THE DEEP-SEA FAUNA. 157 
That none of the palzozoic 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 i 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 
origin 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. Many of the genera have remained un- 
changed, since the separation of the Atlantic from the Pacific 
