BIRDS— YOUNG LIKIi; ADULT FEMALES. 463 



dornis), certain kingfishers (Tanysiptera), Kalij pheasants (Gallo- 

 phasis), and tree-partridges (Arboricola). 



In some analogous cases, namely with birds having a different 

 summer and winter plumage, hut with the two sexes nearly alike, 

 certain closely-allied species can easily be distinguished in their 

 summer or nuptial plumage, yet are indistinguishable in their 

 winter as well as in their immature plumage. This is the case 

 with some of the closely-allied Indian wag-tails or Motacillse. Mr. 

 Swinhoe" informs me that three species of Ardeola, a genus of 

 herons, which represent one another on separate continents, are 

 "most strikingly different" when ornamented with their summer 

 plumes, but are hardly, if at all, distinguishable during the winter. 

 The young also of these three species in their immature plumage 

 closely resemble the adults in their winter dress. This case is all 

 the more interesting, because with two other species of Ardeola 

 both sexes retain, during the winter and summer, nearly the same 

 plumage as that possessed by the three first species during the 

 winter and in their immature state; and this plumage, which is 

 common to several distinct species at different ages and seasons, 

 probably shows us how the progenitors of the genus were colored. 

 In all these cases, the nuptial plumage which we may assume was 

 originally acquired by the adult males during the breeding-season, 

 and transmitted to the adults of both sexes at the corresponding 

 season, has been modified, whilst the winter and immature plum- 

 ages have been left unchanged. 



The question naturally arises, how is it that in these latter cases 

 the winter plumage of both sexes, and in the former cases the 

 plumage of the adult females, as well as the immature plumage 

 of the young, have not been at all affected? The species which 

 represent each other in distinct countries will almost always have 

 been exposed to somewhat different conditions, but we can hardly 

 attribute to this action the modification of the plumage in the 

 males alone, seeing that the females and the young, though 

 similarly exposed, have not been affected. Hardly any fact shows 

 us more clearly how subordinate in importance is the direct action 

 of the conditions of life, in comparison with the accumulation 

 through selection of indefinite variations, than the surprising dif- 

 ference between the sexes of many birds; for both will have con- 

 sumed the same food, and have been exposed to the same climate. 

 Nevertheless we are not precluded from believing that in the 

 course of time new conditions may produce some direct effect 

 either on both sexes or from their constitutional differences chiefly 

 on one sex. We see only that this is subordinate in importance to 



" See, also, Mr. Swinhoe, in 'Ibis,' July, 1863, p. 131; and a previous 

 paper, with an extra.ct from a note by Mr. Blyth, in 'Ibis,' Jan. 1861, 

 p. 25. 



