v COLOURS OF ANIMALS 369 
by natural selection, according to the needs of the animal. 
In birds, too, we have the wonderful clothing of plumage— 
the most highly organised, the most varied, and the most 
expanded of all dermal appendages. The endless processes of 
growth and change during the development of feathers, and 
the enormous extent of this delicately-organised surface, must 
have been highly favourable to the production of varied 
colour-effects, which, when not injurious, have been merely 
fixed for purposes of specific identification, but have often 
been modified or suppressed whenever different tints were 
needed for purposes of protection. 
Selection by Females not a cause of Colour 
To conscious sexual selection—that is, the actual choice by 
the females of the more brilliantly-coloured males or the 
rejection of those less gaily coloured—I believe very little if 
any effect is directly due. It is undoubtedly proved that in 
birds the females do sometimes exert a choice; but the 
evidence of this fact, collected by Mr. Darwin (Descent of Man, 
chap. xiv.), does not prove that colour determines that choice, 
while much of the strongest evidence is directly opposed to 
this view. All the facts appear to be consistent with the 
choice depending on a variety of male characteristics, with 
some of which colour is often correlated. Thus it is the 
opinion of some of the best observers that vigour and liveli- 
ness are most attractive, and these are no doubt usually 
associated with intensity of colour. Again, the display of the 
various ornamental appendages of the male during courtship 
may be attractive; but these appendages, with their bright 
colours or shaded patterns, are due probably to general laws 
of growth, and to that superabundant vitality which we have 
seen to be a cause of colour. But there are many considera- 
tions which seem to show that the possession of these orna- 
mental appendages and bright colours in the male is not an 
important character functionally, and that it has not been 
produced by the action of conscious sexual selection. Amid 
the copious mass of facts and opinions collected by Mr. 
Darwin as to the display of colour and ornaments by the male 
birds, there is a total absence of any evidence that the females, 
as a rule, admire or even notice this display. The hen, the 
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