No. 4.—The cause suggested under this head was the favourite theory of Sir Charles Lyell. That the existing climates are materially influenced by the distribution of land and water, and by the direction of the great ocean currents, physicists and geologists are agreed. It was the view, however, of Sir Charles, that these conditions were of themselves alone adequate to account for all changes of climate which the earth has undergone, though in his latest editions of the “Principles of Geology” he admitted that other causes have probably contributed. In this work he gave imaginary representations of such a distribution of land and water over the globe as, in his view, would produce the extreme of heat and the extreme of cold. In that intended to represent the extreme of heat, the land is all collected in low latitudes on either side of the Equator, so that the higher latitudes and polar regions are occupied entirely by the ocean; while in that intended to represent the extreme of cold, these conditions are reversed.