SCIDUID^— GEOGEAPHIOAL DISTRIBUTION AND VABIATION. 647 



among the Squirrels. Others, however, show the transition that obtains in 

 passing from the moist, fertile prairies of the Mississippi Valley to the dry 

 plains, or from the deserts and mo;'qtainoii8 districts of the interior to the 

 moist region bordering (he Pacific coast north of the piirallel of 40° 

 Spermophilm tridecem-lineatus furnishes a good illustration of the dilTerenccs 

 in color that occur between representatives of the same species living on the 

 moist, fertile prairies and those inhabiting the dry, barren plains, those from 

 Illinois, Wisconsin, Minnesota, and Iowa being much darker than those from 

 Western Nebraska, Western Kansas, and Colorado. Even specimens from 

 Eastern Kansas are much darker than those from the middle and western 

 ]H)rtion8 of the same State. In this species, the color is varied, in passing 

 from the prairief^ to thti plains, not only by the lighter shade of the dark 

 grquud-color, but by this considerably greater breadth of the light spots and 

 stripes in the specimens from the plains. The Spermophilus grammurus 

 group (composed of the S. grammurus, S. Iteecheyi, 8. douglassi, etc., of 

 authors) illustrates not only a similar variation in intensity of color between 

 tlie inhabitants of dry and moist regions, but also a somewhat changed style 

 of coloration. Beginning with the nearly uniformly gray or grizzled type of 

 Texas and Southeastern New Mexico, we pass to the more rufous or reddish 

 phase of the central portions of the Rocky Mountains (in Colorado), which 

 also has an increased amount of hoariness on the sides of the neck and 

 shoulders, to the form west of the Sierra Nevada Mountains, typically repre- 

 senting the Spermophilus beecheyi, in which the hoariness forms broad lateral 

 bands separated by a narrow brown medial stripe. This form in Northern 

 California passes into the so-called Spermophilus dougltissi, which diflfers chiefly 

 from S. beecheyi in having the medial stripe darker, or nearly black. 



" Two of the most instructive and interesting groups of the Sciurida, in 

 this connection, are those of the common Sciurus hudsonius and Tamias 

 guadrivitlatus, [•] the former n>.nging over the northern half of * ^ continent, 

 and the latter extending over the western half of North Amei ■ Eastern 

 Asia. In the Sciurus hutlsonius group, we have at the east ^. all-known 

 Chickaree (S. hudsonius), extending westward to the Plains and northwest- 

 ward to Alaska, with Its brighter and smaller southern form in the Eastern 

 Atlantic States. On the arid plains of the Platte and Upper Missouri Rivers, 

 it presents a markedly paler or more fulvous phase, well illustrated by speci- 



' Timiai tiUMau of the preieut nwuuoir. See potlti, tbo acooar<t of the geuaa TnstiM. 



