The genus _Mustela_ includes the true weasels, the ferrets and minks. The ferrets commonly are treated as a subgenus, _Putorius_, along with the Old World polecat. The minks usually are accorded subgeneric distinction under the name _Lutreola_, and the true weasels comprise the subgenus _Mustela_, the three subgenera together, along with some other subgenera which are mostly monotypic, comprising the genus _Mustela_. Considered in this way, the group of true weasels, subgenus _Mustela_, has a geographic range roughly coextensive with that of the genus _Mustela_. This range includes Asia and Europe, Northern Africa, North America and northern South America. Java has its weasel. Australia and nearly all the oceanic islands lack weasels, and the animals are absent from roughly the southern half of Africa and the southern half of South America. Other small mustelids, weasellike in shape and with corresponding habits and dentition, take the place of true _Mustela_ in the southern half of Africa and in the corresponding part of South America.
In America the subgenus _Mustela_ occurs from the northernmost land in Arctic America southward to Lake Titicaca in the Andes of South America, a distance of approximately 6900 miles. _Felis_, I think, is the only other genus of land mammals in the western hemisphere that has a geographic range as extensive from north to south. _Felis_ does not range so far north but does range farther south. The one species, _Mustela frenata_, ranges from Lake Titicaca northward to about 57° N in British Columbia or for approximately 5000 miles in a north to south direction and from within the Alpine Arctic Life-zone through the Tropical Life-zone. In North America, weasels occur in almost every type of habitat, being absent only in the extremely desert terrain of western Arizona and western Sonora and in adjoining parts of California and Baja California. Even this area, along the Colorado River, may support some weasels; evidence suggesting that it does so is given in the account of _Mustela frenata neomexicana_.
The paleontological record fails to show the precise ancestry of _Mustela_. The genus has been found in deposits of Pleistocene age, but, so far as I can ascertain, not in deposits of earlier times. The Pleistocene remains are not specifically distinct from Recent (living) species, and in only a few instances (see _M. f. latirostra_ and _M. e. angustidens_) are they even subspecifically distinct from the Recent weasel living in the same area today. It is true that fossil remains from deposits of several stages of the Tertiary beds have in the past been identified in the literature as _Mustela_, but most of these identifications were made many years ago when the generic name _Mustela_ was used in a far broader and more inclusive sense than it is today and much of the fossil material was so fragmentary that the generic identity could not be ascertained, at least at that time. Because the generic identity could not be ascertained, the fossil material was tentatively assigned to the genus _Mustela_, the "typical" genus of the family Mustelidae instead of to some other more specialized or less well-known genus of the family. To satisfy my curiosity about these species of "_Mustela_" of a geological age earlier than the Pleistocene I have personally studied nearly all of the original specimens from North America and have found each to be of some genus other than _Mustela_. Also, such study as I have been able to make of the Old World fossils themselves that have been referred to the genus _Mustela_ up to 1938, and my study of the illustrations and descriptions of the others from there lead to the same conclusion; that is to say, none that is true _Mustela_ is known up to now from deposits older than the Pleistocene.