140 LEISURE-TIME STUDIES. 



lie adds, " moreover, no right to calculate on unlimited time 

 for the accumulation of small organic bodies into great 

 masses. ... As well might it be imagined that the British 

 seas would in time become choked up with beds of oysters, 

 -or that the numerous small coral-lines off the inhospitable 

 shores of Terra del Fuego would in time form a solid and 

 expansive coral-reef." 



The causes of the limitation in depth of corals may be 

 summed up by recognising the necessity of a due supply of 

 light and air for maintaining the vitality of the living animals. 

 The living polypes require light as a condition for the exer- 

 cise of their vital functions, and they no less imperatively 

 demand a due supply of the vivifying oxygen ; these essen- 

 tials for vitality being obtainable only in surface-waters, or 

 within a limited depth in the ocean. Recognising the 

 settled and affirmed nature of these two conditions of coral 

 life, we may next proceed to examine the curiously compli- 

 cated problem which the condition of limited depth especi- 

 ally imposes upon the naturalist. How, in other words, 

 when we take into account the limitation in depth of living 

 corals, can we explain the erection of coral-reefs and islands 

 existing in abyssal or unfathomable depths of sea ? 



It is a striking characteristic of scientific procedure that 

 no new or strange fact is long left without an explanation. 

 That the first explanation may not necessarily be correct, 

 but is, on the contrary, more likely to prove untenable when 

 a wider knowledge of the fact or facts is obtained, are state- 

 ments which the history of scientific hypotheses and their 

 verification fully endorses, and which the fate of the first- 

 offered theories of the erection of coral-reefs fully confirms. 

 To appreciate the points which the theories of the erection 

 of coral-reefs include, it becomes necessary to glance, in the 

 first instance, at the various forms which coral-reefs may assume. 

 These reefs may be divided into fringing reefs, barrier reefs, 

 and atolls or lagoon reefs. The nature of the first-named 

 erections is explained by their name. They simply fringe 

 or skirt the margins or coasts of lands, and appear to be mere 



