for digging into the ant-hills, so we shall not be disappointed in our expectation of finding a projecting muzzle unarmed with teeth, and a long tongue provided with a glutinous secretion, to lick up the emmets which are disturbed by the animal's scratching.

In the skeleton of the cape-mole, we may see, from the projecting acromion scapulæ, and a remarkable process of the humerus, that there is a provision for the rotation of the arm, which implies burrowing. But the apparatus seems by no means so perfect as in the mole, implying that it digs in a softer soil than that animal, whilst the possession of gnawing teeth indicates that it lives on roots.

In BIRDS there is altogether a new condition of parts, as there is a new element to contend with. The very peculiar form and structure of their skeleton may be thus accounted for. First, it is necessary that birds, as they are buoyed in the air, be specifically lighter. Secondly, the circumference of their thorax must be extended, and the motions of their ribs limited, that the muscles of the wings may have sufficient space and firmness for their attachment. Both these objects are attained by a modification of the apparatus of breathing. The lungs are highly vascular and spongy, but they are not distended with air. The air is drawn through their substance into the large cavity common to the chest and abdomen; and whilst the great

office of decarbonization of the blood is securely performed, advantage is taken to let the air into all the cavities, even into those of the bones.

From what was said in the introductory chapter, of the weight of the body being a necessary concomitant of muscular strength, we see why birds, by reason of their lightness, as well as by the conformation of their skeleton, walk badly. And, on the other hand, in observing how this lightness is adapted for flight, it is remarkable how small an addition to their body will prevent them rising on the wing. If the griffon-vulture be frightened after his repast, he must disgorge, before he flies; and the condor, in the same circumstances, is taken by the Indians, like a quadruped, by throwing the lasso over it.*

As every one must have observed, the breastbone of birds extends the whole length of the body; and owing to this extension, a lesser degree of motion suffices to respiration. So that a greater surface, necessary for the lodgement and attachment of the muscles of the wings, is obtained, whilst that surface is less disturbed by the action of breathing, and is more steady.

* It is interesting to notice the relations of great functions in the animal economy. Birds are oviparous, because they never could have risen on the wing had they been viviparous; if the full stomach of a carnivorous bird retard its flight, we perceive that it could not have carried its young. The light body, the quillfeathers, the bill, and the laying of eggs, are all necessarily connected.

Another peculiarity of the skeleton of the bird is the consolidation of the vertebræ of the back; a proof, if any were now necessary, that the whole system of bones conforms to that of the extremities, the firmer texture of the bones of the trunk, being a part of the provision for the attachment of the muscles of the wings.*

The vertebræ of the back being fixed in birds, and the pelvis reaching high, there is no motion in the body; indeed, if there were, it would be interrupted by the sternum. We cannot but admire, therefore, the composition of the neck

* The ostrich and cassowary, which are rather runners than fliers, have the spine loose.

and head, and how the extension of the vertebræ, and the length and pliability of the neck, whilst they give to the bill the office of a hand, become a substitution for the loss of motion in the body, by balancing the whole, as in standing, running, or flying. Is it not curious to observe how the whole skeleton is adapted to this one object, the power of the wings.

Whilst the ostrich has no keel in its breastbone, birds of passage are, on dissection, recognisable by the depth of this ridge of the sternum. The reason is that the angle, formed by this process and the body of the bone, affords lodgement for the pectoral muscle, the powerful muscle of the wing. In this sketch of the dissection of the swallow, there is a curious resemblance to the human arm, and we cannot fail to observe, that the pectoral muscle constitutes the greater part of the bulk of the body.* And here we see the correspondence between the strength of this muscle and the rate of flying of the swallow, which is a mile in a minute, for ten hours every day, or six hundred miles a day.† If it be true that birds, when migrating, require a wind that blows against them, it implies an extraordinary

* Borelli makes the pectoral muscles of a bird, exceed in weight all the other muscles taken together; whilst the pectoral muscles of man, are but a seventieth part of the whole mass of the muscles. + Mr. White says truly, that the swift lives on the wing; it eats, drinks, and collects materials for its nest in flying, and never rests but during darkness.

power, as well as continuance of muscular exertion.

We see how Nature completes her work, when the intention is that the animal shall rise buoyant and powerful in the air :-the whole texture of the frame is altered and made light, in a manner consistent with strength. We see also how the mechanism of the anterior extremity is changed, and the muscles of the trunk differently directed. But we are tempted to examine those means, which we would almost say are more awkwardly suited for their purpose, where the system of bones and muscles, peculiar to the quadruped, is preserved, while a power of launching into the air is also given. We have already noticed the structure of the bat as adapted to flight; but there are other animals which enjoy this function in a lesser degree. For example, the flying squirrel (Petromys Volucella), being chased to the end of the bough, spreads out its mantle from one extremity to the other, and drops in the air; but with such a resistance from its extended skin and its tail, that it can direct its flight obliquely downwards, and even turn in the air. But to this end, there is no necessity for any adaptation of the anterior extremity. Among reptiles there is a provision of the same kind, in the Draco fimbriatus; which is capable of creeping to a height and dropping safely to the ground, under the protection of a sort of parachute, formed by