284 CALIFORNIA ACADEMY OF SCIENCES. 



be of any significance whatever, but the white under 

 parts of Micropus may be a form of recognition mark, 

 possibly socialistic in nature. There seems to be a dis- 

 crepancy between the view that light favors the deposi- 

 tion of pigment and the fact that Cypseloides, which is 

 abroad only in the twilight or in cloudy weather, is al- 

 most black in color. There is not an inevitable contra- 

 diction in this, however, because although light may un- 

 doubtedly accelerate the production of pigment, or inten- 

 sify and make more brilliant the colors, nevertheless, if 

 pigment be a waste product, it must necessarily be thrown 

 off after enough has been accumulated in the system, light 

 or no light. Accordingly, if an animal were in a state 

 of increasing vigor and vitality, we should look for an 

 increase in pigmentation and a darkening, but not an 

 increasing brilliancy of color, in the absence of sunlight. 

 The direct action of the sunlight would, however, tend 

 to guide the pigment to particular parts of the body. 

 In most of the swifts it is almost evenly distributed, but 

 in Micropus, which is more diurnal than the rest, it is 

 nearly all on the back, the under parts being left white. 



FAMILY TROCHILID^. The Hummingbirds. 



(7) Adult male more conspicuously colored than 

 female; young similar to adult female. 



Prevailing colors, white, black, metallic reds, greens 

 and blues. 



This genus appears to afford one of the most striking 

 illustrations in the entire bird world of the operation of 

 sexual selection. Mr. Wallace has argued that the un- 

 paralleled beauty and diversity of color and ornament- 

 ation among these feathered gems is due solely or chiefly 

 to the operation of the general laws of growth. To me, 

 however, it seems a significant fact that the beautiful 

 metallic colors are as, a rule, confined to the head and 



