very complex. Plants
appear to be less subject to their attacks--possibly, as has been
suggested, because the acid fluids of the higher vegetable organisms are
less suited for the development of Schizomycetes; nevertheless some are
known to be parasitic on plants. Schizomycetes exist in every part of the
alimentary canal of animals, except, perhaps, where acid secretions
prevail; these are by no means necessarily harmful, though, by destroying
the teeth for instance, certain forms may incidentally be the forerunners
of damage which they do not directly cause.

[Sidenote: History.]

Little was known about these extremely minute organisms before 1860. A. van
Leeuwenhoek figured bacteria as far back as the 17th century, and O. F.
Mueller knew several important forms in 1773, while Ehrenberg in 1830 had
advanced to the commencement of a scientific separation and grouping of
them, and in 1838 had proposed at least sixteen species, distributing them
into four genera. Our modern more accurate though still fragmentary
knowledge of the forms of Schizomycetes, however, dates from F. J. Conn's
brilliant researches, the chief results of which were published at various
periods between 1853 and 1872; Cohn's classification of the bacteria,
published in 1872 and extended in 1875, has in fact dominated the study of
these organisms almost ever since. He proceeded in the main on the
assumption that the forms of bacteria as met with and described by him are
practically constant, at any rate within limits which are not wide:
observing that a minute spherical micrococcus or a rod-like bacillus
regularly produced similar micrococci and bacilli respectively, he based
his classification on what may be considered the constancy of forms which
he called species and genera. As to the constancy of form, however, Cohn
maintained certain reservations which have been ignored by some of his
followers. The fact that Schizomycetes produce spores appeals to have been
discovered by Cohn in 1857, though it was expressed dubiously in 1872;
these spores had no doubt been observed previously. In 1876, however, Cohn
had seen the spores germinate, and Koch, Brefeld, Pratzmowski, van Tieghem,
de Bary and others confirmed the discovery in various species.

The supposed constancy of forms in Cohn's species and genera received a
shock when Lankester in 1873 pointed out that his _Bacterium rubescens_
(since named _Beggiatoa roseo-persicina_, Zopf) passes through con