LIFE PROCESSES IN CAPTIVE GRAY RATS 25 



doubtless be ascribed mainly to the method used in selecting 

 individuals to propagate the race and to inbreeding. 



All individuals in this strain of rats descended from six 

 pairs of wild rats. No new stock has ever been added. Ex- 

 cept in the descendants of one pair of feral animals, few 

 matings were made in early generations between individuals 

 that were very closely related. Rigid selection of only the 

 most vigorous animals as parents of the succeeding genera- 

 tions gradually eliminated descendants of some of the wild 

 rats, thus rendering the stock more homogeneous. For rea- 

 sons given in the final section, all individuals in the twenty- 

 first to the twenty-fifth generations were inbred, brother and 

 sister. This close inbreeding undoubtedly tended to reduce 

 variability and so is responsible, in great measure, for the 

 low body-weight variability in rats of the twenty-fifth gen- 

 eration. 



THE REPRODUCTIVE PERIOD 



When wild animals are brought into captivity, the most 

 marked effect of the changed conditions of life, at first, is on 

 their reproductive activity. Some wild forms never breed 

 after being removed from their natural habitat ; others pro- 

 duce but a small number of young which often must be reared 

 by foster mothers since their own mothers do not care for 

 them. In most cases a considerable period of time elapses 

 before individuals become sufficiently adjusted to their new 

 environment to produce and rear strong, vigorous offspring. 

 The cause of this low fertility in wild animals when first 

 captured is unknown, and, as Darwin (1875) has stated, "we 

 can only infer that it is caused by a change of some kind in 

 the natural conditions of life." 



Thirty-six wild rats (sixteen males and twenty females) 

 were brought to the laboratory to serve as foundation stock 

 for this strain of captive grays. Judging from their size, the 

 youngest of the females were at least 3 months of age when 

 captured, the oldest being probably a year old. None of the 

 females were pregnant when captured, although all of them 



