SOME REASONS FOR THE STUDY OF BIOLOGY 15 



portant part in tlie world in causing and carrying disease. Ani- 

 mals that cause tUseasc are usually tiny, and live upon other 

 animals as parasites; that is, they get their living from their hosts 

 on which they feed. Among the diseases caused by parasitic 

 animals are malaria, yellow fever, the sleeping sickness, and hook- 

 worm disease. Animals also carry disease, especially the flies and 

 mosquitoes; rats and other animals are also well known as 

 spreaders of disease. 



From a money standpoint, animals called insects do much harm. 

 It is estimated that in this country alone they are annually re- 

 sponsible for $800,000,000 worth of damage. 



The Uses of Animals to Man. — We all know the uses man 

 has made of the domesticated animals for food and as beasts of 

 burden. But many other uses are found for animal products, 

 and materials made from animals. Wool, furs, leather, hides, 

 feathers, and silk are examples. The arts make use of ivory, 

 tortoise shell, corals, and mother of pearl; from animals come 

 perfumes and oils, glue, lard, and butter; animals produce honey, 

 wax, milk, eggs, and various other commodities. 



The Conservation of our Natural Resources. — Still another 

 reason why we should study biology is that we may work under- 

 standingly for the conservation of our natural resources, especially 

 our forests. The forest, aside from its beauty and its health- 

 giving properties, holds water in the earth. It keeps the water 

 from drying out of the earth on hot days and from ruiming off on 

 rainy days. Thus a more even supply of water is given to our 

 rivers, and thus freshets are prevented. Countries that have been 

 deforested, such as China, Italy, and parts of France, are now sub- 

 ject to floods, and are in many places barren. On the forests 

 depend our timber, our future water power, and the future com- 

 mercial importance of cities which, like New York, are located 

 at the mouths of our navigable rivers. 



Plants and Animals mutually Helpful. — The study of biology 

 also shows us the interrelation existing between plants and animals 

 on the earth. Most plants and animals stand in an attitude of 

 mutual helpfulness to one another, plants providing food and 

 shelter for animals; animals giving off waste materials useful 

 to plants in the making of food. We also learn that plants and 



