vi.] ON CORAL AND CORAL REEFS. 117 



becomes fixed to the rock, gradually assuming the polype 

 form and growing up to the size of its parent. As the 

 infant polypes of the coral may retain this free and 

 active condition for many hours, or even days, and as aT 

 tidal or other current in the sea may easily flow at the 

 speed of two or even more miles in an hour, it is clear 

 that the embryo must often be transported to very con- 

 siderable distances from the parent. , And it is easily 

 understood how a^ingJe polype, which may give rise 

 to hundreds, or perhaps thousands, of embryos, may, by 

 this process of partly active and partly passive migra- 

 tion, cover an immense surface with its offspring. The 

 masses of coral which may be formed by the assemblages 

 of polypes which spring by budding, or by dividing, 

 from a single polype, occasionally attain very con- 

 siderable dimensions. Such skeletons are sometimes great 

 plates, many feet long and several feet in thickness ; or 

 they may form huge half globes, like the brainstone 

 corals, or may reach the magnitude of stout shrubs, or 

 even small trees. There is reason to believe that such 

 masses as these take a long time to form, and hence that 

 the age a polype tree, or polype turf, may attain, may be 

 considerable. But, sooner or later, the coral polypes, like 

 all other things, die ; the soft flesh decays, while the 

 skeleton is left as a stony mass at the bottom of the sea, 

 where it retains its integrity for a longer or a shorter 

 time, according as its position affords it more or less pro- 

 tection from the wear and tear of the waves. 



The polypes which give rise to the white coral are 

 found, as has been said, in the seas of all parts of the 

 world ; but in the temperate and cold oceans they are 

 scattered and comparatively small in size, so that the 

 skeletons of those which die do not accumulate in any 

 considerable quantity. But it is otherwise in the greater 

 part of the ocean which lies in the warmer parts of the 



