FLORIDA REEFS. 43 



the rate of increase in the wall is greater than that of subsidence in the 

 island, while the latter gradually sinks below the surface, the former rises 

 in proportion, and by the time it has completed its growth the central 

 island has vanished, and there remains only a ring of coral reef, with here 

 and there a break, perhaps, at some spot where the more prosperous growth 

 of the corals has been checked. If, however, as sometimes happens, there 

 is no such break, and the wall is perfectly uninterrupted, the sheet of sea- 

 water so enclosed may be changed to fresh water by the rains that are 

 poured into it. Such a water-basin will remain salt, it is true, in its lower 

 part, and the fact that it is affected by the rise and fall of the tides shows 

 that it is not entirely secluded from communication with the ocean outside ; 

 but the salt water, being heavier, sinks, while the lighter rain-water remains 

 above, and it is to all appearance actually changed into a fresh-water lake. 



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 upon 

 it, and it is at last, perhaps, inhabited by man. There is something very 

 attractive in the idea of these green rings enclosing 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 regulated by altitude, 

 by latitude and longitude, by pressure of atmosphere or pressure of water, 

 by temperature, light, &c., is exceedingly interesting, and presents a most 

 important field of investigation. 



The climatic efiect of different levels of altitude upon the growth of 

 animals and plants is the same as that of different degrees of latitude ; and 



