Interglacial ages were characterized by warmth and aridity as compared to the glacial ages. Glaciers retreated or disappeared, boreal forests became montane in much of the United States, and the lakes in the Great Basin were reduced or disappeared. One can envision that during such times boreal mammals were isolated, their geographic ranges were restricted, and Sonoran mammals expanded their ranges. Evidence is more extensive concerning the number and extent of glacial ages in the eastern than in the western part of North America. This evidence suggests a division of the Pleistocene into four glacial ages and four interglacial ages, the fourth interglacial age corresponding to the present time. More information is available about the Wisconsinan, or last, glacial age, than about the earlier ones, because the last glaciation in many montane areas destroyed evidence of earlier glaciations. The names of currently recognized glacial and interglacial ages of the Pleistocene are listed below. The names of interglacial ages are in Italic type. Wisconsinan _Sangamonian_ Illinoian _Yarmouthian_ Kansan _Aftonian_ Nebraskan We may think of these ages as an alternating series of cool moist and warm dry periods during which boreal mammals, and other organisms, alternately moved southward (disappearing in the glaciated regions) and northward into previously glaciated areas (while disappearing from southern areas except on isolated mountain ranges). _Sorex vagrans_ probably followed this pattern of movement and now is restricted to forested or well-watered places. One possible series of events culminating in the formation of the _Sorex vagrans_ rassenkreis may be thought of as having begun during the Illinoian age. With much of Canada, and perhaps also many areas in the Rockies, Cascades, and the Sierra Nevada covered with glacial ice, the shrew-stock ancestral to _Sorex vagrans_ may well have occupied a more or less continuous range over the Colorado Plateau, the Columbian Plateau, the Great Basin, and in the forests of the Pacific Coast (as well as over part of eastern United States, as will be explained beyond; see fig. 7). At that time the species probably was a continuously interbreeding unit. [Illustration: FIG. 7. Possible distribution in Illinoian (inset) and Sangamonian times of the ancestor of the _Sorex vagrans-ornatus-longirostris-veraepacis_ complex. Approximate southern boundary of Illinoian glaciation marked by heavy line.]