INTEODUCTION 5 



garded the bacteria as animals. Ferdinand Cohn (1872) recog- 

 nized the nature of bacterial spores, showed the close relationship 

 of bacteria to the algae and estabUshed their classification in the 

 plant kingdom. He distinguished six genera — micrococcus, 

 bacterium, bacillus, vibrio, spirillum and spirochaeta. Migula 

 (1897) undertook an extensive revision of bacteriological nomen- 

 clature and classification, basing it upon morphological characters, 

 and his system is doubtless the most satisfactory yet offered. 

 The subject is still in a very unsettled state, nevertheless, and 

 there is no system of classification generally accepted by bac- 

 teriologists. The problem presents so many difficulties and our 

 knowledge of the bacteria is still so incomplete that many authori- 

 ties seem prone to consign systematic classification to the future, 

 and to employ names of sufficient historical prominence to insure 

 their correct interpretation. 



Fermentation and Putrefaction.— The relation of micro- 

 organisms to the decomposition of organic matter, fermentation 

 and putrefaction, was one of the first fields of applied bacteri- 

 ology to be studied. Following the observation of bacteria in 

 saliva by van Leeuwenhoek in 1683, micro-organisms were dis- 

 covered in all sorts of decomposing material. At first, these or- 

 ganisms were regarded as unimportant for the chemical process 

 and interest attached chiefly to the question of their origin, 

 whether by spontaneous generation or from previously living 

 cells. Needham (1745) directing his attention more particularly 

 to this first question, boiled an infusion of meat and, keeping it free 

 from contact with the air, nevertheless observed after some days 

 the presence of "infusoria." Spallanzani (1765) repeated Need- 

 ham's experiments, subjecting hermetically sealed flasks of meat 

 infusion to the temperature of boiling water for one hour, and 

 he found no subsequent development of life and no decomposition 

 of the infusion as long as it remained sealed. While discussion 

 continued concerning the discrepancy between the results of 

 Needham and Spallanzani and concerning the relation which 

 the subsequent exclusion of the air might bear to the absence 



