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70                  PA THOGENIC

and suffered to go unexplained. We have explanations,
but, unfortunately, they are as intricate as the phenomena,
and, though each may possess its grain of truth, not one
will satisfy the demands of the thoughtful student. ^ In
brief review, the theories of immunity are the following- :

1.  THE EXHAUSTION THEORY.—This hypothesis was
advanced by Pasteur in 1880, and suggests that by its
growth in the body the micro-organism  uses up some
substance essential to its life, and that when this sub-
stance is exhausted the microbe can  no longer thrive.
The removal of the necessary material, if complete, will
cause permanent immunity.

As Sternberg points out, were this theory true we must
have within us a material of small-pox, a material of
measles, a material of scarlet fever, etc., to be exhausted
by its appropriate organism. It would necessitate an
almost inconceivably complex body-chemistry and a
rather stable condition of the same.

2.  THE  RETENTION  THEORY.—In   the   same   year
Chauveau  suggested that the growth  of   the  bacteria
in the body might originate some substance prejudicial
to their further and future development.    There seems
to be a large kernel of truth in this, but were it always
the case we would have added to our blood a material
of small-pox, a material of measles, a material of scarlet
fever, etc., so that we would become  saturated with the
excrementitious products of the bacteria, instead of hav-
ing so many substances subtracted from our chemistry.

Roser suggested a relation between   immunity and the
already familiar  phenomenon  of phagocytosis.     Stern-
berg in the United States and Koch in Germany observed
the same thing, but little real attention was paid to the
subject until 1884, when Metschnikoff appeared, with his
careful observations upon the daphnia, as the great cham-
pion of the theory which is now known as " Metschni-
koff 's theory of phagocytosis."

Phagocytosis is the swallowing  or incorporating of