oping in organic infusions; and, secondly, the long-suspected
analogies between the phenomena of fermentation and those of certain
diseases again made themselves felt, as both became better understood.
Needham in 1745 had declared that heated infusions of organic matter were
not deprived of living beings; Spallanzani (1777) had replied that more
careful heating and other precautions prevent the appearance of organisms
in the fluid. Various experiments by Schwann, Helmholtz, Schultz,
Schroeder, Dusch and others led to the refutation, step by step, of the
belief that the more minute organisms, and particularly bacteria, arose _de
novo_ in the special cases quoted. Nevertheless, instances were adduced
where the most careful heating of yolk of egg, milk, hay-infusions, &c.,
had failed,--the boiled infusions, &c., turning putrid and swarming with
bacteria after a few hours.

In 1862 Pasteur repeated and extended such experiments, and paved the way
for a complete explanation of the anomalies; Cohn in 1872 published
confirmatory results; and it became clear that no putrefaction can take
place without bacteria or some other living organism. In the hands of
Brefeld, Burdon-Sanderson, de Bary, Tyndall, Roberts, Lister and others,
the various links in the chain of evidence grew stronger and stronger, and
every case adduced as one of "spontaneous generation" fell to the ground
when examined. No case of so-called "spontaneous generation" has withstood
rigid investigation; but the discussion contributed to more exact ideas as
to the ubiquity, minuteness, and high powers of resistance to physical
agents of the spores of Schizomycetes, and led to more exact ideas of
antiseptic treatments. Methods were also improved, and the application of
some of them to surgery at the hands of Lister, Koch and others has yielded
results of the highest value.

Long before any clear ideas as to the relations of Schizomycetes to
fermentation and disease were possible, various thinkers at different times
had suggested that resemblances existed between the phenomena of certain
diseases and those of fermentation, and the idea that a virus or contagium
might be something of the nature of a minute organism capable of spreading
and reproducing itself had been entertained. Such vague notions began to
take more definite shape as the ferment theory of Cagniard de la Tour
(1828), Schwann (1837) and Pasteur made way, especially in the hands of the
last-named savant