V. You may see that there is, in every variety of circumstance, something that enjoys itself, and has a place to fill up and a part to act amongst the creatures which God has made. But this is not all: the existence of the cold of the Poles, and of the heat of the Tropics, is necessary to the well-being of the whole.

The free air that surrounds the globe, which we breathe, and through which we see, and hear, and smell, and move, is the same in composition in every part of the world. It contains just the same proportions of the two gases, oxygen and nitrogen, in the coldest and the hottest climates, in the deepest valleys and on the tallest mountains. Now, this air, considered on a large scale, is always blowing from the Poles towards the Equator. The more direct rays of the sun heat that portion of the earth's surface which lies in the Torrid Zone, and therefore the air above it is perpetually ascending to the upper regions of the atmosphere, to make room for that which presses upon it from the Poles, because it is cooler and heavier.

It is this process, united with the motion of the earth on its axis, which causes the Trade Winds and Monsoons.

Now, do you not see how necessary the cold Poles are to keep the rest of the world from becoming too hot, by supplying constant currents of fresh air setting in upon the hotter regions from both sides? Then the air, which ascends from the Equator, becomes cool, and travels down again to join the air from the Poles; and so a healthful circulation is kept up, which is necessary for all climates.

VI. It is very interesting to trace the dependance of the various tribes of vegetables and animals on each other, and to observe how one flourishes through the dissolution of another; and still more so to notice the gradual development of the same parts in the kinds, as they ascend one above another. You may see an instance of this in three of the animals that I described to you, which were different enough in other respects, but agreed in being without a bony skeleton, and in having long organs round their mouths to catch their food with.

In the AcalephÆ, or sea-nettles, these organs were merely filaments without sensation, of which the use seemed to be, to entangle little creatures in, which chance might bring in their way. The animals themselves would seem, at first sight, to be very ill able to destroy small crabs and other shell-fish. You would suppose that the struggles they would make with their hard pointed legs, would tear the tender bodies of the AcalephÆ; but the deficiency is supplied by the caustic luminous fluid which paralyzes the victim, and disables it for violent struggles while it is detained by the filaments, before the Acalepha attempts to swallow it.

In the ActiniÆ, you will find the loose disorderly filaments changed into feelers, with wonderfully acute sensation, regularly disposed in a star. When a substance touches the feelers they close in with considerable power, but seem to act more like a mechanical trap,—as the leaves of certain plants catch flies and other insects,—than in direct dependance on the will of the animal. However, the Actinia has the sense of feeling, and has a perception of light and of odours, though without eyes or nose. Having greater muscular strength and more compactures of film, he does not need the destructive fluid with which the Acalepha is furnished. Thus, one sort of power is made to compensate for the loss of another.

The Sepia also catches his food by means of fleshy organs placed round his mouth; but in him you find them possessed of amazing power, moving in strict subjection to his will; never loosely floating about as in the others. Then he tastes, sees, and hears, by means of a tongue, eyes, and ears, distinctly formed.

A similar gradation is observed in the development of the various parts composing the hand and arm in animals with perfect skeletons, in which the bones act as levers. There is a very interesting book[C] written on this subject, which you would do well to read; and you will see in it that all sorts of birds, quadrupeds, and even fishes, have, in their fore feet, wings, and fore fins, elements resembling those of the human hand and arm. Often, in comparing only two animals, you might fail to trace the slightest resemblance; but when one or two or more of other kinds are placed between them, you find a sort of ladder which, in an evident manner, unites the lowest with the highest tribes.

At distant periods of the earth's history, you may see the same sort of organs, and the same dependance of the creatures one on another. The fierce Icthyosaurus, and the sly, long-necked Plesiosaurus, had eyes, ears, tongues, and other parts, the same as our contemporaries; and they ate and digested their food, and moved from place to place, and preyed on each other in no other manner.

If our acquaintance with nature were much greater than it is, we should doubtless be able to bring proofs that there is no sort of stones, of vegetables, or of animals, nor any process or movement of the elements, in which we have not an interest. There is no fact that is not in some way or other connected with the whole, so as to influence its well-being.

VII. Not only are the creatures which inhabit the earth united together by bonds of similarity of structure and appetite, of common wants and enjoyments, and of mutual support; but we are also united with the boundless system of worlds which the night unveils to our view. The principle of gravitation, and the beneficent rays of the sun operating on the planets and their moons, throw over us a plain and obvious tie of brotherhood with the stars that we may see night after night making their way amongst the constellations, as they move in their orbits, distinguished by their steady light from the twinkling multitude of fixed stars. The law that unites us to them is the same as causes a drop of rain to descend, or a weary fly to settle on the earth.

It is not improbable, that the aerolites (if the theory which I hinted at in a former chapter be true) may be fragments of original matter, which have never been appropriated by any globe, and now sometimes pitch on one planet, and sometimes on another. If this be the case, we should be warranted in concluding that the matter of our solar system is everywhere the same, chemically considered, and is, therefore, governed by the same chemical laws; for the aerolites contain no substance which is not to be found far below the surface of the earth.

Some ignorant persons, in all ages of the world, have fancied that the relative positions of the stars to each other, at the moment of the birth of an individual, must have an influence upon his character and the future circumstances of his life. Thus arose what was called the science of astrology, and the practice of casting nativities; and in vulgar conversation it is not unfrequent for people, who do not know the origin of the expression, to "thank their stars," or to talk of their "unlucky stars," which arose from the prevalence of such a belief. This, I need hardly tell you, is all nonsense, from beginning to end; but you may now know that there is, in reality, quite as wonderful a connection, and as direct a one, between yourselves and the stars, as this which was fancied to exist.

VIII. But who can tell how wide the relationship of our earth and everything upon it is extended, through the agency of those wonderful principles Light, Heat, and Electricity? It may seem to you impossible, when I tell you that there is not a blade of grass or a flower on this earth, which may not, in its little degree, affect the climate of a star far beyond the limits of our solar system; but if you will consider the way in which the falling of dews is regulated, you will see that there are grounds for such a notion.

I must first tell you that it is a property of Heat, like Light, to radiate or expand itself in all directions, without limit; so that a heated body is always sending out its heat.

Now, this radiation is influenced by the surfaces of bodies: Heat will radiate more from a black rough surface than from a smooth white one, and a black body will therefore much sooner get cool than a white one, as I had occasion to tell you just now. The heat radiates rapidly from the leaves of vegetables, though from different kinds of plants in very different degrees, while from stones and dry wood it radiates very slowly.

Heat is also reflected, or turned back, by meeting with certain objects; and in this respect, too, it resembles light. You have possibly seen experiments showing this, made with polished metal mirrors.

At night time, when the sun's rays are not present, as the heat radiates quickly from the plants on the ground, each plant becomes thereby cooler than the earth and stones which surround it. This causes the watery particles which the air contains, to condense on its leaves and flowers, in the same manner as you see the moisture in the air of a crowded room, settle on the outside of a glass of cold water; and, what is most wonderful, the surface of each plant is so constructed as to allow the escape of just so much heat, and to receive just so much dew in return, as its peculiar nature requires. You may see, if you look, that some plants always have more dew on them than others.

On a cloudy night, when dews are not so much required, a great part of the heat thus radiated is sent back, being reflected by the clouds. Hence the dew falls less heavily at such a time than on a clear starlight night, when every blade of grass and every little flower sends out its ray of heat to an indefinite extent, and may possibly meet another ray from the Dog-stars or one of the Pleiades!

This is very wonderful; but if you think, you will perceive that it is not more wonderful than that a ray of light should travel so far. Every particle of the surface of these stars does its part in emitting light, and by that the light of our nights is increased; and if Light is affected by such remote influences, why should not Climate be so affected?

IX. As everything has a place to fill up amongst the creatures by which it becomes connected with the universal system, we find innumerable instances of things being most wonderfully provided with powers of retaining the position for which they were created, when circumstances may oppose it. The tendency which every animal has to preserve its own life, supplies abundant illustration. It is this which causes the Arctic animals to change their colour to white in winter, and the swallows to migrate to warmer regions. But there is something selfish in this, as the provision is merely to save the animal's own life. Much more beautiful is it to observe the operation of the affections of parent animals towards their offspring, by which the young, when they are incapable of taking care of themselves, are kept alive and preserved from injury often at the sacrifice of the enjoyment, and even of the life of their parent. You will remember what I told you respecting the love of the old whales towards the suckers. Nothing parts with its own life willingly, but in a great many animals it may be seen that there is a greater regard to the preservation of their race than of their own individual lives.