CHAPTER 2 



THE BASIS OF LIFE 



The chief concern of biology is with living things and the manner 

 in which they function. Before it is possible to discuss the kinds of ani- 

 mals, the manner in which they live, and the functioning of their 

 various organ systems, certain background material must be considered. 

 Among these basic topics are the problem of what distinguishes living 

 materials from nonliving ones, what are the basic units of organization, 

 and finally what are the chemical and physical laws which to some 

 extent explain animal functioning. 



LIVING VERSUS NONLIVING 



Through long years of association, most people have learned to 

 distinguish living from nonliving objects. In general, movement and 

 growth are the chief criteria used ; however, there are other important 

 attributes that distinguish living systems. 



Growth. — While it is true that some nonliving objects increase in 

 size, there is a fundamental difference between their growth and that of 

 living materials. Most nonliving objects increase in size by adding 

 materials to their outer surfaces. Ordinarily these materials are iden- 

 tical to or similar to those of which the object is formed, and are 

 added in an irregular fashion, forming unpredictable shapes. The 

 ever-increasing size of a stalactite in a cave is an example of this type 

 of growth. 



In contrast, living organisms take in, or, as in plants, manufacture 

 food. This food may be quite different from the plant or animal body. 

 By a series of changes, this food is converted into materials like those 

 of the organism. The material taken in is broken into simple units 

 and then rebuilt into the complex compounds of the organism. Growth 

 is an orderly process, one change following another and each related 



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