210 
COLOUR IN NATURE 
CHAP. 
in some fishes and many birds; or in increased 
growth and development of the cuticular structures 
in the male, associated usually with a great develop¬ 
ment of optical colours and of pigment; or as in 
many mammals there may be simply a special 
development of the exoskeleton, not associated with 
a great increase of pigment, or any special brilliancy 
of tint. Beauty in mammals is, however, usually due 
rather to form than to bright colour. 
The Colours of Fishes 
i. Tropical Fishes .—Among the colours of fishes, 
those of the forms inhabiting the neighbourhood 
of the coral reefs of warm seas must be especially 
mentioned. We have already seen how brilliant are 
the tints of the coral polypes themselves, and the 
same brilliancy seems to occur almost universally 
among the inhabitants of the waters round the reef. 
In fishes, however, the brilliant colours are probably 
produced not by pigments but by structure. The 
colours fade with extraordinary rapidity after death 
or removal from the water, and according to those 
who have seen the fish in their natural condition, 
the coloration is at best quaint and striking rather 
than beautiful. Generally speaking, the tints seem 
to be very vivid and hard, with a lack of light and 
shade ; the occurrence of sharply contrasted bands 
or spots on a bright ground is exceedingly common. 
Bright green, blue, red, and yellow seem to be among 
the commonest colours, and there are some striking 
instances of sexual dimorphism. Thus in Ostracion 
ornata the male has a ground colour of grass-green, 
