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mass of animals which undoubtedly flourished upon the great 
submarine plateau at a time when the Gulf Stream found its 
way out of the Gulf of Mexico with less velocity than it now 
has as it passes through the narrow Straits of Bemini. At that 
time it spread itself fan-shaped over the southern part of Florida 
and the Bahama Bank, and flowed. more gently northerly and 
easterly along the coast, with the additional reinforcement of the 
westerly equatorial, flowing north of the Great Antilles to the 
eastward of Cuba. The Bahama Bank then probably consisted 
of a series of banks like Salt Key Bank, separated by channels 
like the Santarem and St. Nicholas, which were undoubtedly 
kept open by the same currents as now form the Old Bahama 
Channel. These channels, like those between the keys on the 
Florida side, have gradually become filled with the detritus 
driven into them by the trade winds, until the bank has been 
formed in its present state of consolidation. Yet it must not 
be forgotten that, while in the western part of the Caribbean, 
the Gulf of Mexico, and Florida a dead level prevails, at 
Havana, Hayti, and Barbados we have reefs elevated to a great 
height, and others at considerable elevation on certain of the 
Greater and Lesser Antilles. 
The somewhat capricious distribution of coral reefs may per- 
haps be explained by the action of the great equatorial currents. 
The larger reefs occur in regions to which these currents bring 
in the track of their course abundant supplies of food for the 
reef-building animals. On the eastern coast of Africa, of Cen- 
tral America, or of Australia, for instance, extensive colonies of 
coral reefs flourish, while on the western coast of the same con- 
tinents, in similar latitudes, but not bathed by such powerful 
equatorial currents, the supply of food seems insufficient for 
more than the isolated patches of corals existing there. 
Other naturalists, as Semper’ and Murray,’ and later, Studer, 
have already attempted to explain the formation of coral reefs, 
in part at least, on grounds differing essentially from those to 
which Darwin ascribed them, and similar in the main to those 
THREE CRUISES OF THE “ BLAKE.” 
1 Semper, C. Zeits. f. Wiss. Zool, No. 107, 1880, X. p. 505, On the Struc- 
1863, XIII. p. 558. ture and Origin of Coral Reefs and 
? Murray, John. Proc. R. S. Edinb., Islands. 
