THEORY OF EVOLUTION 133 



evidence in favor <oi the view that some such 

 interchange probably takes place. 



We find this same process of interchange of 

 characters taking place in each of the other 



J^'IG. 65. Scheme to illustrate double crossing over. 



three groups of Drosophila. An example will 

 show this for the Group II. 



If a black vestigial male is crossed to a gray 

 long- winged female (fig. 66) the offspring are 

 gray long. If an Fi female is back-crossed to 

 a black vestigial male the following kinds of 

 flies are produced : 



