THE LOWER AND UPPER SILURIAN AGES.
By English geologists, the great series of formations which succeeds
to the Cambrian is usually included under the name Silurian System,
first proposed by Sir Roderick Murchison. It certainly, however,
consists of two distinct groups, holding the second and third faunas
of Barrande. The older of the two, usually called the Lower Silurian,
is the Upper Cambrian of Sedgwick, and may properly be called the
_Siluro-Cambrian_. The newer is the true Silurian, or Silurian
proper--the Upper Silurian of Murchison. We shall in this chapter, for
convenience, consider both in connection, using occasionally the term
Lower Silurian as equivalent to Siluro-Cambrian. The Silurian presents
us with a definite physical geography, for the northern hemisphere at
least; and this physical geography is a key to the life conditions of
the time. The North American continent, from its great unbroken area,
affords, as usual, the best means of appreciating this. In this period
the northern currents, acting perhaps in harmony with old Laurentian
outcrops, had deposited in the sea two long submarine ridges, running
to the southward from the extreme ends of the Laurentian nucleus, and
constituting the foundations of the present ridges of the Rocky
Mountains and the Alleghanies. Between these the extensive triangular
area now constituting the greater part of North America, was a shallow
oceanic plateau, sheltered from the cold polar currents by the
Laurentian land on the north, and separated by the ridges already
mentioned from the Atlantic and Pacific. It was on this great plateau
of warm and sheltered ocean that what we call the Silurian fauna
lived; while of the creatures that inhabited the depths of the great
bounding oceans, whose abysses must have been far deeper and at a much
lower temperature, we know little. During the long Silurian periods,
it is true, the great American plateau underwent many revolutions,
sometimes being more deeply submerged, and having clear water tenanted
by vast numbers of corals and shell-fishes, at others rising so as to
become shallow and to receive deposits of sand and mud; but it was
always distinct from the oceanic area without. In Europe, in like
manner, there seems to have been a great internal plateau bounded by
the embryo hills of Western Europe on the west, and harbouring a very
similar assemblage of creatures to those existing in America.
Further, during these long periods there were great changes, from a
fauna of somewhat primordial type up to a new order of things in the
Upper Silurian, tending toward the novelties which were introduced in
the succeeding Devonian and Carboniferous. We may, in the first place,
sketch these changes as they occurred on the two great continental
plateaus, noting as we proceed such hints as can be obtained with
reference to the more extensive oceanic spaces.
Before the beginning of the age, both plateaus seem to have been
invaded by sandy and muddy sediments charged at some periods and
places with magnesian limestone; and these circumstances were not
favourable to the existence or preservation of organic remains. Such
are the Potsdam and Calciferous beds of America and the Tremadoc and
Llandeilo beds of England. The Potsdam and Tremadoc are by their
fossils included in the Cambrian, and may at least be regarded as
transition groups. It is further to be observed, in the case of these
beds, that if we begin at the west side of Europe and proceed
easterly, or at the east side of America and proceed westerly, they
become progressively thinner, the greater amount of material being
deposited at the edges of the future continents; just as on the sides
of a muddy tideway the flats are higher, and the more coarse sediment
deposited near the margin of the channel, and fine mud is deposited at
a greater distance and in thinner beds. The cause, however, on the
great scale of the Atlantic, was somewhat different, ancient ridges
determining the border of the channel. This statement holds good not
only of these older beds, but of the whole of the Silurian, and of the
succeeding Devonian and Carboniferous, all deposited on these same
plateaus. Thus, in the case of the Silurian in England and Wales, the
whole series is more then 20,000 feet thick, but in Russia, it is
less then 1,000 feet. In the eastern part of America the thickness is
estimated at quite as great an amount as in Europe, while in the
region of the Mississippi the Silurian rocks are scarcely thicker then
in Russia, and consist in great part of limestones and fine sediments,
the sandstones and conglomerates thinning out rapidly eastward of the
Appalachian Mountains.
In both plateaus the earlier period of coarse accumulations was
succeeded by one in which was clear water depositing little earthy
sediment, and this usually fine; and in which the sea swarmed with
animal life, from the _débris_ of which enormous beds of limestone
were formed the Trenton limestone of America and the Bala limestone of
Europe. The fossils of this part of the series open up to us the
head-quarters of Lower Silurian life, the second great fauna of
Barrande, that of the Upper Cambrian of Sedgwick; and in America more
especially, the Trenton and its associated limestones can be traced
over forty degrees of longitude; and throughout the whole of this
space its principal beds are composed entirely of comminuted corals,
shells, and crinoids, and studded with organisms of the same kinds
still retaining their forms. Out of these seas, in the European area,
arose in places volcanic islets, like those of the modern Pacific.
In the next succeeding era the clear waters became again invaded with
muddy and sandy sediments, in various alternations, and with
occasional bands of limestone, constituting the Caradoc beds of
Britain and the Utica and Hudson River groups of America. During the
deposition of these, the abounding life of the Siluro-Cambrian
plateaus died away, and a middle group of sandstones and shales, the
Oneida and Medina of America and the Mayhill of England, form the base
of the Upper Silurian.
But what was taking place meanwhile in the oceanic areas separating
our plateaus? These were identical with the basins of the Atlantic and
Pacific, which already existed in this period as depressions of the
earth's crust, perhaps not so deep as at present. As to the deposits
in their deeper portions we know nothing; but on the margin of the
Atlantic area are some rocks which give us at least a little
information.
In the later part of the Cambrian period the enormous thickness of the
Quebec group of North America appears to represent a broad stripe of
deep water parallel to the eastern edge of the American plateau, and
in which an immense thickness of beds of sand and mud was deposited
with very few fossils, except in particular beds, and these of a more
primordial aspect then those of the plateau itself. These rocks no
doubt represent the margin of a deep Atlantic area, over which cold
currents destructive of life were constantly passing, and in which
great quantities of sand and mud, swept from the icy regions of the
North, were continually being laid. The researches of Dr. Carpenter
and Dr. Wyville Thomson show us that there are at present cold areas
in the deeper parts of the Atlantic, on the European side, as we have
long known that they exist at less depths on the American side; and
these same researches, with the soundings on the American banks, show
that sand and gravel may be deposited not merely on shallows, but in
the depths of the ocean, provided that these depths are pervaded by
cold and heavy currents capable of eroding the bottom, and of moving
coarse material. The Quebec group in Canada and the United States, and
the metalliferous Lower Silurian rocks of Nova Scotia and
Newfoundland, destitute of great marine limestones and coral reefs,
evidently represent deep and cold-water areas on the border of the
Atlantic plateau.
At a later period, the beginning of the Upper Silurian, the richly
fossiliferous and exceptional deposits of the Island of Anticosti,
formed in the deep hollow of the Gulf of St. Laurence, show that when
the plateau had become shallowed up by deposition and elevation, and
converted into desolate sand-banks, the area of abundant life was
transferred to the still deep Atlantic basin and its bordering bays,
in which the forms of Lower Silurian life continued to exist until
they were mixed up with those of the Upper Silurian.
If we turn now to these latter rocks, and inquire as to their
conditions on our two great plateaus, we shall find a repetition of
changes similar to those which occurred in the times preceding. The
sandy shallows of the earlier part of this period give place to wide
oceanic areas similar to those of the Lower Silurian; In these we find
vast and thick coral and shell limestones, the Wenlock of England and
Niagara of America, as rich in life as the limestones of the Lower
Silurian, and with the generic and family forms similar, but the
species for the most part different. In America these limestones were
followed by a singularly shallow condition of the plateau, in which
the surface was so raised as at times to be converted into separate
salt lakes in which beds of salt were deposited. On both plateaus
there were alternations of oceanic and shallow conditions, under which
the Lower Helderberg and Ludlow beds, the closing members of the
Silurian, were laid down. Of the Atlantic beds of this period we know
little, except that the great limestones appear to be wanting, and to
be replaced by sandy and muddy deposits, in some parts at least of the
margins of the area. In some portions also of the plateaus and their
margins, extensive volcanic outbursts seem to have occurred; so that
the American plateau presented, at least in parts, the aspect of a
coral sea with archipelagos of volcanic islands, the ejections from
which became mixed with the aqueous deposits forming around them.
Having thus traced the interesting series of geographical conditions
indicated by the Silurian series, we may next take our station on one
of the submerged plateaus, and inquire as to the new forms of life now
introduced to our notice; and in doing so shall include the life of
both the Lower and Upper Silurian.
[Illustration: Fig. 9.--Fragment of Lower Silurian Limestone, sliced
and magnified ten diameters, showing the manner in which it is made up
of fragments of corals, crinoids, and shells. (From a paper oil the
Microscopic Structure of Canadian limestone, "Canadian Naturalist.")]
First, we may remark the vast abundance and variety of corals. The
polyps, close relatives of the common sea-anemone of our coasts, which
build up our modern coral reefs, were represented in the Silurian seas
by a great number of allied yet different forms, equally effectual in
the great work of secreting carbonate of lime in stony masses, and
therefore in the building-up of continents. Let us note some of the
differences. In the first place, whereas our modern coral-workers can
show us but the topmost pinnacles of their creations, peeping above
the surface of the sea in coral reefs and islands, the work of the
coral animals of the Silurian has been finished, by these limestones
being covered with masses of new sediment consolidated into hard rock,
and raised out of the sea to constitute a part of the dry land. In
the Silurian limestones we thus have, not merely the coral reefs, but
the wide beds of comminuted coral, mixed with the remains of other
animals, which are necessarily accumulated in the ocean bed around the
reefs and islands. Further, these beds, which we might find loose and
unconsolidated in the modern sea, have their fragments closely
cemented together in the old limestones. The nature of this difference
can be well seen by comparing a fragment of modern coral or shell
limestone from Bermuda, with a similar fragment of the Trenton
limestone, both being sliced for examination under the microscope. The
old limestone is black or greyish, the modern one is nearly white,
because in the former the organic matter in the animal fragments has
been carbonised or converted into coaly and bituminous matter. The old
limestone is much more dense and compact, partly because its materials
have been more closely compressed by superincumbent weight, but
chiefly because calcareous matter in solution in water has penetrated
all the interstices, and filled them up with a deposit of crystalline
limestone. In examining a slice, however, under the microscope, it
will be seen that the fragments of corals and other organisms are as
distinct and well preserved as in the crumbling modern rock, except
that they are perfectly imbedded in a paste of clear transparent
limestone, or rather calcareous spar, infiltrated between them. I have
examined great numbers of slices of these limestones, ever with new
wonder at the packing of the organic fragments which they present. The
hard marble-like limestones used for building in the Silurian
districts of Europe and America, are thus in most cases consolidated
masses of organic fragments.
In the next place, the animals themselves must have differed somewhat
from their modern successors. This we gather from the structure of
their stony cells, which present points of difference indicating
corresponding difference of detail in the soft parts. Zoologists thus
separate the rugose or wrinkled corals and the tabulate or floored
corals of the Silurian from those of the modern seas. The former must
have been more like the ordinary coral animals; the latter were very
peculiar, more especially in the close union of the cells, and in the
transverse floors which they were in the habit of building across
these cells as they grew in height. They presented, however, all the
forms of our modern corals. Some were rounded and massive in form,
others delicate and branching. Some were solitary or detached, others
aggregative in communities. Some had the individual animals large and
probably showy, others had them of microscopic size. Perhaps the most
remarkable of all is the American _Beatricea_,[H] which grew like a
great trunk of a tree twenty feet or more in height, its solitary
animal at the top like a pillar-saint, though no doubt more
appropriate and comfortable; and multitudes of delicate and encrusting
corals clinging like mosses or lichens to its sides. This creature
belongs to the very middle of the Silurian, and must have lived in
great depths, undisturbed by swell or breakers, and sheltering vast
multitudes of other creatures in its stony colonnades.
[H] First described by Mr. Billings. It has been regarded as a plant,
and as a cephalopod shell; but I believe it was a coral allied to
_Cystiphyllum_.
[Illustration: Fig. 10.--LIFE IN THE SILURIAN AGE.
On the bottom are seen, proceeding from left to right, Corals
(_Stenopora_ and _Beatricea_) and a Gasteropod; _Orthoceras_; Coral
(_Patria_); Crinoids, _Lingulæ_, and Cystideans; a _Trilobite_ and
_Cyrtolites_. In the water is a large _Pterygotus_, and under it a
_Trinucleus_. Further on, are Cephalopods, a Heteropod, and Fishes.
At the surface, _Phyllograptus_, _Graptolithus_, and _Bellerophon_.
On the Land, _Lepidodendron_, _Psilophyton_, and _Prototaxites_.]
Lastly, the Silurian corals nourished in latitudes more boreal then
their modern representatives. In both hemispheres as far north as
Silurian limestones have been traced, well-developed corals have been
found. On the great plateaus sheltered by Laurentian ridges to the
north, and exposed to the sun and to the warmer currents of the
equatorial regions, they nourished most grandly and luxuriantly: but
they lived also north of the Laurentian bands in the Arctic Sea
basins, though probably in the shallower and more sheltered parts.
Undoubtedly the geographical arrangements of the Silurian period
contributed to this. We have already seen how peculiarly adapted to an
exuberant marine life were the submerged continents of the period; and
there was probably little Arctic land producing icebergs to chill the
seas. The great Arctic currents, which then as now flowed powerfully
toward the equator, must have clung to the deeper parts of the ocean
basins, while the return waters from the equator would spread
themselves widely over the surface; so that wherever the Arctic Seas
presented areas a little elevated out of the cold water bottom, there
might be suitable abodes for coral animals. It has been supposed that
in the Silurian period the sea might have derived some appreciable
heat from the crust of the earth below, and astronomical conditions
have been suggested as tending to produce changes of climate; but it
is evident that whatever weight may be due to these causes, the
observed geographical conditions are sufficient to account for the
facts of the case. It is also to be observed, that we cannot safely
infer the requirements as to temperature of Silurian coral animals
from those of the tenants of the modern ocean. In the modern seas many
forms of life thrive best and grow to the greatest size in the colder
seas; and in the later tertiary period there were elephants and
rhinoceroses sufficiently hardy to endure the rigours of an Arctic
climate. So there may have been in the Silurian seas corals of much
less delicate constitution then those now living.
Next to the corals we may place the crinoids, or
stone-lilies--creatures abounding throughout the Silurian seas, and
realizing a new creative idea, to be expanded in subsequent geological
time into all the multifarious types of star-fishes and sea-urchins. A
typical crinoid, such as the _Glyptocrinus_ of the Lower Silurian,
consists of a flexible jointed stem, sometimes several feet in length,
composed of short cylindrical discs, curiously articulated together, a
box-like body on top made up of polygonal pieces attached to each
other at the edges, and five radiating jointed arms furnished with
branches and branchlets, or fringes, all articulated and capable of
being flexed in any direction. Such a creature has more the aspect of
a flower then of an animal; yet it is really an animal, and subsists
by collecting with its arms and drifting into its mouth minute
creatures floating in the water. Another group, less typical, but
abundantly represented in the Silurian seas, is that of the
Cystideans, in which the body is sack-like, and the arms few and
sometimes attached to the body. They resemble the young or larvæ of
crinoids. In the modern seas the crinoids are extremely few, though
dredging in very deep water has recently added to the number of known
species; but in the Silurian period they had their birth, and attained
to a number and perfection not afterwards surpassed. Perhaps the
stone-lilies of the Upper Silurian rocks of Dudley, in England, are
the most beautiful of Palæozoic animals. Judging from the immense
quantities of their remains in some limestones, wide areas of the sea
bottom must have been crowded with their long stalks and flower-like
bodies, presenting vast submarine fields of these stony water-lilies.
Passing over many tribes of mollusks, continued or extended from the
Primordial--and merely remarking that the lamp-shells and the ordinary
bivalve and univalve shell-fishes are all represented largely, more
especially the former group, in the Silurian--we come to the highest
of the Mollusca, represented in our seas by the cuttle-fishes and
nautili, creatures which, like the crinoids, may be said to have had
their birth in the Silurian, and to have there attained to some of
their grandest forms. The modern pearly nautilus shell, well known in
every museum, is beautifully coiled in a disc-like form, and when
sliced longitudinally shows a series of partitions dividing it into
chambers, air-tight, and serving as a float to render the body of the
creature independent of the force of gravity. As the animal grows it
retracts its body toward the front of the shell, and forms new
partitions, so that the buoyancy of the float always corresponds with
the weight of the animal; while by the expansion and contraction of
the body and removal of water from a tube or syphon which traverses
the chambers, or the injection of additional water, slight differences
can be effected, rendering the creature a very little lighter or
heavier then the medium in which it swims. Thus practically delivered
from the encumbrance of weight, and furnished with long flexible arms
provided with suckers, with great eyes and a horny beak, the nautilus
becomes one of the tyrants of the deep, creeping on the bottom or
swimming on the surface at will, and everywhere preying on whatever
animals it can master. Fortunately for us, as well as for the more
feeble inhabitants of the sea, the nautili are not of great size,
though some of their allies, the cuttle-fishes, which, however,
want the floating apparatus, are sufficiently powerful to be
formidable to man. In the Silurian period, however, there were
not only nautili like ours, but a peculiar kind of straight
nautilus--the _Orthoceratites_--which sometimes attained to gigantic
size. The shells of these creatures may be compared to those of nautili
straightened out, the chambers being placed in a direct line in front
of each other. A great number of species have been discovered, many
quite insignificant in size, but others as much as twelve feet in
length and a foot in diameter at the larger end. Indeed, accounts have
been given of individuals of much larger growth. These large
_Orthoceratites_ were the most powerful marine animals known to us in
the Silurian, and must have been in those days the tyrants of the
seas.[I]
[I] Zoologists will observe that I have, in the illustrations given
the Orthoceras the arms rather of a cuttle-fish then of a nautilus.
The form of the outer chamber of the shell, I think, warrants this
view of the structure of the animal, which must have been formed on a
very comprehensive type.
Among the crustaceans, or soft shell-fishes of the Silurian, we meet
with the _Trilobites_, continued from the Primordial in great and
increasing force, and represented by many and beautiful species; while
an allied group of shell-fishes of low organization but gigantic size,
the _Eurypterids_, characteristic of the Upper Silurian, were provided
with powerful limbs, long flexible bodies, and great eyes in the front
of the head, and were sometimes several feet in length. Instead of
being mud grovellers, like the Trilobites and modern king-crabs, these
_Eurypterids_ must have been swimmers, careering rapidly through the
water, and probably active and predaceous. There were also great
multitudes of those little crustaceans which are inclosed in two horny
or shelly valves like a bivalve shell-fish, and the remains of which
sometimes fill certain beds of Silurian shale and limestone.
No remains found in the Silurian rocks have been more fertile sources
of discussion then the so-called _Graptolites_, or written stones--a
name given long ago by Linnæus, in allusion to the resemblance of some
species having rows of cells on one side, to minute lines of writing.
These little bodies usually appear as black coaly stains on the
surface of the rock, showing a slender stem or stalk, with a row of
little projecting cells at one side, or two rows, one on each side.
The more perfect specimens show that, in many of the species at least,
these fragments were branches of a complex organism spreading from a
centre; and at this centre there is sometimes perceived a sort of
membrane connecting the bases of the branches, and for which various
uses have been conjectured. The branches themselves vary much in
different species. They may be simple or divided, narrow, or broad and
leaf-like, with one row of cells, or two rows of cells. Hence arise
generic distinctions into single and double graptolites, leaf and tree
graptolites, net graptolites, and so on. But while it is easy to
recognise these organisms, and to classify them in species and genera,
it is not so easy to say what their affinities are with modern things.
They are exclusively Silurian, disappearing altogether at the close of
this period, and, so far as we know, not succeeded by any similar
creatures serving to connect them with modern forms. Hence the most
various conjectures as to their nature. They have been supposed to be
plants, and have been successively referred to most of the great
divisions of the lower animals. Most recently they have been regarded
by Hall, Nicholson,[J] and others, who have studied them most
attentively, as zoophytes or hydroids allied to the Sertularise, or
tooth-corallines and sea-fir-corallines of our coasts, to the
cell-bearing branches of which their fragments bear a very close
resemblance. In this case, each of the little cells or teeth at the
sides of the fibres must have been the abode of a little polyp,
stretching out its tentacles into the water, and enjoying a common
support and nutrition with the other polyps ranged with it. Still the
mode of life of the community of branching stems is uncertain. In some
species there is a little radicle or spike at the base of the main
stem, which may have been a means of attachment. In others the hollow
central disk has been conjectured to have served as a float. Occurring
as the specimens do usually in shales and slates, which must have been
muddy beds, they could not have been attached to stones or rocks, and
they must have lived in clear water, either seated on the surface of
the mud, attached to sea-weeds, or floating freely by means of hollow
disks filled with air. After much thought on their structure and mode
of occurrence, I am inclined to believe that in their younger stages
they were attached, but by a very slender thread; that at a more
advanced stage they became free, and acquiring a central membranous
disk filled with air, floated by means of this at the surface, their
long branches trailing in the waters below. They would thus be, with
reference to their mode of life, though not to the details of their
structure, prototypes of the modern Portuguese man-of-war, which now
drifts so gaily over the surface of the warmer seas. I have
represented them in this attitude; but in case I should be mistaken,
the reader may imagine it possible that they may be adhering to the
lower surface of floating tangle. The head-quarters of the Graptolites
seem to be in the upper part of the Cambrian, and in the
Siluro-Cambrian, and they are widely distributed in Europe, in
America, and in Australia. This very wide distribution of the species
is probably connected with their floating and oceanic habits.
[J] See also an able paper by Carruthers, in the _Geological
Magazine_, vol. v., p. 64.
Lastly, just as the Silurian period was passing away, we find a new
thing in the earth--vertebrate animals, represented by several species
of shark-like fishes, which came in here as forerunners of the dynasty
of the vertebrates, which from that day to this have been the masters
of the world. These earliest vertebrates are especially interesting as
the first known examples of a plan of structure which culminates only
in man himself. They appear to have had cartilaginous skeletons; and
in this and their shagreen-like skin, strong bony spines, and
trenchant teeth, to have much resembled our modern sharks, or rather
the dog-fishes, for they were of small size. One genus (_Pteraspis_),
apparently the oldest of the whole, belongs, however, to a tribe of
mailed fishes allied to some of those of the old red sandstone. In
both cases the groups of fishes representing the first known
appearance of the vertebrates were allied to tribes of somewhat high
organization in that class; and they asserted their claims to
dominancy by being predaceous and carnivorous creatures, which must
have rendered themselves formidable to their invertebrate
contemporaries. Coprolites, or fossil masses of excrement, which are
found with them, indicate that they chased and devoured orthoceratites
and sea-snails of various kinds, and snapped Lingulæ and crinoids from
their stalks; and we can well imagine that these creatures, when once
introduced, found themselves in rich pasture and increased
accordingly. Space prevents us from following further our pictures of
the animal life of the great Silurian era, the monuments of which were
first discovered by two of England's greatest geologists, Murchison
and Sedgwick. How imperfect such a notice must be, may be learned from
the fact that Dr. Bigsby, in his "Thesaurus Siluricus" in 1868,
catalogues 8,897 Silurian species, of which only 972 are known in the
Primordial.
Our illustration, carefully studied, may do more to present to the
reader the teeming swarms of the Silurian seas then our word-picture,
and it includes many animal forms not mentioned above, more especially
the curved and nautilus-like cuttle-fishes, those singular molluscous
swimmers by fin or float known to zoologists as violet-snails,
winged-snails or pteropods, and carinarias; and which, under various
forms, have existed from the Silurian to the present time. The old
_Lingulæ_ are also there as well as in the Primordial, while the fishes
and the land vegetation belong, as far as we yet know, exclusively to
the Upper Silurian, and point forward to the succeeding Devonian. We
know as yet no Silurian animal that lived on the land or breathed air.
But our knowledge of land plants, though very meagre, is important.
Without regarding such obscure and uncertain forms as the _Eophyton_
of Sweden, Hooker, Page, and Barrande have noticed, in the Upper
Silurian, plants allied to the Lycopods or club-mosses. I have found
in the same deposits another group of plants allied to Lycopods and
pill-worts (Psilophyton), and fragments of wood representing the
curious and primitive type of pine-like trees known as _Prototaxites_.
These are probably only a small instalment of Silurian land plants,
such as a voyager might find floating in the sea on his approach to
some unknown shore, which had not yet risen above his horizon. Time
and careful search will, no doubt, add largely to our knowledge.
In the Silurian, as in the Cambrian, the head-quarters of animal life
were in the sea. Perhaps there was no animal life on the land; but
here our knowledge may be at fault. It is, however, interesting to
observe the continued operation of the creative fiat, "Let the waters
swarm with swarmers" which, beginning to be obeyed in the Eozoic age,
passes down through all the periods of geological time to the "moving
things innumerable" of the modern ocean. Can we infer anything further
as to the laws of creation from these Silurian multitudes of living
things? One thing we can see plainly, that the life of the Silurian is
closely related to that of the Cambrian. The same generic and ordinal
forms are continued. Even some species may be identical. Does this
indicate direct genetic connection, or only like conditions in the
external world correlated with likeness in the organic world? It
indicates both. First, it is in the highest degree probable that many
of the animals of the Lower Silurian are descendants of those of the
Cambrian. Sometimes these descendants may be absolutely unchanged.
Sometimes they may appear as distinct varieties. Sometimes they may
have been regarded as distinct though allied species. The continuance
in this manner of allied forms of life is necessarily related to the
continuance of somewhat similar conditions of existence, while changes
in type imply changed external conditions. But is this all? I think
not; for there are forms of life in the Silurian which cannot be
traced to the Cambrian, and which relate to new and even prospective
conditions, which the unaided powers of the animals of the earlier
period could not have provided for. These new forms require the
intervention of a higher power, capable of correlating the physical
and organic conditions of one period with those of succeeding periods.
Whatever powers may be attributed to natural selection or to any other
conceivable cause of merely genetic evolution, surely prophetic gifts
cannot be claimed for it; and the life of all these geological periods
is full of mute prophecies to be read only in the light of subsequent
fulfilments.
The fishes of the Upper Silurian are such a prophecy. They can claim
no parentage in the older rocks, and they appear at once as kings of
their class. With reference to the Silurian itself, they are of little
consequence; and in the midst of its gigantic forms of invertebrate
life they seem almost misplaced. But they predict the coming Devonian,
and that long and varied reign of vertebrate life which culminates in
man himself. No such prophetic ideas are represented by the giant
crustaceans and cuttle-fishes and swarming graptolites. They had
already attained their maximum, and were destined to a speedy and
final grave in the Silurian, or to be perpetuated only in decaying
families whose poverty is rendered more conspicuous by the contrast
with the better days gone by. The law of creation provided for new
types, and at once for the elevation and degradation of them when
introduced; and all this with reference to the physical conditions not
of the present only but of the future. Such facts, which cannot be
ignored save by the wilfully blind, are beyond the reach of any merely
material philosophy.
The little that we know of Silurian plants is as eloquent of plan and
creation as that which we can learn of animals. I saw not long ago a
series of genealogies in geological time reduced to tabular form by
that ingenious but imaginative physiologist, Haeckel. In one of these
appeared the imaginary derivation of the higher plants from Algæ or
sea-weeds. Nothing could more curiously contradict actual facts. Algæ
were apparently in the Silurian neither more nor less elevated then in
the modern seas, and those forms of vegetable life which may seem to
bridge over the space between them and the land plants in the modern
period, are wanting in the older geological periods, while land plants
seem to start at once into being in the guise of club-mosses, a group
by no means of low standing. Our oldest land plants thus represent one
of the highest types of that cryptogamous series to which they belong,
and moreover are better developed examples of that type then those now
existing. We may say, if we please, that all the connecting links have
been lost; but this is begging the whole question, since no thing 'but
the existence of such links could render the hypothesis of derivation
possible. Further, the occurrence of any number of successive yet
distinct species would not be the kind of chain required, or rather
would not be a chain at all.
Yet in some respects development is obvious in creation. Old forms of
life are often embryonic, or resemble the young of modern animals, but
enlarged and exaggerated, as if they had overgrown themselves and had
prematurely become adult. Old forms are often generalized, or less
specific in their adaptations then those of modern times. There is
less division of labour among them. Old forms sometimes not only rise
to the higher places in their groups, but usurp attributes which in
later times are restricted to their betters. Old forms are often
gigantic in size in comparison with their modern successors, which, if
they could look back on their predecessors, might say, "There were
giants in those days." Some old forms have gone onward in successive
stages of elevation by a regular and constant gradation. Others have
remained as they were through all the ages, Some have no equals in
their groups in modern days. All these things speak of order, but of
order along with development, and this development not evolution;
unless by this term we understand the emergence into material facts of
the plans of the creative mind. These plans we may hope in some degree
to understand, though we may not be able to comprehend the mode of
action of creative power any more then the mode in which our own
thought and will act upon the machinery of our own nerves. Still, the
power is not the less real, that we are ignorant of its mode of
operation. The wind bloweth whither it listeth, and we feel its
strength, though we may not be able to calculate the wind of to-morrow
or the winds of last year. So is the Spirit of God when it breathes
into animals the breath of life, or the Almighty word when it says,
"Let the waters bring forth."