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THE ORIGIN AND EVOLUTION OF LIFE 



The evolution of the articulates^ is believed to be as follows: 

 From a pre-Cambrian annelidan (worm-like) stock arose the 

 trilobites with their chitinous armature and many-jointed 

 bodies. The same stock gave rise also to the chitin-armored 



Fig. 30. Environment. North America in Cambrian Times. 



Theoretic restoration of the North American continent (white), continental seas (gray), 

 and ocean (dark gray) in Upper Cambrian (Lower Saint-Croixian) time, during which 

 there occurred the earhest known great invasion of land by the oceans. This period 

 marks the rise of invertebrate gastropods, limulids, eurypterids, and articulate brach- 

 iopods, and the greatest differentiation of trilobites. The lands were probably all 

 low and the climate warm. Detail from the globe model in the American Museum 

 by Chester A. Reeds and George Robertson, after Schuchert. 



sea-scorpions, or eurypterids, which attained a great size and 

 dominated the seas of Silurian times (Fig. 31). Another line 

 from the same stock is that of the chitin-armored horseshoe 

 crab (Limulus). Out of the eurypterid stock of Silurian times 

 may have come the terrestrial scorpions, fossils of which are 



1 Pirsson, Louis V., and Schuchert, Charles, 1915, p. 608. 



