THEORY OF EVOLUTION 71 



fects as well. The females are almost com- 

 pletely sterile, while the males are fertile. The 

 viability of the stock is poor. When flies with 

 rudimentary wings are put into competition 



Fig. ;U. Mutant race of fruit fly, failed ruiliinentary. 



with wild flies relatively few of the rudimentary 

 flies come through, especially if the culture is 

 crowded. Tlie hind legs are also shortened. 

 All of these efl^ects are the results of a single 

 factor-difference. 



One mav venture the i^uess tliat some of the 

 specific and varietal differences that are char- 



