Fig. 5. Probable present geographic distribution of Sorex vagrans. The range of S. v. vagrans and its derivatives S. v. vancouverensis, S. v. halicoetes, and S. v. paludivagus, is shown by lines slanting in a different direction than those which mark the range of all the other subspecies of S. vagrans. The region in which S. v. vagrans occurs together with other subspecies of S. vagrans is shown by the superposition of one pattern upon the other.
Cases of sympatric existence of two subspecies of one species are known in birds and in reptiles. Notable examples are in the gull, Larus argentatus (Mayr, 1940), in the Old World warbler, Phylloscopus trochiloides (Ticehurst, 1938), and in the great titmouse, Parus major (Rensch, 1933), of the Old World. In the first species the two end-members, the herring gull and the lesser black-backed gull, occur together over an extensive region from northern Europe and the British Isles throughout Fennoscandia. Fitch (1940) described a rassenkreis with overlapping subspecies in the garter snake Thamnophis ordinoides.
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.