On the Origin of Species by Means of Natural Selection (1859)
by Charles Darwin
CHAPTER IX.
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 palæontological
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.
N 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. |
| Palæozoic 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 Palæozoic 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 Palæontological collections. That our
Palæontological collections are very imperfect, is admitted by every
one. The remark of that admirable Palæontologist, 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 Chthamalinæ
(a sub-family of sessile cirripedes) coat the rocks all over the world
in infinite numbers: they are all strictly littoral, with the
exception of a single Mediterranean species, which inhabits deep water
and 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 Palæozoic 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
palæozoic 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 palæontologists, whose
opinions are worthy of much deference, namely Bronn and Woodward, have
concluded that the average duration of each formation is twice or
thrice as long as the average duration of specific forms. But
insuperable difficulties, as it seems to me, prevent us 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
palæozoic beds of North America than in those of Europe; time having
apparently been required for their migration from the American to the
European seas. In examining the latest deposits 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
palæontologist, that very thick deposits are usually barren of
organic remains, except near their upper or lower limits.
It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation
composed of beds of 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
palæontologists have founded their species; and they do this the more
readily if the specimens come from different sub-stages of the same
formation. Some experienced conchologists are now sinking many of the
very fine species of D'Orbigny and others into the rank of varieties;
and on this view we do find the kind of evidence of change which on 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 palæontologists.
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
palæontologists, be ranked as new and distinct species.
If then, there be some degree of truth in these remarks, we have no
right to expect to find in our geological formations, an infinite
number of those fine transitional forms, which on 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 palæontologists, 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 palæontologists,
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
palæontologist, 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 palæontologists 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 palæontologists 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 palæontology it will be seen
that very few species are known from several formations in
Europe. Some few families of fish now have a confined range; the
teleostean 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 palæontological
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 palæozoic or
secondary formation. Hence we may perhaps infer, that during the
palæozoic and secondary periods, neither continents nor continental
islands existed where our oceans now extend; for had they existed
there, palæozoic and secondary formations would in all probability
have been accumulated from sediment derived from their wear and tear;
and would have been at least partially upheaved by the oscillations of
level, which 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 palæontologists, 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.
[ Charles Darwin,
On
the Origin Of Species: A Facsimile of the First Edition,
Cambridge, Massachusetts: Harvard University Press, 1964, pp. 273-311. ]
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