In Aristotle's biological works, there is an apparent conflict between passages which seem to insist that only hypothetical necessity (anagk ex hypotheses) operates in the sublunary world, and passages in which some biological phenomena are explained as simply (hapls) necessary. Parallel to this textual problem lies the claim that explanations in terms of simple necessity render teleological explanations (in some of which Aristotle puts hypothetical necessity to use) superfluous. I argue that the textual conflict is only apparent, and that Aristotle's (...) notion of coincidental sameness allows him to avoid the superfluity problem. (shrink)