In the beginning, the planet upon which we live was (as far as we now know) a large ball of flaming matter, a tiny cloud of smoke in the endless ocean of space. Gradually, in the course of millions of years, the surface burned itself out, and was covered with a thin layer of rocks. Upon these lifeless rocks the rain descended in endless torrents, wearing out the hard granite and carrying the dust to the valleys that lay hidden between the high cliffs of the steaming earth. Finally the hour came when the sun broke through the clouds and saw how this little planet was covered with a few small puddles which were to develop into the mighty oceans of the eastern and western hemispheres. Then one day the great wonder happened. What had been dead, gave birth to life. The first living cell floated upon the waters of the sea. For millions of years it drifted aimlessly with the currents. But during all that time it was developing certain habits that it might survive more easily upon the inhospitable earth. Some of these cells were happiest in the dark depths of the lakes and the pools. They took root in the slimy sediments which had been carried down from the tops of the hills and they became plants. Others preferred to move about and they grew strange jointed legs, like scorpions and began to crawl along the bottom of the sea amidst the plants and the pale green things that looked like jelly-fishes. Still others (covered with scales) depended upon a swimming motion to go from place to place in their search for food, and gradually they populated the ocean with myriads of fishes. [Illustration: THE ASCENT OF MAN] Meanwhile the plants had increased in number and they had to search for new dwelling places. There was no more room for them at the bottom of the sea. Reluctantly they left the water and made a new home in the marshes and on the mud-banks that lay at the foot of the mountains. Twice a day the tides of the ocean covered them with their brine. For the rest of the time, the plants made the best of their uncomfortable situation and tried to survive in the thin air which surrounded the surface of the planet. After centuries of training, they learned how to live as comfortably in the air as they had done in the water. They increased in size and became shrubs and trees and at last they learned how to grow lovely flowers which attracted the attention of the busy big bumble-bees and the birds who carried the seeds far and wide until the whole earth had become covered with green pastures, or lay dark under the shadow of the big trees. [Illustration: THE PLANTS LEAVE THE SEA] But some of the fishes too had begun to leave the sea, and they had learned how to breathe with lungs as well as with gills. We call such creatures amphibious, which means that they are able to live with equal ease on the land and in the water. The first frog who crosses your path can tell you all about the pleasures of the double existence of the amphibian.