pendent of and differing from the cosmic forces; the energy which
living matter gives off is counterbalanced by the energy which it
receives. It undergoes constant change, and there is constant
interchange with the environment. The molecules which compose it are
constantly undergoing change in their number, kind and arrangement.
Atom groups as decomposition products are constantly given off from
it, and in return it receives from without other atom groups with
which it regenerates its substance or increases in amount. All
definitions of life convey this idea of activity. Herbert Spencer
says, "Life is the continuous adjustment of internal relations to
external conditions." The molecules of the substances forming the
living material are large, complex and unstable, and as such they
constantly tend to pass from the complex to the simple, from unstable
to stable equilibrium. The elementary substances which form living
material are known, but it has hitherto not been found possible
artificially so to combine these substances that the resulting mass
will exhibit those activities which we call the phenomena of life. The
distinction between living and nonliving matter is manifest only when
the sum of the activities of the living matter is considered; any
single phenomenon of the living may appear also in the non-living
material. Probably the most distinguishing criterion of living matter
is found in its individuality, which undoubtedly depends upon
differences in structure, whether physical or chemical, between the
different units.

Certain conditions are essential for the continued existence of living
matter. It must be surrounded by a fluid or semi-fluid medium in order
that there may be easy interchange with the environment. It must
constantly receive from the outside a supply of energy in the form of
food, and substances formed as the result of the intracellular
chemical activity must be removed. In the case of many animals it
seems as though the necessity of a fluid environment for living matter
did not apply, for the superficial cells of the skin have no fluid
around them; these cells, however, are dead, and serve merely a
mechanical or protective purpose. All the living cells of the skin and
all the cells beneath this have fluid around them.

Living matter occurs always in the form of small masses called
"cells," which are the living units. The cells vary in form, structure
and size, some being so large that they can