d 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. The ethnic stream may start comparatively pure, but
it becomes mixed on the way. From time to time it leaves behind laggard
elements which in turn make a new racial blend where they stop. Such
were the six thousand Aduatici whom Caesar found in Belgian Gaul. These
were a detachment of the migrating Cimbri, left there in charge of
surplus cattle and baggage while the main body went on to Italy.[142]

[Sidenote: Complex currents of migration.]

A migration rarely involves a single people even at the start. It
becomes contagious either by example or by the subjection of several
neighboring tribes to the same impelling force, by reason of which all
start at or near the same time. We find the Cimbri and Teutons combined
with Celts from the island of Batavia[143] in the first Germanic invasion
of the Roman Empire. Jutes, Saxons and Angles started in close
succession for Britain, and the Saxon group included Frisians.[144] An
unavoidable concomitant of great migrations, especially those of nomads,
is their tendency to sweep into the vortex of their movement any people
whom they brush on the way. Both individuals and tribes are thus caught
up by the current. The general convergence of the central German tribes
towards the Danube frontier of the Roman Empire during the Marcomannic
War drew in its train the Lombards from the lower Elbe down to the
middle Danube and Theiss.[145] The force of the Lombards invading Italy
in 568 included twenty thousand Saxons from Swabia, Gepidae from the
middle Danube, Bulgarians, Slavs from the Russian Ukraine, together with
various tribes from the Alpine district of Noricum and the fluvial
plains of Pannonia. Two centurie