EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 359 



the twenty- third generation only, as the weight records for ani- 

 mals belonging in the twenty-fourth and in the twenty-fifth gen- 

 erations are not yet completed. As all of the animals reached 

 the age of three months, the first mortality record given is that 

 for anunals at six months of age. 



On examining the mortality data for the males, as given in 

 table 10, it is found that comparatively few of the animals in any 

 generation group died before the age of six months, and that over 

 50 per cent of them lived to be more than one year old. A 

 comparison of the corresponding records for the various genera- 

 tion groups shows unmistakably that the animals belonging to 



TABLE 10 



Showing the mortalily at different ages in a group of 236 males and 179 females 

 belonging in the seventh to the twenty-third generations of the A series of 



inbred rats 



the later generations tended to be longer lived than did those in 

 the earlier generations. 



The mortality data for the females of the A series are much like 

 those for the males, the most noticeable difference being found in 

 the records for the first generation group where only 10 per cent 

 of the females lived to be fifteen months of age. Taking the 

 animals of the A series as a whole, about 4 per cent of them died 

 before they reached the age of six months; 20 per cent did not 

 live to the age of nine months ; 50 per cent were dead at the end 

 of one 3^ear, and only about 35 per cent lived to be fifteen months 

 old. 



Mortality data for individuals belonging to various generation 

 groups of the B series are shown in table 11. 



