Man in the Light of Evolution 



life. Muscular locomotion is the dominating 

 element in the life of the animal. But this is 

 pulling the lower visceral organs and spurring 

 the brain to a far higher stage of development 

 and efficiency. 



The lower stages of life were primitively all 

 aquatic, though a few worms now live in damp 

 earth. Some ancient ganoid fish began breath- 

 ing air directly into a lung, jointed legs replaced 

 the fins, and the first amphibian emerged on 

 land. A new mode of locomotion had to be 

 acquired. The legs were weak, the backbone 

 incompletely developed. The animal must have 

 looked helpless and unpromising. Slowly 

 through a vast succession of generations of am- 

 phibia and reptiles the legs strengthened suf- 

 'ficiently to support the body. The writhing 

 form learned to walk and then to run. The legs 

 strengthened and lengthened and it ran faster. 



More rapid locomotion on the land and in the 

 air made larger demands on the internal organs. 

 Heart and lungs enlarged and improved. More 

 oxygen was gained, more fuel was burned, and 

 more heat produced by the increased exercise. 

 The heat radiated less rapidly from the larger 

 body into the less conducting air. The tem- 



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