58 HELEN DEAN KING AND HENEY H. DONALDSON 



recently, that in the albino race the age of the mother has a 

 pronounced influence on the sex ratio in her young. In gen- 

 eral, when the data for litters cast by a considerable number 

 of females are examined, it is found that males tend to pre- 

 dominate in litters cast by young mothers and by those that 

 are from ten to fourteen months old, and that female young 

 are in excess, as a rule, in litters produced when the mothers 

 are at the height of their reproductive activity (from six to 

 nine months of age) and when they are approaching the 

 menopause. In what manner the age of the mother can affect 

 the sex proportions in the young is unknown. It may be 

 that changes occur in the uterus at different age periods that 

 tend to favor the development of embryos of one sex rather 

 than those of the other, or there may be age changes in the 

 amount or in the nature of various hormones that at one 

 time render the ova more readily fertilized by spermatozoa 

 that are male-producing and at another time favor fertiliza- 

 tion by spermatozoa that are female-producing. Whatever 

 these changes are, they tend to balance each other in the long 

 run. When a female breeds at a normal rate throughout 

 reproductive life, the sex ratio in the entire number of off- 

 spring is usually near equality, unless disturbing factors, 

 such as hybridization, produce alterations in the ratio at all 

 periods during the reproductive life of the mother. 



As shown in a previous section of this paper, gray females 

 of the early generations began breeding at an average age 

 of about eight months (table 6). At this age albino females 

 are at the height of their reproductive power and their litters 

 contain, as a rule, an increased number of female young. Con- 

 ditions of captivity, seemingly, eliminated in the gray rats 

 the early period of breeding which seems to favor the de- 

 velopment of male young. The reproductive life of these 

 females, therefore, comprised two periods when female young 

 tend to predominate in the litters cast, and but one period 

 when male young tend to be in excess. Their litters, as shown 

 in table 9, contained a greater proportion of female than of 

 male young. As the generations advanced, an increasing 



