Section 9 of 21 - Table of Contents
CHAPTER IX. THE ANIMALS OF THE COAL-FOREST
We have next to see that when this period of searching adversity
comes--as it will in the next chapter --the animal world also
offers a luxuriant variety of forms from which the higher types
may be selected. This, it need hardly be said, is just what we
find in the geological record. The fruitful, steaming, rich-laden
earth now offered tens of millions of square miles of pasture to
vegetal feeders; the waters, on the other hand, teemed with
gigantic sharks, huge Cephalopods, large scorpion-like and
lobster-like animals, and shoals of armour-plated, hard-toothed
fishes. Successive swarms of vegetarians--Worms, Molluscs, etc.--
followed the plant on to the land; and swarms of carnivores
followed the vegetarians, and assumed strange, new forms in
adaptation to land-life. The migration had probably proceeded
throughout the Devonian period, especially from the calmer shores
of the inland seas. By the middle of the Coal-forest period there
was a very large and varied animal population on the land. Like
the plants, moreover, these animals were of an intermediate and
advancing nature. No bird or butterfly yet flits from tree to
tree; no mammal rears its young in the shelter of the ferns. But
among the swarming population are many types that show a
beginning of higher organisation, and there is a rich and varied
material provided for the coming selection.
The monarch of the Carboniferous forest is the Amphibian. In that
age of spreading swamps and "dim, watery woodlands," the stupid
and sluggish Amphibian finds his golden age, and, except perhaps
the scorpion, there is no other land animal competent to dispute
his rule. Even the scorpion, moreover, would not find the
Carboniferous Amphibian very vulnerable. We must not think of the
smooth-skinned frogs and toads and innocent newts which to-day
represent the fallen race of the Amphibia. They were then heavily
armoured, powerfully armed, and sometimes as large as alligators
or young crocodiles. It is a characteristic of advancing life
that a new type of organism has its period of triumph, grows to
enormous proportions, and spreads into many different types,
until the next higher stage of life is reached, and it is
dethroned by the new-comers.
The first indication--apart from certain disputed impressions in
the Devonian--of the land-vertebrate is the footprint of an
Amphibian on an early Carboniferous mud-flat. Hardened by the
sun, and then covered with a fresh deposit when it sank beneath
the waters, it remains to-day to witness the arrival of the
five-toed quadruped who was to rule the earth. As the period
proceeds, remains are found in great abundance, and we see that
there must have been a vast and varied population of the Amphibia
on the shores of the Carboniferous lagoons and swamps. There were
at least twenty genera of them living in what is now the island
of Britain, and was then part of the British-Scandinavian
continent. Some of them were short and stumpy creatures, a few
inches long, with weak limbs and short tails, and broad,
crescent-shaped heads, their bodies clothed in the fine scaly
armour of their fish-ancestor (the Branchiosaurs). Some (the
Aistopods) were long, snake-like creatures, with shrunken limbs
and bodies drawn out until, in some cases, the backbone had 150
vertebrae. They seem to have taken to the thickets, in the
growing competition, as the serpents did later, and lost the use
of their limbs, which would be merely an encumbrance in winding
among the roots and branches. Some (the Microsaurs) were agile
little salamander-like organisms, with strong, bony frames and
relatively long and useful legs; they look as if they may even
have climbed the trees in pursuit of snails and insects. A fourth
and more formidable sub-order, the Labyrinthodonts--which take
their name from the labyrinthine folds of the enamel in their
strong teeth--were commonly several feet in length. Some of them
attained a length of seven or eight feet, and had plates of bone
over their heads and bellies, while the jaws in their enormous
heads were loaded with their strong, labyrinthine teeth. Life on
land was becoming as eventful and stimulating as life in the
waters.
The general characteristic of these early Amphibia is that they
very clearly retain the marks of their fish ancestry. All of them
have tails; all of them have either scales or (like many of the
fishes) plates of bone protecting the body. In some of the
younger specimens the gills can still be clearly traced, but no
doubt they were mainly lung-animals. We have seen how the fish
obtained its lungs, and need add only that this change in the
method of obtaining oxygen for the blood involved certain further
changes of a very important nature. Following the fossil record,
we do not observe the changes which are taking place in the soft
internal organs, but we must not lose sight of them. The heart,
for instance, which began as a simple muscular expansion or
distension of one of the blood-vessels of some primitive worm,
then doubled and became a two-chambered pump in the fish, now
develops a partition in the auricle (upper chamber), so that the
aerated blood is to some extent separated from the venous blood.
This approach toward the warm-blooded type begins in the
"mud-fish," and is connected with the development of the lungs.
Corresponding changes take place in the arteries, and we shall
find that this change in structure is of very great importance in
the evolution of the higher types of land-life. The heart of the
higher land-animals, we may add, passes through these stages in
its embryonic development.
Externally the chief change in the Amphibian is the appearance of
definite legs. The broad paddle of the fin is now useless, and
its main stem is converted into a jointed, bony limb, with a
five-toed foot, spreading into a paddle, at the end. But the legs
are still feeble, sprawling supports, letting the heavy body down
almost to the ground. The Amphibian is an imperfect, but
necessary, stage in evolution. It is an improvement on the
Dipneust fish, which now begins to dwindle very considerably in
the geological record, but it is itself doomed to give way
speedily before one of its more advanced descendants, the
Reptile. Probably the giant salamander of modern Japan affords
the best suggestion of the large and primitive salamanders of the
Coal-forest, while the Caecilia--snake-like Amphibia with scaly
skins, which live underground in South America--may not
impossibly be degenerate survivors of the curious Aistopods.
Our modern tailless Amphibia, frogs and toads, appear much later
in the story of the earth, but they are not without interest here
on account of the remarkable capacity which they show to adapt
themselves to different surroundings. There are frogs, like the
tree-frog of Martinique, and others in regions where water is
scarce, which never pass through the tadpole stage; or, to be
quite accurate, they lose the gills and tail in the egg, as
higher land-animals do. On the other hand, there is a modern
Amphibian, the axolotl of Mexico, which retains the gills
throughout life, and never lives on land. Dr. Gadow has shown
that the lake in which it lives is so rich in food that it has
little inducement to leave it for the land. Transferred to a
different environment, it may pass to the land, and lose its
gills. These adaptations help us to understand the rich variety
of Amphibian forms that appeared in the changing conditions of
the Carboniferous world.
When we think of the diet of the Amphibia we are reminded of the
other prominent representatives of land life at the time. Snails,
spiders, and myriapods crept over the ground or along the stalks
of the trees, and a vast population of insects filled the air. We
find a few stray wings in the Silurian, and a large number of
wings and fragments in the Devonian, but it is in the Coal-forest
that we find the first great expansion of insect life, with a
considerable development of myriapods, spiders, and scorpions.
Food was enormously abundant, and the insect at least had no
rival in the air, for neither bird nor flying reptile had yet
appeared. Hence we find the same generous growth as amongst the
Amphibia. Large primitive "may-flies" had wings four or five
inches long; great locust-like creatures had fat bodies sometimes
twenty inches in length, and soared on wings of remarkable
breadth, or crawled on their six long, sprawling legs. More than
a thousand species of insects, and nearly a hundred species of
spiders and fifty of myriapods, are found in the remains of the
Coal-forests.
From the evolutionary point of view these new classes are as
obscure in their origin, yet as manifestly undergoing evolution
when they do fully appear, as the earlier classes we have
considered. All are of a primitive and generalised character;
that is to say, characters which are to-day distributed among
widely different groups were then concentrated and mingled in one
common ancestor, out of which the later groups will develop. All
belong to the lowest orders of their class. No Hymenopters (ants,
bees, and wasps) or Coleopters (beetles) are found in the
Coal-forest; and it will be many millions of years before the
graceful butterfly enlivens the landscapes of the earth. The
early insects nearly all belong to the lower orders of the
Orthopters (cockroaches, crickets, locusts, etc.) and Neuropters
(dragon-flies, may-flies, etc.). A few traces of Hemipters (now
mainly represented by the degenerate bugs) are found, but
nine-tenths of the Carboniferous insects belong to the lowest
orders of their class, the Orthopters and Neuropters. In fact,
they are such primitive and generalised insects, and so
frequently mingle the characteristics of the two orders, that one
of the highest authorities, Scudder, groups them in a special and
extinct order, the Palmodictyoptera; though this view is not now
generally adopted. We shall find the higher orders of insects
making their appearance in succession as the story proceeds.
Thus far, then, the insects of the Coal-forest are in entire
harmony with the principle of evolution, but when we try to trace
their origin and earlier relations our task is beset with
difficulties. It goes without saying that such delicate frames as
those of the earlier insects had very little chance of being
preserved in the rocks until the special conditions of the
forest-age set in. We are, therefore, quite prepared to hear that
the geologist cannot give us the slenderest information. He finds
the wing of what he calls "the primitive bug" (Protocimex), an
Hemipterous insect, in the later Ordovician, and the wing of a
"primitive cockroach" (Palaeoblattina) in the Silurian. From
these we can merely conclude that insects were already numerous
and varied. But we have already, in similar difficulties,
received assistance from the science of zoology, and we now
obtain from that science a most important clue to the evolution
of the insect.
In South America, South Africa, and Australasia, which were at
one time connected by a great southern continent, we find a
little caterpillar-like creature which the zoologist regards with
profound interest. It is so curious that he has been obliged to
create a special class for it alone--a distinction which will be
appreciated when I mention that the neighbouring class of the
insects contains more than a quarter of a million living species.
This valuable little animal, with its tiny head, round, elongated
body, and many pairs of caterpillar-like legs, was until a few
decades ago regarded as an Annelid (like the earth-worm). It has,
in point of fact, the peculiar kidney-structures (nephridia) and
other features of the Annelid, but a closer study discovered in
it a character that separated it far from any worm-group. It was
found to breathe the air by means of tracheae (little tubes
running inward from the surface of the body), as the myriapods,
spiders, and insects do. It was, in other words, "a kind of
half-way animal between the Arthropods and the Annelids"
("Cambridge Natural History," iv, p. 5), a surviving kink in the
lost chain of the ancestry of the insect. Through millions of
years it has preserved a primitive frame that really belongs to
the Cambrian, if not an earlier, age. It is one of the most
instructive "living fossils" in the museum of nature.
Peripatus, as the little animal is called, points very clearly to
an Annelid ancestor of all the Tracheates (the myriapods,
spiders, and insects), or all the animals that breathe by means
of trachere. To understand its significance we must glance once
more at an early chapter in the story of life. We saw that a vast
and varied wormlike population must have filled the Archaean
ocean, and that all the higher lines of animal development start
from one or other point in this broad kingdom. The Annelids, in
which the body consists of a long series of connected rings or
segments, as in the earth-worm, are one of the highest groups of
these worm-like creatures, and some branch of them developed a
pair of feet (as in the caterpillar) on each segment of the body
and a tough, chitinous coat. Thus arose the early Arthropods, on
tough-coated, jointed, articulated animals. Some of these
remained in the water, breathing by means of gills, and became
the Crustacea. Some, however, migrated to the land and developed
what we may almost call "lungs"--little tubes entering the body
at the skin and branching internally, to bring the air into
contact with the blood, the tracheae.
In Peripatus we have a strange survivor of these primitive
Annelid-Tracheates of many million years ago. The simple nature
of its breathing apparatus suggests that the trachere were
developed out of glands in the skin; just as the fish, when it
came on land, probably developed lungs from its swimming
bladders. The primitive Tracheates, delivered from the increasing
carnivores of the waters, grew into a large and varied family, as
all such new types do in favourable surroundings. From them in
the course of time were evolved the three great classes of the
Myriapods (millipedes and centipedes), the Arachnids (scorpions,
spiders, and mites), and the Insects. I will not enter into the
much-disputed and Obscure question of their nearer relationship.
Some derive the Insects from the Myriapods, some the Myriapods
from the Insects, and some think they evolved independently;
while the rise of the spiders and scorpions is even more obscure.
But how can we see any trace of an Annelid ancestor in the vastly
different frames of these animals which are said to descend from
it? It is not so difficult as it seems to be at first sight. In
the Myriapod we still have the elongated body and successive
pairs of legs. In the Arachnid the legs are reduced in number and
lengthened, while the various segments of the body are fused in
two distinct body-halves, the thorax and the abdomen. In the
Insect we have a similar concentration of the primitive long
body. The abdomen is composed of a large number (usually nine or
ten) of segments which have lost their legs and fused together.
In the thorax three segments are still distinctly traceable, with
three pairs of legs--now long jointed limbs--as in the
caterpillar ancestor; in the Carboniferous insect these three
joints in the thorax are particularly clear. In the head four or
five segments are fused together. Their limbs have been modified
into the jaws or other mouth-appendages, and their separate
nerve-centres have combined to form the large ring of
nerve-matter round the gullet which represents the brain of the
insect.
How, then, do we account for the wings of the insect? Here we can
offer nothing more than speculation, but the speculation is not
without interest. It may be laid down in principle that the
flying animal begins as a leaping animal. The "flying fish" may
serve to suggest an early stage in the development of wings; it
is a leaping fish, its extended fins merely buoying it, like the
surfaces of an aeroplane, and so prolonging its leap away from
its pursuer. But the great difficulty is to imagine any part of
the smooth-coated primitive insect, apart from the limbs (and the
wings of the insect are not developed from legs, like those of
the bird), which might have even an initial usefulness in buoying
the body as it leaped. It has been suggested, therefore, that the
primitive insect returned to the water, as the whale and seal did
in the struggle for life of a later period. The fact that the
mayfly and dragon-fly spend their youth in the water is thought
to confirm this. Returning to the water, the primitive insects
would develop gills, like the Crustacea. After a time the stress
of life in the water drove them back to the land, and the gills
became useless. But the folds or scales of the tough coat, which
had covered the gills, would remain as projecting planes, and are
thought to have been the rudiment from which a long period of
selection evolved the huge wings of the early dragon-flies and
mayflies. It is generally believed that the wingless order of
insects (Aptera) have not lost, but had never developed, wings,
and that the insects with only one or two pairs all descend from
an ancestor with three pairs.
The early date of their origin, the delicacy of their structure,
and the peculiar form which their larval development has
generally assumed, combine to obscure the evolution of the
insect, and we must be content for the present with these general
indications. The vast unexplored regions of Africa, South
America, and Central Australia, may yet yield further clues, and
the riddle of insect-metamorphosis may some day betray the
secrets which it must hold. For the moment the Carboniferous
insects interest us as a rich material for the operation of a
coming natural selection. On them, as on all other Carboniferous
life, a great trial is about to fall. A very small proportion of
them will survive that trial, and they trill be the better
organised to maintain themselves and rear their young in the new
earth.
The remaining land-life of the Coal-forest is confined to
worm-like organisms whose remains are not preserved, and
land-snails which do not call for further discussion. We may, in
conclusion, glance at the progress of life in the waters. Apart
from the appearance of the great fishes and Crustacea, the
Carboniferous period was one of great stimulation to aquatic
life. Constant changes were taking place in the level and the
distribution of land and water. The aspect of our coal seams
to-day, alternating between thick layers of sand and mud, shows a
remarkable oscillation of the land. Many recent authorities have
questioned whether the trees grew on the sites where we find them
to-day, and were not rather washed down into the lagoons and
shallow waters from higher ground. In that case we could not too
readily imagine the forest-clad region sinking below the waves,
being buried under the deposits of the rivers, and then emerging,
thousands of years later, to receive once more the thick mantle
of sombre vegetation. Probably there was less rising and falling
of the crust than earlier geologists imagined. But, as one of the
most recent and most critical authorities, Professor Chamberlin,
observes, the comparative purity of the coal, the fairly uniform
thickness of the seams, the bed of clay representing soil at
their base, the frequency with which the stumps are still found
growing upright (as in the remarkable exposed Coal-forest surface
in Glasgow, at the present ground-level),* the perfectly
preserved fronds and the general mixture of flora, make it highly
probable that the coal-seam generally marks the actual site of a
Coal-forest, and there were considerable vicissitudes in the
distribution of land and water. Great areas of land repeatedly
passed beneath the waters, instead of a re-elevation of the land,
however, we may suppose that the shallow water was gradually
filled with silt and debris from the land, and a fresh forest
grew over it.
* The civic authorities of Glasgow have wisely exposed and
protected this instructive piece of Coal-forest in one of their
parks. I noticed, however that in the admirable printed
information they supply to the public, they describe the trees as
"at least several hundred thousand years old." There is no
authority in the world who would grant less than ten million
years since the Coal-forest period.
These changes are reflected in the progress of marine life,
though their influence is probably less than that of the great
carnivorous monsters which now fill the waters. The heavy
Arthrodirans languish and disappear. The "pavement-toothed"
sharks, which at first represent three-fourths of the
Elasmobranchs, dwindle in turn, and in the formidable spines
which develop on them we may see evidence of the great struggle
with the sharp-toothed sharks which are displacing them. The
Ostracoderms die out in the presence of these competitors. The
smaller fishes (generally Crossopterygii) seem to live mainly in
the inland and shore waters, and advance steadily toward the
modern types, but none of our modern bony fishes have yet
appeared.
More evident still is the effect of the new conditions upon the
Crustacea. The Trilobite, once the master of the seas, slowly
yields to the stronger competitors, and the latter part of the
Carboniferous period sees the last genus of Trilobites finally
extinguished. The Eurypterids (large scorpion-like Crustacea,
several feet long) suffer equally, and are represented by a few
lingering species. The stress favours the development of new and
more highly organised Crustacea. One is the Limulus or
"king-crab," which seems to be a descendant, or near relative, of
the Trilobite, and has survived until modern times. Others
announce the coming of the long-tailed Crustacea, of the lobster
and shrimp type. They had primitive representatives in the
earlier periods, but seem to have been overshadowed by the
Trilobites and Eurypterids. As these in turn are crushed, the
more highly organised Malacostraca take the lead, and primitive
specimens of the shrimp and lobster make their appearance.
The Echinoderms are still mainly represented by the sea-lilies.
The rocks which are composed of their remains show that vast
areas of the sea-floor must have been covered with groves of
sea-lilies, bending on their long, flexible stalks and waving
their great flower-like arms in the water to attract food. With
them there is now a new experiment in the stalked Echinoderm, the
Blastoid, an armless type; but it seems to have been a failure.
Sea-urchins are now found in the deposits, and, although their
remains are not common, we may conclude that the star-fishes were
scattered over the floor of the sea.
For the rest we need only observe that progress and rich
diversity of forms characterise the other groups of animals. The
Corals now form great reefs, and the finer Corals are gaining
upon the coarser. The Foraminifers (the chalk-shelled, one-celled
animals) begin to form thick rocks with their dead skeletons; the
Radiolaria (the flinty-shelled microbes) are so abundant that
more than twenty genera of them have been distinguished in
Cornwall and Devonshire. The Brachiopods and Molluscs still
abound, but the Molluscs begin to outnumber the lower type of
shell-fish. In the Cephalopods we find an increasing complication
of the structure of the great spiral-shelled types.
Such is the life of the Carboniferous period. The world rejoices
in a tropical luxuriance. Semi-tropical vegetation is found in
Spitzbergen and the Antarctic, as well as in North Europe, Asia,
and America, and in Australasia; corals and sea-lilies flourish
at any part of the earth's surface. Warm, dank, low-lying lands,
bathed by warm oceans and steeped in their vapours, are the
picture suggested-- as we shall see more closely--to the minds of
all geologists. In those happy conditions the primitive life of
the earth erupts into an abundance and variety that are fitly
illustrated in the well-preserved vegetation of the forest. And
when the earth has at length flooded its surface with this
seething tide of life; when the air is filled with a thousand
species of insects, and the forest-floor feels the heavy tread of
the giant salamander and the light feet of spiders, scorpions,
centipedes, and snails, and the lagoons and shores teem with
animals, the Golden Age begins to close, and all the
semi-tropical luxuriance is banished. A great doom is pronounced
on the swarming life of the Coal-forest period, and from every
hundred species of its animals and plants only two or three will
survive the searching test.
Prev
Next
All
Printer Friendly Version | Send this page to a friend | Discuss this Book
Your email address is safe with us. View our Privacy policy.
 |
 |
|
|
The Complete Plays of Gilbert and Sullivan Sections: 50 What's this? Table of Contents |
 FictionShort StoriesPoetryPlaysSci FiPhilosophyReligionBiography |
Body Mass
