<526 THE DESCENT OF MAN 



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 of the plumes of certain herons, 

 lor instance, the Ardea ludovicana.*" But it is doubtful 

 whether the continued development of such feathers is the 

 result of the selection of successive beneficial variations 

 (though this is the most probable view with birds of para- 

 dise) 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 Tnuch m,ore rarely in their summer dress, 

 or they resemble the females alone. Or the young m,ay have an 

 intermediate character ; or, again, they may differ greatly from 

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

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

 depend 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 tKe individuals of the same species 

 pass through at least five distinct states of plumage. W^ith 

 the species, in which the male differs from the female during 

 the summer season alone, or which is rarer, during both sea- 

 sons," the young generally resemble the females — as with 

 the so-called goldfinch of North America, and apparently 

 with the splendid Maluri of Australia," With those species 



because they have not as y«t acquired their fully developed claspers. All such 

 facta are highly interesting, as hearing on one means by which species may 

 undergo great modiflcalions of character. 



•^ Jerdon, "Birds of India," vol. iii. p. 507, on the peacock. Dr. Marshall 

 thinks that the older and more brilliant males of birds of paradise have an ad- 

 vantage over the younger males; see "Archives Neerlandaises, " tom. vi., 1871. 

 On Ardea, Audubon, ibid., vol. iii. p. 139. 



■" For illustrative cases see vol. iv. of Maogillivray's "Hist. Brit. Birds"; 

 on Tringa, etc., pp. 229, 271; on the Machetes, p. 172; on the Gharadrius 

 hiaticula, p. 'll8; on the Gharadrms plmmalis, p. 94. 



*'' For the goldfinch of North Amenoa., Fringilla tristis, Linn., see Audubon, 

 "Ornith. Biography," vol. i. p. 172. For the Maluri, Gould's "Handbook to 

 the Birds of AustraUa," vol. 1. p. 318. 



