themselves and their brethren in the basins of the Yukon and Mackenzie
rivers. Such inevitably come into contrasted climatic conditions, which
further modify the immigrants. [See map page 54.]

Wide digressions differentiate them still further from the parent stock
by landing them amid different ethnic and social groups, by contact with
whom they are inevitably modified. The Namaqua Hottentots, living on the
southern margin of the Hottentot country near the frontier of the
European settlements in Cape Colony, acquired some elements of
civilization, together with a strain of Boer and English blood, and in
some cases even the Dutch vernacular. They were therefore differentiated
from their nomadic and warlike kinsmen in the grasslands north of the
Orange River, which formed the center of the Hottentot area.[220] A view
of the ancient Germans during the first five or six centuries after
Christ reveals differentiation by various contacts in process along all
the ragged borders of the Germanic area. The offshoots who pushed
westward across the Rhine into Belgian Gaul were rapidly Celticized,
abandoning their semi-nomadic life for sedentary agriculture,
assimilating the superior civilization which they found there, and
steadily merging with the native population. They became _Belgae_,
though still conscious of their Teutonic origin.[221] The Batavians, an
offshoot of the ancient Chatti living near the Thuringian Forest,
appropriated the river island between the Rhine and the Waal. There in
the seclusion of their swamps, they became a distinct national unit,
retaining their backward German culture and primitive type of German
speech, which the Chatti themselves lost by contact with the High
Germans.[222] Far away on the southeastern margin of the Teutonic area
the same process of assimilation to a foreign civilization went on a
little later when the Visigoths, after a century of residence on the
lower Danube in contact with the Eastern Empire, adopted the Arian form
of Christianity which had arisen in the Greek peninsula.[223] The border
regions of the world show the typical results of the historical
movement--differentiation from the core or central group through
assimilation to a new group which meets and blends with it along the
frontier.

[Sidenote: Geographic conditions of heterogeneity and homogeneity.]

Entrance into a naturally isolated district, from which subsequent
incursions are debarred, gives conditions for di