I can give here only the briefest abstract of the more important facts.

Change of climate must have had a powerful influence on migration.

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.