THE LOWEB FORMS OF LIFE. 123 



into their present shape by the Coral polyp. This statement 

 is not invalidated but supported by the theory held by some, that 

 these islands are the high peaks of a vast continent gradually sinking 

 into the depths of the sea. If this be true, it involves a previous 

 sinking and a subsequent upheaving, for while all of them are more 

 or less surrounded by reefs, many are formed even to their tops 

 of dead Coral alone. That they are now sinking, I believe there is 

 no doubt ; and the beautiful and generally accepted theory of Mr. 

 Darwin, of the mode by which they were built up, is entirely based 

 upon this fact. The mind is startled when it contemplates the 

 immensity of time which has passed away since the Coral Islands 

 were first formed on the rising continent, and after its history in 

 time, the reefs and lagoons were formed by the same little polyps 

 as it gradually sank again into the unknown depths of the sea. I 

 shall allude to this subject again, but I may remark that no facts in 

 science are better known, and no generalisations more unanswerable 

 than those connected with the formation of coral structures in time- 

 and space. 



In the meantime, let us examine the interesting question, so ably 

 treated by Mr. Darwin, of the modus operandi by which Coral Islands 

 and reefs are formed. 



These islands and reefs are found in the great coral regions in 

 three or four different forms. First, we have islands in mid ocean, 

 with a lake in their centre; these are called "lagoon islands." 

 Secondly, we have islands formed entirely of dead Coral. Thirdly, 

 we have living coral reefs surrounding an island or bordering a. 

 continent, quite in shore, with a small space of still water between 

 the reef and the shore ; these are termed " fringing reefs." Then, 

 fourthly, these reefs are at a greater distance from land, leaving a. 

 large space of still water between them and the shore ; these are 

 termed " barrier reefs." 



Now, before the publication of Mr. Darwin's work on the " Struc- 

 ture and Distribution of Coral Reefs," in 1842, we had no intelligible 

 theory to explain the formation of these reefs. I will briefly detail 

 this theory, referring those who wish for further information to ihe 

 admirable little book above mentioned. 



The two essential facts upon which the theory is built up are, 

 first, that the whole of the land comprising what are known as the 

 South Pacific Islands and the continents bordering thereon, are at 

 the present time, and have been for ages past, gradually sinking ; 

 secondly, that the Coral polyps do not live at greater depths than 

 between twenty and thirty fathoms Agassiz says from twelve to 

 fourteen fathoms. 



Both of these positions have been ably argued by Mr. Darwin, and 



