442 THE DESCENT OF MAN. 



base of the beak and the two others from the corners of the 

 month.* 



l^he colored phimage and certain other ornaments of the 

 adult males are either retained for life or are periodically 

 renewed during the summer and breeding-season. At this 

 same season the beak and naked skin about the head fre- 

 quently change color, as with some herons, ibises, gulls, 

 one of the bell-birds just noticed, etc. In the white ibis, 

 the cheeks, the inflatable skin of the throat, and the basal 

 portion of the beak then become crimson, f In one of the 

 rails, Gallicrex cristatus^ a large red caruncle is developed 

 during this period on the head of the male. So it is with 

 a thin horny crest on the beak of one of the pelicans, P. 

 erythrorhynchus; for, after the breeding-season, these horny 

 crests are shed, like horns from the heads of stags, and the 

 shore of an island in a lake in Nevada was foumd covered 

 with these curious exuviae. J 



Changes of color in the plumage according to the season 

 depend, firstly on a double annual moult, secondly on an 

 actual change of color in the feathers themselves, and 

 thirdly on their dull-colored margins being periodically 

 shed, or on these three processes more or less combined. 

 The shedding of the deciduary margins may be compared 

 with the shedding of their down by very young birds; for 

 the down in most cases arises from the summits of the first 

 true feathers. 



With respect to the birds which annually undergo a 

 double moult, there are, firstly, some kinds, for instance 

 snipes, swallow-plovers (Glareolae), and curlews, in which 

 the two sexes resemble each other and do not change color 

 at any season. I do not know whether the winter plumage 

 is thicker and warmer than the summer plumage, but 

 warmth seems the most probable end attained of a double 

 moult, where there is no change of color. Secondly, there 

 are birds, for instance, certain species of Totanus and other 



*Mr. Sclater, " Intellectual Observer," Jan., 1867. " Waterton's 

 Wanderings," p. 118. See also Mr. Salvin's interesting paper, with 

 a plate, in the " Ibis," 1865, p. 90. 



t " Land and Water," 1867, p. 394. 



tMr. D. G. Elliot, in ' Proc, Zool. Soc," 1869, p. 589. 



^ Nitzsch's " Pterylography," edited by P. L. Sclater. Ray Soc, 

 1867, p. 14. 



