 Another issue confronting direct natural selection accounts is the evident utility of attributing functions to novel traits of organisms developed within a single lifetime such as the capacity of brains to acquire new concepts of kinds of things not previously experienced in the evolutionary lineage, or of the immune system to develop antibodies to new infectious agents. Previously, Millikan (1984) had suggested a notion of “derived proper function” to capture this kind of example. More recently, Bouchard (2013) and Garson (2017) have developed more detailed accounts of derived function, respectively using “differential persistence” and “differential retention” within an organism’s lifespan to play the role that differential reproduction plays in direct natural selection accounts. 