12 THE COLOUKS OF ANIMALS 



CHAPTEB n 



TUE USES OF COLOUR 



I. Non-significant colours 



Colour, as such, is not necessarily of any value to an 

 organism. Organic substances frequently possess a 

 chemical and physical structure which causes certain 

 light-waves to be absorbed ; or the elements of tis- 

 sues may be so arranged that light is scattered, or 

 interference colours are produced. Thus blood is 

 red, fat is white, and the external surface of the air- 

 bladder in certain fishes has a metallic lustre, like 

 silver. In such cases there is no reason why we should 

 inquire as to the use or meaning of the colour in the 

 animal economy; the colour, as such, has no more 

 meaning than it has in a crystal of sulphate of copper 

 or iron. Such colours are the incidental results of 

 chemical or physical structure, which is valuable to 

 the organism on its own account. This argument 

 will be still further enforced if we remember that the 

 colours in question are, strictly speaking, not colours 

 at all. Blood and fat are so constituted that they 

 will be red and white, respectively, in the presence of 



