The Evolution Theory, Vol. 1 of 2

by August Weismann

Fig. 92. Echinoderm-larvæ. A, blastula-stage; the primary mesoderm-cells (M) are being formed at the subsequent invagination-area of the endoderm (Ent). Ekt, the ectoderm. B, gastrula-stage; the archenteron (UD) has been invaginated (Ent), and between it and the ectoderm (Ekt) the mesoderm-cells (Ms) migrate into the gelatinous fluid which fills this cavity. There they attach themselves partly to the ectoderm, and partly to the endoderm. After Selenka.

There are thus three possibilities of development, three kinds of reaction, implied in these cells, which are all outwardly alike, and we can only understand their rôle in the building up of this very symmetrical animal if we assume that of these three only one is in each case liberated, by the specific stimulus exerted by the immediate surroundings of the cell, so that it may become, according to the chance position it takes up after its migration, either a skin-cell, a muscle-cell, or a skeleton-forming cell.

This case may be compared in some respects with the permanent colour-adaptation of those caterpillars, in regard to which Poulton demonstrated that they become almost black if they are reared on blackish-brown bark, light brown on light bark, and green if they are kept among leaves, and in all cases permanently so. In this case also the implicated pigment-cells of the skin may develop in three ways, according to whether this or that quality of the light releases this or that determinant.

But in many cases we do not know the quality of the liberating stimulus, and must content ourselves with imagining it. This is so in the case of dimorphism of the sexes. It is clear that in the males of a species the germ-cells develop quite otherwise than they do in the females, that different determining elements attain to activity in each sex, and since the primary constituents of both sexes must be contained in most animals in the ovum and in the spermatozoon, we must assume that in both there are at once 'ovogenic' and 'spermogenic' determinants, of which, however, only one kind becomes active in a given individual. There are, however, both among plants and animals hermaphrodite individuals, in which both kinds of sexual products are developed simultaneously or successively.

It is not only the primary sexual characters, however, that compel us to the assumption of double determinants in the germ-plasm, the secondary sexual characters do so too. We know very well in relation to ourselves that 'the beautiful soprano voice of the mother may be transmitted through the son to the grand-daughter, and that the black beard of the father may pass through the daughter to the grandson.' Thus both kinds of sexual characters must be present in every sexually differentiated being, some visible, others latent. In animals the determinants are sometimes handed on from germ-plasm to germ-plasm through several generations in a latent state, and only make their appearance again in a subsequent generation. This is the case in the water-fleas (Daphnids) and the plant-lice (Aphides), in which several exclusively female generations succeed one another, and only in the last of them do males occur again side by side with the females.

The germ-plasm of the ovum which is ripe for development must thus contain not only the determinants of the specific ova and sperms of the species, but also those of all the male and female sexual characters, which we discussed at length in the section on sexual selection. I then showed that these secondary sexual characters differ greatly in range and in strength, that among lower animals they are almost entirely absent, and that among higher forms, such Crustaceans, Insects, and Birds, they attain to very different grades of development even among the same species. Thus the birds of Paradise are in most species brilliantly coloured and adorned with decorative feathers only in the male sex, while the females are simply blackish-grey, but there is a single species in which the males are almost as soberly coloured as the females. Conversely, too, we find that in parrots both sexes are usually coloured alike, but a few species exhibit a totally different colouring in the two sexes. In the same way the secondary sex differences may affect only a few parts of the animal or many, while in a few species the sexes are so divergent in structure that almost everything about them may be called different. Examples of this are the dwarf males of most Rotifers, and the males, more minute still in proportion to the females, of the marine worm Bonellia viridis ([p. 227]).

We have now to inquire what theoretical explanation of these facts we can arrive at in accordance with the germ-plasm theory. That double determinants, male and female, for the differently formed parts of the two sexes must be assumed to exist in the germ-plasm has been already said, and we have to suppose that the same stimulus—usually unknown to us—which incites the determinants of the primary sexual characters to activity also liberates those of the secondary characters. But we may safely go a step further and conclude that there are male and female ids, that is, that the male and female determinants belong to different ids. I infer this from the fact that in some groups, such as the Rotifers and certain plant-lice, the ova are sexually differentiated even at the time of their origin. Males and females of these animals arise from different kinds of eggs, which are even externally recognizable. Both develop parthenogenetically, so that fertilization has nothing to do with it; from the first, therefore, they must contain ids which consist of determinants of one sex alone.

If this conclusion be correct, then the sexual equipment of the determinants of the sexual characters must have taken place in the course of phylogeny in such a way that each id was affected in one direction only, and we should thus have to assume male and female ids, even before the separation of the sexes as males and females, and the same conclusion must be extended to the primary sexual characters. Only in this way can we understand the fact that differences between the sexes, at first small, have increased in the course of phylogeny to such complete divergence of structure as is now exhibited in the forms we have named, Bonellia, the Rotifers, and some parasitic worms.