contain traces of life. These first remains are of the simplest description; they are the vestiges of simple plants, called algæ, or marks like the tracks made by worms in the sea mud. There are also the skeletons of the microscopic creatures called Radiolaria. This second series of rocks is called the Proterozoic (beginning of life) series, and marks a long age in the world's history. Lying over and above the Proterozoic rocks is a third series, which is found to contain a considerable number and variety of traces of living things. First comes the evidence of a diversity of shellfish, crabs, and such-like crawling things, worms, seaweeds, and the like; then of a multitude of fishes and of the beginnings of land plants and land creatures. These rocks are called the Palæozoic (ancient life) rocks. They mark a vast era, during which life was slowly spreading, increasing, and developing in the seas of our world. Through long ages, through the earliest Palæozoic time, it was no more than a proliferation of such swimming and creeping things in the water. There were creatures called trilobites; they were crawling things like big sea woodlice that were probably related to the American king-crab of to-day. There were also sea-scorpions, the prefects of that early world. The individuals of certain species of these were nine feet long. These were the very highest sorts of life. There were abundant different sorts of an order of shellfish called brachiopods. There were plant animals, rooted and joined together like plants, and loose weeds that waved in the waters.
It was not a display of life to excite our imaginations. There was nothing that ran or flew or even swam swiftly or skilfully. Except for the size of some of the creatures, it was not very different from, and rather less various than, the kind of life a student would gather from any summer-time ditch nowadays for microscopic examination. Such was the life of the shallow seas through a hundred million years or more in the early Palæozoic period. The land during that time was apparently absolutely barren. We find no trace nor hint of land life. Everything that lived in those days lived under water for most or all of its life.
Between the formation of these Lower Palæozoic rocks in which the sea scorpion and trilobite ruled, and our own time, there have intervened almost immeasurable ages, represented by layers