 It is only once Aristotle begins to distinguish differences among, say, uniform parts that various groupings of animals play an important role in organizing the discussion. And in so far as there are relatively stable general groupings of animals, these are identified by noting their possession of stable correlations among differentiae—for example, there are a large number of different animals all of which have wings, feathers, beaks, and two fleshless legs, and this group of animals has been given the designation  ‘bird’.[10]  In other cases, such groups have not been generally recognized, and Aristotle simply refers to them by means of some of their most important correlated differences—the four-legged and live-bearing animals, for example, corresponding to our land mammals (with one important exception).