Among biologists and philosophers of biology there is no general agreement on a definition of development. Development is not necessarily the history of the individual, or the sequence of changes from egg to adult (adultocentrism). The notion that the adult stage is the target of development is unacceptable, both because it implicitly gives development a purpose, and because it does not apply to the biology of many organisms. In the common use of the term adult, two different notions are confused: adult as reproductively mature stage and as a stage that maintains its morphological organization until the onset of senescence or death. However, reproductive maturity and the presence of definitive morphological condition are not always associated. The divide between developmental processes and mere metabolic changes is not always clear-cut. Modern developmental biology is not the same as the descriptive and experimental biology of the past. Partly owing to strong focus on genetic control and molecular-level processes, most research effort is restricted to a few model species; but these are not necessarily representative of developmental processes in more or less distant relatives.