Characteristics and Classification 

 of Mammals 



Mammals are distinguished from other back-boned animals 

 by having hair, which in a few kinds may be scanty, and by 

 the fact that their females give milk and suckle the young. Also, 

 in all but the egg-laying platypus and echidnas, the young are 

 born alive. The term "mammal" is an attempt in one word to 

 put into common language the idea that all of the thousands of 

 different forms sharing the attributes just mentioned make up 

 a single great class, distinct from the reptiles, birds, amphib- 

 ians, and fishes. The Class Mammalia thus includes the beasts of 

 field and forest, the aquatic whales, seals, and sirenians, the bats, 

 and mankind. This idea is a relatively new one, dating from 

 the Swedish biologist Linnagus in the second half of the eight- 

 eenth century. It has been accepted by most educated people 

 but is still foreign to a large proportion of humanity. 



DISTINCTIVE CHARACTERS OF MAMMALS 



Mammals are grouped into Orders on the evidence of similar- 

 ity of structure, reinforced by what has been discovered about 

 the fossil history of the different groups. The members of or- 

 ders are in most cases obviously more closely allied to each other 

 than to other mammals. Thus, the flesh-eaters belong for the 

 most part to the Order Carnivora (literally, flesh-eaters) ; the 

 monkeys, the gnawing animals, the hoofed animals, the flying 

 animals (bats) likewise are members of distinct orders. But 

 exceptions occur; the pouched animals, including pouchless 



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