Obviously, tallness and dwarfness are not characters which differ in _quality_: they are different degrees of the same thing. Brown eyes and blue eyes are not necessarily different in quality, for we may think of the same kinds of pigment as being present in the iris but mixed in different proportions. But the terminology of this branch of biology appears to suggest that the contrasting characters are, each of them, something quite different from the other: there are “factors” for “tallness,” “dwarfness,” for blue eyes and brown eyes, and so on. These qualities are called “unit-characters,” and they are supposed to possess much the same individuality in the germ-plasm as the “radicles” of the chemist possess in a compound. Sodium chloride, for instance, is not a blend of sodium and chlorine: the two kinds of atoms do not fuse together but are held together merely. The analogy is, however, very imperfect, for in the chemical molecule the characters are not those of either of the constituents but something quite different, whereas in the Mendelian cross the characters remain distinct, but one of them is patent while the other is latent. In the molecule, however, the atoms are regarded by the chemist as lying beside each other in certain positions, and the Mendelian factors are also spoken of as if they lay side by side in the germ-plasm. This terminology is useful, perhaps necessary, in the work of investigation, but we must not forget that it symbolises, rather than describes, the results of experiment. If the factors are identified with certain morphological structures in the nuclei of the germ-cells, obviously all the objections that may be urged against the Weismannian hypothesis as an hypothesis of development apply also to the Mendelian hypotheses as descriptions of a physical process of the transmission of morphological characters.