But why should flowers take such pains to attract insects by serving them with a breakfast of honey, and by hanging out big petals to serve as coloured and perfumed signal-flags? Nature is economical in the expenditure of energy; and as the production of honey and large flowers costs the plant some of its vital energies, we may be sure that this expenditure secures the plant some superior advantage. Sprengel noticed that the insects, while pillaging flowers of their honey, unwittingly brushed off with their wings and feet some of the fertilising dust or pollen, and carried it to the pistil or female part of a flower. But it remained for Darwin to point out what advantage this transference of the pollen secured to the flower. Darwin, says Sir John Lubbock, “was the first clearly to perceive that the essential service which insects perform to flowers consists not only in transferring the pollen from the stamens to the pistil, but in transferring it from the stamens of one flower to the pistil of another. Sprengel had indeed observed in more than one instance that this was the case, but he did not altogether appreciate the importance of the fact. Mr. Darwin however, has not only made it clear from theoretical considerations, but has also proved it, in a variety of cases, by actual experiment. More recently Fritz Müller has even shown that in some cases pollen, if placed on the stigma of the same flower, has no more effect than so much inorganic dust; while, and this is perhaps even more extraordinary, in others, the pollen placed on the stigma of the same flower acted on it like poison”—a curious analogy to the current belief that close intermarriage is injurious to mankind. What Darwin and others have proved by their experiments is that cross-fertilised flowers are more vigorous than those fertilised with their own pollen, and have a more healthy and numerous offspring. With this fact before us we need only apply the usual evolutionary formula to account for the beauty of flowers. It is well known that Nature rarely, if ever, produces two leaves or plants that are exactly alike. There is also a natural tendency in all parts of a plant except the leaves to develop other colours besides green. Now any plant which, owing to chemical causes, favourable position, etc., developed an unusually brilliant colour, would be likely to attract the attention of a winged insect in search of pollen-food. The insect, by alighting on a second flower soon after, would fertilise it with the pollen of the first flower that adhered to its limbs, thus securing to the plant the advantages of cross-fertilisation. Thanks to the laws of heredity, this advantage would be transmitted to the young plants, among which again those most favoured would gain an advantage and a more numerous offspring. And thus the gradual development not only of coloured petals, but of scents and honey, can be accounted for.