20 NATURAL HISTORY 



same and of different species for a place under tlie sun ; literallj'- under 

 the sun, for competition is caused by the limited amount of light that 

 will fall in a given area. Young plants often die because of the shad- 

 ing by the parent plant, and larger plants preempt areas of soil which 

 give little or no space for young growth. Feeding by animals, such 



Effect of sheep grazing upon trees. Thousands of young trees are destroyed 



every year in this way. 



as rabbits or sheep, may change the entire flora of a region, while 

 parasitic organisms injure a vast number of the hosts on which they 

 live. There are also marked cases of partnership between organisms, 

 bacteria in the soil giving and taking from both plants and animals, 

 and helping to create a cycle of food substances which pass through 

 the bodies of both animals and plants. The feeding of animals is their 

 biggest business in life, and the presence of a food supply determines 

 very largely the presence of animals in a given locality. It is said 

 that the oak tree serves as food for over 500 species of insects, the 

 apple for 400, clover and corn, over 200 each. Thus man with his 

 tilling of the soil, destruction of the forest, and domestication of 

 plants and animals has changed the fauna and flora of the land. 



Having discussed the effects of the factors of the environment on 

 living (jrganisms, let us now see how the interaction of these factors 

 affects life in the situations that living things are forced to meet. 

 Animals and plants must be adapted to live either in the water, in 

 air, or on land. The pages that follow show some of these adaptations. 



