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 the sudden appearance of groups of
species. On their sudden appearance in the lowest known fossiliferous
strata.
In the sixth chapter I enumerated the chief objections which might be
justly urged against the views maintained in this volume. Most of them
have now been discussed. One, namely the distinctness of specific
forms, and their not being blended together by innumerable transitional
links, is a very obvious difficulty. I assigned reasons why such links
do not commonly occur at the present day, under the circumstances
apparently most favourable for their presence, namely on an extensive
and continuous area with graduated physical conditions. I endeavoured
to show, that the life of each species depends in a more important
manner on the presence of other already defined organic forms, than on
climate; and, therefore, that the really governing conditions of life
do not graduate away quite insensibly like heat or moisture. I
endeavoured, also, to show that intermediate varieties, from existing
in lesser numbers than the forms which they connect, will generally be
beaten out and exterminated during the course of further modification
and improvement. The main cause, however, of innumerable intermediate
links not now occurring everywhere throughout nature depends
on the very process of natural selection, through which new varieties
continually take the places of and exterminate their parent-forms. But
just in proportion as this process of extermination has acted on an
enormous scale, so must the number of intermediate varieties, which
have formerly existed on the earth, be truly enormous. Why then is not
every geological formation and every stratum full of such intermediate
links? Geology assuredly does not reveal any such finely graduated
organic chain; and this, perhaps, is the most obvious and gravest
objection which can be urged against my theory. The explanation lies,
as I believe, in the extreme imperfection of the geological record.
In the first place it should always be borne in mind what sort of
intermediate forms must, on my theory, have formerly existed. I have
found it difficult, when looking at any two species, to avoid picturing
to myself, forms _directly_ intermediate between them. But this is a
wholly false view; we should always look for forms intermediate between
each species and a common but unknown progenitor; and the progenitor
will generally have differed in some respects from all its modified
descendants. To give a simple illustration: the fantail and pouter
pigeons have both descended from the rock-pigeon; if we possessed all
the intermediate varieties which have ever existed, we should have an
extremely close series between both and the rock-pigeon; but we should
have no varieties directly intermediate between the fantail and pouter;
none, for instance, combining a tail somewhat expanded with a crop
somewhat enlarged, the characteristic features of these two breeds.
These two breeds, moreover, have become so much modified, that if we
had no historical or indirect evidence regarding their origin, it would
not have been possible to have
determined from a mere comparison of their structure with that of the
rock-pigeon, whether they had descended from this species or from some
other allied species, such as C. oenas.
So with natural species, if we look to forms very distinct, for
instance to the horse and tapir, we have no reason to suppose that
links ever existed directly intermediate between them, but between each
and an unknown common parent. The common parent will have had in its
whole organisation much general resemblance to the tapir and to the
horse; but in some points of structure may have differed considerably
from both, even perhaps more than they differ from each other. Hence in
all such cases, we should be unable to recognise the parent-form of any
two or more species, even if we closely compared the structure of the
parent with that of its modified descendants, unless at the same time
we had a nearly perfect chain of the intermediate links.
It is just possible by my theory, that one of two living forms might
have descended from the other; for instance, a horse from a tapir; and
in this case _direct_ intermediate links will have existed between
them. But such a case would imply that one form had remained for a very
long period unaltered, whilst its descendants had undergone a vast
amount of change; and the principle of competition between organism and
organism, between child and parent, will render this a very rare event;
for in all cases the new and improved forms of life will tend to
supplant the old and unimproved forms.
By the theory of natural selection all living species have been
connected with the parent-species of each genus, by differences not
greater than we see between the 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. Professor 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 Professor 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 Professor 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. 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 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, etc., 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, etc., and all our greatest geologists, as Lyell,
Murchison, Sedgwick, etc., 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.