_secondary factors_ of a preservative nature which will perpetuate the
more adaptive organic changes produced by the first influences; it is
clear that the latter are no less essential for evolution than the first
causes for the appearance of variations.

The term "variation" is employed for the natural phenomenon of being or
becoming different. It is an obvious fact that no child is ever exactly
like either of its parents or like any one of its earlier ancestors; while
furthermore in no case does an individual resemble perfectly another of
its own generation or family. This departure from the parental condition,
and the lack of agreement with others even of its closest blood-relatives,
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