18 HELEN DEAN KING 



As the sex ratio for the inbred litters of tho B series was 23 

 points below the norm, while that for corresponding litters of the 

 A series was 18 points above the norm, it would appear that the 

 sex ratio in the rat can be swung by selection farther towards 

 the female side than towards the male side. Moenkhaus ('11) 

 obtained a similar result in his inbreeding experiments with 

 Drosophila. 



The half-inbred litters in the eighth generation of the B series 

 gave a sex ratio nearly 10 points higher than the norm, so here 

 selection was not effective at once in changing the sex ratio. In 

 none of the subsequent generations, however, was the sex ratio in these 

 litters above the norrii, most of them were considerably below it {table 

 5) . When the data were combined in generation groups (table 7) , 

 it was found that the sex ratios for all groups except one (eighth 

 to tenth generations) were very low. For the total of 633 litters 

 the sex ratio was 91.1 cf : 100 9 , thus being 14 points below the 

 norm and 9 points higher than the sex ratio for the inbred litters 

 of this series. 



In each of the inbred series the sex ratios in the half-inbred litters 

 belonging in the eighth to the twenty-fifth generations showed less 

 deviation from the norm than did the sex ratios in the corre- 

 sponding inbred litters, yet in each case the difference between the 

 sex ratio for the inbred group of litters and that for the half- 

 inbred group was less than the difference between the sex ratio 

 for the half-inbred litters and the norm. The possible signifi- 

 cance of these results will be discussed later. 



In order to obtain the sex ratios for the various generations of 

 the inbred strain as a whole, the data for the two series (A, B) 

 were combined as shown in table 8. 



The range of variation in the sex ratios of the litters in the first 

 four generations of the inbred strain was greater than that among 

 all of the other generation groups (table 8) . This result was to be 

 expected, considering the relatively small number of individuals in 

 these generations and the adverse conditions under which the 

 animals lived. When the data were combined, however, the sex 

 ratio obtained (110.3 d" : 100 9) was only 5 points above the 



