Of course we know nothing definite or precise with regard to the units of the germ-plasm, and we cannot tell what is necessary in order that a determinant shall determine a part of the developing body in this way or in that; thus we have no definite idea of the relations subsisting between the variations of the determinants and those of their determinates, but we know at least so much, that hereditary variation of a part is only possible when a corresponding particle in the germ-plasm varies; and we may at least assume that these correspond to each other so far, that a greater development of the one implies a greater development of the other, and that a reversal of these relations is impossible. If the determinant _X_ disappears from the germ-plasm the determinate _X´_ disappears from the soma. It is therefore justifiable to infer from the degree of development of an organ the strength of its determinant, and to assume that plus- and minus-variations in both are correspondingly large. But in addition to the fluctuations in the equilibrium of the germ-plasm which lie at the root of all hereditary variation, we have to take into account something which we have already touched upon briefly--the correlation of the determinants, the influencing of one determinant by those round about it. I have spoken for the sake of brevity of 'the determinant' of a part, although all the large and more important parts must certainly be thought of as represented by several or many, if not, indeed, by whole groups of determinants. Although it is quite out of our power to follow the complex processes of the mutual influences of the determinants upon each other, we can say this at least, _that these influences must exist_, and we have here a faint indication of what must occur in the case of spontaneous variations within the germ-plasm. We must, in the first place, think of the individual determinants as arranged in groups, so that, for instance, the determinants of the right and left half of the body lie together, and therefore are frequently affected together by influences which cause variation, so that both vary in the same direction at the same time. In point of fact, analogous deformities, such as polydactylism of both right and left hands, and even of hands and feet at once, do actually occur. That the right and left hands, the fore- and hind-limbs, are represented in the germ by particular determinants, may be inferred from their frequently different phyletic evolution into different forms of hand and foot, e.g. into flipper and rudimentary hind-leg in the whale, as well as from the cases of particulate inheritance, which are rare, but which undoubtedly do occur, such as when, in Man, there is a maternal blue eye on one side of the head and a paternal brown eye on the other. But almost more striking than the differences between these homologous or homotypic parts are their points of resemblance, and these may probably be in part