46 HELEN DEAN KING 



significant do not always receive biological verification." The 

 more extensive series of data for the guinea pig, given by 

 Kroning ('34), shows that the sex ratio tends to decrease as 

 the age of the mother advances. This investigator explains 

 his findings in accordance with the old Hofacker-Sadler hy- 

 pothesis that the sex ratio in the young is influenced by the 

 relative age of the parents. 



In the large and carefully compiled series of data for 

 Flemish and for Swedish breeds of cattle, summarized by 

 Johansson ('32: table 6), the general trend of the sex ratio 

 in each series is cyclic, and much like that in table 14, except 

 that the ratios are relatively low in the early born young. In 

 Johansson's series, as in these for gray rats, differences be- 

 tween ratios are not important statistically, therefore the 

 findings can be considered merely as suggestive. 



Literature contains numerous references to investigations 

 on the sex ratio in various races of man, based chiefly on data 

 from governmental registrations of vital statistics and on 

 geneological or hospital records. In many of these studies 

 the results accord in showing that the sex ratio is very high 

 for first births and then tends to fall as the age of the mother 

 advances (Busing, 1884; Eosenfeld, '00; Newcomb, '04; Pun- 

 nett, '04; Lewis and Lewis, '06; Siegel, '17; Parkes, '24, etc.). 

 The usual explanation given for this change in the sex ratio 

 is that there is an increase in the prenatal mortality of males 

 as the mothers grow older, but why this increased mortality 

 occurs is not known. Age changes in body metabolism, in the 

 structure of the uterus or in hormone secretions affecting 

 reproductive processes probably affect the vitality of fetal 

 young, and may tend to eliminate more males than females. 

 However, until more light has been shed on the effect of such 

 factors, it w T ill be useless to speculate as to the cause for the 

 change in the sex ratio of the young as the age of the mother 

 advances. 



