I need not dwell here on the further history of such a Coral island, or follow it through the changes by which the summit of its circular wall becomes covered with a fertile soil, a tropical vegetation springs up on it, and it is at last perhaps inhabited by man.  There is something very attractive in the idea of these green rings inclosing sheltered harbors and quiet lakes in mid-ocean, and the subject has lost none of its fascination since the mystery of their existence has been solved by the investigations of several contemporary naturalists who have enabled us to trace the whole story of their structure.  I would refer all who wish for a more detailed account of them to Charles Darwin’s charming little volume on “Coral Reefs,” where their mode of formation is fully described, and also to James D. Dana’s “Geological Report of the United States Exploring Expedition.”

Coral Reefs are found only in tropical regions:  although Polyps, animals of the same class as those chiefly instrumental in their formation, are found in all parts of the globe, yet the Reef-Building Polyps are limited to the Tropics.  We are too apt to forget that the homes of animals are as definitely limited in the water as on the land.  Indeed, the subject of the geographical distribution of animals according to laws that are established by altitude, by latitude and longitude, by pressure of atmosphere or pressure of water, already alluded to in a previous article, is exceedingly interesting, and presents a most important field of investigation.  The climatic effect of different degrees of altitude upon the growth of animals and plants is the same as that of different degrees of latitude; and the slope of a high mountain in the Tropics, from base to summit, presents, in a condensed form, an epitome, as it were, of the same kind of gradation in vegetable growth that may be observed from the Tropics to the Arctics.  At the base of such a mountain we have all the luxuriance of growth characteristic of the tropical forest,—­the Palms, the Bananas, the Bread-trees, the Mimosas; higher up, these give way to a different kind of growth, corresponding to our Oaks, Chestnuts, Maples, etc.; as these wane, on the loftier slopes comes in the Pine forest, fading gradually, as it ascends, into a dwarfish growth of the same kind; and this at last gives way to the low creeping Mosses and Lichens of the greater heights, till even these find a foothold no longer, and the summit of the mountain is clothed in perpetual snow and ice.  What have we here but the same series of changes through which we pass, if, travelling northward from the Tropics, we leave Palms and Pomegranates and Bananas behind, where the Live-Oaks and Cypresses, the Orange-trees and Myrtles of the warmer Temperate Zone come in, and these die out as we reach the Oaks, Chestnuts, Maples, Elms, Nut-trees, Beeches, and Birches of the colder Temperate Zone, these again waning as we enter the Pine forests of the Arctic borders, till, passing out of these, nothing but a dwarf vegetation, a carpet of Moss and Lichen, fit food for the Reindeer and the Esquimaux, greets us, and beyond that lies the region of the snow and ice fields, impenetrable to all but the daring Arctic voyager?