The absence of water, or scarcity of it to a degree that closely approaches absence, in any large area appears to prevent weasels from living there. At any rate, the one sizeable region of North America from which weasels are unknown is the desert of the southwestern United States and adjoining part of northwestern México. More precisely, in western Arizona, the Mohave Desert and the desert of northwestern Sonora, collectors of mammals have repeatedly sought small carnivores without ever finding any weasels.
Degree of moisture is closely correlated with color in weasels. Humidity and cloudiness as well as actual precipitation seem to be involved. Even if we take into account average annual rainfall alone, the darkest-colored weasels are found in the areas of heaviest rainfall and the lightest-colored weasels in areas of lightest rainfall (extreme type of desert where no weasels occur being excepted). In any large region where there is a geographic gradient in rainfall, the transition from light to dark color almost exactly parallels the increase in amount of rainfall. Within a given species the same color reappears in widely separated areas that have the same amount and seasonal distribution of rainfall. This correlation is repeated so often that one can almost certainly say that heavy rainfall, or the associated phenomena of high humidity and cloudiness, acting separately or together, causes an increase in intensity of color. Relative extent of the color of the upper parts and underparts and presence and absence of light facial markings seem also to be correlated, in a more general way, with differences in rainfall. A fuller discussion of the nature and amount of the variation in color is given on page 51.
Temperature seems not to be an important factor in directly limiting the distribution of weasels, since _M. frenata_ occurs from the hottest to some of the coldest parts of the Americas. Do _M. erminea_ and _M. rixosa_ range no farther south, than they do at present, because high temperatures constitute a barrier? No evidence is known to me which provides an answer, one way or the other, to this question. Granting that temperature is unimportant in limiting the distribution of weasels, it seems to cause geographic variation. Increase in mean annual temperature is correlated with decreased size in _M. erminea_ and with increased size in _M. rixosa_. Temperature, it seems, causes the hair to vary; the pelage is harsher and sparser in weasels from tropical regions than in those from boreal regions. Difference in number of hairs is especially well shown on the soles of the feet. In the weasels from the far north, the pads are concealed by hair and in the weasels from the tropical regions the soles are mostly bare. Also, the hair on the soles of the feet is longer in northern than in southern weasels. Furthermore there is seasonal change in length of the hair on the soles of the feet; at a given locality in southern Canada the hair of the white winter coat is so long on the soles of the feet as to obscure completely the palmar and plantar pads whereas the hair of the brown summer coat is shorter and leaves these pads boldly exposed to view. This seasonal change, as would be expected, is most marked in animals of northern regions and is not perceptible in those from the tropics; it is correlated with increase in seasonal change as the distance from the equator increases.