68 Proceedings of the Boyal Physical Society. 



Whether this sensitiveness to cold is due to constitutional 

 causes affecting the whole assemblage of species who have 

 acquired reef-building habits, or whether it is due to a 

 reduction of food-supply, to a combination of these causes, 

 or to others not yet suspected, remains to be proved. But 

 there seems to be no doubt regarding the fact itself, explain 

 it how we may. 



If the animals that form coral reefs cannot maintain an 

 existence at any place where the temperature of the sea- 

 water in which they are found falls, at any time, below 65° 

 Fahr., we can readily understand how it is that no coral reefs 

 occur in marine areas where the surface temperature reaches 

 a high mean, but is subject to periodical falls below the 

 grade mentioned. The influence of the terminal portions of 

 many of the oceanic cold currents in periodically lowering 

 the surface temperature of areas where the temperature is 

 usually high, will illustrate this point. The cold Peru 

 Current, off the west coast of South America, and the 

 Antarctic Drift oft the west coast of Australia, are good 

 examples of the causes referred to. 



On the other hand, if it can be shown that temperature 

 always above the minimum of 65° Fahr. is the chief factor 

 concerned, one can readily understand how it happens in 

 certain areas that corals thrive well down to a certain small 

 depth, and below that can no longer live. The explanation 

 is that the warm water, in this case, forms but a com- 

 paratively small surface film, below which the water is too 

 cold for the welfare of the corals. 



In other cases, the depth of water at the temperature of 

 65° Fahr., or above that, may be much greater, and may, 

 indeed, as is now very well known through the results of the 

 Challenger Expedition, in many cases extend to considerably 

 below 100 fathoms. 



A reference to Dr Buchan's Map No. 3, in the volume of 

 the " Keports of the Challenger Expedition " which deals with 

 Oceanic Circulation, will show that there are five areas over 

 which conditions of temperature suitable for the growth of 

 reef-building corals occur at depths exceeding 100 fathoms. 

 One of these extends into the North Atlantic, north of the 



