These were the principal writers which influenced geology in the days before the Christian era, and it is to be noticed that they did not treat of the construction of the rocks, of the succession of the layers or strata, or of much concerning the ancient history of the globe. But they taught, wisely and admirably, the nature of modern changes, and believed that these and the older ones they could comprehend, were part of a scheme, and were produced by natural causes, in the course of events. Aristoteles, Strabo, and Plinius wrote about the changes which were progressing on the surface of the earth, and compared them, in their reasoning, with changes they presumed had been, but still no great advance was made. In the early part of the sixteenth century a remarkable discussion sprang up about the nature of shells and bones, which were found in layers of earth remote from the sea. The celebrated painter Leonardo da Vinci had seen some of these fossils during his youth, when he planned and carried out some important canals in the north of Italy. He laughed at the fancies of the day about the shells, for some people said that they were made by the stars, and others that they were brought forth naturally, in the layers of earth in which they were found. He wanted to know where the things were being made in the hills, by the stars, at the present time, and stated that, like the rounded stones of gravel, the shells had been in the sea, and that they were of different ages and kinds, and were once alive. But the former living condition of fossils, and the possibility of their being understood, by comparing them with recent or living things, was, perhaps, most strongly put by Steno, a Dane.