The Evolution of Worlds/Chapter 7
CHAPTER VII
A PLANET'S HISTORY
Sun-sustained Stage
TWO stages have characterized the surface history of the Earth,—stages which may be likened to the career of the chick within and without the egg. In the first of them the Earth lay screened from outside influence under a thick shell of cloud, indifferently exclusive of the cold of space or of the heating beams of the Sun. Motherless, the warmth of its own body brooded over it, keeping its heat from dissipating too speedily into space, and so fostering the life that was quickening upon its surface.
The second stage began when the egg-shell broke and the chick lay exposed to the universe about it, to get its living no longer from its little world within, but from the greater one without. One and the same event ended the old life to make possible the new. So soon as the cloud envelope was pierced, both the Earth's own heat escaped and the Sun's rays were permitted to come in.
It is not surprising that under such changed conditions development itself should have changed, too. In fact, the transformation was marked. That its epochal character has failed to impress itself generally on geologists, is perhaps because they look too closely, missing the march of events in the events themselves,
and because, too, of the gradual nature of its processional change. We can recall only De Lapparent as having particularly signalled it; although not only in its cause, but for its effects, it should have delimited two great geologic divisions of time.
Astronomy and geology are each but part of one universal history. The tale each has to tell must prove in keeping with that of the other. If they seem at variance, it behooves us very carefully to scan their respective stories to find the flaw where the apparent incongruity slipped in. Each, too, fittingly supplements the other, and especially must geology look to astronomy for its initial data, since astronomy deals with the beginning of our own Earth.
That study of our Earth in its entirety falls properly within the province of astronomy, is not only deducible from its relationship to the other planets, but demonstrable from the cosmic causes that have been at work upon it, and the inadequacy of anything but cosmic laws to explain them. The ablest geologists to-day are becoming aware of this,—we have one of them at the head of the geology department of the Institute,—while from the curious astronomy at second hand which gets printed in geologic text-books, by eminent men at that, dating from some time before the flood,—of modern ideas,—it seems high time that the connection should be made clear.
For, after all, our Earth too is a heavenly body, in spite of man's doing his best to make it the reverse. It has some right to astronomic regard, even if it is our own mother. At the same time it is quite puerile to consider the universe as bounded by our terrestrial backyard. If man took himself a thought less importantly, he might perceive the humor of so circumscribed a view. Like children we play at being alone in the universe, and then go them one better by believing it too.
I shall, of course, not touch on any matters purely geologic, for fear of committing the very excesses I deplore; mentioning only such points as astronomy has information on, and which, by the sidelights it throws, may help to illuminate the subject.
Thus it certainly is interesting and may to many be a new point of view, that the changes introduced when paleologic times passed into neologic ones were in their fundamental aspects essentially astronomic; which shows how truly astronomic causes are woven into the whole fabric of the Earth. For it was then only, terrestrially speaking, that the year began. The orbital period had existed, of course, from the time the Earth first made the circuit of the Sun. But the year was more a succès d' estime on the Sun's part than one of popular appreciation. As the Sun could not be seen and worked no striking effects upon the Earth, the annual round had no recognizable parts, and one revolution lapsed into the next without demarcation. Only with the clearing of the sky did the seasons come in: to register time by stamping its record on the trees. Before that, summer and winter, spring and autumn, were unknown.
Climate, too, made then its first appearance; climate, named after the sunward obliquity of the Earth, and seeming at times to live down to that characterization. Weather there had been before; pejoratively speaking, nothing but weather. For the downpours in paleologic times must have been exceeded in numbers only by their force. One dull perpetual round of rain was the programme for the day, with absolutely no hope of a happy clearance to-morrow. It was the golden age only for weather prophets whose prognostications could hardly go wrong. With climate, however, it was a very different matter. With polyp corals building reefs almost to the pole (81° 50′),[1] as far north nearly as man has yet by his utmost efforts succeeded in getting, while their fellows were busy at the like industry in the tropics, it is clear that latitude was laughed at and climate even lacked a name.
Another astronomic feature, then for the first time disclosed, was the full significance of the day and the revelation of its cause. While the Earth brooded under perpetual cloud, there could have been but imperfect recognition of day and night. Or perhaps we may put it better by saying that the standard of both was greatly depressed, dull days alternating with nights black as pitch. But the moment the Sun was let in, all this changed, though not in a twinkling. The change came on most gradually. We can see in our mind's eye the first openings in the great welkin permitting the Earth its initial peeps of the world beyond, and how quickly and tantalously they shut in again like a mid-storm morning which dreams of clearing only to find how drowsy it still is. But eventually the clouds parted afresh and farther, and the Earth began to open its eyes to the universe without.
The cause of the clearing, of course, was the falling temperature of the seas. Evaporation went on much less fast as the heat of the water lessened. The whole round of aquatic travel from ocean to air, and back to ocean again, proceeded at an everslackening pace. And here, if it so please geologists, may be found a reconciling of their demands for time to the relative pittance astronomy has been willing to dole them out, a paltry 50 or 100 millions of years, which like all framers of budgets they have declared utterly insufficient. For in early times the forces at work were greater, and by magnifying the means you quicken the process and contract the Earth's earlier eras to reasonable limits.
Upon these various astronomic novelties, the Earth on thus awakening looked for the first time. Such regard altered for good its own internal relations. The wider outlook made impossible the life of the narrower that preceded it. A totally changed set of animals and plants arose, to whom the cosmos bore a different aspect. The Earth ceased to be the self-centred spot it seemed before. As long ago as this had the idea that our globe was the centre of the universe been cosmically exploded. The Earth knew it if man did not.
Its denizens responded. The organisms that already inhabited it proceeded to change their character and crawl out upon the land. For in Devonian times the Earth was the home of fishes. The land was not considered a fit abode by anything but insects, and not over-good by them. But it looked different when the Sun shone. Some maritime dwellers felt tempted to explore, and proceeded in the shape of amphibians to spy
out the land. They have left very readable accounts of their travels in foot-notes by the way. As one should always inspect the original documents, I will reproduce the foot-notes of one early explorer. It is one of the few copies we have, as the type is worn out. But it tells a pretty full story as it stands. The ripple-marks show that a sea beach it was which the discoverer trod in his bold journey of a few feet from home and friends, and the pits in the sandstone that it was raining at the time of his excursion. No Columbus or Hakluyt could have left a record more precise or more eminently trustworthy. The pilgrims found it so good that their eventual collaterals, the great reptiles, actually took possession of the land and held it for many centuries by right of eminent domain. Yet throughout the time of these bold adventurers, their skies were only clearing, as the pitting of the sandstone eloquently states.
It was not till the chalk cliffs of Dover were being laid down that we have evidence that seasons had fully developed, in the shape of the first deciduous trees.[2] Cryptogams, cycads, and, finally, conifers had in turn represented the highest attainments of vegetation, and the last of these had already recognized the seasons by a sort of half-hearted hibernation or annual moulting; deeming it wise not to be off with the old leaves before they were on with the new. But finally the most advanced among them decided unreservedly to accept the winter and go to sleep till spring. The larches and ginkgo trees are descendants of the leaders of this coniferous progressive party.
At the same time color came in. We are not accustomed to realize that nature drew the Earth in grays and greens, and touched it up with color afterward. Only the tempered tints of the rocks and the leaden blue of the sea, subdued by the disheartening welkin overhead to a dull drab, enlivened their abode for the oldest inhabitants. But with Tertiary times entered the brilliantly petalled flowers. Beginning with yellow, these rose through a chromatic scale of beauty from white through red to blue.[3] They decked themselves thus gaudily because the Sun was there to see by, as well as eyes to see. For without the Sun those unconscious horticulturists, the insects, could not have exercised their pictorial profession.
To the entering of the Sun upon the scene this wondrous revolution was due; and once entered, it became the dominant factor in the Earth's organic life. We are in the habit of apostrophizing the Sun as the source of all terrestrial existence. It is true enough to-day, and has been so since man entered on the scene. But it was not always thus. There was a time when the Sun played no part in the world's affairs.
As its heat is now all-important, it becomes an interesting matter to determine the laws governing its amount. That summer is hotter than winter we all know from experience, pleasurable or painful as the case may be. This is due to the fact that the Sun is above the horizon for a greater number of hours in summer and passes more directly overhead. But not so many people are aware that on midsummer day, so far as the Sun is concerned, the north pole should be the hottest place on earth. That Arctic explorers, who have got within speaking acquaintance of it, assure us it is not so, shows that something besides the direct rays of the Sun is involved. Indeed, we learn as much from the extensively advertised thermometers of winter resorts which, judiciously placed, beguile the stranger to sojourn where it is just too cold for comfort. The factor in question is the blanketing character of our air. Now a blanket may keep heat out as well as keep it in. Our air acts in both capacities. It is by no means simply a storer of heat, as many people seem to suppose; it is a heat-stopper as well. What it really is is a temporizer, a buffer to ease the shocks of sudden change like those comfortable, phlegmatic souls who reduce all emotion to a level. For the heating power of the Sun, even at the Earth's distance away, is much greater than appears. Knowledge of this we owe most to Langley, and then to Very, who continued his results to yet a finer determination, the best we have to-day. In consequence we have learnt that the amount of heat we should receive from the Sun, could we get above our air,—the solar constant, as it is called,—would be over three times what it is on the average in our latitude at the surface, and is rising still, so to speak. For as man has gone higher he has found his inferences rising too, and the limit would seem to be not yet. We see then that the air to which we thought ourselves so much indebted, actually begins its kindly offices by shutting off two-thirds of what was coming to us. As it plays, however, something of the same trick to what tries to escape, we are really somewhat beholden to it after all.
But not so much as has been thought. We used to be told that the Moon's temperature even at midday hardly rose above freezing, but Very has found it about 350°F., which even the most chilly of souls might find warm. By the late afternoon, however, he would need his overcoat, and no end of blankets subsequently, for during the long lunar night of fourteen days the temperature must fall appallingly low, to—300°F. or less.
As the determination of temperature is a vital one, not only to any organic existence, but even to inorganic conditions upon a planet, it behooves us to look carefully into the question of the effective heat received from the Sun. Until recently the only criterion in the case was assumed to be distance from the illuminating source, about as efficient a mode of computation as estimating a Russian army by its official roll. For as we saw in our own case, not all that ought to ever gets to the front, to say nothing of what is lost there. Yet on this worse than guesswork astronomic text-books still assert as a fact that the temperature of other bodies—the Moon and Mars, for example—must be excessively low.
Let us now examine into this most interesting problem. It is intricate, of course, but I think you will find it more comprehensible than you imagine. Indeed, I shall be to blame if you do not. For if one knows his subject, he can always explain it, in untechnical language, technical terms being merely a sort of shorthand for the profession. The physical processes involved can be made clear without difficulty, although their quantitative evaluation is less forthrightly demonstrable. Let me, then, give you an epitome of my investigation of the subject.
Consider a ray of light falling on a surface from the
Sun. A part of it is reflected; that is, is instantly thrown off again. By this part the body shines and makes its show in the world, but gets no good itself. Another part is absorbed; this alone goes to heat the body. Now if the visible rays were all that emanated from the Sun, it would be strictly true, and a pretty paradox for believers in the efficacy of distance, that what heated the planet was precisely what seemed not to do so. Unfortunately there are also invisible rays, and these, too, are in part reflected and in part absorbed, and their ratio is different from that of the visible ones. To appreciate them, Langley invented the bolometer, in which heat falling on a strip of metal produces a current of electricity registered by a galvanometer. By thus recording the heat received at different parts of the spectrum and at different heights in our atmosphere, he was able to find how much the air cut off. Very has since determined this still more accurately. By thus determining the depletion in the invisible part of the spectrum joined to what astronomy tells us of the loss in the visible part, we have a value for the whole amount. By knowing, then, the immediate brightness of a planet and approximately the amount of atmosphere it owns, we are enabled to judge how much heat it actually receives. This proves to be, in the case of Mars, more than twice as much as distance alone would lead us to infer.
The second question is how much of this it retains. The temperature of a body at any moment is the balance struck between what it receives and what it radiates. If it gets rid of a great deal of its income, it will clearly be less hot than if it is miserly retentive. To find how much it radiates we may take the difference in temperature between sunset and sunrise, since during this interval the Earth receives no heat from the Sun. In the same way the efficacy of different atmospheric blankets may be judged. Thus the Earth parts with nine centigrade degrees' worth of its store on clear nights, and only four degrees' worth on cloudy ones, before morning. This is at sea-level. By going up a high mountain we get another set of depletions, and from this a relative scale for different atmospheric blankets. This is the principle, and we only have to fill out the skeleton of theory with appropriate numbers to find how warm the body is.
In doing so, we light on some interesting facts. Thus clouds reflect 72 per cent of the visible rays, letting through only 28 per cent of them. We feel chilly when a cloud passes over the Sun. On the other hand, slate reflects only 18 per cent of the visible rays, absorbing all the rest. This is why slate gets so much hotter in the Sun than chalk, and why men wear white in the tropics. White, indeed, is the best color to clothe one's self in the year around, except for the cold effect it has on the imagination, for it keeps one's own heat in as well as keeping the Sun's out. The modest, self-obliterating, white winter habit of the polar hares not only enables them to keep still and escape notice, but keeps them warm while they wait.
Astronomically, the effect is equally striking. Mars, for example, owing to being cloudless and of a duller hue, turns out to have a computed mean temperature nearly equal to the Earth's,—a theoretic deduction which the aspect of the planet most obligingly corroborates. It thus enjoys a comparatively genial old age.
For what is specially instructive in planetary economy is that, on the whole, clear skies add more by what they let in than they subtract by what they let out. If the Earth had no clouds at all, its mean temperature would be higher than it is to-day. Thus as a planet ages a beneficent compensation is brought about, the Sun's heat increasing as its own gives out. Not that the foreign importation, however slight the duty levied on it by the air, ever fully makes up for the loss of the domestic article, but it tempers the refrigeration which inevitably occurs.
The subject of refrigeration leads us to one of the most puzzling and vexed problems of geology: how to account for the great Ice Age of which the manifest sign manuals both in Europe and in America have so intrigued man since he began to read the riddle of the rocks. Upon this, also, planetology throws some light.
If I needed an apology to the geologists for seeming again to trespass on their particular domain, I might refer to the astrocomico expositions put forward to account for the great Ice Age.
We can all remember Croll's "Climate and Time," a book which can hardly be overpraised for its title and which had things worth reading inside, too. It had in consequence no inconsiderable vogue at one time. It undertook to account for glacial epochs on astronomic principles. It called in such grand cosmic conditions and dealt in such imposing periods of time that it fired fancy and almost compelled capitulation by the mere marshalling of its figurative array. Secular change in the eccentricity of the Earth's orbit, combined with progression in the orbital place of the winter's solstice, was supposed to have induced physical changes of climate which accentuated the snowfall in the northern hemisphere and so caused extensive and permanent glaciation there. In other words, long, cold winters followed by short, hot summers in one hemisphere were credited with accumulating a perpetual snow sheet, such as short, warm winters and long, cold summers could not effect.
Now it so happens that these astronomic conditions affecting the Earth several thousand years ago, are in process of action on one of our nearest planetary neighbors at the present time. The orbit of Mars is such that its present eccentricity is greater than what the Earth ever can have had, and the winter solstice of the planet's southern hemisphere falls within 23° of its aphelion point. We have then the conditions for glaciation if these are the astronomic ones supposed, and we should expect a southern polar cap, larger at its maximum and still more so, relatively, at its minimum, than in the opposite hemisphere. Let us now look at the facts, for we have now a knowledge
| Mars | |||
| NORTH POLAR CAP. | SOUTH POLAR CAP. | ||
| At maximum full | extent of white | At maximum | white |
| At minimum | inner circle | At minimum | nothing |
of the Martian polar caps exceeding in some respects what we know of our own. The accompanying diagrams exhibit the state of things at a glance, the maximum and minimum of each cap being represented in a single picture and the two being placed side by side. It will be observed that the southern cap outdoes its antipodal counterpart at its maximum, showing that the longer, colder winter has its effect in snow or hoar-frost deposition. But, on the other hand, instead of excelling it at its minimum, which it should do to produce permanent glaciation, it so far falls short of its fellow that during the last opposition at which it could be well observed, it disappeared entirely. The short, hot summer, then, far exceeded in melting capacity that of the longer but colder one.
Let us now suppose the precipitation to be increased, the winters and summers remaining both in length and temperature what they were before. The amount of snow which a summer of given length and warmth can dispose of is, roughly speaking, a definite quantity. For it depends to a great extent only on its amount of heat. The summer precipitation may be taken as offsetting itself in the two hemispheres alike. If, then, the snow-fall in the winter be for any reason increased daily in both, a time will come when the deposition due the longer winter of the one will exceed what its summer can melt relatively to the other, and a permanent glaciation result in the hemisphere so circumstanced. Increased precipitation, then, not eccentricity of orbit, is the real cause of an Ice Age. And this astronomic deduction we owe not to theoretic conclusions, for which we lack the necessary quantitative data, but wholly to study of our neighbor in space. Had any one informed our geologic colleagues that they must look to the sky for definite information about the cause of an Ice Age, they would probably have been surprised.
With this Martian information, received some years ago, it is pleasing now to see that Earthly knowledge is gradually catching up. For that increased precipitation could account for it, the evidence of pluvial eras in the
equatorial regions, contemporaneous with glacial periods, indicates. But another and probably the chief factor involved was not a generally increased precipitation, potent as that would be, but an increased snow deposit due to temporary elevation of the ground.
For it now appears that there was no glacial epoch. Our early ideas inculcated by text-books at school received a rude shock when it appeared that the glacial epoch was not, as we had been led to believe, a polar phenomenon at all, but a local affair which on the face of it had nothing to do with the pole. For investigation has disclosed that instead of emanating from the
Map showing the glaciated area of North America—the arrows indicating the direction of ice movement—Chamberlin and Salisbury.
pole southward, it proceeded from certain centres, descending thence in all directions, north as much as south. Thus there was a centre in Norway in 65° N. lat. and another in Scotland in 56° N. In North America there were three—the Labradorian in latitude 54° N., the Kerwatin to the northwest of Hudson's Bay in latitude 62° N., and the Cordilleran along the Pacific coast in latitude 58° N. On the other hand, northern Siberia, the coldest region in the world, was not glaciated. That the ice flowed off these centres proves them to have been higher than the sides. But we have further evidence of their then great height from the fact that dead littoral shells have been dredged from 1333 fathoms in the North Atlantic, and the prolongation under water of the fjords of Norway and of land valleys in North America witness to the same subsidence since.
But evidence refuses to stop here. The Alps were then more glaciated than they are now. So was Kilimanjaro and Ruwenzori on the equator; and finally at the same time more ice and snow existed round about the south pole than is the case to-day. Now this is really going too far even for the most ardent believers in the force of eccentricity. For if the astronomic causes postulated were true, they must have produced just the opposite action at the antipodes, to say nothing of the crux of being equally effective at the equator. The theory lies down like the ass between two burdens. Whichever load it chooses to saddle, it must perforce abandon the other.
So it turns out that the Ice Age was not an Ice Age at all but an untoward elevation of certain spots, and is to be relegated to the same limbo of exaggeration of a local incident into a world-wide cataclysm as the deluge. That some geologists will still cling to their former belief I doubt not; for as the philosophic old lady remarked: "There always have been two factions on every subject. Just as there are the suffragists and anti-sufragists now, so there were slaveholders and the anti-slavery people in my time; and even in the days of the deluge, there were the diluvians who were in favor of a flood and the antediluvians who were opposed to it." A tale which has a peculiarly scientific moral, as in science anti and ante seem often interchangeable terms.
When I began the course of lectures that resulted in this volume, I labored under the apprehension that an account of cosmic physics might prove dull. It soon threatened to prove too startling. I therefore hasten to reassure the timid by saying that we are outgrowing ice ages and probably deluges. Elevations of the Earth's crust are likely to be less and less pronounced in the future, and meanwhile such as exist are being slowly worn down. Secondly, the Sun is sure to continue of much the same efficiency for many æons to come. And lastly, the essential ingredient of both prodigies, water, is daily becoming more scarce. To this latter point we now turn, and perhaps when it is explained to him the reader may think that he has been rescued from one fate only to fall into the hands of another.
Geology is necessarily limited in its scope to what has happened; planetology is not so circumscribed in its domain. It may indulge in prognostication of the future, and find countenance for its conclusions in the physiognomy of other worlds. Thus one of the things which it foresees is the relative drying up of our abode. To those whose studies have never led them off this earth, the fact that the oceans are slowly evaporating into space may seem as incredible as would, to one marooned on a desert island, the march of mankind in the meantime. We live on an island in space, but can see something of the islands about us, and our conception of what is coming to our limited habitat can be judged most surely by what we note has happened to others more advanced than ourselves. Just as we look at Jupiter to perceive some likeness of what we once were, the real image of which has travelled by this time far into the depths of space beyond possibility of recall, so must we look to the Moon or Mars if we desire to see some faint adumbration of the pass to which we are likely to come. For from their lack of size they should have preceded us on the road we are bound to travel. Now, both these worlds to-day are water-lacking, in whole or part; the Moon practically absolutely so, Mars so far as any oceans or seas are concerned. We should do wisely then to take note. But we have more definite information than simply their present presentments. For both bear upon their faces marks of having held seas once upon a time. They were once, then, more as we are now. We cannot of course be sure, as we are unable to get near enough to scan their surfaces for signs of erosive action. But so far as we can make out, past seas best explain their appearance.
So sealike, indeed, was their look that the first astronomers to note them took them unhesitatingly for water expanses. Thus the moment the telescope brought the Moon near enough for map making of it we find the
The Moon—Photographed at the Lowell Observatory.
dark patches at once designated as seas. The Sea of Serenity, the Sea of Showers, the Bay of Rainbows, speak still of what once was supposed to be the nature of the dark, smooth, lunar surfaces they name. Suggestively, indeed, in an opera glass do they seem to lap the land. The Lake of Dreams fore-shadowed what was eventually to be thought of them. With increasing optical approach the substance evaporated, but the form remained. It was speedily evident that there was no water there; yet the semblance of its repository still lurked in those shadows and suggests itself to one scanning their surfaces to-day. If they be not old sea bottoms, they singularly mimic the reality in their smooth, sloping floors and their long, curving lines of beach. Their strange uniformity shows that something protected them from volcanic fury while the rest of the lunar face was being corrugated. This preservative points to some superincumbent pressure which can have been no other than water. Lava-flows on such a scale seem inadmissible. What these surfaces show and what they do not show alike hint them sea bottoms once upon a time. In the strange chalk-like hue of the lunar landscape they look like plaster of Paris death-masks of the former seas.
A like history fell to the lot of the surface features of Mars. There too, as soon as the telescope revealed them and their permanency of place, the dark patches upon the planet's face were forthrightly taken for seas, and were so called: the Sea of the Sirens and the Great Red Sea. Such they long continued to be deemed. The seas of Mars held water in theory centuries after the idea of the lunar had vanished into air. At last, ruthless science pricked the pretty bubble analogy had pictured. Being so much farther off than the Moon, it was much later that their true character came out. Come out it has, though, within the last few years. Lines—some of the so-called canals—have been detected crossing the seas, lines persistent in place. This has effectually disposed of any water in them. But here again something of semblance is left behind. They are still the darkest portions of the planet, and their tint changes in places with the progress of the planet's year. That their color is that of vegetation, and that its change obeys the seasons, stamp it for vegetation in fact. Thus these regions must be more humid than the rest of Mars. They must, therefore, be lower. That they are thus lower and possess a modicum of water to-day marks them out for the spots where seas would be, were there any seas to be. As we know of a vera causa which has for ages been tending to deplete them, extrapolation from what is now going on returns them the water they have lost and rehabilitates their ancient aquatic character. To the far-sight of inference, seas they again become in the morning of the ages long ago when Mars itself was young.
Nor is this the end of the evidence. When we compare quantitatively the areas occupied by the quondam seas on Mars and on the Moon, we find reason to increase our confidence in our deduction. For the smaller body, the Moon, should have had less water relatively, at the time when the seas there were laid down, than the larger, Mars. Because from the moment its mass began to collect, it was in process of parting with its gases, water-vapor among the rest, and, as we shall see more in detail in the next chapter, it had from the start less hold on them than Mars. Its oceans, therefore, should have been less extensive than the Martian ones. This is what the present lunar Mare seem to attest. They are less extended than the dark areas of Mars. A fact which becomes the more evident when we remember that the Moon has long turned the same face to the Earth. Her shape, therefore, has been that of an egg, with the apex pointing toward our world. Here the water would chiefly collect. The greater part of the seas she ever had should be on our side of her surface, the one she presents in perpetuity to our gaze.
It is to the heavens that we must look for our surest information on such a cosmic point, because of the long perspective other bodies give us of our own career. Less conclusive, because dependent upon less time, is any evidence our globe can offer. Yet even from it we may learn something; if nothing else, that it does not contradict the story of the sky. To it, therefore, we return, quickened in apprehension by the sights we have elsewhere seen.
The first thing our sharpened sense causes us to note is the spread of deserts even within historic times. Just as deserts show by their latitudinal girdling of the Earth their direct dependence upon the great system of planetary winds, as meteorologists recognizingly call them, so a study of the fringes of these belts discloses their encroachment upon formerly less arid lands. The southern borders of the Mediterranean reveal this all the way from Carthage to Palestine. The disappearance of their former peoples, leaving these lands but scantily inhabited now, points to this; because other regions, as India, which still retain a waterful climate, are as populous as ever. Much of this is doubtless due to the overthrow of dynasties and the ensuing lapse of irrigation, but query: Is it all? For we have still more definite information in the drying up of the streams which have left the aqueducts of Carthage without continuation, as much to water on the one hand as to its drinkers on the other. Men may leave because of lack of water, but water does not leave because of dearth of men to drink.
Recent search around the Caspian by Huntington has disclosed the like degeneration due to encroaching desertism there. Indeed, it is no chance coincidence that just where all the great nations thrived in the morning of the historic times should be precisely where populous peoples no longer exist. For neither increasing cold nor increasing heat is responsible for this, seeing that no general change has occurred in either. Nor were they particularly exposed to extermination by northern hordes of barbarians. Egypt as a world power died a natural death, and Babylonia too; but the common people died of thirst, indirect and unconscious and not wholly of their own choosing. Prehistoric records make this conclusion doubly sure, by lengthening the limit of our observation. Both extinct flora and extinct fauna tell the same tale. In the neighborhood of Cairo petrified forests attest that Egypt was not always a wiped slate, while the unearthed animals of the Fayum bear witness to water where no water is to-day.
Anywhere we wander along these girdling belts we find the same story written for us to read. The great
deserts of New Mexico and Arizona show castellated structures far beyond the means of its present Indian population to inhabit. Yet this retrenchment occurred long before the white man came with his exterminating blight on everything he touched. Nor have we reason to suppose that it arose in consequence of invasion by other alien hordes. Individual communities may thus indeed have perished as the preservation of their domiciles intact leads us to infer, but all did not thus vanish from off the Earth. Here again humanity died or moved away because nature dried the sources of its supply. And here, as elsewhere, we find prehistoric record in the rocks of a once more smiling state of things, strengthening the testimony we deduce from man. The forests, crowning now only the greater heights, are but the shrinking residues of what once clothed the land. The well-named Arid Zone is becoming more so every day.
If from the land evidence of drying up we turn to the marine, we see the same shrinkage at work. It has even been discovered in a lowering of the ocean bed, but as this may so easily be disputed, we turn to one aspect of the situation which cannot so easily be gainsaid,—the bodies of water that have been cut off. That the Dead Sea, the Caspian, the Great Salt Lake, are slowly but surely giving way to land, is patent. If the climate at least were not more arid than before this could not occur; but more than this, if the ocean were not on the whole shrinking, there would be no tendency to leave such arms of itself behind to shrivel up. That the ocean basins are deepening is possible, but we know of one depletion which is not replaced—evaporation into space; and of another bound to come—withdrawal into fissures when the earth shall cease to be too hot.
This gradual withdrawal of the water may seem an unpleasant one to contemplate, but like most things it has its silver lining in the hope it holds out that sometime there shall be no more sea. Those of us who detest the constant going down to the sea in ships hardly more than the occasional going down with them, can take a crumb of comfort in the thought. Unfortunately it partakes of a somewhat far-off realization in our distant descendants, coming a little too late to be of material advantage to ourselves.
But let me not leave the reader wholly disconsolate. For another thought we can take with us in closing our sketch of so much of the Earth's life as brings it well down to to-day,—the thought that it has grown for us a steadily better place to contemplate from the earliest eras to the present time. Indeed, with innate prescience we forbore to appear till the prospect did prove pleasing. Finally, we may palliate prognostication by considering that if its future seem a thought less attractive, we, at least, shall not be there to see.
- ↑ Dana, "Geology."
- ↑ Dana, Geikie, De Lapparent.
- ↑ Cf. Grant Allen.