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 be seen with the naked
eye, while others are so small that they cannot be distinctly seen
with the highest power of the microscope. The living thing or organism
may be composed of a single cell or, in the case of the higher animals
and plants, may be formed of great numbers of cells, those of a
similar character being combined in masses to form organs such as the
liver and brain.