THE INTRODUCTION OF NEW SPECIES. 11 



near their bases will be often very different, repre- 

 sentative species of some genera occurring, and even 

 whole genera being peculiar to one side only, as is 

 remarkably seen in the case of the Andes and Rocky 

 Mountains. A similar phenomenon occurs when an 

 island has been separated from a continent at a very 

 early period. The shallow sea between the Peninsula 

 of Malacca, Java, Sumatra and Borneo was probably 

 a continent or large island at an early epoch, and 

 may have become submerged as the volcanic ranges 

 of Java and Sumatra were elevated. The organic 

 results we see in the very considerable number of 

 species of animals common to some or all of these 

 countries, while at the same time a number of closely 

 allied representative species exist peculiar to each, 

 showing that a considerable period has elapsed since 

 their separation. The facts of geographical distribu- 

 tion and of geology may thus mutually explain each 

 other, in doubtful cases, should the principles here 

 advocated be clearly established. 



In all those cases in which an island has been 

 separated from a continent, or raised by volcanic or 

 coralline action from the sea, or in which a moun- 

 tain-chain has been elevated in a recent geological 

 epoch, the phsenomena of peculiar groups or even 

 of single representative species will not exist. Our 

 own island is an example of this, its separation from 

 the continent being geologically very recent, and we 

 have consequently scarcely a species which is pecu- 

 liar to it; while the Alpine range, one of the most 



