The same remark applies to crossed and self-fertilised
seedlings when these are used as the parent-plants. This want of
correspondence probably depends, at least in part, on the number of
seeds produced being chiefly determined by the number of the
pollen-tubes which reach the ovules, and this will be governed by the
reaction between the pollen and the stigmatic secretion or tissues;
whereas the growth and constitutional vigour of the offspring will be
chiefly determined, not only by the number of pollen-tubes reaching the
ovules, but by the nature of the reaction between the contents of the
pollen-grains and ovules.
There are two other important conclusions which may be deduced from my
observations: firstly, that the advantages of cross-fertilisation do not
follow from some mysterious virtue in the mere union of two distinct
individuals, but from such individuals having been subjected during
previous generations to different conditions, or to their having varied
in a manner commonly called spontaneous, so that in either case their
sexual elements have been in some degree differentiated. And secondly,
that the injury from self-fertilisation follows from the want of such
differentiation in the sexual elements. These two propositions are fully
established by my experiments. Thus, when plants of the Ipomoea and of
the Mimulus, which had been self-fertilised for the seven previous
generations and had been kept all the time under the same conditions,
were intercrossed one with another, the offspring did not profit in the
least by the cross.
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