CHIEF SUB-DIVISIONS OF MAMMALIA. 



Birds in rapidity of locomotion ; but there are few that cannot perform several 

 kinds of progression. Several of their movements require a considerable 

 amount of flexibility in the spine; hence the vertebral column, and -the bony 

 framework of the trunk, are never so much consolidated as they are in Birds. 

 On the other hand, the neck is much less movable ; it never consists of more 

 than seven vertebrae, and these are always present; so that they are sometimes 

 of great length, as in the Giraffe, and sometimes extremely short, as in the 

 Whale, which seems to have no neck at all. In the greatest number of Mam- 

 malia, the body is supported upon all the four extremities, as in Reptiles; 

 being adapted for progression along the surface of the earth. There are some 

 species, however, in which the typical structure has undergone a metamor- 

 phosis, by which it is made to resemble that of a Bird ; whilst in others it is 

 modified, so as to conform to the character of the Fish. In the Bats, the 

 power of motion is almost entirely delegated to the wings, which are com- 

 posed of skin, stretched over a bony framework formed of the widely ex- 

 tended hand; and the sternum has a projecting keel for the attachment of 

 the pectoral muscles, as in Birds. And in the Whale tribe, the power of 

 locomotion is almost completely taken from the extremities, and given back to 

 the trunk, as in Fishes ; for the posterior extremities are entirely absent, and 

 the anterior serve only for guidance; there is this important difference, how- 

 ever, that the tail, which is flattened vertically in Fishes, is flattened horizon- 

 tally in the Cetacea, which require the power of frequently coming to the 

 surface to breathe. 



58. The inferior energy of muscular movement in the Mammalia, is ac- 

 companied by an inferior amount of respiration ; the type of the respiratory 

 apparatus, however, is higher than in Birds, a large extent of surface being 

 comprised within a smaller space. The lungs are confined to the cavity of 

 the thorax; and there is a provision for the regular renewal of the air received 

 into them, by the action of the diaphragm, which here completely separates 

 that cavity from the abdomen. The diminished amount of respiration, again, 

 involves the production of a lower degree of animal heat ; so that the tempera- 

 ture of this class seldom rises above 104. There is, therefore, less need of 

 means for effectually confining the caloric, especially, too, as their greater size 

 causes their radiating surface to be much less, in proportion to their bulk, than 

 is that of Birds ; and accordingly we find them provided only with a covering 

 of hair or fur, which is much less warm than that of feathers, and which is 

 thin and scanty in those which inhabit tropical climates. The chief exception 

 to the last rule is in the case of the Sloths and of some Monkeys, which inhabit 

 situations exposed to the most powerful rays of the sun, and which are covered 

 with a long but thin and coarse hair, the purpose of which is evidently the 

 protection of their skin from the external heat. The inferior energy of the 

 respiration and circulation, involves a diminished activity of the other functions 

 of nutrition, as compared with those of Birds ; and the demand for food appears 

 to be somewhat less constant. Their various organs, however, are developed 

 upon a higher plan ; as we have already observed in regard to those of respi- 

 ration. 



Chief Sub-divisions of Mammalia. 



59. In sub-dividing the truly Viviparous division of the class, so as to sepa- 

 rate Man from the tribes with which he is associated in it, we may be advan- 

 tageously guided, in the first place, by the conformation of the extremities ; 

 since upon* the perfection of the organs of touch, will depend much of the 

 address of an animal, in executing the actions to which it is prompted by its 

 intelligence. The degree of this perfection is estimated, by the number and 



