THE FLORIDA REEFS. 91 
for granted that the most favorable conditions for their support 
exist ; and this condition we assume to be an abundance of food, 
brought to them by the great oceanic currents passing over the 
regions where these submarine plateaux are forming. We know 
as yet too little of the fauna of the oceanic basins to be able to 
affirm how far the population of the bottom depends upon the 
food it receives from oceanic currents. We can only judge by 
analogy. No marine fauna has been explored which equals 
in variety or in the number of its individuals that of the Ca- 
ribbean and of the Gulf of Mexico, from the depth of two 
hundred and fifty to about one thousand fathoms. It has 
proved richest in the districts most favorably situated with re- 
gard to the currents and the food supply they bring in their 
track. It is but natural to extend this effect to other oce- 
anie currents, and in their track we may therefore expect to 
find the most favorable conditions for the support of an im- 
mense fauna. In fact, the question of food is of the utmost 
importance to the distribution, not only of marine, but of ter- 
restrial animals; and the absence or presence of an abundant 
supply of suitable nourishment must of necessity be an all- 
important factor in the character and variety of the fauna of 
any place or period, — far more influential, perhaps, than the 
many obscure physical causes upon which we are so apt to 
explain the distribution of animal life. On the continental 
ledges, where the shore detritus is gradually accumulated, bring- 
ing with it a large amount of animal and vegetable food, we 
find the most populous fauna near the hundred-fathom line. 
When, in addition to the action of the influences which have 
accumulated the shore detritus, we have a continental shore or 
plateau bathed by a great and powerful current, bringing with 
it an abundance of pelagic life, we may expect a superabundant 
supply of food, and consequently a fauna of unusual richness 
aad variety. The fauna of the Pourtalés Plateau, of the hun- 
dred-fathom slope to the westward of the Tortugas, of the 
northeastern slope of the Yucatan plateau, of the windward side 
of the Lesser Antilles, and of the continental slope of the eastern 
coast of the United States below the hundred-fathom line, are 
all examples of such districts supporting a marine fauna of sur- 
