60 HELEN DEAN KING 



with the exceptions noted below. Aside from the rats carry- 

 ing the hooded gene, no mutants were found among the 857 

 individuals comprising the F 2 generation. Any simple re- 

 cessive color genes present in the foundation stock would have 

 manifested their presence in the second generation, unless 

 they were carried by female 1 and her mate, whose offspring 

 could not be inbred. 



It may be of interest to record some data regarding the 

 first pair of wild rats, since their descendants have been domi- 

 nant in the strain from early generations. Female 1 was 

 brought to the laboratory in February, 1919, having been 

 trapped in an abandoned stable in the outskirts of Philadel- 

 phia. She was approximately 6 months of age when captured, 

 and was conspicuous from arrival because of her ferocity. 

 She killed the first and second male with which she was housed, 

 although both were larger than herself. The third attempt 

 to mate her was successful, the male being a young adult cap- 

 tured in March, 1919. Their progeny comprised twenty-five 

 young with the unusual sex ratio of three males and twenty- 

 two females. All of these offspring were reared, with the 

 exception of one female which died when about 2 weeks old, 

 and all were normal grays. Eleven females in these litters 

 were housed with their three brothers for several months, but 

 no young were obtained. Ten females were mated with excess 

 males from litters of the third pair of wild rats. As inbreed- 

 ing the other offspring of wild female 3 and her mate gave 

 fifty-eight normal gray young, this pair of wild rats did not 

 carry genes for any of the recessive color mutations that later 

 appeared in the strain. 



Rats of the second generation were far better adjusted to 

 conditions of captivity than were their parents. Individuals 

 of one litter could be housed with those from other parents, 

 with few casualties resulting. From this time until the twenty- 

 first generation, few matings were made between rats that 

 were very closely related. The young obtained by mating 

 female offspring of the first pair of wild rats with males from 

 litters of the third pair were so noticeably larger and more 



