Chap. XIV. RECAPITULATION. 477 



modification have been the same. We see the full 

 meaning of the wonderful fact, which must have struck 

 every traveller, namely, that on the same continent, 

 under the most diverse conditions, under heat and cold, 

 on mountain and lowland, on deserts and marshes, most 

 of the inhabitants within each great class are plainly 

 related ; for they will generally be descendants of the 

 same progenitors and early colonists. On this same 

 principle of former migration, combined in most cases 

 with modification, we can understand, by the aid of the 

 Glacial period, the identity of some few plants, and the 

 close alliance of many others, on the most distant moun- 

 tains, under the most different climates ; and likewise 

 the close alliance of some of the inhabitants of the sea 

 in the northern and southern temperate zones, though 

 separated by the whole intertropical ocean. Although 

 two areas may present the same physical conditions of 

 life, we need feel no surprise at their inhabitants being 

 widely different, if they have been for a long period 

 completely separated from each other ; for as the rela- 

 tion of organism to organism is the most important of 

 all relations, and as the two areas will have received 

 colonists from some third source or from each other, at 

 various periods and in different proportions, the course 

 of modification in the two areas will inevitably be 

 different. 



On this view of migration, with subsequent modifica- 

 tion, we can see why oceanic islands should be inhabited 

 by few species, but of these, that many should be 

 peculiar. We can clearly see why those animals which 

 cannot cross wide spaces of ocean, as frogs and terrestrial 

 mammals, should not inhabit oceanic islands ; and why, 

 on the other hand, new and peculiar species of bats, 

 which can traverse the ocean, should so often be found 

 on islands far distant from any continent. Such facts 



