126 LOWER INVERTEBRATES. 



The distribution of the coral islands in longitude presents some very interesting 

 facts. As a general thing it may be said that the eastern shores of the continents are 

 richer in coral banks and islands than the western. The eastern coast of North 

 America, for instance, has the Florida reefs, the great Bahama bank, one of the largest 

 in the world, the reefs of Yucatan, Honduras, and Cuba. On the western shore of 

 North and Central America there are no extensive coral reefs. The eastern border 

 of Australia has a wealth of coral life, while the western is almost a desert as far as 

 plantations of these zoopliytes are concerned. The Atlantic coast of Africa has no 

 extensive reefs, while that washed by the Indian Ocean is fringed by very extensive 

 lianks. If in studying this distribution in longitude we consider also the limitations 

 in latitude we find the following law to hold good. While in the longitude of the 

 eastern shores of the continents the reefs extend far from the equator, in that of 

 the western border they are, when found, limited to the immediate vicinity of the 

 equator. An explanation of this curious fact in coral distribution is found in the 

 direction of the great equatorial currents of the Atlantic and Pacific Oceans. These 

 currents cross tlie ocean from east to west, and as they approach the eastern continental 

 borders they divide, one branch flowing to the north the other to the south along the 

 coast lines. Two streams of warm water are thus continually carrj'ing the oceanic 

 isotherm of 68°, which limits the home of the reef-builders, into higher latitudes, and 

 broadening the zone in which these sensitive animals can live. On the western coasts, 

 however, we find an opposite condition of things. The equatorial current is there 

 fed by branches flowing in opposite directions, in which the water is colder since they 

 are setting from higher latitudes towards the equator. The result upon the coral 

 organisms is that they are hemmed into narrow limits on this side of the ocean by the 

 diminution in the breadth of the zone which tliey can inhabit. 



The distribution of corals and the limitation of coral islands by such local phenom- 

 ena as rainfall, volcanic activity, and the like, present many very curious facts. The 

 almost constant changes in the level of the sea floor produced by volcanoes would 

 necessarily destroy plantations of growing corals in the immediate neighborhood. It 

 commonly happens that coral and volcanic islands are found in intimate association, 

 while it is probably true that all oceanic coral islands rest on volcanic bases. Where 

 the volcanic forces are active, the lava poured into the ocean, or the ashes raining 

 from the air at times of great eruptions, generally destroy the coral banks. A good 

 illustration of this fact may be seen in tlie Sandwich Islands, where the most soutliern 

 members of a chain of islands which make up the group are volcanic and destitute of 

 extensive coral banks, while the northern are almost wholly formed of coral. The 

 almost uninterrupted volcanic activity in the southern members of this group has 

 prevented the formation near them of coral banks, while at the north, where active 

 volcanic forces are now unknown, the islands are almost wholly coralline. The eastern 

 members of the Lesser Antilles, as Barbadoes, are coralline, while the western are 

 volcanic. In the island of Guadeloupe we find the same law of local distribution to 

 hold in a single island, the eastern extremity being coralline and the western volcanic. 

 This distribution of corals in the Antilles probably depends upon the direction of the 

 ocean currents in the neighborhood, or perhaps upon the constant direction of the winds 

 by which the ashes from the volcanoes are blown to the leeward, thus killing the growing 

 corals on the western side. It may perhaps be that the profound depths to which the 

 Caribbean Sea sinks to the westward of the lesser Antilles prevents the corals obtain- 

 ing a foothold on that side, while the shallows on the eastern shores are better suited 



