Not a few varieties of the Coral-producing Zoophytes are to be found in actual living reality on our own coasts, where the curious student may examine for himself their habits and general structure. But it is in the warmer regions of the Earth that they are developed in the greatest numbers, and decked in the brightest hues. Those who have seen them through the crystal waters of tropical seas, swarming in countless multitudes on the clear white sand below, speak with enthusiasm of their luxuriant profusion and of their striking beauty. Combining to a picturesque elegance of form a rich variety and pleasing harmony of colors, they present to the eye a scene which has been compared to a magnificent garden, laid out in diverse beds of rare and splendid flowers.
So far we have spoken only of the Polypidom, that is to say, the community of Polyps living together on a common stem of their own construction. Now this Polypidom is the first element of the Coral Reef. In some species of Zoophytes, the Red Coral for instance, the calcareous stem never attains a size greater than that of a diminutive shrub. But in others, and they are very numerous, especially in tropical seas, there seems to be no limit to the growth of the solid stony framework. As the existing generation of Zoophytes is dying out, new individuals are ever budding forth, which continue unceasingly to secrete carbonate of lime, as their predecessors had done before them, from the waters of the ocean; and thus the tree-like form spreads its branching arms on every side, growing upward and outward day by day. The soft gelatinous parts of those generations that have passed away are, in a short time, dissolved, and the stony skeleton alone remains behind. Ages roll on: the calcareous framework, ever increasing in size, becomes at length a formidable rock; and this rock is the Coral Reef.
Let it not be supposed we are here advancing a theory: we are only stating a fact that has been established by close and repeated observations. All the phenomena exhibited in the development of the Polypidom, are exhibited no less plainly in every Coral Reef that has yet been examined. On the surface of the Reef are the living Zoophytes, clinging to the calcareous skeleton which is ever growing larger through the unconscious action of their vital functions; while immediately beneath may be seen the same stony skeleton, already divested of its fleshy integument, and beginning to assume the appearance of compact and massive rock. We can behold, therefore, the mason at work on the upper story of his building, and the structure already finished below. And so we have little less than ocular demonstration that the Coral Reef is the work of the Zoophyte.
It must not be supposed, however, that in every part of the Coral Reef, the form and outline of the stony skeleton are exactly preserved. Fragments of the rock are broken off by the force of the waves, and mixed up with the comminuted shells of oysters, mussels, and other crustaceous animals inhabiting the same waters. In this way a sort of calcareous gravel, sometimes a calcareous paste, is formed, which fills up the interstices, and connects the tree-like coral into a compact rock.
We have yet to explain how the Coral Reefs come, in many cases, to rise above the surface of the ocean, and to form dry land: for it has been found that the reef-building Zoophytes require to be continually immersed in salt water, and therefore, by their own efforts, they cannot raise their structure above the ordinary level of the sea. This question was for a long time involved in obscurity; but it has been cleared up by the actual observations of Naturalists in modern times. The following description, which is given to us by Chamisso, the companion of Kotzebue on his voyages, will convey a good idea of the process by which a sunken reef is often converted into a smiling, fruitful island. “When the reef is of such a height that it remains almost dry at low water, the corals leave off building. Above this line a continuous mass of solid stone is seen, composed of the shells of mollusks and echini, with their broken-off prickles and fragments of coral, united by calcareous sand, produced by the pulverization of shells. The heat of the sun often penetrates the mass of stone when it is dry, so that it splits in many places, and the force of the waves is thereby enabled to separate and lift blocks of coral, frequently six feet long and three or four in thickness, and throw them upon the reef, by which means the ridge becomes at length so high that it is covered only during some seasons of the year by spring tides. After this the calcareous sand lies undisturbed, and offers to the seeds of trees and plants cast upon it by the waves, a soil upon which they rapidly grow, to overshadow its dazzling white surface. Entire trunks of trees, which are carried by the rivers from other countries and islands, find here at length a resting place after their long wanderings: with these come some small animals, such as insects and lizards, as the first inhabitants. Even before the trees form a wood, the sea-birds nestle here; stray land-birds take refuge in the bushes; and, at a much later period, when the work has been long since completed, man appears and builds his hut on the fruitful soil.”[55]
Another question that seems to call for some explanation is suggested by the well-known habits of the Zoophytes themselves. From the observations of Kotzebue and Darwin it appears that those species which are most effective in the construction of Reefs cannot flourish at a greater depth than twenty or thirty fathoms; whereas the coral rocks rise up in many cases from the bottom of an unfathomable ocean. How, then, it may be asked, have the foundations of these wonderful structures been laid? This question opens a wide field for philosophical speculation; and we freely admit that no theory of Coral Reefs can be regarded as complete and satisfactory, which does not furnish a reasonable answer. But so far as the purpose of our argument is concerned, it is quite sufficient if a stratum of solid limestone, twenty fathoms thick, has been formed mainly through the agency of these minute animalcules. And this conclusion, so abundantly demonstrated by facts, is left quite untouched by the difficulty to which we now refer.
It will be interesting, however, to notice in passing the explanation of this phenomenon first suggested by Mr. Darwin, and now very generally accepted. He maintains that the whole Coral Reef—foundations and superstructure alike—is, in most cases, the result entirely of organic agency. The reef-building Zoophyte always begins his labors in water that is comparatively shallow. But as he is building upward, it often happens that the bed of the sea is sinking downward in pretty nearly the same proportion; and thus the reef is ever increasing in height from its original base, while the living mass of Zoophytes on its upper surface remains in about the same depth of water as when the building first began.
This theory is supported by a vast amount of curious and ingenious reasoning. In the first place, there is nothing more remarkable in the physical conformation of the Globe, than the immense predominance of water over land throughout those extensive tracts of ocean where Coral Reefs abound. Now this is just what we should naturally expect if the hypothesis of Mr. Darwin were admitted; for wherever the Crust of the Earth has been subsiding for many ages on a large scale, the domain of the sea must of necessity have been considerably enlarged, and that of the land contracted in proportion. Again, this hypothesis will be found to harmonize most perfectly with all the phenomena of Fringing Reefs, Barrier Reefs, Encircling Reefs, and Lagoon Islands. The Fringing Reef represents, as it were, the first stage of progress. The building operations have just commenced near the shore of some island or continent, and but little space intervenes between the land and the incrusting wall of coral. Then, as the Crust of the Earth gradually subsides, the water encroaches on the land, and forms a channel between it and the reef. Meanwhile the Zoophytes are at work, and the coral rock is growing upward as the foundation on which it rests is sinking downward: each year it is higher from the bed of the sea, and yet no nearer to the surface of the waters. And when at length the channel, which is ever growing wider and wider, has reached a certain limit, the Fringing Reef becomes a Barrier Reef, or if it encompasses an island, an Encircling Reef. Lastly, the Encircling Reef will finally become a Lagoon Island, when the highest peaks of the land it encloses have slowly disappeared beneath the surface of the waters.
In confirmation of this reasoning Mr. Darwin has pointed out numerous examples to illustrate each intermediate stage through which, according to his hypothesis, the Coral Reef must pass in the progress of its construction. He traces the gradual transition from the low bank of coral incrusting a rocky shore to the Encircling Reef that compasses round a lofty island, like Tahiti, with a broad channel between. Then he shows how this channel insensibly becomes wider and wider, encroaching more and more upon the land, until at length only a few high peaks remain above water. Finally he leads us on to the case of a perfect Atoll, within which no trace of land remains to be seen; and the channel, now become a lagoon, is encompassed by a Reef of Coral Rock that rises steeply from an unfathomed ocean.
We do not mean to dwell upon this ingenious speculation, which would carry us too far from the object at which we are aiming. It seems to us, however, that the arguments in its favor are at least deserving of careful consideration; and we may add that they receive new strength from the facts we shall have occasion hereafter to bring forward, when we come to speak of the undulating movements to which the Crust of the Earth has been subject at many different times, and in many different localities, even within the historic period.