in a
small body together, and would often breed together. If the new variety
were successful in its battle for life, it would slowly spread from
a central district, competing with and conquering the unchanged
individuals on the margins of an ever-increasing circle.

It may be worth while to give another and more complex illustration of
the action of natural selection. Certain plants excrete sweet juice,
apparently for the sake of eliminating something injurious from the sap:
this is effected, for instance, by glands at the base of the stipules in
some Leguminosae, and at the backs of the leaves of the common laurel.
This juice, though small in quantity, is greedily sought by insects; but
their visits do not in any way benefit the plant. Now, let us suppose
that the juice or nectar was excreted from the inside of the flowers of
a certain number of plants of any species. Insects in seeking the nectar
would get dusted with pollen, and would often transport it from one
flower to another. The flowers of two distinct individuals of the same
species would thus get crossed; and the act of crossing, as can be fully
proved, gives rise to vigorous seedlings, which consequently would have
the best chance of flourishing and surviving. 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 par