MODERN SCIENTIFIC THOUGHT 93 



course of development the determinants are parceled and reparceled by 

 the repeated divisions of the nuclear chromatin, an element in the cleav- 

 age process that, we have seen, is so striking a phenomenon in develop- 

 ment. Differentiation thus depends not upon the literal expansion of 

 a preexisting whole, but upon the distribution of the preformed deter- 

 minants in the germ that have been inherited from preexisting indi- 

 viduals. 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 deter- 

 minant 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 correspond- 

 ingly, while others will obtain more nourishment and correspondingly 

 strengthen. As the determinants in the germ, so the organs, the char- 

 acters 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 dis- 

 tinguishes 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 pro- 

 ceeds, 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 organi- 

 zation, 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 sub- 

 stance that is based upon ultimate vital units that have individuality, 

 grow and reproduce, various investigators are discovering no such neces- 

 sity 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. 



