THE AGE OF MAMMALS
The Geological Record is not perfect. There are breaks in it such
as have not and may never be filled up; and it is because of these
breaks that some of the divisions are made in geologic time. At
present the earth's crust has only been scratched for fossils.
Great parts of Asia and of Africa and of South America remain to be
explored, and they may in some future generation fill the gaps of
our knowledge and render superfluous some of the divisions which
geologists now place in the eras of the rocks and of the fossils. But
so far as we know at present there were real breaks in the history
of the continents, perhaps not swift or sudden, but wholly changing
the appearance and the life, vegetable and animal, of half a world,
perhaps the whole world at a time. Many geologists believe that the
secret of these changes lies in the core of the earth; and that, to
use our old simile of the golf ball, when the tension and pressures
inside the earth grow too much for its strength something gives
way, and the whole world begins to change, the continents sinking
under the oceans and new lands arising. We shall not again consider
this idea in all its bearings, or ask whether there is any simpler
explanation to be found in the never-ceasing explosive tremors of the
crust; but we shall only say that the last of these great changes
set in at the end of the Chalk age. After that era we arrive at the
period among the rocks which, with all its subdivisions, Eocene,
Miocene, Pliocene, Quaternary, is classed as Cainozoic or Modern.
Let us sum up the changes broadly. The Tertiary period, which now
begins, has been called the Age of Lakes: but this merely means
that there were great lake deposits, and it is true to say that as
contrasted with a period of great waters, the Tertiary is to be
considered as the period of land. That does not mean that there
were in all the hundreds of thousands of years which it embraces no
advances and retreats of the sea, no submergings and uprisings of the
land. There certainly were. But the land was dominant, and it is the
land animals and the land vegetation that are the most important and
progress most. After the earth movements which occurred at the end
of the Mesozoic or Secondary period there appears to have followed a
period of quiet. There was a considerable area of land standing high
above the waters; and there began one of the minor but considerable
encroachments of the sea in North America. It is probable that the
Pacific and the Atlantic joined between North and South America. At
the end of this first period the sea withdrew again, and what is
called the Miocene period began with a lowering of the temperature of
the waters of the Atlantic; and lastly followed the great extension
of the land towards the north, the great withdrawal of the sea of
Pliocene times, and the growing cold which led to the glacial era of
the Pleistocene period. In Europe and in Asia we may note that the
great areas which are now covered by the Alps and the Himalayas were
at the beginning of the Tertiary period still under water and only
a few signs (in the form of islands) of these mighty ranges were
beginning to appear.
Pre-eminently the age which comprises all these periods is the Age
of Mammals. But one of the changes which European geologists first
noticed was the surprising change which took place in the marine
fossils. The animals of the sea which were familiar during the
Chalk period nearly all disappeared and were replaced by new ones.
The great saurian reptiles, from the monsters of the land to the
mososaurus serpents of the sea, disappeared, and most other reptiles
showed profound changes, showing a revolution in the animals of the
land corresponding to that of the sea. Lastly, in this first period,
the Eocene, mammals suddenly appear in force and occupy the first
place among the animals. The vegetation did not change so much as
might have been expected.
Whence came the mammals? That, again, is one of the questions which
time alone can completely answer. But the opinion of most geologists
is that they arose and developed in Asia first of all, and then
spread to other continents. The rise of the mammals, which, unlike
the reptiles, bring forth their young relatively mature and nourish
and protect them, was contributing to the downfall of the reptiles,
though it cannot be considered an actual cause. The mammals' young
had a better chance of living and surviving than had the eggs of
reptiles. Moreover, the mammals began with superior agility and
higher brain-power. It is not surprising, therefore, that the
invasion of the mammals resulted in the clumsy, affectionless,
small-brained reptiles being driven either into extinction, or into
the sedges and rushes, the swamps and lagoons, the coverts of the
jungles, the crevices of the rocks, and the various by-ways which the
mammals cared least to frequent, and that they have been kept there
to this day.
At first the mammals were not very different in habit or type from
one another. Small animals, which, like the shrews, moles, and
hedgehogs lived on insects were among the earliest. There were others
whose toes were turning to hoofs in order to fit them for fleetness;
and there were some curious creatures called _Coryphodons_, which
were like the modern tapir, though they were tusked like boars. The
_Coryphodon_ was a slow beast, with toes like those of an elephant,
though it was much smaller.
In America appeared a small animal not much bigger than a
fox-terrier, which was the ancestor of the horse, and of which we
shall have more to say. The birds increased, and forms like those
of the heron mingled in the swamps with other goose-like birds that
kept in their serrated bills some traces of the teeth of their early
ancestors. Others, like kingfishers, flitted over the streams; and
the emu, ostrich, and moa, as well as the albatross, find their
earliest representatives in the Eocene times.
It is impossible for us to follow, or even to enumerate, all the
varied ancestral lines which sprang up, some of them already
vigorous, in the early Tertiary times, and which developed so
mightily in the successive ages. We can only trace the careers of
a few such as are better and more popularly known, while admitting
that there are many others equally interesting from a scientific or
from any other point of view. From a geologist's point of view the
most important, perhaps, of all the mammal developments was that of
the elephant. The first mammal which geologists discovered that was
like the elephant was the Mastodon, the American variety of which
is called _Tetrabelodon_. But this Mastodon had no proper trunk as
has the elephant. Instead of that he had a very long upper lip which
apparently rested on his projecting upper tusks. Mr. Kipling once
suggested that the elephant's trunk was originally formed by an
accident--an unfortunate young elephant before the days of trunks
having stopped to drink at a pool, and his nose being seized by a
crocodile, who pulled and pulled till the nose lengthened out a
trunk. There certainly was some reason for the elephant's trunk,
which has developed, we do not quite know how, from a long nose. But
a great deal has been found out about the early development of the
elephant by Dr. Andrews of the British Museum.
[Illustration: Evolution of the Head, Proboscis, Nostrils, and Tusks
of the Elephant
The drawings are to the same scale; the nostrils indicated by the
letter N, the upper lips by L, and the tusks by T.
1. Mœritherium of Eocene Libya, with a flexible upper lip and
the small incisor tusks.
2. Palæomastodon of Eocene Libya, with a short proboscis and
powerful upper and lower tusks.
3. Mammoth (Elephas Columbi) from the State of Indiana, with
gigantic upper tusks or ivories, and long proboscis with
nostrils at the tip.
]
Dr. Andrews was travelling in Egypt some years ago, and joined a
party of officers of the great survey of Egypt in a visit to the
Great Western Desert, the rainless, sandy waste west of the Nile, not
very far from what is now called the Fayoum, and where in Roman days
was the great Lake Mœris, now dried up to a mere brine pool, in the
salt water of which the fresh-water fishes of the Nile still live.
The surveying party intended to determine the geological age of these
sands, which stretch for hundreds of miles, often rising into cliffs
which are cut sharp by the wind and show horizontal stratification.
The geologists determined that the sands of this region were of
Eocene and Miocene Age, and from them Dr. Andrews brought home
some very interesting bones. These included the remains of a more
primitive Mastodon than any as yet known, and of an animal which he
called Meritherium, which is a connecting link between elephants
and other mammals. The collection included also remains of great
flesh-eating beasts, and of sea cows, of tortoises, and of a snake
sixty feet long!
However, in regard to the history of elephants, the upshot of Dr.
Andrews' most important discoveries is that we find living here in
the Upper Eocene period (which is older than the age in which the
_Tetrabelodon Mastodon_ was found) an elephant ancestor of the kind
to which Dr. Andrews gave the name of _Palæomastodon_ or "ancient
mastodon." We thus arrive at an ancestral elephant-like creature
which serves to join the elephant stock on to more ordinary mammals.
This beast was not so very big, perhaps about the same size as an
ordinary cart-horse.
Dr. Andrews' further triumph was the additional discovery of the
rather smaller animal which he called the _Meritherium_, and which
was undoubtedly an elephant of sorts, though at first sight it has no
resemblance to one, and probably had no trunk at all. It certainly
had no big tusks; but its teeth make us certain that it belonged
to the elephant family. "Here, then," says Sir E. Ray Lankester,
"we have arrived at a form which undoubtedly was closely related
to the ancestors of all the elephants, if not actually itself that
ancestor, and in it we see the origin of the elephants peculiar
structure. From this comparatively normal pig-like _Meritherium_, the
wonderful elephant, with his upright face, his dependent trunk, and
his huge spreading tusks has been gradually, step by step, produced.
And we have seen some, at least, of the intermediate steps--the
lengthening of the jaws and the increase in the size of the teeth in
the _Palæomastodon_--carried still further on by the _Tetrabelodon_,
and then followed by a shrinkage of the lower jaw and final evolution
of the middle part of the face and upper jaw into the drooping,
wonderful, prehensile trunk."
The long-chinned elephant requires, however, a few moments'
consideration from an altogether different point of view. This
species appears to have had the widest geographical distribution of
any member of the family, of which it may be regarded as the great
colonising or emigrant representative. First developed in North
Africa, where its remains occur in the early Miocene strata of Mogara
and Tunisia, this species ranged right across Europe to the confines
of North-Western India, having probably reached Italy from Africa
by means of a land-bridge by way of Sicily, and thence travelling
through Greece into Asia. On the latter continent it appears to have
given rise to the modern elephants, as remains of the former are
unknown in any other part of the world.
If this be true, it follows that elephants of the modern type
subsequently migrated into Europe and thence to Africa, while in the
other direction they wandered by way of Behring Strait to America.
Hence we are led to the remarkable conclusion that while the first
elephants appeared in Africa, the modern African elephant is of
Asiatic parentage, and a comparatively recent immigrant into the
land of its forefathers. Next to man and the carnivora, elephants
appear to have been the greatest travellers the world has ever
produced, for, starting from their North African birthplace, they
reached by the Behring Strait route nearly to the extremity of South
America, while to the north they penetrated the Arctic circle, and
to the south, on their return journey, reached the coast in the
neighbourhood of Cape Town.
Another great traveller was the horse. The first undoubted
horse-like animal was _Eohippus_, a little creature about eleven
inches in height at the shoulder, and in general rather more like
the flesh-eaters than the horses of the present day. The back was
arched, the head and neck were short, and the limbs of moderate
length, showing no remarkable adaptation for speed. This genus had
a remarkable range, having apparently originated in England (then
a part of Western Europe), and migrated by way of Europe and Asia,
and what is now Behring Strait, to America, where it got as far east
as New Mexico. This migration of _Eohippus_ shifted the scene of
evolution to the western hemisphere, for while examples of it are
continually and continuously found there in succeeding strata it only
appears occasionally in Europe, as if the remains there had been
those of mere emigrants.
Later on the horse developed in America, growing larger till it was
first as big as a collie-dog, with signs of being more adapted for
speed. It then had four toes on its foot. It continued, though very
gradually, to grow larger, and even more gradually its unnecessary
toes grew fewer and fewer till at last they disappeared.
At length appeared the horse which had only one toe. This type,
that of the modern horse, first becomes known in the Upper Pliocene
beds of Europe, and represents the culmination of the race. The
completeness of the record of the evolution of the horse tells us
something of the enormous numbers of ancestral forms which must
have existed in the more than two million years that have elapsed
since the first diminutive horse appeared in North America. While
not strongly given to migration, in the course of time these
animals wandered over the entire world, with the exception of such
inaccessible places as Australia and the Oceanic Islands.... It
would seem that the original stock was of Eurasian derivation, though
the great theatre of the evolutionary drama was soon transferred
to North America, the Eurasian, African, and South American horses
which appear from time to time being in all probability of North
American origin. The ultimate fate of the horses in both North and
South America was extinction, all wild horses of our own time,
including the asses and zebras, being confined to Asia and Africa.
The apparently wild bands of the American western plains, and those
which roam over the pampas of South America, are the descendants of
domestic horses that have escaped from human bondage, largely from
the early Spanish explorers.
The rhinoceroses of to-day, the one-horned Indian variety and the
two-horned African rhinoceros, were preceded by a whole regiment of
rhinoceroses in the Tertiary period. One such was dug out in Fleet
Street during the excavation for the _Daily Chronicle_ office.
This rhinoceros had a hairy coat like the Mammoth which lived much
later, and in Siberia is found sometimes side by side with the later
quadruped. Many of the extinct rhinoceroses had two horns like the
African square-mouthed rhinoceros, which is sometimes misleadingly
called the white rhinoceros. One great extinct beast, the
_Elasmotherium_, allied to them, had a great horn carried on a huge
boss in the middle of its head instead of on the nose, while another
still huger animal called the _Titanotherium_ and found in North
America had a pair of horns perched on either side of its nose. As
large as the rhinoceros but having a very different arrangement of
the bones of its ankles and wrists and very different teeth and horns
are the extraordinary creatures known as _Dinoceras_, whole skeletons
of which have been disinterred from the Eocene strata of Wyoming
in the United States by Professor Marsh. These creatures had three
pairs of horns on the top of the head and a pair of great tusks as
well. Nearly all these animals, though they were more brainy than the
reptiles, had much smaller brains in proportion to their size than
the bulk of the animals which now roam the earth, from which we may
surmise that though a small brain suffices to guide a great animal
machine in established ways, yet in order to learn new things in its
lifetime an animal must have a big brain.
The last great mammal we must mention in this series is the
_Arsinoitherium_, which was found only a few years ago by Dr.
Andrews in Egypt, in the same strata whence he obtained the fossil
ancestors of the elephant. It was so called because it was found
near the palace of Arsinoë, the name of the Egyptian queen of Greek
race. But _Arsinoitherium_ was far from being a graceful ladylike
creature, and, resembling in general appearance a rhinoceros, had two
enormous bones, which grew out of its nose on either side of it. The
bones were hollow and were probably covered with skin in life; and
_Arsinoitherium_ had a wonderful and wonderfully even set of teeth.
To conclude, we must add a representative mammal of this period, the
_Sivatherium_, found in India, and the _Samotherium_, found in the
Isle of Samos, which were like giraffes, and the beginning of the
sloth-like animals, whose appearance we must, however, deal with in
another chapter.
[Illustration: Two Arsinoitheriums (Prehistoric Rhinoceros) at Bay
before a Pack of Hyænodons
The Arsinoitherium stood 5 feet 9 inches at the withers, and measured
9 feet 9 inches from snout to rump. The hyænodons (hyæna toothed)
were no relation of the modern hyæna. They had bodies like the
Tasmanian wolf, and were wonderfully adapted to capture both land and
water living prey.
(Drawn under the direction of Prof. Osborn.)]
The whales whose remains are found in the Pliocene rocks differ
little from those now living; but the observations geologists have
been able to make upon these gigantic remains of the ancient world
are too few to allow of any very precise conclusion. It is certain,
however, that the fossil differs from the existing whale in certain
characteristics perceptible in the bones of the skull. The discovery
of an enormous fragment of a fossil whale, made at Paris in 1779,
in the cellar of a wine merchant in the Rue Dauphine, created a
great sensation. Science pronounced, without much hesitation, on the
true origin of these remains; but the public had some difficulty in
comprehending the existence of a whale in the Rue Dauphine. It was in
digging some holes in his cellars that the wine merchant made this
interesting discovery. His workmen found, under the pick, an enormous
piece of bone buried in a yellow clay. Its complete extraction
caused him a great deal of labour, and presented many difficulties.
Little interested in making further discoveries, our wine merchant
contented himself with raising, with the help of a chisel, a portion
of the monstrous bone. The piece thus detached weighed 227 lbs. It
was exhibited in the wine-shop, where large numbers of the curious
went to see it. Lamanon, a naturalist of that day, who examined it,
conjectured that the bone belonged to the head of a whale. As to the
bone itself, it was purchased for the Teyler Museum, at Haarlem.
Lastly, we must not omit to mention that in the Old World the first
true apes, _Oreopithecus_ and _Dryopithecus_, appeared. The first
of these united some of the characteristics of apes and monkeys;
the second, about the same size, was more closely related to the
chimpanzee and gorilla.