It is well known, for instance, that several species appear somewhat earlier in the palaeozoic beds of North America than in those of Europe; time having apparently been required for their migration from the American to the European seas.
Ongi jakina da, esate baterako, hainbat espezie lehentxeago agertu zirela Ipar Amerikako palezoikoko geruzetan, Europakoetan baino, bistakoa den moduan, Amerikako itsasoetatik Europakoetara etortzeko denbora behar izan zutelako.