The plantar arch is double, but the longitudinal one must be chiefly considered here. It lies under the concave roof of the tarsal bones, seven in number, and the metatarsal bones, and rests in a well-formed foot in front on the heads of the latter, and behind on the inferior surface of the heel-bone. The _astragalus_ alone of these bones in contact with those of the leg, acts like a washer to the ankle joint, and has no muscles attached to it. Three more of the _tarsal_ bones need reference: these are the three _wedge-shaped_ bones which have their bases on the dorsal and their apices directed towards the plantar surface. With such a set of bony tools as this, all the requisites for an arch are at hand. Let the half-tree, half-ground walker become a complete ground-walker, and in the first place the manifest increase of the action of the flexors of 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 enlargement 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 exactly 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 general 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 transverse 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 formation 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.