Domestication, the modification of animals by deliberate human interference with their food and surroundings, with the work or functions they perform, but especially with their breeding. The influence of man on animals extends, however, far beyond those usually regarded as domesticated, and it is not possible to draw a perfectly hard and fast boundary line. Man has exterminated some animals—e.g. birds, and propagated others—e.g. fishes; he has made many become rare, shy, and cunning, while others (e.g. crickets) find shelter in his dwellings; he has kept some captive, like the fish in the pond; tamed others individually for his service, like falcons and cheetahs; he has preserved some artificially from their enemies, because of their rarity, and others because of their utility, but without in any of these cases much modifying them. None of these are in the strict sense domesticated. It is only when a distinct breed has been produced by human interference, in most cases deliberately by artificial selection, that we are justified in calling the result domestication. Strictly 'domesticated animals' correspond to strictly 'cultivated plants;' in both cases the organisms have been modified, more or less fixedly, from their natural or wild state, by changes in food and environment, function and breeding.
Domestication began long before the dawn of history (see AGRICULTURE, Vol. I. p. 98; ANTHROPOLOGY, p. 312; DOG, Vol. IV. p. 36). The domestic animals are discussed under separate articles; but a list of representative forms may be given here. Among lower animals, Silk-moths (see SILK) and hive Bees (q.v.) have been for long controlled, and to a limited extent modified. Among fishes, Goldfish (q.v.) may certainly be regarded as domesticated for decorative purposes. Birds include many illustrations of domestication—pigeons, fowls, ducks, geese, peacocks, turkeys, guinea-fowls, canary-birds, &c. Among mammals, dogs and cats, horses and asses, cattle, sheep and goats, elephants, camels, reindeer, pigs and rabbits, &c., have been domesticated, and have given rise to many different breeds. The complete list is not a long one, though it will probably be increased. To admit of domestication, animals must generally be social and docile in their habits, and must be capable of retaining fertility under changed conditions.
The process of domestication, as far as deliberate control is concerned, is for the most part equivalent to selective breeding. Forms with useful varieties are isolated from the mass, and allowed to breed together, the most desirable results are again selected for breeding, and so on, till a domesticated breed of the same animal is established (see BREED). Different breeds differ from natural species in being usually mutually fertile. In other words, while two domestic races may be externally more different than are two nearly related species in nature, the reproductive elements in the first case cannot differ as they must do in the second. Thus crossing is usually successful between domestic breeds, only rarely between adjacent natural species. When we pass beyond selective breeding to inquire into the conditions of variation, a much more difficult problem is raised. In regard to some changes which crop up in domestic animals, we cannot do more at present than refer them to variations in the unstable germ-cells, and to the intermingling of sexual reproduction. Where the intercrossing is regulated, the importance of the latter is especially obvious. These germinal changes may, however, as the organism grows, find expression in the continually variable rhythm between nutrition and reproduction, between growth and multiplication—the great antithesis of organic life. But while considering this internal aspect, we have at the same time to recognise the importance of external influences, especially of altered climate and diet. These hinder or abet the constitutional or inherited tendencies, and may in course of time bring about important new results. Lastly, it must be remembered how much the habit of life, the normal functions, the daily work of the organisms are often altered under domestication. Some parts are more used, others less; and this is also a source of change (see EVOLUTION). Domesticated forms are more variable than their wild relatives; the males are more variable than the females; and the offspring of hybrids are more unstable than the hybrids themselves.
The results of domestication are very varied. Sometimes the changes induced and cultivated have been comparatively slight, in other cases they have amounted to the evolution of new species. Superficial alterations of colour and skin, hair, and feathers; deeper changes in the less plastic skeletal, muscular, alimentary, and other systems; increased fertility on the one hand, sterility on the other; alteration in mental and emotional characters; the perfecting of a racial characteristic in one case, its loss in another; general progress in some forms, utilitarian degeneration or extraordinary abnormality in others, are abundantly illustrated in Darwin's classic work on variation under domestication. The constant tendency to Atavism (q.v.) or reversion; the danger of carrying selection of a given character too far (see BREED); and the relations to Heredity (q.v.) and Evolution (q.v.) are discussed elsewhere.
See ACCLIMATISATION, CULTIVATED PLANTS, VARIATION; Darwin, Animals and Plants under Domestication (1868); Victor Heyn, Kulturpflanzen und Hausthiere (new ed. by Schrader, 1894); N. S. Shaler, Domesticated Animals (1896).