It is obvious that colonies cannot live for ever, any more than do individuals, and we need to know the fate of a dead colony and how it is replaced by a living one which shall continue the building. So great is the competition among the crowds of the floating young of the fixed animals that any vacant spot is at once appropriated. When a coral colony dies the coloured film of flesh speedily rots away and the snow-white stony skeleton remains, washed clean by waves and currents[46]. In a few days this is covered with a film of the finest green seaweed, invisible among which are the embryos of several orders of animals, e.g. shell-fish, or, there may be the larvae of some other coral. There is a tense struggle for survival among these young creatures, but on a growing reef conditions of course generally favour the coral’s young (otherwise the reef would cease to grow), some of which grow at the expense of nearly everything else and cover the site. Many of the large hemispherical corals live on the reef like loose stones, but on turning them over one may find quite a small shell, or coral branch, attached to the centre of the underside. This is the foundation of the whole, the resting place of the tiny floating larva, the growth of which first covered the stone on which it settled by a vigorous colony which when large enough to be independent of support continued its growth until the mass exceeded by hundreds of times the bulk of the original foundation. The more delicate of coral skeletons, such as those of the porous branched madrepores, rarely survive the death of the polyps that formed them. On losing its coat of living flesh the coral is exposed to the action of boring animals, as well as to the direct solvent action of the sea-water, and many are thus destroyed. Partly they go back into solution, but the greater portion is broken down to mud and sand. In shallow water branched colonies are broken up into pebbles and coarse sand by the waves, and these materials serve to fill in the spaces between the larger colonies and pack the whole together into a solid mass.