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.