If one should remove this vapor from the surrounding atmosphere, a loss of heat would go on at the surface similar to that which takes place in high altitudes, for the atmosphere would then be as powerless to retain heat as a vacuum is. A cold like that at the surface of the moon would be the result. The soil would still receive heat directly from the sun, but even during the daytime this heat would not be retained, and after sunset the earth would be exposed to the glacial cold of space, which appears to be about 273° below zero. Thus vegetable, animal and human life would be impossible, if it had not already become so, through the very disappearance of the water. Certainly we may and must admit that water has not been so essential a condition of life on all the worlds of space as it has been upon our own. The resources of nature are not limited by human observation. There must be, there are, in the limitless realms of space, millions and millions of suns differing from ours, systems of worlds in which other substances, other chemical combinations, other physical and mechanical conditions, other environments, have produced beings absolutely unlike ourselves, living another life, possessed of other senses, differing in organization from ourselves far more than the fish or mollusk of the deep sea differs from the bird or the butterfly. But we are here studying the conditions of terrestrial life, and these conditions are determined by the constitution of the planet itself. [Illustration: FOSSIL SPECIMENS OF THE XXTH. CENTURY. ] The gradual filtration of water into the interior of the earth, keeping pace with the radiation of the earth’s original heat into space, the slow formation of oxides and hydrates, in about eight million years reduced by three-fourths the quantity of water in circulation on the earth’s surface. As a consequence of the disappearance of continental elevations, whose débris, obeying passively the laws of gravity, were slowly carried by the rain, the wind, and the streams to the sea, the earth had become almost level and the seas more shallow; but as evaporation and the formation of aqueous vapor goes on only from the surface and does not depend upon the depth, the atmosphere was still rich in vapor. The conditions of life upon the planet were then similar to those we now observe on Mars; where we see that great oceans have disappeared or have become mere inland seas of slight depth, that the continents are vast plains, that evaporation is active, that a considerable quantity of aqueous vapor still exists, that rains are rare, that snows abound in the polar regions and are almost entirely melted during the summer of each year—in short, a world still habitable by beings analogous to those that people the earth.