A somewhat different and less usual form of interaction between factors may be illustrated by a case in primulas recently worked out by Bateson and Gregory. Like the common primrose, the primula exhibits both pin-eyed and thrum-eyed varieties. In the former the style is long, and the centre of the eye is formed by the end of the stigma which more or less plugs up the opening of the corolla (cf. Fig. 9, A); in the latter the style is short and hidden by the four anthers which spring from higher up in the corolla and form the centre of the eye (cf. Fig. 9, B). The greater part of the "eye" is formed by the greenish-yellow patches on each petal just at the opening {56} of the corolla. In most primulas the eye is small, but there are some in which it is large and extends as a flush over a considerable part of the petals (Fig. 10). Experiments showed that these two pairs of characters behave in simple Mendelian fashion, short style ( = "thrum") being dominant to long style (= "pin") and small eye dominant to large. Besides the normal long and short styled forms, there occurs a third form, which has been termed homostyle. In this form the anthers are placed low down in the corolla tube as they are in the long-styled form, but the style remains short instead of reaching up to the corolla opening (Fig. 9, C). In the course of their experiments Bateson and Gregory crossed a large-eyed homostyle plant with a small-eyed thrum ( = short style). The F_1 plants were all short styled with small eyes. {57} On self-fertilisation these gave an F_2 generation consisting of four types, viz. short styled with small eyes, short styled with large eyes, _long styled_ with small eyes, and _homostyled_ with large eyes. The notable feature of this generation is the appearance of long-styled plants, which, however, occur only in association with the small eye. The proportions in which these four types appeared shows that the presence or absence of but two factors is concerned, and at the same time provides the key to the nature of the homostyled plants. These are potentially long styled, and the position of the anthers is that of normal long-styled plants, but owing to some interaction between the factors the style itself is unable to reach its full development unless the factor for the small eye is present. For this reason long-styled plants with the large eye are always of the homostyle form. What the connecting-link between these apparently unrelated structures may be we cannot yet picture to ourselves, any more than we can picture the relation between flower {58} colour and hairiness in stocks. It is evident, however, that the conception of the interaction of factors, besides clearing up much that is paradoxical in heredity, promises to indicate lines of research which may lead to valuable extensions in our knowledge of the way in which the various parts of the living organism are related to one another.