ditions
which would have been described by most observers influenced by the current
doctrine as so many separate "species" or even "genera,"--that in fact
forms known as _Bacterium_, _Micrococcus_, _Bacillus_, _Leptothrix_, &c.,
occur as phases in one life-history. Lister put forth similar ideas about
the same time; and Billroth came forward in 1874 with the extravagant view
that the various bacteria are only different states of one and the same
organism which he called _Cocco-bacteria septica_. From that time the
question of the pleomorphism (mutability of shape) of the bacteria has been
hotly discussed: but it is now generally agreed that, while a [v.03 p.0158]
certain number of forms may show different types of cell during the various
phases of the life-history,[2] yet the majority of forms are uniform,
showing one type of cell throughout their life-history. The question of
species in the bacteria is essentially the same as in other groups of
plants; before a form can be placed in a satisfactory classificatory
position its whole life-history must be studied, so that all the phases may
be known. In the meantime, while various observers were building up our
knowledge of the morphology of bacteria, others were laying the foundation
of what is known of the relations of these organisms to fermentation and
disease--that ancient will-o'-the-wisp "spontaneous generation" being
revived by the way. When Pasteur in 1857 showed that the lactic
fermentation depends on the presence of an organism, it was already known
from the researches of Schwann (1837) and Helmholtz (1843) that
fermentation and putrefaction are intimately connected with the presence of
organisms derived from the air, and that the preservation of putrescible
substances depends on this principle. In 1862 Pasteur placed it beyond
reasonable doubt that the ammoniacal fermentation of urea is due to the
action of a minute Schizomycete; in 1864 this was confirmed by van Tieghem,
and in 1874 by Cohn, who named the organism _Micrococcus ureae_. Pasteur
and Cohn also pointed out that putrefaction is but a special case of
fermentation, and before 1872 the doctrines of Pasteur were established
with respect to Schizomycetes. Meanwhile two branches of inquiry had
arisen, so to speak, from the above. In the first place, the ancient
question of "spontaneous generation" received fresh impetus from the
difficulty of keeping such minute organisms as bacteria from reaching and
devel