240 BULLETIN OF THE 



what it is in the Northwestern States, while on the Great Plains it is 

 less than one half what it is in the Northeastern States. In the lower 

 part of the Mississippi basin and in the Southeastern States it is much 

 greater than to the northward under the same meridians. Within the 

 tropics, in America and Asia at least, the humidity, as well as the 

 intensity of the solar rays, reaches the maximum, as does the in- 

 tensity of color in both birds and other animals. In Europe, as is 

 well known, the birds from near the Scandinavian coast, where 

 the annual rain-fall reaches forty inches, are darker than in Central 

 Europe, where the yearly rain-fall is only half this amount. So 

 much darker, in fact, are the Scandinavian forms, that by some 

 writers they have been regarded as specifically distinct from their 

 representatives in Southern Germany, the Scandinavian forms of 

 circumpolar species being as dark as their Eastern North American 

 allies. There is again a striking parallelism between the relative 

 humidity of Western Europe and Eastern North America, and the 

 relative depth of color in the representatives of circumpolar species 

 living in these two countries, the rain-fall of the latter region being 

 double that of the former, and the birds of darker and livelier colors. 

 As already intimated, this coincidence is not confined to the birds of 

 these different regions, the same correlation of livelier, brighter, deeper 

 tints with increased humidity being also exhibited by the mammals 

 of these various districts, the Europeo-North American species being 

 higher colored, as a general rule, in Eastern North America than 

 in Europe, as the western forms of the continentally distributed Ameri- 

 can species are often higher colored than the eastern. 



It is a most striking fact that the birds, and even the mammals and 

 reptiles, of the almost rainless districts of Lower California, the Gila 

 and Colorado deserts, are almost all so much paler in color than their 

 relatives of the better-watered neighboring districts, that many of them 

 have been described as distinct species, and the others referred to as 

 strongly marked varieties, they all being characterfzed to a greater or 

 less degree by a faded or bleached aspect. The birds and mammals of 

 the arid plains of the middle region of the continent exhibit also the 

 Same bleached appearance, but in a somewhat less degree. 



I had long suspected that hygrometric conditions had much to do 

 with local variations in color in individuals of the same species, but I 

 was not a little surprised when I came to compare the known areas 



