132 THE TORTUGAS AND FLORIDA REEFS. 



towards the north, as far as the temperature would allow, a supply of food far more 

 abundant than that with which the fauna of the eastern coast was supplied before 

 such a break of continuity existed. As this separation of the Atlantic and Pacific 

 probably took place late in the Cretaceous period, and was perhaps not completed 

 till the Middle Tertiary, we shall naturally expect to find the marine fauna of the 

 earlier geological periods of the Old and the New World to be very similar, and con- 

 sisting of many identical species. These older famife flourished on the shores and 

 continental shelves, either washed by the great equatorial currents or by branches 

 extending both north and south along the then existing continents and continental 

 islands ; and where we now find rich fossiliferous deposits we may feel assured that 

 the beds at the time of their formation lay in the track of a primary or a secon- 

 dary marine current, which supplied an abundance of pelagic food indirectly necessary 

 for the support of any rich marine fauna. 



