But messengers are prepared to bring them. Birds flying over the water sometimes stoop their wings to rest awhile on the rock, and often leave behind them seeds which they have gathered in far distant lands. At first, perhaps, only a few small weeds are seen. These, dying in their turn, improve the soil for their successors, until at length it can support shrubs and undergrowth, the seeds of which are sometimes washed on the shore by the waves, or found hidden in the clefts of some tree which has floated to the island from a distant shore. Last of all arises, like a crown of beauty, the graceful cocoa-nut palm, spreading broad leaves around its tall, slender stem, and making the once barren rock a shady and lovely retreat. The island on which Alexander Selkirk lived is considered volcanic; it is probably formed in some such manner as that which we have described. Madeira, too, and probably St. Helena, are volcanic islands. Pitcairn, the history of which you are now going to read, is also possibly of volcanic origin, and its high crags and sharp peaks seem as if they must have been thrown up by some sudden force; but as it is in the midst of a sea covered with coral islands, and has been supposed by some to be itself partially formed by coral insects, it may be well that you should hear a little of the wonderful growth of coral islands, which, though formed so differently from those of which you have been reading, are yet, when once their tops have risen above the waves, clothed in the same manner with fair growth, to prepare them for the presence of man. Tahiti, which you will hear mentioned in the story of Pitcairn, is a coral island, and they abound in groups, in pairs, or in single islands, through the wide Pacific Ocean. They are formed by myriads of tiny insects, which are connected together, and seem to share a common life. One of these insects fastens itself on some hidden rock; sometimes it may be on an extinct volcano which is not lofty enough to appear above the waves, and on this foundation they begin to build, the insect, as it shapes its cells of coral, filling them with beings like itself, so that every tiny chamber has its inmate. Soon the whole rock is covered below the water with a fine network of delicate coral, and from the tops of the open cells the insects put out their delicate _tentaculae_, or arms, which look like the petals of a flower. By means of the food gathered from the water by these _tentaculae_, all the coral insects are fed. [Illustration: Coral Island. (Page 92.)] Thus each one does its appointed work, laying unseen the foundations of a new land, for the coral growth is still spreading and rising higher and higher, till at length the waves begin to feel its resistance, and to break in white foam around its crests.