THE ANCIENT
LIFE-HISTORY
OF THE EARTH
Chapter 4:
THE BREAKS IN THE GEOLOGICAL AND PALÆONTOLOGICAL RECORD.
The term "contemporaneous" is usually applied by geologists to
groups of strata in different regions which contain the same
fossils, or an assemblage of fossils in which many identical
forms are present. That is to say, beds which contain identical,
or nearly identical, fossils, however widely separated they may
be from one another in point of actual distance, are ordinarily
believed to have been deposited during the same period of the
earth's history. This belief, indeed, constitutes the keystone
of the entire system of determining the age of strata by their
fossil contents; and if we take the word "contemporaneous" in a
general and strictly geological sense, this belief can be accepted
as proved beyond denial. We must, however, guard ourselves against
too literal an interpretation of the word "contemporaneous," and
we must bear in mind the enormously-prolonged periods of time with
which the geologist has to deal. When we say that two groups of
strata in different regions are "contemporaneous," we simply mean
that they were formed during the same geological period, and
perhaps at different stages of that period, and we do not mean to
imply that they were formed at precisely the same instant of time.
A moment's consideration will show us that it is only in the
former sense that we can properly speak of strata being
"contemporaneous;" and that, in point of fact, beds containing
the same fossils, if occurring in widely distant areas, can hardly
be "contemporaneous" in any literal sense; but that the very
identity of their fossils is proof that they were deposited one
after the other. If we find strata containing identical fossils
within the limits of a single geographical region—say in
Europe—then there is a reasonable probability that these beds
are strictly contemporaneous, in the sense that they were deposited
at the same time. There is a reasonable probability of this,
because there is no improbability involved in the idea of an
ocean occupying the whole area of Europe, and peopled throughout
by many of the same species of marine animals. At the present
day, for example, many identical species of animals are found
living on the western coasts of Britain and the eastern coasts
of North America, and beds now in course of deposition off the
shores of Ireland and the seaboard of the state of New York would
necessarily contain many of the same fossils. Such beds would be
both literally and geologically contemporaneous; but the case is
different if the distance between the areas where the strata occur
be greatly increased. We find, for example, beds containing identical
fossils (the Quebec or Skiddaw beds) in Sweden, in the north of
England, in Canada, and in Australia. Now, if all these beds were
contemporaneous, in the literal sense of the term, we should have
to suppose that the ocean at one time extended uninterruptedly
between all these points, and was peopled throughout the vast area
thus indicated by many of the same animals. Nothing, however,
that we see at the present day would justify us in imagining an
ocean of such enormous extent, and at the same time so uniform
in its depth, temperature, and other conditions of marine life,
as to allow the same animals to flourish in it from end to end;
and the example chosen is only one of a long and ever-recurring
series. It is therefore much more reasonable to explain this,
and all similar cases, as owing to the migration of the
fauna, in whole or in part, from one marine area to another.
Thus, we may suppose an ocean to cover what is now the European
area, and to be peopled by certain species of animals. Beds of
sediment—clay, sands, and limestones—will be deposited
over the sea-bottom, and
will entomb the
remains of the animals as fossils. After this has lasted for a
certain length of time, the European area may undergo elevation,
or may become otherwise unsuitable for the perpetuation of its
fauna; the result of which would be that some or all of the
marine animals of the area would migrate to some more suitable
region. Sediments would then be accumulated in the new area to
which they had betaken themselves, and they would then appear,
for the second time, as fossils in a set of beds widely
separated from Europe. The second set of beds would, however,
obviously not be strictly or literally contemporaneous with the
first, but would be separated from them by the period of time
required for the migration of the animals from the one area
into the other. It is only in a wide and comprehensive sense
that such strata can be said to be contemporaneous.
It is impossible to enter further into this subject here; but it
may be taken as certain that beds in widely remote geographical
areas can only come to contain the same fossils by reason of a
migration having taken place of the animals of the one area to
the other. That such migrations can and do take place is quite
certain, and this is a much more reasonable explanation of the
observed facts than the hypothesis that in former periods the
conditions of life were much more uniform than they are at present,
and that, consequently, the same organisms were able to range over
the entire globe at the same time. It need only be added, that
taking the evidence of the present as explaining the phenomena
of the past—the only safe method of reasoning in geological
matters—we have abundant proof that deposits which are
actually contemporaneous, in the strict sense of the term, do
not contain the same fossils, if far removed from one another in
point of distance. Thus, deposits of various kinds are now
in process of formation in our existing seas, as, for example, in
the Arctic Ocean, the Atlantic, and the Pacific, and many of these
deposits are known to us by actual examination and observation
with the sounding-lead and dredge. But it is hardly necessary to
add that the animal remains contained in these deposits—the
fossils of some future period—instead of being identical, are
widely different from one another in their characters.
We have seen, then, that the entire stratified series is capable of
subdivision into a number of definite rock-groups or "formations,"
each possessing a peculiar and characteristic assemblage of fossils,
representing the "life" of the "period" in which the formation
was deposited. We have still to inquire shortly how it came to
pass that two successive formations should
thus be broadly distinguished by their life-forms, and why
they should not rather possess at any rate a majority of identical
fossils. It was originally supposed that this could be explained by
the hypothesis that the close of each formation was accompanied by
a general destruction of all the living beings of the period, and
that the commencement of each new formation was signalised by the
creation of a number of brand-new organisms, destined to figure
as the characteristic fossils of the same. This theory, however,
ignores the fact that each formation—as to which we have
any sufficient evidence—contains a few, at least, of the
life-forms which existed in the preceding period; and it invokes
forces and processes of which we know nothing, and for the supposed
action of which we cannot account. The problem is an undeniably
difficult one, and it will not be possible here to give more than
a mere outline of the modern views upon the subject. Without
entering into the at present inscrutable question as to the manner
in which new life-forms are introduced upon the earth, it may be
stated that almost all modern geologists hold that the living beings
of any given formation are in the main modified forms of others
which have preceded them. It is not believed that any general or
universal destruction of life took place at the termination of
each geological period, or that a general introduction of new
forms took place at the commencement of a new period. It is, on
the contrary, believed that the animals and plants of any given
period are for the most part (or exclusively) the lineal but
modified descendants of the animals and plants of the immediately
preceding period, and that some of them, at any rate, are
continued into the next succeeding period, either unchanged, or
so far altered as to appear as new species. To discuss these
views in detail would lead us altogether too far, but there is
one very obvious consideration which may advantageously receive
some attention. It is obvious, namely, that the great
discordance which is found to subsist between the animal life of
any given formation and that of the next succeeding formation,
and which no one denies, would be a fatal blow to the views just
alluded to, unless admitting of some satisfactory explanation.
Nor is this discordance one purely of life-forms, for there is
often a physical break in the successions of strata as well.
Let us therefore briefly consider how far these interruptions
and breaks in the geological and palæontological record
can be accounted for, and still allow us to believe in some
theory of continuity as opposed to the doctrine of intermittent
and occasional action.
In the first place, it is perfectly clear that if we admit the
conception above mentioned of a continuity of life from the
Laurentian period to the present day, we could never prove
our view to be correct, unless we could produce in evidence
fossil examples of all the kinds of animals and plants
that have lived and died during that period. In order to do
this, we should require, to begin with, to have access to an
absolutely unbroken and perfect succession of all the deposits
which have ever been laid down since the beginning. If, however,
we ask the physical geologist if he is in possession of any such
uninterrupted series, he will at once answer in the negative.
So far from the geological series being a perfect one, it is
interrupted by numerous gaps of unknown length, many of which we
can never expect to fill up. Nor are the proofs of this far to
seek. Apart from the facts that we have hitherto examined only
a limited portion of the dry land, that nearly two-thirds of
the entire area of the globe is inaccessible to geological
investigation in consequence of its being covered by the sea,
that many deposits can be shown to have been more or less
completely destroyed subsequent to their deposition, and that
there may be many areas in which living beings exist where no
rock is in process of formation, we have the broad fact that
rock-deposition only goes on to any extent in water, and that
the earth must have always consisted partly of dry land and
partly of water—at any rate, so far as any period of
which we have geological knowledge is concerned. There
must, therefore, always have existed, at some part or
another of the earth's surface, areas where no deposition of
rock was going on, and the proof of this is to be found in the
well-known phenomenon of "unconformability." Whenever,
namely, deposition of sediment is continuously going on within
the limits of a single ocean, the beds which are laid down succeed
one another in uninterrupted and regular sequence. Such beds are
said to be "conformable," and there are many rock-groups known
where one may pass through fifteen or twenty thousand feet of strata
without a break—indicating that the beds had been deposited
in an area which remained continuously covered by the sea. On
the other hand, we commonly find that there is no such regular
succession when we pass from one great formation to another, but
that, on the contrary, the younger formation rests "unconformably,"
as it is called, either upon the formation immediately preceding
it in point of time, or upon some still older one. The essential
physical feature of this unconformability is that the beds of the
younger formation rest upon a worn and eroded surface formed by the
beds of the older series (fig. 18); and a moment's
consideration will show us what this indicates. It indicates,
Fig. 18.—Section showing strata of Tertiary age (a)
resting upon a worn and eroded surface of White Chalk
(b), the stratification of which is marked by lines of
flint.
beyond the possibility of misconception, that there was an interval
between the deposition of the older series and that of the newer
series of strata; and that during this interval the older beds
were raised above the sea-level, so as to form dry land, and were
subsequently depressed again beneath the waters, to receive upon
their worn and wasted upper surface the sediments of the later
group. During the interval thus indicated, the deposition of rock
must of necessity have been proceeding more or less actively
in other areas. Every unconformity, therefore, indicates that
at the spot where it occurs, a more or less extensive series
of beds must be actually missing; and though we may sometimes
be able to point to these missing strata in other areas, there
yet remains a number of unconformities for which we cannot at
present supply the deficiency even in a partial manner.
It follows from the above that the series of stratified deposits
is to a greater or less extent irremediably imperfect; and in
this imperfection we have one great cause why we can never obtain
a perfect series of all the animals and plants that have lived
upon the globe. Wherever one of these great physical gaps occurs,
we find, as we might expect, a corresponding break in the series
of life-forms. In other words, whenever we find two formations
to be unconformable, we shall always find at the same time that
there is a great difference in their fossils, and that many of
the fossils of the older formation do not survive into the newer,
whilst many of those in the newer are not known to occur in the
older. The cause of this is, obviously,
that the lapse of time, indicated by the unconformability, has
been sufficiently great to allow of the dying out or modification
of many of the older forms of life, and the introduction of new
ones by immigration.
Apart, however, altogether, from these great physical breaks
and their corresponding breaks in life, there are other reasons
why we can never become more than partially acquainted with the
former denizens of the globe. Foremost amongst these is the fact
that an enormous number of animals possess no hard parts of the
nature of a skeleton, and are therefore incapable, under any
ordinary circumstances, of leaving behind them any traces of
their existence. It is true that there are cases in which animals
in themselves completely soft-bodied are nevertheless able to leave
marks by which their former presence can be detected: Thus every
geologist is familiar with the winding and twisting "trails" formed
on the surface of the strata by sea-worms; and the impressions
left by the stranded carcases of Jelly-fishes on the fine-grained
lithographic slates of Solenhofen supply us with an example of how
a creature which is little more than "organised sea-water" may
still make an abiding mark upon the sands of time. As a general
rule, however, animals which have no skeletons are incapable of
being preserved as fossils, and hence there must always have
been a vast number of different kinds of marine animals of which
we have absolutely no record whatever. Again, almost all the
fossiliferous rocks have been laid down in water; and it is a
necessary result of this that the great majority of fossils are
the remains of aquatic animals. The remains of air-breathing
animals, whether of the inhabitants of the land or of the air
itself, are comparatively rare as fossils, and the record of
the past existence of these is much more imperfect than is the
case with animals living in water. Moreover, the fossiliferous
deposits are not only almost exclusively aqueous formations, but
the great majority are marine, and only a comparatively small
number have been formed by lakes and rivers. It follows from the
foregoing that the palæontological record is fullest and most
complete so far as sea-animals are concerned, though even here we
find enormous gaps, owing to the absence of hard structures in
many great groups; of animals inhabiting fresh waters our knowledge
is rendered still further incomplete by the small proportion
that fluviatile and lacustrine deposits bear to marine; whilst
we have only a fragmentary acquaintance with the air-breathing
animals which inhabited the earth during past ages.
Lastly, the imperfection of the palæontological record, due
to the causes above enumerated, is greatly aggravated, especially
as regards the earlier portion of the earth's history, by the fact
that many rocks which contained fossils when deposited have since
been rendered barren of organic remains. The principal cause of
this common phenomenon is what is known as "metamorphism"—that
is, the subjection of the rock to a sufficient amount of heat to
cause a rearrangement of its particles. When at all of a pronounced
character, the result of metamorphic action is invariably the
obliteration of any fossils which might have been originally
present in the rock. Metamorphism may affect rocks of any age,
though naturally more prevalent in the older rocks, and to this
cause must be set down an irreparable loss of much fossil evidence.
The most striking example which is to be found of this is the great
Laurentian series, which comprises some 30,000 feet of
highly-metamorphosed sediments, but which, with one not wholly
undisputed exception, has as yet yielded no remains of living
beings, though there is strong evidence of the former existence
in it of fossils.
Upon the whole, then, we cannot doubt that the earth's crust, so
far as yet deciphered by us, presents us with but a very imperfect
record of the past. Whether the known and admitted imperfections
of the geological and palæontological records are sufficiently
serious to account satisfactorily for the deficiency of direct
evidence recognisable in some modern hypotheses, may be a matter
of individual opinion. There can, however, be little doubt that
they are sufficiently extensive to throw the balance of evidence
decisively in favour of some theory of continuity, as
opposed to any theory of intermittent and occasional action. The
apparent breaks which divide the great series of the stratified
rocks into a number of isolated formations, are not marks of mighty
and general convulsions of nature, but are simply indications of
the imperfection of our knowledge. Never, in all probability,
shall we be able to point to a complete series of deposits, or a
complete succession of life linking one great geological period
to another. Nevertheless, we may well feel sure that such deposits
and such an unbroken succession must have existed at one time.
We are compelled to believe that nowhere in the long series of
the fossiliferous rocks has there been a total break, but that
there must have been a complete continuity of life, and a more
or less complete continuity of sedimentation, from the Laurentian
period to the present day. One generation hands on the lamp of
life to the next, and each system of rocks is the direct offspring
of those which preceded it in time. Though there
has not been continuity in any given area, still the
geological chain could never have been snapped at one point, and
taken up again at a totally different one. Thus we arrive at the
conviction that continuity is the fundamental law of
geology, as it is of the other sciences, and that the lines of
demarcation between the great formations are but gaps in our own
knowledge.
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