The Evolution Theory, Vol. 1 of 2

by August Weismann

Structure of the germ-plasm—Vital affinities—Division—O. Hertwig's chief objections to this theory—Male and female eggs in the Phylloxera show differential division—Dispersal of the germ-plasm in the course of Ontogeny—Active and passive state of the determinants—Predetermination of cells—There are no determinants of characters—Liberation of the determinants—Accessory idioplasm—Herbst's lithium larvæ—Plant galls—Cells with several facultative determinants—Connective tissue in vertebrates—Mesoderm cells of Echinoderms—Sexual dimorphism—Female and male ids—Polymorphism (Papilio merope)—Ants.

I have endeavoured to prove that the germ-substance proper must be looked for in the chromatin of the nucleus of the germ-cell, and more precisely still in those ids or chromosomes which we conceive of as containing the primary constituents (Anlagen) of a complete organism. Such ids in larger or smaller numbers make up the whole germ-plasm of a germ-cell, and each id in its turn consists of primary constituents or determinants, i.e. of vital units, each of which determines the origin and development of a particular part of the organism. We have now to make an attempt to picture to ourselves how these determinants predetermine those cells or cell-groups to which they correspond. In doing so we have to fall back upon mere hypotheses, and in stating any such hypothesis I wish expressly to emphasize that I am only following up one of the possibilities which our imaginative faculty suggests. Nevertheless, to endeavour to form such a conception is certainly not without use, for it is only by elaborating a theory to the utmost that we are able to use it in application to concrete cases, thus stimulating the search for corroboratory or contradictory facts, and leading gradually to a recognition of the gaps or mistakes in the theory.

The first condition that must be fulfilled in order that a determinant may be able to control a cell or cell-group is that it should succeed in getting into it. It must be guided through the numerous cell-divisions of ontogeny so that it shall ultimately come to lie in the cells which it is to control. This presupposes that each determinant has from the very beginning its definite position in relation to the rest, and that the germ-plasm, therefore, is not a mere loose aggregate of determinants, but that it possesses a structure, an architecture, in which the individual determinants have each their definite place. The position of the determinants in relation to one another cannot be due to chance, but depends partly on their historical development from earlier ancestral determinants, partly on internal forces, such as we have already assumed for keeping the determinants together. We may best designate these hypothetical forces 'affinities,' and in order to distinguish them from mere chemical affinities we may call them 'vital.' There must be forces interacting among the different determinants which bind them together into a living whole, the id, which can assimilate, grow, and multiply by division, in the same manner as we were forced to assume for the smaller units, the biophors and single determinants. In the ids, however, we can observe the working of these forces quite directly, since each chromosome splits into two halves of equal size at every nuclear division, and not through the agency of external forces, e.g. the attraction which we may assume to be exerted by the fibrils of the nuclear spindle, but through purely internal forces, often long before the nuclear spindle has been formed at all.

But if the determinants must separate from each other in the course of development so as to penetrate singly into the cells they are to control, the id must not only have the power of dividing into daughter-ids of identical composition, it must also possess the power of dividing under certain influences into dissimilar halves, so that the two daughter-ids contain different complexes of determinants. The first mode of division of the id, and with it of the nucleus and of the cell, I call erbgleich, or integral, the second erbungleich, or differential. The first form of multiplication is the usual one, which we observe everywhere when unicellular organisms divide themselves into two equal daughter-units, or when the cells of multicellular bodies produce their like by division into two. The second is not directly observable, because a dissimilarity of the daughter-cells, as long as it lies only in the idioplasm, cannot be actually seen; it can only be inferred from the different rôle which the two daughter-cells play in the building up of the individual. When, for instance, one of two sister-cells of the embryo gives rise to the cells of the alimentary canal and the other to those of the skin and the nervous system, I infer that the mother-cell divided its nuclear substance in a differential way between the two daughter-cells, so that one contained the determinants of the endoderm, the other those of the ectoderm; or when a red and a black spot lie side by side and under exactly the same conditions on the wing of a butterfly, I conclude that the ancestral cells of these two spots have divided differentially, so that one received the 'red,' the other the 'black' determinants. Our eyes can perceive no difference between the nuclear substance of the two cells, but the same is true of the chromosomes of the paternal and maternal nuclei in the fertilized ovum, although we know in this case that they contain different tendencies. In any case we are not justified in concluding from the apparent similarity of the chromosome-halves in nuclear division that there cannot be differential division. The theoretical possibility that there is such differential division cannot be disputed; indeed, I am inclined to say that it is more easily imagined than the division of the ids into absolutely similar halves. Both are only conceivable on the assumption that there are forces which control the mutual position of the determinants in the ids, that is, on the assumption of 'affinities.' I shall not follow this further, but that there are forces operative within the ids which are still entirely unknown to us is proved at every nuclear division by the spontaneous splitting of the chromosomes.

It has been objected to my theory that such a complex whole as the id could not in any case multiply by division, since there is no apparatus present which can, in the division into two daughter-units, re-establish the architecture disturbed by the growth. But this objection is only valid if we refuse to admit the combining forces, the 'vital affinities' within the ids, and the same is true for the smaller vital units. An ordinary chemical molecule cannot increase by division; if it be forcibly divided it falls into different molecules altogether; it is only the living molecule, that is, the biophor, which possesses this marvellous property of growth and division into two halves similar to itself and to the ancestral molecule, and we may argue from this that in the division of the ids forces of attraction and repulsion must likewise be operative[19].

[19] In my book The Germ-Plasm I have already assumed the existence of 'forces of attraction' in the determinants and biophors, as in the cells. I did not, indeed, enter into details, but I argued on the same basis as now (Germ-Plasm, p. 64, English edition). My critics have overlooked this.

I see no reason why we should not assume the existence of such forces, when we make the assumption that the hundreds of atoms which, according to our modern conceptions, compose the molecule of albumen and determine its nature, are kept by affinities in this definite and exceedingly complex arrangement. Or must we suppose that between the atom-complex of the molecule and the next higher atom-complex of the biophor, determinant, and id there is an absolute line of demarcation, so that we must assume quite different forces in the latter from those we conceive of as operative in the former? The biophor is ultimately only a group of molecules, the determinants a group of biophors, the id a group of determinants, and all the three inferred stages of vital organization only become real units through the forces operating within them and combining them into a whole. What compels the chromatin granules of the resting nucleus to approach each other at the time of cell-division, to unite into a long, band-like thread, and what is it that subsequently causes this thread to break up again into a definite number of pieces? Obviously only internal forces of which we know nothing further than that they are operative.

We shall see later that this assumption of vital affinities must be made not only in regard to the cells, but also in regard to entire organisms whose parts are united by an internal bond, and whose co-ordination is regulated by forces of which we have as yet no secure knowledge. In the meantime we may designate these forces by the name of 'vital affinities.'

It must be admitted, however, that some objections of a fundamental nature have been urged against the assumption of a differential nuclear division of the hereditary substance. O. Hertwig holds that the assumption of differential division is essentially untenable, because it is contradictory to 'one of the first principles of reproduction,' for 'a physiologically fundamental character of every living being is the power of maintaining its species.'

This certainly seems so, but a closer examination shows that this 'principle,' although correct enough when taken in a very general sense, does not really cover the facts, and is therefore incapable of supporting the inferences drawn from it. If the proposition expressed the whole truth there could have been no evolution from the primitive organisms to higher ones, every living being must have simply reproduced exact copies of itself. Whether the transformations of species have been sudden or gradual, whether they have been brought about by large steps or by very small ones, they could only have come about by breaking through this so-called 'principle' of like begetting like. In fact, we may with more justice maintain the exact converse of the principle, and say that 'no living being is able to produce an exact copy of itself,' and this is true not only of sexual, but of asexual reproduction.