ORIGIN OF THE WEST INDIAN (CARIBBEAN) ECHINID FAUNA. 
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 avaiUxble for comparison inchided but little beyond the 
littoral Fauna. Since the results of the deep-sea dredgings have 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 
closel}' allied 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 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 present day 
since 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 "West Indian Fauna a number of Atlantic types, which, as long as the 
Gulf of Mexico and the Caribbean were practically a part of the Pacific, 
probably did not find conditions as suitable to their development as those 
which now exist, and which have existed since their sepai'ation 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 since this separation 
has taken place, it has not been sufficient to effect any very radical change 
in the Echinid Fauna of the two sides of the Isthmus. The principal differ- 
ences are due to the immigration of true Atlantic types into the West Indian 
faunal region during the Tertiary and Post-tertiary period. But 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 
great differences to arise between the Panamic and West Indian Faunae from 
physical causes alone. 
