22 The Story of the Bacteria 



sour. But sooner or later they are used by 

 some animal or plant, and so again enter the 

 domain of life. 



Thus ever in ceaseless alternations between 

 life and death these elemental combinations 

 come and go. And ever since life emerged 

 from its primal simple forms on the earth, 

 the bacteria have silently gone on tearing 

 the worn-out and useless to pieces and turning 

 it over in new combination to other forms of 

 life. 



It was formerly believed that such lowly 

 organisms as the bacteria could spring at once 

 into being wherever in nature the conditions 

 were favorable, but this notion of spontaneous 

 generation, has long since been given up, be- 

 cause it was shown to have depended upon 

 insufficient and crude observation. We now 

 believe that every living thing comes from some 

 pre-existing living thing, be it man, beast, 

 plant, or cell, and this principle holds true as 

 well among the bacteria as among more highly 

 organized beings. 



There is an enormous number of different 

 species of bacteria, each one of which appears to 



