OF NATURAL SELECTION. 81 



its battle for life, it would slowly spread from a central dis< 

 trict, competing with and conquering the unchanged individ- 

 uals on the margins of an ever-increasing circle. 



It may be 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 instance, by glands at the base of the stipules in some 

 Leguminosse, 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 otten trans- 

 port it from one flower to another. The flowers of two dis- 

 tinct 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 pro- 

 duced 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, unin- 

 tentionally 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 like- 

 wise illustrating one step in the separation o£ the sexes 



