Organic Inheritance 17 



offered by three generations of naturalists align 

 themselves under two main ideas only. The 

 first is the idea that the causes of evolution are 

 chiefly from without inward, namely, beginning 

 in the environment of the body and extending: 

 into the germ; this idea is centripetal. The 

 second idea is just the reverse: it is centrif- 

 ugal, namely, that the causes begin in the 

 germ and extend outward and into the body and 

 into the environment. ***** Weismann^s 

 great contribution to thought has been to point 

 out the very sharp distinction which un- 

 doubtedly exists between the hereditary forces 

 and predispositions in the heredity-germ and 

 the visable expression of these forces in the 

 organism. The problem of causes of evolution 

 has become an infinitely more difficult one since 

 Weismann has compelled us to realize that the 

 essential question is the causes of germinal evo- 

 lution rather than the causes of bodily evolu- 

 tion or of environmental evolution. ***** 

 The idea that the germ is an energy complex is 

 an as yet unproved hypothesis ; it has not been 

 demonstrated. The heredity-germ in some re- 



