MODE OF ORIGIN. 25 
show that corals may flourish alike over all parts of the bank, when 
not too deep. The zoophyte can by no means be said to prefer the 
declivity to the central plateau of the submarine bank: on the con- 
trary, the part nearest the surface appears to abound in the largest 
species of corals.* 
A study and comparison of the reefs of different kinds,—fringing, 
barrier, and atoll,—throughout the oceans, is the only philosophical 
mode of arriving at any conclusion on this subject. ‘This course Mr. 
Darwin has happily and successfully pursued, and has arrived, as 
we have reason to believe, at the true theory on the subject. It is 
satisfactory, because it is a simple generalization of facts. ‘The ex- 
plorations of the Expedition afford striking illustrations of his views, 
and elucidate some points which are still deemed obscure, establishing 
the theory on a firm basis of evidence, and exhibiting its complete 
correspondence with observation.f 
Channels nithin barriers.—We may turn again to the chart of the 
Feejee Group, and glance successively at the islands Goro, Angau, 
Nairai, Lakemba, Argo Reef, Exploring Isles, and Nanuku. In 
Goro, the reef closely encircles the land upon whose submarine 
shores it was built up. In the island next mentioned, the reef has 
the same character, but is more distant from the shores, forming what 
has been termed a barrier reef; the name implying a difference in 
position, but none in mode of formation. In the last of the islands 
enumerated, the barrier reef includes a large sea, and the island it 
encloses is but a rocky peak in this sea. 
Can we account for this diversity in the position of barrier reefs, 
and in their extent as compared with the enclosed land? | There is 
evidently one way in which these features might have been produced. 
If, for example, such an island as Angau were very gradually to sub- 
side, from some subterranean cause, two results would take place :— 
the land would slowly disappear, while the coral reef, which is ever 
in constant increase, as has been explained, might retain itself at the 
surface, if the rapidity of subsidence was not beyond a certain rate. 
This subsidence might go on till the last mountain peak remained 
alone above the waters. Should we not then have a Nanuku? Sup- 
pose the subsidence not to have proceeded quite as far as this, it might 
* Lieutenant Nelson, R.N., suggested this hypothesis before the publication of Mr. 
Darwin’s views. See Geol. Trans., vol. v. 122; and Darwin, op. cit. p. 94. 
+ This is given as the conclusion of the author. A different view is offered by Captain 
Wilkes, in the Narrative of the Expedition, iv. 268, 
32 
