VIII 
ORIGIN AND' USES OF COLOUR IN ANIMALS 
189 
are the colours of soap-bubbles, or of steel or glass ou which 
extremely fine lines have been ruled; and these colours often 
produce the effect of metallic lustre, and are the cause of most 
of the metallic hues of birds and insects. 
As colour thus depends on molecular or chemical constitution 
or on the minute surface texture of bodies, and, as the matter 
of which organic beings are composed consists of chemical com¬ 
pounds of great complexity and extreme instability, and is also 
subject to innumerable changes during growth and development, 
we might naturally expect the phenomena of colour to be more 
varied here than in less complex and more stable compounds. 
Yet even in the inorganic world we find abundant and varied 
colours; in the earth and in the water; in metals, gems, and 
minerals ; in the sky and in the ocean ; in sunset clouds and in 
the many-tinted rainbow. Here we can have no question of 
use to the coloured object, and almost as little perhaps in the 
vivid red of blood, in the brilliant colours of red snow and 
other low algae and fungi, or even in the universal mantle of 
green which clothes so large a portion of the earth’s surface. 
The presence of some colour, or even of many brilliant colours, 
in animals and plants would require no other explanation than 
does that of the sky or the ocean, of the ruby or the emerald 
—that is, it would require a purely physical explanation 
only. It is the wonderful individuality of the colours of animals 
and plants that attracts our attention—the fact that the colours 
are localised in definite patterns, sometimes in accordance with 
structural characters, sometimes altogether independent of 
them; while often differing in the most striking and fantastic 
manner in allied species. We are thus compelled to look 
upon colour not merely as a physical but also as a biological 
characteristic, which has been differentiated and specialised 
by natural selection, and must, therefore, find its explanation 
in the principle of adaptation or utility. 
The Constancy of Animal Colour indicates Utility. 
That the colours and markings of animals have been 
acquired under the fundamental law of utility is indicated by 
a general fact which has received very little attention. As a 
rule, colour and marking are constant in each species of wild 
animal, while, in almost every domesticated animal, there arises 
