Strange days and nights those must have been on the earth when the great sea was still too hot for living things to exist in it. The land above the water-line was bare rocks. These were rapidly being crumbled by the action of the air, which was not the mild, pleasant air we know, but was full of destructive gases, breathed out through cracks in the thin crust of the earth from the heated mass below. If you stand on the edge of a lava lake, like one of those on the islands of the Hawaiian group, the stifling fumes that rise might make you feel as if you were back at the beginning of the earth's history, when the solid crust was just a thin film on an unstable sea of molten rock, and this volcano but one of the vast number of openings by which the boiling lava and the condensed gases found their way to the surface. Then the rivers ran black with the waste of the rocky earth they furrowed, and there was no vegetation to soften the bleakness of the landscape. The beginnings of life on the earth are a mystery. Nobody can guess the riddle. The earliest rocks were subjected to great heat. It is not possible that life could have existed in the heated ocean or on the land. Gradually the shores of the seas became filled up with sediment washed down by the rivers. Layer on layer of this sediment accumulated, and it was crumpled by pressure, and changed by heat, so that if any plants or animals had lived along those old shores their remains would have been utterly destroyed. Rocks that lie in layers on top of these oldest, fire-scarred foundations of the earth show the first faint traces of living things. Limestone and beds of iron ore are signs of the presence of life. The first animals and plants lived in the ancient seas. From the traces that are left, we judge that the earliest life forms were of the simplest kind, like some plants and animals that swim in a drop of water. Have you ever seen a drop of pond water under a compound microscope? It is a wonder world you look into, and you forget all the world besides. You are one of the wonderful higher animals, the highest on the earth. You focus on a shapeless creature that moves about and feels and breathes, but hasn't any eyes or mouth or stomach--in fact, it is the lowest form of animal life, and one of the smallest. It is but one of many animal forms, all simple in structure, but able to feed and grow and reproduce their kind.