xi COLOUR AND SONG 315 



air, though its power of flight is due solely to the fact 

 that it could not live without it ; and the horribly 

 cruel instinct that leads a cat to play with a mouse 

 before killing it may be, perhaps, accounted for in 

 the same way. 



Dr. Wallace seeks to explain the facts by an 

 entirely different theory. But before entering into 

 this, it is necessary to clear the ground a little. In- 

 organic things — e.g. gold — have brilliant colours. Mere 

 brightness of colour, therefore, requires no theory to 

 explain it. In animals the pigments are probably 

 waste products derived from their food, and if not 

 employed for this purpose would be of no service 

 at all. It is easy, then, to account for the bright 

 colours of many deep sea animals, where there is no 

 light except from the occasional phosphorescent lamps 

 borne by some of the fishes. Even the bright colours 

 of a butterfly may be no tax upon its strength, for, 

 very probably, they can be as easily produced in the 

 animal system as a dull neutral tint. The difficulty 

 consists partly in the constancy of the colours and 

 patterns, but, chiefly, in the long plumes, the annual 

 production of which must tax the bird's strength, and 

 which are a source of danger to it. The moulting 

 season brings with it an increased rate of mortality 

 among the birds at the Zoological Gardens. How 

 the Argus Pheasant is over- burdened by its plumes, I 

 have already shown. 



With regard to the constancy of colours Dr. Wallace 

 has pointed out that much may be explained on the 

 principle of protective coloration. The hen-bird is 

 exposed to greater danger than the cock-bird, since 



