CHAPTER XIV 

 MAMMALIA 



Of all the classes of the animal kingdom the mammals are the 

 highest, using that term in a broad sense, since it includes man, whose 

 high mental development has given him dominion over the rest of the 

 animate world. Economically it is doubtless the most important 

 class of animals since it includes, among others, our domestic 

 quadrupeds. 



The name mammal has been derived from the presence, in all 

 members of the class, of milk-secreting glands or mammae for the 

 nourishment of the young; these glands vary in development from the 

 rudimentary organs of the Monotremes to the enormous udders of 

 high-grade milk cows. 



The other chief characteristic of the class is the presence, at some 

 period of life, of hairs, which are almost as characteristic of mammals 

 as feathers are of birds; these hairs vary from the soft fur of moles 

 or seals to the long quills of the porcupine; like feathers they are of 

 epidermal origin and like them also they are, in many cases, periodically 

 shed and replaced. 



Other epidermal structures are often highly developed, such as 

 claws, hoofs and teeth, the last-named being almost universally present 

 and in two sets, the "milk" and "permanent" teeth. The variation 

 in bodily size among the Mammalia is enormous, extending from the 

 tiny mice and shrews, 2 or 3 inches long to the largest whales, 

 which may reach a length of 80 feet or more and are probably 

 the bulkiest animals the world has ever produced, though not the 

 longest. 



With very few exceptions the ovum of mammals is of microscopic 

 size and is developed within the uterus of the parents, to which it 

 is attached and from which it derives its nourishment by means of a 

 vascular structure the placenta. 



Unlike the birds but few mammals migrate, but a number of them 



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