216 
SEXUAL SELECTION: BIKDS. 
Part II. 
beauty during many years after they are fully mature ; 
this is the case with the train of the peacock, and with 
the crest and plumes of certain herons ; for instance, the 
Ardea Ludovieana but it is very doubtful whether 
the continued development of such feathers is the 
result of 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 Avith the plumes of birds. 
Class V. When the adults of hoth sexes have a dis~ 
iinct winter and summer flumage, whether or not the male 
differs from the female, the young resemble the adults of 
hoth 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 hoth 
their seasonal fflumages, — 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 the indi- 
viduals 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 resemble the females, — as with 
the so-called goldfinch of North America, and appa- 
rently with the splendid Maluri of Australia.^^ With 
Jerdon, ^ Birds of India/ vol. iii. p. 507, on the peacock. Audu- 
bon, ibid. vol. iii. p. 139, on the Ardea. 
For illustrative cases see vol. iv. of Macgillivray’s * Hist. Brit. 
Birds ; ’ on Trioga, &c., p. 229, 271 ; on the Machetes, p. 172 ; on the 
Charadrius Jiiaticula, p. 118; on the Charadrius pluvialis, p. 94. 
^2 For the goldfinch of N. America, Fringilla tristis, Linn., see 
