30 ZOOLOGY. 



ones, sometimes to such an extent as to greatly change the general aspect of 

 the species, as is the case in the Ortyx virginianus of the Atlantic States, 

 and in other well known species ; also, under the tendency to the increase 

 of dark colors, longitudinal streaks and blotches in a light ground increase 

 in extent and intensity of color. 



" In respect to longitudinal variation, the differences appear to be mainly 

 those of color, and to hold a direct relationship to the humidity of the cli- 

 mate. On the arid plains of the middle and western portion of the conti- 

 nent, the annual rain-fall is less than half that of the eastern half of the 

 continent, while a rain-belt occurs on the Pacific coast stretching northward 

 from near the mouth of the Columbia River to Alaska, over which the 

 annual rain-fall 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 

 re-assume 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 [tortious at 

 least of the Old World ; the Scandinavian forms, for instance, being darker- 

 •red than the conspecific races of Central Europe, and these again darker 

 than those of Northern Africa and the adjacent regions. Humidity alone, 

 or in conjunction 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 



u -, it seems doubtful whether the larger share of modification in intensity 

 of color in birds may not fie due to humidity alone, or to humidity and a. 

 high temperature together, rather than to intensity of light. 



