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 chieiiy 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 effect of different levels of altitude upon the growth of 
animals and plants is the same as that of different degrees of latitude ; and 
