212 INVERTEBRATE PALAEONTOLOGY 



least in accord with present knowledge, to suggest that 

 the course of evolution can be likened to a flowing tide. 

 One wave after another rears its crest and falls in ruin 

 each one rushes up the beach, only to slip back, and 

 yield place to another which may sweep farther or fall 

 short. Amid the welter of advance and retreat the 

 roar of the oncoming breaker and the hiss of its 

 shattered recoil it is hard to see that any lasting 

 progress is achieved. But behind the seeming futility 

 of each foredoomed effort, steadily and invincibly the 

 tide is coming in. 



