i88 THE MIGRATION OF BRITISH BIRDS 



continent to which they are adjacent. The two excep- 

 tions are the Azores and the Madeira groups, which are 

 oceanic islands, and apparently of volcanic or coralline 

 origin. The continental islands resolve themselves 

 again into two distinct classes, viz. those that are of 

 ancient origin and usually separated from the parent 

 land mass by deep seas ; and those that are of recent 

 origin, always situated upon submerged banks that con- 

 nect them with the adjoining continent, and surrounded 

 by shallow seas. The Canary Islands may be taken as 

 the only example of ancient continental islands in the 

 West European Area ; whilst the British Isles, the 

 Channel Isles, Heligoland, the Balearic Isles, Corsica, 

 Sardinia, Sicily, and Malta all belong to the class of 

 recent continental islands. The more ancient islands 

 very often contain the greater number of peculiar 

 species, due to their longer isolation, and are remark- 

 able for the fragmentary character of their fauna. The 

 more recent islands are said to possess {c\w peculiar 

 species, and to exhibit the general characteristics of the 

 fauna and flora of the adjacent land mass. The vol- 

 canic or coralline islands are probably peopled entirely 

 by fortuitous emigration, by accident, and therefore 

 often exhibit a somewhat puzzling mixture of species. 

 In the present chapter it will be my principal aim to 

 show why these Atlantic and Mediterranean islands are, 

 and in all probability will continue to be, so poor in pecu- 

 liar species, and why various other islands in other parts 

 of the world, irrespective of their age and origin, are 

 respectively rich or poor in endemic species — a question 

 which has hitherto, so far as I can determine, never 

 been grappled from the same standpoint as that which 



