Our ape-men forefathers had no obvious natural weapons in the struggle for【C1】______in the open. They had neither the powerful teeth nor the strong claws of the big cats. They could not 【C2】______with the bear, whose strength, speed and claws【C3】______an impressive "small-fire" weaponry. They could not even defend themselves【C4】______running swiftly like the horses, zebras or small animals. If the ape-men had attempted to【C5】______on those terms in the open, they would have been【C6】______to failure and extinction. But they were【C7】______with enormous concealed advantages of a kind not possessed by any of their competitors. In the【C8】______for the pickings of the forest, the ape-men had【C9】______efficient stereoscopic vision and a sense of colour that the animals of the grasslands did not possess. The ability to see clearly at close range 【C10】______the ape-men to study practical problems in a way that lay far 【C11】______the reach of the original inhabitants of the grassland. Good long-distance sight was quite another matter. 【C12】______of long-distance vision had not been a problem for forest-dwelling apes and monkeys because the higher the viewpoint, the greater the range of sight—so【C13】______they had had to do was climb a tree. Out in the open, however, this simple solution was not【C14】______. Climbing a hill would have helped, 【C15】______ in many places the ground was flat. The ape-men【C16】______the only possible solution. They reared up as high as possible on their hind limbs and began to walk upright. This vital change of physical position brought about considerable【C17】______. It was extremely unstable and it meant that the already slow ape-men became slower still. 【C18】______, they persevered and their bone structure gradually became【C19】______ to the new, unstable position that【C20】______them the name Homo Erectus, upright man. 【C3】