 Ernest Nagel (1961) and Carl Hempel (1965) provide early attempts by philosophers of science to directly assimilate functional explanation in biology to more general patterns of explanation. In particular, they both consider functional explanation within the framework of the  Deductive-Nomological account of scientific explanation.  They consider the functional claims to be related to explanations of the presence of a trait in an organism. Their accounts differ primarily on whether to say that a trait T has function F in organism O when T is sufficient to produce F in O (Hempel’s version) or when T is necessary to produce F in O (Nagel’s version). 