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.
[PALEONTOLOGICAL HISTORY]
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.
When, in 1930 (pp. 146-147), I wrote about the taxonomic position of three American genera of fossils (known only from lower jaws), each of which had been previously referred to the genus Mustela, I said that they pertained "to that section of the weasel family (Mustelidae) which comprises the polecats, true weasels, ferrets, minks and martens. The fossil specimens . . . are smaller than any other later Tertiary members of the group yet described, and are more primitive than any of the above mentioned Recent relatives. Of the three extinct genera . . . Miomustela [Lower Pliocene or Upper Miocene of the Lower Madison Valley, Montana] is the most primitive and Martinogale [Pliocene, 18 mi. SE Goodland, Sherman County, Kansas] is the most advanced. This view rests largely on the character of M=1 which in Miomustela has a deeply basined, short, narrow talonid with a thick, high metaconid situated partly posterior to the protoconid. In Martinogale the talonid is incipiently trenchant, long, broad, and it has a lesser developed metaconid which is situated more anterior [ly]. Pliogale [Lower Pliocene, Humboldt County, Nevada] is intermediate in this respect.
"These three forms are of special interest as possible ancestors of the subgenus Mustela, true weasels. No members of this subgenus, nor related forms which can with any degree of certainty be regarded as directly ancestral to them, have yet been described from Miocene or Pliocene deposits. Palaeogale of the Old World and Bunaelurus of North America, each of Oligocene age, have been placed by Schlosser (1888, p. 116) and Matthew (1902, p. 137) as members of the primitive group of mustelids ancestral to Mustela. This course seems logical; and with no truly intermediate links between these forms of the Oligocene on the one hand, and Mustela which first appears in the Pleistocene, on the other, more definite statements about ancestral positions of the small Oligocene forms can hardly be made. The deciding considerations for authors who placed Palaeogale and Bunaelurus as ancestral to Mustela were the absence of a metaconid on M1 and the trenchant talonid of that tooth. These characters are found also in Mustela. On the other hand certain structures in the basicranial region of Palaeogale and more especially of Bunaelurus indicate that these genera possibly are not close to the ancestral form of Mustela . . . Martinogale may stand near the ancestral form of Mustela and . . . Pliogale may be ancestral to Martinogale. Pliogale, in turn, may have had an ancestor similar to Miomustela. If this should prove to be the case, Palaeogale and Bunaelurus might be regarded as an independent branch which displays merely a parallelism to Mustela in the loss of the metaconid on M1 and the development of a trenchant talonid on that tooth. The writer would make it clear that he does not hold such to be the case. The ancestral relation of Martinogale to Mustela is presented merely to show the possibility, and not the special probability, of such an origin for Mustela. Knowledge of the tympanic bullae and other structures of the basicranial region would go far toward answering the question and until these structures are known [in mustelids of the Later Tertiary,] some uncertainty will remain."