The great and striking influence of barriers of all kinds, is intelligible only on the view that the great majority of species have been produced on one side, and have not been able to migrate to the opposite side.
Mota guztietako mugen eragin handi eta harrigarria, espezierik gehienak mugaren alde batean sortu eta beste aldera migratzerik ez izanik bakarrik da ulergarria.