CH. IV] COMPARISON OF ISLANDS. 205 



facts brought forward will at once serve to divide these 

 islands into at least two groups, which are again capable 

 of subdivision. 



Continental Islands. 



In the first category are Great Britain and Madagas- 

 car, which have a copious indigenous fauna of mammalia 

 and amphibia ; to the second category belong the other 

 islands, which have either no mammalia at all or they are 

 extremely few, at most one or two species identical with 

 those found upon the adjacent mainland, and which are 

 absolutely destitute of amphibia of any kind. 



These distributional facts go hand in hand with Geo- 

 logical structure. Great Britain and Madagascar are 

 detached portions of continents, which have been sepa- 

 rated from the mainland for varying periods. The re- 

 maining islands, with the possible exception of the 

 Galapagos, which will be considered later, are not such 

 fragments but have arisen de novo in the ocean by 

 volcanic agency. They have never had any connection 

 with any continent. The relation between the geo- 

 logical structure and the nature of the inhabitants is a 

 real one ; the mammalia, as has been already observed, 

 are not capable of extended migration over the sea on 

 their own account ; it could only be but rarely that a 

 mammal could reach a distant and oceanic island ; and it 

 would be necessary in many cases that two individuals at 

 least should emigrate within a reasonable time of each 

 other. The immense tract of ocean which separates 

 Kerguelen from the nearest continent has proved an 



