 What this distinction misses is the possibility of diachronic aspects of part-whole or interlevel relations. Scientific explanations commonly invoke dynamic (causal) processes involving entities on several levels of organization (Craver and Bechtel 2007). This is one of the core reasons why development is a persisting biological topic for reductionism. During ontogeny there are causal interactions over time among parts and activities (both homogeneous and heterogeneous) to bring about new parts and activities (both homogeneous and heterogeneous), eventually generating an integrated adult (whole) organism.