204 SEA AND LAND 



in their several societies by a process of budding, while 

 the)- multiph' the communities themselves by reproduction 

 from the ^%'g. Thus they are able to grow continuously, 

 after the manner of forest trees, the living resting upon the 

 framework which the dead have left behind them. By means 

 of this double growth they are able to construct vast lime- 

 stone ridges wherever the conditions of the sea favor their 

 development. 



Although solitary corals and communities of insignificant 

 growth plentifully develop far and wide over the sea-floors 

 of tropical and even of temperate regions, the reef-building 

 species can do their characteristic work of construction onh' 

 where tolerably swift moving currents of pure water, having 

 the temperature of the tropical seas, flow against shores or 

 shallows where the water is less than about a hundred and 

 twenty feet in depth. Where such oceanic currents impinge 

 upon a shore or shallow, the germs of the reef-building corals 

 develop upon the bottom, spread over the surface which has 

 less than twenty fathoms of water upon it, and proceed to 

 grow with great rapidity. The evidence goes to show that 

 some of the species may rise above the bottom, at the rate of 

 as much as three inches a year, and it is not improbable that 

 these reefs may grow upward, if the conditions be favorable 

 for rapid growth, to the extent of a foot in from one hundred 

 to two hundred years. This, though slow in terms of human 

 history, is In a geologic sense extremely rapid. As the reef 

 gains in thickness, the outer part of the growth, that which 

 faces the deeper sea. waxes more rapidly, for the colonies of 

 polyps which occupy that position arc most effectively bathed 

 by the ocean currents from the waters of which the animals 

 take their food. The nearer this swiftly growing margin 



