26 HISTORY OF BRITISH ZOOPHYTES. 
formed, for fresh-water kills the little marine polypes. This 
accounts for the commencement of the gap, and it is not 
difficult to account for the continuance of it. It might be 
urged, that when the fringing reef has become a barrier 
reef, and is so far removed from land that the stream in its 
fresh state cannot reach it, then the polypes would resume 
their operations, and would soon fill up the gap. This 
would, doubtless, be the case had there been only the 
influence of the fresh-water as a preventative, but the tide, 
in its constant ebbings and flowings through the gap, would 
always deposit mud and sand, which would be as injurious to 
the polypes as the fresh-water, so that the gap begun by the 
stream continues after the stream has lost its freshness, as 
the rush of the tide and the filth that it carries along with 
it is injurious to the life, and consequently to the work, of 
the polypes. Hence it is that wherever there is a gap in 
a barrier reef, it has been observed that it is opposite to a 
place in the land where a stream enters the sea. 
We trust we may be excused for dwelling so long on the 
mighty works of zoophytes in the Pacific Ocean, as they 
are the kindred of those that inhabit our own seas. Our 
most distinguished naturalists delight to write of them. 
“Bvery one,” says my excellent friend Dr. Johnston, in his 
