Their views, often premature, composed of half-truths, were mingled with glaring errors and fantastic misconceptions, but were none the less germinal. Leibnitz was the first to propose the nebular hypothesis, which was more fully elaborated by Kant and Laplace. Buffon, influenced by the writing of Leibnitz, in his _Théorie de la Terre_, published in 1749, adopted his notion of an original volcanic nucleus and a universal ocean, the latter as he thought leaving the land dry by draining into subterranean caverns. He also dimly saw, or gathered from his reading, that the mountains and valleys were due to secondary causes; that fossiliferous strata had been deposited by ocean currents, and that rivers had transported materials from the highlands to the lowlands. He also states that many of the fossil shells which occur in Europe do not live in the adjacent seas, and that there are remains of fishes and of plants not now living in Europe, and which are either extinct or live in more southern climates, and others in tropical seas. Also that the bones and teeth of elephants and of the rhinoceros and hippopotamus found in Siberia and elsewhere in northern Europe and Asia indicate that these animals must have lived there, though at present restricted to the tropics. In his last essay, _Époques de la Nature_ (1778), he claims that the earth's history may be divided into epochs, from the earliest to the present time. The first epoch was that of fluidity, of incandescence, when the earth and the planets assumed their form; the second, of cooling; the third, when the waters covered the earth, and volcanoes began to be active; the fourth, that of the retreat of the seas, and the fifth the age when the elephants, the hippopotamus, and other southern animals lived in the regions of the north; the sixth, when the two continents, America and the old world, became separate; the seventh and last being the age of man. Above all, by his attractive style and bold suggestions he popularized the subjects and created an interest in these matters and a spirit of inquiry which spread throughout France and the rest of Europe.