546 Chapter XVII 



because it contained some substance injurious to micro-organisms. 

 [570] Very numerous and detailed researches have demonstrated the 

 incorrectness of these hypotheses. There are, of course, certain 

 pathogenic micro-organisms which are very exacting as regards the 

 medium in which they will grow. Some will develop only in the 

 presence of particular substances, whilst others are extremely sensi- 

 tive to the slightest traces of poisons. These, however, are quite 

 the exception. The great majority of pathogenic micro-organisms 

 belonging to the group of bacteria readily adapt themselves to 

 all kinds of culture media, and most of them live and develop 

 freely in the blood or other fluids of refractory organisms. This, 

 therefore, is not the cause of the immunity in such organisms. The 

 cause must be sought for amongst factors more closely connected 

 with life. 



Wishing to penetrate more deeply into these phenomena the 

 hypothesis was put forward that the unharmed organism got rid 

 of the infective micro-organisms by expelling them to the outside 

 along with the excreta. It was maintained for a considerable time 

 that the animal organism possessed the means of causing pathogenic 

 bacteria to pass into the kidneys, whence they were eliminated by 

 the urine. It had to be acknowledged, however, that this elimination 

 never takes place in cases of immunity, and only comes into operation 

 when the animal is ill and the integrity of the renal filter is im- 

 paired. 



The infective micro-organisms, after they have entered into the 

 unharmed organism, remain there for a longer or shorter period, 

 and perish without being expelled. This disappearance of the 

 micro-organisms takes place by the same mechanism that rids 

 the plasmodium of those bacteria which it has managed to ingest 

 during its slow peregrinations over dead leaves or rotten wood. 

 The micro-organisms are absorbed into the refractory organisms 

 as the result of a true act of digestion. It is very remarkable that 

 the gastro-intestinal ingestion, so well provided with means of render- 

 ing the most varied aliments soluble, is generally incapable of 

 digesting pathogenic or other micro-organisms. It is very rare to 

 meet with soluble ferments of the intestinal canal which are capable 

 of digesting microscopic organisms, especially bacteria. Consequently 

 this organ, so rich in digestive diastases, is generally inhabited by 

 a large number of bacteria and other micro-organisms. 



Even in animals whose food contains large numbers of micro- 



