the earth seems to have been a semi-solid or semi-fluid mass formed by the
condensation of the still more fluid or even gaseous nebula out of which
all the planets of the solar system have been formed and of which the sun
is the still fiery core. As soon as the earth had cooled sufficiently its
substances crystallized and wrinkled to form the first mountains and
ridges; between and among these were the basins which soon filled with the
condensing waters to become the earliest lakes and oceans. The wear and
tear of rains and snows and winds so worked upon the surfaces of the
higher regions that sediments of a finer or coarser character like sand
and mud and gravel were washed down into the lower levels. These sediments
were afterwards converted into the first rocks of the so-called stratified
or sedimentary series, as contrasted with the crystalline or plutonic
rocks like the original mass of the earth and the kinds forced to the
surface by volcanic eruptions. Later the earth wrinkled again in various
ways and places so that new ridges and mountains were formed with new
systems of lakes and oceans and rivers; and again the elements continued
to erode and partially destroy the higher masses and to lay down new and
later series of sedimentary rocks upon the old.
It seems scarcely credible that the apparently weak forces of nature like
those we have mentioned are sufficiently powerful to work over the massive
crust of the earth as geology says they have. Our attention is caught, as
a rule, only by the greater things, like the earthquakes at San Francisco
and Valparaiso, and the tidal waves and cyclones of the South Seas; but
the results of these sporadic and local cataclysms are far less than the
effects of the persistent everyday forces of erosion, each one of which
seems so small and futile. When we look at the Rocky Mountains with their
high and rugged peaks, it seems almost impossible that rain and frost and
snow could ever break them up and wear them down so that they would become
like the rounded hills of the Appalachian Mountain chain, yet this is what
will happen unless nature's ways suddenly change to something which they
are not now. A visitor to the Grand Cañon of the Colorado sees a
magnificent chasm over a mile in depth and two hundred miles long which
has actually been carved through layer after layer of solid rock by the
rushing torrents of the river. Perhaps it is easier to estimate the
geological effects of a river in such a case as Niagara. Here we find a
deep gorge below the famous falls, which runs for twenty miles or so to
open out into Lake Ontario. The water passing over the brim of the falls
wears away the edge at a rate which varies somewhat according to the
harder or softer consistency of the rocks, but which, since 1843, has
averaged about 104 inches a year. Knowing this rate, the length of the
gorge, and the character of the rocky walls already carved out, the length
of time necessary for its production can be safely estimated. It is about
30,000 to 40,000 years, not a long period when the whole history of the
earth is taken into account. A similar length of time is indicated for the
recession of the Falls of St. Anthony, of the Mississippi River, an
agreement that is of much interest, for it proves that the two rivers
began to make their respective cuttings when the great ice-sheet receded
to the north at the end of the Glacial epoch.
What has become of the masses washed away during the formation of these
gorges? As gravel and mud and silt the detritus has been carried to the
still waters of the lower levels, to be laid down and later solidified
into sandstone and slate and shale. All over the continents these things
are going on, and indefatigable forces are at work that slowly but surely
shear from the surface almost immeasurable quantities of earth and rock to
be transported far away. In some instances it is possible to find out just
how much effect is produced in a given period of time, especially in the
case of the great river systems. For example, the mass of the fine
particles of mud and silt carried in a given quantity of the water of the
Mississippi as it passes New Orleans can be accurately measured, and a
satisfactory determination can also be made of the total amount of water
carried by in a year. From these figures the amount of materials in
suspension discharged into the Gulf of Mexico becomes known. It is
sufficient to cover one square mile to the depth of 269 feet; in twenty
years it is one cubic mile, or five cubic miles in a century. Turning now
to the other aspect of this process, and the antecedent causes which
produce these effects, it appears that the area of the Mississippi River
basin is 1,147,000 square miles--about one third of the total area of the
United States. Knowing this, and the annual waste from its surface, it is
easy to demonstrate that it will take 6000 years to plane off an average
of one foot of soil and rock from the whole of this immense area. Of
course only an inch or a few inches will be taken from some regions where
the ground is harder or rockier, or where little rain falls, while many
feet will be washed away from other places. The waters of the Hoang-ho
come from about 700,000 square miles of country, from which one foot of
soil is washed away in 1464 years. The Ganges River, draining about
143,000 square miles, carries off a similar depth of eroded materials from
its basin in 823 years! Should we add to the above figures those that
specify the bulk of the chemical substances in solution carried by these
waters, the total would be even greater. We know that in the case of the
Thames River, calcareous substances to the amount of 10,000 tons a year
are carried past London, and all this mineral has been dissolved by
rain-water from the chalky cliffs and uplands of England, so that the land
has become less by this amount. Thus we learn that vast alterations are
being made in the structure of great continents by rain and rivers, as well
as by glaciers and other geological agencies. And at the same time that old
strata are undergoing destruction new ones are in process of construction
at other places, where animal remains can be embedded and preserved as
fossils. The forces at work seem weak, but they continue their operations
through ages that are beyond our comprehension and they accomplish results
of world-building magnitude.
Thus the whole process of geological construction is such that older
exposed strata continually undergo disintegration, but this involves the
destruction of any fossils that they might contain. The very forces that
preserve the relics of extinct animals at one time undo their work at a
later period. There are many other influences besides that destroy the
regularity of rock layers or change their mineralogical characters by
metamorphosis. It is easier to see how volcanic outbursts alter their
neighboring territory. The intense subterranean heat and imprisoned steam
melt the deeper substances of the earth's crust, so that these materials
boil out, as it were, where the pressure is greatest, and where lines of
fracture and lesser resistance can be found. Because so much detritus is
annually added to the ocean floors--enough to raise the levels of the
oceans by inches in a century--it is natural that greater pressures should
be exerted in these areas than in the slowly thinning continental regions.
These are some of the reasons why volcanoes arise almost invariably along
the shores or from the floors of great ocean beds. The chain that extends
from Alaska to Chili within the eastern shore of the Pacific Ocean, and
the many hundreds of volcanoes of the Pacific Islands bring to the surface
vast quantities of eruptive rocks which break up and overlie the
sedimentary strata formed regularly in other ways and at other times. The
volcanoes of the Java region alone have thrown out at least 100 cubic
miles of lava, cinders, and ashes during the last 100 years--twenty times
the bulk of the materials discharged into the Gulf of Mexico by the
Mississippi River in the same period of time.
From these and similar facts, the naturalist finds how agencies of the
present construct new rocks and alter the old; and so in the light of this
knowledge, he proceeds with his task of analyzing the remote past,
confident that the same natural forces have done the work of constructing
the lower geological levels because these earlier products are similar to
those being formed to-day. After learning this much, he must immediately
undertake to arrange the strata according to their ages. This might seem a
difficult or even an impossible task, but the rocks themselves provide him
with sure guidance.
Wherever a river has graven its deep way through an area of hard rocks, as
in the case of Niagara, the walls display on their cut surfaces a series
of lines and planes showing that they are superimposed layers formed
serially by deposits that have differed some or much at different times
according to the circumstances controlling the erosion of their
constituent particles. A layer of several feet in thickness may be
composed of compact shale, while above it will be a zone of limestone, and
again above this another layer of shale. Successive strata like these,
where they are parallel and obviously undisturbed, are evidently arranged
in the order of their formation and age. But by far the most impressive
demonstration of the basic principle of geology employed for the
determination of the relative ages of rocks is the mighty Cañon of the
Colorado. As the traveler stands on the winding rim of this vast chasm,
his eye ranges across 13 miles of space to the opposite walls, which
stretch for scores of miles to the right and left; upon this serried face
he will see zone after zone of yellow and red and gray rock arranged with
mathematical precision and level in the same order as on the steep slopes
beneath him. Plain common sense tells him that the great sheets of rock
stretched continuously at one time between the now separate walls, and
that the various strata of sandstone and limestone were deposited in
successive ages from below upwards in the order of their exposure. When
now he extends his explorations to another state like Utah or Wyoming, he
may find some but not all of the series exhibited in the Grand Cañon,
overlaid or underlaid by other strata which in their turn can be assigned
to definite places in the sequence. By the same method, the geologist
correlates and arranges the rocks not only of different parts of the same
state, or of neighboring states, but even those of widely separated parts
of North America and of different continents. But he learns that he must
refrain from over-hasty conclusions, for he soon finds that the
sedimentary rocks have not been constructed at the same rate in different
places during one and the same epoch, and that rocks formed even at one
period are not always identical in nature. But his guiding principle is
sensible and reasonable, and by employing it with due caution he provides
the palæontologist with the requisite knowledge for his special task,
which is to arrange the extinct animals whose remains are found as fossils
of various earth ages in the order of their succession in time.
CONDENSED TABLE OF PALAEONTOLOGICAL FACTS
__________________________________________________________________________
| | | |
YEARS | NUMBER OF | | | ORDER OF
NECESSARY FOR | FEET IN | GEOLOGICAL | GEOLOGICAL | APPEARANCE OF
FORMATION | THICKNESS | AGE | EPOCH | CHARACTERISTIC
| | | | GROUPS
______________|___________|______________|_______________|________________
| | | |
| | | | M B R A F I
| | | | a i e m i n b
| | | | m r p p s v r
| | Recent | | m d t h h e a
| | or | | a s i i e r t
| | Quaternary | | l l b s t e
| | | | s e i e s
| | | | s a -
______________|___________|______________|_______________|_|_|_|_|_|_|____
| | | | | | | | | |
| | | Pleistocene | | | | | | |
| | Cenozoic | Pliocene | | | | | | |
5,000,000 | 25,000 | or | Miocene | | | | | | |
| | Tertiary | Oligocene | | | | | | |
| | | Eocene | | | | | | |
______________|___________|______________|_______________|_|_|_|_|_|_|____
| | | | | | | | | |
| | Mesozoic | Cretaceous | | | | | | |
4,000,000 | 23,000 | or | Jurassic | | | | | | |
| | Secondary | Triassic | | | | | |
______________|___________|______________|_______________|_____|_|_|_|____
| | | | | | | |
| | | Permian | | | | |
| | Palæozoic | Carboniferous | | | |
21,000,000 | 106,000 | or | Devonian | | |
| | Primary | Silurian | | |
| | | Cambrian | | |
______________|___________|______________|_______________|________________
| | | |
20,000,000 | 30,000 | Azoic | Archæn |
______________|___________|______________|_______________|________________
After what seems an unduly long preparation, we now come to the actual
biological evidence of evolution provided by the results of this division
of zoölogical science. But all of the foregoing is fundamentally part of
this department of knowledge and it is absolutely essential for any one
who desires to understand what the fossils themselves demonstrate.
The oldest sedimentary rocks are devoid of fossil remains and so they are
called the Azoic or Archæan. They comprise about 30,000 feet of strata
which seem to have required at least 20,000,000 years for their formation.
This period is roughly two-fifths of the whole time necessary for the
formation of _all_ the sedimentary rocks, and this proportion holds true
even if the entire period of years should be taken as 100,000,000 instead
of 50,000,000 or less. The earth during this early age was slowly
organizing in chemical and physical respects so that living matter could
be and indeed was formed out of antecedent substances--but this process
does not concern us here. The important fact is that the second major
period, called the Palæozoic, or "age of ancient animals," saw the
evolution of the lowest members of the series,--the invertebrates,--and
the most primitive of the backboned animals, like fishes and amphibia. The
rocks of this long age include about 106,000 feet of strata, demanding
some 21,000,000 or 22,000,000 years for their deposition. Thus it is
proved that the invertebrate animals were succeeded in time by the higher
vertebrates, which is exactly what the evidences of the previous
categories have shown. When we remember that the lower animals are devoid
as a rule of skeletal structures that might be fossilized, and when we
recall the fact that the strata of the palæozoic provided the materials
out of which the upper layers were formed afterwards, we can understand
why the ancient members of the invertebrate groups are not known as well
as the later and higher forms like vertebrates. Yet all the fossils of
these relatively unfamiliar creatures clearly prove that no complex animal
appears upon a geological horizon until after some simple type belonging
to a class from which it may have taken its origin; in brief, there are no
anachronisms in the record, which always corresponds with the record
written by comparative anatomy, wherever the facts enable a comparison to
be made.
But the extinct animals of the third and fourth ages are more interesting
to us, because there are more of them and because they are more like the
well-known organisms of our present era. These two ages are called the
Mesozoic or Secondary, and the Cenozoic or Tertiary. The former is so
named because it was a transitional age of animals that are intermediate
in a general way between the primitive forms of the preceding age and
those of the next period; the latter name means the "recent-animal" age,
when evolution produced not only the larger groups of our present animal
series, but also many of the smaller branches of the genealogical tree
like orders and families to which the species of to-day belong.
Confining our attention to the large vertebrate classes, the testimony of
the rocks proves, as we have said, that fishes appeared first in what are
called the Silurian and Devonian epochs, where they developed into a rich
and varied array of types unequaled in modern times. At that period, they
were the highest existing animals--the "lords of creation," as it were. To
change the figure, their branch constituted the top of the animal tree of
the time, but as other branches grew upwards to bear their twigs and
leaves, as the counterparts of species, the species of the branch of
fishes decreased in number and variety, as do the leaves of a lower part
of a tree when higher limbs grow to overshadow them.
Following the fishes, the amphibia arose during the coal age or
Carboniferous, usurping the proud position of the lower vertebrate class.
The reptiles then appeared and gained ascendancy over the amphibia, to
become in the Mesozoic age the highest and most varied of the existing
vertebrates. At that time there were the great land dinosaurs with a
length of 80 feet, like _Brontosaurus_; aquatic forms like _Ichthyosaurus_
and _Plesiosaurus_, whose mode of evolution from terrestrial to swimming
habits was like that of seals and penguins of far later eras. Flying
reptiles also evolved, to set an example for the bats of the mammalian
class, for both kinds of flying organisms converted their anterior limbs
into wings, although in different ways.
During the Triassic and Jurassic periods of the Mesozoic age, the first
birds and mammals appeared to follow out their diverging and independent
lines of descent. Palæontology makes it possible to trace the origin and
development of many of the different branches that grew out of the
mammalian limb from different places and at different times during the
Mesozoic and the following age, called the Cenozoic, or age of recent
animals. It is unnecessary, however, for us to review more of the details:
the main result is obvious; namely, that the appearance of the great
classes of vertebrates is in the order of comparative anatomy and
embryology. Not only, then, is the fact of evolution rendered trebly sure,
but the general order of events is thrice and independently demonstrated
to be one and the same. Surely we must see that no reasonable explanation
other than evolution can be given for these basic facts and principles.
Turning now to the second division of palæontological evidence, we come to
those groups where abundant materials make it possible to arrange the
animals of successive epochs in series that may be remarkably complete.
For the reasons specified, the backboned animals provide the richest
arrays of these series, and such histories as those of horses and
elephants have taken their places in zoölogical science as classics. But
even among the invertebrates significant cases may be found. For example,
in one restricted locality in Germany the shells of snails belonging to
the genus _Paludina_ have been found in superimposed strata in the order
of their geological sequence. The ample material shows how the several
species altered from age to age by the addition of knobs and ridges to the
surface of the shell, until the fossils in the latest rocks are far
different from their ancestors in the lowermost levels. Yet the
intervening shells fill in the gaps in such a way as to show almost
perfectly how the animals worked out their evolutionary history. This
example illustrates the nature of many other known series of mollusks and
of brachiopods, extending over longer intervals and connecting more widely
separated ages like the Secondary and the present period.
Since the doctrine of evolution and its evidences began to occupy the
thoughts of the intellectual world at large, no fossil forms have received
more attention than the ancient members of the horse tribe. As we have
learned, a modern horse is described by comparative anatomy as a one-toed
descendant of remote five-toed ancestors. When the hoofed animals of
modern times were reviewed as subjects for comparative anatomical study,
the odd-toed forms arranged themselves in a series beginning with an
animal like an elephant with the full number of five digits on each foot
and ending at the opposite extreme with the horse. A reasonable
interpretation of these facts was that the animals with fewer toes had
evolved from ancestors with five digits, of which the outer ones had
progressively disappeared during successive geological periods, while the
middle one enlarged correspondingly. The facts provided by palæontology
sustain this contention with absolutely independent testimony.
Disregarding some problematical five-toed forms like _Phenacodus_, the
first type of undoubted relationship to modern horses is _Hyracotherium_,
a little animal about three feet long that lived during the Eocene period
of the Cenozoic epoch. Its forefeet had four toes each, and its hinder
limbs ended with three toes armed with small hoofs, but one of its
relatives of the same time has a vestige of another digit on the hind
foot. By the geological time mentioned, therefore, the earliest true
horses had already lost some of the toes that their progenitors possessed.
In the Miocene the extinct species, obviously descended from the Eocene
forms, had lost more of their toes; still higher, that is, in the rocks
formed during succeeding periods of time, the animals of this division are
much larger and each of their feet has only three toes, of which the
middle one is the largest while the ones on the sides are small and
withdrawn from the ground so as to appear as useless vestiges. To produce
modern horses and zebras from these nearer ancestors, few additional
changes in the structure of the feet are necessary, for the lateral toes
need only to become a little more reduced and the middle one to enlarge
slightly to give the one-toed limb of modern types, with its splint-like
vestiges still in evidence to show that the ancestor's foot comprised more
of these terminal elements. Comparing the animals of successive periods,
these and other skeletal structures demonstrate that the ancestry of each
group of species is to be found in the animals of the preceding epoch, and
that the whole history of horses is one of natural transformation,--in a
word, of evolution.
No less interesting in their own way are the remains of other hoofed forms
that lead down to the elephants of to-day and to the mammoth and mastodon
of relatively recent geologic times. Common sense would lead to the
conclusion that a form like a modern tapir was the prototype from which
these creatures have arisen, and common sense would lead us to expect that
if any fossils of the ancestors of the modern group of elephants occurred
at all they would be like tapirs. Thus a fossil of much significance in
this connection is _Moeritherium_, whose remains have been found in the
rocks exposed in the Libyan desert, for this creature was practically a
tapir, while at the same time its characters of muzzle and tusk mark it as
very close to the ancestors of the larger woolly elephants of later
geological times, when the trunk had grown considerably and the tusks had
become greatly prolonged. Again the fossil sequence confirms the
conclusions of comparative anatomy, regarding the mode by which certain
modern animals have evolved.
The fossil deer of North America, as well as many other even-toed members
of the group of mammalia possessing hoofs, provide the same kind of
conclusive evidence. The feature of particular interest in the case of
their horns, is a correspondence between the fossil sequence and the order
of events in the life-history of existing species,--that is, between the
results of palæontology and of embryology. Horns of the earliest known
fossil deer have only two prongs; in the rocks above are remains of deer
with additional prongs, and point after point is added as the ancient
history of deer is traced upwards through the rocks to modern species. We
know that the life-history of a modern species of animals reviews the
ancestral record of the species, and what happens during the development
of deer can be directly compared with the fossil series. It is a matter of
common knowledge that the year-old stag has simple spikes as horns, and
that these are shed to be replaced the following year by larger forked
horns. Every year the horns are lost and new ones grow out, and become
more and more elaborately branched as time goes on, thus giving a series
of developmental stages that faithfully repeats the general order of
fossil horns. Even Agassiz, who was a believer in special creation and an
opponent of evolution, was constrained to point out many other instances,
mainly among the invertebrata, where there was a like correspondence
between the ontogeny of existing species and their phylogenetic history as
revealed by the fossil remains of their ancestors.
* * * * *