Civilized man progressively modifies the land which he occupies, removes or reduces obstacles to intercourse, and thereby approximates it to the open plain.

Thus far he facilitates movements.

But while doing this he also places upon the land a dense population, closely attached to the soil, strong to resist incursion, and for economic reasons inhospitable to any marked accession of population from without.

Herein lies the great difference between migration in empty or sparsely inhabited regions, such as predominated when the world was young, and in the densely populated countries of our era.

As the earth grew old and humanity multiplied, peoples themselves became the greatest barriers to any massive migrations, till in certain countries of Europe and Asia the historical movement has been reduced to a continual pressure, resulting in compression of population here, repression there.