17G 



THE GENESIS OF SPECIES. 



[Chap. 



W 



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This shows what great (lifTerences may exist in size, in 

 form, and in function, between parts wiiich 

 are developnientally the same, for all these 

 appendages are modiiications of one connuon 

 kind of structure, which becomes differently 

 modified in dillerent situations; in other words, 

 they are serial homologues. 



The segments of the body, as they follow 

 one behind the other, are also serially alike, 

 as is plainly seen in the abdomen or tail. In 

 the cephalo-thorax of the lobster^ however, 

 this is disguised. It is therefore very inter- 

 esting to find that in the other crustacean 

 before mentioned, the squilla, the segmenta- 

 tion 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 unmistakable serial 

 repetition of parts does not obtain in the 

 highest or back-l)oned animals, the Vertcbrata. 

 1'hus, in n)an and other mammals, nothing of 

 the kind is externally visible, and avc 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 re(x;ive a com- 

 mon name — the ribs. TIkmo also (i. e., in the 

 skeleton) we find a still more remarkable 

 series of similar parts, the joints of the s})ine 

 or backbone (vertebne), which are admitted 

 by all to possess a certain community of structure. 



It is in their limbs, however, that the Vertebrata pre- 



BPINE or OALAQO 

 ALLKNII. 



