The nomenclatural history of this subspecies begins with references in the literature that identify the animal as the Old World species, _Mustela erminea_--an identification which the study here reported upon shows to have been correct in the specific, although not in the subspecific, sense. Richardson, for example, in his "Fauna Boreali-Americana" published in 1829 so identified the animal. In 1838, Bonaparte, basing his description on Richardson's account of 1829, proposed the new name _richardsonii_. Richardson himself, the following year in the "Zoology of Beechey's Voyage," accepted Bonaparte's name and it has been applied to the animal in the central part of the northern timber-belt of North America ever since, except as authors used the name _Mustela erminea_ in the belief that _richardsonii_ was not distinct from _erminea_.
The north and south boundaries of the range assigned to _richardsonii_ varied according to the notions of the particular writer who was employing the name. Until Merriam in 1896 named _arctica_ as distinct, animals from the far north were generally included under the name _richardsonii_ along with populations to which the latter name now is applied. Because _richardsonii_ grades gradually into the smaller _cicognanii_ of more southern occurrence the boundary between the two has been set farther north by one writer and farther south by another, depending probably upon what the writer felt was the halfway point in size. This point of course depended upon the samples selected as typical of _richardsonii_ on the north and _cicognanii_ on the south. Because Bangs, in 1896, took as representative of _richardsonii_ the far northern and hence large-sized animals (now separated as _M. e. arctica_), his halfway point in size between them and the small _cicognanii_ of New England naturally fell farther north than it would have had he used as representative of _richardsonii_ specimens from places south of the range of _arctica_.
In 1903 J. A. Allen proposed the name _Putorius microtis_ for a specimen from Shesley, northwestern British Columbia, a place approximately 50 miles northwest of Telegraph Creek. Considering the great disparity in size between this one specimen and the other larger specimens of normal size, from the general region, available to Allen at that time, it is not surprising that he thought two full species were represented. In 1943 when G. G. Goodwin called to my attention two males, as small as the type of _microtis_ and taken by him approximately 300 miles east of Shesley, in the valley between the Musqwa and Prophet rivers, I for a second time examined all available specimens and data with the possibility in mind that _microtis_ was a species or subspecies distinct from _M. e. richardsonii_, but again concluded that only one subspecies was involved because no character except size was found to distinguish the large from the small individuals of a given sex and there are, preserved from northern British Columbia, individuals of intermediate size. _Putorius microtis_ Allen seems to have been based on an individual of _M. e. richardsonii_ near the lower limit of size for that subspecies and _microtis_ is regarded as a synonym.