444 THE AMERICAN NATURALIST [Vol. LIT 



sume the mature pelage, but I no longer have reason to 

 doubt that the yellow type "breeds true." 



A yellow female gambeli mated to a "pallid" male of 

 the strain discussed above, has given birth to a single 

 offspring, having abundant dark pigment in the skin, 

 eyes and hair. In other words, these two pale, recessive 

 mutants seem to be "complementary" to one another, as 

 were Castle's two yellow races of rats (Castle and 

 Wright, 1915). 



3. Discontinuous Variation in Restricted Pigment Areas. 



I have discussed briefly elsewhere several sorts of color 

 markings, along with limited data which seemed to show 

 that some of -these were inherited in alternative fashion. 

 Other characters of the same type have been added to the 

 list. For example, in the second cage-born generation of 

 gambeli I have found three mice with faces strongly 

 "grizzled" by the presence of white hairs. It is prob- 

 ably no mere coincidence that these three grizzled speci- 

 mens, while not belonging to a single fraternity, are all 

 descended from the same grandparents. Neither the 

 parents nor the grandparents were recorded as having 

 the peculiarity in question, which would hardly have been 

 overlooked if present. Furthermore, the single off- 

 spring which I have obtained from a "grizzled" pair ex- 

 hibits this character plainly, though in a reduced degree. 

 One specimen showing the white-faced condition likewise 

 appeared in the C 2 generation of sonoriensis. 



Again, occasional mice of perhaps all of the races are 

 characterized by having considerable pigment in the skin 

 of the tail. Normally, the skin of this member is nearly 

 or quite devoid of pigment, the dorsal tail-stripe being 

 due to black hairs. Examination of two successive gen- 

 erations of rubidus makes it probable that this caudal skin 

 pigmentation is likewise a hereditary character. 



I shall here discuss only one of the localized pigment 

 variations which were dealt with in my earlier report on 

 color "mutations." This is the occurrence of a white- 

 tipped snout, due partly to the absence of skin pigment 

 and partly to the presence in this region of white hairs. 



