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 centuries later the names of these non-Lombard tribes still survived in certain villages of Italy which had formed their centers.[146] The army which Attila the Hun brought into Gaul was a motley crowd, comprising peoples of probable Slav origin from the Russian steppes, Teutonic Ostrogoths and Gepidae, and numerous German tribes, besides the Huns themselves. When this horde withdrew after the death of Attila, Gepidae and Ostrogoths settled along the middle Danube, and the Slavonic contingent along the Alpine courses of the Drave and Save Rivers.[147] The Vandal migration which in 409 invaded Spain included the Turanian Alans and the German Suevi. The Alans found a