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.