the world's surface, for the forests of warm temperate latitude are, in part, the same as those in Arctic regions. Certain parts of the land appear to have been desert. Life began to change a great deal in Silurian times. The extensive withdrawal of the sea from great stretches of submerged surface reduced the area of shallow water available for the forms of life that had so richly peopled it during the Ordovician period. Then there came an age during which the sea invaded some of the regions of the earth's crust, and again withdrew, leaving behind it great stretches of water which gradually grew more intensely salt. All these things had naturally a great effect on the development of the plants and animals of Silurian times. We cannot in a brief summary of this kind do more than indicate some of the more conspicuous features. Corals began to spread through the clearer seas: and reef building on a great scale took place, generally some distance from the shores of the land. Other life in great abundance and variety gathered upon or about these reefs, and they became rich depositories of the animals of their day. The Crinoids, which, though animals, are sometimes called the lilies of the sea, developed strongly; sea urchins appeared, and forms akin to barnacles. The ancestors of the pearl oyster and the mussel date from Silurian times; and so do the first Ammonites, those creatures known to the youngest collectors of fossils, and deriving their names from the Canaanitish god Ammon, which had a ram's head. Sea scorpions, sand fleas, king crabs, sea squirts, and worms and fishes of various kinds haunted the Silurian seas. The Silurian fish were most of them armed for defence, some with plates of bone; some of them had their tails stiffly joined to their backbones; some had skin like a prickly pear; some were not unlike the modern shark. The plants have left us many records--liverworts, ferns, and club-like mosses. The growing vegetation gave a new impulse to insect life--plant-lice and cockchafers and the scorpions we have named: and the vastness of their numbers is shown by the fact that they have outlasted the changes and vicissitudes of a myriad generations.