Partly laid open to show its chambered structure
(After Günther)
Between one class of animals and another the differences in the condition of the skin are more or less distinctive. In a few amphibians there are some bones in the skin on the under side of the body, though the skin is usually naked, and in frogs is said to transmit air to the blood, so as to exercise a respiratory function of a minor kind. This naked condition, so unlike the armoured skin of the true Reptilia, appears to have been paralleled by a number of extinct groups of fossils of the Secondary rocks, such as Ichthyosaurs and Plesiosaurs, which were aquatic, and probably also by some Dinosauria, which were terrestrial.
Living reptiles are usually defended with some kind of protection to the skin. Among snakes and lizards the skin has commonly a covering of overlapping scales, usually of horny or bony texture. The tortoise and turtle tribe shut up the animal in a true box of bone, which is cased with an armour of horny plates. Crocodiles have a thick skin embedding a less continuous coat of mail. Thus the skin of a reptile does not at first suggest anything which might become an organ of flight; and its dermal appendages, or scales, may seem further removed from the feathers which ensure flying powers to the bird than from the naked skin of a frog.
Although the mode of development of the young and the covering of the skin are conspicuous among important characters by which animals are classified, the brain is an organ of some importance, although of greater weight in the higher Vertebrata than in its lower groups. Reptiles have links in the mode of arrangement of the parts of their brains with fishes and amphibians. The regions of that organ are commonly arranged in pairs of nervous masses, known as (1) the olfactory lobes, (2) the cerebrum, behind which is the minute pineal body, followed by (3) the pair of optic lobes, and hindermost of all (4) the single mass termed the cerebellum. These parts of the brain are extended in longitudinal order, one behind the other in all three groups. The olfactory lobes of the brain in Fishes may be as large as the cerebrum; but among Reptiles and Amphibians they are relatively smaller, and they assume more of the condition found in mammals like the Hare or Mole, being altogether subordinate in size. And the cerebral masses begin to be wider and higher than the other parts of the brain, though they do not extend forward above the olfactory lobes, as is often seen in Mammals. In Crocodiles the cerebral hemispheres have a tendency to a broad circular form. Among Chelonian reptiles that region of the brain is more remarkable for height. Lizards and Ophidians both have this part of the brain somewhat pear-shaped, pointed in front, and elongated. The amphibian brain only differs from the lizard type in degree; and differences between lizards' and amphibian brains are less noticeable than between the other orders of reptiles. The reptilian brain is easily distinguished from that of all other animals by the position and proportions of its regions (see Fig. 19, p. 53).
Birds have the parts of the brain formed and arranged in a way that is equally distinctive. The cerebral lobes are relatively large and convex, and deserve the descriptive name "hemispheres." They are always smooth, as among the lower Mammals, and extend backward so as to abut against the hind brain, termed the cerebellum. This junction is brought about in a peculiar way. The cerebral hemispheres in a bird do not extend backward to override the optic lobes, and hide them, as occurs among adult mammals, but they extend back between the optic lobes, so as to force them apart and push them aside, downward and backward, till they extend laterally beyond the junction of the cerebrum with the cerebellum. The brain of a Bird is never reptilian; but in the young Mammal the brain has a very reptilian aspect, because both have their parts primarily arranged in a line. Therefore the brain appears to determine the boundary between bird and reptile exactly.
The breathing organs of Birds and Reptiles which are associated with these different types of brain are not quite the same. The Frog has a cellular lung which, in the details of the minute sacs which branch and cluster at the terminations of the tubes, is not unlike the condition in a Mammal. In a mammal respiration is aided by the bellows-like action of the muscles connected with the ribs, which encase the cavity where the lungs are placed, and this structure is absent in the Frog and its allies. The Frog, on the other hand, has to swallow air in much the same way as man swallows water. The air is similarly grasped by the muscles, and conveyed by them downward to the lungs. Therefore a Frog keeps its mouth shut, and the animal dies from want of air if its mouth is open for a few minutes.
Crocodiles commonly lie in the sun with their mouths widely open. The lungs in both Crocodiles and Turtles are moderately dense, traversed by great bronchial tubes, but do not differ essentially in plan from those of a Frog, though the great branches of the bronchial tubes are stronger, and the air chambers into which the lung is divided are somewhat smaller. The New Zealand Hatteria has the lungs of this cellular type, though rather resembling the amphibian than the Crocodile. The lungs during life in all these animals attain considerable size, the maximum dimensions being found in the terrestrial tortoises, which owe much of their elevated bulk to the dimensions of the air cells which form the lungs.
The lungs of Serpents and Lizards are formed on a different plan. In both those groups of reptiles the dense cellular tissue is limited to the part of the lung which is nearest to the throat. This network of blood vessels and air cells extends about the principal bronchial tube much as in other animals, but as it extends backward the blood vessels become few until the tubular lung appears in its hinder part, as it extends down the body, almost as simple in structure as the air bladder of a fish. Among Serpents only one of these tubular lungs is commonly present, and the structure has a less efficient appearance as a breathing organ than the single lung of the fish Ceratodus (Fig. 1). The Chameleons are a group of lizards which differ in many ways from most of their nearest kindred, and the lungs, while conforming in general plan to the lizard type in being dense at the throat, and a tubular bladder in the body, give off on both sides a number of short lateral branches like the fingers of a glove (Fig. 18, p. 51).
Thus the breathing organs of reptiles present two or three distinct types which have caused Serpents and Lizards to be associated in one group by most naturalists who have studied their anatomy; while Crocodiles and Chelonians represent a type of lung which is quite different, and in those groups has much in common. These characters of the breathing organs contribute to separate the cold-blooded armoured reptiles from the warm-blooded birds clothed with feathers, as well as from the warm-blooded mammals which suckle their young; for both these higher groups have denser and more elastic spongy lung tissue.
It will be seen hereafter that many birds in the most active development of their breathing organs substantially revert to the condition of the Serpent or Chameleon in a somewhat modified way. Because, instead of having one great bronchial tube expanded to form a vast reservoir of air which can be discharged from the lung in which the reptile has accumulated it, the bird has the lateral branches of the bronchial tubes prolonged so as to pierce the walls of the lung, when its covering membrane expands to form many air cells, which fill much of the cavity of the bird's body (see Fig. 16). Thus the bird appears to combine the characters of such a lung as that of a Crocodile, with a condition which has some analogy with the lung of a Chameleon. It is this link of structure of the breathing organs between reptiles and birds that constitutes one of the chief interests of flying reptiles, for they prove to have possessed air cells prolonged from the lungs, which extended into the bones.
Such are a few illustrations of ways in which reptiles resemble other animals, and differ from them, in the organs by means of which the classification of animals is made. But such an idea is incomplete without noticing that the bony framework of the body associated with such vital organs also shows in its chief parts that reptiles are easily recognised by their bones. I will therefore briefly state how reptiles are defined in some regions of the skeleton, for in tracing the history of reptile life the bones are the principal remains of animals preserved in the rocks; and the soft organs which have perished can only be inferred to have been present from the persistence of durable characteristic parts of the skeleton, which are associated with those soft organs in animals which exist at the present day, and are unknown in other animals in which the skeleton is different.
The manner in which the lower jaw is connected with the skull yields one of the most easily recognised differences between the great groups of vertebrate animals.
In Mammals.—In every mammal—such as the Dog or Sheep—the lower jaw, which is formed of one bone on each side, joins directly on to the head of the animal, and moves upon a bone of the skull which is named the temporal bone. This character is sufficient to prove, by the law of association of soft and hard parts of the body, that such an animal had warm blood and suckled its young.
Comparison to show the articulation with the lower jaw in a mammal and
Pterodactylus Kochi.
The quadrate bone is lettered Q in this Pterodactyle, and comes between the skull and
the lower jaw like the quadrate bone in a bird and in lizards.
In Birds.—In birds a great difference is found in this region of the head. The temporal bone, which it will be more convenient to name the squamosal bone, from its squamous or scale-like form, is still a part of the brain case, and assists in covering the brain itself, exactly as among mammals. But the lower jaw is now made up of five or six bones. And between the hindermost and the squamosal there is an intervening bar of bone, unknown among mammalia, which moves upon the skull by a joint, just as the lower jaw moves upon it. This movable bone unites with parts of the palate and the face, and is known as the quadrate bone. Its presence proves that the animal possessing it laid eggs, and if the face bones join its outer border just above the lower jaw, it proves that the animal possessed hot blood.
In Reptiles.—All reptiles are also regarded as possessing the quadrate bone. But the squamosal bone with which it always unites is in less close union with the brain case, and never covers the brain itself. Serpents show an extreme divergence in this condition from birds, for the squamosal bone appears to be a loose external plate of bone which rests upon the compact brain case and gives attachment to the quadrate bone which is as free as in a bird. Among Lizards the quadrate bone is usually almost as free. In the other division of existing Reptilia, including Crocodiles, the New Zealand lizard-like reptile Hatteria, called Tuatera, and Turtles, the squamosal and quadrate bones are firmly united with the bones of the brain case, face, and palate, so that the quadrate bone has no movement; and the same condition appears in amphibians, such as Toads and Frogs. With these conditions of the quadrate bone are associated cold blood, terrestrial life, and young developed from eggs.
In Fishes.—Bony fishes, and all others in which separate bones build up the skull, differ from Reptiles and Birds much as those animals differ from Mammals. The union of the lower jaw with the skull becomes complicated by the presence of additional bones. The quadrate bone still forms a pulley articulation upon which the lower jaw works, but between it and the squamosal bone is the characteristic bone of the fish known as the hyomandibular, commonly connected with opercular bones and metapterygoid which intervene, and help to unite the quadrate with the brain case. In the Cartilaginous fishes there is only one bone connecting the jaws with the skull on each side. This appears to prove that just as the structure of the arch of bones suspending the jaw may be complicated by the mysterious process called segmentation, which separates a bone into portions, so simplification and variation may result because the primitive divisions of the material cease to be made which exists before bones are formed.
The principal regions of the skull and skeleton all vary in the chief groups of animals with backbones; so that the Reptile may be recognised among fossils, even in extinct groups of animals and occasionally restored from a fragment, to the aspect which characterised it while it lived.
The nature of a reptile is now sufficiently intelligible for something to be said concerning flight, and structures by means of which some animals lift themselves in the air. It is not without interest to remember that, from the earliest periods in human records, representations have been made of animals which were furnished with wings, yet walked upon four feet, and in their typical aspect have the head shaped like that of a bird. They are commonly named Dragons.
The effigy of the dragon survives to the present day in the figure over which St. George triumphs, on the reverse of the British sovereign. In the luxuriant imaginations of ancient Eastern peoples, dating back to prehistoric ages, perhaps 5000 B.C., the dragons present an astonishing constancy of form. In after-times they underwent a curious evolution, as the conception of Babylon and Egypt is traced through Assyria to Greece. The Wings, which had been associated at first with the fore limb of the typical dragon, become characteristic of the Lion, and of the poet's winged Horse, and finally of the Human figure itself, carved on the great columns of the Greek temples of Ephesus. These flying animals are historically descendants of the same common stock with the dragons of China and Japan, which still preserve the aspect of reptiles. Their interest is chiefly in evidence of a latent spirit of evolution in days too remote for its meaning to be now understood, which has carried the winged forms higher and ever higher in grade of organisation, till their wings ceased to be associated with feelings of terror. The Hebrew cherubim are regarded by H. E. Ryle, Bishop of Exeter, as probably Dragons, and the figure of the conventional angel is the human form of the Dragon.
Turning from this reference to the realm of mythology to existing nature, the power of flight is popularly associated with all the chief types of vertebrate animals—fishes, frogs, lizards, birds, and mammals. Many of the animals ill deserve the name of flyers, and most are exceptions to different conditions of existence which control their kindred, but it is convenient to examine for a little the nature of the structures by which this movement in the air, which is not always flight, is made possible. Certain fishes, like the lung-fish Ceratodus, of Queensland, and the mud-fish Lepidosiren, are capable of leaving the water and living on land, and for a time breathe air. But neither these fishes nor Periophthalmus, which runs with rapid movement of its fins and carries the body more or less out of water, or the climbing perch, Anabas, carried out of water over the country by Indian jugglers, ever put on the slightest approach to wings.
The flight of fishes is a kind of parachute support not unlike that by which a folded paper is made to travel in the air. It is chiefly seen in the numerous species of a genus Exocœtus, allied to the gar-pike (Belone), which is common in tropical seas, and usually from a foot to eighteen inches long. They emerge from the water, and for a time support themselves in the air by means of the greatly developed breast fins, which sometimes extend backward to the tail fin. Although these fins appear to correspond to the fore limbs of other animals, they may not be moved at the will of the fish like the wing of a bird. When the flying fishes are seen in shoals in the vicinity of ships, those fins remain extended, so that the fish is said sometimes to travel 200 yards at a speed of fifteen miles an hour, rising twenty feet or more above the surface of the sea, travelling in a straight line, though sometimes influenced by the wind. Here the organ, which is at once a fin and a wing, consists of a number of thin long rods, or rays, which are connected by membrane, and vary in length to form an outline not unlike the wing of a bird which tapers to a point. The interest of these animals is chiefly in the fact that flight is separated from the condition of having lungs with which it is associated in birds, for although the flying fish has an air bladder, there is no duct to connect it with the throat.
The membranes of the foot and hand extend between the metatarsal and metacarpal bones, as well as the bones of the digits.
Among amphibians the organs of flight are also of a parachute kind, but of a different nature. They are seen in certain frogs which frequent trees, and are limited to membranes which extend between the diverging digits of the hand and foot, forming webs as fully developed as in the foot of a swimming bird. As these frogs leap, the membranes are expanded and help to support the weight of the body, so that the animal descends more easily as it moves from branch to branch. There is no evidence that the bones of the digits ever became elongated like the fin rays of the flying fish or the wing bones of a Bat; but the web suggests the basis of such a wing, and the possibilities under which wings may first originate, by elongation of the bones of a webbed hand like that of a Flying Frog.
The Reptilia in their several orders are remarkable for absence of any modification of the arms which might suggest a capacity for acquiring wings, as being latent in their organisation. Crocodiles, Tortoises, and Serpents are alike of the earth, and not of the air. But among Lizards there are small groups of animals in which a limited capacity for movement through the air is developed. It is best known in the family of small lizards named Dragons, represented typically by the species Draco volans found in the Oriental region of the East Indies and Malay Archipelago.
The organ of flight is produced in an unexpected way, by means of the ribs instead of the limbs. The ribs extend outward as far as the arms can stretch, and the first five or six are prolonged beyond the body so as to spread a fold of skin on each side between the arm and the leg. The membrane admits of some movement with the ribs. This arrangement forms a parachute, which enables the animal to move rapidly among branches of trees, extending the structure at will, so that it is used with rapidity too quick to be followed by the eye, as it leaps through considerable distances.
A less singular aid to movement in the air is found in some of the lizards termed Geckos. The so-called Flying Gecko (Platydactylus homalocephalus) has a fringe unconnected with ribs, which extends laterally on the sides of the body and tail, as well as at the back and front of the fore and hind limbs, and between the digits, where the web is sometimes almost as well developed as among Tree Frogs. This is essentially a lateral horizontal frill, extending round the body. Its chief interest is in the circumstance that it includes a membrane which extends between the wrist bones and the shoulder on the front of the arm. That is the only part of the fringe which represents the wing membrane of a bird. The fossil flying reptiles have not only that membrane, but the lateral membranes at the sides of the body and behind the arms.
Other lizards have the skin developed in the direction of the circumference of the body. In the Australian Chlamydosaurus it forms an immense frill round the neck like a mediæval collar. But though such an adornment might break a fall, it could not be regarded as an organ of flight.
The wings of birds, when they are developed so as to minister to flight, are all made upon one plan; but as examples of the variation which the organs contributing to make the fore limb manifest, I may instance the short swimming limb of the Penguin, the practically useless rudiment of a wing found in the Ostrich or Kiwi, and the fully developed wing of the Pigeon. The wings of birds obtain an extensive surface to support the animal by muscular movements of three modifications of structure. First, the bones of the fore limb are so shaped that they cannot, in existing birds, be applied to the ground for support and be used like the limbs of quadrupeds, and are therefore folded up at the sides of the body, and carried in an unused or useless state so long as the animal hops on the ground or walks, balancing its weight on the hind legs. Secondly, there are two small folds of skin, less conspicuous than those on the arms of Geckos; one is between the wrist bones and the shoulder, and the smaller hinder membrane is between the upper arm and the body. These membranous expansions are insignificant, and would in themselves be inadequate to support the body or materially assist its movements. Thirdly, the bird develops appendages to the skin which are familiarly known as feathers, and the large feathers which make the wing are attached to the skin covering the lower arm bone named the ulna, and the other bones which represent the wrist and hand. The area and form of the bird's wing are due to individual appendages to the skin, which are unknown in any other group of animals. Between the extended wing of the Albatross, measuring eleven feet in spread, and the condition in the Kiwi of New Zealand, in which the wing is vanishing, there is every possible variation in size and form. As a rule, the larger the animal the smaller is the wing area. The problem of the origin of the bird's wing is not to be explained by study of existing animals; for the rowing organ of the Penguin, which in itself would never suggest flight, becomes an organ of flight in other birds by the growth upon it of suitable feathers. Anyone who has seen the birds named Divers feeding under water, swimming rapidly with their wings, might never suspect that they were also organs of aerial flight. The Ostrich is even more interesting, for it has not developed flight, and still retains at the extremities of two of the digits the slender claws of a limb which was originally no wing at all, but the support of a four-footed animal (Fig. 46, p. 130).
Flight is also developed among mammals. The Insectivora include several interesting examples of animals which are capable of a certain motion through the air. In the tropical forests of the Malay Archipelago are animals known as Flying Squirrels, Flying Opossums, Flying Lemurs, Flying Foxes, in which the skin extends outward laterally from the sides of the body so as to connect the fore limbs with the hind limbs, and is also prolonged backward from the hind limbs to the tail. The four digits are never elongated; the bones of the fore limb are neither longer nor larger than those of the hind limb, and the foot terminates in five little claws as in other four-footed animals. This condition is adapted for the arboreal life which those animals live, leaping from branch to branch, feeding on fruits and leaves, and in some cases upon insects. These mammals may be compared with the Flying Geckos among reptiles in their parachute-like support by extension of the skin, which gives them one of the conditions of support which contribute to constitute flight.
Bats.—One entire order of mammals—the Bats—not only possess true wings, but are capable of flight which is sustained, and in some cases powerful. The wings are clothed with short hair like the rest of the body, and thus the instrument of flight is unlike that of a bird. The flight of a Bat differs from that of all other animals in being dependent upon a modification of the bones of the fore limb, which, without interfering with the animal's movements as a quadruped, secures an extension of the wing which is not inferior in area to that which the bird obtains by elongation of the bones of the arm and fore-arm and its feathers. The distinctive peculiarity of the Bat's wing is in the circumstance that four of the digits of the hand have their bones prolonged to a length which is often equal to the combined length of the arm and fore-arm. The bones of the digits diverge like the ribs of an umbrella, and between them is the wing membrane, which extends from the sides of the body outward, unites the fore limb with the hind limb, and is prolonged down the tail as in the Flying Foxes. Bats have a small membrane in front of the bones of the arm and fore-arm stretching between the shoulder and the wrist, which corresponds with the wing membrane of a bird; but the remainder of the membranes in Bats' wings are absent in birds, because their function is performed by feathers which give the wing its area. The elongated digits of the Bat's wing are folded together and carried at the sides of the body as though they were a few quill pens attached to its wrist, where the one digit, which is applied to the ground in walking, terminates in a claw.
The organs which support animals in the air are thus seen to be more or less dissimilar in each of the great groups of animals. They fall into three chief types: first, the parachute; secondly, the wing due to the feathers appended to the skin; and thirdly, the wing formed of membrane, supported by enormous elongation of the small bones of the back of the hand and fingers. The two types of true wings are limited to birds and bats; and no living reptile approximates to developing such an organ of flight as a wing. Judged, therefore, by the method of comparing the anatomical structures of one animal with another, which is termed "comparative anatomy," the existence of flying reptiles might be pronounced impossible. But in the light which the revelations of geology afford, our convictions become tempered with modesty; and we learn that with Nature nothing is impossible in development of animal structure.
Late in the eighteenth century, in 1784, a small fossil animal with wings began to be known through the writings of Collini, as found in the white lithographic limestone of Solenhofen in Bavaria, and was regarded by him as a former inhabitant of the sea. The foremost naturalist of the time, the citizen Cuvier—for it was in the days of the French Republic—in 1801, in lucid language, interpreted the animal as a genus of Saurians. That word, so familiar at the present day, was used in the first half of the century to include Lizards and Crocodiles; and described animals akin to reptiles which were manifestly related neither to Serpents nor Turtles. But the term saurian is no longer in favour, and has faded from science, and is interesting only in ancient history of progress. The lizards soon became classed in close alliance with snakes. And the crocodiles, with the Hatteria, were united with chelonians. Most modern naturalists who use the term saurian still make it an equivalent of lizard, or an animal of the lizard kind.
The remains are preserved with the neck arched over the back, and the jaws opened upward
Cuvier defined this fossil from Solenhofen as distinguished by the extreme elongation of the fourth digit of the hand, and from that character invented for the animal the name Pterodactyle. He tells us that its flight was not due to prolongation of the ribs, as among the living lizards named Dragons; or to a wing formed without the digits being distinguishable from each other, as among Birds; nor with only one digit free from the wing, as among Bats; but by having the wing supported mainly by a single greatly elongated digit, while all the others are short and terminate in claws. Cuvier described the amazing animal in detail, part by part; and such has been the influence of his clear words and fame as a great anatomist that nearly every writer in after-years, in French and in English, repeated Cuvier's conclusion, maintained to the end, that the animal is a saurian.
Reconstructed from the scattered bones in Fig. 14, showing the limbs on the left side
Long before fashion determined, as an article of educated belief, that fossil animals exist chiefly to bridge over the gaps between those which still survive, the scientific men of Germany were inclined to see in the Pterodactyle such an intermediate type of life. At first Sömmerring and Wagler would have placed the Pterodactyle between mammals and birds.
Showing positions of the wing membranes with the animal at rest
But the accomplished naturalist Goldfuss, who described another fine skeleton of a Pterodactyle in 1831, saw in this flying animal an indication of the course taken by Nature in changing the reptilian organisation to that of birds and mammals. It is the first flash of light on a dark problem, and its brilliance of inference has never been equalled. Its effects were seen when Prince Charles Bonaparte, the eminent ornithologist, in Italy, suggested for the group the name Ornithosauria; when the profound anatomist de Blainville, in France, placed the short-tailed animal in a class between Reptiles and Birds named Pterodactylia; and Andreas Wagner, of Munich, who had more Pterodactyles to judge from than his predecessors, saw in the fossil animal a saurian in transition to a bird.
But the German interpretation is not uniform, and Hermann von Meyer, the banker-naturalist of Frankfurt a./M., who made himself conversant with all that his predecessors knew, and enlarged knowledge of the Pterodactyles on the most critical facts of structure, continued to regard them as true reptiles, but flying reptiles. Such is the influence of von Meyer that all parts of the world have shown a disposition to reflect his opinions, especially as they practically coincide with the earlier teaching of Cuvier. Owen and Huxley in England, Cope and Marsh in America, Gaudry in France, and Zittel in Germany have all placed the Pterodactyles as flying reptiles. Their judgment is emphatic. But there is weight of competent opinion to endorse the evolutionary teaching of Goldfuss that they rise above reptiles. To form an independent opinion the modern student must examine the animals, weigh their characters bone by bone, familiarise himself, if possible, with some of the rocks in which they are found; to comprehend the conditions under which the fossils are preserved, which have added not a little to the interest in Pterodactyles, and to the difficulty of interpretation.
We may briefly recapitulate the geological history. Those remains of Ornithosaurs which have been mentioned, with a multitude of others which are the glory of the museums of Munich, Stuttgart, Tübingen, Heidelberg, Bonn, Haarlem, and London, have all been found in working the lithographic stone of Bavaria. The whitish yellow limestone forms low, flat-topped hills, now isolated from each other by natural denudation, which has removed the intervening rock. The stone is found at some distance north of the Danube, in a line due north of Augsburg, in the country about Pappenheim, and especially at the villages of Solenhofen, Eichstädt, Kelheim, and Nusplingen. These beds belong to the rocks which are named White Jura limestone in Germany, which is of about the same geological age as the Kimeridge clay in England. Much of it divides into very thin layers, and in these planes of separation the fossils are found. They include the Ammonites lithographicus and a multitude of marine shells, king crabs and other Crustacea, sea-urchins, and other fossils, showing that the deposit was formed in the sea. The preservation of jelly-fish, which so soon disappear when left dry on the beach, shows that the ancient calcareous mud had unusual power of preserving fossils. Into this sea, with its fishes great and small, came land plants from off the land, dragonflies and other insects, tortoises and lizards, Pterodactyles with their flying organs, and birds still clothed with feathers. Sometimes the wing membranes of the flying reptiles are found fully stretched by the wing finger, as in examples to be seen at Munich and in the Yale Museum in Newhaven, in America. At Haarlem there is an example in which the wing membrane appears to be folded much as in the wing of a Bat, when the animal hangs suspended, with the flying membrane bent into a few wide undulations.
The Solenhofen Slate belongs to about the middle period of the history of flying reptiles, for they range through the Secondary epochs of geological time. Remains are recorded in Germany from the Keuper beds at the top of the Trias, which is the bottom division of the Secondary strata; and I believe I have seen fragments of their bones from the somewhat older Muschelkalk of Germany.
In England the remains are found for the first time in the Lower Lias of Lyme Regis, in Dorset, and the Upper Lias of Whitby, in Yorkshire. In Würtemberg they occur on the same horizons. They reappear in England, in every subsequent age, when the conditions of the strata and their fossils give evidence of near proximity to land. In the Stonesfield Slate of Stonesfield, in Oxfordshire, the bones are found isolated, but indicate animals of some size, though not so large as the rare bones of reputed true birds which appear to have left their remains in the same deposit.
At least two Pterodactyles are found in the Oxford clay, known from more or less fragmentary remains or isolated bones; just as they occur in the Kimeridge Clay, Purbeck Limestone, Wealden sandstones, and especially in newer Secondary rocks, named Gault, Upper Greensand, and Chalk, in the south-east of England.
Owing to exceptional facilities for collecting, in consequence of the Cambridge Greensand being excavated for the valuable mineral phosphate of lime it contains, more than a thousand bones are preserved, more or less broken and battered, in the Woodwardian Museum of the University of Cambridge alone. To give some idea of their abundance, it may be stated that they were mostly gathered during two or three years, as a matter of business, by an intelligent foreman of washers of the nodules of phosphate of lime, which, in commerce, are named coprolites. He soon learned to distinguish Pterodactyle bones from other fossils by their texture, and learned the anatomical names of bones from specimens in the University Museum. This workman, Mr. Pond, employed by Mr. William Farren, brought together not only the best of the remains at Cambridge, but most of those in the museums at York and in London, and the thousands of less perfect specimens in public and private collections which passed through the present writer's hands in endeavours to secure for the University useful illustrations of the animal's structure. These fragments, among which there are few entire bones, are valuable, for they have afforded opportunities of examining the articular ends of bones in every aspect, which is not possible when similar organic remains are embedded in rock in their natural connexions.
In England Flying Reptiles disappear with the Chalk. In that period they were widely distributed, being found in Bohemia, in Brazil, and Kansas in the United States, as well as in Kent and other parts of England. They attained their largest dimensions in this period of geological time. One imperfect fragment of a bone from the Laramie rocks of Canada was described, I believe, by Cope, though not identified by him as Ornithosaurian, and is probably newer than other remains.
If this series of animals could all be brought together they would vary greatly in aspect and stature, as well as in structure. Some have the head enormously long, in others it is large and deep, characters which are shared by extinct reptiles which do not fly, and to which some birds may approximate; while in a few the head is small and compact, no more conspicuous, relatively, than the head of a Sparrow. The neck may be slender like that of a Heron, or strong like that of an Eagle; the back is always short, and the tail may be inconspicuous, or as long as the back and neck together. These flying reptiles frequently have the proportions of the limbs similar to those of a Bat, with fore legs strong and hind legs relatively small; while in some the limbs are as long, proportionately, and graceful as those of a Deer. With these differences in proportions of the body are associated great differences in the relative length of the wing and spread of the wing membranes.
The dimensions of the animals have probably varied in all periods of geological time. The smallest, in the Lithographic Slate, are smaller than Sparrows, while associated with them are others in which the drumstick bone of the leg is eight inches long. In the Cambridge Greensand and Chalk imperfect specimens occur, showing that the upper arm bones are larger than those of an Ox. The shaft is one and a half inches in diameter and the ends three inches wide. Such remains may indicate Pterodactyles not inferior in size to the extinct Moas of New Zealand, but with immensely larger heads, animals far larger than birds of flight.
The late Sir Richard Owen, on first seeing these fragmentary remains, said "the flying reptile with outstretched pinions must have appeared like the soaring Roc of Arabian romance, but with the features of leathern wings with crooked claws superinduced, and gaping mouth with threatening teeth." Eventually we shall obtain more exact ideas of their aspect, when the structures of the several regions of the body have been examined. The great dimensions of the stretch of wing, often computed at twenty feet in the larger examples, might lead to expectations of great weight of body, if it were not known that an albatross, with wings spreading eleven feet, only weighs about seventeen pounds.
There is only one safe path which the naturalist may follow who would tell the story of the meaning and nature of an extinct type of animal life, and that is to compare it as fully as possible in its several bones, and as a whole, with other animals, especially with those which survive. It is easy to fix the place in nature of living animals and determine their mutual relations to each other, because all the organs—vital as well as locomotive—are available for comparison. On such evidence they are grouped together into the large divisions of Beasts, Birds, and Reptiles; as well as placed in smaller divisions termed Orders, which are based upon less important modifications of fundamental structures. All these characteristic organs have usually disappeared in the fossil. Hence a new method of study of the hard parts of the skeleton, which alone are preserved, is used in the endeavour to discover how the Flying Reptile or other extinct animal is to be classified, and how it acquired its characters or came into existence.