Ix | THE SEA 361 
now occupy the place which was once the 
highest land. He poimted out that some 
lagoons, as for instance that of Vanikoro, 
contain an island in the middle; while other 
islands, such as Tahiti, are surrounded by a 
margin of smooth water separated from the 
ocean by a coral reef. Now if we suppose 
that Tahiti were to sink slowly it would 
gradually approximate to the condition of 
Vanikoro; and if Vanikoro gradually sank, 
the central island would disappear, while on 
the contrary the growth of the coral might 
neutralise the subsidence of the reef, so that 
we should have simply an atoll with its 
lagoon. The same considerations explain the 
origin of the “barrier reefs,’ such as that 
which runs for nearly a thousand miles, along 
the north-east coast of Australia. Thus 
Darwin’s theory explains the form and the 
approximate identity of altitude of these 
coral islands. But it does more than 
this, because it shows that there are great 
areas in process of subsidence, which though 
slow, is of great importance in physical 
geography. 
