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Natural History: Methods of Study

Atlantic Monthly


A few miles from the southern extremity of Florida, separated from it by a channel, narrow at the eastern end, but widening gradually toward the west, and rendered every year more and more shallow by the accumulation of materials constantly collecting within it, there lies a line of islands called the Florida Keys. They are at different distances from the shore, stretching gradually seaward in the form of an open crescent, from Virginia Key and Key Biscayne, almost adjoining the main-land, to Key West, at a distance of twelve miles from the coast, which does not, however, close the series, for sixty miles farther west stands the group of the Tortugas, isolated in the Gulf of Mexico. Though they seem disconnected, these islands are parts of a submerged Coral Reef, concentric with the shore of the peninsula and continuous underneath the water, but visible above the surface at such points of the summit as have fully completed their growth.

This demands some explanation, since I have already said that no Coral growth can continue after it has reached the line of high-water. But we have not finished the history of a Coral wall, when we have followed it to the surface of the ocean. It is true that its normal growth ceases there, but already a process of partial decay as begun that insures its further increase. Here, as elsewhere, destruction and construction go hand in hand, and the materials that are broken or worn away from one part of the Reef help to build it up elsewhere. The Corals which form the Reef are not the only beings that find their home there: many other animals—Shells, Worms, Crabs, Star-Fishes, Sea-Urchins—establish themselves upon it, work their way into its interstices, and seek a shelter in every little hole and cranny made by the irregularities of its surface. In the Zoological Museum at Cambridge there are some large fragments of Coral Reef which give one a good idea of the populous aspect that such a Reef would present, could we see it as it actually exists beneath the water. Some of these fragments consist of a succession of terraces, as it were, in which are many little miniature caves, where may still be seen the Shells or Sea-Urchins which made their snug and sheltered homes in these recesses of the Reef.

We must not consider the Reef as a solid, massive structure throughout. The compact kinds of Corals, giving strength and solidity to the wall, may be compared to the larger trees in a forest, which give it shade and density; but between these grow all kinds of trailing vines, ferns and mosses, wild flowers and low shrubs, that till the spaces between the larger trees with a thick underbrush. The Coral Reef also has its underbrush of the lighter, branching, more brittle kinds, that fill its interstices and fringe the summit and the sides with their delicate, graceful forms. Such an intricate underbrush of Coral growth affords an excellent retreat for many animals that like its protection better than exposure to the open sea, just as many land-animals prefer the close and shaded woods to the open plain: a forest is not more thickly peopled with Birds, Squirrels, Martens, and the like, than is the Coral Reef with a variety of animals that do not contribute in any way to its growth, but find shelter in its crevices or in its near neighborhood.

But these larger animals are not the only ones that haunt the forest. There is a host of parasites besides, principally insects and their larvae, which bore their way into the very heart of the tree, making their home in the bark and pith, and not the less numerous because hidden from sight. These also have their counterparts in the Reef, where numbers of boring Shells and marine Worms work their way into the solid substance of the wall, piercing it, with holes in every direction, till large portions become insecure, and the next storm suffices to break off the fragments so loosened. Once detached, they are tossed about in the water, crumbled into Coral sand, crushed, often ground to powder by the friction of the rocks and the constant action of the sea.

After a time, an immense quantity of such materials is formed about a Coral Reef; tides and storms constantly throw them up on its surface, and at last a soil collects on the top of the Reef, wherever it has reached the surface of the water, formed chiefly of its own debris, of Coral sand, Coral fragments, even large masses of Coral rock, mingled with the remains of the animals that have had their home about the Reef, with sea-weeds, with mud from the neighboring land, and with the thousand loose substances always floating about in the vicinity of a coast and thrown upon the rocks or shore with every wave that breaks against them. Add to this the presence of a lime-cement in the water, resulting from the decomposition of some of these materials, and we have all that is needed to make a very compact deposit and fertile soil, on which a vegetation may spring up, whenever seeds floating from the shore or dropped by birds in their flight take root on the newly formed island.

There is one plant belonging to tropical or sub-tropical climates that is peculiarly adapted by its mode of growth to the soil of these islands, and contributes greatly to their increase. This is the Mangrove-tree. Its seeds germinate in the calyx of the flower, and, before they drop, grow to be little brown stems, some six or seven inches long and about as thick as a finger, with little rootlets at one end. Such Mangrove-seedlings, looking more like cigars than anything else, float in large numbers about the Reef. I have sometimes seen them in the water about the Florida Reef in such quantities that one would have said some vessel laden with Havana cigars had been wrecked there, and its precious cargo scattered in the ocean.

In consequence of their shape and the development of the root, one end is a little heavier than the other, so that they float unevenly, with the loaded end a little lower than the lighter one. When they are brought by the tide against such a cap of soil as I have described, they become stranded upon it by their heavier end, the rootlets attach themselves slightly to the soil, the advancing and retreating waves move the little plant up and down, till it works a hole in the sand, and having thus established itself more firmly, steadied itself as it were, it now stands upright, and, as it grows, throws out numerous roots, even from a height of several feet above the ground, till it has surrounded the lower part of its stem with a close net-work of roots. Against this natural trellis or screen all sorts of materials collect; sand, mud, and shells are caught in it; and as these Mangrove-trees grow in large numbers and to the height of thirty feet, they contribute greatly to the solidity and compactness of the shores on which they are stranded.

Such caps of soil on the summit of a Coral Reef are of course very insecure till they are consolidated by a long period of accumulation, and they may even be swept completely away by a violent storm. It is not many years since the light-house built on Sand Key for the greater security of navigation along the Reef was swept away with the whole island on which it stood. Thanks to the admirably conducted Investigations of the Coast-Survey, this part of our seaboard, formerly so dangerous on account of the Coral Reefs, is now better understood, and every precaution has been taken to insure the safety of vessels sailing along the coast of Florida.

I cannot deny myself the pleasure of paying a tribute here to the high scientific character of the distinguished superintendent of this survey, who has known so well how to combine the most important scientific aims with the most valuable practical results in his direction of it. If some have hitherto doubted the practical value of such researches,—and unhappily there are always those who estimate intellectual efforts only by their material results,—one would think that these doubts must be satisfied now that the Coast-Survey is seen to be the right arm of our navy. Most of the leaders in our late naval expeditions have been men trained in its service, and familiar with all the harbors, with every bay and inlet of our Southern coasts, from having been engaged in the extensive researches undertaken by Dr. Bache and carried out under his guidance. Many, even, of the pilots of our Southern fleets are men who have been employed upon this work, and owe their knowledge of the coast to their former occupation. It is a singular fact, that at this very time, when the whole country feels its obligation to the men who have devoted so many years of their lives to these investigations, a proposition should have been brought forward in Congress for the suspension of the Coast-Survey on economical grounds. Happily, the almost unanimous rejection of this proposition has shown the appreciation in which the work is held by our national legislature. Even without reference to their practical usefulness, it is a sad sign, when, in the hour of her distress, a nation sacrifices first her intellectual institutions. Then more than ever, when she needs all the culture, all the wisdom, all the comprehensiveness of her best intellects, should she foster the institution that have fostered them, in which they have been trained to do good service to their country in her time of need.

Several of the Florida Keys, such as Key West and Indian Key, are already large, inhabited islands, several miles in extent. The interval between them and the main-land is gradually filling up by a process similar to that by which the islands themselves were formed. The gentle landward slope of the Reef and the channel between it and the shore are covered with a growth of the more branching lighter Corals, such as Sea-Fans, Coral-lines, etc., answering the same purpose as the intricate roots of the Mangrove-tree. All the debris of the Reef, as well as the sand and mud washed from the shore, collect in this net-work of Coral growth within the channel, and soon transform it into a continuous mass, with a certain degree of consistence and solidity. This forms the foundation of the mud-flats which are now rapidly filling the channel and must eventually connect the Keys of Florida with the present shore of the peninsula.

Outside the Keys, but not separated from them by so great a distance as that which intervenes between them and the main-land, there stretches beneath the water another Reef, abrupt, like the first, on its seaward side, but sloping gently toward the inner Reef, and divided from it by a channel. This outer Reef and channel are, however, in a much less advanced state than the preceding ones; only here and there a sand-flat large enough to afford a foundation for a beacon or a lighthouse shows that this Reef also is gradually coming to the surface, and that a series of islands corresponding to the Keys must eventually be formed upon its summit. Some of my readers may ask why the Reef does not rise evenly to the level of the sea, and form a continuous line of land, instead of here and there an island. This is accounted for by the sensitiveness of the Corals to any unfavorable circumstances impeding their growth, as well as by the different rates of increase of the different kinds. Wherever any current from the shore flows over the Reef, bringing with it impurities from the land, there the growth of the Corals will be less rapid, and consequently that portion of the Reef will not reach the surface so soon as other parts, where no such unfavorable influences have interrupted the growth. But in the course of time the outer Reef will reach the surface for its whole length and become united to the inner one by the filling up of the channel between them, while the inner one will long before that time become solidly united to the present shore-bluffs of Florida by the consolidation of the mud-flats, which will one day transform the inner channel into dry land.

What is now the rate of growth of these Coral Reefs? We cannot, perhaps, estimate it with absolute accuracy, since they are now so nearly completed; but Coral growth is constantly springing up wherever it can find a foothold, and it is not difficult to ascertain approximately the rate of growth of the different kinds. Even this, however, would give us far too high a standard; for the rise of the Coral Reef is not in proportion to the height of the living Corals, but to their solid parts which never decompose. Add to this that there are many brittle delicate kinds that have a considerable height when alive, but contribute to the increase of the Reef only so much additional thickness as they would have when broken and crushed down upon its surface. A forest in its decay does not add to the soil of the earth a thickness corresponding to the height of its trees, but only such a thin layer as would be left by the decomposition of its whole vegetation. In the Coral Reef, also, we must allow not only for the deduction of the soft parts, but also for the comminution of all these brittle branches, which would be broken and crushed by the action of the storms and tides, and add, therefore, but little to the Reef in proportion to their size when alive.

The foundations of Fort Jefferson, which is built entirely of Coral rock, were laid on the Tortugas Islands in the year 1846. A very intelligent head-work man watched the growth of certain Corals that established themselves on these foundations, and recorded their rate of increase. He has shown me the rocks on which Corals had been growing for some dozen years, during which they had increased at the rate of about half an inch in ten years. I have collected facts from a variety of sources and localities that confirm this testimony. A brick placed under water in the year 1850 by Captain Woodbury of Tortugas, with the view of determining the rate of growth of Corals, when taken up in 1858 had a crust of Maeandrina upon it a little more than half an inch in thickness. Mr. Allen also sent me from Key West a number of fragments of Maeandrina from the breakwater at Fort Taylor; they had been growing from twelve to fifteen years, and have an average thickness of about an inch. The specimens vary in this respect,—some of them being a little more than an inch in thickness, others not more than half an inch. Fragments of Oculina gathered at the same place and of the same age are from one to three inches in length; but these belong to the lighter, more branching kinds of Corals, which, as we have seen, cannot, from their brittle character, be supposed to add their whole height to the solid mass of the Coral wall. Millepore gives a similar result.

Estimating the growth of the Coral Reef according to these and other data of the same character, it should be about half a foot in a century; and a careful comparison which I have made of the condition of the Reef as recorded in an English survey made about a century ago with its present state would justify this conclusion. But allowing a wide margin for inaccuracy of observation or for any circumstances that might accelerate the growth, and leaving out of consideration the decay of the soft parts and the comminution of the brittle ones, which would subtract so largely from the actual rate of growth, let us double this estimate and call the average increase a foot for every century. In so doing, we are no doubt greatly overrating the rapidity of the progress, and our calculation of the period that must have elapsed in the formation of the Reef will be far within the truth.

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. Astroides, 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:—


Halcyonoids: eight tentacles in pairs, lobed around the margin; always combined in large communities, some of which are free and movable like single animals.

Madrepores: twelve tentacles, alternating in six larger and six smaller ones; frequently a larger top animal standing prominent in the whole community, or on the summit of its branches.

Porites: twelve tentacles, not alternating in size; system of connected beams.

Astraeans: tentacles not definitely limited in number, though usually not exceeding one hundred, and generally much below this number; transverse floors. Maeandrines, generally referred to Astraeans, are higher than the true Astraeans, on account of their compound Polyps.

Fungidae: indefinite multiplication of tentacles; imperfect transverse beams.

Actiniae: indefinite multiplication of tentacles; soft bodies and no transverse beams.

If, now, we compare this structural gradation among Polyps with their geological succession, we shall find that they correspond exactly. The following table gives the geological order in which they have been introduced upon the surface of the earth.


Present, Halcyonoids.
Pliocene, \
Miocene, } Madrepores.
Eocene, /
Cretaceous, \ Porites
Jurassic, } and
Triassic, } Astraeans.
Permian, /
Carboniferous, \
Devonian, } Fungidae
Silurian, /

With regard to the geological position of the Actiniae we can say nothing, because, if their soft, gelatinous bodies have left any impressions in the rocks, none such have ever been found; but their absence is no proof that they did not exist, since it is exceedingly improbable that animals destitute of any hard parts could be preserved.

The position of the Corals on a Reef accords with these series of structural gradation and geological succession. It is true that we do not find the Actiniae in the Reef any more than in the crust of the earth, for the absence of hard parts in their bodies makes them quite unfit to serve as Reef-Builders. Neither do we find the Fungidae, for they, like all low forms, are single, and not confined to one level, having a wider range in depth and extent than other stony Polyps. But the true Reef-Building Polyps follow each other on the Reef in the same order as prevails in their structural gradation and their geological succession; and whether we classify them according to their position on the Reef, or their introduction upon the earth in the course of time, or their relative rank, the result is the same.

It would require an amount of details that would be tedious to many of my readers, were I to add here the evidence to prove that the embryological development of these animals, so far as it is known, and their geographical distribution over the whole surface of our globe, show the same correspondence with the other three series. But this recurrence of the same thought in the history of animals of the same Type, so that, from whatever side we consider them, their creation and existence seem to be guided by one Mind, is so important in the study of Nature, that I shall constantly refer to it in the course of these papers, even though I may sometimes be accused of unnecessary repetition.

What is the significance of these coincidences? They were not sought for by the different investigators, who have worked quite independently, while ascertaining all these facts, without even knowing that there was any relation between the objects of their studies. The succession of fossil Corals has been found in the rocks by the geologist,—the embryologist has followed the changes in the growth of the living Corals,—the zooelogist has traced the geographical distribution and the structural relations of the full-grown animals; but it is only after the results of their separate investigations are collected and compared that the coincidence is perceived, and alt find that they have been working unconsciously to one end. These thoughts in Nature, which we are too prone to call simply facts, when in reality they are the ideal conception antecedent to the very existence of all created beings, are expressed in the objects of our study. It is not the zooelogist who invents the structural relations establishing a gradation between all Polyps,—it is not the geologist who places them in the succession in which he finds them in the rocks,—it is not the embryologist who devises the changes through which the living Polyps pass as he watches their growth; they only read what they see, and when they compare their results they all tell the same story. He who reads most correctly from the original is the best naturalist. What unites all their investigations and makes them perfectly coherent with each other is the coincidence of thought expressed in the facts themselves. In other words, it is the working of the same Intellect through all time, everywhere.

When we observe the practical results of this sequence in the position of Corals on the Reef, we cannot fail to see that it is not a mere accidental difference of structure and relation, but that it bears direct reference to the part these little beings were to play in Creation. It places the solid part of the structure at the base of the Reef,—it fills in the interstices with a lighter growth,—it crowns the summit with the more delicate kinds, that yield to the action of the tides and are easily crushed into the fine sand that forms the soil,—it makes a masonry solid, compact, time-defying, such a masonry as was needed by the great Architect, who meant that these smallest creatures of His hand should help to build His islands and His continents.

Excerpt from "Methods of Study in Natural History" Atlantic Monthly, Vol. 9, No. 56, June, 1862, by various authors.

Keywords: coral reefs, florida keys, key west, natural history


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