The plants which produced flowers
with the largest glands or nectaries, excreting most nectar, would oftenest
be visited by insects, and would oftenest be crossed; and so in the
long-run would gain the upper hand and form a local variety. The flowers,
also, which had their stamens and pistils placed, in relation to the size
and habits of the particular insect which visited them, so as to favour in
any degree the transportal of the pollen, would likewise be favoured. We
might have taken the case of insects visiting flowers for the sake of
collecting pollen instead of nectar; and as pollen is formed for the sole
purpose of fertilisation, its destruction appears to be a simple loss to
the plant; yet if a little pollen were carried, at first occasionally and
then habitually, by the pollen-devouring insects from flower to flower, and
a cross thus effected, although nine-tenths of the pollen were destroyed it
might still be a great gain to the plant to be thus robbed; and the
individuals which produced more and more pollen, and had larger anthers,
would be selected.
When our plant, by the above process long continued, had been rendered
highly attractive to insects, they would, unintentionally on their part,
regularly carry pollen from flower to flower; and that they do this
effectually I could easily show by many striking facts. I will give only
one, as likewise illustrating one step in the separation of the sexes of
plants.
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