120 THE METHOD OF DARWIN. 



reproductive elements become distinct from the 

 parts which develop into the other organs of 

 the body. In this respect Weissmann's hypoth- 

 esis has an additional logical support. It has 

 provoked earnest discussion, and naturalists 

 have taken sides on the subject. Although 

 the germ plasm is itself an assumption, and 

 the hypothesis owes its existence to the known 

 facts of karyokinesis, it has led to further 

 investigations in some directions with fruitful 

 results. Darwin's hypothesis opened the ques- 

 tion under the new light that he had shed upon 

 nature, and the more recent hypotheses, like 

 his own, owe their existence to the " impelling 

 force " of his general theories of descent and 

 natural selection. 



