42 



HELEN DEAN KING 



reliable as those from other sources. In these groups, differ- 

 ences between the ratios are probably not important, except 

 in the data from the U.S.A. Parkes ('26) states that the 

 lower sex ratios in black races seem to be a general phenom- 

 enon that probably has some real basis. He considers it due 

 to greater prenatal mortality in the blacks, among whom abor- 

 tions and stillbirths are greater than in white races. The sex 

 ratio in man, as in other mammals, seems to be one of the 

 characteristics of the race that depends upon a number of 



TABLE 12 

 Sex ratios in white and in colored races of man 



Sex ratios in albino and in gray rats 



factors, some of which are genetic, others physiological or 

 environmental. It is not improbable that the genetic consti- 

 tution of a race may prove to be one of the most important 

 of the various factors that affect the vitality of fetal young 

 and so largely determine the sex ratio among the newborn. 



The hypothesis that litter size affects the sex ratio in poly- 

 tocous mammals is one of long standing, and various series of 

 date have been recorded both for and against it. An arrange- 

 ment of data for gray rats to indicate the relation between 

 litter size and the sex ratio is shown in table 13. 



