The geographic distribution of the species _Sorex vagrans_ is shown in figure 5. The geographic range of the Great Basin subspecies is shown by a different pattern of lines than the other subspecies of _S. vagrans_. In the region in which the geographic range of the Great Basin subspecies overlaps those of the subspecies of the Pacific Coast, the pattern of shading for the Great Basin subspecies is superimposed on the patterns for the other subspecies. ORIGIN OF THE _SOREX VAGRANS_ RASSENKREIS The distribution of the species _Sorex vagrans_ and that of its immediate ancestors obviously has not always been the same; during glacial ages much of the present range of the species in Canada and in some of the higher mountains of the United States was covered with ice and not available to the shrew. Furthermore, large areas that are now too hot and dry to permit the existence of_ S. vagrans_ were at one time habitable. If we are to speculate on the manner in which the _Sorex vagrans_ rassenkreis originated we must inquire into the nature and extent of these climatic changes. The most recent epoch of geological time, the Pleistocene, is known to have been divided into a series of alternating glacial and interglacial ages. During the glacial ages continental and montane glaciers are judged to have covered much of Canada and the northern United States. Concurrently the major storm tracks of the west probably were shifted southward; in any event much of the now arid intermontane west was much better watered than it is today. The increased precipitation, and probably glacial meltwater, formed large lakes in the closed basins of the Great Basin. There were boreal forests at lower elevations than there are today in comparable latitudes and continuous boreal habitat probably connected many of the isolated mountain ranges of the southwest. That probability is supported by the presence of boreal animals and plants on many of these isolated ranges today. A boreal tree squirrel, such as _Tamiasciurus_, could hardly be suspected of crossing a treeless, intermontane desert valley, miles wide. [Illustration: FIGS. 6_a_-6_f_. Fig. 6_a_. _Sorex vagrans pacificus_, 1 mi. N Trinidad, Humboldt Co., California, FC 1442. Fig. 6_b_. _S. v. yaquinae_, Newport, Lincoln Co., Oregon, AW 707. Fig. 6_c_. _S. v. yaquinae_ (near _bairdi_), McKenzie Bridge, Lane Co., Oregon, AW 82. Fig. 6_d_. _S. v. setosus_, Reflection Lake, Jefferson Co., Washington, CMNH 4275. Fig. 6_e_. _S. v. obscurus_, 10 mi. SSW Leadore, Lemhi Co., Idaho, FC 1499. Fig. 6_f_. _S. v. vagrans_, Baker Creek, White Pine Co., Nevada, 88042 (after Hall, 1946:113).]