20 THE STORY OF THE BIRDS 



necessary briefly to glance at the colour with 

 which they are adorned. This too is a most 

 complicated subject, and one which we can only 

 deal with here in a cursory way. In the first 

 place, to what is the colour of a feather due ? 

 Colour is due either to pigment or to peculiarity 

 of structure, or to both these causes combined. 

 These three causes may be thus briefly sum- 

 marised. In the first place colour may be pro- 

 duced by pigment, either in a diffused solution 

 or in the form of pigmented corpuscles. Colours 

 produced in such a way are constant in the sense 

 of not varying in tint in whatever position the 

 light may fall upon them or the eye may view 

 them. These colour pigments are firstly black 

 (Zoomelanin), secondly red (Zoonerythrin), thirdly 

 yellow (Zooxanthin), and fourthly and fifthly 

 peculiar pigments, red (Turacin) and green 

 (Turacoverdin), only known to be present in the 

 plumage of the Plantain-Eaters (Musophagidce). 

 Brown, it may be remarked, is a combination 

 of black and red ; white is not due to pigment, 

 but the appearance of such a feather is due to 

 the countless spaces between its molecules which 

 diffract and reflect the light. The remarkable 

 gloss, irrespective of colour, on so many feathers 

 is due to their highly polished surface. In the 



