ich was extended in 1875; this
scheme has in fact dominated the study of bacteria ever since. Zopf in 1885
proposed a scheme based on the acceptance of extreme views of pleomorphism;
his system, however, was extraordinarily impracticable and was recognized
by him as provisional only. Systems have also been brought forward based on
the formation of arthrospores and endospores, but as explained above this
is eminently unsatisfactory, as arthrospores are not true spores and both
kinds of reproductive bodies are found in one and the same form. Numerous
attempts have been made to construct schemes of classification based on the
power of growing colonies to liquefy gelatine, to secrete coloured
pigments, to ferment certain media with evolution of carbon dioxide or
other gases, or to induce pathological conditions in animals. None of these
systems, which are chiefly due to the medical bacteriologists, has
maintained its position, owing to the difficulty of applying the characters
and to the fact that such properties are physiological and liable to great
fluctuations in culture, because a given organism may vary greatly in such
respects according to its degree of vitality at the time, its age, the mode
of nutrition [v.03 p.0163] and the influence of external factors on its
growth. Even when used in conjunction with purely morphological characters,
these physiological properties are too variable to aid us in the
discrimination of species and genera, and are apt to break down at critical
periods. Among the more characteristic of these schemes adopted at various
times may be mentioned those of Miquel (1891), Eisenberg (1891), and
Lehmann and Neumann (1897). Although much progress has been made in
determining the value and constancy of morphological characters, we are
still in need of a sufficiently comprehensive and easily applied scheme of
classification, partly owing to the existence in the literature of
imperfectly described forms the life-history of which is not yet known, or
the microscopic characters of which have not been examined with sufficient
accuracy and thoroughness. [Sidenote: Fischer's Scheme.] The principal
attempts at morphological classifications recently brought forward are
those of de Toni and Trevisan (1889), Fischer (1897) and Migula (1897). Of
these systems, which alone are available in any practical scheme of
classification, the two most important and most modern are those of Fischer
and Migula. The extended i