Certain highly organized parts of the living organism are not alive. This inherent tendency to self-preservation is essential to the existence of living organisms. Self-preservation is the primary or controlling expresion of life and, normally, is subordinate to no other law except, at times, to the instinct of race preservation, in which case the individual often sacrifices itself for the protection of the young or the flock. Primarily, life seeks to preserve itself and to maintain vital integrity. All the functions of life have reference to this effort at self-preservation either of the individual or of the race.
Nature aims at wholeness.
This is as much true of the single cell as of the complex organism. Nor does the validity of this law of self-presevation depend upon the truth of any particular theory of the nature of life. It is true whatever life may be. If vital power could be manufactured by food, air, water, and exercise, if it is the product of activity, then increased activity would be the best means of increasing the power and the inactivity of sleep would be a waste of time. It makes all the difference whether we are increasing or reducing vital power by increasing vital activity.