d, under the microscope, by the color which a
dye gives to them, and which they refuse to give up when treated with
chemical substances which remove the stain from, or bleach, all the
other tissues which at first have been similarly stained.

The similarity between bacteria and the ordinary plants with which
florists are familiar is, indeed, remarkable. Bacteria grow in animal
and other albuminous fluids; but it is just as essential for them to
have a suitable soil as it is for the corn or wheat that the farmer
plants in his field. By altering the character of the albuminous fluid
in which the micro-organism finds its subsistence, these small plants
can be given a vigorous growth, or may be actually starved to death.
The farmer knows that it is impossible for him to grow the same crop
year after year in the same field, and he is, therefore, compelled to
rotate his crops. So it is with the microscopic plants which we are
considering.

After a time the culture fluid or soil becomes so exhausted of its
needed constituents, by the immense number of plants living in it,
that it is unfit for their life and development. Then this particular
form will no longer thrive; but some other form of bacterium may find
in it the properties required for functional activity, and may grow
vigorously. It is probable that exhaustion or absence of proper soil
is an important agent in protecting man from sickness due to infection
from bacteria. The ever-present bacteria often gain access to man's
blood through external wounds, or through the lungs and digestive
tracts; but unless a soil suited for their development is found in its
fluids, the plants will not grow. If they do not grow and increase in
numbers, they can do little harm.

Again, there are certain bacteria which are so antagonistic to each
other that it is impossible to make them grow in company, or to
co-exist in the blood of the same individual. For example, an animal
inoculated with erysipelas germs cannot be successfully inoculated
immediately afterward with the germs of malignant pustule. This
antagonism is illustrated by the impossibility of having a good crop
of grain in a field overrun with daisies.

On the other hand, however, there are some micro-organisms which
flourish luxuriantly when planted together in the same fluid, somewhat
after the manner of pumpkins and Indian corn growing between the same
fence rails. Others seem unwilling to grow alone, and only flourish