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- 
