THE CARBONIFEROUS AGE.
That age of the world's history which, from its richness in
accumulations of vegetable matter destined to be converted into coal,
has been named the Carboniferous, is in relation to living beings the
most complete and noble of the Palæozoic periods. In it those varied
arrangements of land and water which had been increasing in perfection
in the previous periods, attained to their highest development. In it
the forms of animal and plant life that had been becoming more
numerous and varied from the Eozoic onward, culminated. The Permian
which succeeded was but the decadence of the Carboniferous,
preparatory to the introduction of a new order of things. Thus the
Carboniferous was to the previous periods what the Modern is to the
preceding Tertiary and Mesozoic ages the summation and completion of
them all, and the embodiment of their highest excellence. If the
world's history had closed with the Carboniferous, a naturalist,
knowing nothing further, would have been obliged to admit that it had
already fulfilled all the promise of its earlier years. It is
important to remember this, since we shall find ourselves entering on
an entirely new scene in the Mesozoic period, and since this
character of the Carboniferous, as well as its varied conditions and
products, may excuse us for dwelling on it a little longer then on the
others, On the other hand, the immense economic importance of the coal
formation, and the interesting points connected with it, have made the
Carboniferous more familiar to general readers then most other
geological periods, so that we may select points less common and
well-known for illustration. Popular expositions of geology are,
however, generally so one-sided and so distorted by the prevalent
straining after effect, that the true aspect of this age is perhaps
not much better known then that of others less frequently described.
Let us first consider the Carboniferous geography of the northern
hemisphere; and in doing so we may begin with a fact concerning the
preceding age. One of the most remarkable features of the Newer
Devonian is the immense quantity of red rocks, particularly red
sandstones, contained in it. Red sandstones, it is true, occur in
older formations, but comparatively rarely; their great head-quarters,
both in Europe and America, in so far as the Palæozoic is concerned,
are in the Upper Devonian. Now red sandstone is an infallible mark of
rapid deposition, and therefore of active physical change. If we
examine the grains of sand in a red sandstone, we shall find that they
are stained or coated, externally, with the peroxide of iron, or iron
rust; and that this coating, with perhaps a portion of the same
substance in the intervening cement, is the cause of the colour. In
finer sandstones and red clays the same condition exists, though less
distinctly perceptible. Consequently, if red sands and clays are long
abraded or scoured in water, or are subjected to any chemical agent
capable of dissolving the iron, they cease to be red, and resume their
natural grey or white colour. Now in nature, in addition to mechanical
abrasion, there is a chemical cause most potent in bleaching red
rocks, namely, the presence of vegetable or animal matter in a state
of decay. Without entering into chemical details, we may content
ourselves with the fact that organic matter decaying in contact with
peroxide of iron tends to take oxygen from it, and then to dissolve it
in the state of protoxide, while the oxygen set free aids the decay.
Carrying this fact with us, we may next affirm that iron is so
plentiful in the crust of the earth that nearly all sands and clays
when first produced from the weathering of rocks are stained with it,
and that when this weathering takes place in the air, the iron is
always in the state of peroxide. More especially does this apply to
the greater number of igneous or volcanic rocks, which nearly always
weather brown or red. Now premising that the original condition of
sediment is that of being reddened with iron, and that it may lose
this by abrasion, or by the action of organic matter, it follows that
when sand has been produced by decay of rocks in the air, and when it
is rapidly washed into the sea and deposited there, red beds will
result. For instance, in the Bay of Fundy, whose rapid tides cut away
the red rocks of its shores and deposit their materials quickly, red
mud and sand constitute the modern deposit. On the other hand, when
the red Band and mud are long washed about, their red matter may
disappear; and when the deposition is slow and accompanied with the
presence of organic matter, the red colour is not only removed, but is
replaced by the dark tints due to carbon. Thus, in the Gulf of St.
Lawrence, where red rocks similar to those of the Bay of Fundy are
being more slowly wasted, and deposited in the presence of sea-weeds
and other vegetable substances, the resulting sands and clays are
white and grey or blackened in colour. An intermediate condition is
sometimes observed, in which red beds are stained with grey spots and
lines, where sea-weeds or land-plants have rested on them. I have
specimens of Devonian red shale with the forms of fern leaves, the
substance of which has entirely perished, traced most delicately upon
them in greenish marks.
It follows from these facts that extensive and thick deposits of red
beds evidence sub-aërial decay of rocks, followed by comparatively
rapid deposition in water, and that such red rocks will usually
contain few fossils, not only because of their rapid deposition, but
because the few organic fragments deposited with them will probably
have been destroyed by the chemical action of the superabundant oxide
of iron, which, so to speak, "iron-moulds" them, just as stains of
iron eat holes out of linen. Now when Sir Roderick Murchison tells us
of 10,000 feet in thickness of red iron-stained rocks in the old red
sandstone of England, we can see in this the evidence of rapid aqueous
deposition, going on for a very long time, and baring vast areas of
former land surface. Consequently we have proof of changes of level
and immense and rapid denudation--a conclusion further confirmed by
the apparent unconformity of different members of the series to each
other in some parts of the British Islands, the lower beds having been
tilted up before the newer were deposited. Such was the state of
affairs very generally at the close of the Devonian, and it appears to
have been accompanied with some degree of subsidence of the land,
succeeded by re-elevation at the beginning of the Carboniferous, when
many and perhaps large islands and chains of islands were raised out
of the sea, along whose margins there were extensive volcanic
eruptions, evidenced by the dykes of trap traversing the Devonian, and
the beds of old lava interstratified in the lower part of the
Carboniferous, where also the occurrence of thick beds of conglomerate
or pebble-rock indicates the tempestuous action of the sea.
But a careful study of the Lower Carboniferous beds, where their
margins rest upon the islands of older rocks, shows great varieties in
these old shores. In some places there were shingly beaches; in
others, extensive sand-banks; in others, swampy flats clothed with
vegetation, and sometimes bearing peaty beds, still preserved as small
seams of coal. The bays and creeks swarmed with, fishes. A few
sluggish reptiles crept along the muddy or sandy shores, and out
sea-ward were great banks and reefs of coral and shells in the clear
blue sea. The whole aspect of nature, taken in a general view, in the
Older Carboniferous period, must have much resembled that at present
seen among the islands of the southern hemisphere. And the plants and
animals, though different, were more like those of the modern South
Pacific then any others now living.
As the age wore on, the continents were slowly lifted out of the
water, and the great continental plateaus were changed from coral seas
into swampy flats or low uplands, studded in many places with shallow
lakes, and penetrated with numerous creeks and sluggish streams. In
the eastern continent these land surfaces prevailed extensively, more
especially in the west; and in America they spread both eastward and
westward from the Appalachian ridge, until only a long north and south
Mediterranean, running parallel to the Rocky Mountains, remained of
the former wide internal ocean. On this new and low land, comparable
with the "Sylvas" of the South American continent, flourished the
wondrous vegetation of the Coal period, and were introduced the new
land animals, whose presence distinguishes the close of the Palæozoic.
After a vast lapse of time, in which only slow and gradual subsidence
occurred, a more rapid settlement of the continental areas brought the
greater part of the once fertile plains of the coal formation again
under the waters; and shifting sand-banks and muddy tides engulfed and
buried the remains of the old forests, and heaped on them a mass of
sediment, which, like the weights of a botanical press, flattened and
compressed the vegetable _débris_ preserved in the leaves of the coal
formation strata. Then came on that strange and terrible Permian
period, which, like the more modern boulder-formation, marked the
death of one age and the birth of another.
The succession just sketched is the normal one; but the terms in which
it has been described show that it cannot be universal. There are many
places in which the whole thickness of the Carboniferous is filled
with fossils of the land, and of estuaries and creeks. There are
places, on the other hand, where the deep sea appears to have
continued during the whole period. In America this is seen on the
grandest scale in the absence of the marine members along the western
slopes of the Appalachians, and the almost exclusive prevalence of
marine beds in the far west, where the great Carboniferous
Mediterranean of America spread itself, and continued uninterruptedly
into the succeeding Permian period.
In our survey of the Carboniferous age, though there are peculiarities
in the life of its older, middle, and newer divisions, we may take the
great coal measures of the middle portion as the type of the land life
of the period, and the great limestones of the lower portion as that
of the marine life; and as the former is in this period by far the
most important, we may begin with it. Before doing so, however, to
prevent misapprehension, it is necessary to remind the reader that the
Flora of the Middle Coal Period is but one of a succession of related
floras that reach from the Upper Silurian to the Permian. The meagre
flora of club-mosses and their allies in the Upper Silurian and Lower
Devonian was succeeded by a comparatively rich and varied assemblage
of plants in the Middle Devonian. The Upper Devonian was a period of
decadence, and in the Lower Carboniferous we have another feeble
beginning, presenting features somewhat different from those of the
Upper Devonian. This was the time of the Culm of Germany, the Tweedian
formation of the North of England and South of Scotland, and the Lower
Coal formation of Nova Scotia. It was a period eminently rich in
Lepidodendra. It was followed by the magnificent flora of the Middle
Coal formation, and then there was a time of decadence in the Upper
Coal formation and only a slight revival in the Permian.
In the present condition of our civilization, coal is the most
important product which the bowels of the earth afford to man. And
though there are productive beds of coal in most of the later
geological formations, down to the peats of the modern period, which
are only unconsolidated coals, yet the coal of the Carboniferous age
is the earliest valuable coal in point of time, and by far the most
important in point of quantity. Mineral coal may be defined to be
vegetable matter which has been buried in the strata of the earth's
crust, and there subjected to certain chemical and mechanical changes.
The proof of its vegetable origin will grow upon us as we proceed. The
chemical changes which it has undergone are not very material. Wood or
bark, taken as an example of ordinary vegetable matter, consists of
carbon or charcoal, with the gases hydrogen and oxygen. Coal has
merely parted with a portion of these ingredients in the course of a
slow and imperfect putrefaction, so that it comes to have much less
oxygen and considerably less hydrogen then wood, and it has been
blackened by the disengagement of a quantity of free carbon. The more
bituminous flaming coals have a larger amount of residual hydrogen. In
the anthracite coals the process of carbonisation has proceeded
further, and little remains but charcoal in a dense and compact form.
In cannel coals, and in certain bituminous shales, on the contrary,
the process seems to have taken place entirely under water, by which
putrefaction has been modified, so that a larger proportion then usual
of hydrogen has been retained. The mechanical change which the coal
has experienced consists in the flattening and hardening effect of the
immense pressure of thousands of feet of superincumbent rock, which
has crashed together the cell-walls of the vegetable matter, and
reduced what was originally a pulpy mass of cellular tissue to the
condition of a hard laminated rock. To understand this, perhaps the
simplest way is to compare under the microscope a transverse section
of recent pine-wood with a similar section of a pine trunk compressed
into brown coal or jet. In the one the tissue appears as a series of
meshes with thin woody walls and comparatively wide cavities for the
transmission of the sap. In the other the walls of the cells have been
forced into direct contact, and in some cases have altogether lost
their separate forms, and have been consolidated into a perfectly
compact structureless mass.
With regard to its mode of occurrence, coal is found in beds ranging
in vertical thickness from less then an inch to more then thirty feet,
and of wide horizontal extent. Many such beds usually occur in the
thickness of the coal formation, or "coal measures," as the miners
call it, separated from each other by beds of sandstone and compressed
clay or shale. Very often the coal occurs in groups of several beds,
somewhat close to each other and separated from other groups by
"barren measures" of considerable thickness. In examining a bed of
coal, where it is exposed in a cutting or shore cliff, we nearly
always find that the bed below it, or the "underclay," as it is termed
by miners, is a sort of fossil soil, filled with roots and rootlets.
On this rests the coal, which, when we examine it closely, is found to
consist of successive thin layers of hard coal of different qualities
as to lustre and purity, and with intervening laminae of a dusty
fibrous substance, like charcoal, called "mother coal" by miners, and
sometimes mineral charcoal. Thin partings of dark shale also occur,
and these usually present marks and impressions of the stems and
leaves of plants. Above the coal is its "roof" of hardened clay or
sandstone, and this generally holds great quantities of remains of
plants, and sometimes large stumps of trees with their bark converted
into coal, and the hollow once occupied with wood filled with
sandstone, while their roots spread over the surface of the coal. Such
fossil forests of erect stumps are also found at various levels in the
coal measures, resting directly on under-clays without any coals. A
bed of coal would thus appear to be a fossil bog or swamp.
This much being premised about the general nature of the sooty blocks
which fill our coal-scuttles, we may now transport ourselves into the
forests and bogs of the coal formation, and make acquaintance with
this old vegetation, while it still waved its foliage in the breeze
and drank in the sunshine and showers. We are in the midst of one of
those great low plains formed by the elevation of the former sea bed.
The sun pours down its fervent rays upon us, and the atmosphere, being
loaded with vapour, and probably more rich in carbonic acid then that
of the present world, the heat is as it were accumulated and kept near
the surface, producing a close and stifling atmosphere like that of a
tropical swamp. This damp and oppressive air is, however, most
favourable to the growth of the strange and grotesque trees which
tower over our heads, and to the millions of delicate ferns and
club-mosses, not unlike those of our modern woods, which carpet the
ground. Around us for hundreds of miles spreads a dense and monotonous
forest, with here and there open spaces occupied by ponds and sluggish
streams, whose edges are bordered with immense savannahs of reed-like
plants, springing from the wet and boggy soil. Everything bespeaks a
rank exuberance of vegetable growth; and if we were to dig downward
into the soil, we should find a thick bed of vegetable mould
evidencing the prevalence of such conditions for ages. But the time
will come when this immense flat will meet with the fate which in
modern times befell a large district at the mouth of the Indus.
Quietly, or with earthquake shocks, it will sink under the waters;
fishes and mollusks will swarm where trees grew, beds of sand and mud
will be deposited by the water, inclosing and preserving the remains
of the vegetation, and in some places surrounding and imbedding the
still erect trunks of trees. Many feet of such deposits may be formed,
and our forest surface, with its rich bed of vegetable mould, has been
covered up and is in process of transformation into coal; while in
course of time the shallow waters being filled up with deposit, or a
slight re-elevation occurring, a new forest exactly like the last will
flourish on the same spot. Such changes would be far beyond the
compass of the life even of a Methuselah; but had we lived in the Coal
period, we might have seen all stages of these processes
contemporaneously in different parts of either of the great
continents.
But let us consider the actual forms of vegetation presented to us in
the Coal period, as we can restore them from the fragments preserved
to us in the beds of sandstone and shale, and as we would have seen
them in our imaginary excursion through the Carboniferous forests. To
do this we must first glance slightly at the great subdivisions of
modern plants, which we may arrange in such a way as to give an easy
means for comparison of the aspects of the vegetable kingdom in
ancient and modern times. In doing this I shall avail myself of an
extract from a previous publication of my own on this subject.
"The modern flora of the earth admits of a grand twofold division into
the _Phænogamous_, or flowering and seed-bearing plants, and the
_Cryptogamous_, or flowerless and spore-bearing plants. In the former
series, we have, first, those higher plants which start in life with
two seed-leaves, and have stems with distinct bark, wood, and
pith--the _Exogens_; secondly, those similar plants which begin life
with one seed-leaf only, and have no distinction of bark, wood, and
pith, in the stem--the _Endogens_; and, thirdly, a peculiar group
starting with two or several seed-leaves, and having a stem with bark,
wood, and pith, but with very imperfect flowers, and wood of much
simpler structure then either of the others--the _Gymnosperms_. To the
first of these groups or classes belong most of the ordinary trees of
temperate climates. To the second belong the palms and allied trees
found in tropical climates. To the third belong the pines and cycads.
In the second or Cryptogamous series we have also three classes,--(1.)
The _Acrogens_, or ferns and club-mosses, with stems having true
vessels marked on the sides with cross-bars--the Scalariform vessels.
(2.) The _Anophytes_, or mosses and their allies, with stems and
leaves, but no vessels. (3.) The _Thallophytes_, or lichens, fungi,
sea-weeds, etc., without true stems and leaves.
"In the existing climates of the earth we find these classes of plants
variously distributed as to relative numbers. In some, pines
predominate. In others, palms and tree-ferns form a considerable part
of the forest vegetation. In others, the ordinary exogenous trees
predominate, almost to the exclusion of others. In some Arctic and
Alpine regions, mosses and lichens prevail. In the Coal period we have
found none of the higher Exogens, though one species is known in the
Devonian, and only a few obscure indications of the presence of
Endogens; but Gymnosperms abound, and are highly characteristic. On
the other hand, we have no mosses or lichens, and very few algæ, but a
great number of ferns and Lycopodiaceæ or club-mosses. Thus the coal
formation period is botanically a meeting-place of the lower
Phænogams and the higher Cryptogams, and presents many forms which,
when imperfectly known, have puzzled botanists in regard to their
position in one or other series. In the present world, the flora most
akin to that of the Coal period is that of moist and warm islands in
the southern hemisphere. It is not properly a tropical flora, nor is
it the flora of a cold region, but rather indicative of a moist and
equable climate. In accordance with this is the fact that the equable
but not warm climate of the southern hemisphere at present (which is
owing principally to its small extent of land) enables sub-tropical
plants to extend into high latitudes. In the Coal period this
uniformity was evidently still more marked, since we find similar
plants extending from regions within the Arctic circle to others near
to the tropics. Still we must bear in mind that we may often be
mistaken in reasoning as to the temperature required by extinct
species of plants differing from those now in existence. Further, we
must not assume that the climatal conditions of the northern
hemisphere were in the Coal period at all similar to those which now
prevail. As Sir Charles Lyell has argued, a less amount of land in the
higher latitudes would greatly modify climates, and there is every
reason to believe that in the Coal period there was less land then
now. It has been shown by Tyndall that a very small additional amount
of carbonic acid in the atmosphere would, by obstructing the radiation
of heat from the earth, produce almost the effect of a glass roof or
conservatory, extending over the whole world. There is much in the
structure of the leaves of the coal plants, as well as in the vast
amount of carbon which they accumulated in the form of coal, and the
characteristics of the animal life of the period, to indicate, on
independent grounds, that the Carboniferous atmosphere differed from
that of the present world in this way, or in the presence of more
carbonic acid--a substance now existing in the very minute proportion
of one-thousandth of the whole by weight, a quantity adapted to the
present requirements of vegetable and animal life, but probably not to
those of the Coal period."
Returning from this digression to the forests of the Coal period, we
may first notice that which is the most conspicuous and abundant tree
in the swampy levels--the Sigillaria or seal-tree, so called from the
stamp-like marks left by the fall of its leaves--a plant which has
caused much discussion as to its affinities. Some regard it as a
gymnosperm, others as a cryptogam. Most probably we have under this
name trees allied in part to both groups, and which, when better
known, may bridge over the interval between them. These trees present
tall pillar-like trunks, often ribbed vertically with raised bands,
and marked with rows of scars left by the fallen leaves. They are
sometimes branchless, or divide at top into a few thick limbs, covered
with long rigid grass-like foliage. On their branches they bear long
slender spikes of fruit, and we may conjecture that quantities of
nut-like seeds scattered over the ground around their trunks are
their produce. If we approach one of these trees closely, more
especially a young specimen not yet furrowed by age, we are amazed to
observe the accurate regularity and curious forms of the leaf-scars,
and the regular ribbing, so very different from that of our ordinary
forest trees. If we cut into its stem, we are still further astonished
at its singular structure. Externally it has a firm and hard rind.
Within this is a great thickness of soft cellular inner bark,
traversed by large bundles of tough fibres. In the centre is a core or
axis of woody matter very slender in proportion to the thickness of
the trunk, and still further reduced in strength by a large cellular
pith. Thus a great stem four or five feet in diameter is little else
then a mass of cellular tissue, altogether unfit to form a mast or
beam, but excellently adapted, when flattened and carbonised, to blaze
upon our winter hearth as a flake of coal. The roots of these trees
were perhaps more singular then their stems; spreading widely in the
soft soil by regular bifurcation, they ran out in long snake-like
cords, studded all over with thick cylindrical rootlets, which spread
from them in every direction. They resembled in form, and probably in
function, those cable-like root-stocks of the pond-lilies which run
through the slime of lakes, but the structure of the rootlets was
precisely that of those of some modern Cycads. It was long before
these singular roots were known to belong to a tree. They were
supposed to be the branches of some creeping aquatic plant, and
botanists objected to the idea of their being roots; but at length
their connection with Sigillaria was observed simultaneously by Mr.
Binney, in Lancashire, and by Mr. Richard Brown, in Cape Breton, and
it has been confirmed by many subsequently observed facts. This
connection, when once established, further explained the reason of the
almost universal occurrence of Stigmaria, as these roots were called,
under the coal beds; while trunks of the same plants were the most
abundant fossils of their partings and roofs. The growth of successive
generations of Sigillariæ was, in fact, found to be the principal
cause of the accumulation of a bed of coal. Two species form the
central figures in our illustration.
[Illustration: Fig. 13.--GROUP OF CARBONIFEROUS PLANTS, RESTORED FROM
ACTUAL SPECIMENS.
(_a_) Calamites (type of _C. Suckovii_). (_b_) Lepidofloios, or
Ulodendron. (_c_) Sigillaria (type of _S. reniformis_).
(_d_) (type of _S. elegans_). (_e_) Lepidodendron (type of
_L. corrugatum_). (_f_) Megaphyton (type of _M. magnificum_).
(_g_) Cordaites, or Pychnophyllum (type of _C. borassifolia_).]
Along with the trees last mentioned, we observe others of a more
graceful and branching form, the successors of those Lepidodendra
already noticed in the Devonian, and which still abound in the
Carboniferous, and attain to larger dimensions then their older
relations, though they are certainly more abundant and characteristic
in the lower portions of the carboniferous. Relatives, as already
stated, of our modern club-mosses, now represented only by
comparatively insignificant species, they constitute the culmination
of that type, which thus had attained its acme very long ago, though
it still continues to exist under depauperated forms. They all
branched by bifurcation, sometimes into the most graceful and delicate
sprays. They had narrow slender leaves, placed in close spirals on the
branches. They bore their spores in scaly cones. Their roots were
similar to Stigmaria in general appearance, though differing in
details. In the coal period there were several generic forms of these
plants, all attaining to the dimensions of trees. Like the Sigillariæ,
they contributed to the materials of the coal; and one mode of this
has recently attracted some attention. It is the accumulation of their
spores and spore-cases already referred to in speaking of the
Devonian, and which was in the Carboniferous so considerable as to
constitute an important feature locally in some beds of coal. A
similar modern accumulation of spore-cases of tree-ferns occurs in
Tasmania; but both in the Modern and the Carboniferous, such beds are
exceptional; though wherever spore-cases exist as a considerable
constituent of coal, from their composition they give to it a highly
bituminous character, an effect, however, which is equally produced by
the hard scales supporting the spores, and by the outer epidermal
tissues of plants when these predominate in the coal, more especially
by the thick corky outer bark of Sigillaria. In short, the corky
substance of bark and similar vegetable tissues, from its highly
carbonaceous character, its indestructibility, and its difficult
permeability by water carrying mineral matter in solution, is the best
of all materials for the production of coal; and the microscope shows
that of this the principal part of the coal is actually composed.
In the wide, open forest glades, tree-ferns almost precisely similar
to those of the modern tropics reared their leafy crowns. But among
them was one peculiar type, in which the fronds were borne in pairs
on opposite sides of the stem, leaving when they fell two rows of
large horseshoe-shaped scars marking the sides of the trunk.
Botanists, who have been puzzled with these plants almost as much as
with the Stigmaria, have supposed these scars to be marks of branches,
of cones, and even of aërial roots; but specimens in my collection
prove conclusively that the stem of this genus was a great caudex made
up of the bases of two rows of huge leaves cemented together probably
by intervening cellular tissue. As in the Devonian and in modern
times, the stems of the tree-ferns of the Carboniferous strengthened
themselves by immense bundles of cord-like aërial roots, which look
like enormous fossil brooms, and are known under the name Psaronius.
We have only time to glance at the vast brakes of tall Calamites which
fringe the Sigillaria woods, and stretch far sea-ward over tidal
flats. They were allied to modern Mares' Tails or Equisetums, but were
of gigantic size, and much more woody structure of stem. The Calamites
grew on wet mud and sand-flats, and also in swamps; and they appear to
have been especially adapted to take root in and clothe and mat
together soft sludgy material recently deposited or in process of
deposition. When the seed or spore of a Calamite had taken root, it
probably produced a little low whorl of leaves surrounding one small
joint, from which another and another, widening in size, arose,
producing a cylindrical stem, tapering to a point below. To
strengthen the unstable base, the lower joints, especially if the mud
had been accumulating around the plant, shot out long roots instead of
leaves, while secondary stems grew out of the sides at the surface of
the soil, and in time there was a stool of Calamites, with tufts of
long roots stretching downwards, like an immense brush, into the mud.
When Calamites thus grew on inundated flats, they would, by causing
the water to stagnate, promote the elevation of the surface by new
deposits, so that their stems gradually became buried; but this only
favoured their growth, for they continually pushed out new stems,
while the old buried ones shot out bundles of roots instead of regular
whorls of leaves.
The Calamites, growing in vast fields along the margins of the
Sigillaria forests, must have greatly protected these from the effects
of inundations, and by collecting the mud brought down by streams in
times of flood, must have done much to prevent the intrusion of earthy
deposits among the vegetable matter. Their chief office, therefore, as
coal-producers, seems to have been to form for the Sigillaria forests
those reedy fringes which, when inundations took place, would exclude
mud, and prevent that mixture of earthy matter in the coal which would
have rendered it too impure for use. Quantities of fragments of their
stems can, however, be detected by the microscope in most coals.
The modern Mares' Tails have thin-walled hollow stems, and some of the
gigantic calamites of the coal resembled them in this. But others, to
which the name _Calamodendron_, or Reed-tree, has been given, had
stems with thick woody walls of a remarkable structure, which, while
similar in plan to that of the Mares' Tails, was much more perfect in
its development. Professor Williamson has shown that there were forms
intervening between these extremes; and thus in the calamites and
calamodendrons we have another example of the exaltation in ancient
times of a type now of humble structure; or, in other words, of a
comprehensive type, low in the modern world, but in older periods
taking to itself by anticipation the properties afterward confined to
higher forms. The gigantic club-mosses of the Coal period constitute a
similar example, and it is very curious that both of these types have
been degraded in the modern world, though retaining precisely their
general aspect, while the tree-ferns contemporary with them in the
Palæozoic still survive in all their original grandeur.
Barely in the swampy flats, perhaps more frequently in the uplands,
grew great pines of several kinds; trees capable of doing as good
service for planks and beams as many of their modern successors, but
which lived before their time, and do not appear even to have aided
much in the formation of coal. These pines of the Coal-period seem to
have closely resembled some species still living in the southern
hemisphere; and, like the ferns, they present to us a vegetable type
which has endured through vast periods of time almost unchanged.
Indeed, in the Middle Devonian we have pines almost as closely
resembling those of the Modern world as do those of the Coal period.
It is in accordance with this long duration of the ferns and pines,
that they are plants now of world-wide distribution--suited to all
climates and stations. Capacity to exist under varied conditions is
near akin to capacity to survive cosmical changes. A botanist in the
strange and monstrous woods which we have tried to describe, would
probably have found many curious things among the smaller herbaceous
plants, and might have gathered several precursors of the modern
Exogens and Endogens which have not been preserved to us as fossils,
or are known only as obscure fragments. But incomplete though our
picture necessarily is, and obscured by the dust of time, it may serve
in some degree to render green to our eyes those truly primeval
forests which treasured up for our long winter nights the Palæozoic
sunshine, and established for us those storehouses of heat-giving
material which work our engines and propel our ships and carriages.
Truly they lived not in vain, both as realizing for us a type of
vegetation which otherwise we could not have imagined, and as
preparing the most important of all the substrata of our modern arts
and manufactures. In this last regard even the vegetable waste of the
old coal swamps was most precious to us, as the means of producing the
clay iron ores of the coal measures. I may close this notice of the
Carboniferous forests with a suggestive extract from a paper by
Professor Huxley in the _Contemporary Review_:--
"Nature is never in a hurry, and seems to have had always before her
eyes the adage, 'Keep a thing long enough, and you will find a use for
it.' She has kept her beds of coal for millions of years without being
able to find much use for them; she has sent them down beneath the
sea, and the sea-beasts could make nothing of them: she has raised
them up into dry land and laid the black veins bare, and still for
ages and ages there was no living thing on the face of the earth that
could see any sort of value in them; and it was only the other day, so
to speak, that she turned a new creature oat of her workshop, who by
degrees acquired sufficient wits to make a fire, and then to discover
that the black rock would burn.
"I suppose that nineteen hundred years ago, when Julius Cæsar was good
enough to deal with Britain as we have dealt with New Zealand, the
primeval Briton, blue with cold and woad, may have known that the
strange black stone, of which he found lumps here and there in his
wanderings, would burn, and so help to warm his body and cook his
food. Saxon, Dane, and Norman swarmed into the land. The English
people grew into a powerful nation, and Nature still waited for a
return for the capital she had invested in the ancient club-mosses.
The eighteenth century arrived, and with it James Watt. The brain of
that man was the spore out of which was developed the steam-engine,
and all the prodigious trees and branches of modern industry which
have grown out of this. But coal is as much an essential condition of
this growth and development as carbonic acid is for that of a
club-moss. Wanting the coal, we could not have smelted the iron needed
to make our engines, nor have worked our engines when we had got them.
But take away the engines, and the great towns of Yorkshire and
Lancashire vanish like a dream. Manufactures give place to agriculture
and pasture, and not ten men could live where now ten thousand are
amply supported.
"Thus all this abundant wealth of money and of vivid life is Nature's
investment in club-mosses and the like so long ago. But what becomes
of the coal which is burnt in yielding the interest? Heat comes out of
it, light comes out of it, and if we could gather together all that
goes up the chimney and all that remains in the grate of a
thoroughly-burnt coal fire, we should find ourselves in possession of
a quantity of carbonic acid, water, ammonia, and mineral matters,
exactly equal in weight to the coal. But these are the very matters
with which Nature supplied the club-moss which made the coal. She is
paid back principal and interest at the same time; and she straightway
invests the carbonic acid, the water, and the ammonia in new forms of
life, feeding with them the plants that now live. Thrifty Nature!
surely no prodigal, but most notable of housekeepers!"
All this is true and admirably put. Its one weak point is the poetical
personification of Nature as an efficient planner of the whole. Such
an imaginary goddess is a mere superstition, unknown alike to science
and theology. Surely it is more rational to hold that the mind which
can utilize the coal and understand the manner of its formation, is
itself made in the image and likeness of the Supreme Creative Spirit,
in whom we live and move and have our being, who knows the end from
the beginning, whose power is the origin of natural forces, whose
wisdom is the source of laws and correlations of laws, and whose great
plan is apparent alike in the order of nature of the Palæozoic world
and of the modern world, as well as in the relation of these to each
other.
In the Carboniferous, as in the Devonian age, insects existed, and in
greater numbers. The winged insects of the period, so far as known,
belong to three of the nine or ten orders into which modern insects
are usually divided. Conspicuous among them are representatives of our
well-known domestic pests the cockroaches, which thus belong
geologically to a very old family. The Carboniferous roaches had not
the advantage of haunting our larders, but they had abundance of
vegetable food in the rank forests of their time, and no doubt lived
much as the numerous wild out-of-door species of this family now do.
It is, however, a curious fact that a group of insects created so long
ago, should prove themselves capable of the kind of domestication to
which these creatures attain in our modern days; and that, had we
lived even so far back as the coal period, we might have been liable
to the attacks of this particular kind of pest. Another group,
represented by many species in the coal forests, was that of the
May-flies and shad-flies, or ephemeras, which spend their earlier days
under water, feeding on vegetable matter, and affording food to many
fresh-water fishes--a use which they no doubt served in the coal
period also. Some of them were giants in their way, being probably
seven inches in expanse of wing, and their larvæ must have been choice
morsels to the ganoid fishes, and would have afforded abundant bait
had there been anglers in those days. Another group of insects was
that of the weevils, a family of beetles, whose grubs must have found
plenty of nuts and fruits to devour, without attracting the wrathful
attentions of any gardener or orchardist.
A curious and exceptional little group of creatures in the present
world is that of the galley-worms or millipedes; wingless,
many-jointed, and many-footed crawlers, resembling worms, but more
allied to insects. These animals seem to have swarmed in the coal
forests, and perhaps attained their maximum numbers and importance in
this period, though they still remain, a relic of an ancient
comprehensive type. I have myself found specimens referred by Mr.
Scudder, a most competent entomologist, to two genera and five
species, in a few decayed fossil stumps in Nova Scotia, and several
others have been discovered in other parts of the world. It is not
wonderful that animals like these, feeding on decayed vegetable
matter, should have flourished in the luxuriant Sigillaria swamps. A
few species of scorpions and spiders, very like those of the modern
world, have been found in the coal measures, both in Europe and
America; so that while we know of no enemy of the Devonian insects
except the fishes, we know in addition to these in the Carboniferous
the spiders and their allies, and the smaller reptiles or batrachians
to be noticed in the sequel. With reference to the latter, it is a
curious fact that one of the first fragments of a winged insect found
in the coal-fields of America was a part of a head and some other
remains contained in the coprolites or excrementitious matter of one
of the smaller fossil reptiles. It is perhaps equally interesting that
this head shows one of the compound facetted eyes as perfectly
developed as those of any modern Neuropter, a group of insects
remarkable even in the present world for their large and complex
organs of vision. We may pause here to note that, just as in the
Primordial we already have the Trilobites presenting all the
modifications of which the type is susceptible, so in the
Carboniferous we have in the case of the terrestrial articulates a
similar fact--highly specialised forms like the beetles, the spiders,
and the scorpions, already existing along with comprehensive forms
like the millipedes. Let us formulate the law of creation which the
Primordial trilobites, the Devonian fishes, and the Carboniferous
club-mosses and insects have taught us: it is, that every new type
rapidly attains its maximum of development in magnitude and variety of
forms, and then remains stationary, or even retrogrades, in
subsequent ages. We may connect this with other laws in the sequel.
In the coal measures we also meet, for the first time in our ascending
progress, the land snails so familiar now in every part of the world,
and which are represented by two little species found in the coal
formation of Nova Scotia. The figures of these must speak for
themselves; but the fact of their occurrence here and the mode of
their preservation require some detailed mention. The great province
of the Mollusks we have carried with us since we met with the Lingulæ
in the Primordial, but all its members have been aquatic, and probably
marine. For the first time, in the Carboniferous period, snails emerge
from the waters, and walk upon the ground and breathe air; for, like
the modern land snails, these creatures no doubt had air-sacks instead
of gills. They come suddenly upon us--two species at once, and these
representing two distinct forms of the snail tribe, the elongated and
the rounded. They were very numerous. In the beds where they occur,
probably thousands of specimens, more or less perfect, could be
collected. Were they the first-born of land snails? It would be rash
to affirm this, more especially since in all the coal-fields of the
world no specimens have been found except at one locality in Nova
Scotia;[N] and in all the succeeding beds we meet with no more till we
have reached a comparatively modern time. Yet it is very unlikely
that these creatures were in the coal period limited to one country,
and that, after that period, they dropped out of existence for long
ages, and then reappeared. Still it may have been so.
[N] Bradley has recently announced the discovery of other species in
the coal-field of Illinois.
THE TWO OLDEST LAND SNAILS.
[Illustration: Fig. 14.--_Pupa Vetusta_, Dawson.
(_a_) Natural size, (_b_) Enlarged, (_c_) Apex, enlarged,
(_d_) Sculpture, magnified.]
[Illustration: Fig. 15.--_Conulus Priscus_, Carpenter.
(_a_) Specimen enlarged, (_b_) Sculpture, magnified.]
There are cases of geographical limitation quite as curious now. Here
again another peculiarity meets us. If these are really the oldest
land snails, it is curious that they are so small,--so much inferior
to many of their modern successors even in the same latitudes. The
climate of the coal period must have suited them, and there was plenty
of vegetable food, though perhaps not the richest or most tender.
There is no excuse for them in their outward circumstances. Why, then,
unlike so many other creatures, do they enter on existence in this
poor and sneaking way. We must here for their benefit modify in two
ways the statement broadly made in a previous chapter, that new types
come in under forms of great magnitude. First, we often have, in
advance of the main inroad of a new horde of animals, a few
insignificant stragglers as a sort of prelude to the rest--precursors
intimating beforehand what is to follow. We shall find this to be the
case with the little reptiles of the coal, and the little mammals of
the Trias, preceding the greater forms which subsequently set in.
Secondly, this seems to be more applicable in the case of land animals
then in the case of those of the waters. To the waters was the fiat to
bring forth living things issued. They have always kept to themselves
the most gigantic forms of life; and it seems as if new forms of life
entering on the land had to begin in a small way and took more time to
culminate.
The circumstances in which the first specimens of Carboniferous snails
and gally-worms were found are so peculiar and so characteristic of
the coal formation, that I must pause here to notice them, and to make
of them an introduction to the next group of creatures we have to
consider. In the coal formation in all parts of the world it is not
unusual, as stated already in a previous page, to find erect trees or
stumps of trees, usually Sigillariæ, standing where they grew; and
where the beds are exposed in coast cliffs, or road cuttings, or
mines, these fossil trees can be extracted from the matrix and
examined. They usually consist of an outer cylinder of coal
representing the outer bark, while the space within, once occupied by
the inner bark and wood, is filled with sandstone, sometimes roughly
arranged in layers, the lowest of which is usually mixed with coaly
matter or mineral charcoal derived from the fallen remains of the
decayed wood, a kind of deposit which affords to the fossil botanist
one of the best modes of investigating the tissues of these trees.
These fossil stumps are not uncommon in the roofs of the coal-seams.
In some places they are known to the miners as "coal pipes," and are
dreaded by them in consequence of the accidents which occur from their
suddenly falling after the coal which supported them has been removed.
An old friend and helper of mine in Carboniferous explorations had a
lively remembrance of the fact that one of these old trees, falling
into the mine in which he was working, had crushed his leg and given
him a limp for life; and if he had been a few inches nearer to it
would have broken his back.
The manner in which such trees become fossilized may be explained as
follows:--Imagine a forest of Sigillariæ growing on a low flat. This
becomes submerged by subsidence or inundation, the soil is buried
under several feet of sand or mud, and the trees killed by this agency
stand up as bare and lifeless trunks. The waters subside, and the
trees rapidly decay, the larvæ of wood-boring insects perhaps aiding
in the process, as they now do in the American woods. The dense coaly
outer bark alone resists decomposition, and stands as a hollow
cylinder until prostrated by the wind or by the waters of another
inundation, while perhaps a second forest or jungle has sprung up on
the new surface. When it falls, the part buried in the soil becomes an
open hole, with a heap of shreds of wood and bark in the bottom. Such
a place becomes a fit retreat for gally-worms and land-snails; and
reptiles pursuing such animals, or pursued by their own enemies, or
heedlessly scrambling among the fallen trunks, may easily fall into
such holes and remain as prisoners. I remember to have observed, when
a boy, a row of post-holes dug across a pasture-field and left open
for a few days, and that in almost every hole one or two toads were
prisoners. This was the fate which must have often befallen the
smaller reptiles of the coal forests in the natural post-holes left by
the decay of the Sigillariæ. Yet it may be readily understood that the
combination of circumstances which would effect this result must have
been rare, and consequently this curious fact has been as yet observed
only in the coal formation of Nova Scotia; and in it only in one
locality, and in this in one only out of more then sixty beds in
which erect trees have been found. But these hollow trees must be
filled up in order to preserve their contents; and as inundation and
subsequent decay have been the grave-diggers for the reptiles, so
inundations filled up their graves with sand, to be subsequently
hardened into sandstone, burying up at the same time the newer
vegetation which had grown upon the former surface. The idea that
something interesting might be found in these erect stumps, first
occurred to Sir C. Lyell and the writer while exploring the beautiful
coast cliffs of Western Nova Scotia in 1851; and it was in examining
the fragments scattered on the beach that we found the bones of the
first Carboniferous reptile discovered in America, and the shell of
the oldest known land snail.
These were not, however, the earliest known instances of Carboniferous
reptiles. In 1841, Sir William Logan found footprints of a reptile at
Horton Bluff, in Nova Scotia, in rocks of Lower Carboniferous age. In
1844, Von Dechen found reptilian bones in the coal-field of Saarbruck;
and in the same year Dr. King found reptilian footprints in the
Carboniferous of Pennsylvania. Like Robinson Crusoe on his desert
island, we saw the footprints before we knew the animals that produced
them; and the fact that there were marks on a slab of shale or
sandstone that must have been made by an animal walking on feet, was
as clear and startling a revelation of the advent of a new and higher
form of life, as were the footprints of Man Friday. Within the thirty
years since the discovery of the first slab of footprints, the
knowledge of coal formation reptiles has grown apace. I can scarcely
at present sum up exactly the number of species, but may estimate it
at thirty-five at least. I must, however, here crave pardon of some of
my friends for the use of the word reptile. In my younger days frogs
and toads and newts used to be reptiles; now we are told that they are
more like fishes, and ought to be called Batrachians or Amphibians,
whereas reptiles are a higher type, more akin to birds then to these
lower and more grovelling creatures. The truth is, that the old class
Reptilia bridges over the space between the fishes and the birds, and
it is in some degree a matter of taste whether we make a strong line
at the two ends of it alone, or add another line in the middle. I
object to the latter course, however, in the period of the world's
history of which I am now writing, since I am sure that there were
animals in those days which were batrachians in some points and true
reptiles in others; while there are some of them in regard to which it
is quite uncertain whether they are nearer to the one group or the
other. Although, therefore, naturalists, with the added light and
penetration which they obtain by striding on to the Mesozoic and
Modern periods, may despise my old-fashioned grovellers among the mire
of the coal-swamps, I shall, for convenience, persist in calling them
reptiles in a general way, and shall bring out whatever claims I can
to justify this title for some of them at least.
Perhaps the most fish-like of the whole are the curious creatures from
the coal measures of Saarbruck, first found by Yon Dechen, and which
constitute the genus _Archegosaurus_. Their large heads, short necks,
supports for permanent gills, feeble limbs, and long tails for
swimming, show that they were aquatic creatures presenting many points
of resemblance to the ganoid fishes with which they must have
associated; still they were higher then these in possessing lungs and
true feet, though perhaps better adapted for swimming then even for
creeping.
From these creatures the other coal reptiles diverge, and ascend along
two lines of progress, the one leading to gigantic crocodile-like
animals provided with powerful jaws and teeth, and probably haunting
the margins of the waters and preying on fishes; the other leading to
small and delicate lizard-like species, with well-developed limbs,
large ribs, and ornate horny scales and spines, living on land and
feeding on insects and similar creatures.
[Illustration: Fig. 16.--RESTORATIONS OF BAPHETES, DENDRERPETON.
HYLONOMUS, AND HYLERPETON, WITH CARBONIFEROUS PLANTS IN THE DISTANCE.]
In the first direction we have a considerable number of species found
in the Jarrow coal-field in Ireland, and described by Professor
Huxley. Some of them were like snakes in their general form, others
more like lizards. Still higher stand such animals as _Baphetes_ and
_Eosaurus_ from the Nova Scotia coal-field and _Anthracosaurus_ from
that of Scotland. The style and habits of these creatures it is easy
to understand, however much haggling the comparative anatomists may
make over their bones. They were animals of various size, ranging
from a foot to at least ten feet in length, the body generally
lizard-like in form, with stout limbs and a flattened tail useful in
swimming. Their heads were flat, stout, and massive, with large teeth,
strengthened by the insertion and convolution of plates of enamel. The
fore limbs were probably larger then the hind limbs, the better to
enable them to raise themselves out of the water. The belly was
strengthened by bony plates and closely imbricated scales, to resist,
perhaps, the attacks of fishes from beneath, and to enable them
without injury to drag their heavy bodies over trunks of trees and
brushwood, whether in the water or on the land. Their general aspect
and mode of life were therefore by no means unlike those of modern
alligators; and in the vast swamps of the coal measures, full of ponds
and sluggish streams swarming with fish, such creatures must have
found a most suitable habitat, and probably existed in great numbers,
basking on the muddy banks, surging through the waters, and filling
the air with their bellowings. The most curious point about these
creatures is, that while rigid anatomy regards them as allied in
structure more to frogs and toads and newts then to true lizards, it
is obvious to common sense that they were practically crocodiles; and
even anatomy must admit that their great ribs and breastplates, and
powerful teeth and limbs, indicate a respiration, circulation, and
general vitality, quite as high as those of the proper reptiles.
Hence, it happens that very different views are stated as to their
affinities; questions into which we need not now enter, satisfied with
the knowledge of the general appearance and mode of life of these
harbingers of the reptilian life of the succeeding geological periods.
In the other direction, we find several animals of small size but
better developed limbs, leading to a group of graceful little
creatures, quite as perplexing with regard to affinities as those
first mentioned, but tending towards the smaller lizards of the
modern world. At the top of these I may place the genus _Hylonomus_
from hollow fossil trees of Nova Scotia, of which two species are
represented as restored in our illustration. In these restorations I
have adhered as faithfully as possible to the proportions of parts as
seen in my specimens. Imagine a little animal six or seven inches
long, with small short head, not so flat as those of most lizards, but
with a raised fore-head, giving it an aspect of some intelligence. Its
general form is that of a lizard, but with the hind feet somewhat
large, to aid it in leaping and standing erect, and long and flexible
toes. Its belly is covered with bony scales, its sides with bright and
probably coloured scale armour of horny consistency, and its neck and
back adorned with horny crests, tubercles, and pendants. It runs,
leaps, and glides through the herbage of the coal forests, intent on
the pursuit of snails and insects, its eye glancing and its bright
scales shining in the sun. This is a picture of the best known species
of Hylonomus drawn from the life. Yet the anatomist, when he examines
the imperfectly-ossified joints of its backbone, and the double joint
at the back of its skull, will tell you that it is after all little
better then a mere newt, an ass in a lion's skin, a jackdaw with
borrowed feathers, and that it has no right to have fine scales, or to
be able to run on the land. It may be so; but I may plead in its
behalf, that in the old coal times, when reptiles with properly-made
skeletons had not been created, the next best animals may have been
entitled to wear their clothes and to assume their functions as well.
In short, functionally or officially, our ancient batrachians were
reptiles; in point of rank, as measured by type of skeleton, they
belonged to a lower grade. To this view of the case I think most
naturalists will agree, and they will also admit that the progress of
our views has been in this direction, since the first discovery of
Carboniferous air-breathing vertebrates. In evidence of this I may
quote from Professor Huxley's description of his recently found
species,[O] After noticing the prevalent views that the coal reptiles
were of low organization, he says: "Discoveries in the Nova Scotia
coal-fields first shook this view, which ceased to be tenable when the
great _Anthracosaurus_ of the Scotch coal-field was found to have
well-ossified biconcave vertebrae."
[O] _Geological Magazine_, vol. iii.
The present writer may, however, be suspected of a tendency to extend
forms of life backward in time, since it has fallen to his lot to be
concerned in this process of stretching backward in several cases. He
has named and described the oldest known animal. He has described the
oldest true exogen, and the oldest known pine-tree. He was concerned
in the discovery of the oldest known land snails, and found the oldest
millipedes. He has just described the oldest bituminous bed composed
of spore-cases, and he claims that his genus Hylonomus includes the
oldest animals which have a fair claim to be considered reptiles.
Still this discovery of old things comes rather of fortune and careful
search then of a desire to innovate; and a distinction should be drawn
between that kind of novelty which consists in the development of new
truths, and that which consists in the invention of new fancies, or
the revival of old ones. There is too much of this last at present;
and it would be a more promising line of work for our younger
naturalists, if they would patiently and honestly question nature,
instead of trying to extort astounding revelations by throwing her on
the rack of their own imaginations.
We may pause here a moment to contemplate the greatness of the fact we
have been studying the introduction into our world of the earliest
known vertebrate animals which could open their nostrils and literally
"breathe the breath of life." All previous animals that we know,
except a few Devonian insects, had respired in the water by means of
gills or similar apparatus, Now we not only have the little land
snails, with their imperfect substitutes for lungs, but animals which
must have been able to draw in the vital air into capacious chambered
lungs, and with this power must have enjoyed a far higher and more
active style of vitality; and must have possessed the faculty of
uttering truly vocal sounds. What wondrous possibilities unknown to
these creatures, perhaps only dimly perceived by such rational
intelligences as may have watched the growth of our young world, were
implied in these gifts. It is one of the remarkable points in the
history of creation in Genesis, that this step of the creative work is
emphatically marked. Of all the creatures we have noticed up to this
point, it is stated that God said, "Let the waters bring them
forth"--but it is said that "God created" great reptiles
(_tanninim_).[P] No doubt these "great tanninim" culminate in the
succeeding Mesozoic age, but their first introduction dates as far
back as the Carboniferous; and this introduction was emphatically a
creation, as being the commencement of a new feature among living
beings. What further differences may be implied in the formulæ, "Let
the waters produce" and "God created," we do not know; very probably
he who wrote the words did not fully know. But if we could give a
scientific expression to this difference, and specify the cases to
which its terms apply, we might be able to solve one of the most vexed
questions of biology.
[P] Not "whales," as in our version.
Let us observe, however, that even here, where, if anywhere, we have
actual creation, especial pains are taken to bridge over the gap, and
to prevent any appearance of discontinuity in the work. The ganoid
fishes of the coal period very probably had, like their modern
congeners, well-developed air-bladders, serving to some extent, though
very imperfectly, as lungs. The humbler and more aquatic reptiles of
the period retained the gills, and also some of the other features of
the fishes; so that, like some modern creatures of their class, they
stood, as to respiration, on two stools, and seemed unwilling
altogether to commit themselves to the new mode of life in the
uncongenial element of air. Even the larger and more lizard-like of
the coal reptiles may--though this we do not certainly know, and in
some cases there are reasons for doubting it--have passed the earliest
stage of their lives in the water as gilled tadpoles, in the manner of
our modern frogs. Thus at the very point where one of the greatest
advances of animal life has its origin, we have no sudden stop, but an
inclined plane; and yet, as I have elsewhere endeavoured to show by
arguments which cannot be repeated here,[Q] we have not a shadow of
reason to conclude that, in the coal period, fishes were transmuted
into reptiles.
[Q] "Air-breathers of the Coal Period," p. 77.
But the reader may be wearied with our long sojourn in the
pestilential atmosphere of the coal swamps, and in the company of
their low-browed and squalid inhabitants. Let us turn for a little to
the sea, and notice the animal life of the great coral reefs and shell
beds preserved for us in the Carboniferous limestone. Before doing so,
one point merits attention. The coal formation for the first time
distinctly presents to us the now familiar differences in the
inhabitants of the open sea and those of creeks, estuaries and lakes.
Such distinctions are unknown to us in the Silurian. There all is sea.
They begin to appear in the Devonian, in the shallow fish-banks and
the Anodon-like bivalves found with fossil plants. In the coal period
they become very manifest. The animals found in the shales with the
coal are all, even the aquatic ones, distinct from those of the open
seas of the period. Some of them may have lived in salt or brackish
water, but not in the open sea. They are creatures of still and
shallow waters. It is true that in some coal-fields marine beds occur
in the coal measures with their characteristic fossils, but these are
quite distinct from the usual animal remains of the coal-fields, and
mark occasional overflows of the sea, owing to subsidence of the land.
It is important to notice this geographical difference, marking the
greater specialisation and division of labour, if we may so speak,
that was in the process of introduction.
The sea of the Carboniferous period presented in the main similar
great groups of animals to those of the Devonian, represented however
by different species. We may notice merely some of the salient points
of resemblance or difference. The old types of corals continue in
great force; but it is their last time, for they rapidly decay in the
succeeding Permian and disappear. The Crinoids are as numerous and
beautiful as in any other period, and here for the first time we meet
with the new and higher type of the sea-urchin, in large and beautiful
species. One curious group, that of the _Pentremites_, a sort of
larval form, is known here alone. Among the lamp-shells we may note,
as peculiarly and abundantly Carboniferous, those with one valve very
convex and the other very concave and anchored in the mud by long
spines instead of a peduncle attached to stones and rocks.[R] There
are many beautiful shells allied to modern scallops, and not a few
sea-snails of various sorts. The grand _Orthoceratites_ of the
Silurian diminish in size preparatory to their disappearance in the
Permian, and the more modern type of _Nautilus_ and its allies becomes
prevalent. Among the Crustaceans we may notice the appearance of the
_Limulus_, or king-crab, of which the single little species described
by Woodward from the Upper Silurian may be regarded as merely a
prophecy. It is curious that the Carboniferous king-crabs are very
small, apparently another case of a new form appearing in humble
guise; but as the young of modern king-crabs haunt creeks and swampy
flats, while the adults live in the sea, it may be that only the young
of the Carboniferous species are yet known to us, the specimens found
being mostly in beds likely to be frequented by the young rather then
by the full-grown individuals.
[R] The Productidæ.
The old order of the Trilobites, which has accompanied us from
Primordial times, here fails us, and a few depauperated species alone
remain, the sole survivors of their ancient race--small, unornamented,
and feeble representatives of a once numerous and influential tribe.
How strange that a group of creatures so numerous and apparently so
well adapted to conditions of existence which still continue in the
sea, should thus die out, while the little bivalved crustaceans, which
began life almost as far back and lived on the same sea-floors with
the Trilobites, should still abound in all our seas; and while the
king-crabs, of precisely similar habits with the Trilobites, should
apparently begin to prosper. Equally strange is the fate of the great
swimming Eurypterids which we saw in the Devonian. They also continue,
but in diminished force, in the Carboniferous, and there lay down for
ever their well-jointed cuirasses and formidable weapons, while a few
little shrimp-like creatures, their contemporaries, form the small
point of the wedge of our great tribes of squillas and crabs and
lobsters. Some years ago the late lamented palæontologist, Salter, a
man who scarcely leaves his equal in his department, in conjunction
with Mr. Henry Woodward, prepared a sort of genealogical chart of the
Crustacea on which these facts are exhibited. Some new species have
since been discovered, and a little additional light about affinities
has been obtained; but taken as it stands, the history of the
Crustacea as there shown in one glance, has in it more teaching on the
philosophy of creation then I have been able to find in many ponderous
quartos of tenfold its pretensions. Had Salter been enabled, with the
aid of other specialists like Woodward, to complete similar charts of
other classes of invertebrate animals, scientific palaeontology in
England would have been further advanced then it is likely to be in
the next ten years.
To return to our Trilobites: one of the most remarkable points in
their history is their appearance in full force in the Primordial. In
these rocks we have some of the largest in size--some species of
Paradoxides being nearly two feet long, and some of the very smallest.
We have some with the most numerous joints, others with the fewest;
some with very large tails, others with very small; some with no
ornamentation, others very ornate; some with large eyes, others with
none that have been made out, though it is scarcely probable that they
were wholly blind. They increased in numbers and variety through the
Silurian and Devonian, and then suddenly drop off at the end of the
Lower Carboniferous. Throughout their whole term of existence they
kept rigidly to that type of the mud-plough which the king-crab still
retains, and which renders the anterior extremity so different from
that of the ordinary Crustacea. They constitute one of the few cases
in which we seem to see before us the whole history of an animal type;
and the more we look into that history, the more do we wonder at their
inscrutable introduction, the unity and variety mingled in their
progress, and their strange and apparently untimely end. I have
already referred (page 95) to the use which Barrande makes of this as
an argument against theories of evolution; but must refer to his work
for the details.
One word more I must say before leaving their graves. I have reason to
believe that they were not only the diggers of the burrows, and of
the ladder-tracks and pitted tracks[S] of the Silurian and
Primordial, but that with the strokes of their rounded or spinous
tails, the digging of their snouts, and the hoe-work of their hard
upper lips, or Hypostomes, they made nearly all those strange marks in
the Primordial mud which have been referred to fucoids, and even to
higher plants. The Trilobites worked over all the mud bottoms of the
Primordial, even in places where no remains of them occur, and the
peculiarities of the markings which they left are to be explained only
by a consideration of the structures of individual species.
[S] _Climactichnites_ and _Protichnites_.
I had almost lost sight of the fishes of the Carboniferous period, but
after saying so much of those of the Devonian, it would be unfair to
leave their successors altogether unnoticed. In the Carboniferous we
lose those broad-snouted plate-covered species that form so
conspicuous a feature in the Devonian; and whatever its meaning, it is
surely no accident that these mud-burrowing fishes should decay along
with those crustacean mud-burrowers, the Trilobites. But swarms of
fishes remain, confined, as in the Devonian, wholly to the two orders
of the Gar-fishes (_Ganoids_) and the sharks (_Placoids_). In the
former we have a multitude of small and beautiful species haunting the
creeks and ponds of the coal swamps, and leaving vast quantities of
their remains in the shaly and even coaly beds formed in such places.
Such were the pretty, graceful fishes of the genera _Palæoniscus_ and
_Amblypterus_. Pursuing and feeding on these were larger ganoids,
armed with strong bony scales, and formidable conical or sharp-edged
teeth. Of these were _Rhizodus_ and _Acrolepis_. There were besides
multitudes of sharks whose remains consist almost wholly of their
teeth and spines, their cartilaginous skeletons having perished. One
group was allied to the few species of modern sharks whose mouths are
paved with flat teeth for crushing shells. These were the most
abundant sharks of the Carboniferous--slow and greedy monsters,
haunting shell banks and coral reefs, and grinding remorselessly all
the shell-fishes that came in their way. There were also sharks
furnished with sharp and trenchant teeth, which must have been the
foes of the smaller mailed fishes, pursuing them into creeks and muddy
shallows; and if we may judge from the quantity of their remains in
some of these places, sometimes perishing in their eager efforts. On
the whole, the fishes of the Carboniferous were, in regard to their
general type, a continuation of those of the Devonian, but the sharks
and the scaly ganoids were relatively more numerous. They differed
from our modern fishes in the absence of the ordinary horny-scaled
type to which all our more common fishes belong, and in the prevalence
of that style of tail which has been termed "heterocercal," in which
the continuation of the backbone forms the upper lobe of the tail, a
style which, if we may judge from modern examples, gives more power of
upward and downward movement, and is especially suitable to fishes
which search for food only at the bottom, or only above the surface of
the waters.
Most reluctantly I must here leave one of the most remarkable periods
of the world's history, and reserve to our next chapter the summation
of the history of the older world of life in its concluding stage, the
Permian.