VIII ORIGIN AND 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 eftect 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 Ave 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 cpiestion of 

 n^e to the coloured ol)ject, and almost as little perhaps in the 

 vivid red of blood, in the brilliant colours of red snow and 

 other low alg^e 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 recpiire no other explanation than 

 does that of the sky or the ocean, of the ruby or the emerald 

 — that is, it would require a pui-ely physical explanation 

 only. It is the wonderful indi\aduality of the colours of animals 

 and plants that attracts our attention — the fact that the colours 

 are localised in definite patterns, sometimes in accordance A\dth 

 structural characters, sometimes altogether indef)endent of 

 them ; while often difi'ering 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 diflferentiated 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 



