COLOR MUTATIONS IN MICE OF PEROMYSCUS 303 



two juvenile 'j^ellows/ a darker and a lighter specimen. In 

 comparison with these are shown the corresponding values for 

 darker and lighter specimens of the wild type, the latter extreme 

 being taken from a selected strain of pale or 'buff' animals. The 

 wide differences between both the mean and the extreme values 

 of the two series are sufficiently obvious. On the other hand, 

 there are but trifling differences between the darker 'yellow' and 

 the paler 'normal' individual. 



THE 'PALLID' COLOR VARIETY 



These mice were first referred to as 'partial albinos' (Sumner, 

 '17), but this designation was plainly at variance with cus- 

 tomary usage, so that the non-committal term 'pallid' was later 

 adopted. 



This 'mutation' — if it did arise de novo during these experi- 

 ments — appeared among a lot of Fg hybrids between Peromys- 

 cus maniculatus rubidus and P. m. sonoriensis. Four pallids 

 and seven of the wild type resulted from the mating of an Fi 

 male and his two sisters, each of these last producing two palhds. 

 It is certain that this was no simple segregation phenomenon, 

 due to the recombination of factors regularly present in the two 

 subspecies which were crossed. Up to the present time, more 

 than 300 F. and F3 hybrids between these two races have been 

 reared and no other case of the pallid mutation has come to light. 



Since the paUid-color variety has been described in some de- 

 tail in earlier papers (Sumner, '17, '18), a brief account will suffice 

 here. It is characterized primarily by the lack of most of the 

 black (or sepia) pigment found in normal mice (figs. 5, 9). This 

 lack appears in the absence of all-black (i.e., non-banded) hairs 

 from the pelage, and the extreme reduction of pigment in the 

 basal zone of the others. The latter is of a pale ashy hue in- 

 stead of slate-colored. Furthermore, the eyes are dark red 

 instead of black, the ears are not appreciably pigmented, and 

 the dorsal tail stripe (normally due to dark hairs) is scarcely 

 perceptible. A further peculiarity of this strain is the fact that 

 the eyes are smaller, or at least less protruding, than in the wild 

 type. The pallid mice are pale gray when young, developing 

 a considerable admixture of yellow or orange when adult. 



