A region now impassable to certain organisms from the nature of its climate, might have been a high road for migration, when the climate was different.
I shall, however, presently have to discuss this branch of the subject in some detail.
Changes of level in the land must also have been highly influential: a narrow isthmus now separates two marine faunas; submerge it, or let it formerly have been submerged, and the two faunas will now blend together, or may formerly have blended.
Where the sea now extends, land may at a former period have connected islands or possibly even continents together, and thus have allowed terrestrial productions to pass from one to the other.
No geologist disputes that great mutations of level have occurred within the period of existing organisms.