The geographic variation (subspecific characters) found could be spoken of as two kinds: First, there is the variation which is expressed in a general trend for a long distance, producing, in general, a cline of even slope; and second, that of inconstant trend in any one direction. In his "The Rabbits of North America" Nelson (1909:34-35) has commented on the latter type of variation as follows: "While studying series of specimens from all parts of the vast range occupied by the geographic races of such species as _Sylvilagus floridanus_ and _S. auduboni_, I have been impressed with evidences of fluctuation of both external and skull characters. These fluctuations are somewhat wavelike in character and rise to central points of extreme development and then sink away to intermediate borders beyond which new waves rise. Where the waves of differentiation are pronounced they mark recognizable geographic races. Within the area covered by the larger or geographically broader waves of differentiation (recognized as of subspecific value), smaller waves of differentiation are included, which may represent local variations in intensity of characters of the subspecies, or these characters may diminish and the variation tend in other directions, sometimes even closely reproducing the characters of another subspecies occupying a distinct area." In _Mustela frenata_, much of the geographic variation at first inspection appears to be of this nature. Closer scrutiny, however, reveals that the repetition, at geographic intervals, of several features of color and structure are closely correlated with environmental features which are repeated only at these same places. In _Mustela erminea_, much of the variation is of the first kind, namely, that which can be expressed as long clines of relatively even slope. As several authors have said, zoölogical classification based on this kind of variation is like dividing the spectrum and depends largely upon the standards set, for, theoretically, the possibilities of subdivision are unlimited. Actually, however, none of the clines has an even slope and the possibilities for subdivision therefore are limited. Also, when several features are used, instead of only one feature, the classification is more satisfactory even if the basis is more complex. Some features of structure which provide subspecific characters are mentioned below. Total length, of males, ranges from 598 to 360 mm. in _M. frenata_ and from 336 to 228 mm. in _M. erminea_. There is no cline of sustained slope in _M. frenata_ but in _M. erminea_ there is a progressive decrease in total length from north to south.