TERRAQUEOUSNESS AND TERRESTRIALITY
With the vanishing of its seas we get for the first time solid ground
on which to build our Martian physiography. The change in _venue_ from
oceans to land has produced a complete alteration in our judgment of
the present state of the planet. It destroys the analogy which was
supposed to exist between Mars and our earth, and by abolishing the
actuality of oceans there, seems, metaphorically, to put us at first
all the more at sea in our attempt to understand the planet. But looked
at more carefully, it turns out to explain much that was obscure, and
in so doing gives us at once a wider view of the history of planetary
evolution.
The trait concerned is cosmic. Study of the several planets of our
solar system, notably the Earth, Moon, and Mars, reveals tolerably
legibly an interesting phase of a planet’s career, which apparently
must happen to all such bodies, and evidently has happened or is
happening to these three: the transition of its surface from a
terraqueous to a purely terrestrial condition. The terraqueous state is
well exhibited by our own earth at the moment, where lands and oceans
share the surface between them. The terrestrial is exemplified by both
the Moon and Mars, on whose surfaces no bodies of water at present
exist. That the one state passes by process of development into the
other I shall now give my reasons for believing.
In the first place the appearance of the dark markings both on the Moon
and Mars hints that though seas no longer, they were seas once upon a
time. On the moon, not only does their shape suggest this previous
condition, but the smooth and even look of their surfaces adds to the
cogency of the inference. More important, however, than either of these
characteristics, and confirmatory of both, is the fact that the great
tracts in question seem to lie below the level of the corrugated
surface, which is thickly strewn with volcanic cones. Their level and
their levelness fay in explanation into one another. The first makes
possible the former presence of water; the second speaks of its effect.
For their flat character hints that these areas were held down at the
time when the other parts of the surface were being violently thrown
up. That they can themselves be cooled lava flows, their extent and
position seem enough to negative; to say nothing of the fact that they
should in that case lie above, not below, the general level. Something,
therefore, covered them during the moon’s eruptive youth and
disappeared later. Such superincumbence may well have been water, under
which the now great plains lay then as ocean bottoms. Deep-sea
soundings in our own oceans betray an ocean floor of the same extensive
sort, diversified as on the moon. To call the lunar _maria_ seas may
not be so complete a misnomer after all; but only a resurrecting in
epitaph what was the truth in its day.
Only doubtfully offered here for the Moon, for Mars the inference seems
more sure. Here again the dark regions not only look as they should had
they had an earlier history, but they, too, seem to lie below the level
of the surface round about. When they pass over the terminator they
invariably show as flattenings upon it, as if a slice of the surface
had been pared off. Such profile in such pass is what ground at a lower
level would present. Undoubtedly a part of the seeming depression is
due to relative absence of irradiation consequent upon a more sombre
tint, but loss of light hardly seems capable of the whole effect. In
the case of Mars, then, as with the Moon, a mistaken inference builded
better than it knew, if, indeed, we should rightly consider an
inference to be mistaken which on half data lands us at the right door.
From the aspects of the dark regions we are led, then, to regard Mars
as having passed through that stage of existence in which the earth
finds itself at the moment, the stage at which oceans and seas form a
feature of its landscape and an impediment to subjugation of its
surface in its entirety. What once were ocean beds have become ocean
bottoms devoid of that which originally filled them.
That the process of parting with a watery envelop is an inevitable
concomitant of the evolution of a planet from chaos to world, we do not
have to go so far afield as Mars and the Moon for testimony. Scrutiny
reveals as much in the history of our own globe. Two signposts of the
past, one geologic, the other paleontologic, point unmistakably in this
direction. The geologic guides us the more directly to the goal.
Study of the earth’s surface reveals the preponderating encroachment of
the land upon the sea since both began to be, and demonstrates that,
except for local losses, the oceans have been contracting in size from
archaic times. So much is evidenced by the successive places upon which
marine beds have been laid down. This suggests itself at once as a
theoretic probability to one considering the matter from a cosmic
standpoint, and it is therefore the more interesting and conclusive
that, from an entirely different departure-point, it should have been
one of the pet propositions of the late Professor Dana, who worked out
conclusively the problem for North America, and published charts
detailing the progressive making of that continent.
[Illustration: Map of North America at the close of Archæan time,
showing approximately the areas of dry land. (From Dana’s “Manual of
Geology.”)]
So telling is this reclaiming by nature of land from the sea that it
will be well to follow Dana a little into detail, as the details show
effectively the continuity of the process acting through æons of
geologic time. At the beginning of the Archæan age, or, in other words,
at the epoch when stratified beds were first laid down, the earth
reached a turning-point in its history. Erosion, superficial and
sub-aërial, then set in to help restrict the domain of the sea. At this
juncture North America consisted of a sickle of terrane inclosing
Hudson’s Bay and coming down at its apex to a point not much removed
from where Ottawa now stands, in about latitude 45°—a Labradorian
North America only. This, the kernel of the future continent, curiously
symbolized the form that continent was later to take. For its eastern
edge was roughly parallel to the present Atlantic coastline, although
much within and to the north of it, while its western one was similarly
aligned afar off to the now Pacific slope. Besides this continent
proper, the Appalachian, Rocky Mountain, Sierra Nevada, and Sierra
Madre chains stood out of the ocean in long, narrow ridges of detached
land, outlining in skeleton the bones of the continent that was to be.
The Black Hills of Dakota and other highlands made here and there
islets in the sea.
Much the same backbone-showing of continents yet to be filled out was
true of Europe, Asia, and South America. In Europe the northern
countries constituted all that could be called continental land. Most
of Norway, Sweden, Finland, Lapland, existed then, while the northern
half of Scotland, the outer Hebrides, portions of Ireland, England,
France, and Germany stood out as detached islands. From this, which is
a fair sample of the proportion of land then to land now over the other
continents so far as they are geologically known, we turn to consider
more in detail the history of North America.
By the time the Upper Silurian period came in, the Appalachian
highlands there had been greatly extended and joined to the Labradorian
mainland by continuous territory; otherwise, no important addition had
occurred, though islands emerged in Ohio, Kentucky, and Missouri.
[Illustration: North America at the opening of the Upper Silurian.
(From Dana’s “Manual of Geology.”)]
At the commencement of the Carbonic era what are now the Middle states
had begun to fill up from the north, and Newfoundland, from a small
island in the Upper Silurian, had become a great promontory of
Labrador, while the Eastern states region and Nova Scotia had risen
into being. The movements closing Paleozoic time upheaved from low
islands the Appalachian chain. The earth’s crust here crumpled by
contraction upon itself; and the movement ended, as Dana says, by
making dry land of the whole eastern half of the continent, along
substantially its present lines.
[Illustration: Map of North America after the Appalachian Revolution.
(From Dana’s “Manual of Geology.”)]
Mesozoic time was the period of the making of the West. It was an era
of deposition and coincident subsidence, when the western land had its
nose just above water at one moment to be submerged the next. Though on
the whole this part of the continent was emerging, the fact was that,
synchronously with the sinking of the sea, much of the land from time
to time sank too. The contraction which raised the Appalachian Mountains
at the beginning of the period and that of the Rockies at its close
overdid the necessities of the case and caused subsidence elsewhere.
The southeastern portion of the continent suffered most, the West on
the whole materially gaining. In the Triassic and Jurassic eras the
gain was pronounced; it occurred in the Cretaceous also, but with much
alternation of loss. Finally, at the close of the Cretaceous, the
continent, except for a prolonged Gulf of Mexico and vast internal
lakes, was substantially complete.
[Illustration: North America in the Cretaceous period. (From Dana’s
“Manual of Geology.”)]
The filling up of these lakes and the reclaiming of land from the Gulf
of Mexico constituted the land-making work of Tertiary times. The
extent of the lakes in the Eocene era is held to show that the general
level of the mountain plateau was low and rose later. So that the gain
by the land at this time was greater than the map allows to appear. By
the beginning of the Quaternary epoch the continents had assumed their
present general area, and since then their internal features have alone
suffered change.
[Illustration: Map of North America, showing the parts under water in
the Tertiary Era; the vertically lined is the Eocene. (From Dana’s
“Manual of Geology.”)]
A similar rising from the sea fell to the lot of Europe, though it has
not been detailed with so much care. The skeleton of that continent was
at the beginning of depositary time much what it is to-day, but a great
inland sea occupied the centre of it, which, as time went on, was
gradually silted in and evaporated away, notably during the Upper
Silurian period.
From all this it is pretty clear that, side by side with alternating
risings and sinkings of the land, there was a tolerably steady gain in
the contest by which dry ground dispossessed the sea. We may, of
course, credit this to a general deepening of the ocean bottoms due to
crumpling of the crust, but we may also impute it to a loss of water,
and that the latter is, at least for a part, in the explanation the
condition of the Moon and Mars makes probable.
Paleontology has the same story of reclamation to tell as geology, and
with as much certainty, though its evidence is circumstantial instead
of direct and speaks for the growing importance of the land in the
globe’s economy since the beginning of depositary time, and thus
inferentially to its increasing extent. Fossil remains of the plants
and creatures that have one after the other inhabited the earth show
that the land has been steadily rising both in floral and faunal
estimation as a habitat from the earliest ages to the present day. The
record lies imprinted in the strata consecutively laid down, and except
for gaps reads as directly on in bettering domicile as in evolutionary
development.
In Archæan times we find no undisputed evidence of life either vegetal
or animal. But beds of graphite and of limestone point to the possible
existence of both. Even anthracite has been found in Archæan rocks in
Norway and also in Rhode Island. Whether Dawson’s Eozoon Canadense be a
rhizopod or a crystal, doctors of science disagree. Dana, while
admitting nothing specific, deems it antecedently probable that algæ
and later microscopic fungi related to bacteria existed then, living in
water well up toward the boiling-point. Indeed, it is practically
certain that invertebrate life existed, because of its already
well-developed character in the next era. The like antedating is
inferable for the whole record of the rocks. Relatively their history
is undoubtedly fairly accurate, but absolutely it must be shifted
bodily backward into the next preceding era to correspond with fact not
yet unearthed.
In the Lower Cambrian, when first the existence of life becomes a
certainty, that life, so far as known, was wholly invertebrate and
wholly marine; rhizopods (probably), sponges and corals, echinoderms,
worms, brachiopods, mollusks, and crustaceans grew amid primitive
seaweed and have left their houses in the shape of shells while
perishing themselves. Their tracks too have thus survived. The
trilobites, crustaceans somewhat resembling our horseshoe crab, were
the lords of the Cambrian seas and marked the point to which organic
evolution had then attained. Their aquatic character as well as their
simple type is shown by their thoracic legs having each a natatory
appendage.
In the next era, the Lower Silurian, the fauna and flora were still
marine, although of a higher order than before, and in the Trenton
period, the upper part of the era, the earliest vertebrates, fishes,
come upon the scene: ganoids and possibly sharks. Nothing terrestrial
of this period has yet with certainty been unearthed in America. Europe
would seem to have either been more advanced then or better studied
since, for there the first plant higher than a seaweed has been dug up,
one of a fresh-water genus betokening the land; while in keeping with
this the first insect, an hemipter, also has been disinterred. Both the
geography and the life of the Eopaleozoic period Dana styles
“thalassic.”
Neopaleozoic time, beginning with the Upper Silurian, marked the
emergence of the continents, and following them the emergence of life
from the water on to this land. In the lower beds of the Upper Silurian
in America we find only the aquatic forms of previous strata, but in a
higher one we come in marshes upon plants related to the equiseta or
horsetails. In England land plants appear for the first time in these
latest Silurian beds and in the schists of Angers have been preserved
ferns. In both the old world and the new fossil fishes are found and
the oldest terrestrial species of scorpions. But the great bulk of
forms was still marine; corals, crinoids, brachiopods, trilobites
constituting the principal inhabitants. At this time the seas were
warm, having much the same temperature between 65° and 80° north as
between 30° and 45°; the prevalence of a general temperate tropicality
being shown by the fact that the common tropical chain corals lived in
latitude 82° north.
In the Devonian era, the Old Red Sandstone, fishes grew and multiplied,
increasing in size apparently through the era, and in the last period
of it reaching their culminating point. These pelagic vertebrates much
surpassed in structure the terrestrial population of the time, which
was of a low type and consisted of invertebrates such as myriapods,
spiders, scorpions, and insects; for the land was only making. In the
mid-Devonian, forests of a primitive kind covered such country as there
was, an amphibious land, composed of jungles and widespread marshes.
Tree ferns made the bulk of the vegetation, but among them grew also
cycads and yews. Mammoth may-flies flitted through the gloom of these
old forests, but no vertebrate as yet had left the sea.
Following upon the Old Red Sandstone were laid down the Carbonic
strata, and with the Carbonic entered upon the scene the advance scouts
of an army of progress evolutionarily impelled to spy out the land—the
first amphibians. They made their début in the Subcarboniferous section
of the era, the oldest of the three periods into which the Carbonic is
divided, crawling out of the sea to return again and leaving but
footprints at first on the sands of time. In the second period, the
Coal-measures proper, they ventured so far as to leave their skeletons
on terra firma, or rather infirma, while their tracks there show them
to have been now in great numbers. In this manner the ancestors of the
oldest land inhabitants began to struggle out of the sea. In the
Permian, the third and latest period of paleozoic times, we find their
descendants established in their new habitat, for in it we come upon
the first reptiles. Such possession marks a distinct step up in
function as in fact, for while amphibians visited dry land, reptiles
made it their home. The getting out of the water had now, in the case
of the more evolved forms, become an accomplished fact. The reptiles
were, indeed, the lowest and most generalized of their class,
Rhynchocephalians, “beak-headed” species that by their teeth proclaim
their marine origin and their relationship to the great amphibians that
still felt undecided where to stay. Meanwhile, in Europe dragon-flies,
two feet across, possessed the air; while amphibians there, as here,
ancestrally preceded reptiles in occupying the land.
Mesozoic times were, _par excellence_, the age of monsters; for the
Triassic (the New Red Sandstone), Jurassic and Cretaceous eras marked
the reign of the reptiles. Great dinosaurs sleep still in the Triassic
strata of the Atlantic border and in the Jurassic of the Western
states, to be unearthed from time to time and be given mausolea in our
museums. Gigantic they were and very literally possessed the earth. In
Europe they were substantially as in America during these mesozoic
eras, and showed their dominance by long survival in time as well as
world-wide distribution in space; for they lived all the way from
Kansas to New Zealand and from the Trias to the Upper Cretaceous. It is
supposed by Professor Osborn that many of them, like the herbivorous
brontosaurus, waded in marshes, not wholly unlike in habit to the
modern hippopotamus. Others were land-stalking carnivores, like the
megalosaurs of a little later date. Of enormous size, the largest
exceeded any animal which has ever lived, the whales alone excepted;
the biggest, the atlantosaurus and the brontosaurus, reaching a length
of sixty feet. For all their bulk they had scant brains, just enough to
enable them to feed and wallow, probably. It is interesting to note
that many of the reptiles, the less adventurous, apparently reverted to
the sea. For though the crocodilians existed already in the Trias, the
plesiosaurians did not come in till the Middle Trias in Europe, and the
sea-serpents (mosasaurus) till the Upper Cretaceous.
Though the dinosaurs dominated life in those days, higher forms, their
descendants, unnoticed were gradually creeping in, eventually to
supplant them. For brain was making its way unobtrusively in the
earliest of the mammals, diminutive creatures at first and of the
lowest type. First appearing in the Trias as something approaching the
missing link between reptiles and mammals, they later developed into
monotremes and marsupials, not rising in differentiation above the
latter order to the end of Mesozoic times. And this both in the old
world and the new. In the Jurassic, too, flying lizards and the first
birds appeared, showing their pedigree in their teeth.
With Cenozoic times we come upon the first true or placental mammals
with their culmination up to date in man. In the Eocene they were of a
primitive type; they were also of a comprehensive one, fitted to eat
anything. From this they specialized, some evolving and some on the
whole devolving; the whale, for instance, taking to the water in the
Eocene through the same degenerate proclivity that had characterized
the sea-saurians ages before. The earth was growing colder, though
still fairly warm, and with the fall in temperature the higher types of
life antithetically rose, evolution gradually fitting them to cope with
more advanced conditions. In this manner did the land supplant the sea
as the essential feature of the earth’s surface, first, in coming into
being, and then, by offering conditions fraught with greater
possibilities, as the habitat of the most advanced forms of life, both
plants and animals.
The possibility of advance in evolution was largely due to the fact
that the land did thus supplant the sea. Spontaneous variation, the as
yet unexplained _primum mobile_ in the genesis of species, is probably
to be referred to chemism and is likely later to receive its solution
at the hands of that science. In the meanwhile it is evident that
unless the variation obtain encouragement from the environment no
advance in type occurs. Now the land offers to an organism sufficiently
evolved to benefit by it, opportunities the sea does not possess. First
of these, undoubtedly, is the care it enables to be given to the young.
To cast one’s brood upon the waters is not the best method of insuring
its bringing up. There is too much of the uncertainties of wave and
current to make the process a healthy one, and even when attached to
rocks and seaweed, the attachment to a mother is to be preferred.
Without a period of infancy, when the young is unable to do for itself,
no great development is possible. In the only striking exception, the
case of the whales, dolphins, and porpoises, size has probably counted
for much in the matter, while the development of the cetaceans is far
behind that of the majority of land mammals.
Change of place, not in distance, but in variety, is another factor.
The sea is same as a habitat, one square mile of it being much like
another, except for gradually changing temperature. The land, on the
other hand, from its accidented surface, presents all manner of
diversity in the conditions. And the more varied the conditions to
which the organism is exposed, the greater its own complexity must be
to enable it to meet them.
That the terrestrial stage of planetary development is subsequent to
the terraqueous one, and must of necessity succeed it if the latter
ever exist on a body, follows from the loss of internal heat on the one
hand and from the kinetic theory of gases on the other. To which of the
two to attribute the lion’s share in the business is matter for doubt;
but that both must be concerned in it we may take for certain.
So long as the internal heat suffices to keep the body fluid, the
liquid itself sees to it that all interstices are filled. As the heat
dissipates, the body begins to solidify, starting with the crust. For
cosmic purposes it undoubtedly still remains plastic, but cracks of
relatively small size are both formed and persist. Into these the
surface water seeps. With continued refrigeration the crust thickens,
more cracks are opened and more water given lodgment within, to the
impoverishment of the seas. The process would continue till the
pressure of the crust itself rendered plastic all that lay below,
beyond which, of course, no fissures could be formed. How competent to
swallow all the seas such earth cuticle cracks may be we ignore; for we
cannot be said to know much of the process. We can only infer that to a
certain extent internal absorption of surface seas must mark a stage of
the evolution by which a star becomes a world and then an inert mass,
one of the dark bodies of which space is full.
Of the other means we know more. We are certain that it must take
place, though we are in doubt as to the amount it has already
accomplished. This method of depletion is by the departure of the water
in the form of gas, in consequence of the molecular motions. If we knew
the temperature and the age of Mars and also the amount of atmosphere
originally surrounding it, we could possibly predicate its state.
Reversely, we can infer something as to age and temperature from its
present condition.