One of the enduring puzzles in the life sciences is the description and explanation of change over time. Such change is frequently called "development," and the metaphors of growth and decline, gain and loss have often been employed to characterize change in structure or function of organisms over time. Cells, individuals, groups, and even social systems exhibit change over time. While most change is orderly, regular, and normative, some change is chaotic, irregular, and unpredicted. Growth or decline at the individual level often has antecedents or consequences at the collective group level.