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 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