OCEANIC ISLANDS. igi 



with, these variations will have a chance to develop 

 rapidly. To understand the effect of isolation one 

 needs only to call to mind the result of domestic 

 breeding. The pigeons, for example, have been in 

 a condition of practical isolation in the hands of the 

 breeders, since the varieties are mated together. 

 The result of this is seen in the great abundance of 

 varieties which have arisen. Indeed, so important 

 is this factor of isolation that one naturalist is in- 

 clined to believe it the all-important factor, and to 

 explain the origin of every species by the isolation 

 of a few individuals. Now the isolation of species 

 upon oceanic islands is by no means so complete as 

 it is in the case of the pigeons, for new individuals 

 may from time to time arrive ; and, moreover, each 

 variety which arises from the first immigrants is not 

 separated by itself. Still it is complete enough to 

 enable varieties to develop in a manner impossible 

 on the mainland, where they are subject to severe 

 competition. We may expect, therefore, that the 

 species which arrive upon these islands will become 

 rapidly modified, and soon be so changed as to de- 

 serve the rank of new species, assuming, of course, 

 that species can thus arise by accumulation of varia- 

 tions. And these species will further be closely 

 related to those on the mainland. The extent to 

 which these new species are developed will depend 

 on the completeness of the isolation and the an- 

 tiquity of the island. If time enough is allowed we 

 may find new genera or, in extreme cases, new fami- 

 lies. Imagine, for instance, a single pair of indi- 

 viduals thus introduced into an island far from the 



