158 THE THEORIES OF EVOLUTION 



habit of mind which makes him look at things from 

 the selectionist view-point. In Lamarckian parlance 

 we would speak of the universal influence of environ- 

 ment and mode of life on species, individuals and 

 determinants. 



Weismann has introduced into his biological sj^stem 

 all the strong points of the earlier systems : Darwin's 

 representative particles, Naegeli's separate plasms 

 and elementary characters, De Vries' migration of 

 the particles from the nucleus into the cytoplasm. 

 By combining those ideas in a system of particles 

 which are assigned different ranks in a definite hier- 

 archy, he succeeded in creating a series of units ac- 

 counting for phenomena which the pnrperties of one 

 single category of units could never explain. At the 

 same time, he eliminated some improbable features of 

 the earlier systems : Darwin's circulation of the gem- 

 mules and the uniform constitution of Naegeli's micel- 

 lar strands. He also developed the idea of the 

 germ plasm which had only been hinted at by Jaeger 

 and Nussbaum and he added so much to it that it be- 

 came a truly Weismannian idea. 



The theory of the germ plasm preceded in Weis- 

 mann's mind all his other theories. It is in fact a 

 double theory: On one side we have a theoretical 

 doctrine with all its hypotheses; on the other, a fact, 

 a trivial fact, presented in such a way that it leads to 

 definite conclusions. 



