Popular Science Monthly/Volume 24/March 1884/The Ancestry of Birds
| THE ANCESTRY OF BIRDS. |
By Professor GRANT ALLEN.
SEATED on the dry hill-side here, by the belted blue Mediterranean, I have picked up from the ground a bit of blanched and moldering bone, well cleaned to my hand by the unconscious friendliness of the busy ants; and looking closely at it I recognize it at once, with a sympathetic sigh, for the solid welded tail-piece of some departed British tourist swallow. He came here like ourselves, no doubt, to escape the terrors of an English winter: but among these pine-clad Provençal summits some nameless calamity overtook him, from greedy kestrel or from native sportsman, and left him here, a sheer hulk, for the future contemplation of a wandering and lazy field-naturalist. Fit text, truly, for a sermon on the ancestry of birds; for this solid tail-bone of his tells more strangely than any other part of his whole anatomy the curious story of his evolution from some primitive lizard-like progenitor. Close by here, among the dry rosemary and large-leaved cistus by my side, a few weathered tips of naked basking limestone are peeping thirstily through the arid soil; and on one of these gray lichen-covered masses a motionless gray lizard sits sunning his limbs, in hue and spots just like the lichen itself, so that none but a sharp eye could detect his presence, or distinguish his little curling body from the jutting angles of the rock, to which it adapts itself with such marvelous accuracy. Only the restless sidelong glance from the quick upturned eye suffices to tell one that this is a living animal and not a piece of the lifeless stone on which it "rests like a shadow." A very snake the lizard looks in outline, with only a pair of sprawling fore-legs and a pair of sprawling hind-legs to distinguish him outwardly from his serpentine kin. Yet from some such lizard as this, my swallow and all other birds are ultimately descended; and from such a little creeping four-legged reptile science has to undertake the evolutionary pedigree of the powerful eagle or the broad-winged albatross.
Reptiles are at present a small and dying race. They have seen their best days. But in the great secondary age, as Tennyson graphically puts it, "A monstrous eft was of old the lord and master of earth." At the beginning of that time the mammals had not been developed at all; and even at its close they were but a feeble folk, represented only by weak creatures like the smaller pouched animals of Australia and Tasmania. Accordingly, during the secondary period, the reptiles had things everywhere pretty much their own way, ruling over the earth as absolutely as man and the mammals do now. Like all dominant types for the time being, they split up into many and various forms. In the sea, they became huge paddling enaliosaurians; on the dry land, they became great erect dinosaurians; in the air, they became terrible flying pterodactyls. For a vast epoch they inherited the earth; and then at last they began to fail, in competition with their own more developed descendants, the birds and mammals. One by one they died out before the face of the younger fauna, until at last only a few crocodiles and alligators, a few great snakes, and a few big turtles, remain among the wee skulking lizards and geckos to remind us of the enormous reptilian types that crowded the surface of the liassic oceans.
Long before the actual arrival of true birds upon the scene, however, sundry branches of the reptilian class had been gradually approximating to and foreshadowing the future flying things. Indeed, one may say that at an early period the central reptilian stock, consisting of the long, lithe, four-legged forms like the lizards, still closely allied in shape to their primitive newt-like and eel-like ancestors, began to divide laterally into sundry important branches. Some of them lost their limbs and became serpents; others acquired bony body-coverings and became turtles; but the vast majority went off in one of two directions, either as fish-like sea-saurians, or as bird-like land-saurians. It is with this last division alone that we shall have largely to deal in tracing out the pedigree of our existing birds. Their fossil remains supply us with many connecting links which help us to bridge over the distance between the modern representatives of the two classes. It is true, none of these links can be said to occupy an exactly intermediate place between reptiles and birds; none of them can be regarded as forming an actual part of the ancestry of our own swallows and pigeons: they are rather closely related collateral members of the family than real factors in the central line of descent. But they at least serve to show that, at and before the period when true birds first appeared upon earth, many members of one great reptilian group had made immense advances in several distinct directions toward the perfected avian type.
Clearly, the first step toward the development of a bird must consist in acquiring a more or less upright habit: for the legs must be well differentiated into a large hind pair and a free fore pair, before the last can be further specialized into feathered wings; and the body must have acquired a forward poise before flying becomes a possible mode of locomotion. Such an upright habit is first foreshadowed in the larger-limbed and longer-legged lizards like the dinosaurians, which walked to some extent erect, and more particularly in some highly specialized reptiles like the iguanodon, which had large hind legs and small fore-legs, and could walk or hop on the hind-legs alone, much after the fashion of a kangaroo, or still more of a jerboa or a chinchilla. Now, it is noticeable that the tendency to acquire the most rudimentary form of flying is common among animals of this semi-erect habit, especially when they frequent forests and jump about much from tree to tree. For example, among our modern mammals, the squirrels are a race much given to sitting on their hind-legs and using their paws as hands; while they are also much accustomed to jumping lightly from bough to bough: and some among them, the flying squirrels, have developed a sort of parachute consisting of an extensible skin between the fore and hind legs, which they use to break their fall in descending to the ground. Again, among the lower monkey-like animals, the so-called flying lemur or galeopithecus has hit upon an exactly similar plan; while, in the bats, a membrane which may be fairly called a wing has been evolved to a very high degree of perfection. Everywhere, the habit of living among trees or jumping from rocks tends to produce either parachute or wing-like organs; and in our own time the tendency is very fully displayed among a large number of forestine mammals.
During the secondary ages, however, it was the reptiles which took to thus developing a rudimentary flying-mechanism. Even at the present day there are some modern lizards, the "flying-dragons" of popular natural history, which possess a parachute arrangement of the front ribs, and are so enabled to jump lightly from branch to branch, somewhat in the same manner as the flying-squirrels. But this is an independent and comparatively late development of a flying apparatus among the reptiles, quite distinct in character from those which were in vogue among the real and much more terrible flying-dragons of the liassic and oölitic age. Far the most remarkable of these predecessors of the true birds were the pterodactyls whose bones we still find in our English cliffs at Lyme Regis and Whitby; creatures with a large reptilian head, fierce jaws set with sharp-pointed teeth, and fore-arms prolonged into a great projecting finger so as to support a membranous wing or fold of skin, somewhat analogous to that of the bats. The pterodactyls do not stand anywhere in the regular line of descent toward the true birds; but they are interesting as showing that a general tendency then existed among the higher reptiles toward the development of a flying organ. In these frightful dragons, the organ of flight is formed by an immense prolongation of the last finger on each fore-leg, to a length about as great as that of the rest of the leg all put together. Between this long bony finger and the hind-leg there stretched, in all probability, a featherless wing like a bat's, by means of which the pterodactyl darted through the air and pounced down upon its cowering victims. As in birds, the bones were made very light, and filled with air instead of marrow; and all the other indications of the skeleton show that the creatures were specially designed for the function of flight. Imagine a cross between a vulture and a crocodile, and you have something like a vague mental picture of a pterodactyl.
But at the very time when the terrestrial reptilian type was branching out in one direction toward the ancestors of the pterodactyls, it was branching out in another direction toward the ancestors of the true birds. In the curious lithographic slate of Solenhofen we have preserved for us a great number of fossil forms with an extraordinary degree of perfection; and among these are several which help us on greatly from the reptilian to the avian structure. The lithographic slate is a member of the upper oölitic formation, and it is worked, as its name implies, for the purpose of producing stones for the process of lithography. But the same properties which make the slate in its present condition take so readily the impress of a letter or a sketch made it in its earlier condition take the impress of the various organisms imbedded as they fell in its soft mud. Even the forms and petals of early flowers washed down by floods into the half-formed mud-bank have been thus preserved for us with wonderful minuteness. Most interesting of all for our present purpose, however, are the bones of contemporary reptiles and birds which this Nature-printing rock incloses for the behoof of modern naturalists. One such reptile, known as compsognathus, may be regarded as filling among its own class the place filled among existing mammals by the kangaroo. It was a rather swan-like, erect saurian, standing gracefully on its hind-paws, with its fore-legs free, and probably dragging its round tail behind it on the ground as a support to steady its gait. The neck was long and arched, and the head small and bird-like in shape; but the jaws are armed with sharp and powerful teeth, as in the pterodactyls. Altogether, compsognathus must have looked in outward appearance not at all unlike such birds as the auks and penguins, though its real structural affinities lie rather with the emus and cassowaries. The apteryx or kiwi of New Zealand, which is a bird that does not fly, because it has no wings worth mentioning to fly with, approaches even nearer in the combination of both points to this very bird-like oőlitic reptile.
Even compsognathus himself, however, though very closely allied to the true birds, can not be held to stand as an actual point in the progressive pedigree, because in the very same Solenhofen slates we find a real feathered bird in person. Accordingly, as the two were thus contemporaries, the one could not possibly be the direct ancestor of the other. Nevertheless, it is certainly from some form very closely resembling compsognathus that the true birds are descended. We have only to suppose such a reptile to acquire forestine habits, and to begin jumping freely from tree to tree, in order to set up the series of changes by which a true bird might be produced. But the first historical bird of which we know anything, the archæopteryx of the Solenhofen slate, still remains in many points essentially a reptile. It is only bird-like in two main particulars; its possession of rudimentary wings and its possession of feathers. From the popular point of view, these two particulars are decisive in favor of its being considered a bird; but its anatomical structure is sufficient to make it at least half a reptile; and eminent authorities have differed (with their usual acrimony) as to whether it ought properly to be called a bird-like saurian or a lizard-like bird. There is nothing like a mere question of words such as this to set scientific men or theologians roundly by the ears for half a century together.
Archæopteryx, then, is just compsognathus provided with rude wings and feathers, but in most other respects a good lizard. Unlike all modern birds, it has a long tail composed of twenty separate vertebræ; and opposite each vertebra stand two stout quill-feathers, so that instead of forming a fan, as in our own pigeons and turkeys, they form a long pinnate series like the leaflets of yonder palm-branch. These feathers, like all others, show traces of their origin from the scales of lizards. Moreover, in the jaw are planted some small conical teeth, the like of which of course exist in no living bird. The skeleton is for the most part reptilian; and, though the legs are bird-like, they are not much more so than those of compsognathus, an unmixed reptile. Even the wings are more like the fore-legs, and could only be used for flight by the aid of a side membrane. Accordingly, we may say that we have lithographed for us in archæopteryx a specimen of the intermediate state, when reptiles were just in the very act of passing into birds. The scales and protuberances on the body had already developed into feathers; the fore-legs had already developed into rude and imperfect wings, and the feet had become decidedly bird-like; but as yet there was only a very small breast-bone, the tail remained in internal structure like that of a lizard, the jaws still contained pointed teeth, and the wing ended in a three-toed hand, while flight was probably as rudimentary as in the flying-lemur and the flying-squirrel. Nowhere in the organic series has geology supplied us with a better missing link than this uncouth and half-formed creature, Nature's first tentative rough draft of the beautiful and exquisitely adapted modern birds.
Such an animal, once introduced, was sure to undergo further modification, to fit it more perfectly for its new sphere of action. In the first place, the tail was sure to grow shorter and shorter, by stress of natural selection, because a more fan-like organ would act better as a rudder to steer the flight than the long lizard-like tail of archæopteryx. In the second place, the general bony structure was sure to grow better adapted for flight, by the development of some such feature as the keeled breast-bone, and the general modification of the other parts (especially the wing) into better correspondence with their new function. At the same time, it must not be supposed that all intermediate birds would lose their reptilian features equally and symmetrically. Some for a time might retain one lizard-like peculiarity, say the teeth, and some might retain another, say sundry anatomical points in the structure of the skeleton. It was long indeed before the whole tribe of birds acquired the entire set of traits which we now regard as characteristic of their class. During the intervening period they kept varying in all directions, tentatively, if one may say so, and thus the early forms of birds differ far more among themselves than do any modern members of the feathered kingdom. In other words, when the full bird type was finally evolved, it proved so much better adapted to its airy mode of life than any other and earlier creature that it lived down not only the rude reptilian pterodactyls but also the simpler primeval forms of birds themselves: exactly as civilized European man is now living down not only the elephants and buffaloes but the red Indian and the Australian black fellow as well.
Some of the varying primeval forms have been preserved for us as fossils in the chalk deposits of the Western States, which are of course later in date than the oölitic slates of Solenhofen, where we find the compsognathus and his cousin the archæopteryx. One of these first sketches, the ichthyornis, has a row of teeth in each jaw, and displays another strikingly early reptilian or fish-like peculiarity in the joints of its backbone, which are cup-shaped or hollow on either side, exactly like those of a cod. This strange bird must have resembled an emu in many respects, and it might easily have devoured the large ganoid fish of this period with its formidable jaws. Still more reptilian in some particulars is the hesperornis, also found in the Western American chalk. Hesperornis was a huge swimming ostrich, and it had pointed teeth like a crocodile's, set in a groove running down the jawbone. They were supported on stout fangs, in the same way as the teeth of its reptilian allies, the mosasaurians. Like the ostrich, hesperornis had a broad breast-bone, but this breast-bone was destitute of a keel, as is still the case in all the ostrich family. The wings were also very imperfect, like those of the cassowaries. In its tail, hesperornis resembled its predecessor, archæopteryx, so far as regards the lizard-like separateness of the vertebræ, except at the extreme end, where they were slightly massed together into the first resemblance of a plowshare-bone, such as the one I hold in my hand. Thus these two intermediate birds of the chalk period, though slightly more bird-like than their cousins of the oölitic age, still retained, each in its own way, many unmistakable relics of their descent from reptilian or almost amphibian ancestors. As usual, the further back we go, the more do we find all the lines converging toward a common center.
The primitive teeth died slowly and gradually out as time went on. In the still later eocene deposits of the London clay in the Isle of Sheppey, we find the remains of a true bird, known as odontopteryx, in which the teeth have entirely coalesced with the beak, and have assumed the form of bony projections. Strict biologists will tell us that these projections are not teeth at all, because true teeth are not bony in structure, and are developed from the skin of the gums. But such hair-splitting distinctions are of little value from the evolutionary point of view; the really important fact to observe is this, that while hesperornis has teeth in a groove, reptile-fashion, ichthyornis has teeth in distinct sockets, mammal-fashion, and odontopteryx has them reduced to bony projections from the bill, in a fashion all its own, thus leading the way to modern birds, in which the teeth are wholly wanting and the bill alone remains. Indeed, among our existing kinds there are some which still keep up some dim memory of the odontopteryx stage; for the merganser, a swimming fish-eating bird, has bony ridges on its bill, which help it to grasp its prey; and the South American leaf-cutter has a double set of bony bosses on its beak and palate.
The most apparently distinctive feature of birds lies in the fact that they fly. It is this that gives them their feathers, their wings, and their peculiar bony structure. And yet, truism as such a statement sounds, there are a great many birds that do not fly: and it is among these terrestrial or swimming kinds that we must look for the nearest modern approaches to the primitive bird type. From the very beginning, birds had to endure the fierce competition of the mammals, which had been developed at a slightly earlier period; and they have for the most part taken almost entirely to the air, where alone they possess a distinct superiority over their mammalian compeers. There are certain spots, however, where mammals have been unable to penetrate, as in oceanic islands; and there are certain other spots which were insulated for a long period from the great continents, so that they possessed none of the higher classes of mammals, as in the case of Australia, South America, New Zealand, and South Africa. In these districts, terrestrial birds had a chance which they had not in the great circumpolar land tract, now divided into two portions, North America on the west, and Asia and Europe on the east. It is in Australia and the southern extremities of America and Africa, therefore, that we must look for the most antiquated forms of birds still surviving in the world at the present day.
The decadent and now almost extinct order of struthious birds, to which ostriches and cassowaries belong, supplies us with the best examples of such antique forms. These birds are all distinguished from every other known species, except the transitional Solenhofen creature and a few other old types, by the fact that they have no keel to the flat breast-bone—a peculiarity which at once marks them out as not adapted for flight. Every one whose anatomical studies have been carried on as far as the carving of a chicken or a pheasant for dinner knows that the two halves of the breast are divided by a sharp keel or edge protruding from the breast-bone; but in the ostrich and their allies such a keel is wanting, and the breast-bone is rounded and blunt. At one time these flat-chested birds were widely distributed over the whole world; for they are found in fossil forms from China to Peru; but, as the mammalian race increased and multiplied and replenished the earth, only the best adapted keeled birds were able to hold their own against these four-legged competitors in the great continents. Thus the gigantic ostriches of the Isle of Sheppey and the great divers of the Western States died slowly out, leaving all their modern kindred to inhabit the less progressive southern hemisphere alone. Even there, the monstrous æpyornis, a huge, stalking, wingless bird, disappeared from Madagascar in the tertiary age, while the great moa of New Zealand, after living down to almost historical times, fell a victim at last to that very aggressive and hungry mammal, the Maori himself. This almost reduces the existing struthious types to three small and scattered colonies, in Australasia, South Africa, and South America respectively, though there are still probably a few ostriches left in some remote parts of the Asiatic Continent.
The Australian ostrich kind are in many respects the most archaic and peculiar of all. Strangest among them is the kiwi or apteryx of New Zealand, that almost wholly wingless bird who may be seen any morning at the Zoo, gravely stalking up and down, like an important political prisoner, within the small inclosure to which tyrannical circumstances have temporarily confined him. The kiwi has feathers which closely resemble hair in texture, and his wings are so very rudimentary that they can only be properly observed at a post-mortem examination. His bones have no air-canals, and some of his internal anatomy is very abnormal. The cassowaries of the Papuan district are somewhat more bird-like in type, but they also preserve many antique features, especially in the relative smallness of the head and brain compared with the general size of the whole body. The Australian emus approach more closely to the true ostriches, and their feathers are far more feathery than those of the cassowary. In both these classes, however, the small and functionless wings are destitute of plumes, which are only represented by a few stiff, horny shafts. The true ostriches, including both the familiar African species and the South American rheas, have real wings with real feathers in them, though they can only use them to aid them in running, and not for the purpose of flight. They are therefore the most bird-like of their order, with small wings and very feathery plumes. We may fairly regard all these keelless and often almost wingless birds—the kiwis, cassowaries, emus, and ostriches—as the last survivors of a very ancient group, immediately descended from ancestors not unlike the toothed hesperornis, and never forced by circumstances to develop into the full avian type represented by the swallows, hawks, and herons. All of them are strictly terrestrial in their habits; none of them can fly in even the slightest degree; and the feathers of the most developed among them invariably lack the tiny barbules or small hooks which bind together the cross-barbs in the feathers of the flying bird, so as to form a compact and resisting blade. It is this looseness of the cross-barbs which gives ostrich-plumes their light and fluffy appearance; while, pushed to an extreme in the cassowary and the kiwi, it makes the plumage of those ugly birds approximate in character to the hair of mammals. Though from the human and decorative point of view we may admire the fluffiness of ostrich-plumes, it is obvious that, looked upon as a question of relative development, such loose, floating barbs are far less advanced in type than the firm and tightly interlocked quill-feathers of a goose or a raven, with which alone sustained flight is possible.
Except in such isolated countries where higher mammals do not, or did not till lately, exist, the power of flight, once acquired, was sure to be developed in a high degree. For the possession of feathers gives birds an advantage in this respect which enables even the little sparrows to hold their own in the midst of our crowded cities. Hence all other modern birds, except these lingering, ostrich-like creatures, have keeled breast-bones, which imply their descent from forms adapted to true flight. They are linked to the ostriches, however, and therefore to the still earlier toothed ancestral types, by the South American tinamous, which are intermediate in various anatomical points (too intricate for a lazy man to go into here and now), between the two classes. Put briefly, one may say that these partridge-like Paraguayan birds are ostriches in the bones of their head, but game-birds in those of the breast and body. This line of descent seems to lead us up directly toward the cocks and hens, the pheasants, and the other scrapers. There are more marks of a primitive organization, however, among the penguins, which are almost wingless swimming birds, belonging nearly to the same class as the ducks and geese; and we have reason otherwise to consider the penguins a very early form, since fowls resembling them in many particulars have been unearthed in the upper greensand. Here the wings are reduced to small rudiments, covered with bristly, scale-like feathers, and so rigid that they can be only moved in the mass like fins by a single joint at the base. They are used, in fact, exactly in the same way as the flappers in seals, to assist the bird in diving. The habitual erect attitude of the penguins strongly recalls that of their reptilian ally, compsognathus. From such an incomplete form as this, the gap is not great to the equally erect auks, the guillemots, the grebes, and other web-footed divers, which have short, pointed wings with true quills, but without any extended power of flight. Some species, indeed, can not fly at all, though the puffins and many other kinds can steer their way through the air with comparative ease. Thence to the cormorants, gulls, and ducks the transitions are slight and easy. We are thus led insensibly from almost wingless erect birds, like the penguins, through winged, but mainly swimming forms like the auks and divers, to creatures with such marvelous powers of flight as the frigate-birds, the petrels, and the albatrosses, which pass almost their whole life upon the wing. It must be remembered, however, that in this line of descent the comparatively wingless forms must be regarded as somewhat degenerate representatives of flying ancestors; for the presence of a keeled breast-bone almost conclusively proves hereditary connection with fully-winged progenitors.
By far the greater number of modern birds belong to the still more strictly aërial orders of the perchers, the peckers, and the birds of prey. In almost all these cases, the power of flight is highly developed, and the bird type reaches its highest ideal point of typical excellence. Among the perchers, this perfection of form is best seen in the swallows, whose ceaseless and graceful curved evolutions everybody has seen with his own eyes; while among tropical varieties of the same type the birds-of-paradise, the sun-birds, and the orioles are the most conspicuous. Among the peckers, our own swifts closely simulate the swallow type, while their American relatives, the humming birds, in spite of their small size, possess a power of rapid flitting and of lightly poising themselves in front of flowers which makes them in some ways the very fullest existing embodiment of the avian ideal. To the same order belong also those most intelligent of all birds, the parrots, whose large heads and crafty eyes mark them at once as the opposite pole from the small-browed, dull-eyed, stupid cassowaries. With them must be ranked the toucans, the barbets, the king-fishers, the trogons, and whole hosts of other beautiful southern creatures, among which the feathers have been variously modified into the most exquisite ornamental devices. As for the birds of prey, the eagles, vultures, falcons, hawks, owls, and ospreys must suffice by way of example.
Even among these central groups of birds, which have varied most and developed farthest from the primitive reptilian character, there are many kinds which retain here and there some small and isolated peculiarities of the ancestral forms. For example, among the duck-like birds, as we have already seen, a single group, that of the mergansers, still keeps up some faint memory of the original sharp teeth in the shape of a few horny projections along the edge of the beak. The tooth-billed pigeon of Samoa, a close relation of that early and extinct form the dodo, has also some rudiments of horny teeth; and the South-American leaf-cutters, a primitive set of songless perchers, possess somewhat similar relics of the lost fangs. So, too, our earliest known bird, the archæopteryx, had three free claws on its fore-limb or undeveloped wing; and traces of such claws turn up in sundry unconnected birds even now, no doubt by reversion to the almost forgotten ancestral type. In all modern birds, one of the three fingers which make up the pinion still remains free; and in some species this finger supports an evident claw, sometimes used as a spur for the purpose of fighting. In many thrushes a rudiment of this claw may be perceived in the shape of a small tubercle or knob at the end of the wing, thus pointing back directly to some remote four-footed and claw-bearing reptilian ancestor. Several plovers have spurs, and so has the spur-winged goose; while the horned screamer has two on each wing, which he uses with great effect in battling with his rivals. The Australian brush-turkeys have also the rudiment or last relic of a primitive pinion-claw.
There is another way in which modern birds still partially recall the peculiarities of their reptilian ancestors, and that is in the course of their individual development within the egg. No adult existing bird has all the bones of the tail distinct and separate, like those of the archæopteryx; the last joints are all firmly welded together into a solid expanded piece, known from its queer shape as a plow-share-bone, such as the one which I am holding in my hand as the text for this discourse. The use of the plowshare-bone is to support the fan-like quill-feathers of the tail, and also to shelter the oil-glands with whose contents the birds preen and dress their shining plumage, to secure them against the evil effects of damp or rain. But, while the young chick is in the egg, all its tail-bones still remain separate, as in the ancestral, lizard-like bird and the still earlier ancestral lizard; it is only as the development of the embryo progresses that they become firmly united, as in modern forms. In other words, every young bird begins forming its tail as if it meant to be an archæopteryx, and only afterward so far changes its mind as to become a crow or a sparrow. Similarly, no adult existing bird has true teeth; but the young of certain parrots show in the egg a set of peculiar little swellings inside the jaw, known as dental papillæ, and commonly found as the first stage of teeth in other animals. Moreover, these swellings are actually covered by a thin coat of dentine, the material of which true teeth are made. So here again the young parrot begins its develment as though it meant to start a set of conical fangs in its jaw, like those of the archæopteryx, but afterward changes its mind and contents itself with a bill instead. Such symptoms as these point back surely though remotely to a far-distant reptilian ancestry.
It is worth while noting, too, that the links which bind the birds to the reptiles bind them also in part to the lower mammals. For the lowest existing mammal is that curious Australian creature known to the rough-and-ready classification of the colonists as the water-mole, and rejoicing in the various scientific aliases of the ornithorhynchus and the duck-billed platypus. Unsophisticated English people know the animal best, however, as "the beast with a bill." Now, there are many close resemblances between this strange Australian burrower, on the one hand, and such antiquated forms of birds as the New Zealand kiwi, on the other. In many particulars, too, the water-mole recalls the structure of reptiles, and especially of the ichthyosaurus. In short, it is at once the most bird-like and the most reptile-like of mammals. Hence we may fairly conclude that birds and mammals are both descended by divergent lines from a single common reptilian ancestry. For, on the one hand, the kiwi, an early type of nocturnal bird, preserved for us in isolated New Zealand, has some marked reptilian and mammalian affinities, not only in the external character of its hair-like feathers, but also in the more important structural points of its diaphragm, its movable vertebræ, and its keelless breast-bone, which are questions rather for the professed anatomist than for mere idle loungers basking lazily in the sun on a Provençal hill-side. And, on the other hand, the ornithorhynchus, an early type of burrowing aquatic mammal, preserved for us in isolated Australia, has marked reptilian affinities in its bony structure, and in the teeth implanted on its tongue; while it has also marked resemblances to the ducks and other swimming birds in the external features of its horny bill and webbed feet, besides being still more closely related to them in many of its less obvious anatomical peculiarities.
Birds, then, may be roughly described as reptiles with feathers. Professor Huxley was the first to see the real closeness of the connection between the two groups, and to unite them under a common name as Sauropsida. Strictly speaking, the only constant difference between them, the only one distinctive character of birds as a class, is the possession of feathers; and, if, like uncompromising Karl Vogt, we insist upon calling archæopteryx a reptile, because of its anatomical peculiarities, even this solitary distinction must vanish utterly, leaving us no point of difference at all between the two classes. It must be remembered, of course, that all the other characters which we always have in our mind as part of the abstract idea of a bird are either not constant or not peculiar to birds alone. For instance, we usually think of a bird as a flying animal; but then, on the one hand, many birds, such as the ostriches, kiwis, penguins, and dodos, do not or did not fly at all; and, on the other hand, many other creatures, such as the bats, flying-squirrels, flying-lemurs, pterodactyls, dragon-lizards, and butterflies, do or did once fly just as much as the birds. So with their other peculiarities: their habit of laying eggs descends to them from fish and reptiles; their nest-building propensities, which are wanting in some birds, are found in the Australian water-mole, in field-mice, and even in stickleback; and their horny bill, which is almost confined to them, nevertheless occurs again in the ornithorhynchus and in many turtles. In short, every other apparently distinctive point about birds except the possession of feathers either breaks down on examination or else descends to them directly from early unbird-like ancestors. And the first feathered creature of which we know anything, archæopteryx, was at least as much of a reptile as of a bird.—Longman's Magazine.
