I. LIFE PROCESSES 57 



In the entire series of 9505 individuals the sex ratio was 

 slightly under equality, being 96.99 1.34. Deviations from 

 this ratio in any generation are not great enough to be con- 

 sidered as significant. 



If it is assumed that the sex ratio found in the young of 

 the first generation is about that normal for the race in its 

 wild state, it follows that changed conditions of life in cap- 

 tivity must have influenced the proportion of the sexes in the 

 two succeeding generations, since the sex ratios in these gen- 

 erations were relatively low. Males are seemingly less viable 



Number Moles to 100 Females 

 110 



100 



qo 



Generation 



1 23456781 10 



Chart 10 Showing the sex ratio in different generations of captive gray rats. 



than females at all stages of development, and their mor- 

 tality at birth and during early postnatal life is greater than 

 that among females (King, '15). The most obvious explana- 

 tion, therefore, for the drop in the sex ratio at the second 

 generation is that change in environment rendered fetal mor- 

 tality selective, and that it bore more heavily on male than 

 on female embryos. 



There is, however, another factor which may have affected 

 the sex ratio in the early generations of these rats. It has 

 been shown in a previous publication (King, *24), and the 

 finding confirmed by a large series of data obtained more 



