itself. In the case of an animal it therefore includes other members of
its own kind, and other organisms which prey upon its species or which
serve it as food, as well as the whole series of inorganic influences
which first come to mind when the term is used. For example, the
environment of a lion includes other lions which are either members of its
own family, or else, if they live in the same region, they are its more or
less active rivals and competitors. In the next place, other kinds of
animals exist whose lives are intimately related to the lion's life, such
as the antelopes or zebras that are preyed upon, and the human hunter to
whom the lion itself may fall a victim. In addition, there are the
contrasted influences of inorganic nature which demand certain adjustments
of the lion's activities. Light and darkness, heat and cold, and other
factors have their direct and larger or smaller effects upon the life of a
lion, although these effects are less obvious in this instance than in the
case of lower organisms.

The reality of variations due to the inorganic elements of the environment
is everywhere evident. Those who have spent much time in the sun are aware
that sunburn may result as a product of a factor of this class. The amount
of sunlight falling upon a forest will filter through the tree-tops so as
to cause some of the plants beneath to grow better than others, thus
bringing about variations among individuals that may have sprung from the
myriad seeds of a single parent plant. In times of prolonged drought,
plants cannot grow at the rate which is usual and normal for their
species, and so many variations in the way of inhibited development may
arise.

Then there are the variations of a second class, more complex in nature
than the direct effects of environment,--namely, the functional results of
use and disuse. A blacksmith uses his arm muscles more constantly than do