116 BULLETIN OF THE 



" arcticus " ; and Picoides " americanus " becomes P. dorsalls, through 

 a somewhat similar increase of white in the dorsal plumage. All the 

 Vireos of the Rocky Mountain plateau are paler races of species that 

 range across the continent, the difference in some of them being so 

 great as to give them the character of strongly marked geographical 

 varieties. Most of the Einpidonaces are here also similarly represented ; 

 and farther southward and westward occur pallid forms of Myiarchus 

 and Tyrannies; everywhere establishing the law of pallid races in arid 

 regions, which there represent the brighter conspecific forms of the 

 contiguous moistcr districts. The differences in color between the 

 conspecific forms of arid and of comparatively moist regions is much 

 greater, as a rule, towards the end of the breeding season, or just 

 before the autumnal moult, than after this moult, or in spring speci- 

 mens, or than is observed between young birds of the two forms ; 

 showing most unmistakably the direct influence of the intensely heated 

 dry winds and strongly reflected light upon the colors of birds in semi- 

 desert regions. 



Recent investigations show a rather greater tendency to an enlarge- 

 ment of the bill to the southward along the Pacific slope of the continent 

 than that pointed out in my paper on the Florida Birds as existing to 

 so marked a degree among the birds of the Atlantic States.* Instances 

 are seen in the southern forms of the Chrysomitris psaltria group, in the 

 Carpodacus purpureus group, in the Cardinalis virgmianus group, in 

 ( 'urrirostra " americana" in the rostratus form of Passerculus, in the 

 western forms of Melospiza melodia, in Passerella " schistacea" and in 

 the Pyranga cestiva and P. ludoviciana groups ; it is also well illus- 

 trated by Certhia familiaris, Mniotilta varia, and almost constantly in 

 the J'ireonidce, as well as in numerous other families. 



From the valley of the Columbia River a comparatively narrow belt 

 extends northward along the Pacific coast, where the annual rainfall is 

 nearly double that of any other portion of the continent ; and here the 

 birds (and mammals also), as a general rule, not only reassume the 

 brighter colors of the region east of the Great Plains, but in many 

 cases present a depth of color unequalled eastward in the same lati- 

 tudes, frequently taking on a peculiar deep plumbeous or dusky brown 

 in replacement of ashy or rufous, with a partial obsolescence of spots and 

 streaks, especially marked in several of the fringilline genera. 



* See Bull. Mus. Comp. Zobl., Vol. II, p. 230. 



