KOCH'S POSTULATES 281 



and Pollender, who found in the blood of animals suffering from 

 anthrax the now well-known specific bacillus of that disease. 

 Improvements in the microscope and in methods of cultivation 

 (Koch's plate method in particular) soon brought an army of zealous 

 investigators into the field, and during the last thirty years one 

 disease after another has been traced to a bacterial origin. We may 

 summarise the vast collection of historical, physiological, and patho- 

 logical research extending from 1650 to 1904 in three great periods : 

 The period of detection of living, moving cells (Leeuwenhoek and 

 others in the seventeenth century) ; the period of the discovery of 

 their close relationship to fermentation and putrefaction (Spallanzani, 

 Schulze, Schwann, in the eighteenth century); and, thirdly, the 

 period of appreciation of the role of bacteria in the economy of 

 nature and in the production of disease (Tyndall, Pasteur, Lister, 

 Koch, in the nineteenth). 



But we must look less cursorily at the growth of the idea of 

 bacteria as the cause of disease. More than two hundred years ago 

 Eobert Boyle (1627-91), the philosopher who did so much towards 

 the foundation of the present Eoyal Society, wrote an elaborate treatise 

 on The Pathological Part of Physic. He was one of the earliest 

 scientists to declare that a relationship existed between fermentation 

 and disease. When more accurate knowledge was attained respecting 

 fermentation, great advance was consequently made in the etiology 

 of disease. The preliminary discoveries of Fuchs and others between 

 1840 and 1850 had relation to the existence in diseased tissues of 

 a large number of bacteria. But this was no proof that these germs 

 caused disease. It was not till Davaine had inoculated healthy 

 animals with bacilli from the blood of an anthrax carcase, and had 

 thus reproduced the disease, that reliance could be placed upon that 

 bacillus as the vera causa of anthrax. Too much emphasis cannot 

 be laid upon the idea, that unless a certain organism produces in 

 healthy tissues the disease in question, it cannot be considered as 

 proven that the particular organism is related to the disease as 

 cause to effect. In order to secure a standard by which all investi- 

 gators should test their results, Koch introduced four postulates. 

 Until each of the four has been fulfilled, the final conclusion respect- 

 ing the causal agent in any bacterial disease must be considered sub 

 judice. The postulates are as follows : 



(a) The organism must be demonstrated in the circulation or 

 tissues of the diseased animals. 



(b) The organism thus demonstrated must be cultivated in 

 artificial media outside the body, and successive generations of a 

 pure culture of that organism must be obtained. 



(c) Such pure cultures must, when introduced into a healthy and 

 susceptible animal, produce the specific disease. 



