A situation such as the one here described where well differentiated end members of a chain of subspecies overlap over an extensive geographic range throughout the year without interbreeding--thus reacting toward one another as do full species--so far as I know has not previously been found to exist in mammals. The overlapping end-members of the chain of subspecies of _Sorex vagrans_ really do coexist; specimens of the overlapping subspecies have been taken together at the same localities from California to British Columbia. I have taken a specimen of _S. v. vagrans_ and several of _S. v. setosus_ in the same woodlot at Fort Lewis, Pierce County, Washington. Two subspecies of deer, _Odocoileus hemionus_, in the Sierra Nevada of California, occur together over a sizeable area but for only a part of each year that does not include the breeding season (Cowan, 1936:156-157). In the deer mouse, _Peromyscus maniculatus_, the geographic ranges of several pairs of subspecies meet at certain places without intergradation of the two kinds. In these instances well marked ecological differences exist between the subspecies involved. In western Washington, for example, the geographic range of the lowland subspecies, _P. m. austerus_, interdigitates to the east and west with the range of the montane and coniferous forest-inhabiting subspecies, _P. m. oreas_, and the two kinds have not been shown to intergrade. _Peromyscus maniculatus artemesiae_ and _P. m. osgoodi_ come together without interbreeding in Glacier National Park, Montana. _P. m. artemesiae_ is almost entirely a forest-dwelling subspecies, whereas _osgoodi_ is an inhabitant of open country. The two kinds do not actually occur together ecologically although they occur together in buildings at the edge of the woods (A. Murie, 1933:4-5). [Illustration: 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_.