MAMMALS 57 



our animals those that do not grow a thicker coat in 

 the autumn cannot survive the cold of winter, so among 

 the Arctic hares those have the best chance of surviving 

 which had the strongest shade of white when the fur 

 changed. These favoured ones persisted through most 

 winters, and so would have the largest progeny. But 

 in the southern regions it was no advantage to have 

 a lighter shade at the spring change of the fur ; and on 

 the other hand, there ought to be no change of colour 

 when the earth was still clothed in dazzling white. As 

 natural selection continued its work, a species of hare 

 was produced in the course of time in which the change 

 of coat and colour was proportionate to the length of 

 the winter, so that the animals were in harmony with 

 the prevailing tone at each season. 



Other animals have different shades of colour at 

 different seasons. Even the doe has a lighter colour 

 in summer than in winter, in harmony with the lighter 

 shade of the green-clothed forest. 



Some animals can obtain their food, though often 

 with great trouble, during the winter, but this is 

 impossible for others. These would die if they did 

 not pass the cold period in a long sleep in some warm 

 spot. It is in the summer only that their diet can 

 be had. 



Our hibernating animals pass the winter in nests 

 that are completely closed from within, in the hollow 

 trunks of trees and underneath the ground. There 

 they need no food; they fall into a death-like sleep, 

 and slowly consume their fat. Nature makes this 

 " hunger-cure " possible for them by reducing their 



