This illustration and others in this book are reproduced from Dennis
Hird’s excellent work, “A Picture Book of Evolution,” with the kind
permission of the publishers, Watts & Co., London.]
Millions of ages ago, a collision or an explosion produced a nebula
in the region now occupied by our solar system. That nebula spread
out over thousands of millions of miles of space. Here and there, in
its mighty body, a more solid nucleus drew to itself volumes of the
surrounding substance. In this manner the planets arose, leaving the
sun, with his giant mass, in the center of the field. As the nebula
revolved in one direction, mathematical necessity imparted to all
the forming bodies a whirling motion around the central sphere. The
smaller bodies cooled rapidly, so to speak, and crusted over; but the
sun, owing to his immense bulk, has continued to glow through all
these countless ages, and still sends forth enough light and heat to
illuminate and warm many billions of worlds like ours. How long a time
has elapsed since the earth began to condense from the nebula, the
human mind cannot conceive; but Sir G. H. Darwin, the son of Charles
Darwin, declares it is not unreasonable to suppose that from five
hundred to a thousand million years have passed away since the moon was
detached from the earth.
[Illustration]
[Illustration]
[Illustration: FIG. 7.—SEGMENTATION OF THE FERTILIZED OVUM AND
GASTRULATION.
4, morula; 5, section through blastula showing hollow sphere; 6,
gastrula showing outer layer of cells (epiblast) and inner layer
(hypoblast); the 6 is at the mouth of the cavity (enteron) of the
gastrula. From Dr. D. Kerfoot Shute’s “A First Book in Organic
Evolution.” Courtesy of The Open Court Publishing Co.]
When the moon was born, the earth was in a plastic state, and many
millions of years had yet to elapse before anything like a solid
surface could begin to appear upon it. When, at length, the cooling
globe did begin to form a crust, that crust was from time to time
shattered to fragments by violent eruptions from within. Later, the
immense quantities of hydrogen and oxygen that were in the atmosphere
combined into molecules of water, and torrents of rain settled upon the
hot surface of the earth. Owing to the large admixture of carbonic acid
gas which it contained, the atmosphere was then fifty times heavier
than it is to-day; but notwithstanding this great pressure upon it,
the water that settled on the hot crust of the earth could not remain,
and was sent, hissing, into the air in clouds of steam. The time came,
however, when the cooling surface of the planet no longer offered such
vigorous resistance to the water that fell upon it, and gradually
as the rain fell, the earth became almost entirely covered with a
boiling ocean. But the hot earth, even under the enormous weight of its
ocean and its dense air, was restless, and as time passed away, great
stretches of land emerged. Upon this new land, the rain now poured down
in floods. This caused a great washing of debris into the surrounding
sea. There the debris settled, and beneath the tremendous pressure of
the ocean, it became solidified into the oldest stratified rocks.
[Illustration]
[Illustration: FIG. 8.—CAMBRIAN FOSSILS (above); UPPER SILURIAN FOSSILS
(below).]
The rock pillar (Fig. 6) represents the rock formation of the crust
of the earth. It gives us an idea of what a cross-section of the
earth’s crust would look like, if all the rock deposits could be found
superimposed one above the other in any one place. These rocks, built
up layer upon layer, by sedimentary deposits in the ocean, are believed
by geologists to reach a depth of more than forty miles; and it is held
that it must have required at least a hundred million years—perhaps a
thousand million years—to lay down all the strata, all the series of
layers of rock, that form the crust of the globe.
That these immense depths of rock have been formed by a slow process
of growth, of gradual up-building, is certain. The world was not made
in a day. It has grown through innumerable ages. It is still growing.
At the present time, the rivers of England are carrying away thousands
of tons of land every year and depositing it on the floor of the sea.
Every year the Mississippi river carries four hundred million tons of
solid material into the Gulf of Mexico. This one stream alone displaces
more than a million tons of solid matter every day. Similar things
are occurring, in greater or less degree, in every part of the world.
Everywhere rivers are widening and deepening their channels, cutting
their way into mountains, depositing sediment for rock formations, or
overflowing and fertilizing soil; everywhere mountains and hills are
being worn down by the action of the elements; continents are crumbling
into the ocean; island surfaces are rising above or falling beneath the
waves. Nature is ever flowing. Throughout her infinite domain there is
everlasting movement. Her trademark is eternal change. Nowhere is she
at rest. Her labors never cease. To-day, as in the past, she builds and
destroys. In her endless process of evolution, every day is a day of
creation.
[Illustration: FIG. 9.—THE AMPHIOXUS.
This illustration and several others used in this book are reprinted
from J. A. S. Watson’s “Evolution.” Courtesy of The Frederick A. Stokes
Co.]
These rocks contain fossils, the skeletons of creatures that lived
during the periods when the rocks were being formed. This fact is one
of the keys to the temple of evolution; for when we know the kind of
fossils the different rocks contain, as we rise from the lower to the
higher, we know the kind of creatures that lived in the various periods
when those rocks were formed. Let myth-makers say what they will,
Nature does not lie.
As the crust of the earth cooled, the enveloping ocean gradually lost
its high temperature, and having fallen far below the boiling point,
continued to be warm for ages. In that warm primeval ocean, where the
various elements entered into all sorts of combinations, the necessary
elements combined in the proper proportions, and, as a result of that
happy combination, life was born into the world. Just as the requisite
proportions of charcoal, sulphur and saltpeter unite in making
gunpowder—a high explosive, vastly different from its constituent
elements—so, simple elements, uniting in due proportions, produced the
phenomenon of living matter.
[Illustration: FIG. 10.—THE EARLIEST KNOWN FORM OF FISH (upper); THREE
OTHER EARLY FORMS OF FISHES (lower).]
Whether life is still being evolved from non-living matter, or whether
its origin was confined to a peculiar set of favorable conditions—very
high temperature, and unusual chemical and electrical conditions—in
the early world, remains as yet unknown. On this question, the most
distinguished biologists entertain diverse opinions. Among those who
hold that the evolution of life is still one of the ordinary processes
of Nature’s day’s work may be mentioned Professor Benjamin Moore,
F.R.S., whose views are set forth in his excellent little work, “The
Origin and Nature of Life.”
All living things, from the blade of grass to the giant oak, from the
worm to the philosopher, are composed of cells, and all cells are
composed of protoplasm. Protoplasm is, therefore, the physical basis
of life. This wonderful substance is a compound of carbon, hydrogen,
nitrogen and oxygen, and the story of the evolution of living things
spells the limitless transformations of which this vital substance is
capable.
In the world of to-day, we see the finished forms that have been beaten
out, as it were, upon the anvil of the evolutionary process; and if
we would appreciate the progressive march that life has made, from
the simple forms of early days to the highly developed and complex
creatures of to-day, we must trace the story of evolution through the
numberless ages of the past.
[Illustration: FIG. 11.—LAMPREYS (left). THE MOUTH OF A LAMPREY
(right).]
The first living things consisted of a single cell—the Protozoa. As
this cell grew, it divided into two; the two, having grown somewhat,
divided into four; the four into eight; the eight into sixteen, and so
on. In this way, the growing cells increased their numbers by division,
and what was at first a single cell became an organized group of cells.
Fig. 7 illustrates the process of division and growth from the single
cell—the fertilized ovum—to the gastrula. This division is the law of
life. Every living creature begins its existence in this way.
Gradually, some cells specialized in the performance of the simplest
animal functions, like the catching of food and digestion; and in the
course of time, such simple forms of life as sponges and marine worms
appeared. From flat worms, life advanced to the annelid or ringed
worms—little creatures with a food canal and a body cavity filled with
blood. In the worm begins the first development of the brain. A few
sensitive cells in the fore end of the body, that beginning of mental
life, is a far-flung prophecy of the genius that will one day thrill
the world. Primitive depressions, lined with pigment cells, in the
worm-like head, represent Nature’s first reaching for eyes; two other
sensitive nerve pits are the beginning of the nose; yet a further pair
of susceptible concave surfaces are destined to develop into the ears
of land animals. Creatures with these primitive structures still exist
among Nature’s lowly forms.
[Illustration: FIG. 12.—LUNG FISHES.
Australian Lung Fish, Ceratodus (top); South African Lung Fish,
Protopterus (middle); South American Lung Fish, Lepidosiren (bottom).]
From worms and worm-like creatures, were developed snails and a great
variety of small animals, covered with shells. The Cambrian Rocks, the
earliest rocks that have preserved fossils, have yielded the remains
of some of these shell creatures (Fig. 8, upper). The Upper Silurian
Rocks, belonging to a period much later than the Cambrian, have
entombed the fossils of the molluscs shown in Fig. 8 (lower).
We must understand that by this time living things had been evolving
for millions of years, yet these shelled animals were the highest
forms that had so far appeared. We must understand, too, that only the
skeletons of creatures possessing a bony frame could be preserved as
fossils. The fleshy part of the body, Nature destroyed. Moreover, it
must be realized that of the countless billions of creatures that have
lived, the rocks have preserved the fossil remains of only a few. We
must not expect too much from Nature’s mutilated record. It is enough
to know that the specimens that have been preserved prove the gradual
unfoldment of life, and enable us to interpret the wonderful story of
evolution.
[Illustration: FIG. 13.—THE FLYING FISH.]
There is a small animal known as the Amphioxus (Fig. 9). Standing
midway between the worm and the fish, its distinguishing peculiarity
lies in the fact that it has a rod of cartilage—the notochord—extending
along its back, over which runs a line of nerve cells. This creature,
the child of the worm and the parent of the fish, is of singular
importance, since it foretells the coming of the vertebrates—the
creatures with a backbone. In due time the fishes were evolved from
the Amphioxus. The first fish appeared in the Devonian period, that is
to say, when about one-third of the whole geological series of rocks
had been formed. Fig. 10 (upper) represents a fossil of the earliest
known fish. The skeleton shows a primitive form. The lower specimens
represent other early fishes. Observe the curious worm-like resemblance
of the middle one. There are still fishes of very unfinished form.
Lampreys (Fig. 11) show, as it were, fishes in the making. They have
strangely undeveloped heads, no jaws, and only a crude sucker-like
cavity for a mouth.
The early fishes had no bones in their bodies. Their skeletons were
composed of cartilage. Primitive fishes of to-day—sharks, rays, and
others—have no bones. These fishes continue lines of descent from
ancestors that appeared before the bony frame had been evolved.
Life was born in the sea; it moved from the sea to the land; and when
this advance was made, it was the fishes that led the way. Some fishes
developed lungs and began, tentatively at first, to live on the shore
or in marshes. Life was moving towards the amphibians, and the evidence
of its advance in this direction has been preserved. As the Amphioxus
is the link between the worm and the fish, so lung fishes are links
between the true fishes and the amphibians. Fig. 12 (at the top) shows
the Burnett salmon, of Queensland—a fish with one lung; below are two
mud-fishes of Africa and Brazil—fishes with two lungs. These lung
fishes, or double-breathers, have the characteristics both of the fish
and the frog. To scales and gills and fins and other features of the
fish, they add lungs, nostrils, the beginning of a three-chambered
heart, and other features of the frog. Living in regions from which the
water periodically disappears, these creatures build around themselves
in the dry season a shell of mud and leaves, and there, while awaiting
the return of the water, they breathe air, and live on the fat stored
up in their tails.
[Illustration: FIG. 14.—THE CLIMBING PERCH.]
These lung fishes can walk on their fins; in fact, the fins of some of
them are formed more like legs than fins.
The Flying Fish (Fig. 13) is another variation. This fish can sustain
itself in the air for a hundred yards or more. Yet another curious fish
that will not stay in the water is the Climbing Perch (Fig. 14). This
fish may be seen crossing fields in India, and with the use of its fins
it even climbs trees. These strange fishes are surely links to higher
forms of life.
That the amphibian has been evolved from the fish may be seen in the
evolution of the frog (Fig. 15). Number 1 shows the newly-hatched
tadpoles; 2 and 2a show the branching, external gills; 3 to 8
illustrate further steps in the evolutionary process. The fish-like
tail, so prominent in the early stages, is finally absorbed and we have
the finished frog.
In the evolution of the frog we have a most suggestive illustration of
the transformation of a creature during a single lifetime. The fish
becomes an amphibian; the gilled, water-breathing creature becomes a
lunged, air-breather; a water animal leaves its habitat for a home on
land; a vegetable diet is abandoned for one of flesh. Truly a striking
summary instance of the power of evolution!
[Illustration: FIG. 15.—THE EVOLUTION OF THE FROG.]
All that man has become, all the wealth and worth of the civilization
he has achieved, has been due to the fact that he has possessed a hand
which could obey the command of his brain. Without a hand, without
fingers, man would still be a wild beast of the forest. It was in the
amphibian that Nature first produced the five divisions of the foot,
which, inherited by the reptiles and then by the mammals, in the end
became specialized into the human hand. The beginning of the hand is
seen in the foot of the frog.
In the Carboniferous period, when the coal measures were laid down,
appeared the wedge-headed amphibian, shown in Fig. 16 (above) and
later, in the Permian period, the roof-headed amphibian (Fig. 16,
below) was born into the world. This roof-headed amphibian is all
the more interesting, for from some of these creatures were born the
reptiles, from which, in turn, arose the mammals.
From the amphibians were developed the true reptiles, and these
branched out into many forms. Some lived in the water, some roamed
on the land, some flew in the air. In a warm climate, and where food
abounded, some of these creatures, like the Ceratosaurus (Fig. 17), the
Atlantosaurus and the Diplodocus, grew to a prodigious size. Some were
fifty, some a hundred, some a hundred and fifty feet long; some had
a hundred teeth, and eyes fifteen inches across; some weighed ninety
tons, and made footprints a yard square. It was in the Mesozoic times,
millions of ages ago, when these ungainly monsters were the monarchs of
the earth. Happily, they have long since been extinct, and to-day their
colossal, though harmless, skeletons may be studied in the museums of
the world.
[Illustration: FIG. 16.—THE HEAD OF THE FAMOUS ARCHAEGOSAURUS.
The wedge-headed Amphibian (above); the Branchiosaurus—the roof-headed
Amphibian (below).]
From the reptiles came the birds. The first birds had teeth, claws
on their wings, and bony tails of many joints (Fig. 18). The fossil
remains of two of these reptile-birds—the earliest birds known—were
found, some years ago, in the Jurassic limestone strata of Bavaria.
These creatures had thirty-two teeth, three clawed fingers on each
wing, and a lizard-like tail of twenty joints, with two long feathers
growing out of each vertebra. Occupying the ground midway between the
reptile and the bird, having the characteristics of both—the link
between the four-legged animal and the feathered songster of the
air—the Archaeopteryx, as this ancient bird is called, was about the
size of a crow.
Another line of development led from the reptiles to the mammals—the
hair-clothed creatures that suckle their young. This was the most
promising line of Nature’s advance, for at the end of this line, man
was destined to appear.
[Illustration: FIG. 17.—THE CERATOSAURUS.