‘On the other hand, this course would involve repeated mention of the same attribute, as the same attribute is common to many species, and so far would be somewhat irrational and tedious. Perhaps, then, it will be best to treat generically the universal attributes of the groups that have a common nature and contain closely allied subordinate forms, whether they are groups recognized by a true instinct of mankind, such as Birds and Fishes, or groups not popularly known by a common appellation, but withal composed of closely allied subordinate groups; and only to deal individually with the attributes of a single species, when such species—man, for instance, and any other such, if such there be—stands apart from others, and does not constitute with them a larger natural group. ‘It is generally similarity in the shape of particular organs, or of the whole body, that has determined the formation of the larger groups. It is in virtue of such a similarity that Birds, Fishes, Cephalopoda, and Testacea have been made to form each a separate _genus_ (γένος). For within the limits of each such _genus_, the parts do not differ in that they have no nearer resemblance than that of analogy—such as exists between the bone of man and the spine of fish—but they differ merely in respect of such corporeal conditions as largeness smallness, softness hardness, smoothness roughness, and other similar oppositions, or, in one word, in respect of degree.’[49] [49] _De partibus animalium_, i. 4; 644ᵅ 27. The Aristotelian _genus_ thus differs widely from the term as used in modern biology. In another passage he comes nearer to defining it and the analogy of parts which extends from genus to genus. ‘Groups that differ only in the degree, and in the more or less of an identical element that they possess are aggregated together under a single _genus_; groups whose attributes are not identical but _analogous_ are separated. For instance, bird differs from bird by gradation, or by excess and defect; some birds have long feathers, others short ones, but all are feathered. Bird and Fish are more remote and only agree in having analogous organs; for what in the bird is feather, in the fish is scale. Such _analogies_ can scarcely, however, serve universally as indications for the formation of groups, for almost all animals present analogies in their corresponding parts.’[50] Aristotle nowhere gives to his term _genus_ a rigid application that can be applied throughout the animal kingdom. He uses the word in fact much as we should use the conveniently flexible term _group_, now for a larger and less definite, now for a smaller and more definite collection of species. This varying use of a technical word makes it impossible to draw up a classification based on his _genera_ or indeed with any consistent use of the terms which he actually employs.