ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day—On the
nature of extinct intermediate varieties; on their number—On the lapse
of time, as inferred from the rate of denudation and of deposition
number—On the lapse of time as estimated by years—On the poorness of
our palæontological collections—On the intermittence of geological
formations—On the denudation of granitic areas—On the absence of
intermediate varieties in any one formation—On the sudden appearance of
groups of species—On their sudden appearance in the lowest known
fossiliferous strata—Antiquity of the habitable earth.
In the sixth chapter I enumerated the chief objections which might be
justly urged against the views maintained in this volume. Most of them
have now been discussed. One, namely, the distinctness of specific
forms and their not being blended together by innumerable transitional
links, is a very obvious difficulty. I assigned reasons why such links
do not commonly occur at the present day under the circumstances
apparently most favourable for their presence, namely, on an extensive
and continuous area with graduated physical conditions. I endeavoured
to show, that the life of each species depends in a more important
manner on the presence of other already defined organic forms, than on
climate, and, therefore, that the really governing conditions of life
do not graduate away quite insensibly like heat or moisture. I
endeavoured, also, to show that intermediate varieties, from existing
in lesser numbers than the forms which they connect, will generally be
beaten out and exterminated during the course of further modification
and improvement. The main cause, however, of innumerable intermediate
links not now occurring everywhere throughout nature depends, on the
very process of natural selection, through which new varieties
continually take the places of and supplant their parent-forms. But
just in proportion as this process of extermination has acted on an
enormous scale, so must the number of intermediate varieties, which
have formerly existed, be truly enormous. Why then is not every
geological formation and every stratum full of such intermediate links?
Geology assuredly does not reveal any such finely graduated organic
chain; and this, perhaps, is the most obvious and serious objection
which can be urged against my theory. The explanation lies, as I
believe, in the extreme imperfection of the geological record.
In the first place, it should always be borne in mind what sort of
intermediate forms must, on the theory, have formerly existed. I have
found it difficult, when looking at any two species, to avoid picturing
to myself forms _directly_ intermediate between them. But this is a
wholly false view; we should always look for forms intermediate between
each species and a common but unknown progenitor; and the progenitor
will generally have differed in some respects from all its modified
descendants. To give a simple illustration: the fantail and pouter
pigeons are both descended from the rock-pigeon; if we possessed all
the intermediate varieties which have ever existed, we should have an
extremely close series between both and the rock-pigeon; but we should
have no varieties directly intermediate between the fantail and pouter;
none, for instance, combining a tail somewhat expanded with a crop
somewhat enlarged, the characteristic features of these two breeds.
These two breeds, moreover, have become so much modified, that, if we
had no historical or indirect evidence regarding their origin, it would
not have been possible to have determined from a mere comparison of
their structure with that of the rock-pigeon, C. livia, whether they
had descended from this species or from some other allied species, such
as C. oenas.
So with natural species, if we look to forms very distinct, for
instance to the horse and tapir, we have no reason to suppose that
links directly intermediate between them ever existed, but between each
and an unknown common parent. The common parent will have had in its
whole organisation much general resemblance to the tapir and to the
horse; but in some points of structure may have differed considerably
from both, even perhaps more than they differ from each other. Hence,
in all such cases, we should be unable to recognise the parent-form of
any two or more species, even if we closely compared the structure of
the parent with that of its modified descendants, unless at the same
time we had a nearly perfect chain of the intermediate links.
It is just possible, by the theory, that one of two living forms might
have descended from the other; for instance, a horse from a tapir; and
in this case _direct_ intermediate links will have existed between
them. But such a case would imply that one form had remained for a very
long period unaltered, whilst its descendants had undergone a vast
amount of change; and the principle of competition between organism and
organism, between child and parent, will render this a very rare event;
for in all cases the new and improved forms of life tend to supplant
the old and unimproved forms.
By the theory of natural selection all living species have been
connected with the parent-species of each genus, by differences not
greater than we see between the natural and domestic varieties of the
same species at the present day; and these parent-species, now
generally extinct, have in their turn been similarly connected with
more ancient forms; and so on backwards, always converging to the
common ancestor of each great class. So that the number of intermediate
and transitional links, between all living and extinct species, must
have been inconceivably great. But assuredly, if this theory be true,
such have lived upon the earth.
_On the Lapse of Time, as inferred from the rate of deposition and
extent of Denudation._
Independently of our not finding fossil remains of such infinitely
numerous connecting links, it may be objected that time cannot have
sufficed for so great an amount of organic change, all changes having
been effected slowly. It is hardly possible for me to recall to the
reader who is not a practical geologist, the facts leading the mind
feebly to comprehend the lapse of time. He who can read Sir Charles
Lyell’s grand work on the Principles of Geology, which the future
historian will recognise as having produced a revolution in natural
science, and yet does not admit how vast have been the past periods of
time, may at once close this volume. Not that it suffices to study the
Principles of Geology, or to read special treatises by different
observers on separate formations, and to mark how each author attempts
to give an inadequate idea of the duration of each formation, or even
of each stratum. We can best gain some idea of past time by knowing the
agencies at work; and learning how deeply the surface of the land has
been denuded, and how much sediment has been deposited. As Lyell has
well remarked, the extent and thickness of our sedimentary formations
are the result and the measure of the denudation which the earth’s
crust has elsewhere undergone. Therefore a man should examine for
himself the great piles of superimposed strata, and watch the rivulets
bringing down mud, and the waves wearing away the sea-cliffs, in order
to comprehend something about the duration of past time, the monuments
of which we see all around us.
It is good to wander along the coast, when formed of moderately hard
rocks, and mark the process of degradation. The tides in most cases
reach the cliffs only for a short time twice a day, and the waves eat
into them only when they are charged with sand or pebbles; for there is
good evidence that pure water effects nothing in wearing away rock. At
last the base of the cliff is undermined, huge fragments fall down, and
these remaining fixed, have to be worn away atom by atom, until after
being reduced in size they can be rolled about by the waves, and then
they are more quickly ground into pebbles, sand, or mud. But how often
do we see along the bases of retreating cliffs rounded boulders, all
thickly clothed by marine productions, showing how little they are
abraded and how seldom they are rolled about! Moreover, if we follow
for a few miles any line of rocky cliff, which is undergoing
degradation, we find that it is only here and there, along a short
length or round a promontory, that the cliffs are at the present time
suffering. The appearance of the surface and the vegetation show that
elsewhere years have elapsed since the waters washed their base.
We have, however, recently learned from the observations of Ramsay, in
the van of many excellent observers—of Jukes, Geikie, Croll and others,
that subaërial degradation is a much more important agency than
coast-action, or the power of the waves. The whole surface of the land
is exposed to the chemical action of the air and of the rainwater, with
its dissolved carbonic acid, and in colder countries to frost; the
disintegrated matter is carried down even gentle slopes during heavy
rain, and to a greater extent than might be supposed, especially in
arid districts, by the wind; it is then transported by the streams and
rivers, which, when rapid deepen their channels, and triturate the
fragments. On a rainy day, even in a gently undulating country, we see
the effects of subaërial degradation in the muddy rills which flow down
every slope. Messrs. Ramsay and Whitaker have shown, and the
observation is a most striking one, that the great lines of escarpment
in the Wealden district and those ranging across England, which
formerly were looked at as ancient sea-coasts, cannot have been thus
formed, for each line is composed of one and the same formation, while
our sea-cliffs are everywhere formed by the intersection of various
formations. This being the case, we are compelled to admit that the
escarpments owe their origin in chief part to the rocks of which they
are composed, having resisted subaërial denudation better than the
surrounding surface; this surface consequently has been gradually
lowered, with the lines of harder rock left projecting. Nothing
impresses the mind with the vast duration of time, according to our
ideas of time, more forcibly than the conviction thus gained that
subaërial agencies, which apparently have so little power, and which
seem to work so slowly, have produced great results.
When thus impressed with the slow rate at which the land is worn away
through subaërial and littoral action, it is good, in order to
appreciate the past duration of time, to consider, on the one hand, the
masses of rock which have been removed over many extensive areas, and
on the other hand the thickness of our sedimentary formations. I
remember having been much struck when viewing volcanic islands, which
have been worn by the waves and pared all round into perpendicular
cliffs of one or two thousand feet in height; for the gentle slope of
the lava-streams, due to their formerly liquid state, showed at a
glance how far the hard, rocky beds had once extended into the open
ocean. The same story is told still more plainly by faults—those great
cracks along which the strata have been upheaved on one side, or thrown
down on the other, to the height or depth of thousands of feet; for
since the crust cracked, and it makes no great difference whether the
upheaval was sudden, or, as most geologists now believe, was slow and
effected by many starts, the surface of the land has been so completely
planed down that no trace of these vast dislocations is externally
visible. The Craven fault, for instance, extends for upward of thirty
miles, and along this line the vertical displacement of the strata
varies from 600 to 3,000 feet. Professor Ramsay has published an
account of a downthrow in Anglesea of 2,300 feet; and he informs me
that he fully believes that there is one in Merionethshire of 12,000
feet; yet in these cases there is nothing on the surface of the land to
show such prodigious movements; the pile of rocks on either side of the
crack having been smoothly swept away.
On the other hand, in all parts of the world the piles of sedimentary
strata are of wonderful thickness. In the Cordillera, I estimated one
mass of conglomerate at ten thousand feet; and although conglomerates
have probably been accumulated at a quicker rate than finer sediments,
yet from being formed of worn and rounded pebbles, each of which bears
the stamp of time, they are good to show how slowly the mass must have
been heaped together. Professor Ramsay has given me the maximum
thickness, from actual measurement in most cases, of the successive
formations in _different_ parts of Great Britain; and this is the
result:—
Feet Palæozoic strata (not including igneous beds) 57,154. Secondary
strata 13,190. Tertiary strata 2,240.
that is, very nearly thirteen and three-quarters British miles. Some of
these formations, which are represented in England by thin beds, are
thousands of feet in thickness on the Continent. Moreover, between each
successive formation we have, in the opinion of most geologists, blank
periods of enormous length. So that the lofty pile of sedimentary rocks
in Britain gives but an inadequate idea of the time which has elapsed
during their accumulation. The consideration of these various facts
impresses the mind almost in the same manner as does the vain endeavour
to grapple with the idea of eternity.
Nevertheless this impression is partly false. Mr. Croll, in an
interesting paper, remarks that we do not err “in forming too great a
conception of the length of geological periods,” but in estimating them
by years. When geologists look at large and complicated phenomena, and
then at the figures representing several million years, the two produce
a totally different effect on the mind, and the figures are at once
pronounced too small. In regard to subaërial denudation, Mr. Croll
shows, by calculating the known amount of sediment annually brought
down by certain rivers, relatively to their areas of drainage, that
1,000 feet of solid rock, as it became gradually disintegrated, would
thus be removed from the mean level of the whole area in the course of
six million years. This seems an astonishing result, and some
considerations lead to the suspicion that it may be too large, but if
halved or quartered it is still very surprising. Few of us, however,
know what a million really means: Mr. Croll gives the following
illustration: Take a narrow strip of paper, eighty-three feet four
inches in length, and stretch it along the wall of a large hall; then
mark off at one end the tenth of an inch. This tenth of an inch will
represent one hundred years, and the entire strip a million years. But
let it be borne in mind, in relation to the subject of this work, what
a hundred years implies, represented as it is by a measure utterly
insignificant in a hall of the above dimensions. Several eminent
breeders, during a single lifetime, have so largely modified some of
the higher animals, which propagate their kind much more slowly than
most of the lower animals, that they have formed what well deserves to
be called a new sub-breed. Few men have attended with due care to any
one strain for more than half a century, so that a hundred years
represents the work of two breeders in succession. It is not to be
supposed that species in a state of nature ever change so quickly as
domestic animals under the guidance of methodical selection. The
comparison would be in every way fairer with the effects which follow
from unconscious selection, that is, the preservation of the most
useful or beautiful animals, with no intention of modifying the breed;
but by this process of unconscious selection, various breeds have been
sensibly changed in the course of two or three centuries.
Species, however, probably change much more slowly, and within the same
country only a few change at the same time. This slowness follows from
all the inhabitants of the same country being already so well adapted
to each other, that new places in the polity of nature do not occur
until after long intervals, due to the occurrence of physical changes
of some kind, or through the immigration of new forms. Moreover,
variations or individual differences of the right nature, by which some
of the inhabitants might be better fitted to their new places under the
altered circumstance, would not always occur at once. Unfortunately we
have no means of determining, according to the standard of years, how
long a period it takes to modify a species; but to the subject of time
we must return.
_On the Poorness of Palæontological Collections._
Now let us turn to our richest museums, and what a paltry display we
behold! That our collections are imperfect is admitted by every one.
The remark of that admirable palæontologist, Edward Forbes, should
never be forgotten, namely, that very many fossil species are known and
named from single and often broken specimens, or from a few specimens
collected on some one spot. Only a small portion of the surface of the
earth has been geologically explored, and no part with sufficient care,
as the important discoveries made every year in Europe prove. No
organism wholly soft can be preserved. Shells and bones decay and
disappear when left on the bottom of the sea, where sediment is not
accumulating. We probably take a quite erroneous view, when we assume
that sediment is being deposited over nearly the whole bed of the sea,
at a rate sufficiently quick to embed and preserve fossil remains.
Throughout an enormously large proportion of the ocean, the bright blue
tint of the water bespeaks its purity. The many cases on record of a
formation conformably covered, after an immense interval of time, by
another and later formation, without the underlying bed having suffered
in the interval any wear and tear, seem explicable only on the view of
the bottom of the sea not rarely lying for ages in an unaltered
condition. The remains which do become embedded, if in sand or gravel,
will, when the beds are upraised, generally be dissolved by the
percolation of rain water charged with carbonic acid. Some of the many
kinds of animals which live on the beach between high and low water
mark seem to be rarely preserved. For instance, the several species of
the Chthamalinæ (a sub-family of sessile cirripedes) coat the rocks all
over the world in infinite numbers: they are all strictly littoral,
with the exception of a single Mediterranean species, which inhabits
deep water and this has been found fossil in Sicily, whereas not one
other species has hitherto been found in any tertiary formation: yet it
is known that the genus Chthamalus existed during the Chalk period.
Lastly, many great deposits, requiring a vast length of time for their
accumulation, are entirely destitute of organic remains, without our
being able to assign any reason: one of the most striking instances is
that of the Flysch formation, which consists of shale and sandstone,
several thousand, occasionally even six thousand feet in thickness, and
extending for at least 300 miles from Vienna to Switzerland; and
although this great mass has been most carefully searched, no fossils,
except a few vegetable remains, have been found.
With respect to the terrestrial productions which lived during the
Secondary and Palæozoic periods, it is superfluous to state that our
evidence is fragmentary in an extreme degree. For instance, until
recently not a land-shell was known belonging to either of these vast
periods, with the exception of one species discovered by Sir C. Lyell
and Dr. Dawson in the carboniferous strata of North America; but now
land-shells have been found in the lias. In regard to mammiferous
remains, a glance at the historical table published in Lyell’s Manual,
will bring home the truth, how accidental and rare is their
preservation, far better than pages of detail. Nor is their rarity
surprising, when we remember how large a proportion of the bones of
tertiary mammals have been discovered either in caves or in lacustrine
deposits; and that not a cave or true lacustrine bed is known belonging
to the age of our secondary or palæozoic formations.
But the imperfection in the geological record largely results from
another and more important cause than any of the foregoing; namely,
from the several formations being separated from each other by wide
intervals of time. This doctrine has been emphatically admitted by many
geologists and palæontologists, who, like E. Forbes, entirely
disbelieve in the change of species. When we see the formations
tabulated in written works, or when we follow them in nature, it is
difficult to avoid believing that they are closely consecutive. But we
know, for instance, from Sir R. Murchison’s great work on Russia, what
wide gaps there are in that country between the superimposed
formations; so it is in North America, and in many other parts of the
world. The most skilful geologist, if his attention had been confined
exclusively to these large territories, would never have suspected that
during the periods which were blank and barren in his own country,
great piles of sediment, charged with new and peculiar forms of life,
had elsewhere been accumulated. And if, in every separate territory,
hardly any idea can be formed of the length of time which has elapsed
between the consecutive formations, we may infer that this could
nowhere be ascertained. The frequent and great changes in the
mineralogical composition of consecutive formations, generally implying
great changes in the geography of the surrounding lands, whence the
sediment was derived, accord with the belief of vast intervals of time
having elapsed between each formation.
We can, I think, see why the geological formations of each region are
almost invariably intermittent; that is, have not followed each other
in close sequence. Scarcely any fact struck me more when examining many
hundred miles of the South American coasts, which have been upraised
several hundred feet within the recent period, than the absence of any
recent deposits sufficiently extensive to last for even a short
geological period. Along the whole west coast, which is inhabited by a
peculiar marine fauna, tertiary beds are so poorly developed that no
record of several successive and peculiar marine faunas will probably
be preserved to a distant age. A little reflection will explain why,
along the rising coast of the western side of South America, no
extensive formations with recent or tertiary remains can anywhere be
found, though the supply of sediment must for ages have been great,
from the enormous degradation of the coast rocks and from the muddy
streams entering the sea. The explanation, no doubt, is that the
littoral and sub-littoral deposits are continually worn away, as soon
as they are brought up by the slow and gradual rising of the land
within the grinding action of the coast-waves.
We may, I think, conclude that sediment must be accumulated in
extremely thick, solid, or extensive masses, in order to withstand the
incessant action of the waves, when first upraised and during
subsequent oscillations of level, as well as the subsequent subaërial
degradation. Such thick and extensive accumulations of sediment may be
formed in two ways; either in profound depths of the sea, in which case
the bottom will not be inhabited by so many and such varied forms of
life as the more shallow seas; and the mass when upraised will give an
imperfect record of the organisms which existed in the neighbourhood
during the period of its accumulation. Or sediment may be deposited to
any thickness and extent over a shallow bottom, if it continue slowly
to subside. In this latter case, as long as the rate of subsidence and
supply of sediment nearly balance each other, the sea will remain
shallow and favourable for many and varied forms, and thus a rich
fossiliferous formation, thick enough, when upraised, to resist a large
amount of denudation, may be formed.
I am convinced that nearly all our ancient formations, which are
throughout the greater part of their thickness _rich in fossils_, have
thus been formed during subsidence. Since publishing my views on this
subject in 1845, I have watched the progress of geology, and have been
surprised to note how author after author, in treating of this or that
great formation, has come to the conclusion that it was accumulated
during subsidence. I may add, that the only ancient tertiary formation
on the west coast of South America, which has been bulky enough to
resist such degradation as it has as yet suffered, but which will
hardly last to a distant geological age, was deposited during a
downward oscillation of level, and thus gained considerable thickness.
All geological facts tell us plainly that each area has undergone
numerous slow oscillations of level, and apparently these oscillations
have affected wide spaces. Consequently, formations rich in fossils and
sufficiently thick and extensive to resist subsequent degradation, will
have been formed over wide spaces during periods of subsidence, but
only where the supply of sediment was sufficient to keep the sea
shallow and to embed and preserve the remains before they had time to
decay. On the other hand, as long as the bed of the sea remained
stationary, _thick_ deposits cannot have been accumulated in the
shallow parts, which are the most favourable to life. Still less can
this have happened during the alternate periods of elevation; or, to
speak more accurately, the beds which were then accumulated will
generally have been destroyed by being upraised and brought within the
limits of the coast-action.
These remarks apply chiefly to littoral and sublittoral deposits. In
the case of an extensive and shallow sea, such as that within a large
part of the Malay Archipelago, where the depth varies from thirty or
forty to sixty fathoms, a widely extended formation might be formed
during a period of elevation, and yet not suffer excessively from
denudation during its slow upheaval; but the thickness of the formation
could not be great, for owing to the elevatory movement it would be
less than the depth in which it was formed; nor would the deposit be
much consolidated, nor be capped by overlying formations, so that it
would run a good chance of being worn away by atmospheric degradation
and by the action of the sea during subsequent oscillations of level.
It has, however, been suggested by Mr. Hopkins, that if one part of the
area, after rising and before being denuded, subsided, the deposit
formed during the rising movement, though not thick, might afterwards
become protected by fresh accumulations, and thus be preserved for a
long period.
Mr. Hopkins also expresses his belief that sedimentary beds of
considerable horizontal extent have rarely been completely destroyed.
But all geologists, excepting the few who believe that our present
metamorphic schists and plutonic rocks once formed the primordial
nucleus of the globe, will admit that these latter rocks have been
stripped of their covering to an enormous extent. For it is scarcely
possible that such rocks could have been solidified and crystallised
while uncovered; but if the metamorphic action occurred at profound
depths of the ocean, the former protecting mantle of rock may not have
been very thick. Admitting then that gneiss, mica-schist, granite,
diorite, &c., were once necessarily covered up, how can we account for
the naked and extensive areas of such rocks in many parts of the world,
except on the belief that they have subsequently been completely
denuded of all overlying strata? That such extensive areas do exist
cannot be doubted: the granitic region of Parime is described by
Humboldt as being at least nineteen times as large as Switzerland.
South of the Amazon, Boue colours an area composed of rocks of this
nature as equal to that of Spain, France, Italy, part of Germany, and
the British Islands, all conjoined. This region has not been carefully
explored, but from the concurrent testimony of travellers, the granitic
area is very large: thus Von Eschwege gives a detailed section of these
rocks, stretching from Rio de Janeiro for 260 geographical miles inland
in a straight line; and I travelled for 150 miles in another direction,
and saw nothing but granitic rocks. Numerous specimens, collected along
the whole coast, from near Rio de Janeiro to the mouth of the Plata, a
distance of 1,100 geographical miles, were examined by me, and they all
belonged to this class. Inland, along the whole northern bank of the
Plata, I saw, besides modern tertiary beds, only one small patch of
slightly metamorphosed rock, which alone could have formed a part of
the original capping of the granitic series. Turning to a well-known
region, namely, to the United States and Canada, as shown in Professor
H.D. Rogers’ beautiful map, I have estimated the areas by cutting out
and weighing the paper, and I find that the metamorphic (excluding the
“semi-metamorphic”) and granite rocks exceed, in the proportion of 19
to 12.5, the whole of the newer Palæozoic formations. In many regions
the metamorphic and granite rocks would be found much more widely
extended than they appear to be, if all the sedimentary beds were
removed which rest unconformably on them, and which could not have
formed part of the original mantle under which they were crystallised.
Hence, it is probable that in some parts of the world whole formations
have been completely denuded, with not a wreck left behind.
One remark is here worth a passing notice. During periods of elevation
the area of the land and of the adjoining shoal parts of the sea will
be increased and new stations will often be formed—all circumstances
favourable, as previously explained, for the formation of new varieties
and species; but during such periods there will generally be a blank in
the geological record. On the other hand, during subsidence, the
inhabited area and number of inhabitants will decrease (excepting on
the shores of a continent when first broken up into an archipelago),
and consequently during subsidence, though there will be much
extinction, few new varieties or species will be formed; and it is
during these very periods of subsidence that the deposits which are
richest in fossils have been accumulated.
_On the Absence of Numerous Intermediate Varieties in any Single
Formation._
From these several considerations it cannot be doubted that the
geological record, viewed as a whole, is extremely imperfect; but if we
confine our attention to any one formation, it becomes much more
difficult to understand why we do not therein find closely graduated
varieties between the allied species which lived at its commencement
and at its close. Several cases are on record of the same species
presenting varieties in the upper and lower parts of the same
formation. Thus Trautschold gives a number of instances with Ammonites,
and Hilgendorf has described a most curious case of ten graduated forms
of Planorbis multiformis in the successive beds of a fresh-water
formation in Switzerland. Although each formation has indisputably
required a vast number of years for its deposition, several reasons can
be given why each should not commonly include a graduated series of
links between the species which lived at its commencement and close,
but I cannot assign due proportional weight to the following
considerations.
Although each formation may mark a very long lapse of years, each
probably is short compared with the period requisite to change one
species into another. I am aware that two palæontologists, whose
opinions are worthy of much deference, namely Bronn and Woodward, have
concluded that the average duration of each formation is twice or
thrice as long as the average duration of specific forms. But
insuperable difficulties, as it seems to me, prevent us from coming to
any just conclusion on this head. When we see a species first appearing
in the middle of any formation, it would be rash in the extreme to
infer that it had not elsewhere previously existed. So again, when we
find a species disappearing before the last layers have been deposited,
it would be equally rash to suppose that it then became extinct. We
forget how small the area of Europe is compared with the rest of the
world; nor have the several stages of the same formation throughout
Europe been correlated with perfect accuracy.
We may safely infer that with marine animals of all kinds there has
been a large amount of migration due to climatal and other changes; and
when we see a species first appearing in any formation, the probability
is that it only then first immigrated into that area. It is well known,
for instance, that several species appear somewhat earlier in the
palæozoic beds of North America than in those of Europe; time having
apparently been required for their migration from the American to the
European seas. In examining the latest deposits, in various quarters of
the world, it has everywhere been noted, that some few still existing
species are common in the deposit, but have become extinct in the
immediately surrounding sea; or, conversely, that some are now abundant
in the neighbouring sea, but are rare or absent in this particular
deposit. It is an excellent lesson to reflect on the ascertained amount
of migration of the inhabitants of Europe during the glacial epoch,
which forms only a part of one whole geological period; and likewise to
reflect on the changes of level, on the extreme change of climate, and
on the great lapse of time, all included within this same glacial
period. Yet it may be doubted whether, in any quarter of the world,
sedimentary deposits, _including fossil remains_, have gone on
accumulating within the same area during the whole of this period. It
is not, for instance, probable that sediment was deposited during the
whole of the glacial period near the mouth of the Mississippi, within
that limit of depth at which marine animals can best flourish: for we
know that great geographical changes occurred in other parts of America
during this space of time. When such beds as were deposited in shallow
water near the mouth of the Mississippi during some part of the glacial
period shall have been upraised, organic remains will probably first
appear and disappear at different levels, owing to the migrations of
species and to geographical changes. And in the distant future, a
geologist, examining these beds, would be tempted to conclude that the
average duration of life of the embedded fossils had been less than
that of the glacial period, instead of having been really far greater,
that is, extending from before the glacial epoch to the present day.
In order to get a perfect gradation between two forms in the upper and
lower parts of the same formation, the deposit must have gone on
continuously accumulating during a long period, sufficient for the slow
process of modification; hence, the deposit must be a very thick one;
and the species undergoing change must have lived in the same district
throughout the whole time. But we have seen that a thick formation,
fossiliferous throughout its entire thickness, can accumulate only
during a period of subsidence; and to keep the depth approximately the
same, which is necessary that the same marine species may live on the
same space, the supply of sediment must nearly counterbalance the
amount of subsidence. But this same movement of subsidence will tend to
submerge the area whence the sediment is derived, and thus diminish the
supply, whilst the downward movement continues. In fact, this nearly
exact balancing between the supply of sediment and the amount of
subsidence is probably a rare contingency; for it has been observed by
more than one palæontologist that very thick deposits are usually
barren of organic remains, except near their upper or lower limits.
It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation
composed of beds of widely different mineralogical composition, we may
reasonably suspect that the process of deposition has been more or less
interrupted. Nor will the closest inspection of a formation give us any
idea of the length of time which its deposition may have consumed. Many
instances could be given of beds, only a few feet in thickness,
representing formations which are elsewhere thousands of feet in
thickness, and which must have required an enormous period for their
accumulation; yet no one ignorant of this fact would have even
suspected the vast lapse of time represented by the thinner formation.
Many cases could be given of the lower beds of a formation having been
upraised, denuded, submerged, and then re-covered by the upper beds of
the same formation—facts, showing what wide, yet easily overlooked,
intervals have occurred in its accumulation. In other cases we have the
plainest evidence in great fossilised trees, still standing upright as
they grew, of many long intervals of time and changes of level during
the process of deposition, which would not have been suspected, had not
the trees been preserved: thus Sir C. Lyell and Dr. Dawson found
carboniferous beds 1,400 feet thick in Nova Scotia, with ancient
root-bearing strata, one above the other, at no less than sixty-eight
different levels. Hence, when the same species occurs at the bottom,
middle, and top of a formation, the probability is that it has not
lived on the same spot during the whole period of deposition, but has
disappeared and reappeared, perhaps many times, during the same
geological period. Consequently if it were to undergo a considerable
amount of modification during the deposition of any one geological
formation, a section would not include all the fine intermediate
gradations which must on our theory have existed, but abrupt, though
perhaps slight, changes of form.
It is all-important to remember that naturalists have no golden rule by
which to distinguish species and varieties; they grant some little
variability to each species, but when they meet with a somewhat greater
amount of difference between any two forms, they rank both as species,
unless they are enabled to connect them together by the closest
intermediate gradations; and this, from the reasons just assigned, we
can seldom hope to effect in any one geological section. Supposing B
and C to be two species, and a third, A, to be found in an older and
underlying bed; even if A were strictly intermediate between B and C,
it would simply be ranked as a third and distinct species, unless at
the same time it could be closely connected by intermediate varieties
with either one or both forms. Nor should it be forgotten, as before
explained, that A might be the actual progenitor of B and C, and yet
would not necessarily be strictly intermediate between them in all
respects. So that we might obtain the parent-species and its several
modified descendants from the lower and upper beds of the same
formation, and unless we obtained numerous transitional gradations, we
should not recognise their blood-relationship, and should consequently
rank them as distinct species.
It is notorious on what excessively slight differences many
palæontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking many of the
very fine species of D’Orbigny and others into the rank of varieties;
and on this view we do find the kind of evidence of change which on the
theory we ought to find. Look again at the later tertiary deposits,
which include many shells believed by the majority of naturalists to be
identical with existing species; but some excellent naturalists, as
Agassiz and Pictet, maintain that all these tertiary species are
specifically distinct, though the distinction is admitted to be very
slight; so that here, unless we believe that these eminent naturalists
have been misled by their imaginations, and that these late tertiary
species really present no difference whatever from their living
representatives, or unless we admit, in opposition to the judgment of
most naturalists, that these tertiary species are all truly distinct
from the recent, we have evidence of the frequent occurrence of slight
modifications of the kind required. If we look to rather wider
intervals of time, namely, to distinct but consecutive stages of the
same great formation, we find that the embedded fossils, though
universally ranked as specifically different, yet are far more closely
related to each other than are the species found in more widely
separated formations; so that here again we have undoubted evidence of
change in the direction required by the theory; but to this latter
subject I shall return in the following chapter.
With animals and plants that propagate rapidly and do not wander much,
there is reason to suspect, as we have formerly seen, that their
varieties are generally at first local; and that such local varieties
do not spread widely and supplant their parent-form until they have
been modified and perfected in some considerable degree. According to
this view, the chance of discovering in a formation in any one country
all the early stages of transition between any two forms, is small, for
the successive changes are supposed to have been local or confined to
some one spot. Most marine animals have a wide range; and we have seen
that with plants it is those which have the widest range, that oftenest
present varieties, so that, with shells and other marine animals, it is
probable that those which had the widest range, far exceeding the
limits of the known geological formations in Europe, have oftenest
given rise, first to local varieties and ultimately to new species; and
this again would greatly lessen the chance of our being able to trace
the stages of transition in any one geological formation.
It is a more important consideration, leading to the same result, as
lately insisted on by Dr. Falconer, namely, that the period during
which each species underwent modification, though long as measured by
years, was probably short in comparison with that during which it
remained without undergoing any change.
It should not be forgotten, that at the present day, with perfect
specimens for examination, two forms can seldom be connected by
intermediate varieties, and thus proved to be the same species, until
many specimens are collected from many places; and with fossil species
this can rarely be done. We shall, perhaps, best perceive the
improbability of our being enabled to connect species by numerous,
fine, intermediate, fossil links, by asking ourselves whether, for
instance, geologists at some future period will be able to prove that
our different breeds of cattle, sheep, horses, and dogs are descended
from a single stock or from several aboriginal stocks; or, again,
whether certain sea-shells inhabiting the shores of North America,
which are ranked by some conchologists as distinct species from their
European representatives, and by other conchologists as only varieties,
are really varieties, or are, as it is called, specifically distinct.
This could be effected by the future geologist only by his discovering
in a fossil state numerous intermediate gradations; and such success is
improbable in the highest degree.
It has been asserted over and over again, by writers who believe in the
immutability of species, that geology yields no linking forms. This
assertion, as we shall see in the next chapter, is certainly erroneous.
As Sir J. Lubbock has remarked, “Every species is a link between other
allied forms.” If we take a genus having a score of species, recent and
extinct, and destroy four-fifths of them, no one doubts that the
remainder will stand much more distinct from each other. If the extreme
forms in the genus happen to have been thus destroyed, the genus itself
will stand more distinct from other allied genera. What geological
research has not revealed, is the former existence of infinitely
numerous gradations, as fine as existing varieties, connecting together
nearly all existing and extinct species. But this ought not to be
expected; yet this has been repeatedly advanced as a most serious
objection against my views.
It may be worth while to sum up the foregoing remarks on the causes of
the imperfection of the geological record under an imaginary
illustration. The Malay Archipelago is about the size of Europe from
the North Cape to the Mediterranean, and from Britain to Russia; and
therefore equals all the geological formations which have been examined
with any accuracy, excepting those of the United States of America. I
fully agree with Mr. Godwin-Austen, that the present condition of the
Malay Archipelago, with its numerous large islands separated by wide
and shallow seas, probably represents the former state of Europe, while
most of our formations were accumulating. The Malay Archipelago is one
of the richest regions in organic beings; yet if all the species were
to be collected which have ever lived there, how imperfectly would they
represent the natural history of the world!
But we have every reason to believe that the terrestrial productions of
the archipelago would be preserved in an extremely imperfect manner in
the formations which we suppose to be there accumulating. Not many of
the strictly littoral animals, or of those which lived on naked
submarine rocks, would be embedded; and those embedded in gravel or
sand would not endure to a distant epoch. Wherever sediment did not
accumulate on the bed of the sea, or where it did not accumulate at a
sufficient rate to protect organic bodies from decay, no remains could
be preserved.
Formations rich in fossils of many kinds, and of thickness sufficient
to last to an age as distant in futurity as the secondary formations
lie in the past, would generally be formed in the archipelago only
during periods of subsidence. These periods of subsidence would be
separated from each other by immense intervals of time, during which
the area would be either stationary or rising; whilst rising, the
fossiliferous formations on the steeper shores would be destroyed,
almost as soon as accumulated, by the incessant coast-action, as we now
see on the shores of South America. Even throughout the extensive and
shallow seas within the archipelago, sedimentary beds could hardly be
accumulated of great thickness during the periods of elevation, or
become capped and protected by subsequent deposits, so as to have a
good chance of enduring to a very distant future. During the periods of
subsidence, there would probably be much extinction of life; during the
periods of elevation, there would be much variation, but the geological
record would then be less perfect.
It may be doubted whether the duration of any one great period of
subsidence over the whole or part of the archipelago, together with a
contemporaneous accumulation of sediment, would _exceed_ the average
duration of the same specific forms; and these contingencies are
indispensable for the preservation of all the transitional gradations
between any two or more species. If such gradations were not all fully
preserved, transitional varieties would merely appear as so many new,
though closely allied species. It is also probable that each great
period of subsidence would be interrupted by oscillations of level, and
that slight climatical changes would intervene during such lengthy
periods; and in these cases the inhabitants of the archipelago would
migrate, and no closely consecutive record of their modifications could
be preserved in any one formation.
Very many of the marine inhabitants of the archipelago now range
thousands of miles beyond its confines; and analogy plainly leads to
the belief that it would be chiefly these far-ranging species, though
only some of them, which would oftenest produce new varieties; and the
varieties would at first be local or confined to one place, but if
possessed of any decided advantage, or when further modified and
improved, they would slowly spread and supplant their parent-forms.
When such varieties returned to their ancient homes, as they would
differ from their former state in a nearly uniform, though perhaps
extremely slight degree, and as they would be found embedded in
slightly different sub-stages of the same formation, they would,
according to the principles followed by many palæontologists, be ranked
as new and distinct species.
If then there be some degree of truth in these remarks, we have no
right to expect to find, in our geological formations, an infinite
number of those fine transitional forms, which, on our theory, have
connected all the past and present species of the same group into one
long and branching chain of life. We ought only to look for a few
links, and such assuredly we do find—some more distantly, some more
closely, related to each other; and these links, let them be ever so
close, if found in different stages of the same formation, would, by
many palæontologists, be ranked as distinct species. But I do not
pretend that I should ever have suspected how poor was the record in
the best preserved geological sections, had not the absence of
innumerable transitional links between the species which lived at the
commencement and close of each formation, pressed so hardly on my
theory.
_On the sudden Appearance of whole Groups of allied Species._
The abrupt manner in which whole groups of species suddenly appear in
certain formations, has been urged by several palæontologists—for
instance, by Agassiz, Pictet, and Sedgwick, as a fatal objection to the
belief in the transmutation of species. If numerous species, belonging
to the same genera or families, have really started into life at once,
the fact would be fatal to the theory of evolution through natural
selection. For the development by this means of a group of forms, all
of which are descended from some one progenitor, must have been an
extremely slow process; and the progenitors must have lived long before
their modified descendants. But we continually overrate the perfection
of the geological record, and falsely infer, because certain genera or
families have not been found beneath a certain stage, that they did not
exist before that stage. In all cases positive palæontological evidence
may be implicitly trusted; negative evidence is worthless, as
experience has so often shown. We continually forget how large the
world is, compared with the area over which our geological formations
have been carefully examined; we forget that groups of species may
elsewhere have long existed, and have slowly multiplied, before they
invaded the ancient archipelagoes of Europe and the United States. We
do not make due allowance for the enormous intervals of time which have
elapsed between our consecutive formations, longer perhaps in many
cases than the time required for the accumulation of each formation.
These intervals will have given time for the multiplication of species
from some one parent-form: and in the succeeding formation, such groups
or species will appear as if suddenly created.
I may here recall a remark formerly made, namely, that it might require
a long succession of ages to adapt an organism to some new and peculiar
line of life, for instance, to fly through the air; and consequently
that the transitional forms would often long remain confined to some
one region; but that, when this adaptation had once been effected, and
a few species had thus acquired a great advantage over other organisms,
a comparatively short time would be necessary to produce many divergent
forms, which would spread rapidly and widely throughout the world.
Professor Pictet, in his excellent Review of this work, in commenting
on early transitional forms, and taking birds as an illustration,
cannot see how the successive modifications of the anterior limbs of a
supposed prototype could possibly have been of any advantage. But look
at the penguins of the Southern Ocean; have not these birds their front
limbs in this precise intermediate state of “neither true arms nor true
wings?” Yet these birds hold their place victoriously in the battle for
life; for they exist in infinite numbers and of many kinds. I do not
suppose that we here see the real transitional grades through which the
wings of birds have passed; but what special difficulty is there in
believing that it might profit the modified descendants of the penguin,
first to become enabled to flap along the surface of the sea like the
logger-headed duck, and ultimately to rise from its surface and glide
through the air?
I will now give a few examples to illustrate the foregoing remarks, and
to show how liable we are to error in supposing that whole groups of
species have suddenly been produced. Even in so short an interval as
that between the first and second editions of Pictet’s great work on
Palæontology, published in 1844-46 and in 1853-57, the conclusions on
the first appearance and disappearance of several groups of animals
have been considerably modified; and a third edition would require
still further changes. I may recall the well-known fact that in
geological treatises, published not many years ago, mammals were always
spoken of as having abruptly come in at the commencement of the
tertiary series. And now one of the richest known accumulations of
fossil mammals belongs to the middle of the secondary series; and true
mammals have been discovered in the new red sandstone at nearly the
commencement of this great series. Cuvier used to urge that no monkey
occurred in any tertiary stratum; but now extinct species have been
discovered in India, South America and in Europe, as far back as the
miocene stage. Had it not been for the rare accident of the
preservation of footsteps in the new red sandstone of the United
States, who would have ventured to suppose that no less than at least
thirty different bird-like animals, some of gigantic size, existed
during that period? Not a fragment of bone has been discovered in these
beds. Not long ago, palæontologists maintained that the whole class of
birds came suddenly into existence during the eocene period; but now we
know, on the authority of Professor Owen, that a bird certainly lived
during the deposition of the upper greensand; and still more recently,
that strange bird, the Archeopteryx, with a long lizard-like tail,
bearing a pair of feathers on each joint, and with its wings furnished
with two free claws, has been discovered in the oolitic slates of
Solenhofen. Hardly any recent discovery shows more forcibly than this
how little we as yet know of the former inhabitants of the world.
I may give another instance, which, from having passed under my own
eyes has much struck me. In a memoir on Fossil Sessile Cirripedes, I
stated that, from the large number of existing and extinct tertiary
species; from the extraordinary abundance of the individuals of many
species all over the world, from the Arctic regions to the equator,
inhabiting various zones of depths, from the upper tidal limits to
fifty fathoms; from the perfect manner in which specimens are preserved
in the oldest tertiary beds; from the ease with which even a fragment
of a valve can be recognised; from all these circumstances, I inferred
that, had sessile cirripedes existed during the secondary periods, they
would certainly have been preserved and discovered; and as not one
species had then been discovered in beds of this age, I concluded that
this great group had been suddenly developed at the commencement of the
tertiary series. This was a sore trouble to me, adding, as I then
thought, one more instance of the abrupt appearance of a great group of
species. But my work had hardly been published, when a skilful
palæontologist, M. Bosquet, sent me a drawing of a perfect specimen of
an unmistakable sessile cirripede, which he had himself extracted from
the chalk of Belgium. And, as if to make the case as striking as
possible, this cirripede was a Chthamalus, a very common, large, and
ubiquitous genus, of which not one species has as yet been found even
in any tertiary stratum. Still more recently, a Pyrgoma, a member of a
distinct subfamily of sessile cirripedes, has been discovered by Mr.
Woodward in the upper chalk; so that we now have abundant evidence of
the existence of this group of animals during the secondary period.
The case most frequently insisted on by palæontologists of the
apparently sudden appearance of a whole group of species, is that of
the teleostean fishes, low down, according to Agassiz, in the Chalk
period. This group includes the large majority of existing species. But
certain Jurassic and Triassic forms are now commonly admitted to be
teleostean; and even some palæozoic forms have thus been classed by one
high authority. If the teleosteans had really appeared suddenly in the
northern hemisphere at the commencement of the chalk formation, the
fact would have been highly remarkable; but it would not have formed an
insuperable difficulty, unless it could likewise have been shown that
at the same period the species were suddenly and simultaneously
developed in other quarters of the world. It is almost superfluous to
remark that hardly any fossil-fish are known from south of the equator;
and by running through Pictet’s Palæontology it will be seen that very
few species are known from several formations in Europe. Some few
families of fish now have a confined range; the teleostean fishes might
formerly have had a similarly confined range, and after having been
largely developed in some one sea, have spread widely. Nor have we any
right to suppose that the seas of the world have always been so freely
open from south to north as they are at present. Even at this day, if
the Malay Archipelago were converted into land, the tropical parts of
the Indian Ocean would form a large and perfectly enclosed basin, in
which any great group of marine animals might be multiplied; and here
they would remain confined, until some of the species became adapted to
a cooler climate, and were enabled to double the southern capes of
Africa or Australia, and thus reach other and distant seas.
From these considerations, from our ignorance of the geology of other
countries beyond the confines of Europe and the United States, and from
the revolution in our palæontological knowledge effected by the
discoveries of the last dozen years, it seems to me to be about as rash
to dogmatize on the succession of organic forms throughout the world,
as it would be for a naturalist to land for five minutes on a barren
point in Australia, and then to discuss the number and range of its
productions.
_On the sudden Appearance of Groups of allied Species in the lowest
known Fossiliferous Strata._
There is another and allied difficulty, which is much more serious. I
allude to the manner in which species belonging to several of the main
divisions of the animal kingdom suddenly appear in the lowest known
fossiliferous rocks. Most of the arguments which have convinced me that
all the existing species of the same group are descended from a single
progenitor, apply with equal force to the earliest known species. For
instance, it cannot be doubted that all the Cambrian and Silurian
trilobites are descended from some one crustacean, which must have
lived long before the Cambrian age, and which probably differed greatly
from any known animal. Some of the most ancient animals, as the
Nautilus, Lingula, &c., do not differ much from living species; and it
cannot on our theory be supposed, that these old species were the
progenitors of all the species belonging to the same groups which have
subsequently appeared, for they are not in any degree intermediate in
character.
Consequently, if the theory be true, it is indisputable that before the
lowest Cambrian stratum was deposited long periods elapsed, as long as,
or probably far longer than, the whole interval from the Cambrian age
to the present day; and that during these vast periods the world
swarmed with living creatures. Here we encounter a formidable
objection; for it seems doubtful whether the earth, in a fit state for
the habitation of living creatures, has lasted long enough. Sir W.
Thompson concludes that the consolidation of the crust can hardly have
occurred less than twenty or more than four hundred million years ago,
but probably not less than ninety-eight or more than two hundred
million years. These very wide limits show how doubtful the data are;
and other elements may have hereafter to be introduced into the
problem. Mr. Croll estimates that about sixty million years have
elapsed since the Cambrian period, but this, judging from the small
amount of organic change since the commencement of the Glacial epoch,
appears a very short time for the many and great mutations of life,
which have certainly occurred since the Cambrian formation; and the
previous one hundred and forty million years can hardly be considered
as sufficient for the development of the varied forms of life which
already existed during the Cambrian period. It is, however, probable,
as Sir William Thompson insists, that the world at a very early period
was subjected to more rapid and violent changes in its physical
conditions than those now occurring; and such changes would have tended
to induce changes at a corresponding rate in the organisms which then
existed.
To the question why we do not find rich fossiliferous deposits
belonging to these assumed earliest periods prior to the Cambrian
system, I can give no satisfactory answer. Several eminent geologists,
with Sir R. Murchison at their head, were until recently convinced that
we beheld in the organic remains of the lowest Silurian stratum the
first dawn of life. Other highly competent judges, as Lyell and E.
Forbes, have disputed this conclusion. We should not forget that only a
small portion of the world is known with accuracy. Not very long ago M.
Barrande added another and lower stage, abounding with new and peculiar
species, beneath the then known Silurian system; and now, still lower
down in the Lower Cambrian formation, Mr Hicks has found South Wales
beds rich in trilobites, and containing various molluscs and annelids.
The presence of phosphatic nodules and bituminous matter, even in some
of the lowest azotic rocks, probably indicates life at these periods;
and the existence of the Eozoon in the Laurentian formation of Canada
is generally admitted. There are three great series of strata beneath
the Silurian system in Canada, in the lowest of which the Eozoon is
found. Sir W. Logan states that their “united thickness may possibly
far surpass that of all the succeeding rocks, from the base of the
palæozoic series to the present time. We are thus carried back to a
period so remote, that the appearance of the so-called primordial fauna
(of Barrande) may by some be considered as a comparatively modern
event.” The Eozoon belongs to the most lowly organised of all classes
of animals, but is highly organised for its class; it existed in
countless numbers, and, as Dr. Dawson has remarked, certainly preyed on
other minute organic beings, which must have lived in great numbers.
Thus the words, which I wrote in 1859, about the existence of living
beings long before the Cambrian period, and which are almost the same
with those since used by Sir W. Logan, have proved true. Nevertheless,
the difficulty of assigning any good reason for the absence of vast
piles of strata rich in fossils beneath the Cambrian system is very
great. It does not seem probable that the most ancient beds have been
quite worn away by denudation, or that their fossils have been wholly
obliterated by metamorphic action, for if this had been the case we
should have found only small remnants of the formations next succeeding
them in age, and these would always have existed in a partially
metamorphosed condition. But the descriptions which we possess of the
Silurian deposits over immense territories in Russia and in North
America, do not support the view that the older a formation is the more
invariably it has suffered extreme denudation and metamorphism.
The case at present must remain inexplicable; and may be truly urged as
a valid argument against the views here entertained. To show that it
may hereafter receive some explanation, I will give the following
hypothesis. From the nature of the organic remains which do not appear
to have inhabited profound depths, in the several formations of Europe
and of the United States; and from the amount of sediment, miles in
thickness, of which the formations are composed, we may infer that from
first to last large islands or tracts of land, whence the sediment was
derived, occurred in the neighbourhood of the now existing continents
of Europe and North America. This same view has since been maintained
by Agassiz and others. But we do not know what was the state of things
in the intervals between the several successive formations; whether
Europe and the United States during these intervals existed as dry
land, or as a submarine surface near land, on which sediment was not
deposited, or as the bed of an open and unfathomable sea.
Looking to the existing oceans, which are thrice as extensive as the
land, we see them studded with many islands; but hardly one truly
oceanic island (with the exception of New Zealand, if this can be
called a truly oceanic island) is as yet known to afford even a remnant
of any palæozoic or secondary formation. Hence, we may perhaps infer,
that during the palæozoic and secondary periods, neither continents nor
continental islands existed where our oceans now extend; for had they
existed, palæozoic and secondary formations would in all probability
have been accumulated from sediment derived from their wear and tear;
and would have been at least partially upheaved by the oscillations of
level, which must have intervened during these enormously long periods.
If, then, we may infer anything from these facts, we may infer that,
where our oceans now extend, oceans have extended from the remotest
period of which we have any record; and on the other hand, that where
continents now exist, large tracts of land have existed, subjected, no
doubt, to great oscillations of level, since the Cambrian period. The
coloured map appended to my volume on Coral Reefs, led me to conclude
that the great oceans are still mainly areas of subsidence, the great
archipelagoes still areas of oscillations of level, and the continents
areas of elevation. But we have no reason to assume that things have
thus remained from the beginning of the world. Our continents seem to
have been formed by a preponderance, during many oscillations of level,
of the force of elevation. But may not the areas of preponderant
movement have changed in the lapse of ages? At a period long antecedent
to the Cambrian epoch, continents may have existed where oceans are now
spread out, and clear and open oceans may have existed where our
continents now stand. Nor should we be justified in assuming that if,
for instance, the bed of the Pacific Ocean were now converted into a
continent we should there find sedimentary formations, in recognisable
condition, older than the Cambrian strata, supposing such to have been
formerly deposited; for it might well happen that strata which had
subsided some miles nearer to the centre of the earth, and which had
been pressed on by an enormous weight of superincumbent water, might
have undergone far more metamorphic action than strata which have
always remained nearer to the surface. The immense areas in some parts
of the world, for instance in South America, of naked metamorphic
rocks, which must have been heated under great pressure, have always
seemed to me to require some special explanation; and we may perhaps
believe that we see in these large areas the many formations long
anterior to the Cambrian epoch in a completely metamorphosed and
denuded condition.
The several difficulties here discussed, namely, that, though we find
in our geological formations many links between the species which now
exist and which formerly existed, we do not find infinitely numerous
fine transitional forms closely joining them all together. The sudden
manner in which several groups of species first appear in our European
formations, the almost entire absence, as at present known, of
formations rich in fossils beneath the Cambrian strata, are all
undoubtedly of the most serious nature. We see this in the fact that
the most eminent palæontologists, namely, Cuvier, Agassiz, Barrande,
Pictet, Falconer, E. Forbes, &c., and all our greatest geologists, as
Lyell, Murchison, Sedgwick, &c., have unanimously, often vehemently,
maintained the immutability of species. But Sir Charles Lyell now gives
the support of his high authority to the opposite side, and most
geologists and palæontologists are much shaken in their former belief.
Those who believe that the geological record is in any degree perfect,
will undoubtedly at once reject my theory. For my part, following out
Lyell’s metaphor, I look at the geological record as a history of the
world imperfectly kept and written in a changing dialect. Of this
history we possess the last volume alone, relating only to two or three
countries. Of this volume, only here and there a short chapter has been
preserved, and of each page, only here and there a few lines. Each word
of the slowly-changing language, more or less different in the
successive chapters, may represent the forms of life, which are
entombed in our consecutive formations, and which falsely appear to
have been abruptly introduced. On this view the difficulties above
discussed are greatly diminished or even disappear.