=Prehistoric Science.=--Scholars have examined the débris and have asked themselves what men have left them. From their skeletons, they have tried to construct their physical appearance; from their tools, the kind of life they led. They have determined that these instruments resemble those used by certain savages today. The study of all these objects constitutes a new science, Prehistoric Archæology.[1] =The Four Ages.=--Prehistoric remains come down to us from very diverse races of men; they have been deposited in the soil at widely different epochs since the time when the mammoth lived in western Europe, a sort of gigantic elephant with woolly hide and curved tusks. This long lapse of time may be divided into four periods, called Ages: 1. The Rough Stone Age. 2. The Polished Stone Age. 3. The Bronze Age. 4. The Iron Age. The periods take their names from the materials used in the manufacture of the tools,--stone, bronze, iron. These epochs, however, are of very unequal length. It may be that the Rough Stone Age was ten times as long as the Age of Iron. THE ROUGH STONE AGE =Gravel Débris.=--The oldest remains of the Stone Age have been found in the gravels. A French scholar found between 1841 and 1853, in the valley of the Somme, certain sharp instruments made of flint. They were buried to a depth of six metres in gravel under three layers of clay, gravel, and marl which had never been broken up. In the same place they discovered bones of cattle, deer, and elephants. For a long time people made light of this discovery. They said that the chipping of the flints was due to chance. At last, in 1860, several scholars came to study the remains in the valley of the Somme and recognized that the flints had certainly been cut by men. Since then there have been found more than 5,000 similar flints in strata of the same order either in the valley of the Seine or in England, and some of them by the side of human bones. There is no longer any doubt that men were living at the epoch when the gravel strata were in process of formation. If the strata that cover these remains have always been deposited as slowly as they are today, these men whose bones and tools we unearth must have lived more than 200,000 years ago.