266 COLOUR IN NATURE chap. 



three, the first occur equally in long quill-feathers 

 and in the short contour-feathers ; they have no 

 obvious relation to the structure of the coloured 

 parts, and as already seen, little is known of their 

 meaning or course of evolution. 



(2) The optical or structural colours are divided 

 into subjective and objective. The changing sub- 

 jective colours occur only in the barbules, and require 

 the presence of a large amount of dark pigment for 

 their full manifestation. Objective colours like green 

 and blue are confined to the barbs and do not occur 

 in the barbules, and (3) the bright pigments occupy 

 the same position. In general terms, therefore, we 

 may say the barbules always contain a certain amount 

 of dark pigment, and when this is in excess and the 

 structure is modified metallic colours arise. The 

 barbs, on the other hand, may contain dark pigment, 

 may show objective optical colours, or may contain 

 bright pigments. The variations which produce these 

 colour phenomena are much commoner in the general 

 feathers of the body than in quills ; they do not 

 usually occur simultaneously, and the appearance of 

 any one set of colours is associated with an increased 

 development of the special region of the feather with 

 which the colour is associated, as of the barbs, a 

 portion of the barbules, and so on. It may be that 

 this is in part the explanation of the fact that, apart 

 from the development of markings, quill-feathers are 

 slow to vary in colour, and are rarely brilliant. 

 Colour brilliancy is associated with a special de- 

 velopment of some individual region of the feather, 

 and it is essential for the purposes of flight that there 

 should be a harmonious development of all the parts 



