SECTION I — INTRODUCTION 



CHAPTER I 

 HOW OUR KNOWLEDGE OF MICROSCOPIC LIFE BEGAN 



The mother of a large family of children well knows that 

 she has her hands full in keeping them out of mischief. Gen- 

 erally by the time the children have grown up she feels she 

 knows as much on the subject as anyone ; and quite possibly 

 in a practical way she does, even in these days of specialists 

 in child feeding, children's diseases, child psychology, and 

 other studies relating to children. If she lives in the country 

 her household may include cats, dogs, and possibly other 

 animals, and she knows that keeping them in their proper 

 places is almost as much of a task as it is with her own children. 



But what about the invisible members of the household, 

 the bacteria, yeasts, and molds? Does she often stop to 

 realize what a problem they can be ? They are omnipresent ; 

 they are capable of doing more mischief than a cat in the 

 larder, more actual harm than a vicious dog ; they cannot be 

 taught obedience like a child or a dog ; nor can they be thrown 

 outdoors like a cat that is in the way. They are too small to 

 be seen and can be recognized only by what they do; and 

 for such tiny creatures they can do a lot, either for good or 

 for bad. The housewife has to learn something about them 

 in a practical way whether or not she knows anj^hing about 

 their actual nature. She has to know that milk will sour 



