THEORY OF EVOLUTION 133 



evidence in favor of 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 



FIG. 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 F x female is back-crossed to 

 a black vestigial male the following kinds of 

 flies are produced : 



