Breed, a term applied to varieties of animals which have been developed more or less directly under human control. Since organisms are liable to change, even the encroachment of civilisation on their old habitats is liable to cause variation. When useful animals are subjugated or tamed by man, when they are brought under the changed conditions of domesticated life, and above all, when their coupling is controlled by human selection, distinct breeds arise. Some breeds have doubtless arisen unintentionally, or as we say naturally, others have from the first been subject to man's purposeful improvement. Some have been formed very slowly and insensibly (e.g. race-horse, greyhound, gamecock, carrier-pigeon), while others have appeared as sudden, more or less abnormal and pathological, variations (e.g. ancon and mawchamp sheep, turnspits, pug-dogs, short-faced tumbler-pigeons). The slowly formed varieties, arising, like most of the species in nature, by steady progress in some definite direction, are more permanent; the more sudden sports and fancy forms are less fixed, and are more apt to revert to the primitive type, or to vary again in some new direction. From zoological analogies, from the occurrence of reversions, and from definite historical evidence, it is now regarded as certain that very many of the domestic varieties or breeds with which we are familiar have descended from a common stock. The domesticated cattle, dogs, fowls, horses, pigs, sheep, &c., are regarded zoologically as variations of one or a few species. The old notion of a distinct origin for each distinct breed is no longer seriously maintained. Their history in general terms has always been the same: wild forms were domesticated, were allowed to breed among themselves, further variations were selected as more fit, reversions were thrown out, and desired forms stereotyped by controlled coupling.
History.—Domestication began in the early savage state. Only a few animals were tried, because few were capable of taming, few promised direct usefulness, and only a few could endure domestication without loss of fertility. The animals thus domesticated in different localities varied very greatly in their new conditions, and as the fitter variations were selected for breeding, numerous sub-breeds arose, and the variety as a whole came to vary more and more markedly from the original wild forms. In some cases, different wild species—e.g. of dog, ox, pig—were in this way subjugated and changed, and thus we have distinct domestic species. In other cases the same wild species would in its domestication in diverse localities experience different changes, and thus different domestic varieties of the same species occur in various countries. As the animals continued—with increased rapidity indeed—to vary under the manifold influences of domestication, the breeder might set apart one set of variations for one purpose, another set for another, and thus a breed became split up into various sub-breeds. For the detailed history of the different breeds, the reader must consult the various articles dealing with domesticated animals—e.g. CATTLE, DOG, HORSE, SHEEP.—What is true of animals is also true of plants, whether cereals, vegetables, or decorative flowers, though the word breed is not usually extended to vegetable organisms. De Candolle's treatise on the Origin of Cultivated Plants (Inter. Science Series, 1884) may be profitably consulted on this subject.
Theory of Breeds.—While the whole subject of domestication will be elsewhere discussed (see DOMESTICATION), some general facts in the formation of breeds must be noticed. (a) The whole process is started by the liability of organisms to change, either (1) in the line of their constitutional tendencies (e.g. increase of fat, hair, decoration, strength, swiftness), or (2) as the direct result of novel external conditions, or (3) in consequence of the increased use or disuse of various structures. (b) The process is sustained by restricting the pairing to organisms of similar constitution. If a variety were allowed to pair with another of a very different tendency, the result would be that the desired variation would in all probability be less marked in the offspring. The secret of breeding is of course to couple those forms from which a continuance of the peculiar features of the breed may be most probably expected. Sexual reproduction, being the mingling of different experiences, is in itself an abundant source of variation, hence the importance of pairing similar forms. To analyse the results of domestication is a complex problem, for the influence of constitutional variability, of restricted or selected sexual range, of changed environment, of altered function, have all to be kept in view. The process of breed-forming is only a special case of what goes on in nature's species-forming, with those important differences, that the life is artificially conditioned, the habits artificially restricted, the sexual reproduction artificially selected. By artificial, we describe conditions which do not find any accurate parallel in nature when unrestricted by human interference. In drawing analogies between the origin of breeds and the origin of species, it must also be remembered that the number and range of domesticated animals is comparatively small, that the variations desired by man concern for the most part the vegetative or nutritive as opposed to the reproductive system, and that in many cases the domesticated males are only in exceptional forms allowed to become reproductive at all.
Variations of Breeds.—While many of the peculiarities which distinguish the various breeds and sub-breeds are very slight, 'no part of the organisation escapes the tendency to vary.' The skin and its outgrowths of hair, feathers, horns, wattles, &c., vary extremely in form, texture, colour, and degree of development. The wool of sheep, the plumes of fowls and canaries, the horns of cattle, the wattle of the carrier-pigeon, are familiar examples. The webbing of the feet of water-dogs is also an instructive case. The muscular system also varies, especially in response to increased use or disuse. What happens in men following different trades, happens similarly in domesticated animals, such as beasts of burden, when habituated for generations to special services. The drooping of the ears is also an illustration of frequent muscular change. The skeleton, too, varies, as is well pointed out by Darwin in the case of pig, niata ox, rabbit, fowls, pigeons, &c. By attending to such trivial characters as the form of the comb or the plume of feathers on the head, the breeder has given ridges to the skull. The various breeds of short-legged animals (e.g. sheep and dogs) illustrate considerable modifications of the skeleton. The teeth also exhibit many variations in domestic animals. The alimentary system and other internal organs are not such that the breeder can direct much attention to the increase or decrease of their tendencies, but they do vary for all that. The lungs and liver of improved breeds of cattle are found by Professor Tanner 'to be considerably reduced in size when compared with those possessed by animals having perfect liberty.' In the case of the pouter pigeon, the zeal of the breeder for the peculiar form characteristic of that variety of bird, has enormously increased the width of the esophagus. The changed conditions of life frequently render the reproductive organs more or less impotent. Many other modifications of internal structure, even of the brain, have been recorded. For a detailed account of the various direct and indirect influences of man's control of animals, Darwin's work on the Variation of Plants and Animals under Domestication must be consulted, and also such works on the structure of our domestic animals as are referred to under special articles. It hardly needs to be pointed out that the characters due directly to breeding—i.e. to intentional control—must be distinguished from those which are simply the secondary results of that change of life which is expressed in domestication.
Practical Breeding.—On this subject works on animal culture must obviously be consulted. The general theory is simple, the practical details numerous. Since many desired variations are still unstable, the breeder has constantly to be on the watch that these do not revert under changed conditions, or become washed out by careless coupling. To preserve a given peculiarity, similar forms must be paired, and yet this 'breeding in and in' may be pursued too far to the degeneration of the stock. Too great attention to non-reproductive characters, in the anxiety to spread and stereotype some desired feature, may tend to general reproductive impotency. When a variation crops up which is novel and desirable, the breeder has to segregate the varying forms, and by controlling the range of reproduction, manufacture a new sub-breed. That this has been repeatedly done is demonstrated by the history of breeds. It seems tolerably certain, however, that only constitutional variations can be transmitted to the offspring, and not such as may be impressed from without upon the parent. The latter may indeed become part of the hereditary capital if the external condition inciting change persist through a series of generations. When this is so, the externally impressed variation becomes deeply rooted, saturates through the organism, becomes part of the constitution, affects the reproductive elements, and may be then transmitted. See HEREDITY.
The whole subject of breeds is one of immense practical and theoretical importance. Without our existing breeds of domesticated animals, from silkworms to horses, human life would be a very different matter. And while the experimental energy of breeders has in some cases become an enthusiastic craze, this same energy has vastly increased the wealth of those civilised countries where the subject receives most attention. The body of experimental fact now accumulated has raised the practice of breeding from the more or less haphazard affair of earlier days to a practical art, in which skill and patient care can (within certain limits) insure any desired result. To the biologist again, the subject is one of unbounded interest. For, in the annals of breeding, as Darwin's patient investigations first emphatically showed, he has the record of a vast series of experiments on the origin of species. On the problems of variation, of environmental influence, and of hereditary transmission, the facts accumulated by breeders have shed and are shedding a great flood of light. See CANARY, CATTLE, DOG, DOMESTICATION, ENVIRONMENT, FOWL, HEREDITY, HORSE, HYBRID, PIGEON, SHEEP, VARIATION, &c.