978 EVOLUTIONAL MODIFICATIONS 



lower priinati's that has conimittcd these animals to the relatively low 

 level of diflerentiatlon attained b\- them, /-or this rtason (l.wy are still a})es. 

 It seems impossible to eseape the eonelusion that the evolution of a human 

 foot eventually freed the hand for all the complex purposes to w hieh it has 

 been applied. The development of this foot doubtless had its inception in 

 some pedal structure specialized for arboreal life. The main factors producing; 

 the modidcation were eversion and elongation ot the loot, formation of a 

 broad supportiufi; plantar surface, loss ol ]:)rehensile lunctions m the threat 

 toe and distal migration of the hallux. It is generally belie\c'd that such 

 develo]:>nu'nt of the foot is of primary importance in furthering the assump- 

 tion ol the erect posture, and thus eventually leading on to all ol the exten- 

 sne modifications necessary to the dc\"elopmcnt ol the human hand. 



The following stages of postural e\'()lution passing from the quadruped to 

 human bipedal locomotion are based on tlu' critical analysis of Gregory and 

 Morton. Those gradual changes which linally ga\e rise to the human foot 

 had their incejjtion in the Eocene. They were lirst manifest in some terrestrial 

 quadrupeds, which in the course of certain bclia\ioral readjustments began 

 to live in the trees. This arboreal lilc im|)oscd upon them new necessities in 

 locomotion, it had mcst ])ronounccd ellccts upon the lore- and hindpaws. 

 in order to climb and mo\e along the branches, a claw ing grip was necessary 

 for this clinging t\ pe ol transit. Long, sharp claws de\ eloped in coiisec|uence 

 ol such ad|ustincnt. The digits of xUv manus were short. The [)alm ol the 

 hand was well padded. The |)olk-\ was short, but not opposable. 1 here was 

 as yet no sc|uatting or hall-sittmg p( stiire. The toes were also short and 

 clawed, tlu' lu'cl t'le\att'd, tlu' plantai' surface of tlu' foot was padded and 

 the great toe well dcNcloped. These arboreal t|uadrupeds had, howexer, 

 made but imperfect adaptation to trc'c life. Tiieir moxcments were slow 

 and tht'ir range of acti\it\' corri'spoiulinglx limited. The tree shrew is a 

 living representati\e of such subprimate ciuadrupeds, while certain Eocene 



