20 HISTORY 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. We refer to the theory of the 
philosophical-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 im 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 three 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 
