130 THE TORTUGAS AND FLORIDA REEFS. 



dently been heaped up to its highest point by the influence of the waves or winds. 

 Furthermore, we see growing to the westward of the Tortugas a knoll similar to 

 that which has formed the Tortugas themselves, and which will form, in the course 

 of time, an island or a series of islands like them, to the westward. It is further 

 evident, also, that the great Alacran Reef (Plate V.) has been built up in a similar 

 way, and that its peculiar atoll shape is entirely due to the action of the prevailing 

 winds and currents, and not to any subsidence of the great Yucatan Plateau. 



The character of the Fauna and Flora of the Tortugas is interesting as corroborat- 

 ing the comparatively recent age at which the reef has been formed. We find, 

 as we go north along the keys, that the nearer we come to the mainland of Florida, 

 the greater do we find the number of plants characteristic of the mainland. As 

 we reach islands more or less inaccessible, or islands merely formed by flats which 

 have reached low-water mark, we find the vegetation to consist almost entirely of 

 mangroves. Yet at the Tortugas, in spite of the narrow channel which separates 

 them from the Marquesas, I saw but a single diminutive mangrove plant, while a 

 few Bay-cedars, as they are called, a Hop-vine with a thick white flower, and Ber- 

 muda grass, have alone found their way there, in spite of the fact that the Tortugas 

 are in the direct line of the prevailing winds from the Marquesas. One of the 

 species of land shells common at Key West has already found its way to the Tor- 

 tugas. The group is visited by pelicans, cranes, humming-birds, plovers, and a few 

 land birds. It being the winter season, the insects were few in number. No 

 terrestrial reptiles have as yet been found on the Tortugas, while at Key West 

 we find many of the frogs, toads, hzards, and snakes characteristic of the southern 

 spit of the mainland, all this showing that the Tortugas reefs have not been above 

 the level of the sea long enough to have received as yet the fauna or flora charac- 

 teristic of the more northern line of keys. 



The explanation given in this paper of the formation of huge deposits of lime- 

 stone from the limestone carcasses of Invertebi'ates, takes it 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 as yet been explored which equals in variety or 

 in the number of its individuals that of the Caribbean and of the Gulf of Mexico from 

 the depth of two hundred and fifty to about one thousand fathoms. It has proved 



