HABITAT OF THE EARLY VERTEBRATES 407 



floating or swimming forms, the corals, the chrinoids, the echi- 

 noids, and many other sea forms of ancient history and long 

 opportunity, have not made an effective entrance into the 

 streams during geologic time ; and this is probably not 

 wholly, and perhaps not chiefly, due to the sweetness of the 

 waters. 



A compact form of body presents obvious advantages, except 

 .as environment or food or locomotion requires some departure 

 from it, and the vast majority of animals are more or less rotund, 

 and their locomotive devices are adjusted to this form. But the 

 rotund form offers much resistance to rapid currents and unfits 

 the animal for effective stream life unless it persistently hugs the 

 bottom. Neither the rotund floaters and swimmers like the 

 ancient cephalopods, nor the cilliated spawn of the sessile forms 

 are well adapted to resist the unceasing pressure of a rapid 

 stream, and these are practically absent from river faunas. 



There is only one conspicuous type that is facilely suited to 

 free life, independent of the bottom, in swift streams, and that is 

 the fish-form. The form and the motion of the typical fish are 

 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 con- 

 fervae. 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 vegetation in a pulsating current, it is the pressure of the 

 pulses of water against the sides of the rope that give the incur- 

 vations. 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 



