GEOGRAPHICAL DISTRIBUTION. 299 



CHAPTER IV. 



GEOGRAPHICAL DISTRIBUTION OF 

 CORAL REEFS AND ISLANDS. 



The distribution of coral reefs over the globe depends on 

 the following circumstances, arising from the habitudes of 

 polyps already explained. 



1. The temperature of the ocean. 



2. The character of coasts as regards (a) the depth of 

 water, — (b) the nature of the shores, — (c) the presence of 

 streams. 



3. Liability to exposure to destructive agents, such as vol- 

 canic heat. 



It has been stated (p. 108) that reef-growing corals will flour- 

 ish in the hottest seas of the equator, and over the ocean, wher- 

 ever the average temperature of the waters during the coldest 

 month of winter, is not below 68 °F. The isothermal line of 

 this temperature (or isocryme) forms, therefore, the boundary 

 line of the coral-reef seas. Other corals not forming reefs 

 grow in colder seas (p. 109), but to those we do not now refer. 



This line traverses the oceans between the parallels 26° and 

 30°, or in general near 28° But, as has been stated, in the 

 vicinity of the continents it undergoes remarkable flexures 

 from the influence of oceanic currents, the polar currents bending 

 it toward the equator, while the tropical cause a divergence. 

 From a comparison of the thermometrical observations of va- 



