162 THREE CRUISES OF THE **' BLAKE.'* 



from the European seas, and of their limitation to those regions 

 of the Afi'ican, Indian, Australian, Pacific, and West Indian 

 seas which most faithfully represent at the present day the con- 

 ditions which formerly made it possible for coral reefs to thrive 

 so far to the north as the British Islands. At the same time, 

 they give a natural explanation of the cosmopolitan nature of 

 many of the species of older geological periods. I have already 

 referred to this when speaking of the Echini of the two sides 

 of Panama. The history of coral reefs forms one of the most 

 suggestive aids in tracing the persistency of species and types 

 from the earlier geological times. The identity of some of the 

 cainozoic deep-sea corals with those now living at great depths 

 shows us that, with advancing knowledge, the distinctions be- 

 tween the marine fauna of the miocene and pliocene and the 

 fauna of to-day are constantly narrowed. 



It becomes evident that a large number of the species now 

 li\dng must have flourished before the important changes in the 

 physical geography which distinguish the present period from 

 the later tertiaries had taken place. We recognize the main 

 outlines of the bathymetrical faunal divisions as clearly as we 

 trace a tropical, a temperate, and an arctic fauna and flora along 

 a mountain slope within the Hmits of the tropics.^ They con- 

 sist of a Httoral fauna, all light, motion, and heat ; a continental 

 fauna, with superabundance of food and an equable tempera- 

 ture; and a deep-sea fauna, having a cold, unvaried temperature, 

 deriving its food largely from pelagic animals and plants. It 

 is however impossible to determine zones of depths except in 

 the most general way, because representatives of nearly all the 

 principal groups characteristic of the deep sea find their way up 

 to higher levels, and vice versa. 



The fauna found at great depths in the ocean is peculiar, and 

 appears to contain many species of extensive geographical range, 

 and to be made up of a smaller number of representative species 

 than is common in areas of lesser depth. We lose the geo- 

 graphical limits we are accustomed to find in lesser depths, and 



^ Oersted was perhaps the first to at- he recognized extended but little beyond 

 tempt a subdivision of the littoral range low-water mark, 

 of marine animals and plants. The belts 



