The earth is one kingdom, the water another, and there is the great republic of the gases surrounding us on every side; only we can't see it, because its inhabitants have the fairy gift of being invisible to us. Each of these kingdoms has products to export, and is all ready to trade with the others, if only some one will supply the means; just as the Frenchmen might stand on their shores, and hold out to us wines and prunes and silks and muslins, and we might stand on our shores, and hold out gold and silver to them, and yet could make no exchange, because there were no ships to carry the goods across. "Ah," you may say, "that is not at all the case here; for the earth, the air, and the water are all close to each other, and close to us, and there is no need of ships; we can exchange hand to hand." But here comes a difficulty. Read carefully, and I think you will understand it. Here is Ruth, a little growing girl, who wants phosphate of lime to build bones with; for as she grows, of course her bones must grow too. Very well, I answer, there is plenty of phosphate of lime in the earth; she can have all she wants. Yes, but does Ruth want to eat earth?--do you?--does anybody? Certainly not: so, although the food she needs is close beside her, even under her feet, she cannot get it any more than we can get the French goods, excepting by means of the carrying trade. Where now are the little ships that shall bring to Ruth the phosphate of lime she needs, and cannot reach, although it lies in her own father's field? Let me show you how her father can build the ships that will bring it to her. He must go out into that field, and plant wheat-seeds, and as they grow, every little ear and kernel gathers up phosphate of lime, and becomes a tiny ship freighted with what his little daughter needs. When that wheat is ground into flour, and made into bread, Ruth will eat what she couldn't have been willing to taste, unless the useful little ships of the wheat-field had brought it to her. Now let us send to the republic of the gases for some supplies, for we cannot live without carbon and oxygen; and although we do breathe in oxygen with every breathe we draw, we also need to receive it in other ways: so the sugar-cane and the maple-trees engage in the carrying trade for us, taking in carbon and oxygen by their leaves, and sending it through their bodies, and when it reaches us it is sugar,--and a very pleasant food to most of you, I dare say. But we cannot take all we need of these gases in the form of sugar, and there are many other ships that will bring it to us. The corn will gather it up, and offer it in the form of meal, or of cornstarch puddings; or the grass will bring it to the cow, since you and I refuse to take it from the grass ships. But the cow offers it to us again in the form of milk, and we do not think of refusing; or the butcher offers it to us in the form of beef, and we do not say "no."