105 RELATIONS OF PLANTS TO ANIMALS 



ships of insects and flowers are another example of a symbiotic 

 partnership if we use the term in its widest and best sense, as, 

 indeed, is the give and take in our whole social life. Everyday 

 life in a community or in a household must be give and take ; 

 we must each sacrifice for others if we expect to have them do the 

 same for us. The sacrifices made by parents must be matched in 

 equal willingness to help on the part of the children if our social 

 life is to continue. Thus we see that much of the foundation of 

 society is built on this idea of mutual helpfulness. 



Summary. — Just as plants and animals in a balanced aquarium 

 show a relationship of give and take, so plants and animals on the 

 earth show this same relation. The living together of organisms 

 of different kinds for mutual helpfulness is called symbiosis and is 

 shown in an exact way in some forms of animal life, such as the 

 hermit crab and sea anemone. But the fundamental idea is at 

 the basis of a happy home life and the life of society. 



Problem Questions 



1. Can you make a balance sheet illustrating the relationg existing in a 

 balanced aquarium ? 



2. What would be the condition of the balance sheet if the aquarium were 

 put in a dark room ? If several extra snails and fish were introduced ? 



3. What are meant by the oxygen cycle, the carbon cycle, the nitrogen 

 cycle? Make diagrams to illustrate. 



Problem and Project References 



Hunter, Laboratory Problems in Civic Biology. American Book Company. 



Downing, Our Living World: A Source Book of Biological Nature Study. Uni- 

 versity of Chicago Press. 



Eggelin and Ehrenberg, The Fresh Water Aquarium. Henry Holt and Com- 

 pany. 



Needham, General Biology. Comstock Publishing Company. 



Needham and Lloyd, Life of Inland Waters. Comstock Publishing Company. 



Ward and Whipple, Manual of Freshwater Biology. John Wiley and Sons. 



Woodruff, Foundations of Biology. The Macmillan Company. 



