MAMMALS 



331 



Much has been written of the usefulness of wild birds to man, and 

 this usefulness can hardly be exaggerated. Perhaps their greatest 

 service, aside from the pleasure they give us, is in the destruction of 

 insects that attack our food plants and trees. 



345. Mammals. Mammals are so called because they 

 produce milk to nourish their young. The young are 

 brought forth alive, but require a period of care before they 

 are mature. While reptiles have a covering of plates, and 

 birds one of feathers, the skin of mammals forms a covering 

 of hair. The nails, claws, hoofs, and horns of animals, 

 including the quills of the porcupine, are all modified hair. 



Mammals are all air-breathing, and live chiefly on the 

 land. Whales and porpoises are mammals that live in 

 the sea. The whale has lost its hind limbs, although there 

 are remnants of them under the surface of its body. The 

 whale's front limbs have 

 been changed to flaps, 

 to meet its needs as a 

 swimmer. The bat, also 

 a mammal, has learned 

 to fly. 



A strange creature, the 

 duckbill, which lives in 

 Australia, is like birds 

 and reptiles, on the one 

 hand, and like mammals, 

 on the other. It lays 

 eggs, but it also forms Fl - 27L 



. . An Opossum. The Field Museum. 



milk for its young. One 



group of mammals includes kangaroos and opossums (Fig. 



271). These bring forth their young alive, but the young 



