426 BIOLOGY AND HUMAN LIFE 



of materials in the vast cycles of life. In common with other 

 animals and with plants we discharge carbon dioxid and nitrog- 

 enous wastes. Some of these are used by green plants directly ; 

 others find their way into the green-plant organisms only after 

 having gone through the protoplasm of various bacteria, pro- 

 tozoa, and larger animals. After a long chain of chemical 

 changes some of the elements come back to us in new com- 

 binations (see Fig. 176). 



Many plants and animals serve us as a means of keeping 

 alive the organisms in which we have a direct interest ; bacteria 

 make it possible for soil to increase our crops, and field crops 

 are used as fodder to increase our supplies of useful animals. 

 Moreover, the forest and the forest floor, very complex groups 

 of many organisms, are of vital importance to us in keeping us 

 continuously supplied with water (see section 394), although 

 many of us never see these organisms. 



Less directly, but still vitally, we depend upon many insects 

 for the distribution of pollen for important wild plants as 

 well as for many cultivated plants ; and we depend upon in- 

 sects and birds and other animals to keep in check various 

 enemies of the forest, of the crop plants, and of other useful 

 organisms. 



Finally, we make direct use of many animals for immediate 

 service, especially in transportation. Besides the horse, man 

 has depended upon the camel, the ox, the elephant, the reindeer, 

 the goat, and the dog to help carry his burdens and transport 

 his wares. From many animals we also get direct service in the 

 form of companionship and amusement. 



Thus we may see that living things outside our own species 

 are important to us, not only because we could not maintain 

 physical life without them but also because they have played a 

 large part in making possible man's civilization and his spiritual 

 development and satisfaction. 



