atives,
are two familiar forms of variation. As a rule, the degree to which a
given organism is said to vary in a given character is most conveniently
measured by the difference between its actual condition and the general
average of its species, even though there is no such thing as a specimen
of average nature in all of its qualities. In brief, then, variation means
the existence of some differences between an individual and its parents,
its fraternity, and, in a wider sense, all others of its species.

Passing now to the causes of variation, all of the countless deviations of
living things can be referred to three kinds of primary factors; namely,
the _environmental_, _functional_, and _congenital_ influences that work
upon the organism in different ways and at different times during its
life. We shall learn that the evolutionary values of these three classes
are by no means equal, but we take a long step forward when we realize
that among the things we see every day are facts demonstrating the reality
of three kinds of natural powers quite able to change the characters of
organic mechanisms.

The "environment" of an organism is everything outside the creature
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