CiiAr. XVI. Birds — Young like both Adults. 



485 



soon be lost, as the male3 would prevail, which retained their immature 

 dress for the longest period, and thus the character of the species 

 would ultimately be modified. 39 If, on the other hand, the young never 

 succeeded in obtaining a female, + he habit of' early reproduction would 

 perhaps be sooner or later eliminated, ti\>in 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 ihe train 

 of the peacock, with some of the birds of paradise, and with the crest 

 and plumes of certain herons, for instance, the Arclea ludovieana. i0 

 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 paradise) or merely of con- 

 tinuous 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 pievail with the plumes of birds. 



Class V. Tl7ie-H 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 

 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. Jn 

 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 s; asons, 41 the young generally resemble the females, 

 — as with the so-called goldfinch of North America, and apparently 

 with the splendid Malnri of Australia. 42 With ihose species, the sexes 

 of which are alike during both the summer and winter, the young may 



39 Other animals, belonging to 

 quite distinct classes, are either 

 habitually or occasionally capable 

 of breeding before they have fully 

 acquired their adult characters. 

 This is the case with the young 

 males of the salmon. Several am- 

 phibians have been known to breed 

 whilst retaining their larval struc- 

 ture. Fritz Miiller has shewn 

 (' Facts and Arguments for Darwin,' 

 Eng. trans. 1869, p. 79) that the 

 males of several amphipod crusta- 

 ceans become sexually mature whilst 

 young ; and I infer that this is a 

 case of premature breeding, because 

 thev have not as vet acouired their, 

 fully-developed claspers. All such 

 facts are highly interesting, as 

 bearing on one means by which 

 species may undergo great modifi- 

 cations of character. 



40 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 advantage over 

 the younger males; see 'Archives 

 Neerlandaises,' torn. vi. 1871. — On 

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

 139. 



41 For illustrative cases see vol. 

 iv. of Macgillivray's 'Hist. Brit. 

 Birds ;' on f ringa, &c, pp. 229, 271 ; 

 on the Machetes, p. 172; on the 

 C/taradrius hiaticida, p. 118 ; on the 

 Charadrius pluvialis, p. 94. 



r - For the goldfinch of N\ America, 

 Fringiila tristis, Linn., see Audubon. 

 ' Ornith. Biography,' vol. i. p. 172. 

 For the Maluri, Gould's ' Handbook 

 tc the Birds of Australia,' vol.. i. p 

 318. 



