coloured died out. I have tried to explain, by the same sympathetic selection of colour, the wonderful fact that the majority of pelagic animals—that is, of those which live on the surface of the open sea—are bluish, or completely colourless and transparent, like glass and water itself. Such colourless, glassy animals are met with in the most different classes. To them belong, among fish, the Helmicthyidæ, through whose crystalline bodies the words of a book can be read; among the molluscs, the finned snails (Heteropods) and sea-butterflies, or whales-food (Pteropods); among worms, the Salpæ, Alciope, and Sagitta; further, a great number of pelagic crabs (Crustacea), and the greater part of the Medusæ Umbrella-jellies, (Discomedusæ); Comb-jellies, (Ctenophora). All of these pelagic animals, which float on the surface of the ocean, are transparent and colourless, like glass and like the water itself, while their nearest kin live at the bottom of the ocean, and are coloured and opaque like the inhabitants of the land. This remarkable fact, like the sympathetic colouring of the inhabitants of the earth, can be explained by natural selection. Among the ancestors of the pelagic glass-like animals which showed a different degree of colourlessness and transparency, those that were the most colourless and transparent must have been most favoured in the active struggle for life which takes place on the surface of the ocean. They were enabled to approach their prey the most easily unobserved, and were themselves least observed by their enemies. Hence they could preserve and propagate themselves more easily than their more coloured and opaque relatives; and finally, by accumulative adaptation and transmission by inheritance, through natural selection, in the course of many generations their bodies would attain that degree of crystal-like transparency and colourlessness which we at present admire in them. (Gen. Morph. ii. 242.)