But if their
flowers are visited by insects, they will intercross; and this will give
to their sexual elements during a considerable number of generations a
sufficient amount of differentiation for a cross to be beneficial.
Moreover, seeds are frequently exchanged or procured from other gardens
having a different kind of soil; and the individuals of the same
cultivated species will thus be subjected to a change of conditions. If
the flowers are not visited by our native insects, or very rarely so, as
in the case of the common and sweet pea, and apparently in that of the
tobacco when kept in a hothouse, any differentiation in the sexual
elements caused by intercrosses will tend to disappear. This appears to
have occurred with the plants just mentioned, for they were not
benefited by being crossed one with another, though they were greatly
benefited by a cross with a fresh stock.
I have been led to the views just advanced with respect to the causes of
the differentiation of the sexual elements and of the variability of our
garden plants, by the results of my various experiments, and more
especially by the four cases in which extremely inconstant species,
after having been self-fertilised and grown under closely similar
conditions for several generations, produced flowers of a uniform and
constant tint. These conditions were nearly the same as those to which
plants, growing in a garden clear of weeds, are subjected, if they are
propagated by self-fertilised seeds on the same spot.
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