74 VERTEBRATE ZOOLOGY 



a close imitation of the form and motion of wisps of water-grass 

 passively shaped and gracefully waved by the pulsations of the 

 current. The rhythmical undulations of the lamprey, which 

 perhaps best illustrates the primitive vertebrate form, and is itself 

 archaic in structure, are an almost perfect embodiment in the 

 active voice of the passive undulations of ropes of river confervse. 

 The movement of the fish is produced by alternate rhythmical 

 contractions of the side muscles, by which the pressure of the 

 fish's body is brought to bear in successive waves against the water 

 of the incurved sections. In the movement of a rope of vegeta- 

 tion in the pulsating current, it is the pressure of the pulses of the 

 water against the sides of the rope that give the incurvations. 

 The two phenomena are natural reciprocals in the active and 

 passive voices. 



" The development in the fish of a rhythmical system of motion 

 responsive to the rhythm impressed upon it by its persistent 

 environment and duly adjusted to it in pulse and force, is a 

 natural mode of neutralizing the current force and securing 

 stability of position or motion against the current, as desired. 

 Beyond question the form and movement of the typical fish are 

 admirably adapted to motion in static water and that has been 

 thought a sufficient reason for the evolution of the form, and so 

 possibly it may be, but fishes in static water have not as uniformly 

 retained the attenuated spindle-like form and the extreme lat- 

 eral flexibility as have those of running water. Among 'these 

 latter it is rare that any great departure from typical lines and 

 from ample flexibility has taken place, while it is common in sea 

 fishes. Among the latter not a few have lost both the typical 

 form and the flexibility. The porcupine fish, the sea-horse, the 

 flounders, and many others are examples of such retrogressive 

 evolution, which is doubtless advantageous to them within 

 their special spheres in quiet waters, but would quite unfit them 

 for life in a swift stream. And if the view be extended to include 

 the low degenerate forms, like the Ascidians (tunicates), that 

 are by some authors classed as chordates, the statement finds 

 further emphasis. 



"It is not difficult for the imagination to picture a lowly 

 aggregate of animal cells, still plastic and undeterminate in or- 

 ganization, brought under the influence of a persistent current 



