 This shift in attention was a precursor to understanding the philosophy of science through the lens of mechanisms. Darden, building on the work of Machamer, Darden, and Craver (2000), has more recently returned to the question of how Mendelian and molecular genetics are related and viewed it through this lens (Darden 2005). Rather than understanding the relationship as one of reduction, she suggests they can be understood as relating via a focus on different working entities (often at different size levels) that operate at different times. Thus, the relation was one of integration of sequentially operating chromosomal and molecular hereditary mechanisms rather than reduction. (For an alternative but still integrative reading of the relationship between classical genetics and molecular biology that focuses on their shared functional units, see Baetu 2010.)