 For reasons that will become clear, the relationship between the books of the Historia Animalium dealing with generation and On the Generation of Animals (GA) is somewhat more complex. GA has a reasonably clear organization of its own; but its central (though not only) project of explaining the causal roles of the male and female in sexual generation and the causality of embryological development raises a host of topics and problems well beyond the scope of HA. But a study of how the two works are related is nonetheless illuminating. This discussion of Aristotle’s biological practice will thus close by considering the causal explanation of embryological development and in particular the explanation for the early appearance of the heart, with a final look at the application of this explanation to the case of embryonic birds.