and the question will naturally arise whether we can assign any
definite time to the various critical events recorded in the
evolution of the earth. At present we can only make more or less
plausible estimates. Thus, from a consideration of the thickness of
the sedimentary crust, and the rate at which sediments are now being
deposited, it has been asserted that the interval which separates
us from the close of the Archæan era may amount to about twenty-six
millions of years. Professor Joly, basing his argument on the undoubted
fact that the ocean derives the greater part of its salt from the
dissolved material contributed to it by rivers, comes to the conclusion
that the ocean first came into existence about one hundred millions
of years ago. As regards the birth of the moon, Sir George Darwin has
given a minimum limit of fifty-four millions of years, but he adds that
it may have taken place many hundreds of millions of years before this.
Lord Kelvin has attempted to determine the time which has elapsed since
the earth first acquired a solid crust. If we only knew the rate at
which the earth is cooling we might calculate back to this time with
some assurance of certainty, always, however, on the assumption that
the earth is simply a hot body cooling like any other hot body--such,
say, as a red-hot cannonball. But a few years ago it began to be
seriously suspected that this assumption was a very doubtful one, for
a new element--radium--was discovered in 1898, which possesses the
remarkable property of spontaneously liberating heat, and this not in
small quantities, but at an astonishing rate. One gramme of radium, for
example, gives out enough heat in one hour to raise the temperature of
one gramme of water to boiling point; hour after hour, year in, year
out, this wonderful substance is setting free the energy it contains,
and will continue to do so until, some thousands of years hence,
it has exhausted its store. If this element should happen to exist
in sufficient quantity within the earth, then the earth could not be
said to be cooling just like a piece of hot iron, and the increase of
temperature we experience as we descend towards the interior of the
earth might possibly be due to the heat set free from radium. Indeed,
the argument is not confined to the earth; it may apply also to the
sun, and much of the heat we derive from that luminary may be provided
by bursting atoms of radium. This was pointed out by Sir George Darwin
and Professor Joly in 1903.
It became obviously a question of the first importance to discover
what proportion of the earth’s crust consists of radium, and an
investigation was undertaken for this purpose by the Hon. R. J.
Strutt, who finds that the rocks composing the earth’s crust contain
a superabundance of radium--sufficient, if this element is uniformly
distributed through the whole earth in the same proportion as it occurs
at the surface, not only to make good the heat which is radiated away
into space, but actually to raise the temperature of our planet, which,
on this evidence, should, therefore, be growing not colder, but hotter.
This is a result as disconcerting at first sight as it is astonishing,
and its effects are very wide-reaching. Of course, it completely
destroys the validity of Lord Kelvin’s argument, but it also deprives
the nebular hypothesis of one of its cherished lines of evidence--a
loss which the force of the general argument enables us to bear with
equanimity.
[Sidenote: On the Eve of great Events]
In any case, the vast body of facts bearing on the history of the earth
suffices to show that its temperature cannot be rising. Mr. Strutt
has, therefore, imagined that the radium is not uniformly distributed
throughout the mass of the planet, and supposes that it is restricted
to an external zone forty-five miles in thickness; this would suffice
to maintain the earth at its existing temperature. If, however, we
admit a restriction of this kind, we are in no way bound to fix the
limit at forty-five miles. All we can say is that we do not know how
far downwards the radium reaches--for aught we know five miles, or even
less, is as likely a limit as forty-five miles. Professor Joly, indeed,
maintains that the radium we meet with is not proper to the earth at
all, but comes from the sun.
Radium is a short-lived element, its existence being limited to a
few thousand years; but as fast as it decays it is reproduced at the
expense of another element--uranium--the lifetime of which is measured
by hundreds of millions of years.
The last quarter of a century has proved fertile in great
discoveries--more so than any corresponding period in the past. As a
result, the whole world of scientific thought has been thrown into
commotion; old-established theories, and even the most fundamental
notions, seem to be in a state of flux. Under the stimulus of new ideas
great questions, such as the constitution of matter, the origin of
species, and the birth of worlds are being re-investigated with renewed
energy, and we seem to be on the eve of great events.
WILLIAM JOHNSON SOLLAS
FOUR PERIODS OF THE EARTH’S DEVELOPMENT
A Postscript to Professor Sollas’s Chapter on the Wonderful Story
of the World’s Birth, beginning on page 79
The earth was once “a fluid haze of light.” The whole solar system
once formed a vast nebula, consisting of glowing gas, or a swarm of
meteoroids. Our planet was slowly shaped into a globe out of this
primitive nebula.
This globe was at first intensely hot, and probably liquid. A solid
crust formed on the surface as heat was lost by radiation, and this
crust consisted of the oldest rocks of igneous formation like the
granites and gneisses. During this Archæan or Eozoic Period, the earth
acquired its atmosphere and its oceans, and it is probable that the
mysterious origin of life took place.
The later history of the earth since the stratified rocks began to
appear, and life existed, is divided into four main periods, of which
the first is known as Primary, or Palæozoic.
The First Period of the Earth
CAMBRIAN SYSTEM. The rocks formed in the Cambrian Age are
mainly grits, quartzites, and conglomerates, with shales, schists, and
limestones. The earth was then mostly covered by seas, and the first
well-defined forms of life were of marine origin.
SILURIAN SYSTEM. The Silurian rocks are mostly sandstones,
shales, and slates deposited in the seas. The first vertebrates made
their appearance as fishes, whilst insects began to flutter in the air,
and occasionally to alight on the emerging land.
DEVONIAN SYSTEM. This was the age of the old red sandstone.
Fishes reached a high state of development, whilst the first traces
appeared of land vegetation, ferns and lycopods.
CARBONIFEROUS SYSTEM. This system is exceptionally important,
because its chief rock is coal, the fossilised remains of the luxuriant
vegetation which grew in tropical swamps. The first terrestrial
animals, true air breathers, now appeared.
PERMIAN SYSTEM. The last of the primary systems gave us the
new red sandstone, distinguished from the old by lying above the coal
measures. The Permian Age was apparently unfavourable to life, and is
only notable for the first appearance of the land reptiles into which
the amphibians developed.
The Second Period of the Earth
The Secondary Period marks the emergence of the dry land into
importance greater than that of the sea.
TRIASSIC SYSTEM. The Triassic rocks chiefly consist of
sandstones and hardened clays laid down in shallow sea basins. Land
vegetation now first began to assume a modern type, with conifers and
cycads. The seas were still richly peopled, and the land first gave a
home to huge reptiles, or dinosaurs.
JURASSIC SYSTEM. This system is marked by a great variety
of limestones, the product of dead sea creatures. It is essentially
the age of reptiles. The ichthyosaurus disputed the seas with the
plesiosaurus; the pterodactyl ruled the air; whilst on land, huge
monsters like the brontosaur and diplodocus browsed on tropical
vegetation. From these reptiles the birds were developing, whilst small
marsupials, the oldest of the great mammalian race, skipped under the
branches.
CRETACEOUS SYSTEM. This was the age of the great chalk
deposits. The birds, now emerging from their reptilian ancestry,
dominated its life, and the first modern plants appeared on the land.
The Third Period of the Earth
The Tertiary Period marks the true beginning of modern geological
history, when the great outlines of geography were laid down, and
the first representatives of modern plants and animals made their
appearance.
EOCENE SYSTEM. The Eocene rocks are mainly limestones, with
sandstone and hardened clays. We owe them to the sea and its organisms.
Modern evergreen trees now first appeared. The mammals come to the
front, with the tapir-like palæotherium and the first recognisable
ancestor of the horse.
MIOCENE SYSTEM. The Miocene Age was a mountain-building
period, when the great chain which runs from the Alps into Central
Asia received its final uplift. Deciduous trees, like the beech and
elm, now made their appearance. The giant mastodon and the formidable
sabre-toothed tiger roamed the Miocene forest, and true apes--man’s
first forerunners--mopped and mowed in the boughs.
PLIOCENE SYSTEM. The last of the Tertiary ages set the final
stamp on the geological moulding of the earth’s crust. Its plants were
transitional to the flora of modern Europe. Great herds of herbivora
now appeared.
The Fourth Period of the Earth
The Quaternary Period is that in which we are still living. Its
outstanding feature is the appearance of man.
PLEISTOCENE OR GLACIAL SYSTEM. Its essential feature was the
appearance of glacial conditions over most of the northern hemisphere,
when great ice sheets rubbed our land into shape. The vegetation was
Arctic, and only animals like the reindeer and the hairy mammoth could
endure the cold.
HUMAN OR RECENT SYSTEM. The precise antiquity of man is still
uncertain, but it was only after the close of the Glacial Period that
he made his home in Europe, where he shared a precarious existence
with mammoth, cave-bear, and rhinoceros. Man developed through the
_Palæolithic_ and _Neolithic_ ages of stone implements to the _Bronze_
and _Iron_ ages, when metal was first worked. In the last of these we
live.
GEOLOGICAL CLOCK OF THE WORLD’S LIFE
This page is an effort, based on Professor Lester Ward’s calculations
in “Pure Sociology,” to show the comparative length of each geological
period, and the thin white line between Tertiary and Archæan indicates
the period of human history. Thin as this line is--and we could not
show it thinner--it is too thick, and out of proportion to the rest
of the clock. If we assume that from the beginning of the world--from
its first forming into a solid sphere--to the present, time may be
represented by a day of twenty-four hours, the time occupied by human
history does not exceed twelve seconds. This is reckoning human history
as ten thousand years. There is, of course, no possibility of obtaining
more than relative figures for such a scheme as this, which should be
regarded in connection with the previous page and the chart of the
Beginnings of Life, facing page 96
[Illustration: The thin white line between the Tertiary and the Archæan
periods represents the duration of human history]
TABLE SHOWING PROPORTIONS OF YEARS AND HOURS
Geological Periods | Years | Hours
------------------------+-------------+---------
Archæan | 18,000,000 | 6
Laurentian | 18,000,000 | 6
Cambrian | 6,000,000 | 2
Silurian | 6,000,000 | 2
Devonian | 6,000,000 | 2
Carboniferous | 6,000,000 | 2
Triassic | 3,000,000 | 1
Jurassic | 3,000,000 | 1
Cretaceous | 3,000,000 | 1
Tertiary and Quaternary | 3,000,000 | 1
------------------------+-------------+---------
The Quaternary Period | 72,000,000 = 24
is that in which we live|
TERTIARY AND QUATERNARY PERIODS
At a rough guess, three million years may be allowed for the Tertiary
and Quaternary periods
--------------------+-----------+------+------+------
Geological Periods | Years | Hrs. | Min. | Sec.
--------------------+-----------+------+------+------
Tertiary | 2,600,000 | -- | 52 | --
Pleistocene | 300,000 | -- | 6 | --
Human | 100,000 | -- | 2 | --
+-----------+------+------+------
Total | 3,000,000 | 1 | -- | --
--------------------+-----------+------+------+------
Human History | 10,000 == == 12
HOW LIFE BECAME POSSIBLE ON THE EARTH
BY DR. ALFRED RUSSEL WALLACE
Early writers on the relation of man and animated nature to the
material universe not only assumed that the latter existed for the
former, but that both alike were the results of special acts of
creation.
Furthermore, they usually took it for granted that all things were
created very much in the condition in which we now see them, and that
any changes that have since taken place are but slight superficial
modifications of a permanent and unchanging whole. Not only were the
sun and moon and stars created as appanages of the earth, but the earth
itself in all its details of sea and land, hills and valleys, mountains
and precipices, swamps and deserts, was made and fashioned just as we
now see it, and every feature of its surface was supposed to have some
purpose in connection with man.
[Sidenote: The Old Ideas of Creation]
These purposes we could, in some cases, understand, while in others
they seemed wholly unintelligible, and much ingenuity was bestowed
by the natural theologian and others to explain more and more of the
observed facts from this point of view. The same opinions prevailed in
regard to the infinite variety of animals and plants, each individual
species being supposed to have been an independent creation, and all to
have some definite and preordained purpose in relation to mankind.
These views, however absurd they seem to most people now, were almost
universally held so recently as during the seventeenth and eighteenth
centuries, and were thus coincident with one of the most brilliant
epochs of our literature and our dawning science. It was only towards
the beginning of the nineteenth century, when geology became widely
studied and its results were fully appreciated, that the more rational
conception of a very slow development of the earth’s surface during
countless ages began to be generally accepted.
[Sidenote: Changing Conditions of the Earth]
The grand nebular hypothesis of Laplace came to reinforce the views of
the geologists, by showing how the earth itself may have originated
as a gaseous or molten globe; and its slow process of cooling, with
the reaction of the interior and exterior on each other, served to
elucidate the facts of the heated interior, as shown by hot springs and
volcanoes, as well as many of the phenomena presented by the distorted
and metamorphosed strata which formed its crust. Hence it gradually
came to be perceived that the condition of the earth, with all its
endless variations of surface, of continents and oceans, of seas and
islands, of vast plateaux and lofty mountain ranges and extensive low
plains, with their ravines and cataracts, their great lakes and stately
rivers, was subject to perpetual change from that remote epoch when it
seems to have been actually the case that “the earth was without form
and void,” and that owing to the greater density of the vapour-laden
atmosphere, “darkness was upon the face of the deep.”
[Sidenote: Changing Forms of Life]
Another field of geological research forced us to the conclusion that
the same continued process of change had affected the forms of life
upon the earth. When carefully investigated, the crust was found
to abound in the fossilised remains of animals and plants. Careful
study of these showed that the oldest of all were of comparatively
simple structure, and that the higher forms only appeared in more
recent epochs; while the highest of all were probably very little
older than man himself. It is only during the last half century that
the theory of Evolution has been elaborated and has become generally
accepted as applicable to the whole of the vast cosmic process--from
the development of the nebulæ into stars and suns and systems, with a
corresponding development of planets from an early condition of intense
heat, through a more or less lengthy period of cooling and contraction,
to an ultimate state of refrigeration, the earlier and later stages
being alike unsuited to the existence of life.
[Sidenote: Theory of Natural Selection]
More important still, the discovery of the theory of Natural Selection
by Darwin--and at a later period by myself--has led to a satisfactory
explanation of the successive appearance of higher and more complex
forms of life, and also of that wonderfully minute and complex
_adaptation_ of every species to its conditions of existence and to
its organic as well as its inorganic environment, which all other
theories--even the most recent--have failed to grapple with.
[Sidenote: Wonderful Complexity of the Universe]
The logical completeness as well as the extreme simplicity of this
explanation of organic evolution has led great numbers of thoughtful
but ill-informed persons to reject it, because it seems to render
unnecessary the existence of a primary intelligent cause; while
another equally large but, as I think, equally ill-informed class--the
so-called monists--use it to demonstrate the non-existence, or, at all
events, the needlessness, of any such cause. Both alike err, because
they fail to take cognisance of the fact that every form of evolution,
and pre-eminently that of the organic world, is an explanation of a
process of change, a law of development, not in any sense or by any
possibility an explanation of fundamental laws, causes, or origins.
It presupposes the existence not only of matter--itself a thing whose
nature is becoming more and more mysterious and unthinkable with the
advance of physical science--but of all the vast complex of laws
and forces which act upon it--mechanical, physical, chemical, and
electrical laws and forces--all more or less dependent on the still
more mysterious, all-pervading ether. Thus, the universe in its purely
physical and inorganic aspect is now seen to be such an overwhelmingly
complex organism as to suggest to most minds some vast intelligent
power pervading and sustaining it.
Persons to whom this seems a logical necessity will not be much
disturbed by the dilemma of the agnostics--that, however wonderful the
material universe may be, a being who could bring it into existence
must be more wonderful, and that they prefer to hold the lesser
marvel to be self-existent rather than the greater. When, however,
we pass from the inorganic to the organic world, governed by a new
set of laws, and apparently by some regulating and controlling forces
altogether distinct from those at work in inorganic nature; and when,
further, we see that these organisms originated at some definite epoch
when the earth had become adapted to sustain them, and thereafter
developed into two great branches of non-sentient and sentient
life, the latter gradually acquiring higher and higher senses and
faculties till it culminated in man--a being whose higher intellectual
and moral nature seems adapted for, even to call for, indefinite
development--this logical necessity for some higher intelligence to
which he himself owes his existence, and which alone rendered the
origin of sentient life possible, will seem still more irresistible.
[Sidenote: Mind Behind the World]
The preceding remarks are intended to suggest that the theory of
evolution, combined with the quite recent and very startling advances
in physical science, so far from making the universe around us more
intelligible as a self-sustaining and self-existent whole, has really
rendered it less so, by showing that it is infinitely more complex
than we had formerly supposed; and further, that matter itself,
instead of being, as was once believed, a comparatively simple thing,
eternal and indestructible, is in all its various forms subject to
decay and disintegration. We now see that the only thing known to us
that we can conceive as having unending existence is mind itself; and,
just as Darwin’s theory of Natural Selection has opened up to us an
infinite field of study and admiration in the forms and colours and
mutual relations of the various species of animals and plants, so does
modern science open up to us new and unfathomable depths in the inner
structure of matter and of the cosmos, and thus compels us more and
more to recognise a mental rather than a mere physical substratum to
account for its existence.
There is, however, another set of relations which have been hitherto
very little studied--those between the organic and the inorganic
worlds in their broader aspects. These are now found to be very much
more complex and more remarkable than is usually supposed, and they
also have an important bearing upon the great problem of the origin
and destiny of man. This is a subject which opens up a variety of
considerations of extreme interest, showing that the exact adaptations
of our earth--and presumably of any other planets--to enable it to
sustain organic life, from its first appearance and through its long
course of development, is as varied and complex and as much beyond
the possibilities of chance coincidences as are any of the individual
adaptations of animals and plants to their immediate environment. Most
of these latter adaptations have been made known to us by Darwin and
his followers, and they have excited the admiration and astonishment
of all lovers of Nature. When the antecedent and grander relations of
planet to life are studied with equal care, these also will, I believe,
excite deeper admiration, still more profound astonishment, because
any secondary laws that could have brought them about are less easy to
discover, or even to imagine.
[Sidenote: Essential Conditions of Life]
Before we can form any adequate idea of the nature of a world which
shall be able to support and develop organic life, we must consider
what are the special conditions that alone render such life possible.
We, of course, refer to the whole of the organic world, from the lowest
to the highest, not to the few exceptional cases in which life may be
possible under conditions that would be fatal to the higher as well as
to most of the lower forms.
[Sidenote: The Miracle of Human Life]
The one striking speciality of the higher animals--and to a less
degree of the higher plants--is that of continuous, all-pervading
motion, every portion of their substance being in a state of flux:
each particle itself moving, growing, living and dying, and being
replaced by other particles of the same nature and fulfilling the
same functions. To keep up this growth, and to enable every part of
the structure to be continually renewed, food is required. This is
taken into the stomach of animals in the solid or liquid form, is
then decomposed and recomposed, that which is useless or superfluous
being thrown off by the intestines, while what is needed for growth
is transformed into blood and by a wonderfully intricate system of
branching tubes is carried to every part of the body, furnishing
nourishment and repair alike to bone and muscle, to all the internal
organs and all the outward integuments, and to that marvellously
complex nervous system which also permeates every part of the body and
is essential to the higher manifestations of life--to the exertion of
force, voluntary motion, and, apparently, to thought itself. Add to
this the constant influx of air, which at once purifies the blood and
supplies animal heat, and is so important that its cessation for a
few minutes is usually fatal, and we have a machine so complex in its
structure and mode of action that the most elaborate of human machines
is but as a grain of sand to a world in comparison.
[Sidenote: Basis of Physical Life]
Now the very possibility of such a material organism as this depends
upon a highly complex form of matter termed protoplasm, which is at
once extremely plastic and of extreme instability, and is yet capable
of secreting or building up its atoms into such solid and apparently
durable forms as bone, horn, and hair, besides the various liquids and
semi-solids which build up the organism. This fundamental organic
substance consists of only four chemical elements--nitrogen, hydrogen,
oxygen and carbon, and almost all animal and vegetable structures and
products have the same elemental constitution, though with such widely
different characteristics. Four other elements--sulphur, lime, silicon,
and phosphorus--also occur in small quantities in organic tissues,
to supply special needs; but these are not essential to all forms of
life, and are only taken up and utilised by the living protoplasm when
required. Protoplasm is undoubtedly the basis of physical life, yet
it only exists in, and is produced by, living organisms. The moment
such an organism dies, disorganisation and decay set in, and the whole
mass becomes gradually changed into more stable compounds, or into its
constituent elements. It appears, therefore, that some agency--usually
termed “vital force”--must be at work, first to produce this wonderful
compound, then to form it into “cells”--the physiological units of
all organisms--and afterwards to direct the energies supplied by heat
and light so as to build up the excessively complex structures, with
all their wonderful powers and potentialities, which we term animals
and plants. All this seems to imply not “a force” only, but very many
forces, all of which must have some kind of mind in or behind them,
to direct these forces to such infinitely varied yet perfectly defined
ends.
[Sidenote: A Marvel of Every Day]