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intelligence, are naturally placed at the head of the animal creation. No other class possesses the same interest for man, for it is the class of which he is himself a member. It is the class which contains his favourite domesticated animals, such as the dog, the horse, the ox, and the sheep. It is the class on which he is chiefly dependent for the conveniences and even the necessaries of life. And, to omit other particulars, it is the class in whose structure he finds the most striking exhibitions of creative wisdom. The nervous system, especially, attains in most of the mammals a perfection elsewhere unknown. As we ascend from the lower to the higher orders, the brain becomes more and more fully developed, till, in the case of man, it forms the connecting link between the material and the spiritual, between the mortal and the immortal.
The human body is, indeed, the noblest type of vertebrate structure. For this reason, and also on account of the intrinsic interest and importance of the subject, a description of some of its parts and functions will occupy several subsequent lessons.
THE COVERING OF ANIMALS.
mammals and birds is Now, it is worthy of
Ir has been stated that the blood of warm; of reptiles and fishes, cold. notice that warm-blooded animals are provided by nature with some kind of covering, to prevent the heat of their bodies from escaping too rapidly, so as to reduce them to a temperature inconsistent with the healthy exercise of their functions.
It has to be remarked, however, that such coverings are in many cases armour as well as clothing, intended for protection as well as warmth. Nor is this protection bestowed upon warm-blooded animals only; on the contrary, fishes and reptiles are nearly all armed with scales, which, while they protect, do not encumber or overburden them. There is, indeed, scarcely any part of the structure of animals
more worthy of admiration than their covering, whether we look to its variety, or its suitableness to their several natures. We have bristles, hair, wool, fur, feathers, quills, prickles, scales; yet, in this diversity both of material and form, we cannot change one animal's coat for another, without evidently changing it for the worse.
Of the higher animals, man is the only one that is naked, and the only one that can clothe himself. This is one of the properties which render him an animal of all climates, and of all seasons. He can adapt the warmth or lightness of his covering to the temperature of his habitation. Had he been born with a fleece on his back, although he might have been comforted by its warmth in high latitudes, it would have oppressed him by its weight and heat, as the species spread towards the equator.
What art, however, does for man, nature has in many instances done for those animals which are incapable of employing art. Their clothing, of its own accord, changes with their necessities. This is particularly the case with that large tribe of quadrupeds which are covered with fur. Every dealer in hare-skins and rabbit-skins knows how much the fur is thickened by the approach of winter. It seems to be a part of the same constitution and the same design, that wool in hot countries degenerates, as it is called, but in truth (most happily for the animal's ease) passes into hair; whilst, on the contrary, hair on the dogs of the polar regions is turned into wool, or something very like it.
The covering of birds, and its adaptation to their necessities, can scarcely escape the notice of the most superficial observer. Its lightness, its smoothness, its warmth, the disposition of the feathers all inclined backwards, the down about their stems, the overlapping of their edges, their different configuration in different parts of the body, not to mention the variety of their colours, constitute a vestment so beautiful, and so appropriate to the life which the animal is to lead, that, if we had never seen it, we should have had no conception of anything so perfect. Nor can we even now imagine anything more so.
Scarcely less remarkable is the apparatus provided for keeping this beautiful garment in good order. From a gland, or bag, situated near the tail, the bird extracts, by the pressure of its bill, an oily fluid with which to dress itself. You may often see it engaged in this process, working with its bill among its feathers. The feathers, smeared with oil, throw off the rain which falls upon them, without being wet or injured by it. It is a curious circumstance, that swimming birds, such as ducks, geese, and swans, not only have a thicker and warmer covering on those parts of the body that are exposed to the water, but are also abundantly supplied with this dressing oil; whereas little or none of it is found in hens and other domestic fowl, which usually have the means of artificial shelter.
Such are some of the minute, yet beautiful adaptations, which are everywhere met with through the works of nature. While we think with admiring gratitude of the benevolence of Him who doeth all this, is it not comforting to reflect that we, too, are under the protection of the same gracious hand?
QUESTIONS FOR EXAMINATION.
Of what substance is the skeleton of vertebrate animals composed? What are its essential parts? What are the two central portions of the nervous system? How are they protected? What is the use of the nervous system? of the limbs? Which vertebrates have no limbs? How are mammals distinguished? How birds? How reptiles? How fishes? Which classes are warm-blooded? which cold? which breathe by gills? Why are mammals the most interesting to us? Why have animals coverings? Which of the higher animals has no natural covering? Is the want of it a gain or a loss? Show how. What is the usual covering of cold-blooded animals? Why is it suitable for them? Describe the effect of climate on the covering of animals. What constitutes the excellence of a bird's covering? How do birds keep their feathers in order?
TIDES, or the alternate flowing and ebbing of the sea, are produced by the attraction of the moon and sun, but principally by the attraction of the moon. For the moon being so much nearer to the earth than the sun, has a much greater attractive influence on its waters than the sun.
The ancients looked upon the flowing and ebbing of the tides as one of the greatest mysteries in nature; and but for the more than human intellect of Newton, it is probable that it would have remained a mystery to this day.
The phenomenon of the tides has been observed in every part of the earth which has been washed by the sea. For about six hours the sea gradually swells, so that it enters the mouths of harbours and rivers, and comes nearer to the coasts. This is called Flood Tide. For about twelve minutes it rests or remains in equilibrio; during which it is said to be high water. It then begins to ebb, and continues to do so for about six hours, when it pauses again for about twelve minutes; during which it is said to be low water. It then begins to flow again for six hours; and so on alternately. Hence, in every twenty-four hours and fifty minutes there are two tides. If the moon were stationary, the same part of our globe would return under or opposite to it every twenty-four hours, and there would, in consequence, be two tides every twenty-four hours; but while the earth is turning once upon its axis, the moon moves forward in her orbit 13 ̊, and hence it takes the earth about fifty minutes more to bring the same meridian under or opposite to the moon.
As the earth turns round on its axes, it presents every part of its surface, in succession, to the moon, which, in accordance with the law of gravity, exerts a greater attractive influence upon those parts of the earth's surface that are turned towards her, or nearest, than upon those that are turned from her, or most remote. Hence, as seas pass under the moon-or, as is commonly said, when the moon comes to the meridian of the place the fluid particles of which they are composed, being more easily separated and attracted than particles of earth, are drawn more strongly towards her, which causes them to swell and bulge out, till the impulse is overcome by the attraction of the other watery particles, as they are brought by the rotation of the earth, under the more direct influence of the moon.
While the water is thus attracted and heaped up on the side of the earth which is nearest to the moon, it is at the
same time equally elevated on the other side of the earth, or the side which is farthest from the moon; and hence there are always two tides at the same time, one on the side of the earth next to the moon, and the other on the opposite side.
It is evident that the tides will be greatest at that point of the earth's surface which is nearest to the moon, or where the latter is vertical. She is so between the tropics; and accordingly the tides are there greatest, and they diminish as we approach either pole. It is further to be remarked, that the tide is not highest when the moon is on the meridian of the place. From two to three hours elapse before the waters are raised, in consequence of the law of inertia, or a disposition which every body has to continue in the condition of motion or rest in which it happens to be placed.
That the moon should attract and raise up the waters of the earth that are under her, is easy to conceive; but that the same cause should, at the same time, raise them up on the opposite side of the globe, seems strange and incredible. The general principle is, that as those parts of the earth which are nearest to the moon are more strongly attracted towards her than the parts which are more remote, the sea which covers the surface of the earth on the side farthest from the moon is less strongly attracted than the land which is under it, and which is consequently nearer to the moon. Hence the body of the earth being more strongly attracted than the waters which cover its side farthest from the moon, is drawn away from these waters, and the same result is produced as if they had risen in tides.
The following diagrams will illustrate more clearly the action of the sun and moon upon the waters. Suppose that the earth were a regular and uniform sphere covered with water. If no external body or influence operated on this system, it is clear that the waters would, in obedience to the law of gravitation, arrange themselves regularly and uniformly around the earth, forming a coating or bed everywhere of the same depth. Let an external body, M, the