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The Story of Evolution

Joseph McCabe

Section 14 of 21 - Table of Contents
CHAPTER XIV.  IN THE DAYS OF THE CHALK

In accordance with the view of the later story of the earth which
was expressed on an earlier page, we now come to the second of
the three great revolutions which have quickened the pulse of
life on the earth. Many men of science resent the use of the word
revolution, and it is not without some danger. It was once
thought that the earth was really shaken at times by vast and
sudden cataclysms, which destroyed its entire living population,
so that new kingdoms of plants and animals had to be created. But
we have interpreted the word revolution in a very different
sense. The series of changes and disturbances to which we give
the name extended over a period of hundreds of thousands of
years, and they were themselves, in some sense, the creators of
new types of organisms. Yet they are periods that stand out
peculiarly in the comparatively even chronicle of the earth. The
Permian period transformed the face of the earth; it lifted the
low-lying land into a massive relief, drew mantles of ice over
millions of miles of its surface, set volcanoes belching out fire
and fumes in many parts, stripped it of its great forests, and
slew the overwhelming majority of its animals. On the scale of
geological time it may be called a revolution.

It must be confessed that the series of disturbances which close
the Secondary and inaugurate the Tertiary Era cannot so
conveniently be summed up in a single formula. They begin long
before the end of the Mesozoic, and they continue far into the
Tertiary, with intervals of ease and tranquillity. There seems to
have been no culminating point in the series when the uplifted
earth shivered in a mantle of ice and snow. Yet I propose to
retain for this period--beginning early in the Cretaceous (Chalk)
period and extending into the Tertiary--the name of the
Cretaceous Revolution. I drew a fanciful parallel between the
three revolutions which have quickened the earth since the
sluggish days of the Coal-forest and the three revolutionary
movements which have changed the life of modern Europe. It will
be remembered that, whereas the first of these European
revolutions was a sharp and massive upheaval, the second
consisted in a more scattered and irregular series of
disturbances, spread over the fourth and fifth decades of the
nineteenth century; but they amounted, in effect, to a
revolution.

So it is with the Cretaceous Revolution. In effect it corresponds
very closely to the Permian Revolution. On the physical side it
includes a very considerable rise of the land over the greater
part of the globe, and the formation of lofty chains of
mountains; on the botanical side it means the reduction of the
rich Mesozoic flora to a relatively insignificant population, and
the appearance and triumphant spread of the flowering plants, on
the zoological side it witnesses the complete extinction of the
Ammonites, Deinosaurs, and Pterosaurs, an immense reduction of
the reptile world generally, and a victorious expansion of the
higher insects, birds, and mammals; on the climatic side it
provides the first definite evidence of cold zones of the earth
and cold seasons of the year, and seems to represent a long, if
irregular, period of comparative cold. Except, to some extent,
the last of these points, there is no difference of opinion, and
therefore, from the evolutionary point of view, the Cretaceous
period merits the title of a revolution. All these things were
done before the Tertiary period opened.

Let us first consider the fundamental and physical aspect of this
revolution, the upheaval of the land. It began about the close of
the Jurassic period. Western and Central Europe emerged
considerably from the warm Jurassic sea, which lay on it and had
converted it into an archipelago. In North-western America also
there was an emergence of large areas of land, and the Sierra and
Cascade ranges of mountains were formed about the same time. For
reasons which will appear later we must note carefully this rise
of land at the very beginning of the Cretaceous period.

However, the sea recovered its lost territory, or compensation
for it, and the middle of the Cretaceous period witnessed a very
considerable extension of the waters over America, Europe, and
southern Asia. The thick familiar beds of chalk, which stretch
irregularly from Ireland to the Crimea, and from the south of
Sweden to the south of France, plainly tell of an overlying sea.
As is well known, the chalk consists mainly of the shells or
outer frames of minute one-celled creatures (Thalamophores) which
float in the ocean, and form a deep ooze at its bottom with their
discarded skeletons. What depth this ocean must have been is
disputed, and hardly concerns us. It is clear that it must have
taken an enormous period for microscopic shells to form the thick
masses of chalk which cover so much of southern and eastern
England. On the lowest estimates the Cretaceous period, which
includes the deposit of other strata besides chalk, lasted about
three million years. And as people like to have some idea of the
time since these things happened, I may add that, on the lowest
estimate (which most geologists would at least double), it is
about three million years since the last stretches of the
chalk-ocean disappeared from the surface of Europe.

But while our chalk cliffs conjure up a vision of England lying
deep--at least twenty or thirty fathoms deep-- below a warm
ocean, in which gigantic Ammonites and Belemnites and sharks ply
their deadly trade, they also remind us of the last phase of the
remarkable life of the earth's Middle Ages. In the latter part of
the Cretaceous the land rises. The chalk ocean of Europe is
gradually reduced to a series of inland seas, separated by masses
and ridges of land, and finally to a series of lakes of brackish
water. The masses of the Pyrenees and Alps begin to rise; though
it will not be until a much later date that they reach anything
like their present elevation. In America the change is even
greater. A vast ridge rises along the whole western front of the
continent, lifting and draining it, from Alaska to Cape Horn. It
is the beginning of the Rocky Mountains and the Andes. Even
during the Cretaceous period there had been rich forests of
Mesozoic vegetation covering about a hundred thousand square
miles in the Rocky Mountains region. Europe and America now begin
to show their modern contours.

It is important to notice that this great uprise of the land and
the series of disturbances it entails differ from those which we
summed up in the phrase Permian Revolution. The differences may
help us to understand some of the changes in the living
population. The chief difference is that the disturbances are
more local, and not nearly simultaneous. There is a considerable
emergence of land at the end of the Jurassic, then a fresh
expansion of the sea, then a great rise of mountains at the end
of the Cretaceous, and so on. We shall find our great
mountain-masses (the Pyrenees, Alps, Himalaya, etc.) rising at
intervals throughout the whole of the Tertiary Era. However, it
suffices for the moment to observe that in the latter part of the
Mesozoic and early part of the Tertiary there were considerable
upheavals of the land in various regions, and that the Mesozoic
Era closed with a very much larger proportion of dry land, and a
much higher relief of the land, than there had been during the
Jurassic period. The series of disturbances was, says Professor
Chamberlin, "greater than any that had occurred since the close
of the Palaeozoic."

From the previous effect of the Permian upheaval, and from the
fact that the living population is now similarly annihilated or
reduced, we should at once expect to find a fresh change in the
climate of the earth. Here, however, our procedure is not so
easy. In the Permian age we had solid proof in the shape of vast
glaciated regions. It is claimed by continental geologists that
certain early Tertiary beds in Bavaria actually prove a similar,
but smaller, glaciation in Europe, but this is disputed. Other
beds may yet be found, but we saw that there was not a general
upheaval, as there had been in the Permian, and it is quite
possible that there were few or no ice-fields. We do not, in
fact, know the causes of the Permian icefields. We are thrown
upon the plant and animal remains, and seem to be in some danger
of inferring a cold climate from the organic remains, and then
explaining the new types of organisms by the cold climate. This,
of course, we shall not do. The difficulty is made greater by the
extreme disinclination of many recent geologists, and some recent
botanists who have too easily followed the geologists, to admit a
plain climatic interpretation of the facts. Let us first see what
the facts are.

In the latter part of the Jurassic we find three different zones
of Ammonites: one in the latitude of the Mediterranean, one in
the latitude of Central Europe, and one further north. Most
geologists conclude that these differences indicate zones of
climate (not hitherto indicated), but it cannot be proved, and we
may leave the matter open. At the same time the warm-loving
corals disappear from Europe, with occasional advances. It is
said that they are driven out by the disturbance of the waters,
and, although this would hardly explain why they did not spread
again in the tranquil chalk-ocean, we may again leave the point
open.

In the early part of the Cretaceous, however, the Angiosperms
(flowering plants) suddenly break into the chronicle of the
earth, and spread with great rapidity. They appear abruptly in
the east of the North American continent, in the region of
Virginia and Maryland. They are small in stature and primitive in
structure. Some are of generalised forms that are now unknown;
some have leaves approaching those of the oak, willow, elm,
maple, and walnut; some may be definitely described as fig,
sassafras, aralia, myrica, etc. Eastern America, it may be
recalled, is much higher than western until the close of the
Cretaceous period. The Angiosperms do not spread much westward;
they appear next in Greenland, and, before the middle of the
Cretaceous, in Portugal. They have travelled over the North
Atlantic continent, or what remains of it. The process seems very
rapid as we write it, but it must be remembered that the first
half of the Cretaceous period means a million or a million and a
half years.

The cycads, and even the conifers, shrink before the higher type
of tree. The landscape, in Europe and America, begins to wear a
modern aspect. Long before the end of the Cretaceous most of the
modern genera of Angiosperm trees have developed. To the fig and
sassafras are now added the birch, beech, oak, poplar, walnut,
willow, ivy, mulberry, holly, laurel, myrtle, maple, oleander,
magnolia, plane, bread-fruit, and sweet-gum. Most of the American
trees of to-day are known. The sequoias (the giant Californian
trees) still represent the conifers in great abundance, with the
eucalyptus and other plants that are now found only much further
south. The ginkgoes struggle on for a time. The cycads dwindle
enormously. Of 700 specimens in one early Cretaceous deposit only
96 are Angiosperms; of 460 species in a later deposit about 400
are Angiosperms. They oust the cycads in Europe and America, as
the cycads and conifers had ousted the Cryptogams. The change in
the face of the earth would be remarkable. Instead of the groves
of palm-like cycads, with their large and flower-like
fructifications, above which the pines and firs and cypresses
reared their sombre forms, there were now forests of
delicate-leaved maples, beeches, and oaks, bearing nutritious
fruit for the coming race of animals. Grasses also and palms
begin in the Cretaceous; though the grasses would at first be
coarse and isolated tufts. Even flowers, of the lily family
(apparently), are still detected in the crushed and petrified
remains.

We will give some consideration later to the evolution of the
Angiosperms. For the moment it is chiefly important to notice a
feature of them to which the botanist pays less attention. In his
technical view the Angiosperm is distinguished by the structure
of its reproductive apparatus, its flowers, and some recent
botanists wonder whether the key to this expansion of the
flowering plants may not be found in a development of the insect
world and of its relation to vegetation. In point of fact, we
have no geological indication of any great development of the
insects until the Tertiary Era, when we shall find them deploying
into a vast army and producing their highest types. In any case,
such a view leaves wholly unexplained the feature of the
Angiosperms which chiefly concerns us. This is that most of them
shed the whole of their leaves periodically, as the winter
approaches. No such trees had yet been known on the earth. All
trees hitherto had been evergreen, and we need a specific and
adequate explanation why the earth is now covered, in the
northern region, with forests of trees which show naked boughs
and branches during a part of the year.

The majority of palaeontologists conclude at once, and quite
confidently, from this rise and spread of the deciduous trees,
that a winter season has at length set in on the earth, and that
this new type of vegetation appears in response to an appreciable
lowering of the climate. The facts, however, are somewhat
complex, and we must proceed with caution. It would seem that any
general lowering of the temperature of the earth ought to betray
itself first in Greenland, but the flora of Greenland remains far
"warmer," so to say, than the flora of Central Europe is to-day.
Even toward the close of the Cretaceous its plants are much the
same as those of America or of Central Europe. Its fossil remains
of that time include forty species of ferns, as well as cycads,
ginkgoes, figs, bamboos, and magnolias. Sir A. Geikie ventures to
say that it must then have enjoyed a climate like that of the
Cape or of Australia to-day. Professor Chamberlin finds its flora
like that of "warm temperate" regions, and says that plants which
then flourished in latitude 72 degrees are not now found above
latitude 30 degrees.

There are, however, various reasons to believe that it is unsafe
to draw deductions from the climate of Greenland. There is, it is
true, some exaggeration in the statement that its climate was
equivalent to that of Central Europe. The palms which flourished
in Central Europe did not reach Greenland, and there are
differences in the northern Molluscs and Echinoderms which--like
the absence of corals above the north of England--point to a
diversity of temperature. But we have no right to expect that
there would be the same difference in temperature between
Greenland and Central Europe as we find to-day. If the warm
current which is now diverted to Europe across the Atlantic--the
Gulf Stream--had then continued up the coast of America, and
flowed along the coast of the land that united America and
Europe, the climatic conditions would be very different from what
they are. There is a more substantial reason. We saw that during
the Mesozoic the Arctic continent was very largely submerged,
and, while Europe and America rise again at the end of the
Cretaceous, we find no rise of the land further north. A
difference of elevation would, in such a world, make a great
difference in temperature and moisture.

Let us examine the animal record, however, before we come to any
conclusion. The chronicle of the later Cretaceous is a story of
devastation. The reduction of the cyeads is insignificant beside
the reduction or annihilation of the great animals of the
Mesozoic world. The skeletons of the Deinosaurs become fewer and
fewer as we ascend the upper Cretaceous strata. In the uppermost
layer (Laramie) we find traces of a last curious expansion--the
group of horned reptiles, of the Triceratops type, which we
described as the last of the great reptiles. The Ichthyosaurs and
Plesiosaurs vanish from the waters. The "sea-serpents"
(Mososaurs) pass away without a survivor. The flying dragons,
large and small, become entirely extinct. Only crocodiles,
lizards, turtle, and snakes cross the threshold of the Tertiary
Era. In one single region of America (Puerco beds) some of the
great reptiles seem to be making a last stand against the
advancing enemy in the dawn of the Tertiary Era, but the exact
date of the beds is disputed, and in any case their fight is soon
over. Something has slain the most formidable race that the earth
had yet known, in spite of its marvellous adaptation to different
environments in its innumerable branches.

We turn to the seas, and find an equal carnage among some of its
most advanced inhabitants. The great cuttlefish-like Belemnites
and the whole race of the Ammonites, large and small, are
banished from the earth. The fall of the Ammonites is
particularly interesting, and has inspired much more or less
fantastic speculation. The shells begin to assume such strange
forms that observers speak occasionally of the "convulsions" or
"death-contortions" of the expiring race. Some of the coiled
shells take on a spiral form, like that of a snail's shell. Some
uncoil the shell, and seem to be returning toward the primitive
type. A rich eccentricity of frills and ornamentation is found
more or less throughout the whole race. But every device --if we
may so regard these changes--is useless, and the devastating
agency of the Cretaceous, whatever it was, removes the Ammonites
and Belemnites from the scene. The Mollusc world, like the world
of plants and of reptiles, approaches its modern aspect.

In the fish world, too, there is an effective selection in the
course of the Cretaceous. All the fishes of modern times, except
the large family of the sharks, rays, skates, and dog-fishes
(Elasmobranchs), the sturgeon and chimaera, the mud-fishes, and a
very few other types, are Teleosts, or bony-framed fishes--the
others having cartilaginous frames. None of the Teleosts had
appeared until the end of the Jurassic. They now, like the
flowering plants on land, not only herald the new age, but
rapidly oust the other fishes, except the unconquerable shark.
They gradually approach the familiar types of Teleosts, so that
we may say that before the end of the Cretaceous the waters
swarmed with primitive and patriarchal cod, salmon, herring,
perch, pike, bream, eels, and other fishes. Some of them grew to
an enormous size. The Portheus, an American pike, seems to have
been about eight feet long; and the activity of an eight-foot
pike may be left to the angler's imagination. All, however, are,
as evolution demands, of a generalised and unfamiliar type: the
material out of which our fishes will be evolved.

Of the insects we have very little trace in the Cretaceous. We
shall find them developing with great richness in the following
period, but, imperfect as the record is, we may venture to say
that they were checked in the Cretaceous. There were good
conditions for preserving them, but few are preserved. And of the
other groups of invertebrates we need only say that they show a
steady advance toward modern types. The sea-lily fills the rocks
no longer; the sea-urchin is very abundant. The Molluscs gain on
the more lowly organised Brachiopods.

To complete the picture we must add that higher types probably
arose in the later Cretaceous which do not appear in the records.
This is particularly true of the birds and mammals. We find them
spreading so early in the Tertiary that we must put back the
beginning of the expansion to the Cretaceous. As yet, however,
the only mammal remains we find are such jaws and teeth of
primitive mammals as we have already described. The birds we
described (after the Archaeopteryx) also belong to the
Cretaceous, and they form another of the doomed races. Probably
the modern birds were already developing among the new vegetation
on the higher ground.

These are the facts of Cretaceous life, as far as the record has
yielded them, and it remains for us to understand them. Clearly
there has been a great selective process analogous to, if not
equal to, the winnowing process at the end of the Palaeozoic. As
there has been a similar, if less considerable, upheaval of the
land, we are at once tempted to think that the great selective
agency was a lowering of the temperature. When we further find
that the most important change in the animal world is the
destruction of the cold-blooded reptiles, which have no concern
for the young, and the luxuriant spread of the warm-blooded
animals, which do care for their young, the idea is greatly
confirmed. When we add that the powerful Molluscs which are
slain, while the humbler Molluscs survive, are those which--to
judge from the nautilus and octopus--love warm seas, the
impression is further confirmed. And when we finally reflect that
the most distinctive phenomenon of the period is the rapid spread
of deciduous trees, it would seem that there is only one possible
interpretation of the Cretaceous Revolution.

This interpretation--that cold was the selecting agency --is a
familiar idea in geological literature, but, as I said, there are
recent writers who profess reserve in regard to it, and it is
proper to glance at, or at least look for, the alternatives.

Before doing so let us be quite clear that here we have nothing
to do with theories of the origin of the earth. The Permian
cold--which, however, is universally admitted--is more or less
entangled in that controversy; the Cretaceous cold has no
connection with it. Whatever excess of carbon-dioxide there may
have been in the early atmosphere was cleared by the
Coal-forests. We must set aside all these theories in explaining
the present facts.

It is also useful to note that the fact that there have been
great changes in the climate of the earth in past time is beyond
dispute. There is no denying the fact that the climate of the
earth was warm from the Arctic to the Antarctic in the Devonian
and Carboniferous periods: that it fell considerably in the
Permian: that it again became at least "warm temperate"
(Chamberlin) from the Arctic to the Antarctic in the Jurassic,
and again in the Eocene: that some millions of square miles of
Europe and North America were covered with ice and snow in the
Pleistocene, so that the reindeer wandered where palms had
previously flourished and the vine flourishes to-day; and that
the pronounced zones of climate which we find today have no
counterpart in any earlier age. In view of these great and
admitted fluctuations of the earth's temperature one does not see
any reason for hesitating to admit a fall of temperature in the
Cretaceous, if the facts point to it.

On the other hand, the alternative suggestions are not very
convincing. We have noticed one of these suggestions in
connection with the origin of the Angiosperms. It hints that this
may be related to developments of the insect world. Most probably
the development of the characteristic flowers of the Angiosperms
is connected with an increasing relation to insects, but what we
want to understand especially is the deciduous character of their
leaves. Many of the Angiosperms are evergreen, so that it cannot
be said that the one change entailed the other. In fact, a
careful study of the leaves preserved in the rocks seems to show
the deciduous Angiosperms gaining on the evergreens at the end of
the Cretaceous. The most natural, it not the only, interpretation
of this is that the temperature is falling. Deciduous trees shed
their leaves so as to check their transpiration when a season
comes on in which they cannot absorb the normal amount of
moisture. This may occur either at the on-coming of a hot, dry
season or of a cold season (in which the roots absorb less).
Everything suggests that the deciduous tree evolved to meet an
increase of cold, not of heat.

Another suggestion is that animals and plants were not
"climatically differentiated "until the Cretaceous period; that
is to say, that they were adapted to all climates before that
time, and then began to be sensitive to differences of climate,
and live in different latitudes. But how and why they should
suddenly become differentiated in this way is so mysterious that
one prefers to think that, as the animal remains also suggest,
there were no appreciable zones of climate until the Cretaceous.
The magnolia, for instance, flourished in Greenland in the early
Tertiary, and has to live very far south of it to-day. It is much
simpler to assume that Greenland changed--as a vast amount of
evidence indicates--than that the magnolia changed.

Finally, to explain the disappearance of the Mesozoic reptiles
without a fall in temperature, it is suggested that they were
exterminated by the advancing mammals. It is assumed that the
spreading world of the Angiospermous plants somewhere met the
spread of the advancing mammals, and opened out a rich new
granary to them. This led to so powerful a development of the
mammals that they succeeded in overthrowing the reptiles.

There are several serious difficulties in the way of this theory.
The first and most decisive is that the great reptiles have
practically disappeared before the mammals come on the scene.
Only in one series of beds (Puerco) in America, representing an
early period of the Tertiary Era, do we find any association of
their remains; and even there it is not clear that they were
contemporary. Over the earth generally the geological record
shows the great reptiles dying from some invisible scourge long
before any mammal capable of doing them any harm appears; even if
we suppose that the mammal mainly attacked the eggs and the
young. We may very well believe that more powerful mammals than
the primitive Mesozoic specimens were already developed in some
part of the earth--say, Africa--and that the rise of the land
gave them a bridge across the Mediterranean to Europe. Probably
this happened; but the important point is that the reptiles were
already almost extinct. The difficulty is even greater when we
reflect that it is precisely the most powerful reptiles
(Deinosaurs) and least accessible reptiles (Pterosaurs,
Ichthyosaurs, etc.) which disappear, while the smaller land and
water reptiles survive and retreat southward-- where the mammals
are just as numerous. That assuredly is not the effect of an
invasion of carnivores, even if we could overlook the absence of
such carnivores from the record until after the extinction of the
reptiles in most places.

I have entered somewhat fully into this point, partly because of
its great interest, but partly lest it be thought that I am
merely reproducing a tradition of geological literature without
giving due attention to the criticisms of recent writers. The
plain and common interpretation of the Cretaceous
revolution--that a fall in temperature was its chief devastating
agency--is the only one that brings harmony into all the facts.
The one comprehensive enemy of that vast reptile population was
cold. It was fatal to the adult because he had a three-chambered
heart and no warm coat; it was fatal to the Mesozoic vegetation
on which, directly or indirectly, he fed; it was fatal to his
eggs and young because the mother did not brood over the one or
care for the other. It was fatal to the Pterosaurs, even if they
were warm-blooded, because they had no warm coats and did not
(presumably) hatch their eggs; and it was equally fatal to the
viviparous Ichthyosaurs. It is the one common fate that could
slay all classes. When we find that the surviving reptiles
retreat southward, only lingering in Europe during the renewed
warmth of the Eocene and Miocene periods, this interpretation is
sufficiently confirmed. And when we recollect that these things
coincide with the extinction of the Ammonites and Belemnites, and
the driving of their descendants further south, as well as the
rise and triumph of deciduous trees, it is difficult to see any
ground for hesitating.

But we need not, and must not, imagine a period of cold as
severe, prolonged, and general as that of the Permian period. The
warmth of the Jurassic period is generally attributed to the low
relief of the land, and the very large proportion of
water-surface. The effect of this would be to increase the
moisture in the atmosphere. Whether this was assisted by any
abnormal proportion of carbon-dioxide, as in the Carboniferous,
we cannot confidently say. Professor Chamberlin observes that,
since the absorbing rock-surface was greatly reduced in the
Jurassic, the carbon-dioxide would tend to accumulate in its
atmosphere, and help to explain the high temperature. But the
great spread of vegetation and the rise of land in the later
Jurassic and the Cretaceous would reduce this density of the
atmosphere, and help to lower the temperature.

It is clear that the cold would at first be local. In fact, it
must be carefully realised that, when we speak of the Jurassic
period as a time of uniform warmth, we mean uniform at the same
altitude. Everybody knows the effect of rising from the warm,
moist sea-level to the top of even a small inland elevation.
There would be such cooler regions throughout the Jurassic, and
we saw that there were considerable upheavals of land towards its
close. To these elevated lands we may look for the development of
the Angiosperms, the birds, and the mammals. When the more
massive rise of land came at the end of the Cretaceous, the
temperature would fall over larger areas, and connecting ridges
would be established between one area and another. The Mesozoic
plants and animals would succumb to this advancing cold. What
precise degree of cold was necessary to kill the reptiles and
Cephalopods, yet allow certain of the more delicate flowering
plants to live, is yet to be determined. The vast majority of the
new plants, with their winter sleep, would thrive in the cooler
air, and, occupying the ground of the retreating cycads and
ginkgoes would prepare a rich harvest for the coming birds and
mammals.
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