which for 20 or 30 generations had shown itself to be
attenuated, that he accepted the idea that these varia-
tions depended on one single vibrio and its culture
medium.

It was a great step indeed; but beyond this there was
nothing, and in order to see farther it was necessary
to consider the virus diseases. The latter presented
facts analogous to those of Coze and Feltz, and Davaine.
It was known that there were benign epidemics of smallpox
and others that were deadly, that the severity was
variable in the course of the same epidemic, and generally
diminished as it drew to a close. It was also known from
the practise of smallpox inoculation, resorted to before the
time of Jenner, that inoculation from a benign case of
smallpox ordinarily produced a smallpox still more
benign, but this was not always true, for sometimes
the inoculated patient died.

The vaccine introduced by Jenner had been a wonder-
ful discovery, but it had made the veil still thicker
behind which the virus diseases lay concealed. With
it variations in virulence were scarcely to be feared.
After being very clearly diminished in passing from
the cow to the man, the virulence of the vaccine was
maintained very constantly from arm to arm, for a
long series of generations. But if there was something
immutable in the severity of the disease or in its period
of evolution, there was, on the contrary, great variation
in the duration of the immunity which it produced.
So that, to sum up, the ideas which seem to us to-day
the most closely related, the most coherent, were at
that time scattered and contradictory, and no one
attempted to correlate them.

It is here that Pasteur experienced the benefit of
his former studies and of facts which he alone knew,
since he had published them only in part, and in that

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