CHAPTER XXXII 



THE PHYSICAL ENVIRONMENT OF ORGANISMS 



We have seen that all organisms share a kinship based upon descent from 

 a common ancestry far back in geologic time. In this kinship we have the 

 clue to the essential similarity of protoplasmic composition and cell 

 structure throughout the living world, as well as to the more detailed 

 resemblances that are shown by organisms with a less remote common 

 ancestor. 



There is another sense in which all forms of life are related. This rela- 

 tionship consists in the intricately interwoven dependencies, competitions, 

 and exploitations that exist among all forms of life, through their neces- 

 sity of maintaining themselves by the capture and expenditure of energy. 

 Here the relationship is primarily an economic one, far too complex to be 

 known in full detail, yet clear enough in at least two respects to be un- 

 doubtable. Part of our knowledge of this relationship comes from a 

 consideration of the "energy cycle," in which we can trace the main- 

 tenance of all life to utilization of energy that is ultimately derived from 

 the sun. Another part comes from the incomplete but considerable portion 

 of the economic and social interplay among organisms that biologists 

 have been able to trace and to some extent to measure. 



THE ENERGY CYCLE IN THE ORGANIC WORLD 



All being alive involves a constant expenditure of energy, and precisely 

 as in any engine, a part of this energy is never again available for any 

 life processes. With no significant exception, all this energy upon which 

 life depends comes from the sun. Until it is finally dissipated beyond the 

 use of any protoplasmic device, it goes through a series of transformations 

 and interchanges that support the entire organic world. In the sense that 

 the energy from the sun is gradually lost and must be continuously 

 replaced, the process is noncyclic; but in its turnover and reutilization 

 of raw materials and in the often long series of transferences of energy 

 from one organism to another — always with some loss of total usable 

 energy — a cyclic relationship is well marked. 



The original capture of energy. Roughly, about one-millionth of 

 the sun's constant output of energy falls upon the earth in the form of 



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