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But since Mr. Darwin
showed that plants gained both in vigour and in fertility by being
crossed with other individuals of the same species, and that this
crossing was usually effected by insects which, in search of nectar or
pollen, carried the pollen from one plant to the flowers of another
plant, almost every detail is found to have a purpose and a use. The
shape, the size, and the colour of the petals, even the streaks and
spots with which they are adorned, the position in which they stand, the
movements of the stamens and pistil at various times, especially at the
period of, and just after, fertilisation, have been proved to be
strictly adaptive in so many cases that botanists now believe that all
the external characters of flowers either are or have been of use to the
species.
It has also been shown, by Kerner and other botanists, that another set
of characteristics have relation to the prevention of ants, slugs, and
other animals from reaching the flowers, because these creatures would
devour or injure them without effecting fertilisation.


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