While these great changes in the planets were taking place, humanity had continued to advance; for progress is the supreme law. Terrestrial life, which began with the rudimentary protozoans, without mouths, blind, deaf, mute and almost wholly destitute of sensation, had acquired successively the marvellous organs of sense, and had finally reached its climax in man, who, having also grown more perfect with the lapse of centuries, had risen from his primitive savage condition as the slave of nature to the position of a sovereign who ruled the world by mind, and who had made it a paradise of happiness, of pure contemplation, of knowledge and of pleasure.

Men had attained that degree of intelligence which enabled them to live wisely and tranquilly. After a general disarmament had been brought about, so rapid an increase in public riches and so great an amelioration in the well-being of every citizen was observed, that the efforts of intelligence and labor, no longer wasted by this intellectual suicide, had been directed to the conquest of new forces of nature and the constant improvement of civilization. The human body had become insensibly transformed, or more exactly, transfigured.

Nearly all men were intelligent. They remembered with a smile the childish ambitions of their ancestors whose aspiration was to be someone rather than something, and who had struggled so feverishly for outward show. They had learned that happiness resides in the soul, that contentment is found only in study, that love is the sun of the heart, that life is short and ought not to be lived superficially; and thus all were happy in the possession of liberty of conscience, and careless of those things which one cannot carry away.

Woman had acquired a perfect beauty. Her form had lost the fullness of the Greek model and had become more slender; her skin was of a translucent whiteness; her eyes were illuminated by the light of dreams; her long and silky hair, in whose deep chestnut were blended all the ruddy tints of the setting sun, fell in waves of rippling light; the heavy animal jaw had become idealized, the mouth had grown smaller, and in the presence of its sweet smile, at the sight of its dazzling pearls between the soft rose of the lips, one could not understand how lovers could have pressed such fervent kisses upon the lips of women of earlier times, specimens of whose teeth, resembling those of animals, had been preserved in the museums of ethnography. It really seemed as if a new race had come into existence, infinitely superior to that to which Aristotle, Kepler, Victor Hugo, Phryne, or Diana of Poictiers had belonged.

Thanks to the progress in physiology, hygiene, and antiseptic science, as well as to the general well-being and intelligence of the race the duration of human life had been greatly prolonged, and it was not unusual to see persons who had attained the age of 150 years. Death had not been conquered, but the secret of living without growing old had been found, and the characteristics of youth were retained beyond the age of one hundred.

But one fatherland existed on the planet, which, like a chorus heard above the chords of some vast harmony, marched onward to its high destiny, shining in the splendor of intellectual supremacy.

The internal heat of the globe, the light and warmth of the sun, terrestrial magnetism, atmospheric electricity, inter-planetary attraction, the psychic forces of the human soul, the unknown forces which preside over destinies,—all these science had conquered and controlled for the benefit of mankind. The only limits to its conquests were the limitations of the human faculties themselves, which, indeed, are feeble, especially when we compare them with those of certain extra-terrestrial beings.

All the results of this vast progress, so slowly and gradually acquired by the toil of centuries, must, in obedience to a law, mysterious and inconceivable for the petty race of man, reach at last their apogee, when further advance becomes impossible. The geometric curve which represents this progress of the race, falls as it rises: starting from zero, from the primitive nebulous cosmos, ascending through the ages of planetary and human history to its lofty summit, to descend thereafter into a night that knows no morrow.

Yes! all this progress, all this knowledge, all this happiness and glory, must one day be swallowed up in oblivion, and the voice of history itself be forever silenced. Life had a beginning: it must have an end. The sun of human hopes had risen, had ascended victoriously to its meridian, it was now to set and to disappear in endless night. To what end then all this glory, all this struggling, all these conquests, all these vanities, if light and life must come to an end?

Martyrs and apostles, in every cause, have poured out blood upon the earth, destined also in its turn to perish.

Everything is doomed to decay, and death must remain the final sovereign of the world. Have you ever thought, in viewing a village cemetery, how small it is, to contain the generations buried there from time immemorable? Man existed before the last glacial epoch, which dates back 200,000 years; and the age of man extends over a period of more than 250,000 years. Written history dates from yesterday. Cut and polished flints have been found at Paris, proving the presence of man on the banks of the Seine long before the first historic record of the Gauls. The Parisians of the close of the nineteenth century walk upon ground consecrated by more than ten thousand years of ancestry. What remains of all who have swarmed in this forum of the world? What is left of the Romans, the Greeks, and the Asiatics, whose empires lasted for centuries? What remains of the millions who have existed? Not even a handful of ashes.

A human being dies every second, or about 86,000 a day, and an equal number, or to speak more exactly, a little more than 86,000 are born daily. This figure, true for the nineteenth century, applies to a long period, if we increase it proportionately to the time. The population of the globe has increased from epoch to epoch. In the time of Alexander there were perhaps a thousand million living beings on the surface of the earth. At the end of the nineteenth century fifteen hundred million; in the twenty-second century two thousand million; in the twenty-ninth three thousand million; at its maximum the population of the globe had reached one hundred thousand million. Then it had begun to decrease.

Of the innumerable human bodies which have lived, not one remains. All have been resolved into their elements, which have again formed new individuals.

All that fills the passing day—labor, pleasure, grief and happiness—vanishes with it into oblivion. Time flies, and the past exists no longer; what has been, has disappeared in the gulf of eternity. The visible world is vanishing every instant. Only the invisible is real and enduring.

During the ten million years of history, the human race, surviving generation after generation, as if it were a real thing, had been greatly modified from both a physical and moral point of view. It had always remained master of the world, and no new race had aspired to its sovereignty; for races do not come down from heaven or rise from hell; no Minerva is born full-armed, no Venus awakes full-grown in a shell of pearl on the seashore; everything grows, and the human race, with its long line of ancestry, was from the very beginning the natural result of the vital evolution of the planet. Under the law of progress, it had emerged from the limbo of animalism, and by the continued action of this same law of progress it had become gradually perfected, modified and refined.

But the time had come when the conditions of terrestrial life began to fail; when humanity, instead of advancing, was itself to enter upon its downward path.

The internal heat of the globe, still considerable in the nineteenth century, although it had ceased to have any effect upon surface temperature, which was maintained solely by the sun, had slowly diminished, and the earth had, at last, become entirely cold. This had not directly influenced the physical conditions of terrestrial life, which continued to depend upon the atmosphere and solar heat. The cooling of the earth cannot bring about the end of the world.

Imperceptibly, from century to century, the earth’s surface had become levelled. The action of the rain, snow, frost and solar heat upon the mountains, the waters of torrents, rivulets and rivers, had slowly carried to the sea the dÉbris of every continental elevation. The bottom of the sea had risen, and in nine million years the mountains had almost entirely disappeared. Meanwhile, the planet had grown old faster than the sun; the conditions favorable to life had disappeared more rapidly than the solar light and heat.

This conception of the planet’s future conforms to our present knowledge of the universe. Doubtless, our logic is radically incomplete, puerile even, in comparison with the real and eternal Truth, and might be justly compared with that of two ants talking together about the history of France. But, confessing the modesty which befits the finite in presence of the infinite, and acknowledging our nothingness as compared with the universe, we cannot avoid the necessity of appearing logical to ourselves; we cannot assume that the abdication of reason is a better proof of wisdom than the use of it. We believe that an intelligent order presides over the universe and controls the destiny of worlds and their inhabitants; that the larger members of the solar system must last longer than the lesser ones, and, consequently, that the life of each planet is not equally dependent upon the sun, and cannot, therefore, continue indefinitely, any more than the sun itself. Moreover, direct observation confirms this general conception of the universe. The earth, an extinct sun, has cooled more rapidly than the sun. Jupiter, so immense, is still in its youth. The moon, smaller than Mars, has reached the more advanced stages of astral life, perhaps even has reached its end. Mars, smaller than the earth, is more advanced than the earth and less so than the moon. Our planet, in its turn, must die before Jupiter, and this, also, must take place before the sun becomes extinct.

Consider, in fact, the relative sizes of the earth and the other planets. The diameter of Jupiter is eleven times that of the earth, and the diameter of the sun about ten times that of Jupiter. The diameter of Saturn is nine times that of the earth. It seems to us, therefore, natural to believe that Jupiter and Saturn will endure longer than our planet, Venus, Mars or Mercury, those pigmies of the system!

Events justified these deductions of science. Dangers lay in wait for us in the immensity of space; a thousand accidents might have befallen us, in the form of comets, extinct or flaming suns, nebulÆ, etc. But the planet did not perish by an accident. Old age awaited the earth, as it waits for all other things, and it grew old faster than the sun. It lost the conditions necessary for life more rapidly than the central luminary lost its heat and its light.

During the long periods of its vital splendor, when, leading the chorus of the worlds, it bore on its surface an intelligent race, victors over the blind forces of nature, a protecting atmosphere, beneath which went on all the play of life and happiness, guarded its flourishing empires. An essential element of nature, water, regulated terrestrial life; from the very beginning this element had entered into the composition of every substance, vegetable, animal and human. It formed the active principle of atmospheric circulation; it was the chief agent in the changes of climate and seasons; it was the sovereign of the terrestrial state.

From century to century the quantity of water in the sea, the rivers and the atmosphere diminished. A portion of the rain water was absorbed by the earth, and did not return to the sea; for, instead of flowing into the sea over impermeable strata, and so forming either springs or subterranean and submarine watercourses, it had filtered deeper within the surface, insensibly filling every void, every fissure, and saturating the rocks to a great depth. So long as the internal heat of the globe was sufficient to prevent the indefinite descent of this water, and to convert it into vapor, a considerable quantity remained upon the surface; but the time came when the internal heat of the globe was entirely dispersed in space and offered no obstacle to infiltration. Then the surface water gradually diminished; it united with the rocks, in the form of hydrates, and thus disappeared from circulation.

Indeed, were the loss of the surface water of the globe to amount only to a few tenths of a millimeter yearly, in ten million years none would remain.

This vapor of water in the atmosphere had made warmth and life possible; with its disappearance came cold and death. If at present the aqueous vapor of the atmosphere should disappear, the heat of the sun would be incapable of maintaining animal and vegetable life; life which, moreover, could not exist, inasmuch as vegetables and animals are chiefly composed of water.^[4]

The invisible vapor of water, distributed through the atmosphere, exercises the greatest possible influence on temperature. In quantity this vapor seems almost negligible, since oxygen and nitrogen alone form ninety-nine and one-half per cent. of the air we breathe; and the remaining one-half of one per cent. contains, besides the vapor of water, carbonic acid, ammonia and other substances. There is scarcely more than a quarter of one per cent. of aqueous vapor. If we consider the constituent atoms of the atmosphere, the physicist tells us that for two hundred atoms of oxygen and nitrogen there is scarcely one of water-vapor; but this one atom has eighty times more absorptive energy than the two hundred others.

The radiant heat of the sun, after traversing the atmosphere, warms the surface of the earth. The heat waves reflected from the warmed earth are not lost in space. The aqueous vapor atoms, acting like a barrier, turn them back and preserve them for our benefit.

This is one of the most brilliant and the most fruitful discoveries of modern physics. The oxygen and nitrogen molecules of dry air do not oppose the radiation of heat; but, as we have just said, one molecule of water-vapor possesses eighty times the absorptive energy of the other two hundred molecules of dry air, and consequently such a molecule is sixteen thousand times more efficacious in so far as the conservation of heat is concerned. So that it is the vapor of water and not the air, properly speaking, which regulates the conditions of life upon the earth.

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.

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 sea-water, 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 roots, seeking in every way to absorb the humidity necessary for life. Species which had not been able to adjust themselves to the new conditions had vanished; the rest were transformed. Not a tree or a plant with which we are familiar was to be seen. There were no oaks, nor ashes, nor elms, nor willows, and the landscape bore no resemblance to that of today. Rudimentary species of cryptogams only survived.

Like changes had taken place in the animal kingdom. Animal forms had been greatly modified. The wild species had either disappeared or been domesticated. The scarcity of water had modified the food of herbivora as well as carnivora. The most recent species, evolved from those which preceded them, were smaller, with less fat and a larger skeleton. The number of plants had sensibly decreased. Less of the carbonic acid of the air was absorbed, and a proportionally greater quantity existed in the atmosphere. As for the human race, its metamorphosis was so absolute that it was with an astonishment bordering on incredulity that one saw in geological museums fossil specimens of men of the twentieth or one hundredth century, with great brutal teeth and coarse intestines; it was difficult to admit that organisms so gross could really be the ancestors of intellectual man.

Though millions of years had passed, the sun still poured upon the earth almost the same quantity of heat and light. At most, the loss had not exceeded one-tenth. The only difference was that the sun appeared a little yellower and a little smaller.

The moon still revolved about the earth, but more slowly. Its distance from the earth had increased and its apparent diameter had diminished. At the same time the period of the earth’s rotation had lengthened. This slower rotatory motion of the earth, increase in the distance of the moon, and lengthening of the lunar month, were the results of the friction of the tides, whose action resembled that of a brake. If the earth and the moon last long enough, and there are still oceans and tides, calculation would enable us to predict that the time would come when the periodic time of the earth’s rotation would finally equal the lunar month, so that there would be but five and one-quarter days in the year: the earth would then always present the same side to the moon. But this would require more than 150 million years. The period of which we are speaking, ten million years, is but a fifteenth of the above; and the time of the earth’s rotation, instead of being seventy times, was only four and one-half times greater than it now is, or about 110 hours.

These long days exposed the earth to the prolonged action of the sun, but except in those regions where its rays were normal to the surface, that is to say in the equatorial zone between the two tropical circles, this exposure availed nothing; the obliquity of the ecliptic had not changed; the inclination of the axis of the earth being the same, about two degrees, and the changes in the eccentricity of the earth’s orbit had produced no sensible effect upon the seasons or the climate.

The human form, food, respiration, organic functions, physical and intellectual life, ideas, opinions, religion, science, language—all had changed. Of present man almost nothing survived.