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 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