THEORY OF EVOLUTION 



135 



Oaootos of Hale 



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

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

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

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

 Crossing over takes place in the Fj female but not in the 

 F, male. 



value under the same conditions is obtained 

 from the same pair of characters. 



