40 COLOR AND CLIMATE. which we can show the cause of a given structure ; but color responds more quickly to the influence of sur- roundings, and in many cases we can point to cause and effect with some certainty. This is best illustrated by the relation between climate and color. Briefly, it has been found that birds are darkest in humid regions and palest in arid regions. This at first thought seems of small moment, but in reality it is one of the most important facts established by ornithologists. It is an undeniable demonstration of " evolution by environment " — that is, the bird's color is in part due to the conditions under which it lives. For example, our common Song Sparrow, which in- habits the greater part of North America, varies so greatly in color in different parts of its range that no less than eleven subspecies or geographical races are known to ornithologists. The extremes are found in the arid deserts of Arizona, where the annual rainfall aver- ages eight inches, and on the humid Pacific coast from Washington to Alaska, where the annual rainfall averages about eighty inches. The Arizona Song Sparrows are pale, sandy colored birds, while those from Alaska are dark, sooty brown. One would imagine them to be different species ; but unlike as are these extremes, they, Avith the other nine races in this group, are found to intergrade in those re- gions where the climatic conditions themselves undergo a change. That is, as we pass from an arid into a humid region, the birds gradually get darker as the average rainfall increases. If now we study other birds living in these regions, we find that many of them, especially the resident species, are similarly affected by the prevailing climatic influ- ences — that is, many Arizona birds are bleached and faded in appearance, while all the thirty odd Northwest
