26 ZOOLOGY. 



regions of extreme cloudiness, while the dry regions are comparatively cloud- 

 less districts. 



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

 is another phase of color-variation that requires in this connection a pass- 

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

 species of mammal may be expected to present melanistic individuals ; in- 

 stances 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 pre- 

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

 some other tint than black, as Professor Baird 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 Sciuridae, 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 Spermopliiles, while in others they are some- 

 times the common, if not the prevalent, form over a considerable area, as 

 occurs in Sciurus cinereus and Sciurus carolinensis. Melanism is often of 

 frequent occurrence in Sciurus aberti and in Spcrrnopliilus gramnmrus, which 

 presents a melanistic form both in Texas and in Lower California. Spermo- 

 pliilus parryi has also a black race along the Youkon River, and frequent 

 instances of melanism are 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 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 cm-rent as 

 more or less well established species, and also to consequent changes in 

 nomenclature." 



