Western Canada and Alaska were populated by shrews which originated in the habitable parts of the Rocky Mountains and Colorado Plateau during Wisconsinan time (as opposed to shrews originating, as subspecies, in the Great Basin or on the Pacific Coast). These shrews differentiated into the currently recognized subspecies of the west coast and coastal islands of British Columbia and Alaska in response to the different environments in these places, many of which were isolated; the subspecies _isolatus_, _mixtus_, _setosus_, _longicauda_, _elassodon_, _prevostensis_, _malitiosus_, and _alaskensis_ are thought to have originated in this fashion after the areas now occupied by them were freed of Wisconsinan ice. This group of shrews from the Rocky Mountains probably came into contact with the Pacific coastal segment of the species somewhere in northwestern Oregon. The clinal decrease in size from _S. v. pacificus_ to _S. v. setosus_ seems steepest in this area. Upon the establishment of this contact reproductive continuity was resumed, probably because the temporal separation of the two stocks involved was not so great as, say, that between _S. v. vagrans_ and _S. v. pacificus_, and in addition the morphological differentiation was not so great. On the eastern side of the Rockies the montane stock moved northeastward, occupying suitable territory opened up by the dissolution of the Laurentide ice sheet. Still later changes in the character of the northern plains owing to desiccation divided the range of the species and isolated _S. v. soperi_ in Manitoba and central Saskatchewan and a population of _S. v. obscurus_, in the Cypress Hills. A number of semi-isolated stocks in central Montana became differentiated as a recognizable subspecies there. A number of other boreal mammals have geographic ranges which resemble that of _Sorex vagrans_, except that the geographic ranges of subspecies do not overlap. Because of the general similarities of these geographic ranges, it is pertinent to examine the reasons suggested by students to account for the present geographic distributions of some of these other boreal species.