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 on islands far distant from any continent.
Argi ikusten dugu zergatik ezin den bizi irla ozeanikoetan itsas arlo zabalik zeharkatzeko gai ez den animali talderik, adibidez, igelik ez lehorreko ugaztunik; eta, aitzitik, zergatik topatu ahal diren itsasoa zeharkatzeko gauza diren saguzarren espezie berri eta bereziak edozein kontinentetik urrun dauden irletan.