134 CORAL AND ATOLLS 



corals, exhibiting probably one form of growth only, and one 

 that is represented by an intermediate type to-day. 



With the origin of the heaped-up debris that forms the 

 island ring, and with the formation of the barrier above the 

 level of the tides, a gradual change occurred, and in the place 

 of one uniform environment, an infinity of diverse habitats 

 was produced. 



The rough water of the barrier, the smooth water of the 

 lagoon, the silting water of the inlets, and the clear water of 

 the ocean, were marked off from one another ; and the em- 

 bryos of the originally similar corals had to grow dissimilar to 

 adapt their vegetative types to the new-formed habitats. 



In this way the present infinity of types was brought 

 about, and wherever the environment is changing to-day new 

 types are developing to conform with its demands. 



