352 A TEXTBOOK OF THEORETICAL BOTANY 



by Devaine in 1863. These observations were taken up and extended by the 

 great work of Pasteur (Fig. 340), in whose hands the " germ theory " of 

 disease developed into an accepted part of medical science. The number 

 of organisms reputed to cause diseases, of the most varied kinds, extended 



Fig. 340. — Portrait of Louis Pasteur. 



rapidly, and the multiplication of doubtful claims led Robert Koch, the 

 discoverer of the bacillus of tuberculosis, to formulate four " postulates " 

 or conditions which must be fulfilled before such a claim could be accepted. 

 These postulates are recognized to be the foundation stones of clinical 

 Bacteriology. They are : — 



1. The organism must always be associated with the disease. 



2. It must be isolated in pure culture. 



3. A pure culture inoculated into a healthy and susceptible animal must 



reproduce the disease. 



4. It must be re-isolated in pure culture from the inoculated animal. 



At the present time a very large number of diseases, especially fevers, 

 are proved to be of bacterial origin, and the ancient mystery of infection is 

 known to depend on the transference of living Bacteria from man to man, 

 either directly or indirectly. To enter further into the ramifications of medical 

 Bacteriology would, however, be impossible in a textbook of Botany. 



Pasteur's interest in Bacteria sprang from his long controversy on the 

 subject of spontaneous generation. It was for long supposed that putre- 

 faction was a spontaneous phenomenon, and when it became known that 

 decomposing matter swarmed with microscopic organisms they were 



