THE HAIRY COAT 
Another distinctive peculiarity of the mammals is their 
hairy covering — something that belongs to no other class of 
animals, and which gives an immense advantage. 
A hair is a threadlike outgrowth from a minute nipple sunken in the 
skin, which continues to secrete the horny substance at its root, 
as the hair wears away at its point. Hairs vary in form and 
structure and consequently in appearance, giving such differences in pelage 
as the velvety fur of the mole, the long wavy coat of the skunk, the bristles of 
the pig, the spines of the porcupine, or the scale armor of pangolin and 
armadillo. All of these are modifications of hairs, and it seems likely that 
claws, finger nails, and sheath horns are of similar structure. 
Hair. 
The purpose of the hairy coat is mainly protection from the 
weather; and those mammals (of cold climates) which have a 
close woolly under fur, usually have also an outer thatch of 
longer, coarser hairs to shed the rain. Hair makes a warm 
covering, not only because of the air with which it is filled, 
but because of the air entangled among it. Watch a muskrat 
swimming. You will see that his body is covered with glisten- 
ing bubbles held between the under and outer fur, and that he 
comes out of the water with a perfectly dry skin. Thus hair 
not only keeps dampness and cold out, but the bodily heat in, 
as you well know who wear this covering at second-hand in the 
form of ‘furs’; and the coats of mammals vary in character 
and density not only with the climate in which they dwell, 
but with the season, the heavier winter coat loosening and 
falling out in the spring, but growing again in the autumn. 
A semiannual shedding of the hair is characteristic of most 
northern animals, at least, as is familiarly shown by horses and 
dogs. This quality of warmth in the hair is probably the secret 
of the supremacy attained by the mammalian race. Its 
beginnings were early in the Mesozoic or Secondary age of 
geologic history, when dry land was gradually rising out of 
the seas and swamps into extensive areas, and the climate of 
the world was slowly cooling. At that time the ruling animals 
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