To repeat in other words. All enduring forms establish a _modus vivendi_ with their surroundings. They can do this because both they and the surroundings are plastic within certain undefined but somewhat narrow limits. They are plastic because they can to some extent change their habits, and changed habit, if persisted in, involves corresponding change, however slight, in the organs employed; but their plasticity depends in great measure upon their failure to perceive that they are moulding themselves. If a change is so great that they are seriously incommoded by its novelty, they are not likely to acquiesce in it kindly enough to grow to it, but they will make no difficulty about the miracle involved in accommodating themselves to a difference of only two or three per cent. {72a} As long as no change exceeds this percentage, and as long, also, as fresh change does not supervene till the preceding one is well established, there seems no limit to the amount of modification which may be accumulated in the course of generations—provided, of course, always, that the modification continues to be in conformity with the instinctive habits and physical development of the organism in their collective capacity. Where the change is too great, or where an organ has been modified cumulatively in some one direction, until it has reached a development too seriously out of harmony with the habits of the organism taken collectively, then the organism holds itself excused from further effort, throws up the whole concern, and takes refuge in the liquidation and reconstruction of death. It is only on the relinquishing of further effort that this death ensues; as long as effort endures, organisms go on from change to change, altering and being altered—that is to say, either killing themselves piecemeal in deference to the surroundings or killing the surroundings piecemeal to suit themselves. There is a ceaseless higgling and haggling, or rather a life-and-death struggle between these two things as long as life lasts, and one or other or both have in no small part to re-enter into the womb from whence they came and be born again in some form which shall give greater satisfaction.