Finally, with the waning of Wisconsinan ice, the species again was able to occupy northern and montane areas as it had during Sangamonian times. Again dessication of the Great Basin caused drastic restriction of shrew habitat. The small, marsh-dwelling kind of wandering shrew which had developed there around the lakes of Wisconsinan time occupied suitable habitat all the way to the Pacific coast where its range came into contact with that of the western arm of the Sangamonian "U."-pattern of shrew distribution (see fig. 9). The animals of this western segment and the new arrivals from the east were by this time so different from one another that the two kinds lived in the same areas without interbreeding. The descendants of the original western arm now are known as _Sorex vagrans sonomae_, _S. v. pacificus_, _S. v. yaquinae_, and _S. v. bairdi_. The newcomers from the east are known as _S. v. vagrans_, _S. v. halicoetes_, _S. v. paludivagus_ and _S. v. vancouverensis_. In addition to occupying the Pacific Coast from San Francisco Bay north to the Fraser Delta, the Great Basin subspecies populated the Columbia Plateau and the western foothills of the central and northern Rockies. By so doing that subspecies came into secondary contact with its own parent stock with which it was still in reproductive continuity in Utah. In some places in British Columbia differentiation between the two kinds had proceeded to such an extent that some reproductive isolation was effected, but in many other places the two interbred. The Rocky Mountain form spread north and west and occupied the Cascades and coastal lowlands in southwestern British Columbia and in Washington. Here the differentiation between the Rocky Mountain subspecies and the Great Basin subspecies was great enough to cause complete reproductive isolation. [Illustration: FIG. 9. Probable changes in the distribution of _Sorex vagrans_ concurrent with and following the dissipation of Wisconsinan ice. Dark arrows in Washington, Idaho, Oregon, and California, shows_ S. v. vagrans_.] Deglaciation of the Sierra Nevada opened it up for reoccupation from the east by _Sorex vagrans_ of the Great Basin. In response to the montane environment the subspecies _obscuroides_, resembling the subspecies _obscurus_ of the Rockies, developed. Desiccation of the intermontane parts of New Mexico, Arizona, and Chihuahua, left "marooned" populations of _Sorex vagrans_ on suitable mountain ranges. In this way _Sorex vagrans orizabae_ may have been isolated in southern Mexico. The isolated populations of Arizona and New Mexico differentiated _in situ_ into the subspecies _monticola_ and _neomexicanus_.