Masterpieces of Science 



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 mountains, and in the northern 

 and southern temperate zones; and likewise the 

 close alliance of some of the inhabitants of the 

 sea in the northern and southern temperate 

 latitudes, though separated by the whole inter- 

 tropical ocean. Although two countries may 

 present physical conditions as closely similar 

 as the same species ever acquire, we need feel 

 no surprise at their inhabitants being widely 

 different, if they have been for a long period 

 completely sundered from each other; for as the 

 relation of organism to organism is the most 

 important of all relations, and as the two coun- 

 tries will have received colonists at various 

 periods and in different proportions, from some 

 other country or from each other, the course of 

 modification in the two areas will inevitably have 

 been different. 



On this view of migration, with subsequent 

 modification, we see why oceanic islands are 

 inhabited by only few species, but of these, why 

 many are peculiar or endemic forms. We 

 clearly see why species belonging to those groups 

 of animals which cannot cross wide spaces of the 

 ocean, as frogs and terrestrial mammals, do not 

 inhabit oceanic islands; and why, on the other 

 hand, new and peculiar species of bats, animals 

 which can traverse the ocean, are often found 

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