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 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.]