176 



THE GENESIS OF SPECIES. 



[CHAP. 



This shows what great differences may exist in size, in 

 form, and in function, between parts which 

 are developmentally the same, for all these 

 appendages are modifications of one common 

 kind of structure, which becomes differently 

 modified in different 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-boned animals, the Vertebrata. 

 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 

 SPINE OF GAI.AGO series of similar parts, the joints of the spine 

 or backbone (vertebrae), which are admitted 

 by all to possess a certain community of structure. 



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



