But if you go down to a dry dock and look at the hull of a ship which has come in to be cleaned and scraped, you will see that it is entirely covered by seaweeds and shells. That ship has been driven through the water perhaps at ten miles an hour or more, and yet those delicate-looking seaweeds have held on! It is more surprising still if you can get some of them and examine them with a microscope, for amongst them are tiny, delicate, graceful little fronds and sprays which one would think consisted of nothing but jelly. Yet they have been able to thrive and grow on the ship's hull while it has been hurrying day and night through the sea, in calm or in tempest, and in currents of hot or cold water. Those seaweeds were called by Horace _Algæ inutiles_, or useless seaweeds; but are they useless? Go down to a little pool and watch them waving in the water. Could anything be more beautiful than these little graceful red, yellow, or brown sprays? All sorts of seaslugs, shrimps, and minute animals of weird and wonderful design are clearly living on them. Fishes live upon these animals, and fishes are an extremely useful and excellent food for man. CHAPTER XIII ROCKS, STONES, AND SCENERY An old wall--Beautiful colours--Insects--Nature's chief aim--Hard times of lichens--Age of lichens--Crusts--Mosses--Lava flows of great eruptions--Colonizing plants--Krakatoa--Vesuvius--Greenland volcanoes--Sumatra--Shale-heaps--Foreigners on railway lines--Plants keep to their own grounds--Precipices and rocks--Plants which change the scenery--Cañons in America. At first sight, and when one is striding along at some four miles an hour, there seems to be nothing at all interesting in an old wall. But if one stops and carefully examines the stones, there is a great deal that is interesting. Rocks and walls possess a fascination of their own. Probably at least 2000 British plants are _only_ found upon them, and yet of these, the vast majority are so small and inconspicuous that an ordinary person never perceives a single one of them. It is perhaps on rocks or old walls near the sea that this stone flora is most richly developed. The nearly circular orange-yellow patches of the Lichen _Physcia parietina_ are quite distinct and conspicuous. But any old wall, provided it is well out in the country, is pretty sure to be interesting. At first it seems to have only a dull grey or neutral tint. But if one goes to four or five feet distance, one discovers that many shades of brown, red, white, and black go to make this grey. But the extraordinary beauty of such a wall is only visible when one peers and scrutinizes the surface very slowly and carefully with the eyes six or seven inches away from it.