462 THE DESCENT OF MAN. 



Heliothrix 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 having 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 con- 

 spicuously 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 entirely resemble the adult 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 conclusion 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 un- 

 explained cause, and transmitted in their diminished state to their 

 male offspring alone, when arrived at the corresponding age of 

 maturity. The belief that in the present class the male alone has 

 been modified, as far as the differences between the male and the 

 female together with her young are concerned, is strongly sup- 

 ported by some remarkable facts recorded by Mr. Blyth,'' with re- 

 spect to closely-allied species which represent each other in dis- 

 tinct countries. For with several of these representative species 

 the adult males have undergone a certain amount of change and 

 can be distinguished; the females and the young from the distinct 

 countries being indistinguishable, and therefore absolutely un- 

 changed. This is the case with certain Indian chats (Thamno- 

 bia), with certain honey-suckers (Nectarinia), shrikes (Tephro- 



* I owe this information to Mr. Gould, who showed me the speci- 

 mens; see, also, his 'Introduction to the TrochiUdae,' 1861, p. 120. 



' MacgilUvray, 'I-Iist. Brit. Birds,' vol. v. pp. 207-214. 



' See his admirahle paper in the 'Journal of the Asiatic Soc. of Ben- 

 gal,' vol. xix. 1850, p. 223; see, also, Jerdon, 'Birds of India,' vol. i. 

 introduction, p. xxix. In regard to Tanysiptera, Prof. Schlegel told 

 Mr. Clyth that he could distinguish several distinct races, solely by 

 comparing the adult males. 



