The Encyclopedia Americana (1920)/Breeding
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|Edition of 1920. See also Animal husbandry and Plant breeding on Wikipedia, and the disclaimer.|
BREEDING, the process of organic procreation. In this article the physiology of the matter is not to be considered, however, but only the methods and results as applied to the reproduction of domesticated animals and plants on varietal lines in accordance with human requirements or fancy — that is, selective breeding. When men first began to keep flocks and herds and pet-animals, which were subjugated rather than domesticated as yet, reproduction was no doubt left wholly to nature, as still is the case on the vast cattle ranches of South Africa, Argentina and in America. But very early in the history of pastoral life (which, doubtless, was the first rise above savagery) selective influences must have begun to operate, especially in the case of those animals which are most closely associated with their owners, and therefore most constantly under observation and control. Not only from the earliest times of Egyptian and Oriental record, but long previous, as shown by sub-fossil remains, distinct varieties of dogs, horses, camels and cattle had been produced from their original wild stocks. This might have come about in part naturally as a result of migration of bands to novel climates and food, to inbreeding and to other influences entailed by even a semi-domesticated condition; yet, no doubt, a rough sort of selective breeding began with the earliest domestication of live stock, poultry, etc. Favorites would be preserved when, in times of stress, others were killed for food. Owners fancying bigness, or speed, or a special color or some other quality in certain of their animals would try to get more of that particular kind. In both cases the saving of these better ones would, when they came to breed, strengthen the collection until a noticeable strain had been evolved without any definite intention or action by the owner.
It is probable, moreover, that observation and reasoning — in neither of which faculties was the primitive herdsman deficient within the limits of his experience — would soon suggest to him some advantageous choice of mates among his animals; and there is no question that selective breeding was understood in a practical way at the dawn of history. It is most strikingly manifested in the care taken in ancient as in modern times in southwestern Asia, and in northern Africa, to keep pure and perfect the standard breeds of horses and camels. Little progress was made in Europe in this direction, for various reasons, previous to or during the Middle Ages, and no one with influence made any study of the principles that underlie that proverb of contented credulity — “Like begets like.” Men were accustomed to choose as progenitors the best specimens available of the type they preferred, and were thankful when they got satisfactory progeny. The first attempt at scientific treatment of the matter was that of Robert Bakewell in England, who, about the middle of the 18th century, studied and formulated the peculiarities of certain breeds of cattle, and showed how a male and a female excelling alike in the one or more selected characteristics must be used if the offspring were to progress along that desired line. Thus was begun the now familiar practice of systematically “breeding for points” — that is for the standard characteristics by which a breed (that is, an artificial variety) of any animal is distinguished. Such a breed, when finally attained, continues to perpetuate its distinctive form, or, as they say, “breeds true,” by virtue of the laws of heredity. These laws cannot be discoursed of here, but a few words of explanation may be permitted. While it is true in general that “like begets like,” it is not so in all particulars. No two young are precisely like their parents, or exact duplicates of one another. Individual variations appear in size, color, proportions and temperament. Some differences are rarely noticeable, for some physical features are apparently too fundamental to suffer any perceivable alteration — teeth or eyes (except as to color) for example; other more superficial and plastic features frequently show diversities. The art of the breeder, who desires an improvement in a certain kind of domestic animal, either for increased usefulness, or for some fanciful end, is to recognize and profit by emphasizing some one of the plastic features, and to cultivate it at the expense of other features. To do this he selects a pair that show a little more than others the desired qualify. Some of their offspring are likely to exhibit this quality better than others of the litter, and these are mated. So progressive generations go on advancing the desired feature and eliminating undesirable ones. See Heredity; Mendel's Law.
Now the theory of this is that the qualities of the parents are transmitted to the young in virtually the same proportion as they exist in the parents, but not with exact equality. Some young, either of the same or in successive births, will get a little more of quality A, others a little more of quality B, than the rest. If in the next generation one with a superabundance of quality A mates with one having an excess of quality B, the balance will be restored; and this is what usually happens in nature, so that variations are almost sure to be speedily extinguished. If, however, sovereign man sees to it that A mates with another A, then their young will be doubly endowed with that quality; and if the process is continued that quality will finally predominate, and will be maintained so long as outside mating is prevented.
In practice, however, the matter is not quite so simple. Some animals resist change, or may be modified up to a certain point, and no further in that direction, as in the well-known case of the blue Andalusian chickens, which will not breed true. It must be remembered, also, that negative as well as positive characters are transmissible, the weaknesses as well as the strength. Sometimes a fault seems inseparable from the sought-for virtue, as in the prevalent deafness of white cats.
The evidence as to inheritance of disease is too conflicting to be determinative. Probably no disease or its resulting lesion is transmitted; but a weakness of resistance, which amounts to a tendency, or at least a susceptibility to that disease, often develops in the descendants of animals afflicted with certain ailments, as ring-bone in horses. Domestication has an effect on the procreative ability, or, at any rate, on the fecundity of animals, sometimes reducing it, but more often increasing it, owing, no doubt, to safer conditions and better food than the same creatures had in the wilderness; and the production of special breeds has occasionally been followed by exaggerations of this tendency. In varieties produced by hybridizing it is almost a rule that they are sterile. The more perfect in show-points is a kennel of toy-dogs the more delicate in health are they, and the harder it is for them to beget and nurture their puppies.
The mere production of a fixed artificial variety of animal is one thing, and the making of a strong, useful and valuable breed is quite another. It is probable that few new ones of practical importance will be effected.
Plant-breeding, or the establishment of new forms of plants, depends on essentially the same principles as have been outlined. The principal difficulty in producing a new variety of flower or fruit is to prevent mixture by the introduction of foreign pollen — the fertilizing element; but this may be guarded against. One source of a new variety is by seizing upon some conspicuous accidental variation in a plant, called by horticulturists a “sport,” isolating it, and perpetuating it by some of the methods presently to be mentioned. Many well-known flowers and fruits have originated from such sports. “On an ordinary orange tree, not so many years ago, a single bud produced a branch which bore only seedless fruit, . . . correlated with the presence of a tiny accessory orange embedded almost wholly in the larger; that branch was grafted, as were the resulting branches; and this is the origin of . . . the navel orange” (Ganong).
Reproduction by grafting or by cuttings is one method, then, of plant-breeding; but more commonly the new variety is originated or brought to perfection, or both, by artificial pollination — transferring by human agency desirable pollen to the stigma of the plant to be affected, whence it descends and fertilizes the seed. This is done in most cases by touching the anthers of a ripe flower with a soft, camel's-hair brush, and then touching the stigma to be affected, to which the conveyed pollen will stick. In some special cases the pollen of the plant's own flower is used; this ensures non-variability, and is called “in-breeding”; but as a rule the pollen from another plant of the same kind is taken. This is “cross-pollination,” and tends to make an equally true and more vigorous stock, with an increased tendency to (controllable) variation. Thirdly, the breeder may pollinate a given stigma from a different variety, or even (but not always) from a different species of plant. This is “hybridization,” and the result is a “hybrid” — the source of many of the remarkable varieties of plants that Luther Burbank and other scientific gardeners and experimenters have given us. It is the rule, however, as among animals, that these hybrids do not produce seeds, and must be perpetuated by planting bulbs or cuttings, or in the case of trees, by grafting, generation after generation. Success in these methods can be obtained only be preventing all access to the stigma, and thence to the seeds of the plant operated on, of any pollen except of the one kind needed. This is most usually accomplished by simply enclosing the blossom to be treated in a cap or bag of paper or light gauze, so that no flying pollen or pollen-loaded insects can reach it.
In this review of methods the writer has necessarily given an impression of extreme simplicity, but the road to success abounds in difficulties and often tries the patience of the experimenter by its length.
Bibliography.— Darwin, ‘Variations of Plants and Animals under Domestication’ (London 1885); Castle, ‘Heredity in Relation to Evolution and Animal Breeding’ (New York 1911); Davenport, ‘Principles of Breeding’ (Boston 1907); Ewart, ‘The Principles of Breeding and the Origin of Domesticated Breeds of Animals’ (Annual Report of the Bureau of Animal Industry, United States Department of Agriculture, 1910); Kronacher, ‘Grundzüge der Züchtungs-Biologie’ (Berlin 1912); Marshall, ‘Breeding Farm Animals’ (Chicago 1911); Pearl, ‘Inheritance in Poultry’ (several Bulletins of the Maine Experiment Station); Wilson, ‘Principles of Stock Breeding’ (London 1912).