OCHOTONA PRINCEPS.--The descent of the Wisconsin ice separated the western pikas into two populations. One was confined to the Cascade-Sierra Nevada system and another to the Rocky Mountains. Two races are now found in the Cascades of Washington. One, _brunnescens_, inhabits the higher Cascades and another, _fenisex_, the eastern edge of the Cascades. The range of _brunnescens_ extends southward into Oregon while the range of _fenisex_ extends farther north in British Columbia than does that of _brunnescens_. The principal difference between the two races is the smaller size and paler color of _fenisex_. In Washington, _fenisex_ occupies a slightly more arid habitat than _brunnescens_. Further, _fenisex_ lives in talus principally of basaltic rock while _brunnescens_ lives in talus of granitic rock. The basaltic talus is more finely fractured, offering smaller crevices in which pikas can conceal themselves. The granitic rock, on the other hand, forms talus composed of fragments of large size. Freshly fractured granite is pale, whitish gray. After weathering, however, it becomes blackish as the more soluble, pale feldspars are removed, leaving the black hornblend and biotite exposed. Freshly fractured Columbian basalt is blackish but, after weathering, becomes rusty, reddish brown. In Washington the two races maintain their distinctiveness because selective factors in the basalt talus of the eastern Cascades favor the smaller size and paler color of _fenisex_ while in the higher Cascades, selective factors in the granitic batholith favor larger size and darker color. The present range of _brunnescens_ in Oregon indicates that this race was the pre-Wisconsin resident of the Cascades of at least southern Washington. Probably _fenisex_ evolved in the arid interior of British Columbia. In color _fenisex_ is intermediate between the dark _brunnescens_ and the pale _cuppes_ of the western spurs of the Rocky Mountains. Probably _fenisex_ was forced southward into the eastern Cascades by the Wisconsin ice and inhabited the area east of the range of _brunnescens_. This eastern area was not occupied by _brunnescens_ because the basaltic talus was of small size. After the retreat of the ice, _fenisex_ (adapted to more arid habitat than _brunnescens_), spread northward to the relatively dry valleys of the interior of British Columbia. The distinctness of _brunnescens_ and _fenisex_ is maintained by selective factors of the habitats they occupy but the zone of intergradation between the two is broad. A tendency towards paleness is noticeable in specimens from as far west as the eastern side of Mount Rainier. The pikas in the Rocky Mountain area have given rise to a number of races. The pale, small race of northern Idaho, _cuppes_, entered northeastern Washington with other members of the Great Basin Fauna after the retreat of the ice.