I20 THE ORIGIN AND EVOLUTION OF LIFE 



a more complicated system of action, reaction, and interaction 

 with the environment and within the organism, were, first, 

 a more efficient locomotion in the quest of food, in the capture 

 of food, and in the escape from enemies, giving rise in some 

 cases to skeletal structures of various types; second, the evolu- 

 tion of offensive and defensive weapons and armature; third, 

 various chemical modes of offense and defense; fourth, protec- 

 tion and concealment by methods of burrowing.^ 



There are heavy protective coverings for slowly moving 

 and sessile animals. In contrast we find swiftly moving types 

 (c. g., Sagitta and other chaetognaths) with the lines of modern 

 submarines, whose mechanical means of propulsion resemble 

 those of the most primitive darting fishes. Other types, such 

 as the Crustacea, have skeletal parts for the triple purposes of 

 defense, offense, and locomotion, some being adapted to less 

 swift motion. In Palaeozoic time they include the slowly 

 moving, bottom-living, armored types of trilobites. Then 

 there are other slowly moving, bottom-living forms, such as 

 the brachiopods and gastropods, with very dense armature of 

 phosphate and carbonate of lime. Finally, there are pelagic 

 or surface-floating t}q3es, such as the jellyfishes, which are 

 chemically protected by the poisonous secretions of their 

 "sting-cells." 



This highly varied life of mid-Cambrian time affords abun- 

 dant evidence that in pre-Cambrian time certain of the inver- 

 tebrates had already passed through first, second, and even 

 third phases of form in adaptation to as many different life 

 zones. 



Our first actual knowledge of such extremely ancient adap- 

 tations dates back to the pre-Cambrian and is afforded by Wal- 

 cott's discovery- in the Greyson shales of the Algonkian Belt 



1 R. W. Miner. " Walcott, Charles D., 1899, pp. 235-244. 



