86 ILLUSTRATIONS OF THE ACTION 



It maybe worth while to give another and more complex 

 illustration of the action of natural selection. Certain 

 plants excrete sweet juice, apparently for the sake of elim- 

 inating something injurious from the sap: this is effected, 

 for instaTice, by glands at the base of the stipules in some 

 LeguminosfB, 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 ben- 

 efit 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 flovy-ers 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 par- 

 ticular 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 

 unmtentionally 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 



