n the soil are due to this
co-operation of oxygen-protecting species with anaerobic ones, _e.g._
_Tetanus_.

[Illustration: FIG. 21.--A plate-culture colony of a species of
_Bacillus--Proteus_ (Hauser)--on the fifth day. The flame-like processes
and outliers are composed of writhing filaments, and the contours are
continually changing while the colony moves as a whole. Slightly magnified.
(H. M. W.)]

[Sidenote: Activity of bacteria.]

Astonishment has been frequently expressed at the powerful activities of
bacteria--their rapid growth and dissemination, the extensive and profound
decompositions and fermentations induced by them, the resistance of their
spores to dessication, heat, &c.--but it is worth while to ask how far
these properties are really remarkable when all the data for comparison
with other organisms are considered. In the first place, the extremely
small size and isolation of the vegetative cells place the protoplasmic
contents in peculiarly favourable circumstances for action, and we may
safely conclude that, weight for weight and molecule for molecule, the
protoplasm of bacteria is brought into contact with the environment at far
more points and over a far larger surface than is that of higher organisms,
whether--as in plants--it is distributed in thin layers round the
sap-vacuoles, or--as in animals--is bathed in fluids brought by special
mechanisms to irrigate it. Not only so, the isolation of the cells
facilitates the exchange of liquids and gases, the passage in of food
materials and out of enzymes and products of metabolism, and thus each unit
of protoplasm obtains opportunities of immediate action, the results of
which are removed with equal rapidity, not attainable in more complex
multi-cellular organisms. To put the matter in another way, if we could
imagine all the living cells of a large oak or of a horse, having given up
the specializations of function impressed on them during evolution and
simply carrying out the fundamental functions of nutrition, growth, and
multiplication which mark the generalized activities of the bacterial cell,
and at the same time rendered as accessible to the environment by isolation
and consequent extension of surface, we should doubtless find them exerting
changes in the fermentable fluids necessary to their life similar to those
exerted by an equal mass of bacteria, and that in proportion to their
approximation in size to the latter. Ciliary movements, which un