292 F. B. SUMNER AND H. H. COLLINS 



The 'a' strain of yellows {fig. 2) 



The three chief independent strains of yellows will be desig- 

 nated as 'a/ '6/ and 'c/ respectively, according to the order in 

 which they appeared in our stock. It is the 'a' strain which has 

 been referred to in earlier publications. 



As already stated, this strain appeared among the offspring 

 cf two males and three females, these five being derived from 

 a single pair of wild individuals (P9 46 and P c^ 16). 

 The parents and grandparents were all of normal appearance. 

 Unless the 'mutation' appeared for the first time in the germ- 

 cells of the parent generation, these five parents of the original 

 yellows must have all been heterozygous, since each gave rise to 

 at least one yellow. The total number of their offspring (exclud- 

 ing those dying very young) was 24, of which 8 (1 cf, 5 9 and 

 2 of unknow^n sex) were yellows, while 16 (8 cf, 5 9, and 3?) 

 were of the wild color. Furthermore, three of these parents, 

 when mated later to yellows, gave rise to 5 yellows and 5 of the 

 wild color. If they were not actually heterozygous in origin, 

 they must, at least, have been producing 'yellow' and normal 

 gametes in about equal numbers. Two matings of yellows of 

 this strain resulted in 10 offspring, all yellow. For the most 

 part, however, the matings of these mice were made with non- 

 yellows or with yellows of another strain, as will be described 

 below. 



As previously stated, these mice ''are of a peculiar yellow- 

 brown hue, probably lying betw^een the 'cinnamon buff' and the 

 'clay color' of Ridgway, and not unlike the most highly colored 

 parts of the hair in P. m. sonoriensis."^ The peculiar hue was 

 attributed to two causes: 1) the larger number of banded 

 ('agouti') hairs, in proportion to the all-black, and, 2) the greater 

 proportional extent of the yellow region on these banded hairs. ■• 

 As regards the first point, it should be added that strictly 'all- 

 black' hairs (i.e., those entirely devoid of a paler cross-band) 



3 Sumner ('17). — Such a comparison with any set of color standards is ad- 

 mittedly extremely crude, since the pelage is very far from being a uniformly 

 tinted surface. 



4 Sumner ('18). 



