 The dual framework ‘proximate vs. ultimate’ provides a qualitative distinction of biological causality (for related distinctions, see J. Baker 1938; Tinbergen 1951, 1963). It holds that biologists who study proximate causes ask how questions about individual developmental processes. Thus, functional biologists interested in such proximate causes study how systems work. Instead, evolutionary biologists that study ultimate causes ask why questions, like why phylogenesis has produced particular evolutionary functions. According to this distinction, at least on the surface, proximate causes resemble Aristotelian efficient causes while ultimate causes resemble Aristotelian final causes. To illustrate this distinction, Mayr (1961) draws on an example of avian migration. Migration can be studied by asking how bird migrate (i.e., how they develop skills like navigation) or why they migrate (i.e., due to what selective advantage). These two investigations are understood to be both important and complementary. However, they should be treated as distinct from one another.