STUDIES IN FERMENTATION 73 



material to which its special ferment had not been 

 added. Both kinds of fermentation often occur in 

 the same material in which bits of cheese or other 

 decomposable substances are placed or which have 

 simply been exposed to the air. Still other kinds 

 of fermentations might occur, the products ob- 

 tained varying greatly according to circumstances 

 and often for reasons that appear quite accidental 

 or capricious. Pasteur recognized in these varied 

 phenomena the influence of mixtures of different 

 organisms which might have been introduced in 

 different proportions from the floating matter of 

 the air or from the rotten cheese or meat used to 

 start up the fermentation. He observed in such 

 cases yeast plants along with the organisms found 

 in lactic acid fermentation, and also other organ- 

 isms, some of which were bacteria, while others 

 were forms that he referred to as "infusoria." 



"We may compare," Pasteur says, "what goes on 

 in fermentation with what is produced in a field in 

 which one sows no seed. One soon sees it inhab- 

 ited by diverse plants and insects which are mu- 

 tually destructive." Pure fermentations were found 

 to contain predominantly but one characteristic 

 kind of organism, while the mixed fermentations 

 contained two or more kinds in variable propor- 



