It has already been proved, that a cross between plants
which have been self-fertilised or intercrossed during several
generations, having been kept all the time under closely similar
conditions, does not benefit the offspring; and on the other hand, that
a cross between plants that have been subjected to different conditions
benefits the offspring to an extraordinary degree. We may therefore
conclude that some degree of differentiation in the sexual system is
necessary for the full fertility of the parent-plants and for the full
vigour of their offspring. It seems also probable that with those plants
which are capable of complete self-fertilisation, the male and female
elements and organs already differ to an extent sufficient to excite
their mutual interaction; but that when such plants are taken to another
country, and become in consequence self-sterile, their sexual elements
and organs are so acted on as to be rendered too uniform for such
interaction, like those of a self-fertilised plant long cultivated under
the same conditions. Conversely, we may further infer that plants which
are self-sterile in their native country, but become self-fertile under
changed conditions, have their sexual elements so acted on, that they
become sufficiently differentiated for mutual interaction.
We know that self-fertilised seedlings are inferior in many respects to
those from a cross; and as with plants in a state of nature pollen from
the same flower can hardly fail to be often left by insects or by the
wind on the stigma, it seems at first sight highly probable that
self-sterility has been gradually acquired through natural selection in
order to prevent self-fertilisation.
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