EVOLUTION OF STRUCTURES 



15 



sides contract alternately to produce a rhythmical wave 

 passing along the entire series of segments and giving the 

 trunk an undulatory movement. 



Should this elongate body now acquire a more fish-like 

 form, in attaining, for example, the power of more rapid 

 movement, it is obvious that this simple type of meta- 

 merism would undergo a series of changes. Every change 

 of outward form would be reflected on the parts not only 

 of each, but of all segments in their common relationships. 

 To perform more perfectly the functions of their location, 

 adjacent segments might become enlarged, folded, or 

 blended, and cause the most puzzling complications of 

 their component structures. One region of the body might 

 thus appear to develop at the expense of another, as in the 

 evolution of fin structures (cf. pp. 32-44), where a vertical 

 fin fold, representing the sum of the dorsal and ventral out- 

 growths of the hinder body segments, becomes reduced to 

 the lappet-like dorsal and ventral fins ; the intervening 

 substance of the fin web becoming drawn to the points 

 where greater rigidity is required. 



The simple metameral character of the lamprey acquires 

 an especial interest when the different groups of fishes are 

 examined ; for it is found that all exhibit clearly body 

 segments and segmental structures in the most varied 

 stages of complexity. To trace metamerism seems, accord- 

 ingly, a mode of determining to what degree the differ- 

 ent groups have diverged from a common stem ; and to 

 compare the sums of the archaic metameral characters in 

 the different types of fishes may perhaps be looked upon 

 as one of the safest aids in determining their genetic posi- 

 tion. From the conditions of segmentation the lampreys 

 must certainly be given a lowly rank ; even with due allow- 

 ance for degeneration of structures they are clearly more 



