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DIVISIONS OF THE ANIMAL KINGDOM. XXXI 



hension and progression on land, and their fore -extremities, 

 expanded, covered with strong feathers, and moved by pow- 

 erful muscles, serve as the organs of flight. Their jaws ter- 

 minate in a pointed beak ; and their necks are long and flex- 

 ible, so that by moving it, they may vary the centre of gra- 

 vity of the body, bringing it forwards when in flight to be 

 more under the wings, and backwards above the limbs of 

 support when at rest. The external air permeates the body, 

 passing from the lungs even into the bones, so that the body 

 may be rendered buoyant. Their respiratory action is strong, 

 their blood warm, and their movements are agile and power- 

 ful. Impregnation takes place within the body, and the egg, 

 when protruded, is covered by a calcareous shell ; the heat 

 required to hatch it being usually supplied by the body of the 

 parent. In birds, the nervous system is more developed than 

 in the tribes below them, and their intelligence may be be- 

 lieved to be superior. In them we first find animals resign- 

 ing their natural wildness, changing their form and instincts 

 with the new conditions in which they are placed, and thus 

 submitting themselves to true domestication. 



The Mammalia derive their distinctive name from mam- 

 ma, a breast, having glands by which the female is en- 

 abled to supply milk to the young. The mammalia are, most 

 of them, inhabitants of land, but some of them are formed to 

 live wholly in water, and some of them live partly in water, 

 and partly on land. The greater number are quadrupeds, 

 the members of both extremities being limbs, formed to sup- 

 port the animal, and serve the purpose of locomotion, and in 

 numerous cases of prehension. The monkey tribes possess 

 four members, having hands, but their natural motion is on 

 all-fours. Man possesses but two limbs of support, and is 

 formed to walk erect, his upper extremities or arms being 

 left free for the various uses to which they are to be applied. 

 All the mammalia bring forth their young alive, and so are 

 termed viviparous. They are divided into various groups, 

 hich may be termed Orders. 



