050 MONOGRAPHS OF NORTH AMERICAN RODENTIA. 



than the northern, darker form, or than the fuscous type of the northwest 

 coast. 



''As corroborative evidence that these varied types of coloration are but 

 geographical races, it becomes interesting to observe that the light and dark 

 and the fulvous and rufous forms, respectively, of the different species, occur 

 over the same areas. With the fuscous type of Tamias quadrivit talus occur 

 the dark types of Sciurus hudsonius, and the dark- backed form of Spermo- 

 phUus grammurus, and also a peculiar dusky form of Arctomys and of Lepus, 

 and a dark form of SpermophUus richardsoni. On the Plains occur pallid 

 forms of Sciurus ' ludovicianus\ Sciurus hudsonius, Tamias quadrivittatus, and 

 SpermophUus richardsoni. With the fulvous type of Tamias quadrivittatus 

 occurs a rufous form of SpermophUus grammurus ; but the form of Sciurus 

 hudsonius, occurring over the same area, presents the exceptional condition 

 of a minimum amount of rufous." 



"In addition to the tendency to cliange of color with locality, there is 

 another phase of color variation that requires, in this connection, a passing 

 notice, — namely, melanism. It is now well known that almost every species 

 of Mammal may be expected to present melanistic individuals, instances of 

 its occurrence in the majority of the North American species being now well 

 established. Indeed, the very fact of a melanistic phase of coloration may 

 be looked upon as almost a priori evidence that the individuals presenting it 

 belong to a melanistic race of some species whose normal color is some other 

 tint than black, as Professor Band long since remarked in respect to the 

 American Squirrels. It has been supposed that the tendency to melanism is 

 more prevalent at the northward ; but such does not appear to be necessarily 

 the case. Among the Sciuridce, for instance, a group rather remarkable for 

 a tendency to melanistic varieties, the black and dusky forms are as often 

 southern as northern. In some species, melanistic individuals are as rare as 

 are the cases of albinism, as in Sciurus hudsonius, the species of Tamias, and 

 in many of the SpermophUi, while in others they are sometimes the common, 

 if not the prevalent, form over a considerable area, as occurs in Sciurus caro- 

 linensis and Sciurus cinereus. Melanism is akso of frequent occurrence in 

 Sciurus aberti [*] and in SpermophUus grammurus; the latter presents a melan- 

 istic form both in Texas and Lower California. SpermophUus parryi has also 

 a black race along the Yukon River, and frequent instances of melanism are 



['Melanism of this species Large); predominates over the normal coloration iu Colorado.] 



