Length of tail varies from as little as a half to as much as seven-tenths of the length of the head and body in _M. frenata_, the subspecies _neomexicana_ having the long tail and the two subspecies _arthuri_ and _primulina_ having short tails. The geographic ranges of _primulina_ and _neomexicana_ are contiguous. In _M. erminea_ there is likewise no variation of a clinal nature in length of tail and furthermore the variation is much less than in _M. frenata_. In length of hind foot, which in males varies from 49 mm. in northern populations of _M. erminea_ to 28 mm. in southern populations, the same cline is seen as in the total length of animals of this species. In _M. frenata_, however, there are several decreases and increases along any straight line which can be drawn through the geographic range of the species. The range of variation in males is 41 mm. (_M. f. arizonensis_) to 59 mm. (_M. f. macrophonius_). Weight of the entire animal is an excellent measure of size but weights are unavailable for many subspecies. In _M. frenata_, the two subspecies _texensis_ and _macrophonius_ probably are the heaviest and _effera_, _arizonensis_ and _helleri_ probably are the lightest. Geographically the variation in weight behaves in approximately the same way as does the measurement of total length. In _M. erminea_ the variation in weight of males is from 206 grams in northern animals to 58 grams in southernmost populations, there being a relatively constant gradient geographically. Degree of hairiness of the foot-soles in _M. frenata_ clearly is linked with the temperature; in regions of high average temperature the hairiness is least and in regions of low average temperature it is most. The decrease in hairiness is accomplished in two ways, namely, smaller breadth and decreased length of individual hairs and decrease in number of hairs on a given area of dermal surface. This correlation holds throughout the entire north to south range of the species. Corresponding differences are found on the same latitude where topographic diversity in an east to west direction produces northern conditions at high altitudes and southern conditions at low altitudes. The conclusion seems unavoidable that climate, directly or indirectly, determines the degree of hairiness. Less careful observations were made on the hairiness of the soles of the feet in other species but it is clear that the northern species _M. erminea_ has the most hair on the foot-soles and that _M. africana_, the tropical weasel, has the least. In this regard, _M. frenata_ is intermediate as it is also in geographic position. [Illustration: FIGS. 11-15. Dorsal views of adult skulls of each sex of five subspecies of the ermine, _Mustela erminea_, to show secondary sexual variation and geographic variation in size of the skull. Males on the left and females on the right. All × 1.