HAIRY COVERINGS OF THE MAMMALIA. 97 



slightly curled. Wool is a kind of long hair, very fine, and 

 twisted in every direction ; and lastly, Down is composed of hairs 

 of extreme fineness and softness, and is principally found hidden 

 under a more or less dense covering of common hair. 



77. The colour of hair varies much, but is almost always 

 some modification of white, black, brownish-red, or yellow ; it 

 appears to depend on the existence of a coloured fatty matter, 

 which is soluble in boiling alcohol. When this oil is extracted 

 by the action of the liquid just mentioned, the hair always 

 becomes of a grayish yellow. In white hair we also find a 

 white oil, which in red hair is replaced by a reddish oil ; and in 

 black hair, the existence of an oil, tinged of a blackish-blue by 

 sulphuret of iron, has been observed. Sometimes the hairs pre- 

 serve the same colour through their entire length ; sometimes 

 their hue is deeper at the point than at the base ; and sometimes, 

 also, they present a series of white and coloured rings. More- 

 over, their colour varies nearly always in different parts of the 

 body ; and the general arrangement of these tints is character- 

 istic of particular species of Mammals. These colours are gene- 

 rally much deeper on the upper than on the under surface of the 

 body ; and when they form spots, these are nearly always dis- 

 posed symmetrically on each side ; provided, at least, that the 

 animals have not been domesticated, for then their markings often 

 present very great irregularity. The colour and marking are 

 usually the same in both sexes, and in general vary but little 

 at different ages. In some species, however, the young have 

 spots, and a variety of shades, which disappear in the adult ; 

 and it' frequently happens that the colour of the Mammalia 

 changes with the seasons. 



78. The hairs generally fall off at a certain period of the 

 year, and are replaced by others ; this moulting chiefly takes 

 place in Spring and Autumn. Sometimes this is effected without 

 the colour being altered ; at other times it causes very consider- 

 able changes both in the colour, the quantity, and the nature 

 of the hair. Thus our common Squirrel (Fig. 48), whose hair 

 is deep-red in summer, becomes a beautiful blue-grey in winter. 

 In this latter season, the fur of the Mammalia is generally much 



