MAMMALIA. 471 



tusks, and in the Narwhal one is commonly sup- 

 pressed, while the other grows into a long, spirally 

 twisted, straight tusk, that projects like a horn in 

 front of its head. The Whale has no teeth ; but a 

 series of horny plates, parallel to each other, depends 

 from the upper jaw, and constitutes the valuable 

 substance called whalebone. In the Ant-eaters, and 

 some others of the Edentata, there are no teeth at all, 

 while the Armadillo has ninety-six, and some of the 

 Dolphins have a hundred and fifty. 



All the parts of animal structure are in beautiful 

 harmony with each other, and with the habits and 

 instincts of the species. The short and powerful jaw 

 in the Cats (Felidse), the lacerating teeth, the muscu- 

 lar fore-limbs, their freedom of motion, the sharp, 

 curved talons, the flexibility of the spine, and the 

 straight and simple digestive canal, equally indicate 

 activity and testify to the possession of sanguinary 

 and carnivorous propensities. In the Camels, the 

 prominent lips, the structure of the teeth, the broad 

 spongy soles of the feet, the callous pads on the 

 limbs, the complex digestive apparatus, and the 

 water-cells, all point out a creature fitted for feeding 

 on coarse and thorny herbage, and for traversing sandy 

 deserts. Neither of these animals could exchange 

 any portion of its structure with the other, without 

 serious derangement of the whole. This correspond- 

 ence of part with part, and the adaptation of eveiy 

 organ to the mode of life prescribed is so exactly 

 maintained, that a skilful comparative anatomist can, 

 from a single tooth or bone, build up in imagination 

 the whole structure of an animal which he never 

 saw, indicate its form, and pronounce with consider- 

 able confidence upon its food, its habits, and its 

 manner of life. 



The classification of the Mammalia is based upon 

 the structure of their teeth and feet. These are the 

 organs that most affect the conditions of existence 

 under which each is found. 



There is, however, one group, almost limited in 



