The Story of Germ Life | Page 9

physiological
characters. Disease bacteria, for instance, under certain conditions lose
their powers of developing disease. Species which sour milk, or others
which turn gelatine green, may lose their characters. Now, since it is
upon just such physiological characters as these that we must depend in
order to separate different species of bacteria from each other, it will be
seen that great confusion and uncertainty will result in our attempts to
define species. Further, it has been proved that there is sometimes more
or less of a metamorphosis in the life history of certain species of
bacteria. The same species may form a short rod, or a long thread, or
break up into spherical spores, and thus either a short rod, or a thread,
or a spherical form may belong to the same species. Other species may
be motile at one time and stationary at another, while at a third period it
is a simple mass of spherical spores. A spherical form, when it
lengthens before dividing, appears as a short rod, and a short rod form
after dividing may be so short as to appear like a spherical organism.
With all these reasons for confusion, it is not to be wondered at that no
satisfactory classification of bacteria has been reached, or that different
bacteriologists do not agree as to what constitutes a species, or whether
two forms are identical or not. But with all the confusion there is
slowly being obtained something like system. In spite of the fact that
species may vary and show different properties under different
conditions, the fundamental constancy of species is everywhere
recognised to-day as a fact. The members of the same species may
show different properties under different conditions, but it is believed
that under identical conditions the properties will be constant. It is no
more possible to convert one species into another than it is among the
higher orders of plants. It is believed that bacteria do form a group of
plants by themselves, and are not to be regarded as stages in the history
of higher plants. It is believed that, together with a considerable amount
of variability and an occasional somewhat long life history with
successive stages, there is also an essential constancy. A systematic
classification has been made which is becoming more or less
satisfactory. We are constantly learning more and more of the
characters, so that they can be recognised in different places by
different observers. It is the conviction of all who work with bacteria

that, in spite of the difficulties, it is only a matter of time when we shall
have a classification and description of bacteria so complete as to
characterize the different species accurately.
Even with our present incomplete knowledge of what characterizes a
species, it is necessary to use some names. Bacteria are commonly
given a generic name based upon their microscopic appearance. There
are only a few of these names. Micrococcus, Streptococcus,
Staphylococcus, Sarcina, Bacterium, Bacillus, Spirillum, are all the
names in common use applying to the ordinary bacteria. There are a
few others less commonly used. To this generic name a specific name
is commonly added, based upon some physiological character. For
example, Bacillus typhosus is the name given to the bacillus which
causes typhoid fever. Such names are of great use when the species is a
common and well-known one, but of doubtful value for less-known
species It frequently happens that a bacteriologist makes a study of the
bacteria found in a certain locality, and obtains thus a long list of
species hitherto unknown. In these cases it is common simply to
number these species rather than name them. This method is frequently
advisable, since the bacteriologist can seldom hunt up all
bacteriological literature with sufficient accuracy to determine whether
some other bacteriologist may not have found the same species in an
entirely different locality. One bacteriologist, for example, finds some
seventy different species of bacteria in different cheeses. He studies
them enough for his own purposes, but not sufficiently to determine
whether some other person may not have found the same species
perhaps in milk or water. He therefore simply numbers them--a method
which conveys no suggestion as to whether they may be new species or
not. This method avoids the giving of separate names to the same
species found by different observers, and it is hoped that gradually
accumulating knowledge will in time group together the forms which
are really identical, but which have been described by different
observers.
WHERE BACTERIA ARE FOUND.
There are no other plants or animals so universally found in Nature as

the bacteria. It is this universal presence, together with their great
powers of multiplication, which renders them of so much importance in
Nature. They exist almost everywhere on the surface of the earth. They