480 THE DESCENT OP MAN. 



the young never succeeded in obtaining a female, the habit of 

 early reproduction would perhaps be sooner or later eliminated, 

 from being superfluous and entailing waste of power. 



The plumage of certain birds goes on increasing in beauty 

 during many years after they are fully mature; this is the case 

 with the train of the peacock, with some of the birds of paradise, 

 and with the crest and plumes of certain herons, for Instance, the 

 Ardea ludovicana.* But it is doubtful whether the continued de- 

 velopment of such feathers is the result of the selection of suc- 

 cessive beneficial variations (though this is the most probable 

 view with birds of paradise) or merely of continuous growth. 

 Most fishes continue increasing in size, as long as they are in 

 good health and have plenty of food; and a somewhat similar 

 law may prevail with the plumes of birds. 



Class V. When the adults of both sexes have a distinct winter 

 and summer plumage, whether or not the male differs from the 

 female, the young resemble the adults of both sexes in their 

 winter dress, or much more rarely in their summer dress, or 

 they resemble the females alone. Or the young may have an 

 intermediate character; or, again, they may ditt'er greatly from 

 the adults in both their seasonal plumages. — The cases in this 

 class are singularly complex; nor is this surprising, as they de- 

 pend on inheritance, limited in a greater or less degree in three 

 different ways, namely, by sex, age, and the season of the year. 

 In some cases the individuals of the same species pass through at 

 least five distinct states of plumage. With the species, in which 

 the male differs from the female during the summer season alone, 

 or, which is rarer, during both seasons," the young generally re- 

 semble the females, — as with the so-called goldfinch of North 

 America, and apparently with the splendid Maluri of Australia.'" 

 With those species, the sexes of which are alike during both 

 the summer and winter, the young may resemble the adults, first- 

 ly, in their winter dress; secondly, and this is of much rarer 



they have not as yet acquired their fully-developed claspers. All 

 such facts are highly interesting, as bearing on one means by which 

 species may undergo great modifications of character. 



*'^ Jerdon, 'Birds of India,' vol. iii. p. 507, on the peacock. I>r. 

 Marshall thinks that the older p^nd more brilliant males of birds of 

 paradise have an advantage over the younger males; see 'Archives 

 Neerlandaises,' tom. vl. 1871.— On Ardea, Audubon, Ibid. vol. iii. p. 139. 



■•i For Illustrative cases see vol. iv. of Macgillivray's 'Hist. Brit. 

 Birds;' on Trlnga, &c., pp. 229, 271; on the Machetes, p. 172; on the 

 Charadrius hiaticula, p. 118; on the Charadrius pluvialis, p. 94. 



"For the goldfinch of N. America, iringilla tristis, Linn., see Au- 

 dubon. 'Ornith. Biography,' vol. i. p. 172. For the Maluri, Gould's 

 'Handbook to the Birds of Australia,' vol. i. p. 318. 



