Geological Contemporaneity and Persistent Types of Life
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Thomas H. Huxley >> Geological Contemporaneity and Persistent Types of Life
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GEOLOGICAL CONTEMPORANEITY AND PERSISTENT TYPES OF LIFE.*
by Thomas H. Huxley
[footnote] *The Anniversary Address to the Geological
Society for 1862.
MERCHANTS occasionally go through a wholesome, though troublesome and
not always satisfactory, process which they term "taking stock." After
all the excitement of speculation, the pleasure of gain, and the pain
of loss, the trader makes up his mind to face facts and to learn the
exact quantity and quality of his solid and reliable possessions.
The man of science does well sometimes to imitate this procedure; and,
forgetting for the time the importance of his own small winnings, to
re-examine the common stock in trade, so that he may make sure how far
the stock of bullion in the cellar--on the faith of whose existence so
much paper has been circulating--is really the solid gold of truth.
The Anniversary Meeting of the Geological Society seems to be an
occasion well suited for an undertaking of this kind--for an inquiry,
in fact, into the nature and value of the present results of
paleontological investigation; and the more so, as all those who have
paid close attention to the late multitudinous discussions in which
paleontology is implicated, must have felt the urgent necessity of some
such scrutiny.
First in order, as the most definite and unquestionable of all the
results of paleontology, must be mentioned the immense extension and
impulse given to botany, zoology, and comparative anatomy, by the
investigation of fossil remains. Indeed, the mass of biological facts
has been so greatly increased, and the range of biological speculation
has been so vastly widened, by the researches of the geologist and
paleontologist, that it is to be feared there are naturalists in
existence who look upon geology as Brindley regarded rivers. "Rivers,"
said the great engineer, "were made to feed canals"; and geology, some
seem to think, was solely created to advance comparative anatomy.
Were such a thought justifiable, it could hardly expect to be received
with favour by this assembly. But it is not justifiable. Your
favourite science has her own great aims independent of all others; and
if, notwithstanding her steady devotion to her own progress, she can
scatter such rich alms among her sisters, it should be remembered that
her charity is of the sort that does not impoverish, but "blesseth him
that gives and him that takes."
Regard the matter as we will, however, the facts remain. Nearly 40,000
species of animals and plants have been added to the Systema Naturae by
paleontologic research. This is a living population equivalent to that
of a new continent in mere number; equivalent to that of a new
hemisphere, if we take into account the small population of insects as
yet found fossil, and the large proportion and peculiar organization of
many of the Vertebrata.
But, beyond this, it is perhaps not too much to say that, except for the
necessity of interpreting paleontologic 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 paleontology, 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, paleontology 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 a 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 paleontology is pushed further.
Those whose business it is to acquaint themselves specially with the
works of paleontologists, 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 paleontology 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 paleontologists 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 paleontology.
Every one is aware that Professor Bronn's 'Untersuchungen' and Professor
Pictet's 'Traite de Paleontologie' are works of standard authority,
familiarly consulted by every working paleontologist. 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 paleontologists and
geologists, not only on the Continent but in this country, as
expressing some of the best-established results of paleontology.
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 coeval 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 paleontology able to succeed where physical geology fails? Standard
writers on paleontology, 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 paleontology, 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 paleontology 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 Paleozoic 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
paleontologist 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 paleontologist.
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 paleontologist 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 paleontology 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.* But, pending the construction of a surer foundation than
paleontology 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 paleontologic facts are justifiable.
[footnote] *"le plus grand service qu'on puisse rendre a 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*, 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 paleontology, and to inquire
what they tell us.
[footnote] *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.