Stages hi Hmnan Evolution 



of most mammals. They were taught by hard- 

 ship and danger and spurred by necessity. 



The eadlest mammals were somewhat short- 

 legged, and brought the heel to the ground when 

 they walked, as we do now. The brain was still 

 small compared with that of later forms. But 

 the respiratory surface of the lining of the lungs 

 was very large, supplying plenty of oxygen. The 

 lung capacity is the best test of the power of an 

 animal, as of the proficiency of a child. The 

 muscles, though heavy, were powerful. The 

 animal was still somewhat below medium size. 



Most of them seem to have lived on the 

 ground, but our ancestors took to the trees, 

 tempted by food or encouraged to climb by 

 stronger enemies. While they remained small 

 they could probably run along the branches like 

 squirrels. But as they increased In size, the hind 

 legs were used to support and push the body, the 

 forelegs to grasp and pull. This difference 

 in use gradually brought about the difference In 

 structure between hand and foot. The fingers 

 became longer and freer than the toes, the thumb 

 more independent and opposable. Thus the ar- 

 boreal life of our ancestors gave us our hands 

 and fingers, our means of grasping and of using 



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