 I have highlighted the language of nature, necessity, possibility (and impossibility), and being for the sake of in this passage in order to highlight the contrast with the discussion of the same organic correlations in the corresponding HA passage. Here the goal is explanation—parts are present and have the character they do primarily due to the conditional necessity imposed by the organism’s functional requirements. One sees here the Aristotle that so impressed the great French naturalist Georges Cuvier: Aristotle is not only systematically discussing the adaptive functions of each of these organs; he is also displaying the complex way in which the internal parts of animals constitute an organic system.