Such analogies can scarcely, however, serve universally as indications for the formation of groups, for almost all animals present analogies in their corresponding parts." 1 It is thus similarity in form and structure which determines the formation of the main groups. Within each group the parts differ only in degree, in largeness or smallness, softness and hardness, smoothness or roughness, and the like (loc. cit., i., 4, 644 b ). These passages show that Aristotle had some conception of homology as distinct from analogy. He did not, however, develop the idea. What Aristotle sought in the variety of animal structure, and what he found, were not homologies, but rather communities of function, parts with the same attributes. His interest was all in organs, in functioning parts, not in the mere spatial relationship of parts. This comes out clearly in his treatise On the Parts of Animals, which is subsequent to, and the complement of, hisHistory of Animals, The latter is a description of the variety of animal form, the former is a treatise on the functions of the parts.He describes the plan of the De Partibus Animalium as follows:"We have, then, first to describe the common functions, common, that is, to the whole animal kingdom, or to certain large groups, or to members of a species. In other words, we have to describe the attributes common to all animals, or to assemblages, like the class of Birds, '-' I.e Rt^nc Animal, i., p. 6, 1817. It has much affinity with the similar conceptions of Goethe and the German transcendentalists. ' Ueber Enhvickelungsgeschichte dcr Thiere, i., p. xvii., 1828. LAW OF PARALLELISM 91 ologic" 2 pts., 1806-7) as forming the turning-point in our Haeckel, 250-1, 254 Criticism of this idea, 303, 304,
