The Evolution Theory, Vol. 2 of 2

by August Weismann

If we are able to recognize all the more prominent features of the organization in Cetaceans and Birds as due to adaptation, we must conclude that, in the rest of the great groups of the animal kingdom, the main and essential parts of the structure are adaptations to the conditions of life, even although the relations between external circumstances and internal organization are not so readily recognizable. For if there were an internal evolutionary force at all, we should be able to recognize its operation in the origin of the races of Cetaceans and Birds; but if there be no such power, then even in cases where the conditions of life are not so conspicuously divergent as in Cetaceans and Birds, we must refer the typical structure of the group to adaptation. Thus everything about organisms depends upon adaptation, not only the main features of the organization, but the little details in as far as they possess selection-value; it is only what lies below this level that is determined by internal factors alone, by germinal selection; but this is not an imperative force in the sense in which the term is used by Nägeli and his successors, for it is capable of being guided; it does not necessarily lead to an invariable and predetermined goal, but it can be directed according to circumstances into many different paths. But it is precisely this that constitutes the main problem of the evolution theory—how development due to internal causes can, at the same time, bring about adaptation to external circumstances.

This lecture had been transcribed so far, and was ready for the press, when I received the first volume of a new work by De Vries, in which that distinguished botanist develops new views in regard to the transformation of species, based upon numerous experiments, carried on for many years on the variation of plants. As not only his views, but the interesting facts he sets forth, seem to contradict the conclusions as to the transmutation of organisms which I have been endeavouring to establish, I cannot refrain from saying something on the subject.

De Vries does not believe that the transformation of species can depend on the cumulative summation of minute 'individual' variations; he distinguishes between 'variations' and 'mutations,' and attributes only to the latter the power of changing the character of a species. He regards the former as mere fluctuating deviations which may be increased by artificial selection, and may even, with difficulty, if carefully and purely bred for a long period, be made use of to give character to a new breed, but which play no part at all in the natural course of phylogeny. As regards phylogeny, he maintains that only 'mutations' have any influence, that is, the larger or smaller saltatory variations which crop up suddenly and which have from the very first a tendency to be purely transmitted, that is, to breed true.

The facts upon which these views are mainly based are observations on and breeding experiments with a species of evening primrose (Œnothera) which was found in quantities on a fallow potato-field at Hilversum in Holland. It had been cultivated previously in a neighbouring garden, and had sown itself thence in the field. The numerous specimens of this Œnothera lamarckiana growing there were in a state of marked 'fluctuating' variability, but in addition there grew among them two strongly divergent forms which must have arisen from the others, and which led De Vries to bring the parent stock under cultivation, in the hope that it would yield new forms, in the Botanical Gardens at Amsterdam. This hope was fulfilled; in the second cultivated generation there were, among the 15,000 plants, ten which represented two divergent forms, and in the succeeding generations these forms were repeated several times and in many cases, and five other new forms cropped up, most of them in several specimens and in different generations of the original stock. All these new forms, which De Vries calls 'elementary species,' breed true, that is to say, when they are fertilized with their own pollen they yield seed which gives rise to the same 'elementary species.' The differences between the new forms are usually manifold, and of the same kind as those between the 'elementary' species of the wild Linnæan species. But, according to De Vries, what we have been accustomed since the time of Linné to call a 'species' is a collective category, whose components are these 'elementary' species which De Vries has observed in his experiments with Œnothera. In other species, such as Viola tricolor and Draba verna, true-breeding varieties have long been known to botanists, and these have been studied carefully and tested experimentally, especially by A. Jordan, and more recently by De Bary.

All 'species,' according to the Linnæan conception, consist, De Vries maintains, of a larger or smaller number (in Draba there are two hundred) of these 'elementary' species, and these arise, as is proved by the case of Œnothera, by saltatory or discontinuous 'variations' which occur periodically and suddenly break up a species into many new species, because the variations of the germ-plasm, which are for a time merely latent, suddenly find expression in the descendants of one individual or another. According to this view, species must be the outcome of purely internal causes of development, which reveal themselves as 'mutations,' that is as saltatory variations, which are stable and transmissible from the very first, and among which the struggle for existence decides which shall survive and which shall be eliminated. For the mutations themselves occur in no particular direction; they are sometimes advantageous, sometimes indifferent, sometimes even injurious (for instance, when one sex is left out), and so it is always only a fraction of the mutations, often only a few, which prove themselves capable of permanent existence. Thus 'species do not arise through the struggle for existence, but they are eliminated by it' (p. 150); natural selection does nothing more than weed out what is unfit for existence, it does not exercise any selective, in the sense of directive, influence on the survivors. A difference in the nature of variations was previously maintained by the American palæontologist Scott, though for different reasons and also with a different meaning. He believed that variations in a definite direction were necessary to explain the direct course of development which many animal groups, such as the horses and the ruminants, have actually followed, and which he thought could not be ascribed to cumulative adaptation to the conditions of life. The 'mutations' of De Vries are not distinguished from the 'fluctuating' variations by following a definite direction, but in that they are strictly heritable, that they 'breed true.' It is true that 'fluctuating' individual differences are also transmissible, and can be increased by artificial selection, but they lack one thing that would make them component parts of a natural species, namely, constancy; they do not breed true, and are therefore never independent of selection, but require to be continually selected out afresh in order that they may be kept pure. They form 'breeds,' not species, and if left to themselves they soon revert to the characters of the parent species, as is well known of the numerous 'ennobled races' among our cereals. De Vries therefore denies absolutely that a new species could be developed by natural selection from 'fluctuating' variations, and not alone because there is no constancy of character, but also because the capacity of the character for being increased is very limited. Usually nothing more can be achieved than doubling of the original character, and then progress becomes more difficult and finally ceases altogether.

These are incisive conclusions, based upon an imposing array of weighty facts. I readily admit that I have rarely read a scientific book with as much interest as De Vries's Mutationstheorie. Nevertheless I believe that one might be carried away too far by De Vries, for he obviously overestimates the value of his facts, interesting and important as these undoubtedly are, and under the influence of what is new he overlooks what lies before him—the other aspect of the transmutation of species, to which the attention of most observers since Darwin and Wallace has been almost exclusively devoted—I mean the origin of adaptations. Not that he does not mention these, he assumes in regard to his mutations 'a selection working in a constant direction,' and seeks to interpret them in terms of it, but as the mutations occur from purely internal reasons—I mean without any connexion with the necessity for a new adaptation—and occur only in a small percentage of individuals, and in no definite direction, they cannot possibly suffice to explain adaptation, which seems to dominate the whole organic world. But this is precisely the point at which many botanists cease to understand the zoologists, because among plants there are fewer adaptations than among animals; or, in any case, adaptations in plants are not so readily demonstrated as among animals, which not infrequently seem to us to be entirely built up of adaptations.

In this book, and in this chapter itself, I have discussed adaptations and their origin so much already that I need only refer to these pages for convincing evidence that we cannot think of them as being brought about by the accumulation and augmentation of individually occurring saltatory 'mutations.' Not even if we assume that the leaps of mutation can be increased in the course of generations; in short, even if we say that mutations are all those variations which breed true and lead to the development of species, while variations are those which do not. This would only be playing with words, so let us say that the fluctuating variations are really different in their nature, that is, in their causes, from mutations. De Vries lays great stress on the fact that these two kinds of variations must be sharply distinguished from one another, and this may have been useful or necessary for the first investigation of the facts before him, for we must first analyse and then recombine, but that variations and mutations are in reality different in nature can assuredly not be assumed, since innumerable adaptations can only have arisen through the augmentation of individual variations. These must therefore be able to become 'pure breeding,' even although they may not have done so in the cases of artificial selection which have hitherto been observed. How is it possible that chance mutations, in no particular direction, occurring only rarely and in a small percentage of individuals, can explain the origin of the leaf-marking of a Kallima or an Anæa—the shifting of the original wing-nervures to form leaf-veins, and the exact correlation of these veins across the surfaces of both pairs of wings? And even if we were to admit that a mutation might have occurred which caused the veins of the anterior and posterior wings to meet exactly by chance, that would still not be a leaf-adaptation, for there would still be wanting the instinct which compels the butterfly, when it settles down, to hold the wings in such a position that the two pictures on the anterior and posterior wings fit into each other. Correlated mutations of the nervous system suited to this end are required, but that is too much to attribute to happy chance! The same holds true in regard to the whole leaf-picture on the two wings, for it could not possibly have arisen as a whole by a sudden mutation. The whole litany of objections which have been urged throughout several decades against the Darwin-Wallace theory of natural selection, which were based on the improbability that chance variations not in a definite direction should yield suitable material for the necessary adaptations, may be urged much more strongly against mutations, which make their appearance in much smaller numbers and with less diversity.

But it is—as we have already seen—in regard to the necessity which exists almost everywhere for the co-adaptation of numerous variations of the most different parts, that the 'mutation theory' breaks down utterly. The kaleidoscopic picture, the mutation, is implicit from the first, and must be accepted or rejected just as it is in the struggle for existence; but harmonious adaptation requires a gradual, simultaneous, or successive purposive variation of all the parts concerned, and this can be secured only through the fluctuating variations which are always occurring, and are increased by germinal selection and guided by personal selection.

Many naturalists, and especially many botanists, regard adaptation as something secondary, something given to species by the way, to improve the conditions of their existence, but not affecting their nature—comparable perhaps to the clothing worn by man to protect himself from cold; but that is hardly the real state of the matter.

The deep-sea expedition conducted by Chun in 1898 and 1899 made many interesting discoveries in regard to animals living in the depths of the ocean, all of which exhibit peculiar adaptations to the special conditions of their life, and especially to the darkness of the great depths. One of the most striking of these discoveries was that of the luminous organs which are found not in all but in a great many animals living on the bottom of the abyssal area, and also among the animals occurring at various levels above the floor of the abyss. These are sometimes glands which secrete a luminous substance, but sometimes complex organs, 'lanterns' which are controlled by the will of the animal, and suddenly evolve a beam of light and project it in a particular direction, like an electric searchlight. These organs have a most complex structure, composed of nerves and lenses, which focus the light, and on the whole are not unlike eyes. That this sort of structure should have arisen all at once through a 'mutation' is inconceivable; it can have originated only from simple beginnings by a gradual increase of its structure along with continual strict selection among the variations which cropped up. They all depend upon complicated 'harmonious' adaptation, and cannot possibly have been derived from mutations, that is, from ready-made structural 'constellations,' unless we are to call in the aid of the miraculous. But lanterns of this kind are found in many different kinds of animals—in Schizopod Crustaceans, in shrimps, in fishes of different genera and families. Many fishes have long rows of luminous organs on the sides and on the belly, and these probably serve to light up the sea-floor and facilitate the finding of food; in others the luminous organs are placed upon the snout just above the wide voracious mouth, and in that position they have undoubtedly the significance attributed to them by Chun, namely, that they attract small animals, just as the electric lamps allure all sorts of nocturnal animals, and especially insects, in large numbers to their destruction. But not fishes only, but molluscs, e.g. the Cephalopods of the great depths, have developed luminous organs, and one species of Cephalopod has about twenty large luminous organs, like gleaming jewels, ultramarine, ruby-red, sky-blue and silvery, while in another the whole surface of the belly is dotted over with little pearl-like luminous organs. Even if we cannot be quite clear as to the special use of these lanterns of deep-sea animals, there can be no doubt that they are adaptations to the darkness of the great depths, and when we find the same adaptations (in a physiological sense) in many animals belonging to the most diverse groups, there is no possibility of referring them to sudden mutations which have arisen all at once in these groups with no relation to utility, and yet have not occurred in any animals living in the light. Only 'variations' progressing and combining in the direction of utility can give us the key to an explanation of the origin of such structures.