MAMMALS 311 



Various Adaptations. Mammals include about 2500 different 

 species, which, compared with insects is a small number, yet their 

 habitat and mode of life varies so widely that they are a splendid 

 illustration of the modification of homologous parts for different 

 functions. 



Limbs. All mammals have two pairs of limbs, usually provided 

 with five toes; some are modified for flight (bats), some for 

 swimming (seals, whales), some for rapid land locomotion (horse, 

 deer), some for climbing (squirrel), or for burrowing (mole), for 

 attack and defense (cat, tiger), for jumping (kangaroo), for 

 prehension (apes, man). 



Teeth. In the same way the teeth may vary in structure and 

 use, there being usually four kinds present, the incisors, canines, 

 premolars, and molars. In some animals they are adapted for 

 tearing prey (tiger, lion), some for gnawing (rat, beaver), some for 

 grinding vegetable foods (horse, cow). All are of similar origin 

 and are merely different forms of the same organs. 



Body Covering. The body covering also varies greatly. The 

 hairs of the dog or horse, the wool of the sheep, the quills of the 

 porcupine and the scales of the armadillo, are all of similar origin. 

 Claws, hoofs and nails, horns, bristles, manes and tails are also 

 developed from epidermal structures. 



Four Important Orders. The mammals of North America 

 represent eight orders out of eleven, the three remaining orders 

 being found in Australia or the tropics. From this number we 

 shall study only four, the rodents, ungulates, carnivora, and 

 primates. 



The Rodents (gnawers) include many of our commonest animals, 

 the rabbits, porcupines, guinea-pigs, chipmunks, squirrels, beavers, 

 rats, mice, and woodchucks. All these forms have teeth especially 

 adapted for gnawing: the front teeth (incisors) are chisel shaped, 

 strong, and provided with a continuously growing root, so that 

 they replace themselves as fast as they wear off. Also the front 

 edge is harder than the rear edge, so that they are self sharpening 

 since the cutting edge is always worn thin. These tooth adapta- 

 tions together with strong jaws and powerful jaw muscles fit the 



