THEORY OF EVOLUTION 



135 



FIG. 66. Cross between black vestigial and gray long flies. 

 Two pairs of factors involved in the second group. The F t fe- 

 male is back crossed (to right) to black vestigial male; and 

 the F, male is back crossed to black vestigial female (to left). 

 Crossing over takes place in the F t female but not in the 

 Fj male. 



value under the same conditions is obtained 

 from the same pair of characters. 



