90 THE COMMON SENSE OF THE MILK QUESTION 



III 



Most persons have at some time in their lives been 

 fascinated by the observation of a drop of water 

 through a microscope. They have seen many forms 

 of animal life which were quite invisible to the naked 

 eye, and some forms of plant life of a still lower order 

 than the animalcula. These are bacteria, exceedingly 

 minute little organisms of plant life, belonging to the 

 fungi order. Infinitesimal as they frequently are, 

 the bacteriologist in his laboratory finds that they 

 differ very materially from each other, that there are 

 classes of bacteria as distinct as the classes among 

 the higher forms of life. Some are in the form of tiny 

 spheres, others like little rods, and still others like 

 miniature corkscrews. In short, there are many 

 variations in shape and general appearance observ- 

 able when bacteria are closely examined through a 

 very powerful microscope. 



Not only is it possible to note physical differences 

 in bacteria, but their characters have, to some extent, 

 been likewise studied. While it is not yet known just 

 what effect many of them have upon our food, or 

 when they are taken into our system,' other varieties 

 have been more successfully studied. This is espe- 

 cially true of the varieties of bacteria which are called 

 "pathogenic," that is, disease-producing. The chief 

 reason why we know more concerning the pathogenic 



