Chap. XIII.] LAST AND PRESENT CHAPTEES. 199 



communication which allowed certain 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 



rapidly, there would ensue in the two or more regions, 



independently of their physical conditions, infinitely 



diversified conditions 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 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 batra- 



chians and terrestrial mammals, should be absent from 



oceanic islands, whilst 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 inhabitants of an archipelago, 



though specifically distinct on the several islets, should 

 35 



