NATURAL SELECTION 1495 



conquering the unchanged individuals on the mar- 

 gins of an ever-increasing circle. 



It may be worth while to give another and more 

 complex illustration of the action of natural selec- 

 tion. 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 in- 

 sects; 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 cer- 

 tain number of plants of any species. Insects in seek- 

 ing 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 cross- 

 ing, 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 favor in any degree the transportal of 

 the pollen, would likewise be favored. We might 

 have taken the case of insects visiting flowers for the 

 sake of collecting pollen instead of nectar; and as 



J VOL. IV. 



