76 THREE CRUISES OF THE " BLAKE." 



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 Bernini. 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 suppHes 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 naturahsts, 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 



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. 



