274 PASTEUR : THE HISTORY OF A MIND 



reading of the works of Jenner and his followers had 

 left a profound impression on the mind of the master, 

 and by correlating incessantly in his thoughts the 

 teachings of the books and those of the laboratory, he 

 had formed a general impression which I desire to 

 summarize, relying not simply on my own recollections, 

 but also on that of his collaborators at this memorable 

 time. 



On the subject of variation in power of microbes to 

 attack there existed only the curious results obtained by 

 Coze and Feltz in 1869, confirmed since then by Davaine 

 for the anthrax bacteridium, and especially for the dis- 

 ease of Leplat and Jaillard. The virus increased in 

 strength by passage through the organism. The blood 

 of the first animal inoculated was fatal to a second only 

 in a dose, let us say, of one-tenth of a drop. The fatal 

 dose decreased little by little with successive animal 

 passages to that of a hundredth, a thousandth, a millionth 

 of a drop. This fact was the only one of its kind. It 

 was eminently curious and suggestive. It would have 

 been more so if there had not been needed, in 

 order to realize it, the cooperation of the organism, 

 at cross purposes with which everything becomes ob- 

 scure. Men so little dreamed of ascribing the increased 

 virulence to its true origin, the microbe itself, that when 

 Pasteur, in his study of the septic vibrio, finds cultures 

 which prove to be unequally active in animals, his first 

 thought is that he has two or several septic vibrios of 

 unequal virulence, which the cultures have separated 

 more or less completely. Under this impression he 

 carried on investigations for a long time without result. 

 It was only when he discovered that a simple change 

 in culture method, namely, the substitution of a blood 

 serum slightly charged with coagulated fibrine for Liebig's 

 bouillon, suddenly increased the virulence of a vibrio 



