EVOLUTION OF THE COLORS OF BIRDS. 233 



But how, it may be asked, can this view be reconciled 

 with the undoubted bleaching effect of sunlight upon 

 desert forms? It seems to be a general rule that dry- 

 ness as well as heat is necessary to produce the pallor. 

 Sea coast species are generally darker than inland forms, 

 even among such as the savanna sparrows( Ammodramus) 

 which live much of the time on the sands of the beach. 

 However, it may be said that moisture in the air means 

 much foggy and cloudy weather when the sunlight i& 

 obscured, while dryness means that the sun is constantly 

 shining. One fact which should be borne in mind is 

 that the moist sea coast climate does not produce more 

 brilliant colors, but rather darker and duller hues than 

 the normal. The facts seem to show that the maximum 

 of sunlight and moisture together favors the develop- 

 ment of the most pure and brilliant coloration, sunlight 

 without moisture has a tendency to burn and bleach, 

 while moisture without sunlight produces darker and 

 duller colors. Nor need this be particularly difficult to 

 understand. The intense heat and dryness of the desert 

 might well leave the plumage in a state bordering on 

 pathological, so drying and parching the feathers that 

 pigment could not well be deposited. The heat and 

 dryness may also have an influence upon the structure 

 of the feather itself, leaving it in a condition analogous 

 to the "worn" plumage at the close of the breeding 

 season, which is much paler. This burnt or worn ap- 

 pearance of the feathers of desert birds may often be 

 noticed. On the other hand, moisture alone might well 

 have a direct influence on the pigment cells in making 

 them darker, or it might be a condition favorable to the 

 deposition of more pigment. 



A complete discussion of the relation between climate 

 and color demands a few preliminary remarks on the 

 general principles of the geographical distribution of 



