Page - 106 - in The Origin of Species

106 ORIGIN OF SPECIES 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 in- stance, by glands af the base of the stipules in some Legu- minosse, 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 seed- lings, which consequently would have the best chance of flour- ishing and surviving. The plants which produced flowers with the largest glands or nectaries, excreting much 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 visit- ing 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, unin- tentionally 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 like- wise illustrating one step in the separation of the sexes of