Page:Popular Science Monthly Volume 82.djvu/97

Rh course of development the determinants are parceled and reparceled by the repeated divisions of the nuclear chromatin, an element in the cleavage process that, we have seen, is so striking a phenomenon in development. Differentiation thus depends not upon the literal expansion of a preexisting whole, but upon the distribution of the preformed determinants in the germ that have been inherited from preexisting individuals. And this distribution takes place, by nuclear division, in such a way that the right determinant always finds itself ultimately in the right place, that is, in the same relative position that that sort of determinant occupied in the parent.

The germ, then, is not only the abiding place of an enormous and complex assemblage of determinants, but these determinants are living morphological units. Not only that. They struggle for existence, according to the conception, just as organisms do. The basis of this struggle lies in inequalities in the food distribution in the germ, whereby some determinants will obtain less nourishment and weaken correspondingly, while others will obtain more nourishment and correspondingly strengthen. As the determinants in the germ, so the organs, the characters which they determine, vary.

By means of this ingenious application of the theory of natural selection to the vital units of which living substance is composed, the determinant hypothesis obtains a theory of variation which at once distinguishes it from the preformation theory of Bonnet. It goes still farther. Even the biophors vary—those ultimate vital units of which the determinants are the first aggregates.

With this liberal provision for variation, the determinant hypothesis would appear to have approached very close to modern conceptions of epigenesis. Certain fundamental differences, however, still persist. Whatever the provision for variation in the germ, differentiation proceeds, according to the determinant hypothesis, by the segregation of determinants already present in the germ; and these determinants are vital morphological units. According to the most advanced epigenetic theory, differentiation proceeds from a relatively simple germinal organization, not by the segregation of hypothetical vital units, but by means of progressive changes of a physico-chemical nature.

Just here appears the characteristic of the determinant hypothesis most significant for us. While the great inventor of the determinants finds it fundamentally necessary to assume a structure for living substance that is based upon ultimate vital units that have individuality, grow and reproduce, various investigators are discovering no such necessity in the facts. What is necessary is a hypothesis that will work. One of the strongest objections to the determinant hypothesis is, that, paradoxically enough, the chief researches it has stimulated are those which have been guided by the assumption that it would not work.