individual possesses this power, but all individuals do not exhibit it in
its most complete form. In some cases this faculty may not be exhibited at
the top of the main stem, although developed in lateral branches: in
others it begins too late for full development. Much depends upon
nourishment and cultivation, but almost always the horticulturist has to
single out the best individuals and to reject those which do not come up to
the standard.

The internal causes are of a historical nature. The external ones may be
defined as nourishment and environment. In some cases nutrition is the
main factor, as, for instance, in fluctuating variability, but in natural
selection environment usually plays the larger part.

The internal or historical causes are constant during the life-time of a
species, using the term species in its most limited sense, as designating
the so-called elementary species or the units out of which the ordinary
species are built up. These historical causes are simply the specific
characters, since in the origin of a species one or more of these must have
been changed, thus producing the characters of the new type. These changes
must, of course, also be due partly to internal and partly to external
causes.

In contrast to these changes of the internal causes, the ordinary
variability which is exhibited during the life-time of a species is called
fluctuating variability. The name mutations or mutating variability is
then given to the changes in the specific characters. It is desirable to
consider these two main divisions of variability separately.

In the case of fluctuations the internal causes, as well as the external
ones, are often apparent. The specific characters may be designated as the