84 THE SHORE 



plants on any land vary proportionately to its size, 

 temperature, varieties of soil, and rainfall ; the exact 

 influences of each of these cannot, of course, be exactly 

 stated. In the case of islands formed by submarine 

 eruptions, their proximity to other lands, and the 

 presence or absence of currents and winds, suitable 

 for the transport of the seeds and germs of plants and 

 animals, are of primary importance. Hawaii, Samoa, 

 and Tahiti were probably all formed by eruptions, and 

 to some degree they illustrate the influence of distance 

 and size already mentioned. Mauritius, Reunion, and 

 Rodriguez are probably further examples ; but the 

 former may have been connected to the Seychelles, 

 and so to Africa ; and Rodriguez was partially formed 

 by an upheaval of the sea-floor. The Atlantic Islands — 

 the Canaries, Cape Verdes, and Azores — are probably 

 all of similar origin, though some of them have been 

 supposed to represent the remains of a continent 

 connecting Africa to South America. 



It will be now clear that the importance of the study 

 of islands to a large degree depends on the light which 

 they throw on the existence of former continents — on 

 the distribution of land and sea in past geological 

 periods. An island may exist where we are practically 

 certain that there was formerly continental land, but 

 it by no means follows that it was ever part of that 

 continent, since it may have been formed subsequently 

 to the submersion of the continent. Yet it is in islands 

 that we might expect to find our best evidence of the 

 existence of former connecting land masses. 



The interest of our third class of islands — coral islands 

 — is again different, and lies in the question, how far 

 do they indicate the sites of former land masses which 

 have disappeared by subsidence or from other causes ? 

 Some of these islands have been elevated to great 



