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Viewpoint of 

 Modem Ecology 



Every living thing is surrounded by materials and forces which con- 

 stitute its environment and from which it must derive its needs. 

 Contact with the environment is inescapable. Protoplasm, the essen- 

 tial constituent of an organism, is a dynamic substance, requiring a 

 continuous exchange of energy and materials. In order to remain 

 alive the organism, too, must secure energy and materials, and these 

 can be procured only by exchange with the outside world. Thus the 

 animal or plant cannot live completely sealed in an impervious skin or 

 shell but requires from its surroundings: (1) a supply of energy; (2) a 

 supply of materials; (3) a removal of waste products. The exchanges 

 that the organism has with its environment may be thought of as an 

 "external physiology," and they are just as essential as its internal 

 physiological adjustments. 



The supplying of the vital needs of the organism is by no means 

 the only action of the environment. Since the animal or plant must 

 leave its borders open to foreign trade, as it were, the possibility 

 exists that harmful materials will enter the organism or destructive 

 influences act upon it. For example, algae in a river must be suffi- 

 ciently permeable to take in the water needed for their metabolism, 

 but, if they drift into the sea, the higher salinity will cause a fatal loss 

 of water from their tissues. Animals in the environment of these 

 algae tend to add materials to the water which serve as plant nutri- 

 ents, but these same animals may also feed upon the algae and 

 eventually destroy the population. 



In order for the life of an organism to continue, the environment 

 must be satisfactory on two counts : ( 1 ) it must provide the minimum 

 requirements for life; (2) it must contain no influence incompatible 

 with life. If there is too little water, as in the Sahara Desert, or too 

 little oxygen, as at the top of Mt. Everest, or if nutrients are not avail- 

 able, as on rocky plateaux, animals or plants cannot obtain their 



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