112 ESSAYS ON BACTERIOLOGY. 



layers of the soil, the waters from clean springs, and 

 deep wells free from surface drainage, are compara- 

 tively or quite sterile. The unexposed tissues of 

 healthy bodies are also normally free from germs. 



Though it is now known that many diseases are 

 caused by bacteria, it must not be supposed that all 

 the bacteria are enemies of the higher forms of life. 

 On the contrary, many of them play a beneficent role 

 in promoting the disintegration of dead organic ma- 

 terial, which would otherwise, by accumulation, make 

 the world one vast graveyard; in preparing, by such 

 changes, the food for other living beings, and in ren- 

 dering the soil suitable for the growth of higher 

 plants. We fear the microbes which produce disease, 

 yet life would soon be impossible to higher organisms 

 without the helpful work of others. In all our study 

 of bacteria let us not forget that they may" be the 

 friends as well as the enemies of mankind. It is, how- 

 ever, the disease-producing micro-organisms which en- 

 gage the chief study of the physician, for these it is 

 with which he has to contend. These it is of whose 

 presence he must learn, in the field of diagnosis, to be 

 forewarned. 



The bacteria, like other living things, perform the 

 ordinary functions of life. They breathe; they eat, 

 taking in food substances, changing, digesting and ap- 

 propriating them to their needs, excreting the wdste 

 products. They multiply and reproduce their kind. 

 Like other living beings, also, their existence, and 



