LIFE IN THE PAST
Anyone who earnestly studies plants and animals as they exist in the
world to-day cannot help wondering how the earth began and where it
got its life. This is the true end and aim of geological study. The
history of man seems to run back into a far distant and gloomy past.
Except for the poetical account in Genesis and the traditions of
various peoples throughout the world, real history fades away into an
earlier time of which there are no written records. When the delvers
in the Mesopotamian plain talk to us of kingdoms running back through
seven or eight or nine thousand years, we seem to be getting back to
the beginnings of things. But seven or eight or nine thousand years
are as nothing in comparison with the age of the earth, which runs
back into a past so limitless that no man can safely assign any set
figure to it. In a recent paper, Dr. Walcott, of the Smithsonian
Institution, says that the antiquity of the earth must be measured not
in millions, for they are too short, nor hundreds of millions, for
this carries us too far, but must surely be measured in tens of
millions of years.
When we attempt to study the past we find its various epochs unequally
clear to us. In human history only quite modern times are absolutely
clear. The history of the Middle Ages is distinct enough for us to
build for ourselves a picture of the time with reasonable hope of
gaining a correct view of the state of affairs. Back of this comes the
long stretch of the Dark Ages, in which here and there we have bright
spots, but it will perhaps long be impossible to portray clearly the
life of the people. Getting back to the Romans, things once more
become reasonably plain, as is true also in the case of Greek history.
Back of this stretches the Egyptian with fair precision, and, older
than it, the Babylonian and Chaldean. But these past three have not
left nearly so definite an account for us as did the later
civilizations of Greece and Rome.
When we try to go back of these we must change our method of study
entirely. Writing is absent, and all we know of earlier men must be
inferred from a few pictures that were daubed on the rocks or carved
in ivory or bone, from tools made of stone or bone, from a few metal
or stone ornaments, or from the bones of the men themselves. Even so,
the history fades out without telling us its own beginnings. It is
quite as impossible for history to write its origins as it is for man,
from his own knowledge, to describe his birth.
What is true of the human story is quite as true of that of the earth.
Recent steps are very plain. We may read them with considerable
confidence. As we go deeper into the rocks and find older fossils, the
evidence becomes less certain. The animals differed enough from those
of to-day for us to be less sure what they were like. As we keep on
moving backward through time, and downward through the rocks, we find,
after a while, strata in which there are evidences of life that
existed long ago, but in which these traces are so altered that it is
impossible to tell what sort of living things existed; we learn only
that they were alive. Going back still further, these fade out. There
is no knowing when the earth began; there is no knowing when life
began upon the earth. It is not meant that men have not wondered, even
reckoned carefully, as to how long ago each of these events occurred.
Many speculations have proved entirely useless, a few remain as yet
neither confirmed nor disproved, and of such we shall speak.
For the last hundred years the theory of the earth's origin suggested
by the Marquis Pierre Simon De La Place, of France, near the end of
the eighteenth century, has held almost undisputed sway among men who
were willing to consider the question as open to human solution. This
theory is known as La Place's Nebular Hypothesis. When men began to
study the heavenly bodies with the newly invented telescope, new ideas
naturally sprang up. Among the objects which the glass disclosed were
the nebulae, which are great clouds of fire mist, glowing masses of
gas. They are scarcely visible to the naked eye, but are among the
most interesting objects in the heavens when seen through a telescope.
The other suggestive heavenly body was our sister planet, Saturn.
Besides having a full complement of moons, Saturn has around it, as
distant as we would expect moons to be, three great rings. These look
very much as if one's hat, with an enormously wide brim, should have
the connection between the rim and the hat broken out completely, but
the rim should still float around the hat without touching it and
should steadily revolve as it stood there. The rings of Saturn are not
solid like the suggested hat rim. They are evidently made up of a
great number of very small particles, each moving around the center of
Saturn. But the great cloud of them is spread out flat. At the
distance which Saturn is from the earth they look as if they made a
solid sheet. Furthermore, they do not form, as it were, one continuous
hat rim, but it is as if the rim were broken into three circular
sections, each bigger than the one inside it and separated from the
next by an area nearly as wide as the ring itself.
With such material in the heavens to guide him, La Place suggested
that the sun had once been an enormous fire mist scattered over an
area billions of miles in diameter. This gaseous material, by the
attraction of its particles for each other, began to condense and
contract. When the plug is pulled from a washbasin the particles of
water, in moving toward the center, in order to get out of the basin,
invariably set up a rotary motion. As the particles of this diffused
nebula began to gather together they, too, gave to the mass a rotary
movement. This grew more and more rapid, with greater contraction,
until the particles on the outer edge of the rotating mass had just so
much speed that the least bit more would make them tend to fly off as
mud would fly from a revolving wheel. When this point was reached
there was a balance of forces which made the outermost portion remain
as a ring while the rest contracted away from it, leaving it behind.
It was La Place's idea that this process had repeated itself, and ring
after ring had been left behind. Finally the sun condensed and grew
into a ball, occupying the center of the system. At varying distances
from it were to be found either rings or planets which had been formed
out of such rings. For La Place suggested that in a ring like this
the material could not be quite evenly distributed. While every
particle in the ring kept revolving around the sun, those in front of
the densest part were slowly held back by the attraction of the
thicker portion, while those behind it in rotation had their speed
hastened until finally all the material in the ring had collected at
one spot and a new planet was born. La Place believed that these
planets formed their moons in exactly the same way, and that Saturn
was simply a planet not all of whose moons had yet been formed. He
believed that this happy accident served to tell us how the universe
had been created.
Of course, so detailed a theory concerning anything of which we know
so little has always had much ridicule thrown upon it, and yet no
truly competing theory has been proposed until very recent times.
Within a few years a Planetesimal Theory has been announced, and is
gaining considerable prominence, although it is too early yet to say
whether it will supersede La Place's idea. In this theory, also, the
suggestion comes from the heavenly bodies. With the increasing study
of the nebulae, many forms of these interesting bodies have been
discovered. A very common type consists of a great coherent central
mass, with two or more arms extending from opposite sides in the form
of a spiral. This is as if gaseous revolving nebulae had come into
comparatively close proximity to a passing body. The visitor, by its
attraction, drew from the nebula a wisp of gas. The revolving motion
of the nebula gave to the attracted arm the spiral form.
These twisted arms are not equally dense throughout, but have
thickened knots here and there in their course. The Planetesimal
Theory suggests that these thickened knots are embryo planets and the
central portion of the nebulae an embryo sun. After all the material in
such a body has condensed either around the knots or about the central
mass a new solar system will be complete. As before stated, neither of
these theories can be said to be demonstrated. Each of them has points
in its favor and each has its difficulties. It is pleasant to know
what men have clearly thought concerning such questions, but for a man
not a trained geologist neither will carry much conviction. He will
still rest with his own early conclusion that whichever shall prove to
be true, for him his old formula is still valid, "in the beginning God
made the heavens and the earth." He will no longer think of God as
having shaped the balls with his own hand and thrown them into space;
he will no longer dream that it all occurred within a week not more
than six thousand years ago; but still to him will come the reverent
conviction that, whatever the plan by which it was accomplished, it
was still God's plan and God carried it out.
Now that we have tried to stretch our imagination back to the origin
of our globe, the question not unnaturally comes to our mind, how long
ago did all this happen? Is there any possible means of telling when
the history of the earth began? All such attempts lead either to
indefinite or to uncertain conclusions. Each man who essays the
problem approaches it from a different side and ends with a different
result. But no matter what the method of approach, all are agreed on
at least one point, the enormous length of time, as counted in years,
through which the earth has lasted.
One great mathematician worked on the basis of the rate of the present
cooling of the earth. Counting backward to the time when the earth's
surface must have been hotter, according to La Place's idea, he
decided that our globe has been cool enough for the existence of life
upon it for a period of somewhere in the neighborhood of one hundred
million years. Those who try to study the rate at which mud is being
deposited in our bays and at the mouth of our rivers, and who hence
try to deduce how long it has taken to produce the thickness of all
the stratified rock we know, arrive at a figure larger, rather than
smaller, than that mentioned above. The same is true of those who try
to count the age of the earth by the rate at which the present rivers
are carrying away their river basins, and hence who calculate how long
it has taken the rivers of the globe to wash away all the rocks which
it is quite clear have been carried out. Still others have attempted
to solve the problem by seeing how much salt the rivers are carrying
into the sea, and consequently how long it must have taken the sea to
become as salt as it is. A very late attempt has been based on the
alteration in the minerals that show radio-activity. Conservative
estimates, based on all of these, would give us a figure on which we
must not count with any exactness, but which will serve at least to
mark the present trend of opinion. We may put this figure at one
hundred millions of years.
The following table gives us the names of the periods into which the
geologist has divided the past history of the earth. The first column
gives a simple name, which, in each case, is a translation of the
technical name the geologist gives to the era. This technical name is
also given in parenthesis. The second column shows the number of years
ago at which this period may be placed, while the third column gives a
series of names most of which are in use in geology and which are
intended to indicate the stage of advancement of the higher animals in
that particular period. Some of these names are perhaps giving way to
later terms, but all of them will be understood by any geologist.
Most of them will serve to keep very clearly before the mind of the
ungeological the period which he is studying. Like all such tables,
this must be read from the bottom up. This arrangement is used because
the oldest rocks in the series are naturally at the bottom and the
newest rocks are on the top, though occasionally a region is
sufficiently upset partly to reverse the order.
TABLE OF GEOLOGICAL TIMES
------------+------------------------+---------------------------
| MILLIONS OF YEARS AGO | STAGES OF ANIMAL
ERAS | (VERY UNCERTAIN) | DEVELOPMENT
------------+------------------------+---------------------------
| | Age of Man
Recent Life | | (Quaternary)
(Cenozoic) | 0 to 5 | Age of Mammals
| | (Tertiary)
------------+------------------------+---------------------------
Middle Life | |
(Mesozoic) | 5 to 10 | Age of Reptiles
------------+------------------------+---------------------------
| | Age of Amphibians
| | (Carboniferous)
Ancient Life| | Age of Fishes
(Palaeozoic)| 10 to 25 | (Devonian)
| | Age of Invertebrates
| | (Silurian and Cambrian)
------------+------------------------+---------------------------
Dawn Life | | Earliest Animals and
(Eozoic) | 25 to 50 | Plants
------------+------------------------+---------------------------
Having seen what the scientist supposes to be the method of formation
of the earth itself, it will be interesting next to consider what the
biologist surmises as to the origin of the life upon the earth. Here
again two explanations hold. The one, and distinctly the older of the
two, says that at some time in the far distant past, under conditions
which are rarely if ever duplicated, out of the lifeless material of
the globe were produced simple and low forms of life. These could not
properly be called either animal or plant, but partook somewhat of the
nature of both. Of this there is at present no evidence whatever. The
only reason we have for suggesting it is that, if we understand the
past conditions on the earth, there was a time when life was
impossible. Now we find life. Hence it must have arisen. This of
itself, of course, furnishes no proof, but leads us to try to imagine
how the transition might have come about. Every scientist who believes
in this form of origin holds that if the exact conditions are repeated
the result will occur once more. He may believe that no such
repetition is possible, but he is confident that, if it could be, life
would arise again from lifeless matter.
This process of life arising from matter that is not alive is known as
Spontaneous Generation. Two hundred years ago it was supposed to occur
frequently. It was common belief that the beautiful pickerel weed
which borders our Northern lakes, after freezing, went into a sort of
protoplasmic slime out of which pickerel were produced. The eelgrass
of the river was supposed to yield eels in a similar fashion. The dead
bodies of animals were supposed to turn into maggots. Such crude ideas
of spontaneous generation are no longer possible. The whole science of
bacteriology absolutely presupposes the impossibility of spontaneous
generation in the flasks and test tubes of the laboratory. One or two
men of otherwise good standing in science still maintain that they are
getting new life in their own test tubes, but they fail utterly to
persuade the scientific world. I think it is a fair statement of the
position of science to-day to say that there is no evidence whatever
of spontaneous generation, excepting the presence of life upon the
globe.
Not all has been said, however, on this question. The chemist is
learning in the laboratory to produce many substances which, until
very recent times, were produced only in the bodies of animals or
plants. Dye-stuffs were originally gotten almost entirely from animal
or plant material. At present the great majority of them are made in
the laboratory, and in not a few cases they not only imitate the color
of the older material, but actually have identically the same
composition and constitution. The laboratory-made material is exactly
like that made by the animals or the plants.
The same is true with regard to a large number of the fruit flavors.
These are due to the presence of ethereal oils in the plant, and their
exact counterparts can now be produced in the laboratory, and can
serve every purpose of the fruit flavor itself. Alcohol has been
produced artificially, and alcohols, which nature never dreamed of
making, so far as we can tell, but which are made on her plan, are
manufactured by the chemist. Last of all, sugar has recently been
built up by the chemist, though the method at present is so expensive
that it cannot possibly compete with the production of the commodity
from the cane and the beet. As in the case of alcohol, all the sugars
that nature makes can now be made artificially, and others of the same
general plan which she seems not to have as yet devised can be
produced within the laboratory.
Attempts have been made to manufacture proteids, but these have as yet
eluded the efforts of the chemist. He is beginning, however, to come
nearer understanding their composition, and when he once clearly
comprehends that he may be able to reproduce them.
One of the German chemists is convinced that the nuclein in the
nucleus of the cell is not a very complicated compound. Under such
conditions it is not a matter of surprise that the physiological
chemist should be constantly dreaming that he may at some time produce
living matter in the laboratory. To the ordinary mind it scarcely
seems possible. We are so entirely sure that life is not amenable to
physics or chemistry that we can hardly conceive of the possibility of
its originating out of matter in the test tube. If it does so come,
and when it does so come, this will not prove that life is a less
noble and less wonderful thing than we thought. It will only prove
that chemistry and physics are more noble and more wonderful than we
dreamed.
There is another way of approaching this life problem, though it seems
to be rather a begging of the question than a solution of it. Of
recent years it has been discovered that even the very low
temperatures obtained by evaporating liquid air, say three hundred
degrees below zero, Fahrenheit, do not kill seeds or spores of mold.
The space between the planets is undoubtedly extremely cold. We have
always supposed it to be entirely too cold for life to exist in it.
But we laid little stress on the fact because we had no thought of any
possible life existing there. But the discovery that seeds and spores
can live uninjured through extreme cold has led to an interesting
suggestion. This is that when the earth became adapted to the presence
of life it was infected by germs transported on meteors from some
other system. According to this theory, organic dust through space is
ready to infect any planet which offers the conditions under which
life may arise. Of course this theory does not explain the origin of
life. It pushes back that origin a little farther or supposes that
life is as old as matter itself. Again we may leave to the scientist
the discussion and the elaboration of this or any other theory he may
promulgate concerning the origin of life. When he has established
clearly the process and can produce life we will accept his
explanation; meanwhile, we will always be interested in his attempts
to solve the problem, but still our simple formula, "in the beginning
God," serves our present needs and will satisfy us better than any as
yet unverified hypothesis.
When we find through scientific investigation how life arises we will
simply know how God created it in the beginning.
The next step in the understanding of early life is to study under the
microscope the simplest forms which we can find in existence to-day.
This, while far easier of execution than the problems which we have
thus far considered, is still not without serious difficulties. But
every day brings us nearer to the understanding of the structure of
living things. Life the scientist cannot see. All he can study is
living matter. Whether life can exist separate from living things is a
problem outside the range of his, at least present, possibilities.
Therefore, concerning it he has no answer whatever to give. But when
we come to study living things we find that all life is associated
with protoplasm. This apparently foamy, jellylike, transparent
material is the only living substance in all the world. Animals and
plants are larger or smaller collections of the little masses of
protoplasm which we know as cells. The lowest animals are each made up
of but a single cell. This consists of a small mass of protoplasm
surrounded almost always by a thicker skin or covering, known as the
cell wall and enclosing a complicated kernel known as the nucleus. The
protoplasm seems to be the living substance itself. The cell wall is
not a simple dead scum on the outside of the protoplasm, but is itself
able to do certain things which can only, so far as we know, be done
by living substances. For instance, of two materials dissolved in the
water in which the cell floats, the wall may permit one to soak into
the animal and keep the other out. The one allowed to enter will
usually be found good to be used for food by the cell. The nucleus
seems to store within itself the record of its past history and thus
enable the cell to do in the future what its ancestors did in the
past.
Such simple cells can exhibit in very low form all the activities the
higher animals show in much more elaborate development. A one-celled
animal can move about, can recognize the proximity of food, can engulf
its food and digest it, can build up its own substance out of the
digested food, can absorb oxygen, can use this oxygen in the burning
of its own substance to produce its own activities, can act in
response to sensation gained from outside, can throw off its waste
matter produced by its own activities, and can grow. When the proper
time comes its nucleus can split in two, the cell itself enclosing the
nucleus can separate into two cells, each of which can grow to the
size of the parent cell and repeat its life. This is as simple an
animal as we have yet discovered. Every kitchen drain swarms with such
creatures. On a summer day the stagnant pools are full of them. The
simplest microscope will show them clearly. This is life in its lowest
terms with which we are acquainted. With such life, it seems to us,
the animal and plant world must have started their existence, when
first the earth began to teem with living matter.
If, then, we may form any judgment concerning the first living things
upon the globe by considering the simplest creatures that live here
to-day, certain facts seem clear. In the first place, life began in
the water, and for a long time was only to be found in the water.
Single cells are so small and dry out so easily that it is necessary
to their existence that they should be kept entirely moist by the
presence of water all about them. It is true many of them will stand
drying, but while they are thus dried they can scarcely be said to be
much more than just alive. They are utterly inactive, or, as we say,
they are dormant. In such conditions they become covered with a tough
skin, almost a shell, and their protoplasm is itself nearly dry. Under
these circumstances the life processes hardly continue at all. The
protozoa, as these small animals are called, tolerate drought for a
time; but they only live, in any sense worth calling living, when
water is abundant and is neither very warm nor very cold. It is safe
to say that the early life of the world formed in the oceans of the
time. So absolutely is the habit fixed upon cells of protoplasm that
even to-day the activities of the cells of higher animals depend upon
the presence of moisture. The cells of our own bodies are to-day
living, as it were, in an ocean. Everyone can remember far enough back
to recall some time at which a tear slipped from his own eye onto his
own tongue; we know our tears are salt. The tongue has tasted,
undoubtedly, the perspiration from the lip on more than one summer
day; this perspiration tasted as salt as the tear itself. The lymph
that constitutes the "water" of a so-called "water blister" is also
salty, and even the little blood one gets into his mouth in trying
nature's method of stanching the flow from a cut finger gives the
impression that it contains a little salt. Every fluid of the body is
salty, and every cell of the body is bathed in salt water. It is too
long since the ancestors of our cells swam in the seas of the Eozoic
time for us to assert with any positiveness that the ancestral habit
is responsible for this trait in the descendants. Sure it is that
to-day our cells, like their ancestors of old, live in water, and this
water is slightly salty--as were probably the Archaean seas.
The geologist tries as best he may to build up the geography of the
earth in the past. He endeavors to judge from the rocks as he now
finds them, where the seas, the bays, the dry land, and the mountains
of earlier geological times lay. The present aspect of the earth is
very recent, and earlier ages must have shown an entirely different
distribution of land and water. The North American continent was
certainly very much smaller than it is now. The first known lands lay
close to the Atlantic seaboard and probably extended out into the
water some distance beyond the present shoreline. The stretch of
continent was narrow, and grew narrower as it went southward. In what
is now the Canadian district, a considerable expanse probably existed
in very early times. Then a great internal sea, shallower than the
Atlantic, stretched its unbroken sheet over almost the entire area now
occupied by the United States, while only a comparatively small hump
of earth, ending in a narrower strip, lay where the great Western
plateau now rears its enormous bulk.
A large portion of the history of the North American continent, with
its developing animals and plants, is tied up with the gradual
shrinkage of this interior sea. Slowly across the Canadian district,
the Eastern and Western lands became connected with each other, while
the waters progressively were pushed down the continent, which was
steadily growing from the east and from the north, though less slowly
from the west, into this internal sea. To-day only the Gulf of Mexico
remains as evidence of the broad stretch that once extended through to
the Arctic Ocean and west beyond the present position of the Rocky
Mountains.
How this great Eastern backbone of the continent was produced, what
sort of animals lived while these rocks were being formed, or whether
this preceded entirely the existence of life upon the earth, no man
to-day may surely say. In the oldest of the rocks there are beds of
graphite, from which lead pencils are made. This substance is believed
by the geologists to be, like coal, the remains of vegetable life. But
these early rocks have been so heated and baked, so twisted and bent,
that whatever forms of life they once held are now obliterated, or so
altered as to give us no idea of what may have been their character.
So far as anyone can now see, this past history is wiped out forever
and it will be impossible for men ever to demonstrate the character of
this early life. Speculations, more or less certain, will arise. They
may, after a while, seem so clear as to receive the acceptance of the
scientific mind. Yet the truth remains that the early history of the
earth, so far as animals and plants are concerned, is probably lost
forever.
The most striking feature concerning the earliest layers of rocks in
which good fossils are found abundantly is the complexity of the life.
With the exception of the backboned animals, every important branch of
the animal kingdom is represented, and it is just possible that we
have even earlier forms of the vertebrates themselves. This, to the
evolutionist, is very disconcerting. To find the great groups all well
developed at least twenty-five million years ago and to find only
fossils built on the same lines since almost nonplusses him. When the
geologist tells him what an enormous length of time preceded the rocks
in which he finds these fossils and how absolutely these earlier
strata have been altered by the later geological activities he easily
understands why it is impossible to find fossils in them. As a
consequence, the evolutionist is forced to believe that all the
earliest animals have left no clear traces behind them. Life as he
first surely knows it is already extremely varied and quite well
developed in some of its groups. The early animals were as well
adapted to the times in which they lived as are the great majority of
the animals of to-day. The reader must not infer this to mean that
the animals of those days were like our present animals. They were
not. No one traveling in a far country could find there animals as
strange to him as would be those of the earlier stratified rocks. In
these there were no fishes as we know them to-day, not a single member
of the frog and salamander class, not a reptile, not a bird, not a
mammal, and probably no air-living insects. It is highly doubtful
whether there was any animal living upon the land and breathing the
air twenty-five million years ago.
We start our study, then, at the period known as the Palaeozoic era,
the era of the ancient life of the globe, beginning twenty-five
million and ending ten million years ago. The first of the three
sections into which this period of life is divided is known as the
Silurian age, the age of invertebrates. The word invertebrate is an
unscientific but convenient term under which we embrace all the
animals below those having backbones. This period is called the age of
invertebrates because, although there is an enormous wealth of animal
and plant life in the Silurian, there are no backboned animals except
the lowest kinds of fishes. It was supposed for a long time that even
fishes were absent. Now we know they existed, but they were small and
inconspicuous. In this period corals were wonderfully abundant,
particularly in the great internal sea which spread over what is now
known as the Mississippi Valley. Everywhere over this region must have
grown in the shallow water great numbers of creatures called crinoids
or stone lilies. They were attached to the bottom by slender stems,
sometimes many feet long. These stems are jointed, and when they
became fossilized the sections were apt to separate, with the result
that over a wide area in the Mississippi Valley it is very common to
find these little segments which look not unlike checkers. At the end
of the stem was a rounded head, with a mouth at the top, and around
the mouth were branched, feathery arms. The creatures must have been
exquisitely beautiful, but they have completely disappeared from the
face of the earth, with the exception of a very few, found in the
obscurity of the almost fathomless depths of the great ocean. Here
they remain as peculiar relics, only preserved by the unvarying
conditions in the deep sea from the extinction that has met their
sisters.
Those who are familiar with our seacoast will know an interesting
creature known as the horseshoe crab, or king crab, though in reality
it is not a crab at all. It is rather more nearly related to the
spiders than the crabs, though no one but a technical zooelogist could
possibly associate them together. The ancestors of these king crabs
were the finest and best developed animals in this early Palaeozoic
time. These creatures had bodies jointed like the tail of a lobster.
They were wide and flat, instead of narrow and rounded like a lobster,
and each joint of the body was highest in the middle and distinctly
lower at the two sides, thus forming three regions along their backs.
This structure gives to these creatures the name of trilobites. These
animals were the kings of the early ocean. They had an interesting
habit of curling up nose to tail before they died, and, as a result, a
large proportion of all the trilobite fossils we find are curled in
this peculiar manner.
After these forms the most abundant fossils we find in Silurian times
were creatures that at first sight looked as if they might be related
to the clams. These are known as lampshells, because one shell
projects beyond the other and curls up at the tip so as to resemble
the clay lamps which are dug out of old Roman towns. The lampshells
also have nearly disappeared in modern times. Simple creatures
belonging with our present crab and snail had begun to make their
appearance, but they were not as abundant as we find them later on.
The third group of the mollusks to which the nautilus and squid of
to-day belong is very abundantly represented in the Silurian by
fossils with coiled-up shells. As for the plant life of the time, it
is exceedingly difficult to say much about it. There must have been
nothing but marine plants, and these must have been on the general
line of the seaweeds. Little can be definitely said concerning them.
The next period of the Palaeozoic is known as the Devonian age, or the
age of fishes. Now the backboned animals first make their clear and
unmistakable appearance. There are remains in the Silurian which show
that there must have been a few fishes at that time. The Devonian is
so full of them and they are so well developed and so diversified that
this period is definitely known as the "age of fishes." They do not
closely resemble the fishes of to-day, but anyone would recognize most
of them for what they are. Their bodies were covered, not so much with
scales as with heavy plates, often arranged like tiles, those on the
forward half of the animal being often larger than those surrounding
the rest of the body. The creature was encased, as it were, in armor.
These were the rulers of the Devonian seas. The land, as yet, was
probably nearly without animal life, the creatures thus far being
almost confined to the water. A few insects make their appearance and
a few thousand-leggers are running around among the lowly plants; a
few spider-like animals have arisen; there are a few snails that have
left the water and taken to the land. Altogether only the dawn of a
land fauna is to be noticed. In the Devonian the plants are creeping
up upon the ground. Ferns are growing about everywhere, though they
are not exactly our ferns, but are rather a sort of intermediate form
between these and the present seed plants.
Now comes an entire change in the history of the world. By some means
a rise in the bottom seems to have cut off a great part of the
internal sea from the outer ocean and to have converted it into a
widespread shallow bay, much like the sounds which lie back of the
islands that line the Atlantic Coast from New Jersey to Florida. Just
as this coastal region to-day is covered with salt marshes, so the
whole internal sea of the Carboniferous period was converted into a
great swamp. Sometimes an oscillation of the crust of the earth
brought this marsh above the surface of the sea and a luxuriant growth
of plants spread over it. Then a sinking of the bottom allowed the mud
and sand to wash down the shores, and spread out over the marsh, and
enclose the muck of the marsh under a layer of sand or clay. Another
lift of the bottom would start the swamp growing once more, and a
series of alternations between marsh land and sound seems to have
followed. The plants of this period are not the plants of to-day,
though we still have some very degenerate representatives of them. The
common horse-tail, with its angular, slender, leaflike branches and
its club-shaped spore-bearing body, is a modern degenerate descendant
of the treelike calamites of the Carboniferous forest. A creeping
evergreen, known by the name of clubmoss, is in like manner the modern
degenerate remnant of the scalestem and sealstem, which were the great
trees of the forests of the coal period.
All over the surface of the marsh, between these big trees, grew the
ferns. While the coal itself was formed generally from the scalestems
and sealstems, the most common fossils found in the shales that lie
upon the coal beds are the ferns which covered the surface of the
marsh.
It is believed by many geologists that this great luxuriant forest
points to a time when the climate was far warmer than it is to-day,
when the air was moist and heavily laden with carbon dioxide, and when
a great mass of clouds practically enveloped the earth. In this way
only do most geologists account for the enormous wealth of vegetation
in the Carboniferous period and for the abundance of plants up to the
Arctic Ocean, of the kinds that now grow chiefly in the tropics. But
of recent years a few geologists point to the fact that the peat bogs
of to-day, which seem to be the beginnings of future coal deposits,
are found almost entirely in cold countries. Hence it is a serious
matter to attempt to describe the climate of any part of the
Palaeozoic era. Certainly of the climate earlier than the Carboniferous
it is very risky to say anything definite.
The forests of the coal period seem actually to have cleared the air;
at least now we begin to find creatures related to our salamanders and
frogs moving about among the stumps of the marshes. These amphibians
are evidently the descendants of some of the fishes of the Devonian
times. Among these fishes were some which bear a great resemblance to
a few found in South America, in Africa and Australia to-day, and
which we know as lungfish. Anyone who has cleaned our fresh water
fishes in preparation for the table will remember that inside of them
there is a long slender bladder filled with air. This bladder assists
in making the fish light, hence making it easier for it to support
itself in the water. In certain swampy regions these lungfish swim
freely in the water of the marshes. When the dry season comes,
however, the water evaporates, draining the marshes completely. This
would prove the death of most fishes. The lungfish have a curious
habit which keeps them over the dry season. They cover themselves with
a coat of mud, inside of which there is a lining of slime produced
from their bodies. In such cocoon-like cases they survive the drought.
The means by which they breathe during this dry season is
interesting. The swim-bladder which we have just described in other
fishes is, with this lungfish, peculiarly spongy in its walls,
presenting a large surface full of blood vessels which absorb the air
on the inside of the bladder. This air the fish changes with moderate
frequency, the result being that the swim-bladder serves him exactly
as the lung serves a higher animal. To this fact he owes his name of
lungfish.
We sometimes gain much light concerning the past history of any
particular form of animal by studying the development of that animal
in the egg, or, in the case of the mammals, before birth. It is an
interesting fact that when the lung begins to form in the embryo it
starts as a simple sac which is an offspring from the gullet, and
occupies the position of the swim-bladder of the fish. This sac later
divides into two, and develops into the lungs of the animal. This
assures the zooelogist that the origin of the lungs in the higher
animals is found in the swim-bladder of the so-called lungfish. In
this Silurian time certain of these lungfish were perhaps trapped in
the basin in the marsh by the uplifting of the border. The waters
becoming progressively shallower and more crowded, these fishes took
to the land, their fins developing into awkward limbs which slowly
became more perfect.
To state the fact in this simple fashion is to make it seem far less
probable than is really the case. The simple forms of the life of
lowly creatures, as well as the simple character of the legs and feet
in the salamander class, make the explanation not so unlikely as would
at first sight appear. Suffice it to say that the scientist now
believes that out of the lungfish of the Devonian came the amphibians
of the Carboniferous period.
At the end of the coal period came the greatest change the face of the
globe had seen for many millions of years. Slowly the continent rose
on both sides of the old interior sea. A great plateau formed in the
region of the Alleghenies and another in the western district, though
this latter uplift was to be completely washed away, and later to rise
again into the Rocky Mountains and the Sierras. With the uplift at the
edges of the continent came a steady rise of the internal marshes,
until what had previously been swamp land became progressively first
dry land and, in the western part, even desert, in that respect being
somewhat like what it is now.
The amphibians of to-day (animals like the salamander and frog) all
lay their eggs in the water and their young have a tadpole stage. This
doubtless was true of the amphibians of the coal period. With the
beginning of the Mesozoic, or "middle life" period, a change and a
progression comes over the animal world. The tadpole life of the frog
is a rather lengthened one, while the toad has learned to crowd its
tadpole life within a few weeks. It would seem as if, in the earlier
times of the Mesozoic, this same change of habit had been going on.
With the drying up of the swamp, some of the amphibians crowded their
tadpole stage further and further back, until it was completely
accomplished before their young left the egg. An examination of the
development of the reptile in the egg will show a stage very similar
to the fish and to the amphibians, but this is all experienced before
the reptile emerges from the egg. The reptilian egg, unlike that of
the frog, is covered with a shell, packed away under the surface of
the ground, and left to its own fate. If, as most geologists believe,
the climate of the Mesozoic was distinctly warm, this habit of the
parent of forsaking the egg was not a serious matter. However the
creatures arose, it is certain that in this Mesozoic age reptiles
roamed the forests, swam the seas, and even flew in the air. Probably
at no other time in the earth's history has any one class of animals
so completely dominated the situation as did the reptiles of this age.
They were not only abundant; they were frequently enormously large.
Their skeletons are among the most interesting that we find to-day.
Gigantic lizards, seventy feet long and eighteen feet high at the
shoulders, dragged their heavy bodies through the marshy edges of the
lakes. Out upon the land others, not quite so heavy nor so large,
roamed about, some of them feeding upon the soft vegetation, others
having teeth fitted to tear down their herbivorous cousins. In some of
them the hind legs and tail were very heavy and the front legs so
light that it is quite clear they must have hopped around as do the
kangaroos to-day. Others of these reptiles went back to the sea, lost
the leglike development of their limbs and regained the flipper form,
though the bones of the fingers and toes are singularly
distinguishable in the paddle.
Strangest of all, a considerable group of these wonderful reptiles
lengthened their little fingers, sometimes to three or four feet in
length, and had a skin stretched from these fingers over to the body
in such a fashion as to give them wings not unlike those of the bat.
In the wing of the bat, however, four of the fingers of the hand run
through the membrane and support it. In the pterodactyl, as these
flying reptiles are called, the middle finger supports the web, while
the remaining fingers can still be used to clasp objects or serve the
animal to lift himself, as the bat can do with his thumbs.
Meanwhile an entire change is coming over the plant world. The last
third of this age of reptiles is known as the Cretaceous or chalk
period. Now, for the first time, the forests begin to take on more of
the character of our forests of to-day. Plants like our willow and
beech, poplar and sassafras appear in great abundance. Their broad
leaves serve better than those of any earlier plants to catch the
sunlight. But in addition they offered such effective evaporating
surface that they cast off rapidly the moisture obtained from the
ground by the plant. Accordingly in the winter season, when the water
in the ground is frozen and not available for plant purposes, they
were forced to throw away their leaves. It is quite possible that up
to and including the time of the Carboniferous, plants were all
evergreen. There had been before this little variation in climate over
the globe. Life in the Cretaceous begins to take on distinctly its
modern form.
Among the reptiles of the forest there appear to have been a few small
creatures which to an observer of those times, if there could have
been an observer, would have seemed of the utmost insignificance
compared with their giant cousins.
These little creatures climbed up into the trees to escape their
enemies. There were some in whom the skin, in front of the elbow and
behind the wrist, was loose, and stretched across the joint a little
like the wing of a bat. This reptile, climbing into the trees to
escape its enemies, found that this loose flap of skin served it
nicely, and sailed out of the trees in a manner not unlike that of
the flying squirrel of to-day. Among these experimenters in aviation,
certain forms produced scales which became elongated and finally slit
up along the side. These slit scales slowly developed into the
feathers of the birds of to-day. Whether the steps by which the change
occurred have been correctly stated or not, the result is sure. In the
rocks of the chalk period we find the remains of an interesting
creature. If nothing but its bones had been found it would have been
called a reptile. It had a long tail, it had claws on its front limbs;
it had teeth in its mouth; it had a flexible backbone. All of these
are reptilian rather than bird characters. Yet on the rocks
surrounding these bones are the unmistakable impressions of the
feathers of the wings and of the tail. Nothing in the world to-day has
feathers excepting the birds, and in this "ancient winged thing," for
this is the significance of its name--archaeopteryx--we have perhaps
the most remarkable link in the world between two distinct sections of
the animal kingdom. Here is a creature half reptile, half bird;
perhaps one-third reptile and two-thirds bird. It was about the size
of the crow. A little later unmistakable bird skeletons will appear,
but still their jaws are provided with long conical teeth.
Still more interesting from our standpoint is another set of primitive
animals, utterly insignificant in appearance, but of momentous
importance on account of their later history. Among these reptiles
were a few small creatures perhaps not much bigger than mice or moles.
Their teeth were a little more complicated and specialized than the
teeth of their reptilian cousins. Between their scales were small and
sparse hairs. Almost nothing but their jaws remain to-day to tell us
anything about them. But in this humble little creature of the
Mesozoic, utterly insignificant beside the tremendous reptiles of the
time, we discern the ancestor of the mammals. These were the
progenitors of the horses and cows, of the cats and dogs, of the
monkeys and apes, of the men of to-day.
During this chalk period, which forms the last portion of the age of
reptiles, life for the first time grew to look much as it does to-day.
Now, apparently, the cold of winter and the heat of summer followed
each other in regular succession. There have been colder and warmer
periods at various times in the previous history of the earth, but
undoubtedly they were more uniformly cold or uniformly warm than now.
Ages were warm, or ages were cold, but now the earth clearly shows the
annual alternations of summer and winter, and for the first time
clearly shows the bands of climate on the earth which we know as
zones.
In the chalk period this new factor of cold works mightily in favor
of the mammals. Their reptilian ancestors were cold blooded. When the
climate was warm they were active; when the climate was cold they were
sluggish. With the continuation of the annual alternations of cold and
warm weather that had now set in upon the earth, the little birds and
mammals had in their warm blood an advantage which, in the long run,
enables them not simply to compete with their reptile forefathers, but
to outdistance them absolutely in the race. Here and there, on earth
to-day, exist a few big reptiles like the crocodiles and the boa
constrictors. But they are few and comparatively insignificant among
the multitudinous population of the globe and are confined to the
hotter portions of the earth. For the most part, the reptiles now play
an insignificant and unobtrusive part. The little molelike creatures,
practically unnoticed between their feet in the later Mesozoic, have
come to supplant them entirely, and almost to rival them in size.
While the reptiles have grown steadily smaller, the mammals have
steadily become larger.
While there is no land mammal to-day as big as the heaviest of the
reptiles in the Mesozoic, the whale, which is one of the mammals that
has again taken to the ocean, surpasses in size even those gigantic
creatures. There never lived in the world before a creature quite so
big as the biggest of our whales. Size, however, is not the most
important point in any animal. Speed, sagacity, variability, and power
of adaptation, these are the qualities which the world prizes, and
these the new mammals possessed.
The next geological era is the Cenozoic, or period of modern life.
This is divided into two quite distinct sections, the Tertiary and the
Quaternary. This era began about five million years ago, roughly
speaking, and is still going on. The greater half of it is known as
the Tertiary. It was during this time that the mammals came to their
own. At first these creatures belonged to what the scientist knows as
generalized types. They are jacks-of-all-trades. The student of early
animal life finds in the little Phenacodus, which was scarcely bigger
than a good-sized setter dog, the beginnings from which many forms
have subsequently developed. This creature showed points of structure
which to-day may be seen in such diversified animals as the dog, the
horse, the rabbit, and the monkey. It is not, of course, suggested
that Phenacodus was the immediate ancestor of any of these. But there
were no animals in those times more like these I have mentioned than
was Phenacodus, and from forms like it in main features all of these
other animals have since been derived, each species of animal having
become adapted to one particular kind of life. The development of
diversified situations on the earth, the varieties of climate, the
variation between marsh and upland, between valley and plateau,
furnish a complexity of environment into each niche of which a new
form of animal fitted itself.
With the increased complexity of mammals comes the submergence of the
reptiles and amphibians to-day. In all sorts of situations we find
mammals. The old-fashioned continent of Australia is separated from
everything about it by deep water, impassable to any animal which
lives upon it. In this secluded country evolution is very slow and
animals are very antiquated. We still find there mammals with the
ancient habit of laying eggs in a hollow in the ground, though after
these eggs are hatched the young are nursed on the milk of the mother.
But on the great continental stretches, where competition is keen,
where the animal must battle for his life against a wide field of
other animals, where migration into new situations is possible, the
rapidity of the development has been very much greater.
It is in such a situation that man has arisen. In the extreme
southeastern portion of Asia, and on the islands lying close to the
coast, his highest non-human relatives, members of the ape family,
have reached their best development. These, of course, are not man's
ancestors. They are the less progressive members who are left behind
entirely in the race. Whether we have to-day any traces of the steps
by which man arose from the animal beneath him is vigorously disputed.
Eminent scientists will be found on both sides of this question.
Many scientific writers to-day take it for granted that one form,
discovered in Java, while it may not be in the absolutely direct line,
must be very close indeed to the line of ascent toward man out of the
apelike forms. A scientist by the name of DuBois, working in the banks
of a stream in south-central Java, found a thigh bone which seemed to
him exceedingly human in its general character and yet not absolutely
like the human thigh bone. The oncoming of the rainy season raised the
water in the river so that DuBois could not continue his search.
Returning a year later, and digging back deeper into this bank, he
found a skull cap and two molar teeth which seemed to him to belong to
the thigh bone, although they lay several yards farther back, but at
the same level in the bank.
When these bones were subsequently presented to a meeting of European
scientists by DuBois, he claimed to have found the "missing link" for
which there was so eager a demand. Some of the best anatomists of the
meeting, notably Virchow, laughed at his claim and said that the skull
cap was simply that of a human idiot, and could be duplicated in any
large asylum. A committee of twelve naturalists was appointed to
report upon DuBois' find. Of this committee three asserted the bones
to be those of a low-grade man, three insisted that they belonged to a
high ape, of a type somewhat higher than any we know to-day, but still
distinctly an ape. Six members of the committee of twelve agreed that
the remains were those of a creature higher than an ape and lower than
any normal man, and represented, in their opinion, a stage distinctly
along the line of development out of the apes and into man.
This so-called "Java find" is known in science by the name of
Pithecanthropus, which means the ape-man. Whether we look upon this
fossil as a serious find or not, it is very certain that in the caves
of Europe belonging to the Quaternary period we find abundant
evidences of primitive man. The older these evidences are, the more
likely they are to be distinctly below the grade of man of to-day, in
the size and shape of the brain case and in the length and massiveness
of the jaw.
There are probably more races than one represented among these skulls.
Some of them are surely well-deserving of the title of low brow. Their
heavy ridges over the eyes, their small foreheads, their massive,
heavy-set jaws show a race of men far less endowed mentally and much
better endowed in the matter of brute force than the men of to-day.
These skeletons, or parts of skeletons, are turning up every year, and
we are just beginning to know much about them. Capable men are
studying them with much care. The next fifty years may not improbably
make the history of the ascent of man as clear as is now that of the
horse, to which we shall refer later.
The whole question of the descent of man from the lower animals, or
his ascent from them, as Drummond aptly termed it, is to most people
so entirely repugnant as to set them at once, and finally, against all
willingness to consider the question of Evolution. This, however, does
not solve the problem. Even though truth be horribly unpalatable, it
is still to be believed if it is only the truth. There is practically
no doubt left among scientific men of the origin of man in lower
forms. The evidences grow more and more complete year by year, and
from every line of investigation. Whether we study his anatomy, his
embryology, his history, his language, or his civilization, all
indications point in the same direction. Constant discoveries indicate
the fact of an enormously long development from a very humble form. If
this proves to be true and remains unpalatable, the fault lies in the
palate and not in the truth. Gradually we are coming to understand
that there is no reason why this truth should be unpalatable. We
consider a rise from humble conditions to be the glory of our heroes;
we esteem it an added charm in their strength that they should have
developed from untoward surroundings. It is not a disgrace to man to
have descended from the apes. It is to the glory of man that he should
have ascended from forms not much more promising-looking than the apes
of to-day. We must repeat, however, that the apes were the
unprogressive members, and hence we must not judge man's ancestors too
harshly. It must have been in them to rise. But the great glory in the
thought of the humble ancestry lies in the possibilities of his
future. If out of a creature not materially unlike the gibbering ape
of to-day there should have come, under the guiding hand of an
Almighty God, creatures with the endowments and capabilities of man of
to-day, then this is only an earnest and foretaste of that which may
be expected in the future. A time will come when man shall have risen
to heights as far above anything he now is as to-day he stands above
the ape. Even then there seems no end. With Infinite Power as the
agent, and limitless time in which to work, man would be limiting God
to an extent unwarranted by the history of the past to imagine that
His process had stopped to-day, and that man, with his many
imperfections of body, of mind, and of morals, should be the best that
is yet to come. There cling to him still the limitations and dregs of
his brute life. Often the brute in him comes to the surface. Little by
little he is coming to be dominated by the qualities God has last
given him. Slowly the brute shall sink away, slowly the divine in him
shall advance, until such heights are attained as we to-day can
scarcely imagine. As we can scarcely conceive the beginnings of this
process, so we can with difficulty imagine its end. This only can be
seen by the Eternal through whom it shall all come to pass, and by
whom all will in time be accomplished.