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 Astreea 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 Astraeacea, Madreporacea, and 

 the Gemmiporidae among the Caryophyllacea, are, with i^'^- excep- 

 tions, confined to the coral-reef seas,t and to within twenty fathoms 

 of the surface. The Caryophyllidse 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 Caryophyllidaj, 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 almo.st 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 j>ecM&r to the region specified at top. 



* The evidences on this point will be presented in the Report on Coral Islands, 

 t The exceptions belong mostl_v to the genus Euphyllia. 



