o point out the
open doorways through which the affirmers of such transmutations had
allowed error to march in upon them. [Footnote: 'Those who wish for an
illustration of the care necessary in these researches, and of the
carelessness with which they have in some cases been conducted, will
do well to consult the Rev. W. H. Dallinger's excellent 'Notes on
Heterogenesis' in the October number of the Popular Science Review.]

The great source of error here has been already alluded to in this
discourse. The observers worked in an atmosphere charged with the
germs of different organisms; the mere accident of first possession
rendering now one organism, now another, triumphant. In different
stages, moreover, of its fermentative or putrefactive changes, the
same infusion may so alter as to be successively taken possession of
by different organisms. Such cases have been adduced to show that the
earlier organisms must have been transformed into the later ones,
whereas they are simply cases in which different germs, because of
changes in the infusion, render themselves valid at different times.

By teaching us how to cultivate each ferment in its purity--in other
words, by teaching us how to rear the individual organism apart from
all others,--Pasteur has enabled us to avoid all these errors. And
where this isolation of a particular organism has been duly effected
it grows and multiplies indefinitely, but no change of it into another
organism is ever observed. In Pasteur's researches the Bacterium
remained a Bacterium, the Vibrio a Vibrio, the Penicillium a
Penicillium, and the Torula a Torula. Sow any of these in a state of
purity in an appropriate liquid; you get it, and it alone, in the
subsequent crop. In like manner, sow small-pox in the human body,
your crop is small-pox. Sow there scarlatina, and your crop is
scarlatina. Sow typhoid virus, your crop is typhoid--cholera, your
crop is cholera. The disease bears as constant a relation to its
_contagium_ as the microscopic organisms just enumerated do to their
germs, or indeed as a thistle does to its seed. No wonder then, with
analogies so obvious and so striking, that the conviction is spreading
and growing daily in strength, that reproductive parasitic life is at
the root of epidemic disease--that living ferments finding lodgment in
the body increase there and multiply, directly ruining the tissue on
which they subsist, or destroying life indirectly by the gene