the pedigree of this insignificant shell-fish. We Englishmen are proud
to have an ancestor who was present at the Battle of Hastings. The
ancestors of _Terebratulina caput serpentis_ may have been present at a
battle of _Ichthyosauria_ in that part of the sea which, when the chalk
was forming, flowed over the site of Hastings. While all around has
changed, this _Terebratulina_ has peacefully propagated its species from
generation to generation, and stands to this day, as a living testimony
to the continuity of the present with the past history of the globe.
Up to this moment I have stated, so far as I know, nothing but
well-authenticated facts, and the immediate conclusions which they force
upon the mind.
But the mind is so constituted that it does not willingly rest in facts
and immediate causes, but seeks always after a knowledge of the remoter
links in the chain of causation.
Taking the many changes of any given spot of the earth's surface, from
sea to land and from land to sea, as an established fact, we cannot
refrain from asking ourselves how these changes have occurred. And when
we have explained them--as they must be explained--by the alternate slow
movements of elevation and depression which have affected the crust of
the earth, we go still further back, and ask, Why these movements?
I am not certain that any one can give you a satisfactory answer to
that question. Assuredly I cannot. All that can be said, for certain,
is, that such movements are part of the ordinary course of nature,
inasmuch as they are going on at the present time. Direct proof may be
given, that some parts of the land of the northern hemisphere are at
this moment insensibly rising and others insensibly sinking; and there
is indirect, but perfectly satisfactory, proof, that an enormous area
now covered by the Pacific has been deepened thousands of feet, since
the present inhabitants of that sea came into existence.
Thus there is not a shadow of a reason for believing that the physical
changes of the globe, in past times, have been effected by other than
natural causes.
Is there any more reason for believing that the concomitant
modifications in the forms of the living inhabitants of the globe have
been brought about in other ways?
Before attempting to answer this question, let us try to form a distinct
mental picture of what has happened in some special case.
The crocodiles are animals which, as a group, have a very vast
antiquity. They abounded ages before the chalk was deposited; they
throng the rivers in warm climates, at the present day. There is a
difference in the form of the joints of the back-bone, and in some minor
particulars, between the crocodiles of the present epoch and those which
lived before the chalk; but, in the cretaceous epoch, as I have already
mentioned, the crocodiles had assumed the modern type of structure.
Notwithstanding this, the crocodiles of the chalk are not identically
the same as those which lived in the times called "older tertiary,"
which succeeded the cretaceous epoch; and the crocodiles of the older
tertiaries are not identical with those of the newer tertiaries, nor are
these identical with existing forms. I leave open the question whether
particular species may have lived on from epoch to epoch. But each epoch
has had its peculiar crocodiles; though all, since the chalk, have
belonged to the modern type, and differ simply in their proportions, and
in such structural particulars as are discernible only to trained eyes.
How is the existence of this long succession of different species of
crocodiles to be accounted for?
Only two suppositions seem to be open to us--Either each species of
crocodile has been specially created, or it has arisen out of some
pre-existing form by the operation of natural causes.
Choose your hypothesis; I have chosen mine. I can find no warranty for
believing in the distinct creation of a score of successive species of
crocodiles in the course of countless ages of time. Science gives no
countenance to such a wild fancy; nor can even the perverse ingenuity of
a commentator pretend to discover this sense, in the simple words in
which the writer of Genesis records the proceedings of the fifth and
sixth days of the Creation.
On the other hand, I see no good reason for doubting the necessary
alternative, that all these varied species have been evolved from
pre-existing crocodilian forms, by the operation of causes as completely
a part of the common order of nature, as those which have effected the
changes of the inorganic world.
Few will venture to affirm that the reasoning which applies to
crocodiles loses its force among other animals, or among plants. If one
series of species has come into existence by the operation of natural
causes, it seems folly to deny that all may have arisen in the same way.
A small beginning has led us to a great ending. If I were to put the bit
of chalk with which we started into the hot but obscure flame of burning
hydrogen, it would presently shine like the sun. It seems to me that
this physical metamorphosis is no false image of what has been the
result of our subjecting it to a jet of fervent, though nowise
brilliant, thought to-night. It has become luminous, and its clear rays,
penetrating the abyss of the remote past, have brought within our ken
some stages of the evolution of the earth. And in the shifting "without
haste, but without rest" of the land and sea, as in the endless
variation of the forms assumed by living beings, we have observed
nothing but the natural product of the forces originally possessed by
the substance of the universe.
FOOTNOTES:
[29] See Appendix to Captain Dayman's "Deep Sea Soundings in the North
Atlantic Ocean, between Ireland and Newfoundland, made in H.M.S.
_Cyclops_. Published by order of the Lords Commissioners of the
Admiralty, 1858." They have since formed the subject of an elaborate
Memoir by Messrs. Parker and Jones, published in the _Philosophical
Transactions_ for 1865.
[30] During the cruise of H.M.S. _Bull-dog_, commanded by Sir Leopold
M'Clintock, in 1860, living star-fish were brought up, clinging to the
lowest part of the sounding-line, from a depth of 1,260 fathoms, midway
between Cape Farewell, in Greenland, and the Rockall banks. Dr. Wallich
ascertained that the sea-bottom at this point consisted of the ordinary
_Globigerina_ ooze, and that the stomachs of the star-fishes were full
of _Globigerinæ_. This discovery removes all objections to the existence
of living _Globigerinæ_ at great depths, which are based upon the
supposed difficulty of maintaining animal life under such conditions;
and it throws the burden of proof upon those who object to the
supposition that the _Globigerinæ_ live and die where they are found.
[31] I have recently traced out the development of the "coccoliths" from
a diameter of 1/7000th of an inch up to their largest size (which is
about 1/1600th), and no longer doubt that they are produced by
independent organisms, which, like the _Globigerinæ_, live and die at
the bottom of the sea.
[32] "Elements of Geology," by Sir Charles Lyell, Bart. F.R.S., p. 23.
X.
GEOLOGICAL CONTEMPORANEITY AND PERSISTENT TYPES OF LIFE.
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
palæontological investigation; and the more so, as all those who have
paid close attention to the late multitudinous discussions in which
palæontology 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 palæontology, 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
palæontologist, 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 Naturæ by
palæontological 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 palæontological 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 minutiæ of
osteology, were it not that in such minutiæ 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 palæontology, 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, palæontology 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 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 palæontology is pushed further.
Those whose business it is to acquaint themselves specially with the
works of palæontologists, 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 palæontology 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 palæontologists 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 palæontology.
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 palæontologist. 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 palæontologists and geologists,
not only on the Continent but in this country, as expressing some of the
best-established results of palæontology. 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 faunas
and floræ 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 phænomena; 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 palæontology able to succeed where physical geology fails? Standard
writers on palæontology, 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 _primâ 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 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 faunæ 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.