I. LIFE PROCESSES 59 



number of females began breeding at an earlier age. The 

 reproductive life of these females included all four periods in 

 which the sex ratio tends to change with the age of the 

 mother. In the young born to these females the sex ratio 

 was near equality (table 9). 



Changed conditions of life in captivity apparently had no 

 direct effect on the sex ratio in the litters cast by gray 

 females. They may be considered to have influenced the ratio 

 indirectly, however, by first shortening and then lengthening 

 the average span of reproductive life in the females. From 

 the evidence given it seems more probable that the variations 

 in the sex ratio found in the different generations of these 

 rats can be ascribed to physiological changes due to the ad- 

 vancing age of the mothers rather than to differential mor- 

 tality during fetal life. 



MORTALITY 



The scope of this study did not include a determination 

 of the normal duration of life in captive gray rats, as it 

 seemed more important to ascertain the condition of the 

 central nervous system and of various other organs before 

 they had been seriously affected by senescence or by dise.-i--. 

 Most of the rats, therefore, were killed and dissected when 

 they reached the age of twenty months. Data regarding the 

 mortality at various age periods and the chief causes of 

 death are given in the present section. 



In the entire series of 9505 young born in the first ten 

 generations, only eighteen individuals, ten males and eight 

 females, were found dead at birth. The mortality at this 

 time, about 0.19 per cent, was therefore very low, an<l much 

 less than that usually found in stock Albinos, where the still 

 born form about 2 per cent of all the offspring cast (Kinir. 

 '21). Since all of the litters could not be examined at the time 

 of birth, there is the possibility that the number of stillborn 

 was greater than the records show and that such individuals 

 were eaten bv the mothers. 



