Chap. XVI.] SEASONAL CHANGES OF PLUMAGE. 20? 



the selection of successive beneficial variations, 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 dis- 

 tinct 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 hi both 

 their seasonal plumages. — The cases in this class are 

 singularly complex ; nor is this surprising, as they dej)end 

 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, 41 the young generally resemble the 

 females — as with the so-called goldfinch of North Amer- 

 ica, and apparently with the splendid Maluri of Aus- 

 tralia. 42 With the species, the sexes of which are alike 

 during both the summer and winter, the young may re- 

 semble the adults, firstly, in their winter dress ; secondly, 

 which occurs much more rarely, in their summer dress ; 

 thirdly, they may be intermediate between these two 

 6tates; and, fourthly, they may differ greatly from the 



41 For illustrative cases see vol. iv. of Macgillivray's ' Hist. Brit. 

 Birds;' on Tringa, etc., pp. 229, 271; on the Machetes, p. 172; ou the 

 C'haradrius hiaticula, p. 118; on the Charadrius phtvialis, p. 94. 



45 For the goldfinch of North America, Fringilla tristis, Linn., see Au- 

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

 book of the Birds of Australia,' vol. i. p. 318. 



