SPIRAZINES 



INTRODUCTION 



The various forms of life which are encoun- 

 tered in nature exhibit such a variety of appear- 

 ances and such a profusion of details that the 

 casual observer is often too bewildered by the 

 complexity of his surroundings to gain a clear 

 conception of life processes in their entirety and 

 to distinguish what is fundamental and indis- 

 pensible from what is superficial and unessential. 

 The presence of life is usually recognized by 

 characteristic bodily form, spontaneous mobility, 

 and responsiveness to external stimuli, but these 

 properties can all be closely imitated artificially 

 and in the lower forms of life are often entirely 

 absent, so that they must be regarded as secon- 

 dary characteristics which have developed in the 

 course of evolution and not as primary attributes 

 of life itself. 



Every living organism is unique in that it con- 

 stitutes a self-contained entity having its own 

 laws of action, and which is capable, under favor- 

 able conditions, of producing others like itself. 

 The phenomenon of self-perpetuation or repro- 

 duction is exhibited by every living organism 

 regardless of its rank in the plant or animal 



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