THE ANCIENT
LIFE-HISTORY
OF THE EARTH
Chapter 5:
CONCLUSIONS TO BE DRAWN FROM FOSSILS.
We have already seen that geologists have been led by the study
of fossils to the all-important generalisation that the vast
series of the Fossiliferous or Sedimentary Rocks may be divided
into a number of definite groups or "formations," each of which is
characterised by its organic remains. It may simply be repeated here
that these formations are not properly and strictly characterised
by the occurrence in them of any one particular fossil. It may be
that a formation contains some particular fossil or fossils not
occurring out of that formation, and that in this way an observer
may identify a given group with tolerable certainty. It very
often happens, indeed, that some particular stratum, or sub-group
of a series, contains peculiar fossils, by which its existence
may be determined in various localities. As before remarked,
however, the great formations are characterised properly by the
association of certain fossils, by the predominance of certain
families or orders, or by an assemblage of fossil remains
representing the "life" of the period in which the formation
was deposited.
Fossils, then, enable us to determine the age of the deposits
in which they occur. Fossils further enable us to come to very
important conclusions as to the mode in which the fossiliferous
bed was deposited, and thus as to the condition of the particular
district or region occupied by the fossiliferous bed at the time
of the formation of the latter. If, in the first place, the bed
contain the remains of animals such as now inhabit rivers, we
know that it is "fluviatile" in its origin, and that it must at
one time have either formed an actual riverbed, or been deposited
by the overflowing of an ancient stream. Secondly, if the bed
contain the remains of shellfish, minute crustaceans, or fish,
such as now inhabit lakes,
we know that it is "lacustrine," and was deposited beneath the
waters of a former lake. Thirdly, if the bed contain the remains
of animals such as now people the ocean, we know that it is
"marine" in its origin, and that it is a fragment of an old
sea-bottom.
We can, however, often determine the conditions under which a bed
was deposited with greater accuracy than this. If, for example, the
fossils are of kinds resembling the marine animals now inhabiting
shallow waters, if they are accompanied by the detached relics
of terrestrial organisms, or if they are partially rolled and
broken, we may conclude that the fossiliferous deposit was laid
down in a shallow sea, in the immediate vicinity of a coast-line,
or as an actual shore-deposit. If, again, the remains are those
of animals such as now live in the deeper parts of the ocean,
and there is a very sparing intermixture of extraneous fossils
(such as the bones of birds or quadrupeds, or the remains of
plants), we may presume that the deposit is one of deep water.
In other cases, we may find, scattered through the rock, and
still in their natural position, the valves of shells such as
we know at the present day as living buried in the sand or mud
of the sea-shore or of estuaries. In other cases, the bed may
obviously have been an ancient coral-reef, or an accumulation of
social shells, like Oysters. Lastly, if we find the deposit to
contain the remains of marine shells, but that these are dwarfed
of their fair proportions and distorted in figure, we may conclude
that it was laid down in a brackish sea, such as the Baltic, in
which the proper saltness was wanting, owing to its receiving
an excessive supply of fresh water.
In the preceding, we have been dealing simply with the remains
of aquatic animals, and we have seen that certain conclusions
can be accurately reached by an examination of these. As regards
the determination of the conditions of deposition from the remains
of aerial and terrestrial animals, or from plants, there is not
such an absolute certainty. The remains of land-animals would,
of course, occur in "sub-aerial" deposits—that is, in beds,
like blown sand, accumulated upon the land. Most of the remains
of land-animals, however, are found in deposits which have been
laid down in water, and they owe their present position to the
fact that their former owners were drowned in rivers or lakes,
or carried out to sea by streams. Birds, Flying Reptiles, and
Flying Mammals might also similarly find their way into aqueous
deposits; but it is to be remembered that many birds and mammals
habitually spend a great part of their time in the water, and
that these might therefore be naturally expected to present
themselves as fossils in
Sedimentary Rocks. Plants, again, even when undoubtedly such as
must have grown on land, do not prove that the bed in which they
occur was formed on land. Many of the remains of plants known to
us are extraneous to the bed in which they are now found, having
reached their present site by falling into lakes or rivers, or
being carried out to sea by floods or gales of wind. There are,
however, many cases in which plants have undoubtedly grown on
the very spot where we now find them. Thus it is now generally
admitted that the great coal-fields of the Carboniferous age
are the result of the growth in situ
of the plants which compose coal, and that these grew on vast
Fig. 19.—Erect Tree containing Reptilian remains.
Coal-measures, Nova Scotia. (After Dawson.)
marshy or partially submerged tracts of level alluvial land. We
have, however, distinct evidence of old land-surfaces, both in
the Coal-measures and in other cases (as, for instance, in the
well-known "dirt-bed" of the Purbeck series). When, for example,
we find the erect stumps of trees standing at right angles to
the surrounding strata, we know that the surface through which
these send their roots was at one time the surface of the dry
land, or, in other words, was an ancient soil (fig. 19).
In many cases fossils enable us to come to important conclusions
as to the climate of the period in which they lived but only a
few instances of this can be here adduced. As fossils in the
majority of instances are the remains of marine animals, it is
mostly the temperature of the sea which can alone be determined
in this way; and it is important to remember that, owing to the
existence of heated currents, the marine climate of a given area
does not necessarily imply a correspondingly warm climate in
the neighbouring land. Land-climates can only be determined by
the remains of land-animals or land-plants, and these are
comparatively rare as fossils. It is also important to remember
that all conclusions on this
head are really based upon the present distribution of animal
and vegetable life on the globe, and are therefore liable to be
vitiated by the following considerations:—
a. Most fossils are extinct, and it is not certain that
the habits and requirements of any extinct animal were exactly
similar to those of its nearest living relative.
b. When we get very far back in time, we meet with groups
of organisms so unlike anything we know at the present day as to
render all conjectures as to climate founded upon their supposed
habits more or less uncertain and unsafe.
c. In the case of marine animals, we are as yet very far
from knowing the exact limits of distribution of many species within
our present seas; so that conclusions drawn from living forms as
to extinct species are apt to prove incorrect. For instance, it
has recently been shown that many shells formerly believed to
be confined to the Arctic Seas have, by reason of the extension
of Polar currents, a wide range to the south; and this has thrown
doubt upon the conclusions drawn from fossil shells as to the
Arctic conditions under which certain beds were supposed to have
been deposited.
d. The distribution of animals at the present day is certainly
dependent upon other conditions beside climate alone; and the causes
which now limit the range of given animals are certainly such as
belong to the existing order of things. But the establishment of
the present order of things does not date back in many cases to
the introduction of the present species of animals. Even in the
case, therefore, of existing species of animals, it can often
be shown that the past distribution of the species was different
formerly to what it is now, not necessarily because the climate
has changed, but because of the alteration of other conditions
essential to the life of the species or conducing to its extension.
Still, we are in many cases able to draw completely reliable
conclusions as to the climate of a given geological period, by
an examination of the fossils belonging to that period. Among
the more striking examples of how the past climate of a region
may be deduced from the study of the organic remains contained in
its rocks, the following may be mentioned: It has been shown that
in Eocene times, or at the commencement of the Tertiary period,
the climate of what is now Western Europe was of a tropical or
sub-tropical character. Thus the Eocene beds are found to contain
the remains of shells such as now inhabit tropical seas, as, for
example, Cowries and Volutes; and with these are the fruits of
palms, and the remains of other tropical plants. It has been shown,
again, that in Miocene times, or
about the middle of the Tertiary period, Central Europe was
peopled with a luxuriant flora resembling that of the warmer
parts of the United States, and leading to the conclusion that
the mean annual temperature must have been at least 30°
hotter than it is at present. It has been shown that, at the
same time, Greenland, now buried beneath a vast ice-shroud, was
warm enough to support a large number of trees, shrubs, and other
plants, such as inhabit temperate regions of the globe. Lastly,
it has been shown upon physical as well as palæontological
evidence, that the greater part of the North Temperate Zone, at
a comparatively recent geological period, has been visited with
all the rigours of an Arctic climate, resembling that of
Greenland at the present day. This is indicated by the occurrence
of Arctic shells in the superficial deposits of this period,
whilst the Musk-ox and the Reindeer roamed far south of their
present limits.
Lastly, it was from the study of fossils that geologists learnt
originally to comprehend a fact which may be regarded as of cardinal
importance in all modern geological theories and
speculations—namely, that the crust of the earth is liable
to local elevations and subsidences. For long after the remains of
shells and other marine animals were for the first time observed
in the solid rocks forming the dry land, and at great heights
above the sea-level, attempts were made to explain this almost
unintelligible phenomenon upon the hypothesis that the fossils in
question were not really the objects they represented, but were
in truth mere lusus naturœ, due to some "plastic virtue
latent in the earth." The common-sense of scientific men, however,
soon rejected this idea, and it was agreed by universal consent
that these bodies really were remains of animals which formerly
lived in the sea. When once this was admitted, the further steps
were comparatively easy, and at the present day no geological
doctrine stands on a firmer basis than that which teaches us
that our present continents and islands, fixed and immovable as
they appear, have been repeatedly sunk beneath the ocean.
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