. . . The universe exists only as a delicate matching of immense forces. Change, and life itself, are the result of miniscule imbalances. . .
 
. . . Galactic stability was no more than the delicate matching of rotational kinetic energy and gravitational potential energy, creating and maintaining spiral arms and central hubs and dark-matter halos.
 
Life was not exempt from the principle. It had learned the lesson early. Successful species sat on the narrow line between exact replication, which permitted no adaptation to a changing environment, and too-imperfect replication, which resulted in large error rates and nonviable offspring. Sex was no more than an ingenious attempt to solve the problem, by permitting generational variability within the constraints of the exact duplication of genetic material. Within every cell of every organism the same struggle went on, a fine balance between uncontrolled combustion, which could kill, and the too-slow enzymatic release of energy, which in a competitive world was just as fatal. . .
 
- Charles Sheffield (1935-2002)
Cold as Ice, p. 147, © 1992 TomDoherty Associates, Inc.