search of food. Most of them are capable of a lofty and longcontinued flight, as the pelican, petrel, cormorant, albatross, swan, wild goose and duck; whilst others, from the shortness of their wings, can scarcely raise themselves into the air, but are princi pally confined to the surface of the water, as the sea-diver, guillemot, penguin, and the auk.

There are some points of comparison between the several orders of Birds which are worthy of notice in connection with the views that have been already presented, and which serve further to illustrate them. The Accipitres (Fig. 23), for example, require great strength in the head, neck, and claws. These are accordingly short; for were the neck long, like that of most birds, with a large and strong head at the end of it, it would require an undue • proportion of muscular power to manage it. The same is true of the claws and legs. They are short or they could not have been strong; and in seizing and carrying away their prey they derive a great advantage from the mechanical contrivance formerly alluded to, which fixes them involuntarily to the roost while sleeping. When an eagle seizes his prey and wishes to rise with it into the air, he draws the body downward on the legs. This occasions the firm grasping of it by his talons, and he thus lifts and carries it off without any special care.

With the Accipitres may be contrasted the Waders, Grallæ, or Shore Birds. These are also birds of prey, but of so different a kind that they require a different structure. Their prey consists of animals that make little resistance and require no great strength to conquer, namely, animals inhabiting the water. They agree with the Accipitres in their great powers of wing, but stand at the opposite end of the scale in the conformation of their feet, legs, and neck. These are long, enabling them to wade in the water and then seize on their food at the bottom. The Accipitres require strength, without length of neck and legs, the Waders, length without great strength, in order to secure the prey on which they respectively feed. But although the Waders do not require strength to seize on their prey, they do need a power of standing for a long time in order to watch for it. Were this done by muscular power, their limbs must have been large and heavy, and thus the balance necessary to flight be destroyed. To avoid this there

is a simple contrivance at the kneejoint, by which the bones are as it were bolted together. They can thus stand a long time, even on one leg, without fatigue; and sometimes, when standing on one foot in the water, they hold a stone in the claws of the other to steady themselves against the motion of the waves.

Strength for standing can thus be given, but not strength for moving. Hence in the ostriches and cassowaries which are like the Waders in their general shape, but differ entirely in their habits, being intended to run instead of flying-the legs are not only very long but also very strong and large and endowed

with an immense quantity of muscular substance.

Fig. 30.

Heron.

These form a link between the Waders and the Gallinaceous birds, the peacock, turkey, and common fowl. In these the powers of motion are more equally distributed between the wings and legs, though the advantage is in many in favor of the latter, especially when domesticated. But in the Passeres or Sparrow tribe, who are both good flyers and good walkers, but not remarkable in either respect, though to this there are some exceptions, - there is a more equal division of power. Among the Climbers, too, the powers are very equally divided; but in the Palmipedes or web-footed birds a very different arrangement is necessary in the relative construction of the neck and legs.

The neck is longer than the legs and sometimes very much longer, and the latter are placed very far back in the body for the convenience of swimming. The long neck answers the same purpose as in the Waders, namely, to enable them to seek their prey at the bottom of shallow waters, over which, however, they swim, instead of wading through. In proportion as they are more perfectly adapted for swimming they are less so for walking, but not for flying, because the apparatus for flying is not interfered

with by the provision for swimming, whilst that for walking is. Hence they are notoriously awkward on land, though they are capable, many of them, of long flight, and move with great beauty and grace upon the water. They find it extremely difficult in walking to preserve their balance, and are only enabled to do so by throwing the body back upon the heels and arching the neck backward, so as to bring the weight of the head but little in advance of the body.

Auk.

In consequence of their peculiarities of structure, some of the birds of this order can maintain themselves on land only in a sort of erect posture, not like the gallinaceous birds on the toes, but by throwing themselves backward and resting upon the whole of the shank. They can hardly move upon land at all, and do not even have the power of flight, their motions being almost exclusively confined to the water, where, however, they swim and dive with great perfection, their stumps of wings serving the purpose of oars. Such are the guille

mots and auks.

SECTION III.

CLASS III. REPTILES.

The class of Reptiles, including the tortoises, lizards, serpents, toads, and frogs, have cold blood, and a circulation and respiration less perfect than those of the preceding classes. In Reptiles, only a part of the blood received from the body by the heart is sent to the lungs, to be subjected to the influence of the air; whilst the remainder, having been mixed with a portion which has undergone the change that takes place in respiration, is

:

returned again into the circulation. The first three orders of the animals of this class have two auricles to the heart, but only one ventricle into the left auricle, the red blood from the lungs is poured, and into the right, the black blood from the body. From the auricles, the two kinds of blood are immediately transferred to the ventricle, where they are mixed together; and this mingled mass is, by the contraction of the ventricle, sent through two distinct vessels, in part to the lungs, and in part to the body.

The vessels of Reptiles, then, are not filled with pure red blood, like those of the Mammalia and Birds, but with an imperfect fluid, not so well adapted to give them a high degree of life and vigor. Hence, as the animal heat is always in proportion to the quantity of respiration, they are cold-blooded. Their lungs are not so large; their circulation is more languid; they consume less air, and are capable of living for a longer time without it. They are, in general, sluggish and indolent in their habits of life, obtuse in their sensations, and slow in their digestion. In cold Fig. 32.

In this figure, the lower shell is removed so as to display the principal parts of the internal skeleton.

vc, vertebræ of the neck; vd, of the back; c, ribs; cs, sternal ribs or marginal pieces of the carapace; o, scapula; cl, clavicle; co, coracoid bone; b, pelvis; f, femur; t, tibia; p, fibula.

countries, they pass the greater part of the winter in a dormant state. Their brain is small, and their nervous system imperfect and of less influence than in the preceding classes. They produce their young by means of eggs, but take no pains themselves to hatch them. They have less intelligence, fewer faculties, and less instinct, than either quadrupeds or birds. They are arranged in four orders.

I. Chelonia, the Tortoises, are distinguished by the peculiar structure of their ribs, sternum, and vertebræ. These are so arranged as to form a complete covering, consisting of an upper and under shell, joined together at their sides; which permits only their head, tail, and four extremities, to be extended without it. The upper shell is formed by the extension and enlargement of the ribs and part of the backbone, and the lower shell by an alteration in the form of the sternum; called respectively the carapace and plastron. These parts are, however, covered by a substance of a horny texture, known familiarly as tortoise or turtle shell, and entirely different from bone in its nature and texture. Their other bones are not essentially different from those of other vertebral animals. Thus a part of their skeleton is, in fact, on the outside of their bodies. They have no teeth; but their jaws are armed with a tough, horny substance, which supplies their place. Their stomach is simple and strong; their intestines are long, and they are capable of going a great length of time without food. All the various species of the tortoise and turtle belong to this order.

II. Sauria, the Lizards. This order includes a very considerable variety, among which are the crocodile, the alligator, the chameleon, the true lizards, and the dragons. The greater part of them have four feet, but a few are possessed of only two. They have nails and teeth, and their skin is covered with scales.

The Crocodile is the most celebrated animal of this order. It is from twenty to thirty feet in length, including the tail, and is covered with a coat of scales, which, on the back, form an armor proof against a bullet, and have an appearance like that of carved work. It deposits its eggs in the sand, where the greater part of them are destroyed by birds and an animal called the ichneumon. Their eggs resemble, a good deal, those of the domestic goose, and are of about the same size; the young, when first hatched, are of