1872.] 215 ia [Allen. 
bly 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 peninsular of Lower California and the al- 
most 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 northtvard, 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 portions, at 
least, of the Old World; the Scandinavian forms, for instance, being 
darker colored 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 seabeaches, and desert interior regions, it seems 
doubtful whether the larger share of modification in intensity of 
color in birds may not be 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 southward, 
it seems not unreasonable to suppose that the increase of tempera- 
ture in stimulating the circulation in these exposed members may 
have something to do with it, especially in view of the evidence af- 
forded by mammals, which in general present climatic modifications 
parallel with those of birds. 
Whatever may be the cause of the above modifications of structure 
and color at different localities, we certainly find the following coin- 
cidences: I. In accordance with the increase in the intensity of 
color in individuals of the same species from the north southward, in 
the northern hemisphere, the brighter colored species in genera rep- 
resented in both the temperate and tropical regions occur, as a gen- 
eral rule, at the southward ; the same fact holding good also for 
sub-families. In cosmopolitan genera, families, etc., the tropical spe- 
cies are almost always brighter colored than the extra-tropical ones. 
All the most gorgeously colored families of birds are either exclu- 
sively tropical or semi-tropical, with generally the outlying species 
