302 PASTEUR: THE HISTORY OF A MIND 



would always cause the same reaction, which was a 

 surer means of recognition than their microscopical 

 appearance. Transferred in the same way to a living 

 creature they would produce a definite disease, that is, 

 one which was always the same when the avenue of 

 entrance was the same, and which became thereby a sort 

 of morbid entity: thus bringing us back, in the experi- 

 mental field, to the oldest conceptions of medicine. 

 Studies on the flacherie scarcely modified this point 

 of view. They had shown merely that the microbe, 

 in order to become active, sometimes needed to be aided 

 by external conditions. But when it did act, it always 

 produced the same results. 



In short, when nearly 60 years of age, Pasteur dis- 

 covers facts which are not in accord with this old concep- 

 tion. These relate to the attenuation of virus. One 

 and the same microbial species can invest itself, accord- 

 ing to the culture conditions, with characters which 

 render it unrecognizable to one who has not followed 

 it closely through all of its transitions. I have stated 

 above how Pasteur had endeavored to convince himself 

 that there were in his cultures of the septic vibrio two 

 species of unequal virulence, which the culture conditions 

 enabled him to separate. He refused to admit that these 

 culture conditions could produce them. The same may 

 be said of the chicken cholera. It was the struggle be- 

 tween the old spirit and the new, and one must admire 

 the readiness with which Pasteur abandoned his first 

 conceptions when experiment had taught him that they 

 were not in accord with the facts. 



It was with ardor and without regret that he threw 

 himself into this new path, divining the resources which 

 he would find there for attacking the greatest and most 

 delicate problems of pathology. He could henceforth 

 take up again his old idea of conflict, no longer that 



