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, _Epoques 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.
But notwithstanding the crude and uncritical nature of the writings of the second half of the eighteenth century, resulting from the lack of that more careful and detailed observation which characterizes our day, there was during this period a widespread interest in physical and natural science, and it led up to that more exact study of nature which signalizes the nineteenth century.

"More new truths concerning the external world," says Buckle, "were discovered in France during the latter half of the eighteenth century than during all preceding periods put together."[66] As Perkins[67] says: "Interest in scientific study, as in political investigation, seemed to rise suddenly from almost complete inactivity to extraordinary development.