20 COPEIA 



habit of locomotion. The forked form is used almost 

 without exception by those species which swim con- 

 tinually through extensive stretches of open water. 

 The water slipping along the fishes flanks must escape 

 backward in the middle line of its tail. Side to side 

 motion of a square caudal would interfere with the 

 backward flow of the water and impede the fish. Ob- 

 viously the forked fin is better adapted for this work 

 as the central impeding portion is eliminated and the 

 lobes brace against comparatively stationary water. 

 A narrow peduncle is also an advantage and we find 

 the peduncle tending to be more and more narrow 

 particularly in its vertical diameter, and it is often 

 strengthened by keels in its horizontal diameter, that 

 is the plane of its motion. It is interesting that the 

 same type is approached by the mackerels, a free 

 swimming more or less pelagic off-shoot of the Per- 

 coid, stem, by the mackerel-sharks, most active swim- 

 mers of the sharks, which have a very different heter- 

 ocercal caudal as a basis of variation, and the Cetacea, 

 among mammals. The Cetacea are, interestingly 

 enough, adapted to motion in a different plane, mov- 

 ing their forked caudal up and down instead of from 

 side to side, and with the peduncles narrowed hori- 

 zontally instead of vertically. 



Certain fishes, except when alarmed, propel 

 themselves not by the caudal, but by the breast-fins. 

 Such are the wrasse-parrotfish group, which slip in 

 and out among rocks and the crevices of coral-reefs. 

 These have usually squarish or rounded caudals, quite 

 different from the firm forked ones of more actively 

 free-swimming fishes. Indeed the relative forking 

 is a fair criterion of the amount of swimming that a 

 fish does, the minnows with forked caudals, being 

 more active swimmers than the killifish group with 

 rounded, the sea running salmon having a more 

 forked caudal than the brook inhabiting trout, the 

 old trout a squarer tail than the young, to whom the 



