Citellus townsendii.—The Townsend ground squirrel probably entered the Yakima Valley area from Oregon. The differences between it and its relatives in Oregon indicate a considerable period of isolation but one far shorter than the period during which washingtoni is presumed to have been isolated from townsendii.
Citellus columbianus.—The Columbian ground squirrel might have been forced southward in the Rocky Mountain area by the Wisconsin glaciation, might have lived in southeastern Washington since then, and might have invaded northeastern Washington in the Recent with other species of the Rocky Mountain Fauna.
Citellus beecheyi.—This ground squirrel is known to have entered Washington about 1915 from Oregon.
Citellus saturatus.—The mantled ground squirrel of the Cascades probably evolved, from the lateralis stock, as a separate species while isolated in the southern Cascades during Vashon Time. It is a poorly differentiated species and may actually be instead a strongly marked subspecies.
Citellus lateralis.—The golden-mantled ground squirrels of northeastern and southeastern Washington are closely similar. It is deduced that connectens of southeastern Washington developed the differences that characterize it while isolated, from the main stock, in the Blue Mountains area of Washington and Oregon.
The race found in extreme northeastern Washington (tescorum) probably reached that area in relatively recent times. Its range in Washington is more restricted than that of several other members of the Rocky Mountain Fauna; areas of suitable habitat west of the Columbia River are not inhabited by these ground squirrels. Its range in Washington is almost exactly that of (Marmota monax).
Tamias minimus.—The least chipmunk of the Yakima Valley is the same race (scrutator) as that occupying the Great Basin area of Oregon and Nevada. It must have crossed the Columbia in relatively recent times. Had it been resident in the isolated Yakima Valley area for any considerable period of time, the development of distinctive racial characters there would be expected. Perhaps, then, it has not been resident there as long as has the Townsend ground squirrel which, though closely related to the ground squirrel of eastern Oregon, is racially distinct.
The least chipmunk of the Columbian Plateau is thought to be racially distinct from its relatives in the Yakima Valley and eastern Oregon. Probably it reached the Plateau very early in the Recent. It has probably not been separated from the parent stock as long as has the ground squirrel (Citellus washingtoni) of the plateau. The ground squirrel is specifically rather than racially distinct.
Tamias amoenus.—The distributional picture of the yellow pine chipmunks in Washington is complex. (Fig. 81.) Certain habits of these mammals doubtless have modified what was probably the original postglacial distribution of the species. Chipmunks are diurnal and natural selective factors for color possibly operate more strongly on animals active by day than on nocturnal animals. Yellow pine chipmunks are neither forest nor desert inhabitants. Indeed, dense forests or open deserts serve as barriers to their distribution. They prefer brush lands, open woods, and other habitats where there is food and cover but abundant sunlight. In such habitats they are almost independent of altitude, temperature and humidity. They live in the Olympic Mountains where rainfall is heavy and humidity high. They live and breed at considerable altitudes in the Cascades, even in the crater of Mount Rainier, where snow, ice and freezing conditions exist the year around. On the other extreme, they occupy the low, open pine forests and brush lands at the lower edge of the Arid Transition Life-zone where temperatures, in summer, are high and rainfall scarce.
We find in the present distribution of the species in the Cascade-Sierra Nevada chain and the Rocky Mountains, indication that the species had a wide geographic range over western North America previous to the Vashon-Wisconsin glacial interval. Probably the range of the species extended in an arc, from the Rocky Mountains across northern Washington to the Cascades, around the basaltic plateau desert in eastern Washington and Oregon. Presumably the descent of the Vashon-Wisconsin glaciers broke this arc into two parallel geographic ranges, the Rocky Mountains and the Cascade-Sierra Nevada chain, with a desert area between.