CHAPTER X - ON THE IMPERFECTION
OF THE GEOLOGICAL RECORD
On the Absence of Numerous Intermediate Varieties in any Single
Formation
From these several 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 much 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. Several cases are on record of the same species
presenting varieties in the upper and lower parts of the same
formation; thus, Trautschold gives a number of instances with
ammonites; and Hilgendorf has described a most curious case of ten
graduated forms of Planorbis multiformis in the successive beds of a
fresh-water formation in Switzerland. Although each formation has
indisputably required a vast number of years for its deposition,
several reasons can be given why each should not commonly include a
graduated series of links between the species which lived at its
commencement and close; but I cannot assign due proportional weight to
the following considerations.
Although each formation may mark a very long lapse of years, each
probably 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 from 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 last layers
have been deposited, it would be equally rash to suppose that it
then became 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.
We may safely infer that with marine animals of all kinds there
has been a large amount of migration due to 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 appear
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 in 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 epoch, which forms only a part of one
whole geological period; and likewise to reflect on the changes of
level, on the extreme change of climate, and on the great 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 best flourish: for we know that great
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 migrations of
species and to geographical changes. And in the distant future, a
geologist, examining these beds, would 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
continuously accumulating during a long period, sufficient for the
slow process of modification; hence the deposit must be a very thick
one; and the species, undergoing change must have lived in the same
district throughout the whole time. But we have seen that a thick
formation, fossiliferous throughout its entire thickness, can
accumulate only during a period of subsidence; and to keep the depth
approximately the same, which is necessary that the same marine
species may live on the same space, the supply of sediment must nearly
counterbalance the amount of subsidence. But this same movement of
subsidence will tend to submerge the area whence the sediment is
derived, and thus diminish the supply, whilst the downward movement
continues. In fact, this nearly exact balancing between the supply
of sediment and the amount of subsidence is probably a rare
contingency; for it has been observed by more than one
palaeontologist, that very thick deposits are usually barren of
organic remains, except near their upper or lower limits.
It would seem that each separate formation, like the whole pile of
formations in any country, has generally been intermittent in its
accumulation. When we see, as is so often the case, a formation
composed of beds of widely different mineralogical composition, we may
reasonably suspect that the process of deposition has been more or
less interrupted. Nor will the closest inspection of a formation
give us any idea of the length of time which its deposition may have
consumed. Many instances could be given of beds only a few feet in
thickness, representing formations, which are 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 even
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 not
have been suspected, had not the trees been preserved: thus Sir C.
Lyell and Dr. 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 occurs at the bottom, middle, and top of a formation, the
probability is that it has not lived on the same spot during the whole
period of deposition, but has disappeared and reappeared, perhaps many
times, during the same geological period. Consequently if it were to
undergo a considerable amount of modification during the deposition of
any one geological formation, a section would not include all the fine
intermediate gradations which must on our theory have existed, but
abrupt, though perhaps 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 the
closest 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 older and 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 closely
connected by intermediate varieties with either one or both forms. Nor
should it be forgotten, as before explained, that A might be the
actual progenitor of B and C, and yet would not necessarily be
strictly intermediate between them in all respects. So that we might
obtain the parent-species, and its several modified descendants from
the lower and upper beds of the same formation, and unless we obtained
numerous transitional gradations, we should not recognise their
blood-relationship, and should consequently rank them as distinct
species.
It is notorious on what excessively slight differences many
palaeontologists have founded their species; and they do this the more
readily if the specimens come from different substages 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 the theory we ought to find. Look again at the later
tertiary deposits, which include many shells believed by the
majority of naturalists to be identical with existing species; but
some excellent naturalists as Agassiz and Pictet, maintain that all
these tertiary species are specifically distinct, though the
distinction is admitted to be very slight; so that here, unless we
believe that these eminent naturalists have been misled by their
imaginations, and that these late tertiary species really present no
difference whatever from their living. representatives, or unless we
admit, in opposition to the judgment of most naturalists, that these
tertiary species are all truly distinct from the recent, we have
evidence of the frequent occurrence of slight modifications of the
kind required. It we look to rather wider intervals of time, namely,
to distinct but consecutive stages of the same great formation, we
find that the embedded fossils, though universally ranked as
specifically different, yet are far more closely related to each other
than are the species found in more widely separated formations; so
that here again we have undoubted evidence of change in the
direction required by the theory; but to this latter subject I shall
return in the following chapter.
With animals and plants that propagate rapidly and do not wander
much, 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 probable that those which had
the widest range, far exceeding the limits of the known geological
formations in Europe, 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 trace the stages of
transition in any one geological formation.
It is a more important consideration, leading to the same result, as
lately insisted on by Dr. Falconer, namely, that the period during
which each species underwent modification, though long as measured
by years, was probably short in comparison with that during which it
remained without undergoing any change.
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 are collected from many places; and with fossil
species this can rarely be done. 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 are
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 by the future
geologist only by his discovering in a fossil state numerous
intermediate gradations; and such success is improbable in the highest
degree.
It has been asserted over and over again, by writers who believe
in the immutability of species, that geology yields no linking
forms. This assertion, as we shall see in the next chapter, is
certainly erroneous. As Sir J. Lubbock has remarked, "Every species is
a link between other allied forms." If we take a genus having a
score of species, recent and extinct, and destroy four-fifths of them,
no one doubts that the remainder will stand much more distinct from
each other. If the extreme forms in the genus happen to have been thus
destroyed, the genus itself will stand more distinct from other allied
genera. What geological research has not revealed, is the former
existence of infinitely numerous gradations, as fine as existing
varieties, connecting together nearly all existing and extinct
species. But this ought not to be expected; yet this has been
repeatedly advanced as a most serious objection against my views.
It may be worth while to sum up the foregoing remarks on the
causes of the imperfection of the geological record under an imaginary
illustration. The Malay Archipelago is 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, whilst most of our formations were accumulating.
The Malay Archipelago is one of the richest regions 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 extremely imperfect manner
in the formations which we suppose to be there accumulating. 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.
Formations rich in fossils of many kinds, and of thickness
sufficient to last to an age as distant in futurity as the secondary
formations lie in the past, would generally be formed in the
archipelago only during periods of subsidence. These periods of
subsidence would be separated from each other by immense intervals
of time, during which the area would be either stationary or rising;
whilst rising, the fossiliferous formations on the steeper shores
would be destroyed, almost as soon as accumulated, by the incessant
coast-action, as we now see on the shores of South America. Even
throughout the extensive and shallow seas within the archipelago,
sedimentary beds could hardly be accumulated of great thickness during
the periods of elevation, or become capped and protected by subsequent
deposits, so as to have a good chance of enduring to a very distant
future. 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 less 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 all fully preserved, transitional varieties would merely appear as
so many new, though closely allied 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 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 plainly leads to
the belief that it would be chiefly these far ranging species,
though only some of them, which would oftenest produce new
varieties; and the varieties would at first 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, and as they would
be found embedded in slightly different sub-stages of the same
formation, they would, according to the principles followed by many
palaeontologists, be ranked as new and distinct species.
If then there be some degree of truth in these remarks, we have no
right to expect to find, in our geological formations, an infinite
number of those fine transitional forms which, on our theory, 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, and such assuredly we do find- some more distantly, some
more closely, related to each other; and these links, let them be ever
so close, if found in different stages of the same formation, would,
by many palaeontologists, be ranked as distinct species. But I do
not pretend that I should ever have suspected how poor was the
record in the best preserved geological sections, had not the
absence of innumerable transitional links between the species which
lived at the commencement and close of each formation, pressed so
hardly on my theory.
Previous section | Next section
