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SEXUAL SELECTION: BIEDS. 
Part II. 
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 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 shewn 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 par- 
rots, kingfishers, pigeons, &c., the young of both sexes 
resemble the adult female.^ We see the same 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 longer tail 
than that of the male ; now the young of both sexes 
2 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 males have blue 
tails, and the females brown ones ; and Mr. K. 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. p. 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 Palseornis 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 Columba ])asserina. 
