GEOGRAPHICAL DISTRIBUTION. 103 
102. ‘The range of these reef-forming corals in depth is singularly 
small. ‘Twenty or perhaps sixteen fathoms will include very nearly 
all the species of the Madrepore and Astrea tribes.* ‘Temperature 
has little or no influence in occasioning this limit, as 68° F. will not 
be found under the equator short of a depth of one hundred fathoms. 
Light and pressure, the latter affecting the amount of air for aeration, 
are probably the principal causes. ‘The waves, moreover, not reach- 
ing, when most powerful, to a greater depth than fifteen or twenty 
fathoms, cannot aid in renewing the expended air below, as they do 
at the surface. 
In recapitulation we state that the Astraacea, Madreporacea, and 
the Gemmiporide among the Caryophyllacea, are, with few excep- 
tions, confined to the coral-reef seas, and to within twenty fathoms 
of the surface. ‘The Caryophyllide extend from the equator to the 
frigid zone, and some species occur at a depth of two hundred 
fathoms or more. The Alcyonaria have an equally wide range with 
the Caryophyllide, and probably reach still farther towards the poles. 
The Hydroidea range from the equator to the polar regions, but are 
most abundant in the waters of the temperate zone. 
103. Besides the above-mentioned limiting causes, there are others 
of importance, one of which only may be alluded to in this place, the 
remaining belonging more properly to the Geological Report on 
Coral Reefs and Islands. The cause referred to, is that proceeding 
from original sites or centres of distribution. There is sufficient 
evidence that such centres of distribution, as regards zoophytes, are 
to be recognised. ‘The species of corals in the West Indies are, in 
many respects, peculiar, and not one can with certainty be identified 
with any of the East Indies. The central parts of the Pacific Ocean 
appear to be almost as peculiar in the corals they afford. But few 
from the Feejees have been found to be identical with those of the 
Indian Ocean. A more complete acquaintance with the corals of these 
different seas may multiply the number of identical species; but ob- 
servations, thus far made, seem sufficient to establish as a fact that a 
large part of zoophytes are confined to a small longitudinal range. 
This will be seen from the following table, exhibiting, in a general 
manner, as far as known, their geographical distribution. Each 
column gives the number peculiar to the region specified at top. 
* The evidences on this point will be presented in the Report on Coral Islands. 
} The exceptions belong mostly to the genus Euphyllia. 
