The Evolution Theory, Vol. 2 of 2

by August Weismann

Of course we know nothing definite or precise with regard to the units of the germ-plasm, and we cannot tell what is necessary in order that a determinant shall determine a part of the developing body in this way or in that; thus we have no definite idea of the relations subsisting between the variations of the determinants and those of their determinates, but we know at least so much, that hereditary variation of a part is only possible when a corresponding particle in the germ-plasm varies; and we may at least assume that these correspond to each other so far, that a greater development of the one implies a greater development of the other, and that a reversal of these relations is impossible. If the determinant X disappears from the germ-plasm the determinate X´ disappears from the soma. It is therefore justifiable to infer from the degree of development of an organ the strength of its determinant, and to assume that plus- and minus-variations in both are correspondingly large.

But in addition to the fluctuations in the equilibrium of the germ-plasm which lie at the root of all hereditary variation, we have to take into account something which we have already touched upon briefly—the correlation of the determinants, the influencing of one determinant by those round about it. I have spoken for the sake of brevity of 'the determinant' of a part, although all the large and more important parts must certainly be thought of as represented by several or many, if not, indeed, by whole groups of determinants. Although it is quite out of our power to follow the complex processes of the mutual influences of the determinants upon each other, we can say this at least, that these influences must exist, and we have here a faint indication of what must occur in the case of spontaneous variations within the germ-plasm. We must, in the first place, think of the individual determinants as arranged in groups, so that, for instance, the determinants of the right and left half of the body lie together, and therefore are frequently affected together by influences which cause variation, so that both vary in the same direction at the same time. In point of fact, analogous deformities, such as polydactylism of both right and left hands, and even of hands and feet at once, do actually occur. That the right and left hands, the fore- and hind-limbs, are represented in the germ by particular determinants, may be inferred from their frequently different phyletic evolution into different forms of hand and foot, e.g. into flipper and rudimentary hind-leg in the whale, as well as from the cases of particulate inheritance, which are rare, but which undoubtedly do occur, such as when, in Man, there is a maternal blue eye on one side of the head and a paternal brown eye on the other. But almost more striking than the differences between these homologous or homotypic parts are their points of resemblance, and these may probably be in part referred to their disposition side by side and common history in the germ-substance, although a far larger proportion of them are probably due to their adaptation to similar functions, and are therefore to be regarded as a phenomenon of convergence within the same organism.

We have already seen that the first increase in the growth of one determinant means a withdrawal of nourishment, however slight, from its neighbours; this can, of course, be equalized again if the claims on the common nutritive stream from another quarter are at the same time diminished; but it is possible that the claims from another quarter may also be increased, and the withdrawal will then be more marked, and the determinants being thus injured from two directions at once will sink downwards with greater rapidity. But it is also conceivable that the majority of determinants of a part may vary upwards, and, by their combined increased power of assimilation, direct towards themselves such a greatly increased stream of nourishment that the whole organ—for instance, a particular feather in a bird—varies in an upward direction, and becomes larger and larger, as we see in the case of many decorative feathers; or that certain determinants vary only as far as some of their biophors are concerned, and similarly for their determinates, as when a group of scales on a butterfly's wing that had previously been black turn out a brilliant blue. It can probably also happen that such variations within the determinants are transmitted to neighbouring determinants because the nutritive conditions which caused the first to vary have extended to those about them. The increase of brightly coloured spots in birds and butterflies gives us ground for concluding that there are processes of this kind within the germ-plasm.

I will refrain from following this idea into greater detail, and translating the observable relations and variations of the fully-formed parts of the body into the language of the germ-plasm; but so much may be taken as certain, that multitudinous inter-relations and influences exist between the elements of the germ-plasm, and that one variation brings another in its train, so that—usually at a very slow rate, that is, in the course of generations and of species-forming, definite variations occur from purely intra-germinal reasons—variations which as far as they remain outside the limits of good or bad may of themselves change the character of a species, but which when they are seized upon by personal selection may, by sifting and combination of the ids, be led on to still higher development.

If we consider further that the variation of a part must depend not only on the quality of the external stimulus but also upon the constitution, the reacting power of the part, we shall understand that similar nutritive variations may cause two different determinants to vary in different ways, and when we reflect that every nutritive change must extend from the point from which it started with diminishing strength in a particular direction, we have a further factor in the variation of determinants and one which influences even similar determinants differently.

Finally, if we remember that determinants of different constitution will also extract different ingredients from the nutritive stream and thus set up in it different kinds of chemical change, thus causing an altered supply of nutritive substances to flow to the neighbour determinants, we get some insight into a very complex and delicate but perfectly definite set of processes, into a mechanism which we can certainly only guess at, but whose results lie plainly before us in the spontaneous variations of the organism. We understand in principle the possibility of saltatory variation, as a more or less widespread, more or less marked disturbance of the species-type in this or that group of characters, and we may acknowledge that those 'kaleidoscopic variations' which Eimer supposed to be the sole basis of the transformation of species, and which have been brought to the foreground again quite recently by De Vries[20], are probably factors in transmutation operative within a limited sphere.

[20] See end of chap. xxxiii.

But we must think of all these struggles and mutual influencings as taking place on the smallest possible scale, so that it is only by long summation that they can produce any visible effect, and we must never forget the essential significance of the plurality of ids, for these 'spontaneous' variations may take place in a different and quite independent manner in each individual id. If this were not so no intervention of personal selection would be possible, natural selection would not exist, and the adaptation of the organism from the single cell up to the whole would remain wholly unexplained. The whole crop of spontaneous germ-variations, whenever it ceases to be 'indifferent,' and becomes either 'good' or 'bad,' comes under the shears of personal selection and under its almost sovereign sway.

On the other hand, the sudden first appearance of a saltatory variation takes place quite independently of personal selection, depending on similar variations in a number of ids, which remain latent until they have by the process of reducing division which precedes amphimixis, chanced to attain a majority. In sudden bud-variations we may perhaps suppose that reducing division occurring in some still unverified abnormal manner is the reason why the germinal variation suddenly makes itself visible—a supposition previously suggested as the explanation of the reversion of these sports.

The rarity of bud-variation is thus explained, while the greater frequency of saltatory variations in plants propagated by seed may be accounted for by the regular occurrence of reducing division in sexual reproduction. But that the same or similar variations may occur in several, it may be in many, ids at the same time must depend upon similar general influences which affect the plant as a whole, as happens through cultivation, manuring, and so on. I shall return to this when discussing the influence of the environment.