ORIGIN OF THE WEST INDIAN (CARIBBEAN) ECHINID l'.\l NA 



The resemblance of the Fauna of the Gulf of Mexico and of the Caribbean 

 to that of the Pacific has been noticed by former writers, even at a time 

 when the materials available for comparison included bul little beyond the 

 [ittoral Fauna. Since the results of the deep-sea dredgings Lave become 

 known, the extent of this resemblance has become quite striking. In fact, 

 the deep-sea Fauna of the Caribbean and of the Gulf of Mexico is far more 

 closely allied to that of the Pacific than to thai of the Atlantic. Before the 

 Cretaceous period, the Gulf of .Mexico and the Caribbean were undoubtedly 

 m ,,,.,.,. communication with the Pacific than with the Atlantic Ocean; so 

 that, while probably before that time the Fauna of these seas contained a 

 number of Atlantic types, yet the characteristic genera were common to the 

 Pacific. Many of the genera have remained unchanged to the presenl day 

 B ince the absolute separation of the Atlantic and of the Pacific by the Isth- 

 mus of Panama and the .Mexican Plateau, while there have been added to 

 the Wesl Indian Fauna a number of Atlantic types, which, as long as the 

 Gulf of Mexico and the Caribbean were practically a pari of the Pacific, 

 probably did not find conditions as Buitable to their development as those 

 which now exist, and which have existed since their separation from the time 

 they became merely extensions of the equatorial Atlantic district 



This explanation gives us an apparently good reason for the mixed char- 

 acter of the Fauna of the West Indian seas, showing us at the same time 

 that, however long a period of time may have elapsed Bince this separation 

 has taken place, il has not been sufficient to effect any very radical change 

 i„ the Echinid Fauna of the two sides f the Isthmus. The principal differ- 

 ences are due to the immigration of true Atlantic types into the Wesl Indian 

 faunal region during the Tertiarj and Post-tertiary period. Bui as the prin- 

 cipal physical conditions of the sea in the tropical regions of the two sides 

 of the Isthmus appear to be so nearly identical, we could not expect any 

 greal differences to arise between the Panamic and Wesl Indian Faunass from 

 physical causes alone. 



