INFLUENCE OF ENVIRONMENT 349 



germ cells probably lead to certain general 

 modifications of the adult, which may be larger 

 or smaller, stronger or weaker, according as 

 the germ is well or poorly nourished, but it is 

 incredible that the environment which pro- 

 duces rickets, or hypertrophied heart, or loss 

 of sight in one generation should modify the 

 germ cells in such a peculiar and definite way 

 that they should give rise in the next genera- 

 tion to these particular peculiarities, in the ab- 

 sence of the extrinsic cause which first pro- 

 duced them. The inheritance of acquired 

 characters is incredible, because the egg is a 

 cell and not an adult organism; and in this 

 case there is no sufficient evidence that the 

 thing which is incredible really does happen. 

 If specific changes of environment produced 

 specific changes in heredity we should expect 

 to find that where plants or animals are grafted 

 together each would modify more or less the 

 hereditary constitution of the other. But this 

 does not occur. Everybody knows that when 

 a branch of a particular kind of fruit tree is 

 grafted upon a tree of a different variety the 

 quality of the fruit borne by that branch is 



