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The Story of Evolution by Joseph McCabe

Part 4 out of 6

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modification of the pelvis, breast-bone, and clavicle are a
natural outcome of flight.

In the Chalk period we find a large number of bird remains, of
about thirty different species, and in some respects they resume
the story of the evolution of the bird. They are widely removed
from our modern types of birds, and still have teeth in the jaws.
They are of two leading types, of which the Ichthyornis and
Hesperornis are the standard specimens. The Ichthyornis was a
small, tern-like bird with the power of flight strongly
developed, as we may gather from the frame of its wings and the
keel-shaped structure of its breast-bone. Its legs and feet were
small and slender, and its long, slender jaws had about twenty
teeth on each side at the bottom. No modern bird has teeth;
though the fact that in some modern species we find the teeth
appearing in a rudimentary form is another illustration of the
law that animals tend to reproduce ancestral features in their
development. A more reptilian character in the Ichthyornis group
is the fact that, unlike any modern bird, but like their reptile
ancestors, they had biconcave vertebrae. The brain was relatively
poor. We are still dealing with a type intermediate in some
respects between the reptile and the modern bird. The gannets,
cormorants, and pelicans are believed to descend from some branch
of this group.

The other group of Cretaceous birds, of the Hesperornis type,
show an actual degeneration of the power of flight through
adaptation to an environment in which it was not needed, as
happened, later, in the kiwi of New Zealand, and is happening in
the case of the barn-yard fowl. These birds had become divers.
Their wings had shrunk into an abortive bone, while their
powerful legs had been peculiarly fitted for diving. They stood
out at right angles to the body, and seem to have developed
paddles. The whole frame suggests that the bird could neither
walk nor fly, but was an excellent diver and swimmer. Not
infrequently as large as an ostrich (five to six feet high), with
teeth set in grooves in its jaws, and the jaws themselves joined
as in the snake, with a great capacity of bolting its prey, the
Hesperornis would become an important element in the life of the
fishes. The wing-fingers have gone, and the tail is much
shortened, but the grooved teeth and loosely jointed jaws still
point back to a reptilian ancestry.

These are the only remains of bird-life that we find in the
Mesozoic rocks. Admirably as they illustrate the evolution of the
bird from the reptile, they seem to represent a relatively poor
development and spread of one of the most advanced organisms of
the time. It must be understood that, as we shall see, the latter
part of the Chalk period does not belong to the depression, the
age of genial climate, which I call the Middle Ages of the earth,
but to the revolutionary period which closes it. We may say that
the bird, for all its advances in organisation, remains obscure
and unprosperous as long as the Age of Reptiles lasts. It awaits
the next massive uplift of the land and lowering of temperature.

In an earlier chapter I hinted that the bird and the mammal may
have been the supreme outcomes of the series of disturbances
which closed the Primary Epoch and devastated its primitive
population. As far as the bird is concerned, this may be doubted
on the ground that it first appears in the upper or later
Jurassic, and is even then still largely reptilian in character.
We must remember, however, that the elevation of the land and the
cold climate lasted until the second part of the Triassic, and it
is generally agreed that the bird may have been evolved in the
Triassic. Its slow progress after that date is not difficult to
understand. The advantage of a four-chambered heart and warm coat
would be greatly reduced when the climate became warmer. The
stimulus to advance would relax. The change from a coat of scales
to a coat of feathers obviously means adaptation to a low
temperature, and there is nothing to prevent us from locating it
in the Triassic, and indeed no later known period of cold in
which to place it.

It is much clearer that the mammals were a product of the Permian
revolution. They not only abound throughout the Jurassic, in
which they are distributed in more than thirty genera, but they
may be traced into the Triassic itself. Both in North America and
Europe we find the teeth and fragments of the jaws of small
animals which are generally recognised as mammals. We cannot, of
course, from a few bones deduce that there already, in the
Triassic, existed an animal with a fully developed coat of fur
and an apparatus, however crude, in the breast for suckling the
young. But these bones so closely resemble the bones of the
lowest mammals of to-day that this seems highly probable. In the
latter part of the long period of cold it seems that some reptile
exchanged its scales for tufts of hair, developed a
four-chambered heart, and began the practice of nourishing the
young from its own blood which would give the mammals so great an
ascendancy in a colder world.

Nor can we complain of any lack of evidence connecting the mammal
with a reptile ancestor. The earliest remains we find are of such
a nature that the highest authorities are still at variance as to
whether they should be classed as reptilian or mammalian. A skull
and a fore limb from the Triassic of South Africa (Tritylodon and
Theriodesmus) are in this predicament. It will be remembered that
we divided the primitive reptiles of the Permian period into two
great groups, the Diapsids and Synapsids (or Theromorphs). The
former group have spread into the great reptiles of the Jurassic;
the latter have remained in comparative obscurity. One branch of
these Theromorph reptiles approach the mammals so closely in the
formation of the teeth that they have received the name "of the
Theriodonts", or "beast-toothed" reptiles. Their teeth are, like
those of the mammals, divided into incisors, canines (sometimes
several inches long), and molars; and the molars have in some
cases developed cusps or tubercles. As the earlier remains of
mammals which we find are generally teeth and jaws, the
resemblance of the two groups leads to some confusion in
classifying them, but from our point of view it is not unwelcome.
It narrows the supposed gulf between the reptile and the mammal,
and suggests very forcibly the particular branch of the reptiles
to which we may look for the ancestry of the mammals. We cannot
say that these Theriodont reptiles were the ancestors of the
mammals. But we may conclude with some confidence that they bring
us near to the point of origin, and probably had at least a
common ancestor with the mammals.

The distribution of the Theriodonts suggests a further idea of
interest in regard to the origin of the mammals. It would be
improper to press this view in the present state of our
knowledge, yet it offers a plausible theory of the origin of the
mammals. The Theriodonts seem to have been generally confined to
the southern continent, Gondwana Land (Brazil to Australia), of
which an area survives in South Africa. It is there also that we
find the early disputed remains of mammals. Now we saw that,
during the Permian, Gondwana Land was heavily coated with ice,
and it seems natural to suppose that the severe cold which the
glacial fields would give to the whole southern continent was the
great agency in the evolution of the highest type of the animal
world. From this southern land the new-born mammals spread
northward and eastward with great rapidity. Fitted as they were
to withstand the rigorous conditions which held the reptiles and
amphibia in check, they seemed destined to attain at once the
domination of the earth. Then, as we saw, the land was revelled
once more until its surface broke into a fresh semi-tropical
luxuriance, and the Deinosaurs advanced to their triumph. The
mammals shrank into a meagre and insignificant population, a
scattered tribe of small insect-eating animals, awaiting a fresh
refrigeration of the globe.

The remains of these interesting early mammals, restricted, as
they generally are, to jaws and teeth and a few other bones that
cannot in themselves be too confidently distinguished from those
of certain reptiles, may seem insufficient to enable us to form a
picture of their living forms. In this, however, we receive a
singular and fortunate assistance. Some of them are found living
in nature to-day, and their distinctly reptilian features would,
even if no fossil remains were in existence, convince us of the
evolution of the mammals.

The southern continent on which we suppose the mammals to have
originated had its eastern termination in Australia. New Zealand
seems to have been detached early in the Mesozoic, and was never
reached by the mammals. Tasmania was still part of the Australian
continent. To this extreme east of the southern continent the
early mammals spread, and then, during either the Jurassic or the
Cretaceous, the sea completed its inroad, and severed Australia
permanently from the rest of the earth. The obvious result of
this was to shelter the primitive life of Australia from invasion
by higher types, especially from the great carnivorous mammals
which would presently develop. Australia became, in other words,
a "protected area," in which primitive types of life were
preserved from destruction, and were at the same time sheltered
from those stimulating agencies which compelled the rest of the
world to advance. "Advance Australia" is the fitting motto of the
present human inhabitants of that promising country; but the
standard of progress has been set up in a land which had remained
during millions of years the Chinese Empire of the living world.
Australia is a fragment of the Middle Ages of the earth, a
province fenced round by nature at least three million years ago
and preserving, amongst its many invaluable types of life,
representatives of that primitive mammal population which we are
seeking to understand.

It is now well known that the Duckbill or Platypus
(Ornithorhyncus) and the Spiny Anteater (Echidna) of Australia
and Tasmania--with one representative of the latter in New
Guinea, which seems to have been still connected--are
semi-reptilian survivors of the first animals to suckle their
young. Like the reptiles they lay tough-coated eggs and have a
single outlet for the excreta, and they have a reptilian
arrangement of the bones of the shoulder-girdle; like the
mammals, they have a coat of hair and a four-chambered heart, and
they suckle the young. Even in their mammalian features they are,
as the careful research of Australian zoologists has shown, of a
transitional type. They are warm-blooded, but their temperature
is much lower than that of other mammals, and varies appreciably
with the temperature of their surroundings.* Their apparatus for
suckling the young is primitive. There are no teats, and the milk
is forced by the mother through simple channels upon the breast,
from which it is licked by the young. The Anteater develops her
eggs in a pouch. They illustrate a very early stage in the
development of a mammal from a reptile; and one is almost tempted
to see in their timorous burrowing habits a reminiscence of the
impotence of the early mammals after their premature appearance
in the Triassic.

* See Lucas and Le Soulf's Animals of Australia, 1909.

The next level of mammal life, the highest level that it attains
in Australia (apart from recent invasions), is the Marsupial. The
pouched animals (kangaroo, wallaby, etc.) are the princes of
pre-human life in Australia, and represent the highest point that
life had reached when that continent was cut off from the rest of
the world. A few words on the real significance of the pouch,
from which they derive their name, will suffice to explain their
position in the story of evolution.

Among the reptiles the task of the mother ends, as a rule, with
the laying of the egg. One or two modern reptiles hatch the eggs,
or show some concern for them, but the characteristic of the
reptile is to discharge its eggs upon the warm earth and trouble
no further about its young. It is a reminiscence of the warm
primitive earth. The bird and mammal, born of the cooling of the
earth, exhibit the beginning of that link between mother and
offspring which will prove so important an element in the higher
and later life of the globe. The bird assists the development of
the eggs with the heat of her own body, and feeds the young. The
mammal develops the young within the body, and then feeds them at
the breast.

But there is a gradual advance in this process. The Duckbill lays
its eggs just like the reptile, but provides a warm nest for them
at the bottom of its burrow. The Anteater develops a temporary
pouch in its body, when it lays an egg, and hatches the egg in
it. The Marsupial retains the egg in its womb until the young is
advanced in development, then transfers the young to the pouch,
and forces milk into its mouth from its breasts. The real reason
for this is that the Marsupial falls far short of the higher
mammals in the structure of the womb, and cannot fully develop
its young therein. It has no placenta, or arrangement by which
the blood-vessels of the mother are brought into connection with
the blood-vessels of the foetus, in order to supply it with food
until it is fully developed. The Marsupial, in fact, only rises
above the reptile in hatching the egg within its own body, and
then suckling the young at the breast.

These primitive mammals help us to reconstruct the mammal life of
the Mesozoic Epoch. The bones that we have are variously
described in geological manuals as the remains of Monotremes,
Marsupials, and Insectivores. Many of them, if not most, were no
doubt insect-eating animals, but there is no ground for supposing
that what are technically known as Insectivores (moles and
shrews) existed in the Mesozoic. On the other hand, the lower jaw
of the Marsupial is characterised by a peculiar hooklike process,
and this is commonly found in Mesozoic jaws. This circumstance,
and the witness of Australia, permit us, perhaps, to regard the
Jurassic mammals as predominantly marsupial. It is more difficult
to identify Monotreme remains, but the fact that Monotremes have
survived to this day in Australia, and the resemblance of some of
the Mesozoic teeth to those found for a time in the young
Duckbill justify us in assuming that a part of the Mesozoic
mammals correspond to the modern Monotremes. Not single specimen
of any higher, or placental, mammal has yet been found in the
whole Mesozoic Era.

We must, however, beware of simply transferring to the Mesozoic
world the kinds of Monotremes and Marsupials which we know in
nature to-day. In some of the excellent "restorations" of
Mesozoic life which are found in recent illustrated literature
the early mammal is represented with an external appearance like
that of the Duckbill. This is an error, as the Duckbill has been
greatly modified in its extremities and mouth-parts by its
aquatic and burrowing habits. As we have no complete skeletons of
these early mammals we must abstain from picturing their external
appearance. It is enough that the living Monotreme and Marsupial
so finely illustrate the transition from a reptilian to a
mammalian form. There may have been types more primitive than the
Duckbill, and others between the Duckbill and the Marsupial. It
seems clear, at least, that two main branches, the Monotremes and
Marsupials, arose from the primitive mammalian root. Whether
either of these became in turn the parent of the higher mammals
we will inquire later. We must first consider the fresh series of
terrestrial disturbances which, like some gigantic sieve, weeded
out the grosser types of organisms, and cleared the earth for a
rapid and remarkable expansion of these primitive birds and

We have attended only to a few prominent characters in tracing
the line of evolution, but it will be understood that an advance
in many organs of the body is implied in these changes. In the
lower mammals the diaphragm, or complete partition between the
organs of the breast and those of the abdomen, is developed. It
is not a sudden and mysterious growth, and its development in the
embryo to-day corresponds to the suggestion of its development
which the zoologist gathers from the animal series. The ear also
is now fully developed. How far the fish has a sense of hearing
is not yet fully determined, but the amphibian certainly has an
organ for the perception of waves of sound. Parts of the
discarded gill-arches are gradually transformed into the three
bones of the mammal's internal ear; just as other parts are
converted into mouth cartilages, and as--it is believed--one of
the gill clefts is converted into the Eustachian tube. In the
Monotreme and Marsupial the ear-hole begins to be covered with a
shell of cartilage; we have the beginning of the external ear.
The jaws, which are first developed in the fish, now articulate
more perfectly with the skull. Fat-glands appear in the skin, and
it is probably from a group of these that the milk-glands are
developed. The origin of the hairs is somewhat obscure. They are
not thought to be, like the bird's feathers, modifications of the
reptile's scales, but to have been evolved from other structures
in the skin, possibly under the protection of the scales.

My purpose is, however, rather to indicate the general causes of
the onward advance of life than to study organs in detail--a vast
subject--or construct pedigrees. We therefore pass on to consider
the next great stride that is taken by the advancing life of the
earth. Millions of years of genial climate and rich vegetation
have filled the earth with a prolific and enormously varied
population. Over this population the hand of natural selection is
outstretched, as it were, and we are about to witness another
gigantic removal of older types of life and promotion of those
which contain the germs of further advance. As we have already
explained, natural selection is by no means inactive during these
intervening periods of warmth. We have seen the ammonites and
reptiles, and even the birds and mammals, evolve into hundreds of
species during the Jurassic period. The constant evolution of
more effective types of carnivores and their spread into new
regions, the continuous changes in the distribution of land and
water, the struggle for food in a growing population, and a dozen
other causes, are ever at work. But the great and comprehensive
changes in the face of the earth which close the eras of the
geologist seem to give a deeper and quicker stimulus to its
population and result in periods of especially rapid evolution.
Such a change now closes the Mesozoic Era, and inaugurates the
age of flowering plants, of birds, and of mammals.


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

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

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

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


We have already traversed nearly nine-tenths of the story of
terrestrial life, without counting the long and obscure Archaean
period, and still find ourselves in a strange and unfamiliar
earth. With the close of the Chalk period, however, we take a
long stride in the direction of the modern world. The Tertiary
Era will, in the main, prove a fresh period of genial warmth and
fertile low-lying regions. During its course our deciduous trees
and grasses will mingle with the palms and pines over the land,
our flowers will begin to brighten the landscape, and the forms
of our familiar birds and mammals, even the form of man, will be
discernible in the crowds of animals. At its close another mighty
period of selection will clear the stage for its modern actors.

A curious reflection is prompted in connection with this division
of the earth's story into periods of relative prosperity and
quiescence, separated by periods of disturbance. There was--on
the most modest estimate--a stretch of some fifteen million years
between the Cambrian and the Permian upheavals. On the same
chronological scale the interval between the Permian and
Cretaceous revolutions was only about seven million years, and
the Tertiary Era will comprise only about three million years.
One wonders if the Fourth (Quaternary) Era in which we live will
be similarly shortened. Further, whereas the earth returned after
each of the earlier upheavals to what seems to have been its
primitive condition of equable and warm climate, it has now
entirely departed from that condition, and exhibits very
different zones of climate and a succession of seasons in the
year. One wonders what the climate of the earth will become long
before the expiration of those ten million years which are
usually assigned as the minimum period during which the globe
will remain habitable.

It is premature to glance at the future, when we are still some
millions of years from the present, but it will be useful to look
more closely at the facts which inspire this reflection. From
what we have seen, and shall further see, it is clear that, in
spite of all the recent controversy about climate among our
geologists, there has undeniably been a progressive refrigeration
of the globe. Every geologist, indeed, admits "oscillations of
climate," as Professor Chamberlin puts it. But amidst all these
oscillations we trace a steady lowering of the temperature.
Unless we put a strained and somewhat arbitrary interpretation on
the facts of the geological record, earlier ages knew nothing of
our division of the year into pronounced seasons and of the globe
into very different climatic zones. It might plausibly be
suggested that we are still living in the last days of the
Ice-Age, and that the earth may be slowly returning to a warmer
condition. Shackleton, it might be observed, found that there has
been a considerable shrinkage of the south polar ice within the
period of exploration. But we shall find that a difference of
climate, as compared with earlier ages, was already evident in
the middle of the Tertiary Era, and it is far more noticeable

We do not know the causes of this climatic evolution-- the point
will be considered more closely in connection with the last
Ice-Age--but we see that it throws a flood of light on the
evolution of organisms. It is one of the chief incarnations of
natural selection. Changes in the distribution of land and water
and in the nature of the land-surface, the coming of powerful
carnivores, and other agencies which we have seen, have had their
share in the onward impulsion of life, but the most drastic
agency seems to have been the supervention of cold. The higher
types of both animals and plants appear plainly in response to a
lowering of temperature. This is the chief advantage of studying
the story of evolution in strict connection with the geological
record. We shall find that the record will continue to throw
light on our path to the end, but, as we are now about to
approach the most important era of evolution, and as we have now
seen so much of the concrete story of evolution, it will be
interesting to examine briefly some other ways of conceiving that

We need not return to the consideration of the leading schools of
evolution, as described in a former chapter. Nothing that we have
seen will enable us to choose between the Lamarckian and the
Weismannist hypothesis; and I doubt if anything we are yet to see
will prove more decisive. The dispute is somewhat academic, and
not vital to a conception of evolution. We shall, for instance,
presently follow the evolution of the horse, and see four of its
toes shrink and disappear, while the fifth toe is enormously
strengthened. In the facts themselves there is nothing whatever
to decide whether this evolution took place on the lines
suggested by Weismann, or on the lines suggested by Lamarck and
accepted by Darwin. It will be enough for us merely to establish
the fact that the one-toed horse is an evolved descendant of a
primitive five-toed mammal, through the adaptation of its foot to
running on firm ground, its teeth and neck to feeding on grasses,
and so on.

On the other hand, the facts we have already seen seem to justify
the attitude of compromise I adopted in regard to the Mutationist
theory. It would be an advantage in many ways if we could believe
that new species arose by sudden and large variations (mutations)
of the young from the parental type. In the case of many organs
and habits it is extremely difficult to see how a gradual
development, by a slow accentuation of small variations, is
possible. When we further find that experimenters on living
species can bring about such mutations, and when we reflect that
there must have been acute disturbances in the surroundings of
animals and plants sometimes, we are disposed to think that many
a new species may have arisen in this way. On the other hand,
while the palaeontological record can never prove that a species
arose by mutations, it does sometimes show that species arise by
very gradual modification. The Chalk period, which we have just
traversed, affords a very clear instance. One of our chief
investigators of the English Chalk, Dr. Rowe, paid particular
attention to the sea-urchins it contains, as they serve well to
identify different levels of chalk. He discovered, not merely
that they vary from level to level, but that in at least one
genus (Micraster) he could trace the organism very gradually
passing from one species to another, without any leap or
abruptness. It is certainly significant that we find such cases
as this precisely where the conditions of preservation are
exceptionally good. We must conclude that species arise,
probably, both by mutations and small variations, and that it is
impossible to say which class of species has been the more

There remain one or two conceptions of evolution which we have
not hitherto noticed, as it was advisable to see the facts first.
One of these is the view--chiefly represented in this country by
Professor Henslow--that natural selection has had no part in the
creation of species; that the only two factors are the
environment and the organism which responds to its changes. This
is true enough in the sense that, as we saw, natural selection is
not an action of nature on the "fit," but on the unfit or less
fit. But this does not in the least lessen the importance of
natural selection. If there were not in nature this body of
destructive agencies, to which we apply the name natural
selection, there would be little--we cannot say no--evolution.
But the rising carnivores, the falls of temperature, etc., that
we have studied, have had so real, if indirect, an influence on
the development of life that we need not dwell on this.

Another school, or several schools, while admitting the action of
natural selection, maintain that earlier evolutionists have made
nature much too red in tooth and claw. Dr. Russel Wallace from
one motive, and Prince Krapotkin from another, have insisted that
the triumphs of war have been exaggerated, and the triumphs of
peace, or of social co-operation, far too little appreciated. It
will be found that such writers usually base their theory on life
as we find it in nature to-day, where the social principle is
highly developed in many groups of animals. This is most
misleading, since social co-operation among animals, as an
instrument of progress, is (geologically speaking) quite a recent
phenomenon. Nearly every group of animals in which it is found
belongs, to put it moderately, to the last tenth of the story of
life, and in some of the chief instances the animals have only
gradually developed social life.* The first nine-tenths of the
chronicle of evolution contain no indication of social life,
except--curiously enough--in such groups as the Sponges, Corals,
and Bryozoa, which are amongst the least progressive in nature.
We have seen plainly that during the overwhelmingly greater part
of the story of life the predominant agencies of evolution were
struggle against adverse conditions and devouring carnivores; and
we shall find them the predominant agencies throughout the
Tertiary Era.

* Thus the social nature of man is sometimes quoted as one of the
chief causes of his development. It is true that it has much to
do with his later development, but we shall see that the
statement that man was from the start a social being is not at
all warranted by the facts. On the other hand, it may be pointed
out that the ants and termites had appeared in the Mesozoic. We
shall see some evidence that the remarkable division of labour
which now characterises their life did not begin until a much
later period, so that we have no evidence of social life in the
early stages.

Yet we must protest against the exaggerated estimate of the
conscious pain which so many read into these millions of years of
struggle. Probably there was no consciousness at all during the
greater part of the time. The wriggling of the worm on which you
have accidentally trodden is no proof whatever that you have
caused conscious pain. The nervous system of an animal has been
so evolved as to respond with great disturbance of its tissue to
any dangerous

or injurious assault. It is the selection of a certain means of
self-preservation. But at what level of life the animal becomes
conscious of this disturbance, and "feels pain," it is very
difficult to determine. The subject is too vast to be opened
here. In a special investigation of it* I concluded that there is
no proof of the presence of any degree of consciousness in the
invertebrate world even in the higher insects; that there is
probably only a dull, blurred, imperfect consciousness below the
level of the higher mammals and birds; and that even the
consciousness of an ape is something very different from what
educated Europeans, on the ground of their own experience, call
consciousness. It is too often forgotten that pain is in
proportion to consciousness. We must beware of such fallacies as
transferring our experience of pain to a Mesozoic reptile, with
an ounce or two of cerebrum to twenty tons of muscle and bone.

* "The Evolution of Mind" (Black), 1911.

One other view of evolution, which we find in some recent and
reputable works (such as Professor Geddes and Thomson's
"Evolution," 1911), calls for consideration. In the ordinary
Darwinian view the variations of the young from their parents are
indefinite, and spread in all directions. They may continue to
occur for ages without any of them proving an advantage to their
possessors. Then the environment may change, and a certain
variation may prove an advantage, and be continuously and
increasingly selected. Thus these indefinite variations may be so
controlled by the environment during millions of years that the
fish at last becomes an elephant or a man. The alternative view,
urged by a few writers, is that the variations were "definitely
directed." The phrase seems merely to complicate the story of
evolution with a fresh and superfluous mystery. The nature and
precise action of this "definite direction" within the organism
are quite unintelligible, and the facts seem explainable just as
well--or not less imperfectly--without as with this mystic
agency. Radiolaria, Sponges, Corals, Sharks, Mudfishes,
Duckbills, etc., do not change (except within the limits of their
family) during millions of years, because they keep to an
environment to which they are fitted. On the other hand, certain
fishes, reptiles, etc., remain in a changing environment, and
they must change with it. The process has its obscurities, but we
make them darker, it seems to me, with these semi-metaphysical

It has seemed advisable to take this further glance at the
general principles and current theories of evolution before we
extend our own procedure into the Tertiary Era. The highest types
of animals and plants are now about to appear on the stage of the
earth; the theatre itself is about to take on a modern
complexion. The Middle Ages are over; the new age is breaking
upon the planet. We will, as before, first survey the Tertiary
Era as a whole, with the momentous changes it introduces, and
then examine, in separate chapters, the more important phases of
its life.

It opens, like the preceding and the following era, with "the
area of land large and its relief pronounced." This is the
outcome of the Cretaceous revolution. Southern Europe and
Southern Asia have risen, and shaken the last masses of the Chalk
ocean from their faces; the whole western fringe of America has
similarly emerged from the sea that had flooded it. In many
parts, as in England (at that time a part of the Continent),
there is so great a gap between the latest Cretaceous and the
earliest Tertiary strata that these newly elevated lands must
evidently have stood out of the waters for a prolonged period. On
their cooler plains the tragedy of the extinction of the great
reptiles comes to an end. The cyeads and ginkgoes have shrunk
into thin survivors of the luxuriant Mesozoic groves. The oak and
beech and other deciduous trees spread slowly over the successive
lands, amid the glare and thunder of the numerous volcanoes which
the disturbance of the crust has brought into play. New forms of
birds fly from tree to tree, or linger by the waters; and strange
patriarchal types of mammals begin to move among the bones of the
stricken reptiles.

But the seas and the rains and rivers are acting with renewed
vigour on the elevated lands, and the Eocene period closes in a
fresh age of levelling. Let us put the work of a million years or
so in a sentence. The southern sea, which has been confined
almost to the limits of our Mediterranean by the Cretaceous
upheaval, gradually enlarges once more. It floods the north-west
of Africa almost as far as the equator; it covers most of Italy,
Turkey, Austria, and Southern Russia; it spreads over Asia Minor,
Persia, and Southern Asia, until it joins the Pacific; and it
sends a long arm across the Franco-British region, and up the
great valley which is now the German Ocean.

From earlier chapters we now expect to find a warmer climate, and
the record gives abundant proof of it. To this period belongs the
"London Clay," in whose thick and--to the unskilled
eye--insignificant bed the geologist reads the remarkable story
of what London was two or three million years ago. It tells us
that a sea, some 500 or 600 feet deep, then lay over that part of
England, and fragments of the life of the period are preserved in
its deposit. The sea lay at the mouth of a sub-tropical river on
whose banks grew palms, figs, ginkgoes, eucalyptuses, almonds,
and magnolias, with the more familiar oaks and pines and laurels.
Sword-fishes and monstrous sharks lived in the sea. Large turtles
and crocodiles and enormous "sea-serpents" lingered in this last
spell of warmth that Central Europe would experience. A primitive
whale appeared in the seas, and strange large tapir-like
mammals--remote ancestors of our horses and more familiar
beasts--wandered heavily on the land. Gigantic primitive birds,
sometimes ten feet high, waded by the shore. Deposits of the
period at Bournemouth and in the Isle of Wight tell the same
story of a land that bore figs, vines, palms, araucarias, and
aralias, and waters that sheltered turtles and crocodiles. The
Parisian region presented the same features.

In fact, one of the most characteristic traces of the southern
sea which then stretched from England to Africa in the south and
India in the east indicates a warm climate. It will be remembered
that the Cretaceous ocean over Southern Europe had swarmed with
the animalcules whose dead skeletons largely compose our
chalk-beds. In the new southern ocean another branch of these
Thalamophores, the Nummulites, spreads with such portentous
abundance that its shells--sometimes alone, generally with other
material--make beds of solid limestone several thousand feet in
thickness. The pyramids are built of this nummulitic limestone.
The one-celled animal in its shell is, however, no longer a
microscopic grain. It sometimes forms wonderful shells, an inch
or more in diameter, in which as many as a thousand chambers
succeed each other, in spiral order, from the centre. The beds
containing it are found from the Pyrenees to Japan.

That this vast warm ocean, stretching southward over a large part
of what is now the Sahara, should give a semitropical aspect even
to Central Europe and Asia is not surprising. But this genial
climate was still very general over the earth. Evergreens which
now need the warmth of Italy or the Riviera then flourished in
Lapland and Spitzbergen. The flora of Greenland--a flora that
includes magnolias, figs, and bamboos--shows us that its
temperature in the Eocene period must have been about 30 degrees
higher than it is to-day.* The temperature of the cool Tyrol of
modern Europe is calculated to have then been between 74 and 81
degrees F. Palms, cactuses, aloes, gum-trees, cinnamon trees,
etc., flourished in the latitude of Northern France. The forests
that covered parts of Switzerland which are now buried in snow
during a great part of the year were like the forests one finds
in parts of India and Australia to-day. The climate of North
America, and of the land which still connected it with Europe,
was correspondingly genial.

* The great authority on Arctic geology, Heer, who makes this
calculation, puts this flora in the Miocene. It is now usually
considered that these warmer plants belong to the earlier part of
the Tertiary era.

This indulgent period (the Oligocene, or later part of the
Eocene), scattering a rich and nutritious vegetation with great
profusion over the land, led to a notable expansion of animal
life. Insects, birds, and mammals spread into vast and varied
groups in every land. Had any of the great Mesozoic reptiles
survived, the warmer age might have enabled them to dispute the
sovereignty of the advancing mammals. But nothing more formidable
than the turtle, the snake, and the crocodile (confined to the
waters) had crossed the threshold of the Tertiary Era, and the
mammals and birds had the full advantage of the new golden age.
The fruits of the new trees, the grasses which now covered the
plains, and the insects which multiplied with the flowers
afforded a magnificent diet. The herbivorous mammals became a
populous world, branching into numerous different types according
to their different environments. The horse, the elephant, the
camel, the pig, the deer, the rhinoceros gradually emerge out of
the chaos of evolving forms. Behind them, hastening the course of
their evolution, improving their speed, arms, and armour, is the
inevitable carnivore. He, too, in the abundance of food, grows
into a vast population, and branches out toward familiar types.
We will devote a chapter presently to this remarkable phase of
the story of evolution.

But the golden age closes, as all golden ages had done before it,
and for the same reason. The land begins to rise, and cast the
warm shallow seas from its face. The expansion of life has been
more rapid and remarkable than it had ever been before, in
corresponding periods of abundant food and easy conditions; the
contraction comes more quickly than it had ever done before.
Mountain masses begin to rise in nearly all parts of the world.
The advance is slow and not continuous, but as time goes on the
Atlas, Alps, Pyrenees, Apennines, Caucasus, Himalaya, Rocky
Mountains, and Andes rise higher and higher. When the geologist
looks to-day for the floor of the Eocene ocean, which he
recognises by the shells of the Nummulites, he finds it 10,000
feet above the sea-level in the Alps, 16,000 feet above the
sea-level in the Himalaya, and 20,000 feet above the sea-level in
Thibet. One need not ask why the regions of London and Paris
fostered palms and magnolias and turtles in Tertiary times, and
shudder in their dreary winter to-day.

The Tertiary Era is divided by geologists into four periods: the
Eocene, Oligocene, Miocene, and Pliocene. "Cene" is our barbaric
way of expressing the Greek word for "new," and the
classification is meant to mark the increase of new (or modern
and actual) types of life in the course of the Tertiary Era. Many
geologists, however, distrust the classification, and are
disposed to divide the Tertiary into two periods. From our point
of view, at least, it is advisable to do this. The first and
longer half of the Tertiary is the period in which the
temperature rises until Central Europe enjoys the climate of
South Africa; the second half is the period in which the land
gradually rises, and the temperature falls, until glaciers and
sheets of ice cover regions where the palm and fig had

The rise of the land had begun in the first half of the Tertiary,
but had been suspended. The Pyrenees and Apennines had begun to
rise at the end of the Eocene, straining the crust until it
spluttered with volcanoes, casting the nummulitic sea off large
areas of Southern Europe. The Nummulites become smaller and less
abundant. There is also some upheaval in North America, and a
bridge of land begins to connect the north and south, and permit
an effective mingling of their populations. But the advance is,
as I said, suspended, and the Oligocene period maintains the
golden age. With the Miocene period the land resumes its rise. A
chill is felt along the American coast, showing a fall in the
temperature of the Atlantic. In Europe there is a similar chill,
and a more obvious reason for it. There is an ascending movement
of the whole series of mountains from Morocco and the Pyrenees,
through the Alps, the Caucasus, and the Carpathians, to India and
China. Large lakes still lie over Western Europe, but nearly the
whole of it emerges from the ocean. The Mediterranean still sends
an arm up France, and with another arm encircles the Alpine mass;
but the upheaval continues, and the great nummulitic sea is
reduced to a series of extensive lakes, cut off both from the
Atlantic and Pacific. The climate of Southern Europe is probably
still as genial as that of the Canaries to-day. Palms still
linger in the landscape in reduced numbers.

The last part of the Tertiary, the Pliocene, opens with a slight
return of the sea. The upheaval is once more suspended, and the
waters are eating into the land. There is some foundering of land
at the south-western tip of Europe; the "Straits of Gibraltar"
begin to connect the Mediterranean with the Atlantic, and the
Balearic Islands, Corsica, and Sardinia remain as the mountain
summits of a submerged land. Then the upheaval is resumed, in
nearly every part of the earth.

Nearly every great mountain chain that the geologist has studied
shared in this remarkable movement at the end of the Tertiary
Era. The Pyrenees, Alps, Himalaya, etc., made their last ascent,
and attained their present elevation. And as the land rose, the
aspect of Europe and America slowly altered. The palms, figs,
bamboos, and magnolias disappeared; the turtles, crocodiles,
flamingoes, and hippopotamuses retreated toward the equator. The
snow began to gather thick on the rising heights; then the
glaciers began to glitter on their flanks. As the cold increased,
the rivers of ice which flowed down the hills of Switzerland,
Spain, Scotland, or Scandinavia advanced farther and farther over
the plains. The regions of green vegetation shrank before the
oncoming ice, the animals retreated south, or developed Arctic
features. Europe and America were ushering in the great Ice-Age,
which was to bury five or six million square miles of their
territory under a thick mantle of ice.

Such is the general outline of the story of the Tertiary Era. We
approach the study of its types of life and their remarkable
development more intelligently when we have first given careful
attention to this extraordinary series of physical changes. Short
as the Era is, compared with its predecessors, it is even more
eventful and stimulating than they, and closes with what
Professor Chamberlin calls "the greatest deformative movements in
post-Cambrian history." In the main it has, from the evolutionary
point of view, the same significant character as the two
preceding eras. Its middle portion is an age of expansion,
indulgence, exuberance, in which myriads of varied forms are
thrown upon the scene, its later part is an age of contraction,
of annihilation, of drastic test, in which the more effectively
organised will be chosen from the myriads of types. Once more
nature has engendered a vast brood, and is about to select some
of her offspring to people the modern world. Among the types
selected will be Man.


AS we approach the last part of the geological record we must
neglect the lower types of life, which have hitherto occupied so
much of our attention, so that we may inquire more fully into the
origin and fortunes of the higher forms which now fill the stage.
It may be noted, in general terms, that they shared the opulence
of the mid-Tertiary period, produced some gigantic specimens of
their respective families, and evolved into the genera, and often
the species, which we find living to-day. A few illustrations
will suffice to give some idea of the later development of the
lower invertebrates and vertebrates.

Monstrous oysters bear witness to the prosperity of that ancient
and interesting family of the Molluscs. In some species the
shells were commonly ten inches long; the double shell of one of
these Tertiary bivalves has been found which measured thirteen
inches in length, eight in width, and six in thickness. In the
higher branch of the Mollusc world the naked Cephalopods
(cuttle-fish, etc.) predominate over the nautiloids--the shrunken
survivors of the great coiled-shell race. Among the sharks, the
modern Squalodonts entirely displace the older types, and grow to
an enormous size. Some of the teeth we find in Tertiary deposits
are more than six inches long and six inches broad at the base.
This is three times the size of the teeth of the largest living
shark, and it is therefore believed that the extinct possessor of
these formidable teeth (Carcharodon megalodon) must have been
much more than fifty, and was possibly a hundred, feet in length.
He flourished in the waters of both Europe and America during the
halcyon days of the Tertiary Era. Among the bony fishes, all our
modern and familiar types appear.

The amphibia and reptiles also pass into their modern types,
after a period of generous expansion. Primitive frogs and toads
make their first appearance in the Tertiary, and the remains are
found in European beds of four-foot-long salamanders. More than
fifty species of Tertiary turtles are known, and many of them
were of enormous size. One carapace that has been found in a
Tertiary bed measures twelve feet in length, eight feet in
width, and seven feet in height to the top of the back. The
living turtle must have been nearly twenty feet long. Marine
reptiles, of a snake-like structure, ran to fifteen feet in
length. Crocodiles and alligators swarmed in the rivers of Europe
until the chilly Pliocene bade them depart to Africa.

In a word, it was the seven years of plenty for the whole living
world, and the expansive development gave birth to the modern
types, which were to be selected from the crowd in the subsequent
seven years of famine. We must be content to follow the evolution
of the higher types of organisms. I will therefore first describe
the advance of the Tertiary vegetation, the luxuriance of which
was the first condition of the great expansion of animal life;
then we will glance at the grand army of the insects which
followed the development of the flowers, and at the accompanying
expansion and ramification of the birds. The long and interesting
story of the mammals must be told in a separate chapter, and a
further chapter must be devoted to the appearance of the human

We saw that the Angiosperms, or flowering plants, appeared at the
beginning of the Cretaceous period, and were richly developed
before the Tertiary Era opened. We saw also that their precise
origin is unknown. They suddenly invade a part of North America
where there were conditions for preserving some traces of them,
but we have as yet no remains of their early forms or clue to
their place of development. We may conjecture that their
ancestors had been living in some elevated inland region during
the warmth of the Jurassic period.

As it is now known that many of the cycad-like Mesozoic plants
bore flowers--as the modern botanist scarcely hesitates to call
them--the gap between the Gymnosperms and Angiosperms is very
much lessened. There are, however, structural differences which
forbid us to regard any of these flowering cycads, which we have
yet found, as the ancestors of the Angiosperms. The most
reasonable view seems to be that a small and local branch of
these primitive flowering plants was evolved, like the rest, in
the stress of the Permian-Triassic cold; that, instead of
descending to the warm moist levels with the rest at the end of
the Triassic, and developing the definite characters of the
cycad, it remained on the higher and cooler land; and that the
rise of land at the end of the Jurassic period stimulated the
development of its Angiosperm features, enlarged the area in
which it was especially fitted to thrive, and so permitted it to
spread and suddenly break into the geological record as a fully
developed Angiosperm.

As the cycads shrank in the Cretaceous period, the Angiosperms
deployed with great rapidity, and, spreading at various levels
and in different kinds of soils and climates, branched into
hundreds of different types. We saw that the oak, beech, elm,
maple, palm, grass, etc., were well developed before the end of
the Cretaceous period. The botanist divides the Angiosperms into
two leading groups, the Monocotyledons (palms, grasses, lilies,
orchises, irises, etc.) and Dicotyledons (the vast majority), and
it is now generally believed that the former were developed from
an early and primitive branch of the latter. But it is impossible
to retrace the lines of development of the innumerable types of
Angiosperms. The geologist has mainly to rely on a few stray
leaves that were swept into the lakes and preserved in the mud,
and the evidence they afford is far too slender for the
construction of genealogical trees. The student of living plants
can go a little further in discovering relationships, and, when
we find him tracing such apparently remote plants as the apple
and the strawberry to a common ancestor with the rose, we foresee
interesting possibilities on the botanical side. But the
evolution of the Angiosperms is a recent and immature study, and
we will be content with a few reflections on the struggle of the
various types of trees in the changing conditions of the
Tertiary, the development of the grasses, and the evolution of
the flower. In other words, we will be content to ask how the
modern landscape obtained its general vegetal features.

Broadly speaking, the vegetation of the first part of the
Tertiary Era was a mixture of sub-tropical and temperate forms, a
confused mass of Ferns, Conifers, Ginkgoales, Monocotyledons, and
Dicotyledons. Here is a casual list of plants that then grew in
the latitude of London and Paris: the palm, magnolia, myrtle,
Banksia, vine, fig, aralea, sequoia, eucalyptus, cinnamon tree,
cactus, agave, tulip tree, apple, plum, bamboo, almond, plane,
maple, willow, oak, evergreen oak, laurel, beech, cedar, etc. The
landscape must have been extraordinarily varied and beautiful and
rich. To one botanist it suggests Malaysia, to another India, to
another Australia.

It is really the last gathering of the plants, before the great
dispersion. Then the cold creeps slowly down from the Arctic
regions, and begins to reduce the variety. We can clearly trace
its gradual advance. In the Carboniferous and Jurassic the
vegetation of the Arctic regions had been the same as that of
England; in the Eocene palms can flourish in England, but not
further north; in the Pliocene the palms and bamboos and
semi-tropical species are driven out of Europe; in the
Pleistocene the ice-sheet advances to the valleys of the Thames
and the Danube (and proportionately in the United States), every
warmth-loving species is annihilated, and our grasses, oaks,
beeches, elms, apples, plums, etc., linger on the green southern
fringe of the Continent, and in a few uncovered regions, ready to
spread north once more as the ice creeps back towards the Alps or
the Arctic circle. Thus, in few words, did Europe and North
America come to have the vegetation we find in them to-day.

The next broad characteristic of our landscape is the spreading
carpet of grass. The interest of the evolution of the grasses
will be seen later, when we shall find the evolution of the
horse, for instance, following very closely upon it. So striking,
indeed, is the connection between the advance of the grasses and
the advance of the mammals that Dr. Russel Wallace has recently
claimed ("The World of Life," 1910) that there is a clear
purposive arrangement in the whole chain of developments which
leads to the appearance of the grasses. He says that "the very
puzzling facts" of the immense reptilian development in the
Mesozoic can only be understood on the supposition that they were
evolved "to keep down the coarser vegetation, to supply animal
food for the larger Carnivora, and thus give time for higher
forms to obtain a secure foothold and a sufficient amount of
varied form and structure" (p. 284).

Every insistence on the close connection of the different strands
in the web of life is welcome, but Dr. Wallace does not seem to
have learned the facts accurately. There is nothing "puzzling"
about the Mesozoic reptilian development; the depression of the
land, the moist warmth, and the luscious vegetation of the later
Triassic and the Jurassic amply explain it. Again, the only
carnivores to whom they seem to have supplied food were reptiles
of their own race. Nor can the feeding of the herbivorous
reptiles be connected with the rise of the Angiosperms. We do not
find the flowering plants developing anywhere in those vast
regions where the great reptiles abounded; they invade them from
some single unknown region, and mingle with the pines and
ginkgoes, while the cyeads alone are destroyed.

The grasses, in particular, do not appear until the Cretaceous,
and do not show much development until the mid-Tertiary; and
their development seems to be chiefly connected with physical
conditions. The meandering rivers and broad lakes of the
mid-Tertiary would have their fringes of grass and sedge, and, as
the lakes dried up in the vicissitudes of climate, large areas of
grass would be left on their sites. To these primitive prairies
the mammal (not reptile) herbivores would be attracted, with
important results. The consequences to the animals we will
consider presently. The effect on the grasses may be well
understood on the lines so usefully indicated in Dr. Wallace's
book. The incessant cropping, age after age, would check the
growth of the larger and coarser grasses give opportunity to the
smaller and finer, and lead in time to the development of the
grassy plains of the modern world. Thus one more familiar feature
was added to the landscape in the Tertiary Era.

As this fresh green carpet spread over the formerly naked plains,
it began to be enriched with our coloured flowers. There were
large flowers, we saw, on some of the Mesozoic cycads, but their
sober yellows and greens--to judge from their descendants--would
do little to brighten the landscape. It is in the course of the
Tertiary Era that the mantle of green begins to be embroidered
with the brilliant hues of our flowers.

Grant Allen put forward in 1882 ("The Colours of Flowers") an
interesting theory of the appearance of the colours of flowers,
and it is regarded as probable. He observed that most of the
simplest flowers are yellow; the more advanced flowers of simple
families, and the simpler flowers of slightly advanced families,
are generally white or pink; the most advanced flowers of all
families, and almost all the flowers of the more advanced
families, are red, purple, or blue; and the most advanced flowers
of the most advanced families are always either blue or
variegated. Professor Henslow adds a number of equally
significant facts with the same tendency, so that we have strong
reason to conceive the floral world as passing through successive
phases of colour in the Tertiary Era. At first it would be a
world of yellows and greens, like that of the Mesozoic
vegetation, but brighter. In time splashes of red and white would
lie on the face of the landscape; and later would come the
purples, the rich blues, and the variegated colours of the more
advanced flowers.

Why the colours came at all is a question closely connected with
the general story of the evolution of the flower, at which we
must glance. The essential characteristic of the flower, in the
botanist's judgment, is the central green organ which you
find--say, in a lily--standing out in the middle of the floral
structure, with a number of yellow-coated rods round it. The
yellow rods bear the male germinal elements (pollen); the central
pistil encloses the ovules, or female elements. "Angiosperm"
means "covered-seed plant," and its characteristic is this
protection of the ovules within a special chamber, to which the
pollen alone may penetrate. Round these essential organs are the
coloured petals of the corolla (the chief part of the flower to
the unscientific mind) and the sepals, often also coloured, of
the calyx.

There is no doubt that all these parts arose from modifications
of the leaves or stems of the primitive plant; though whether the
bright leaves of the corolla are directly derived from ordinary
leaves, or are enlarged and flattened stamens, has been disputed.
And to the question why these bright petals, whose colour and
variety of form lend such charm to the world of flowers, have
been developed at all, most botanists will give a prompt and very
interesting reply. As both male and female elements are usually
in one flower, it may fertilise itself, the pollen falling
directly on the pistil. But fertilisation is more sure and
effective if the pollen comes from a different individual--if
there is "cross fertilisation." This may be accomplished by the
simple agency of the wind blowing the pollen broadcast, but it is
done much better by insects, which brush against the stamens, and
carry grains of the pollen to the next flower they visit.

We have here a very fertile line of development among the
primitive flowers. The insects begin to visit them, for their
pollen or juices, and cross-fertilise them. If this is an
advantage, attractiveness to insects will become so important a
feature that natural selection will develop it more and more. In
plain English, what is meant is that those flowers which are more
attractive to insects will be the most surely fertilised and
breed most, and the prolonged application of this principle
during hundreds of thousands of years will issue in the immense
variety of our flowers. They will be enriched with little stores
of honey and nectar; not so mysterious an advantage, when we
reflect on the concentration of the juices in the neighbourhood
of the seed. Then they must "advertise" their stores, and the
strong perfumes and bright colours begin to develop, and ensure
posterity to their possessors. The shape of the corolla will be
altered in hundreds of ways, to accommodate and attract the
useful visitor and shut out the mere robber. These utilities,
together with the various modifying agencies of different
environments, are generally believed to have led to the
bewildering variety and great beauty of our floral world.

It is proper to add that this view has been sharply challenged by
a number of recent writers. It is questioned if colours and
scents do attract insects; though several recent series of
experiments seem to show that bees are certainly attracted by
colours. It is questioned if cross-fertilisation has really the
importance ascribed to it since the days of Darwin. Some of these
writers believe that the colours and the peculiar shape which the
petals take in some flowers (orchises, for instance) have been
evolved to deter browsing animals from eating them. The theory is
thus only a different application of natural selection; Professor
Henslow, on the other hand, stands alone in denying the
selection, and believing that the insects directly developed the
scents, honeys, colours, and shapes by mechanical irritation. The
great majority of botanists adhere to the older view, and see in
the wonderful Tertiary expansion of the flowers a manifold
adaptation to the insect friends and insect foes which then
became very abundant and varied.

Resisting the temptation to glance at the marvellous adaptations
which we find to-day in our plant world-- the insect-eating
plants, the climbers, the parasites, the sensitive plants, the
water-storing plants in dry regions, and so on--we must turn to
the consideration of the insects themselves. We have already
studied the evolution of the insect in general, and seen its
earlier forms. The Tertiary Era not only witnessed a great
deployment of the insects, but was singularly rich in means of
preserving them. The "fly in amber" has ceased to be a puzzle
even to the inexpert. Amber is the resin that exuded from
pine-like trees, especially in the Baltic region, in the Eocene
and Oligocene periods. Insects stuck in the resin, and were
buried under fresh layers of it, and we find them embalmed in it
as we pick up the resin on the shores of the Baltic to-day. The
Tertiary lakes were also important cemeteries of insects. A great
bed at Florissart, in Colorado, is described by one of the
American experts who examined it as "a Tertiary Pompeii." It has
yielded specimens of about a thousand species of Tertiary
insects. Near the large ancient lake, of which it marks the site,
was a volcano, and the fine ash yielded from the cone seems to
have buried myriads of insects in the water. At Oeningen a
similar lake-deposit has, although only a few feet thick, yielded
900 species of insects.

Yet these rich and numerous finds throw little light on the
evolution of the insect, except in the general sense that they
show species and even genera quite different from those of
to-day. No new families of insects have appeared since the
Eocene, and the ancient types had by that time disappeared. Since
the Eocene, however, the species have been almost entirely
changed, so that the insect record, from its commencement in the
Primary Era, has the stamp of evolution on every page of it.
Unfortunately, insects, especially the higher and later insects,
are such frail structures that they are only preserved in very
rare conditions. The most important event of the insect-world in
the Tertiary is the arrival of the butterflies, which then appear
for the first time. We may assume that they spread with great
rapidity and abundance in the rich floral world of the
mid-Jurassic. More than 13,000 species of Lepidoptera are known
to-day, and there are probably twice that number yet to be
classified by the entomologist. But so far the Tertiary deposits
have yielded only the fragmentary remains of about twenty
individual butterflies.

The evolutionary study of the insects is, therefore, not so much
concerned with the various modifications of the three pairs of
jaws, inherited from the primitive Tracheate, and the wings,
which have given us our vast variety of species. It is directed
rather to the more interesting questions of what are called the
"instincts" of the insects, the remarkable metamorphosis by which
the young of the higher orders attain the adult form, and the
extraordinary colouring and marking of bees, wasps, and
butterflies. Even these questions, however, are so large that
only a few words can be said here on the tendencies of recent

In regard to the psychic powers of insects it may be said, in the
first place, that it is seriously disputed among the modern
authorities whether even the highest insects (the ant, bee, and
wasp) have any degree whatever of the intelligence which an
earlier generation generously bestowed on them. Wasmann and
Bethe, two of the leading authorities on ants, take the negative
view; Forel claims that they show occasional traces of
intelligence. It is at all events clear that the enormous
majority of, if not all, their activities--and especially those
activities of the ant and the bee which chiefly impress the
imagination--are not intelligent, but instinctive actions. And
the second point to be noted is that the word "instinct," in the
old sense of some innate power or faculty directing the life of
an animal, has been struck out of the modern scientific
dictionary. The ant or bee inherits a certain mechanism of nerves
and muscles which will, in certain circumstances, act in the way
we call "instinctive." The problem is to find how this mechanism
and its remarkable actions were slowly evolved.

In view of the innumerable and infinitely varied forms of
"instinct" in the insect world we must restrict ourselves to a
single illustration--say, the social life of the ants and the
bees. We are not without indications of the gradual development
of this social life. In the case of the ant we find that the
Tertiary specimens--and about a hundred species are found in
Switzerland alone, whereas there are only fifty species in the
whole of Europe to-day-- all have wings and are, apparently, of
the two sexes, not neutral. This seems to indicate that even in
the mid-Tertiary some millions of years after the first
appearance of the ant, the social life which we admire in the
ants today had not yet been developed. The Tertiary bees, on the
other hand, are said to show some traces of the division of
labour (and modification of structure) which make the bees so
interesting; but in this case the living bees, rising from a
solitary life through increasing stages of social co-operation,
give us some idea of the gradual development of this remarkable

It seems to me that the great selective agency which has brought
about these, and many other remarkable activities of the insects
(such as the storing of food with their eggs by wasps), was
probably the occurrence of periods of cold, and especially the
beginning of a winter season in the Cretaceous or Tertiary age.
In the periods of luxuriant life (the Carboniferous, the
Jurassic, or the Oligocene), when insects swarmed and varied in
every direction, some would vary in the direction of a more
effective placing of the eggs; and the supervening period of cold
and scarcity would favour them. When a regular winter season set
in, this tendency would be enormously increased. It is a parallel
case to the evolution of the birds and mammals from the reptiles.
Those that varied most in the direction of care for the egg and
the young would have the largest share in the next generation.
When we further reflect that since the Tertiary the insect world
has passed through the drastic disturbance of the climate in the
great Ice-Age, we seem to have an illuminating clue to one of the
most remarkable features of higher insect life.

The origin of the colour marks' and patterns on so many of the
higher insects, with which we may join the origin of the
stick-insects, leaf-insects, etc., is a subject of lively
controversy in science to-day. The protective value of the
appearance of insects which look almost exactly like dried twigs
or decaying leaves, and of an arrangement of the colours of the
wings of butterflies which makes them almost invisible when at
rest, is so obvious that natural selection was confidently
invoked to explain them. In other cases certain colours or marks
seemed to have a value as "warning colours," advertising the
nauseousness of their possessors to the bird, which had learned
to recognise them; in other cases these colours and marks seemed
to be borrowed by palatable species, whose unconscious "mimicry"
led to their survival; in other cases, again, the patterns and
spots were regarded as "recognition marks," by which the male
could find his mate.

Science is just now passing through a phase of acute
criticism--as the reader will have realised by this time--and
many of the positions confidently adopted in the earlier
constructive stage are challenged. This applies to the protective
colours, warning colours, mimicry, etc., of insects. Probably
some of the affirmations of the older generation of evolutionists
were too rigid and extensive; and probably the denials of the new
generation are equally exaggerated. When all sound criticism has
been met, there remains a vast amount of protective colouring,
shaping, and marking in the insect world of which natural
selection gives us the one plausible explanation. But the
doctrine of natural selection does not mean that every feature of
an animal shall have a certain utility. It will destroy animals
with injurious variations and favour animals with useful
variations; but there may be a large amount of variation,
especially in colour, to which it is quite indifferent. In this
way much colour-marking may develop, either from ordinary
embryonic variations or (as experiment on butterflies shows) from
the direct influence of surroundings which has no vital
significance. In this way, too, small variations of no selective
value may gradually increase until they chance to have a value to
the animal.*

* For a strong statement of the new critical position see Dewar
and Finn's "Making of Species," 1909, ch. vi.

The origin of the metamorphosis, or pupa-stage, of the higher
insects, with all its wonderful protective devices, is so obscure
and controverted that we must pass over it. Some authorities
think that the sleep-stage has been evolved for the protection of
the helpless transforming insect; some believe that it occurs
because movement would be injurious to the insect in that stage;
some say that the muscular system is actually dissolved in its
connections; and some recent experts suggest that it is a
reminiscence of the fact that the ancestors of the metamorphosing
insects were addicted to internal parasitism in their youth. It
is one of the problems of the future. At present we have no
fossil pupa-remains (though we have one caterpillar) to guide us.
We must leave these fascinating but difficult problems of insect
life, and glance at the evolution of the birds.

To the student of nature whose interest is confined to one branch
of science the record of life is a mysterious Succession of
waves. A comprehensive view of nature, living and non-living,
past and present, discovers scores of illuminating connections,
and even sees at times the inevitable sequence of events. Thus if
the rise of the Angiospermous vegetation on the ruins of the
Mesozoic world is understood in the light of geological and
climatic changes, and the consequent deploying of the insects,
especially the suctorial insects, is a natural result, the
simultaneous triumph of the birds is not unintelligible. The
grains and fruits of the Angiosperms and the vast swarms of
insects provided immense stores of food; the annihilation of the
Pterosaurs left a whole stratum of the earth free for their

We saw that a primitive bird, with very striking reptilian
features, was found in the Jurassic rocks, suggesting very
clearly the evolution of the bird from the reptile in the cold of
the Permian or Triassic period. In the Cretaceous we found the
birds distributed in a number of genera, but of two leading
types. The Ichthyornis type was a tern-like flying bird, with
socketed teeth and biconcave vertebrae like the reptile, but
otherwise fully evolved into a bird. Its line is believed to
survive in the gannets, cormorants, pelicans, and frigate-birds
of to-day. The less numerous Hesperornis group were large and
powerful divers. Then there is a blank in the record,
representing the Cretaceous upheaval, and it unfortunately
conceals the first great ramification of the bird world. When the
light falls again on the Eocene period we find great numbers of
our familiar types quite developed. Primitive types of gulls,
herons, pelicans, quails, ibises, flamingoes, albatrosses,
buzzards, hornbills, falcons, eagles, owls, plovers, and
woodcocks are found in the Eocene beds; the Oligocene beds add
parrots, trogons, cranes, marabouts, secretary-birds, grouse,
swallows, and woodpeckers. We cannot suppose that every type has
been preserved, but we see that our bird-world was virtually
created in the early part of the Tertiary Era.

With these more or less familiar types were large ostrich-like
survivors of the older order. In the bed of the sea which covered
the site of London in the Eocene are found the remains of a
toothed bird (Odontopteryx), though the teeth are merely sharp
outgrowths of the edge of the bill. Another bird of the same
period and region (Gastornis) stood about ten feet high, and must
have looked something like a wading ostrich. Other large waders,
even more ostrich-like in structure, lived in North America; and
in Patagonia the remains have been found of a massive bird, about
eight feet high, with a head larger than that of any living
animal except the elephant, rhinoceros, and hippopotamus

The absence of early Eocene remains prevents us from tracing the
lines of our vast and varied bird-kingdom to their Mesozoic
beginnings. And when we appeal to the zoologist to supply the
missing links of relationship, by a comparison of the structures
of living birds, we receive only uncertain and very general
suggestions.* He tells us that the ostrich-group (especially the
emus and cassowaries) are one of the most primitive stocks of the
bird world, and that the ancient Dinornis group and the recently
extinct moas seem to be offshoots of that stock. The remaining
many thousand species of Carinate birds (or flying birds with a
keel [carina]-shaped breast-bone for the attachment of the flying
muscles) are then gathered into two great branches, which are
"traceable to a common stock" (Pycraft), and branch in their turn
along the later lines of development. One of these lines--the
pelicans, cormorants, etc.--seems to be a continuation of the
Ichthyornis type of the Cretaceous, with the Odontopteryx as an
Eocene offshoot; the divers, penguins, grebes, and petrels
represent another ancient stock, which may be related to the
Hesperornis group of the Cretaceous. Dr. Chalmers Mitchell thinks
that the "screamers" of South America are the nearest
representatives of the common ancestor of the keel-breasted
birds. But even to give the broader divisions of the 19,000
species of living birds would be of little interest to the
general reader.

* The best treatment of the subject will be found in W. P.
Pycraft's History of Birds, 1910.

The special problems of bird-evolution are as numerous and
unsettled as those of the insects. There is the same dispute as
to "protective colours" and "recognition marks", the same
uncertainty as to the origin of such instinctive practices as
migration and nesting. The general feeling is that the annual
migration had its origin in the overcrowding of the regions in
which birds could live all the year round. They therefore pushed
northward in the spring and remained north until the winter
impoverishment drove them south again. On this view each group
would be returning to its ancestral home, led by the older birds,
in the great migration flights. The curious paths they follow are
believed by some authorities to mark the original lines of their
spread, preserved from generation to generation through the
annual lead of the older birds. If we recollect the Ice-Age which
drove the vast majority of the birds south at the end of the
Tertiary, and imagine them later following the northward retreat
of the ice, from their narrowed and overcrowded southern
territory, we may not be far from the secret of the annual

A more important controversy is conducted in regard to the
gorgeous plumage and other decorations and weapons of the male
birds. Darwin, as is known, advanced a theory of "sexual
selection" to explain these. The male peacock, to take a concrete
instance, would have developed its beautiful tail because,
through tens of thousands of generations, the female selected the
more finely tailed male among the various suitors. Dr. Wallace
and other authorities always disputed this aesthetic sentiment
and choice on the part of the female. The general opinion today
is that Darwin's theory could not be sustained in the range and
precise sense he gave to it. Some kind of display by the male in
the breeding season would be an advantage, but to suppose that
the females of any species of birds or mammals had the definite
and uniform taste necessary for the creation of male characters
by sexual selection is more than difficult. They seem to be
connected in origin rather with the higher vitality of the male,
but the lines on which they were selected are not yet understood.

This general sketch of the enrichment of the earth with flowering
plants, insects, and birds in the Tertiary Era is all that the
limits of the present work permit us to give. It is an age of
exuberant life and abundant food; the teeming populations
overflow their primitive boundaries, and, in adapting themselves
to every form of diet, every phase of environment, and every
device of capture or escape, the spreading organisms are moulded
into tens of thousands of species. We shall see this more clearly
in the evolution of the mammals. What we chiefly learn from the
present chapter is the vital interconnection of the various parts
of nature. Geological changes favour the spread of a certain type
of vegetation. Insects are attracted to its nutritious
seed-organs, and an age of this form of parasitism leads to a
signal modification of the jaws of the insects themselves and to
the lavish variety and brilliance of the flowers. Birds are
attracted to the nutritious matter enclosing the seeds, and, as
it is an advantage to the plant that its seeds be scattered
beyond the already populated area, by passing through the
alimentary canal of the bird, and being discharged with its
excrements, a fresh line of evolution leads to the appearance of
the large and coloured fruits. The birds, again, turn upon the
swarming insects, and the steady selection they exercise leads to
the zigzag flight and the protective colour of the butterfly, the
concealment of the grub and the pupa, the marking of the
caterpillar, and so on. We can understand the living nature of
to-day as the outcome of that teeming, striving, changing world
of the Tertiary Era, just as it in turn was the natural outcome
of the ages that had gone before.


In our study of the evolution of the plant, the insect, and the
bird we were seriously thwarted by the circumstance that their
frames, somewhat frail in themselves, were rarely likely to be
entombed in good conditions for preservation. Earlier critics of
evolution used, when they were imperfectly acquainted with the
conditions of fossilisation, to insinuate that this fragmentary
nature of the geological record was a very convenient refuge for
the evolutionist who was pressed for positive evidence. The
complaint is no longer found in any serious work. Where we find
excellent conditions for preservation, and animals suitable for
preservation living in the midst of them, the record is quite
satisfactory. We saw how the chalk has yielded remains of
sea-urchins in the actual and gradual process of evolution.
Tertiary beds which represent the muddy bottoms of tranquil lakes
are sometimes equally instructive in their fossils, especially of
shell-fish. The Paludina of a certain Slavonian lake-deposit is a
classical example. It changes so greatly in the successive levels
of the deposit that, if the intermediate forms were not
preserved, we should divide it into several different species.
The Planorbis is another well-known example. In this case we have
a species evolving along several distinct lines into forms which
differ remarkably from each other.

The Tertiary mammals, living generally on the land and only
coming by accident into deposits suitable for preservation,
cannot be expected to reveal anything like this sensible advance
from form to form. They were, however, so numerous in the
mid-Tertiary, and their bones are so well calculated to survive
when they do fall into suitable conditions, that we can follow
their development much more easily than that of the birds. We
find a number of strange patriarchal beasts entering the scene in
the early Eocene, and spreading into a great variety of forms in
the genial conditions of the Oligocene and Miocene. As some of
these forms advance, we begin to descry in them the features,
remote and shadowy at first, of the horse, the deer, the
elephant, the whale, the tiger, and our other familiar mammals.
In some instances we can trace the evolution with a wonderful
fullness, considering the remoteness of the period and the
conditions of preservation. Then, one by one, the abortive, the
inelastic, the ill-fitted types are destroyed by changing
conditions or powerful carnivores, and the field is left to the
mammals which filled it when man in turn began his destructive

The first point of interest is the origin of these Tertiary
mammals. Their distinctive advantage over the mammals of the
Mesozoic Era was- the possession by the mother of a placenta (the
"after-birth" of the higher mammals), or structure in the womb by
which the blood-vessels of the mother are brought into such
association with those of the foetus that her blood passes into
its arteries, and it is fully developed within the warm shelter
of her womb. The mammals of the Mesozoic had been small and
primitive animals, rarely larger than a rat, and never rising
above the marsupial stage in organisation. They not only
continued to exist, and give rise to their modern representatives
(the opossum, etc.) during the Tertiary Era, but they shared the
general prosperity. In Australia, where they were protected from
the higher carnivorous mammals, they gave rise to huge
elephant-like wombats (Diprotodon), with skulls two or three feet
in length. Over the earth generally, however, they were
superseded by the placental mammals, which suddenly break into
the geological record in the early Tertiary, and spread with
great vigour and rapidity over the four continents.

Were they a progressive offshoot from the Mesozoic Marsupials, or
Monotremes, or do they represent a separate stock from the
primitive half-reptile and half-mammal family? The point is
disputed; nor does the scantiness of the record permit us to tell
the place of their origin. The placental structure would be so
great an advantage in a cold and unfavourable environment that
some writers look to the northern land, connecting Europe and
America, for their development. We saw, however, that this
northern region was singularly warm until long after the spread
of the mammals. Other experts, impressed by the parallel
development of the mammals and the flowering plants, look to the
elevated parts of eastern North America.

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