Tin Coiiinnj of Verfi l>r<it,x ami Itise of 



their turn and give place to still higher types 

 of animals. 



These changes in life which took place to- 

 ward, or just after, the close of the Paleozoic 

 era have a direct relation to the changes that 

 occurred in the continent itself. Early North 

 America consisted mainly of land now repre- 

 sented by its northeastern and eastern portions, 

 with a few narrow strips following the general 

 trend of the mountain ranges of the West, while, 

 save for one or two islands in the Central States, 

 the sea rolled over the greater part of what 

 is now the United States. Here were vast 

 stretches of water sufficiently shallow for fishes 

 and invertebrates to thrive, with great gulfs 

 and sheltered areas furnishing the best of con- 

 ditions for their increase and prosperity. And 

 here they flourished while the slowly rising 

 land underwent the changes that fitted it for 

 the abode of plants and for the animal life that 

 could not exist without them. Paleozoic time 

 closed with the great upheaval that formed the 

 greater part of the United States east of the 

 Mississippi and created the great Appalachian 



111 



