504 THE DESCENT OF MaN 



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

 with these curious exuviae." 



Changes of color in the plumage according to the sea- 

 son depend, first, on a double annual moult; secondly, on 

 an actual change of color in the feathers themselyes; 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, first, some kinds, for instance, 

 snipes, swallow-plovers (Griareolse), 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, wherf 

 there is no change of color. Secondly, there are birds, for in 

 stance, certain species of Totanus and other Grrallatores, the 

 sexes of which resemble each other, but in which the sum- 

 mer and winter plumage differ slightly in color. The differ- 

 ence, however, in these cases is so small that it can hardly 

 be an advantage to them; and it may, perhaps, be attributed 

 to the direct action of the different conditions to which the 

 birds are exposed during the two seasons. Thirdly, there 

 are many other birds the sexes of which are alike, but which 

 are widely different in their summer and winter plumage. 

 Fourthly, there are birds the sexes of which differ from 

 each other in color; but the females, though moulting 

 twice, retain the same colors throughout the year, while 

 the males undergo a change of color, sometimes a great 

 one, as with certain bustards. Fifthly and lastly, there 

 are birds the sexes of which differ from each other in 



" Mr. D. G. Elliott, in "Proe. Zool. Soc," 1869, p. 589. 



'8 Nitusoh's "Pterylography," edited by P. L. Selater. Kay Soc, 186'7, p. 14 



