Many hybrid varieties are quite constant, and remain true to their type
if no further crosses are made; many others are artificially propagated
only in a vegetative way, and for this reason are always found true.
Hybrid varieties as a rule were formerly confused with pure varieties,
and in many instances our knowledge as to their origin is quite
insufficient for sharp distinctions. To every student of nature it is
obvious, that crossing and pure variability are wholly distinct groups
of phenomena, which should never be treated under the same head, or
under the same name.
Leaving aside polymorphy, we may now discuss those cases of variability,
in which the changes themselves, and not only their final results play a
part. Of such changes two types exist. First, the ever-recurring
variability, never absent in any large group of individuals, and
determining the differences which are always to be seen between parents
and their children, or between the children themselves. This type is
commonly called "individual variability" and since this term also has
still other meanings, it has of late become customary to use instead the
term "fluctuating variability." [191] And to avoid the repetition of the
latter word it is called "fluctuation." In contrast to these
fluctuations are the so-called sports or single varieties, not rarely
denominated spontaneous variations, and for which I propose to use the
term "mutations.
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