The discussions of this work are exclusively concerned with the evolution of society and of psychic characteristics.  But even in this limited field we have not attempted to cover the whole ground.  We have given our chief attention to the interdependence of social phenomena and psychic characteristics.  The causes of evolution in the social order have not been the main subject under discussion.

Segregation is the essential condition on which divergent evolution is dependent.  Many forms of segregation may be specified, under each of which evolution proceeds on a different principle.  In brief, it may be said that biological segregation prevents the swamping of incipient organic divergences, by preventing the intermarriage of those possessing such divergences, while social segregation prevents the swamping of incipient social divergences and their corresponding incipient psychic characteristics by preventing the inter-association of those having such tendencies.

Biologically segregated groups undergo divergent biological evolution through segregated marriage, producing distinct physiological unities or racial types.  These racial types are now relatively fixed and can be appreciably modified only by the intermarriage of different races.

Socially segregated groups undergo divergent social evolution through the segregated social intercourse of the members of each group, producing distinct civilizational and psychic unities.  The differences between these social or psychic groups are relatively plastic and are the subject of constant variation.  The modification of the social and psychic characteristics of a group takes place through a change in the physical or social environment of the group, or through the rise of strong personalities within the group.

Biologically distinct groups may thus be unified biologically only by intermarriage, while socially physically distinct groups may be unified socially and psychically without intermarriage, but exclusively through association.

The psychic defects of the offspring of interracial marriages may be largely due to the defective social heredity transmitted by the parents, rather than to mixed intrinsic inheritance.

The term “race soul” is a convenient, though delusive, because highly figurative, expression for the psychic unity of a social group.  The unity is due entirely to the more or less complete possession by the individual members of the group, of common ideas, ideals, methods of thought, emotions, volitions, customs, institutions, arts, and beliefs.

Each individual is molded psychically to the type of the social group in which he is reared.  The “race soul” is thus imposed on the individual by conscious and unconscious education.

The psychic evolution of social groups is divergent so long as isolation is fairly complete, but becomes convergent in proportion to association.  Perfect association produces complete psychic unity, though it should be noted that perfect association of geographically separated social groups is practically unattainable.