to buoy itself in the air, whilst leaping from one tree to another It can leap in this way to the distance of a hundred yards.

The Merian Opossum is remarkable for its method of carrying its young on its back, where they fix themselves by twisting their tails closely about that of their parent, clinging with their claws to its fur.

The Kangaroo (Fig. 17) is the largest animal of this order, and the largest quadruped which has been discovered in Australia. It is sometimes six feet in height, and is distinguished by the great disproportion in length between its fore and hind legs; the former being only one foot and a half long, but the latter three feet and a half. In consequence of this, they cannot walk upon all fours without difficulty, but leap with great power and to a prodigious distance, sometimes twenty feet, and to the height of nine feet. They sit upon their hind legs whilst at rest, seldom using the fore legs, except for supporting themselves when stooping to drink, for conveying food to the mouth, and for digging in the earth. But although disproportionately long, as has just been observed, when full grown, the hind legs of the kangaroo at birth are not so large or so strong as the fore legs, which are more necessary, in order to favor the motions of the little animal while in the pouch.

X. Monotremata. Of these the most remarkable is the Ornithorhynchus, or as it was at first called, the Duck-billed Opossum. Naturalists were at first startled, when this animal was made known to them, by the strange combination which it exhibits of the characteristics of the quadruped and the bird. It has a bill like a duck, a fourchette or wishing-bone, and spurs upon its hind feet; the limbs of an aquatic quadruped, the body of the otter, and the fur of the mole. Its young are produced in a very imperfect and unformed state, like those of the Marsupial animals; and though it has no pouch, it has the marsupial bones.

The Echidna, or Porcupine Anteater, is another animal of this order, resembling it in certain characteristic traits, but widely differing in many others, and in habits and modes of life. It is covered with spines like the porcupine; its mouth and jaws are constructed like those of the anteaters; but its relation to its young is like that of the ornithorhynchus, and it has, like this

animal, spurs upon its hind feet. Like this animal, also, it is only found in Australia and some of the islands of that part of With a few exceptions, this is, also, true of the

the world. Marsupialia.

Birds are more uniform in their structure than animals of any other class. The reason of this uniformity may be probably attributed to the peculiar character of the motions for which they are chiefly intended. The muscular structure, even of quadrupeds, intended to move only on the surface of the earth, is largely devoted to progressive motion, though they are still capable of many other uses of their limbs. But in Birds, the element in which their principal movements are performed, renders it necessary that a vastly greater proportion of muscular power should be devoted to this function, and, of course, a less proportion to all others. When we consider how powerful an effort is required for us to leap but a few feet from the earth, we can easily understand how great must be that by which a bird conveys itself with ease and rapidity through the air.

So prominent are the provisions for this species of locomotion in Birds, that it is found, on examination, that their whole structure is modified in order to adapt them to this, as if it were the

.

principal object. Not that a particular kind of locomotion actually is, in birds, or in any other animal, the principal object of existence; it always is, and must be, subordinate to other purposes. But in an animal whose kind of motion requires, comparatively, such enormous power as flying, those other purposes which are more important as it respects the nourishment, growth, exercise of the senses, and animal enjoyment, are modified in their mode of performance, so as not to interfere with this, Cut off a bird's wings, and all these great ends of life may be answered just as well as if he could still fly; and yet, such is the predominating influence which the preparation for this function has had upon his structure, that all the remaining organs exhibit indications of this influence, as much as those which have been removed.

Hence it is, that the structure of Birds as a class, connected with their peculiar mode of locomotion, affords some of our most striking examples of the adaptation of structure to purpose, and of a designing mind in creation.

Let us look at the modifications of their structure, and the purposes they are intended to answer. Their respiratory apparatus is much more extensive, and they consume a much greater amount of air than other animals. This is rendered necessary by the great amount of muscular power which is expended in flying. Where there is great muscular action, there is required a corresponding quantity of blood to support that action. This renders necessary a greater exposure of the blood to the influence of air in the lungs. Lavoisier tells us that two sparrows consumed as much oxygen from a portion of air as a guinea pig, an animal many times as large. The air, in Birds, not only penetrates the lungs, but into cells among the viscera, among the muscles, and in the cavities of the bones. This not only makes respiration more extensive, but renders the body lighter in proportion to its bulk and strength. In those birds which fly highest and longest, this provigon is most ample, and is found chiefly in and about the wings; whilst in those which fly badly, like the domestic fowl, or do not fly at all, like the ostrich, it is less extensive, and is transferred from the wings to the legs. Through all these cavities the air circulates as it does through the lungs.

There is a further purpose answered by the hollowness of the

bones, beside that of increasing the respiratory surface. It is an object in the bird to combine strength with lightness, and the strongest form into which any given quantity of material can be put is that of a hollow cylinder. For this reason the bones of birds are hollow cylinders. Had they been made on precisely the same plan with those of the Mammalia, it would have been necessary, in order to give them the requisite strength, that they should have been much heavier.

In the construction of the skeleton the same predominating purpose is manifest. The general plan is the same as in Mammalia, but in every part are found modifications intended to adapt the animal for flight. The backbone in the Mammalia is composed of separate pieces, movable upon each other, constituting a flexible column. In Birds, these pieces are all consolidated, to give a firm steady base to support the violent actions of the wings. The breastbone, instead of being small and narrow, is large, broad, and flat, with a projecting ridge or keel in the middle, the whole presenting a very extensive surface for the insertion and reception of the large muscles which move the wings.

Fig. 22.

Skeleton of Falcon.

In the act of flying, a very nice adjustment of the mechanical relations of different parts is necessary. The body is to be very accurately balanced upon the points of insertion of the two wings, as upon pivots, whilst in order to preserve the vertical position, the whole apparatus requires to be ballasted, as much as a ship in the water, or it would be constantly liable to misplacement, in the course of the various and sudden changes of position to which the animal is liable. This is effected by placing the point of

suspension above the greatest portion of the weight of the body, especially of the large breastbone with its projecting keel and large mass of muscular flesh. The equilibrium of the body is otherwise preserved by the head and neck before, and the feet and legs behind. These usually correspond in weight, but where they do not, or when, during the act of flying, the balance is destroyed, such is the freedom of motion of these parts, that by a change in their direction, it may be thrown forward or backward. An example of this effort at maintaining the equilibrium, is seen in birds with very long necks and long legs, who in flying throw the former very far forward and the latter backward.

The anterior limbs being used for flying are good for nothing else; consequently the bird is a biped. Still it is not, like man, intended for the erect posture, but for one somewhat horizontal; and consequently the toes are made very long, and project like radii, in order to form a large base to support the body. In man, the length of the foot is about twelve inches, and its average breadth not over three and one half inches, giving an area of forty-two square inches, as a base to support the body, whose height is nearly six feet. In the common domestic fowl the extent of surface is about ten square inches, while the height of the body is from twelve to eighteen. In this way, it is enabled with two limbs to maintain a posture nearly horizontal.*

Birds, then, though bipeds, are bipeds of a very different kind from man. Man is intended for an erect, birds for a horizontal, or rather, perhaps, an oblique, posture. Hence arise certain of their modifications of structure. It is necessary that the centre of gravity should be underneath the shoulders, in order to flying, and as far back as and between the feet, in order to walking and standing. Hence, as the joints of the hip-bones are very far back on the trunk, the feet are, by the bending of the joints, thrown very far forward in order to maintain the balance of the body. This is necessary even when the head is erect, but when the head is thrown forward and downward so as to reach the ground, the

*In a male bird measured for this purpose, whose height was found to be nearly a foot and a half, the area covered by the foot was nearly ten square inches and its length from before backward no less than five inches. So that, while the height of the bird is but one fourth that of man, the length of its foot is nearly one half, in that direction in which a greater support is required.