on.]

Differentiation results not only from contrasted geographic conditions,
but also from segregation. A moving or expanding throng in search of
more and better lands drops off one group to occupy a fertile valley or
plain, while the main body goes on its way, till it reaches a
satisfactory destination or destinations. The tendency to split and
divide, characteristic of primitive peoples, is thus stimulated by
migration and expansion. Each offshoot, detached from the main body,
tends to diverge from the stock type. If it reaches a naturally isolated
region, where its contact without is practically cut off, it grows from
its own loins, emphasizes its group characteristic by close in-breeding,
and tends to show a development related to biological divergence under
conditions of isolation. Since man is essentially a gregarious animal,
the size of every such migrating band will always prevent the evolution
of any sharply defined variety, according to the standard of biology.
Nevertheless, the divergent types of men and societies developed in
segregated regions are an echo of the formation of new species under
conditions of isolation which is now generally acknowledged by
biological science. Isolation was recognized by Darwin as an occasional
factor in the origin of species and especially of divergence; in
combination with migration it was made the basis of a theory of
evolution by Moritz Wagner in 1873;[217] and in recent years has come to
be regarded as an essential in the explanation of divergence of types,
as opposed to differentiation.[218]

[Sidenote: Differentiation and digression.]

The traditions of the Delaware Indians and Sioux in the north of the
United States territory, and of the Creeks in the south, commence with
each stock group as a united body, which, as it migrates, splits into
tribes and sends out offshoots developing different dialects. Here was
tribal differentiation after entry into the general stock area, the
process going on during migration as well as after the tribes had become
established in their respective habitats. Culture, however, made little
progress till after they became sedentary and took up agriculture to
supplement the chase.[219] Tribes sometimes wander far beyond the limits
of their stock, like the Iroquoian Cherokees of East Tennessee and North
Carolina or the Athapascan Navajos and Apaches of arid New Mexico and
Arizona, who had placed twenty or thirty degrees of latitude between