62 HELEN DEAN KING AND HENKY H. DONALDSON 



lengthen the average span of life in these gray rats, since 

 at both twelve and at twenty months of age the mortality 

 was less in individuals of the later generations than in those 

 of the earlier generations. 



Data given in table 10 show that there was practically no 

 difference in the mortality of males and of females in the 

 series as a whole. In both sexes^ at twelve months, the mor- 

 tality was about 4 per cent; at twenty months it was about 

 38 per cent. The mortality in males and females of the dif- 

 ferent generations at twenty months is shown by graphs in 

 chart 11 constructed from data in table 10. 



Percent Mortality 



go 



60 



40 



\ / ^>v ^ ^ 



Moles 



^^^\ ^\^ 



20 



1 2345678S10 



Chart 11 Showing the percentage of mortality at twenty months in males and 

 females of the different generations of captive Grays. 



In chart 11 the graph for the females starts slightly 

 higher than that for males, but drops below at the third gen- 

 eration. Subsequently the graphs cross and recross, but 

 after the sixth generation the graph for females runs below 

 that for males until the end of its course. In the later gen- 

 erations, therefore, the mortality was less in females than 

 in males. 



Such data as have been obtained indicate that males are 

 less viable than females at birth and during early postnatal 

 life. During adolescence and early maturity there is ap- 

 parently little difference in the mortality of the two sexes. 

 In later life, however, mortality among males again seems 

 to be greater than that among females. Lung disease} which 



