After all such allowances, there remains a residue unexplained by them, and inconsistent with the general theory of advantage in race-intermixture.
The solution of this problem is to be found in the operation of an obscure but certain law of heredity which has been demonstrated by the best modern observers.
This reads that in the struggle for transmission between contrary characteristics in the parents, any trait, mental or physical, may be passed down separately, independently of others.
Thus, on the physical side, the father may have red, the mother black, hair. The children will inherit, not a blended colour, but some the red, some the black hair. Or, let us say, one parent has marked musical ability, the other none. Some of the children will have as much as the gifted parent, the others be devoid of the faculty.
It is essential, also, to remember that it is the inferior race only which reaps the psychical advantage. Compared to the parent of the higher race, the children are a deteriorated product. Only when contrasted with the average of the lower race can they be expected to take some precedence. The mixture, if general and continued through generations, will infallibly entail a lower grade of power in the descendant. The net balance of the two accounts will show a loss when compared with the result of unions among the higher race alone.
This consideration has led a recent writer, Dr. Reibmayr, to a theory of ethnic mental development which merits close attention.
A family, tribe, caste, or race, to preserve and increase its faculties must sedulously avoid intermarriage with one of inferior gifts. The value of “breeding in-and-in” is familiar to all interested in the improvement of the lower animals. This was attained in primitive life by the tribal law of endogamous marriages, by which a man must take his wife within the tribe, but not of his immediate blood.
The superiority which this developed led to the subjection of other tribes, and this, through capture and enslavement of the women, to intermixture of blood, with its above mentioned first consequences: deterioration of power in the captors, and, next, elevation of the lower, conquered tribe.
The former was sometimes counteracted by the maintenance of purity of blood in a portion of the community, which thus became the ruling class; and if this did not take place, the tribe itself soon fell beneath the sway of some neighbour which had maintained its lineage more purely.
Thus, says Dr. Reibmayr, the history of human mental development is, in fact, the history of human hybridity and its necessary consequences.