THE STORY OF THE BACTERIA. 21 



over the rest to their earth neighbors, who 

 have got higher up the scale of being, but not 

 yet so far as not to need absolutely and hourly 

 oxygen, hydrogen, nitrogen, and carbon, to 

 keep their life's furnaces a-going. 



Milk is a most excellent food for many forms 

 of bacteria, and among those which are com- 

 monly present in milk is one which causes it to 

 become sour when left to itself. Other forms 

 of bacteria develop those peculiar chemical 

 compounds which give to cheese its special 

 and varying flavors. It is, in fact, a very motley 

 group of chemical substances which these bac- 

 teria set free in feeding themselves on nature's 

 waste organic materials. Sometimes they are 

 very bad smelling gases, sometimes aromatic 

 substances, sometimes they are sweet, some- 

 times they are sour. But sooner or later they 

 are used by some animal or plant, and so again 

 enter the domain of life. Thus ever in cease- 

 less 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 



