138 LEISURE-TIME STUDIES. 



question of temperature is a highly important one, inasmuch 

 as the condition of the sea as to warmth will be found to 

 regulate the distribution in space of the corals. The geo- 

 graphy of these animals, in short, is bounded by well-defined 

 lines or degrees of temperature ; and the statement that 

 reef-building corals will not as a rule flourish and grow in 

 seas the temperature of which falls below 68 Fahr., may be 

 taken as a summary of what has been ascertained on this 

 point. We must, therefore, look to equatorial seas, as those 

 in which the typical development of reef-building corals 

 occurs ; and a ready mode of stating the broad facts of the 

 distribution of coral life consists in our selecting the equator 

 as a natural centre of our globe, and in measuring off a 

 band of 1800 miles in breadth on each side of that line. 

 A broad band or area some 3600 miles in breadth, en- 

 compassing our globe, and having the equator for its centre, 

 will thus be found to include in its course the chief regions 

 of coral growth. But, as Mr. Dana remarks, whilst the 

 distribution of corals depends to a very great extent upon 

 temperature, " regional peculiarities" also " exist that are 

 not thus accounted for." 



Whilst the Pacific and Indian Oceans form great reposi- 

 tories of coral-reefs existing within the limits just mentioned, 

 and whilst the Red Sea, the N. E. coast of Australia, and the 

 coast of Florida also exemplify great areas of coral develop- 

 ment, certain other oceanic tracts exist, from which coral- 

 reefs are wholly absent. Mr. Darwin thus informs us that 

 " no coral-reefs were observed during the surveying voyages 

 of the Beagle on the west coast of South America south 

 of the equator, or round the Galapagos Islands. It appears 

 also," he continues, "that there are none on this coast north 

 of the equator." The western coast of Africa is singularly 

 free from coral-reefs ; and it may be laid down as a rule of 

 the widest possible kind, that coral-reefs are not found near 

 the estuaries of great rivers, a result clearly due to the mixed 

 or brackish character of the water in such situations. It 

 may be shown that the absence of reefs on the western 



