HEREDITY, VARIATION AND DEATH 27 



gemmules, which somehow find their way to the reproductive 

 cells. And thus the germ-plasm is formed in part, and might, 

 by a slight extension of the theory, be formed wholly from the 

 body. The hypothesis, thus extended, would give us alternating 

 generations of persons and reproductive cells. This theory of 

 Darwin's was only intended to be provisional a means of 

 accounting for the supposed inheritance of acquired character- 

 istics and there are insurmountable objections to it. How can 

 these gemmules find their way to their destination ? How are 

 they worked into the organisation of the germ-plasm ? And 

 would they not be assimilated like other matter ingested ? 



There is, besides, the theory of epigenesis, the supporters of Epigenesis 

 which maintain that Weismann's preformation, the idea that one 

 cell can contain potentially the whole future animal, is absurd. 

 According to them the embryo is shaped by external influences : 

 the cells get their characters from their position relatively to one 

 another, from the temperature, food and all that is summed up in 

 environment. And so the thorough-going advocates of epigenesis, 

 if such there be, would have us believe in the differentiation of 

 identical embryos into a host of different species ! When we 

 remember what minute differences are hereditary, environment is 

 a wretched reed to lean upon. Weismann speaks of a family 

 known to him in which a small depression no bigger than a pin's 

 head appears in the skin in front of the left ear in each generation. 

 And this small depression must somehow be represented in the 

 germ-plasm. No environment can cause it. 



The attempt to formulate the theory of epigenesis has given 

 strength to the theory of the continuity of the germ-plasm. I 

 recommend anyone who is unconvinced to read Hertwig's 

 Biological Problem of To-Day. It is an attempt to overthrow 

 Weismannism and put epigenesis in its place. The destructive 

 part of the book raises some formidable difficulties as one might 

 expect. But in the constructive part there is an almost complete 

 surrender of the position. After an elaborate attempt to show 

 that the environment guides the development of the embryo, 

 comes the confession that each animal has its characteristic 

 germ-plasm and that the environment alone cannot decide 



