als have been grown for succeeding years in the same place, growth
finally diminishes not from the exhaustion of the soil, but from the
accumulation in it of substances produced by the plants. Beneath
certain trees, as the Norway maple, grass will not grow, and it has
been shown that the tree produces substances which inhibit the growth
of grass. When bacteria are grown in a culture flask, growth ceases
long before the nutritive material has been consumed, from the
accumulation of waste products in the fluid. The other class of toxic
substances, called endotoxines, are not secretion products, but are
contained in the bacterial substance and become active by the
destruction and disintegration of the bacteria. They can be
artificially produced by grinding up masses of bacteria, and in the
body the destruction and solution of bacteria which is constantly
taking place sets them free. The toxines and the endotoxines are of an
albuminous nature, and act only when they come in contact with the
living cells within the body. When taken into the alimentary canal
they are either not absorbed or so changed by the digestive fluids as
to be innocuous. Many of the ordinary food substances, even a material
apparently so simple as the white of an egg, are highly injurious if
they reach the tissues in an unchanged form.

By means of these substances the bacteria produce such changes in
their environment within the body that this becomes adapted to their
parasitic existence. In symbiosis the bacteria probably undergo
changes by which they become adapted to the environment, and in
parasitism the environment becomes adapted to them. In the same way
man can change his immediate environment by means of clothing,
artificial heating, etc., and adapt it to his needs; or by hardening
his body he can adapt it to the environment. The pathogenic bacterium
finds the living tissue hostile, its cells devour him, the tissue
fluids destroy him, and by means of the toxines he changes the
environment from that of living to dead tissue, or in other ways so
alters it that it is no longer hostile. The parasite has also means of
passive defence comparable to the armor of the warrior in the past. It
may form a protective mantle called a capsule around itself, which
serves to protect it from the action of the body fluids. Such capsule
formation is a very common thing in the pathogenic organisms, and they
are found only when these are growing in the body and do n