Ohap. XVI. Birds — Young like Adult Females. 469 



gpecies at different ages and seasons, probably shews us how 

 the progenitors of the genus were coloured. 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 

 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 con- 

 ditions, but w r e can hardly attribute to this action the modi- 

 fication of the plumage in the males alone, seeing that the 

 females and the young, though similarly exposed, have not been 

 affected. Hardly any fact shews us more clearly how subor- 

 dinate in importance is the direct action of the conditions of 

 life, in comparison with the accumulation through selection ol 

 indefinite variations, 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 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 the 

 accumulated results of selection. Judging, however, from a 

 wide-spread analogy, when a species migrates into a new 

 country (and this must precede the formation of representative- 

 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 sexual selection, 

 which depends on an element liable to change — the taste or 

 admiration of the female — will have had new shades of colour 

 or other differences to act on and accumulate ; and as sexual 

 selection is alw r ays at work, it would (from what we know 

 of the results on domesfic animals of man's unintentional 

 selection), be surprising if animals inhabiting separate districts, 

 which can never cross and thus blend their newly-acquired 

 characters, w r ere not, after a sufficient lapse of time, differently 

 modified. These remarks likewise apply to the nuptial or 

 hummer plumage, whether confined to the males or common to 

 both sexes. 



Although the females of the above closely-allied or repre- 

 sentative species, together with their young, differ hardly at all 



