ment of their organs around a central spot, at which the mouth

is situated. The animals of this division are

commonly denom

inated Zoophytes. The organs of sensation, of locomotion, of circulation, of respiration, and the nervous system, are very imperfectly developed, and cannot be always detected, and in many of the lower tribes scarce any trace of organization is to be discovered. Starfish, sea-anemone, polypes, sunfish, coral animals,

infusoria.

The first two classes are distinguished by having a fixed temperature above the ordinary temperature of the medium in which they live. The last two by having a temperature varying but little from that of such medium. Hence the former are called warm-blooded, and the latter cold-blooded.

Of the warm-blooded animals, those of the first class produce their young alive, and nourish them during infancy by their own milk, secreted by organs called mamma or breasts. Hence their name. This includes man, quadrupeds, seals, whales. Those of the second class, or Birds, produce their young by means of eggs, hatched usually by the heat of their own bodies, and support them by food, which they provide for them as soon as they come out of the egg. These two classes resemble each other also in the general structure of the organs of respiration and circulation.

* A different disposition of the classes of Vertebral animals has been suggested, increasing their number to nine or ten, by a subdivision of those of Reptiles and Fishes. Doubtless there may be sufficient grounds for this new arrangement, but they are such as can only be intelligible to well-instructed naturalists, and it has seemed preferable, therefore, to adhere to the old and generally understood divisions

The first of the classes of cold-blooded animals, Reptiles, contains those which breathe air only and cannot exist without it, as lizards, frogs, tortoises, serpents; the second, or Fishes, those which receive the air through the medium of water by means of branchiæ or gills.

SECTION I.

CLASS I. -MAMMALIA.

Of the structure of man, the most perfect example of this class, an account has already been given, and we shall get the most intelligible idea of that of the other Mammalia by tracing the degree and manner in which they depart from him.

We have already adverted to the manner in which the structure of man is modified in the formation of the monkey tribe, p. 33, (Fig. 6.) The locomotion of these animals is chiefly that of climbing. On level ground they scramble along rapidly enough, but clumsily and imperfectly, whilst among the branches of trees they make their way with astonishing ease and rapidity. This is owing partly to the presence of hands upon all four of their extremities, but quite as much to the different distribution of force between the upper and the lower limbs. Man is comparatively a poor climber. His chief strength is in his legs. It is with difficulty that he raises his body with his arms. The monkey's greatest strength is in his arms, and the advantage which this gives him in climbing will be seen at once.

proper carnivorous animals, like the cat tribe, and in the gnawing tribe, like the squirrel. These, although differing from each other in so many particulars, are yet somewhat alike in the manner in which they differ from man. The fore limbs are brought nearer together and are directed forward; the chest is narrow, especially at its upper part; and the shoulder and arms have much less freedom and variety of motion. The fingers are not separate, but enveloped and confined by the skin; there is no thumb, and claws are substituted for nails. The wrist and heel are removed farther from the fingers, and, as a consequence, motion, which is wholly on the four extremities, is performed upon the ends of the

Fig. 11.

Jerboa.

fingers and toes instead of the hand and foot. Hence the arm and thigh are both shorter; and the elbow, or the joint corresponding to the elbow, and the knee, are carried up toward the body. The fore limbs are still, however, capable of performing some of the offices of hands, as we see in the common cat, and more especially in the squirrel. The panther and jerboa are also examples of the same structure.

Still considerable freedom of motion remains. Many of these animals use their fore paws as hands with some dexterity, and are excellent climbers, as the squirrels, though they have not the power of grasping which distinguishes the monkeys. Some of them naturally, and others by force of education, are capable of assuming the erect posture, by throwing themselves backward upon their heels and bending the knee and hip joints at very acute angles.

All the animals of these tribes are not constructed alike; for, whilst there are some not far removed from the monkeys in their structure and capacity for motion, others approach to the more complete quadrupeds.

As familiar examples of these last, namely, the complete quad

—

rupeds, we may take the ruminating and the single hoofed animals, the camel, (fig. 12,) the ox, and the horse. In these the chest is still more contracted, the arms or fore legs are brought more closely together, all freedom of motion in the shoulder joint is lost; the bones of the fingers are united into a solid mass, and form a round limb terminated by one or more hoofs. The same takes place in the hind limb. The heel is further raised from the ground, and the animal can no longer throw himself back upon it, but, as in a horse that rears, always supports himself on the ends of the toes.

In this figure the single line drawn around the skeleton indicates the outline of the living animal; vc, the vertebræ of the neck; vd, of the back; vl, of the loins; vs, of the sacrum; vq, of the tail; c, the ribs; o, shoulder-blade or scapula; h, arm; cu, forearm, a single bone corresponding to the two bones between the elbow and wrist in man,between these two bones is the joint corresponding to the elbow; ca, bones of the wrist; mc, of the hand; ph, of the fingers; fe, of the thigh; ro, of the knee; ti, of the leg, in place of two in man; ta, of the ankle; mt, toes.

By a comparison of this skeleton with that of man (p. 23) and with the account of the gradual transition of form which takes place from the structure of man to that of the complete quadruped, a conception will be obtained of the changes there referred to.

We have noticed only the stronger distinctions of formation. In every case there are species lying between, in which a gradual

transition of structure may be traced. To notice them, would lead to a too minute detail.

These variations are all connected with the kinds of motion required by the necessities of the animals in which they are found. There are others in which a greater departure, from the residence and habits of man requires a still greater departure from his structure. Some Mammalia, as the bats, are intended to move in air; others, as the seals and whales, in water.

It would have been difficult to imagine how the same fundamental structure, which, by no essential modification of plan, has been transformed from a man to an ox, could, without greater violence to the plan, be converted into a bat. But this is exactly what has been done. Examine the skeleton of this animal, and you find it is essentially built like that of man. The parts are the same; the proportions only are altered. It is chiefly to be observed, that all four of its limbs are put in requisition for the purpose of flying; just as in monkeys for climbing, and in quadrupeds for walking and running; the fore limbs being reserved

cl, clavicle; h, humerus; cu, ulna; r, radius; ca, carpus, or wrist; po, thumb; mc, metacarpus; ph, fingers; o, scapula; f, thigh; ti, leg.

By continuing the comparison with the human skeleton, the points in which that of the bat at once differs and corresponds will be easily perceived. The most noteworthy variation is that which has been effected in the arm and hand. The bone between the elbow

and hand is prolonged to a considerable, and those of the fingers to a very great extent, and are spread out like the sticks of an umbrella, so as to give support to the membrane of the wings, whilst those of the arm and shoulder are larger and stronger. If, especially, the relative development of the upper and lower extremities be contrasted with that of man, it will clearly appear how singularly these parts have been modified in order to accommodate them to the purposes of flight.