462 BIOLOGY AND HUMAN LIFE 



ucts, as is nearly all of our leather. The hairs of mammals are 

 used in a great variety of ways, and appear daily in our tooth- 

 brushes and other cleaning devices, as .well as in paintbrushes 

 and artists' brushes. Quills are modified hairs. The bones of 

 mammals are converted into buttons and handles for knives, 

 umbrellas, and other small objects, as is also the horn of various 

 hoofed animals. Ivory is considered the most valuable of these 

 hard materials and is obtained from tusks (the teeth of ele- 

 phants). A number of the smaller mammals destroy insects 

 and so help in keeping these in check. Of the domestic mam- 

 mals those that are brought to the large packing houses are sys- 

 tematically used up to the last hair ; every part of the steer or 

 sheep or pig is used for something or other ; finally, the blood 

 and the trimmings that are of no other direct use are converted 

 into fertilizer or feed for poultry. 



The mammals have rendered us splendid service as experi- 

 mental material ; with their aid we can discover important facts 

 about the workings of our body, and especially about the inter- 

 action between parasites of various kinds and the human body. 

 The smallpox vaccine which we get by means of the calf, the 

 antitoxin for diphtheria by means of the horse, and the treat- 

 ment for rabies by means of the rabbit tell only part of the story. 

 The guinea pigs have been of the greatest help in these studies 

 and experiments, because their blood, in its reactions to para- 

 sites, is in many ways like human blood. 



From the very fact that mammals are efficient animals we 

 should expect to find many enemies among them. All the 

 wild animals that prey upon human beings (wolves and some 

 members of the cat family) or upon our domestic animals are 

 gradually being pushed farther and farther from the edge of 

 our clearings in the world's wilderness, although in parts of 

 Africa and of Asia they still remain formidable enemies. But 

 we have not succeeded in getting rid of our worst mammalian 

 enemy— the rat. This animal destroys more buildings, earth- 

 works, stored grain, prepared food, clothing, and other organic 

 material than any other species. Moreover, in its filthy habits 



