General Conclusions 59 



only briefly mentioned by Nageli,^^ and the whole pre- 

 sentation of this subject shows what great difficulties the 

 hypotheses of the solid composition of the idioplasm en- 

 counters. 



Nageli's theory tells us as little as any other theory 

 about growth through assimilation and the multiplication 

 of the material bearers of heredity. That the properties 

 of those elements are determined by their molecular 

 structure is just as little an advantage of his theory; it is 

 a conclusion derived from our most general conceptions, 

 which can be applied with the same right to the hypotheti- 

 cal units of every theory of heredity. But how that mole- 

 cular structure explains the hereditary factors, we, of 

 course, learn as little here as by any other theory. It is a 

 weak point of Nageli's work that these hitherto unex- 

 plained facts are not clearly designated as such, and that 

 the common basis of the various theories is not simply 

 mentioned as such. 



8. General Considerations 



To m}^ mind the above briefly sketched theories clearly 

 prove that the fundamental thought of pangenesis, that 

 is, of different material bearers for the individual hered- 

 itary characters cannot be avoided. Spencer, who wrote 

 before Darwin, did not have this thought, and it was im- 

 possible for him to give a satisfactory explanation of the 

 differentiation of organs. Weismann's theory, as we have 

 already seen, led its originator himself in that direction, 

 and forced him to admit, more or less clearly, a divisibility 

 of the germ-plasm in this sense. And Nageli's idioplasm 

 is, on the whole, built up from those elements. 



The more carefully we look into these theories in de- 



27Loc. cit. pp. 215-220. 



