Popular Science Monthly/Volume 13/October 1878/Bird or Reptile-Which?
|BIRD OR REPTILE-WHICH?|
TO most people it may appear not only easy enough to distinguish, but even a matter of some difficulty not to be able to identify, a bird from a reptile or from any other animal whatsoever. No one would hesitate for a moment to assign to the bird tribe, on seeing them even for the first time, forms differing from each other so much as the "wingless" apteryx of New Zealand and the strong-pinioned albatross; the marvelously tinted humming-bird and the raw-necked vulture; or the fleet ostrich and the stolid hornbill; for in each individual the eye at once perceives one character at least common to the whole assemblage which is wanting in all other groups. Yet the question to be discussed in this paper of bird or not-bird, and in particular of bird or reptile, is, as we shall see below, one not without serious difficulty.
In order to a more easy comprehension of the question, let us shortly, and with as few technicalities of expression as possible, pass in review the chief characters of the groups we have placed in apposition.
Birds may be characterized generally as feathered bipeds, whose mouth is modified into a longer or shorter beak incased in a horny sheath, sometimes serrated along the margin, but never presenting true teeth; whose fore-limbs assume the form of wings more or less developed, and having the hind-limbs supported on, at most, four toes, the innermost, however, in many birds being so imperfectly developed as not to reach the ground.
Every one who has handled a living bird knows that it is warm-blooded; and whoever, while not neglecting the "main chance," when dining on partridge or fowl, has nevertheless not been too absorbed to mark the prominent points that distinguished the skeletal remains of his feast from those of a hare, for instance, is aware that along the centre of the breastbone there runs a high crest for the attachment of the wing-muscles; that the collar-bones unite to form the bone of destiny with which he has been familiar from his youth as the "merry-thought;" that the haunch-bone, which incloses the bowels and gives attachment to the hind-limbs, differs from a higher quadruped's in being composed, not of two bones (each of which is in reality made up of three bones ossified together), one on each side articulating with yet separate from the spine, and touching each other in the median line beneath, but of these elements and several vertebræ in addition, consolidated into one, having the margins free and separated by a considerable space from each other below; and that instead of a tail, commonly so called, the rear of the spinal column is brought up by what is known as the "ploughshare" bone formed by the union of several of its segments into a terminal mass for the support of the rudder-quills and of the oil-gland. Several very marked characteristics are to be seen in the hind-limb, to which, without entering deeply into osteological details, we may draw attention. Opening into the hollow shafts of the stronger bones—a character common to those of the wing and parts of the spine—there are to be found small pores, the air-passages by which the air-sacs, themselves extensions of the air-tubes of the lungs, are prolonged into the bones. In the skull also we find numerous air-cavities; these, however, are filled, not from the lung air-system, but from the nasal and ear chambers. No one who has examined the leg-bone, often called the "drumstick" (technically known as the tibia), of a common fowl, can have failed to observe the great ridge, or prominent crest, on the front of its upper extremity, or how easily the pulley-shaped articular surface of its lower end separates off from the shaft in the young bird, especially if the bone has been boiled or macerated for some time in water. This peculiarity vanishes when the fowl attains to its full growth; but till then the separation remains, as if to assert the right of the extremity to be considered, what in reality it is, a separate and distinct bone, the sole representative of a colony of ossicles (corresponding to the bones of the heel in the human foot) once existing in its grandsires at this spot, which for reasons of expediency has here coalesced with its long neighbor. On its outer side the leg-bone is always accompanied by a very slender bone, known as the fibula, attached only at its upper end, tapering gradually to a point about the middle of its fellow. Lastly, to the leg-bone immediately succeeds the hock-bone, the beautiful conformation of whose lower end into the resemblance of a triple pulley, for the articulation of the toes, is a mark by which we can unhesitatingly say that it belonged to a bird.
Bearing in mind these peculiarities, for whose detection no very deep scrutiny is required, which are but a few, yet sufficient for our present purpose, of the more striking characteristics to which the members of the Avian family more or less closely conform, we shall now for a little turn our attention to that other division of the animal kingdom with which we have in the title of this article contrasted the bird.
The reptiles are a wonderfully interesting group on account not only of the marvelous variety of their habits and modes of life, but also of their manifold diversity of form. Our country, in common with the rest of Northern Europe, can claim to be the habitat of but few examples of this tribe, whose home is under warmer latitudes; and consequently only limited opportunities present themselves to the European student for becoming acquainted with their habits and animated forms, unless he happens to live within reach of the menageries of the Zoölogical Societies of London, Berlin, Paris, or Amsterdam; those who are unfortunately distant from such interesting educational centres must make their acquaintance in a mummified or skeletonized form in museums. It cannot but strike the visitor to any zoölogical collection where the vertebrated section is well represented that the cases devoted to the reptilian group contain forms so divergent as the tortoise and the lizard, the snake and the alligator. If, however, the eye be permitted to pass to the sections on either hand—on the one side, to the amphibious animals, such as the frogs and newts, and on the other, to the birds—it is impossible not to perceive that the contrast is very great. A careless or inexperienced classifier might, perchance, be tempted to relegate the lizard to a place among the amphibia, near to the newts, or vice versa; but the most unobservant of men could never locate a snake among the birds, nor set a turtle or a crocodile on the same shelf with the swallow or the golden-crested wren.
The first and lowest link of the reptilian segment in the great chain of animal existences commences just above the highest of the amphibian assemblage, and is constituted by the river and mud loving tortoises and the turtles of the warmer seas; while the highest now living embraced the Crocodilian family, in whose membership are included the alligators and jacars of the New World, the crocodiles of the Ganges and the gavials of Northern Africa. The gap between these extremes is filled up by various intermediate gradations. To the tortoises succeeds, according to our best classifiers, a powerful race of long-necked ancient mariners—the plesiosaurs—which hunted their prey by the sea-coasts of the geological middle ages, where they left their bones, the sole testimony to the existence of their race, which became extinct before the chalk-cliffs of England were completed, however long ago that may be. After them comes the large group of the true lizards, comprising, along with several extinct orders, the chameleons, the lizards, and the geckos, both the latter being familiar enough to Continental travelers on sunny spots in Southern Europe; the geckos, especially, attracting attention by their habit of running on ceilings and perpendicular walls, by their sucker-formed toes. The next cohort embraces the serpents—the pythons and boas, endued with a power of crushing almost unsurpassed in the animal kingdom; and the rattlesnakes and cobras, carrying swift and certain death in the lightning stroke of their head. The next place is assigned to the great fish-lizards, or ichthyosaurs, which frequented the deeper waters of the same seas as the plesiosaurs, of whose existence also all knowledge would have perished forever, since they died out leaving no representative to continue their line, had not the kindly mud of the bottom preserved for us fragments of their history in their disjointed bones. Advancing from these "dragons of the prime" we again reach the crocodiles, the most specialized of modern reptiles.
Although between the highest and the lowest of these forms there is nothing like the close bond of union which connects the most distantly related of the birds, yet these diverse families have many characters in common, separating them from the other divisions of the animal world. Their bodies are protected by modifications of the skin into scales, enormous rugosities of almost impenetrable horny plates or flat shields of various forms. No reptile ever has feathers, for, on account of a peculiarity of the circulation of the blood by which the aerated and unaërated portions mingle together, they are cold-blooded, and therefore do not require so heat-conserving a covering for the body. Most reptiles possess two pairs of legs, of which the fore limb conforms much more closely to the hind in structure than is the case between the anterior and posterior extremities of the bird. On these they crawl rather than walk, their bellies, which are dragged along the ground, assisting in the support of the body; some have both pairs adapted for aquatic life, while others are entirely devoid of progressional appendages. No member of the class can be called a true volant, notwithstanding that a few, such as the flying dragons of the Philippine Islands, are able, by means of membranous expansions of the skin, to sustain themselves in the air while passing from one tree or support to another. With the exception of the tortoises, the majority are carnivorous and possess powerful jaws set with strong, sharp teeth.
So much lies on the surface.
From an examination of the chief points of their internal framework we learn that the "collar-bones" do not unite to form a "merry-thought;" nor does the breastbone develop a median keel. In general the tail is more or less elongated, but its terminal segments do not unite to form a "ploughshare" bone. The leg-bone of the reptile differs from the bird's in having a well-developed fibula lying parallel to it throughout its whole length; it does not present a strongly-marked crest at its upper end, nor is the articular surface of the narrow lower extremity formed by the coalescence with the shaft of a separate bone into a pulley-shaped termination. The coalescence never takes place at all; but each retains a separate existence throughout life. In the situation of the hock in the bird the reptile has at least four distinct bones to which are articulated as many toes; and, lastly, the haunch-bone, instead of being a consolidated mass, is composed of two halves, one on each side, articulating with, but not united by, bony tissue to the spine, and meeting each other below—a character in which the struthious birds, such as the ostriches, agree. It may be remarked, also, that in their keelless breastbone, as well as in the disunion of their collar-bones, these birds present other similitudes to the reptiles.
Every student of osteology is well aware that all bones in their embryonic condition are composed of cartilage, wherein, as the animal grows older, bony spots or "centres" appear, whence the ossification spreads till the whole structure is converted into bone. Among the higher animals these centres are seen only during the earlier years of life, while with increasing age their outlines, becoming gradually fainter, are at length entirely lost. But among the reptiles many of the bones either continue throughout life with their component parts unsolidified together, or else indicate by clear marks their lines of union, so that it is always easy to tell the number and configuration of which each is composed.
Thus far the characters which separate a reptile from a bird stand so widely apart—the interval between the highest living crocodile and the nearest living bird (represented by such forms as the New Zealand kiwis, the mooruk of Australia, the cassowary of the Moluccas, and the rheas or ostriches of South America) is of such enormous magnitude— that it would seem needless to entertain any fear of mistaking a member of the former group for one of the latter. Meanwhile let us withhold any decided opinion.
On November 29, 1871, a letter to Prof. Dana, dated from San Francisco, written by Prof. O. C. Marsh, of Yale College, New Haven, Connecticut, announced the discovery of a portion of a large headless skeleton in the upper chalk formation of Western Kansas, consisting of the nearly entire posterior limbs, portions of the haunch-bones, several segments from the neck and tail of the spinal column, and numerous ribs all in excellent preservation. The long leg-bone exhibited on the front aspect of its upper extremity the large crest which, as we have already pointed out, is a remarkably Avian character; along its shaft lay a fibula developed as among the diving birds of the present day, to whose thigh-bone also that of the fossil bore considerable resemblance. The "hock-bone," in presenting a trifid pulley-shaped lower end, was birdlike; while in the oblique arrangement of these divisions it again claimed affinities with the divers, whose toes are articulated in this manner to facilitate the forward stroke of their feet through the water, The external division, however, which projects beyond the other two, and is twice the size of either, is developed in a way unknown in any recent or fossil bird, and the bones of the toe supported by it are peculiarly articulated to produce rigidity and prevent flexion, except in one direction, in order by the interlocking of the bones to increase the strength of the joints during the act of swimming; for the whole limb is unquestionably adapted for rapid motion through water. The haunch-bone presents some resemblance to what is seen among the reptiles, in the permanence as separate bones of some of the portions of which it is composed, and in its not being firmly joined to the spine by bony union as in ordinary birds.
The examination so far of these interesting remains proved that the skeleton was certainly a bird's. On comparing its various bones with the corresponding ones in existing representatives, its affinities, notwithstanding considerable divergences from all known recent or ancient species and genera, were evidently with the swimming-birds, of which it is the largest known exponent, and of these it most resembled the great northern diver, near which, for a time, it received a niche with the appellation of Royal Bird-of-the-Dawn (Hesperornis regalis).
On September 26, 1872, Silliman's American Journal of Science announced the disinterment of another skeleton from the chalk of Kansas, "one of the most interesting of recent discoveries in paleontology." The remains included, among other bones, a number of biconcave vertebræ, that is, having the bodies, or solid central piece of the spinal segments, cup-shaped at both ends, a configuration which obtains, as every one has observed, in the divisions of the backbone of the common cod. This characteristic of the spine is frequent enough among reptiles; but it never occurs among birds met with nowadays. If among them there be any tendency that way, as there is in a few birds, the concavity is invariably found in the posterior end, the rarest form of vertebræ among reptiles. "The neck, back, and tail vertebræ preserved, all show this character, the ends of their bodies (centra) resembling those in the plesiosaurs;" notwithstanding the strongly non-Avian description of the spine, all the other bones—the prominently keeled breastbone, the collar-bone united to form a "merry-thought," as well as the leg-and long wing-bones—exhibit those marks which we have found to be most typical of the bird tribe. The wings were large in proportion to the posterior extremities; and the lower end of the leg-bones is incurved as in swimming-birds. Prof. Marsh, therefore, judging from their relative proportion, concludes that the bones belonged to a bird about the size of a pigeon, in many respects resembling the aquatic birds. He has christened it Ichthyornis dispar.
In October of the same year this indefatigable geologist once more announced through the pages of Silliman a new "find" from his favorite and fruitful mine in Upper Kansas. This time it was "a new reptile from the cretaceous. . . a very small saurian, which differs widely from any hitherto discovered." The only remains found on this occasion were two lower jaws, nearly perfect, and with many of the teeth in good preservation. The jaws resemble in general form those of an extinct family of marine reptiles whose remains were first found in the chalk formation near Maestricht; but apart from their very diminutive size they present several features which no species of that group has been observed to possess. Noticeably, the teeth are implanted in distinct sockets, and are directed obliquely backward. There are apparently twenty in each jaw, all compressed, with very acute summits. Then there is no distinct groove on the inner surface of the jaws as in all known Mososauroids—as the family of Maestricht reptiles is named. "Clearly," says Prof. Marsh, "the specimen indicates a new genus."
A more careful removal of the surrounding shale brought to light a fact that enormously enhanced the importance and value of this "most interesting of recent discoveries in paleontology." The jaws, which had been accredited to "a new genus" of reptiles, belonged most undoubtedly, from the position in which they were found with reference to the other bones, to the Ichthyornis dispar, which owned the spine with double cup-shaped segments. Here was a dilemma! The ichthyornis had on what seemed reliable data been adjudged a bird; but not only was no bird ever known to have teeth set in sockets, but no bird had ever yet differed so far from its fellows as to affect teeth at all, not to mention the fact of its having resuscitated the fashion of a by-gone day in having its spinal vertebræ cupped at both ends. When it lived, was this creature, in which the types have become so strangely mixed, a reptile, or after all a bird? was a question that for a time made the brows of the philosophers anxious even in the midst of their happiness at the new discovery. They finally declared for the latter. There was, therefore, no resource left but to extend the boundaries which had hitherto confined the avian territory, and institute a new sub-class for its reception, whereat the ornithologists were greatly pleased and cordially welcomed the tooth bills among their feathered friends.
Among the treasures which on December 7, 1872, Prof. Marsh and his Yale College explorers brought back to New Haven, as the results of their autumn reaping among the Rocky Mountains, was the nearly entire skeleton, containing all the missing bones, of the royal hesperornis and of another bi-concave vertebrated bird.
The breastbone of the gigantic diver of the chalk is thin and weak, and entirely without a keel; in front it resembles the ostrich's or that of the apteryx of New Zealand—a group of birds presenting the greatest range in time and also the widest geographical distribution over the globe—but in some respects it approaches to the penguin's also. The wing-bones are diminutive, and the wings are rudimentary and useless as organs of flight. The bones that girdle the thigh clearly exhibit a resemblance to the corresponding bones of a cassowary; yet, although avian in type, they are peculiar and present some well-marked reptilian proclivities.
Furnished with these bones alone, and judging from his experience of bird architecture, in plan hitherto undeviated from, no ornithologist would have hesitated to relegate the remains to a place among the birds; and, had he been asked to restore the missing portions, he would in all probability have devised some cross between the corresponding parts of the divers, of the dabchicks (for their knee-cap resembles that of the hesperornis), and of the ostrich-like birds, adding thereto a tail somewhat after the model of the penguin's. Certain it is, however, he would never have approached the features presented by the actual bones. This primeval bird possessed a skull in its general form like that of the great northern diver, but with a less pointed beak. The jawbones, however, though they were originally covered with a horny bill as in ordinary birds, are widely different. They are massive and have throughout their length a deep groove which was thickly set with sharp-pointed teeth—evidence of carnivorous habits—their crowns covered with enamel and supported on stout fangs. In form of crown and base they most resemble the teeth of the reptiles found in the Maestricht beds, to which we have referred above, as well as in the method of replacement, for some of the teeth preserved have the crowns of their successors implanted in cavities in their fangs. This peculiarity in the manner of teeth-shedding is characteristic of some reptiles, each of whose teeth is merely a hollow cone tilled in the interior with a soft pulp which supplies the material for the external bony layer. When the tooth becomes worn and useless, a new one is formed beneath the shell of the first by the pulp in the interior, which gradually ousts the old from its socket. In addition to these, the hesperornis possessed other reptilian characters. While the formation of the spinal column in the neck and back is of the true avian type, the structure of the tail, where there have been discovered no fewer than twelve segments, is very peculiar, and differs entirely from anything hitherto seen in birds. The bones of its middle and posterior portions have very long and horizontally flattened processes which prevent all motion in a lateral direction: a peculiarity from which we may certainly infer that, like the beaver's, this appendage was moved vertically, and doubtless was an efficient aid in diving, perhaps compensating for want of wings, which the penguins use with such wonderful dexterity in swimming under water. The last three or four bones are firmly united together, forming a flat terminal mass analogous to, but quite unlike, the "ploughshare" bone of modern birds.
Here, again, is another form half doubtful whether to assume the reptilian or the avian garb, a protestant against the hard and fast lines within which the various groups of the animal kingdom have hitherto been confined. The hesperornis certainly approaches the ichthyornis so far as to come under the new sub-class instituted for the reception of that bird; but, inasmuch as it differs in having its teeth not in sockets but set in a groove, and since, rejecting the conservative bi-concavity in the matter of spinal segments, it has adopted a newer and more high-class "cut," it has been necessary to give to each the honor of heading a separate section.
Though no living bird has so long a tail as this bird-of-the-dawn, yet there was in 1862 disinterred from the lithographic slates of Solenhofen part of the skeleton of a feathered biped—the archæopteryx (the existence of which was foreshadowed by the discovery of a feather the year before), exhibiting in most of the bones preserved the marks of a true bird. In the length of its tail, however, it is peculiar. This appendage contains the enormous number of twenty distinct bones gradually decreasing in size to the last, and each supporting a pair of quill feathers. To the skeleton no head is attached; but a portion of a small separate jaw on the same slab has been the subject of much controversy as to whether it belongs to the accompanying bones or not. Hermann von Meyer, the illustrious anatomist and paleontologist, holds that there can be little doubt but that they are parts of one and the same skeleton. If this be so, these remains belonged to a toothed bird; and Prof. Marsh thinks that probably it possessed bi-concave segments in its backbone, indicating, therefore, some alliance with the ichthyornis. The structure of its wings, Prof. Huxley points out, differs in some very remarkable respects from that which they present in a true bird. In the archæopteryx the upper arm-bone is like that of a bird, and the two bones of the forearm are more or less like those of a bird; but the fingers, which in all modern avian representatives are fused together into one mass, are not bound together—they are free. What the number may have been is uncertain, but several, if not all, of them were terminated by a strong-curved claw; so that in the archæopteryx we have an animal which to a certain extent occupies a midway place between a bird and a reptile. It is a bird in so far as its foot and sundry other parts of its skeleton are concerned; it is essentially and thoroughly a bird by its feathers: but it is much more properly a reptile in the fact that the region which represents the hand has separate bones with claws resembling those which terminate the fore-limb of a reptile. Teeth and a long tail, moreover, have certainly been considered hitherto non-avian characteristics.
More recently in our own country there has been brought to light from the London clay, in the island of Sheppey, a skull with the margins of the jawbones armed with larger alternating with smaller denticulations. It has been submitted to the examination of Prof. Owen, facile princeps among the restorers of osteological remains, who concludes that it belonged "to a warm-blooded feathered biped with wings"—to a bird, in fact—"and further, that it was web-footed and a fish-eater, and that in the catching of its slippery prey it was assisted by the peculiar armature of its jaws." Many living birds, such as the mergansers or saw-bills, have denticulations on the borders of the horny covering of the bill; but no modern bird has ever the underlying bone elevated into ridges or denticulations like those seen in the London-clay fossil. On the palate, however, of the rare Phytotome, a South American perching bird belonging to the group of the Leaf cutters, which bears in its structure many "marks of ancientness," we find two rows of bony denticulations, the remains of what are apparently but recently lost teeth, if we calculate time by the geological horologe, and which may be faint memorials of the dental arrangement seen in the chameleon. Certainly, "they are not the less of interest, seeing that as yet we have nothing else intervening between them and the teeth of the Sheppey fossil." How far this fossil may have resembled any of the avian remains which we have described above, we must wait to know. To conjecture would be dangerous, considering how wide of the mark would have been, in all likelihood, the restoration, had any been attempted, of the hesperornis, whose true structure when revealed so greatly surprised the most experienced naturalists. All that can at present be said is, that the owner of the solitary skull could not have claimed a place within the old avian province. It is interesting, however, in affording a suggestion as to the possible steps by which the toothbills, as regards the armature of their jaws, may have passed into modern toothless birds.
The Stonesfield slates have yielded up an almost entire skeleton of a wonderful extinct form, unique as yet, described under the name of Compsognathus, which possesses a singularly long neck supporting a head whose structure is light, and, except in the possession of teeth, birdlike. Its anterior limbs are small, while the leg-bone of its very long hind-limb exhibits the prominent crest of which we have so often spoken, a ridge on its outer side for the fibula, and the pulley-shaped articular surface of its lower end identical in conformation with that seen in the bird. This skeleton diverges from the bird type, however, in the absence of a "merry-thought," and in having the single hock-bone of the bird replaced by three distinct bones, fitting immovably together, of which the trifid extremity of a fowl's, for example, indicates the coalescence. The haunch-bone, moreover, indicates relationship with the reptiles, in its form and in the manner in which it unites below with its fellow of the opposite side—a feature in which it agrees with the arrangement of the corresponding bone in the crocodiles and in the rheas. This strange creature, bird or reptile, "must, without doubt," Prof. Huxley remarks, "have hopped or walked on its hind limbs after the manner of a bird, to which its long neck, slight head, and small anterior limbs, must have given it an extraordinary resemblance." There is reason for believing that it was possessed of a long tail, which must have greatly helped to support it in the erect position.
The extinct group to which this singular Stonesfield fossil has been assigned contains some of the largest known terrestrial animals, such as the carnivorous giant-lizard (Megalosaurus), thirty feet in length, whose structure in many points resembles that of the bird, especially in the form of its hip-girdle and hind-limbs, on which, in the late Prof. Phillips's opinion, it moved with free steps, sometimes, if not habitually, claiming a curious analogy, if not some degree of affinity, with the ostrich. Another example is the still more gigantic herb-eating iguanodon, from beds in Sussex, taller than an elephant and vaster in size, wherein, also, are mingled avian and reptilian characters. In the form of its vertebra, which, except in the neck, are double-cup-shaped, it is reptilian—in the absence of collar-bones it is non-avian; but in the formation of its three-toed hind limbs, which are larger than the fore, as well as of the supporting haunch-bone, it is distinctly birdlike. Again, it is unbirdlike in regard to its teeth, whose general form and crenated edges are somewhat like the iguanas', which now frequent the tropical woods of America and of the West Indies; but they differ from them in having a flat surface on the crown of the tooth, worn down evidently by the process of mastication, whereas the herbivorous reptiles of the present day clip and gnaw off, but do not chew, the vegetable productions on which they feed.
On the same sands at Hastings there have been found large impressions of the three-toed foot of some biped, the length of whose stride was so great that it is impossible not to conclude that they were made by the hind-feet of one or other of the seventy monsters whose bones have been found scattered about within the narrow area of what was once the banks and delta of a great Wealden river, and which, like the giant-lizards, probably walked occasionally, if not always, on their hind limbs with their fore-feet elevated in front. The question again arises, nor is it easy to answer, whether these forms should be called reptilian birds or avian reptiles.
In the northern gallery of the British Museum there is a very instructive specimen of a reptile, the frilled lizard of Australia, caught near Port Nelson while perching on the stem of a tree. Its long tail recalls at once the same appendage in the kangaroos, inasmuch as by its position in the stuffed specimen the creature would seem to use it as a support to its body. Its fore feet are much smaller than its hind, and an Australian resident, to whom the specimen was shown in presence of Dr. Günther and himself—so Dr. Woodward tells in a paper read before the Geological Society remarked—that it not merely sits up occasionally, but habitually runs on the ground on its hind-legs without allowing its fore-paws to touch the earth. The edges of its jawbones are elevated into enamel-tipped denticulations, which remind us of those in the jawbone of the Sheppey fossil. In the same slates which have given us the long-tailed reptilian bird and the long-necked, birdlike lizard, there has been found a three-toed bipedal track which "reminded me," said Dr. Woodward, "at once of what the frilled lizard or the compsognathus might produce under favorable conditions. The slab presents a median track formed by the tail drawn along on the ground; the two hind-feet with outspread toes leave their mark, while the forepaws just touch the ground, leaving a dot-like impression on either side of the median line. The median track is alternately stronger and fainter. Since the tail of the archæopteryx is bordered all the way by feathers, it will at once be seen that it could not leave behind a clean, simple furrow, but a broad smudge composed of many lines, while the tail of a lizard, progressing by hops and supporting itself on its hind limbs and tail, would produce just such an impression.
There is yet another interesting group of extinct forms to which we would refer shortly, termed "winged reptiles, or flying dragons." In the Woodwardian Museum, at Cambridge, there is a large collection of these bones, belonging to many species, from the soft marl in the neighborhood of that town, about which there have been entertained the most diverse opinions by the most eminent naturalists. They have been variously held to belong to bats, to forms between birds and mammals, to reptiles, and even to dolphins. Prof. Huxley finds in them great resemblances to birds; Prof. Owen thinks that they are reptilian remains; while Prof. Seeley, judging from the form of the cranium, is of opinion that these flying dragons "had a brain indistinguishable from a bird's."
They are all remarkable for their great proportionate length of head and neck, for in some the lizard-like and in others the birdlike length of tail, and for the large size of the fore-limb, which, quite unlike the same extremity in a bird, was terminated by four digits, whereof three were clawed, while the clawless fourth or little finger was enormously elongated to support the outer edge of an expansion of the integument like the wing of a bat. The bones of the hind-limb and of the haunch differ widely from the bird type; nevertheless, air-passages, such as characterize no other kind of skeleton, are met with in the bones of the head, of the spine, and of the fore and hind limbs, often coinciding identically in situation with those in birds, and indicate, according to Prof. Seeley, a system of air-circulation from the lungs similar to what is found in birds. From this he argues the existence in these gigantic volants of warm blood, and of a heart similar to the bird's in construction. They have the breastbone broad, strongly keeled, and unlike that of other reptiles; there is evidence also that the jaws were incased in a horny sheath. On these considerations, therefore, it is held that, as far as the skeleton indicates, their differences from birds are much less than the differences between the several orders of mammals or reptiles. The same paleontologist has made careful casts of the interior of the skull, and, from the position of certain lobes whose distance or proximity distinguishes the brains of modern birds and reptiles, he says in an interesting paper on the subject in the Linnæan Society's "Transactions" for 1876: "The resemblance of form and arrangement of parts between this fossil animal's brain and the brain of a bird amounts, as far as the evidence goes, to absolute identity; no more perfect specimen could add to the force of the conclusion that its brain is an avian brain of a typical structure. Since brain and lungs are organs of incomparably greater value in questions of organization than fore and hind limbs—organs in which, according to Prof. Huxley, they depart most widely from the bird type—the flying dragons on the whole are very reptilian birds rather than very avian reptiles."
The Solenhofen stone preserves not only bones and hard parts, but even the cutaneous characters of its old inhabitants. It shows casts of the down and feathers, impressions of the fine foldings or wrinkles of thin expansions of naked skin, as well as of delicate tendons. Prof. Owen, therefore, thinks that if the flying dragons had possessed any plumose clothing it would in all probability have been preserved, and, as no such indications (but contrariwise, several genera undoubtedly had their body-covering hardened into bony scales, sometimes produced into prodigious spines) have been discovered, though the Oolitic mud has entombed the greatest number and variety of these beings, he concludes that they were cold-blooded, as other reptiles are; whereas, if they had been warm-blooded, they would have possessed feathers, as their contemporary the archæopteryx did; for the constant correlative structure with hot-bloodedness is a non-conducting covering for the body. Prof. Huxley, on the other hand, differing from this anatomist, thinks that, judging from the air-passages in their bones, they were warm-blooded, but that, nevertheless, they were reptiles with special modifications for special purposes.
It would, therefore, appear that we are again face to face with a group which the most eminent authorities are far from agreed whether to regard as reptiles or as birds.
We have now passed in review various remarkable forms—living birds and living reptiles, separated by an immeasurable distance from each other, and forms which have so mingled the characters of both as to present great difficulties to their being included among the members of either group. Starting from the groveling crocodile, we have seen that there existed gigantic crocodile-like forms, such as the giant-lizard and the iguanodon, that walked, sometimes at least, oil their hind limbs; others, like the long-necked, long-tailed compsognathus from the Solenhofen slates, that hopped on the ground after the manner of a bird; then "flying dragons," with birdlike brain and bones that cleft the air with their twenty-feet expanse of wing; next, undoubted birds, with toothed bills, the one with reptilian vertebræ, the other with a beaver-like tail; while last of all, omitting the imperfectly known Sheppey fossil, the feathered archæopteryx whose twenty caudal segments bar its entrance to every existing family of birds.
Without by any means asserting—what is not only far from being ascertained fact, but is indeed very improbable; for we are not in a position to state that they appeared on the earth intermediately between the two groups—that these forms are the direct terms in the series of progressions from reptiles to birds, we can, in their intelligent contemplation, without overstraining the imagination or violating our reason, picture still more modified forms wherein the reptilian and the avian types would so harmoniously blend that we should find it impossible to say, "At this point the line between reptiles and birds must be drawn." There can be no reasonable doubt but that the remains, which only through the circumstance of a happy burial have been preserved to us from the second great era of the world's history till now, are no more than a very few examples with many a blank between of the fauna which has lived and died, whose tombs no man knoweth. Moreover, it seems easy enough to believe, after studying these forms, that, could any human eye have followed from that day to this the waxing and waning of the various animal groups, he could have constructed for us a marvelous chain of existences between reptiles and birds, the conformation of whose unknown links we can almost fabricate in our minds, between which no abrupt transitions harshly jarring would occur, no stepping-stones too wide to stride across; and, handing on to us, besides, the traditions of a still earlier time, he could have pictured to us the whole of living Nature, each varied offshoot fitly joined together, sloping gently back along the vast converging lines of ordinary generation to one grand starting-point, wherein till the fullness of time every living thing, from the microscopic diatom to the giant sequoia, and from the shapeless amœba to the stateliest of bipeds—
Sleeps in the plain eggs of the nightingale."