Selection and the coupling of animals is the practical application of the principles of breed ing as briefly enumerated, and its successful accomplishment. requires the ability to conceive or comprehend an ideal and to maintain or develop it.. In the coupling of animals several systems or practices arc recognized, depending upon the relationship of the sire and the dam, as in-and-in breeding, line breeding, crosshreed ing, etc. In-and-in breeding, or close breeding, is the coupling of animals nearly related. Re garding the closeness of relationship implied in these terms, it not only appears that no definite rule has been established, but also that different writers use them with a somewhat different shade of meaning. This probably accounts large ly for the difference of opinion of writers regard ing the merits of the practice. In theory it rests upon the belief (I) that to produce the best animals the best obtainable stock should be selected and bred together, and this repeated with the offspring over and over again, thus maintaining the excellences of the family free from the intermixture of any less excellent blood, and fixing the desirable characters; and (2) that the in-and-in bred animals are prepotent over others. The object sought is to establish desired forms or qualities with 'the greatest certainty and in the least time. Those who object to close breeding hold that it carries with it danger of loss of constitutional vigor and fertility in the offspring, loss of size, and sometimes malforma tion. This danger undoubtedly depends upon the extent to which the practice is carried and upon the character of the animal. The more purely bred and uniform in type the stock be comes, the greater does the probable danger be come; while with miscellaneously bred stock the evil effects are comparatively slow in showing themselves. All the great breeders have prac ticed close breeding to a greater or less extent, apparently for the purpose of retaining and fix ing certain desirable characteristics that have been developed by modified conditions. They evidently intended to breed together animals of the same qualities, regardless of relationship. The practice is generally conceded to be an im portant means of improvement, when judiciously fol lowed.
Closely allied to in-and-in breeding is line breeding, Which, as applied to a distinct system, is comparatively new. Historically it is an off shoot of in-and-in breeding. It consists in breed ing within a few closely related stocks or fam ilies, no ainmals being interbred which are not closely connected in the general lines of their blood. The practice tends to bring about uni formity in a herd, reducing the animals to a single type: but it is claimed that carried too far, it le-ads to retrogression rather than im provement, as every fault and defect becomes fixed, and deterioration in size and vigor are liable to result.
Natural or miscellaneous breeding is held by conservative writers to be the safest, course in the long run. In this the breeder does not con fine himself to any family or strain of blood, selects the best individual animals obtain able, the sole object being to secure the best pos sible offspring. it avoids any danger there may be in close breeding, and the infusion of fresh foreign blood is held to increase the vigor of the offspring. This system of breeding frequently implies the crossing of representatives of dif ferent families of the same breed, and hence is sometimes referred to as 'making a cross,' 'cross breeding,' etc. Strictly speaking. however, cross breeding is the coupling of animals of distinct breeds. A cross between au animal of a recog nized breed and a native or unimproved stock is spoken of as 'grading up.' and the produet as a 'grade.' In the past cross-breeding has been prac ticed for deriving new breeds or the improvement of a breed in some particular. At present, it is most commonly used as an occasional remedy for some fault. There is a general belief that crossing usually results in increased size and vigor and greater fertility, although if cross breeding is carried so far as to unite distinct species, fertility may be entirely lost. A familiar example of this is the crossing of the ass and the mare, resulting in the mule, which is a sterile hybrid.
The control of the sex of the offspring has been a matter of speculation ever since the days of Aristotle, and a long list of rules have been ad vanced which have found some measure of favor for a time. It still remains a matter of specu lation. however, and in the present state of knowledge is entirely beyond the control of the breeder. The case appears to be similar with regard to the relative influence of the male and the female on the progeny. A variety of theories have been advanced and stoutly contended for— some to the effect that one parent controls the external appearance and the other the dis position. others that the outward form and organs of locomotion are chiefly determined by the male, while the vital organs. size, and vigor are determined by the female. But we have yet to discover how the natures of the parents are mingled, and apparently have made very little advance in accurate knowledge. For a more detailed discussion of the theories, principles, and practice of breeding animals, the reader is referred to Darwin, Variations of Plants and Ani mals Under Domestication (London, 1SS5) ; Mies. Stock Breeding (New York. 1875) ; and \Vartield, Theory and Practice of Cattle Breed ing (New York. ISSO). The different breeds of live stock are considered under CATTLE; HORSE; SWINE; SHEEp; POULTRY. etc. See, also, HERED ITY.