SEXUAL SELECTION 605 



winter and summer, nearly the same plumage as that pos- 

 sessed 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 modi- 

 fied, 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 dis- 

 tinct 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 sim- 

 ilarly 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. Neverthe- 

 less, 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 impor- 

 tance to the accumulated results of selection. Judging, how- 

 ever, from a widespread analogy, when a species migrates into 

 a new country (and this must precede the formation of repre- 

 sentative species), the changed conditions to which they will 

 almost always have been exposed will cause them to undergo 

 a certain amount of fluctuating variability. In this case sex- 



