Home >> Encyclopedia Americana, Volume 8 >> Cromer to Curves Of Double Curvature >> Cross Fertilization in Ani Mals

Cross-Fertilization in Ani Mals and in Man

in-and-inbreeding, races, animals, means, close, result, breeds, breeding and favorite

CROSS-FERTILIZATION IN ANI MALS AND IN MAN. In animals and man, cross-fertilization means the crossing of dividuals of different races or breeds in con• tradistinction 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 in tervals, seldom or never results in evil effect. It is the continuous in-and-inbreeding of closely related individuals, generation after genera tion, without 'intermission, that is believed by some to result in delicacy of constitution, pre disposition 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 pri marily in any beneficial result of in-and-in breeding 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 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 re sorted 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 evi dently been to secure in the offspring a pre dominance of their most highly valued char acters.' 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 pre vented 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 per petuate 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 render ing 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-grand daughter, 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 off spring. The majority of our best breeds of animals have been very closely in-and-inbred without noticeable deterioration in any direc tion 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 in terbred very closely, as has sometimes occurred, there is said to be a great gain in vigor as a result of intermarriage with a distinctly differ ent 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 attrib uted 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, Bens or phratry) but must intermarry with an individual of some other of the minor divisions of 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. Inter racial 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, ticularly 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 re sults on the one hand from too close inbreed ing 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 con sult bibliographies under these articles.