PREFACE 



Life in the country, like that in the city, has been 

 rendered less simple by inventions and scientific in- 

 vestigations. A whole series of new problems have 

 arisen within a single generation and have called forth 

 more or less successiui attempts at their solution. 

 Among these new problems we may properly include 

 that of the bacteria, those minute living things that 

 float in the air that we breathe, that exist for our 

 weal or woe in the water that we drink, that perform a 

 mighty work in the soil and thereby make it possible 

 for generations of plants and of animals to come and go. 



The extreme smallness of the bacteria renders them 

 invisible to the naked eye, and makes it difficult for 

 the layman to think of them as definite living beings 

 entrusted with an important task in the continuing 

 of life. The deepening current of human existence now 

 forces us to study the bacteria and other microorgan- 

 isms. In so far as they are dangerous to our health 

 and happiness we must learn to defend ourselves ; 

 we must learn to destroy them or to render them harm- 

 less. In so far as they are beneficial, we must learn to 



(vii) 



