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.
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.
This epoch marked the apogee of the human race. Thenceforward the conditions of life grew less favorable, and from century to century, from generation to generation, underwent marked change. Vegetable and animal species, the human race itself, everything in short, became transformed. But whereas, hitherto, these metamorphoses had enriched, embellished and perfected life, the day had come when decadence was to begin.
During more than a hundred thousand years it was insensible, for the parabolic curve of life did not suddenly fall away from its highest point. Humanity had reached a degree of civilization, of intellectual greatness, of physical and moral well-being, of scientific, artistic and industrial perfection, incomparably beyond anything of which we know. For several million years the central heat of the globe had been utilized in winter for general warming purposes by towns, villages, manufactories and every variety of industry. When this failing source of heat had finally become exhausted, the heat of the sun had been stored subject to the wants of the race, hydrogen had been extracted from seawater, the energy of waterfalls, and subsequently that of the tides, had been transformed into light and heat, and the entire planet had become the plaything of science, which disposed at will of all its elements. The human senses, perfected to a degree which we should now qualify as supernatural, and those newly acquired, mentioned above, become with the lapse of time more highly developed; humanity released more and more from the empire of matter; a new system of alimentation; the spirit governing the body and the gross appetites of former times forgotten; the psychic faculties in perpetual play, acting at a distance over the entire surface of the globe, communicating under certain conditions with even the inhabitants of Mars and Venus; apparatus which we cannot imagine replacing those optical instruments with which physical astronomy had begun its investigations; the whole world made new in its perceptions and interests; an enlightened social condition from which envy and jealousy, as well as robbery, suffering and murder had disappeared—this, indeed, was a real humanity of flesh and bone like our own, but as far above it in intellectual supremacy as we are above the simians of the tertiary epoch.
Human intelligence had so completely mastered the forces of nature that it seemed as if so glorious an era never could come to an end. The decrease in the amount of water, however, commenced to alarm even the most optimistic. The great oceans had disappeared. The crust of the Earth, once so thin and mobile, had gradually increased in thickness, and, notwithstanding the internal pressure, the Earth had become almost completely solidified. Oscillations of the surface were no longer possible, for it had become entirely rigid. The seas which remained were confined to the tropics. The poles were frozen. The continents of olden times, where so many other foci of civilization had shone so brilliantly, were immense deserts. Step by step humanity had migrated towards the tropical zone, still watered by streams, lakes and seas. There were no more mountains, no more condensers of snow.
As the quantity of water and rainfall diminished, and, as the springs failed and the aqueous vapor of the atmosphere grew less, vegetation had entirely changed its aspect, increasing the volume of its leaves and the length of its
