56 THE BOOK OF NATURE STUDY 



Attention should be drawn to the fact that the direction 

 of the hair varies much on different parts of the body in 

 most mammals. In the horse, for instance, most of the hair 

 slopes backwards on the trunk, and downwards on the legs. 

 But on the chest, behind the arm and on the flanks, near the 

 hind-leg, the hair forms " whorls/' or slopes directly upwards, 

 as may be seen by an inspection of the nearest cab-horse. 



The length of the hair is more or less directly related to the 

 temperature of the animal's environment ; thus the tiger in India, 

 for example, has relatively short hair, the tiger of the cold regions 

 of Manchuria has an extremely thick coat ; even in England 

 many species, such as squirrels, for example, develop a thick, 

 long-haired coat for the winter. 



In some animals the hair attains a great length and a peculiar 

 texture, known as wool, as in the sheep, for example ; but in 

 hot countries the sheep are short-haired. 



The common hedgehog of our countryside exhibits to per- 

 fection a very remarkable modification of this hairy covering. 

 Herein, the upper surface of the body is covered with a really for- 

 midable armature of stiff and sharp-pointed spines, which form 

 the only protection which this otherwise defenceless creature pos- 

 sesses. When threatened, instead of running away, it rolls 

 up till the body forms a prickly ball such as few enemies care 

 to attack, and in this position it remains till danger has passed. 

 The porcupine possesses an even greater development of these 

 spines, which attain to a very considerable length and thick- 

 ness ; here, however, the fore-part of the body is hairy, but the 

 hairs become gradually thicker and longer towards the back, 

 so that a perfect gradation between hair and spines may be 

 found. The porcupine, however, does not roll up after the 

 fashion of the hedgehog when threatened, but turns its back 

 towards the foe, and sets all its spear-like shafts on end a 

 device which appears to be quite effective. In passing it may 

 be remarked that the hedgehog and porcupine are in no way 

 related. Each has, as it were, hit upon the same method of 

 defence ; and this is true of certain species of mice, whose backs 

 bristle with spines, though these are not very long. At one 

 time it was believed that the porcupine possessed the power of 



