XX.] COMPARATIVE MORPHOLOGY. 243 



quadruped are plainly not only analogous but homologous ; 

 they are analogous as to function, being all of them organs 

 of support and motion ; and they are homologous in their 

 relation to the spine, and in the number and relative posi- 

 tion of their constituent bones. In man, the arms are as 

 plainly homologous in the same sense with the legs, both in 

 their relation to the spine, and in the number and relative 

 position of their constituent bones ; indeed, as remarked in 

 the last chapter, the resemblance of the hands and feet is 

 carried much further than is demanded by any mechanical 

 necessity. But the arms and the legs are not analogous with 

 respect to function — in commoner language, they do dif- 

 ferent work. This, however, is not a very good example of 

 the kind ; for every man now and then uses his arms as legs. 

 A much better example is that of the nippers of crabs and Legs and 

 lobsters, which, like the arms of man, are evidently homo- cmstacea 

 logons with the legs, and yet are not capable of being used 

 as legs. And the jaws of the Crustacea, and of all other jaws of 

 Articulata, are homologous with their legs. This is made ■^'^ti^^'il^ta. 

 tolerably evident by the fact that, unlike the jaws of the 

 Vertebrata, they open horizontally instead of vertically. 



Thus, organs belonging to the same individual may be 

 homologous in position and structure without being ana- 

 logous in function. The converse case is that of organs 

 which are analogous in function without being homo- 

 logous. The best instance I can remember of this is that Fins of 

 of the jBns of fishes. The pectoral pair and the ventral ^^^^'^^^ 

 pair are homologous with each other, and homologous with 

 the limbs of the higher Vertebrata. The caudal fin is, of 

 course, not homologous with the pectoral and ventral fins ; 

 but it is homologous with the tail of the higher Vertebrata. 

 And the dorsal fin is not homologous either with any 

 other of the fish's fins, or with any organ whatever in 

 any other vertebrate class.^ 



1 I believe the only organ in any other part of the animal kingdom that 

 resembles the dorsal fin of the fish in position, is the so-called " foot" of the 

 gasteropodous Molliisca, which in the Heteropoda is flattened into a vertical 

 fin. This fin in the Heteropoda is ventral, not dorsal ; but the ventral 

 side in the MoUusca and Articulata corresponds to the dorsal side in the 



r2 



