114 ILLUSTRATIONS OF THE ACTION OF [Chap. IV. 



with and conquering the unchanged individuals on the 

 margins of an ever- increasing circle. 



O CD 



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, 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. Xow, 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 



