 Chapter
9 - 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 vast lapse of time, as inferred from the rate of
deposition and of denudation *
On the poorness of our palaeontological collections *
On the intermittence of geological formations *
On the absence of intermediate varieties in any one formation
*
On their sudden appearance in the lowest known fossiliferous
strata
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 exterminate
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 on the earth, 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 gravest 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 my 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 have 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, 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 ever existed directly intermediate between
them, 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 my 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 will tend to supplant the old and unimproved.
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 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 species; 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 this earth.
On the lapse of Time. Independently of our not
finding fossil remains of such infinitely numerous connecting
links, it may be objected, that time will not have sufficed
for so great an amount of organic change, all changes having
been effected very slowly through natural selection. It
is hardly possible for me even to recall to the reader,
who may not be 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, yet does
not admit how incomprehensibly 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 each stratum. A man must
for years examine for himself great piles of superimposed
strata, and watch the sea at work grinding down old rocks
and making fresh sediment, before he can hope to comprehend
anything of the lapse of time, the monuments of which we
see around us.
It is good to wander along lines of sea-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 reason
to believe that pure water can effect little or 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 reduced in size they
can be rolled about by the waves, and then 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.
He who most closely studies the action of the sea on our
shores, will, I believe, be most deeply impressed with the
slowness with which rocky coasts are worn away. The observations
on this head by Hugh Miller, and by that excellent observer
Mr. Smith of Jordan Hill, are most impressive. With the
mind thus impressed, let any one examine beds of conglomerate
many thousand feet in thickness, which, though probably
formed at a quicker rate than many other deposits, yet,
from being formed of worn and rounded pebbles, each of which
bears the stamp of time, are good to show how slowly the
mass has been accumulated. Let him remember Lyell's profound
remark, that the thickness and extent of sedimentary formations
are the result and measure of the degradation which the
earth's crust has elsewhere suffered. And what an amount
of degradation is implied by the sedimentary deposits of
many countries! Professor Ramsay has given me the maximum
thickness, in most cases from actual measurement, in a few
cases from estimate, of each formation in different parts
of Great Britain; and this is the result:-
Feet
Palaeozoic strata (not including igneous beds) 57,154
Secondary strata 13,190
Tertiary strata 2,240
making altogether 72,584 feet; 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, enormously long blank periods. So that the lofty
pile of sedimentary rocks in Britain, gives but an inadequate
idea of the time which has elapsed during their accumulation;
yet what time this must have consumed! Good observers have
estimated that sediment is deposited by the great Mississippi
river at the rate of only 600 feet in a hundred thousand
years. This estimate may be quite erroneous; yet, considering
over what wide spaces very fine sediment is transported
by the currents of the sea, the process of accumulation
in any one area must be extremely slow.
But the amount of denudation which the strata have in
many places suffered, independently of the rate of accumulation
of the degraded matter, probably offers the best evidence
of the lapse of time. I remember having been much struck
with the evidence of denudation, 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 still more plainly told 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, the surface of the
land has been so completely planed down by the action of
the sea, that no trace of these vast dislocations is externally
visible.
The Craven fault, for instance, extends for upwards of
30 miles, and along this line the vertical displacement
of the strata has varied from 600 to 3000 feet. Prof. Ramsay
has published an account of a downthrow in Anglesea of 2300
feet; and he informs me that he fully believes there is
one in Merionethshire of 12,000 feet; yet in these cases
there is nothing on the surface to show such prodigious
movements; the pile of rocks on the one or other side having
been smoothly swept away. The consideration of these facts
impresses my mind almost in the same manner as does the
vain endeavour to grapple with the idea of eternity.
I am tempted to give one other case, the well-known one
of the denudation of the Weald. Though it must be admitted
that the denudation of the Weald has been a mere trifle,
in comparison with that which has removed masses of our
Palaeozoic strata, in parts ten thousand feet in thickness,
as shown in Prof. Ramsay's masterly memoir on this subject.
Yet it is an admirable lesson to stand on the North Downs
and to look at the distant South Downs; for, remembering
that at no great distance to the west the northern and southern
escarpments meet and close, one can safely picture to oneself
the great dome of rocks which must have covered up the Weald
within so limited a period as since the latter part of the
Chalk formation. The distance from the northern to the southern
Downs is about 22 miles, and the thickness of the several
formations is on an average about 1100 feet, as I am informed
by Prof. Ramsay. But if, as some geologists suppose, a range
of older rocks underlies the Weald, on the flanks of which
the overlying sedimentary deposits might have accumulated
in thinner masses than elsewhere, the above estimate would
be erroneous; but this source of doubt probably would not
greatly affect the estimate as applied to the western extremity
of the district. If, then, we knew the rate at which the
sea commonly wears away a line of cliff of any given height,
we could measure the time requisite to have denuded the
Weald. This, of course, cannot be done; but we may, in order
to form some crude notion on the subject, assume that the
sea would eat into cliffs 500 feet in height at the rate
of one inch in a century. This will at first appear much
too small an allowance; but it is the same as if we were
to assume a cliff one yard in height to be eaten back along
a whole line of coast at the rate of one yard in nearly
every twenty-two years. I doubt whether any rock, even as
soft as chalk, would yield at this rate excepting on the
most exposed coasts; though no doubt the degradation of
a lofty cliff would be more rapid from the breakage of the
fallen fragments. On the other hand, I do not believe that
any line of coast, ten or twenty miles in length, ever suffers
degradation at the same time along its whole indented length;
and we must remember that almost all strata contain harder
layers or nodules, which from long resisting attrition form
a breakwater at the base. Hence, under ordinary circumstances,
I conclude that for a cliff 500 feet in height, a denudation
of one inch per century for the whole length would be an
ample allowance. At this rate, on the above data, the denudation
of the Weald must have required 306,662,400 years; or say
three hundred million years.
The action of fresh water on the gently inclined Wealden
district, when upraised, could hardly have been great, but
it would somewhat reduce the above estimate. On the other
hand, during oscillations of level, which we know this area
has undergone, the surface may have existed for millions
of years as land, and thus have escaped the action of the
sea: when deeply submerged for perhaps equally long periods,
it would, likewise, have escaped the action of the coast-waves.
So that in all probability a far longer period than 300
million years has elapsed since the latter part of the Secondary
period.
I have made these few remarks because it is highly important
for us to gain some notion, however imperfect, of the lapse
of years. During each of these years, over the whole world,
the land and the water has been peopled by hosts of living
forms. What an infinite number of generations, which the
mind cannot grasp, must have succeeded each other in the
long roll of years! Now turn to our richest geological museums,
and what a paltry display we behold!
On the poorness of our Palaeontological collections.
That our Palaeontological collections are very imperfect,
is admitted by every one. The remark of that admirable Palaeontologist,
the late Edward Forbes, should not be forgotten, namely,
that numbers of our 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 will decay and disappear
when left on the bottom of the sea, where sediment is not
accumulating. I believe we are continually taking a most
erroneous view, when we tacitly admit to ourselves 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 enormous 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.
I suspect that but few of the very many animals which live
on the beach between high and low watermark are preserved.
For instance, the several species of the Chthamalinae (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 has been found fossil in Sicily,
whereas not one other species has hitherto been found in
any tertiary formation: yet it is now known that the genus
Chthamalus existed during the chalk period. The molluscan
genus Chiton offers a partially analogous case.
With respect to the terrestrial productions which lived
during the Secondary and Palaeozoic periods, it is superfluous
to state that our evidence from fossil remains is fragmentary
in an extreme degree. For instance, not a land shell is
known belonging to either of these vast periods, with one
exception discovered by Sir C. Lyell in the carboniferous
strata of North America. I n regard to mammiferous remains,
a single glance at the historical table published in the
Supplement to 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 palaeozoic
formations.
But the imperfection in the geological record mainly 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. 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 exclusively confined 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 each
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
has been derived, accords with the belief of vast intervals
of time having elapsed between each formation.
But 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 scantily 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 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, safely 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. Such thick and extensive accumulations of sediment
may be formed in two ways; either, in profound depths of
the sea, in which case, judging from the researches of E.
Forbes, we may conclude that the bottom will be inhabited
by extremely few animals, and the mass when upraised will
give a most imperfect record of the forms of life which
then existed; or, sediment may be accumulated 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 life, and
thus a fossiliferous formation thick enough, when upraised,
to resist any amount of degradation, may be formed.
I am convinced that all our ancient formations, which
are 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 certainly 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, may 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 could not have been
accumulated in the shallow parts, which are the most favourable
to life. Still less could this have happened during the
alternate periods of elevation; or, to speak more accurately,
the beds which were then accumulated will have been destroyed
by being upraised and brought within the limits of the coast-action.
Thus the geological record will almost necessarily be
rendered intermittent. I feel much confidence in the truth
of these views, for they are in strict accordance with the
general principles inculcated by Sir C. Lyell; and E. Forbes
independently arrived at a similar conclusion.
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 most 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 the productions on the shores of a continent
when first broken up into an archipelago), and consequently
during subsidence, though there will be much extinction,
fewer new varieties or species will be formed; and it is
during these very periods of subsidence, that our great
deposits rich in fossils have been accumulated. Nature may
almost be said to have guarded against the frequent discovery
of her transitional or linking forms.
From the foregoing 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 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. Some cases
are on record of the same species presenting distinct varieties
in the upper and lower parts of the same formation, but,
as they are rare, they may be here passed over. Although
each formation has indisputably required a vast number of
years for its deposition, I can see several reasons why
each should not include a graduated series of links between
the species which then lived; but I can by no means pretend
to assign due proportional weight to the following considerations.
Although each formation may mark a very long lapse of
years, each perhaps is short compared with the period requisite
to change one species into another. I am aware that two
palaeontologists, 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 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 uppermost layers have been deposited, it would be equally
rash to suppose that it then became wholly 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.
With marine animals of all kinds, we may safely infer
a large amount of migration during 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 appeared somewhat earlier in the palaeozoic 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 of 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 period, which forms only a part of one whole
geological period; and likewise to reflect on the great
changes of level, on the inordinately great change of climate,
on the prodigious 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 flourish;
for we know what vast 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 migration
of species and to geographical changes. And in the distant
future, a geologist examining these beds, might 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 accumulating for a very long period,
in order to have given sufficient time for the slow process
of variation; hence the deposit will generally have to be
a very thick one; and the species undergoing modification
will have had to live on the same area throughout this whole
time. But we have seen that a thick fossiliferous formation
can only be accumulated during a period of subsidence; and
to keep the depth approximately the same, which is necessary
in order to enable the same species to live on the same
space, the supply of sediment must nearly have counterbalanced
the amount of subsidence. But this same movement of subsidence
will often tend to sink 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 palaeontologist, 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 different mineralogical
composition, we may reasonably suspect that the process
of deposition has been much interrupted, as a change in
the currents of the sea and a supply of sediment of a different
nature will generally have been due to geographical changes
requiring much time. Nor will the closest inspection of
a formation give any idea of the time which its deposition
has consumed. Many instances could be given of beds only
a few feet in thickness, representing formations, 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 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 never even have been suspected,
had not the trees chanced to have been preserved: thus,
Messrs Lyell and Dawson found carboniferous beds 1400 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 occur at the bottom,
middle, and top of a formation, the probability is that
they have not lived on the same spot during the whole period
of deposition, but have disappeared and reappeared, perhaps
many times, during the same geological period. So that if
such species were to undergo a considerable amount of modification
during any one geological period, a section would not probably
include all the fine intermediate gradations which must
on my theory have existed between them, but abrupt, though
perhaps very 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 close 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 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 most
closely connected with either one or both forms by intermediate
varieties. Nor should it be forgotten, as before explained,
that A might be the actual progenitor of B and C, and yet
might not at all necessarily be strictly intermediate between
them in all points of structure. So that we might obtain
the parent-species and its several modified descendants
from the lower and upper beds of a formation, and unless
we obtained numerous transitional gradations, we should
not recognise their relationship, and should consequently
be compelled to rank them all as distinct species.
It is notorious on what excessively slight differences
many palaeontologists 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 my theory
we ought to find. Moreover, if we look to rather wider intervals,
namely, to distinct but consecutive stages of the same great
formation, we find that the embedded fossils, though almost
universally ranked as specifically different, yet are far
more closely allied to each other than are the species found
in more widely separated formations; but to this subject
I shall have to return in the following chapter.
One other consideration is worth notice: with animals
and plants that can propagate rapidly and are not highly
locomotive, 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-forms 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 probably those which have had the widest range, far
exceeding the limits of the known geological formations
of Europe, which 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 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 have been collected
from many places; and in the case of fossil species this
could rarely be effected by palaeontologists. 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 have 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 only by the future geologist discovering
in a fossil state numerous intermediate gradations; and
such success seems to me improbable in the highest degree.
Geological research, though it has added numerous species
to existing and extinct genera, and has made the intervals
between some few groups less wide than they otherwise would
have been, yet has done scarcely anything in breaking down
the distinction between species, by connecting them together
by numerous, fine, intermediate varieties; and this not
having been effected, is probably the gravest and most obvious
of all the many objections which may be urged against my
views. Hence it will be worth while to sum up the foregoing
remarks, under an imaginary illustration. The Malay Archipelago
is of 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, when most of our
formations were accumulating. The Malay Archipelago is one
of the richest regions of the whole world 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
excessively imperfect manner in the formations which we
suppose to be there accumulating. I suspect that 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.
In our archipelago, I believe that fossiliferous formations
could be formed of sufficient thickness to last to an age,
as distant in futurity as the secondary formations lie in
the past, only during periods of subsidence. These periods
of subsidence would be separated from each other by enormous
intervals, during which the area would be either stationary
or rising; whilst rising, each fossiliferous formation would
be destroyed, almost as soon as accumulated, by the incessant
coast-action, as we now see on the shores of South America.
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 least 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 fully
preserved, transitional varieties would merely appear as
so many distinct species. It is, also, probable that each
great period of subsidence would be interrupted by oscillations
of level, and that slight climatal changes would intervene
during such lengthy periods; and in these cases the inhabitants
of the archipelago would have to 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
leads me to believe that it would be chiefly these far-ranging
species which would oftenest produce new varieties; and
the varieties would at first generally 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, they would, according to the principles
followed by many palaeontologists, 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 my theory assuredly 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, some more
closely, some more distantly related to each other; and
these links, let them be ever so close, if found in different
stages of the same formation, would, by most palaeontologists,
be ranked as distinct species. But I do not pretend that
I should ever have suspected how poor a record of the mutations
of life, the best preserved geological section presented,
had not the difficulty of our not discovering innumerable
transitional links between the species which appeared 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 palaeontologists, for instance, by Agassiz,
Pictet, and by none more forcibly than by Professor 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 all at once,
the fact would be fatal to the theory of descent with slow
modification through natural selection. For the development
of a group of forms, all of which have descended from some
one progenitor, must have been an extremely slow process;
and the progenitors must have lived long ages before their
modified descendants. But we continually over-rate 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. 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 of the United States.
We do not make due allowance for the enormous intervals
of time, which have probably elapsed between our consecutive
formations, longer perhaps in some 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 or some few parent-forms; and in the succeeding
formation such 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; but that when this had 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 be able to
spread rapidly and widely throughout the world.
I will now give a few examples to illustrate these remarks;
and to show how liable we are to error in supposing that
whole groups of species have suddenly been produced. I may
recall the well-known fact that in geological treatises,
published not many years ago, the great class of mammals
was 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 one true mammal has 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 even as
far back as the eocene stage. The most striking case, however,
is that of the Whale family; as these animals have huge
bones, are marine, and range over the world, the fact of
not a single bone of a whale having been discovered in any
secondary formation, seemed fully to justify the belief
that this great and distinct order had been suddenly produced
in the interval between the latest secondary and earliest
tertiary formation. But now we may read in the Supplement
to Lyell's 'Manual,' published in 1858, clear evidence of
the existence of whales in the upper greensand, some time
before the close of the secondary period.
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 have stated that, from the 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 50
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 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 thought one more instance of the abrupt appearance
of a great group of species. But my work had hardly been
published, when a skilful palaeontologist, M. Bosquet, sent
me a drawing of a perfect specimen of an unmistakeable sessile
cirripede, which he had himself extracted from the chalk
of Belgium. And, as if to make the case as striking as possible,
this sessile cirripede was a Chthamalus, a very common,
large, and ubiquitous genus, of which not one specimen has
as yet been found even in any tertiary stratum. Hence we
now positively know that sessile cirripedes existed during
the secondary period; and these cirripedes might have been
the progenitors of our many tertiary and existing species.
The case most frequently insisted on by palaeontologists
of the apparently sudden appearance of a whole group of
species, is that of the teleostean fishes, low down in the
Chalk period. This group includes the large majority of
existing species. Lately, Professor Pictet has carried their
existence one sub-stage further back; and some palaeontologists
believe that certain much older fishes, of which the affinities
are as yet imperfectly known, are really teleostean. Assuming,
however, that the whole of them did appear, as Agassiz believes,
at the commencement of the chalk formation, the fact would
certainly be highly remarkable; but I cannot see that it
would be an insuperable difficulty on my theory, unless
it could likewise be shown that the species of this group
appeared suddenly and simultaneously throughout the world
at this same period. It is almost superfluous to remark
that hardly any fossil-fish are known from south of the
equator; and by running through Pictet's palaeontology 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 fish might formerly have
had a similarly confined range, and after having been largely
developed in some one sea, might 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 and similar considerations, but chiefly from
our ignorance of the geology of other countries beyond the
confines of Europe and the United States; and from the revolution
in our palaeontological ideas on many points, which the
discoveries of even the last dozen years have effected,
it seems to me to be about as rash in us to dogmatize on
the succession of organic beings throughout the world, as
it would be for a naturalist to land for five minutes on
some one 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 graver. I allude to
the manner in which numbers of species of the same group,
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 have descended from one
progenitor, apply with nearly equal force to the earliest
known species. For instance, I cannot doubt that all the
Silurian trilobites have descended from some one crustacean,
which must have lived long before the Silurian age, and
which probably differed greatly from any known animal. Some
of the most ancient Silurian animals, as the Nautilus, Lingula,
&c., do not differ much from living species; and it
cannot on my theory be supposed, that these old species
were the progenitors of all the species of the orders to
which they belong, for they do not present characters in
any degree intermediate between them. If, moreover, they
had been the progenitors of these orders, they would almost
certainly have been long ago supplanted and exterminated
by their numerous and improved descendants.
Consequently, if my theory be true, it is indisputable
that before the lowest Silurian stratum was deposited, long
periods elapsed, as long as, or probably far longer than,
the whole interval from the Silurian age to the present
day; and that during these vast, yet quite unknown, periods
of time, the world swarmed with living creatures.
To the question why we do not find records of these vast
primordial periods, I can give no satisfactory answer. Several
of the most eminent geologists, with Sir R. Murchison at
their head, are convinced that we see in the organic remains
of the lowest Silurian stratum the dawn of life on this
planet. Other highly competent judges, as Lyell and the
late E. Forbes, dispute this conclusion. We should not forget
that only a small portion of the world is known with accuracy.
M. Barrande has lately added another and lower stage to
the Silurian system, abounding with new and peculiar species.
Traces of life have been detected in the Longmynd beds beneath
Barrande's so-called primordial zone. The presence of phosphatic
nodules and bituminous matter in some of the lowest azoic
rocks, probably indicates the former existence of life at
these periods. But the difficulty of understanding the absence
of vast piles of fossiliferous strata, which on my theory
no doubt were somewhere accumulated before the Silurian
epoch, is very great. If these most ancient beds had been
wholly worn away by denudation, or obliterated by metamorphic
action, we ought to find only small remnants of the formations
next succeeding them in age, and these ought to be very
generally in a metamorphosed condition. But the descriptions
which we now 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 it has
suffered the extremity of 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 existing continents
of Europe and North America. But we do not know what was
the state of things in the intervals between the 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 again
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
not one oceanic island is as yet known to afford even a
remnant of any palaeozoic or secondary formation. Hence
we may perhaps infer, that during the palaeozoic and secondary
periods, neither continents nor continental islands existed
where our oceans now extend; for had they existed there,
palaeozoic 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 we may fairly conclude
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 earliest silurian 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 have we any right
to assume that things have thus remained from eternity?
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 immeasurably antecedent
to the silurian 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 formations older than the silurian
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 bare 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 silurian epoch in a completely metamorphosed
condition.
The several difficulties here discussed, namely our not
finding in the successive formations infinitely numerous
transitional links between the many species which now exist
or have existed; the sudden manner in which whole groups
of species appear in our European formations; the almost
entire absence, as at present known, of fossiliferous formations
beneath the Silurian strata, are all undoubtedly of the
gravest nature. We see this in the plainest manner by the
fact that all the most eminent palaeontologists, namely
Cuvier, Owen, Agassiz, Barrande, Falconer, E. Forbes, &c.,
and all our greatest geologists, as Lyell, Murchison, Sedgwick,
&c., have unanimously, often vehemently, maintained
the immutability of species. But I have reason to believe
that one great authority, Sir Charles Lyell, from further
reflexion entertains grave doubts on this subject. I feel
how rash it is to differ from these great authorities, to
whom, with others, we owe all our knowledge. Those who think
the natural geological record in any degree perfect, and
who do not attach much weight to the facts and arguments
of other kinds even in this volume, will undoubtedly at
once reject my theory. For my part, following out Lyell's
metaphor, I look at the natural 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, in which the history
is supposed to be written, being more or less different
in the interrupted succession of chapters, may represent
the apparently abruptly changed forms of life, entombed
in our consecutive, but widely separated formations. On
this view, the difficulties above discussed are greatly
diminished, or even disappear.
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