Cross-sectional and longitudinal investigations of development have resulted in an accumulation of large masses of data in which differences or changes of particular variables have been plotted against chronological age, revealing the various growth trends reported in texts and handbooks such as those edited by Mussen (1970) on child psychology or by Birren (1959) on aging. Despite the wealth of data collected, the records are insufficient for at least three reasons: (1) Almost none of the reports have been addressed to the question of why organisms grow and change, but have remained descriptive and prescientific. (2) The uncritical application of physical time scales has been harmful for the development of theoretical interpretations of growth. (3) Without exception, individual and cultural changes have been confounded.