I THE COLOURS OF ORGANISMS 13 



quisitely varying tints, and it is to them that many 

 birds and insects owe their wonderful flashing beauty. 

 These colours glow with all the tints of the rainbow, 

 and change with every changing ray of light. Such 

 metallic colours are of course not uncommon pheno- 

 mena in the inorganic world, and are displayed, for 

 example, with extraordinary brilliancy on a polished 

 slab of the mineral Labradorite ; among organisms, 

 although suggested elsewhere, they attain their 

 maximum brilliancy in birds and insects. In birds 

 their degree of development varies greatly, for we 

 find them ranging from the dull greenish gloss of 

 some of the female humming-birds to the gorgeous 

 colouring of many of the males in humming-birds 

 and birds of Paradise. Among birds metallic colour 

 is apparently always associated with the presence in 

 the feathers of dull brown or black pigments, which 

 are necessary for the production of the colours. It 

 is also associated with a modification of the feather 

 structure which in many cases renders the feathers 

 unfitted for the purpose of flight. Here, as in the 

 case of objective structural colours, the exact physical 

 causation of the colours is unknown. 



In insects the colours are equally bright, but have 

 had little attention bestowed upon them ; it is still 

 doubtful whether the colours in them are or are not 

 associated with the presence of dark pigment. 



4. The presence of a pigment is not, however, 

 essential to the production of structural colours ; the 

 common earthworm, for example, exhibits a faint 

 iridescence which is due to the presence of numerous 

 fine lines on its colourless cuticle, these fine lines 

 producing interference of light. Although the colour- 



