20 HISTOKY OF BRITISH ZOOPHYTES. 



a hundred miles. These would indeed have been the craters 

 of tremendous volcanoes ! Instead of enumerating the 

 various theories, we shall merely give a short account of 

 one which has been much countenanced, and which cer- 

 tainly seems the most plausible and satisfactory of any with 

 which we are acquainted, AA"e refer to the theory of the 

 philoso])hical-minded Darwin, first, we believe, brought for- 

 ward in his interesting journal of the Voyages of the Bea- 

 gle, and afterwards more fully brought out in a separate 

 publication. He divides reefs into three classes : first, 

 fringing reefs; second, barrier reefs; third, atolls. The 

 fringing reef is that which is near to the shore, and along 

 the shore of an island or of a continent. The barrier reef 

 is along the shore of a continent or around an island, but 

 at the distance it may be of many miles from continent or 

 island. The barrier reef encloses an island, with some 

 miles of sea betwixt the reef and the island. The atoll 

 encloses only water, and the enclosed space is often called a 

 lagoon. In order to understand Mr. Darwin's theory it is 

 necessary to remember his tlu-ee kinds of reefs, though the 

 fringing reef, the barrier reef, and the atoll are only dif- 

 ferent phases of the same thing. We are to bear also in 

 mind that though the sea is proverbially changeable, the 



