We are at the beginning of geological history, and yet the corals are a large and flourishing class, coral-reefs are growing as nowadays, and the corals themselves, though of course of altogether different forms, are essentially the same down to the first syllable of recorded time. But having proofs of evolution which are independent of the geological record over these vast aeons, we may safely carry back the process into those times represented by rocks so ancient that no fossil trace of life is found in them, to the times when the lowest fish-like vertebrata were not, and the simple polyp was the highest product of life upon the earth. We know that most probably there really was such a time, but to imagine it is like trying to comprehend the solar system by arithmetic. We may speculate and wonder at the first beginnings of life, but I, for one, prefer to leave it to each reader’s imagination. CHAPTER VIII THE BUILDING OF REEFS Sea-water, besides containing comparatively large quantities of common salt, contains several other substances in solution in less quantity. One of these is the limestone[45] which the coral polyp extracts and renders solid as its stony skeleton, and of which, in essentially the same way, the “shell fish,” whether oyster, winkles or crabs make their hard coverings. Another constituent is magnesium carbonate, a substance rather similar to limestone, to which we refer later. Having examined the individual stone formed by the growth of a coral colony we must consider how such stones are aggregated to form a reef.