The flow of the lakes into one another followed. Innumerable lakelets developed into rivers or chains of lakes on the surface of the young planet, continually becoming larger bodies of water, till they developed into the vast irregular oceans of to-day. This evolution is of great importance from a geological point of view, because it leads the way to the origin of the ocean basins and the great platforms of land which we call continents. It is easy to see that because of the weight of water in the depressions the earth under the waters tended to become more and more depressed, so that the water areas tended to grow larger and deeper. The wash of earths from the land tended to build its borders out into the water basins, but the deepening and spreading of the water basins is believed to have been the most marked feature of the earth's early growth. All this time the earth was growing in diameter and circumference.[9] When this growth ceased other causes and effects came into play, and the proportions of sea and land became better balanced. [Footnote 9: For reasons which are a little too complex to be considered here. We can only indicate the general line of reasoning by saying that the central heat as it moved outwards from the rocks nearer the surface expanded them.] There is nothing in our human knowledge to tell us with certainty when or how life first appeared on the earth. We have already spoken of animal and vegetable remains that for ages are preserved in the rocks. But clearly no such remains could ever be found in the volcanic or molten rocks of the earliest stages of the earth's life. Think for a moment of what the simplest forms of life are. A great deal has been heard of microbes and bacteria during recent years, and we may therefore assume that every one has some knowledge of the structure of these simplest living things. They may be compared to tiny bladders of jelly--so small that the microscope is necessary in order to see them, and sometimes so much smaller than this that the best microscopes cannot distinguish them. Such forms of life are called "unicellular organisms," because they consist of a single cell, which contains the jelly-like substance called protoplasm, and a smaller body, smaller even than these tiny cells themselves, which is called the nucleus.