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 
