THE ANCIENT
LIFE-HISTORY
OF THE EARTH
Chapter 1:
THE SCOPE AND MATERIALS OF PALÆONTOLOGY.
The study of the rock-masses which constitute the crust of the
earth, if carried out in the methodical and scientific manner of
the geologist, at once brings us, as has been before remarked, in
contact with the remains or traces of living beings which formerly
dwelt upon the globe. Such remains are found, in greater or less
abundance, in the great majority of rocks; and they are not only of
great interest in themselves, but they have proved of the greatest
importance as throwing light upon various difficult problems in
geology, in natural history, in botany, and in philosophy. Their
study constitutes the science of Palæontology; and though it
is possible to proceed to a certain length in geology and zoology
without much palæontological knowledge, it is hardly possible
to attain to a satisfactory general acquaintance with either of
these subjects without having mastered the leading facts of the
first. Similarly, it is not possible to study palæontology
without some acquaintance with both geology and natural history.
Palæontology, then, is the science which treats of the living
beings, whether animal or vegetable, which have inhabited the earth
during past periods of its history. Its object is to elucidate,
as far as may be, the structure, mode of existence, and habits
of all such ancient forms of life; to determine their position
in the scale of organised beings; to lay down the geographical
limits within which they flourished; and to fix the period of
their advent and disappearance. It is the ancient life-history
of the earth; and were its record complete, it would furnish
us with a detailed knowledge of the form and relations of all
the animals and plants which have at any period flourished upon
the land-surfaces of the globe or inhabited its waters; it would
enable us to determine precisely their succession in time; and
it would place in our hands an unfailing key to the problems of
evolution. Unfortunately, from causes which will be subsequently
discussed, the palæontological record is extremely imperfect,
and our knowledge is interrupted
by gaps, which not only bear a large proportion to our solid
information, but which in many cases are of such a nature that
we can never hope to fill them up.
FOSSILS.—The remains of animals or vegetables which we now
find entombed in the solid rock, and which constitute the working
material of the palæontologist, are termed "fossils,"[3] or
"petrifactions." In most cases, as can be readily understood,
fossils are the actual hard parts of animals and plants which
were in existence when the rock in which they are now found was
being deposited. Most fossils, therefore, are of the nature of
the shells of shell-fish, the skeletons of coral-zoophytes, the
bones of vertebrate animals, or the wood, bark, or leaves of
plants. All such bodies are more or less of a hard consistence
to begin with, and are capable of resisting decay for a longer
or shorter time—hence the frequency with which they occur
in the fossil condition. Strictly speaking, however, by the term
"fossil" must be understood "any body, or the traces of the
existence of any body, whether animal or vegetable, which
has been buried in the earth by natural causes" (Lyell). We shall
find, in fact, that many of the objects which we have to study
as "fossils" have never themselves actually formed parts of any
animal or vegetable, though they are due to the former existence
of such organisms, and indicate what was the nature of these.
Thus the footprints left by birds, or reptiles, or quadrupeds
upon sand or mud, are just as much proofs of the former existence
of these animals as would be bones, feathers, or scales, though
in themselves they are inorganic. Under the head of fossils,
therefore, come the footprints of air-breathing vertebrate animals;
the tracks, trails, and burrows of sea-worms, crustaceans, or
molluscs; the impressions left on the sand by stranded jelly-fishes;
the burrows in stone or wood of certain shell-fish; the "moulds"
or "casts" of shells, corals, and other organic remains; and
various other bodies of a more or less similar nature.
FOSSILISATION.—The term "fossilisation" is applied to all those
processes through which the remains of organised beings may pass
in being converted into fossils. These processes are numerous
and varied; but there are three principal modes of fossilisation
which alone need be considered here. In the first instance, the
fossil is to all intents and purposes an actual portion of the
original organised being—such as a bone, a shell, or a piece
of wood. In some rare instances, as in the case of the body of
the Mammoth discovered embedded in ice at the mouth of the Lena
in Siberia, the fossil may be preserved
almost precisely in its original condition, and even with its soft
parts uninjured. More commonly, certain changes have taken place
in the fossil, the principal being the more or less total removal
of the organic matter originally present. Thus bones become light
and porous by the removal of their gelatine, so as to cleave to
the tongue on being applied to that organ; whilst shells become
fragile, and lose their primitive colours. In other cases, though
practically the real body it represents, all the cavities of the
fossil, down to its minutest recesses, may have become infiltrated
with mineral matter. It need hardly be added, that it is in the
more modern rocks that we find the fossils, as a rule, least
changed from their former condition; but the original structure
is often more or less completely retained in some of the fossils
from even the most ancient formations.
In the second place, we very frequently meet with fossils in
the state of "casts" or moulds of the original organic body.
What occurs in this case will be readily understood if we imagine
any common bivalve shell, as an Oyster, or Mussel, or Cockle,
embedded in clay or mud. If the clay were sufficiently soft and
fluid, the first thing would be that it would gain access to the
interior of the shell, and would completely fill up the space
between the valves. The pressure, also, of the surrounding matter
would insure that the clay would everywhere adhere closely to
the exterior of the shell. If now we suppose the clay to be in
any way hardened so as to be converted into stone, and if we were
to break up the stone, we should obviously have the following
state of parts. The clay which filled the shell would form an
accurate cast of the interior of the shell, and the clay
outside would give us an exact impression or cast of
the exterior of the shell (fig. 1). We should have, then,
Fig. 1.—Trigonia longa, showing casts of the exterior
and interior of the shell.—Cretaceous (Neocomian).
two casts, an interior and an exterior, and the two would be
very different to one another, since the inside of a shell is
very unlike the outside. In the case, in fact, of many univalve
shells, the interior cast or "mould" is so unlike the exterior
cast, or unlike the shell itself, that it may be difficult to
determine the true origin of the former.
It only remains to add that there is sometimes a further
complication. If the rock be very porous and permeable by
water, it may happen that the original shell is
entirely dissolved away, leaving the interior cast loose,
like the kernel of a nut, within the case formed by the exterior
cast. Or it may happen that subsequent to the attainment of this
state of things, the space thus left vacant between the interior
and exterior cast—the space, that is, formerly occupied by
the shell itself—may be filled up by some foreign mineral
deposited there by the infiltration of water. In this last case
the splitting open of the rock would reveal an interior cast,
an exterior cast, and finally a body which would have the exact
form of the original shell, but which would be really a much
later formation, and which would not exhibit under the microscope
the minute structure of shell.
In the third class of cases we have fossils which present with
the greatest accuracy the external form, and even sometimes the
internal minute structure, of the original organic body, but
which, nevertheless, are not themselves truly organic, but have
been formed by a "replacement" of the particles of the primitive
organism by some mineral substance. The most elegant example of
this is afforded by fossil wood which has been "silicified" or
converted into flint (silex). In such cases we have fossil
wood which presents the rings of growth and fibrous structure of
recent wood, and which under the microscope exhibits the minutest
vessels which characterise ligneous tissue, together with the even
more minute markings of the vessels (fig. 2). The whole, however,
Fig. 2.—Microscopic section of the silicified
wood of a Conifer (Sequoia) cut in the long direction
of the fibres. Post-tertiary? Colorado. (Original.)
Fig. 3.—Microscopic section of the wood of the
common Larch (Abies larix), cut in the long direction
of the fibres. In both the fresh and the fossil wood (fig. 2)
are seen the discs characteristic of coniferous wood. (Original.)
instead of being composed of the original carbonaceous matter of
the wood, is now converted into flint. The only explanation that
can be given
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of this by no means rare phenomenon, is that the wood must have
undergone a slow process of decay in water charged with silica or
flint in solution. As each successive particle of wood was removed
by decay, its place was taken by a particle of flint deposited
from the surrounding water, till ultimately the entire wood was
silicified. The process, therefore, resembles what would take
place if we were to pull down a house built of brick by successive
bricks, replacing each brick as removed by a piece of stone of
precisely the same size and form. The result of this would be that
the house would retain its primitive size, shape, and outline, but
it would finally have been converted from a house of brick into a
house of stone. Many other fossils besides wood—such as shells,
corals, sponges, &c.—are often found silicified; and this
may be regarded as the commonest form of fossilisation by replacement.
In other cases, however, though the principle of the process is the
same, the replacing substance may be iron pyrites, oxide of iron,
sulphur, malachite, magnesite, talc, &c.; but it is rarely that
the replacement with these minerals is so perfect as to preserve the
more delicate details of internal structure.
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