144 LEISURE-TIME STUDIES. 



itself formed, as we have seen, a subject of debate amongst 

 the earlier writers, so the " reason why " the coral-polypes 

 aggregated themselves together in the form of atolls and 

 barrier reefs, constituted one of the knotty points of early 

 biological science. The same mysterious " instinct," which 

 was supposed to be the cause of their secreting lime, was 

 credited with being the directing impulse in causing the 

 selection of admirable sites for the coral-reefs. They formed 

 a great protecting reef, according to Flinders, that they might 

 work in safety under its shelter, and that the leeward aspect 

 of the reef might form a kind of nursery-ground, whence "their 

 infant colonies might be safely sent forth." In this case the 

 coral-polypes are credited with the possession of intelligence 

 of no mean order, and are presumed to co-operate together 

 for an end and in a manner utterly unknown to be repre- 

 sented in any other group of animals. Such a theory,, 

 moreover, leaves untouched the essential question of the 

 causes in virtue of which coral-reefs should assume the 

 characteristic forms observed in the atolls and barrier reefs, 

 and the want of an explanation of the latter points suggested 

 a theory whose simplicity is unfortunately its only recom- 

 mendation. It was believed that, taking into account the 

 imitation in depth of living corals, these animals obtained 

 a basis and foundation in land which lay submerged some 

 120 or 150 feet in the sea; so that every coral-reef was 

 regarded as simply presenting us with a coral top to solid 

 land. The circular form of the atoll was ingeniously ac- 

 counted for on the supposition that the coral-polypes had 

 built around the rim of a volcanic crater, and that the break 

 in the coral ring affording entrance to the lagoon was repre- 

 sented by a fissure of greater or less extent in the continuity 

 of the crater's margin. The plausibility of this theory be- 

 comes sadly weakened if we subject its supporters to the 

 cross-examination of the physical geographer. For the 

 stability of the ideas thus ventilated, it would require to be 

 proved firstly that submarine plateaus or ridges existed not 

 only in great profusion in the coral regions, but also that 



