Varieties have to be freed by selection from all such impurities, since
isolation is a means which is quite impracticable under ordinary
circumstances. Isolation is a scientific requirement that should never
be neglected in experiments, indeed it may be said to be the first and
most important requisite for all exact research in questions of
variability and inheritance. But in cultivating large fields of allied
varieties for commercial purposes, it is impossible to grow them at such
distances from each other [197] as to prevent cross-pollination by the
visits of bees.
This purification must be done in nearly every generation. The oldest
varieties are to be subjected to it as well as the latest. There is no
regular amelioration, no slow progression in the direction of becoming
free from these admixtures. Continuous selection is indispensable to
maintain the races in the degree of purity which is required in
commerce, but it does not lead to any improvement. Nor does it go so far
as to become unnecessary in the future. This shows that there must be a
continuous source of impurities, which in itself is not neutralized by
selection, but of which selection can only eliminate the deteriorating
elements.
The same selection is usually applied to new varieties, when they
occasionally arise.
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