32 HELEN DEAN KING 



as well as that in individuals of early generations born in 

 captivity, was due to disturbances of the nervous system in- 

 duced by fear and confinement within a limited space. The 

 nervous tension thus produced probably influenced the activity 

 of many organs of the body, especially those concerned with 

 secretion, and thus indirectly affected the reproductive organs 

 so that they were unable to function normally. As the ner- 

 vous tension was lessened, after a relatively short period of 

 captivity, the rats began to breed at a more normal rate. 



During the first eight generations sterility in females de- 

 creased from 37.29 per cent to 5.88 per cent. It rose to 18.64 

 per cent at the ninth generation, because of the prevalence of 

 ovarian tumors and infections of the genital tract at that time. 

 Only five of the 161 females reared in the tenth to the twelfth 

 generation did not breed, and in these cases sterility was 

 caused by diseases that affected the reproductive organs. In 

 later generations all females reared were fertile. By this 

 time the rats had lost their fear of man and were so well 

 adjusted to their new environment that restriction to repro- 

 duction induced by removal from their natural habitat had 

 disappeared. 



LITTEK SIZE 



Data for litter size in the second to the twenty-sixth genera- 

 tion are given in table 9. Litters of the first generation are 

 not included in this table because of the probability that the 

 data obtained did not cover the entire litter output of the wild 

 females that bred while in captivity. Data for five genera- 

 tions are grouped together to facilitate their analysis. 



The range in litter size was from one to fifteen, with the 

 mean at six. In each of the first three groups of table 9, over 

 half of the litters were of medium size, containing from five 

 to seven young. The last two groups comprise a relatively 

 smaller number of medium sized litters and more litters be- 

 low or above this magnitude. This difference in the distribu- 

 tion of the various sized litters in the groups was due, doubt- 

 less, to tHe extension of the reproductive period and the 

 increased fertility of females in the later generations (table 8). 



