PREVIEW 519 



and many other commodities. But both plants and animals have, 

 in another sense from the above, an economic value. If plants, 

 such as weeds, destroy our crops by taking their place, or if animals, 

 such as coyotes, destroy sheep by killing them, then they are harm- 

 ful in a " dollars and cents " way. 



We have already learned that man plays a very important part 

 in disturbing the balance of life as it exists on the earth. This has 

 been brought about by the increased population and the conse- 

 quent necessary increase in food and other supplies. Through 

 planting crops which have nitrogen-fixing bacteria associated with 

 them, it has become possible for the earth to supply more crops. 

 By irrigating large areas of practically desert land man has been 

 able to raise large crops of grains, vegetables, and fruits. 



Man is also constantly finding new uses for animal products. 

 Fishes, such as the dogfish, which were formerly unmarketable, 

 because they were not thought good to eat, are now an article of 

 food under the name of the grayfish. This is only one instance of 

 how man, as the thinking animal, exploits other forms for his own 

 benefit. More people on the earth means a need for more food. 

 Man has come to realize the way in which he has been wasting the 

 living things which he needs and he is emphasizing methods of 

 conservation as well as the use for food of plants and animals that 

 formerly were not considered as fit for food. 



Those of us who live in farming communities are aware of the 

 harm done by many insects and know, too, that our bird friends do 

 a good deal to help make it possible for the farmer to raise his crops. 

 But those of us who do not know the birds as friendly fighters in 

 our behalf should have some evidence along this line. Moreover, 

 all of us ought to know a few common birds so we may be able to 

 recognize them. 



Birds not only eat insects but some of them eat weed seeds, thus 

 keeping these pests somewhat more under control. Even the 

 birds which do eat crops make up for this by feeding in part upon 

 insects or harmful rodents. 



To understand the value of birds better a few examples of in- 

 sect damage will be given and, when possible, it will be shown 

 how insects are controlled by the birds which feed upon them. 



