 The full explanation goes on much longer, but we need not follow it out for the purposes of making two important points about the relationship of GA to HA. The first is that the organization of GA dictates that for the explanation of facts of wide generality such as this one, we must look in book II—notice that he claims that this explanation for the large size of eyes early in development and for their subsequent contraction applies to all land, air and water animals. The second point to be stressed is the relentlessly causal/explanatory focus of the GA discussion, a feature (properly) completely absent in the corresponding discussions in HA. As impressive as the descriptive accuracy of the embryological passage cited earlier from HA is, we again need to remember what is absent from that discussion. For the equally impressive embryological theory, the reader of Aristotle’s biology must turn to the Generation of Animals.