THEORY OF EVOLUTION 



133 



evidence in favor of the view that some such 

 interchange probably takes i^lace. 



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 gra}^ 

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

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

 a ])lack vestigial male the following kinds of 

 flies are produced : 



