Nevertheless, I am aware that it will take many
years to remove this prejudice.
There is hardly anything more wonderful in nature than the sensitiveness
of the sexual elements to external influences, and the delicacy of their
affinities. We see this in slight changes in the conditions of life
being favourable to the fertility and vigour of the parents, while
certain other and not great changes cause them to be quite sterile
without any apparent injury to their health. We see how sensitive the
sexual elements of those plants must be, which are completely sterile
with their own pollen, but are fertile with that of any other individual
of the same species. Such plants become either more or less self-sterile
if subjected to changed conditions, although the change may be far from
great. The ovules of a heterostyled trimorphic plant are affected very
differently by pollen from the three sets of stamens belonging to the
same species. With ordinary plants the pollen of another variety or
merely of another individual of the same variety is often strongly
prepotent over its own pollen, when both are placed at the same time on
the same stigma. In those great families of plants containing many
thousand allied species, the stigma of each distinguishes with unerring
certainty its own pollen from that of every other species.
There can be no doubt that the sterility of distinct species when first
crossed, and of their hybrid offspring, depends exclusively on the
nature or affinities of their sexual elements.
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