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.
Barrett-Hamilton in 1904 named the animal at "Fort Simpson, British Columbia" Putorius arcticus imperii. Preble (1908:232) pointed out that Fort Simpson on the Mackenzie undoubtedly was the place intended, and arranged imperii as a synonym of M. e. richardsonii. The type specimen of imperii was stated to have been received from B. M. Ross who is known to have collected specimens, including specimens of this species (now in U. S. Nat. Mus.), at Fort Simpson on the Mackenzie. I know of no Fort Simpson in British Columbia. If, as seems improbable, Port Simpson, British Columbia, was the place that Barrett-Hamilton intended to designate (where so far as I know Ross did not collect), the name imperii still would seem to be a synonym of richardsonii because richardsonii seems to be the race of weasel at Port Simpson. In proposing the name Putorius arcticus imperii, Barrett-Hamilton stressed that the weasel, which he was naming, was a subspecies of P. arcticus, gave characters which applied perfectly to richardsonii but made no reference to richardsonii. Barrett-Hamilton did not refer to richardsonii possibly because he relied on Merriam's classification of 1896 wherein richardsonii is treated as a species distinct from arctica. Merriam, it will be remembered, held that slight degree of morphological difference rather than intergradation was the criterion for subspecies. Although I have no record of having examined the type specimen of imperii I have but little hesitancy in treating it as a synonym, and would have no hesitancy at all in so doing if the type was certainly known to have been obtained at Fort Simpson on the Mackenzie.
The name Mustela cicognanii mortigena Bangs, 1913, proposed for the ermine of Newfoundland, is placed as a synonym of richardsonii only after repeated, detailed comparisons. In advance of study I supposed that the isolation of the ermine, in Newfoundland, had contributed to its differentiation, which, however, the original describer, Bangs, indicated was slight. Bangs was a careful worker and I am confident that the differences he described really existed between his specimens. Material more nearly adequate than he had from the mainland, shows the males, so far as my measurements and comparisons go, to be in nowise different from those in Newfoundland. Females in Newfoundland may have, on the average, slightly longer hind feet than on the opposite mainland but I am not certain that they do and even if there is a slight difference in this regard as suggested by available data, I think it insufficient basis, alone, for according subspecific status to the insular animal.
The name richardsonii was based by Bonaparte on Richardson's description which in turn was drawn from a specimen taken at Fort Franklin, that thus becomes the type locality. It is fortunate that Preble, in 1903, succeeded in taking specimens there because the place is near the belt of intergradation between arctica and richardsonii. Of Preble's two adult males (see Preble, 1908:232) I have examined no. 133847, which is in transitional pelage and therefore gives no clue in so far as coloration is concerned, as to affinities with arctica versus richardsonii. Specimens in the summer pelage are much to be desired from Fort Franklin. Regardless of what their coloration may be, specimen no. 133847, in external measurements and most certainly in cranial features is of the race to the south and not the race that Merriam named arctica. Because all specimens from localities to the south of Fort Franklin likewise differ from arctica of the barren grounds, considerable additional confidence is felt in allocating the name richardsonii to the animal which ranges from Fort Franklin southward rather than to the one, here designated arctica, that occurs to the northward of Fort Franklin.
Although in most structural features richardsonii is a sort of average for the American races of the species, it is the extreme in high degree of sexual dimorphism. The difference in size between the males and females is greater than in any other race except possibly M. e. kadiacensis in which so little is known of the female that the difference between the two sexes cannot be accurately judged. It will aid in understanding the high degree of secondary sexual difference in richardsonii to visualize two kinds of weasels distributed over the northern half of the continent, thinking now of the geographic area in America occupied by the whole species Mustela erminea of which the subspecies richardsonii is only a part. One of the two kinds of weasel is the male ermine and the other the female. The decrease in size of the male, as measured by the weight of the skull, is in the ratio of 7 in the north to 2 in the south. This decrease is gradual whereas the corresponding decrease from 3 to 1 in the female is not gradual; half of the decrease in the female occurs in the short north to south distance comprised in the belt of intergradation, along the northern boundary of richardsonii, between it and arctica. As a result richardsonii is composed of females with medium sized skulls and males with relatively large skulls, the ratio by weight being approximately 5 to 2. The disproportion in races of ermines both to the north and to the south is less. Actually in the north (arctica) the approximate ratio by weight is 2-1/3:1; in richardsonii, 2-1/2:1; in the south (muricus), 1-2/5:1. Indicated in still another way in richardsonii the skull of the female is 56 per cent lighter than that of the male and the skull of the male is 127 per cent heavier than that of the female. Intergradation with races whose ranges border on that of richardsonii is complete. On the northern boundary of the range of richardsonii along the western shore of Hudsons Bay for perhaps a hundred miles north of Eskimo Point, there are intergrades with arctica. As judged by their lesser size, individuals of this population are influenced by the semplei-stock. Otherwise, intergradation on the northern boundary, with arctica, is abrupt whereas intergradation at the south, between richardsonii and cicognanii, is gradual. Intergradation is similarly gradual between richardsonii on the one hand and bangsi and invicta on the other. By speaking of the intergradation as abrupt, it is intended, in this instance, to indicate that in a relatively narrow belt, between the geographic ranges of arctica and richardsonii, ermines intermediate in color-pattern, shape of skull, and size, bridge the gap between the ermine of the tundra (arctica) and that in the forest belt (richardsonii). It may be added that the degree of difference between the two subspecies just mentioned is approximately twice as much as between richardsonii and cicognanii. The intergradation between cicognanii and richardsonii is gradual. By gradual it is meant that the change from one kind to the other is achieved in a wider area where ermines from locality A do not differ appreciably from those taken at, say, locality B, 50 miles farther south, although ermines from A and those from a third locality, C, say, 130 miles south, clearly show differences indicative of geographic variation.