preserve an action on a living species. Let us take for
example the most attenuated, that which is barely able
to kill a guinea-pig a day old: if we inoculate its blood
into a guinea-pig of the same age, that of the second
animal into a third, and so on, we shall shortly see the
virulence of the bacteridium return little by little.
Soon we shall be able to kill with it guinea-pigs three or
four days old, a week, a month old, and finally sheep.
By successive cultures in living media, the bacteridium
has been restored to its original virulence.

It is justifiable to form out of these facts a general rule,
in accordance with our theory. A microbe introduced
into the body of an animal is not living under the same
conditions as one sown in an inert vessel; it is subjected
to the pressing alternative of living or dying, of being
victorious or vanquished. Vanquished, its history is
soon written; victorious, it will come out of the struggle
strengthened, that is to say, having complied with the
conditions of its new medium, it is better prepared to
accommodate itself therein anew. If it is transferred
several times from individual to individual of the same
race, without having been influenced by external con-
ditions in the interim between two passages, we may
expect to see its virulence augmented and in some degree
fixed for the race and for the customary mode of trans-
mission in this race. Thus the bacteridium of sheep an-
thrax, for example, living for a long time on our soil, is
acclimated to some degree in the race which shelters it,
and its virulence varies little from one subject to another,
and from one year to another for the same country,
The same thing is true, to a certain extent, for Jenner's
vaccine, if it is transferred directly from arm to arm on
unvaccinated healthy individuals, and if it is carefully
preserved between the two operations. The same thing
is also true for the virus of rabies administered by tre-

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