NATURAL SELECTION 137 



a_beneficial nature, the original form would soon be sup- 

 planted by the modified form, through the survival of 

 the fittest. 



To the effects of intercrossing in eliminating variations 

 of all kinds, I shall have to recur; but it may be here 

 remarked that most animals and plants keep to their 

 proper homes, and do not needlessly wander about; we 

 see this even with migratory birds, which almost always 

 return to the same spot. Consequently each newl"- . 

 formed variety would generally be at first local, as 

 seems to be the common rule with varieties in a state 

 of nature; so that 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 central 

 district, competing with and conquering the unchanged 

 individuals 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 eliminating 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 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 indi- 

 viduals of the same species would thus get crossed; and 



