stages in HMman Evolution 



sects move swiftly and have good Instincts, 

 though lacking in intelligence. The skeleton is 

 exactly suited to the needs of small animals. 

 Very few of its possessors have attained any- 

 thing like large size. 



The internal skeleton was at first a rod of 

 cartilage running along the back or near the 

 dorsal surface of some primitive fish. This was 

 later replaced by a backbone. The muscles 

 formed two great masses on its right and left 

 sides. They propel the body swiftly through the 

 water by vigorous sweeps of the tail. More fuel 

 and oxygen were needed, hence the digestive and 

 respiratory system were improved through vig- 

 orous exercise. More waste was produced and 

 the kidneys became more complex to meet the 

 emergency. The blood flowed more swiftly to 

 carry oxygen and food to all the organs and to 

 remove their waste. A powerful sacklike heart 

 with thick muscular walls arose. Complex eyes 

 and ears result from the adaptation of the body 

 to swifter locomotion. The brain enlarged rap- 

 idly and became far more complex and effective. 

 There is little or no thinking, but much per- 

 ception and action. Only the most vigorous, 

 powerful, alert, and agile can win the prizes of 



31 



