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.
The essential elements constituting national unity are psychic and social, not biological. Racial unity is biological. The same race may accordingly separate into different social and psychic groups. And members of different races may belong to the same social psychic group.
The so-called "race soul" of many sociologists is, therefore, a fiction and indicates mental confusion. The term refers not to the racial unity of inherent psychic nature, but only to the social unity of socially inherited psychic characteristics. Groups thus socially unified may or may not be racially homogeneous. In point of fact no race is strictly homogeneous biologically, nor is any social group completely unified psychically.
In sociology as in biology function produces organism, that is to say, activity produces the organ or faculty fitted to perform the activity.[[DN]] The psychic characteristics differentiating social groups are chiefly, and perhaps exclusively, due to diverse social activities. These activities are determined by innumerable causes, geographical, climatic, economic, political, intellectual, emotional, and personal.
The plasticity of a psychic group is due to the plasticity of the infant mind and brain, which is wonderfully capable of acquiring the language, thought forms, and differentiating characteristics of any group in which it may be reared. To what extent this plasticity extends only carefully conducted experiments can show. In the higher Asiatic and European races we find it to be much greater than is generally supposed to be the case, but it is not improbable that the lowest races possess it in a much lower degree.
The relative fixity of a psychic group is due to the fact that in full-grown adults, who form the majority of every group, function has produced structure. Body, brain, and mind have "set" or crystallized in the mold provided by the social order. Influences sufficiently powerful to transform the young have little effect on the adult. The relative fixity of a psychic group is also due to the difficulty—well-nigh impossibility—of bringing new psychic influences to bear on all members of the group simultaneously. The majority, being oblivious to the new psychic forces, maintain the old psychic régime. The difficulty of reform, of transforming a social order, is principally due to these two causes.