Coloration of head and foreparts in ten subspecies of long-tailed weasel, Mustela frenata. All figures are of males, approximately × 1/2.

In regions of heavy rainfall (see figs. 2 and 3) there is an increase in pigmentation and extent of blackish color backward over the neck and a decrease in extent of the white facial markings. In regions progressively more arid (see figs. 3 to 7) there is a decrease in pigmentation and extent of blackish color and an increase in extent of the white facial markings.

As shown by rearing mammals from humid regions in arid regions, and vice versa, the color is not visibly altered in one or a few generations; the color is an hereditary character. Beginning with the southernmost subspecies (fig. 1) and continuing northward to the northern subspecies (fig. 10) there is a darkening, next a lightening, and finally a darkening closely conforming to amounts of precipitation in the geographic regions concerned. A fuller discussion of this correlation is given on page 51.