CHAPTER III 



GENERAL CHARACTERS OF LIVING ORGANISMS 



CHARACTERS OF ORGANISMS IN RELATION 

 TO ENVIRONMENT 



All organisms have the power of self -maintenance ; 

 i.e., of maintaining their identity and a certain constancy 

 of structure, chemical composition, and activity in 

 spite of continual changes in their surroundings and in 

 their own living substance. The degree of environmental 

 change to which different organisms are exposed varies 

 greatly, and many cells of higher animals pass their 

 whole life in media which are automatically secured 

 against all but slight variation. On the other hand, 

 protoplasmic activity, implying chemical change, is 

 uninterrupted during life; and, as already pointed out, 

 is largely the expression of chemical reactions, chiefly 

 oxidative in nature, by which energy is freed. In all 

 organisms part of the energy thus freed takes such a 

 form that the organism is enabled to maintain itself in 

 equilibrium with its surroundings, grow, and eventually 

 reproduce itself. A curious and highly characteristic 

 cycle of activity is thus shown; thus the animal uses 

 its muscular energy, derived from the oxidation of 

 carbohydrate, to secure more carbohydrate and other 

 materials which serve as sources of vital energy; and 

 this cycle, regulated in accordance with the varying 

 physiological requirements, is repeated continually 

 throughout life. Such facts illustrate the general depend- 

 ence of life upon the interchange of material and energy 



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