THE DEVONIAN AGE.
Paradoxical as it may appear, this period of geological history has
been held as of little account, and has even been by some geologists
regarded as scarcely a distinct age, just because it was one of the
most striking and important of the whole. The Devonian was an age of
change and transition, in both physical and organic existence; and an
age which introduced, in the Northern hemisphere at least, more varied
conditions of land and water and climate then had previously existed.
Hence, over large areas of our continents, its deposits are irregular
and locally diverse; and the duration and importance of the period are
to be measured rather by the changes and alterations of previous
formations, and the ejection of masses of molten rock from beneath,
then by a series of fossiliferous deposits. Nevertheless, in some
regions in North America and Eastern Europe, the formations of this
era are of vast extent and volume, those of North America being
estimated at the enormous thickness of 15,000 feet, while they are
spread over areas of almost continental breadth.
At the close of the Upper Silurian, the vast continental plateaus of
the northern hemisphere were almost wholly submerged. No previous
marine limestone spreads more widely then that of the Upper Silurian,
and in no previous period have we much less evidence of the existence
of dry land; yet before the end of the period we observe, in a few
fragments of land plants scattered here and there in the marine
limestones--evidence that islands rose amid the waste of waters. As it
is said that the sailors of Columbus saw the first indications of the
still unseen Western Continent in drift canes, and fragments of trees
floating in mid ocean, so the voyager through the Silurian seas finds
his approach to the verdant shores of the Devonian presaged by a few
drift plants borne from shores yet below the horizon. The small
remains of land in the Upper Silurian were apparently limited to
certain clusters of islands in the north-eastern part of America and
north-western part of Europe, with perhaps some in the intervening
Atlantic On these limited surfaces grew the first land plants
certainly known to us--herbs and trees allied to the modern
club-mosses, and perhaps forests of trees allied to the pines, though
of humbler type; and this wide Upper Silurian sea, with archipelagos
of wooded islands, may have continued for a long time. But with the
beginning of the Devonian, indications of an unstable condition of the
earth's crust began to develop themselves. New lands were upheaved;
great shallow, muddy, and sandy flats were deposited around them the
domains of corals and sea-weeds were contracted and on banks, and in
shallows and estuaries, there swarmed shoals of fishes of many
species, and some of them of most remarkable organization. On the
margins of these waters stretched vast swamps, covered with a rank
vegetation.
But the period was one of powerful igneous activity. Volcanoes poured
out their molten rocks over sea and land, and injected huge dykes of
trap into the newly-formed beds. The land was shaken with earthquake
throes, and was subject to many upheavals and subsidences. Violent
waves desolated the coasts, throwing sand and gravel over the flats,
and tearing up newly-deposited beds; and poisonous exhalations, or
sudden changes of level, often proved fatal to immense shoals of
fishes. This was the time of the Lower Devonian, and it is marked,
both in the old world and the new, by extensive deposits of sandstones
and conglomerates.
But the changes going on at the surface were only symptomatic of those
occurring beneath. The immense accumulations of Silurian sediment had
by this time so overweighted certain portions of the crust, that great
quantities of aqueous sediment had been pressed downward into the
heated bowels of the earth, and were undergoing, under an enormous
weight of superincumbent material, a process of baking and
semi-fusion. This process was of course extremely active along the
margins of the old Silurian plateaus, and led to great elevation of
land, while in the more central parts of the plateaus the oceanic
conditions still continued; and in the Middle Devonian, in America at
least, one of the most remarkable and interesting coral limestones in
the world--the corniferous limestone--was deposited. In process of
time, however, these clear waters became shallow, and were invaded by
muddy sediments; and in the Upper Devonian the swampy flats and muddy
shallows return in full force, and in some degree anticipate the still
greater areas of this kind which existed in the succeeding Coal
formation.
Such is a brief sketch of the Devonian, or, as it may be better called
in America, from the vast development of its beds on the south side of
Lake Erie, the _Erian_ formation. In America the marine beds of the
Devonian were deposited on the same great continental plateau which
supported the seas of the Upper and Lower Silurian, and the beds were
thicker towards the east and thinned towards the west, as in the case
of the older series. But in the Devonian there was much, land in the
north-east of America; and on the eastern margin of this land, as in
Gaspé and New Brunswick, the deposits throughout the whole period were
sandstones and shales, without the great coral limestones of the
central plateau. Something of the same kind occurred in Europe, where,
however, the area of Devonian sea was smaller. There the fossiliferous
limestones of the Middle Devonian in Devon, in the Eifel district, in
France and in Russia, represent the great corniferous limestone of
America; while the sandstones of South Wales, of Ireland, and of
Scotland, resemble the local conditions of Gaspé and New Brunswick,
and belonged to a similar area in the north-west of Europe, in which
shallow water and land conditions prevailed during the whole of the
Devonian, and which was perhaps connected with the corresponding
region in Eastern America by a North Atlantic archipelago, now
submerged. This whole subject is so important to the knowledge of the
Devonian, and of geology in general, that I may be pardoned for
introducing it here in a tabular form, taking the European series from
Etheridge's excellent and exhaustive paper in the "Journal of the
Geological Society."
DEVONIAN OF ERIAN.
DIVISIONS. CENTRAL AREAS.
Devon. Rhen. Prussia. New York.
{Pilton group:-- Clymenia, Cypridina, Chemung and Portage.
{ Brown calcareous etc. Shales, Sandstones
Upper { shales, brown and limestones, and and shales.
{ yellow sandstone. sandstones. Plants and marine
{ Land plants and Plants and marine shells.
{ marine shells. shells.
{Ilfracombe group:-- Eifel limestone, Hamilton shales,
{ Grey and red Calceola shales, and Corniferous
Middle { sandstones and etc. or cherty
{ flags, calcareous Corals, shells, limestone.
{ slates and etc. Many corals and
{ limestones, with shells, also
{ corals, etc. plants.
{Lynton group:-- Coblentz and Schoharie and
{ Bed and purple Wissenbach shales, Caudagalli grits.
Lower { sandstones. Marine Rhenish greywacke, Oriskany
{ shells, etc. Spinier sandstones.
{ sandstone. Marine shells.
{ Marine shells.
DIVISIONS. MARGINAL AREAS.
Scotland. Ireland. Gaspé and New Brunswick.
{Yellow and red Yellow and red Red and grey
{ sandstones. sandstones, etc. sandstones, grits
Upper {Fishes and plants. Plants, fishes, and shales, and
{ etc. conglomerates of
{ Gaspé and Mispeck.
{ Plants.
{Red shales and Grits and Grey and Red
{ sandstones, and sandstones of sandstones, and
Middle { conglomerates. Dingle. grey and dark
{Caithness flags. shales. Gaspé
{Fishes and plants. and St. John.
{ Many plants and
{ fishes.
{Flagstones, shales Glengariff grits, Sandstone and
{ and conglomerates. etc. conglomerate.
Lower {Fishes and plants. Gaspé and St.
{ John.
{ Plants and fishes.
A glance at this table suffices to show that when we read Hugh
Miller's graphic descriptions of the Old Red Sandstone of Scotland,
with its numerous and wonderful fishes, we have before us a formation
altogether distinct from that of Devonshire or the Eifel. But the one
represents the shallow, and the other the deeper seas of the same
period. We learn this by careful tracing of the beds to their junction
with, corresponding series, and by the occasional occurrence of the
characteristic fishes of the Scottish strata in the English and German
beds. In like manner a geologist who explores the Gaspé sandstones or
the New Brunswick shales has under his consideration a group of beds
very dissimilar from that which he would have to study on the shores
of Lake Erie. But here again identity of relations to the Silurian
below and the carboniferous above, shows the contemporaneousness of
the beds, and this is confirmed by the occurrence in both series of
some of the same plants and shells and fishes.
It will further be observed that it is in the middle that the greatest
difference occurs. Sand and mud and pebble-banks were almost universal
over our two great continental plateaus in the Older and Newer
Devonian. But in the Middle there were in some places deeper waters
with coral reefs, in others shallow flats and swamps rich in
vegetation. Herein we see the greater variety and richness of the
Devonian. Had we lived in that age, we should not have seen great
continents like those that now exist, but we could have roamed over
lovely islands with breezy hills and dense lowland jungles, and we
could have sailed over blue coral seas, glowing below with all the
fanciful forms and brilliant colours of polyp life, and filled with
active and beautiful fishes. Especially did all these conditions
culminate in the Middle Devonian, when what are now the continental
areas of the northern hemisphere must have much resembled the present
insular and oceanic regions of the South Pacific.
Out of the rich and varied life of the Devonian I may select for
illustration its corals, its crustaceans, its fishes, its plants, and
its insects.
[Illustration: Fig. 11.--CORALS, FISHES, AND CRUSTACEANS OF THE
DEVONIAN
In the foreground are Corals of the genera _Favosites_, _Michelina_,
_Phillipsatrea_, _Zaphrentis_, _Blothrophyllum_, and _Syringopora_,
and the seaweed Spirephyton; also Fishes of the genera _Cephalaspis_
and _Pterichthys_. Above are _Pterygotus_ and _Dinichtys_, with Fishes
of the genera _Diplacanthus_, _Osteolepis_, _Holoptychius_,
_Pteraspis_, _Coccosteus_, etc. The distant land had _Lepidodendra_,
Pines and Tree-ferns.]
The central limestones of the Devonian may be regarded as the
head-quarters of the peculiar types of coral characteristic of the
Palæozoic age. Here they were not only vastly numerous, but present
some of their grandest and also their most peculiar forms. Edwards and
Haime, in their "Monograph of British Fossil Corals" in 1854,
enumerate one hundred and fifty well-ascertained species, and the
number has since been largely increased; I have no doubt that my
friend Dr. Bigsby, in his forth-coming "Thesaurus Devonicus," will
more then double it. In the Devonian limestones of England, as for
instance at Torquay, the specimens, though abundant and well preserved
as to their internal structure, are too firmly imbedded in the rock to
show their external forms. In the Devonian of the continent of Europe
much finer specimens occur; but, perhaps, in no part of the world is
there so clear an exhibition of them as in the Devonian limestones of
the United States and Canada. Sir Charles Lyell thus expresses his
admiration of the exposure of these corals, which he saw at the falls
of the Ohio, near Louisville. He says, "Although the water was not at
its lowest, I saw a grand display of what may be termed an ancient
coral-reef, formed by zoophytes which flourished in a sea of earlier
date then the Carboniferous period. The ledges of horizontal
limestone, over which the water flows, belong to the Devonian group,
and the softer parts of the stone have decomposed and wasted away, so
that the harder calcareous corals stand out in relief. Many branches
of these zoophytes project from their erect stems precisely as if they
were living. Among other species I observed large masses, not less
then five feet in diameter, of _Favosites Gothlandica_, with its
beautiful honeycomb structure well displayed. There was also the
cup-shaped _Cyathophyllum_, and the delicate network of _Fenestella_,
and that elegant and well-known European species of fossil, the chain
coral, _Catenipora escharoides_, with a profusion of others which it
would be tedious to all but the geologist to enumerate. Although
hundreds of fine specimens have been detached from these rocks to
enrich the museums of Europe and America, another crop is constantly
working its way out under the action of the stream, and of the sun and
rain in the warm season when the channel is laid dry."[K] These
limestones have been estimated to extend, as an almost continuous
coral reef, over the enormous area of five hundred thousand square
miles of the now dry and inland surface of the great American
continental plateau. The limestones described by Sir Charles are known
in the Western States as the "Cliff limestone." In the State of New
York and in Western Canada the "Corniferous limestone," so called from
the masses of hornstone, like the flint of the English chalk,
contained in it, presents still more remarkable features. The corals
which it contains have been replaced by the siliceous or flinty
matter in such a manner that, when the surrounding limestone weathers
away, they remain projecting in relief in all the beauty of their
original forms. Not only so, but on the surface of the country they
remain as hard siliceous stones, and may be found in ploughing the
soil and in stone fences and roadside heaps, so that tons of them
could often be collected over a very limited space. When only partly
disengaged from the matrix, the process may be completed by immersing
them in a dilute acid. The beauty of these specimens when thus
prepared is very great not at all inferior to that of modern corals,
which they often much resemble in general form, though differing in
details of structure. One of the most common forms is that of the
_Favosites_, or honeycomb coral, presenting regular hexagonal cells
with transverse floors or tabulæ. Of these there are several species,
usually flat or massive in form; but one species, _F. polymorpha_,
branches out like the modern stag-horn corals. Another curious form,
_Michelina_, looks exactly like a mass of the papery cells of the
great American hornet in a petrified state, and the convex floors
simulate the covers of the cells, so that it is quite common to find
them called fossil wasps' nests. Some of the largest belong to the
genus _Phillipsastrea_ or _Smithia_, which Hugh Miller has
immortalized by comparing its crowded stars, with confluent rays, to
the once-popular calico pattern known as "Lane's net"--a singular
instance of the accidental concurrence of a natural and artificial
design. Another very common type is that of the conical _Zaphrentis_,
with a deep cut at top to lodge the body of the animal, whose
radiating chambers are faithfully represented by it's delicate
lamellæ. Perhaps the most delicate of the whole is the _Syringopora_,
with its cylindrical worm-like pipes bound together by transverse
processes, and which sometimes can be dissolved out in all its fragile
perfection by the action of an acid on a mass of Corniferous limestone
filled with these corals in a silicified state.
[K] "Travels in North America." second series.
These Devonian corals, like those of the Silurian, belong to the great
extinct groups of Tabulate and Rugose corals; groups which present, on
the one hand, points of resemblance to the ordinary coral animals of
the modern seas, and, on the other, to those somewhat exceptional
corals, the Millepores, which are produced by another kind of polyp,
the Hydroids. Some of them obviously combine properties belonging to
both, as, for example, the radiating partitions with the arrangement
of the parts in multiples of four, the horizontal floors, and the
external solid wall; and this fact countenances the conclusion that in
these old corals we have a group of high and complex organization,
combining properties now divided between two great groups of animals,
neither of them probably, either in their stony skeletons or the soft
parts of the animal, of as high organization as their Paleozoic
predecessors. This sort of disintegration of composite types, or
dissolution of old partnerships, seems to have been no unusual
occurrence in the history of life.[L]
[L] Verril has suggested that the Tabulata may be divided into two
groups, one referable to Actinoids, the other to Hydroids.
If the Devonian witnessed the culmination of the Palæozoic corals, its
later stages saw the final decadence of the great dynasty of the
Trilobites. Of these creatures there are in the Devonian some large
and ornate species, remarkable for their spines and tubercles; as if
in this, the latter day of their dominion, they had fallen into habits
of luxurious decoration unknown to their sterner predecessors, and at
the same time had found it necessary to surround their now disputed
privileges with new safeguards of defensive armour. Not improbably the
decadence of the Trilobites may have been connected with the
introduction of the numerous and formidable fishes of the period.
But while the venerable race of the Trilobites was preparing to fight
its last and unsuccessful battle, another and scarcely less ancient
tribe of crustaceans, the Eurypterids, already strong in the Silurian,
was armed with new and formidable powers. The _Pterygotus anglicus_,
which should have been named _scoticus_, since its head-quarters are
in Scotland, was in point of size the greatest of known crustaceans,
recent or fossil. According to Mr. Henry Woodward, who has published
an admirable description and figures of the creature in the
Palæontographical Society's Memoirs, it must have been six feet in
length, and nearly two feet in breadth. Its antennæ were, unlike the
harmless feelers of modern Crustacea, armed with powerful claws. Two
great eyes stood in the front of the head, and two smaller ones on the
top. It had four pairs of great serrated jaws, the largest as wide as
a man's hand. At the sides were a pair of powerful paddles, capable of
urging it swiftly through the water as it pursued its prey; and when
attacked by any predaceous fish, it could strike the water with its
broad tail, terminated by a great flat "telson," and retreat backward
with the rapidity of an arrow. Woodward says it must have been the
"shark of the Devonian seas;" rather, it was the great champion of the
more ancient family of the lobsters, set to arrest, if possible, the
encroachments of the coming sharks.
The Trilobites and Eurypterids constitute a hard case for the
derivationists. Unlike those Melchisedeks, the fishes of the Silurian,
which are without father or mother, the Devonian crustaceans may boast
of their descent, but they have no descendants. No distinct link
connects them with any modern crustaceans except the Limuli, or
horse-shoe crabs; and here the connection is most puzzling, for while
there seems some intelligible resemblance between the adult
Eurypterids and the horse-shoe, or king-crabs, the latter, in their
younger state, rather resemble Trilobites, as Dr. Packard has recently
shown. Thus the two great tribes of Eurypterids and Trilobites have
united in the small modern group of king-crabs, while on the other
hand, there are points of resemblance, as already stated, between
Trilobites and Isopods, and the king-crabs had already begun to exist,
since one species is now known in the Upper Silurian. So puzzling are
these various relationships, that one naturalist of the derivationist
school has recently attempted to solve the difficulty by suggesting
that the Trilobites are allied to the spiders! Thus nature sports with
our theories, showing us in some cases, as in the corals and fishes,
partnerships split up into individuals, and in others distinct lines
of being converging and becoming lost in one slender thread. Barrande,
the great palæontologist of Bohemia, has recently, in an elaborate
memoir on the Trilobites, traced these and other points through all
their structures and their whole succession in geological time thereby
elaborating a most powerful inductive argument against the theory of
evolution, and concluding that, so far from the history of these
creatures favouring such a theory, it seems as if expressly contrived
to exclude its possibility.
But, while the gigantic Eurypterids and ornate Trilobites of the
Devonian were rapidly approaching their end, a few despised little
crustaceans,--represented by the _Amphipeltis_ of New Brunswick and
_Kampecaris_ of Scotland,--were obscurely laying the foundation of a
new line of beings, that of the Stomapods, destined to culminate in
the Squillas and their allies, which, however different in structure,
are practically the Eurypterids of the modern ocean. So change the
dynasties of men and animals.
"Thou takest away their breath, they die,
They return to their dust;
Thou sendest forth Thy Spirit,
They are created;
Thou renewest the form of the earth."
The reign of fishes began in the Upper Silurian, for in the rocks of
this age, more especially in England, several species have been found.
They occur, however, only in the newer beds of this formation, and are
not of large size, nor very abundant. It is to be observed that, in so
far as the fragments discovered can be interpreted, they indicate the
existence already of two distinct types of fishes, the Ganoids, or
gar-fishes, protected with bony plates and scales, and the Placoids,
or shark-like fishes; and that in the existing world these fishes are
regarded as occupying a high place in their class. Further, these two
groups of fishes are those which throughout a large portion of
geological time continue to prevail to the exclusion of other types,
the ordinary bony fishes having been introduced only in comparatively
recent periods. With the Devonian, however, there comes a vast
increase to the finny armies; and so characteristic are these that the
Devonian has been called the age of fishes _par excellence_, and we
must try, with the help of our illustration, to paint these old
inhabitants of the waters as distinctly as we can. Among the most
ancient and curious of these fishes are those singular forms covered
with broad plates, of which the _Pteraspis_ of the Upper Silurian is
the herald, and which are represented in the Lower Devonian by several
distinct genera. Of these, one of the most curious is the
_Cephalaspis_, or buckler-head, distinguished by its broad flat head,
rounded in front and prolonged at the sides into two great spines,
which project far beyond the sides of the comparatively slender body.
This fish, it may be mentioned, is the type of a family highly
characteristic of the Lower Devonian, as well as of the Upper
Silurian, and all of which are provided with large plate-like cephalic
coverings, sometimes with a long snout in front, and, in so far as is
known, a comparatively weak body and tail. They were all probably
ground-living creatures, feeding on worms and shell-fishes, and
"rooting" for these in the mud, or burrowing therein for their safety.
In these respects they have a most curious analogy to the Trilobites,
which in habits they must have greatly resembled, though belonging by
their structure to an entirely different and much higher class. So
close is this resemblance, that their head-shields used to be mistaken
for those of Trilobites. The case is one of those curious analogies
which often occur in nature, and which must always be distinguished
from the true affinities which rest on structural resemblances.
Another group of small fishes, likewise cuirassed in bony armour of
plates, may be represented by the _Pterichthys_, with its two strong
bony fins at the sides, which may have served for swimming, but
probably also for defence, and for creeping on or shovelling up the
mud at the bottom of the sea. But, besides the Ganoids which were
armed in plated cuirasses, there were others, active and voracious,
clad in shining enamelled scales, like the bony pikes of the American
rivers and the _Polypterus_ of the Nile. Some of these, like the
_Diplacanthus_, or "double-spine" were of small size, and chiefly
remarkable for their sharp defensive bony spines. Others, like
_Holoptychius_ (wrinkled-scale) and _Osteolepis_ (bone-scale), were
strongly built, and sometimes of great size. One Russian species of
_Asterolepis_ (star-scale) is supposed to have been twenty feet in
length, and furnished with strong and trenchant teeth in two rows.
These great fishes afford a good reason for the spines and
armour-plates of the contemporary trilobites and smaller fishes. Just
as man has been endeavouring to invent armour impenetrable to shot,
for soldiers and for ships, and, on the other hand, shot and shells
that can penetrate any armoury so nature has always presented the
spectacle of the most perfect defensive apparatus matched with the
most perfect weapons for destruction. In the class of fishes, no age
of the world is more eminent in these respects then the Devonian.[M]
In addition to these fishes, there were others, represented
principally by their strong bony spines, which must have been allied
to some of the families of modern sharks, most of them, however,
probably to that comparatively harmless tribe which, furnished with
flat teeth, prey upon shell-fishes. There are other fishes difficult
to place in our systems of classification; and among these an eminent
example is the huge _Dinichthys_ of Newberry, from the Hamilton group
of Ohio. The head of this creature is more then three feet long and
eighteen inches broad, with the bones extraordinarily strong and
massive. In the upper jaw, in addition to strong teeth, there were in
front two huge sabre-shaped tusks or incisors, each nearly a foot
long; and corresponding to these in the massive lower jaw were two
closely joined conical tusks, fitting between those of the upper jaw.
No other fish presents so frightful an apparatus for destruction; and
if, as is probable, this was attached to a powerful body, perhaps
thirty feet in length, and capable of rapid motion through the water,
we cannot imagine any creature so strong or so well armed as to cope
with the mighty _Dinichthys_.
[M] Many of these were discovered and successfully displayed and
described by Hugh Miller, and are graphically portrayed in his
celebrated work on the "Old Red Sandstone," published in 1841.
The difference between the fishes of the Devonian and those of the
modern seas is well marked by the fact that, while the ordinary bony
fishes now amount to probably 9,000 species, and the ganoid fishes to
less then thirty, the finny tribes of the Devonian are predominantly
ganoids, and none of the ordinary type are known. To what is this
related, with reference to conditions of existence? Two explanations,
different yet mutually connected, may be suggested. One is that armour
was especially useful in the Devonian as a means of defence from the
larger predaceous species, and the gigantic crustaceans of the period.
that this was the case may be inferred from the conditions of
existence of some modern ganoids. The common bony pike of Canada
(_Lepidosteus_), frequenting shallow and stagnant waters, seems to be
especially exposed to injury from its enemies. Consequently, while it
is rare to find an ordinary fish showing any traces of wounds, a large
proportion of the specimens of the bony pike which I have examined
have scars on their scales, indicating injuries which they have
experienced, and which possibly, to fishes not so well armed, might
have proved fatal. Again, in the modern Amia, or mud-fish, in the bony
pike and _Polypterus_, there is an extremely large air-bladder, amply
supplied with blood-vessels, and even divided into cells or chambers,
and communicating with the mouth by an "air-duct." This organ is
unquestionably in function a lung, and enables the animal to dispense
in some degree with the use of its gills, which of course depend for
their supply of vital air on the small quantity of oxygen dissolved in
the water. Hence, by the power of partially breathing air, these
fishes can live in stagnant and badly aerated waters, where other
fishes would perish. In the case of the _Amia_, the grunting noises
which it utters, its habit of frequenting the muddy creeks of swamps,
and its possession of gill-cleaners, correspond with this view. It is
possible that the Devonian fishes possessed this semi-reptilian
respiration; and if so, they would be better adapted then other fishes
to live in water contaminated with organic matter in a state of decay,
or in waters rich in carbonic acid or deficient in oxygen. Possibly
the palæozoic waters, as well as the palæozoic atmosphere, were less
rich in pure oxygen then those of the present world; and it is certain
that, in many of the beds in which the smaller Devonian fishes abound,
there was so much decaying vegetable matter as to make it probable
that the water was unfit for the ordinary fishes. Thus, though at
first sight the possession of external armour and means to respire
air, in the case of these peculiar fishes, may seem to have no direct
connection with each other, their obvious correlation in some modern
ganoids may have had its parallel on a more extensive scale among
their ancient relatives. Just as the modern gar-fish, by virtue of its
lungs, can live in stagnant shallows and hunt frogs, but on that
account needs strong armour to defend it against the foes that assail
it in such places; so in the Devonian the capacity to inhabit
unaërated water and defensive plates and scales may have been alike
necessary, especially to the feebler tribes of fishes. We shall find
that in the succeeding carboniferous period there is equally good
evidence of this.
We have reserved little space for the Devonian plants and insects; but
we may notice both in a walk through a Devonian forest, in which we
may include the vegetation of the several subordinate periods into
which this great era was divisible. The Devonian woods were probably,
like those of the succeeding carboniferous period, dense and dark,
composed of but few species of plants, and these somewhat monotonous
in appearance, and spreading out into broad swampy jungles,
encroaching on the shallow bays and estuaries. Landing on one of these
flats, we may first cast our eyes over a wide expanse, covered with
what at a distance we might regard as reeds or rushes. But on a near
approach they appear very different; rising in slender, graceful
stems, they fork again and again, and their thin branches are sparsely
covered with minute needle-like leaves, while the young shoots curl
over in graceful tresses, and the older are covered with little oval
fruits, or spore-cases; for these plants are cryptogamous, or
flowerless. This singular vegetation stretches for miles along the
muddy flats, and rises to a height of two or three feet from a knotted
mass of cylindrical roots or root-stocks, twining like snakes through
and over the soil. This plant may, according as we are influenced by
its fruit or structure, be regarded as allied to the modern
club-mosses or the modern pill-worts. It is _Psilophyton_, in every
country one of the most characteristic plants of the period, though,
when imperfectly preserved, often relegated by careless and unskilled
observers to the all-engulfing group of fucoids. A little further
inland we see a grove of graceful trees, forking like _Psilophyton_,
but of grander dimensions, and with the branches covered with linear
leaves, and sometimes terminated by cones. These are _Lepidodendra_,
gigantic club-mosses, which were developed to still greater
dimensions in the coal period. Near these we may see a still more
curious tree, more erect in its growth, with rounded and somewhat
rigid leaves and cones of different form, and with huge cable-like
roots, penetrating the mud, and pitted with the marks of long
rootlets. This is _Cyclostigma_, a plant near to the _Lepidodendron_,
but distinct, and peculiar to the Devonian. Some of its species attain
to the dimensions of considerable trees; others are small and shrubby.
Another small tree, somewhat like the others, but with very long
shaggy leaves, and its bark curiously marked with regular
diamond-shaped scars, is the _Leptophleum_. All these plants are
probably allied to our modern club-mosses, which are, however, also
represented by some low and creeping species cleaving to the ground. A
little further, and we reach a dense clump of _Sigillariæ_, with tall
sparsely forking stems, and ribbed with ridges holding rows of
leaf-scars a group of plants which we shall have further occasion to
notice in the coal formation; and here is an extensive jungle of
_Calamites_, gigantic and overgrown mares'-tails, allies of the modern
equisetums.
[Illustration: Fig. 12.--VEGETATION OF THE DEVONIAN.
To the left are _Calamites_; next to these, _Leptophleum_; in the
centre are _Lepidodendron_, _Sigillaria_, and a Pine. Below are
_Psilophyton_, _Cordaites_, Ferns, and _Asterophyllites_.]
Amidst these trees, every open glade is filled with delicate ferns of
marvellous grace and beauty; and here and there a tree-fern rears its
head, crowned with its spreading and graceful leaves, and its trunk
clad with a shaggy mass of aërial roots--an old botanical device, used
in these ancient times, as well as now, to strengthen and protect the
stems of trees not fitted for lateral expansion. Beyond this mass of
vegetation, and rising on the slopes of the distant hills, we see
great trees that look like pines. We cannot approach them more nearly;
but here on the margin of a creek we see some drift-trunks, that have
doubtless been carried down by a land flood. One of them is certainly
a pine, in form and structure of its wood very like those now living
in the southern hemisphere; it is a _Dadoxylon_. Another is different,
its sides rough and gnarled, and marked with huge irregular ridges;
its wood loose, porous, and stringy, more like the bark of modern
pines, yet having rings of growth and a true bark of its own, and
sending forth large branches and roots. It is the strange and
mysterious _Prototaxites_, one of the wonders of the Devonian land,
and whose leaves and fruits would be worth their weight in gold in our
museums, could we only procure them. A solitary fragment further
indicates that in the yet unpenetrated solitudes of the Devonian
forests there may be other trees more like our ordinary familiar
friends of the modern woods; but of these we know as yet but little.
What inhabitants have these forests? All that we yet know are a few
large insects, relatives of our modern May-flies, flitting with broad
veined wings over the stagnant waters in which their worm-like larvæ
dwell, and one species at least assuming one of the properties of the
grasshopper tribe, and enlivening the otherwise silent groves with a
cricket-like chirp, the oldest music of living things that geology as
yet reveals to us; and this, not by the hearing of the sound itself,
but by the poor remains of the instrument attached to a remnant of a
wing from the Devonian shales of New Brunswick.
A remarkable illustration of the abundance of certain plants in the
Devonian, and also of the slow and gradual accumulation of some of its
beds, is furnished by layers of fossil spore-cases, or the minute sacs
which contain the microscopic germs of club-mosses and similar plants.
In the American forests, in spring, the yellow pollen-grains of
spruces and pines sometimes drift away in such quantities in the
breeze that they fall in dense showers, popularly called showers of
sulphur; and this vegetable sulphur, falling in lakes and ponds, is
drifted to the shore in great sheets and swathes. The same thing
appears to have occurred in the Devonian, not with the pollen of
flowering plants, but with the similar light spores and spore-cases of
species of Lepidodendron and allied trees. In a bed of shale, at
Kettle Point, Lake Huron, from 12 to 14 feet thick, not only are the
surfaces of the beds dotted over with minute round spore-cases, but,
on making a section for the microscope, the substance of each layer is
seen to be filled with them; and still more minute bodies, probably
the escaped spores, are seen to fill up their interstices. The
quantity of these minute bodies is so great that the shale is
combustible, and burns with much flame. A bed of this nature must have
been formed in shallow and still water, on the margin of an extensive
jungle or forest; and as the spore-cases are similar to those of the
Lepidodendra of the coal-measures, the trees were probably of this
kind. Year after year, as the spores became ripe, they were wafted
away, and fell in vast quantities into the water, to be mixed with the
fine mud there accumulating. When we come to the coal period, we shall
see that such beds of spore-cases occur there also, and that they have
even been supposed to be mainly instrumental in the accumulation of
certain beds of coal. Their importance in this respect may have been
exaggerated, but the fact of their occurrence in immense quantities in
certain coals and shales is indisputable.
This is but a slender sketch of the Devonian forests: but we shall
find many of the same forms of plants in the carboniferous period
which succeeds. With one thought we may close. We are prone to ask for
reasons and uses for things, but sometimes we cannot be satisfied. Of
what use were the Devonian forests? They did not, like those of the
coal formation, accumulate rich beds of coal for the use of man.
Except possibly a few insects, we know no animals that subsisted on
their produce, nor was there any rational being to admire their
beauty. Their use, except as helping us in these last days to complete
the order of the vegetable kingdom as it has existed in geological
time, is a mystery. We can but fall back on that ascription of praise
to Him "who liveth for ever and ever," on the part of the heavenly
elders who cast down their crowns before the throne and say, "Thou art
worthy, Lord, to receive the glory, and the honour, and the might;
because Thou didst create all things, and by reason of _Thy will_ they
are and were created."