GLACIAL-CONTROL THEORY OF CORAL REEFS. 169' 



The numbers in bold type show the minimum mean monthly 

 temperatures; slightly lower are, of course, the respective absolute 

 minima. 



Remembering the 20° lower limit for reef corals, the reader observes 

 that a mere general fall of only 6° C. in the minima must cause a very 

 extensive destruction of the living animals. ^^ Further the favorable 

 temperature conditions of the western tropical Pacific and w^estern 

 tropical Atlantic are partly due to the westward driving of abundant 

 warm water by the trade winds. In the Glacial period the trade-wind 

 belt must have been much narrower than now, and the effect of these 

 winds correspondingly less. At the same time the general storminess, 

 correlated with rapid shifts of cold currents, was greater than at 

 present. Finally, the Pleistocene extension of the Antarctic ice-cap 

 must have caused some narrowing of the sea south of Cape Horn, with 

 the probable result of increasing the volume of the cold Humboldt 

 current, which now distinctly lowers the temperature of the central 

 Pacific. 



For various reasons, therefore, the temperature conditions for lusty 

 coral growth during the Glacial period are not fully suggested even 

 by the statement that the mean annual temperature was then lower 

 than at present by a half-dozen or more degrees. That lowering was 

 but one of several associated causes for the inhibition of coral-reef 

 growth. The writer believes it is not an extreme view to hold that 

 practically the entire area now occupied by the oceanic archipelagoes 

 and by the great barrier reefs of Australia and New Caledonia was, 

 during the maximum Pleistocene glaciation, bereft of reefs growing 

 rapidly enough to resist destruction by the waves. Though meager, 

 slow growth of corals may have been possible in the open ocean, they 

 could only thrive in sheltered ba^^s or seas, especially those along the 

 eastern continental borders within the tropics. In these localities 

 the corals perpetuated their kind, rendering possible a future, more 

 favorable existence in the open ocean. The resulting Pleistocene 

 reefs are, of course, now completely submerged. A likely place for 

 their development was in the southern part of the Red sea, which, 

 in the Glacial period as now, was doubtless particularly warm. The 

 vast, rough plateaus at 60 m. to 120 m. below the surface of that sea, 

 may represent places where reef corals then flourished. 



The tropical Atlantic water is to-day cooler than that of the tropical 



18 J. D. Dana (Corals and Coral Islands, New York, 3rd ed., p. 108 (1890) 

 states "that the temperature of 68° F. (20°C.) is a temporary extreme — • not 

 that under which the polyps will flourish." 



