XIII 
THE COLOURS OF BIRDS 
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structural colour occurs, it is here green and not blue. 
Many of the feathers of the back are black, cross- 
barred with yellow, and here the terminal cross-bar 
is wholly or in part replaced by green. 
General Characters of Colours of Birds 
These illustrations of colour phenomena in birds, 
if they do not explain the development of bright 
colour, may perhaps at least shed some light on 
the problem. They show that the development of 
brilliant colour and structural modification go hand 
in hand ; that brilliant pigmental colours tend to be 
confined to the barbs and are often associated with 
the suppression of the barbules ; that melanin pig¬ 
ments may be present in large amount in both barbs 
and barbules ; and that their presence in the latter is 
often associated with a structural modification which 
gives rise to optical colours ; that the closeness of 
the association between the deposition of pigment in 
any region of a feather and the special development 
of that region is such as to prevent in the general 
case the feathers of flight acquiring great brilliancy 
of colour. Facts of this kind surely tend to prove 
the definiteness of variation ; they should, at least, 
be allowed for by those who discuss the questions 
connected with the origin of colour. 
Meaning of Colour in Birds 
As to the meaning of the various types of colour 
in the physiology of birds we can say very little. 
