A second group of facts, which indicates that old-established characters have less tendency to overstep the limits of the neutral 'variation-playground,' is to be found in the experience of breeders, and especially that of gardeners who have brought wild plants under cultivation in order to procure varieties. It has been proved that the wild plants often exhibit no hereditary variations for a long series of generations, notwithstanding the greatly altered conditions of life, but that then a moment comes in which isolated variations crop up, which may then be intensified by the manipulations of the breeder to form sport-species with large conspicuously coloured flowers, or with some other distinctive character. Darwin called this a shattering of the constitution of the plant; but the stable and slowly varying 'constitution' simply means that in old-established and well-adapted species the determinants possess only a very restricted 'variation-playground,' and because of their firmly based harmonious correlation are not easily and never very quickly induced to overstep its limits in any marked degree. Let us now apply all this to the institution of amphimixis and amphigony, and it is immediately obvious that these determinants of the germ-plasm which control the characters relating to sexual reproduction must be _more stable and less variable than all others which a species possesses, for they are infinitely older_. They are older than all species-characters, older than the characters of the genus, of the family, of the class, and indeed of the whole series or phylum to which a higher animal, a vertebrate, for instance, belongs. We cannot wonder, therefore, that amphigony has persisted through hundreds and thousands of generations, even if it had not been reinforced in the germ-plasm during this period by selection. We should rather wonder that an institution so primaeval, and so firmly engrained in the germ-plasm, can ever be departed from, even when its abandonment is to the advantage of the species, as has happened in parthenogenesis.