THE TORTUGAS AND FLORIDA REEFS. 131 



richest in the districts most favorably situated with regard to the currents and the food 

 supply they bring in their track. It is but natural to extend this effect to other 

 great oceanic currents, and in their track we may therefore expect to find the most 

 favorable conditions for the support of an immense fauna. In fact, the question of 

 food is of the utmost importance to the distribution, not only of marine, but of terres- 

 trial animals, and the absence or presence of an abundant supply of suitable nour- 

 ishment must of necessity be an all-important factor in the character and variety 

 of the fauna of any place or period, — far more influential, perlmps, 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 accu- 

 mulated, bringing; with it a large amount of animal and vegetable food, we find 



_,.*. & ,,*„« *.„ ^ ""o 



the most populous fauna near the hundred-fathom line. When, in addition to the 

 action of the influences which have accumulated the shore detritus, w have a con- 

 tinental 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 and variety. The fauna of the Pourtalea 

 Plateau, of the hundred-fathom slope to the westward of the Tortugas, of the 

 northeastern slope of the Yucatan Plateau, of the windward side of the Lesser An- 

 tilles, 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 surpassing richness. In a similar way, we may expect to find in the track 

 of the great Pacific equatorial current also the most favorable condition- for the 

 support of a rich and varied marine fauna. The "Challenger" found, perhaps, no 

 richer field than that off the coast of Japan, which lies directly in the track of the 

 Japanese current, and may be considered as the Pacific counterpart of the Florida 

 and Caribbean fauna. 



In past geological times the effect of the currents in determining the distribution 

 of the marine invertebrates must have been as marked as it is at the pr sent day. A? 

 long as we had a great equatorial current running practically unbroken round the 

 world, and only slightly deflected by the great continental islands of Central America 

 and of the East Indies, which stood in the path of this great equatorial belt, it was 

 natural that we should have a very extensive geographical range for all the tropical 

 marine forms. It was only after the complete shutting off or comparative isolation 

 of the Atlantic from the Pacific that different physical conditions began to exist 

 simultaneously, which were of the greatest importance in reducing the supply of 

 food to the animals on the west coast of the continental barriers, and in extending 



