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.
All change is _pro tanto_ death or _pro tanto_ birth. Change is the common substratum which underlies both life and death; life and death are not two distinct things absolutely antagonistic to one another; in the highest life there is still much death, and in the most complete death there is still not a little life. _La vie_, says Claud Bernard, { 73a} _c’est la mort_: he might have added, and perhaps did, _et la mort ce n’est que la vie transformée_. Life and death are the extreme modes of something which is partly both and wholly neither; this something is common, ordinary change; solve any change and the mystery of life and death will be revealed; show why and how anything becomes ever anything other in any respect than what it is at any given moment, and there will be little secret left in any other change. One is not in its ultimate essence more miraculous that another; it may be more striking—a greater _congeries_ of shocks, it may be more credible or more incredible, but not more miraculous; all change is _quâ_ us absolutely incomprehensible and miraculous; the smallest change baffles the greatest intellect if its essence, as apart from its phenomena, be inquired into.