RIVER OF LIFE 45 



of food or broke the surface of the water. Not even crabs or 

 lobsters existed. Instead, the trilobites, forerunners of the 

 Crustacea, crawled, burrowed, or swam sluggishly in the shal- 

 low waters, looking somewhat like smaller three-lobed varieties 

 of the horseshoe crab, which lives today along the Atlantic 

 coast of North America, almost a living fossil itself. 



The trilobites, which existed in a great variety of species, 

 were at times enormously abundant, and were at least as com- 

 mon on the sea floor as lobsters and crabs are today. Yet, after 

 more than 200 million years, they finally ceased to exist, in the 

 Permian, before the start of the Appalachian uplift. Just what 

 happened in those far distant days to bring about the complete 

 annihilation of entire classes of animals is something we shall 

 probably never know for sure. The trilobites had certainly sur- 

 vived fluctuating movements of the sea floor, and were adapted 

 to a variety of temperatures. Perhaps, some think, a race of 

 animals may grow too old for continued adaptation to chang- 

 ing conditions, and may no longer be able to compete with 

 more variable, more adaptable, and more vigorous races. An- 

 other probable factor in the case of the trilobites was the ap- 

 pearance, in the period before their extinction, of an abundance 

 of voracious fishes and of the powerful carnivorous relatives 

 of the octopus and squid. 



Other ancient forms of sea life have appeared in the chang- 

 ing pattern of life in the ocean, have flourished for a greater or 

 lesser time, and have become extinct. Relatives of the trilo- 

 bites, the sea scorpions or eurypterids, lived for nearly 100 mil- 

 lion years and vanished. Some of them reached the colossal 

 length of nine feet before finally disappearing. Before making 

 their exit, however, it is believed that these monstrous crea- 

 tures left behind them a group of air-breathing descendants 

 from which came the scorpions and millipedes of today. 



Still other groups of creatures have existed without radical 

 changes from the Cambrian to the present. Early in the story 



