12 MR GOODCHILD ON , 
Whether it is from the fact that these are both com- 
paratively cold currents, or that the courses of the currents 
in question are towards and not away from any centres 
where the ova of reef-building corals are set free, the fact 
remains that there are no coral reefs at all on the west 
coast of Africa. Equally marked is the effect, so far as 
coral reefs are concerned, produced by the California 
Current off the west coast of North America and by the 
Peru Current off the south. In these cases also it results 
that not a single coral reef is to be found on the whole 
west coast of America. It must not, however, be supposed 
from this that corals of no kinds exist; on the contrary, 
many genera, and a large number of species, not of the reef- 
building kinds, exist in more or less abundance and at 
almost all depths. Off the east coast of North America the 
influence of the cold Labrador Current produces similar 
effects upon the reef corals even as far south as Florida. 
The courses of these cold currents are determined by at 
least four factors:—(1) The difference in specific gravity 
between cold water and warm ; (2) the lowering of the sea 
level within the Tropics, arising from the quantity of sea- 
water evaporated being in excess of the rainfall; (3) the 
influence of the earth’s rotation, which carries the land east- 
ward faster than it does the waters of the sea; (4) the influence 
of the prevailing winds, which blow off-shore on the west and 
on-shore on the east, and therefore cause the warm surface- 
waters to drift to leeward, and also permit of the uprise of 
the colder substrata of the sea-water to the surface. 
There are, however, as might be expected, and as the 
recent deep-sea investigations have proved, some oceanic 
areas where the warmth suitable for the growth of coral 
reefs extends to a considerable depth below the surface. 
The following are two cases of the kind which specially 
concern the student of coral reefs, In the chief coral tract 
of the Pacific, the Challenger results have shown that a 
mean temperature of 60° Fahr. exists, to a depth of a hundred 
fathoms, over an area nearly equal in extent to the whole of 
Europe. The same temperature at that depth is found also 
in the south-west of the Indian Ocean, over an area nearly 
equal to that of Australia. If we are correct in supposing 
