Now the amazing thing about the corals is that the polyps have entered into an alliance with certain microscopic plants which come to live in their bodies, and they feed upon the starchy products these plants form in sunlight, and even upon the plants themselves. So intimate is the union of these strange partners that neither can live without the other, the coral has lost its independence, and in fact as well as in appearance leads the life of a plant. At the first sight of a coral sea one wonders what takes the place of the great beds of brown and green weed which fringe British shores, and are a source of the oxygen essential to animal life. The discovery of these plant partners of the corals gives the answer. This easy life, this evasion of the necessity of capturing prey, is doubtless the reason for the degeneration of the polyp structure noticed above.
One of the greatest interests of these lowly forms of life is their place in the evolution of the higher. We have left all that is familiar, the creatures with heads and limbs, far behind, on the surface as it were, and are groping among the foundations of the edifice of creation. It is difficult indeed to express how very far down we are without some description of the rest of the series. But this is impossible; I am asked to give means of understanding what a coral is, and should not be thanked for giving in reply a treatise on Zoology. Let us take two steps only of the process of evolution, and let these short lengths give an expression of the whole descent.
Consider the vast interval of time and changes of structure involved in the evolution of man from his ape-like ancestor. How many thousands of years, what vast advances! How far above the purely animal is the lowest savage, and how far above that the best of civilised man! And yet even in the case of the brain, the development of which is man’s main advance, the man’s brain is but the further development of the ape’s[43], which has already gone the greater part of the way manwards from the condition found in ordinary animals.
Now we and the apes together are derived from some fish-like ancestor. We all had gill bars fundamentally like those of a fish at one stage of our existences. It is a vast descent through the reptiles to the amphibia and then to the fishes[44]! And the fishes again are our second step illustrative of the vast changes involved. Fish are just fish to the ordinary man, and yet the fact is that the difference between man and ape is just nothing to that between the ordinary higher fish, the kinds that come in after the soup, and the sharks. The shark family have not yet attained the possession of true bone, for instance, and their brain development is almost rudimentary. But we are already in the dim beginnings of geological history, for sharks essentially like those we now know were living when almost the earliest of rocks were being laid down as mud in primeval oceans. These were times incredibly remote, when land animals were not in existence, when plants were represented only by seaweeds, the whole land a desert but for possibly some creeping films of vegetation adapted to life on damp soil ashore, times long ages before those strange reptiles Iguanodon, Diplodocus, the whale-like Ichthyosaurus, the giant ferns and lycopods of the coal measures, whose fossil remains remind us of nightmare worlds which have passed away, had ever come into being.
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.
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.