PKINCIPAL ATTEMPTS AT EXPLANATION 163 



life and the plan of evolution and determination of its 

 limits, etc. 



(2) Neo- Darwinism. 



(i) Doctrine. As chief advocate in this direction 

 ranks Prof. Weismann (Freiburg i/B.). According to 

 him, natural selection acts not on the complete, fully- 

 grown organisms themselves, but on the ( determinants ' 

 i.e. on the hypothetically smallest material parts in 

 the nuclear cells. Each quality of the organism has, 

 according to him, a determining part in the sexual cells. 

 The determining parts (= determinants) show that 

 variability and that divergence which Darwin imputed 

 to the organism itself. Those best nourished prevail 

 over the weaker. They evince, however, that influence 

 in the grown organism only if the change in the deter- 

 minants concerned has reached a sufficient grade, so 

 that they then, in the organism, immediately produce 

 a perfected new organ as the first expression of their 

 influence, since it is only when the organism has the new 

 organ in a complete form that natural selection sets in. 



(ii) Criticism. The determinants are hypothetical 

 creations ; if, however, they exist they must also develop 

 according to plan and harmoniously and always in the 

 same direction, in order (for example) to found a poison 

 apparatus, just as must be the case in a grown organism. 



By ' Weismannism/ therefore, nothing is gained, 

 and the whole question is referred to the microscopic 

 stage, from which we learn no more than from the 

 matured organism. 



M 2 



