EFFECTS OF INBREEDING ON FERTILITY AND VIGOR 355 



days old; while Lantz ('10) reports a case in which an albino rat 

 was said to have produced a litter at the age of fifty-six days. 

 This last case is certainly a remarkable one, and its parallel has 

 not been found among the 50,000 rats bred in our colony. 



The last section of table 9 shows that, after the tenth genera- 

 tion, there was no marked change in the proportion of females 

 that bred at three and at four months of age, respectively. 

 Nearly 24 per cent of the total number of females used for breed- 

 ing cast their first litter by the time they were three months old; 

 over 60 per cent of them bred for the first time when they were 

 between ninety and one hundred and twenty days of age; while 

 about 13 per cent did not breed until after they were four months 

 old. The latter group was made up, for the most part, of females 

 that were born in the summer or autumn. 



Although Diising ('84) states that inbred animals tend to 

 mature very early, I do not think that inbreeding alone was 

 responsible for the fact that relatively more of the females in the 

 later than in the earlier generations of these inbred rats bred at 

 three months of age. In these experiments, when two or more 

 females of a litter were reared as possible breeding stock, the first 

 female that became pregnant was the one taken to continue the 

 line, provided she fulfilled all requirements as to size and vigor. 

 Thus the manner in which breeding females were selected pre- 

 served those individuals that tended to breed at an early age, 

 and this tendency to early maturity, if heritable, must have been 

 retained in the stock and intensified to some extent through 

 continued brother and sister matings. Inbreeding, aided by 

 selection, would thus seem to be the factor involved in producing 

 a strain of rats in which the females attained sexual maturity at a 

 relatively early age. 



D. Sterility 



Sterility occurs normally in the Albino, as in other strains of 

 rats, and therefore it might be expected to appear at times 

 in any strain, regardless of whether the animals were inbred or 

 outbred. Crampe ('84) states that of 221 Albino females which 

 he selected for breeding forty-six, or 20.8 per cent, were sterile. 



