I. LIFE PROCESSES 



61 



for the second to the tenth generations are combined in 

 groups of three generations. In the males of the first group 

 (second to fourth generations) the mortality at twelve 

 months was 5 per cent, which was about twice as great as that 

 in the second group (fifth to seventh generations). The mor- 

 tality in the third group of males (eighth to tenth genera- 

 tions) was 0.6 per cent higher than that in the second group. 



TABLE 10 

 Showing mortality in different generations of captive gray rat* 



In the females, at twelve months, the change in mortality 

 as the generations advanced was relatively slight, falling 

 from 4.1 to 3.1 per cent from the first to the third group. 

 The mortality at twenty months for males of the first 

 group was 44 per cent, and it dropped subsequently about 

 14 per cent. In the females at this age the mortality showed 

 a marked decrease as the generations advanced, being 26.9 

 per cent less in the third than in the first group. From these 

 findings it is evident that conditions of captivity tendril t 



