VI 



EVOLUTION IN NATURE 



Some biologists still find the idea of mutations, 

 even with no attempt to explain the underlying 

 cause, an adequate explanation of evolution. The 

 occurrence of mutations and of the selective proc- 

 esses which may act upon them is too well estab- 

 lished to cause any deep concern; even though the 

 idea of evolution by such means demands a tre- 

 mendous range of mutations and an almost equally 

 great destruction involving the mutant individuals 

 which fail to encounter a satisfactory environment 

 or to receive an adequate heritage, we must admit 

 the remote possibility that it points the way to 

 some evolutionary change of positive adaptive 

 value. So far as I am aware it would be difficult to 

 predict adaptive value in the mutations which 

 have been observed, but Muller points out in a 

 recent article that the abundance of mutations in 

 Drosophila meets one of these conditions, and fur- 

 ther that most of these mutations are of a sort 

 which would lead to destruction rather than to 

 survival in the natural state. ^ This obliging little 

 fly gives abundant evidence of mutation and de- 



1 Scientific Monthly, Vol. XL (6), pp. 481-505, 1929. 



143 



