another dip beneath the sea and has come up again, some geologist
applies this doctrine, in comparing the strata laid bare by the upheaval
of the bottom, say, of St. George's Channel with what may then remain of
the Suffolk Crag. Reasoning in the same way, he will at once decide the
Suffolk Crag and the St. George's Channel beds to be contemporaneous;
although we happen to know that a vast period (even in the geological
sense) of time, and physical changes of almost unprecedented extent,
separate the two.
But if it be a demonstrable fact that strata containing more than 60 or
70 per cent. of species of Mollusca in common, and comparatively close
together, may yet be separated by an amount of geological time
sufficient to allow of some of the greatest physical changes the world
has seen, what becomes of that sort of contemporaneity the sole evidence
of which is a similarity of facies, or the identity of half a dozen
species, or of a good many genera?
And yet there is no better evidence for the contemporaneity assumed by
all who adopt the hypotheses of universal faunæ and floræ, of a
universally uniform climate, and of a sensible cooling of the globe
during geological time.
There seems, then, no escape from the admission that neither physical
geology, nor palæontology, possesses any method by which the absolute
synchronism of two strata can be demonstrated. All that geology can
prove is local order of succession. It is mathematically certain that,
in any given vertical linear section of an undisturbed series of
sedimentary deposits, the bed which lies lowest is the oldest. In any
other vertical linear section of the same series, of course,
corresponding beds will occur in a similar order; but, however great may
be the probability, no man can say with absolute certainty that the beds
in the two sections were synchronously deposited. For areas of moderate
extent, it is doubtless true that no practical evil is likely to result
from assuming the corresponding beds to be synchronous or strictly
contemporaneous; and there are multitudes of accessory circumstances
which may fully justify the assumption of such synchrony. But the moment
the geologist has to deal with large areas, or with completely separated
deposits, the mischief of confounding that "homotaxis" or "similarity of
arrangement," which _can_ be demonstrated, with "synchrony" or "identity
of date," for which there is not a shadow of proof, under the one common
term of "contemporaneity" becomes incalculable, and proves the constant
source of gratuitous speculations.
For anything that geology or palæontology are able to show to the
contrary, a Devonian fauna and flora in the British Islands may have
been contemporaneous with Silurian life in North America, and with a
Carboniferous fauna and flora in Africa. Geographical provinces and
zones may have been as distinctly marked in the Palæozoic epoch as at
present, and those seemingly sudden appearances of new genera and
species, which we ascribe to new creation, may be simple results of
migration.
It may be so; it may be otherwise. In the present condition of our
knowledge and of our methods, one verdict--"not proven, and not
proveable"--must be recorded against all the grand hypotheses of the
palæontologist respecting the general succession of life on the globe.
The order and nature of terrestrial life, as a whole, are open
questions. Geology at present provides us with most valuable
topographical records, but she has not the means of working them up into
a universal history. Is such a universal history, then, to be regarded
as unattainable? Are all the grandest and most interesting problems
which offer themselves to the geological student essentially insoluble?
Is he in the position of a scientific Tantalus--doomed always to thirst
for a knowledge which he cannot obtain? The reverse is to be hoped; nay,
it may not be impossible to indicate the source whence help will come.
In commencing these remarks, mention was made of the great obligations
under which the naturalist lies to the geologist and palæontologist.
Assuredly the time will come when these obligations will be repaid
tenfold, and when the maze of the world's past history, through which
the pure geologist and the pure palæontologist find no guidance, will be
securely threaded by the clue furnished by the naturalist.
All who are competent to express an opinion on the subject are, at
present, agreed that the manifold varieties of animal and vegetable form
have not either come into existence by chance, nor result from
capricious exertions of creative power; but that they have taken place
in a definite order, the statement of which order is what men of science
term a natural law. Whether such a law is to be regarded as an
expression of the mode of operation of natural forces, or whether it is
simply a statement of the manner in which a supernatural power has
thought fit to act, is a secondary question, so long as the existence of
the law and the possibility of its discovery by the human intellect are
granted. But he must be a half-hearted philosopher who, believing in
that possibility, and having watched the gigantic strides of the
biological sciences during the last twenty years, doubts that science
will sooner or later make this further step, so as to become possessed
of the law of evolution of organic forms--of the unvarying order of that
great chain of causes and effects of which all organic forms, ancient
and modern, are the links. And then, if ever, we shall be able to begin
to discuss, with profit, the questions respecting the commencement of
life, and the nature of the successive populations of the globe, which
so many seem to think are already answered.
The preceding arguments make no particular claim to novelty; indeed they
have been floating more or less distinctly before the minds of
geologists for the last thirty years; and if, at the present time, it
has seemed desirable to give them more definite and systematic
expression, it is because palæontology is every day assuming a greater
importance, and now requires to rest on a basis the firmness of which is
thoroughly well assured. Among its fundamental conceptions, there must
be no confusion between what is certain and what is more or less
probable.[33] But, pending the construction of a surer foundation than
palæontology now possesses, it may be instructive, assuming for the
nonce the general correctness of the ordinary hypothesis of geological
contemporaneity, to consider whether the deductions which are ordinarily
drawn from the whole body of palæontological facts are justifiable.
The evidence on which such conclusions are based is of two kinds,
negative and positive. The value of negative evidence, in connexion with
this inquiry, has been so fully and clearly discussed in an address from
the chair of this Society,[34] which none of us have forgotten, that
nothing need at present be said about it; the more, as the
considerations which have been laid before you have certainly not tended
to increase your estimation of such evidence. It will be preferable to
turn to the positive facts of palæontology, and to inquire what they
tell us.
We are all accustomed to speak of the number and the extent of the
changes in the living population of the globe during geological time as
something enormous; and indeed they are so, if we regard only the
negative differences which separate the older rocks from the more
modern, and if we look upon specific and generic changes as great
changes, which from one point of view they truly are. But leaving the
negative differences out of consideration, and looking only at the
positive data furnished by the fossil world from a broader point of
view--from that of the comparative anatomist who has made the study of
the greater modifications of animal form his chief business--a surprise
of another kind dawns upon the mind; and under _this_ aspect the
smallness of the total change becomes as astonishing as was its
greatness under the other.
There are two hundred known orders of plants; of these not one is
certainly known to exist exclusively in the fossil state. The whole
lapse of geological time has as yet yielded not a single new ordinal
type of vegetable structure.[35]
The positive change in passing from the recent to the ancient animal
world is greater, but still singularly small. No fossil animal is so
distinct from those now living as to require to be arranged even in a
separate class from those which, contain existing forms. It is only
when we come to the orders, which may be roughly estimated at about a
hundred and thirty, that we meet with fossil animals so distinct from
those now living as to require orders for themselves; and these do not
amount, on the most liberal estimate, to more than about 10 per cent, of
the whole.
There is no certainly known extinct order of Protozoa; there is but one
among the Clenterata--that of the rugose corals; there is none among
the Mollusca; there are three, the Cystidea, Blastoidea, and
Edrioasterida, among the Echinoderms; and two, the Trilobita and
Eurypterida, among the Crustacea; making altogether five for the great
sub-kingdom of Annulosa. Among Vertebrates there is no ordinally
distinct fossil fish: there is only one extinct order of Amphibia--the
Labyrinthodonts; but there are at least four distinct orders of
Reptilia, viz. the Ichthyosauria, Plesiosauria, Pterosauria, Dinosauria,
and perhaps another or two. There is no known extinct order of Birds,
and no certainly known extinct order of Mammals, the ordinal
distinctness of the "Toxodontia" being doubtful.
The objection that broad statements of this kind, after all, rest
largely on negative evidence is obvious, but it has less force than may
at first be supposed; for, as might be expected from the circumstances
of the case, we possess more abundant positive evidence regarding Fishes
and marine Mollusks than respecting any other forms of animal life; and
yet these offer us, through the whole range of geological time, no
species ordinarily distinct from those now living; while the far less
numerous class of Echinoderms presents three, and the Crustacea two,
such orders, though none of these come down later than the Palæozoic
age. Lastly, the Reptilia present the extraordinary and exceptional
phænomenon of as many extinct as existing orders, if not more; the four
mentioned maintaining their existence from the Lias to the Chalk
inclusive.
Some years ago one of your Secretaries pointed out another kind of
positive palæontological evidence tending towards the same
conclusion--afforded by the existence of what he termed "persistent
types" of vegetable and of animal life.[36] He stated, on the authority
of Dr. Hooker, that there are Carboniferous plants which appear to be
generically identical with some now living; that the cone of the Oolitic
_Araucaria_ is hardly distinguishable from that of an existing species;
that a true _Pinus_ appears in the Purbecks and a _Juglans_ in the
Chalk; while, from the Bagshot Sands, a _Banksia_, the wood of which is
not distinguishable from that of species now living in Australia, had
been obtained.
Turning to the animal kingdom, he affirmed the tabulate corals of the
Silurian rocks to be wonderfully like those which now exist; while even
the families of the Aporosa were all represented in the older Mesozoic
rocks.
Among the Mollusca similar facts were adduced. Let it be borne in mind
that _Avicula_, _Mytilus_, _Chiton_, _Natica_, _Patella_, _Trochus_,
_Discina_, _Orbicula_, _Lingula_, _Rhynchonella_, and _Nautilus_, all of
which are existing _genera_, are given without a doubt as Silurian in
the last edition of "Siluria;" while the highest forms of the highest
Cephalopods are represented in the Lias by a genus, _Belemnoteuthis_,
which presents the closest relation to the existing _Loligo_.
The two highest groups of the Annulosa, the Insecta and the Arachnida,
are represented in the Coal, either by existing genera, or by forms
differing from existing genera in quite minor peculiarities.
Turning to the Vertebrata, the only palæozoic Elasmobranch Fish of which
we have any complete knowledge is the Devonian and Carboniferous
_Pleuracanthus_, which differs no more from existing Sharks than these
do from one another.
Again, vast as is the number of undoubtedly Ganoid fossil Fishes, and
great as is their range in time, a large mass of evidence has recently
been adduced to show that almost all those respecting which we possess
sufficient information, are referable to the same sub-ordinal groups as
the existing _Lepidosteus_, _Polypterus_, and Sturgeon; and that a
singular relation obtains between the older and the younger Fishes; the
former, the Devonian Ganoids, being almost all members of the same
sub-order as _Polypterus_, while the Mesozoic Ganoids are almost all
similarly allied to _Lepidosteus_.[37]
Again, what can be more remarkable than the singular constancy of
structure preserved throughout a vast period of time by the family of
the Pycnodonts and by that of the true Coelacanths: the former
persisting, with but insignificant modifications, from the Carboniferous
to the Tertiary rocks, inclusive; the latter existing, with still less
change, from the Carboniferous rocks to the Chalk, inclusive?
Among Reptiles, the highest living group, that of the Crocodilia, is
represented, at the early part of the Mesozoic epoch, by species
identical in the essential characters of their organization with those
now living, and differing from the latter only in such matters as the
form of the articular facets of the vertebral centra, in the extent to
which the nasal passages are separated from the cavity of the mouth by
bone, and in the proportions of the limbs.
And even as regards the Mammalia, the scanty remains of Triassic and
Oolitic species afford no foundation for the supposition that the
organization of the oldest forms differed nearly so much from some of
those which now live as these differ from one another.
It is needless to multiply these instances; enough has been said to
justify the statement that, in view of the immense diversity of known
animal and vegetable forms, and the enormous lapse of time indicated by
the accumulation of fossiliferous strata, the only circumstance to be
wondered at is, not that the changes of life, as exhibited by positive
evidence, have been so great, but that they have been so small.
Be they great or small, however, it is desirable to attempt to estimate
them. Let us, therefore, take each great division of the animal world in
succession, and, whenever an order or a family can be shown to have had
a prolonged existence, let us endeavour to ascertain how far the later
members of the group differ from the earlier ones. If these later
members, in all or in many cases, exhibit a certain amount of
modification, the fact is so far, evidence in favour of a general law of
change; and, in a rough way, the rapidity of that change will be
measured by the demonstrable amount of modification. On the other hand,
it must be recollected that the absence of any modification, while it
may leave the doctrine of the existence of a law of change without
positive support, cannot possibly disprove all forms of that doctrine,
though it may afford a sufficient refutation of many of them.
The PROTOZOA.--The Protozoa are represented throughout the
whole range of geological series, from the Lower Silurian formation to
the present day. The most ancient forms recently made known by Ehrenberg
are exceedingly like those which now exist: no one has ever pretended
that the difference between any ancient and any modern Foraminifera is
of more than generic value; nor are the oldest Foraminifera either
simpler, more embryonic, or less differentiated, than the existing
forms.
The CLENTERATA.--The Tabulate Corals have existed from the
Silurian epoch to the present day, but I am not aware that the ancient
_Heliolites_ possesses a single mark of a more embryonic or less
differentiated character, or less high organization, than the existing
_Heliopora_. As for the Aporose Corals, in what respect is the Silurian
_Paloeocydus_ less highly organized or more embryonic than the modern
_Fungia_, or the Liassic Aporosa than the existing members of the same
families?
The _Mollusca_.--In what sense is the living _Waldheimia_ less
embryonic, or more specialized, than the palæozoic _Spirifer_; or the
existing _Rhynchonellæ_, _Craniæ_, _Discinæ_, _Lingulæ_, than the
Silurian species of the same genera? In what sense can _Loligo_ or
_Spirula_ be said to be more specialized, or less embryonic, than
_Belemnites_; or the modern species of Lamellibranch and Gasteropod
genera, than the Silurian species of the same genera?
The ANNULOSA.--The Carboniferous Insecta and Arachnida are
neither less specialized, nor more embryonic, than those that now live,
nor are the Liassic Cirripedia and Macrura; while several of the
Brachyura, which appear in the Chalk, belong to existing genera; and
none exhibit either an intermediate, or an embryonic, character.
The VERTEBRATA.--Among fishes I have referred to the
Coelacanthini (comprising the genera _Coelacanthus_, _Holophagus_,
_Undina_, and _Macropoma_) as affording an example of a persistent type;
and it is most remarkable to note the smallness of the differences
between any of these fishes (affecting at most the proportions of the
body and fins, and the character and sculpture of the scales),
notwithstanding their enormous range in time. In all the essentials of
its very peculiar structure, the _Macropoma_ of the Chalk is identical
with the _Coelacanthus_ of the Coal. Look at the genus _Lepidotus_,
again, persisting without a modification of importance from the Liassic
to the Eocene formations, inclusive.
Or among the Teleostei--in what respect is the _Beryx_ of the Chalk more
embryonic, or less differentiated, than _Beryx lineatus_ of King
George's Sound?
Or to turn to the higher Vertebrata--in what sense are the Liassic
Chelonia inferior to those which now exist? How are the Cretaceous
Ichthyosauria, Plesiosauria, or Pterosauria less embryonic, or more
differentiated, species than those of the Lias?
Or lastly, in what circumstance is the _Phascolotherium_ more embryonic,
or of a more generalized type, than the modern Opossum; or a
_Lophiodon_, or a _Palæotherium_, than a modern _Tapirus_ or _Hyrax_?
These examples might be almost indefinitely multiplied, but surely they
are sufficient to prove that the only safe and unquestionable testimony
we can procure--positive evidence--fails to demonstrate any sort of
progressive modification towards a less embryonic, or less generalized,
type in a great many groups of animals of long-continued geological
existence. In these groups there is abundant evidence of variation--none
of what is ordinarily understood as progression; and, if the known
geological record is to be regarded as even any considerable fragment of
the whole, it is inconceivable that any theory of a necessarily
progressive development can stand, for the numerous orders and families
cited afford no trace of such a process.
But it is a most remarkable fact, that, while the groups which have been
mentioned, and many besides, exhibit no sign of progressive
modification, there are others, coexisting with them, under the same
conditions, in which more or less distinct indications of such a process
seem to be traceable. Among such indications I may remind you of the
predominance of Holostome Gasteropoda in the older rocks as compared
with that of Siphonostome Gasteropoda in the later. A case less open to
the objection of negative evidence, however, is that afforded by the
Tetrabranchiate Cephalopoda, the forms of the shells and of the septal
sutures exhibiting a certain increase of complexity in the newer genera.
Here, however, one is met at once with the occurrence of _Orthoceras_
and _Baculites_ at the two ends of the series, and of the fact that one
of the simplest genera, _Nautilus_, is that which now exists.
The Crinoidea, in the abundance of stalked forms in the ancient
formations as compared with their present rarity, seem to present us
with a fair case of modification from a more embryonic towards a less
embryonic condition. But then, on careful consideration of the facts,
the objection arises that the stalk, calyx, and arms of the palæozoic
Crinoid are exceedingly different from the corresponding organs of a
larval _Comatula_; and it might with perfect justice be argued that
_Actinocrinus_ and _Eucalyptocrinus_, for example, depart to the full as
widely, in one direction, from the stalked embryo of _Comatula_, as
_Comatula_ itself does in the other.
The Echinidea, again, are frequently quoted as exhibiting a gradual
passage from a more generalized to a more specialized type, seeing that
the elongated, or oval, Spatangoids appear after the spheroidal
Echinoids. But here it might be argued, on the other hand, that the
spheroidal Echinoids, in reality, depart further from the general plan
and from the embryonic form than the elongated Spatangoids do; and that
the peculiar dental apparatus and the pedicellariæ of the former are
marks of at least as great differentiation as the petaloid ambulacra and
semitæ of the latter.
Once more, the prevalence of Macrurous before Brachyurous Podophthalmia
is, apparently, a fair piece of evidence in favour of progressive
modification in the same order of Crustacea; and yet the case will not
stand much sifting, seeing that the Macrurous Podophthalmia depart as
far in one direction from the common type of Podophthalmia, or from any
embryonic condition of the Brachyura, as the Brachyura do in the other;
and that the middle terms between Macrura and Brachyura--the
Anomura--are little better represented in the older Mesozoic rocks than
the Brachyura are.
None of the cases of progressive modification which are cited from among
the Invertebrata appear to me to have a foundation less open to
criticism than these; and if this be so, no careful reasoner would, I
think, be inclined to lay very great stress upon them. Among the
Vertebrata, however, there are a few examples which appear to be far
less open to objection.
It is, in fact, true of several groups of Vertebrata which have lived
through a considerable range of time, that the endoskeleton (more
particularly the spinal column) of the older genera presents a less
ossified, and, so far, less differentiated, condition than that of the
younger genera. Thus the Devonian Ganoids, though almost all members of
the same sub-order as _Polypterus,_ and presenting numerous important
resemblances to the existing genus, which possesses biconcave vertebræ,
are, for the most part, wholly devoid of ossified vertebral centra. The
Mesozoic Lepidosteidæ, again, have, at most, biconcave vertebræ, while
the existing _Lepidosteus_ has Salamandroid, opisthocoelous, vertebræ.
So, none of the Palæozoic Sharks have shown themselves to be possessed
of ossified vertebræ, while the majority of modern Sharks possess such
vertebræ. Again, the more ancient Crocodilia and Lacertilia have
vertebræ with the articular facets of their centra flattened or
biconcave, while the modern members of the same group have them
procoelous. But the most remarkable examples of progressive
modification of the vertebral column, in correspondence with geological
age, are those afforded by the Pycnodonts among fish, and the
Labyrinthodonts among Amphibia.