- On the absence
of intermediate varieties at
the present day
- On
the nature of extinct intermediate
varieties; on their number
- On
the vast lapse of time,
as inferred from the rate of
deposition and
of denudation
- On
the poorness of our palaeontological
collections
- On
the intermittence of
geological formations
- On
the absence of intermediate
varieties in any
one formation
- On
their sudden appearance
in the lowest known
fossiliferous strata
IN the sixth chapter I enumerated
the chief objections which might
be justly urged against the views
maintained in this volume. Most
of them have now been discussed.
One, namely the distinctness
of specific forms, and their
not being blended together by
innumerable transitional links,
is a very obvious difficulty.
I assigned reasons why such links
do not commonly occur at the
present day, under the circumstances
apparently most favourable for
their presence, namely on an
extensive and continuous area
with graduated physical conditions.
I endeavoured to show, that the
life of each species depends
in a more important manner on
the presence of other already
defined organic forms, than on
climate; and, therefore, that
the really governing conditions
of life do not graduate away
quite insensibly like heat or
moisture. I endeavoured, also,
to show that intermediate varieties,
from existing in lesser numbers
than the forms which they connect,
will generally be beaten out
and exterminated during the course
of further modification and improvement.
The main cause, however, of innumerable
intermediate links not now occurring
everywhere throughout nature
depends on the very process of
natural selection, through which
new varieties continually take
the places of and exterminate
their parent-forms. But just
in proportion as this process
of extermination has acted on
an enormous scale, so must the
number of intermediate varieties,
which have formerly existed on
the earth, be truly enormous.
Why then is not every geological
formation and every stratum full
of such intermediate links? Geology
assuredly does not reveal any
such finely graduated organic
chain; and this, perhaps, is
the most obvious and gravest
objection which can be urged
against my theory. The explanation
lies, as I believe, in the extreme
imperfection of the geological
record.
In the first place it should
always be borne in mind what
sort of intermediate forms must,
on my theory, have formerly existed.
I have found it difficult, when
looking at any two species, to
avoid picturing to myself, forms directly intermediate
between them. But this is a wholly
false view; we should always
look for forms intermediate between
each species and a common but
unknown progenitor; and the progenitor
will generally have differed
in some respects from all its
modified descendants. To give
a simple illustration: the fantail
and pouter pigeons have both
descended from the rock-pigeon;
if we possessed all the intermediate
varieties which have ever existed,
we should have an extremely close
series between both and the rock-pigeon;
but we should have no varieties
directly intermediate between
the fantail and pouter; none,
for instance, combining a tail
somewhat expanded with a crop
somewhat enlarged, the characteristic
features of these two breeds.
These two breeds, moreover, have
become so much modified, that
if we had no historical or indirect
evidence regarding their origin,
it would not have been possible
to have determined from a mere
comparison of their structure
with that of the rock-pigeon,
whether they had descended from
this species or from some other
allied species, such as C. oenas.
So with natural species, if
we look to forms very distinct,
for instance to the horse and
tapir, we have no reason to suppose
that links ever existed directly
intermediate between them, but
between each and an unknown common
parent. The common parent will
have had in its whole organisation
much general resemblance to the
tapir and to the horse; but in
some points of structure may
have differed considerably from
both, even perhaps more than
they differ from each other.
Hence in all such cases, we should
be unable to recognise the parent-form
of any two or more species, even
if we closely compared the structure
of the parent with that of its
modified descendants, unless
at the same time we had a nearly
perfect chain of the intermediate
links.
It is just possible by my theory,
that one of two living forms
might have descended from the
other; for instance, a horse
from a tapir; and in this case direct intermediate
links will have existed between
them. But such a case would imply
that one form had remained for
a very long period unaltered,
whilst its descendants had undergone
a vast amount of change; and
the principle of competition
between organism and organism,
between child and parent, will
render this a very rare event;
for in all cases the new and
improved forms of life will tend
to supplant the old and unimproved.
By the theory of natural selection
all living species have been
connected with the parent-species
of each genus, by differences
not greater than we see between
the varieties of the same species
at the present day; and these
parent-species, now generally
extinct, have in their turn been
similarly connected with more
ancient species; and so on backwards,
always converging to the common
ancestor of each great class.
So that the number of intermediate
and transitional links, between
all living and extinct species,
must have been inconceivably
great. But assuredly, if this
theory be true, such have lived
upon this earth.
On the lapse of Time. Independently
of our not finding fossil remains
of such infinitely numerous connecting
links, it may be objected, that
time will not have sufficed for
so great an amount of organic
change, all changes having been
effected very slowly through
natural selection. It is hardly
possible for me even to recall
to the reader, who may not be
a practical geologist, the facts
leading the mind feebly to comprehend
the lapse of time. He who can
read Sir Charles Lyell's grand
work on the Principles of Geology,
which the future historian will
recognise as having produced
a revolution in natural science,
yet does not admit how incomprehensibly
vast have been the past periods
of time, may at once close this
volume. Not that it suffices
to study the Principles of Geology,
or to read special treatises
by different observers on separate
formations, and to mark how each
author attempts to give an inadequate
idea of the duration of each
formation or even each stratum.
A man must for years examine
for himself great piles of superimposed
strata, and watch the sea at
work grinding down old rocks
and making fresh sediment, before
he can hope to comprehend anything
of the lapse of time, the monuments
of which we see around us.
It is good to wander along
lines of sea-coast, when formed
of moderately hard rocks, and
mark the process of degradation.
The tides in most cases reach
the cliffs only for a short time
twice a day, and the waves eat
into them only when they are
charged with sand or pebbles;
for there is reason to believe
that pure water can effect little
or nothing in wearing away rock.
At last the base of the cliff
is undermined, huge fragments
fall down, and these remaining
fixed, have to be worn away,
atom by atom, until reduced in
size they can be rolled about
by the waves, and then are more
quickly ground into pebbles,
sand, or mud. But how often do
we see along the bases of retreating
cliffs rounded boulders, all
thickly clothed by marine productions,
showing how little they are abraded
and how seldom they are rolled
about! Moreover, if we follow
for a few miles any line of rocky
cliff, which is undergoing degradation,
we find that it is only here
and there, along a short length
or round a promontory, that the
cliffs are at the present time
suffering. The appearance of
the surface and the vegetation
show that elsewhere years have
elapsed since the waters washed
their base.
He who most closely studies
the action of the sea on our
shores, will, I believe, be most
deeply impressed with the slowness
with which rocky coasts are worn
away. The observations on this
head by Hugh Miller, and by that
excellent observer Mr. Smith
of Jordan Hill, are most impressive.
With the mind thus impressed,
let any one examine beds of conglomerate
many thousand feet in thickness,
which, though probably formed
at a quicker rate than many other
deposits, yet, from being formed
of worn and rounded pebbles,
each of which bears the stamp
of time, are good to show how
slowly the mass has been accumulated.
Let him remember Lyell's profound
remark, that the thickness and
extent of sedimentary formations
are the result and measure of
the degradation which the earth's
crust has elsewhere suffered.
And what an amount of degradation
is implied by the sedimentary
deposits of many countries! Professor
Ramsay has given me the maximum
thickness, in most cases from
actual measurement, in a few
cases from estimate, of each
formation in different parts
of Great Britain; and this is
the result:-
Feet
Palaeozoic strata (not including
igneous beds) 57,154
Secondary strata 13,190
Tertiary strata 2,240
making altogether 72,584 feet;
that is, very nearly thirteen
and three-quarters British miles.
Some of these formations, which
are represented in England by
thin beds, are thousands of feet
in thickness on the Continent.
Moreover, between each successive
formation, we have, in the opinion
of most geologists, enormously
long blank periods. So that the
lofty pile of sedimentary rocks
in Britain, gives but an inadequate
idea of the time which has elapsed
during their accumulation; yet
what time this must have consumed!
Good observers have estimated
that sediment is deposited by
the great Mississippi river at
the rate of only 600 feet in
a hundred thousand years. This
estimate may be quite erroneous;
yet, considering over what wide
spaces very fine sediment is
transported by the currents of
the sea, the process of accumulation
in any one area must be extremely
slow.
But the amount of denudation
which the strata have in many
places suffered, independently
of the rate of accumulation of
the degraded matter, probably
offers the best evidence of the
lapse of time. I remember having
been much struck with the evidence
of denudation, when viewing volcanic
islands, which have been worn
by the waves and pared all round
into perpendicular cliffs of
one or two thousand feet in height;
for the gentle slope of the lava-streams,
due to their formerly liquid
state, showed at a glance how
far the hard, rocky beds had
once extended into the open ocean.
The same story is still more
plainly told by faults, those
great cracks along which the
strata have been upheaved on
one side, or thrown down on the
other, to the height or depth
of thousands of feet; for since
the crust cracked, the surface
of the land has been so completely
planed down by the action of
the sea, that no trace of these
vast dislocations is externally
visible.
The Craven fault, for instance,
extends for upwards of 30 miles,
and along this line the vertical
displacement of the strata has
varied from 600 to 3000 feet.
Prof. Ramsay has published an
account of a downthrow in Anglesea
of 2300 feet; and he informs
me that he fully believes there
is one in Merionethshire of 12,000
feet; yet in these cases there
is nothing on the surface to
show such prodigious movements;
the pile of rocks on the one
or other side having been smoothly
swept away. The consideration
of these facts impresses my mind
almost in the same manner as
does the vain endeavour to grapple
with the idea of eternity.
I am tempted to give one other
case, the well-known one of the
denudation of the Weald. Though
it must be admitted that the
denudation of the Weald has been
a mere trifle, in comparison
with that which has removed masses
of our Palaeozoic strata, in
parts ten thousand feet in thickness,
as shown in Prof. Ramsay's masterly
memoir on this subject. Yet it
is an admirable lesson to stand
on the North Downs and to look
at the distant South Downs; for,
remembering that at no great
distance to the west the northern
and southern escarpments meet
and close, one can safely picture
to oneself the great dome of
rocks which must have covered
up the Weald within so limited
a period as since the latter
part of the Chalk formation.
The distance from the northern
to the southern Downs is about
22 miles, and the thickness of
the several formations is on
an average about 1100 feet, as
I am informed by Prof. Ramsay.
But if, as some geologists suppose,
a range of older rocks underlies
the Weald, on the flanks of which
the overlying sedimentary deposits
might have accumulated in thinner
masses than elsewhere, the above
estimate would be erroneous;
but this source of doubt probably
would not greatly affect the
estimate as applied to the western
extremity of the district. If,
then, we knew the rate at which
the sea commonly wears away a
line of cliff of any given height,
we could measure the time requisite
to have denuded the Weald. This,
of course, cannot be done; but
we may, in order to form some
crude notion on the subject,
assume that the sea would eat
into cliffs 500 feet in height
at the rate of one inch in a
century. This will at first appear
much too small an allowance;
but it is the same as if we were
to assume a cliff one yard in
height to be eaten back along
a whole line of coast at the
rate of one yard in nearly every
twenty-two years. I doubt whether
any rock, even as soft as chalk,
would yield at this rate excepting
on the most exposed coasts; though
no doubt the degradation of a
lofty cliff would be more rapid
from the breakage of the fallen
fragments. On the other hand,
I do not believe that any line
of coast, ten or twenty miles
in length, ever suffers degradation
at the same time along its whole
indented length; and we must
remember that almost all strata
contain harder layers or nodules,
which from long resisting attrition
form a breakwater at the base.
Hence, under ordinary circumstances,
I conclude that for a cliff 500
feet in height, a denudation
of one inch per century for the
whole length would be an ample
allowance. At this rate, on the
above data, the denudation of
the Weald must have required
306,662,400 years; or say three
hundred million years.
The action of fresh water on
the gently inclined Wealden district,
when upraised, could hardly have
been great, but it would somewhat
reduce the above estimate. On
the other hand, during oscillations
of level, which we know this
area has undergone, the surface
may have existed for millions
of years as land, and thus have
escaped the action of the sea:
when deeply submerged for perhaps
equally long periods, it would,
likewise, have escaped the action
of the coast-waves. So that in
all probability a far longer
period than 300 million years
has elapsed since the latter
part of the Secondary period.
I have made these few remarks
because it is highly important
for us to gain some notion, however
imperfect, of the lapse of years.
During each of these years, over
the whole world, the land and
the water has been peopled by
hosts of living forms. What an
infinite number of generations,
which the mind cannot grasp,
must have succeeded each other
in the long roll of years! Now
turn to our richest geological
museums, and what a paltry display
we behold!
On the poorness of our Palaeontological
collections. That our Palaeontological
collections are very imperfect,
is admitted by every one. The
remark of that admirable Palaeontologist,
the late Edward Forbes, should
not be forgotten, namely, that
numbers of our fossil species
are known and named from single
and often broken specimens,
or from a few specimens collected
on some one spot. Only a small
portion of the surface of the
earth has been geologically
explored, and no part with
sufficient care, as the important
discoveries made every year
in Europe prove. No organism
wholly soft can be preserved.
Shells and bones will decay
and disappear when left on
the bottom of the sea, where
sediment is not accumulating.
I believe we are continually
taking a most erroneous view,
when we tacitly admit to ourselves
that sediment is being deposited
over nearly the whole bed of
the sea, at a rate sufficiently
quick to embed and preserve
fossil remains. Throughout
an enormously large proportion
of the ocean, the bright blue
tint of the water bespeaks
its purity. The many cases
on record of a formation conformably
covered, after an enormous
interval of time, by another
and later formation, without
the underlying bed having suffered
in the interval any wear and
tear, seem explicable only
on the view of the bottom of
the sea not rarely lying for
ages in an unaltered condition.
The remains which do become
embedded, if in sand or gravel,
will when the beds are upraised
generally be dissolved by the
percolation of rain-water.
I suspect that but few of the
very many animals which live
on the beach between high and
low watermark are preserved.
For instance, the several species
of the Chthamalinae (a sub-family
of sessile cirripedes) coat
the rocks all over the world
in infinite numbers: they are
all strictly littoral, with
the exception of a single Mediterranean
species, which inhabits deep
water and has been found fossil
in Sicily, whereas not one
other species has hitherto
been found in any tertiary
formation: yet it is now known
that the genus Chthamalus existed
during the chalk period. The
molluscan genus Chiton offers
a partially analogous case.
With respect to the terrestrial
productions which lived during
the Secondary and Palaeozoic
periods, it is superfluous to
state that our evidence from
fossil remains is fragmentary
in an extreme degree. For instance,
not a land shell is known belonging
to either of these vast periods,
with one exception discovered
by Sir C. Lyell in the carboniferous
strata of North America. I n
regard to mammiferous remains,
a single glance at the historical
table published in the Supplement
to Lyell's Manual, will bring
home the truth, how accidental
and rare is their preservation,
far better than pages of detail.
Nor is their rarity surprising,
when we remember how large a
proportion of the bones of tertiary
mammals have been discovered
either in caves or in lacustrine
deposits; and that not a cave
or true lacustrine bed is known
belonging to the age of our secondary
or palaeozoic formations.
But the imperfection in the
geological record mainly results
from another and more important
cause than any of the foregoing;
namely, from the several formations
being separated from each other
by wide intervals of time. When
we see the formations tabulated
in written works, or when we
follow them in nature, it is
difficult to avoid believing
that they are closely consecutive.
But we know, for instance, from
Sir R. Murchison's great work
on Russia, what wide gaps there
are in that country between the
superimposed formations; so it
is in North America, and in many
other parts of the world. The
most skilful geologist, if his
attention had been exclusively
confined to these large territories,
would never have suspected that
during the periods which were
blank and barren in his own country,
great piles of sediment, charged
with new and peculiar forms of
life, had elsewhere been accumulated.
And if in each separate territory,
hardly any idea can be formed
of the length of time which has
elapsed between the consecutive
formations, we may infer that
this could nowhere be ascertained.
The frequent and great changes
in the mineralogical composition
of consecutive formations, generally
implying great changes in the
geography of the surrounding
lands, whence the sediment has
been derived, accords with the
belief of vast intervals of time
having elapsed between each formation.
But we can, I think, see why
the geological formations of
each region are almost invariably
intermittent; that is, have not
followed each other in close
sequence. Scarcely any fact struck
me more when examining many hundred
miles of the South American coasts,
which have been upraised several
hundred feet within the recent
period, than the absence of any
recent deposits sufficiently
extensive to last for even a
short geological period. Along
the whole west coast, which is
inhabited by a peculiar marine
fauna, tertiary beds are so scantily
developed, that no record of
several successive and peculiar
marine faunas will probably be
preserved to a distant age. A
little reflection will explain
why along the rising coast of
the western side of South America,
no extensive formations with
recent or tertiary remains can
anywhere be found, though the
supply of sediment must for ages
have been great, from the enormous
degradation of the coast-rocks
and from muddy streams entering
the sea. The explanation, no
doubt, is, that the littoral
and sub-littoral deposits are
continually worn away, as soon
as they are brought up by the
slow and gradual rising of the
land within the grinding action
of the coast-waves.
We may, I think, safely conclude
that sediment must be accumulated
in extremely thick, solid, or
extensive masses, in order to
withstand the incessant action
of the waves, when first upraised
and during subsequent oscillations
of level. Such thick and extensive
accumulations of sediment may
be formed in two ways; either,
in profound depths of the sea,
in which case, judging from the
researches of E. Forbes, we may
conclude that the bottom will
be inhabited by extremely few
animals, and the mass when upraised
will give a most imperfect record
of the forms of life which then
existed; or, sediment may be
accumulated to any thickness
and extent over a shallow bottom,
if it continue slowly to subside.
In this latter case, as long
as the rate of subsidence and
supply of sediment nearly balance
each other, the sea will remain
shallow and favourable for life,
and thus a fossiliferous formation
thick enough, when upraised,
to resist any amount of degradation,
may be formed.
I am convinced that all our
ancient formations, which are
rich in fossils, have thus been
formed during subsidence. Since
publishing my views on this subject
in 1845, I have watched the progress
of Geology, and have been surprised
to note how author after author,
in treating of this or that great
formation, has come to the conclusion
that it was accumulated during
subsidence. I may add, that the
only ancient tertiary formation
on the west coast of South America,
which has been bulky enough to
resist such degradation as it
has as yet suffered, but which
will hardly last to a distant
geological age, was certainly
deposited during a downward oscillation
of level, and thus gained considerable
thickness.
All geological facts tell us
plainly that each area has undergone
numerous slow oscillations of
level, and apparently these oscillations
have affected wide spaces. Consequently
formations rich in fossils and
sufficiently thick and extensive
to resist subsequent degradation,
may have been formed over wide
spaces during periods of subsidence,
but only where the supply of
sediment was sufficient to keep
the sea shallow and to embed
and preserve the remains before
they had time to decay. On the
other hand, as long as the bed
of the sea remained stationary,
thick deposits could not have
been accumulated in the shallow
parts, which are the most favourable
to life. Still less could this
have happened during the alternate
periods of elevation; or, to
speak more accurately, the beds
which were then accumulated will
have been destroyed by being
upraised and brought within the
limits of the coast-action.
Thus the geological record
will almost necessarily be rendered
intermittent. I feel much confidence
in the truth of these views,
for they are in strict accordance
with the general principles inculcated
by Sir C. Lyell; and E. Forbes
independently arrived at a similar
conclusion.
One remark is here worth a
passing notice. During periods
of elevation the area of the
land and of the adjoining shoal
parts of the sea will be increased,
and new stations will often be
formed; all circumstances most
favourable, as previously explained,
for the formation of new varieties
and species; but during such
periods there will generally
be a blank in the geological
record. On the other hand, during
subsidence, the inhabited area
and number of inhabitants will
decrease (excepting the productions
on the shores of a continent
when first broken up into an
archipelago), and consequently
during subsidence, though there
will be much extinction, fewer
new varieties or species will
be formed; and it is during these
very periods of subsidence, that
our great deposits rich in fossils
have been accumulated. Nature
may almost be said to have guarded
against the frequent discovery
of her transitional or linking
forms.
From the foregoing considerations
it cannot be doubted that the
geological record, viewed as
a whole, is extremely imperfect;
but if we confine our attention
to any one formation, it becomes
more difficult to understand,
why we do not therein find closely
graduated varieties between the
allied species which lived at
its commencement and at its close.
Some cases are on record of the
same species presenting distinct
varieties in the upper and lower
parts of the same formation,
but, as they are rare, they may
be here passed over. Although
each formation has indisputably
required a vast number of years
for its deposition, I can see
several reasons why each should
not include a graduated series
of links between the species
which then lived; but I can by
no means pretend to assign due
proportional weight to the following
considerations.
Although each formation may
mark a very long lapse of years,
each perhaps is short compared
with the period requisite to
change one species into another.
I am aware that two palaeontologists,
whose opinions are worthy of
much deference, namely Bronn
and Woodward, have concluded
that the average duration of
each formation is twice or thrice
as long as the average duration
of specific forms. But insuperable
difficulties, as it seems to
me, prevent us coming to any
just conclusion on this head.
When we see a species first appearing
in the middle of any formation,
it would be rash in the extreme
to infer that it had not elsewhere
previously existed. So again
when we find a species disappearing
before the uppermost layers have
been deposited, it would be equally
rash to suppose that it then
became wholly extinct. We forget
how small the area of Europe
is compared with the rest of
the world; nor have the several
stages of the same formation
throughout Europe been correlated
with perfect accuracy.
With marine animals of all
kinds, we may safely infer a
large amount of migration during
climatal and other changes; and
when we see a species first appearing
in any formation, the probability
is that it only then first immigrated
into that area. It is well known,
for instance, that several species
appeared somewhat earlier in
the palaeozoic beds of North
America than in those of Europe;
time having apparently been required
for their migration from the
American to the European seas.
In examining the latest deposits
of various quarters of the world,
it has everywhere been noted,
that some few still existing
species are common in the deposit,
but have become extinct in the
immediately surrounding sea;
or, conversely, that some are
now abundant in the neighbouring
sea, but are rare or absent in
this particular deposit. It is
an excellent lesson to reflect
on the ascertained amount of
migration of the inhabitants
of Europe during the Glacial
period, which forms only a part
of one whole geological period;
and likewise to reflect on the
great changes of level, on the
inordinately great change of
climate, on the prodigious lapse
of time, all included within
this same glacial period. Yet
it may be doubted whether in
any quarter of the world, sedimentary
deposits, including fossil remains,
have gone on accumulating within
the same area during the whole
of this period. It is not, for
instance, probable that sediment
was deposited during the whole
of the glacial period near the
mouth of the Mississippi, within
that limit of depth at which
marine animals can flourish;
for we know what vast geographical
changes occurred in other parts
of America during this space
of time. When such beds as were
deposited in shallow water near
the mouth of the Mississippi
during some part of the glacial
period shall have been upraised,
organic remains will probably
first appear and disappear at
different levels, owing to the
migration of species and to geographical
changes. And in the distant future,
a geologist examining these beds,
might be tempted to conclude
that the average duration of
life of the embedded fossils
had been less than that of the
glacial period, instead of having
been really far greater, that
is extending from before the
glacial epoch to the present
day.
In order to get a perfect gradation
between two forms in the upper
and lower parts of the same formation,
the deposit must have gone on
accumulating for a very long
period, in order to have given
sufficient time for the slow
process of variation; hence the
deposit will generally have to
be a very thick one; and the
species undergoing modification
will have had to live on the
same area throughout this whole
time. But we have seen that a
thick fossiliferous formation
can only be accumulated during
a period of subsidence; and to
keep the depth approximately
the same, which is necessary
in order to enable the same species
to live on the same space, the
supply of sediment must nearly
have counterbalanced the amount
of subsidence. But this same
movement of subsidence will often
tend to sink the area whence
the sediment is derived, and
thus diminish the supply whilst
the downward movement continues.
In fact, this nearly exact balancing
between the supply of sediment
and the amount of subsidence
is probably a rare contingency;
for it has been observed by more
than one palaeontologist, that
very thick deposits are usually
barren of organic remains, except
near their upper or lower limits.
It would seem that each separate
formation, like the whole pile
of formations in any country,
has generally been intermittent
in its accumulation. When we
see, as is so often the case,
a formation composed of beds
of different mineralogical composition,
we may reasonably suspect that
the process of deposition has
been much interrupted, as a change
in the currents of the sea and
a supply of sediment of a different
nature will generally have been
due to geographical changes requiring
much time. Nor will the closest
inspection of a formation give
any idea of the time which its
deposition has consumed. Many
instances could be given of beds
only a few feet in thickness,
representing formations, elsewhere
thousands of feet in thickness,
and which must have required
an enormous period for their
accumulation; yet no one ignorant
of this fact would have suspected
the vast lapse of time represented
by the thinner formation. Many
cases could be given of the lower
beds of a formation having been
upraised, denuded, submerged,
and then re-covered by the upper
beds of the same formation, facts,
showing what wide, yet easily
overlooked, intervals have occurred
in its accumulation. In other
cases we have the plainest evidence
in great fossilised trees, still
standing upright as they grew,
of many long intervals of time
and changes of level during the
process of deposition, which
would never even have been suspected,
had not the trees chanced to
have been preserved: thus, Messrs
Lyell and Dawson found carboniferous
beds 1400 feet thick in Nova
Scotia, with ancient root-bearing
strata, one above the other,
at no less than sixty-eight different
levels. Hence, when the same
species occur at the bottom,
middle, and top of a formation,
the probability is that they
have not lived on the same spot
during the whole period of deposition,
but have disappeared and reappeared,
perhaps many times, during the
same geological period. So that
if such species were to undergo
a considerable amount of modification
during any one geological period,
a section would not probably
include all the fine intermediate
gradations which must on my theory
have existed between them, but
abrupt, though perhaps very slight,
changes of form.
It is all-important to remember
that naturalists have no golden
rule by which to distinguish
species and varieties; they grant
some little variability to each
species, but when they meet with
a somewhat greater amount of
difference between any two forms,
they rank both as species, unless
they are enabled to connect them
together by close intermediate
gradations. And this from the
reasons just assigned we can
seldom hope to effect in any
one geological section. Supposing
B and C to be two species, and
a third, A, to be found in an
underlying bed; even if A were
strictly intermediate between
B and C, it would simply be ranked
as a third and distinct species,
unless at the same time it could
be most closely connected with
either one or both forms by intermediate
varieties. Nor should it be forgotten,
as before explained, that A might
be the actual progenitor of B
and C, and yet might not at all
necessarily be strictly intermediate
between them in all points of
structure. So that we might obtain
the parent-species and its several
modified descendants from the
lower and upper beds of a formation,
and unless we obtained numerous
transitional gradations, we should
not recognise their relationship,
and should consequently be compelled
to rank them all as distinct
species.
It is notorious on what excessively
slight differences many palaeontologists
have founded their species; and
they do this the more readily
if the specimens come from different
sub-stages of the same formation.
Some experienced conchologists
are now sinking many of the very
fine species of D'Orbigny and
others into the rank of varieties;
and on this view we do find the
kind of evidence of change which
on my theory we ought to find.
Moreover, if we look to rather
wider intervals, namely, to distinct
but consecutive stages of the
same great formation, we find
that the embedded fossils, though
almost universally ranked as
specifically different, yet are
far more closely allied to each
other than are the species found
in more widely separated formations;
but to this subject I shall have
to return in the following chapter.
One other consideration is
worth notice: with animals and
plants that can propagate rapidly
and are not highly locomotive,
there is reason to suspect, as
we have formerly seen, that their
varieties are generally at first
local; and that such local varieties
do not spread widely and supplant
their parent-forms until they
have been modified and perfected
in some considerable degree.
According to this view, the chance
of discovering in a formation
in any one country all the early
stages of transition between
any two forms, is small, for
the successive changes are supposed
to have been local or confined
to some one spot. Most marine
animals have a wide range; and
we have seen that with plants
it is those which have the widest
range, that oftenest present
varieties; so that with shells
and other marine animals, it
is probably those which have
had the widest range, far exceeding
the limits of the known geological
formations of Europe, which have
oftenest given rise, first to
local varieties and ultimately
to new species; and this again
would greatly lessen the chance
of our being able to trace the
stages of transition in any one
geological formation.
It should not be forgotten,
that at the present day, with
perfect specimens for examination,
two forms can seldom be connected
by intermediate varieties and
thus proved to be the same species,
until many specimens have been
collected from many places; and
in the case of fossil species
this could rarely be effected
by palaeontologists. We shall,
perhaps, best perceive the improbability
of our being enabled to connect
species by numerous, fine, intermediate,
fossil links, by asking ourselves
whether, for instance, geologists
at some future period will be
able to prove, that our different
breeds of cattle, sheep, horses,
and dogs have descended from
a single stock or from several
aboriginal stocks; or, again,
whether certain sea-shells inhabiting
the shores of North America,
which are ranked by some conchologists
as distinct species from their
European representatives, and
by other conchologists as only
varieties, are really varieties
or are, as it is called, specifically
distinct. This could be effected
only by the future geologist
discovering in a fossil state
numerous intermediate gradations;
and such success seems to me
improbable in the highest degree.
Geological research, though
it has added numerous species
to existing and extinct genera,
and has made the intervals between
some few groups less wide than
they otherwise would have been,
yet has done scarcely anything
in breaking down the distinction
between species, by connecting
them together by numerous, fine,
intermediate varieties; and this
not having been effected, is
probably the gravest and most
obvious of all the many objections
which may be urged against my
views. Hence it will be worth
while to sum up the foregoing
remarks, under an imaginary illustration.
The Malay Archipelago is of about
the size of Europe from the North
Cape to the Mediterranean, and
from Britain to Russia; and therefore
equals all the geological formations
which have been examined with
any accuracy, excepting those
of the United States of America.
I fully agree with Mr Godwin-Austen,
that the present condition of
the Malay Archipelago, with its
numerous large islands separated
by wide and shallow seas, probably
represents the former state of
Europe, when most of our formations
were accumulating. The Malay
Archipelago is one of the richest
regions of the whole world in
organic beings; yet if all the
species were to be collected
which have ever lived there,
how imperfectly would they represent
the natural history of the world!
But we have every reason to
believe that the terrestrial
productions of the archipelago
would be preserved in an excessively
imperfect manner in the formations
which we suppose to be there
accumulating. I suspect that
not many of the strictly littoral
animals, or of those which lived
on naked submarine rocks, would
be embedded; and those embedded
in gravel or sand, would not
endure to a distant epoch. Wherever
sediment did not accumulate on
the bed of the sea, or where
it did not accumulate at a sufficient
rate to protect organic bodies
from decay, no remains could
be preserved.
In our archipelago, I believe
that fossiliferous formations
could be formed of sufficient
thickness to last to an age,
as distant in futurity as the
secondary formations lie in the
past, only during periods of
subsidence. These periods of
subsidence would be separated
from each other by enormous intervals,
during which the area would be
either stationary or rising;
whilst rising, each fossiliferous
formation would be destroyed,
almost as soon as accumulated,
by the incessant coast-action,
as we now see on the shores of
South America. During the periods
of subsidence there would probably
be much extinction of life; during
the periods of elevation, there
would be much variation, but
the geological record would then
be least perfect.
It may be doubted whether the
duration of any one great period
of subsidence over the whole
or part of the archipelago, together
with a contemporaneous accumulation
of sediment, would exceed the
average duration of the same
specific forms; and these contingencies
are indispensable for the preservation
of all the transitional gradations
between any two or more species.
If such gradations were not fully
preserved, transitional varieties
would merely appear as so many
distinct species. It is, also,
probable that each great period
of subsidence would be interrupted
by oscillations of level, and
that slight climatal changes
would intervene during such lengthy
periods; and in these cases the
inhabitants of the archipelago
would have to migrate, and no
closely consecutive record of
their modifications could be
preserved in any one formation.
Very many of the marine inhabitants
of the archipelago now range
thousands of miles beyond its
confines; and analogy leads me
to believe that it would be chiefly
these far-ranging species which
would oftenest produce new varieties;
and the varieties would at first
generally be local or confined
to one place, but if possessed
of any decided advantage, or
when further modified and improved,
they would slowly spread and
supplant their parent-forms.
When such varieties returned
to their ancient homes, as they
would differ from their former
state, in a nearly uniform, though
perhaps extremely slight degree,
they would, according to the
principles followed by many palaeontologists,
be ranked as new and distinct
species.
If then, there be some degree
of truth in these remarks, we
have no right to expect to find
in our geological formations,
an infinite number of those fine
transitional forms, which on
my theory assuredly have connected
all the past and present species
of the same group into one long
and branching chain of life.
We ought only to look for a few
links, some more closely, some
more distantly related to each
other; and these links, let them
be ever so close, if found in
different stages of the same
formation, would, by most palaeontologists,
be ranked as distinct species.
But I do not pretend that I should
ever have suspected how poor
a record of the mutations of
life, the best preserved geological
section presented, had not the
difficulty of our not discovering
innumerable transitional links
between the species which appeared
at the commencement and close
of each formation, pressed so
hardly on my theory.
On the sudden appearance
of whole groups of Allied Species. The
abrupt manner in which whole
groups of species suddenly
appear in certain formations,
has been urged by several palaeontologists,
for instance, by Agassiz, Pictet,
and by none more forcibly than
by Professor Sedgwick, as a
fatal objection to the belief
in the transmutation of species.
If numerous species, belonging
to the same genera or families,
have really started into life
all at once, the fact would
be fatal to the theory of descent
with slow modification through
natural selection. For the
development of a group of forms,
all of which have descended
from some one progenitor, must
have been an extremely slow
process; and the progenitors
must have lived long ages before
their modified descendants.
But we continually over-rate
the perfection of the geological
record, and falsely infer,
because certain genera or families
have not been found beneath
a certain stage, that they
did not exist before that stage.
We continually forget how large
the world is, compared with
the area over which our geological
formations have been carefully
examined; we forget that groups
of species may elsewhere have
long existed and have slowly
multiplied before they invaded
the ancient archipelagoes of
Europe and of the United States.
We do not make due allowance
for the enormous intervals
of time, which have probably
elapsed between our consecutive
formations, longer perhaps
in some cases than the time
required for the accumulation
of each formation. These intervals
will have given time for the
multiplication of species from
some one or some few parent-forms;
and in the succeeding formation
such species will appear as
if suddenly created.
I may here recall a remark
formerly made, namely that it
might require a long succession
of ages to adapt an organism
to some new and peculiar line
of life, for instance to fly
through the air; but that when
this had been effected, and a
few species had thus acquired
a great advantage over other
organisms, a comparatively short
time would be necessary to produce
many divergent forms, which would
be able to spread rapidly and
widely throughout the world.
I will now give a few examples
to illustrate these remarks;
and to show how liable we are
to error in supposing that whole
groups of species have suddenly
been produced. I may recall the
well-known fact that in geological
treatises, published not many
years ago, the great class of
mammals was always spoken of
as having abruptly come in at
the commencement of the tertiary
series. And now one of the richest
known accumulations of fossil
mammals belongs to the middle
of the secondary series; and
one true mammal has been discovered
in the new red sandstone at nearly
the commencement of this great
series. Cuvier used to urge that
no monkey occurred in any tertiary
stratum; but now extinct species
have been discovered in India,
South America, and in Europe
even as far back as the eocene
stage. The most striking case,
however, is that of the Whale
family; as these animals have
huge bones, are marine, and range
over the world, the fact of not
a single bone of a whale having
been discovered in any secondary
formation, seemed fully to justify
the belief that this great and
distinct order had been suddenly
produced in the interval between
the latest secondary and earliest
tertiary formation. But now we
may read in the Supplement to
Lyell's 'Manual,' published in
1858, clear evidence of the existence
of whales in the upper greensand,
some time before the close of
the secondary period.
I may give another instance,
which from having passed under
my own eyes has much struck me.
In a memoir on Fossil Sessile
Cirripedes, I have stated that,
from the number of existing and
extinct tertiary species; from
the extraordinary abundance of
the individuals of many species
all over the world, from the
Arctic regions to the equator,
inhabiting various zones of depths
from the upper tidal limits to
50 fathoms; from the perfect
manner in which specimens are
preserved in the oldest tertiary
beds; from the ease with which
even a fragment of a valve can
be recognised; from all these
circumstances, I inferred that
had sessile cirripedes existed
during the secondary periods,
they would certainly have been
preserved and discovered; and
as not one species had been discovered
in beds of this age, I concluded
that this great group had been
suddenly developed at the commencement
of the tertiary series. This
was a sore trouble to me, adding
as I thought one more instance
of the abrupt appearance of a
great group of species. But my
work had hardly been published,
when a skilful palaeontologist,
M. Bosquet, sent me a drawing
of a perfect specimen of an unmistakeable
sessile cirripede, which he had
himself extracted from the chalk
of Belgium. And, as if to make
the case as striking as possible,
this sessile cirripede was a
Chthamalus, a very common, large,
and ubiquitous genus, of which
not one specimen has as yet been
found even in any tertiary stratum.
Hence we now positively know
that sessile cirripedes existed
during the secondary period;
and these cirripedes might have
been the progenitors of our many
tertiary and existing species.
The case most frequently insisted
on by palaeontologists of the
apparently sudden appearance
of a whole group of species,
is that of the teleostean fishes,
low down in the Chalk period.
This group includes the large
majority of existing species.
Lately, Professor Pictet has
carried their existence one sub-stage
further back; and some palaeontologists
believe that certain much older
fishes, of which the affinities
are as yet imperfectly known,
are really teleostean. Assuming,
however, that the whole of them
did appear, as Agassiz believes,
at the commencement of the chalk
formation, the fact would certainly
be highly remarkable; but I cannot
see that it would be an insuperable
difficulty on my theory, unless
it could likewise be shown that
the species of this group appeared
suddenly and simultaneously throughout
the world at this same period.
It is almost superfluous to remark
that hardly any fossil-fish are
known from south of the equator;
and by running through Pictet's
palaeontology it will be seen
that very few species are known
from several formations in Europe.
Some few families of fish now
have a confined range; the teleostean
fish might formerly have had
a similarly confined range, and
after having been largely developed
in some one sea, might have spread
widely. Nor have we any right
to suppose that the seas of the
world have always been so freely
open from south to north as they
are at present. Even at this
day, if the Malay Archipelago
were converted into land, the
tropical parts of the Indian
Ocean would form a large and
perfectly enclosed basin, in
which any great group of marine
animals might be multiplied;
and here they would remain confined,
until some of the species became
adapted to a cooler climate,
and were enabled to double the
southern capes of Africa or Australia,
and thus reach other and distant
seas.
From these and similar considerations,
but chiefly from our ignorance
of the geology of other countries
beyond the confines of Europe
and the United States; and from
the revolution in our palaeontological
ideas on many points, which the
discoveries of even the last
dozen years have effected, it
seems to me to be about as rash
in us to dogmatize on the succession
of organic beings throughout
the world, as it would be for
a naturalist to land for five
minutes on some one barren point
in Australia, and then to discuss
the number and range of its productions.
On the sudden appearance
of groups of Allied Species
in the lowest known fossiliferous
strata. There is another
and allied difficulty, which
is much graver. I allude to
the manner in which numbers
of species of the same group,
suddenly appear in the lowest
known fossiliferous rocks.
Most of the arguments which
have convinced me that all
the existing species of the
same group have descended from
one progenitor, apply with
nearly equal force to the earliest
known species. For instance,
I cannot doubt that all the
Silurian trilobites have descended
from some one crustacean, which
must have lived long before
the Silurian age, and which
probably differed greatly from
any known animal. Some of the
most ancient Silurian animals,
as the Nautilus, Lingula, &c.,
do not differ much from living
species; and it cannot on my
theory be supposed, that these
old species were the progenitors
of all the species of the orders
to which they belong, for they
do not present characters in
any degree intermediate between
them. If, moreover, they had
been the progenitors of these
orders, they would almost certainly
have been long ago supplanted
and exterminated by their numerous
and improved descendants.
Consequently, if my theory
be true, it is indisputable that
before the lowest Silurian stratum
was deposited, long periods elapsed,
as long as, or probably far longer
than, the whole interval from
the Silurian age to the present
day; and that during these vast,
yet quite unknown, periods of
time, the world swarmed with
living creatures.
To the question why we do not
find records of these vast primordial
periods, I can give no satisfactory
answer. Several of the most eminent
geologists, with Sir R. Murchison
at their head, are convinced
that we see in the organic remains
of the lowest Silurian stratum
the dawn of life on this planet.
Other highly competent judges,
as Lyell and the late E. Forbes,
dispute this conclusion. We should
not forget that only a small
portion of the world is known
with accuracy. M. Barrande has
lately added another and lower
stage to the Silurian system,
abounding with new and peculiar
species. Traces of life have
been detected in the Longmynd
beds beneath Barrande's so-called
primordial zone. The presence
of phosphatic nodules and bituminous
matter in some of the lowest
azoic rocks, probably indicates
the former existence of life
at these periods. But the difficulty
of understanding the absence
of vast piles of fossiliferous
strata, which on my theory no
doubt were somewhere accumulated
before the Silurian epoch, is
very great. If these most ancient
beds had been wholly worn away
by denudation, or obliterated
by metamorphic action, we ought
to find only small remnants of
the formations next succeeding
them in age, and these ought
to be very generally in a metamorphosed
condition. But the descriptions
which we now possess of the Silurian
deposits over immense territories
in Russia and in North America,
do not support the view, that
the older a formation is, the
more it has suffered the extremity
of denudation and metamorphism.
The case at present must remain
inexplicable; and may be truly
urged as a valid argument against
the views here entertained. To
show that it may hereafter receive
some explanation, I will give
the following hypothesis. From
the nature of the organic remains,
which do not appear to have inhabited
profound depths, in the several
formations of Europe and of the
United States; and from the amount
of sediment, miles in thickness,
of which the formations are composed,
we may infer that from first
to last large islands or tracts
of land, whence the sediment
was derived, occurred in the
neighbourhood of the existing
continents of Europe and North
America. But we do not know what
was the state of things in the
intervals between the successive
formations; whether Europe and
the United States during these
intervals existed as dry land,
or as a submarine surface near
land, on which sediment was not
deposited, or again as the bed
of an open and unfathomable sea.
Looking to the existing oceans,
which are thrice as extensive
as the land, we see them studded
with many islands; but not one
oceanic island is as yet known
to afford even a remnant of any
palaeozoic or secondary formation.
Hence we may perhaps infer, that
during the palaeozoic and secondary
periods, neither continents nor
continental islands existed where
our oceans now extend; for had
they existed there, palaeozoic
and secondary formations would
in all probability have been
accumulated from sediment derived
from their wear and tear; and
would have been at least partially
upheaved by the oscillations
of level, which we may fairly
conclude must have intervened
during these enormously long
periods. If then we may infer
anything from these facts, we
may infer that where our oceans
now extend, oceans have extended
from the remotest period of which
we have any record; and on the
other hand, that where continents
now exist, large tracts of land
have existed, subjected no doubt
to great oscillations of level,
since the earliest silurian period.
The coloured map appended to
my volume on Coral Reefs, led
me to conclude that the great
oceans are still mainly areas
of subsidence, the great archipelagoes
still areas of oscillations of
level, and the continents areas
of elevation. But have we any
right to assume that things have
thus remained from eternity?
Our continents seem to have been
formed by a preponderance, during
many oscillations of level, of
the force of elevation; but may
not the areas of preponderant
movement have changed in the
lapse of ages? At a period immeasurably
antecedent to the silurian epoch,
continents may have existed where
oceans are now spread out; and
clear and open oceans may have
existed where our continents
now stand. Nor should we be justified
in assuming that if, for instance,
the bed of the Pacific Ocean
were now converted into a continent,
we should there find formations
older than the silurian strata,
supposing such to have been formerly
deposited; for it might well
happen that strata which had
subsided some miles nearer to
the centre of the earth, and
which had been pressed on by
an enormous weight of superincumbent
water, might have undergone far
more metamorphic action than
strata which have always remained
nearer to the surface. The immense
areas in some parts of the world,
for instance in South America,
of bare metamorphic rocks, which
must have been heated under great
pressure, have always seemed
to me to require some special
explanation; and we may perhaps
believe that we see in these
large areas, the many formations
long anterior to the silurian
epoch in a completely metamorphosed
condition.
The several
difficulties here discussed,
namely our not finding
in the successive formations
infinitely numerous transitional
links between the many species
which now exist or have existed;
the sudden manner in which whole
groups of species appear in our
European formations; the almost
entire absence, as at present
known, of fossiliferous formations
beneath the Silurian strata,
are all undoubtedly of the gravest
nature. We see this in the plainest
manner by the fact that all the
most eminent palaeontologists,
namely Cuvier, Owen, Agassiz,
Barrande, Falconer, E. Forbes, &c.,
and all our greatest geologists,
as Lyell, Murchison, Sedgwick, &c.,
have unanimously, often vehemently,
maintained the immutability of
species. But I have reason to
believe that one great authority,
Sir Charles Lyell, from further
reflexion entertains grave doubts
on this subject. I feel how rash
it is to differ from these great
authorities, to whom, with others,
we owe all our knowledge. Those
who think the natural geological
record in any degree perfect,
and who do not attach much weight
to the facts and arguments of
other kinds even in this volume,
will undoubtedly at once reject
my theory. For my part, following
out Lyell's metaphor, I look
at the natural geological record,
as a history of the world imperfectly
kept, and written in a changing
dialect; of this history we possess
the last volume alone, relating
only to two or three countries.
Of this volume, only here and
there a short chapter has been
preserved; and of each page,
only here and there a few lines.
Each word of the slowly-changing
language, in which the history
is supposed to be written, being
more or less different in the
interrupted succession of chapters,
may represent the apparently
abruptly changed forms of life,
entombed in our consecutive,
but widely separated formations.
On this view, the difficulties
above discussed are greatly diminished,
or even disappear. |