It is true that we cannot learn _anything directly_ in regard to the intimate structure of the germ-plasm, and even in regard to the vital processes going on within it we can only guess a very little, but so much we may say--that its living parts are nourished, and that they multiply. But it follows from this that nourishment in a dissolved state must penetrate between its vital particles, and that whether the determinants grow, and at what rate they do so, depends mainly on the amount of nourishment which reaches them. As long as the germ-cells multiply by division the determinants have no other function but to grow; a part of their substance undergoes oxidation and thereby yields the supply of energy necessary to assimilation, that is, to the formation of new living substance. If each kind of determinant always secured the same quantity of nourishment, all would grow in the same degree, that is, in exact proportion to their power of assimilation. But we know that in less minute conditions which we can observe more directly, there is nowhere absolute equality, that all vital processes are subject to fluctuations; any little obstacles in the current of the nutritive fluid, or in its composition, may cause poorer nutrition of one part, better of another. We may therefore assume that there are similar irregularities and differences in the minute and unobservable conditions of the germ-plasm likewise, and the result must be a slight shifting of the position of equilibrium as regards size and strength in the determinant system; for the less well-nourished determinants will grow more slowly, will fail to attain to the size and strength of their neighbours, and will multiply more slowly. But the vigour of growth does not depend only on the influence of nourishment; one cell grows quickly, another slowly in the same nutritive fluid; it depends in great part on the cell's power of assimilation. In the same way the assimilating power of the determinants and their affinity for nourishment will vary with their constitution, and a weaker determinant will remain smaller than a stronger one, even when the stream of nourishment is the same. It seems to me that it is upon the unequal nutrition of the determinants conditioned by the chances of the food-supply that individual hereditary variability ultimately depends. If, for instance, the determinant _A_ receives poorer nourishment at a particular time than the determinant _B_, it will grow more slowly, remain weaker, and then, when the germ-cells develop into an animal, the part to which it gives rise will be weaker than it usually is in other individuals.