Section 17 of 21 - Table of Contents
CHAPTER XVII. THE ORIGIN OF OUR MAMMALS
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
career.
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.
Such evidence as there is seems rather to suggest that South
Africa was the cradle of the placental mammals. We shall find
that many of our mammals originated in Africa; there, too, is
found to-day the most primitive representative of the Tertiary
mammals, the hyrax; and there we find in especial abundance the
remains of the mammal-like reptiles (Theromorphs) which are
regarded as their progenitors. Further search in the unexplored
geological treasures and dense forests of Africa is needed. We
may provisionally conceive the placental mammals as a group of
the South African early mammals which developed a fortunate
variation in womb-structure during the severe conditions of the
early Mesozoic. In this new structure they would have no
preponderant advantage as long as the genial Jurassic age
favoured the great reptiles, and they may have remained as small
and insignificant as the Marsupials. But with the fresh upheaval
and climatic disturbance at the end of the Jurassic, and during
the Cretaceous, they spread northward, and replaced the dying
reptiles, as the Angiosperms replaced the dying cycads. When they
met the spread of the Angiosperm vegetation they would receive
another great stimulus to development.
They appear in Europe and North America in the earliest
Cretaceous. The rise of the land had connected many hitherto
isolated regions, and they seem to have poured over every bridge
into all parts of the four continents. The obscurity of their
origin is richly compensated by their intense evolutionary
interest from the moment they enter the geological record. We
have seen this in the case of every important group of plants and
animals, and can easily understand it. The ancestral group was
small and local; the descendants are widely spread. While,
therefore, we discover remains of the later phases of development
in our casual cuttings and quarries, the ancestral tomb may
remain for ages in some unexplored province of the geological
world. If this region is, as we suspect, in Africa, our failure
to discover it as yet is all the more intelligible.
But these mammals of the early Tertiary are still of such a
patriarchal or ancestral character that the student of evolution
can dispense with their earlier phase. They combine in their
primitive frames, in an elementary way, the features which we now
find distributed in widely removed groups of their descendants.
Most of them fall into two large orders: the Condylarthra, the
ancestral herbivores from which we shall find our horses, oxen,
deer, elephants, and hogs gradually issuing, and the Creodonta,
the patriarchal carnivores, which will give birth to our lions
and tigers, wolves and foxes, and their various cousins. As yet
even the two general types of herbivore and carnivore are so
imperfectly separated that it is not always possible to
distinguish between them. Nearly all of them have the five-toed
foot of the reptile ancestor; and the flat nails on their toes
are the common material out of which the hoof of the ungulate and
the claw of the carnivore will be presently fashioned. Nearly all
have forty-four simply constructed teeth, from which will be
evolved the grinders and tusks of the elephant or the canines of
the tiger. They answer in every respect to the theory that some
primitive local group was the common source of all our great
mammals. With them are ancestral forms of Edentates (sloths,
etc.) and Insectivores (moles, etc.), side-branches developing
according to their special habits; and before the end of the
Eocene we find primitive Rodents (squirrels, etc.) and
Cheiroptera (bats).
From the description of the Tertiary world which we have seen in
the last chapter we understand the rapid evolution of the
herbivorous Condylarthra. The rich vegetation which spreads over
the northern continents, to which they have penetrated, gives
them an enormous vitality and fecundity, and they break into
groups, as they increase in number, adapted to the different
conditions of forest, marsh, or grass-covered plain. Some of
them, swelling lazily on the abundant food, and secure for a time
in their strength, become the Deinosaurs of their age, mere
feeding and breeding machines. They are massive, sluggish,
small-brained animals, their strong stumpy limbs terminating in
broad five-toed feet. Coryphodon, sometimes as large as an ox, is
a typical representative. It is a type fitted only for prosperous
days, and these Amblypoda, as they are called, will disappear as
soon as the great carnivores are developed.
Another doomed race, or abortive experiment of early mammal life,
were the remarkable Deinocerata ("terrible-horned" mammals). They
sometimes measured thirteen feet in length, but had little use
for brain in the conditions in which they were developed. The
brain of the Deinoceras was only one-eighth the size of the brain
of a rhinoceros of the same bulk; and the rhinoceros is a
poor-brained representative of the modern mammals. To meet the
growing perils of their race they seem to have developed three
pairs of horns on their long, flat skulls, as we find on them
three pairs of protuberances. A late specimen of the group,
Tinoceras, had a head four feet in length, armed with these six
horns, and its canine teeth were developed into tusks sometimes
seven or eight inches in length. They suggest a race of powerful
but clumsy and grotesque monsters, making a last stand, and
developing such means of protection as their inelastic nature
permitted. But the horns seem to have proved a futile protection
against the advancing carnivores, and the race was extinguished.
The horns may, of course, have been mainly developed by, or for,
the mutual butting of the males.
The extinction of these races will remind many readers of a
theory on which it is advisable to say a word. It will be
remembered that the last of the Deinosaurs and the Ammonites also
exhibited some remarkable developments in their last days. These
facts have suggested to some writers the idea that expiring races
pass through a death-agony, and seem to die a natural death of
old age like individuals. The Trilobites are quoted as another
instance; and some ingenious writers add the supposed
eccentricities of the Roman Empire in its senile decay and a
number of other equally unsubstantial illustrations.
There is not the least ground for this fantastic speculation. The
destruction of these "doomed races" is as clearly traceable to
external causes as is the destruction of the Roman Empire; nor,
in fact, did the Roman Empire develop any such eccentricities as
are imagined in this superficial theory. What seem to our eye the
"eccentricities" and "convulsions" of the Ceratopsia and
Deinocerata are much more likely to be defensive developments
against a growing peril, but they were as futile against the new
carnivores as were the assegais of the Zulus against the
European. On the other hand, the eccentricities of many of the
later Trilobites--the LATEST Trilobites, it may be noted, were
chaste and sober specimens of their race, like the last Roman
patricians--and of the Ammonites may very well have been caused
by physical and chemical changes in the sea-water. We know from
experiment that such changes have a disturbing influence,
especially on the development of eggs and larvae; and we know
from the geological record that such changes occurred in the
periods when the Trilobites and Ammonites perished. In fine, the
vast majority of extinct races passed through no "convulsions"
whatever. We may conclude that races do not die; they are killed.
The extinction of these races of the early Condylarthra, and the
survival of those races whose descendants share the earth with us
to-day, are quite intelligible. The hand of natural selection lay
heavy on the Tertiary herbivores. Apart from overpopulation,
forcing groups to adapt themselves to different regions and
diets, and apart from the geological disturbances and climatic
changes which occurred in nearly every period, the shadow of the
advancing carnivores was upon them. Primitive but formidable
tigers, wolves, and hyenas were multiplying, and a great
selective struggle set in. Some groups shrank from the battle by
burrowing underground like the rabbit; some, like the squirrel or
the ape, took refuge in the trees; some, like the whale and seal,
returned to the water; some shrank into armour, like the
armadillo, or behind fences of spines, like the hedgehog; some,
like the bat, escaped into the air. Social life also was probably
developed at this time, and the great herds had their sentinels
and leaders. But the most useful qualities of the large
vegetarians, which lived on grass and leaf, were acuteness of
perception to see the danger, and speed of limb to escape it. In
other words, increase of brain and sense-power and increase of
speed were the primary requisites. The clumsy early Condylarthra
failed to meet the tests, and perished; the other branches of the
race were more plastic, and, under the pressure of a formidable
enemy, were gradually moulded into the horse, the deer, the ox,
the antelope, and the elephant.
We can follow the evolution of our mammals of this branch most
easily by studying the modification of the feet and limbs. In a
running attitude--the experiment may be tried--the weight of the
body is shifted from the flat sole of the foot, and thrown upon
the toes, especially the central toes. This indicates the line of
development of the Ungulates (hoofed animals) in the struggle of
the Tertiary Era. In the early Eocene we find the Condylarthra
(such as Phenacodus) with flat five-toed feet, and such a mixed
combination of characters that they "might serve very well for
the ancestors of all the later Ungulata" (Woodward). We then
presently find this generalised Ungulate branching into three
types, one of which seems to be a patriarchal tapir, the second
is regarded as a very remote ancestor of the horse, and the third
foreshadows the rhinoceros. The feet have now only three or four
toes; one or two of the side-toes have disappeared. This
evolution, however, follows two distinct lines. In one group of
these primitive Ungulates the main axis of the limb, or the
stress of the weight, passes through the middle toe. This group
becomes the Perissodactyla ("odd-toed" Ungulates) of the
zoologist, throwing out side-branches in the tapir and the
rhinoceros, and culminating in the one-toed horse. In the other
line, the Artiodactyla (the "even-toed" or cloven-hoofed
Ungulates), the main axis or stress passes between the third and
fourth toes, and the group branches into our deer, oxen, sheep,
pigs, camels, giraffes, and hippopotamuses. The elephant has
developed along a separate and very distinctive line, as we shall
see, and the hyrax is a primitive survivor of the ancestral
group.
Thus the evolutionist is able to trace a very natural order in
the immense variety of our Ungulates. He can follow them in
theory as they slowly evolve from their primitive Eocene ancestor
according to their various habits and environments; he has a very
rich collection of fossil remains illustrating the stages of
their development; and in the hyrax (or "coney") he has one more
of those living fossils, or primitive survivors, which still
fairly preserve the ancestral form. The hyrax has four toes on
the front foot and three on the hind foot, and the feet are flat.
Its front teeth resemble those of a rodent, and its molars those
of the rhinoceros. In many respects it is a most primitive and
generalised little animal, preserving the ancestral form more or
less faithfully since Tertiary days in the shelter of the African
Continent.
The rest of the Ungulates continued to develop through the
Tertiary, and fortunately we are enabled to follow the
development of two of the most interesting of them, the horse and
the elephant, in considerable detail. As I said above, the
primitive Ungulate soon branches into three types which dimly
foreshadow the tapir, the horse, and the rhinoceros, the three
forms of the Perissodactyl. The second of these types is the
Hyracotherium. It has no distinct equine features, and is known
only from the skull, but the authorities regard it as the
progenitor (or representative of the progenitors) of the
horse-types. In size it must have been something like the rabbit
or the hyrax. Still early in the Eocene, however, we find the
remains of a small animal (Eohippus), about the size of a fox,
which is described as "undoubtedly horse-like." It had only three
toes on its hind feet, and four on its front feet; though it had
also a splint-bone, representing the shrunken and discarded fifth
toe, on its fore feet. Another form of the same period
(Protorohippus) shows the central of the three toes on the hind
foot much enlarged, and the lateral toes shrinking. The teeth,
and the bones and joints of the limbs, are also developing in the
direction of the horse.
In the succeeding geological period, the Oligocene, we find
several horse-types in which the adaptation of the limbs to
running on the firm grassy plains and of the teeth to eating the
grass continues. Mesohippus has lost the fourth toe of the fore
foot, which is now reduced to a splintbone, and the lateral toes
of its hind foot are shrinking. In the Miocene period there is a
great development of the horse-like mammals. We have the remains
of more than forty species, some continuing the main line of
development on the firm and growing prairies of the Miocene, some
branching into the softer meadows or the forests, and giving rise
to types which will not outlive the Tertiary. They have three
toes on each foot, and have generally lost even the rudimentary
trace of the fourth toe. In most of them, moreover, the lateral
toes--except in the marsh-dwelling species, with spreading
feet--scarcely touch the ground, while the central toe is
developing a strong hoof. The leg-bones are longer, and have a
new type of joint; the muscles are concentrated near the body.
The front teeth are now chopping incisors, and the grinding teeth
approach those of the modern horse in the distribution of the
enamel, dentine, and cement. They are now about the size of a
donkey, and must have had a distinctly horsy appearance, with
their long necks and heads and tapering limbs. One of them,
Merychippus, was probably in the direct line of the evolution of
the horse. From Hipparion some of the authorities believe that
the zebras may have been developed. Miohippus, Protohippus, and
Hypohippus, varying in size from that of a sheep to that of a
donkey, are other branches of this spreading family.
In the Pliocene period the evolution of the main stem culminates
in the appearance of the horse, and the collateral branches are
destroyed. Pliohippus is a further intermediate form. It has only
one toe on each foot, with two large splint bones, but its hoof
is less round than that of the horse, and it differs in the shape
of the skull and the length of the teeth. The true horse (Equus)
at length appears, in Europe and America, before the close of the
Tertiary period. As is well known, it still has the rudimentary
traces of its second and fourth toes in the shape of splint
bones, and these bones are not only more definitely toe-shaped in
the foal before birth, but are occasionally developed and give us
a three-toed horse.
From these successive remains we can confidently picture the
evolution, during two or three million years, of one of our most
familiar mammals. It must not, of course, be supposed that these
fossil remains all represent "ancestors of the horse." In some
cases they may very well do so; in others, as we saw, they
represent sidebranches of the family which have become extinct.
But even such successive forms as the Eohippus, Mesohippus,
Miohippus, and Pliohippus must not be arranged in a direct line
as the pedigree of the horse. The family became most extensive in
the Miocene, and we must regard the casual fossil specimens we
have discovered as illustrations of the various phases in the
development of the horse from the primitive Ungulate. When we
recollect what we saw in an earlier chapter about the evolution
of grassy plains and the successive rises of the land during the
Tertiary period, and when we reflect on the simultaneous advance
of the carnivores, we can without difficulty realise this
evolution of our familiar companion from a hyrax-like little
animal of two million years ago.
We have not in many cases so rich a collection of intermediate
forms as in the case of the horse, but our fossil mammals are
numerous enough to suggest a similar development of all the
mammals of to-day. The primitive family which gave birth to the
horse also gave us, as we saw, the tapir and the rhinoceros. We
find ancestral tapirs in Europe and America during the Tertiary
period, but the later cold has driven them to the warm swamps of
Brazil and Malaysia. The rhinoceros has had a long and
interesting history. From the primitive Hyrochinus of the Eocene,
in which it is dimly foreshadowed, we pass to a large and varied
family in the later periods. In the Oligocene it spreads into
three great branches, adapted, respectively, to life on the
elevated lands, the lowlands, and the water. The upland type
(Hyracodon) was a light-limbed running animal, well illustrating
the close relation to the horse. The aquatic representative
(Metamynodon) was a stumpy and bulky animal. The intermediate
lowland type was probably the ancestor of the modern animal. All
three forms were yet hornless. In the Miocene the lowland type
(Leptaceratherium, Aceratherium, etc.) develops vigorously, while
the other branches die. The European types now have two horns,
and in one of the American species (Diceratherium) we see a
commencement of the horny growths from the skull. We shall see
later that the rhinoceros continued in Europe even during the
severe conditions of the glacial period, in a branch that
developed a woolly coat.
There were also in the early Tertiary several sidebranches of the
horse-tapir-rhinoceros family. The Palaeotheres were more or less
between the horse and the tapir in structure; the Anoplotheres
between the tapir and the ruminant. A third doomed branch, the
Titanotheres, flourished vigorously for a time, and begot some
strange and monstrous forms (Brontops, Titanops, etc.). In the
larger specimens the body was about fourteen feet long, and stood
ten feet from the ground. The long, low skull had a pair of horns
over the snout. They perished like the equally powerful but
equally sluggish and stupid Deinocerata. The Tertiary was an age
of brain rather than of brawn. As compared with their early
Tertiary representatives' some of our modern mammals have
increased seven or eight-fold in brain-capacity.
While the horses and tapirs and rhinoceroses were being gradually
evolved from the primitive types, the Artiodactyl branch of the
Ungulates--the pigs, deer, oxen, etc.--were also developing. We
must dismiss them briefly. We saw that the primitive herbivores
divided early in the Eocene into the "odd-toed" and "even-toed"
varieties; the name refers, it will be remembered, not to the
number of toes, but to the axis of stress. The Artiodactyl group
must have quickly branched in turn, as we find very primitive
hogs and camels before the end of the Eocene. The first hog-like
creature (Homacodon) was much smaller than the hog of to-day, and
had strong canine teeth, but in the Oligocene the family gave
rise to a large and numerous race, the Elotheres. These
"giant-pigs," as they have been called, with two toes on each
foot, flourished vigorously for a time in Europe and America, but
were extinguished in the Miocene, when the true pigs made their
appearance. Another doomed race of the time is represented by the
Hyopotamus, an animal between the pig and the hippopotamus; and
the Oreodontids, between the hog and the deer, were another
unsuccessful branch of the early race. The hippopotamus itself
was widespread in Europe, and a familiar form in the rivers of
Britain, in the latter part of the Tertiary.
The camel seems to be traceable to a group of primitive North
American Ungulates (Paebrotherium, etc.) in the later Eocene
period. The Paebrotherium, a small animal about two feet long, is
followed by Pliauchenia, which points toward the llamas and
vicunas, and Procamelus, which clearly foreshadows the true
camel. In the Pliocene the one branch went southward, to develop
into the llamas and vicunas, and the other branch crossed to
Asia, to develop into the camels. Since that time they have had
no descendants in North America.
The primitive giraffe appears suddenly in the later Tertiary
deposits of Europe and Asia. The evidence points to an invasion
from Africa, and, as the region of development is unknown and
unexplored, the evolution of the giraffe remains a matter of
speculation. Chevrotains flourished in Europe and North America
in the Oligocene, and are still very primitive in structure,
combining features of the hog and the ruminants. Primitive deer
and oxen begin in the Miocene, and seem to have an earlier
representative in certain American animals (Protoceras), of which
the male has a pair of blunt outgrowths between the ears. The
first true deer are hornless (like the primitive muskdeer of Asia
to-day), but by the middle of the Miocene the males have small
two-pronged antlers, and as the period proceeds three or four
more prongs are added. It is some confirmation of the
evolutionary embryonic law that we find the antlers developing in
this way in the individual stag to-day. A very curious race of
ruminants in the later Tertiary was a large antelope
(Sivatherium) with four horns. It had not only the dimensions,
but apparently some of the characters, of an elephant.
The elephant itself, the last type of the Ungulates, has a
clearer line of developments. A chance discovery of fossils in
the Fayum district in Egypt led Dr. C. W. Andrews to make a
special exploration, and on the remains which he found he has
constructed a remarkable story of the evolution of the elephant.*
It is clear that the elephant was developed in Africa, and a
sufficiently complete series of remains has been found to give a
good idea of the origin of its most distinctive features. In the
Eocene period there lived in the Egyptian region an animal,
something like the tapir in size and appearance, which had its
second incisors developed into small tusks and--to judge from the
nasal opening in the skull--a somewhat prolonged snout. This
animal (Moeritherium) only differed from the ordinary primitive
Ungulate in these incipient elephantine features. In the later
Eocene a larger and more advanced animal, the Palaeomastodon,
makes its appearance. Its tusks are larger (five or six inches
long), its molars more elephantine, the air-cells at the back of
the head more developed. It would look like a small elephant,
except that it had a long snout, instead of a flexible trunk, and
a projecting lower jaw on which the snout rested.
*See this short account, "Guide to the Elephants in the British
Museum," 1908.
Up to the beginning of the Miocene, Africa was, as we saw, cut
off from Europe and Asia by the sea which stretched from Spain to
India. Then the land rose, and the elephant passed by the new
tracts into the north. Its next representative, Tetrabelodon, is
found in Asia and Europe, as well as North Africa. The frame is
as large as that of a medium-sized elephant, and the increase of
the air-cells at the back of the skull shows that an increased
weight has to be sustained by the muscles of the neck. The
nostrils are shifted further back. The tusks are from twenty to
thirty inches long, and round, and only differ from those of the
elephant in curving slightly downward, The chin projects as far
as the tusks. The neck is shorter and thicker, and, as the animal
increases in height, we can understand that the long
snout--possibly prehensile at its lower end--is necessary for the
animal to reach the ground. But the snout still lies on the
projecting lower jaw, and is not a trunk. Passing over the many
collateral branches, which diverge in various directions, we next
kind that the chin is shortening (in Tetrabelodon longirostris),
and, through a long series of discovered intermediate forms, we
trace the evolution of the elephant from the mastodon. The long
supporting skin disappears, and the enormous snout becomes a
flexible trunk. Southern Asia seems to have been the province of
this final transformation, and we have remains of some of these
primitive elephants with tusks nine and a half feet long. A later
species, which wandered over Central and Southern Europe before
the close of the Tertiary, stood fifteen feet high at the
shoulder, while the mammoth, which superseded it in the days of
early man, had at times tusks more than ten feet in length.
It is interesting to reflect that this light on the evolution of
one of our most specialised mammals is due to the chance opening
of the soil in an obscure African region. It suggests to us that
as geological exploration is extended, many similar discoveries
may be made. The slenderness of the geological record is a defect
that the future may considerably modify.
From this summary review of the evolution of the Ungulates we
must now pass to an even briefer account of the evolution of the
Carnivores. The evidence is less abundant, but the characters of
the Carnivores consist so obviously of adaptations to their
habits and diet that we have little difficulty in imagining their
evolution. Their early Eocene ancestors, the Creodonts, gave rise
in the Eocene to forms which we may regard as the forerunners of
the cat-family and dog-family, to which most of our familiar
Carnivores belong. Patriofelis, the "patriarchal cat," about five
or six feet in length (without the tail), curiously combines the
features of the cat and the seal-family. Cyonodon has a wolf-like
appearance, and Amphicyon rather suggests the fox. Primitive
weasels, civets, and hyaenas appear also in the Eocene. The
various branches of the Carnivore family are already roughly
represented, but it is an age of close relationships and
generalised characters.
In the Miocene we find the various groups diverging still further
from each other and from the extinct stocks. Definite wolves and
foxes abound in America, and the bear, civet, and hyaena are
represented in Europe, together with vague otter-like forms. The
dog-family seems to have developed chiefly in North America. As
in the case of the Ungulates, we find many strange side-branches
which flourished for a time, but are unknown to-day. Machoerodus,
usually known as "the sabre-toothed tiger," though not a tiger,
was one of the most formidable of these transitory races. Its
upper canine teeth (the "sabres") were several inches in length,
and it had enormously distensible jaws to make them effective.
The great development of such animals, with large numbers of
hyaenas, civets, wolves, bears, and other Carnivores, in the
middle and later Tertiary was probably the most effective agency
in the evolution of the horse and deer and the extinction of the
more sluggish races. The aquatic branch of the Carnivores (seals,
walruses, etc.) is little represented in the Tertiary record. We
saw, however, that the most primitive representatives of the
elephant-stock had also some characters of the seal, and it is
thought that the two had a common origin.
The Moeritherium was a marsh-animal, and may very well have been
cousin to the branch of the family which pushed on to the seas,
and developed its fore limbs into paddles.
The Rodents are represented in primitive form early in the Eocene
period. The teeth are just beginning to show the characteristic
modification for gnawing. A large branch of the family, the
Tillodonts, attained some importance a little later. They are
described as combining the head and claws of a bear with the
teeth of a rodent and the general characters of an ungulate. In
the Oligocene we find primitive squirrels, beavers, rabbits, and
mice. The Insectivores also developed some of the present types
at an early date, and have since proved so unprogressive that
some regard them as the stock from which all the placental
mammals have arisen.
The Cetacea (whales, porpoises, etc.) are already represented in
the Eocene by a primitive whale-like animal (Zeuglodon) of
unknown origin. Some specimens of it are seventy feet in length.
It has large teeth, sometimes six inches long, and is clearly a
terrestrial mammal that has returned to the waters. Some forms
even of the modern whale develop rudimentary teeth, and in all
forms the bony structure of the fore limbs and degenerate relic
of a pelvis and back limbs plainly tell of the terrestrial
origin. Dolphins appear in the Miocene.
Finally, the Edentates (sloths, anteaters, and armadilloes) are
represented in a very primitive form in the early Eocene. They
are then barely distinguishable from the Condylarthra and
Creodonta, and seem only recently to have issued from a common
ancestor with those groups. In the course of the Tertiary we find
them--especially in South America, which was cut off from the
North and its invading Carnivores during the Eocene and
Miocene--developed into large sloths, armadilloes, and anteaters.
The reconnection with North America in the Pliocene allowed the
northern animals to descend, but gigantic sloths (Megatherium)
and armadilloes (Glyptodon) flourished long afterwards in South
America. The Megatherium attained a length of eighteen feet in
one specimen discovered, and the Glyptodon often had a dorsal
shield (like that of the armadillo) from six to eight feet long,
and, in addition, a stoutly armoured tail several feet long.
The richness and rapidity of the mammalian development in the
Tertiary, of which this condensed survey will convey some
impression, make it impossible to do more here than glance over
the vast field and indicate the better-known connections. It will
be seen that evolution not only introduces a lucid order and
arrangement into our thousands of species of living and fossil
mammals, but throws an admirable light on the higher animal world
of our time. The various orders into which the zoologist puts our
mammals are seen to be the branches of a living tree, approaching
more and more closely to each other in early Tertiary times, in
spite of the imperfectness of the geological record. We at last
trace these diverging lines to a few very primitive, generalised,
patriarchal groups, which in turn approach each other very
closely in structure, and plainly suggest a common Cretaceous
ancestor. Whether that common ancestor was an Edentate, an
Insectivore, or Creodont, or something more primitive than them
all, is disputed. But the divergence of nearly all the lines of
our mammal world from those patriarchal types is admirably clear.
In the mutual struggle of carnivore and herbivore, in adaptation
to a hundred different environments (the water, the land, and the
air, the tree, the open plain, the underground, the marsh, etc.)
and forms of diet, we find the descendants of these patriarchal
animals gradually developing their distinctive characters. Then
we find the destructive agencies of living and inorganic nature
blotting out type after type, and the living things that spread
over the land in the later Tertiary are found to be broadly
identical with the living things of to-day. The last great
selection, the northern Ice-Age, will give the last touches of
modernisation.
Prev
Next
All
Your email address is safe with us. View our Privacy policy.
 |
 |
|
|
Category: Plays Sections: 12 What's this? Table of Contents |
 FictionShort StoriesPoetryPlaysSci FiPhilosophyReligionBiography |
FIND JOBS
