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 Linnzeus’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- 
276 
