390 The Descent of Man. Takt IL 



large red caruncle is developed during this period ou the heat! 

 of the male. So it is with a thin horny crest on the beak oi 

 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 

 fimnd covered with these curious exuvise." 



Changes of colour in the plumage according to the seasoD 

 il spend, firstly on a double annual moult, secondly on an actual 

 change of colour in the feathers theniselves, and thirdly on their 

 dull-coloured margins being periodically shed, or on these three 

 processes more or less combined. The shedding of the de- 

 ciduary 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 fii'st true feathers.'* 



With respect to the birds which annually undergo a double 

 moult, there are, firstly, some kinds, for instance snipes, swallow- 

 plovers (Glareolse), anil curlews, in which the two sexes resemble 

 each other, and do not change colour 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 colour. 

 Secondly, there are birds, for instance, certain species of Totanus 

 and other Grallatores, the sexes of which resemble each other, 

 but in which the summer and winter plumage differ slightly in 

 colour. The difference, 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, thete are birds, the sexes of which differ from each 

 other in colour; but the females, though moulting twice, retain 

 the same colours throughout the year, whilst the males undergo a 

 change of colour, sometimes a great one, as with certain bustards. 

 Fifthly and lastly, there are birds the sexes of which differ from 

 each other in both their summer and winter plumage ; but the 

 male undergoes a greater amount of change at each recurrent 

 *iason than the female— of which the ruff {Machetes pugnax) 

 offers a good instance. 



With respect to the cause or p-.u-po'-u of the differences in 

 colour between the summer and winter plumage, this may in 

 ijcsme instances, as with the ptarmigan,'" serve during both 



" Mr. D. G. Elliot, in 'P.-oc. edited by V L. Sclater. Rav Soc. 

 Jool. Soc' 1869, p. 589. 1867, p 14. 



" yitTsch's ' riorylcgraphy,' " The briiwn mottled iumiDPi 



