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.