 Although we might be inclined to simply prohibit the terminology of functional homology, its widespread use in molecular and developmental biology should at least make us  pause.[18]  While it is important to recognize this pervasive practice, some occurrences may be illicit. Swapping structurally homologous genes between species to rescue mutant or null phenotypes is not a genuine criterion of functional homology, especially when there is little or no attention to establishing a phylogenetic context. This makes a number of claims of functional homology suspect. To not run afoul of the conceptual tension, explicit attention must be given to the meaning of “function.” Biological practice harbors at least four separate meanings of function (Wouters 2003, 2005): activity (what something does), causal role (contribution to a capacity), fitness advantage or viability (value of having something), and selected effect or etiology (origination and maintenance via natural selection). Debate has raged about which of them (if any) is most appropriate for different aspects of biological and psychological reasoning or most general in scope (i.e., what makes them all function concepts?) (see discussion in Garson 2016). Here the issue is whether we can identify a legitimate concept of homology of function.