56 P. W. WHITING AND HELEN DEAN KING 



female, and one adult female; and three adult dilutes — two 

 males and one female. 



On May 13, one adult wild- type male. 



In all, nine dilutes and twenty-two of the wild type were cap- 

 tured in the waste-paper house. 



Fifteen other wild rats were trapped during this time a short 

 distance from the waste-paper house, but none of them were 

 dilutes. For several years rats have been caught in large num- 

 bers in the near vicinity for study by investigators at The Wistar 

 Institute and at the Zoological Laboratory of the University of 

 Pennsylvania, but in no case were there any dilutes. It is very 

 probable, then, that the mutation occurred not long before the 

 dilute segregates appealed. It is also probable that some de- 

 gree of inbreeding must exist among wild rats, since the dilutes 

 seemed to be restricted to the waste-paper house and were there 

 in relatively large proportion. Dilutes were reported to have 

 been seen about the waste-paper house during the following sum- 

 mer, and one of them, a young male, was caught in August and 

 kept for a while at the Zoological Laboratory. 



The rats taken in the spring of 1916 at the waste-paper house 

 were kept at The Wistar Institute. The two adult dilute fe- 

 males failed to breed, although paired with dilute males for some 

 six months. The three adult males were crossed with a number 

 of females of different colors. An attempt was made also to 

 breed from the wild-type rats taken in the waste-paper house by 

 crossing them with Albinos. Of the twelve wild-type females 

 tested only one produced offspring, and she gave dilutes as well 

 as wild-type young in the first generation. Five wild-type males 

 sired litters by Albino mothers, but only one of these had dilute 

 offspring. Thus of wild-type rats that bred two out of seven 

 carried the factor for dilution. 



EXPERIMENTAL DATA 



Dilute males crossed with black-hooded females sired seven 

 litters consisting of fifty-three individuals, twenty-seven males 

 and twenty-six females. All of these rats were gray Irish, show- 

 ing that the new form of dilution is completely recessive and that 



