virulent bacteridium, others with the attenuated bacteri-
dium. The first develops and kills the sheep. The
second, after a period of growth made with more or less
difficulty and causing a transitory illness of the sheep,
abandons the struggle and leaves the animal more or
less vaccinated. This is one method of studying the
influence of the bacteridium alone.

Now elt us take a normal sheep and a vaccinated
sheep, into which we inoculate a very virulent strain of
the anthrax bacteridium. It kills the first and has no
effect on the second. Here we have a way to study the
influence of immunity acquired by a former vaccination.

Let us take now a French sheep and an Algerian sheep;
let us inoculate both of them with a light dose of a viru-
lent strain of the bacteridium. The first will die, the
second will resist, after an illness in general benign. In
this we have the influence of race or of natural immunity.

The French sheep has a natural immunity for the
attenuated bacteridium; the Algerian sheep, a natural
immunity against the virulent disease; the vaccinated
sheep an acquired immunity, more or less marked; the
dog, a natural and absolute immunity. In all the cases,
the natural or acquired immunity, when it is complete, is
correlative with the non-development of the bacteridium,
which instead of invading the tissues, remains confined
to the point of inoculation or its vicinity, and finally
perishes there.

What is the cause of this non-development of a
living cell which has been sown? This question is what
the inquiry led to! We see that it was precise. It was
already a conquest only to be able to state it thus. Until
that time it had been necessary to bow down without
seeking to penetrate the mystery. What reply, in fact,
can be given to this general question: Why is the sheep
sensitive to anthrax, and the dog not sensitive? Why

-313-