360 Comparative Anatomy — Its History, Aim, and Method 



find that a certain organ of the ancient ancestral animal had become 

 so changed that we could not recognize it in the recent animal. Jaw- 

 bones may have become ear-bones. It is imaginable, too, that some 

 organ of the ancient animal may, in the course of long evolution, have 

 undergone degeneration and completely disappeared. Then, in the 

 later history of the same race, a new organ may have appeared and 

 become so like the earlier organ that, not knowing the intervening 

 history, we would naturally make the mistake of regarding the recent 

 organ as the "same" as the apparently corresponding organ of the 

 remote ancestor. Some whales have a median dorsal fin which seems 

 to correspond to a median fin of a fish. They do, in fact, correspond in 

 function, but they cannot be the same if, as the evidence indicates, the 

 immediate ancestors of whales were land mammals. 



For the purpose of working out the genetic relationships of animals, 

 we need some way of measuring the genealogic distance between 

 animals. From the foregoing discussion, it appears that comparison 

 of animals as wholes gives us no adequate measure. They must be 

 compared part by part. Further, it becomes clear that the only measure 

 significant for our purpose is derived from comparison of an organ of 

 one animal with that organ which, in another animal, is genetically 

 identical with the first — the same organs, however different in position, 

 form, or function. 



Analogy and Homology 



It is not surprising that the early comparative anatomists were con- 

 fused by the fact that organs of different animals might correspond in 

 some ways and not in others. It was Geoffroy Saint-Hilaire who first 

 gave special attention to this problem. Convinced of the "unity of 

 plan" of all vertebrates, he asserted in his "Theorie des Analogues" 

 that the same parts or units of structure occur in all vertebrates and 

 he designated the corresponding parts as "analogues" (Fig. 289). For 

 recognition of "analogous" parts in different animals, he depended not 

 so much on form and structure but mainly on position and especially 

 "position" in the sense of the spatial relations of a part to neighboring 

 parts and their attachments to one another — the "principle of connec- 

 tions." He was not always consistent, however, and some of his "anal- 

 ogies" depended more on function than on position and connections. 

 He did not clearly recognize genetic relationship as being the basis for 

 his "analogies." 



Richard Owen, in the preface to his "Comparative Anatomy and 

 Physiology of Vertebrates " (1866), criticized Saint-Hilaire's use of the 

 word "analogy." Owen pointed out that "analogy" signifies "a like- 

 ness of ratios" — that is, similarity of relation. Therefore it may prop- 



