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 ebbiugs 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 dwelKng 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. 

 " Every one,^^ says my excellent friend Dr. Johnston, in his 



