THE ANCIENT
LIFE-HISTORY
OF THE EARTH
Chapter 6:
THE BIOLOGICAL RELATIONS OF FOSSILS.
Not only have fossils, as we have seen, a most important bearing
upon the sciences of Geology and Physical Geography, but they
have relations of the most complicated and weighty character with
the numerous problems connected with the study of living beings,
or in other words, with the science of Biology. To such an extent
is this the case, that no adequate comprehension of Zoology and
Botany, in their modern form, is so much as possible without
some acquaintance with the types of animals and plants which have
passed away. There are also numerous speculative questions in
the domain of vital science, which, if soluble at all, can only
hope to find their key in researches carried out on extinct
organisms. To discuss fully the biological relations of fossils
would, therefore, afford matter for a separate treatise; and all
that can be done here is to indicate very cursorily the principal
points to which the attention of the palæontological student
ought to be directed.
In the first place, the great majority of fossil animals and
plants are "extinct"—that is to say, they belong to species
which are no longer in existence at the present day. So far,
however, from there being any truth in the old view that there
were periodic destructions of all the living beings in existence
upon the earth, followed by a corresponding number of new creations
of animals and plants, the actual facts of the case show that
the extinction of old forms and the introduction of new forms
have been processes constantly going on throughout the whole
of geological time. Every species seems to come into being at
a certain definite point of time, and to finally disappear at
another definite point; though there are few instances indeed,
if there are any, in which our present knowledge would permit
us safely to fix with precision the times of entrance and exit.
There are, moreover, marked differences in the actual time during
which different species remained in existence, and therefore
corresponding differences in their "vertical range," or, in other
words, in the actual amount and thickness of strata through which
they present themselves as fossils. Some species are found to
range through two or even three formations, and a few have an even
more extended life. More commonly the species which begin in the
commencement of a great formation die
out at or before its close, whilst those which are introduced for
the first time near the middle or end of the formation may either
become extinct, or may pass on into the next succeeding formation.
As a general rule, it is the animals which have the lowest and
simplest organisation that have the longest range in time, and
the additional possession of microscopic or minute dimensions
seems also to favour longevity. Thus some of the Foraminifera
appear to have survived, with little or no perceptible alteration,
from the Silurian period to the present day; whereas large and
highly-organised animals, though long-lived as individuals,
rarely seem to live long specifically, and have, therefore,
usually a restricted vertical range. Exceptions to this, however,
are occasionally to be found in some "persistent types," which
extend through a succession of geological periods with very
little modification. Thus the existing Lampshells of the genus
Lingula are little changed from the Lingulœ
which swarmed in the Lower Silurian seas; and the existing Pearly
Nautilus is the last descendant of a clan nearly as ancient. On
the other hand, some forms are singularly restricted in their
limits, and seem to have enjoyed a comparatively brief lease
of life. An example of this is to be found in many of the
Ammonites—close allies of the Nautilus—which
are often confined strictly to certain zones of strata, in some
cases of very insignificant thickness.
Of the causes of extinction amongst fossil animals and
plants, we know little or nothing. All we can say is, that the
attributes which constitute a species do not seem to be
intrinsically endowed with permanence, any more than the attributes
which constitute an individual, though the former may
endure whilst many successive generations of the latter have
disappeared. Each species appears to have its own life-period,
its commencement, its culmination, and its gradual decay; and
the life-periods of different species may be of very different
duration.
From what has been said above, it may be gathered that our existing
species of animals and plants are, for the most part, quite of modern
origin, using the term "modern" in its geological acceptation.
Measured by human standards, the majority of existing animals
(which are capable of being preserved as fossils) are known to
have a high antiquity; and some of them can boast of a pedigree
which even the geologist may regard with respect. Not a few of
our shellfish are known to have commenced their existence at some
point of the Tertiary period; one Lampshell
(Terebratulina caput-serpentis) is believed to have
survived since the Chalk; and some of the Foraminifera date,
at any rate, from the Carboniferous period. We learn from this
the additional fact that our existing animals and plants do not
constitute an assemblage of organic forms which were introduced
into the world collectively and simultaneously, but that they
commenced their existence at very different periods, some being
extremely old, whilst others may be regarded as comparatively
recent animals. And this introduction of the existing fauna and
flora was a slow and gradual process, as shown admirably
by the study of the fossil shells of the Tertiary period. Thus,
in the earlier Tertiary period, we find about 95 per cent of the
known fossil shells to be species that are no longer in existence,
the remaining 5 per cent being forms which are known to live in
our present seas. In the middle of the Tertiary period we find
many more recent and still existing species of shells, and the
extinct types are much fewer in number; and this gradual
introduction of forms now living goes on steadily, till, at the
close of the Tertiary period, the proportions with which we
started may be reversed, as many as 90 or 95 per cent of the
fossil shells being forms still alive, while not more than 5
per cent may have disappeared.
All known animals at the present day may be divided into some
five or six primary divisions, which are known technically as
"sub-kingdoms." Each of these sub-kingdoms [9] may be
regarded as representing a certain type or plan of structure,
and all the animals comprised in each are merely modified forms
of this common type. Not only are all known living animals thus
reducible to some five or six fundamental plans of structure,
but amongst the vast series of fossil forms no one has yet been
found—however unlike any existing animal—to possess
peculiarities which would entitle it to be placed in a new
sub-kingdom. All fossil animals, therefore, are capable of
being referred to one or other of the primary divisions of the
animal kingdom. Many fossil groups have no closely-related group
now in existence; but in no case do we meet with any grand
structural type which has not survived to the present day.
The old types of life differ in many respects from those now
upon the earth; and the further back we pass in time, the more
marked does this divergence become. Thus, if we were to compare
the animals which lived in the Silurian seas with
those inhabiting our present oceans, we should in most
instances find differences so great as almost to place us in
another world. This divergence is the most marked in the
Palæozoic forms of life, less so in those of the Mesozoic
period, and less still in the Tertiary period. Each successive
formation has therefore presented us with animals becoming
gradually more and more like those now in existence; and though
there is an immense and striking difference between the Silurian
animals and those of to-day, this difference is greatly reduced
if we compare the Silurian fauna with the Devonian; that
again with the Carboniferous; and so on till we reach the present.
It follows from the above that the animals of any given formation
are more like those of the next formation below, and of the next
formation above, than they are to any others; and this fact of
itself is an almost inexplicable one, unless we believe that
the animals of any given formation are, in part at any rate, the
lineal descendants of the animals of the preceding formation,
and the progenitors, also in part at least, of the animals of the
succeeding formation. In fact, the palæontologist is so
commonly confronted with the phenomenon of closely-allied forms of
animal life succeeding one another in point of time, that he is
compelled to believe that such forms have been developed from
some common ancestral type by some process of "evolution."
On the other hand, there are many phenomena, such as the apparently
sudden introduction of new forms throughout all past time, and
the common occurrence of wholly isolated types, which cannot
be explained in this way. Whilst it seems certain, therefore,
that many of the phenomena of the succession of animal life in
past periods can only be explained by some law of evolution, it
seems at the same time certain that there has always been some
other deeper and higher law at work, on the nature of which it
would be futile to speculate at present.
Not only do we find that the animals of each successive formation
become gradually more and more like those now existing upon the
globe, as we pass from the older rocks into the newer, but we also
find that there has been a gradual progression and development in
the types of animal life which characterise the geological
ages. If we take the earliest-known and oldest examples of any
given group of animals, it can sometimes be shown that these
primitive forms, though in themselves highly organised, possessed
certain characters such as are now only seen in the young
of their existing representatives. In technical language, the
early forms of life in some
instances possess
"embryonic" characters, though this does not prevent them
often attaining a size much more gigantic than their nearest living
relatives. Moreover, the ancient forms of life are often what is
called "comprehensive types"—that is to say, they possess
characters in combination such as we nowadays only find separately
developed in different, groups of animals. Now, this permanent
retention of embryonic characters and this "comprehensiveness" of
structural type are signs of what a zoologist considers to be a
comparatively low grade of organisation; and the prevalence of
these features in the earlier forms of animals is a very striking
phenomenon, though they are none the less perfectly organised so
far as their own type is concerned. As we pass upwards in the
geological scale, we find that these features gradually disappear,
higher and ever higher forms are introduced, and "specialisation"
of type takes the place of the former comprehensiveness. We shall
have occasion to notice many of the facts on which these views are
based at a later period, and in connection with actual examples.
In the meanwhile, it is sufficient to state, as a widely-accepted
generalisation of palæontology, that there has been in the past
a general progression of organic types, and that the appearance
of the lower forms of life has in the main preceded that of the
higher forms in point of time.
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