188 SEXUAL SELECTION : BIP.DS. Part IL 



manner, from the adult female. Innumerable instances 

 in all Orders could be given ; it will suffice to call to 

 mind the common pheasant, duck, and house-sparrow. 

 The cases under this class graduate into others. Thus 

 the two sexes when adult may differ so slightly, and the 

 young so slightly from the adults, that it is doubtful 

 whether such cases ought to come under the present, or 

 under the third or fourth classes. So again the young 

 of both sexes, instead of being quite alike, may diff'er 

 in a slight degree from each other, as in our sixth class. 

 These transitional cases, however, are few in number, 

 or at least are not strongly pronounced, in comparison 

 with those which come strictly under the present class. 

 The force of the present law is well shewn in those 

 groups, in w^hich, as a general rule, the two sexes and 

 the young are all alike ; for when the male in these 

 groups does diff'er from the female, as with certain par- 

 rots, kingfishers, pigeons, &c., the young of both sexes 

 resemble the adult female.^ We see the saine fact ex- 

 hibited still more clearly in certain anomalous cases ; 

 thus the male of Heliothrix auriculata (one of the hum- 

 ming-birds) differs conspicuously from the female in 

 having a splendid gorget and fine ear-tufts, but the 

 female is remarkable from having a much loDger tail 

 than that of the male ; now the young of both sexes 



^ See, for instance, Mr. Gould's account (' Handbook of the Birds of 

 Australia," vol. i. p. 133) of Cyanalcyon (one of the Kingfishers) in which, 

 however, the young male, though resembling the adult female, is less 

 brilliantly coloured. In some species of Dacelo the mules have blue 

 tails, and the females brown ones ; and Mr. R. B, Sharpe informs me 

 that the tail of the 3'oung male of D. Gauih'chaudi is at first brown. 

 Mr. Gould has described (ibid. vol. ii. p. 11, 20, 37) 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 Paheornis rosa, in which the young are more like the 

 female than the male. See Audubon (' Ornith. Biograph.' vol. ii. p. 

 475) on the two sexes and the young of Columha passerina. 



