As the crust of the earth cooled, the enveloping ocean gradually lost its high temperature, and having fallen far below the boiling point, continued to be warm for ages. In that warm primeval ocean, where the various elements entered into all sorts of combinations, the necessary elements combined in the proper proportions, and, as a result of that happy combination, life was born into the world. Just as the requisite proportions of charcoal, sulphur and saltpeter unite in making gunpowder—a high explosive, vastly different from its constituent elements—so, simple elements, uniting in due proportions, produced the phenomenon of living matter. [Illustration: FIG. 10.—THE EARLIEST KNOWN FORM OF FISH (upper); THREE OTHER EARLY FORMS OF FISHES (lower).] Whether life is still being evolved from non-living matter, or whether its origin was confined to a peculiar set of favorable conditions—very high temperature, and unusual chemical and electrical conditions—in the early world, remains as yet unknown. On this question, the most distinguished biologists entertain diverse opinions. Among those who hold that the evolution of life is still one of the ordinary processes of Nature’s day’s work may be mentioned Professor Benjamin Moore, F.R.S., whose views are set forth in his excellent little work, “The Origin and Nature of Life.” All living things, from the blade of grass to the giant oak, from the worm to the philosopher, are composed of cells, and all cells are composed of protoplasm. Protoplasm is, therefore, the physical basis of life. This wonderful substance is a compound of carbon, hydrogen, nitrogen and oxygen, and the story of the evolution of living things spells the limitless transformations of which this vital substance is capable. In the world of to-day, we see the finished forms that have been beaten out, as it were, upon the anvil of the evolutionary process; and if we would appreciate the progressive march that life has made, from the simple forms of early days to the highly developed and complex creatures of to-day, we must trace the story of evolution through the numberless ages of the past. [Illustration: FIG. 11.—LAMPREYS (left). THE MOUTH OF A LAMPREY (right).] The first living things consisted of a single cell—the Protozoa. As this cell grew, it divided into two; the two, having grown somewhat, divided into four; the four into eight; the eight into sixteen, and so on. In this way, the growing cells increased their numbers by division, and what was at first a single cell became an organized group of cells. Fig. 7 illustrates the process of division and growth from the single cell—the fertilized ovum—to the gastrula. This division is the law of life. Every living creature begins its existence in this way.