314 F. B. SUMNER AND H. H. COLLINS 



variability in their coat color and an even wider variability in 

 their eye color. In respect to the latter, they range from a 

 pink only slightly darker than that of the albinos to a shade only 

 slightly paler than the full black of the wild type. Indeed, both 

 of these extremes have been found among the derivatives of an 

 albino-pallid cross. Whether these differences are due to 'modi- 

 fying factors' or to the 'contamination of factors,' or whether 

 they are 'purely somatic' (whatever that may mean!) we are 

 unable to conjecture at present. 



13. There are included in the preceding pages the results of 

 numerous color analyses made by the senior author with the aid 

 of a Hess-Ives tint-photometer. The principal (macroscopic) 

 differences between the 'yellow' and 'pallid' mutants and 'wild- 

 type' mice of the subspecies gambeli were found to be due to 

 different proportions of black, white and a 'color' of tolerably 

 constant quality. The spectral position of this last, as judged 

 by the red : green ratio, was found not to differ very widely in 

 any of these forms. This may be taken as evidence of a consid- 

 erable degree of uniformity in the 'yellow' pigment of the hair, 

 though the latter is doubtless not the only factor concerned in 

 the gross results. Small, though well-marked, differences were 

 found, on the other hand, even in this red : green ratio, the most 

 noteworthy case being that of the 'a' and 'b' strains of yellows. 



14. To what degree the color mutations here discussed cor- 

 respond with those which have been described for house-mice 

 or other domesticated rodents, we cannot state with certainty. 

 The authors have not had the opportunity to make the neces- 

 sary comparisons either with living specimens or skins of such 

 races. 



On first thought, it might seem that our 'yellows' are of the 

 same type as the familiar recessive, black-eyed yellows, whose 

 condition is attributed to the replacement of the 'extension' 

 factor, 'E', by its recessive allelomorph, 'e' (Castle, '20, p. 124). 

 It must be recalled, however, that the black pigment in our races 

 is far from being restricted to the eyes, but is present in full 

 intensity in the bases of the body hairs, in the ears, feet, tail, 

 and some other parts. 



