HOMOLOGY AND ANALOGY 9 



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." 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 small- 

 ness, 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, his 

 History 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, 

 of closely allied groups differentiated by gradation, or 

 to groups like Man not .differentiated into subordinate 

 groups. In the first case the common attributes may be 

 called analogous, in the second generic, in the third specific " 

 (i., 5, 645 b , trans. Ogle). The alimentary canal is a good 

 example of a part which is "analogous" throughout the 

 animal kingdom, for "all animals possess in common those 

 parts by which they take in food, and into which they receive 

 it " (Cresswell, loc. cit., p. 6). 



The De Partibus Animalium becomes in form a com- 

 parative organography, but the emphasis is always on function 

 and community of function. Thus he treats of bone, " fish- 

 spine," and cartilage together (De Partibus, ii., 9, 65 5 a ), 

 because they have the same function, though he says 



1 De Partibus Animalium, i., 4, 644% trans. W. Ogle, Oxford, 1911. 



