  Several genetic programming systems are created, each solving a different
problem. In these systems, the median number of generations G needed to evolve
a working program is measured. The behavior of G is observed as the difficulty
of the problem is increased. In these systems, the density D of working
programs in the universe of all possible programs is measured. The relationship
G ~ 1/sqrt(D) is observed to approximately hold for two program-like systems.
For parallel systems (systems that look like several independent programs
evolving in parallel), the relationship G ~ 1/(n ln n) is observed to
approximately hold. Finally, systems that are anti-parallel are considered.
