cesses
and exuberance of destructive microbes, and we are now able to see
that it is in adopting her methods that our best hope of increasing
that protection lies. Nature is satisfied if the efficacy of her
defence is sufficient to save enough individuals to carry on the race.
Man desires in the case of his own fellows to out-do Nature and to
save all.

A century and a half ago, before the true character of infective
disease was understood, it was observed that an individual who was
attacked by the smallpox and recovered became incapable of receiving
the infection again. He was "protected" or "immune." The practice of
"inoculation" was introduced from the East by Lady Montague. The
infectious matter was introduced from a smallpox patient into the
person to be protected by rubbing it into a scarified part of the
skin. A much less severe attack of smallpox was thus produced than
that which usually followed the natural infection, which (though we do
not know precisely its mode of entrance) is more widely spread through
the blood. At the same time the condition of "immunity" after the
attack was brought about with equal efficacy. When Jenner introduced
inoculation with "cowpox" for the purpose of establishing "immunity"
in the vaccinated person, inoculation with smallpox itself was a very
usual practice. It was open to the objection that sometimes an
unexpectedly violent attack of the disease was produced, resulting in
death, and that the active infection was kept alive and ever present
in the community. The notion with regard to the mode in which
"immunity" was produced by either the Montacutian or Jennerian
inoculation was, even after the general knowledge of microbes as the
living contagion of disease had been arrived at, that the mild attack
due to inoculation "used up" something in the blood--in fact,
exhausted the soil, so that the infective matter or microbe could no
longer flourish in the blood. And this view was accepted as the
explanation of the "immunity" to the anthrax disease conferred on
cattle and sheep by Pasteur's inoculations of weakened, but still
actively growing, cultures of the anthrax bacillus. Another theory was
that they produced something in the blood by their own life-processes
which checked their further growth, just as yeast will not grow in
wort in which it has produced 8 per cent. of alcohol, and as a fire
may be choked by its own smoke or ashes.

We now know that both these explanations of