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.
CHAPTER X. THE PERMIAN REVOLUTION
In an earlier chapter it was stated that the story of life is a story
of gradual and continuous advance, with occasional periods of more
rapid progress. Hitherto it has been, in these pages, a slow and even
advance from one geological age to another, one level of organisation
to another. This, it is true, must not be taken too literally. Many
a period of rapid change is probably contained, and blurred out of
recognition, in that long chronicle of geological events. When a region
sinks slowly below the waves, no matter how insensible the subsidence
may be, there will often come a time of sudden and vast inundations,
as the higher ridges of the coast just dip below the water-level and
the lower interior is flooded. When two invading arms of the sea meet
at last in the interior of the sinking continent, or when a land-barrier
that has for millions of years separated two seas and their populations
is obliterated, we have a similar occurrence of sudden and far-reaching
change. The whole story of the earth is punctuated with small cataclysms.
But we now come to a change so penetrating, so widespread, and so
calamitous that, in spite of its slowness, we may venture to call
it a revolution.
Indeed, we may say of the remaining story of the earth that it is
characterised by three such revolutions, separated by millions of
years, which are very largely responsible for the appearance of higher
types of life. The facts are very well illustrated by an analogy drawn
from the recent and familiar history of Europe.
The socio-political conditions of Europe in the eighteenth century,
which were still tainted with feudalism, were changed into the socio-political
conditions of the modern world, partly by a slow and continuous evolution,
but much more by three revolutionary movements. First there was the
great upheaval at the end of the eighteenth century, the tremors of
which were felt in the life of every country in Europe. Then, although,
as Freeman says, no part of Europe ever returned entirely to its former
condition, there was a profound and almost universal reaction. In
the 'thirties and 'forties, differing in different countries, a second
revolutionary disturbance shook Europe. The reaction after this upheaval
was far less severe, and the conditions were permanently changed to
a great extent, but a third revolutionary movement followed in the
next generation, and from that time the evolution of socio-political
conditions has proceeded more evenly.
The story of life on the earth since the Coal-forest period is similarly
quickened by three revolutions. The first, at the close of the Carboniferous
period, is the subject of this chapter. It is the most drastic and
devastating of the three, but its effect, at least on the animal world,
will be materially checked by a profound and protracted reaction.
At the end of the Chalk period, some millions of years later, there
will be a second revolution, and it will have a far more enduring
and conspicuous result, though it seem less drastic at the time. Yet
there will be something of a reaction after a time, and at length
a third revolution will inaugurate the age of man. If it is clearly
understood that instead of a century we are contemplating a period
of at least ten million years, and instead of a decade of revolution
we have a change spread over a hundred thousand years or more, this
analogy will serve to convey a most important truth.
The revolutionary agency that broke into the comparatively even chronicle
of life near the close of the Carboniferous period, dethroned its
older types of organisms, and ushered new types to the lordship of
the earth, was cold. The reader will begin to understand why I dwelt
on the aspect of the Coal-forest and its surrounding waters. There
was, then, a warm, moist earth from pole to pole, not even temporarily
chilled and stiffened by a few months of winter, and life spread luxuriantly
in the perpetual semi-tropical summer. Then a spell of cold so severe
and protracted grips the earth that glaciers glitter on the flanks
of Indian and Australian hills, and fields of ice spread over what
are now semitropical regions. In some degree the cold penetrates the
whole earth. The rich forests shrink slowly into thin tracts of scrubby,
poverty-stricken vegetation. The loss of food and the bleak and exacting
conditions of the new earth annihilate thousands of species of the
older organisms, and the more progressive types are moulded into fitness
for the new environment. It is a colossal application of natural selection,
and amongst its results are some of great moment.
In various recent works one reads that earlier geologists, led astray
by the nebular theory of the earth's origin, probably erred very materially
in regard to the climate of primordial times, and that climate has
varied less than used to be supposed. It must not be thought that,
in speaking of a "Permian revolution," I am ignoring or
defying this view of many distinguished geologists. I am taking careful
account of it. There is no dispute, however, about the fact that the
Permian age witnessed an immense carnage of Carboniferous organisms,
and a very considerable modification of those organisms which survived
the catastrophe, and that the great agency in this annihilation and
transformation was cold. To prevent misunderstanding, nevertheless,
it will be useful to explain the controversy about the climate of
the earth in past ages which divides modern geologists.
The root of the difference of opinion and the character of the conflicting
parties have already been indicated. It is a protest of the "Planetesimalists"
against the older, and still general, view of the origin of the earth.
As we saw, that view implies that, as the heavier elements penetrated
centreward in the condensing nebula, the gases were left as a surrounding
shell of atmosphere. It was a mixed mass of gases, chiefly oxygen,
hydrogen, nitrogen, and carbon-dioxide (popularly known as "carbonic
acid gas"). When the water-vapour settled as ocean on the crust,
the atmosphere remained a very dense mixture of oxygen, nitrogen,
and carbon-dioxide--to neglect the minor gases. This heavy proportion
of carbon-dioxide would cause the atmosphere to act as a glass-house
over the surface of the earth, as it does still to some extent. Experiment
has shown that an atmosphere containing much vapour and carbon-dioxide
lets the heat-rays pass through when they are accompanied by strong
light, but checks them when they are separated from the light. In
other words, the primitive atmosphere would allow the heat of the
sun to penetrate it, and then, as the ground absorbed the light, would
retain a large proportion of the heat. Hence the semi-tropical nature
of the primitive earth, the moisture, the dense clouds and constant
rains that are usually ascribed to it. This condition lasted until
the rocks and the forests of the Carboniferous age absorbed enormous
quantities of carbon-dioxide, cleared the atmosphere, and prepared
an age of chill and dryness such as we find in the Permian.
But the planetesimal hypothesis has no room for this enormous percentage
of carbon-dioxide in the primitive atmosphere. Hinc illoe lachrymoe:
in plain English, hence the acute quarrel about primitive climate,
and the close scanning of the geological chronicle for indications
that the earth was not moist and warm until the end of the Carboniferous
period. Once more I do not wish to enfeeble the general soundness
of this account of the evolution of life by relying on any controverted
theory, and we shall find it possible to avoid taking sides.
I have not referred to the climate of the earth in earlier ages, except
to mention that there are traces of a local "ice-age" about
the middle of the Archaean and the beginning of the Cambrian. As these
are many millions of years removed from each other and from the Carboniferous,
it is possible that they represent earlier periods more or less corresponding
to the Permian. But the early chronicle is so compressed and so imperfectly
studied as yet that it is premature to discuss the point. It is, moreover,
unnecessary because we know of no life on land in those remote periods,
and it is only in connection with life on land that we are interested
in changes of climate here. In other words, as far as the present
study is concerned, we need only regard the climate of the Devonian
and Carboniferous periods. As to this there is no dispute; nor, in
fact, about the climate from the Cambrian to the Permian.
As the new school is most brilliantly represented by Professor Chamberlin,*
it will be enough to quote him. He says of the Cambrian that, apart
from the glacial indications in its early part, "the testimony
of the fossils, wherever gathered, implies nearly uniform climatic
conditions . . . throughout all the earth wherever records of the
Cambrian period are preserved" (ii, 273). Of the Ordovician he
says: " All that is known of the life of this era would seem
to indicate that the climate was much more uniform than now throughout
the areas where the strata of the period are known" (ii, 342).
In the Silurian we have "much to suggest uniformity of climate"--in
fact, we have just the same evidence for it--and in the Devonian,
when land-plants abound and afford better evidence, we find the same
climatic equality of living things in the most different latitudes.
Finally, "most of the data at hand indicate that the climate
of the Lower Carboniferous was essentially uniform, and on the whole
both genial and moist" (ii, 518). The "data," we may
recall, are in this case enormously abundant, and indicate the climate
of the earth from the Arctic regions to the Antarctic. Another recent
and critical geologist, Professor Walther ("Geschichte der Erde
und des Lebens," 1908), admits that the coal-vegetation shows
a uniformly warm climate from Spitzbergen to Africa. Mr. Drew ("The
Romance of Modern Geology," 1909) says that " nearly all
over the globe the climate was the same--hot, close, moist, muggy"
(p. 219).
* An apology is due here in some measure. The work
which I quote as of Professor Chamberlin ("Geology," 1903)
is really by two authors, Professors Chamberlin and Salisbury. I merely
quote Professor Chamberlin for shortness, and because the particular
ideas I refer to are expounded by him in separate papers. The work
is the finest manual in modern geological literature. I have used
it much, in conjunction with the latest editions of Geikie, Le Conte,
and Lupparent, and such recent manuals as Walther, De Launay, Suess,
etc., and the geological magazines.
The exception which Professor Chamberlin has in mind when he says
"most of the data" is that we find deposits of salt and
gypsum in the Silurian and Lower Carboniferous, and these seem to
point to the evaporation of lakes in a dry climate. He admits that
these indicate, at the most, local areas or periods of dryness in
an overwhelmingly moist and warm earth. It is thus not disputed that
the climate of the earth was, during a period of at least fifteen
million years (from the Cambrian to the Carboniferous), singularly
uniform, genial, and moist. During that vast period there is no evidence
whatever that the earth was divided into climatic zones, or that the
year was divided into seasons. To such an earth was the prolific life
of the Coal-forest adapted.
It is, further, not questioned that the temperature of the earth fell
in the latter part of the Carboniferous age, and that the cold reached
its climax in the Permian. As we turn over the pages of the geological
chronicle, an extraordinary change comes over the vegetation of the
earth. The great Lepidodendra gradually disappear before the close
of the Permian period; the Sigillariae dwindle into a meagre and expiring
race; the giant Horsetails (Calamites) shrink, and betray the adverse
conditions in their thin, impoverished leaves. New, stunted, hardy
trees make their appearance: the Walchia, a tree something like the
low Araucarian conifers in the texture of its wood, and the Voltzia,
the reputed ancestor of the cypresses. Their narrow, stunted leaves
suggest to the imagination the struggle of a handful of pines on a
bleak hill-side. The rich fern-population is laid waste. The seed-ferns
die out, and a new and hardy type of fern, with compact leaves, the
Glossopteris, spreads victoriously over the globe; from Australia
it travels northward to Russia, which it reaches in the early Permian,
and westward, across the southern continent, to South America. A profoundly
destructive influence has fallen on the earth, and converted its rich
green forests, in which the mighty Club-mosses had reared their crowns
above a sea of waving ferns, into severe and poverty-stricken deserts.
No botanist hesitates to say that it is the coming of a cold, dry
climate that has thus changed the face of the earth. The geologist
finds more direct evidence. In the Werribee Gorge in Victoria I have
seen the marks which Australian geologists have discovered of the
ice-age which put an end to their Coal-forests. From Tasmania to Queensland
they find traces of the rivers and fields of ice which mark the close
of the Carboniferous and beginning of the Permian on the southern
continent. In South Africa similar indications are found from the
Cape to the Transvaal. Stranger still, the geologists of India have
discovered extensive areas of glaciation, belonging to this period,
running down into the actual tropics. And the strangest feature of
all is that the glaciers of India and Australia flowed, not from the
temperate zones toward the tropics, but in the opposite direction.
Two great zones of ice-covered land lay north and south of the equator.
The total area was probably greater than the enormous area covered
with ice in Europe and America during the familiar ice-age of the
latest geological period.
Thus the central idea of this chapter, the destructive inroad of a
colder climate upon the genial Carboniferous world, is an accepted
fact. Critical geologists may suggest that the temperature of the
Coal-forest has been exaggerated, and the temperature of the Permian
put too low. We are not concerned with the dispute. Whatever the exact
change of temperature was, in degrees of the thermometer, it was admittedly
sufficient to transform the face of the earth, and bring a mantle
of ice over millions of square miles of our tropical and subtropical
regions. It remains for us to inquire into the causes of this transformation.
It at once occurs to us that these facts seem to confirm the prevalent
idea, that the Coal-forests stripped the air of its carbon-dioxide
until the earth shivered in an atmosphere thinner than that of to-day.
On reflection, however, it will be seen that, if this were all that
happened, we might indeed expect to find enormous ice-fields extending
from the poles--which we do not find--but not glaciation in the tropics.
Others may think of astronomical theories, and imagine a shrinking
or clouding of the sun, or a change in the direction of the earth's
axis. But these astronomical theories are now little favoured, either
by astronomers or geologists. Professor Lowell bluntly calls them
"astrocomic" theories. Geologists think them superfluous.
There is another set of facts to be considered in connection with
the Permian cold.
As we have seen several times, there are periods when, either owing
to the shrinking of the earth or the overloading of the sea-bottoms,
or a combination of the two, the land regains its lost territory and
emerges from the ocean. Mountain chains rise; new continental surfaces
are exposed to the sun and rain. One of the greatest of these upheavals
of the land occurs in the latter half of the Carboniferous and the
Permian. In the middle of the Carboniferous, when Europe is predominantly
a flat, low-lying land, largely submerged, a chain of mountains begins
to rise across its central part. From Brittany to the east of Saxony
the great ridge runs, and by the end of the Carboniferous it becomes
a chain of lofty mountains (of which fragments remain in the Vosges,
Black Forest, and Hartz mountains), dragging Central Europe high above
the water, and throwing the sea back upon Russia to the north and
the Mediterranean region to the south. Then the chain of the Ural
Mountains begins to rise on the Russian frontier. By the beginning
of the Permian Europe was higher above the water than it had ever
yet been; there was only a sea in Russia and a southern sea with narrow
arms trailing to the northwest. The continent of North America also
had meantime emerged. The rise of the Appalachia and Ouachita mountains
completes the emergence of the eastern continent, and throws the sea
to the west. The Asiatic continent also is greatly enlarged, and in
the southern hemisphere there is a further rise, culminating in the
Permian, of the continent ("Gondwana Land") which united
South America, South Africa, the Antarctic land, Australia and New
Zealand, with an arm to India.
In a word, we have here a physical revolution in the face of the earth.
The changes were generally gradual, though they seem in some places
to have been rapid and abrupt (Chamberlin); but in summary they amounted
to a vast revolution in the environment of animals and plants. The
low-lying, swampy, half-submerged continents reared themselves upward
from the sea-level, shook the marshes and lagoons from their face,
and drained the vast areas that had fostered the growth of the Coal-forests.
It is calculated (Chamberlin) that the shallow seas which had covered
twenty or thirty million square miles of our continental surfaces
in the early Carboniferous were reduced to about five million square
miles in the Permian. Geologists believe, in fact, that the area of
exposed land was probably greater than it is now.
This lifting and draining of so much land would of itself have a profound
influence on life-conditions, and then we must take account of its
indirect influence. The moisture of the earlier period was probably
due in the main to the large proportion of sea-surface and the absence
of high land to condense it. In both respects there is profound alteration,
and the atmosphere must have become very much drier. As this vapour
had been one of the atmosphere's chief elements for retaining heat
at the surface of the earth, the change will involve a great lowering
of temperature. The slanting of the raised land would aid this, as,
in speeding the rivers, it would promote the circulation of water.
Another effect would be to increase the circulation of the atmosphere.
The higher and colder lands would create currents of air that had
not been formed before. Lastly, the ocean currents would be profoundly
modified; but the effect of this is obscure, and may be disregarded
for the moment.
Here, therefore, we have a massive series of causes and effects, all
connected with the great emergence of the land, which throw a broad
light on the change in the face of the earth. We must add the lessening
of the carbon dioxide in the atmosphere. Quite apart from theories
of the early atmosphere, this process must have had a great influence,
and it is included by Professor Chamberlin among the causes of the
world-wide change. The rocks and forests of the Carboniferous period
are calculated to have absorbed two hundred times as much carbon as
there is in the whole of our atmosphere to-day. Where the carbon came
from we may leave open. The Planetesimalists look for its origin mainly
in volcanic eruptions, but, though there was much volcanic activity
in the later Carboniferous and the Permian, there is little trace
of it before the Coal-forests (after the Cambrian). However that may
be, there was a considerable lessening of the carbon-dioxide of the
atmosphere, and this in turn had most important effects. First, the
removal of so much carbon-dioxide and vapour would be a very effective
reason for a general fall in the temperature of the earth. The heat
received from the sun could now radiate more freely into space. Secondly,
it has been shown by experiment that a richness in carbon-dioxide
favours Cryptogamous plants (though it is injurious to higher plants),
and a reduction of it would therefore be hurtful to the Cryptogams
of the Coal-forest. One may almost put it that, in their greed, they
exhausted their store. Thirdly, it meant a great purification of the
atmosphere, and thus a most important preparation of the earth for
higher land animals and plants.
The reader will begin to think that we have sufficiently "explained"
the Permian revolution. Far from it. Some of its problems are as yet
insoluble. We have given no explanation at all why the ice-sheets,
which we would in a general way be prepared to expect, appear in India
and Australia, instead of farther north and south. Professor Chamberlin,
in a profound study of the period (appendix to vol. ii, "Geology"),
suggests that the new land from New Zealand to Antarctica may have
diverted the currents (sea and air) up the Indian Ocean, and caused
a low atmospheric pressure, much precipitation of moisture, and perpetual
canopies of clouds to shield the ice from the sun. Since the outer
polar regions themselves had been semi-tropical up to that time, it
is very difficult to see how this will account for a freezing temperature
in such latitudes as Australia and India. There does not seem to have
been any ice at the Poles up to that time, or for ages afterwards,
so that currents from the polar regions would be very different from
what they are today. If, on the other hand, we may suppose that the
rise of "Gondwana Land" (from Brazil to India) was attended
by the formation of high mountains in those latitudes, we have the
basis, at least, of a more plausible explanation. Professor Chamberlin
rejects this supposition on the ground that the traces of ice-action
are at or near the sea-level, since we find with them beds containing
marine fossils. But this only shows, at the most, that the terminations
of the glaciers reached the sea. We know nothing of the height of
the land from which they started.
For our main purpose, however, it is fortunately not necessary to
clear up these mysteries. It is enough for us that the Carboniferous
land rises high above the surface of the ocean over the earth generally.
The shallow seas are drained off its surface; its swamps and lagoons
generally disappear; its waters run in falling rivers to the ocean.
The dense, moist, warm atmosphere that had so long enveloped it is
changed into a thinner mantle of gas, through which, night by night,
the sun-soaked ground can discharge its heat into space. Cold winds
blow over it from the new mountains; probably vast regions of it are
swept by icy blasts from the glaciated lands. As these conditions
advance in the Permian period, the forests wither and shrink. Of the
extraordinarily mixed vegetation which we found in the Coal-forests
some few types are fitted to meet the severe conditions. The seed-bearing
trees, the thin, needle-leafed trees, the trees with stronger texture
of the wood, are slowly singled out by the deepening cold. The golden
age of Cryptogams is over. The age of the Cycad and the Conifers is
opening. Survivors of the old order linger in the warmer valleys,
as one may see to-day tree-ferns lingering in nooks of southern regions
while an Antarctic wind is whistling on the hills above them; but
over the broad earth the luscious pasturage of the Coal-forest has
changed into what is comparatively a cold desert. We must not, of
course, imagine too abrupt a change. The earth had been by no means
all swamp in the Carboniferous age. The new types were even then developing
in the cooler and drier localities. But their hour has come, and there
is great devastation among the lower plant population of the earth.
It follows at once that there would be, on land, an equal devastation
and a similar selection in the animal world. The vegetarians suffered
an appalling reduction of their food; the carnivores would dwindle
in the same proportion. Both types, again, would suffer from the enormous
changes in their physical surroundings. Vast stretches of marsh, with
teeming populations, were drained, and turned into firm, arid plains
or bleak hill-sides. The area of the Amphibia, for instance, was no
less reduced than their food. The cold, in turn, would exercise a
most formidable selection. Before the Permian period there was not
on the whole earth an animal with a warm-blooded (four-chambered)
heart or a warm coat of fur or feathers; nor was there a single animal
that gave any further care to the eggs it discharged, and left to
the natural warmth of the earth to develop. The extermination of species
in the egg alone must have been enormous.
It is impossible to convey any just impression of the carnage which
this Permian revolution wrought among the population of the earth.
We can but estimate how many species of animals and plants were exterminated,
and the reader must dimly imagine the myriads of living things that
are comprised in each species. An earlier American geologist, Professor
Le Conte, said that not a single Carboniferous species crossed the
line of the Permian revolution. This has proved to be an exaggeration,
but Professor Chamberlin seems to fall into an exaggeration on the
other side when he says that 300 out of 10,000 species survived. There
are only about 300 species of animals and plants known in the whole
of the Permian rocks (Geikie), and most of these are new. For instance,
of the enormous plant-population of the Coal-forests, comprising many
thousands of species, only fifty species survived unchanged in the
Permian. We may say that, as far as our knowledge goes, of every thirty
species of animals and plants in the Carboniferous period, twenty-eight
were blotted out of the calendar of life for ever; one survived by
undergoing such modifications that it became a new species, and one
was found fit to endure the new conditions for a time. We must leave
it to the imagination to appreciate the total devastation of individuals
entailed in this appalling application of what we call natural selection.
But what higher types of life issued from the womb of nature after
so long and painful a travail? The annihilation of the unfit is the
seamy side, though the most real side, of natural selection. We ignore
it, or extenuate it, and turn rather to consider the advances in organisation
by which the survivors were enabled to outlive the great chill and
impoverishment.
Unfortunately, if the Permian period is an age of death, it is not
an age of burials. The fossil population of its cemeteries is very
scanty. Not only is the living population enormously reduced, but
the areas that were accustomed to entomb and preserve organisms--the
lake and shore deposits--are also greatly reduced. The frames of animals
and plants now rot on the dry ground on which they live. Even in the
seas, where life must have been much reduced by the general disturbance
of conditions, the record is poor. Molluscs and Brachiopods and small
fishes fill the list, but are of little instructiveness for us, except
that they show a general advance of species. Among the Cephalopods,
it is true, we find a notable arrival. On the one hand, a single small
straight-shelled Cephalopod lingers for a time with the ancestral
form; on the other hand, a new and formidable competitor appears among
the coiled-shell Cephalopods. It is the first appearance of the famous
Ammonite, but we may defer the description of it until we come to
the great age of Ammonites.
Of the insects and their fortunes in the great famine we have no direct
knowledge; no insect remains have yet been found in Permian rocks.
We shall, however, find them much advanced in the next period, and
must conclude that the selection acted very effectively among their
thousand Carboniferous species.
The most interesting outcome of the new conditions is the rise and
spread of the reptiles. No other sign of the times indicates so clearly
the dawn of a new era as the appearance of these primitive, clumsy
reptiles, which now begin to oust the Amphibia. The long reign of
aquatic life is over; the ensign of progress passes to the land animals.
The half-terrestrial, half-aquatic Amphibian deserts the water entirely
(in one or more of its branches), and a new and fateful dynasty is
founded. Although many of the reptiles will return to the water, when
the land sinks once more, the type of the terrestrial quadruped is
now fully evolved, and from its early reptilian form will emerge the
lords of the air and the lords of the land, the birds and the mammals.
To the uninformed it may seem that no very great advance is made when
the reptile is evolved from the Amphibian. In reality the change implies
a profound modification of the frame and life of the vertebrate. Partly,
we may suppose, on account of the purification of the air, partly
on account of the decrease in water surface, the gills are now entirely
discarded. The young reptile loses them during its embryonic life--as
man and all the mammals and birds do to-day--and issues from the egg
a purely lung-breathing creature. A richer blood now courses through
the arteries, nourishing the brain and nerves as well as the muscles.
The superfluous tissue of the gill-structures is used in the improvement
of the ear and mouth-parts; a process that had begun in the Amphibian.
The body is raised up higher from the ground, on firmer limbs; the
ribs and the shoulder and pelvic bones-- the saddles by which the
weight of the body is adjusted between the limbs and the backbone--are
strengthened and improved. Finally, two important organs for the protection
and nurture of the embryo (the amnion and the allantois) make their
appearance for the first time in the reptile. In grade of organisation
the reptile is really nearer to the bird than it is to the salamander.
Yet these Permian reptiles are so generalised in character and so
primitive in structure that they point back unmistakably to an Amphibian
ancestry. The actual line of descent is obscure. When the reptiles
first appear in the rocks, they are already divided into widely different
groups, and must have been evolved some time before. Probably they
started from some group or groups of the Amphibia in the later Carboniferous,
when, as we saw, the land began to rise considerably. We have not
yet recovered, and may never recover, the region where the early forms
lived, and therefore cannot trace the development in detail. The fossil
archives, we cannot repeat too often, are not a continuous, but a
fragmentary, record of the story of life. The task of the evolutionist
may be compared to the work of tracing the footsteps of a straying
animal across the country. Here and there its traces will be amply
registered on patches of softer ground, but for the most part they
will be entirely lost on the firmer ground. So it is with the fossil
record of life. Only in certain special conditions are the passing
forms buried and preserved. In this case we can say only that the
Permian reptiles fall into two great groups, and that one of these
shows affinities to the small salamander-like Amphibia of the Coal-forest
(the Microsaurs), while the other has affinities to the Labyrinthodonts.
A closer examination of these early reptiles may be postponed until
we come to speak of the "age of reptiles." We shall see
that it is probable that an even higher type of animal, the mammal,
was born in the throes of the Permian revolution. But enough has been
said in vindication of the phrase which stands at the head of this
chapter; and to show how the great Primary age of terrestrial life
came to a close. With its new inhabitants the earth enters upon a
fresh phase, and thousands of its earlier animals and plants are sealed
in their primordial tombs, to await the day when man will break the
seals and put flesh once more on the petrified bones.
CHAPTER XI. THE MIDDLE AGES OF THE EARTH
The story of the earth from the beginning of the Cambrian period to
the present day was long ago divided by geologists into four great
eras. The periods we have already covered--the Cambrian, Ordovician,
Silurian, Devonian, Carboniferous, and Permian--form the Primary or
Palaeozoic Era, to which the earlier Archaean rocks were prefixed
as a barren and less interesting introduction. The stretch of time
on which we now enter, at the close of the Permian, is the Secondary
or Mesozoic Era. It will be closed by a fresh upheaval of the earth
and disturbance of life-conditions in the Chalk period, and followed
by a Tertiary Era, in which the earth will approach its modern aspect.
At its close there will be another series of upheavals, culminating
in a great Ice-age, and the remaining stretch of the earth's story,
in which we live, will form the Quaternary Era.
In point of duration these four eras differ enormously from each other.
If the first be conceived as comprising sixteen million years--a very
moderate estimate--the second will be found to cover less than eight
million years, the third less than three million years, and the fourth,
the Age of Man, much less than one million years; while the Archaean
Age was probably as long as all these put together. But the division
is rather based on certain gaps, or "unconformities," in
the geological record; and, although the breaches are now partially
filled, we saw that they correspond to certain profound and revolutionary
disturbances in the face of the earth. We retain them, therefore,
as convenient and logical divisions of the biological as well as the
geological chronicle, and, instead of passing from one geological
period to another, we may, for the rest of the story, take these three
eras as wholes, and devote a few chapters to the chief advances made
by living things in each era. The Mesozoic Era will be a protracted
reaction between two revolutions: a period of low-lying land, great
sea-invasions, and genial climate, between two upheavals of the earth.
The Tertiary Era will represent a less sharply defined depression,
with genial climate and luxuriant life, between two such upheavals.
The Mesozaic ("middle life") Era may very fitly be described
as the Middle Ages of life on the earth. It by no means occupies a
central position in the chronicle of life from the point of view of
time or antiquity, just as the Middle Ages of Europe are by no means
the centre of the chronicle of mankind, but its types of animals and
plants are singularly transitional between the extinct ancient and
the actual modern types. Life has been lifted to a higher level by
the Permian revolution. Then, for some millions of years, the sterner
process of selection relaxes, the warm bosom of the earth swarms again
with a teeming and varied population, and a rich material is provided
for the next great application of drastic selective agencies. To a
poet it might seem that nature indulges each succeeding and imperfect
type of living thing with a golden age before it is dismissed to make
place for the higher.
The Mesozoic opens in the middle of the great revolution described
in the last chapter. Its first section, the Triassic period, is at
first a mere continuation of the Permian. A few hundred species of
animals and hardy plants are scattered over a relatively bleak and
inhospitable globe. Then the land begins to sink once more. The seas
spread in great arms over the revelled continents, the plant world
rejoices in the increasing warmth and moisture, and the animals increase
in number and variety. We pass into the Jurassic period under conditions
of great geniality. Warm seas are found as far north and south as
our present polar regions, and the low-lying fertile lands are covered
again with rich, if less gigantic, forests, in which hordes of stupendous
animals find ample nourishment. The mammal and the bird are already
on the stage, but their warm coats and warm blood offer no advantage
in that perennial summer, and they await in obscurity the end of the
golden age of the reptiles. At the end of the Jurassic the land begins
to rise once more. The warm, shallow seas drain off into the deep
oceans, and the moist, swampy lands are dried. The emergence continues
throughout the Cretaceous (Chalk) period. Chains of vast mountains
rise slowly into the air in many parts of the earth, and a new and
comparatively rapid change in the vegetation--comparable to that at
the close of the Carboniferous--announces the second great revolution.
The Mesozoic closes with the dismissal of the great reptiles and the
plants on which they fed, and the earth is prepared for its new monarchs,
the flowering plants, the birds, and the mammals.
How far this repeated levelling of the land after its repeated upheavals
is due to a real sinking of the crust we cannot as yet determine.
The geologist of our time is disposed to restrict these mysterious
rises and falls of the crust as much as possible. A much more obvious
and intelligible agency has to be considered. The vast upheaval of
nearly all parts of the land during the Permian period would naturally
lead to a far more vigorous scouring of its surface by the rains and
rivers. The higher the land, the more effectively it would be worn
down. The cooler summits would condense the moisture, and the rains
would sweep more energetically down the slopes of the elevated continents.
There would thus be a natural process of levelling as long as the
land stood out high above the water-line, but it seems probable that
there was also a real sinking of the crust. Such subsidences have
been known within historic times.
By the end of the Triassic--a period of at least two million years--the
sea had reconquered a vast proportion of the territory wrested from
it in the Permian revolution. Most of Europe, west of a line drawn
from the tip of Norway to the Black Sea, was under water--generally
open sea in the south and centre, and inland seas or lagoons in the
west. The invasion of the sea continued, and reached its climax, in
the Jurassic period. The greater part of Europe was converted into
an archipelago. A small continent stood out in the Baltic region.
Large areas remained above the sea-level in Austria, Germany, and
France. Ireland, Wales, and much of Scotland were intact, and it is
probable that a land bridge still connected the west of Europe with
the east of America. Europe generally was a large cluster of islands
and ridges, of various sizes, in a semi-tropical sea. Southern Asia
was similarly revelled, and it is probable that the seas stretched,
with little interruption, from the west of Europe to the Pacific.
The southern continent had deep wedges of the sea driven into it.
India, New Zealand, and Australia were successively detached from
it, and by the end of the Mesozoic it was much as we find it to-day.
The Arctic continent (north of Europe) was flooded, and there was
a great interior sea in the western part of the North American continent.
This summary account of the levelling process which went on during
the Triassic and Jurassic will prepare us to expect a return of warm
climate and luxurious life, and this the record abundantly evinces.
The enormous expansion of the sea--a great authority, Neumayr, believes
that it was the greatest extension of the sea that is known in geology--and
lowering of the land would of itself tend to produce this condition,
and it may be that the very considerable volcanic activity, of which
we find evidence in the Permian and Triassic, had discharged great
volumes of carbon-dioxide into the atmosphere.
Whatever the causes were, the earth has returned to paradisiacal conditions.
The vast ice-fields have gone, the scanty and scrubby vegetation is
replaced by luscious forests of cycads, conifers, and ferns, and warmth-loving
animals penetrate to what are now the Arctic and Antarctic regions.
Greenland and Spitzbergen are fragments of a continent that then bore
a luxuriant growth of ferns and cycads, and housed large reptiles
that could not now live thousands of miles south of it. England, and
a large part of Europe, was a tranquil blue coral-ocean, the fringes
of its islands girt with reefs such as we find now only three thousand
miles further south, with vast shoals of Ammonites, sometimes of gigantic
size, preying upon its living population or evading its monstrous
sharks; while the sunlit lands were covered with graceful, palmlike
cycads and early yews and pines and cypresses, and quaint forms of
reptiles throve on the warm earth or in the ample swamps, or rushed
on outstretched wings through the purer air.
It was an evergreen world, a world, apparently, of perpetual summer.
No trace is found until the next period of an alternation of summer
and winter--no trees that shed their leaves annually, or show annual
rings of growth in the wood--and there is little trace of zones of
climate as yet. It is true that the sensitive Ammonites differ in
the northern and the southern latitudes, but, as Professor Chamberlin
says, it is not clear that the difference points to a diversity of
climate. We may conclude that the absence of corals higher than the
north of England implies a more temperate climate further north, but
what Sir A. Geikie calls (with slight exaggeration) "the almost
tropical aspect" of Greenland warns us to be cautious. The climate
of the mid-Jurassic was very much warmer and more uniform than the
climate of the earth to-day. It was an age of great vital expansion.
And into this luxuriant world we shall presently find a fresh period
of elevation, disturbance, and cold breaking with momentous evolutionary
results. Meantime, we may take a closer look at these interesting
inhabitants of the Middle Ages of the earth, before they pass away
or are driven, in shrunken regiments, into the shelter of the narrowing
tropics.
The principal change in the aspect of the earth, as the cold, arid
plains and slopes of the Triassic slowly yield the moist and warm
ow-lying lands of the Jurassic, to consists in the character of the
vegetation. It is wholly intermediate in its forms between that of
the primitive forests and that of the modern world. The great Cryptogams
of the Carboniferous world--the giant Club-mosses and their kindred--have
been slain by the long period of cold and drought. Smaller Horsetails
(sometimes of a great size, but generally of the modern type) and
Club-mosses remain, but are not a conspicuous feature in the landscape.
On the other hand, there is as yet-- apart from the Conifers--no trace
of the familiar trees and flowers and grasses of the later world.
The vast majority of the plants are of the cycad type. These-- now
confined to tropical and subtropical regions--with the surviving ferns,
the new Conifers, and certain trees of the ginkgo type, form the characteristic
Mesozoic vegetation.
A few words in the language of the modern botanist will show how this
vegetation harmonises with the story of evolution. Plants are broadly
divided into the lower kingdom of the Cryptogams (spore-bearing) and
the upper kingdom of the Phanerogams (seed-bearing). As we saw, the
Primary Era was predominantly the age of Cryptogams; the later periods
witness the rise and supremacy of the Phanerogams. But these in turn
are broadly divided into a less advanced group, the Gymnosperms, and
a more advanced group, the Angiosperms or flowering plants. And, just
as the Primary Era is the age of Cryptogams, the Secondary is the
age of Gymnosperms, and the Tertiary (and present) is the age of Angiosperms.
Of about 180,000 species of plants in nature to-day more than 100,000
are Angiosperms; yet up to the end of the Jurassic not a single true
Angiosperm is found in the geological record.
This is a broad manifestation of evolution, but it is not quite an
accurate statement, and its inexactness still more strongly confirms
the theory of evolution. Though the Primary Era was predominantly
the age of Cryptogams, we saw that a very large number of seed-bearing
plants, with very mixed characters, appeared before its close. It
thus prepares the way for the cycads and conifers and ginkgoes of
the Mesozoic, which we may conceive as evolved from one or other branch
of the mixed Carboniferous vegetation. We next find that the Mesozoic
is by no means purely an age of Gymnosperms. I do not mean merely
that the Angiosperms appear in force before its close, and were probably
evolved much earlier. The fact is that the Gymnosperms of the Mesozoic
are often of a curiously mixed character, and well illustrate the
transition to the Angiosperms, though they may not be their actual
ancestors. This will be clearer if we glance in succession at the
various types of plant which adorned and enriched the Jurassic world.
The European or American landscape--indeed, the aspect of the earth
generally, for there are no pronounced zones of climate--is still
utterly different from any that we know to-day. No grass carpets the
plains; none of the flowers or trees with which we are familiar, except
conifers, are found in any region. Ferns grow in great abundance,
and have now reached many of the forms with which we are acquainted.
Thickets of bracken spread over the plains; clumps of Royal ferns
and Hartstongues spring up in moister parts. The trees are conifers,
cycads, and trees akin to the ginkgo, or Maidenhair Tree, of modern
Japan. Cypresses, yews, firs, and araucarias (the Monkey Puzzle group)
grow everywhere, though the species are more primitive than those
of today. The broad, fan-like leaves and plum-like fruit of the ginkgoales,
of which the temple-gardens of Japan have religiously preserved a
solitary descendant, are found in the most distant regions. But the
most frequent and characteristic tree of the Jurassic landscape is
the cycad.
The cycads--the botanist would say Cycadophyta or Cycadales, to mark
them off from the cycads of modern times--formed a third of the whole
Jurassic vegetation, while to-day they number only about a hundred
species in 180,000, and are confined to warm latitudes. All over the
earth, from the Arctic to the Antarctic, their palm-like foliage showered
from the top of their generally short stems in the Jurassic. But the
most interesting point about them is that a very large branch of them
(the Bennettiteae) went far beyond the modern Gymnosperm in their
flowers and fruit, and approached the Angiosperms. Their fructifications
"rivalled the largest flowers of the present day in structure
and modelling" (Scott), and possibly already gave spots of sober
colour to the monotonous primitive landscape. On the other hand, they
approached the ferns so much more closely than modern cycads do that
it is often impossible to say whether Jurassic remains must be classed
as ferns or cycads.
We have here, therefore, a most interesting evolutionary group. The
botanist finds even more difficulty than the zoologist in drawing
up the pedigrees of his plants, but the general features of the larger
groups which he finds in succession in the chronicle of the earth
point very decisively to evolution. The seed-bearing ferns of the
Coal-forest point upward to the later stage, and downward to a common
origin with the ordinary spore-bearing ferns. Some of them are "altogether
of a cycadean type" (Scott) in respect of the seed. On the other
hand, the Bennettiteae of the Jurassic have the mixed characters of
ferns, cycads, and flowering plants, and thus, in their turn, point
downward to a lower ancestry and upward to the next great stage in
plant-development. It is not suggested that the seed-ferns we know
evolved into the cycads we know, and these in turn into our flowering
plants. It is enough for the student of evolution to see in them so
many stages in the evolution of plants up to the Angiosperm level.
The gaps between the various groups are less rigid than scientific
men used to think.
Taller than the cycads, firmer in the structure of the wood, and destined
to survive in thousands of species when the cycads would be reduced
to a hundred, were the pines and yews and other conifers of the Jurassic
landscape. We saw them first appearing, in the stunted Walchias and
Voltzias, during the severe conditions of the Permian period. Like
the birds and mammals they await the coming of a fresh period of cold
to give them a decided superiority over the cycads. Botanists look
for their ancestors in some form related to the Cordaites of the Coal-forest.
The ginkgo trees seem to be even more closely related to the Cordaites,
and evolved from an early and generalised branch of that group. The
Cordaites, we may recall, more or less united in one tree the characters
of the conifer (in their wood) and the cycad (in their fruit).
So much for the evolutionary aspect of the Jurassic vegetation in
itself. Slender as the connecting links are, it points clearly enough
to a selection of higher types during the Permian revolution from
the varied mass of the Carboniferous flora, and it offers in turn
a singularly varied and rich group from which a fresh selection may
choose yet higher types. We turn now to consider the animal population
which, directly or indirectly, fed upon it, and grew with its growth.
To the reptiles, the birds, and the mammals, we must devote special
chapters. Here we may briefly survey the less conspicuous animals
of the Mesozoic Epoch.
The insects would be one of the chief classes to benefit by the renewed
luxuriance of the vegetation. The Hymenopters (butterflies) have not
yet appeared. They will, naturally, come with the flowers in the next
great phase of organic life. But all the other orders of insects are
represented, and many of our modern genera are fully evolved. The
giant insects of the Coal-forest, with their mixed patriarchal features,
have given place to more definite types. Swarms of dragon-flies, may-flies,
termites (with wings), crickets, and cockroaches, may be gathered
from the preserved remains. The beetles (Coleopters) have come on
the scene in the Triassic, and prospered exceedingly. In some strata
three-fourths of the insects are beetles, and as we find that many
of them are wood-eaters, we are not surprised. Flies (Dipters) and
ants (Hymenopters) also are found, and, although it is useless to
expect to find the intermediate forms of such frail creatures, the
record is of some evolutionary interest. The ants are all winged.
Apparently there is as yet none of the remarkable division of labour
which we find in the ants to-day, and we may trust that some later
period of change may throw light on its origin.
Just as the growth of the forests--for the Mesozoic vegetation has
formed immense coal-beds in many parts of the world, even in Yorkshire
and Scotland--explains this great development of the insects, they
would in their turn supply a rich diet to the smaller land animals
and flying animals of the time. We shall see this presently. Let us
first glance at the advances among the inhabitants of the seas.
The most important and stimulating event in the seas is the arrival
of the Ammonite. One branch of the early shell-fish, it will be remembered,
retained the head of its naked ancestor, and lived at the open mouth
of its shell, thus giving birth to the Cephalopods. The first form
was a long, straight, tapering shell, sometimes several feet long.
In the course of time new forms with curved shells appeared, and began
to displace the straight-shelled. Then Cephalopods with close-coiled
shells, like the nautilus, came, and--such a shell being an obvious
advantage-- displaced the curved shells. In the Permian, we saw, a
new and more advanced type of the coiled-shell animal, the Ammonite,
made its appearance, and in the Triassic and Jurassic it becomes the
ogre or tyrant of the invertebrate world. Sometimes an inch or less
in diameter, it often attained a width of three feet or more across
the shell, at the aperture of which would be a monstrous and voracious
mouth.
The Ammonites are not merely interesting as extinct monsters of the
earth's Middle Ages, and stimulating terrors of the deep to the animals
on which they fed. They have an especial interest for the evolutionist.
The successive chambers which the animal adds, as it grows, to the
habitation of its youth, leave the earlier chambers intact. By removing
them in succession in the adult form we find an illustration of the
evolution of the elaborate shell of the Jurassic Ammonite. It is an
admirable testimony to the validity of the embryonic law we have often
quoted--that the young animal is apt to reproduce the past stages
of its ancestry--that the order of the building of the shell in the
late Ammonite corresponds to the order we trace in its development
in the geological chronicle. About a thousand species of Ammonites
were developed in the Mesozoic, and none survived the Mesozoic. Like
the Trilobites of the Primary Era, like the contemporary great reptiles
on land, the Ammonites were an abortive growth, enjoying their hour
of supremacy until sterner conditions bade them depart. The pretty
nautilus is the only survivor to-day of the vast Mesozoic population
of coiled-shell Cephalopods.
A rival to the Ammonite appeared in the Triassic seas, a formidable
forerunner of the cuttle-fish type of Cephalopod. The animal now boldly
discards the protecting and confining shell, or spreads over the outside
of it, and becomes a "shell-fish" with the shell inside.
The octopus of our own time has advanced still further, and become
the most powerful of the invertebrates. The Belemnite, as the Mesozoic
cuttle-fish is called, attained so large a size that the internal
bone, or pen (the part generally preserved), is sometimes two feet
in length. The ink-bags of the Belemnite also are sometimes preserved,
and we see how it could balk a pursuer by darkening the waters. It
was a compensating advantage for the loss of the shell.
In all the other classes of aquatic animals we find corresponding
advances. In the remaining Molluscs the higher or more effective types
are displacing the older. It is interesting to note that the oyster
is fully developed, and has a very large kindred, in the Mesozoic
seas. Among the Brachiopods the higher sloping-shoulder type displaces
the square-shoulder shells. In the Crustacea the Trilobites and Eurypterids
have entirely disappeared; prawns and lobsters abound, and the earliest
crab makes its appearance in the English Jurassic rocks. This sudden
arrival of a short-tailed Crustacean surprises us less when we learn
that the crab has a long tail in its embryonic form, but the actual
line of its descent is not clear. Among the Echinoderms we find that
the Cystids and Blastoids have gone, and the sea-lilies reach their
climax in beauty and organisation, to dwindle and almost disappear
in the last part of the Mesozoic. One Jurassic sea-lily was found
to have 600,000 distinct ossicles in its petrified frame. The free-moving
Echinoderms are now in the ascendant, the sea-urchins being especially
abundant. The Corals are, as we saw, extremely abundant, and a higher
type (the Hexacoralla) is superseding the earlier and lower (Tetracoralla).
Finally, we find a continuous and conspicuous advance among the fishes.
At the close of the Triassic and during the Jurassic they seem to
undergo profound and comparatively rapid changes. The reason will,
perhaps, be apparent in the next chapter, when we describe the gigantic
reptiles which feed on them in the lakes and shore-waters. A greater
terror than the shark had appeared in their environment. The Ganoids
and Dipneusts dwindle, and give birth to their few modern representatives.
The sharks with crushing teeth diminish in number, and the sharp-toothed
modern shark attains the supremacy in its class, and evolves into
forms far more terrible than any that we know to-day. Skates and rays
of a more or less modern type, and ancestral gar-pikes and sturgeons,
enter the arena. But the most interesting new departure is the first
appearance, in the Jurassic, of bony-framed fishes (Teleosts). Their
superiority in organisation soon makes itself felt, and they enter
upon the rapid evolution which will, by the next period, give them
the first place in the fish world.
Over the whole Mesozoic world, therefore, we find advance and the
promise of greater advance. The Permian stress has selected the fittest
types to survive from the older order; the Jurassic luxuriance is
permitting a fresh and varied expansion of life, in preparation for
the next great annihilation of the less fit and selection of the more
fit. Life pauses before another leap. The Mesozoic earth--to apply
to it the phrase which a geologist has given to its opening phase--welcomes
the coming and speeds the parting guest. In the depths of the ocean
a new movement is preparing, but we have yet to study the highest
forms of Mesozoic life before we come to the Cretaceous disturbances.
CHAPTER XII. THE AGE OF REPTILES
From one point of view the advance of life on the earth seems to proceed
not with the even flow of a river, but in the successive waves of
an oncoming tide. It is true that we have detected a continuous advance
behind all these rising and receding waves, yet their occurrence is
a fact of some interest, and not a little speculation has been expended
on it. When the great procession of life first emerges out of the
darkness of Archaean times, it deploys into a spreading world of strange
Crustaceans, and we have the Age of Trilobites. Later there is the
Age of Fishes, then of Cryptogams and Amphibia, and then of Cycads
and Reptiles, and there will afterwards be an Age of Birds and Mammals,
and finally an Age of Man. But there is no ground for mystic speculation
on this circumstance of a group of organisms fording the earth for
a few million years, and then perishing or dwindling into insignificance.
We shall see that a very plain and substantial process put an end
to the Age of the Cycads, Ammonites, and Reptiles, and we have seen
how the earlier dynasties ended.
The phrase, however, the Age of Reptiles, is a fitting and true description
of the greater part of the Mesozoic Era, which lies, like a fertile
valley, between the Permian and the Chalk upheavals. From the bleak
heights of the Permian period, or--more probably--from its more sheltered
regions, in which they have lingered with the ferns and cycads, the
reptiles spread out over the earth, as the summer of the Triassic
period advances. In the full warmth and luxuriance of the Jurassic
they become the most singular and powerful army that ever trod the
earth. They include small lizard-like creatures and monsters more
than a hundred feet in length. They swim like whales in the shallow
seas; they shrink into the shell of the giant turtle; they rear themselves
on towering hind limbs, like colossal kangaroos; they even rise into
the air, and fill it with the dragons of the fairy tale. They spread
over the whole earth from Australia to the Arctic circle. Then the
earth seems to grow impatient of their dominance, and they shrink
towards the south, and struggle in a diminished territory. The colossal
monsters and the formidable dragons go the way of all primitive life,
and a ragged regiment of crocodiles, turtles, and serpents in the
tropics, with a swarm of smaller creatures in the fringes of the warm
zone, is all that remains, by the Tertiary Era, of the world-conquering
army of the Mesozoic reptiles.
They had appeared, as we said, in the Permian period. Probably
Continua
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