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.
[Illustration: EARTH AS SEEN FROM ABOVE—PHOTOGRAPHED BY DR. LOWELL AT
AN ALTITUDE OF 5500 FEET.]
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′),[22] 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.
[22] Dana. “Geology.”
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 ever slackening 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.
[Illustration: TRACKS OF SAUROPUS PRIMÆVUS (× ½). I. LEA.—DANA, “MANUAL
OF GEOLOGY.”]
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 footnotes by the way. As one should always inspect
the original documents, I will reproduce the footnotes 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.[23] 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.[24] 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.
[23] Dana, Geikie, De Lapparent.
[24] Cf. Grant Allen.
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.
[Illustration: ADVENTURES OF A HEAT RAY.]
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.
[Illustration: MARS.
NORTH POLAR CAP. SOUTH POLAR CAP.
At maximum full extent of white At maximum white
At minimum inner circle At minimum nothing]
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
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 snowfall 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.
[Illustration: GLACIAL MAP OF EURASIA—AFTER JAMES GEIKIE.]
[Illustration: MAP SHOWING THE GLACIATED AREA OF NORTH AMERICA—THE
ARROWS INDICATING THE DIRECTION OF ICE MOVEMENT—CHAMBERLIN AND
SALISBURY.]
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 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 fiords 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-suffragists 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.
[Illustration: THE MOON—PHOTOGRAPHED AT THE LOWELL OBSERVATORY.]
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
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.
[Illustration: PETRIFIED BRIDGE, THIRD PETRIFIED FOREST, NEAR ADAMANA,
ARIZONA—PHOTOGRAPH BY HARVEY.]
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.