The Evolution Theory, Vol. 2 of 2

by August Weismann

The first advance to a higher stage of life must have been brought about by multiplication, since accumulations of Biophoridæ, unintegrated but connected masses, would be formed.

In this way the threshold of microscopical visibility would gradually be reached and crossed, but—to argue from the modern Baccilli—long before that time a differentiation of the biophors on the principle of division of labour would have taken place within a colony of Biophoridæ. This first step towards higher organization must probably have taken enormous periods of time, for before any differentiation could occur and bring any advantage the unintegrated aggregates of Biophors must first have become orderly, and have formed themselves into a stable association with definite form and definite structure, somewhat analogous to the spherical cell-colonies of Magosphæra or Pandorina. Only then was the further step made of a differentiation of the individual biophors forming the colony, and this is comparable to the species of Volvox among the lower Algæ. The gradual ascent of these colonies of biophors must, then, be referred to the principles to which we attribute the ascent of the higher forms of life to ever-higher and ever-new differentiations; the principles of division of labour and selection.

These differentiated colonies of biophors have brought us nearer to the lowest known organisms, among which there are some whose existence we can only infer from their pathological effects, since we have not been able to make them visible. The bacillus of measles has never yet been seen, but we cannot doubt its existence, and we must assume that there are bacilli of such exceeding smallness that we shall never be able to see them, even with the most improved methods of staining and the strongest lenses.

These non-nucleated Monera lead on to the stage of nucleus-formation, and this at once implies the cell. As, on our view, the nucleus is primarily a storehouse of 'primary constituents' (Anlagen), its origin must have begun at the moment at which the differentiation of the cell-body reached such a degree of differentiation of its parts that a mechanical division into two halves was no longer possible, and that the two products of division, if they were each to develop to a new and intact whole, required a reserve of primordia (Anlagen) to give rise to the missing parts. As this higher differentiation would bring about a superiority over the lower forms of life, in that they would make possible the utilization of new conditions of life, but on the other hand could only survive if the differentiation of a reserve of primary constituents, that is, a nucleus, were introduced at the same time, the development of the nucleus can be ranged under the principle of utility to which we traced back the evolution of all higher and more differentiated forms of life. But it would scarcely be profitable to try to follow out in detail the first steps in the progress of organization under the control of selective processes, since we know far too little about the life of the simplest organisms to be able to judge how far their differentiations are of use in improving their capacity for life.

That would be a bold undertaking even in regard to unicellular organisms, and it is only in the case of multicellular organisms that we can speak with greater certainty and really recognize the changing of the external conditions, in the most general and comprehensive sense, as the fundamental cause of the lasting variations of organic forms. We can here distinguish with certainty between the direct and the indirect effect of external influences, and we see how these sources of variation interact upon each other. The lowest and deepest root of variation is without doubt the direct effect of changed conditions. Without this the indirect effect would have had no lever with which to work, for the primitive beginnings of variation would be absent, and an accumulation of these through personal selection could not take place. It is a primitive character of living substance to be variable, that is, to be able to respond to some extent to changed external conditions, and to vary in accordance with them, or—as we might also say—to be able to exist in many very similar but not identical combinations of substances, and we must imagine that even the first biophors which arose through spontaneous generation were different according to the conditions under which, and the substances from which, they originated. And from each of these slightly different beginnings there must, in the course of multiplication by fission, have been produced a whole genealogical tree of divergent variations of the primitive Biophoridæ, since it is inconceivable that all the descendants would remain constantly under the same conditions of life under which they originated. For every persistent change in the conditions of existence, and especially of nutrition, must have involved a variation in the constitution of the organism, whose vital processes, and especially the repair of its body, depended on these conditions.

But the external influences to which the descendants of a particular form of life were subject never remained permanently the same. Not only did the surface of the earth and its climatic conditions change in the course of time with the cooling of the earth, but mountains arose and were levelled again, old land-surfaces sank out of sight or emerged again, and so on; all that, of course, played its part in the transformation of the forms of life, but did so to any considerable extent only at a later stage, when there were already highly differentiated organisms. These unknown primitive beginnings of life must have been forced to diverge into different variations through the different conditions of the same place in which they lived.

Let us think of the simplest microscopic Monera on the mud of the sea-coast, equipped with the faculty of plant-like assimilation, and we shall see that their unlimited multiplication would cause differences in nutrition, for those lying uppermost would be in a stronger light than those below, and would, therefore, be better nourished, and, consequently, would transmit the variations thus induced to their progeny which arose by fission. Thus it is conceivable that even the more or less favourable position as regards light would bring about the origin of two different races from the same parent form, and as it is conceivable in the case of light, so is it also in regard to all the influences which cause variation in the organism.

We have already seen that variations in the lowest (non-nucleated) forms of life caused by the direct influence of the vital processes may be directly transmitted to the descendants, but that in all those whose bodies have already differentiated into a germ- or idioplasmic-substance, in contrast to a somatic substance in the more restricted sense, this hereditary transmission is only possible in the case of the variations of the germ-plasm, and hereditary variations of the species can only arise by the circuitous route of influencing the germ-plasm. The body (soma) can be caused to change by external influences, by the use or disuse of an organ, but variations of this kind are not transmitted; they do not become a lasting possession of the species, but cease with the individual; they are transient changes.

Thus it was only through those external influences—including those from the soma of the organism itself—which affected the germ-substance, either as a whole or in certain of its primary constituents, that hereditarily transmissible variations of the organism arose, and we have already discussed in detail how particular variational tendencies may arise through the struggle of the parts within the germ-plasm, which may give an advantage to certain groups of primary constituents. And these tendencies are of themselves sufficient to cause the specific type to vary further and further in given directions.

Nevertheless, the infinite diversity of the forms of life could never have been brought about in this way alone, if there had not been another—the indirect—effect of the changeful external influences.