The Origin of Species
Chapter 9: On the Imperfection of the Geological Record
by Charles Darwin
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.