THE ANIArAL ANCESTRY OF MAN 79 



digits. In the typical South American monkeys, on the 

 contrary, the skeleton is highly speciahzed for arboreal hfe. 

 The Hmbs are long, giving the animal a long reach and the 

 long cyhndrical tail is unusually thick and muscular, com- 

 prising many spirally-wrapped muscles and tendons which 

 enable it to coil up hke a watch-spring and to wrap itself 

 around branches. Its flexible tip even functions as a sort of 

 fifth hand. In general the skeleton .of the South American 

 monkeys is radically different in leading features from that 

 of man and every bone of it is readily distinguished from its 

 human homologue. 



In all Primates great skill in balancing the body and in 

 judging distances in leaping and climbing are obviously 

 necessary, so that in the comparative study of the brains 

 of lemurs, apes, monkeys and man, neurologists have 

 found an increasingly high degree of development of all 

 those parts of the brain that serve first to correlate the 

 sense of vision with the senses of balance and of bodily 

 posture, and secondly, to initiate the appropriate stimuli 

 to the muscular system so that precision of movement and 

 balance may be habitual. 



This great skill in balancing, together with the possession 

 of grasping hands and feet, early led both the New World 

 and the Old World divisions of the Primates to use one or 

 the other of the four extremities in grasping for objects of 

 food, while the remaining three were employed in main- 

 taining the body in its always unstable equihbrium. The 

 habit of sitting upright, which enabled both hands to be 

 used in the manipulation of the food, led in the Old World 

 division to the development of special pads called ischial 

 callosities at the hind end of the pelvis. Again, the habit of 

 sitting upright in the ancestors of the anthropoid division 

 of the Old World series, together with the increasing length 

 of the limbs, finally resulted in the peculiar method of 

 climbing which not until our own time has received a name, 

 notwithstanding its literally revolutionary significance in 

 the history of man. This habit of "brachiation" (or swinging 

 by the arms), as it was aptly named by Sir Arthur Keith, 

 rescued us from monkey-hood and by turning the backbone 

 of our ancestor up on end it literally set him on his feet 



