THE ANCIENT
LIFE-HISTORY
OF THE EARTH
Chapter 23:
THE SUCCESSION OF LIFE UPON THE GLOBE.
In conclusion, it may not be out of place if we attempt to summarise,
in the briefest possible manner, some of the principal results
which may be deduced as to the succession of life upon the earth
from the facts which have in the preceding portion of this work
been passed in review. That there was a time when the earth was
void of life is universally admitted, though it may be that the
geological record gives us no direct evidence of this. That the
globe of to-day is peopled with innumerable forms of life whose
term of existence has been, for the most part, but as it were
of yesterday, is likewise an assertion beyond dispute. Can we
in any way connect the present with the remote past, and can we
indicate even imperfectly the conditions and laws under which the
existing order was brought about? The long series of fossiliferous
deposits, with their almost countless organic remains, is the
link between what has been and what is; and if any answer to the
above question can be arrived at, it will be by the careful and
conscientious study of the facts of Palæontology. In the present
state of our knowledge, it may be safely said that anything like
a dogmatic or positive opinion as to the precise sequence of
living forms upon the globe, and still more as to the manner in
which this sequence may have been brought about, is incapable of
scientific proof. There are, however, certain general deductions
from the known facts which may be regarded as certainly established.
In the first place, it is certain that there has been a
succession of life upon the earth, different specific and
generic types succeeding one another in successive periods. It
follows from this, that the animals and plants with which we are
familiar as living, were not always upon the earth, but that they
have been preceded by numerous races more or less differing from
them. What is true of the species of animals and plants, is true
also of the higher zoological divisions; and it is, in the second
place, quite certain that there has been a similar succession
in the order of appearance of the primary groups ("sub-kingdoms,"
"classes," &c.) of animals and vegetables. These great groups did
not all come into existence at once, but they made their appearance
successively. It is true that we cannot be said to be certainly
acquainted with the first absolute
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appearance of any great group of animals. No one dare assert
positively that the apparent first appearance of Fishes in the
Upper Silurian is really their first introduction upon the earth:
indeed, there is a strong probability against any such supposition.
To whatever extent, however, future discoveries may push back the
first advent of any or of all of the great groups of life, there is
no likelihood that anything will be found out which will materially
alter the relative succession of these groups as at present
known to us. It is not likely, for example, that the future has
in store for us any discovery by which it would be shown that
Fishes were in existence before Molluscs, or that Mammals made their
appearance before Fishes. The sub-kingdoms of Invertebrate animals
were all represented in Cambrian times—and it might therefore
be inferred that these had all come simultaneously into
existence; but it is clear that this inference, though incapable
of actual disproof, is in the last degree improbable. Anterior
to the Cambrian is the great series of the Laurentian, which,
owing to the metamorphism to which it has been subjected, has so
far yielded but the singular Eozoön. We may be certain,
however, that others of the Invertebrate sub-kingdoms besides
the Protozoa were in existence in the Laurentian period; and we
may infer from known analogies that they appeared successively,
and not simultaneously.
When we come to smaller divisions than the sub-kingdoms—such
as classes, orders, and families—a similar succession of groups
is observable. The different classes of any given sub-kingdom, or
the different orders of any given class, do not make their
appearance together and all at once, but they are introduced upon
the earth in succession. More than this, the different classes
of a sub-kingdom, or the different orders of a class, in the
main succeed one another in the relative order of their zoological
rank—the lower groups appearing first and the higher groups
last. It is true that in the Cambrian formation—the
earliest series of sediments in which fossils are abundant—we
find numerous groups, some very low, others very high, in the
zoological scale, which appear to have simultaneously
flashed into existence. For reasons stated above, however, we
cannot accept this appearance as real; and we must believe that
many of the Cambrian groups of animals really came into being long
before the commencement of the Cambrian period. At any rate, in the
long series of fossiliferous deposits of later date than the Cambrian
the above-stated rule holds good as a broad generalisation—that
the lower groups, namely, precede the higher in point of time;
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and though there are apparent exceptions to the rule, there are none
of such a nature as not to admit of explanation. Some of the leading
facts upon which this generalisarion is founded will be enumerated
immediately; but it will be well, in the first place, to consider
briefly what we precisely mean when we speak of "higher" and "lower"
groups.
It is well known that naturalists are in the habit of "classifying"
the innumerable animals which now exist upon the globe; or, in
other words, of systematically arranging them into groups. The
precise arrangement adopted by one naturalist may differ in minor
details from that adopted by another; but all are agreed as to the
fundamental points of classification, and all, therefore, agree in
placing certain groups in a certain sequence. What, then, is the
principle upon which this sequence is based? Why, for example, are
the Sponges placed below the Corals; these below the Sea-urchins;
and these, again, below the Shell-fish? Without entering into
a discussion of the principles of zoological classification,
which would here be out of place, it must be sufficient to say
that the sequence in question is based upon the relative type
of organisation of the groups of animals classified. The
Corals are placed above the Sponges upon the ground that, regarded
as a whole, the plan or type of structure of a Coral is
more complex than that of a Sponge. It is not in the slightest
degree that the Sponge is in any respect less highly organised
or less perfect, as a Sponge, than is the Coral as a Coral. Each
is equally perfect in its own way; but the structural pattern
of the Coral is the highest, and therefore it occupies a higher
place in the zoological scale. It is upon this principle, then,
that the primary subdivisions of the animal kingdom (the so-called
"sub-kingdoms") are arranged in a certain order. Coming, again, to
the minor subdivisions (classes, orders, &c.) of each sub-kingdom,
we find a different but entirely analogous principle employed as
a means of classification. The numerous animals belonging to
any given sub-kingdom are formed upon the same fundamental plan
of structure; but they nevertheless admit of being arranged in
a regular series of groups. All the Shell-fish, for example,
are built upon a common plan, this plan representing the ideal
Mollusc; but there are at the same time various groups of the
Mollusca, and these groups admit of an arrangement in a
given sequence. The principle adopted in this case is simply of
the relative elaboration of the common type. The Oyster
is built upon the same ground-plan as the Cuttle-fish; but this
plan is carried out with much greater elaboration, and with many
more complexities, in the latter than in the former: and
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in accordance with this, the Cephalopoda constitute a
higher group than the Bivalve Shell-fish. As in the case of
superiority of structural type, so in this case also, it is not
in the least that the Oyster is an imperfect animal. On
the contrary, it is just as perfectly adapted by its
organisation to fill its own sphere and to meet the exigencies
of its own existence as is the Cuttle-fish; but the latter lives
a life which is, physiologically, higher than the former, and its
organisation is correspondingly increased in complexity.
This being understood, it may be repeated that, in the main,
the succession of life upon the globe in point of time
has corresponded with the relative order of succession of the
great groups of animals in zoological rank; and some of
the more striking examples of this may be here alluded to. Amongst
the Echinoderms, for instance, the two orders generally
admitted to be the "lowest" in the zoological scale—namely,
the Crinoids and the Cystoids—are likewise the
oldest, both, appearing in the Cambrian, the former slowly dying
out as we approach the Recent period, and the latter disappearing
wholly before the close of the Palæozoic period. Amongst the
Crustaceans, the ancient groups of the Trilobites, Ostracodes,
Phyllopods, Eurypterids, and Limuloids, some of which exist at the
present day, are all "low" types; whereas the highly-organised
Decapods do not make their appearance till near the close of the
Palæozoic epoch, and they do not become abundant till we reach
Mesozoic times. Amongst the Mollusca, those Bivalves which
possess breathing-tubes (the "siphonate" Bivalves) are generally
admitted to be higher than those which are destitute of these
organs (the "asiphonate" Bivalves); and the latter are especially
characteristic of the Palæozoic period, whilst the former abound
in Mesozoic and Kainozoic formations. Similarly, the Univalves
with breathing-tubes and a corresponding notch in the mouth of the
shell ("siphonostomatous" Univalves) are regarded as higher in the
scale than the round-mouthed vegetable-eating Sea-snails, in which
no respiratory siphons exist ("holostomatous" Univalves); but the
latter abound in the Palæozoic rocks—whereas the former
do not make their appearance till the Jurassic period, and their
higher groups do not seem to have existed till the close of the
Cretaceous. The Cephalopods, again—the highest of all
the groups of Mollusca—are represented in the Palæozoic
rocks exclusively by Tetrabranchiate forms, which constitute the
lowest of the two orders of this class; whereas the more highly
specialised Dibranchiates do not make their appearance till the
commencement of the Mesozoic. The Palæozoic
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Tetrabranchiates, also, are of a much simpler type than the highly
complex Ammonitidœ of the Mesozoic.
Similar facts are observable amongst the Vertebrate animals.
The Fishes are the lowest class of Vertebrates, and they are the
first to appear, their first certain occurrence being in the
Upper Silurian; whilst, even if the Lower Silurian and Upper
Cambrian "Conodonts" were shown to be the teeth of Fishes, there
would still remain the enormously long periods of the Laurentian
and Lower Cambrian, during which there were Invertebrates, but no
Vertebrates. The Amphibians, the next class in zoological
order, appears later than the Fishes, and is not represented
till the Carboniferous; whilst its highest group (that of the
Frogs and Toads) does not make its entrance upon the scene till
Tertiary times are reached. The class of the Reptiles,
again, the next in order, does not appear till the Permian, and
therefore not till after Amphibians of very varied forms had
been in existence for a protracted period. The Birds seem
to be undoubtedly later than the Reptiles; but, owing to the
uncertainty as to the exact point of their first appearance, it
cannot be positively asserted that they preceded Mammals, as they
should have done. Finally, the Mesozoic types of Mammals are
mainly, if not exclusively, referable to the Marsupials,
one of the lowest orders of the class; whilst the higher orders
of the "Placental" Quadrupeds are not with certainty known to
have existed prior to the commencement of the Tertiary period.
Facts of a very similar nature are offered by the succession of
Plants upon the globe. Thus the vegetation of the Palæozoic
period consisted principally of the lowly-organised groups of
the Cryptogamous or Flowerless plants. The Mesozoic formations,
up to the Chalk, are especially characterised by the naked-seeded
Flowering plants—the Conifers and the Cycads; whilst the
higher groups of the Angiospermous Exogens and Monocotyledons
characterise the Upper Cretaceous and Tertiary rocks.
Facts of the above nature—and they could be greatly
multiplied—seem to point clearly to the existence of some
law of progression, though we certainly are not yet in a position
to formulate this law, or to indicate the precise manner in which
it has operated. Two considerations, also, must not be overlooked.
In the first place, there are various groups, some of them highly
organised, which make their appearance at an extremely ancient
date, but which continue throughout geological time almost
unchanged, and certainly unprogressive. Many of these "persistent
types" are known—such as various of the
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Foraminifera, the Lingulœ, the Nautili,
&c.; and they indicate that under given conditions, at present
unknown to us, it is possible for a life-form to subsist for an
almost indefinite period without any important modification of
its structure. In the second place, whilst the facts above
mentioned point to some general law of progression of the great
zoological groups, it cannot be asserted that the primeval types
of any given group are necessarily "lower," zoologically
speaking, than their modern representatives. Nor does this seem to
be at all necessary for the establishment of the law in question.
It cannot be asserted, for example, that the Ganoid and Placoid
Fishes of the Upper Silurian are in themselves less highly organised
than their existing representatives; nor can it even be asserted
that the Ganoid and Placoid orders are low groups of the
class Pisces. On the contrary, they are high groups; but
then it must be remembered that these are probably not really
the first Fishes, and that if we meet with Fishes at some future
time in the Lower Silurian or Cambrian, these may easily prove
to be representatives of the lower orders of the class. This
question cannot be further entered into here, as its discussion
could be carried out to an almost unlimited length; but whilst
there are facts pointing both ways, it appears that at present
we are not justified in asserting that the earlier types of each
group—so far as these are known to us, or really are without
predecessors—are necessarily or invariably more
"degraded" or "embryonic" in their structure than their more
modern representatives.
It remains to consider very briefly how far Palæontology
supports the doctrine of "Evolution," as it is called; and this,
too, is a question of almost infinite dimensions, which can but
be glanced at here. Does Palæontology teach us that the
almost innumerable kinds of animals and plants which we know to
have successively flourished upon the earth in past times were
produced separately and wholly independently of each other, at
successive periods? or does it point to the theory that a large
number of these supposed distinct forms, have been in reality
produced by the slow modification of a comparatively small number
of primitive types? Upon the whole, it must be unhesitatingly
replied that the evidence of Palæontology is in favour of
the view that the succession of life-forms upon the globe has
been to a large extent regulated by some orderly and
constantly-acting law of modification and evolution. Upon no
other theory can we comprehend how the fauna of any given
formation is more closely related to that of the formation next
below in the series, and to that of
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the formation next above, than to that of any other series of
deposits. Upon no other view can we comprehend why the
Post-Tertiary Mammals of South America should consist
principally of Edentates, Llamas, Tapirs, Peccaries, Platyrhine
Monkeys, and other forms now characterising this continent;
whilst those of Australia should be wholly referable to the
order of Marsupials. On no other view can we explain the common
occurrence of "intermediate" or "transitional" forms of life,
filling in the gaps between groups now widely distinct.
On the other hand, there are facts which point clearly to the
existence of some law other than that of evolution, and probably
of a deeper and more far-reaching character. Upon no theory of
evolution can we find a satisfactory explanation for the constant
introduction throughout geological time of new forms of life, which
do not appear to have been preceded by pre-existent allied types;
The Graptolites and Trilobites have no known predecessors, and leave
no known successors. The Insects appear suddenly in the Devonian,
and the Arachnides and Myriapods in the Carboniferous, under
well-differentiated and highly-specialised types. The Dibranchiate
Cephalopods appear with equal apparent suddenness in the older
Mesozoic deposits, and no known type of the Palæozoic period can
be pointed to as a possible ancestor. The Hippuritidœ
of the Cretaceous burst into a varied life to all appearance
almost immediately after their first introduction into existence.
The wonderful Dicotyledonous flora of the Upper Cretaceous period
similarly surprises us without any prophetic annunciation from
the older Jurassic.
Many other instances could be given; but enough has been said
to show that there is a good deal to be said on both sides, and
that the problem is one environed with profound difficulties.
One point only seems now to be universally conceded, and that
is, that the record of life in past time is not interrupted by
gaps other than those due to the necessary imperfections of the
fossiliferous series, to the fact that many animals are incapable
of preservation in a fossil condition, or to other causes of a
like nature. All those who are entitled to speak on this head
are agreed that the introduction of new and the destruction of
old species have been slow and gradual processes, in no sense of
the term "catastrophistic." Most are also willing to admit that
"Evolution" has taken place in the past, to a greater or less
extent, and that a greater or less number of so-called species of
fossil animals are really the modified descendants of pre-existent
forms. How this process of evolution has
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been effected, to what extent it has taken place, under what
conditions and laws it has been carried out, and how far it may
be regarded as merely auxiliary and supplemental to some deeper
law of change and progress, are questions to which, in spite of
the brilliant generalisations of Darwin, no satisfactory answer
can as yet be given. In the successful solution of this
problem—if soluble with the materials available to our
hands—will lie the greatest triumph that Palæontology
can hope to attain; and there is reason to think that, thanks to
the guiding-clue afforded by the genius of the author of the
'Origin of Species,' we are at least on the road to a sure,
though it may be a far-distant, victory.
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