66 HELEN DEAN KING 



similar disparity in the sex ratios of white and colored races 

 of man is noted (table 12). 



The sex ratio was not influenced by litter size (table 13). 



Data given suggest, but give no definite proof, that there 

 was a cyclic change in the sex ratio of the young as the age 

 of the mother advanced (table 14, fig. 13). 



Mortality at birth was low in all generations, and only 265 

 stillbirths were recorded for the entire strain. Mortality was 

 low, also, during the first year of life, averaging 4.2 per cent 

 for males and 3.4 per cent for females: 79.9 per cent males 

 and 76.6 per cent females lived to the age of 20 months. The 

 mortality rate among older rats decreased as the generations 

 advanced, indicating that under conditions of captivity the 

 life span in both sexes was lengthened (table 15). The chief 

 causes of death were pneumonia, which was more prevalent 

 among males, and tumors which occurred mainly in females. 



At the twenty-fifth generation gray rats had lost the high 

 nervous tension and fear of man displayed by individuals in 

 early generations. Females then took excellent care of their 

 young and would serve as foster mothers if certain precau- 

 tions were taken. Two of the outstanding traits in rats of 

 early generations still persisted: adults continued to show 

 antipathy to strangers placed in their cage, and exceptionally 

 large rats, particularly males, were yet prone to attack small 

 individuals of the same sex. Gray rats of later generations 

 could be rendered as tame as are stock albinos if they were 

 handled frequently during early life. It was then possible 

 to use these rats for any kind of experimental work. 



Several mutations affecting the color or the structure of 

 the hair appeared in this strain of gray rats. Gray hooded 

 rats were developed by selective breeding among the descend- 

 ants of one pair of wild rats in the foundation stock that 

 carried the hooded gene (figs. 14 to 20). Black, albino and 

 ruby-eyed dilute mutants appeared in the twelfth to the four- 

 teenth generations. Curly, a dominant mutation previously 

 unknown in rats, emerged in the seventeenth generation (figs. 

 21 to 26). Another new mutation, cinnamon, was found in 

 the twenty-second generation. 



