142 ROSCOE R. HYDE 



cultures. Moenkhaus found in his strains that steriHty was 

 practically confined to the males. He attempted to harmonize 

 Castle's results with his own by the assumption that each had 

 used a different measure of productiveness. Both investigators 

 are in agreement that inbreeding can not be the cause of sterility, 

 and that sterility is amenable to selection. From Castle's paper 

 I gather that he considers low productivity and complete sterility 

 in the female as related and that sterility in this case is related to 

 egg structure. Moenkhaus seems to imply that sterility is due 

 to some condition of the sperm. Evidence that bears on these 

 questions is given in the following pages. 



I wish to state in the beginning that my results warrant me in 

 making a sharp distinction between fertility and sterility as it 

 actually exists in the strains that I have used. Fertility, as I 

 have found it, does not grade into complete sterility. There may 

 however be gradations in fertility. The total sterility that ap- 

 pears in my cultures bears no relation to low fertility. Sterility 

 as it affects the flies in my strains is due to a different condition 

 from that operating to reduce the fertility. This statement is 

 certainly true of the sterility as it affects the female. The dis- 

 tinction paves the way for bringing the results of other investi- 

 gators under a common point of view. Failure to distinguish 

 between sterility and fertility has I suspect led to confusion, cer- 

 tainly in the work of Castle, and probably in the results of Moenk- 

 haus, because as I shall show the two things may relate to quite 

 different phenomena. The inheritance of sterility and fertility 

 in Drosophila must be separately treated if any progress is to be 

 made. 



In a culture of Drosophila, which I had been inbreeding and to 

 which I shall refer as the inbred stock, there appeared an increas- 

 ing number of sterile pairs. The sterility affected primarily the 

 females as was evident by testing them with other individuals. 

 In the sixth generation which had descended from a single pair of 

 grandparents of the fourth generation, 51 pairs in a total of 105 

 pairs were sterile. There were 47 sterile females, 3 sterile males 

 and 1 questionable case. 



