It does not affect matters to discover cases in which we cannot recognize this relation between the regenerative capacity of a part and its importance or its liability to injury. Such instances do not lessen the convincingness of the positive cases, since we do not know the exact conditions which may lead to the increase of regenerative capacity in a part, and, above all, since we do not know the rate at which such an increase may take place. If adaptation in general depends upon processes of selection, these processes must also be able to give rise to an increase in the power of regeneration. On the other hand, it by no means follows that the disappearance of a faculty of regeneration which was once present in a part, but which has become superfluous in the course of time, must take place immediately through natural selection. For it is the very essence of natural selection that it only furthers what is useful, and only removes what is injurious; over what is indifferent it has no power at all. Thus it follows that the faculty of regeneration, when it has once been present in a part, cannot be set aside by natural selection (personal selection), for it is in no way injurious to its possessor. If it gradually decreases and becomes extinct notwithstanding this, when it is of no further use, as seems to be to some extent the case in regard to the legs and tail of the blind Proteus, that must depend on other processes, on those which generally bring about the gradual disappearance of disused parts or capacities. We shall attempt to probe to the roots of these processes later on; for the present let it suffice us to know that, according to our experience, they go on with exceeding slowness, and that it has taken whole geological periods to eliminate the legs of the snake-ancestors so completely as has been done from the structure of most of our modern snakes, while the Proteus which migrated into the caves of Krain as far back as the Cretaceous period is indeed blind, but still retains its eyes under the skin, though in a degenerate condition.