18 HISTORY 



characteristics placing them without question among 

 the plants, while others with equal definiteness belong 

 to the animals. The line between is by no means 

 sharp, and much difference of opinion exists among 

 investigators as to the borderline forms. 



HISTORY 



The existence of more or less independent forms of 

 life invisible to the naked eye was first proven about 

 two and one-half centuries ago by Van Leeuwenhoek 

 and Kircher, who actually saw and described what 

 were called animalculse. The first conception of the 

 existence of such microscopic forms cannot be accred- 

 ited to these observers, since so long ago as in the 

 fourth century B.C. Aristotle suggested the possibility. 



As might be expected, these single-celled bodies were 

 not seen until the development of lens-making per- 

 mitted accurate enlargement. The greatest advances 

 have been made, furthermore, since the perfection 

 of the compound microscope in the early years of 

 the nineteenth century. It is also noteworthy that 

 those who might be considered the founders of this 

 science, so important to physicians, were botanists 

 and chemists. The most important consideration for 

 the early observers was the relation that these minute 

 bodies bore to the spoiling of food and water. Indeed, 

 most physicians of the past and not a few of the present 

 have discredited the relation of bacteria to disease. 

 The first opinion upon the relation of specific disease- 

 producing bacteria came in the middle of the eighteenth 

 century, but such a theory could not be proven until 



