The Glacial period, as measured by years, must have been very long; and when we remember over what vast spaces some naturalised plants and animals have spread within a few centuries, this period will have been ample for any amount of migration.
Glaziaraldiak, urtez neurturik, oso luzea izan behar izan du; eta gogoratzen badugu, zenbait menderen buruan zein hedadura amaigabeetan zehar hedabanatu diren landare eta animalia bertakotu batzuk, zeinahi migraziotarako denborarik aski izan dela ikusiko dugu.