Man in the Light of Evolution 



tools. The use of the legs for pushing and of 

 the arms for pulling accustomed the ape to a 

 more nearly upright position.^ 



Our muscular system is composed of parts or 

 successive additions which are of very different 

 age and origin. The muscles of our trunk are 

 inherited from the tubular body wall of worms. 

 Shoulder and thigh muscles were developed by 

 fish to move the fins. Arms and legs grew 

 stronger through a long series of generations of 

 amphibia and reptiles. Hands and fingers were 

 developed by arboreal mammals. They mature 

 in the same order in the human child to-day. 

 Our muscles grow younger as we pass from the 

 trunk outward to the fingers or downward to 

 the toes. The muscles of the neck are very old. 

 Those of the jaws are younger, those of tongue 

 and lips and the muscles of expression in our 

 face are younger still. 



Hence physiologists distinguish between cen- 

 tral muscles of the trunk and peripheral muscles 

 of the wrist, and especially of the fingers. The 

 muscles of the arm are central when viewed in 



^ See Heineman, T. W., "Psychic and Economic Results 

 of Man's Physical Uprightness." 



36 



