The Transition Life-zone is the principal life-zone in Washington. It is divisible into three subdivisions: Humid, Arid-timbered and Arid-grasslands (Fig. 11) subdivisions. The Humid and Arid-timbered subdivisions of the Transition life-zone are closely related in some respects but different in others. They are separated by the Cascade Mountains. All of the Transition Life-zone west of the Cascades belongs to the Humid subdivision and the timbered Transition Life-zone east of the Cascades belongs to the Arid-timbered subdivision. [Illustration: FIG. 11. Life-zones of Washington. Arctic-alpine not shaded. A. Hudsonian and Canadian (mapped together). B. Forested Transition (Humid and Arid subdivisions). C. Arid-grasslands of the Transition. D. Upper Sonoran.] The Arid-grasslands are of minor geographic extent. Although this subdivision is relatively distinct as concerns the distribution of plants, insects and birds, it is of little importance as concerns the distribution of mammals. For the most part, the mammals occupying it are more representative of surrounding areas. Large parts of the Arid-grasslands have been taken over for agriculture, especially wheat raising. Perhaps the greatest extent of the Arid-grasslands existing in a natural state is along the eastern Cascade Mountains and along the eastern side of the Columbian Plateau. These are truly transition areas, situated where the arid pine forests are replaced by open, sagebrush desert. [Illustration: FIG. 12. Arctic-alpine Life-zone, Mount Rainier, Washington: Cowlitz Glacier from elevation of 9,500 feet. (Fish and Wildlife Service photo by Victor B. Scheffer. No. 900.)] The Upper Sonoran Life-zone includes the desert areas of Washington (figure 11 D). Its principal extent is the central Columbian Plateau. From the central Columbian Plateau, fingerlike projections of desert extend along the principal valleys. The Arctic-alpine Life-zone occurs in the high Olympic Mountains and on the higher peaks of the Cascades. This is shown on the life-zone map, Fig. 11, as white, unshaded areas. The mammalian faunas of the Hudsonian and Canadian life-zones resemble each other closely. The boundary between them is too complex to permit separating them on a small-scale map. Consequently they are mapped together on Fig. 11 as A. THE HUMID SUBDIVISION OF THE TRANSITION LIFE-ZONE This subdivision is remarkably uniform in composition over western Washington. The greatest difference is in precipitation. Rainfall along the coast is heavier than that in the interior. Consequently vegetation is more dense and luxuriant along the coast.