=The Cause of the Mendelian Ratio.--=Concerning the cause of this peculiar ratio of inheritance in crossed forms Mendel suggested a simple explanation. Animals or plants that can be cross-bred, obviously must be forms that produce a new individual from the union of two germ-cells, one of which is provided by each parent. Mendel's idea was that there must be some process of segregation going on in the developing germ-cells of each hybrid whereby the factors for the two qualities are set apart in different cells with the result that half of the germ-cells of a given individual will contain the determiner of one character and half, the determiner of the other. That is, a given germ-cell carries a factor for one or the other of the two alternate characters but not the factors for both. In a plant, for example, in the male line, half of the pollen grains would bear germ-cells carrying the determiner of one character and half, that of the other. Similarly, in the female line, half of the ovules would contain the determiner of the one character and half, that of the other. Likewise in animals as regards such pairs of characters there would be two classes of germ-cells in the male and two in the female. In the case of the blue Andalusian fowls under discussion this would mean that half of the mature germ-cells of the male carry the black-producing factor, and half carry the white-producing factor, and the same is true of the germ-cells of the female. Thus when two such crossed forms are mated, there are, by the laws of chance, four possible combinations, namely: (1) white-determining sperm-cells and white-determining ovum; (2) white-determining sperm-cells and black-determining ovum; (3) black-determining sperm-cells and white-determining ovum; and (4) black-determining sperm-cells and black-determining ovum. Manifestly, the first combination can only give white offspring; the second, white and black, gives blue (by such a cross the original blues were established); likewise, the third, black and white, gives blue; and the fourth combination can only give black offspring. This matter may be graphically represented by the following formulæ in which B indicates the determiner of Black in the germ-cell and W the determiner of White: [male] signifies male; [female] female. IN THE ORIGINAL PARENTS W × B = WB = Blue IN THE HYBRIDS [male] [female] germ- germ- cells cells [male] [female] W----W W × W = WW = White \/ W × B = WB = Blue /\ or B × W = BW = Blue B----B B × B = BB = Black