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. The eastern species _Sorex longirostris_ has many similarities with shrews of the _ornatus-vagrans_ stock. _S. l. longirostris_ is close in many ways to _S. nanus_. Indeed, the differences between the species _S. nanus_, _S. ornatus_, and _S. longirostris_ seem to me to be of the same magnitude and indicate a similar period of differentiation from a common ancestor. The ancestor of _S. longirostris_ may have gained access to the eastern United States in the Illinoian Age _via_ the northern Great Plains south of the glacial boundary (fig. 7). The ancestor of _Sorex veraepacis_ of southern Mexico probably reached that area in Illinoian time as part of the ancestral _vagrans_ stock and probably attained its differentiation during the Sangamonian interval. All the kinds of shrews so far discussed, including the _S. vagrans_ complex, might thus be thought of as having had a common ancestor in the Illinoian Age. This entire group of shrews has the third unicuspid smaller than the fourth, a pigmented ridge from the apex to the cingulum of each upper unicuspid, and, in most individuals, lacks a post-mandibular foramen in the lower jaw (Findley, 1953:636-637). The pigment is not always prominent in _S. longirostris_.