I. LIFE PROCESSES 69 



related with the growth rate, as is the case in the albino rat 

 and in man. 



6. Evidence presented indicates that variability in the body 

 weight of rats during postnatal life is due, in part, to differ- 

 ences in the genetic constitution of the individuals. 



7. The average age at which females of the early genera- 

 tions cast their first litters was seven months. With the ad- 

 vance of the generations breeding began at an earlier age; 

 consequently, the average length of the reproductive period 

 was increased by about two months (table 6, chart 7). 



8. The average number of litters cast by gray females of 

 the first nine generations was 3.69. Deviations from this 

 mean in any generation were too small to be significant 

 (table 7). 



9. The range in litter size was from one to fourteen, with 

 the average 6.1. This average had not changed appreciably 

 at the end of ten generations (table 7, chart 8). 



10. Sterility was very high (70 per cent) in the wild females 

 which formed the foundation stock for this series of captive 

 Grays. It decreased steadily, how r ever, as the generations 

 advanced, falling to 5.88 per cent at the eighth generation 

 (table 8, chart 9). Sterility was due chiefly to diseases affect- 

 ing the reproductive organs or to decreased vitality resulting 

 from lung infection. 



11. In the entire series of individuals the sex ratio was 

 96.99 1.34. Deviations from this ratio in any generation 

 were not statistically important, and they are ascribed, ten- 

 tatively, to physiological changes due to the advancing age 

 of the mothers rather than to differential mortality during 

 fetal life. 



12. Mortality at birth was very low in the first ten genera- 

 tions of captive Grays (0.19 per cent). It increased during 

 the suckling period, fell to a low point during adolescence and 

 early maturity, and showed a marked rise after the animals 

 were a year old. 



13. In the series as a whole the mortality was about the 

 same in the two sexes at the age periods noted, being about 



