202 THE EVOLUTION OF CONTINUITY 



The mantle-sac of the Cephalopod is equivalent to the 

 ccelom or abdominal cavity of the typical Fish, in that 

 both represent the space enclosed by a continuous series of 

 medusoid walls ; the viscera lying suspended as we would 

 expect a continuous manubrial structure to be in a medusoid 

 series. We have suggested that in contrast with the 

 primitive Fish the primitive Cephalopod evolved from a 

 stationary or attached type of strobilar plan, and we have 

 to presume that on full development the organism became 

 humbly motile. This might well have happened as a result 

 of gill-formation, and the strain on the organism's " moor- 

 ings " during the expulsion of respiratory water. 



The Cephalopod could never hope to evolve rapid 

 continuous movement like that of the typical Fish, for it 

 evolved two separate locomotive mechanisms, which acted 

 in opposite directions. It did its best with fin-fringes, 

 assisted by grasping tentacles, to develop forward move- 

 ment, but the contractions necessary for respiration of the 

 walls of the mantle-sac have more or less defeated its 

 purpose. It may be remarked that in the typical Fish we 

 find the water current of expiration trying to defeat voluntary 

 movement in a different way ; for if the Fish desires to 

 remain motionless, or to move backwards, it has to counter- 

 act the forward drive of gill-contraction. This can be well 

 observed in the case of a stationary goldfish in a glass 

 globe. 



The main road of Evolution leading to Man has not 

 passed through any primitive Cephalopod type, but through 

 one whose features were roughly those of the present-day 

 typical Fish. And it is well here to repeat that the truth 

 seems to be that no living organism in this world represents 

 an actual past type on the main road of Evolution. In the 

 light of the Continuity they exhibit we can clearly recognise 

 through known organisms what the outstanding features of 

 the main road have been, but the actual road has been wiped 

 out. Moreover, the side-roads have also been wiped out ; 

 and as Man is all that remains to represent the main road, 

 so is a given animal or plant species all that remains of the 

 actual side-road leading to it. The road of the Evolution 

 of Continuity has, however, been preserved to the present 

 day. Side-path evolution has been a matter of modification 



