Of course the coral polypes cannot build above the high-tide mark; but the surf which beats upon them piles up their broken fragments just as a sea-beach is piled up, and hammers them together with that water hammer which is heavier and stronger than any you have ever seen in a smith's forge. And then, as is the fashion of lime, the whole mass sets and becomes hard, as you may see mortar set; and so you have a low island a few feet above the sea. Then sea-birds come to it, and rest and build; and seeds are floated thither from far lands; and among them almost always the cocoa- nut, which loves to grow by the sea-shore, and groves of cocoa palms grow up upon the lonely isle. Then, perhaps, trees and bushes are drifted thither before the trade-wind; and entangled in their roots are seeds of other plants, and eggs or cocoons of insects; and so a few flowers and a few butterflies and beetles set up for themselves upon the new land. And then a bird or two, caught in a storm and blown away to sea finds shelter in the cocoa-grove; and so a little new world is set up, in which (you must remember always) there are no four-footed beasts, nor snakes, nor lizards, nor frogs, nor any animals that cannot cross the sea.
And on some of those islands they may live (indeed there is reason to believe they have lived), so long, that some of them have changed their forms, according to the laws of Madam How, who sooner or later fits each thing exactly for the place in which it is meant to live, till upon some of them you may find such strange and unique creatures as the famous cocoa-nut crab, which learned men call Birgus latro. A great crab he is, who walks upon the tips of his toes a foot high above the ground. And because he has often nothing to eat but cocoa-nuts, or at least they are the best things he can find, cocoa-nuts he has learned to eat, and after a fashion which it would puzzle you to imitate. Some say that he climbs up the stems of the cocoa-nut trees, and pulls the fruit down for himself; but that, it seems, he does not usually do.
What he does is this: when he finds a fallen cocoa-nut, he begins tearing away the thick husk and fibre with his strong claws; and he knows perfectly well which end to tear it from, namely, from the end where the three eye-holes are, which you call the monkey's face, out of one of which you know, the young cocoa-nut tree would burst forth. And when he has got to the eye-holes, he hammers through one of them with the point of his heavy claw. So far, so good: but how is he to get the meat out? He cannot put his claw in. He has no proboscis like a butterfly to insert and suck with.
He is as far off from his dinner as the fox was when the stork offered him a feast in a long-necked jar. What then do you think he does? He turns himself round, puts in a pair of his hind pincers, which are very slender, and with them scoops the meat out of the cocoa-nut, and so puts his dinner into his mouth with his hind feet. And even the cocoa-nut husk he does not waste; for he lives in deep burrows which he makes like a rabbit; and being a luxurious crab, and liking to sleep soft in spite of his hard shell, he lines them with a quantity of cocoa-nut fibre, picked out clean and fine, just as if he was going to make cocoa-nut matting of it. And being also a clean crab, as I hope you are a clean little boy, he goes down to the sea every night to have his bath and moisten his gills, and so lives happy all his days, and gets so fat in his old age that he carries about his body nearly a quart of pure oil.
That is the history of the cocoa-nut crab. And if any one tells me that that crab acts only on what is called "instinct"; and does not think and reason, just as you and I think and reason, though of course not in words as you and I do: then I shall be inclined to say that that person does not think nor reason either.
Then were there many coral-reefs in Britain in old times?
Yes, many and many, again and again; some whole ages older than this, a bit of which you see, and some again whole ages newer.
But look: then judge for yourself. Look at this geological map.
Wherever you see a bit of blue, which is the mark for limestone, you may say, "There is a bit of old coral-reef rising up to the surface." But because I will not puzzle your little head with too many things at once, you shall look at one set of coral-reefs which are far newer than this bit of Dudley limestone, and which are the largest, I suppose, that ever were in this country; or, at least, there is more of them left than of any others.
Look first at Ireland. You see that almost all the middle of Ireland is coloured blue. It is one great sheet of old coral-reef and coral-mud, which is now called the carboniferous limestone.
You see red and purple patches rising out of it, like islands--and islands I suppose they were, of hard and ancient rock, standing up in the middle of the coral sea.
But look again, and you will see that along the west coast of Ireland, except in a very few places, like Galway Bay, the blue limestone does not come down to the sea; the shore is coloured purple and brown, and those colours mark the ancient rocks and high mountains of Mayo and Galway and Kerry, which stand as barriers to keep the raging surf of the Atlantic from bursting inland and beating away, as it surely would in course of time, the low flat limestone plain of the middle of Ireland. But the same coral-reefs once stretched out far to the westward into the Atlantic Ocean; and you may see the proof upon that map. For in the western bays, in Clew Bay with its hundred islands, and Galway Bay with its Isles of Arran, and beautiful Kenmare, and beautiful Bantry, you see little blue spots, which are low limestone islands, standing in the sea, overhung by mountains far aloft.