534 THE DESCENT OF MAN. 



indistinguishable in their winter as well as in their imma- 

 ture plumage. This is the case with some of the closely 

 allied Indian wag-tails or Motacillae. Mr. Swinhoe * 

 informs me that three species of Ardeola. a genus of 

 herons, which represent one another on separate conti- 

 nents, are " most strikingly different " when ornamented 

 with their summer plumes, but are hardly, if at all, dis- 

 tinguishable 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 spe- 

 cies 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 corre- 

 sponding season, has been modified, while the winter and 

 immature plumages 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 simi- 

 larly 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 vari- 

 ations, than the surprising difference between the sexes of 

 many birds; for both will have consumed the same food, 

 and have been exposed to the same climate. Nevertheless 

 we are not precluded from believing that in the course of 



*See also Mr. Swinhoe, in "Ibis," July, 1863, p. 131; and a pre- 

 vious paper, with an extract from a note by Mr. Blyth, in " Ibis," 

 Jan, 18W, p. 34. 



