Bathymetrical Distribution of Reef-huilcling Corals. 67 



when the subject comes to be more carefully investigated, 

 that the depth in question, instead of being one of tolerable 

 uniformity, as it appears to be generally assumed is the case, 

 is one that is subject to a very wide range. What is true of 

 one particular area by no means necessarily applies even to 

 the areas adjacent, and may be very far from the truth in 

 connection with other parts of the ocean. 



In all probability other factors than mere depth below 

 the surface will have to be taken into account in discussing 

 any problems connected with the bathymetrical distribution 

 of reef-building corals. That such must be the case is 

 obvious from a consideration of the various causes which 

 limit the growth of the adult coral at or near the surface. 

 Amongst these causes, the salinity of the water, its com- 

 parative freedom from suspended particles of terrigenous 

 origin, the degree of oxygenation of the water, the nature 

 and extent of the plankton present, and perhaps also the 

 amount of light, are all of them factors which need to be 

 taken into account in dealing with the adult corals, as much 

 as they do in connection with their earlier stages as free- 

 swimming, pelagic fry. As in the case of the earlier stages in 

 the life-history of the coral animals, so with their welfare when 

 adult, the important factor of temperature plays a considerable 

 part. It seems to be proved beyond question that in both their 

 earlier and their later stages of life, coral animals are very 

 susceptible to cold. If at any time, and under any circum- 

 stances, the temperature of the sea- water by which they are 

 surrounded falls below 65° Fahr., the corals cannot thrive. 

 It is, of course, obvious that this remark does not apply to 

 corals of every kind, for many species of corals can thrive in 

 water whose temperature is only a trifle above the freezing 

 point. These same corals also do well at ocean depths down 

 to five miles, at which great depth, notwithstanding the 

 increased solvent power produced by the great pressure, 

 their coralla long remain uncorroded. But it seems to be 

 a well-established fact that reef-building species of coral 

 cannot thrive in water whose temperature at any time falls 

 below 65° Fahr. It is not the maximum, nor the mean, but 

 the minimum temperature which influences their welfare. 



