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air ; non-green plants and animals, on the other hand, are con- 

 tinually consuming organic matter and by returning carbon 

 dioxide to the air keep up the supply of raw material necessary 

 for the manufacture of fresh organic material by green plants. 

 From the fact that the average percentage of carbon dioxide 

 in the air remains practically constant, we see that a balance 

 is maintained in nature between this work of construction on 

 the one hand and of destruction on the other, a continuance 

 of life being thus assured. 



We may then divide all living organisms into the following 

 two great classes : 



(1) Green Plants obtaining their carbonaceous food from 



the air. 



(2) Non-green Plants and Animals obtaining their carbon- 



aceous food directly or indirectly from green plants. 

 Now although, as ha s been shown above, each of these 

 great classes as a whole depends upon the other for its exis- 

 tence, yet we know that in nature individual organisms, owing 

 to their great powers of increase and to the limited quantity of 

 food materials available, are continually being injured and 

 destroyed in the struggle for . existence by other organisms. 

 Regarding their relations then to any particular organism or 

 group of organisms, all others may be classed as follows : 

 Competitors. (i) Competitors. Those which are indirectly injurious owing 

 to their competition for the same necessaries of life. 



Parasite?. (2) Parasites. Those which are directly injurious owing 



to their obtaining some or all of their necessaries 

 of life directly from the living bodies of organisms. 

 The organism from which a parasite obtains its 

 supplies is called a host. 



Symbionta. ^3) Symbionts. Those which not only are not injurious but 



are actually beneficial and are hence more or less 

 necessary for the existence of the organisms bene- 

 fited. 



The phenomenon of two organisms living together with 

 benefit to one or both of them is termed symbiosis. If both or- 

 ganisms are benefited it is called reciprocal symbiosis ; if only 

 one organism is benefited it is called antagonistic symbiosis, as 

 in the case of a parasite and its host. Symbiosis also may be 

 close when the organisms live in intimate connection with 

 each other, or distant when there is no direct or fixed union 

 between them. It is of course often impossible to insist on 

 these distinctions ; organisms, which at one period of their 

 life may be beneficial (symbionts), at other times may be 



