114 ILLUSTRATIONS OF THE ACTION OP [Chap. IV. 



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 com- 

 plex 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, hy glands at the base of the 

 stipules in some Leguminosffi, 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 indi- 

 viduals 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 nec- 

 taries, 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 insects 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 



