A SHORT HISTORY OF THE WORLD BY H. G. WELLS
5.THE AGE OF THE COAL SWAMPS
The land during this Age of Fishes was apparently quite lifeless. Crags and uplands of barren rock lay under the sun and rain. There was no real soil—for as yet there were no earthworms which help to make a soil, and no plants to break up the rock particles into mould; there was no trace of moss or lichen. Life was still only in the sea.
Over this world of barren rock played great changes of climate. The
causes of these changes of climate were very complex and they have
still to be properly estimated. The changing shape of the earth’s
orbit, the gradual shifting of the poles of rotation, changes in the
shapes of the continents, probably even fluctuations in the warmth of
the sun, now conspired to plunge great areas of the earth’s surface
into long periods of cold and ice and now again for millions of years
spread a warm or equable climate over this planet. There seem to have
been phases of great internal activity in the world’s history, when in
the course of a few million years accumulated upthrusts would break out
in lines of volcanic eruption and upheaval and rearrange the mountain
and continental outlines of the globe, increasing the depth of the sea
and the height of the mountains and exaggerating the extremes of
climate. And these would be followed by vast ages of comparative
quiescence, when frost, rain and river would wear down the mountain
heights and carry great masses of silt to fill and raise the sea
bottoms and spread the seas, ever shallower and wider, over more and
more of the land. There have been “high and deep” ages in the world’s
history and “low and level” ages. The reader must dismiss from his
mind any idea that the surface of the earth has been growing steadily
cooler since its crust grew solid. After that much cooling had been
achieved, the internal temperature ceased to affect surface conditions.
There are traces of periods of superabundant ice and snow, of “Glacial
Ages,” that is, even in the Azoic period.
It was only towards the close of the Age of Fishes, in a period of
extensive shallow seas and lagoons, that life spread itself out in any
effectual way from the waters on to the land. No doubt the earlier
types of the forms that now begin to appear in great abundance had
already been developing in a rare and obscure manner for many scores of
millions of years. But now came their opportunity.
Plants no doubt preceded animal forms in this invasion of the land, but
the animals probably followed up the plant emigration very closely. The
first problem that the plant had to solve was the problem of some
sustaining stiff support to hold up its fronds to the sunlight when the
buoyant water was withdrawn; the second was the problem of getting
water from the swampy ground below to the tissues of the plant, now
that it was no longer close at hand. The two problems were solved by
the development of woody tissue which both sustained the plant and
acted as water carrier to the leaves. The Record of the Rocks is
suddenly crowded by a vast variety of woody swamp plants, many of them
of great size, big tree mosses, tree ferns, gigantic horsetails and the
like. And with these, age by age, there crawled out of the water a
great variety of animal forms. There were centipedes and millipedes;
there were the first primitive insects; there were creatures related to
the ancient king crabs and sea scorpions which became the earliest
spiders and land scorpions, and presently there were vertebrated
animals.
Some of the earlier insects were very large. There were dragon flies in
this period with wings that spread out to twenty-nine inches.
In various ways these new orders and genera had adapted themselves to
breathing air. Hitherto all animals had breathed air dissolved in
water, and that indeed is what all animals still have to do. But now in
divers fashions the animal kingdom was acquiring the power of supplying
its own moisture where it was needed. A man with a perfectly dry lung
would suffocate to-day; his lung surfaces must be moist in order that
air may pass through them into his blood. The adaptation to air
breathing consists in all cases either in the development of a cover to
the old-fashioned gills to stop evaporation, or in the development of
tubes or other new breathing organs lying deep inside the body and
moistened by a watery secretion. The old gills with which the
ancestral fish of the vertebrated line had breathed were inadaptable to
breathing upon land, and in the case of this division of the animal
kingdom it is the swimming bladder of the fish which becomes a new,
deep-seated breathing organ, the lung. The kind of animals known as
amphibia, the frogs and newts of to-day, begin their lives in the water
and breathe by gills; and subsequently the lung, developing in the same
way as the swimming bladder of many fishes do, as a baglike outgrowth
from the throat, takes over the business of breathing, the animal comes
out on land, and the gills dwindle and the gill slits disappear. (All
except an outgrowth of one gill slit, which becomes the passage of the
ear and ear-drum.) The animal can now live only in the air, but it
must return at least to the edge of the water to lay its eggs and
reproduce its kind.
All the air-breathing vertebrata of this age of swamps and plants
belonged to the class amphibia. They were nearly all of them forms
related to the newts of to-day, and some of them attained a
considerable size. They were land animals, it is true, but they were
land animals needing to live in and near moist and swampy places, and
all the great trees of this period were equally amphibious in their
habits. None of them had yet developed fruits and seeds of a kind that
could fall on land and develop with the help only of such moisture as
dew and rain could bring. They all had to shed their spores in water,
it would seem, if they were to germinate.
It is one of the most beautiful interests of that beautiful science,
comparative anatomy, to trace the complex and wonderful adaptations of
living things to the necessities of existence in air. All living
things, plants and animals alike, are primarily water things. For
example all the higher vertebrated animals above the fishes, up to and
including man, pass through a stage in their development in the egg or
before birth in which they have gill slits which are obliterated before
the young emerge. The bare, water-washed eye of the fish is protected
in the higher forms from drying up by eyelids and glands which secrete
moisture. The weaker sound vibrations of air necessitate an ear-drum.
In nearly every organ of the body similar modifications and adaptations
are to be detected, similar patchings-up to meet aerial conditions.
This Carboniferous age, this age of the amphibia, was an age of life in
the swamps and lagoons and on the low banks among these waters. Thus
far life had now extended. The hills and high lands were still quite
barren and lifeless. Life had learnt to breathe air indeed, but it
still had its roots in its native water; it still had to return to the
water to reproduce its kind.