stages in Human Evolution 



different planes of life and show very different 

 degrees of activity. 



The first substage is well represented by 

 worms. These vary greatly in form and struc- 

 ture. The higher worm is a cylindrical animal, 

 like the earthworm, moving with one end fore- 

 most. Some day there will be a head at the 

 front end. It has developed layers of muscles, 

 and is using them for locomotion as well as di- 

 rectly for gaining food. It consists essentially 

 of two parallel tubes, one within the other. 

 The outer tube is the muscular wall of the body, 

 the inner the intestine running straight from the 

 mouth to the hind end of the body. Between 

 the intestine and the body wall is a space, the 

 perivisceral cavity, in which circulatory and ner- 

 vous and other systems are safely packed away. 

 This plan of structure was attained only after 

 many experiments and comparative failures. 

 But it proved so advantageous that it descended 

 to and characterized the trunk of all higher 

 animals. 



A muscular fiber is a microscopic engine 

 which produces energy by the oxidization or 

 combustion of food material, as coal must be 

 burned in the fire box of the locomotive to make 



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