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 pomt to cause and 

 eifeet 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 jiarts 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, with the other nme 

 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 



