Weismann s theory of Heredity (1891). 57 



the sense of never having had any of its hereditary 

 endowments in any way affected by the general body- 

 tissues in which it resides, the following important 

 consequences, it will be remembered, arise. The 

 process of organic evolution must have been exclusively 

 due to a natural selection of favourable variations 

 occurring within the limits of this substance itself; 

 and therefore the so-called Lamarckian factors can 

 never have played any part at all in the evolution of 

 any but the unicellular organisms. On the other hand, 

 if this substance has not been thus perpetually con- 

 tinuous, but more or less formed anew at each ontogeny 

 by the general body-tissues in which it resides, natural 

 selection has probably been in some corresponding 

 degree assisted in its work of organic evolution by the 

 Lamarckian factors, with the result that the experi- 

 ences of parents count for something in the congenital 

 endowments of their offspring. So much for the 

 first of the two differences between germ-plasm and 

 gemmules, or the difference which arises from the 

 perpetual continuity of germ-plasm. 



Touching the second difference, or that which arises 

 from the absolute stability of germ-plasm, it will be 

 remembered how from this character there arises 

 another important chain of consequences. Namely, 

 individual variations of the congenital kind can only 

 be due to admixtures of different masses of germ- 

 plasm in every act of sexual fertilization ; natural 

 selection is therefore dependent, for the possibility 

 of its working, upon the sexual methods of propa- 

 gation ; hence, natural selection is without any juris- 

 diction among the unicellular organisms, where the 

 Lamarckian factors hold exclusive sway ; and hence, 



