illus subtilis_, Cohn, and _Spirillum undula_, Ehrenb.
B. _Planococcus citreus_ (Menge) Migula.
C. _Pseudomonas pyocyanea_ (Gessard), Migula.
D. _P. macroselmis_, Migula.
E. _P. syncyanea_ (Ehrenb.), Migula.
F. _Bacillus typhi_, Gaffky.
G. _B. vulgaris_ (Hauser), Migula.
H. _Microspira Comma_ (Koch), Schroeter.
J, K. _Spirillum rubrum_, Esmarsch.
L, M. _S. undula_ (Mueller), Ehrenb. (_All after Migula._) ]

[Sidenote: Distribution in Time.]

That bacteria have existed from very early periods is clear from their
presence in fossils; and although we cannot accept all the conclusions
drawn from the imperfect records of the rocks, and may dismiss as absurd
the statements that geologically immured forms have been found still
living, the researches of Renault and van Tieghem have shown pretty clearly
that large numbers of bacteria existed in Carboniferous and Devonian times,
and probably earlier.

[Sidenote: Distribution in Space.]

Schizomycetes are ubiquitous as saprophytes in still ponds and ditches, in
running streams and rivers, and in the sea, and especially in drains, bogs,
refuse heaps, and in the soil, and wherever organic infusions are allowed
to stand for a short time. Any liquid (blood, urine, milk, beer, &c.)
containing organic matter, or any solid food-stuff (meat preserves,
vegetables, &c.), allowed to stand exposed to the air soon swarms with
bacteria, if moisture is present and the temperature not abnormal. Though
they occur all the world over in the space, air and on the surface of
exposed bodies, it is not to be supposed that they are by any means equally
distributed, and it is questionable whether the bacteria suspended in the
air ever exist in such enormous quantities as was once believed. The
evidence to hand shows that on heights and in open country, especially in
the north, there may be few or even no Schizomycetes detected in the air,
and even in towns their distribution varies greatly; sometimes they appear
to exist in minute clouds, as it were, with interspaces devoid of any, but
in laboratories and closed spaces where their cultivation has been promoted
the air may be considerably laden with them. Of course the distribution of
bodies so light and small is easily influenced by movements, rain, wind,
changes of temperature, &c. As parasites, certain Schizomycetes inhabit and
prey upon the organs of man and animals in varying degrees, and the
conditions for their growth and distribution are then