118 CRITIQUES AND ADDRESSES. [vi. 



world, comprised within a distance of about 1,800 miles 

 oil each side of the equator. Within the zone thus 

 bounded, by far the greater part of the ocean is inhabited 

 by coral polypes, which not only form very strong and 

 large skeletons, but associate together into great masses, 

 like the thickets and the meadow turf, or, better still, the 

 accumulations of peat, to which plants give rise on the 

 dry land. These masses of stony matter, heaped up 

 beneath the waters of the ocean, become as dangerous 

 to mariners as so much ordinary rock, and to these, 

 as to common rock ridges, the seaman gives the name 

 of "reefs." 



Such coral reefs cover many thousand square miles in 

 the Pacific and in the Indian Oceans. There is one reef, 

 or rather great series of reefs, called the Barrier Reef, 

 which stretches, almost continuously, for more than 1,100 

 miles off the east coast of Australia. Multitudes of the 

 island in the Pacific are either reefs themselves, or are 

 surrounded by reefs. The Red Sea is in many parts 

 almost a maze of such reefs ; and they abound no less in 

 the West Indies, along the coast of Florida, and even as 

 far north as the Bahama Islands. But it is a very 

 remarkable circumstance that, within the area of what 

 we may call the " coral zone," there are no coral reefs 

 upon the west coast of America, nor upon the west coast 

 of Africa ; and it is a general fact that the reefs are 

 interrupted, or absent, opposite the mouths of great 

 rivers. The causes of this apparent caprice in the distri- 

 bution of coral reefs are not far to seek. The polypes 

 which fabricate them require for their vigorous growth a 

 temperature which must not fall below 68 Fahrenheit 

 all the year round, and this temperature is only to be 

 found within the distance on each side of the equator 

 which has been mentioned, or thereabouts. But even 

 within the coral zone this degree of warmth is not every- 



