The same methods of selection that establish a breed are used to maintain it, or to establish strains.  In ordinary breeding there are two other principles that are sometimes called into play.  One is prepotency, the other is inbreeding.  By prepotent we mean having unusual power to transmit characters to offspring.  Suppose a breeder has five yards headed by five cock birds.  The male in yard two he does not consider quite as fine as the bird in yard one, but in the fall he finds the offspring of bird from two much better than the offspring from yard one.  The breeder should keep the prepotent sire and his offspring rather than the more perfect male, who fails to stamp his traits upon his get.

Normally a child has two parents, four grandparents, and eight great-grandparents.  Now, when cousins marry, the great-grandparents of the offspring are reduced to six.  The mating of brother and sister cuts the grandparents to two, and the great-grandparents to four.  Mating of parent and offspring makes a parent and grandparent identical and likewise eliminates ancestry.  Inbreeding means the reduction of the number of branches in the ancestral tree, and this means the reduction of the number of chances to get variation, be they good or bad.

Inbreeding simply intensifies whatever is there.  It does not necessarily destroy the vitality, but if close inbreeding is practiced long enough, sooner or later some little existing weakness or peculiarity would become intensified and may prove fatal to the strain.  For illustration, suppose we began inbreeding brother and sister with a view of keeping it up indefinitely.  Now, in the original blood, a tendency for the predominance of one sex over the other undoubtedly exists and would be intensified until there would come a generation all of one sex, which, of course, terminates our experiment.

Inbreeding has always been tabooed by the people generally.  Meanwhile the clever stock breeders have combined inbreeding with selection and have won the show prizes and sold the people “new blood” at fancy prices.

Unintelligent inbreeding as practiced on many a farm, results in run down stock, not so much from inbreeding as from lack of selection.  Out-crossing or mixing in of new blood is better than hit-or-miss inbreeding.  Intelligent inbreeding is better still.

Scientific Theories of Breeding.

The main tenet of Darwin’s theory of racial inheritance or evolution, was that changes in animal life, wild or domestic, were brought about by the addition of very slight, perhaps imperceptible, variations.  He argued that the giraffe with the longest neck could browse on higher leaves in time of drought and hence left offspring with slightly longer necks than the previous generation.

Upon this theory the ordinary breeding by selection is based.  In case of breeding for show room, the breeder’s eye, or the judge’s score card, is the tape with which to measure the length of the giraffe’s neck.  This principle can be applied equally well, even better, to characteristics where accurate measurement may be used.