The Story of Evolution
by
Joseph McCabe
Part 3 out of 6
vertebrae. They seem to have taken to the thickets, in the
growing competition, as the serpents did later, and lost the use
of their limbs, which would be merely an encumbrance in winding
among the roots and branches. Some (the Microsaurs) were agile
little salamander-like organisms, with strong, bony frames and
relatively long and useful legs; they look as if they may even
have climbed the trees in pursuit of snails and insects. A fourth
and more formidable sub-order, the Labyrinthodonts--which take
their name from the labyrinthine folds of the enamel in their
strong teeth--were commonly several feet in length. Some of them
attained a length of seven or eight feet, and had plates of bone
over their heads and bellies, while the jaws in their enormous
heads were loaded with their strong, labyrinthine teeth. Life on
land was becoming as eventful and stimulating as life in the
waters.
The general characteristic of these early Amphibia is that they
very clearly retain the marks of their fish ancestry. All of them
have tails; all of them have either scales or (like many of the
fishes) plates of bone protecting the body. In some of the
younger specimens the gills can still be clearly traced, but no
doubt they were mainly lung-animals. We have seen how the fish
obtained its lungs, and need add only that this change in the
method of obtaining oxygen for the blood involved certain further
changes of a very important nature. Following the fossil record,
we do not observe the changes which are taking place in the soft
internal organs, but we must not lose sight of them. The heart,
for instance, which began as a simple muscular expansion or
distension of one of the blood-vessels of some primitive worm,
then doubled and became a two-chambered pump in the fish, now
develops a partition in the auricle (upper chamber), so that the
aerated blood is to some extent separated from the venous blood.
This approach toward the warm-blooded type begins in the
"mud-fish," and is connected with the development of the lungs.
Corresponding changes take place in the arteries, and we shall
find that this change in structure is of very great importance in
the evolution of the higher types of land-life. The heart of the
higher land-animals, we may add, passes through these stages in
its embryonic development.
Externally the chief change in the Amphibian is the appearance of
definite legs. The broad paddle of the fin is now useless, and
its main stem is converted into a jointed, bony limb, with a
five-toed foot, spreading into a paddle, at the end. But the legs
are still feeble, sprawling supports, letting the heavy body down
almost to the ground. The Amphibian is an imperfect, but
necessary, stage in evolution. It is an improvement on the
Dipneust fish, which now begins to dwindle very considerably in
the geological record, but it is itself doomed to give way
speedily before one of its more advanced descendants, the
Reptile. Probably the giant salamander of modern Japan affords
the best suggestion of the large and primitive salamanders of the
Coal-forest, while the Caecilia--snake-like Amphibia with scaly
skins, which live underground in South America--may not
impossibly be degenerate survivors of the curious Aistopods.
Our modern tailless Amphibia, frogs and toads, appear much later
in the story of the earth, but they are not without interest here
on account of the remarkable capacity which they show to adapt
themselves to different surroundings. There are frogs, like the
tree-frog of Martinique, and others in regions where water is
scarce, which never pass through the tadpole stage; or, to be
quite accurate, they lose the gills and tail in the egg, as
higher land-animals do. On the other hand, there is a modern
Amphibian, the axolotl of Mexico, which retains the gills
throughout life, and never lives on land. Dr. Gadow has shown
that the lake in which it lives is so rich in food that it has
little inducement to leave it for the land. Transferred to a
different environment, it may pass to the land, and lose its
gills. These adaptations help us to understand the rich variety
of Amphibian forms that appeared in the changing conditions of
the Carboniferous world.
When we think of the diet of the Amphibia we are reminded of the
other prominent representatives of land life at the time. Snails,
spiders, and myriapods crept over the ground or along the stalks
of the trees, and a vast population of insects filled the air. We
find a few stray wings in the Silurian, and a large number of
wings and fragments in the Devonian, but it is in the Coal-forest
that we find the first great expansion of insect life, with a
considerable development of myriapods, spiders, and scorpions.
Food was enormously abundant, and the insect at least had no
rival in the air, for neither bird nor flying reptile had yet
appeared. Hence we find the same generous growth as amongst the
Amphibia. Large primitive "may-flies" had wings four or five
inches long; great locust-like creatures had fat bodies sometimes
twenty inches in length, and soared on wings of remarkable
breadth, or crawled on their six long, sprawling legs. More than
a thousand species of insects, and nearly a hundred species of
spiders and fifty of myriapods, are found in the remains of the
Coal-forests.
From the evolutionary point of view these new classes are as
obscure in their origin, yet as manifestly undergoing evolution
when they do fully appear, as the earlier classes we have
considered. All are of a primitive and generalised character;
that is to say, characters which are to-day distributed among
widely different groups were then concentrated and mingled in one
common ancestor, out of which the later groups will develop. All
belong to the lowest orders of their class. No Hymenopters (ants,
bees, and wasps) or Coleopters (beetles) are found in the
Coal-forest; and it will be many millions of years before the
graceful butterfly enlivens the landscapes of the earth. The
early insects nearly all belong to the lower orders of the
Orthopters (cockroaches, crickets, locusts, etc.) and Neuropters
(dragon-flies, may-flies, etc.). A few traces of Hemipters (now
mainly represented by the degenerate bugs) are found, but
nine-tenths of the Carboniferous insects belong to the lowest
orders of their class, the Orthopters and Neuropters. In fact,
they are such primitive and generalised insects, and so
frequently mingle the characteristics of the two orders, that one
of the highest authorities, Scudder, groups them in a special and
extinct order, the Palmodictyoptera; though this view is not now
generally adopted. We shall find the higher orders of insects
making their appearance in succession as the story proceeds.
Thus far, then, the insects of the Coal-forest are in entire
harmony with the principle of evolution, but when we try to trace
their origin and earlier relations our task is beset with
difficulties. It goes without saying that such delicate frames as
those of the earlier insects had very little chance of being
preserved in the rocks until the special conditions of the
forest-age set in. We are, therefore, quite prepared to hear that
the geologist cannot give us the slenderest information. He finds
the wing of what he calls "the primitive bug" (Protocimex), an
Hemipterous insect, in the later Ordovician, and the wing of a
"primitive cockroach" (Palaeoblattina) in the Silurian. From
these we can merely conclude that insects were already numerous
and varied. But we have already, in similar difficulties,
received assistance from the science of zoology, and we now
obtain from that science a most important clue to the evolution
of the insect.
In South America, South Africa, and Australasia, which were at
one time connected by a great southern continent, we find a
little caterpillar-like creature which the zoologist regards with
profound interest. It is so curious that he has been obliged to
create a special class for it alone--a distinction which will be
appreciated when I mention that the neighbouring class of the
insects contains more than a quarter of a million living species.
This valuable little animal, with its tiny head, round, elongated
body, and many pairs of caterpillar-like legs, was until a few
decades ago regarded as an Annelid (like the earth-worm). It has,
in point of fact, the peculiar kidney-structures (nephridia) and
other features of the Annelid, but a closer study discovered in
it a character that separated it far from any worm-group. It was
found to breathe the air by means of tracheae (little tubes
running inward from the surface of the body), as the myriapods,
spiders, and insects do. It was, in other words, "a kind of
half-way animal between the Arthropods and the Annelids"
("Cambridge Natural History," iv, p. 5), a surviving kink in the
lost chain of the ancestry of the insect. Through millions of
years it has preserved a primitive frame that really belongs to
the Cambrian, if not an earlier, age. It is one of the most
instructive "living fossils" in the museum of nature.
Peripatus, as the little animal is called, points very clearly to
an Annelid ancestor of all the Tracheates (the myriapods,
spiders, and insects), or all the animals that breathe by means
of trachere. To understand its significance we must glance once
more at an early chapter in the story of life. We saw that a vast
and varied wormlike population must have filled the Archaean
ocean, and that all the higher lines of animal development start
from one or other point in this broad kingdom. The Annelids, in
which the body consists of a long series of connected rings or
segments, as in the earth-worm, are one of the highest groups of
these worm-like creatures, and some branch of them developed a
pair of feet (as in the caterpillar) on each segment of the body
and a tough, chitinous coat. Thus arose the early Arthropods, on
tough-coated, jointed, articulated animals. Some of these
remained in the water, breathing by means of gills, and became
the Crustacea. Some, however, migrated to the land and developed
what we may almost call "lungs"--little tubes entering the body
at the skin and branching internally, to bring the air into
contact with the blood, the tracheae.
In Peripatus we have a strange survivor of these primitive
Annelid-Tracheates of many million years ago. The simple nature
of its breathing apparatus suggests that the trachere were
developed out of glands in the skin; just as the fish, when it
came on land, probably developed lungs from its swimming
bladders. The primitive Tracheates, delivered from the increasing
carnivores of the waters, grew into a large and varied family, as
all such new types do in favourable surroundings. From them in
the course of time were evolved the three great classes of the
Myriapods (millipedes and centipedes), the Arachnids (scorpions,
spiders, and mites), and the Insects. I will not enter into the
much-disputed and Obscure question of their nearer relationship.
Some derive the Insects from the Myriapods, some the Myriapods
from the Insects, and some think they evolved independently;
while the rise of the spiders and scorpions is even more obscure.
But how can we see any trace of an Annelid ancestor in the vastly
different frames of these animals which are said to descend from
it? It is not so difficult as it seems to be at first sight. In
the Myriapod we still have the elongated body and successive
pairs of legs. In the Arachnid the legs are reduced in number and
lengthened, while the various segments of the body are fused in
two distinct body-halves, the thorax and the abdomen. In the
Insect we have a similar concentration of the primitive long
body. The abdomen is composed of a large number (usually nine or
ten) of segments which have lost their legs and fused together.
In the thorax three segments are still distinctly traceable, with
three pairs of legs--now long jointed limbs--as in the
caterpillar ancestor; in the Carboniferous insect these three
joints in the thorax are particularly clear. In the head four or
five segments are fused together. Their limbs have been modified
into the jaws or other mouth-appendages, and their separate
nerve-centres have combined to form the large ring of
nerve-matter round the gullet which represents the brain of the
insect.
How, then, do we account for the wings of the insect? Here we can
offer nothing more than speculation, but the speculation is not
without interest. It may be laid down in principle that the
flying animal begins as a leaping animal. The "flying fish" may
serve to suggest an early stage in the development of wings; it
is a leaping fish, its extended fins merely buoying it, like the
surfaces of an aeroplane, and so prolonging its leap away from
its pursuer. But the great difficulty is to imagine any part of
the smooth-coated primitive insect, apart from the limbs (and the
wings of the insect are not developed from legs, like those of
the bird), which might have even an initial usefulness in buoying
the body as it leaped. It has been suggested, therefore, that the
primitive insect returned to the water, as the whale and seal did
in the struggle for life of a later period. The fact that the
mayfly and dragon-fly spend their youth in the water is thought
to confirm this. Returning to the water, the primitive insects
would develop gills, like the Crustacea. After a time the stress
of life in the water drove them back to the land, and the gills
became useless. But the folds or scales of the tough coat, which
had covered the gills, would remain as projecting planes, and are
thought to have been the rudiment from which a long period of
selection evolved the huge wings of the early dragon-flies and
mayflies. It is generally believed that the wingless order of
insects (Aptera) have not lost, but had never developed, wings,
and that the insects with only one or two pairs all descend from
an ancestor with three pairs.
The early date of their origin, the delicacy of their structure,
and the peculiar form which their larval development has
generally assumed, combine to obscure the evolution of the
insect, and we must be content for the present with these general
indications. The vast unexplored regions of Africa, South
America, and Central Australia, may yet yield further clues, and
the riddle of insect-metamorphosis may some day betray the
secrets which it must hold. For the moment the Carboniferous
insects interest us as a rich material for the operation of a
coming natural selection. On them, as on all other Carboniferous
life, a great trial is about to fall. A very small proportion of
them will survive that trial, and they trill be the better
organised to maintain themselves and rear their young in the new
earth.
The remaining land-life of the Coal-forest is confined to
worm-like organisms whose remains are not preserved, and
land-snails which do not call for further discussion. We may, in
conclusion, glance at the progress of life in the waters. Apart
from the appearance of the great fishes and Crustacea, the
Carboniferous period was one of great stimulation to aquatic
life. Constant changes were taking place in the level and the
distribution of land and water. The aspect of our coal seams
to-day, alternating between thick layers of sand and mud, shows a
remarkable oscillation of the land. Many recent authorities have
questioned whether the trees grew on the sites where we find them
to-day, and were not rather washed down into the lagoons and
shallow waters from higher ground. In that case we could not too
readily imagine the forest-clad region sinking below the waves,
being buried under the deposits of the rivers, and then emerging,
thousands of years later, to receive once more the thick mantle
of sombre vegetation. Probably there was less rising and falling
of the crust than earlier geologists imagined. But, as one of the
most recent and most critical authorities, Professor Chamberlin,
observes, the comparative purity of the coal, the fairly uniform
thickness of the seams, the bed of clay representing soil at
their base, the frequency with which the stumps are still found
growing upright (as in the remarkable exposed Coal-forest surface
in Glasgow, at the present ground-level),* the perfectly
preserved fronds and the general mixture of flora, make it highly
probable that the coal-seam generally marks the actual site of a
Coal-forest, and there were considerable vicissitudes in the
distribution of land and water. Great areas of land repeatedly
passed beneath the waters, instead of a re-elevation of the land,
however, we may suppose that the shallow water was gradually
filled with silt and debris from the land, and a fresh forest
grew over it.
* The civic authorities of Glasgow have wisely exposed and
protected this instructive piece of Coal-forest in one of their
parks. I noticed, however that in the admirable printed
information they supply to the public, they describe the trees as
"at least several hundred thousand years old." There is no
authority in the world who would grant less than ten million
years since the Coal-forest period.
These changes are reflected in the progress of marine life,
though their influence is probably less than that of the great
carnivorous monsters which now fill the waters. The heavy
Arthrodirans languish and disappear. The "pavement-toothed"
sharks, which at first represent three-fourths of the
Elasmobranchs, dwindle in turn, and in the formidable spines
which develop on them we may see evidence of the great struggle
with the sharp-toothed sharks which are displacing them. The
Ostracoderms die out in the presence of these competitors. The
smaller fishes (generally Crossopterygii) seem to live mainly in
the inland and shore waters, and advance steadily toward the
modern types, but none of our modern bony fishes have yet
appeared.
More evident still is the effect of the new conditions upon the
Crustacea. The Trilobite, once the master of the seas, slowly
yields to the stronger competitors, and the latter part of the
Carboniferous period sees the last genus of Trilobites finally
extinguished. The Eurypterids (large scorpion-like Crustacea,
several feet long) suffer equally, and are represented by a few
lingering species. The stress favours the development of new and
more highly organised Crustacea. One is the Limulus or
"king-crab," which seems to be a descendant, or near relative, of
the Trilobite, and has survived until modern times. Others
announce the coming of the long-tailed Crustacea, of the lobster
and shrimp type. They had primitive representatives in the
earlier periods, but seem to have been overshadowed by the
Trilobites and Eurypterids. As these in turn are crushed, the
more highly organised Malacostraca take the lead, and primitive
specimens of the shrimp and lobster make their appearance.
The Echinoderms are still mainly represented by the sea-lilies.
The rocks which are composed of their remains show that vast
areas of the sea-floor must have been covered with groves of
sea-lilies, bending on their long, flexible stalks and waving
their great flower-like arms in the water to attract food. With
them there is now a new experiment in the stalked Echinoderm, the
Blastoid, an armless type; but it seems to have been a failure.
Sea-urchins are now found in the deposits, and, although their
remains are not common, we may conclude that the star-fishes were
scattered over the floor of the sea.
For the rest we need only observe that progress and rich
diversity of forms characterise the other groups of animals. The
Corals now form great reefs, and the finer Corals are gaining
upon the coarser. The Foraminifers (the chalk-shelled, one-celled
animals) begin to form thick rocks with their dead skeletons; the
Radiolaria (the flinty-shelled microbes) are so abundant that
more than twenty genera of them have been distinguished in
Cornwall and Devonshire. The Brachiopods and Molluscs still
abound, but the Molluscs begin to outnumber the lower type of
shell-fish. In the Cephalopods we find an increasing complication
of the structure of the great spiral-shelled types.
Such is the life of the Carboniferous period. The world rejoices
in a tropical luxuriance. Semi-tropical vegetation is found in
Spitzbergen and the Antarctic, as well as in North Europe, Asia,
and America, and in Australasia; corals and sea-lilies flourish
at any part of the earth's surface. Warm, dank, low-lying lands,
bathed by warm oceans and steeped in their vapours, are the
picture suggested-- as we shall see more closely--to the minds of
all geologists. In those happy conditions the primitive life of
the earth erupts into an abundance and variety that are fitly
illustrated in the well-preserved vegetation of the forest. And
when the earth has at length flooded its surface with this
seething tide of life; when the air is filled with a thousand
species of insects, and the forest-floor feels the heavy tread of
the giant salamander and the light feet of spiders, scorpions,
centipedes, and snails, and the lagoons and shores teem with
animals, the Golden Age begins to close, and all the
semi-tropical luxuriance is banished. A great doom is pronounced
on the swarming life of the Coal-forest period, and from every
hundred species of its animals and plants only two or three will
survive the searching test.
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
they had been developed during the later Carboniferous, since we
find them already branched into three orders, with many
sub-orders, in the Permian. The stimulating and selecting
disturbances which culminated in the Permian revolution had begun
in the Carboniferous. Their origin is not clear, as the
intermediate forms between them and the amphibia are not found.
This is not surprising, if we may suppose that some of the
amphibia had, in the growing struggle, pushed inland, or that, as
the land rose and the waters were drained in certain regions,
they had gradually adopted a purely terrestrial life, as some of
the frogs have since done. In the absence of water their frames
would not be preserved and fossilised. We can, therefore,
understand the gap in the record between the amphibia and the
reptiles. From their structure we gather that they sprang from at
least two different branches of the amphibia. Their remains fall
into two great groups, which are known as the Diapsid and the
Synapsid reptiles. The former seem to be more closely related to
the Microsauria, or small salamander-like amphibia of the
Coal-forest; the latter are nearer to the Labyrinthodonts. It is
not suggested that these were their actual ancestors, but that
they came from the same early amphibian root.
We find both these groups, in patriarchal forms, in Europe, North
America, and South Africa during the Permian period. They are
usually moderate in size, but in places they seem to have found
good conditions and prospered. A few years ago a Permian bed in
Russia yielded a most interesting series of remains of Synapsid
reptiles. Some of them were large vegetarian animals, more than
twelve feet in length; others were carnivores with very powerful
heads and teeth as formidable as those of the tiger. Another
branch of the same order lived on the southern continent,
Gondwana Land, and has left numerous remains in South Africa. We
shall see that they are connected by many authorities with the
origin of the mammals.* The other branch, the Diapsids, are
represented to-day by the curiously primitive lizard of New
Zealand, the tuatara (Sphenodon, or Hatteria), of which I have
seen specimens, nearly two feet in length, that one did not care
to approach too closely. The Diapsids are chiefly interesting,
however, as the reputed ancestOrs of the colossal reptiles of the
Jurassic age and the birds.
* These Synapsid reptiles are more commonly known as Pareiasauria
or Theromorpha.
The purified air of the Permian world favoured the reptiles'
being lung-breathers, but the cold would check their expansion
for a time. The reptile, it is important to remember' usually
leaves its eggs to be hatched by the natural warmth of the
ground. But as the cold of the Permian yielded to a genial
climate and rich vegetation in the course of the Triassic, the
reptiles entered upon their memorable development. The amphibia
were now definitely ousted from their position of dominance. The
increase of the waters had at first favoured them, and we find
more than twenty genera, and some very large individuals, of the
amphibia in the Triassic. One of them, the Mastodonsaurus, had a
head three feet long and two feet wide. But the spread of the
reptiles checked them, and they shrank rapidly into the poor and
defenceless tribe which we find them in nature to-day.
To follow the prolific expansion of the reptiles in the
semi-tropical conditions of the Jurassic age is a task that even
the highest authorities approach with great diffidence. Science
is not yet wholly agreed in the classification of the vast
numbers of remains which the Mesozoic rocks have yielded, and the
affinities of the various groups are very uncertain. We cannot be
content, however, merely to throw on the screen, as it were, a
few of the more quaint and monstrous types out of the teeming
Mesozoic population, and describe their proportions and
peculiarities. They fall into natural and intelligible groups or
orders, and their features are closely related to the differing
regions of the Jurassic world. While, therefore, we must abstain
from drawing up settled genealogical trees, we may, as we review
in succession the monsters of the land, the waters, and the air,
glance at the most recent and substantial conjectures of
scientific men as to their origin and connections.
The Deinosaurs (or "terrible reptiles"), the monarchs of the land
and the swamps, are the central and outstanding family of the
Mesozoic reptiles. As the name implies, this group includes most
of the colossal animals, such as the Diplodocus, which the
illustrated magazine has made familiar to most people.
Fortunately the assiduous research of American geologists and
their great skill and patience in restoring the dead forms enable
us to form a very fair picture of this family of medieval giants
and its remarkable ramifications.*
* See, besides the usual authorities, a valuable paper by Dr. R.
S. Lull, "Dinosaurian Distribution" (1910).
The Diapsid reptiles of the Permian had evolved a group with
horny, parrot-like beaks, the Rhyncocephalia (or "beak-headed"
reptiles), of which the tuatara of New Zealand is a lingering
representative. New Zealand seems to have been cut off from the
southern continent at the close of the Permian or beginning of
the Triassic, and so preserved for us that very interesting relic
of Permian life. From some primitive level of this group, it is
generally believed, the great Deinosaurs arose. Two different
orders seem to have arisen independently, or diverged rapidly
from each other, in different parts of the world. One group seems
to have evolved on the "lost Atlantis," the land between Western
Europe and America, whence they spread westward to America,
eastward over Europe, and southward to the continent which still
united Africa and Australia. We find their remains in all these
regions. Another stock is believed to have arisen in America.
Both these groups seem to have been. more or less biped, rearing
themselves on large and powerful hind limbs, and (in some cases,
at least) probably using their small front limbs to hold or grasp
their food. The first group was carnivorous, the second
herbivorous; and, as the reptiles of the first group had four or
five toes on each foot, they are known as the Theropods (or
"beast-footed" ), while those of the second order, which had
three toes, are called the Ornithopods (or "bird-footed"). Each
of them then gave birth to an order of quadrupeds. In the
spreading waters and rich swamps of the later Triassic some of
the Theropods were attracted to return to an amphibiOus life, and
became the vast, sprawling, ponderous Sauropods, the giants in a
world of giants. On the other hand, a branch of the vegetarian
Ornithopods developed heavy armour, for defence against the
carnivores, and became, under the burden of its weight, the
quadrupedal and monstrous Stegosauria and Ceratopsia. Taking this
instructive general view of the spread of the Deinosaurs as the
best interpretation of the material we have, we may now glance at
each of the orders in succession.
The Theropods varied considerably in size and agility. The
Compsognathus was a small, active, rabbit-like creature, standing
about two feet high on its hind limbs, while the Megalosaurs
stretched to a length of thirty feet, and had huge jaws armed
with rows of formidable teeth. The Ceratosaur, a
seventeen-foot-long reptile, had hollow bones, and we find this
combination of lightness and strength in several members of the
group. In many respects the group points more or less
significantly toward the birds. The brain is relatively large,
the neck long, and the fore limbs might be used for grasping, but
had apparently ceased to serve as legs. Many of the Theropods
were evidently leaping reptiles, like colossal kangaroos, twenty
or more feet in length when they were erect. It is the general
belief that the bird began its career as a leaping reptile, and
the feathers, or expanded scales, on the front limbs helped at
first to increase the leap. Some recent authorities hold,
however, that the ancestor of the bird was an arboreal reptile.
To the order of the Sauropods belong most of the monsters whose
discovery has attracted general attention in recent years.
Feeding on vegetal matter in the luscious swamps, and having
their vast bulk lightened by their aquatic life, they soon
attained the most formidable proportions. The admirer of the
enormous skeleton of Diplodocus (which ran to eighty feet) in the
British Museum must wonder how even such massive limbs could
sustain the mountain of flesh that must have covered those bones.
It probably did not walk so firmly as the skeleton suggests, but
sprawled in the swamps or swam like a hippopotamus. But the
Diplodocus is neither the largest nor heaviest of its family. The
Brontosaur, though only sixty feet long, probably weighed twenty
tons. We have its footprints in the rocks to-day, each impression
measuring about a square yard. Generally, it is the huge
thigh-bones of these monsters that have survived, and give us an
idea of their size. The largest living elephant has a femur
scarcely four feet long, but the femur of the Atlantosaur
measures more than seventy inches, and the femur of the
Brachiosaur more than eighty. Many of these Deinosaurs must have
measured more than a hundred feet from the tip of the snout to
the end of the tail, and stood about thirty feet high from the
ground. The European Sauropods did not, apparently, reach the
size of their American cousins-- so early did the inferiority of
Europe begin--but our Ceteosaur seems to have been about fifty
feet long and ten feet in height. Its thigh-bone was sixty-four
inches long and twenty-seven inches in circumference at the
shaft. And in this order of reptiles, it must be remembered, the
bones are solid.
To complete the picture of the Sauropods, we must add that the
whole class is characterised by the extraordinary smallness of
the brain. The twenty-ton Brontosaur had a brain no larger than
that of a new-born human infant. Quite commonly the brain of one
of these enormous animals is no larger than a man's fist. It is
true that, as far as the muscular and sexual labour was
concerned, the brain was supplemented by a great enlargement of
the spinal cord in the sacral region (at the top of the thighs).
This inferior "brain" was from ten to twenty times as large as
the brain in the skull. It would, however, be fully occupied with
the movement of the monstrous limbs and tail, and the sex-life,
and does not add in the least to the "mental" power of the
Sauropods. They were stupid, sluggish, unwieldy creatures,
swollen parasites upon a luxuriant vegetation, and we shall
easily understand their disappearance at the end of the Mesozoic
Era, when the age of brawn will yield to an age of brain.
The next order of the Deinosaurs is that of the biped
vegetarians, the Ornithopods, which gradually became heavily
armoured and quadrupedal. The familiar Iguanodon is the chief
representative of this order in Europe. Walking on its three-toed
hind limbs, its head would be fourteen or fifteen feet from the
ground. The front part of its jaws was toothless and covered with
horn. It had, in fact, a kind of beak, and it also approached the
primitive bird in the structure of its pelvis and in having five
toes on its small front limbs. Some of the Ornithopods, such as
the Laosaur, were small (three or four feet in height) and
active, but many of the American specimens attained a great size.
The Camptosaur, which was closely related to the Iguanodon in
structure, was thirty feet from the snout to the end of the tail,
and the head probably stood eighteen feet from the ground. One of
the last great representatives of the group in America, the
Trachodon, about thirty feet in length, had a most extraordinary
head. It was about three and a half feet in length, and had no
less than 2000 teeth lining the mouth cavity. It is conjectured
that it fed on vegetation containing a large proportion of
silica.
In the course of the Jurassic, as we saw, a branch of these
biped, bird-footed vegetarians developed heavy armour, and
returned to the quadrupedal habit. We find them both in Europe
and America, and must suppose that the highway across the North
Atlantic still existed.
The Stegosaur is one of the most singular and most familiar
representatives of the group in the Jurassic. It ran to a length
of thirty feet, and had a row of bony plates, from two to three
feet in height, standing up vertically along the ridge of its
back, while its tail was armed with formidable spikes. The
Scleidosaur, an earlier and smaller (twelve-foot) specimen, also
had spines and bony plates to protect it. The Polacanthus and
Ankylosaur developed a most effective armour-plating over the
rear. As we regard their powerful armour, we seem to see the
fierce-toothed Theropods springing from the rear upon the
poor-mouthed vegetarians. The carnivores selected the
vegetarians, and fitted them to survive. Before the end of the
Mesozoic, in fact, the Ornithopods became aggressive as well as
armoured. The Triceratops had not only an enormous skull with a
great ridged collar round the neck, but a sharp beak, a stout
horn on the nose, and two large and sharp horns on the top of the
head. We will see something later of the development of horns.
The skulls of members of the Ceratops family sometimes measured
eight feet from the snout to the ridge of the collar. They were,
however, sluggish and stupid monsters, with smaller brains even
than the Sauropods.
Such, in broad outline, was the singular and powerful family of
the Mesozoic Deinosaurs. Further geological research in all parts
of the world will, no doubt, increase our knowledge of them,
until we can fully understand them as a great family throwing out
special branches to meet the different conditions of the crowded
Jurassic age. Even now they afford a most interesting page in the
story of evolution, and their total disappearance from the face
of the earth in the next geological period will not be
unintelligible. We turn from them to the remaining orders of the
Jurassic reptiles.
In the popular mind, perhaps, the Ichthyosaur and Plesiosaur are
the typical representatives of that extinct race. The two
animals, however, belong to very different branches of the
reptile world, and are by no means the most formidable of the
Mesozoic reptiles. Many orders of the land reptiles sent a branch
into the waters in an age which, we saw, was predominantly one of
water-surface. The Ichthyosauria ("fish-reptiles") and
Thalattosauria ("sea-reptiles") invaded the waters at their first
expansion in the later Triassic. The latter groups soon became
extinct, but the former continued for some millions of years, and
became remarkably adapted to marine life, like the whale at a
later period.
The Ichthyosaur of the Jurassic is a remarkably fish-like animal.
Its long tapering frame--sometimes forty feet in length, but
generally less than half that length--ends in a dip of the
vertebral column and an expansion of the flesh into a strong
tail-fin. The terminal bones of the limbs depart more and more
from the quadruped type, until at last they are merely rows of
circular bony plates embedded in the broad paddle into which the
limb has been converted. The head is drawn out, sometimes to a
length of five feet, and the long narrow jaws are set with two
formidable rows of teeth; one specimen has about two hundred
teeth. In some genera the teeth degenerate in the course of time,
but this merely indicates a change of diet. One fossilised
Ichthyosaur of the weaker-toothed variety has been found with the
remains of two hundred Belemnites in its stomach. It is a flash
of light on the fierce struggle and carnage which some recent
writers have vainly striven to attenuate. The eyes, again, which
may in the larger animals be fifteen inches in diameter, are
protected by a circle of radiating bony plates. In fine, the
discovery of young developed skeletons inside the adult frames
has taught us that the Ichthgosaur had become viviparous, like
the mammal. Cutting its last connection with the land, on which
it originated it ceased to lay eggs, and developed the young
within its body.
The Ichthyosaur came of the reptile group which we have called
the Diapsids. The Plesiosaur seems to belong to the Synapsid
branch. In the earlier Mesozoic we find partially aquatic
representatives of the line, like the Nothosaur, and in the later
Plesiosaur the adaptation to a marine life is complete. The skin
has lost its scales, and the front limbs are developed into
powerful paddles, sometimes six feet in length. The neck is drawn
out until, in some specimens, it is found to consist of
seventy-six vertebrae: the longest neck in the animal world. It
is now doubted, however, if the neck was very flexible, and, as
the jaws were imperfectly joined, the common picture of the
Plesiosaur darting its snake-like neck in all directions to seize
its prey is probably wrong. It seems to have lived on small food,
and been itself a rich diet to the larger carnivores. We find it
in all the seas of the Mesozoic world, varying in length from six
to forty feet, but it is one of the sluggish and unwieldy forms
that are destined to perish in the coming crisis.
The last, and perhaps the most interesting, of the doomed
monsters of the Mesozoic was the Pterosaur, or "flying reptile."
It is not surprising that in the fierce struggle which is
reflected in the arms and armour of the great reptiles, a branch
of the family escaped into the upper region. We have seen that
there were leaping reptiles with hollow bones, and although the
intermediate forms are missing, there is little doubt that the
Pterosaur developed from one or more of these leaping Deinosaurs.
As it is at first small, when it appears in the early Jurassic
--it is disputed in the late Triassic--it probably came from a
small and agile Deinosaur, hunted by the carnivores, which relied
on its leaping powers for escape. A flapperlike broadening of the
fore limbs would help to lengthen the leap, and we must suppose
that this membrane increased until the animal could sail through
the air, like the flying-fish, and eventually sustain its weight
in the air. The wing is, of course, not a feathery frame, as in
the bird, but a special skin spreading between the fore limb and
the side of the body. In the bat this skin is supported by four
elongated fingers of the hand, but in the Pterosaur the fifth (or
fourth) finger alone--which is enormously elongated and
strengthened--forms its outer frame. It is as if, in flying
experiments, a man were to have a web of silk stretching from his
arm and an extension of his little finger to the side of his
body.
From the small early specimens in the early Jurassic the flying
reptiles grow larger and larger until the time of their
extinction in the stresses of the Chalk upheaval. Small
Pterosaurs continue throughout the period, but from these
bat-like creatures we rise until we come to such dragons as the
American Pteranodon, with a stretch of twenty-two feet between
its extended wings and jaws about four feet long. There were
long-tailed Pterosaurs (Ramphorhyncus), sometimes with a
rudder-like expansion of the end of the tail, and short-tailed
Pterosaurs (Pterodactyl), with compact bodies and keeled breasts,
like the bird. In the earlier part of the period they all have
the heavy jaws and numerous teeth of the reptile, with four or
five well-developed fingers on the front limbs. In the course of
time they lose the teeth--an advantage in the distribution of the
weight of the body while flying--and develop horny beaks. In the
gradual shaping of the breast-bone and head, also, they
illustrate the evolution of the bird-form.
But the birds were meantime developing from a quite different
stock, and would replace the Pterosaurs at the first change in
the environment. There is ground for thinking that these flying
reptiles were warm-blooded like the birds. Their hollow bones
seem to point to the effective breathing of a warm-blooded
animal, and the great vitality they would need in flying points
toward the same conclusion. Their brain, too, approached that of
the bird, and was much superior to that of the other reptiles.
But they had no warm coats to retain their heat, no clavicle to
give strength to the wing machinery, and, especially in the later
period, they became very weak in the hind limbs (and therefore
weak or slow in starting their flight). The coming selection will
therefore dismiss them from the scene, with the Deinosaurs and
Ammonites, and retain the better organised bird as the lord of
the air.
There remain one or two groups of the Mesozoic reptiles which are
still represented in nature. The turtle-group (Chelonia) makes
its appearance in the Triassic and thrives in the Jurassic. Its
members are extinct and primitive forms of the thick-shelled
reptiles, but true turtles, both of marine and fresh water,
abound before the close of the Mesozoic. The sea-turtles attain
an enormous size. Archelon, one of the primitive types, measured
about twelve feet across the shell. Another was thirteen feet
long and fifteen feet from one outstretched flipper to the other.
In the Chalk period they form more than a third of the reptile
remains in some regions. They are extremely interesting in that
they show, to some extent, the evolution of their characteristic
shell. In some of the larger specimens the ribs have not yet
entirely coalesced.
The Crocodilians also appear in the later Triassic, abound in the
Jurassic, and give way before the later types, the true
Crocodiles, in the Cretaceous. They were marine animals with
naked skin, a head and neck something like that of the
Ichthyosaur, and paddles like those of the Plesiosaur. Their back
limbs, however, were not much changed after their adaptation to
life in the sea, and it is concluded that they visited the land
to lay their eggs. The Teleosaur was a formidable narrow-spouted
reptile, somewhat resembling the crocodiles of the Ganges in the
external form of the jaws. The modern crocodiles, which replaced
this ancient race of sea-crocodiles, have a great advantage over
them in the fact that their nostrils open into the mouth in its
lower depths. They can therefore close their teeth on their prey
under water and breathe through the nose.
Snakes are not found until the close of the Mesozoic, and do not
figure in its characteristic reptile population. We will consider
them later. But there was a large group of reptiles in the later
Mesozoic seas which more or less correspond to the legendary idea
of a sea-serpent. These Dolichosaurs ("long reptiles") appear at
the beginning of the Chalk period, and develop into a group, the
Mososaurians, which must have added considerably to the terrors
of the shore-waters. Their slender scale-covered bodies were
commonly twenty to thirty feet in length. The supreme
representative of the order, the Mososaur, of which about forty
species are known, was sometimes seventy-five feet long. It had
two pairs of paddles--so that the name of sea-serpent is very
imperfectly applicable --and four rows of formidable teeth on the
roof of its mouth. Like the Deinosaurs and Pterosaurs, the order
was doomed to be entirely extinguished after a brief supremacy in
its environment.
From this short and summary catalogue the reader will be able to
form some conception of the living inhabitants of the Mesozoic
world. It is assuredly the Age of Reptiles. Worms, snails, and
spiders were, we may assume, abundant enough, and a great variety
of insects flitted from tree to tree or sheltered in the fern
brakes. But the characteristic life, in water and on land, was
the vast and diversified family of the reptiles. In the western
and the eastern continent, and along the narrowing bridge that
still united them, in the northern hemisphere and the southern,
and along every ridge of land that connected them, these sluggish
but formidable monsters filled the stage. Every conceivable
device in the way of arms and armour, brute strength and means of
escape, seemed to be adopted in their development, as if they
were the final and indestructible outcome of the life-principle.
And within a single geological period the overwhelming majority
of them, especially the larger and more formidable of them, were
ruthlessly slain, leaving not a single descendant on the earth.
Let us see what types of animals were thus preferred to them in
the next great application of selective processes.
CHAPTER XIII. THE BIRD AND THE MAMMAL
In one of his finest stories, Sur La Pierre Blanche, Anatole
France has imagined a group of Roman patricians discussing the
future of their Empire. The Christians, who are about to rise to
power on their ruin, they dismiss with amiable indifference as
one of the little passing eccentricities of the religious life of
their time. They have not the dimmest prevision, even as the
dream of a possibility, that in a century or two the Empire of
Rome will lie in the dust, and the cross will tower above all its
cities from York to Jerusalem. If we might for a moment endow the
animals of the Mesozoic world with AEsopian wisdom, we could
imagine some such discussion taking place between a group of
Deinosaur patricians. They would reflect with pride on the
unshakable empire of the reptiles, and perhaps glance with
disdain at two types of animals which hid in the recesses or fled
to the hills of the Jurassic world. And before another era of the
earth's story opened, the reptile race would be dethroned, and
these hunted and despised and feeble eccentricities of Mesozoic
life would become the masters of the globe.
These two types of organisms were the bird and the mammal. Both
existed in the Jurassic, and the mammals at least had many
representatives in the Triassic. In other words, they existed,
with all their higher organisation, during several million years
without attaining power. The mammals remained, during at least
3,000,000 years, a small and obscure caste, immensely
overshadowed by the small-brained reptiles. The birds, while
making more progress, apparently, than the mammals, were far
outnumbered by the flying reptiles until the last part of the
Mesozoic. Then there was another momentous turn of the wheel of
fate, and they emerged from their obscurity to assume the
lordship of the globe.
In earlier years, when some serious hesitation was felt by many
to accept the new doctrine of evolution, a grave difficulty was
found in the circumstance that new types--not merely new species
and new genera, but new orders and even sub-classes--appeared in
the geological record quite suddenly. Was it not a singular
coincidence that in ALL cases the intermediate organisms between
one type and another should have wholly escaped preservation? The
difficulty was generally due to an imperfect acquaintance with
the conditions of the problem. The fossil population of a period
is only that fraction of its living population which happened to
be buried in a certain kind of deposit under water of a certain
depth. We shall read later of insects being preserved in resin
(amber), and we have animals (and even bacteria) preserved in
trees from the Coal-forests. Generally speaking, however, the
earth has buried only a very minute fraction of its
land-population. Moreover, only a fraction of the earth's
cemeteries have yet been opened. When we further reflect that the
new type of organism, when it first appears, is a small and local
group, we see what the chances are of our finding specimens of it
in a few scattered pages of a very fragmentary record of the
earth's life. We shall see that we have discovered only about ten
skeletons or fragments of skeletons of the men who lived on the
earth before the Neolithic period; a stretch of some hundreds of
thousands of years, recorded in the upper strata of the earth.
Whatever serious difficulty there ever was in this scantiness of
intermediate types is amply met by the fact that every fresh
decade of search in the geological tombs brings some to light. We
have seen many instances of this-- the seed-bearing ferns and
flower-bearing cycads, for example, found in the last decade--and
will see others. But one of the most remarkable cases of the kind
now claims our attention. The bird was probably evolved in the
late Triassic or early Jurassic. It appears in abundance, divided
into several genera, in the Chalk period. Luckily, two
bird-skeletons have been found in the intermediate period, the
Jurassic, and they are of the intermediate type, between the
reptile and the bird, which the theory of evolution would
suggest. But for the fortunate accident of these two birds being
embedded in an ancient Bavarian mud-layer, which happened to be
opened, for commercial purposes, in the second half of the
nineteenth century, critics of evolution--if there still were any
in the world of science--might be repeating to-day that the
transition from the reptile to the bird was unthinkable in theory
and unproven in fact.
The features of the Archaeopteryx ("primitive bird") have been
described so often, and such excellent pictorial restorations of
its appearance may now be seen, that we may deal with it briefly.
We have in it a most instructive combination of the characters of
the bird and the reptile. The feathers alone, the imprint of
which is excellently preserved in the fine limestone, would
indicate its bird nature, but other anatomical distinctions are
clearly seen in it. "There is," says Dr. Woodward, "a typical
bird's 'merrythought' between the wings, and the hind leg is
exactly that of a perching bird." In other words, it has the
shoulder-girdle and four-toed foot, as well as the feathers, of a
bird. On the other hand, it has a long tail (instead of a
terminal tuft of feathers as in the bird) consisting of
twenty-one vertebrae, with the feathers springing in pairs from
either side; it has biconcave vertebrae, like the fishes,
amphibia, and reptiles; it has teeth in its jaws; and it has
three complete fingers, free and clawed, on its front limbs.
As in the living Peripatus, therefore, we have here a very
valuable connecting link between two very different types of
organisms. It is clear that one of the smaller reptiles--the
Archaeopteryx is between a pigeon and a crow in size--of the
Triassic period was the ancestor of the birds. Its most
conspicuous distinction was that it developed a coat of feathers.
A more important difference between the bird and the reptile is
that the heart of the bird is completely divided into four
chambers, but, as we saw, this probably occurred also in the
other flying reptiles. It may be said to be almost a condition of
the greater energy of a flying animal. When the heart has four
complete chambers, the carbonised blood from the tissues of the
body can be conveyed direct to the lungs for purification, and
the aerated blood taken direct to the tissues, without any
mingling of the two. In the mud-fish and amphibian, we saw, the
heart has two chambers (auricles) above, but one (ventricle)
below, in which the pure and impure blood mingle. In the reptiles
a partition begins to form in the lower chamber. In the turtle it
is so nearly complete that the venous and the arterial blood are
fairly separated; in the crocodile it is quite complete, though
the arteries are imperfectly arranged. Thus the four-chambered
heart of the bird and mammal is not a sudden and inexplicable
development. Its advantage is enormous in a cold climate. The
purer supply of blood increases the combustion in the tissues,
and the animal maintains its temperature and vitality when the
surrounding air falls in temperature. It ceases to be
"cold-blooded."
But the bird secures a further advantage, and here it outstrips
the flying reptile. The naked skin of the Pterosaur would allow
the heat to escape so freely when the atmosphere cooled that a
great strain would be laid on its vitality. A man lessens the
demand on his vitality in cold regions by wearing clothing. The
bird somehow obtained clothing, in the shape of a coat of
feathers, and had more vitality to spare for life-purposes in a
falling temperature. The reptile is strictly limited to one
region, the bird can pass from region to region as food becomes
scarce.
The question of the origin of the feathers can be discussed only
from the speculative point of view, as they are fully developed
in the Archaeopteryx, and there is no approach toward them in any
other living or fossil organism. But a long discussion of the
problem has convinced scientific men that the feathers are
evolved from the scales of the reptile ancestor. The analogy
between the shedding of the coat in a snake and the moulting of a
bird is not uninstructive. In both cases the outer skin or
epidermis is shedding an old growth, to be replaced by a new one.
The covering or horny part of the scale and the feather are alike
growths from the epidermis, and the initial stages of the growth
have certain analogies. But beyond this general conviction that
the feather is a development of the scale, we cannot proceed with
any confidence. Nor need we linger in attempting to trace the
gradual modification of the skeleton, owing to the material
change in habits. The horny beak and the reduction of the toes
are features we have already encountered in the reptile, and the
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