242 THE GEOLOGY OF BVKUADOS. 



time when the materials available for comparison included but 

 little beyond the littoral fauna. From the results of the deep-sea 

 dredgings we have become quite familiar with the extent of this 

 resemblance. In fact the deep-sea fauna of the Caribbean and 

 Gulf of Mexico is far more closely related to that of the Pacific 

 than to that of the Atlantic. Before the Cretaceous period, the 

 Gulf of Mexico and the Caribbean were undoubtedly in freer com- 

 munication with the Pacific than with the Atlantic Ocean, so that, 

 notwithstanding the presence of a number of Atlantic types, the 

 characteristic genera were common to the Pacific. Many of the 

 genera have remained unchanged since the separation of the 

 Atlantic from the Pacific by the elevation of the Isthmus of Panama 

 and the Mexican Plateau." 



In this and succeeding passages, Prof. Agassiz constantly assumes 

 that the elevation of the Isthmus took place at the close of the 

 Cretaceous period, but there is nothing among the few geological 

 facts which he mentions that can be regarded as a basis for such a 

 view. Moreover, the resemblance of the Caribbean to the Pacific 

 fauna would not be exi)lained by intercommunication in Cretaceous 

 times. It is surely an indication of free communication between 

 the two oceans at a much more recent date, so recently, in fact, that 

 the Atlantic forms have not yet been able to displace the descendants 

 of the Pacific types. 



Finally, we are led to consider how far the existence of such 

 geographical conditions in the Caribbean region must have afi'ected 

 the physical conditions of other regions, and particularly those of 

 the North Atlantic, by preventing the present diversion of the great 

 Equatorial current and the consequent formation of a Gulf-stream. 



As everyone knows, the Gulf-stream is produced by the impinge- 

 ment of the Equatorial current upon the coast of Central America ; 

 but w^hen, instead of the present continuous coast-line, there were 

 only a series of islands with broad channels of 300-fathoms' depth 

 between them, the great equatorial current must have passed 

 through these channels into the Pacific Ocean. 



It is not unlikely that the islands caused a separation of the 

 current into two branches, one curving southward to join the 

 Equatorial current of the South Pacific, the other passing in a W.J^.W. 

 direction to join the north Equatorial current. There is some posi- 

 tive evidence for the existence of the south-westerly current over 

 the Isthmus of Panama in the West-Indian element which is obser- 

 vable in the fauna and flora of the Galapagos Islands. Mr. Alfred 

 "Wallace * has explained the peculiarities of these islands on this 

 very hypothesis. He says: — "These facts are explained by the 

 past history of the American continent, its separation at various 

 epochs by arms of the sea uniting the two oceans across what is 

 now Central America (the last separation being of recent date, as 

 shown by the identical species of fishes on both sides of the isthmus), 

 and the influence of the Glacial epoch in driving the temperate 

 flora southward along the mountain-plateau. At the time when 



* 'Island Life,' p. 277. 



