The Evolution Theory, Vol. 2 of 2

by August Weismann

There must then be something else at work which causes the minus-variations in a disused organ to preponderate persistently and permanently over the plus-variations, and this something can lie nowhere else than where the roots of all hereditary variations are to be found—in the germ-plasm. This train of thought leads us to the discovery of a process which we must call selection between the elements of the germ-plasm, or, as I have named it shortly, Germinal Selection.

If the substance of the germ-plasm is—as we assumed—composed of heterogeneous living particles, which have dissimilar rôles in the building up of the organism, there must of necessity be among them a definite labile state of equilibrium, which cannot be disturbed without modifying in some way the structure of the organism itself which arises from the germ-plasm. But if our further view be correct, that these individual and different living units of the germ-plasm are 'determinants,' that is, are the primary constituents of particular parts of the organism, in the sense that these parts could not arise if their determinants were absent from the germ-plasm, and that they would be different if the determinants were differently composed, we can draw far-reaching deductions.

It is true that we cannot learn anything directly in regard to the intimate structure of the germ-plasm, and even in regard to the vital processes going on within it we can only guess a very little, but so much we may say—that its living parts are nourished, and that they multiply. But it follows from this that nourishment in a dissolved state must penetrate between its vital particles, and that whether the determinants grow, and at what rate they do so, depends mainly on the amount of nourishment which reaches them. As long as the germ-cells multiply by division the determinants have no other function but to grow; a part of their substance undergoes oxidation and thereby yields the supply of energy necessary to assimilation, that is, to the formation of new living substance.

If each kind of determinant always secured the same quantity of nourishment, all would grow in the same degree, that is, in exact proportion to their power of assimilation. But we know that in less minute conditions which we can observe more directly, there is nowhere absolute equality, that all vital processes are subject to fluctuations; any little obstacles in the current of the nutritive fluid, or in its composition, may cause poorer nutrition of one part, better of another. We may therefore assume that there are similar irregularities and differences in the minute and unobservable conditions of the germ-plasm likewise, and the result must be a slight shifting of the position of equilibrium as regards size and strength in the determinant system; for the less well-nourished determinants will grow more slowly, will fail to attain to the size and strength of their neighbours, and will multiply more slowly.

But the vigour of growth does not depend only on the influence of nourishment; one cell grows quickly, another slowly in the same nutritive fluid; it depends in great part on the cell's power of assimilation. In the same way the assimilating power of the determinants and their affinity for nourishment will vary with their constitution, and a weaker determinant will remain smaller than a stronger one, even when the stream of nourishment is the same.

It seems to me that it is upon the unequal nutrition of the determinants conditioned by the chances of the food-supply that individual hereditary variability ultimately depends. If, for instance, the determinant A receives poorer nourishment at a particular time than the determinant B, it will grow more slowly, remain weaker, and then, when the germ-cells develop into an animal, the part to which it gives rise will be weaker than it usually is in other individuals.

These primary inequalities in the equipment of the determinants which are caused by a passing inequality in the food-stream are, of course, so slight that we are unable to observe their consequences. They must persist for a considerable time before they become observable, but they may persist for a long time, and their effect must then mount up, because every diminution in the strength of the determinant also signifies a lessened power of assimilation, and growth becomes slower for the twofold reason that passive and active nutrition decrease at the same time. In the less minute conditions observable in the histological elements of the body we know that function strengthens the organ, and that disuse weakens it, and we are justified in applying this proposition also to these more intimate conditions and minuter vital units. Thus, in the course of the multiplication of the germ-cells, the less vigorously working determinant, A, will gradually, but very slowly, become weaker, that is, of diminished power of assimilation, presupposing of course that the intra-germinal food-stream does not become stronger again at the same place—a possibility to which I shall subsequently refer. But while one determinant may be slowly becoming weaker, its neighbour, on the other hand, may be varying on an ascending scale, just because the former is, on account of its diminished power of assimilation, no longer able to exhaust completely the food-stream which flows to it.

The determinants are thus in constant motion, here ascending, there descending, and it is in these fluctuations of the equilibrium of the determinant-system that I see the roots of all hereditary variation, while in the fact that the variation-directions of particular determinants must continue the same without limit as long as they meet with no obstacle lies the possibility of the adaptation of the organism to changing conditions, the increase and transformation of one part, the degeneration and disappearance of another, in short, the processes of natural selection. The reason why such variation movements must continue until they meet some resistance is that every chance upward or downward movement—due, that is, to mere passive fluctuation in the food-supply, at the same time strengthens or weakens the determinant, and makes it either more or less capable of attracting nourishment to itself; in the former case an increasingly strong stream of food will be directed towards it, in the latter more and more of the available food-supply will be withdrawn from it by its neighbour-determinants on all sides; in the former the determinant will go on increasing in strength as long as it can go on attracting more nourishment, in the latter it will continue to become weaker until it disappears altogether. To the ascending progression, as is evident, there are limits set, not only by the amount of food which can circulate through the whole id, but also by the neighbour determinants, which will sooner or later resist the withdrawal of nourishment from them; but for the descending progression there are no limits except total disappearance, and this is actually reached in all cases in which the determinants are related to a part which has become useless. But both these movements, the upward and the downward alike, are quite independent of natural selection, i.e. of personal selection; they are processes of a unique kind which run their course purely in accordance with intra-germinal laws. Whether a determinant 'ascends' or 'descends' depends solely upon the play of forces within the germ-plasm, not upon whether the direction of the variation in question is useful or prejudicial, or on whether the organ in question, the determinate, is of value or otherwise. In this fact lies the great importance of this play of forces within the germ-plasm, that it gives rise to variations quite independently of the relations of the organism to the external world. In many cases, of course, personal selection intervenes, but even then it cannot directly effect the rising or falling of the individual determinants—these are processes quite outside of its influence—but it can, by eliminating the bearers of unfavourably varying determinants, set a limit to further advance in such directions. This we shall consider in more detail later on. Personal selection operates by removing unfavourably varying individuals from the genealogical tree of the species, but at the same time the determinants which are varying unfavourably are also removed, and their variation is thus put a stop to for all time.

I have called these processes which are ceaselessly going on within the germ-plasm, Germinal Selection, because they are analogous to those processes of selection which we already know in connexion with the larger vital units, cells, cell-groups and persons. If the germ-plasm be a system of determinants, then the same laws of struggle for existence in regard to food and multiplication must hold sway among its parts which hold sway between all systems of vital units—among the biophors which form the protoplasm of the cell-body, among the cells of a tissue, among the tissues of an organ, among the organs themselves, as well as among the individuals of a species and between species which compete with one another.