No
innate tendency to run out and no changing influence of the climate are
required for an adequate explanation of the facts.
In the observation quoted, what astonishes us most, is the great
rapidity of the change, and the short time necessary for the offspring
of the accidental crosses to completely supplant the introduced type. In
the lecture on the selection of elementary species, closely analogous
cases were described. One of them was the wild oat or _Avena fatua_
which rapidly supplants the cultivated oats in bad years in parts of the
fields. Other instances were the experiments of Risler with the
"Galland" wheat and the observation of Rimpau on "Rivett's bearded"
wheat.
Before leaving the question of vicinism and its bearing on the general
belief of the instability of varieties, which when tested with due care,
prove to be quite stable, it may be as well to consider the phenomena
from another point of view. Our present knowledge of the effects of
crosses between varieties enables us to formulate some general rules,
which may be used to calculate, and in some way to predict, the nature
of the impurities which necessarily attend the cultivation of allied
species in close vicinity. And this mode of cultivation being in almost
universal use in the larger nurseries, [208] we may, by this discussion,
arrive at a more scientific estimation of the phenomena of vicinism,
hitherto described.
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