THE AGE OF INVERTEBRATES OF THE SEA.
If the middle portion of the Laurentian age was really a time of
exuberant and abounding life, either this met with strange reverses in
succeeding periods, or the conditions of preservation have been such as
to prevent us from tracing its onward history. Certain it is, that
according to present appearances we have a new beginning in the
Cambrian, which introduces the great PalA|ozoic age, and few links of
connection are known between this and the previous Eozoic.
At the beginning of the PalA|ozoic we have reason to believe that our
continents were slowly subsiding under the sea, after a period of
general continental elevation which was consequent on the crumbling of
the earthaEuro(TM)s crust at the close of the Eozoic; and on the new sea-bottoms
formed by this subsidence came in, slowly at first, but in
ever-increasing swarms, the abundant and varied life of the early
PalA|ozoic.
In the oldest portion of the Cambrian series in Wales, Hicks has
catalogued species of no less than seventeen genera, embracing
Crustaceans, the representatives of our crabs and lobsters, bivalve and
univalve shell-fishes of different types, worms, sea-stars, zoophytes,
and sponges. If we could have walked on the shores of the old Cambrian
sea, or cast our dredge or trawl into its depths, we should have found
representatives of most of the humbler forms of sea life still extant,
though of specific forms strange to us. Perhaps the nearest approach to
such experience which we can make is to examine the group of Cambrian
animals delineated in Fig. 28, and to notice, under the guidance of the
geologist above named, the sections seen at St. DavidaEuro(TM)s, in South Wales.
[Illustration: FIG. 28.aEuro"Group of Cambrian Animals (from Nicholson).
_a_, _Arenicolites didymus_, worm tubes. _b_, _Lingulella ferruginea_.
_c_, _Theca Davidii_. _d_, _Modiolopsis solvensis_. _e_, _Orthis
Hicksii_. _f_, _Obolella sagittalis_. _g_, _Hymenocaris vermicauda_.
_h_, Trilobite, _Olenus micrurus_.]
Here we find a nucleus of ancient rocks supposed to be Laurentian,
though in mineral character more nearly akin to the Huronian, but which
have hitherto afforded no trace of fossils. Resting unconformably on
these is a series of partially altered rocks, regarded as Lower
Cambrian, and also destitute of organic remains. These have a thickness
of almost 1,000 feet, and they are succeeded by 3,000 feet more of
similar rocks, still classed as Lower Cambrian, but which have afforded
fossils. The lowest bed which contains indications of life is a red
shale, perhaps a deep-sea bed, and possibly itself partly of organic
origin, by that strange process of decomposition or dissolution of
foraminiferal ooze and volcanic fragments, going on in the depths of the
modern ocean, and described by Dr. Wyville Thomson as occurring over
large areas in the South Pacific. The species are two _LingulellA|_, a
_Discina_ and a _Leperditia_. Supposing these to be all, it is
remarkable that we have no Protozoa or Corals or Echinoderms, and that
the types of Brachiopods and Crustaceans are of comparatively modern
affinities. Passing upward through another 1,000 feet of barren
sandstone, we reach a zone in which no less than five genera of
Trilobites are found, along with Pteropods and a sponge. Thus it is that
life comes in at the base of the Cambrian in Wales, and it may be
regarded as a fair specimen of the facts as they appear in the earlier
fossiliferous beds succeeding the Laurentian. Taking the first of these
groups of fossils, we may recognise in the _Leperditia_ a two-valved
Crustacean closely allied to forms still living in the seas and fresh
waters. The LingulellA|, whether we regard them as molluscoids, or, with
Professor Morse, as singularly specialised worms, represent a peculiar
and distinct type, handed down, through all the vicissitudes of the
geological ages, to the present day. The Pteropods and the sponge are
very similar to forms now living. The Trilobites are an extinct group,
but closely allied to some modern Crustaceans. Had the primordial life
begun with species altogether inscrutable and unexampled in succeeding
ages, this would no doubt have been mysterious; but next to this is the
mystery of the oldest forms of life being also among the newest.
Whatever the origin of these creatures, they represent families which
have endured till now in the struggle for existence without either
elevation or degradation. Yet, though thus vast in their duration, they
seem to have swarmed in together and in great numbers, in the Cambrian,
without any previous preparation. From the Cambrian onward, throughout
the whole PalA|ozoic, there is no decided break in the continuity of
marine life; and already in the Silurian period the sea was tenanted
with all the forms of invertebrate life it yet presents, and these in a
teeming abundance not surpassed in any succeeding age. Let us now, in
accordance with our plan, select some of these ancient inhabitants of
the waters and trace their subsequent history.
Remains of sea-weeds are undoubtedly present in the Cambrian rocks. One
of the lowest beds in Sweden has been named from their abundance the
Fucoidal Sandstone; and wherever fossiliferous Cambrian rocks occur,
some traces, more or less obscure, of these plants may be found. Nearly
all that we can say of them, however, is, that, in so far as their
remains give any information, they are very like the plants of the same
group that now abound in our seas. In the fucoidal sandstone of Sweden
certain striated or ribbed bodies have been found, which have even been
regarded as land plants;[8] but they seem rather to be trails or marks
left by sea-weeds dragged by currents over a muddy bottom. The plants of
the sea thus precede those of the land, and they begin on the same level
as to structure that they have since maintained. I agree with Nathorst,
however, in holding that the Bilobites and many other forms believed by
some to be sea-weeds, are really trails and tracks of animals.[9]
The Foraminifera of the PalA|ozoic we have noticed in the last chapter;
but we now find a new type of ProtozoanaEuro"that of the Sponge. Sponges as
they exist at present may be defined to be composite animals, made up of
a great number of one-celled or gelatinous zoids, provided with
vibrating threads or cilia, and so arranged that currents of water are
driven through passages or canals in the mass, by the action of the
cilia, bringing food and aerated water for respiration. To support these
soft sarcodic sponge-masses, they secrete fibres of horny matter and
needles (spiculA|) of flint or of limestone, forming complicated fibrous
and spicular skeletons, often of great beauty. They abound in all seas,
and some species are found in fresh waters.
[Illustration: FIG. 29.aEuro"Portion of skeleton of Hexactinellid Sponge
(_CA"loptychium_). Magnified. After Zittel.]
With the exception of a very few species destitute of skeleton, and
which we cannot expect to find in a fossil state, the sponges may be
roughly divided into three groups: 1, those with corneous or horny
skeleton, like our common washing sponges; 2, those with skeletons
composed of silicious needles of various forms and arrangement; 3, those
with calcareous spicules. Of these, the second or silicious group has
precedence in point of time, beginning in the Early Cambrian, and
continuing to the present. Two of its subdivisions are especially
interesting in their range. The first is that of the Lattice-sponges
(_HexactinellidA|_), in which the spicules have six rays placed at right
angles, and are attached to each other by their points, so as to form a
very regular network (Fig. 29). The second is that of the Stone-sponges
(_LithistidA|_), in which the spicules are four-rayed or irregular, and
are united by the branching root-like ends of the rays. The most
beautiful of all sponges, the Venus Flower-basket (_Euplectella_), is a
modern Hexactinellid, and the wonderful weaving of its spicules is as
marvellous a triumph of constructive skill as its general form is
graceful. The Lithistids are less beautiful, but are the densest and
most compact of sponges, and are represented by several species in the
modern seas. Both of these types go back to the Early Cambrian, and have
continued side by side to the present day, as representatives of two
distinct geometrical methods for the construction of a spicular
skeleton.
[Illustration: FIG. 30.aEuro"_Protospongia fenestrata_ (Salter). Menevian
group.
_a_, Fragment showing the spicules partially displaced. _b_, Portion
enlarged.]
[Illustration: FIG. 31.aEuro"_Astylospongia prA|morsa_ (Roemer). Niagara
group.aEuro"After Hall.
_a_, Spicules magnified.]
[Illustration: FIG. 32.aEuro"Spicules of Lithistid sponge (_Trichospongia_
of Billings). From the Cambrian of Labrador.]
Many years ago the keen eye of the late lamented Salter detected in a
stain on the surface of a slab of Cambrian slate the remains of a
sponge; and minute examination showed that its spicules crossed each
other, and formed lattice-work on the hexactinellid plan. Salter boldly
named it _Protospongia_ (the first sponge), and it is still the earliest
that we know (Fig. 30). Thus the type whose skeleton is the most perfect
in a mechanical point of view takes the lead. It is continued in the
Silurian in many curious forms, of which the stalkless sponges
(_Astylospongia_) are the most common (Fig. 31). It perhaps attains its
maximum in the Cretaceous, from which the beautiful example in Fig. 29
is taken, and it still flourishes, giving us the most beautiful of all
recent forms. Before the close of the Cambrian there were other sponges
of the Lithistid type. Fig. 32 represents a group of spicules from the
Calciferous (Lowest Silurian or Upper Cambrian) of Mingan,[10] and which
probably belong to a large Lithistid sponge of that early time. The
Lithistids have been recognised in the Upper Silurian and Carboniferous,
and continuing upward to the Cretaceous, there become vastly numerous,
while their modern representatives are by no means few. The silicious
sponges with simple spicules appear to have existed as far back as the
Siluro-Cambrian, and there is believed to be almost as early evidence of
horny or corneous sponges. The calcareous sponges have been recognised
as far back as the Silurian.[11] Thus from the close of the PalA|ozoic
all the types of sponges seem to have existed side by side; and in the
Cretaceous period, when such large areas of our continents were deeply
submerged, they attained a wonderful development, perhaps not equalled
in any other era of the earthaEuro(TM)s history.
[Illustration: FIG. 33.aEuro"_Oldhamia antiqua_ (Forbes).]
[Illustration: FIG. 34.aEuro"_Dictyonema sociale_. Enlarged. _Lingula_
flags.aEuro"After Salter.]
Sponges may be regarded as the highest or most complex of the Protozoa
or the lowest of the Coelenterates. We have no links wherewith to
connect them with the lower Protozoa of the Eozoic period; and through
their long history, though very numerous in genera and species, they
show no closer relationship with the Foraminifera below, and the Corals
above, than do their successors in the modern seas. They thus stand very
much apart; and modern studies of their development and minute
structures do not seem to remove them from this isolation. Though we are
treating here of inhabitants of the sea, it may be proper to mention
that Geinitz has described two species from the Permian which he
believed to be early precursors of the SpongillA|, or fresh-water
sponges; but more recently he seems to regard them as probably AlgA|.
Young has, however, recently found true spicules of _Spongilla_ in the
Purbeck beds.[12]
[Illustration: FIG. 35.aEuro"_Dictyonema Websteri_ (Dn). Niagara formation.
_a_, Enlarged portion (_Acadian Geology_).]
[Illustration: FIG. 36.aEuro"Group of modern Hydroids allied to Graptolites.
Magnified, and natural size.
_a_, _Sertularia_. _b_, _Tubularia_. _c_, _Campanularia._]
[Illustration: FIG. 37.aEuro"Silurian GraptolitidA|.
_a_, _Graptolithus_. _b_, _Diplograpsus_. _c_, _Phyllograpsus_. _d_,
_Tetragrapsus_. _e_, _Didymograpsus_.]
[Illustration: FIG. 38.aEuro"Central portion of Graptolite, with membrane,
or float (_Dichograpsus octobrachiatus_, Hall).]
[Illustration: FIG. 39aEuro"_Ptilodictya acuta_ (Hall). Bryozoan.
Siluro-Cambrian.]
A stage higher than the sponges are those little polyp-like animals with
sac-like bodies and radiating arms or tentacles, which form minute horny
or calcareous cells, and bud out into branching communities, looking to
untrained eyes like delicate sea-weedsaEuro"the sea-firs and sea-mosses of
our coasts (Fig. 36). These belong to a very old group, for in the
oldest Cambrian we have a form referred to this type (Fig. 33), and in
the Upper Cambrian another still more decided example (Fig. 34).[13]
This style of life, once introduced, must have increased in variety and
extended itself with amazing rapidity, for in the Siluro-Cambrian age we
find it already as characteristic as in our modern seas, and so abundant
that vast thicknesses of shale are filled and blackened with the
_dA(C)bris_ of forms allied to the sea-firs, and masses of limestone
largely made up of the more calcareous forms of the sea-mosses. As
examples of the former we may take the _Graptolites_, so named from
their resemblance to lines of writing, and of which several forms are
represented in Fig. 37. The little teeth on the sides of these were
cells, inhabited probably by polyps, like those represented in the
modern _Sertularia_ in Fig. 36. Some of them were probably attached to
the bottom. In others the branches radiated from a central film which
may have been a hollow vesicle or float, enabling them to live at the
surface of the water (Fig. 38). These Graptolites are specially
characteristic of the Upper Cambrian and Lower Silurian. The netted ones
(_Dictyonema_), as may be seen from Figs. 34 and 35, came in before the
close of the Cambrian, and continue unchanged to the Silurian, where
they disappear. The branching forms, seen in Fig. 37, have scarcely so
great a range. They thus form most certain marks of the period to which
they belong, and being oceanic and probably floaters, they diffused
themselves so rapidly that they appear to indicate the same geological
time in countries so widely separated as Europe, North America, and
Australia. It is curious, too, that while the Graptolites thus mark a
definite geological time, and seem to disappear abruptly and without
apparent cause, they are the first link in the long chain of the
Hydroids, which, though under different family forms, continue to this
day, apparently neither better nor worse than their perished PalA|ozoic
relatives. There is a group of little Stony Corals (_MonticuliporidA|_),
which were possibly also the cells of Hydroids, that have a similar
history. They are the only known Corals that date so far back as the
Upper Cambrian; and they continue under very similar forms all through
the PalA|ozic, and are represented by the millepore corals of the present
day. Fig. 40 represents a form found at the base of the Siluro-Cambrian,
and Fig. 41 shows forms characteristic of the Carboniferous Limestone.
[Illustration: FIG. 39_a_.aEuro"_Fenestella Lyelli_ (Dawson). A
Carboniferous Bryozoan.]
[Illustration: FIG. 40.aEuro"_Chaetetes fibrosa_. A tubulate coral with
microscopic cells. Siluro-Cambrian.]
If we turn now to the sea-mosses (Bryozoa), we have a group of minute
polyp-like animals inhabiting cells not unlike those of the Hydroids,
and which form plant-like aggregates. But the animals themselves are so
different in structure that they are considered to be nearer allies of
the bivalve shell-fishes than of the Corals. They are, in short, so
different, that the most ardent evolutionist would scarcely hold a
community of origin between them and such creatures as the Graptolites
and Millepores, though an ordinary observer might readily confound the
one with the other. These animals appear at the beginning of the
Siluro-Cambrian, and such forms as that represented in Fig. 39, very
closely allied to some now living, are large constituents of some of the
limestones of that period. Other forms, like that represented in Fig.
39_a_, are very characteristic of the Carboniferous. These animals,
individually small, though complicated in structure and branching into
communities, scarcely ever of any great magnitude, humble creatures
which have never played any great part in the world, have, nevertheless,
been so persistent that, though specific and generic forms have been
changed, the group may be said to be in the modern seas exactly what it
was in those of the early PalA|ozoic, nor can it be affirmed to have
originated in anything different, or to have produced anything.
[Illustration: FIG. 41.aEuro"_a_, _Stenopora exilis_ (Dawson). _b_,
_Chaetetes tumidus_ (Edwards and Haine). Carboniferous.]
The true Stony Corals (_Anthozoa_) are as yet unknown in the Cambrian.
They entered on the stage in immense abundance in the Siluro-Cambrian,
where considerable limestones are largely composed of their remains,
mixed, however, and sometimes overpowered with those of Bryozoa and
Hydroids. An ordinary coral, such as those of which coral reefs are
builtaEuro"the red coral, used for ornament is not quite similaraEuro"is the
skeleton of an animal constructed on the plan of a sea anemone; with a
central stomach surrounded by radiating chambers, and having above a
crown of tentacles. The stony coral surrounds and protects the soft body
of the animal, and may either be a single cell, for one animal, or an
aggregation of such cells, constituting a rounded or branching mass.
The modern star coral, represented in Fig. 42, is an instance of the
latter condition. It shows nineteen or twenty animals, each with a
central mouth and fringe of short tentacles, aggregated together, and
two of them showing the spontaneous division by which the number of
animals in the mass is progressively increased. The living coral shows
only the soft animals and the animal matter connecting them; but if dead
there would be a white stony mass with a star-like cell or depression
corresponding to each animal.
[Illustration: FIG. 42.aEuro"Living Anthozoan Coral (_AstrA|a_).]
In their general plan, the oldest Corals were precisely of this
character, but they presented some differences in detail, which have
caused them to be divided into two groups, which are eminently
characteristic of the PalA|ozoic ageaEuro"the tabulate or floored corals, and
the rugose or wrinkled corals. In the former (Fig. 43) the cells are
usually small and thin-walled, often hexagonal, like a honeycomb, and
are floored across at intervals with tabulA| or horizontal plates. A few
modern corals present a similar arrangement,[14] but this kind of
structure was far more prevalent in the PalA|ozoic. In the second type
the animals are usually larger and often solitary, the cell has strongly
marked radiating plates, while the horizontal floors are absent or
subordinate, and there is usually a thick external rind or outer coat
(Figs. 44, 45). In general plan, these rugose corals closely resemble
those of our modern reefs; but they differ in their details of
structure, and only a very few modern forms from the deep sea are
regarded as actual modern representatives.[15] One curious point of
difference is that their radiating laminA| begin with four, and increase
by multiples of that number, while in modern corals the numbers are six
and multiples of six; a change of mathematical relation not easily
accounted for, and which assimilates them to Hydroids on the one hand,
and to a higher group, the Alcyonids, on the other, both of which prefer
four and eight to six, or have had these numbers chosen for them. In the
Mesozoic period the tabulate and rugose corals were replaced by others,
the porous and solid corals of the modern seas; but, in so far as we
know, the animals producing these, though differing in some details,
were neither more nor less elevated than their predecessors, and they
took up precisely the same rA'le as reef-builders in the sea, though with
probably more tendency to the accumulation of great masses of coral
limestone in particular spots.
[Illustration: FIG. 43.aEuro"Tabulate Corals.
_a_, _Halisites_, and _b_, _Favosites_. Upper Silurian.]
[Illustration: FIG. 44.aEuro"Rugose Coral (_Heliophyllum Halli_). Devonian.]
[Illustration: FIG. 44_a_.aEuro"_Zaphrentis prolifica_ (Billings). Devonian.]
[Illustration: FIG. 45.aEuro"Rugose Corals.
_a_, _Zaphrentis Minas_ (Dn.), and _b_, _Cyathophyllum Billingsi_ (Dn.).
Carboniferous.]
Leaving the corals, we may turn to the sea-stars and sea-urchins. These
merely put in an appearance in the Early Cambrian, but become vastly
multiplied in the Silurian, where the stalked feather stars (Crinoids)
(Fig. 46) seem to have covered great areas of sea-bottom, and multiplied
so rapidly that thick sheets of limestone are largely made up of the
fragments of their skeletons. The ordinary star-fishes appear first in
the Silurian (Fig. 47). The sea-urchins begin in the Upper Silurian, the
early species having numerous and loosely attached plates, like some of
those now found in the deep sea[16] (Fig. 48).
[Illustration: FIG. 46.aEuro"Modern Crinoid (_Rhisocrinus Lofotensis_).aEuro"After
Sars.]
[Illustration: FIG. 47.aEuro"_PalA|aster Niagarensis_ (Hall). One of the
oldest star fishes.]
[Illustration: FIG. 48.aEuro"_PalA|chinus ellipticus_ (McCoy). One of the
oldest types of sea-urchins.]
The most curious history in this group is that of the feather-stars. In
the Early Cambrian they are represented by a few species known to us
only in fragments, and these belong to a humble group (Cystideans)
resembling the larval or immature condition of the higher Crinoids. Fig.
49 shows one of these animals of somewhat later age. They have few or
rudimentary arms and short stalks, and want the beautiful radial
symmetry of the typical star-fishes. In the Silurian these creatures are
reinforced by a vast number of beautiful and perfect feather-stars
(Figs. 50, 51). These continue to increase in number and beauty, and
apparently culminate in the Mesozoic, where gigantic forms exist, some
of them probably having more complicated skeletons, in so far as number
of distinct parts is concerned, than any other animals. Buckland has
calculated that in a crinoid similar to that in Fig. 52 there are no
less than 150,000 little bones, and 300,000 contractile bundles of
fibres to move them. In the modern seas the feather-stars have somewhat
dwindled both in numbers and complexity, and are mostly confined to the
depths of the ocean. On the other hand, the various types of ordinary
star-fishes and sea-urchins have increased in number and importance. We
thus find in this group a certain advance and improvement from the
Cystideans of the Early PalA|ozoic to the sea-urchins and their allies.
This advance is not, however, along one line for the Cystideans
continue unimproved to the end. The Crinoids culminate in the Mesozoic,
and are not known to give origin to anything higher. The star-fishes and
sea-urchins commence independently, before the culmination of the
Crinoids, and, though greatly increased in number and variety, still
adhere very closely to their original types.
[Illustration: FIG. 49.aEuro"_Pleurocystites squamosus_. Siluro-Cambrian.
After Billings.]
[Illustration: FIG. 50.aEuro"_Heterocrinus simplex_ (Meek). One of the least
complex crinoids of that period. Siluro-Cambrian.]
[Illustration: FIG. 51.aEuro"Body of _Glyptocrinus_. Siluro-Cambrian.]
The great sub-kingdom of the Mollusca, including the bivalve and
univalve shell-fishes, makes its first appearance in the Cambrian, where
its earliest representatives belong to a group, the Arm-bearers or Lamp
shells (Brachiopods), held by some to be allied to worms as much as to
mollusks. The oldest of all these shells are allies of the modern
_LingulA|_ (Fig. 54), some of the earliest of which are shown in Fig.
55. The modern _Lingula_ is protected by a delicate two-valved shell,
composed, unlike that of most other mollusks, of phosphate of lime or
bone earth. It lives on sand-banks, attached by its long flexible stalk,
which it buries like a root in the bottom. Its food consists of
microscopic organisms, drifted to its mouth by cilia placed on two
arm-like processes, from which the group derives its name. In the modern
world about one hundred species of Brachiopods are known, belonging to
about twenty genera, some of which differ considerably from the LingulA|.
The genus _Terebratula_, represented at Fig. 56, is one of the most
common modern as well as fossil forms, and has the valves unequal, with
a round opening in one of them for the stalk, which is attached to some
hard object, and there is an internal shelly loop for supporting the
arms.
[Illustration: FIG. 52.aEuro"_Extracrinus Briareus_. Reduced. Jurassic.]
[Illustration: FIG. 53.aEuro"_Pentacrinus caput-medusA|_. Reduced. Modern.]
[Illustration: FIG. 54.aEuro"_Lingula anatina_. With flexible muscular
stalk. Modern.]
[Illustration: FIG. 55.aEuro"Cambrian and Silurian LingulA|.
_a_, _Lingulella Matthewi_ (Hartt). Acadian group. _b_, _Lingula
quadrata_ (Hall). Siluro-Cambrian. _c_, _Lingulella prima_ (Hall).
Potsdam. _d_, _Lingulella antiqua_ (Hall). Potsdam.]
These curious, and in the modern seas, exceptional shells, were dominant
in the PalA|ozoic period. Upwards of three thousand fossil species are
known, of which a large proportion belong to the Cambrian and Silurian,
nine genera appearing in the Cambrian, and no less than fifty-two in the
Silurian. The history of these creatures is very remarkable. The
LingulA|, which are the first to appear, continue unchanged and with the
same phosphatic shells to the present day. Morse, who has carefully
studied an American species, remarks in illustration of this, that it is
exceedingly tenacious of life, bearing much change of depth,
temperature, etc., without being destroyed. The genus _Discina_, which
is nearly as old, also continues throughout geological time. The genus
_Orthis_ (Fig. 57), which appears at the same time with the last,
becomes vastly abundant in Silurian times, but dies out altogether
before the end of the PalA|ozoic. _Rhynchonella_ (Fig. 58), which comes
in a little later, near the beginning of the Siluro-Cambrian, continues
to this day. _Spirifer_ and _Productus_ (Figs. 59 and 60) appear later, and
die out at the close of the PalA|ozoic. So strange and inscrutable are
the fortunes of these animals, which on the whole have lost in the
battle of life, that their place in nature is vastly less important than
it was. It has been suggested that if any group of creatures could throw
light upon the theory of descent with modification, it would be these;
but Davidson, who has perhaps studied them more thoroughly than any
other naturalist, found them as silent on the subject as the sponges or
the corals. In a series of papers published in the _Geological
Magazine_, a short time before his death, he remarked as follows:
[Illustration: FIG. 56.aEuro"_Terebratula sacculus_ (Martin).
Carboniferous.]
[Illustration: FIG. 57.aEuro"Brachiopods; genus _Orthis_.
_a_, _O. Billingsi_ (Hartt). Lower Cambrian. _b_, _O. pectinella_
(Hall). Siluro-Cambrian. _c_, _O. lynx_ (Eichwald). Siluro-Cambrian.]
[Illustration: FIG. 58.aEuro"_Rhynchonella increbrescens_ (Hall).
Siluro-Cambrian.]
[Illustration: FIG. 59.aEuro"_Spirifer mucronatus_ (Conrad). Devonian.]
[Illustration: FIG. 59_a_.aEuro"_Athyris subtilita_ (Hall). Carboniferous.
_a_, _b_, Exteriors. _c_, Interior, showing spirals.]
aEurooeWe find that the large number of genera made their first appearance
during the PalA|ozoic periods, and since they have been decreasing in
number to the present period. We will leave out of question the species,
for they vary so little that it is often very difficult to trace really
good distinctive characters between them; it is different with the
genera, as they are, or should be, founded on much greater and more
permanent distinctions. Thus, for example, the family _SpiriferidA|_
includes genera which are all characterised by a calcified spiral lamina
for the support of the brachial appendages; and, however varied these
may be, they always retain the distinctive characters of the group from
their first appearance to their extinction. The Brachiopodist labours
under the difficulties of not being able to determine what are the
simplest, or which are the highest families into which either of the two
great groups of his favourite class is divided; so far, then, he is
unable to point out any evidence favouring progressive development in
it. But, confining himself to species, he sees often before him great
varietal changes, so much so as to make it difficult for him to define
the species; and it leads him to the belief that such groups were not of
independent origin, as was universally thought before Darwin published
his great work on the _Origin of Species_. But in this respect the
Brachiopoda reveal nothing more than other groups of the organic
kingdoms.
[Illustration: FIG. 60.aEuro"_Productus cora_ (DaEuro(TM)Orbigny). Carboniferous.]
aEurooeNow, although certain genera, such as _Terebratula_, _Rhynchonella_,
_Crania_, and _Discina_, have enjoyed a very considerable geological
existence, there are genera, such as _Stringocephalus_, _Uncites_,
_Porambonites_, _Koninckina_, and several others, which made their
appearance very suddenly and without any warning; after a while they
disappeared in a similar abrupt manner, having enjoyed a comparatively
short existence. They are all possessed of such marked and distinctive
internal characters that we cannot trace between them and associated or
synchronous genera any evidence of their being either modifications of
one or the other, or of being the result of descent with modification.
Therefore, although far from denying the possibility or probability of
the correctness of the Darwinian theory, I could not conscientiously
affirm that the Brachiopoda, as far as I am at present acquainted with
them, would be of much service in proving it. The subject is worthy of
the continued and serious attention of every well-informed man of
science. The sublime Creator of the universe has bestowed on him a
thinking mind; therefore all that can be discovered is legitimate.
Science has this advantage, that it is continually on the advance, and
is ever ready to correct its errors when fresh light or new discoveries
make such necessary.aEuroŁ The late Joachim Barrande, the great
palA|ontologist of Bohemia, bears similar testimony.
[Illustration: FIG. 61.aEuro"Group of Older PalA|ozoic Lamellibranchs.aEuro"After
Billings.
1, _Cucullea opima_. 2, _Nucula oblonga_. 3, _Nucula lineata_. 4,
_Cypricardia truncata_. 5, _Tellina ovata_. 6, _Nucula bellatula_. 7,
_Modiola concentrica_.]
The ordinary bivalves, like the mussels and cockles, now so very
plentiful on our coasts, are rare in the Cambrian and Silurian, and for
the first time make a somewhat conspicuous appearance in the Upper
Silurian and Devonian. But from the first they resemble very closely
their modern successors, though on the whole neither so large nor so
ornate (Fig. 61). Their fortunes have thus been precisely the opposite
of those of the Brachiopods, though in neither case is there very
marked elevation or deterioration in the individual animals. A very
similar statement may be made as to the sea-snails, whether the curious
winged snails (Pteropods) or the ordinary crawlers (Gastropods). The
former come in early, and are represented by PalA|ozoic forms finer than
any now extant. The genus _Conularia_ (Fig. 62) presents some Silurian
species six inches or more in length, which are giants in comparison
with any now living. The forms of more ordinary Gastropods from the
Silurian represented in Fig. 63 will suffice to show that their styles
are not very dissimilar from those still extant.[17] As in the case of
the ordinary bivalves, however, the modern Gastropods much exceed in
numbers and magnitude those of the PalA|ozoic.
[Illustration: FIG. 62.aEuro"_Conularia planicostata_ (Dn.). A Carboniferous
Pteropod.]
[Illustration: FIG. 63.aEuro"Silurian Sea-snails. Canada.
_a_, _Murchisonia bicincta_ (Hall). _b_, _Pleurotomaria umbilicatula_
(Hall). _c_, _Murchisonia gracilis_ (Hall). _d_, _Bellerophon
sulcatinus_ (Billings).]
The highest group of Mollusks, represented in the modern ocean by the
Nautili and Cuttle-fishes, has a history so strange and eventful, and so
different from what might have been anticipated, that it perhaps
deserves a more detailed notice, more especially as Barrande has
recently directed marked attention to it in his magnificent work on the
PalA|ontology of Bohemia.
The Cuttle-fishes and Squids and their allies are, in the modern seas, a
most important group (Fig. 64). The great numbers in which the smaller
species appear on many coasts, and the immense size and formidable
character of others; their singular apparatus of arms, bearing suckers,
their strange forms, and the inky secretion with which they can darken
the water, have at all times attracted popular attention. The great
complexity of their structures, and the fact that in many points they
stand quite at the head of the invertebrates of the sea, and approach
most nearly to the elevation of the true fishes, have secured to them
the attention of naturalists. Some of these animals have shelly internal
supports, and one genus, that of the Argonauts, or Paper Nautili, has an
external protective shell. Allied, though more distantly, to the
Cuttle-fishes, are the true Nautili, represented in the modern sea
principally by the Pearly Nautilus, though there are two other species,
both of them very rare. The modern pearly nautilus (Fig. 65) may be
regarded as a peculiar kind of cuttle-fish provided with a discoidal
shell for protection, and also for floatage. The shell is divided into a
number of chambers by partitions. Of these the animal inhabits the last
and largest. The others are empty, and are connected with the body of
the animal only by a pipe, or siphuncle, with membranous walls and
filled with fluid. Thus provided, the nautilus, when in the water, has
practically no weight, and can move up or down in the sea with the
greatest facility, using its sucker-bearing arms and horny beak to seize
and devour the animals on which it preys. The buoyancy of the shell
seems exactly adapted to the weight of the animal; and this proportion
is kept up by the addition of new air-chambers as the body increases in
size. In the modern seas this singular little group stands entirely
isolated, and its individuals are so rare that it is difficult to
procure perfect specimens for collections, though its mechanical
structure and advantages for the struggle for existence seem of the
highest order. But in the old world of past geological time the case was
altogether different.
[Illustration: FIG. 64.aEuro"Squid (_Loligo_).]
[Illustration: FIG. 65.aEuro"Pearly Nautilus (_Nautilus pompilius_).
_a_, Mantle. _b_, Its dorsal fold. _c_, Hood. _o_, Eye. _t_, Tentacles.
_f_, Funnel. _g_, Air chambers. _h_, Siphuncle.]
The Nautiloid shell-fishes burst suddenly upon us in the beginning of
the Siluro-Cambrian, or Lower Silurian, BarrandeaEuro(TM)s second fauna; and
this applies to all the countries where they have been studied. In this
formation alone about 450 species are known, and in the Silurian these
increase to 1,200; and here the group culminates. It returns in the
Devonian to about the same number with the Lower Silurian, diminishes in
the Carboniferous to 350, and in the Mesozoic, where the Nautiloid forms
are replaced by others of the type of the Ammonites, becomes largely
reduced. In the Tertiary there are but nineteen species, and, as already
stated, in the modern world _three_. These statements do not, however,
represent the whole truth. In the PalA|ozoic, in addition to the genus
_Nautilus_, we have a great number of other genera, some with perfectly
straight shells, like _Orthoceras_ (Fig. 66), others bent
(_Cyrtoceras_), others differing in the style of siphuncle, or aperture,
or chambers (_Endoceras_, _Gomphoceras_, _Lituites_, Figs. 67 to 69), or
inflated into sac-like forms (_Ascoceras_). There is, besides, the
family of the _GoniatidA|_ (Fig. 70), with the chambers thrown into
angular folds and the siphuncle at the back. Further, some of the early
forms, as the OrthoceratidA|, attain to gigantic dimensions, being six
feet or more in length, and nearly a foot in diameter. Thus the idea
that we should naturally form from the study of the Nautilus, that it
represents a type suited for much more varied and important adaptations
than those that we now see, is more than realised in those PalA|ozoic
ages when these animals seem to have been the lords of the seas.
[Illustration: FIG. 66.aEuro"_Orthoceras_. Siluro-Cambrian. The dotted line
shows the position of the siphuncle.]
[Illustration: FIG. 67.aEuro"_Gomphoceras._]
[Illustration: FIG. 68.aEuro"_Lituites._]
[Illustration: FIG. 69.aEuro"_Nautilus Avonensis_ (Dn.). Carboniferous.
_a_, Shell, reduced. _b_, Section, showing siphuncle.]
[Illustration: FIG. 70.aEuro"_Goniatites crenistria_ (Philips).
Carboniferous.]
[Illustration: FIG. 71.aEuro"_Ceratites nodosus_ (Schloth). Triassic.]
When we leave the PalA|ozoic and enter the Mesozoic, though the Nautiloid
shells still abound, we find them superseded, in great part, by a nobler
form, that of the _AmmonitidA|_ (Figs. 71, 72). These are remarkable for
the ornate markings on the surfaces of their shells, and for the
beautifully waved edges of the partitions (Fig. 72_a_), which, by giving
a much more complete support to the sides of the shell, must have
contributed greatly to the union of lightness and strength so important
to the utility of the shell as a float. This type admits of all the same
variety of straight, bent, and curled forms with the simpler Nautiloid
type, and some of the species are of great size, Ammonites being known
three feet or more in diameter. These animals, unknown in the PalA|ozoic,
appear in numerous species in the Early Mesozoic, culminate in hundreds
of beautiful species in the middle of that era, and disappear for ever
at its close, leaving no modern successors. Many and beautiful species
of Ammonites and their allies have been obtained from the Mesozoic rocks
of British Columbia and other parts of the west coast of North America,
perfectly representing this group as it occurs at the same period in
Europe, and closely resembling the Mesozoic Ammonites of India. These
animals have all perished, yet the Atlantic and the Pacific roll
between, apparently with conditions as favourable for their comfortable
existence as those of any previous time. They perished long ago, at the
dawn of the Tertiary; yet the genus Nautilus, one of the oldest and
least improved of the whole, survived, and still testifies to the
wonderful contrivance embodied in these animals.
[Illustration: FIG. 72.aEuro"_Ammonites Jason_ (Reinecke). Jurassic.]
[Illustration: FIG. 72_a_.aEuro"Suture of _Ammonites componens_ (Meek), of
British Columbia. Showing the complicated folding of the edges of the
chambers to give strength to the shell. Cretaceous.]
[Illustration: FIG. 73.aEuro"Cretaceous AmmonitidA|.
_a_, _Baculites_. _b_, _Ancyloceras_. _c_, _Crioceras_. _d_,
_Turrilites_.]
These are merely general considerations, but Barrande, in his _A%tudes
GA(C)nA(C)rales_, goes much farther. He sums up all the known facts in the
most elaborate manner, considering first the embryonic characters of the
shell in the different genera, then their distribution in space and
time, then all the different parts and characters of the shells in the
different groupsaEuro"the whole with reference to any possible derivation of
the species; and he finds that all leads to the result that in every
respect these shells seem to have been so introduced as to make any
theory of evolution with respect to them altogether untenable. In his
concluding sentence this greatest of PalA|ozoic palA|ontologists affirms
that, aEurooeThe theoretical evolution of the Cephalopods is, like that of the
Trilobites, a mere figment of imagination, without any foundation in
fact.aEuroŁ[18]
[Illustration: FIG. 74.aEuro"_Belemnite._aEuro"After Philips.]
[Illustration: FIG. 74_a_.aEuro"_Belemnoteuthis antiquus._ Supposed to be a
Belemnite, with soft parts preserved.aEuro"Jurassic.aEuro"After Mantell.]
I have reserved no space to notice the geological history of the other
and higher group of Cephalopods, including the true Cuttles and Squids.
This is perhaps less to be regretted, as, from the absence of external
shells, they are likely to be much less perfectly known as fossils. So
far as known, they are vastly younger than the Nautiloids, for no
examples whatever have been found in the PalA|ozoic. They appear
abundantly in the Mesozoic, but are there represented principally by an
extinct group of squids (Belemnites and their allies, Figs. 74, 74_a_),
remarkable for the great and complicated development of their internal
support, which has a chambered float as well as a solid sheath. This
family becomes extinct at the close of the Mesozoic, though the cuttles
as a whole perhaps culminate in the modern.
[Illustration: FIG. 75.aEuro"Cambrian Trilobites.
_a_, _Paradoxides_. _b_, _Dikellocephalus_. _c_, _Conocoryphe_ (head).
_d_, _Agnostus_ (head and tail).]
The remarkable group of the Trilobites had precedence in order of time
of the Nautiloid shell-fishes. No animal structures can well be more
dissimilar than those of the two great groups of aquatic animals which
popular speech confounds under the name of aEurooeshell-fishes.aEuroŁ Take a whelk
and a crab, for example, and compare their general forms, the structure
of their shells, and their organs of motion, and it is scarcely possible
to imagine any two animals more unlike; and when we examine their
anatomy in detail this difference does not diminish. They have, it is
true, corresponding parts, and these parts serve similar uses, but in
plan of structure they are wholly different. Yet both animals may live
in the same pool, and may subsist on nearly the same food. If we attempt
to find some common type which both resemble, we may trace the structure
of the crab back to those of some of the marine worms with which it has
some affinity, and those of the whelk to such creatures as the
_Lingula_, which are supposed to have a resemblance to the worms. But
still the two types, that of the Mollusk and the Articulate, are
distinct even from their first appearance in the egg, nor have either
any close affinities with the Protozoa, the Hydroids, or the Corals.
[Illustration: FIG. 76.aEuro"Transverse section of _Calymene_. A Silurian
Trilobite.aEuro"After Wolcott.
_a_, Dorsal shell. _b_, Visceral cavity. _c_, Legs. _d_,
EpipoditeaEuro"gill-cleaner or palp. _e_, Spiral gills.]
Both types meet us in the Early Cambrian, but while the Mollusk is there
represented only by low forms, the Articulate is then not only in the
humble guise of the worm, but in the complex and highly organised form
of the Trilobite (Figs. 28 and 75). What older phases they may have
passed through we know not; but in the Lower Cambrian we have various
forms of these animals, including some of the largest known as well as
some of the smallest; some of the most complex in number of parts as
well as some of the simplest. These animals, in short, seem to have
appeared at once all over the world fully formed, and in a variety of
generic and specific forms; and nothing short of a very large faith in
the imperfection of the geological record can suffice to account for
their evolution.
[Illustration: FIG. 76_a_.aEuro"Burrows of Trilobite and of modern
King-crab. The Trilobite burrow is known as Ruschinites, and has been
supposed to be a sea-weed of the kind called _Bilobites_.]
A Trilobite is a creature in whose structure the number three is
dominant. Seen from above, it presents three divisions from front to
rear:aEuro"first, a cephalic shield or head-piece; secondly, a thorax,
divided into several segments movable upon each other; and thirdly, a
tail-piece or pygidium, which, when brought against the head by the
rolling up of the body segments, effectually covers the lower parts.
This lower portion was until lately little known; but the discoveries of
Billings and of Wolcott have enabled us to restore the jaws under the
head, the jointed legs and spiral gills under the thorax, and thus to
complete the structure of the animal, and understand better its
relations to modern crabs and shrimps (Fig. 76). Of these it certainly
comes nearest to the King-crabs and Horseshoe-crabs, a somewhat limited
group at present, and one which reaches back in geological time only to
the Upper Silurian, when the Trilobites had perhaps already passed their
culmination.
Constructed as above described, the Trilobite could swim, as is
supposed, usually on its back or side. It could crawl on the bottom.
Using its snout as a shovel, it could burrow like a modern King-crab
(Fig. 76_a_); and when pressed by danger some species could roll
themselves into balls and defy their enemies.
[Illustration: FIG. 77.aEuro"Silurian Trilobites.
_a_, _Isotelus_. _b_, _Homalonotus_. _c_, _Calymene_.]
This type of animal, entering on the stage in full force in the Older
Cambrian, continues under many forms through the whole PalA|ozoic age,
dying out finally in the Carboniferous. Figs. 77 and 78 show a few of
the forms of the Silurian, Devonian, and Carboniferous.
Contemporaneously with the dawn of the Trilobite group, appear some
small shrimp-like forms (Fig. 28),[19] and others with bivalve shells
(Fig. 79), which are closely allied to modern forms,[20] and, like the
_LingulA|_, persist through the succeeding formations with little more
than specific changeaEuro"presenting in this a strange contrast to the
Trilobites. While the latter were still flourishing, about the close of
the Lower Silurian, a remarkable group of large and highly-developed
creatures, allied to the Trilobites, but suited for rapid swimming
rather than creeping, was introduced; and in the Upper Silurian and
Devonian these creatures[21] attained to gigantic sizes, exceeding,
probably, any modern Crustaceans, and were tyrants of the seas.
_Pterygotus anglicus_ (Fig. 80) is supposed to have attained the length
of six feet. Yet these noble representatives of the Crustaceans became
extinct in the Carboniferous. On the other hand, a few small king-crabs
appear in the Upper Silurian, and this type still continues, and seems
to culminate as to size in modern times; so diverse have been the
fortunes of these various groups.
[Illustration: FIG. 78.aEuro"Devonian and Carboniferous Trilobites.
_a_, _Phaceps latifrons_ (Bronn). _b_, _Philipsia Howi_ (Billings)
(tail).]
[Illustration: FIG. 79.aEuro"PalA|ozoic Ostracod Crustaceans. Magnified.
_a_, _Bairdia_. _b_, _Cytherella inflata_ (Jones). _c_, _Cythere_.
Carboniferous. _d_, _Beyrichia Jonesii_ (Dn.). Carboniferous. _e_,
_Beyrichia pustulosa_ (Hall). Silurian.]
The higher, or decapod Crustaceans, now familiar to us in the modern
crabs and lobsters, are first found in a few small species in the
Devonian[22] and Carboniferous, and they are accompanied in the Devonian
by at least one species of the allied group of the Stomapods (Figs. 81,
82).
[Illustration: FIG. 80.aEuro"_Pterygotus anglicus_. Reduced.aEuro"After Page and
Woodward.]
[Illustration: FIG. 81.aEuro"_Amphipeltis paradoxus_ (Salter). A Devonian
Stomapod from New Brunswick.]
[Illustration: FIG. 82.aEuro"_AnthropalA|mon Hilliana_ (Dn). A Carboniferous
Decapod from Nova Scotia. The carapace only.]
The PalA|ozoic age of geology is thus emphatically an age of
invertebrates of the sea. In this period they were dominant in the
waters, and until toward its close almost without rivals. We shall find,
however, that in the Upper Silurian, fishes made their appearance, and
in the Carboniferous amphibian reptiles, and that, before the close of
the PalA|ozoic, vertebrate life in these forms had become predominant.
We shall also see that just as the leading groups of Mollusks and
Crustaceans seem to have had no ancestors, so it is with the groups of
Vertebrates which take their places. It is also interesting to observe
that already in the PalA|ozoic all the types of invertebrate marine life
were as fully represented as at present, and that this swarming marine
life breaks upon us in successive waves as we proceed upward from the
Cambrian. Thus the progress of life is not gradual, but intermittent,
and consists in the sudden and rapid influx of new forms destined to
increase and multiply in the place of those which are becoming effete
and ready to vanish away or to sink to a lower place. Farther, since the
great waves of aquatic life roll in with each great subsidence of the
land, a fact which coincides with their appearance in the limestones of
the successive periods, it follows that it is not struggle for
existence, but expansion under favourable circumstances and the opening
up of new fields of migration that is favourable to the introduction of
new species. The testimony of palA|ontology on this point, which I have
elsewhere adduced at length,[23] in my judgment altogether subverts the
prevalent theory of aEurooesurvival of the fittest,aEuroŁ and shows that the
struggle for existence, so far from being a cause of development and
improvement, has led only to decay and extinction, whereas the advent of
new and favourable conditions, and the removal of severe competition,
are the circumstances favourable to introduction of new and advanced
species. This testimony of the invertebrates of the sea we shall find is
confirmed by other groups of living beings, to be noticed in the
sequel.[24]
* * * * *
NOTE.aEuro"The term aEurooeSiluro-Cambrian,aEuroŁ as used in this and the next chapter,
is synonymous with aEurooeOrdovicianaEuroŁ of Lapworth, which is now coming into
somewhat general use.
[Illustration: CORDAITES, OF THE GROUP OF DORY-CORDAITES. BRANCH
RESTORED.aEuro"After GrandaEuro(TM) Eury.]