660 



MONOGEAPHS OF NORTH AMERICAN RODBNTIA. 



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

 coust. 



" 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 quadrivittatiu occur 

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

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

 and a dark form of Spermophiius rkhardsoni. On the Plains occur pallid 

 forms of Sciurus ^udovicianus\ Sciurus hudsonius, Tamias quadrivittatus, and 

 Spermophiius richardsoni. With the fulvous type of Tamias quadrivittatus 

 occurs a rufous form of Spermophiius 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 change 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 

 lint than black, as Professor Buird 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 Sciurida, for instance, a group rather remarkable for 

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

 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 also of frequent occurrence in 

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

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

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



['Melanism of III is speciea largely predominate* ovor (he normal coluratiou in Colorado.] 



