90 
CONDITIONS FAVOURABLE TO 
Ch. IV. 
be thought would entirely remove the injurious effects 
both of the fresh water and the sediment, will receive 
a simple explanation when we discuss the origin of 
barrier-reefs. 
In the vegetable kingdom every different station 
has its peculiar group of plants, and similar relations 
appear to prevail with corals. We have already de- 
scribed the great difference between the corals within 
the lagoon of an atoll and those on its outer margin. 
The corals, also, on the margin of Keeling Island oc- 
curred in zones : thus the Porites and Millcpora compla- 
nata grow to a large size, only where they are washed by 
a heavy sea, and are killed by a short exposure to the 
air ; whereas, three species of Nullipora also live amidst 
the breakers, but are able to survive uncovered for a 
part of each tide : at greater depths, a strong Madre- 
pora and Millepora alcicornis are the commonest kinds; 
the former appearing to be confined to this part : be- 
neath the zone of massive corals, minute encrusting 
corallines and other organic bodies live. If we com- 
pare the external margin of the reef at Keeling atoll 
with that on the leeward side of Mauritius, which are 
very differently circumstanced, we shall find a corre- 
sponding difference in the appearance of the corals. At 
the latter place, the genus Madrepora is preponderant 
over every other kind ; and beneath the zone of massive 
corals there are large beds of Seriatopora. There is 
also a marked difference, according to Captain Moresby , 1 
1 Captain Moresby on the Northern Maldiva Atolls. Geograph. 
Journ., vol. v. p. 401. 
