The transference of a whole people from its native or adopted seat to a new habitat, as in the _Voelkerwanderungen_, empties the original district, which then becomes a catchment basin for various streams of people about its rim; and in the new territory it dislodges a few or all of the occupants, and thereby starts up a fresh movement as the original one comes to rest.
Nor is this all.

A torrent that issues from its source in the mountains is not the river which reaches the sea.

On its long journey from highland to lowland it receives now the milky waters of a glacier-fed stream, now a muddy tributary from agricultural lands, now the clear waters from a limestone plateau, while all the time its racing current bears a burden of soil torn from its own banks.

Now it rests in a lake, where it lays down its weight of silt, then goes on, perhaps across an arid stretch where its water is sucked up by the thirsty air or diverted to irrigate fields of grain.

So with those rivers of men which we call migrations.