CHAPTER XLIII 

 OUR BALANCED WORLD 



This old world of ours is. like a giant scale which is 

 continually being kept in balance. Or it is like a column 

 of constantly changing figures that always add to the same 

 total. 



We have seen how energy is continually changing its 

 form, yet no energy is ever lost. We have seen how mat- 

 ter is separated into its elements and these are recombined 

 into other substances, yet in all these changes no matter 

 actually ceases to exist. 



There is an astonishing balance between the various 

 forms of life on earth in which every animal and plant has 

 a part. Man is learning more and more to control this 

 balance, fighting harmful plants and animals and protecting 

 useful ones. The study of our balanced world and of 

 man's relation to it is a fascinating one, which casts a new 

 light on the laws of science we have come to know. 



A Balanced Aquarium. An aquarium is a water-filled tank 

 or globe in which water animals and plants are kept. A balanced 

 aquarium is one in which the materials needed for the continued 

 life of both animals and plants are mutually provided. This means 

 that the animals provide material which the plants are able to 

 make into food for their own nourishment and the plants in turn 

 provide nourishment for the animals. An aquarium containing 

 green water plants, such as water hyacinths and Elodea, and fish, 

 snails, and water bugs is said to be balanced when each organism 

 thrives. 



In a balanced aquarium the plants serve as food for the 

 animals. They also throw off oxygen which is used by the ani- 

 mals. In return for this the animals excrete carbon dioxide and 

 nitrogenous matter, both of which are used by the plants as raw 

 material for the production of new food. Thus all the necessi- 

 ties for continued life are provided by the organisms within the 



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