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outside the sphere of their mental apprehension as
chimerical or else theoretical, and in consequence unworthy
of consideration; but if their reasoning capacity could be
only a little extended, if they could lift up, as it were, the
fringe of the curtain that bounds their mental horizon, they
would clearly see that those things which are true in theory
are also true in practice—that the theory of to-day is the
practice of to-morrow—that theory is based upon fact and
differs fundamentally from mere conjecture or hypothesis.
Until the theory of the poisonous effects of sewer gas had a
practical demonstration in repeated illnesses and frequent
deaths among those who had been exposed to its influence,
builders continued to disregard the teaching of science, and
to construct the drains of houses in such manner as though
an atmosphere pervaded by sewer gas was a thing to be
desired, all remonstrance to the contrary notwithstanding.
All infectious diseases are caused by the agency of microbes.
The germ theory of disease is no longer a conjecture, nor is
it founded upon the research or statement of any one class
of observers. Any scientific man trained in the methods of
research may verify for himself the fact of the existence, and
trace the growth and development, of these minute enemies
to mankind.
The species of microbes which cause disease are numerous;
every specific disease is caused by its specific microbe, and
therefore it is not in accord with scientific teaching to suppose
that one specific disease may be metamorphosed into another.
Yet this inconsistency in reasoning is a prevalent one—
one frequently hears that an ulcerated throat has turned into
diphtheria, or a pneumonia into typhoid. When the farmer
sows a field, if the seed be wheat, the crop will be also
wheat, if he sow oats he may expect, and that reasonably, to
reap oats; if reasoning is to be relied upon at all, it must be