"The cessation of selection when working alone (as it probably does during the first centuries of its action upon structures or colours which do not entail any danger to, or perceptible drain upon, the nutritive resources of the organism) cannot cause degeneration below, probably, some 10 to 20 per cent. But if from the first the cessation of selection has been assisted by the _reversal_ of selection (on account of the degenerating structure having originally been of a size sufficient to entail a perceptible drain on the nutritive resources of the organism, having now become a source of danger, and so forth), the two principles acting together will continue to reduce the ever-diminishing structure down to the point at which its presence is no longer a perceptible disadvantage to the species. When that point is reached, the reversal of selection will terminate, and the cessation of selection will not then be able of itself to reduce the organ through more than at most a very few further percentages of its original size. But, after this point has been reached, the now total absence of selection, either for or against the organ, will sooner or later entail this further and most important consequence, a failure of heredity as regards the organ. So long as the organ was of use, its efficiency was constantly _maintained_ by the _presence_ of selection--which is merely another way of saying that selection was constantly maintaining the force of heredity as regards that organ. But as soon as the organ ceased to be of use, selection ceased to maintain the force of heredity; and thus, sooner or later, that force began to waver or fade. Now it is this wavering or fading of the force of heredity, thus originally due to the cessation of selection, that in turn co-operates with the still continued cessation of selection in reducing the structure below the level where its reduction was left by the actual reversal of selection. So that from that level downwards the cessation of selection, and the consequent failing of heredity, act and react in their common work of causing obsolescence. In the case of newly added characters, the force of heredity will be less than in that of more anciently added characters; and thus we can understand the long endurance of 'vestiges' characteristic of the higher taxonomic divisions, as compared with those characteristic of the lower. But in all cases, if time enough be allowed under the cessation of selection, the force of heredity will eventually fall to zero, when the hitherto obsolescent structure will finally become obsolete. In cases of newly added and comparatively trivial characters, with regard to which reversal of selection is not likely to take place (e.g.