382 



How Living Things Affect One Another unit vii 



F'iG. 7, 1, 1 Sl.teep are muong our most useful an'mials. How are they useful to us? How 

 7nay a large flock such as this chajige the environment? (u. s. forest service) 



increase or decrease. The changes take 

 place without any interference from 

 man. They are the results of changes that 

 are brought about by the organisms 

 themselves. 



Man's relationships differ. Our popula- 

 tion increases out of all proportion to 

 that of other organisms. In recent years 

 it has grown at the rate of 25,000,000 

 persons a year. This affects our relation- 

 ships with other organisms. We destroy 

 organisms and the pendulum does not 

 swing back as in the rabbit-lynx relation- 

 ship. Thus, problems are created for us, 

 as you will read later in this unit. Our 

 relationships, too, are complex and very 

 numerous. We are not only a link in a 

 food chain, but we make use of innumer- 

 able organisms in countless other ways. 



Our use of animals. We are dependent 

 on animals in many ways even in this age 

 of machines. We use animals and their 

 products for food. We make leather 

 from skins; we use furs; wc get wool for 

 clothing and feathers or down for pil- 

 lows and comforters; and we use hooves 



and horns to make glue. For ages we 

 have used animals as beasts of burden. 

 The list is long: horses, donkeys, oxen, 

 dogs, elephants, llamas, camels, and 

 others. But there is still another impor- 

 tant use of animals: modern vise of ani- 

 mals for the purpose of experimentation. 

 The guinea pigs, white rats and mice, 

 monkeys, dogs, and cats used by biolo- 

 gists and physicians have saved many 

 millions of human lives. 



And not least is the value of bees and 

 other insects in cross-pollinating flowers. 

 Without this many seeds would fail to 

 develop, many plant species would be- 

 come extinct. You can add to this list by 

 doinc^ Exercise i . 



Man and higher plants. l{vcr\'thing we 

 eat unless it is animal food or salt comes 

 from plants. We may eat the root, the 

 stem, the leaves, the flower or the fruit 

 w ith its seeds as vegetables. We use the 

 seeds of certain plants such as wheat, rice, 

 oats, barley, corn, and others as cereals 

 or grains. They are conccnrrarcd foods 

 containing little water and much nourish- 



