SCIURIDH—GEOGRAPHICAL DISTRIBUTION AND VARIATION. 651 
well. known in all the species of Arctomys. In numerous instances, these 
melanistic individuals and melanistic forms have been described as distinct 
species, while in reality they are generally so sporadic in their occurrence as 
to render them hardly worthy of recognition, even as varieties. 
“The gradual increase of our knowledge in respect to the character of 
these melanistic forms, and especially in regard to the extent and character 
of geographical variation, necessarily leads to the modification of our views 
in respect to the status of many forms that have formerly passed current 
as more or less well-established species, and also to consequent changes 
in nomenclature. The representatives of few groups are more variable in 
respect to color, even among individuals of the same species inhabiting the 
same locality, than the arboreal Squirrels. Add to this the considerable 
amount of geographical variation that obtains among them, and the very con- 
siderable changes attendant upon season in respect to the character of the 
pelage, and we shall no longer feel surprised at the profusion of synonyms 
that attach to many of the species.” * 
In several species of North American Sciurz, a tendency to fulvous- or 
rufous-bellied forms is noticeable. This is commonly developed at the south- 
ward, but in one instance occurs in the moist region of the Pacific coast, 
north of the fortieth parallel, namely, in Sciwrus hudsonius var. douglassi. 
It also prevails to a marked degree in all three of the varieties of Sciurus 
niger (including the S. “cinereus”, S. vulpinus”, and S. “ludovicianus”), but 
especially in that (var. /udovicianus) inhabiting the Mississippi Valley, where, 
as already noted, there is a marked increase in the intensity of the rufous of 
the ventral surface in passing southward from the Great Lakes to the Gulf. 
S. colligi, which ranges northward to Arizona, also runs into a rufous-bellied 
phase in Mexico, while all of the other. species found south of the United 
States are either always orange or dark rufous below, or present this colora- 
tion as the prevalent or normal one; white- or grayish-bellied specimens 
of any species from the warmer parts of the two Americas being exceptional. 
Not only, also, is there a greater tendency to redness on the ventral surface, 
but this color often involves the outer (as well as the inner) surface of the 
limbs, and frequently extends high up on the sides of the body, especially 
anteriorly. The dorsal surface is also quite generally suffused with either 
bright fulvous or rufous, while the middle of the lower surface of the tail is, 
* Proc. Bost. Soc. Nat. Hist. vol. xvi, 1874, pp. 277-282, 284, 285. 
