194 J. D. Dana on an Isothermal Oceanic Chart, 



Their abundance at the Hawaiian Islands, as at Oahu, is 

 hence a consequence of their hardier character, and not a 

 mere region peculiarity independent of temperature. There 

 are grounds, therefore, for drawing a line between the Ha- 

 waiian Islands and the Feejees ; and as the temperature at 

 the latter sinks to 74J° F. some parts of the year, 74° F. is 

 taken as the limiting temperature. The Feejee seas are ex- 

 ceedingly prolific and varied in tropical species. The corals 

 grow in great luxuriance, exceeding in extent and beauty 

 anything elsewhere observed by the writer in the tropics. 

 The ocean between 74° F. north of the equator, and 74° F. 

 south, is therefore the proper tropical or torrid region of 

 zoological life. 



With respect to the line of 80 F., we are not satisfied 

 that it is of much importance as regards the distribution of 

 species. The range from the hottest waters of the ocean, 

 88° to 74° F., is but fourteen degrees, and there are probably 

 few species occurring within the region that demand a less 

 range. Still, investigations hereafter made may shew that 

 the hot waters limited by the isocryme of 80° include some 

 peculiar species. At Sydney Island and Fakaafo, within this 

 hot area, there appeared to be among corals a rather greater 

 prevalence than usual of the genus Manopora, which, as these 

 are tender species, may perhaps shew that the waters are 

 less favorable for hardier corals than those of the Feejees, 

 where the range of temperature is from 70° to 80° F. ; but 

 this would be a hasty conclusion, without more extended 

 observations. The author was on these islands only for a few 

 hours, and his collections were afterwards lost at the wreck 

 of the Peacock, just as the vessel was terminating the voyage 

 by entering the Columbia River. 



It is unnecessary to remark particularly upon the fitness 

 of the other isocrymals for the purposes of illustrating the 

 geographical distribution of marine species, as this will be- 

 come apparent from the explanations on the following pages. 



The regions thus bounded require, for convenience of de- 

 signation, separate names, and the following are therefore 

 proposed. They constitute three larger groups : the first, 

 the Torrid zone or Coral-reef seas, including all below the 



