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 smal] 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, 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 
traded in appearance, while all the thirty odd Northwest 
