The ecological requirements of jumping mice, genus Zapus, and the subspecies of Sorex vagrans that dwell in hydroseres are essentially similar. The species Zapus princeps lives in the Rocky Mountains, the Great Basin, the Sierra Nevada, and north to Yukon (Krutzsch, 1954:395). Its geographic range is similar to that of the montane and basin segments of S. vagrans. The species Z. trinotatus occurs along the Pacific coast and in the Cascades north to southwestern British Columbia. Its distribution thus coincides in general with that of the large red coastal subspecies of S. vagrans. Krutzsch (1954:368-369) thought that these two kinds of jumping mice were first separated by the formation of the Cascade Mountains and the Sierra Nevada and finally by Pleistocene glaciation. The Sierran jumping mouse (Zapus princeps), as is the Sierran vagrant shrew, is more closely related to the jumping mouse of the Great Basin and of the Rocky Mountains than it is to the jumping mouse (Z. trinotatus) of the Pacific Coast, just as the Sierran vagrant shrew is related to the shrew of the Great Basin and Rocky Mountains. The jumping mouse also is limited in its distribution by hydrosere communities, not by forests.
In western North America there are two species of water or marsh shrews: Sorex palustris and S. bendiri. They have been placed in separate subgenera, but, as pointed out beyond, are closely related and here are placed in the same subgenus. The species palustris is found throughout the Rocky Mountains, north into Alaska, across the Great Basin into the Sierra Nevada, and west to the Pacific coast in Washington. The species bendiri is found from northwestern California north along the Pacific coast to southwestern British Columbia and east to the Cascades. Where the ranges of the two species overlap in western Washington they do not interbreed so far as is known, and are somewhat different in their ecology, bendiri being a lowland, and palustris being a montane, species. The two species probably were separated in a glacial period as seems to have been the case with the wandering shrews. Also, the water shrew of the Sierra Nevada is derived from that of the Great Basin and Rocky Mountains. Sorex palustris is tied closely in its distribution to hydrosere communities and is not dependent upon the presence of forests.
Red-backed mice, genus Clethrionomys, occur throughout the Rocky Mountains and west to the Cascades in Washington as the species C. gapperi. The species C. californicus is found along the Pacific Coast from California north to the Olympic Peninsula. Where the ranges of the two species meet in Washington they seem not to intergrade. In some glacial interval these two species may have evolved in the same manner as has been described for the species of Zapus and those of Tamiasciurus. No Clethrionomys are found in the Sierra Nevada, nor are red-backed mice found in the boreal islands of the Great Basin. It is not known why Clethrionomys californicus does not occur in the Sierra Nevada. Some boreal birds have distributional patterns similar to those of the mammalian examples cited above. One kind of sapsucker, Sphyrapicus varius nuchalis, occurs in the Rocky Mountains north into British Columbia and west to the Cascades and Sierra Nevada. A related kind, S. varius ruber, occurs along the Pacific Coast from California north into British Columbia. Recently Howell (1952) has shown that some intergradation takes place between ruber and nuchalis in Washington and British Columbia, although they do not intergrade freely. Previously the two kinds were thought not to intergrade and were regarded as two species. The two kinds intergrade also in northeastern California, although in that state S. v. daggeti, rather than S. v. ruber, is involved in the intergradation. Howell considered the two kinds to be conspecific with one another as well as with the eastern S. varius. He attributed a measure of the distinctness of nuchalis and ruber to their separation during a glacial period, but felt that the separation was much older than Wisconsinan. Whatever the time of separation, the pattern seems clear: nuchalis and ruber (as well as varius) were separated into montane, coastal, and eastern segments respectively, probably by glaciation (it seems to me in the Pleistocene), and have since re-established contact with one another.
The grouse genus Dendrogapus is divided into a Great Basin species, D. obscurus, which extends northward into British Columbia, and a Rocky Mountain species, D. fuliginosus, that is found in the Sierra Nevada and northward along the coast and Cascades into British Columbia. Although the two kinds have at times been considered conspecific, they differ in voice, hooting mechanism, and characters of the downy young, and so far no actual intergradation between the two has been shown (Grinnell and Miller, 1944:113). These grouse thus seem to offer additional evidence for a Pleistocene, possibly Wisconsinan, separation of the boreal fauna into a Rocky Mountain and a Pacific coastal segment.
A notable sidelight on these data is the frequency with which species in the Sierra Nevada have their closest relatives in the Rocky Mountains, rather than in the geographically nearer Cascades or coastal areas. This similarity in fauna of the Sierra Nevada and the Rockies was noted long ago by Merriam (1899:86).
RELATIONSHIPS WITH OTHER SPECIES
During the Sangamonian interval, isolated segments of the once widespread ancestral Sorex vagrans quite possibly persisted in such places as the Sierra Nevada, coastal southern California, the mountains of Arizona, New Mexico, and southern Mexico, and in the Black Hills (see [fig. 6]). One might expect that by Wisconsinan time these populations would have become reproductively isolated from their parent stock. They would therefore have remained specifically distinct when Wisconsinan Sorex vagrans, reoccupied these outlying areas, and may still be found isolated in places peripheral to the range of the ancestral species.
