190. CLIMBING is merely walking upon an inclined or even upright surface. It is usually accomplished by means of sharp nails; and hence many carnivorous animals climb with great facility, such as the cat tribe, the lizards; and many birds, the woodpecker, for instance. Others employ their arms for this purpose, like the bears when they climb a tree; or their hands, and even their tails, like the monkeys; or their beaks, like the parrots. Lastly, there are some whose natural mode of progression is climbing. Such are the sloths, with their arms so long, that, when placed upon the ground, they move very awkwardly; and yet their struc ture is by no means defective, for in their accustomed move ments upon trees they can use their limbs with very great adroitness.

191. Most quadrupeds can both walk, trot, gallop, and leap; birds walk and leap; lizards neither leap nor gallop, but only walk and run, and some of them with great rapidity. No insect either trots or gallops, but many of them leap. Yet their leaping is not always the effect of the muscular force of their legs, as with the flea and grasshopper; but some of them leap by means of a spring, in the form of a hook, attached to the tail, which they bend beneath the body, and which, when let loose, propels them to a great distance, as in the Podurellæ. Still others leap by means of a spring, attached beneath the breast, which strikes against the abdomen when the body is bent; as the spring-beetles, (Elaters.)

192. FLIGHT is accomplished by the simultaneous action of the two anterior limbs, the wings, as leaping is by that of the two hinder limbs. The wings being expanded, strike and compress the air, which thus becomes a support, for the mcment, upon which the bird is sustained. But as this support very soon yields, owing to the slight density of the air, it follows that the bird must make the greater and more

rap id efforts to compensate for this disadvantage. Hence it requires a much greater expenditure of strength to fly than to walk; and, therefore, we find the great mass of muscles in birds concentrated about the breast, (Fig. 30.) To facilitate its progress, the bird, after each flap of the wings, brings them against the body, so as to present as little surface as possible to the air; for a still further diminution of resistance, all birds have the anterior part of the body very slender. Their flight would be much more difficult if they had large heads and short necks.

193. Some quadrupeds, such as the flying-squirrel and Galeopithecus, have a fold of the skin at the sides, which may be extended by the legs, and which enables them to leap from branch to branch with more security. But this is not flight, properly speaking, since none of the peculiar operations of flight are performed. There are also some fishes, whose pectoral fins are so extended as to enable them to dart from the water, and sustain themselves for a considerable time in the air; and hence they are called flying-fish. But this is not truly flight."

194. SWIMMING is the mode of locomotion employed by the greater part of the aquatic animals. Most animals which live in the water swim with more or less facility. Swimming has this in common with flight, that the medium in which it is performed, the water, becomes also the support, and read ily yields also to the impulse of the fins. Only, as water is much more dense than air, and as the body of most aquatic animals is of very nearly the same specific gravity as water, it follows that, in swimming, very little effort is requisite to keep the body from sinking. The whole power of the muscles is consequently employed in progression, and hence swimming requires vastly less muscular force than flying.

195. Swimming is accomplished by means of various organs designated under the general term, fins, although in an

anatomical point of view these may represent very different parts. In the Whales, the anterior extremities and the tail are transformed into fins. In Fishes, the pectoral fins, which represent the arms, and the ventral fins, which represent the legs, are employed for swimming, but they are not the principal organs; for it is by the tail, or caudal fin, that progression is principally effected. Hence the progression of the fish is precisely that of a boat under the sole guidance of the sculling-oar. In the same manner as a succession of strokes alternately right and left propels the boat straight forwards, so the fish advances by striking alternately right and left. To advance obliquely, it has only to strike a little more strongly in the direction opposite to that which he wishes to take. The Whales, on the contrary, swim by striking the water up and down; and it is the same with a few fishes also, such as the rays and the soles. The airbladder facilitates the rising and sinking of the fish, by enabling it to vary the specific weight of the body.

196. Most land animals swim with more or less ease, by simply employing the ordinary motions of walking or leaping. Those which frequent the water, like the beaver, or which feed on marine animals, as the otter and duck, have webbed feet; that is to say, the fingers are united by a membrane, which, when expanded, acts as a paddle.

197. There is also a large number of invertebrate animals in which swimming is the principal or the only mode of progression. Lobsters swim by means of their tail, and, like the Whales, strike the water up and down. Other crustacea have a pair of legs fashioned like oars; as the posterior legs in sea-crabs, for example. Many insects likewise, swim with their legs, which are abundantly fringed with hairs to give them surface; as the little water boatmen, (Gyrinus, Dytiscus,) whose mazy dances on the summer streams every one must have observed. The cuttle-fish uses its long ten

tacles as oars, (Fig. 47;) and some star-fishes (Comatula, Euryale) use their arms with great adroitness, (Fig. 151.) Finally, there are some insects which have their limbs constructed for running on the surface of water, as the waterspiders, (Ranatra, Hydrometra.)

198. A large number of animals have the faculty of mov ing both in the air and on land, as is the case with most birds, and a great proportion of insects. Others move with equal facility, and by the same members, on land and in water, as some of the aquatic birds and most of the reptiles, which latter have even received the name Amphibia, on this account. There are some which both walk, fly, and swim, as the ducks and water-hens; but they do not excel in either mode of progression.

199. However different the movements and offices performed by the limbs may appear to us, according to the element in which they act, we see that they are none the less the effect of the same mechanism. The contraction of the same set of muscles causes the leg of the stag to bend for leaping, the wing of the bird to flap in the air, the arm of the mole to excavat: the earth, and the fin of the whale to strike the water.

CHAPTER SIXTH.

NUTRITION.

200. THE second class of the functions of animals are those which relate to the maintenance of life and the perpetuation of the species; the functions of vegetative life, (59.)

201. The increase of the volume of the body must re quire additional materials. There is also an incessant waste of particles which, having become unfit for further use, are carried out of the system. Every contraction of a muscle expands the energy of some particles, whose place must be supplied. These supplies are derived from every natural source, the animal, vegetable, and even the mineral kingdoms; and are received under every variety of solid, liquid. and gaseous form. Thus, there is a perpetual interchange of substance between the animal body and the world around. The conversion of these supplies into a suitable material, its distribution to all parts, and the appropriation of it to the growth and sustenance of the body, is called NUTRITION in the widest sense of that term.

202. In early life, during the period of growth, the amount of substances appropriated is greater than that which is lost At a later period, when growth is completed, an equilibrium between the matters received and those rejected is established. At a still later period, the equilibrium is again disturbed, more is rejected than is retained, decrepitude begins, and at last the organism becomes exhausted, the functions cease, and death ensues. 70

203. The solids and fluids taken into the body as food are