56 



HELEN DEAN KING AND HENRY H. DONALDSON 



The sex distribution and the sex ratio in the entire series 

 of 1539 litters comprised in the first ten generations of captive 

 Grays are given in table 9. 



As shown in table 9, litters forming the first generation 

 of captive gray rats, which were cast by wild females of 

 unknown ages, contained a total of 139 young, among which 

 the sexes were very evenly distributed, the ratio being 98.57 

 males to 100 females. In the second generation the ratio 

 dropped to 81.95 males to 100 females. The difference of 

 16.62 11.94 between the ratios for the first and second gen- 



TABLE 9 

 Showing sex distribution and sex ratio in different generations of captive gray rats 



erations is considerable, but it cannot be deemed as statisti- 

 cally important because of the size of its probable error. In 

 succeeding generations the sex ratio rose gradually, reach- 

 ing its maximum, 104.14 males to 100 females, in the sixth 

 generation, after which it declined slightly. 



The changes in the sex ratio with the advance of the gener- 

 ations are shown graphically in chart 10. 



Aside from its sharp decline at the second generation, the 

 graph in chart 10 shows only minor fluctuations and tends to 

 run at a level which indicates a nearly equal proportion of 

 the sexes. 



