. From about 1870 onwards the "germ theory of disease" has
passed into acceptance. P. F. O. Rayer in 1850 and Davaine had observed the
bacilli in the blood of animals dead of anthrax (splenic fever), and
Pollender discovered them anew in 1855. In 1863, imbued with ideas derived
from Pasteur's researches on fermentation, Davaine reinvestigated the
matter, and put forth the opinion that the anthrax bacilli caused the
splenic fever; this was proved to result from inoculation. Koch in 1876
published his observations on Davaine's bacilli, placed beyond doubt their
causal relation to splenic fever, discovered the spores and the saprophytic
phase in the life-history of the organism, and cleared up important points
in the whole question (figs. 7 and 9). In 1870 Pasteur had proved that a
disease of silkworms was due to an organism of the nature of a bacterium;
and in 1871 Oertel showed that a _Micrococcus_ already known to exist in
diphtheria is intimately concerned in producing that disease. In 1872,
therefore, Cohn was already justified in grouping together a number of
"pathogenous" Schizomycetes. Thus arose the foundations of the modern "germ
theory of disease;" and, in the midst of the wildest conjectures and the
worst of logic, a nucleus of facts was won, which has since grown, and is
growing daily. Septicaemia, tuberculosis, glanders, fowl-cholera, relapsing
fever, and other diseases are now brought definitely within the range of
biology, and it is clear that all contagious and infectious diseases are
due to the action of bacteria or, in a few cases, to fungi, or to protozoa
or other animals.

[Illustration: FIG. 2.--The various phases of germination of spores of
_Bacillus ramosus_ (Fraenkel), as actually observed in hanging drops under
very high powers.

A. The spore sown at 11 A.M., as shown at a, had swollen (b) perceptibly by
noon, and had germinated by 3.30 P.M., as shown at c: in d at 6 P.M., and e
at 8.30 P.M.; the resulting filament is segmenting into bacilli as it
elongates, and at midnight (f) consisted of twelve such segments.

B, C. Similar series of phases in the order of the small letters in each
case, and with the times of observation attached. At f and g occurs the
breaking up of the filament into rodlets.

D. Germinating spores in various stages, more highly magnified, and showing
the different ways of escape of the filament from the spore-membrane. (H.
M. W.) ]

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