 Etiological theories of biological function need to allow for the fact that ancestral traits might have been selected for something other than the present function of descendent traits. For example, a penguin’s flippers and an emu’s vestigial wings no longer have the function of flight, even though ancestral forelimbs were selected for flight. Griffiths (1993) and Godfrey-Smith (1994) offer “modern history” versions of the etiological theory, according to which functions are determined by recent selection. Note that selection does not cease when traits “go to fixation” if on-going maintenance selection is still weeding out fresh harmful mutations as they arise. However, selection does require some variation and Schwartz (1999) suggests that a continuing usefulness supplement is needed, which kicks in if variation is absent for a time. In the absence of any variation, the trait retains its function if it is still   adaptive.[3] 