168 THEORY OF EVOLUTION 



FIG. 83 b. Series of wings of different length shown by trun- 

 cate stock of D. ampelophila. 



bred from individuals having the shorter 

 wings until at last a stock was obtained in 

 which some of the individuals had wings much 

 shorter than the body. By means of linkage 

 experiments it was shown that at least three 

 factors were present that modified the wings. 

 These were isolated by means of their linkage 

 relations, and their mutual influence on the pro- 

 duction of truncate wings was shown. 



