101 



Distribution. — The influence of physical agents on Hving beings 

 determines the geographical distribution of the species : thus 

 light, heat, and temperature, latitude and longitude, atmospheric 

 pressure and water pressure, individually and collectively, affect 

 the life, growth, and development, as well as the decadence and 

 destruction, of all organised bodies. The Actinozoa, it would 

 appear, are remarkably sensitive to the influence of these agents; 

 for although numerous genera of this class inhabit the waters 

 of every region of the globe, still it has been ascertained that 

 the true reef-building Polyps are hmited to the seas of the 

 tropics. Most of the high islands between the parallels of 28° 

 north and south of the Equator, and also the borders of the 

 continents within the same limits, are fringed with Coral reefs, 

 provided the other conditions necessary to their development 

 are present. Should the slope of the rocks below the water be 

 steep, the Polyps cannot grow far from the shore ; but if the 

 slope is gentle, they make a wide fringing reef around the coast, 

 the outer limit of which is determined by the depth of the 

 water. Where the bottom is muddy, and rivers pour fresh water 

 in any great abundance into the sea, there the reef-building 

 Polyps are absent. According to Dana's observations, wherever 

 cold oceanic currents invade tropical seas, and lower the mean 

 temperature of the water in the coldest winter month below 

 68° Fahrenheit, there likewise reef-building Polyps cannot live. 

 For this reason, and others perhaps not fully explained, — there 

 are no Coral reefs on the West Coast of South America, or 

 round the Galapagos Islands, in consequence of the cooling 

 influence of that branch of the Grreat Antarctic or Humboldt's 

 ciurent, which sweeps along the western shores of that continent. 



The Bermuda Islands, in 32° 15' N., are the most northern 

 limits of Coral reefs at present known ; their distance from the 

 Equator is, however, entirely compensated by the increased 

 temperature of the ocean, derived from the Gulf-stream which 

 flows around their western shores. In the Red Sea there are 

 Coral reefs in lat. 30°. In the Pacific Ocean, the Loo-Choo 

 Islands, in latitude 27° N., at the north-east of the Isle of 

 Formosa, have reefs on their shores, and there is an AtoU at 

 the north-west of the Archipelago of the Sandwich Islands, in 

 28° 30'. 



