Different cohorts within a species fulfill different functions required for survival of the gene pool as a whole. Younger cohorts provide the flexibility necessary for the species to adapt to changes in its existing environment and to expand into new niches; older cohorts, the specialization necessary for maintaining the gene pool within those environmental niches to which the species has already adapted – thus providing a secure base from which the younger can venture out. A high rate of environmental change demands rapid adaptation by the younger cohort hence rapid differentiation of the cohort-structure of the species itself. Evidence is drawn from cellular biology, animal ethology, and human psychology. The ‘heredity-environment’ controversy and the ‘generation gap’ are considered as special instances of this dialectical relationship between cohorts at different levels of developmental analysis.