£80 SEXUAL SELECTION: BIRDS. [Part II 



ders 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, in- 

 stead of being quite alike, may differ 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 shown in those 

 groups, in which, as a general rule, the two sexes and the 

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

 does differ from the female, as with certain parrots, king- 

 fishers, pigeons, etc., the young of both sexes resemble 

 the adult female. 8 We see the same fact exhibited still 

 more clearly in certain anomalous cases ; thus the male 

 of Heliothrix auriculata (one of the humming-birds) differs 

 conspicuously 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 



2 See, for instance, Mr. Gould's account (' Hand-book 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 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. Gauclichaitdi is at first brown. Mr. Gould 

 has described (ibid. vol. ii. pp. 14, 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 Palccornit 

 rosa, in which the young are more like the female than the male. -See 

 Audubon (' Ornith. Bio^raph.' vol. ii. p. 475) on the two sexes and the 

 young of Columba passcriua. 



