: CORALS AND CORAL REEFS. 11 
eventually assume the form of the adult and subside to the 
sea bottom, where they take up their stand on some solid 
object. Another process of natural selection now comes into 
play. Corals have very different habits in regard to the 
temperature of the water in which they can thrive. Some 
few can flourish in water whose temperature is not much 
above the freezing point; while others, to do at all well, 
require to be surrounded by water whose temperature never 
falls below 68° Fahr. This matter of temperature is a 
factor of the very first importance in connection with coral 
reefs ; for while certain corals appear indifferent in regard 
to what the temperature of the water may be—and, therefore, 
are able to live on the sea bottom at all depths from a few 
fathoms down to five miles—there are others, including the 
corals associated with coral reefs, that cease to thrive when- 
ever the temperature of the water falls much, and are killed 
if at any time of the year they are surrounded by water 
whose temperature happens to be below 68° Fahr. It is a 
well-known fact that the warmer currents of the sea are 
limited to within a few fathoms of the surface—the depth 
to which a given temperature (say 68° Fahr.) extends vary- 
ing considerably in different parts of the world, but being in 
no case very great. Even in those oceanic areas where the 
surface temperature rises to between 70° and 80° Fahr., the 
water at this temperature forms what may be described asa 
mere surface-film, which floats above deep currents of sea- 
water, whose temperature is considerably lower, and much 
too cold to permit warmth-loving animals like reef-building 
corals to flourish. 
To appreciate the influence of these submarine currents of 
cold water, we have but to turn to a good physical atlas and 
study the relation of the cold Arctic Labrador Current to the 
east of Georgia and Florida in relation to the Gulf Stream. 
Or, again, the influence of the cold Antarctic stream of water 
known as the Benguela Current, which rises to the surface 
off the west coast of South Africa, and which flows at first 
northward close to the coast and then westward, until it 
gradually merges into the South Equatorial Current. A 
similar current (uot, however, of so low a temperature 
initially) sets in along the coast of North-Western Africa. 
