Chap. XIII.] LAST AND PRESENT CHAPTERS. 199 



allowed certain forms and not others to enter, either 

 in greater or lesser numhers; 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 rapidly, there would 

 ensue in the two or more regions, independently of 

 their physical conditions, infinitely diversified con- 

 ditions of life, — ^there would be an almost endless 

 amount of organic 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 principles we can understand, as I 

 have endeavoured to show, why oceanic islands should 

 have few inhabitants, but that of these, a large propor- 

 tion should be endemic or peculiar; and why, in rela- 

 tion 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 terrestrial mammals, should be absent 

 from oceanic islands, whilst the most isolated islands 

 should possess their own peculiar species of aerial mam- 

 mals 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 inhabitants of an archipelago, 

 though specifically distinct on the several islets, should 

 be closely related to each other; and should likewise 



