GEOGRAPHICAL DISTRIBUTION 367 



inhabited by very different forms of life ; for according 

 to the length of time which has elapsed since new in- 

 habitants entered one region ; according to the nature 

 of the 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 in more or less direct competition with each other 

 and with the aborigines ; and according as the immi- 

 grants were capable of varying more or less rapidly, 

 there would ensue in different regions, independently of 

 their physical conditions, infinitely diversified condi- 

 tions of life, — there would be an almost endless amount 

 of organic action and reaction, — and we should find, as 

 we do find, some groups of beings greatly, and some 

 only slightly modified, — some developed in great force, 

 some existing in scanty numbers — in the different great 

 geographical provinces of the world. 



On these same principles, we can understand, as I have 

 endeavoured to show, why oceanic islands should have 

 tew inhabitants, but of these a great number should be 

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

 of migration, one group of beings, even within the same 

 class, should have all its species endemic, and another 

 group should have all its species common to other 

 quarters 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 possess their own peculiar species of 

 aerial mammals or bats. We can see why there should 

 be some relation between the presence of mammals, in 

 a more or less modified condition, and the depth of the 

 sea between an island 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 likewise be related, 

 but less closely, to those of the nearest continent or 

 other source whence immigrants were probably de- 

 rived. We can see why in two areas, however distant 

 from each other, there should be a correlation, in 

 the presence of identical species, of varieties, of 



