228 THE THEORIES OF EVOLUTION 



servations and experiments, but their general postu- 

 late, the theoretical impossibility of the transmission 

 of such characters. We have explained elsewhere 

 how their thesis of the distinction between soma and 

 germ plasm precluded all idea of transmission. 



Our present knowledge of the mechanism of hered- 

 ity does not enable us to refute Weismann's assertions 

 positively. For instance, we do not know exactly 

 how the growth of the camel's callouses, due to fric- 

 tion, can influence the germ cell so as to produce in 

 the offspring a similar modification. 



It may be that the chemical composition of the pro- 

 toplasm, very little known yet, will some day give a 

 solution of the problem. For the present we must 

 confine ourselves to very general and perhaps vague 

 considerations; the most important thing, however, is 

 to point out a method to follow. We will therefore 

 present once more a thesis developed in another work 

 of ours 12 and suggested to the author by Sachs' the- 

 ory of "formative substances." What distinguishes 

 the ovum from the other cells of the organism, is the 

 fact that in the course of embryogenetic development, 

 the cells to which it gives rise, all alike at first, become 

 gradually differentiated and specialised and lose when 

 specialised the faculty of creating an entire organism, 

 while the germ cells retain that faculty. This differ- 

 ence seems to preclude the possibility for the ovum 



12 Yves Delage. L'HSrediti, etc., pp. 829-843, 1903. 



