VIIL] 



HOMOLOGIES. 



183 



"(if," 



to find that iu the other crustacean before mentioned, 

 the Squilla, the segmentation of the body is 

 more completely preserved, and even the first 

 three segments, which go to compose the head, 

 remain permanently distinct. 



Such an obvious and unmistakeable serial 

 repetition of parts does not obtain in the 

 hiohest, or backboned animals, the Verte- 

 brata. Thus in. man, and other mammals, 

 nothing of the kind is externally visible, and 

 we have to penetrate to his skeleton to find 

 such a series of homologous parts. 



There, indeed, we discover a number of 

 pairs of bones, each pair so obviously resem- 

 bling the others that they all receive a com- 

 mon name — the ribs. There also {i.e. in the 

 skeleton) we find a still more remarkable 

 series of similar parts, the joints of the spine 

 or backbone (vertebra?), which are admitted 

 by all to possess a certain community of 

 structure. 



It is in their limbs, however, that the 

 Vertebrata present their most conspicuous 

 example of serial homology — almost the only 

 serial homology noticeable externally. 



The facts of serial homology seem hardly 

 to have excited the amount of interest they 

 certainly merit. 



Very many writers, indeed, have occupied 

 themselves with investigations and specula- 

 tions as to what portions of the leg and foot answer 

 to what parts of the arm and hand, a question which 



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SPINE OF GALA60 

 ALLENII. 



