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).
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.
[Illustration: FIG. 10. Probable distribution of _S. veraepacis_, _S. longirostris_, and the _S. ornatus_ group (stipple) and of their Wisconsinan ancestors (lines). Heavy line indicates limits of Wisconsinan glaciation.]
In fact, we do find species closely related to _Sorex vagrans_ in just such places today (fig. 10). Probably _Sorex ornatus_, including members of the _ornatus_ group such as _S. trigonirostris_, _S. sinuosus_, _S. willeti_, _S. tenellus_, and _S. nanus_, and also _S. veraepacis_, arose by separation from the ancestral _vagrans_ stock in Sangamonian time. Probably the eastern _S. longirostris_ arose in a like manner. The ancestor of _S. ornatus_ may have been isolated in southwestern California during Sangamonian time, spread north and south during the Wisconsinan age, and afterward given rise to _S. trigonirostris_ and the modern _S. ornatus_ complex of California and Baja California. In at least one place reproductive isolation between _ornatus_ and the invading _S. vagrans_ has broken down (Rudd, 1953); the place is a salt marsh along San Pablo Bay, where a hybrid population between _S. vagrans_ and _S. sinuosus_, an _ornatus_ derivative, has formed. _Sorex tenellus_ may have been isolated in the Sierra Nevada in the Sangamonian interval, moved into the valleys east of the mountains during the Wisconsinan age, and become restricted to its present range since the retreat of the last ice. _Sorex nanus_ may have occurred in the Black Hills and isolated mountains of Arizona and New Mexico during the Sangamonian interval and remained in these general areas during the Wisconsinan age. Its present range is peripheral to the main body of the Rockies and the Colorado Plateau.