The Evolution Theory, Vol. 1 of 2

by August Weismann

Many deformities of particular parts in Man and the higher animals may be referred to imperfect or inhibited nutrition of the part in question during embryonic development; the determinants alone cannot make the part, they must have a supply of formative material, and according as this material is afforded more abundantly or more scantily the part will turn out larger or smaller. In the same way the pressure conditions of the surrounding parts must in many cases have a furthering or inhibiting influence, or may even determine the shape. But it is quite possible, indeed even probable, that other specific influences are exerted by the cells or cell-aggregates surrounding an organ which is in process of being formed, just as the stake on which a twining plant is growing may prompt it to coil. If the stake be absent, the predetermined twining of the plant cannot attain to more than very imperfect expression, if indeed it finds any. The spirally coiled sheath of muscle-cells which occurs so often around blood-vessels in worms, Echinoderms, and Vertebrates is probably due to similar processes, that is, on the one hand, to a specific mode of reaction characteristic of these cells, and predetermined from the germ; on the other hand, to the external influence of the cell-surroundings without which the determination of the muscle-cell is not liberated, that is, is not excited to activity.

Fig. 93. The development of a limb in the pupa of a Fly (Sarcophaga carnaria). A, apex of the limb from a pupa four days old; the jointing is hinted at; hy, hypodermis; ps, pupal sheath; ph, phagocytes; tr, tracheal branch. B, the same on the fifth day; the lumen of the limb is quite filled with phagocytes (ph); the last tarsal joint (t⁵) is beginning to show a bifid apex. C, the same on the seventh day; the claws (Kr) and the adhesive lobes (hl) are formed.

But even if every determinant requires a stimulus to liberate it, whether this stimulus consists in currents of particular nutritive fluids, in contact with other cells, or, conversely, on the removal of some pressure previously exerted on the cell by its surroundings, the material cause of a structure is to be sought for not in these conditions of its appearance, but in the primary constituents which have been handed on to the relevant cell or cell-group from the germ, in other words, through its determinants. How, for instance, could the blunt rounded knob of the rough and clumsily jointed sac of cells which represents the insect's leg at the beginning of the pupal period ([Fig. 93], A) be incited to thicken, to constrict at the root (B), and to form a joint-surface, to broaden out at the end, and produce two sharply cut points (C), which become incurved and form claws (kr), while beneath these a broad flat lobe (hl) grows forward, and with its regularly disposed cells gradually forms the characteristic adhesive organ of the fly—how could all this happen if there were not contained within these cells special formative forces which determine them not only in their form and the rest of their constitution, but above all in their power of multiplication? No special external stimulus affects the still unfinished knob of the fly's leg unless it be the removal of pressure; but this operates regularly, and cannot be the cause of the growth, at definite places, of claws and adhesive lobes with all their characteristically placed hairs.

We require to assume that each of the cells composing the primary rudiment of the limb possessed a determining power which made it grow and multiply under the given conditions of nutrition and pressure in a prescribed manner and at a prescribed rate; and we must make the same assumption in regard to all the daughter and grand-daughter-cells, and so on. The strictest regulation of the power of multiplication of each of the implicated cells is a necessary condition of the constant production of the same two claws and adhesive lobes, the same form of tarsal joint, the same regular covering of hair, and so on. This exact determination of the cells can only take place through material vital particles, and it is these which I call determinants.

I have already said so much about the assumed 'determinants' of the germ-plasm that it might perhaps be supposed that we have now exhausted the topic; but the assumption of such 'primary constituents' is so fundamental, not only for my own germ-plasm theory of to-day and to-morrow, but also—unless I am much mistaken—for all future theories of development and inheritance. In point of fact, the conception of determinants has as yet penetrated so little into the consciousness of biologists, that I cannot remain content with what I have already said, but must endeavour to test and to corroborate my thesis by additional illustrations.

As far as I am aware, only a few zoologists have expressly and unconditionally agreed with the assumption of determinants; on the other hand, several biologists have rejected it as fanciful and untenable, while others have set it aside as a useless playing with ideas. The last, I am inclined to believe, have not taken the trouble to think out what the idea is. It has even been objected that there can be no determinants because we can see nothing of them, and that they must therefore be pure figments of the imagination, invented to explain facts which could be explained much more easily and simply in some other way. From the very first I have stated emphatically that they have not been, and never will be seen, because they lie far below the limit of visibility, and thus can at best only become visible when they are collected in large aggregates like chromatin granules. Nor have I any objections to make if any one chooses to describe all the details of their activity as mere hypotheses, such, for instance, as their distribution during development, their 'maturation,' their migration from the nucleus, and the manner in which they control the cell. All this is really an imaginative picture which may be correct to a certain degree, but may also be erroneous; no formal proof of it can be obtained at present; and I am content if it be simply admitted to be possible. On the other hand, the existence of determinants seems to me to be, in the sense indicated, indubitable and demonstrable.

Let us return for a moment to the claws and adhesive lobes which are developed on the foot of the fly. It may perhaps be thought that it is possible to do without the assumption of determinants for these parts, by assuming that although 'external' influences in the ordinary sense could not possibly have determined that certain cells of the apex of the leg should form claws and others adhesive lobes, the result might be due to the differences of intercellular pressure within the apical knob; these may have been stronger in one direction, weaker in another, thus prompting the cells to grow here into claws and there into adhesive lobes. If we had merely to explain from the constitution of the germ-plasm the ontogeny or development of these parts in an individual fly there might perhaps be no radical objection to this view, though it would hardly be possible to explain the assumed differences in pressure otherwise than as due to a different intensity of growth in the cells in the various regions of the limb-apex, which again would have to be referred to differences in the germ-plasm. But when we reflect that these parts vary hereditarily and independently of other parts, and owe their present form to their power of doing so, and that they are differently formed in every genus and species, we see at once that they must be represented in the germ-plasm by particular vital particles, which are the roots of their transmissible variability, that is, which must have previously undergone a corresponding variation if the relevant parts themselves are to vary. Without previous variation of the determinants of the germ no transmissible independent deviation on the part of the claws or adhesive lobes of the animal is conceivable.

All the opponents of my theory have overlooked this fact; both Oscar Hertwig and Kassowitz have forgotten that a theory of development is not a theory of heredity; they only aim at the former, and they therefore dispute the logical necessity for an assumption of determinants.

But as this is the very foundation of the theory, let me further submit the following considerations in its favour.