the least memorable is the promulgation of the first well-founded
conceptions of the high antiquity of the globe. Some six thousand
years had previously been usually believed to comprise the whole life
of the planet, and indeed of the entire universe. When the curtain
was then first raised that had veiled the history of the earth, and
men, looking beyond the brief span within which they had supposed
that history to have been transacted, beheld the records of a long
vista of ages stretching far away into a dim illimitable past, the
prospect vividly impressed their imagination. Astronomy had made known
the immeasurable fields of space; the new science of geology seemed
now to reveal boundless distances of time. The more the terrestrial
chronicles were studied the farther could the eye range into an
antiquity so vast as to defy all attempts to measure or define it. The
progress of research continually furnished additional evidence of the
enormous duration of the ages that preceded the coming of man, while,
as knowledge increased, periods that were thought to have followed
each other consecutively were found to have been separated by prolonged
intervals of time. Thus the idea arose and gained universal acceptance
that, just as no boundary could be set to the astronomer in his free
range through space, so the whole of bygone eternity lay open to the
requirements of the geologist. Playfair, re-echoing and expanding
Hutton's language, had declared that neither among the records of the
earth nor in the planetary motions can any trace be discovered of the
beginning or of the end of the present order of things; that no symptom
of infancy or of old age has been allowed to appear on the face of
Nature, nor any sign by which either the past or the future duration of
the universe can be estimated; and that although the Creator may put
an end, as He no doubt gave a beginning, to the present system, such a
catastrophe will not be brought about by any of the laws now existing,
and is not indicated by anything which we perceive. This doctrine was
naturally espoused with warmth by the extreme uniformitarian school,
which required an unlimited duration of time for the accomplishment
of such slow and quiet cycles of change as they conceived to be alone
recognisable in the records of the earth's past history.
It was Lord Kelvin who, in the writings to which I have already
referred, first called attention to the fundamentally erroneous nature
of these conceptions. He pointed out that from the high internal
temperature of our globe, increasing inwards as it does, and from
the rate of loss of its heat, a limit may be fixed to the planet's
antiquity. He showed that so far from there being no sign of a
beginning, and no prospect of an end to the present economy, every
lineament of the solar system bears witness to a gradual dissipation
of energy from some definite starting-point. No very precise data were
then, or indeed are now, available for computing the interval which
has elapsed since that remote commencement, but he estimated that the
surface of the globe could not have consolidated less than twenty
millions of years ago, for the rate of increase of temperature inwards
would in that case have been higher than it actually is; nor more than
400 millions of years ago, for then there would have been no sensible
increase at all. He was inclined, when first dealing with the subject,
to believe that from a review of all the evidence then available, some
such period as 100 millions of years would embrace the whole geological
history of the globe.
It is not a pleasant experience to discover that a fortune which one
has unconcernedly believed to be ample has somehow taken to itself
wings and disappeared. When the geologist was suddenly awakened by
the energetic warning of the physicist, who assured him that he had
enormously overdrawn his account with past time, it was but natural
under the circumstances that he should think the accountant to be
mistaken, who thus returned to him dishonoured the large drafts he had
made on eternity. He saw how wide were the limits of time deducible
from physical considerations, how vague the data from which they had
been calculated. And though he could not help admitting that a limit
must be fixed beyond which his chronology could not be extended, he
consoled himself with the reflection that after all a hundred millions
of years was a tolerably ample period of time, and might possibly have
been quite sufficient for the transaction of all the prolonged sequence
of events recorded in the crust of the earth. He was therefore disposed
to acquiesce in the limitation thus imposed upon geological history.
But physical inquiry continued to be pushed forward with regard to the
early history and the antiquity of the earth. Further consideration
of the influence of tidal friction in retarding the earth's rotation,
and of the sun's rate of cooling, led to sweeping reductions of the
time allowable for the evolution of the planet. The geologist found
himself in the plight of Lear when his bodyguard of one hundred knights
was cut down. 'What need you five-and-twenty, ten, or five?' demands
the inexorable physicist, as he remorselessly strikes slice after
slice from his allowance of geological time. Lord Kelvin is willing, I
believe, to grant us some twenty millions of years, but Professor Tait
would have us content with less than ten millions.
In scientific as in other mundane questions there may often be two
sides, and the truth may ultimately be found not to lie wholly with
either. I frankly confess that the demands of the early geologists
for an unlimited series of ages were extravagant, and even, for their
own purposes, unnecessary, and that the physicist did good service
in reducing them. It may also be freely admitted that the latest
conclusions, from physical considerations of the extent of geological
time, require that the interpretation given to the record of the rocks
should be rigorously revised, with the view of ascertaining how far
that interpretation may be capable of modification or amendment. But we
must also remember that the geological record constitutes a voluminous
body of evidence regarding the earth's history which cannot be ignored,
and must be explained in accordance with ascertained natural laws. If
the conclusions derived from the most careful study of this record
cannot be reconciled with those drawn from physical considerations, it
is surely not too much to ask that the latter should be also revised.
It was well said by Huxley that the mathematical mill is an admirable
piece of machinery, but that the value of what it yields depends upon
the quality of what is put into it. That there must be some flaw in the
physical argument I can, for my own part, hardly doubt, though I do not
pretend to be able to say where it is to be found. Some assumption, it
seems to me, has been made, or some consideration has been left out
of sight, which will eventually be seen to vitiate the conclusions,
and which when duly taken into account will allow time enough for any
reasonable interpretation of the geological record.
In problems of this nature, where geological data capable of numerical
statement are so needful, it is hardly possible to obtain trustworthy
computations of time. We can only measure the rate of changes in
progress now, and infer from these changes the length of time required
for the completion of results achieved by the same processes in the
past. There is fortunately one great cycle of movement which admits of
careful investigation, and which has been made to furnish valuable
materials for estimates of this kind. The universal degradation of
the land, so notable a characteristic of the earth's surface, has
been regarded as an extremely slow process. Though it goes on without
ceasing, yet from century to century it seems to leave hardly any
perceptible trace on the landscapes of a country. Mountains and plains,
hills and valleys, appear to wear the same familiar aspect which is
indicated in the oldest pages of history. This obvious slowness in one
of the most important departments of geological activity, doubtless
contributed in large measure to form and foster a vague belief in the
vastness of the antiquity required for the evolution of the earth.
But, as geologists eventually came to perceive, the rate of degradation
of the land is capable of actual measurement. The amount of material
worn away from the surface of any drainage-basin and carried in
the form of mud, sand, or gravel, by the main river into the sea,
represents the extent to which that surface has been lowered by waste
in any given period of time. But denudation and deposition must be
equivalent to each other. As much material must be laid down in
sedimentary accumulations as has been mechanically removed, so that in
measuring the annual bulk of sediment borne into the sea by a river, we
obtain a clue not only to the rate of denudation of the land, but also
to the rate at which the deposition of new sedimentary formations takes
place.
As might be expected, the activities involved in the lowering of the
surface of the land are not everywhere equally energetic. They are
naturally more vigorous where the rainfall is heavy, where the daily
range of temperature is large, and where frosts are severe. Hence
they are obviously much more effective in mountainous regions than on
plains; and their results must constantly vary, not only in different
basins of drainage, but even, and sometimes widely, within the same
basin. Actual measurement of the proportion of sediment in river water
shows that while in some cases the lowering of the surface of the land
may be as much as 1/730 of a foot in a year, in others it falls as low
as 1/6800. In other words, the rate of deposition of new sedimentary
formations, over an area of seafloor equivalent to that which has
yielded the sediment, may vary from one foot in 730 years to one foot
in 6800 years.
If now we take these results and apply them as measures of the length
of time required for the deposition of the various sedimentary
masses that form the outer part of the earth's crust, we obtain some
indication of the duration of geological history. On a reasonable
computation these stratified masses, where most fully developed, attain
a united thickness of not less than 100,000 feet. If they were all laid
down at the most rapid recorded rate of denudation, they would require
a period of seventy-three millions of years for their completion. If
they were laid down at the slowest rate they would demand a period of
not less than 680 millions.
But it may be argued that all kinds of terrestrial energy are growing
feeble, that the most active denudation now in progress is much less
vigorous than that of bygone ages, and hence that the stratified part
of the earth's crust may have been put together in a much briefer
space of time than modern events might lead us to suppose. Such
arguments are easily adduced and look sufficiently specious, but no
confirmation of them can be gathered from the rocks. On the contrary,
no one can thoughtfully study the various systems of stratified
formations without being impressed by the fulness of their evidence
that, on the whole, the accumulation of sediment has been extremely
slow. Again and again we encounter groups of strata composed of thin
paper-like laminæ of the finest silt, which evidently settled down
quietly and at intervals on the sea bottom. We find successive layers
covered with ripple-marks and sun-cracks, and we recognise in them
memorials of ancient shores where sand and mud tranquilly gathered as
they do in sheltered estuaries at the present day. We can see no proof
whatever, nor even any evidence which suggests, that on the whole the
rate of waste and sedimentation was more rapid during Mesozoic and
Palæozoic time than it is to-day. Had there been any marked difference
in this rate from ancient to modern times, it would be incredible that
no clear proof of it should have been recorded in the crust of the
earth.
But in actual fact the testimony in favour of the slow accumulation and
high antiquity of the geological record is much stronger than might be
inferred from the mere thickness of the stratified formations. These
sedimentary deposits have not been laid down in one unbroken sequence,
but have had their continuity interrupted again and again by upheaval
and depression. So fragmentary are they in some regions, that we can
easily demonstrate the length of time represented there by still
existing sedimentary strata to be vastly less than the time indicated
by some of the gaps in the series.
There is yet a further and impressive body of evidence furnished by the
successive races of plants and animals which have lived upon the earth
and have left their remains sealed up within its rocky crust. No one
now believes in the exploded doctrine that successive creations and
universal destructions of organic life are chronicled in the stratified
rocks. It is everywhere admitted that, from the remotest times up to
the present day, there has been an onward march of development, type
succeeding type in one long continuous progression. As to the rate
of this evolution precise data are wanting. There is, however, the
important negative argument furnished by the absence of evidence of
recognisable specific variations of organic forms since man began to
observe and record. We know that within human experience a few species
have become extinct, but there is no conclusive proof that a single new
species has come into existence, nor are appreciable variations readily
apparent in forms that live in a wild state. The seeds and plants found
with Egyptian mummies, and the flowers and fruits depicted on Egyptian
tombs, are easily identified with the vegetation of modern Egypt. The
embalmed bodies of animals found in that country show no sensible
divergence from the structure or proportions of the same animals at the
present day. The human races of Northern Africa and Western Asia were
already as distinct when portrayed by the ancient Egyptian artists as
they are now, and they do not seem to have undergone any perceptible
change since then. Thus a lapse of four or five thousand years has not
been accompanied by any recognisable variation in such forms of plant
and animal life as can be tendered in evidence. Absence of sensible
change in these instances is, of course, no proof that considerable
alteration may not have been accomplished in other forms more exposed
to vicissitudes of climate and other external influences. But it
furnishes at least a presumption in favour of the extremely tardy
progress of organic variation.
If, however, we extend our vision beyond the narrow range of human
history, and look at the remains of the plants and animals preserved in
those younger formations which, though recent when regarded as parts of
the whole geological record, must be many thousands of years older than
the very oldest of human monuments, we encounter the most impressive
proofs of the persistence of specific forms. Shells which lived in
our seas before the coming of the Ice-Age present the very same
peculiarities of form, structure, and ornament which their descendants
still possess. The lapse of so enormous an interval of time has not
sufficed seriously to modify them. So too with the plants and the
higher animals which still survive. Some forms have become extinct, but
few or none which remain display any transitional gradations into new
species. We must admit that such transitions have occurred, that indeed
they have been in progress ever since organised existence began upon
our planet, and are doubtless taking place now. But we cannot detect
them on the way, and we feel constrained to believe that their march
must be excessively slow.
There is no reason to think that the rate of organic evolution has ever
seriously varied; at least no proof has been adduced of such variation.
Taken in connection with the testimony of the sedimentary rocks, the
inferences deducible from fossils entirely bear out the opinion that
the building up of the stratified crust of the earth has been extremely
gradual. If the many thousands of years which have elapsed since
the Ice-Age have produced no appreciable modification of surviving
plants and animals, how vast a period must have been required for that
marvellous scheme of organic development which is chronicled in the
rocks!
After careful reflection on the subject, I affirm that the geological
record furnishes a mass of evidence which no arguments drawn from other
departments of Nature can explain away, and which, it seems to me,
cannot be satisfactorily interpreted save with an allowance of time
much beyond the narrow limits which recent physical speculation would
concede.[70]
I have reserved for final consideration a branch of the history of the
earth which, while it has become, within the lifetime of the present
generation, one of the most interesting and fascinating departments
of geological inquiry, owed its first impulse to the far-seeing
intellects of Hutton and Playfair. With the penetration of genius these
illustrious teachers perceived that if the broad masses of land and the
great chains of mountains owe their origin to stupendous movements
which from time to time have convulsed the earth, their details of
contour must be mainly due to the eroding power of running water. They
recognised that as the surface of the land is continually worn down, it
is essentially by a process of sculpture that the physiognomy of every
country has been developed, valleys being hollowed out and hills left
standing, and that these inequalities in topographical detail are only
varying and local accidents in the progress of the one great process of
the degradation of the land.
From the broad and guiding outlines of theory thus sketched we have
now advanced amid ever-widening multiplicity of detail into a fuller
and nobler conception of the origin of scenery. The law of evolution
is written as legibly on the landscapes of the earth as on any other
page of the Book of Nature. Not only do we recognise that the existing
topography of the continents, instead of being primeval in origin, has
gradually been developed after many precedent mutations, but we are
enabled to trace these earlier revolutions in the structure of every
hill and glen. Each mountain-chain is thus found to be a memorial of
successive stages in geographical evolution. Within certain limits,
land and sea have changed places again and again. Volcanoes have broken
out and have become extinct in many countries long before the advent of
man. Whole tribes of plants and animals have meanwhile come and gone,
and in leaving their remains behind them as monuments at once of the
slow development of organic types, and of the prolonged vicissitudes of
the terrestrial surface, have furnished materials for a chronological
arrangement of the earth's topographical features. Nor is it only
from the organisms of former epochs that broad generalisations may be
drawn regarding revolutions in geography. The living plants and animals
of to-day have been discovered to be eloquent of ancient geographical
features that have long since vanished. In their distribution they tell
us that climates have changed, that islands have been disjoined from
continents, that oceans once united have been divided from each other,
or once separate have now been joined; that some tracts of land have
disappeared, while others for prolonged periods of time have remained
in isolation. The present and the past are thus linked together not
merely by dead matter, but by the world of living things, into one vast
system of continuous progression.
In this marvellous increase of knowledge regarding the transformations
of the earth's surface, one of the most impressive features, to my
mind, is the power now given to us of perceiving the many striking
contrasts between the present and former aspects of topography and
scenery. We seem to be endowed with a new sense. What is seen by the
bodily eye--mountain, valley, or plain--serves but as a veil, beyond
which, as we raise it, visions of long-lost lands and seas rise before
us in a far-retreating vista. Pictures of the most diverse and opposite
character are beheld, as it were, through each other, their lineaments
subtly interwoven and even their most vivid contrasts subdued into one
blended harmony. Like the poet, 'we see, but not by sight alone'; and
the 'ray of fancy' which, as a sunbeam, lightened up his landscape, is
for us broadened and brightened by that play of the imagination which
science can so vividly excite and prolong.
Admirable illustrations of this modern interpretation of scenery are
supplied by the district wherein we are now assembled. On every side of
us rise the most convincing proofs of the reality and potency of that
ceaseless sculpture by which the elements of landscape have been carved
into their present shapes. Turn where we may, our eyes rest on hills,
that project above the lowland, not because they have been upheaved
into these positions, but because their stubborn materials have enabled
them better to withstand the degradation which has worn down the softer
strata into the plains around them. Inch by inch the surface of the
land has been lowered, and each hard rock successively laid bare has
communicated its own characteristics of form and colour to the scenery.
If, standing on the Castle Rock, the central and oldest site in
Edinburgh, we allow the bodily eye to wander over the fair landscape,
and the mental vision to range through the long vista of earlier
landscapes which science here reveals to us, what a strange series of
pictures passes before our gaze! The busy streets of to-day seem to
fade away into the mingled copsewood and forest of prehistoric time.
Lakes that have long since vanished gleam through the woodlands, and
a rude canoe pushing from the shore startles the red deer that had
come to drink. While we look, the picture changes to a polar scene,
with bushes of stunted Arctic willow and birch, among which herds of
reindeer browse and the huge mammoth makes his home. Thick sheets of
snow are draped all over the hills around, and far to the north-west
the distant gleam of glaciers and snow-fields marks the line of the
Highland mountains. As we muse on this strange contrast to the living
world of to-day, the scene appears to grow more Arctic in aspect,
until every hill is buried under one vast sheet of ice, 2,000 feet or
more in thickness, which fills up the whole midland valley of Scotland
and creeps slowly eastward into the basin of the North Sea. Here the
curtain drops upon our moving pageant, for in the geological record of
this part of the country an enormous gap occurs before the coming of
the Ice-Age.
When once more the spectacle resumes its movement the scene is found to
have utterly changed. The familiar hills and valleys of the Lothians
have disappeared. Dense jungles of a strange vegetation--tall reeds,
club-mosses, and tree-ferns--spread over the steaming swamps that
stretch for leagues in all directions. Broad lagoons and open seas are
dotted with little volcanic cones which throw out their streams of lava
and showers of ashes. Beyond these, dimmer in outline and older in
date, we descry a wide lake or inland sea, covering the whole midland
valley and marked with long lines of active volcanoes, some of them
several thousand feet in height. And still further and fainter over
the same region, we may catch a glimpse of that still earlier expanse
of sea which in Silurian times overspread most of Britain. But beyond
this scene our vision fails. We have reached the limit across which no
geological evidence exists to lead the imagination into the primeval
darkness beyond.