"For, ere the beech and elm have cast their leaf

Deciduous, when now November dark

Checks vegetation in the torpid plant,

Expos'd to his cold breath, the task begins."-CowPER.

they are designed for cutting or grinding. The relation of the jaw, and the muscular forces by which it is moved, requires a closer examination.

In herbivorous animals, which have to grind down their food by constant trituration, the jaw is fixed to the skull, so as to allow the former to have a rotatory movement; but such a movement would be useless to carnivorous animals, where the grinding operation is not required.

In carnivorous animals the jaw is locked in the cavity of the skull by ligaments, in the same manner as the parts of a hinge are fixed together. The cavity is deep and elongated, and the articulating surface of the jaw-bone corresponds, so that the joint can have only a hinge-like motion.

327. This is remarkably conspicuous in the condycles of the lower jaw of the seaotter. The jaw of the sea-wolf is composed of several pieces, instead of being one entire bone; and these pieces are connected by ligaments, so that a greater freedom of motion is allowed, and the concussion to the brain arising from the reduction of crabs, muscles, and other shell-fish upon which the animal feeds, is diminished, the jar being broken by being divided over a number of bones.

628. Why do some animals undergo a state of torpor during the winter?

Because during the winter they cannot produce more heat than is sufficient to raise their temperature from 20° to 26° above the surrounding atmosphere. It follows, therefore, that while in the hottest part of summer their temperature is nearly the same as that of other warm-blooded animals, it falls to a much lower point in the cold season; and whenever the depression of temperature attains a certain limit, the circulation and respiration decrease in frequency and energy, so that the animal falls into a state of torpor, or lethargic sleep, which continues until the temperature of the atmosphere is sufficiently elevated to re-establish the activity of the vital functions.

"The migration of birds from a hotter to a colder country, or a colder to a hotter, according to the seasons of the year, as their nature is, I know not how to give an account of it, it is so strange and admirable."-RAY.

629. Why are certain species of animals destined to perform their functions only periodically?

Because it has been allotted to them to check superfluities and remove nuisances.

When vegetation develops its vast powers of reproduction, there issue forth from their winter retreats innumerable creatures that live variously upon the roots, leaves, or seeds.

When the vitality of vegetation diminishes, the natural office of these creatures ends; and they return again to their torpid condition.

During the season when animal existence is vigorous, and the destinies of nature are being fulfilled, the great harvest of death strews millions of bodies upon the face of nature. Then the scavengers are busy day and night; and either devour upon the surface, or bury in the earth, those substances which would otherwise diffuse pestilential influences.

630. Why do some portions of the animal creation migrate?

For two reasons: first, the welfare of the migrating animal is promoted by finding milder regions, and a continual supply of food; second, the blessings of creation are thus diffused, by seasonable visitations of creatures useful to man, to those localities where he stands in need of them.

631. Had the Creator so willed, all these animals might have been organised so as not to require a warmer or colder climate for the breeding or rearing of their young; but His will was, that some of His best gifts should thus oscillate, as it were, between two points, that the benefit they conferred might be more widely distributed, and not become the sole property of the inhabitants of one climate. Thus the all-wise and beneficent Being has so organised certain classes of animals, and circumstanced them, as to be directed annually, by some pressing want, to seek distant climates, and, after a certain period, to return again to their former quarters; and that this instinct should be productive of so much good to mankind, and, at the same time, be necessary, under its present circumstances, for the preservation or Propagation of the species of these several animals.*

* Partington's "Cyclopædia."

"The wisdom of the Deity, as testified in the works of creation, surpasses all idea we have of wisdom, drawn from the highest intellectual operations of the highest class of intelligent beings with whom we are acquainted."-PALEY.

632. How is the wisdom of Providence shown in so constituting the lower animals that they can exist for a long time on a limited supply of air and moisture?

Because animals thus circumstanced would otherwise find it impossible to exist during the long intervals that many of them are periodically or occasionally enclosed in inaccessible places; so that when confined in solid rocks, or sealed up in the hearts of trees, so long as the smallest quantity of air or moisture is supplied them, they live for an indefinite period of time.

633. One of the most remarkable accounts of the long duration of the vital principle in animals is mentioned by Dr. Silliman, who, on the authority of Professor Eaton, of New York, states that the diluvial deposit through which the Erie canal was made, contains ridges of hard compact gravel, and that on cutting through one of these near Rome village, sixteen miles west of Utica, the workmen found several hundreds of live molluscous animals. The workmen fried and ate them. He adds: "I was assured they were taken alive forty-two feet deep in the deposit. Several of the shells are now before me. The deposit is diluvial. These animals must have been there from the time of the deluge; for the earth in which they were is too compact for them to have been produced by a succession of generations. These fresh-water clans of three thousand years old precisely resemble the species which now inhabit the fresh water of that district; therefore the lives of these animals have been greatly prolonged by their exclusion from light and air for more than three thousand years." A toad was buried in a flower-pot for twenty years, and when taken out was found to be healthy and increased in size. That snails can exist for a long period by means of the exclusion of air and the retention of moisture, which they are enabled to accomplish by a sort of door at the aperture of the shell, has been proved by Mr. Simon, who mentions the circumstance of having had one in his cabinet for fifteen years; and, for aught he knew, it might have been in his father's possession many years before, as it was in his collection of fossils. Speaking of this snail, he says it had come out four several times, in the presence of different people, each of whom assured him that they saw it. A day or two after this, he brought the identical shell, as he declared, into the presence of several other persons, that they might try if the snail would again make its appearance. After the shell had lain ten minutes in a glass of warm water, the snail began to appear, and in five minutes more they perceived half the body fairly pushed out from the cavity of the shell. It afterwards crawled about, erected its horns, and seemed in perfect health.

"If chance at length he find a greensward smooth,

And faithful to the foot his spirits rise,

He cherups brisk his ear-erecting steed,

And winds his way with pleasure and with ease."-Cowper.

634. Why are the ears of some animals turned forwards, and those of others backwards?

The external ears of beasts of prey, as lions, tigers, and wolves, have their trumpet-part or concavity standing forward, to seize the sounds which are before them—namely, the sounds of the animals which they pursue or watch. The ears of animals of flight are turned backward, to give notice of the approach of an enemy from behind, that he may not steal upon them unawares. (See 392.)

635. Why, in animals of great speed, is the shoulder connected to the trunk by the agency of muscles, and not by a collar-bone?

Because, if animals possessing great speed had been formed with a collar-bone, it could not have withstood the shock from the descent of the whole weight of the animal when thrown forwards ; and even though the structure of the fore legs had been as powerful as the posterior extremities, they would have suffered fracture or dislocation. This beautiful provision not only serves to diminish the shock of descending, but contributes to the elasticity of the anterior extremities.

636. Why are the knee-joints of the hind legs of most swimming animals turned round, as in a person who is knock-kneed?

Because by this mode of articulation the hind legs form a kind of swimming sail; and, in consequence of this they act more horizontally, and thereby impel the animal forward in the water with more velocity and with less exertion.

637. Animals which are constructed principally for walking on the land make the chief exertion with the fore feet when they swim; and, therefore, they are sooner fatigued than when they move even faster upon land; but quadruped animals having a regular swimming habit, impel themselves chiefly by means of the hind feet, and on this account they are no more fatigued in water than they are on land.

"Whose snout hath rooted up

The fruitful vineyard of the commonwealth."

BEAUMONT AND FLETCHER.

The reason of this will be readily understood by those who are aware how much more easily a boat is pulled by oars nearly on a level with the water than when the oars have to dip deeply into the water, and are used near the bows.

638. Why have certain species of animals sharp-pointed snouts?

to live at the bottom of burrows hollowed in the banks of rivers, also exhibit a similar conformation. (See 466.)

639. Why is the fur of some animals changed in the winter from a dark to a white colour?

Because, although the darker colours absorb heat to a greater degree than the lighter ones, so that dark-coloured clothing is much warmer than light-coloured, where the wearer is exposed to the sun's rays, the radiation of heat is also much greater from dark than from light-coloured surfaces, and consequently the animal heat from within is more completely retained by a white than by a dark covering. The temperature of an animal, therefore, having white fur, would continue more equable than that of one clothed in darker colours, although the latter would experience a greater degree of warmth when exposed to the sun's rays. Another reason may be that the mottled browns, which form the principal colours of the animals alluded to, although well adapted for their concealment