ARTIFICIAL SELECTION Artificial selection is definable as the methodical or uncon scious choice by man of certain individual animals or plants as parents of the next generation. It is made possible by the facts that related individuals maintained under the same conditions exhibit dissimilarities, and that certain of these dissimilarities are hereditary. Man cannot es yet produce new hereditary characters at will, but as and when these appear he can, by appropriate breeding practices, incorporate or eliminate them from his stock. When a conspicuous and advantageous new inherited character appears, selection is reduced to the preservation of the individual or individuals exhibiting it and to the use of these for further breeding. But in most instances a new desirable character is at first only faintly pronounced ; it is a slight difference in the degree of expression of a character already existing in the stock, a slightly finer quality of wool, or a slightly increased quantity or improved quality of milk, and then patience, the finest powers of discrim ination and the soundest of judgments must be exercised during many years if this improvement is to be maintained. For such selection there must always be a clearly predetermined object in view, the breeder must be able to define his aim and to discrim inate between slight differences. In the absence of any precise method of assessing the kind and degree of improvement, judg ment can be acquired only by long experience.
Unconscious selection is the practice of preserving the more valued and of eliminating the less valued individuals without any thought of altering the general characterization of the stock. Methodical selection is the conscious and deliberate practice of modifying the general characterization of a stock according to some predetermined standard through the careful choice of cer tain individuals as the parents of the next generation and through the control of the coatings of these individuals. Save that in one case man acts deliberately and purposefully and in the other unintentionally, there is little difference between the two kinds of artificial selection : each blends into the other. In both cases man preserves those individuals which promise to be most useful or attractive to him, and neglects the others.
In methodical selection the choice of the individual for further breeding is determined by (I) its appearance, (2) the record of its ancestry (pedigree) and (3) the record of its progeny (the progeny test).
Remarkable results have been secured by this mass selection— the choice for breeding of all or many of those individuals exhibit ing variation in the desired direction. This is the method of the grower of corn who, at harvest time, selects the best ears from the whole yield and from these rears his next year's crop. This selection, on the basis of appearance, though slow and uncertain, has produced the improved varieties of plants and animals. It is effective because most of the characters upon which it is em ployed are dependent on a large number of hereditary factors, which can be sorted out and accumulated by selection of this kind. The most rapid and permanent results of such selection are those which take the form of the sorting out of pure lines from a mixed population of self-fertilized plants, for in this case a single selec tion separates the inherited differences at once. This accomplishes immediately all that is possible, for selection within a pure line is ineffective. (See PURE LINE.) In animals the nearest approach to a pure line is a closely inbred strain comparatively homozy gous for the characters for which selection is practised. It is abundantly demonstrated that appearance alone is not a reliable guide to breeding ability. Individuals which look alike may possess different breeding records, and the only real tests for breeding ability are the performances of their relatives (pedigree) and more especially the records of their progeny (the progeny test).
Many domestic animals and plants afford striking proofs of the power of artificial selection. Thus from a wolf-like ancestor have arisen the greyhound and the bull-dog, the Pekinese and the great dane. Similarly, artificial selection has been the means of perfecting the thoroughbred, the Shetland, and the shire horse. Amongst plants, most, if not all, cultivated species have been more or less transformed by selection, roses and hyacinths affording notable instances. (See ANIMAL BREEDING, MENDELISM, PURE