After more than two thousand years, the Ionian philosophers "come to their own" again. Xenophanes of Colophon has been referred to as arriving, five centuries B. C., at a true explanation of the imprints of plants and animals in rocks. Pythagoras, who lived before him, may, if Ovid, writing near the Christian era, is to be trusted, have reached some sound conclusions about the action of water in the changes of land and sea areas. But we are on surer ground when we meet the geographer Strabo, who lived in the reign of Augustus. Describing the countries in which he travelled, he notes their various features, and explains the causes of earthquakes and allied phenomena. Then eleven hundred years pass before we find any explanation of like rational character supplied. This was furnished by the Arabian philosopher, Avicenna, whose theory of the origin of mountains is the more marvellous when we remember what intellectual darkness surrounded him. He says that "mountains may be due to two different causes. Either they are effects of upheavals of the crust of the earth, such as might occur during a violent earthquake, or they are the effect of water, which, cutting for itself a new route, has denuded the valleys, the strata being of different kinds, some soft, some hard. The winds and waters disintegrate the one, but leave the other intact. Most of the eminences of the earth have had this latter origin. It would require a long period of time for all such changes to be accomplished, during which the mountains themselves might be somewhat diminished in size. But that water has been the main cause of these effects is proved by the existence of fossil remains of aquatic and other animals on many mountains" (cf. Osborn's From the Greeks to Darwin, p. 76). A similar explanation of fossils was given by the engineer-artist Leonardo de Vinci in the fifteenth century, and by the potter Bernard Palissy, in the sixteenth century; but thence onward, for more than a hundred years, the earth was as a sealed book to man. The earlier chapters of its history, once reopened, have never been closed again. Varied as were the theories of the causes which wrought manifold changes on its surface, they agreed in demanding a far longer time-history than the Church was willing to allow. If the reasoning of the geologists was sound, the narrative in Genesis was a myth. Hence the renewal of struggle between the Christian Church and Science, waged, at first, over the six days of the Creation.