The potential versatility of form and potential adaptive zone of a higher taxon or body plan depend on the number and range of independent morphogenetic parameters. The actual versatility and adaptive zone of a given higher taxon are determined by interactions of members of that taxon with species belonging to groups with a different body plan. More recently evolved higher taxa tend to exhibit a greater potential versatility of form than related more ancient taxa. In the course of earth history, more potentially versatile taxa have tended to replace less versatile groups in the same or similar adaptive zone. This increase in potential versatility is not the inevitable consequence of the decrease in number of higher taxa through geologic time, but is the result of strong selection favoring some plans of organization over others which may or may not have become extinct. Increasing potential versatility allows for greater homeostasis, efficiency, and exploitation of continuously available resources, and integration of structures and functions. New body plans may arise either from stocks of small adult body size and with relatively simple integration patterns, or else by a process of correlative progression of structures and functions from a relatively specialized ancestral stock.