460 SCIENTIFIC THOUGHT. 



passed over in a history of Thought. Moreover, it has 

 made itself felt by giving rise to two separate views of 

 the cause of variation i.e., of that phenomenon in the 

 living creation on which the entire modern theory of 

 descent is founded. 



If it be true that the preservation of the species, the 

 continuity of living forms, is dependent on the germ- 

 plasma, whereas the somatic plasma, from this point of 

 view, only serves individual ends and is a receptacle or 

 temporary dwelling-place for the germs which it trans- 

 mits but does not create, the experiences of the body, its 

 changes and development, can have little or no influence 

 on the hidden germs and their further history. Thus 

 56. Weismann is led to a denial of the influence of en- 



Weismann v. 



Lamarck, vironment, of habit and acquired characters, except in 

 those cases where, as in the lower organisms, no dif- 

 ferentiation has set in between the germinal and the 

 personal substance. This amounts to a negation of those 

 modifying influences which Lamarck emphasised, and 

 which play such a great part in the theories elaborated 

 by Darwin, Haeckel, and especially by Herbert Spencer. 

 On the other side, it has led Weismann to lay a much 

 greater weight upon sexual selection and the effects 

 of crossing in the process of descent and the pheno- 

 mena of heredity. But for sexual selection, and the 

 endless combinations of different germ - plasmas, there 

 would, according to Weismann, be no variation, and 

 hence no development of the higher forms of life. The 

 controversy turns mainly upon the inheritance of acquired 

 characters, of which indeed no genuine and authenti- 



