28 HELEN DEAN KING 



slightly higher than the norm. The results of these various series 

 of experiments would seem to indicate that inbreeding per se has 

 little, if any, effect on the sex ratio. 



Moenkhaus' ('11) extensive series of inbreeding experiments on 

 Drosophila so closely parallel my own experiments on the rat, 

 both in the manner in which the experiments were conducted and 

 in the results obtained, that a brief resume of his work must be 

 given here. 



In order to obtain the normal sex ratio in Drosophila, Moenk- 

 haus ascertained the sex of 26,933 imagos that developed from 

 eggs laid by wild flies, and found among them a sex ratio of 88.8 

 cf : 100 9 . In this species, therefore, there is normally an excess 

 of females, as other investigators (Rawls, '13 ; Hyde, '14 ; Warren, 

 '18) have noted. The experiments were conducted in the fol- 

 lowing way: "Two pairs were selected from nature, the one 

 showing a high, the other a low female ratio. These were se- 

 lected as the parents of the two strains to be developed. From 

 among the offspring of each of these two pairs a number of single 

 matings were made. From among these the pair that showed the 

 most favorable ratio in the desired direction was selected to con- 

 tinue the strain. The same process was repeated as often as 

 desired." 



In this way Moenkhaus developed two inbred strains in one of 

 which the individuals showed a high sex ratio, in the other a low 

 sex ratio. The results of this part of the investigation showed that 

 "it is possible to develop a strain with a high female ratio much 

 more easily and pronouncedly than a male strain." Moenkhaus 

 then made reciprocal crosses between the two strains in order to 

 determine, "first, whether the maternal or the paternal elements 

 had an equal share in the control of this ratio, and second, 

 whether this ratio was determined in the process of fertilization." 

 The experiments showed, in a most decided way, that "the male 

 has little or no influence in determining the sex ratio in this species. 

 In most of the cases the ratio of the offspring falls pretty closely 

 around that of the strain from which the females were taken. 

 . . . It is not certain, however, that the sex ratio is estabhshed 

 before fertilization, since the experiments do not with certainty 



