142 THE THEORY OF EVOLUTION 



and deep water is commonly taken as the 100-fathom 

 depth; this is not an arbitrary selection, but the 

 reasons for the choice cannot be discussed here. 

 Several of the best known of the continental islands, 

 such as Great Britain and Ireland, Borneo, Sumatra, 

 Java, Madagascar, have been mentioned in this lec- 

 ture. Their animals and plants are those of a con- 

 tinent and the degree of difference from the neigh- 

 bouring mainland is an indication of the date of 

 disjunction. In a few cases continental islands rise 

 from great depths of water, as is true of the Greater 

 Antilles, and some are far from any land, like New 

 Zealand and the Seychelles. Oceanic islands, on 

 the other hand, are all either volcanic and have been 

 built up from the sea-floor by the material ejected 

 in the eruptions of submarine volcanoes, or they are 

 coral reefs. As a rule they rise from the profoundest 

 depths of the sea and are far removed from any land 

 and they appear never to have formed part of any 

 continent. Examples of this class are the Azores, 

 Madeira, the Canaries, Cape Verdes, Hawaiian 

 Islands and the thousands which dot the South 

 Pacific. 



The two classes of islands differ as radically in 

 their animals as they do in structure. The con- 

 tinental islands may and frequently do contain 

 everything that an equal area of the parent main- 

 land had at the time of separation, though in this 

 respect there is naturally a difference between large 

 and small islands. Sometimes it is difficult to un- 



