532 THE DESCENT OF MAN. 



given; it will suffice to call to mind the common pheasant, 

 (luck and house-sparrow. The cases under this class gradu- 

 ate into others. Thus the two sexes when adult may differ 

 BO 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 the two sexes, instead of being quite alike, 

 may differ in a slight degree from each other, as in our 

 sixth class. These transitional cases, however, are few, 

 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 in these groups the male does 

 differ from the female, as with certain parrots, kingfishers, 

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

 adult female.* We see the same fact exhibited still 

 more clearly in certain anomalous cases; thus the male of 

 HelioVirix 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 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, f Again, 

 the plumage of the male goosander (Mergus mergansei') is 

 more conspicuously colored than that of the female, with 

 the scapular and secondary wing- feathers much longer; but 



* See, for instance, Mr. Gould's account (" Hand-book to 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. gmtdichaudi 

 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 Palceornis 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 Co- 

 lumba passerina. 



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

 mens; see also his " Introduction to the Trochilidae," 1861, p. 120. 



