THE EARTH AS THE ABODE OF LIFE
In the last chapter we spoke of the formation of the atmosphere of
the earth and of the growth of the oceans. We must now consider the
formation of these more closely, and we must distinguish between the
great vaporous clouds which rolled about the earth in its molten
state and the settled atmosphere which formed about it when it grew
cooler.
After the earth had begun to solidify it was at first covered with
a collection of porous fragments of rock covering the earth like a
shell and containing the elements of water. Such materials in general
appearance would be not unlike the pumice stone which is expelled
from volcanoes to-day. Those who have never had the fortune to see
volcanic eruptions for themselves usually imagine that the volcano
throws out nothing but fire and smoke. As a matter of fact it throws
out vast quantities of vapour, of which, according to Sir Archibald
Geikie, 999 parts in 1000 are steam. At the great eruption of Mount
Pelée the cloud of steam continually arising from the volcano for
months in succession was several cubic miles in measurement.
Consequently it will be seen that the porous volcanic rocks with
which the young earth was covered contained all the materials for
water-manufacture within themselves. As the water began to form,
squeezed out of the porous rocks as we can squeeze it out of a sponge
(or as we might steam it out if we put the moist sponge in an oven),
it gathered itself into reservoirs underground. As it increased in
bulk it rose nearer to the surface; because, of course, owing to
the heat of the inner portions of the earth it could never succeed
in sinking below a certain depth. Doubtless it first appeared at
the bottom of the pits which had been sunk by volcanoes or volcanic
action. There must have been innumerable depressions in the earth's
surface as widespread and deeper than those which we can perceive
on the rugged surface of the Moon. We may gain an idea on a very
minute scale of what the first pits of water were like from the
examples (formed, however, at a much later period and probably in a
different way) of the crater lakes that are left to-day. Some curious
examples of "crater lakes" are to be found in the Eifel district
of Germany, an ancient volcanic region which lies in the triangle
formed by the junction of the Rhine and the Moselle at Coblenz. One
of the pleasing peculiarities of this district is that, owing to
the volcanic nature of the soil, the neighbourhood is seldom dusty,
even in August or September, after the dry continental summer. It is
well worth visiting for its castles as well as for its crater lakes
and other volcanic relics, and it is the scene of R. L. Stevenson's
romance _Prince Otto_. The chief crater lakes are between Daun and
Manderscheid. There are, of course, many other and larger crater
lakes in existence, but we select these because they are so easily
accessible.
The flow of the lakes into one another followed. Innumerable lakelets
developed into rivers or chains of lakes on the surface of the young
planet, continually becoming larger bodies of water, till they
developed into the vast irregular oceans of to-day. This evolution is
of great importance from a geological point of view, because it leads
the way to the origin of the ocean basins and the great platforms of
land which we call continents. It is easy to see that because of the
weight of water in the depressions the earth under the waters tended
to become more and more depressed, so that the water areas tended
to grow larger and deeper. The wash of earths from the land tended
to build its borders out into the water basins, but the deepening
and spreading of the water basins is believed to have been the most
marked feature of the earth's early growth. All this time the earth
was growing in diameter and circumference.[9] When this growth ceased
other causes and effects came into play, and the proportions of sea
and land became better balanced.
[Footnote 9: For reasons which are a little too complex to be
considered here. We can only indicate the general line of reasoning
by saying that the central heat as it moved outwards from the rocks
nearer the surface expanded them.]
There is nothing in our human knowledge to tell us with certainty
when or how life first appeared on the earth. We have already spoken
of animal and vegetable remains that for ages are preserved in the
rocks. But clearly no such remains could ever be found in the
volcanic or molten rocks of the earliest stages of the earth's life.
Think for a moment of what the simplest forms of life are. A great
deal has been heard of microbes and bacteria during recent years,
and we may therefore assume that every one has some knowledge of the
structure of these simplest living things. They may be compared to
tiny bladders of jelly--so small that the microscope is necessary
in order to see them, and sometimes so much smaller than this that
the best microscopes cannot distinguish them. Such forms of life are
called "unicellular organisms," because they consist of a single
cell, which contains the jelly-like substance called protoplasm, and
a smaller body, smaller even than these tiny cells themselves, which
is called the nucleus.
These are the simplest forms of life. But all the higher forms of
life, and we may say, roughly speaking, any form of life that the
unaided eye can see, is made up not of one cell but of many cells.
A human being, for example, is made up of uncounted millions of
cells; and millions of cells go to the formation of a worm, a fish,
a gnat, or indeed to the formation of the simplest well-known animal
that the ordinary person could name. Similarly millions of cells go
to the formation of a leaf or a twig. These higher forms of life
are called "multicellular organisms," because they have many cells,
and most often many different kinds of cells. For instance, in the
body of a man there are different kinds of cells to form the skin,
or the lining of the mouth, or the substance of the eyes, or the
red or white corpuscles of the blood, or the grey matter of the
brain, or the roots of the hair--to name only a few. Thus we see how
complicated the structure of animals has become since life first
made its appearance on the earth. The cells joined themselves to
form tissues; and the tissues joined themselves to form organs; and
these things had to happen before anything like a complete animal of
the higher type, or even a complete vegetable, made its appearance.
Suppose by some great cataclysm, not so great as that which we have
imagined in an earlier chapter, but still world-wide in its effects,
the whole world should once again be swept by a great outbreak of
lava and molten rocks, which of all the living things would leave
traces of its existence? Perhaps a few of the animals with great
bones, or the great trees, might leave an impress of themselves
in the depths of the overwhelming rocks, just as we can stamp the
impress of the skin of our finger-tips on hot sealing-wax; but it is
fairly evident that all the soft-tissued animals and vegetables would
disappear entirely and leave not a trace behind--certainly no trace
that anybody could recognise many millions of years afterwards. It is
still more certain that the simplest forms of life, "the unicellular
organisms," would leave no trace at all. We know that side by side
with the complicated organisms that we can see the simplest organisms
exist now; and must have existed at the beginning of life. Yet when
we examine the records of living things in the rocks which were
formed in the youth of the world, and go back right to the earliest
of these forms of life that have ever been discovered, we find that
such specimens are all of the rather higher (if not of the highest
forms of life). From that we infer that life must have existed many
ages before the period of such remains, though, as we should have
expected, all examples of the earlier forms have disappeared.
Where did this life come from? Lord Kelvin once rashly committed
himself to the notion that life might have been brought to the earth
on one of those flying pieces of rock, which we have already spoken
of, which are named meteorites, or on a fragment of some other
planetary body that had been cast out into space. The speculation is
not so wildly improbable as it has sometimes been considered to be,
because recent researches have shown that it is not impossible for
life to survive at the very low temperatures which a meteorite would
experience on its way through space, and also that the time which a
small body would occupy in travelling, let us say from Mars to the
earth, would not be too great for the prolonged existence of some
germ of life on the meteorite. On the other hand, there is nothing
in known meteorites to suggest that they came from worlds where
conditions exist suitable for life as we know it; and, moreover, even
if we shut our eyes to these improbabilities, we are no nearer to
a solution of the problem of where and how life began. To say that
it was brought to earth on a meteorite is merely to throw back the
problem another stage, for we have still to ask how life began on the
meteorite and on the planet from which it came. The indirect evidence
regarding the probable beginning of the era of life on the earth is
also extremely difficult to examine, and we can only say that the
best geological authority leans to the idea that the conditions
when life would have been possible on the earth were finished long
before the earth had finished growing, and that these conditions may
have existed when the earth was about the size of Mars. Consequently
the first beginnings of life may have existed at depths hundreds
of miles below the earth's surface of to-day. The life was then,
however, only of the very simplest kind; it was probably vegetable
life. Probably also the first life appeared in the ocean, though it
is not altogether unlikely that it may have begun, and have gone on
developing, in fresh water--in those great pits which were first
formed by volcanoes and which afterwards became lakes and then seas.
For our purpose, however, it will be sufficient to say that life
began in the great bodies of water which were accumulating on the
globe, and which owing to the washing down by rain and rivers and
stream and wave action of land materials were becoming "saltier," or
more highly charged with mineral salts of various kinds. The early
forms of life were of the nature of jelly-fish, or simple organisms
which were permeated by the fluid in which they dwelt. The sea was
then warmer than it is now, and there are reasons for believing that
it was something above 100° F., perhaps higher--perhaps rather hotter
than we should now care to bathe in. It was also at the beginning
an ocean which was much less salt and had much less lime in it than
now. Its water was a good deal "softer." It was, however, becoming
much more hard, more like the Dead Sea, which, as everybody knows,
is a body of water so charged with mineral salts and accumulations
that a bather cannot sink in it, and will emerge from his bath
encrusted with salty deposits. As the early ocean became more and
more saturated (with lime and carbonates, etc.), the more vigorous of
the living forms in the water began to resist the change in various
ways. They tried to meet it, or to alter themselves so as not to be
incommoded by it.
This is a very familiar occurrence in natural life and evolution.
Perhaps the commonest example of it that we can select is the
formation of corns on the human feet and hands. A corn, properly
considered, is the defence raised by the skin against unusual or
discommoding pressure or friction. When a boot is too tight, or
when a plough handle or a cricket bat or a golf club is continually
clasped tightly, a "callosity" or horn-like defence is formed. In
some cases a blister is antecedent to the corn; and we may regard
this not only as showing the need for the hardening of the skin, but
as being a stage preliminary to it. There are many other instances.
Hair is formed as a protection to the body, and owing to nature's
economies is not usually formed when clothing takes its place or
heat renders it unnecessary. If the heat is too great, or the light
beating on the unprotected skin is too strong, then another form
of protection takes the place of hair. Why is it that races living
near the Equator wear "the burnished livery of the burning sun,"
and show black or brown pigmentation of the skin? It is because
this pigmentation arrests the penetration of the rays of the sun
more effectively than a garment. Lately Dr. Sambon has pointed
out that the white linen clothes which Europeans generally wear
in the tropics, though they look cool, are not sufficient screens
against the rays of light and heat, and has suggested that white
men's clothes, to be properly protective against the sun, ought to
be woven of threads of two colours, so that the garments should be
white outside and black inside. Apply these principles to animals in
sea-water, who were distressingly affected by conditions to which
they had never been subjected. What would happen? The weaker would
probably be killed by the change in the conditions, just as some
fresh-water fishes and animalculæ would be killed now if plunged into
sea-water. The stronger would, however, become acclimatised, and
would in the course of successive generations struggle to adapt their
bodies to the new conditions.
Thus the living organisms in the earth's early sea contrived to cut
themselves off from being bathed with the novel carbonaceous water.
They cut themselves off from it in the course of generations by
closing themselves off from it with skin or membrane. Many of them
stirred up their cells to secrete lime and exude it so as to form for
themselves a more or less impervious covering or shell. Finally, as
they grew to like the mineral water less, they continually made fresh
experiments to avoid it, and the more enterprising and adventurous
got out of the ocean altogether, and migrated to the air or the land,
perhaps by way of the shore sands and muds. This period, when the
ocean seems to have passed its best stage for supporting all forms of
life, appears to have been that which is geologically known as the
Cambrian. After this period there was a wealth of ferns, of animals
able to leave hard traces of themselves in the fossil records. Before
this period there was no physiological need for either skin or shell.
But once the skin and shell had been developed, primarily as a
defence against the sea-water, their great advantages for purposes of
the struggle for life among all forms of animals soon made themselves
felt, and so they were retained.