hant, the snake, and the sacred cow, so this age
should erect its temples to the guinea-pig. From one of the most
trifling and unimportant,--kept merely as a pet and curiosity by the
small boys of all ages,--he has become, after the horse, the cow, the
pig, and the sheep, easily our most useful and important domestic
animal. It may be urged that he deserves no credit, since his
sacrifice--though of inestimable value--was entirely involuntary on his
own part; but this should only make us the more deeply bound to
acknowledge our obligation to him.

By a stern necessity of fate, which no one regrets more keenly than the
laboratory workers themselves, the guinea-pig has had to be used as a
stepping-stone for every inch of this progress. Upon it were conducted
every one of the experiments whose results widened our knowledge, until
we found that this bacillus and no other would cause diphtheria; that
instead of getting, like many other disease-germs, into the blood, it
chiefly limited itself to growing and multiplying upon a comparatively
small patch of the body-surface, most commonly of the throat; that most
of its serious and fatal results upon the body were produced, not by the
entrance of the germs themselves into the blood, but by the absorption
of the toxins or poisons produced by them on the moist surface of the
throat, just as the yeast plant will produce alcohol in grape juice or
sweet cider.

Here was a most important clew. It was not necessary to fight the germs
themselves in every part of the body, but merely to introduce some
ferment or chemical substance which would have the power of neutralizing
their poison. Instantly attention was turned in this direction, and it
was quickly found that if a guinea-pig were injected with a very small
dose of the diphtheria toxin and allowed to recover, he would then be
able to throw off a still larger dose, until finally, after a number of
weeks, he could be given a dose which would have promptly killed him in
the beginning of the experiments, but which he now readily resisted and
recovered from. Evidently some substance was produced in his blood which
was a natural antidote for the toxin, and a little further search
quickly resulted in discovering and filtering out of his body the now
famous antitoxin. A dose of this injected into another guinea-pig
suffering from diphtheria would promptly save its life.

Could this antitoxin be obtained in sufficient amounts to protect the