I THE INTRODUCTION OF NEW SPECIES 9 
have not hitherto received any, even a conjectural explana- 
tion. The Galapagos are a volcanic group of high antiquity, 
and have probably never been more closely connected with 
the continent than they are at present. They must have 
been first peopled, like other newly-formed islands, by the 
action of winds and currents, and at a period sufficiently 
remote to have had the original species die out, and the modi- 
fied prototypes only remain. In the same way we can account 
for the separate islands having each their peculiar species, 
either on the supposition that the same original emigration 
peopled the whole of the islands with the same species from 
which differently modified prototypes were created, or that the 
islands were successively peopled from each other, but that 
new species have been created in each on the plan of the pre- 
existing ones. St. Helena is a similar case of a very ancient 
island having obtained an entirely peculiar, though limited, 
flora. On the other hand, no example is known of an island 
which can be proved geologically to be of very recent origin 
(late in the Tertiary, for instance), and yet possesses generic 
or family groups, or even many species peculiar to itself. 
When a range of mountains has attained a great eleva- 
tion, and has so remained during a long geological period, 
the species of the two sides at and near their bases will be 
often very different, representative 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 repre- 
sentative species exist peculiar to each, showing that a con- 
siderable period has elapsed since their separation. The facts 
of geographical distribution and of geology may thus mutu- 
ally explain each other in doubtful cases, should the prin- 
ciples here advocated be clearly established. 
