There is little apparent resemblance between the elegant feathered warbler which makes the woods re-echo to its cheerful song, and the crawling reptile which is apt to inspire feelings of disgust when the more potent sensation of terror is absent—between the familiar Swallow, which builds its house of clay under the eaves of your roof, or the warbler whose nest, with its young progeny, carefully watched by the father of the brood in the silent watches of the night, is now threatened by the Serpent which has glided so silently into the bush, its huge mouth already open to swallow the whole family, while the despairing and fascinated parents have nothing but their slender bills to oppose to their formidable foe. "Placed side by side," says Professor Huxley, "a Humming-bird and a Tortoise, or an Ostrich and a Crocodile, offer the strongest contrast; and a Stork seems to have little but its animality in common with the Snake which it swallows." Nevertheless, unlike as they are in outward appearance, there is sufficient resemblance in their internal economy to bring them together in most attempts at a classification of the Animal Kingdom. The air-bladder which exists between the digestive canal and kidneys in some fishes, becomes vascular with the form and cellular structure of lungs in reptiles; the heart has two auricles, the ventricle in most is imperfectly divided, and more or less of the venous blood is mixed with the arterial which circulates over the body; but retaining their gills and being therefore transitional in structure, they are also cold-blooded. In birds, the lungs are spongy, the cavity of the air-bags becoming obliterated by the multiplication of vascular cellules; the heart is four-chambered, transmitting venous blood to the lungs, and pure arterial blood to the body; the temperature is raised and maintained at 90° to 100° Fahr.

Thus Reptiles, like Birds, breathe the common air by means of their lungs, but respiration is much less active. "Although," remarks Professor Owen, "the heart of Birds resembles in some particulars that of Reptiles, the four cavities are as distinct as in the Mammalia, but they are relatively stronger, their valvular mechanism is more perfect, and the contractions of this organ are more forcible and frequent in birds, in accordance with their more extended respiration and their more energetic muscular action." It is true, as Professor Huxley informs us, that the pinion of a bird, which corresponds with the human hand or the fore paw of a reptile, has three points representing three fingers: no reptile has so few.1 The breast-bone of a bird is converted into membrane-bone: no such conversion takes place in reptiles. The sacrum is formed by a number of caudal and dorsal vertebrÆ. In reptiles the organ is constituted by one or two sacral vertebrÆ.

In other respects the two classes present many obvious differences, but these are more superficial than would be suspected at first glance. And Professor Huxley believes that, structurally, "reptiles and birds do really agree much more closely than birds with mammals, or reptiles with amphibians."

While most existing birds differ thus widely from existing reptiles, the cursorial or struthious genera, comprising the Ostrich, Nandou, Emu, Cassowary, Apteryx, and the recently extinct Dinornis of New Zealand, come nearer to the reptiles in structure than any others. All of these birds are remarkable for the shortness of their wings, the absence of a crest or keel upon the breast-bone, and some peculiarities of the skull, which render them more peculiarly reptilian. But the gap between reptiles and birds is only slightly narrowed by their existence, and is somewhat unsatisfactory to those who advocate the development theory, which asserts that all animals have proceeded, by gradual modification, from a common stock.

Traces had been discovered in the Mesozoic formations of certain Ornitholites, which were too imperfect to determine the affinities of the bird. But the calcareous mud of the ancient sea-bottom, which has hardened into the famous lithographic slate of Solenhofen, revealed to Hermann von Meyer, in 1861, first the impression of a feather, and, in the same year, the independent discovery of the skeleton of the bird itself, which Von Meyer had named ArchÆopteryx lithographicus. This relic of a far-distant age now adorns the British Museum.

On the other hand, the same writer points out certain peculiarities in the single reptile found also among the Solenhofen slates, which has been described and named Compsognathus longipes by the late Andreas Wagner. This reptile he declares "to be a still nearer approximation to the missing link between reptiles and birds," thus narrowing the gap between the two classes.

While we think it proper to point to these structural resemblances of one class of the animal creation to others very different in their external appearance, it is necessary to guard ourselves and our readers from adopting the inferences sometimes deduced from them; that "these infinitely diversified forms are merely the final terms in an immense series of changes which have been brought about in the course of immeasurable time, by the operation of causes more or less similar to those which are at work at the present day." Domestication and other causes have no doubt produced changes in the form of many animals; but none from which this inference can be drawn, except in the imagination of ingenious men who strain the facts to support a preconceived hypothesis. In spite of the innumerable forms which the pigeon assumes by cross-breeding and domestication, it still remains a pigeon; the dog is still a dog, and so with other animals. Nor does it seem to us to be necessary, or calculated to advance our knowledge in natural history, to form theories which can only disturb our existing systems without supplying a better. Systems are necessary for the purpose of arrangement and identification; but it should never be forgotten that all classifications are artificial—a framework or cabinet, into the partitions of which many facts may be stowed away, carefully docketed for future use. "Theories," says Le Vaillant, "are more easily made and more brilliant probably than observations; but it is by observation alone that science can be enriched." A bountiful Creator appears to have adopted one general plan in the organization of all the vertebrate creation; and, in order to facilitate their study, naturalists have divided them into classes, orders, and genera, formed on the differences which exist in the structure of their vital functions. The advantages of this are obvious, but it does not involve the necessity of fathoming what is unfathomable, of explaining what is to man inexplicable in the works of God.2

In previous volumes of this series3 we have endeavoured to give the reader some general notions of the form, life, and manners of the branches of the animal kingdom known as Zoophytes, Mollusca, Articulata, and Pisces. We now continue the superior sub-kingdom (to which the fishes also belong) of the Vertebrated Animals, so called from the osseous skeleton which encircles their bodies, in which the vertebral column, surmounted by the cranium, its appendage, forms the principal part.

The presence of a solid frame in this series of animals admits of their attaining a size which is denied to any of the others. The skeleton being organized in such a manner as to give remarkable vigour and precision to all their movements.

In the vertebrated animals the nervous system is also more developed. There is, consequently, a more exquisite sensibility in them than in the classes whose history we have hitherto discussed. They possess five senses, more or less fully developed, a heart, a circulation, and their blood is red.

We have now to deal with a class advanced above that of fishes, that of Reptilia, which is divided as follows:—

Amphibia—(Batrachia, Cuv.)

Animals having ribs or processes, or short, slight, and free vertebrÆ, forming a series of separate centrums, deeply cupped at both ends, one of which is converted by ossification in the mature animal into a ball, which may be the front one, as in the Surinam Toad, Pipa, or the hind ones in the Frogs and Toads, Rana. The skin is nude, limbs digitate, gills embryonal,—permanent in some, in most lost in metamorphosis,—to be succeeded by pulmonary respiration,—or both; a heart with one ventricle and two auricles. They consist of:—

Chelonia, or Turtles.

Distinguished by the double shield in which their bodies are enclosed, whether they are terrestrial, fresh-water, or marine.

Lacertilia.

Having a single transverse process on each side, single-headed ribs, two external nostrils, eyes with movable lids; body covered with horny, sometimes bony, scales.

Lacerta—the Monitors, Crocodiles, Lizards; having ambulatory limbs.
Anguis—Ophisaurus, Bimanus, Chalcides, Seps; limbs abortive; no sacrum.

Ophidia.

Having numerous vertebrÆ with single-headed hollow ribs, no visible limbs, eyelids covered by an immovable transparent lid; body covered by horny scales. It includes:—

Crocodilia.

Teeth in a single row, implanted in distinct sockets; body depressed, elongated, protected on the back by solid shield; tail longer than the trunk, compressed laterally, and furnished with crests above. The several families are:—