populations which seem to represent intergrades between the two subspecies. Isolated areas of Canadian Life-zone, even though surrounded with Transition Life-zone, often harbor a population of _obscurus_, whereas the streams in the nearby dry valleys harbor populations of _vagrans_. Farther south in the Rocky Mountain chain, _obscurus_ seemingly intergrades regularly with _vagrans_. This intergradation is seen in populations from several localities in Utah. There the lower elevations west of the Wasatch and Uinta mountains are inhabited by _S. v. vagrans_, the higher elevations by _obscurus_ and where the ranges of the two abut intergrading populations occur. In these series of intergrades there are specimens which, using size as a subspecific criterion, would unhesitatingly be assigned, as individuals, to _obscurus_, and others would be assigned to _vagrans_, but these individuals represent extremes of a normally variable population. At Cuddy Mountain, Idaho, the two subspecies seemingly abut without intergradation; anyhow the available specimens from this locality are referable to one or the other subspecies and none is intermediate. The situation just described understandably has been the source of much anguish to students who sought to identify shrews from the Rocky Mountains. The reason for the relationship just described has been discussed at length in a previous section. In the Rocky Mountains of Wyoming and Colorado the subspecies _S. v. obscurus_ ranges almost uninterruptedly over relatively large areas, but southward in New Mexico and southwestward into Utah and Arizona, suitable boreal habitat becomes insular in nature and obscurus there is confined to the higher mountains. With one exception, once the shrew populations become 'insular' in this region they become smaller and show intergradation with _Sorex vagrans monticola_. The exception is the population in the Sacramento Mountains of southeastern New Mexico which is larger than _obscurus_ and has been rightly recognized as a distinct subspecies, _neomexicanus_. Almost without exception the range of typical _Sorex vagrans obscurus_ is sympatric with that of _Sorex cinereus_, usually the subspecies _S. c. cinereus_. So close is this correspondence that the presence of _S. cinereus_ comes near to being a useful aid in identifying _S. v. obscurus_. In areas where individuals of _obscurus_ show intergradation with _vagrans_, _Sorex cinereus_ is absent or rare. The implication is that as the species _S. vagrans_ approaches the size of the species S. cinereus, competition between the two increases with resultant displacement of _cinereus_. _Specimens examined._--Total number, 982.