POLYPES. 231 



With islands thus formed, and others in the 

 several stages of their formation, Torres Strait is 

 nearly choked up. The time will come it may be 

 ten thousand or ten million years, but come it 

 must when New Holland and New Guinea, and all 

 the little groups of islets and reefs to the north and 

 north-west of them, will either be united in one 

 great continent, or be separated only by deep chan- 

 nels, in which the strength or velocity of the 

 currents may perhaps obstruct the silent and un- 

 observed agency of these insignificant, but most 

 efficacious labourers. 



FIG. 29. Fragment of Hadrepora muricata. 



Madrepora muricata, L. (Fig. 29), is the species 

 which contributes most largely to the formation of 

 reefs ; it is often sold for ornaments, particularly in 



