C. { swimming { 12. Eel species Eels, snake eels, electric =Osseous= { bladder { _Enchelygenes_ eels, etc.

=Fish= { _Physostomi_ { +Teleostei+ { { VII. Osseous Fish { 13. _Stichobranchii_ Perch, wra.s.se, turbot, { without an air { etc.

{ pa.s.sage to the { 14. _Plectognathi_ Trunk fish, globe fish, { swimming { etc.

{ bladder { 15. _Lophobranchii_ Pipe fish, sea horses, { _Physoclisti_ { etc.

PEDIGREE OF THE NON-AMNIONATE CRANIOTA.

Plectognathi Anura | Lophobranchia Peromela | | | | | | | | Sozura -------v-----/ Labyrinthodonta | | | | | | Stichobranchia | | | +Physoclisti+ --v-----/ | | | | Enchelygenes | Ganocephala Sozobranchia | | +Phractamphibia+ +Lissamphibia+ | | | | --v---/ | | | -------v-------/ | | Thrissogenes +Amphibia+ +Physostomi+ Semaeopteri | +Teleostei+ | | | | Protopteri | | Fulcrati | | | Plesiosauria Pycnoscolopes | | | | Icthyosauria | | | | | | | | Closcolopes Efulcri | | | | | +Cycliferi+ +Rhombiferi+ | | | | (Cycloganoides) (Rhomboganoides) | | | | | | | | | | ---------v--------/ | | ----v-----/ | +Dipneusta+ | | Placoderma | | Simosauria Sturiones | | | +Halisauria+ | | | | | Cephalaspidae | Rajacei -----------v--------/ | | | | | | | Amphipneumona -----v-------/ | | | | | Pamphracti | | +Tabuliferi+ | | (Placoganoides) | Chimaeracei | +Ganoides+ | +Holocephali+ | | | | | Squalacei | | | | | | | --------v---------/ | | | | | +Plagiostomi+ | | | | | -------------v---------------------/ | +Selachii+ +Fish+ +Amphirrhina+ | Cyclostoma | +Monorrhina+ | | -------v---------/ | +Craniota+

Genuine fish are divided into three distinct sub-cla.s.ses, namely, Primaeval fish, Ganoid fish, and Osseous fish. The oldest of these, where the original form has been most faithfully preserved, is that of the _Primaeval fish_ (Selachii). Of these there still exist Sharks (Squali), and Rays (Rajae), which are cla.s.sed together as cross-mouthed fishes (Plagiostomi), and the strange and grotesquely formed Sea-cats, or _Chimaeracei_ (Holocephali). These primary fish of the present day, which are met with in all seas, are only poor remains of the prevailing animal groups, rich in forms, which the Selachii formed in the earlier periods of the earth's history, and especially during the palaeolithic period.

Unfortunately all Primaeval fish possess a cartilaginous, never a completely osseous skeleton, which is but little, if at all, capable of being petrified. The only hard parts of the body which could be preserved in a fossil state, are the teeth and fin-spikes. These are found in the older formations in such quant.i.ties, varieties, and sizes, that we may, with certainty, infer a very considerable development of Primaeval fish in those remote ages. They are even found in the Silurian strata, which contain but few remains of other Vertebrata, such as Enamelled fish (and these only in the most recent part, that is, in the upper Silurian). By far the most important and interesting of the three orders of Primaeval fish are Sharks; of all still living double-nostriled animals, they are probably most closely allied to the original primary form of the whole group, namely, to the Proselachii. Out of these Proselachii, which probably differed but little from genuine Sharks, Enamelled fish, and the present Primaeval fish, in all probability, developed in one direction, and the Dipneusta, Sea-dragons, and Amphibia in another.

The _Ganoid_, or _Enamelled fish_ (Ganoides), in regard to their anatomy stand midway between the Primaeval and the Osseous fish. In many characteristics they agree with the former, and in many others with the latter. Hence, we infer that genealogically they form the transition from Primaeval to Osseous fish. The Ganoids are for the most part extinct, and more nearly so than the Primaeval fish, whereas they were developed in great force during the entire palaeolithic and mesolithic periods. Ganoid fish are divided into three legions according to the form of their external covering, namely, Mailed, Angular-scaled, and Round-scaled. The _Mailed Ganoid fish_ (Tabuliferi) are the oldest, and are directly allied to the Selachii, out of which they originated.

Fossil remains of them, though rare, are found even in the upper Silurian (Pteraspis ludensis of the Ludlow strata). Gigantic species of them, coated with strong bony plates, are found in the Devonian system.

But of this legion there now lives only the small order of Sturgeons (Sturiones), including the Spade-sturgeons (Spatularidae), and those Sturgeons (Accipenseridae) to which belong, among others, the Huso, which yields isingla.s.s, or sturgeon's sound, and the Caviar-sturgeon, whose eggs we eat in the shape of caviar, etc. Out of the mailed Ganoid fish, the angular and round-scaled ones probably developed as two diverging branches. The _Angular-scaled Ganoid fish_ (Rhombiferi)-which can be distinguished at first sight from all other fish by their square or rhombic scales-are at present represented only by a few survivors, namely, the Finny Pike (Polypterus) in African rivers (especially the Nile), and by the Bony Pike (Lepidosteus) in American rivers. Yet during the palaeolithic and the first half of the mesolithic epochs this legion formed the most numerous group of fishes. The third legion, that of _Round-scaled Ganoid fish_ (Cycliferi), was no less rich in forms, and lived princ.i.p.ally during the Devonian and Coal periods. This legion, of which the Bald Pike (Amia), in North American rivers, is the only survivor, was especially important, inasmuch as the third sub-cla.s.s of fish, namely, Osseous fish, developed out of it.

_Osseous fish_ (Teleostei) include the greater portion of the fish of the present day. Among these are by far the greater portion of marine fish, and all of our fresh-water fish except the Ganoid fish just mentioned. This cla.s.s is distinctly proved by numerous fossils to have arisen about the middle of the Mesolithic epoch out of Ganoid fish, and moreover out of the Round-scaled, or Cycliferi. The Thrissopidae of the Oolitic period (Thrissops, Leptolepis, Tharsis), which are most closely allied to the herrings of the present day, are probably the oldest of all Osseous fish, and have directly arisen out of Round-scaled Ganoid fish, closely allied to the existing Amia. In the older Osseous fish of the legion called _Physostomi_, as also in the Ganoides, the swimming bladder throughout life was connected with the throat by a permanent air pa.s.sage (a kind of windpipe). This is still the case with all the fish belonging to this legion, namely, with herrings, salmon, carp, shad, eels, etc. However, during the chalk period this air pa.s.sage, in some of the Physostomi, became constricted and closed, and the swimming bladder was thus completely separated from the throat. Hence there arose a second legion of Osseous fish, the _Physoclisti_, which did not attain their actual development until the tertiary epoch, and soon far surpa.s.sed the Physostomi in variety. To this legion belong most of the sea fish of the present day, especially the large families of the Turbot, Tunny, Wra.s.se, Crowfish, etc., further, the Lock-jaws (Plectognathi), Trunk fish, and Globe-fish and the Bushy-gills (Lophobranchi), viz., Pipe-fish, and Sea-horses. There are, however, only very few Physoclisti among our river fish, for instance, Perch and Sticklebacks; the majority of river fish are Physostomi.

Midway between genuine Fish and Amphibia is the remarkable cla.s.s of _Mud-fish_, or _Scaly Sirens_ (Dipneusta, or Protopteri). There now exist only a few representatives of this cla.s.s, namely, the American Mud-fish (Lepidosiren paradoxa) in the region of the river Amazon, and the African Mud-fish (Protopterus annectens) in different parts of Africa. A third large Salamander-fish (Ceratodus Fosteri) has lately been discovered in Australia. During the dry season, that is in summer, these strange animals bury themselves in a nest of leaves in the dry mud, and then breathe air through lungs like the Amphibia. But during the wet season, in winter, they live in rivers and bogs, and breathe water through gills like fish. Externally, they resemble fish of the eel kind, and are like them covered with scales; in many other characteristics also-in their internal structure, their skeleton, extremities, etc.-they resemble Fish more than Amphibia. But in certain features they resemble the Amphibia, especially in the formation of their lungs, nose, and heart. There is consequently an endless dispute among zoologists, as to whether the Mud-fish are genuine Fish or Amphibia. Distinguished zoologists have expressed themselves in favour of both opinions. But in fact, owing to the complete blending of characteristics which they present, they belong neither to the one nor to the other cla.s.s, and are probably most correctly dealt with as a special cla.s.s of Vertebrata, forming the transition between Fishes and Amphibians. The still living Dipneusta are probably the last surviving remains of a group which was formerly rich in forms, but has left no fossil traces on account of the want of a solid skeleton. In this respect, these animals are exactly like the Monorrhina and the Leptocardia. However, teeth are found in the Trias which resemble those of the living Ceratodus. Possibly the extinct Dipneusta of the palaeolithic period, which developed in the Devonian epoch out of primaeval fish, must be looked upon as the primary forms of the Amphibia, and thus also of all higher Vertebrata. At all events the unknown forms of transition-from Primaeval fish to Amphibia-were probably very like the Dipneusta.

A very peculiar cla.s.s of Vertebrate animals, long since extinct, and which appears to have lived only during the secondary epoch, is formed by the remarkable _Sea-dragons_ (Halisauria, or Enaliosauria, also called Nexipoda, or Swimming-footed animals). These formidable animals of prey inhabited the mesolithic oceans in great numbers, and were of most peculiar forms, sometimes from thirty to forty feet in length. From many and excellently preserved fossil remains and impressions, both of the entire body of Sea-dragons as well as of single parts, we have become very accurately acquainted with the structure of their bodies.

They are usually cla.s.sed among Reptiles, whilst some anatomists have placed them in a much lower rank, as directly allied to Fish.

Gegenbaur's recently published investigations, which place the structure of their limbs in a true light, have led to the surprising conclusion that the Sea-dragons form quite an isolated group, differing widely both from Reptiles and Amphibia as well as from Fish. The skeleton of their four legs, which are transformed into short, broad, paddling fins (like those of fish and whales) furnishes us with a clear proof that the Halisauria branched off from the main-stock of Vertebrata at an earlier period than the Amphibia. For Amphibia, as well as the three higher cla.s.ses of Vertebrata, are all derived from a common primary form, which possessed only _five_ toes or fingers on each leg. But the Sea-dragons have (either distinctly developed or in a rudimentary condition as parts of the skeleton of the foot) more than five fingers, as have also the Selachians or Primaeval fish. On the other hand, they breathed air through lungs, like the Dipneusta, although they always swam about in the sea. They, therefore, perhaps, in conjunction with the Dipneusta, branched off from the Selachii, but did not develop into higher Vertebrata; they form an extinct lateral line of the pedigree, which has died out.

The more accurately known Sea-dragons are cla.s.sed into three orders, distinct enough one from the other, namely, _Primaeval Dragons_, _Fish Dragons_, and _Serpent Dragons_. The _Primaeval Dragons_ (Simosauria) are the oldest Sea-dragons, and lived only during the Trias period. The skeletons of many different genera of them are met with in the German limestone known as "Muschel-kalk." They seem upon the whole to have been very like the Plesiosauria, and are, consequently, sometimes united with them into one order as Sauropterygia. The _Serpent Dragons_ (Plesiosauria) lived in the oolitic and chalk periods together with the Ichthyosauria. They were characterised by an uncommonly long thin neck, which was frequently longer than the whole body, and carried a small head with a short snout. When their arched neck was raised they must have looked very like a swan; but in place of wings and legs they had two pairs of short, flat, oval-paddling fins.

The body of the _Fish Dragons_ (Ichthyosauria) was of an entirely different form; these animals may be opposed to the two preceding orders under the name of Fish-finners (Ichthyopterygia). They possessed a very long extended body, like a fish, and a heavy head with an elongated, flat snout, but a very short neck. Externally, they were probably very like porpoises. Their tail was very long, whereas it was very short in the members of the preceding orders. Also both pairs of paddling fins are broader and show very different structure from that seen in the other two orders. Probably the Fish Dragons and Serpent Dragons developed as two diverging branches out of the Primaeval Dragons; but it is also possible that the Plesiosauria alone originated out of the Simosauria, and that the Ichthyosauria were lower off-shoots from the common stock. At all events, they must all be directly, or indirectly derived from the Selachii, or Primaeval fish.

The succeeding cla.s.ses of Vertebrata, the _Amphibia_ and the _Amniota_ (Reptiles, Birds, and Mammals), owing to the characteristic structure which they all exhibit of five toes to each foot, may all be derived from a common primary form, which originated from the Selachii, and which possessed five toes on each of its four limbs. When we find a less number of toes than five, we can show that the missing ones must have been lost in the course of time by adaptation. The oldest known Vertebrata with five toes are the _Batrachias_ (Amphibia). We divide this cla.s.s into two sub-cla.s.ses, namely, mailed Batrachians and naked Batrachians, the first of which is distinguished by the body being covered with bony plates or scales.

The first and elder sub-cla.s.s of Amphibia consists of the _Mailed Batrachians_ (Phractamphibia), the oldest land living Vertebrata of which fossil remains exist. Well-preserved fossil remains of them occur in the coal, especially of those with _Enamelled heads_ (Ganocephala), which are most closely allied to fish, namely, the Archegosaurus of Saarbruck, and the Dendrerpeton of North America. There then follow at a later period the gigantic _Labyrinth-toothed animals_ (Labyrinthodonta), which are represented in the Permian system by Zygosaurus, but at a later period, more especially in the Trias, by Mastodonsaurus, Trematosaurus, Capitosaurus, etc. The shape of these formidable rapacious animals seems to have been between that of crocodiles, salamanders, and frogs, but in their internal structure they were more closely related to the two latter, while by their solid coat of mail, formed of strong bony plates, they resembled the first animals. These gigantic mailed Batrachians seem to have become extinct towards the end of the Tria.s.sic period. No fossil remains of mailed Batrachia are known during the whole of the subsequent periods. However, the still living blind Snakes, or _Caeciliae_ (Peromela)-small-scaled Phractamphibia of the form and the same mode of life as the earth-worm-prove that this sub-cla.s.s continued to exist, and never became completely extinct.

The second sub-cla.s.s of Amphibia, the _naked Batrachia_ (Lissamphibia), probably originated even during the primary and secondary epochs, although fossil remains of them are first found in the tertiary epoch.

They are distinguished from mailed Batrachia by possessing a naked smooth, and slimy skin, entirely without scales or coat of mail. They probably developed either out of a branch of the Phractamphibia, or out of the same common root with them. The ontogeny of the three still living orders of naked Batrachia-the gilled Batrachia, tailed Batrachia, and frog Batrachia-distinctly repeats the historical course of development of the whole sub-cla.s.s. The oldest forms are the gilled Batrachia (Sozobranchia), which retain throughout life the original primary form of naked Batrachia, and possess a long tail, together with water-breathing gills. They are most closely allied to the Dipneusta, from which, however, they differ externally by the absence of the coat of scales. Most gilled Batrachia live in North America: among others of the cla.s.s is the Axolotl, or Siredon, already mentioned. (Compare above, vol. i. p. 241.) In Europe the order is only represented by one form, the celebrated "Olm" (Proteus anguinus), which inhabits the grotto of Adelsberg and other caves in Carinthia, and which, from living in the dark, has acquired rudimentary eyes which can no longer see (vol. i. p.

13). The order of Tailed Batrachia (Sozura) have developed out of the gilled Batrachia by the loss of external gills; the order includes our black and yellow spotted land Salamander (Salamandra maculata), and our nimble aquatic Salamanders (Tritons). Many of them-for instance, the celebrated giant Salamanders in j.a.pan (Cryptobranchus j.a.ponicus)-still retain the gill-slits, although the gills themselves have disappeared.

All of them, however, retain the tail throughout life. Tritons occasionally-when forced to remain in water always-retain their gills, and thus remain at the same stage of development as gilled Batrachia.

(Compare above, vol. i. p. 241.) The third order, the _tailless_ or _frog-like Batrachia_ (Anura), during their metamorphosis, not only lose their gills, with which in early life (as so-called tadpoles) they breathe in water, but also the tail with which they swim about. During their ontogeny, therefore, they pa.s.s through the course of development of the whole sub-cla.s.s, they being at first _Gilled Batrachia_, then _Tailed Batrachia_, and finally _Frog-like Batrachia_. The inference from this is evidently, that _Frog-like Batrachia_ developed at a later period out of _Tailed Batrachia_, as the latter had developed out of _Gilled Batrachia_ which originally existed alone.

In pa.s.sing from the Amphibia to the next cla.s.s of Vertebrata, namely, Reptiles, we observe a very considerable advance in the progress of organization. All the double-nostriled animals (Amphirrhina) up to this time considered, and more especially the two larger cla.s.ses of Fish and Batrachia, agree in a number of important characteristics, which essentially distinguish them from the three remaining cla.s.ses of Vertebrata-Reptiles, Birds, and Mammals. During the embryological development of these latter, a peculiarly delicate covering, the _first ftal membrane_, or _amnion_, which commences at the navel, is formed round the embryo; this membrane is filled with the amnion-water, and encloses the embryo or germ in the form of a bladder. On account of this very important and characteristic formation, we may comprise the three most highly developed cla.s.ses of Vertebrata under the term _Amnion-animals_ (Amniota). The four cla.s.ses of double-nostriled animals which we have just considered, in which the amnion is wanting (as is the case in all lower Vertebrate animals, single-nostriled and skull-less animals), may on the other hand be opposed to the others as _amnion-less animals_ (Anamnia).

The formation of the ftal membrane, or amnion, which distinguishes reptiles, birds, and mammals from all other Vertebrata, is evidently a very important process in their ontogeny, and in the phylogeny which corresponds with it. It coincides with a series of other processes, which essentially determine the higher development of Amnionate animals.

The first of these important processes is the _total loss of gills_, for which reason the Amniota, under the name of _Gill-less animals_ (Ebranchiata), were formerly opposed to all other Vertebrate animals which breathed through gills (Branchiata). In all the Vertebrate already discussed, we found that they either always breathed through gills, or at least did so in early life, as in the case of Frogs and Salamanders.

On the other hand, we never meet with a Reptile, Bird, or Mammal which at any period of its existence breathes through gills, and the gill-arches and openings which do exist in the embryos, are, during the course of the ontogeny, changed into entirely different structures, viz., into parts of the jaw-apparatus and the organ of hearing. (Compare above, vol. i. p. 307.) All Amnionate animals have a so-called cochlea in the organ of hearing, and a "round window" corresponding with it.

These parts are wanting in the Amnion-less animals; moreover, their skull lies in a straight line with the axis of the vertebral column. In Amniotic animals the base of the skull appears bent in on the abdominal side, so that the head sinks upon the breast. (Plate III. Fig. _C_, _D_, _G_, _H_.) The organs of tears at the side of the eye also first develop in the Amniota.

The question now is, When did this important advance take place in the course of the organic history of the earth? When did the common ancestor of all Amniota develop out of a branch of the Non-amniota, to wit, out of the branch of the Amphibia?

To this question, the fossil remains of Vertebrata do not give us a very definite, but still they do give an approximate, answer. For with the exception of two lizard-like animals found in the Permian system (the Proterosaurus and Rhopalodon), all the fossil remains of Amniota, as yet known, belong to the _secondary_, _tertiary_, and _quaternary epochs_.

With regard to the two Vertebrata just named, it is still doubtful whether they are genuine reptiles, or perhaps Amphibia of the salamander kind. Their skeleton alone is known to us, and even this not perfectly.

Now as we know nothing of the characteristic features of their soft parts, it is quite possible that the Proterosaurus and Rhopalodon were non-amnionate animals more closely allied to Amphibia than to Reptiles; possibly they belonged to the transition form between the two cla.s.ses.

But, on the other hand, as undoubted fossil remains of Amniota have been found as early as the Trias, it is probable that the _main cla.s.s of Amniota_ first developed in the Trias, that is, in the beginning of the Mesolithic epoch. As we have already seen, this very period is evidently one of the most important turning points in the organic history of the earth. The palaeolithic fern forests were then replaced by the pine forests of the Trias period; important transformations then took place in many of the cla.s.ses of Invertebrata. Articulated marine lilies (Colocrina) developed out of the plated ones (Phatnocrina.) The Autechinidae, or sea-urchins with only twenty rows of plates, took the place of the palaeolithic Palechinidae, the sea-urchins with more than twenty rows of plates. The Cystideae, Blastoideae, Trilobita, and other characteristic groups of Invertebrata of the primary period became extinct. It is no wonder that transforming conditions of adaptation powerfully influenced the Vertebrate tribes also in the beginning of the Trias period, and caused the origin of Amniotic animals.

If, however, the two Lizard and Salamander-like animals of the Permian system, the Proterosaurus and Rhopalodon, are considered genuine Reptiles, and consequently the most ancient Amniota, then the origin of this main cla.s.s must necessarily have taken place in the preceding period, towards the end of the primary, namely, in the Permian period.

However, all other remains of Reptiles, which were formerly believed to have been found in the Permian and the Coal system, or even in the Devonian system, have been proved to be either not remains of Reptiles at all, or to belong to a more recent date (for the most part to the Trias). (Compare Plate XIV.)

The common hypothetical primary form of all Amniotic animals, which we may call _Protamnion_, and which was possibly nearly related to the Proterosaurus, very probably stood upon the whole midway between salamanders and lizards, in regard to its bodily formation. Its descendants divided at an early period into two different lines, one of which became the common primary form of Reptiles and Birds, the other the primary form of Mammals.

Of all the three cla.s.ses of Amniota, _Reptiles_ (Reptilia, or Pholidota, also called Sauria in the widest sense), remain at the lowest stage of development, and differ least from their ancestors, the Amphibia. Hence they were formerly universally included among them, although their whole organization is much more like that of Birds than Amphibia. There now exist only four orders of Reptiles, namely,-Lizards, Serpents, Crocodiles, and Tortoises. They, however, form but a poor remnant of the exceedingly various and highly developed host of Reptiles which lived during the Mesolithic, or Secondary epoch, and predominated over all other Vertebrata. The immense development of Reptiles during the Secondary epoch is so characteristic that we could as well name it after those animals as after the Gymnosperms (p. 111). Twelve of the twenty-seven sub-orders, given on the accompanying table, and four of the eight orders, belong exclusively to the secondary period. These mesolithic groups are marked by an asterisk. All the orders, with the exception of Serpents, are found fossilized even in the Jura and Trias periods.

[Ill.u.s.tration: _Pl. XIV._

[Horizontal axis:]

Branches, Cla.s.ses, and Sub-Cla.s.ses, of the Vertebrate Stem.

Prochordata Evertebrate Forefathers of the Vertebrate

Skull-less (Acrania) or Tube-hearted Fish, (Leptocardia)

Single nostrilled (Monorrhina) or Round-mouthed Fishes (Cyclostoma)

Anamnia { Paired-nostrilled forms or Amphirrhina { with gills, without Amnion.

Fish, Pisces

Primeval Fish, Selachii.

Enamelled Fish, Ganoides.

Bony Fish, Teleostei.

Mud Fish, Dipneusta.

Sea-Dragons, Halisauria.

Frogs and Newts, Amphibia.

Amniota { Paired nostrilled or Amphirrhina { with Amnion, without gills.

Reptiles, Reptilia.

Primaeval Reptiles, Tocosauria.

Lizards, Lacertilia.

Snakes, Ophidia.

Crocodiles, Crocodilia.