produces them in excess. The receptors so produced pass into the
blood, where they combine with the toxine which has been absorbed; the
combination is a stable one, and the toxine is thus prevented from
combining with the tissue cells. The antitoxine which is formed during
the disease, and the production of which in the horse can be
enormously stimulated by the injection of toxine, represents merely
the excess of cell receptors, and when the serum of the horse
containing them is injected in a case of diphtheria the same
combination takes place as in the case of receptors provided by the
patient. In the case of the destruction of bacteria in the blood by
the action of amboceptor and complement, the amboceptor must be able
to combine with both the bacterial cell and the complement which
brings about its destruction, and just as antitoxine is formed so new
amboceptors may be formed.

Few hypotheses have been advanced in science which are more ingenious,
in better accord with the facts, have had greater importance in
enabling the student to grasp the intricacies of an obscure problem,
and which have had an equal influence in stimulating research. The
immunity which results from disease in accordance with this theory, is
due not to conditions preventing the entrance of organisms into the
body, but to greater aptitude on the part of the cells to produce
these protective substances having once learned to do so. An
individual need not practise for many years, having once learned them,
those combinations of muscular action used in swimming; but the habit
at once returns when he falls into the water.

Infectious diseases and recovery are phases of the struggle for
existence between parasite and host, and illustrate the power of
adaptation to environment which is so striking a characteristic of