THE EARTH IN ITS RELATION TO THE DEVELOPMENT AND MAINTENANCE OF LIFE
The first circumstance to be considered in relation to the habitability of
a planet is its distance from the sun. We know that the heating power of
the sun upon our earth is ample for the development of life in an almost
infinite variety of forms; and we have a large amount of evidence to show
that, were it not for the equalising power of air and water, distributed as
they are with us, the heat received from the sun would be sometimes too
great and sometimes too little. In some parts of Africa, Australia, and
India, the sandy soil becomes so hot that an egg can be cooked by placing
it just below the surface. On the other hand, at an elevation of about
12,000 feet in lat. 40° it freezes every night, and throughout the day in
all places sheltered from the sun. Now, both these temperatures are adverse
to life, and if either of them persisted over a considerable portion of the
earth, the development of life would have been impossible. But the heat
derived from the sun is inversely as the square of the distance, so that at
half the distance we should have four times as much heat, and at twice the
distance only one-fourth of the heat. Even at two-thirds of the distance we
should receive more than twice as much heat; and, considering the facts as
to the extreme sensitiveness of protoplasm and the coagulation of albumen,
it seems certain that we are situated in what has been termed the temperate
zone of the solar system, and that we could not be removed far from our
present position without endangering a considerable portion of the life now
existing upon the earth, and in all probability rendering the actual
development of life, through all its phases and gradations, impossible.
THE OBLIQUITY OF THE ECLIPTIC
The effect of the obliquity of the earth's equator to its path round the
sun, upon which depend our varying seasons and the inequality of day and
night throughout all the temperate zones, is very generally known. But it
is not usually considered that this obliquity is of any great importance as
regards the suitability of the earth for the development and maintenance of
life; and it seems to have been passed over as an accident hardly worth
notice, because almost any other obliquity or none at all would have been
equally advantageous. But if we consider what the direction of the earth's
axis might possibly have been, we shall find that it is really a matter of
great importance from our present point of view.
Let us suppose, first, that the earth's axis was, like that of Uranus,
almost exactly in the plane of its orbit or directed towards the sun. There
can be little doubt that such a position would have rendered our world
unfitted for the development of life. For the result would be the most
tremendous contrasts of the seasons; at mid-winter, on one half the globe,
arctic night and more than arctic cold would prevail; while on the other
half there would be a midsummer of continuous day with a vertical sun and
such an amount of heat as nowhere exists with us. At the two equinoxes the
whole globe would enjoy equal day and night, all our present tropics and
part of the sub-tropical zone having the sun at noon so near to the zenith
as to have the essential of a tropical climate. But the change to about a
month of constant sunshine or a month of continuous night would be so
rapid, that it seems almost impossible that either vegetable or animal life
would ever have developed under such terrible conditions.
The other extreme direction of the earth's axis, exactly at right angles to
the plane of the orbit, would be much more favourable, but would still have
its disadvantages. The whole surface from equator to poles would enjoy
equal day and night, and every part would receive the same amount of
sun-heat all the year round, so that there would be no change of seasons;
but the heat received would vary with the latitude. In our latitude the
sun's altitude at noon all the year would be less than 40°, the same as now
occurs at the equinoxes, and we might therefore have a perpetual spring as
regards temperature. But the constancy of the heat in the equatorial and
tropical regions and of cold towards the poles would lead to a more
constant and more rapid circulation of air, and we should probably
experience such continuous north-westerly winds as to render our climate
always cold and probably very damp. Near the poles the sun would always be
on, or close to, the horizon, and would give so little heat that the sea
might be perpetually frozen and the land deeply snow-buried; and these
conditions would probably extend into the temperate zone, and possibly so
far south as to render life impossible in our latitudes, since whatever
results arose would be due to permanent causes, and we know how powerful
are snow and ice to extend their sway over adjacent areas if not
counteracted by summer heat or warm moist winds. On the whole, therefore,
it seems probable that this position of the earth's axis would result in a
much smaller portion of its surface being capable of supporting a luxuriant
and varied vegetable and animal life than is now the case; while the
extreme uniformity of conditions everywhere present might be so
antagonistic to the great law of rhythm that seems to pervade the universe,
and be in other ways so unfavourable, that life-development would probably
have taken quite a different course from that which it has taken.
It appears almost certain, therefore, that some intermediate position of
the axis would be the most favourable; and that which actually exists seems
to combine the advantage of change of seasons with good climatical
conditions over the largest possible area. We know that during the greater
part of the epoch of life-development this area was much greater than at
present, since a luxuriant vegetation of deciduous and evergreen trees and
shrubs extended up to and within the Arctic Circle, leading to the
formation of coal-beds both in palæozoic and tertiary times; the extremely
favourable conditions for organic life which then prevailed over so large a
portion of the globe's surface, and which persisted down to a
comparatively recent epoch, lead to the conclusion that no more favourable
degree of obliquity was possible than that which we actually possess. A
short account of the evidence on this interesting subject will now be
given.
PERSISTENCE OF MILD CLIMATES THROUGH GEOLOGIC TIME
The whole of the geological evidence goes to show that in remote ages the
climate of the earth was generally more uniform, though perhaps not warmer,
than it is now, and this can be best explained by a slightly different
distribution of sea and land, which allowed the warm waters of the tropical
oceans to penetrate into various parts of the continents (which were then
more broken up than they are now), and also to extend more freely into the
Arctic regions. So soon as we go back into the tertiary period, we find
indications of a warmer climate in the north temperate zone; and when we
reach the middle of that period, we find abundant indications, both in
plant and animal remains, of mild climates near to the Arctic Circle, or
actually within it.
On the west coast of Greenland, in 70° N. lat., there are found abundance
of fossil plants very beautifully preserved, among which are many different
species of oaks, beeches, poplars, plane-trees, vines, walnuts, plums,
chestnuts, sequoias, and numerous shrubs--137 species in all, indicating a
vegetation such as now grows in the north temperate parts of America and
Eastern Asia. And even further north, in Spitzbergen, in N. lat. 78° and
79°, a somewhat similar flora is found, not quite so varied, but with oaks,
poplars, birches, planes, limes, hazels, pines, and many aquatic plants
such as may now be found in West Norway and in Alaska, nearly twenty
degrees further south.
Still more remote, in the Cretaceous period, fossil plants have been found
in Greenland, consisting of ferns, cycads, conifers, and such trees and
shrubs as poplars, sassafras, andromedas, magnolias, myrtles, and many
others, similar in character and often identical in species with fossils of
the same period found in Central Europe and the United States, indicating a
widespread uniformity of climate, such as would be brought about by the
great ocean-currents carrying the warm waters of the tropics into the
Arctic seas.
Still further back, in the Jurassic period, we have proofs of a mild
climate in East Siberia and at Andö in Norway just within the Arctic
Circle, in numerous plant remains, and also remains of great reptiles
allied to those found in the same strata in all parts of the world. Similar
phenomena occur in the still earlier Triassic period; but we will pass on
to the much more remote Carboniferous period, during which most of the
great coal-beds of the world were formed from a luxuriant vegetation,
consisting mostly of ferns, giant horse-tails, and primitive conifers. The
luxuriance of these plants, which are often found beautifully preserved and
in immense quantities, is supposed to indicate an atmosphere in which
carbonic acid gas was much more abundant than now; and this is rendered
probable by the small number and low type of terrestrial animals,
consisting of a few insects and amphibia.
But the interesting point is, that true coal-beds, with similar fossils to
those of our own coal-measures, are found at Spitzbergen and at Bear Island
in East Siberia, both far within the Arctic Circle, again indicating a
great uniformity of climate, and probably a denser and more vapour-laden
atmosphere, which would act as a blanket over the earth and preserve the
heat brought to the Arctic seas by the ocean currents from the warmer
regions.
The still earlier Silurian rocks are also found abundantly in the Arctic
regions, but their fossils are entirely of marine animals. Yet they show
the same phenomena as regards climate, since the corals and cephalopodous
mollusca found in the Arctic beds closely resemble those of all other parts
of the earth.[16]
Many other facts indicate that throughout the enormous periods required for
the development of the varied forms of life upon the earth, the great
phenomena of nature were but little different from those that prevail in
our own times. The slow and gentle processes by which the various vegetable
and animal remains were preserved are shown by the perfect state in which
many of the fossils exist. Often trunks of trees, cycads, and tree-ferns
are found standing erect, with their roots still imbedded in the soil they
grew in. Large leaves of poplars, maples, oaks, and other trees are often
preserved in as perfect a state as if gathered by a botanist and dried
between paper for his herbarium, and the same is especially the case with
the beautiful ferns of the Permian and Carboniferous periods. Throughout
these and most other formations well-preserved ripple-marks are found in
the solidified mud or sand of old seashores, differing in no respect from
similar marks to be found on almost every coast to-day. Equally interesting
are the marks of rain-drops preserved in the rocks of almost all ages. Sir
Charles Lyell has given illustrations of recent impressions of rain-drops
on the extensive mud-flats of Nova Scotia, and also an illustration of
rain-drops on a slab of shale from the carboniferous formation of the same
country; and the two are as much alike as the prints of two different
showers a few days apart. The general size and form of the drops are almost
identical, and imply a great similarity in the general atmospheric
conditions.
We must not forget that this presence of rain throughout geological time
implies, as we have seen in our last chapter, a constant and universal
distribution of atmospheric dust. The two chief sources of this dust--the
total quantity of which in the atmosphere must be enormous--are volcanoes
and deserts, and we are therefore sure that these two great natural
phenomena have always been present. Of volcanoes we have ample independent
evidence in the presence of lavas and volcanic ashes, as well as actual
stumps or cores of old volcanoes, through all geological formations; and we
can have little doubt that deserts also were present, though perhaps not
always so extensive as they are now. It is a very suggestive fact that
these two phenomena, usually held to be blots on the fair face of nature,
and even to be opposed to belief in a beneficent Creator, should now be
proved to be really essential to the earth's habitability.
Notwithstanding this prevalence of warm and uniform conditions, there is
also evidence of considerable changes of climate; and at two periods--in
the Eocene and in the remote Permian--there are even indications of
ice-action, so that some geologists believe that there were then actual
glacial epochs. But it seems more probable that they imply only local
glaciation, owing to there having been high land and other suitable
conditions for the production of glaciers in certain areas.
The whole bearing of the geological evidence indicates the wonderful
continuity of conditions favourable for life, and for the most part of
climatal conditions more favourable than those now prevailing, since a
larger extent of land towards the North Pole was available for an abundant
vegetation, and in all probability for an equally abundant animal life. We
know, too, that there was never any total break in life-development; no
epoch of such lowering or raising of temperature as to destroy all life; no
such general subsidence as to submerge the whole land-surface. Although the
geological record is in parts very imperfect, yet it is, on the whole,
wonderfully complete; and it presents to our view a continuous progress,
from simple to complex, from lower to higher. Type after type becomes
highly specialised in adaptation to local or climatal conditions, and then
dies out, giving room for some other type to arise and be specialised in
harmony with the changed conditions. The general character of the inorganic
change appears to have been from more insular to more continental
conditions, accompanied by a change from more uniform to less uniform
climates, from an almost sub-tropical warmth and moisture, extending up to
the Arctic Circle, to that diversity of tropical, temperate, and cold
areas, capable of supporting the greatest possible variety in the forms of
life, and which seems especially adapted to stimulate mankind to
civilisation and social development by means of the necessary struggle
against, and utilisation of, the various forces of nature.
WATER, ITS AMOUNT AND DISTRIBUTION ON THE EARTH
Although it is generally known that the oceans occupy more than two-thirds
of the whole surface of the globe, the enormous bulk of the water in
proportion to the land that rises above its surface is hardly ever
appreciated. But as this is a matter of the greatest importance, both as
regards the geological history of the globe and the special subject we are
here discussing, it will be necessary to enter into some details in regard
to it.
According to the best recent estimates, the land area of the globe is 0.28
of the whole surface, and the water area 0.72. But the mean height of the
land above the sea-level is found to be 2250 feet, while the mean depth of
the seas and oceans is 13,860 feet; so that though the water area is two
and a half times that of the land, the mean depth of the water is more
than six times the mean height of the land. This is, of course, due to the
fact that lowlands occupy most of the land-area, the plateaus and high
mountains a comparatively small portion of it; while, though the greatest
depths of the oceans about equal the greatest heights of the mountains, yet
over enormous areas the oceans are deep enough to submerge all the
mountains of Europe and temperate North America, except the extreme summits
of one or two of them. Hence it follows that the bulk of the oceans, even
omitting all the shallow seas, is more than thirteen times that of the land
above sea-level; and if all the land-surface and ocean-floors were reduced
to one level, that is, if the solid mass of the globe were a true oblate
spheroid, the whole would be covered with water about two miles deep. The
diagram here given will render this more intelligible and will serve to
illustrate what follows.
[Illustration: _Diagram of proportionate mean height of Land and depth of
Oceans_. _Land_ _Area. .28 of area of Globe._ _Ocean_ _Area .72 of area of
Globe._]
In this diagram the lengths of the sections representing land and ocean are
proportionate to their areas, while the thickness of each is proportionate
to their mean height and mean depth respectively. Hence the two sections
are in correct proportion to their cubic contents.
A mere inspection of this diagram is sufficient to disprove the old idea,
still held by a few geologists and by many biologists, that oceans and
continents have repeatedly changed places during geological times, or that
the great oceans have again and again been bridged over to facilitate the
distribution of beetles or birds, reptiles or mammals. We must remember
that although the diagram shows the continents and oceans as a whole, yet
it also shows, with quite sufficient accuracy, the proportions of each of
the great continents to the oceans which are adjacent to them. It must also
be borne in mind that there can be no elevation on a large scale without a
corresponding subsidence elsewhere; because if there were not a vast
unsupported hollow would be left beneath the rising land or in some part
adjacent to it.
Now, looking at the diagram and at a chart or globe, try to imagine the
ocean-bottom rising gradually, to form a continent joining Africa with
South America or with Australia (both of which are demanded by many
biologists): it is clear that, while such an elevation was going on, either
some continental land or some other part of the ocean-bed must sink to a
corresponding amount. We shall then see, that if such changes of elevation
on a continental scale have taken place again and again at different
periods, it would have been almost impossible, on every occasion, to avoid
a whole continent being submerged (or even all the continents) in order to
equalise subsidence with elevation while new continents were being raised
up from the abyssal depths of the ocean. We conclude, therefore, that with
the exception of a comparatively narrow belt around the continents, which
may be roughly indicated by the thousand fathom line of soundings, the
great ocean depths are permanent features of the earth's surface. It is
this stability of the general distribution of land and water that has
secured the continuity of life upon the earth. Had the great oceanic
basins, on the other hand, been unstable, changing places with the land at
various periods of geological time, they would, almost certainly, again and
again have swallowed up the land in their vast abysses, and have thus
destroyed all the organic life of the world.
There are many confirmatory proofs of this view (which is now widely
accepted by geologists and physicists), and a few of them may be briefly
stated.
1. None of the continents present us with marine deposits of any one
geological age and occupying a large part of the surface of each, as must
have been the case had they ever been sunk deep beneath the ocean and again
elevated; neither do any of them contain extensive formations corresponding
to the deep oceanic clays and oozes, which again they must have done had
they been at any time raised up from the ocean depths.
2. All the continents present an almost complete and continuous series of
rocks of _all_ geological ages, and in each of the great geological periods
there are found fresh water and estuarine deposits, and even old
land-surfaces, demonstrating continuity of continental or insular
conditions.
3. All the great oceans possess, scattered over them, a few or many islands
termed 'oceanic,' and characterised by a volcanic or coralline structure,
with no ancient stratified rocks in anyone of them; and in none of these
is there found a single indigenous land mammal or amphibian. It is
incredible that, if these oceans had ever contained extensive continents,
and if these oceanic islands are--as even now they are often alleged to
be--parts of these now submerged continents, not one fragment of any of the
old stratified rocks, which characterise all existing continents, should
remain to show their origin. In the Atlantic we find the Azores, Madeira,
and St. Helena; in the Indian Ocean, Mauritius, Bourbon, and Kerguelen
Island; in the Pacific, the Fiji, Samoan, Society, Sandwich, and Galapagos
Islands, all without exception telling us the same tale, that they have
been built up from the ocean depths by submarine volcanoes and coralline
growths, but have never formed part of continental areas.
4. The contours of the floors of all the great oceans, now fairly well
known through the soundings of exploring vessels and for submarine
telegraph lines, also give confirmatory evidence that they have never been
continental land. For if any part of them were a sunken continent, that
part must have retained some impress of its origin. Some of the numerous
mountain ranges which characterise _every_ continent would have remained.
We should find slopes of from 20° to 50° not uncommon, while valleys
bordered by rocky precipices, as in Lake Lucerne and a hundred others, or
isolated rock-walled mountains like Roraima, or ranges of precipices as in
the Ghâts of India or the Fiords of Norway, would frequently be met with.
But not a single feature of this kind has ever been found in the ocean
abysses. Instead of these we have vast plains, which, if the water were
removed, would appear almost exactly level, with no abrupt slopes
anywhere. When we consider that deposits from the land never reach these
remote ocean depths, and that there is no wave-action below a few hundred
feet, these continental features once submerged would be indestructible;
and their total absence is, therefore, itself a demonstration that none of
the great oceans are on the sites of submerged continents.
HOW OCEAN DEPTHS WERE PRODUCED
It is a very difficult problem to determine how the vast basins which are
filled by the great oceans, especially that of the Pacific, were first
produced. When the earth's surface was still in a molten state, it would
necessarily take the form of a true oblate spheroid, with a compression at
the poles due to its speed of rotation, which is supposed to have been very
great. The crust formed by the gradual cooling of such a globe would be of
the same general form, and, being thin, would easily be fractured or bent
so as to accommodate itself to any unequal stresses from the interior. As
the crust thickened and the whole mass slowly cooled and contracted,
fissures and crumpling would occur, the former serving as outlets for
volcanic activities whose results are found throughout all geological ages;
the latter producing mountain chains in which the rocks are almost always
curved, folded, or even thrust over each other, indicating the mighty
forces due to the adjustments of a solid crust upon a shrinking fluid or
semi-fluid interior.
But during this whole process there seem to be no forces at work that could
lead to the production of such a feature as the Pacific, a vast depression
covering nearly one-third of the whole surface of the globe. The Atlantic
Ocean, being smaller and nearly opposite to the Pacific, but approximately
of equal depth, may be looked upon as a complementary phenomenon which will
be probably explained as a result of the same causes as the vaster cavity.
So far as I am aware, there is only one suggested cause of the formation of
these great oceans that seems adequate; and as that cause is to some extent
supported by quite independent astronomical evidence, and also directly
bears upon the main subject of the present volume, it must be briefly
considered.
A few years ago, Professor George Darwin, of Cambridge, arrived at a
certain conclusion as to the origin of the moon, which is now comparatively
well known by Sir Robert Ball's popular account of it in his small volume,
_Time and Tide_. Briefly stated, it is as follows. The tides produce
friction on the earth and very slowly increase the length of our day, and
also cause the moon to recede further from us. The day is lengthened only
by a small fraction of a second in a thousand years, and the moon is
receding at an equally imperceptible rate. But as these forces are
constant, and have always acted on the earth and moon, as we go back and
back into the almost infinite past we come to a time when the rotation of
the earth was so rapid that gravity at the equator could hardly retain its
outer portion, which was spread out so that the form of the whole mass was
something like a cheese with rounded edges. And about the same epoch the
distance of the moon is found to have been so small that it was actually
touching the earth. All this is the result of mathematical calculation from
the known laws of gravitation and tidal effects; and as it is difficult to
see how so large a body as the moon could have originated in any other way,
it is supposed that at a still earlier period the moon and earth were one,
and that the moon separated from the parent mass owing to centrifugal force
generated by the earth's rapid rotation. Whether the earth was liquid or
solid at this epoch, and exactly how the separation occurred, is not
explained either by Professor Darwin or Sir Robert Ball; but it is a very
suggestive fact that, quite recently, it has been shown, by means of the
spectroscope, that double stars of short period _do_ originate in this way
from a single star, as already described in our sixth chapter; but in these
cases it seems probable that the parent star is in a gaseous state.
These investigations of Professor G. Darwin have been made use of by the
Rev. Osmond Fisher (in his very interesting and important work, _Physics of
the Earth's Crust_) to account for the basins of the great oceans, the
Pacific being the chasm left when the larger portion of the mass of the
moon parted from the earth.
Adopting, as I do, the theory of the origin of the earth by meteoric
accretion of solid matter, we must consider our planet as having been
produced from one of those vast rings of meteorites which in great numbers
still circulate round the sun, but which at the much earlier period now
contemplated were both more numerous and much more extensive. Owing to
irregularities of distribution in such a ring and through disturbance by
other bodies, aggregations of various size would inevitably occur, and the
largest of these would in time draw in to itself all the rest, and thus
form a planet. During the early stages of this process the particles would
be so small and would come together so gradually, that little heat would be
produced, and there would result merely a loose aggregation of cold matter.
But as the process went on and the mass of the incipient planet became
considerable--perhaps half that of the earth--the rest of the ring would
fall in with greater and greater velocity; and this, added to the
gravitative compression of the growing mass might, when nearly its present
size, have produced sufficient heat to liquefy the outer layers, while the
central portion remained solid and to some extent incoherent, with probably
large quantities of heavy gases in the interstices. When the amount of the
meteoric accretions became so reduced as to be insufficient to keep up the
heat to the melting-point, a crust would form, and might have reached about
half or three-fourths of its present thickness when the moon became
separated.
Let us now try to picture to ourselves what happened. We should have a
globe somewhat larger than our earth is now, both because it then contained
the material of the moon and also because it was hotter, revolving so
rapidly as to be very greatly flattened at the poles; while the equatorial
belt bulged out enormously, and would probably have separated in the form
of a ring with a very slight increase of the time of rotation, which is
supposed to have been about four hours. This globe would have a
comparatively thin crust, beneath which there was molten rock to an
unknown depth, perhaps a few hundreds, perhaps more than a thousand miles.
At this time the attraction of the sun acting on the molten interior
produced tides in it, causing the thin crust to rise and fall every two
hours, but to so small an extent--only about a foot or so--as not
necessarily to fracture it; but it is calculated that this slight rhythmic
undulation coincided with the normal period of undulation due to such a
large mass of heavy liquid, and so tended to increase the instability due
to rapid rotation.
The bulk of the moon is about one-fiftieth part that of the earth, and an
easy calculation shows us that, taking the area of the Pacific, Atlantic,
and Indian Oceans combined as about two-thirds that of the globe, it would
require a thickness (or depth) of about forty miles to furnish the material
for the moon. We must, of course, assume that there were some inequalities
in the thickness of the crust and in its comparative rigidity, so that when
the critical moment came and the earth could no longer retain its
equatorial protuberance against the centrifugal force due to rotation
combined with the tidal undulations caused by the sun, instead of a
continuous ring slowly detaching itself, the crust gave way in two or more
great masses where it was weakest, and as the tidal wave passed under it
and a quantity of the liquid substratum rose with it, the whole would break
up and collect into a sub-globular mass a short distance from the earth,
and continue revolving with it for some time at about the same rate as the
surface had rotated. But as tidal action is always equal on opposite sides
of a globe, there would be a similar disruption there, forming, it may be
supposed, the Atlantic basin, which, as may be seen on a small globe, is
almost exactly opposite a part of the Central Pacific. So soon as these two
great masses had separated from the earth, the latter would gradually
settle down into a state of equilibrium, and the molten matter of the
interior, which would now fill the great oceanic basins up to a level of a
few mile below the general surface would soon cool enough to form a thin
crust. The larger portion of the nascent moon would gradually attract to
itself the one or more smaller portions and form our satellite; and from
that time tidal friction by both moon and sun would begin to operate and
would gradually lengthen our day and, more rapidly, our month in the way
explained in Sir Robert Ball's volume.
A very interesting point may now be referred to, because it seems
confirmatory of this origin of the great ocean basins. In Mr. Osmond
Fisher's work it is explained how the variations in the force of gravity,
at numerous points all over the world, have been determined by observations
with the pendulum, and also how these variations afford a measure of the
thickness of the solid crust, which is of less specific gravity than the
molten interior on which it rests. By this means a very interesting result
was obtained. The observations on numerous oceanic islands proved that the
sub-oceanic crust was considerably more dense than the crust under the
continents, but also thinner, the result being to bring the average mass of
the sub-oceanic crust and oceans to an equality with that of the
continental crust, and this causes the whirling earth to be in a state of
balance, or equilibrium. Now, both the thinness and the increased density
of the crust seem to be well explained by this theory of the origin of the
oceanic basins. The new crust would necessarily for a long time be thinner
than the older portion, because formed so much later, but it would very
soon become cool enough to allow the aqueous vapour of the atmosphere and
that given off through fissures from the molten interior to collect in the
ocean basins, which would thenceforth be cooled more rapidly and kept at a
uniform temperature and also under a uniform pressure, and these conditions
would lead to the steady and continuous increase of thickness, with a
greater compactness of structure than in the continental areas. It is no
doubt to this uniformity of conditions, with a lowering of the bottom
temperature throughout the greater part of geological time, till it has
become only a few degrees above the freezing-point, that we owe the
remarkable persistence of the vast and deep ocean basins on which, as we
have seen, the continuity of life on the earth has largely depended.
There is one other fact which lends some support to this theory of the
origin of the ocean basins--their almost complete symmetry with regard to
the equator. Both the Atlantic and Pacific basins extend to an equal
distance north and south of the equator, an equality which could hardly
have been produced by any cause not directly connected with the earth's
rotation. The polar seas which are coterminous with the two great oceans
are very much shallower, and cannot, therefore, be considered as forming
part of the true oceanic basins.
WATER AS AN EQUALISER OF TEMPERATURE
The importance of water in regulating the temperature of the earth is so
great that, even if we had enough water on the land for all the wants of
plants and animals, but had no great oceans, it is almost certain that the
earth could not have produced and sustained the various forms of life which
it now possesses.
The effect of the oceans is twofold. Owing to the great specific heat of
water, that is, its property of absorbing heat slowly but to a large
amount, and giving it out with equal slowness, the surface-waters of the
oceans and seas are heated by the sun so that by the evening of a bright
day they have become quite warm to a depth of several feet. But air has
much less specific heat than water, a pound of water in cooling one degree
being capable of warming four pounds of air one degree; but as air is 770
times as light as water, it follows that the heat from one cubic foot of
water will warm more than 3000 cubic feet of air as much as it cools
itself. Hence the enormous surface of the seas and oceans, the larger part
of which is within the tropics, warms the whole of the lower and denser
portions of the air, especially during the night, and this warmth is
carried to all parts of the earth by the winds, and thus ameliorates the
climate. Another quite distinct effect is due to the great ocean currents,
like the Gulf Stream and the Japan Current, which carry the warm water of
the tropics to temperate and arctic regions, and thus render many countries
habitable which would otherwise suffer the rigour of an almost arctic
winter. These currents are, however, directly due to the winds, and
properly belong to the section on the atmosphere.
The other equalising action, due primarily to the great area of the seas
and oceans, is a result of the vast evaporating surface from which the land
derives almost all its water in the form of rain and rivers; and it is
quite evident that if there were not sufficient water-surface to produce an
ample supply of vapour for this purpose, arid districts would occupy more
and more of the earth's surface. How much water-surface is necessary for
life we do not know; but if the proportions of water and land-surfaces were
reversed, it seems probable that the larger proportion of the earth might
be uninhabitable. The vapour thus produced has also a very great effect in
equalising temperature; but this also is a point which will come better
under our next chapter on the atmosphere.
* * * * *
There are, however, some matters connected with the water-supply of the
earth, and its relation to the development of life, that call for a few
remarks here. What has determined the total quantity of water on the earth
or on other planets does not appear to be known; but presumably it would
depend, partially or wholly, on the mass of the planet being sufficient to
enable it to retain by its gravitative force the oxygen and hydrogen of
which water is composed. As the two gases are so easily combined to form
water, but can only be separated under special conditions, its quantity
would be dependent on the supply of hydrogen, which is but rarely found on
the earth in a free state. The important fact, however, is, that we do
possess so great a quantity of water, that if the whole surface of the
globe was as regularly contoured as are the continents, and merely wrinkled
with mountain chains, then the existing water would cover the whole globe
nearly two miles deep, leaving only the tops of high mountains above its
surface as rows of small islands, with a few larger islands formed by what
are now the high plateaus of Tibet and the Southern Andes.
Now there seems no reason why this distribution of the water should not
have occurred--in fact it seems probable that it would have occurred, had
it not been for the fortunate coincidence of the formation of enormously
deep ocean basins. So far as I am aware, no sufficient explanation of the
formation of these basins has been given but that of Mr. Osmond Fisher, as
here described, and that depends upon three unique circumstances: (1) the
formation of a satellite at a very late period of the planet's development
when there was already a rather thick crust; (2) the satellite being far
larger in proportion to its primary than any other in the solar system; and
(3) its having been produced by fission from its primary on account of
extremely rapid rotation, combined with solar tides in its molten interior,
and a rate of oscillation of that molten interior coinciding with the tidal
period.[17]
Whether this very remarkable theory of the origin of our moon is the true
one, and if so, whether the explanation it seems to afford of the great
oceanic basins is correct, I am not mathematician enough to judge. The
tidal theory of the origin of the moon, as worked out mathematically by
Professor G.H. Darwin, has been supported by Sir Robert Ball and accepted
by many other astronomers; while the researches of the Rev. Osmond Fisher
into the _Physics of the Earth's Crust_, together with his mathematical
abilities and his practical work as a geologist, entitle his opinion on the
question of the mode of origin of the ocean basins to the highest respect.
And, as we have seen, the existence of these vast and deep ocean basins,
produced by the agency of a series of events so remarkable as to be quite
unique in the solar system, played an important part in rendering the earth
fit for the development of the higher forms of animal life, while without
them it seems not improbable that the conditions would have been such as to
render any varied forms of terrestrial life hardly possible.
FOOTNOTES:
[16] For a fuller account of this Arctic fauna and flora see the works of
Sir C. Lyell, Sir A. Geikie, and other geologists. A full summary of it is
also given in the author's _Island Life_.
[17] Professor G.H. Darwin states that it is nearly certain that no other
satellite nor any of the planets originated in the same way as the moon.