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 



