The Evolution Theory, Vol. 2 of 2

by August Weismann

LECTURE XXXIV

ORIGIN OF THE SPECIFIC TYPE (continued)

Illustration of phyletic evolution by an analogy—Reconciliation of Nägeli and Darwin—Unity of the specific type furthered by climatic variation—By natural selection: illustration from aquatic animals—Direct path of evolution—Natural selection works in association with amphigony—Influence of isolation in defining the specific type—Duration of the periods of constancy—The Siberian pine-jays—Species are, so to speak, 'variable crystals'—Gradual increase of constancy and decrease of reversions—Physiological segregation of species through mutual sterility—Romanes's physiological selection—Breeds of domestic animals mutually fertile, presumably therefore 'amiktic' species also—Mutual fertility in plant species—Mutual sterility certainly not a condition of the splitting up of species—Splitting up of species without amphigony—Lichens—Splitting up of species apart from isolation and mutual sterility, Lepus variabilis.

Our train of thought in the last lecture brought us back again to the so-called 'indifferent' characters, whose occurrence is so often used as evidence against natural selection, as a proof that evolution is guided essentially by internal forces alone. But this objection was based on a fallacy, for the fact that the first small variations are due to internal processes in the germ-plasm does not imply that the whole further course of evolution is determined by these alone, any more than the fact that a sleigh requires a push to start it on its descent down an inclined plane would imply that its rapid descent is due only to the force of the push, and not at the same time to the attraction of gravity. But the analogy is not quite sound, for the processes within the germ-plasm which condition and direct variation do not merely give them the first shove off; they are associated with every onward step in the evolutionary path of the species, they impel it further and further, and without these continual impulses the progressive movement would cease altogether. We have seen that, for internal reasons, germinal selection continues to impel the varying determinants further along the path on which they have started, and thus gives them cumulative strength, and that it is in this way that adaptations to the conditions of life are brought about. The evolution of the character of a species may thus be compared to the course of a sleigh upon a level snow-surface which it could traverse in any direction, but it is moved only by the impulses received through germinal selection. The conditions of life to which the varying parts have to adapt themselves may be thought of as the distant goal, and the processes in the germ-plasm which give rise to variation in a definite direction may be compared to numerous human beings scattered irregularly over the surface of the snow. If the sleigh receives from one of these a push which chances to be in the direction of the goal, it rushes on towards this and ultimately reaches it if the person pushing continues to push in the same direction. So far, then, it seems as if the transformation of the part concerned depended upon germinal selection alone, but we must not forget that the germ-plasm does not contain only one determinant for every part of the body, but as many determinants as there are ids. We must therefore increase the number of our sleighs, and now it is obvious that the pushers of the sleighs, that is, germinal selection, may push one sleigh on toward the goal, but others in the opposite or in any other direction. If we assume that all the sleighs which have taken a wrong direction must reach dangerous ground, and ultimately plunge into an abyss, but that from a neighbouring point sleighs were being dispatched to replace all that came to grief, that these in their turn might attempt to reach the goal, it would ultimately come about that the requisite number of sleighs would arrive at the goal—that is to say, that the new adaptation would be attained.

The abysses represent the elimination of the less favourable variational tendencies, and the constant replacing of sleighs represents the intermingling of fresh ids through amphimixis. If all the sleighs run in the wrong direction they all come to grief, that is, the individual concerned is eliminated with all the ids of its germ-plasm—it disappears altogether from the ranks of the species. But if only a portion of them run in the right direction, care is taken that in following generations, that is, in the continuation of the sleigh-race, this portion combines with those of another group which are also running in the right direction—that is, with the half number of ids from another germ-plasm in amphimixis.

It is not possible to follow the analogy further, but perhaps it may serve to illustrate how germinal selection may be the only impelling force in the organisms, and yet only a small part of its results are determined by itself, and by far the larger part by external conditions. We understand how a variation in a definite direction can exist, and yet it is not that which creates species, genera, orders, and classes; it is the selection and combination of the variational tendencies by the conditions of life, which occurs at every step. There was no variational tendency leading from terrestrial mammals to Cetaceans, but there was a variational tendency moving the nostrils upwards towards the forehead, the hind-limbs towards diminution, the lungs towards lengthening, the tail towards broadening out. But each of these variational tendencies was always only one of several possibilities, and that the particular path which led towards the 'goal' was followed was due to the fact that the others plunged into the abyss to which the wrong paths led, that is to say, they were weeded out by selection. Thus germinal selection offers a possibility of reconciliation between Nägeli's and Darwin's interpretations, which seem so directly contradictory; for the former referred everything to the hypothesis of an internal evolutionary force, the latter rejected this, and regarded natural selection as the main, if not the exclusive factor in evolution. The internal struggles for food, which we have assumed as occurring in the germ-plasm, represent an internal force, though not in the sense of Nägeli, who thought of determining influence operative from first to last, but still an impelling force, which determines the direction of variation for the individual determinants, and must therefore do the same for the whole evolution up to a certain point; for it is only the possible variations of the determinants in a germ-plasm which can be chosen, selected, combined, and increased by natural selection, and every germ-plasm cannot give rise to all sorts of variations; the determinants contained in it condition what is possible and what is not, and this is an important limitation to the efficacy of natural selection, and to a certain extent also implies a guiding and determining power on the part of the internal mainspring, to wit, germinal selection.

The essential difference between Darwin's view of the transformation of forms and my own lies in the fact that Darwin conceived of natural selection as working only with variations which are not only due to chance themselves, but the intensification of which also depends in its turn solely upon natural selection, while, according to my view, natural selection works with variational tendencies which become intensified through internal causes, and are simply accumulated by natural selection in an ever-growing majority of ids in a germ-plasm through the selection of individuals.

This affects our view of the establishment of a specific type in so far that my intra-germinal variational tendencies are not necessarily, and not always due to chance, though they are so in most cases. If certain determinants are impelled to vary in a particular direction through climatic or any other influences, as we have seen to be the case, for instance, with the climatic varieties of many Lepidoptera, then the corresponding determinants in all the individuals must vary in the same direction, and thus all the individuals of the species which are subject to the same influence must undergo the same variation. Transformations of this kind have exactly the appearance of resulting from 'an internal evolutionary force,' such as Nägeli assumed, and the unity of the specific type will not be disturbed by them.

Nor will this occur, as far as I can see, if the transformation of a species depends solely upon new adaptations and their internal consequences, for if a particular organism has to adapt itself to special new conditions, it will usually be able to do so only in one way, and thus natural selection will always allow the same suitable variational tendencies to survive and reproduce, so that the unity of the specific type will not be permanently disturbed in this way either. The more advantageous the new conditions of life prove, and the more diverse the ways in which they can be utilized, the more rapidly will the species first adapted to them multiply, and the more will their descendants be impelled to adapt themselves specially to the different possibilities of utilizing the new situation, and thus, from a parent species adapted in general to the new conditions, there arise forms adapted to its more detailed possibilities. I must refer again to the previous instance of the Cetaceans which originated from vegetarian littoral, or fluviatile mammals, and have evolved since the Triassic period into a very considerable number of species-groups. All are alike in their general adaptation, and these adaptations to the conditions of life of aquatic animals—the fish-like form, the flippers, the peculiarities of the respiratory organs and the organs of hearing—when once acquired would not and could not be lost again; but each of the modern groups of whales has its particular sphere of life, which it effectively exploits by means of subordinate adaptations. Thus there are the dolphins with their bill-like jaws and the two rows of conical teeth, their active temperament, rapid movements, and diet of fish; and the whalebone whales with their enormous gape, the sieve-apparatus of whalebone-plates, and a diet of small molluscs and the like. But each of these groups has split up into species, and if we again regard the principle of adaptation as determining and directing evolution, we are no nearer being able to prove the assumption in regard to individual cases, for we know too little about the conditions of life to be able to demonstrate that the peculiarities of structure are actually adaptations to these. Theoretically, however, it is quite easy to suppose that adaptation to a particular sphere of life was the guiding factor in their evolution, and if this be so—as we have already proved that it is in regard to the two chief groups and the whole class—then the harmony of the structure must be due solely to the continued selection of the fittest. We require no further principle of explanation for the establishment of a specific type.

This 'type' is thus not reached by any indefinite varying of the parent form in all directions, but in general it is reached by the most direct and shortest way. The parent form must indeed have become to some extent fluctuating, since not only the variational tendencies 'aiming at the goal,' but others as well, must have emerged in the germ-plasm; but gradually these others would occur less and less frequently, being always weeded out afresh by selection, until the great majority of the individuals would follow the same path of evolution, under the guidance of germinal selection, which continues to work in the direction that has once been taken. After a short period of variation, which need not, of course, involve the whole organism, but may refer only to certain parts of it, a steady direct progress in the direction of the 'goal,' that is, of perfect adaptation, will set in, as we have seen in the case of the Planorbis snails of Steinheim.