Is Aristotle committed, as a theoretical matter, to fixed species in biology? The answer seems to be a resounding no, if we were to infer his theoretical commitments from the actual practice found in his biological works. The answer, however, is far from clear, if we turn to the ‘philosophical discussion of biology’ found in Book 1 of Parts of Animals. In fact, I shall note that its context suggests that, contrary to some recent interpretations put forward, the phrase τὰ ἔσχατα εἴδη at 644a23 is best translated and understood as ‘infima species’, and that such a reading implies that Aristotle favours the methodology that reflects his theoretical commitment to fixed species (at least in Book 1 of Part. an.).