Dragons of the Air Part 3

Resemblances and differences in the bones are easily over-estimated in importance as evidence of pedigree relationship. The Mammalia show, by means of such skeletons as are exhibited in any Natural History Museum, how small is the importance to be attached to even the existence of any group of bones in determining its grade of organisation. The whole Whale tribe suckle their young and conform to the distinctive characters in brain and lungs which mark them as being mammals. But if there is one part of the skeleton more than another which distinguishes the Mammalia, it is the girdle of bones at the hips which supports the hind limbs. It is characterised by the bone named the ilium being uniformly directed forward. Yet in the Whale tribe the hip-girdle and the hind limb which it usually supports are so faintly indicated as to be practically lost; while the fore limb becomes a paddle without distinction of digits, and is therefore devoid of hoofs or claws, which are usual terminations of the extremities in mammals. Yet this swimming paddle, with its ill-defined bones--sometimes astonishing in number, as well as in fewness of the finger bones--is represented by the burrowing fore limb of the Mole, which lives underground; by the elongated hoofed legs of the Giraffe, which lives on plains; and the extended arm and finger bones of the Bat, which are equally mammals with the Whale. From such comparison it is seen that no proportion, or form, or length, or use of the bones of the limbs, or even the presence of limbs, is necessarily characteristic of a mammal. No limitation can be placed upon the possible diversity of form or development of bones in unknown animals, when they are considered in the light of such experience of varied structural conditions in living members of a single cla.s.s.

What is true for the limbs and the bony arches which support them is true for the backbone also, for the ribs, and to some extent for the skull. The neck in the Whale is shortened almost beyond recognition. In the Giraffe the same seven vertebrae are elongated into a marvellous neck; so that in the technical definition of a mammal both are said to have seven neck vertebrae. Yet exceptions show a capacity for variation.

One of the Sloths reduces the number to six, while another has nine vertebrae in the neck; proving that there is no necessary difference between a mammal and a reptile when judged by a character which is typically so distinctive of mammals as the number of the neck bones.

The skull varies too, though to a less extent. The Great Ant-eater of South America is a mammal absolutely without teeth. The Porpoises have a simple unvarying row of conical teeth with single roots extending along the jaw. And the dental armature of the jaws, and relative dimensions of the skull bones, exhibit such diversity, in evidence of what may be parted with or acquired, that recognition of the many reptilian structures and bones in the skull of Ornithorhynchus, the Australian Duckbill, demonstrates that the difficulties in recognising an animal by its bones are real, unless we can discover the Animal Type to which the bones belong; and that there is very little in osteology which may not be lost without affecting an animal's grade of organisation.

VARIATION IN SKIN COVERING OF MAMMALS

Even the covering of the body varies in the same cla.s.s, or even order of animals, so that the familiar growth on the skin is never its only possible covering. The Indian ant-eater, named Manis, which looks like a gigantic fir-cone, the Armadillo, which sheathes the body in rings of bone, bearing only a scanty development of hair, are examples of mammalian hair, as singular as the quills of a Porcupine, the horn of a Rhinoceros, or the growth of hair of varying length and stoutness on different parts of the body in various animals, or the imperfect development of hair in the marine Cetacea. Among living animals it is enough for practical purposes to say that a mammal is clothed with hair, but in a fossil state the hair must usually be lost beyond recognition from its fineness and shortness of growth.

VARIATION IN SKIN COVERING OF BIRDS

No Cla.s.s of living animals is more h.o.m.ogeneous than Birds; and well-preserved remains prove that, at least as far back in time as the Upper Oolites, birds were clothed with feathers of essentially the same mode of growth and appearance as the feathers of living birds. There may, therefore, be no ground for a.s.suming that the covering was ever different, though some regions of the skin are free from feathers. Yet the variations from fine under-down to the scale-like feathers on the wings of a Penguin, or the great feathers in the wings of birds of flight, or the double quill of the Ostrich group, are calculated to yield dissimilar impressions in a fossil state, even if the fine down would be preserved in any stratum.

VARIATION IN THE BONES OF BIRDS

Osteologically there is less variety in the skeleton of birds than in other great groups of animals. The existing representatives do not exhaust its capability for modification. The few specimens of birds. .h.i.therto found in the Secondary strata have rudely removed many differences in the bones which separated living birds from reptiles; so that if only the older fossil birds were known, and the Tertiary and living birds had not existed, a bird might have been defined as an animal having its jaw armed with teeth, instead of devoid of teeth; with vertebrae cupped at both ends, instead of with a saddle-shaped articulation which in front is concave from side from side, and convex from above downwards; in which the bones of the hand are separate, so that three digits terminating in claws can be applied to the ground, instead of the metacarpal bones being united in a solid ma.s.s with clawless digits; and in which the tail is elongated like the tail of a lizard. Yet the limits to variation are not to be formulated till Nature has exhausted all her resources in efforts to preserve organic types by adapting them to changed circ.u.mstances. Birds may be regarded theoretically as equally capable with mammals of parting with almost every distinctive structure in the skeleton by which it is best known.

Even the living frigate bird blends the early joints of the backbone into a compact ma.s.s like a sacrum. The Penguin has a cup-and-ball articulation in the early dorsal vertebrae, with the ball in front. And the genus Cypselus has the upper arm bone almost as broad as long, unlike the bird type. Such examples prove that we are apt to accept the predominant structures in an animal type as though they were universal, and forget that inferences based, like those of early investigators, on limited materials may be re-examined with advantage.

VARIATION IN THE BONES OF REPTILES

The true Reptilia, notwithstanding some strong resemblances to Birds in technical characters of the skeleton, display among their surviving representatives an astonishing diversity in the bony framework of the body, exceeding that of the mammalia. This unlooked-for capacity for varying the plan of construction of the skeleton is in harmony with the diversity of structure in groups of extinct animals to which the name reptiles has also been given. The interval in form is so vast between Serpent and Tortoise, and so considerable in structure of the skeleton between these and the several groups of Lizards, Crocodiles, and Hatteria, that any other diversity could not be more surprising. And the inference is reasonable that just as mammals live in the air, in the sea, on the earth, and burrow under the earth, similar modes of existence might be expected for birds and reptiles, though no bird is yet known to have put on the aspect of a fish, and no reptiles have been discovered which roamed in herds like antelopes, or lived in the air like birds or bats, unless these fossil flying animals prove on examination to justify the name by which they are known.

Comparative study of structure in this way demolishes the prejudice, born of experience of the life which now remains on earth, that the ideas of Reptile and of Flight are incongruous, and not to be combined in one animal. The comparative study of the parts of animals does not leave the student in a chaos of possibilities, but teaches us that organic structures, which mark the grades of life, have only a limited scope of change; while Nature flings away every part of the skeleton which is not vital, or changes its form with altering circ.u.mstances of existence, enforced by revolutions of the Earth's surface in geological time, in her efforts to save organisms from extinction and pa.s.s the grade of life onward to a later age.

The bones are only of value to the naturalist as symbols, inherited or acquired, and vary in value as evidence of the nature and a.s.sociation of those vital organs which differentiate the great groups of the vertebrata.

These distinctive structures, which separate Mammals, Birds, and Reptiles, are sometimes demonstrated by the impress of their existence left on the bones; or sometimes they may be inferred from the characters of the skeleton as a whole.

CHAPTER VII

INTERPRETATION OF PTERODACTYLES BY THEIR SOFT PARTS

THE ORGANS WHICH FIX AN ANIMAL'S PLACE IN NATURE

We shall endeavour to ascertain what marks of its grade of organisation the Pterodactyle has to show. The organs which are capable of modifying the bones are probably limited to the kidneys, the brain, and the organs of respiration. It may be sufficient to examine the latter two.

PNEUMATIC FORAMINA IN PTERODACTYLES

[Ill.u.s.tration: FIG. 15. HEAD OF THE HUMERUS OF THE PTERODACTYLE ORNITHOCHEIRUS

Showing position of the pneumatic foramen on the ulnar side of the bone as in a bird]

Hermann von Meyer, the historian of the Ornithosaurs of the Lithographic Slate, as early as 1837 described some Pterodactyle bones from the Lias of Franconia, which showed that air was admitted into the interior of the bones by apertures near their extremities, which, from this circ.u.mstance, are known as pneumatic foramina. He drew the inference, naturally enough, that such a structure is absolute proof that the Pterodactyle was a flying animal. It was not quite the right form in which the conclusion should have been stated, because the Ostrich and other birds which do not fly have the princ.i.p.al bones pneumatic.

Afterwards, in 1859, the larger bones which Professor Sedgwick, of Cambridge, transmitted to Sir Richard Owen established this condition as characteristic of the Flying Reptiles of the Cambridge Greensand. It was thus found as a distinctive structure of the bones both at the beginning and the close of the geological history of these animals. Von Meyer remarks that the supposition readily follows that in the respiratory process there was some similarity between Pterodactyles and Birds. This cautious statement may perhaps be due to the circ.u.mstance that in many animals air cavities are developed in the skull without being connected with organs of respiration. It is well known that the bulk of the Elephant's head is due to the brain cavity being protected with an envelope formed of large air cells. Small air cells are seen in the skulls of oxen, pigs, and many other mammals, as well as in the human forehead. The head of a bird like the Owl owes something of its imposing appearance to the way in which its ma.s.s is enlarged by the dense covering of air cells in the bones above the brain, like that seen in some Cretaceous Pterodactyles. Nor are the skulls of Crocodiles or Tortoises exceptions to the general rule that an animal's head bones may be pneumatic without implying a pneumatic prolongation of air from the lungs. The mere presence of air cells without specification of the region of the skeleton in which they occur is not remarkable. The holes by which air enters the bones are usually much larger in Pterodactyles than in Birds, but the entrance to the air cell prolonged into the bones is the same in form and position in both groups. So far as can be judged by this character, there is no difference between them. The importance of the comparison can only be appreciated by examining the bones side by side. In the upper arm bone of a bird, on what is known as the ulnar border, near to the shoulder joint, and on the side nearest to it, is the entrance to the air cell in the humerus. In the Pterodactyle the corresponding foramen has the same position, form, and size, and is not one large hole, but a reticulation of small perforations, one beyond another, exactly such as are seen in the entrance to the air cell in the bone of a bird, in which the pneumatic character is found. For it is not every bird of flight which has this pneumatic condition of the bones; and Dr. Crisp stated that quite a number of birds--the Swallow, Martin, Snipe, Canary, Wood-wren and Willow-wren, Whinchat, Glossy-starling, Spotted-fly-catcher, and Black-headed Bunting--have no air in their bones. And it is well known that in many birds, especially water birds, it is only the upper bones of the limbs which are pneumatic, while the smaller bones retain the marrow.

LUNGS AND AIR CELLS

[Ill.u.s.tration: FIG. 16. LUNGS OF THE BIRD APTERYX PARTLY OPENED ON THE RIGHT-HAND SIDE

The circles are openings of the bronchial tubes on the surface of the lung. The notches on the inner edges of the lungs are impressions of the ribs (After R. Owen)]

[Ill.u.s.tration: FIG. 17. THE BODY OF AN OSTRICH LAID OPEN TO SHOW THE AIR CELLS WHICH EXTEND THROUGH ITS LENGTH (After Georges Roche)]

It may be well to remember that the lungs of a bird are differently conditioned from those of any other animal. Instead of hanging freely suspended in the cone-shaped chamber of the thorax formed by the ribs and sternum, they are firmly fixed on each side, so that the ribs deeply indent them and hold them in place. The lungs have the usual internal structure, being made up of branching cells. The chief peculiarity consists in the way in which the air pa.s.ses not only into them, but through them. The air tube of the throat of a bird, unlike that of a man, has the organ of voice, not at the upper end in the form of a larynx, but at the lower end, forming what is termed a syrinx. There is no evidence of this in a fossil state, although in a few birds the rings of the trachaea become ossified, and are preserved. But below the syrinx the trachaea divides into two bronchi, tubes which carry the ringed character into the lungs for some distance, and these give off branches termed bronchial tubes, the finer subdivisions from which, in their cl.u.s.tered minute branching sacs, make up the substance of the lung.

There is nothing exceptional in that. But towards the outer or middle part of the ventral or under surface of the lungs, four or five rounded openings are seen on each side. Each of these openings resembles the entrance of the air cell into a bone, since it displays several smaller openings which lead to it. Each opening from the lung leads to an air cell. Those cells may be regarded as the blowing out of the membrane which covers the lungs into a film which holds air like a ma.s.s of soap bubbles, until the whole cavity of the body of a bird from neck to tail is occupied by sacculated air cells, commonly ten in number, five on each side, though two frequently blend at the base of the neck in the region of the #V#-shaped bone named the clavicle or furculum, popularly known as the merry-thought. Most people have seen some at least of these semi-transparent bladder-like air cells beneath the skin in the abdominal region of a fowl. The cells have names from their positions, and on each side one is abdominal, two are thoracic, one clavicular, and one cervical, which last is at the base of the neck. The clavicular and abdominal air cells are perhaps the most interesting. The air cell termed clavicular sends a process outward towards the arm, along with the blood vessels which supply the arm. Thus this air cell, entering the region of the axilla or arm-pit, enters the upper arm bone usually on its under side, close to the articular head of the humerus, and in the same way the air may pa.s.s from bone to bone through every bone in the fore limb. The hind limbs similarly receive air from the abdominal air cell, which supplies the femur and other bones of the leg, the sacrum, and the tail. But the joints of the backbone in front of the sacrum receive their air from the cervical air sac. The air cells are not limited to the bones, but ramify through the body, and in some cases extend among the muscles. A bird may be said to breathe not only with its lungs, but with its whole body. And it is even affirmed that respiration has been carried on through a broken arm bone when the throat was closed, and the bird under water.

Birds differ greatly in the extent to which the aircell system prolonged from the lungs is developed, some having the air absent from every bone, while others, like the Swift, are reputed to have air in every bone of the body.

Comparison shows that in so far as the bones are the same in Bird and Ornithosaur, the evidence of the air cells entering them extends to resemblance, if not coincidence, in every detail. No living group of animals except birds has pneumatic limb bones, in relation to the lungs; so that it is reasonable to conclude that the identical structures in the bones were due to the same cause in both the living and extinct groups of animals. It is impossible to say that the lungs were identical in Birds and Pterodactyles, but so far as evidence goes, there is no ground for supposing them to have been different.

THE LUNGS OF REPTILES

[Ill.u.s.tration: FIG. 18. THE SIDE OF THE BODY OF A CHAMELEON

Ribs removed to show the sacculate branched form of the lung]

There is nothing comparable to birds, either in the lungs of living reptiles or in their relation to the bones. The Chameleon is remarkable in that the lung is not a simple bladder prolonged through the whole length of the body cavity, as in a serpent, but it develops a number of large lateral branches visible when the body is laid open. Except near the trachaea, where the tissue has the usual density of a lizard lung, the air cell is scarcely more complicated than the air bladder of a fish, and does not enter into any bone of the skeleton. And although many fishes like the Loach have the swim bladder surrounded by bone connected with the head, it offers no a.n.a.logy to the pneumatic condition of the bones in the Pterodactyle.

THE FORM OF THE BRAIN CAVITY

[Ill.u.s.tration: FIG. 19. THE FORM OF THE BRAIN]

But the ident.i.ty of the pneumatic foramina in Birds and Flying Reptiles is not a character which stands by itself as evidence of organisation, for a mould of the form of the brain case contributes evidence of another structural condition which throws some light on the nature of Ornithosaurs. Among many of the lower animals, such as turtles, the brain does not fill the chamber in the dry skull, in which the same bones are found as are moulded upon the brain in higher animals. For the brain case in such reptiles is commonly an envelope of cartilage, as among certain fishes; and except among serpents, the Ophidia, the bones do not completely close the reptilian brain case in front. The brain fills the brain case completely among birds. A mould from its interior is almost as definite in displaying the several parts of which it is formed as the actual brain would be. And the chief regions of the brain in a bird--cerebrum, optic lobes, cerebellum--show singularly little variation in proportion or position. The essential fact in a bird's brain, which separates it absolutely from all other animals, is that the pair of nerve ma.s.ses known as the optic lobes are thrust out at the sides, so that the large cerebral hemispheres extend partly over them as they extend between them to abut against the cerebellum. This remarkable condition has no parallel among other vertebrate animals. In Fishes, Amphibians, Reptiles, and Mammals the linear succession of the several parts of the brain is never departed from; and any appearance of variation from it among mammals is more apparent than real, for the linear succession may be seen in the young calf till the cerebral hemispheres grow upward and lop backward, so as to hide the relatively small brain ma.s.ses which correspond to the optic lobes of reptiles, extending over these corpora-quadrigemina, as they are named, so as to cover more or less of the ma.s.s of the cerebellum. From these conditions of the brain and skull, it would not be possible to mistake a mould from the brain case of a bird for that of a reptile, though in some conditions of preservation it is conceivable that the mould of the brain of a bird might be distinguished with difficulty from that of the brain in the lowest mammals. Taken by itself, the avian form of brain in an animal would be as good evidence that its grade of organisation was that of a bird as could be offered.

THE BRAIN IN SOLENHOFEN PTERODACTYLES

It happens that moulds of the brain of Pterodactyles, more or less complete, are met with of all geological ages--Lia.s.sic, Oolitic, and Cretaceous. The Solenhofen Slate is the only deposit in Europe in which Pterodactyle skulls can be said to be fairly numerous. They commonly have the bones so thin as to show the form of the upper surface of the mould of the brain, or the bones have scaled off the mould, or remain in the counterpart slab of stone, so as to lay bare the shape of the brain ma.s.s.

In the Museum at Heidelberg a skull of this kind is seen in the long-tailed genus of Pterodactyles named Rhamphorhynchus. It shows the large rounded cerebral hemispheres, which extend in front of cerebral ma.s.ses of smaller size a little below them in position, which perhaps are as like the brain of a monotreme mammal as a bird.

The short-tailed Pterodactylus described by Cuvier has the cerebral hemispheres very similar to those of a bird, but the relations of the hinder parts of the brain to each other are less clear.

The first specimen to show the back of the brain was found by Mr. John Francis Walker, M.A., in the Cambridge Greensand. I was able to remove the thick covering of cellular bone which originally extended above it, and thus expose evidence that in the mutual relations of the fore and hind parts of the brain bird and ornithosaur were practically identical.

Another Cambridge Greensand skull showed that in the genus Ornithocheirus the optic lobes of the brain are developed laterally, as in birds. That skull was isolated and imperfect. But about the same time the late Rev. W. Fox, of Brixton, in the Isle of Wight, obtained from Wealden beds another skull, with jaws, teeth, and the princ.i.p.al bones of the skeleton, which showed that the Wealden Pterodactyle Ornithodesmus had a similar and bird-like brain. In 1888 Mr. E. T. Newton, F.R.S., obtained a skull from the Upper Lias, uncrushed and free from distortion. This made known the natural mould of the brain, which shows the cerebral hemispheres, optic lobes, and cerebellum more distinctly than in the specimens previously known. In some respects it recalls the Heidelberg brain of Rhamphorhynchus in the apparently transverse subdivision of the optic lobes, but it is unmistakably bird-like, and quite unlike any reptile.