458 BIOLOGY AND HUMAN LIFE 



birds even more than they do amphibians. The scales on the 

 legs of birds, the beaks of turtles, and the bony plates around 

 the eyes are some of the similarities. 



From an economic point of view most reptiles are harmless 

 and even useful in that they destroy insects, mice, snails, and 

 other pests. Some are valuable as food, particularly the turtles 

 in this country and the iguana (lizards) in South America. The 

 tortoise shell is a valuable material for combs and other orna- 

 mental objects; the skin of crocodiles, alligators, and some 

 snakes is made into a tough and beautiful leather. There are a 

 few poisonous snakes. In this country four are worth noting. 

 The copperhead and the water moccasin, as well as the rattler, 

 can be recognized by the head, which is rather distinct from the 

 trunk and somewhat triangular. The coral, or bead, snake of 

 the southeastern states is recognized by the bright red bands 

 with yellow borders. Antitoxins have been prepared as effective 

 remedies for snake bite, but they have to be used very soon if 

 they are to be of any help. In India poisonous snakes take a 

 large toll of human life every year, but in this country they are 

 really not very serious. 



339. Birds. There are nearly 20,000 species of birds in the 

 world, and nearly 1000 in North America. They are warm- 

 blooded — warmer than mammals even — and are characteris- 

 tically covered with feathers. There are three kinds of feathers 

 in birds: (i) down; (2) contour feathers, which cover most of 

 the body ; and (3) the quill feathers. In addition there are hair- 

 like bristles about the beak. All birds lay eggs which hatch 

 outside the mother's body, but usually with the help of heat 

 furnished by one or both parents. Living species of birds are 

 without teeth (although extinct forms had teeth), and the jaws 

 are typically beaks. There is a wide range in size, from hum- 

 ming birds hardly two inches long to ostriches standing eight 

 feet or more in height and averaging four hundred pounds. As 

 with other vertebrates, fossil remains show that larger forms 

 existed in past ages. The birds also show striking adaptations 

 to a great variety of living conditions. 



