198 INITIATIVE IN EVOLUTION 



the leg will pull to an unusual extent on the tendo achillis and 

 heel-bone, leading, in accordance with a well-known law, to steady- 

 enlargement of the parts near to which it is attached. The greater 

 amount of weight thrown henceforth on the heel tends in just the 

 same direction, indeed, to general enlaigement of the whole bone. 

 The astragalus being in No Man's Land, so to speak, takes less part 

 in the change than any other tarsal bone. The wedge-shaped 

 bones are exactty so constructed as to retreat a little in a dorsal 

 direction as the modified walking increases under the action of 

 certain muscles which will later be mentioned. This, in conjunction 

 with the projection backwards of the heel and the geneial growth 

 of the bone, permits, as far as the bony parts go, a gradual hollowing 

 out of the originally flat plantar surface, and the increasing eversion 

 of the foot places more weight on the front pier of the arch, that is, 

 the heads of the metatarsal bones. The squeezing-up process of 

 the smaller tarsal bones contributes also to the formation of the 

 tran verse arch. 



The ligaments need no new invention on his part but only a 

 more human degree of development, and in particular the calcaneo- 

 navicular ligament and internal lateral of the ankle undergo in the 

 human foot great development, and the long plantar ligament, 

 originally part of the tendon of the gastrocnemius, comes in to 

 the aid of the arch and goes to bind it together, so that these 

 humbler structures follow in the wake of the changing and enlarging 

 bones. 



The plantar fascia, though a powerful protective armour for 

 the deeper parts of the sole, cannot be held to enter into the forma- 

 tion of the arch. The initiative in this process lies with the muscles, 

 and, even if neither gorilla C. himself, nor his descendants, had 

 altered the muscles of his foot and just given up climbing for 

 walking, there were muscles strong enough and appropriate for 

 modifying very profoundly his simian foot, though he might not 

 have arrived at an arch. He or they might have become long- 

 distance walkers, but never sprinters. 



If the sole of the dissected foot is observed it is seen that 

 the plantar arch lies approximately over a triangle of which the 

 base is formed by the transverse adductor muscle of the great toe, 

 across the heads of the metatarsal bones, and the two sides by the 

 oblique adductor of the great toe and the short flexor of the little toe. 

 It extends, of course, somewhat further back under the heel-bone, 

 but this is its highest part. 



In the changing foot the tibialis posticus, which was originally 

 a flexor of the metatarsal bones, obtains a secondary attachment 



