The Story of Germ Life | Page 8

The Oscillariae are, however, true plants, and are of
a green colour. Bacteria are therefore to- day looked upon as a low type

of plant which has no chlorophyll, [Footnote: Chlorophyll is the green
colouring matter of plants.] but is related to Oscillariae. The absence of
the chlorophyll has forced them to adopt new relations to food, and
compels them to feed upon complex foods instead of the simple ones,
which form the food of green plants. We may have no hesitation, then,
in calling them plants. It is interesting to notice that with this idea their
place in the organic world is reduced to a small one systematically.
They do not form a class by themselves, but are simply a subclass, or
even a family, and a family closely related to several other common
plants. But the absence of chlorophyll and the resulting peculiar life has
brought about a curious anomaly. Whereas their closest allies are
known only to botanists, and are of no interest outside of their
systematic relations, the bacteria are familiar to every one, and are
demanding the life attention of hundreds of investigators. It is their
absence of chlorophyll and their consequent dependence upon complex
foods which has produced this anomaly.
CLASSIFICATION OF BACTERIA.
While it has generally been recognised that bacteria are plants, any
further classification has proved a matter of great difficulty, and
bacteriologists find it extremely difficult to devise means of
distinguishing species. Their extreme simplicity makes it no easy
matter to find points by which any species can be recognised. But in
spite of their similarity, there is no doubt that many different species
exist. Bacteria which appear to be almost identical, under the
microscope prove to have entirely different properties, and must
therefore be regarded as distinct species. But how to distinguish them
has been a puzzle. Microscopists have come to look upon the
differences in shape, multiplication, and formation of spores as
furnishing data sufficient to enable them to divide the bacteria into
genera. The genus Bacillus, for instance, is the name given to all
rod-shaped bacteria which form endogenous spores, etc. But to
distinguish smaller subdivisions it has been found necessary to fall
back upon other characters, such as the shape of the colony produced in
solid gelatine, the power to produce disease, or to oxidize nitrites, etc.
Thus at present the different species are distinguished rather by their

physiological than their morphological characters. This is an
unsatisfactory basis of classification, and has produced much confusion
in the attempts to classify bacteria. The problem of determining the
species of bacteria is to-day a very difficult one, and with our best
methods is still unsatisfactorily solved. A few species of marked
character are well known, and their powers of action so well
understood that they can be readily recognised; but of the great host of
bacteria studied, the large majority have been so slightly experimented
upon that their characters are not known, and it is impossible, therefore,
to distinguish many of them apart. We find that each bacteriologist
working in any special line commonly keeps a list of the bacteria which
he finds, with such data in regard to them as he has collected. Such a
list is of value to him, but commonly of little value to other
bacteriologists from the insufficiency of the data. Thus it happens that a
large part of the different species of bacteria described in literature to-
day have been found and studied by one investigator alone. By him
they have been described and perhaps named. Quite likely the same
species may have been found by two or three other bacteriologists, but
owing to the difficulty of comparing results and the incompleteness of
the descriptions the identity of the species is not discovered, and they
are probably described again under different names. The same process
may be repeated over and over again, until the same species of
bacterium will come to be known by several different names, as it has
been studied by different observers.
VARIATION OF BACTERIA.
This matter is made even more confusing by the fact that any species of
bacterium may show more or less variation. At one time in the history
of bacteriology, a period lasting for many years, it was the prevalent
opinion that there was no constancy among bacteria, but that the same
species might assume almost any of the various forms and shapes, and
possess various properties. Bacteria were regarded by some as stages in
the life history of higher plants. This question as to whether bacteria
remain constant in character for any considerable length of time has
ever been a prominent one with bacteriologists, and even to-day we
hardly know what the final answer will be. It has been demonstrated

beyond peradventure that some species may change their