Selection (apart pick), the choice (for perpetuation by reproduction) from a mixed population, of the individuals possessing in common a certain character or a certain degree of some character. Two kinds of selection may be distinguished: (1) natural selection, in which choice is made automatically by the failure to reproduce (through death or some other cause) of the individuals who are not "fit" to pass the tests of the environment (vitality, disease resistance, speed, success in mating, or what not); and (2) artificial selection, in which the choice is made consciously by man, as a livestock breeder.
Sex-Limited, a term applied to traits which differ in the two sexes, because influenced by the hormones of the reproductive glands. Example, the beard.
Sex-Linked, a term applied to traits which are connected with sex accidentally and not physiologically in development. The current explanation is that such traits happen to be in the same chromosome as the determiner of maleness or femaleness, as the case may be. Color-blindness is the classical example in man.
Sexual Selection, the conscious or unconscious preference by individuals of one sex, or by that sex as a whole, for individuals of the other sex who possess some particular attribute or attributes in a degree above or below the average of their sex. If the deviation of the chosen character is in the same direction (plus or minus) as in the chooser, the mating is called assortative; if in one direction independent of the characteristic of the chooser, it is called preferential.
Soma (body), the body as distinguished from the germ-plasm. From this point of view every individual consists of only two parts,—germ-plasm and soma or somatoplasm.
Trait, a term used by geneticists as a synonym of "character."
Unit-Character, in Mendelian heredity a character or alternative difference of any kind, which is apparently not capable of subdivision in heredity, but is inherited as a whole, and which is capable of becoming associated in new combinations with other characters. The term is now going out of use, as it makes for clearer thinking about heredity to fix the attention on the factors of the germ-cells instead of on the characters of the adult.
Variation, a deviation in the size, shape, or other feature of a character or trait, from the mean or average of that character in the species.
Vestigial (footstep), a term applied to a character which at some time in the evolutionary history of the species possessed importance, or functioned fully, but which has now lost its importance or its original use, so that it remains a mere souvenir of the past, in a degenerated condition. Example, the muscles which move a man's ears.
Zygote (yolk), the fertilized egg-cell; the united cell formed by the union of the ovum and spermatozoön after fertilization.