196 HEREDITY AND SEX 



(mainly females), but we must be careful to distinguish 

 between the appearance of sterile individuals in these 

 cultures and the lessened fertility that may be shown 

 by the stock in general. The recent work of Hyde on 

 these same flies has shown that the appearance of 

 sterile individuals may be an entirely different question 

 from that of a decrease in general fertility. The 

 latter again may be due to a number of quite different 

 conditions. Castle and his co-workers found that the 

 sterile individuals could be eliminated if in each genera- 

 tion the offspring were selected from pairs that had not 

 produced sterile individuals. Hyde has found, in 

 fact, that one kind at least of sterile females owe their 

 sterility to a definitely inherited factor that can be 

 eliminated as can any other Mendelian recessive 

 trait. Moenkhaus, who has also extensively studied the 

 problem of inbreeding in these flies has likewise found 

 that his strains could be maintained at their normal 

 rate of propagation by selecting from the more fertile 

 pairs. 



If we eliminate from the discussion the occurrence 

 of sterile individuals, the question still remains whether 

 the output of the fertile pairs decreases if inbreeding 

 is carried on through successive generations. There 

 is some substantial evidence to show that this really 

 takes place, as the following figures taken from Hyde's 

 results show. 



At the end of thirteen generations the fertility 

 of the stock was reduced by half, as determined in this 



