Chapter IX On the
Imperfection of the Geological Record
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.
2 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.
3 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.
4 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 forms.
5 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.
6 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.
7 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.
8 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:-
9 Feet Palaeozoic strata (not including igneous
beds)..57,154 Secondary strata................................13,190 Tertiary
strata..................................2,240
10 --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.
11 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.
12 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.
13 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.
14 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.
15 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!
16 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.
17 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. In 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.
18 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.
19 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.
20 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.
21 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.
22 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.
23 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.
24 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.
25 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.
26 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.
27 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.
28 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.
29 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.
30 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.
31 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.
32 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.
33 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.
34 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!
35 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.
36 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.
37 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.
38 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.
39 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.
40 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.
41 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.
42 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.
43 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.
44 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.
45 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.
46 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.
47 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.
48 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.
49 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.
50 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.
51 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 given 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.