Note especially the geographic variation in decreasing size of the skull from north to south in each sex, and that the secondary sexual variation in size of skull is less in ermines with small skulls than in those with large skulls.] [Illustration: FIG. 16. Map showing the localities where the skulls, represented in figures 11-15, were obtained.] The maximum length of facial and carpal vibrissae is attained in _M. erminea_ in the far north. In weasels from north of the Arctic Circle the longest facial vibrissae extend posteriorly beyond the posterior border of the ear. In the tropical weasel, _M. africana_, the facial vibrissae do not extend posteriorly beyond the ear and the carpal vibrissae are not so long as the distance between their bases and the apical pad of the first digit. The correlation of long vibrissae with low temperature, is mentioned here merely because length and density of pelage were under consideration. The most obvious and most exact correlation between change in climate and change in the animal is furnished by color. This is well shown in the one species, _Mustela frenata_, to which the following remarks apply unless indication is given to the contrary. The color of the upper parts varies from bay (blackish brown) in _M. f. panamensis_ to buckthorn brown (light brown) in _M. f. neomexicana_. The color of the head varies from solid brown (white chin excepted) to contrasting black and white markings. Dark color of the upper parts is associated with a large area of this color; the enlargement of this area is at the expense of the area of light color on the underparts. In the weasels of darkest color the upper parts occupy four-fifths of the circumference of the body (as measured in the anterior lumbar region) but in the lightest-colored weasels the upper parts comprise only two-thirds of the total circumference. In these light-colored animals the color of the underparts extends onto the underside of the tail and down the insides of the legs and over the feet whereas in the animals with the darkest upper parts the entire tail, feet, and legs below the knees ordinarily are of the same dark color as the upper parts. The length of the black tip on the tail varies inversely with the length of the tail, probably because the lightest-colored weasel has the longest tail. In some subspecies the black brush is almost half as long as the tail-vertebrae but in others is less than a fourth as long as the tail-vertebrae.