CHAPTER XIII 



THE WORK OF PASTEUR, KOCH, AND OTHERS 



The knowledge of bacteria, those minutest forms of life, 

 has exerted a profound influence upon the development of 

 general biology. There are many questions relating to bac- 

 teria that are strictly medical, but other phases of their life 

 and activities are broadly biological, and some of those 

 broader aspects will next be brought under consideration. 



The bacteria were first described by Leeuwenhoek in 

 1687, twelve years after his discovery of the microscopic 

 animalcula now called protozoa. They are so infinitesimal 

 in size that under his microscope they appeared as mere 

 specks, and, naturally, observation of these minute organ- 

 isms was suspended until nearly the middle of the nineteenth 

 century, after the improvement of microscope lenses. It is 

 characteristic of the little knowledge of bacteria in Linnaeus's 

 period that he grouped them into an order, with other micro- 

 scopic forms, under the name chaos. 



At first sight, the bacteria appear too minute to figure 

 largely in human affairs, but a great department of natural 

 science — bacteriology — has been opened by the study of their 

 activities, and it must be admitted that the development of 

 the science of bacteriology has been of great practical im- 

 portance. The knowledge derived from experimental studies 

 of the bacteria has been the chief source of light in an obscure 

 domain which profoundly affects the well-being of mankind. 

 To the advance of such knowledge we owe the germ-theory 

 of disease and the ability of medical men to cope with con- 



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