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 coöperation 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

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