4 MICRO-ORGANISMS AND DISEASE [intro. 



organisms ; to go on cultivating them from one cultivation 

 to another for several successive generations, in order to 

 obtain them free of every kind of matter derived from the 

 animal body from which they have been taken in the first 

 instance. (3) After having thus cultivated the micro- 

 organisms for several successive generations it is necessary 

 to re-introduce them into the body of a healthy animal 

 susceptible to the disease, and in this way to show that this 

 animal becomes affected with the same disease as the one 

 from which the organisms were originally derived. (4) And, 

 finally, it is necessary that in this so affected new animal the 

 same micro-organisms should again be found. A particular 

 micro-organism may probably be the cause of a particular 

 disease, but that really and unmistakably it is so can only be 

 inferred with certainty when every one of these desiderata 

 has been satisfied. 



Now, at the time when Koch laid down these principles, 

 which being of the nature of exactness were accepted by 

 all, there still existed amongst medical men a considerable 

 number who doubted that microbes have any primary 

 relation to disease, the doctrine of contagium vivum was 

 still to them an unproven view, although the practice in 

 sanitary science had long accepted the correctness of that 

 view. And it may be said to have been one of the most 

 complete and exact achievements of Koch to have been the 

 first who by exact methods conclusively demonstrated the 

 correctness of the view in the case of malignant anthrax. 

 The same principles led him to the demonstration of the 

 causative relation between certain microbes and septi- 

 cfemic infections in animals, and he may be said to have 

 crowned the edifice by his brilliant discovery of the tubercle 

 bacilli. In all these instances it was possible for him to 

 absolutely and by exact methods and without cavil to 



