EFFECTS OF INBREEDING ON THE SEX RATIO 29 



entirely exclude the male influence. ' ' In his summary Moenkhaus 

 states: ''The sex ratio is one of the qualities that is, like color, an 

 inherent character of this creature, strongly transmissible and 

 amenable to the process of selection. . . . Sex is probably 

 very little, if at all, influenced at fertilization in this species, but 

 it is probably determined much earlier and by the female."^ 

 Moenkhaus' conclusions regarding the character of the sex 

 ratio and its amenability to selection are as applicable to the rat 

 as they are to Drosophila, judging from the results of my in- 

 breeding experiments on the former species. Neither of these 

 investigations, however, give any information regarding the 

 causes that condition sex, although each seems to indicate that 

 the female takes quite as important a part in this process as does 

 the male. 



In the inbred strain, after the animals for breeding were selected 

 in each generation according to a definite plan, the two series 

 (A and B) became two separate lines as regards the sex-propor- 

 tions among the young. In the one line (A) the litters coiitained, 

 as a rule, an excess of males; in the other line (B) there was a cor- 

 responding excess of females. Between these two lines there was 

 no very marked difference as regards the size of the individuals 

 at a given age, their fertility or longevity, as the data given in 

 previous papers have shown (King, '18, '18 a). Generation after 

 generation, as far as the experiments have been carried, the sex 

 ratios in the inbred lines have remained distinct, and the varia- 

 tions from the norm have been in the same direction in each 

 generation of each series. These results are definite enough, and 

 they are based on data from a sufficiently large number of animals. 

 I think, to warrant the conclusion that in the rat the sex ratio is 

 to a certain extent at least, a character that is amenable to 

 selection. 



1 Warren ('18) has recently repeated Moenkhaus' selection experiments on 

 Drosophila, and concludes that the sex ratio in this form is "not readily, if at all, 

 modifiable by selection." Warren believes that the modified sex ratios found in 

 two of his three series of experiments were due to 'chance variation,' and he 

 attributes the anomalous sex ratios obtained by Moenkhaus to the action of a 

 sex-linked lethal factor — the explanation offered by Morgan ('14 a) to account for 

 the unusually low sex ratios found in several strains of Drosophila. 



