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The Story of Evolution by Joseph McCabe

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This etext was scanned with OmniPage Pro OCR software donated by
Caere by Dianne Bean, Chino Valley, AZ. from a 1921 edition.




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

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





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

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

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.


To the mind of the vast majority of earlier observers the phrase
"foundations of the universe" would have suggested something
enormously massive and solid. From what we have already seen we
are prepared, on the contrary, to pass from the inconceivably
large to the inconceivably small. Our sun is, as far as our
present knowledge goes, one of modest dimensions. Arcturus and
Canopus must be thousands of times larger than it. Yet our sun is
320,000 times heavier than the earth, and the earth weighs some
6,000,000,000,000,000,000,000 tons. But it is only in resolving
these stupendous masses into their tiniest elements that we can
reach the ultimate realities, or foundations, of the whole.

Modern science rediscovered the atoms of Democritus, analysed the
universe into innumerable swarms of these tiny particles, and
then showed how the infinite variety of things could be built up
by their combinations. For this it was necessary to suppose that
the atoms were not all alike, but belonged to a large number of
different classes. From twenty-six letters of the alphabet we
could make millions of different words. From forty or fifty
different "elements" the chemist could construct the most varied
objects in nature, from the frame of a man to a landscape. But
improved methods of research led to the discovery of new
elements, and at last the chemist found that he had seventy or
eighty of these "ultimate realities," each having its own very
definite and very different characters. As it is the experience
of science to find unity underlying variety, this was profoundly
unsatisfactory, and the search began for the great unity which
underlay the atoms of matter. The difficulty of the search may be
illustrated by a few figures. Very delicate methods were invented
for calculating the size of the atoms. Laymen are apt to
smile--it is a very foolish smile--at these figures, but it is
enough to say that the independent and even more delicate methods
suggested by recent progress in physics have quite confirmed

Take a cubic millimetre of hydrogen. As a millimetre is less than
1/25th of an inch, the reader must imagine a tiny bubble of gas
that would fit comfortably inside the letter "o" as it is printed
here. The various refined methods of the modern physicist show
that there are 40,000 billion molecules (each consisting of two
atoms of the gas) in this tiny bubble. It is a little universe,
repeating on an infinitesimal scale the numbers and energies of
the stellar universe. These molecules are not packed together,
moreover, but are separated from each other by spaces which are
enormous in proportion to the size of the atoms. Through these
empty spaces the atoms dash at an average speed of more than a
thousand miles an hour, each passing something like 6,000,000,000
of its neighbours in the course of every second. Yet this
particle of gas is a thinly populated world in comparison with a
particle of metal. Take a cubic centimetre of copper. In that
very small square of solid matter (each side of the cube
measuring a little more than a third of an inch) there are about
a quadrillion atoms. It is these minute and elusive particles
that modern physics sets out to master.

At first it was noticed that the atom of hydrogen was the
smallest or lightest of all, and the other atoms seemed to be
multiples of it. A Russian chemist, Mendeleeff, drew up a table
of the elements in illustration of this, grouping them in
families, which seemed to point to hydrogen as the common parent,
or ultimate constituent, of each. When newly discovered elements
fell fairly into place in this scheme the idea was somewhat
confidently advanced that the evolution of the elements was
discovered. Thus an atom of carbon seemed to be a group of 12
atoms of hydrogen, an atom of oxygen 16, an atom of sulphur 32,
an atom of copper 64, an atom of silver 108, an atom of gold 197,
and so on. But more correct measurements showed that these
figures were not quite exact, and the fraction of inexactness
killed the theory.

Long before the end of the nineteenth century students were
looking wistfully to the ether for some explanation of the
mystery. It was the veiled statue of Isis in the scientific
world, and it resolutely kept its veil in spite of all progress.
The "upper and limpid air" of the Greeks, the cosmic ocean of
Giordano Bruno, was now an established reality. It was the
vehicle that bore the terrific streams of energy from star to
planet across the immense reaches of space. As the atoms of
matter lay in it, one thought of the crystal forming in its
mother-lye, or the star forming in the nebula, and wondered
whether the atom was not in some such way condensed out of the
ether. By the last decade of the century the theory was
confidently advanced--notably by Lorentz and Larmor-- though it
was still without a positive basis. How the basis was found, in
the last decade of the nineteenth century, may be told very

Sir William Crookes had in 1874 applied himself to the task of
creating something more nearly like a vacuum than the old
air-pumps afforded. When he had found the means of reducing the
quantity of gas in a tube until it was a million times thinner
than the atmosphere, he made the experiment of sending an
electric discharge through it, and found a very curious result.
From the cathode (the negative electric point) certain rays
proceeded which caused a green fluorescence on the glass of the
tube. Since the discharge did not consist of the atoms of the
gas, he concluded that it was a new and mysterious substance,
which he called "radiant matter." But no progress was made in the
interpretation of this strange material. The Crookes tube became
one of the toys of science--and the lamp of other investigators.

In 1895 Rontgen drew closer attention to the Crookes tube by
discovering the rays which he called X-rays, but which now bear
his name. They differ from ordinary light-waves in their length,
their irregularity, and especially their power to pass through
opaque bodies. A number of distinguished physicists now took up
the study of the effect of sending an electric discharge through
a vacuum, and the particles of "radiant matter" were soon
identified. Sir J. J. Thomson, especially, was brilliantly
successful in his interpretation. He proved that they were tiny
corpuscles, more than a thousand times smaller than the atom of
hydrogen, charged with negative electricity, and travelling at
the rate of thousands of miles a second. They were the
"electrons" in which modern physics sees the long-sought
constituents of the atom.

No sooner had interest been thoroughly aroused than it was
announced that a fresh discovery had opened a new shaft into the
underworld. Sir J. J. Thomson, pursuing his research, found in
1896 that compounds of uranium sent out rays that could penetrate
black paper and affect the photographic plate; though in this
case the French physicist, Becquerel, made the discovery
simultaneously' and was the first to publish it. An army of
investigators turned into the new field, and sought to penetrate
the deep abyss that had almost suddenly disclosed itself. The
quickening of astronomy by Galilei, or of zoology by Darwin, was
slight in comparison with the stirring of our physical world by
these increasing discoveries. And in 1898 M. and Mme. Curie made
the further discovery which, in the popular mind, obliterated all
the earlier achievements. They succeeded in isolating the new
element, radium, which exhibits the actual process of an atom
parting with its minute constituents.

The story of radium is so recent that a few lines will suffice to
recall as much as is needed for the purpose of this chapter. In
their study of the emanations from uranium compounds the Curies
were led to isolate the various elements of the compounds until
they discovered that the discharge was predominantly due to one
specific element, radium. Radium is itself probably a product of
the disintegration of uranium, the heaviest of known metals, with
an atomic weight some 240 times greater than that of hydrogen.
But this massive atom of uranium has a life that is computed in
thousands of millions of years. It is in radium and its offspring
that we see most clearly the constitution of matter.

A gramme (less than 15 1/2 grains) of radium contains-- we will
economise our space--4x10 (superscript)21 atoms. This tiny mass
is, by its discharge, parting with its substance at the rate of
one atom per second for every 10,000,000,000 atoms; in other
words, the "indestructible" atom has, in this case, a term of
life not exceeding 2500 years. In the discharge from the radium
three elements have been distinguished. The first consists of
atoms of the gas helium, which are hurled off at between 10,000
and 20,000 miles a second. The third element (in the order of
classification) consists of waves analogous to the Rontgen rays.
But the second element is a stream of electrons, which are
expelled from the atom at the appalling speed of about 100,000
miles a second. Professor Le Bon has calculated that it would
take 340,000 barrels of powder to discharge a bullet at that
speed. But we shall see more presently of the enormous energy
displayed within the little system of the atom. We may add that
after its first transformation the radium passes, much more
quickly, through a further series of changes. The frontiers of
the atomic systems were breaking down.

The next step was for students (notably Soddy and Rutherford) to
find that radio-activity, or spontaneous discharge out of the
atomic systems, was not confined to radium. Not only are other
rare metals conspicuously active, but it is found that such
familiar surfaces as damp cellars, rain, snow, etc., emit a
lesser discharge. The value of the new material thus provided for
the student of physics may be shown by one illustration. Sir J.
J. Thomson observes that before these recent discoveries the
investigator could not detect a gas unless about a billion
molecules of it were present, and it must be remembered that the
spectroscope had already gone far beyond ordinary chemical
analysis in detecting the presence of substances in minute
quantities. Since these discoveries we can recognise a single
molecule, bearing an electric charge.

With these extraordinary powers the physicist is able to
penetrate a world that lies immeasurably below the range of the
most powerful microscope, and introduce us to systems more
bewildering than those of the astronomer. We pass from a
portentous Brobdingnagia to a still more portentous Lilliputia.
It has been ascertained that the mass of the electron is the
1/1700th part of that of an atom of hydrogen, of which, as we
saw, billions of molecules have ample space to execute their
terrific movements within the limits of the letter "o." It has
been further shown that these electrons are identical, from
whatever source they are obtained. The physicist therefore
concludes-- warning us that on this further point he is drawing a
theoretical conclusion--that the atoms of ordinary matter are
made up of electrons. If that is the case, the hydrogen atom, the
lightest of all, must be a complex system of some 1700 electrons,
and as we ascend the scale of atomic weight the clusters grow
larger and larger, until we come to the atoms of the heavier
metals with more than 250,000 electrons in each atom.

But this is not the most surprising part of the discovery. Tiny
as the dimensions of the atom are, they afford a vast space for
the movement of these energetic little bodies. The speed of the
stars in their courses is slow compared with the flight of the
electrons. Since they fly out of the system, in the conditions we
have described, at a speed of between 90,000 and 100,000 miles a
second, they must be revolving with terrific rapidity within it.
Indeed, the most extraordinary discovery of all is that of the
energy imprisoned within these tiny systems, which men have for
ages regarded as "dead" matter. Sir J. J. Thomson calculates
that, allowing only one electron to each atom in a gramme of
hydrogen, the tiny globule of gas will contain as much energy as
would be obtained by burning thirty-five tons of coal. If, he
says, an appreciable fraction of the energy that is contained in
ordinary matter were to be set free, the earth would explode and
return to its primitive nebulous condition. Mr. Fournier d'Albe
tells us that the force with which electrons repel each other is
a quadrillion times greater than the force of gravitation that
brings atoms together; and that if two grammes of pure electrons
could be placed one centimetre apart they would repel each other
with a force equal to 320 quadrillion tons. The inexpert
imagination reels, but it must be remembered that the speed of
the electron is a measured quantity, and it is within the
resources of science to estimate the force necessary to project
it at that speed.*

* See Sir J. J. Thomson, "The Corpuscular Theory of Matter"
(1907) and--for a more elementary presentment--"Light Visible and
Invisible" (1911); and Mr. Fournier d'Albe, "The Electron Theory"
(2nd. ed., 1907).

Such are the discoveries of the last fifteen years and a few of
the mathematical deductions from them. We are not yet in a
position to say positively that the atoms are composed of
electrons, but it is clear that the experts are properly modest
in claiming only that this is highly probable. The atom seems to
be a little universe in which, in combination with positive
electricity (the nature of which is still extremely obscure),
from 1700 to 300,000 electrons revolve at a speed that reaches as
high as 100,000 miles a second. Instead of being crowded
together, however, in their minute system, each of them has, in
proportion to its size, as ample a space to move in as a single
speck of dust would have in a moderate-sized room (Thomson). This
theory not only meets all the facts that have been discovered in
an industrious decade of research, not only offers a splendid
prospect of introducing unity into the eighty-one different
elements of the chemist, but it opens out a still larger prospect
of bringing a common measure into the diverse forces of the

Light is already generally recognised as a rapid series of
electro-magnetic waves or pulses in ether. Magnetism becomes
intelligible as a condition of a body in which the electrons
revolve round the atom in nearly the same plane. The difference
between positive and negative electricity is at least partly
illuminated. An atom will repel an atom when its equilibrium is
disturbed by the approach of an additional electron; the
physicist even follows the movement of the added electron, and
describes it revolving 2200 billion times a second round the
atom, to escape being absorbed in it. The difference between good
and bad conductors of electricity becomes intelligible. The atoms
of metals are so close together that the roaming electrons pass
freely from one atom to another, in copper, it is calculated, the
electron combines with an atom and is liberated again a hundred
million times a second. Even chemical action enters the sphere of

However these hypotheses may fare, the electron is a fact, and
the atom is very probably a more or less stable cluster of
electrons. But when we go further, and attempt to trace the
evolution of the electron out of ether, we enter a region of pure
theory. Some of the experts conceive the electron as a minute
whirlpool or vortex in the ocean of ether; some hold that it is a
centre of strain in ether; some regard ether as a densely packed
mass of infinitely small grains, and think that the positive and
negative corpuscles, as they seem to us, are tiny areas in which
the granules are unequally distributed. Each theory has its
difficulties. We do not know the origin of the electron, because
we do not know the nature of ether. To some it is an elastic
solid, quivering in waves at every movement of the particles; to
others it is a continuous fluid, every cubic millimetre of which
possesses "an energy equivalent to the output of a
million-horse-power station for 40.000,000 years" (Lodge); to
others it is a close-packed granular mass with a pressure of
10,000 tons per square centimetre. We must wait. It is little
over ten years since the vaults were opened and physicists began
to peer into the sub-material world. The lower, perhaps lowest,
depth is reserved for another generation.

But it may be said that the research of the last ten years has
given us a glimpse of the foundations of the universe. Every
theory of the electron assumes it to be some sort of nodule or
disturbed area in the ether. It is sometimes described as "a
particle of negative electricity" and associated with "a particle
of positive electricity" in building up the atom. The phrase is
misleading for those who regard electricity as a force or energy,
and it gives rise to speculation as to whether "matter" has not
been resolved into "force." Force or energy is not conceived by
physicists as a substantial reality, like matter, but an abstract
expression of certain relations of matter or electrons.

In any case, the ether, whether solid or fluid or granular,
remains the fundamental reality. The universe does not float IN
an ocean of ether: it IS an ocean of ether. But countless myriads
of minute disturbances are found in this ocean, and set it
quivering with the various pulses which we classify as forces or
energies. These points of disturbance cluster together in systems
(atoms) of from 1000 to 250,000 members, and the atoms are
pressed together until they come in the end to form massive
worlds. It remains only to reduce gravitation itself, which
brings the atoms together, to a strain or stress in ether, and we
have a superb unity. That has not yet been done, but every theory
of gravitation assumes that it is a stress in the ether
corresponding to the formation of the minute disturbances which
we call electrons.

But, it may be urged, he who speaks of foundations speaks of a
beginning of a structure; he who speaks of evolution must have a
starting-point. Was there a time when the ether was a smooth,
continuous fluid, without electrons or atoms, and did they
gradually appear in it, like crystals in the mother-lye? In
science we know nothing of a beginning. The question of the
eternity or non-eternity of matter (or ether) is as futile as the
question about its infinity or finiteness. We shall see in the
next chapter that science can trace the processes of nature back
for hundreds, if not thousands, of millions of years, and has
ground to think that the universe then presented much the same
aspect as it does now, and will in thousands of millions of years
to come. But if these periods were quadrillions, instead of
millions, of years, they would still have no relation to the idea
of eternity. All that we can say is that we find nothing in
nature that points to a beginning or an end.*

* A theory has been advanced by some physicists that there is
evidence of a beginning. WITHIN OUR EXPERIENCE energy is being
converted into heat more abundantly than heat is being converted
into other energy. This would hold out a prospect of a paralysed
universe, and that stage would have been reached long ago if the
system had not had a definite beginning. But what knowledge have
we of conversions of energy in remote regions of space, in the
depths of stars or nebulae, or in the sub-material world of which
we have just caught a glimpse? Roundly, none. The speculation is

One point only need be mentioned in conclusion. Do we anywhere
perceive the evolution of the material elements out of electrons,
just as we perceive the devolution, or disintegration, of atoms
into electrons? There is good ground for thinking that we do. The
subject will be discussed more fully in the next chapter. In
brief, the spectroscope, which examines the light of distant
stars and discovers what chemical elements emitted it, finds
matter, in the hottest stars, in an unusual condition, and seems
to show the elements successively emerging from their fierce
alchemy. Sir J. Norman Lockyer has for many years conducted a
special investigation of the subject at the Solar Physics
Observatory, and he declares that we can trace the evolution of
the elements out of the fiery chaos of the young star. The
lightest gases emerge first, the metals later, and in a special
form. But here we pass once more from Lilliputia to
Brobdingnagia, and must first explain the making of the star


The greater part of this volume will be occupied with the things
that have happened on one small globe in the universe during a
certain number of millions of years. It cannot be denied that
this has a somewhat narrow and parochial aspect. The earth is,
you remember, a million times smaller than the sun, and the sun
itself is a very modest citizen of the stellar universe. Our
procedure is justified, however, both on the ground of personal
interest, and because our knowledge of the earth's story is so
much more ample and confident. Yet we must preface the story of
the earth with at least a general outline of the larger story of
the universe. No sensible man is humbled or dismayed by the
vastness of the universe. When the human mind reflects on its
wonderful scientific mastery of this illimitable ocean of being,
it has no sentiment of being dwarfed or degraded. It looks out
with cold curiosity over the mighty scattering of worlds, and
asks how they, including our own world, came into being.

We now approach this subject with a clearer perception of the
work we have to do. The universe is a vast expanse of ether, and
somehow or other this ether gives rise to atoms of matter. We may
imagine it as a spacious chamber filled with cosmic dust;
recollecting that the chamber has no walls, and that the dust
arises in the ether itself. The problem we now approach is, in a
word: How are these enormous stretches of cosmic dust, which we
call matter, swept together and compressed into suns and planets?
The most famous answer to this question is the "nebular
hypothesis." Let us see, briefly, how it came into modern

We saw that some of the ancient Greek speculators imagined their
infinite number of atoms as scattered originally, like dust,
throughout space and gradually coming together, as dust does, to
form worlds. The way in which they brought their atoms together
was wrong, but the genius of Democritus had provided the germ of
another sound theory to the students of a more enlightened age.
Descartes (1596-1650) recalled the idea, and set out a theory of
the evolution of stars and planets from a diffused chaos of
particles. He even ventured to say that the earth was at one time
a small white-hot sun, and that a solid crust had gradually
formed round its molten core. Descartes had taken refuge in
Sweden from his persecutors, and it is therefore not surprising
that that strange genius Swedenborg shortly afterwards developed
the same idea. In the middle of the eighteenth century the great
French naturalist, Buffon, followed and improved upon Descartes
and Swedenborg. From Buffon's work it was learned by the German
philosopher Kant, who published (1755) a fresh theory of the
concentration of scattered particles into fiery worlds. Then
Laplace (1749-1827) took up the speculation, and gave it the form
in which it practically ruled astronomy throughout the nineteenth
century. That is the genealogy of the famous nebular hypothesis.
It did not spring full-formed from the brain of either Kant or
Laplace, like Athene from the brain of Zeus.

Laplace had one great advantage over the early speculators. Not
only was he an able astronomer and mathematician, but by his time
it was known that nebulae, or vast clouds of dispersed matter,
actually existed in the heavens. Here was a solid basis for the
speculation. Sir William Herschel, the most assiduous explorer of
the heavens, was a contemporary of Laplace. Laplace therefore
took the nebula as his starting-point.

A quarter of an ounce of solid matter (say, tobacco) will fill a
vast space when it is turned into smoke, and if it were not for
the pressure of the atmosphere it would expand still more.
Laplace imagined the billions of tons of matter which constitute
our solar system similarly dispersed, converted into a fine gas,
immeasurably thinner than the atmosphere. This nebula would be
gradually drawn in again by gravitation, just as the dust falls
to the floor of a room. The collisions of its particles as they
fell toward the centre would raise its temperature and give it a
rotating movement. A time would come when the centrifugal force
at the outer ring of the rotating disk would equal the
centripetal (or inward) pull of gravity, and this ring would be
detached, still spinning round the central body. The material of
the ring would slowly gather, by gravitation, round some denser
area in it; the ring would become a sphere; we should have the
first, and outermost, planet circling round the sun. Other rings
would successively be detached, and form the rest of the planets;
and the sun is the shrunken and condensed body of the nebula.

So simple and beautiful a theory of the solar system could not
fail to captivate astronomers, but it is generally rejected
to-day, in the precise form which Laplace gave it. What the
difficulties are which it has encountered, and the modifications
it must suffer, we shall see later; as well as the new theories
which have largely displaced it. It will be better first to
survey the universe from the evolutionary point of view. But I
may observe, in passing, that the sceptical remarks one hears at
times about scientific theories contradicting and superseding
each other are frivolous. One great idea pervades all the
theories of the evolution of worlds, and that idea is firmly
established. The stars and their planets are enormous
aggregations of cosmic dust, swept together and compressed by the
action of gravitation. The precise nature of this cosmic dust--
whether it was gas, meteorites and gas, or other particles-- is
open to question.

As we saw in the first chapter, the universe has the word
evolution written, literally, in letters of fire across it. The
stars are of all ages, from sturdy youth to decrepit age, and
even to the darkness of death. We saw that this can be detected
on the superficial test of colour. The colours of the stars are,
it is true, an unsafe ground to build upon. The astronomer still
puzzles over the gorgeous colours he finds at times, especially
in double stars: the topaz and azure companions in beta Cygni,
the emerald and red of alpha Herculis, the yellow and rose of eta
Cassiopeiae, and so on. It is at the present time under
discussion in astronomy how far these colours are objective at
all, or whether, if they are real, they may not be due to causes
other than temperature. Yet the significance of the three
predominating colours--blue-white, yellow, and red--has been
sustained by the spectroscope. It is the series of colours
through which a white-hot bar of iron passes as it cools. And the
spectroscope gives us good ground to conclude that the stars are

When a glowing gas (not under great pressure) is examined by the
spectroscope, it yields a few vertical lines or bars of light on
a dark background; when a glowing liquid or solid is examined, it
gives a continuous rainbow-like stretch of colour. Some of the
nebulae give the former type of spectrum, and are thus known to
be masses of luminous gas; many of the nebulae and the stars have
the latter type of spectrum. But the stretch of light in the
spectrum of a star is crossed, vertically, by a number of dark
lines, and experiment in the laboratory has taught us how to
interpret these. They mean that there is some light-absorbing
vapour between the source of light and the instrument. In the
case of the stars they indicate the presence of an atmosphere of
relatively cool vapours, and an increase in the density of that
atmosphere--which is shown by a multiplication and broadening of
the dark lines on the spectrum--means an increase of age, a loss
of vitality, and ultimately death. So we get the descending scale
of spectra. The dark lines are thinnest and least numerous in the
blue stars, more numerous in the yellow, heavy and thick in the
red. As the body of the star sinks in temperature dense masses of
cool vapour gather about it. Its light, as we perceive it, turns
yellow, then red. The next step, which the spectroscope cannot
follow, will be the formation of a scum on the cooling surface,
ending, after ages of struggle, in the imprisonment of the molten
interior under a solid, dark crust. Let us see how our sun
illustrates this theory.

It is in the yellow, or what we may call the autumnal, stage.
Miss Clerke and a few others have questioned this, but the
evidence is too strong to-day. The vast globe, 867,000 miles in
diameter, seems to be a mass of much the same material as the
earth--about forty elements have been identified in it--but at a
terrific temperature. The light-giving surface is found, on the
most recent calculations, to have a temperature of about 6700
degrees C. This surface is an ocean of liquid or vaporised
metals, several thousand miles in depth; some think that the
brilliant light comes chiefly from clouds of incandescent carbon.
Overlying it is a deep layer of the vapours of the molten metals,
with a temperature of about 5500 degrees C.; and to this
comparatively cool and light-absorbing layer we owe the black
lines of the solar spectrum. Above it is an ocean of red-hot
hydrogen, and outside this again is an atmosphere stretching for
some hundreds of thousands of miles into space.

The significant feature, from our point of view, is the
"sun-spot"; though the spot may be an area of millions of square
miles. These areas are, of course, dark only by comparison with
the intense light of the rest of the disk. The darkest part of
them is 5000 times brighter than the full moon. It will be seen
further, on examining a photograph of the sun, that a network or
veining of this dark material overspreads the entire surface at
all times. There is still some difference of opinion as to the
nature of these areas, but the evidence of the spectroscope has
convinced most astronomers that they are masses of cooler vapour
lying upon, and sinking into, the ocean of liquid fire. Round
their edges, as if responding to the pressure of the more
condensed mass, gigantic spurts and mountains of the white-hot
matter of the sun rush upwards at a rate of fifty or a hundred
miles a second, Sometimes they reach a height of a hundred, and
even two hundred, thousand miles, driving the red-hot hydrogen
before them in prodigious and fantastic flames. Between the black
veins over the disk, also, there rise domes and columns of the
liquid fire, some hundreds of miles in diameter, spreading and
sinking at from five to twenty miles a second. The surface of the
sun--how much more the interior !--is an appalling cauldron of
incandescent matter from pole to pole. Every yard of the surface
is a hundred times as intense as the open furnace of a Titanic.
From the depths and from the surface of this fiery ocean, as, on
a small scale, from the surface of the tropical sea, the vapours
rise high into the extensive atmosphere, discharge some of their
heat into space, and sink back, cooler and heavier, upon the

This is a star in its yellow age, as are Capella and Arcturus and
other stars. The red stars carry the story further, as we should
expect. The heavier lines in their spectrum indicate more
absorption of light, and tell us that the vapours are thickening
about the globe; while compounds like titanium oxide make their
appearance, announcing a fall of temperature. Below these, again,
is a group of dark red or "carbon" stars, in which the process is
carried further. Thick, broad, dark lines in the red end of the
spectrum announce the appearance of compounds of carbon, and a
still lower fall of temperature. The veil is growing thicker; the
life is ebbing from the great frame. Then the star sinks below
the range of visibility, and one would think that we can follow
the dying world no farther. Fortunately, in the case of Algol and
some thirty or forty other stars, an extinct sun betrays its
existence by flitting across the light of a luminous sun, and
recent research has made it probable that the universe is strewn
with dead worlds. Some of them may be still in the condition
which we seem to find in Jupiter, hiding sullen fires under a
dense shell of cloud; some may already be covered with a crust,
like the earth. There are even stars in which one is tempted to
see an intermediate stage: stars which blaze out periodically
from dimness, as if the Cyclops were spending his last energy in
spasms that burst the forming roof of his prison. But these
variable stars are still obscure, and we do not need their aid.
The downward course of a star is fairly plain.

When we turn to the earlier chapters in the life of a star, the
story is less clear. It is at least generally agreed that the
blue-white stars exhibit an earlier and hotter stage. They show
comparatively little absorption, and there is an immense
preponderance of the lighter gases, hydrogen and helium. They
(Sirius, Vega, etc.) are, in fact, known as "hydrogen stars," and
their temperature is generally computed at between 20,000 and
30,000 degrees C. A few stars, such as Procyon and Canopus, seem
to indicate a stage between them and the yellow or solar type.
But we may avoid finer shades of opinion and disputed classes,
and be content with these clear stages. We begin with stars in
which only hydrogen and helium, the lightest Of elements, can be
traced; and the hydrogen is in an unfamiliar form, implying
terrific temperature. In the next stage we find the lines of
oxygen, nitrogen, magnesium, and silicon. Metals such as iron and
copper come later, at first in a primitive and unusual form.
Lastly we get the compounds of titanium and carbon, and the
densely shaded spectra which tell of the thickly gathering
vapours. The intense cold of space is slowly prevailing in the
great struggle.

What came before the star? It is now beyond reasonable doubt that
the nebula--taking the word, for the moment, in the general sense
of a loose, chaotic mass of material--was the first stage.
Professor Keeler calculated that there are at least 120,000
nebulae within range of our telescopes, and the number is likely
to be increased. A German astronomer recently counted 1528 on one
photographic plate. Many of them, moreover, are so vast that they
must contain the material for making a great number of worlds.
Examine a good photograph of the nebula in Orion. Recollect that
each one of the points of light that are dotted over the expanse
is a star of a million miles or more in diameter (taking our sun
as below the average), and that the great cloud that sprawls
across space is at least 10,000 billion miles away; how much more
no man knows. It is futile to attempt to calculate the extent of
that vast stretch of luminous gas. We can safely say that it is
at least a million times as large as the whole area of our solar
system; but it may run to trillions or quadrillions of miles.

Nearly a hundred other nebulae are known, by the spectroscope, to
be clouds of luminous gas. It does not follow that they are
white-hot, and that the nebula is correctly called a "fire-mist."
Electrical and other agencies may make gases luminous, and many
astronomers think that the nebulae are intensely cold. However,
the majority of the nebulae that have been examined are not
gaseous, and have a very different structure from the loose and
diffused clouds of gas. They show two (possibly more, but
generally two) great spiral arms starting from the central part
and winding out into space. As they are flat or disk-shaped, we
see this structure plainly when they turn full face toward the
earth, as does the magnificent nebula in Canes Venatici. In it,
and many others, we clearly trace a condensed central mass, with
two great arms, each apparently having smaller centres of
condensation, sprawling outward like the broken spring of a
watch. The same structure can be traced in the mighty nebula in
Andromeda, which is visible to the naked eye, and it is said that
more than half the nebulae in the heavens are spiral. Knowing
that they are masses of solid or liquid fire, we are tempted to
see in them gigantic Catherine-wheels, the fireworks of the gods.
What is their relation to the stars?

In the first place, their mere existence has provided a solid
basis for the nebular hypothesis, and their spiral form
irresistibly suggests that they are whirling round on their
central axis and concentrating. Further, we find in some of the
gaseous nebulae (Orion) comparatively void spaces occupied by
stars, which seem to have absorbed the nebulous matter in their
formation. On the other hand, we find (in the Pleiades) wisps and
streamers of nebulous matter clinging about great clusters of
stars, suggesting that they are material left over when these
clustered worlds crystallised out of some vast nebula; and
enormous stretches of nebulous material covering regions (as in
Perseus) where the stars are as thick as grains of silver. More
important still, we find a type of cosmic body which seems
intermediate between the star and the nebula. It is a more or
less imperfectly condensed star, surrounded by nebular masses.
But one of the most instructive links of all is that at times a
nebula is formed from a star, and a recent case of this character
may be briefly described.

In February, 1901, a new star appeared in the constellation
Perseus. Knowing what a star is, the reader will have some dim
conception of the portentous blaze that lit up that remote region
of space (at least 600 billion miles away) when he learns that
the light of this star increased 4000-fold in twenty-eight hours.
It reached a brilliance 8000 times greater than that of the sun.
Telescopes and spectroscopes were turned on it from all parts of
the earth, and the spectroscope showed that masses of glowing
hydrogen were rushing out from it at a rate of nearly a thousand
miles a second. Its light gradually flickered and fell, however,
and the star sank back into insignificance. But the photographic
plate now revealed a new and most instructive feature. Before the
end of the year there was a nebula, of enormous extent, spreading
out on both sides from the centre of the eruption. It was
suggested at the time that the bursting of a star may merely have
lit up a previously dark nebula, but the spectroscope does not
support this. A dim star had dissolved, wholly or partially, into
a nebula, as a result of some mighty cataclysm. What the nature
of the catastrophe was we will inquire presently.

These are a few of the actual connections that we find between
stars and nebulae. Probably, however, the consideration that
weighs most with the astronomer is that the condensation of such
a loose, far-stretched expanse of matter affords an admirable
explanation of the enormous heat of the stars. Until recently
there was no other conceivable source that would supply the sun's
tremendous outpour of energy for tens of millions of years except
the compression of its substance. It is true that the discovery
of radio-activity has disclosed a new source of energy within the
atoms themselves, and there are scientific men, like Professor
Arrhenius, who attach great importance to this source. But,
although it may prolong the limited term of life which physicists
formerly allotted to the sun and other stars, it is still felt
that the condensation of a nebula offers the best explanation of
the origin of a sun, and we have ample evidence for the
connection. We must, therefore, see what the nebula is, and how
it develops.

"Nebula" is merely the Latin word for cloud. Whatever the nature
of these diffused stretches of matter may be, then, the name
applies fitly to them, and any theory of the development of a
star from them is still a "nebular hypothesis." But the three
theories which divide astronomers to-day differ as to the nature
of the nebula. The older theory, pointing to the gaseous nebulae
as the first stage, holds that the nebula is a cloud of extremely
attenuated gas. The meteoritic hypothesis (Sir N. Lockyer, Sir G.
Darwin, etc.), observing that space seems to swarm with meteors
and that the greater part of the nebulae are not gaseous,
believes that the starting-point is a colossal swarm of meteors,
surrounded by the gases evolved and lit up by their collisions.
The planetesimal hypothesis, advanced in recent years by
Professor Moulton and Professor Chamberlin, contends that the
nebula is a vast cloud of liquid or solid (but not gaseous)
particles. This theory is based mainly on the dynamical
difficulties of the other two, which we will notice presently.

The truth often lies between conflicting theories, or they may
apply to different cases. It is not improbable that this will be
our experience in regard to the nature of the initial nebula. The
gaseous nebulae, and the formation of such nebulae from disrupted
stars, are facts that cannot be ignored. The nebulae with a
continuous spectrum, and therefore--in part, at least--in a
liquid or solid condition, may very well be regarded as a more
advanced stage of condensation of the same; their spiral shape
and conspicuous nuclei are consistent with this. Moreover, a
condensing swarm of meteors would, owing to the heat evolved,
tend to pass into a gaseous condition. On the tether hand, a huge
expanse of gas stretched over billions of miles of space would be
a net for the wandering particles, meteors, and comets that roam
through space. If it be true, as is calculated, that our 24,000
miles of atmosphere capture a hundred million meteors a day, what
would the millions or billions of times larger net of a nebula
catch, even if the gas is so much thinner? In other words, it is
not wise to draw too fine a line between a gaseous nebula and one
consisting of solid particles with gas.

The more important question is: How do astronomers conceive the
condensation of this mixed mass of cosmic dust? It is easy to
reply that gravitation, or the pressure of the surrounding ether,
slowly drives the particles centre-ward, and compresses the dust
into globes, as the boy squeezes the flocculent snow into balls;
and it is not difficult for the mathematician to show that this
condensation would account for the shape and temperature of the
stars. But we must go a little beyond this superficial statement,
and see, to some extent, how the deeper students work out the

* See, especially, Dr. P. Lowell, "The Evolution of Worlds"
(1909). Professor S. Arrhenius, "Worlds in the Making" (1908),
Sir N. Lockyer, "The Meteorite Hypothesis" (1890), Sir R. Ball,
"The Earth's Beginning" (1909), Professor Moulton, "The
Astrophysical Journal (October, 1905), and Chamberlin and
Salisbury, "Geology," Vol. II. (1903).

Taking a broad view of the whole field, one may say that the two
chief difficulties are as follows: First, how to get the whole
chaotic mass whirling round in one common direction; secondly,
how to account for the fact that in our solar system the
outermost planets and satellites do not rotate in the same
direction as the rest. There is a widespread idea that these
difficulties have proved fatal to the old nebular hypothesis, and
there are distinguished astronomers who think so. But Sir R. Ball
(see note), Professor Lowell (see note), Professor Pickering
(Annals of Harvard College Observatory, 53, III), and other high
authorities deny this, and work out the newly discovered
movements on the lines of the old theory. They hold that all the
bodies in the solar system once turned in the same direction as
Uranus and Neptune, and the tidal influence of the sun has
changed the rotation of most of them. The planets farthest from
the sun would naturally not be so much affected by it. The same
principle would explain the retrograde movement of the outer
satellites of Saturn and Jupiter. Sir R. Ball further works out
the principles on which the particles of the condensing nebula
would tend to form a disk rotating on its central axis. The
ring-theory of Laplace is practically abandoned. The spiral
nebula is evidently the standard type, and the condensing nebula
must conform to it. In this we are greatly helped by the current
theory of the origin of spiral nebulae.

We saw previously that new stars sometimes appear in the sky, and
the recent closer scrutiny of the heavens shows this occurrence
to be fairly frequent. It is still held by a few astronomers that
such a cataclysm means that two stars collided. Even a partial or
"grazing " collision between two masses, each weighing billions
of tons, travelling (on the average) forty or fifty miles a
second--a movement that would increase enormously as they
approach each other--would certainly liquefy or vaporise their
substance; but the astronomer, accustomed to see cosmic bodies
escape each other by increasing their speed, is generally
disinclined to believe in collisions. Some have made the new star
plunge into the heart of a dense and dark nebula; some have
imagined a shock of two gigantic swarms of meteors; some have
regarded the outflame as the effect of a prodigious explosion. In
one or other new star each or any of these things may have
occurred, but the most plausible and accepted theory for the new
star of 1901 and some others is that two stars had approached
each other too closely in their wandering. Suppose that, in
millions of years to come, when our sun is extinct and a firm
crust surrounds the great molten ball, some other sun approaches
within a few million miles of it. The two would rush past each
other at a terrific speed, but the gravitational effect of the
approaching star would tear open the solid shell of the sun, and,
in a mighty flame, its molten and gaseous entrails would be flung
out into space. It has long been one of the arguments against a
molten interior of the earth that the sun's gravitational
influence would raise it in gigantic tides and rend the solid
shell of rock. It is even suspected now that our small earth is
not without a tidal influence on the sun. The comparatively near
approach of two suns would lead to a terrific cataclysm.

If we accept this theory, the origin of the spiral nebula becomes
intelligible. As the sun from which it is formed is already
rotating on its axis, we get a rotation of the nebula from the
first. The mass poured out from the body of the sun would, even
if it were only a small fraction of its mass, suffice to make a
planetary system; all our sun's planets and their satellites
taken together amount to only 1/100th of the mass of the solar
system. We may assume, further, that the outpoured matter would
be a mixed cloud of gases and solid and liquid particles; and
that it would stream out, possibly in successive waves, from more
than one part of the disrupted sun, tending to form great spiral
trails round the parent mass. Some astronomers even suggest that,
as there are tidal waves raised by the moon at opposite points of
the earth, similar tidal outbursts would occur at opposite points
on the disk of the disrupted star, and thus give rise to the
characteristic arms starting from opposite sides of the spiral
nebula. This is not at all clear, as the two tidal waves of the
earth are due to the fact that it has a liquid ocean rolling on,
not under, a solid bed.

In any case, we have here a good suggestion of the origin of the
spiral nebula and of its further development. As soon as the
outbursts are over, and the scattered particles have reached the
farthest limit to which they are hurled, the concentrating action
of gravitation will slowly assert itself. If we conceive this
gravitational influence as the pressure of the surrounding ether
we get a wider understanding of the process. Much of the
dispersed matter may have been shot far enough into space to
escape the gravitational pull of the parent mass, and will be
added to the sum of scattered cosmic dust, meteors, and close
shoals of meteors (comets) wandering in space. Much of the rest
will fall back upon the central body But in the great spiral arms
themselves the distribution of the matter will be irregular, and
the denser areas will slowly gather in the surrounding material.
In the end we would thus get secondary spheres circling round a
large primary.

This is the way in which astronomers now generally conceive the
destruction and re-formation of worlds. On one point the new
planetesimal theory differs from the other theories. It supposes
that, since the particles of the whirling nebula are all
travelling in the same general direction, they overtake each
other with less violent impact than the other theories suppose,
and therefore the condensation of the material into planets would
not give rise to the terrific heat which is generally assumed. We
will consider this in the next chapter, when we deal with the
formation of the planets. As far as the central body, the sun, is
concerned, there can be no hesitation. The 500,000,000
incandescent suns in the heavens are eloquent proof of the
appalling heat that is engendered by the collisions of the
concentrating particles.

In general outline we now follow the story of a star with some
confidence. An internal explosion, a fatal rush into some dense
nebula or swarm of meteors, a collision with another star, or an
approach within a few million miles of another star, scatters, in
part or whole, the solid or liquid globe in a cloud of cosmic
dust. When the violent outrush is over, the dust is gathered
together once more into a star. At first cold and attenuated, its
temperature rises as the particles come together, and we have,
after a time, an incandescent nucleus shining through a thin veil
of gas--a nebulous star. The temperature rises still further, and
we have the blue-hot star, in which the elements seem to be
dissociated, and slowly re-forming as the temperature falls.
After, perhaps, hundreds of millions of years it reaches the
"yellow" stage, and, if it has planets with the conditions of
life, there may be a temporary opportunity for living things to
enjoy its tempered energy. But the cooler vapours are gathering
round it, and at length its luminous body is wholly imprisoned.
It continues its terrific course through space, until some day,
perhaps, it again encounters the mighty cataclysm which will make
it begin afresh the long and stormy chapters of its living

Such is the suggestion of the modern astronomer, and, although we
seem to find every phase of the theory embodied in the varied
contents of the heavens, we must not forget that it is only a
suggestion. The spectroscope and telescopic photography, which
are far more important than the visual telescope, are
comparatively recent, and the field to be explored is enormous.
The mist is lifting from the cosmic landscape, but there is still
enough to blur our vision. Very puzzling questions remain
unanswered. What is the origin of the great gaseous nebulae? What
is the origin of the triple or quadruple star? What is the
meaning of stars whose light ebbs and flows in periods of from a
few to several hundred days? We may even point to the fact that
some, at least, of the spiral nebulae are far too vast to be the
outcome of the impact or approach of two stars.

We may be content to think that we have found out some truths, by
no means the whole truth, about the evolution of worlds.
Throughout this immeasurable ocean of ether the particles of
matter are driven together and form bodies. These bodies swarm
throughout space, like fish in the sea; travelling singly (the
"shooting star"), or in great close shoals (the nucleus of a
comet), or lying scattered in vast clouds. But the inexorable
pressure urges them still, until billions of tons of material are
gathered together. Then, either from the sheer heat of the
compression, or from the formation of large and unstable atomic
systems (radium, etc.), or both, the great mass becomes a
cauldron of fire, mantled in its own vapours, and the story of a
star is run. It dies out in one part of space to begin afresh in
another. We see nothing in the nature of a beginning or an end
for the totality of worlds, the universe. The life of all living
things on the earth, from the formation of the primitive microbes
to the last struggles of the superman, is a small episode of that
stupendous drama, a fraction of a single scene. But our ampler
knowledge of it, and our personal interest in it, magnify that
episode, and we turn from the cosmic picture to study the
formation of the earth and the rise of its living population.


The story of the evolution of our solar system is, it will now be
seen, a local instance of the great cosmic process we have
studied in the last chapter. We may take one of the small spiral
nebulae that abound in the heavens as an illustration of the
first stage. If a still earlier stage is demanded, we may suppose
that some previous sun collided with, or approached too closely,
another mighty body, and belched out a large part of its contents
in mighty volcanic outpours. Mathematical reasoning can show that
this erupted material would gather into a spiral nebula; but, as
mathematical calculations cannot be given here, and are less safe
than astronomical facts, we will be content to see the early
shape of our solar system in a relatively small spiral nebula,
its outermost arm stretching far beyond the present orbit of
Neptune, and its great nucleus being our present sun in more
diffused form.

We need not now attempt to follow the shrinking of the central
part of the nebula until it becomes a rounded fiery sun. That has
been done in tracing the evolution of a star. Here we have to
learn how the planets were formed from the spiral arms of the
nebula. The principle of their formation is already clear. The
same force of gravitation, or the same pressure of the
surrounding ether, which compresses the central mass into a fiery
globe, will act upon the loose material of the arms and compress
it into smaller globes. But there is an interesting and acute
difference of opinion amongst modern experts as to whether these
smaller globes, the early planets, would become white-hot bodies.

The general opinion, especially among astronomers, is that the
compression of the nebulous material of the arms into globes
would generate enormous heat, as in the case of the sun. On that
view the various planets would begin their careers as small suns,
and would pass through those stages of cooling and shrinking
which we have traced in the story of the stars. A glance at the
photograph of one of the spiral nebulae strongly confirms this.
Great luminous knots, or nuclei, are seen at intervals in the
arms. Smaller suns seem to be forming in them, each gathering
into its body the neighbouring material of the arm, and rising in
temperature as the mass is compressed into a globe. The
spectroscope shows that these knots are condensing masses of
white-hot liquid or solid matter. It therefore seems plain that
each planet will first become a liquid globe of fire, coursing
round the central sun, and will gradually, as its heat is
dissipated and the supply begins to fail, form a solid crust.

This familiar view is challenged by the new "planetesimal
hypothesis," which has been adopted by many distinguished
geologists (Chamberlin, Gregory, Coleman, etc.). In their view
the particles in the arms of the nebula are all moving in the
same direction round the sun. They therefore quietly overtake the
nucleus to which they are attracted, instead of violently
colliding with each other, and much less heat is generated at the
surface. In that case the planets would not pass through a
white-hot, or even red-hot, stage at all. They are formed by a
slow ingathering of the scattered particles, which are called
"planetesimals" round the larger or denser masses of stuff which
were discharged by the exploding sun. Possibly these masses were
prevented from falling back into the sun by the attraction of the
colliding body, or the body which caused the eruption. They would
revolve round the parent body, and the shoals of smaller
particles would gather about them by gravitation. If there were
any large region in the arm of the nebula which had no single
massive nucleus, the cosmic dust would gather about a number of
smaller centres. Thus might be explained the hundreds of
planetoids, or minor planets, which we find between Mars and
Jupiter. If these smaller bodies came within the sphere of
influence of one of the larger planets, yet were travelling
quickly enough to resist its attraction, they would be compelled
to revolve round it, and we could thus explain the ten satellites
of Saturn and the eight of Jupiter. Our moon, we shall see, had a
different origin.

We shall find this new hypothesis crossing the familiar lines at
many points in the next few chapters. We will consider those
further consequences as they arise, but may say at once that,
while the new theory has greatly helped us in tracing the
formation of the planetary system, astronomers are strongly
opposed to its claim that the planets did not pass through an
incandescent stage. The actual features of our spiral nebulae
seem clearly to exhibit that stage. The shape of the
planets--globular bodies, flattened at the poles--strongly
suggests that they were once liquid. The condition in which we
find Saturn and Jupiter very forcibly confirms this suggestion;
the latest study of those planets supports the current opinion
that they are still red-hot, and even seems to detect the glow of
their surfaces in their mantles of cloud. These points will be
considered more fully presently. For the moment it is enough to
note that, as far as the early stages of planetary development
are concerned, the generally accepted theory rests on a mass of
positive evidence, while the new hypothesis is purely
theoretical. We therefore follow the prevailing view with some

Those of the spiral nebulae which face the earth squarely afford
an excellent suggestion of the way in which planets are probably
formed. In some of these nebulae the arms consist of almost
continuous streams of faintly luminous matter; in others the
matter is gathering about distinct centres; in others again the
nebulous matter is, for the most part, collected in large glowing
spheres. They seem to be successive stages, and to reveal to us
the origin of our planets. The position of each planet in our
solar system would be determined by the chance position of the
denser stuff shot out by the erupting sun. I have seen Vesuvius
hurl up into the sky, amongst its blasts of gas and steam,
white-hot masses of rock weighing fifty tons. In the far fiercer
outburst of the erupting sun there would be at least thinner and
denser masses, and they must have been hurled so far into space
that their speed in travelling round the central body, perhaps
seconded by the attraction of the second star, overcame the
gravitational pull back to the centre. Recollect the force which,
in the new star in Perseus, drove masses of hydrogen for millions
of miles at a speed of a thousand miles a second.

These denser nuclei or masses would, when the eruption was over,
begin to attract to themselves all the lighter nebulous material
within their sphere of gravitational influence. Naturally, there
would at first be a vast confusion of small and large centres of
condensation in the arms of the nebula, moving in various
directions, but a kind of natural selection--and, in this case,
survival of the biggest--would ensue. The conflicting movements
would be adjusted by collisions and gravitation, the smaller
bodies would be absorbed in the larger or enslaved as their
satellites, and the last state would be a family of smaller suns
circling at vast distances round the parent body. The planets,
moreover, would be caused to rotate on their axes, besides
revolving round the sun, as the particles at their inner edge
(nearer the sun) would move at a different speed from those at
the outer edge. In the course of time the smaller bodies, having
less heat to lose and less (or no) atmosphere to check the loss,
would cool down, and become dark solid spheres, lit only by the
central fire.

While the first stage of this theory of development is seen in
the spiral nebula, the later stages seem to be well exemplified
in the actual condition of our planets. Following, chiefly, the
latest research of Professor Lowell and his colleagues, which
marks a considerable advance on our previous knowledge, we shall
find it useful to glance at the sister-planets before we approach
the particular story of our earth.

Mercury, the innermost and smallest of the planets, measuring
only some 3400 miles in diameter, is, not unexpectedly, an
airless wilderness. Small bodies are unable to retain the gases
at their surface, on account of their feebler gravitation. We
find, moreover, that Mercury always presents the same face to the
sun, as it turns on its axis in the same period (eighty-eight
days) in which it makes a revolution round the sun. While,
therefore, one half of the globe is buried in eternal darkness,
the other half is eternally exposed to the direct and blistering
rays of the sun, which is only 86,000,000 miles away. To
Professor Lowell it presents the appearance of a bleached and
sun-cracked desert, or "the bones of a dead world." Its
temperature must be at least 300 degrees C. above that of the
earth. Its features are what we should expect on the nebular
hypothesis. The slowness of its rotation is accounted for by the
heavy tidal influence of the sun. In the same way our moon has
been influenced by the earth, and our earth by the sun, in their
movement of rotation.

Venus, as might be expected in the case of so large a globe
(nearly as large as the earth), has an atmosphere, but it seems,
like Mercury, always to present the same face to the sun. Its
comparative nearness to the sun (67,000,000 miles) probably
explains this advanced effect of tidal action. The consequences
that the observers deduce from the fact are interesting. The
sun-baked half of Venus seems to be devoid of water or vapour,
and it is thought that all its water is gathered into a rigid
ice-field on the dark side of the globe, from which fierce
hurricanes must blow incessantly. It is a Sahara, or a desert far
hotter than the Sahara, on one side; an arctic region on the
other. It does not seem to be a world fitted for the support of
any kind of life that we can imagine.

When we turn to the consideration of Mars, we enter a world of
unending controversy. With little more than half the diameter of
the earth, Mars ought to be in a far more advanced stage of
either life or decay, but its condition has not yet been
established. Some hold that it has a considerable atmosphere;
others that it is too small a globe to have retained a layer of
gas. Professor Poynting believes that its temperature is below
the freezing-point of water all over the globe; many others, if
not the majority of observers, hold that the white cap we see at
its poles is a mass of ice and snow, or at least a thick coat of
hoar-frost, and that it melts at the edges as the springtime of
Mars comes round. In regard to its famous canals we are no nearer
agreement. Some maintain that the markings are not really an
objective feature; some hold that they are due to volcanic
activity, and that similar markings are found on the moon; some
believe that they are due to clouds; while Professor Lowell and
others stoutly adhere to the familiar view that they are
artificial canals, or the strips of vegetation along such canals.
The question of the actual habitation of Mars is still open. We
can say only that there is strong evidence of its possession of
the conditions of life in some degree, and that living things,
even on the earth, display a remarkable power of adaptation to
widely differing conditions.

Passing over the 700 planetoids, which circulate between Mars and
Jupiter, and for which we may account either by the absence of
one large nucleus in that part of the nebulous stream or by the
disturbing influence of Jupiter, we come to the largest planet of
the system. Here we find a surprising confirmation of the theory
of planetary development which we are following. Three hundred
times heavier than the earth (or more than a trillion tons in
weight), yet a thousand times less in volume than the sun,
Jupiter ought, if our theory is correct, to be still red-hot. All
the evidence conspires to suggest that it is. It has long been
recognised that the shining disk of the planet is not a solid,
but a cloud, surface. This impenetrable mass of cloud or vapour
is drawn out in streams or belts from side to side, as the giant
globe turns on its axis once in every ten hours. We cannot say
if, or to what extent, these clouds consist of water-vapour. We
can conclude only that this mantle of Jupiter is "a seething
cauldron of vapours" (Lowell), and that, if the body beneath is
solid, it must be very hot. A large red area, at one time 30,000
miles long, has more or less persisted on the surface for several
decades, and it is generally interpreted, either as a red-hot
surface, or as a vast volcanic vent, reflecting its glow upon the
clouds. Indeed, the keen American observers, with their powerful
telescopes, have detected a cherry-red glow on the edges of the
cloud-belts across the disk; and more recent observation with the
spectroscope seems to prove that Jupiter emits light from its
surface analogous to that of the red stars. The conspicuous
flattening of its poles is another feature that science would
expect in a rapidly rotating liquid globe. In a word, Jupiter
seems to be in the last stage of stellar development. Such, at
some remote time, was our earth; such one day will be the sun.

The neighbouring planet Saturn supports the conclusion. Here
again we have a gigantic globe, 28,000 miles in diameter, turning
on its axis in the short space of ten hours; and here again we
find the conspicuous flattening of the poles, the trailing belts
of massed vapour across the disk, the red glow lighting the edges
of the belts, and the spectroscopic evidence of an emission of
light. Once more it is difficult to doubt that a highly heated
body is wrapped in that thick mantle of vapour. With its ten
moons and its marvellous ring-system--an enormous collection of
fragments, which the influence of the planet or of its nearer
satellites seems to have prevented from concentrating--Saturn has
always been a beautiful object to observe; it is not less
interesting in those features which we faintly detect in its

The next planet, Uranus, 32,000 miles in diameter, seems to be
another cloud-wrapt, greatly heated globe, if not, as some think,
a sheer mass of vapours without a liquid core. Neptune is too dim
and distant for profitable examination. It may be added, however,
that the dense masses of gas which are found to surround the
outer planets seem to confirm the nebular theory, which assumes
that they were developed in the outer and lighter part of the
material hurled from the sun.

From this encouraging survey of the sister-planets we return with
more confidence to the story of the earth. I will not attempt to
follow an imaginative scheme in regard to its early development.
Take four photographs --one of a spiral nebula without knots in
its arms, one of a nebula like that in Canes Venatici, one of the
sun, and one of Jupiter--and you have an excellent illustration
of the chief stages in its formation. In the first picture a
section of the luminous arm of the nebula stretches thinly across
millions of miles of space. In the next stage this material is
largely collected in a luminous and hazy sphere, as we find in
the nebula in Canes Venatici. The sun serves to illustrate a
further stage in the condensation of this sphere. Jupiter
represents a later chapter, in which the cooler vapours are
wrapped close about the red-hot body of the planet. That seems to
have been the early story of the earth. Some 6,000,000,000
billion tons of the nebulous matter were attracted to a common
centre. As the particles pressed centreward, the temperature
rose, and for a time the generation of heat was greater than its
dissipation. Whether the earth ever shone as a small white star
we cannot say. We must not hastily conclude that such a
relatively small mass would behave like the far greater mass of a
star, but we may, without attempting to determine its
temperature, assume that it runs an analogous course.

One of the many features which I have indicated as pointing to a
former fluidity of the earth may be explained here. We shall see
in the course of this work that the mountain chains and other
great irregularities of the earth's surface appear at a late
stage in its development. Even as we find them to-day, they are
seen to be merely slight ridges and furrows on the face of the
globe, when we reflect on its enormous diameter, but there is
good reason to think that in the beginning the earth was much
nearer to a perfectly globular form. This points to a liquid or
gaseous condition at one time, and the flattening of the sphere
at the poles confirms the impression. We should hardly expect so
perfect a rotundity in a body formed by the cool accretion of
solid fragments and particles. It is just what we should expect
in a fluid body, and the later irregularities of the surface are
accounted for by the constant crumpling and wearing of its solid
crust. Many would find a confirmation of this in the phenomena of
volcanoes, geysers, and earthquakes, and the increase of the
temperature as we descend the crust. But the interior condition
of the earth, and the nature of these phenomena, are much
disputed at present, and it is better not to rely on any theory
of them. It is suggested that radium may be responsible for this
subterraneous heat.

The next stage in the formation of the earth is necessarily one
that we can reach only by conjecture. Over the globe of molten
fire the vapours and gases would be suspended like a heavy
canopy, as we find in Jupiter and Saturn to-day. When the period
of maximum heat production was passed, however, the radiation
into space would cause a lowering of the temperature, and a scum
would form on the molten surface. As may be observed on the
surface of any cooling vessel of fluid, the scum would stretch
and crack; the skin would, so to say, prove too small for the
body. The molten ocean below would surge through the crust, and
bury it under floods of lava. Some hold that the slabs would sink
in the ocean of metal, and thus the earth would first solidify in
its deeper layers. There would, in any case, be an age-long
struggle between the molten mass and the confining crust, until
at length--to employ the old Roman conception of the activity of
Etna--the giant was imprisoned below the heavy roof of rock.

Here again we seem to find evidence of the general correctness of
the theory. The objection has been raised that the geologist does
not find any rocks which he can identify as portions of the
primitive crust of the earth. It seems to me that it would be too
much to expect the survival at the surface of any part of the
first scum that cooled on that fiery ocean. It is more natural to
suppose that millions of years of volcanic activity on a
prodigious scale would characterise this early stage, and the
"primitive crust" would be buried in fragments, or dissolved
again, under deep seas of lava. Now, this is precisely what we
find, The oldest rocks known to the geologist--the Archaean
rocks--are overwhelmingly volcanic, especially in their lower
part. Their thickness, as we know them, is estimated at 50,000
feet; a thickness which must represent many millions of years.
But we do not know how much thicker than this they may be. They
underlie the oldest rocks that have ever been exposed to the gaze
of the geologist. They include sedimentary deposits, showing the
action of water, and even probable traces of organic remains, but
they are, especially in their deeper and older sections,
predominantly volcanic. They evince what we may call a volcanic
age in the early story of the planet.

But before we pursue this part of the story further we must
interpolate a remarkable event in the record--the birth of the
moon. It is now generally believed, on a theory elaborated by Sir
G. Darwin, that when the formation of the crust had reached a
certain depth--something over thirty miles, it is calculated--it
parted with a mass of matter, which became the moon. The size of
our moon, in comparison with the earth, is so exceptional among
the satellites which attend the planets of our solar system that
it is assigned an exceptional origin. It is calculated that at
that time the earth turned on its axis in the space of four or
five hours, instead of twenty-four. We have already seen that the
tidal influence of the sun has the effect of moderating the
rotation of the planets. Now, this very rapid rotation of a
liquid mass, with a thin crust, would (together with the
instability occasioned by its cooling) cause it to bulge at the
equator. The bulge would increase until the earth became a
pear-shaped body. The small end of the pear would draw further
and further away from the rest--as a drop of water does on the
mouth of a tap--and at last the whole mass (some 5,000,000,000
cubic miles of matter) was broken off, and began to pursue an
independent orbit round the earth.

There are astronomers who think that other cosmic bodies, besides
our moon, may have been formed in this way. Possibly it is true
of some of the double stars, but we will not return to that
question. The further story of the moon, as it is known to
astronomers, may be given in a few words. The rotational movement
of the earth is becoming gradually slower on account of tidal
influence; our day, in fact, becomes an hour longer every few
million years. It can be shown that this had the effect of
increasing the speed, and therefore enlarging the orbit, of the
moon, as it revolved round the earth. As a result, the moon drew
further and further away from the earth until it reached its
present position, about 240,000 miles away. At the same time the
tidal influence of the earth was lessening the rotational
movement of the moon. This went on until it turned on its axis in
the same period in which it revolves round the earth, and on this
account it always presents the same face to the earth.

Through what chapters of life the moon may have passed in the
meantime it is impossible to say. Its relatively small mass may
have been unable to keep the lighter gases at its surface, or its
air and water may, as some think, have been absorbed. It is
to-day practically an airless and waterless desert, alternating
between the heat of its long day and the intense cold of its long
night. Careful observers, such as Professor Pickering, think that
it may still have a shallow layer of heavy gases at its surface,
and that this may permit the growth of some stunted vegetation
during the day. Certain changes of colour, which are observed on
its surface, have been interpreted in that sense. We can hardly
conceive any other kind of life on it. In the dark even the gases
will freeze on its surface, as there is no atmosphere to retain
the heat. Indeed, some students of the moon (Fauth, etc.) believe
that it is an unchanging desert of ice, bombarded by the
projectiles of space.

An ingenious speculation as to the effect on the earth of this
dislodgment of 5,000,000,000 cubic miles of its substance is
worth noting. It supposes that the bed of the Pacific Ocean
represents the enormous gap torn in its side by the delivery of
the moon. At each side of this chasm the two continents, the Old
World and the New, would be left floating on their molten ocean;
and some have even seen a confirmation of this in the lines of
crustal weakness which we trace, by volcanoes and earthquakes, on
either side of the Pacific. Others, again, connect the shape of
our great masses of land, which generally run to a southern
point, with this early catastrophe. But these interesting
speculations have a very slender basis, and we will return to the
story of the development of the earth.

The last phase in preparation for the appearance of life would be
the formation of the ocean. On the lines of the generally
received nebular hypothesis this can easily be imagined, in broad
outline. The gases would form the outer shell of the forming
planet, since the heavier particles would travel inward. In this
mixed mass of gas the oxygen and hydrogen would combine, at a
fitting temperature, and form water. For ages the molten crust
would hold this water suspended aloft as a surrounding shell of
cloud, but when the surface cooled to about 380 degrees C.
(Sollas), the liquid would begin to pour on it. A period of
conflict would ensue, the still heated crust and the frequent
volcanic outpours sending the water back in hissing steam to the
clouds. At length, and now more rapidly, the temperature of the
crust would sink still lower, and a heated ocean would settle
upon it, filling the hollows of its irregular surface, and
washing the bases of its outstanding ridges. From that time
begins the age-long battle of the land and the water which, we
shall see, has had a profound influence on the development of

In deference to the opinion of a number of geologists we must
glance once more at the alternative view of the planetesimal
school. In their opinion the molecules of water were partly
attracted to the surface out of the disrupted matter, and partly
collected within the porous outer layers of the globe. As the
latter quantity grew, it would ooze upwards, fill the smaller
depressions in the crust, and at length, with the addition of the
attracted water, spread over the irregular surface. There is an
even more important difference of opinion in regard to the
formation of the atmosphere, but we may defer this until the
question of climate interests us. We have now made our globe, and
will pass on to that early chapter of its story in which living
things make their appearance.

To some it will seem that we ought not to pass from the question
of origin without a word on the subject of the age of the earth.
All that one can do, however, is to give a number of very
divergent estimates. Physicists have tried to calculate the age
of the sun from the rate of its dissipation of heat, and have
assigned, at the most, a hundred million years to our solar
system; but the recent discovery of a source of heat in the
disintegration of such metals as radium has made their
calculations useless. Geologists have endeavoured, from
observation of the action of geological agencies to-day, to
estimate how long it will have taken them to form the stratified
crust of the earth; but even the best estimates vary between
twenty-five and a hundred million years, and we have reason to
think that the intensity of these geological agencies may have
varied in different ages. Chemists have calculated how long it
would take the ocean, which was originally fresh water, to take
up from the rocks and rivers the salt which it contains to-day;
Professor Joly has on this ground assigned a hundred million
years since the waters first descended upon the crust. We must be
content to know that the best recent estimates, based on positive
data, vary between fifty and a hundred million years for the
story which we are now about to narrate. The earlier or
astronomical period remains quite incalculable. Sir G. Darwin
thinks that it was probably at least a thousand million years
since the moon was separated from the earth. Whatever the period
of time may be since some cosmic cataclysm scattered the material
of our solar system in the form of a nebula, it is only a
fraction of that larger and illimitable time which the evolution
of the stars dimly suggests to the scientific imagination.


[The scale of years adopted--50,000,000 for the stratified
rocks--is merely an intermediate between conflicting estimates.]


Quaternary Holocene 500,000 years

Tertiary Pliocene 5,500,000 years
or Miocene
Cenozoic Oligocene

Secondary Cretaceous 7,200,000 years
or Jurassic 3,600,000 "
Mesozoic Triassic 2,500,000 "

Primary Permian 2,800,000 years
or Carboniferous 6,200,000 "
Palaeozoic Devonian 8,000,000 "
Silurian 5,400,000 "
Ordovician 5,400,000 "
Cambrian 8,000,000 "

Archaean Keweenawan Unknown (probably
Animikie at least
Huronian 50,000,000 years)


There is, perhaps, no other chapter in the chronicle of the earth
that we approach with so lively an interest as the chapter which
should record the first appearance of life. Unfortunately, as far
as the authentic memorials of the past go, no other chapter is so
impenetrably obscure as this. The reason is simple. It is a
familiar saying that life has written its own record, the
long-drawn record of its dynasties and its deaths, in the rocks.
But there were millions of years during which life had not yet
learned to write its record, and further millions of years the
record of which has been irremediably destroyed. The first volume
of the geological chronicle of the earth is the mass of the
Archaean (or "primitive") rocks. What the actual magnitude of
that volume, and the span of time it covers, may be, no geologist
can say. The Archaean rocks still solidly underlie the lowest
depth he has ever reached. It is computed, however, that these
rocks, as far as they are known to us, have a total depth of
nearly ten miles, and seem therefore to represent at least half
the story of the earth from the time when it rounded into a
globe, or cooled sufficiently to endure the presence of oceans.

Yet all that we read of the earth's story during those many
millions of years could be told in a page or two. That section of
geology is still in its infancy, it is true. A day may come when
science will decipher a long and instructive narrative in the
masses of quartz and gneiss, and the layers of various kinds,
which it calls the Archaean rocks. But we may say with confidence
that it will not discover in them more than a few stray syllables
of the earlier part, and none whatever of the earliest part, of
the epic of living nature. A few fossilised remains of somewhat
advanced organisms, such as shell-fish and worms, are found in

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