v COLOURS OF ANIMALS 377 



Darwin evidently considers to be the strongest argument in 

 favour of conscious selection by the female. This display is, 

 no doubt, a very interesting and important phenomenon ; but 

 it may, I believe, be satisfactorily explained on the general 

 principles here laid down, without calling to our aid a purely 

 hypothetical choice exerted by the female bird. 



At pairing-time the male is in a state of excitement, and 

 full of exuberant energy. Even unornamental birds flutter 

 their wings or spread them out, erect their tails or crests, 

 and thus give vent to the nervous excitability with which 

 they are overcharged. It is not improbable that crests and 

 other erectile feathers may be primarily of use in frightening 

 away enemies, since they are generally erected when angry 

 or during combat. Those individuals who were most pug- 

 nacious and defiant, and who brought these erectile plumes 

 most frequently and most powerfully into action, would 

 tend to leave them further developed in some of their 

 descendants. If, in the course of this development, colour 

 appeared and we have already shown that such develop- 

 ments of plumage are a very probable cause of colour 

 we have every reason to believe it would be most vivid in 

 these most pugnacious and energetic individuals; and as 

 these would always have the advantage in the rivalry 

 for mates (to which advantage the excess of colour and 

 plumage might sometimes conduce), there seems nothing to 

 prevent a progressive development of these ornaments in all 

 dominant races ; that is, wherever there was such a surplus of 

 vitality, and such complete adaptation to conditions, that the 

 inconvenience or danger produced by such ornaments was so 

 comparatively small as not to affect the superiority of the 

 race over its nearest allies. 



But if those portions of the plumage which were originally 

 erected under the influence of anger or fear became largely 

 developed and brightly coloured, the actual display under 

 the influence of jealousy or sexual excitement becomes quite 

 intelligible. The males, in their rivalry with each other, 

 would endeavour to excel their enemies as far as voluntary 

 exertion would enable them to do so, just as they endeavour 

 to rival each other in song, even sometimes to the point of 

 causing their own destruction. 



