It is true that vegetative varieties as a
rule run back, when propagated by seeds; they are an obvious instance of
inconstancy. In the second place we have considered the group of
inconstant or sporting varieties, which of course we must exclude when
studying the stability of other types. However, even these sporting
varieties are unstable only to a certain degree, and in a broader sense
will prove to be as true to their character as the most constant types.
Having separated these two groups, which include also the wide range of
hybrid forms, we may next consider only those varieties of pure origin,
and ordinarily propagated by seeds, [155] which have been discussed in
former chapters. Their general character lies in their fidelity to type,
and in the fact that this is single, and not double, as in the sporting
varieties.
But the current belief is, that they are only true to their
peculiarities to a certain degree, and that from time to time, and not
rarely, they revert to the type from which they have arisen. Such
reversion is supposed to prove that they are mere varieties, and at the
same time to indicate empirically the species from which they have
sprung.
In the next lecture we shall examine critically the evidence on which
this assumption rests. Before doing so however, it will be necessary to
collate the cases in which there is no reversion at all, or in which the
reversion is absent at least in experimental and pure sowings.
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