Vv 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. 
