IO2 CHARLES DARWIN 



nests in immense quantities. The number of mice, 

 again, is greatly affected by the proportion of cats in 

 the neighbourhood ; so that Colonel Newman, who paid 

 much attention to this subject, found humble-bees most 

 numerous in the neighbourhood of villages and small 

 towns, an effect which he attributed to the abundance 

 of cats, and the consequent scarcity of the destructive 

 field-mice. Yet here once more, who could suppose 

 beforehand that the degree to which the purple clover 

 set its seeds was in part determined by the number of 

 cats kept in houses in the surrounding district ? 



One of Darwin's own favourite examples of the 

 action of natural selection, which he afterwards ex- 

 panded largely in his work on Orchids and in several 

 other volumes, is that which relates to the origin of 

 conspicuous flowers. Many plants have a sweet excre- 

 tion, which is eliminated sometimes even by the leaves, 

 as in the case of the common laurel. This juice, though 

 small in quantity, is eagerly sought and eaten by insects. 

 Now let us suppose that, in some variety of an incon- 

 spicuous flower, similar nectar was produced in the 

 neighbourhood of the petals and stamens. Insects, in 

 seeking the nectar, would dust their bodies over with 

 the pollen, and would carry it away with them to the 

 next flower visited. This would result in an act of 

 crossing ; and that act, as Darwin afterwards abundantly 

 proved in a separate and very laborious treatise, gives 

 rise to exceptionally vigorous seedlings, which would 

 therefore have the best chance of flourishing and sur- 

 viving in the struggle for existence. The flowers which 

 produced most honey would oftenest be visited, and 

 oftenest crossed ; so that they would finally form a ne\v 



