THE TERTIARY
=The Cenozoic era.= The last stages of the Cretaceous are marked by a
decadence of the reptiles. By the end of that period the reptilian
forms characteristic of the time had become extinct one after another,
leaving to represent the class only the types of reptiles which
continue to modern times. The day of the ammonite and the belemnite
also now drew to a close, and only a few of these cephalopods were
left to survive the period. It is therefore at the close of the
Cretaceous that the line is drawn which marks the end of the Middle
Age of geology and the beginning of the Cenozoic era, the era of
modern life,--the Age of Mammals.
In place of the giant reptiles, mammals now become masters of the
land, appearing first in generalized types which, during the long ages
of the era, gradually evolve to higher forms, more specialized and
ever more closely resembling the mammals of the present. In the
atmosphere the flying dragons of the Mesozoic give place to birds and
bats. In the sea, whales, sharks, and teleost fishes of modern types
rule in the stead of huge swimming reptiles. The lower vertebrates,
the invertebrates of land and sea, and the plants of field and forest
take on a modern aspect, and differ little more from those of to-day
than the plants and animals of different countries now differ from one
another. From the beginning of the Cenozoic era until now there is a
steadily increasing number of species of animals and plants which have
continued to exist to the present time.
The Cenozoic era comprises two divisions,--the _Tertiary_ period and
the _Quaternary_ period.
In the early days of geology the formations of the entire geological
record, so far as it was then known, were divided into three
groups,--the _Primary_, the _Secondary_ (now known as the Mesozoic),
and the _Tertiary_, When the third group was subdivided into two
systems, the term Tertiary was retained for the first system of the
two, while the term _Quaternary_ was used to designate the second.
=Divisions of the Tertiary.= The formations of the Tertiary are
grouped in three divisions,--the _Pliocene_ (more recent), the
_Miocene_ (less recent), and the _Eocene_ (the dawn of the recent).
Each of these epochs is long and complex. Their various subdivisions
are distinguished each by its own peculiar organisms, and the changes
of physical geography recorded in their strata. In the rapid view
which we are compelled to take we can note only a few of the most
conspicuous events of the period.
=Physical geography of the Tertiary in eastern North America.= The
Tertiary rocks of eastern North America are marine deposits and occupy
the coastal lowlands of the Atlantic and Gulf states (Fig. 260). In
New England, Tertiary beds occur on the island of Martha's Vineyard,
but not on the mainland; hence the shore line here stood somewhat
farther out than now. From New Jersey southward the earliest Tertiary
sands and clays, still unconsolidated, leave only a narrow strip of
the edge of the Cretaceous between them and the Triassic and
crystalline rocks of the Piedmont oldland; hence the Atlantic shore
here stood farther in than now, and at the beginning of the period the
present coastal plain was continental delta. A broad belt of Tertiary
sea-laid limestones, sandstones, and shales surrounds the Gulf of
Mexico and extends northward up the Mississippi embayment to the mouth
of the Ohio River; hence the Gulf was then larger than at present, and
its waters reached in a broad bay far up the Mississippi valley.
Along the Atlantic coast the Mesozoic peneplain may be traced
shoreward to where it disappears from view beneath an unconformable
cover of early Tertiary marine strata. The beginning of the Tertiary
was therefore marked by a subsidence. The wide erosion surface which
at the close of the Mesozoic lay near sea level where the Appalachian
Mountains and their neighboring plateaus and uplands now stand was
lowered gently along its seaward edge beneath the Tertiary Atlantic to
receive a cover of its sediments.
As the period progressed slight oscillations occurred from time to
time. Strips of coastal plain were added to the land, and as early as
the close of the Miocene the shore lines of the Atlantic and Gulf
states had reached well-nigh their present place. Louisiana and
Florida were the last areas to emerge wholly from the sea,--Florida
being formed by a broad transverse upwarp of the continental delta at
the opening of the Miocene, forming first an island, which afterwards
was joined to the mainland.
=The Pacific coast.= Tertiary deposits with marine fossils occur along
the western foothills of the Sierra Nevadas, and are crumpled among
the mountain masses of the Coast Ranges; it is hence inferred that the
Great Valley of California was then a border sea, separated from the
ocean by a chain of mountainous islands which were upridged into the
Coast Ranges at a still later time. Tertiary marine strata are spread
over the lower Columbia valley and that of Puget Sound, showing that
the Pacific came in broadly there.
=The interior of the western United States.= The closing stages of the
Mesozoic were marked, as we have seen, by the upheaval of the Rocky
Mountains and other western ranges. The bases of the mountains are now
skirted by widespread Tertiary deposits, which form the highest strata
of the lofty plateaus from the level of whose summits the mountains
rise. Like the recent alluvium of the Great Valley of California (p.
101), these deposits imply low-lying lands when they were laid, and
therefore at that time the mountains rose from near sea level. But the
height at which the Tertiary strata now stand--five thousand feet
above the sea at Denver, and twice that height in the plateaus of
southern Utah--proves that the plateaus of which the Tertiary strata
form a part have been uplifted during the Cenozoic. During their
uplift, warping formed extensive basins both east and west of the
Rockies, and in these basins stream-swept and lake-laid waste gathered
to depths of hundreds and thousands of feet, as it is accumulating at
present in the Great Valley of California and on the river plains of
Turkestan (p. 103). The Tertiary river deposits of the High Plains
have already been described (p. 100). How widespread are these ancient
river plains and beds of fresh-water lakes may be seen in the map of
Figure 260.
[Illustration: Fig. 340. View in the Bad Lands of South Dakota]
=The Bad Lands.= In several of the western states large areas of
Tertiary fresh-water deposits have been dissected to a maze of hills
whose steep sides are cut with innumerable ravines. The deposits of
these ancient river plains and lake beds are little cemented and
because of the dryness of the climate are unprotected by vegetation;
hence they are easily carved by the wet-weather rills of scanty and
infrequent rains. These waterless, rugged surfaces were named by the
early French explorers the _Bad Lands_ because they were found so
difficult to traverse. The strata of the Bad Lands contain vast
numbers of the remains of the animals of Tertiary times, and the large
amount of barren surface exposed to view makes search for fossils easy
and fruitful. These desolate tracts are therefore frequently visited
by scientific collecting expeditions.
=Mountain making in the Tertiary.= The Tertiary period included epochs
when the earth's crust was singularly unquiet. From time to time on
all the continents subterranean forces gathered head, and the crust
was bent and broken and upridged in lofty mountains.
The Sierra Nevada range was formed, as we have seen, by strata
crumpling at the end of the Jurassic. But since that remote time the
upfolded mountains had been worn to plains and hilly uplands, the
remnants of whose uplifted erosion surfaces may now be traced along
the western mountain slopes. Beginning late in the Tertiary, the
region was again affected by mountain-making movements. A series of
displacements along a profound fault on the eastern side tilted the
enormous earth block of the Sierras to the west, lifting its eastern
edge to form the lofty crest and giving to the range a steep eastern
front and a gentle descent toward the Pacific.
The Coast Ranges also have had a complex history with many
vicissitudes. The earliest foldings of their strata belong to the
close of the Jurassic, but it was not until the end of the Miocene
that the line of mountainous islands and the heavy sediments which had
been deposited on their submerged flanks were crushed into a
continuous mountain chain. Thick Pliocene beds upon their sides prove
that they were depressed to near sea level during the later Tertiary.
At the close of the Pliocene the Coast Ranges rose along with the
upheaval of the Sierra, and their gradual uplift has continued to the
present time.
The numerous north-south ranges of the Great Basin and the Mount Saint
Elias range of Alaska were also uptilted during the Tertiary.
During the Tertiary period many of the loftiest mountains of the
earth--the Alps, the Apennines, the Pyrenees, the Atlas, the Caucasus,
and the Himalayas--received the uplift to which they owe most of their
colossal bulk and height, as portions of the Tertiary sea beds now
found high upon their flanks attest. In the Himalayas, Tertiary marine
limestones occur sixteen thousand five hundred feet above sea level.
=Volcanic activity in the tertiary.= The vast deformations of the
Tertiary were accompanied on a corresponding scale by outpourings of
lava, the outburst of volcanoes, and the intrusion of molten masses
within the crust. In the Sierra Nevadas the Miocene river gravels of
the valleys of the western slope, with their placer deposits of gold,
were buried beneath streams of lava and beds of tuff (Fig. 258).
Volcanoes broke forth along the Rocky Mountains and on the plateaus of
Utah, New Mexico, and Arizona.
Mount Shasta and the immense volcanic piles of the Cascades date from
this period. The mountain basin of the Yellowstone Park was filled to
a depth of several thousand feet with tuffs and lavas, the oldest
dating as far back as the beginning of the Tertiary. Crandall
volcano (Fig. 263) was reared in the Miocene and the latest eruptions
of the Park are far more recent.
[Illustration: Fig. 341. Lava Plateau with Lava Domes in the
Distance]
=The Columbia and Snake River lavas.= Still more important is the
plateau of lava, more than two hundred thousand square miles in area,
extending from the Yellowstone Park to the Cascade Mountains, which
has been built from Miocene times to the present.
Over this plateau, which occupies large portions of Idaho, Washington,
and Oregon, and extends into northern California and Nevada, the
country rock is basaltic lava. For thousands of square miles the
surface is a lava plain which meets the boundary mountains as a lake
or sea meets a rugged and deeply indented coast. The floods of molten
rock spread up the mountain valleys for a score of miles and more, the
intervening spurs rising above the lava like long peninsulas, while
here and there an isolated peak was left to tower above the inundation
like an island off a submerged shore.
The rivers which drain the plateau--the Snake, the Columbia, and their
tributaries--have deeply trenched it, yet their canyons, which reach the
depth of several thousand feet, have not been worn to the base of the
lava except near the margin and where they cut the summits of mountains
drowned beneath the flood. Here and there the plateau has been deformed.
It has been upbent into great folds, and broken into immense blocks of
bedded lava, forming mountain ranges, which run parallel with the
Pacific coast line. On the edges of these tilted blocks the thickness of
the lava is seen to be fully five thousand feet. The plateau has been
built, like that of Iceland (p. 242), of innumerable overlapping sheets
of lava. On the canyon walls they weather back in horizontal terraces
and long talus slopes. One may distinguish each successive flow by its
dense central portion, often jointed with large vertical columns, and
the upper portion with its mass of confused irregular columns and
scoriaceous surface. The average thickness of the flows seems to be
about seventy-five feet.
The plateau was long in building. Between the layers are found in
places old soil beds and forest grounds and the sediments of lakes.
Hence the interval between the flows in any locality was sometimes
long enough for clays to gather in the lakes which filled depressions
in the surface. Again and again the surface of the black basalt was
reddened by oxidation and decayed to soil, and forests had time to
grow upon it before the succeeding inundation sealed the sediments and
soils away beneath a sheet of stone. Near the edges of the lava plain,
rivers from the surrounding mountains spread sheets of sand and gravel
on the surface of one flow after another. These pervious sands,
interbedded with the lava, become the aquifers of artesian wells.
In places the lavas rest on extensive lake deposits, one thousand feet
deep, and Miocene in age as their fossils prove. It is to the middle
Tertiary, then, that the earliest flows and the largest bulk of the
great inundation belong. So ancient are the latest floods in the
Columbia basin that they have weathered to a residual yellow clay from
thirty to sixty feet in depth and marvelously rich in the mineral
substances on which plants feed.
In the Snake River valley the latest lavas are much younger. Their
surfaces are so fresh and undecayed that here the effusive eruptions
may well have continued to within the period of human history. Low
lava domes like those of Iceland mark where last the basalt outwelled
and spread far and wide before it chilled (Fig. 341). In places small
mounds of scoria show that the eruptions were accompanied to a slight
degree by explosions of steam. So fluid was this superheated lava that
recent flows have been traced for more than fifty miles.
The rocks underlying the Columbia lavas, where exposed to view, are
seen to be cut by numerous great dikes of dense basalt, which mark the
fissures through which the molten rock rose to the surface.
The Tertiary included times of widespread and intense volcanic action
in other continents as well as in North America. In Europe,
Vesuvius (p. 231) and Etna began their career as submarine volcanoes in
connection with earth movements which finally lifted Pliocene deposits
in Sicily to their present height,--four thousand feet above the sea.
Volcanoes broke forth in central France and southern Germany, in
Hungary and the Carpathians. Innumerable fissures opened in the crust
from the north of Ireland and the western islands of Scotland to the
Faroes, Iceland, and even to arctic Greenland; and here great plateaus
were built of flows of basalt similar to that of the Columbia River.
In India, at the opening of the Tertiary, there had been an outwelling
of basalt, flooding to a depth of thousands of feet two hundred
thousand square miles of the northwestern part of the peninsula (Fig.
342), and similar inundations of lava occurred where are now the
table-lands of Abyssinia. From the middle Tertiary on, Asia Minor,
Arabia, and Persia were the scenes of volcanic action. In Palestine
the rise of the uplands of Judea at the close of the Eocene, and the
downfaulting of the Jordan valley (p. 221) were followed by volcanic
outbursts. In comparison with the middle Tertiary, the present is a
time of volcanic inactivity and repose.
[Illustration: Fig. 342. Map showing the Lava Sheet
(shaded area) of Western India]
=Erosion of Tertiary mountains and plateaus.= The mountains and
plateaus built at various times during the Tertiary and at its
commencement have been profoundly carved by erosive agents. The Sierra
Nevada Mountains have been dissected on the western slope by such
canyons as those of King's River and the Yosemite. Six miles of strata
have been denuded from parts of the Wasatch Mountains since their rise
at the beginning of the era. From the Colorado plateaus, whose uplift
dates from the same time, there have been stripped off ten thousand
feet of strata over thousands of square miles, and the colossal canyon
of the Colorado has been cut after this great denudation had been
mostly accomplished.
On the eastern side of the continent, as we have seen, a broad
peneplain had been developed by the close of the Cretaceous. The
remnants of this old erosion surface are now found upwarped to various
heights in different portions of its area. In southern New England it
now stands fifteen hundred feet above the sea in western
Massachusetts, declining thence southward and eastward to sea level at
the coast. In southwestern Virginia it has been lifted to four
thousand feet above the sea. Manifestly this upwarp occurred since the
peneplain was formed; it is later than the Mesozoic, and the vast
dissection which the peneplain has suffered since its uplift must
belong to the successive cycles of Cenozoic time.
Revived by the uplift, the streams of the area trenched it as deeply
as its elevation permitted, and reaching grade, opened up wide valleys
and new peneplains in the softer rocks. The Connecticut valley is
Tertiary in age, and in the weak Triassic sandstones has been widened
in places to fifteen miles. Dating from the same time are the valleys
of the Hudson, the Susquehanna, the Delaware, the Potomac, and the
Shenandoah.
In Pennsylvania and the states lying to the south the Mesozoic
peneplain lies along the summits of the mountain ridges. On the
surface of this ancient plain, Tertiary erosion etched out the
beautifully regular pattern of the Allegheny mountain ridges and their
intervening valleys. The weaker strata of the long, regular folds were
eroded into longitudinal valleys, while the hard Paleozoic sandstones,
such as the Medina (p. 335) and the Pocono (p. 350), were left in
relief as bold mountain walls whose even crests rise to the common
level of the ancient plain. From Virginia far into Alabama the great
Appalachian valley was opened to a width in places of fifty miles and
more, along a belt of intensely folded and faulted strata where once
was the heart of the Appalachian Mountains. In Figure 70, the summit of
the Cumberland plateau (ab) marks the level of the Mesozoic peneplain,
while the lower erosion levels are Tertiary and Quaternary in age.
[Illustration: Fig. 343. Diagram of the Allegheny Mountains,
Pennsylvania
From Davis' Elementary Physical Geography]
Life of the Tertiary Period
=Vegetation and climate.= The highest plants in structure, the
_dicotyls_ (such as our deciduous forest trees) and the _monocotyls_
(represented by the palms), were introduced during the Cretaceous. The
vegetable kingdom reached its culmination before the animal kingdom,
and if the dividing line between the Mesozoic and the Cenozoic were
drawn according to the progress of plant life, the Cretaceous instead
of the Tertiary would be made the opening period of the modern era.
The plants of the Tertiary belonged, for the most part, to genera
now living; but their distribution was very different from that of
the flora of to-day. In the earlier Tertiary, palms flourished over
northern Europe, and in the northwestern United States grew the
magnolia and laurel, along with the walnut, oak, and elm. Even in
northern Greenland and in Spitzbergen there were lakes covered with
water lilies and surrounded by forests of maples, poplars, limes, the
cypress of our southern states, and noble sequoias similar to the
"big trees" and redwoods of California. A warm climate like that of
the Mesozoic, therefore, prevailed over North America and Europe,
extending far toward the pole. In the later Tertiary the climate
gradually became cooler. Palms disappeared from Europe, and everywhere
the aspect of forests and open lands became more like that of to-day.
Grasses became abundant, furnishing a new food for herbivorous
animals.
=Animal life of the Tertiary.= Little needs to be said of the Tertiary
invertebrates, so nearly were they like the invertebrates of the
present. Even in the Eocene, about five per cent of marine shells were
of species still living, and in the Pliocene the proportion had risen
to more than one half.
Fishes were of modern types. Teleosts were now abundant. The ocean
teemed with sharks, some of them being voracious monsters seventy-five
feet and even more in length, with a gape of jaw of six feet, as
estimated by the size of their enormous sharp-edged teeth.
Snakes are found for the first time in the early Tertiary. These
limbless reptiles, evolved by degeneration from lizardlike ancestors,
appeared in nonpoisonous types scarcely to be distinguished from those
of the present day.
=Mammals of the early Tertiary.= The fossils of continental deposits
of the earliest Eocene show that a marked advance had now been made in
the evolution of the Mammalia. The higher mammals had appeared, and
henceforth the lower mammals--the monotremes and the marsupials--are
reduced to a subordinate place.
[Illustration: Fig. 344. Phenacodus]
These first true mammals were archaic and generalized in structure.
Their feet were of the primitive type, with five toes of about equal
length. They were also _plantigrades_,--that is, they touched the
ground with the sole of the entire foot from toe to heel. No foot had
yet become adapted to swift running by a decrease in the number of
digits and by lifting the heel and sole so that only the toes touch
the ground,--a tread called _digitigrade_. Nor was there yet any foot
like that of the cats, with sharp retractile claws adapted to seizing
and tearing the prey. The forearm and the lower leg each had still
two separate bones (ulna and radius, fibula and tibia), neither pair
having been replaced with a single strong bone, as in the leg of the
horse. The teeth also were primitive in type and of full number. The
complex heavy grinders of the horse and elephant, the sharp cutting
teeth of the carnivores, and the cropping teeth of the grass eaters
were all still to come.
Phenacodus is a characteristic genus of the early Eocene, whose
species varied in size from that of a bulldog to that of an animal a
little larger than a sheep. Its feet were primitive, and their five
toes bore nails intermediate in form between a claw and a hoof. The
archaic type of teeth indicates that the animal was omnivorous in
diet. A cast of the brain cavity shows that, like its associates of
the time, its brain was extremely small and nearly smooth, having
little more than traces of convolutions.
The long ages of the Eocene and the following epochs of the Tertiary
were times of comparatively rapid evolution among the Mammalia.
The earliest forms evolved along diverging lines toward the various
specialized types of hoofed mammals, rodents, carnivores,
proboscidians, the primates, and the other mammalian orders as we know
them now. We must describe the Tertiary mammals very briefly, tracing
the lines of descent of only a few of the more familiar mammals of the
present.
=The horse.= The pedigree of the horse runs back into the early Eocene
through many genera and species to a five-toed,[3] short-legged ancestor
little bigger than a cat. Its descendants gradually increased in stature
and became better and better adapted to swift running to escape their
foes. The leg became longer, and only the tip of the toes struck the
ground. The middle toe (digit number three), originally the longest of
the five, steadily enlarged, while the remaining digits dwindled and
disappeared. The inner digit, corresponding to the great toe and thumb,
was the first to go. Next number five, the little finger, was also
dropped. By the end of the Eocene a three-toed genus of the horse
family had appeared, as large as a sheep. The hoof of digit number
three now supported most of the weight, but the slender hoofs of
digits two and four were still serviceable. In the Miocene the stature
of the ancestors of the horse increased to that of a pony. The feet
were still three-toed, but the side hoofs were now mere dewclaws and
scarcely touched the ground. The evolution of the family was completed
in the Pliocene. The middle toe was enlarged still more, the side toes
were dropped, and the palm and foot bones which supported them were
reduced to splints.
[3] Or, more accurately, with four perfect toes and a
rudimentary fifth corresponding to the thumb.
[Illustration: Fig. 345. Development of Forefoot (A), the
Forearm (B), the Molar (C), of the Horse Family]
While these changes were in progress the radius and ulna of the fore
limb became consolidated to a single bone; and in the hind limb the
fibula dwindled to a splint, while the tibia was correspondingly
enlarged. The molars, also gradually lengthened, and became more and
more complex on their grinding surface; the neck became longer; the
brain steadily increased in size and its convolutions became more
abundant. The evolution of the horse has made for greater fleetness
and intelligence.
=The rhinoceros and tapir.= These animals, which are grouped with the
horse among the _odd-toed_ (perissodactyl) mammals, are now verging
toward extinction. In the rhinoceros, evolution seems to have taken
the opposite course from that of the horse. As the animal increased in
size it became more clumsy, its limbs became shorter and more massive,
and, perhaps because of its great weight, the number of digits were
not reduced below the number three. Like other large herbivores, the
rhinoceros, too slow to escape its enemies by flight, learned to
withstand them. It developed as its means of defense a nasal horn.
Peculiar offshoots of the line appeared at various times in the
Tertiary. A rhinoceros, semiaquatic in habits, with curved tusks,
resembling in aspect the hippopotamus, lived along the water courses
of the plains east of the Rockies, and its bones are now found by the
thousands in the Miocene of Kansas. Another developed along a line
parallel to that of the horse, and herds of these light-limbed and
swift-footed running rhinoceroses ranged the Great Plains from the
Dakotas southward.
The tapirs are an ancient family which has changed but little since it
separated from the other perissodactyl stocks in the early Tertiary.
At present, tapirs are found only in South America and southern
Asia,--a remarkable distribution which we could not explain were it
not that the geological record shows that during Tertiary times tapirs
ranged throughout the northern hemisphere, making their way to South
America late in that period. During the Pleistocene they became
extinct over all the intervening lands between the widely separated
regions where now they live. The geographic distribution of animals,
as well as their relationships and origins, can be understood only
through a study of their geological history.
[Illustration: Fig. 346. A Tertiary Mastodon]
[Illustration: Fig. 347. Head of Dinothere]
=The proboscidians.= This unique order of hoofed mammals, of which the
elephant is the sole survivor, has been traced back to the close of
the Eocene. In the middle and later Tertiary it was represented by
huge creatures so nearly akin to the mastodons of the Pleistocene that
they are often included in that genus. The Tertiary _Mastodon_ was
furnished with a long, flexible proboscis, and armed with two pairs of
long, straight ivory tusks, the pair of the lower jaw being smaller.
The _Dinothere_ was a curious offshoot of the line, which developed in
the Miocene in Europe. In this immense proboscidian, whose skull was
three feet long, the upper pair of tusks had disappeared, and those of
the lower jaw were bent down with a backward curve in walrus fashion.
[Illustration: Fig. 348. Crown of Mastodon Tooth]
In the true _elephants_, which do not appear until near the close of
the Tertiary, the lower jaw loses its tusks and the grinding teeth
become exceedingly complex in structure. The grinding teeth of the
mastodon had long roots and low crowns crossed by four or five peaked
enameled ridges. In the teeth of the true elephants the crown has
become deep, and the ridges of enamel have changed to numerous
upright, platelike folds, their interspaces filled with cement. The
two genera--Mastodon and Elephant--are connected by species whose
teeth are intermediate in pattern. The proboscidians culminated in the
Pliocene, when some of the giant elephants reached a height of
fourteen feet.
[Illustration: Fig. 349. Tooth of an Extinct Elephant, the Mammoth]
[Illustration: Fig. 350. Evolution of the Artiodactyl Foot,
Illustrated by Existing Families
_A_, pig; _B_, roebuck; _C_, sheep; _D_, camel]
=The artiodactyls= comprise the hoofed Mammalia which have an even
number of toes, such as cattle, sheep, and swine. Like the
perissodactyls, they are descended from the primitive five-toed
plantigrade mammals of the lowest Eocene. In their evolution, digit
number one was first dropped, and the middle pair became larger and
more massive, while the side digits, numbers two and five, became
shorter, weaker, and less serviceable. The _four-toed artiodactyls_
culminated in the Tertiary; at present they are represented only by
the hippopotamus and the hog. Along the main line of the evolution of
the artiodactyls the side toes, digits two and five, disappeared,
leaving as proof that they once existed the corresponding bones of
palm and sole as splints. The _two-toed artiodactyls_, such as the
camels, deer, cattle, and sheep, are now the leading types of the
herbivores.
_Swine and peccaries_ are two branches of a common stock, the first
developing in the Old World and the second in the New. In the Miocene
a noticeable offshoot of the line was a gigantic piglike brute, a root
eater, with a skull a yard in length, whose remains are now found in
Colorado and South Dakota.
=Camels and llamas.= The line of camels and llamas developed in North
America, where the successive changes from an early Eocene ancestor,
no larger than a rabbit, are traced step by step to the present forms,
as clearly as is the evolution of the horse. In the late Miocene some
of the ancestral forms migrated to the Old World by way of a land
connection where Bering Strait now is, and there gave rise to the
camels and dromedaries. Others migrated into South America, which had
now been connected with our own continent, and these developed into
the llamas and guanacos, while those of the race which remained in
North America became extinct during the Pleistocene.
Some peculiar branches of the camel stem appeared in North America. In
the Pliocene arose a llama with the long neck and limbs of a giraffe,
whose food was cropped from the leaves and branches of trees. Far more
generalized in structure was the _Oreodon_, an animal related to the
camels, but with distinct affinities also with other lines, such as
those of the hog and deer. These curious creatures were much like the
peccary in appearance, except for their long tails. In the middle
Eocene they roamed in vast herds from Oregon to Kansas and Nebraska.
=The ruminants.= This division of the artiodactyls includes antelopes,
deer, oxen, bison, sheep, and goats,--all of which belong to a common
stock which took its rise in Europe in the upper Eocene from ancestral
forms akin to those of the camels. In the Miocene the evolution of the
two-toed artiodactyl foot was well-nigh completed. Bonelike growths
appeared on the head, and the two groups of the ruminants became
specialized,--the deer with bony antlers, shed and renewed each year,
and the ruminants with hollow horns, whose two bony knobs upon the
skull are covered with permanent, pointed, horny sheaths.
The ruminants evolved in the Old World, and it was not until the later
Miocene that the ancestors of the antelope and of some deer found
their way to North America. Mountain sheep and goats, the bison and
most of the deer, did not arrive until after the close of the
Tertiary, and sheep and oxen were introduced by man.
The hoofed mammals of the Tertiary included many offshoots from the
main lines which we have traced. Among them were a number of genera of
clumsy, ponderous brutes, some almost elephantine in their bulk.
=The carnivores.= The ancestral lines of the families of the flesh
eaters--such as the cats (lions, tigers, etc.), the bears, the hyenas,
and the dogs (including wolves and foxes)--converge in the creodonts
of the early Eocene,--an order so generalized that it had affinities
not only with the carnivores but also with the insect eaters, the
marsupials, and the hoofed mammals as well. From these primitive flesh
eaters, with small and simple brains, numerous small teeth, and
plantigrade tread, the different families of the carnivores of the
present have slowly evolved.
=Dogs and bears.= The dog family diverged from the creodonts late in
the Eocene, and divided into two branches, one of which evolved the
wolves and the other the foxes. An offshoot gave rise to the family
of the bears, and so closely do these two families, now wide apart,
approach as we trace them back in Tertiary times that the Amphicyon,
a genus doglike in its teeth and bearlike in other structures, is
referred by some to the dog and by others to the bear family. The
well-known plantigrade tread of bears is a primitive characteristic
which has survived from their creodont ancestry.
=Cats.= The family of the cats, the most highly specialized of all the
carnivores, divided in the Tertiary into two main branches. One, the
saber-tooth tigers (Fig. 351), which takes its name from their long,
saberlike, sharp-edged upper canine teeth, evolved a succession of
genera and species, among them some of the most destructive beasts of
prey which ever scourged the earth. They were masters of the entire
northern hemisphere during the middle Tertiary, but in Europe during
the Pliocene they declined, from unknown causes, and gave place to the
other branch of cats,--which includes the lions, tigers, and leopards.
In the Americas the saber-tooth tigers long survived the epoch.
[Illustration: Fig. 351. Saber-Tooth Tiger]
=Marine mammals.= The carnivorous mammals of the sea--whales, seals,
walruses, etc.--seem to have been derived from some of the creodonts
of the early Tertiary by adaptation to aquatic life. Whales evolved
from some land ancestry at a very early date in the Tertiary; in the
marine deposits of the Eocene are found the bones of the Zeuglodon, a
whalelike creature seventy feet in length.
=Primates.= This order, which includes lemurs, monkeys, apes, and man,
seems to have sprung from a creodont or insectivorous ancestry in
the lower Eocene. Lemur-like types, with small, smooth brains, were
abundant in the United States in the early Tertiary, but no primates
have been found here in the middle Tertiary and later strata. In
Europe true monkeys were introduced in the Miocene, and were abundant
until the close of the Tertiary, when they were driven from the
continent by the increasing cold.
=Advance of the mammalia during the tertiary.= During the several
millions of years comprised in Tertiary time the mammals evolved from
the lowly, simple types which tenanted the earth at the beginning of
the period, into the many kinds of highly specialized mammals of the
Pleistocene and the present, each with the various structures of the
body adapted to its own peculiar mode of life. The swift feet of the
horse, the horns of cattle and the antlers of the deer, the lion's
claws and teeth, the long incisors of the beaver, the proboscis of the
elephant, were all developed in Tertiary times. In especial the brain
of the Tertiary mammals constantly grew larger relatively to the
size of body, and the higher portion of the brain--the cerebral
lobes--increased in size in comparison with the cerebellum. Some
of the hoofed mammals now have a brain eight or ten times the size
of that of their early Tertiary predecessors of equal bulk. Nor
can we doubt that along with the increasing size of brain went a
corresponding increase in the keenness of the senses, in activity
and vigor, and in intelligence.