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.])