EVOLUTION OF THE COLORS OF BIRDS. 



247 



assume a completely blue plumage. This may be ac- 

 counted for by supposing that the rainy northwest coast 

 region was the original home of the species. The pig- 

 ment would there be darkened, but as the species spread 

 toward the south the tendency would be not towards 

 darkness so much as intensification and purity, which 

 would result in a complete intense blue plumage at the 

 southern limit of its range. The differences in color 

 between the various forms may be tabulated as follows: 



Mr. J. A. Allen has drawn particular attention to the 

 relation between climate and geographical races, both as 

 to the modifications in the bill, feet and tail, and of 

 color. * He calls attention to the increase in the inten- 

 sity of color to the southward and also to the " increase 

 in the extent of dusky or black markings at the expense 

 of the intervening lighter or white ones; or conversely; 

 the reduction in size of white spots and bars." He draws 

 attention to the extreme pallor of desert forms as con- 

 trasted with the same species in a moist climate, and 

 says: " This coincidence of bright and pale tints, with 

 the relative humidity of the locality, is certainly sug- 

 gestive, if not demonstrative, of the relation of cause 

 and effect between these two phenomena, since the same 



* cf. Boston Soc. Proc., 1872, pp. 15, 212, 219. 

 Bull. Mus. Coinp. Zool. ii, pp. 229, 247. 

 Am. Jour. Sci. and Arts, xii, 1866. Baird on Geographical races. 



