantage, is the outcome. This can,
of course, only be determined in each particular case from a
knowledge of the whole life of a.species and of the enemies
that are likely to injure it.

Hudson has recorded! a number of cases of this death-

1“The Naturalist in La Plata.”

410 Evolution and Adaptation

feigning instinct in higher animals, and attributes it to violent
emotion, or fear, that produces a sort of swoon. He describes
the gaucho boys’ method, in La Plata, of catching the silver-
bill by throwing a stick or a stone at it, and then rushing
toward the bird, “when it sits perfectly still, disabled by
fear, and allows itself to be taken.” He also states that one
of the foxes (Canis azar@) and one of the opossums (Didelphys
azar) “are strangely subject to the death-simulating swoon.”

Hudson remarks that it seems strange that animals so
well prepared to defend themselves should possess this “‘safe-
guard.” When caught or run down by dogs, the fox fights
savagely at first, but after a time its efforts stop, it relaxes,
and it drops to the ground. The animal appears dead, and
Hudson states that the dogs are “constantly taken in by it.”
He has seen the gauchos try the most barbarous tricks on a
captive fox in this condition, and, despite the mutilations to
which it was subjected, it did not wince. If, however, the
observer draws a little away from the animal, “a slight open-
ing of the eye may be detected, and finally, when left to
himself, he does not recover and start up like an animal that
has been stunned, but cautiously raises his head at first and
only gets up when his foes are at a safe distance.” Hudson,
coming once suddenly upon a young fox, saw it swoon at his
approach, and although it was lashed with a whip it did not
move.

The common partridge of the pampas of La Plata (Hothura
maculosa) shows this death-feigning instinct in a very marked
degree. “When captured, after a few violent struggles to
escape, it drops its head, gasps two or three times, and to all
appearance dies.” But if it is released it is off in an instant.
The animal is excessively timid, and if frightened, may actually
die simply from terror. If. they are chased, and can find no
thicket or burrow into which to escape, “they actually drop
down dead on the plain. Probably when they feign death in
their captor’s hand they are in reality very near to death.”

Tropisms and Instincts as Adaptations 411

In this latter instance it must appear very improbable that
we are dealing with an instinct that has been built up by
slow degrees on account of the benefit accruing at each stage
to the individual. In fact, it appears that the instinct is in
this case of really no use at all to the animal, for there can
scarcely be any question of an escape by this action. Yet so
far as we can judge it is the same instinct shown by other
animals, and it is not logical to account for its origin in one
case on the grounds of its usefulness, when we cannot apply
the explanation in the other cases. If this be admitted, we
have another illustration of the importance of keeping apart |
the origin of an instinct or of a structure and the fact of its ,
usefulness or non-usefulness to the organism. Thus under
certain conditiéns this death-feigning instinct might really
be of use to the animal, while under other conditions and in
other animals it may be of no advantage at all, and in still
other conditions it may be a positive injury to its possessor.
Perhaps we need not go outside of our own experience to find
a parallel case, for the state of fright into which imminent
danger may throw an individual may deprive him for the
moment of the proper use of those very mental qualities of
which he stands in this crisis in greatest need.

The peculiar behavior of cattle caused by the smell of blood
is another case of an instinct whose usefulness to its possess-
ors is far from apparent. It is known that cattle and horses
and several wild animals become violently excited by the
smell of blood. Hudson gives a vivid account of a scene
witnessed by himself, the animals congregating, “and moving
around in a dense mass, bellowing continually.” Those ani-
mals that forced their way into the centre of the mass where
the blood was “pawed the earth and dug it up with their
horns, and trampled each other down in their frantic excite-
ment.”

This action leads us to a consideration of the behavior
of animals toward companions in distress. ‘ Herbivorous

412 Evolution and Adaptation

animals at such times will trample and gore the distressed
one to death. In the case of wolves and other savage-
tempered carnivorous species the distressed fellow is fre-
quently torn to pieces and devoured on the spot.” If any
one will be bold enough to claim in this case that this habit
has been acquired because of advantage to the pack, ze.
if it be imagined that the pack gains more by feeding on
a weak member than by letting him take his chances of
recovery, it may be pointed out in reply that cattle also
destroy their weak or injured, but do not devour them, and
the same statement holds for birds, where the same instinct
has often been observed. Romanes has suggested that the
instinct of destroying the weak or injured members is of use
because such members are a source of danger to the rest of the
herd; but Hudson points out that it is not so much the weak
and sickly members of the herd that are attacked in this
way, as those that are injured, and concludes, “ the instinct
is not only useless, but actually detrimental.” He suggests
that these “wild abnormal movements of social animals” are
a, sort of aberration, so “ that in turning against a distressed
fellow they oppose themselves to the law of being.” Yet
whether we gain anything by calling this action aberrant or
abnormal, the important fact remains that it is a definite
response under certain external conditions, and is shown by
all the individuals of the species.

The preceding illustrations of reactions that go to make
up the so-called instincts of animals may be separated into
those that are essential to the life of the individual or of the
race, those that are of some apparent use, although not
absolutely essential, and a few of no use at all, and fewer
still that appear to be even injurious. If the latter reactions
take place only rarely, as appears often to be the case,
they are not sufficiently harmful to cause the destruction of
the race. The evidence points to the conclusion, I believe,
that the origin of these tropisms and instincts cannot be

Tropisms and Instincts as Adaptations 413

accounted for on the ground of their benefit to the indi-
vidual or to the race; and it does not seem reasonable to
make up one explanation for the origin of those that are
essential, and another for those that are of little use or even
of no use at all.

From what has been already said more than once, while
discussing each particular case, the simplest course appears
to be in all instances to look upon these instincts as having
appeared independently of the use to which they may be
put, and not as having been built up by selection of the
individual variations that happen to give an organism some
advantage over its fellows in a life and death struggle. It |
appears reasonable to deal with the origin of tropisms and
instincts in general in the same way as in dealing with
structures; for, after all, the tropism is only the outcome
of some material or structural basis in the organism.

No attempt has been made here to interpret the more com-
plex reactions of the nervous system, for until we can get
some insight into the meaning of the simpler processes, we
are on safer ground in dealing with these first.

CHAPTER XII
SEX AS AN ADAPTATION

In what sense may the separation of all the individuals of
a species into two kinds of individuals, male and female, be
called an adaptation? Does any advantage result to the species
that would not come from a non-sexual method of reproduc-
tion? Many attempts have been made to answer these ques-
tions, but with what success I shall now try to show.

There are four principal questions that must be con-
sidered : —

I. The different kinds of sexual individuals in the animal
and plant kingdoms.

II. The historical question as to the evolution of separate
sexes.

III. The factors that determine the sex in each individual
developing from an egg.

IV. The question as to whether any advantage is gained
by having each new individual produced by the union of two
germ-cells, or by having the germ-cells carried by two kinds
of individuals.

While our main problem is concerned with the last of these
topics, yet there would be little hope of giving a complete an-
swer to it unless we could get some answer to the first three
questions.

Tue DirFeEReNT Kinps oF SExuaL INDIVIDUALS

Amongst the unicellular animals and plants the fusion of
two (or more) individuals into a single one is generally re-
garded as the simplest, and possibly also the most primitive,

414

Sex as an Adaptation 415

method of sexual reproduction. Two amcebas, or ameceba-
like bodies, thus flow together, as it were, to produce a new
individual.

In the more highly specialized unicellular animals, the
processes ate different. Thus in vorticella, a small, active
individual unites with a larger fixed individual. The proto-
plasm fuses into a common mass, and a very complicated
series of changes is passed through by the nucleus. In
paramoecium, a free-swimming form very much like vorti-
cella, two individuals that are alike unite only temporarily,
and after an interchange of nuclear material they separate.

In the lower plants, and more especially in some of the
simple aggregates or colonial forms, there are found a num-
ber of stages between species in which the uniting individuals
are alike, and those in which they are different. There are
several species whose individuals appear to be exactly alike ;
and other species in which the only apparent difference
between the individuals that fuse together is one of size;
and still other species in which there are larger resting or
passive individuals, and smaller active individuals that unite
with the larger ones. In several of the higher groups, in-
cluding the green algee and seaweeds, we find similar series,
which give evidence of having arisen independently of each
other. If we are really justified in arranging the members
of these groups in series, beginning with the simpler cases
and ending with those showing a complete differentiation
into two kinds of germ-cells, we seem to get some light as to
the way in which the change has come about. It should not
be forgotten, however, that it does not follow because we can
arrange such a series without any large gaps in its con-
tinuity, that the more complex conditions have been gradu-
ally formed in exactly this way from the simplest conditions.

So far we have spoken mainly of those cases in which the
forms are unicellular, or of many-celled species in which all
the cells of the individual resolve themselves into one or the

416 Evolution and Adaptation

other kind of germ-cells. This occurs, however, only in the
lowest forms. A step higher we find that only a part of the
cells of the colony are set aside for purposes of reproduction.
The cells surrounding these germ-cells may form distinct
organs, which may show certain differences according to
whether they contain male or female germ-cells. When
these two kinds of cells are produced by two separate indi-
viduals, the individuals themselves may be different in other
parts of the body, as well as in the reproductive organs.

When this condition is reached, we have individuals that
we call males and females, because, although they do not
themselves unite to form new individuals, they produce one
or the other kind of germ-cell. It is the germ-cells alone
that now combine to form the new individual.

Amongst living groups of animals we find no such complete
series of forms as exist in plants, and the transition from the
one-celled to the many-celled forms is also more abrupt. On
the other hand, we find an astonishing variety of ways in
which the reproduction is accomplished, and several ways in
which the germ-cells are carried by the sexual individuals.
Let us examine some of the more typical conditions under
the following headings: (1) sexes separate; (2) sexes united
in the same individual; (3) parthenogenetic forms; (4) ex-
ceptional methods of propagation.

1. Sexes Separate ; Unisexual Forms.’ — Although the ani-
mals with which we are more familiar have the sexes sepa-
rate, this is far from being universal amongst animals and
plants; and, in fact, can scarcely be said to be even the rule.
When the sexes are separate they may be externally alike,
and this is especially true for those species that do not unite,
but set free their eggs and spermatozoa in the water, as fish,
frogs, corals, starfish, jellyfish, and many other forms. In
other animals there are sometimes other secondary differ-

1 Geddes and Thompson’s “ The Evolution of Sex * has been freely used in the
preparation of this part of this chapter.

Sex as an Adaptation 417

ences in the sexes besides those connected with the organs
of reproduction. Such differences are found, as we have
seen, in insects, in some spiders, crustaceans, and in many
birds and mammals. In a few cases the difference between
the sexes is very great, especially when the female is par-
asitic and the male free, as in some of the crustaceans. In
some other cases the male is parasitic on the female. Thus
in Bonellia the male is microscopic in size, being in length
only one-hundredth part of the female. In Hydatina senta
the male is only about a third as large as the female. It has
no digestive tract, and lives only a few days. In another
rotifer the males are mere sacs enclosing the male reproduc-
tive organs.

2. Hermaphroditic Forms.—There*are many species of
animals and plants in which each individual contains both
the male and the female organs of reproduction, and there
are whole groups in which only these hermaphroditic forms
occur. Thus in the ctenophors the eggs develop along
one side of each radial canal and spermatozoa along the
other. The group of flatworms is almost exclusively her-
maphroditic. The earthworms and the leeches have only
these bisexual forms, and in the mollusks, while a few groups
have separate sexes, yet certain groups of gasteropods and
of bivalve forms are entirely hermaphroditic.

In the common garden snail, although there are two sets
of sexual ducts closely united, yet from the same reproduc-
tive sac both eggs and sperm are produced. The barnacles
and the ascidians are for the most part hermaphroditic forms.
Many other examples might be cited, but these will suffice
to show that it is by no means unusual in the animal kingdom
for the same individual to produce both male and female germ-
cells. However, one of the most striking facts in this connec-
tion is that self-fertilization seldom takes place, so that the
result is the same in certain respectsas though separate sexes ex-
isted. This point will come up later for further consideration.

2E

418 Evolution and Adaptation

3. Parthenogenetic Reproduction.— It has long been known
that, in some cases, eggs that are not fertilized will begin
to develop and may even produce new individuals. Tichomi-
roff showed that by rubbing with a brush the unfertilized
eggs of the silkworm moth, a larger percentage would
produce caterpillars than if they were not rubbed. During
the last few years it has been shown that the development
of a non-fertilized egg may be started in a number of ways.
Such, for example, as by certain solutions of salt or of
sugar, by subjecting the eggs to cold, or by simply shaking
them.

There are certain groups of animals in which the males
appear only at regular (in others at irregular) intervals. In
their absence the females produce eggs that develop without
being fertilized, z.e. parthenogenetically. The following exam-
ples will serve to show some of the principal ways in which
this “virgin reproduction” takes place. In the group of
rotifers the males are generally smaller than the females
and are usually also degenerate. In some species, although
degenerate males are present, they are unnecessary, since
parthenogenesis is the rule. In still other species no males
exist and the eggs develop, therefore, without being fertilized.
In some of the lower crustaceans parthenogenesis occurs in
varying degrees. In Apus males may be entirely absent at
‘times in certain localities, and at other times a few, or even
very many, males may appear. Some species of ostracod
crustaceans seem to be purely parthenogenetic ; others repro-
duce by means of fertilized eggs; and others by an alterna-
tion of the two processes. The crustaceans of the genus
Daphnia produce two kinds of eggs. The summer eggs
are small, and have a thin shell. These eggs develop with-
out being fertilized, but in the autumn both male and female
individuals develop from these unfertilized eggs, and the
eggs of the female, the so-called winter eggs, are fertilized.
These are also larger than the summer eggs, have thicker

Sex as an Adaptation 419

shells, and are much more resistant to unfavorable condi-
tions. They give rise in the following spring to females
only, and these are the parthenogenetic individuals that con-
tinue to produce during the summer new parthenogenetic
eggs.

It is within the group of insects that some of the most
remarkable cases of parthenogenesis that we know are
found. In the moth, Psyche helix, only females are present,
as a rule, but rarely males have been found. In another
moth, Solenobia tringuetrella, the female reproduces by par-
thenogenesis, but at times males appear and may then be
even more numerous than the females. In the gall-wasps °
parthenogenetic generations may alternate with a sexual
generation, and it is interesting to note that the sexual and
the parthenogenetic generations are so different that they
were supposed to belong to separate species, until it was
found that they were only alternate generations of the same
species.

The aphids or plant-lice reproduce during the summer
by parthenogenesis, but in the autumn winged males and
females appear, and fertilized winter eggs are produced.
From these eggs there develop, in the following spring, the
wingless parthenogenetic summer forms, which produce the
successive generations of the wingless forms. As many
as fourteen summer broods may be produced. By keeping
the aphids in a warm temperature and supplying them with
plenty of moist food, it has been possible to continue the
parthenogenetic reproduction of the wingless forms for
years. As many as fifty successive broods have been pro-
duced in this way. It has not been entirely determined
whether it is the temperature or a change in the amount,
or kind, of food that causes the appearance of the winged
males and females, although it seems fairly certain that
diminution in the food, or in the amount of water contained
in it, is the chief cause of the change.

420 Evolution and Adaptation

In the honey-bee the remarkable fact has been well estab-
lished that fertilized eggs give rise only to females (queens
and workers), while unfertilized eggs develop into males.
Whether a fertilized egg becomes a queen or a worker
(sterile female) depends solely on the kind of food that is
given to the young larva, and this is determined, in a sense,
entirely by the bees themselves.

In plants also there are many cases of parthenogenesis
known. Some species of Chara when kept under certain
conditions produce only female organs, and seem to produce
new plants parthenogenetically. In this case it appears
‘that the same conditions that caused the plants to produce
only female organs may also lead to the development
of the egg-cells without fertilization. In fact it is only by
a combination of this kind that parthenogenesis could arise.
The result is similar when the eggs of insects produce only
females whose eggs are capable of parthenogenetic develop-
ment. Ifa case should arise in which only females appeared
whose eggs did not possess the power of parthenogenetic
development, the species would die out.

In the green alga, Spirogyra, it has been found that if
conjugation of two cells is prevented, a single cell may be-
come a parthenogenetic cell. In a number of parasitic fungi
the male organs appear to be degenerate, and from the
female organs parthenogenetic development takes place. A
small number of flowering plants are also capable of par-
thenogenetic reproduction.

There is a peculiarity in the development of the partheno-
genetic eggs of animals that will be more fully discussed later,
but may be mentioned here. Ordinarily an egg that becomes
fertilized gives off two polar bodies, but in a number of cases
in which parthenogenetic development occurs it has been found
that only one polar body is given off. It is supposed that in
such cases one polar body is retained, and that it plays the
same part as the entrance of the spermatozoon of the male.

Sex as an Adaptation 421

4. Exceptional Cases. — Occasionally in a species that is uni-
sexual an individual is found that is bisexual. The male of
the toad, Pelobates fuscus, has frequently a rudimentary ovary
in front of the testis. The same thing has been found in sev-
eral species of fish. In Serranus, a testis is present in the wall
of the ovary, and the eggs are said to be fertilized by the sper-
matozoa of the same individual. In frogs it has been occasion-
ally found that ovary and testis may be associated in the same
individual, or a testis may be present on one side, and a testis
with an anterior ovarian portion on the other. Cases like
these lead up to those in which the body itself may also show
a mosaic of sex-characters, and it is noticeable that when
this occurs there is nearly always a change in the reproduc-
tive organs also. Thus butterflies have been found with the
wings and the body of one side colored like the male and the
other side like the female. Similar cases have also been found
in bees and ants. Bees have been found with the anterior part
of the body of one sex and posterior part of another!

The preceding cases illustrate, in different ways, the fact
that in the same individual both kinds of reproductive organs
may suddenly appear, although it is the rule in such species
that only one set develops. Conversely, there are cases
known, especially amongst plants, in which individuals, that
usually produce male and female organs (or more strictly
spores of two kinds from which these organs develop), pro-
duce under special conditions only one or the other kind.
Facts like these have led to the belief that each individual is
potentially bisexual, but in all unisexual forms one sex
predominates, and the other remains latent. This idea has
been the starting-point for nearly all modern theories of sex.

An excellent illustration of this theory is found in those
cases in which the same individual may be male at one time
and female at another. For instance, it is said that in one of
the species of starfish (Asterina gibbosa) the individuals at
Roscoff are males for one or two years, and then become

422 Evolution and A daptation

females. At Banyuls they are males for the first two or three
years, and then become females; while at Naples some are
always males, others females, some hermaphrodites, others
transitional as in the cases just given. In one of the isopod
crustaceans, Angiostomum, the young individuals are males
and the older females. In Myzostomum glabrum the young
animal is at first hermaphroditic, then there is a functional
male condition, followed by a hermaphroditic condition, and
finally a functional female phase, during which the male repro-
ductive organs disappear.

The flowers of most of the flowering plants have both sta-
mens and pistils, which contain the two kinds of spores out
of which the male and female germ-cells are formed. The
stamens become mature before the pistils, as a rule, but in
some cases the reverse is the case. This difference in the
time of ripening of the two organs is often spoken of as an
adaptation which prevents self-fertilization. The latter is
supposed to be less advantageous than cross-fertilization.
This question will be more fully considered later.

Before we come to an examination of the question of the
adaptations involved in the cases in which the sexes are
separate, and the different times at which the sex-cells are
ripened, it will be profitable first to examine the question as
to what determines in the egg or young whether a male or a
female or a hermaphroditic form shall arise.

THE DETERMINATION OF SEX

A large number of views have been advanced as to what
determines whether an egg will give rise to a male or to a female
individual. The central question is whether the fertilized egg
has its sex already determined, or whether it is indifferent;
and if the latter, what external factor or factors determine
the sex of the embryo. Let us first examine the view that
some external factor determines the sex of the individual, and

Sex as an Adaptation Azz

then the evidence pointing in the opposite direction. Among
the different causes suggested as determining the sex of the
embryo, that of the condition of the egg itself at the time of
fertilization has been imagined to be an important factor in
the result. Another similar view holds that the condition of
the spermatozoon plays the same réle. For instance, it has
been suggested that if the egg is fertilized soon after it leaves
the ovary, it produces a female, but if the fertilization is de-
layed, a male is produced. It has also been suggested that
the relative age of the male and the female parents produces
an effect in determining the sex of the young. There is
no satisfactory evidence, however, showing that this is really
the case.

Another view suggested is that the sex is determined by
the more vigorous parent; but again there is no proof that
this is the case, and it would be a difficult point to establish,
since as Geddes and Thompson point out, what is meant by
greater vigor is capable of many interpretations. Some-
what similar is the idea that if the conditions are favorable,
the embryo develops further, as it were, and becomes a
male; but there are several facts indicating that this view
is untenable.

Diising maintains that several of these factors may play a
part in determining the sex of the embryo, and if this be true,
the problem becomes a very complex one. He also suggests
that there are self-regulative influences of such a kind that,
when one sex becomes less numerous, the conditions imposed
in consequence on the other sex are such as to bring the
number back to the normal condition; but this idea is far
from being established. The fact that in some species there
are generally more individuals of one sex than of the other
shows that this balance is not equally adjusted in such forms.

Of far greater value than these speculations as to the origin
of sex are the experiments that appear to show that nutrition
is an important factor in determining sex. Some of the

424 Evolution and Adaptation

earlier experiments in this direction are those of Born and of
Yung. By feeding one set of tadpoles with beef, Yung found
the percentage of females that developed to be greatly in-
creased, and a similar increase was observed when the tad-
poles were fed on the flesh of fish. An even greater effect
was produced by using the flesh of frogs, the percentage
rising to 92 females in every hundred. These results have
been given a different interpretation by Pfliiger and by
others, and, as will be pointed out later, there is a possible
source of error that may invalidate them.

Somewhat similar results have been obtained by Nussbaum
for one of the rotifers. He found that if the rotifer is abun-
dantly fed in early life, it produces female eggs, that is, larger
eggs that become females; while if sparingly fed, it produces
only small eggs, from which males develop. It has been
claimed also in mammals, and even in man, that sex is to
some extent determined by the nourishment of the individual.

Some experiments made by Mrs. Treat with caterpillars
seemed to show that if the caterpillars were well nourished
more female moths were produced, and if starved before
pupation more males emerged. But Riley has pointed out
that since the larger female caterpillars require more food
they will starve sooner than the males, and, in consequence,
it may appear that proportionately more male butterflies are
born when the caterpillars are subjected to a starvation diet.
This point of view is important in putting us on our guard
against hastily supposing that food may directly determine
sex. Unless the entire number of individuals present at the
beginning of the experiment is taken into account, the results
may be misleading, because the conditions may be more fatal
to one sex than to the other.

In some of the hymenopterous insects, the bees for example,
it has been discovered that the sex of the embryo is deter-
mined by the entrance, or lack of entrance, of the spermato-
zoon. In the honey-bee all the fertilized eggs produce

Sex as an Adaptation 425

females and the unfertilized eggs males. The same relation
is probably true also in the case of ants and of wasps.
In the saw-flies, the conditions are very remarkable. Sharp
gives the following account of some of these forms:!— “It
is a rule in this family that males are very much less nu-
merous than females, and there are some species in which no
males have been discovered. This would not be of itself
evidence of the occurrence of parthenogenesis, but this has
been placed beyond doubt by taking females bred in confine-
ment, obtaining unfertilized eggs from them, and rearing the
larvee produced from the eggs. This has been done by nu-
merous observers with curious results. In many cases the
parthenogenetic progeny, or a portion of it, dies without
attaining full maturity. This may or may not be due to con-
stitutional weakness, arising from the parthenogenetic state.
Cameron, who has made extensive observations on this subject,
thinks that the parthenogenesis does involve constitutional
weakness, fewer of the parthenogenetic young reaching
maturity. This, he suggests, may be compensated for —
when the parthenogenetic progeny is all of the female sex —
by the fact that all those that grow up are producers of eggs.
In many cases the parthenogenetic young of Tenthredinidz
are of the male sex, and sometimes the abnormal progeny is
of both sexes. In the case of one species —the common
‘currant-fly, Mematus ribesii—the parthenogenetic progeny
is nearly, but not quite always, entirely of the male sex; this
has been ascertained again and again, and it is impossible to
suggest in these cases any advantage to the species to com-
pensate for constitutional parthenogenetic weakness. On the
whole, it appears most probable that the parthenogenesis, and
the special sex produced by it, whether male or female, are
due to physiological conditions of which we know little, and
that the species continues in spite of the parthenogenesis
rather than profits by it. It is worthy of remark that one of
1 “The Cambridge Natural History,’ Vol. V, “Insects,” by David Sharp.

426 Evolution and Adaptation

the species in which parthenogenesis with the production of
males occurs — Vematus ribesii —is perhaps the most abun-
dant of saw-flies.”

It has been pointed out that in a number of species of
animals and plants only parthenogenetic females are present
at certain times. In a sense this means a preponderance of
one sex, but since the eggs are adapted only to this kind of
development, it may be claimed that the conditions in such
cases are somewhat different from those in which eggs that
would be normally fertilized may develop in the absence of
fertilization. Nevertheless, it is generally supposed that the
actual state of affairs is about the same. It is usually as-
sumed, and no doubt with much probability, that these
parthenogenetic forms have evolved, from a group which
originally had both male and female forms. One of the most
striking facts in this connection is that in the groups to which
these parthenogenetic species belong there are, as a rule,
other species with occasional parthenogenesis, and in some of
these the males are also fewer in number than the females.

In the aphids, the parthenogenetic eggs give rise during
the summer to parthenogenetic females, but in the autumn
the parthenogenetic eggs give rise without fertilization both
to males and to females. It appears, therefore, that we can
form no general rule as to a relation between fertilization
and the determination of sex. While in certain cases, as in
the bees, there appears to be a direct connection between
these two, in other cases, as in that of the aphids just men-
tioned, there is no such relation apparent.

Geddes and Thompson have advocated a view in regard to
sex which at best can only serve as a sort of analogy under
which the two forms of sex may be considered, rather than
as a legitimate explanation of the phenomenon of sex. They
rest their view on the idea that living material is continually
breaking down and building up. An animal in which there
is an excess of the breaking-down process is a male, and

Sex as an Adaptation 427

one that is more constructive is a female. Furthermore,
whichever process is in the excess during development deter-
mines the sex of the individual. Thus, if conditions are very
favorable, there will be more females produced ; but if, on the
other hand, there is an excess of the breaking-down process,
males are produced. So far, the process is conceived as a
purely physiological one, but to this the authors then apply
the selection hypothesis, which, they suppose, acts as a sort
of break or regulation of the physiological processes, or in
other words as a directive agent. They state: “Yet the
sexual dimorphism, in the main, and in detail, has an adaptive
significance, also securing the advantages of cross-fertiliza-
tion and the like, and is, therefore, to some extent the result
of the continual action of natural selection, though this may,
of course, check variation in one form as well as favor it in
another.” Disregarding this last addition, with which Geddes
and Thompson think it necessary to burden their theory, let
us return to the physiological side of the hypothesis. Their
idea appears to me a sort of symbolism rather than a
scientific attempt to explain sex. If their view had a real
value, it ought to be possible to determine the sex of the
developing organism with precision by regulating the condi-
tions of its growth, and yet we cannot do this, nor do the
authors make any claim of being able to do so. The hy-
pothesis lacks the only support that can give it scientific
standing, the proof of experiment.

There have been made, from time to time, a number of
attempts to show that the sex of the embryo is predeter-
mined in the egg, and is not determined later by external
circumstances. In recent years this view has come more to
the front, despite the apparent experimental evidence which
seemed in one or two cases to point to the opposite view.
One of the most complete analyses of the question is that
.of Cuénot, who has attempted to show that the sex of the
embryo is determined in the egg, before or at the time of

428 Evolution and Adaptation

fertilization. He has also examined critically the evidence
that appeared to show that external conditions, acting on the
embryo, may determine the sex, and has pointed out some
possible sources of error that had been overlooked. The
best-known case is that of the tadpole of the frog, but Cuénot
shows not only that there are chances of error in this experi-
ment as carried out, but also, by his own experiments and
observations, that the facts themselves are not above suspi-
cion. He points out that at the age at which some of the tad-
poles were when the examination was made, it was not always
possible to tell definitely the sex of the individual, and least
of all by means of the size alone of the reproductive organs,
as was supposed, in one case at least, to be sufficient. In his
own experiments he did not find an excess of one sex over
the other as a result of feeding.

Cuénot points out that Brocadello found that the larger
eggs laid by the silkworm give rise to from 88 to 95 per cent
of females, and the small eggs to from 88 to 92 per cent of
males. Joseph has confirmed this for Ocneria dispar, and Cuénot
himself also reached this conclusion. Korschelt found that
the large eggs of Dinophilus produced females and the small
ones males. Cuénot experimented with three species of flies,
and found that when the maggots were well nourished the
number of the individuals of the two sexes was about equal,
and when poorly nourished there were a few more females in
two cases, and in another about the same number of males
and females.

It has been claimed that the condition of nourishment of
the mother may determine the number of eggs of a particular
sex, but Cuénot found, in three species of flies which he
raised, that there was a slight response in the opposite di-
rection. He concludes that the condition of the mother is
not a factor in the determination of sex.

The first egg of the two laid in each set by the pigeon is
said, as a rule, to produce a male, and the second a female.

Sex as an Adaptation 429

Both Flourens and Cuénot found this to be the case in the
few instances that they examined, but Cuénot has shown that
this does not always happen. Even when this occurs, it has
not been determined whether the result depends on something
in the egg itself, that causes a male egg to be set free first, or
on some external condition that determines that the first egg
shall become a male. It has been claimed that the age of
the spermatozoon might in this and in other cases determine
the result; but Gerbe has shown that if the domestic hen is
isolated for fifteen days after union with the male, she will
continue to produce fertile eggs from which both sexes are
produced, without showing any relation between the time
the eggs are laid and the particular sex that develops.

Cuénot does not discuss whether sex is determined by the
nucleus or by the protoplasm, but if, as he thinks probable,
the size of the egg is a determining factor, it would appear
that the protoplasm must be the chief agent. Even if this
were the case it would still be possible that the size of the
egg itself might be connected with some action on the part
of the nucleus. If, as seems probable, identical twins come
from halves of the same egg, then, since they are of the same
sex, the absolute amount of protoplasm cannot be a factor
in sex determination.

As a basis for the discussion that follows, certain processes
that take place during the maturation divisions of the egg
and of the spermatozoon must be briefly noticed.: After the
egg leaves the ovary it extrudes a minute body called the
first polar body (Fig. 6 B, C, D). This process of extrusion
is really a cell division accompanied by the regular mitotic
division of the nucleus; but since one of the products of the
division, the polar body, is extremely small, the meaning of
the process was not at first understood. The half of the
nucleus, that remains in the egg, divides again, and one of its
halves is thrown out into a second polar body (Fig. 6 E,
F, G). Meanwhile, the first polar body has divided into two

430 Evolution and Adaptation

equal parts, so that we find now three polar bodies and the
egg (Fig. 6 G). A strictly analogous process takes place
in the formation of the spermatozoa (Fig. 7 B-F). The
mother-cell of the spermatozoon divides into two parts, which
are equal in this case (Fig. 7 B-D). Each of these then

Fic. 6.— Diagram showing the maturation of the egg.

divides again (Fig. 7 E, F), producing four cells that are
comparable to the three polar bodies and the mature egg.
Each of the four becomes a functional spermatozoon (Fig.
7G,H). Thus while in the maturation of the egg only the
egg itself is capable of development, in the case of the male
cells all four products of the two maturation divisions are
functional.

Sex as an Adaptation 431

Now, in certain cases of parthenogenesis, it has been found
that one of the polar bodies may not be given off, but, remain-
ing in the egg, its nucleus reunites with the egg nucleus, and
thus takes the place of the spermatozoon, which does exactly
the same thing when it fertilizes the egg, z.c. the nucleus of

Fic. 7.— Diagram showing the maturation of the spermatozoon.

the spermatozoon unites with the nucleus of the egg. This
fact in regard to the action of the polar body in fertilization
is not as surprising as appears at first sight, for if each of the
polar bodies is equivalent to a spermatozoon, the fertilization
of the egg by one of its own polar bodies conforms to theory.

There is a considerable body of evidence showing that
in many eggs at one of the two maturation divisions the

432 Evolution and Adaptation

chromatin rods derived from the nucleus are divided crosswise
(Fig. 6 B, C). The same thing occurs at one of the two
divisions in the formation of the spermatozoon (Fig. 7 B, C).
At the other division to form the other polar body (or the
other sperm-cell) the chromatin rods appear to be split
lengthwise, as in ordinary cell division (Fig. 6 E, F, G).
In recent years the cross-diviston of the chromatin rods has
attracted a great deal of notice, and Weismann in particular
drew attention to the possible importance of this kind of
division.

There is another fact that gives this division especial sig-
nificance. It has been discovered that the number of chro-
mosomes that appears in each dividing cell of the organism
is a constant number, but it has also been discovered that
the egg, before extruding its polar bodies, and the mother-
cell of the spermatozoon (Figs. 6, 7 B), contain exactly
half of the number of chromosomes that are characteristic
of the body-cells of the same animal (Figs. 6,7 A). Now
there is good evidence to show that the reduction in number
is due to the chromosomes uniting sometimes end to end in
pairs, as shown in Figures A and B. Furthermore, it has
been suggested that at one of the maturation divisions, when
the chromosomes divide crosswise, the united chromosomes
are separated (Figs. 6, 7 B, C), so that one remains in the
egg and the other goes out into the polar body. The same
thing is supposed to occur at one of the maturation divisions
of the sperm mother-cell. A further consideration of capital
importance in this connection has been advocated by Mont-
gomery and by Sutton, namely, that, when the chromosomes
unite in pairs, a chromosome from one parent unites with
one from the other parent. Consequently at one of the two
reduction divisions maternal and paternal chromosomes may
separate again, some to go to one cell, some to the other.

When the spermatozoon enters the egg it brings into the
egg as many new chromosomes as the egg itself possesses at

Sex as an Adaptation 433

this time, and the two nuclei, uniting into a single one, fur-
nish the total number of chromosomes characteristic of the
animal that develops from the egg. At first the chromosomes
that are brought in by the spermatozoon lie at one side of
the fused nucleus, and those from the egg itself at the other
side. This arrangement appears, however, in some cases
at least, to be lost later. At every division of the nucleus,
each chromosome divides and sends a half to each of the
daughter-nuclei. Thus every cell in the body contains as
many paternal as maternal chromosomes. This statement
also applies.to the first cells that go into the reproductive
organs, some of which become the mother-cells of the germ-
cells. Later, however, in the history of the germ-cells, —
just before the maturation divisions, —these chromosomes
are supposed to unite in pairs, end to end, as explained
above, to give the reduced number. Later there follows
the separation of these paired chromosomes at one of the
two maturation divisions. If at this time all the paternal
chromosomes should pass to one pole, and all the maternal
to the other, the germ-cell ceases to be mixed, and becomes
purely paternal or maternal. If this ever occurs, the problem
of heredity may become simplified, and even the question of
sex may be indirectly involved; but it has not been established
that, when the reduced number of chromosomes is formed,
there is a strict union between the paternal and maternal chro-
mosomes, and if not, the subsequent separation is probably
not along these lines. If, however, the chromosomes contain
different qualities, as Boveri believes, there may be two kinds
of eggs, and two kinds of spermatozoa in regard to each
particular character. It is this last assumption only that is
made in Mendel’s theory of the purity of the germ-cells.
Several attempts have been made at different times to
connect the facts in regard to the extrusion of the polar
bodies with those involved in the determination of sex.
Minot suggested, in 1877, that the egg ejects by means of the
2F

434 Evolution and Adaptation

polar bodies its male elements, which are again received in
the fertilization of the egg by the spermatozoon. The same
idea has also been expressed by others. It has been objected
to this view that one polar body ought to suffice, and that
no similar throwing out of part of its substance is found in
the process of formation of the spermatozoon, which should,
on the hypothesis, throw out its female elements. It
would seem, on first thought, that this view might find sup-
port in the idea expressed above, namely, that in one of
the polar bodies half of the chromosomes pass out, so that
there is conceivably a separation of the maternal from the
paternal. If this were the case also in the spermatozoa, then
two of each four would be paternal and two maternal. This
is, however, a very different thing from supposing them to
be male and female, for it by no means follows, because
the chromosomes correspond to those of the father or of
the mother in the sum of their characters, that they are,
therefore, also male or female in regard to sex.

It has been pointed out already, that in most partheno-
genetic eggs only one polar body is extruded. There are,
it is true, a few apparent exceptions to this rule, but in most
cases it is certain that only one is extruded. In several
cases the beginning of the formation of the second matura-
tion division of the nucleus takes place, but after the chro-
mosomes have divided they come together again in the
nucleus. If each polar body be interpreted as equivalent
to a spermatozoon, then this result is rather a process of
self-fertilization than true parthenogenesis. It is, neverthe-
less, true that in some cases development seems to go on
. after both polar bodies have been extruded. Moreover, it
has been found possible to cause the eggs of the sea-urchin
to begin their development by artificial solutions after they
have extruded both polar bodies. A single spermatozoon
may also produce an embryo if it enters a piece of egg-pro-
toplasm without a nucleus. The last instance is a case of

Sex as an Adaptation 435

male parthenogenesis, and if the theory of the equivalency
of spermatozoon and egg be correct, this is what should
occur.

Quite recently, Cuénot, Beard, Castle, and Lenhossek have
contended that the differentiation of sex is the outcome of inter-
nal factors. They think that the view that sex is determined
by external agents is fundamentally erroneous. The fallacies
that have given rise to this conception, Castle points out, are,
first, that in animals that reproduce sometimes by partheno-
genesis and sometimes by fertilized eggs, the former process
is favored by good nutrition and the latter by poor nutrition.
This only means, in reality, Castle thinks, that parthenogenetic
reproduction is favored by external conditions, and this kind
of reproduction, he thinks, is a thing sad generis, and not
to be compared to the formation of more females in the
sexual forms of reproduction. There is no proof, how-
ever, that this is anything more than a superficial distinction,
and it ignores the fact that in ordinary cases the females
sometimes lay parthenogenetic eggs which differ, as far as we
can see, from eggs that are destined to be fertilized in no
important respect. More significant, it seems to me, is the
fact that only parthenogenetic females develop the following
spring from the fertilized eggs of the last generation of the
autumn series, whose origin is described to be due to lack
of food. We find, in the case of aphids, that unfertilized
parthenogenetic eggs and also fertilized eggs give rise to
females only, while a change in the amount of food causes
the parthenogenetic eggs to give rise both to males and to
females. This point is not, I think, fully met by Castle, for
even if the change in food does not, as he claims, cause only
one sex to appear, yet lack of food does seem to account for
the appearance of the males at least.

The other fallacy, mentioned by Cuénot, is that the excess
of males that has been observed when the food supply is
limited is due to the early death of a larger percentage

436 Evolution and Adaptation

of females, which require more food, but this still fails to
account for the excess of females when more food is given,
provided Yung’s experiments on tadpoles are correct. It
may be, however, in the light of Pfitiger’s results, that there
has been some mistake in the experiments themselves.

We may now proceed to examine Castle’s argument,
attempting to show in what way sex is predetermined in the
embryo. His hypothesis rests on the three following prem-
ises: “(1) the idea of Darwin, that in animals and plants of
either sex the characters of the opposite sex are latent ; (2) the
idea of Mendel, that in the formation of the gametes [germ-
cells] of hybrids a segregation of the parental characters
takes place, and when in fertilization different segregated
characters meet, one will dominate, the other become latent
or recessive; (3) the idea of Weismann, that in the matura-
tion of egg and spermatozoon a segregation is attended by a
visible reduction in the number of chromosomes in the
germinal nuclei.”

Expressed in a somewhat more general way, Castle suggests
that each egg and each spermatozoon is either a male or a
female germ-cell (and not a mixture of the two), and when a
female egg is fertilized by a male spermatozoon, or vice versa,
the individual is a sexual hybrid with one sex dominating
and the other latent. The assumption that there are two
kinds of. eggs, male and female, and two kinds of spermatozoa,
male and female, is not supported by any direct or experi-
mental evidence. Moreover, in order to carry out the
hypothesis, it is necessary to make the further assumption
that a female egg can only be fertilized by a male spermato-
zoon, and a male egg by a female spermatozoon. While
such a view is contrary to all our previous ideas, yet it must
be admitted that there are no facts which disprove directly
that such a selection on the part of the germ-cells takes place.
If these two suppositions be granted, then Castle’s hypothesis
is as follows :—

Sex as an Adaptation 437

In order that half of the individuals shall become males
and half females it is necessary to assume that in some
individuals the male element dominates and in others the
female, and since each fertilized egg contains both male
and female elements, it is necessary to assume that either
the egg or the spermatozoon contains the dominating ele-
ment.

Castle supposes that in hermaphroditic organisms the two
characters “ exist in the balanced relationship in which they
were received from the parents,” but, as has just been stated,
in unisexual forms one or the other sex dominates, except of
course in those rare cases, as in the bees and ants, where
half of the body may bear the characters of one sex, and the
other half that of the other sex.

In parthenogenetic species the female character is supposed
to be uniformly stronger, so that it dominates in every contest,
“for the fertilized egg in such species develops invariably
into a female.” Under certain circumstances, as Castle
points out, the parthenogenetic female produces both males
and females, and this is also true in the occasional develop-
ment of the unfertilized egg of the silkworm moth, and of
the gypsy moth, in which both male and female individuals
are produced by parthenogenesis. These facts show that
even in unfertilized eggs both sexes are potentially present ;
but this might be interpreted to mean that some eggs are
male and some female, rather than that each egg has the
possibility of both kinds of development. If, however, one
polar body is retained in these parthenogenetic eggs, then er
hypothese each egg would contain the potentialities of both
sexes (if the polar body were of the opposite sex character).
It seems necessary to make this assumption because in some
parthenogenetic forms males and females may be produced
later by each individual, as in the aphids, and this could not
occur if we assume that some parthenogenetic eggs are purely
male and some female.

438 Evolution and Adaptation ’

Castle assumes, in fact, that in animals like daphnids and
rotifers one polar body only is extruded, and the other (the
second) is retained in the egg, and hence the potentiality of
producing males is present. In the honey-bee, on the con-
trary, Castle assumes that both polar bodies are extruded
in the unfertilized egg (and there are some observations that
support this idea), and since only males are produced from
these, he believes it is the female element that has been sent
out into the second polar body. This hypothesis is necessary,
because Castle assumes that when both elements are pres-
ent in the bee’s eggs, the female element dominates. ‘“ Hence,
if the egg which has formed two polar cells develops with-
out fertilization, it must develop into a male. But if such an
egg is fertilized, it invariably forms a parthenogenetic female
?(o), that is, an individual in which the male character is
recessive. Accordingly the functional spermatozoon must
in such cases invariably bear the female character, and this is
invariably dominant over the male character when the two
meet in fertilization.”

If it should prove generally true that the size of the egg
is one of the factors determining the sex, we have still the
further question to consider as to whether some eggs are
bigger because they are already female, or whether all eggs
that go beyond a certain size are females, and all those that
fail to reach this are males. If this is the case, an animal
might produce more females if the external conditions were
favorable to the growth of the eggs, and if in some cases
these large eggs were capable of developing, parthenogenetic
races might become established. Should, however, the condi-
tions for nutrition become less favorable, some of the eggs
might fall below the former size and produce males. It is
not apparent, however, why all the fertilized autumn eggs of
the aphids should give rise to females, for although these
eggs are known to be larger than the summer eggs, yet they
are produced under unfavorable conditions.

Sex as an Adaptation 439

The preceding discussion will show how far we still are
from knowing what factors determine sex. Castle’s argu-
ment well illustrates how many assumptions must be made
in order to make possible the view that sex is a predeter-
mined quality of each germ-cell. Even if these assumptions
were admissible, we still return to the old idea that the fer-
tilized egg has both possibilities, and something determines
which shall dominate. Until we have ascertained definitely
by experimental work whether the sex in some forms can be
determined by external conditions, it is almost worthless to
speculate further. Whatever decision is reached, the conclu-
sion will have an immediate bearing on the question to be
next discussed. Meanwhile, we can at least examine some
of the theories that have been advanced as to what advan-
tage, if any, has been gained by having the individuals of
many classes divided into two kinds, male and female.

SEX AS A PHENOMENON OF ADAPTATION

Of what advantage is it to have the individuals of many
species separated into males and females? It is obviously
a disadvantage from the point of view of propagation to have
half of the individuals incapable of producing young, and the
other half also incapable of doing so, as a rule, unless the
eggs are fertilized by the other sex. Is there any compen-
sation gained because each new individual arises from two
parents instead of from one? Many answers have been
attempted to these questions.

At the outset it should be recognized that we are by no
means forced to assume, as is so often done, that because
there is this separation of the sexes it must have arisen on
account of its advantage to the species. Whether the result
may be of some benefit regardless of how it arose, may
be an entirely different question. It would be extremely
difficult to weigh the relative advantages (if there are any)

440 Evolution and Adaptation

and disadvantages (that are obvious as pointed out above),
nor is it probable that in this way we can hope to get a final
answer to our problem. We may begin by examining some
of the modern hypotheses that have been advanced in this
connection.

Darwin has brought together a large number of facts
which appear to show the beneficial effects of the union of
germ-cells from two different individuals. Conversely, it is
very generally believed, both by breeders and by some experi-
menters, that self-fertilization in the case of hermaphroditic
forms leads, in many cases, though apparently not in all, to
the production of less vigorous offspring. Darwin’s general
position is that it is an advantage to the offspring to have
been derived from two parents rather than to have come
from the union of the germ-cells of the same individual, and
he sees, in the manifold contrivances in hermaphroditic ani-
mals and plants to insure cross-fertilization, an adaptation for
this purpose.

This question of whether self-fertilization is less advan-
tageous than cross-fertilization is, however, a different ques-
tion from that of whether 2on-sexual methods of reproduction
are less advantageous than sexual ones. Since some plants,
like the banana, have been propagated for a very long time
solely by non-sexual methods without any obvious detriment
to them, it is at first sight not easy to see what other advan-
tage could be gained by the sexual method. The case of the
banana shows that some forms do not require a sexual
method of propagation. Other forms, however, are so con-
stituted, as we find them, that they cannot reproduce at the
present time except by the sexual method. In other words,
the latter are now adapted, as it were, to the sexual method,
and there is no longer any choice between the two methods.
The question of whether a non-sexual form might do better
if it had another method of propagation is not, perhaps, a
profitable question to discuss.

Sex as an Adaptation 441

What we really need to know is whether or not the sexual
method was once acquired, because it was an advantage
to a particular organism, or to the species to reproduce in
this way. It is assumed by many writers that this was the
case, but whether they have sufficient ground for forming
such an opinion is our chief concern here. On the other
hand, it is conceivable, at least, that if the sexual method
once became established, it might continue without respect to
any superiority it gave over other methods, and might finally
become a necessary condition for the propagation of particu-
lar species. Thus the method would become essential to
propagation without respect to whether the species lost more
than it gained. Whichever way the balance should turn, it
might make little difference, so long as the species was still
able to propagate itself.

Brooks made the interesting and ingenious suggestion that
the separation of the sexes has been brought about as a sort
of specialization of the individuals in two directions. The
male cells are supposed to accumulate the newly acquired
characters, and represent, therefore, the progressive element
in evolution. The female cells are the conservative element,
holding on to what has been gained in the past. It does not
seem probable, in the light of more recent work, that this
is the function of the two sexes, and it is unlikely that we
could account for the origin of the two sexes through the
supposed advantage that such a specialization might bring
about. A number of writers, Galton, Van Beneden, Biitschli,
Maupas, and others, have looked at the process of sexual re-
production as a sort of renewal of youth, or rejuvenescence
of the individuals. There is certainly a good deal in the
process to suggest that something of this sort takes place,
although we must be on our guard against assuming that
the rejuvenescence is anything more than the fulfilment of a
necessary stage in the life history. Weismann has ridiculed
this suggestion on the ground that it is inconceivable that

442 Evolution and Adaptation

two organisms, decrepid with old age, could renew their
youth by uniting. Two spent rockets, he says, cannot be
imagined to form a new one by combining. There is ap-
parent soundness in this argument, if the implication is taken
in a narrow physical sense. If, on the other hand, the egg is
so constituted that at a certain stage in its development an
outside change is required to introduce a new phase, then
the conception of rejuvenescence does not appear in quite
so absurd a light.

This hypothesis of rejuvenescence is based mainly on cer-
tain processes that take place in the life history of some of
the unicellular animals. Let us now see what this evidence is.
The results of certain experiments carried out by Maupas on
some of the ciliate protozoans have been fruitful in arousing
discussion as to the ultimate meaning of the sexual process.
Maupas’ experiments consisted in isolating single individuals,
and in following the history of the descendants that were
produced non-sexually by division. He found that the de-
scendants of an individual kept on dividing, but showed no
tendency to unite with each other. After a large number of
generations had been passed through (in Stylonychia pustu-
fata, between 128 and 175; in Leucophys patila, 300 to 450;
and in Onychodromus grandis, 140 to 230 generations), the
division began to slow down, and finally came to a stand-
still. Maupas found that if he took one of these run-down
individuals, and placed it with another in the same condition
from another culture, that had had a different parentage, the
two would unite and the so-called process of conjugation take
place. This process consists for the species used, in the tem-
porary union and partial fusion of the protoplasm of the two
individuals, of an interchange of micronuclei, and of a fusion,
in each individual, of the micronucleus received from the
other individual with one of its own. The. individuals then

separate, and a new nucleus (or nuclei) is formed out of the
fused pair.

Sex as an Adaptation 443

The individuals in question, in which this interchange of
micronuclei has taken place, undergo a change, and behave
differently from what they did before. They feed, become
larger and less vacuolated, and are more active. They soon
begin once more to divide. Maupas found that an individual
that has conjugated will run through a new cycle of divisions,
which will, however, after a time also slow down, unless con-
jugation with another individual having a different history
takes place. If conjugation is prevented, the individual will
die after a time. These results seemed to show that the
division phase of the life history cannot go on indefinitely,
and that through conjugation the individual is again brought
back to the starting-point.

Quite recently Calkins has carried out a somewhat similar
series of experiments, which have an important bearing on
the interpretation of Maupas’ results. The experiment of
isolating an individual and tracing the career of its descen-
dants was repeated with the following results: two series
were started, the original forms coming from different locali-
ties. Of their eight descendants four of each were isolated.
The remaining four of each set were kept together as stock
material. The rate of division was taken as the measure of
vitality. The animals divided more or less regularly from
February to July. After each division (or sometimes after
two divisions) the individuals were separated. About the
30th of July the paramoecia began to die “at an alarming
rate, indicating that a period of depression had apparently
set in, or degeneration in Maupas’ sense.’ Up to this time
the animals had been living in hay infusion, renewed every
few days, from which they obtained the bacteria on which
they feed. Calkins tried the effect of putting the weakened
parameecia into a new environment. Infusion of vegetables
gave no good results, but meat infusions proved successful.
“The first experiment with the latter was with teased liver,
which was added to the usual hay infusion. The result was

444 Evolution and Adaptation

very gratifying, for the organisms began immediately to grow
and to divide, the rate of division rising from five to nine divi-
sions in successive ten-day periods.” This beneficial effect
was not lasting, however, and after ten days the paramoecia
began to die off faster than before, and the renewed applica-
tion of the liver extract failed to revive them. A number of
other extracts were then tried without effect. Finally they
were transferred to the clear extract of lean beef in tap water.
The effect of this medium was interesting, for, although it
restored the weakened vitality, there was no rapid increase
in the rate of division, as when first treated with the teased
liver. The infusoria were, however, now large and vigorous,
and did not die unless transferred from the beef medium to
the usual hay infusion. “When this was attempted, they
would become abnormally active and would finally die. The
division rate gradually increased during the month of August
until, in the last ten days, they averaged six generations.
Finally, in September, the attempts to get them back on the
old diet of hay infusion were successful, and then the divi-
sion rate went up at once to twelve times in ten days, and a
month later they were dividing at the rate of fifty times a
month.”

“These cultures went on well until December, when the
paramoecia began to die again. They were saved once more
with the beef extract, and when returned later to the hay
infusion continued through another cycle of almost three
months. Some of these were treated, once a week for
twenty-four hours, with the beef extract, and while the two
sets ran a parallel course at first, those kept continuously in
the hay infusion died after a time, but those that had been
put once a week into the beef extract (which had been
stopped, however, in March) continued their high rate of
division throughout the period of decline of their sister cells,
and did not show signs of diminished vitality until the first
period in June.” At this time their rate of division increased

Sex as an Adaptation 445

rapidly. They were put back into the beef extract, but it
failed now to have a beneficial effect, and the animals con-
tinued to die at a rapid rate. To judge from the appearance
of the organisms the new decline was due to a different cause;
for, while in the former periods the food vacuoles contained
undigested food, at this period the interior was free from food
masses. The protoplasm became granular and different from
that of a healthy individual. None of the former remedies
were now of any avail. ‘When the last of the B-series stock
had died in the five hundred and seventieth generation (June
16th), it looked as though the cultures were about to come to
an end.” Extract of the brain and of the pancreas were then
tried. To this a favorable response took place at once. The
organisms became normal in appearance and began to divide.
After forty-eight hours’ treatment they were returned to
the usual hay infusion. Here they continued to multiply and
reached on June 28th the six hundred and sixty-fifth genera-
tion.

There can be no doubt that the periods of depression that
appear in these infusoria kept in cultures can be successfully
passed if the animals are introduced into a new environment.
Without a change of this sort they will die. Calkins
thinks that the effect is produced, not by the new kind of
food that is supplied, but by the presence of certain chemi-
cal compounds. The beef extract “does not have a direct
stimulating effect upon the digestive process and upon divi-
sion, for, while the organisms are immersed in it, there is a
very slow division rate; when transferred again to the hay
infusion, however, they divide more rapidly than before.”

This brings us back to the idea of the “ renewal of youth”
through conjugation. Maupas claimed that union of individ-
uals having the same immediate descent is profitless. Calkins
suggests that this is due to the similarity in the chemical
composition of the protoplasm of the two individuals. When
in nature two individuals that have lived under somewhat

446 Evolution and Adaptation

different conditions conjugate, the result should be benefi-
cial, since there takes place the commingling of different
protoplasms.

Calkins’s work has shown that by means of certain sub-
stances much the same effect can be produced as that which
is supposed to follow from the conjugation of two unrelated
individuals. The presumption, therefore, is in favor of the
view that the two results may be brought about in the same
way, although we should be careful against a too ready accep-
tation of this plausible argument; for we have ample evi-
dence to show that many closely similar (if not identical)
responses of organisms may be brought about by very
different agencies. The experiments seem to indicate that
paramcecium might go on indefinitely reproducing by divi-
sion, provided its environment is changed from time to time.
If this is true, it is conceivable that the same thing is accom-
plished through conjugation. In the light of this possible
interpretation much of the mystery connected with the term
rejuvenescence is removed, for we see that there is nothing
in the process itself except that it brings the organism into a
new relation with other substances. Difficult as it assuredly
is to understand how this benefits the animal, the experi-
mental fact shows, nevertheless, that such a change is for
its good. That there is really nothing in the process of
conjugation itself apart from the difference in the constitu-
tion of the conjugating individuals is shown by the result
that the union of individuals having the same history and
kept under the same conditions is of no benefit.

Can we apply this same conception to the process of
fertilization in the higher animals and plants? Is the sub-
stance of which their bodies are made of such a sort that it
cannot go on living indefinitely under the same conditions,
but must at times be supplied with a new environment? If
this could be established, we could see the advantage of
sexual reproduction over the non-sexual method. It would

Sex as an Adaptation 447

be extremely rash at present to make a generalization of this
kind, for there are many forms known in which the only
method of propagation that exists is the non-sexual one. In
other words, there are no grounds for the assumption that
this is a necessary condition for all kinds of protoplasm, but
only for certain kinds.

In the insects, crustaceans, rotifers, and in some plants there
are a few species whose egg develops without fertilization.
This makes it appear probable that the particular kind of
protoplasm of these animals does not absolutely require
union from time to time with the protoplasm of another
individual having a somewhat different constitution.

There is also an interesting parallel between the effects of
solutions on the protozoans in Calkins’s experiments and cer-
tain results that have been obtained in artificial partheno-
genesis. It has been stated, that by brushing the unfertilized
eggs of the silkworm moth a larger percentage will develop
parthenogenetically ; and more recently it has been shown by
Matthews that by agitation of the water in which the un-
fertilized eggs of the starfish have been placed many of
them will begin their development. It was first shown by
Richard Hertwig that by putting the unfertilized eggs of the
sea-urchin in strychnine solutions, they will begin to segment,
and I obtained the samé results much better by placing the
eggs.in solutions of magnesium chloride. Loeb then suc-
ceeded in carrying the development to a later stage by using
a different strength of the same solution, as well as by other
solutions. Under the most favorable circumstances some of
the eggs may produce larve that seem normal in all respects,
but whether they can develop into adult sea-urchins has not
yet been shown.

These results indicate that one at least of the factors of
fertilization is the stimulus given to the egg. On the other
hand, the lack of vigor shown by many eggs that have been
artificially fertilized indicates that some other result is also

448 Evolution and Adaptation

accomplished by the normal method of fertilization that is
here absent. This may mean no more than that as yet we
have not found all the conditions necessary to supply the
place of the spermatozoon.

In our study of the phenomena of adaptation we have
found that sometimes the adaptation is for the benefit of the
individual and at other times for the benefit of the species.
May it not be true also that the process of sexual reproduc-
tion has more to do with a benefit conferred on the race
rather than on the individual? In fact, Weismann has
elaborated a view based on the conception that the process
of sexual reproduction is beneficial to the race rather than to
the individual. His idea, however, is not so much that the
result is of direct benefit to a particular species, as it is ad-
vantageous to the formation of new species from the original
one. In a sense this amounts, perhaps, to nearly the same
thing, but in another sense the idea involves a somewhat
different point of view.

According to his view “the deeper significance of conjuga-
tion” and of sexual reproduction is concerned “with the
mingling of the hereditary tendencies of two individuals.”
In this way, through the different combinations that are
formed, variations which he supposes are indispensable for the
action of natural selection originate. The purpose of the
sexual process is solely, according to Weismann, to supply
the variations for natural selection. If it be asked how this
process has been acquired for the purpose of supplying nat-
ural selection with the material on which it can work, we find
the following reply given by Weismann. “But if amphi-
mixis [by which he means the union of sex-cells from different
individuals] is not absolutely necessary, the rarity of purely
parthenogenetic reproduction shows that it must have a wide-
spread and deep significance. Its benefits are not to be
sought in the single individual; for organisms can arise by
agamic methods, without thereby suffering any loss of vital

Sex as an Adaptation 449

energy ; amphimixis must rather be advantageous for the
maintenance and modification of species. As soon as we
admit that amphimixis confers some such benefits, it is clear
that the latter must be augmented, as the method appears
more frequently in the course of generations ; hence we are
led to inquire how nature can best have undertaken to give
this amphimixis the widest possible range in the organic
world.” Nature, Weismann says, could find no more effec-
tual means of bringing about the union of the sexual cells
than by rendering them incapable of developing alone.
“The male germ-cells, being specially adapted for seeking
and entering the ovum, are, as a rule, so ill provided with nu-
triment that their unaided development into an individual
would be impossible ; but with the ovum it is otherwise, and
accordingly the ‘reduction division’ removes half the germ-
plasm and the power of developing is withdrawn.” It can
scarcely be claimed, in the light of more recent discoveries,
that the reduction division takes place in order to prevent the
development of the ovum, for how then could we explain the
corresponding division of the male germ-cells?

Whatever means has been employed to bring about the pro-
cess of sexual reproduction, the guiding principle is supposed
by Weismann to be natural selection as stated in the following
paragraph: “If we regard amphimixis as an adaptation of
the highest importance, the phenomenon can be explained in a
simple way. I only assume that amphimixis is of advantage
in the phyletic development of life, and furthermore that it is
beneficial in maintaining the level of adaptation, which has
been once attained, in every single organism; for this is as
dependent upon the continuous activity of natural selection as
the coming of new species. According to the frequency
with which amphimixis recurs in the life of a species, is the
efficiency with which the species is maintained; since so
much the more easily will it adapt itself to new conditions of
life, and thus become modified.”

2G

450 Evolution and Adaptation

Thus we reach the somewhat startling conclusion that
through natural selection the germ-cells and their protozoan
prototypes have been rendered incapable, through natural
selection, of reproducing by non-sexual methods, in order
that variations may be supplied for the farther action of
this same process of natural selection. The speculation has
the appearance of arguing in a circle, although if it were
worth the attempt an ingenious mind might perhaps succeed
in showing that such a thing is not logically inconceivable.

It seems strange that a claim of this sort should have been
made, when it is so apparent that the most immediate effect
of intercrossing is to swamp all variations that depart from
the average. Even if it were true that new combinations of
characters would arise through the union of the germ-cells of
two different animals, it is certainly true that in the case of fluc-
tuating variations this new combination would be lost by later
crossing with average individuals. Moreover, it is well known
that variations occur amongst forms that are produced asex-
ually. On the whole, it does not seem to be a satisfactory
solution of the problem to assume that sexual reproduction
has been acquired in order to supply natural selection with
material on which it may work.

Our examination of the suggestions that have been made
and of the speculation indulged in, as to what benefit the
process of sexual reproduction confers on the animals and
plants that make use of this method of propagation, has failed
to show convincingly that any advantage to the individual or
to the species is the outcome. This may mean, either that
there is no advantage, or that we have as yet failed to
understand the meaning of the phenomenon. The only light
that has been thrown on the question is that a certain amount
of renewed vigor is a consequence of this process, but we
cannot explain how this takes place. There is also the sug-
gestion that the union of different cells produces the same
beneficial effect as a change in the conditions of life produces

Sex as an Adaptation 451

on the organism. The bad effects of close interbreeding that
seem sometimes to follow is explicable on this view. This,
it seems to me, is the most plausible solution of the question
that has been advanced; but, even if this should prove to be
the correct view, we need not assume that the process has
been acquired on account of this advantage, for there is
nothing to show that it has been acquired in this way.

CHAPTER XIII
SUMMARY AND GENERAL CONCLUSIONS

Tue question of the origin of the adaptations with which
the last three chapters have so largely dealt is one of the
most difficult problems in the whole range of biology, and
yet it is one whose immense interest has tempted philoso-
phers in the past, and will no doubt continue to excite the
imagination of biologists for many years to come. No pre-
tence has been made in the preceding pages to account for
the cause of a single useful variation. We have examined
the evidence, and from this we believe the assumption justi-
fied that such variations do sometimes appear. The more
fundamental question as to the origin of these variations has
not been taken up, except in those cases in which the adap-
tive response appeared directly in connection with a known
external cause. But these kinds of responses do not appear
to have been the source of the other adaptations of the
organic world. Our discussion has been largely confined to
the problem of the widespread occurrence of adaptation in
living things, and to the most probable kinds of known
variations that could have given rise to these adaptations.
But, to repeat, we have made no attempt to account for the
causes or the origin of the different kinds of variation.

Nageli, in speaking of the methods of the earlier theorists
in Germany, remarks with much acumen: “We might have
expected that after the period of the Nature-philosophizers,
which in Germany crippled the best forces that might have
been used for the advance of the science, we should have
learnt something from experience, and have carefully guarded

452

Summary and General Conclusions 453

the field of real scientific work from philosophical speculation.
But the outcome has shown that, in general, the philosophi-
cal, philological, and zesthetic expression always gets the upper
hand, and a fundamental and exact treatment of scientific
questions remains limited to a small circle. The public at
large always shows a distinct preference for the so-called
idealistic, poetic, and speculative modes of expression.” The
truth of this statement can scarcely be doubted when in our
own time we have seen more than once the same method
employed with great public applause. Nowhere is this more
apparent than in the writings of many of the followers of
Darwin in respect to the adaptations of living things. To
imagine that a particular organ is useful to its possessor, and
to account for its origin because of the imagined benefit con-
ferred, is the general procedure of the followers of this school.
Although protests have from time to time been raised against
this unwarrantable way of settling the matter, they have been
largely ignored and forgotten. The fallacy of the argument
has, for example, been admirably pointed out by Bateson in
the following statement :! “In examining cases of variation
I have not thought it necessary to speculate on the useful-
ness or harmfulness of the variations described. For reasons
given in Section II such speculation, whether applied to nor-
mal structures or to variation, is barren and profitless. If
any one is curious on these questions of Adaptation, he may
easily thus exercise his imagination. In any case of Varia-
tion there are a hundred ways in which it may be beneficial
or detrimental. For instance, if the ‘hairy’ variety of the
moor-hen became established on an island, as many strange
varieties have been, I do not doubt that ingenious persons
would invite us to see how the hairiness fitted the bird in
some special way for life in that island in particular. Their
contention would be hard to deny, for on this class of specu-
lation the only limitations are those of the ingenuity of the
1 “ Materials for the Study of Variation.”

454 Evolution and Adaptation

author. While the only test of utility is the success of the
organism, even this does not indicate the utility of one part
of the economy, but rather the net fitness of the whole.”
Keeping in mind the admonitions contained in the two pre-
ceding quotations, let us pass in review and attempt to ana-
lyze more fully the different points that have been considered
in the preceding chapters.

It has been pointed out that the evidence in favor of the
theory of evolution appears to establish this theory with great
probability, although a closer examination shows that we are
almost completely in the dark as to how.the process has
come about. For example, we have not yet been able to
determine whether the great groups of animals and plants
owe their resemblance to descent from a single original spe-
cies or from a large number of species. The former view is
more plausible, because on it we appear to be furnished with
a better explanation of resemblances as due to divergence
of character. Yet even here a closer scrutiny of the homol-
ogies of comparative anatomy shows that this explanation
may be more apparent than real. If discontinuous variation
represents the steps by which evolution has taken place, the
artificiality of the explanation is apparent, at least to a certain
degree.

Admitting that the theory of evolution is the most prob-
able view that we have to account for the facts, we next
meet with two further questions, — the origin of species and
the meaning of adaptation. These are two separate and dis-
tinct questions, and not one and the same as the Darwinian
theory claims. The fact that all organisms are more or
less adapted to live in some environment appears from our
examination to have no direct connection with the origin of
the adaptation, for, in the first place, it seems probable that,
in general, organisms do not respond adaptively to the envi-
ronment and produce new species in this way; and, in the
second place, there is no evidence to show that variation

Summary and General Conclusions 455

from internal causes is so regulated that only adaptive struc-
tures arise (although only adaptive ones may survive).

Our general conclusion is then as follows: A species does not
arise from another one because it is better adapted. Selection,
in other words, does not account for the origin of new species ;
and adaptation cannot be taken as the measure of a species.

It may sound like a commonplace to state that only those
individuals survive and propagate themselves that can find
some place in nature where they can exist and leave descen-
dants; and yet this statement may contain all that it is
necessary to assume, in order to account for the fact that
organisms are, on the whole, adapted. Let us see how this
view differs from the Darwinian statement of the origination
of new forms through a process of natural selection.

According to Darwin’s view of the origin of species, each
new species is gradually formed out of an older one, because
of the advantage that the new individual may have over the
parent form. Each step forward is acquired, because it
better adapts the organism to the old, or to a new set of
conditions. In contrast to this, I have urged that the for-
mation of the new species is, as a rule, quite independent
of its adaptive value in regard to the parent species. But
after it has appeared, its survival will depend upon whether
it can find a place in nature where it can exist and leave
descendants. If it should be well adapted to an environment,
it will be represented in it by a large number of individuals.
If it is poorly adapted, it may only barely succeed in existing,
and leave correspondingly fewer descendants. If its adap-
tiveness falls below a certain point, it can never get a perma-
nent foothold, however often it may appear. Thus the test
of survival determines which species can remain in existence,
and which cannot, but new species are not manufactured in
this way. How far subsequent variations may be supposed
to be determined by the survival of certain species and the
destruction of others will be discussed presently.

456 Evolution and Adaptation

The difference between the two points of view that we
are contrasting can be best brought out by considering
the two other kinds of selection which Darwin supposes to
have been at work ; namely, artificial and sexual selection.

Darwin thinks that the results of artificial selection are
brought about by the breeder picking out fluctuating varia-
tions. It appears that he has probably overestimated the
extent to which this process can be carried ; for while there
can be no doubt that a certain standard, or fixity of type,
can be obtained by selecting fluctuating variations, yet it
now seems quite certain that the extent to which this can be
carried is very limited. It appears that other factors have
also played an important réle ; amongst these the occasional
appearance of discontinuous variation, also the bringing under
cultivation of the numerous “smaller species” of De Vries,
or the so-called “single variations” of Darwin. Further,
the effects of intercrossing in all combinations of the above
forms of variations, followed by the selection of certain of
the new forms obtained, has been largely employed, and
also the direct influence of food and of other external con-
ditions, which may be necessary to keep the race up to a
certain standard, have played a part in some cases. The
outcome is, therefore, by no means so simple as one might
infer from Darwin’s treatment of the subject in his “ Origin
of Species.” For these reasons, as well as for others that
have been given, it will be evident that the process of arti-
ficial selection cannot be expected to give a very clear idea
of how natural selection could act.

It is, however, the process of sexual selection that brings
out in the strongest contrast the difference between Darwin's
main idea of natural selection and the law of the survival
of species. In sexual selection the competition is supposed
to be always between the individuals of the same species
and of the same sex. There can be no doubt in one’s
mind, after reading “The Descent of Man,” that Darwin

Summary and General Conclusions -457

held firmly to the belief that the. individual differences, or
fluctuating variations, furnish the material for selection.
In this way it could never happen that two competing
species could exterminate each other, because in the one
the males were better adorned, or killed each other off on
a larger scale, owing to the presence of special weapons of
warfare. It is clear that on the law of the survival of
species, secondary sexual characters cannot be supposed to
have evolved because of their value. Their origin is totally
inexplicable on this view. In fact, the presence of the
ornaments must -be in some cases injurious to the existence
of the species. The interpretation of this means, I think,
that individual competition cannot be as severe as Darwin
believed, and cannot lead to the results that he imagined
it does. For this reason it seemed important to make as
careful an examination of the claims of the theory of sexual
selection as possible, and I hope that the outcome of the
examination has shown quite definitely that the theory is
incompetent to account for the facts that it claims to explain.
It is certain in this case that we are dealing with a phe-
nomenon that must be studied quite apart from any selective
value that the secondary sexual organs may have. If this is
granted, it will be seen that there is here a wide field for
experimental investigation that is practically untouched.

It is evident that the first step that will clear the way toa
fuller understanding of the problem of evolution must be a
more thorough examination of the question of variation.
Darwin himself fully appreciated this fact, yet until within
the last fifteen years the study of variation has been largely
neglected. Witha fuller knowledge of the nature of fluctuat-
ing variation as the outcome of the studies of Galton, Pear-
son, De Vries, and others, and with a fuller knowledge of the
possibilities of discontinuous variation as emphasized by
‘Bateson and by De Vries, and, further, with a better knowl-
edge of some of the laws of inheritance in. these cases, we

458 Evolution and Adaptation

have begun to get a different conception of how evolution has
come about. It may be well, therefore, to go once more over
the main points in regard to the different kinds of variation.

While it has been found that no two individuals of a
species are exactly alike, yet, taken as a group, the variations
appear as though they followed the law of chance. The
descendants of the group show the same differences. Thus
the group as a whole appears constant, while the individuals
fluctuate continually in all directions. This is what we
understand by fluctuating variation. If the external condi-
tions are changed, a new “mode”? may appear, but the change
is generally only a temporary one, and lasts only as long as
the new conditions remain. Thus, while the direct influence
of the environment may show for a time, the result is tran-
sient. Even if it were permanent, there is no evidence that
the adaptation of organisms could be accounted for in this
way unless the response were useful. It appears that this
sometimes really occurs, especially in responses to tempera-
ture, to moisture, to the amount of salts in solution, to
poisonous substances, etc. In this way, one kind of adapta-
tion is brought about, but there is no evidence that the great
number of structural adaptations have thus arisen.

The Lamarckian principle of the inheritance of acquired
characters has also been supposed by many writers to be an
important source of adaptive variation. An examination of
this theory is not found to inspire confidence. We have, there-
fore, eliminated this hypothesis on the ground that it lacks
evidence in its favor, and also because it appears improbable
that in this way many of the adaptations in organisms could
have been acquired.

Finally, there is the group of discontinuous variations. Of
these there may be several kinds, and there is some evidence
showing that there are such. For the present we may in-
clude all the different sorts under the term mutation, mean-
ing that the new character or group of characters suddenly

Summary and General Conclusions 459

appears, and is inherited in its new form. From the re-
sults of De Vries it appears that mutations are sometimes
scattering, at least in the case of the evening primrose.
From such scattering mutations, the smaller species or
varieties (in so far as these do not depend on local con-
ditions) arise. There is here an important point of agree-
ment with Darwin’s idea in regard to evolution, inasmuch as
he supposed that varieties are incipient species. Our point
of view is different, however, in that we do not suppose these
varieties (mutations) to have been gradually formed out of
fluctuating variations by a process of selection, but to have
arisen at once by a single mutation. It also appears that in
some cases a single new mutation may develop in a species.
We may suppose that the new form might in such a case
supplant the parent species by absorbing it, or both may go
on living side by side, as will be more likely the case if they
are adapted to somewhat different conditions.

A number of writers have supposed that evolution marches
steadily forward toward its final goal, which may even
lead in some cases to the final but inevitable destruction
of the species. By certain writers this view has been called
orthogenesis, although at other times the idea is not so much
that there is advance in a straight line, as advance in all
directions. This appears to be Nageli’s view. It gives a
splendid picture of the organic world, as irresistibly marching
toward its goal, —a relentless process in some cases, leading
to final annihilation, a beneficent process in other cases,
leading to the fullest perfection of form of which the type is
capable. Compared with the vacillating progress which is
supposed to be the outcome of individual selection, this view
of progression has a grandeur that appeals directly to the imag-
ination. We must be guided, however, by evidence, rather
than by sentiment. The case will, moreover, bear closer
scrutiny. If evolution has indeed taken place by the survi-
val of a series of mutations, whose origin has no connection

460 Evolution and Adaptation

with their value, does not this in the end amount to nearly
the same thing as maintaining that evolution of organisms
has been a steady progress forward, —a progress undirected
by external forces, but the outcome of internal development ?
Admitting that innumerable creations have been lopped off,
because they could find no foothold, yet, as Nageli points out,
the result is that, instead of a dense tangle of forms, there
has been left relatively few that have been found capable of
existing, — those that have found some place in which they
can live and leave progeny. From this point of view it
may appear, at first thought, that the idea of evolution
through mutations involves a fundamentally different view
from that of the Darwinian school of selection; but in so far
as selection also depends on the spontaneous appearance of
fluctuating variations, the same point of view is to some ex-
tent involved, —only the steps are supposed to be smaller.
This point is usually ignored and passed over in silence by
the Darwinians, but, as Wigand has pointed out, it makes
very little difference whether the stages in the process of
evolution are imagined to be very small or somewhat larger,
so long as they are spontaneous. Selection does not do more
than determine the survival of what is offered to it, and does
not create anything new.

It is true that if the fluctuating variations that are selected
be connected by very slight differences with an almost con-
tinuous series of other forms, and if little by little such a
series be advanced in a given direction by selection, we get
the idea of a continuity, whose advance is determined by selec-
tion. It is this conception that appears to give the theory of
natural selection a creative power, which in reality it does
not possess, and certainly not in the modified form in which
the theory was finally left by Darwin. For Darwin found
himself forced to admit that, unless a very considerable num-
ber of individuals varied at the same time and in the same
direction, the formation of new species could not take place, and

Summary and General Conclusions 461

this idea of many individuals varying at the same time, and
in the same direction, at once involves the conception that
evolution moves forward by some force residing in the organ-
ism, driving it forwards or backwards. Instability comes,
perhaps, nearer to expressing this idea than any other term,
and yet to evolve from a protozoan to a man implies the idea
of something more than simple unstableness.

The idea that Weismann has touched upon in this connec-
tion, namely, that the survival of a given form determines the
future course of evolution for that form, is very plausible, and
also fits in well with the results of our experience in the field
of the inheritance of variations. We see new variations or mu-
tations departing in some or in many characters from the
original type, apparently by new combinations or perturba-
tions of those already present. We never expect to see a
bird emerge from the egg of an alligator. Thus it appears
that by the survival of certain forms the future course of
evolution is determined in so far as the new types of muta-
tion are thereby limited. Weismann means, however, that
in this way new plus or minus steps will be indefinitely deter-
mined amongst the new fluctuating variations, but this state-
ment is contradicted by our experience of the results of
artificial selection. The upper limit does not keep on pushing
out indefinitely in the direction determined by the first selec-
tion, but is soon brought to a standstill. So that, as far as
Weismann’s hypothesis is concerned, the idea appears to have
no special value. On the other hand, this idea may be fruitful
if applied to mutations, but here unfortunately we have not
sufficient experience to guide us, and we do not know defi-
nitely whether a new character that appears as a mutation
will be more likely, in subsequent mutations, to go on increas-
ing in some of the descendants. Thus, while the mutation
theory must assume that some new characters will go on
heaping up, we lack the experimental evidence to show that
this really occurs. It would be also equally important to

462 Evolution and Adaptation

determine, whether, if after several mutations have succes-
sively appeared in the same direction, there would be an
established tendency to go on in the same direction in some
of the future mutations. But here again we must wait until
we have more data before we attempt to build up a theory
on such a basis.

The attacks on the Darwinian school by the followers of
the modern school of experimentalists are with few exceptions
based on the assumption that the natural selectionists pre-
tend that their principle is a sort of creative force, —a factor
in evolution in the sense of being an active agent. This
assumption of the selectionists has led many of them to ig-
nore a fundamental weakness of their theory, namely, the
origin of the variations themselves, although Darwin did
not overlook or ignore this side of the problem, or fail to
realize its importance, as have some of his more ardent, but
less critical, followers. They have contented themselves, asa
rule, with pointing out that certain structures are useful, and
this has seemed to them sufficient proof that the structures
must have been acquired because of their value. In contrast
to this complacency of the selectionists, we find here and
there naturalists who have, from time to time, insisted that
the scientific problem of evolution is not to be found in the
principle of selection, but in the origin of the variations
themselves. _ It will be clear, from what has been said, that
this is our position also, and for us adaptation itself does not
appear to be any more a problem that can be examined by
scientific methods, than the lack of adaptation. The causes
of the change of whatever kind should be our immediate quest.
The destruction of the unfit, because they can find no place
where they can exist, does not explain the origin of the fit.

Over and beyond the primary question of the ovigiz of the
adaptive, or non-adaptive, structure is the fact that we find
that the great majority of animals and plants show distinct
evidence of being suited or adapted to live in a special envi-

Summary and General Conclusions 463

ronment, z.¢. their structure and their responses are such that
they can live and leave descendants behind them. I can see
but two ways in which to account for this condition, either
(1) teleologically, by assuming that only adaptive, variations
arise, or (2) by the survival of only those mutations that are
sufficiently adapted to get a foothold. Against the former
view is to be urged that the evidence shows quite clearly
that variations (mutations) arise that are not adaptive. On
the latter view the dual nature of the problem that we have
to deal with becomes evident, for we assume that, while the
origin of the adaptive structures must be due to purely
physical principles in the widest sense, yet whether an organ-
ism that arises in this way shall persist depends on whether
it can find a suitable environment. This latter is in one
sense selection, although the word has come to have a differ-
ent significance, and, therefore, I prefer to use the term
survival of species.

The origin of a new form and its survival after it has
appeared have been often confused by the Darwinian school
and have given the critics of this school a fair chance for ridi-
culing the selection theory. The Darwinian school has sup-
posed that it could explain the origin of adaptations on the
basis of their usefulness. In this it seems to me they are
wrong. Their opponents, on the other hand, have, I believe,
gone too far when they state that the present condition of
animals and plants can be explained without. applying the
test of survival, or in a broad sense the principle of selection
amongst species.

It will be clear, therefore, in spite of the criticism that I
have not hesitated to apply to many of the phases of the selec-
tion theory, especially in relation to the selection of the indi-
viduals of a species, that I am not unappreciative of the great
value of that part of Darwin’s idea which claims that the con-
dition of the organic world, as we find it, cannot be accounted
for entirely without applying the principle of selection in one

464 Evolution and Adaptation

form or another. This idea will remain, I think, a most
important contribution to the theory of evolution, We may
sum up our position categorically in the following statements:

Animals and plants are not changed in this or in that part
in order to become better adjusted to a given environment,
as the Darwinian theory postulates. Species exist that are
in some respects very poorly adapted to the environment in
which they must live. If competition were as severe as the
selection theory assumes, this imperfection would not exist.

In other cases a structure may be more perfect than the
requirements of selection demand. We must admit, there-
fore, that we cannot measure the organic world by the meas-
ure of utility alone. If it be granted that selection is not a
moulding force in the organic world, we can more easily |
understand how both less perfection and greater perfection
may be present than the demands of survival require.

If we suppose that new mutations and “definitely” in-
herited variations suddenly appear, some of which will find
an environment to which they are more or less well fitted,
we can see how evolution may have gone on without assum-
ing new species have been formed through a process of
competition. Nature’s supreme test is survival. She makes
new forms to bring them to this test through mutation, and
does not remodel old forms through a process of individual
selection.

INDEX

Acclimatization, 319. Beard, 210, 211, 216.

Acorn, 24. Beard, J., 435.

Acracids, 160. Bee, 2, 3, 19, 143, 179, 303, 350, 406, 420,
Adaptation, definition of, 1. 421, 425, 438.

Adjustments, individual, 12, Beethoven, 218.

Agassiz, I, 44, 61. Beetles, 182, 183, 189.

Agelzeus, 173. Bell-bird, 198.

Alcohol, 13. Beneden, Van, 441.

Algze, red, 9. Berbura goat, 208.

Alkaloids, 13. Biogenetic Law, 71.

Allen, 173, 307-310, Birds, 6, 407; definition of group, 36;
Allolobophora, 380. evolution of, 41; instincts of young, 4;
Alpheus, 344. nest, 4; of paradise, 6; teeth of, 301;

Ammophila, 5.
Ammotragus, 208,
Ampelopsis, 403.
Amphimixis, 448-449. .
Amphioxus, 399.

Ancon race, 315-316.
Angiostomum, 422.
Anguillidze, 320.

Annelids, 19, 20.

Anolis, 10, 194.

Ant-eater, 227, 228.
Antelope, 6, 206, 208,
.Antitoxin, 14.

Ants, 141-146, 354, 386, 4097.
Aphids, 384-386, 419, 426.
Apus, 418.

Archzeopteryx, 41, 42, 53, 54+
Ardea, 200,

Argus pheasant, 199.
Aristolochia Clematitis, 10, 12, 12,
Arsenic, 13.

Artemia, 306,

Ascidians, 417.

Askenasy, 161.

Aspalax, 227.

Asterina, 421-422.
Autenrieth, 58.

Baboon, 209.

Bacteria, 14, 15, 111, 398.

Baer, Von, 60, 61, 74, 75+

Bamboo, 313.

Barnacles, 417.

Bartlett, 209, 220.

Bat, 2.

Bates, 183, 186.

Bateson, 273, 278, 453.
2H

variation in, 309-312.
Blind animals, 354.
Blow-fly, 383.

Bonellia, 353, 417+
Born, 424.

Bos, 206.

Boveri, 433.

Bovide, 207.
Branchipus, 306.
Brocadello, 428.
Brooks, 441.
Brown-Séquard, 232, 241, 250-257.
Buffon, 300.

Bull, 207, 315.
Biitschli, 441.
Butterfly, 3, 184, 389.

Cactus, Io.

Caffein, 13.

California salmon, 19.
Calkins, 443-447.
Callionymus, I91.
Calocalanus, 177.
Cameron, 425.
Canestrini, 178.
Canidz, 308.

Canis, 410.
Carbonnier, 190, 192
Cardamine, 335.
Cardinalis, 173.
Cardium, 305.
Cassowary, 202.
Castle, 148, 321, 435, 438
Caterpillar, 5, 8, 186.
Cattle, 411.

Cats, 209.
Cercopithecus, 208.

465

466

Cervus, 304.

Chara, 420.

Charrin, 257.

Chick, 57, 406.

Child, 72.

Chinese pheasants, 6.
Chlorophyl, 9.

Cicadas, 187, 188.

Ciona, 148.

Classification, 31-36.
Classification, scheme of, 38.
Cockatoo, 6.

Colaptes, 310.

Colias, 185.

Colonial forms, 127.

Color, 18, 19, 24, 133, 375+
Coloration, 5, 6, 7, 23, 357-360.
Columba livia, 76.

Comb of bees, 4.
Communal marriages, 210.
Competition, 104, 112, 119, 120, I21, 122,123.
Compositz, 130.

Cones, 310.

Conklin, 72.

Cope, 49, 259.

Copridz, 183.
Coral-snakes, 194.
Correlated variation, 94.
Correlation, 134.

Cottus, I9I.

Crab, 15, 248, 344, 354»
Crickets, 188.

Crocodiles, 193.

Crosby, 398.
Cross-fertilization, 21.
Crossing of species, 148, 149, 150.
Crystal, 29.

Cryptocerus, 144.
Ctenophors, 417.

Cuckoo, 139, 140, 141.
Cuénot, 427-428, 435.
Culicidze, 188.
Cunningham, 257-260,
Cuvier, 44, 301.
Cynocephalus, 209.
Cypridopsis, 392-393.
Cyprinodonts, 190.

Cypris, 320.

Dall, 260,

Dallinger, 320.

Danaids, 160,

Dances, 195.

Daphnia, 305, 418.

Darwin, C., numerous references through-
out the text.

Darwin, Erasmus, 223,

Lndex

Date-palm, 313.
Davenport, 264, 266, 321.
Dean, 358.

Death, 370.

Death, feigning, 410, 411.
Deer, 309.

Degeneration, 165.
Delamare, 257.

Descent theory, 31-35,
Desmarest, 206.
Desmodium, 403.
Dianthus, 149.
Didelphys, 410.
Dimorphism, 360.
Dingoes, 314.

Dinophilus 428.

Diptera, 180, 188,
Divergence of character, 127, 128,
Dog, 226.

Draba, 288, 289, 290, 292, 294.
Draco, 194.

Dragonet, IgI.

Drill, 209.

Ducks, 94, 314.

Diising, 423.

Dutrochet, 320.

Earthworm, 380, 382, 383, 384, 417.
Echidna, 54.

Eciton, 144.

Egerton, 204.

Egg, 429-430, 432.

Eggs, number of, 19.

Egypt, animals of, 225.

Egyptian remains of animals, 43, 44.
Eimer, 158, 260.

Eisig, 72.

Electric organs, 22, 132, 372.
Elephant, 110-111, 206, 304.
Emu, 202. -

Entoscolax, 353.

Epihippus, so.

Equus, 50.

Eristales, 188.

Esmeralda, 182.

Euploids, 160.

Eustephanus, 201,
Evolution, 29.

Ewart, 238.

Exercise, 12.

External conditions, 130,
Eye, 13, 131, 132.

Eye, evolution of, 131, 132,
Eye, of flatfish, 137.

Fayal Islands, 124.
Felidae, 308.

Lndex

Felis, 308.

Fish, change of color, 16.

Fishes, 7.

Fishes, secondary sexual character of, 190.
Flatfish, 137, 138.

Flatworms, 417.

Fleischmann, 45-57.

Flounders, 228, 346, 347.

Flowers, 9, 17, 21, 342, 399, 422, 429.
Fly, 428.

Foot of horse, 47.

Forel, 5.

Fossil horses, 52.

Foxes, 210, 410.

Franqueiros cattle, 315.

Frogs, 193, 320, 382, 405.

Frogs, cross-fertilization, 150.

Fruit, down of, 133.

Fundulus, 16,

Galton, 236, 270-272, 289, 441.
Gavials, 301.

Geddes and Thompson, 417, 423, 426, 427.
Geer, De, 178.

Gegenbaur, 49.

Gelasimus, 177.

Geoffroy St.-Hilaire, 44, 67, 300-303.
Geological evidence, 39.
Gerbe, 429.

Germinal selection, 154.
Gibbon, 213.

Gill clefts, 62, 63, 64, 73.
Giraffe, 6, 203, 229, 248-249,
Glacier, 28.

Glowworm, 23.

Goat, 206, 208. °

Gonionema, 399.

Gorilla, 205,

Gothic period, 47, 48.

Gould, 197.

Graba, 124, 125.

Grafting, 153.

Grasshoppers, 8, 188.

Gray, 126.

Greyhound, 134.

Growth of plants, 17.
Guillemots, 124.

Guinea pigs, 232.

Giinther, Igo.

Gymnotus, 132.

Haeckel, 48, 49, 56, 79, 71, 79, 80, 82.
Hartman, 187.

Heart, 66, 67.

Heliconids, 160.

Helix, 344, 345-346.

Hemiptera, 181.

467

Heredity, 270.
‘Hermaphroditic animals, 126.
Hertwig, O., 78, 79, 80, 81, 82, 83.
Hertwig, R., 447.
Hieracium, 330, 331.
Hildebrand, 148.

Hill, 252.

Hipparion, 51.
Hippeastrum, 148.

His, 71, 72.

Holmes, 72. ,

Hornbills, 21g.

Horns, 229.

Horse, 42, 224.
Horse-chestnut, 24.
Hothura, 410.

Hottentots, 212.

Hudson, 140, 195, 409-412.
Humming-birds, 6, 197, 228.
Hurst, 75, 76, 77, 78.
Huxley, 49, 156, 233.

Hyatt, 259.

Hybrids, 149, 239.
Hydatina, 417.

Hydroides, 348.

Hylobates, 205.
Hymenoptera, 181,

Tee, 28.

Ichneumonidz, 181.
Idioplasm, 335.
Immunity, 13.

India cattle, 208.
Infanticide, 25.

Inorganic adaptations, 26.
Insectivorous plants, 10,
Insects, coloration of, 7; wingless, 228.
Instinct, 25, 139, 140, 141.
Trish elk, 247.

Jackson, 260.
Jaguar, 209.
Japanese cock, 163.
Jennings, 395.
Jonghe, 314.
Jordan, 292.
Joseph, 428.

Junco, 311.

Kallima, 7, 161, 162, 358.
Kangaroo, 229, 351.
Katydid, 8.

Kent, W. Saville, 191.
Kidneys, 66.

Kielmeyer, 58.

Kirby, 232.

Kiwi, 354.

468

KGlreuter, 149.
Korschelt, 428,

Labidocera, 393.
Lamarck, 146, 222-233.
Lamarckian factor, 94, 205, 211, 222, 458.
Lang, 345.

Law of Biogenesis, 30.
Leaf, resemblance to, 7.
Leaves, closing of, 11.
Leeches, 417.
Leguminosae, 124.
Leidy, 46.

Length of life, 20,
Lenhossek, 435.
Leopard, 209.
Lepidoptera, 172,
Leptothrix, 320.
Leucophys, 442.
Lichen, 9.

Lillie, 72.

Limbs of vertebrates, 46,
Linaria, gor.

Linnzean species, 85.
Linnzeus, 191.

Lion, 6.

Lizards, 7, 16, 17, 193.
Lobelia, 148.

Lobster, 343.
Lockwood, 138.
Locusts, 188.

Loeb, 383-392, 447.
Lomaria, 290.

Lowell lectures, 61.
Lumbriculus, 15.
Luminous organs, 133.
Lymnza, 305, 322.
Lythrum, 365-370.

Machines, 26, 27, 28.
McIntosh, 176.
McNeill, 204.
Macropus, 192.
Malva, 4or.
Mammaiia, origin, 54, 202.
Man, 2Io,

Marsh, 49.

Matthews, 447.
Mauchamp, 315.
Maupas, 441, 442, 445.
May-flies, 19, 353, 389.

Mead, 72.
Meckel, 59, 60.
Melanism, 209.

Melospiza, 311.

Mendel, 278-286, 433, 436.
Mesohippus, 51.

Mimosa, 404.

Lndex

Minnow, 16.
Minot, 433.
Mirabilis, 149, 150.
Mivart, 136, 137.
Mole, 1, 2, 18, 227.
Mole-cricket, 1, 2.
Molothrus, 140.
Monkeys, 207, 208.
Mons, Van, 332.
Montgomery, 432.
Moor-hen, 453.
Moquin-Tandon, 303.
Morton, Lord, 238.
Moschus, 206,
Moths, 184, 387, 388.
Moussu, 257.
Mozart, 218.
Mulberry, 313.
Miiller, 182, 188.
Miller, Fritz, 148.
Muscles, 12.
Mycetes, 205.
Myzostomum, 422,

Nageli, 161, 325-339, 459.

Natural selection, 104-107, 108, I0g, 110,
etc.; definition of, 117.

Nauplius, 69.

Nectar, 124.

Nectar-feeding insects, 126, 127.

Nectarines, 134.

Negroes, 212,

Nematode, number of eggs, I10.

Nematus, 425.

Nemertian worms, 176.

Neo-Lamarckians, 240, 259-260,

Nepenthes, Io.

Nephela, 178.

Nest of birds, 4, 407-408.

Neuters, 142.

Nicotine, 13.

Nostocs, 320.

Notochord, 64, 65.

Nussbaum, 424.

Ocneria, 428.
Cnothera, 294-297.
Oken, 56, 58.

Old age, at, 25.
Onites, 232,
Onychodromus, 442,
Opossum, 410.

Organs of little use, 22,
“ Origin of Species,” 129.
Ornithorynchus, 54.
Orobanchia, 330.
Osborn, 259.

Index

‘Oscillaria, 320.
Ostrich, 203, 354.
Oxalis, 290, 404.
Oxen, 304.
Oxide, 29.

Packard, 231, 260,

Paludina, 320, 322.

Pangenesis, 233-240.

Papilio, 158, 360, 388; polyxenes, 3.

Paradisea, 197.

Paramzecium, 395-398, 442-447.

Parasitism, 352-353.

Parker, 393.

Parrots, 6.

Partridge, 410.

Passerella, 311.

Passiflora, 148.

Pavo, 317.

Peach, 134.

Peacock, 200, 317-318.

Peafowl, 198. *

Pearson, 265, 267, 268-270,

Peas, 281-286.

Peckham, 178, 408.

Pelobates, 421.

Pfliiger, 424, 430.

Phosphorescent organs, 22, 133.

Physa, 320, 322.

Pigeons, selection in, 102,

Pipilo, 311.

Pisum, 278.

Pithecia, 208.

Planaria, 380.

Planarians, 394.

Plants, 403, 415; color of, 24; influence of
light, 17.

Plato, 304.

Plover, 202.

Poisons, 13, 14, 15,°18, 20, 377.

Polar bear, 6.

Pollen, 2, 125.

Polygon, 262,

Porthesia, 389.

Primula, 361-365.

Prionide, 182.

Probosces of insects, 127.

Protective coloration, 5, 6, 16, 158, 159.

Proteus, 227.

Protohippus, 51.

Przibram, 347.

Psyche, 419.

Ptarmigan, 5.

Pyrodes, 182.

Quetelet, 289.
Quiscalus, major, 173.

469

Rabbit, Porto Santo, 316-317.
Rabbits, 304.

Rabbits in Australia, 112,
Race-horse, 134.
Ranunculus, 305.
Ray-florets, 135.

Rays, electric organs of, 22,
Réaumur, 388.
Recapitulation theory, 58-83.
Reduction division, 432-433.
Regencration, 15, 16, 27, 379.
Regulations, 27, 28.
Reproductive organs, 19.
Reptiles, fossil, 52, 53.
Reugger, 205.
Rhododendron, 330.
Rhyncheea, 201.

Riley, 424.

Rivers, 28.

Robinia, 404.

Romanes, 132, 250-256, 412,
Rose, 307.

Rothert, 398.

Rotifers, 118, 353, 424.
Roulin, 304.

Roundworms, 176, 353.
Rudimentary organs, 22.
Ryder, 260.

Sacculina, 353.

Sachs, 10.

Salmon, I9.

Salter, 314.

Salvin, 201.

Saphirina, 176,

Savages, 210.

Saw-flies, 425.

Scarlet tanager, 198.

Schaefer, 244.

Sclater, 198.

Scops, 312.

Scott, 148, 259.

Sea-anemone, 341.

Sea-urchin, 341.

Secondary sexual characters, 21.
Selection, 116.

Selection, artificial, 91, 92, 96, 97, 98.
Self-fertilization, 126.

Semper, 260.

Setchel, 320.

Sexual characters, secondary, 372-374.
Sexual selection, 167.

Sharp, 350, 425.

Sheep, 208.

Sherrington, 244.

Shrew mice, 206..

Silkworm, 428, 447.

470

Silver-bill, 410,

Sirex, 181.

Siricidze, 181.

Sitaria, 194.

Skin, thickening of, 12, 13.

Skull, 37, 65.

Skunk, 3.

Slaves of ants, 141.

Sleep in plants, 404.

Sloth, 229.

Snail, 417.

Snails, color of, 23.

Snakes, 14, 193-194, 227.

Snowy owl, 6.

Solenobia, 419.

Soles, 137, 228.

Sparassus, 178.

Sparrow, 200; English, 112.

Species, 31, 32, 33; adaptation for good of,
1g; sharp separation of, 131.

Spencer, 240-246, 247, 290.

Spermatozoa, 150, 430-433.

Sphinx, 186, 388.

Spiders, 177-178, 179, 406; web, 3.

Spirogyra, 420.

Spontaneous variability, 134.

Spores, 322.

Squilla, 177.

Squirrels, 210.

Stag-beetle, 179.

Stags, 203-204, 219.

Sterility, 147-152.

Strasburger, 395.

Stridulating organs, 188, 189.

Struggle for existence, Iog, IIo,

Stylonychia, 442.

Survival of the fittest, 107, 108, 109, 117.

Sutton, 432.

Swallow, 115.

Sweating, 12,

Tadpole, 321, 428.

Tail, a

Tanager, 6.

Tapeworm, 353; number of eggs, 110,
Taraxacum, 305.

Tear-sacs, 206.

Teeth, bird's, 67, 68.

Telegony, 95, 234, 237, 238, 239.
Tenthredinidee, 181, 425.
Termite, number of eggs, 110.
Termitidze, 350.

Thrush, 115.

Tipule, 188.

Toad, 7.

Torpedo, 132,

Towle, 392.

Lndex

Transitional forms, 42,
Transmutation theory, 31,.34-
Traquair, 138.

Treadwell, 72.

Treat, 424.

Tree-frogs, 7.

Trichina, 353.

Trifolium, 404,

_| Triton, 193.

Turkeys, 314.
Turnix, 201, 202,
Turtles, 193.

Umbelliferze, 135.
Uria lacrymans, 124.

Utricularia, Io.

Vanessa, 360.

‘Variability, 92, 93, 95, 96, 318-319.

Variation, 261, 340.

Variation, fluctuating, 100, 118, 123.
Variation under domestication, 136.
Varieties, 106, 107, 148.

Varigny, De, 303-306, 314-315, 322.
Venus fly-trap, 9.

Verbascum, 148, 149.

Vertebrates, evolution of, 40, 45.
Vilmorin, 303, 314.

Vinson, 178.

Vries, De, 97, 278, 289-298, 340.
Vulpine, 209. :

Wallace, 7, 162, 186, 202, 221, 249,
Walrus, 203.

Walsh, 181.

Walther, 59.

Wasp, 3, 5, 408, 409.

Waterton, 198.

Web, spider's, 3, 4.

Weir, 171.

Weismann, 154-166, 441, 448-450.
Westwood, 188.

Whale, 227, 301.

Wilson, E. B., 72.

Wing of bat, 2.

Wolf, 308, 376.

Wolves, 412.

Women, 210.

Woodpecker, 228,

Wounds, healing of, 15.

Yarrell, 138.
Yung, 424, 436.

Zebu cattle, 208,
Zeleny, 348.
Zoéa, 69, 70,

REGENERATION

By THOMAS HUNT MORGAN, Ph.D.

Professor of Biology in Bryn Mawr College ; author of “ The Development
of the Frog's Egg,” ete.

Cloth 8vo $3.00 net

“This volume is the outcome of five lectures on ‘ Regeneration and Experi-
mental Embryology,” given in Columbia University, in January, 1900. The sub-
jects dealt with in the lectures are here more fully treated and are supplemented
by the discussion of a number of related topics. During the last few years the
problems connected with the regeneration of organisms have interested a large
number of biologists, and much new work has been done in this field, especially
in connection with the regenerative phenomena of the egg and early embryo.
The development of isolated cells or blastomeres has, for instance, aroused wide-+
spread interest. It has become clearer, as new discoveries have been made, that
the latter phenomena are only special cases of the general phenomena of regen-
eration in organisms, so that the results have been treated from this point of view
in the present volume, which, however, has a wider bearing than simply as a
treatment of the problems of regeneration.” — From the Author's Preface.

“The presentation is thorough and comprehensive; besides original researches
of the author, it involves a great deal of recapitulation and verification of experi-
ments performed by others, with analyses and criticisms of the conclusions drawn
from the phenomena or of theories advanced in explanation. Dr. Morgan is for-
tunate in possessing experience, depth of insight, and a judicial habit of mind that
give him special fitness for his task.” — The Nation.

“Tt is rare indeed to find « book which contains so large an amount of re-
search work and which is at the same time of such general interest and impor-
tance. The book will undoubtedly take a prominent place among the standard
biological works of the world.” — E. G. C. in Sctence.

THE MACMILLAN COMPANY

66 FIFTH AVENUE, NEW YORK
I

THE DEVELOPMENT OF THE
FROG’S EGG

An Introduction to Experimental Embryology

By THOMAS HUNT MORGAN, Ph.D.

Professor of Biology in Bryn Mawr College

Cloth 8vo $1.60 net

“Professor Morgan’s book gives us a much-needed text-book
for both student and instructor, and it should stimulate and
greatly aid investigation by pointing out the wide field the frog’s
egg still offers to embryological research.”

— American Journal of Science.

“A clear, succinct, and comprehensive account of all the
known phases of the fertilization and development of the frog’s
egg. ... That the statements are clear and intelligible as possi-
ble, the reader may feel sure. ... The medical student should
master it... . The general student or the reader who is inter-
ested in the matter of the physical basis of heredity will find
here the fundamental facts regarding the first beginnings of life
and the structure of the egg as well as the sperm cell.”

— The Independent.

THE MACMILLAN COMPANY

66 FIFTH AVENUE, NEW YORK
2