_Geographic variation._--In the Old World four subspecies are currently recognized (see Allen, 1933:316) and the same number is here recognized in North America. Length of the tail, length of head and body and hind foot, breadth of the rostral part of the skull in relation to its length, and position on the side of the head of the line of demarcation between the dark color of the upper parts and the white underparts, are the features in which geographic variation has been detected. The general impression is that the amount of geographic variation is much less than in _Mustela frenata_ and only slightly less than in _Mustela erminea_ of the same geographic area.
_Nomenclature._--It is exceptional for a species which occurs in both the Old-and New-World to take its specific name from New World material, especially if the name was proposed as recently as 1896; most circumboreal species take their names from descriptions of European specimens. Although the least weasel, _Mustela rixosa_ (Bangs) 1896, seems now to be an exception, it may yet turn out that the first available name was based on European material. Zimmermann (1943) shows that the least weasel actually was named on the basis of European material long before 1896 and concludes that the name _Putorius minutus_ Pomel, 1853, based on a specimen from France, is the first available name.
Because _Putorius_ nowadays is relegated to subgeneric rank under the generic name _Mustela_, we have for consideration the name-combination _Mustela minuta_ (Pomel). Unfortunately for Zimmermann's conclusion, _Mustela minuta_ Pomel is not available because it is preoccupied by _Mustela minuta_ Gervais [= _Palaeogale minuta_ (Gervais), 1848-1852--see Simpson, 1946: 2, 12], a name applied to another species of small mustelid from the Oligocene or lower Miocene deposits of Europe.
Some other early names thought by Zimmermann (1943:290) to have been based on the dwarf weasel of Europe are judged to be _nomina nuda_ and therefore are to be ignored.
The name _Mustela minor_ Nilsson 1820 was thought by Miller (1912:402) to be a renaming, and hence a synonym, of _Mustela nivalis_ Linnaeus. If that is the case the name does not apply to the dwarf weasel. If the name _Mustela minor_ Nilsson was instead based on the dwarf weasel, the name might still be unavailable, depending on rulings on secondary homonyms, because the name might be preoccupied by _[Lutra] minor_ Erxleben 1777 which is a synonym of _[Mustela] lutreola_ Linnaeus 1766. Two names seemingly available for weasels, and in use for them today, which might replace _rixosa_ as the name of the species, are, first, _Mustela boccamela_ Bechstein, 1801, of Sardinia [= _Mustela nivalis boccamela_ of Miller, 1912, 405] and second, _Putorius numidicus_ Pucheran, 1855, of Morocco and Algeria [= _Mustela numidica_ of Allen, G. M., 1939, 183]. As they stand in the current literature, _Mustela numidica_ is a species distinct from the dwarf weasel and the other name, _Mustela nivalis boccamela_, is an insular subspecies of the mouse weasel. Zimmermann (1943:292), however, implies that _M. numidica_ may belong to the dwarf weasel group when he says "Ob auch _iberica_ BARR.-HAM. als Unterart zu _minuta_ POM. zu stellen ist, soll hier nicht untersucht werden, ebensowenig die von CABRERA vermutete Zugehörigkeit der grossen nordafrikanischen _M. numidica_ PUCH. zur '_iberica_-Gruppe'." The answer to this problem requires a taxonomic, rather than a nomenclatural, decision. Whether either _M. numidica_ or _M. boccamela_ are conspecific with the dwarf weasel I cannot at this time ascertain for want of adequate specimens. Because these two names, _M. boccamela_, and _M. numidica_, are assigned to kinds of weasels which are currently regarded as specifically distinct from the dwarf weasel, and because all the other names which certainly have been assigned to Old World populations of the dwarf weasel before 1896, so far as I know, are _nomina nuda_ or are preoccupied, the next available name, _Mustela rixosa_ (Bangs, 1896), is here employed.