Parts of Animals Book 1 is mainly concerned with a discussion of the norms (horoi) that govern natural (biological) inquiry. In the present chapter I examine one of those norms, which concerns “how one ought to carry out an investigation of animals” (PA 1.1.639b3–5). Aristotle examines two alternative methods. The first recommends investigating animals species by species (e.g. sparrow, finch, raven). The second begins by grouping species into wider kinds (e.g. bird) and studies those features that belong to them as members of those wider kinds before going on to study those variations that differentiate one form of that kind from another (e.g. variations in beak shape). While scholars have been tempted to conclude that Aristotle rejects the first method outright, I argue that he thinks both approaches are important tools in the biologist’s tool-kit (PA 1.4.644b1–6). In the final section of the chapter I show how this discussion helps bring into focus the broader controversy surrounding the relation between the scientific theory presented in the Posterior Analytics and Aristotle’s scientific practice in the biological works.