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.