The Evolution Theory, Vol. 2 of 2

by August Weismann

THE ORIGIN AND THE EXTINCTION OF SPECIES

Adaptation does not depend upon chance—The case of eyes—Of leaf-mimicry—All persistent change depends ultimately on selection—Mutual sterility without great significance—Relative isolation (Lepus variabilis)—Influence of hybridization—Decadence of species—Differences in the duration of decadence—Natural death of individuals—Extinction due to excessive variability (Emery)?—Machairodus as interpreted by Brandes—Lower types more capable of adaptation than higher—Flightless birds—Disturbance of insular fauna and flora by cultivation—The big game of Central Europe.

In the polar hare we have a case in which the adaptations to the life conditions both of time and space are recognizable as the effect of definite causes, and thus as a necessity; but the same must be true everywhere even in regard to the most complex adaptations which seem to depend entirely upon chance; everywhere adaptation results of necessity—if it is possible at all with the given organization of the species—as certainly as the adaptation dress of the hare depends on the length of the winter, and in point of fact not less certainly than the blue colour of starch on the addition of iodine. The most delicate adaptations of the vertebrate eye to the task set for it by life in various groups have been gradually brought about as the necessary results of definite causes, just in the same way as the complex protective markings and colouring on the wing of the Kallima and other leaf-mimicking butterflies.

That adaptations can be regarded as mechanically necessitated is due to the fact that in every process of adaptation the same direction of variation on the part of the determinants concerned is guaranteed, since personal selection eliminates those which vary in a wrong direction, so that only those varying in a suitable direction survive, and they then continue to vary in the same direction. But the greatest difference between our conception of natural selection and that of Darwin lies in this: that Darwin regarded its intervention as dependent upon chance, while we consider it as necessary and conditioned by the upward and downward intra-germinal fluctuation of the determinants. Appropriate variational tendencies not only may present themselves, they must do so, if the germ-plasm contains determinants at all by whose fluctuations in a plus or minus direction the appropriate variation is attainable.

That a horse should grow wings is beyond the limits of the possibilities of equine variation—there are no determinants which could present variations directed towards this goal; but that any multicellular animal which lives in the light should develop eyes lies within the variational possibilities of its ectoderm determinants, and in point of fact almost all such animals do possess eyes, and eyes, too, whose functional capacity may be increased in any direction, and which are adaptable and modifiable in any manner in accordance with the requirements of the case. As soon as the determinants of the most primitive eye came into existence, they formed the fundamental material by whose plus- or minus-variations all the marvellous eye structures might be brought about, which we find in the different groups of the Metazoa, from a mere spot sensitive to light to a shadowy perception of a moving body, and from that again to the distinct recognition of a clear image, which we are aware of in our own eyes. And what wonderful special adaptations of the eye to near and to distant vision, to vision in the dusk and at night, or in the great ocean-depths, to recognition of mere movement or the focussing of a clear image, have been interpolated in the course of this evolution!

All such adaptations are possible, because they can proceed from variations of determinants which are in existence; and in the same way it is possible, at every stage of the evolution of organisms, for eyes to degenerate again, whether they have been high up or low down in the scale of gradations of this perhaps the most delicate of all our sense-organs. As soon as a species migrated permanently from the light into perfect darkness its eyes began to degenerate. We know blind flat worms, blind water-fleas and Isopods, also blind insects and higher Crustaceans, and even blind fishes and amphibians, the eyes of which are now to be found at very different levels of degeneration, as Eigenmann has recently shown in regard to several species of cave-dwelling salamanders of the State of Ohio. In all these cases it is only necessary for the determinants of the eye to continue to vary in the minus direction, and the disappearance of the eye must be gradually brought about.

We must picture upward development in quite a similar way. The forest butterflies of the Tropics could not possibly all have their under surfaces coloured like a leaf if the protective pattern depended solely upon the chance of a useful variation presenting itself. It always presented itself through the fluctuations of the determinants, and thus the appropriate colourings were not merely able to develop, but of necessity did so in gradually increasing perfection. If chance played any part in the matter, it would be quite unintelligible why the protective colouring should occur only where it acts as a protection, and why, for instance, it should not appear sometimes upon the upper surface of the butterfly wing, or upon the posterior wings which are covered when the butterfly is at rest. We have already studied in detail the precision with which the coloration is localized on minute points and corners of the wing: this can only be understood if natural selection works with the certainty of a perfect mechanism. Chance only comes into the matter in so far as it depends upon chance whether the relevant determinants in one id or another are to vary in the direction of plus or minus; but as the germ-plasm contains many ids, and chance may decide it differently in each of these, the presence of a majority of determinants varying in a desirable direction does not depend upon chance, for if they are not contained in one individual they are in another. It is only necessary that they should be present in some, and that these should be selected for reproduction.

We must therefore regard natural selection, that is to say, personal selection, as a mechanical process of development, which begins with the same certainty and works 'in a straight line' towards its 'goal,' just as any principle of development might be supposed to do. Fundamentally it is after all a purely internal force which gives rise to evolution, the power of the most minute vital units to vary under changing influences, and it is only the guidance of evolution along particular paths that is essentially left to personal selection, which brings together what is useful and thus determines the direction of further evolution. If we bear in mind that even the minutest variations of the biophors and determinants express nothing more or less than reactions to changed external conditions in the direction of adaptation, and that the same is true of each of the higher categories of vital units, whether they be called cell, tissue, organ, person, or corm, we see that the whole evolution of the forms of life upon the earth depends upon adaptations following each other in unbroken succession, and fitting into each other in the most complex way. The whole evolution is made possible by the power of variation of the living units of every grade, and called forth and directed by the ceaseless changes of the external influences. I said years ago that everything in organic evolution depended upon selection, for every lasting change in a vital unit means adaptation to changed external influences, and implies a preference in favour of the parts of the unit concerned, which are thereby more fitly disposed.

In this sense we can also say that the species is a complex of adaptations, for we have seen that it depends upon the co-operation of different grades of selective processes, that in many cases it is produced solely by germinal selection, but that in very many more personal selection plays the chief part, whether in bringing about sexual adaptations, or adaptations to the conditions of existence.