The Evolution Theory, Vol. 2 of 2

by August Weismann

Different modes and grades of selection—Changes due to the influences of environment—Superfluity and lack of food—The horses and cattle of the Falkland Islands—Angora animals—Protection against cold in Arctic and marine mammals—Plant-galls—Nägeli's Hieracium experiments—Experiments with Polyommatus phlæas—Artificially produced Vanessa-aberrations—Vöchting's experiments on the influence of light in the production of flower-forms—Heliotropism and other tropisms—Primary and secondary reactions of organisms—Herbst's 'lithium larvæ'—Schmankewitsch's experiments with Artemia—Poulton's caterpillars with facultative colour adaptation—Colour-change in fishes, chamæleon, &c.—Actual scope of those influences which directly produce organic changes.

Through a long series of lectures we have devoted our attention to those phenomena which bear some relation to the processes of selection; we have attempted to gain clearness in regard to the modes and stages of these, and we reached the result that all variations which have taken place in organisms since the first appearance of living matter are directed by processes of selection, that is, their direction and duration are determined by these processes, although they may have their roots in external influences. But it is not to be supposed that this guidance is due solely to that one kind of selection which, with Darwin and Wallace, we designate 'natural selection'; on the contrary, we must regard this as only one of the different modes of the processes of selection, necessarily occurring between all living units which are equivalent to one another, and which, therefore, must maintain a continual struggle with one another for space and food. If the expression 'natural selection' were not already so firmly fixed in its meaning, I should propose that it should be employed in the most general sense for all the processes of selection collectively, but we must keep to its original meaning and use it only for personal selection.

We have seen that processes of selection take place even between the elements of the germ-plasm in all organisms which possess a germ-plasm as distinguished from the mass of the body, and that through these processes there arise those hereditary individual variations which, under some circumstances, form the basis of transformations in the species.

Obviously this may come about in a twofold manner: firstly, a variation movement originating in the germ-plasm may go on increasing till it attains to selection-value, and then 'personal selection' steps in, and seeks to make it the common property of the species. But it is obviously also conceivable that variational tendencies arising in the germ-plasm may never attain to selection-value at all, and then in most cases they will only continue to exist through a longer or shorter series of generations as individual distinguishing characters, without being transmitted to a larger number of individuals or becoming a constant character of the species. Their persistence will depend essentially on the chance of mingling with other individuals, and on the halving of the germ-plasm which precedes sexual reproduction. Sooner or later these individual peculiarities disappear again, as may often be observed in the case of abnormalities or morbid tendencies in man, in as far as these do not weaken vitality. In the latter case they attain selection-value, though only negatively.

But even quite indifferent germinal variations, which neither raise nor lower the individual's power of survival, may, under some circumstances, increase and lead to permanent variations of all the individuals of a species, and this happens when they are conditioned by external influences which affect all the individuals of a species, or of the particular colony concerned, and it is this kind of organismal change which we shall now study for a little in detail.

The ordinary never-ceasing, always active germinal selection depends, we must assume, upon intra-germinal fluctuations of nutrition, or inequalities in the nutritive stream which circulates within the germ-plasm. The variations which it produces may, therefore, be different in each individual, since these fluctuations are a matter of chance and may affect the determinants A in one individual and the determinants B, C, or X in another, or alternating groups of these. Or it may be that the homologous determinants A may vary in a plus direction in one individual, and in a minus direction in another, while in a third they may remain unchanged, and although the same direction of variation of a determinant N may occur in many individuals, it will certainly not do so in all, and still less will it occur in all along with the same combination of fluctuations in the rest of the determinants. It is only if this occurs that the variation can become a specific character.

We might expect on a priori grounds that not only the chance fluctuations of nutrition within the germ-plasm would cause its elements to vary in this or that direction, but that there would also be influences of a more general kind, especially those of nutrition and climate, which would in the first place affect the body as a whole, but with it also the germ-plasm, and which would therefore bring about variations, either in all or only in certain determinants. In this case all the individuals would vary in the same way, because all would be similarly affected by the same causes of change.

This is actually the case; it is indubitable that external influences, such as those emanating from the environment or media in which species live, are able to cause direct variation of the germ-plasm, that is, permanent, because hereditary variations. We have already referred to this process and called it 'induced germinal selection.'

That such influences of environment may bring about changes in individual organisms is obvious enough; that, for instance, good nutrition makes the body strong and vigorous, that too abundant food makes it fat and causes degeneration, that insufficient food lessens its stamina and vigour, are well-known facts. We have to inquire, on the one hand, to what extent such influences are able to cause changes in the individual body in the course of a lifetime, and, on the other hand, more particularly, how far such changes or modifications of the soma can call forth corresponding variations in the determinant system of the germ-cells, and whether and under what circumstances they may be transmitted; for where this is not the case there can be no permanent hereditary variation of the whole species, and the variation will only persist as long as the conditions which gave rise to it endure, and will disappear again with these.

The influence of nutrition as a cause of variation has often been over-estimated. The old statement which has gone the round of the textbooks since the time of John Hunter, that the stomach of carnivores may be transformed by vegetable diet into a herbivore stomach, is absolutely unproved. Brandes at least, who not only subjected all the statements in the literature on this point to a critical investigation, but also instituted experiments of his own, regards the statement as altogether unfounded. All the 'cases' cited, in which the stomach of a gull or of an owl fed on grain became transformed into an organ with stronger muscles and covered with horny plates, depend, according to Brandes, upon inexact observation. There can therefore be no question of any inheritance of this fictitious stomach-transformation, and the idea that such a fundamental histological adaptation as the alleged transformation of the stomach of the grain-eating bird should arise as a direct effect of the food is wholly without foundation.