14 
COLOUR IN NATURE 
CHAP. 
ing is very slight in the earthworm, it is well known 
that some of the marine worms, eg. the sea-mouse 
( Aphrodite ), are covered with numerous bristles which 
exhibit brilliant iridescent colours. Again, the colours 
of mother-of-pearl are of course produced by structure 
only, without any assistance from pigment. For con¬ 
venience of reference, Structural Colours may perhaps 
be arranged as follows, retaining Gadow’s distinction 
of objective and subjective colours. 
Structural Colours :— 
1. Those not dependent upon the presence of a 
pigment. 
(a) Due to total reflection ; white colour of 
some flowers, of some feathers, of hair, etc. 
(/ 3 ) Due to striation of the surface, occurrence 
of thin plates, etc. ; iridescence of bristles 
and cuticle of worms, of mother-of-pearl, etc. 
2. Those dependent on the presence of a pigment. 
(a) Objective structural colours ; blue and 
green feathers. 
(/ 3 ) Subjective structural colours; metallic 
colours of many birds and insects. 
Structural colours are of extreme interest, not 
only on account of their wonderful beauty, but also 
on account of the difficult questions connected with 
their origin. It is to some extent possible to corre¬ 
late pigment-production with the physiology of the 
organism, but this seems extremely difficult in the 
case of structural coloration. We may note, how¬ 
ever, that structural colouring attains its greatest 
perfection among birds and butterflies, and both 
groups are noted for the extraordinary development 
of their cuticular structures. The delicate beauty of 
