58 FROM FISH TO PHILOSOPHER 



disappeared before half the next geologic period had 

 elapsed, but their eroded roots along the St. Lawrence 

 and Hudson River valleys show the Ordovician strata 

 turned almost on end. 



After the Taconic revolution the seas again spread 

 across the continents in the Silurian period, and a warm 

 climate carried coral reefs to the Arctic Circle, while in- 

 vertebrates were prevalent in all parts of the world 

 where fossil beds are found. The most distinctive new- 

 comers among the invertebrates were the eurypterids 

 or 'sea scorpions,' which had been derived from the 

 Ordovician trilobites; although confined to a few limited 

 horizons of a fresh-water origin, the eurypterids are com- 

 mon fossils where they do occur. Land plants also made 

 their first appearance, and possibly the first air-breathing 

 animals, the scorpions, derived from a primitive euryp- 

 terid stem. It is interesting that these should have made 

 the advance from aquatic to terrestrial Hfe at least one 

 geologic period if not two periods ahead of the verte- 

 brates, but these terrestrial invertebrates were much 

 smaller animals and continued to breathe by means of 

 modified gills. In the abundant marine deposits of the 

 Silurian, a period forty million years in length, inverte- 

 brate evolution was going on apace; but there is still no 

 trace of marine vertebrates. 



Though it is not invariable, the pressure of natural 

 selection is frequently of such intensity as to seem to 

 operate not through its subtler modes but by the very 

 threat of death, by calling forth new adaptations as the 

 only alternative to sudden and complete extinction. And 

 it appears to have been such a catastrophe-impending 

 climax that finally forced the vertebrates to seek refuge 

 in the sea. At the close of the Silurian the earth began to 

 heave again in another disturbance that raised a range 

 of mountains higher than the present Alps, and extend- 

 ing in a great curve four thousand miles long from the 

 north of Greenland eastward through Spitsbergen, south 

 through Norway, and westward into Scotland and north- 



