THE EVOLUTION OF INVERTEBRATES 



295 



There is very little direct evidence to indicate the charac- 

 ter of the life in the Archaean era, but in all probability it 

 was of the simplest structure, like some of the single-cell 

 organisms of to-day. Although fossil remains of that time are 

 wanting, beds of limestone and graphite formed then, point 

 to the existence of life, for similar beds formed in later eras 

 are known to have been made through the agency of organ- 



FIG. 147. North America toward the Close of the Age of Invertebrates 

 (From Dana's Manual of Geology] 



isms (polyps, mollusks, etc., and plants). Fig. 146 shows the 

 North American continent as it probably existed at the close, 

 of the Archaean era. The main body of land was a V-shaped 

 mass lying north of the present Great Lakes and the St. 

 Lawrence River. Archaean islands lay in the position of parts 

 of subsequent eastern and western highlands. 



The Age of Invertebrates. Following the Archaean era, 

 several succeeding geological periods may be, for our purpose, 



