SEXUAL SELECTION 603 



clearly in certain anomalous cases; thus the male ot Helio- 

 thrix auriculata (one of the humming-birds) differs con- 

 spicuously from the female in having a splendid gorget 

 and fine ear-tufts, but the female is remarkable from hav- 

 ing a much longer tail than that of the male; now the 

 young of both sexes resemble (with the exception of 

 the breast being spotted with bronze) the adult female in 

 all other respects, including the length of her tail, so that 

 the tail of the male actually becomes shorter as he reaches 

 maturity, which is a most unusual circumstance/ Again, 

 the plumage of the male goosander {Mergus merganser) is 

 more conspicuously colored than that of the female, with 

 the scapular and secondary wing-feathers much longer; but 

 differently from what occurs, as far as I know, in any other 

 bird, the crest of the adult male, though broader than that 

 of the female, is considerably shorter, being only a little 

 above an inch in length ; the crest of the female being two 

 and a half inches long. Now the young of both sexes en- 

 tirely resemble the adnlt female, so that their crests are 

 actually of greater length, though narrower, than in the 

 adult male.* 



When the young and the females closely resemble each 

 other, and both differ from the males, the most obvious con- 

 clusion is that the males alone have been modified. Even 

 in the anomalous cases of the Heliothrix and Mergus, it is 

 probable that originally both adult sexes were furnished — • 

 the one species with a much elongated tail, and the other 

 with a much elongated crest — these characters having since 

 been partially lost by the adult males from some unexplained 



colored. In some species of Dacelo the males have blue tails, and the females 

 brown ones ; and Mr. R. B. Sharpe informs me that the tail of the young male 

 of D. gaudichaudi is at first brown. Mr. Gould has described (ibid., vol. ii. 

 pp. 14, 20, St) the sexes and the young of certain black Cockatoos and of the 

 King Lory, with which the same rule prevails. Also Jerdon ("Birds of India," 

 vol. i. p. 260) on the PaUsornis rosa, in which the young are more like the 

 female than the male. See Audubon ("Oruith. Biography," vol. ii. p. 475) on 

 the two sexes and the young of Columia passerina. 



' I owe this information to Mr. Gould, who showed me the specimens; 

 see, also, his "Introduction to the Trochilidse, " 1861, p. 120. 



* Macgillivray, "Hist. British Birds," vol. v. pp. 207-214 



