The extent of the color of the head, as well as the intensity of the color there, varies markedly and is correlated with climatic conditions. The extent and intensity of this dark color is greater in weasels inhabiting regions of heavy rainfall than in those inhabiting regions of sparse rainfall. Considering the geographic range of each subspecies of _Mustela frenata_, that of _M. f. panamensis_ has the maximum of rainfall. Reference to the colored plate (1) will show that in _M. f. panamensis_ (2) the black of the head is extended over all of the upper parts. _M. f. macrura_ (1) of Perú, to the southward, is from an area of lesser rainfall and is correspondingly lighter colored. Returning to _panamensis_ (2) as a starting point and proceeding northward to the range of _nicaraguae_ (3), which also has lesser rainfall, thence another step northward to Guatamala, which has still less rainfall, the weasel there, _M. f. goldmani_ (4) has the black extending posteriorly only to the shoulders. _M. f. leucoparia_ (5) from Michoacán, and _M. f. frenata_ (6) from Tamaulipas are from progressively more northern and also progressively drier regions. In _M. f. frenata_ (6) the dark color extends posteriorly only to the ears and is blackish rather than black. In _M. f. neomexicana_ (7) of the extremely arid parts of Durango, Arizona, and New Mexico the dark marking of the head is confined to a brown spot on the nose. Its geographic range is the most arid of those of all of the subspecies. The contrast between _neomexicana_ (7) and _panamensis_ (2) illustrates the great range of geographic variation in color which occurs in the one species. Continuing from the geographic range of _neomexicana_ (specimen from Safford, Arizona) northwesterly 480 miles to Riverside, California (see 8, _latirostra_), 430 miles north to Point Reyes, California (see 9, _munda_), and finally 570 miles north to Tillamook, Oregon (see 10, _altifrontalis_), each place with more rainfall than the one farther south, another correlation of increasingly dark coloration with increasing amount of rainfall is illustrated. This geographic variation, it should be remembered, is all within one species. It is the more significant still when we remember that the same correlation, with never an exception, occurs at hundreds of places within the geographic range of the species. A particular feature of climate, namely rainfall, and possibly therefore humidity, is concerned in this correlation. The same correlation, heavy rainfall and dark color, is shown also in the other species of North American weasels. The conclusion is unavoidable that climate, directly or indirectly, determines or influences the color of weasels.