plants, but, as similar results have been obtained again and again with animals, we may substitute animals for plants. When white cattle are mated with red, their progeny (first crosses) are roan; but when these roans are mated together, a quarter of their progeny return in colour to the white parent race and a quarter to the red parent race, while the remainder are roans again, like their parents. Mendel's theory explains such phenomena. He conceived the idea that an animal carries, from its very beginning, determinants which are going to decide, one its eventual colour, another its size, another its length of limb, and so on; and that a half of each determinant is inherited from each parent Each determinant is therefore, as it were, bicellular, bouble-barrelled.
A roan animal's red parent carries a double-barrelled determinant for redness, which we may represent thus: , while its white parent carries another for whiteness, which we may represent thus: .
When a red animal is mated with a white, their determinants meet, and the young, taking a half of that offered by each parent, starts off with a determinant, one half of which is for redness, the other half for whiteness, thus: , and
