The Encyclopedia Americana (1920)/Cross-Fertilization in Animals and in Man
CROSS-FERTILIZATION IN ANIMALS AND IN MAN. In animals and man, cross-fertilization means the crossing of individuals of different races or breeds in contradistinction to in-and-inbreeding which is generally regarded as leading to evil results. Inbreeding, interbreeding or close breeding, which means the breeding together of closely related animals at rather distant or long intervals, seldom or never results in evil effect. It is the continuous in-and-inbreeding of closely related individuals, generation after generation, without intermission, that is believed by some to result in delicacy of constitution, predisposition to disease, lack of fecundity, etc. It must be admitted that breeders who have used in-and-inbreeding the most have done so as a means to an end, and not because they believe primarily in any beneficial result of in-and-inbreeding in itself. This is the surest and best way to render a character prepotent — i.e., to isolate pure Mendelian characteristics. (See Heredity). It is used, therefore, as a means of strengthening the transmitting power or prepotency of a character, which otherwise in most instances would be lost. Miles states that “From a careful examination of the pedigrees . . . that may be found in the herd-books and breeding-registers, representing the practice of breeders of acknowledged reputation, it will be found that in-and-inbreeding has only been resorted to in the case of some favorite animal or animals that were superior in certain respects to the average members of the herd or family which they represent, and the object has evidently been to secure in the offspring a predominance of their most highly valued characters.” In most instances the older original character is more strongly hereditary, and it is only by in-and-inbreeding that a new character can be rendered stable and prepotent and prevented from being swamped and lost. In regard to the belief that in-and-inbreeding leads to sterility and predisposition to disease, a careful consideration of the evidence at command leaves the student in doubt as to the conclusion to be drawn. The facts seem to indicate that close breeding or in-and-inbreeding may be very detrimental in some cases, as it tends to perpetuate any constitutional defects that may have been produced by other agencies; therefore the best animals, free from constitutional weakness or disease should be selected as mates. When used judiciously, in-and-inbreeding forms an important means of securing improvements and is the only known means of fixing and rendering slight variations hereditary.
The majority of our various breeds of cattle have been brought up and improved as a result of very close inbreeding. As an illustration, the famous shorthorn bull, Favorite, was bred to his daughter, granddaughter and great-granddaughter, and the product of the last union was matched with the bull Wellington, having 62.5 per cent of the blood of Favorite. Clarissa, the offspring of the last union, was bred with the bull Lancaster, having 68.75 per cent of the blood of Favorite and gave very valuable offspring. The majority of our best breeds of animals have been very closely in-and-inbred without noticeable deterioration in any direction except possibly in fecundity. Darwin says that “Although by careful selection of the best animals, close inter-breeding may be long carried on with cattle, yet the good effects of a cross between almost any two breeds is at once shown by the greater size and vigor of the offspring; and authorities agree that ‘crossing distinct breeds certainly improves cattle for the butcher.’ ”
In the case of man, where families have interbred very closely, as has sometimes occurred, there is said to be a great gain in vigor as a result of intermarriage with a distinctly different family, a fact long recognized and acted upon in the seeking of American heiresses in marriage by scions of ancient European families of distinction. The hardihood and general vigor of the Americans as a nation is commonly attributed to the great intermixture of peoples of many different nationalities; and the whole human race, apparently from the beginning has acted on this principle by the almost universal practice of exogamy. This is the rule, held all but sacred among savages everywhere, that a man or woman must not marry one of his or her own group (clan, gens or phratry) but must intermarry with an individual of some other of the minor divisions ot the tribe. The theory is that all members of each such minor divisions are children of the same (ancestral) mother. The world-wide prohibition of, and repulsion against, marriage within certain degrees of consanguinity, minister to the same end. Interracial marriage, or miscegenation, the marriage of individuals of distinct races, as a whole, results very disastrously both as to physical and mental characteristics. The result of such a union is a hybrid, frequently sterile, mainly intermediate in characters between the two races, and usually in large measure a social outcast. Such half-breeds or hybrids are in general inferior to the pure parental races, particularly in physical vigor, though mentally they may be equal or possibly superior. In crosses, for instance, of the negro and white races, the offspring commonly show a tendency toward sterility and are in general weak in constitution.
In conclusion it may be stated that injury results on the one hand from too close inbreeding and on the other hand from crossing races too distinct; but that the crossing of slightly distinct strains, and of individuals reared under different conditions, is beneficial. See Breeding; Exogamy; Heredity; Mendel's Law, and consult bibliographies under these articles.