VIII ORIGIN AN'D USES OF COLOUR IN ANIMALS 189 



are the colours of soap-bubbles, or of steel or glass on 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 Cjuestion 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 algre 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 sj^ecialised 

 by natural selection, and must, therefore, find its exjjlanation 

 in the principle of adaptation or utility. 



The Constanc!/ 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, Avhile, in almost every domesticated animal, there arises 



