162 THREE CRUISES OF THE “ BLAKE." 
from the European seas, and of their limitation to those regions 
of the African, Indian, Australian, Pacific, and West Indian 
seas which most faithfully represent at the present day the con- 
ditions which formcrly 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 
living 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 limits of the tropics." They con- 
sist of a littoral 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 
1 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 
