THE EARTH FOR MANKIND 



533 



f 



-^Oxygen- 



-Carbon Dioxid.«- 



:^: 



Green 



Plants 



Animals 



-> FOO D 



The total amount of animal life in the pond is limited by the 



total amount* of plant life. This fundamental interrelation of 



chlorophyl life and non-chlorophyl life sets the limit to the total 



amount of protoplasm that the pond can support. In an 



aquarium we can bring about a balance between the amount 



of plant life and the 



amount of animal 



life, so that it may 



continue indefinitely 



without the addition 



of anything from the 



outside except fresh 



water to make up for 



evaporation. 



This balance be- 

 tween one kind of 

 life and other kinds 

 of Hfe extends also 

 to life on land and 

 in the air. A single 

 species of animal 

 could not live by it- 

 self. Neither man 

 nor the ameba could 

 live unless there 

 were at least some 

 green plants also. 

 The balance of life 



Fungi 



and 



Bacteria 



Water 



Soil Materials-*- 



I 



Fig. 221. The interrelations of organisms 



The green plants, using water and carbon dioxid and 

 salts from the soil, are the source of all food and the 

 source of much oxygen derived from the decomposi- 

 tion of carbon dioxid (during photosynthesis). The 

 food is used by animals and by lower plants (fungi 

 and bacteria), and in the end the substance of the 

 animals is also used by the fungi and bacteria. The 

 carbon dioxid given off by the animals and by the 

 fungi and bacteria sooner or later finds its way back 

 to the green plants through the air or water. Other 

 wastes given off by these organisms are the raw ma- 

 terial which in time becomes the food that green 

 plants absorb from the soil 



includes the fact 



that the many different kinds of plants and animals fit into a 

 total that makes possible the largest amount of protoplasm. 

 385. The struggle for existence. Every animal is constantly 

 destro3ang other life (or using material from organisms that 

 have already died), and it is also in danger of being destroyed 

 by some other living thing — plant or animal. It is therefore 

 impossible for every new individual organism to reach its full 



