40 COLOR AND CLIMATE. 



wliicli we can sliow tlie cause of a given structure ; but 

 color I'esponds more quickly to tlie 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 j^alest 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 hj environment " — that is, the bird's color is 

 in part due to the conditions under which it lives. 



For example, our common Song Sjjarrow, which in- 

 habits the greater part of IS^oi'th America, varies so 

 greatly in color in different jjarts 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 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 



