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The Story of Evolution
THE STORY OF EVOLUTION BY JOSEPH McCABE
1912
PREFACE
An ingenious student of science once entertained his generation with a
theory of how one might behold again all the stirring chapters that
make up the story of the earth. The living scene of our time is lit by
the light of the sun, and for every few rays that enter the human eye,
and convey the image of it to the human mind, great floods of the
reflected light pour out, swiftly and indefinitely, into space.
Imagine, then, a man moving out into space more rapidly than light,
his face turned toward the earth. Flashing through the void at, let us
say, a million miles a second, he would (if we can overlook the
dispersion of the rays of light) overtake in succession the light that
fell on the French Revolution, the Reformation, the Norman Conquest,
and the faces of the ancient empires. He would read, in reverse order,
the living history of man and whatever lay before the coming of man.
Few thought, as they smiled over this fairy tale of science, that some
such panoramic survey of the story of the earth, and even of the
heavens, might one day be made in a leisure hour by ordinary mortals;
that in the soil on which they trod were surer records of the past
than in its doubtful literary remains, and in the deeper rocks were
records that dimly lit a vast abyss of time of which they never
dreamed. It is the supreme achievement of modern science to have
discovered and deciphered these records. The picture of the past which
they afford is, on the whole, an outline sketch. Here and there the
details, the colour, the light and shade, may be added; but the
greater part of the canvas is left to the more skilful hand of a
future generation, and even the broad lines are at times uncertain.
Yet each age would know how far its scientific men have advanced in
constructing that picture of the growth of the heavens and the earth,
and the aim of the present volume is to give, in clear and plain
language, as full an account of the story as the present condition of
our knowledge and the limits of the volume will allow. The author has
been for many years interested in the evolution of things, or the way
in which suns and atoms, fishes and flowers, hills and elephants, even
man and his institutions, came to be what they are. Lecturing and
writing on one or other phase of the subject have, moreover, taught
him a language which the inexpert seem to understand, although he is
not content merely to give a superficial description of the past
inhabitants of the earth.
The particular features which, it is hoped, may give the book a
distinctive place in the large literature of evolution are, first,
that it includes the many evolutionary discoveries of the last few
years, gathers its material from the score of sciences which confine
themselves to separate aspects of the universe, and blends all these
facts and discoveries in a more or less continuous chronicle of the
life of the heavens and the earth. Then the author has endeavoured to
show, not merely how, but why, scene succeeds scene in the chronicle
of the earth, and life slowly climbs from level to level. He has taken
nature in the past as we find it to-day: an interconnected whole, in
which the changes of land and sea, of heat and cold, of swamp and
hill, are faithfully reflected in the forms of its living population.
And, finally, he has written for those who are not students of
science, or whose knowledge may be confined to one branch of science,
and used a plain speech which assumes no previous knowledge on the
reader's part.
For the rest, it will be found that no strained effort is made to
trace pedigrees of animals and plants when the material is scanty;
that, if on account of some especial interest disputable or
conjectural speculations are admitted, they are frankly described as
such; and that the more important differences of opinion which
actually divide astronomers, geologists, biologists, and
anthropologists are carefully taken into account and briefly
explained. A few English and American works are recommended for the
convenience of those who would study particular chapters more closely,
but it has seemed useless, in such a work, to give a bibliography of
the hundreds of English, American, French, German, and Italian works
which have been consulted.
CONTENTS
I. THE DISCOVERY OF THE UNIVERSE II. THE FOUNDATIONS OF THE
UNIVERSE III. THE BIRTH AND DEATH OF WORLDS IV. THE PREPARATION
OF THE EARTH V. THE BEGINNING OF LIFE VI. THE INFANCY OF THE
EARTH VII. THE PASSAGE TO THE LAND VIII. THE COAL-FOREST IX. THE
ANIMALS OF THE COAL-FOREST X. THE PERMIAN REVOLUTION XI. THE
MIDDLE AGES OF THE EARTH XII. THE AGE OF REPTILES XIII. THE BIRD
AND THE MAMMAL XIV. IN THE DAYS OF THE CHALK XV. THE TERTIARY ERA
XVI. THE FLOWER AND THE INSECT XVII. THE ORIGIN OF OUR MAMMALS
XVIII. THE EVOLUTION OF MAN XIX. MAN AND THE GREAT ICE-AGE XX.
THE DAWN OF CIVILISATION XXI. EVOLUTION IN HISTORY INDEX
THE STORY OF EVOLUTION
CHAPTER I. THE DISCOVERY OF THE UNIVERSE
The beginning of the victorious career of modern science was very
largely due to the making of two stimulating discoveries at the close
of the Middle Ages. One was the discovery of the earth: the other the
discovery of the universe. Men were confined, like molluscs in their
shells, by a belief that they occupied the centre of a comparatively
small disk--some ventured to say a globe--which was poised in a
mysterious way in the middle of a small system of heavenly bodies. The
general feeling was that these heavenly bodies were lamps hung on a
not too remote ceiling for the purpose of lighting their ways. Then
certain enterprising sailors--Vasco da Gama, Maghalaes,
Columbus--brought home the news that the known world was only one side
of an enormous globe, and that there were vast lands and great peoples
thousands of miles across the ocean. The minds of men in Europe had
hardly strained their shells sufficiently to embrace this larger earth
when the second discovery was reported. The roof of the world, with
its useful little system of heavenly bodies, began to crack and
disclose a profound and mysterious universe surrounding them on every
side. One cannot understand the solidity of the modern doctrine of the
formation of the heavens and the earth until one appreciates this
revolution.
Before the law of gravitation had been discovered it was almost
impossible to regard the universe as other than a small and compact
system. We shall see that a few daring minds pierced the veil, and
peered out wonderingly into the real universe beyond, but for the
great mass of men it was quite impossible. To them the modern idea of
a universe consisting of hundreds of millions of bodies, each weighing
billions of tons, strewn over billions of miles of space, would have
seemed the dream of a child or a savage. Material bodies were "heavy,"
and would "fall down" if they were not supported. The universe, they
said, was a sensible scientific structure; things were supported in
their respective places. A great dome, of some unknown but compact
material, spanned the earth, and sustained the heavenly bodies. It
might rest on the distant mountains, or be borne on the shoulders of
an Atlas; or the whole cosmic scheme might be laid on the back of a
gigantic elephant, and--if you pressed--the elephant might stand on
the hard shell of a tortoise. But you were not encouraged to press.
The idea of the vault had come from Babylon, the first home of
science. No furnaces thickened that clear atmosphere, and the
heavy-robed priests at the summit of each of the seven-staged temples
were astronomers. Night by night for thousands of years they watched
the stars and planets tracing their undeviating paths across the sky.
To explain their movements the priest-astronomers invented the solid
firmament. Beyond the known land, encircling it, was the sea, and
beyond the sea was a range of high mountains, forming another girdle
round the earth. On these mountains the dome of the heavens rested,
much as the dome of St. Paul's rests on its lofty masonry. The sun
travelled across its under-surface by day, and went back to the east
during the night through a tunnel in the lower portion of the vault.
To the common folk the priests explained that this framework of the
world was the body of an ancient and disreputable goddess. The god of
light had slit her in two, "as you do a dried fish," they said, and
made the plain of the earth with one half and the blue arch of the
heavens with the other.
So Chaldaea lived out its 5000 years without discovering the universe.
Egypt adopted the idea from more scientific Babylon. Amongst the
fragments of its civilisation we find representations of the firmament
as a goddess, arching over the earth on her hands and feet, condemned
to that eternal posture by some victorious god. The idea spread
amongst the smaller nations which were lit by the civilisation of
Babylon and Egypt. Some blended it with coarse old legends; some, like
the Persians and Hebrews, refined it. The Persians made fire a purer
and lighter spirit, so that the stars would need no support. But
everywhere the blue vault hemmed in the world and the ideas of men. It
was so close, some said, that the birds could reach it. At last the
genius of Greece brooded over the whole chaos of cosmical
speculations.
The native tradition of Greece was a little more helpful than the
Babylonian teaching. First was chaos; then the heavier matter sank to
the bottom, forming the disk of the earth, with the ocean poured round
it, and the less coarse matter floated as an atmosphere above it, and
the still finer matter formed an "aether" above the atmosphere. A
remarkably good guess, in its very broad outline; but the solid
firmament still arched the earth, and the stars were little undying
fires in the vault. The earth itself was small and flat. It stretched
(on the modern map) from about Gibraltar to the Caspian, and from
Central Germany--where the entrance to the lower world was located--to
the Atlas mountains. But all the varied and conflicting culture of the
older empires was now passing into Greece, lighting up in succession
the civilisations of Asia Minor, the Greek islands, and then Athens
and its sister states. Men began to think.
The first genius to have a glimpse of the truth seems to have been the
grave and mystical Pythagorus (born about 582 B.C.). He taught his
little school that the earth was a globe, not a disk, and that it
turned on its axis in twenty-four hours. The earth and the other
planets were revolving round the central fire of the system; but the
sun was a reflection of this central fire, not the fire itself. Even
Pythagoras, moreover, made the heavens a solid sphere revolving, with
its stars, round the central fire; and the truth he discovered was
mingled with so much mysticism, and confined to so small and retired a
school, that it was quickly lost again. In the next generation
Anaxagoras taught that the sun was a vast globe of white-hot iron, and
that the stars were material bodies made white-hot by friction with
the ether. A generation later the famous Democritus came nearer than
any to the truth. The universe was composed of an infinite number of
indestructible particles, called "atoms," which had gradually settled
from a state of chaotic confusion to their present orderly arrangement
in large masses. The sun was a body of enormous size, and the points
of light in the Milky Way were similar suns at a tremendous distance
from the earth. Our universe, moreover, was only one of an infinite
number of universes, and an eternal cycle of destruction and
re-formation was running through these myriads of worlds.
By sheer speculation Greece was well on the way of discovery. Then the
mists of philosophy fell between the mind of Greece and nature, and
the notions of Democritus were rejected with disdain; and then, very
speedily, the decay of the brilliant nation put an end to its feverish
search for truth. Greek culture passed to Alexandria, where it met the
remains of the culture of Egypt, Babylonia, and Persia, and one more
remarkable effort was made to penetrate the outlying universe before
the night of the Middle Ages fell on the old world.
Astronomy was ardently studied at Alexandria, and was fortunately
combined with an assiduous study of mathematics. Aristarchus (about
320-250 B.C.) calculated that the sun was 84,000,000 miles away; a
vast expansion of the solar system and, for the time, a remarkable
approach to the real figure (92,000,000) Eratosthenes (276-196 B.C.)
made an extremely good calculation of the size of the earth, though he
held it to be the centre of a small universe. He concluded that it was
a globe measuring 27,000 (instead of 23,700) miles in circumference.
Posidonius (135-51 B.C.) came even nearer with a calculation that the
circumference was between 25,000 and 19,000 miles; and he made a
fairly correct estimate of the diameter, and therefore distance, of
the sun. Hipparchus (190-120 B.C.) made an extremely good calculation
of the distance of the moon.
By the brilliant work of the Alexandrian astronomers the old world
seemed to be approaching the discovery of the universe. Men were
beginning to think in millions, to gaze boldly into deep abysses of
space, to talk of vast fiery globes that made the earth insignificant
But the splendid energy gradually failed, and the long line was closed
by Ptolemaeus, who once more put the earth in the centre of the
system, and so imposed what is called the Ptolemaic system on Europe.
The keen school-life of Alexandria still ran on, and there might have
been a return to the saner early doctrines, but at last Alexandrian
culture was extinguished in the blood of the aged Hypatia, and the
night fell. Rome had had no genius for science; though Lucretius gave
an immortal expression to the views of Democritus and Epicurus, and
such writers as Cicero and Pliny did great service to a later age in
preserving fragments of the older discoveries. The curtains were once
more drawn about the earth. The glimpses which adventurous Greeks had
obtained of the great outlying universe were forgotten for a thousand
years. The earth became again the little platform in the centre of a
little world, on which men and women played their little parts,
preening themselves on their superiority to their pagan ancestors.
I do not propose to tell the familiar story of the revival at any
length. As far as the present subject is concerned, it was literally a
Renascence, or re-birth, of Greek ideas. Constantinople having been
taken by the Turks (1453), hundreds of Greek scholars, with their old
literature, sought refuge in Europe, and the vigorous brain of the
young nations brooded over the ancient speculations, just as the
vigorous young brain of Greece had done two thousand years before.
Copernicus (1473-1543) acknowledges that he found the secret of the
movements of the heavenly bodies in the speculations of the old Greek
thinkers. Galilei (1564-1642) enlarged the Copernican system with the
aid of the telescope; and the telescope was an outcome of the new
study of optics which had been inspired in Roger Bacon and other
medieval scholars by the optical works, directly founded on the Greek,
of the Spanish Moors. Giordano Bruno still further enlarged the
system; he pictured the universe boldly as an infinite ocean of liquid
ether, in which the stars, with retinues of inhabited planets, floated
majestically. Bruno was burned at the stake (1600); but the curtains
that had so long been drawn about the earth were now torn aside for
ever, and men looked inquiringly into the unfathomable depths beyond.
Descartes (1596-1650) revived the old Greek idea of a gradual
evolution of the heavens and the earth from a primitive chaos of
particles, taught that the stars stood out at unimaginable distances
in the ocean of ether, and imagined the ether as stirring in gigantic
whirlpools, which bore cosmic bodies in their orbits as the eddy in
the river causes the cork to revolve.
These stimulating conjectures made a deep impression on the new age. A
series of great astronomers had meantime been patiently and
scientifically laying the foundations of our knowledge. Kepler
(1571-1630) formulated the laws of the movement of the planets; Newton
(1642-1727) crowned the earlier work with his discovery of the real
agency that sustains cosmic bodies in their relative positions. The
primitive notion of a material frame and the confining dome of the
ancients were abandoned. We know now that a framework of the most
massive steel would be too frail to hold together even the moon and
the earth. It would be rent by the strain. The action of gravitation
is the all-sustaining power. Once introduce that idea, and the great
ocean of ether might stretch illimitably on every side, and the
vastest bodies might be scattered over it and traverse it in
stupendous paths. Thus it came about that, as the little optic tube of
Galilei slowly developed into the giant telescope of Herschel, and
then into the powerful refracting telescopes of the United States of
our time; as the new science of photography provided observers with a
new eye--a sensitive plate that will register messages, which the
human eye cannot detect, from far-off regions; and as a new
instrument, the spectroscope, endowed astronomers with a power of
perceiving fresh aspects of the inhabitants of space, the horizon
rolled backward, and the mind contemplated a universe of colossal
extent and power.
Let us try to conceive this universe before we study its evolution. I
do not adopt any of the numerous devices that have been invented for
the purpose of impressing on the imagination the large figures we must
use. One may doubt if any of them are effective, and they are at least
familiar. Our solar system--the family of sun and planets which had
been sheltered under a mighty dome resting on the hill-tops--has
turned out to occupy a span of space some 16,000,000,000 miles in
diameter. That is a very small area in the new universe. Draw a
circle, 100 billion miles in diameter, round the sun, and you will
find that it contains only three stars besides the sun. In other
words, a sphere of space measuring 300 billion miles in
circumference--we will not venture upon the number of cubic
miles--contains only four stars (the sun, alpha Centauri, 21,185
Lalande, and 61 Cygni). However, this part of space seems to be below
the average in point of population, and we must adopt a different way
of estimating the magnitude of the universe from the number of its
stellar citizens.
Beyond the vast sphere of comparatively empty space immediately
surrounding our sun lies the stellar universe into which our great
telescopes are steadily penetrating. Recent astronomers give various
calculations, ranging from 200,000,000 to 2,000,000,000, of the number
of stars that have yet come within our faintest knowledge. Let us
accept the modest provisional estimate of 500,000,000. Now, if we had
reason to think that these stars were of much the same size and
brilliance as our sun, we should be able roughly to calculate their
distance from their faintness. We cannot do this, as they differ
considerably in size and intrinsic brilliance. Sirius is more than
twice the size of our sun and gives out twenty times as much light.
Canopus emits 20,000 times as much light as the sun, but we cannot
say, in this case, how much larger it is than the sun. Arcturus,
however, belongs to the same class of stars as our sun, and
astronomers conclude that it must be thousands of times larger than
the sun. A few stars are known to be smaller than the sun. Some are,
intrinsically, far more brilliant; some far less brilliant.
Another method has been adopted, though this also must be regarded
with great reserve. The distance of the nearer stars can be positively
measured, and this has been done in a large number of cases. The
proportion of such cases to the whole is still very small, but, as far
as the results go, we find that stars of the first magnitude are, on
the average, nearly 200 billion miles away; stars of the second
magnitude nearly 300 billion; and stars of the third magnitude 450
billion. If this fifty per cent increase of distance for each lower
magnitude of stars were certain and constant, the stars of the eighth
magnitude would be 3000 billion miles away, and stars of the sixteenth
magnitude would be 100,000 billion miles away; and there are still two
fainter classes of stars which are registered on long-exposure
photographs. The mere vastness of these figures is immaterial to the
astronomer, but he warns us that the method is uncertain. We may be
content to conclude that the starry universe over which our great
telescopes keep watch stretches for thousands, and probably tens of
thousands, of billions of miles. There are myriads of stars so remote
that, though each is a vast incandescent globe at a temperature of
many thousand degrees, and though their light is concentrated on the
mirrors or in the lenses of our largest telescopes and directed upon
the photographic plate at the rate of more than 800 billion waves a
second, they take several hours to register the faintest point of
light on the plate.
When we reflect that the universe has grown with the growth of our
telescopes and the application of photography we wonder whether we may
as yet see only a fraction of the real universe, as small in
comparison with the whole as the Babylonian system was in comparison
with ours. We must be content to wonder. Some affirm that the universe
is infinite; others that it is limited. We have no firm ground in
science for either assertion. Those who claim that the system is
limited point out that, as the stars decrease in brightness, they
increase so enormously in number that the greater faintness is more
than compensated, and therefore, if there were an infinite series of
magnitudes, the midnight sky would be a blaze of light. But this
theoretical reasoning does not allow for dense regions of space that
may obstruct the light, or vast regions of vacancy between vast
systems of stars. Even apart from the evidence that dark nebulae or
other special light-absorbing regions do exist, the question is under
discussion in science at the present moment whether light is not
absorbed in the passage through ordinary space. There is reason to
think that it is. Let us leave precarious speculations about
finiteness and infinity to philosophers, and take the universe as we
know it.
Picture, then, on the more moderate estimate, these 500,000,000 suns
scattered over tens of thousands of billions of miles. Whether they
form one stupendous system, and what its structure may be, is too
obscure a subject to be discussed here. Imagine yourself standing at a
point from which you can survey the whole system and see into the
depths and details of it. At one point is a single star (like our
sun), billions of miles from its nearest neighbour, wearing out its
solitary life in a portentous discharge of energy. Commonly the stars
are in pairs, turning round a common centre in periods that may occupy
hundreds of days or hundreds of years. Here and there they are
gathered into clusters, sometimes to the number of thousands in a
cluster, travelling together over the desert of space, or trailing in
lines like luminous caravans. All are rushing headlong at
inconceivable speeds. Few are known to be so sluggish as to run, like
our sun, at only 8000 miles an hour. One of the "fixed" stars of the
ancients, the mighty Arcturus, darts along at a rate of more than 250
miles a second. As they rush, their surfaces glowing at a temperature
anywhere between 1000 and 20,000 degrees C., they shake the environing
space with electric waves from every tiny particle of their body at a
rate of from 400 billion to 800 billion waves a second. And somewhere
round the fringe of one of the smaller suns there is a little globe,
more than a million times smaller than the solitary star it attends,
lost in the blaze of its light, on which human beings find a home
during a short and late chapter of its history.
Look at it again from another aspect. Every colour of the rainbow is
found in the stars. Emerald, azure, ruby, gold, lilac, topaz,
fawn--they shine with wonderful and mysterious beauty. But, whether
these more delicate shades be really in the stars or no, three colours
are certainly found in them. The stars sink from bluish white to
yellow, and on to deep red. The immortal fires of the Greeks are
dying. Piercing the depths with a dull red glow, here and there, are
the dying suns; and if you look closely you will see, flitting like
ghosts across the light of their luminous neighbours, the gaunt frames
of dead worlds. Here and there are vast stretches of loose cosmic dust
that seems to be gathering into embryonic stars; here and there are
stars in infancy or in strenuous youth. You detect all the chief
phases of the making of a world in the forms and fires of these
colossal aggregations of matter. Like the chance crowd on which you
may look down in the square of a great city, they range from the
infant to the worn and sinking aged. There is this difference,
however, that the embryos of worlds sprawl, gigantic and luminous,
across the expanse; that the dark and mighty bodies of the dead rush,
like the rest, at twenty or fifty miles a second; and that at
intervals some appalling blaze, that dims even the fearful furnaces of
the living, seems to announce the resurrection of the dead. And there
is this further difference, that, strewn about the intermediate space
between the gigantic spheres, is a mass of cosmic dust--minute grains,
or large blocks, or shoals consisting of myriads of pieces, or
immeasurable clouds of fine gas--that seems to be the rubbish left
over after the making of worlds, or the material gathering for the
making of other worlds.
This is the universe that the nineteenth century discovered and the
twentieth century is interpreting. Before we come to tell the fortunes
of our little earth we have to see how matter is gathered into these
stupendous globes of fire, how they come sometimes to have smaller
bodies circling round them on which living things may appear, how they
supply the heat and light and electricity that the living things need,
and how the story of life on a planet is but a fragment of a larger
story. We have to study the birth and death of worlds, perhaps the
most impressive of all the studies that modern science offers us.
Indeed, if we would read the whole story of evolution, there is an
earlier chapter even than this; the latest chapter to be opened by
science, the first to be read. We have to ask where the matter, which
we are going to gather into worlds, itself came from; to understand
more clearly what is the relation to it of the forces or energies
--gravitation, electricity, etc.--with which we glibly mould it into
worlds, or fashion it into living things; and, above all, to find out
its relation to this mysterious ocean of ether in which it is found.
Less than half a century ago the making of worlds was, in popular
expositions of science, a comparatively easy business. Take an
indefinite number of atoms of various gases and metals, scatter them
in a fine cloud over some thousands of millions of miles of space, let
gravitation slowly compress the cloud into a globe, its temperature
rising through the compression, let it throw off a ring of matter,
which in turn gravitation will compress into a globe, and you have
your earth circulating round the sun. It is not quite so simple; in
any case, serious men of science wanted to know how these convenient
and assorted atoms happened to be there at all, and what was the real
meaning of this equally convenient gravitation. There was a greater
truth than he knew in the saying of an early physicist, that the atom
had the look of a "manufactured article." It was increasingly felt, as
the nineteenth century wore on, that the atoms had themselves been
evolved out of some simpler material, and that ether might turn out to
be the primordial chaos. There were even those who felt that ether
would prove to be the one source of all matter and energy. And just
before the century closed a light began to shine in those deeper
abysses of the submaterial world, and the foundations of the universe
began to appear.