Se aero 
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 
