LAWS OF ADAPTATION.
Laws of Indirect or Potential Adaptation.—Individual
Adaptation.—Monstrous or Sudden Adaptation.—Sexual
Adaptation.—Laws of Direct or Actual
Adaptation.—Universal Adaptation.—Cumulative
Adaptation.—Cumulative Influence of External Conditions
of Existence and Cumulative Counter-Influence of the
Organism.—Free Will.—Use and Non-use of
Organs.—Practice and Habit.—Correlative
Adaptation.—Correlation of Development.—Correlation of
Organs.—Explanation of Indirect or Potential Adaptation
by the Correlation of the Sexual Organs and of the other
parts of the Body.—Divergent Adaptation.—Unlimited or
Infinite Adaptation.
In the last chapter we reduced into two groups the phenomena of
Adaptation or Variation, which, in connection and interaction with the
phenomena of Heredity, produce all the endless variety of forms in
animals and plants—first, the group of indirect or potential, and
secondly, the group of direct or actual Adaptation. We shall occupy
ourselves with a closer examination of the different laws which we can
discover in these two groups of the phenomena of variation. Let us first
take into consideration the remarkable and very important, although
hitherto much neglected, phenomena of indirect variation.
_Indirect or potential adaptation_ manifests itself, it will be
remembered, in the striking and exceedingly important fact that organic
individuals experience transformations and assume forms in consequence
of changes of nutrition which have not operated on them themselves, but
upon their parental organism. The transforming influence of the external
conditions of existence, of climate, of nutrition, etc., shows its
effects here not directly in the transformation of the organism itself,
but indirectly in that of its descendants. (Gen. Morph. ii. 202.)
As the principal and most universal of the laws of indirect variation
must be mentioned _the law of individual adaptation_, or the important
proposition that all organic individuals from the commencement of their
individual existence are unequal, although often very much alike. As a
proof of this proposition, I may at once point to the fact, that in the
human race in general all brothers and sisters, all children of the same
parents, are unequal from their birth. No one will venture to assert
that two children at their birth are perfectly alike; that the size of
the individual parts of their bodies, the number of hairs on their
heads, the number of cells composing their outer skins or epidermis, the
number of blood-cells are the same in both children, or that both
children have come into the world with the same abilities or talents.
But what more specially proves this law of individual difference, is the
fact that in the case of those animals which produce several young ones
at a time,—for instance, dogs and cats,—all the young of each birth
differ from one another more or less strikingly in size and colour of
the individual parts of the body, or in strength, etc. Now this law is
universal. All organic individuals from their beginning are
distinguished by certain, though often extremely minute, differences,
and the cause of these individual differences, though in detail usually
utterly unknown to us, depends partly or entirely on certain influences
which the organs of propagation in the parental organism have undergone.
A second law of indirect adaptation, which we shall call _the law of
monstrous or sudden adaptation_, is of less importance and less general
than the law of individual adaptation. Here the divergences of the
child-organism from the parental form are so striking that, as a rule,
we may designate them as monstrosities. In many cases they are produced,
as has been proved by experiments, by the parental organism having been
subject to a certain treatment, and placed under peculiar conditions of
nutrition; for example, when air and light are withdrawn from it, or
when other influences powerfully acting upon its nutrition are changed
in a certain way. The new condition of existence causes a strong and
striking modification of form, not directly of the organism itself, but
only of that of its descendants. The mode of this influence in detail we
cannot discover, and we can only in a very general way detect a causal
connection between the abnormal formation of the child and a certain
change in the conditions of existence of its parents exerting a special
influence upon the organs of propagation in the latter. The previously
mentioned phenomenon of albinism probably belongs to this group of
abnormal or sudden variations, also the individual cases of human beings
with six fingers and toes, the case of the hornless cattle, as well as
those of sheep and goats with four or six horns. The abnormal deviation
in all these cases probably owes its origin to a cause which at first
only affected the reproductive system of the parental organism, the egg
of the mother or the sperm of the father.
A third curious manifestation of indirect adaptation may be termed _the
law of sexual adaptation_. Under this name we indicate the remarkable
fact that certain influences, which act upon the male organs of
propagation only, affect the structure of the male descendants, and in
like manner other influences, which act upon the female organs of
propagation only, manifest their effect only in the change of structure
of the female descendants. This remarkable phenomenon is still very
obscure, and has not as yet been investigated, but is probably of great
importance in regard to the origin of “secondary sexual
characteristics,” to which we have already made allusion.
All the phenomena of sexual, monstrous, and individual adaptation, which
we may comprise under the name of the laws of _indirect or potential
adaptation_, are as yet very little known to us in their real nature and
in their deeper causal connection. Only this much we can at present
maintain with certainty, that numerous and important transformations in
organic forms owe their existence to this process. Many and striking
variations of form solely depend on causes which at first only affect
the nutrition of the parental organism, and specially its organs of
propagation. Evidently the relations in which the sexual organs stand to
other parts of the body are of the greatest importance. We shall have
more to say of these presently, when we speak of the law of correlative
adaptation. How powerfully the variations in the conditions of life and
nutrition affect the propagation of organisms is rendered obvious by the
remarkable fact that numerous wild animals which we keep in our
zoological gardens, and exotic plants which are grown in our botanical
gardens, are no longer able to reproduce themselves. This is the case,
for example, with most birds of prey, parrots, and monkeys. The
elephant, also, and the animals of prey of the bear genus, in captivity
hardly ever produce young ones. In like manner many plants in a
cultivated state become sterile. The two sexes may indeed unite, but no
fructification, or no development of the fructified germ, takes place.
From this it follows with certainty that the changed mode of nutrition
in the cultivated state is able completely to destroy the capability of
reproduction, and therefore to exercise the greatest influence upon the
sexual organs. In like manner other adaptations or variations of
nutrition in the parental organism may cause, not indeed a complete want
of descendants, but still important changes in their form.
Much better known than the phenomena of indirect or potential adaptation
are those of _direct or actual adaptation_, to the consideration of
which we now turn our attention. To them belong all those changes of
organisms which are generally considered to be the results of practice,
habit, training, education, etc.; also those changes of organic forms
which are effected directly by the influence of nutrition, of climate,
and other external conditions of existence. As has already been remarked
in direct or actual adaptation, the transforming influence of the
external cause affects the form of the organism itself, and does not
only manifest itself in that of the descendants. (Gen. Morph. ii. 207.)
We may place _the law of universal adaptation_ at the head of the
different laws of direct or actual adaptation, because it is the chief
and most comprehensive among them. It may be briefly explained in the
following proposition: “All organic individuals become unequal to one
another in the course of their life by adaptation to different
conditions of life, although the individuals of one and the same species
remain mostly very much alike.” A certain inequality of organic
individuals, as we have seen, was already to be assumed in virtue of the
law of individual (indirect) adaptation. But, beyond this, the original
inequality of individuals is afterwards increased by the fact that every
individual, during its own independent life, subjects and adapts itself
to its own peculiar conditions of existence. All different individuals
of every species, however like they may be in their first stages of
life, become in the further course of their existence less like to one
another. They deviate from one another in more or less important
peculiarities, and this is a natural consequence of the different
conditions under which the individuals live. There are no two single
individuals of any species which can complete their life under exactly
the same external circumstances. The vital conditions of nutrition, of
moisture, air, light; further, the vital conditions of society, the
inter-relations with surrounding individuals of the same or other
species, are different in every individual being; and this difference
first affects the functions, and later changes the form of every
individual organism. If the children of a human family show, even at the
beginning, certain individual inequalities which we may consider as the
consequence of individual (indirect) adaptation, they will appear still
more different at a later period of life, when each child has passed
through different experiences, and has adapted itself to different
conditions of life. The original difference of the individual processes
of development, evidently becomes greater the longer the life lasts and
the more various the external conditions which influence the separate
individuals. This may be demonstrated in the simplest manner in man, as
well as in domestic animals and cultivated plants, in which the vital
conditions may be arbitrarily modified. Two brothers, of whom one is
brought up as a workman and the other as a priest, develop quite
differently in body as well as in mind; in like manner, two dogs of one
and the same birth, of which one is trained as a sporting dog and the
other chained up as a watch dog. The same observation may also readily
be made as to organic individuals in a natural state. If, for instance,
one carefully compares all the trees in a fir or beech forest, which
consists of trees of a single species, one finds that among all the
hundreds or thousands of trees, there are not two individual trees
completely agreeing in size of trunk and other parts, in the number of
branches, leaves, etc. Everywhere we find individual inequalities which,
in part at least, are merely the consequences of the different
conditions of life under which the trees have developed. It is true we
can never say with certainty how much of this dissimilarity in all the
individuals of every species may have originally been caused by indirect
individual adaptation, and how much of it acquired under the influence
of direct or universal adaptation.
A second series of phenomena of direct adaptation, which we may comprise
under _the law of cumulative adaptation_, is no less important and
general than universal adaptation. Under this name I include a great
number of very important phenomena, which are usually divided into two
quite distinct groups. Naturalists, as a rule, have distinguished,
first, those variations of organisms which are produced directly by the
permanent influence of external conditions (by the constant action of
nutrition, of climate, of surroundings, etc.), and secondly, those
variations which arise from habit and practice, from accustoming
themselves to definite conditions of life, and from the use and non-use
of organs. The latter influences have been set forth especially by
Lamarck as important causes of the change of organic forms, while the
former have for a very long time been recognized as such more generally.
The sharp distinction usually made between these two groups of
cumulative adaptation, and which even Darwin still maintains, disappears
as soon as we reflect more accurately and deeply upon the real nature
and causal foundation of these two, apparently very different, series of
adaptations. We then arrive at the conviction that in both cases there
are always two different active causes to be dealt with: on the one hand
the _external influence_ or _action_ of adaptative conditions of life,
and on the other hand the _internal reaction of the organism_ which
subjects and adapts itself to that condition of life. If cumulative
adaptation is considered from the first point of view alone, and the
transforming actions of the permanent external conditions of life are
traced to those conditions solely, then the principal stress is laid
unduly upon the external factor, and the necessary internal reaction of
the organism is not taken into proper consideration. If, on the other
hand, cumulative adaptation is unjustly regarded solely in relation to
its second factor, and the transforming action of the organism itself,
its reaction against the external influences, its change by practice,
habit, use, or non-use of organs, is put into the foreground, then we
forget that this reaction is first called into play by the action of
external conditions of existence. Hence it seems that the distinction
made between these two groups lies only in the different manner of
viewing them, and I believe that they can, with full justice, be
considered as one. The most essential fact in these phenomena of
cumulative adaptation is that the change of the organism which manifests
itself first in the functions, and at a later period in the form, is the
result either of long enduring, or of often repeated, influences of an
external cause. The smallest cause, by cumulation of its action, can
attain the greatest results.
There are innumerable examples of this kind of direct adaptation. In
whatever direction we may examine the life of animals and plants, we
discover on all hands evident and undeniable changes of this kind. Let
me first mention some of those phenomena of adaptation occasioned
directly by nutrition itself. Every one knows that the domestic animals
which are bred for certain purposes can be variously modified, according
to the different quantity and quality of the food given to them. If a
farmer in breeding sheep wishes to produce fine wool, he gives them
different food from what he would give if he wished to obtain good flesh
or an abundance of fat. Choice race and carriage horses receive better
food than dray and cart horses. Even the bodily form of man—for
example, the amount of fat—is quite different according to his
nutrition. Food containing much nitrogen produces little fat, that
containing little nitrogen produces a great deal of fat. People who, by
means of Banting’s system, at present so popular, wish to become thin
eat only meat and eggs—no bread, no potatoes. The important variations
that can be produced among cultivated plants, solely by changing the
quantity and quality of nourishment, are well known. The same plant
acquires an altogether different appearance, according as it is placed
in a dry and warm place, exposed to the sunlight or placed in a cool
damp spot in the shade. Many plants, if transferred to the sea shore,
get in a short space of time thick, fleshy leaves, and the same plants
placed in a particularly dry and hot locality get thin hairy leaves. All
these variations arise directly from the cumulative influence of changed
nutrition.
But it is not only the quantity and quality of the articles of nutrition
which affect and powerfully change and transform the organism, but it is
affected also by all the other external conditions of existence, above
all by its nearest organic surroundings, the society of friendly or
hostile organisms. One and the same kind of tree develops itself quite
differently in an open locality, where it is free on all sides, and in a
forest where it must adapt itself to its surroundings, where it is
pressed on all sides by its nearest neighbours, and is forced to shoot
upwards. In the former case, the branches of the tree spread widely out;
in the latter, the trunk extends upwards, and the top of the tree
remains small and contracted. How powerfully all these circumstances,
and how powerfully the hostile or friendly influence of surrounding
organisms, of parasites, etc., affect every animal and every plant, is
so well known, that it appears superfluous to quote further examples.
The change of form, or transformation which is thereby effected, is
never solely the direct result of the external influence, but must
always be traced to the corresponding reaction, and to the activity of
the organism itself, which consists in contracting a habit, or practice,
and in the use or non-use of organs. The fact that these latter
phenomena, as a rule, have been considered distinct from the former, is
owing first to the one-sided manner of viewing them already mentioned,
and secondly to the wrong notion which has been formed as to the nature
and the influence of the activity of the will in animals.
The activity of the will, which is the organ of habit, of practice, of
the use or non-use of organs among animals, is, like every other
activity of the animal soul, dependent upon material processes in the
central nervous system, upon peculiar motions which emanate from the
albuminous matter of the ganglion cells, and the nervous fibres
connected with them. The will, as well as the other mental activities,
in higher animals, in this respect is different from that of men only in
quantity, not in quality. The will of the animal, as well as that of
man, is never free. The widely spread dogma of the freedom of the will
is, from a scientific point of view, altogether untenable. Every
physiologist who scientifically investigates the activity of the will in
man and animals, must of necessity arrive at the conviction that _in
reality the will is never free_, but is always determined by external or
internal influences. These influences are for the most part ideas which
have been either formed by Adaptation or by Inheritance, and are
traceable to one or other of these two physiological functions. As soon
as we strictly examine the action of our own will, without the
traditional prejudice about its freedom, we perceive that every
apparently free action of the will is the result of previous ideas,
which are based on notions inherited or otherwise acquired, and are
therefore, in the end, dependent on the laws of Adaptation and
Inheritance. The same also applies to the action of the will in all
animals. As soon as their will is considered in connection with their
mode of life, in its relation to the changes which the mode of life is
subject to from external conditions, we are at once convinced that no
other view is possible. Hence the changes of the will which follow the
changes of nutrition, and which, in the form of practice, habit, etc.,
produce variations in structure, must be reckoned among the other
material processes of cumulative adaptation.
Whilst an animal’s will is adapting itself to changed conditions of
existence by the acquisition of new habits, practices, etc., it not
unfrequently effects the most remarkable transformations of the organic
form. Numerous instances of this may be found everywhere in animal life.
Thus, for example, many organs in domestic animals are suppressed, when
in consequence of a changed mode of life they cease to act. Ducks and
fowls in a wild state fly exceedingly well, but lose this facility more
or less in a cultivated state. They accustom themselves to use their
legs more than their wings, and in consequence the muscles and skeleton
used in flying are essentially changed in their development and form.
Darwin has proved this by a very careful comparative measurement and
weighing of the respective parts of the skeleton in the different races
of domestic ducks, which are all descended from the wild duck (_Anas
boschas_). The bones of the wings in tame ducks are weaker, the bones of
the legs, on the other hand, are more strongly developed than in wild
ducks. In ostriches and other running birds which have become completely
unaccustomed to fly, the consequence is that their wings are entirely
crippled and degenerate into mere “rudimentary organs” (p. 12). In many
domestic animals, especially in many races of dogs and rabbits, we find
that in the cultivated state they have acquired pendulous ears. This is
simply a consequence of a diminished use of the auricular muscles. In a
wild state these animals have to exert their ears very much in order to
discover an approaching foe, and this is accompanied by a strong
development of the muscular apparatus, which keeps the outer ears in an
upright position, and by which they can turn them in all directions. In
a domestic state the same animals no longer require to listen so
attentively, they prick up or turn their ears only a little; the
auricular muscles cease to be used, gradually become weakened, and the
ears hang down flabbily, or become rudimentary.
As in these cases the function, and consequently the form also, of the
organ becomes degenerated through disuse, so, on the other hand, it
becomes more developed by greater use. This is particularly striking if
we compare the brain, and the mental activity belonging to it, in wild
animals and those domestic animals which are descended from them. The
dog and horse, which are so vastly improved by cultivation, show an
extraordinary degree of mental development, in comparison with their
wild original ancestors, and evidently the change in the bulk of the
brain, which is connected with it, is mainly determined by persistent
exercise. It is also well known how quickly and powerfully muscles grow
and change their form by continual practice. Compare, for example, the
arms and legs of a trained gymnast with those of an immovable book-worm.
How powerfully external influences affect the habits of animals and
their mode of life, and in this way still further change their forms, is
very strikingly shown in many cases among amphibious animals and
reptiles. Our commonest indigenous snake, the ringed snake, lays eggs
which require three weeks’ time to develop. But when it is kept in
captivity, and no sand is strewn in the cage, it does not lay its eggs,
but retains them until the young ones are developed. The difference
between animals producing living offspring and those laying eggs is here
effaced simply by the change of the ground upon which the animal lives.
The water-salamanders, or tritons, which have been artificially made to
retain their original gills, are extremely interesting in this respect.
The tritons are amphibious animals, nearly akin to frogs, and possess,
like the latter, in their youth external organs of
respiration—gills—with which they, while living in water, breathe the
air dissolved in the water. At a later date a metamorphosis takes place
in tritons, as in frogs. They leave the water, lose their gills, and
accustom themselves to breathe with their lungs. But if they are
prevented from doing this by being kept shut up in a tank, they do not
lose their gills. The gills remain, and the water salamander continues
through life in that low stage of development, beyond which its lower
relations, the gilled salamanders, or Sozobranchiata, never pass. The
gilled salamander attains its full size, its sexual development, and
reproduces itself without losing its gills.
Great interest was caused a short time ago, among zoologists, by the
axolotel (Siredon pisciformis), a gilled salamander from Mexico, nearly
related to the triton; it had already been known for a long time, and
been bred on a large scale in the zoological garden in Paris. This
animal possesses external gills, like the young salamander, but retains
them all its life, like all other Sozobranchiata. This gilled salamander
generally remains in the water, with its aquatic organs of respiration,
and also propagates itself there. But in the Paris garden, unexpectedly
from among hundreds of these animals, a small number crept out of the
water on to the dry land, lost their gills, and changed themselves into
gill-less salamanders, which are not to be distinguished from a
North-American genus of tritons (Amblystoma), and breathe only through
lungs. In this exceedingly curious case we can directly follow the great
stride from water-breathing to air-breathing animals, a stride which can
indeed be observed every spring in the individual history of development
of frogs and salamanders. Just as every separate frog and every separate
salamander transforms itself from an amphibious animal breathing through
gills, at a later period into one breathing through lungs, so the whole
group of frogs and salamanders have arisen from animals breathing
through gills, and akin to the Siredon. The Sozobranchiata have remained
up to the present day in that low stage of development. Ontogeny here
explains phylogeny; the history of the development of individuals
explains that of the whole group (p. 10).
To the law of accumulative adaptation there closely follows a third law
of direct or actual adaptation, _the law of correlative adaptation_.
According to this important law, actual adaptation not only changes
those parts of the organism which are directly affected by its
influence, but other parts also not directly affected by it. This is the
consequence of organic solidarity, and especially of the unity of the
nutrition existing among all the parts of every organism. If, for
example, the hairiness of the leaves increases in a plant by its being
transferred to a dry locality, then this change reacts upon the
nutrition of other parts, and it may result in a shortening of the parts
of the stalk, and produce a more contracted form of the whole plant. In
some races of pigs and dogs—for example, in the Turkish dog—which by
adaptation to a warmer climate have more or less lost their hair, the
teeth also have degenerated. Whales and Endentata (armadillos), which by
their curious skin-covering are removed from the other mammals, also
show the greatest deviations in the formation of their teeth. Further,
those races of domestic animals (oxen and pigs) which have acquired
short legs have, as a rule, also a short and compact head. Among other
examples, the races of pigeons which have the longest legs are also
characterized by the longest beaks. The same correlation between the
length of the legs and beaks is universal in the order of stilted-birds
(Grallatores), in storks, cranes, snipe, etc. The correlations which
thus exist between different parts of the organism are most remarkable,
but their real cause is unknown to us. In general, we can of course say,
the changes of nutrition affecting an individual part must necessarily
react on the other parts, because the nutrition of every organism is a
connected, centralized activity. But why just this or that part should
exhibit this or that particular correlation is in most cases quite
unknown to us. We know a great number of such correlations in nutrition;
they are especially seen in those changes of animals and plants which
give rise to an absence of pigment (noticed previously)—in albinoes.
The want of the usual colouring matter goes hand in hand with certain
changes in the formation of other parts; for example, of the muscular
and osseous system, consequently of organic systems which are not at all
ultimately connected with the system of the outer skin. Very frequently
albinoes are more feebly developed, and consequently the whole structure
of the body is more delicate and weak than in coloured animals of the
same species. The organs of the senses and nervous system are in like
manner curiously affected when there is this want of pigment. White cats
with blue eyes are nearly always deaf. White horses are distinguished
from coloured horses by their special liability to form sarkomatous
tumours. In man, also, the degree of the development of pigment in the
outer skin greatly influences the susceptibility of the organism for
certain diseases; so that, for instance, Europeans with a dark
complexion, black hair, and brown eyes become more easily acclimatized
to tropical countries, and are less subject to the diseases there
prevalent (inflammation of the liver, yellow fever, etc.) than Europeans
of white complexion, fair hair, and blue eyes. (Compare above, p. 150.)
Among these correlations in the formation of different organs, those are
specially remarkable which exist between the sexual organs and other
parts of the body. No change of any part reacts so powerfully upon the
other parts of the body as a certain treatment of the sexual organs.
Farmers who wish to obtain an abundant formation of fat in pigs, sheep,
etc., remove the sexual organs by cutting them out (castration), and
this is indeed done to animals of both sexes. The result is an
excessive development of fat. The same is done to the singers in certain
religious corporations. These unfortunates are castrated in early youth,
in order that they may retain their high boyish voices. In consequence
of this mutilation of the genitals, the larynx remains in its youthful
stage of development. The muscular tissues of the body remain at the
same time weakly developed, while below the skin an abundance of fat
accumulates. But this mutilation also powerfully reacts upon the
development of the nervous system, the energy of the will, etc., and it
is well known that human castrates, or eunuchs, as well as castrated
animals, are utterly deficient in the special psychical character which
distinguishes the male sex. Man is a man, both in body and soul, solely
through his male generative glands.
These most important and influential correlations between the sexual
organs and the other parts of the body, especially the brain, are found
equally in both sexes. This might be expected even _à priori_, because
in most animals the two kinds of organs develop themselves from the same
foundation, and at the beginning are not different. In man, as in the
rest of the vertebrate animals, the male and female organs in the
original state of the germ are entirely the same, and the differences of
the two sexes only gradually arise in the course of embryonic
development (in man, in the ninth week of embryonic life), by one and
the same gland developing in the female as the ovary, and in the male as
the testicle. Every change of the female ovary, therefore, has a no less
important reaction upon the whole female organism than every change of
the testicle has upon the male organism. Virchow has expressed the
importance of this correlation in his admirable essay on “Das Weib und
die Zelle” (“Woman and the Cell”), in the following words:—“Woman is
woman only by her sexual glands; all the peculiarities of her body and
mind, of her nutrition and her nervous activity, the sweet delicacy and
roundness of her limbs, the peculiar formation of the pelvis, the
development of the breasts, the continuance of the high voice, that
beautiful ornament of hair on her head, with the scarcely perceptible
soft down on the rest of the skin—then again, the depth of feeling, the
truth of her direct perceptions, her gentleness, devotion, and
fidelity—in short, all the feminine qualities which we admire and
honour in a true woman are but a dependence of the ovary. Take this
ovary away, and the man-woman stands before us—a loathly abortion.”
The same close correlation between the sexual organs and the other parts
of the body occurs among plants as generally as among animals. If one
wishes to obtain an abundance of fruit from a garden plant, the growth
of the leaves is curtailed by cutting off some of them. If, on the other
hand, an ornamental plant with a luxuriance of large and beautiful
leaves is desired, then the development of the blossoms and fruit is
prevented by cutting off the flower buds. In both cases one system of
organs develops at the cost of the others. Thus, also, most variations
in the formation of leaves in wild plants result in corresponding
transformations of the generative parts or blossoms. The great
importance of this “compensation of development,” of this “correlation
of parts,” has been already set forth by Goethe, by Geoffroy St.
Hilaire, and other nature-philosophers. It rests mainly upon the fact
that direct or actual adaptation cannot produce an important change in a
single part of the body, without at the same time affecting the whole
organism.
The correlative adaptation between the reproductive organs and the other
parts of the body deserves a very special consideration, because it is,
above all others, likely to throw light upon the obscure and mysterious
phenomena of indirect or potential adaptation, which have already been
considered. For just as every change of the sexual organs powerfully
reacts upon the rest of the body, so on the other hand every important
change in another part of the body must necessarily more or less react
on the sexual organs. This reaction, however, will only become
perceptible in the formation of the offspring which arise out of the
changed generative parts. It is, in fact, precisely those remarkable and
imperceptible changes of the genital system (in themselves utterly
insignificant changes)—changes of the eggs and the sperm—brought about
by such correlations, which have the greatest influence upon the
formation of the offspring, and all the phenomena of indirect or
potential adaptation previously mentioned may in the end be traced to
correlative adaptation.
A further series of remarkable examples of correlative adaptation is
furnished by the different animals and plants which become degenerated
through parasitic life or parasitism. No other change in the mode of
life so much affects the shapes of organisms as the adoption of a
parasitical life. Plants thereby lose their green leaves; as, for
instance, our native parasitical plants, Orobanche, Lathræa, Monotropa.
Animals which originally have lived freely and independently, but
afterwards adopt a parasitical mode of life on other animals or plants,
in the first place cease to use their organs of motion and their organs
of sense. The loss of this activity is succeeded by the loss of the
organs themselves, and thus we find, for example, many crabs, or
crustacea, which in their youth possess a tolerably high degree of
organization, viz., legs, antennæ, and eyes, in old age completely
degenerate, living as parasites, without eyes, without apparatus of
motion, and without antennæ. The lively, active form of youth, has
become a shapeless, motionless lump. Only the most necessary organs of
nutrition and propagation retain their activity; all the rest of the
body has degenerated. Evidently these complete transformations are, to a
large extent, the direct consequences of cumulative adaption, of the
non-use and defective exercise of the organs, but a great portion of
them must certainly be attributed also to correlative adaptation.
(Compare Plate X. and XI.)
A seventh law of adaptation, the fourth in the group of direct
adaptation, is _the law of divergent adaptation_. By this law we
indicate the fact that parts originally formed alike have developed in
different ways under the influence of external conditions. This law of
adaptation is extremely important for the explanation of the phenomenon
of division of labour, or polymorphism. We can see this very easily in
our own selves; for instance, in the activity of our two hands. We
usually accustom our right hand to quite different work from that which
we give our left, and in consequence of the different occupation there
arises a different formation of the two hands. The right hand, which we
use much more than the left, shows a stronger development of the nerves,
muscles, and bones. The same applies to the whole arm. In most human
beings the bones and flesh of the right arm are, in consequence of their
being more employed, stronger and heavier than those of the left arm.
Now, as the special use of the right arm has been adopted and
transmitted by inheritance for thousands of years among Europeans, the
stronger shape and size of the right arm have already become hereditary.
P. Harting, an excellent Dutch naturalist, has shown by measuring and
weighing newly-born children, that even in them the right arm is more
developed than the left.
According to the same law of divergent adaptation, both eyes also
frequently develop differently. If, for example, a naturalist accustoms
himself always to use one eye for the microscope (it is better to use
the left), then that eye will acquire a power different from that of the
other, and this division of labour is of great advantage. The one eye
will become more short-sighted, and better suited for seeing things near
at hand; the other eye becomes, on the contrary, more long-sighted, more
acute for looking at an object in the distance. If, on the other hand,
the naturalist alternately uses both eyes for the microscope, he will
not acquire the short-sightedness of the one eye and the compensatory
degree of long-sight in the other, which is attained by a wise
distribution of these different functions of sight between the two eyes.
Here then again the function, that is the activity, of originally
equally-formed organs can become divergent by habit; the function reacts
again upon the form of the organ, and thus we find, after a long
duration of such an influence, a change in the more delicate parts and
the relative growth of the divergent organs, which in the end becomes
apparent even in their coarser outlines.
Divergent adaptation can very easily be perceived among plants,
especially in creepers. Branches of one and the same creeping plant,
which originally were formed alike, acquire a completely different form
and extent, a completely different degree of curvature and diameter of
spiral winding, according as they twine themselves round a thinner or a
thicker bar. The divergent change of form of parts originally identical
in form, which tending in different directions develop themselves under
different external conditions, can be distinctly demonstrated in many
other examples. As this divergent adaptation interacts with progressive
inheritance, it becomes the cause of a division of labour among the
different organs.
An eighth and last law of adaptation we may call _the law of unlimited
or infinite adaptation_. By it we simply mean to express that we know of
no limit to the variation of organic forms occasioned by the external
conditions of existence. We can assert of no single part of an organism,
that it is no longer variable, or that if it were subjected to new
external conditions it would not be changed by them. It has never yet
been proved by experience that there is a limit to variation. If, for
example, an organ degenerates from non-use, this degeneration ends
finally in a complete disappearance of the organ, as is the case with
the eyes of many animals. On the other hand, we are able, by continual
practice, habit, and the ever-increasing use of an organ, to bring it to
a degree of perfection which we should at the beginning have considered
to be impossible. If we compare the uncivilized savages with civilized
nations, we find among the former a development of the organs of
sense—sight, smell, and hearing—such as civilized nations can hardly
conceive of. On the other hand, the brain, that is mental activity,
among more civilized nations is developed to a degree of which the wild
savages have no idea.
There appears indeed to be a limit given to the adaptability of every
organism, by the “type” of its tribe or phylum; that is, by the
essential fundamental qualities of this tribe, which have been inherited
from a common ancestor, and transmitted by conservative inheritance to
all its descendants. Thus, for example, no vertebrate animal can acquire
the ventral nerve-chord of articulate animals, instead of the
characteristic spinal marrow of the vertebrate animals. However, within
this hereditary primary form, within this inalienable type, the degree
of adaptability is unlimited. The elasticity and fluidity of the organic
form manifests itself, within the type, freely in all directions, and to
an unlimited extent. But there are some animals, as, for example, the
parasitically degenerate crabs and worms, which seem to pass even the
limit of type, and have forfeited all the essential characteristics of
their tribe by an astonishing degree of degeneration. As to the
adaptability of man, it is, as in all other animals, also unlimited, and
since it is manifested in him above all other animals, in the
modifications of the brain, there can be absolutely no limit to the
knowledge which man in a further progress of mental cultivation may not
be able to exceed. The human mind, according to the law of unlimited
adaptation, enjoys an infinite perspective of becoming ever more and
more perfect.
These remarks are sufficient to show the extent of the phenomena of
Adaptation, and the great importance to be attached to them. The laws of
Adaptation, or the facts of Variation caused by the influence of
external conditions, are just as important as the laws of Inheritance.
All phenomena of Adaptation, in the end, can be traced to conditions of
nutrition of the organism, in the same way as the phenomena of
Inheritance are referable to conditions of reproduction; but the latter,
as well as the former, may further be traced to chemical and physical,
that is to mechanical, causes. According to Darwin’s Theory of Selection
the new forms of organisms, the transformations which artificial
selection produces in the state of cultivation, and which natural
selection produces in the state of nature, arise solely by the
interaction of such causes.