118 



animal population of this tract, the main phenomena are 

 almost self-evident. Should any of its isolated frag- 

 ments chance to contain a portion of one of those limited 

 areas which a species of slow progressive powers had 

 succeeded in colonizing, it would of course harbour (pro- 

 vided that the other portion has disappeared) what would 

 now be denned as endemic. Numbers of these small 

 areas, or, in other words, of the species which had over- 

 spread them, would in all probability be lost for ever ; 

 whilst the occurrence of any of the surviving ones in 

 more than a single island would manifestly depend on 

 the proximity of the islands inter se. Those forms which 

 had diffused themselves over the whole original con- 

 tinent would now be found in all the detachments of the 

 cluster; whilst others, which had wandered over the 

 greater portion of it only, might be traceable perhaps in 

 every island except a few. 



Such are the primary facts which suggest themselves, 

 whilst discussing the question of isolation as regulating 

 the distribution of the Annulose tribes. Its after ejfectSj 

 on their external configuration and development, we 

 have examined in a preceding chapter of this treatise; 

 and we have also lately intimated what might be a few 

 of the presumptive consequences of a subsidence (in a 

 general sense), apart from the still more important 

 principle of isolation. Before, however, we dismiss these 

 brief and elementary reflexions on the upward and down- 

 ward movements which geology testifies to have occurred, 

 at various epochs, on the earth's surface, I shall per- 



