GEOGRAPHICAL DISTRIBUTION: 255 



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, — (fi) the nature of the shores, — {c) the presence of 

 streams. 



3. Liability to exposure to destructive agents, such as 

 volcanic heat. 



It has been stated (p. 255) that reef-growmg corals will 

 flourish in the hottest seas of the equator, and over the ocean, 

 wherever 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. 255), 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 



