We must not consider the Reef as a solid, massive structure throughout.  The compact kinds of Corals, giving strength and solidity to the wall, may be compared to the larger trees in a forest, which give it shade and density; but between these grow all kinds of trailing vines, ferns and mosses, wild flowers and low shrubs, that till the spaces between the larger trees with a thick underbrush.  The Coral Reef also has its underbrush of the lighter, branching, more brittle kinds, that fill its interstices and fringe the summit and the sides with their delicate, graceful forms.  Such an intricate underbrush of Coral growth affords an excellent retreat for many animals that like its protection better than exposure to the open sea, just as many land-animals prefer the close and shaded woods to the open plain:  a forest is not more thickly peopled with Birds, Squirrels, Martens, and the like, than is the Coral Reef with a variety of animals that do not contribute in any way to its growth, but find shelter in its crevices or in its near neighborhood.

But these larger animals are not the only ones that haunt the forest.  There is a host of parasites besides, principally Insects and their larvae, which bore their way into the very heart of the tree, making their home in the bark and pith, and not the less numerous because hidden from sight.  These also have their counterparts in the Reef, where numbers of boring Shells and marine Worms work their way into the solid substance of the wall, piercing it, with holes in every direction, till large portions become insecure, and the next storm suffices to break off the fragments so loosened.  Once detached, they are tossed about in the water, crumbled into Coral sand, crushed, often ground to powder by the friction of the rocks and the constant action of the sea.

After a time, an immense quantity of such materials is formed about a Coral Reef; tides and storms constantly throw them up on its surface, and at last a soil collects on the top of the Reef, wherever it has reached the surface of the water, formed chiefly of its own debris, of Coral sand, Coral fragments, even large masses of Coral rock, mingled with the remains of the animals that have had their home about the Reef, with sea-weeds, with mud from the neighboring land, and with the thousand loose substances always floating about in the vicinity of a coast and thrown upon the rocks or shore with every wave that breaks against them.  Add to this the presence of a lime-cement in the water, resulting from the decomposition of some of these materials, and we have all that is needed to make a very compact deposit and fertile soil, on which a vegetation may spring up, whenever seeds floating from the shore or dropped by birds in their flight take root on the newly formed island.