This inherent divergence is so profound that two well-marked groups become incapable of mental unity. They may be separated by an imaginary line, and have been for generations under like climatic and cultural conditions, but the imprint of the divergence is ineradicable. If they have the same religion, they will understand it differently; the same events will impress them differently; their feeling and their hopes will be asunder. While this is true, it is also true that a new stimulus to progress is created by the union of divergent lines of thought. The resultant is a fresh element in mental life, a new birth independent of either parent. Such unions are brought about either by similarity or contrast. There is a species of elective affinity between certain lines of psychical development which at once unites them as they approach each other. There is also a similar union induced by contrasted psychical states. We say familiarly that “opposites attract each other,” and it is a maxim drawn from frequent experience. The rapid changes from social freedom to military tyranny in the mercurial population of some states seem more gratifying to the ethnic spirit than a continued stable government. Parallel variations lead to similarity in products. They are “homoplastic,” to use the term of the evolutionist. Primitive tribes, developing under the same general conditions of environment, are strikingly alike in culture. Divergent variations are “heteroplastic,” that is, they lead to new products, and hence are the higher activities in all that makes for advancement. Whatever multiplies them stimulates the growth of culture. 2. _Variation in Circles or Curves._—Both parallel and divergent evolution are expressions of continuity of progress in lines, extending from point to point, intersecting to produce other lines of new directions. Such a rectilinear scheme is the simplest that we can sketch of human advancement; and for many purposes it is sufficiently correct. It does not, however, fully express the geometrical representation of such agencies as we are considering. Professor Baldwin has justly remarked that there is a “circular activity” in all progress. Its influence is not aimed solely at a point ahead, but extends itself in all directions. The reception of a new and true idea in the human mind may be likened to the introduction of a ray of sunlight into a darkened room. Its chief force is seen in the linear shaft of light, but the illumination extends in some degree to the whole space. Johannes Schmidt has shown that the distribution of the early Aryan dialects and religions was not from the point of common origin by right lines of migration in different directions, but should be represented diagrammatically by a series of irregular circles and ellipses, overlapping each other. The tendency to variation arises in some centre and spreads from it in a series of curves. These meeting others lead to an “interlinking” of cultural areas.