432 GENERAL ZOOLOGY 



these germs have been killed, a fluid containing the dead 

 bacteria is injected into the body of a man. The presence 

 of the dead germs in this instance causes the blood to manu- 

 facture antitoxins for this particular germ. As a conse- 

 quence of this treatment the individual becomes immune to 

 typhoid. 



Rabies, scarlet fever, and lockjaw are others of the list 

 of dreaded diseases from which immunity may be secured. 

 Experiments on animals have thus relieved man from some 

 of his most dangerous enemies. 



Fur-Farming. As the country has become more densely 

 populated the natural supply of furs taken in the wilds by 

 trappers has steadily decreased. In recent years an entirely 

 new industry has been developed. Various kinds of wild 

 and domesticated animals are being bred and raised in 

 captivity for furs. Fox-farming has become an important 

 industry in many localities. Skunks, minks, and rabbits, as 

 well as many other animals, are being raised for their skins. 



Mammals as Pests. Next to the insects the mammals in- 

 clude some of man's worst enemies. In the frontier days of 

 our own country wolves, bears, foxes, and coyotes caused 

 great damage to flocks and herds of domestic animals. 

 Skunks, minks, and weasels prey upon birds, including do- 

 mestic fowls. Rabbits and other rodents destroy trees, 

 shrubs, and vegetable gardens. Coyotes, prairie dogs, and 

 woodchucks by their burrows damage fields and pasture 

 lands and frequently are the cause of injury to horses and 

 cattle which step into their burrows. Rats have become so 

 destructive that it is estimated that every rat eats at least 

 two dollars' worth of food a year. In addition the rats are 

 a necessary link in the chain for spreading one of the most 

 dreaded of human diseases, — bubonic plague. 



Wild-Life Sanctuaries. In addition to the bird protection 

 mentioned earlier, our national parks and forests give pro- 



