SEC. Ill EXTERIOR PARTS OF BIRDS 133 



for particular groups of birds, since birds very closely related differ 

 greatly in resj^ect to their changes of plumage ; and the subject 

 has not yet received the attention its interest and importance 

 should claim for it. The physiological processes involved are 

 analogous to those concerned in the shedding of the hair of 

 mammals and the casting of the cuticle of reptiles. 



Plumag-e-ehanges with Sex, Age, and Season. — Aside from 

 any consideration of the way in which plumage changes, whether 

 by moult or otherwise, the fact remains that most birds of the same 

 species diifer more or less from one another according to certain 

 circumstances. The dissimilarity is not only in coloration, though 

 this is the usual and most pronounced difference, but also in the 

 degree of the development of plumes — their size, form, and texture. 

 Since young birds are those which have not come to sexual vigour ; 

 since breeding recurs at regular periods of the year ; and since 

 males and females usually differ in plumage, nearly all the various 

 dresses worn by different individuals of the same species are cor- 

 related with the conditions of the reproductive system. As the 

 internal generative organs represent of course the essential or 

 jjrimari) sexual characters, all those features of the plumage just 

 indicated may be properly classed as secondary sexual characters. 

 These are of great importance, not only in practical ornithology, 

 but as the basis of some of the soundest views that have been 

 advanced respecting the evolution of specific characters in this class 

 of animals. The generalisations may be made : that when the 

 sexes are strikingly different in plumage, the young at first resemble 

 the female ; when the adults are alike, the young are different from 

 either ; when seasonal changes are great, the young resemble the 

 fall plumage of the parents ; and further, that when the adults of 

 two related species of the same genus are nearly alike, the young 

 are usually intermediate, their specific characters not being fully 

 developed. Specific characters are often to be found only in the 

 male, the females of two related species being scarcely distinguish- 

 able, though the males may be told apart at a glance. Extraordinary 

 developments of feathers, as to size, shape, and colour, are often 

 confined to one sex, usually the male. The more richly, exten- 

 sively, or peculiarly the male is adorned, the simpler the female in 

 comparison, as the peacock and peahen. Darwin has formulated 

 the several categories of secondary sexual characters, giving the 

 following rules or classes of cases : " 1. When the adult male is 

 more beautiful or conspicuous than the adult female, the young of 

 both sexes in their first jDlumage closely resemble the adult female, 

 as -with the common fowl and peacock, or, as occasionally occurs, 

 they resemble her much more closely than they do the adult male. 

 2, When the adult female is more conspicuous than the adult male. 



