In the last resort, then, it is mainly on mud that the life of humanity in all countries bases itself. Every great plain is the alluvial deposit of a great river, ultimately derived from a great mountain chain. The substance consists as a rule of the debris of torrents, which is often infertile, owing to its stoniness and its purely mineral character; but wherever it has lain long enough to be covered by earth-worms with a deep black layer of vegetable mould, there the resulting soil shows the surprising fruitfulness one gets (for example) in Lombardy, where twelve crops a year are sometimes taken from the meadows. Everywhere and always the amount and depth of the mud is the measure of possible fertility; and even where, as in the Great American Desert, want of water converts alluvial plains into arid stretches of sand-waste, the wilderness can be made to blossom like the rose in a very few years by artificial irrigation. The diversion of the Arkansas River has spread plenty over a vast sage scrub; the finest crops in the world are now raised over a tract of country which was once the terror of the traveller across the wild west of America. THE GREENWOOD TREE. It is a common, not to say a vulgar error, to believe that trees and plants grow out of the ground. And of course, having thus begun by calling it bad names, I will not for a moment insult the intelligence of my readers by supposing them to share so foolish a delusion. I beg to state from the outset that I write this article entirely for the benefit of Other People. You and I, O proverbially Candid and Intelligent One, it need hardly be said, are better informed. But Other People fall into such ridiculous blunders that it is just as well to put them on their guard beforehand against the insidious advance of false opinions. I have known otherwise good and estimable men, indeed, who for lack of sound early teaching on this point went to their graves with a confirmed belief in the terrestrial origin of all earthly vegetation. They were probably victims of what the Church in its succinct way describes and denounces as Invincible Ignorance. Now, the reason why these deluded creatures supposed trees to grow out of the ground, instead of out of the air, is probably only because they saw their roots there.