With respect to the
second of these two propositions, namely, that self-fertilisation is
generally injurious, we have abundant evidence. The structure of the
flowers in such plants as Lobelia ramosa, Digitalis purpurea, etc.,
renders the aid of insects almost indispensable for their fertilisation;
and bearing in mind the prepotency of pollen from a distinct individual
over that from the same individual, such plants will almost certainly
have been crossed during many or all previous generations. So it must
be, owing merely to the prepotency of foreign pollen, with cabbages and
various other plants, the varieties of which almost invariably
intercross when grown together. The same inference may be drawn still
more surely with respect to those plants, such as Reseda and
Eschscholtzia, which are sterile with their own pollen, but fertile with
that from any other individual. These several plants must therefore have
been crossed during a long series of previous generations, and the
artificial crosses in my experiments cannot have increased the vigour of
the offspring beyond that of their progenitors. Therefore the difference
between the self-fertilised and crossed plants raised by me cannot be
attributed to the superiority of the crossed, but to the inferiority of
the self-fertilised seedlings, due to the injurious effects of
self-fertilisation.
With respect to the first proposition, namely, that cross-fertilisation
is generally beneficial, we likewise have excellent evidence.
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