CHIEF SUB-DIVISIONS OF MAMMALIA. 65 



ence, however, that the tail, which is flattened vertically in Fishes, is flattened 

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

 the surface to breathe. 



47. 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 beino- 

 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 tempe- 

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

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

 average 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 Mammals inhabiting 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 this 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 respiration. 



11. Chief Sub-divisions of Mammalia. 



48. 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 

 mobility of the fingers, and by the degree in which their extremities are en- 

 veloped by the nail, claw, or hoof, that terminates them. When the fingers 

 are partly absent, or are consolidated together, and a hoof envelopes all that 

 portion which touches the ground, it is obvious that the sensibility must be 

 blunted, whilst, at the same time, the member becomes incapacitated for pre- 

 hension. The opposite extreme is where (as in Man) a thin nail covers only 

 one side of the extremity of the finger, leaving the other possessed of all its 

 delicacy ; where several such fingers exist, of which one can be opposed to 

 the rest, so as to render prehension more perfect, and to perform a great va- 

 riety of actions : and where the plane of the whole hand can be turned in 

 any position, by the nature of its attachment to the fore-arm. Between these 

 there are many intermediate gradations. By these characters, the viviparous 

 Mammalia may be divided into the Unguiculated, which have separate fin- 

 gers, terminated by distinct nails or claws ; and the Ungulated, in which the 

 ringers are more or less consolidated, and inclosed at their extremity in a hard 

 hoof. Hoofed animals are necessarily Herbivorous, inasmuch as the con- 

 tormation of their feet precludes the possibility of their seizing a living prey ; 

 and they have flat-crowned grinding teeth for triturating their food. The 

 summits of these teeth are usually not covered by a smooth coat of enamel, 

 but present a series of elevations and depressions ; these are occasioned by 



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