xxi 
lost in the regions of the clouds : such are the madre- 
porous islands so numerous in the South-eastern 
Ocean, which Polypi are every day augmenting by 
their inconceivable labours. Some authors have 
imagined that these isles were only the summits of 
submarine mountains, that had been covered by Po- 
lypidoms ; but these submarine mountains, the almost 
perpetual source of subterranean fires, are found in 
all latitudes, and greatly vary in their extent and in 
their forms : the madreporic islands, on the contrary, 
exist only within the tropics, and present forms con- 
stantly analogous to each other, and are never over- 
turned by earthquakes or volcanic eruptions. They 
are described by most modern voyagers who have 
traversed those seas. 
If we compare this polypean mass, that rises from 
the immeasurable depths of the ocean to its surface, 
and that shoots into air under jthe form of mountains, 
to that Melobesia, to that Cellepora, which has only 
the appearance of a white spot, a small deposite of 
calcareous particles, how striking is the difference ! 
what an infinity of intermediates must exist between 
the two extremes ! Generally speaking, the Cellular 
Polypidoms, as well as the Calciferous and Car- 
noid, seldom exceed a metre in height ; usually 
they are much smaller. The Corticiferous are some- 
