Note: Project Gutenberg also has an HTML version of this
file which includes the original illustrations.
See 63540-h.htm or 63540-h.zip:
(https://www.gutenberg.org/cache/epub/63540/pg63540-images.html)
or
(https://www.gutenberg.org/files/63540/63540-h.zip)
Images of the original pages are available through
Internet Archive. See
https://archive.org/details/evolutionadaptat00morg
Transcriber’s note:
Text that was in italics is enclosed by underscores
(_italics_).
Text that was in bold face is enclosed by by equal
signs (=bold=).
A caret character is used to denote superscription. A
single character following the caret is superscripted
(example: J^1).
EVOLUTION AND ADAPTATION
------------------------------------------------------------------------
[Illustration: Publisher's logo (The Macmillan Company)]
------------------------------------------------------------------------
EVOLUTION AND ADAPTATION
by
THOMAS HUNT MORGAN, Ph.D.
New York
The Macmillan Company
London: Macmillan & Co., Ltd.
1908
All rights reserved
Copyright, 1903,
by The Macmillan Company.
Set up and electrotyped. Published October, 1903. Reprinted January,
1908.
Norwood Press
J. S. Cushing Co.—Berwick & Smith Co.
Norwood, Mass., U.S.A.
------------------------------------------------------------------------
TO
Professor William Keith 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 question at once suggests itself: How has such a relation
been brought about? Is it due to something inherent in the living 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 arrangement, 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,” 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 problèmes 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 recognized 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
conclusion 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 acceptation 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 that if any one will undertake to question modern zoologists
and botanists concerning their relation to the Darwinian theory, he will
find that, while professing _in a general way_ to hold this theory, most
biologists have many reservations and doubts, which they either keep to
themselves or, at any rate, do not allow to interfere either with their
teaching of the Darwinian doctrine or with the applications that they
may make of it in their writings. The claim of the opponents 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 environment 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 underlying
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 regenerative 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 to regenerate; and it seemed to
me, therefore, desirable to reëxamine the whole question of adaptation,
for might it not prove true here, also, that the theory of natural
selection was inapplicable? This was my starting-point. The results of
my examination are given in the following pages.
I am deeply indebted to Professor G. H. Parker and to Professor E. G.
Conklin for advice and friendly criticism; and in connection with the
revision of the proof I am under many obligations to Professor Joseph W.
Warren and to Professor E. A. Andrews. Without their generous help I
should scarcely have ventured into a field so full of pitfalls.
Bryn Mawr, Penn., June 10, 1903.
------------------------------------------------------------------------
CONTENTS
CHAPTER I
PAGE
The Problem of Adaptation 1
– Structural Adaptations 1
– Adaptations for the Good of the 19
Species
– Organs of Little Use to the 22
Individual
– Changes in the Organism that are 25
of No Use to the Individual or to
the Race
– Comparison with Inorganic 26
Phenomena
CHAPTER II
The Theory of Evolution 30
– Evidence in Favor of the 32
Transmutation Theory
– – Evidence from Classification 32
and from Comparative Anatomy
– – The Geological Evidence 39
– – Evidence from Direct 43
Observation and Experiment
– – Modern Criticism of the Theory 44
of Evolution
CHAPTER III
The Theory of Evolution (continued) 58
– The Evidence from Embryology 58
– – The Recapitulation Theory 58
– Conclusions 84
CHAPTER IV
Darwin’s Theories of Artificial and 91
of Natural Selection
– The Principle of Selection 91
– Variation and Competition in 104
Nature
– The Theory of Natural Selection 116
CHAPTER V
The Theory of Natural Selection 129
(continued)
– Objections to the Theory of 129
Natural Selection
– Sterility between Species 147
– Weismann’s Germinal Selection 154
CHAPTER VI
Darwin’s Theory of Sexual Selection 167
– Sexual Selection 167
– General Criticism of the Theory 213
of Sexual Selection
CHAPTER VII
The Inheritance of Acquired 222
Characters
– Lamarck’s Theory 222
– Darwin’s Hypothesis of Pangenesis 233
– The Neo-Lamarckian School 240
CHAPTER VIII
Continuous and Discontinuous 261
Variation and Heredity
– Continuous Variation 261
– Heredity and Continuous Variation 270
– Discontinuous Variation 272
– Mendel’s Law 278
– The Mutation Theory of De Vries 287
– Conclusions 297
CHAPTER IX
Evolution as the Result of External 300
and Internal Factors
– The Effect of External Influences 300
– Responsive Changes in the 319
Organism that adapt it to the New
Environment
– Nägeli’s Perfecting Principle 325
CHAPTER X
The Origin of the Different Kinds 340
of Adaptations
– Form and Symmetry 340
– Mutual Adaptation of Colonial 350
Forms
– Degeneration 352
– Protective Coloration 357
– Sexual Dimorphism and Trimorphism 360
– Length of Life as an Adaptation 370
– Organs of Extreme Perfection 371
– Secondary Sexual Organs as 372
Adaptations
– Individual Adjustments as 375
Adaptations
– Color Changes as Individual 375
Adaptations
– Increase of Organs through Use 376
and Decrease through Disuse
– Reactions of the Organism to 377
Poisons, etc.
– Regeneration 379
CHAPTER XI
Tropisms and Instincts as 382
Adaptations
CHAPTER XII
Sex as an Adaptation 414
– The Different Kinds of Sexual 414
Individuals
– The Determination of Sex 422
– Sex as a Phenomenon of Adaptation 439
CHAPTER XIII
Summary and General Conclusions 452
INDEX 465
------------------------------------------------------------------------
EVOLUTION AND ADAPTATION
CHAPTER I
THE PROBLEM OF ADAPTATION
Between an organism and its environment there takes place a constant
interchange of energy and of material. This is, in general, also true
for all bodies whether living or lifeless; but in the living organism
this relation is a peculiar one; first, because the plant or the animal
is so constructed that it is suited to a particular set of physical
conditions, and, second, because it may so respond to a change in the
outer world that it further adjusts itself to changing conditions,
_i.e._ 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 following 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 foreleg 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, 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 adaptation. 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 adaptations. 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, hanging 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 protection 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 of the outer surface of
the tibia is arched over by stiff hairs. The pollen collected from the
stamens of flowers is stowed away in this receptacle by means of the
other pairs of legs. The structure is unique, and is not found in any
other insects except the bees. It is, moreover, present only in the
worker bees, and is absent in the queen and the males.
The preceding cases, in which the adapted parts are used for the
ordinary purposes of life of the individual, are not essentially
different from the cases in which the organ is used to protect the
animal from its enemies. The bad taste of certain insects is supposed to
protect them from being eaten by birds. Cases like this of passive
protection grade off in turn into those in which, by some reflex or
voluntary act, the animal protects itself. The bad-smelling horns of the
caterpillar of the black swallow-tailed butterfly (_Papilio polyxenes_)
are thrust out when the animal is touched, and it is believed that they
serve to protect the caterpillar from attack. The fœtid 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
extermination 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 construction 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 characteristic 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 is the means of securing the
insects used for food, it fulfils a purpose necessary for the welfare of
the spider.
The making of the nests by birds appears to be also in large part an
instinctive act; although some writers are inclined to think that memory
of the nest in which the young birds lived plays a part in their
actions, and imitation of the old birds at the time of nest-building
may, perhaps, also enter into the result. It has been stated that the
first nest built by young birds is less perfect than that built by older
birds, but this may be due to the bird’s learning something themselves
in building their nests, _i.e._ to the perfecting of the instinct in the
individual that makes use of it. In any case much remains that must be
purely instinctive. The construction of the comb by bees appears to be
largely, perhaps 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 formation 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 larvæ, 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 disturbed, the birds at once squat down on
the ground, and remain perfectly quiet until the danger is past. Their
resemblance to the ground is so perfect that they are almost invisible
so long as they remain quiet. If, instead of remaining still, they were
to attempt to run away when disturbed, they would be much more easily
seen.
Certain solitary wasps (_Ammophila_) have the habit of stinging
caterpillars and spiders, and dragging them to their nests, where they
are stored away for the future use of the young that hatch from the eggs
laid by the wasp on the body of the prey. As a result of the sting which
the wasp administers to the caterpillar, the latter is paralyzed, and
cannot escape from the hole in which it is stored, where it serves as
food for the young wasp that emerges from the egg. It was originally
claimed by Forel that the wasp stings the caterpillar in such a way that
the central nervous system is always pierced, and many subsequent
naturalists have marvelled 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 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 northern 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 approach 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 extremely 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
color between the organism and its surroundings, and in neglecting all
others, is, as has been already said, a point to be further examined.
Not only among mammals and birds have many cases of protective
coloration been described by writers dealing with this subject, but in
nearly every group of the animal kingdom similar cases have been
recognized. The green and brown color of lizards may protect them, the
green color of many frogs is supposed to conceal them as they sit
amongst the plants on the edge of a stream or pond. The gray-brown color
of the toad has been described as a resemblance to the dry ground, while
the brilliant green of several tree-frogs conceals them very effectively
amongst the leaves. Many fishes are brilliantly colored, and it has even
been suggested that those living amongst corals and sea-anemonies have
acquired their colors as a protection, but Darwin states that they
appeared to him very conspicuous even in their highly colored
environment.
Amongst insects innumerable cases of adaptive coloration have been
described. In fact this is the favorite group for illustrating the
marvels of protective coloration. A few examples will here serve our
purpose. The oft-cited case of the butterfly _Kallima_ is, apparently, a
striking instance of protective resemblance. When at rest the wings are
held together over the back, as in nearly all butterflies, so that only
the under surface is exposed. This surface has an unquestionably close
resemblance to a brown leaf. It is said on no less authority than that
of Wallace that when this butterfly alights on a bush it is almost
impossible to distinguish 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 this information there is danger that the
case may prove too much, for, if it should turn out that this remarkable
case is accidental the view in regard to the resemblance may be
endangered.
Amongst caterpillars there are many cases of remarkable resemblances in
color between the animal and its surroundings. The green color of many
of those forms that remain on the leaves of the food-plant during the
day will give, even to the most casual observer, the impression that the
color is for the purpose of concealment; and that it does serve to
conceal the animal there can be no doubt. But even from the point of
view of those who maintain that this color has been acquired because of
its protective value it must be admitted that the color is insufficient,
because some of these same green caterpillars are marvellously armed
with an array of spines which are also supposed to be a protection
against enemies. Equally well protected are the brown and mottled
geometrid caterpillars. These have, moreover, the striking and unusual
habit of fixing themselves by the posterior pairs of false legs, and
standing still and rigid in an oblique position on the twigs to which
they are affixed. So close is their resemblance to a short twig, that
even when their exact position is known it is very difficult to
distinguish them.
Grasshoppers that alight on the ground are, in many cases, so similar to
the surface of the ground that unless their exact location is known they
easily escape attention, while the green color of the katydid, a member
of the same group of orthoptera, protects it from view in the green
foliage of the trees where it lives. The veinlike wings certainly
suggest a resemblance to a leaf, but whether there is any necessity for
so close an imitation may be questioned.
There can be little doubt in some of these cases that the color of the
animal may be a protection to it, but as has been hinted already, it is
another question whether it acquired these colors because of their
usefulness. Nevertheless, if the color is useful to its possessor, it is
an adaptation in our sense of the word, without regard to the way in
which it has been acquired. Even, for instance, if the resemblance 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 coloration 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 resemblance is of any use to the lichen. Nor does the color of
the marine red algæ 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 adaptation 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. 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 interior 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 estimated 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.”
[Illustration:
Fig. 1.—The fertilization of _Aristolochia Clematitis_.
A, portion of stem with flowers in axil of leaf in different stages.
B and C, longitudinal sections of two flowers, before and after
fertilization. (After Sachs.)]
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
B a small fly has entered, and has brought in upon its back some pollen
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
sticking to its back will be rubbed against the stigmatic surface. “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, 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 unfertilized flowers stand erect with widely
open mouths. As soon as they have been fertilized they bend down, as
seen in 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
doing 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 somewhat different
sense from that usually employed. The result is of no direct advantage
to the animal, except possibly in saving 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 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 circumstances, 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 suddenly 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 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
inconceivable that the nicely graded series of doses of increasing
strength necessary to bring about the immunity could ever be acquired.
Hence we find here a case of response in an adaptive direction that
could not have been the outcome of experience in the past. It is
important to emphasize this capacity of organisms to adapt themselves to
certain conditions entirely new to them.
These cases lead at once to cases of immunity to certain bacterial
diseases. An animal may become immune to a particular disease in several
ways. First, by having the disease itself, which renders it immune for a
longer or a shorter period afterwards; or, second, by having a mild form
of the disease as in the case of smallpox, where immunity is brought
about by vaccination, _i.e._ by giving the individual a mild form of
smallpox; or, third, by introducing into the blood an antidote, in the
form, for example, of antitoxin, which has been made by another animal
itself immune to the disease. The first two classes of immunity may be
looked upon as adaptations which are of the highest importance to the
organism; 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 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 usefulness of this power is
obvious when it is remembered how exposed most animals are to injuries.
By repairing the injury the animal can better carry on its normal
functions. Moreover, the presence of the wound would give injurious
bacteria a ready means of entering the body. In fact, an intact skin is
one of the best preventives to the entrance of bacteria.
Not only have most organisms the power of repairing injuries, but many
animals have also the closely related power of regenerating new parts if
the old ones are lost. If a crab loses its leg, a new one is
regenerated. If a fresh-water worm (_Lumbriculus_) is cut into pieces,
each piece makes a new head at its anterior end and a new tail at the
posterior end. In this way as many new worms are produced as there are
pieces. And while in a strict sense it cannot be claimed that this power
of regeneration is of any use to the original worm, since the original
worm, as such, no longer exists, yet since it has not died but has
simply changed over into several new worms, the process is of use
inasmuch as by this means the pieces can remain in existence.
We need not discuss here the relative importance to different animals of
this power of regeneration, but it may be stated, 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, concerned with the relative
importance of the different adaptations, 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
background 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 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 pupæ 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
surface 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 extraordinary 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 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
necessary 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 structures 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 supposed 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
adaptations as the special cases that have been selected for
illustration. 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 mammals 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 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 environment. 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 reproduction 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 individual 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; 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 reproductive 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 lengthening of the life period has been acquired on account of its
use to the species. When, however, as in the case of the vertebrates,
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 as 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 number of cases
where there are accurate data, that the individual 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 process
takes place, and these animals appear to have the power of reproducing
themselves indefinitely.
The phenomenon of old age (apart from its possible connection 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 characters are supposed to be of use to the
individual in conquering its rivals, or in attracting the females. They
may be considered 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 entering the flowers bring about
cross-fertilization. Thus while the nectar seems to be of no immediate
service to the plant itself, it is useful to the species in bringing
about the fertilization of the flowers. The odors of flowers also serve
to attract insects, and their presence is one of the means by which
insects find the flowers. This also is of advantage to the race.
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 presence 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 usually 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 rudimentary 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 considered 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
developed 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,
ctenophores, copepods, pyrosoma, etc., cannot be of use to these animals
in protecting them from attack. In the case of certain bacteria it seems
quite evident that the production of light can be of no use as such to
them. The production of light may be only a sort of by-product of
changes going on in the organism, and have no relation to outside
conditions. In certain cases, as in the glowworm, it has been supposed
that the display may serve to bring the sexes together; but since the
phosphorescent organs are also present in the larval stages of the
glowworm, and since even the egg itself is said to be phosphorescent, it
is improbable, in these stages at least, that the presence of the light
is of service to the organism.
It has been pointed out that the colors of certain animals may serve to
conceal them and may be regarded as an adaptation; but it is also true
that in many cases the color of the whole animal or the color of special
parts can be of little if any direct use. While it is difficult to show
that the wonderful patterns and magnificent coloration of many of the
larger animals are not of service to the animal, however sceptical we
may be on the subject, yet in the case of many microscopical forms that
are equally brilliantly colored there can be little doubt that the
coloration can be of no special service to them. If it be admitted that
in these small forms the color and the color patterns are not
protective, we should at least be on our guard in ascribing off-hand to
larger forms a protective value in their coloration, unless there is
actual proof that it serves some purpose.
We also see in other cases that the presence of color need not be
connected with any use that it bears as such to the animal. For
instance, the beautiful colors on the inside of the shells of many
marine snails and of bivalve mollusks, can be of no use to the animal
that makes the shell, because as long as the animal is alive this color
cannot be seen from the outside. This being the case let us not jump too
readily to the conclusion that when other shells are colored on the
outer surface that this must be of use to the mollusk.
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 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 imaginative explanations of this sort, that have
no facts of experience 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 foothold in the body.
We have already discussed the possible relation of death as an
adaptation, but the changes just mentioned take place independently of
their relation to the death of the organism as a whole, and show that
some of the normal 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 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 challenges 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 that its integrity is maintained. Most
machines have no such regulative power, although, in a sense, the
fly-wheel of an engine regulates the speed, and a water-bath, with a
thermostat, regulates itself to a fixed temperature; but even this
comparison lacks one of the essential features of the regulation 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 individuals 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 parts
of the body mainly in that they are made of different compounds having
different properties, as the above comparisons have shown. But the
regulations of the machine have been added to it by man on account of
their usefulness 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 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 acquired 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 individual, 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 subsequent
chapters this will be fully discussed, but before we take up this topic,
it will be necessary to reach some understanding 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.
------------------------------------------------------------------------