64 CHARACTERS OF VERTEBRATE ANIMALS 



(sebaceous glands), secreting an oily substance that may be 

 regarded as a sort of natural pomatum. After a hair has attained 

 a certain length it falls out, and is succeeded by another hair grow- 

 ing from the same follicle. Many animals at stated periods of the 

 year undergo a sort of moult in preparation for the growth of a 

 new coat. This is the case, for example, with horses, which lose 

 their hair on the approach of winter and develop a thicker covering. 



There is very great variation as to the amount of hair present, 

 some aquatic forms, such as whales, being almost devoid of any 

 such covering, which on the other hand is extraordinarily well 

 developed in such creatures as sheep and goats. There may also 

 be a localization of well-developed hair, as m the mane of horse or 

 lion, and the beard of man and many apes. Not only are there 

 these differences in amount of hair, but also in kind and colour. 

 Both these points are illustrated by the races of mankind, which 

 present such differing types as the fair wavy hair of many Euro- 

 peans, the black straight hair of the Chinese, and the frizzly wool 

 of negroes. These differences are not merely ones of colour, but 

 shape, for the hairs of a Chinaman are almost perfectly cylindrical, 

 while the wavy hairs are flattened to some extent, and the wool is 

 much flattened, as in curling wood-shavings. In the last case, too, 

 the hairs are set obliquely in the skin. Even in a human being 

 we can distinguish between the coarser hair of the head and the 

 fine downy hair covering most parts of the body; but in many 

 mammals such a distinction is much better marked, as in the fur- 

 seal, which possesses coarse comparatively long hair, together 

 with the soft close-set finer sort which makes the skin valuable 

 commercially. A more violent contrast is seen in a hedgehog 

 (fig. 52), where we find not only ordinary hair, but also spines, 

 which are simply very large strong protective hairs, and this is 

 carried a step further in porcupines. 



The epidermis of Mammals also produces other structures, 

 which are more or less allied to hairs. The horn of a rhinoceros 

 is of this nature, and may be considered as a mass of fused hairs, 

 a view which is confirmed by examination of sections under the 

 microscope. The fringed scales of some armadilloes are to be 

 looked upon in a similar light, while such structures as claws, nails, 

 and the hollow horns of cattle are more remotely related. 



Mammals exhibit the greatest variety in the coloration of 

 the hairy covering of the body, and some of the colour-schemes 



