sure that any animals lived in them, are called the "archæan," which
is Greek for ancient. They were probably mud and sand and limestone
when first made, but they have been changed to mica schists, gneiss,
granite, marble, and other crystalline rocks. When any rock becomes
crystalline, the fossils dissolve and disappear, as coins lose their
stamp and form when they are melted in the jeweller's gold-pot.
These ancient rocks that lie deepest in the earth are very thick, and
must have taken a great time in building; great continents must have
been worn down by rain and waves in order to supply the waste out of
which they were made. It is tolerably certain that they took as much
time during their making as has been required for all the other times
since they were formed. During the vast ages of this archæan the life
of our earth began to be. We first find many certain evidences of life
in the rocks which lie on top of the archæan rock, and are known as
the Cambriani and Silurian periods. There we have creatures akin to
our corals and crabs and worms, and others that are the distant
kindred of the cuttle-fishes and of our lamp-shells. There were no
backboned animals, that is to say, no land mammals, reptiles, or
fishes at this stage of the earth's history. It is not likely that
there was any land life except of plants and those forms like the
lowest ferns, and probably mosses. Nor is it likely that there were
any large continents as at the present time, but rather a host of
islands lying where the great lands now are, the budding tops of the
continents just appearing above the sea.
Although the life of this time was far simpler than at the present
day, it had about as great variety as we would find on our present
sea-floors. There were as many different species living at the same
time on a given surface.
The Cambrian and Silurian time--the time before the coming of the
fishes--must have endured for many million years without any great
change in the world. Hosts of species lived and died; half a dozen
times or more the life of the earth was greatly changed. New species
came much like those that had gone before, and only a little gain here
and there was perceptible at any time. Still, at the end of the
Silurian, the life of the world had climbed some steps higher in
structure and in intelligence.
[Illustration: FIG. 1. NORTH AMERICA IN CAMBRIAN TIME.]
The next set of periods is known as the Devonian. It is marked by the
rapid extension of the fishes; for, although the fishes began in the
uppermost Silurian, they first became abundant in this time. These,
the first strong-jawed tyrants of the sea, came all at once, like a
rush of the old Norman pirates into the peaceful seas of Great
Britain. They made a lively time among the sluggish beings of that
olden sea. Creatures that were able to meet feebler enemies were swept
away or compelled to undergo great changes, and all the life of the
oceans seems to have a spur given to it by these quicker-formed and
quicker-willed animals. In this Devonian section of our rocks we have
proofs that the lands were extensively covered with forests of low
fern trees, and we find the first trace of air-breathing animals in
certain insects akin to our dragon-flies. In this stage of the earth's
history the fishes grew constantly more plentiful, and the seas had a
great abundance of corals and crinoids. Except for the fishes, there
were no very great changes in the character of the life from that
which existed in the earlier time of the Cambrian and Silurian. The
animals are constantly changing, but the general nature of the life
remains the same as in the earlier time.
[Illustration: FIG. 2. RANICEPS LYELLI--COAL TIME SALAMANDER.]
In the Carboniferous or coal-bearing age, we have the second great
change in the character of the life on the earth. Of the earlier
times, we have preserved only the rocks formed in the seas. But rarely
do we find any trace of the land life or even of the life that lived
along the shores. In this Carboniferous time, however, we have very
extensive sheets of rocks which were formed in swamps in the way shown
in the earlier part of this book. They constitute our coal-beds,
which, though much worn away by rain and sea, still cover a large part
of the land surface. These beds of coal grew in the air, and, although
the swamps where they were formed had very little animal life in them,
we find some fossils which tell us that the life of the land was
making great progress; there are new insects, including beetles,
cockroaches, spiders, and scorpions, and, what is far more important,
there are some air-breathing, back-boned animals, akin to the
salamanders and water-dogs of the present day. These were nearly as
large as alligators, and of much the same shape, but they were
probably born from the egg in the shape of tadpoles and lived for a
time in the water as our young frogs, toads, and salamanders do. This
is the first step upwards from the fishes to land vertebrates; and we
may well be interested in it, for it makes one most important advance
in creatures through whose lives our own existence became possible.
Still, these ancient woods of the coal period must have had little of
the life we now associate with the forests; there were still no birds,
no serpents, no true lizards, no suck-giving animals, no flowers, and
no fruits. These coal-period forests were sombre wastes of shade, with
no sound save those of the wind, the thunder, and the volcano, or of
the running streams and the waves on the shores.
In the seas of the Carboniferous time, we notice that the ancient life
of the earth is passing away. Many creatures, such as the trilobites,
die out, and many other forms such as the crinoids or sea lilies
become fewer in kind and of less importance. These marks of decay in
the marine life continue into the beds just after the Carboniferous,
known as the Permian, which are really the last stages of the
coal-bearing period.
When with the changing time we pass to the beds known as the Triassic,
which were made just after the close of the Carboniferous time, we
find the earth undergoing swift changes in its life. The moist climate
and low lands that caused the swamps to grow so rapidly have ceased to
be, and in their place we appear to have warm, dry air, and higher
lands.
On these lands of the Triassic time the air-breathing life made very
rapid advances. The plants are seen to undergo considerable changes.
The ferns no longer make up all the forests, but trees more like the
pines began to abound, and insects became more plentiful and more
varied.
[Illustration: FIG. 3. CYCAS CIRCINALIS, AKIN TO HIGHEST PLANTS OF
COAL TIME.]
Hitherto the only land back-boned animal was akin to our salamanders.
Now we have true lizards in abundance, many of them of large size.
Some of them were probably plant-eaters, but most were flesh-eaters;
some seem to have been tenants of the early swamps, and some dwelt in
the forests.
The creatures related to the salamanders have increased in the variety
of their forms to a wonderful extent. We know them best by the tracks
which they have left on the mud stones formed on the borders of lakes
or the edge of the sea. In some places these footprints are found in
amazing numbers and perfection. The best place for them is in the
Connecticut Valley, near Turner's Falls, Mass. At this point the red
sandstone and shale beds, which are composed of thin layers having a
total thickness of several hundred feet, are often stamped over by
these footprints like the mud of a barnyard. From the little we can
determine from these footprints, the creatures seem to have been
somewhat related to our frogs, but they generally had tails, and,
though provided with four legs, were in the habit of walking on the
hind ones alone like the kangaroo. A few of these tracks are shown in
the figure on this page.
[Illustration: FIG. 4. FOOT-PRINTS, CONNECTICUT SANDSTONES.]
These strange creatures were of many different species. Some of them
must have been six or seven feet high, for their steps are as much as
three feet apart, and seem to imply a creature weighing several
hundred pounds. Others were not bigger than robins. Strangely enough,
we have never found their bones nor the creatures on which they fed,
and but for the formation of a little patch of rocks here and there we
should not have had even these footprints to prove to us that such
creatures had lived in the Connecticut Valley in this far-off time.
[Illustration: FIG. 5. FOOT-PRINT, TURNER'S FALLS.]
But these wonderful forms are less interesting than two or three
little fossil jaw-bones that prove to us that in this Triassic time
the earth now bore another animal more akin to ourselves, in the shape
of a little creature that gave suck to its young. Once more life takes
a long upward step in this little opossum-like animal, perhaps the
first creature whose young was born alive. These little creatures
called Microlestes or Dromatherium, of which only one or two different
but related species have been found in England and in North Carolina,
appear to have been insect-eaters of about the size and shape of the
Australian creature shown in Fig. 7. So far we know it in but few
specimens,--altogether only an ounce or two of bones,--but they are
very precious monuments of the past.
[Illustration: FIG. 6. DROMATHERIUM SYLVESTRE AND TEETH OF MICROLESTES
ANTIQUUS.]
In this Triassic time the climate appears to have been rather dry, for
in it we have many extensive deposits of salt formed by the
evaporation of closed lakes, of seas, such as are now forming on the
bottom of the Dead Sea, and the Great Salt Lake of Utah, and a hundred
or more other similar basins of the present day.
[Illustration: FIG. 7. MYRMECOBIUS.]
In the sea animals of this time we find many changes. Already some of
the giant lizard-like animals, which first took shape on the land, are
becoming swimming-animals. They changed their feet to paddles, which,
with the help of a flattened tail, force them through the water.
The fishes on which these great swimming lizards preyed are more like
the fishes of our present day than they were before. The trilobites
are gone, and of the crinoids only a remnant is left. Most of the
corals of the earlier days have disappeared, but the mollusks have not
changed more than they did at several different times in the earliest
stages of the earth's history.
[Illustration: FIG. 8. ICHTHYOSAURUS AND PLESIOSAURUS.]
After the Trias comes a long succession of ages in which the life of
the world is steadily advancing to higher and higher planes; but for a
long time there is no such startling change as that which came in the
passage from the coal series of rocks to the Trias. This long set of
periods is known to geologists as the age of reptiles. It is well
named, for the kindred of the lizards then had the control of the
land. There were then none of our large fish to dispute their control,
so they shaped themselves to suit all the occupations that could give
them a chance for a living. Some remained beasts of prey like our
alligators, but grew to larger size; some took to eating the plants,
and came to walk on their four legs as our ordinary beasts do, no
longer dragging themselves on their bellies as do the lizard and
alligator, their lower kindred. Others became flying creatures like
our bats, only vastly larger, often with a spread of wing of fifteen
or twenty feet. Yet others, even as strangely shaped, dwelt with the
sharks in the sea.
[Illustration: FIG. 9. REPTILES OF JURASSIC PERIOD.]
In this time of the earth's history we have the first bird-like forms.
They were feathered creatures, with bills carrying true teeth, and
with strong wings; but they were reptiles in many features, having
long, pointed tails such as none of our existing birds have. They show
us that the birds are the descendants of reptiles, coming off from
them as a branch does from the parent tree. The tortoises began in
this series of rocks. At first they are marine or swimming forms, the
box-turtles coming later. Here too begin many of the higher insects.
Creatures like moths and bees appear, and the forests are enlivened
with all the important kinds of insects, though the species were very
different from those now living.
In the age of reptiles the plants have made a considerable advance.
Palms are plenty; forms akin to our pines and firs abound, and the old
flowerless group of ferns begins to shrink in size, and no longer
spreads its feathery foliage over all the land as before. Still there
were none of our common broad-leaved trees; the world had not yet
known the oaks, birches, maples, or any of our hard-wood trees that
lose their leaves in autumn; nor were the flowering plants, those with
gay blossoms, yet on the earth. The woods and fields were doubtless
fresh and green, but they wanted the grace of blossoms, plants, and
singing-birds. None of the animals could have had the social qualities
or the finer instincts that are so common among animals of the present
day. There were probably no social animals like our ants and bees, no
merry singing creatures; probably no forms that went in herds. Life
was a dull round of uncared-for birth, cruel self-seeking, and of
death. The animals at best were clumsy, poorly-endowed creatures, with
hardly more intelligence than our alligators.
The little thread of higher life begun in the Microlestes and
Dromatherium, the little insect-eating mammals of the forest, is
visible all through this time. It held in its warm blood the powers of
the time to come, but it was an insignificant thing among the mighty
cold-blooded reptiles of these ancient lands. There are several
species of them, but they are all small, and have no chance to make
headway against the older masters of the earth.
The Jurassic or first part of the reptilian time shades insensibly
into the second part, called the Cretaceous, which immediately follows
it. During this period the lands were undergoing perpetual changes;
rather deep seas came to cover much of the land surfaces, and there is
some reason to believe that the climate of the earth became much
colder than it had been, at least in those regions where the great
reptiles had flourished. It may be that it is due to a colder climate
that we owe the rapid passing away of this gigantic reptilian life of
the previous age. The reptiles, being cold-blooded, cannot stand even
a moderate winter cold, save when they are so small that they can
crawl deep into crevices in the rocks to sleep the winter away,
guarded from the cold by the warmth of the earth. At any rate these
gigantic animals rapidly ceased to be, so that by the middle of the
Cretaceous period they were almost all gone, except those that
inhabited the sea; and at the end of this time they had shrunk to
lizards in size. The birds continue to increase and to become more
like those of our day; their tails shrink away, their long bills lose
their teeth; they are mostly water-birds of large size, and there are
none of our songsters yet; still they are for the first time perfect
birds, and no longer half-lizard in their nature.
The greatest change in the plants is found in the coming of the
broad-leaved trees belonging to the families of our oaks, maples, etc.
Now for the first time our woods take on their aspect of to-day; pines
and other cone-bearers mingle with the more varied foliage of
nut-bearing or large-seeded trees. Curiously enough, we lose sight of
the little mammals of the earlier time. This is probably because there
is very little in the way of land animals of this period preserved to
us. There are hardly any mines or quarries in the beds of this age to
bring these fossils to light. In the most of the other rocks there is
more to tempt man to explore them for coal ores or building stones.
In passing from the Cretaceous to the Tertiary, we enter upon the
threshold of our modern world. We leave behind all the great wonders
of the old world, the gigantic reptiles, the forests of tree ferns,
the seas full of ammonites and belemnites, and come among the no less
wonderful but more familiar modern forms. We come at once into lands
and seas where the back-boned animals are the ruling beings. The
reptiles have shrunk to a few low forms,--the small lizards, the
crocodiles and alligators, the tortoises and turtles, and, as if to
mark more clearly the banishment of this group from their old empire,
the serpents, which are peculiarly degraded forms of reptiles which
have lost the legs they once had, came to be the commonest reptiles of
the earth.
The first mammals that have no pouches now appear. In earlier times,
the suck-giving animals all belonged to the group that contains our
opossums, kangaroos, etc. These creatures are much lower and feebler
than the mammals that have no pouches. Although they have probably
been on the earth two or three times as long as the higher mammals,
they have never attained any eminent success whatever; they cannot
endure cold climates; none of them are fitted for swimming as are the
seals and whales, or for flying as the bats, or for burrowing as the
moles; they are dull, weak things, which are not able to contend with
their stronger, better-organized, higher kindred. They seem not only
weak, but unable to fit themselves to many different kinds of
existence.
In the lower part of the Tertiary rocks, we find at once a great
variety of large beasts that gave suck to their young. It is likely
that these creatures had come into existence in a somewhat earlier
time in other lands, where we have not been able to study the fossils;
for to make their wonderful forms slowly, as we believe them to have
been made, would require a very long time. It is probable that during
the Cretaceous time, in some land where we have not yet had a chance
to study the rocks, these creatures grew to their varied forms, and
that in the beginning of the Tertiary time, they spread into the
regions where we find their bones.
Beginning with the Tertiary time, we find these lower kinsmen of man,
through whom man came to be. The mammals were marked by much greater
simplicity and likeness to each other than they now have. There were
probably no monkeys, no horses, no bulls, no sheep, no goats, no
seals, no whales, and no bats. All these animals had many-fingered
feet. There were no cloven feet like those of our bulls, and no solid
feet as our horses have. Their brains, which by their size give us a
general idea of the intelligence of the creature, are small; hence we
conclude that these early mammals were less intelligent than those of
our day.
It would require volumes to trace the history of the growth of these
early mammals, and show how they, step by step, came to their present
higher state. We will take only one of the simplest of these changes,
which happens to be also the one which we know best. This is the
change that led to the making of our common horses, which seem to have
been brought into life on the continent of North America. The most
singular thing about our horses is that the feet have but one large
toe or finger, the hoof, the hard covering of which is the nail of
that extremity. Now it seems hard to turn the weak, five-fingered
feet of the animals of the lower Tertiary--feet which seem to be
better fitted for tree-climbing than anything else--into feet such as
we find in the horse. Yet the change is brought about by easy stages
that lead the successive creatures from the weak and loose-jointed
foot of the ancient forms to the solid, single-fingered horse's hoof,
which is wonderfully well-fitted for carrying a large beast at a swift
speed, and is so strong a weapon of defence that an active donkey can
kill a lion with a well-delivered kick.
[Illustration: FIG. 10. FEET OF TERTIARY MAMMALS.]
The oldest of these creatures that lead to the horses is called
_Eohippus_ or beginning horse. This fellow had on the forefeet four
large toes, each with a small hoof and fifth imperfect one, which
answered to the thumb. The hind feet had gone further in the change,
for they each had but three toes, each with hoofs, the middle-toed
hoof larger and longer than the others. A little later toward our day
we find another advance in the _Orohippus_, when the little imperfect
thumb has disappeared, and there are only four toes on the forefeet
and three on the hind.
Yet later we have the _Mesohippus_ or half-way horse. There are still
three toes on the hind foot, but one more of the fingers of the
forefeet has disappeared. This time it is the little finger that goes,
leaving only a small bone to show that its going was by a slow
shrinking. The creature now has three little hoofs on each of its
feet.
Still nearer our own time comes the _Miohippus_, which shows the two
side hoofs on each foot shrinking up so that they do not touch the
ground, but they still bear little hoofs. Lastly, about the time of
man's coming on the earth, appears his faithful servant, the horse, in
which those little side hoofs have disappeared, leaving only two
little "splint" bones to mark the place where these side hoofs belong.
Thus, step by step, our horses' feet were built up; while these parts
were changing, the other parts of the animals were also slowly
altering. They were at first smaller than our horses,--some of them
not as large as an ordinary Newfoundland dog; others as small as
foxes.
[Illustration: FIG. 11. DEVELOPMENT OF HORSES'S FOOT.]
As if to remind us of his old shape, our horses now and then, but
rarely, have, in place of the little splint bones above the hoof, two
smaller hoofs, just like the foot of _Miohippus_. Sometimes these are
about the size of a silver dollar, on the part that receives the shoe
when horses are shod.
In this way, by slow-made changes, the early mammals pass into the
higher. Out of one original part are made limbs as different as the
feet of the horse, the wing of a bat, the paddle of a whale, and the
hand of man. So with all the parts of the body the forms change to
meet the different uses to which they are put.
At the end of this long promise, which was written in the very first
animals, comes man himself, in form closely akin to the lower animals,
but in mind immeasurably apart from them. We can find every part of
man's body in a little different shape in the monkeys, but his mind is
of a very different quality. While his lower kindred cannot be made to
advance in intelligence any more than man himself can grow a horse's
foot or a bat's wing, he is constantly going higher and higher in his
mental and moral growth.
So far we have found but few traces of man that lead us to suppose
that he has been for a long geological time on the earth, yet there is
good evidence that he has been here for a hundred thousand years or
more. It seems pretty clear that he has changed little in his body in
all these thousands of generations. The earliest remains show us a
large-brained creature, who used tools and probably had already made a
servant of fire, which so admirably aids him in his work.
Besides the development of this wonderful series of animals, that we
may call in a certain way our kindred, there have been several other
remarkable advances in this Tertiary time, this age of crowning
wonders in the earth's history. The birds have gone forward very
rapidly; it is likely that there were no songsters at the first part
of this period, but these singing birds have developed very rapidly in
later times. Among the insects the most remarkable growth is among the
ants, the bees, and their kindred. These creatures have very wonderful
habits; they combine together for the making of what we may call
states, they care for their young, they wage great battles, they keep
slaves, they domesticate other insects, and in many ways their acts
resemble the doings of man. Coming at about the same time as man,
these intellectual insects help to mark this later stage of the earth
as the intellectual period in its history. Now for the first time
creatures are on the earth which can form societies and help each
other in the difficult work of living.
Among the mollusks, the most important change is in the creation of
the great, strong swimming squids, the most remarkable creatures of
the sea. Some of these have arms that can stretch for fifty feet from
tip to tip.