[_First published in_ The Universal Review _for July,_ 1859.]
That proclivity to generalization which is possessed in greater or less
degree by all minds, and without which, indeed, intelligence cannot
exist, has unavoidable inconveniences. Through it alone can truth be
reached; and yet it almost inevitably betrays into error. But for the
tendency to predicate of every other case, that which has been found in
the observed cases, there could be no rational thinking; and yet by this
indispensable tendency, men are perpetually led to found, on limited
experience, propositions which they wrongly assume to be universal or
absolute. In one sense, however, this can scarcely be regarded as an
evil; for without premature generalizations the true generalization
would never be arrived at. If we waited till all the facts were
accumulated before trying to formulate them, the vast unorganized mass
would be unmanageable. Only by provisional grouping can they be brought
into such order as to be dealt with; and this provisional grouping is
but another name for premature generalization. How uniformly men follow
this course, and how needful the errors are as steps to truth, is well
illustrated in the history of Astronomy. The heavenly bodies move round
the Earth in circles, said the earliest observers: led partly by the
appearances, and partly by their experiences of central motions in
terrestrial objects, with which, as all circular, they classed the
celestial motions from lack of any alternative conception. Without this
provisional belief, wrong as it was, there could not have been that
comparison of positions which showed that the motions are not
representable by circles; and which led to the hypothesis of epicycles
and eccentrics. Only by the aid of this hypothesis, equally untrue, but
capable of accounting more nearly for the appearances, and so of
inducing more accurate observations--only thus did it become possible
for Copernicus to show that the heliocentric theory is more feasible
than the geocentric theory; or for Kepler to show that the planets move
round the sun in ellipses. Yet again, without the aid of Kepler's more
advanced theory of the Solar system, Newton could not have established
that general law from which it follows, that the motion of a heavenly
body is not necessarily in an ellipse, but may be in any conic section.
And lastly, it was only after the law of gravitation had been verified,
that it became possible to determine the actual courses of planets,
satellites, and comets; and to prove that, in consequence of
perturbations, their orbits always deviate, more or less, from regular
curves. In these successive theories we may trace both the tendency men
have to leap from scanty data to wide generalizations, that are either
untrue or but partially true; and the necessity there is for such
transitional generalizations as steps to the final one.
In the progress of geological speculation, the same laws of thought are
displayed. We have dogmas that were more than half false, passing
current for a time as universal truths. We have evidence collected in
proof of these dogmas; by and by a colligation of facts in antagonism
with them; and eventually a consequent modification. In conformity with
this improved hypothesis, we have a better classification of facts; a
greater power of arranging and interpreting the new facts now rapidly
gathered together; and further resulting corrections of hypothesis.
Being, as we are at present, in the midst of this process, it is not
possible to give an adequate account of the development of geological
science as thus regarded: the earlier stages are alone known to us. Not
only, however, is it interesting to observe how the more advanced views
now received respecting the Earth's history, have been evolved out of
the crude views which preceded them; but we shall find it extremely
instructive to observe this. We shall see how greatly the old ideas
still sway both the general mind and the minds of geologists themselves.
We shall see how the kind of evidence that has in part abolished these
old ideas, is still daily accumulating, and threatens to make other like
revolutions. In brief, we shall see whereabouts we are in the
elaboration of a true theory of the Earth; and, seeing our whereabouts,
shall be the better able to judge, among various conflicting opinions,
which best conform to the ascertained direction of geological discovery.
It is needless here to enumerate the many speculations which were in
earlier ages propounded by acute men--speculations some of which
contained portions of truth. Falling in unfit times, these speculations
did not germinate; and hence do not concern us. We have nothing to do
with ideas, however good, out of which no science grew; but only with
those which gave origin to the existing system of Geology. We therefore
begin with Werner.
Taking for data the appearances of the Earth's crust in a narrow
district of Germany; observing the constant order of superposition of
strata, and their respective physical characters; Werner drew the
inference that strata of like characters succeeded each other in like
order over the entire surface of the Earth. And seeing, from the
laminated structure of many formations and the organic remains contained
in others, that they were sedimentary; he further inferred that these
universal strata had been in succession precipitated from a chaotic
menstruum which once covered our planet. Thus, on a very incomplete
acquaintance with a thousandth part of the Earth's crust, he based a
sweeping generalization applying to the whole of it. This Neptunist
hypothesis, mark, borne out though it seemed to be by the most
conspicuous surrounding facts, was quite untenable if analyzed. That a
universal chaotic menstruum should deposit a series of numerous
sharply-defined strata, differing from one another in composition, is
incomprehensible. That the strata so deposited should contain the
remains of plants and animals, which could not have lived under the
supposed conditions, is still more incomprehensible. Physically absurd,
however, as was this hypothesis, it recognized, though under a distorted
form, one of the great agencies of geological change--the action of
water. It served also to express the fact, that the formations of the
Earth's crust stand in some kind of order. Further, it did a little
towards supplying a nomenclature, without which much progress was
impossible. Lastly, it furnished a standard with which successions of
strata in various regions could be compared, the differences noted, and
the actual sections tabulated. It was the first provisional
generalization; and was useful, if not indispensable, as a step to truer
ones.
Following this rude conception, which ascribed geological phenomena to
one agency, acting during one primeval epoch, there came a
greatly-improved conception, which ascribed them to two agencies, acting
alternately during successive epochs. Hutton, perceiving that
sedimentary deposits were still being formed at the bottom of the sea
from the detritus carried down by rivers; perceiving, further, that the
strata of which the visible surface chiefly consists, bore marks of
having been similarly formed out of pre-existing land; and inferring
that these strata could have become land only by upheaval after their
deposit; concluded that throughout an indefinite past, there had been
periodic convulsions, by which continents were raised, with intervening
eras of repose, during which such continents were worn down and
transformed into new marine strata, fated to be in their turns elevated
above the surface of the ocean. And finding that igneous action, to
which sundry earlier geologists had ascribed basaltic rocks, was in
countless places a cause of disturbance, he taught that from it resulted
these periodic convulsions. In this theory we see:--first, that the
previously-recognized agency of water was conceived to act, not as by
Werner, after a manner of which we have no experience, but after a
manner daily displayed to us; and secondly, that the igneous agency,
before considered only as originating special formations, was recognized
as a universal agency, but assumed to act in an unproved way. Werner's
sole process Hutton developed from the catastrophic and inexplicable
into the uniform and explicable; while that antagonistic second process,
of which he first adequately estimated the importance, was regarded by
him as a catastrophic one, and was not assimilated to known
processes--not explained. We have here to note, however, that the facts
collected and provisionally arranged in conformity with Werner's theory,
served, after a time, to establish Hutton's more rational theory--in so
far, at least, as aqueous formations are concerned; while the doctrine
of periodic subterranean convulsions, crudely as it was conceived by
Hutton, was a temporary generalization needful as a step towards the
theory of igneous action.
Since Hutton's time, the development of geological thought has gone
still further in the same direction. These early sweeping doctrines have
received additional qualifications. It has been discovered that more
numerous and more heterogeneous agencies have been at work, than was at
first believed. The conception of igneous action has been rationalized,
as the conception of aqueous action had previously been. The gratuitous
assumption that vast elevations suddenly occurred after long intervals
of quiescence, has grown into the consistent theory, that islands and
continents are the accumulated results of successive small upheavals,
like those experienced in ordinary earthquakes. To speak more
specifically, we find, in the first place, that instead of assuming the
denudation produced by rain and rivers to be the sole means of wearing
down lands and producing their irregularities of surface, geologists now
see that denudation is only a part-cause of such irregularities; and
further, that the new strata deposited at the bottom of the sea, are not
the products of river-sediment solely, but are in part due to the
actions of waves and tidal currents on the coasts. In the second place,
we find that Hutton's conception of upheaval by subterranean forces, has
not only been modified by assimilating these subterranean forces to
ordinary earthquake-forces; but modern inquiries have shown that,
besides elevations of surface, subsidences are thus produced; that local
upheavals, as well as the general upheavals which raise continents, come
within the same category; and that all these changes are probably
consequent on the progressive collapse of the Earth's crust upon its
cooling and contracting nucleus. In the third place, we find that beyond
these two great antagonistic agencies, modern Geology recognizes sundry
minor ones: those of glaciers and icebergs, those of coral-polypes;
those of _Protozoa_ having siliceous or calcareous shells--each of which
agencies, insignificant as it seems, is found capable of slowly working
terrestrial changes of considerable magnitude. Thus, then, the recent
progress of Geology has been a still further departure from primitive
conceptions. Instead of one catastrophic cause, once in universal
action, as supposed by Werner--instead of one general continuous cause,
antagonized at long intervals by a catastrophic cause, as taught by
Hutton; we now recognize several causes, all more or less general and
continuous. We no longer resort to hypothetical agencies to explain the
phenomena displayed by the Earth's crust; but we are day by day more
clearly perceiving that these phenomena have arisen from forces like
those now at work, which have acted in all varieties of combination,
through immeasurable periods of time.
* * * * *
Having thus briefly traced the evolution of geologic science, and noted
its present form, let us go on to observe the way in which it is still
swayed by the crude hypotheses it set out with; so that even now,
doctrines long since abandoned as untenable in theory, continue in
practice to mould the ideas of geologists, and to foster sundry beliefs
that are logically indefensible. We shall see, both how those simple
sweeping conceptions with which the science commenced, are those which
every student is apt at first to seize hold of, and how several
influences conspire to maintain the twist thus resulting--how the
original nomenclature of periods and formations necessarily keeps alive
the original implications; and how the need for arranging new data in
some order, results in their being thrust into the old classification,
unless their incongruity with it is very glaring. A few facts will best
prepare the way for criticism.
Up to 1839 it was inferred, from their crystalline character, that the
metamorphic rocks of Anglesea were more ancient than any rocks of the
adjacent main land; but it has since been shown that they are of the
same age with the slates and grits of Carnarvon and Merioneth. Again,
slaty cleavage having been first found only in the lowest rocks, was
taken as an indication of the highest antiquity: whence resulted serious
mistakes; for this mineral characteristic is now known to occur in the
Carboniferous system. Once more, certain red conglomerates and grits on
the north-west coast of Scotland, long supposed from their lithological
aspect to belong to the Old Red Sandstone, are now identified with the
Lower Silurians. These are a few instances of the small trust to be
placed in mineral qualities, as evidence of the ages or relative
positions of strata. From the recently-published third edition of
_Siluria_, may be culled numerous facts of like implication. Sir R.
Murchison considers it ascertained, that the siliceous Stiper stones of
Shropshire are the equivalents of the Tremadock slates of North Wales.
Judged by their fossils, Bala slate and limestone are of the same age as
the Caradoc sandstone, lying forty miles off. In Radnorshire, the
formation classed as upper Llandovery rock, is described at different
spots, as "sandstone or conglomerate," "impure limestone," "hard coarse
grits," "siliceous grit"--a considerable variation for so small an area
as that of a county. Certain sandy beds on the left bank of the Towy,
which Sir R. Murchison had, in his _Silurian System_, classed as Caradoc
sandstone (evidently from their mineral characters), he now finds, from
their fossils, belong to the Llandeilo formation. Nevertheless,
inferences from mineral characters are still habitually drawn and
received. Though _Siluria_, in common with other geological works,
supplies numerous proofs that rocks of the same age are often of
widely-different composition a few miles off, while rocks of
widely-different ages are often of similar composition; and though Sir
R. Murchison shows us, as in the case just cited, that he has himself in
past times been misled by trusting to lithological evidence; yet his
reasoning all through _Siluria_, shows that he still thinks it natural
to expect formations of the same age to be chemically similar, even in
remote regions. For example, in treating of the Silurian rocks of South
Scotland, he says:--"When traversing the tract between Dumfries and
Moffat, in 1850, it occurred to me, that the dull reddish or purple
sandstone and schist to the north of the former town, which so resembled
the bottom rocks of Longmynd, Llanberis, and St. David's, would prove to
be of the same age;" and further on, he again insists upon the fact that
these strata "are absolutely of the same composition as the bottom rocks
of the Silurian region." On this unity of mineral character it is, that
this Scottish formation is concluded to be contemporaneous with the
lowest formations in Wales; for the scanty paleontological evidence
suffices for neither proof nor disproof. Now, had there been a
continuity of like strata in like order between Wales and Scotland,
there might have been little to criticize in this conclusion. But since
Sir R. Murchison himself admits, that in Westmoreland and Cumberland,
some members of the system "assume a lithological aspect different from
what they maintain in the Silurian and Welsh region," there seems no
reason to expect mineralogical continuity in Scotland. Obviously,
therefore, the assumption that these Scottish formations are of the same
age with the Longmynd of Shropshire, implies the latent belief that
certain mineral characters indicate certain eras. Far more striking
instances, however, of the influence of this latent belief remain to be
given. Not in such comparatively near districts as the Scottish lowlands
only, does Sir R. Murchison expect a repetition of the Longmynd strata;
but in the Rhenish provinces, certain "quartzose flagstones and grits,
like those of the Longmynd," are seemingly concluded to be of
contemporaneous origin, because of their likeness. "Quartzites in
roofing-slates with a greenish tinge that reminded us of the lower
slates of Cumberland and Westmoreland," are evidently suspected to be of
the same age. In Russia, he remarks that the carboniferous limestones
"are overlaid along the western edge of the Ural chain by sandstones and
grits, which occupy much the same place in the general series as the
millstone grit of England;" and in calling this group, as he does, the
"representative of the millstone grit," Sir R. Murchison clearly shows
that he thinks likeness of mineral composition some evidence of
equivalence in time, even at that great distance. Nay, on the flanks of
the Andes and in the United States, such similarities are looked for,
and considered as significant of certain ages. Not that Sir R. Murchison
contends theoretically for this relation between lithological character
and date. For on the page from which we have just quoted (_Siluria_,
p. 387), he says, that "whilst the soft Lower Silurian clays and sands
of St. Petersburg have their equivalents in the hard schists and quartz
rocks with gold veins in the heart of the Ural mountains, the equally
soft red and green Devonian marls of the Valdai Hills are represented on
the western flank of that chain by hard, contorted, and fractured
limestones." But these, and other such admissions, seem to go for
little. While himself asserting that the Potsdam-sandstone of North
America, the Lingula-flags of England, and the alum-slates of
Scandinavia are of the same period--while fully aware that among the
Silurian formations of Wales, there are oolitic strata like those of
secondary age; yet his reasoning is more or less coloured by the
assumption, that formations of like qualities probably belong to the
same era. Is it not manifest, then, that the exploded hypothesis of
Werner continues to influence geological speculation?
"But," it will perhaps be said, "though individual strata are not
continuous over large areas, yet systems of strata are. Though within a
few miles the same bed gradually passes from clay into sand, or thins
out and disappears, yet the group of strata to which it belongs does not
do so; but maintains in remote regions the same relations to other
groups."
This is the generally-current belief. On this assumption the received
geological classifications appear to be framed. The Silurian system, the
Devonian system, the Carboniferous system, etc., are set down in our
books as groups of formations which everywhere succeed each other in a
given order; and are severally everywhere of the same age. Though it may
not be asserted that these successive systems are universal; yet it
seems to be tacitly assumed that they are. In North and South America,
in Asia, in Australia, sets of strata are assimilated to one or other of
these groups; and their possession of certain mineral characters and a
certain order of superposition are among the reasons assigned for so
assimilating them. Though, probably, no competent geologist would
contend that the European classification of strata is applicable to the
globe as a whole; yet most, if not all geologists, write as though it
were. Among readers of works on Geology, nine out of ten carry away the
impression that the divisions, Primary, Secondary and Tertiary, are of
absolute and uniform application; that these great divisions are
separable into subdivisions, each of which is definitely distinguishable
from the rest, and is everywhere recognizable by its characters as such
or such; and that in all parts of the Earth, these minor systems
severally began and ended at the same time. When they meet with the term
"Carboniferous era," they take for granted that it was an era
universally carboniferous--that it was, what Hugh Miller indeed actually
describes it, an era when the Earth bore a vegetation far more luxuriant
than it has since done; and were they in any of our colonies to meet
with a coal-bed, they would conclude that, as a matter of course, it was
of the same age as the English coal-beds.
Now this belief that geologic "systems" are universal, is no more
tenable than the other. It is just as absurd when considered _a priori_;
and it is equally inconsistent with the facts. Though some series of
strata classed together as Oolite, may range over a wider district than
any one stratum of the series; yet we have but to ask what were the
circumstances under which it was deposited, to see that the Oolitic
series, like one of its individual strata, must be of local origin; and
that there is not likely to be anywhere else, a series which
corresponds, either in its characters or in its commencement and
termination. For the formation of such a series implies an area of
subsidence, in which its component beds were thrown down. Every area of
subsidence is necessarily limited; and to suppose that there exist
elsewhere groups of beds completely answering to those known as Oolite,
is to suppose that, in contemporaneous areas of subsidence, like
processes were going on. There is no reason to suppose this; but good
reason to suppose the reverse. That in contemporaneous areas of
subsidence throughout the globe, the conditions would cause the
formation of Oolite, is an assumption which no modern geologist would
openly make. He would say that the equivalent series of beds found
elsewhere, would probably be of dissimilar mineral character. Moreover,
in these contemporaneous areas of subsidence, the processes going on
would not only be different in kind; but in no two cases would they be
likely to agree in their commencements and terminations. The
probabilities are greatly against separate portions of the Earth's
surface beginning to subside at the same time, and ceasing to subside at
the same time--a coincidence which alone could produce equivalent groups
of strata. Subsidences in different places begin and end with utter
irregularity; and hence the groups of strata thrown down in them can but
rarely correspond. Measured against each other in time, their limits
must disagree. On turning to the evidence, we find that it daily tends
more and more to justify these _a priori_ positions. Take, as an
example, the Old Red Sandstone system. In the north of England this is
represented by a single stratum of conglomerate. In Herefordshire,
Worcestershire, and Shropshire, it expands into a series of strata from
eight to ten thousand feet thick, made up of conglomerates, red, green,
and white sandstones, red, green, and spotted marls, and concretionary
limestones. To the south-west, as between Caermarthen and Pembroke,
these Old Red Sandstone strata exhibit considerable lithological
changes; on the other side of the Bristol Channel, they display further
changes in mineral characters; while in South Devon and Cornwall, the
equivalent strata, consisting chiefly of slates, schists, and
limestones, are so wholly different, that they were for a long time
classed as Silurian. When we thus see that in certain directions the
whole group of deposits thins out, and that its mineral characters
change within moderate distances; does it not become clear that the
whole group of deposits was a local one? And when we find, in other
regions, formations analogous to these Old Red Sandstone or Devonian
formations, is it certain--is it even probable--that they severally
began and ended at the same time with them? Should it not require
overwhelming evidence to make us believe as much?
Yet so strongly is geological speculation swayed by the tendency to
regard the phenomena as general instead of local, that even those most
on their guard against it seem unable to escape its influence. At page
158 of his _Principles of Geology_, Sir Charles Lyell says:--
"A group of red marl and red sandstone, containing salt and gypsum,
being interposed in England between the Lias and the Coal, all
other red marls and sandstones, associated some of them with salt,
and others with gypsum, and occurring not only in different parts
of Europe, but in North America, Peru, India, the salt deserts of
Asia, those of Africa--in a word, in every quarter of the globe,
were referred to one and the same period.... It was in vain to urge
as an objection the improbability of the hypothesis which implies
that all the moving waters on the globe were once simultaneously
charged with sediment of a red colour. But the rashness of
pretending to identify, in age, all the red sandstones and marls in
question, has at length been sufficiently exposed, by the discovery
that, even in Europe, they belong decidedly to many different
epochs."
Nevertheless, while in this and many kindred passages Sir C. Lyell
protests against the bias here illustrated, he seems himself not
completely free from it. Though he utterly rejects the old hypothesis
that all over the Earth the same continuous strata lie one upon another
in regular order, like the coats of an onion, he still writes as though
geologic "systems" do thus succeed each other. A reader of his _Manual_
would certainly suppose him to believe, that the Primary epoch ended,
and the secondary epoch began, all over the world at the same time--that
these terms really correspond to distinct universal eras. When he
assumes, as he does, that the division between Cambrian and Lower
Silurian in America, answers chronologically to the division between
Cambrian and Lower Silurian in Wales--when he takes for granted that
the partings of Lower from Middle Silurian, and of Middle Silurian from
Upper, in the one region, are of the same dates as the like partings in
the other region; does it not seem that he believes geologic "systems"
to be universal, in the sense that their separations were in all places
contemporaneous? Though he would, doubtless, disown this as an article
of faith, is not his thinking unconsciously influenced by it? Must we
not say that, though the onion-coat hypothesis is dead, its spirit is
traceable, under a transcendental form, even in the conclusions of its
antagonists?
* * * * *
Let us now consider another leading geological doctrine,--the doctrine
that strata of the same age contain like fossils; and that, therefore,
the age and relative position of any stratum may be known by its
fossils. While the theory that strata of like mineral characters were
everywhere deposited simultaneously, has been ostensibly abandoned,
there has been accepted the theory that in each geologic epoch similar
plants and animals existed everywhere; and that, therefore, the epoch to
which any formation belongs may be known by the organic remains
contained in the formation. Though, perhaps, no leading geologist would
openly commit himself to an unqualified assertion of this theory, yet it
is tacitly assumed in current geological reasoning.
This theory, however, is scarcely more tenable than the other. It cannot
be concluded with any certainty, that formations in which similar
organic remains are found, were of contemporaneous origin; nor can it be
safely concluded that strata containing different organic remains are of
different ages. To most readers these will be startling propositions;
but they are fully admitted by the highest authorities. Sir Charles
Lyell confesses that the test of organic remains must be used "under
very much the same restrictions as the test of mineral composition." Sir
Henry de la Beche, who variously illustrates this truth, remarks on the
great incongruity there must be between the fossils of our carboniferous
rocks and those of the marine strata deposited at the same period. But
though, in the abstract, the danger of basing positive conclusions on
evidence derived from fossils, is recognized; yet, in the concrete, this
danger is generally disregarded. The established convictions respecting
the ages of strata, have been formed in spite of it; and by some
geologists it seems altogether ignored. Throughout his _Siluria_, Sir R.
Murchison habitually assumes that the same, or kindred, species, lived
in all parts of the Earth at the same time. In Russia, in Bohemia, in
the United States, in South America, strata are classed as belonging to
this or that part of the Silurian system, because of the similar fossils
contained in them--are concluded to be everywhere contemporaneous if
they enclose a proportion of identical or allied forms. In Russia the
relative position of a stratum is inferred from the fact that, along
with some Wenlock forms, it yields the _Pentamerus oblongus_. Certain
crustaceans called _Eurypteri_, being characteristic of the Upper Ludlow
rock, it is remarked that "large Eurypteri occur in a so-called black
grey-wacke slate in Westmoreland, in Oneida County, New York, which will
probably be found to be on the parallel of the Upper Ludlow rock:" in
which word "probably," we see both how dominant is this belief of
universal distribution of similar creatures at the same period, and how
apt this belief is to make its own proof, by raising the expectation
that the ages are identical when the forms are alike. Besides thus
interpreting the formations of Russia, England, and America, Sir R.
Murchison thus interprets those of the antipodes. Fossils from Victoria
Colony, he agrees with the Government-surveyor in classing as of Lower
Silurian or Llandovery age: that is, he takes for granted that when
certain crustaceans and mollusks were living in Wales, certain similar
crustaceans and mollusks were living in Australia. Yet the
improbability of this assumption may be readily shown from Sir R.
Murchison's own facts. If, as he points out, the fossil crustaceans of
the uppermost Silurian rocks in Lanarkshire are, "with one doubtful
exception," all "distinct from any of the forms known on the same
horizon in England;" how can it be fairly presumed that the forms
existing on the other side of the Earth during the Silurian period, were
nearly allied to those existing here? Not only, indeed, do Sir R.
Murchison's conclusions tacitly assume this doctrine of universal
distribution, but he distinctly enunciates it. "The mere presence of a
graptolite," he says, "will at once decide that the enclosing rock is
Silurian;" and he says this, notwithstanding repeated warnings against
such generalizations. During the progress of Geology, it has over and
over again happened that a particular fossil, long considered
characteristic of a particular formation, has been afterwards discovered
in other formations. Until some twelve years ago, Goniatites had not
been found lower than the Devonian rocks; but now, in Bohemia, they have
been found in rocks classed as Silurian. Quite recently, the
_Orthoceras_, previously supposed to be a type exclusively palaeozoic,
has been detected along with mesozoic Ammonites and Belemnites. Yet
hosts of such experiences fail to extinguish the assumption, that the
age of a stratum may be determined by the occurrence in it of a single
fossil form. Nay, this assumption survives evidence of even a still more
destructive kind. Referring to the Silurian system in Western Ireland,
Sir R. Murchison says, "in the beds near Maam, Professor Nicol and
myself collected remains, some of which would be considered Lower, and
others Upper, Silurian;" and he then names sundry fossils which, in
England, belong to the summit of the Ludlow rocks, or highest Silurian
strata; "some, which elsewhere are known only in rocks of Llandovery
age"--that is, of middle Silurian age; and some, only before known in
Lower Silurian strata, not far above the most ancient fossiliferous
beds. Now what do these facts prove? Clearly, they prove that species
which in Wales are separated by strata more than twenty thousand feet
deep, and therefore seem to belong to periods far more remote from each
other, were really co-existent. They prove that the mollusks and
crinoids held to be characteristic of early Silurian strata, and
supposed to have become extinct long before the mollusks and crinoids of
the later Silurian strata came into existence, were really flourishing
at the same time with these last; and that these last possibly date back
to as early a period as the first. They prove that not only the mineral
characters of sedimentary formations, but also the collections of
organic forms they contain, depend, to a great extent, on local
circumstances. They prove that the fossils met with in any series of
strata, cannot be taken as representing anything like the whole Flora
and Fauna of the period they belong to. In brief, they throw great doubt
upon numerous geological generalizations.
Notwithstanding facts like these, and notwithstanding his avowed opinion
that the test of organic remains must be used "under very much the same
restrictions as the test of mineral composition," Sir Charles Lyell,
too, considers sundry positive conclusions to be justified by this test:
even where the community of fossils is slight and the distance great.
Having decided that in various places in Europe, middle Eocene strata
are distinguished by Nummulites; he infers, without any other assigned
evidence, that wherever Nummulites are found--in Morocco, Algeria,
Egypt, in Persia, Scinde, Cutch, Eastern Bengal, and the frontiers of
China--the containing formation is Middle Eocene. And from this
inference he draws the following important corollary:--
"When we have once arrived at the conviction that the nummulitic
formation occupies a middle place in the Eocene series, we are
struck with the comparatively modern date to which some of the
greatest revolutions in the physical geography of Europe, Asia, and
northern Africa must be referred. All the mountain chains, such as
the Alps, Pyrenees, Carpathians, and Himalayas, into the
composition of whose central and loftiest parts the nummulitic
strata enter bodily, could have had no existence till after the
Middle Eocene period."--_Manual_, p. 232.
A still more marked case follows on the next page. Because a certain bed
at Claiborne in Alabama, which contains "_four hundred_ species of
marine shells," includes among them the _Cardita planicosta_, "and _some
others_ identical with European species, or very nearly allied to them,"
Sir C. Lyell says it is "highly probable the Claiborne beds agree in age
with the central or Bracklesham group of England." When we find
contemporaneity alleged on the strength of a community no greater than
that which sometimes exists between strata of widely-different ages in
the same country, it seems as though the above-quoted caution had been
forgotten. It appears to be assumed for the occasion, that species which
had a wide range in space had a narrow range in time; which is the
reverse of the fact. The tendency to systematize overrides the evidence,
and thrusts Nature into a formula too rigid to fit her endless variety.
"But," it may be urged, "surely, when in different places the order of
superposition, the mineral characters, and the fossils, agree, it may
safely be concluded that the formations thus corresponding date back to
the same time. If, for example, the United States display a succession
of Silurian, Devonian, and Carboniferous systems, lithologically similar
to those known here by those names, and characterized by like fossils,
it is a fair inference that these groups of strata were severally being
deposited in America while their equivalents were being deposited here."
On this position, which seems a strong one, we have, in the first place,
to remark, that the evidence of correspondence is always more or less
suspicious. We have already adverted to the several "idols"--if we may
use Bacon's metaphor--to which geologists unconsciously sacrifice, when
interpreting the structures of unexplored regions. Carrying with them
the classification of strata existing in Europe, and assuming that
groups of strata in other parts of the world must answer to some of the
groups of strata known here, they are necessarily prone to assert
parallelism on insufficient evidence. They scarcely entertain the
previous question, whether the formations they are examining have or
have not any European equivalents; but the question is--with which of
the European series shall they be classed?--with which do they most
agree?--from which do they differ least? And this being the mode of
inquiry, there is apt to result great laxity of interpretation. How lax
the interpretation really is, may be readily shown. When strata are
discontinuous, as between Europe and America, no evidence can be derived
from the order of superposition, apart from mineral characters and
organic remains; for, unless strata can be continuously traced, mineral
characters and organic remains afford the only means of classing them as
such or such. As to the test of mineral characters, we have seen that it
is almost worthless; and no modern geologist would dare to say it should
be relied on. If the Old Red Sandstone series in mid-England, differs
wholly in lithological aspect from the equivalent series in South Devon,
it is clear that similarities of texture and composition cannot justify
us in classing a system of strata in another quarter of the globe with
some European system. The test of fossils is the only one that remains;
and with how little strictness this test is applied, one case will show.
Of forty-six species of British Devonian corals, only six occur in
America; and this, notwithstanding the wide range which the _Anthozoa_
are known to have. Similarly of the _Mollusca_ and _Crinoidea_, it
appears that, while there are sundry genera found in America which are
found here, there are scarcely any of the same species. And Sir Charles
Lyell admits that "the difficulty of deciding on the exact parallelism
of the New York subdivisions, as above enumerated, with the members of
the European Devonian, is very great, so few are the species in common."
Yet it is on the strength of community of fossils, that the whole
Devonian series of the United States is assumed to be contemporaneous
with the whole Devonian series of England. And it is partly on the
ground that the Devonian of the United States corresponds in time with
our own Devonian, that Sir Charles Lyell concludes the superjacent
coal-measures of the two countries to be of the same age. Is it not,
then, as we said, that the evidence in these cases is very suspicious?
Should it be replied, as it may fairly be, that this correspondence from
which the synchronism of distant formations is inferred, is not a
correspondence between particular species or particular genera, but
between the general characters of the contained assemblages of
fossils--between the _facies_ of the two Faunas; the rejoinder is, that
though such correspondence is a stronger evidence of synchronism it is
still an insufficient one. To infer synchronism from such
correspondence, involves the postulate that throughout each geologic era
there has habitually existed a recognizable similarity between the
groups of organic forms inhabiting all the different parts of the Earth;
and that the causes which have in one part of the Earth changed the
organic forms into those which characterize the next era, have
simultaneously acted in all other parts of the Earth, in such ways as to
produce parallel changes of their organic forms. Now this is not only a
large assumption to make; but it is an assumption contrary to
probability. The probability is, that the causes which have changed
Faunas have been local rather than universal; that hence while the
Faunas of some regions have been rapidly changing, those of others have
been almost quiescent; and that when those of others have been changed,
it has been, not in such ways as to maintain parallelism, but in such
ways as to produce divergence.
Even supposing, however, that districts some hundreds of miles apart,
furnished groups of strata which completely agreed in their order of
superposition, their mineral characters, and their fossils, we should
still have inadequate proof of contemporaneity. For there are
conditions, very likely to occur, under which such groups might differ
widely in age. If there be a continent of which the strata crop out on
the surface obliquely to the line of coast--running, say,
west-north-west, while the coast runs east and west--it is clear that
each group of strata will crop out on the beach at a particular part of
the coast; that further west the next group of strata will crop out on
the beach; and so continuously. As the localization of marine plants and
animals, is in a considerable degree determined by the natures of the
rocks and their detritus, it follows that each part of this coast will
have its more or less distinct Flora and Fauna. What now must result
from the action of the waves in the course of a geologic epoch? As the
sea makes slow inroads on the land, the place at which each group of
strata crops out on the beach will gradually move towards the west: its
distinctive fish, mollusks, crustaceans, and sea-weeds, migrating with
it. Further, the detritus of each of these groups of strata will, as the
point of outcrop moves westwards, be deposited over the detritus of the
group in advance of it. And the consequence of these actions, carried on
for one of those enormous periods which a geologic change takes, will be
that, corresponding to each eastern stratum, there will arise a stratum
far to the west, which, though occupying the same position relatively to
other beds, formed of like materials, and containing like fossils, will
yet be perhaps a million years later in date.
* * * * *
But the illegitimacy, or at any rate the great doubtfulness, of many
current geological inferences, is best seen when we contemplate
terrestrial changes now going on; and ask how far such inferences are
countenanced by them. If we carry out rigorously the modern method of
interpreting geological phenomena, which Sir Charles Lyell has done so
much to establish--that of referring them to causes like those at
present in action--we cannot fail to see how improbable are sundry of
the received conclusions.
Along each shore which is being worn away by the waves, there are being
formed mud, sand, and pebbles. This detritus has, in each locality, a
more or less special character; determined by the nature of the strata
destroyed. In the English Channel it is not the same as in the Irish
Channel; on the east coast of Ireland it is not the same as on the west
coast; and so throughout. At the mouth of each great river, there is
being deposited sediment differing more or less from that deposited at
the mouths of other rivers in colour and quality; forming strata which
are here red, there yellow, and elsewhere brown, grey, or dirty white.
Besides which various formations, going on in deltas and along shores,
there are some much wider, and still more strongly contrasted,
formations. At the bottom of the AEgean Sea, there is accumulating a bed
of Pteropod shells, which will eventually, no doubt, become a calcareous
rock. For some hundreds of thousands of square miles, the ocean-bed
between Great Britain and North America, is being covered with a stratum
of chalk; and over large areas in the Pacific, there are going on
deposits of coralline limestone. Thus, there are at this moment being
produced in different places multitudinous strata differing from one
another in lithological characters. Name at random any part of the
sea-bottom, and ask whether the deposit there taking place is like the
deposit taking place at some distant part of the sea-bottom, and the
almost-certainly correct answer will be--No. The chances are not in
favour of similarity, but against it--many to one against it.
In the order of superposition of strata there is being established a
like variety. Each region of the Earth's surface has its special history
of elevations, subsidences, periods of rest: and this history in no case
fits chronologically with the history of any other portion. River
deltas are now being thrown down on formations of different ages: some
very ancient, some quite modern. While here there has been deposited a
series of beds many hundreds of feet thick, there has elsewhere been
deposited but a single bed of fine mud. While one region of the Earth's
crust, continuing for a vast epoch above the surface of the ocean, bears
record of no changes save those resulting from denudation; another
region of the Earth's crust gives proof of sundry changes of level, with
their several resulting masses of stratified detritus. If anything is to
be judged from current processes, we must infer, not only that
everywhere the succession of sedimentary formations differs more or less
from the succession elsewhere; but also that in each place, there exist
groups of strata to which many other places have no equivalents.
With respect to the organic bodies imbedded in formations now in
progress, a like truth is equally manifest, if not more manifest. Even
along the same coast, within moderate distances, the forms of life
differ very considerably; and they differ much more on coasts that are
remote from one another. Again, dissimilar creatures which are living
together near the same shore, do not leave their remains in the same
beds of sediment. For instance, at the bottom of the Adriatic, where the
prevailing currents cause the deposits to be here of mud, and there of
calcareous matter, it is proved that different species of co-existing
shells are being buried in these respective formations. On our own
coasts, the marine remains found a few miles from shore, in banks where
fish congregate, are different from those found close to the shore,
where littoral species flourish. A large proportion of aquatic creatures
have structures which do not admit of fossilization; while of the rest,
the great majority are destroyed, when dead, by various kinds of
scavengers. So that no one deposit near our shores can contain anything
like a true representation of the Fauna of the surrounding sea; much
less of the co-existing Faunas of other seas in the same latitude; and
still less of the Faunas of seas in distant latitudes. Were it not that
the assertion seems needful, it would be almost absurd to say, that the
organic remains now being buried in the Dogger Bank, can tell us next to
nothing about the fish, crustaceans, mollusks, and corals, which are
being buried in the Bay of Bengal. Still stronger is the argument in the
case of terrestrial life. With more numerous and greater contrasts
between the types inhabiting one continent and those inhabiting another,
there is a far more imperfect registry of them. Schouw marks out on the
Earth more than twenty botanical regions, occupied by groups of forms so
distinct, that, if fossilized, geologists would scarcely be disposed to
refer them all to the same period. Of Faunas, the Arctic differs from
the Temperate; the Temperate from the Tropical; and the South Temperate
from the North Temperate. Nay, in the South Temperate Zone itself, the
two regions of South Africa and South America are unlike in their
mammals, birds, reptiles, fishes, mollusks, insects. The shells and
bones now lying at the bottoms of lakes and estuaries in these several
regions, have certainly not that similarity which is usually looked for
in those of contemporaneous strata; and the recent forms exhumed in any
one of these regions would very untruly represent the present Flora and
Fauna of the Earth. In conformity with the current style of geological
reasoning, an exhaustive examination of deposits in the Arctic circle,
might be held to prove that though at this period there were sundry
mammals existing, there were no reptiles; while the absence of mammals
in the deposits of the Galapagos Archipelago, where there are plenty of
reptiles, might be held to prove the reverse. And at the same time, from
the formations extending for two thousand miles along the great
barrier-reef of Australia--formations in which are imbedded nothing but
corals, echinoderms, mollusks, crustaceans, and fish, along with an
occasional turtle, or bird, or cetacean--it might be inferred that there
lived in our epoch neither terrestrial reptiles, nor terrestrial
mammals. The mention of Australia, indeed, suggests an illustration
which, even alone, would amply prove our case. The Fauna of this region
differs widely from any that is found elsewhere. On land, all the
indigenous mammals, except bats, belong to the lowest, or implacental
division; and the insects are singularly different from those found
elsewhere. The surrounding seas contain numerous forms which are more or
less strange; and among the fish there exists a species of shark, which
is the only living representative of a genus that flourished in early
geologic epochs. If, now, the modern fossiliferous deposits of Australia
were to be examined by one ignorant of the existing Australian Fauna;
and if he were to reason in the usual manner; he would be very unlikely
to class these deposits with those of the present time. How, then, can
we place confidence in the tacit assumption that certain formations in
remote parts of the Earth are referable to the same period, because the
organic remains contained in them display a certain community of
character? or that certain others are referable to different periods,
because the _facies_ of their Faunas are different?
"But," it will be replied, "in past eras the same, or similar, organic
forms were more widely distributed than now." It may be so; but the
evidence adduced by no means proves it. The argument by which this
conclusion is reached, runs a risk of being quoted as an example of
reasoning in a circle. As already pointed out, between formations in
remote regions the accepted test of equivalence is community of fossils.
If, then, the contemporaneity of remote formations is concluded from the
likeness of their fossils; how can it be said that similar plants and
animals were once more widely distributed, because they are found in
contemporaneous strata in remote regions? Is not the fallacy manifest?
Even supposing there were no such fatal objection as this, the evidence
commonly assigned would still be insufficient. For we must bear in mind
that the community of organic remains usually thought sufficient proof
of correspondence in time, is a very imperfect community. When the
compared sedimentary beds are far apart, it is scarcely expected that
there will be many species common to the two: it is enough if there be
discovered a considerable number of common genera. Now had it been
proved that throughout geologic time, each genus lived but for a short
period--a period measured by a single group of strata--something might
be inferred. But what if we learn that many of the same genera continued
to exist throughout enormous epochs, measured by several vast systems of
strata? "Among molluscs, the genera _Avicula_, _Modiola_, _Terebratula_,
_Lingula_, and _Orbicula_, are found from the Silurian rocks upwards to
the present day." If, then, between the lowest fossiliferous formations
and the most recent, there exists this degree of community; must we not
infer that there will probably often exist a great degree of community
between strata that are far from contemporaneous?
Thus the reasoning from which it is concluded that similar organic forms
were once more widely spread than now, is doubly fallacious; and,
consequently, the classifications of foreign strata based on the
conclusion are untrustworthy. Judging from the present distribution of
life, we cannot expect to find similar remains in geographically remote
strata of the same age; and where, between the fossils of geographically
remote strata, we do find much similarity, it is probably due rather to
likeness of conditions than to contemporaneity. If from causes and
effects such as we now witness, we reason back to the causes and effects
of past epochs, we discover inadequate warrant for sundry of the
received doctrines. Seeing, as we do, that in large areas of the Pacific
this is a period characterized by abundance of corals; that in the North
Atlantic it is a period in which a great chalk-deposit is being formed;
and that in the valley of the Mississippi it is a period of new
coal-basins--seeing also, as we do, that in one extensive continent this
is peculiarly an era of implacental mammals, and that in another
extensive continent it is peculiarly an era of placental mammals; we
have good reason to hesitate before accepting these sweeping
generalizations which are based on a cursory examination of strata
occupying but a tenth part of the Earth's surface.
* * * * *
At the outset, this article was to have been a review of the works of
Hugh Miller; but it has grown into something much more general.
Nevertheless, the remaining two doctrines which we propose to criticize,
may conveniently be treated in connexion with his name, as that of one
who fully committed himself to them. And first, a few words respecting
his position.
That he was a man whose life was one of meritorious achievement, every
one knows. That he was a diligent and successful working geologist,
scarcely needs saying. That with indomitable perseverance he struggled
up from obscurity to a place in the world of literature and science,
shows him to have been highly endowed in character and intelligence. And
that he had a remarkable power of presenting his facts and arguments in
an attractive form, a glance at any of his books will quickly prove. By
all means, let us respect him as a man of activity and sagacity, joined
with a large amount of poetry. But while saying this we must add, that
his reputation stands by no means so high in the scientific world as in
the world at large. Partly from the fact that our Scotch neighbours are
in the habit of blowing the trumpet rather loudly before their
notabilities--partly because the charming style in which his books are
written has gained him a large circle of readers--partly, perhaps,
through a praiseworthy sympathy with him as a self-made man; Hugh Miller
has met with an amount of applause which, little as we wish to diminish
it, must not be allowed to blind the public to his defects as a man of
science. The truth is, he was so far committed to a foregone conclusion,
that he could not become a philosophical geologist. He might be aptly
described as a theologian studying geology. The dominant idea with which
he wrote, may be seen in the titles of two of his books--_Footprints of
the Creator_,--_The Testimony of the Rocks_. Regarding geological facts
as evidence for or against certain religious conclusions, it was
scarcely possible for him to deal with geological facts impartially. His
ruling aim was to disprove the Development Hypothesis, the assumed
implications of which were repugnant to him; and in proportion to the
strength of his feeling, was the one-sidedness of his reasoning. He
admitted that "God might as certainly have _originated_ the species by a
law of development, as he _maintains_ it by a law of development;--the
existence of a First Great Cause is as perfectly compatible with the one
scheme as with the other." Nevertheless, he considered the hypothesis at
variance with Christianity; and therefore combated with it. He
apparently overlooked the fact, that the doctrines of geology in
general, as held by himself, had been rejected by many on similar
grounds; and that he had himself been repeatedly attacked for his
anti-Christian teachings. He seems not to have perceived that, just as
his antagonists were wrong in condemning as irreligious, theories which
he saw were not irreligious; so might he be wrong in condemning, on like
grounds, the Theory of Evolution. In brief, he fell short of that
highest faith which knows that all truths must harmonize; and which is,
therefore, content trustfully to follow the evidence whithersoever it
leads.
Of course it is impossible to criticize his works without entering on
this great question to which he chiefly devoted himself. The two
remaining doctrines to be here discussed, bear directly on this
question; and, as above said, we propose to treat them in connexion with
Hugh Miller's name, because, throughout his reasonings, he assumes
their truth. Let it not be supposed, however, that we shall aim to
prove what he has aimed to disprove. While we purpose showing that his
geological arguments against the Development Hypothesis are based on
invalid assumptions; we do not purpose showing that the geological
arguments urged in support of it are based on valid assumptions. We hope
to make it apparent that the geological evidence at present obtained, is
insufficient for either side; further, that there seems little
probability that sufficient evidence will ever be obtained; and that if
the question is eventually decided, it must be decided on other than
geological grounds.
* * * * *
The first of the current doctrines to which we have just referred, is,
that there occur in the serial records of former life on our planet, two
great blanks; whence it is inferred that, on at least two occasions, the
previously existing inhabitants of the Earth were almost wholly
destroyed, and a different class of inhabitants created. Comparing the
general life on the Earth to a thread, Hugh Miller says:--
"It is continuous from the present time up to the commencement of
the Tertiary period; and then so abrupt a break occurs, that, with
the exception of the microscopic diatomaceae, to which I last
evening referred, and of one shell and one coral, not a single
species crossed the gap. On its farther or remoter side, however,
where the Secondary division closes, the intermingling of species
again begins, and runs on till the commencement of this great
Secondary division; and then, just where the Palaeozoic division
closes, we find another abrupt break, crossed, if crossed at
all,--for there still exists some doubt on the subject,--by but two
species of plant."
These breaks are supposed to imply actual new creations on the surface
of our planet--supposed not by Hugh Miller only, but by the majority of
geologists. And the terms Palaeozoic, Mesozoic, and Cainozoic, are used
to indicate these three successive systems of life. It is true that some
accept this belief with caution; knowing how geologic research has been
all along tending to fill up what were once thought wide gaps. Sir
Charles Lyell points out that "the hiatus which exists in Great Britain
between the fossils of the Lias and those of the Magnesian Limestone,
is supplied in Germany by the rich fauna and flora of the Muschelkalk,
Keuper, and Bunter Sandstein, which we know to be of a date precisely
intermediate." Again he remarks that "until lately the fossils of the
coal-measures were separated from those of the antecedent Silurian group
by a very abrupt and decided line of demarcation; but recent discoveries
have brought to light in Devonshire, Belgium, the Eifel, and Westphalia,
the remains of a fauna of an intervening period." And once more, he
says, "we have also in like manner had some success of late years in
diminishing the hiatus which still separates the Cretaceous and Eocene
periods in Europe." To which let us add that, since Hugh Miller penned
the passage above quoted, the second of the great gaps he refers to has
been very considerably narrowed by the discovery of strata containing
Palaeozoic genera and Mesozoic genera intermingled. Nevertheless, the
occurrence of two great revolutions in the Earth's Flora and Fauna
appears still to be held by many; and geologic nomenclature habitually
assumes it.
Before seeking a solution of the problem thus raised, let us glance at
the several minor causes which produce breaks in the geological
succession of organic forms; taking first, the more general ones which
modify climate, and, therefore, the distribution of life. Among these
may be noted one which has not, we believe, been named by writers on the
subject. We mean that resulting from a certain slow astronomic rhythm,
by which the northern and southern hemispheres are alternately subject
to greater extremes of temperature. In consequence of the slight
ellipticity of its orbit, the Earth's distance from the sun varies to
the extent of some 3,000,000 of miles. At present, the aphelion occurs
at the time of our northern summer; and the perihelion during the summer
of the southern hemisphere. In consequence, however, of that slow
movement of the Earth's axis which produces the precession of the
equinoxes, this state of things will in time be reversed: the Earth
will be nearest to the sun during the summer of the northern hemisphere,
and furthest from it during the southern summer or northern winter. The
period required to complete the slow movement producing these changes,
is nearly 26,000 years; and were there no modifying process, the two
hemispheres would alternately experience this coincidence of summer with
relative nearness to the sun, during a period of 13,000 years. But there
is also a still slower change in the direction of the axis major of the
Earth's orbit; from which it results that the alternation we have
described is completed in about 21,000 years. That is to say, if at a
given time the Earth is nearest to the sun at our mid-summer, and
furthest from the sun at our mid-winter; then, in 10,500 years
afterwards, it will be furthest from the sun at our mid-summer, and
nearest at our mid-winter. Now the difference between the distances from
the sun at the two extremes of this alternation, amounts to
one-thirtieth; and hence, the difference between the quantities of heat
received from the sun on a summer's day under these opposite conditions
amounts to one-fifteenth. Estimating this, not with reference to the
zero of our thermometers, but with reference to the temperature of the
celestial spaces, Sir John Herschel calculates "23 deg. Fahrenheit, as the
least variation of temperature under such circumstances which can
reasonably be attributed to the actual variation of the sun's distance."
Thus, then, each hemisphere has at a certain epoch, a short summer of
extreme heat, followed by a long and very cold winter. Through the slow
change in the direction of the Earth's axis, these extremes are
gradually mitigated. And at the end of 10,500 years, there is reached
the opposite state--a long and moderate summer, with a short and mild
winter. At present, in consequence of the predominance of sea in the
southern hemisphere, the extremes to which its astronomical conditions
subject it, are much ameliorated; while the great proportion of land in
the northern hemisphere, tends to exaggerate such contrast as now
exists in it between winter and summer: whence it results that the
climates of the two hemispheres are not widely unlike. But 10,000 years
hence, the northern hemisphere will undergo annual variations of
temperature far more marked than now.
In the last edition of his _Outlines of Astronomy_, Sir John Herschel
recognizes this as an element in geological processes; regarding it as
possibly a part-cause of those climatic changes indicated by the records
of the Earth's past. That it has had much to do with those larger
changes of climate of which we have evidence, seems unlikely, since
there is reason to think that these have been far slower and more
lasting; but that it must have entailed a rhythmical exaggeration and
mitigation of the climates otherwise produced, seems beyond question.
And it seems also beyond question that there must have been a consequent
rhythmical change in the distribution of organisms--a rhythmical change
to which we here wish to draw attention, as one cause of minor breaks in
the succession of fossil remains. Each species of plant and animal has
certain limits of heat and cold within which only it can exist; and
these limits in a great degree determine its geographical position. It
will not spread north of a certain latitude, because it cannot bear a
more northern winter, nor south of a certain latitude, because the
summer heat is too great; or else it is indirectly restrained from
spreading further by the effect of temperature on the humidity of the
air, or on the distribution of the organisms it lives upon. But now,
what will result from a slow alteration of climate, produced as above
described? Supposing the period we set out from is that in which the
contrast of seasons is least marked, it is manifest that during the
progress towards the period of most violent contrast, each species of
plant and animal will gradually change its limits of distribution--will
be driven back, here by the winter's increasing cold, and there by the
summer's increasing heat--will retire into those localities that are
still fit for it. Thus during 10,000 years, each species will ebb away
from certain regions it was inhabiting; and during the succeeding 10,000
years will flow back into those regions. From the strata there forming,
its remains will disappear; they will be absent from some of the
superposed strata; and will be found in strata higher up. But in what
shapes will they re-appear? Exposed during the 21,000 years of their
slow recession and their slow return, to changing conditions of life,
they are likely to have undergone modifications; and will probably
re-appear with slight differences of constitution and perhaps of
form--will be new varieties or perhaps new sub-species.
To this cause of minor breaks in the succession of organic forms--a
cause on which we have dwelt because it has not been taken into
account--we must add sundry others. Besides these periodically-recurring
changes of climate, there are the irregular ones produced by
redistributions of land and sea; and these, sometimes less, sometimes
greater, in degree, than the rhythmical changes, must, like them, cause
in each region emigrations and immigrations of species; and consequent
breaks, small or large as the case may be, in the paleontological
series. Other and more special geological changes must produce other and
more local blanks in the succession. By some inland elevation the
natural drainage of a continent is modified; and instead of the sediment
previously brought down to the sea by it, a great river brings down
sediment unfavourable to various plants and animals living in its delta:
whereupon these disappear from the locality, perhaps to re-appear in a
changed form after a long epoch. Upheavals or subsidences of shores or
sea-bottoms, involving deviations of marine currents, remove the
habitats of many species to which such currents are salutary or
injurious; and further, this redistribution of currents alters the
places of sedimentary deposits, and thus stops the burying of organic
remains in some localities, while commencing it in others. Had we space,
many more such causes of blanks in our paleontological records might be
added. But it is needless here to enumerate them. They are admirably
explained and illustrated in Sir Charles Lyell's _Principles of
Geology_.
Now, if these minor changes of the Earth's surface produce minor breaks
in the series of fossilized remains; must not great changes produce
great breaks? If a local upheaval or subsidence causes throughout its
small area the absence of some links in the chain of fossil forms; does
it not follow that an upheaval or subsidence extending over a large part
of the Earth's surface, must cause the absence of a great number of such
links throughout a very wide area?
When during a long epoch a continent, slowly sinking, gives place to a
far-spreading ocean some miles in depth, at the bottom of which no
deposits from rivers or abraded shores can be thrown down; and when,
after some enormous period, this ocean-bottom is gradually elevated and
becomes the site for new strata; it is clear that the fossils contained
in these new strata are likely to have but little in common with the
fossils of the strata below them. Take, in illustration, the case of the
North Atlantic. We have already named the fact that between this country
and the United States, the ocean-bottom is being covered with a deposit
of chalk--a deposit which has been forming, probably, ever since there
occurred that great depression of the Earth's crust from which the
Atlantic resulted in remote geologic times. This chalk consists of the
minute shells of _Foraminifera_, sprinkled with remains of small
_Entomostraca_, and probably a few Pteropod-shells; though the sounding
lines have not yet brought up any of these last. Thus, in so far as all
high forms of life are concerned, this new chalk-formation must be a
blank. At rare intervals, perhaps, a polar bear, drifted on an iceberg,
may have its bones scattered over the bed; or a dead, decaying whale
may similarly leave traces. But such remains must be so rare, that this
new chalk-formation, if accessible, might be examined for a century
before any of them were disclosed. If now, some millions of years hence,
the Atlantic-bed should be raised, and estuary deposits or shore
deposits laid upon it, these would contain remains of a Flora and a
Fauna so distinct from everything below them, as to appear like a new
creation.
Thus, along with continuity of life on the Earth's surface, there not
only _may_ be, but there _must_ be, great gaps in the series of fossils;
and hence these gaps are no evidence against the doctrine of Evolution.
* * * * *
One other current assumption remains to be criticized; and it is the one
on which, more than on any other, depends the view taken respecting the
question of development.
From the beginning of the controversy, the arguments for and against
have turned upon the evidence of progression in organic forms, found in
the ascending series of our sedimentary formations. On the one hand,
those who contend that higher organisms have been evolved out of lower,
joined with those who contend that successively higher organisms have
been created at successively later periods, appeal for proof to the
facts of Paleontology; which, they say, countenance their views. On the
other hand, the Uniformitarians, who not only reject the hypothesis of
development, but deny that the modern forms of life are higher than the
ancient ones, reply that the paleontological evidence is at present very
incomplete; that though we have not yet found remains of
highly-organized creatures in strata of the greatest antiquity, we must
not assume that no such creatures existed when those strata were
deposited; and that, probably, search will eventually disclose them.
It must be admitted that thus far, the evidence has gone in favour of
the latter party. Geological discovery has year after year shown the
small value of negative facts. The conviction that there are no traces
of higher organisms in earlier strata, has resulted not from the absence
of such traces, but from incomplete examination. At p. 460 of his
_Manual of Elementary Geology_, Sir Charles Lyell gives a list in
illustration of this. It appears that in 1709, fishes were not known
lower than the Permian system. In 1793 they were found in the subjacent
Carboniferous system; in 1828 in the Devonian; in 1840 in the Upper
Silurian. Of reptiles, we read that in 1710 the lowest known were in the
Permian; in 1844 they were detected in the Carboniferous; and in 1852 in
the Upper Devonian. While of the Mammalia the list shows that in 1798
none had been discovered below the Middle Eocene: but that in 1818 they
were discovered in the Lower Oolite; and in 1847 in the Upper Trias.
The fact is, however, that both parties set out with an inadmissible
postulate. Of the Uniformitarians, not only such writers as Hugh Miller,
but also such as Sir Charles Lyell,[27] reason as though we had found
the earliest, or something like the earliest, strata. Their antagonists,
whether defenders of the Development Hypothesis or simply
Progressionists, almost uniformly do the like. Sir R. Murchison, who is
a Progressionist, calls the lowest fossiliferous strata, "Protozoic."
Prof. Ansted uses the same term. Whether avowedly or not, all the
disputants stand on this assumption as their common ground.
Yet is this assumption indefensible, as some who make it very well know.
Facts may be cited against it which show that it is a more than
questionable one--that it is a highly improbable one; while the evidence
assigned in its favour will not bear criticism.
Because in Bohemia, Great Britain, and portions of North America, the
lowest unmetamorphosed strata yet discovered, contain but slight traces
of life, Sir R. Murchison conceives that they were formed while yet few,
if any, plants or animals had been created; and, therefore, classes them
as "Azoic." His own pages, however, show the illegitimacy of the
conclusion that there existed at that period no considerable amount of
life. Such traces of life as have been found in the Longmynd rocks, for
many years considered unfossiliferous, have been found in some of the
lowest beds; and the twenty thousand feet of superposed beds, still
yield no organic remains. If now these superposed strata throughout a
depth of four miles, are without fossils, though the strata over which
they lie prove that life had commenced; what becomes of Sir R.
Murchison's inference? At page 189 of _Siluria_, a still more conclusive
fact will be found. The "Glengariff grits," and other accompanying
strata there described as 13,500 feet thick, contain no signs of
contemporaneous life. Yet Sir R. Murchison refers them to the Devonian
period--a period which had a large and varied marine Fauna. How then,
from the absence of fossils in the Longmynd beds and their equivalents,
can we conclude that the Earth was "azoic" when they were formed?
"But," it may be asked, "if living creatures then existed, why do we not
find fossiliferous strata of that age, or an earlier age?" One reply is,
that the non-existence of such strata is but a negative fact--we have
not found them. And considering how little we know even of the
two-fifths of the Earth's surface now above the sea, and how absolutely
ignorant we are of the three-fifths below the sea, it is rash to say
that no such strata exist. But the chief reply is, that these records of
the Earth's earlier history have been in great part destroyed, by
agencies which are ever tending to destroy such records.
It is an established geological doctrine, that sedimentary strata are
liable to be changed, more or less profoundly, by igneous action. The
rocks originally classed as "transition," because they were
intermediate in character between the igneous rocks found below them,
and the sedimentary strata found above them, are now known to be nothing
else than sedimentary strata altered in texture and appearance by the
intense heat of adjacent molten matter; and hence are renamed
"metamorphic rocks." Modern researches have shown, too, that these
metamorphic rocks are not, as was once supposed, all of the same age.
Besides primary and secondary strata which have been transformed by
igneous action, there are similarly-changed deposits of tertiary
origin--deposits changed, even as far as a quarter of a mile from the
point of contact with neighbouring granite. By this process fossils are
of course destroyed. "In some cases," says Sir Charles Lyell, "dark
limestones, replete with shells and corals, have been turned into white
statuary marble, and hard clays, containing vegetable or other remains,
into slates called mica-schist or hornblende-schist; every vestige of
the organic bodies having been obliterated." Again, it is fast becoming
an acknowledged truth that igneous rock, of whatever kind, is the
product of sedimentary strata which have been completely melted. Granite
and gneiss, which are of like chemical composition, have been shown, in
various cases, to pass one into the other; as at Valorsine, near Mont
Blanc, where the two, in contact, are observed to "both undergo a
modification of mineral character. The granite still remaining
unstratified, becomes charged with green particles; and the talcose
gneiss assumes a granitiform structure without losing its
stratification." In the Aberdeen-granite, lumps of unmelted gneiss are
abundant; and we can ourselves bear witness that the granite on the
banks of Loch Sunart yields proofs that, when molten, it contained
incompletely-fused clots of sedimentary strata. Nor is this all. Fifty
years ago, it was thought that all granitic rocks were primitive, or
existed before any sedimentary strata; but it is now "no easy task to
point out a single mass of granite demonstrably more ancient than all
the known fossiliferous deposits." In brief, accumulated evidence shows,
that by contact with, or proximity to, the molten matter of the Earth's
nucleus, all beds of sediment are liable to be actually melted, or
partially fused, or so heated as to agglutinate their particles; and
that according to the temperature they have been raised to, and the
circumstances under which they cool, they assume the forms of granite,
porphyry, trap, gneiss, or rock otherwise altered. Further, it is
manifest that though strata of various ages have been thus changed, yet
the most ancient strata have been so changed to the greatest extent;
both because they have been nearer to the centre of igneous agency; and
because they have been for longer periods liable to be affected by it.
Whence it follows, that sedimentary strata passing a certain antiquity,
are unlikely to be found in an unmetamorphosed state; and that strata
much earlier than these are certain to have been melted up. Thus if,
throughout a past of indefinite duration, there had been at work those
aqueous and igneous agencies which we see still at work, the state of
the Earth's crust might be just what we find it. We have no evidence
which puts a limit to the period throughout which this formation and
destruction of strata has been going on. For aught the facts prove, it
may have been going on for ten times the period measured by our whole
series of sedimentary deposits.
Besides having, in the present appearances of the Earth's crust, no data
for fixing a commencement to these processes--besides finding that the
evidence permits us to assume such commencement to have been
inconceivably remote, as compared even with the vast eras of geology; we
are not without positive grounds for inferring the inconceivable
remoteness of such commencement. Modern geology has established truths
which are irreconcilable with the belief that the formation and
destruction of strata began when the Cambrian rocks were formed; or at
anything like so recent a time. One fact from _Siluria_ will suffice.
Sir R. Murchison estimates the vertical thickness of Silurian strata in
Wales, at from 26,000 to 27,000 feet, or about five miles; and if to
this we add the vertical depth of the Cambrian strata, on which the
Silurians lie conformably, there results, on the lowest computation, a
total depth of some seven miles. Now it is held by geologists, that this
vast series of formations must have been deposited in an area of gradual
subsidence. These beds could not have been thus laid one on another in
regular order, unless the Earth's crust had been at that place sinking,
either continuously or by small steps. Such an immense subsidence,
however, must have been impossible without a crust of great thickness.
The Earth's molten nucleus tends ever, with enormous force, to assume
the form of a regular oblate spheroid. Any depression of its crust below
the surface of equilibrium, and any elevation of its crust above that
surface, have to withstand immense resistances. It follows inevitably
that, with a thin crust, nothing but small elevations and subsidences
would have been possible; and that, conversely, a subsidence of seven
miles implies a crust of great strength, or, in other words, of great
thickness. Indeed, if we compare this inferred subsidence in the
Silurian period, with such elevations and depressions as our existing
continents and oceans display, we see no evidence that the Earth's crust
was appreciably thinner then than now. What are the implications? If, as
geologists generally admit, the Earth's crust has resulted from that
slow cooling which is even still going on--if we see no sign that at the
time when the earliest Cambrian strata were formed, this crust was
appreciably thinner than now; we are forced to conclude that the era
during which it acquired that great thickness possessed in the Cambrian
period, was enormous as compared with the interval between the Cambrian
period and our own. But during the incalculable series of epochs thus
implied, there existed an ocean, tides, winds, waves, rain, rivers. The
agencies by which the denudation of continents and filling up of seas
have all along been carried on, were as active then as now. Endless
successions of strata must have been formed. And when we ask--Where are
they? Nature's obvious reply is--They have been destroyed by that
igneous action to which so great a part of our oldest-known strata owe
their fusion or metamorphosis.
Only the last chapter of the Earth's history has come down to us. The
many previous chapters, stretching back to a time immeasurably remote,
have been burnt; and with them all the records of life we may presume
they contained. The greater part of the evidence which might have served
to settle the Development-controversy, is for ever lost; and on neither
side can the arguments derived from Geology be conclusive.
"But how happen there to be such evidences of progression as exist?" it
may be asked. "How happens it that, in ascending from the most ancient
strata to the most recent strata, we _do_ find a succession of organic
forms, which, however irregularly, carries us from lower to higher?"
This question seems difficult to answer. Nevertheless, there is reason
for thinking that nothing can be safely inferred from the apparent
progression here cited. And the illustration which shows as much, will,
we believe, also show how little trust is to be placed in certain
geological generalizations that appear to be well established. With this
somewhat elaborate illustration, to which we now pass, our criticisms
may fitly conclude.
* * * * *
Let us suppose that in a region now covered by wide ocean, there begins
one of those great and gradual upheavals by which new continents are
formed. To be precise, let us say that in the South Pacific, midway
between New Zealand and Patagonia, the sea-bottom has been little by
little thrust up toward the surface, and is about to emerge. What will
be the successive phenomena, geological and biological, which are
likely to occur before this emerging sea-bottom has become another
Europe or Asia? In the first place, such portions of the incipient land
as are raised to the level of the waves, will be rapidly denuded by
them: their soft substance will be torn up by the breakers, carried away
by the local currents, and deposited in neighbouring deeper water.
Successive small upheavals will bring new and larger areas within reach
of the waves; fresh portions will each time be removed from the surfaces
previously denuded; and further, some of the newly-formed strata, being
elevated nearly to the level of the water, will be washed away and
re-deposited. In course of time the harder formations of the upraised
sea-bottom will be uncovered. These, being less easily destroyed, will
remain permanently above the surface; and at their margins will arise
the usual breaking down of rocks into beach-sand and pebbles. While in
the slow course of this elevation, going on at the rate of perhaps two
or three feet in a century, most of the sedimentary deposits produced
will be again and again destroyed and reformed; there will, in those
adjacent areas of subsidence which accompany areas of elevation, be more
or less continuous successions of sedimentary deposits lying on the
pre-existing ocean bed. And now, what will be the character of these
strata, old and new? They will contain scarcely any traces of life. The
deposits that had previously been slowly formed at the bottom of this
wide ocean, would be sprinkled with fossils of but few species. The
oceanic Fauna is not a rich one; its hydrozoa do not admit of
preservation; and the hard parts of its few kinds of molluscs and
crustaceans and insects are mostly fragile. Hence, when the ocean-bed
was here and there raised to the surface--when its strata of sediment
with their contained organic fragments were torn up and long washed
about by the breakers before being re-deposited--when the re-deposits
were again and again subject to this violent abrading action by
subsequent small elevations, as they would mostly be; what few fragile
organic remains they contained, would be in nearly all cases destroyed.
Thus such of the first-formed strata as survived the repeated changes of
level, would be practically "azoic;" like the Cambrian of our
geologists. When by the washing away of the soft deposits, the hard
sub-strata had been exposed in the shape of rocky islets, and a footing
had thus been furnished, the pioneers of a new life might be expected to
make their appearance. What would they be? Not any of the surrounding
oceanic species, for these are not fitted for a littoral life; but
species flourishing on some of the far-distant shores of the Pacific. Of
such, the first to establish themselves would be sea-weeds and
zoophytes; because the most readily conveyed on floating wood, &c., and
because when conveyed they would find fit food. It is true that
Cirrhipeds and Lamellibranchs, subsisting on the minute creatures which
everywhere people the sea, would also find fit food. But the chances of
early colonization are in favour of species which, multiplying by
agamogenesis, can people a whole shore from a single germ; and against
species which, multiplying only by gamogenesis, must be introduced in
considerable numbers that some may propagate. Thus we infer that the
earliest traces of life left in the sedimentary deposits near these new
shores, will be traces of life as humble as that indicated in the most
ancient rocks of Great Britain and Ireland. Imagine now that the
processes above indicated, continue--that the emerging lands become
wider in extent, and fringed by higher and more varied shores; and that
there still go on those ocean-currents which, at long intervals, convey
from far distant shores immigrant forms of life. What will result? Lapse
of time will of course favour the introduction of such new forms:
admitting, as it must, of those combinations of fit conditions, which
can occur only after long intervals. Moreover, the increasing area of
the islands, individually and as a group, implies increasing length of
coast, and therefore a longer line of contact with the streams and waves
which bring drifting masses bearing germs of fresh life. And once more,
the comparatively-varied shores, presenting physical conditions which
change from mile to mile, will furnish suitable habitats for more
numerous species. So that as the elevation proceeds, three causes
conspire to introduce additional marine plants and animals. To what
classes will the increasing Fauna be for a long period confined? Of
course, to classes of which individuals, or their germs, are most liable
to be carried far away from their native shores by floating sea-weed or
drift-wood; to classes which are also least likely to perish in transit,
or from change of climate; and to those which can best subsist around
coasts comparatively bare of life. Evidently then, corals, annelids,
inferior molluscs, and crustaceans of low grade, will chiefly constitute
the early Fauna. The large predatory members of these classes, will be
later in establishing themselves; both because the new shores must first
become well peopled by the creatures they prey on, and because, being
more complex, they, or their ova, must be less likely to survive the
journey, and the change of conditions. We may infer, then, that the
strata deposited next after the almost "azoic" strata, would contain the
remains of invertebrata, allied to those found near the shores of
Australia and South America. Of such invertebrate remains, the lower
beds would furnish comparatively few genera, and those of relatively low
types; while in the upper beds the number of genera would be greater,
and the types higher: just as among the fossils of our Silurian system.
As this great geologic change slowly advanced through its long history
of earthquakes, volcanic disturbances, minor upheavals and
subsidences--as the extent of the archipelago became greater and its
smaller islands coalesced into larger ones, while its coast-line grew
still longer and more varied, and the neighbouring sea more thickly
inhabited by inferior forms of life; the lowest division of the
vertebrata would begin to be represented. In order of time, fish would
naturally come later than the lower invertebrata; both as being less
likely to have their ova transported across the waste of waters, and as
requiring for their subsistence a pre-existing Fauna of some
development. They might be expected to make their appearance along with
the predaceous crustaceans; as they do in the uppermost Silurian rocks.
And here, too, let us remark, that as, during this long epoch we have
been describing, the sea would have made great inroads on some of the
newly-raised lands which had remained stationary; and would probably in
some places have reached masses of igneous or metamorphic rocks; there
might, in course of time, arise by the decomposition and denudation of
such rocks, local deposits coloured with oxide of iron, like our Old Red
Sandstone. And in these deposits might be buried the remains of the fish
then peopling the neighbouring sea.
Meanwhile, how would the surfaces of the upheaved masses be occupied? At
first their deserts of naked rocks would bear only the humblest forms of
vegetal life, such as we find in grey and orange patches on our own
rugged mountain sides; for these alone could flourish on such surfaces,
and their spores would be the most readily transported. When, by the
decay of such protophytes, and that decomposition of rock effected by
them, there had resulted a fit habitat for mosses; these, of which the
germs might be conveyed in drifted trees, would begin to spread. A soil
having been eventually thus produced, it would become possible for
plants of higher organization to find roothold; and as the archipelago
and its constituent islands grew larger, and had more multiplied
relations with winds and waters, such higher plants might be expected
ultimately to have their seeds transferred from the nearest lands. After
something like a Flora had thus colonized the surface, it would become
possible for insects to exist; and of air-breathing creatures, insects
would manifestly be among the first to find their way from elsewhere.
As, however, terrestrial organisms, both vegetal and animal, are less
likely than marine organisms to survive the accidents of transport from
distant shores; it is inferable that long after the sea surrounding
these new lands had acquired a varied Flora and Fauna, the lands
themselves would still be comparatively bare; and thus that the early
strata, like our Silurians, would afford no traces of terrestrial life.
By the time that large areas had been raised above the ocean, we may
fairly suppose a luxuriant vegetation to have been acquired. Under what
circumstances are we likely to find this vegetation fossilized? Large
surfaces of land imply large rivers with their accompanying deltas; and
are liable to have lakes and swamps. These, as we know from extant
cases, are favourable to rank vegetation; and afford the conditions
needful for preserving it in coal-beds. Observe, then, that while in the
early history of such a continent a carboniferous period could not
occur, the occurrence of a carboniferous period would become probable
after long-continued upheavals had uncovered large areas. As in our own
sedimentary series, coal-beds would make their appearance only after
there had been enormous accumulations of earlier strata charged with
marine fossils.
Let us ask next, in what order the higher forms of animal life would
make their appearance. We have seen how, in the succession of marine
forms, there would be something like a progress from the lower to the
higher: bringing us in the end to predaceous molluscs, crustaceans, and
fish. What are likely to succeed fish? After marine creatures, those
which would have the greatest chance of surviving the voyage would be
amphibious reptiles; both because they are more tenacious of life than
higher animals, and because they would be less completely out of their
element. Such reptiles as can live in both fresh and salt water, like
alligators; and such as are drifted out of the mouths of great rivers on
floating trees, as Humboldt says the Orinoco alligators are; might be
early colonists. It is manifest, too, that reptiles of other kinds would
be among the first vertebrata to people the new continent. If we
consider what will occur on one of those natural rafts of trees, soil,
and matted vegetable matter, sometimes swept out to sea by such currents
as the Mississippi, with a miscellaneous living cargo; we shall see that
while the active, hot-blooded, highly-organized creatures will soon die
of starvation and exposure, the inert, cold-blooded ones, which can go
long without food, will live perhaps for weeks; and so, out of the
chances from time to time occurring during long periods, reptiles will
be the first to get safely landed on foreign shores: as indeed they are
even now known sometimes to be. The transport of mammalia being
comparatively precarious, must, in the order of probability, be longer
postponed; and would, indeed, be unlikely to occur until by the
enlargement of the new continent, the distances of its shores from
adjacent lands had been greatly diminished, or the formation of
intervening islands had increased the chances of survival. Assuming,
however, that the facilities for immigration had become adequate; which
would be the first mammals to arrive and live? Not large herbivores; for
they would be soon drowned if by any accident carried out to sea. Not
the carnivora; for these would lack appropriate food, even if they
outlived the voyage. Small quadrupeds frequenting trees, and feeding on
insects, would be those most likely both to be drifted away from their
native lands and to find fit food in a new one. Insectivorous mammals,
like in size to those found in the Trias and the Stonesfield slate,
might naturally be looked for as the pioneers of the higher vertebrata.
And if we suppose the facilities of communication to be again increased,
either by a further shallowing of the intervening sea and a consequent
multiplication of islands, or by an actual junction of the new continent
with an old one, through continued upheavals; we should finally have an
influx of the larger and more perfect mammals.
Now rude as is this sketch of a process that would be extremely
elaborate and involved, and open as some of its propositions are to
criticisms which there is no space here to meet; no one will deny that
it represents something like the biologic history of the supposed new
continent. Details apart, it is manifest that simple organisms, able to
flourish under simple conditions of life, would be the first successful
immigrants; and that more complex organisms, needing for their existence
the fulfilment of more complex conditions, would afterwards establish
themselves in something like an ascending succession. At the one extreme
we see every facility. The new individuals can be conveyed in the shape
of minute germs; immense numbers of these are perpetually being carried
in all directions to great distances by ocean-currents--either detached
or attached to floating bodies; they can find nutriment wherever they
arrive; and the resulting organisms can multiply asexually with great
rapidity. At the other extreme, we see every difficulty. The new
individuals must be conveyed in their adult forms; their numbers are, in
comparison, utterly insignificant; they live on land, and are very
unlikely to be carried out to sea; when so carried, the chances are
immense against their escape from drowning, starvation, or death by
cold; if they survive the transit, they must have a pre-existing Flora
or Fauna to supply their special food; they require, also, the
fulfilment of various other physical conditions; and unless at least two
individuals of different sexes are safely landed, the race cannot be
established. Manifestly, then, the immigration of each successively
higher order of organisms, having, from one or other additional
condition to be fulfilled, an enormously-increased probability against
it, would naturally be separated from the immigration of a lower order
by some period like a geologic epoch. And thus the successive
sedimentary deposits formed while this new continent was undergoing
gradual elevation, would seem to furnish clear evidence of a general
progress in the forms of life. That lands thus raised up in the midst of
a wide ocean, would first give origin to unfossiliferous strata; next,
to strata containing only the lowest marine forms; next to strata
containing only the higher marine forms, ascending finally to fish; and
that the strata above these would contain reptiles, then small mammals,
then great mammals; seems to us demonstrable. And if the succession of
fossils presented by the strata of this supposed new continent, would
thus simulate the succession presented by our own sedimentary series;
must we not conclude that our own sedimentary series very possibly
records nothing more than the phenomena accompanying one of these great
upheavals? The probability of this conclusion being admitted, it must be
admitted that the facts of Paleontology can never suffice either to
prove or disprove the Development Hypothesis; but that the most they can
do is to show whether the last few pages of the Earth's biologic
history, are or are not in harmony with this hypothesis--whether the
existing Flora and Fauna can or can not be affiliated upon the Flora and
Fauna of the most recent geologic times.