726 



THE AMERICAN NATURALIST 



[Vol. XLIX 



fer from one cage to another, the character of the germ- 

 plasm of its descendants, and so of the race, depends very 

 largely upon what mates are transferred to the same cage 

 with it. This is where the selection comes in and there is 

 nothing " mysterious" about it either. 



The idea that selection can bring about no change in the 

 germ-plasm of the race ' 1 except by sorting over what is 

 already there, ' ' to which Pearl gives expression, rests on 

 the assumption that the germ-plasm never changes. What 

 ground have we for such an assumption % No more than 

 for the idea of the unchangableness of species, which for- 

 merly prevailed. Even Johannsen admits that large 

 germinal changes (" mutations") sometimes occur. He 

 himself records having observed them. Why should we 

 be so skeptical about the occurrence of minor germinal 

 changes ? It is easy to overlook them when purely somatic 

 changes are associated with them and outnumber them as 

 they possibly do in Johannsen 's beans and Pearl's fowls 

 but a single clearly established case should suffice to estab- 

 lish their existence and their importance in evolution. 



