THE PROBLEM OF IMMUNITY 301 



modality and its direction, it was necessary that the 

 idea of disease also be changed, and we have already 

 seen the efforts made to ascribe physico-chemical origins 

 to pathology. In this conception the old idea of strife 

 had entirely disappeared, and although, for Virchow, 

 a tumor was a physiological development misplaced in 

 time and space, that is to say, produced where it ought not 

 be, and at a time which was not its own, it was difficult 

 to see therein anything which resembled the conception 

 that made the disease something at war with the vital 

 force. 



It is for this reason that the physiologists were so 

 opposed to the microbian doctrines. The microbe, 

 producing a chemical phenomenon or causing a disease, 

 was the sudden reappearance of the vital force in regions 

 from which it was desired to eliminate it. The idea of 

 the microbe brought back in the clearest manner the 

 idea of conflict, of strife for the necessities of life, of the 

 struggle for existence. Such is the idea which Pasteur, 

 more than any one else, was instrumental in introduc- 

 ing into science and into pathology. 



This idea in its turn underwent some transformations 

 in his mind. At the time of the publication of his 

 Etudes sur la maladie des vers a soie, the microbe was for 

 him a pathogenic cause external to the organism, func- 

 tioning simply and in some measure irresistibly. In 

 order to be rid of the disease, the parasite must be cUs- 

 posed of. This is what Pasteur had done for the corpus- 

 cle of the pebrine. It was what had been done before 

 him for the muscardine fungus and the itch mite. 



This rather absurd conception of bacterial diseases 

 was for Pasteur in perfect accord with what he then knew 

 of microbes. He believed that the bacterial species 

 were nearly constant in form and possessed immutable 

 properties. Transferred from medium to medium they 



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