14 COLOUR IN NATURE chap. 



ing is very slight in the earthworm, it is well known 

 that some of the marine worms, e.g. 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. 



(yS) 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 



