86 THEORY OF EVOLUTION 



that have been accumulated in wild species are 

 of significance in the maintenance of the species, 

 or at least we are led to infer that even though 

 the visible character that we attend to may not 

 itself be important, one at least of the other 

 effects of the factors that represent these char- 

 acters is significant. It is, of course, hardly to 

 be expected that any random change in as com- 

 plex a mechanism as an insect would improve 

 the mechanism, and as a matter of fact it is 

 doubtful whether any of the mutant types so 

 far discovered are better adapted to those con- 

 ditions to which a fly of this structure and hab- 

 its is already adjusted. But this is beside the 

 mark, for modern genetics shows very posi- 

 tively that adaptive characters are inherited in 

 exactly the same way as are those that are not 

 adaptive; and I have already pointed out that 

 we cannot study a single mutant factor without 

 at the same time studying one of the factors 

 responsible for normal characters, for the two 

 together constitute the Mendelian pair. 



And, finally, I want to urge on your atten- 

 tion a question that we are to consider in more 

 detail in the last lecture. Evolution of wild 



