EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 363 



the final records for the two groups show a difference of 22.2 per 

 cent in favor of the inbred animals. In the female groups the 

 span of life in the inbreds also tended to be longer than that in 

 the controls, but the difference was not quite as marked as in the 

 case of the males: the final records show a difference of only 12.7 

 per cent. 



It appears, from the above comparison of data for stock and 

 inbred rats, that continued inbreeding, under favorable environ- 

 mental conditions and with the aid of selection, cannot only lessen 

 the tendency to early death caused by malnutrition, but that it 

 can extend the average span of life in the rat considerably beyond 

 that found in the stock controls. Constitutional vigor, as judged 

 by the longevity of the individuals, is therefore not invariably 

 lessened by continued inbreeding. 



In table 10 and in table 11 it will be noted that the mortality 

 data for the first generation group indicate that the span of life 

 in the females, particularly^ in the B series, was much shorter 

 than that in the males. The reason for this ' selective mortality' 

 is not clear, although it may be that the females were not able to 

 throw off the effects of malnutrition quite as readily as were the 

 males. In both inbred series, after the tenth generation, the 

 mortality in the females at any age period was practically the 

 same as that in the corresponding group of males. Data given 

 in table 12 show that stock females tended to live longer than 

 stock males : a reversed relation seemed to hold for the inbred rats. 

 Taking the inbred colony as a whole, I am inclined to the opinion 

 that the females, as a rule, tend to live longer than do the males. 

 More males than females usually die as the result of a sudden, 

 sharp change in temperature, and the impression one gets from 

 working daih^ with the animals is that the males are far more sus- 

 ceptible to pneumonia than are the females, and that they are 

 sooner attacked by various parasitic pests, such as lice and ear- 

 mites. White ('14) states that in India the bubonic plague is a 

 more fatal disease to male than to female rats, thus indicating 

 that the female is stronger, constitutionally, than the male. 

 These results are in accord with the findings for the human race : 

 census reports and various statistical tables that have been com- 



