the Introduction of New Species. 189 



of the two sides at and near their bases will be often very dif- 

 ferent^ 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 

 phsenomenon 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 consi- 

 derable number of species of animals common to some or all of 

 thes3 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 distribution and of geology may thus mu- 

 tually 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 b}' volcanic or coralline action from the 

 sea, or in which a mountain-chain has been elevated, m 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 geo- 

 logically very recent, and we have consequently scarcely a species 

 which IS peculiar to it ; while the Alpine range, one of the most 

 recent mountain elevations, separates faunas and floras Avhich 

 scarcely differ more than may be due to climate and latitude 

 alone. 



The series of facts alluded to in Proposition 3, of closely alhed 

 species in rich groups being found geographically near each 

 other, is most striking and important. Mr. Lovell Reeve has 

 well exemplified it in his able and interesting paper on the Distri- 

 bution of the Bulimi. It is also seen in the Humming-birds 

 and Toucans, little groups of two or three closely allied species 

 being often found in the same or closely adjoining districts, as 

 we have had the good fortune of personally verifying. Fishes 

 give evidence of a similar kind : each great river has its peculiar 

 genera, and in more extensive genera its groups of closely allied 

 species. But it is the same throughout Nature; every class and 

 order of animals will contribute similar facts. Hitherto no 

 attempt has been made to explain these singular phenomena, or 

 to show how they have arisen. ^Vhy are the genera of Palms 

 and of Orchids in almost every case confined to one hemisphere? 

 Why are the closely allied species of brown-backed Trogons all 

 found in the East, and the green-backed in the West ? Why 

 are the Macaws and the Cockatoos similarly restricted ? Insects 



