PEDIGREE OF THE NON-AMNIONATE CRANIOTA.

Plectognathi Anura
│
Lophobranchia Peromela│
│
│
 │
│
│
│
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Sozura

Labyrinthodonta│
│
Stichobrancia
Physoclisti		 │
│
│
│
│
│
Enchelygenes│

│
│
│
│
│
 Ganocephala
Phractamphibia		Sozobranchia
Lissamphibia
│
│
 │
│

│
 Amphibia
Thrissogenes
Physostomi
Teleostei
Semæopteri
 │
│
│
│
Fulcrati│
Protopteri │
 Plesiosauria
Pycnoscolopes│
│
│
 │
 Icthyosauria│
│
│
│
│
 │
 │
│
Cœloscolopes
Cycliferi
Cycloganoides)

Efulcri
Rhombiferi
(Rhomboganoides)		│
│
│
│
 │
│
│
│
 │
│
│
│
│
│
│
│
│
│
│
 │
 │
│

Placoderma		Dipneusta │
│
 │
│
│
│

Sturiones
│
│		│
│
│
│
 │
│
│
│
 │
│
│
│

Simosauria
Halisauria
│
│
 │
 │
 │
Cephalaspidæ│
Rajacei│

│
│
 Amphipneumona
Pamphracti
Tabuliferi
(Placoganoides)
Ganoides		│
│
│
│
│
│
│
│
 │
│
│
│
│
│
│
│
Chimæracei
Holocephali		 │
│
Squalacei│
│
 │
│
│
│
 │

Plagiostomi		│
│
 │
│
│
│
 │
│
│
 │

Selachii
Fish
Amphirrhina		 │
│
│
│
│
│
│
 Cyclostoma
Monorrhina

Craniota

Genuine fish are divided into three distinct sub-classes, namely, Primæval fish, Ganoid fish, and Osseous fish. The oldest of these, where the original form has been most faithfully preserved, is that of the Primæval fish (Selachii). Of these there still exist Sharks (Squali), and Rays (Rajæ), which are classed together as cross-mouthed fishes (Plagiostomi), and the strange and grotesquely formed Sea-cats, or Chimæracei (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 palæolithic period. Unfortunately all Primæval 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 quantities, varieties, and sizes, that we may, with certainty, infer a very considerable development of Primæval 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 Primæval 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 Primæval 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 Primæval 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 Primæval to Osseous fish. The Ganoids are for the most part extinct, and more nearly so than the Primæval fish, whereas they were developed in great force during the entire palæolithic 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 (Spatularidæ), and those Sturgeons (Accipenseridæ) to which belong, among others, the Huso, which yields isinglass, 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 palæolithic 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 principally 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-class 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 class 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 Thrissopidæ 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 passage (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 passage, 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 surpassed 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, Wrasse, 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 class of Mud-fish, or Scaly Sirens (Dipneusta, or Protopteri). There now exist only a few representatives of this class, 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 class, and are probably most correctly dealt with as a special class 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 palæolithic period, which developed in the Devonian epoch out of primæval 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 Primæval fish to Amphibia—were probably very like the Dipneusta.

A very peculiar class 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 classed 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 classes 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 Primæval 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 classed into three orders, distinct enough one from the other, namely, Primæval Dragons, Fish Dragons, and Serpent Dragons. The Primæval 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 Primæval 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 Primæval fish.

The succeeding classes 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 class into two sub-classes, 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-class 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 Triassic period. No fossil remains of mailed Batrachia are known during the whole of the subsequent periods. However, the still living blind Snakes, or Cæciliæ (Peromela)—small-scaled Phractamphibia of the form and the same mode of life as the earth-worm—prove that this sub-class continued to exist, and never became completely extinct.

The second sub-class 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-class. 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 class 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 Japan (Cryptobranchus Japonicus)—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 pass through the course of development of the whole sub-class, 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 passing from the Amphibia to the next class 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 classes of Fish and Batrachia, agree in a number of important characteristics, which essentially distinguish them from the three remaining classes of Vertebrata—Reptiles, Birds, and Mammals. During the embryological development of these latter, a peculiarly delicate covering, the first fœtal 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 classes of Vertebrata under the term Amnion-animals (Amniota). The four classes 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 fœtal 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 classes. 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 class 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 palæolithic fern forests were then replaced by the pine forests of the Trias period; important transformations then took place in many of the classes of Invertebrata. Articulated marine lilies (Colocrina) developed out of the plated ones (Phatnocrina.) The Autechinidæ, or sea-urchins with only twenty rows of plates, took the place of the palæolithic Palechinidæ, the sea-urchins with more than twenty rows of plates. The Cystideæ, Blastoideæ, 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 class 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 mid-way 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 classes 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.

Haeckel-History of CreationPl. XIV.

In the first order, that of Primary Reptiles, or Primary Creepers (Tocosauria), we class the extinct Thecodontia of the Trias, together with those Reptiles which we may look upon as the common primary form of the whole class. To the latter, which we may call Primæval Reptiles (Proreptilia), the Proterosaurus of the Permian system very probably belongs. The seven remaining orders must be considered as diverging branches, which have developed in different directions out of that common primary form. The Thecodontia of the Trias, the only positively known fossil forms of Tocosauria, were Lizards which seem to have been like the still living monitor lizards (Monitor, Varanus).

SYSTEMATIC SURVEY
Of the 8 Orders and 27 Sub-orders of Reptiles.
(Those groups marked with * became extinct even during the Secondary Period.)
Orders
of
Reptiles.		Sub-orders
of
Reptiles.		Systematic Name
of the
Sub-orders.		A Generic Name
as
an example.
I. Primary
Reptiles
Tocosauria

1.Primæval reptiles1.Proreptilia*(Proterosaurus?)
2. 2.Thecodontia*Palæosaurus
II. Lizards
Lacertilia

3.Cleft-tongued3.Fissilingues Monitor
4.Thick-tongued4.Crassilingues Iguana
5.Short-tongued5.Brevilingues Anguis
6.Ringed lizards6.Glyptodermata Amphisbæna
7.Chameleons7.Vermilingues Chamæleo
III. Serpents
Ophidia

8.Adders8.Aglyphodonta Coluber
9.Tree serpents9.Opisthoglypha Dipsas
10. 10.Proteroglypha Hydrophis
11.Vipers11.Solenoglypha Vipera
12.Worm serpents12.Opoterodonta Typhlops
IV. Crocodiles
Crocodilia

13.Amphicœla13.Teleosauria*Teleosaurus
14.Opisthocœla14.Steneosauria*Steneosaurus
15.Prosthocœla15.Alligatores Alligator
V. Tortoises
Chelonia

16.Sea tortoises16.Thalassita Chelone
17.River tortoises17.Potamita Trionyx
18.Marsh tortoises18.Elodita Emys
19.Land tortoises19.Chersita Testudo
VI. Flying
Reptiles
Pterosauria*

20.Long-tailed
Flying lizards		20.Rhamphorhynchi*Rhamphorhynchus
21.Short-tailed
Flying lizards		21.Pterodactyli*Pterodactylus
VII. Dragons
Dinosauria*

22.Giant dragons22.Harpagosauria*Megalosaurus
23.Elephantine dragons23.Therosauria*Iguanodon
VIII. Beaked
Reptiles
Anomodontia*

24.Dog-toothed24.Cynodontia*Dicynodon
25.Toothless25.Cryptodontia*Udenodon
26.Kangaroo reptiles26.Hypsosauria*Compsognathus
27.Bird reptiles27.Tocornithes*(Tocornis)

Of the four orders of reptiles now existing, and which, moreover, have alone represented the class since the beginning of the tertiary epoch, that of Lizards (Lacertilia) is probably most closely allied to the extinct Primary Reptiles, and especially through the monitors already named. The class of Serpents (Ophidia) developed out of a branch of the order of lizards, and this probably not until the beginning of the tertiary epoch. At least we at present only know of fossil remains of serpents from the tertiary strata. Crocodiles (Crocodilia) existed much earlier; the Teleosauria and Steneosauria belonging to the class are found fossil in large quantities even in the Jura; but the still living alligators are first met with in a fossil state in the chalk and tertiary strata. The most isolated of the four existing orders of reptiles consists of the remarkable group of Tortoises (Chelonia); fossils of these strange animals are first met with in the Jura. In some characteristics they are allied to Amphibia, in others, to Crocodiles, and by certain peculiarities even to Birds, so that their true position in the pedigree of Reptiles is probably far down at the root. The extraordinary resemblance of their embryos to Birds, manifested even at later stages of the ontogenesis, is exceedingly striking.

The four extinct orders of Reptiles show among one another, and, with the four existing orders just mentioned, such various and complicated relationships, that in the present state of our knowledge we are obliged to give up the attempt at establishing their pedigree. The most deviating and most curious forms are the Flying Reptiles (Pterosauria); flying lizards, in which the extremely elongated fifth finger of the hand served to support an enormous flying membrane. They probably flew about, in the secondary period, much in the same way as the bats of the present day. The smallest flying lizards were about the size of a sparrow; the largest, however, with a breadth of wing of more than sixteen feet, exceeded the largest of our living flying birds in stretch of wing (condor and albatross). Numerous fossil remains of them, of the long-tailed Rhamphorhynchia and of the short-tailed Pterodactylæ are found in all the strata of the Jura and Chalk periods, but in these only.

Not less remarkable and characteristic of the Mesolithic epoch was the group of Dragons (Dinosauria, or Pachypoda). These colossal reptiles, which attained a length of more than fifty feet, are the largest inhabitants of the land which have ever existed on our globe; they lived exclusively in the secondary epoch. Most of their remains are found in the lower cretaceous system, more especially in the Wealden formations of England. The majority of them were fearful beasts of prey (the Megalosaurus from twenty to thirty, the Pelorosaurus from forty to fifty feet in length). The Iguanodon, however, and some others lived on vegetable food, and probably played a part in the forests of the chalk period similar to that of the unwieldy but smaller elephants, hippopotami, and rhinoceroses of the present day.

The Beaked Reptiles (Anomodontia), likewise also long since extinct, but of which very many remarkable remains are found in the Trias and Jura, were perhaps closely related to the Dragons. Their jaws, like those of most Flying Reptiles and Tortoises, had become changed into a beak, which either possessed only degenerated rudimentary teeth, or no teeth at all. In this order, if not in the preceding one, we must look for the primary parents of the bird class, which we may call Bird Reptiles (Tocornithes). Probably very closely related to them was the curious, kangaroo-like Compsognathus from the Jura, which in very important characteristics already shows an approximation to the structure of birds.

The class of Birds (Aves), as already remarked, is so closely allied to Reptiles in internal structure and by embryonal development, that they undoubtedly originated out of a branch of this class. Even a glance at Plates [II]. and [III]. will show that the embryos of birds at a time when they already essentially differ from the embryos of Mammals, are still scarcely distinguishable from those of Tortoises and other Reptiles. The cleavage of the yolk is partial in the case of Birds and Reptiles, in Mammals it is total. The red blood-cells of the former possess a kernel, those of the latter do not. The hair of Mammals develops in closed follicles in the skin, but the feathers of birds and also the scales of reptiles develop in hillocks on the skin. The lower jaw of the latter is much more complicated than that of Mammals; the latter do not possess the quadrate bone of the former. Whereas in Mammals (as in the case of Amphibia) the connection between the skull and the first neck vertebra is formed by two knobbed joints, or condyles, in Birds and Reptiles those have become united into a single condyle. The two last classes may therefore justly be united into one group as Monocondylia, and contrasted to Mammals, or Dicondylia.

The deviation of Birds from Reptiles, in any case, first took place in the mesolithic epoch, and this moreover probably during the Trias. The oldest fossil remains of birds are found in the upper Jura (Archæopteryx). But there existed, even in the Trias period, different Saurians (Anomodonta) which in many respects seem to form the transition from the Tocosauria to the primary ancestors of Birds, the hypothetical Tocornithes. Probably these Tocornithes were scarcely distinguishable from other beaked lizards in the system, and were closely related to the kangaroo-like Compsognathus from the Jura of Solenhofen. Huxley classes the latter with the Dinosauria, and believes them to be the nearest relations to the Tocornithes.

The great majority of Birds—in spite of all the variety in the colouring of their beautiful feathery dress, and in the formation of their beaks and feet—are of an exceedingly uniform organization, in much the same way as are the class of insects. The bird form has adapted itself on all sides to the external conditions of existence, without having thereby in any way essentially deviated from the strict hereditary type of its characteristic structure. There are only two small groups, the feather-tailed birds (Saururæ) and those of the ostrich kind, which differ considerably from the usual type of bird, namely, from those with keel-shaped breasts (Carinatæ), and hence the whole class may be divided into three sub-classes.

The first sub-class, the Reptile-tailed, or Feather-tailed Birds (Saururæ), are as yet known only through a single, and that an imperfect, fossil impression, which, however, in being the oldest and also a very peculiar fossil bird, is of great importance. This fossil is the Primæval Griffin, or Archæopteryx lithographica, of which as yet only one specimen has been found in the lithographic slate at Solenhofen, in the Upper Jura system of Bavaria. This remarkable bird seems on the whole to have been of the size and form of a large raven, especially as regards the legs, which are in a good state of preservation; head and breast unfortunately are wanting. The formation of the wings deviates somewhat from that of other birds, but that of the tail still more so. In all other birds the tail is very short and composed of but few short vertebræ; the last of these have grown together into a thin, bony plate standing perpendicularly, upon which the rudder-feathers of the tail are attached in the form of a fan. The Archæopteryx, however, has a long tail like a lizard, composed of numerous (20) long thin vertebræ, and on every vertebra are attached the strong rudder-feathers in twos, so that the whole tail appears regularly feathered. This same formation of the tail part of the vertebral column occurs transiently in the embryos of other birds, so that the tail of the Archæopteryx evidently represents the original form of bird-tail inherited from reptiles. Large numbers of similar birds with lizard-tails probably lived during the middle of the secondary period; accident has as yet, however, only revealed this one fossil.

The Fan-tailed, or Keel-breasted birds (Carinatæ), which form the second sub-class, comprise all living Birds of the present day, with the exception of those of the ostrich kind, or Ratitæ. They probably developed out of Feather-tailed Birds during the first half of the secondary period, namely, in the Jura or Chalk period, by the hinder tail vertebræ growing together, and by the tail becoming shortened. Only very few remains of them are known from the secondary period, and these moreover only out of the last section of it, namely, from the Chalk. These remains belong to a swimming bird of the albatross species, and a wading bird like a snipe. All the other fossil remains of birds as yet known have been found in the tertiary strata.

The Bushy-tailed, or Ostrich-like Birds (Ratitæ), also called Running Birds (Cursores), the third and last sub-class, is now represented only by a few living species, by the African ostrich with two toes, the American and Australian ostrich with three toes, by the Indian cassowary and the four-toed kiwi, or Apteryx, in New Zealand. The extinct giant birds of Madagascar (Æpyornis) and the New Zealand Dinornis, which were much larger than the still living ostriches, also belong to this group. The Birds of the ostrich kind—by giving up the habit of flying, by the degeneration of the muscles for flying resulting from this, and of the breast bone which serves as their support, and by the corresponding stronger development of the hinder legs for running—have probably arisen out of a branch of the Keel-breasted birds. But possibly, as Huxley thinks, they may be the nearest relations of the Dinosauria and of the Reptiles akin to them, especially of the Compsognathus; at all events, the common primary form of all Birds must be looked for among the extinct Reptiles.