It is probable also that in the absence of degenerative tendencies the higher qualities of mind and body are similarly intensified by marriage between highly endowed members of the same family.  Dr. Reibmayr believes that inbreeding is necessary to the higher evolution of the race:  “A settled abode, natural protection from race mixture and the development of a closely inbred social class are the basic conditions of every culture period.”  But inbreeding must not be carried too far:  “In the course of generations the ruling class begins to degenerate mentally and physically, until not only is the class destroyed, but for lack of capable leadership the people (Volk) itself is subjugated and a crossing of blood again takes place."[95]

[Footnote 95:  Trans. from Insucht und Vermischung beim Menschen, p. 46.]

In the breeding of animals the closest inbreeding is frequently resorted to in order to improve the stock, and many examples can be given of the closest possible inbreeding for generations without apparent detriment, but it is universally admitted that the animals selected for such inbreeding must be sound constitutionally, and free from disease.  After a certain number of generations however, degeneration apparently sets in.  The number of generations through which inbreeding may be carried varies with the species, and the purpose for which the animals are bred.  Where they are bred primarily for their flesh, as for beef, mutton or pork, it can be pursued farther and closer than where they are bred for achievement in which a special strength is required—­for instance in the breeding of race horses.  This would indicate that the more delicate brain and nervous system is sooner affected than the lower bodily functions.

In man, however, freedom from hereditary taint cannot so easily be secured.  Individuals cannot be selected scientifically for breeding purposes.  Furthermore, the human body is more delicately constructed than that of the lower animals, and the nervous system is more highly developed and specialized, so that it is reasonable to suppose that in man degeneration would set in earlier in the process of inbreeding, and that it would be impossible to breed as closely as with the lower animals.  Instances are well known, however, where incestuous unions have been productive of healthy offspring, and successive generations of offspring of incestuous connection are not unknown; but, although statistics are lacking, it seems to be very often true that children of such unions are degenerate.  It may be that the reason for this is that with the laws and social sentiments now prevailing in all civilized communities, only degenerates ever contract incestuous alliances.  Desirable as it may be from a social point of view that this strong sentiment against incest should continue, it is not yet proven that even the closest blood relationship between the parents is directly injurious to the offspring.  The “instinctive horror of incest” is a myth, for although a horror of incest does very properly exist in civilized, and in some tribal societies, it is purely a matter of custom and education, and not at all a universal law.