NATURAL SELECTION 73 



similarly modified individuals would soon exist in a small body 

 together, and would often breed together. If the new variety were 

 successful in its battle for life, it would slowly spread from a cen- 

 tral district, competing with and conquering the unchanged indi- 

 viduals on the margins of an ever-increasing circle. 



It may be worth while to give another and more complex illus- 

 tration of the action of natural selection. Certain plants excrete 

 sweet juice, apparently for the sake of eliminating something in- 

 jurious 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. 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 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 pollen is formed for 

 the sole purpose of fertilization, 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, unintentionally 

 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 likewise illustrating one step in the 

 separation of the sexes of plants. Some holly-trees bear only male 

 flowers, which have four stamens producing a rather small quan- 



