GEOGRAPHICAL VARIATION IN NORTH AMERICAN BIRDS. 537 
In respect to longitudinal variation, the differences appear to be 
mainly those of color, and to hold a direct relationship to the hu- 
midity of the climate. On the arid plains of the middle and 
western portions of the continent the annual rainfall is less than 
half that of the eastern half of the continent, while a rainy belt 
occurs on the Pacific coast, stretching northward from near the 
mouth of the Columbia River to Alaska, over which the annual 
rainfall is double that of any portion of the eastern half of the 
continent. Taking the species that present a nearly continental 
range, we find that almost invariably they pass gradually into the 
pallid forms of the interior at the eastern edge of the arid plains, 
the greatest pallor being developed in the driest regions, as the 
peninsula of Lower California and the almost rainless belt along 
the Colorado River, and northward along the eastern base of the 
Sierra Nevada Mountains; that on the Pacific slope they again 
reassume nearly the tints of the eastern form, but more to the 
northward, over the above-mentioned rainy region, they acquire 
a depth of color far in excess of what the species presents in the 
Atlantic region. This coincidence of bright and pale tints, with 
the relative humidity of the locality is certainly suggestive, if not 
demonstrative, of the relation of cause and effect between these 
two phenomena, since the same rule is traceable, over large por- 
tions, at least, of the Old World; the Scandinavian forms, for 
instance, being darker colored than the conspecific races of Cen- 
tral Europe, and these again darker than those of Northern 
Africa and the adjacent regions. Humidity alone, or in con- 
junction with greater intensity of light, seems equally well to 
account for the increase of color to the southward. Yet, from the 
well known bleaching effect of sunlight, intensified by reflection, 
upon the colors of animals living upon sandy islands, and sea- 
beaches, and desert interior regions, it seems doubtful whether the 
larger share of modification in intensity of color in birds may not 
due to humidity alone, or to humidity and a high temperature 
together, rather than to intensity of light.* 
In regard to the enlargement of peripheral parts at the south- 
ward, it seems not unreasonable to suppose that the increase of 
temperature in stimulating the circulation in these exposed mem- 
bers may have something to do with it, especially in view of the 
<i on this point further remarks by the same writer in Proc. Bost. Soc. Nat. Hist. 
xvi, June, 1874 
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