THE FIRST AIR-BREATHERS.
Were our experience limited to the animals whose remains are found in
the earlier PalA|ozoic rocks, we might be unable to conceive the
possibility of an animal capable of living and breathing in the thin and
apparently uncongenial medium of air. More especially would this appear
doubtful if our experience of the atmosphere presented it to us as
loaded with carbonic acid, and less rich in vital air than it is at
present. Even the mechanical difficulties of the case might strike us as
considerable, in our ignorance of the capabilities of limbs. Still, as
time wore on, we should find this problem worked out along three
distinct lines of advancementaEuro"those of the Mollusk, the Arthropod, and
the Vertebrate, and in each of these with different machinery, related
to the previous locomotive and water-breathing apparatus of the type.
Respiration under water depends, not on the water itself, but on the
small percentage of free oxygen which it contains, and this is utilised
for the aA"ration of the blood of animals, by that wonderful and often
extremely beautiful apparatus of delicate fibres or laminA| penetrated
with blood-vessels, which we call a gill. Except those lowest creatures
which aA"rate their blood merely at the general surface of the body, all
animals capable of respiration in water are provided with gills in some
form, though in many of the humbler types, like that of the familiar
Oyster, the gills are used for the double purpose of aA"rating the blood
and, by their minute vibrating threads or cilia, drifting food to the
mouth.
In the great group of radiated animals, the _Protozoa_, _CA"lenterata_,
and _Echinodermata_, no air-breathing creature exists, or, in so far as
is known, has existed, so that this vast group of animals is limited
altogether to the waters; and this is undoubtedly one mark of its
inferiority.
In the sub-kingdom of the Mollusks the highest class, that of the
Cuttle-fishes and Nautili, has been, singularly enough, rejected as
unfit for this promotion, though it was early introduced, and attains to
a high development of muscular energy and nervous power. The group next
in order, that of the Snails and their allies, alone ventures in some of
its families to assume the _rA'le_ of air-breathing. As might be
expected, in creatures of this stamp the simplest means are employed to
effect the result. In the sub-aquatic species the gills are contained in
a chamber, where they are protected and kept supplied with water. In the
air-breathing species, this gill-chamber is merely emptied of its
contents and converted into an air-sac or functional lung. Thus a rude
and imperfect method of air-breathing is contrived, which scarcely
separates the animals that possess it from their aquatic relatives, but
which nevertheless gives to us the beautiful and varied groups of the
Land-snails and of the air-breathing fresh-water Snails.
In the worms and Crustaceans the gills are placed at the sides of the
body, and connected with its several segments. But the Crustaceans, like
the Cuttle-fishes, though the highest aquatic type, never become
air-breathers. It is true some of them, like the Land-crabs, live in the
air; but they retain their gills, and have to carry with them a supply
of water to keep these moist.
But in order to elevate the Annulose type to the true dignity of air
breathing, three new classes had to be introduced, differing altogether
in their details of structure; and all three seem to have been placed on
the earth about the same time. They are: First, the Myriapods, or
Gallyworms and Centipedes; secondly, the Insects; and thirdly, the
Arachnidans, or Spiders and Scorpions.
In the Myriapods a system of air-tubes, kept open by elastic spiral
fibres, penetrates the body by lateral pores, thus retaining the
resemblance to the lateral respiration of the Crustaceans and worms. In
the Insects, where this type of structure rises to its highest
mechanical perfection, and where the animal is enabled to be not merely
an air-breather, but a flier, the same system of lateral pores and
internal air-tubes is adopted, and is so extended and ramified as to
give a very perfect respiration. In the Spiders and Scorpions the system
is the same, except that in the latter and a part of the former the
whole or a part of the tracheal system becomes expanded into
air-chambers simulating true lungs.
Among the Vertebrates, the fishes are breathers by gills attached to
arches at the sides of the neck. But already in the Devonian we have
reason to believe that there were fishes having the swimming-bladder
opening into the back of the mouth to receive air, and divided into
chambers, so as to constitute an imperfect lung. And here we have not,
as in the lower types, an adaptation of the old water-breathing organs,
but an entirely new apparatus. In the next grade of Vertebrates we find,
as in the Frogs, Water-lizards, etc., that the young are aquatic and
breathe by gills, while the adults acquire lungs, sometimes retaining
their gills also, but in the higher forms parting with them. Thus in the
vertebrates alone we have true lungs, distinct structurally from gills;
and these lungs attain to their highest perfection in the birds and
mammals.
[Illustration: FIG. 123.aEuro"Wings of Devonian Insects. Middle Devonian of
New Brunswick.
_a_, _Platephemera antiqua_ (Scudder).
_b_, _Homothetus fossilis_ (Scudder).
_c_, _Lithentomum Harttii_ (Scudder).
_d_, _Xenoneura antiquorum_ (Scudder).]
The oldest air breathers at present known are Scorpions and insects
allied to the modern May-flies, which have been found in the Silurian.
Next to these, and more important in number and variety, are the
insects of the Erian plant beds of New Brunswick. They were discovered
by the late lamented Prof. C. F. Hartt in the plant-bearing shales of
the Middle Devonian (Fig. 123). The beds containing them hold also a
species of _Eurypterus_, an obscure Trilobite, and a Crustacean allied
to the modern Stomapods,[42] besides a shell which may possibly be that
of a Land-snail, to be mentioned in the sequel. They are also
exceedingly rich in beautifully-preserved remains of Devonian plants.
The collection made by Prof. Hartt is limited to a few fragments of
wings; but these, in the skilful hands of Mr. Scudder, have proved to be
rich in geological interest. One is a gigantic _Ephemera_ or May-fly,
which must have been five inches in the expanse of the wings, which are
more complex in their venation than those of its modern allies (Fig.
123, _a_). Another presents peculiarities between those of the May-flies
and Dragon-flies (Fig. 123, _b_). A third is a Neuropter, not belonging
to any known family, but allied to some in the Coal-formation (Fig. 123,
_c_). A fourth (Fig. 123, _d_) is a small and delicate wing, supposed
to have belonged to an animal having some points of resemblance to the
modern crickets. Two others are represented by mere fragments of wings,
insufficient to determine their affinities with certainty. No other
insects of this age have been discovered elsewhere; but it is to be
borne in mind that no other locality rich in Devonian plants has
probably been so thoroughly explored. The hard slaty ridges containing
these fossils are well exposed on the coast near the city of St. John,
and Messrs. Hartt and Matthew of that city, acting, I believe, in
concert with and aided by the Natural History Society of the place, not
only searched superficially, but removed by blasting large portions of
the richest beds, and examined every fragment with the greatest care.
Their primary object was fossil plants, of which they obtained
magnificent collections; and it is scarcely possible that the insects
could have been found but for the exhaustive methods of exploration
employed.
It is interesting to observe, respecting these oldest insects, that they
all belong to those families which have jaws, and not suctorial
apparatus, that they are not of those which undergo a complete
metamorphosis, and that their modern congeners pass their larval stage
in the water. Thus the waters gave birth to the first insects, and their
earliest families were not of those which suck honied juices or the
blood of animals, or which pass through a worm-like infancy. These
groups belong apparently to much later times.
On one of the specimens collected by Messrs. Hartt and Matthew, and
placed by them in my hands, is a spiral form which in every particular
of external marking resembles a genus of modern West Indian
Land-snails.[43] I have hesitated to describe it, as the structure is
lost and the form imperfect; but I cannot help regarding it as an
indication that this group of land animals also will be traced back to
the Devonian age.
Ascending from the Devonian to the Carboniferous, we at once find
ourselves in the midst of air-breathers of various types. Here are
Myriapods, insects of several orders, Spiders, Scorpions, Land-snails,
and Batrachian reptiles, and these of many species, and found in many
localities widely separated. We can thus people those dark, luxuriant
forests, to which we owe our most valuable beds of coal, with many forms
of life; and as most of these belong to tribes likely to multiply
abundantly where food was plentiful, we can imagine multitudes of Snails
and Millepedes feeding on succulent or decaying vegetable matter, swarms
of insects flitting through the air in the sunnier spots, while their
larvA| luxuriated in decaying masses of leaves or wood, or peopled the
pools and streams. In like manner, in imagination we can render these
old woods vocal with the trill of crickets and with the piping or
booming of smaller and larger Batrachians. Let us now, in accordance
with our plan, inquire as to the nature of these early air-breathers and
the fortunes of their families in the geological history.
[Illustration: FIG. 124.aEuro"Land-snail (_Pupa vetusta_, Dawson). From the
Coal-formation.
_a_, Natural size. _b_, Magnified. _c_, Apex. _d_, Sculpture. Enlarged.]
[Illustration: FIG. 125.aEuro"Land-snail (_Zonites (Conulus) priscus_,
Carpenter). From the Coal-formation.
_a_, Shell. Enlarged; the line below shows the natural size. _b_,
Sculpture. Enlarged.]
The Land-snails known as yet in the Carboniferous are limited to five or
six species, belonging to four genera, all American and related to
existing American forms. The two earliest known are represented in Figs.
124 and 125.[44] One of them is a _Pupa_, or elongated Land-snail, so
similar to modern forms that it does not merit a generic distinction,
and is indeed very near to some existing West Indian species. The other
is in like manner a member of the modern genus _Zonites_. These are from
the Coal-formation of Nova Scotia, and the Pupa must have been very
abundant, as it has been found in considerable numbers in a layer of
shale, and in the stumps of erect trees, in beds separated from each
other by a thickness of 2,000 feet of strata. The _Zonites_ is much more
rare. A second Pupa is found in Nova Scotia, and two species occur in
the Coal-field of Illinois. One of these is a Pupa still smaller than
_P. vetusta_, and, like some modern species, with a tooth-like process
on the inner lip. The other has been placed in a new genus,[45] but is
very near to some of the smaller American Snails still living. Its most
special character is a plate extending from the inner lip over half the
aperture, a contrivance for protection still seen in some modern forms.
Thus the Land-snails come on the stage in at least three generic forms,
similar to those which still live, but all of small size, indicating
perhaps that the conditions were less favourable for such creatures than
those of the temperate and warmer climates at present. It may seem a
small step in advance for Sea-snails to lose their gills and to become
Land-snails, and this without any elevation of their general structure;
but it must be borne in mind that we have here not only the dropping of
the gills for an air-sac, but profound changes in teeth, mucous glands,
shell, and other particulars, to fit them for new food and new habits.
It is also singular that the Land-snails at once appear instead of the
intermediate forms of the air-breathing fresh-water snails. These last
may, however, yet be found.
The Millepedes, like the Land-snails, were first found in the
Coal-formation of Nova Scotia, but species have since been discovered
not only in Illinois, but also in Great Britain and in Bohemia. In Nova
Scotia alone two genera and five distinct species have been found, all
in the interior of erect trees, to which these creatures probably
resorted for food and shelter (Fig. 126). All the species yet known are
allied to the modern Gallyworms, though presenting special features
which seem to separate them as a distinct family,[46] and were probably
vegetable-feeders. Some of the species have the peculiarity, unknown
among their modern successors, of being armed with long spines.[47] The
moist, equable climate and exuberant vegetation of the Coal-period would
naturally be very favourable to Millepedes, and it is likely that the
discoveries made as yet give but a faint idea of their actual abundance.
It is not improbable that they subsequently declined, as we know of none
between the Carboniferous and the Jurassic, and they do not seem to have
improved up to the modern period. The Carnivorous Myriapods, however, or
Centipedes proper, a higher and essentially distinct type, are not known
until much more recent times.
[Illustration: FIG. 126.aEuro"Millepedes. From the Coal-formation.
_a_, _Xylobius sigillariA|_ (Dawson). _b_, _Archiulus xylobioides_
(Scudder). Anterior segments. Enlarged, _c_, _X. farctus_ (Scudder).
Caudal portion. Enlarged.]
The insects of the Carboniferous as yet known, belong to three out of
the ten or more orders into which the class is divided. One of these is
represented by a number of species of Cockroach, another by May-flies
and a Dragon-fly, and another by some weevil-like Beetles. The Cockroach
is characterised by Huxley as one of the aEurooeoldest, least modified, and in
many ways most instructive forms of insects;aEuroŁ and both he and Rolleston
take its anatomy as typical of that of the class. That these creatures
should have abounded in the Coal-period we need not wonder, when we
consider the habits of those that infest our houses, and when we further
bear in mind the number of species, some of them two inches in length,
that exist in tropical climates. So many species of this family have
been found in the Coal-formation on both sides of the Atlantic,[48] that
we may fairly regard them as constituting one of its most characteristic
features, and as probably the oldest representatives of the order to
which they belong[49] (Fig. 127). There were also in the Coal-period
insects allied to the Locusts and to the Mantids, a carnivorous group.
One of the latter (_Lithomantis_), described by Woodward, is a
magnificent insect, not unlike some modern tropical species. It was
found in the Coal-formation of Scotland. A still larger species,
probably the largest insect known, has been described by Brongniart. The
May-flies (_EphemeridA|_) are represented in the Carboniferous by several
very large species. That of which the wing is shown in Fig. 128 must
have been seven inches in expanse of wings. The habits of the modern
May-flies show us how animals of this group, living as larvA| in the
streams and lakes, must have afforded large supplies of food to fishes,
and when mature must have emerged from the waters in countless myriads,
filling the air for the brief term of their existence in the perfect
state. The May-flies represent another insect order.[50] The
Coal-measures of Saarbruck have afforded several species allied to the
white ants (_Termites_), insects which must have found abundant scope
for their activity in the dead trees of the carboniferous forests. The
occurrence of beetles,[51] especially of the weevil family, which have
as yet been found only in Europe, might have been expected, considering
the habits and modern distribution of this group. It has been asserted
that moths[52] have been found in the Carboniferous; but the proof of
this, so far as known to me, is the occurrence of leaves, noticed by
Sternberg, with markings similar to those made by the larvA| of minute
leaf-mining moths. This, however, is uncertain evidence. If we consider
the orders of insects not found in the Coal-formation, we can perceive
good reasons for the absence of some of them. Those containing the lice
and fleas, and other minute and parasitic insects, we can scarcely
expect to find. The bees and wasps, and the butterflies and moths, are
little likely to have been present where there were scarcely any
flowering plants; but such groups as those of the two-winged flies, the
plant-bugs and the ants, we might have expected, but for the fact of
their being highly specialised forms, and for that reason likely to have
appeared later.[53] There are, indeed, as yet no haustellate or
suctorial insects known in this early period. Plausible theories of the
phylogeny of insects are not wanting; but they do not well suit the
known facts as to their first appearance; and perhaps we may venture
without much blame to apply to the insects of the Coal-period the remark
made by Wollaston with reference to the rich insect fauna of the
isolated rock of St. Helena: aEurooeTo a mind which, like my own, can accept
the doctrine of creative acts as not necessarily aEuro~unphilosophical,aEuro(TM) the
mysteries [of the existence of these species in an island so remote from
other lands], however great, become at least conceivable; but those
which are not able to do this may, perhaps, succeed in elaborating some
special theory of their own, which, even if it does not satisfy all the
requirements of the problem, may at least prove convincing to
themselves.aEuroŁ
[Illustration: FIG. 127.aEuro"Wings of Cockroaches. From the Coal-formation.
_a_, _Archimulacris Acadicus_ (Scudder). _b_, _Blattina Bretonensis_
(Scudder). _c_, _B. Hesri_ (Scudder).]
[Illustration: FIG. 128.aEuro"Wing of May-fly (_Haplophlebium Barnesii_,
Scudder). From the Coal-formation.]
[Illustration: FIG. 129.aEuro"A Jurassic Sphinx-moth (_Sphinx Snelleri_,
Weyenburgh).]
[Illustration: FIG. 130.aEuro"An Eocene Butterfly (_Prodryas persephone_,
Scudder). From Colorado.]
The suctorial insects make their first certain appearance in the
Jurassic; and the magnificent Sphinx Moth in Fig. 129 is an example of
the magnitude and perfection to which that tribe attained in the age of
the Solenhofen slate; though Weyenburgh, who describes it, fancies that
he sees evidence that it may, unlike any modern moths, have been
provided with a sting. The most perfect and beautiful fossil butterfly
known to me is that represented in Fig. 130, from a photograph kindly
given to me by Mr. Scudder. It is from the Tertiary rocks of Western
America, and is laid out in stone as neatly as if prepared by an
entomologist, while its preservation is so perfect that even the
microscopic scales on the wings can be made out. It belongs to one of
the highest types of modern butterflies, that to which the _VanessA|_
belong, but with some points of structure pointing to the lower group of
the aEurooeSkippersaEuroŁ (_HesperiadA|_). Scudder remarks that while the fore-wings
resemble those of the former group, the hind-wings look more like those
of the latter; and this seems to be a common character of two or three
others of the few fossil species known, none of which are older than the
Tertiary.
[Illustration: FIG. 131.aEuro"Abdominal part of a Carboniferous
Scorpion.[54]]
We know too little of the spiders and scorpions of the Carboniferous to
say more than that they closely resemble modern forms. Two of the
scorpions are represented in Figs. 131 and 132; and the only spider
certainly known, which is from Silesia, is said to belong to the group
of the hunting or trap-door spiders (_Lycosa_).[55]
The Batrachians of the Coal are its most characteristic and remarkable
air-breathers,aEuro"especially so as the precursors of the reptiles of the
Mesozoic age. Cope in a recent summary enumerates no less than
thirty-nine genera and about one hundred species; and to these have to
be added at least a dozen more recently discovered in Europe; though it
was only in 1841 that the first indications of such creatures were
found, and were then regarded by geologists with the same scepticism
which some of them still apply to _Eozoon_. The first trace ever observed
of batrachians in the Carboniferous consisted of a series of small but
well-marked footprints found by the late Sir W. E. Logan in the Lower
Carboniferous shales of Horton Bluff, in Nova Scotia. In that year this
painstaking geologist had examined the coal-fields of Pennsylvania and
Nova Scotia, with the view of following up his important discovery of
the _StigmariA|_, or roots of _Sigillaria_, as accompaniments of the
coal-underclays. On his return he read a paper, detailing his
observations, before the Geological Society of London. In this he
mentioned the footprints in question; but the paper was published only
in abstract, and the importance of the discovery was overlooked for a
time, the anatomists evidently being shy to acknowledge the validity of
the evidence for a fact so unexpected. Fig. 133 is a representation of
another slab subsequently found in beds of the same age in Nova Scotia,
and which may serve to indicate the nature of Sir WilliamaEuro(TM)s discovery.
In consequence of the neglect of this first hint by the London
geologists, the discovery of bones of a batrachian by von Dechen at
Saarbruck in 1844, and that of footprints by King in Pennsylvania in the
same year, are usually represented as the first facts of this kind. My
own earliest discovery of reptilian bones in Nova Scotia was made in
1844, though not published till some time afterward, and was followed
up by further collections in company with Sir Charles Lyell in 1851, at
which time also the earliest land-snail was found, and in the following
year the first millepede. Since that time the progress of discovery has
been astonishingly rapid, and has extended over most of the principal
coal-areas on both sides of the Atlantic.
[Illustration: FIG. 132.aEuro"Carboniferous Scorpion (_Eoscorpius
carbonarius_, Meek and Worthen). Illinois.]
[Illustration: FIG. 133.aEuro"Footprints of one of the oldest known
Batrachians, probably a species of _Dendrerpeton_. From the Lower
Carboniferous of ParrsboroaEuro(TM), Nova Scotia. Upper figure natural size.]
We may, for convenience, call these animals reptiles, but they are
regarded as belonging to that lower grade of reptilian animals, the
Amphibians or Batrachians, which includes the modern frogs and newts and
water-lizards.[56] Still it would be doing great injustice to the
carboniferous reptiles not to say, that while related to this low type,
they presented a much greater range of organisation than it shows at
present, evincing a capability to fill most of the places now occupied
by the true reptiles. Some of them were aquatic, and with limbs
rudimentary or little developed, but many of them walked on the land,
and were powerful and predaceous creatures. They had large and complex
teeth, they were protected by external bony plates, and some of them had
in addition a beautiful covering of horny plates and spines, and
ornamental lappets. Many had well-developed ribs, indicating a condition
of respiration much in advance of that in the ribless batrachians. Some
of them attained to size and strength rivalling those of the modern
alligators, while some of the smallest species exhibit characters
approaching in some respects to the lizards.
Perhaps the most fish-like of these animals are those first discovered
by von Dechen (_Archegosaurus_, Fig. 134). Their long heads, short
necks, supports for gills, feeble limbs and long flat tail, show that
they were aquatic creatures presenting many points of resemblance to the
Ganoid fishes which must have been their companions. Yet they show what
no fish can exhibit, fore and hind limbs with proper toes, and the
complete series of bones that appear in our own arms and legs, while
they must have had true lungs and breathed through nostrils. So
different are they from the fish in details, that a single limb bone, a
vertebra, a rib, or a fragment of a skull bone, suffices to distinguish
them. Much has been said recently of the genesis of limbs; and here, as
far as now known, we have the first true limbs; but it is scarcely too
much to say that the feet of _Archegosaurus_ differ more from the fins
of any carboniferous fish than they do from the human hand; while it is
certain that the feet which made the impressions represented in Fig.
133, on the lowest beds of the Carboniferous, or that from the upper
coal-formation represented in Fig. 139, were not less typical or
perfectly formed feet than those of modern lizards.
Leaving these fish-like forms, we find the remainder of the
carboniferous reptiles to diverge from them along three lines.
[Illustration: FIG. 134.aEuro"_Archegosaurus Decheni._ Head and anterior
limb reduced. Coal-field of Saarbruck.]
[Illustration: FIG. 135.aEuro"_Ptyonius._ A Snake-like Amphibian.
Coal-measures of Ohio.aEuro"After Cope.]
The first leads to snake-like creatures, destitute of limbs, and which
must have been functionally the representatives of the serpents in the
PalA|ozoic, though batrachian in their affinities (Fig. 135). They are
found both in Europe and America; and Huxley describes one from Ireland
more than twenty-one inches long, and with over one hundred
vertebrA|.[57] Some extraordinary traces are found on the sandstones of
the coal-formation,[58] which appear to indicate that there may have
been species of this type much larger than any represented by
skeletons, and with bodies perhaps six inches in diameter. It is not
unlikely that they had the habits of the modern water snakes.
[Illustration: FIG. 136.aEuro"A large Carboniferous Labyrinthodont
(_Baphetes planiceps_, Owen).
_a_, Anterior part of the skull, viewed from beneath. One-sixth natural
size, _b_, One of the largest teeth, natural size.]
A second line leads upward to large crocodile-like creatures, with
formidable teeth, strong bony armour, and well-developed limbs
(_Labyrinthodontia_, Figs. 136, 137). Some of them must have attained a
length of ten feet. They were lizard-like in form, could walk well, as
is seen from the footprints of some of the species which present a
considerable stride, and moved over mud without the belly touching the
ground. Their tails were long, and probably useful in swimming. Their
heads were flat and massive, and their teeth were strengthened by a
remarkable folding inward of the outer plate of enamel (Fig. 137 _b_).
The belly was protected by bony plates and closely imbricated scales. In
some of the species at least the upper parts were clothed with horny
scales, and the throat and sides were ornamented with pendent scaly
fringes or lappets.[59] Their general aspect and mode of life must have
resembled those of modern alligators; and in the vast swamps of the
Coal-period, full of ponds and sluggish streams swarming with fish, they
must have found a most suitable abode. While rigid anatomy may ally
these animals rather with the batrachians than the true reptiles, it is
evident that their great size, their capacity for walking with the body
borne well above the ground, their bony and scaly armour, their powerful
teeth and their capacious chests, with well-developed ribs, indicate
conditions of respiration and general vitality quite comparable with
those of the highest modern members of the class Reptilia.
[Illustration: FIG. 137.aEuro"_Baphetes planiceps_ (Owen).
_a_, Fragment of maxillary bone showing sculpture, four outer teeth, and
one inner tooth. Natural size. _b_, Section of inner tooth. Magnified,
_c_, Dermal scale. Natural size.]
The third line of progress leads to some slender and beautiful creatures
(_Microsauria_), chiefly known to us by remains found in erect trees,
and which resembled in form and habits the smaller modern lizards. They
have simple teeth, a well-developed brain-case, limbs of some length,
and bony and scaly armour, the latter in some cases highly ornate.[60]
They were probably the most thoroughly terrestrial, and the most active
of the coal batrachians, if indeed they were not strictly intermediate
between them and the lizards proper. Fig. 138 shows some fragments of
one of these animals; and the animal represented in Fig. 139, recently
figured by Fritsch, probably belongs to this group.
[Illustration: FIG. 138.aEuro"A lizard-like Amphibian (_Hylonomus
aciedentatus_).
_a_, Maxillary bone; enlarged.
_b_, Mandible; enlarged.
_c_, Teeth; magnified, showing front and side view of
ordinary tooth and grooved anterior tooth.
_d_, Section of tooth; magnified.
_e_, Scale; natural size and magnified.
_f_, Pelvic bone (?); natural size.
_g_, Rib; natural size.
_h_, Scapular bone (?); natural size.
_i_, Palate; natural size.]
[Illustration: FIG. 139.aEuro"_Stelliosaurus longicostatus_ (Fritsch). Upper
Coal-formation of Bohemia.]
The Labyrinthodonts of the Carboniferous continue upward into the
Permian, where they meet with the true reptiles; and in the earlier
Mesozoic some of the largest and most typical examples are found.[61]
But here their reign ceases, and they give place to reptiles of more
elevated type, whose history we must consider in the next chapter.
Nothing can be more remarkable than the apparently sudden and
simultaneous incoming of the batrachian reptiles in the Coal-formation.
As if at a given signal, they came up like the frogs of Egypt everywhere
and in all varieties of form. If, as evolutionists suppose, they were
developed from fishes, this must have been by some sudden change,
occurring at once all over the world, unless indeed some great and
unknown gap separates the Devonian from the CarboniferousaEuro"a supposition
which seems quite contrary to factaEuro"or unless in some region yet
unexplored this change was proceeding, and at a particular time its
products spread themselves over the worldaEuro"a supposition equally
improbable. In short, the hypothesis of evolution, as applied to these
animals, is surrounded with geological improbabilities.
A remarkable picture of the conditions of PalA|ozoic land life is
presented by the occurrence of remains of reptiles, millepedes and
land-snails in such erect trees as that represented in Fig. 140. In the
now celebrated section of the South Joggins in Nova Scotia, trees of
this kind occur at more than sixty different levels; but only in one of
these have they as yet been found to be rich in animal remains.
Fortunately this bed is so well exposed and so abundant in trees, that I
have myself, within a few years, removed from it about twenty of them,
the greater number affording remains of land animals.
[Illustration: FIG. 140.aEuro"Section showing the position of an erect
_Sigillaria_, containing remains of land animals.
1. Underclay, with rootlets of _Stigmaria_, resting on gray shale,
with two thin coaly seams.
2. Gray sandstone, with erect trees, _Calamites_, and other stems:
9 feet.
3. Coal, with erect tree on its surface: 6 inches.
4. Underclay with _Stigmaria_ rootlets.
_a_, _Calamites_.
_b_, Stem of plant undetermined.
_c_, _Stigmaria_ roots.
_d_, Erect trunk, 9 feet high.]
The history of one of these trees may be shortly stated thus. It was a
_Sigillaria_, perhaps two feet in diameter, and its stem had a dense and
imperishable outer bark, a soft cellular inner bark liable to rapid
decay, and a slender woody axis not very durable. It grew on the surface
of a swamp, now represented by a bed of coal. By inundations and by
subsidence, this swamp was exposed to the invasion of muddy and sandy
sediment, and this went on accumulating until the stem of the tree was
buried to the height of about nine feet, before which time it was no
doubt killed. After a time the top decayed and fell, leaving the buried
stump imbedded in the sandy soil, which had now become dry, or nearly
so. The trunk decayed, its inner bark and axis rotting away and falling
in shreds into the bottom of the cylindrical hole, about nine feet deep,
once occupied by the stem, and now kept open like a shaft or well by the
hard resisting outer bank. The ground around this opening became clothed
with ferns and reed-like _Calamites_, partly masking and concealing it.
And now millepedes and land snails made the buried trunk a home, or fell
into it in their wanderings; and small reptiles sporting around, in
pursuit of prey, or themselves pursued, stumbled into the open pitfall,
and were unable to extricate themselves, though I have found in some of
the layers in these trees trails which show that these imprisoned
reptiles had wearily wandered round and round, in the vain search for
means of exit, till they died of exhaustion and famine. The bones of
these dead reptiles, shells of land-snails and crusts of millepedes,
accumulated in these natural coffins, and became mixed with vegetable
debris falling into them, and with thin layers of mud washed in by the
rains; and this process continued so long that a layer of six inches to
a foot in thickness, full of bones, was sometimes produced. At length a
new change supervened, the area was again inundated and drifted over
with sand, and the hollow trunk was filled to the top and buried under
many feet of sediment, never to be re-opened till, after the whole had
been hardened into sandstone and elevated to form a part of the modern
coast, when the old tree and its forest companions which had shared the
same fate with it, are made to yield up their treasures to the
geologist. This history is no fancy picture. It represents the results
of long and careful study of the beds holding these erect trees, and of
the laborious extraction of great numbers of them, and the breaking-up
of their contents into thin flakes, to be carefully examined with the
lens under a bright light in search of the relics they contained. Fig.
11 in Chap. I. represents the extraction of one of these trees, which
happened to be partially exposed by the wasting of the cliff; but many
others had to be laboriously mined out of the rock by blasting with
gunpowder.
[Illustration: FIG. 140_a_.aEuro"Section of base of erect _Sigillaria_,
containing remains of land animals.
_a_, Mineral charcoal. _b_, Dark-coloured sandstone, with plants, bones,
&c. _c_, Gray sandstone, with Calamites and Cordaites.]
It is evident that the combination of circumstances referred to above
could not often occur; and it is therefore not wonderful that only in
one place and one bed has evidence of it been found, and that even in
this some of the trees have been filled up at once by sand and clay, or
so crushed by falling in or lateral pressure, that they could receive no
animal remains. In one respect this is a striking evidence of the
imperfection of the geological record, since, but for what may be called
a fortunate accident, many of the most interesting inhabitants of the
coal forests might have been altogether unknown to us. On the other
hand, it shows how strange and unexpected are the ways in which the
relics of the old world have been preserved for our inspection, and that
there is probably scarcely any animal or plant that has ever lived of
which some fragment does not exist, did we know where to look for it.
It may be well to remark, in closing this chapter, how many new forms of
life, air-breathing and otherwise, make their first appearance in the
Carboniferous, and have continued to prevail until now. Here we find the
first specimens of Amphibians, Spiders, Myriapods, Orthopterous and
Coleopterous Insects, and of the Crabs among ten-footed Crustaceans. In
the latter group Woodward has recently described the oldest known crab,
from the Coal-formation of Belgium.
[Illustration: INHABITANTS OF THE ENGLISH SEAS IN THE AGE OF REPTILES.
_Pliosaurus_, _Ichthyosaurus_, _Plesiosaurus_, _Mososaurus_, and
_Teleosaurus_.]