The Struggle for Existence 141 



foam, it is due to the complete reflection of the light from in- 

 numerable minute surfaces of crystals or bubbles. 



The mountain hare may escape the fox the more readily be- 

 cause its whiteness makes it so inconspicuous against a back- 

 ground of snow ; and yet, at other times, we have seen the creature 

 standing out like a target on the dark moorland. So it cuts both 

 ways. The ermine has almost no enemies except the gamekeeper, 

 but its winter whiteness may help it to sneak upon its victims, 

 such as grouse or rabbit, when there is snow upon the ground. In 

 both cases, however, the probability is that the constitutional 

 rhythm which leads to white hair in winter has been fostered and 

 fixed for a reason quite apart from protection. The fact is that 

 for a warm-blooded creature, whether bird or mammal, the physio- 

 logically best dress is a white one, for there is less radiation of the 

 precious animal heat from white plumage or white pelage than 

 from any other colour. The quality of warm-bloodedness is a 

 prerogative of birds and mammals, and it means that the body 

 keeps an almost constant temperature, day and night, year in and 

 year out. This is effected by automatic internal adjustments 

 which regulate the supply of heat, chiefly from the muscles, to the 

 loss of heat, chiefly through the skin and from the lungs. The 

 chief importance of this internal heat is that it facilitates the 

 smooth continuance of the chemical processes on which life de- 

 pends. If the temperature falls, as in hibernating mammals 

 (whose warm-bloodedness is imperfect), the rate of the vital 

 process is slowed down sometimes dangerously. Thus we see 

 how the white coat helps the life of the creature. 



3 

 Rapid Colour-change 



Bony flat-fishes, like plaice and sole, have a remarkable 

 power of adjusting their hue and pattern to the surrounding 

 gravel and sand, so that it is difficult to find them even when 

 we know that they are there. It must be admitted that they 



