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, w^hich 

 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: 



Coast form. 



^ o white 



spot 



Interior 



form . 

 Post ocular 

 spot present 



Variety. 



Stelleri 



Frontalis . . . 



3. Annectens . . 



4. Macrolopha . 



5. Diademata. . 



Inconspicuous blue streaks. 

 Conspicuous blue streaks. . 



Indi.stinct light blue 



Bluish white 



Pure white 



Deep blue 



Secondaries and 

 Tail. 



Deep Berlin blue. 



Deep azure, light- 

 er than above. 



Very deep Berlin 

 blue. 



Bright blue. 



Bright blue. 

 Deep blue. 



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, 187'2, pp. 15, 212, 219. 

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

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



