THE DEVELOPMENT 7 



chemists from early days are the phenomena of fermenta- 

 tion, putrefaction, and decay. It has long been known, 

 for example, that if fruit juices are kept under the right 

 conditions fermentation will occur and alcohol (spirits of 

 wine) is formed. This was early regarded by the chemists 

 as purely a chemical transformation. During the first half 

 of the nineteenth century, however, it was repeatedly ob- 

 served that alcoholic fermentation was always accompanied 

 by the development of minute or microscopic cells which 

 were called yeasts. It was conjectured by those who ob- 

 served these living cells that they might be the cause of the 

 fermentation. Chemists opposed this conjecture and ar- 

 gued that these minute objects were the results and not the 

 cause of the fermentation. Here again it was necessary 

 to take the subject into the laboratory in order to decide 

 whether microorganisms caused fermentation or were the 

 result of fermentation. The great German chemist Liebig 

 argued for the latter, and Pasteur for the former concep- 

 tion. The controversy between these two schools of thought 

 waged for many years, but was at last settled conclusively 

 by the laboratory experiments of Pasteur. This investiga- 

 tor demonstrated beyond reasonable doubt that practically 

 all fermentation, putrefaction, and decay, are due to the 

 presence and growth of microorganisms, for the most part 

 bacteria, yeasts, and molds. In the course of the contro- 

 versy, however, many methods of laboratory work were de- 

 veloped, and many new species of microorganisms dis- 

 covered. 



Controversy over the Theory of Pleomorphism. It is 

 comparatively uncommon in nature to find single kinds or 

 species of bacteria growing by themselves apart from all 

 other species, or as the modern bacteriologist terms it "in 

 pure cultures." This fact was not recognized by some of 

 the earlier investigators, and led to a long argument or 

 controversy over the theory of pleomorphism. About 1870 



