Section 11 of 21 - Table of Contents
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.
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