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Chapter 6
Development Among Mammals
ONLY a portion of the mammal species that existed a few million years ago have
survived to this day. Yet the species that have survived are so numerous that any
adequate history of their development would require several volumes. The various
stages of progress of some kinds are as yet unknown; but there are a vast number
which have left their bones at successive stages of development where they were
covered by mud, sand, dust, or volcanic debris, to become fossils.
These constitute an indelible record by which modern mammals may be traced back
unerringly through their genealogical chain to more primitive stock; link after link
being revealed as new remains are uncovered, until the chain disappears in the
primitive creatures of the Triassic period some 170 million years ago. Here,
however, limitations of space will permit me to mention, and most briefly, the
progress of only a few forms. And for that purpose I have selected a dozen species of
mammals with which everyone is somewhat familiar, for they can be seen at any time
on the farm or in the zoo: the dog, lion, elephant, horse, camel, ape, rhinoceros,
porcupine, bear, pig, deer and bison.
It should be understood that only a few small and insignificant primitive mammals
existed up to the commencement of the Cenozoic era about 55 million years ago.
These minute creatures were the forerunners of modern marsupials. Their remains
are found in the Triassic of North Carolina. These Allothera, as they are called, are
also found in the Triassic of Europe and the Jurassic of South Africa.
That they must have had a terrible time trying to survive is indicated by the record
that for 115 million years, more than twice as long as since their enemies, the
dinosaurs, disappeared and gave them a chance really to progress, they remained
little, sly creatures which were constantly on the dodge to keep from becoming food
for the big lizards that dominated the world. But as soon as the dinosaurs disappeared
they started rapid progress. And it may be assumed that the conditions under which
they lived, with their lives in danger every moment, previous to this time had
sharpened their wits and given them considerable intelligence. The dinosaurs, on the
other hand, are noted for the minuteness of their brain in comparison to their brawn.
Even before the extinction of the dinosaurs, the little ancestors of modern mammals
had made some progress, for in the Cretaceous period of about 90 million years ago
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there are found the fossils of numerous minute marsupials which had developed from
the older stock. And apparently contemporaneous with them, as indicated by their
fossils, were also the first placental mammals, the undoubted ancestors of all modern
forms. But all of both types, until the dinosaurs vanished, were constantly forced to
hide, and given no opportunity to develop size.
The mammals of today all are of the placental type, with the exception of the
opossums of both Americas, the Coenolestes of Ecuador and Colombia, and certain
primitive types which live in Australia.
In the Triassic of South Africa a group of reptiles has been found which very closely
approaches the form of the most primitive mammals. Three such primitive
mammals, which have made little progress above the mentioned advanced reptiles,
survive in Australia even to the present time. They are the duckmole, the spiny
anteater, and the proechidna, which is an anteater without a vernacular name.
These mammals have pouches in which, like the marsupials, they place their young.
But as do the reptiles, and unlike the marsupials, they lay eggs. They have never
reached the stage of caring for their young by retaining them in their body until they
are mature enough to make the hazards of external life less dangerous. The eggs they
lay are large and contain much yolk. In the case of the duckmole, after the eggs hatch
there are no teats for the young to suck, but the milk oozes out of numerous pores on
the front of the mother's body and is licked off by the young. In the case of the
anteater the egg is placed in a temporarily developed pouch and when it hatches milk
oozes out into the pouch.
These mammals belong to the group lower than the marsupials. They are known as
Monotremes. They seem to represent the transition stage between lizards and
placental mammals. It appears that the desire to protect their young from the rigors of
difficult environment had stimulated extrasensory perception and psychokinesis,
and that psychokinesis guided by extrasensory perception had actually taken a long
step to attain this desire; for, unlike the young of reptiles, their young are provided
nourishment, the forerunner of true milk, until they are husky enough to forage for
themselves. But as yet they have not provided the fuller protection that the placental
mammals have devised for their offspring.
Neither have the marsupials, although they have made a decided advance in this
respect over the monotremes. They no longer lay eggs, but give birth to their young.
But unlike higher orders of mammals, the young are born prematurely, so helpless in
fact that they cannot even suck. Thus the marsupials have only partially solved the
problem. The mother takes the young one in her mouth and places it in the skin pouch
and adjusts its mouth over the teat and then injects the milk down its gullet. A special
arrangement of the glottis in the young enables it to breathe while milk is thus being
pumped down it, without danger of strangling.
The kangaroo is the most familiar marsupial of today. With the exception of the
dingo--a dog presumably reaching there through a human agency--all the
mammals of Australia are monotremes or marsupials. This is due to the fact that
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Australia was cut off from the balance of the world before the placental mammals
developed. In Australia there are marsupials that have solved most of the other
problems solved by various species of more developed mammals. Some are similar
to squirrels, some to our wolves, some to our hoofed animals, some to our burrowers,
etc. Being isolated from the rest of the world, the development of mammals in
Australia and the adjoining islands has been along independent lines. The other
continents having repeatedly been connected by land since placental mammals came
into existence, have forms of life with much more in common.
That marsupial mammals and placental mammals solved the problem of adaptation
to similar environmental conditions along almost identical lines, even though the
problem could have been solved in different ways, was probably as much due to
extrasensory perception and the inner plane weather favoring this particular solution,
as to the outer plane conditions.
The Placental Mammals
--The placental mammals, to which group man belongs, made a distinct advance
over the marsupials. Premature young are under a great handicap. The death rate due
to enemies and climatic conditions is high. It is a decided advantage that the young
shall have reached an advanced stage of growth at birth, and thus soon be able to
endure severe weather and follow the mother about, or perhaps remain hidden by her
without perishing.
This desire for greater protection for the young brought psychokinesis into play in
the development of the placenta; a tissue that links the unborn young and the mother
in a prolonged partnership. By means of this tissue the blood vessels of the young are
brought into close contact with the blood vessels of the mother, and thus absorb from
her dissolved nutritive matter, oxygen, water, and the necessary salts; also giving to
her in return the dissolved waste products. One of the most important functions in
this long sleep of the unborn is that it enables it to be born with a well developed
brain.
Even to glimpse the development of the various present day species of mammals
from more ancient and primitive types it becomes necessary to know something of
the interrelations of land areas throughout the world at the times when important
mammalian modifications were made. I will, therefore, make brief mention of the
changes in land areas that took place at different times. Due to shrinkage of the earth,
and to the impact of inner plane weather, land areas rise and sink. There are periods in
which portions of continents sink below the sea, islands and other land areas
disappear, and land bridges that previously have connected continents subside
leaving no way open for long lapses of time by which land creatures or plants from
one continent can find their way to another.
There are various ways of accurately determining the existing land areas at any
period of the past. For instance, the shell fish and other marine forms that are able to
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migrate only along a coast line and never across deep bodies of water, are entirely
different along the warm Asiatic coast near Japan from those occupying the coast
along the Arctic Ocean. When land rises across Bering Strait, making a complete
land bridge between Asia and America, the Arctic waters no longer flow southward
along the west American coast. Instead, the waters are warmed by the Japanese
Current, and the shore life from the Japanese region spreads along the continuous
shore line following the land bridge, and is found all along it and well down into
California.
When this land bridge subsides, and instead of leaving a small shallow strait, as
today, the Arctic Ocean is given free access to the North Pacific, these cold waters
exterminate the Asiatic form of coast life that are suited to warm waters, and we find
the fossils of the purely Arctic types. There is then no longer a route of migration
open between Asia and North America, and the shell fish of the Arctic follow the cold
waters well south along the western coast of North America. Thus, without taking the
space to go into detail, it will be understood that when I speak of land connections
between continents being made or broken, or of land areas being raised or lowered, I
am not referring to tradition, but making the assertion upon sound and fully accepted
scientific opinion.
In the Cretaceous period, something more than 55 million years ago, it is quite certain
that all the continents of the world were connected by land; for dinosaurs, which were
strictly land animals, have left their fossils in the non marine Cretaceous rocks of
every continent. Because there were both marsupial and placental mammals during
this period, it is possible that the latter also found their way to all the continents,
although no fossils of placental mammals belonging to this period have thus far been
found in Australia or South America. South America was then connected with
Australia by land across Antarctica, which then had a mild climate; and there is
strong evidence that this connection, and a land connection between Brazil and West
Africa, existed into Basal Eocene times.
Eocene Period
--The first period of the Cenozoic era (meaning era of modern life) was the Eocene,
which commenced about 55 million years ago. It is usually divided into two sections,
the lower being called Basal Eocene, and the upper merely Eocene. In Basal Eocene
times both Europe and Asia were well connected by land at the north with North
America. South America was then cut off from North America, and continued thus
cut off well into the Cenozoic era, the Pacific and Atlantic being connected across
what is now Central America. The Culebra Cut of Panama, through formation
belonging to the Eocene period, reveals marine shells which would not have been
present if it had then been a land area.
The climate during the previous period, the Cretaceous, was very warm even into the
Arctic regions; warm climate plants being found in Greenland and Alaska. In Eocene
times it was somewhat cooler, but still genial, as shown by the innumerable remains
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of great crocodiles and large palm trees in Wyoming and Idaho. North America in
Basal Eocene extended further to the east than today, and Florida and the North Gulf
Coast were submerged; but otherwise this continent was in shape very much as now.
The Appalachian Mountains, older than the western ranges, were already worn down
almost to a plain with a few peaks sticking up, as peaks thus stick up at present in
large areas of Texas, the highest being in North Carolina. The Rocky Mountains and
the Sierra were in existence, thought much lower than today, and the eastern coast
and the interior were vast plains. The continent was largely forested; willows,
poplars, sycamores, oaks, and other modern trees being mingled with conifers.
England was temperate in climate.
Lower Eocene rocks of northwestern New Mexico, and Upper Eocene rocks of
eastern Wyoming, South Dakota, and Montana, have yielded numerous fossil
mammals; but for the most part they are types that later became extinct, leaving no
descendants. All are small, no mammal of say 50 million years ago as large as a sheep
ever having been found. The ancestors of all modern mammals, then very small and
primitive, had their origin probably in Cretaceous times on land well toward the
North Pole, according to Wortman as a result of his studies of the fossils at Yale.
From thence, as the then sub tropical climate cooled, they were forced south into
Europe and North America, where they displaced more primitive forms that were
less adapted to environment than are those of today.
In Eocene times proper, there was a submergence of the Atlantic Coast and Gulf
Coast of North America, and the Gulf extended up the Mississippi Valley as a long
arm of the sea. On the Pacific another long, narrow arm of the sea extended up the
great valley of California to Oregon and Washington. The Mediterranean covered
most of Southern Europe, the Pyrenees, Alps, and such great ranges not yet having
been lifted. Europe was completely separated from Asia by a strait east of the Ural
Mountains joining the Mediterranean with the Arctic Ocean. America and Europe
were well joined by land, and the wave of mammal migration coming down into both
continents from the north gave them mammals that were more nearly identical than at
any time since. This is particularly true of the lowest formation of the Eocene proper,
where innumerable fossil mammals have been found. Before the close of the Eocene,
as shown by the Uinta formation in Green River Valley, northeastern Utah and
northwestern Colorado, the mammals had become totally different from those of
Europe; for the land connecting these two continents had sunk below the sea.
In Basal Eocene the carnivorous mammals were represented by the creodonts;
nothing similar to the dog family or the cat family yet having evolved. These
creodonts were flesh eating mammals, with teeth very much like cats, but with heavy
tails and blunt claws. The first known member of the dog family in North America is
a very small fox like creature living about 40 million years ago found in Upper
Eocene. It is believed to be a direct descendant from the primitive creodonts. It also
appeared in Europe at about the same date. Furthermore, about this period there is
found in the Libyan Desert of Egypt an animal about three and one half feet high. It is
the Moeritherium which, although having no proboscis as yet, shows by the nasal
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openings the beginning of the development which finally supplied the elephant with
a trunk. Fortunately, the intermediate links between this short faced creature and the
elephant of today, to which it is the ancestor, have been found; showing a gradual
development in size and length of proboscis.
The horse, on the other hand, had its origin, and most of its development in America.
Eohippus,--the dawn horse, is found in the very earliest Eocenes laid down about 55
million years ago, in Wyoming and New Mexico. It was a small graceful creature
twelve inches high at the withers, with an arched back. It had much the proportions of
a fox terrier, except that the feet were already beginning to be modified from toes into
hoofs. The hand had four complete toes, each with a hoof like nail. The foot had three
such toes, although a splint shows where its ancestor had another toe which had
atrophied. Animals which require speed run on their toes, not flat footed. This lifts
the outer toes on either side from the ground somewhat, tending to place the weight
on the middle and longer toe. Thus the middle toes become stronger and better
adapted to carrying the weight, and fortified against the impact of the ground; while
the outer toes, not getting much use, fail to grow.
In Middle Eocene, about 10 million years later, is found in Wyoming and New
Mexico, Orohippus, the mountain horse. By that time this ancestral horse had
developed in size to thirteen and one half inches tall. The splint remnant of the fourth
toe had been entirely lost, the outer finger of the hand had been shortened, and the
teeth had been modified. In Upper Eocene, another 5 million years later, is found
Epihippus, still larger than Orohippus, yet with three toes behind and four in front,
but with the middle toe of each front foot becoming quite dominant. From the
vestigial structures of these early horses it is quite certain that they descended, as did
all present day mammals, from a five toed ancestor. The five toed horse, however,
while frequently mentioned, so far has not been found.
Strange as it may seem to some, North America is the original home of the camel,
where he remained and developed for some 35 million years, only migrating to the
Old World in Pliocene times. Before the Upper Eocene the camel is not to be
distinguished from other small primitive mammals. But in Upper Eocene
distinguishing characteristics were far enough advanced that in Protylopus, a
creature the size of a jack rabbit in the Uinta formation, we can recognize the ancestor
of modern camels, the intermediate steps being represented by a very complete series
of fossils.
The Primate mammals, to which apes, monkeys, and man belong, there is much
evidence to show, developed from the same small insect eating mammals of the
Cretaceous period from which the carnivorous mammals developed. The
descendants of this primitive Cretaceous insect eater diverged into two branches:
those which were strictly flesh eating became the carnivorous mammals, and those
which adopted a strict diet of nuts and fruit which took them into the trees, developed
into the Primates. The remains of these first Primates are found in the Lower Eocene
of both Europe and America, and the transitional forms between the insect eaters and
the Lemurs are found in North American Basal Eocene of about 50 million years ago.
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The first recognized Primate, such as Pelycodus, is found in the Wasatch formation
of the Great Basin, some 45 million years old. In Upper Eocene formation of the
Green River Valley, the Lemur, Northarctus, is abundant These lemurs, so abundant
in the northern hemisphere during the Eocene, have since that time been found only
in Madagascar, tropical Africa, and southern Asia, where very similar lemurs still
exist at present.
The earliest tapir is a mammal the size of a coyote, found in the Wasatch formation of
the Eocene of the Great Basin of North America, of about 45 million years ago. Most
of the development of the tapir was accomplished in North America. It migrated into
Europe during the Oligocene period, about 25 million years ago, but only reached
South America where alone it exists as a much larger animal today, in Pleistocene
times, less than a million years ago.
Found in the Lower Eocene of North America, of about 50 million years ago, is a
small animal that in the course of several million years developed into a huge
creature with two horns on its nose, resembling in many respects the rhinoceros. This
creature, the Titanotheres, very numerous at one time, is not the ancestor of the
rhinoceros; for it became extinct. Why it died out no one knows; perhaps from
disease, as there was no predatory animal at that time large enough to menace it. It is
believed, however, that the true rhinoceroses had their origin and much of their
development in North America. The earliest form is a small fleet mammal without
the characteristic horn developed, from the Wind River formation of the Eocene
period, some 45 million years old. From this developed a number of distinct forms,
increasing in size as time rolled on, and finally becoming extinct in America in lower
Pliocene, about three and one half million years ago.
Those animals which have an even number of toes like the ox, deer and pig, are called
artiodactyls. The oldest members of this group so far found are from the Lower
Eocene of the Great Basin. One of these little creatures the size of a rabbit is the
ancestor of such mammals as chew the cud, as do sheep, bison and cattle. Another,
somewhat larger, is the ancestor of the pig. Most of the development of the
artiodactyls took place in the old world, to which they migrated very early.
Oligocene Period
--The second period of the Cenozoic era is the Oligocene, commencing about 35
million years ago. It was marked by the rising of a land bridge between America and
Europe, across which mammals migrated both ways. This is shown by the close
similarity of the mammals found in the White River deposits of the early half of this
period in northeast Colorado, western Nebraska, eastern Wyoming and South
Dakota, with those of Europe. The American climate was warmer than at present,
although the gradual cooling that finally in the Pleistocene brought a glacial age set in
during the second half of the period. In Europe the climate was sub tropical, being
even warmer than Eocene times; palms growing in Germany. The latter half of the
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Oligocene witnessed a great change; for the land bridge between Europe and
America again subsided.
As a consequence the mammals of the two continents, as shown by the John Day
deposits of eastern Oregon, being isolated from each other, and not able through
interbreeding to disseminate to each other their developed characters, developed
each along lines that were demanded by peculiarities of climate. Hence, while early
Oligocene deposits reveal mammals in America and Europe that are almost
identical, by late Oligocene the mammals of the two continents had so developed
along divergent lines as to be quite dissimilar. Fortunately, Oligocene formation,
both early and late, yield vast quantities of fossils.
In North American Oligocene formation there have been found many different kinds
of primitive cats, some of which undoubtedly were the ancestors of modern lions,
cats, leopards, etc.; but none of which had developed into a near likeness of these.
The claws, however, in some species had become well developed, and two broad
groups were emerging, one with teeth gradually becoming more and more like those
of present day species, and the other with the upper canine teeth greatly elongated.
These became the sabre toothed group, the most terrible cats of all time, and the
dominant beasts of prey down to less than a million years ago.
In Oligocene times the dog group had quite distinctly separated from the cat group,
although both certainly had a common ancestor in the creodonts of the previous
period. The dog, represented by several species of Daphaenus, the largest dog of that
time, was hardly larger than a coyote. The backbone greatly resembled that of the
cats, and the claws were somewhat retractile, which is a cat feature. The teeth were
small and had only partly developed the shearing edges characteristic of modern
species, but the skull was dog like, rather than like that of the cats.
In both Africa and India the descendants of the more primitive elephant of the
preceding period had developed in size, in the height of the skull, and in the length of
the proboscis. As found in Oligocene deposits its lower jaw had elongated and short
tusks had developed both in the upper jaw and in the lower jaw. It, therefore, had four
tusks, instead of two as in modern species. This Oligocene ancestor of the modern
elephant is called Palaeomastodon.
In North America, in the lower Oligocene formation is found Mesohippus, a horse
which now had developed to the size of a coyote, and in somewhat later formation to
the size of a sheep. It no longer had four toes in front, as did its Eocene ancestor, but
had three toes in front and three behind, the middle toe having developed to much
greater strength and the missing toe being represented by a splint. In Upper
Oligocene, representing several million years later, is found Miohippus, which is
very much like its ancestor, Mesohippus, except that it is much larger.
The camel of the lower Oligocene, about 35 million years ago, as shown by the White
River deposits of North America, had developed to the size of a sheep, although
more slender and fleet of foot. In the John Day deposits of eastern Oregon,
representing Upper Oligocene some 15 million years later, the camels had
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developed into several branches: the giraffe camels, the gazelle camels, and the two
other groups which ultimately became the llamas and the true camels.
The Primates--monkeys, apes, etc.--became extinct in North America at the close
of the Eocene period, although they existed in South America, and still continue
there. They also became extinct in Europe during the Oligocene period, continuing
their development in Southern Asia and Africa. The transitional form between
Northarctus of the Eocene and the man like apes was discovered in the Oligocene of
Egypt. It is Propliopithecus, which lived about 30 million years ago, and greatly
resembles the gibbons of the present day.
There are a great many fossils of primitive rhinoceroses in the White River beds of
North American Oligocene. There were three different groups, one of which was
light and fleet of foot, another which was massive and slow, and still another which
was the ancestor of modern species. Some of these had horns, and others had none,
but they were larger than their Eocene ancestors, although not nearly so large as
rhinoceroses at present. Tapirs also occur, larger and with longer proboscises than
previously.
In this John Day formation, belonging to the Oligocene period, are also found
primitive peccaries, and primitive giant pigs. There are found in this formation,
likewise, innumerable small hoofed mammals that were developing into deer like,
ox like, and sheep like forms. The ox, nevertheless, mostly developed in Europe.
Miocene Period
--The third period of the Cenozoic era is the Miocene period, commencing about 19
million years ago. The Atlantic Coast and Gulf region were again submerged, a cool
current from the north driving out tropical forms along the eastern coast of America
and replacing them with northern forms. On the west coast the sea again extended up
the California Valley, leaving the tops of the Coast Range as small islands, Miocene
formations in California being as much as from 5 thousand to 7 thousand feet thick.
There was a good connection between America and Europe, but the bridge across
Bering Sea was broken. Also, for the first time in the Cenozoic era, early in Miocene
the principal elevation of the Coast Range took place, and the Sierra and Plateaus of
Arizona and Utah were elevated higher than previously. It was warmer than at
present, although not warm enough for crocodiles to be found far north. It was even
warmer in Europe, due to the continent being broken and intersected by warm arms
of the sea, the climate being like that of India. It was also a great period of mountain
making in Europe.
This period is chiefly notable for the marked development both in size and number of
the mammals. Many kinds of mammals in the Miocene period grew to much greater
size than their present day representatives, and there were far more genera and
species than now. Practically all families of mammals now on earth are represented
by more primitive Miocene forms, and many groups then prevalent have since
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become extinct. It was the period of greatest mammalian abundance, there being
great numbers of hares, mice, pocket gophers, squirrels, marmots and beavers, as
well as numerous larger animals.
There were many kinds of dogs, some of which were as large as any existing bear;
truly formidable beasts. There were also true cats, developed from the more
primitive forms of the Oligocene period, but they were not so large as the lion and
tiger of today. The sabre tooth tigers were likewise present, having made notable
advances, but not yet so large as the huge beasts that became extinct at the close of the
ice age. The fossil remains of weasels, martins, otters, and raccoons have been found;
but these too are represented by more primitive animals than those bearing the same
name today.
The elephants of the preceding period had continued to increase in size, and had
developed along several lines. One, the Dinotherium, found in Europe and India, had
lost its upper tusks, and instead had a lower jaw that with its tusks bent abruptly
downward. The four tusked mastodons increased in size and in tusk development,
one genera, the Trilophodon found in France, not nearly so large as the Indian
Elephant, had an enormously lengthened lower jaw. It represents an intermediate
stage toward the development of the modern elephant. This was the first elephant to
reach America, coming by way of Asia in early Miocene times, about 18 million
years ago. The members of the four tusked mastodon group are called Tetrabelodons.
The considerable elevation of land areas and the growing aridity of North America
during Miocene times caused much of the forested regions to lose their trees and
shrubbery and become broad plains covered with coarse grasses. Animals, such as
the horses, that previously had fed upon the succulent browse of the trees, were
compelled to turn to the harsher and harder food found on the plains. Also, due to the
drying up of the rivers, it became necessary to travel immense distances for water.
The result of this was a pronounced modification in the teeth and in the feet. There
were a number of horses in Miocene times. They had increased in size to that of small
ponies. Most of them were three toed, although the outer toe scarcely touched the
ground, the middle toe bearing almost the whole weight, being much larger and a
well formed hoof.
Merrychippus, a horse of the Middle Miocene, about 15 million years ago, is the
intermediate link between the browsing type and the grazing type of horse. The milk
teeth in this species are short crowned and have little cement, as in previous horses;
but the permanent teeth are higher crowned and quite heavily cemented. In Upper
Miocene, developed from Merrychippus, is found Protohippus, still having three
toes, but with teeth much more similar to those of the modern horse.
The camel of Lower Miocene times, Protomoryx, had made a decided advance over
the camel of the previous period. For one thing, it is larger. All the teeth are present,
which is not the case with modern camels; and the two toes, instead of having
cushions as at present, are armed with sharp hoofs like those of a deer. The teeth also
indicate a change to make them suitable for grazing rather than for browsing. The
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first tooth reduction is shown in Procamelus, a descendant belonging to the Upper
Miocene, showing changes in the shape of the foot and various other desert
adaptations.
Descending from the ape like creatures of the preceding period, Sivapithecus, found
in India in Middle Miocene deposits some 15 million years old, while still an ape, has
numerous man like characteristics. From this common stock, about 18 million years
ago, the Orangutan branched off, and since that time has developed away from its
human characteristics. The Chimpanzee and the Gorilla branched from the common
stock in Middle Miocene, some 15 million years ago. Instead of developing human
characteristics, they developed other qualities, the Chimpanzee becoming less
human than previously, and the Gorilla retaining its structure about such man like
characteristics as were common to the original stock in Middle Miocene times. Since
Middle Miocene times the living apes have mostly been developing along lines
almost the opposite of those followed by the ancestors of man.
The bears are descended from the same ancestors as the dogs, the intermediate links
having been found. Nothing that had developed far enough to be called a bear is
present before late Miocene times. Its fossils are found in Europe, and it did not reach
America until the next period.
The history of the porcupines is also quite well known. They, however, underwent
their development in South America, where today no less than 6 families and 29
genera are known. Some are no larger than a rat, some are large; in fact the largest
living rodent, the Wart Hog, belongs to this group. Some live in trees, some in water,
and some in the ground, and some have long prehensile tails that they use as an extra
hand. It was not until Miocene times, when a land bridge was formed between North
and South America, that porcupines are found in North America, and then only a
single species, as now, represented by the present day Short Tailed Porcupine.
The rhinoceroses of Miocene times were abundant both in North America and in
Europe, and show considerable advance in size and structure over those of the
preceding period. The ox and most of the deer underwent their development chiefly
in the Old World, but the prong horn antelope and the Virginia deer have their
ancestors well represented in American Lower Miocene, and can be traced
accurately from that time to the present. Pigs were numerous in Miocene times, but
for some reason not clearly known, but which no doubt was influenced by inner plane
weather, the giant pigs, huge creatures as large as a horse, did not persist beyond the
Miocene but, together with many other mammals, became extinct.
Pliocene Period
--The fourth period of the Cenozoic era, the Pliocene period, commenced about 7
million years ago. It was cooler than the previous period, and a complete land
connection existed between North America and Asia across what is now Bering Sea,
giving a marine fauna on the Pacific Coast like that of Japan.
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Early in the Pliocene the true wolves developed, some of immense size, and by
Middle Pliocene, about 4 million years ago, the modern genus of dogs had become
established. Many cats are found in the formation, some of which are very large; both
such cats as became lions and tigers, and those of the sabre tooth group. Of the
elephant group of this period, now practically as large as modern genera, there was
one, Tetralophodon, found in Nebraska, with four tusks, and a lower jaw six feet
long. Another type, found over most of America, is Dibelodon, very much like a
mastodon except that its teeth were not so developed. The mastodons, differing from
the elephants in their teeth and in some other features, also were present in Pliocene
times; and in America the Imperial Elephant, that then roamed the hills about Los
Angeles and had a very wide range, was larger than any present day species.
The land bridge between America and Asia enabled horses and camels to migrate
from America to Asia, and enabled elephants, now becoming somewhat similar to
modern species, to migrate to America. The first of the modern horses is found both
in America and Asia in Upper Pliocene, about 2 million years old. The feet have one
toe each, but the splints are also prominent enough to be "dew claws." The camels of
this time are somewhat more advanced in structure than those of the previous period,
being represented by a mammal called Pliauchenia.
Bears reached America during this period, and the native pigs, called Platygonus,
were numerous and more highly developed in many respects than the modern
American descendant, the peccary. There was a great abundance of rhinoceroses,
four or five kinds native to America, and several others that migrated in Miocene
times from the Old World. Several different kinds of ox and sheep are also present
and the prong horn antelope and Virginia deer; but the moose, caribou and wapiti
developed in Europe and did not reach America until the next period.
Nothing has been found of the Primates in North America belonging to this Pliocene
period, with the exception of a single tooth found in Nebraska of a man like ape, or an
ape like man, called Hesperopithecus. In Bechuanaland, 80 miles north of
Kimberley, in South Africa, late in 1924, Dr. Raymond Dart, professor of anatomy at
the University of Witwatersrand discovered in a limestone cliff which at the time was
thought to date well back in the Pliocene, the remains of a man like ape, which he
called Australopithecus africanus (Southern man like ape of Africa). Some even yet
are inclined to place this find back as far as the beginning of the Pliocene, 7 million
years ago, basing their opinion chiefly on geological evidence. But in the 24 years
since he made this find Dr. Dart, and Dr. Broom, chief paleontologist of the
Transvaal Museum were able to find fossils representing possibly 15 individuals.
And chiefly because of their high intelligence, as they were using fire, Dr. Dart now
wishes he had named these Transvaal pygmies, which are believed by some to be the
ancestors of modern man, Homunculus (little man). And according to an article by
him in the Autumn, 1948, issue of American Journal of Anthropology, he places
them contemporaneous with the Men of Java, who lived at the commencement of the
Pleistocene period, about one million years ago.
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Pleistocene Period
--The fifth period of the Cenozoic era is the Pleistocene period which commenced
about one million years ago. Due to considerable elevation in land areas at the end of
the Pliocene period, the climate of the entire Northern Hemisphere was greatly
cooled, ushering in the glacial age. The Pleistocene is the age of ice during which the
ice sheet came down from the north reaching as far south as 40° latitude, and over
New York attaining a depth of 10,000 feet. During the 970,000 years of the
Pleistocene the ice came down no less than four distinct times, each time again
receding. Between these intrusions of ice there were long interglacial periods in
which the climate usually grew even warmer than it is now.
At the commencement of the Pleistocene period had there been a primitive explorer
roaming over America he would have found all the southern part of what is now the
United States covered with such spruce and pine as at present grow in Canada. Had
he gone north through this thick forest, he would have been stopped abruptly by such
a wall of ice as is now to be seen when approaching Antarctica. It extended from what
is now New York City to the State of Oregon, covering all of Canada and at least half
of the United States.
In addition to the elevation of land areas, climatic changes are also influenced by the
relation of the earth to the sun. Not only by the variation of the inclination of the
earth's axis to the ecliptic, but also by the distance of the earth from the sun. The
earth's orbit fluctuates considerably. It was elongated some 100,000 years ago, then
was almost circular about 50,000 years ago, became elongated again in 20,000 B.C.
and now is once more moving toward the circular. It will reach its nearest circular
shape in about 20,000 years. Except as influenced by terrestrial conditions, the
weather gets colder when the orbit elongates, and warmer when it is more circular.
Therefore, we are now moving toward a warmer climate which will be at its hottest
about 20,000 years from now, when it is expected tropical vegetation will grow on
the shores of the Arctic Ocean. But, according to George Gamow, professor of
physics at George Washington University, large masses of ice will again start
creeping down from the north, by about the year 50,000 A.D., as the fifth glacial
advance, and will completely cover all the cities of Canada and northern United
States.
The animals of the Pleistocene period, though different in size and in other respects
from those of the present, would readily be recognized as the general group to which
they belong if seen today. The American Mastodon ranged the forests, and of the true
elephants there were three species: the Mammoth, the Colombian Elephant, and the
Imperial Elephant. There were giant wolves, and there were true cats, some of large
size, as well as the Sabre Tooth Tiger, which was more massive than any living tiger.
There were camels that were considerably larger than those of the present. There
were also several species of true horses, with a single hoof on each foot, and there
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were bison that were much larger than any existing species. Moose, caribou and
wapiti had reached America from Europe, and there were huge cave bears in various
parts of the world. Furthermore, there were huge ground sloths, and armored
glyptodonts, and a number of other mammals that have since become extinct. But in
addition to these extinct forms there were those with which we are familiar, such as
the peccary, mink, weasel, martin, skunk, otter, badger, wolverine, raccoon, fox,
coyote, puma, etc., all present in America. And as there were rigorous climatic
changes, the competition between forms and the struggle to survive must have been
exceedingly intense.
Such a brief survey of the development of mammals is not altogether satisfactory, yet
at least I trust it shows conclusively that all existing forms have developed from more
primitive preexisting mammals. In many cases complete series of fossils have been
found showing the chief steps by which changes in structure were made, so that to
doubt their genealogy is to doubt the evidence of one's eyes.
A study of these fossils indicates that whenever members of a single species
separated over a long period of time with an impassable barrier between such as the
destruction of a land bridge between two continents, or in some cases by a desert, or a
broad river, or a mountain range; that the members of each region developed along
different lines. No longer able to commingle and breed, and thus impart developing
qualities each to the other; being in a different environment, they each develop
qualities called for by that environment. Both in structure and in habit they become
further and further apart, until they become distinct species.
We have witnessed the creation of breeds by artificial selection; for all the tame
pigeons--fantails, pouters, carriers, etc.--are known to be derived from the rock
pigeon; and all domestic chickens are known to be descendants of the Jungle Fowl of
India. There are instances, also, within the knowledge of man of new species being
the result of isolation. Rabbits, for instance, were turned loose by mariners on certain
islands off the southwest coast of Europe several hundred years ago. These rabbits,
although of the same stock as rabbits on the mainland, are today a totally different
species, quite incapable of breeding with the rabbits of the mainland.