SCIURIDiE— GEOGRAPHICAL DISTRIBUTION AND VARIATION. 649 



size. Beginning at the northward, we find that specimens from as far south 

 as Pembina, and thence northward, are quite [indistinguishable from speci- 

 mens from Northeastern Asia, or the so-called Tamias 'pallasi' {T. paMasi 

 Baird = T. striatus of most European authors). This form is found to only 

 a limited extent south of the northern boundary of the United States, where, 

 on the plains of the Upper Missouri, it passes into the blanched, pallid form 

 of T. quadrivittatus (T. quadrivittatus var. pallidus nobis, — see beyond), and 

 further westward into the true T. quadrivittatus of the Rocky Mountains, and 

 still further westward into the so-called T. townsendi of tlie Pacific coast. 

 In this group, the greatest pallor is reached on the plains of the Yellowstone, 

 and in the deserts of Nevada, Utah, and Arizona. In the central portions of 

 the Rocky Mountains (Colorado and portions of New Mexico), a form is devel- 

 oped, distinguished by its generally bright, strong colors, but especially for 

 the rich fulvous tints of the sides of the body, to which there is but a slight 

 tendency either in the northern form or the pallid form of the plains. Both, 

 however, very gradually pass into the rufous-sided type, the pallid form 

 wherever the plains approach the mountains (as along the eastern base of 

 the Rocky Mountains, the Uintah, Sierra Nevada, and others of the more 

 southern ranges), gradually becoming fulvous, while the darker northern form 

 grades into the larger fulvous race of the more northern portions of the Rocky 

 Mountains in Montana and Idaho. This larger fulvous race west of the main 

 divide soon begins to assume a duller, more fuscous shade, deepening finally 

 into the very fuscous form ( T. townsendi) of the region between the Cascade 

 Range and the Pacific coast. In this form, the general color increases so 

 much in depth as to become dusky yellowish-brown, and both the light and 

 the dark stripes become obscure, and occasionally almost entirely obsolete, 

 through the gradual accession of color. Between the extreme phase of this 

 fuscous type and the extreme phase of the pallid type of the Plains, in which 

 the stripes are sometimes again partially obsolete, through the extreme light- 

 ness of the general color, the differences are very great indeed. Yet in 

 placing the scores of specimens I have had the opportunity of examining in 

 a geographical series, or arranging them simply according to their localities, 

 a most thorough and minute intergradation becomes at once apparent. The 

 difference in size, too, between northern and southern specimens, is also 

 unusually great; the pale, southern form of the Plains, and the extremely 

 bright, fulvous form of Colorado and New Mexico, being very much smaller 



