Assuming, then, and we must all start with this proposition, that aquatic plants certainly, and warm or hot water plants probably, were the first living things upon the earth, what are the next steps in the history of the plant kingdom? The answer to that question involves a few simple facts in geology and, particularly, in the making of fossils, which must be understood before we can see those steps or their significance. The geological changes which have resulted in the present condition of the earth’s surface are described in the volume devoted to that subject, and will not be repeated here. But some mention must be made of the formation of fossil plants, particularly as it is upon the evidence of these that the story of the development of plant life is literally written in the rocks. If a leaf or twig drops into shallow water with a clay or mud bottom, it ultimately sinks, and if then a film of clay or silt is brought down by freshets or what not, it will bury the leaf or twig, of course, filling in every slight depression. If then the buried object were raised so that it dried out and could be split open, we should find a perfect impression of the veins and other outward characters of the leaf etched in the clay. This is often such a perfect process that every detail of the leaf is left in the mud impression, and only the opportunity for this impression to become hardened into rock is needed for us to have a fossil. For these are merely the final hardened rock stages of a process that began as we have indicated, and the thousands of fossils that have been dug out of the earth prove how common the conditions for their formation must have been in certain periods of the world’s history.