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 phenomena 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 
