The outer Reef, still incomplete, as I have stated, and therefore of course somewhat lower than the inner one, measures about seventy feet in height. Allowing a foot of growth for every century, not less than seven thousand years must have elapsed since this Reef began to grow. Some miles nearer the main-land are the Keys, or the inner Reef; and though this must have been longer in the process of formation than the outer one, since its growth is completed, and nearly the whole extent of its surface is transformed into islands, with here and there a narrow break separating them, yet, in order to keep fully within the evidence of the facts, I will allow only seven thousand years for the formation of this Reef also, making fourteen thousand for the two.
This brings us to the shore-bluffs, consisting simply of another Reef exactly like those already described, except that the lapse of time has united it to the main-land by the complete filling up and consolidation of the channel which once divided it from the extremity of the peninsula, as a channel now separates the Keys from the shore-bluffs, and the outer Reef, again, from the Keys. These three concentric Reefs, then, the outer Reef, the Keys, and the shore-bluffs, if we measure the growth of the two latter on the same low estimate by which I have calculated the rate of progress of the former, cannot have reached their present condition in less than twenty thousand years. Their growth must have been successive, since, as we have seen, all Corals need the fresh action of the open sea upon them, and if either of the outer Reefs had begun to grow before the completion of the inner one, it would have effectually checked the growth of the latter. The absence of an incipient Reef outside of the outer Reef shows these conclusions to be well founded. The islands capping these three do not exceed in height the level to which the fragments accumulated upon their summits may have been thrown by the heaviest storms. The highest hills of this part of Florida are not over ten or twelve feet above the level of the sea, and yet the luxuriant vegetation with which they are covered gives them an imposing appearance.
But this is not the end of the story. Travelling inland from the shore-bluffs, we cross a low, flat expanse of land, the Indian hunting-ground, which brings us to a row of elevations called the Hummocks. This hunting-ground, or Everglade as it is also called, is an old channel, changed first to mud-flats and then to dry land by the same kind of accumulation that is filling up the present channels, and the row of hummocks is but an old Coral Reef with the Keys or islands of past days upon its summit. Seven such Reefs and channels of former times have already been traced between the shore-bluffs and Lake Okee-cho-bee, adding some fifty thousand years to our previous estimate. Indeed, upon the lowest calculation, based upon the facts thus far ascertained as to their growth, we cannot suppose that less than seventy thousand years have elapsed since the Coral Reefs already known to exist in Florida began to grow. When we remember that this is but a small portion of the peninsula, and that, though we have not yet any accurate information as to the nature of its interior, yet the facts already ascertained in the northern part of this State, formed like its Southern extremity of Coral growth, justify the inference that the whole peninsula is formed of successive concentric Reefs, we must believe that hundreds of thousands of years have elapsed since its formation began. Leaving aside, however, all that part of its history which is not susceptible of positive demonstration in the present state of our knowledge, I will limit my results to the evidence of facts already within our possession; and these give us as the lowest possible estimate a period of seventy thousand years for the formation of that part of the peninsula which extends south of Lake Okee-cho-bee to the present outer Reef.
So much for the duration of the Reefs themselves. What, now, do they tell us of the permanence of the Species by which they were formed? In these seventy thousand years has there been any change in the Corals living in the Gulf of Mexico? I answer, most emphatically, No. Astraeans, Porites, Maeandrinas, and Madrepores were represented by exactly the same Species seventy thousand years ago as they are now. Were we to classify the Florida Corals from the Reefs of the interior, the result would correspond exactly to a classification founded upon the living Corals of the outer Reef to-day. There would be among the Astraeans the different species of Astraea proper, forming the close round heads,—the Mussa, growing in smaller stocks, where the mouths coalesce and run into each other as in the Brain-Corals, but in which the depressions formed by the mouths are deeper,—and the Caryophyllians, in which the single individuals stand out more distinctly from the stock; among Porites, the P. Astroïdes, with pits resembling those of the Astraeans in form, though smaller in size, and growing also in solid heads, though these masses are covered with club-shaped protrusions, instead of presenting a smooth, even surface like the Astraeans,—and the P. Clavaria, in which the stocks are divided in short, stumpy branches, with club-shaped ends, instead of growing in close, compact heads; among the Maeandrinas we should have the round heads we know as Brain-Corals, with their wavy lines over the surface, and the Manacina, differing again from the preceding by certain details of structure; among the Madrepores we should have the Madrepora prolifera, with its small, short branches, broken up by very frequent ramifications, the M. cervicornis, with longer and stouter branches and less frequent ramifications, and the cup-like M. palmata, resembling an open sponge in form. Every Species, in short, that lives upon the present Reef is found in the more ancient ones. They all belong to our own geological period, and we cannot, upon the evidence before us, estimate its duration at less than seventy thousand years, during which time we have no evidence of any change in Species, but on the contrary the strongest proof of the absolute permanence of those Species whose past history we have been able to trace.
Before leaving the subject of the Coral Reefs, I would add a few words on the succession of the different kinds of Polyp Corals on a Reef as compared with their structural rank and also with their succession in time, because we have here another of those correspondences of thought, those intellectual links in Creation, which give such coherence and consistency to the whole, and make it intelligible to man.
The lowest in structure among the Polyps are not Corals, but the single, soft-bodied Actiniae. They have no solid parts, and are independent in their mode of existence, never forming communities, like the higher members of the class. It might at first seem strange that independence, considered a sign of superiority in the higher animals, should here be looked upon as a mark of inferiority. But independence may mean either simple isolation, or independence of action; and the life of a single Polyp is no more independent in the sense of action than that of a community of Polyps. It is simply not connected with or related to the life of any others. The mode of development of these animals tells us something of the relative inferiority and superiority of the single ones and of those that grow in communities. When the little Polyp Coral, the Astraean or Madrepore, for instance, is born from the egg, it is as free as the Actinia, which remains free all its life. It is only at a later period, as its development goes on, that it becomes solidly attached to the ground, and begins its compound life by putting forth new beings like itself as buds from its side. Since we cannot suppose that the normal development of any being can have a retrograde action, we are justified in believing that the loss of freedom is in fact a stage of progress in these lower animals, and their more intimate dependence on each other a sign of maturity.
There are, however, structural features by which the relative superiority of these animals may be determined. In proportion as the number of their parts is limited and permanent, their structure is more complicated; and the indefinite multiplication of identical parts is connected with inferiority of structure. Now in these lowest Polyps, the Actiniae, the tentacles increase with age indefinitely, never ceasing to grow while life lasts, new chambers being constantly added to correspond with them, till it becomes impossible to count their numbers. Next to these come the true Fungidae. They are also single, and though they are stony Corals, they have no share in the formation of Reefs. In these, also, the tentacles multiply throughout life, though they are usually not so numerous as in the Actiniae. But a new feature is added to the complication of their structure, as compared with Actiniae, in the transverse beams which connect their vertical partitions, though they do not stretch across the animal so as to form perfect floors, as in some of the higher Polyps. These transverse beams or floors must not be confounded with the horizontal floors alluded to in a former article as characteristic of the ancient Acalephian Corals, the Rugosa and Tabulata. For in the latter these floors stretch completely across the body, uninterrupted by vertical partitions, which, if they exist at all, pass only from floor to floor, instead of extending unbroken through the whole height of the body, as in all Polyps. Where, on the contrary, transverse floors exist in true Polyps, they never cut the vertical partitions in their length, but simply connect their walls, stretching wholly or partially from wall to wall.
In the Astraeans, the multiplication of tentacles is more definite and limited, rising sometimes to ninety and more, though often limited to forty-eight in number, and the transverse floors between the vertical partitions are more complete than in the Fungidae. The Porites have twelve tentacles only, never more and never less; and in them the whole solid frame presents a complicated system of connected beams. The Madrepores have also twelve tentacles, but they have a more definite character than those of the Porites, on account of their regular alternation in six smaller and six larger ones; in these also the transverse floors are perfect, but exceedingly delicate. Another remarkable feature among the Madrepores consists in the prominence of one of the Polyps on the summit of the branches, showing a kind of subordination of the whole community to these larger individuals, and thus sustaining the view expressed above, that the combination of many individuals into a connected community is among Polyps a character of superiority when contrasted with the isolation of the Actiniae;. In the Sea-Fans, the Halcyonoids, as they are called in our classification, the number of tentacles is always eight, four of which are already present at the time of their birth, arranged in pairs, while the other four are added later. Their tentacles are lobed all around the margin, and are much more complicated in structure than those of the preceding Polyps.
According to the relative complication of their structure, these animals are classified in the following order:—