THE DEEP-SEA FAUNA. 157 



That none of the palseozoic forms are found in the deep sea 

 seems to indicate, as has been suggested by Moseley, that its 

 first inhabitants date back no farther than the cretaceous period. 

 Of course, there must have been pelagic animals, and foramini- 

 fera may have lived at great depths in the track of the currents, 

 but probably no invertebrates of a period older than the jura 

 and chalk existed, or if they did exist they did not wander far 

 from the continental shelf. Their distribution was then, as to- 

 day, mainly a question of food. The animals of those times 

 lived upon the coast shelf, and while they and their predecessors 

 remained as fossils in the littoral beds of the earlier formations, 

 their successors, belonging either to the same or to allied genera, 

 passed over into the following period. 



The littoral belt is perhaps the most important portion of the 

 sea floor, since within its limits the greatest changes of light, 

 heat, and motion occur. To the modifications which under 

 such influences have taken place during past ages, and are still 

 going on, in this limited area, we may attribute the gradual 

 migration of the littoral fauna into the deeper waters, and into 

 the less favored portions of the continental belt and the abyssal 

 region. This succession we see going on around our shores, 

 and in some groups of animals it has been traced with consid- 

 erable detail. I may take as an example the history of the 

 orioin of the West Indian echinid fauna. 



The resemblance of the fauna of the Gulf of Mexico and of 

 the Caribbean to that of the Pacific was noticed by writers, even 

 at a 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 ex- 

 tent of this resemblance. In fact, the deep-sea fauna of the 

 Caribbean and of the 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 communication with the Pacific than with 

 the Atlantic Ocean ; so that, notwithstanding the presence of a 

 number of Atlantic types, the characteristic genera were com- 

 mon to the Pacific. Manv of the oenera have remained un- 

 changed, since the separation of the Atlantic fi-om the Pacific 



