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.