XIII THE COLOURS OF BIRDS 283 



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. 



