Every one has heard of coral reefs. They are one of the best and most familiar examples of the way in which great masses of solid rock can be built up by the dead bodies of animals. In the warmer seas of the earth, and notably in the track of the great ocean currents, various kinds of coral polyps, as they are called, take root on the edges and summits of submerged rocks and peaks, as well as on the shelving shores of islands. The coral polyp is a jelly-like creature, but it has a hard chalky skeleton inside its transparent body. It is a great colonist, with no liking for a solitary life, but with, on the contrary, a great fancy for its neighbours; in fact, the polyps grow and thrive in clumps, and the clumps unite to form communities, and the communities increase to colonies and nations, till they unite to form what is called a reef. The coral polyps are rather exigent in the choice of their residential neighbourhoods. They cannot live at a greater depth than fifteen or twenty fathoms, and in defiance of the inclinations which rule human beings, they have the strongest distaste for sun and air; in fact, they die when exposed to it. Now when the polyp dies its skeleton remains behind it, and millions upon millions of these coral skeletons make a layer of coral. These layers of coral gradually lift the generations of polyps upwards to the surface of the water. But as we have seen, the living polyps die when they get so far, and consequently the reef then spreads outwards. On the outer edges of the reef the coral polyps flourish in the most vigorous way. There they are as completely provided for as in a County Council Utopia. The breakers bring them the food on which they live; the water and the climate suit them exactly. The only blot on their lives are the occasional storms which break off fragments of the coral foundation on which they live. But even this, while it is disastrous to the individual polyp, is for the good of the community, because these blocks as they roll down form a new foundation on which new generations of polyps can grow and feed. Moreover, it is better for the polyp to take the risks of these evictions than to vegetate _inside_ the reef, for there in the calmer water he will not have enough to eat, and will dwindle and die. Thus the tendency of all reefs must be to grow seawards, and to increase in breadth. Perhaps their breadth may tell us roughly how old they are. But there is another possibility to be taken into consideration, which is that while the polyps are building the sea bottom or island foundation may be slowly sinking. In that case it is quite likely that the coral builders might just keep pace with the subsiding foundations of their home, and build up a great thickness of coral rock during the countless years of change.