STRUCTURAL SERIES.
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.
GEOLOGICAL SUCCESSION.
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.
[Illustration: SUCCESSION ON THE REEF.]
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 zoölogist 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 zoölogist 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.