208 THE ORIOIX OF SPECIES 



of the coiuiiiunication wliicli iillowed certtiin forms and 

 not others to enter, either in greater or lesser numbers; 

 according or not as those which entered happened to 

 come into more or less direct competition with each 

 other and with the aborigines: and according as the 

 immigrants were capable of varying more or less rapidl}', 

 there would ensue in the two or more regions, independ- 

 ently of their physical conditions, infinitely diversified 

 conditions of life — there would be an almost endless 

 amount of orsanic action and reaction — and we should 

 find some groups of beings greatly, and some only 

 slightly modified — some developed in great force, some 

 existing in scanty numbers — and this we do find in the 

 several great geographical provinces of the world. 



On these same j^rinciples we can understand, as I have 

 endeavored to show, why oceanic islands should have few 

 inhabitants, but that of these, a large proportion should 

 be endemic or peculiar; and why, in relation to the 

 means of migration, one group of beings should have 

 all its species peculiar, and another group, even within 

 the same class, should have all its species the same with 

 those in an adjoining quarter of the world. We can see 

 why whole groups of organisms, as batrachians and ter- 

 restrial mammals, should be absent from oceanic islands, 

 while the most isolated islands should possess their own 

 peculiar species of aerial mammals or bats. We can see 

 why, in islands, there should be some relation between 

 the presence of mammals, in a more or less modified 

 condition, and the depth of the sea between such islands 

 and the mainland. We can clearly see why all the in- 

 habitants of an archipelago, though specifically distinct 

 on the several islets, should be closely related to each 



