No. 615] INHERIT AX ( K IX PKHOMYscrs 



191 



hair both of Mus and Peromyscus, I can not feel sure of 

 any sharp distinction between the black and the brown 

 pigments. It is true that the axial pigment bodies of the 

 basal portions of the hair are nearly dead black, while 

 most of the superficial pigment at the distal ends is dis- 

 tinctly brown. But all gradations occur in the axial pig- 

 ment of the transitional zones, and these gradations ap- 

 pear to be due not merely to differences in the density of 

 the clusters of granules, but to gradations in the depth 

 of color of the individual granules themselves. Without 

 having made any careful chemical tests, I am disposed to 

 believe that black and brown, in the hair of mice, are due 

 merely to different degrees of aggregation of a single 

 pigment. On the other hand, this dark pigment seems to 

 differ, chemically and otherwise, from the various shades 

 of yellow. 



The differences in the color of mice of different sub- 

 species and of different parts of the pelage of a single 

 individual appear to be due to two chief causes: (1) the 

 relative length of the pale zone, in relation to the rest of 

 the hair; and (2) the proportionate numbers of the all- 

 dark and of the banded hairs; probably also to (3) the 

 depth of shade of the yellow pigment in the pale zones, 

 and (4) the degree of concentration of the superficial pig- 

 ment at the distal ends. In some of the "mutants," as 

 will be pointed out below, certain other factors contribute 

 to the differences shown. 



Of importance for our general viewpoint is the fact that 

 no one of the geographic races which has been examined 

 possesses any type of hair which is wholly lacking in any 

 other race. It would be impossible from a single hair, 

 or even a small group of hairs, to say from what sort of 

 mouse they were taken. 



When viewed on the ventral side, these four races of 

 mice likewise present characteristic differences. They 

 form a graded series in respect to the whiteness of the 

 pelage, which is purest in the desert race and least so in 

 that from the redwoods. The differences are found to 



