It is an interesting question—When two races or subraces are crossed, do they ever produce a homogeneous and true subrace, exhibiting a true and stable blend of the qualities of the parental stocks? Or does the blend always remain imperfect, with many individuals in whom the qualities of one or other of the parental stocks predominate? The answer seems to be that a stable subrace may be formed in this way, though usually not until free intermarriage has gone on for many generations. According to the most recent doctrine of heredity, the Mendelian, every human being is a mosaic or patchwork of unit qualities, organs, or capacities, each of which is inherited wholly from one of the parents and not at all from the other. If this view is well founded, it follows that there can be no true blending of these unit qualities. But still the mosaic may be so finely grained and the unit qualities derived from the two parents so closely interwoven, that each individual may present an intimate mixture of the parental qualities, may represent for all practical purposes a blending of the two stocks. CHAPTER XVIII RACIAL CHANGES DURING THE HISTORIC PERIOD We have found reason to believe that national character, as expressed in the collective mental life of any people, is only to be understood and explained when we take into account the native or racial mental qualities of the people; and we have seen reason to think that these racial qualities were in the main formed in the prehistoric or race-making period; we have noted some of the principal attempts to throw light on the prehistoric moulding of races. But these racial qualities, although very persistent, are not unalterable. We must, therefore, consider whether, and in what ways, the racial mental qualities of a people may have been changed during the nation-making or historic period. What are the factors which determine such changes? What is their influence on the destiny of nations? The most diverse opinions are still held in regard to the question of the extent and nature of changes of innate mental qualities of peoples during the historic period, the period during which a people, or a branch of a people, attains political unity and becomes a nation. There is no doubt that the moulding power of physical environment tends to become greatly diminished during this more settled period of the life of a people, and that, in so far as changes take place, they are determined principally by racial substitutions and by social selections within a people, rather than by the mere struggle for survival of individuals or of family groups against the inclemency of nature or against other individuals and groups.