136 



THE ORIGIN AND EVOLUTION OF LIFE 



invasion of life from the waters, the first conquest of the terres- 

 trial environment being attained by the scorpions, shell-fish, 

 worms, and insects. 



This is an instance of the constant dispersion of animal 

 forms into new environments in search of their food-supply, 



the chief instinctive 

 cause of all migration. 

 This impulse is con- 

 stantly acting and react- 

 ing throughout geologic 

 time with the migration 

 of the environment, 

 which is graphically pre- 

 sented by Huntington's 

 chart (Fig. ;2^;^), from the 

 researches of Barrell, 

 Schuchert, and others. 

 The periodic readjust- 

 ment of the earth crust 

 of North America^ is 

 witnessed in fourteen 

 periods of mountain- 

 making (oblique lines), 

 concluding with the Appalachian Range, the Sierra Nevada 

 (Sierran), the Rocky Mountains (Laramide), and the Pacific 

 Coast Range. 



Between these relatively short periods of mountain up- 

 heaval came'- periods of continental depression and oceanic 

 invasion (horizontal lines) when the continent was more or 

 less flooded by the oceans. There are certainly twelve and 

 probably not less than seventeen periods of continental flood- 



' Pirsson, Louis V., and Schuchert, Charles, 1915, p. 979. ° Op. ciL, p. 98::. 



Fig. 34. Fossil Starfishes. 



A portion of petrified sea bottom of Devonian age, 

 showing fossil starfishes associated with and 

 devouring bivalves as starfishes attack oyster- 

 beds at the present time. Hamilton group, 

 Saugerties, N. Y. After John INI. Clarke. 



