necessity of interpreting palaeontological facts, the laws of distribution
would have received less careful study; while few comparative anatomists
(and those not of the first order) would have been induced by mere love
of detail, as such, to study the minutiae of osteology, were it not that
in such minutiae lie the only keys to the most interesting riddles offered
by the extinct animal world.
These assuredly are great and solid gains. Surely it is matter for no
small congratulation that in half a century (for palaeontology, though it
dawned earlier, came into full day only with Cuvier) a subordinate branch
of biology should have doubled the value and the interest of the whole
group of sciences to which it belongs.
But this is not all. Allied with geology, palaeontology has established
two laws of inestimable importance: the first, that one and the same area
of the earth's surface has been successively occupied by very different
kinds of living beings; the second, that the order of succession
established in one locality holds good, approximately, in all.
The first of these laws is universal and irreversible; the second is an
induction from a vast number of observations, though it may possibly, and
even probably, have to admit of exceptions. As a consequence of the
second law, it follows that a peculiar relation frequently subsists
between series of strata containing organic remains, in different
localities. The series resemble one another not only in virtue of a
general resemblance of the organic remains in the two, but also in virtue
of a resemblance in the order and character of the serial succession in
each. There is a resemblance of arrangement; so that the separate terms
of each series, as well as the whole series, exhibit a correspondence.
Succession implies time; the lower members of an undisturbed series of
sedimentary rocks are certainly older than the upper; and when the notion
of age was once introduced as the equivalent of succession, it was no
wonder that correspondence in succession came to be looked upon as a
correspondence in age, or "contemporaneity." And, indeed, so long as
relative age only is spoken of, correspondence in succession _is_
correspondence in age; it is _relative_ contemporaneity.
But it would have been very much better for geology if so loose and
ambiguous a word as "contemporaneous" had been excluded from her
terminology, and if, in its stead, some term expressing similarity of
serial relation, and excluding the notion of time altogether, had been
employed to denote correspondence in position in two or more series of
strata.
In anatomy, where such correspondence of position has constantly to be
spoken of, it is denoted by the word "homology" and its derivatives; and
for Geology (which after all is only the anatomy and physiology of the
earth) it might be well to invent some single word, such as "homotaxis"
(similarity of order), in order to express an essentially similar idea.
This, however, has not been done, and most probably the inquiry will at
once be made--To what end burden science with a new and strange term in
place of one old, familiar, and part of our common language?
The reply to this question will become obvious as the inquiry into the
results of palaeontology is pushed further.
Those whose business it is to acquaint themselves specially with the
works of palaeontologists, in fact, will be fully aware that very few, if
any, would rest satisfied with such a statement of the conclusions of
their branch of biology as that which has just been given.
Our standard repertories of palaeontology profess to teach us far higher
things--to disclose the entire succession of living forms upon the
surface of the globe; to tell us of a wholly different distribution of
climatic conditions in ancient times; to reveal the character of the
first of all living existences; and to trace out the law of progress from
them to us.
It may not be unprofitable to bestow on these professions a somewhat more
critical examination than they have hitherto received, in order to
ascertain how far they rest on an irrefragable basis; or whether, after
all, it might not be well for palaeontologists to learn a little more
carefully that scientific "ars artium," the art of saying "I don't know."
And to this end let us define somewhat more exactly the extent of these
pretensions of palaeontology.
Every one is aware that Professor Bronn's "Untersuchungen" and Professor
Pictet's "Traité de Paléontologie" are works of standard authority,
familiarly consulted by every working palaeontologist. It is desirable to
speak of these excellent books, and of their distinguished authors, with
the utmost respect, and in a tone as far as possible removed from carping
criticism; indeed, if they are specially cited in this place, it is
merely in justification of the assertion that the following propositions,
which may be found implicitly, or explicitly, in the works in question,
are regarded by the mass of palaeontologists and geologists, not only on
the Continent but in this country, as expressing some of the best-
established results of palaeontology. Thus:--
Animals and plants began their existence together, not long after the
commencement of the deposition of the sedimentary rocks; and then
succeeded one another, in such a manner, that totally distinct faunae and
florae occupied the whole surface of the earth, one after the other, and
during distinct epochs of time.
A geological formation is the sum of all the strata deposited over the
whole surface of the earth during one of these epochs: a geological fauna
or flora is the sum of all the species of animals or plants which
occupied the whole surface of the globe, during one of these epochs.
The population of the earth's surface was at first very similar in all
parts, and only from the middle of the Tertiary epoch onwards, began to
show a distinct distribution in zones.
The constitution of the original population, as well as the numerical
proportions of its members, indicates a warmer and, on the whole,
somewhat tropical climate, which remained tolerably equable throughout
the year. The subsequent distribution of living beings in zones is the
result of a gradual lowering of the general temperature, which first
began to be felt at the poles.
It is not now proposed to inquire whether these doctrines are true or
false; but to direct your attention to a much simpler though very
essential preliminary question--What is their logical basis? what are the
fundamental assumptions upon which they all logically depend? and what is
the evidence on which those fundamental propositions demand our assent?
These assumptions are two: the first, that the commencement of the
geological record is coëval with the commencement of life on the globe;
the second, that geological contemporaneity is the same thing as
chronological synchrony. Without the first of these assumptions there
would of course be no ground for any statement respecting the
commencement of life; without the second, all the other statements cited,
every one of which implies a knowledge of the state of different parts of
the earth at one and the same time, will be no less devoid of
demonstration.
The first assumption obviously rests entirely on negative evidence. This
is, of course, the only evidence that ever can be available to prove the
commencement of any series of phenomena; but, at the same time, it must
be recollected that the value of negative evidence depends entirely on
the amount of positive corroboration it receives. If A.B. wishes to prove
an _alibi_, it is of no use for him to get a thousand witnesses simply to
swear that they did not see him in such and such a place, unless the
witnesses are prepared to prove that they must have seen him had he been
there. But the evidence that animal life commenced with the Lingula-
flags, _e.g._, would seem to be exactly of this unsatisfactory
uncorroborated sort. The Cambrian witnesses simply swear they "haven't
seen anybody their way"; upon which the counsel for the other side
immediately puts in ten or twelve thousand feet of Devonian sandstones to
make oath they never saw a fish or a mollusk, though all the world knows
there were plenty in their time.
But then it is urged that, though the Devonian rocks in one part of the
world exhibit no fossils, in another they do, while the lower Cambrian
rocks nowhere exhibit fossils, and hence no living being could have
existed in their epoch.
To this there are two replies: the first that the observational basis of
the assertion that the lowest rocks are nowhere fossiliferous is an
amazingly small one, seeing how very small an area, in comparison to that
of the whole world, has yet been fully searched; the second, that the
argument is good for nothing unless the unfossiliferous rocks in question
were not only _contemporaneous_ in the geological sense, but
_synchronous_ in the chronological sense. To use the _alibi_ illustration
again. If a man wishes to prove he was in neither of two places, A and B,
on a given day, his witnesses for each place must be prepared to answer
for the whole day. If they can only prove that he was not at A in the
morning, and not at B in the afternoon, the evidence of his absence from
both is nil, because he might have been at B in the morning and at A in
the afternoon.
Thus everything depends upon the validity of the second assumption. And
we must proceed to inquire what is the real meaning of the word
"contemporaneous" as employed by geologists. To this end a concrete
example may be taken.
The Lias of England and the Lias of Germany, the Cretaceous rocks of
Britain and the Cretaceous rocks of Southern India, are termed by
geologists "contemporaneous" formations; but whenever any thoughtful
geologist is asked whether he means to say that they were deposited
synchronously, he says, "No,--only within the same great epoch." And if,
in pursuing the inquiry, he is asked what may be the approximate value in
time of a "great epoch"--whether it means a hundred years, or a thousand,
or a million, or ten million years--his reply is, "I cannot tell."
If the further question be put, whether physical geology is in possession
of any method by which the actual synchrony (or the reverse) of any two
distant deposits can be ascertained, no such method can be heard of; it
being admitted by all the best authorities that neither similarity of
mineral composition, nor of physical character, nor even direct
continuity of stratum, are _absolute_ proofs of the synchronism of even
approximated sedimentary strata: while, for distant deposits, there seems
to be no kind of physical evidence attainable of a nature competent to
decide whether such deposits were formed simultaneously, or whether they
possess any given difference of antiquity. To return to an example
already given: All competent authorities will probably assent to the
proposition that physical geology does not enable us in any way to reply
to this question--Were the British Cretaceous rocks deposited at the same
time as those of India, or are they a million of years younger or a
million of years older?
Is palaeontology able to succeed where physical geology fails? Standard
writers on palaeontology, as has been seen, assume that she can. They take
it for granted, that deposits containing similar organic remains are
synchronous--at any rate in a broad sense; and yet, those who will study
the eleventh and twelfth chapters of Sir Henry De La Beche's remarkable
"Researches in Theoretical Geology," published now nearly thirty years
ago, and will carry out the arguments there most luminously stated, to
their logical consequences, may very easily convince themselves that even
absolute identity of organic contents is no proof of the synchrony of
deposits, while absolute diversity is no proof of difference of date. Sir
Henry De La Beche goes even further, and adduces conclusive evidence to
show that the different parts of one and the same stratum, having a
similar composition throughout, containing the same organic remains, and
having similar beds above and below it, may yet differ to any conceivable
extent in age.
Edward Forbes was in the habit of asserting that the similarity of the
organic contents of distant formations was _prima facie_ evidence, not of
their similarity, but of their difference of age; and holding as he did
the doctrine of single specific centres, the conclusion was as legitimate
as any other; for the two districts must have been occupied by migration
from one of the two, or from an intermediate spot, and the chances
against exact coincidence of migration and of imbedding are infinite.
In point of fact, however, whether the hypothesis of single or of
multiple specific centres be adopted, similarity of organic contents
cannot possibly afford any proof of the synchrony of the deposits which
contain them; on the contrary, it is demonstrably compatible with the
lapse of the most prodigious intervals of time, and with the
interposition of vast changes in the organic and inorganic worlds,
between the epochs in which such deposits were formed.
On what amount of similarity of their faunae is the doctrine of the
contemporaneity of the European and of the North American Silurians
based? In the last edition of Sir Charles Lyell's "Elementary Geology" it
is stated, on the authority of a former President of this Society, the
late Daniel Sharpe, that between 30 and 40 per cent. of the species of
Silurian Mollusca are common to both sides of the Atlantic. By way of due
allowance for further discovery, let us double the lesser number and
suppose that 60 per cent. of the species are common to the North American
and the British Silurians. Sixty per cent. of species in common is, then,
proof of contemporaneity.
Now suppose that, a million or two of years hence, when Britain has made
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 hypothesis of universal faunae and florae, 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 palaeontology, 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 many other vertical
linear sections 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 palaeontology 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 Palaeozoic 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
provable"--must be recorded against all the grand hypotheses of the
palaeontologist 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 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 palaeontologist.
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 palaeontologist 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 palaeontology 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.[1] But,
pending the construction of a surer foundation than palaeontology 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 palaeontological facts are justifiable.
[Footnote 1: "Le plus grand service qu'on puisse rendre à la science est
d'y faire place nette avant d'y rien construire."--CUVIER.]
The evidence on which such conclusions are based is of two kinds,
negative and positive. The value of negative evidence, in connection with
this inquiry, has been so fully and clearly discussed in an address from
the chair of this Society,[2] 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 palaeontology, and to inquire what they tell us.
[Footnote 2: Anniversary Address for 1851, _Quart. Journ. Geol. Soc._
vol. vii.]
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.[3]
[Footnote 3: See Hooker's _Introductory Essay to the Flora of Tasmania_,
p. xxiii.]
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 Coelenterata--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.