164 EVOLUTION AND GENETICS 



good deal of evidence that is significant and un- 

 favorable. Darwin explained the eyeless condition 

 of many cave animals as a result of disuse. Recently 

 Pa3aie has bred fifty generations of flies in total 

 darkness and has found that their reaction to light 

 had been in no way impaired. Darwin suggested that 

 the wingless condition of some insects living on 

 islands was due in part to disuse. Now, there have 

 appeared in our laboratory cultures of flies raised 

 in milk bottles, of three different kinds having no 

 wings. These appeared as single individuals with the 

 wings entirely absent, from j)arents whose wings 

 had not decreased visibly in size in their long con- 

 finement. Each of the new types arose by a muta- 

 tion; and the inheritance of the wingless condition 

 shows that they owe their peculiarity to a change in 

 a single hereditary element, and are, in this respect, 

 comparable to the four hundred other mutant types 

 that have also arisen, whose new characters have no 

 conceiA^able relation to their confinement. 



It is more difficult to obtain definite information 

 as to whether or not the use of a part that increases 

 its size or improves its functions is inherited. Imagi- 

 nary cases of this sort are abundant, but since 

 other explanations will cover them they do not serve 

 our present j^urposes. There are no measurements, 

 so far as I know, to prove or to disprove the claim 

 that the children of blacksmiths have stronger arms 



