There are two forces continually at work within us, one toward destruction and disintegration, and the other toward construction and upbuilding. The common physiological terms for these activities are "waste" and "repair," and we observe them as one of the distinct manifestations of the universal laws of growth, progress, and evolution.
History moves in cycles. Even the life of nations depends somewhat upon this same principle of the interplay of the positive and negative forces of life.
Life and death in changes of seasons
We see the same thing in the changes of the seasons upon the face of the earth. Throughout autumn and winter there is a process of decay, death, and disintegration; leaves fall; plants and vegetables die; fruits ripen, fall and decay. This process continues until former beautiful and symmetrical bodies of matter are thoroughly disintegrated, and the particles once composing them are separated into their original elements, to be appropriated in new manifestations of life in springtime and summer.
Human body compared to a machine
We are inclined to think of the human body as a machine—a marvelous, intricate, and complex mechanism which serves our will and our desires; as a tool with which we work out our earthly destiny. But unlike man-made machines, it is self-repairing, self-adjusting, and contains within itself the forces of construction, which are constantly tending toward perfection, while our industrial machines are constantly tending toward their own disintegration and destruction.
Constant changes in body-tissue
Every movement of the body, conscious or unconscious, even thought and emotion, use up some part of the body-tissue which must be replaced by new material. This constant change in the texture and the make-up of the body we call "metabolism," involving the functions of digestion, absorption, assimilation, and elimination.
While we may regard the body as a machine, there are many points in which the favorite comparison to a steam-engine is not exact.
Favorite comparison of the body with the steam-engine