_Mustela erminea_ is regarded as having undergone some progressive change in structure, but less than the other weasels, in the period of time when the weasels were evolving from the stem form. The least weasel, _Mustela rixosa_, seems to be an ancient type and to judge from the size and proportions of its parts, was differentiated from the _erminea_ stem at a time earlier than were the other American Recent species of weasels. In size, in reduction of the tail, and in proportions of the skull, _M. rixosa_ is, in each instance, the most aberrant of all the weasels, _Mustela nivalis_ of Europe and western Asia included. This aberrancy results from the retention of certain primitive features, in the teeth and basicranial region, and from specialization in proportions of the skull. The skull is long, deep, and narrow. These proportions probably are adaptations permitting the animal to follow the smaller kinds of mice into their burrows. In most of that part of North America where _erminea_ and _rixosa_ occur together, _erminea_ is a much larger animal and takes as prey almost all kinds of land vertebrates that it is powerful enough to kill. These include varying hares and ptarmigans. The least weasel, _rixosa_, can hardly manage such large prey and lives on the smaller rodents. _Mustela rixosa_ may eat numbers of insects (see page 176 beyond),--a kind of food which _Mustela erminea_ is not known to eat. Apparently the two species are able to live in the same areas because each eats a somewhat different kind of food than does the other and hence they do not compete to the point where one is crowded out by the other. This is the case in the latitudes where the two species of weasels are of different bodily size, but in the southernmost latitudes where these two species occur, _erminea_ becomes almost as small as _rixosa_ and only one of the species, to the exclusion of the other, occurs in a given area. All through the Rocky Mountains, south of Montana and in the territory west of these mountains all the way to the Pacific Coast, only the small subspecies of _erminea_ is to be found. In the Alleghenies of the eastern United States only _rixosa_ occurs. In New England where _erminea_ approaches the size of _rixosa_, the latter is unknown. Probably this exclusiveness results from competition for food, although competition for dens, safe breeding places and other requirements of life may be involved.