The species _erminea_ invaded the western United States and in the process of invasion probably developed there the small size appropriate to permit _erminea_ to live in that latitude before it could do the same thing in the Appalachian region. Later than _erminea_, the least weasel, _Mustela rixosa_, which was small to begin with, also spread southward from the holarctic region, stopped short in the western United States at the northern boundary of the area in which _erminea_ was of small size, but in the Appalachian region of the eastern United States continued on southward to the limits of temperature tolerant for it because _erminea_ had not yet penetrated into that region and no other small carnivore was there to offer competition. The long-tailed weasel, _Mustela frenata_, occurs mostly south of the regions inhabited by the ermine, and mostly south of the region inhabited by the least weasel which appears to live as well with _frenata_ as with _erminea_. It is true that _erminea_ and _frenata_ occur in the same region, but this is a relatively narrow belt across the United States; and from within it a person cannot go far either north or south without reaching a region in which only one of the two species occurs. Exception has to be made for the Rocky Mountains and the Sierra Nevada, where _erminea_ is of exceptionally small size. In these mountains and in the boreal mountainous parts of the intervening region of the United States, _erminea_ and the large-sized _frenata_ occur together over a wide area. Presumably the two occupy different ecologic niches, much as _rixosa_ and _frenata_ probably do where they occur together. Most of the geographic range of the long-tailed weasel, _M. frenata_, is in the temperate region. Structurally, this species is the most advanced of the American weasels. Its dentition is the most highly specialized for cutting. M1 is relatively small and the inner lobe is slightly larger than the outer lobe. The skull, throughout, is more modeled than in the other species; the rostrum, the lower jaws and the teeth--all parts of the offensive equipment--are well developed relative to the corresponding structures in other weasels; the basicranial region exhibits an advanced stage of development in that the tympanic bullae show the maximum degree of inflation. Also, they are thrust far out of the braincase, thereby providing more room for the relatively larger brain which is protected by a more solidly built braincase than in _erminea_.