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 expanded 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 excretion, 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 inconspicuous 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 surviving 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 new species. The more brightly coloured among them, again, would be more readily discriminated than the less brightly coloured; and this would give them such an advantage that in the long run, as we actually see, almost all habitually insect-fertilised flowers would come to have brilliant petals. The germ of this luminous idea, once more, is to be found in Sprengel's remarkable work on the fertilisation of flowers--a work far in advance of its time in many ways, and to which Darwin always expressed his deep obligations; but, as in so many other instances, while Sprengel looked upon all the little modifications and adaptations of flower and insect to one another as the result of distinct creative design, Darwin looked upon them as the result of natural selection, working upon the basis of indeterminate spontaneous variations.