Aim and Method of Comparative Anatomy 



379 



Fig. 299. Melanogrammus aeglefinus (Linnaeus), the haddock. (Drawing by 

 H. L. Todd. Courtesy, Jordan & Evermann: "Fishes of North and Middle 

 America," Washington, The Smithsonian Institution.) 



merit to the rear of the skull (see p. 133). Dissection of the cod's paired 

 fins reveals that the larger posterior fin has all the skeletal features of a 

 typical pectoral fin, while the smaller anterior fin has the much simpler 

 skeleton of a pelvic fin, without attachment to any other skeletal part. 

 Internal structure and skeletal connections therefore indicate that the 

 cod's posterior fin is the pectoral. Highly significant is the fact that 

 the nerves which pass from the spinal cord to the small anterior fin 

 arise from the cord posterior to the nerves which pass to the large 

 posterior fin. This crossing of the two sets of nerves indicates that the 

 small fin "belongs" behind the large fin and is truly pelvic — the 

 nervous "anchorage" holds fast. 



Assuming genetic continuity of fishes, we are required to imagine 

 that, in the ancestry of the cod, the pelvic fins have somehow "mi- 

 grated" to their extraordinary position anterior to the pectorals. The 

 pelvic fins of fishes are usually much less strongly developed than the 

 pectorals. The pelvic girdle is merely embedded in the muscle of the 

 ventral body-wall and has no connection with the vertebral column. 

 It may be imagined, therefore, that the pelvic fins are comparatively 

 free to shift their position. In most fishes the chief propelling organ is 

 the tail. The paired fins are used for balancing and guiding. This being 

 so, the pelvic fins may become mechanically more effective by shifting 

 forward. But these suggestions offer no clue to the evolutionary process 

 whereby functional advantage was gained by shifting the fins forward, 

 nor do they explain why pelvic fins should have moved so far forward 

 in the cod family and less or not at all in other fishes. The immediate 

 point of importance is that position, in the sense of mere spatial rela- 

 tion, is not infallible as a criterion of homology. The pelvic fins of the 



