_THE IMPERFECTION OF THE GEOLOGICAL RECORD._
Complaints of the imperfection of the geological record are rife among
those biologists who expect to find continuous series of fossils
representing the gradual transmutation of species. No doubt these gaps
are in some cases portentous, and unfortunately they often occur just
where it is most essential to certain general conclusions that they
should be filled up. Instead, however, of making vague lamentations
on the subject, it is well to inquire to what causes these gaps may
be due, to what extent they invalidate the completeness of geological
history for scientific purposes, and how they may best be filled.
Here we may first remark that it is not so much the physical record of
geology that is imperfect as the organic record. Ever since the time
of Hutton and Playfair we have learned that the processes of mineral
detrition and deposition are continuous, and have been so throughout
geological time. The erosion of the land is constantly going on, every
shower carries its tribute of earthy matter toward the sea, and every
wave that strikes against a beach or cliff does some work toward the
grinding of shells, pebbles or stone. Thus, everywhere around our
continents there is a continuous deposition of beds of earthy matter,
and it is this which, when elevated into new land, has given us our
chronological series of geological formations. True, the elevating
process is not continuous, but, so far as we know, intermittent; but
it has been so often repeated that we have no reason to doubt that the
wasting continents afford a complete series of aqueous deposits, since
the time when the dry land first appeared.
In recent years the _Challenger_ expedition and similar dredgings have
informed us of still another continuity of deposition in the depths
of the ocean. There, where no detritus from the land, or only a very
little fine volcanic ash or pumice has ever reached, we have, going on
from age to age, a deposit of the hard parts of abyssal animals and
of those that swim in the open sea; so that if it were possible to
bore or sink a shaft in some parts of the ocean, we should find not
only a continuous bed, but a continuous series of pelagic life from
the Laurentian to the present day. Thus we have continuous physical
records, could we but reach or completely put them together, and
eliminate the disturbing influence of merely local vicissitudes. It is
when we begin to search the geological formations for fossils, that
imperfection in our record first becomes painfully manifest.
In the case of many groups of marine animals, as, for example, the
shell-fish and the corals, and I may add the bivalve crustaceans, so
admirably worked up by my friend Prof. Rupert Jones, we have very
complete series. With the and snails the case is altogether different.
As stated in another paper of this series, a few species of these
animals appear in the later Palæozoic age, and after that they have no
successors known to us in all the great periods covered by the Permian,
the Trias, and the earlier Jurassic. A few air-breathing water-snails
appear in the upper Jurassic, and true land snails are not met with
again until the Tertiary. Were there no land snails in this vast lapse
of time? Have we two successive creations, so to speak, of these
creatures at distant intervals? Were they only diminished in numbers
and distribution in the intervening time? Is the hiatus owing merely
to the unlikelihood of such shells being preserved? Or is it owing to
the lack of diligence and care in collecting?
In this particular case we are, no doubt, disposed to say that the
series must have been continuous. But we cannot be sure of this. In
whatever way a few species of land snails were so early introduced in
the time of the Devonian or of the Coal formation, if from physical
vicissitudes or lack of proper pabulum they became extinct, there is
no reason known to us why, when circumstances again became favourable,
they should not be reintroduced in the same manner as at first, whether
by development from allied types or otherwise. The fact that the few
Devonian and Carboniferous species are very like those that still
exist, perhaps makes against this supposition, but does not exclude it.
If we suppose that new forms of life of low grade are introduced from
time to time in the course of the geological ages, and if we adopt the
Darwinian hypothesis of evolution, we arrive, as Naegeli has so well
pointed out, at the strange paradox, that the highest forms of life
must be the oldest of all, since they will be the descendants of the
earliest of the lower animals, whereas the animals now of low grade
may have been introduced later, and may not have had time to improve.
But all our attempts to reduce nature to one philosophic expression
necessarily lead to such paradoxes.
On the other hand, the chances of the preservation of land snails
in aqueous deposits are vastly less than those in favour of the
preservation of aquatic species. The first Carboniferous species
found[15] had been preserved in the very exceptional circumstances
afforded by the existence of hollow trunks of Sigillariæ on the borders
of the Coal formation flats, and the others subsequently found were
in beds no doubt receiving the drainage of neighbouring land areas.
Still it is not uncommon on the modern sea-shore, anywhere near the
mouths of rivers, to find a few fresh-water shells here and there.
The carbonaceous beds of the Trias, the fossil soils of the Portland
series, the estuarine Wealden beds would seem to be as favourably
situated as those of the coal formation for preserving land shells,
though possibly the flora of the Mesozoic was less suitable for feeding
such creatures than that of the Coal period, and they may consequently
have become few and local. After all, perhaps more diligent collecting
and more numerous collectors might succeed, and may succeed in the
future, in filling this and similar gaps.
[15] _Pupa vetusta_ of the Nova Scotia coal formation.
It is a great mistake to suppose that discoveries of this kind are made
by chance. It is only by the careful and painstaking examination of
much material that the gaps in the geological record can be filled up,
and I propose in the sequel of this article to note a few instances, in
a country where the range of territory is altogether out of proportion
to the number of observers, and which have come within my own knowledge.
It was not altogether by accident that Sir C. Lyell and the writer
discovered a few reptilian bones and a land-snail in breaking up
portions of the material filling an erect Sigillaria in the South
Joggins coal measures. We were engaged in a deliberate survey of the
section, to ascertain as far as might be the conditions of accumulation
of coal, and one point which occurred to us was to inquire as to the
circumstances of preservation of stumps of forest trees in an erect
position, to trace their roots into the soils on which they stood,
and to ascertain the circumstances in which they had been buried, had
decayed, and had been filled with mineral matter. It was in questioning
these erect trees on such subjects and this not without some digging
and hammering that we made the discovery referred to.
But we found such remains only in one tree, and they were very
imperfect, and indicated only two species of batrachians and one
land-snail. There the discovery might have rested. But I undertook to
follow it up. In successive visits to the coast, a large number of
trees standing in the cliff and reefs, or fallen to the shore, were
broken up and examined, the result being to discover that, with one
unimportant exception, the productive trees were confined to one of
the beds at Coal Mine Point, that from which the original specimens
had been obtained. Attention was accordingly concentrated on this, and
as many as thirty trees were at different times extracted from it, of
which rather more than one-half proved more or less productive. By
these means bones representing about sixty specimens and twelve species
were extracted, besides numerous remains of land shells, millipedes,
and scorpions. In this way a very complete idea was obtained of the
land life, or at least of the smaller land animals, of this portion
of the coal formation of Nova Scotia. It is not too much to say that
if similar repositories could be found in the succeeding formations,
and properly worked when found, our record of the history of land
quadrupeds might be made very complete.
When in 1855 I changed my residence from Nova Scotia to Montreal,
and so was removed to some distance from the carboniferous rocks
which I had been accustomed to study, I naturally felt somewhat out
of place in a Cambro-Silurian district, more especially as my friend
Billings had already almost exhausted its fossils. I found, however,
a congenial field in the Pleistocene shell beds; more especially as
I had given some attention to recent marine animals when on the sea
coast. The very perfect series of Pleistocene deposits in the St.
Lawrence valley locally contain marine shells from the bottom of
the till or boulder clay up to the overlying sands and gravels. The
assemblage was a more boreal one than that on the coast of Nova Scotia,
though many of the species were the same, and both the climatal and
bathymetrial conditions differed in different parts of the Pleistocene
beds themselves. The gap in the record here could at that time be
filled up only by collecting recent shells. In addition to what
could be obtained by exchanging with naturalists who had collected
in Greenland, Labrador, and Norway, I employed myself, summer after
summer, in dredging both on the south and north shore of the St.
Lawrence, until able at length to discover in a living state, but under
different conditions as to temperature and depth, nearly every species
found in the beds on the land, from the lower boulder clay to the top
of the formation, and from the sea-level to the beds six hundred feet
high on the hills. Not only so: I could ascertain in certain places and
conditions all the peculiar varieties of the species, and the special
modes of life which they indicated. Thus, in the cases of the Peter
Redpath Museum, and in notes on the Post-pliocene of Canada, the gap
between the Modern and the Glacial age was completely filled up in so
far as Canadian marine species are concerned. The net result was, as I
have elsewhere stated, that no change other than varietal had occurred.
In studying the fossil plants of the Carboniferous, so abundant in
the fine exposures of the coal formation in Nova Scotia, two defects
struck me painfully. One was the fragmentary and imperfect state of
the specimens procurable. Another was the question, What preceded
these plants in the older rocks? The first of these was to be met only
by thorough exploration. When a fragment of a plant was disclosed it
was necessary to inquire if more existed in the same bed, and to dig,
or blast away or break up the rock, until some remaining portions
were disclosed. In this way it has been possible to obtain entire
specimens of many trees of the Carboniferous; and to such an extent has
the laborious and somewhat costly process been effectual, that more
species of carboniferous trees are probably known in their entire forms
from the Coal formations of Nova Scotia than from any other part of
the world. I have been amused to find that so little are experiences
of this kind known to some of my _confrères_ abroad, that they are
disposed to look with scepticism on the information obtained by this
laborious but certain process, and to suppose that they are being
presented with imaginary "restorations." I think it right here to copy
a remark of a German botanist, who has felt himself called to criticise
my work: "Dawson's description of the genus (_Psilophyton_) rests
chiefly on the impression made on him in his repeated researches," etc.
"He puts us off with an account of the general idea which he has drawn
from the study of them." This is the remark of a closet naturalist,
with reference to the kind of work above referred to, which, of course,
cannot be represented in its entirety in figures or hand specimens.[16]
[16] Solms-Laubach, "Fossil Botany." A pretentious book, which should
not have been translated into English without thorough revision and
correction.
As to the precursors of the Carboniferous flora, in default of
information already acquired, I proceeded to question the Erian or
Devonian rocks of Canada, in which Sir William Logan had already found
remains of plants which had not, however, been studied or described.
Laboriously coasting along the cliffs of Gaspé and the Baie des
Chaleurs, digging into the sandstones of Eastern Maine, and studying
the plants collected by the New York Survey, I began to find that there
was a rich Devonian flora, and that, like that of the Carboniferous,
it presented different stages from the base to the summit of the
formation. But here a great advance was made in a somewhat unexpected
way. My then young friends, the late Prof. Hartt and Mr. Matthew, of
St. John, had found a few remains of plants in the Devonian, or at
least pre-Carboniferous beds of St. John, which were placed in my
hands for description. They were so novel and curious that inquiry was
stimulated, and these gentlemen, with some friends of similar tastes,
explored the shales exposed in the reefs near St. John, and when they
found the more productive beds, broke them up by actual quarrying
operations in such a way that they soon obtained the richest Devonian
plant collections ever known. I think I may truly say that these young
and enthusiastic explorers worked the St. John plant-beds in a manner
previously unexampled in the world. Their researches were not only thus
rewarded, but incidentally they discovered the first known Devonian
insects, which could not have been found by a less painstaking process,
and one of them discovered what I believe to be the oldest known
land shell. Still more, their studies led to the separation from the
Devonian beds of the Underlying Cambrian slates, previously confounded
with them; and this, followed up by the able and earnest work of Mr.
Matthew, has carried back our knowledge of the older rocks in Canada
several stages, or as far as the earliest Cambrian previously known in
Europe, but not before fully recognised in America, and has discovered
in these old rocks the precursors of many forms of life not previously
traced so far back.
The moral of these statements of fact is that the imperfections of the
record will yield only to patient and painstaking work, and that much
is in the power of local amateurs. I would enforce this last statement
by a reference to a little research, in which I have happened to take
part at a summer resort on the Lower St. Lawrence, at which I have from
time to time spent a few restful vacation weeks. Little Metis is on the
Quebec Group of Sir William Logan, that peculiar local representative
of the lower part of the Cambro-Silurian and Upper Cambrian formations
which stretches along the south side of the St. Lawrence all the way
from Quebec to Cape Rosier, near Gaspé, a distance of five hundred
miles. This great series of rocks is a jumble of deposits belonging
at that early time to the marginal area of what is now the American
continent, and indicating the action not merely of ordinary causes
of aqueous deposit, but of violent volcanic ejections, accompanied
perhaps by earthquake waves, and not improbably by the action of heavy
coast ice. The result is that mud rocks now in the form of black, grey,
and red shales and slates alternate with thick and irregular beds of
hard sandstone, sometimes so coarse that it resembles the angular
_débris_ of the first treatment of quartz in a crusher. With these
sandstones are thick and still more irregular conglomerates formed of
pebbles and boulders of all sizes, up to several feet in diameter, some
of which are of older limestones containing Cambrian fossils, while
others are of quartzite or of igneous or volcanic rocks.
The whole formation, as presented at Metis, is of the most unpromising
character as regards fossils, and after visiting the place for ten
years, and taking many long walks along the shore and into the
interior, and scrutinising every exposure, I had found nothing more
interesting than a few fragments of graptolites, little zoophytes,
ancient representatives of our sea mosses, and which are quite
characteristic of several portions of the Quebec Group. With these were
some marks of fucoids and tracks or burrows of worms. The explorers of
the Geological Survey had been equally unsuccessful.
Quite accidentally a new light broke upon these unpromising rocks.
My friend, Dr. Harrington, strolling one day on the shore, sat down
to rest on a stone, and picked up a piece of black slate lying at
his feet. He noticed on it some faintly traced lines which seemed
peculiar. He put it in his pocket and showed it to me. On examination
with a lens it proved to have on it a few spicules of a hexactinellid
sponge--little crosses forming a sort of mesh or lattice-work similar
to that which Salter had many years before found in the Cambrian
rocks of Wales, and had named _Protospongia_--the first sponge. The
discovery seemed worth following up, and we took an early opportunity
of proceeding to the place, where, after some search, we succeeded in
tracing the loose pieces to a ledge of shale on the beach, where there
was a little band, only about an inch thick, stored with remains of
sponges, a small bivalve shell and a slender branching seaweed. This
was one small layer in reefs of slate more than one hundred feet thick.
We subsequently found two other thin layers, but less productive.
Tools and workmen were procured, and we proceeded to quarry in the
reef, taking out at low tide as large slabs as possible of the most
productive layer, and carefully splitting these up. The results, as
published in the Transactions of the Royal Society of Canada,[17]
show more than twelve species of siliceous sponges belonging to six
genera, besides fragments indicating other species, and all of these
living at one time on a very limited space of what is practically a
single surface of muddy sea-bottom.[18] The specimens show the parts
of these ancient sponges much more perfectly than they were previously
known, and indeed, enable many of them to be perfectly restored. They
for the first time connect the modern siliceous sponges of the deep
sea with those that flourished on the old sea-bottom of the early
Cambro-Silurian, and thus bridge over a great, gap in the history of
this low form of life, showing that the principles of construction
embodied in the remarkable and beautiful siliceous sponges, like
Euplectella, the "Venus flower-basket," now dredged from the deep sea,
were already perfectly carried out in this far-back beginning of life.
This little discovery further indicates that portions of the older
Palæozoic sea-bottoms were as well stored with a varied sponge life
as those of any part of the modern ocean. I figure[19] a number of
species, remains of all of which may be gathered from a few yards of a
single surface at Little Metis. The multitude of interesting details
embodied in all this it is impossible to enter into here, but may be
judged of from the forms reproduced. These examples tend to show that
the imperfection of the record may not depend on the record itself, but
on the incompleteness of our work. We must make large allowance for
imperfect collecting, and especially for the too prevalent habit of
remaining content with few and incomplete specimens, and of grudging
the time and labour necessary to explore thoroughly the contents of
special beds, and to work out all the parts of forms found more or less
in fragments.
[17] Additional collections made in 1892 show two or three additional
species, one of them the type of a new and remarkable genus.
[18] 1889, section iv. p. 39.
[19] Frontispiece to chapter.
The point of all this at present is that patient work is needed to fill
up the breaks in our record. A collector passing along the shore at
Metis might have picked up a fragment of a fossil sponge, and recorded
it as a fossil, or possibly described the fragment. This fact alone
would have been valuable, but to make it bear its full fruit it was
necessary to trace the fragment to its source, and then to spend time
and labour in extracting from the stubborn rock the story it had to
tell. Instances of this kind crowd on my memory as coming within my own
experience and observation. It is hopeful to think that the record is
daily becoming less imperfect; it is stimulating to know that so much
is only waiting for investigation. The history never can be absolutely
complete. Practically, to us it is infinite. Yet every series of facts
known may be complete in itself for certain purposes, however many gaps
there may be in the story. Even if we cannot find a continuous series
between the snails of the Coal formation or the sponges of the Quebec
Group and their successors to-day, we can at least see that they are
identical in plan and structure, and can note the differences of detail
which fitted them for their places in the ancient or the modern world.
Nor need we be too discontented if the order of succession, such as
it is, does not exactly square with some theories we may have formed.
Perhaps it may in the end lead us to greater and better truths.
Another subject which merits attention here is the evidence which mere
markings or other indications may sometimes give as to the existence
of unknown creatures, and thus may be as important to us as the
footprints of Friday to Robinson Crusoe. As I have been taking Canadian
examples, I may borrow one here from Mr. Matthew, of St. John, New
Brunswick.
He remarks in one of his papers the manner in which the Trilobites
of the early Cambrian are protected with defensive spines, and asks
against what enemies they were intended to guard. That there were
enemies is further proved by the occurrence of Coprolites or masses
of excrement, oval or cylindrical in form, and containing fragments
of shells of Trilobites, of Pteropods (Hyolithes) and of Lingula.
There must therefore have been marine animals of considerable size,
which preyed on Trilobites. Dr. Hunt and myself have recorded similar
facts from the Upper Cambrian and Cambro-Silurian of the Province of
Quebec. No remains, however, are known of animals which could have
produced such coprolites, except, indeed, some of the larger worms of
the period, and they seem scarcely large enough. In these circumstances
Mr. Matthew falls back on certain curious marks or scratches with
which large surfaces of these old rocks are covered, and which he
names Ctenichnites or "Comb tracks." These markings seem to indicate
the rapid motion of some animal touching the bottom with fins or other
organs; and as we know no fishes in these old rocks, the question
recurs, What could it have been? From the form and character of the
markings Mr. Matthew infers (1) That these animals lived in "schools,"
or were social in their habits; (2) That they had a rapid, direct,
darting motion; (3) That they had three or four (at least) flexible
arms; (4) That these arms were furnished with hooks or spines; (5)
That the creatures swam with an easy motion, so that sometimes the
arms of one side touched the bottom, sometimes those of the other.
These indications point to animals allied to the modern squids or
cuttle-fishes, and as these animals may have had no hard parts capable
of preservation, except their horny beaks, nothing might remain to
indicate their presence except these marks on the bottom. Mr. Matthew
therefore conjectures that there may have been large cuttle-fishes in
the Cambrian. Since, however, these are animals of very high rank in
their class, and are not certainly known to us till a very much later
period, their occurrence in these old rocks would be a very remarkable
and unexpected fact.
A discovery made by Walcott in the Western States since Mr. Matthew's
paper was written, throws fresh light on the question. Remains of
fishes have been found by the former in the Cambro Silurian rocks
nearly as far back as Mr. Matthew's comb-tracks. Besides this, Pander
in Russia has found in these old rocks curious teeth, which he refers
conjecturally to fishes (Conodonts). Why may there not have been in
the Cambrian large fishes having, like the modern sharks, cartilage or
gristle instead of bone--perhaps destitute of scales, and with small
teeth which have not yet been detected. The fin rays of such fishes may
have left the comb tracks, and in support of this I may say that there
are in the Lower Carboniferous of Horton Bluff, in Nova Scotia, very
similar tracks in beds holding many remains of fishes. Whichever view
we adopt we see good evidence that there were in the early Cambrian
animals of higher grade than we have yet dreamt of. Observe, however,
that if we could complete the record in this point it would only give
us higher forms of life at an earlier time, and so push farther back
their possible development from lower forms. I fear, indeed, that I
can hold out little hopes to the evolutionists that a more complete
geological record would help them in any way. It would possibly only
render their position more difficult.
But the saddest of all the possible defects of the geological record
is that it may want the beginning, and be like the Bible of some of
the German historical critics, from which they eliminate as mythical
everything before the time of the later Hebrew kings. Our attention is
forcibly called to this by the condition of the fauna of the earliest
Cambrian rocks. The discoveries in these in Wales, in Norway, and in
America show us that the seas of this early period swarmed with animals
representing all the great types of invertebrate marine life. We have
here highly organized Crustaceans, Worms, Mollusks and other creatures
which show us that in that early age all these distinct forms of life
were as well separated from each other as in later times, that eyes
of different types, jointed limbs with nerves and muscles, and a vast
variety of anatomical contrivances were as highly developed as at any
subsequent time.[20] To a Darwinian evolutionist this means nothing
less than that these creatures must have existed through countless ages
of development from their imagined simple ancestral form or forms how
long it is impossible to guess, since, unless change was more speedy
in the infancy of the earth, the term of ages required must have far
exceeded that from the Cambrian to the Modern. Yet, to represent all
this we have absolutely nothing except Eozoon in its solitary grandeur,
and a few other forms, possibly of Protozoa and worms. An imaginary
phylogeny of animal life from Monads to Trilobites would be something
as long as the whole geological history. Yet it would be almost wholly
imaginary, for the record of the rocks tells little or nothing. In face
of such an imperfection as this, geologists should surely be humble,
and make confession of ignorance to any extent that may be desired. Yet
we may at least, with all humility and self-abasement, ask our critics
how they know that this great blank really exists, and whether it may
not be possible that the swarming life of the early Cambrian may, after
all, have appeared suddenly on the stage in some way as yet unknown to
us and to them.
[20] Walcott and Matthew record more than 160 species of 67 genera,
including Sponges, Zoophytes, Echinoderms, Brachiopods, Bivalve and
Univalve shellfishes, Trilobites and other Crustaceans from the Lower
Cambrian of the United States of America and Canada alone; and these
are but a portion of the inhabitants of the early Cambrian seas. There
is a rich Scandinavian fauna of the same early date, and in England and
Wales, Sailer, Hicks and Lapworth have described many fossils of the
basal Cambrian. From year to year, also, discoveries of fossil remains
are being made, both in America and Europe, in beds of older date than
those previously known to be fossiliferous. At present, however, these
remains are still few and imperfectly known, and it is not in all
cases certain whether the beds in which they occur are pre-Cambrian or
belong to the lowest members of that great system. It is unfortunate
that so many of the strata between the Laurentian and the Cambrian seem
to be of a character little likely to contain fossils; being littoral
deposits produced in times of much physical disturbance. Yet there must
have been contemporaneous beds of a different character, which may yet
be discovered.
References:--"Fossil Sponges from the Quebec Group of Little Metis,
Lower St. Lawrence": _Transactions Royal Society of Canada_,
1890. "Rèsumè of the Carboniferous Land Shells of North America":
_American Journal of Science_, 1880. "Burrows and Tracks of
Invertebrate Animals": _Journal Geological Society of London_,
1890. "Notes on the Pleistocene of Canada": _Canadian Naturalist_,
1876. "Air-breathers of the Coal Period ": _Ibid._, 1863.
_THE HISTORY OF THE NORTH ATLANTIC._
DEDICATED TO THE MEMORY OF
PROF. JOHN PHILLIPS, OF OXFORD,
One of the most able, earnest, and genial of
English Geologists;
and of other Eminent Scientific Men, now passed away,
who supported him as
President of the British Association, at its
Meeting in Birmingham, in 1865.
Distribution of Land and Water--Causes of Irregularities of the
Surface Crust and Interior--Position of Continents--Past History of the
Atlantic--Its Relations to Life--Its Future
[Illustration: Map of the North Atlantic, showing depths from 4,000
fathoms upward (after the Challenger Survey).]