These primary inequalities in the equipment of the determinants which are caused by a passing inequality in the food-stream are, of course, so slight that we are unable to observe their consequences. They must persist for a considerable time before they become observable, but they may persist for a long time, and their effect must then mount up, because every diminution in the strength of the determinant also signifies a lessened power of assimilation, and growth becomes slower for the twofold reason that passive and active nutrition decrease at the same time. In the less minute conditions observable in the histological elements of the body we know that function strengthens the organ, and that disuse weakens it, and we are justified in applying this proposition also to these more intimate conditions and minuter vital units. Thus, in the course of the multiplication of the germ-cells, the less vigorously working determinant, _A_, will gradually, but very slowly, become weaker, that is, of diminished power of assimilation, presupposing of course that the intra-germinal food-stream does not become stronger again at the same place--a possibility to which I shall subsequently refer. But while one determinant may be slowly becoming weaker, its neighbour, on the other hand, may be varying on an ascending scale, just because the former is, on account of its diminished power of assimilation, no longer able to exhaust completely the food-stream which flows to it. The determinants are thus in constant motion, here ascending, there descending, and it is in these fluctuations of the equilibrium of the determinant-system that I see the roots of all hereditary variation, while in the fact that the variation-directions of particular determinants must continue the same without limit as long as they meet with no obstacle lies the possibility of the adaptation of the organism to changing conditions, the increase and transformation of one part, the degeneration and disappearance of another, in short, the processes of natural selection. The reason why such variation movements must continue until they meet some resistance is that every chance upward or downward movement--due, that is, to mere passive fluctuation in the food-supply, at the same time strengthens or weakens the determinant, and makes it either more or less capable of attracting nourishment to itself; in the former case an increasingly strong stream of food will be directed towards it, in the latter more and more of the available food-supply will be withdrawn from it by its neighbour-determinants on all sides; in the former the determinant will go on increasing in strength as long as it can go on attracting more nourishment, in the latter it will continue to become weaker until it disappears altogether. To the ascending progression, as is evident, there are limits set, not only by the amount of food which can circulate through the whole id, but also by the neighbour determinants, which will sooner